Steel sheet for hot stamping member and method of producing same
View Patent ↗A steel sheet for a hot stamping member contains, as a chemical composition, 0.10 mass % to 0.35 mass % of C; 0.01 mass % to 1.0 mass % of Si; 0.3 mass % to 2.3 mass % of Mn; 0.01 mass % to 0.5 mass % of Al; limited to 0.03 mass % or less of P; limited to 0.02 mass % or less of S; limited to 0.1 mass % or less of N; and a balance consisting of Fe and unavoidable impurities, in which a standard deviation of diameters of iron carbides which are contained in a region from a surface to a ¼ thickness position of the steel sheet is less than or equal to 0.8 μm.
1. A steel sheet for producing a hot stamping member, the steel sheet comprising, as a chemical composition:
0.10 mass % to 0.35 mass % of C;
0.01 mass % to 1.0 mass % of Si;
0.3 mass % to 2.3 mass % of Mn;
0.01 mass % to 0.5 mass % of Al;
limited to 0.03 mass % or less of P;
limited to 0.02 mass % or less of S;
limited to 0.1 mass % or less of N; and
a balance of Fe and unavoidable impurities,
wherein a standard deviation of diameters of iron carbides which are contained in a region from a surface to a ¼ thickness position of the steel sheet is less than or equal to 0.8 μm and an average value of a size of the iron carbides which are contained in the region from the surface to the ¼ thickness position of the steel sheet is 1.0 μm or more and 2.0 μm or less.
2. The steel sheet for producing a hot stamping member according to claim 1 ,
wherein the chemical composition further contains one or more selected from the group consisting of
0.01 mass % to 2.0 mass % of Cr;
0.001 mass % to 0.5 mass % of Ti;
0.001 mass % to 0.5 mass % of Nb;
0.0005 mass % to 0.01 mass % of B;
0.01 mass % to 1.0 mass % of Mo;
0.01 mass % to 0.5 mass % of W;
0.01 mass % to 0.5 mass % of V;
0.01 mass % to 1.0 mass % of Cu; and
0.01 mass % to 5.0 mass % of Ni.
3. The steel sheet for producing a hot stamping member according to claim 2 ,
wherein an Al plating layer having a plating thickness of 50 μm or less is formed on the surface.
4. The steel sheet for producing a hot stamping member according to claim 2 ,
wherein a Zn plating layer having a plating thickness of 30 μm or less is formed on the surface.
5. The steel sheet for producing a hot stamping member according to claim 2 ,
wherein a Zn—Fe alloy layer having a plating thickness of 45 μm or less is formed on the surface.
6. The steel sheet for producing a hot stamping member according to claim 1 ,
wherein an Al plating layer having a plating thickness of 50 μm or less is formed on the surface.
7. The steel sheet for producing a hot stamping member according to claim 1 ,
wherein a Zn plating layer having a plating thickness of 30 μm or less is formed on the surface.
8. The steel sheet for producing a hot stamping member according to claim 1 ,
wherein a Zn—Fe alloy layer having a plating thickness of 45 μm or less is formed on the surface.
9. A method of producing a steel sheet for a hot stamping member according to claim 1 , the method comprising:
performing a recrystallization annealing process in which a cold-rolled steel sheet is heated such that a change d/dt (ΔT/Δt; ° C./s 2 ) in a rise rate of a steel sheet temperature from 300° C. to a maximum temperature S satisfies following expression 1 and such that the maximum temperature S is 720° C. to 820° C., wherein:
T represents the steel sheet temperature (° C.), t represents time (second), and ΔT/Δt represents the rise rate (° C./s) of the steel sheet temperature for a time Δt (second) during the heating of the recrystallization annealing process; and
the cold-rolled steel sheet contains, as a chemical composition,
0.10 mass % to 0.35 mass % of C;
0.01 mass % to 1.0 mass % of Si;
0.3 mass % to 2.3 mass % of Mn;
0.01 mass % to 0.5 mass % of Al;
limited to 0.03 mass % or less of P;
limited to 0.02 mass % or less of S;
limited to 0.1 mass % or less of N; and
a balance of Fe and unavoidable impurities,
−0.20≦ d/dt (Δ T/Δt )<0 (Expression 1).
10. The method of producing a steel sheet for a hot stamping member according to claim 9 ,
wherein the chemical composition further contains one or more selected from the group consisting of
0.01 mass % to 2.0 mass % of Cr;
0.001 mass % to 0.5 mass % of Ti;
0.001 mass % to 0.5 mass % of Nb;
0.0005 mass % to 0.01 mass % of B;
0.01 mass % to 1.0 mass % of Mo;
0.01 mass % to 0.5 mass % of W;
0.01 mass % to 0.5 mass % of V;
0.01 mass % to 1.0 mass % of Cu; and
0.01 mass % to 5.0 mass % of Ni.
11. The method of producing a steel sheet for a hot stamping member according to claim 9 ,
wherein the change d/dt (ΔT/Δt) is two times a coefficient of a variable of second degree when a temperature is read at a time interval of 10 seconds or shorter from a temperature history during the heating of the recrystallization annealing process and then a polynomial approximation curve of the second degree is determined such that a coefficient of determination R 2 is greater than or equal to 0.99.
12. The method of producing a steel sheet for a hot stamping member according to claim 9 , after the recrystallization annealing process, further comprising:
dipping the cold-rolled steel sheet in an Al bath to form an Al plating layer having a plating thickness of 50 μm or less on a surface of the cold-rolled steel sheet.
13. The method of producing a steel sheet for a hot stamping member according to claim 9 , after the recrystallization annealing process, further comprising:
dipping the cold-rolled steel sheet in a Zn bath to form a Zn plating layer having a plating thickness of 30 μm or less on a surface of the cold-rolled steel sheet.
14. The method of producing a steel sheet for a hot stamping member according to claim 9 , after the recrystallization annealing process, further comprising:
dipping the cold-rolled steel sheet in a Zn bath to form a Zn plating layer on a surface of the cold-rolled steel sheet; and
heating the cold-rolled steel sheet to 600° C. or lower to form a Zn—Fe alloy layer having a plating thickness of 45 μm or less on a surface of the cold-rolled steel sheet.
15. The method of producing a steel sheet for a hot stamping member according to claim 10 ,
wherein the change d/dt (ΔT/Δt) is two times a coefficient of a variable of second degree when a temperature is read at a time interval of 10 seconds or shorter from a temperature history during the heating of the recrystallization annealing process and then a polynomial approximation curve of the second degree is determined such that a coefficient of determination R 2 is greater than or equal to 0.99.
16. The method of producing a steel sheet for a hot stamping member according to claim 10 , after the recrystallization annealing process, further comprising:
dipping the cold-rolled steel sheet in an Al bath to form an Al plating layer having a plating thickness of 50 μm or less on a surface of the cold-rolled steel sheet.
17. The method of producing a steel sheet for a hot stamping member according to claim 10 , after the recrystallization annealing process, further comprising:
dipping the cold-rolled steel sheet in a Zn bath to form a Zn plating layer having a plating thickness of 30 μm or less on a surface of the cold-rolled steel sheet.
18. The method of producing a steel sheet for a hot stamping member according to claim 10 , after the recrystallization annealing process, further comprising:
dipping the cold-rolled steel sheet in a Zn bath to form a Zn plating layer on a surface of the cold-rolled steel sheet; and
heating the cold-rolled steel sheet to 600° C. or lower to form a Zn—Fe alloy layer having a plating thickness of 45 μm or less on a surface of the cold-rolled steel sheet.