ZnAlMg-Coated Metal Sheet with Improved Flexibility and Corresponding Manufacturing Process

Abstract

A process for the manufacture of a pre-painted sheet. The process includes supplying a steel substrate, depositing a metallic coating on at least one face by hot-dipping of the substrate in a bath including 4.4% to 5.25% by weight aluminum and 0.3% to 0.56% by weight magnesium. The rest of the bath includes exclusively zinc, unavoidable impurities resulting from the process and optionally one or more additional elements including Si, Ti, Ca, Mn, La, Ce and Bi. The content by weight of each additional element in the metallic coating is less than 0.3% and the presence of nickel is excluded. The process further includes solidifying the metallic coating, surface preparation of the metallic coating and painting of the metallic coating. The present invention further provides a pre-painted sheet.

What is claimed is: 
     
         1 . A process for the manufacture of a pre-painted sheet comprising the steps of:
 supplying a steel substrate;   depositing a metallic coating on at least one face by hot-dipping the steel substrate in a bath comprising:
 4.4% to 5.25% by weight aluminum, 
 0.3% to 0.56% by weight magnesium, 
 0.0 to less than 0.3% by weight Si, 
 0.0 to less than 0.3% by weight Ti, 
 0.0 to less than 0.3% by weight Ca, 
 0.0 to less than 0.3% by weight Mn, 
 0.0 to less than 0.3% by weight La, 
 0.0 to less than 0.3% by weight Ce, 
 0.0 to less than 0.3% by weight Bi, 
 a remainder of the bath including zinc and unavoidable impurities resulting from the process, the presence of nickel being excluded; 
   solidifying the metallic coating;   skin-pass treating the steel substrate with the metallic coating to provide a roughness;   surface preparation of the metallic coating; and   painting the metallic coating with a paint film comprising a melamine cross-linked polyester, with the exclusion of cataphoretic paints, thereby providing the pre-painted sheet;   wherein the paint film has a thickness between 1 and 200 μm, and   the metallic coating has a thickness less than or equal to 25 μm.   
     
     
         2 . The manufacturing process of  claim 1 , wherein the paint film has a thickness between 13 and 20 μm, the metallic coating has a thickness between 12 and 20 μm, aluminum content of the bath is from 4.6 to 5.1% by weight, and magnesium content of the bath is 0.45 to 0.55%. 
     
     
         3 . The manufacturing process according  claim 1 , wherein paint film is formed by two successive layers of paints. 
     
     
         4 . The manufacturing process according  claim 3 , wherein the two successive layers of paint consist of a primer layer of the melamine cross-linked polyester containing anti-corrosion pigments, and a finish layer of paint of the melamine cross-linked polyester. 
     
     
         5 . The manufacturing process according  claim 1 , wherein the substrate is a sheet, and the painting is before the sheet is bent. 
     
     
         6 . The manufacturing process according  claim 5 , wherein the paint film has a thickness between 13 and 20 μm, the metallic coating has a thickness between 12 and 20 μm. 
     
     
         7 . The manufacturing process according to  claim 6  wherein aluminum content of the bath is from 4.6 to 5.1% by weight and magnesium content of the bath is 0.45 to 0.55%. 
     
     
         8 . The manufacturing process according to  claim 7 , wherein the solidification of the metallic coating takes place at a cooling rate from 2° C./sec to 35° C./sec. 
     
     
         9 . The manufacturing process according to  claim 8 , wherein the cooling rate is from 2° C./sec to 30° C./sec. 
     
     
         10 . The manufacturing process according to  claim 9 , wherein the cooling rate is between 15 and 35° C./s. 
     
     
         11 . The manufacturing method according to  claim 8 , wherein the metallic coating is free from spangles visible to the naked eye. 
     
     
         12 . The manufacturing process according to  claim 7 , wherein a propensity of the metallic coating to cracking is reduced by the paint film, as determined by a T-bend test as defined in standard EN13523-7 dated April 2001, and the propensity of the metallic coating to cracking includes a reduction in an average crack width in the metallic coating. 
     
     
         13 . The manufacturing process according to  claim 1 , wherein the bath is at a temperature from 370° C. to 470° C. 
     
     
         14 . The manufacturing process according to  claim 1 , wherein aluminum content of the bath is from 4.75 to 5.25% by weight. 
     
     
         15 . The manufacturing process according  claim 1 , wherein magnesium content of the bath is from 0.44 to 0.56% by weight. 
     
     
         16 . The manufacturing process according  claim 1 , wherein the bath includes at least one element selected from a group consisting of Si, Ti, Ca, Mn, La, Ce and Bi. 
     
     
         17 . The manufacturing process according  claim 1 , wherein the bath consists of aluminum, magnesium, zinc and unavoidable impurities. 
     
     
         18 . The manufacturing process according to  claim 1 , wherein the surface preparation comprises a step selected from the group consisting of rinsing, degreasing, a conversion treatment, and combinations thereof. 
     
     
         19 . A process for the manufacture of a pre-painted sheet comprising the steps of:
 supplying a steel substrate;   depositing a metallic coating on at least one face by hot-dipping the steel substrate in a bath comprising:
 4.4% to 5.25% by weight aluminum, 
 0.3% to 0.56% by weight magnesium, 
 0.0 to less than 0.3% by weight Si, 
 0.0 to less than 0.3% by weight Ti, 
 0.0 to less than 0.3% by weight Ca, 
 0.0 to less than 0.3% by weight Mn, 
 0.0 to less than 0.3% by weight La, 
 0.0 to less than 0.3% by weight Ce, 
 0.0 to less than 0.3% by weight Bi, 
 a remainder of the bath including zinc and unavoidable impurities resulting from the process, the presence of nickel being excluded; 
   solidifying the metallic coating, the solidification of the metallic coating taking place at a cooling rate greater than or equal to 15° C./s between the beginning of solidification and the end of solidification of the metallic coating;   surface preparation of the metallic coating; and   painting the metallic coating with a paint film comprising at least one layer of paint comprising at least one polymer selected from the group consisting of melamine cross-linked polyesters, isocyanate cross-linked polyesters, polyurethanes and halogenated derivatives of vinyl polymers, with the exclusion of cataphoretic paints, thereby providing the pre-painted sheet;   wherein the paint film has a thickness between 1 and 200 μm, and the metallic coating has a thickness less than or equal to 25 μm.   
     
     
         20 . The manufacturing process according  claim 19 , wherein the substrate is a sheet, and the painting is before the sheet is bent. 
     
     
         21 . The manufacturing process according  claim 20 , wherein paint film is formed by two successive layers of paints. 
     
     
         22 . The manufacturing process according  claim 21 , wherein the two successive layers of paint consist of a primer layer of the melamine cross-linked polyester containing anti-corrosion pigments, and a finish layer of paint of the melamine cross-linked polyester. 
     
     
         23 . The manufacturing process according to  claim 20 , wherein aluminum content of the bath is from 4.6 to 5.1% by weight and magnesium content of the bath is 0.45 to 0.55%. 
     
     
         24 . The manufacturing process according  claim 23 , wherein the paint film has a thickness between 13 and 20 μm, and the metallic coating has a thickness between 12 and 20 μm. 
     
     
         25 . The manufacturing process according to  claim 24 , wherein the cooling rate is from 15° C./sec to 30° C./sec. 
     
     
         26 . The manufacturing process according to  claim 24 , wherein a propensity of the metallic coating to cracking is reduced by the paint film, as determined by a T-bend test as defined in standard EN13523-7 dated April 2001, and the propensity of the metallic coating to cracking includes a reduction in an average crack width in the metallic coating. 
     
     
         27 . The manufacturing process according to  claim 24 , wherein the metallic coating is free from spangles visible to the naked eye. 
     
     
         28 . The manufacturing method according to  claim 19 , wherein the surface prepared steel substrate with the metallic coating is coiled before the surface preparation and the painting. 
     
     
         29 . A process for the manufacture of a pre-painted sheet comprising the steps of:
 supplying a steel substrate;   providing a bath comprising:
 4.4% to 5.6% by weight aluminum, 
 0.3% to 0.56% by weight magnesium, 
 0.0 to less than 0.3% by weight Si, 
 0.0 to less than 0.3% by weight Ti, 
 0.0 to less than 0.3% by weight Ca, 
 0.0 to less than 0.3% by weight Mn, 
 0.0 to less than 0.3% by weight La, 
 0.0 to less than 0.3% by weight Ce, 
 0.0 to less than 0.3% by weight Bi, 
 a remainder of the bath including zinc and unavoidable impurities resulting from the process, the presence of nickel being excluded; 
   depositing a metallic coating on at least one face by hot-dipping the steel substrate in the bath;   solidifying the metallic coating by cooling;   surface preparation of the metallic coating; and   painting the metallic coating with a paint film comprising at least one layer of paint comprising at least one polymer selected from the group consisting of melamine cross-linked polyesters, isocyanate cross-linked polyesters, polyurethanes and halogenated derivatives of vinyl polymers, with the exclusion of cataphoretic paints, thereby providing the pre-painted sheet;   wherein the paint film has a thickness between 1 and 200 μm, and   the metallic coating has a thickness less than or equal to 25 μm.   
     
     
         30 . The manufacturing process according to  claim 29 , wherein the at least one polymer in the film is a melamine cross-linked polymer. 
     
     
         31 . The manufacturing process according  claim 30 , wherein the paint film has a thickness between 13 and 20 μm, the metallic coating has a thickness between 12 and 20 μm. 
     
     
         32 . The manufacturing process according to  claim 31 , wherein aluminum content of the bath is from 4.6 to 5.1% by weight and magnesium content of the bath is 0.45 to 0.55%. 
     
     
         33 . The manufacturing process according  claim 31 , wherein the paint film is formed by two successive layers of paints. 
     
     
         34 . The manufacturing process according  claim 33 , wherein the two successive layers of paint consist of a primer layer of the melamine cross-linked polyester containing anti-corrosion pigments, and a finish layer of paint of the melamine cross-linked polyester. 
     
     
         35 . The manufacturing process according to  claim 32 , wherein the cooling rate is from 15° C./sec to 30° C./sec. 
     
     
         36 . The manufacturing process according to  claim 35 , wherein the cooling rate is from 2° C./sec to 30° C./sec. 
     
     
         37 . The manufacturing process according to  claim 36 , wherein the cooling rate is between 15 and 35° C./s. 
     
     
         38 . The manufacturing process according to  claim 32 , wherein the metallic coating is free from spangles visible to the naked eye. 
     
     
         39 . The manufacturing process according  claim 32 , wherein the substrate is a sheet, and the painting is before the sheet is bent. 
     
     
         40 . The manufacturing process according to  claim 39 , wherein aluminum content of the bath is from 4.6 to 5.1% by weight and magnesium content of the bath is 0.45 to 0.55%. 
     
     
         41 . The manufacturing process according to  claim 40 , wherein a propensity of the metallic coating to cracking is reduced by the paint film, as determined by a T-bend test as defined in standard EN13523-7 dated April 2001, and the propensity of the metallic coating to cracking includes a reduction in an average crack width in the metallic coating. 
     
     
         42 . The manufacturing process according to  claim 40 , wherein the surface preparation of the metallic coating comprises an application of mechanical forces to the metallic coating. 
     
     
         43 . The manufacturing process according  claim 1 , wherein the solidifying is by cooling under natural convection. 
     
     
         44 . The manufacturing process according  claim 19 , wherein the bath consists of:
 4.4% to 5.25% by weight aluminum,   0.3% to 0.56% by weight magnesium,   0.0 to less than 0.3% by weight Si,   0.0 to less than 0.3% by weight Ti,   0.0 to less than 0.3% by weight Ca,   0.0 to less than 0.3% by weight Mn,   0.0 to less than 0.3% by weight La,   0.0 to less than 0.3% by weight Ce,   0.0 to less than 0.3% by weight Bi,   the remainder of the bath including zinc and unavoidable impurities resulting from the process, the presence of nickel being excluded.