| 摘要 |
PROBLEM TO BE SOLVED: To produce a high-carbon hot-rolled steel plate having a tensile strength of≥440 MPa and a hole expansion ratioλof≥70%, further having excellent stretch flange formability satisfying a stretch of≥35%, or further having excellent ductility. SOLUTION: The high-carbon hot-rolled steel plate has a composition comprising 0.20 to 0.48% C,≤0.1% Si, 0.20 to 0.60% Mn,≤0.02% P,≤0.01% S,≤0.1% sol.Al,≤0.005% N, 0.001 to 0.005% B and 0.05 to 0.3% Cr, and the balance iron with inevitable impurities, and a structure in which the average grain size of ferrite is≤6μm, the average grain size of carbides is 0.1 to <1.20μm, and the volume ratio of ferrite grains substantially containing no carbides is≤10%. In a manufacturing method using the same, the steel having the composition is hot-rolled under the conditions where the finishing temperature is (an Ar<SB>3</SB>point-10°C) or higher, the cooling rate is >120°C/s, the cooling stopping temperature is≤620°C and the coiling temperature is≤600°C, and is thereafter annealed at an annealing temperature of 640°C to an Ac<SB>1</SB>point. COPYRIGHT: (C)2006,JPO&NCIPI
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