High-strength cold-rolled steel sheet, hot-dipped galvanized steel sheet, alloyed hot-dipped galvanized steel sheet, and methods for producing of these
A high-strength cold-rolled steel sheet according to one aspect of the present invention contains, in % by mass: C: 0.070 to 0.140%; Si: 0.80 to 1.80%; and Mn: 1.80 to 2.80%, with the balance being iron and unavoidable impurities; in area ratio in a structure observed with a scanning electron microscope, bainite, tempered martensite, and hard phases composed of at least one of cementite and MA made up of quenched martensite and retained austenite combined together account for 85% or more in total, the hard phase that has a breadth of 0.4 μm or less and a length of 1.2 μm or more accounts for 0.5% or more, the hard phase that has a breadth of 1.2 μm or more accounts for 5.0% or less, and structures other than the bainite, the tempered martensite, and the hard phase account for 15% or less; and in volume ratio measured by X-ray diffractometry, retained austenite accounts for 3.0% or more and 7.0% or less.
1 . A cold-rolled steel sheet comprising, in % by mass:
C: 0.070 to 0.140%;
Si: 0.80 to 1.80%; and
Mn: 1.80 to 2.80%,
in area ratio in a structure observed with a scanning electron microscope,
bainite, tempered martensite, and hard phases composed of at least one of cementite and MA made up of quenched martensite and retained austenite combined together account for 85% or more in total,
the hard phase that has a breadth of 0.4 μm or less and a length of 1.2 μm or more accounts for 0.5% or more,
the hard phase that has a breadth of 1.2 μm or more accounts for 5.0% or less, and
structures other than the bainite, the tempered martensite, and the hard phases account for 15% or less; and
in volume ratio measured by X-ray diffractometry, retained austenite accounts for 3.0% or more and 7.0% or less,
wherein the cold-rolled steel sheet has a tensile strength of 900 MPa or more.
2 . The cold-rolled steel sheet according to claim 1 , further comprising, in % by mass:
one or more members selected from the group consisting of:
Al: 0.015 to 0.60%,
Cr: more than 0% and 0.60% or less,
Ti: 0.010 to 0.040%,
B: 0.0015 to 0.0040%,
Cu: more than 0% and 0.30% or less,
Ni: more than 0% and 0.30% or less,
Mo: more than 0% and 0.30% or less,
V: more than 0% and 0.30% or less,
Nb: more than 0% and 0.040% or less, and
Ca: more than 0% and 0.0050% or less.
3 . The cold-rolled steel sheet according to claim 1 , further comprising as unavoidable impurities, in % by mass:
P: more than 0% and 0.015% or less, and
S: more than 0% and 0.0050% or less.
4 . The cold-rolled steel sheet according to claim 1 , having a stretch-flangeability (hole expansion ratio λ) of 60% or more.
5 . The cold-rolled steel sheet according to claim 1 , having a VDA bending angle of 100° or more.