| 摘要 |
PURPOSE:To manufacture steel sheet having superior tensile strength, ductility and secondary workability, by heat treating steel sheet having a suitable compsn. under a specific condition. CONSTITUTION:Steel sheet contg. by weight 0.12-0.70%, 0.4-1.8% Si, 0.2-2.5% Mn, 0.01-0.07% Sol, Al, <0.02% T.N is heated to >=AC3 temp. at 1-100 deg.C/sec rate, held and annealed for <=3min, favorably <=40sec. Next, the sheet is cooled up to 350-500 deg.C range at 1-200 deg.C/sec rate, aging treated at the temp. for 30sec-10min, thereafter cooled to 150-250 deg.C range at <=50 deg.C/sec rate, further cooled thereafter to normal temp. by arbitrary method, without being limited especially. The title sheet having composite structure composed of at least 1-50% ferrite phase and >=10% residual austenitic phase is obtd.
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