| 摘要 |
PROBLEM TO BE SOLVED: To prevent surface ruggedness defect after press forming, occurring in a cold rolled extra low carbon steel sheet prepared by continuous annealing. SOLUTION: At the time of forming a steel product while providing a composition containing, by weight, <=0.01% C, (48/14)×N to 0.1% Ti, and <=0.1% Nb and satisfying C*>0 and C*=C-(12/48)×Ti*-(12/93)×Nb [where Ti*= Ti-(48/14)×N-(48/32)×S is satisfied when Ti-(48/14)×N-(48/32)×S>=0, and Ti*=0 is satisfied when Ti-(48/14)×N-(48/32)×S<0], cooling is carried out at <=20 deg.C/s cooling rate through the temp. range from T1 ( deg.C) to T2 ( deg.C) represented by T1 deg.C=100×logC*+650 and T2 ( deg.C)=2×(YS-120) [where YS is the yield point (N/mm<2> ) of the steel sheet after annealing] in the course of cooling after soaking heating at recrystallization annealing. A cast slab of extra low carbon steel is hot-rolled, cold-rolled, and continuously annealed, by which the steel sheet product is produced.
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