| 摘要 |
<p>PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel sheet of a relatively low alloy which has excellent workability, and to provide a production method therefor. SOLUTION: Steel containing 0.06 to 0.25% C, <=2% Si, 0.5 to 2.5% Mn, and <=0.1% sol.Al is hot-rolled so as to be finished at an Ar3 point or higher, and, within 2 sec, cooling is started, the cooling rate is controlled to >150 deg.C/s, and the cooling is stopped at <=350 deg.C to produce a high strength hot rolled steel sheet. Alternatively, steel, further containing one or more metals selected from Ti, Nb, V, and Zr by 0.01 to 0.2% in total, or containing one or more metals selected from <=1% Cr and <=1% Mo can be used. It is also possible that a crude bar or the stock to be rolled is subjected to induction heating at the inlet side of a finish rolling mill or between stands, the variation width of the cooling stopping temperature is controlled to <=60 deg.C, and the draft in the final stand is controlled to <30%. The high strength hot rolled steel sheet has a structure essentially consisting of martensite, and in which the variation of tensile strength in the coil is±8% from a central value.</p> |
