HOT-FORMED STEEL SHEET MEMBER
A hot-formed steel sheet member having a chemical composition includes, in terms of mass %, from 0.08 to 0.16% of C, 0.19% or less of Si, from 0.40 to 1.50% of Mn, 0.02% or less of P, 0.01% or less of S, from 0.01 to 1.0% of sol. Al, 0.01% or less of N, from 0.25 to 3.00% of Cr, from 0.01 to 0.05% of Ti, from 0.001 to 0.01% of B, and a reminder consisting of Fe and impurities, wherein a total volume fraction of martensite, tempered martensite, and bainite is 50% or more, and a volume fraction of ferrite is 3% or less, an average grain size of prior γ grains is 10 μm or less, and a number density of residual carbides which are present is 4×10 3 per mm 2 or less.
1 . A hot-formed steel sheet member having a chemical composition consisting of, in terms of mass %, from 0.08 to 0.16% of C, 0.19% or less of Si, from 0.40 to 1.50% of Mn, 0.02% or less of P, 0.01% or less of S, from 0.01 to 1.0% of sol. Al, 0.01% or less of N, from 0.25 to 3.00% of Cr, from 0.01 to 0.05% of Ti, from 0.001 to 0.01% of B, from 0 to 0.50% of Nb, from 0 to 2.0% of Ni, from 0 to 1.0% of Cu, from 0 to 1.0% of Mo, from 0 to 1.0% of V, from 0 to 0.005% of Ca, and a remainder consisting of Fe and impurities,
wherein a total volume fraction of martensite, tempered martensite, and bainite is 50% or more, and a volume fraction of ferrite is 3% or less,
an average grain size of prior γ grains is 10 μm or less, and
a number density of residual carbides which are present is 4×10 3 per mm 2 or less.
2 . The hot-formed steel sheet member according to claim 1 , wherein the chemical composition comprises one or more selected from the group consisting of, in terms of mass %, from 0.003 to 0.50% of Nb, from 0.01 to 2.0% of Ni, from 0.01 to 1.0% of Cu, from 0.01 to 1.0% of Mo, from 0.01 to 1.0% of V, and from 0.001 to 0.005% of Ca.
3 . The hot-formed steel sheet member according to claim 1 , wherein a value of a cleanliness level of steel specified by JIS G 0555 (2003) is 0.08% or less.
4 . The hot-formed steel sheet member according to claim 1 , wherein a segregation degree α of Mn represented by the following formula (i) is 1.6 or less,
α=[maximum Mn concentration (mass %) at a central part of a sheet thickness]/[average Mn concentration (mass %) at a ¼ depth position of the sheet thickness from a surface] (i).
5 . The hot-formed steel sheet member according to claim 1 , wherein the steel sheet member has a surface on which a plating layer is formed.
6 . The hot-formed steel sheet member according to claim 1 , wherein the steel sheet member has tensile strength of 1.0 GPa or more.
7 . A hot-formed steel sheet member having a chemical composition comprising, in terms of mass %, from 0.08 to 0.16% of C, 0.19% or less of Si, from 0.40 to 1.50% of Mn, 0.02% or less of P, 0.01% or less of S, from 0.01 to 1.0% of sol. Al, 0.01% or less of N, from 0.25 to 3.00% of Cr, from 0.01 to 0.05% of Ti, from 0.001 to 0.01% of B, from 0 to 0.50% of Nb, from 0 to 2.0% of Ni, from 0 to 1.0% of Cu, from 0 to 1.0% of Mo, from 0 to 1.0% of V, from 0 to 0.005% of Ca, and a remainder comprising Fe and impurities,
wherein a total volume fraction of martensite, tempered martensite, and bainite is 50% or more, and a volume fraction of ferrite is 3% or less,
an average grain size of prior γ grains is 10 μm or less, and
a number density of residual carbides which are present is 4×10 3 per mm 2 or less.
8 . The hot-formed steel sheet member according to claim 7 , wherein the chemical composition comprises one or more, in terms of mass %, from 0.003 to 0.50% of Nb, from 0.01 to 2.0% of Ni, from 0.01 to 1.0% of Cu, from 0.01 to 1.0% of Mo, from 0.01 to 1.0% of V, or from 0.001 to 0.005% of Ca.