EP2195471A1

EP2195471A1 - Anti-corrosion coating with improved adhesion

Info

Publication number: EP2195471A1
Application number: EP08802862A
Authority: EP
Inventors: Werner BRANDSTÄTTER; Siegfried Kolnberger; Josef Faderl; Martin Peruzzi
Original assignee: Voestalpine Stahl GmbH
Current assignee: Voestalpine Stahl GmbH
Priority date: 2007-10-10
Filing date: 2008-10-10
Publication date: 2010-06-16
Also published as: DE102007048504A1; DE202008017609U1; WO2009049836A1; DE102007048504B4

Abstract

The invention relates to an anti-corrosion coating for steel sheets, which are subjected to heating to or above the austenization temperature and to quench hardening for hardening purposes, wherein the coating is a coating comprising primarily zinc and that is applied onto the steel sheet during a hot dipping process. The invention is characterized in that the coating comprises 0.1 to 5% aluminum and 0.2 to 2% magnesium, in addition to zinc and possibly present inevitable impurities.

Description

Corrosion protection coating with improved adhesion

The invention relates to a corrosion protection coating with improved paint adhesion.

It is known from WO 2005/021822 of the Applicant that, in order to protect a cathodic anticorrosive layer, oxygen-affine elements are added to the metal forming the cathodic protective layer within certain limits in order to provide protection of the cathodic protective layer during curing of a component produced with the cathodically protected metal. To harden such components, they must be heated above the austenizing temperature of the base metal, in this case steel. In particular, in hochhärtbaren steels, this temperature is above 800 ⁰ C. At such temperatures, the most cathodic protection layers are destroyed by evaporation or oxidation, such that a component treated in this way would have no cathodic protection after curing. The addition of the oxygen-affine elements results in the oxygen-oxygen elements diffusing from the composition of the cathodic protective layer to the surface where they form a very fine protective layer. This very fine protective layer may, for example, consist of aluminum oxide. Investigations have shown that in the main oxygen-type element used, namely aluminum in the zinc coating, an aluminum oxide layer forms on the surface, which is very hard, vitreous and smooth and which apparently hampers good paint adhesion. If you paint such hardened hot-dip galvanized sheets, the paint adheres poorly to the sheet. Possibly the paint dissolves together with aluminum oxide layer or the paint binds badly on the alumina. One way to improve paint adhesion is therefore to previously remove the poorly adherent oxide film.

DE 697 30 212 T2 discloses a hot-dip coated steel sheet with excellent corrosion properties and a process for its production which has a zinc-aluminum-magnesium coating, the coating containing, in addition to zinc, 4-10% by weight of aluminum and 1 - 4 wt .-% magnesium.

From GB 1125 965 A a zinc coating and a method of applying it to sheet steel are known, whereby the zinc coating may contain 3-5% by weight of aluminum and 1 to 4% by weight of magnesium. This coating should also provide good corrosion resistance.

WO 2006 002 843 A1 discloses a steel sheet with a hot dip coating based on zinc, this hot dip coating additionally comprising 0.3 to 2.3% by weight of magnesium and 0.6 to 2.3% by weight of aluminum may contain.

From DE 32 42 625 Al a process for the production of hot-dip galvanized steel sheets is known, wherein the galvanizing bath with respect. The lead content should be strictly limited and also 0.35-3% aluminum and 0.15-1% magnesium may be included.

US Pat. No. 6,379,820 B1 likewise discloses a hot-dip coated steel sheet, the coating metal or the coating comprising a zinc-aluminum-magnesium coating with 4-10% by weight of aluminum and 1-4% by weight. Magnesium is. The composition of the coating should have an influence especially on crystalline aluminum phases in the metal.

From JP 58 189 363 A a coated steel sheet is also known, which is coated with a zinc alloy. Said zinc alloy may contain 0.5-5% by weight of aluminum and 0.01-2% by weight of magnesium and additionally lead. A steel strip coated in this way is to be subjected to a diffusion process after the coating in order to enrich the iron content in the layer.

The object of the invention is to provide a corrosion protection coating which, on the one hand, provides reliable protection against oxidation and evaporation of the zinc layer during curing and, secondly, enables good paint adhesion without post-processing of the corrosion protection layer.

The object is achieved with a coating having the features of claim 1. Advantageous developments are characterized in the subclaims.

Another object of the invention is to provide a method for producing a hardened sheet metal component with a corrosion protection coating with improved adhesion. This object is achieved by a method having the features of claim 3. Advantageous developments are characterized in the dependent claims.

Another object of the invention is to provide a hardened sheet metal component having a corrosion protection coating with improved adhesion.

This object is achieved with a hardened sheet metal component with the features of claim 5.

According to the invention, it has been found that a defined admixing of magnesium to an aluminum-containing zinc coating evidently intervenes in the formation of the hard layer from the aluminum oxide in such a way that no vitreous, very hard and smooth aluminum oxide surface is formed, but a uniformly structured layer which gives a good lacquer adhesion results.

In addition, can be achieved with such a defined admixing a layer which assumes a darker color during annealing, so that in addition the emissivity increases, which inventively coated sheets can be heated more quickly to temperatures of about 900 ⁰ C, as hot-dip galvanized sheets without magnesium supplement.

According to the invention, the coating of a steel sheet is formed by providing the steel sheet in a conventional dip-dip coating process with a metal coating containing 0.1 to 5% aluminum and 0.2 to 2% magnesium, balance zinc. Such a coating results in a very good adhesion of the oxide layer which forms and a very good lability.

The explanation for the good adhesion of the oxide layer is the following: The magnesium is in the group of network converters. It forms crystalline oxides and, moreover, has a greater tendency than aluminum to form oxides. As a result, the aluminum is no longer possible when heated to 900 ⁰ C to form a covering glassy network of alumina. A solid-state reaction forms well-adhering oxide. It may also be possible to form a compound MgAl ₂ O ₄ , the spinel with zinc oxide.

In contrast, a coating with only an aluminum additive reacts in a different way.

After melting of the zinc at 420 ⁰ C, a glassy, a few nanometers thick aluminum oxide layer forms on the surface. Subsequently, the decomposition of Zn-Fe phase into zinc-rich melt and solid Zn-Fe phase causes stresses on the surface. At defects in the aluminum oxide layer, alumina rapidly forms by diffusion of aluminum from the zinc-rich melt to the surface. However, it shows first zinc oxidation in the form of ZnO clusters. Subsequently, trenches are formed in the Zn-Fe layer, which consists largely of melt and zinc-saturated α-iron. The aluminum oxide layer already has a thickness of up to 100 nm at this time. As a result of the consumption of the molten zinc, it subsequently lies like a corrugated roof on the zinc-saturated α-iron grains. Where the alumina still adheres well to the substrate, however, tensions occur during rapid cooling. After the hard The aluminum oxide layer with the ZnO clusters therefore adheres very poorly to the substrate.

The invention will be explained by way of example with reference to a drawing. It show here:

FIG. 1 shows a diagram of the surface of hardened hot-dip-galvanized sheet metal according to the invention with magnesium in the layer;

FIG. 2: a scanning electron micrograph of the

Surface of the annealed sheet with the coating according to the invention;

Figure 3 is a schematic of the surface of a prior art hot dip dip galvanized sheet having a poorly adherent oxide layer;

FIG. 4 shows a scanning electron micrograph of the surface of the annealed sheet according to FIG. 3;

Figure 5: the appearance of the heated and heated to 900 ⁰ C sheets with a sheet coated according to the invention (left), and a sheet metal according to the prior art (right);

Figure 6: the paint adhesion to a annealed according to the invention coated sheet (left) and a sheet metal according to the prior art (right) after corrosive removal;

Figure 7 is a photomicrograph of the cross-section of annealed sheet of the prior art; FIG. 8 shows a cross section of the annealed sheet coated in accordance with the invention;

FIG. 9: GDOES analysis of the hot-dip galvanized and non-heat treated sheet according to the invention;

Figure 10: GDOES analysis of the hot-dip galvanized and non-heat treated sheet of the prior art;

Figure 11: GDOES analysis of the inventively coated and heated to 900 ⁰ C sheet;

Figure 12: GDOES analysis of the heated to 900 ⁰ C and cured sheet according to the prior art;

FIG. 13: technical data of the trial painting of the sample sheets;

FIG. 14: arrangement of the bonding of sample strips;

FIG. 15: tensile shear strengths of the hardened sheet according to the invention and a comparison sheet according to the prior art;

FIG. 16 shows the emissivity of a steel sheet coated in accordance with the invention during annealing in the radiation furnace, which is only 895.degree.

FIG. 17: chemical composition of the steel 592519 used for the samples;

FIG. 18: chemical composition of a further test steel 600354; FIG. 19: the scanning electron micrograph of the

Surface of a hardened sheet according to FIG. 18;

FIG. 20: a table describing the test results relating to the weldability of sheets according to the invention.

According to the invention, a coating is used for the anticorrosion coating 1 of a steel sheet 2, which consists essentially of zinc and in addition to zinc contains aluminum in contents of 0.1 to 5% and magnesium in contents of 0.2 to 2%.

Within these mixing ranges, the ratio of aluminum to magnesium can be freely adjusted, with all possible mixtures a success can be achieved.

The result of the annealing treatment can be seen in FIGS. 1 and 2. One recognizes the steel substrate 2 and on the steel substrate 2, an iron-zinc layer 3, which may consist of different iron-zinc phases. As the top layer 4, an oxide layer 4 is present at the top, which follows the wavy or rough structure of the iron-zinc layer 3 on the surface. In the region of the depressions 5 of the roughness, cracks 6 are recognized in the iron-zinc layer 3.

In Figure 2 can be seen very well the rugged surface of a coated according to the invention and then annealed sheet. Here, a well-adherent oxide layer has formed during annealing, which may be several microns thick depending on the annealing conditions. It is believed that magnesium, as a network transducer, forms crystalline oxides and has a greater tendency to form oxides than aluminum. As a result, the aluminum is no longer possible when heated to 900 ⁰ C or above. lent to forming a covering, glassy alumina network. A solid-state reaction between the iron-zinc layer and the oxide layer forms the well-adhering oxide, it being assumed that spinels may also be formed with the zinc oxide.

In Figures 3 and 4, a conventional known coating 10 is shown, where in contrast to the coating according to the invention 1 is a more or less smooth closed Al exists θ ₃ layer 11 _2, which has in the range of Mikroris- sen Zinkoxidausblühungen 12, which are clearly visible in particular in FIG. This structure could be determined by means of Auger electron spectroscopy. This formation of a continuous layer is explained as follows.

After melting of the zinc at 420 ⁰ C, a glassy, a few nanometers thick aluminum oxide layer forms on the surface. Subsequently, the decomposition of Zn-Fe phase into zinc-rich melt and solid Zn-Fe phase causes stresses on the surface. At defects in the aluminum oxide layer, alumina rapidly forms by diffusion of aluminum from the zinc-rich melt to the surface. However, it shows first zinc oxidation in the form of ZnO clusters. Subsequently, trenches are formed in the Zn-Fe layer, which consists largely of melt and zinc-saturated α-iron. The aluminum oxide layer already has a thickness of up to 100 nm at this time. As a result of the consumption of the molten zinc, it subsequently lies like a corrugated roof on the zinc-saturated α-iron grains. Where the alumina still adheres well to the substrate, however, tensions occur during rapid cooling. After hardening, the aluminum oxide layer with the ZnO clusters therefore adheres very poorly to the substrate. In contrast to the invention, in which the oxide layer follows well the waviness of the iron-zinc layer, in the prior art adhesion areas form in which the layer adheres to the iron-zinc layer and free areas under which cavities are located.

The formation of oxide during the hardening of hot dip galvanized sheets with and without inventive magnesium addition to the zinc bath can be compared by means of GDOES analyzes before and after curing.

According to the invention, hot-dip galvanized sheets (magnesium addition to the zinc bath of 1.1% by mass) and conventionally hot dip-galvanized sheets without magnesium addition to the zinc bath were analyzed by glow discharge spectroscopy before and after hardening.

It was found that the layer structure of the two sheets after hot dip galvanizing is about the same. In hot-dip galvanizing, an aluminum-rich phase forms between the steel and the zinc layer in both sheets, which is necessary for a good formability of the boards to form components. At the surface is in both cases only a thin oxide layer. The oxygen signal O is hardly recognizable. In metal sheets according to the invention magnesium is incorporated in the zinc layer due to the addition of magnesium to the zinc bath (compare FIG. 9 and FIG. 10).

After hardening of the two sheets arise especially in the oxide layer different sputtering profiles. The oxide layer of the heated and cured sheet according to the invention is thicker and is completely removed only after about 50 s. Compared to the thinner oxide layer of the non-inventive sheet, the aluminum signal is present on the surface. while zinc and magnesium are clearly embedded in the oxide layer.

The aluminum according to the invention could not form a covering aluminum oxide layer during the heating, but it also oxidized magnesium and zinc. This oxide layer adheres very well to the substrate.

On conventional hot-dip galvanized sheet without added magnesium formed when heated to 900 ⁰ C, a well-protective and relatively thin aluminum oxide layer. However, this thin oxide layer, which consists almost exclusively of aluminum oxide, adheres very poorly to the substrate (see FIG. 11 and FIG. 12).

FIG. 7 shows the transverse section of the hardened conventionally hot-dip-galvanized sheet. The Zn-Fe layer is about

25 microns thick and has a structure of bright, zinc-rich phase and dark zinc-containing α-Fe grains. The oxide layer is in

Cross section not visible and consists of an approximately

150 nm thick aluminum oxide layer.

FIG. 8 shows the transverse section of the hardened sheet according to the invention. The Zn-Fe layer is again about 25 microns thick and has again a structure of bright, zinc-rich phase and zinc-containing α-Fe mixed crystals. In the zinc-rich areas containing magnesium and aluminum, some are pores. The dark surface of the cured sheet according to the invention is an oxide layer up to 4 μm thick.

Both FIGS. 7 and 8 show that even with a coating according to the invention, although it obviously reacts differently or gives a different cover layer, a substantially identical phase composition of the zinc-iron oxide is obtained. Produces phases on the surface of the steel sheet, which is important for a cathodic corrosion protection.

The invention will be further explained by way of examples.

1. Sample preparation and hardening of the sheets

Applicant's hot-dip galvanizing simulator has zinc-plated phs-ultraform plates of 592519 steel with an alloy containing about 0.2% aluminum and 1.1% magnesium in addition to zinc. The result was a sheet steel coated on both sides 592519 ZMg 190, hereinafter referred to as sheet A.

Further, in the hot-dip galvanizing simulator of the Applicant, phs-ultraform boards of steel 592519 were zinc-plated with an alloy containing only about 0.2% aluminum and no magnesium in addition to zinc. The result was a conventional hot dip galvanized sheet steel 592519 Z 200 coated on both sides, referred to hereafter as sheet B.

Blanks of the size 100 mm × 100 mm of the two 1.5 mm sheets A and B were annealed in a 910 ^° C. hot radiation oven for 5 minutes and then cooled between steel plates and thus hardened. Figure 5 shows the two annealed sheets, left the hot-dip galvanized sheet A with magnesium added, the brighter conventional hot dip-galvanized sheet B.

Painting the hardened sheets

After curing, the two panels A and B were phosphated and KT-painted as in the car manufacturer.

FIG. 13 shows the process for coating metal sheets by means of automotive phosphating and KT painting by the applicant. Of the Painting process is similar to the process of painting bodies with car manufacturers.

Examination of paint adhesion against corrosive removal

In a paint adhesion test, you scratch a so-called cross-hatch cut into the surface, press an adhesive tape firmly over the scratches and then pull it off. In this case, no paint should be removed.

Paint adhesion to conventionally hot dip galvanized samples from corrosive aging, i. the sheets B; is mediocre. The insufficient paint adhesion of the samples is due to the formation of the poorly adhering aluminum oxide layer. The parting line paint-sheet is the oxide layer. The aluminum oxide on hardened, hot-dip-galvanized blanks is not removed during phosphating. There are wide areas without phosphate coating.

The paint adhesion before corrosion aging of the sheet B was rated on a scale of 0 to 5 with 3. The value 0 would not mean a lacquer finish after cross-hatching, a value of 3 means that even large areas within the cross-hatch showed a poor lacquer adhesion and could be removed with adhesive tape.

Paint adhesion to magnesium-containing hot-dip galvanized samples, i. the sheets A, is very good. The paint adhesion before corrosion aging of the sheet A was rated 0 on a scale of 0 to 5, i. no paint was peeled off with the adhesive tape.

Examination of paint adhesion after corrosive aging The sheets of type A and B were phosphated, KT-coated and stored in the VDA-Wechselklimatest DIN 621-45 for 10 weeks under corrosive conditions. Then the paint adhesion was tested by three methods.

1. A tape removal after crosshatching.

2. The corrosive infiltration of the paint on a scratched area prior to corrosive removal.

3. An area that has been bombarded with steel shot before and after corrosion removal.

The result for all 3 methods was that the paint adhesion to the cured conventional hot-dip galvanized sheet B is insufficient. The cross hatch rating was 5, the corrosive infiltration of the paint was not only at the scribe but across the entire sheet surface and in that area which was bombarded with steel shot, the paint almost completely dissolved (see Figure 6).

Sheet A, the sheet containing magnesium in the zinc layer, shows excellent paint adhesion both in the crosshatch test (rating: 0), at the scribe and also in the area which was bombarded with steel shot.

Figure 6 shows the samples A and B after the corrosive aging and after the tests (without cross-hatching).

2. Comparison of the adhesive suitability of hardened, conventionally hot dipped galvanized sheets with hardened hot dipped galvanized sheets made by adding magnesium to the zinc bath From the hardened sheets of variants A and B strips of dimension 100 mm x 25 mm were produced. Two such strips were joined at the ends with a crash-stable structural adhesive Betamate 1496. The glued surface of the structural adhesive was 25 mm × 10 mm with an adhesive layer thickness of 0.2 mm. FIG. 14 shows the basic form of the bond.

The adhesive bond was tested by means of a tensile shear test according to DIN EN 1465 with a subsequent assessment of the fracture pattern.

FIG. 15 shows the tensile shear strength of the adhesive bonds. The graph shows the results of the individual experiments, the mean and the standard deviation of the mean for the two variants.

The tensile shear strength of the adhesive bonds of the hot-dip galvanized sheets by means of magnesium addition to the zinc bath was on average 33 MPa (sheet A) in the cured state. The tensile shear strengths of the cured conventional hot-dip galvanized samples were only 14 MPa (plate B). A tensile shear strength of 14 MPa is insufficient, especially since the tensile shear strength drops even further after any corrosive stress. The reason for the low adhesive adhesion is the poor adhesion of the aluminum oxide to the substrate, which also manifests itself in an adhesive failure of the adhesive bond. In contrast, the tensile shear strength of the sheet with the magnesium-containing zinc coating is close to the adhesive strength itself. Therefore, a cohesive failure was also detectable from the fracture pattern.

3. Comparison of the weldability of hot-dip galvanized sheets by adding magnesium to the zinc bath were made with other hardened galvanized sheets

Hot-dip galvanized sheets, prepared with a magnesium additive for zinc bath, conventional hot-dip galvanized sheets without addition of magnesium to the zinc bath and electrolytically galvanized sheets without magnesium or aluminum in the zinc coating were heated in a radiation oven to about 900 ⁰ C and then cured.

Experiments for resistance spot welding of hardened galvanized sheets showed the following:

The hardened conventionally hot dip galvanized sheets showed the largest weld area and were well weldable. The hardened hot dip galvanized sheets with magnesium in the zinc coating showed a wide welding range and could be welded well. The hardened electrolytically galvanized sheets could not be welded by resistance spot welding because a very thick zinc oxide layer was formed during annealing. The zinc oxide layer therefore formed very thick and dense (several microns thick, very compact) because there was no aluminum and magnesium which could protect the zinc from oxidation.

When measuring the contact resistance of the sheets, it was found that a hardened hot-dip galvanized sheet with zinc-coated magnesium had a contact resistance of 6.3mΩ on average with a standard deviation of 2, 1 for 10 measurements.

The contact resistance of comparable hardened electrolytically galvanized sheets was at an average of about 11.8 milliohms in 10 measurements, ie about twice as high. The higher the Contact resistance of the sheet or the oxide layer, the smaller the so-called welding area and the worse the weldability in resistance spot welding.

This also explains the better weldability of the cured magnesium-containing hot-dip galvanized sheets compared to the hardened sheets with an original pure zinc coating.

FIG. 20 shows a table which reproduces the results of tests, the coverage of the coating according to the invention being 120 g / m ² . In each case, the weldability was assessed at different furnace temperatures and different residence times. It can be seen that, regardless of the furnace temperature, a very good weldability is achieved and the contact resistance of 4.3 mΩ on average is well achieved.

4. Heating behavior due to the color change of the sheets when heated

FIG. 16 shows the emissivity of sheet A during annealing in the 895 ° C hot-air furnace. The emissivity increases up to a temperature of 700 ⁰ C to ~ 0,2 - 0,3 before it steeply increases to a value close to 1 at an annealing temperature close to the oven temperature of 895 ° C. The surface after annealing and hardening is black.

While conventionally hot-dip galvanized sheets remain bright when heated in 900 ⁰ C hot furnace and the emissivity in the furnace rises only slowly over a value of about 0.6, hot-dip galvanized sheets with magnesium in the zinc coating heat up faster because their emissivity at 900 ⁰ C near 1 lies. The reason for the low emissivity of conventional hot-dip galvanized sheets at 900 ⁰ C is that the thin aluminum oxide layer is considered to be transparent and the underlying bright zinc-rich phases reflects the heat radiation rather than absorbed. However, when hot-dip galvanized sheet metal with magnesium in the zinc coating is heated, dark Zn-Mg-Al oxides are formed, which absorb heat radiation well and thus rapidly heat the sheet. See also Figure 5.

Hardening a sheet of steel 600354 with a magnesium-containing, hot-dip galvanized coating

A 1.5 mm thick steel collar number 600354/2 was galvanized on a hot-dip galvanizing line in a hot-dip galvanizing bath containing magnesium. The aluminum content in the zinc bath was about 4%, the magnesium content about 1%. The zinc coating was about 14 μm thick on both sides, i. This was followed by a Zn-Al-Mg suspension of ZnAlMg 200.

When 7-minute heating the boards of the collar 600,354 in 900 ⁰ C hot oven radiation, the surface is colored dark again. Subsequently, the sheet was rapidly cooled between steel plates. The result was a typical phase structure of zinc-rich and zinc-poor Zn-Fe phases. FIG. 19 shows the scanning electron micrograph of the surface of the annealed sheet, which again shows that the oxide layer protrudes into the Zn-Fe phases.

The paint adhesion before and after corrosion aging was excellent and was judged by cross hatch with 0. The sheet showed a high tensile shear strength when glued with Betamate 1496 and was well resistant to welding. At the fair The contact resistance of the sheet was found to be 6.3 mΩ with a standard deviation of 2.1 for 10 measurements.

In the invention, it is advantageous that by selective selection of a coating composition for the corrosion protection coating of a hardenable steel sheet after curing, a corrosion protection layer is achieved which has a very good paint adhesion, a very good bondability and a good weldability compared to a known coating results. The addition of magnesium to a defined extent significantly improves a developing oxidation barrier for a zinc-iron layer.

In addition, by the invention, the emissivity and thus the heatability of a thus coated sheet can be significantly improved.

List of reference numbers:

1 corrosion protection coating

2 sheet steel

3 iron-zinc layer

4 oxide layer

5 wells

6 cracks

10 coating according to the prior art

11 Al ₂ O ₃ layer

12 zinc oxide efflorescence

Claims

claims

1. An anticorrosion coating for steel sheets, which are subjected to the purpose of curing a heating to or above the Austeni- tisierungsstemperatur and a quench hardening, wherein the coating is a substantially zinc-containing, by hot-dip method applied to the steel sheet, characterized in that the coating in addition to zinc and optionally present unavoidable impurities 0.1 to 5% aluminum and 0.2 to 2% magnesium.

2. corrosion protection coating according to claim 1, characterized in that the corrosion protection coating is applied in a thickness of 5 to 25 microns on the steel.

3. A method for producing a corrosion protection coating for steel sheets, which are subjected to the purpose of curing a heating to or above the austenitizing temperature and a quench hardening, in particular a corrosion protection coating according to claim 1 or 2, characterized in that a coating which contains substantially zinc in Hot dip method is applied to the steel sheet and a coating is used, in addition to zinc and possibly present unavoidable Impurities 0.1 to 5% aluminum and 0.2 to 2% magnesium.

4. The method according to claim 3, characterized in that the corrosion protection coating is applied in a thickness of 5 to 25 microns on the sheet.

5. Hardened sheet metal component with a corrosion protection coating according to claim 1 or 2, produced by a method according to one of claims 3 or 4.