| 摘要 |
PROBLEM TO BE SOLVED: To provide a method for producing a high toughness and high tensile strength steel in a class of >=590 MPa ss by direct quenching and tempering which can deal with steels of every thickens, and causes little placement of restrictions on the heating and cooling capabilities of equipment used for tempering heat treatment. SOLUTION: A steel having a composition containing, by mass, 0.05 to 0.35% C, 0.05 to 0.6% Si, 0.8 to 2% Mn and 0.002 to 0.07% sol.Al, and the balance substantially Fe is heated, and is thereafter rolled so as to be finished in the temperature range of the Ar3 point or higher. After that, the steel is cooled from a state in which the thickness average temperature of the sheet is >=Ar3 point till the thickness average temperature of the sheet thickness reaches <=500 deg.C at the average cooling rate of >=2 deg.C/sec. Then, the steel sheet is subjected to tempering at the maximum arrival temperature of >=500 deg.C and at the heating rate of >=0.5 deg.C/sec in the temperature range of >=500 deg.C so that the tempering parameter TP is controlled to <=21,000.
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