Method for providing a Zn—Al—Mg coating, and such coating

Abstract

A Zn—Al—Mg coating on a steel strip. Specific amounts of Mg and Al are chosen, in a relationship wherein the amount of Al is higher than the amount of Mg. Also provided is a method for providing a steel strip having a Zn—Al—Mg coating provided by hot dip coating with a coating bath, wherein the thickness of the coating is controlled using gas knifes above the coating bath.

Claims

1. A steel strip having a Zn—Al—Mg coating provided by hot dip coating, wherein the coating has a composition consisting of: 1.1-1.4 weight % magnesium; 1.9-2.3 weight % aluminium; optional <0.3 weight % of Sn, Bi, Sb, Ln, Ce, Ti, Sc, Sr and/or B in total; the remainder consisting of zinc and unavoidable impurities; wherein the Al and Mg contents satisfy the relationship: Al (in weight %) >1.28 Mg (in weight %)+0.25 with the proviso that if Al is 2.0 weight % then Mg may be 1.4 weight %, wherein the coating weight of a one-sided coating layer is 35 to less than 45 g/m.sup.2, and wherein the coating does not contain binary (Zn+MgZn.sub.2) microstructures, wherein the coating has an absence of a visible beach defect; wherein the strip has a roughness Ra<1.5 μm before entering a coating bath for the hot dip coating, wherein the coated steel strip is made by a process comprising controlling the thickness of the coating using gas knifes above a coating bath that provides the alloy for the coating and wherein Z is a value in a range of 5 to 8 mm that meets an equation Z<0.6 e.sup.0.064CW, wherein Z is the distance between each gas knife and the steel strip in mm, and wherein CW is the weight of the one-sided coating layer in g/m.sup.2 and wherein each said gas knife has a distance above the coating bath of at most 500 mm.

2. The steel strip according to claim 1, wherein the coating weight on each side of the metal strip is 40 to less than 45 g/m.sup.2 and no surface defects are present at all.

3. The steel strip according to claim 2, wherein Z is in a range from 5 to 8 mm and wherein each said gas knife has a distance above the coating bath of at most 400 mm.

4. The steel strip according to claim 2 wherein the coated steel strip has a length between 1000 and 4000 m and the coating is homogeneous.

5. The steel strip according to claim 1, wherein the coating weight on each side of the metal strip 35 to less than 40 g/m.sup.2, and no surface defects are present at all, wherein Z is the distance between each gas knife and the steel strip in mm, and wherein Z is in a range from 5 to 7.5 mm, wherein the gas knifes have a distance above the bath of at most 400 mm.

6. The steel strip according to claim 1, wherein Z is in a range from 5 to 7 mm.

7. The steel strip according to claim 2, wherein the coating has a composition in which 2.0-2.3 weight % aluminium is present.

8. The steel strip according to claim 5, wherein the coating has a composition in which 2.0-2.3 weight % aluminium is present.

9. The steel strip according to claim 1, wherein the coating has a uniformly distributed (Al+Zn+MgZn2) ternary eutectic structure.

10. The steel strip according to claim 1, wherein the coating has no Ti, wherein the coating weight of the one-sided coating layer is 37.8 to less than 45 g/m.sup.2 and 2.0-2.3 weight % aluminium is present.

11. The steel strip according to claim 1, wherein the coating has a composition in which 1.1-1.3 weight % magnesium is present.

12. The steel strip according to claim 1, wherein in the coating composition 1.2-1.4 weight % magnesium is present.

13. The steel strip according to claim 1, wherein the steel strip is a carbon steel strip, wherein in the coating composition 1.2-1.4 weight % magnesium is present and 2.0-2.3 weight % aluminium is present, wherein the coating weight of the one-sided coating layer is 37.8 to less than 45 g/m.sup.2.

14. The steel strip according to claim 1, wherein the coating weight of the one-sided coating layer is 37.8 to 43.6 g/m.sup.2.

15. The steel strip according to claim 14, wherein in the coating composition 1.1-1.2 weight % magnesium is present.

16. The steel strip according to claim 1, wherein the optional total weight of Sn, Bi, Sb, Ln, Ce, Ti, Sc, Sr and/or B is <0.1 weight %.

17. The steel strip according to claim 1, wherein the coating weight on each side of the metal strip is <37.8 to 43.6 g/m.sup.2.

18. The steel strip according to claim 1, wherein Z is in a range from 5 to 8 mm and wherein each said gas knife has a distance above the coating bath of at most 400 mm.

19. A method for providing a Zn—Al—Mg coating on a steel strip of claim 1 using a hot dip coating process with a coating bath, wherein the thickness of the coating is controlled using gas knifes above the coating bath, wherein to obtain a good appearance without a beach defect the coating on the steel strip is given a composition consisting of: 1.2-1.5 weight % magnesium; between 1.8 and 2.4 weight % aluminium; optional <0.3 weight % of Sn, Bi, Sb, Ln, Ce, Ti, Sc, Sr and/or B in total; the remainder consisting of zinc and unavoidable impurities; wherein the Al and Mg contents satisfy the relationship: Al (in weight %) >1.28 Mg (in weight %)+0.25 with the proviso that if Al is 2.0 weight % then Mg may be 1.4 weight %, and wherein Z<0.6 e.sup.0.064CW wherein Z is the distance between each gas knife and the steel strip in mm, and wherein CW is the weight of the one-sided coating layer in g/m.sup.2, wherein CW 35 to <45 g/m.sup.2, wherein the gas knifes have a distance above the bath of at most 500 mm.

20. The method according to claim 19, wherein CW<40 g/m.sup.2.

21. The method according to claim 1, wherein the gas knifes have a distance above the bath of at most 250 mm.

22. The method according to claim 1, wherein the strip has a roughness Ra<1.0 μm before entering the coating bath.

23. The method according to claim 19, wherein the steel strip is a carbon steel strip.

24. The method according to claim 19, wherein the coated steel strip has a width between 400 and 2500 mm and a gauge between 0.5 and 5 mm.

25. The method according to claim 19, wherein the coated steel strip has a length between 400 and 40,000 m.

Description

(1) FIG. 1 shows the surface of coated sheets, where for manufacturing the left-hand sheet the gas knives were placed at a height of 200 mm above the coating bath, and where for manufacturing the right-hand sheet the gas knives were places at a height of 300 mm above the coating bath. In both cases, the coating weight was approximately 35 g/m2. The distance between each gas knife and the strip was 6 mm.

(2) FIG. 1 clearly shows that the upper sheet has a better surface appearance than the lower sheet. Since all other manufacturing parameters are the same, FIG. 1 thus shows that the height of the gas knifes above the coating bath is a determining factor for the surface appearance of the sheets produced, and notably that a gas knifes height of 200 mm above the bath surface provides a better surface than a height of 300 mm.