Patent ID: 8911574
Filing Date: 2014-12-16
Classification: C21D,C22C,C23G

Abstract:
1. A method for manufacturing high-Si cold rolled steel sheets having excellent chemical conversion properties which comprises: 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 use of 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.; subsequently heating the cold rolled steel sheet with use of 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 with the proviso that 450° C.≦Ta° C.≦550° C. and 650° C.≦Tb° C.≦800° C.