| 摘要 |
PROBLEM TO BE SOLVED: To produce a cold-rolled steel sheet having≥440 MPa tensile strength and≥0.85 yield ratio. SOLUTION: A steel slab having a component composition containing 0.03-0.15% C, 0.02-0.20% Nb, etc., is reheated to the temperature of 1,100-1,300°C and a hot-rolling is applied at Ar<SB>3</SB>transforming point or higher of the finish temperature and this hot-rolled sheet is wound at the temperature of 700°C or lower. Successively, after pickling and cold-rolling, this cold-rolled steel sheet is heated and held at the temperature of (Ac<SB>3</SB>transforming point) to (Ac<SB>3</SB>transforming point+200)°C for≥30s, cooled at the cooling speed of≥10°C/s and held at 500-700°C for≥60s and again, this steel sheet is cooled. As the other way, after cold-rolling and the reheating is performed and thereafter, the steel sheet is cooled till the rapidly cooling starting temperature at 650-750°C, at 10-30°C/s cooling speed, and successively, the rapid-cooling is performed from the above rapid-cooling starting temperature till the rapid-cooling stopping temperature of≤450°C at the cooling speed exceeding 30°C/s and successively, the reheating is performed to the temperature at 500-700°C and after holding it for≥60s, again, the cooling can be performed. COPYRIGHT: (C)2009,JPO&INPIT
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