| 摘要 |
high-strength cold-rolled steel sheet having excellent ductility and stretch flangeability includes: a chemical composition consisting, in mass%, C: 0.06 to 0.3'36, Si: 0.4 to 2.5%, Mn: 0.6 to 3.5%, P: at most 0.1%, S: at most 0.05%, Ti: 0 to 0.08%, Nb: 0 to 0.04%, a total content of Ti and Nb: 0 to 0.10%, sol.Al: 0 to 2.0%, Cr: 0 to l%, Mo: 0 to 0.3%, V: 0 to 0.3%, B: 0 to 0.005%, Ca: 0 to 0.003%, REM: 0 to 0.003% and the remainder of Fe and impurities; a microstructure having a main phase of ferrite which comprising at least 40 area%, and a second phase of a low-temperature transformation phase consisting either or both of martensite and bainite which comprising at least 10 area% in total and retained austenite (y) at least which comprising 3 area%, and an average grain diameter of ferrite having a tilt angle of at least 15° is at most 5.0 µm, an average grain diameter of the low-temperature transformation-produced phase is at most 2.0 µm, an average grain diameter of lump-like retained y having an aspect ratio of less than 5 is at most 1.5 µm, and an area fraction of the lump-like retained y relative to all the retained y is at least 50%. |
