| 摘要 |
A hot rolled steel sheet, which has a chemical composition, in mass %, that C: 0.01 to 0.2 %, Si: 0.01 to 2 %, Mn: 0.1 to 2 %, P: 0.1 % or less, S: 0.03 % or less, Al: 0.001 to 0.1 %, N: 0.01 % or less, and the balance; Fe and inevitable impurities, has a microstructure being composed mainly of a unifo rm continuous cooling transformation structure and having an average grain diameter of greater than 8 .mu.m and 30 .mu.m or less; and a method for producing the hot rolled steel sheet which comprises a step of subjecting a steel billet having the above composition to rough rolling, to make a rough bar, a step of subjecting the rough bar to a finish rolling under a conditio n involving an ending temperature of (the Ar3 transformation temperature there of + 50~C) or higher, to prepare a rolled material, and a step comprising starting to cool the rolled material at the Ar3 transformation temperature thereof or higher after the elapse of 0.5 sec or more from the end of the finish rolling, cooling it at a cooling rate of 80~C/sec or higher at least in a temperature region of from the Ar3 transformation temperature to 500~C, an d further cooling it to a temperature region of 500~C or lower, followed by winding it up as a hot-rolled steel sheet. |
