Hot-formed member and manufacturing method of same

1. A hot-formed member having a chemical composition comprising, by mass %,

C: 0.05% to 0.40%,

Si: 0.5% to 3.0%,

Mn: 1.2% to 8.0%,

P: 0.05% or less,

S: 0.01% or less,

sol. Al: 0.001% to 2.0%,

N: 0.01% or less,

Ti: 0% to 1.0%,

Nb: 0% to 1.0%,

V: 0% to 1.0%,

Cr: 0% to 1.0%,

Mo: 0% to 1.0%,

Cu: 0% to 1.0%,

Ni: 0% to 1.0%,

Ca: 0% to 0.01%,

Mg: 0% to 0.01%,

REM: 0% to 0.01%,

Zr: 0% to 0.01%,

B: 0% to 0.01%,

Bi: 0% to 0.01%, and

the balance of Fe and impurities,

wherein the hot-formed member has a metallographic microstructure which contains an austenite of 10 area % to 40 area % and in which the total number density of particles of the austenite and particles of a martensite is equal to or greater than 1.0 piece/μm 2 , and

wherein a tensile strength is 900 MPa to 1300 MPa.

2. The hot-formed member according to claim 1 ,

wherein the chemical composition includes one or two or more selected from the group consisting of, by mass %,

Ti: 0.003% to 1.0%,

Nb: 0.003% to 1.0%,

V: 0.003% to 1.0%,

Cr: 0.003% to 1.0%,

Mo: 0.003% to 1.0%,

Cu: 0.003% to 1.0%, and

Ni: 0.003% to 1.0%.

3. The hot-formed member according to claim 2 ,

wherein the chemical composition includes one or two more selected from the group consisting of, by mass %,

Ca: 0.0003% to 0.01%,

Mg: 0.0003%, to 0.01%,

REM: 0.0003% to 0.01%, and

Zr: 0.0003% to 0.01%.

4. The hot-formed member according to claim 2 ,

wherein the chemical composition includes, by mass %, B: 0.0003% to 0.01%.

5. The hot-formed member according to claim 2 ,

wherein the chemical composition includes, by mass %, Bi: 0.0003% to 0.01%.

6. The hot-formed member according to claim 1 ,

wherein the chemical composition includes one or two or more selected from the group consisting of, by mass %,

Ca: 0.0003% to 0.01%,

Mg: 0.0003% to 0.01%,

REM: 0.0003% to 0.01%, and

Zr: 0.0003% to 0.01%.

7. The hot-formed member according to claim 1 ,

wherein the chemical composition includes, by mass %, B: 0.0003% to 0.01%.

8. The hot-formed member according to claim 1 ,

wherein the chemical composition includes, by mass %, Bi: 0.0003% to 0.01%.

9. A manufacturing method of a hot-formed member, the method comprising:

heating a base steel sheet having a chemical composition which is same as the chemical composition of the hot-formed member according to claim 1 and in which a Mn content is 2.4 mass % to 8.0 mass %, and having a metallographic microstructure in which the total area ratio of one or both of a bainite and a martensite is equal to or greater than 70 area %, and particles of a cementite are present at a number density equal to or greater than 1.0 number/μm 2 , to a temperature region which is equal to or higher than 670° C. and lower than 780° C. and is lower than an Ac 3 temperature;

then holding the temperature of the base steel sheet in the temperature region which is equal to or higher than 670° C. and lower than 780° C. and is lower than an Ac 3 temperature for 2 minutes to 20 minutes;

then performing a hot forming with respect to the base steel sheet; and

then cooling the base steel sheet under conditions in which an average cooling rate in a temperature region of 600° C. to 150° C. is from 5° C./sec to 500° C./sec.

10. A manufacturing method of a hot-formed member, the method comprising:

heating a base steel sheet having a chemical composition which is same as the chemical composition of the hot-formed member according to claim 1 and in which a Mn content equal to or more than 1.2 mass % and less than 2.4 mass %, and having a metallographic microstructure in which the total area ratio of one or both of a bainite and a martensite is equal to or greater than 70 area %, and particles of a cementite are present at a number density equal to or greater than 1.0 number/μm 2 , to a temperature region which is equal to or higher than 670° C. and lower than 780° C. and is lower than an Ac 3 temperature;

then holding the temperature of the base steel sheet in the temperature region which is equal to or higher than 670° C. and lower than 780° C. and is lower than an Ac 3 temperature for 2 minutes to 20 minutes;

then performing a hot forming with respect to the base steel sheet; and

then cooling the base steel sheet under conditions in which an average cooling rate in a temperature region of 600° C. to 500° C. is from 5° C./sec to 500° C./sec and the average cooling rate in a temperature region lower than 500° C. and equal to or higher than 150° C. is from 5° C./sec and 20° C./sec.