Mitigating liquid metal embrittlement in zinc-coated press hardened steels

Abstract

Methods of reducing liquid metal embrittlement (LME) in zinc-coated high-strength steel alloys are provided. In one variation, the method includes decarburizing an exposed surface of a high-strength steel alloy to form a decarburized surface layer. The decarburized surface layer has a thickness of less than or equal to about 50 micrometers. The decarburized surface layer may have greater than or equal to about 80 volume % ferrite. The method also includes applying a zinc-based coating to the decarburized surface layer. A blank is formed from the high-strength steel alloy. The method also includes heating and press hardening the blank to form a press-hardened component having an ultimate tensile strength of greater than or equal to about 1,100 MPa that is substantially free of liquid metal embrittlement.

Description

INTRODUCTION[0001]This section provides background information related to the present disclosure which is not necessarily prior art.[0002]Press-hardened steel (PHS), also referred to as “hot-stamped steel” or “boron-steel” (e.g., 22MnB5 alloy), is one of the strongest steels used for automotive body structural applications, typically having tensile strength properties on the order of about 1,400 megapascals (MPa) or higher. Such steel has desirable properties, including forming steel components with significant increases in strength-to-weight ratios. PHS components have become ever more prevalent in various industries and applications, including general manufacturing, construction equipment, automotive or other transportation industries, home or industrial structures, and the like.[0003]For example, when manufacturing vehicles, especially automobiles, continual improvement in fuel efficiency and performance is desirable, thus PHS components have been increasingly used. PHS component...

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Benefits of technology

[0003]For example, when manufacturing vehicles, especially automobiles, continual improvement in fuel efficiency and performance is desirable, thus PHS components have been increasingly used. PHS components are often used for forming load-bearing components, like door beams, which usually require high-strength materials. Thus, the finished state of these steels are designed to have high strength and enough ductility to resist external forces, for example, to resist intrusion into the passenger compartment while maintaining load so as to provide protection to the occupants. Moreover, galvanized PHS components may provide cathodic protection.

[0023]In another variation, the present disclosure further provides a method of reducing liquid metal embrittlement (LME) in zinc-coated high-strength steel. The method includes decarburizing an exposed surface of a high-strength steel alloy to form a decarburized surface layer. The decarburized surface layer has a thickness of less than or equal to about 50 micrometers. The decarburized surface layer includes greater than or equal to about 80 volume % ferrite. The high-strength steel alloy is then hot-dip galvanized in a heated zinc galvanization bath to form a zinc-based coating over the decarburized surface layer. A blank is formed from the high-strength steel alloy. The blank is heated for austenitization and press hardened to form a press-hardened component having an ultimate tensile strength of greater than or equal to about 1,300 MPa to less than or equal to about 2,000 MPa. The press-hardened component is substantially free of liquid metal embrittlement.

Problems solved by technology

However, liquid metal embrittlement (LME) may occur when a metallic system is exposed to a liquid metal, such as zinc, during forming at high temperature, resulting in potential cracking and a reduction of total elongation or diminished ductility of a material.

LME may also result in decreased ultimate tensile strength.

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