| 摘要 |
A method for producing a high tensile steel sheet, which comprises casting a steel having a chemical composition, in mass %, that C: 0.02 to 0.18%, Si: 0.05 to 0.5 %, Mn: 0.5 to 2.0 %, Al: 0.005 to 0.1 %, N: 0.0005 to 0.008 %, P: 0.03 % or less, S: 0.03 % or less, P: 0.03 % or less, and the balance: Fe and inevitable impurities, subjecting the resultant steel to hot rolling so as for the sheet to have a prescribed sheet thickness, without cooling it to its Ar3 transformation point or lower or after re-heating it to its Ac3 transformation point or higher, subsequently subjecting the rolled sheet to the direct quenching or accelerated cooling from its Ar3 transformation point or higher, to cool it to 400°C or lower, and then tempering the cooled sheet to such a degree that a portion of the sheet being central in the thickness direction has a highest temperature of 520°C or higher, with an average rate of the temperature rise in the above portion of the sheet of 1°C/s or higher for the range from 460°C to a prescribed tempering temperature being its Ac1 transformation point or lower, by the use of a heating apparatus provided being directly connected to the same production line as that for a roller and an apparatus for direct quenching or accelerated cooling. The above method allows the production of a high tensile strength sheet which is superior to a conventional sheet in the balance between tensile strength and toughness before PWHT and after PWHT and has a tensile strength of 570 MPa (N/mm2) or more, by adopting, in the tempering treatment for a quenched and tempered steel sheet, a specific value as the rate of temperature rise in a portion of the steel sheet being central in the thickness direction.
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