METAL-COATED STEEL STRIP

Abstract

A steel strip having a coating of a metal alloy on at least one surface of the strip is disclosed. The metal alloy contains aluminium, zinc, silicon, and magnesium as the major elements. The metal alloy also contains strontium and/or calcium and unavoidable impurities and optionally other elements that are present as deliberate alloying elements. The concentration of magnesium is at least 1 wt. % and the concentration of (i) strontium or (ii) calcium or (iii) strontium and calcium is greater than 50 ppm.

Claims

1. A steel strip having a coating of a metal alloy on at least one surface of the strip, wherein the metal alloy contains aluminium, zinc, silicon, and magnesium as the major elements and also contains strontium and/or calcium and unavoidable impurities and optionally other elements that are present as deliberate alloying elements, and wherein the concentration of aluminium is 40 to 60 wt. %, the concentration of zinc is 40 to 60 wt. %, the concentration of silicon is 0.3 to 3 wt. %, the concentration of magnesium is at least 1 wt. % and the concentration of (i) strontium or (ii) calcium or (iii) strontium and calcium together is greater than 50 ppm.

2. The steel strip defined in claim 1 wherein the concentration of (i) strontium or (ii) calcium or (iii) strontium and calcium together is less than X, where X is selected from 0.2 wt. %, 150 ppm, and 100 ppm.

3.-4. (canceled)

5. The steel strip defined in claim 1 wherein the magnesium concentration is less than X, where X is selected from 10 wt. %, 5 wt. %, 3 wt. %, and 0.5 wt. %.

6.-8. (canceled)

9. The steel strip defined in claim 1 wherein the magnesium concentration is between X and Y, where X is selected from 1 wt. % and 1.5 wt. % and Y is selected from 5 wt. % and 3 wt. %.

10. (canceled)

11. The steel strip defined in claim 1 wherein the aluminium, zinc, silicon, and magnesium alloy is a titanium diboride-modified alloy as defined herein.

12. The steel strip defined in claim 1 wherein the aluminium, zinc, silicon, and magnesium alloy contains any one or more of indium, tin, beryllium, titanium, copper, nickel, cobalt, and manganese.

13. The steel strip defined in claim 1 wherein the aluminium, zinc, silicon, and magnesium alloy does not contain vanadium and/or chromium as deliberate alloy elements—as opposed to being present in trace amounts for example due to contamination in the molten bath.

14. The steel strip defined in claim 1 wherein the concentration of iron is less than 1 wt. %.

15. The steel strip defined in claim 1 wherein the coating has small spangles as defined herein.

16. The steel strip defined in claim 1 wherein the strip is coated on one or both sides thereof.

17. The steel strip defined in claim 1 wherein the coating has a coating mass of less than 80 g/m.sup.2 of metal alloy on the or each side of the strip.

18. The steel strip defined in claim 1 wherein the coating has an average coating thickness of less than 20 micrometers on the or each side of the strip.

19. A method of forming a corrosion-resistant coating of a metal alloy on at least one surface of a steel strip, wherein the metal alloy coating contains aluminium, zinc, silicon, and magnesium as the major elements and also contains strontium, or calcium, or strontium and calcium, and unavoidable impurities, and optionally other elements that are present as deliberate alloying elements, wherein the concentration of aluminium is about 55 wt. %, the concentration of silicon is about 1.5 wt. %, the concentration of magnesium is greater than 1.5 wt. % and less than 3 wt. % and the concentration of (i) strontium, or (ii) calcium, or (iii) strontium and calcium together is greater than 50 ppm and less than 150 ppm, and the balance is zinc; the method includes the steps of successively passing the steel strip through a heat treatment furnace and a molten bath that contains the metal alloy by (a) heat treating the steel strip in the heat treatment furnace; and (b) hot-dip coating the strip in the molten bath and forming a coating of the metal alloy on the steel strip; wherein the metal alloy coating provides galvanic protection to the steel strip.

20.-21. (canceled)

22. The method defined claim 19 wherein step (b) includes forming the coating with a coating mass of less than 80 g/m.sup.2 of metallic coating on the or each side of the strip, and/or forming the coating with an average coating thickness of less than 20 micrometers on the or each side of the strip.

23. (canceled)

24. The method defined in claim 19 wherein step (b) includes forming the coating with small spangles less than 0.5 mm, measured using the intercept distance method as described in the Australian Standard AS1733.

25. Cold formed products made from the metal alloy coated steel strip defined in claim 1.

26.-27. (canceled)

28. The method defined in claim 19, wherein the metal alloy coating contains as deliberate alloying elements any one or more of indium, tin, beryllium, titanium, copper, nickel, cobalt and manganese.

29. The method defined in claim 19, wherein the metal alloy coating does not contain vanadium or chromium as deliberate alloying elements.

30. (canceled)

31. The method defined in claim 19 wherein the coating contains strontium at a concentration of greater than 50 ppm and less than 150 ppm.

32.-33. (canceled)

34. The method defined in claim 19 wherein the coating contains calcium at a concentration of greater than 50 ppm and less than 150 ppm.

35.-36. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0065] The FIGURE is a schematic drawing of one embodiment of a continuous production line for producing steel strip coated with Al—Zn—Si—Mg alloy in accordance with the method of the present invention.

DETAILED DESCRIPTION

[0066] With reference to the FIGURE, in use, coils of cold rolled steel strip are uncoiled at an uncoiling station 1 and successive uncoiled lengths of strip are welded end to end by a welder 2 and form a continuous length of strip.

[0067] The strip is then passed successively through an accumulator 3, a strip cleaning section 4 and a furnace assembly 5.

[0068] The furnace assembly 5 includes a preheater, a preheat reducing furnace, and a reducing furnace.

[0069] The strip is heat treated in the furnace assembly 5 by careful control of process variables including: (i) the temperature profile in the furnaces, (ii) the reducing gas concentration in the furnaces, (iii) the gas flow rate through the furnaces, and (iv) strip residence time in the furnaces (i.e. line speed).

[0070] The process variables in the furnace assembly 5 are controlled so that there is removal of iron oxide residues from the surface of the strip and removal of residual oils and iron fines from the surface of the strip.

[0071] The heat treated strip is then passed via an outlet snout downwardly into and through a bath containing a molten metal alloy held in a coating pot 6 and is coated with the metal alloy.

[0072] The metal alloy is an Al—Zn—Si—Mg coating alloy that contains:

[0073] (a) at least 0.5 wt. % and less than 10 wt. % magnesium to contribute to corrosion resistance of the coating,

[0074] (b) titanium didiborides to minimise spangle size of the coating, and

[0075] (c) more than 50 ppm and less than 0.2 wt. % strontium and calcium together to minimise the number of the above-described surface defects.

[0076] Preferably the metal alloy does not contain vanadium and/or chromi

[0077] Typically, the metal alloy contains incidental impurities, such as iron.

[0078] The metal alloy is maintained molten in the coating pot by use of heating inductors (not shown).

[0079] Within the bath the strip passes around a sink roll and is taken upwardly out of the bath. Both surfaces of the strip are coated with the metal alloy in the bath as it passes through the bath.

[0080] The coating that forms on the strip in the molten bath is in the form of the metal alloy.

[0081] The coating has a comparatively smaller number of the above-described surface defects due to the strontium and calcium.

[0082] The coating has small spangles due to the titanium diboride.

[0083] After leaving the molten bath 6 the coated strip passes vertically through a gas wiping station (not shown) at which its coated surfaces are subjected to jets of wiping gas to control the thickness of the coating.

[0084] The coated strip is then passed through a cooling section 7 and subjected to forced cooling.

[0085] The cooled, coated strip, is then passed through a rolling section 8 that conditions the surface of the coated strip.

[0086] The coated strip is thereafter coiled at a coiling station 10.

[0087] Many modifications may be made to the preferred embodiment described above without departing from the spirit and scope of the present invention.