| 摘要 |
PROBLEM TO BE SOLVED: To provide a method of producing a high strength low yield ratio cold rolled steel sheet excellent in weldability and having a TS of 980 MPa or more, an El of 15% or more and a YS of 590 to 800 MPa.SOLUTION: The method of producing the high strength low yield ratio cold rolled steel sheet applies a continuous annealing to a cold rolled steel sheet containing, by mass%, C of 0.05 to 0.12%, Si of 0.2 to 2.5%, Mn of 2.0 to 3.3%, Al of 0.1% or less and Ti of 0.025 to 0.100% and the balance of Fe with inevitable impurity, where the contents of C and M satisfy expression (1). The continuous annealing is applied to the cold rolled steel sheet in such a way that the temperature of the steel sheet is raised in a heating range of 300 to 700°C at an average temperature raising rate satisfying expression (2), heating the steel sheet for 5 to 600 s to a temperature range of 750 to 900°C and then cooling the steel sheet in a temperature range of 750 to 500°C at an average cooling rate satisfying expression (3). 0.36≥(%C)×(%Mn)≥0.13 (1), the average temperature rasing rate(°C/s)≥0.24/(%Ti) (2), 75≥the average cooling rate(°C/s)≥100×(%Ti)+13 (3). |
