HOT-ROLLED STEEL SHEET HAVING EXCELLENT STRAIN AGING PROPERTY AND METHOD FOR PRODUCING THE SAME

摘要

PROBLEM TO BE SOLVED: To provide a hot rolled steel sheet which has excellent press workability, and exhibits excellent strain aging property, that is, can be markedly increased in its tensile strength by a heat treatment, after press forming, at a temperature around that for a conventional baking finish, and thus can be suitably used as a steel sheet for an automobile, to provide a hot-rolled steel sheet in which fatigue characteristics are improved in addition to the strain aging property, and to provide a method for producing the same.;SOLUTION: A hot-rolled steel sheet which comprises, by mass, 0.01 to 0.2% C, ≤2.0% Si, ≤3.0% Mn, ≤0.1.% P, ≤0.02% S, ≤0.1% Al and ≤0.02% N, and the balance Fe with inevitable impurities, and has a nontempered martensite phase as the main phase and a ferrite phase as a second phase in the area ratio of 1 to 30%, and also, the average grain size of the ferrite phase is 20 μm or less.;COPYRIGHT: (C)2007,JPO&INPIT