Heat-treated steel sheet member and method for producing the same
A heat-treated steel sheet member having a composition including, by mass %: C: 0.05 to 0.50%; Si: 0.50 to 5.0%; Mn: 1.5 to 4.0%; P: 0.05% or less; S: 0.05% or less; N: 0.01% or less; Ti: 0.01 to 0.10%; B: 0.0005 to 0.010%; Cr: 0 to 1.0%; Ni: 0 to 2.0%; Cu; 0 to 1.0%; Mo: 0 to 1.0%; V: 0 to 1.0%; Ca: 0 to 0.01%; Al: 0 to 1.0%; Nb: 0 to 1.0%; REM: 0 to 0.1%; and the balance: Fe and impurities. The steel sheet member has a microstructure comprising mainly martensite and retained austenite of which a volume ratio is 0.2 to 1.0%, a number density of retained carbide in the steel sheet member having circle-equivalent diameters of 0.1 mm or larger is 4.0×10 3 /mm 2 or lower, a tensile strength is 1.4 GPa or higher, and a yield ratio is 0.65 or higher.
1. A heat-treated steel sheet member having a chemical composition comprising, by mass %:
C: 0.05 to 0.50%;
Si: 0.50 to 5.0%;
Mn: 1.5 to 4.0%;
P: 0.05% or less;
S: 0.05% or less;
N: 0.01% or less;
Ti: 0.01 to 0.10%;
B: 0.0005 to 0.010%;
Cr: 0 to 1.0%;
Ni: 0 to 2.0%;
Cu: 0 to 1.0%;
Mo: 0 to 1.0%;
V: 0 to 1.0%;
Ca: 0 to 0.01%;
Al: 0 to 1.0%;
Nb: 0 to 1.0%;
REM: 0 to 0.1%; and
the balance: Fe and impurities, wherein
the steel sheet member has a steel micro-structure comprising: martensite of which a volume ratio is 95% or higher; and retained austenite of which a volume ratio is 0.2 to 1.0%,
a number density of retained carbide being present in the steel sheet member and having circle-equivalent diameters of 0.1 μm or larger is 4.0×10 3 /mm 2 or lower,
the steel sheet member has a tensile strength is 1.4 GPa or higher, and
the steel sheet member has a yield ratio is 0.65 or higher.
2. The heat-treated steel sheet member according to claim 1 , wherein the chemical composition contains, by mass %, one or more elements selected from:
Cr: 0.01 to 1.0%;
Ni: 0.1 to 2.0%;
Cu: 0.1 to 1.0%;
Mo: 0.1 to 1.0%;
V: 0.1 to 1.0%;
Ca: 0.001 to 0.01%;
Al: 0.01 to 1.0%;
Nb: 0.01 to 1.0%; and
REM: 0.001 to 0.1%.
3. The heat-treated steel sheet member according to claim 1 , wherein an Mn segregation degree α expressed by a following formula (ii) is 1.6 or lower:
α=[Maximum Mn concentration (mass %) at sheet-thickness center portion]/[Average Mn concentration (mass %) in ¼sheet-thickness depth position from surface] (ii).
4. The heat-treated steel sheet member according to claim 1 , wherein a value of an index of cleanliness of steel specified in JIS G 0555 (2003) is 0.10% or lower.
5. The heat-treated steel sheet member according to claim 2 , wherein an Mn segregation degree α expressed by a following formula (ii) is 1.6 or lower:
α=[Maximum Mn concentration (mass %) at sheet-thickness center portion]/[Average Mn concentration (mass %) in ¼sheet-thickness depth position from surface] (ii).
6. The heat-treated steel sheet member according to claim 2 , wherein a value of an index of cleanliness of steel specified in JIS G 0555 (2003) is 0.10% or lower.
7. The heat-treated steel sheet member according to claim 3 , wherein a value of an index of cleanliness of steel specified in JIS G 0555 (2003) is 0.10% or lower.
8. The heat-treated steel sheet member according to claim 5 , wherein a value of an index of cleanliness of steel specified in JIS G 0555 (2003) is 0.10% or lower.
9. A method for producing a heat-treated steel sheet member, the method comprising:
heating a steel sheet, which has a chemical composition as defined below, a maximum height roughness Rz on a surface of 3.0 to 10.0 μm, and a number density of carbide having circle-equivalent diameters of 0.1 μm or larger is 8.0×10 3 /mm 2 or lower, up to a temperature range from an Ac 3 point to the Ac 3 point+200° C. at an average temperature rise rate of 5° C./s or higher;
subsequently, cooling the steel sheet from the temperature range down to an Ms point at an upper critical cooling rate or higher; and
subsequently, cooling the steel sheet from the Ms point down to 100° C. at an average cooling rate of 60° C./s or higher, to result in the heat-treated steel sheet member wherein
the chemical composition of the steel sheet comprises, by mass %:
C: 0.05 to 0.50%;
Si: 0.50 to 5.0%;
Mn: 1.5 to 4.0%;
P: 0.05% or less;
S: 0.05% or less;
N: 0.01% or less;
Ti: 0.01 to 0.10%;
B: 0.0005 to 0.010%;
Cr: 0 to 1.0%;
Ni: 0 to 2.0%;
Cu: 0 to 1.0%;
Mo: 0 to 1.0%;
V: 0 to 1.0%;
Ca: 0 to 0.01%;
Al: 0 to 1.0%;
Nb: 0 to 1.0%;
REM: 0 to 0.1%; and
the balance: Fe and impurities, wherein
the steel sheet member has:
a steel micro-structure comprising: martensite of which a volume ratio is 95% or higher; and retained austenite of which a volume ratio is 0.2 to 1.0%,
a number density of retained carbide being present in the steel sheet member and having circle-equivalent diameters of 0.1 μm or larger is 4.0×10 3 /mm 2 or lower,
a tensile strength is 1.4 GPa or higher, and
a yield ratio is 0.65 or higher.
10. The method for producing a heat-treated steel sheet member according to claim 9 , wherein the chemical composition contains, by mass %, one or more elements selected from:
Cr: 0.01 to 1.0%;
Ni: 0.1 to 2.0%;
Cu: 0.1 to 1.0%;
Mo: 0.1 to 1.0%;
V: 0.1 to 1.0%;
Ca: 0.001 to 0.01%;
Al: 0.01 to 1.0%;
Nb: 0.01 to 1.0%; and
REM: 0.001 to 0.1%.
11. The method for producing a heat-treated steel sheet member according to claim 9 , wherein an Mn segregation degree α expressed by a following formula (ii) is 1.6 or lower:
α=[Maximum Mn concentration (mass %) at sheet-thickness center portion]/[Average Mn concentration (mass %) in ¼sheet-thickness depth position from surface] (ii).
12. The method for producing a heat-treated steel sheet member according to claim 9 , wherein a value of an index of cleanliness of steel specified in JIS G 0555 (2003) is 0.10% or lower.
13. The method for producing a heat-treated steel sheet member according to claim 9 , wherein the steel sheet is subjected to hot forming after being heated up to the temperature range and before being cooled down to the Ms point.