| 摘要 |
PROBLEM TO BE SOLVED: To provide a steel sheet which has excellent press formability before hardening and rapid heating/hardening property and in which martensitic structure can be obtained after hardening even in the case of low heating temperature and also to provide its manufacturing method. SOLUTION: The steel sheet has a chemical composition consisting of, by mass, 0.06 to 0.25% C,≤0.3% Si, 0.5 to 1.5% Mn,≤0.03% P,≤0.02% S,≤0.1% sol.Al,≤0.01% N and the balance Fe with impurities and also containing, if necessary, proper amounts of one or more elements among B, Ti, Cr, Mo, W and Ni and also has a structure in which a main phase is composed of ferrite phase and a second phase is contained, and further, the area ratio of a band-like structure composed of the above second phase is made to≤6.0%. The steel sheet can be manufactured by hot rolling a steel ingot or a steel slab having the above chemical composition, carrying out cold rolling at≥55% cold rolling rate (CR) and then annealing the resultant steel sheet at a soaking temperature (TA) satisfying inequality A<SB>c1</SB><(TA)<A<SB>c1</SB>+(A<SB>c3</SB>-A<SB>c1</SB>)(CR-10)/100. COPYRIGHT: (C)2007,JPO&INPIT
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