METHOD OF FORMING AN ALLOY COATING ON A STRIP

Abstract

A hot dip method of forming an AlZnSiMg alloy coating on a strip is disclosed. The method includes controlling the conditions in the molten bath to minimise the top dross layer in the molten bath. In particular, the method includes controlling top dross formation by including Ca and/or Sr in the coating alloy in the bath.

Claims

1.-24. (canceled)

25. A method of minimizing a top dross layer in a molten bath containing a layer of molten AlZnSiMg alloy, the method comprising: providing molten AlZnSiMg alloy in a molten bath, the alloy comprising the following ranges in % by weight: Al: 45% to 60%; Zn: 35% to 50%; Si: 1.2% to 2.5%; and Mg: 1.0% to 3.0%; passing a steel strip from a heat treatment furnace into the molten bath wherein the molten alloy forms a coating of the alloy on the strip; removing the coated strip from the molten bath including passing the coated strip through a top dross layer on the molten bath; monitoring a Ca concentration and/or a Sr concentration in the molten bath; and controlling the composition of the molten bath by adding Ca and/or Sr to the bath to achieve: (a) a concentration of Sr of more than 150 ppm and less than 1250 ppm; or (b) a concentration of Ca of more than 150 ppm and less than 500 ppm; or (c) a combined concentration of Sr and Ca of more than 150 ppm, wherein the concentration of Sr is less than 1250 ppm and the concentration of Ca is less than 500 ppm; to minimize the top dross layer in the molten bath.

26. The method of claim 25, wherein controlling the conditions in the molten bath minimizes entrainment of any one or more of molten metal, gas, and intermetallic particles in oxide films in the top dross layer.

27. The method of claim 25, wherein controlling the composition of the bath comprises maintaining a concentration of more than 200 ppm Ca in the bath.

28. The method of claim 25, wherein controlling the composition of the bath comprises maintaining a concentration of more than 200 ppm Sr in the bath.

29. The method of claim 25, wherein controlling the composition of the bath comprises maintaining a concentration of more than 1000 ppm Sr in the bath.

30. The method of claim 25, wherein the Ca and/or Sr are added on a continuous or a periodic basis or by inclusion of Ca and/or Sr in Al and/or Zn ingots that are provided as feed materials for the molten bath.

31. The method of claim 25, wherein the molten AlZnSiMg alloy comprises more than 1.3% by weight Mg.

32. The method of claim 25, wherein the molten AlZnSiMg alloy comprises more than 1.5% by weight Mg.

33. The method of claim 25, wherein the molten AlZnSiMg alloy comprises more than 2.5% by weight Mg.

34. The method of claim 25, wherein the molten AlZnSiMg alloy comprises the following ranges in % by weight: Al: 53%; Zn: 43%; Si: 1.5%; and Mg: 2%.

35. The method of claim 25, wherein controlling the composition of the bath comprises maintaining a concentration of more than 200 ppm of Sr and Ca in the bath.

36. The method of claim 25, wherein the mass of top dross layer generated is less than 400 grams per 15 kg of molten alloy after 2 hours.

37. The method of claim 25, wherein the mass of top dross layer generated is less than 200 grams per 15 kg of molten alloy after 3 hours.

Description

[0078] The present invention is described further by way of example with reference to the accompanying drawings of which:

[0079] 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;

[0080] FIG. 2 is a graph of the mass of dross versus time for molten AlZnSi alloy baths with and without Mg and with and without Ca in experiments on dross generation carried out by the applicant;

[0081] FIG. 3 is a graph of the mass of dross versus time for molten AlZnSi alloy baths with and without Mg and with and without Sr in experiments on dross generation carried out by the applicant;

[0082] FIG. 4 presents selected results from the experimental work summarised in FIGS. 2 and 3 that highlights the impact of Ca and Sr on top dross generation;

[0083] FIG. 5 is a graph of the mass of dross versus Ca content in AlZnSiMg alloy baths after process times of 1 and 3 hours; and

[0084] FIG. 6 is a graph of the mass of dross generated versus time during the course of a line trial carried out by the applicant.

[0085] 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.

[0086] 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.

[0087] 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).

[0088] 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.

[0089] 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.

[0090] 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.

[0091] The coated strip is then passed through a cooling section 7 and subjected to forced cooling.

[0092] The cooled, coated strip is then passed through a rolling section 8 that conditions the surface of the coated strip.

[0093] The coated strip is thereafter coiled at a coiling station 10.

[0094] As is indicated above, the applicant has found that AlZnSiMg alloy coating baths generate substantially greater amounts of top dross in the baths than is the case with conventional 55% AlZn alloy baths in the coating lines of the applicant.

[0095] As discussed above, the applicant has conducted a number of laboratory experiments and line trials to determine whether it is possible to reduce the amount of dross generated in an AlZnSiMg alloy bath. As discussed above, the applicant found that it was possible to significantly reduce the level of top dross by the addition of Ca or Sr to AlZnSiMg alloys in coating baths.

[0096] The experimental results on the effect of Ca and Sr additions to coating baths on the level of top dross generation in AlZnSiMg alloy coating baths are summarized in FIGS. 2 to 5.

[0097] The experimental work was carried out on the following alloy compositions, in wt. % for (a) an AlZn alloy (referred to as AZ in the Figures) and (b) an AlZnMg alloy (referred to as MAZ in the Figures) and (c) these AZ and MAZ alloys plus parts per million (ppm) Ca and Sr additions to these compositions: [0098] AZ: 55Al-43Zn-1.5Si0.5Fe [0099] MAZ: 53Al-43Zn-2Mg-1.5Si0.5Fe [0100] MAZ+236 ppm Ca. [0101] MAZ+90 ppm Ca. [0102] MAZ+400 ppm Ca. [0103] MAZ+500 ppm Sr. [0104] MAZ+750 ppm Sr. [0105] MAZ+800 ppm Sr.

[0106] It is noted that the concentrations of Ca and Sr are the concentrations of these elements in the metallic parts of molten baths.

[0107] 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: [0108] A melting furnace with clay graphite crucible. [0109] A variable speed overhead mechanical stirrer with a support stand. [0110] 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. [0111] Stainless steel impellor shaft. [0112] Impellor machined from high density sintered boron nitride ceramic.

[0113] The dross collector cup and the impellor were fabricated from a high temperature material that is non-wetting to molten AZ and MAZ alloys. The sintered boron nitride ceramic of these components provided excellent non-wetting characteristics and high temperature stability in the coating bath.

[0114] 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 600? 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.

[0115] The experiments were conducted for durations of 0.5, 1.2, and 3 hrs.

[0116] After each experiment the dross collected was removed and weighed and the results are plotted as shown in FIGS. 2 to 5.

[0117] FIGS. 2 to 4 are graphs of the mass of dross versus time for the molten alloy baths, with the FIG. 2 results focusing on the results for the Ca alloys and the FIG. 3 results focusing on the results for the Sr alloys and the FIG. 4 results highlighting selected results for Ca and Sr from FIGS. 2 and 3.

[0118] FIG. 5 is a graph of the mass of dross versus Ca content in molten alloy baths after process times of 1 and 3 hours.

[0119] FIGS. 2 to 5 clearly show that the level of top dross generated in an AlZnSiMg alloy bath can be significantly reduced by additions of Ca or Sr to MAZ alloy coating baths. More particularly, FIGS. 2 to 5 show that: [0120] (a) MAZ alloy coating baths generate significantly higher amounts of top dross that AZ alloy coating baths, and [0121] (b) the amount of top dross decreases significantly with increasing amounts of Ca and Sr in the MAZ alloys.

[0122] The results shown in FIGS. 2 to 5 were further confirmed for Ca in a line trial conducted for approximately 2 weeks. The line trial was carried out on the above-mentioned AZ alloy to which Mg and Ca were added at different points in time during the course of the line trial. FIG. 6 shows the dross collected during the line trial and that the results are consistent with what was observed in the laboratory work. In particular, FIG. 6 shows that there was a substantial increase in the amount of dross generated in the molten bath with the addition of Mg to the bath and a substantial decrease in the amount of dross as a consequence of the addition of Ca to the bath.

[0123] As is indicated above, the applicant attributes the reduction in the dross level to reduction in the entrainment of molten metal, gas, and intermetallic particles in the oxide film in the molten bath (i.e. in the top dross layer in the bath) that resulted from (a) changes to the apparent surface tension at the liquid metal/oxide interface as a result of the Ca and Sr additions and (b) changes in the nature of the oxide film as a result of the Ca and Sr additions. The changes in the nature of the oxide film reduced the level of oxide stringers formed, which in turn assists in an overall reduction in liquid droplet entrainment. The changes in the entrainment lead to reductions in the level of top dross generation in molten AlZnSiMg alloys.

[0124] Ca and Sr are examples of elements that can be added to a molten bath of an AlZnSiMg alloy to reduce the entrainment of molten metal, gas, and intermetallic particles in the oxide film in the bath and thereby reduce the level of dross in the bath. Other bath additions include, by way of example, rare earth elements such as yttrium and combinations of rare earths and calcium and strontium and calcium/strontium.

[0125] In practice, the Ca and/or Sr may be added to the bath as required. It could be by way of specific additions of Ca and/or Sr 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.

[0126] Many modifications may be made to the present invention described above without departing from the spirit and scope of the invention.