Steel sheet, hot-dip galvanized steel sheet, galvannealed steel sheet, and manufacturing methods therefor
A steel sheet according to an aspect of the present invention has predetermined chemical composition, in which a structure at a thickness ¼ portion includes, in terms of volume ratios, tempered martensite: 70% or more and one or both of ferrite and bainite: a total of less than 20%, in the structure at the thickness ¼ portion, a volume ratio of residual austenite is less than 10%, a volume ratio of fresh martensite is 10% or less, a volume ratio of pearlite is 10% or less, and a total volume ratio of the residual austenite, the fresh martensite, and the pearlite is 15% or less, a number density of iron-based carbides having a major axis of 5 nm or more in the tempered martensite at the thickness ¼ portion is 5×10 7 particles/mm 2 or more, a ratio of the number of ε-type carbides in the number of the iron-based carbides having the major axis of 5 nm or more at the thickness ¼ portion is 20% or more, and a tensile strength is 780 MPa or higher.
1. A steel sheet, comprising, as chemical composition, by mass %:
C: 0.05% to 0.40%,
Si: 0.05% to 3.00%,
Mn: 1.50% or more and less than 3.50%,
P: 0.04% or less,
S: 0.01% or less,
N: 0.01% or less,
O: 0.006% or less,
Al: 0% to 2.00%,
Cr: 0% to 1.00%,
Mo: 0% to 1.00%,
Ni: 0% to 1.00%,
Cu: 0% to 1.00%
Nb: 0% to 0.30%,
Ti: 0% to 0.30%,
V: 0% to 0.50%
B: 0% to 0.01%
Ca: 0% to 0.04%,
Mg: 0% to 0.04%,
REM: 0% to 0.04%, and
a remainder comprising Fe and impurities,
wherein a structure at a thickness ¼ portion includes, in terms of volume ratios,
tempered martensite: 70% or more and
one or both of ferrite and bainite: a total of less than 20%,
wherein in the structure at the thickness ¼ portion, a volume ratio of residual austenite is less than 10%, a volume ratio of fresh martensite is 10% or less, a volume ratio of pearlite is 10% or less, and a total volume ratio of the residual austenite, the fresh martensite, and the pearlite is 15% or less,
wherein a number density of iron-based carbides having a major axis of 5 nm or more in the tempered martensite at the thickness ¼ portion is 5×10 7 particles/mm 2 or more,
wherein a ratio of the number of ε-type carbides with respect to the number of the iron-based carbides having the major axis of 5 nm or more at the thickness ¼ portion is 20% or more, and
wherein a tensile strength is 780 MPa or higher.
2. The steel sheet according to claim 1 ,
wherein the chemical composition of the steel sheet includes, by mass %, one or more selected from the group consisting of
Cr: 0.05% to 1.00%,
Mo: 0.01% to 1.00%,
Ni: 0.05% to 1.00%, and
Cu: 0.05% to 1.00%.
3. The steel sheet according to claim 1 ,
wherein the chemical composition of the steel sheet includes, by mass %, one or more selected from the group consisting of
Nb: 0.005% to 0.30%,
Ti: 0.005% to 0.30%, and
V: 0.005% to 0.50%.
4. The steel sheet according to claim 1 ,
wherein the chemical composition of the steel sheet includes, by mass %,
B: 0.0001% to 0.01%.
5. The steel sheet according to claim 1 ,
wherein the chemical composition of the steel sheet includes, by mass %, one or more selected from the group consisting of
Ca: 0.0005% to 0.04%,
Mg: 0.0005% to 0.04%, and
REM: 0.0005% to 0.04%.
6. The steel sheet according to claim 1 ,
wherein an average major axis of the iron-based carbides is 350 nm or less.
7. A hot-dip galvanized steel sheet,
wherein a hot-dip galvanized layer including 15 mass % or less of Fe and a remainder comprising Zn, Al, and impurities is formed on a surface of the steel sheet according to claim 1 .
8. A galvannealed steel sheet,
wherein a galvannealed layer including 15 mass % or less of Fe and a remainder comprising Zn, Al, and impurities is formed on a surface of the steel sheet according to claim 1 .
9. The steel sheet according to claim 2 ,
wherein the chemical composition of the steel sheet includes, by mass %, one or more selected from the group consisting of
Nb: 0.005% to 0.30%,
Ti: 0.005% to 0.30%, and
V: 0.005% to 0.50%.
10. The steel sheet according to claim 2 ,
wherein the chemical composition of the steel sheet includes, by mass %,
B: 0.0001% to 0.01%.
11. The steel sheet according to claim 3 ,
wherein the chemical composition of the steel sheet includes, by mass %,
B: 0.0001% to 0.01%.
12. The steel sheet according to claim 2 ,
wherein the chemical composition of the steel sheet includes, by mass %, one or more selected from the group consisting of
Ca: 0.0005% to 0.04%,
Mg: 0.0005% to 0.04%, and
REM: 0.0005% to 0.04%.
13. The steel sheet according to claim 3 ,
wherein the chemical composition of the steel sheet includes, by mass %, one or more selected from the group consisting of
Ca: 0.0005% to 0.04%,
Mg: 0.0005% to 0.04%, and
REM: 0.0005% to 0.04%.
14. The steel sheet according to claim 4 ,
wherein the chemical composition of the steel sheet includes, by mass %, one or more selected from the group consisting of
Ca: 0.0005% to 0.04%,
Mg: 0.0005% to 0.04%, and
REM: 0.0005% to 0.04%.
15. The steel sheet according to claim 2 ,
wherein an average major axis of the iron-based carbides is 350 nm or less.
16. The steel sheet according to claim 3 ,
wherein an average major axis of the iron-based carbides is 350 nm or less.
17. The steel sheet according to claim 4 ,
wherein an average major axis of the iron-based carbides is 350 nm or less.
18. The steel sheet according to claim 5 ,
wherein an average major axis of the iron-based carbides is 350 nm or less.
19. The steel sheet according to claim 1 ,
wherein tempered martensite is, in terms of volume ratios, 75% or more.
20. The steel sheet according to claim 1 ,
wherein one or both of ferrite and bainite is, in terms of volume ratios, less than 10% in a total.
21. The steel sheet according to claim 1 ,
wherein the number density of the iron-based carbides is 10×10 7 particles/mm 2 or more.
22. The steel sheet according to claim 1 ,
wherein the number density of the iron-based carbides is 30×10 7 particles/mm 2 or more.