| 摘要 |
PROBLEM TO BE SOLVED: To improve the material quality at the head and the tail end of a high yield ratio type hot rolled high strength steel plate excellent in formability and to remove the dispersion of material quality in a coil. SOLUTION: A steel slab, which has a chemical composition containing, by weight ratio, 0.16-<0.30% C, 0.5-3.0% Si, 0.5-3.0% Mn, <=0.020% P, <=0.010% S, 0.005-0.100% Al, and Fe as essential components and satisfying Si+Mn=>1.5 to 6.0%, is rolled by a continuous hot rolling process. The resultant steel plate has a microstructure constituted of three phases of ferrite, bainite, and retained austenite, and, in this steel plate, the ratio between the volume occupancy of ferrite (VF) and the grain size of ferrite (dF) is >=7 and fluctuation in a coil is <4 and, further, the volume occupancy of retained austenite of <=2μm is >=5% and fluctuation in a coil is <5%. As characteristics, this steel plate has >=60% yield ratio(YR), >=2000(kgf/mm<2> .%) strength- ductility balance [(tensile strength)×(total elongation)], >=1.1, spreading ratio (d/d0 ), and >=10% uniform elongation, and further, the fluctuation of TS in a coil, the fluctuation of TS×TEl, and the fluctuation of d/d0 are regulated to <101kgf/mm<2> ), <500(kgf/ mm<2> .%), and <0.2, respectively.
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