| 摘要 |
PROBLEM TO BE SOLVED: To provide a hot rolled steel sheet for automobile use, having superfine grains, increased in strength, excellent in press formability, and also excellent in press formability in a laser jointed part, and its manufacturing method. SOLUTION: A steel, having composition which contains, by weight, 0.01-0.2% C, <=2.0% Si, <=2.1% Mn, <=0.5% P, 0.03-0.3% Ti, and <=0.1% Al and in which carbon equivalent Ceq is regulated to <=0.35, is used. This steel is repeatedly subjected to reduction in the dynamic recrystallization low temperature region. After hot rolling, rapid cooling is performed and then skin pass rolling is applied. By this procedure, the resultant steel sheet can be provided with a structure consisting of ferrite with <4μm average grain size and a fine secondary phase dispersed into island-like state. Further, surface roughness Ra is regulated to 0.8-1.5μm, and press formability including that in a laser jointed part can be improved.
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