Hot pressed member and method of manufacturing same

Abstract

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 first region having: a microstructure including a martensite phase of 80.0% or more in volume fraction and a retained austenite phase of 3.0% or more and 20.0% or less in volume fraction; and a dislocation density of 1.0×1016/m2 or more; and a second region having a microstructure including a ferrite phase of 30.0% or more and 60.0% or less in volume fraction, a retained austenite phase of 10.0% or more and 70.0% or less in volume fraction, and a martensite phase of 30.0% or less in volume fraction.

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 first region having: a microstructure including a martensite phase of 80.0% or more in volume fraction and a retained austenite phase of 3.0% or more and 20.0% 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; and   a second region having: a microstructure including a ferrite phase of 30.0% or more and 60.0% or less in volume fraction, a retained austenite phase of 10.0% or more and 70.0% or less in volume fraction, and a martensite phase of 30.0% or less in volume fraction; and a tensile property of 780 MPa or more in tensile strength TS and 15.0% or more in uniform elongation uEl.   
     
     
         2 . The hot pressed member according to  claim 1 ,
 wherein, in the microstructure in the second region, a mean grain size of the ferrite phase is 10 μm or less, a mean grain size of a secondary phase is 10 μm or less, and Mns/Mnα is 1.5 or more, where Mns is a Mn concentration in the secondary phase and Mnα is a Mn concentration in the ferrite phase.   
     
     
         3 . 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%.   
     
     
         4 . The hot pressed member according to  claim 1 , comprising
 a coated layer on a surface thereof.   
     
     
         5 . The hot pressed member according to  claim 4 ,
 wherein the coated layer is any of a zinc or zinc alloy coated layer and an aluminum or aluminum alloy coated layer.   
     
     
         6 . The hot pressed member according to  claim 5 ,
 wherein the zinc or zinc alloy coated layer contains Ni: 10 mass % to 25 mass %.   
     
     
         7 . A method of manufacturing a hot pressed member, the method comprising:
 heating a steel sheet to a first temperature that is an Ac1 point or more and an Ac3 point or less, retaining the steel sheet at the first temperature for 1 hr or more and 48 hr or less, and then cooling the steel sheet to obtain a blank steel sheet, the steel sheet 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;   performing a heating process of separately heating a first region of the blank steel sheet to a second temperature that is the Ac3 point or more and 1000° C. or less and heating a second region of the blank steel sheet to a third temperature that is the Ac1 point or more and not more than a temperature obtained by subtracting 20° C. from the Ac3 point; and   thereafter performing a hot press forming process of simultaneously press forming and quenching the blank steel sheet using a press tool for forming, to obtain a hot pressed member.   
     
     
         8 . 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%.   
     
     
         9 . The method of manufacturing a hot pressed member according to  claim 7 , further comprising
 forming a coated layer on a surface of the 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.