Hot pressed member and method of manufacturing same

Abstract

A hot pressed member having all of: high strength of 1500 MPa or more in tensile strength TS; high ductility of 6.0% or more in uniform elongation uEl; and excellent heat treatment hardenability of increasing in yield stress YS by 150 MPa or more when subjected to heat treatment (baking finish) is provided. A hot pressed member comprises: a predetermined chemical composition (in particular, low C of 0.090% or more and less than 0.30% and high Mn of 3.5% or more and less than 11.0%); a microstructure including a martensite phase of 70.0% or more in volume fraction and a retained austenite phase of 3.0% or more and 30.0% or less in volume fraction; and a dislocation density of 1.0×10 16 /m 2 or more.

1 . A hot pressed member comprising:
 a chemical composition containing, in mass %,   C: 0.090% or more and less than 0.30%,   Mn: 3.5% or more and less than 11.0%,   Si: 0.01% to 2.5%,   P: 0.05% or less,   S: 0.05% or less,   Al: 0.005% to 0.1%, and   N: 0.01% or less,   with a balance consisting of Fe and incidental impurities;   a microstructure including a martensite phase of 70.0% or more in volume fraction and a retained austenite phase of 3.0% or more and 30.0% or less in volume fraction;   a tensile property of 1500 MPa or more in tensile strength TS and 6.0% or more in uniform elongation uEl; and   a dislocation density of 1.0×10 16 /m 2  or more.   
     
     
         2 . The hot pressed member according to  claim 1 ,
 wherein the chemical composition further contains, in mass %, one or more groups selected from   A group: one or more selected from Ni: 0.01% to 5.0%, Cu: 0.01% to 5.0%, Cr: 0.01% to 5.0%, and Mo: 0.01% to 3.0%,   B group: one or more selected from Ti: 0.005% to 3.0%, Nb: 0.005% to 3.0%, V: 0.005% to 3.0%, and W: 0.005% to 3.0%,   C group: one or more selected from REM: 0.0005% to 0.01%, Ca: 0.0005% to 0.01%, and Mg: 0.0005% to 0.01%,   D group: Sb: 0.002% to 0.03%, and   E group: B: 0.0005% to 0.05%.   
     
     
         3 . The hot pressed member according to  claim 1 , comprising
 a coated layer on a surface thereof.   
     
     
         4 . The hot pressed member according to  claim 3 ,
 wherein the coated layer is any of a zinc or zinc alloy coated layer and an aluminum or aluminum alloy coated layer.   
     
     
         5 . The hot pressed member according to  claim 4 ,
 wherein the zinc or zinc alloy coated layer contains Ni: 10 mass % to 25 mass %.   
     
     
         6 . A method of manufacturing a hot pressed member, the method comprising:
 heating a slab and hot rolling the slab to obtain a hot rolled steel sheet, the slab having a chemical composition containing, in mass %,   C: 0.090% or more and less than 0.30%,   Mn: 3.5% or more and less than 11.0%,   Si: 0.01% to 2.5%,   P: 0.05% or less,   S: 0.05% or less,   Al: 0.005% to 0.1%, and   N: 0.01% or less,   with a balance consisting of Fe and incidental impurities;   heating the hot rolled steel sheet to a first temperature that is an Ac1 point or more and an Ac3 point or less, retaining the hot rolled steel sheet at the first temperature for 1 hr or more and 48 hr or less, and then cooling the hot rolled steel sheet to obtain a first blank steel sheet;   performing a heating process of heating the first blank steel sheet to a second temperature that is the Ac3 point or more and 1000° C. or less and retaining the first blank steel sheet at the second temperature for 900 sec or less; and   thereafter performing a hot press forming process of simultaneously press forming and quenching the first blank steel sheet using a press tool for forming, to obtain a hot pressed member.   
     
     
         7 . The method of manufacturing a hot pressed member according to  claim 6 , further comprising:
 cold rolling the first blank steel sheet to obtain a cold rolled steel sheet, before the heating process; and   annealing the cold rolled steel sheet to obtain a second blank steel sheet, the annealing including heating the cold rolled steel sheet to a temperature that is the Ac1 point or more and the Ac3 point or less, retaining the cold rolled steel sheet at the temperature, and then cooling the cold rolled steel sheet,   wherein the heating process and the hot press forming process are performed on the second blank steel sheet instead of the first blank steel sheet.   
     
     
         8 . The method of manufacturing a hot pressed member according to  claim 6 ,
 wherein the chemical composition further contains, in mass %, one or more groups selected from   A group: one or more selected from Ni: 0.01% to 5.0%, Cu: 0.01% to 5.0%, Cr: 0.01% to 5.0%, and Mo: 0.01% to 3.0%,   B group: one or more selected from Ti: 0.005% to 3.0%, Nb: 0.005% to 3.0%, V: 0.005% to 3.0%, and W: 0.005% to 3.0%,   C group: one or more selected from REM: 0.0005% to 0.01%, Ca: 0.0005% to 0.01%, and Mg: 0.0005% to 0.01%,   D group: Sb: 0.002% to 0.03%, and   E group: B: 0.0005% to 0.05%.   
     
     
         9 . The method of manufacturing a hot pressed member according to  claim 6 , further comprising
 forming a coated layer on a surface of the first blank steel sheet before the heating process.   
     
     
         10 . The method of manufacturing a hot pressed member according to  claim 9 ,
 wherein the coated layer is any of a zinc or zinc alloy coated layer and an aluminum or aluminum alloy coated layer.   
     
     
         11 . The method of manufacturing a hot pressed member according to  claim 10 ,
 wherein the zinc or zinc alloy coated layer contains Ni: 10 mass % to 25 mass %.   
     
     
         12 . The method of manufacturing a hot pressed member according to  claim 9 ,
 wherein a coating weight of the coated layer is 10 g/m 2  to 90 g/m 2  per side.   
     
     
         13 . The method of manufacturing a hot pressed member according to  claim 7 ,
 wherein the chemical composition further contains, in mass %, one or more groups selected from   A group: one or more selected from Ni: 0.01% to 5.0%, Cu: 0.01% to 5.0%, Cr: 0.01% to 5.0%, and Mo: 0.01% to 3.0%,   B group: one or more selected from Ti: 0.005% to 3.0%, Nb: 0.005% to 3.0%, V: 0.005% to 3.0%, and W: 0.005% to 3.0%,   C group: one or more selected from REM: 0.0005% to 0.01%, Ca: 0.0005% to 0.01%, and Mg: 0.0005% to 0.01%,   D group: Sb: 0.002% to 0.03%, and   E group: B: 0.0005% to 0.05%.   
     
     
         14 . The method of manufacturing a hot pressed member according to  claim 7 , further comprising
 forming a coated layer on a surface of the second blank steel sheet before the heating process.   
     
     
         15 . The method of manufacturing a hot pressed member according to  claim 14 ,
 wherein the coated layer is any of a zinc or zinc alloy coated layer and an aluminum or aluminum alloy coated layer.   
     
     
         16 . The method of manufacturing a hot pressed member according to  claim 15 ,
 wherein the zinc or zinc alloy coated layer contains Ni: 10 mass % to 25 mass %.   
     
     
         17 . The method of manufacturing a hot pressed member according to  claim 14 ,
 wherein a coating weight of the coated layer is 10 g/m 2  to 90 g/m 2  per side.