Steel sheet for hot stamping and hot-stamping formed body

Abstract

This steel sheet for hot stamping has a predetermined chemical composition and has a microstructure in which Sα+SGB, which is a total of an area ratio Sα of ferrite and an area ratio SGB of a granular bainite, is 10% or more and less than 50% and SGB/Sα, which is a ratio between the area ratio SGB of the granular bainite and the area ratio Sα of the ferrite, is 0.30 to 0.70. In addition, a hot-stamping formed body manufactured using this steel sheet for hot stamping has a predetermined chemical composition and a microstructure in which an average grain size of prior austenite grains is 5 to 25 μm, and a standard deviation of grain sizes of the prior austenite grains is 0.1 to 2.0 μm, and a tensile strength of the hot-stamping formed body is 2,200 MPa or more.

1 . A steel sheet for hot stamping consisting of, as a chemical composition, by mass %:
 C: more than 0.40% and 0.70% or less;   Si: 0.010% to 1.30%;   Mn: more than 0.60% and 3.00% or less;   P: 0.100% or less;   S: 0.0100% or less;   N: 0.0130% or less;   O: 0.0200% or less;   Al: 0.0010% to 0.500%;   Cr: 0.010% to 0.80%;   Nb: 0% to 0.100%;   Ti: 0% to 0.100%;   B: 0% to 0.0100%;   Mo: 0% to 1.00%;   Co: 0% to 2.00%;   Ni: 0% or more and less than 3.00%;   Cu: 0% to 1.00%;   V: 0% to 1.00%;   W: 0% to 1.000%;   Ca: 0% to 0.010%;   Mg: 0% to 1.000%;   REM: 0% to 1.000%;   Sb: 0% to 1.000%;   Zr: 0% to 1.000%;   Sn: 0% to 1.000%;   As: 0% to 0.100%; and   a remainder of Fe and impurities,   wherein the steel sheet for hot stamping has a microstructure in which S α +S GB , which is a total of an area ratio S α  of ferrite and an area ratio S GB  of a granular bainite, is 10% or more and less than 50%, and   S GB /S α , which is a ratio between the area ratio S GB  of the granular bainite and the area ratio S α  of the ferrite, is 0.30 to 0.70.   
     
     
         2 . The steel sheet for hot stamping according to  claim 1 , wherein the steel sheet for hot stamping contains, as the chemical composition, by mass %, one or more of:
 Nb: 0.001% to 0.100%;   Ti: 0.010% to 0.100%;   B: 0.0015% to 0.0100%;   Mo: 0.05% to 1.00%;   Co: 0.05% to 2.00%;   Ni: 0.01% or more and less than 3.00%;   Cu: 0.01% to 1.00%;   V: 0.01% to 1.00%;   W: 0.001% to 1.000%;   Ca: 0.001% to 0.010%;   Mg: 0.001% to 1.000%;   REM: 0.001% to 1.000%;   Sb: 0.005% to 1.000%;   Zr: 0.001% to 1.000%;   Sn: 0.001% to 1.000%; and   As: 0.001% to 0.100%.   
     
     
         3 . A hot-stamping formed body consisting of, as a chemical composition, by mass %:
 C: more than 0.40% and 0.70% or less;   Si: 0.010% to 1.30%;   Mn: more than 0.60% and 3.00% or less;   P: 0.100% or less;   S: 0.0100% or less;   N: 0.0130% or less;   O: 0.0200% or less;   Al: 0.0010% to 0.500%;   Cr: 0.010% to 0.80%;   Nb: 0% to 0.100%;   Ti: 0% to 0.100%;   B: 0% to 0.0100%;   Mo: 0% to 1.00%;   Co: 0% to 2.00%;   Ni: 0% or more and less than 3.00%;   Cu: 0% to 1.00%;   V: 0% to 1.00%;   W: 0% to 1.000%;   Ca: 0% to 0.010%;   Mg: 0% to 1.000%;   REM: 0% to 1.000%;   Sb: 0% to 1.000%;   Zr: 0% to 1.000%;   Sn: 0% to 1.000%;   As: 0% to 0.100%; and   a remainder of Fe and impurities,   wherein the hot-stamping formed body has a microstructure in which an average grain size of prior austenite grains is 5 to 25 μm,   a standard deviation of grain sizes of the prior austenite grains is 0.1 to 2.0 μm, and   a tensile strength of the hot-stamping formed body is 2,200 MPa or more.   
     
     
         4 . The hot-stamping formed body according to  claim 3 , wherein the hot-stamping formed body contains, as the chemical composition, by mass %, one or more of:
 Nb: 0.001% to 0.100%;   Ti: 0.010% to 0.100%;   B: 0.0015% to 0.0100%;   Mo: 0.05% to 1.00%;   Co: 0.05% to 2.00%;   Ni: 0.01% or more and less than 3.00%;   Cu: 0.01% to 1.00%;   V: 0.01% to 1.00%;   W: 0.001% to 1.000%;   Ca: 0.001% to 0.010%;   Mg: 0.001% to 1.000%;   REM: 0.001% to 1.000%;   Sb: 0.005% to 1.000%;   Zr: 0.001% to 1.000%;   Sn: 0.001% to 1.000%; and   As: 0.001% to 0.100%.   
     
     
         5 . The hot-stamping formed body according to  claim 3 , wherein an area ratio of the prior austenite grains having an average grain size of 0.5 to 3.0 μm is 60% or less. 
     
     
         6 . The hot-stamping formed body according to  claim 4 , wherein an area ratio of the prior austenite grains having an average grain size of 0.5 to 3.0 μm is 60% or less. 
     
     
         7 . A steel sheet for hot stamping comprising, as a chemical composition, by mass %:
 C: more than 0.40% and 0.70% or less;   Si: 0.010% to 1.30%;   Mn: more than 0.60% and 3.00% or less;   P: 0.100% or less;   S: 0.0100% or less;   N: 0.0130% or less;   O: 0.0200% or less;   Al: 0.0010% to 0.500%;   Cr: 0.010% to 0.80%;   Nb: 0% to 0.100%;   Ti: 0% to 0.100%;   B: 0% to 0.0100%;   Mo: 0% to 1.00%;   Co: 0% to 2.00%;   Ni: 0% or more and less than 3.00%;   Cu: 0% to 1.00%;   V: 0% to 1.00%;   W: 0% to 1.000%;   Ca: 0% to 0.010%;   Mg: 0% to 1.000%;   REM: 0% to 1.000%;   Sb: 0% to 1.000%;   Zr: 0% to 1.000%;   Sn: 0% to 1.000%;   As: 0% to 0.100%; and   a remainder of Fe and impurities,   wherein the steel sheet for hot stamping has a microstructure in which S α +S GB , which is a total of an area ratio S α  of ferrite and an area ratio S GB  of a granular bainite, is 10% or more and less than 50%, and   S GB /S α , which is a ratio between the area ratio S GB  of the granular bainite and the area ratio S α  of the ferrite, is 0.30 to 0.70.