Hot-pressed steel sheet member and method for producing same

Abstract

Disclosed is a hot-pressed steel sheet member having a tensile strength of 1780 MPa or more and excellent bending collapsibility. The hot-pressed steel sheet member has: a specific chemical composition where a ratio of a C content in mass % to a Nb content in mass %, C/Nb, is from 22 to 100; a microstructure in which an average grain size of prior austenite grains is 8 μm or less, a volume fraction of martensite is 90% or more, and a solute C content is 25% or less of a total C content; and a tensile strength of 1780 MPa or more.

1 . A hot-pressed steel sheet member, comprising:
 a chemical composition containing, in mass %,   C: 0.30% or more and less than 0.50%,   Si: 0.01% or more and 2.0% or less,   Mn: 0.5% or more and 3.5% or less,   Nb: 0.001% or more and 0.10% or less,   P: 0.05% or less,   S: 0.01% or less,   Al: 0.01% or more and 1.00% or less, and   N: 0.01% or less,   
       with the balance being Fe and inevitable impurities, where a ratio of a C content in mass % to a Nb content in mass %, C/Nb, is from 22 to 100;
 a microstructure in which an average grain size of prior austenite grains is 8 μm or less, a volume fraction of martensite is 90% or more, and a solute C content is 25% or less of a total C content; and 
 a tensile strength of 1780 MPa or more. 
 
     
     
         2 . The hot-pressed steel sheet member according to  claim 1 , wherein the chemical composition further contains, in mass %, at least one selected from the group consisting of
 Mo: 0.35% or less,   Cr: 0.35% or less,   Ti: 0.15% or less,   B: 0.0050% or less,   Ca: 0.005% or less,   V: 0.05% or less,   Cu: 0.50% or less,   Ni: 0.50% or less,   Sn: 0.50% or less,   Zn: 0.10% or less,   Co: 0.10% or less,   Zr: 0.10% or less,   Ta: 0.10% or less, and   W: 0.10% or less.   
     
     
         3 . The hot-pressed steel sheet member according to  claim 1 , further comprising, on a surface thereof, an Al or Al alloy coated or plated layer or a Zn or Zn alloy coated or plated layer. 
     
     
         4 . A method for producing a hot-pressed steel sheet member, comprising:
 heating a cold-rolled steel sheet to a heating temperature of Ac3 transformation temperature or higher and 1000 ° C. or lower, the cold-rolled steel sheet comprising a chemical composition containing, in mass%,   C: 0.30% or more and less than 0.50%,   Si: 0.01% or more and 2.0% or less,   Mn: 0.5% or more and 3.5% or less,   Nb: 0.001% or more and 0.10% or less,   P: 0.05% or less,   S: 0.01% or less,   Al: 0.01% or more and 1.00% or less, and   N: 0.01% or less,   
       with the balance being Fe and inevitable impurities, where a ratio of a C content in mass % to a Nb content in mass %, C/Nb, is from 22 to 100;
 hot pressing the heated cold-rolled steel sheet to obtain a hot-pressed steel sheet; 
 cooling the hot-pressed steel sheet to Mf point or lower; and 
 subjecting the cooled hot-pressed steel sheet to heat treatment under a set of conditions including a heating temperature of 50° C. to 300° C. and a holding time of 5 seconds to 3600 seconds. 
 
     
     
         5 . The method for producing a hot-pressed steel sheet member according to  claim 4 , wherein the chemical composition further contains, in mass %, at least one selected from the group consisting of
 Mo: 0.35% or less,   Cr: 0.35% or less,   Ti: 0.15% or less,   B: 0.0050% or less,   Ca: 0.005% or less,   V: 0.05% or less,   Cu: 0.50% or less,   Ni: 0.50% or less,   Sn: 0.50% or less,   Zn: 0.10% or less,   Co: 0.10% or less,   Zr: 0.10% or less,   Ta: 0.10% or less, and   W: 0.10% or less.   
     
     
         6 . The hot-pressed steel sheet member according to  claim 2 , further comprising, on a surface thereof, an Al or Al alloy coated or plated layer or a Zn or Zn alloy coated or plated layer.