WO2009055843A1

WO2009055843A1 - Metal-coated steel strip

Info

Publication number: WO2009055843A1
Application number: PCT/AU2008/001593
Authority: WO
Inventors: Qiyang Liu
Original assignee: BlueScope Steel Ltd
Current assignee: BlueScope Steel Ltd
Priority date: 2007-10-29
Filing date: 2008-10-29
Publication date: 2009-05-07
Anticipated expiration: 2010-04-29

Abstract

A coated steel strip that comprises a metal coating on at least one surface of the strip is disclosed. The coated strip is formed by contacting the strip with molten aluminium-zinc-silicon alloy and allowing the molten alloy to solidify on the strip. The coating comprises an intermetallic alloy layer on the steel strip and an aluminium-zinc-silicon alloy layer on the intermetallic alloy layer. The coating is characterised by Θ-phase (FeAl3) rather than α -phase (FeAlSi) or other phases as a stable intermetallic phase in the coating.

Description

METAL-COATED STEEL STRIP

The present invention relates to steel strip that has a corrosion-resistant metal coating of an aluminium- zinc- silicon alloy that is formed on the strip by coating the strip in a molten bath of the coating alloy.

The present invention relates particularly to metal coated steel strip of the type described in the preceding paragraph that has a corrosion-resistant metal coating with small spangles, i.e. a coating with an average spangle size of the order of less than 0.5mm measured using the average intercept distance method as described in Australian Standard AS1733.

In broad terms, the term "spangles" is understood herein to mean the visual manifestation of the grains that form when molten metal solidifies to form metal coated steel strip.

In more specific terms, as used herein as the definition, the term "spangle" is understood to mean a group of typically 6 shiny or dull sectors observed on a coating surface, with each of the sectors having an approximately triangular shape with two straight boundaries extending radially outwardly from a centre of the spangle and a curved boundary that defines a part of an outer perimeter of the spangle. Each spangle is a single dendritic grain that has nucleated at the spangle centre and has grown radially outwardly from the centre.

The present invention relates more particularly but not exclusively to metal coated steel strip of the type described above that has a corrosion-resistant metal coating of an aluminium- zinc-silicon alloy that has a relatively low concentration of silicon and has small spangles .

The present invention relates particularly but not exclusively to metal coated steel strip that can be cold formed (e.g. by roll forming) into an end-use product, such as roofing products.

Conventional aluminium- zinc-silicon alloys used to coat steel strip generally comprise the following ranges in % by weight of the elements aluminium, zinc and silicon:

aluminium: 45.0-60.0; zinc: 37.0-46.0; and silicon: 1.2-2.3.

Conventional aluminium- zinc- silicon alloys may also contain other elements, such as, by way of example, any one or more of iron, vanadium, magnesium, and chromium. These other elements may be deliberate additions or unavoidable impurities.

Conventionally, an aluminium- zinc- silicon alloy coating on steel strip is formed using a hot-dip metal coating method.

In the conventional hot-dip metal coating method steel strip passes through one or more heat treatment furnaces and thereafter into and through a bath of molten aluminium-zinc-silicon alloy contained in a coating pot. A coating of molten aluminium-zinc-silicon alloy forms on the steel strip as the strip moves through the bath. Initially, the coating is in molten alloy form. However, the molten alloy quickly solidifies and forms a solid coating after the coated steel strip emerges from the coating pot . Typically, the solid coating comprises an intermetallic layer on the steel strip and an aluminium- zinc- silicon alloy layer on the intermetallic layer. The intermetallic layer comprises iron from the steel strip and aluminium, zinc, and silicon from the coating pot.

Consequently, in addition to aluminium, zinc, and silicon, typically the solid coating comprises up to 2.5% by weight iron derived from the steel strip.

In a widely used conventional method the strip moves downwardly into the bath and around one or more sink rolls in the bath and thereafter upwardly from the bath.

It has also been proposed to provide an opening in a bottom wall of a coating pot and to move strip vertically upwardly through the opening into the bath and thereafter from the bath. This so-called "levitation" method relies on the use of an electromagnetic plugging means that prevents molten aluminium-zinc-silicon alloy flowing downwardly from the pot via the opening.

The applicant has carried out research and development work to optimize the composition of aluminium- zinc-silicon alloys used to form coatings on steel strip for given end-use applications and to optimize coating practices for forming such coatings on steel strip.

The present invention was made in the course of the research and development work of the applicant that focused on forming coatings with small and/or no spangles.

The term "small spangles" is understood herein to mean metal coated strip that has spangles that are less than 0.5 mm, preferably less than 0.2 mm, measured using the average intercept distance method as described in Australian Standard AS1733. The control of spangle size is important from aesthetic and functional viewpoints.

The applicant found in the course of the research and development work that reducing the silicon concentration of an aluminium-zinc-silicon alloy to be well below the conventional 1.2 wt% minimum mentioned above had advantages in terms of forming small or no spangles.

It is known that aluminium-zinc-silicon alloy coatings with relatively high aluminium contents (as in the production of GALVALUME ^® coated steel), i.e. 55 wt.% or more Al, depend on silicon additions to prevent a strongly exothermic reaction during metallic coating in which the entire solid coating becomes an alloy of aluminium, zinc and iron. Such coatings would be highly brittle and commercially useless. It is also known that without silicon additions the exothermic reaction is so spectacular as to heat steel substrates such that it glows bright red, and on occasion the coating may actually show combustion. The strongly exothermic reaction has been a driving force for maintaining silicon concentrations of at least 1.2 wt.% in conventional aluminium-zinc-silicon alloys used in coating pots.

However, the applicant has found evidence, via a research project funded by the applicant and research and development work of the applicant, that supports a conclusion that:

(a) an aluminium-zinc-silicon alloy layer of a solid coating on a steel strip nucleates from an intermetallic layer of the coating and, hence, the phase structure of the intermetallic layer is an important factor in the nucleation and growth and ultimate size of spangles in the solid coating; (b) the monoclinic 0-phase (FeAl₃) promotes spangle nucleation by providing a greater number of nucleation sites, resulting in a smaller spangle size than is the case with other intermetallic phases, such as the HCP α-phase (FeAlSi) ;

(c) the intermetallic phase that forms as a stable phase is the monoclinic 0-phase (FeAl₃) when molten alloy coatings have low silicon concentrations, i.e. concentrations typically below 1.2 wt.%, and whereas other intermetallic phases, such as the HCP a-phase (FeAlSi) , become stable when molten alloy coatings have higher silicon concentrations, i.e. concentrations typically above 1.2 wt.%; and

(d) hence, selecting the composition, such as the silicon concentration, of the aluminium-zinc-silicon alloy used to form the coating, to promote the formation of 0-phase (FeAl₃) rather than α-phase (FeAlSi) or other phases as the stable intermetallic phase on the steel strip during solidification of the aluminium- zinc-silicon alloy on the steel strip makes it possible to control spangle nucleation and ultimately the spangle size, i.e. to achieve small spangles (or no spangles) without the need of grain refiner additions, such as titanium diboride.

A coated steel strip in accordance with the present invention comprises a metal coating on at least one surface of the strip which is formed by contacting the steel strip with molten aluminium-zinc-silicon alloy and allowing the molten alloy to solidify on the strip, with the coating comprising an intermetallic alloy layer on the steel strip and an aluminium-zinc-silicon alloy layer on the intermetallic alloy layer, and the coating being characterised by 0-phase (FeAl₃) rather than α-phase

(FeAlSi) or other phases as a stable intermetallic phase in the coating. A coated steel strip in accordance with the present invention comprises a metal coating on at least one surface of the strip which is formed by contacting the steel strip with molten aluminium-zinc-silicon alloy and allowing the molten alloy to solidify on the strip, with the coating comprising an intermetallic alloy layer on the steel strip and an aluminium- zinc- silicon alloy layer on the intermetallic alloy layer, and the coated steel strip being characterised in that the composition of the aluminium- zinc-silicon alloy is selected to promote the formation of 0-phase (FeAl₃) rather than α-phase (FeAlSi) or other phases as a stable intermetallic phase that forms on the steel strip during solidification of the molten aluminium- zinc-silicon alloy on the steel strip.

Preferably the composition of the aluminium- zinc- silicon alloy is selected to comprise less than 1.2 wt.% silicon to promote the formation of 0-phase (FeAl₃) rather than α-phase (FeAlSi) or other phases as the stable intermetallic phase that forms on the steel strip during solidification of the molten aluminium-zinc-silicon alloy on the steel strip.

A coated steel strip in accordance with the present invention comprises a coating on at least one surface of the strip which is formed by contacting the steel strip with molten aluminium- zinc-silicon alloy and allowing the molten alloy to solidify on the strip, with the coating comprising an intermetallic alloy layer on the steel strip and an aluminium- zinc- silicon alloy layer on the intermetallic alloy layer, and the coated steel strip being characterised in that the aluminium- zinc- silicon alloy contains less than 1.2 wt.% silicon.

Preferably the silicon concentration is less than 1.0 wt.%. Preferably the silicon concentration is less than 0.85 wt. %.

Preferably the silicon concentration is less than

0.7 wt. %.

Preferably the silicon concentration is less than 0.6 wt. %.

More preferably the silicon concentration is less than 0.5 wt . % .

More preferably the silicon concentration is less than 0.3 wt.%.

Preferably the aluminium concentration is at least 45 wt.%.

Typically, the aluminium concentration is at least 50 wt.%.

Typically, the aluminium-zinc-silicon alloy comprises the following ranges in % by weight of the elements aluminium and zinc:

aluminium: 45.0-60.0; and zinc: 37.0-46.0.

The aluminium-zinc-silicon alloy may also contain other elements by way of deliberate addition. For example, the aluminium- zinc-silicon alloy may also contain 0.5-8 wt.% magnesium.

In addition to suppressing growth of the intermetalliσ alloy layer, the magnesium addition to the aluminium-zinc-silicon alloy improves the corrosion resistance of the aluminium-zinc-silicon layer of the solid coating .

Preferably the magnesium concentration is less than 8 wt.%.

Preferably the magnesium concentration is less than 3 wt.%.

Preferably the magnesium concentration is at least 0.5 wt.%.

Preferably the magnesium concentration is between 1 wt.% and 3 wt.%.

More preferably the magnesium concentration is between 1.5 wt . % and 2.5 wt . % .

The aluminium-zinc-silicon alloy may contain other elements as impurities or as deliberate additions.

Preferably the aluminium-zinc-silicon alloy contains strontium and/or calcium as a deliberate addition.

The strontium and/or calcium addition to the aluminium- zinc -silicon alloy substantially reduces the number of surface defects described by the applicant as "rough coating" and "pinhole - uncoated" defects and compensates for the increased number of such surface defects that appear to be caused by magnesium.

The strontium and the calcium may be added separately or in combination.

Preferably the concentration of (i) strontium or

(ii) calcium or (iii) strontium and calcium together is at least 2ppm. Preferably the concentration of (i) strontium or (ii) calcium or (iii) strontium and calcium together is less than 0.2 wt.%.

Preferably the concentration of (i) strontium or (ii) calcium or (iii) strontium and calcium together is less than lOOppm.

More preferably the concentration of (i) strontium or (ii) calcium or (iii) strontium and calcium together is no more than 50ppm.

Preferably the aluminium-zinc-silicon alloy does not contain vanadium and/or chromium as deliberate alloy elements - as opposed to being present in trace amounts for example as unavoidable impurities due to contamination in the molten bath.

According to the present invention there is also provided cold formed products made from the above-described metal coated steel strip.

According to the present invention there is also provided a method of forming a coating on at least one surface of a steel strip, with the coating comprising an intermetallic alloy layer on the steel strip and an aluminium- zinc-silicon alloy layer on the intermetallic alloy layer, and the method comprising hot-dip coating the steel strip by contacting the steel strip with a molten aluminium- zinc-silicon alloy and allowing the molten alloy to solidify on the strip and thereby forming the coating, with the composition of the aluminium-zinc-silicon alloy being selected to promote the formation of 0-phase (FeAl₃) rather than αr-phase (FeAlSi) or other phases as a stable intermetallic phase on the steel strip during solidification of the molten aluminium-zinc-silicon alloy on the steel strip.

Preferably the composition of the aluminivim- zinc- silicon alloy is selected to comprise less than 1.2 wt.%, preferably less that 0.8,wt.%, and more preferably less than 0.6 wt.%, silicon to promote the formation of 0-phase (FeAl₃) rather than α-phase (FeAlSi) or other phases as the stable intermetallic phase that forms on the steel strip during solidification of the molten aluminium- zinc-silicon alloy on the steel strip.

Preferably the method comprises hot-dip coating the steel strip in a bath of molten aluminium-zinc-silicon alloy by moving steel strip upwardly through an opening in a base of the coating pot. The use of such a levitation method makes it possible to avoid using stainless steel pot gear such as sink rolls that may be the subject of attack by the low silicon aluminium-zinc-silicon alloy.

Preferably the method comprises hot-dip coating the steel strip with a residence time of the steel strip in contact with the molten aluminium- zinc- silicon alloy of less than 0.7 seconds. Operating with a short residence time of less than 0.7 seconds, compared to residence times of at least 1 second with conventional hot-dip coating methods, is an important feature because it makes it possible to avoid potential issues that may arise with longer residence times. These potential issues include a thicker intermetallic layer that may result in a poor coating ductility.

Preferably the residence time is less than 0.5 seconds .

Preferably the residence time is at least 0.2 seconds . Preferably the residence time is at least 0.3 seconds .

Preferably the method comprises forming the coating with an intermetallic layer with a thickness of 1-2 micrometres .

The present invention is described further with reference to the following Figures, of which:

Figure 1 is a series of 3 photomicrographs that show the visual surface appearances of steel strip samples coated with Al-Zn-Si alloys having different silicon concentrations;

Figure 2 is a graph of spangle size versus silicon concentration in Al-Zn-Si coating alloys that illustrates the relationship between silicon concentration and spangle size; and

Figure 3 is a series of 6 photomicrographs that show the detailed spangle features on the surfaces of steel strip samples coated with Al-Zn-Si alloys having different silicon concentrations .

The present invention is based on an external research project funded by the applicant and on internal research work carried out by the applicant.

A series of coated steel strip samples were formed by coating steel substrates with (a) molten Al-Zn-Si alloys having low silicon concentrations (0.28, 0.50, 0.70, and 0.85 wt.%) in accordance with the present invention and (b) molten conventional Al-Zn-Si alloys having higher silicon concentrations (1.0 and 1.5 wt.%), and allowing the coatings to solidify on the substrates. The compositions of two of the alloys are set out below, with the concentrations in wt.%

Alloy A - in accordance with the present invention

Al - 55.5 Zn - 43.6 Si - 0.28 Fe - 0.48 V - 0.009 Ti - 0.005

Alloy B - conventional alloy

Al - 57.6

Zn - 40.4

Si - 1.5

Fe - 0.53 V - 0.009

Ti - 0.006

The compositions of the alloys were determined by inductively coupled plasma spectroscopy (ICP) technology.

It is noted that the vanadium and the titanium in the above alloys are unavoidable impurities, these elements being on or very close to the detection limit of 0.005 wt.% of the ICP used by the applicant.

The photomicrographs of Figure 1 show the surfaces of three samples, one having 0.28 wt.% Si in accordance with the present invention, and the other two samples being conventional higher silicon coatings with (a) 1.5 wt.% Si and (b) 1.5 wt.% Si and titanium diboride. It is evident from the photomicrographs of Figure 1 that the low silicon Al-Zn-Si alloy in accordance with the present invention produced coatings with a significant proportion of small spangles compared to the 1.5 wt.% silicon Al-Zn-Si alloy coating without the titanium diboride. It is noted that the 1.5 wt.% silicon Al-Zn-Si alloy coating with the titanium diboride addition had a small spangle size because the titanium boride acted as a grain refiner.

The conclusion that can be drawn from the photomicrographs of Figure 1 is that, in terms of forming small spangles, the low silicon Al-Zn-Si alloy is the preferred option.

This conclusion is reinforced by the graph of spangle size versus bath silicon concentration. In particular, the graph illustrates that bath silicon concentrations of 0.7 wt.% and less make it possible to form coatings with small spangle sizes around 0.2-0.3 mm. By way of comparison, the graph illustrates that bath silicon concentrations of greater than 1.0 wt.% form coatings with large spangle sizes of at least 0.6 mm. It is noted that, whilst not circled, the 0.8 wt.% sample, having a spangle size of around 0.4 mm is considered to be in the small spangle category.

The detailed, characteristic features associated with the variation in spangle size achieved by variations in the silicon concentrations of Al-Zn-Si alloys are illustrated in Figure 3. In particular, it is apparent from the Figure that the proportion of small spangles or no spangles decreases with increasing silicon concentrations in the alloys. Specifically, the 0.28 wt.% Al-Zn-Si alloy is substantially a plurality of small equiaxed grains that are not spangles at all according to the definition of the applicant and 3 small spangles. In addition, the 0.5 wt.% Al-Zn-Si alloy has fewer small equiaxed grains and more small spangles than the 0.28 wt.% Al-Zn-Si alloy, and so on. The applicant believes that the variations in appearance are due to a transition in the intermetallic layer over a range of silicon concentrations, with the intermetallic layer being substantially the 0-phase (FeAl₃) at low silicon concentrations (such as 0.28 wt.%) and substantially the α-phase (FeAlSi) at high silicon concentrations (such as 1.3 wt.%) and a mixture of these phases at silicon concentrations between these extremes.

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

By way of example, whilst the above experimental work focuses on the significance of silicon concentration to achieve a small spangle size or no spangles, the present invention is not so limited and extends to any means by which it is possible to form 0-phase (FeAl₃) rather than a- phase (FeAlSi) in the intermetallic layer of the coating.

Claims

1. A coated steel strip comprises a metal coating on at least one surface of the strip which is formed by contacting the steel strip with molten aluminium-zinc- silicon alloy and allowing the molten alloy to solidify on the strip, with the coating comprising an intermetallic alloy layer on the steel strip and an aluminium-zinc- silicon alloy layer on the intermetallic alloy layer, and the coating being characterised by 0-phase (FeAl₃) rather than a-phase (FeAlSi) or other phases as a stable intermetallic phase in the coating.

2. A coated steel strip comprises a metal coating on at least one surface of the strip which is formed by contacting the steel strip with molten aluminium- zinc- silicon alloy and allowing the molten alloy to solidify on the strip, with the coating comprising an intermetallic alloy layer on the steel strip and an aluminium- zinc- silicon alloy layer on the intermetallic alloy layer, and the coated steel strip being characterised in that the composition of the aluminium- zinc-silicon alloy is selected to promote the formation of 0-phase (FeAl₃) rather than a- phase (FeAlSi) or other phases as a stable intermetallic phase that forms on the steel strip during solidification of the molten aluminium-zinc-silicon alloy on the steel strip.

3. The coated steel strip defined in claim 1 or claim 2 wherein the composition of the aluminium-zinc- silicon alloy comprises less than 1.2 wt.% silicon to promote the formation of θ-phase (FeAl₃) rather than a- phase (FeAlSi) or other phases as the stable intermetallic phase that forms on the steel strip during solidification of the molten aluminium-zinc-silicon alloy on the steel strip.

4. A coated steel strip comprises a coating on at least one surface of the strip which is formed by contacting the steel strip with molten aluminium- zinc- silicon alloy and allowing the molten alloy to solidify on the strip, with the coating comprising an intermetallic alloy layer on the steel strip and an aluminium- zinc- silicon alloy layer on the intermetallic alloy layer, and the coated steel strip being characterised in that the aluminium- zinc-silicon alloy contains less than 1.2 wt.% silicon.

5. The coated steel strip defined in any one of the preceding claims wherein the silicon concentration is less than 1.0 wt . % .

6. The coated steel strip defined in any one of the preceding claims wherein the silicon concentration is less than 0.85 wt.%.

7. The coated steel strip defined in any one of the preceding claims wherein the silicon concentration is less than 0.7 wt . % .

8. The coated steel strip defined in any one of the preceding claims wherein the silicon concentration is less than 0.6 wt . % .

9. The coated steel strip defined in any one of the preceding claims wherein the silicon concentration is less than 0.5 wt.%.

10. The coated steel strip defined in any one of the preceding claims wherein the silicon concentration is less than 0.3 wt . % .

11. The coated steel strip defined in any one of the preceding claims wherein the aluminium concentration is at least 45 wt.%.

12. The coated steel strip defined in any one of the preceding claims wherein the aluminium concentration is at least 50 wt.%.

13. The coated steel strip defined in any one of the preceding claims wherein the aluminium-zinc-silicon alloy comprises the following ranges in % by weight of the elements aluminium and zinc:

aluminium: 45.0-60.0; and zinc: 37.0-46.0.

14. The coated steel strip defined in any one of the preceding claims wherein the aluminium-zinc-silicon alloy contain other elements by way of deliberate addition.

15. A cold formed products made from the metal coated steel strip defined in any one of the preceding claims .

16. A method of forming a coating on at least one surface of a steel strip, with the coating comprising an intermetallic alloy layer on the steel strip and an aluminium- zinc- silicon alloy layer on the intermetallic alloy layer, and the method comprising hot-dip coating the steel strip by contacting the steel strip with a molten aluminium-zinc-silicon alloy and allowing the molten alloy to solidify on the strip and thereby forming the coating, with the composition of the aluminium-zinc-silicon alloy being selected to promote the formation of 0-phase (PeAl₃) rather than α-phase (PeAlSi) or other phases as a stable intermetallic phase on the steel strip during solidification of the molten aluminium-zinc-silicon alloy on the steel strip.

17. The method defined in claim 14 wherein the composition of the aluminium-zinc-silicon alloy is selected to comprise less than 1.2 wt.%, preferably less that 0.8,wt.%, and more preferably less than 0.6 wt.%, silicon to promote the formation of έ?-phase (FeAIs) rather than a- phase (FeAlSi) or other phases as the stable intermetallic phase that forms on the steel strip during solidification of the molten aluminium-zinc-silicon alloy on the steel strip.

18. The method defined in claim 16 or claim 17 comprises hot-dip coating the steel strip with a residence time of the steel strip in contact with the molten aluminium-zinc-silicon alloy of less than 0.7 seconds.

19. The method defined in any one of claims 16 to 18 comprises hot-dip coating the steel strip with a residence time of the steel strip in contact with the molten aluminium- zinc-silicon alloy of less than 0.5 seconds.

20. The method defined in any one of claims 14 to 19 comprises forming the coating with an intermetallic layer with a thickness of 1-2 micrometers.