Steel sheet for hot pressing, and method of manufacturing the hot-pressed steel sheet member

Abstract

A steel sheet for hot pressing includes a specific chemical composition and further includes a steel structure in which an area ratio of ferrite in a region ranging from a surface to 100 μm in depth is 30% to 90% and an area ratio of pearlite including an average grain diameter of 5 μm or more in a region excluding the region ranging from the surface to 100 μm in depth is 10% to 70%.

1 . A steel sheet for hot pressing, comprising: 
       a chemical composition represented by, in mass %:
 C: 0.11% to 0.35%; 
 Si: 0.5% to 2.0%; 
 Mn: 1.0% to 3.0%; 
 sol. Al: 0.001% to 1.0%; 
 P: 0.05% or less; 
 S: 0.01% or less; 
 N: 0.01% or less; 
 Ti: 0% to 0.20%; 
 Nb: 0% to 0.20%; 
 V: 0% to 0.20%; 
 Cr: 0% to 1.0%; 
 Mo: 0% to 1.0%; 
 Cu: 0% to 1.0%; 
 Ni: 0% to 1.0%; 
 Ca: 0% to 0.01%; 
 Mg: 0% to 0.01%; 
 REM: 0% to 0.01%; 
 Zr: 0% to 0.01%; 
 B: 0% to 0.01%; 
 Bi: 0% to 0.01%; and 
 balance: Fe and impurities; and 
 
       an internal oxide layer including a thickness of 30 μm or less; and 
       a steel structure in which an area ratio of ferrite in a region ranging from a surface to 100 μm in depth is 30% to 90% and an area ratio of pearlite including an average grain diameter of 5 μm or more in a region excluding the region ranging from the surface to 100 μm in depth is 10% to 70%. 
     
     
         2 . The steel sheet for hot pressing according to  claim 1 , wherein the chemical composition comprises one or more selected from the group consisting of, in mass %:
 Ti: 0.003% to 0.20%;   Nb: 0.003% to 0.20%;   V: 0.003% to 0.20%;   Cr: 0.005% to 1.0%;   Mo: 0.005% to 1.0%;   Cu: 0.005% to 1.0%; and   Ni: 0.005% to 1.0%.   
     
     
         3 . The steel sheet for hot pressing according to  claim 1 , wherein the chemical composition comprises one or more selected from the group consisting of, in mass %:
 Ca: 0.0003% to 0.01%;   Mg: 0.0003% to 0.01%;   REM: 0.0003% to 0.01%; and   Zr: 0.0003% to 0.01%.   
     
     
         4 . The steel sheet for hot pressing according to  claim 1 , wherein the chemical composition comprises, in mass %, B: 0.0003% to 0.01%. 
     
     
         5 . The steel sheet for hot pressing according to  claim 1 , wherein the chemical composition comprises, in mass %, Bi: 0.0003% to 0.01%. 
     
     
         6 . A method of manufacturing a hot-pressed steel sheet member, comprising:
 a step of heating the steel sheet for hot pressing according to  claim 1  in a temperature zone of 720° C. to an Ac 3  point;   a step of performing a decarburization treatment of reducing a C content on a surface of the steel sheet for hot pressing by 0.0005 mass % to 0.015 mass % after the heating; and   a step of hot pressing and cooling down to an Ms point at an average cooling rate of 10° C./second to 500° C./second after the decarburization treatment.   
     
     
         7 . The method of manufacturing the hot-pressed steel sheet member according to  claim 6 , wherein the step of performing a decarburization treatment comprises performing air cooling for 5 seconds to 50 seconds.