METAL-COATED STEEL STRIP

Abstract

An Al—Zn—Si—Mg alloy coated strip that has Mg.sub.2Si particles in the coating microstructure is disclosed. The distribution of Mg.sub.2Si particles is such that the surface of the coating has only a small proportion of Mg.sub.2Si particles or is at least substantially free of any Mg.sub.2Si particles.

Claims

1. A hot-dip coating method to minimise coating thickness variations and control distribution characteristics of Mg2Si phases when coating a corrosion-resistant Al—Zn—Si—Mg alloy on a steel strip to lower risk of mottling defect on the surface of the coating, the method comprising: passing a steel strip through a hot dip coating bath that contains aluminum, zinc, silicon, and magnesium to form an alloy coating on the steel strip having, in weight %, 40 to 60% aluminum; 40 to 60% zinc; 0.3 to 3% silicon; and 0.3 to 10% magnesium, and controlling a coating thickness and a short range coating thickness variation of the coating, including: passing the coated steel strip through a coating thickness control station to control the coating thickness to be greater than 7 μm and less than 30 μm from the steel strip and to control the short range coating thickness variation to be no more than 40% in a 5 mm diameter section of the coating, and cooling the coated steel strip, wherein the coating has a microstructure comprising Mg.sub.2Si particles, and wherein controlling the coating thickness and the short range coating thickness variation controls the distribution characteristics of Mg2Si phase such that the surface of the coating has no more than 10 wt % of Mg.sub.2Si particles to thereby lower risk of mottling defect on the surface of the coating.

2. The method defined in claim 1 wherein the coating thickness variation is no more than 30% in a 5 mm diameter section of the coating.

3. The method defined in claim 1 wherein, for a coating thickness of 22 μm, the maximum thickness in a region of the coating greater than 1 mm in diameter is 27 μm.

4. The method defined in claim 1 wherein the hot dip coating bath comprises one or more of strontium, iron, vanadium and chromium and the coating comprises one or more of strontium, iron, vanadium and chromium.

5. The method defined in claim 1 wherein the surface of the coating has no Mg.sub.2Si particles.

6. The method defined in claim 1 wherein the surface of the coating is substantially free of Mg.sub.2Si particles.

7. The method defined in claim 1 wherein the coating thickness control station is a gas knife or gas wiping station.

8. The method defined in claim 1 wherein cooling the coated steep strip occurs at a cooling rate of less than 80° C./sec and greater than 11° C./sec for coating masses up to 75 grams per square meter of strip surface per side, or at a cooling rate of less than 50° C./sec and greater than 11° C./sec for coating masses of 75-100 grams per square meter of strip surface per side.

Description

[0073] FIG. 1 summarises the results of one set of experiments carried out by the applicant that illustrate the present invention.

[0074] 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 during solidification and coating thickness variations discussed above.

[0075] The mottling that results from such a coating composition is identified by the arrow in the top plan view. It is evident from the cross-section that Mg.sub.2Si particles are distributed throughout the coating thickness. This is a problem for the reasons stated above.

[0076] 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—Zn-1.5% Si-2.0% Mg alloy and 500 ppm Sr. A complete absence of mottling is evident from the top plan view. In addition, 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.sub.2Si particles, with the Mg.sub.2Si particles being confined to a central band of the coating. This is advantageous for the reasons stated above.

[0077] The photomicrographs of the FIGURE illustrate clearly the benefits of the addition of Sr to an Al—Zn—Si—Mg coating alloy.

[0078] 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.

[0079] 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 substrates.

[0080] The purpose of these trials was to investigate the impact of cooling rates and coating masses on mottling in the surface of the coatings.

[0081] The trials covered a range of coating masses from 60 to 100 grams per square metre surface per side of strip, with cooling rates up to 90° C./sec.

[0082] The applicant found two factors that affected the coating microstructure, particularly the distribution of Mg.sub.2Si particles in the coatings, in the trials.

[0083] The first factor is the effect of the cooling rate of the strip exiting the coating bath before completing the coating solidification. The applicant found that controlling the cooling rate makes it possible to avoid mottling.

[0084] 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 Standard AS1397-2001), if the cooling rate is greater than 80° C./sec, Mg.sub.2Si particles formed on the surface of the coating. In particular, when the cooling rate was greater than 100° C./sec, mottling occurred.

[0085] The applicant also found that for the same coating it is not desirable that the cooling rate be too low, particularly below 11° 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, which compromises the corrosion performance of the coating.

[0086] Therefore, for a AZ150 class coating, under the experimental conditions tested, the cooling rate should be controlled to be in a range of 11-80° C./sec to avoid mottling on the surface.

[0087] On the other hand, the applicant also found that for a AZ200 class coating, if the cooling rate was greater than 50° C./sec, Mg.sub.2Si particles formed on the surface of the coating and mottling occurred.

[0088] Therefore, for a AZ200 class coating, under the experimental conditions tested, a cooling rate in a range of 11-50° C./sec is desirable.

[0089] The second important factor found by the applicant is the uniformness of coating thickness across the strip surface.

[0090] The applicant found that the coating on the strip surface normally had thickness variations that are (a) long range (across the entire strip width, measured by the “weight-strip-weight” method on a 50 mm diameter disc) and (b) short range (across every 25 mm length in the strip width direction, measured in the cross-section of the coating under a microscope with 500× magnification). In a production situation, the long range thickness variation is normally regulated to meet the minimum coating mass requirements as defined in relevant national standards. In a production situation, as far is the applicant is aware, there is no regulation for short range thickness variation, as long as the minimum coating mass requirements as defined in relevant national standards are met.

[0091] However, the applicant found that short range coating thickness variations could be very high, and special operational measures had to be applied to keep the variations under control. It was not uncommon in the experimental work for the coating thickness to change by a factor of two or more over a distance as short as 5 mm, even when the product perfectly met the minimum coating mass requirements as defined in relevant national standards. This short range coating thickness variation had a pronounced impact on the Mg.sub.2Si particles in the surface of coatings.

[0092] By way of example, the applicant found that for a AZ150 class coating even in the desirable cooling rate ranges as described above, if the short range coating thickness variation was greater than 40% above the nominal coating thickness within a distance of 5 mm across the strip surface, Mg.sub.2Si particles formed on the surface of the coating and thereby increased the risk of mottling.

[0093] Therefore, under the experimental conditions tested, the short range coating thickness variation should be controlled to no greater than 40% above the nominal coating thickness within a distance of 5 mm across the strip surface to avoid mottling.

[0094] 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 the Mg.sub.2Si 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.

[0095] When an Al—Zn—Si—Mg alloy coating is cooled to a temperature in the vicinity of 560° C., the α-Al phase is the first phase to nucleate. The α-Al phase then grows into a dendritic form. As the α-Al phase grows, Mg and Si, along with other solute elements, are rejected into the molten liquid phase and thus the remaining molten liquid in the interdendritic regions is enriched in Mg and Si.

[0096] When the enrichment of Mg and Si in the interdendritic regions reaches a certain level, the Mg.sub.2Si phase starts to form, which also corresponds to a temperature around 465° C. For simplification, it will be assumed that an interdendritic region near the outer 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 and Si is the same in region A as in region B.

[0097] At or below 465° C., the Mg.sub.2Si 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 phase will preferably nucleate at a site whereupon the resultant system free energy is the minimum. The Mg.sub.2Si 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-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.sub.2Si phase, which favours the nucleation of Mg.sub.2Si phase by minimizing any increase in system free energy. In comparison, for the Mg.sub.2Si phase to nucleate on the surface oxide of the coating in region A, the increase in system free energy would have been greater.

[0098] Upon nucleation in region B, the Mg.sub.2Si phase grows upwardly, along the molten liquid channels in the interdendritic regions, towards region A. At the growth front of the Mg.sub.2Si phase (region C), the molten liquid phase becomes depleted in Mg and Si (depending on the partition coefficients of Mg and Si between the liquid phase and the Mg.sub.2Si 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 will diffuse from region A to region C. Note that the growth of the α-Al phase in region A means that region A is always enriched in Mg and Si and the tendency for the Mg.sub.2Si phase to nucleate in region A always exists because the liquid phase is “undercooled” with regard to the Mg.sub.2Si phase.

[0099] Whether the Mg.sub.2Si 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 Si being rejected into that region by the α-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.sub.2Si nucleation/growth process has to be completed at a temperature around 380° C., before the L.fwdarw.Al—Zn eutectic reaction takes place, wherein L depicts the molten liquid phase.

[0100] The applicant has found that controlling the balance between the time available for diffusion and the diffusion distance for Mg and Si can control the subsequent nucleation or growth of the Mg.sub.2Si phase or the final distribution of the Mg.sub.2Si phase in the coating thickness direction.

[0101] In particular, the applicant has found that for a set coating thickness, the cooling rate should be regulated to a particular range, and more particularly not to exceed a threshold temperature, to avoid the risk for the Mg.sub.2Si 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 cooling rate will drive the α-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.sub.2Si phase to nucleate, in region A (which is undesirable).

[0102] On the other hand, for a set cooling rate, a thicker coating (or a thicker local coating region) will 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 and Si, or a higher risk for the Mg.sub.2Si phase to nucleate, in region A (which is undesirable).

[0103] Practically, the applicant has found that, to achieve the distribution of Mg.sub.2Si particles of the present invention, i.e. to avoid mottling defect on the surface of a coated strip, 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° C./sec for coating 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 Mg.sub.2Si particles of the present invention.

[0104] The applicant has also found that, when Sr is present in a coating bath, the above described kinetics of Mg.sub.2Si nucleation can be significantly influenced. At 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 coating surface. Such changes alter significantly the preferential nucleation sites for the Mg.sub.2Si phase and, as a result, the distribution pattern of the Mg.sub.2Si phase in the coating thickness direction. In particular, the applicant has found that, Sr at concentrations 250-3000 ppm in the 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 increase in system free energy would otherwise be generated. Instead, the Mg.sub.2Si phase can only nucleate at the central region of the coating in the thickness direction, resulting in a coating structure that is substantially free of Mg.sub.2Si at both the coating outer surface region and the region near the steel surface. Therefore, Sr additions in the range 250-3000 ppm are proposed as one of the effective means to achieve a desired distribution of Mg.sub.2Si particles in a coating.

[0105] Many modifications may be made to the present invention as described above without departing from the spirit and scope of the invention.

[0106] 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) cooling rates (for a given coating mass) and (c) control of short range coating thickness variation as means for achieving a desired distribution of Mg.sub.2Si particles in coatings, i.e. at least substantially no Mg.sub.2Si 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.sub.2Si particles in the coating.