HIGH STRENGTH COLD ROLLED STEEL SHEET REDUCED IN DISPERSION IN MECHANICAL CHARACTERISTIC, AND ITS MANUFACTURING METHOD

摘要

PROBLEM TO BE SOLVED: To provide a high strength cold rolled steel sheet reduced in dispersion in mechanical properties and its manufacturing method.;SOLUTION: The high strength cold rolled steel sheet has a composition which consists of 0.06 to 0.15% C, 0.5 to 1.5% Si, 1.5 to 3.0% Mn, ≤0.05% P, ≤0.01% S, 0.5 to 1.5% Al and the balance Fe with inevitable impurities and in which the value of A, defined by A=Si+9×Al, satisfies 6.0≤A≤20.0. Moreover, the steel sheet has a structure which is composed of a dual-phase structure of ferrite and martensite and in which the area ratio of the ferrite is 40 to 90%. When manufacturing this steel sheet, recrystallization annealing/tempering treatment is carried out by carrying out: holding at a temperature ranging from the Ac₁ point and the Ac₃ point for ≥10 sec; cooling down to 500 to 750°C at ≤20°C/sec cooling rate; rapid cooling down to ≤100°C at ≥100°C/sec cooling rate; and tempering at 300 to 500°C.;COPYRIGHT: (C)2007,JPO&INPIT