HOT-DIP GALVANISED STEEL SHEET

Abstract

The present disclosure relates to a hot-dip-coated steel sheet having a ZnMgAl coating which includes aluminum at between 0.1 and 8.0 wt %, magnesium at between 0.1 and 8.0 wt %, the balance being zinc and unavoidable impurities, wherein the coating comprises zinc grains and further phases of magnesium and/or aluminum and also eutectic structures including at least intermetallic zinc-magnesium phases, wherein a native oxide layer is formed on the coating. In accordance with the present disclosure, the coating beneath the native oxide layer has an area fraction of at least 35% in which there is an average nanohardness of at least 4 GPa.

Claims

1. A hot-dip-coated steel sheet having a ZnMgAl coating which includes aluminum at between 0.1 and 8.0 wt %, magnesium at between 0.1 and 8.0 wt %, the balance being zinc and unavoidable impurities, wherein the coating comprises zinc grains and further phases of magnesium and/or aluminum and also eutectic structures including at least intermetallic zinc-magnesium phases, wherein the coating comprises a native oxide layer being formed on the coating, and a coating beneath the native oxide layer having an area fraction of at least 35% in which there is an average nanohardness of at least 4 GPa.

2. The steel sheet as claimed in claim 1, wherein the coating in a depth of 20 nm beneath the native oxide layer has an area fraction of at least 35% in which there is an average nanohardness of at least 3 GPa.

3. The steel sheet as claimed in claim 1, wherein the coating in a depth of 40 nm beneath the native oxide layer has an area fraction of at least 35% in which there is an average nanohardness of at least 2.5 GPa.

4. The steel sheet as claimed in claim 2 wherein the coating in a depth of 70 nm beneath the native oxide layer has an area fraction of at least 35% in which there is an average nanohardness of at least 2 GPa.

5. The steel sheet as claimed in claim 4 wherein the coating includes aluminum and magnesium at in each case at least 0.5 wt %.

6. The steel sheet as claimed in claim 5 wherein aluminum and magnesium in the coating are limited to in each case not more than 3.5 wt %.

7. The steel sheet as claimed in claim 1 wherein the coating has a thickness of between 2 and 20 ?m.

8. The steel sheet as claimed in claim 7 wherein the coating bears an impressed deterministic or stochastic surface structure.

9. The steel sheet as claimed in claim 1 wherein the hard regions of the surface of the coating that are exposed after treatment with an inorganic acid have a developed boundary area ratio Sdr of at least 5.5%, based on an AFM scan region of 5?5 ?m.sup.2.

10. The steel sheet as claimed in claim 1 wherein the steel sheet has a surface-coveringly homogeneous phosphate layer with zinc phosphate crystals of up to 3 ?m in size.

11. The steel sheet as claimed in claim 3 wherein the coating in a depth of 70 nm beneath the native oxide layer has an area fraction of at least 35% in which there is an average nanohardness of at least 2 GPa.

12. The steel sheet as claimed in claim 11 wherein the coating includes aluminum and magnesium at in each case at least 0.5 wt %.

13. The steel sheet as claimed in claim 12 wherein aluminum and magnesium in the coating are limited to in each case not more than 3.5 wt %.

Description

IN THE DRAWING

[0035] FIG. 1) shows nanoindenter-generated hardness mappings in a depth of 20 nm, 40 nm and 70 nm within a standard ZnMgAl coating,

[0036] FIG. 2) shows nanoindenter-generated hardness mappings in a depth of 20 nm, 40 nm and 70 nm within a standard ZnMgAl coating according to one inventive embodiment, and

[0037] FIG. 3) shows respective SEM micrographs of the surface before and after treatment with an inorganic acid for a standard ZnMgAl coating, left-hand images, and respective SEM micrographs of the surface before and after treatment with an inorganic acid of a ZnMgAl coating according to one inventive embodiment, right-hand images.

[0038] Samples of a conventional steel sheet of grade DC04 with a thickness of 0.7 mm were coated in the laboratory with a ZnMgAl coating in a hot-dip simulator, with one cohort of the samples being passed through a first melt bath with Al=1.8 wt %, Mg=1.4 wt %, balance zinc and unavoidable impurities, and the other cohort of the samples through a second melt bath with Al=5.4 wt %, Mg=4.8 wt %, balance zinc and unavoidable impurities. The samples were withdrawn from the melt bath and passed to a stripping apparatus, which acted on both sides on the liquid melt on the samples and stripped off superfluous melt, with a gas stream adjusted in the stripping apparatus, such that after the solidification of the coating the thickness on all the samples was 7 ?m. The coating of the samples as a result of the first melt had a composition of Al=1.6 wt % and Mg=1.1 wt %, balance zinc and unavoidable impurities. The coating of the samples as a result of the second melt had a composition of Al=4.6 wt % and Mg=4.1 wt %, balance zinc and unavoidable impurities. Stripping took place in an inert atmosphere with 5% H2, balance N2 and unavoidable constituents, and the stripping gas used was N2. The cohort of the samples (1) which had passed through the first melt was cooled conventionally by the inert atmosphere and, owing to the acting gas stream, with a cooling rate of around 7? C./s. The other cohort of the samples (2) which had passed through the first melt was cooled actively with a cooling rate>20? C./s. Analogously, a cohort of the samples (3) from the second melt was cooled conventionally, and the other cohort of the samples (4) was cooled at a cooling rate>20? C./s.

[0039] Formed on all the samples (1) to (4), on the surface of the coating, was a native (magnesium-rich and aluminum-rich) oxide layer which on average for all the samples (1) to (4) was determined at around 8 nm via x-ray photoelectron spectroscopy, independently of the composition of the coating and of the cooling rate.

[0040] The various samples (1) to (4) were indented using a Hysitron TI Premier nanoindenter from Bruker. The analysis was conducted as already described above. The result was a locationally resolved and depth-resolved representation (nanoindentation), referred to as hardness mappings, in a depth of 20 nm, 40 nm and also 70 nm beneath the native oxide layer, over an analyzed area of 65?65 ?m.sup.2 of the local nanohardness; cf. FIG. 1 for a representation on average of the measured samples (1) and FIG. 2 for a representation on average of the measured samples (2). In terms of result, samples (3) and (4) were situated in the same order of magnitude as the results for samples (2). Using the nanoindenter and the Oliver & Pharr evaluation method, therefore, it is possible to determine the average nanohardness and the corresponding area fraction accordingly, generally over an area of 65?65 ?m.sup.2, as a function of location and depth.

[0041] Owing to the structure of the different phases within the coating, an increase in hardness through multiplication of the hard intermetallic zinc-magnesium phases can be ensured in the coating of the invention, this increase in hardness being manifested positively in turn in the corrosion and forming properties. The average nanohardness of at least 3 GPa in a depth of 20 nm beneath the native oxide layer of the coating is represented with an area fraction of at least 35%. In a depth of 40 nm beneath the native oxide layer of the coating, the coating has an area fraction of at least 35% in which there is an average nanohardness of at least 2.5 GPa. Further, in a depth of 70 nm beneath the oxide layer of the coating, the coating has an area fraction of at least 35% in which there is an average nanohardness of at least 2 GPa. In this regard, compare implementations in the corresponding planes/depths in FIGS. 1 and 2. Very readily apparent is the increase in the area fraction with an average nanohardness of at least 4 GPa in a depth of 20 nm for the samples (2) in FIG. 2, in comparison to the area fractions in the depth of 20 nm for the samples (1) in FIG. 1.

[0042] Samples (1) to (4) underwent further investigation by being treated at their surfaces with an inorganic acid under laboratory conditions. Here, the samples were degreased with alkaline cleaner and then immersed for 5 s in a solution with 12 ml/l sulfuric acid which had a temperature of 20? C. This was followed by rinsing with water and isopropanol. The entire experiments were conducted under standard air atmosphere. SEM micrographs were used to capture the conditions before and after the treatment with the inorganic acid on samples (1) and (2) see FIG. 3with the result that even in the condition before the acid treatment (micrographs in FIG. 3 top, sample (1) on the left and sample (2) on the right) in accordance with the invention there is a multiplication and, in conjunction therewith, a higher fraction of hard intermetallic zinc-magnesium (MgZn2 and/or Mg2Zn11) phases in evidence, with a relatively fine eutectic structure, top right-hand micrograph in FIG. 3, by comparison with the standard, top left-hand micrograph in FIG. 3. Going hand in hand with this, there is also a reduction in evidence in the soft zinc grains (Zn) by comparison with the prior art.

[0043] In the acidic medium, however, the magnesium in the intermetallic zinc-magnesium phases dissolves preferentially, and so such acid treatment of the coating of the invention leaves behind a comparatively more aluminum-rich surface. The aluminum at the surface of the coating, moreover, has the advantage that it is more readily soluble by alkaline process media such as cleaners or adhesives and hence allows the surface of the coating to be activated more effectively by such process media.

[0044] The treatment of the surface with an inorganic acid therefore has the effect, firstly, of chemically removing the original native (magnesium-rich and aluminum-rich) oxide layer, and secondly of dissolving parts of the underlying intermetallic zinc-magnesium phases from the eutectic; see bottom micrographs in FIG. 3. This leads to a marked roughening of the surface in the region of the intermetallic zinc-magnesium phases (in the nanometer range) and is accompanied by a corresponding growth of surface which is reflected in turn in improved keying of coating materials/adhesives at the surface and also, generally, in improved reactivity; see invention in the bottom right-hand micrograph in comparison to the standard in the bottom left-hand micrograph in FIG. 3. As a result of the corresponding surface-covering occupation (area fraction at least 35% or more) with eutectic phases, the chemical treatment and the associated enlargement in area on the surfaces of the coatings of the invention are much higher than conventional surfaces with less eutectic phases. The acid-treated eutectic at the surface of the coating has an average roughness of at least 7.5 nm, measured by means of AFM. The average roughness of the acid-treated eutectic at the surface of the conventional coating is less than 4.9 nm.

[0045] The higher area fraction of the eutectic, more particularly of the intermetallic zinc-magnesium phases and of the relatively fine microstructuresee right-hand micrographs in FIG. 3improve the corrosion control mechanism in that it allows the surface-covering formation of an even denser corrosion film. The demonstration of the trend toward improved corrosion properties was investigated in different electrolytes (NaCl solution, borate solution), and accordingly it was possible to observe that there was a trend toward lower corrosion propensity with regard to the surface of the coating of the invention. Factors suggesting this are a lower corrosion potential and also a lower limiting current density for oxygen diffusion, relative to the borate buffer.