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.

What is claimed is: 
     
         1 . A method of reducing liquid metal embrittlement (LME) in zinc-coated high-strength steel, the method comprising:
 decarburizing an exposed surface of a high-strength steel alloy to form a decarburized surface layer having a thickness of less than or equal to about 50 micrometers and comprising greater than or equal to about 80 volume % ferrite;   applying a zinc-based coating to the decarburized surface layer;   forming a blank from the high-strength steel alloy; and   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.   
     
     
         2 . The method of  claim 1 , the decarburizing occurs at a temperature of greater than or equal to about 700° C. in an environment comprising nitrogen and water. 
     
     
         3 . The method of  claim 1 , wherein the zinc-based coating comprises passing the blank through a zinc galvanization bath at a temperature of greater than or equal to about 420° C. to less than or equal to about 480° C. 
     
     
         4 . The method of  claim 1 , wherein the decarburized surface layer has a thickness of greater than or equal to about 20 micrometers to less than or equal to about 50 micrometers. 
     
     
         5 . The method of  claim 1 , wherein the decarburized surface layer comprises greater than or equal to about 90 volume % ferrite. 
     
     
         6 . The method of  claim 1 , wherein the steel alloy comprises carbon at less than or equal to about 0.4 weight %. 
     
     
         7 . The method of  claim 1 , wherein the steel alloy comprises:
 manganese at greater than or equal to about 0.2 weight % to less than or equal to about 2.0 weight %;   carbon at greater than or equal to about 0.15 weight % to less than or equal to about 0.4 weight %; and   silicon at greater than 0.1 weight % to less than or equal to about 1 weight %.   
     
     
         8 . The method of  claim 1 , wherein the heating occurs at a temperature of greater than or equal to about 800° C. to less than or equal to about 950° C. for austenitization of the high-strength steel alloy. 
     
     
         9 . The method of  claim 1 , further comprising quenching the press-hardened component to below room temperature after the press hardening. 
     
     
         10 . The method of  claim 1 , wherein the high-strength steel alloy has a first side and a second side opposite to the first side, wherein after the decarburizing, the first side has a first decarburized surface layer and the second side has a second decarburized surface layer that sandwich a central region. 
     
     
         11 . The method of  claim 10 , wherein the heating austenitizes the high-strength steel alloy and the method further comprises quenching the high-strength steel alloy blank after the heating and press hardening so that the central region comprises greater than or equal to about 98 volume % martensite. 
     
     
         12 . The method of  claim 1 , wherein the press-hardened component has an ultimate tensile strength of greater than or equal to about 1,300 MPa to less than or equal to about 2,000 MPa. 
     
     
         13 . A method of reducing liquid metal embrittlement (LME) in zinc-coated high-strength steel, the method comprising:
 decarburizing an exposed surface of a high-strength steel alloy to form a decarburized surface layer having a thickness of less than or equal to about 50 micrometers and comprising greater than or equal to about 80 volume % ferrite;   hot dip galvanizing the high-strength steel alloy in a heated zinc galvanization bath to form a zinc-based coating over the decarburized surface layer;   forming a blank from the high-strength steel alloy;   heating the blank to austenitize the high-strength steel alloy; and   press hardening the blank 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 that is substantially free of liquid metal embrittlement.   
     
     
         14 . The method of  claim 13 , wherein the decarburizing occurs at a temperature of greater than or equal to about 700° C. in an environment comprising nitrogen and water. 
     
     
         15 . The method of  claim 13 , wherein the zinc-based coating comprises passing the blank through a zinc galvanization bath at a temperature of greater than or equal to about 420° C. to less than or equal to about 480° C. 
     
     
         16 . The method of  claim 13 , wherein the decarburized surface layer has a thickness of greater than or equal to about 20 micrometers to less than or equal to about 50 micrometers. 
     
     
         17 . The method of  claim 13 , wherein the decarburized surface layer comprises greater than or equal to about 90 volume % ferrite. 
     
     
         18 . The method of  claim 13 , wherein the steel alloy comprises:
 manganese at greater than or equal to about 0.2 weight % to less than or equal to about 2.0 weight %;   carbon at greater than or equal to about 0.15 weight % to less than or equal to about 0.4 weight %; and   silicon at greater than 0.1 weight % to less than or equal to about 1 weight %.   
     
     
         19 . The method of  claim 13 , wherein the heating occurs at a temperature of greater than or equal to about 800° C. to less than or equal to about 950° C. 
     
     
         20 . The method of  claim 13 , wherein the high-strength steel alloy has a first side and a second side opposite to the first side, wherein after the decarburizing, the first side has a first decarburized surface layer and the second side has a second decarburized surface layer that sandwich a central region, wherein the method further comprises quenching the blank after the heating and press hardening so that the central region comprises greater than or equal to about 98 volume % martensite.