HOT ROLLED STEEL SHEET FOR HIGH STRENGTH COLD ROLLED STEEL SHEET, METHOD FOR PRODUCING THE SAME, AND METHOD FOR PRODUCING HIGH STRENGTH COLD ROLLED STEEL SHEET

摘要

PROBLEM TO BE SOLVED: To produce a high strength cold rolled steel sheet in which variation of the material in the longitudinal direction and variation in sheet thickness are reduced, thereby; to achieve prevention of trouble and the improvement of yield upon production; to improve workability ad operability upon press working; and further to improve the quality of a product after the press working.

SOLUTION: The hot rolled steel sheet for a high strength cold rolled steel sheet has a microstructure where the total area fraction of bainite and/or bainitic ferrite is ≥70%, the area fraction of polygonal ferrite is ≤30%, and the average crystal grain size of the polygonal ferrite is ≤10 μm.

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