| 摘要 |
PURPOSE:To obtain a cold rolled steel sheet excellent in press formability as well as in spot weldability, by incorporating respectively prescribed amounts of C, Si, Mn, P, S, Ti, Nb, B and Al and regulating surface roughness so that it satisfies either of two specific conditions. CONSTITUTION:The titled cold-rolled steel sheet which has a composition consisting of, by weight, <=0.04% C, <=0.1% Si, <=0.5% Mn, <=0.025% P, <=0.025% S, 0.01-0.04% Ti, 0.001-0.010% Nb, 0.0001-0.0010% B, 0.01-0.10% Al, and the balance Fe and in which surface roughness is regulated so that it satisfies either of the two undermentioned conditions represented by (I) and (II) is provided. In this invention, the surface roughness of a steel sheet is increased, the contact area between steel sheets is reduced, and electric resistance at the time of welding is increased, by which the value of welding current is reduced. Moreover, as to the component system of the steel sheet, a dead-soft C-type steel is used as a principal component and, besides the addition of Ti and Nb, trace amounts of B are further added to refine the structure in a spot weld zone, and, as a result, joining strength can be improved to a greater extent.
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