| 摘要 |
The invention provides high-strength hot rolled steel sheets with excellent stretch flangeability which have small variations in mechanical properties in individual coils. The chemical composition includes, by mass%, C: more than 0.010% and not more than 0.06%, Si: not more than 0.3%, Mn: not more than 0.8%, P: not more than 0.03%, S: not more than 0.02%, Al: not more than 0.1%, N: not more than 0.01% and Ti: 0.05 to 0.10%, the balance being Fe and inevitable impurities. Variations in strength from place to place in a coil are decreased by minimally reducing the Si and Mn contents to suppress the occurrence of problems such as segregation. Further, the microstructure of the steel sheets is configured such that a ferrite phase represents an area ratio of not less than 95%, the ferrite crystal grains have an average grain size of not less than 1 µm, and the ferrite crystal grains contain TiC with an average particle size of not more than 7 nm dispersed in the crystal grains. According to these configurations, high-strength hot rolled steel sheets may be obtained which ensure a yield strength of not less than 530 MPa. |
