| 摘要 |
A semi-manufactured steel material has a chemical composition including, by mass%, C: 0.055% to 0.15%, Si: not more than 0.2%, Mn: not more than 1.3%, P: not more than 0.03%, S: not more than 0.007%, A1: not more than 0.1%, N: not more than 0.01%, and Ti: 0.14% to 0.30%, the balance comprising Fe and inevitable impurities. In the chemical composition, 1.0 ‰¤ ([C]/12)/([Ti*]/48) is satisfied ([C], [S], [N] and [Ti]: contents (mass%) of the respective elements, and [Ti*] = [Ti] - 3.4 × [N] - 1.5 × [S]), and the contents of niobium and boron as impurities are limited to Nb: less than 0.03% and B: less than 0.0005%. The semi-manufactured steel material is heated to a temperature T (°C) that is not less than 1150°C and satisfies [Ti*] < 10 {-700/(T+ 273) + 2.75} /[C], and is held at the temperature range of not less than 1150°C for 15 minutes or more. The semi-manufactured steel material is then subjected to hot rolling which is performed in such a manner that the total reduction ratio in a temperature range of 980°C and below is not more than 40% and the finish rolling temperature is not less than 880°C. The steel sheet is cooled within 3 seconds after the completion of finish rolling, at a cooling rate of 40°C/sec. to 200°C/sec., and is coiled at a temperature of 500°C to 680°C, thereby producing a high strength hot rolled steel sheet having an excellent tensile strength of not less than 850 MPa and a small anisotropy in mechanical properties. |
