Metal-coated steel strip

Abstract

An Al—Zn—Si—Mg alloy coated strip that has Mg 2 Si particles in the coating microstructure is disclosed. The distribution of Mg 2 Si particles is such that the surface of the coating has only a small proportion of Mg 2 Si particles or is at least substantially free of any Mg 2 Si particles.

1 - 26 . (canceled) 
     
     
         27 . An Al—Zn—Si—Mg alloy coated steel strip comprising a coating of an Al—Zn—Si—Mg alloy, the Al—Zn—Si—Mg alloy coating comprising:
 40 to 60 wt % aluminum; 40 to 60 wt % zinc; 0.3 to 3 wt % silicon; and 0.3 to 10 wt % magnesium; 
 a thickness greater than 7 μm and less than 30 μm and a coating thickness variation being no more than 40% in a 5 mm diameter section of the coating; and 
 a microstructure comprising Mg 2 Si particles, 
 wherein the surface of the coating has no more than 10 wt % of Mg 2 Si particles to lower risk of mottling defect on the surface. 
 
     
     
         28 . The Al—Zn—Si—Mg alloy coated steel strip defined in  claim 27 , wherein the coating thickness variation is no more than 30% in a 5 mm diameter section of the coating. 
     
     
         29 . The Al—Zn—Si—Mg alloy coated steel strip defined in  claim 27 , wherein, for a coating thickness of 22 μm, the maximum thickness in a region of the coating greater than 1 mm in diameter is 27 μm. 
     
     
         30 . The Al—Zn—Si—Mg alloy coated steel strip defined in  claim 27 , wherein the coating comprises one or more of strontium, iron, vanadium and chromium. 
     
     
         31 . The Al—Zn—Si—Mg alloy coated steel strip defined in  claim 27 , wherein the surface of the coating has no Mg 2 Si particles. 
     
     
         32 . The Al—Zn—Si—Mg alloy coated steel strip defined in  claim 27 , wherein the surface of the coating is at least substantially free of Mg 2 Si particles. 
     
     
         33 . A hot-dip coating method for forming a coating of a corrosion-resistant Al—Zn—Si—Mg alloy on a steel strip with lower risk of mottling defect on the surface of the coating, the method comprising:
 passing a steel strip through a hot dip coating bath that comprises aluminum, zinc, silicon, and magnesium to form an alloy coating on the steel strip having, in weight %, 40 to 60% aluminum; 40 to 60% zinc; 0.3 to 3% silicon; and 0.3 to 10% magnesium, and 
 cooling the coated steel strip exiting the hot dip coating bath during solidification of the coating 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 and at a cooling rate of less than 50° C./sec and greater than 11° C./sec for coating masses of 75-100 grams per square meter of strip surface per side, 
 wherein the coating having a coating thickness greater than 7 μm and less than 30 μm and a coating thickness variation being no more than 40% in a 5 mm diameter section of the coating, and 
 wherein the coating having a microstructure comprising Mg 2 Si particles such that the surface of the coating has no more than 10 wt % of Mg 2 Si particles to lower risk of mottling defect on the surface. 
 
     
     
         34 . The method defined in  claim 33 , wherein the coating thickness variation is no more than 30% in a 5 mm diameter section of the coating. 
     
     
         35 . The method defined in  claim 33 , wherein, for a coating thickness of 22 μm, the maximum thickness in a region of the coating greater than 1 mm in diameter is 27 μm. 
     
     
         36 . The method defined in  claim 33 , wherein the hot dip coating bath comprises one or more of strontium, iron, vanadium and chromium and the coating comprises one or more of strontium, iron, vanadium and chromium. 
     
     
         37 . The method defined in  claim 33 , wherein the surface of the coating has no Mg 2 Si particles. 
     
     
         38 . The method defined in  claim 33 , wherein the surface of the coating is at least substantially free of Mg 2 Si particles.