Kobe Steel, Ltd.

Kosuke Shibata

Toshio Murakami

Atsuhiro Shiraki

Katsura Kajihara

Kenji Saito

Michiharu Nakaya

Yukihiro Utsumi

Muneaki Ikeda

Fumio Yuse

Junichiro Kinugasa

Elijah Kakiuchi

Takahiro Ozawa

Toshiya Nakata

Keisuke Nakata

A high-strength steel sheet with excellent delayed fracture resistance, having a tensile strength of 1700 MPa or larger, including a predetermined component composition, having a martensite structure whose ratio accounts for 95 area % or more of the entire metallographic structure, and having a transition metal carbide whose ratio accounts for 0.8 volume % or more of the entire metallographic structure.

High-strength steel sheet having excellent delayed fracture resistance

Provided is an ultra-high-strength steel sheet having a component composition that includes specific amounts of each of C, Mn, and Al and a remainder of iron and unavoidable impurities, and in which the amounts of each of P, S, and N among the unavoidable impurities are limited to a specific amount. The ultra-high-strength steel sheet includes 2 area% or more of a region having a structure that includes 90% or more of martensite and 0.5% or more of residual austentite by area ratio relative to the entire structure, the local Mn concentration in said region being at least 1.1 times that of the Mn content of the entire steel sheet. The ultra-high-strength steel sheet has a tensile strength of 1470 MPa or more.

Ultra-high-strength steel sheet having excellent delayed fracture resistance at cut end thereof

A high strength cold-rolled steel sheet has a component composition containing specific amounts of C, Si, Mn, P, S, N and Al, respectively with a remainder being iron and inevitable impurities. The steel sheet contains 95% or more of martensite in terms of area ratio, and contains 5% or less (inclusive of 0%) of residual austenite and ferrite in terms of a total area ratio. An average size of a carbide is 60 nm or less in terms of an equivalent circle diameter, and a number density of the carbide having the equivalent circle diameter of 25 nm or more is 5.0&#xd7;10pieces or less per mm. The steel sheet has a yield strength of 1,180 MPa or more and a tensile strength of 1,470 MPa or more.

High-strength cold-rolled steel sheet having excellent bendability

The present disclosure relates to a high-strength hot-dip galvannealed steel sheet excellent in bake hardenability and bendability and having a component composition containing, in mass %, C: from 0.05 to 0.30%, Si: from 0.5 to 3.0%, Mn: from 0.2 to 3.0%, P: from 0 to 0.10%, S: from 0 to 0.010%, N: form 0 to 0.010%, and Al: from 0.001 to 0.10%, with the remainder being iron and unavoidable impurities. The steel sheet has a steel structure containing, in terms of area percentage, martensite: form 50 to 85% and ferrite: 0% or more and less than 5%, with the remainder being bainite. The steel sheet has a dislocation density of 5.0&#xd7;10mor more, a solute carbon amount of 0.08 mass % or more and a tensile strength of 1180 MPa or more.

High-strength hot-dip galvannealed steel sheet having excellent bake hardening property and bendability

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Hyogo, Japan

Kosuke Shibata