Patent Publication Number: US-2018044752-A1

Title: HIGH-STRENGTH COLD-ROLLED STEEL SHEET HAVING EXCELLENT FORMABILITY AND COLLISION CHARACTERISTICS AND HAVING TENSILE STRENGTH OF 980 MPa OR MORE, AND METHOD FOR PRODUCING SAME

Description:
TECHNICAL FIELD 
     The present invention relates to a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more and being excellent in formability and crashworthiness and to a method for producing the same. In further detail, the present invention relates to the high-strength cold-rolled steel sheet described above, a high-strength electrogalvanized steel sheet having an electrogalvanized layer formed on a surface of the high-strength cold-rolled steel sheet, a high-strength hot-dip galvanized steel sheet having a hot-dip galvanized layer formed on a surface of the high-strength cold-rolled steel sheet, and a high-strength hot-dip galvannealed steel sheet having a hot-dip galvannealed layer formed on a surface of the high-strength cold-rolled steel sheet, and to a method for producing the same. 
     BACKGROUND ART 
     In order to achieve fuel cost reduction of automobiles, transport aircrafts and the like, it is desired to reduce the weight of the automobiles, transport aircrafts and the like. In order to achieve weight reduction, it is effective, for example, to reduce the sheet thickness by using a high-strength steel sheet. However, when the steel sheet is made to have a higher strength, the steel sheet comes to have poorer ductility and stretch-flangeability, thereby degrading the formability into a product shape. 
     Also, in steel parts for automobiles, a steel sheet whose surface has been subjected to galvanization such as electrogalvanization (which may hereafter be denoted as EG), hot-dip galvanizing (which may hereafter be denoted as GI), or hot-dip galvannealing (which may hereafter be denoted as GA), which may hereafter be comprehensively referred to as galvanized steel sheet, is often used from the viewpoint of corrosion resistance. In these galvanized steel sheets as well, increase in strength and formability is demanded in the same manner as in the above high-strength steel sheet. 
     For example, Patent Literature 1 discloses a hot-dip galvannealed steel sheet having a metal structure in which martensite and retained austenite are mixedly present in ferrite and having a tensile strength TS of 490 to 880 MPa by reinforcement of the complex structure thereof, thus having a good press formability. 
     Also, Patent Literature 2 discloses a high-strength cold-rolled steel sheet having a TS (Tensile Strength) of 590 MPa or more and being excellent in formability, specifically, with TS×EL (EL: Elongation, elongation) being 23000 MPa % or more, and being excellent in corrosion resistance after application even in a severe environment such as a hot salt water test, a salt water spraying test, or a combined cyclic corrosion test. The metal structure of this steel sheet is a structure including ferrite, retained austenite, bainite, and/or martensite. It is described that the retained austenite has a function of enhancing ductility of the steel sheet, the function being known as a TRIP effect. 
     In the meantime, it is demanded that the steel parts for automobiles are excellent in crashworthiness which is an ability to efficiently absorb an impact generated when the automobiles come into collision. There is known, for example, Patent Literature 3 as a technique for improving the crashworthiness. Patent Literature 3 discloses a high-strength galvanized steel sheet having a maximum tensile strength of 900 MPa or more and being excellent in collision absorption energy in which a dynamic/static ratio as large as that of a steel sheet of 590 MPa class and a maximum tensile strength of 900 MPa or more are compatible with each other, as well as a method for producing the same. This production method is characterized in that, after performing galvanization, cooling is performed, and rolling is performed with use of a roll having a roughness (Ra) of 3.0 or less. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent No. 3527092 
     Patent Literature 2: Japanese Patent No. 5076434 
     Patent Literature 3: Japanese Patent No. 5487916 
     SUMMARY OF INVENTION 
     Problems to be Solved by the Invention 
     According to the techniques disclosed in Patent Literatures 1 and 2, the formability of a steel sheet can be improved. However, no consideration is made on the crashworthiness. In contrast, according to the technique disclosed in Patent Literature 3, the crashworthiness of the steel sheet can be improved. However, no consideration is made on the formability as evaluated by ductility and stretch-flangeability. 
     The present invention has been made in view of the aforementioned circumstances, and an object thereof is to provide a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more, having good formability as evaluated by ductility and stretch-flangeability, and having excellent crashworthiness. Another object of the present invention is to provide a high-strength electrogalvanized steel sheet having an electrogalvanized layer on a surface of the high-strength cold-rolled steel sheet, a high-strength hot-dip galvanized steel sheet having a hot-dip galvanized layer on a surface of the high-strength cold-rolled steel sheet, and a high-strength hot-dip galvannealed steel sheet having a hot-dip galvannealed layer on a surface of the high-strength cold-rolled steel sheet. Still another object of the present invention is to provide a method for producing a high-strength cold-rolled steel sheet, a high-strength hot-dip galvanized steel sheet, and a high-strength hot-dip galvannealed steel sheet having the above properties in combination. 
     Means for Solving the Problems 
     A high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more according to the present invention that has solved the aforementioned problems is a steel sheet containing, in mass %, C: 0.10% or more to 0.5% or less, Si: 1.0% or more to 3% or less, Mn: 1.5% or more to 7% or less, P: more than 0% to 0.1% or less, S: more than 0% to 0.05% or less, Al: 0.005% or more to 1% or less, N: more than 0% to 0.01% or less, and O: more than 0% to 0.01% or less, with a balance being iron and inevitable impurities. Further, the gist lies in that a metal structure at a position of ¼ of a sheet thickness satisfies (1) to (4) below. The term “MA” is an abbreviation for Martensite-Austenite Constituent. 
     (1) When the metal structure is observed with a scanning electron microscope, an area ratio of ferrite relative to a whole of the metal structure is more than 10% to 65% or less, with a balance being a hard phase including quenched martensite and retained austenite and including at least one selected from the group consisting of bainitic ferrite, bainite, and tempered martensite.
 
(2) When the metal structure is measured by X-ray diffractometry, a volume ratio V γ  of retained austenite relative to the whole of the metal structure is 5% or more to 30% or less.
 
(3) When the metal structure is observed with an optical microscope, an area ratio V MA  of an MA structure, in which quenched martensite and retained austenite are combined, relative to the whole of the metal structure is 3% or more to 25% or less, and an average circle-equivalent diameter of the MA structure is 2.0 μm or less.
 
(4) A ratio V MA /V γ  of the area ratio V MA  of the MA structure to the volume ratio V γ  of the retained austenite satisfies a formula (i) below:
 
       0.50≦ V   MA   /V   γ ≦1.50  (i).
 
     The steel sheet may further contain, as other elements, in mass %: 
     (a) at least one selected from the group consisting of Cr: more than 0% to 1% or less and Mo: more than 0% to 1% or less,
 
(b) at least one selected from the group consisting of Ti: more than 0% to 0.15% or less, Nb: more than 0% to 0.15% or less, and V: more than 0% to 0.15% or less,
 
(c) at least one selected from the group consisting of Cu: more than 0% to 1% or less and Ni: more than 0% to 1% or less,
 
(d) B: more than 0% to 0.005% or less,
 
(e) at least one selected from the group consisting of Ca: more than 0% to 0.01% or less, Mg: more than 0% to 0.01% or less, and REM: more than 0% to 0.01% or less, and the like.
 
     A high-strength electrogalvanized steel sheet having an electrogalvanized layer on a surface of the high-strength cold-rolled steel sheet, a high-strength hot-dip galvanized steel sheet having a hot-dip galvanized layer on a surface of the high-strength cold-rolled steel sheet, and a high-strength hot-dip galvannealed steel sheet having a hot-dip galvannealed layer on a surface of the high-strength cold-rolled steel sheet are also comprised within the scope of the present invention. 
     The high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more and being excellent in formability and crashworthiness according to the present invention can be produced by subjecting a steel satisfying a component composition described above to hot rolling with a rolling rate at a final stand of finish rolling being 5 to 25% and with a finish rolling end temperature being the Ar 3  point or higher to 900° C. or lower, coiling with a coiling temperature being 600° C. or lower, and cooling to room temperature; cold rolling; heating, at an average heating rate of 10° C./second or more, to a temperature region of 800° C. or higher and lower than the Ac 3  point, and soaking by holding in the temperature region for 50 seconds or more; cooling at an average cooling rate of 10° C./second or more, to an arbitrary cooling stop temperature T° C. that lies in a temperature range of 50° C. or higher and the Ms point or lower; and heating and holding in a temperature region of higher than the cooling stop temperature T° C. and 550° C. or lower for 50 seconds or more, and thereafter cooling to room temperature. 
     A high-strength hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more and being excellent in formability and crashworthiness according to the present invention can be produced by subjecting a steel satisfying a component composition described above to hot rolling with a rolling rate at a final stand of finish rolling being 5 to 25% and with a finish rolling end temperature being the Ar 3  point or higher to 900° C. or lower, coiling with a coiling temperature being 600° C. or lower, and cooling to room temperature; cold rolling; heating, at an average heating rate of 10° C./second or more, to a temperature region of 800° C. or higher and lower than the Ac 3  point, and soaking by holding in the temperature region for 50 seconds or more; cooling at an average cooling rate of 10° C./second or more, to an arbitrary cooling stop temperature T° C. that lies in a temperature range of 50° C. or higher and the Ms point or lower; and heating and holding in a temperature region of higher than the cooling stop temperature T° C. and 550° C. or lower for 50 seconds or more, and after performing hot-dip galvanizing within a holding time, cooling to room temperature. 
     A high-strength hot-dip galvannealed steel sheet having a tensile strength of 980 MPa or more and being excellent in formability and crashworthiness according to the present invention can be produced by subjecting a steel satisfying a component composition described above to hot rolling with a rolling rate at a final stand of finish rolling being 5 to 25% and with a finish rolling end temperature being the Ar 3  point or higher to 900° C. or lower, coiling with a coiling temperature being 600° C. or lower, and cooling to room temperature; cold rolling; heating, at an average heating rate of 10° C./second or more, to a temperature region of 800° C. or higher and lower than the Ac 3  point, and soaking by holding in the temperature region for 50 seconds or more; cooling at an average cooling rate of 10° C./second or more, to an arbitrary cooling stop temperature T° C. that lies in a temperature range of 50° C. or higher and the Ms point or lower; and heating and holding in a temperature region of higher than the cooling stop temperature T° C. and 550° C. or lower for 50 seconds or more, and after performing hot-dip galvanizing within a holding time, further performing an alloying treatment and thereafter cooling to room temperature. 
     Effects of the Invention 
     According to the present invention, the component composition and the metal structure are suitably controlled, so that a high-strength cold-rolled steel sheet, a high-strength electrogalvanized steel sheet, a high-strength hot-dip galvanized steel sheet, and a high-strength hot-dip galvannealed steel sheet having a tensile strength of 980 MPa or more and being excellent both in formability as evaluated by ductility and stretch-flangeability and in crashworthiness can be provided. The high-strength cold-rolled steel sheet, the high-strength electrogalvanized steel sheet, the high-strength hot-dip galvanized steel sheet, and the high-strength hot-dip galvannealed steel sheet according to the present invention is particularly excellent in ductility among the formability properties. The present invention can also provide a method for producing the high-strength cold-rolled steel sheet, the high-strength electrogalvanized steel sheet, the high-strength hot-dip galvanized steel sheet, and the high-strength hot-dip galvannealed steel sheet described above. The high-strength cold-rolled steel sheet, the high-strength electrogalvanized steel sheet, the high-strength hot-dip galvanized steel sheet, and the high-strength hot-dip galvannealed steel sheet according to the present invention are extremely useful in the fields of industry such as automobiles. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic descriptive view showing one example of a heat treatment pattern performed in the Examples. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The present inventors have repeatedly made eager studies in order to improve all of ductility, stretch-flangeability, and crashworthiness in a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more. As a result, the present inventors have found out that, in order to improve the ductility while ensuring the tensile strength by setting a ferrite fraction in the metal structure to be within a predetermined range and setting the balance structure to be a hard phase, it is effective to generate a predetermined amount of ferrite and to appropriately control a ratio V MA /V γ  of an area ratio V MA  of an MA structure, in which quenched martensite and retained austenite are combined, to a volume ratio V γ  of retained austenite relative to the whole of the metal structure and that, in order to improve the stretch-flangeability, it is effective to make the MA structure finer and, in order to improve the crashworthiness, it is effective to make the MA structure finer and to appropriately control the above ratio V MA /V γ , thereby completing the present invention. 
     First, the metal structure characterizing the present invention will be described. 
     The high-strength cold-rolled steel sheet according to the present invention is characterized in that the metal structure at a position of ¼ of the sheet thickness satisfies (1) to (4) below. 
     (1) When the metal structure is observed with a scanning electron microscope, the area ratio of ferrite relative to the whole of the metal structure is more than 10% and 65% or less, with the balance being a hard phase including quenched martensite and retained austenite and including at least one selected from the group consisting of bainitic ferrite, bainite, and tempered martensite.
 
(2) When the metal structure is measured by X-ray diffractometry, the volume ratio V γ  of retained austenite relative to the whole of the metal structure is 5% or more and 30% or less.
 
(3) When the metal structure is observed with an optical microscope, the area ratio V MA  of an MA structure, in which quenched martensite and retained austenite are combined, relative to the whole of the metal structure is 3% or more and 25% or less, and an average circle-equivalent diameter of the MA structure is 2.0 μm or less.
 
(4) The volume ratio V γ  of the retained austenite and the area ratio V MA  of the MA structure satisfy a formula (i) below:
 
       0.50≦ V   MA   /V   γ ≦1.50  (i).
 
     The observation of the above metal structure is carried out all at the position of ¼ of the sheet thickness, as representing the steel sheet. 
     Methods of measuring the fractions in the metal structure as defined in the above (1) to (3) may differ from, each other, so that a sum of the fractions may exceed 100%. In other words, in the above (1), the metal structure is observed with a scanning electron microscope, so that the measured area ratio is a ratio obtained when the whole of the metal structure is assumed to be 100%. The area ratio measured with use of a scanning electron microscope includes that of quenched martensite and retained austenite as an area ratio of the hard phase. On the other hand, in the above (2), the retained austenite fraction in the metal structure is calculated by X-ray diffractometry, while in the above (3), the area ratio of the MA structure in which quenched martensite and retained austenite are combined is observed with an optical microscope. For this reason, the fraction of retained austenite and quenched martensite is measured in a duplicated manner by a plurality of methods. Also, hereafter, the retained austenite may be denoted as retained γ. Accordingly, a sum of the fractions in the metal structure as defined in the above (1) to (3) may exceed 100%. Also, the structure in which quenched martensite and retained γ are combined may be denoted as MA structure. 
     (1) In the present invention, the area ratio of ferrite relative to the whole of the metal structure is set to be more than 10% to 65% or less when the metal structure is observed with a scanning electron microscope. Ferrite is a structure that particularly enhances ductility among the formability properties of the steel sheet. In order that such an effect may be exhibited, the area ratio of ferrite is set to be more than 10% in the present invention. The area ratio of ferrite is preferably 15% or more, more preferably 20% or more. However, when ferrite is excessive in amount, the strength of the steel sheet decreases, so that a tensile strength of 980 MPa or more cannot be ensured. Accordingly, in the present invention, the area ratio of ferrite is set to be 65% or less. The area ratio of ferrite is preferably 60% or less, more preferably 50% or less. 
     The balance of the above metal structure is a hard phase including quenched martensite and retained γ as an essential structure and including at least one selected from the group consisting of bainitic ferrite, bainite, and tempered martensite. These hard phases constitute a structure that is harder than ferrite, so that, by generating a predetermined amount of ferrite and making the balance structure be a hard phase, the strength of the steel sheet can be enhanced to be 980 MPa or more. The reason why quenched martensite and retained γ are contained as an essential structure is, as described later, for the purpose of generating a predetermined amount of an MA structure in which quenched martensite and retained γ are combined. 
     In addition to ferrite and the hard phase, the above metal structure may contain at least one selected from the group consisting of pearlite and cementite. A sum area ratio of pearlite and cementite is not particularly limited as long as the effect of the present invention is not deteriorated; however, the sum area ratio is preferably, for example, 20% or less. The sum area ratio is more preferably 15% or less, still more preferably 10% or less. 
     The area ratio of the above metal structure may be calculated by performing observation with a scanning electron microscope after the position of ¼ of the sheet thickness is corroded with nital, and the observation magnification may be set to be, for example, 1000 times. 
     (2) In the present invention, when the metal structure is measured by X-ray diffractometry, the volume ratio V γ  of retained γ relative to the whole of the metal structure is set to be 5% or more to 30% or less. The retained γ produces an effect of suppressing concentration of strain by receiving the strain so as to be deformed and transformed into martensite when the steel sheet is processed, thereby promoting hardening of the deformed portion during the processing. For this reason, the strength-elongation balance of the steel sheet is enhanced, and the ductility can be improved. In order that such an effect may be exhibited, it is necessary that the volume ratio of retained γ is set to be 5% or more. The volume ratio of retained γ is preferably 6% or more, more preferably 7% or more. However, when the volume ratio of retained γ increases excessively, the stretch-flangeability becomes deteriorated. Accordingly, the volume ratio of retained γ is set to be 30% or less in the present invention. The volume ratio of retained γ is preferably 25% or less, more preferably 20% or less. 
     The above volume ratio of retained γ may be determined by measuring the position of ¼ of the sheet thickness by X-ray diffractometry. The retained γ exists between the laths of bainitic ferrite or by being included in the MA structure. The above effect by the retained γ is exhibited irrespective of the existence form, so that, in the present invention, the volume ratio was determined by calculating a sum of the amounts of all the retained γ measured by X-ray diffractometry irrespective of the existence form. 
     (3) In the present invention, when the metal structure is observed with an optical microscope, the area ratio V MA  of the MA structure relative to the whole of the metal structure is set to be 3% or more to 25% or less. The above MA structure is a structure that enhances the strength-elongation balance of the steel sheet and can improve the ductility. In order that such an effect may be exhibited, it is necessary that the area ratio of the MA structure is set to be 3% or more. The area ratio of the MA structure is preferably 4% or more, more preferably 5% or more. However, when the area ratio of the MA structure increases excessively, the crashworthiness becomes deteriorated. Accordingly, the area ratio of the MA structure is set to be 25% or less in the present invention. The area ratio of the MA structure is preferably 23% or less, more preferably 20% or less. 
     Also, in the present invention, the average circle-equivalent diameter of the MA structure is set to be 2.0 μm or less. By making the MA structure be finer, the stretch-flangeability and the crashworthiness can be enhanced. In order that such an effect may be exhibited, it is necessary that the average circle-equivalent diameter of the MA structure is set to be 2.0 m or less. The average circle-equivalent diameter of the MA structure is preferably 1.8 μm or less, more preferably 1.5 μm or less. According as the MA structure becomes finer, the stretch-flangeability and the crashworthiness will be better, so that a lower limit of the average circle-equivalent diameter of the MA structure is not particularly limited; however, from an industrial point of view, the lower limit is about 0.1 μm. 
     The above MA structure is a structure in which quenched martensite and retained γ are combined. The quenched martensite means a structure in a state in which untransformed austenite is transformed into martensite during the process in which the steel sheet is cooled from the heating temperature down to room temperature. By observation with an optical microscope, the quenched martensite can be distinguished from the tempered martensite that has been tempered by a heating treatment. In other words, when the metal structure is observed with an optical microscope after being subjected to LePera corrosion, the quenched martensite is observed to be white whereas the tempered martensite is observed to be gray. 
     The quenched martensite and the retained γ are hardly distinguished from each other by observation with an optical microscope, so that the structure in which quenched martensite and retained γ are combined is measured as the MA structure in the present invention. 
     The area ratio of the above MA structure is a value as measured at the position of ¼ of the sheet thickness of the steel sheet. 
     The average circle-equivalent diameter of the MA structure is a value determined by calculating a circle-equivalent diameter based on the area of each MA structure for all the MA structures that are recognized in the field of observation and calculating an average of the obtained circle-equivalent diameters. 
     (4) In the present invention, it is important that the ratio V MA /V γ  of the area ratio V MA  of the MA structure to the volume ratio V γ  of the retained γ satisfies the following formula (i): 
       0.50≦ V   MA   /V   γ ≦1.50  (i).
 
     The ductility and the crashworthiness are rendered compatible with each other when the value of the above ratio V MA /V γ  is controlled to satisfy the above formula (i). In other words, as described above, the retained γ is positively generated in the present invention in order to enhance the strength-elongation balance that constitutes an index of ductility. As a result of this, the MA structure is inevitably formed in the steel sheet. Further, upon further studies on the strength-elongation balance, it has been found out that, when a predetermined amount of retained γ is generated, it is good to control the area ratio V MA  of the MA structure so that the value of the above ratio V MA /V γ  may become 0.50 or more. The value of the above ratio V MA /V γ  is preferably 0.55 or more, more preferably 0.60 or more. However, when the value of the above ratio V MA N/V γ  becomes excessively large, the MA structure is excessively generated. The quenched martensite that exists in the MA structure is a very hard structure, so that, when the MA structure is excessively generated, cracks are liable to be generated at the interface to other structures at the time of collision, and accordingly, the crashworthiness is rather deteriorated. Therefore, in the present invention, the value of the above ratio V MA /V γ  is set to be 1.50 or less in order to reduce the area ratio of quenched martensite in the MA structure to ensure the crashworthiness. The value of the above ratio V MA /V γ  is preferably 1.40 or less, more preferably 1.30 or less. 
     As shown above, the metal structure of the high-strength cold-rolled steel sheet that characterizes the present invention has been described. 
     Next, the component composition of the high-strength cold-rolled steel sheet according to the present invention will be described. Hereafter, “%” with regard to the component composition of a steel sheet means “mass %”. 
     [C: 0.10% or More to 0.5% or Less] 
     C is an element that is necessary for ensuring the tensile strength of 980 MPa or more and for enhancing the stability of retained γ to ensure a predetermined amount of the retained γ. In the present invention, the C amount is set to be 0.10% or more. The C amount is preferably 0.12% or more, more preferably 0.15% or more. However, when the C amount is excessively large, the strength after hot rolling increases, so that cracks may be generated during the cold rolling, or the weldability of a final product may decrease. Accordingly, the C amount is set to be 0.5% or less. The C amount is preferably 0.40% or less, more preferably 0.30% or less, and still more preferably 0.25% or less. 
     [Si: 1.0% or More to 3% or Less] 
     Si is an element that acts as a solute-strengthening element and contributes to a higher strength of the steel. Also, Si suppresses generation of carbide and effectively acts for generation of ferrite and retained γ, so that Si is an element that is necessary for ensuring an excellent strength-elongation balance. In the present invention, the Si amount is set to be 1.0% or more. The Si amount is preferably 1.2% or more, more preferably 1.35% or more, and still more preferably 1.5% or more. However, when the Si amount is excessively large, a considerable scale is formed during the hot rolling to generate scale marks on the surface of the steel sheet, thereby degrading the surface property. Also, the pickling property is degraded as well. Accordingly, the Si amount is set to be 3% or less. The Si amount is preferably 2.8% or less, more preferably 2.6% or less. 
     [Mn: 1.5% or More to 7% or Less] 
     Mn is an element that contributes to a higher strength of the steel sheet by enhancing the hardenability. Further, Mn is an element that is necessary for stabilizing γ to generate retained γ. In the present invention, the Mn amount is set to be 1.5% or more. The Mn amount is preferably 1.6% or more, more preferably 1.7% or more, still more preferably 1.8% or more, and furthermore preferably 2.0% or more. However, when the Mn amount is excessively large, the strength after hot rolling increases, so that cracks may be generated during the cold rolling, or the weldability of the final product may decrease. Also, when Mn is added in an excessively large amount, Mn is segregated to deteriorate the ductility and the stretch-flangeability. Accordingly, the Mn amount is set to be 7% or less. The Mn amount is preferably 5.0% or less, more preferably 4.0% or less, and still more preferably 3.0% or less. 
     [P: More than 0% to 0.1% or Less] 
     P is an impurity element that is inevitably contained and, when contained in an excessively large amount, deteriorates the weldability of the final product. Accordingly, the P amount is set to be 0.1% or less in the present invention. The P amount is preferably 0.08% or less, more preferably 0.05% or less. The smaller the P amount is, the better it is. However, it is industrially difficult to set the P amount to be 0%. A lower limit of the P amount is 0.0005% from the industrial point of view. 
     [S: More than 0% to 0.05% or Less] 
     As with P, S is an impurity element that is inevitably contained and, when contained in an excessively large amount, deteriorates the weldability of the final product. Also, S forms sulfide-based inclusions in the steel sheet, thereby causing deterioration of the ductility and the stretch-flangeability of the steel sheet. Accordingly, the S amount is set to be 0.05% or less in the present invention. The S amount is preferably 0.01% or less, more preferably 0.005% or less. The smaller the S amount is, the better it is. However, it is industrially difficult to set the S amount to be 0%. A lower limit of the S amount is 0.0001% from the industrial point of view. 
     [Al: 0.005% or More to 1% or Less] 
     Al is an element that acts as a deoxidizer. In order that such an action may be exhibited, the Al amount is set to be 0.005% or more in the present invention. The Al amount is more preferably 0.01% or more. However, when the Al amount is excessively large, the weldability of the final product is considerably deteriorated. Accordingly, the Al amount is set to be 1% or less in the present invention. The Al amount is preferably 0.8% or less, more preferably 0.6% or less. 
     [N: More than 0% to 0.01% or Less] 
     N is an impurity element that is inevitably contained and, when N is contained in an excessively large amount, nitride is deposited in a large amount to deteriorate the ductility, stretch-flangeability, and crashworthiness. Accordingly, the N amount is set to be 0.01% or less in the present invention. The N amount is preferably 0.008% or less, more preferably 0.005% or less. Since nitride in a small amount contributes to a higher strength of the steel sheet, the N amount may be 0.001% or more. 
     [O: More than 0% to 0.01% or Less] 
     O is an impurity element that is inevitably contained and, when contained in an excessively large amount, deteriorates the ductility and the crashworthiness. Accordingly, the O amount is set to be 0.01% or less in the present invention. The O amount is preferably 0.005% or less, more preferably 0.003% or less. The smaller the O amount is, the better it is. However, it is industrially difficult to set the O amount to be 0%. A lower limit of the O amount is 0.0001% from the industrial point of view. 
     The cold-rolled steel sheet according to the present invention satisfies the aforementioned component composition, and the balance is made of iron and inevitable impurities. The inevitable impurities may include the above-mentioned elements such as P, S, N, and O, which may be brought into the steel depending on the circumstances of raw materials, facility materials, production equipment, and the like, and may also include tramp elements such as Pb, Bi, Sb, and Sn. 
     The cold-rolled steel sheet of the present invention may further contain, as other elements, 
     (a) at least one selected from the group consisting of Cr: more than 0% to 1% or less and Mo: more than 0% to 1% or less,
 
(b) at least one selected from the group consisting of Ti: more than 0% to 0.15% or less, Nb: more than 0% to 0.15% or less, and V: more than 0% to 0.15% or less,
 
(c) at least one selected from the group consisting of Cu: more than 0% to 1% or less and Ni: more than 0% to 1% or less,
 
(d) B: more than 0% to 0.005% or less,
 
(e) at least one selected from the group consisting of Ca: more than 0% to 0.01% or less, Mg: more than 0% to 0.01% or less, and REM: more than 0% to 0.01% or less, and the like.
 
     These elements of (a) to (e) may be contained either alone or in an arbitrary combination. The reason why such ranges have been set is as follows. 
     [(a) at Least One Selected from the Group Consisting of Cr: More than 0% to 1% or Less and Mo: More than 0% to 1% or Less] 
     Cr and Mo are each an element that acts to improve the strength of the steel sheet by enhancing hardenability. In order that such an action may be effectively exhibited, the amount of each of Cr and Mo is preferably set to be 0.1% or more, more preferably 0.3% or more. However, when these elements are contained in an excessively large amount, the ductility and the stretch-flangeability decrease. Also excessive addition leads to higher costs. Accordingly, when Cr or Mo is contained alone, the amount is preferably 1% or less, more preferably 0.8% or less, still more preferably 0.5% or less. Cr and Mo may be used either alone or in combination. When Cr and Mo are used in combination, it is preferable that each amount is within the above range of the content when used alone, and a sum of the contents of Cr and Mo is 1.5% or less. 
     [(b) at Least One Selected from the Group Consisting of Ti: More than 0% to 0.15% or Less, Nb: More than 0% to 0.15% or Less, and V: More than 0% to 0.15% or Less] 
     Ti, Nb, and V are each an element that acts to improve the strength of the steel sheet by forming carbide and nitride in the steel sheet and to make prior γ grains finer. In order that such an action may be effectively exhibited, the amount of each of Ti, Nb, and V is preferably set to be 0.005% or more, more preferably 0.010% or more. However, when these elements are contained in an excessively large amount, carbide is deposited at the grain boundary, so that the stretch-flangeability and the crashworthiness of the steel sheet are deteriorated. Accordingly, in the present invention, the amount of each of Ti, Nb, and V is preferably set to be 0.15% or less, more preferably 0.12% or less, and still more preferably 0.10% or less. These elements may be used either alone or in combination of two or more that are arbitrarily selected. 
     [(c) at Least One Selected from the Group Consisting of Cu: More than 0% to 1% or Less and Ni: More than 0% to 1% or Less] 
     Cu and Ni are each an element that acts effectively for generation and stabilization of retained γ. Also, Cu and Ni act to improve the corrosion resistance of the steel sheet. In order that such an action may be effectively exhibited, the amount of each of Cu and Ni is preferably set to be 0.05% or more, more preferably 0.10% or more. However, when Cu is contained in an excessively large amount, the hot formability is deteriorated. Accordingly, when Cu is added alone, the amount of Cu is preferably set to be 1% or less, more preferably 0.8% or less, and still more preferably 0.5% or less. On the other hand, when Ni is contained in an excessively large amount, a higher cost is invited, so that the amount of Ni is preferably set to be 1% or less, more preferably 0.8% or less, and still more preferably 0.5% or less. Cu and Ni may be used either alone or in combination. When Cu and Ni are used in combination, the above action is more likely to be exhibited, and also, by incorporation of Ni, the deterioration of hot formability caused by addition of Cu is more likely to be suppressed. When Cu and Ni are used in combination, a sum of the amounts of Cu and Ni is preferably set to be 1.5% or less, more preferably 1.0% or less. 
     [(d) B: More than 0% to 0.005% or Less] 
     B is an element that improves hardenability and is an element that acts to allow austenite to exist stably down to room temperature. In order that such an action may be effectively exhibited, the amount of B is preferably set to be 0.0005% or more, more preferably 0.0010% or more, and still more preferably 0.0015% or more. However, when B is contained in an excessively large amount, boride may be generated to deteriorate the ductility. Accordingly, the amount of B is preferably set to be 0.005% or less. The amount of B is more preferably 0.004% or less, still more preferably 0.0035% or less. 
     [(e) at Least One Selected from the Group Consisting of Ca: More than 0% to 0.01% or Less, Mg: More than 0% to 0.01% or Less, and REM: More than 0% to 0.01% or Less] 
     Ca, Mg, and REM are elements that act to finely disperse the inclusions in the steel sheet. In order that such an action may be effectively exhibited, the amount of each of Ca, Mg, and REM is preferably set to be 0.0005% or more, more preferably 0.0010% or more. However, when these elements are added in an excessively large amount, the castability, hot formability, and the like may be deteriorated. Accordingly, the amount of each of Ca, Mg, and REM is preferably set to be 0.01% or less, more preferably 0.008% or less, and still more preferably 0.007% or less. These elements may be used either alone or in combination of two or more that are arbitrarily selected. In the present invention, REM is an abbreviation for Rare earth metal (rare earth element), and is meant to include lanthanoid elements which are fifteen elements from La to Lu, and Sc and Y. 
     As shown above, the high-strength cold-rolled steel sheet according to the present invention has been described. 
     An electrogalvanized layer, a hot-dip galvanized layer, or a hot-dip galvannealed layer may be formed on a surface of the high-strength cold-rolled steel sheet. In other words, the scope of the present invention includes a high-strength electrogalvanized steel sheet (which may hereafter be referred to as EG steel sheet) having an electrogalvanized layer formed on a surface of the high-strength cold-rolled steel sheet, a high-strength hot-dip galvanized steel sheet (which may hereafter be referred to as GI steel sheet) having a hot-dip galvanized layer formed on a surface of the high-strength cold-rolled steel sheet, and a high-strength hot-dip galvannealed steel sheet (which may hereafter be referred to as GA steel sheet) having a hot-dip galvannealed layer formed on a surface of the high-strength cold-rolled steel sheet. 
     Next, a method for producing the high-strength cold-rolled steel sheet according to the present invention will be described. 
     The high-strength cold-rolled steel can be produced by subjecting a steel satisfying a component composition described above to hot rolling with a rolling rate at a final stand of finish rolling being 5 to 25% and with a finish rolling end temperature being the Ar 3  point or higher and 900° C. or lower, coiling with a coiling temperature being 600° C. or lower, and cooling to room temperature; cold rolling; heating, at an average heating rate of 10° C./second or more, to a temperature region of 800° C. or higher and lower than the Ac 3  point, and soaking by holding in the temperature region for 50 seconds or more; cooling at an average cooling rate of 10° C./second or more, to an arbitrary cooling stop temperature T° C. that lies in a temperature range of 50° C. or higher and the Ms point or lower; and heating and holding in a temperature region of higher than the cooling stop temperature ToC and 550° C. or lower for 50 seconds or more, and thereafter cooling to room temperature. 
     Hereafter, the steps will be sequentially described. 
     [Rolling Rate at a Final Stand of Finish Rolling: 5 to 25%] 
     First, a steel satisfying the aforementioned component composition is heated in accordance with a conventional method. A heating temperature is not particularly limited; however, the heating temperature is preferably set to be, for example, 1000 to 1300° C. When the heating temperature is lower than 1000° C., solid solution of carbide is insufficiently formed, and a sufficient strength is hardly obtained. On the other hand, when the heating temperature is higher than 1300° C., the structure of the hot-rolled steel sheet becomes coarse, and also the MA structure of the cold-rolled steel sheet is liable to become coarse. As a result, the crashworthiness tends to decrease. 
     After the heating, hot rolling is carried out. In the present invention, it is important that the rolling rate at a final stand of finish rolling is set to be 5 to 25%. When the rolling rate is less than 5%, the austenite grain size after hot rolling becomes coarse, and the average circle-equivalent diameter of the MA structure in the cold-rolled steel sheet after annealing becomes large. As a result, the stretch-flangeability decreases. Accordingly, in the present invention, it is necessary that the rolling rate is set to be 5% or more. The rolling rate is preferably 6% or more, more preferably 7% or more, and still more preferably 8% or more. However, when the rolling rate exceeds 25%, the average circle-equivalent diameter of the MA structure also becomes large, leading to deterioration of the stretch-flangeability and crashworthiness. The mechanism therefor is not clear; however, this seems to be because the structure after hot rolling is made non-homogeneous. In the present invention, it is necessary that the rolling rate is set to be 25% or less. The rolling rate is preferably 23% or less, more preferably 20% or less. 
     [Finish Rolling End Temperature: Ar 3  Point or Higher and 900° C. or Lower] 
     When the finish rolling end temperature is lower than the temperature of the Ar 3  point, the steel sheet structure after hot rolling becomes non-homogeneous, and the stretch-flangeability decreases. On the other hand, when the finish rolling end temperature exceeds 900° C., recrystallization of austenite occurs to make the crystal grains become coarse, and the average circle-equivalent diameter of the MA structure in the cold-rolled steel sheet becomes large. As a result, the stretch-flangeability decreases. Accordingly, in the present invention, it is necessary that the finish rolling end temperature is set to be 900° C. or lower. The finish rolling end temperature is preferably 890° C. or lower, more preferably 880° C. or lower. 
     The temperature of the Ar 3  point was calculated on the basis of the following formula (ii). In the formula, brackets [ ] indicate the content of each element (mass %), and calculation may be made by assuming that the content of an element that is not contained in the steel sheet is 0 mass %. 
       Ar 3  point(° C.)=910−310×[C]−80×[Mn]−20×[Cu]−15×[Cr]−55×[Ni]−80×[Mo]   (ii)
 
     [Coiling Temperature: 600° C. or Lower] 
     When the coiling temperature exceeds 600° C., the crystal grains become coarse, and the average circle-equivalent diameter of the MA structure in the cold-rolled steel sheet becomes large. As a result, the stretch-flangeability decreases. Accordingly, in the present invention, the coiling temperature is set to be 600° C. or lower. The coiling temperature is preferably 580° C. or lower, more preferably 570° C. or lower, and still more preferably 550° C. or lower. 
     [Cold Rolling] 
     After the hot rolling, the steel sheet may be coiled, cooled to room temperature, pickled by a conventional method in accordance with the needs, and subsequently cold-rolled by a conventional method. The cold rolling rate in the cold rolling may be set to be, for example, 30 to 80%. 
     [Annealing] 
     After the cold rolling, annealing is carried out by heating, at an average heating rate of 10° C./sec or more, to a temperature region of 800° C. or higher and lower than the Ac 3  point, and soaking by holding in the temperature region for 50 seconds or more. When the average heating rate of heating to the above temperature region after the cold rolling is lower than 10° C./sec, the austenite grains grow and become coarse during the heating, so that the average circle-equivalent diameter of the MA structure in the cold-rolled steel sheet becomes large, and the stretch-flangeability decreases. Accordingly, in the present invention, the average heating rate is set to be 10° C./sec or more. The average heating rate is preferably 12° C./sec or more, more preferably 15° C./sec or more. An upper limit of the above average heating rate is not particularly limited; however, the average heating rate is typically about 100° C./sec at the maximum. 
     By setting the soaking temperature to be 800° C. or higher and lower than the Ac 3  point, a desired ferrite amount can be ensured. When the soaking temperature is lower than 800° C., reverse transformation to austenite becomes insufficient, and the processed structure remains in the cold-rolled steel sheet, so that the formability decreases. Accordingly, in the present invention, the soaking temperature is set to be 800° C. or higher. The soaking temperature is preferably 805° C. or higher, more preferably 810° C. or higher. However, when the soaking temperature is higher than the temperature of the Ac 3  point, a desired ferrite amount cannot be ensured, and the ductility is deteriorated. Accordingly, in the present invention, the soaking temperature is set to be lower than the temperature of the Ac 3  point. The soaking temperature is preferably (Ac 3  point−10° C.) or lower, more preferably (Ac 3  point−20° C.) or lower. 
     When the soaking time is less than 50 seconds, the processed structure remains in the cold-rolled steel sheet, and the ductility is deteriorated. Accordingly, in the present invention, the soaking time is set to be 50 seconds or more. The soaking time is preferably 60 seconds or more. An upper limit of the soaking time is not particularly limited; however, when the soaking time is too long, concentration of Mn into the austenite phase proceeds, and the Ms point may decrease, leading to increase or coarsening of the MA structure. Accordingly, the soaking time is preferably set to be 3600 seconds or less, more preferably 3000 seconds or less. 
     Regarding the soaking holding in the above temperature region, the steel sheet need not be thermostatically held at the same temperature, so that the steel sheet may be heated and cooled in a fluctuating manner within the above temperature region. 
     The temperature of the aforementioned Ac 3  point can be calculated on the basis of the following formula (iii) disclosed in “The Physical Metallurgy of Steels” (William C. Leslie, published by Maruzen Co., Ltd. on May 31, 1985, page 273). In the formula, brackets [ ] indicate the content of each element (mass %), and calculation may be made by assuming that the content of an element that is not contained in the steel sheet is 0 mass %. 
       Ac 3 (° C.)=910−203×[C] 1/2 −15.2×[Ni]+44.7×[Si]+104×[V]+31.5×[Mo]+13.1×[W]−(30×[Mn]+11×[Cr]+20×[Cu]−700×[P]−400×[Al]−120×[As]−400×[Ti])   (iii)
 
     [Cooling] 
     After the above soaking holding, the steel sheet is cooled to an arbitrary cooling stop temperature T° C. that lies in a temperature range of 50° C. or higher and the Ms point or lower. By cooling down to this temperature range, untransformed austenite can be transformed to martensite and hard bainite phase, and the MA structure also can be made finer. During this period, martensite exists as quenched martensite immediately after the transformation; however, the martensite is tempered while being reheated and held in a later step and remains as tempered martensite. This tempered martensite does not give adverse effects on any of the ductility, stretch-flangeability, and crashworthiness of the steel sheet. However, when the above cooling stop temperature T exceeds the Ms point, martensite is not generated, and the MA structure generated in the reheating holding step at a high temperature becomes coarse, so that the local deformation capability decreases, and the stretch-flangeability cannot be improved. Accordingly, in the present invention, the cooling stop temperature T is set to be equal to or lower than the temperature of the Ms point. The cooling stop temperature T is preferably (Ms point−20° C.) or lower, more preferably (Ms point−50° C.) or lower. On the other hand, when the cooling stop temperature T is lower than 50° C., retained γ and the MA structure are generated only in a slight amount, so that the ductility cannot be improved. Accordingly, in the present invention, a lower limit of the cooling stop temperature T is set to be 50° C. or higher. The cooling stop temperature T is preferably 60° C. or higher, more preferably 70° C. or higher. 
     The temperature of the aforementioned Ms point can be calculated on the basis of the following formula (iv). In the formula, brackets [ ] indicate the content of each element (mass %), and calculation may be made by assuming that the content of an element that is not contained in the steel sheet is 0 mass %. Also, Vf in the formula indicates the area ratio of ferrite relative to the whole of the metal structure. 
       Ms point(° C.)=561−474×[C]/(1−Vf/100)−33×[Mn]−17×[Ni]−17×[Cr]−21×[Mo]   (iv)
 
     After performing the above soaking and holding, it is important that an average cooling rate down to the cooling stop temperature T that lies in the above temperature range is set to be 10° C./sec or more. Excessive generation of ferrite can be suppressed by appropriately controlling the cooling rate down to the cooling stop temperature T after soaking and holding. In other words, when the average cooling rate is lower than 10° C./sec, ferrite is excessively generated during the cooling, and the tensile strength decreases. Accordingly, in the present invention, the average cooling rate is set to be 10° C./sec or more. The average cooling rate is preferably 15° C./sec or more, more preferably 20° C./sec or more. An upper limit of the above average cooling rate is not particularly limited, and the steel sheet may be cooled by cooling with water or cooling with oil. 
     [Reheating Step] 
     After the steel sheet is cooled down to an arbitrary cooling stop temperature T° C. in the temperature range of 50° C. or higher and the Ms point or lower, it is important that the steel sheet is reheated to a temperature region of higher than the cooling stop temperature T° C. and 550° C. or lower, and the steel sheet is held in this temperature region for 50 seconds or more. By reheating to the temperature region of higher than the cooling stop temperature T° C. and 550° C. or lower, the hard phase such as martensite can be tempered, and untransformed austenite can be transformed to bainitic ferrite or bainite. When the reheating is not carried out, the balance between the amounts of generation of retained γ and the MA structure becomes degraded, and the ratio V MA /V γ  of the area ratio V MA  of the MA structure to the volume ratio V γ  of the retained γ cannot be controlled to be within an appropriate range. As a result, the crashworthiness cannot be improved. Further, the hard phase cannot be tempered, and dislocation at a high density is generated. Accordingly, in the present invention, the steel sheet is reheated to a temperature exceeding the cooling stop temperature T after the steel sheet is cooled to the cooling stop temperature T. The reheating temperature is preferably (T+20° C.) or higher, more preferably (T+30° C.) or higher, and still more preferably (T+50° C.) or higher. However, when the reheating temperature exceeds 550° C., retained γ and the MA structure are generated only in a slight amount, so that the tensile strength decreases, and the value of TS×λ becomes smaller, making it impossible to improve the stretch-flangeability. Accordingly, in the present invention, the reheating temperature is set to be 550° C. or lower. The reheating temperature is preferably 520° C. or lower, more preferably 500° C. or lower, and still more preferably 450° C. or lower. 
     Here, in the present invention, “reheating” means, as it is stated, heating, that is, raising the temperature from the above cooling stop temperature T. Accordingly, the reheating temperature is a temperature higher than the above cooling stop temperature T. Therefore, even if the reheating temperature is, for example, within a temperature region of 50° C. or higher and 550° C. or lower, this does not fall under the category of the reheating of the present invention if the cooling stop temperature T and the reheating temperature are the same as each other or if the reheating temperature is lower than the cooling stop temperature T. 
     After the steel sheet is reheated to the temperature region of higher than the cooling stop temperature T° C. and 550° C. or lower, the steel sheet is held in the temperature region for 50 seconds or more. When the reheating holding time is less than 50 seconds, the MA structure is excessively generated, and the ductility cannot be improved. Further, the MA structure becomes coarse, and the average circle-equivalent diameter cannot be appropriately controlled, so that the stretch-flangeability cannot be improved either. Also, the ratio V MA /V γ  of the area ratio V MA  of the MA structure to the volume ratio V γ  of the retained γ cannot be appropriately controlled, so that the crashworthiness cannot be improved either. Furthermore, the hard phase cannot be sufficiently tempered, and also the transformation of untransformed austenite to bainitic ferrite or bainite does not proceed sufficiently. Accordingly, in the present invention, the reheating holding time is set to be 50 seconds or more. The reheating holding time is preferably 80 seconds or more, more preferably 100 seconds or more, and still more preferably 200 seconds or more. An upper limit of the reheating holding time is not particularly limited. However, when the holding time is long, the productivity decreases, and the tensile strength tends to decrease. From such viewpoints, the reheating holding time is preferably set to be 1500 seconds or less, more preferably 1000 seconds or less. 
     After the steel sheet is reheated and held, the steel sheet is cooled to room temperature. An average cooling rate during the cooling is not particularly limited; however, the average cooling rate is preferably, for example, 0.1° C./sec or more, more preferably 0.4° C./sec or more. Further, the average cooling rate is preferably, for example, 200° C./sec or less, more preferably 150° C./sec or less. 
     [Plating Treatment] 
     After the reheating holding, the high-strength cold-rolled steel sheet according to the present invention obtained by cooling to room temperature may be subjected to electrogalvanization, hot-dip galvanizing, or hot-dip galvannealing in accordance with a conventional method. 
     The electrogalvanization may be carried out, for example, by subjecting the above high-strength cold-rolled steel sheet to energization while immersing the steel sheet into a zinc solution of 50 to 60° C. (particularly 55° C.) so as to perform an electrogalvanization treatment. The plating adhesion amount is not particularly limited and may be, for example, about 10 to 100 g/m 2  per one surface. 
     The hot-dip galvanizing may be carried out, for example, by immersing the above high-strength cold-rolled steel sheet into a hot-dip galvanizing bath of 300° C. or higher and 550° C. or lower, so as to perform a hot-dip galvanizing treatment. The plating time may be suitably adjusted so that a desired plating adhesion amount can be ensured. The plating time is preferably set to be, for example, 1 to 10 seconds. 
     The hot-dip galvannealing may be carried out by performing an alloying treatment after the above hot-dip galvanizing. The alloying treatment temperature is not particularly limited; however, when the alloying treatment temperature is too low, the alloying does not proceed sufficiently, so that the alloying treatment temperature is preferably 450° C. or higher, more preferably 460° C. or higher, and still more preferably 480° C. or higher. However, when the alloying treatment temperature is too high, the alloying proceeds too much, and the Fe concentration in the plating layer becomes high, thereby deteriorating the plating adhesion property. From such a viewpoint, the alloying treatment temperature is preferably 550° C. or lower, more preferably 540° C. or lower, and still more preferably 530° C. or lower. The alloying treatment time is not particularly limited and may be adjusted so that the hot-dip galvanized layer may be alloyed. The alloying treatment time is preferably, for example, 10 to 60 seconds. 
     A high-strength hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more and being excellent in formability and crashworthiness according to the present invention can also be produced by subjecting a steel satisfying a component composition described above to hot rolling with a rolling rate at a final stand of finish rolling being 5 to 25% and with a finish rolling end temperature being the Ar 3  point or higher and 900° C. or lower, coiling with a coiling temperature being 600° C. or lower, and cooling to room temperature; cold rolling; heating, at an average heating rate of 10° C./second or more, to a temperature region of 800° C. or higher and lower than the Ac 3  point, and soaking by holding in the temperature region for 50 seconds or more; cooling at an average cooling rate of 10° C./second or more, to an arbitrary cooling stop temperature T° C. that lies in a temperature range of 50° C. or higher and the Ms point or lower; and heating and holding in a temperature region of higher than the cooling stop temperature T° C. and 550° C. or lower for 50 seconds or more, and after performing hot-dip galvanizing within a holding time, cooling to room temperature. In other words, the steps until heating to the temperature region of higher than the cooling stop temperature T° C. and 550° C. or lower are the same as those of the above-described method for producing a high-strength cold-rolled steel sheet according to the present invention, so that the hot-dip galvanizing and the holding for 50 seconds or more that is carried out in the above temperature region of higher than the cooling stop temperature T° C. and 550° C. or lower may be simultaneously carried out in the same step. 
     The hot-dip galvanizing may be carried out within the holding time in the reheating temperature region, that is, in the temperature region of higher than the cooling stop temperature T° C. and 550° C. or lower, and a conventional method can be adopted as a specific plating method. For example, the steel sheet heated to the temperature region of higher than the cooling stop temperature T° C. and 550° C. or lower may be immersed into a plating bath adjusted to have a temperature within the range of higher than the cooling stop temperature TOC and 550° C. or lower, so as to perform a hot-dip galvanizing treatment. The plating time may be suitably adjusted so that a desired plating amount can be ensured within the time of the reheating holding. The plating time is preferably set to be, for example, 1 to 10 seconds. 
     There are the following three patterns of (I) to (III) as a combination of the hot-dip galvanizing treatment; and only the heating and without performing the plating treatment, in the reheating. 
     (I) Only the heating is carried out, and thereafter, the hot-dip galvanizing treatment is carried out. 
     (II) The hot-dip galvanizing treatment is carried out, and thereafter, only the heating is carried out. 
     (III) Only the heating is carried out, and thereafter, the hot-dip galvanizing treatment is carried out, and further, only the heating is carried out, in this order. 
     The reheating temperature at which only the heating is carried out and the temperature of the plating bath used for performing the hot-dip galvanizing may be different from each other. In the present invention, heating or cooling may be carried out from one temperature to the other temperature. Furnace heating, induction heating, or the like may be adopted as a method for the heating. 
     A high-strength hot-dip galvannealed steel sheet having a tensile strength of 980 MPa or more and being excellent in formability and crashworthiness according to the present invention can also be produced by subjecting a steel satisfying a component composition described above to hot rolling with a rolling rate at a final stand of finish rolling being 5 to 25% and with a finish rolling end temperature being the Ar 3  point or higher and 900° C. or lower, coiling with a coiling temperature being 600° C. or lower, and cooling to room temperature; cold rolling; heating, at an average heating rate of 10° C./second or more, to a temperature region of 800° C. or higher and lower than the Ac 3  point, and soaking by holding in the temperature region for 50 seconds or more; cooling at an average cooling rate of 10° C./second or more, to an arbitrary cooling stop temperature T° C. that lies in a temperature range of 50° C. or higher and the Ms point or lower; and heating and holding in a temperature region of higher than the cooling stop temperature T° C. and 550° C. or lower for 50 seconds or more, and after performing hot-dip galvanizing within a holding time, further performing an alloying treatment and thereafter cooling to room temperature. In other words, the steps until heating to the temperature region of higher than the cooling stop temperature TOC and 550° C. or lower are the same as those of the above-described method for producing a high-strength cold-rolled steel sheet according to the present invention, so that the hot-dip galvanizing and the holding for 50 seconds or more that is carried out in the above temperature region of higher than the cooling stop temperature T° C. and 550° C. or lower may be simultaneously carried out in the same step, and thereafter the hot-dip galvanized layer may be alloyed, followed by cooling down to room temperature. 
     The alloying treatment temperature is not particularly limited; however, when the alloying treatment temperature is too low, the alloying does not proceed sufficiently, so that the alloying treatment temperature is preferably 450° C. or higher, more preferably 460° C. or higher, and still more preferably 480° C. or higher. However, when the alloying treatment temperature is too high, the alloying proceeds too much, and the Fe concentration in the plating layer becomes high, thereby deteriorating the plating adhesion property. From such a viewpoint, the alloying treatment temperature is preferably 550° C. or lower, more preferably 540° C. or lower, and still more preferably 530° C. or lower. 
     The alloying treatment time is not particularly limited and may be adjusted so that the hot-dip galvanized layer may be alloyed. The alloying treatment time is preferably, for example, 10 to 60 seconds. The alloying treatment is carried out after performing the hot-dip galvanizing treatment for a predetermined period of time within the temperature region of higher than the cooling stop temperature T° C. and 550° C. or lower, so that the time needed for the alloying treatment is not included in the holding time within the temperature region of higher than the cooling stop temperature T° C. and 550° C. or lower. 
     After performing the hot-dip galvanizing within the holding time in the temperature region of higher than the cooling stop temperature T° C. and 550° C. or lower and performing the alloying treatment in accordance with the needs, the steel sheet may be cooled down to room temperature. The average cooling rate during the cooling is not particularly limited; however, the average cooling rate is preferably, for example, 0.1° C./sec or more, more preferably 0.4° C./sec or more. Further, the average cooling rate is preferably, for example, 200° C./sec or less, more preferably 150° C./sec or less. 
     The high-strength cold-rolled steel sheet according to the present invention has a tensile strength of 980 MPa or more. The tensile strength is preferably 1000 MPa or more, more preferably 1010 MPa or more. Further, the above high-strength cold-rolled steel sheet is excellent in formability as evaluated by ductility and stretch-flangeability, and also is excellent in crashworthiness. 
     The ductility can be evaluated by strength--elongation balance. In the present invention, those in which a product of the tensile strength TS (MPa) and the elongation EL (%) is 17000 MPa·% or more are rated as acceptable. The value of TS×EL is preferably 17100 MPa·% or more, more preferably 17200 MPa·% or more. 
     The stretch-flangeability can be evaluated by strength-hole expansion ratio balance. In the present invention, those in which a product of the tensile strength TS (MPa) and the hole expansion ratio λ (%) is 20000 MPa·% or more are rated as acceptable. The value of TS×λ is preferably 21000 MPa·% or more, more preferably 22000 MPa·% or more. 
     The crashworthiness can be evaluated by strength-VDA bending angle balance. In the present invention, those in which a product of the tensile strength TS (MPa) and the VDA bending angle (°) is 90000 MPa·° or more are rated as acceptable. The value of TS×VDA bending angle is preferably 90500 MPa·° or more, more preferably 91000 MPa·° or more. 
     The thickness of the high-strength cold-rolled steel sheet according to the present invention is not particularly limited; however, the steel sheet is preferably a thin steel sheet having a thickness of, for example, 6 mm or less. 
     The present application claims the rights of priority based on Japanese Patent Application No. 2015-071438 filed on Mar. 31, 2015 and Japanese Patent Application No. 2015-225507 filed on Nov. 18, 2015. The entire contents of the specifications of Japanese Patent Application No. 2015-071438 and Japanese Patent Application No. 2015-225507 are incorporated in the present application by reference. 
     EXAMPLES 
     Hereafter, the present invention will be described more specifically by way of Examples; however, the invention is not limited by the following Examples and can be carried out while including additional modifications within a scope conforming to the gist disclosed heretofore and hereinafter, all such modifications being encompassed within the technical scope of the invention. 
     A steel containing the components given in the following Table 1 with the balance being iron and inevitable impurities was prepared by ingot-making and subjected to hot rolling, cold rolling, and continuous annealing to produce a cold-rolled steel sheet. In the following Table 1, “−” means that the corresponding element is not contained. The following Table 1 also show the temperature of the Ar 3  point calculated on the basis of the above formula (ii) and the temperature of the Ac 3  point calculated on the basis of the above formula (iii). Further,  FIG. 1  shows one example of a heat treatment pattern that was carried out in the continuous annealing. In  FIG. 1 , the reference sign  1  denotes a heating step,  2  a soaking step,  3  a cooling step,  4  a reheating holding step, and  5  a cooling stop temperature. 
     [Hot Rolling] 
     A slab obtained by ingot-making was heated to 1250° C., and hot rolling was carried out to a sheet thickness of 2.3 mm with the rolling reduction in the final stand of finish rolling being set to be a rolling reduction shown in the following Table 2-1 or 2-2 and with the finish rolling end temperature being set to be a temperature shown in the following Table 2-1 or 2-2. After the hot rolling, the steel sheet was cooled down to a coiling temperature shown in the following Table 2-1 or 2-2 at an average cooling rate of 30° C./sec, followed by coiling. After the coiling, the steel sheet was cooled in air to room temperature, so as to produce a hot-rolled steel sheet. 
     [Cold Rolling] 
     After the obtained hot-rolled steel sheet was pickled to remove surface scale, cold rolling was carried out to produce a cold-rolled steel sheet having a thickness of 1.2 mm. 
     [Continuous Annealing] 
     The obtained cold-rolled steel sheet was subjected to continuous annealing based on the heat treatment pattern shown in  FIG. 1 . That is, the obtained cold-rolled steel sheet was heated as a heating step at an average heating rate shown in the following Table 2-1 or 2-2 up to the soaking temperature shown in the following Table 2-1 or 2-2, and was held at the soaking temperature as a soaking step. The following Table 2-1 or 2-2 shows the soaking time. Further, the following Table 2-1 or 2-2 shows a value calculated by subtracting the soaking temperature from the temperature of the Ac 3  point. 
     After the soaking, the steel sheet was cooled as a cooling step at an average cooling rate shown in the following Table 2-1 or 2-2 down to the cooling stop temperature T° C. shown in the following Table 2-1 or 2-2. 
     After the cooling, the steel sheet was heated to the reheating temperature shown in the following Table 2-1 or 2-2 and was held at the reheating temperature as a reheating holding step, followed by cooling down to room temperature to produce a test sample material. The following Table 2-1 or 2-2 shows the reheating holding time. Also, the following Table 2-1 or 2-2 shows a value calculated by subtracting the cooling stop temperature T from the reheating temperature. 
     Nos. 8 and 11 shown in the following Table 2-1 are samples in which the reheating holding step was not carried out after the cooling was stopped at the cooling stop temperature T shown in the following Table 2-1. That is, in No. 8, the steel sheet was cooled with the cooling stop temperature T set to be 480° C., and thereafter cooled to 350° C., which was lower than that temperature, and held at 350° C. for 300 seconds. For the sake of convenience, the following Table 2-1 gives 350° C. in the section of the reheating temperature and gives 300 seconds in the section of the reheating holding time. In No. 11, the steel sheet was cooled with the cooling stop temperature T set to be 330° ° C., and thereafter cooled to 300° C., which was lower than that temperature, and held at 300° C. for 300 seconds. For the sake of convenience, the following Table 2-1 gives 300° C. in the section of the reheating temperature and gives 300 seconds in the section of the reheating holding time. 
     [Electrogalvanization] 
     No. 2 shown in the following Table 2-1 is a sample in which the above test sample material was immersed into a galvanizing bath of 55° C. to perform an electrogalvanization treatment and thereafter washed with water and dried to produce an electrogalvanized steel sheet. The electrogalvanization treatment was carried out with an electric current density set to be 40 A/dm 2 . The galvanizing adhesion amount was 40 g/m 2  per one surface. In the electrogalvanization treatment, washing treatments such as degreasing with alkaline aqueous solution immersion, washing with water, and pickling or the like were carried out as appropriate, so as to produce a test sample material having an electrogalvanized layer on the surface of the cold-rolled steel sheet. In the following Table 2-1, the section of classification for No. 2 gives “EG”. 
     [Hot-Dip Galvanizing] 
     No. 36 shown in the following Table 2-2 is a sample in which the above test sample material was immersed into a hot-dip galvanizing bath of 460° C. to perform a hot-dip galvanizing treatment, thereby to produce a hot-dip galvanized steel sheet. The hot-dip galvanizing adhesion amount was 30 g/m 2  per one surface. In the following Table 2-2, the section of classification for No. 36 gives “GI”. 
     [Hot-Dip Galvannealing] 
     No. 18 shown in the following Table 2-1 is a sample in which the above test sample material was immersed into a hot-dip galvanizing bath of 460° C. to perform a hot-dip galvanizing treatment, followed by heating to 500° C. to perform an alloying treatment, thereby to produce a hot-dip galvannealed steel sheet. The hot-dip galvannealing adhesion amount was 30 g/m 2  per one surface. In the following Table 2-1, the section of classification for No. 18 gives “GA”. 
     Test sample materials in which none of the electrogalvanization treatment, hot-dip galvanizing treatment, and hot-dip galvannealing treatment was carried out are denoted as “cold-rolled” in the section of classification in the following Tables 2-1 and 2-2. 
     With respect to the obtained test sample materials, a metal structure was observed by the following procedure. 
     [Observation of Metal Structure] 
     (Area Ratio of Ferrite and Hard Phase) 
     After the cross-section of the obtained test sample material was polished, the test sample material was subjected to nital corrosion, followed by performing observation at the position of ¼ of the sheet thickness in three fields of view at a magnification of 1000 times with a scanning electron microscope, so as to capture a photomicrograph image. The observation field of view was such that one field of view had a size of 100 μm×100 μm. With the lattice interval set to be 5 μm, the area ratio of ferrite was measured by the point counting method with the number of lattice points being 20×20, and an average value Vf of the three fields of view was calculated. The calculation results are shown in the following Tables 3-1 and 3-2. The area ratio of ferrite was calculated by excluding the area ratio of the hard phase that existed in the ferrite phase. 
     Further, the Ms point was calculated in accordance with the above formula (iv) based on the component composition shown in the following Table 1 and the average area ratio Vf of ferrite shown in the following Tables 3-1 and 3-2. The results are shown in the following Tables 2-1 and 2-2. The following Tables 2-1 and 2-2 also show a value obtained by subtracting the temperature of the Ms point from the cooling stop temperature T. 
     In a similar manner, a sum area ratio of pearlite and cementite was measured by the point counting method, and an average value of the three fields of view was calculated. The calculation results are shown in the following Tables 3-1 and 3-2. The sum area ratio of pearlite and cementite is denoted as “other structures” in the following Tables 3-1 and 3-2. 
     In the present Examples, the structure other than ferrite, pearlite, and cementite calculated by the point counting method was assumed to be a hard phase. In other words, a value obtained by subtracting the area ratio of ferrite and the sum area ratio of pearlite and cementite from 100% was calculated as an area ratio of the hard phase. The results are shown in the following Tables 3-1 and 3-2. 
     As a result of observation of a specific structure constituting the hard phase, it was found out that the hard phase included quenched martensite and retained γ and included at least one selected from the group consisting of bainitic ferrite, bainite, and tempered martensite. 
     (Volume Ratio V γ  of Retained γ) 
     The obtained test sample material was polished down to the position of ¼ of the sheet thickness with use of a sandpaper of #1000 to #1500, and further the surface was subjected to electrolytic polishing down to the depth of 10 to 20 μm, followed by measuring the volume ratio V γ  of retained γ with use of an X-ray diffractometer. Specifically, “RINT 1500” manufactured by Rigaku Corporation was used as the X-ray diffractometer and, with use of a Co target, a power of 40 kV-200 mA was output to measure the range of 40° to 130° in terms of 2θ. The volume ratio V γ  of retained γ was quantitated on the basis of the obtained bcc (α) diffraction peaks (110), (200), and (211) and fcc (γ) diffraction peaks (111), (200), (220), and (311). The results are shown in the following Tables 3-1 and 3-2. 
     (Area Ratio V MA  and Average Circle-Equivalent Diameter of MA Structure) 
     After the cross-section of the obtained test sample material was polished, the test sample material was subjected to LePera corrosion, followed by performing observation at the position of ¼ of the sheet thickness in three fields of view at a magnification of 1000 times with an optical microscope, so as to capture a photomicrograph image. The observation field of view was such that one field of view had a size of 100 μm×100 μm. The portion whitened by LePera corrosion was regarded as the MA structure. With the lattice interval set to be 5 μm, the area ratio of the MA structure was measured by the point counting method with the number of lattice points being 20×20, and an average value of the three fields of view was calculated. The calculation results are shown in the following Tables 3-1 and 3-2. 
     Upon subjecting the photomicrograph image captured with the optical microscope to image analysis, the average circle-equivalent diameter d of each MA structure was calculated, and an average value was determined. The results are shown in the following Tables 3-1 and 3-2. 
     (Ratio of Area Ratio V MA  of MA Structure to Volume Ratio V γ  of Retained γ) 
     The ratio V MA /V γ  of the area ratio V MA  of the MA structure to the volume ratio V γ  of the retained γ was calculated on the basis of the volume ratio V γ  of the retained γ and the area ratio V MA  of the MA structure calculated by the above-described procedure. The calculation results are shown in the following Tables 3-1 and 3-2. 
     Next, with respect to the obtained test sample material, the mechanical properties, ductility, stretch-flangeability, and crashworthiness were evaluated by the following procedure. 
     [Evaluation of Mechanical Properties and Ductility] 
     A No. 5 test piece defined in JIS Z2201 was cut out so that the direction perpendicular to the rolling direction of the obtained test sample material would be a longitudinal direction. With use of this test piece, a tensile test was carried out so as to measure the tensile strength TS and the elongation EL. The measurement results are shown in the following Tables 3-1 and 3-2. 
     In the present Examples, the samples in which the tensile strength was 980 MPa or more were evaluated as having a high strength and being acceptable, whereas the samples in which the tensile strength was less than 980 MPa were evaluated as having an insufficient strength and being a reject. 
     Also, the value of tensile strength TS×elongation EL was calculated on the basis of the measured values of tensile strength TS and elongation EL. The calculation results are shown in the following Tables 3-1 and 3-2. The value of TS×EL indicates a strength-elongation balance and serves as an index for evaluating the ductility. 
     In the present Examples, the samples in which the value of TS×EL was 17000 MPa·% or more were evaluated as having an excellent ductility and being acceptable, whereas the samples in which the value of TS×EL was less than 17000 MPa·% were evaluated as having a poor ductility and being a reject. 
     [Evaluation of Stretch-Flangeability] 
     In order to evaluate the stretch-flangeability of the test sample material, a hole expansion test was carried out according to the Japan Iron and Steel Federation Standard JFS T 1001, so as to measure the hole expansion ratio λ. The measurement results are shown in the following Tables 3-1 and 3-2. 
     Also, the value of tensile strength TS×hole expansion ratio λ was calculated on the basis of the measured values of tensile strength TS and hole expansion ratio λ. The calculation results are shown in the following Tables 3-1 and 3-2. The value of TS×λ indicates a strength-hole expansion ratio balance and serves as an index for evaluating the stretch-flangeability. 
     In the present Examples, the samples in which the value of TS×λ was 20000 MPa·% or more were evaluated as having an excellent stretch-flangeability and being acceptable, whereas the samples in which the value of TS×λ was less than 20000 MPa·% were evaluated as having a poor stretch-flangeability and being a reject. 
     [Evaluation of Crashworthiness] 
     It is disclosed in the following literature that the crashworthiness is correlated to a bending angle.
     Literature: P. Larour, H. Pauli, T. Kurz, T. Hebesberger: “Influence of post uniform tensile and bending properties on the crash behaviour of AHSS and press-hardening steel grades”, IDDRG2010   

     Accordingly, a bending test was carried out under the following conditions on the basis of the VDA standard (VDA238-100) defined by the German Association of the Automotive Industry. The displacement at the maximum load measured by the bending test was converted into an angle according to the VDA standard, so as to determine the bending angle. The conversion results are shown in the following Tables 3-1 and 3-2. 
     (Measurement Conditions) 
     Test method: support with rolls, pressing-in of punch 
     Roll diameter: φ30 mm 
     Punch shape: tip end R=0.4 mm 
     Distance between rolls: 2.9 mm 
     Punch pressing-in speed: 20 mm/min 
     Test piece dimension: 60 mm×60 mm 
     Bending direction: direction perpendicular to the rolling direction 
     Testing machine: SIMAZU AUTOGRAPH 20 kN 
     Also, the value of tensile strength TS×VDA bending angle° was calculated on the basis of the values of the tensile strength TS measured in the tensile test and the VDA bending angle. The calculation results are shown in the following Tables 3-1 and 3-2. 
     In the present Examples, the samples in which the value of TS×VDA was 90000 MPa·° or more were evaluated as having an excellent crashworthiness and being acceptable, whereas the samples in which the value of TS×VDA was less than 90000 MPa·° were evaluated as having a poor crashworthiness and being a reject. 
     On the basis of the above results, samples satisfying all of the requirements: the value of TS being 980 MPa or more, the value of TS×EL being 17000 MPa·% or more, the value of TS×λ being 20000 MPa·% or more, and the value of TS×VDA being 90000 MPa·° or more were regarded as the present invention examples and listed as being acceptable in the totaltotal evaluation section of the following Tables 3-1 and 3-2. On the other hand, samples in which one or more of the value of TS, the value of TS×EL, the value of TS×λ, and the value of TS×VDA failed to satisfy the above acceptance standard were regarded as the comparative examples and listed as being a reject in the total evaluation section of the following Tables 3-1 and 3-2. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                 Steel 
                 Component (mass %) 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 type 
                 C 
                 Si 
                 Mn 
                 P 
                 S 
                 Al 
                 Cr 
                 Mo 
                 Ti 
                 Nb 
                 V 
               
               
                   
               
               
                 A 
                 0.16 
                 1.98 
                 2.05 
                 0.03 
                 0.001 
                 0.01 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 B 
                 0.22 
                 1.86 
                 2.06 
                 0.02 
                 0.001 
                 0.03 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 C 
                 0.19 
                 1.55 
                 2.34 
                 0.03 
                 0.001 
                 0.01 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 D 
                 0.16 
                 2.55 
                 2.05 
                 0.03 
                 0.003 
                 0.01 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 E 
                 0.17 
                 2.20 
                 1.84 
                 0.04 
                 0.003 
                 0.04 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 F 
                 0.20 
                 1.96 
                 2.53 
                 0.01 
                 0.001 
                 0.02 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 G 
                 0.12 
                 2.17 
                 2.53 
                 0.03 
                 0.002 
                 0.02 
                 0.2 
                 — 
                 — 
                 — 
                 — 
               
               
                 H 
                 0.28 
                 1.98 
                 2.09 
                 0.02 
                 0.002 
                 0.04 
                 — 
                 0.5 
                 — 
                 — 
                 — 
               
               
                 I 
                 0.42 
                 1.73 
                 2.35 
                 0.04 
                 0.001 
                 0.04 
                 — 
                 — 
                 0.10 
                 — 
                 — 
               
               
                 J 
                 0.22 
                 1.08 
                 2.02 
                 0.02 
                 0.002 
                 0.04 
                 — 
                 — 
                 — 
                 0.08 
                 — 
               
               
                 K 
                 0.23 
                 1.37 
                 2.08 
                 0.04 
                 0.002 
                 0.03 
                 — 
                 — 
                 — 
                 — 
                 0.13 
               
               
                 L 
                 0.23 
                 2.32 
                 1.66 
                 0.03 
                 0.003 
                 0.03 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 M 
                 0.20 
                 2.04 
                 1.87 
                 0.01 
                 0.003 
                 0.02 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 N 
                 0.20 
                 2.03 
                 2.86 
                 0.01 
                 0.002 
                 0.03 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 O 
                 0.17 
                 2.16 
                 4.13 
                 0.02 
                 0.001 
                 0.10 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 P 
                 0.14 
                 2.42 
                 6.27 
                 0.03 
                 0.003 
                 0.15 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 Q 
                 0.07 
                 1.84 
                 2.17 
                 0.03 
                 0.003 
                 0.03 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 R 
                 0.18 
                 0.55 
                 2.06 
                 0.02 
                 0.001 
                 0.05 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 S 
                 0.16 
                 1.65 
                 1.10 
                 0.03 
                 0.001 
                 0.03 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Ar 3   
                 Ac 3   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Steel 
                 Component (mass %) 
                 point 
                 point 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 type 
                 Cu 
                 Ni 
                 B 
                 Ca 
                 Mg 
                 REM 
                 N 
                 O 
                 (° C.) 
                 (° C.) 
               
               
                   
               
               
                   
                 A 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.004 
                 0.002 
                 697 
                 881 
               
               
                   
                 B 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.003 
                 0.001 
                 676 
                 861 
               
               
                   
                 C 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.003 
                 0.001 
                 663 
                 845 
               
               
                   
                 D 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.004 
                 0.001 
                 696 
                 906 
               
               
                   
                 E 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.004 
                 0.001 
                 710 
                 914 
               
               
                   
                 F 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.005 
                 0.001 
                 647 
                 847 
               
               
                   
                 G 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.005 
                 0.001 
                 668 
                 888 
               
               
                   
                 H 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.004 
                 0.001 
                 616 
                 874 
               
               
                   
                 I 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.004 
                 0.001 
                 593 
                 870 
               
               
                   
                 J 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.002 
                 0.001 
                 680 
                 832 
               
               
                   
                 K 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.003 
                 0.001 
                 673 
                 865 
               
               
                   
                 L 
                 0.20 
                 0.20 
                 — 
                 — 
                 — 
                 — 
                 0.002 
                 0.001 
                 692 
                 893 
               
               
                   
                 M 
                 — 
                 — 
                 0.0015 
                 — 
                 — 
                 — 
                 0.005 
                 0.001 
                 697 
                 869 
               
               
                   
                 N 
                 — 
                 — 
                 — 
                 0.0022 
                 — 
                 — 
                 0.003 
                 0.001 
                 620 
                 844 
               
               
                   
                 O 
                 — 
                 — 
                 — 
                 — 
                 0.0030 
                   
                 0.003 
                 0.001 
                 526 
                 852 
               
               
                   
                 P 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.0025 
                 0.003 
                 0.001 
                 364 
                 835 
               
               
                   
                 Q 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.004 
                 0.002 
                 716 
                 908 
               
               
                   
                 R 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.003 
                 0.001 
                 691 
                 822 
               
               
                   
                 S 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.003 
                 0.001 
                 771 
                 902 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 2-1 
               
             
            
               
                   
               
               
                   
                   
                   
                   
                   
                 Annealing step 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Reheating holding 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Re- 
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                   
                 Cooling 
                   
                 heating 
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Cool- 
                   
                 temper 
                   
                   
               
               
                   
                   
                 Hot rolling step 
                   
                   
                   
                   
                 Aver- 
                 Cool- 
                   
                 ing  
                   
                 ature- 
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Finish 
                   
                   
                 Heating 
                 Soaking 
                 age 
                 ing 
                   
                 stop 
                   
                 cooling 
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 rolling  
                 Final 
                   
                 Average 
                 Soak- 
                 Ac 3 - 
                   
                 cool- 
                 stop 
                   
                 temper- 
                 Re- 
                 stop 
                 Re- 
                   
               
               
                   
                   
                 end 
                 stand 
                 Coiling 
                 heating 
                 ing 
                 soaking 
                 Soak- 
                 ing 
                 temper- 
                   
                 ature 
                 heating 
                 temper- 
                 heating 
                   
               
               
                   
                   
                 temper- 
                 rolling 
                 temper- 
                 rate 
                 temper- 
                 temper- 
                 ing 
                 rate 
                 ature 
                 Ms 
                 T-Ms 
                 temper- 
                 ature 
                 holding 
                   
               
               
                   
                 Steel 
                 ature 
                 rate 
                 ature 
                 (° C./ 
                 ature 
                 ature 
                 time 
                 (° C./ 
                 T 
                 point 
                 point 
                 ature 
                 T 
                 time 
                 Classifi- 
               
               
                 No. 
                 type 
                 (° C.) 
                 (%) 
                 (° C.) 
                 sec) 
                 (° C.) 
                 (° C.) 
                 (sec) 
                 sec) 
                 (° C.) 
                 (° C.) 
                 (° C.) 
                 (° C.) 
                 (° C.) 
                 (sec) 
                 cation 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                  1 
                 A 
                  880 
                 15 
                 550 
                 15 
                 820 
                 61 
                 300 
                 15 
                 250 
                 349 
                 −99 
                 450 
                 200 
                 600 
                 Cold-rolled 
               
               
                  2 
                 A 
                  880 
                 15 
                 550 
                 15 
                 840 
                 41 
                 300 
                 15 
                 200 
                 383 
                 −183 
                 380 
                 180 
                 600 
                 EG 
               
               
                  3 
                 A 
                 1000 
                 15 
                 550 
                 15 
                 820 
                 61 
                 300 
                 15 
                 300 
                 323 
                 −23 
                 420 
                 120 
                 600 
                 Cold-rolled 
               
               
                  4 
                 A 
                  880 
                 40 
                 550 
                 15 
                 820 
                 61 
                 300 
                 15 
                 300 
                 352 
                 −52 
                 450 
                 150 
                 600 
                 Cold-rolled 
               
               
                  5 
                 A 
                  880 
                  3 
                 550 
                 15 
                 820 
                 61 
                 300 
                 15 
                 250 
                 344 
                 −94 
                 420 
                 170 
                 400 
                 Cold-rolled 
               
               
                  6 
                 B 
                  880 
                 20 
                 500 
                 15 
                 820 
                 41 
                 300 
                 15 
                 190 
                 310 
                 −120 
                 420 
                 230 
                 600 
                 Cold-rolled 
               
               
                  7 
                 B 
                  880 
                 20 
                 550 
                 15 
                 900 
                 −39 
                 300 
                 20 
                  90 
                 386 
                 −296 
                 380 
                 290 
                 600 
                 Cold-rolled 
               
               
                  8 
                 B 
                  880 
                 20 
                 500 
                 15 
                 850 
                 11 
                 300 
                 15 
                 480 
                 372 
                 108 
                 350 
                 −130 
                 300 
                 Cold-rolled 
               
               
                  9 
                 C 
                  880 
                 15 
                 500 
                 15 
                 840 
                 5 
                 300 
                 15 
                 300 
                 364 
                 −64 
                 420 
                 120 
                 600 
                 Cold-rolled 
               
               
                 10 
                 C 
                  880 
                 15 
                 500 
                 15 
                 840 
                 5 
                 300 
                 15 
                  25 
                 369 
                 −344 
                 480 
                 455 
                 600 
                 Cold-rolled 
               
               
                 11 
                 C 
                  880 
                 30 
                 500 
                 15 
                 820 
                 25 
                 300 
                 15 
                 330 
                 350 
                 −20 
                 300 
                 −30 
                 300 
                 Cold-rolled 
               
               
                 12 
                 D 
                  880 
                 15 
                 500 
                 15 
                 850 
                 56 
                 300 
                 20 
                 150 
                 387 
                 −237 
                 380 
                 230 
                 600 
                 Cold-rolled 
               
               
                 13 
                 D 
                  880 
                  7 
                 500 
                 15 
                 850 
                 56 
                 300 
                 20 
                 200 
                 384 
                 −184 
                 440 
                 240 
                 600 
                 Cold-rolled 
               
               
                 14 
                 D 
                  880 
                 15 
                 500 
                 15 
                 880 
                 26 
                 300 
                 20 
                 250 
                 402 
                 −152 
                 450 
                 200 
                 5 
                 Cold-rolled 
               
               
                 15 
                 E 
                  880 
                 10 
                 550 
                 20 
                 850 
                 64 
                 300 
                 20 
                 250 
                 387 
                 −137 
                 400 
                 150 
                 600 
                 Cold-rolled 
               
               
                 16 
                 E 
                  880 
                 10 
                 550 
                  3 
                 850 
                 64 
                 300 
                 20 
                 250 
                 382 
                 −132 
                 450 
                 200 
                 600 
                 Cold-rolled 
               
               
                 17 
                 F 
                  880 
                 15 
                 550 
                 15 
                 830 
                 17 
                 300 
                 15 
                 170 
                 353 
                 −183 
                 360 
                 190 
                 600 
                 Cold-rolled 
               
               
                 18 
                 F 
                  880 
                 15 
                 550 
                 15 
                 810 
                 37 
                 300 
                 15 
                 170 
                 337 
                 −167 
                 440 
                 270 
                 300 
                 GA 
               
               
                 19 
                 F 
                  880 
                 15 
                 550 
                 15 
                 810 
                 37 
                 300 
                 15 
                 400 
                 345 
                 55 
                 450 
                 50 
                 600 
                 Cold-rolled 
               
               
                 20 
                 G 
                  880 
                 10 
                 550 
                 15 
                 820 
                 68 
                 300 
                 15 
                 300 
                 392 
                 −92 
                 450 
                 150 
                 600 
                 Cold-rolled 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 2-2 
               
             
            
               
                   
               
               
                   
                   
                   
                   
                   
                 Annealing step 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Reheating holding 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Re- 
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                   
                 Cooling 
                   
                 heating 
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Cool- 
                   
                 temper- 
                   
                   
               
               
                   
                   
                 Hot rolling step 
                   
                   
                   
                   
                 Aver- 
                 Cool- 
                   
                 ing  
                   
                 ature 
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Finish 
                   
                   
                 Heating 
                 Soaking 
                 age 
                 ing 
                   
                 stop 
                   
                 cooling 
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 rolling  
                 Final 
                   
                 Average 
                 Soak- 
                 Ac 3 - 
                   
                 cool- 
                 stop 
                   
                 temper- 
                 Re- 
                 stop 
                 Re- 
                   
               
               
                   
                   
                 end 
                 stand 
                 Coiling 
                 heating 
                 ing 
                 soaking 
                 Soak- 
                 ing 
                 temper- 
                   
                 ature 
                 heating 
                 temper- 
                 heating 
                   
               
               
                   
                   
                 temper- 
                 rolling 
                 temper- 
                 rate 
                 temper- 
                 temper- 
                 ing 
                 rate 
                 ature 
                 Ms 
                 T-Ms 
                 temper- 
                 ature 
                 holding 
                   
               
               
                   
                 Steel 
                 ature 
                 rate 
                 ature 
                 (° C./ 
                 ature 
                 ature 
                 time 
                 (° C./ 
                 T 
                 point 
                 point 
                 ature 
                 T 
                 time 
                 Classifi- 
               
               
                 No. 
                 type 
                 (° C.) 
                 (%) 
                 (° C.) 
                 sec) 
                 (° C.) 
                 (° C.) 
                 (sec) 
                 sec) 
                 (° C.) 
                 (° C.) 
                 (° C.) 
                 (° C.) 
                 (° C.) 
                 (sec) 
                 cation 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 21 
                 H 
                 880 
                 15 
                 550 
                 15 
                 820 
                 54 
                 300 
                 15 
                 170 
                 283 
                 −113 
                 450 
                 280 
                 600 
                 Cold-rolled 
               
               
                 22 
                 I 
                 880 
                 20 
                 550 
                 15 
                 850 
                 20 
                 300 
                 20 
                 100 
                 257 
                 −157 
                 500 
                 400 
                 600 
                 Cold-rolled 
               
               
                 23 
                 J 
                 880 
                 15 
                 600 
                 15 
                 810 
                 22 
                 300 
                 10 
                 300 
                 362 
                 −62 
                 450 
                 150 
                 600 
                 Cold-rolled 
               
               
                 24 
                 K 
                 880 
                 15 
                 550 
                 15 
                 820 
                 45 
                 300 
                 10 
                 200 
                 350 
                 −150 
                 420 
                 220 
                 600 
                 Cold-rolled 
               
               
                 25 
                 K 
                 880 
                 15 
                 550 
                 15 
                 820 
                 45 
                 300 
                 10 
                 300 
                 346 
                 −46 
                 600 
                 300 
                 1000 
                 Cold-rolled 
               
               
                 26 
                 L 
                 880 
                 15 
                 550 
                 15 
                 830 
                 63 
                 300 
                 20 
                 250 
                 297 
                 −47 
                 420 
                 170 
                 600 
                 Cold-rolled 
               
               
                 27 
                 L 
                 880 
                 15 
                 550 
                 15 
                 830 
                 63 
                 300 
                  1 
                 300 
                 106 
                 194 
                 420 
                 120 
                 600 
                 Cold-rolled 
               
               
                 28 
                 M 
                 880 
                 15 
                 550 
                 15 
                 840 
                 29 
                 300 
                 20 
                 250 
                 333 
                 −83 
                 440 
                 190 
                 600 
                 Cold-rolled 
               
               
                 29 
                 M 
                 880 
                 15 
                 700 
                 15 
                 840 
                 29 
                 300 
                 20 
                 250 
                 321 
                 −71 
                 460 
                 210 
                 600 
                 Cold-rolled 
               
               
                 30 
                 N 
                 880 
                 15 
                 600 
                 15 
                 810 
                 34 
                 300 
                 10 
                 200 
                 345 
                 −145 
                 400 
                 200 
                 600 
                 Cold-rolled 
               
               
                 31 
                 O 
                 880 
                 15 
                 600 
                 15 
                 810 
                 42 
                 300 
                 10 
                 190 
                 325 
                 −135 
                 480 
                 290 
                 600 
                 Cold-rolled 
               
               
                 32 
                 P 
                 880 
                 15 
                 600 
                 15 
                 805 
                 30 
                 300 
                 10 
                 150 
                 278 
                 −128 
                 500 
                 350 
                 600 
                 Cold-rolled 
               
               
                 33 
                 Q 
                 880 
                 15 
                 500 
                 15 
                 840 
                 68 
                 300 
                 20 
                 200 
                 443 
                 −243 
                 420 
                 220 
                 600 
                 Cold-rolled 
               
               
                 34 
                 R 
                 880 
                 15 
                 500 
                 15 
                 810 
                 12 
                 300 
                 20 
                 200 
                 403 
                 −203 
                 420 
                 220 
                 600 
                 Cold-rolled 
               
               
                 35 
                 S 
                 880 
                 15 
                 500 
                 15 
                 830 
                 72 
                 300 
                 10 
                 200 
                 247 
                 −47 
                 480 
                 280 
                 600 
                 Cold-rolled 
               
               
                 36 
                 E 
                 880 
                 10 
                 550 
                 20 
                 880 
                 34 
                 400 
                 20 
                 200 
                 401 
                 −201 
                 440 
                 240 
                 200 
                 GI 
               
               
                 37 
                 F 
                 880 
                 15 
                 550 
                  7 
                 830 
                 17 
                 300 
                 15 
                 320 
                 344 
                 −24 
                 460 
                 140 
                 600 
                 Cold-rolled 
               
               
                 38 
                 E 
                 880 
                 10 
                 550 
                 20 
                 820 
                 94 
                 300 
                  7 
                 300 
                 204 
                 96 
                 400 
                 100 
                 300 
                 Cold-rolled 
               
               
                 39 
                 C 
                 880 
                 15 
                 500 
                 15 
                 830 
                 15 
                 300 
                 15 
                 210 
                 356 
                 −146 
                 370 
                 160 
                 100 
                 Cold-rolled 
               
               
                 40 
                 B 
                 880 
                 15 
                 500 
                 10 
                 840 
                 21 
                 300 
                 30 
                 220 
                 335 
                 −115 
                 380 
                 160 
                 600 
                 Cold-rolled 
               
               
                 41 
                 A 
                 880 
                 15 
                 550 
                 20 
                 830 
                 51 
                 100 
                 20 
                 220 
                 380 
                 −160 
                 420 
                 200 
                 300 
                 Cold-rolled 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 3-1 
               
             
            
               
                   
               
               
                   
                   
                 Average 
                   
                   
               
               
                   
                   
                 circle- 
                   
                   
               
               
                   
                 Structure fraction 
                 equivalent 
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 MA 
                 diameter 
                   
                   
               
               
                   
                 Ferrite 
                 Hard 
                 Other 
                 Retained γ 
                 structure 
                 of MA 
                   
                 Material properties 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Vf 
                 phase 
                 structures 
                 V γ   
                 V MA   
                 structure 
                   
                 TS 
                 EL 
                 λ 
                 VDA 
                 TS × EL 
                 TS × λ 
                 TS × VDA 
                 Total 
               
               
                 No. 
                 (area %) 
                 (area %) 
                 (area %) 
                 (vol %) 
                 (area %) 
                 (μm) 
                 V MA /V γ   
                 (MPa) 
                 (%) 
                 (%) 
                 (°) 
                 (MPa · %) 
                 (MPa · %) 
                 (MPa · °) 
                 evaluation 
               
               
                   
               
               
                  1 
                 48 
                 50 
                 2 
                 12 
                 15 
                 1.5 
                 1.25 
                 1022 
                 22 
                 24 
                 92 
                 22484 
                 24528 
                  94024 
                 Acceptable 
               
               
                  2 
                 32 
                 65 
                 3 
                  8 
                  8 
                 1.2 
                 1.00 
                 1216 
                 17 
                 31 
                 76 
                 20672 
                 37696 
                  92416 
                 Acceptable 
               
               
                  3 
                 56 
                 41 
                 3 
                 14 
                 18 
                 2.5 
                 1.29 
                 1051 
                 20 
                 17 
                 86 
                 21020 
                 17867 
                  90386 
                 Reject 
               
               
                  4 
                 47 
                 48 
                 5 
                 16 
                 22 
                 2.8 
                 1.38 
                 1073 
                 22 
                 15 
                 79 
                 23606 
                 16095 
                  84767 
                 Reject 
               
               
                  5 
                 50 
                 47 
                 3 
                 12 
                 15 
                 2.2 
                 1.25 
                 1031 
                 20 
                 18 
                 89 
                 20620 
                 18558 
                  91759 
                 Reject 
               
               
                  6 
                 42 
                 57 
                 1 
                 17 
                 20 
                 1.1 
                 1.18 
                 1055 
                 26 
                 32 
                 98 
                 27430 
                 33760 
                 103390 
                 Acceptable 
               
               
                  7 
                  1 
                 97 
                 2 
                  5 
                  6 
                 0.7 
                 1.20 
                 1281 
                 10  
                 42 
                 82 
                 12810 
                 53802 
                 105042 
                 Reject 
               
               
                  8 
                 12 
                 84 
                 4 
                 13 
                 24 
                 1.9 
                 1.85 
                 1057 
                 19 
                 21 
                 75 
                 20083 
                 22197 
                  79275 
                 Reject 
               
               
                  9 
                 24 
                 73 
                 3 
                 13 
                 18 
                 1.2 
                 1.38 
                 1036 
                 19 
                 22 
                 94 
                 19684 
                 22792 
                  97384 
                 Acceptable 
               
               
                 10 
                 21 
                 76 
                 3 
                  3 
                  1 
                 0.7 
                 0.33 
                  992 
                 15 
                 42 
                 91 
                 14880 
                 41664 
                  90272 
                 Reject 
               
               
                 11 
                 32 
                 65 
                 3 
                  7 
                 12 
                 2.3 
                 1.71 
                 1354 
                 13  
                 18 
                 61 
                 17602 
                 24372 
                  82594 
                 Reject 
               
               
                 12 
                 28 
                 68 
                 4 
                 10 
                 13 
                 1.1 
                 1.30 
                 1246 
                 17 
                 26 
                 75 
                 21182 
                 32396 
                  93450 
                 Acceptable 
               
               
                 13 
                 30 
                 68 
                 2 
                 12 
                 15 
                 1.6 
                 1.25 
                 1053 
                 20 
                 21 
                 88 
                 21060 
                 22113 
                  92664 
                 Acceptable 
               
               
                 14 
                 16 
                 83 
                 1 
                 14 
                 35 
                 3.4 
                 2.50 
                 1521 
                  7 
                  8 
                 55 
                 10647 
                 12168 
                  83655 
                 Reject 
               
               
                 15 
                 29 
                 66 
                 5 
                 13 
                 16 
                 1.4 
                 1.23 
                 1041 
                 21 
                 26 
                 92 
                 21861 
                 27066 
                  95772 
                 Acceptable 
               
               
                 16 
                 32 
                 66 
                 2 
                 12 
                 17 
                 2.4 
                 1.42 
                  992 
                 21 
                 17 
                 91 
                 20832 
                 16864 
                  90272 
                 Reject 
               
               
                 17 
                 26 
                 71 
                 3 
                  9 
                  7 
                 1.0 
                 0.78 
                 1222 
                 17  
                 31 
                 78 
                 20774 
                 37882 
                  95316 
                 Acceptable 
               
               
                 18 
                 34 
                 64 
                 2 
                 13 
                 14 
                 1.2 
                 1.08 
                 1062 
                 22 
                 28 
                 86 
                 23364 
                 29736 
                  91332 
                 Acceptable 
               
               
                 19 
                 30 
                 66 
                 4 
                 13 
                 16 
                 2.3 
                 1.23  
                 1034 
                 18 
                 15 
                 88 
                 18612 
                 15510 
                  90992 
                 Reject 
               
               
                 20 
                 31 
                 67 
                 2 
                 10 
                 11 
                 1.0 
                 1.10 
                 1029 
                 17 
                 31 
                 91 
                 17493 
                 31899 
                  93639 
                 Acceptable 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 3-2 
               
             
            
               
                   
               
               
                   
                   
                 Average 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                   
                 circle- 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 Structure fraction 
                 equivalent 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 MA 
                 diameter 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 Ferrite 
                 Hard 
                 Other 
                 Retained γ 
                 structure 
                 of MA 
                   
                 Material properties 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Vf 
                 phase 
                 structures 
                 V γ   
                 V MA   
                 structure 
                   
                 TS 
                 EL 
                 λ 
                 VDA 
                 TS × EL 
                 TS × λ 
                 TS × VDA 
                 Total 
               
               
                 No. 
                 (area %) 
                 (area %) 
                 (area %) 
                 (vol %) 
                 (area %) 
                 (μm) 
                 V MA /V γ   
                 (MPa) 
                 (%) 
                 (%) 
                 (°) 
                 (MPa · %) 
                 (MPa · %) 
                 (MPa · °) 
                 evaluation 
               
               
                   
               
               
                 21 
                 33 
                 65 
                  2 
                 18 
                 21 
                 1.2 
                 1.17 
                 1238 
                 18 
                 22 
                  76 
                 22284 
                 27236 
                  94088 
                 Acceptable 
               
               
                 22 
                 13 
                 86 
                  1 
                 22 
                 24 
                 1.2 
                 1.09 
                 1421 
                 14 
                 21 
                  65 
                 19894 
                 29841 
                  92365 
                 Acceptable 
               
               
                 23 
                 21 
                 77 
                  2 
                 11 
                 15 
                 1.6 
                 1.36 
                 1053 
                 17 
                 23 
                  87 
                 17901 
                 24219 
                  91611 
                 Acceptable 
               
               
                 24 
                 24 
                 74 
                  2 
                 12 
                 15 
                 0.9 
                 1.25 
                 1017 
                 20 
                 24 
                 100 
                 20340 
                 24408 
                 101700 
                 Acceptable 
               
               
                 25 
                 26 
                 60 
                 14 
                  2 
                  2 
                 0.9 
                 1.00 
                  819 
                 21 
                 16 
                  82 
                 17199 
                 13104 
                  67158 
                 Reject 
               
               
                 26 
                 48 
                 49 
                  3 
                 16 
                 19 
                 1.0 
                 1.19  
                 1004 
                 25 
                 28 
                  95 
                 25100 
                 28112 
                  95380 
                 Acceptable 
               
               
                 27 
                 73 
                 22 
                  5 
                 11 
                 16 
                 1.5 
                 1.45 
                  921 
                 29 
                 16 
                 101 
                 26709 
                 14736 
                  93021 
                 Reject 
               
               
                 28 
                 42 
                 56 
                  2 
                 13 
                 16 
                 1.6 
                 1.23 
                  997 
                 25 
                 26 
                  92 
                 24925 
                 25922 
                  91724 
                 Acceptable 
               
               
                 29 
                 46 
                 50 
                  4 
                 12 
                 17 
                 2.3 
                 1.42 
                 1011 
                 21 
                 17 
                  91 
                 21231 
                 17187 
                  92001 
                 Reject 
               
               
                 30 
                 23 
                 75 
                  2 
                  9 
                 10 
                 0.9 
                 1.11  
                 1194 
                 16 
                 32 
                  77 
                 19104 
                 38208 
                  91938 
                 Acceptable 
               
               
                 31 
                 18 
                 82 
                  0 
                 13 
                 17 
                 1.5 
                 1.31 
                 1432 
                 14 
                 23 
                  65 
                 20048 
                 32936 
                  93080 
                 Acceptable 
               
               
                 32 
                 12 
                 88 
                  0 
                 12 
                 17 
                 1.7 
                 1.42 
                 1501 
                 13 
                 25 
                  61 
                 19513 
                 37525 
                  91561 
                 Acceptable 
               
               
                 33 
                 32 
                 66 
                  2 
                  3 
                  5 
                 1.0 
                 1.67 
                 1023 
                 15 
                 34 
                  97 
                 15345 
                 34782 
                  99231 
                 Reject 
               
               
                 34 
                  8 
                 89 
                  3 
                  5 
                  6 
                 1.1 
                 1.20 
                 1038 
                 14 
                 56 
                 102 
                 14532 
                 58128 
                 105876 
                 Reject 
               
               
                 35 
                 72 
                 20 
                  8 
                 11 
                 14 
                 1.7 
                 1.27 
                  938 
                 25 
                 15 
                  97 
                 23450 
                 14070 
                  90986 
                 Reject 
               
               
                 36 
                 19 
                 77 
                  4 
                 11 
                 14 
                 1.6 
                 1.27  
                 1005 
                 20 
                 31 
                  94 
                 20100 
                 31155 
                  94470 
                 Acceptable 
               
               
                 37 
                 31 
                 66 
                  3 
                 10 
                 11 
                 2.4 
                 1.10  
                 1027 
                 17 
                 17 
                  93 
                 17459 
                 17459 
                  95511 
                 Reject 
               
               
                 38 
                 73 
                 22 
                  5 
                 13 
                 16 
                 1.8 
                 1.23 
                  932 
                 22 
                 19 
                 102 
                 20504 
                 17708 
                  95064 
                 Reject 
               
               
                 39 
                 29 
                 69 
                  2 
                 10 
                  9 
                 1.0 
                 0.90 
                 1248 
                 15 
                 34 
                  76 
                 18720 
                 42432 
                  94848 
                 Acceptable 
               
               
                 40 
                 33 
                 64 
                  3 
                 13 
                 16 
                 1.5 
                 1.23 
                 1214 
                 16 
                 27 
                  76 
                 19424 
                 32778 
                  92264 
                 Acceptable 
               
               
                 41 
                 34 
                 63 
                  3 
                 10 
                 12 
                 1.3 
                 1.20 
                 1064 
                 18 
                 27 
                  87 
                 19152 
                 28728 
                  92568 
                 Acceptable 
               
               
                   
               
            
           
         
       
     
     From Tables 1, 2-1, 2-2, 3-1, and 3-2, the following considerations can be made. 
     In Tables 3-1 and 3-2, all of the samples rated as “acceptable” in the total evaluation section are steel sheets satisfying the requirements defined in the present invention, and all of the value of TS×EL, the value of TS×λ, and the value of TS×VDA determined in accordance with the tensile strength TS satisfy the acceptance standard values. It will be understood that these steel sheets have good formability as evaluated by ductility and stretch-flangeability, and are excellent in ductility in particular, and also in crashworthiness. 
     In contrast, the samples rated as “reject” in the total evaluation section are steel sheets that do not satisfy one or more of the requirements defined in the present invention, and at least one of ductility, stretch-flangeability, and crashworthiness could not be improved. The details are as follows. 
     No. 3 is a sample in which the MA structure was coarsened because the finish rolling end temperature was too high. As a result, the value of TS×λ was small, so that the stretch-flangeability could not be improved. 
     No. 4 is a sample in which the MA structure was coarsened because the rolling reduction at the final stand during the finish rolling was too high and exceeded the range defined in the present invention. As a result, the value of TS×λ was small, so that the stretch-flangeability could not be improved. Also, the value of TS×VDA was small, so that the crashworthiness could not be improved. 
     No. 5 is a sample in which the MA structure was coarsened because the rolling reduction at the final stand during the finish rolling was too low and was below the range defined in the present invention. As a result, the value of TS×λ was small, so that the stretch-flangeability could not be improved. 
     No. 7 is a sample in which a ferrite amount within the range defined in the present invention could not be ensured because the soaking was carried out at a high temperature exceeding the temperature region of 800° C. or higher and lower than the Ac 3  point. As a result, the value of TS×EL was small, so that the ductility could not be improved. 
     No. 8 is a sample in which the value of V MA /V γ  was too large because the cooling stop temperature T after the soaking was too high and exceeded the temperature region of 50° C. or higher and the Ms point or lower and because the reheating holding was not carried out after the cooling. As a result, the value of TS×VDA was small, so that the crashworthiness could not be improved. 
     No. 10 is a sample in which a predetermined amount of retained γ and the MA structure could not be ensured because the cooling stop temperature T after the soaking was below 50° C., so that the value of V MA /V γ  was small and below the defined range. As a result, the value of TS×EL was small, so that the ductility could not be improved. 
     No. 11 is a sample in which the MA structure was coarsened and the value of V MA /V γ  was too large because the rolling reduction at the final stand during the finish rolling was too high and exceeded the range defined in the present invention and because the reheating holding was not carried out after the cooling. As a result, the value of TS×VDA was small, so that the crashworthiness could not be improved. 
     No. 14 is a sample in which the MA structure was coarsened because the reheating holding time was too short. As a result, the value of TS×λ was small, so that the stretch-flangeability could not be improved. Further, the MA structure was generated excessively. As a result, the value of TS×EL was small, so that the ductility could not be improved. Also, the value of V MA /V γ  was too large. As a result, the value of TS×VDA was small, so that the crashworthiness was deteriorated. 
     Nos. 16 and 37 are samples in which the MA structure was coarsened because the average heating rate after the coiling was too small. As a result, the value of TS×λ was small, so that the stretch-flangeability could not be improved. 
     No. 19 is a sample in which the MA structure was coarsened because the cooling stop temperature T after the soaking was too high and exceeded the temperature region of 50° C. or higher and the Ms point or lower. As a result, the value of TS×λ was small, so that the stretch-flangeability could not be improved. 
     No. 25 is a sample in which decomposition of austenite occurred and a predetermined amount of retained γ and the MA structure could not be ensured because the reheating temperature carried out after the cooling was too high. As a result, TS was small. Also, the value of TS×κ was small, so that the stretch-flangeability could not be improved. Further, the value of TS×VDA was small, so that the crashworthiness could not be improved. 
     Nos. 27 and 38 are samples in which ferrite was excessively generated because the average cooling rate after the soaking was too small. As a result, TS was small. Also, the value of TS×λ was small, so that the stretch-flangeability could not be improved. 
     No. 29 is a sample in which the MA structure was coarsened because the coiling temperature was too high. As a result, the value of TS×λ was small, so that the stretch-flangeability could not be improved. 
     No. 33 is a sample in which the C amount was too small, so that a retained γ amount within the range defined in the present invention could not be ensured, and the value of V MA /V γ  was so large as to exceed the range defined in the present invention. As a result, the value of TS×EL was small, so that the ductility was deteriorated. 
     No. 34 is a sample in which the Si amount was too small, so that a ferrite amount within the range defined in the present invention could not be ensured. As a result, the value of TS×EL was small, so that the ductility was deteriorated. 
     No. 35 is a sample in which the Mn amount was too small, so that the hardenability was insufficient, and ferrite was excessively generated. As a result, TS was low. Further, the value of TS×λ was small, so that the stretch-flangeability was deteriorated. 
     REFERENCE SIGNS 
     
         
           1  Heating step 
           2  Soaking step 
           3  Cooling step 
           4  Reheating holding step 
           5  Cooling stop temperature