Heat-treated steel sheet member and method for producing the same

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:

mainly martensite;

and retained austenite of which a volume ratio is 5.0% or higher,

retained carbides are present in the steel sheet member, and a number density of retained carbides, each having a circle-equivalent diameter of 0.1 μm or larger, is 4.0×10 3 /mm 2 or lower,

a value of strain induced transformation parameter k expressed by a following formula (i) is less than 20.0 when following mechanical properties are measured using a sheet specimen specified in ASTM E8,

a tensile strength is 1.4 GPa or higher, and

a total elongation is 8.0% or higher:

k =(log f γ0 −log f γ (0.02))/0.02  (i)

where meaning of each symbol in the above formula is as follows:

f γ0 : Volume ratio of retained austenite present in the steel sheet member

f γ (0.02): Volume ratio of retained austenite present in the steel sheet member after a true strain of 0.02 is applied to and unloaded from the member.

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 ¼ sheet-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 315 G 0555 (2003) is 0.10% or lower.

5. A method for producing a heat-treated steel sheet member according to claim 1 , the method comprising:

heating a steel sheet 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 5° C./s or lower, wherein

the steel sheet subjected to the heating and cooling steps has:

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;

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 of 8.0×10 3 /mm 2 or lower.

6. The method for producing a heat-treated steel sheet member according to claim 5 , 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%.

7. The method for producing a heat-treated steel sheet member according to claim 5 , wherein an Mn segregation degree a 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 ¼ sheet-thickness depth position from surface]  (ii).

8. The method for producing a heat-treated steel sheet member according to claim 5 , wherein a value of an index of cleanliness of steel specified in NS G 0555 (2003) is 0.10% or lower.

9. The method for producing a heat-treated steel sheet member according to claim 5 , 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.

10. 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 ¼ sheet-thickness depth position from surface]  (ii).

11. The heat-treated steel sheet member according to claim 2 , wherein a value of an index of cleanliness of steel specified in MS G 0555 (2003) is 0.10% or lower.

12. The heat-treated steel sheet member according to claim 3 , wherein a value of an index of cleanliness of steel specified in MS G 0555 (2003) is 0.10% or lower.

13. The heat-treated steel sheet member according to claim 10 , wherein a value of an index of cleanliness of steel specified in MS G 0555 (2003) is 0.10% or lower.