Patent Publication Number: US-2022228244-A1

Title: Hot-stamped article

Description:
TECHNICAL FIELD 
     The present invention relates to a hot-stamped article. Priority is claimed on Japanese Patent Application No. 2019-101988, filed May 31, 2019, the content of which is incorporated herein by reference. 
     BACKGROUND ART 
     In recent years, there has been a demand for the weight reduction of automotive bodies from the viewpoint of environmental protection and resource saving, and the application of high-strength steel sheets to automotive parts has been accelerating. Automotive parts are manufactured by press forming, and an increase in the strength of steel sheets does not only increase forming loads but also degrades formability, which creates a problem with the formability of high-strength steel sheets into parts with a complicated shape. In order to solve such a problem, the application of hot stamping techniques, in which a steel sheet is heated to a high temperature in an austenite region where the steel sheet softens and then formed by pressing, is underway. Hot stamping is drawing attention as a technique in which a quenching treatment is carried out in a die at the same time as pressing, thereby satisfying both formability into automotive parts and the securement of the strength of automotive parts. 
     However, in hot-stamped articles of the related art manufactured by hot stamping, a hard structure (mainly martensite) is formed throughout the entire region in the sheet thickness direction, and thus the dislocation density is high. Since a high dislocation density also increases hydrogen embrittlement susceptibility, a small amount of hydrogen may cause hydrogen embrittlement cracking. Therefore, there are cases where improvement in hydrogen embrittlement resistance becomes an objective. 
     Patent Document 1 discloses a technique in which the cooling rate from finishing rolling through coiling in a hot rolling process is controlled to control the crystal orientation difference in bainite to become 5° to 14°, thereby improving deformability such as stretch-flangeability. 
     Patent Document 2 discloses a technique in which manufacturing conditions for finishing rolling through coiling in a hot rolling process are controlled to control the strength of a specific crystal orientation group out of ferrite crystal grains, thereby improving local deformability. 
     Patent Document 3 discloses a technique in which a heat treatment is carried out on a steel sheet for hot stamping to form ferrite in the surface layer, thereby reducing the number of pores that are generated in the interface between ZnO and the steel sheet or the interface between ZnO and a Zn-based plating layer during heating before hot pressing and improving perforation corrosion resistance or the like. 
     However, in the above-described techniques, there are cases where a sufficient strength and sufficient hydrogen embrittlement resistance cannot be obtained. 
     CITATION LIST 
     Patent Document 
     [Patent Document 1] 
     PCT International Publication No. WO 2016/132545 
     [Patent Document 2] 
     Japanese Unexamined Patent Application, First Publication No. 2012-172203 
     [Patent Document 3] 
     Japanese Patent No. 5861766 
     Non-Patent Document 
     [Non-Patent Document 1] 
     T. Ungar and three coauthors, Journal of Applied Crystallography (1999), Volume 32 (PP. 992 to 1002) 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     The present invention has been made in consideration of the problem of the related art, and an objective of the present invention is to provide a hot-stamped article having an excellent strength and excellent hydrogen embrittlement resistance. 
     Means for Solving the Problem 
     The present inventors carried out intensive studies regarding a method for solving the above-described problem and consequently obtained the following findings. 
     The present inventors carried out studies regarding the hydrogen embrittlement resistance of hot-stamped articles. As a result, the present inventors found that, when the metallographic structure of a surface layer region of a base steel sheet that configures a hot-stamped article includes 90.0% or more of martensite in terms of area percentage and the concentration of Ni in prior austenite grain boundaries in the surface layer region is 5.5 mass % or more, the hydrogen embrittlement resistance of the hot-stamped article improves. 
     Furthermore, the present inventors found that, in order to obtain the above-described metallographic structure in the surface layer region of the base steel sheet that configures the hot-stamped article, in the surface layer region in the steel sheet for hot stamping before hot stamping, it is necessary to set the average dislocation density to 4×10 15  m/m 3  or more, to set the proportion of the crystal grains of one or more kinds of unauto-tempered martensite and lower bainite to 15.0% or more in terms of area percentage, and to carry out hot stamping under a predetermined condition. 
     The present invention has been made by further progressing studies based on the above-described findings, and the gist thereof is as described below. 
     [1] A hot-stamped article according to one aspect of the present invention having a base steel sheet containing, as chemical components, by mass %, 
     C: 0.15% or more and less than 0.70%, 
     Si: 0.005% or more and 0.250% or less, 
     Mn: 0.30% or more and 3.00% or less, 
     sol. Al: 0.0002% or more and 0.500% or less, 
     P: 0.100% or less, 
     S: 0.1000% or less, 
     N: 0.0100% or less, 
     Nb: 0% or more and 0.150% or less, 
     Ti: 0% or more and 0.150% or less, 
     Mo: 0% or more and 1.000% or less, 
     Cr: 0% or more and 1.000% or less, 
     B: 0% or more and 0.0100% or less, 
     Ca: 0% or more and 0.0100% or less, and 
     REM: 0% or more and 0.30% or less 
     with a remainder including Fe and impurities and 
     a plating layer on a surface of the base steel sheet, the plating layer being attached to a single surface in an amount of 10 g/m 2  or more and 90 g/m 2  or less and having a Ni content of 10 mass % or more and 25 mass % or less with a remainder including Zn and impurities 
     in which a metallographic structure of a surface layer region that is a region from the surface of the base steel sheet to a depth of 50 μm includes 90.0% or more of martensite in terms of area percentage, and 
     the concentration of Ni in a prior austenite grain boundary in the surface layer region is 5.5 mass % or more. 
     [2] The hot-stamped article according to [1] may further contain, in the base steel sheet, as chemical components, by mass %, one or more selected from the group consisting of 
     Nb: 0.010% or more and 0.150% or less, 
     Ti: 0.010% or more and 0.150% or less, 
     Mo: 0.005% or more and 1.000% or less, 
     Cr: 0.005% or more and 1.000% or less, 
     B: 0.0005% or more and 0.0100% or less, 
     Ca: 0.0005% or more and 0.0100% or less, and 
     REM: 0.0005% or more and 0.30% or less. 
     Effects of the Invention 
     According to the above-described aspect of the present invention, it is possible to provide a hot-stamped article having an excellent strength and excellent hydrogen embrittlement resistance. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a view showing a test piece that is used for the measurement of the concentration of Ni in prior austenite grain boundaries. 
         FIG. 2  is a view showing a test piece used for the evaluation of the hydrogen embrittlement resistance of examples. 
     
    
    
     EMBODIMENT FOR IMPLEMENTING THE INVENTION 
     In a hot-stamped article according to the present embodiment, the metallographic structure of a surface layer region that is a region from a surface of a base steel sheet that configures the hot-stamped article to a depth of 50 μm includes 90.0% or more of martensite in terms of area percentage, and the concentration of Ni in prior austenite grain boundaries in the surface layer region is 5.5 mass % or more. Such characteristics make it possible to obtain a hot-stamped article having an excellent strength and excellent hydrogen embrittlement resistance. In the present embodiment, the expression “having an excellent strength” refers to the fact that the (maximum) tensile strength is 1500 MPa or higher. 
     As a result of intensive studies, the present inventors found that a hot-stamped article having the above-described metallographic structure can be obtained by the following method. 
     As a first stage, in a hot rolling process, cooling is initiated within five seconds from the end of finishing rolling in a manner that the average cooling rate on the surface of the base steel sheet reaches 80° C./s or faster, and the base steel sheet is cooled to a temperature region of lower than 500° C. and coiled. Even after being coiled, the base steel sheet is continuously cooled with water to room temperature (approximately 40° C. or lower). As described above, compared with the related art, the average cooling rate is set to be fast and the coiling temperature is set to be low, whereby it is possible to suppress the generation of a carbide, ferritic transformation, and bainitic transformation. This makes it possible in the metallographic structure of the surface layer region in the steel sheet for hot stamping to set the proportion of the crystal grains of one or two kinds of unauto-tempered martensite and lower bainite to 15.0% or more in terms of area percentage and to set the average dislocation density of the surface layer region to 4×10 15  m/m 3  or more. 
     As a second stage, a Zn-based plating layer containing 10 to 25 mass % of Ni is formed on the surface of the base steel sheet such that the amount attached to the single surface reaches 10 to 90 g/m 2 , thereby producing a steel sheet for hot stamping. 
     As a third stage, the average heating rate of heating before hot stamping is controlled, thereby diffusing Ni in the plating layer disposed on the surface of the base steel sheet into the surface layer region of the base steel sheet. 
     Usually, in hot-rolled steel sheets having a high dislocation density that contain 0.15 mass % or more of C, have a metallographic structure including martensite, and are not tempered, ductility, toughness, and hydrogen embrittlement resistance deteriorate. Additionally, in the case of carrying out cold rolling after coiling, since the above-described hot-rolled steel sheets do not have excellent ductility, cracking is likely to occur. Therefore, it is usual that the above-described hot-rolled steel sheets are tempered after hot rolling and before post processes. In order to improve the bendability and hydrogen embrittlement resistance of hot-rolled steel sheets, it is important to improve the ductility of the surface layer region, and thus there are also cases where a treatment for softening the surface layer region (for example, a surface layer decarburization treatment) is carried out on the above-described steel sheets. 
     In addition, usually, when steel sheets containing 0.15 mass % or more of C are hot-stamped, there are cases where the hot-stamped articles are not excellent in terms of hydrogen embrittlement resistance. 
     However, in the present embodiment, the metallographic structure of the surface layer region in the steel sheet for hot stamping is put into a preferred state, and Ni in the plating layer disposed on the surface of the base steel sheet is diffused into the surface layer region of the base steel sheet by heating before hot stamping, whereby it is possible to improve the hydrogen embrittlement resistance of the hot-stamped article even without tempering after hot stamping. 
     The metallographic structure of the surface layer region in the steel sheet for hot stamping that is applied to the hot-stamped article according to the present embodiment includes 15.0% or more of the crystal grains of one or two kinds of unauto-tempered martensite and lower bainite in terms of area percentage. In prior austenite grain boundaries of unauto-tempered martensite and lower bainite, since the number of grain boundary segregation elements such as C or precipitates is small, Ni easily diffuses. 
     Therefore, in the case of setting the average heating rate of heating before hot stamping to be fast, it is possible to preferentially diffuse Ni into the prior austenite grain boundaries. The present inventors found that, when the average heating rate before hot stamping is set to 100° C./s or faster and slower than 200° C./s, and Ni is preferentially diffused into the prior austenite grain boundaries in the surface layer region of the base steel sheet, these prior austenite grain boundaries serve as an obstacle to hydrogen intrusion, and it is possible to improve the hydrogen embrittlement resistance of the hot-stamped article. 
     Hereinafter, the hot-stamped article according to the present embodiment and a method for manufacturing the same will be described in detail. First, reasons for limiting the chemical composition of the base steel sheet that configures the hot-stamped article according to the present embodiment will be described. 
     Numerical limitation ranges to be described below include the lower limit value and the upper limit value in the range. Numerical values with an expression of “less than” or “more than” do not include the numerical value in the numerical range. Regarding the chemical composition, “%” indicates “mass %” in all cases. 
     The base steel sheet that configures the hot-stamped article according to the present embodiment contains, as chemical components, by mass %, C: 0.15% or more and less than 0.70%, Si: 0.005% or more and 0.250% or less, Mn: 0.30% or more and 3.00% or less, sol. Al: 0.0002% or more and 0.500% or less, P: 0.100% or less, S: 0.1000% or less, N: 0.0100% or less with a remainder including Fe and an impurity. 
     “C: 0.15% or More and Less Than 0.70%” 
     C is an important element for obtaining a tensile strength of 1500 MPa or higher in the hot-stamped article. When the C content is less than 0.15%, martensite becomes soft, and it is difficult to obtain a tensile strength of 1500 MPa or higher. In addition, when the C content is less than 0.15%, in the metallographic structure of the surface layer region in the steel sheet for hot stamping that is applied to the hot-stamped article according to the present embodiment, the area ratio of unauto-tempered martensite and lower bainite becomes small. Therefore, the C content is set to 0.15% or more. The C content is preferably 0.20% or more and more preferably 0.30% or more. On the other hand, when the C content is 0.70% or more, a coarse carbide is generated, breakage is likely to occur, and the bendability and hydrogen embrittlement resistance of the hot-stamped article deteriorate. Therefore, the C content is set to less than 0.70%. The C content is preferably 0.50% or less and more preferably 0.45% or less. 
     “Si: 0.005% or More and 0.250% or Less” 
     Si is an element that is contained to secure hardenability. When the Si content is less than 0.005%, the above-described effect cannot be obtained, and, in the steel sheet for hot stamping, there are cases where the dislocation density decreases and cases where unauto-tempered martensite and lower bainite cannot be obtained, which makes it impossible to obtain a desired metallographic structure in the hot-stamped article. Therefore, the Si content is set to 0.005% or more. Even when more than 0.250% of Si is contained, the above-described effect is saturated, and thus the Si content is set to 0.250% or less. The Si content is preferably 0.210% or less. 
     “Mn: 0.30% or More and 3.00% or Less” 
     Mn is an element that contributes to improvement in the strength of the hot-stamped article by solid solution strengthening. When the Mn content is less than 0.30%, the solid solution strengthening capability is poor, martensite becomes soft, and it is difficult to obtain a tensile strength of 1500 MPa or higher in the hot-stamped article. Therefore, the Mn content is set to 0.30% or more. The Mn content is preferably 0.50% or more or 0.70% or more. On the other hand, when the Mn content is set to more than 3.00%, coarse inclusions are generated in steel, breakage is likely to occur, and the bendability and hydrogen embrittlement resistance of the hot-stamped article deteriorate. Therefore, the lower limit is set to 3.00%. The Mn content is preferably 2.50% or less or 2.00% or less. 
     “sol. Al (Acid-soluble Al): 0.0002% or More and 0.500% or Less” 
     Al is an element having an action of deoxidizing molten steel to make the steel sound (suppressing the generation of a defect such as a blowhole in steel). When the sol. Al content is less than 0.0002%, since molten steel is not sufficiently deoxidized, and the above-described effect cannot be obtained, the sol. Al content is set to 0.0002% or more. The sol. Al content is preferably 0.0010% or more or 0.0020% or more. On the other hand, when the sol. Al content exceeds 0.500%, a coarse oxide is generated in steel, and the bendability and hydrogen embrittlement resistance of the hot-stamped article deteriorate. Therefore, the sol. Al content is set to 0.500% or less. The sol. Al content is preferably 0.400% or less or 0.300% or less. 
     “P: 0.100% or Less” 
     P is an element that is segregated in grain boundaries and degrades the strength of the grain boundaries. When the P content exceeds 0.100%, the strength of grain boundaries significantly decreases, and the bendability and hydrogen embrittlement resistance of the hot-stamped article deteriorate. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.050% or less. The lower limit of the P content is not particularly limited. However, when the P content is decreased to lower than 0.0001%, the dephosphorization cost increases significantly, which is not preferable economically, and thus the lower limit of the P content may be set to 0.0001% in actual operation. 
     “S: 0.1000% or Less” 
     S is an element that forms an inclusion in steel. When the S content exceeds 0.1000%, a large amount of an inclusion is generated in steel, and the bendability and hydrogen embrittlement resistance of the hot-stamped article deteriorate. Therefore, the S content is set to 0.1000% or less. The S content is preferably 0.0050% or less. The lower limit of the S content is not particularly limited. However, when the S content is decreased to lower than 0.00015%, the desulfurization cost increases significantly, which is not preferable economically, and thus the lower limit of the S content may be set to 0.00015% in actual operation. 
     “N: 0.0100% or Less” 
     N is an impurity element and an element that forms a nitride in steel to degrade the toughness and hydrogen embrittlement resistance of the hot-stamped article. When the N content exceeds 0.0100%, a coarse nitride is generated in steel to significantly degrade the bendability and hydrogen embrittlement resistance of the hot-stamped article. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0075% or less. The lower limit of the N content is not particularly limited. However, when the N content is decreased to lower than 0.0001%, the denitrification cost increases significantly, which is not preferable economically, and thus the lower limit of the N content may be set to 0.0001% in actual operation. 
     The remainder of the chemical composition of the base steel sheet that configures the hot-stamped article according to the present embodiment is Fe and impurities. As the impurities, an exemplary example is an element that is inevitably contained by accident from a steel raw material or a scrap and/or in a steel making process and is permitted to an extent that the properties of the hot-stamped article according to the present embodiment are not impaired. 
     In addition, the Ni content of the base steel sheet that configures the hot-stamped article according to the present embodiment is less than 0.005%. Since Ni is an expensive element, in the present embodiment, it is possible to suppress the cost at a low level compared with a case where the Ni content is set to 0.005% or more by intentionally including Ni. 
     The base steel sheet that configures the hot-stamped article according to the present embodiment may contain the following elements as arbitrary elements. In a case where the following arbitrary elements are not contained, the content thereof is 0%. 
     “Nb: 0% or more and 0.150% or Less” 
     Nb is an element that contributes to improvement in the strength of the hot-stamped article by solid solution strengthening and thus may be contained as necessary. In a case where Nb is contained, the Nb content is preferably set to 0.010% or more in order to reliably exhibit the above-described effect. The Nb content is more preferably 0.035% or more. On the other hand, even when more than 0.150% of Nb is contained, the above-described effect is saturated, and thus the Nb content is preferably set to 0.150% or less. The Nb content is more preferably 0.120% or less. 
     “Ti: 0% or More and 0.150% or Less” 
     Ti is an element that contributes to improvement in the strength of the hot-stamped article by solid solution strengthening and thus may be contained as necessary. In a case where Ti is contained, the Ti content is preferably set to 0.010% or more in order to reliably exhibit the above-described effect. The Ti content is preferably 0.020% or more. On the other hand, even when more than 0.150% of Ti is contained, the above-described effect is saturated, and thus the Ti content is preferably set to 0.150% or less. The Ti content is more preferably 0.120% or less. 
     “Mo: 0% or More and 1.000% or Less” 
     Mo is an element that contributes to improvement in the strength of the hot-stamped article by solid solution strengthening and thus may be contained as necessary. In a case where Mo is contained, the Mo content is preferably set to 0.005% or more in order to reliably exhibit the above-described effect. The Mo content is more preferably 0.010% or more. On the other hand, even when more than 1.000% of Mo is contained, the above-described effect is saturated, and thus the Mo content is preferably set to 1.000% or less. The Mo content is more preferably 0.800% or less. 
     “Cr: 0% or More and 1.000% or Less” 
     Cr is an element that contributes to improvement in the strength of the hot-stamped article by solid solution strengthening and thus may be contained as necessary. In a case where Cr is contained, the Cr content is preferably set to 0.005% or more in order to reliably exhibit the above-described effect. The Cr content is more preferably 0.100% or more. On the other hand, even when more than 1.000% of Cr is contained, the above-described effect is saturated, and thus the Cr content is preferably set to 1.000% or less. The Cr content is more preferably 0.800% or less. 
     “B: 0% or More and 0.0100% or Less” 
     B is an element that is segregated in grain boundaries to improve the strength of the grain boundaries and is thus contained as necessary. In a case where B is contained, the B content is preferably set to 0.0005% or more in order to reliably exhibit the above-described effect. The B content is preferably 0.0010% or more. On the other hand, even when more than 0.0100% of B is contained, the above-described effect is saturated, and thus the B content is preferably set to 0.0100% or less. The B content is more preferably 0.0075% or less. 
     “Ca: 0% or More and 0.0100% or Less” 
     Ca is an element having an action of deoxidizing molten steel to make steel sound. In order to reliably exhibit this action, the Ca content is preferably set to 0.0005% or more. On the other hand, even when more than 0.0100% of Ca is contained, the above-described effect is saturated, and thus the Ca content is preferably set to 0.0100% or less. 
     “REM: 0% or More and 0.30% or Less” 
     REM is an element having an action of deoxidizing molten steel to make steel sound. In order to reliably exhibit this action, the REM content is preferably set to 0.0005% or more. On the other hand, even when more than 0.30% of REM is contained, the above-described effect is saturated, and thus the REM content is preferably set to 0.30% or less. 
     In the present embodiment, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoids, and the REM content refers to the total amount of these elements. 
     The above-described chemical composition of the hot-stamped article may be measured by an ordinary analysis method. For example, the chemical composition may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S may be measured using an infrared absorption method after combustion, and N may be measured using an inert gas melting-thermal conductivity method. The chemical composition is preferably analyzed after the plating layer on the surface is removed by machining. 
     Next, the metallographic structure of the steel sheet that configures the steel sheet for hot stamping that is applied to the hot-stamped article according to the present embodiment and the plating layer will be described. 
     &lt;Steel Sheet for Hot Stamping&gt; 
     “In metallographic structure of surface layer region that is region from surface of base steel sheet to depth of 50 μm, 15.0% or more of crystal grains of one or two kinds of unauto-tempered martensite and lower bainite in terms of area percentage are included and average dislocation density of surface layer region is 4×10 15  m/m 3  or more” 
     In the metallographic structure of the surface layer region that is a region from the surface of the base steel sheet to a depth of 50 μm, the proportion of the crystal grains of one or two kinds of unauto-tempered martensite and lower bainite is set to 15.0% or more in terms of area percentage, and the average dislocation density of the surface layer region is set to 4×10 15  m/m 3  or more, whereby it is possible to diffuse Ni in the plating layer into the surface layer region of the steel sheet by heating before hot stamping. The upper limit of the average dislocation density of the surface layer region is not particularly limited and the average dislocation density of the surface layer region may be, for example, 5×10 17  m/m 3  or less or 1×10 18  m/m 3  or less. 
     In the case of controlling the average heating rate of heating before hot stamping to 100° C./s or faster and slower than 200° C./s, Ni in the plating layer preferentially diffuses into the prior austenite grain boundaries in the surface layer region of the base steel sheet that configures the hot-stamped article. As a result, hydrogen intrusion into the prior austenite grain boundaries is suppressed, which makes it possible to improve the hydrogen embrittlement resistance of the hot-stamped article. 
     In order to obtain the above-described effect, the proportion of the crystal grains of one or two kinds of unauto-tempered martensite and lower bainite in the surface layer region is set to 15.0% or more in terms of area percentage. The proportion of the crystal grains thereof is preferably 20.0% or more in terms of area percentage. From the viewpoint of suppressing the occurrence of cracking at the time of cold rolling in post processes, the proportion of the crystal grains thereof may be set to 30.0% or more in terms of area percentage. The upper limit of the proportion of the crystal grains of one or two kinds of unauto-tempered martensite and lower bainite in the metallographic structure of the surface layer region is not particularly limited and may be, for example, 50% or less or 90% or less in terms of area percentage. In addition, as a remaining structure other than the unauto-tempered martensite and the lower bainite, the metallographic structure in the surface layer region may include one or more of ferrite, upper bainite, residual austenite, and martensite that has been auto-tempered. 
     The metallographic structure of the central portion of the base steel sheet is not particularly limited, but is normally one or more of ferrite, upper bainite, lower bainite, martensite, residual austenite, an iron carbide, and an alloy carbide. Here, the central portion of the base steel sheet refers to a portion ranging from a position 0.2 mm apart from one surface of the base steel sheet in the sheet thickness central direction to a position 0.2 mm apart from the other surface of the base steel sheet in the sheet thickness central direction. 
     “Measurement of Area Fraction of Crystal Grains of Unauto-tempered Martensite and Lower Bainite” 
     A method for measuring the area fraction of the unauto-tempered martensite and the lower bainite in the surface layer region of the base steel sheet that configures the steel sheet for hot stamping that is applied to the hot-stamped article according to the present embodiment will be described. 
     First, a sample is cut out from an arbitrary position 50 mm or more apart from the end face of the steel sheet for hot stamping such that a rolling-direction section (sheet thickness section) perpendicular to the surface can be observed. While also depending on a measuring instrument, the size of the sample is set to a size large enough to observe approximately 10 mm in the rolling direction. A measurement surface of the sample that corresponds to the above-described rolling-direction cross section is polished using silicon carbide paper #600 to #1500, and then the measurement surface is mirror-finished using a liquid in which diamond powder having particle diameters of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water. Next, the measurement surface is polished for eight minutes at room temperature using colloidal silica containing no alkaline solution to remove strain present on the surface layer of the sample. After that, the measurement surface is sputtered with argon ion beams using a cross section polisher manufactured by JEOL Ltd. At this time, the argon ion beams are radiated to the measurement surface from all directions using a specimen rotary holder manufactured by JEOL Ltd. for the purpose of suppressing the generation of streaky unevenness on the measurement surface. 
     At an arbitrary position of the measurement surface in the rolling direction, a 50 μm-long region from the interface between the plating layer and the surface of the base steel sheet to a depth of 50 μm is measured by the electron backscatter diffraction method at measurement intervals of 0.1 μm, thereby obtaining crystal orientation information. 
     For the measurement, an instrument including a thermal field emission-type scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.) and an EBSD detector (DVC 5-type detector manufactured by TSL) is used. At this time, the degree of vacuum in the instrument is set to 9.6×10 −5  Pa or less, the accelerating voltage is set to 15 kV, the irradiation current level is set to 13, and the irradiation time of the electron beam is set to 0.5 seconds/point. The obtained crystal orientation information is analyzed using a “Grain Average Image Quality” function provided to software “OIM Analysis (registered trademark)” accompanied by the EBSD analyzer. With this function, it is possible to digitalize the definition of the crystal orientation information as an IQ value and to discriminate a structure that is not auto-tempered. The unauto-tempered martensite and the lower bainite have a poor crystallinity and thus have a small IQ value. A region showing an IQ value calculated to be 60000 or less with the “Grain Average Image” function is defined as the unauto-tempered martensite and the lower bainite, and the area fraction thereof is calculated. The area percentage of the crystal grains of the unauto-tempered martensite and the lower bainite in the surface layer region is obtained by the above-described method. 
     “Measurement of Average Dislocation Density” 
     Next, a method for measuring the average dislocation density in the surface layer region will be described. The average dislocation density can be measured by the 
     X-ray diffraction method or transmission electron microscopic observation and is measured using the X-ray diffraction method in the present embodiment. 
     First, a sample is cut out from an arbitrary position 50 mm or more apart from the end face of the base steel sheet. While also depending on a measuring instrument, the size of the sample is set to a size of approximately 20 mm×20 mm. The thickness of 25 μm is reduced from each of the front surface and the rear surface of the sample using a mixed solution of distilled water (48%), a hydrogen peroxide solution (48%), and hydrofluoric acid (4%), thereby reducing a total of 50 μm of the thickness. Therefore, regions 25 μm deep from the surfaces of the sample before the thickness reduction are exposed. X-ray diffraction measurement is carried out on these exposed surfaces to specify a plurality of diffraction peaks of body-centered cubic lattices. The average dislocation density is analyzed from the half widths of these diffraction peaks, thereby obtaining the average dislocation density of the surface layer region. Regarding the analysis method, a modified Williamson-Hall method described in Non-Patent Document 1 is used. 
     “Plating Layer Being Attached to Single Surface in Amount of 10 g/m 2  or More and 90 g/m 2  or Less and Having Ni Content of 10 Mass % or More and 25 Mass % or Less With Remainder Including Zn and Impurity” 
     The steel sheet for hot stamping that is applied to the hot-stamped article according to the present embodiment has a plating layer on the surface of the base steel sheet that configures the steel sheet for hot stamping, the plating layer being attached to a single surface in an amount of 10 g/m 2  or more and 90 g/m 2  or less and having a Ni content of 10 mass % or more and 25 mass % or less with a remainder including Zn and an impurity. This makes it possible to diffuse Ni into the surface layer region during heating before hot stamping. 
     When the amount of the plating layer attached to a single surface is less than 10 g/m 2  or the Ni content in the plating layer is less than 10 mass %, the amount of Ni that concentrates in the surface layer region of the base steel sheet becomes small, and it is not possible to obtain a desired metallographic structure in the surface layer region after hot stamping. On the other hand, in a case where the amount attached to a single surface exceeds 90 g/m 2  or the Ni content in the plating layer exceeds 25 mass %, Ni concentrates excessively in the interface between the plating layer and the base steel sheet, the adhesion between the plating layer and the base steel sheet deteriorates, it becomes difficult for Ni in the plating layer to diffuse into the surface layer region of the base steel sheet, and it is not possible to obtain a desired metallographic structure in the hot-stamped article after hot stamping. 
     The amount of the plating layer attached to a single surface is preferably 30 g/m 2  or more and more preferably 40 g/m 2  or more. In addition, the amount of the Ni plating layer attached to a single surface is preferably 80 g/m 2  or less and more preferably 60 g/m 2  or less. 
     The amount of a plate attached in the steel sheet for hot stamping and the Ni content in the plating layer are measured by the following methods. 
     The amount of the plate attached is measured by collecting a test piece from an arbitrary position in the steel sheet for hot stamping according to the testing method described in JIS H 0401: 2013. Regarding the Ni content in the plating layer, a test piece is collected from an arbitrary position in the steel sheet for hot stamping according to the testing method described in JIS K 0150: 2005, and the Ni content at a ½ position of the total thickness of the plating layer is measured, thereby obtaining the Ni content in the plating layer in the steel sheet for hot stamping. 
     The sheet thickness of the steel sheet for hot stamping that is applied to the hot-stamped article according to the present embodiment is not particularly limited, but is preferably set to 0.5 to 3.5 mm from the viewpoint of the weight reduction of automotive bodies. 
     Next, the hot-stamped article according to the present embodiment will be described. 
     &lt;Hot-stamped Article&gt; 
     “Metallographic Structure of Surface Layer Region That is Region From Surface of Base Steel Sheet to Depth of 50 μm Includes 90.0% or More of Martensite in Terms of Area Percentage” 
     When the proportion of martensite in the metallographic structure of the surface layer region that is a region from the surface of the base steel sheet to a depth of 50 μm is 90.0% or more, it is possible to obtain an excellent strength and excellent hydrogen embrittlement resistance in the hot-stamped article. The proportion of the martensite is preferably as high as possible. 
     As a remaining structure other than the martensite in the surface layer region, one or more of ferrite, upper bainite, lower bainite, and residual austenite may be included. 
     “Method for Measuring Metallographic Structure” 
     A method for measuring the metallographic structure of the surface layer region that is a region from the surface of the steel sheet to a depth of 50 μm will be described. 
     First, samples are cut out from an arbitrary position 50 mm or more apart from the end face of the hot-stamped article such that a rolling-direction cross section (sheet thickness cross section) perpendicular to the surface can be observed. While also depending on a measuring instrument, the size of the sample is set to a size large enough to observe approximately 10 mm in the rolling direction. 
     A measurement surface that corresponds to the rolling-direction cross section of the sample is polished using silicon carbide paper #600 to #1500, then, mirror-finished using a liquid in which diamond powder having particle diameters of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water, and Nital-etched. Next, a region within 50 μm from the end portion of the measurement surface on a side toward the surface of the base steel sheet is observed as an observation visual field using a thermal field emission-type scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.). The area fraction of the martensite can be obtained as the total of the area fractions of tempered martensite and fresh martensite. The tempered martensite is an aggregate of lath crystal grains and is differentiated as a structure in which there are two or more extension directions of an iron carbide. The fresh martensite is not sufficiently etched by Nital etching and thus can be differentiated from other structures that are etched. Here, since the residual austenite is also, similar to the fresh martensite, not sufficiently etched, the area percentage of the fresh martensite is obtained from the difference between the area fraction of the structures that are not etched by Nital etching and the area fraction of the residual austenite calculated above. The area fraction of the martensite in the surface layer region is obtained by calculating the total of the area percentages of the tempered martensite and the fresh martensite obtained by the above-described method. 
     A measurement surface of the sample (a sample different from the sample used for the measurement of the area fraction of the martensite) is polished using silicon carbide paper #600 to #1500, and then the measurement surface is mirror-finished using a liquid in which diamond powder having particle diameters of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water. Next, the measurement surface is polished for eight minutes at room temperature using colloidal silica containing no alkaline solution to remove strain present on the surface layer of the sample. At an arbitrary position of the measurement surface of the sample in the rolling direction, a 50 μm-long region from the surface of the base steel sheet to a depth of 50 μm is measured by the electron backscatter diffraction method at measurement intervals of 0.1 μm, thereby obtaining crystal orientation information. For the measurement, an instrument including a thermal field emission-type scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.) and an EBSD detector (DVC 5-type detector manufactured by TSL) is used. At this time, the degree of vacuum in the instrument is set to 9.6×10 −5  Pa or less, the accelerating voltage is set to 15 kV, the irradiation current level is set to 13, and the irradiation time of the electron beam is set to 0.01 seconds/point. The area fraction of the residual austenite is calculated from the obtained crystal orientation information using a “Phase Map” function provided to software “OIM Analysis (registered trademark)” accompanied by the EBSD analyzer, thereby obtaining the area fraction of the residual austenite in the surface layer region. A structure having an fcc structure as the crystal structure is determined as the residual austenite. 
     “The Concentration of Ni in Prior Austenite Grain Boundaries in Surface Layer Region is 5.5 Mass % or More” 
     When the concentration of Ni in the prior austenite grain boundaries in the surface layer region is 5.5 mass % or more, it is possible to obtain an excellent strength and excellent hydrogen embrittlement resistance in the hot-stamped article. The concentration of Ni is preferably 7.0 mass % or more. The concentration of Ni is preferably as high as possible, but it is difficult to set to the concentration of Ni to 12.0 mass % or more in normal actual operation, and thus the substantial upper limit of the concentration of Ni is 12.0 mass %. 
     “Method for Measuring Concentration of Ni in Prior Austenite Grain Boundaries in Surface Layer Region” 
     A method for measuring the concentration of Ni in the prior austenite grain boundaries in the surface layer region will be described. 
     A test piece having dimensions shown in  FIG. 1  is produced from the central portion of the hot-stamped article after a heat treatment. In a cut at the central portion of the test piece, the bonding portion at the bottom of the cut is controlled from 100 μm to 200 μm by inserting a wire cutter. Next, the test piece is immersed in a 40% ammonium thiocyanate solution for 24 to 48 hours. After the end of the immersion, galvanization is carried out on the front and rear surfaces of the test piece within 0.5 hours. After the galvanization, an Auger electron emission spectroscopic analysis is carried out on the test piece within 1.5 hours. The kind of a device for carrying out the Auger electron emission spectroscopic analysis is not particularly limited. The test piece is set in the analyzing device, and the prior austenite grain boundaries are exposed by breaking the test piece from the cut portion in a vacuum of 9.6×10 −5  Pa or less. An electron beam is radiated to the prior austenite grain boundaries exposed in a 50 μm region in the surface layer in the sheet thickness direction at an accelerating voltage of 1 to 30 kV, and the concentration (mass %) of Ni in the grain boundaries is measured. The concentration of Ni is measured at 10 or more prior austenite grain boundaries. The measurement is completed within 30 minutes from the breakage in order to prevent the contamination of the grain boundaries. The average value of the obtained concentrations (mass %) of Ni is calculated, thereby obtaining the concentration of Ni in the prior austenite grain boundaries in the surface layer region. 
     “Plating Layer Being Attached to Single Surface in Amount of 10 g/m 2  or More and 90 g/m 2  or Less and Having Ni Content of 10 Mass % or More and 25 Mass % or Less With Remainder Including Zn and Impurities” 
     The steel sheet for hot stamping that is applied to the hot-stamped article according to the present embodiment has a plating layer on the surface of the base steel sheet that configures the hot-stamped article, the plating layer being attached to a single surface in an amount of 10 g/m 2  or more and 90 g/m 2  or less and having a Ni content of 10 mass % or more and 25 mass % or less with a remainder including Zn and an impurity. 
     When the amount of the plating layer attached to a single surface of the base steel sheet is less than 10 g/m 2  or the Ni content in the plating layer is less than 10 mass %, the amount of Ni that concentrates in the surface layer region of the base steel sheet becomes small, and it is not possible to obtain a desired metallographic structure in the surface layer region after hot stamping. On the other hand, in a case where the amount of the plating layer attached to a single surface of the base steel sheet exceeds 90 g/m 2  or the Ni content in the plating layer exceeds 25 mass %, Ni concentrates excessively in the interface between the plating layer and the base steel sheet, the adhesion between the plating layer and the base steel sheet deteriorates, it becomes difficult for Ni in the plating layer to diffuse into the surface layer region of the base steel sheet, and it is not possible to obtain a desired metallographic structure in the hot-stamped article. 
     The amount of the Ni plating layer attached to a single surface of the base steel sheet is preferably 30 g/m 2  or more and more preferably 40 g/m 2  or more. In addition, the amount of the Ni plating layer attached to a single surface of the base steel sheet is preferably 80 g/m 2  or less and more preferably 60 g/m 2  or less. 
     The amount of the plate attached in the hot-stamped article and the Ni content in the plating layer are measured by the following methods. 
     The amount of the plate attached is measured by collecting a test piece from an arbitrary position in the hot-stamped article according to the testing method described in JIS H 0401: 2013. Regarding the Ni content in the plating layer, a test piece is collected from an arbitrary position in the hot-stamped article according to the testing method described in JIS K 0150: 2005, and the Ni content at a ½ position of the total thickness of the plating layer is measured, thereby obtaining the Ni content in the plating layer in the hot-stamped article. 
     Next, a preferred method for manufacturing the steel sheet for hot stamping that is applied to the hot-stamped article according to the present embodiment will be described. 
     &lt;Method for Manufacturing Steel Sheet for Hot Stamping&gt; 
     A steel piece (steel) that is to be subjected to hot rolling may be, for example, a steel piece manufactured by an usual method such as a continuously cast slag or a thin slab caster as long as the steel piece is manufactured by a normal method. Rough rolling may also be carried out by a usual method and is not particularly limited. 
     “Finishing Rolling” 
     In the final rolling (final pass) of finishing rolling, it is necessary to carry out the finishing rolling in a temperature range of the A 3  point or higher at a rolling reduction of smaller than 20%. When the finishing rolling is carried out at a temperature of lower than the A 3  point or the rolling reduction is 20% or larger in the final rolling of the finishing rolling, ferrite is formed in the surface layer region, and it is not possible to set the proportion of the crystal grains of one or two kinds of the unauto-tempered martensite and the lower bainite to 15.0% or more in terms of area percentage. The A 3  point is represented by Formula (1). 
         A   3  point=850+10×( C+N )× Mn+ 350 ×Nb+ 250 ×Ti+ 40 ×B+ 10 ×Cr + 100 ×Mo   (1)
 
     In Formula (1), C, N, Mn, Nb, Ti, B, Cr, and Mo indicate the amounts (mass %) of the individual elements. 
     “Cooling” 
     After the end of the finishing rolling, cooling at an average cooling rate of 80° C./s or faster is initiated within five seconds, and the base steel sheet is cooled to a temperature range of lower than 500° C. and coiled. In addition, even after being coiled, the base steel sheet is continuously cooled with water to room temperature. In a case where the cooling initiation time exceeds five seconds, a case where the average cooling rate is slower than 80° C./s, or a case where the coiling initiation temperature is higher than 500° C., ferrite, bainite, and upper bainite are likely to be formed, and it is impossible to set the proportion of the crystal grains of one or two kinds of the unauto-tempered martensite and the lower bainite to 15.0% or more in terms of area percentage in the surface layer region. The average cooling rate at this time is calculated from a change in the temperature of the surface of the steel sheet and indicates the average cooling rate while the temperature reaches the coiling initiation temperature from the finishing rolling temperature. 
     “Plating” 
     On the hot-rolled steel sheet as it is or after cold rolling, a plating layer being attached to a single surface in an amount of 10 g/m 2  or more and 90 g/m 2  or less and having a Ni content of 10 mass % or more and 25 mass % or less with a remainder including Zn and an impurity is formed to obtain the steel sheet for hot stamping. In a case where cold rolling is carried out before plating, the rolling reduction in the cold rolling is not particularly limited, but is preferably set to 40% to 60% from the viewpoint of the shape stability of the steel sheet. The manufacturing of the steel sheet for hot stamping may additionally include a well-known manufacturing method such as pickling or tempered rolling before the plating. However, when tempering is carried out at a Ms point temperature of—15° C. or higher, it is not possible to set the proportion of the crystal grains of one or two kinds of the unauto-tempered martensite and the lower bainite to 15.0% or more in terms of area percentage and to set the average dislocation density to 4×10 15  m/m 3  or more in the surface layer region, and consequently, it is not possible to obtain a hot-stamped article having a desired metallographic structure. Therefore, in a case where there is a need to carry out tempering before the plating for the reason of a high C content or the like, the tempering is carried out at a temperature of lower than the Ms point—15° C. The Ms point is represented by Formula (2). 
         Ms= 493−300 ×C− 33.3 ×Mn− 11.1 ×Si− 22.2 ×Cr− 16.7 ×Ni− 11.1 ×Mo    (2)
 
     In Formula (2), C, Mn, Si, Cr, Ni, and Mo indicate the amounts (mass %) of the individual elements. 
     Next, a method for manufacturing the hot-stamped article according to the present embodiment using the steel sheet for hot stamping manufactured by the above-described method will be described. 
     &lt;Method for Manufacturing hot-stamped Article&gt; 
     The hot-stamped article is manufactured by heating the steel sheet for hot stamping obtained as described above in a temperature range of 500° C. or higher and the A 3  point or lower at an average heating rate of 100° C./s or faster and slower than 200 ° C./s, then, holding the steel sheet for hot stamping at a temperature of the A 3  point or higher and the A 3  point+150° C. or lower, hot-stamping the steel sheet for hot stamping such that the elapsed time from the initiation of the heating to the initiation of forming reaches a predetermined time or shorter, and cooling the steel sheet for hot stamping to room temperature. 
     In addition, in order to adjust the strength of the hot-stamped article, a softened region may be formed by tempering a partial region or the entire region of the hot-stamped article at a temperature of lower than the Ms point—15° C. 
     The steel sheet for hot stamping is heated in a temperature range of 500° C. or higher and the A 3  point or lower at an average heating rate of 100° C./s or faster and slower than 200° C./s, and the elapsed time from the initiation of the heating to the initiation of the forming is set to shorter than 240 seconds, whereby it is possible to obtain a desired metallographic structure in the surface layer region. This makes it possible to obtain excellent hydrogen embrittlement resistance in the hot-stamped article. The average heating rate is preferably 120° C./s or faster. The average heating rate is set to slower than 200° C./s since transformation into austenite is accelerated while a carbide that is contained in the steel sheet for hot stamping is not completely dissolved, and the deterioration of the hydrogen embrittlement resistance of the hot-stamped article is caused. The average heating rate is preferably slower than 180° C./s. The elapsed time from the initiation of the heating to the initiation of the forming is preferably set to 120 seconds or longer and 180 seconds or shorter. 
     The holding temperature during the hot stamping is preferably set to the A 3  point +10° C. or higher and the A 3  point +150° C. or lower. In addition, the average cooling rate after the hot stamping is preferably set to 10° C./s or faster. 
     EXAMPLES 
     Next, examples of the present invention will be described. Conditions in the examples are examples of conditions adopted to confirm the feasibility and effect of the present invention, and the present invention is not limited to these examples of conditions. The present invention is capable of adopting a variety of conditions as long as the objective of the present invention is achieved without departing from the gist of the present invention. 
     Steel pieces manufactured by casting molten steel having a chemical composition shown in Tables 1 and 2 were hot-rolled, cold-rolled, and plated under conditions shown in Tables 3 and 4, thereby obtaining steel sheets for hot stamping shown in Tables 3 and 4. The obtained steel sheets for hot stamping were hot-stamped by carrying out a heat treatment shown in Tables 5 and 6, thereby obtaining hot-stamped articles shown in Tables 5 and 6. A partial softened region was formed by irradiating a part of the hot-stamped article with a laser, cooling the irradiated portion to lower than the Ms—15° C., and tempering the irradiated portion. 
     Underlined values in the tables indicate that the values are outside the scope of the present invention, the preferred manufacturing conditions are not satisfied, or property values are not preferable. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Steel 
                 Chemical composition (mass %) with remainder of Fe and impurities 
                 A 3   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 No. 
                 C 
                 Si 
                 Mn 
                 P 
                 S 
                 sol. Al 
                 N 
                 Nb 
                 Ti 
                 Mo 
                 Cr 
                 B 
                 Ca 
                 REM 
                 (° C.) 
                 Note 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 0.16 
                 0.250 
                 1.10 
                 0.006 
                 0.0020 
                 0.030 
                 0.0026 
                   
                   
                 0.130 
                   
                   
                 865 
                 Invention steel 
               
               
                 2 
                 0.44 
                 0.250 
                 1.80 
                 0.010 
                 0.0090 
                 0.400 
                 0.0040 
                   
                   
                   
                   
                 0.03 
                 858 
                 Invention steel 
               
               
                 3 
                 0.23 
                 0.250 
                 1.20 
                 0.010 
                 0.0100 
                 0.030 
                 0.0050 
                   
                 0.020 
                   
                 0.200 
                   
                 860 
                 Invention steel 
               
               
                 4 
                 
                   0.08 
                 
                 0.220 
                 0.81 
                 0.008 
                 0.0009 
                 0.044 
                 0.0026 
                   
                   
                   
                   
                   
                 851 
                 Comparative steel 
               
               
                 5 
                 0.16 
                 0.150 
                 0.71 
                 0.011 
                 0.0006 
                 0.043 
                 0.0037 
                   
                   
                   
                   
                   
                 851 
                 Invention steel 
               
               
                 6 
                 0.31 
                 0.250 
                 0.80 
                 0.015 
                 0.0011 
                 0.041 
                 0.0039 
                   
                   
                   
                   
                   
                 853 
                 Invention steel 
               
               
                 7 
                 0.36 
                 0.180 
                 0.81 
                 0.005 
                 0.0005 
                 0.045 
                 0.0037 
                   
                   
                   
                   
                   
                 853 
                 Invention steel 
               
               
                 8 
                 0.44 
                 0.250 
                 0.71 
                 0.015 
                 0.0007 
                 0.034 
                 0.0042 
                   
                   
                   
                   
                   
                 853 
                 Invention steel 
               
               
                 9 
                 0.67 
                 0.190 
                 0.71 
                 0.014 
                 0.0003 
                 0.037 
                 0.0035 
                   
                   
                   
                   
                   
                 855 
                 Invention steel 
               
               
                 10 
                 
                   0.78 
                 
                 0.250 
                 0.90 
                 0.014 
                 0.0011 
                 0.031 
                 0.0026 
                   
                   
                   
                   
                   
                 857 
                 Comparative steel 
               
               
                 11 
                 0.36 
                 
                   0.002 
                 
                 0.86 
                 0.005 
                 0.0003 
                 0.041 
                 0.0032 
                   
                   
                   
                   
                   
                 853 
                 Comparative steel 
               
               
                 12 
                 0.38 
                 0.007 
                 0.83 
                 0.005 
                 0.0011 
                 0.050 
                 0.0030 
                   
                   
                   
                   
                   
                 853 
                 Invention steel 
               
               
                 13 
                 0.37 
                 0.210 
                 0.72 
                 0.011 
                 0.0007 
                 0.030 
                 0.0041 
                   
                   
                   
                   
                   
                 853 
                 Invention steel 
               
               
                 14 
                 0.37 
                 0.240 
                 0.90 
                 0.015 
                 0.0007 
                 0.047 
                 0.0037 
                   
                   
                   
                   
                   
                 853 
                 Invention steel 
               
               
                 15 
                 0.37 
                 0.150 
                 
                   0.15 
                 
                 0.005 
                 0.0003 
                 0.035 
                 0.0030 
                   
                   
                   
                   
                   
                 851 
                 Comparative steel 
               
               
                 16 
                 0.44 
                 0.170 
                 0.44 
                 0.007 
                 0.0005 
                 0.049 
                 0.0029 
                   
                   
                   
                   
                   
                 852 
                 Invention steel 
               
               
                 17 
                 0.36 
                 0.240 
                 0.82 
                 0.010 
                 0.0011 
                 0.035 
                 0.0038 
                   
                   
                   
                   
                   
                 853 
                 Invention steel 
               
               
                 18 
                 0.37 
                 0.180 
                 1.29 
                 0.007 
                 0.0010 
                 0.030 
                 0.0028 
                   
                   
                   
                   
                   
                 855 
                 Invention steel 
               
               
                 19 
                 0.37 
                 0.150 
                 1.99 
                 0.009 
                 0.0005 
                 0.035 
                 0.0042 
                   
                   
                   
                   
                   
                 857 
                 Invention steel 
               
               
                 20 
                 0.38 
                 0.170 
                 2.89 
                 0.007 
                 0.0005 
                 0.046 
                 0.0037 
                   
                   
                   
                   
                   
                 861 
                 Invention steel 
               
               
                 21 
                 0.38 
                 0.150 
                 
                   3.15 
                 
                 0.012 
                 0.0009 
                 0.036 
                 0.0042 
                   
                   
                   
                   
                   
                 862 
                 Comparative steel 
               
               
                 22 
                 0.38 
                 0.240 
                 0.82 
                  0.0004 
                 0.0007 
                 0.045 
                 0.0026 
                   
                   
                   
                   
                   
                 853 
                 Invention steel 
               
               
                 23 
                 0.36 
                 0.160 
                 0.90 
                 0.009 
                 0.0006 
                 0.030 
                 0.0038 
                   
                   
                   
                   
                   
                 853 
                 Invention steel 
               
               
                 24 
                 0.36 
                 0.150 
                 0.77 
                 0.094 
                 0.0010 
                 0.043 
                 0.0033 
                   
                   
                   
                   
                   
                 853 
                 Invention steel 
               
               
                 25 
                 0.37 
                 0.190 
                 0.84 
                 
                   0.123 
                 
                 0.0010 
                 0.033 
                 0.0032 
                   
                   
                   
                   
                   
                 853 
                 Comparative steel 
               
               
                 26 
                 0.36 
                 0.200 
                 0.75 
                 0.009 
                 0.0002 
                 0.047 
                 0.0045 
                   
                   
                   
                   
                   
                 853 
                 Invention steel 
               
               
                 27 
                 0.37 
                 0.150 
                 0.81 
                 0.013 
                 0.0003 
                 0.031 
                 0.0029 
                   
                   
                   
                   
                   
                 853 
                 Invention steel 
               
               
                 28 
                 0.37 
                 0.190 
                 0.89 
                 0.008 
                 0.0022 
                 0.044 
                 0.0032 
                   
                   
                   
                   
                   
                 853 
                 Invention steel 
               
               
                 29 
                 0.36 
                 0.230 
                 0.80 
                 0.007 
                 0.0900 
                 0.049 
                 0.0030 
                   
                   
                   
                   
                   
                 853 
                 Invention steel 
               
               
                 30 
                 0.36 
                 0.190 
                 0.72 
                 0.006 
                 
                   0.1334 
                 
                 0.045 
                 0.0025 
                   
                   
                   
                   
                   
                 853 
                 Comparative steel 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Steel 
                 Chemical composition (mass %) with remainder of Fe and impurities 
                 A 3   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 No. 
                 C 
                 Si 
                 Mn 
                 P 
                 S 
                 sol. Al 
                 N 
                 Nb 
                 Ti 
                 Mo 
                 Cr 
                 B 
                 Ca 
                 REM 
                 (° C.) 
                 Note 
               
               
                   
               
               
                 31 
                 0.38 
                 0.230 
                 0.79 
                 0.013 
                 0.0008 
                   0.0001   
                 0.0027 
                   
                   
                   
                   
                   
                   
                   
                 853 
                 Comparative steel 
               
               
                 32 
                 0.38 
                 0.160 
                 0.85 
                 0.010 
                 0.0009 
                  0.0003 
                 0.0033 
                   
                   
                   
                   
                   
                   
                   
                 853 
                 Invention steel 
               
               
                 33 
                 0.35 
                 0.200 
                 0.72 
                 0.014 
                 0.0007 
                  0.0029 
                 0.0042 
                   
                   
                   
                   
                   
                   
                   
                 853 
                 Invention steel 
               
               
                 34 
                 0.37 
                 0.160 
                 0.73 
                 0.006 
                 0.0006 
                 0.031 
                 0.0026 
                   
                   
                   
                   
                   
                   
                   
                 853 
                 Invention steel 
               
               
                 35 
                 0.35 
                 0.240 
                 0.83 
                 0.009 
                 0.0008 
                 0.494 
                 0.0034 
                   
                   
                   
                   
                   
                   
                   
                 853 
                 Invention steel 
               
               
                 36 
                 0.37 
                 0.240 
                 0.84 
                 0.011 
                 0.0007 
                 
                   0.581 
                 
                 0.0040 
                   
                   
                   
                   
                   
                   
                   
                 853 
                 Comparative steel 
               
               
                 37 
                 0.37 
                 0.220 
                 0.89 
                 0.007 
                 0.0007 
                 0.035 
                 0.0001 
                   
                   
                   
                   
                   
                   
                   
                 853 
                 Invention steel 
               
               
                 38 
                 0.38 
                 0.150 
                 0.89 
                 0.009 
                 0.0008 
                 0.038 
                 0.0073 
                   
                   
                   
                   
                   
                   
                   
                 853 
                 Invention steel 
               
               
                 39 
                 0.38 
                 0.190 
                 0.71 
                 0.007 
                 0.0007 
                 0.039 
                 0.0090 
                   
                   
                   
                   
                   
                   
                   
                 853 
                 Invention steel 
               
               
                 40 
                 0.36 
                 0.210 
                 0.73 
                 0.008 
                 0.0003 
                 0.035 
                 
                   0.0160 
                 
                   
                   
                   
                   
                   
                   
                   
                 853 
                 Comparative steel 
               
               
                 41 
                 0.37 
                 0.230 
                 0.87 
                 0.009 
                 0.0006 
                 0.031 
                 0.0025 
                 0.012 
                   
                   
                   
                   
                   
                   
                 857 
                 Invention steel 
               
               
                 42 
                 0.36 
                 0.170 
                 0.70 
                 0.009 
                 0.0009 
                 0.046 
                 0.0030 
                 0.032 
                   
                   
                   
                   
                   
                   
                 864 
                 Invention steel 
               
               
                 43 
                 0.37 
                 0.220 
                 0.73 
                 0.008 
                 0.0004 
                 0.033 
                 0.0038 
                 0.120 
                   
                   
                   
                   
                   
                   
                 895 
                 Invention steel 
               
               
                 44 
                 0.37 
                 0.230 
                 0.90 
                 0.009 
                 0.0011 
                 0.044 
                 0.0044 
                   
                 0.013 
                   
                   
                   
                   
                   
                 857 
                 Invention steel 
               
               
                 45 
                 0.35 
                 0.170 
                 0.89 
                 0.011 
                 0.0007 
                 0.043 
                 0.0028 
                   
                 0.036 
                   
                   
                   
                   
                   
                 862 
                 Invention steel 
               
               
                 46 
                 0.36 
                 0.170 
                 0.88 
                 0.007 
                 0.0004 
                 0.031 
                 0.0033 
                   
                 0.140 
                   
                   
                   
                   
                   
                 888 
                 Invention steel 
               
               
                 47 
                 0.36 
                 0.210 
                 0.80 
                 0.005 
                 0.0003 
                 0.037 
                 0.0035 
                   
                   
                 0.006 
                   
                   
                   
                   
                 854 
                 Invention steel 
               
               
                 48 
                 0.37 
                 0.200 
                 0.78 
                 0.009 
                 0.0010 
                 0.031 
                 0.0026 
                   
                   
                 0.012 
                   
                   
                   
                   
                 854 
                 Invention steel 
               
               
                 49 
                 0.38 
                 0.160 
                 0.82 
                 0.015 
                 0.0009 
                 0.031 
                 0.0041 
                   
                   
                 0.980 
                   
                   
                   
                   
                 951 
                 Invention steel 
               
               
                 50 
                 0.36 
                 0.230 
                 0.77 
                 0.011 
                 0.0008 
                 0.043 
                 0.0038 
                   
                   
                   
                 0.006 
                   
                   
                   
                 853 
                 Invention steel 
               
               
                 51 
                 0.35 
                 0.160 
                 0.70 
                 0.005 
                 0.0006 
                 0.047 
                 0.0026 
                   
                   
                   
                 0.009 
                   
                   
                   
                 853 
                 Invention steel 
               
               
                 52 
                 0.37 
                 0.250 
                 0.83 
                 0.006 
                 0.0010 
                 0.033 
                 0.0039 
                   
                   
                   
                 0.960 
                   
                   
                   
                 863 
                 Invention steel 
               
               
                 53 
                 0.37 
                 0.150 
                 0.70 
                 0.015 
                 0.0008 
                 0.031 
                 0.0044 
                   
                   
                   
                   
                 0.0006 
                   
                   
                 853 
                 Invention steel 
               
               
                 54 
                 0.36 
                 0.230 
                 0.86 
                 0.005 
                 0.0003 
                 0.050 
                 0.0044 
                   
                   
                   
                   
                 0.0011 
                   
                   
                 853 
                 Invention steel 
               
               
                 55 
                 0.36 
                 0.160 
                 0.74 
                 0.015 
                 0.0006 
                 0.034 
                 0.0044 
                   
                   
                   
                   
                 0.0090 
                   
                   
                 853 
                 Invention steel 
               
               
                 56 
                 0.36 
                 0.160 
                 0.78 
                 0.015 
                 0.0006 
                 0.037 
                 0.0039 
                   
                   
                   
                   
                   
                 0.0080 
                   
                 853 
                 Invention steel 
               
               
                 57 
                 0.36 
                 0.190 
                 0.80 
                 0.010 
                 0.0006 
                 0.034 
                 0.0027 
                   
                   
                   
                   
                   
                   
                 0.28 
                 853 
                 Invention steel 
               
               
                 58 
                 
                   0.14 
                 
                 0.110 
                 0.84 
                 0.011 
                 0.0008 
                 0.036 
                 0.0031 
                   
                   
                   
                   
                   
                   
                   
                 851 
                 Comparative steel 
               
               
                 59 
                 0.20 
                 0.210 
                 1.32 
                 0.012 
                 0.0007 
                 0.028 
                 0.0027 
                   
                   
                   
                   
                   
                   
                   
                 853 
                 Invention steel 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
             
            
               
                   
                 Finishing rolling 
                   
                   
                 Heat 
               
            
           
           
               
               
               
               
            
               
                   
                 Final 
                   
                 treatment 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 rolling 
                   
                 Cooling 
                 Coiling 
                 Cold 
                 before 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 reduction of 
                 Finishing 
                 Time until 
                   
                 Coiling 
                 rolling 
                 plating 
               
               
                   
                 Steel 
                 finishing 
                 rolling 
                 initiation 
                 Cooling 
                 initiation 
                 Rolling 
                 Heating 
               
               
                 Steel 
                 sheet 
                 rolling 
                 temperature 
                 of cooling 
                 rate 
                 temperature 
                 reduction 
                 temperature 
               
               
                 No. 
                 No. 
                 (%) 
                 (° C.) 
                 (s) 
                 (° C./s) 
                 (° C.) 
                 (%) 
                 (° C.) 
               
               
                   
               
               
                  1 
                 1 
                 6 
                 910 
                 3 
                   38   
                 
                   571 
                 
                 41 
                 N/A 
               
               
                  2 
                 2 
                 5 
                 921 
                 2 
                   32   
                 
                   596 
                 
                 40 
                 N/A 
               
               
                  3 
                 3 
                 5 
                 934 
                 3 
                   51   
                 397 
                 50 
                 
                   773 
                 
               
               
                   4   
                 4 
                 9 
                 954 
                 1 
                 126 
                 372 
                 53 
                 N/A 
               
               
                  5 
                 5 
                 6 
                 884 
                 4 
                 115 
                 369 
                 58 
                 N/A 
               
               
                  6 
                 6 
                 8 
                 917 
                 2 
                 133 
                 345 
                 45 
                 N/A 
               
               
                  7 
                 7 
                 10 
                 901 
                 1 
                 123 
                 330 
                 44 
                 N/A 
               
               
                  8 
                 8 
                 10 
                 897 
                 4 
                 103 
                 266 
                 49 
                 N/A 
               
               
                  9 
                 9 
                 6 
                 924 
                 1 
                 115 
                 261 
                 53 
                 N/A 
               
               
                 
                   10 
                 
                 10 
                 10 
                 898 
                 1 
                 115 
                 316 
                 43 
                 N/A 
               
               
                 
                   11 
                 
                 11 
                 8 
                 905 
                 1 
                 142 
                 292 
                 55 
                 N/A 
               
               
                 12 
                 12 
                 5 
                 887 
                 4 
                 118 
                 353 
                 60 
                 N/A 
               
               
                 13 
                 13 
                 5 
                 926 
                 3 
                 146 
                 390 
                 47 
                 N/A 
               
               
                 14 
                 14 
                 9 
                 890 
                 3 
                 134 
                 283 
                 55 
                 N/A 
               
               
                 
                   15 
                 
                 15 
                 8 
                 933 
                 1 
                 123 
                 329 
                 53 
                 N/A 
               
               
                 16 
                 16 
                 6 
                 885 
                 4 
                 115 
                 329 
                 52 
                 N/A 
               
               
                 17 
                 17 
                 9 
                 900 
                 3 
                 133 
                 379 
                 45 
                 N/A 
               
               
                 18 
                 18 
                 7 
                 876 
                 4 
                 134 
                 250 
                 42 
                 N/A 
               
               
                 19 
                 19 
                 10 
                 942 
                 1 
                 105 
                 315 
                 59 
                 N/A 
               
               
                 20 
                 20 
                 10 
                 960 
                 3 
                 125 
                 273 
                 49 
                 N/A 
               
               
                 
                   21 
                 
                 21 
                 5 
                 895 
                 2 
                 147 
                 343 
                 41 
                 N/A 
               
               
                 22 
                 22 
                 8 
                 931 
                 3 
                 123 
                 354 
                 43 
                 N/A 
               
               
                 23 
                 23 
                 10 
                 961 
                 1 
                 138 
                 272 
                 48 
                 N/A 
               
               
                 24 
                 24 
                 10 
                 955 
                 2 
                 124 
                 330 
                 45 
                 N/A 
               
               
                 
                   25 
                 
                 25 
                 5 
                 901 
                 1 
                 104 
                 310 
                 50 
                 N/A 
               
               
                 26 
                 26 
                 5 
                 892 
                 4 
                 134 
                 329 
                 52 
                 N/A 
               
               
                 27 
                 27 
                 10 
                 938 
                 3 
                 104 
                 251 
                 41 
                 N/A 
               
               
                 28 
                 28 
                 7 
                 953 
                 1 
                 149 
                 372 
                 60 
                 N/A 
               
               
                 29 
                 29 
                 10 
                 943 
                 1 
                 126 
                 382 
                 41 
                 N/A 
               
               
                 
                   30 
                 
                 30 
                 7 
                 948 
                 1 
                 148 
                 260 
                 44 
                 N/A 
               
               
                 
                   31 
                 
                 31 
                 5 
                 895 
                 1 
                 115 
                 328 
                 52 
                 N/A 
               
               
                 32 
                 32 
                 6 
                 918 
                 1 
                 126 
                 341 
                 52 
                 N/A 
               
               
                 33 
                 33 
                 6 
                 885 
                 4 
                 106 
                 314 
                 47 
                 N/A 
               
               
                 34 
                 34 
                 5 
                 883 
                 3 
                 149 
                 266 
                 45 
                 N/A 
               
               
                 35 
                 35 
                 6 
                 947 
                 1 
                 131 
                 387 
                 57 
                 N/A 
               
               
                 
                   36 
                 
                 36 
                 8 
                 877 
                 3 
                 109 
                 291 
                 47 
                 N/A 
               
               
                 37 
                 37 
                 9 
                 947 
                 2 
                 118 
                 365 
                 45 
                 N/A 
               
               
                 38 
                 38 
                 6 
                 924 
                 4 
                 145 
                 393 
                 45 
                 N/A 
               
               
                 39 
                 39 
                 9 
                 938 
                 3 
                 143 
                 354 
                 51 
                 N/A 
               
               
                 
                   40 
                 
                 40 
                 5 
                 943 
                 1 
                 144 
                 395 
                 40 
                 N/A 
               
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Steel sheet for hot stamping 
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Unauto- 
                   
                   
               
               
                   
                   
                   
                 Ni 
                   
                 tempered 
               
               
                   
                   
                 Amount 
                 content 
                 Average 
                 martensite 
               
               
                   
                   
                 of plate 
                 in plating 
                 dislocation 
                 and lower 
                 Sheet 
               
               
                   
                 Steel 
                 attached 
                 layer 
                 density 
                 bainite 
                 thickness 
               
               
                   
                 No. 
                 (g/m 2 ) 
                 (mass %) 
                 (10 14  m/m 3 ) 
                 (area %) 
                 (mm) 
                 Note 
               
               
                   
                   
               
               
                   
                  1 
                 39 
                 15 
                   1   
                   4.9   
                 1.6 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                  2 
                 55 
                 12 
                   29   
                   2.1   
                 1.6 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                  3 
                 41 
                 12 
                   21   
                   6.3   
                 1.6 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                   4   
                 56 
                 15 
                   3   
                   9.8   
                 1.6 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                  5 
                 50 
                 17 
                  41 
                 18.4 
                 2.0 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                  6 
                 41 
                 17 
                  96 
                 21.2 
                 1.9 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                  7 
                 54 
                 25 
                 187 
                 21.9 
                 1.5 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                  8 
                 57 
                 16 
                 345 
                 18.5 
                 1.7 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                  9 
                 40 
                 17 
                 948 
                 21.3 
                 1.6 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 
                   10 
                 
                 53 
                 18 
                 1120  
                 24.7 
                 1.3 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 
                   11 
                 
                 48 
                 18 
                 155 
                 18.1 
                 1.3 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 12 
                 58 
                 15 
                 230 
                 19.9 
                 1.6 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 13 
                 48 
                 15 
                 223 
                 22.3 
                 1.6 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 14 
                 46 
                 15 
                 219 
                 17.6 
                 1.8 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 
                   15 
                 
                 58 
                 18 
                   11   
                   3.7   
                 1.6 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 16 
                 51 
                 17 
                 372 
                 22.0 
                 1.3 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 17 
                 43 
                 18 
                 157 
                 22.7 
                 1.2 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 18 
                 52 
                 17 
                 184 
                 16.6 
                 1.7 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 19 
                 50 
                 17 
                 229 
                 20.3 
                 1.9 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 20 
                 45 
                 17 
                 217 
                 22.4 
                 2.0 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 
                   21 
                 
                 45 
                 17 
                 192 
                 24.4 
                 1.6 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 22 
                 60 
                 17 
                 178 
                 20.3 
                 1.8 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 23 
                 47 
                 15 
                 159 
                 18.4 
                 2.0 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 24 
                 60 
                 15 
                 189 
                 18.9 
                 1.2 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 
                   25 
                 
                 58 
                 15 
                 237 
                 20.9 
                 1.6 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 26 
                 60 
                 16 
                 206 
                 21.2 
                 1.2 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 27 
                 52 
                 17 
                 164 
                 19.2 
                 1.5 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 28 
                 50 
                 18 
                 172 
                 23.4 
                 1.8 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 29 
                 53 
                 17 
                 201 
                 21.5 
                 1.6 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 
                   30 
                 
                 51 
                 16 
                 237 
                 16.9 
                 1.2 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 
                   31 
                 
                 46 
                 16 
                 152 
                 18.5 
                 1.8 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 32 
                 40 
                 15 
                 214 
                 22.4 
                 1.2 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 33 
                 43 
                 18 
                 181 
                 18.0 
                 1.5 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 34 
                 46 
                 15 
                 156 
                 19.9 
                 1.8 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 35 
                 51 
                 15 
                 151 
                 22.3 
                 1.9 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 
                   36 
                 
                 47 
                 15 
                 206 
                 16.1 
                 1.6 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 37 
                 52 
                 16 
                 171 
                 21.4 
                 1.7 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 38 
                 46 
                 15 
                 212 
                 23.1 
                 1.8 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 39 
                 60 
                 16 
                 173 
                 22.2 
                 1.2 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 
                   40 
                 
                 60 
                 16 
                 220 
                 23.7 
                 1.8 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
             
            
               
                   
                 Finishing rolling 
                   
                   
                 Heat 
               
            
           
           
               
               
               
               
            
               
                   
                 Final 
                   
                 treatment 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 rolling 
                   
                 Cooling 
                 Coiling 
                 Cold 
                 before 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 reduction 
                 Finishing 
                 Time until 
                   
                 Coiling 
                 rolling 
                 plating 
               
               
                   
                 Steel 
                 of finishing 
                 rolling 
                 initiation 
                 Cooling 
                 initiation 
                 Rolling 
                 Heating 
               
               
                 Steel 
                 sheet 
                 rolling 
                 temperature 
                 of cooling 
                 rate 
                 temperature 
                 reduction 
                 temperature 
               
               
                 No. 
                 No. 
                 (%) 
                 (° C.) 
                 (s) 
                 (° C./s) 
                 (° C.) 
                 (%) 
                 (° C.) 
               
               
                   
               
               
                 41 
                 41 
                 9 
                 921 
                 2 
                 114 
                 394 
                 58 
                 N/A 
               
               
                 42 
                 42 
                 9 
                 898 
                 1 
                 106 
                 330 
                 48 
                 N/A 
               
               
                 43 
                 43 
                 10  
                 952 
                 4 
                 131 
                 376 
                 55 
                 N/A 
               
               
                 44 
                 44 
                 5 
                 902 
                 4 
                 119 
                 291 
                 52 
                 N/A 
               
               
                 45 
                 45 
                 6 
                 929 
                 4 
                 131 
                 342 
                 42 
                 N/A 
               
               
                 46 
                 46 
                 10  
                 929 
                 3 
                 117 
                 316 
                 54 
                 N/A 
               
               
                 47 
                 47 
                 9 
                 896 
                 4 
                 149 
                 269 
                 55 
                 N/A 
               
               
                 48 
                 48 
                 10  
                 928 
                 2 
                 110 
                 331 
                 46 
                 N/A 
               
               
                 49 
                 49 
                 9 
                 987 
                 2 
                 102 
                 386 
                 58 
                 N/A 
               
               
                 50 
                 50 
                 5 
                 953 
                 2 
                 109 
                 343 
                 43 
                 N/A 
               
               
                 51 
                 51 
                 9 
                 928 
                 4 
                 101 
                 316 
                 46 
                 N/A 
               
               
                 52 
                 52 
                 10  
                 939 
                 4 
                 106 
                 338 
                 56 
                 N/A 
               
               
                 53 
                 53 
                 5 
                 936 
                 4 
                 144 
                 276 
                 57 
                 N/A 
               
               
                 54 
                 54 
                 10  
                 876 
                 1 
                 104 
                 315 
                 51 
                 N/A 
               
               
                 55 
                 55 
                 9 
                 944 
                 2 
                 108 
                 338 
                 40 
                 N/A 
               
               
                 56 
                 56 
                 9 
                 883 
                 3 
                 138 
                 344 
                 50 
                 N/A 
               
               
                 57 
                 57 
                 9 
                 916 
                 4 
                 127 
                 263 
                 47 
                 N/A 
               
               
                 7 
                 58 
                 7 
                 887 
                 
                   7 
                 
                 137 
                 255 
                 40 
                 N/A 
               
               
                 7 
                 59 
                 10  
                 890 
                 4 
                   72   
                 260 
                 47 
                 N/A 
               
               
                 7 
                 60 
                 9 
                 900 
                 2 
                  88 
                 379 
                 54 
                 N/A 
               
               
                 7 
                 61 
                 6 
                 946 
                 2 
                 176 
                 325 
                 47 
                 N/A 
               
               
                 7 
                 62 
                 8 
                 963 
                 2 
                 113 
                 
                   536 
                 
                 56 
                 N/A 
               
               
                 7 
                 63 
                 9 
                 905 
                 3 
                 134 
                 308 
                 51 
                 N/A 
               
               
                 7 
                 64 
                 6 
                 911 
                 3 
                 118 
                 220 
                 59 
                 N/A 
               
               
                 7 
                 65 
                 5 
                 908 
                 3 
                 139 
                  94 
                 45 
                 N/A 
               
               
                 7 
                 66 
                 8 
                 881 
                 4 
                 137 
                  42 
                 56 
                 N/A 
               
               
                 7 
                 67 
                 10  
                 874 
                 2 
                 108 
                 365 
                 43 
                 320 
               
               
                 7 
                 68 
                 6 
                 898 
                 2 
                 119 
                 386 
                 0 
                 N/A 
               
               
                 7 
                 69 
                 10  
                 901 
                 1 
                 123 
                 330 
                 44 
                 N/A 
               
               
                 7 
                 70 
                 10  
                 901 
                 1 
                 123 
                 330 
                 44 
                 N/A 
               
               
                 2 
                 71 
                   40   
                 901 
                 5 
                  82 
                 418 
                 51 
                 N/A 
               
               
                 9 
                 72 
                 9 
                 932 
                 2 
                   58   
                 403 
                 53 
                 N/A 
               
               
                 58 
                 73 
                 8 
                 946 
                 3 
                  87 
                 486 
                 49 
                 N/A 
               
               
                 59 
                 74 
                 9 
                 957 
                 3 
                 102 
                 283 
                 50 
                 N/A 
               
               
                   
               
            
           
           
               
               
               
            
               
                   
                 Steel sheet for hot stamping 
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Unauto- 
                   
                   
               
               
                   
                   
                   
                 Ni 
                   
                 tempered 
               
               
                   
                   
                 Amount 
                 content 
                 Average 
                 martensite 
               
               
                   
                   
                 of plate 
                 in plating 
                 dislocation 
                 and lower 
                 Sheet 
               
               
                   
                 Steel 
                 attached 
                 layer 
                 density 
                 bainite 
                 thickness 
               
               
                   
                 No. 
                 (g/m 2 ) 
                 (mass %) 
                 (10 14  m/m 3 ) 
                 (area %) 
                 (mm) 
                 Note 
               
               
                   
                   
               
               
                   
                 41 
                 45 
                 16 
                 169 
                 23.1 
                 2.0 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 42 
                 58 
                 15 
                 229 
                 18.9 
                 1.8 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 43 
                 59 
                 15 
                 233 
                 23.2 
                 1.6 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 44 
                 45 
                 16 
                 219 
                 19.7 
                 1.2 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 45 
                 42 
                 18 
                 223 
                 22.4 
                 1.5 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 46 
                 58 
                 17 
                 219 
                 21.5 
                 2.0 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 47 
                 42 
                 17 
                 226 
                 17.5 
                 1.5 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 48 
                 48 
                 17 
                 223 
                 20.5 
                 1.8 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 49 
                 58 
                 17 
                 176 
                 23.4 
                 1.3 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 50 
                 42 
                 17 
                 198 
                 19.7 
                 1.6 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 51 
                 51 
                 15 
                 216 
                 19.2 
                 1.6 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 52 
                 60 
                 16 
                 197 
                 21.5 
                 1.6 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 53 
                 49 
                 17 
                 239 
                 19.9 
                 1.9 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 54 
                 40 
                 18 
                 211 
                 18.3 
                 1.4 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 55 
                 54 
                 16 
                 209 
                 21.6 
                 1.7 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 56 
                 44 
                 15 
                 181 
                 22.6 
                 1.7 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 57 
                 46 
                 16 
                 166 
                 18.9 
                 1.5 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 54 
                 25 
                   21   
                   8.9   
                 1.9 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 54 
                 25 
                   34   
                   3.9   
                 1.3 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 54 
                 25 
                 213 
                 19.7 
                 1.8 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 54 
                 25 
                 216 
                 21.3 
                 1.4 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 54 
                 25 
                   28   
                   1.8   
                 1.8 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 54 
                 25 
                 238 
                 25.1 
                 1.3 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 54 
                 25 
                 174 
                 28.6 
                 1.9 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 54 
                 25 
                 204 
                 39.4 
                 1.4 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 54 
                 25 
                 206 
                 46.8 
                 1.6 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 54 
                 25 
                  94 
                 15.3 
                 1.5 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 54 
                 25 
                 203 
                 21.5 
                 2.8 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 54 
                   8   
                 187 
                 21.9 
                 1.5 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 54 
                 11 
                 187 
                 21.9 
                 1.5 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 2 
                 47 
                 15 
                   2   
                   3.1   
                 1.4 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 9 
                 41 
                 16 
                 721 
                 
                   12.6 
                 
                 1.4 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 58 
                 55 
                 14 
                   29   
                 44.4 
                 1.6 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 59 
                 45 
                 17 
                  71 
                 19.7 
                 1.6 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 5 
               
               
                   
                   
               
             
            
               
                   
                 Heat treatment process during hot stamping 
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Elapsed 
                   
                   
                   
               
               
                   
                   
                   
                   
                   
                 time from 
               
               
                   
                   
                   
                   
                   
                 initiation 
               
               
                   
                   
                   
                   
                   
                 of heating 
                   
                   
                 Amount 
               
               
                   
                 Steel 
                   
                 Heating 
                 Holding 
                 to initiation 
                 Tempering 
                 Partial 
                 of plate 
               
               
                 Steel 
                 sheet 
                 Manufacture 
                 rate 
                 temperature 
                 of forming 
                 temperature 
                 softened 
                 attached 
               
               
                 No. 
                 No. 
                 No. 
                 (° C./s) 
                 (° C.) 
                 (s) 
                 (° C.) 
                 region 
                 (g/m 2 ) 
               
               
                   
               
               
                  1 
                   1   
                 B1 
                 130 
                 879 
                 173 
                 N/A 
                 No 
                 39 
               
               
                  2 
                   2   
                 B2 
                 139 
                 900 
                 176 
                 N/A 
                 No 
                 55 
               
               
                  3 
                   3   
                 B3 
                 145 
                 906 
                 150 
                 N/A 
                 No 
                 41 
               
               
                   4   
                   4   
                 B4 
                 145 
                 948 
                 160 
                 N/A 
                 No 
                 56 
               
               
                  5 
                  5 
                 B5 
                 165 
                 871 
                 132 
                 N/A 
                 No 
                 50 
               
               
                  6 
                  6 
                 B6 
                 142 
                 915 
                 160 
                 N/A 
                 No 
                 41 
               
               
                  7 
                  7 
                 B7 
                 130 
                 928 
                 154 
                 N/A 
                 No 
                 54 
               
               
                  8 
                  8 
                 B8 
                 143 
                 929 
                 161 
                 N/A 
                 No 
                 57 
               
               
                  9 
                  9 
                 B9 
                 146 
                 868 
                 135 
                 247 
                 No 
                 40 
               
               
                 
                   10 
                 
                 
                   10 
                 
                 B10 
                 147 
                 894 
                 152 
                 N/A 
                 No 
                 53 
               
               
                 
                   11 
                 
                 
                   11 
                 
                 B11 
                 139 
                 950 
                 148 
                 N/A 
                 No 
                 48 
               
               
                 12 
                 12 
                 B12 
                 129 
                 952 
                 167 
                 N/A 
                 No 
                 58 
               
               
                 13 
                 13 
                 B13 
                 180 
                 923 
                 123 
                 N/A 
                 No 
                 48 
               
               
                 14 
                 14 
                 B14 
                 154 
                 904 
                 152 
                 N/A 
                 No 
                 46 
               
               
                 
                   15 
                 
                 
                   15 
                 
                 B15 
                 174 
                 929 
                 140 
                 N/A 
                 No 
                 58 
               
               
                 16 
                 16 
                 B16 
                 157 
                 945 
                 129 
                 N/A 
                 No 
                 51 
               
               
                 17 
                 17 
                 B17 
                 180 
                 947 
                 148 
                 N/A 
                 No 
                 43 
               
               
                 18 
                 18 
                 B18 
                 128 
                 916 
                 157 
                 N/A 
                 No 
                 52 
               
               
                 19 
                 19 
                 B19 
                 166 
                 914 
                 131 
                 N/A 
                 No 
                 50 
               
               
                 20 
                 20 
                 B20 
                 133 
                 897 
                 165 
                 N/A 
                 No 
                 45 
               
               
                 
                   21 
                 
                 
                   21 
                 
                 B21 
                 160 
                 873 
                 166 
                 N/A 
                 No 
                 45 
               
               
                 22 
                 22 
                 B22 
                 120 
                 943 
                 149 
                 N/A 
                 No 
                 60 
               
               
                 23 
                 23 
                 B23 
                 151 
                 944 
                 143 
                 N/A 
                 No 
                 47 
               
               
                 24 
                 24 
                 B24 
                 136 
                 942 
                 178 
                 N/A 
                 No 
                 60 
               
               
                 
                   25 
                 
                 
                   25 
                 
                 B25 
                 168 
                 900 
                 143 
                 N/A 
                 No 
                 58 
               
               
                 26 
                 26 
                 B26 
                 124 
                 912 
                 126 
                 N/A 
                 No 
                 60 
               
               
                 27 
                 27 
                 B27 
                 149 
                 931 
                 130 
                 N/A 
                 No 
                 52 
               
               
                 28 
                 28 
                 B28 
                 139 
                 931 
                 145 
                 N/A 
                 No 
                 50 
               
               
                 29 
                 29 
                 B29 
                 128 
                 950 
                 137 
                 N/A 
                 No 
                 53 
               
               
                 
                   30 
                 
                 
                   30 
                 
                 B30 
                 143 
                 924 
                 130 
                 N/A 
                 No 
                 51 
               
               
                 
                   31 
                 
                 
                   31 
                 
                 B31 
                 127 
                 878 
                 132 
                 N/A 
                 No 
                 46 
               
               
                 32 
                 32 
                 B32 
                 159 
                 944 
                 153 
                 N/A 
                 No 
                 40 
               
               
                 33 
                 33 
                 B33 
                 157 
                 871 
                 124 
                 N/A 
                 No 
                 43 
               
               
                 34 
                 34 
                 B34 
                 142 
                 886 
                 131 
                 N/A 
                 No 
                 46 
               
               
                 35 
                 35 
                 B35 
                 125 
                 924 
                 123 
                 N/A 
                 No 
                 51 
               
               
                 
                   36 
                 
                 
                   36 
                 
                 B36 
                 138 
                 912 
                 163 
                 N/A 
                 No 
                 47 
               
               
                 37 
                 37 
                 B37 
                 142 
                 920 
                 146 
                 N/A 
                 No 
                 52 
               
               
                 38 
                 38 
                 B38 
                 126 
                 913 
                 168 
                 N/A 
                 No 
                 46 
               
               
                 39 
                 39 
                 B39 
                 163 
                 932 
                 169 
                 N/A 
                 No 
                 60 
               
               
                 
                   40 
                 
                 
                   40 
                 
                 B40 
                 124 
                 914 
                 140 
                 N/A 
                 No 
                 60 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Surface layer region 
                 Mechanical properties 
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Ni content 
                   
                 Concentration of 
                   
                 Evaluation 
                   
               
               
                   
                   
                 in plating 
                   
                 Ni in prior γ 
                 Tensile 
                 of hydrogen 
               
               
                   
                 Steel 
                 layer 
                 Martensite 
                 grain boundaries 
                 strength 
                 embrittlement 
               
               
                   
                 No. 
                 (mass %) 
                 (area %) 
                 (mass %) 
                 (MPa) 
                 resistance 
                 Note 
               
               
                   
                   
               
               
                   
                  1 
                 15 
                 97.9 
                 
                   1.7 
                 
                 1601 
                 
                   NG 
                 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                  2 
                 12 
                 97.8 
                 
                   1.0 
                 
                 2505 
                 
                   NG 
                 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                  3 
                 12 
                 96.1 
                 
                   2.1 
                 
                 1664 
                 
                   NG 
                 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                   4   
                 15 
                 95.3 
                 6.0 
                 
                   1081 
                 
                 — 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                  5 
                 17 
                 91.3 
                 7.1 
                 1632 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                  6 
                 17 
                 97.6 
                 6.5 
                 1990 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                  7 
                 25 
                 90.2 
                 9.7 
                 2136 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                  8 
                 16 
                 96.3 
                 6.5 
                 2575 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                  9 
                 17 
                 95.3 
                 6.7 
                 1866 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 
                   10 
                 
                 18 
                 93.0 
                 7.3 
                 
                   1445 
                 
                 — 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 
                   11 
                 
                 18 
                 90.1 
                 6.8 
                 2260 
                 
                   NG 
                 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 12 
                 15 
                 94.0 
                 5.9 
                 2011 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 13 
                 15 
                 98.6 
                 5.8 
                 2024 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 14 
                 15 
                 92.4 
                 5.9 
                 2271 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 
                   15 
                 
                 18 
                 90.0 
                 7.6 
                 
                   1363 
                 
                 — 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 16 
                 17 
                 91.1 
                 6.5 
                 2727 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 17 
                 18 
                 93.3 
                 7.3 
                 2078 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 18 
                 17 
                 90.6 
                 7.1 
                 2280 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 19 
                 17 
                 92.7 
                 6.5 
                 2028 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 20 
                 17 
                 94.3 
                 7.0 
                 2173 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 
                   21 
                 
                 17 
                 96.2 
                 7.1 
                 2459 
                 
                   NG 
                 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 22 
                 17 
                 96.8 
                 6.5 
                 1927 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 23 
                 15 
                 92.9 
                 5.9 
                 2099 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 24 
                 15 
                 92.3 
                 6.0 
                 2055 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 
                   25 
                 
                 15 
                 99.0 
                 5.9 
                 2134 
                 
                   NG 
                 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 26 
                 16 
                 95.8 
                 6.5 
                 2300 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 27 
                 17 
                 95.1 
                 6.7 
                 2257 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 28 
                 18 
                 98.0 
                 7.1 
                 2141 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 29 
                 17 
                 97.4 
                 7.1 
                 2201 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 
                   30 
                 
                 16 
                 92.9 
                 6.3 
                 2217 
                 
                   NG 
                 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 
                   31 
                 
                 16 
                 94.6 
                 6.7 
                 2287 
                 
                   NG 
                 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 32 
                 15 
                 97.1 
                 5.7 
                 2210 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 33 
                 18 
                 91.7 
                 7.5 
                 2083 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 34 
                 15 
                 93.5 
                 6.2 
                 2192 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 35 
                 15 
                 92.9 
                 5.9 
                 1995 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 
                   36 
                 
                 15 
                 96.0 
                 6.0 
                 2280 
                 
                   NG 
                 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 37 
                 16 
                 98.1 
                 6.1 
                 2118 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 38 
                 15 
                 98.0 
                 6.2 
                 2084 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 39 
                 16 
                 98.1 
                 6.2 
                 2137 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 
                   40 
                 
                 16 
                 95.0 
                 6.5 
                 2157 
                 
                   NG 
                 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                   
                 TABLE 6 
               
               
                   
                   
               
             
            
               
                   
                 Heat treatment process during hot stamping 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Elapsed time 
                   
                   
                   
               
               
                   
                   
                   
                   
                   
                 from initiation 
               
               
                   
                   
                   
                   
                   
                 of heating 
                   
                   
                 Amount 
               
               
                   
                 Steel 
                   
                 Heating 
                 Holding 
                 to initiation 
                 Tempering 
                 Partial 
                 of plate 
               
               
                 Steel 
                 sheet 
                 Manufacture 
                 rate 
                 temperature 
                 of forming 
                 temperature 
                 softened 
                 attached 
               
               
                 No. 
                 No. 
                 No. 
                 (° C./s) 
                 (° C.) 
                 (s) 
                 (° C.) 
                 region 
                 (g/m 2 ) 
               
               
                   
               
               
                 41 
                 41 
                 B41 
                 172 
                 869 
                 141 
                 N/A 
                 No 
                 45 
               
               
                 42 
                 42 
                 B42 
                 132 
                 895 
                 127 
                 N/A 
                 No 
                 58 
               
               
                 43 
                 43 
                 B43 
                 162 
                 962 
                 130 
                 N/A 
                 No 
                 59 
               
               
                 44 
                 44 
                 B44 
                 164 
                 941 
                 158 
                 N/A 
                 No 
                 45 
               
               
                 45 
                 45 
                 B45 
                 168 
                 937 
                 156 
                 N/A 
                 No 
                 42 
               
               
                 46 
                 46 
                 B46 
                 144 
                 932 
                 134 
                 N/A 
                 No 
                 58 
               
               
                 47 
                 47 
                 B47 
                 177 
                 928 
                 132 
                 N/A 
                 No 
                 42 
               
               
                 48 
                 48 
                 B48 
                 120 
                 946 
                 174 
                 N/A 
                 No 
                 48 
               
               
                 49 
                 49 
                 B49 
                 153 
                 1016  
                 142 
                 N/A 
                 No 
                 58 
               
               
                 50 
                 50 
                 B50 
                 136 
                 913 
                 120 
                 N/A 
                 No 
                 42 
               
               
                 51 
                 51 
                 B51 
                 164 
                 930 
                 152 
                 N/A 
                 No 
                 51 
               
               
                 52 
                 52 
                 B52 
                 158 
                 945 
                 141 
                 N/A 
                 No 
                 60 
               
               
                 53 
                 53 
                 B53 
                 158 
                 864 
                 134 
                 N/A 
                 No 
                 49 
               
               
                 54 
                 54 
                 B54 
                 158 
                 944 
                 168 
                 N/A 
                 No 
                 40 
               
               
                 55 
                 55 
                 B55 
                 174 
                 913 
                 146 
                 N/A 
                 No 
                 54 
               
               
                 56 
                 56 
                 B56 
                 151 
                 907 
                 121 
                 N/A 
                 No 
                 44 
               
               
                 57 
                 57 
                 B57 
                 167 
                 872 
                 147 
                 N/A 
                 No 
                 46 
               
               
                 7 
                 
                   58 
                 
                 B58 
                 178 
                 886 
                 151 
                 N/A 
                 No 
                 54 
               
               
                 7 
                 
                   59 
                 
                 B59 
                 151 
                 888 
                 124 
                 N/A 
                 No 
                 54 
               
               
                 7 
                 60 
                 B60 
                 132 
                 936 
                 173 
                 N/A 
                 No 
                 54 
               
               
                 7 
                 61 
                 B61 
                 149 
                 897 
                 125 
                 N/A 
                 No 
                 54 
               
               
                 7 
                 
                   62 
                 
                 B62 
                 128 
                 900 
                 141 
                 N/A 
                 No 
                 54 
               
               
                 7 
                 63 
                 B63 
                 124 
                 905 
                 136 
                 N/A 
                 No 
                 54 
               
               
                 7 
                 64 
                 B64 
                 176 
                 907 
                 150 
                 N/A 
                 No 
                 54 
               
               
                 7 
                 65 
                 B65 
                 175 
                 863 
                 161 
                 N/A 
                 No 
                 54 
               
               
                 7 
                 66 
                 B66 
                 125 
                 934 
                 165 
                 N/A 
                 No 
                 54 
               
               
                 7 
                 67 
                 B67 
                 155 
                 889 
                 136 
                 N/A 
                 No 
                 54 
               
               
                 7 
                 68 
                 B68 
                 131 
                 927 
                 135 
                 N/A 
                 No 
                 54 
               
               
                 7 
                 
                   69 
                 
                 B69 
                 171 
                 881 
                 147 
                 N/A 
                 No 
                 54 
               
               
                 7 
                 70 
                 B70 
                 144 
                 930 
                 172 
                 N/A 
                 No 
                 54 
               
               
                 7 
                  7 
                 B71 
                   92   
                 926 
                 136 
                 N/A 
                 No 
                 54 
               
               
                 7 
                  7 
                 B72 
                 108 
                 952 
                 124 
                 N/A 
                 No 
                 54 
               
               
                 7 
                  7 
                 B73 
                 190 
                 894 
                 146 
                 N/A 
                 No 
                 54 
               
               
                 7 
                  7 
                 B74 
                 155 
                 
                   796 
                 
                 147 
                 N/A 
                 No 
                 54 
               
               
                 7 
                  7 
                 B75 
                 152 
                   1059   
                 150 
                 N/A 
                 No 
                 54 
               
               
                 7 
                  7 
                 B76 
                 154 
                 944 
                 144 
                 N/A 
                 No 
                 54 
               
               
                 7 
                  7 
                 B77 
                 122 
                 905 
                 
                   249 
                 
                 N/A 
                 No 
                 54 
               
               
                 7 
                  7 
                 B78 
                 142 
                 946 
                 154 
                 N/A 
                 Yes 
                 54 
               
               
                 2 
                 
                   71 
                 
                 B79 
                 188 
                 897 
                 174 
                 N/A 
                 No 
                 47 
               
               
                 9 
                 
                   72 
                 
                 B80 
                 161 
                 908 
                 126 
                 N/A 
                 No 
                 41 
               
               
                 58 
                 73 
                 B81 
                 121 
                 896 
                 123 
                 N/A 
                 No 
                 55 
               
               
                 59 
                 74 
                 B82 
                 148 
                 908 
                 131 
                 N/A 
                 No 
                 45 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Surface layer region 
                 Mechanical properties 
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Ni content 
                   
                 Concentration 
                   
                 Evaluation 
                   
               
               
                   
                   
                 in plating 
                   
                 of Ni in prior γ 
                 Tensile 
                 of hydrogen 
               
               
                   
                 Steel 
                 layer 
                 Martensite 
                 grainboundaries 
                 strength 
                 embrittlement 
               
               
                   
                 No. 
                 (mass %) 
                 (area %) 
                 (mass %) 
                 (MPa) 
                 resistance 
                 Note 
               
               
                   
                   
               
               
                   
                 41 
                 16 
                 92.6 
                 6.5 
                 2369 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 42 
                 15 
                 95.7 
                 5.8 
                 2396 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 43 
                 15 
                 95.5 
                 6.2 
                 2133 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 44 
                 16 
                 97.3 
                 6.4 
                 2325 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 45 
                 18 
                 96.3 
                 7.5 
                 2115 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 46 
                 17 
                 95.7 
                 7.1 
                 2461 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 47 
                 17 
                 97.7 
                 6.5 
                 2402 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 48 
                 17 
                 90.4 
                 7.1 
                 2433 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 49 
                 17 
                 92.6 
                 7.1 
                 2386 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 50 
                 17 
                 90.0 
                 7.1 
                 2280 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 51 
                 15 
                 94.9 
                 5.9 
                 2502 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 52 
                 16 
                 97.8 
                 6.1 
                 2202 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 53 
                 17 
                 94.5 
                 6.8 
                 2097 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 54 
                 18 
                 98.8 
                 6.9 
                 2129 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 55 
                 16 
                 96.1 
                 6.6 
                 2178 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 56 
                 15 
                 91.5 
                 6.1 
                 2038 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 57 
                 16 
                 90.5 
                 6.1 
                 2300 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 25 
                 95.3 
                 
                   2.8 
                 
                 2205 
                 
                   NG 
                 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 25 
                 95.3 
                 
                   3.3 
                 
                 2142 
                 
                   NG 
                 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 25 
                 97.5 
                 6.1 
                 2019 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 25 
                 98.1 
                 9.4 
                 2226 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 25 
                 96.2 
                 
                   3.1 
                 
                 2098 
                 
                   NG 
                 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 25 
                 93.0 
                 9.4 
                 2116 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 25 
                 94.6 
                 9.7 
                 2058 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 25 
                 95.5 
                 11.6  
                 1999 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 25 
                 94.2 
                 11.4  
                 2181 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 25 
                 90.9 
                 8.9 
                 2333 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 25 
                 90.2 
                 9.1 
                 2039 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 
                   8 
                 
                 97.2 
                 
                   3.2 
                 
                 2017 
                 
                   NG 
                 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 11 
                 97.3 
                 5.6 
                 2026 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 25 
                 98.9 
                 
                   5.1 
                 
                 2110 
                 
                   NG 
                 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 25 
                 93.0 
                 5.8 
                 2080 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 25 
                 92.5 
                 9.5 
                 2182 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 25 
                 
                   79.2 
                 
                 
                   1.8 
                 
                 
                   1276 
                 
                 — 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 25 
                 97.7 
                 9.9 
                 2059 
                 
                   NG 
                 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 25 
                 91.3 
                 5.7 
                 2121 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 25 
                 95.6 
                 
                   5.1 
                 
                 2101 
                 
                   NG 
                 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 7 
                 25 
                 92.9 
                 9.1 
                 2077 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 2 
                 15 
                 91.3 
                 
                   0.8 
                 
                 2427 
                 
                   NG 
                 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 9 
                 16 
                 93.1 
                 
                   4.2 
                 
                 2547 
                 
                   NG 
                 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 58 
                 14 
                 92.8 
                 8.1 
                 
                   1489 
                 
                 — 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                 59 
                 17 
                 94.1 
                 6.9 
                 1579 
                 OK 
                 Invention 
               
               
                   
                   
                   
                   
                   
                   
                   
                 steel 
               
               
                   
                   
               
            
           
         
       
     
     The metallographic structures, the average dislocation densities, and the concentrations (amounts) of Ni of the steel sheets for hot stamping and the hot-stamped articles were measured by the above-described measurement methods. In addition, the mechanical properties of the hot-stamped articles were evaluated by the following methods. 
     “Tensile Strength” 
     Regarding the tensile strength of the hot-stamped article, a No. 5 test piece described in JIS Z 2201: 2011 was produced from an arbitrary position in the hot-stamped article, and the tensile strength was obtained according to the testing method described in JIS Z 2241: 2011. In a case where the tensile strength was lower than 1500 MPa, the tensile strength was evaluated as a failure, and the test described below was not carried out. 
     “Hydrogen Embrittlement Resistance” 
     The hydrogen embrittlement resistance of the hot-stamped article was evaluated by the following method.  FIG. 2  shows the shape of a test piece used for the evaluation of the hydrogen embrittlement resistance. A nominal stress of 1100 MPa, which was calculated by dividing the applied load by the cross-sectional area of the bottom of the cut, was imparted to a test piece shown in  FIG. 2  imparted with V notches, then, a constant load test in which electrolytic hydrogen charging was carried out on a 3 mass % NaCl aqueous solution at a current density of 0.1 mA/cm 2  for 48 hours was carried out at room temperature, and the hydrogen embrittlement resistance was determined depending on the presence or absence of breakage. In the tables, cases where no breakage occurred are indicated as pass (OK), and cases where breakage occurred were indicated as failure (NG). R 10  shown in  FIG. 2  indicates that the curvature radius is 10 mm. 
     In Tables 5 and 6, in cases where the tensile strength was 1500 MPa or higher and the hydrogen embrittlement resistance was pass (OK), the strength and the hydrogen embrittlement resistance were considered to be excellent, and the steels were determined as invention steel. In cases where any one of the two performances described above was not satisfied, the steels were determined as comparative steel. 
     From Tables 5 and 6, it is found that hot-stamped articles having a chemical composition, a plate composition, and a metallographic structure within the scope of the present invention have an excellent strength and excellent hydrogen embrittlement resistance. 
     On the other hand, it is found that hot-stamped articles having any one or more of a chemical composition and a metallographic structure outside the scope of the present invention are poor in terms of one or more of strength and hydrogen embrittlement resistance. 
     INDUSTRIAL APPLICABILITY 
     According to the present invention, it is possible to provide a hot-stamped article having an excellent strength and excellent hydrogen embrittlement resistance.