| 摘要 |
PROBLEM TO BE SOLVED: To provide a high-carbon hot-rolled steel sheet excellent in stretch flange formability and further excellent in ductility. SOLUTION: The high-carbon hot-rolled steel sheet 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, 0.005 to 0.05% Ti, 0.0005 to 0.003% B and 0.05 to 0.3% Cr, and satisfying Ti-(48/14)N≥0.005, 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 carbide is 0.1 to <1.20μm, and the volume ratio of ferrite grains substantially free from carbide is≤5%. In the manufacturing method, the steel having the above composition is hot-rolled under the conditions where the finishing temperature is (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 Ac<SB>1</SB>point. COPYRIGHT: (C)2005,JPO&NCIPI
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