| 摘要 |
PROBLEM TO BE SOLVED: To obtain a high tensile cold-rolled steel sheet having excellent toughness and shape-fixability while securing≥750 N/mm<SP>2</SP>class high strength by using a low component base steel. SOLUTION: A steel slab having a composition by mass of 0.08 to 0.18% C, 1.00 to 2.0% Si, 1.5 to 3.0% Mn,≤0.03% P,≤0.005% S, and 0.01 to 0.1% Al and 1.05 to 1.10 Mn degree of segregation defined by the formula (1) is hot-rolled. After subsequent cold-rolling, this rolled steel is heated to 750 to 870°C for≥60 sec in two-phase range or single-phase range at a continuous annealing line. Mn degree of segregation=(Mn concentration at the central part of the slab-the base Mn concentration)/(Base Mn concentration) (1). After cooling in the temperature zone of 720 to 600°C at≤10°C/s average cooling rate, this steel sheet is cooled to 350 to 460°C at≥10°C/s average cooling rate, and after keeping for 30 sec to 20 min in this range, this steel sheet is cooled to room temperature to obtain a five-phase structure of polygonal ferrite + acicular ferrite+ bainite + retained austenite + martensite. COPYRIGHT: (C)2005,JPO&NCIPI
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