| 摘要 |
PROBLEM TO BE SOLVED: To provide a high tensile, hot rolled steel plate excellent in press processability and strain aging hardening characteristics there the tensile strength is extremely increased by a heat treatment at relatively low temperatures after press forming, and to provide its manufacturing method. SOLUTION: A steel slab having a composition in which C, Si and Mn are made to <=0.15% C, <=2.0% Si and <=3.0% Mn and P, S, Al and N are respectively adjusted and which contains 0.5-3.0% Cu or <=2.0%, in total, of one or more elements among Cr, Mo and W is hot rolled while regulating FDT to a temperature not lower than the Ar3 transformation point. After the completion of rolling, the resultant steel plate is cooled at >=5 deg.C/s cooling rate to the temperature region between the Ar3 and the Ar1 transformation point, air cooled or cooled slowly in this temperature region, cooled at a rate of >=5 deg.C/s again and coiled at <=550 deg.C, by which a composite structure containing ferrite and >=2% martensite by area ratio is formed. By this method, the steel plate excellent in press formability and having superior strain aging hardening characteristics as high as to give >=80 MPaΔTS can be obtained.
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