Abstract:
High tensile strength steels that have both favorable delayed fracture resistance and a tensile strength of 600 MPa or higher and are suitably used in construction machinery, tanks, penstocks, and pipelines, as well as methods for manufacturing such steels are provided. The safety index of delayed fracture resistance (%) is 100×(X 1 /X 0 ), where X 0 : reduction of area of a specimen substantially free from diffusible hydrogen, and X 1 : reduction of area of a specimen containing diffusible hydrogen.

Description:
RELATED APPLICATIONS 
     This is a §371 of International Application No. PCT/JP2008/052002, with an international filing date of Jan. 31, 2008 (WO 2008/093897 A1, published Aug. 7, 2008), which is based on Japanese Patent Application Nos. 2007-021573, filed Jan. 31, 2007, and 2007-086296, filed Mar. 29, 2007. 
     TECHNICAL FIELD 
     This disclosure relates to high tensile strength steels having favorable delayed fracture resistance and those having favorable delayed fracture resistance with the tensile strength thereof being 600 MPa or higher, in particular, 900 MPa or higher, as well as methods for manufacturing such steels. 
     BACKGROUND 
     Recently, in the fields involving the use of steels, such as construction machinery (e.g., moves and chassis for cranes), tanks, penstocks, and pipelines, the increasing size of structures urges steels to be stronger and also the use environment of such steels has been becoming progressively harsher. 
     However, strengthening of steels and a harsher use environment are generally known to increase the susceptibility of steels to delayed fractures. For example, in the field of high tensile bolts, JIS (Japanese Industrial Standards) B 1186 stipulates that the use of F11T bolts (tensile strength: 1100 to 1300 N/mm 2 ) should be avoided whenever possible, indicating that the use of high strength steels is limited. 
     In response to this, methods for manufacturing steels with favorable delayed fracture resistance have been proposed in publications including Japanese Unexamined Patent Application Publication No. H3-243745, Japanese Unexamined Patent Application Publication No. 2003-73737, Japanese Unexamined Patent Application Publication No. 2003-239041, Japanese Unexamined Patent Application Publication No. 2003-253376, and Japanese Unexamined Patent Application Publication No. 2003-321743. These methods are based on various techniques, such as optimization of components, strengthening of grain boundaries, decreasing the size of crystal grains, the use of hydrogen-trapping sites, control of structural morphology, and fine dispersion of carbides. 
     However, the methods described in the publications listed above, including Japanese Unexamined Patent Application Publication No. H3-243745, Japanese Unexamined Patent Application Publication No. 2003-73737, Japanese Unexamined Patent Application Publication No. 2003-239041, Japanese Unexamined Patent Application Publication No. 2003-253376, and Japanese Unexamined Patent Application Publication No. 2003-321743, do not produce sufficiently strong steels achieving a delayed fracture resistance level that is required in applications where they are exposed to a severely corrosive environment. Thus, steels having both better delayed fracture resistance and a high level of tensile strength, in particular, a tensile strength of 900 MPa or higher, and methods for manufacturing such steels are demanded. 
     Delayed fractures reportedly occur when hydrogen able to diffuse in steel at room temperature, namely so-called “diffusible hydrogen,” gathers at a stress concentration zone and reaches the threshold limit value of the material. This threshold limit value depends on material strength, its structure, and other parameters. 
     In general, a delayed fracture of high strength steels starts from non-metallic inclusions, such as MnS, and grows along grain boundaries, such as prior austenite grain boundaries. 
     Thus, ways of improving delayed fracture resistance include reduction of the amount of non-metallic inclusions, such as MnS, and strengthening of prior austenite grain boundaries. 
     It could therefore be helpful to provide a high tensile strength steel having delayed fracture resistance better than that of known steels with the tensile strength thereof being 600 MPa or higher, in particular, 900 MPa or higher, as well as a method for manufacturing such a steel. 
     SUMMARY 
     We discovered that high tensile strength steels having delayed fracture resistance better than those of known steels can be obtained by the following principles: reduction of the amount of P and S that are impurity elements as well as extension of crystal grains and introduction of deformation bands via rolling of non-recrystallization regions can prevent the formation of MnS, non-metallic inclusions; a decrease in the covering density of grain boundaries of P, which is an impurity element, segregated in prior austenite grain boundaries, which may be followed by reduction of the amount of cementite precipitations formed in the boundaries of laths, can prevent a decrease in the strength of the prior austenite grain boundaries. 
     We thus provide: 
     1. A high tensile strength steel having favorable delayed fracture resistance, containing elements C: 0.02 to 0.25%, Si: 0.01 to 0.8%, Mn: 0.5 to 2.0%, Al: 0.005 to 0.1%, N: 0.0005 to 0.008%, P: 0.02% or lower, and S: 0.004% or lower, all in percent by mass, and Fe and unavoidable impurities as the balance, wherein the average aspect ratio of prior austenite grains calculated over the entire thickness is at least three; 
     2. The high tensile strength steel according to 1, wherein S: 0.003% or lower and the cementite covering ratio measured at boundaries of laths is 50% or lower; 
     3. The high tensile strength steel having favorable delayed fracture resistance according to 1 or 2, further containing one or more of Mo: 1% or lower, Nb: 0.1% or lower, V: 0.5% or lower, Ti: 0.1% or lower, Cu: 2% or lower, Ni: 4% or lower, Cr: 2% or lower, and W: 2% or lower, all in percent by mass; 
     4. The high tensile strength steel having favorable delayed fracture resistance according to 1 to 3, further containing one or more of B: 0.003% or lower, Ca: 0.01% or lower, REM: 0.02% or lower, and Mg: 0.01% or lower; 
     5. The high tensile strength steel having. favorable delayed fracture resistance according to any one of 1 to 4, wherein, hydrogen is charged into the steel and the hydrogen contained in the steel is sealed by zinc galvanizing, the safety index of delayed fracture resistance calculated using the formula described below being at least 75% when a slow strain rate test is performed with the strain rate set to 1×10 −3 /s or lower:
 
Safety index of delayed fracture resistance (%)=100×( X   1   /X   0 )
         where X 0 : reduction of area of a specimen substantially free from diffusible hydrogen, and   X 1 : reduction of area of a specimen containing diffusible hydrogen;       

     6. The high tensile strength steel according to 5, wherein the safety index of delayed fracture resistance is at least 80%; 
     7. A method for manufacturing the high tensile strength steel having favorable delayed fracture resistance according to 5, including a step of casting steel having the composition according to any one of 1 to 4, a step of protecting the steel from cooling to the Ar 3  transformation temperature or lower or heating the steel to a temperature equal to or higher than the Ac 3  transformation temperature once again, a step of hot rolling to achieve a predetermined steel thickness including rolling conducted with the rolling reduction for non-recrystallization regions set to 30% or higher, a step of cooling the steel from a temperature equal to or higher than the Ar 3  transformation temperature to a temperature equal to or lower than 350° C. at a cooling rate of 1° C./s or higher, and a step of tempering the steel at a temperature equal to or lower than the Ac 1  transformation temperature; 
     8. The method according to 7, in which the steel is tempered at a temperature equal to or lower than the Ac 1  transformation temperature, for manufacturing the high tensile strength steel having favorable delayed fracture resistance according to 6, wherein a heating apparatus installed in a manufacturing line having a rolling mill and a cooling apparatus is used to heat the steel from 370° C. to a predetermined tempering temperature equal to or lower than the Ac 1  transformation while maintaining the average heating rate for heating the middle of the steel thickness at 1° C./s or higher so that the maximum tempering temperature at the middle of the steel thickness is 400° C. or higher; and 
     9. The method according to 8, in which the steel is tempered at a temperature equal to or lower than the Ac 1  transformation temperature, for manufacturing the high tensile strength steel having favorable delayed fracture resistance according to 6, wherein the steel is heated from a tempering initiation temperature to 370° C. with the average heating rate for heating the middle of the steel thickness maintained at 2° C./s or higher. 
     We enable manufacturing high tensile strength steels having excellent delayed fracture resistance with the tensile strength thereof being 600 MPa or higher, in particular, 900 MPa or higher, and thus has very high industrial applicability. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 : A schematic diagram of a martensite structure. 
         FIG. 2 : Schematic diagrams and transmission electron microscope (TEM) images (extracted replicas) showing cementite precipitations formed in the boundaries of laths during slow-heating tempering and rapid-heating tempering. 
     
    
    
     DETAILED DESCRIPTION 
     Component Compositions 
     The following are reasons for selection of the components. The percentages representing the content ratios of chemical components are all in percent by mass. C: 0.02 to 0.25% 
     C ensures strength. C contained at a content ratio lower than 0.02% would have an insufficient effect, whereas C contained at a content ratio higher than 0.25% would result in reduced toughness of the base material and weld-heat-affected zones and significantly deteriorated weldability. Therefore, the content ratio of C should be in the range of 0.02 to 0.25% and is preferably in the range of 0.05 to 0.20%. 
     Si: 0.01 to 0.8% 
     Si is used as a deoxidizing material and a reinforcing element in a steel-making process. Si contained at a content ratio lower than 0.01% would have an insufficient effect, whereas Si contained at a content ratio higher than 0.8% would make grain boundaries brittle, thereby promoting the development of delayed fractures. Therefore, the content ratio of Si should be in the range of 0.01 to 0.8% and is preferably in the range of 0.1 to 0.5%. 
     Mn: 0.5 to 2.0% 
     Mn ensures strength and, during the tempering step, is concentrated in cementite to prevent coarsening thereof by diffusing as substitutional atoms to limit the cementite growth rate. Mn contained at a content ratio lower than 0.5% would have an insufficient effect, whereas Mn contained at a content ratio higher than 2.0% would result in reduced toughness of weld-heat-affected zones and significantly deteriorated weldability. Therefore, the content ratio of Mn should be in the range of 0.5 to 2.0% and is preferably in the range of 0.7 to 1.8%. 
     Al: 0.005 to 0.1% 
     Al is added as a deoxidizing material also having the effect of downsizing the diameters of crystal grains. Al contained at a content ratio lower than 0.005% would have an insufficient effect, whereas Al contained at a content ratio higher than 0.1% would increase the risk of surface flaws of resulting steels. Therefore, the content ratio of Al should be in the range of 0.005 to 0.1% and is preferably in the range of 0.01 to 0.05%. 
     N: 0.0005 to 0.008% 
     N binds to Ti or the like to form nitrides that reduce the size of resulting structures, thereby improving the toughness of the base material and weld-heat-affected zones. N contained at a content ratio lower than 0.0005% would result in insufficient downsizing of the resulting structures, whereas N contained at a content ratio higher than 0.008% would lead to an increased amount of a solid solution of N, thereby reducing the toughness of the base material and weld-heat-affected zones. Therefore, the content ratio of N should be in the range of 0.0005 to 0.008% and is preferably in the range of 0.001 to 0.005%. 
     P: 0.02% or Lower 
     P, which is an impurity element, is often segregated in crystal grain boundaries such as prior austenite grains during the tempering process. P contained at a content ratio higher than 0.02% would result in weakened bonds between adjacent crystal grains, thereby reducing low-temperature toughness and delayed fracture resistance. Therefore, the content ratio of P should be 0.02% or lower and is preferably 0.015% or lower. 
     S: 0.004% or Lower 
     S, which is an impurity element, often forms non-metallic inclusions, MnS. S contained at a content ratio higher than 0.004% would produce a vast amount of inclusions and thus reduce ductile fracture resistance, thereby deteriorating low-temperature toughness and delayed fracture resistance. Therefore, the content ratio of S should be 0.004% or lower and is preferably 0.003% or lower. 
     The following components may also be added if desired. 
     Mo: 1% or Lower 
     Mo has the effect of improving quenching properties and strength and forms carbides that trap diffusible hydrogen and enhance delayed fracture resistance. To achieve these effects, the content ratio of Mo is preferably 0.05% or higher. However, the addition of Mo at a content ratio higher than 1% would be uneconomic. Therefore, when Mo is added, the content ratio thereof should be 1% or lower and is preferably 0.8% or lower. It should be noted that Mo has the effect of improving temper softening resistance and thus, to ensure a strength of 900 MPa or higher, the content ratio thereof is preferably 0.2% or higher. 
     Nb: 0.1% or Lower 
     Nb is a microalloying element that improves strength, and forms carbides, nitrides, and carbonitrides that trap diffusible hydrogen and enhance delayed fracture resistance. To achieve these effects, the content ratio of Nb is preferably 0.01% or higher. However, the addition of Nb at a content ratio higher than 0.1% would result in reduced toughness of weld-heat-affected zones. Therefore, when Nb is added, the content ratio thereof should be 0.1% or lower and is preferably 0.05% or lower. 
     V: 0.5% or Lower 
     V is a microalloying element that improves strength, and forms carbides, nitrides, and carbonitrides that trap diffusible hydrogen and enhance delayed fracture resistance. To achieve these effects, the content ratio of V is preferably 0.02% or higher. However, the addition of V at a content ratio higher than 0.5% would result in reduced toughness of weld-heat-affected zones. Therefore, when V is added, the content ratio thereof should be 0.5% or lower and is preferably 0.1% or lower. 
     Ti: 0. 1% or Lower 
     When hot-rolled or welded, Ti forms TiN to prevent the growth of austenite grains, thereby improving the toughness of the base material and weld-heat-affected zones, and forms carbides, nitrides, and carbonitrides that trap diffusible hydrogen and enhance delayed fracture resistance. To achieve these effects, the content ratio of Ti is preferably 0.005% or higher. However, the addition of Ti at a content ratio higher than 0.1% would result in reduced toughness of weld-heat-affected zones. Therefore, when Ti is added, the content ratio thereof should be 0.1% or lower and is preferably 0.05% or lower. 
     Cu: 2% or Lower 
     Cu has the effect of improving strength through solid solution strengthening and precipitation strengthening. To achieve this effect, the content ratio of Cu is preferably 0.05% or higher. However, the addition of Cu at a content ratio higher than 2% would increase the risk of hot tearing that occurs during heating slabs or welding. Therefore, when Cu is added, the content ratio thereof should be 2% or lower and is preferably 1.5% or lower. 
     Ni: 4% or Lower 
     Ni has the effect of improving toughness and quenching properties. To achieve this effect, the content ratio of Ni is preferably 0.3% or higher. However, the addition of Ni at a content ratio higher than 4% would be uneconomic. Therefore, when Ni is added, the content ratio thereof should be 4% or lower and is preferably 3.8% or lower. 
     Cr: 2% or Lower 
     Cr has the effect of improving strength and toughness and is excellent in terms of high-temperature strength properties. Furthermore, during the tempering step, Cr is concentrated in cementite to prevent coarsening thereof by diffusing as substitutional atoms to limit the cementite growth rate. Thus, it is preferable to add Cr whenever possible for the purposes of improving strength, preventing coarsening of cementite, and, in particular, achieving a tensile strength of 900 MPa or higher, at a content ratio of 0.3% or higher. However, the addition of Cr at a content ratio higher than 2% would result in reduced weldability. Therefore, when Cr is added, the content ratio thereof should be 2% or lower and is preferably 1.5% or lower. 
     W: 2% or Lower 
     W has the effect of improving strength. To achieve this effect, the content ratio of W is preferably 0.05% or higher. However, the addition of W at a content ratio higher than 2% would result in reduced weldability. Therefore, when W is added, the content ratio thereof should be 2% or lower. 
     B: 0.003% or Lower 
     B has the effect of improving quenching properties. To achieve this effect, the content ratio of B is preferably 0.0003% or higher. However, the addition of B at a content ratio higher than 0.003% would result in reduced toughness. Therefore, when B is added, the content ratio thereof should be 0.003% or lower. 
     Ca: 0.01% or Lower 
     Ca is an element essential to control the morphology of sulfide inclusions. To achieve this effect, the content ratio of Ca is preferably 0.0004% or higher. However, the addition of Ca at a content ratio higher than 0.01% would result in reduced cleanliness and delayed fracture resistance. Therefore, when Ca is added, the content ratio thereof should be 0.01% or lower. 
     REM: 0.02% or Lower 
     REM (note: REM is an abbreviation representing Rare Earth Metal) forms REM (rare-earth metal) oxysulfides, namely REM (O, S), in steel to reduce the amount of solid solution S at crystal grain boundaries, thereby improving SR (stress relief) cracking resistance (in other words, PWHT (post welded heat treatment) cracking resistance). To achieve this effect, the content ratio of REM is preferably 0.001% or higher. However, the addition of REM at a content ratio higher than 0.02% would cause material deterioration due to significant deposition of REM oxysulfides on precipitated crystal bands. Therefore, when REM is added, the content ratio thereof should be 0.02% or lower. 
     Mg: 0.01% or Lower 
     Mg is used as a hot metal desulfurization agent in some cases. To achieve this effect, the content ratio of Mg is preferably 0.001% or higher. However, the addition of Mg at a content ratio higher than 0.01% would result in reduced cleanliness. Therefore, when Mg is added, the content ratio thereof should be 0.01% or lower. 
     Microstructure 
     The following are reasons for selection of the microstructure. 
     The representative structures of the high strength steel are martensite and bainite. In particular, a martensite structure has, as shown in the schematic structure diagram of  FIG. 1 , a fine and complex morphology in which a plurality of four kinds of characteristic structure units (prior austenite, packets, blocks, and laths) are layered. The packets described herein are defined as regions each consisting of a population of parallel laths having the same habit plane. The blocks consist of a population of parallel laths having the same orientation. 
     The average aspect ratio of prior austenite grains calculated over the entire steel thickness (in  FIG. 1 , the ratio a/b between the major axis a and the minor axis b of the prior austenite grain) is at least three and preferably at least four. 
     The aspect ratio of prior austenite grains being at least three reduces the grain boundary covering ratio of P segregated in prior austenite grain boundaries, packet boundaries, or the like, thereby improving low-temperature toughness and delayed fracture resistance, and such microstructures distributing over the entire steel thickness provide homogenous steel having the properties described above. 
     To measure the aspect ratio of prior austenite grains, prior austenite grains are developed using, for example, picric acid, and then image analysis is performed to simply average aspect ratios of, for example, 500 or more prior austenite grains. 
     The state in which the average aspect ratio of prior austenite grains calculated over the entire thickness is at least three means that the average aspect ratio calculated from values obtained at the following positions is at least three and preferably at least four: 1 mm in depth from the surface of steel, positions located at ¼, ½, and ¾ of the steel thickness, and 1 mm in depth from the back surface of the steel. 
     In addition to the findings described above, we found that reducing the ratio of cementite precipitating in the boundaries between many fine laths generated in the blocks illustrated in  FIG. 1  (hereinafter, referred to as the cementite covering ratio of lath boundaries) to 50% or lower particularly prevents a decrease in the strength. of prior austenite grain boundaries and thus improves delayed fracture resistance. Preferably, the cementite covering ratio of lath boundaries is 30% or lower.  FIG. 2  includes schematic diagrams and TEM images showing cementite precipitations formed in the boundaries of laths. 
     The cementite covering ratio of lath boundaries is determined by imaging a structure developed using nital (a solution of nitric acid and an alcohol) with a scanning electron microscope as shown in  FIG. 2 ; analyzing, for example, 50 or more laths in the obtained image in terms of the lengths of formed cementite precipitations along the lath boundaries (L Cementite ) and the lengths of the lath boundaries (L Lath ); dividing the sum of the lengths of cementite along the lath boundaries by the sum of the lengths of the lath boundaries; and then multiplying the quotient by 100. 
     Safety Index of Delayed Fracture Resistance 
     Hydrogen is charged into the steel and the hydrogen contained in the steel is sealed by zinc galvanizing, the safety index of delayed fracture resistance calculated using the formula described below being at least 75% and preferably at least 80% when a slow strain rate test is performed with the strain rate set to 1×10 −3 /s or lower:
 
Safety index of delayed fracture resistance (%)=100×( X   1   /X   0 )
         where X 0 : reduction of the area of a specimen substantially free from diffusible hydrogen, and   X 1 : reduction of the area of a specimen containing diffusible hydrogen.       

     The safety index of delayed fracture resistance is a quantitative measure of delayed fracture resistance of steel, and the higher this index is, the better the delayed fracture resistance is. In the practical use of steel under normal atmospheric conditions, the safety index of delayed fracture resistance for sufficiently high delayed fracture resistance is 75% or higher and preferably 80% or higher. In some cases, however, steels having a tensile strength less than 1200 MPa would be used under harsh conditions such as a corrosive environment and lower temperatures or be difficult to process. Therefore, it is desirable that the safety index of delayed fracture resistance is 80% or higher and more preferably 85% or higher. 
     Manufacturing Conditions 
     We provide various forms of steels such as steel plates, steel shapes, and steel bars. The temperature specifications described in the manufacturing conditions are applicable to temperatures measured at the center of steel. As for steel plates, the center of the steel is taken as the middle of the steel thickness. As for steel shapes, it is taken as the middle of the steel thickness measured at a site to which selected properties are given. As for steel bars, it is taken as the middle of diameter. It should be noted that the surroundings of the center of steel experience temperature changes similar to those at the center, and thus the scope of the temperature specifications is not limited to the center itself 
     Cast Conditions 
     Our steels are effective regardless of casting conditions used to manufacture steels, and thus particular limitations on cast conditions are unnecessary. Any method can be used in manufacturing of cast slabs from liquid steel and rolling of the cast slabs to produce billets. Examples of methods that can be used to melt steel include converter processes and electric furnace processes, and examples of methods that can be used to produce slabs include continuous casting and ingot-based methods. 
     Hot-Rolling Conditions 
     In rolling of cast slabs to produce billets, the cast slabs may be protected from cooling to the Ar 3  transformation temperature or lower or allowed to cool and then heated to a temperature equal to or higher than the Ac 3  transformation temperature once again before the start of hot rolling. This is because effectiveness is ensured whenever rolling is started as long as the temperature at that time is in the range described above. 
     The rolling reduction for non-recrystallization regions is 30% or higher and preferably 40% or higher, and rolling is finished at a temperature equal to or higher than the Ar 3  transformation temperature. The reason why non-recrystallization regions are rolled with the rolling reduction being 30% or higher is because hot rolling performed in this way leads to extension of austenite grains and, at the same time, introduces deformation bands, thereby reducing the grain boundary covering ratio of P segregated in the grain boundaries during the tempering process. Higher aspect ratios of prior austenite grains would reduce effective grain sizes (sizes of grains that are fracture appearance units or, more specifically, packets) and the grain boundary covering ratios of P covering the prior austenite grains, packet boundaries, or the like, thereby improving delayed fracture resistance. 
     No particular limitation is imposed on formulae used to calculate the Ar 3  transformation temperature (° C.) and the Ac 3  transformation temperature (° C.). For example, Ar 3 =910−310C−80Mn−20Cu−15Cr−55Ni−80Mo, and Ac 3 =854−180C+44Si−14Mn−17.8Ni−1.7Cr. In these formulae, each of the elements represents the content ratio (percent by mass) thereof in the steel. 
     Post-Hot-Rolling Cooling Conditions 
     After the completion of hot rolling, the steel is forcedly cooled from a temperature equal to or higher than the Ar 3  transformation temperature to a temperature of 350° C. or lower at a cooling rate of 1° C./s or higher to ensure the strength and toughness of the base material. The reason why the forced-cooling initiation temperature is equal to or higher than the Ar 3  transformation temperature is because steel plates should consist of austenite phases only in the start of cooling. Cooling started when the temperature is lower than the Ar 3  transformation temperature would result in unevenly tempered structures and reduced toughness and delayed fracture resistance. The reason why steel plates are cooled to a temperature of 350° C. or lower is because such a low temperature is required to complete transformation from austenite to martensite or bainite, thereby improving the toughness and delayed fracture resistance of the base material. The cooling rate used in this process is 1° C./s or higher and preferably 2° C./s or higher. It should be noted that the cooling rate is defined as the average cooling rate obtained by dividing the temperature difference required in cooling the steel after hot rolling it from a temperature equal to or higher than the Ar 3  transformation temperature to a temperature of 350° C. or lower by the time required in this cooling process. 
     Tempering Conditions 
     The tempering process is performed at a certain temperature that makes the maximum temperature at the middle of the steel thickness equal to or lower than the Ac 1  transformation temperature. The reason why the maximum temperature should be equal to or lower than the Ac 1  transformation temperature is because, when it exceeds the Ac 1  transformation temperature, austenite transformation significantly reduces strength. Meanwhile, in this tempering process, an on-line heating apparatus installed in a manufacturing line having a rolling mill and a cooling apparatus and after the cooling apparatus is preferably used. This shortens the time required in the process including rolling, quenching, and tempering, thereby improving the productivity. 
     In this tempering process, the heating rate is preferably 0.05° C./s or higher. A heating rate lower than 0.05° C./s would increase the amount of P segregated in prior austenite grains, packet boundaries, or the like during tempering, thereby deteriorating low-temperature toughness and delayed fracture resistance. In addition, in slow heating where the heating rate for tempering is 2° C./s or lower, the time for which the tempering temperature is maintained is preferably 30 min or shorter because such a tempering time would prevent the growth of precipitations such as cementite and improve the productivity. 
     More preferred tempering conditions are rapid-heating conditions where the average heating rate for heating the middle of the steel thickness from 370° C. to a certain temperature equal to or lower than the Ac 1  transformation temperature is 1° C./s or higher and the maximum temperature at the middle of the steel thickness is 400° C. or higher. 
     The reason why the average heating rate is 1° C./s or higher is because such a heating rate would reduce the grain boundary covering density of P, an impurity element segregated in prior austenite grain boundaries, packet boundaries, or the like, and achieve lath boundaries with a reduced amount of cementite precipitations, which are shown in  FIG. 2  providing the comparison between the slow-heating tempering and the rapid-heating tempering in terms of the schematic diagram and the TEM image showing cementite precipitations formed in the boundaries of laths. 
     More effective prevention of grain boundary segregation of P in prior austenite grain boundaries, packet boundaries, or the like would be preferably achieved by performing rapid heating where the average heating rate at the middle of the steel thickness for heating from the tempering initiation temperature to 370° C. is 2° C./s or higher in addition to the above-described rapid heating process, where the average heating rate at the middle of the steel thickness for heating from 370° C. to a certain tempering temperature equal to or lower than the Ac 1  transformation temperature is 1° C./s or higher. 
     The reason why the average heating rate at the middle of the steel thickness for heating from the tempering initiation temperature to 370° C. is 2° C./s or higher is because segregation of P in prior austenite grain boundaries, packet boundaries, or the like is particularly promoted in this temperature range. 
     Meanwhile, when the average heating rate at the middle of the steel thickness for heating from 370° C. to a certain tempering temperature equal to or lower than the Ac 1  transformation temperature is 1° C./s or higher and the average heating rate at the middle of the steel thickness for heating from the tempering initiation temperature to 370° C. is 2° C./s or higher, the time for which the tempering temperature is maintained is preferably 60 s or shorter because such a tempering time would prevent a decrease in productivity and deterioration of delayed fracture resistance due to coarsening of precipitations such as cementite. In addition, the heating rate is defined as the average heating rate obtained by dividing the temperature difference required in reheating the steel to a certain temperature so that the maximum temperature at the middle of the steel thickness is equal to or lower than the Ac 1  transformation temperature after cooling it by the time required in this reheating process. 
     The average cooling rate for cooling the tempered steel from the tempering temperature to 200° C. is preferably 0.05° C./s or higher to prevent coarsening of precipitations during this cooling process. 
     Meanwhile, the heating method for tempering may be induction heating, energization heating, infra-red radiant heating, furnace heating, or any other heating method. 
     The tempering apparatus may be a heating apparatus installed in a manufacturing line that is different from one having a rolling mill and a direct quenching apparatus or that installed in a manufacturing line having a rolling mill and a direct quenching apparatus so as to be directly connected to them. None of these heating apparatuses spoils the advantageous effect. 
     EXAMPLE 1 
     Tables 1 and 2 show the chemical compositions of the steels used in this example, whereas Tables 3 and 4 show the steel manufacturing conditions and aspect ratios of prior austenite grains. 
     Steels A to Z and AA to II whose chemical compositions are shown in Tables 1 and 2 were melted and cast into slabs (slab dimensions: 100 mm in height×150 mm in width×150 mm in length). The obtained slabs were heated in a furnace to the heating temperatures shown in Tables 3 and 4 and then hot-rolled with the rolling reduction for non-recrystallization regions set to the values shown in Tables 3 and 4 to produce steel plates. After the hot-rolling process, the steel plates were directly quenched with the direct quenching initiation temperatures, direct quenching termination temperatures, and cooling rates set to the values shown in Tables 3 and 4 and then tempered using solenoid type induction heating apparatus with the tempering initiation temperatures, tempering temperatures, and tempering times set to the values shown in Tables 3 and 4. The direct quenching was completed by forcedly cooling (cooling in water) the individual steel plates to a temperature of 350° C. or lower at a cooling rate of 1° C./s or higher. 
     The average heating rates at the middle of the steel thickness were achieved by controlling the threading rates of the steel plates. In addition, each steel plate was moved back and forth in the solenoid type induction heating apparatus while being heated so that its temperature was maintained in the range ±5° C. of the target heating temperature. 
     The cooling process after heating for tempering was completed by performing air cooling under the conditions shown in Tables 3 and 4. The temperatures, such as tempering temperatures and quenching temperatures, at the middle of the thickness of each steel plate were determined by heat transfer calculation based on temperatures dynamically measured on the surface thereof using an emission pyrometer. 
     Tables 5 and 6 show the yield strength, tensile strength, fracture appearance transition temperatures (vTrs), and safety indices of delayed fracture resistance of the obtained steel plates. 
     Each cooling rate was the average cooling rate for cooling from the direct quenching initiation temperature to the direct quenching termination temperature measured at the middle of the thickness of the steel plate. 
     For the tests described later, three specimens were sampled from the midpoint of the longitudinal axis of each steel plate, and additional three specimens were sampled from the position located at ¼ of the width of each steel plate. 
     The aspect ratios of prior austenite grains were determined by etching the structures of the specimens with picric acid, imaging each specimen using an optical microscope at 1 mm in depth from the surface thereof, positions located at ¼, ½, and ¾ of the thickness thereof, and 1 mm in depth from the back surface thereof, measuring the aspect ratios of approximately 500 prior austenite grains, and then averaging the aspect ratio measurements. 
     The yield strength and tensile strength were measured using specimens for the overall thickness tensile test according to JIS Z2241. The toughness was evaluated using the Charpy pendulum impact test according to JIS Z2242, in which vTrs of specimens sampled from the middle of the thickness of each steel plate was measured. 
     The safety indices of delayed fracture resistance were evaluated using rod-like specimens in the following way: hydrogen was charged into the specimens by cathodic hydrogen charging so that the amount of diffusible hydrogen contained in each specimen was approximately 0.5 mass ppm; the hydrogen was sealed by zinc galvanizing of the surface of each specimen; tensile tests of the specimens were performed with the strain rate set to 1×10 −6 /s and the reductions of area of the fractured specimens were measured; and then the same tensile tests were performed using other specimens, into which no hydrogen was charged. The obtained results were used to evaluate the safety indices of delayed fracture resistance in accordance with the following formula:
 
Safety index of delayed fracture resistance (%)=100×( X   1   /X   0 )
         where X 0 : reduction of area of a specimen substantially free from diffusible hydrogen, and   X 1 : reduction of area of a specimen containing diffusible hydrogen.       

     The target vTrs was set to −40° C. or lower for steels having a tensile strength less than 1200 MPa and −30° C. or lower for steels having a tensile strength of 1200 MPa or higher. On the other hand, the target safety index of delayed fracture resistance was set to 80% or higher for steels having a tensile strength less than 1200 MPa and 75% or higher for steels having a tensile strength of 1200 MPa or higher. 
     As is clear in Tables 3 and 4, the steel plates 18 to 20, in which the rolling reduction for non-recrystallization regions deviated from our range, had the aspect ratios of prior austenite grains deviating from our range. 
     Furthermore, as is clear in Tables 5 and 6, the steel plates 1 to 17 and 33 to 39 (our examples) were produced under manufacturing conditions falling within our range to have a chemical component and the aspect ratio of prior austenite grains falling within our ranges, and showed favorable vTrs and a high safety index of delayed fracture resistance. 
     However, in the comparative steel plates 18 to 32 and 40 to 44 (comparative examples), at least one of vTrs and the safety index of delayed fracture resistance deviated from the target range thereof described above. The following are specific explanations of these comparative examples. 
     The steel plates 29 to 32 and 40 to 44 produced with the composition deviating from our range showed vTrs and/or the safety index of delayed fracture resistance being short of the target value. 
     The steel plates 18 to 20 produced with the rolling reduction for non-crystallization regions deviating from our range showed the safety index of delayed fracture resistance being short of the target value. 
     The steel plates 21 to 23 produced with the direct quenching initiation temperature deviating from our range showed vTrs and the safety index of delayed fracture resistance being short of the target value. 
     The steel plate 24 produced with the direct quenching termination temperature deviating from our range showed vTrs and the safety index of delayed fracture resistance being short of the target value. 
     The steel plate 25 produced with the cooling rate and direct quenching termination temperature deviating from our ranges showed vTrs and the safety index of delayed fracture resistance being short of the target value. 
     The steel plates 26 to 28 produced with the tempering temperature deviating from our range showed vTrs and the safety index of delayed fracture resistance being short of the target value. 
     EXAMPLE 2 
     As with those produced in Example 1, steel plates were produced. More specifically, Steels A to Z and AA to II whose chemical compositions are shown in Tables 7 and 8 were melted and cast into slabs, and the obtained slabs were heated in a furnace and then hot-rolled to produce the steel plates. After the hot-rolling process, the steel plates were directly quenched and then tempered using solenoid type induction heating apparatus. The direct quenching was completed by forcedly cooling (cooling in water) the individual steel plates to a temperature of 350° C. or lower at a cooling rate of 1° C./s or higher. 
     The aspect ratios of prior austenite grains were determined in the same manner as Example 1, except that approximately 550 prior austenite grains were used to calculate the average aspect ratio. 
     The cementite covering ratios of lath boundaries were determined by imaging structures etched using nital with a scanning electron microscope at the position located at ¼ of the thickness of each specimen; analyzing the boundaries of approximately 60 laths in terms of the lengths of formed cementite precipitations along the lath boundaries (L Cementite ) and the lengths of the lath boundaries (L Lath ); dividing the sum of the lengths of cementite along the lath boundaries by the sum of the lengths of the lath boundaries; and then multiplying the quotient by 100. 
     Additionally, the yield strength, tensile strength, and safety indices of delayed fracture resistance were determined in the same manner as Example 1. 
     The target vTrs was set to −40° C. or lower for steels having a tensile strength less than 1200 MPa and −30° C. or lower for steels having a tensile strength of 1200 MPa or higher. On the other hand, the target safety index of delayed fracture resistance was set to 85% or higher for steels having a tensile strength less than 1200 MPa and 80% or higher for steels having a tensile strength of 1200 MPa or higher. 
     Tables 9 and 10 show the manufacturing conditions, aspect ratios of prior austenite grains, and cementite covering ratios of laths of the individual steel plates, and Tables 11 and 12 show the yield strength, tensile strength, fracture appearance transition temperatures (vTrs), and safety indices of delayed fracture resistance of the obtained steel plates. 
     It should be noted that, in Tables 9 to 12, our examples consist of steel plates meeting our requirements, whereas the comparative examples consist of those deviating from those requirements. The steel plates 1 to 17 and 41 to 47 are our examples in which the heating rate for heating from the tempering initiation temperature to 370° C. was 2° C./s or higher. 
     The steel plates 35 and 36 are close to our requirements, namely the requirement that the heating rate for heating from the tempering initiation temperature to 370° C. should be 2° C./s or higher and they meet others of our requirements and thus are classified into our examples. 
     As is clear in Tables 9 and 10, the steel plates 18 to 20, in which the rolling reduction for non-recrystallization regions deviated from our range, had the aspect ratio of prior austenite grains and cementite covering ratios of laths deviating from our ranges. 
     The steel plates 26 to 28 produced with the tempering temperature deviating from our range showed the cementite covering ratio of laths deviating from our range. 
     Furthermore, the steel plates 30 and 32 to 34 produced with the average heating rate for heating the middle of the steel thickness from the tempering initiation temperature to 370° C. and/or the average heating rate for heating the middle of the steel thickness from 370° C. to the tempering temperature deviating from our ranges showed the cementite covering ratio of laths deviating from our range. 
     Meanwhile, as is clear in Tables 11 and 12, the steel plates 1 to 17, 35, and 36 (our examples) were produced under manufacturing conditions falling within our range to have a chemical composition, the aspect ratio of prior austenite grains, and the cementite covering ratio of laths falling within our ranges, and showed favorable vTrs and a high safety index of delayed fracture resistance. 
     The comparison between the steel plates 4 and 35, both of which fall within our scope and are identical to each other except for the difference in the average heating rate for heating the middle of the steel thickness from the tempering initiation temperature to 370° C., revealed that the steel plate 4 produced with the average heating rate for heating the middle of the steel thickness from the tempering initiation temperature to 370° C. being higher than 2° C./s was better in terms of vTrs and the safety index of delayed fracture resistance than the steel plate 35. This is the case also for the comparison between the steel plates 12 and 36. 
     However, in the comparative steel plates 18 to 34, 37 to 46, and 48 to 52 (comparative examples), at least one of vTrs and the safety index of delayed fracture resistance deviated from the target range thereof described above. The following are specific explanations of these comparative examples. 
     The steel plates 37 to 40 and 48 to 52 produced with the composition deviating from our range showed vTrs and the safety index of delayed fracture resistance being short of the target value. 
     The steel plates 18 to 20 produced with the rolling reduction for non-crystallization regions deviating from our range showed the safety index of delayed fracture resistance being short of the target value. 
     The steel plates 21 to 23 produced with the direct quenching initiation temperature deviating from our range showed vTrs and/or the safety index of delayed fracture resistance being short of the target value. 
     The steel plates 24 and 25 produced with the direct quenching termination temperature deviating from our range showed vTrs being short of the target value. 
     The steel plates 26 to 28 produced with the tempering temperature deviating from our range showed vTrs and/or the safety index of delayed fracture resistance being short of the target value. 
     The steel plates 29 to 34 produced with the average heating rate for heating the middle of the steel thickness from 370° C. to the tempering temperature deviating from our range showed vTrs and/or the safety index of delayed fracture resistance being short of the target value. 
     Industrial Applicability 
     The steels disclosed herein are high tensile strength steels having excellent delayed fracture resistance with the tensile strength thereof being 600 MPa or higher, in particular, 900 MPa or higher, and thus has very high industrial applicability. 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 (mass %) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Steels 
                 C 
                 Si 
                 Mn 
                 P 
                 S 
                 Cu 
                 Ni 
                 Cr 
                 Mo 
                 Nb 
                 V 
                 Ti 
               
               
                   
               
               
                 A 
                 0.05 
                 0.19 
                 1.34 
                 0.011 
                 0.0019 
                 0.00 
                 0.00 
                 0.03 
                 0.05 
                 0.020 
                 0.034 
                 0.000 
               
               
                 B 
                 0.08 
                 0.26 
                 1.43 
                 0.018 
                 0.0022 
                 0.00 
                 0.00 
                 0.03 
                 0.19 
                 0.021 
                 0.035 
                 0.000 
               
               
                 C 
                 0.10 
                 0.31 
                 1.08 
                 0.014 
                 0.0038 
                 0.00 
                 0.00 
                 0.06 
                 0.09 
                 0.019 
                 0.008 
                 0.010 
               
               
                 D 
                 0.12 
                 0.38 
                 1.48 
                 0.014 
                 0.0018 
                 0.02 
                 0.01 
                 0.49 
                 0.38 
                 0.017 
                 0.041 
                 0.012 
               
               
                 E 
                 0.12 
                 0.40 
                 1.51 
                 0.012 
                 0.0019 
                 0.02 
                 0.01 
                 0.26 
                 0.40 
                 0.020 
                 0.000 
                 0.010 
               
               
                 F 
                 0.13 
                 0.41 
                 1.51 
                 0.014 
                 0.0023 
                 0.00 
                 0.00 
                 0.51 
                 0.41 
                 0.020 
                 0.042 
                 0.013 
               
               
                 G 
                 0.14 
                 0.41 
                 1.55 
                 0.014 
                 0.0022 
                 0.00 
                 1.09 
                 0.50 
                 0.43 
                 0.020 
                 0.000 
                 0.011 
               
               
                 H 
                 0.15 
                 0.41 
                 1.52 
                 0.014 
                 0.0019 
                 0.30 
                 0.30 
                 0.51 
                 0.21 
                 0.020 
                 0.042 
                 0.013 
               
               
                 I 
                 0.15 
                 0.41 
                 1.21 
                 0.014 
                 0.0037 
                 0.00 
                 0.00 
                 0.51 
                 0.69 
                 0.020 
                 0.000 
                 0.013 
               
               
                 J 
                 0.16 
                 0.42 
                 1.19 
                 0.005 
                 0.0019 
                 0.26 
                 0.28 
                 0.34 
                 0.65 
                 0.019 
                 0.044 
                 0.012 
               
               
                 K 
                 0.16 
                 0.27 
                 1.35 
                 0.002 
                 0.0009 
                 0.26 
                 0.24 
                 0.53 
                 0.52 
                 0.022 
                 0.052 
                 0.013 
               
               
                 L 
                 0.17 
                 0.37 
                 1.12 
                 0.009 
                 0.0010 
                 0.05 
                 0.06 
                 0.51 
                 0.69 
                 0.022 
                 0.041 
                 0.012 
               
               
                 M 
                 0.17 
                 0.20 
                 1.35 
                 0.005 
                 0.0018 
                 0.00 
                 0.40 
                 0.35 
                 0.25 
                 0.022 
                 0.050 
                 0.000 
               
               
                 N 
                 0.17 
                 0.22 
                 1.45 
                 0.015 
                 0.0009 
                 0.00 
                 1.32 
                 0.35 
                 0.21 
                 0.015 
                 0.035 
                 0.000 
               
               
                 O 
                 0.18 
                 0.35 
                 1.75 
                 0.004 
                 0.0007 
                 0.20 
                 0.20 
                 0.45 
                 0.30 
                 0.019 
                 0.008 
                 0.010 
               
               
                 P 
                 0.21 
                 0.33 
                 1.09 
                 0.014 
                 0.0012 
                 0.02 
                 0.01 
                 0.55 
                 0.69 
                 0.020 
                 0.041 
                 0.012 
               
               
                   
               
             
          
           
               
                   
                 Remarks 
               
             
          
           
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Ar 3   
                 Ac 1   
                   
               
               
                 Steels 
                 B 
                 W 
                 Ca 
                 REM 
                 Mg 
                 Al 
                 T.N 
                 (° C.) 
                 (° C.) 
                 Remarks 
               
               
                   
               
               
                 A 
                 0.0000 
                 — 
                 — 
                 — 
                 — 
                 0.031 
                 0.0032 
                 783 
                 709 
                 Example 
               
               
                 B 
                 0.0000 
                 — 
                 — 
                 — 
                 — 
                 0.028 
                 0.0029 
                 755 
                 709 
                 Example 
               
               
                 C 
                 0.0010 
                 — 
                 — 
                 — 
                 — 
                 0.022 
                 0.0037 
                 785 
                 716 
                 Example 
               
               
                 D 
                 0.0012 
                 — 
                 0.0017 
                 — 
                 — 
                 0.030 
                 0.0030 
                 716 
                 722 
                 Example 
               
               
                 E 
                 0.0013 
                 — 
                 — 
                 — 
                 — 
                 0.027 
                 0.0031 
                 715 
                 717 
                 Example 
               
               
                 F 
                 0.0010 
                 — 
                 — 
                 — 
                 — 
                 0.032 
                 0.0037 
                 708 
                 723 
                 Example 
               
               
                 G 
                 0.0015 
                 — 
                 — 
                 — 
                 — 
                 0.024 
                 0.0024 
                 641 
                 706 
                 Example 
               
               
                 H 
                 0.0010 
                 — 
                 — 
                 — 
                 — 
                 0.032 
                 0.0030 
                 695 
                 718 
                 Example 
               
               
                 I 
                 0.0010 
                 — 
                 — 
                 0.0025 
                 — 
                 0.032 
                 0.0030 
                 704 
                 727 
                 Example 
               
               
                 J 
                 0.0012 
                 — 
                 — 
                 — 
                 0.0015 
                 0.028 
                 0.0046 
                 688 
                 719 
                 Example 
               
               
                 K 
                 0.0015 
                 — 
                 0.0032 
                 — 
                 — 
                 0.052 
                 0.0035 
                 684 
                 719 
                 Example 
               
               
                 L 
                 0.0013 
                 — 
                 0.0019 
                 — 
                 — 
                 0.027 
                 0.0037 
                 701 
                 726 
                 Example 
               
               
                 M 
                 0.0000 
                 — 
                 — 
                 0.0019 
                 — 
                 0.031 
                 0.0032 
                 702 
                 711 
                 Example 
               
               
                 N 
                 0.0000 
                 — 
                 — 
                 — 
                 — 
                 0.028 
                 0.0029 
                 647 
                 697 
                 Example 
               
               
                 O 
                 0.0010 
                 0.20 
                 — 
                 — 
                 — 
                 0.022 
                 0.0037 
                 668 
                 714 
                 Example 
               
               
                 P 
                 0.0012 
                 — 
                 0.0015 
                 — 
                 — 
                 0.030 
                 0.0030 
                 693 
                 728 
                 Example 
               
               
                   
               
               
                 Note 1: 
               
               
                 The symbol * means that the parameter deviates from the range specified in the present invention. 
               
               
                 Note 2: 
               
               
                 Ar 3  = 91-310C—80Mn—20Cu—15Cr—55Ni—80Mo (the elements represent content ratios in mass percent) 
               
               
                 Note 3: 
               
               
                 Ac 1  = 723-14Mn + 22Si—14.4Ni + 23.3Cr (the elements represent content ratios in mass percent) 
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 (mass %) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Steels 
                 C 
                 Si 
                 Mn 
                 P 
                 S 
                 Cu 
                 Ni 
                 Cr 
                 Mo 
                 Nb 
                 V 
                 Ti 
                 B 
               
               
                   
               
               
                 Q 
                 0.23 
                 0.45 
                 1.52 
                 0.018 
                 0.0015 
                 0.02 
                 1.34 
                 0.45 
                 0.45 
                 0.020 
                 0.000 
                 0.010 
                 0.0013 
               
               
                 R 
                 0.12 
                 0.38 
                 1.48 
                 0.025* 
                 0.0018 
                 0.02 
                 0.01 
                 0.49 
                 0.38 
                 0.017 
                 0.041 
                 0.012 
                 0.0012 
               
               
                 S 
                 0.14 
                 0.41 
                 1.55 
                 0.014 
                 0.0043* 
                 0.00 
                 1.09 
                 0.50 
                 0.43 
                 0.020 
                 0.000 
                 0.011 
                 0.0015 
               
               
                 T 
                 0.15 
                 0.41 
                 1.52 
                 0.031* 
                 0.0019 
                 0.30 
                 0.30 
                 0.51 
                 0.21 
                 0.020 
                 0.042 
                 0.013 
                 0.0010 
               
               
                 U 
                 0.17 
                 0.37 
                 1.12 
                 0.032* 
                 0.0042* 
                 0.05 
                 0.06 
                 0.51 
                 0.69 
                 0.022 
                 0.041 
                 0.012 
                 0.0013 
               
               
                 X 
                 0.03 
                 0.26 
                 1.31 
                 0.010 
                 0.0009 
                 0.01 
                 0.03 
                 0.56 
                 0.05 
                 0.012 
                 0.031 
                 0.001 
                 0.0003 
               
               
                 Y 
                 0.17 
                 0.67 
                 1.81 
                 0.006 
                 0.0008 
                 1.98 
                 3.91 
                 0.63 
                 0.72 
                 0.018 
                 0.043 
                 0.012 
                 0.0015 
               
               
                 Z 
                 0.24 
                 0.32 
                 1.92 
                 0.003 
                 0.0006 
                 1.95 
                 3.95 
                 0.51 
                 0.95 
                 0.016 
                 0.042 
                 0.015 
                 0.0013 
               
               
                 AA 
                 0.18 
                 0.02 
                 1.12 
                 0.005 
                 0.0003 
                 1.66 
                 3.81 
                 0.36 
                 0.86 
                 0.022 
                 0.045 
                 0.012 
                 0.0010 
               
               
                 BB 
                 0.20 
                 0.75 
                 1.08 
                 0.006 
                 0.0004 
                 1.82 
                 3.56 
                 0.48 
                 0.89 
                 0.019 
                 0.046 
                 0.012 
                 0.0011 
               
               
                 CC 
                 0.23 
                 0.41 
                 0.60 
                 0.004 
                 0.0003 
                 1.91 
                 3.78 
                 0.39 
                 0.88 
                 0.021 
                 0.045 
                 0.010 
                 0.0013 
               
               
                 DD 
                 0.15 
                 0.42 
                 1.20 
                 0.006 
                 0.0006 
                 0.00 
                 0.01 
                 0.51 
                 0.41 
                 0.019 
                 0.042 
                 0.012 
                 0.0012 
               
               
                 EE 
                 0.27* 
                 0.53 
                 1.12 
                 0.006 
                 0.0004 
                 1.61 
                 3.23 
                 0.68 
                 0.78 
                 0.021 
                 0.043 
                 0.011 
                 0.0012 
               
               
                 FF 
                 0.22 
                 0.85* 
                 1.08 
                 0.005 
                 0.0005 
                 1.55 
                 3.16 
                 0.51 
                 0.77 
                 0.022 
                 0.041 
                 0.009 
                 0.0011 
               
               
                 GG 
                 0.18 
                 0.42 
                 2.11* 
                 0.003 
                 0.0003 
                 1.51 
                 2.84 
                 0.53 
                 0.63 
                 0.021 
                 0.038 
                 0.011 
                 0.0012 
               
               
                 HH 
                 0.21 
                 0.51 
                 1.32 
                 0.004 
                 0.0005 
                 0.13 
                 0.26 
                 0.36 
                 0.64 
                 0.022 
                 0.041 
                 0.009 
                 0.0011 
               
               
                 II 
                 0.22 
                 0.48 
                 1.16 
                 0.005 
                 0.0004 
                 0.16 
                 0.28 
                 0.38 
                 0.65 
                 0.019 
                 0.043 
                 0.008 
                 0.0012 
               
               
                   
               
             
          
           
               
                   
                 Remarks 
               
             
          
           
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Ar 3   
                 Ac 1   
                   
               
               
                   
                 Steels 
                 W 
                 Ca 
                 REM 
                 Mg 
                 Al 
                 T.N 
                 (° C.) 
                 (° C.) 
                 Remarks 
               
               
                   
                   
               
               
                   
                 Q 
                 0.15 
                 — 
                 — 
                 — 
                 0.027 
                 0.0031 
                 600 
                 703 
                 Example 
               
               
                   
                 R 
                 — 
                 0.0017 
                 — 
                 — 
                 0.030 
                 0.0030 
                 716 
                 722 
                 Comparative Example 
               
               
                   
                 S 
                 — 
                 — 
                 — 
                 — 
                 0.024 
                 0.0024 
                 641 
                 706 
                 Comparative Example 
               
               
                   
                 T 
                 — 
                 — 
                 — 
                 — 
                 0.032 
                 0.0030 
                 695 
                 718 
                 Comparative Example 
               
               
                   
                 U 
                 — 
                 0.0019 
                 — 
                 — 
                 0.027 
                 0.0037 
                 701 
                 726 
                 Comparative Example 
               
               
                   
                 X 
                 — 
                 — 
                 — 
                 — 
                 0.035 
                 0.0034 
                 782 
                 723 
                 Example 
               
               
                   
                 Y 
                 — 
                 0.0005 
                 — 
                 — 
                 0.031 
                 0.0032 
                 391 
                 671 
                 Example 
               
               
                   
                 Z 
                 — 
                 0.0012 
                 — 
                 0.0012 
                 0.028 
                 0.0035 
                 342 
                 658 
                 Example 
               
               
                   
                 AA 
                 — 
                 0.0016 
                 — 
                 — 
                 0.031 
                 0.0034 
                 448 
                 661 
                 Example 
               
               
                   
                 BB 
                 — 
                 0.0017 
                 — 
                 — 
                 0.032 
                 0.0035 
                 451 
                 684 
                 Example 
               
               
                   
                 CC 
                 — 
                 0.0018 
                 — 
                 — 
                 0.028 
                 0.0033 
                 468 
                 678 
                 Example 
               
               
                   
                 DD 
                 — 
                 0.0093 
                 — 
                 — 
                 0.026 
                 0.0038 
                 727 
                 727 
                 Example 
               
               
                   
                 EE 
                 — 
                 0.0014 
                 — 
                 — 
                 0.025 
                 0.0034 
                 454 
                 688 
                 Comparative Example 
               
               
                   
                 FF 
                 — 
                 0.0012 
                 — 
                 — 
                 0.028 
                 0.0033 
                 481 
                 693 
                 Comparative Example 
               
               
                   
                 GG 
                 — 
                 0.0013 
                 — 
                 — 
                 0.031 
                 0.0034 
                 441 
                 674 
                 Comparative Example 
               
               
                   
                 HH 
                 — 
                 0.0003* 
                 — 
                 — 
                 0.033 
                 0.0032 
                 666 
                 720 
                 Comparative Example 
               
               
                   
                 II 
                 — 
                 0.0108* 
                 — 
                 — 
                 0.031 
                 0.0028 
                 673 
                 722 
                 Comparative Example 
               
               
                   
                   
               
               
                   
                 Note 1: 
               
               
                   
                 The symbol * means that the parameter deviates from the range specified in the present invention. 
               
               
                   
                 Note 2: 
               
               
                   
                 Ar 3  = 910-310C—80Mn—20Cu—15Cr—55Ni—80Mo (the elements represent content ratios in mass percent) 
               
               
                   
                 Note 3: 
               
               
                   
                 Ac 1  = 723-14Mn + 22Si—14.4Ni + 23.3Cr (the elements represent content ratios in mass percent) 
               
             
          
         
       
     
     
       
         
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
             
             
               
                   
                   
                   
                   
                 Rolling 
                 Direct 
                 Direct 
                   
                   
               
               
                   
                   
                   
                   
                 reduction for 
                 quenching 
                 quenching 
               
               
                   
                   
                   
                 Heating 
                 non- 
                 initiation 
                 termination 
               
               
                   
                   
                 Thickness 
                 temperature 
                 recrystallization 
                 temperature 
                 temperature 
                 Cooling rate 
                 Tempering initiation 
               
               
                 No. 
                 Steels 
                 (mm) 
                 (° C.) 
                 regions (%) 
                 (° C.) 
                 (° C.) 
                 (° C./s) 
                 temperature (° C.) 
               
               
                   
               
             
          
           
               
                 1 
                 A 
                 25 
                 1170 
                 35 
                 840 
                 180 
                 30 
                 160 
               
               
                 2 
                 B 
                 12 
                 1150 
                 30 
                 820 
                 350 
                 80 
                 330 
               
               
                 3 
                 C 
                 25 
                 1130 
                 55 
                 840 
                 320 
                 30 
                 300 
               
               
                 4 
                 D 
                 12 
                 1100 
                 60 
                 830 
                 230 
                 80 
                 210 
               
               
                 5 
                 E 
                 25 
                 1050 
                 60 
                 820 
                 170 
                 30 
                 150 
               
               
                 6 
                 F 
                 12 
                 1200 
                 70 
                 830 
                 230 
                 80 
                 210 
               
               
                 7 
                 G 
                 25 
                 1100 
                 60 
                 830 
                 130 
                 30 
                 110 
               
               
                 8 
                 H 
                 50 
                 1130 
                 60 
                 820 
                 180 
                 10 
                 160 
               
               
                 9 
                 I 
                 12 
                 1150 
                 80 
                 830 
                 190 
                 80 
                 170 
               
               
                 10 
                 J 
                 25 
                 1150 
                 60 
                 830 
                 200 
                 30 
                 180 
               
               
                 11 
                 K 
                 50 
                 1130 
                 60 
                 850 
                 90 
                 10 
                  70 
               
               
                 12 
                 L 
                 60 
                 1150 
                 60 
                 850 
                 150 
                 8 
                 130 
               
               
                 13 
                 M 
                 6 
                 1100 
                 60 
                 730 
                 140 
                 150 
                 120 
               
               
                 14 
                 N 
                 12 
                 1100 
                 60 
                 750 
                 240 
                 80 
                 Room temperature 
               
               
                 15 
                 O 
                 25 
                 1100 
                 60 
                 760 
                 130 
                 30 
                 110 
               
               
                 16 
                 P 
                 60 
                 1110 
                 60 
                 710 
                 110 
                 8 
                 Room temperature 
               
               
                 17 
                 Q 
                 6 
                 1090 
                 60 
                 810 
                 210 
                 150 
                 190 
               
               
                 18 
                 A 
                 25 
                 1170 
                  25* 
                 840 
                 180 
                 30 
                 160 
               
               
                 19 
                 B 
                 12 
                 1150 
                  20* 
                 820 
                 350 
                 80 
                 330 
               
               
                 20 
                 C 
                 25 
                 1130 
                  25* 
                 840 
                 320 
                 30 
                 300 
               
               
                 21 
                 D 
                 12 
                 1100 
                 60 
                  705* 
                 230 
                 75 
                 210 
               
               
                 22 
                 E 
                 25 
                 1050 
                 60 
                  700* 
                 170 
                 25 
                 150 
               
               
                   
               
             
          
           
               
                   
                   
                   
                 Average 
                   
                   
                   
                   
               
               
                   
                   
                   
                 heating rate 
               
               
                   
                   
                   
                 for heating 
               
               
                   
                   
                   
                 the middle 
               
               
                   
                   
                   
                 of the steel 
               
               
                   
                   
                   
                 thickness 
               
               
                   
                   
                   
                 from the 
                   
                 Average 
               
               
                   
                   
                   
                 tempering 
                 Time for 
                 cooling rate 
               
               
                   
                   
                   
                 initiation 
                 which the 
                 for cooling 
               
               
                   
                   
                   
                 temperature 
                 tempering 
                 from the 
               
               
                   
                   
                   
                 to the 
                 temperature 
                 maintained 
                 Aspect ratio 
               
               
                   
                   
                 Tempering 
                 tempering 
                 is 
                 tempering 
                 of prior 
               
               
                   
                   
                 temperature 
                 temperature 
                 maintained 
                 temperature 
                 austenite 
               
               
                   
                 No. 
                 (° C.) 
                 (° C./s) 
                 (s) 
                 to 200° C. (° C./s) 
                 grains 
                 Remarks 
               
               
                   
                   
               
               
                   
                 1 
                 540 
                 0.5 
                 600 
                 0.3 
                 3.5 
                 Example 
               
               
                   
                 2 
                 610 
                 1.0 
                 600 
                 0.6 
                 3.3 
                 Example 
               
               
                   
                 3 
                 570 
                 0.5 
                 600 
                 0.3 
                 13.2 
                 Example 
               
               
                   
                 4 
                 550 
                 1.0 
                 600 
                 0.6 
                 9.8 
                 Example 
               
               
                   
                 5 
                 590 
                 0.5 
                 1200 
                 0.3 
                 7.5 
                 Example 
               
               
                   
                 6 
                 640 
                 1.0 
                 2400 
                 0.6 
                 12.3 
                 Example 
               
               
                   
                 7 
                 680 
                 0.5 
                 3600 
                 0.3 
                 17.3 
                 Example 
               
               
                   
                 8 
                 600 
                 0.2 
                 300 
                 0.2 
                 6.5 
                 Example 
               
               
                   
                 9 
                 630 
                 1.0 
                 600 
                 0.6 
                 17.3 
                 Example 
               
               
                   
                 10 
                 600 
                 0.5 
                 600 
                 0.3 
                 15.3 
                 Example 
               
               
                   
                 11 
                 580 
                 0.2 
                 600 
                 0.2 
                 10.9 
                 Example 
               
               
                   
                 12 
                 550 
                 0.2 
                 600 
                 0.1 
                 5.3 
                 Example 
               
               
                   
                 13 
                 410 
                 2.0 
                 600 
                 1.3 
                 16.9 
                 Example 
               
               
                   
                 14 
                 460 
                 1.0 
                 60 
                 0.6 
                 11.9 
                 Example 
               
               
                   
                 15 
                 480 
                 0.5 
                 600 
                 0.3 
                 12.3 
                 Example 
               
               
                   
                 16 
                 510 
                 0.2 
                 600 
                 0.1 
                 5.4 
                 Example 
               
               
                   
                 17 
                 430 
                 2.0 
                 600 
                 1.3 
                 17.9 
                 Example 
               
               
                   
                 18 
                 540 
                 0.5 
                 600 
                 0.3 
                 2.5* 
                 Comparative Example 
               
               
                   
                 19 
                 610 
                 1.0 
                 600 
                 0.6 
                 2.3* 
                 Comparative Example 
               
               
                   
                 20 
                 570 
                 0.5 
                 600 
                 0.3 
                 1.7* 
                 Comparative Example 
               
               
                   
                 21 
                 550 
                 1.0 
                 600 
                 0.6 
                 9.8 
                 Comparative Example 
               
               
                   
                 22 
                 590 
                 0.5 
                 1200 
                 0.3 
                 7.5 
                 Comparative Example 
               
               
                   
                   
               
               
                   
                 Note 1: 
               
               
                   
                 The symbol * means that the parameter deviates from the range specified in the present invention. 
               
               
                   
                 Note 2: 
               
               
                   
                 Ranges specified in the present invention are as follows: rolling reduction for non-recrystallization regions: 30% or higher; direct quenching initiation temperature: Ar 3  transformation temperature or higher; direct quenching termination temperature: 350° C. or lower; cooling rate: 1° C./s or higher; tempering temperature: Ac 1  transformation temperature or lower 
               
             
          
         
       
     
     
       
         
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
             
             
               
                   
                   
                   
                   
                 Rolling 
                   
                   
                   
                   
               
               
                   
                   
                   
                   
                 reduction for 
                 Direct 
                 Direct 
               
               
                   
                   
                   
                   
                 non- 
                 quenching 
                 quenching 
                   
                 Tempering 
               
               
                   
                   
                   
                 Heating 
                 recrystallization 
                 initiation 
                 termination 
                   
                 initiation 
               
               
                   
                   
                 Thickness 
                 temperature 
                 regions 
                 temperature 
                 temperature 
                 Cooling rate 
                 temperature 
               
               
                 No. 
                 Steels 
                 (mm) 
                 (° C.) 
                 (%) 
                 (° C.) 
                 (° C.) 
                 (° C./s) 
                 (° C.) 
               
               
                   
               
               
                 23 
                 F 
                 12 
                 1200 
                 70 
                  690* 
                 230 
                 75 
                 210 
               
               
                 24 
                 G 
                 25 
                 1100 
                 60 
                 830 
                  400* 
                 35 
                 110 
               
               
                 25 
                 H 
                 50 
                 1130 
                 60 
                 820 
                  450* 
                 0.8* 
                 160 
               
               
                 26 
                 I 
                 12 
                 1150 
                 80 
                 830 
                 190 
                 80 
                 170 
               
               
                 27 
                 J 
                 25 
                 1150 
                 60 
                 830 
                 200 
                 30 
                 180 
               
               
                 28 
                 K 
                 50 
                 1130 
                 60 
                 850 
                  90 
                 10 
                 70 
               
               
                 29 
                 R* 
                 35 
                 1100 
                 60 
                 830 
                 200 
                 15 
                 180 
               
               
                 30 
                 S* 
                 50 
                 1050 
                 60 
                 850 
                 150 
                 10 
                 130 
               
               
                 31 
                 T* 
                 50 
                 1050 
                 60 
                 850 
                 150 
                 10 
                 130 
               
               
                 32 
                 U* 
                 60 
                 1200 
                 60 
                 850 
                 150 
                 8 
                 130 
               
               
                 33 
                 X 
                 25 
                 1160 
                 30 
                 830 
                 230 
                 30 
                 210 
               
               
                 34 
                 Y 
                 6 
                 1120 
                 65 
                 670 
                  80 
                 150 
                 60 
               
               
                 35 
                 Z 
                 25 
                 1110 
                 75 
                 640 
                 100 
                 30 
                 80 
               
               
                 36 
                 AA 
                 12 
                 1120 
                 70 
                 650 
                 120 
                 80 
                 100 
               
               
                 37 
                 BB 
                 32 
                 1130 
                 75 
                 720 
                 100 
                 18 
                 80 
               
               
                 38 
                 CC 
                 20 
                 1150 
                 70 
                 680 
                 100 
                 50 
                 80 
               
               
                 39 
                 DD 
                 32 
                 1100 
                 60 
                 830 
                 230 
                 18 
                 210 
               
               
                 40 
                 EE* 
                 16 
                 1100 
                 75 
                 700 
                 100 
                 60 
                 80 
               
               
                 41 
                 FF* 
                 8 
                 1110 
                 70 
                 680 
                 100 
                 120 
                 80 
               
               
                 42 
                 GG* 
                 12 
                 1120 
                 60 
                 670 
                 100 
                 80 
                 80 
               
               
                 43 
                 HH* 
                 12 
                 1120 
                 60 
                 830 
                 200 
                 80 
                 180 
               
               
                 44 
                 II* 
                 12 
                 1120 
                 60 
                 830 
                 200 
                 80 
                 180 
               
               
                   
               
             
          
           
               
                   
                   
                 Average heating rate 
                   
                   
                   
                   
               
               
                   
                   
                 for heating the 
                   
                 Average cooling 
               
               
                   
                   
                 middle of the steel 
                 Time 
                 rate for cooling 
               
               
                   
                   
                 thickness from the 
                 for which 
                 from the 
               
               
                   
                   
                 tempering initiation 
                 the tempering 
                 maintained 
                 Aspect ratio 
               
               
                   
                 Tempering 
                 temperature to the 
                 temperature 
                 tempering 
                 of prior 
               
               
                   
                 temperature 
                 tempering 
                 is maintained 
                 temperature to 
                 austenite 
               
               
                 No. 
                 (° C.) 
                 temperature (° C./s) 
                 (s) 
                 200° C. (° C./s) 
                 grains 
                 Remarks 
               
               
                   
               
               
                 23 
                 640 
                 1.0 
                 2400 
                 0.6 
                 12.3 
                 Comparative Example 
               
               
                 24 
                 680 
                 0.5 
                 3600 
                 0.3 
                 17.3 
                 Comparative Example 
               
               
                 25 
                 600 
                 0.2 
                 300 
                 0.2 
                 6.5 
                 Comparative Example 
               
               
                 26 
                  740* 
                 1.0 
                 600 
                 0.6 
                 17.3 
                 Comparative Example 
               
               
                 27 
                  730* 
                 0.5 
                 600 
                 0.3 
                 15.3 
                 Comparative Example 
               
               
                 28 
                  730* 
                 0.2 
                 600 
                 0.2 
                 10.9 
                 Comparative Example 
               
               
                 29 
                 490 
                 0.3 
                 600 
                 0.2 
                 10.7 
                 Comparative Example 
               
               
                 30 
                 520 
                 0.2 
                 600 
                 0.2 
                 4.9 
                 Comparative Example 
               
               
                 31 
                 520 
                 0.2 
                 600 
                 0.2 
                 5.5 
                 Comparative Example 
               
               
                 32 
                 500 
                 0.2 
                 600 
                 0.1 
                 6.3 
                 Comparative Example 
               
               
                 33 
                 520 
                 0.5 
                 10 
                 0.3 
                 3.5 
                 Example 
               
               
                 34 
                 500 
                 2.0 
                 10 
                 1.3 
                 12.5 
                 Example 
               
               
                 35 
                 500 
                 0.5 
                 10 
                 0.3 
                 16.1 
                 Example 
               
               
                 36 
                 520 
                 1.0 
                 10 
                 0.6 
                 14.1 
                 Example 
               
               
                 37 
                 500 
                 0.4 
                 10 
                 0.2 
                 16.3 
                 Example 
               
               
                 38 
                 520 
                 0.6 
                 60 
                 0.4 
                 14.5 
                 Example 
               
               
                 39 
                 560 
                 0.4 
                 600 
                 0.2 
                 8.3 
                 Example 
               
               
                 40 
                 520 
                 0.8 
                 10 
                 0.5 
                 16.7 
                 Comparative Example 
               
               
                 41 
                 520 
                 1.5 
                 10 
                 0.9 
                 17.6 
                 Comparative Example 
               
               
                 42 
                 500 
                 1.0 
                 10 
                 0.6 
                 6.5 
                 Comparative Example 
               
               
                 43 
                 500 
                 1.0 
                 10 
                 0.6 
                 6.3 
                 Comparative Example 
               
               
                 44 
                 500 
                 1.0 
                 10 
                 0.6 
                 6.5 
                 Comparative Example 
               
               
                   
               
               
                 Note 1: 
               
               
                 The symbol * means that the parameter deviates from the range specified in the present invention. 
               
               
                 Note 2: 
               
               
                 Ranges specified in the present invention are as follows: rolling reduction for non-recrystallization regions: 30% or higher; direct quenching initiation temperature: Ar 3  transformation temperature or higher; direct quenching termination temperature: 350° C. or lower; cooling rate: 1° C./s or higher; tempering temperature: Ac 1  transformation temperature or lower 
               
             
          
         
       
     
     
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 vTrs at the middle of 
                 Safety index of 
                   
               
               
                   
                   
                 Thickness 
                 Yield strength 
                 Tensile strength 
                 the steel thickness 
                 delayed fracture 
               
               
                 No. 
                 Steels 
                 (mm) 
                 (MPa) 
                 (MPa) 
                 (° C.) 
                 resistance (%) 
                 Remarks 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 A 
                 25 
                 573 
                 648 
                 −105  
                 93 
                 Example 
               
               
                 2 
                 B 
                 12 
                 601 
                 678 
                 −116  
                 89 
                 Example 
               
               
                 3 
                 C 
                 25 
                 801 
                 868 
                 −78 
                 91 
                 Example 
               
               
                 4 
                 D 
                 12 
                 1023 
                 1048 
                 −68 
                 89 
                 Example 
               
               
                 5 
                 E 
                 25 
                 1006 
                 1027 
                 −69 
                 85 
                 Example 
               
               
                 6 
                 F 
                 12 
                 1056 
                 1061 
                 −59 
                 83 
                 Example 
               
               
                 7 
                 G 
                 25 
                 1013 
                 1052 
                 −59 
                 85 
                 Example 
               
               
                 8 
                 H 
                 50 
                 1014 
                 1019 
                 −52 
                 84 
                 Example 
               
               
                 9 
                 I 
                 12 
                 1083 
                 1197 
                 −42 
                 81 
                 Example 
               
               
                 10 
                 J 
                 25 
                 1197 
                 1247 
                 −42 
                 85 
                 Example 
               
               
                 11 
                 K 
                 50 
                 1232 
                 1267 
                 −41 
                 79 
                 Example 
               
               
                 12 
                 L 
                 60 
                 1017 
                 1057 
                 −48 
                 86 
                 Example 
               
               
                 13 
                 M 
                 6 
                 1257 
                 1263 
                 −49 
                 80 
                 Example 
               
               
                 14 
                 N 
                 12 
                 1357 
                 1376 
                 −41 
                 79 
                 Example 
               
               
                 15 
                 O 
                 25 
                 1327 
                 1387 
                 −39 
                 78 
                 Example 
               
               
                 16 
                 P 
                 60 
                 1287 
                 1298 
                 −36 
                 79 
                 Example 
               
               
                 17 
                 Q 
                 6 
                 1356 
                 1387 
                 −35 
                 78 
                 Example 
               
               
                 18 
                 A 
                 25 
                 476 
                 553 
                 −42 
                  46* 
                 Comparative Example 
               
               
                 19 
                 B 
                 12 
                 529 
                 607 
                 −58 
                  42* 
                 Comparative Example 
               
               
                 20 
                 C 
                 25 
                 815 
                 823 
                 −59 
                  38* 
                 Comparative Example 
               
               
                 21 
                 D 
                 12 
                 831 
                 867 
                  −29* 
                  66* 
                 Comparative Example 
               
               
                 22 
                 E 
                 25 
                 923 
                 941 
                  −31* 
                  59* 
                 Comparative Example 
               
               
                   
               
               
                 Note 1: 
               
               
                 The symbol * means that the parameter deviates from the range specified in the present invention. 
               
               
                 Note 2: 
               
               
                 Ranges specified in the present invention are as follows: 1. vTrs at the middle of the steel thickness (° C.): −40° C. or lower for steel plates with a tensile strength lower than 1200 MPa; −30° C. or lower for steel plates with a tensile strength of 1200 MPa or higher: 2. Safety index of delayed fracture resistance: 80% or higher for steel plates with a tensile strength lower than 1200 MPa; 75% or higher for steel plates with a tensile strength of 1200 MPa or higher 
               
             
          
         
       
     
     
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 6 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 vTrs at the middle of 
                   
                   
               
               
                   
                   
                 Thickness 
                 Yield strength 
                 Tensile strength 
                 the steel thickness 
                 Safety index of delayed 
               
               
                 No. 
                 Steels 
                 (mm) 
                 (MPa) 
                 (MPa) 
                 (° C.) 
                 fracture resistance (%) 
                 Remarks 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 23 
                 F 
                 12 
                 982 
                 991 
                 −38* 
                 52* 
                 Comparative Example 
               
               
                 24 
                 G 
                 25 
                 923 
                 956 
                 −31* 
                 78* 
                 Comparative Example 
               
               
                 25 
                 H 
                 50 
                 937 
                 952 
                 −27* 
                 76* 
                 Comparative Example 
               
               
                 26 
                 I 
                 12 
                 983 
                 1063 
                 −27* 
                 68* 
                 Comparative Example 
               
               
                 27 
                 J 
                 25 
                 1101 
                 1157 
                 −29* 
                 62* 
                 Comparative Example 
               
               
                 28 
                 K 
                 50 
                 1127 
                 1151 
                 −27* 
                 53* 
                 Comparative Example 
               
               
                 29 
                 R* 
                 35 
                 1017 
                 1041 
                 −31* 
                 43* 
                 Comparative Example 
               
               
                 30 
                 S* 
                 50 
                 1007 
                 1047 
                 −27* 
                 42* 
                 Comparative Example 
               
               
                 31 
                 T* 
                 50 
                 1009 
                 1012 
                 −23* 
                 36* 
                 Comparative Example 
               
               
                 32 
                 U* 
                 60 
                 1021 
                 1061 
                 −15* 
                 39* 
                 Comparative Example 
               
               
                 33 
                 X 
                 25 
                 562 
                 627 
                 −102  
                 96  
                 Example 
               
               
                 34 
                 Y 
                 6 
                 1380 
                 1457 
                 −42  
                 78  
                 Example 
               
               
                 35 
                 Z 
                 25 
                 1421 
                 1512 
                 −46  
                 77  
                 Example 
               
               
                 36 
                 AA 
                 12 
                 1358 
                 1583 
                 −48  
                 80  
                 Example 
               
               
                 37 
                 BB 
                 32 
                 1391 
                 1623 
                 −42  
                 79  
                 Example 
               
               
                 38 
                 CC 
                 20 
                 1413 
                 1678 
                 −43  
                 81  
                 Example 
               
               
                 39 
                 DD 
                 32 
                 1071 
                 1112 
                 −63  
                 88  
                 Example 
               
               
                 40 
                 EE* 
                 16 
                 1378 
                 1563 
                 −26* 
                 56* 
                 Comparative Example 
               
               
                 41 
                 FF* 
                 8 
                 1341 
                 1532 
                 −25* 
                 63* 
                 Comparative Example 
               
               
                 42 
                 GG* 
                 12 
                 1328 
                 1419 
                 −23* 
                 65* 
                 Comparative Example 
               
               
                 43 
                 HH* 
                 12 
                 1151 
                 1238 
                 −41  
                 68* 
                 Comparative Example 
               
               
                 44 
                 II* 
                 12 
                 1168 
                 1241 
                 −28* 
                 53* 
                 Comparative Example 
               
               
                   
               
               
                 Note 1: 
               
               
                 The symbol * means that the parameter deviates from the range specified in the present invention. 
               
               
                 Note 2: 
               
               
                 Ranges specified in the present invention are as follows: 1. vTrs at the middle of the steel thickness (° C.): −40° C. or lower for steel plates with a tensile strength lower than 1200 MPa; −30° C. or lower for steel plates with a tensile strength of 1200 MPa or higher: 2. Safety index of delayed fracture resistance: 80% or higher for steel plates with a tensile strength lower than 1200 MPa; 75% or higher for steel plates with a tensile strength of 1200 MPa or higher 
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 7 
               
               
                   
               
               
                 (mass %) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Steels 
                 C 
                 Si 
                 Mn 
                 P 
                 S 
                 Cu 
                 Ni 
                 Cr 
                 Mo 
                 Nb 
                 V 
                 Ti 
               
               
                   
               
               
                 A 
                 0.05 
                 0.19 
                 1.34 
                 0.011 
                 0.0019 
                 0.00 
                 0.00 
                 0.03 
                 0.05 
                 0.020 
                 0.034 
                 0.000 
               
               
                 B 
                 0.08 
                 0.26 
                 1.43 
                 0.018 
                 0.0022 
                 0.00 
                 0.00 
                 0.03 
                 0.19 
                 0.021 
                 0.035 
                 0.000 
               
               
                 C 
                 0.10 
                 0.31 
                 1.08 
                 0.014 
                 0.0029 
                 0.00 
                 0.00 
                 0.06 
                 0.09 
                 0.019 
                 0.008 
                 0.010 
               
               
                 D 
                 0.12 
                 0.38 
                 1.48 
                 0.014 
                 0.0018 
                 0.02 
                 0.01 
                 0.49 
                 0.38 
                 0.017 
                 0.041 
                 0.012 
               
               
                 E 
                 0.12 
                 0.40 
                 1.51 
                 0.012 
                 0.0019 
                 0.02 
                 0.01 
                 0.26 
                 0.40 
                 0.020 
                 0.000 
                 0.010 
               
               
                 F 
                 0.13 
                 0.41 
                 1.51 
                 0.014 
                 0.0023 
                 0.00 
                 0.00 
                 0.51 
                 0.41 
                 0.020 
                 0.042 
                 0.013 
               
               
                 G 
                 0.14 
                 0.41 
                 1.55 
                 0.014 
                 0.0022 
                 0.00 
                 1.09 
                 0.50 
                 0.43 
                 0.020 
                 0.000 
                 0.011 
               
               
                 H 
                 0.15 
                 0.41 
                 1.52 
                 0.014 
                 0.0019 
                 0.30 
                 0.30 
                 0.51 
                 0.21 
                 0.020 
                 0.042 
                 0.013 
               
               
                 I 
                 0.15 
                 0.41 
                 1.21 
                 0.014 
                 0.0027 
                 0.00 
                 0.00 
                 0.51 
                 0.69 
                 0.020 
                 0.000 
                 0.013 
               
               
                 J 
                 0.16 
                 0.42 
                 1.19 
                 0.005 
                 0.0019 
                 0.26 
                 0.28 
                 0.34 
                 0.65 
                 0.019 
                 0.044 
                 0.012 
               
               
                 K 
                 0.16 
                 0.27 
                 1.35 
                 0.002 
                 0.0009 
                 0.26 
                 0.24 
                 0.53 
                 0.52 
                 0.022 
                 0.052 
                 0.013 
               
               
                 L 
                 0.17 
                 0.37 
                 1.12 
                 0.009 
                 0.0010 
                 0.05 
                 0.06 
                 0.51 
                 0.69 
                 0.022 
                 0.041 
                 0.012 
               
               
                 M 
                 0.17 
                 0.20 
                 1.35 
                 0.005 
                 0.0018 
                 0.00 
                 0.40 
                 0.35 
                 0.25 
                 0.022 
                 0.050 
                 0.000 
               
               
                 N 
                 0.17 
                 0.22 
                 1.45 
                 0.015 
                 0.0009 
                 0.00 
                 1.32 
                 0.35 
                 0.21 
                 0.015 
                 0.035 
                 0.000 
               
               
                 O 
                 0.18 
                 0.35 
                 1.75 
                 0.004 
                 0.0007 
                 0.20 
                 0.20 
                 0.45 
                 0.30 
                 0.019 
                 0.008 
                 0.010 
               
               
                 P 
                 0.21 
                 0.33 
                 1.09 
                 0.014 
                 0.0012 
                 0.02 
                 0.01 
                 0.55 
                 0.69 
                 0.020 
                 0.041 
                 0.012 
               
               
                   
               
             
          
           
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Remarks 
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Ar 3   
                 Remarks Ac 1   
               
               
                 Steels 
                 B 
                 W 
                 Ca 
                 REM 
                 Mg 
                 Al 
                 T.N 
                 (° C.) 
                 (° C.) 
               
               
                   
               
               
                 A 
                 0.0000 
                 — 
                 — 
                 — 
                 — 
                 0.031 
                 0.0032 
                 783 
                 709 
               
               
                 B 
                 0.0000 
                 — 
                 — 
                 — 
                 — 
                 0.028 
                 0.0029 
                 755 
                 709 
               
               
                 C 
                 0.0010 
                 — 
                 — 
                 — 
                 — 
                 0.022 
                 0.0037 
                 785 
                 716 
               
               
                 D 
                 0.0012 
                 — 
                 0.0017 
                 — 
                 — 
                 0.030 
                 0.0030 
                 716 
                 722 
               
               
                 E 
                 0.0013 
                 — 
                 — 
                 — 
                 — 
                 0.027 
                 0.0031 
                 715 
                 717 
               
               
                 F 
                 0.0010 
                 — 
                 — 
                 — 
                 — 
                 0.032 
                 0.0037 
                 708 
                 723 
               
               
                 G 
                 0.0015 
                 — 
                 — 
                 — 
                 — 
                 0.024 
                 0.0024 
                 641 
                 706 
               
               
                 H 
                 0.0010 
                 — 
                 — 
                 — 
                 — 
                 0.032 
                 0.0030 
                 695 
                 718 
               
               
                 I 
                 0.0010 
                 — 
                 — 
                 0.0025 
                 — 
                 0.032 
                 0.0030 
                 704 
                 727 
               
               
                 J 
                 0.0012 
                 — 
                 — 
                 — 
                 0.0015 
                 0.028 
                 0.0046 
                 688 
                 719 
               
               
                 K 
                 0.0015 
                 — 
                 0.0032 
                 — 
                 — 
                 0.052 
                 0.0035 
                 684 
                 719 
               
               
                 L 
                 0.0013 
                 — 
                 0.0019 
                 — 
                 — 
                 0.027 
                 0.0037 
                 701 
                 726 
               
               
                 M 
                 0.0000 
                 — 
                 — 
                 0.0019 
                 — 
                 0.031 
                 0.0032 
                 702 
                 711 
               
               
                 N 
                 0.0000 
                 — 
                 — 
                 — 
                 — 
                 0.028 
                 0.0029 
                 647 
                 697 
               
               
                 O 
                 0.0010 
                 0.20 
                 — 
                 — 
                 — 
                 0.022 
                 0.0037 
                 668 
                 714 
               
               
                 P 
                 0.0012 
                 — 
                 0.0015 
                 — 
                 — 
                 0.030 
                 0.0030 
                 693 
                 728 
               
               
                   
               
               
                 Note 1: 
               
               
                 The symbol * means that the parameter deviates from the range specified in the present invention. 
               
               
                 Note 2: 
               
               
                 Ar 3  (° C.) = 910-310C—80Mn—20Cu—15Cr—55Ni—80Mo 
               
               
                 Note 3: 
               
               
                 Ac 1  (° C.) = 723-14Mn + 22Si—14.4Ni + 23.3Cr 
               
             
          
         
       
     
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 8 
               
               
                   
                   
               
               
                   
                 (mass %) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Steels 
                 C 
                 Si 
                 Mn 
                 P 
                 S 
                 Cu 
                 Ni 
                 Cr 
                 Mo 
                 Nb 
                 V 
                 Ti 
               
               
                   
               
               
                 Q 
                 0.23 
                 0.45 
                 1.52 
                 0.018 
                 0.0015 
                 0.02 
                 1.34 
                 0.45 
                 0.45 
                 0.020 
                 0.000 
                 0.010 
               
               
                 R 
                 0.12 
                 0.38 
                 1.48 
                 0.025* 
                 0.0018 
                 0.02 
                 0.01 
                 0.49 
                 0.38 
                 0.017 
                 0.041 
                 0.012 
               
               
                 S 
                 0.14 
                 0.41 
                 1.55 
                 0.014 
                 0.0043* 
                 0.00 
                 1.09 
                 0.50 
                 0.43 
                 0.020 
                 0.000 
                 0.011 
               
               
                 T 
                 0.15 
                 0.41 
                 1.52 
                 0.031* 
                 0.0019 
                 0.30 
                 0.30 
                 0.51 
                 0.21 
                 0.020 
                 0.042 
                 0.013 
               
               
                 U 
                 0.17 
                 0.37 
                 1.12 
                 0.032* 
                 0.0042* 
                 0.05 
                 0.06 
                 0.51 
                 0.69 
                 0.022 
                 0.041 
                 0.012 
               
               
                 X 
                 0.03 
                 0.26 
                 1.31 
                 0.010 
                 0.0009 
                 0.01 
                 0.03 
                 0.56 
                 0.05 
                 0.012 
                 0.031 
                 0.001 
               
               
                 Y 
                 0.17 
                 0.67 
                 1.81 
                 0.006 
                 0.0008 
                 1.98 
                 3.91 
                 0.63 
                 0.72 
                 0.018 
                 0.043 
                 0.012 
               
               
                 Z 
                 0.24 
                 0.32 
                 1.92 
                 0.003 
                 0.0006 
                 1.95 
                 3.95 
                 0.51 
                 0.95 
                 0.016 
                 0.042 
                 0.015 
               
               
                 AA 
                 0.18 
                 0.02 
                 1.12 
                 0.005 
                 0.0003 
                 1.66 
                 3.81 
                 0.36 
                 0.86 
                 0.022 
                 0.045 
                 0.012 
               
               
                 BB 
                 0.20 
                 0.75 
                 1.08 
                 0.006 
                 0.0004 
                 1.82 
                 3.56 
                 0.48 
                 0.89 
                 0.019 
                 0.046 
                 0.012 
               
               
                 CC 
                 0.23 
                 0.41 
                 0.60 
                 0.004 
                 0.0003 
                 1.91 
                 3.78 
                 0.39 
                 0.88 
                 0.021 
                 0.045 
                 0.010 
               
               
                 DD 
                 0.15 
                 0.42 
                 1.20 
                 0.006 
                 0.0006 
                 0.00 
                 0.01 
                 0.51 
                 0.41 
                 0.019 
                 0.042 
                 0.012 
               
               
                 EE 
                 0.27* 
                 0.53 
                 1.12 
                 0.006 
                 0.0004 
                 1.61 
                 3.23 
                 0.68 
                 0.78 
                 0.021 
                 0.043 
                 0.011 
               
               
                 FF 
                 0.22 
                 0.85* 
                 1.08 
                 0.005 
                 0.0005 
                 1.55 
                 3.16 
                 0.51 
                 0.77 
                 0.022 
                 0.041 
                 0.009 
               
               
                 GG 
                 0.18 
                 0.42 
                 2.11* 
                 0.003 
                 0.0003 
                 1.51 
                 2.84 
                 0.53 
                 0.63 
                 0.021 
                 0.038 
                 0.011 
               
               
                 HH 
                 0.21 
                 0.51 
                 1.32 
                 0.004 
                 0.0005 
                 0.13 
                 0.26 
                 0.36 
                 0.64 
                 0.022 
                 0.041 
                 0.009 
               
               
                 II 
                 0.22 
                 0.48 
                 1.16 
                 0.005 
                 0.0004 
                 0.16 
                 0.28 
                 0.38 
                 0.65 
                 0.019 
                 0.043 
                 0.008 
               
               
                   
               
             
          
           
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Remarks 
                 Remarks 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Ar 3   
                 Ac 1   
               
               
                   
                 Steels 
                 B 
                 W 
                 Ca 
                 REM 
                 Mg 
                 Al 
                 T.N 
                 (° C.) 
                 (° C.) 
               
               
                   
                   
               
               
                   
                 Q 
                 0.0013 
                 0.15 
                 — 
                 — 
                 — 
                 0.027 
                 0.0031 
                 600 
                 703 
               
               
                   
                 R 
                 0.0012 
                 — 
                 0.0017 
                 — 
                 — 
                 0.030 
                 0.0030 
                 716 
                 722 
               
               
                   
                 S 
                 0.0015 
                 — 
                 — 
                 — 
                 — 
                 0.024 
                 0.0024 
                 641 
                 706 
               
               
                   
                 T 
                 0.0010 
                 — 
                 — 
                 — 
                 — 
                 0.032 
                 0.0030 
                 695 
                 718 
               
               
                   
                 U 
                 0.0013 
                 — 
                 0.0019 
                 — 
                 — 
                 0.027 
                 0.0037 
                 701 
                 726 
               
               
                   
                 X 
                 0.0003 
                 — 
                 — 
                 — 
                 — 
                 0.035 
                 0.0034 
                 782 
                 723 
               
               
                   
                 Y 
                 0.0015 
                 — 
                 0.0005 
                 — 
                 — 
                 0.031 
                 0.0032 
                 391 
                 671 
               
               
                   
                 Z 
                 0.0013 
                 — 
                 0.0012 
                 — 
                 0.0012 
                 0.028 
                 0.0035 
                 342 
                 658 
               
               
                   
                 AA 
                 0.0010 
                 — 
                 0.0016 
                 — 
                 — 
                 0.031 
                 0.0034 
                 448 
                 661 
               
               
                   
                 BB 
                 0.0011 
                 — 
                 0.0017 
                 — 
                 — 
                 0.032 
                 0.0035 
                 451 
                 684 
               
               
                   
                 CC 
                 0.0013 
                 — 
                 0.0018 
                 — 
                 — 
                 0.028 
                 0.0033 
                 468 
                 678 
               
               
                   
                 DD 
                 0.0012 
                 — 
                 0.0093 
                 — 
                 — 
                 0.026 
                 0.0038 
                 727 
                 727 
               
               
                   
                 EE 
                 0.0012 
                 — 
                 0.0014 
                 — 
                 — 
                 0.025 
                 0.0034 
                 454 
                 688 
               
               
                   
                 FF 
                 0.0011 
                 — 
                 0.0012 
                 — 
                 — 
                 0.028 
                 0.0033 
                 481 
                 693 
               
               
                   
                 GG 
                 0.0012 
                 — 
                 0.0013 
                 — 
                 — 
                 0.031 
                 0.0034 
                 441 
                 674 
               
               
                   
                 HH 
                 0.0011 
                 — 
                 0.0003* 
                 — 
                 — 
                 0.033 
                 0.0032 
                 666 
                 720 
               
               
                   
                 II 
                 0.0012 
                 — 
                 0.0108* 
                 — 
                 — 
                 0.031 
                 0.0028 
                 673 
                 722 
               
               
                   
                   
               
               
                   
                 Note 1: 
               
               
                   
                 The symbol * means that the parameter deviates from the range specified in the present invention. 
               
               
                   
                 Note 2: 
               
               
                   
                 Ar 3  (° C.) = 910-310C—80Mn—20Cu—15Cr—55Ni—80Mo 
               
               
                   
                 Note 3: 
               
               
                   
                 Ac 1  (° C.) = 723-14Mn + 22Si—14.4Ni + 23.3Cr 
               
             
          
         
       
     
     
       
         
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 9 
               
               
                   
               
             
             
               
                   
                   
                   
                   
                   
                 Direct 
                 Direct 
                   
                   
               
               
                   
                   
                   
                   
                 Rolling reduction 
                 quenching 
                 quenching 
               
               
                   
                   
                   
                 Heating 
                 for non- 
                 initiation 
                 termination 
                   
                 Tempering 
               
               
                   
                   
                 Thickness 
                 temperature 
                 recrystallization 
                 temperature 
                 temperature 
                 Tempering initiation 
                 temperature 
               
               
                 No. 
                 Steels 
                 (mm) 
                 (° C.) 
                 regions (%) 
                 (° C.) 
                 (° C.) 
                 temperature (° C.) 
                 (° C.) 
               
               
                   
               
               
                 1 
                 A 
                 25 
                 1170 
                 35 
                 840 
                 180 
                 160 
                 540 
               
               
                 2 
                 B 
                 12 
                 1150 
                 30 
                 820 
                 350 
                 330 
                 610 
               
               
                 3 
                 C 
                 25 
                 1130 
                 55 
                 840 
                 320 
                 300 
                 570 
               
               
                 4 
                 D 
                 12 
                 1100 
                 60 
                 830 
                 230 
                 210 
                 550 
               
               
                 5 
                 E 
                 25 
                 1050 
                 60 
                 820 
                 170 
                 150 
                 590 
               
               
                 6 
                 F 
                 12 
                 1200 
                 70 
                 830 
                 230 
                 210 
                 640 
               
               
                 7 
                 G 
                 25 
                 1100 
                 60 
                 830 
                 130 
                 110 
                 680 
               
               
                 8 
                 H 
                 50 
                 1130 
                 60 
                 820 
                 180 
                 160 
                 600 
               
               
                 9 
                 I 
                 12 
                 1150 
                 80 
                 830 
                 190 
                 170 
                 630 
               
               
                 10 
                 J 
                 25 
                 1150 
                 60 
                 830 
                 200 
                 180 
                 600 
               
               
                 11 
                 K 
                 50 
                 1130 
                 60 
                 850 
                  90 
                  70 
                 580 
               
               
                 12 
                 L 
                 60 
                 1150 
                 60 
                 850 
                 150 
                 130 
                 550 
               
               
                 13 
                 M 
                 6 
                 1100 
                 60 
                 730 
                 140 
                 120 
                 410 
               
               
                 14 
                 N 
                 12 
                 1100 
                 60 
                 750 
                 240 
                 Room temperature 
                 460 
               
               
                 15 
                 O 
                 25 
                 1100 
                 60 
                 760 
                 130 
                 110 
                 480 
               
               
                 16 
                 P 
                 60 
                 1110 
                 60 
                 710 
                 110 
                 Room temperature 
                 510 
               
               
                 17 
                 Q 
                 6 
                 1090 
                 60 
                 810 
                 210 
                 190 
                 430 
               
               
                 18 
                 A 
                 25 
                 1170 
                  25* 
                 840 
                 180 
                 160 
                 540 
               
               
                 19 
                 B 
                 12 
                 1150 
                  20* 
                 820 
                 350 
                 330 
                 610 
               
               
                 20 
                 C 
                 25 
                 1130 
                  25* 
                 840 
                 320 
                 300 
                 570 
               
               
                 21 
                 D 
                 12 
                 1100 
                 60 
                  705* 
                 230 
                 210 
                 550 
               
               
                 22 
                 E 
                 25 
                 1050 
                 60 
                  700* 
                 170 
                 150 
                 590 
               
               
                 23 
                 F 
                 12 
                 1200 
                 70 
                  690* 
                 230 
                 210 
                 640 
               
               
                 24 
                 G 
                 25 
                 1100 
                 60 
                 830 
                  400* 
                 110 
                 680 
               
               
                 25 
                 H 
                 50 
                 1130 
                 60 
                 820 
                  450* 
                 160 
                 600 
               
               
                 26 
                 I 
                 12 
                 1150 
                 80 
                 830 
                 190 
                 170 
                  740* 
               
               
                   
               
             
          
           
               
                   
                   
                 Average 
                   
                   
                   
                   
                   
               
               
                   
                 Average 
                 heating rate 
                   
                 Average 
               
               
                   
                 heating rate 
                 for heating 
                   
                 cooling rate 
               
               
                   
                 for heating 
                 the middle 
                   
                 for cooling 
               
               
                   
                 the middle of 
                 of the steel 
                   
                 from the 
               
               
                   
                 the steel thickness 
                 thickness from 
                   
                 maintained 
                 Aspect 
               
               
                   
                 from the tempering 
                 370° C. to the 
                 Time for which 
                 tempering 
                 ratio of 
                 Cementite 
               
               
                   
                 initiation 
                 tempering 
                 the tempering 
                 temperature 
                 prior 
                 covering 
               
               
                   
                 temperature 
                 temperature 
                 temperature is 
                 to 200° C.  
                 austenite 
                 rate of 
               
               
                 No. 
                 to 370° C. (° C./s) 
                 (° C./s) 
                 maintained (s) 
                 (° C./s) 
                 grains 
                 laths 
                 Classification 
               
               
                   
               
               
                 1 
                 6.0 
                 8.0 
                 0 
                 0.3 
                 3.5 
                  5 
                 Example 
               
               
                 2 
                 12.5 
                 14.5 
                 0 
                 0.6 
                 3.3 
                  7 
                 Example 
               
               
                 3 
                 6.0 
                 8.0 
                 0 
                 0.3 
                 13.2 
                 12 
                 Example 
               
               
                 4 
                 12.5 
                 14.5 
                 0 
                 0.6 
                 9.8 
                 15 
                 Example 
               
               
                 5 
                 6.0 
                 8.0 
                 0 
                 0.3 
                 7.5 
                 24 
                 Example 
               
               
                 6 
                 12.5 
                 14.5 
                 0 
                 0.6 
                 12.3 
                 34 
                 Example 
               
               
                 7 
                 6.0 
                 8.0 
                 0 
                 0.3 
                 17.3 
                 40 
                 Example 
               
               
                 8 
                 3.0 
                 5.0 
                 60 
                 0.2 
                 6.5 
                 26 
                 Example 
               
               
                 9 
                 12.5 
                 14.5 
                 0 
                 0.6 
                 17.3 
                 25 
                 Example 
               
               
                 10 
                 6.0 
                 8.0 
                 0 
                 0.3 
                 15.3 
                 30 
                 Example 
               
               
                 11 
                 3.0 
                 5.0 
                 60 
                 0.2 
                 10.9 
                 26 
                 Example 
               
               
                 12 
                 2.5 
                 4.5 
                 0 
                 0.1 
                 5.3 
                 19 
                 Example 
               
               
                 13 
                 25.0 
                 27.0 
                 0 
                 1.3 
                 16.9 
                 11 
                 Example 
               
               
                 14 
                 12.5 
                 14.5 
                 0 
                 0.6 
                 11.9 
                 23 
                 Example 
               
               
                 15 
                 6.0 
                 8.0 
                 0 
                 0.3 
                 12.3 
                 37 
                 Example 
               
               
                 16 
                 2.5 
                 4.5 
                 0 
                 0.1 
                 5.4 
                 40 
                 Example 
               
               
                 17 
                 25.0 
                 27.0 
                 0 
                 1.3 
                 17.9 
                 35 
                 Example 
               
               
                 18 
                 6.0 
                 8.0 
                 0 
                 0.3 
                 2.5* 
                  55* 
                 Comparative Example 
               
               
                 19 
                 12.5 
                 14.5 
                 0 
                 0.6 
                 2.3* 
                  52* 
                 Comparative Example 
               
               
                 20 
                 6.0 
                 8.0 
                 0 
                 0.3 
                 1.7* 
                  53* 
                 Comparative Example 
               
               
                 21 
                 12.5 
                 14.5 
                 0 
                 0.6 
                 8.8 
                 14 
                 Comparative Example 
               
               
                 22 
                 6.0 
                 8.0 
                 0 
                 0.3 
                 7.1 
                 23 
                 Comparative Example 
               
               
                 23 
                 12.5 
                 14.5 
                 0 
                 0.6 
                 11.2 
                 32 
                 Comparative Example 
               
               
                 24 
                 6.0 
                 8.0 
                 0 
                 0.3 
                 16.6 
                 38 
                 Comparative Example 
               
               
                 25 
                 3.0 
                 5.0 
                 60 
                 0.2 
                 6.2 
                 24 
                 Comparative Example 
               
               
                 26 
                 12.5 
                 14.5 
                 0 
                 0.6 
                 17.0 
                  56* 
                 Comparative Example 
               
               
                   
               
               
                 Note: 
               
               
                 The symbol * means that the parameter deviates from the range specified in the present invention. 
               
             
          
         
       
     
     
       
         
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 10 
               
               
                   
               
             
             
               
                   
                   
                   
                   
                 Rolling 
                 Direct 
                 Direct 
                   
                   
               
               
                   
                   
                   
                   
                 reduction for 
                 quenching 
                 quenching 
               
               
                   
                   
                   
                 Heating 
                 non- 
                 initiation 
                 termination 
                   
                 Tempering 
               
               
                   
                   
                 Thickness 
                 temperature 
                 recrystallization 
                 temperature 
                 temperature 
                 Tempering initiation 
                 temperature 
               
               
                 No. 
                 Steels 
                 (mm) 
                 (° C.) 
                 regions (%) 
                 (° C.) 
                 (° C.) 
                 temperature (° C.) 
                 (° C.) 
               
               
                   
               
             
          
           
               
                 27 
                 J 
                 25 
                 1150 
                 60 
                 830 
                 200 
                 180 
                  730* 
               
               
                 28 
                 K 
                 50 
                 1130 
                 60 
                 850 
                  90 
                  70 
                  730* 
               
               
                 29 
                 L 
                 60 
                 1150 
                 60 
                 850 
                 150 
                 130 
                 550 
               
               
                 30 
                 M 
                 6 
                 1100 
                 60 
                 730 
                 140 
                 120 
                 410 
               
               
                 31 
                 N 
                 12 
                 1100 
                 60 
                 750 
                 240 
                 Room temperature 
                 460 
               
               
                 32 
                 O 
                 25 
                 1100 
                 60 
                 760 
                 130 
                 110 
                 480 
               
               
                 33 
                 P 
                 60 
                 1110 
                 60 
                 710 
                 110 
                 Room temperature 
                 510 
               
               
                 34 
                 Q 
                 6 
                 1090 
                 60 
                 810 
                 210 
                 190 
                 430 
               
               
                 35 
                 D 
                 12 
                 1100 
                 60 
                 830 
                 230 
                 210 
                 550 
               
               
                 36 
                 L 
                 60 
                 1150 
                 60 
                 850 
                 150 
                 130 
                 550 
               
               
                 37 
                 R 
                 35 
                 1100 
                 60 
                 830 
                 200 
                 180 
                 490 
               
               
                 38 
                 S 
                 50 
                 1050 
                 60 
                 850 
                 150 
                 130 
                 520 
               
               
                 39 
                 T 
                 50 
                 1050 
                 60 
                 850 
                 150 
                 130 
                 520 
               
               
                 40 
                 U 
                 60 
                 1200 
                 60 
                 850 
                 150 
                 130 
                 500 
               
               
                 41 
                 X 
                 25 
                 1160 
                 30 
                 830 
                 230 
                 810 
                 520 
               
               
                 42 
                 Y 
                 6 
                 1120 
                 65 
                 670 
                 80 
                 850 
                 500 
               
               
                 43 
                 Z 
                 25 
                 1110 
                 75 
                 640 
                 100 
                 620 
                 500 
               
               
                 44 
                 AA 
                 12 
                 1120 
                 70 
                 650 
                 120 
                 630 
                 520 
               
               
                 45 
                 BB 
                 32 
                 1130 
                 75 
                 720 
                 100 
                 700 
                 500 
               
               
                 46 
                 CC 
                 20 
                 1150 
                 70 
                 680 
                 100 
                 660 
                 520 
               
               
                 47 
                 DD 
                 32 
                 1100 
                 60 
                 830 
                 230 
                 810 
                 560 
               
               
                 48 
                 EE 
                 16 
                 1100 
                 75 
                 700 
                 100 
                 680 
                 520 
               
               
                 49 
                 FF 
                 8 
                 1110 
                 70 
                 680 
                 100 
                 660 
                 520 
               
               
                 50 
                 GG 
                 12 
                 1120 
                 60 
                 670 
                 100 
                 650 
                 500 
               
               
                 51 
                 HH 
                 12 
                 1120 
                 60 
                 830 
                 200 
                 810 
                 500 
               
               
                 52 
                 II 
                 12 
                 1120 
                 60 
                 830 
                 200 
                 810 
                 500 
               
               
                   
               
             
          
           
               
                   
                 Average 
                   
                   
                   
                   
                   
                   
               
               
                   
                 heating rate 
               
               
                   
                 for heating 
                   
                   
                 Average 
               
               
                   
                 the middle 
                   
                   
                 cooling rate 
               
               
                   
                 of the steel 
                   
                   
                 for cooling 
               
               
                   
                 thickness from 
                 Average heating rate 
                 Time for 
                 from the 
               
               
                   
                 the tempering 
                 for heating the middle 
                 which the 
                 maintained 
                 Aspect 
               
               
                   
                 initiation 
                 of the steel thickness 
                 tempering 
                 tempering 
                 ratio of 
                 Cementite 
               
               
                   
                 temperature 
                 from 370° C. to the 
                 temperature 
                 temperature 
                 prior 
                 covering 
               
               
                   
                 to 370° C. 
                 tempering 
                 is maintained 
                 to 200° C. 
                 austenite 
                 rate of 
               
               
                 No. 
                 (° C./s) 
                 temperature (° C./s) 
                 (s) 
                 (° C./s) 
                 grains 
                 laths 
                 Classification 
               
               
                   
               
               
                 27 
                 6.0 
                 8.0 
                 0 
                 0.3 
                 15.1 
                  61* 
                 Comparative Example 
               
               
                 28 
                 3.0 
                 5.0 
                 60 
                 0.2 
                 10.2 
                  63* 
                 Comparative Example 
               
               
                 29 
                 2.5 
                 0.8* 
                 0 
                 0.1 
                 5.3 
                 39 
                 Comparative Example 
               
               
                 30 
                 25.0 
                 0.9* 
                 0 
                 1.3 
                 16.9 
                  52* 
                 Comparative Example 
               
               
                 31 
                 12.5 
                 0.7* 
                 0 
                 0.6 
                 11.9 
                 42 
                 Comparative Example 
               
               
                 32 
                 1.5 
                 0.6* 
                 0 
                 0.3 
                 12.3 
                  55* 
                 Comparative Example 
               
               
                 33 
                 1.1 
                 0.6* 
                 0 
                 0.1 
                 5.4 
                  61* 
                 Comparative Example 
               
               
                 34 
                 1.2 
                 0.8* 
                 0 
                 1.3 
                 17.9 
                  53* 
                 Comparative Example 
               
               
                 35 
                 1.5 
                 14.5 
                 0 
                 0.6 
                 9.8 
                 23 
                 Example 
               
               
                 36 
                 1.0 
                 4.5 
                 0 
                 0.1 
                 5.3 
                 32 
                 Example 
               
               
                 37 
                 4.3 
                 6.3 
                 0 
                 0.2 
                 10.7 
                 41 
                 Comparative Example 
               
               
                 38 
                 3.0 
                 5.0 
                 0 
                 0.2 
                 4.9 
                 45 
                 Comparative Example 
               
               
                 39 
                 3.0 
                 5.0 
                 0 
                 0.2 
                 5.5 
                 23 
                 Comparative Example 
               
               
                 40 
                 2.5 
                 4.5 
                 0 
                 0.1 
                 6.3 
                  56* 
                 Comparative Example 
               
               
                 41 
                 5.0 
                 7.0 
                 10 
                 0.3 
                 3.5 
                 25 
                 Example 
               
               
                 42 
                 20.0 
                 22.0 
                 0 
                 1.3 
                 12.5 
                 21 
                 Example 
               
               
                 43 
                 5.0 
                 7.0 
                 10 
                 0.3 
                 16.1 
                 25 
                 Example 
               
               
                 44 
                 10.0 
                 12.0 
                 10 
                 0.6 
                 14.1 
                 21 
                 Example 
               
               
                 45 
                 3.0 
                 5.0 
                 0 
                 0.2 
                 16.3 
                 32 
                 Example 
               
               
                 46 
                 5.0 
                 7.0 
                 0 
                 0.4 
                 14.5 
                 26 
                 Example 
               
               
                 47 
                 3.0 
                 5.0 
                 0 
                 0.2 
                 8.3 
                 31 
                 Example 
               
               
                 48 
                 8.0 
                 10.0 
                 0 
                 0.5 
                 16.7 
                 34 
                 Comparative Example 
               
               
                 49 
                 15.0 
                 17.0 
                 0 
                 0.9 
                 17.6 
                 19 
                 Comparative Example 
               
               
                 50 
                 10.0 
                 12.0 
                 0 
                 0.6 
                 6.5 
                 32 
                 Comparative Example 
               
               
                 51 
                 10.0 
                 12.0 
                 0 
                 0.6 
                 6.3 
                 23 
                 Comparative Example 
               
               
                 52 
                 10.0 
                 12.0 
                 10 
                 0.6 
                 6.5 
                 26 
                 Comparative Example 
               
               
                   
               
               
                 Note: 
               
               
                 The symbol * means that the parameter deviates from the range specified in the present invention. 
               
             
          
         
       
     
     
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 11 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 vTrs at the 
                 Safety index of 
                   
               
               
                   
                   
                   
                 Yield 
                 Tensile 
                 middle of the 
                 delayed 
               
               
                   
                   
                 Thickness 
                 strength 
                 strength 
                 steel thickness 
                 fracture 
               
               
                 No. 
                 Steels 
                 (mm) 
                 (MPa) 
                 (MPa) 
                 (° C.) 
                 resistance (%) 
                 Classification 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 A 
                 25 
                 596 
                 667 
                 −121  
                 100  
                 Example 
               
               
                 2 
                 B 
                 12 
                 611 
                 695 
                 −131  
                 99 
                 Example 
               
               
                 3 
                 C 
                 25 
                 812 
                 888 
                 −93 
                 100  
                 Example 
               
               
                 4 
                 D 
                 12 
                 1037 
                 1061 
                 −81 
                 98 
                 Example 
               
               
                 5 
                 E 
                 25 
                 1015 
                 1041 
                 −83 
                 99 
                 Example 
               
               
                 6 
                 F 
                 12 
                 1112 
                 1115 
                 −73 
                 97 
                 Example 
               
               
                 7 
                 G 
                 25 
                 1069 
                 1100 
                 −76 
                 97 
                 Example 
               
               
                 8 
                 H 
                 50 
                 1025 
                 1034 
                 −63 
                 96 
                 Example 
               
               
                 9 
                 I 
                 12 
                 1151 
                 1253 
                 −53 
                 95 
                 Example 
               
               
                 10 
                 J 
                 25 
                 1251 
                 1314 
                 −51 
                 90 
                 Example 
               
               
                 11 
                 K 
                 50 
                 1296 
                 1312 
                 −49 
                 91 
                 Example 
               
               
                 12 
                 L 
                 60 
                 1051 
                 1097 
                 −56 
                 98 
                 Example 
               
               
                 13 
                 M 
                 6 
                 1315 
                 1317 
                 −66 
                 89 
                 Example 
               
               
                 14 
                 N 
                 12 
                 1410 
                 1426 
                 −56 
                 88 
                 Example 
               
               
                 15 
                 O 
                 25 
                 1399 
                 1415 
                 −49 
                 89 
                 Example 
               
               
                 16 
                 P 
                 60 
                 1333 
                 1348 
                 −41 
                 85 
                 Example 
               
               
                 17 
                 Q 
                 6 
                 1410 
                 1451 
                 −66 
                 82 
                 Example 
               
               
                 18 
                 A 
                 25 
                 523 
                 601 
                 −59 
                  53* 
                 Comparative Example 
               
               
                 19 
                 B 
                 12 
                 538 
                 623 
                 −63 
                  49* 
                 Comparative Example 
               
               
                 20 
                 C 
                 25 
                 783 
                 852 
                 −67 
                  41* 
                 Comparative Example 
               
               
                 21 
                 D 
                 12 
                 927 
                 953 
                  −39* 
                  73* 
                 Comparative Example 
               
               
                 22 
                 E 
                 25 
                 936 
                 951 
                  −36* 
                  75* 
                 Comparative Example 
               
               
                 23 
                 F 
                 12 
                 1037 
                 1039 
                 −41 
                  67* 
                 Comparative Example 
               
               
                 24 
                 G 
                 25 
                 986 
                 1012 
                  −38* 
                 97 
                 Comparative Example 
               
               
                 25 
                 H 
                 50 
                 953 
                 967 
                  −34* 
                 96 
                 Comparative Example 
               
               
                 26 
                 I 
                 12 
                 1053 
                 1149 
                  −32* 
                 95 
                 Comparative Example 
               
               
                   
               
               
                 Note: 
               
               
                 The symbol * means that the parameter deviates from the range specified in the present invention. 
               
               
                 Note 2: 
               
               
                 Ranges specified in the present invention are as follows: 1. vTrs at the middle of the steel thickness (° C.): −40° C. or lower for steel plates with a tensile strength lower than 1200 MPa: −30° C. or lower for steel plates with a tensile strength of 1200 MPa or higher: 2. Safety index of delayed fracture resistance: 85% or higher for steel plates with a tensile strength lower than 1200 MPa; 80% or higher for steel plates with a tensile strength of 1200 MPa or higher 
               
             
          
         
       
     
     
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 12 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 vTrs at 
                   
                   
               
               
                   
                   
                   
                   
                   
                 the middle 
               
               
                   
                   
                   
                   
                   
                 of the 
               
               
                   
                   
                   
                 Yield 
                 Tensile 
                 steel 
                 Safety index of 
                   
               
               
                   
                   
                 Thickness 
                 strength 
                 strength 
                 thickness 
                 delayed fracture 
               
               
                 No. 
                 Steels 
                 (mm) 
                 (MPa) 
                 (MPa) 
                 (° C.) 
                 resistance (%) 
                 Classification 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 27 
                 J 
                 25 
                 1153 
                 1213 
                 −33 
                 67* 
                 Comparative Example 
               
               
                 28 
                 K 
                 50 
                 1183 
                 1203 
                 −35 
                 69* 
                 Comparative Example 
               
               
                 29 
                 L 
                 60 
                 1012 
                 1053 
                  −23* 
                 83* 
                 Comparative Example 
               
               
                 30 
                 M 
                 6 
                 1213 
                 1216 
                  −28* 
                 81  
                 Comparative Example 
               
               
                 31 
                 N 
                 12 
                 1308 
                 1327 
                  −25* 
                 78* 
                 Comparative Example 
               
               
                 32 
                 O 
                 25 
                 1297 
                 1323 
                  −24* 
                 72* 
                 Comparative Example 
               
               
                 33 
                 P 
                 60 
                 1216 
                 1218 
                  −26* 
                 68* 
                 Comparative Example 
               
               
                 34 
                 Q 
                 6 
                 1309 
                 1311 
                 −35 
                 73* 
                 Comparative Example 
               
               
                 35 
                 D 
                 12 
                 1039 
                 1058 
                 −75 
                 95  
                 Example 
               
               
                 36 
                 L 
                 60 
                 1048 
                 1093 
                 −47 
                 93  
                 Example 
               
               
                 37 
                 R 
                 35 
                 1031 
                 1063 
                  −38* 
                 64* 
                 Comparative Example 
               
               
                 38 
                 S 
                 50 
                 1061 
                 1105 
                  −34* 
                 61* 
                 Comparative Example 
               
               
                 39 
                 T 
                 50 
                 1015 
                 1023 
                  −29* 
                 53* 
                 Comparative Example 
               
               
                 40 
                 U 
                 60 
                 1049 
                 1099 
                  −23* 
                 55* 
                 Comparative Example 
               
               
                 41 
                 X 
                 25 
                 589 
                 661 
                 −112  
                 98  
                 Example 
               
               
                 42 
                 Y 
                 6 
                 1411 
                 1473 
                 −51 
                 88  
                 Example 
               
               
                 43 
                 Z 
                 25 
                 1459 
                 1539 
                 −53 
                 82  
                 Example 
               
               
                 44 
                 AA 
                 12 
                 1371 
                 1606 
                 −55 
                 86  
                 Example 
               
               
                 45 
                 BB 
                 32 
                 1403 
                 1641 
                 −47 
                 86  
                 Example 
               
               
                 46 
                 CC 
                 20 
                 1451 
                 1712 
                 −51 
                 90  
                 Example 
               
               
                 47 
                 DD 
                 32 
                 1115 
                 1143 
                 −70 
                 92  
                 Example 
               
               
                 48 
                 EE 
                 16 
                 1405 
                 1589 
                 −32 
                 62* 
                 Comparative Example 
               
               
                 49 
                 FF 
                 8 
                 1369 
                 1551 
                 −34 
                 72* 
                 Comparative Example 
               
               
                 50 
                 GG 
                 12 
                 1351 
                 1441 
                 −32 
                 71* 
                 Comparative Example 
               
               
                 51 
                 HH 
                 12 
                 1179 
                 1251 
                 −52 
                 72* 
                 Comparative Example 
               
               
                 52 
                 II 
                 12 
                 1181 
                 1269 
                 −39 
                 62* 
                 Comparative Example 
               
               
                 27 
                 J 
                 25 
                 1153 
                 1213 
                 −33 
                 67* 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 28 
                 K 
                 50 
                 1183 
                 1203 
                 −35 
                 69* 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 29 
                 L 
                 60 
                 1012 
                 1053 
                  −23* 
                 83* 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 30 
                 M 
                 6 
                 1213 
                 1216 
                  −28* 
                 81  
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 31 
                 N 
                 12 
                 1308 
                 1327 
                  −25* 
                 78* 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 32 
                 O 
                 25 
                 1297 
                 1323 
                  −24* 
                 72* 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 33 
                 P 
                 60 
                 1216 
                 1218 
                  −26* 
                 68* 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 34 
                 Q 
                 6 
                 1309 
                 1311 
                 −35 
                 73* 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 35 
                 D 
                 12 
                 1039 
                 1058 
                 −75 
                 95  
                 Example 
               
               
                 36 
                 L 
                 60 
                 1048 
                 1093 
                 −47 
                 93  
                 Example 
               
               
                 37 
                 R 
                 35 
                 1031 
                 1063 
                  −38* 
                 64* 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 38 
                 S 
                 50 
                 1061 
                 1105 
                  −34* 
                 61* 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 39 
                 T 
                 50 
                 1015 
                 1023 
                  −29* 
                 53* 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 40 
                 U 
                 60 
                 1049 
                 1099 
                  −23* 
                 55* 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 41 
                 X 
                 25 
                 589 
                 661 
                 −112  
                 98  
                 Example 
               
               
                 42 
                 Y 
                 6 
                 1411 
                 1473 
                 −51 
                 88  
                 Example 
               
               
                 43 
                 Z 
                 25 
                 1459 
                 1539 
                 −53 
                 82  
                 Example 
               
               
                 44 
                 AA 
                 12 
                 1371 
                 1606 
                 −55 
                 86  
                 Example 
               
               
                 45 
                 BB 
                 32 
                 1403 
                 1641 
                 −47 
                 86  
                 Example 
               
               
                 46 
                 CC 
                 20 
                 1451 
                 1712 
                 −51 
                 90  
                 Example 
               
               
                 47 
                 DD 
                 32 
                 1115 
                 1143 
                 −70 
                 92  
                 Example 
               
               
                 48 
                 EE 
                 16 
                 1405 
                 1589 
                 −32 
                 62* 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 49 
                 FF 
                 8 
                 1369 
                 1551 
                 −34 
                 72* 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 50 
                 GG 
                 12 
                 1351 
                 1441 
                 −32 
                 71* 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 51 
                 HH 
                 12 
                 1179 
                 1251 
                 −52 
                 72* 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 52 
                 II 
                 12 
                 1181 
                 1269 
                 −39 
                 62* 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                   
               
               
                 Note: 
               
               
                 The symbol * means that the parameter deviates from the range specified in the present invention. 
               
               
                 Note 2: 
               
               
                 Ranges specified in the present invention are as follows: 1. vTrs at the middle of the steel thickness (° C.): −40° C. or lower for steel plates with a tensile strength lower than 1200 MPa; −30° C. or lower for steel plates with a tensile strength of 1200 MPa or higher: 2. Safety index of delayed fracture resistance: 85% or higher for steel plates with a tensile strength lower than 1200 MPa; 80% or higher for steel plates with a tensile strength of 1200 MPa or higher