AU2014253547A1

AU2014253547A1 - Metal-coated steel strip

Info

Publication number: AU2014253547A1
Application number: AU2014253547A
Authority: AU
Inventors: Qiyang Liu; Wayne Renshaw; Joe Williams
Original assignee: BlueScope Steel Ltd
Current assignee: BlueScope Steel Ltd
Priority date: 2008-03-13
Filing date: 2014-10-24
Publication date: 2014-11-13

Abstract

An Al-Zn-Si-Mg alloy coated strip that has Mg 2 Si particles in the coating microstructure is disclosed. The 5 distribution of Mg 2 Si particles is such that a surface region of the coating has only a small proportion of Mg 2 Si particles or is at least substantially free of any Mg 2 Si particles. 5890523_1 (GHMatters) P77207.AU.2 PETAK 24/10/14 Strontium-free Strontium addition Figure 1 Sr additions in a 55%AI-Zn-1.5%Si-2.O%Mg coating eliminate the surface mottling defect and change the distribution pattern of the Mg2Si phase in the coating thickness direction. mammimnam

Description

- 1 METAL-COATED STEEL STRIP The present invention relates to strip, typically steel strip, which has a corrosion-resistant metal alloy 5 coating. The present invention relates particularly to a corrosion-resistant metal alloy coating that contains aluminium-zinc-silicon-magnesium as the main elements in 10 the alloy, and is hereinafter referred to as an "Al-Zn-Si Mg alloy" on this basis. The 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 to cover alloys that 15 contain such other elements and the other elements may be deliberate alloying additions or unavoidable impurities. The present invention relates particularly but not exclusively to steel strip that is coated with the 20 above-described Al-Zn-Si-Mg alloy and can be cold formed (e.g. by roll forming) into an end-use product, such as roofing products. Typically, the Al-Zn-Si-Mg alloy comprises the 25 following ranges in % by weight of the elements aluminium, zinc, silicon, and magnesium: Aluminium: 40 to 60 % Zinc: 40 to 60 % 30 Silicon: 0.3 to 3% Magnesium 0.3 to 10 % Typically, the corrosion-resistant metal alloy coating is formed on steel strip by a hot dip coating 35 method. 5890523_1 (GHMatters) P77207.AU.2 PETAK 24/10/14 - 2 In the conventional hot-dip metal coating method, steel strip generally passes through one or more heat treatment furnaces and thereafter into and through a bath of molten metal alloy held in a coating pot. The heat 5 treatment furnace that is adjacent a coating pot has an outlet snout that extends downwardly to a location below the upper surface of the bath. The metal alloy is usually maintained molten in 10 the coating pot by the use of heating inductors. The strip usually exits the heat treatment furnaces via an outlet end section in the form of an elongated furnace exit chute or snout that dips into the bath. Within the bath the strip passes around one or more sink rolls and is 15 taken upwardly out of the bath and is coated with the metal alloy as it passes through the bath. After leaving the coating bath the metal alloy coated strip passes through a coating thickness control 20 station, such as a gas knife or gas wiping station, at which its coated surfaces are subjected to jets of wiping gas to control the thickness of the coating. The metal alloy coated strip then passes through 25 a cooling section and is subjected to forced cooling. The cooled metal alloy coated strip may thereafter be optionally conditioned by passing the coated strip successively through a skin pass rolling section 30 (also known as a temper rolling section) and a tension levelling section. The conditioned strip is coiled at a coiling station. A 55%Al-Zn alloy coating is a well known metal 35 alloy coating for steel strip. After solidification, a 55%Al-Zn alloy coating normally consists of a-Al dendrites 5890523_1 (GHMatters) P77207.AU.2 PETAK 24/10/14 - 3 and a B-Zn phase in the inter-dendritic regions of the coating. It is known to add silicon to the coating alloy 5 composition to prevent excessive alloying between the steel substrate and the molten coating in the hot-dip coating method. A portion of the silicon takes part in a quaternary alloy layer formation but the majority of the silicon precipitates as needle-like, pure silicon 10 particles during solidification. These needle-like silicon particles are also present in the inter-dendritic regions of the coating. It has been found by the applicant that when Mg 15 is included in a 55%Al-Zn-Si alloy coating composition, Mg brings about certain beneficial effects on product performance, such as improved cut-edge protection, by changing the nature of corrosion products formed. 20 However, it has also been found by the applicant that Mg reacts with Si to form a Mg 2 Si phase and that the formation of the Mg 2 Si phase compromises the above mentioned beneficial effects of Mg in a number of ways. 25 By way of example, the Mg 2 Si phase forms as large particles in relation to typical coating thicknesses and can provide a path for rapid corrosion where particles extend from a coating surface to an alloy layer adjacent the steel strip. 30 By way of further example, the Mg 2 Si particles tend to be brittle and sharp particles and provide both an initiation and propagation path for cracks that form on bending of coated products formed from coated strip. 35 Increased cracking compared to Mg-free coatings can result in more rapid corrosion of the coatings. 5890523_1 (GHMatters) P77207.AU.2 PETAK 24/10/14 - 4 The above description is not to be taken as an admission of the common general knowledge in Australia or elsewhere. 5 The present invention is an Al-Zn-Si-Mg alloy coated strip that has Mg 2 Si particles in the coating microstructure with the distribution of Mg 2 Si particles being such that a surface region of the coating has only a small proportion of Mg 2 Si particles or is at least 10 substantially free of any Mg 2 Si particles. The term "surface region" is understood herein to mean a region that extends inwardly from the exposed surface of a coating. 15 The applicant has found that the above-described distribution of Mg 2 Si particles in the coating microstructure provides significant advantages and can be achieved by any one or more of: 20 (a) strontium additions in the coating alloy; (b) selection of the cooling rate during solidification of coated strip for a given coating mass 25 (i.e. coating thickness) exiting a coating bath; and (c) minimising variations in coating thickness. According to the present invention there is 30 provided an Al-Zn-Si-Mg alloy coated steel strip that comprises a coating of an Al-Zn-Si-Mg alloy on a steel strip, with the microstructure of the coating comprising Mg 2 Si particles, and with the distribution of the Mg 2 Si particles being such that there is only a small proportion 35 of Mg 2 Si particles or at least substantially no Mg 2 Si particles in a surface region of the coating. 5890523_1 (GHMatters) P77207.AU.2 PETAK 24/10/14 - 5 The small proportion of Mg 2 Si particles in the surface region of the coating may be no more than 10 wt.% of the Mg 2 Si particles. 5 Typically, the Al-Zn-Si-Mg alloy comprises the following ranges in % by weight of the elements aluminium, zinc, silicon, and magnesium: Aluminium: 40 to 60 % 10 Zinc: 40 to 60 % Silicon: 0.3 to 3% Magnesium 0.3 to 10 % The Al-Zn-Si-Mg alloy may also contain other 15 elements, such as, by way of example any one or more of iron, vanadium, chromium, and strontium. Preferably the surface region has a thickness that is at least 5% of the total thickness of the coating. 20 Preferably the surface region has a thickness that is less than 30% of the total thickness of the coating. 25 More preferably the surface region has a thickness that is less than 20% of the total thickness of the coating. More preferably the surface region has a 30 thickness that is 5-30% of the total thickness of the coating. Preferably at least a substantial proportion of the Mg 2 Si particles are in a central region of the coating. 35 5890523_1 (GHMatters) P77207.AU.2 PETAK 24/10/14 - 6 The substantial proportion of the Mg 2 Si particles in the central region of the coating may be at least 80 wt.% of the Mg 2 Si particles. 5 Typically, the coating thickness is less than 30pn. Preferably the coating thickness is greater than 7pm. 10 The coating microstructure may also include a region that is adjacent the steel strip that has only a small proportion of Mg 2 Si particles or is at least substantially free of any Mg 2 Si particles, whereby the 15 Mg 2 Si particles in the coating microstructure are at least substantially confined to a central or core region of the coating. Preferably the coating contains more than 250 ppm 20 Sr, with the Sr addition promoting the formation of the above distribution of Mg 2 Si particles in the coating. Preferably the coating contains more than 500 ppm Sr. 25 Preferably the coating contains more than 1000 ppm Sr. Preferably the coating contains less than 30 3000 ppm Sr. The Al-Zn-Si-Mg-Sr alloy coating may contain other elements as deliberate additions or as unavoidable impurities. 35 Preferably there are minimal coating thickness variations. 5890523_1 (GHMatters) P77207.AU.2 PETAK 24/10/14 - 7 According to the present invention there is also provided a hot-dip coating method for forming a coating of a corrosion-resistant Al-Zn-Si-Mg alloy on a steel strip 5 that is characterised by passing the steel strip through a hot dip coating bath that contains Al, Zn, Si, Mg, and more than 250 ppm Sr and optionally other elements and forming an alloy coating on the strip that has Mg 2 Si particles in the coating microstructure with the 10 distribution of the Mg 2 Si particles being such that there is only a small proportion of Mg 2 Si particles or substantially no Mg 2 Si particles in a surface region of the coating. 15 Preferably the coating contains more than 500 ppm Sr. Preferably the coating contains more than 1000 ppm Sr. 20 Preferably the molten bath contains less than 3000 ppm Sr. The Al-Zn-Si-Mg-Sr alloy coating may contain 25 other elements as deliberate additions or as unavoidable impurities. According to the present invention there is also provided a hot-dip coating method for forming a coating of 30 a corrosion-resistant Al-Zn-Si-Mg alloy on a steel strip that is characterised by passing the steel strip through a hot dip coating bath that contains Al, Zn, Si, and Mg and optionally other elements and forming an alloy coating on the strip, and cooling coated strip exiting the coating 35 bath during solidification of the coating at a rate that is controlled so that the distribution of Mg 2 Si particles in the coating microstructure is such that there is only a 5890523_1 (GHMatters) P77207.AU.2 PETAK 24/10/14 - 8 small proportion of Mg 2 Si particles or substantially no Mg 2 Si particles in a surface region of the coating. The small proportion of Mg 2 Si particles in the 5 surface region of the coating may be no more than 10 wt.% of the Mg 2 Si particles. Preferably the method comprises selecting the cooling rate for coated strip exiting the coating bath to 10 be at less than a threshhold cooling rate. In any given situation, the selection of the required cooling rate is related to the coating thickness (or coating mass). 15 Preferably the method comprises selecting the cooling rate for coated strip exiting the coating bath to be less than 80 0 C/sec for coating masses up to 75 grams per square metre of strip surface per side. 20 Preferably the method comprises selecting the cooling rate for coated strip exiting the coating bath to be less than 50 0 C/sec for coating masses 75-100 grams per square metre of strip surface per side. 25 Typically, the method comprises selecting the cooling rate for coated strip exiting the coating bath to at least 11 0 C/sec. 30 The coating bath and the coating on steel strip coated in the bath may contain Sr. According to the present invention there is also provided a hot-dip coating method for forming a coating of 35 a corrosion-resistant Al-Zn-Si-Mg alloy on a steel strip that is characterised by passing the steel strip through a hot dip coating bath that contains Al, Zn, Si, and Mg and 5890523_1 (GHMatters) P77207.AU.2 PETAK 24/10/14 - 9 optionally other elements and forming an alloy coating on the strip with minimal variation in the thickness of the coating so that the distribution of Mg 2 Si particles in the coating microstructure is such that there is only a small 5 proportion of Mg 2 Si particles or substantially no Mg 2 Si particles in a surface region of the coating. Preferably the coating thickness variation should be no more than 40% in any given 5 mm diameter section of 10 the coating. More preferably the coating thickness variation should be no more than 30% in any given 5 mm diameter section of the coating. 15 In any given situation, the selection of an appropriate thickness variation is related to the coating thickness (or coating mass). 20 By way of example, for a coating thickness of 22pm, preferably the maximum thickness in any given 5 mm diameter section of the coating should be 27pm. Preferably the method comprises selecting the 25 cooling rate during solidification of coated strip exiting the coating bath to be less than a threshhold cooling rate. The coating bath and the coating on steel strip 30 coated in the bath may contain Sr. The hot-dip coating method may be the conventional method described above or any other suitable method. 35 The advantages of the invention include the following advantages. 5890523_1 (GHMatters) P77207.AU.2 PETAK 24/10/14 - 10 * Enhanced corrosion resistance. The Mg 2 Si distribution of the present invention eliminates direct corrosion channels from the 5 coating surface to steel strip that occurs with a conventional Mg 2 Si distribution. As a result, the corrosion resistance of the coating is markedly enhanced. 10 e Improved coating ductility. Mg 2 Si particles at the coating surface and adjacent to the steel strip are effective crack initiation sites when the coating undergoes a high strain fabrication. The Mg 2 Si distribution of the 15 present invention eliminates such crack initiation sites altogether or substantially reduces the total number of crack initiation sites, resulting in a significantly improved coating ductility. 20 * The addition of Sr allows the use of higher cooling rates, reducing the length of cooling equipment required after the pot. 25 Example The applicant has carried out laboratory experiments on a series of 55%Al-Zn-1.5%Si-2.0%Mg alloy compositions having up to 3000 ppm Sr coated on steel 30 substrates. The purpose of these experiments was to investigate the impact of Sr on the distribution of Mg 2 Si particles in the coatings. 35 5890523_1 (GHMatters) P77207.AU.2 PETAK 24/10/14 - 11 Figure 1 summarises the results of one set of experiments carried out by the applicant that illustrate the present invention. 5 The left hand side of the Figure comprises a top plan view of a coated steel substrate and a cross-section through the coating with the coating comprising a 55%Al Zn-1.5%Si-2.0%Mg alloy with no Sr. The coating was not formed having regard to the selection of cooling rate 10 during solidification discussed above. It is evident from the cross-section that Mg 2 Si particles are distributed throughout the coating thickness. This is a problem for the reasons stated 15 above. The right hand side of the Figure comprises a top plan view of a coated steel substrate and a cross-section through the coating, with the coating comprising a 55%Al 20 Zn-1.5%Si-2.0%Mg alloy and 500 ppm Sr. The cross-section illustrates upper and lower regions at the coating surface and at the interface with the steel substrate that are completely free of Mg 2 Si particles, with the Mg 2 Si particles being confined to a central band of the coating. 25 This is advantageous for the reasons stated above. The photomicrographs of the Figure illustrate clearly the benefits of the addition of Sr to an Al-Zn-Si Mg coating alloy. 30 The laboratory experiments found that the microstructure shown in the right hand side of the Figure were formed with Sr additions in the range of 250-3000 ppm. 35 5890523_1 (GHMatters) P77207.AU.2 PETAK 24/10/14 - 12 The applicant has also carried out line trials on 55%Al-Zn-1.5%Si-2.0%Mg alloy composition (not containing Sr) coated on steel strip. 5 The purpose of these trials was to investigate the impact of cooling rates and coating masses on the distribution of Mg 2 Si particles in the coatings. The experiments covered a range of coating masses 10 from 60 to 100 grams per square metre surface per side of strip, with cooling rates up to 90 0 C/sec. The applicant found two factors that affected the coating microstructure, particularly the distribution of 15 Mg 2 Si particles in the coatings. The first factor is the effect of the cooling rate of the strip exiting the coating bath before completing the coating solidification. The applicant 20 found that controlling the cooling rate is important. By way of example, the applicant found that for a AZ150 class coating (or 75 grams of coating per square metre surface per side of strip - refer to Australia 25 Standard AS1397-2001), if the cooling rate is greater than 80 0 C/sec, Mg 2 Si particles formed in the surface region of the coating. The applicant also found that for the same 30 coating it is not desirable that the cooling rate be too low, particularly below 11 0 C/sec, as in this case the coating develops a defective "bamboo" structure, whereby the zinc-rich phases forms a vertically straight corrosion path from the coating surface to the steel interface, 35 which compromises the corrosion performance of the coating. 5890523_1 (GHMatters) P77207.AU.2 PETAK 24/10/14 - 13 Therefore, for a AZ150 class coating, under the experimental conditions tested, the cooling rate should be controlled to be less than 80 0 C/sec and typically in a range of 11-80 0 C/sec. 5 On the other hand, the applicant also found that for a AZ200 class coating, if the cooling rate was greater than 50 0 C/sec, Mg 2 Si particles formed on the surface of the coating. 10 Therefore, for a AZ200 class coating, under the experimental conditions tested, a cooling rate of less than 50 0 C/sec and typically in a range of 11-50 0 C/sec is desirable. 15 The research work carried out by the applicant on the solidification of Al-Zn-Si-Mg coatings, which is extensive and is described in part above, has helped the applicant to develop an understanding of the formation of 20 the Mg 2 Si phase in a coating and the factors affecting its distribution in the coating. Whilst the applicant does not wish to be bound by the following discussion, this understanding is as set out below. 25 When an Al-Zn-Si-Mg alloy coating is cooled to a temperature in the vicinity of 560 0 C, the a-Al phase is the first phase to nucleate. The a-Al phase then grows into a dendritic form. As the a-Al phase grows, Mg and Si, along with other solute elements, are rejected into the 30 molten liquid phase and thus the remaining molten liquid in the interdendritic regions is enriched in Mg and Si. When the enrichment of Mg and Si in the interdendritic regions reaches a certain level, the Mg 2 Si 35 phase starts to form, which also corresponds to a temperature around 465 0 C. For simplification, it will be assumed that an interdendritic region near the outer 5890523_1 (GHMatters) P77207.AU.2 PETAK 24/10/14 - 14 surface of the coating is region A and another interdendritic region near the quaternary intermetallic alloy layer at the steel strip surface is region B. It will also be assumed that the level of enrichment in Mg 5 and Si is the same in region A as in region B. At or below 465 0 C, the Mg 2 Si phase has the same tendency to nucleate in region A as in region B. However, the principles of physical metallurgy teach us that a new 10 phase will preferably nucleate at a site whereupon the resultant system free energy is the minimum. The Mg 2 Si phase would normally nucleate preferably on the quaternary intermetallic alloy layer in region B provided the coating bath does not contain Sr (the role of Sr with Sr 15 containing coatings is discussed below). The applicant believes that this is in accordance with the principles stated above, in that there is a certain similarity in crystal lattice structure between the quaternary intermetallic alloy phase and the Mg 2 Si phase, which 20 favours the nucleation of Mg 2 Si phase by minimizing any increase in system free energy. In comparison, for the Mg 2 Si phase to nucleate on the surface oxide of the coating in region A, the increase in system free energy would have been greater. 25 Upon nucleation in region B, the Mg 2 Si phase grows upwardly, along the molten liquid channels in the interdendritic regions, towards region A. At the growth front of the Mg 2 Si phase (region C), the molten liquid 30 phase becomes depleted in Mg and Si (depending on the partition coefficients of Mg and Si between the liquid phase and the Mg 2 Si phase), compared with that in region A. Thus a diffusion couple forms between region A and region C. In other words, Mg and Si in the molten liquid phase 35 will diffuse from region A to region C. Note that the growth of the a-Al phase in region A means that region A is always enriched in Mg and Si and the tendency for the 5890523_1 (GHMatters) P77207.AU.2 PETAK 24/10/14 - 15 Mg 2 Si phase to nucleate in region A always exists because the liquid phase is "undercooled" with regard to the Mg 2 Si phase. 5 Whether the Mg 2 Si phase is to nucleate in region A, or Mg and Si are to keep diffusing from region A to region C, will depend on the level of Mg and Si enrichment in region A, relevant to the local temperature, which in turn depends on the balance between the amount of Mg and 10 Si being rejected into that region by the a-Al growth and the amount of Mg and Si being moved away from that region by the diffusion. The time available for the diffusion is also limited, as the Mg 2 Si nucleation/growth process has to be completed at a temperature around 380 0 C, before the 15 L-+Al-Zn eutectic reaction takes place, wherein L depicts the molten liquid phase. The applicant has found that controlling this balance can control the subsequent nucleation or growth of 20 the Mg 2 Si phase or the final distribution of the Mg 2 Si phase in the coating thickness direction. In particular, the applicant has found that for a set coating thickness, the cooling rate should be 25 regulated to a particular range, and more particularly not to exceed a threshhold temperature, to avoid the risk for the Mg 2 Si phase to nucleate in region A. This is because for a set coating thickness (or a relatively constant diffusion distance between regions A and C), a higher 30 cooling rate will drive the a-Al phase to grow faster, resulting in more Mg and Si being rejected into the liquid phase in region A and a greater enrichment of Mg and Si, or a higher risk for the Mg 2 Si phase to nucleate, in region A (which is undesirable). 35 On the other hand, for a set cooling rate, a thicker coating (or a thicker local coating region) will 5890523_1 (GHMatters) P77207.AU.2 PETAK 24/10/14 - 16 increase the diffusion distance between region A and region C, resulting in a smaller amount of Mg and Si being able to move from region A to region C by the diffusion within a set time and in turn a greater enrichment of Mg 5 and Si, or a higher risk for the Mg 2 Si phase to nucleate, in region A (which is undesirable). Practically, the applicant has found that, to achieve the distribution of Mg 2 Si particles of the present 10 invention, i.e. to avoid nucleation of the Mg 2 Si phase in region A, the cooling rate for coated strip exiting the coating bath has to be in a range of 11-80'C/sec for coating masses up to 75 grams per square metre of strip surface per side and in a range 11-50 0 C/sec for coating 15 masses of 75-100 grams per square metre of strip surface per side. The short range coating thickness variation also has to be controlled to be no greater than 40% above the nominal coating thickness within a distance of 5 mm across the strip surface to achieve the distribution of 20 Mg 2 Si particles of the present invention. The applicant has also found that, when Sr is present in a coating bath, the above described kinetics of Mg 2 Si nucleation can be significantly influenced. At 25 certain Sr concentration levels, Sr strongly segregates into the quaternary alloy layer (i.e. changes the chemistry of the quaternary alloy phase). Sr also changes the characteristics of surface oxidation of the molten coating, resulting in a thinner surface oxide on the 30 coating surface. Such changes alter significantly the preferential nucleation sites for the Mg 2 Si phase and, as a result, the distribution pattern of the Mg 2 Si phase in the coating thickness direction. In particular, the applicant has found that, Sr at concentrations 250-3000ppm in the 35 coating bath makes it virtually impossible for the Mg2Si phase to nucleate on the quaternary alloy layer or on the surface oxide, presumably due to the very high level of 5890523_1 (GHMatters) P77207.AU.2 PETAK 24/10/14 - 17 increase in system free energy would otherwise be generated. Instead, the Mg 2 Si phase can only nucleate at the central region of the coating in the thickness direction, resulting in a coating structure that is 5 substantially free of Mg 2 Si at both the coating outer surface region and the region near the steel surface. Therefore, Sr additions in the range 250-3000ppm are proposed as one of the effective means to achieve a desired distribution of Mg 2 Si particles in a coating. 10 Many modifications may be made to the present invention as described above without departing from the spirit and scope of the invention. 15 In this context, whilst the above description of the present invention focuses on (a) the addition of Sr to Al-Zn-Si-Mg coating alloys, (b) regulating cooling rates (for a given coating mass) and (c) minimising variations in coating thickness as means for achieving a desired 20 distribution of Mg 2 Si particles in coatings, i.e. at least substantially no Mg 2 Si particles in the surface of a coating, the present invention is not so limited and extends to the use of any suitable means to achieve the desired distribution of Mg 2 Si particles in the coating. 25 5890523_1 (GHMatters) P77207.AU.2 PETAK 24/10/14

Claims

1. An Al-Zn-Si-Mg alloy coated steel strip that comprises a coating of an Al-Zn-Si-Mg alloy on a steel 5 strip, with the microstructure of the coating comprising Mg 2 Si particles, and with the distribution of the Mg 2 Si particles being such that there is only a small proportion of Mg 2 Si particles or at least substantially no Mg 2 Si particles in a surface region of the coating. 10

2. The alloy coated steel strip defined in claim 1 wherein the small proportion of Mg 2 Si particles in the surface region of the coating is no more than 10 wt.% of the Mg 2 Si particles. 15

3. The alloy coated steel strip defined in claim 1 or claim 2 wherein the Al-Zn-Si-Mg alloy comprises the following ranges in % by weight of the elements aluminium, zinc, silicon, and magnesium: 20 Aluminium: 40 to 60 % Zinc: 40 to 60 % Silicon: 0.3 to 3% Magnesium 0.3 to 10 % 25

4. The alloy coated steel strip defined in any one of the preceding claims wherein the surface region has a thickness that is at least 5% of the total thickness of the coating. 30

5. The alloy coated steel strip defined in any one of the preceding claims wherein the surface region has a thickness that is less than 30% of the total thickness of the coating. 35

6. The alloy coated steel strip defined in any one of the preceding claims wherein at least a substantial 5890523_1 (GHMatters) P77207.AU.2 PETAK 24/10/14 - 19 proportion of the Mg 2 Si particles are in a central region of the coating.

7. The alloy coated steel strip defined in claim 6 5 wherein the substantial proportion of the Mg 2 Si particles in the central region of the coating is at least 80 wt.% of the Mg 2 Si particles.

8. The alloy coated steel strip defined in any one 10 of the preceding claims wherein the coating thickness is less than 30pn.

9. The alloy coated steel strip defined in any one of the preceding claims wherein the coating thickness is 15 greater than 7pm.

10. The alloy coated steel strip defined in any one of the preceding claims wherein the coating microstructure comprises a region that is adjacent the steel strip that 20 has only a small proportion of Mg 2 Si particles or is at least substantially free of any Mg 2 Si particles, whereby the Mg 2 Si particles in the coating microstructure are at least substantially confined to a central or core region of the coating. 25

11. The alloy coated steel strip defined in any one of the preceding claims wherein the coating contains more than 250 ppm Sr, with the Sr addition promoting the formation of the above distribution of Mg 2 Si particles in 30 the coating.

12. The alloy coated steel strip defined in claim 11 wherein the coating contains more than 500 ppm Sr. 35

13. The alloy coated steel strip defined in claim 11 wherein the coating contains more than 1000 ppm Sr. 5890523_1 (GHMatters) P77207.AU.2 PETAK 24/10/14 - 20

14. The alloy coated steel strip defined in any one of the preceding claims wherein the coating contains less than 3000 ppm Sr. 5

15. The alloy coated steel strip defined in any one of the preceding claims wherein there are minimal coating thickness variations.

16. A hot-dip coating method for forming a coating of 10 a corrosion-resistant Al-Zn-Si-Mg alloy on a steel strip that is characterised by passing the steel strip through a hot dip coating bath that contains Al, Zn, Si, Mg, and more than 250 ppm Sr and optionally other elements and forming an alloy coating on the strip that has Mg 2 Si 15 particles in the coating microstructure with the distribution of the Mg 2 Si particles being such that there is only a small proportion of Mg 2 Si particles or substantially no Mg 2 Si particles in a surface region of the coating. 20

17. The method defined in claim 16 wherein the coating contains more than 500 ppm Sr.

18. The method defined in claim 16 or claim 17 25 wherein the coating contains more than 1000 ppm Sr.

19. The method defined in any one of claims 16 to 18 wherein the molten bath contains less than 3000 ppm Sr. 30

20. A hot-dip coating method for forming a coating of a corrosion-resistant Al-Zn-Si-Mg alloy on a steel strip that is characterised by passing the steel strip through a hot dip coating bath that contains Al, Zn, Si, and Mg and optionally other elements and forming an alloy coating on 35 the strip, and cooling coated strip exiting the coating bath during solidification of the coating at a rate that is controlled so that the distribution of Mg 2 Si particles 5890523_1 (GHMatters) P77207.AU.2 PETAK 24/10/14 - 21 in the coating microstructure is such that there is only a small proportion of Mg 2 Si particles or substantially no Mg 2 Si particles in a surface region of the coating. 5

21. The method defined in claim 20 comprises selecting the cooling rate for coated strip exiting the coating bath to be at less than a threshhold cooling rate.

22. The method defined in claim 20 or claim 21 10 comprises selecting the cooling rate for coated strip exiting the coating bath to be less than 80'C/sec for coating masses up to 75 grams per square metre of strip surface per side. 15

23. The method defined in any one of claims 20 to 22 comprises selecting the cooling rate for coated strip exiting the coating bath to be less than 50'C/sec for coating masses 75-100 grams per square metre of strip surface per side. 20

24. The method defined in any one of claims 20 to 23 comprises selecting the cooling rate for coated strip exiting the coating bath to at least 11 0 C/sec.

25 25. A hot-dip coating method for forming a coating of a corrosion-resistant Al-Zn-Si-Mg alloy on a steel strip that is characterised by passing the steel strip through a hot dip coating bath that contains Al, Zn, Si, and Mg and optionally other elements and forming an alloy coating on 30 the strip with minimal variation in the thickness of the coating so that the distribution of Mg 2 Si particles in the coating microstructure is such that there is only a small proportion of Mg 2 Si particles or substantially no Mg 2 Si particles in a surface region of the coating. 35 5890523_1 (GHMatters) P77207.AU.2 PETAK 24/10/14 - 22

26. The method defined in claim 25 wherein the coating thickness variation is no more than 40% in any given 5 mm diameter section of the coating. 5

27. The method defined in claim 25 or claim 26 wherein the coating thickness variation is no more than 30% in any given 5 mm diameter section of the coating.

28. The method defined in any one of claims 25 to 27 10 comprises selecting the cooling rate during solidification of coated strip exiting the coating bath to be less than a threshhold cooling rate. 5890523_1 (GHMatters) P77207.AU.2 PETAK 24/10/14