| 摘要 |
The invention provides methods for manufacturing high-Si cold rolled steel sheets which achieve good chemical conversion properties and exhibit a tensile strength of not less than 590 MPa and excellent workability with TS x EL being not less than 18000 MPa·%. A method includes a step of heating a cold rolled steel sheet that has a chemical composition containing C at 0.05 to 0.3 mass%, Si at 0.6 to 3.0 mass%, Mn at 1.0 to 3.0 mass%, P at not more than 0.1 mass%, S at not more than 0.05 mass%, Al at 0.01 to 1 mass%, and N at not more than 0.01 mass%, with the balance being represented by Fe and inevitable impurities, with a direct flame burner (A) having an air ratio of not more than 0.89 when the temperature of the cold rolled steel sheet that is being increased is in the temperature range of not less than 300°C and less than Ta°C, a step of subsequently heating the cold rolled steel sheet with a direct flame burner (B) having an air ratio of not less than 0.95 when the temperature of the cold rolled steel sheet is in the temperature range of not less than Ta°C and less than Tb°C, and subsequently soak-annealing the cold rolled steel sheet in a furnace having an atmospheric gas composition which has a dew point of not more than -25°C and contains 1 to 10 volume% of H 2 and the balance of N 2 .
450°C ‰¤ Ta°C ‰¤ 550°C and 650°C ‰¤ Tb°C ‰¤ 800°C. |
