Jfe Steel Corporation

Kazuaki Tsuchimoto

Akira Matsuzaki

Takeshi Matsuda

Katsuya Hoshino

Shinichi Furuya

Shinji Otsuka

Kentaro Hata

A steel sheet mainly suitable for strength members of automobiles or building materials, which has a tensile strength of 1,180 MPa or more, and which is excellent in delayed fracture resistance and primary rust prevention performance. The steel sheet includes a coating, placed on a surface of a cold-rolled steel sheet with a tensile strength of 1,180 MPa or more, containing one or more metalates selected from molybdates and tungstates and a P compound. The sum of the coating weights of the metalates in terms of Mo and W is 10 mg/mto 1,000 mg/mand is preferably 50 mg/mto 1,000 mg/m. The coating weight of the P compound in terms of P is 10 mg/mto 1,000 mg/m.

High-strength cold-rolled steel sheet

Provided is a method for manufacturing a zinc-based coated steel sheet having excellent press formability. The method includes an oxide layer-forming step of bringing the zinc-based coated steel sheet into contact with an acidic solution, holding the zinc-based coated steel sheet for 1 second to 60 seconds, and then water-washing the zinc-based coated steel sheet and a neutralization step of holding a zinc-based oxide layer formed in the oxide layer-forming step for 0.5 seconds or more in such a state that a surface of the zinc-based oxide layer is in contact with an alkaline aqueous solution, water-washing the zinc-based oxide layer, and then drying the zinc-based oxide layer. The acidic solution contains HFNa and/or HFK in a total amount of 0.10 g/L to 5.0 g/L.

Method for manufacturing zinc-based coated steel sheet

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Tokyo, Japan

Kazuaki Tsuchimoto