Hot-formed member and method of manufacturing same

Abstract

A hot-formed member according to the present invention has a predetermined chemical composition, a metallographic microstructure in which, in terms of area %, ferrite is 10% to 90%, unrecrystallized ferrite is 0% to 2.0%, martensite is 10% to 90%, the total area ratio of the ferrite and the martensite is 90% to 100%, and the average grain size of the ferrite is 0.5 μm to 5.0 μm, and the tensile strength of 900 MPa to 1800 MPa.

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.