Method for producing a high-strength steel sheet and high strength cold rolled steel sheet having the above ductility and bendability good tensile maximum stress 900MPa, the method of producing a high strength galvanized steel sheet

摘要

PROBLEM TO BE SOLVED: To provide a high strength steel sheet, and to provide a method for producing the same.;SOLUTION: The high strength steel sheet excellent in ductility and bendability and having the maximum tensile stress of ≥900 MPa is composed of a steel comprising, by mass, 0.07 to 0.25% C, 0.3 to 2.50% Si, 1.5 to 3.0% Mn, 0.005 to 0.09% Ti, 0.0001 to 0.01% B, 0.001 to 0.03% P, 0.0001 to 0.01% S, ≤2.5% Al, 0.0005 to 0.0100% N and 0.0005 to 0.007% O, and the balance iron with inevitable impurities. The steel sheet structure consists essentially of ferrite and includes martensite, and a structural homogeneity index being an index indicating the homogeneity of the steel sheet given by the standard deviation of the Rockwell hardness in the surface of the steel sheet is ≤0.4.;COPYRIGHT: (C)2011,JPO&INPIT