| 摘要 |
PROBLEM TO BE SOLVED: To provide a cold rolled steel sheet for galvannealing and a galvannealed steel sheet using a Ti- and Nb-added steel sheet as a basis-material sheet and causing no striped pattern occurring in a galvannealed steel sheet, and their manufacturing method. SOLUTION: A slab, having a composition containing 0.005-0.0030% C, 0.0005-0.0040% N, 0.005-0.05% Ti and <=0.05% Nb, is used. The slab is heated under the condition of 1,000-<1,100 deg.C heating temperature and <=3 h in-furnace time at >=1,000 deg.C. Then hot rolling is performed while regulating the surface temperature on the inlet side of a first stand at finish rolling to (Ar3 transformation point -100 deg.C) to 1,000 deg.C and also regulating finish rolling finishing temperature to a value not lower than the Ar3 transformation point, followed by acid pickling and cold rolling. In this way, the ratio between the average grain size r (nm) of precipitates and the area ratio f (%) of the precipitates, r/f, in the surface layer part at a depth of 1.0-3.0μm from the surface is regulated to >=2,000 (nm/%).
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