| 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 an d inevitable impurities, subjecting the resultant steel to hot rolling so as f or the sheet to have a prescribed sheet thickness, without cooling it to its Ar 3 transformation point or lower or after re-heating it to its Ac3 transformati on 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 th e 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 an d 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 o f the steel sheet being central in the thickness direction.
|
