| 摘要 |
PROBLEM TO BE SOLVED: To provide a method of producing high toughness, high tensile strength steel of≥780 MPa which can be produced on line without adding large quantities of alloy elements and without performing complicated heat treatment, particularly the one which has a high uniform elongation and a low yield ratio. SOLUTION: Steel containing, by mass, 0.05 to 0.3% C, 0.01 to 0.5% Si, 0.2 to 2.5% Mn,≤0.05% P and≤0.05% S is heated at 900 to 1,250°C, and is thereafter subjected to rolling at a cumulative draft of≥50% at an Ar3 point or higher. Directly after that, the steel is subjected to accelerated cooling from the Ar3 point or higher to 600°C at a cooling rate of≥20°C/s. The accelerated cooling is temporarily interrupted, and the surface of the steel is recuperated to the temperature range of (Ms point - 100) to (Ms point + 200)°C. After that, the steel is again subjected to accelerated cooling to≤200°C at a cooling rate of≥20°C/s, so that the high toughness, high tensile strength steel having a tensile strength of≥780 MPa is produced. COPYRIGHT: (C)2004,JPO
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