High-strength hot-formed steel sheet member
A high-strength hot-formed steel sheet member exhibiting both a consistent hardness and delayed-fracture resistance, and is characterized in that: the high-strength hot-formed steel sheet member has a prescribed chemical composition; the degree of Mn segregation α (=[maximum Mn concentration (mass %) at the sheet center in the thickness direction]/[average Mn concentration (mass %) at a depth of ¼ of the total thickness of the sheet from the surface]) is less than or equal to 1.6; the steel purity value as defined in JIS G 0555 (2003) is less than or equal to 0.08%; the average grain size for prior γ grains is less than or equal to 10 μm; and the number density of the residual carbides is less than or equal to 4×10 3 particles/mm 2 .
1. A high-strength hot-formed steel sheet member having a chemical composition comprising, by mass %,
C: 0.25 to 0.40%,
Si: 0.005 to 0.14%,
Mn: 1.50% or less,
P: 0.02% or less,
S: 0.005% or less,
sol. Al: 0.0002 to 1.0%,
N: 0.01% or less,
Cr: 0.25 to 3.00%,
Ti: 0.01 to 0.05%,
Nb: 0.01 to 0.50%,
B: 0.001 to 0.01%, and
a balance of Fe and unavoidable impurities;
a total of content of Mn and content of Cr of 1.5 to 3.5%;
an Mn segregation ratio α represented by the following formula (i) of 1.6 or less;
a value of cleanliness of steel prescribed by JIS G 0555 (2003) of 0.08% or less;
having an average grain size of prior γ-grains of 10 μm or less; and
a number density of residual carbides present of 4×10 3 /mm 2 or less:
α=[Maximum Mn concentration at center part in sheet thickness(mass %)]/[Average Mn concentration at position of ¼ sheet thickness depth from surface(mass %)] (i).
2. The high-strength hot-formed steel sheet member according to claim 1 wherein said chemical composition further includes, by mass %, one or more elements selected from
Ni: 0 to 3.0%,
Cu: 0 to 1.0%,
Mo: 0 to 2.0%,
V: 0 to 0.1%, and
Ca: 0 to 0.01%.
3. The high-strength hot-formed steel sheet member according to claim 2 having a plating layer at the surface of said steel sheet.
4. The high-strength hot-formed steel sheet member according to claim 2 wherein said steel sheet member has a tensile strength of 1.7 GPa or more.
5. The high-strength hot-formed steel sheet member according to claim 1 having a plating layer at the surface of said steel sheet.
6. The high-strength hot-formed steel sheet member according to claim 5 wherein said steel sheet member has a tensile strength of 1.7 GPa or more.
7. The high-strength hot-formed steel sheet member according to claim 1 wherein said steel sheet member has a tensile strength of 1.7 GPa or more.