| 摘要 |
PROBLEM TO BE SOLVED: To provide a hot rolled steel sheet and a cold rolled steel sheet having excellent formability and hardenability, i.e., whose hardenability is good, and to which high tensile strength can be imparted after quenching. SOLUTION: The steel sheet has a composition containing, by mass, 0.05 to 0.50% C, <=0.1% Si, <0.80% Mn, <=0.02% P, <=0.01% S, 0.05 to 2.5% Cr, 0.0005 to 0.0040% B, <=0.0050% N, <=0.04% Ti and <=0.1% sol-Al, and the balance iron with inevitable impurities. In the case of the hot rolled steel sheet, the contents of C, Mn and Cr satisfy the inequality of 150>=(11800 Cr%+10600 Mn%-2250)×C%-1050>=0...(1), and in the case of the cold rolled steel sheet, the contents of C, Mn and Cr satisfy the inequality of 150>=(11800 Cr%+10600 Mn%-1300)×C%-1350>=0...(2).
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