METAL-COATED STEEL STRIP

Abstract

A method of forming an Al—Zn—Si—Mg alloy coating on a strip that includes dipping strip into a bath of molten Al—Zn—Si—Mg alloy and forming a coating of the alloy on the strip, with the Al—Zn—Si—Mg 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.

Description

BACKGROUND OF THE INVENTION

The present invention relates to the production of strip, typically steel strip, which has a corrosion-resistant metal alloy coating that contains aluminium-zinc-silicon-magnesium as the main elements in the alloy in the following ranges in % by weight:

Al: 2 to 19%

Si: 0.1 to 2%

Mg: 1 to 10%

Zn: 80 to 97%

The above alloy is described and claimed in Australian patent 758643 entitled “Plated steel product, plated steel sheet and precoated steel sheet having excellent resistance to corrosion” in the name of Nippon Steel Corporation.

The above alloy is hereinafter referred to as an “Al—Zn—Si—Mg alloy”.

The Al—Zn—Si—Mg alloy coating may contain other elements that are present as deliberate alloying additions or as unavoidable impurities. Hence, the phrase “Al—Zn—Si—Mg alloy” is understood herein to cover alloys that contain such other elements as deliberate alloying additions or as unavoidable impurities. The other elements may include by way of example any one or more of Fe, Ti, Cu, Ni, Co, Ca, Mn, Be, Sr, Ca, Cr, and V.

SUMMARY OF THE INVENTION

In particular, the present invention relates to a hot-dip metal coating method of forming an Al—Zn—Si—Mg alloy coating on a strip that includes dipping uncoated strip into a bath of molten Al—Zn—Si—Mg alloy and forming a coating of the alloy on the strip.

The present invention is concerned with minimising the amount of top dross in the alloy coating bath. Top dross is undesirable from the viewpoints of cost of production and coating quality, as is discussed further below.

The term “top dross” is herein understood to include any one or more of the following components on or near the surface of the molten bath:

• • (a) an oxide film on the surface of a molten bath,
  • (b) molten metal droplets covered by an oxide film,
  • (c) gas bubbles having an oxide film as the wall of the bubbles,
  • (d) intermetallic particles that are formed in the coating bath, including particles covered by an oxide film, and
  • (e) combinations of any two or more of gas, molten metal, and intermetallic particles covered by an oxide film.

Items (b), (c), (d), and (e) can be described as the result of entrainment of molten metal, gas, and intermetallic particles in the oxide film on or near the surface of the molten bath.

International application PCT/AU2011/000069 entitled “Metal-Coated Steel Strip” in the name of the applicant is concerned with minimising the amount of top dross in an alloy coating bath of an alloy that contains aluminium-zinc-silicon-magnesium as the main elements in the alloy. The invention described and claimed in the International application is based on laboratory work and line trials on coating bath alloy compositions containing, 53% Al, 43% Zn, 2% Mg, 1.5% Si, and 0.5% Fe, with the percentages being percentages by weight, and different amounts of Ca and Sr in the baths carried out by the applicant. The coating alloys were coated onto steel strip. The invention was made during the course of a research and development project that investigated the addition of Mg to a known corrosion resistant metal coating composition, namely 55% Al—Zn—Si, that is used widely in Australia and elsewhere for building products, particularly profiled wall and roofing sheets. At the time the invention of the International application was made, the addition of Mg to this known composition of 55% Al—Zn—Si coating composition had been proposed in the patent literature for a number of years, see for example US patent 6,635,359 in the name of Nippon Steel Corporation, but Al—Zn—Si—Mg coatings on steel strip were not commercially available in Australia. More particularly, it had been established that when Mg is included in a 55% Al—Zn coating composition, Mg brings about certain beneficial effects on product performance, such as improved cut-edge protection. However, the applicant found in the research and development project that Mg-containing molten 55% Al—Zn coating metal is susceptible to increased levels of top dross generation compared to molten 55% Al—Zn coating metal that does not contain Mg. During a line trial involving hot-dip metal coating a Mg-containing 55% Al—Zn alloy onto a steel strip conducted by the applicant it was shown that the level of top dross generated in the coating bath was 6 to 8 times that of the top dross formed in a 55% Al—Zn alloy coating bath without Mg addition. This amount of top dross generated has a significant impact on the cost of production of Mg-containing 55% Al—Zn alloy coated steel and product quality. The applicant found in the research and development project that the amount of top dross could be greatly reduced by the addition of Ca and/or Sr to a coating bath.

The applicant has now found that there is substantial top dross generated in hot dip coating steel strip in a molten bath containing the above-described Al—Zn—Si—Mg alloy containing in % by weight: Al: 2 to 19% , Si: 0.1 to 2% , Mg: 1 to 10% , and Zn: 80 to 97% and that the top dross is undesirable in terms of cost of production and product quality. The applicant had not anticipated that top dross would have been as significant an issue with the above-described Al—Zn—Si—Mg alloy.

The above discussion is not to be taken as an admission of the common general knowledge in Australia and elsewhere.

The applicant has been able to reduce the top dross levels in molten Al—Zn—Si—Mg alloy baths containing in % by weight: Al: 2 to 19% , Si: 0.1 to 2% , Mg: 1 to 10% , and Zn: 80 to 97% by the addition to molten baths of Ca, and the reduction in top dross levels has lead to benefits in terms of production costs and product quality.

According to the present invention there is provided a method of forming an Al—Zn—Si—Mg alloy coating on a strip that includes dipping strip into a bath of molten Al—Zn—Si—Mg alloy and forming a coating of the alloy on the strip, with the Al—Zn—Si—Mg 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.

The Al—Zn—Si—Mg alloy may contain other elements that are present as deliberate alloying additions or as unavoidable impurities. Hence, the phrase “Al—Zn—Si—Mg alloy” is understood herein to cover alloys that contain such other elements as deliberate alloying additions or as unavoidable impurities. The other elements may include by way of example any one or more of Fe, Ti, Cu, Ni, Co, Ca, Mn, Be, Sr, Cr, and V.

The composition of the bath may include more than 50 ppm Ca. It is noted that all references to ppm in the specification are references to ppm by weight.

The composition of the bath may include more than 100 ppm Ca.

The composition of the bath may include more than 200 ppm Ca.

The composition of the bath may include more than 250 ppm Ca.

The composition of the bath may include more than 300 ppm Ca.

The composition of the bath may include less than 2000 ppm Ca.

The composition of the bath may include less than 1500 ppm Ca.

The composition of the bath may include less than 1000 ppm Ca.

It is noted that the references to amounts of elements such as Ca as part of the composition of a molten bath are understood herein to be references to the concentrations of the elements in the molten metal layer of the bath as opposed to the top dross layer in the bath. The reason for this is that it is the standard practice of the applicant to measure bath concentrations in the molten metal layers of molten baths.

It is also noted that the applicant found that Ca tends to segregate to the top dross layer of molten baths and, as a consequence the top dross layer becomes enriched with respect to Ca when compared to the metal layer. Specifically, if there is “x” wt. % of Ca in the molten metal layer of a molten bath, there will be a higher concentration of the element in the top dross layer of the bath. For example, the applicant found in laboratory work that in a bath with a nominal bath composition of 90 ppm Ca, the Ca content of the top dross layer increased to 100 ppm Ca. Similarly, the applicant found that in a bath with a nominal composition of 400 ppm Ca, the top dross layer was enriched substantially to 600 ppm. In practice, this means that, if it is required that there be “x” wt. % of Ca in the molten metal layer of a molten bath, it will be necessary to add an amount of Ca that is greater than “x” wt. % in the total bath to compensate for the higher concentration of Ca that will segregate to the top dross layer.

The Ca may be added to the 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 in Al and/or Zn ingots that are provided as feed materials for the bath.

The method may include controlling the concentration of Ca in the bath to minimise the top dross layer in the molten bath.

The method may include 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.

In a situation in which the Ca is part of ingots of other elements that are in the composition in the bath, the method may include selecting any one or more of the sizes of the ingots, the timing of the addition of the ingots, and the sequence of the addition of the ingots to maintain the concentration of Ca substantially constant or within a preferred range of +or −10% for the elements.

The Al—Zn—Si—Mg alloy may include more than 8% by weight Al.

The Al—Zn—Si—Mg alloy may include more than 10% by weight Al.

The Al—Zn—Si—Mg alloy may include less than 15% by weight Al.

The Al—Zn—Si—Mg alloy may include less than 12% by weight Al.

The Al—Zn—Si—Mg alloy may include more than 0.3% by weight Mg.

The Al—Zn—Si—Mg alloy may include more than 1% by weight Mg.

The Al—Zn—Si—Mg alloy may include more than 2% by weight Mg.

The Al—Zn—Si—Mg alloy may include more than 2.5% by weight Mg.

The Al—Zn—Si—Mg alloy may include more than 3% by weight Mg.

The Al—Zn—Si—Mg alloy may include less than 5% by weight Mg.

The Al—Zn—Si—Mg alloy may include less than 4% by weight Mg.

The Al—Zn—Si—Mg alloy may include more than 0.15% by weight Si.

The Al—Zn—Si—Mg alloy may include less than 1.2% by weight Si.

The Al—Zn—Si—Mg alloy may include less than 1% by weight Si.

The Al—Zn—Si—Mg alloy may include less than 0.25% by weight Si.

According to the present invention there is also provided an Al—Zn—Si—Mg alloy coating on a strip produced by the above-described method.

BRIEF DESCRIPTION OF THE DRAWINGS

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

FIG. 1 is a schematic drawing of one embodiment of a continuous production line for producing steel strip coated with an Al—Zn—Si—Mg alloy in accordance with the method of the present invention; and

FIG. 2 is a graph of the mass of dross versus Ca concentration for molten Al—Zn—Si—Mg alloy baths with and without Ca in experiments on dross generation carried out by the applicant.

DETAILED DESCRIPTION

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.

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.

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).

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.

The heat treated strip is then passed via an outlet snout downwardly into and through a molten bath containing an Al—Zn—Si—Mg alloy held in a coating pot 6 and is coated with Al—Zn—Si—Mg alloy. The Al—Zn—Si—Mg 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 Al—Zn—Si—Mg alloy as it passes through the bath.

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.

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

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

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

As is indicated above, the applicant has found that Al—Zn—Si—Mg 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.

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 Al—Zn—Si—Mg 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% .

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 Al—Zn—Si—Mg alloys in coating baths.

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 Al—Zn—Si—Mg alloy coating baths is summarized in FIG. 2.

The experimental work was carried out on the following alloy compositions, in wt. % for (a) an Al—Zn—Si—Mg alloy and (b) this alloy plus parts per million (ppm) Ca additions to the composition:

• • Alloy: Al: 11.2% Al; Mg: 3% ; Si: 0.19% ; Zn: balance; and unavoidable impurities
  • Alloy+500 ppm (0.05 wt. % ) Ca.
  • Alloy+750 ppm (0.075 wt. % ) Ca.
  • Alloy+1500 ppm (0.15 wt. % ) Ca.

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

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:

• • A melting furnace with clay graphite crucible.
  • A variable speed overhead mechanical stirrer with a support stand.
  • 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.
  • Stainless steel impellor shaft.
  • Impellor machined from high density sintered boron nitride ceramic.

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.

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.

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

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.

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

FIG. 2 clearly shows that the level of top dross generated in an Al—Zn—Si—Mg 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.

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.

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

Claims

1. A method of forming an Al—Zn—Si—Mg alloy coating on a strip that includes dipping the strip into a bath of molten Al—Zn—Si—Mg alloy and forming a coating of the alloy on the strip, with the Al—Zn—Si—Mg 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 Al—Zn—Si—Mg 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 Al—Zn—Si—Mg alloy includes more than 8% by weight Al.

11. The method defined in claim 1 wherein the Al—Zn—Si—Mg alloy includes less than 15% by weight Al.

12. The method defined in claim 1 wherein the Al—Zn—Si—Mg alloy includes more than 0.3% by weight Mg.

13. The method defined in claim 1 wherein the Al—Zn—Si—Mg alloy includes more than 2% by weight Mg.

14. The method defined in claim 1 wherein the Al—Zn—Si—Mg alloy includes less than 5% by weight Mg.

15. The method defined in claim 1 wherein the Al—Zn—Si—Mg alloy includes more than 0.15% by weight Si.