STEEL SHEET FOR HOT STAMPING, METHOD FOR PRODUCTION THEREOF, AND HOT STAMPING STEEL MATERIAL
A hot stamping steel material, which secures good hydrogen embrittlement resistance even when the steel sheet after hot stamping is subjected to processing leading to remaining of stress, such as piercing and which is easily practicable, wherein the steel sheet has the chemical composition of: C: 0.18 to 0.26%; Si: more than 0.02% and not more than 0.05%; Mn: 1.0 to 1.5%; P: 0.03% or less; S: 0.02% or less; Al: 0.001 to 0.5%; N: 0.1% or less;O: 0.001 to 0.02%; Cr: 0 to 2.0%; Mo: 0 to 1.0%; V: 0 to 0.5%; W: 0 to 0.5%; Ni: 0 to 5.0%; B: 0 to 0.01%; Ti: 0 to 0.5%; Nb: 0 to 0.5%; Cu: 0 to 1.0%; and balance: Fe and impurities, in terms of % by mass, the concentration of a Mn-containing inclusion is not less than 0.010% by mass and less than 0.25% by mass, and the number ratio of a Mn oxide to the inclusion having a maximum length of 1.0 to 4.0 pm is 10.0% or more.
1 . A method for production of a steel sheet for hot stamping, the method comprising:
hot-rolling a steel piece having a chemical composition of:
C: 0.18 to 0.26%;
Si: more than 0.02% and not more than 0.05%;
Mn: 1.0 to 1.5%;
P: 0.03% or less;
S: 0.02% or less;
Al: 0.001 to 0.5%;
N: 0.1% or less;
O: 0.0010 to 0.020%;
Cr: 0 to 2.0%;
Mo: 0 to 1.0%;
V: 0 to 0.5%;
W: 0 to 0.5%;
Ni: 0 to 5.0%;
B: 0 to 0.01%;
Ti: 0 to 0.5%;
Nb: 0 to 0.5%;
Cu: 0 to 1.0%; and
balance: Fe and impurities, in terms of % by mass;
coiling the steel piece at a temperature of 690° C. or higher to form a hot-rolled steel sheet; and
cold-rolling the hot-rolled steel sheet at a draft of 10 to 90% to form a cold-rolled steel sheet.
2 . The method for production of a steel sheet for hot stamping according to claim 1 , wherein the chemical composition comprises one or more selected from the group consisting of
Cr: 0.01 to 2.0%;
Mo: 0.01 to 1.0%;
V: 0.01 to 0.5%;
W: 0.01 to 0.5%;
Ni: 0.01 to 5.0%; and
B: 0.0005 to 0.01%, in terms of % by mass.
3 . The method for production of a steel sheet for hot stamping according to claim 1 , wherein the chemical composition comprises one or more selected from the group consisting of
Ti: 0.001 to 0.5%;
Nb: 0.001 to 0.5%; and
Cu: 0.01 to 1.0%, in terms of % by mass.
4 . The method for production of a steel sheet for hot stamping according to claim 1 , further comprising,
immersing the cold-rolled sheet in an aluminum hot-dipping bath to form an aluminum hot-dipping layer on the surface of the cold-rolled steel sheet.
5 . The method for production of a steel sheet for hot stamping according to claim 1 , further comprising,
immersing the cold-rolled sheet in a hot-dip galvanizing bath to form a hot-dip galvanized layer on the surface of the cold-rolled steel sheet.
6 . The method for production of a steel sheet for hot stamping according to claim 1 , further comprising,
immersing the cold-rolled sheet in a hot-dip galvanizing bath, and then heating the cold-rolled sheet at a temperature of 600° C. or lower to form an alloyed hot-dip galvanized layer on the surface of the cold-rolled steel sheet.