METAL-COATED STEEL STRIP

Abstract

A method of forming an AlZnSiMg alloy coating on a strip that includes dipping strip into a bath of molten AlZnSiMg alloy and forming a coating of the alloy on the strip, with the AlZnSiMg alloy containing in % by weight: Al: 2 to 19%, Si: 0.1 to 2%, Mg: 1 to 10%, and Zn: 80 to 97%, and with the bath having a molten metal layer and a top dross layer on the metal layer, and the method including providing Ca in the composition of the bath to minimise the top dross layer in the molten bath.

Claims

1. A method of forming an AlZnSiMg alloy coating on a strip that includes dipping the strip into a bath of molten AlZnSiMg alloy and forming a coating of the alloy on the strip, with the AlZnSiMg alloy containing in % by weight: Al: 2 to 19%, Si: 0.1 to 2%, Mg: 1 to 10%, and Zn: 80 to 97%, and with the bath having a molten metal layer and a top dross layer on the metal layer, and the method including providing Ca in the composition of the bath to minimise the top dross layer in the molten bath.

2. The method defined in claim 1 wherein the AlZnSiMg alloy contains other elements that are present as deliberate alloying additions or as unavoidable impurities.

3. The method defined in claim 1 wherein the composition of the bath includes more than 50 ppm Ca.

4. The method defined in claim 1 wherein the composition of the bath includes more than 100 ppm Ca.

5. The method defined in claim 1 wherein the composition of the bath includes more than 200 ppm Ca.

6. The method defined in claim 1 wherein the composition of the bath includes less than 2000 ppm Ca.

7. The method defined in claim 1 includes adding Ca by way of specific additions of Ca compounds on a continuous or a periodic basis.

8. The method defined in claim 1 includes adding Ca in Al and/or Zn ingots that are provided as feed materials for the bath.

9. The method defined in claim 1 and further including controlling the composition of the bath to minimise the top dross layer in the bath by periodically monitoring the concentration of Ca that is in the bath and adding Ca as required to maintain the bath composition for the element.

10. The method defined in claim 1 wherein the AlZnSiMg alloy includes more than 8% by weight Al.

11. The method defined in claim 1 wherein the AlZnSiMg alloy includes less than 15% by weight Al.

12. The method defined in claim 1 wherein the AlZnSiMg alloy includes more than 0.3% by weight Mg.

13. The method defined in claim 1 wherein the AlZnSiMg alloy includes more than 2% by weight Mg.

14. The method defined in claim 1 wherein the AlZnSiMg alloy includes less than 5% by weight Mg.

15. The method defined in claim 1 wherein the AlZnSiMg alloy includes more than 0.15% by weight Si.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] The present invention is described further by way of example with reference to the accompanying drawings of which:

[0056] FIG. 1 is a schematic drawing of one embodiment of a continuous production line for producing steel strip coated with an AlZnSiMg alloy in accordance with the method of the present invention; and

[0057] FIG. 2 is a graph of the mass of dross versus Ca concentration for molten AlZnSiMg alloy baths with and without Ca in experiments on dross generation carried out by the applicant.

DETAILED DESCRIPTION

[0058] With reference to FIG. 1, 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.

[0059] The strip is then passed successively through an accumulator 3, a strip cleaning section 4 and a furnace assembly 5. The furnace assembly 5 includes a preheater, a preheat reducing furnace, and a reducing furnace.

[0060] 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).

[0061] 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.

[0062] The heat treated strip is then passed via an outlet snout downwardly into and through a molten bath containing an AlZnSiMg alloy held in a coating pot 6 and is coated with AlZnSiMg alloy. The AlZnSiMg alloy is maintained molten in the coating pot by use of heating inductors (not shown). 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 AlZnSiMg alloy as it passes through the bath.

[0063] After leaving the coating bath 6 the 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.

[0064] The coated strip is then passed through a cooling section 7 and subjected to forced cooling.

[0065] The cooled, coated strip is then passed through a rolling section 8 that conditions the surface of the coated strip.

[0066] The coated strip is thereafter coiled at a coiling station 10.

[0067] As is indicated above, the applicant has found that AlZnSiMg alloy coating baths containing in % by weight: Al: 2 to 19%, Si: 0.1 to 2%, Mg: 1 to 10%, and Zn: 80 to 97% generate substantial amounts of top dross in the baths that is undesirable in terms of production costs and product quality.

[0068] As discussed above, the applicant conducted a number of laboratory experiments to determine whether it is possible to reduce the amount of dross generated in AlZnSiMg alloy baths having compositions, in % by weight, of: Al: 2 to 19%, Si: 0.1 to 2%, Mg: 1 to 10%, and Zn: 80 to 97%.

[0069] As discussed above, the applicant found that it was possible to significantly reduce the level of top dross by the addition of Ca to such AlZnSiMg alloys in coating baths.

[0070] The experimental results for experiments of 3 hours duration on the effect of Ca additions to coating baths on the level of top dross generation in AlZnSiMg alloy coating baths is summarized in FIG. 2.

[0071] The experimental work was carried out on the following alloy compositions, in wt. % for (a) an AlZnSiMg alloy and (b) this alloy plus parts per million (ppm) Ca additions to the composition: [0072] Alloy: Al: 11.2% Al; Mg: 3%; Si: 0.19%; Zn: balance; and unavoidable impurities [0073] Alloy +500 ppm (0.05 wt. %) Ca. [0074] Alloy +750 ppm (0.075 wt. %) Ca. [0075] Alloy +1500 ppm (0.15 wt. %) Ca.

[0076] It is noted that the concentrations of Ca are the concentrations of these elements in the metallic parts of molten baths.

[0077] In the experimental work the top dross generation was simulated using a laboratory melting furnace and an overhead mechanical stirrer. The laboratory set-up consisted of the following components: [0078] A melting furnace with clay graphite crucible. [0079] A variable speed overhead mechanical stirrer with a support stand. [0080] Dross collector cup machined from high density sintered boron-nitride ceramic and having a series of drainage holes in the bottom of the cup and a series of upstanding handles to allow the cup to be positioned and removed from the crucible. [0081] Stainless steel impellor shaft. [0082] Impellor machined from high density sintered boron nitride ceramic.

[0083] The dross collector cup and the impellor were fabricated from a high temperature material that is non-wetting to the coating alloy tested in the experimental work. The sintered boron nitride ceramic of these components provided excellent non-wetting characteristics and high temperature stability in the coating bath.

[0084] For each experiment, 15 kg of the coating alloy of a required composition was formed in the crucible and held at the process temperature of 460 C. The dross collector cup was then inserted into the molten bath and was retained in the bath until the melt temperature reached the process temperature. Then the shaft impellor assembly was lowered into the bath until the impellor just touched the surface of the melt. The stirrer motor was then switched on and the stirring speed was adjusted to 60 RPM. This experimental set-up resulted in shearing of the surface of the bath without creating a vortex so that at each revolution of the impellor a fresh melt was continuously exposed to air to generate dross. The dross generated was pushed to the side of the crucible and accumulated on the side of the crucible. At the end of each experiment the accumulated dross was removed from the crucible by lifting the dross collector cup from the crucible and allowing excess entrained bath metal to drain into the crucible via holes in the dross collector cup. What was left in the dross collector cup comprised the entrained bath metal and dross intermetallic particles covered with oxide film. This retained material was the top dross generated in each experiment.

[0085] The experiments were conducted for durations of 0.5, 1, 2, and 3 hrs.

[0086] After each experiment the dross collected was removed and weighed and the results are plotted for the 3 hour experiments as shown in FIG. 2.

[0087] FIG. 2 is a graph of the mass of dross generated versus Ca concentration for the molten alloy baths.

[0088] FIG. 2 clearly shows that the level of top dross generated in an AlZnSiMg alloy bath can be significantly reduced by additions of Ca to coating baths. More particularly, FIG. 2 shows that the amount of top dross decreases significantly with increasing amounts of Ca in the coating baths.

[0089] In practice, the Ca may be added to a coating bath as required. It could be by way of specific additions of Ca compounds on a continuous or a periodic basis. It could also be by way of the inclusion of Ca and/or in Al and/or Zn ingots that are provided as feed materials for the bath.

[0090] Many modifications may be made to the present invention described above without departing from the spirit and scope of the invention.