HOT-FORMED MEMBER AND METHOD OF MANUFACTURING SAME

1 - 8 . (canceled)

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

0.10% to 0.40% of C;

0% to 2.0% of Si;

1.0% to 3.0% of Mn;

0.05% or less of P;

0.01% or less of S;

0.001% to 1.0% of sol. Al;

0.050% to 0.30% of Ti;

0.01% or less of N;

0% to 0.4% of Nb;

0% to 0.4% of V;

0% to 1.0% of Cr;

0% to 1.0% of Mo;

0% to 1.0% of Cu;

0% to 1.0% of Ni;

0% to 0.01% of Ca;

0% to 0.01% of Mg;

0% to 0.01% of REM;

0% to 0.01% of Zr;

0% to 0.01% of B;

0% to 0.01% of Bi; and

the balance of Fe and impurities,

wherein the hot-formed member has a metallographic microstructure which has, in terms of area %, 10% to 90% of a ferrite, 0% to 2.0% of an unrecrystallized ferrite, 10% to 90% of a martensite, in which a total area ratio of the ferrite and the martensite is 90% to 100%, and in which an average grain size of the ferrite is 0.5 μm to 5.0 μm; and

a tensile strength is 900 MPa to 1800 MPa.

10 . The hot-formed member according to claim 9 ,

wherein the chemical composition contains, by mass %, one or two or more selected from the group consisting of 0.003% to 0.4% of Nb, 0.003% to 0.4% of V, 0.005% to 1.0% of Cr, 0.005% to 1.0% of Mo, 0.005% to 1.0% of Cu, and 0.005% to 1.0% of Ni.

11 . The hot-formed member according to claim 9 or 10 ,

wherein the chemical composition contains, by mass %, one or two or more selected from the group consisting of 0.0003% to 0.01% of Ca, 0.0003% to 0.01% of Mg, 0.0003% to 0.01% of REM, and 0.0003% to 0.01% of Zr.

12 . The hot-formed member according to claim 9 or 10 ,

wherein the chemical composition contains, by mass %, 0.0003% to 0.01% of B.

13 . The hot-formed member according to claim 11 ,

wherein the chemical composition contains, by mass %, 0.0003% to 0.01% of B.

14 . The hot-formed member according to claim 9 or 10 ,

wherein the chemical composition contains, by mass %, 0.0003% to 0.01% of Bi.

15 . The hot-formed member according to claim 11 ,

wherein the chemical composition contains, by mass %, 0.0003% to 0.01% of Bi.

16 . The hot-formed member according to claim 12 ,

wherein the chemical composition contains, by mass %, 0.0003% to 0.01% of Bi.

17 . The hot-formed member according to claim 13 ,

wherein the chemical composition contains, by mass %, 0.0003% to 0.01% of Bi.

18 . A method of manufacturing a hot-formed member including:

heating a base steel sheet having the same chemical composition as that of the hot-formed member according to claim 9 and a metallographic microstructure in which the amount of an unrecrystallized ferrite is 0 area % to 2.0 area % and an average grain size of a ferrite is 0.5 μm to 7.0 μm to a temperature range of 720° C. to lower than an Ac 3 temperature;

then maintaining a temperature of the base steel sheet for 1 minute to 20 minutes in the temperature range of 720° C. to lower than the Ac 3 temperature;

then hot-forming the base steel sheet; and

then cooling the base steel sheet under conditions in which an average cooling rate is 20° C./sec to 500° C./sec in a temperature range of 600° C. to 150° C.

19 . A method of manufacturing a hot-formed member including:

heating a base steel sheet having the same chemical composition as that of the hot-formed member according to claim 9 and a metallographic microstructure in which an unrecrystallized ferrite is more than 2.0 area % and an average grain size of a ferrite is 0.5 μm to 7.0 μm to a temperature range of an Ac 3 temperature to the Ac 3 temperature+100° C.;

then maintaining a temperature of the base steel sheet for 30 seconds or longer and shorter than 20 minutes in the temperature range of the Ac 3 temperature to the Ac 3 temperature+100° C.;

then hot-forming the base steel sheet; and

then cooling the base steel sheet under conditions in which an average cooling rate is 3° C./sec to 20° C./sec in a temperature range of the Ac 3 temperature to 600° C.

20 . The method of manufacturing a hot-formed member according to claim 18 or 19 ,

wherein the base steel sheet is one selected from the group consisting of a cold-rolled steel sheet, a hot-dip galvanized steel sheet, and a galvannealed steel sheet.