| 摘要 |
PROBLEM TO BE SOLVED: To provide a high tensile strength cold rolled steel sheet excellent in a balance between strength and elongation as well as in surface characteristic and having >=340 MPa tensile strength, and its manufacture. SOLUTION: The steel sheet contains, by weight, <=0.20% C, <=2.0% Si, <=3.0% Mn, <=0.15% P, <=0.05% S, <=0.01% N, 0.010-0.50% Ti, and >=0.0005% Ca and/or metallic REM and also contains Al in an amount satisfying (1) wt.% Ti/wt.% Al>=5 or (2) Al<=0.010 wt.% and wt.% Ti/wt.% Al<5 and in which C, Si, Mn, and P are contained satisfying the relation of 9(wt.%C/12)+2(wt.%Si/28)+3(wt.%Mn/55)+30(wt.%P/31)>=0.05, and further, oxide inclusions with a size of <=50μm are contained by 0.002-0.015 wt.%. At the time of its manufacture, a slab is heated and soaked at >=900 deg.C, finish rolling is finished at >=650 deg.C, coiling is performed at 400 to 750 deg.C, cold rolling is done, and the resultant sheet is subjected to recrystallization annealing at 700 to 920 deg.C.
|
