| 摘要 |
<P>PROBLEM TO BE SOLVED: To achieve a high-tension, cold-rolled steel sheet excellent in ductility, work hardening property and stretch flange formability. <P>SOLUTION: The cold-rolled steel sheet comprises, by mass%, >0.020% but <0.30% C, >0.10% but <3.00% Si and >1.00% but ≤3.50% Mn, optionally further comprises one or more chosen from the group consisting of Ti, Nb, V, Cr, Mo, B, Ca, Mg, REM and Bi, and has a metal structure comprising a phase produced by low-temperature transformation as a main phase and residual austenite as a second phase. The residual austenite accounts for >4.0% but <25.0% of the entire structure by a volume rate and has an average grain size of <0.80 μm, wherein the number density N<SB POS="POST">R</SB>of residual austenite grains with a grain size of ≥1.2 μm is ≤3.0×10<SP POS="POST">-2</SP>pieces/μm<SP POS="POST">2</SP>and the average grain size of grains having bcc structure and grains having bct structure surrounded by a grain boundary with an orientation difference angle of ≥15° is ≤7.0 μm. <P>COPYRIGHT: (C)2013,JPO&INPIT |
