HOT ROLLED HIGH STRENGTH STEEL SHEET FOR HIGH WORKING EXCELLENT IN FATIGUE CHARACTERISTIC AND THERMOSOFTENING RESISTANCE BY HOT ROLLING CONTINUOUS PROCESS AND ITS PRODUCTION

摘要

PROBLEM TO BE SOLVED: To produce a hot rolled high strength steel sheet for high working usable for automobile parts such as wheels, members or the like and industrial machine parts. ;SOLUTION: This steel sheet contents, as chemical components, by weight 0.03 to 0.20% C, 1.0 to 5.0% Si, 0.5 to 3.5% Mn, 0.003 to 0.050% Al, ≤0.05% P, ≤0.01% S, and the balance Fe. Then, its microstructure is composed of ferrite as primary phases and secondary phases, in which the occupying volume rate of the ferrite is regulated to ≥50%, the fluctuation in the coil is regulated to ≤20%, the ratio of micro-Vickers hardness between the ferrite and the secondary phases (the hardness of the secondary phases/the hardness of the ferrite) is regulated to ≤1.5, the fluctuation in the coil is regulated to ≤0.3, tensile strength ≥400MPa, the tensile strength × the total elongation ≥18,000MPa%, the tensile strength × uniform elongation ≥12,000MPa.%, fatigue limit ratio ≥0.50 and thermosoftening resistance (ΔTS)≤55 MPa are regulated, the fluctuation of the tensile strength in the coil is regulated to 100MPa, the fluctuation of the tensile strength × the total elongation is regulated to 3,000MPa∫N%, and the fluctuation of the tensile strength × the uniform elongation in the coil is regulated to 2,000MPa.%.;COPYRIGHT: (C)1998,JPO