High-strength cold rolled steel sheet having excellent formability, and plated steel sheet
To provide a high-strength cold rolled steel sheet that has well-balanced tensile strength and elongation as well as well-balanced tensile strength and stretch-flangeability, and a plated steel sheet manufactured by plating the steel sheet. The high-strength cold rolled steel sheet contains: 0.10 to 0.28% of C, 1.0 to 2.0% of Si, 1.0 to 3.0% of Mn, and 0.03 to 0.10% of Nb in terms of % by mass, wherein the content of Al is controlled to 0.5 or less, the content of P is controlled to 0.15% or less, and the content of S is controlled to 0.02% or less, and wherein residual austenite accounts for 5 to 20%, bainitic ferrite accounts for 50% or more, and polygonal ferrite accounts for 30% or less (containing 0%), of the entire structure, and wherein a mean number of residual austenite blocks is 20 or more as determined when the random area (15 μm×15 μμm) is observed by EBSP (electron back scatter diffraction pattern).
1 . A high-strength cold rolled steel sheet, which contains:
0.10 to 0.28% of C,
1.0 to 2.0% of Si,
1.0 to 3.0% of Mn, and
0.10% or less (containing 0%) of Nb in terms of % by mass,
wherein the content of Al is controlled to 0.5 or less,
the content of P is controlled to 0.15% or less, and
the content of S is controlled to 0.02% or less,
and wherein residual austenite accounts for 5 to 20%,
bainitic ferrite accounts for 50% or more, and
polygonal ferrite accounts for 30% or less (containing 0%), of the entire structure,
and wherein a mean number of residual austenite blocks is 20 or more as determined when the random area (15 μm×15 μm) is observed by EBSP (electron back scatter diffraction pattern).
2 . The high-strength cold rolled steel sheet according to claim 1 , which contains 0.03 to 0.10% of Nb.
3 . The high-strength cold rolled steel sheet according to claim 2 , which contains 0.05 to 0.10% of Nb.
4 . The high-strength cold rolled steel sheet according to claim 1 , further containing: at least one of the group consisting of
1.0% or less (more than 0%) of Mo,
0.5% or less (more than 0%) of Ni, and
0.5% or less (more than 0%) of Cu.
5 . The high-strength cold rolled steel sheet according to claim 1 , further containing: at least one of the group consisting of
0.003% or less (more than 0%) of Ca, and
0.003% or less (more than 0%) of REM.
6 . The high-strength cold rolled steel sheet according to claim 1 , further containing: at least one of the group consisting of
0.1% or less (more than 0%) of Ti or
0.1% or less (more than 0%) of V.
7 . The high-strength cold rolled steel sheet according to claim 1 , wherein the content of C in the residual austenite is 0.8% or more.
8 . The high-strength cold rolled steel sheet according to claim 1 , wherein the bainitic ferrite accounts for 70% or more of the entire structure.
9 . A plated steel sheet manufactured by plating the cold rolled steel sheet according to claim 1.