High-strength plated steel sheet having excellent plating properties, workability, and delayed fracture resistance, and method for producing same

Abstract

The high-strength plated steel sheet of the present invention has a plated layer on the surface of a base steel sheet and contains predetermined steel components. The steel sheet includes, in the order from the interface of the base steel sheet and the plated layer towards the base steel sheet: a soft layer having a Vickers hardness that is 90% or less of the Vickers hardness at a portion t/4 of the base steel sheet, where t is a sheet thickness of the base steel sheet: and a hard layer containing martensite, bainite, and ferrite in predetermined ranges. The average depth D of the soft layer is 20 μm or greater, and the average depth d of an internal oxide layer is 4 μm or greater and smaller than D.

Description

TECHNICAL FIELD[0001]The present invention relates to a high-strength plated steel sheet having a tensile strength of 980 MPa or higher and having excellent plalability; excellent formability in terms or bendability, hole expandability, and balance between strength and ductility; and excellent delayed fracture resistance, and also relates to a method for producing the steel sheet. The term plated steel sheet of the present invention denotes both of a hot-dip galvanized steel sheet and a hot-dip galvannealed steel sheet.BACKGROUND ART[0002]Hot-dip galvanized steel sheets and hot-dip galvannealed steel sheets that are widely used in the fields of automobiles, transport aircrafts and the like, are required to have, in addition to high strength, also excellent formability in terms of bendability, hole expandability (stretch flangeability), and balance between strength and ductility, and to exhibit moreover excellent delayed fracture resistance.[0003]Adding a substantial amount of streng...

AI Technical Summary

Benefits of technology

[0022]The plated steel sheet of the present invention is configured to have, sequentially from an interface of a plated layer and a base steel sheet towards the base steel sheet; an internal oxide layer having at least one oxide selected from the group consisting of Si and Mn; a soft layer including the region of the internal oxide layer; and a hard layer which is other than the soft layer (and which contains martensite and bainite: 60 area % or more to less than 95 area % and polygonal ferrite; more than 5 area % to 40 area % or less, as a matrix phase structure). In partictilar, the internal oxide layer is utilized as a hydrogen trap site by controlling the average depth d of the internal oxide layer to be 4 μm or greater, and accordingly, it becomes possible to suppress hydrogen embrittlement effectively, and to obtain a high-strength plated steel sheet having a tensile strength of 980 Pa or higher and being excellent in all of formability in terms of bendability, hole expandability, and balance between strength and ductility; and delayed fracture resistance. Preferably, the relationship between the average depth d of the internal oxide layer and the average depth D of the soft layer at includes the region of the internal oxide layer is controlled properly, whereby bendability and delayed fracture resistance in particular are further enhanced.

Problems solved by technology

However, Si and Mn are readily oxidizable elements, and hot-dip galvanizing wettability is significantly impaired by Si oxides, Mn oxides, and complex oxides of Si and Mn, that are formed on the surface.

This poor wettability gives rise to problems such as bare spots and the like.

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