Metal-coated steel strip

Abstract

An Al—Zn—Si—Mg alloy coated strip that has Mg2Si particles in the coating microstructure is disclosed. The distribution of Mg2Si particles is such that a surface region of the coating has only a small proportion of Mg2Si particles or is at least substantially free of any Mg2Si particles.

1 .- 28 . (canceled) 
     
     
         29 . A hot-dip coating method for forming a corrosion-resistant and cracking-resistant Al—Zn—Si—Mg alloy coated steel strip, the method comprising:
 passing a steel strip through a hot dip coating bath that contains Al, Zn, Si, and Mg to form an alloy coating on the steel strip, the alloy coating comprising, in weight %, 40 to 60% aluminum, 40 to 60% zinc, 0.3 to 3% silicon, 0.3 to 10% magnesium; 
 passing the steel strip with alloy coating through a coating thickness control station and controlling a short range coating thickness variation of the alloy coating on the steel strip to be no greater than 40% in a 5 mm diameter section of the coating; and 
 cooling the alloy coating to form the Al—Zn—Si—Mg alloy coated steel strip; 
 wherein the alloy coating comprises Mg 2 Si particles, and 
 wherein controlling the short range coating thickness variation controls a microstructure distribution of the Mg 2 Si particles in the alloy coating such that there is no more than 10 wt. % of the Mg 2 Si particles in a surface region of the coating to form a corrosion-resistant and cracking-resistant Al—Zn—Si—Mg alloy coated steel strip. 
 
     
     
         30 . The method of  claim 29 , wherein the coating thickness variation is no more than 30% in a 5 mm diameter section of the coating. 
     
     
         31 . The method of  claim 29 , wherein passing the steel strip with alloy coating through the coating thickness control station further controls a coating thickness of the alloy coating to be greater than 7 μm and less than 30 μm. 
     
     
         32 . The method of  claim 29 , wherein the hot dip coating bath further comprises one or more of strontium, iron, vanadium, and chromium, and the alloy coating comprises one or more of strontium, iron, vanadium, and chromium. 
     
     
         33 . The method of  claim 29 , wherein the surface region of the coating comprises substantially no Mg 2 Si particles. 
     
     
         34 . The method of  claim 29 , wherein the surface region of the coating comprises no Mg 2 Si particles. 
     
     
         35 . The method of  claim 29 , wherein the coating thickness control station is a gas knife or gas wiping station. 
     
     
         36 . The method of  claim 29 , wherein cooling the alloy coating occurs at a cooling rate of less than 80° C./sec and greater than 11° C./sec for coating masses up to 75 grams per square meter of strip surface per side, or at a cooling rate of less than 50° C./sec and greater than 11° C./sec for coating masses 75 to 100 grams per square meter of strip surface per side.