METHOD COMPRISING HOT ROLLING, COOLING, COILING, COLD ROLLING AND ANNEALING PROCESSES TO MANUFACTURE HIGH CARBON HOT ROLLED STEEL SHEET AND COLD ROLLED STEEL SHEET, AND HIGH CARBON HOT ROLLED STEEL SHEET AND COLD ROLLED STEEL SHEET MANUFACTURED THEREBY

摘要

PURPOSE: To provide high carbon hot rolled steel sheet and cold rolled steel sheet having excellent ductility and stretch-flange formability by enabling perforated cross section not to be broken easily, maintaining tensile strength of not less than 440 MPa and satisfying a burring ratio =70% and elongation of 35% or more, and methods for manufacturing the high carbon hot rolled steel sheet and cold rolled steel sheet. CONSTITUTION: The high carbon hot rolled steel sheet substantially comprises 0.20 to 0.48 mass% of C, 0.1 mass% or less of Si, 0.20 to 0.60 mass% of Mn, 0.02 mass% or less of P, 0.01 mass% or less of soluble Al, 0.005 mass% or less of N, 0.001 to 0.005 mass% of B, 0.05 to 0.30 mass% of Cr and the balance being Fe and inevitable impurities and has ferrite structure having an average particle diameter of 6 m or less and carbide having an average particle diameter of 0.1 m to less than 1.2 m, wherein the ferrite structure comprises ferrite particles in which carbide is not substantially contained, and a volume ratio of the ferrite particles in which carbide is not substantially contained is 10% or less. The cold rolled steel sheet substantially comprises 0.20 to 0.58 mass% of C, 0.1 mass% or less of Si, 0.20 to 0.60 mass% of Mn, 0.02 mass% or less of P, 0.01 mass% or less of soluble Al, 0.005 mass% or less of N, 0.001 to 0.005 mass% of B, 0.05 to 0.30 mass% of Cr and the balance being Fe and inevitable impurities and has ferrite structure having an average particle diameter of 6 m or less and carbide having an average particle diameter of 0.1 m to less than 1.2 m, wherein the ferrite structure comprises ferrite particles in which carbide is not substantially contained, and a volume ratio of the ferrite particles in which carbide is not substantially contained is 15% or less.