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 . An Al—Zn—Si—Mg alloy coated steel strip that comprises a coating of an Al—Zn—Si—Mg alloy on a steel strip, with the microstructure of the coating comprising Mg 2 Si particles, and with the distribution of the Mg 2 Si particles being such that there is only a small proportion of Mg 2 Si particles or at least substantially no Mg 2 Si particles in the surface of the coating. 
     
     
         2 . The alloy coated steel strip defined in  claim 1  wherein the small proportion of Mg 2 Si particles in the surface region of the coating is no more than 10 wt. % of the Mg 2 Si particles. 
     
     
         3 . The alloy coated steel strip defined in  claim 1  wherein the Al—Zn—Si—Mg alloy comprises the following ranges in % by weight of the elements aluminium, zinc, silicon, and magnesium: 
       
         
           
                 
                 
                 
               
                     
                     
                 
                     
                   Aluminium: 
                   40 to 60% 
                 
                     
                   Zinc: 
                   40 to 60% 
                 
                     
                   Silicon: 
                    0.3 to 3% 
                 
                     
                   Magnesium 
                   0.3 to 10%  
                 
                     
                     
                 
             
                
               
               
                
                
                
                
                
               
            
           
         
       
     
     
         4 . The alloy coated steel strip defined in  claim 1  wherein the coating thickness is less than 30 μm 
     
     
         5 . The alloy coated steel strip defined in  claim 1  wherein the coating thickness is greater than 7 μm. 
     
     
         6 . The alloy coated steel strip defined in  claim 1  wherein the coating contains more than 250 ppm Sr, with the Sr addition promoting the formation of the above distribution of Mg 2 Si particles in the coating. 
     
     
         7 . The alloy coated steel strip defined in  claim 6  wherein the coating contains more than 500 ppm Sr. 
     
     
         8 . The alloy coated steel strip defined in  claim 6  wherein the coating contains more than 1000 ppm Sr. 
     
     
         9 . The alloy coated steel strip defined in  claim 1  wherein the coating contains less than 3000 ppm Sr. 
     
     
         10 . A hot-dip coating method for forming a coating of a corrosion-resistant Al—Zn—Si—Mg alloy on a steel strip that is characterised by passing the steel strip through a hot dip coating bath that contains Al, Zn, Si, Mg, and more than 250 ppm Sr and optionally other elements and forming an alloy coating on the strip that has Mg 2 Si particles in the coating microstructure with the distribution of the Mg 2 Si particles being such that there is only a small proportion of Mg 2 Si particles or substantially no Mg 2 Si particles in the surface of the coating. 
     
     
         11 . The method defined in  claim 10  wherein the small proportion of Mg 2 Si particles in the surface region of the coating is no more than 10 wt. % of the Mg 2 Si particles. 
     
     
         12 . The method defined in  claim 10  or  claim 11  wherein the coating contains more than 500 ppm Sr. 
     
     
         13 . The method defined in  claim 12  wherein the coating contains at least 1000 ppm Sr. 
     
     
         14 . The method defined in  claim 12  wherein the molten bath contains less than 3000 ppm Sr. 
     
     
         15 . A hot-dip coating method for forming a coating of a corrosion-resistant Al—Zn—Si—Mg alloy on a steel strip that is characterised by passing the steel strip through a hot dip coating bath that contains Al, Zn, Si, and Mg and optionally other elements and forming an alloy coating on the strip, and cooling coated strip exiting the coating bath during solidification of the coating at a rate that is controlled so that the distribution of Mg 2 Si particles in the coating microstructure is such that there is only a small proportion of Mg 2 Si particles or substantially no Mg 2 Si particles in the surface of the coating. 
     
     
         16 . The method defined in  claim 15  wherein the small proportion of Mg 2 Si particles in the surface region of the coating is no more than 10 wt. % of the Mg 2 Si particles. 
     
     
         17 . The method defined in  claim 15  or  claim 16  comprises selecting the cooling rate for coated strip exiting the coating bath to be less than a threshhold cooling rate. 
     
     
         18 . The method defined in  claim 15  comprises selecting the cooling rate for coated strip exiting the coating bath to be less than 80° C./sec for coating masses up to 75 grams per square metre of strip surface per side. 
     
     
         19 . The method defined in  claim 15  comprises selecting the cooling rate for coated strip exiting the coating bath to be less than 50° C./sec for coating masses of 75-100 grams per square metre of strip surface per side. 
     
     
         20 . The method defined in  claim 15  comprises selecting the cooling rate to be at least 11° C./sec. 
     
     
         21 . The method defined in  claim 15  comprises, for a coating having an average thickness of 22 μm, selecting the cooling rate for coated strip exiting the coating bath during solidification as follows:
 (a) 55° C./sec in a temperature range of 600-530° C., 
 (b) 70° C./sec in a temperature range of 530-500° C., and 
 (c) 80° C./sec in a temperature range of 500-300° C. 
 
     
     
         22 . A hot-dip coating method for forming a coating of a corrosion-resistant Al—Zn—Si—Mg alloy on a steel strip that is characterised by passing the steel strip through a hot dip coating bath that contains Al, Zn, Si, and Mg and optionally other elements and forming an alloy coating on the strip with minimal variation in the thickness of the coating so that the distribution of Mg 2 Si particles in the coating microstructure is such that there is only a small proportion of Mg 2 Si particles or substantially no Mg 2 Si particles in the surface of the coating. 
     
     
         23 . The method defined in  claim 22  wherein the coating thickness variation is no more than 40% in any given 5 mm diameter section of the coating. 
     
     
         24 . The method defined in  claim 22  wherein the coating thickness variation is no more than 30% in any given 5 mm diameter section of the coating. 
     
     
         25 . The method defined in  claim 22  wherein, for a coating thickness of 22 μm, the maximum thickness in any region of the coating greater than 1 mm in diameter is 27 mm. 
     
     
         26 . The method defined in  claim 22  comprises selecting the cooling rate during solidification of coated strip exiting the coating bath to be less than a threshhold cooling rate.