Jfe Steel Corporation

Kenji Tsuzumi

Minoru Tanaka

Yoshiyasu Kawasaki

Katsutoshi Takashima

Yuji Miki

Michitaka Sakurai

Shingo Arakawa

Hideya Masaki

Masaki Hideya

A mold powder which prevents surface defects from occurring on a surface of a cast slab of Al-containing hypo-peritectic steel having Al: 0.2% to 2.0%, by mass %, and, in a hypo-peritectic region, C: 0.08% to 0.17%, by mass %. The mold powder includes CaO, SiO, NaO, LiO, F, and C. LiO/NaO is 0.6 or more, 1.0+0.05×Al≤CaO/SiO≤2.0−0.35×Al, 10<LiO+0.5×NaO+0.8×F<20, and 1.00≤F/(LiO+0.5×NaO+1.46)≤1.24 are satisfied where Al is content by mass % of molten steel, and respective contents of the remaining elements are by mass %. A viscosity of the mold powder at 1,300° C. is in a range of 0.05 Pa·s to 0.20 Pa·s, and a crystallization temperature of the mold powder is in a range of 1,100° C. to 1,250° C.

Mold powder for continuous casting of A1-containing sub-peritectic steel and continuous casting method

A hot-pressed member has a predetermined chemical composition. In the hot-pressed member, a steel sheet has a microstructure in which a prior austenite average grain diameter is 8 μm or less, and martensite is present in a volume fraction of 95% or greater in a region within 30 μm of a surface; a Ni diffusion region having a thickness of 0.5 μm or greater exists in a surface layer; a standard deviation of Vickers hardness values is 35 or less; Mndif (mass %) in a sheet thickness direction ≤0.20, where Mndif (mass %) is a degree of Mn segregation; and a tensile strength is 1780 MPa or greater.

Hot-pressed member, cold-rolled steel sheet for hot-pressed member, and method for producing the same

A high strength steel sheet having excellent surface quality and formability with a tensile strength of 980 MPa or more and a TS-El balance of 30000 MPa % or more is provided. A high strength steel sheet comprises: a chemical composition containing C: 0.08% to 0.30%, Si: 2.0% or less, Mn: more than 3.0% and 10.0% or less, P: 0.05% or less, S: 0.01% or less, Al: 1.5% or less, Ti: 0.010% to 0.300%, and N: 0.0020% to 0.0100% in a range satisfying 1.1&#x2264;(Ti+Mn/400)/(0.01+5N)&#x2264;6.0; and a microstructure including a retained austenite phase and a ferrite phase, wherein a ratio &#x3b3;Mn/&#x3b1;Mn of an average Mn concentration (&#x3b3;Mn) of the retained austenite phase to an average Mn concentration (&#x3b1;Mn) of the ferrite phase is 1.5 or more.

Material for high strength steel sheets, hot rolled material for high strength steel sheets, hot-rolled and annealed material for high strength steel sheets, high strength steel sheet, high strength hot-dip-coated steel sheet, high strength electroplated steel sheet, and method of manufacturing same

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Tokyo, Japan

Kenji Tsuzumi