METHOD FOR PRODUCING HIGH-Si COLD ROLLED STEEL SHEET HAVING EXCELLENT CHEMICAL CONVERSION TREATABILITY

摘要

Disclosed is a method for producing a high-Si cold rolled steel sheet having favorable chemical conversion treatability, a tensile strength of at least 590 MPa, TS×EL of at least 18,000 MPa°%, and excellent workability. The disclosed method includes: a step for using a direct-fired burner (A) having an air ratio of no more than 0.89 to heat the temperature of the cold rolled steel sheet to a temperature range of at least 300°C and less that Ta°C when raising the temperature of the cold rolled steel sheet, which has an elemental composition comprising 0.05-0.3 mass% C, 0.6-3.0 mass% Si, 1.0-3.0 mass% Mn, no more than 0.1 mass% P, no more than 0.05 mass% S, 0.01-1 mass% Al, no more than 0.01 mass% N, the remainder being Fe and unavoidable impurities; and a step for, continuing thereafter, using another direct-fired burner (B) having an air ratio of at least 0.95 to heat the temperature of the cold rolled steel sheet to a temperature range of at least Ta°C and less than Tb°C; and continuing thereafter, the cold rolled steel sheet is soaked/annealed in a furnace having a gas composition that is 1-10 vol% H2, the remainder being N2, and having a dew point of no greater than -25°C. 450°C = Ta°C = 550°C, 650°C = Tb°C = 800°C