High-strength cold-rolled steel sheet
A high-strength cold-rolled steel sheet which is a steel sheet having a tensile strength of 980 MPa or more has a predetermined chemical component composition, a metallographic microstructure of the steel sheet contains, by area ratio, ferrite and granular bainite: 10% or more and 50% or less in total, one or both of upper bainite and lower bainite: 10% or more and 50% or less in total, tempered martensite: more than 0% and 30% or less, retained austenite: 5% or more, and one or more of pearlite, cementite, and martensite: 0% to 10% in total, and an area ratio of the ferrite with respect to a total area ratio of the ferrite and the granular bainite is 25% or less.
1. A high-strength cold-rolled steel sheet comprising, as a chemical composition, by mass %:
C: 0.15% or more and 0.30% or less;
P: 0.040% or less;
S: 0.0100% or less;
N: 0.0100% or less;
O: 0.0060% or less;
one or both of Si and Al: 0.70% or more and 2.50% or less in total;
one or both of Mn and Cr: 1.50% or more and 3.50% or less in total;
Mo: 0% or more and 1.00% or less;
Ni: 0% or more and 1.00% or less;
Cu: 0% or more and 1.00% or less;
Nb: 0% or more and 0.30% or less;
Ti: 0% or more and 0.30% or less;
V: 0% or more and 0.30% or less;
B: 0% or more and 0.0050% or less;
Ca: 0% or more and 0.0400% or less;
Mg: 0% or more and 0.0400% or less;
REM: 0% or more and 0.0400% or less; and
a remainder of Fe and impurities
wherein a metallographic microstructure of the steel sheet contains, by area ratio,
one or both of ferrite and granular bainite: 10% or more and 50% or less in total,
one or both of upper bainite and lower bainite: 10% or more and 50% or less in total,
tempered martensite: more than 0% and 30% or less,
retained austenite: 5% or more, and
one or more of pearlite, cementite, and martensite: 0% to 10% in total,
an area ratio of the ferrite with respect to a total area ratio of the ferrite and the granular bainite is 25% or less, if the area ratio is determined from a cross-section of the steel sheet in a rolling direction or a cross-section in a direction perpendicular to the rolling direction at a magnification of 1,000 to 50,000 times and the structure is observed by electron back scattering diffraction (EBSD), X-ray diffraction, and a scanning electron microscope after corrosion using a nital reagent or a lepera liquid, and
the steel sheet has a tensile strength of 980 MPa or more, when the tensile strength is measured by collecting a test piece in a direction perpendicular to the rolling direction of the steel sheet and by performing a tensile test according to Japanese Industrial Standards JISZ2242.
2. The high-strength cold-rolled steel sheet according to claim 1 ,
wherein the martensite is contained in an amount of 3% or less by area ratio in the metallographic microstructure.
3. The high-strength cold-rolled steel sheet according to claim 1 ,
wherein the chemical composition contains, by mass %, one or more selected from
Mo: 0.01% or more and 1.00% or less,
Ni: 0.05% or more and 1.00% or less,
Cu: 0.05% or more and 1.00% or less,
Nb: 0.005% or more and 0.30% or less,
Ti: 0.005% or more and 0.30% or less,
V: 0.005% or more and 0.30% or less,
B: 0.0001% or more and 0.0050% or less,
Ca: 0.0005% or more and 0.0400% or less,
Mg: 0.0005% or more and 0.0400% or less, and
REM: 0.0005% or more and 0.0400% or less.
4. The high-strength cold-rolled steel sheet according to claim 1 , wherein
a hot-dip galvanized layer is formed on a surface of the steel sheet.
5. The high-strength cold-rolled steel sheet according to claim 1 , wherein
an alloyed hot-dip galvannealed layer is formed on a surface of the steel sheet.
6. The high-strength cold-rolled steel sheet according to claim 2 ,
wherein the chemical composition contains, by mass %, one or more selected from
Mo: 0.01% or more and 1.00% or less,
Ni: 0.05% or more and 1.00% or less,
Cu: 0.05% or more and 1.00% or less,
Nb: 0.005% or more and 0.30% or less,
Ti: 0.005% or more and 0.30% or less,
V: 0.005% or more and 0.30% or less,
B: 0.0001% or more and 0.0050% or less,
Ca: 0.0005% or more and 0.0400% or less,
Mg: 0.0005% or more and 0.0400% or less, and
REM: 0.0005% or more and 0.0400% or less.
7. The high-strength cold-rolled steel sheet according to claim 2 , wherein
a hot-dip galvanized layer is formed on a surface of the steel sheet.
8. The high-strength cold-rolled steel sheet according to claim 3 , wherein
a hot-dip galvanized layer is formed on a surface of the steel sheet.
9. The high-strength cold-rolled steel sheet according to claim 6 , wherein
a hot-dip galvanized layer is formed on a surface of the steel sheet.
10. The high-strength cold-rolled steel sheet according to claim 2 , wherein
an alloyed hot-dip galvannealed layer is formed on a surface of the steel sheet.
11. The high-strength cold-rolled steel sheet according to claim 3 , wherein
an alloyed hot-dip galvannealed layer is formed on a surface of the steel sheet.
12. The high-strength cold-rolled steel sheet according to claim 6 , wherein
an alloyed hot-dip galvannealed layer is formed on a surface of the steel sheet.