| 摘要 |
PROBLEM TO BE SOLVED: To obtain a hot-rolled steel sheet excellent in workability by allowing it to have a specified compsn., also allowing the contents of specified elements to have a specified relation and allowing it to have a metallic structure composed of bainite of a specified volume fractional rate as the main phase and at least one kind among pearlite, ferrite or the like as a 2nd phase, and in which the bainitic phase has the specified average grain size. SOLUTION: The steel sheet having a compsn. contg., by weight, 0.05 to 0.20% C, 0.05 to 0.50% Si, 1.0 to 3.5% Mn, <=0.05% P, <=0.01% S, 0.005 to 0.30% Nb, 0.001 to 0.10% Ti, 0.01 to 1.0% Cr, <=0.10% Al, and the balance Fe with inevitable impurities, and in which each content of Si, P, Cr, Ti, Nb and Mn satisfies the relation expressed by the inequality is prepd. Moreover, its metallic structure is the one composed of bainite of 60 to <90% in a volume fractional ratio as the main phase and at least one kind among pearlite, ferrite, residual austenite and martensite as a 2nd phase, and in which the average grain size of the bainitic phase is controlled to <4μm.
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