| 摘要 |
A high strength cold rolled steel sheet having excellent stretch flangeability is provided. A steel having a composition containing C: 0.050% to 0.090%, Mn: 1.5% to 2.0%, Ti: 0.005% to 0.050%, and Nb: 0.020% to 0.080% on a percent by mass basis is subjected to a hot rolling step, a cold rolling step, and an annealing step. Here, the annealing step is specified to be a step having a maximum achieving temperature: 800°C to 900°C, two-step heating, and two-step cooling. The two-step heating includes first-step heating to heat to a temperature range of (maximum achieving temperature - (10°C to 50°C)) at an average temperature raising rate: 0.5°C to 5.0°C/s and second-step heating in which the temperature raising time from the that temperature range to the maximum achieving temperature is specified to be 30 to 150 s. Meanwhile, the two-step cooling includes first-step cooling to cool from the maximum achieving temperature at a cooling rate of 10°C to 40°C/s and second-step cooling to cool to a temperature range of 400°C to 500°C at a cooling rate of 0.2 to 0.8 times the first-step cooling rate for a cooling time of 0.2 to 0.8 times the total cooling time. After the cooling is finished, staying is performed in the second-step cooling stop temperature range for 100 to 1,000 s. Consequently, a microstructure including appropriate microstructure fractions of ferrite, bainite, martensite, and retained ³ can be established and a high strength cold rolled steel sheet having excellent stretch flangeability is produced. |
