| 摘要 |
PROBLEM TO BE SOLVED: To provide a high tension cold-rolled steel sheet having fine grain structure and excellent mechanical characteristics, particularly, fatigue characteristic. SOLUTION: After heating a steel blank containing in the ranges satisfying the following inequlities (1), (2), (3) respectively, particularly C, Si, Mn, Ni. Ti and Nb in the steel components to≥1,200°C, hot-rolling is completed at≥Ar<SB>3</SB>point. Successively, this steel is wound up in the temperature range from 650 to 750°C and after pickling, the cold-rolling is applied. Thereafter, recrystallizing annealing is applied at predicted value temperature of A<SB>3</SB>to (A<SB>3</SB>+ 30)(°C) and then, the annealed steel is cooled to predicted value temperature A<SB>1</SB>+ 50°C at≥5°C/s cooling speed and successively, after holding it for≥60 sec in the temperature range of (A<SB>1</SB>+ 50) to (A<SB>1</SB>+ 20)°C, this steel is cooled at least to Ms point at rate of≥3°C/s. The inquality (1) is : 637.5 + 4930(Ti<SP>*</SP>+ (48/93)×[%Nb]) > A<SB>1</SB>, wherein Ti<SP>*</SP>is used as effective Ti content for precipitating TiC. The inquality (2) is : A<SB>3</SB>< 860. The inquality (3) is : [%Mn] + [%Ni]≥1.3. COPYRIGHT: (C)2004,JPO
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