CN115279945A

CN115279945A - Method for forming conditional zinc layer by electrolytic galvanizing of steel strip

Info

Publication number: CN115279945A
Application number: CN202180020478.9A
Authority: CN
Inventors: M·弗莱彻德尔; E·沙辛格
Original assignee: Voestalpine Stahl GmbH
Current assignee: Voestalpine Stahl GmbH
Priority date: 2020-02-28
Filing date: 2021-03-01
Publication date: 2022-11-01
Also published as: EP4110971A1; EP3872231A1; WO2021170861A1; CA3172957A1

Abstract

The invention relates to a method for the surface treatment of a steel strip covered with an electrolytically deposited corrosion protection layer of a zinc alloy, the steel strip being subjected to elevated temperatures in order to modify the structure of the steel strip, characterized in that during the electrolytic deposition of the steel strip tin is deposited or tin is deposited together with zinc in such a way that after the electrolytic deposition tin is formed at or near the surface or tin is applied in metallic form or in ionic form.

Description

Method for forming conditional zinc layer by electrolytic galvanizing of steel strip

Technical Field

The invention relates to a method for electrolytically galvanising a steel strip, wherein a zinc anti-corrosion layer is treated.

Background

It has long been known to provide a protective layer for metal sheets, particularly metal strips, which corrode under normal use conditions.

Typically, the corrosion protection layer on the metal strip may be an organic coating, for example, paint. These paints easily contain corrosion inhibitors.

It is also known to protect the metal strips by means of a metal layer. Such a metal layer may comprise an electrochemically more noble metal or an electrochemically less base metal.

Where the metal layer comprises an electrochemically more noble metal or a self-passivating metal such as aluminum, we refer to it as a barrier protective layer. For example, when aluminum is applied to steel, the steel material may be subject to corrosion if the barrier protective layer is no longer present in certain areas, for example, due to mechanical damage. A common barrier protective layer for steel is the above-mentioned aluminium layer, usually applied by means of hot dip coating.

If an electrochemically more basic metal is used as the protective layer, we refer to it as a cathodic corrosion protection coating. Because if the corrosion-resistant coating is mechanically damaged to expose the steel, the electrochemically more base metal corrodes before the steel when it is exposed to the corrosive conditions, and itself corrodes.

The most common cathodic protective coating on steel is a zinc layer.

There are many known galvanization methods. One common galvanization method is so-called hot dip galvanization (also called batch galvanization). In this case, steel (e.g. steel strip and steel wire) or steel sheet (e.g. steel components) is continuously immersed in a bath of molten zinc (the melting point of zinc is 419.5 ℃) at a temperature of about 450 to 600 ℃. The zinc bath generally contains at least 98.0% by weight of zinc according to DIN EN ISO 1461. A tough alloy layer of iron and zinc is formed on the steel surface, which is covered by a firmly adhering pure zinc layer, the composition of which corresponds to the composition of the zinc bath. The thickness of the zinc layer on the continuously galvanized steel strip is 5 to 40 μm. In assemblies formed by galvanising steel sheets, the thickness of the zinc layer may be 50 to 150 μm.

In electrolytic galvanizing (galvanization), a steel strip or sheet is not dipped into a galvanizing bath, not into a zinc electrolyte. In this case, the steel to be galvanized is introduced into the solution as the cathode, the purer the better the zinc electrode as the anode. The current is conducted through the electrolyte solution. In this case, the zinc (oxidation stage + II) is reduced in ionic form to metallic zinc which deposits on the steel surface. Thinner zinc layers can be obtained with electrolytic galvanization compared to hot galvanization. The zinc layer thickness is then proportional to the intensity and duration of the current, wherein a zinc layer thickness profile is formed over the entire workpiece, depending on the geometry of the workpiece and the anode.

Careful pretreatment of the surface is required to ensure adhesion and uniformity of the zinc layer. For example, the pretreatment may be degreasing, caustic washing, acid washing, rinsing, and/or descaling. After galvanization, one or more post-treatments can be carried out, such as phosphating, oiling, or application of organic coatings (cathodic dip coating, CIP for short).

Typically, this involves not only depositing a pure metal coating. Also, many known alloys can be deposited. In addition to pure aluminum coatings, coatings comprising aluminum and zinc, and coatings comprising a small amount of aluminum in addition to primarily zinc; other elements such as zinc, nickel, chromium, magnesium, and other elements, and mixtures thereof, may also be included.

It is known to provide at least a portion of a vehicle body with high strength to ensure sufficient strength in the event of a collision, particularly in order to reduce the weight of the vehicle body. The weight savings is due to the fact that high strength steel grades can be used and have relatively thin wall thicknesses and therefore are lighter in weight.

Even when high-strength steel grades are used, there are different methods and an extremely wide range of steel grades available.

The use of high strength steel grades is particularly common due to quench hardening. A common steel grade that can be hardened by quench hardening is the so-called boron manganese steel, e.g. the most common 22MnB5, and derivatives of such steels, e.g. 22MnB8 and 30MnB8.

This steel grade can be easily formed in the unhardened state and cut to the desired dimensions.

Basically, there are two different processes, in particular in the construction of vehicle bodies, for producing these steel grades in the desired shape and hardening them.

The first older process is the so-called press hardening. In press quenching, a flat sheet stock material is cut from a thin strip of quench hardenable alloy steel, such as 22MnB5 or similar manganese boron steels. The sheet stock material is then heated to the point where the steel structure is in the form of gamma iron or austenite. In order to achieve this structure, the so-called austenitizing temperature Ac3 must therefore be exceeded, at least when complete austenitization is required.

Depending on the steel, this temperature may be between 820 and 900 ℃; such steel strips are heated, for example, to approximately 900-930 c and maintained at this temperature until the structural change is complete.

This steel strip is then transferred in the hot state into a press comprising an upper tool and a lower tool each having a corresponding shape, the hot steel strip being shaped into the desired shape in a single pressure stroke. Energy is quickly removed from the steel by the contact of the hot steel material with the relatively cold, in particular cooled, pressure tool, i.e. the forming tool. In particular, the heat must be removed quickly enough to exceed the so-called critical hardening rate, typically in the range of 20 to 25 ° kelvin per second.

If cooling is performed at such a rate, the austenitic structure does not change back to the ferritic initial structure; instead, a martensitic structure is obtained. Since austenite dissolves much more carbon in its structure than martensite, the carbon precipitation phenomenon causes lattice distortion, resulting in a high hardness of the final product. It can be said that rapid cooling stabilizes the martensitic state. This makes it possible to achieve a hardness and tensile strength Rm of more than 1500 MPa. The hardness profile may also be established by suitable measures that do not require a more detailed discussion, such as full or partial reheating.

Another relatively new method of producing hardened steel parts, particularly for vehicle body structures, is form hardening, which was developed by the applicant. In form hardening, a flat steel strip is cut out of the steel strip and then formed in the cold state. In particular, the forming is not performed by a single pressure stroke, but rather as is customary in conventional press lines, for example in a five-step process. This process enables the production of significantly more complex shapes, and thus ultimately complex-shaped parts, such as the B-pillar or longitudinal member of an automobile.

In order to harden this completely formed component, the component is then likewise austenitized in a furnace and transferred in the austenitized state into a forming tool. The forming tool has the contour of the final part. Preferably, the preformed part is shaped before heating in such a way that after heating and thus also after thermal expansion has taken place, the part already corresponds as far as possible to the final dimensions of the hardened part. The austenitized blank is placed in the forming tool in an austenitized state and the forming tool is closed. In this case, the component is preferably contacted and held in a clamping manner on all sides by the shaping tool and heat is likewise removed by contact with the shaping tool, so that a martensitic structure results.

In the clamped state, no shrinkage occurs. The finally hardened part with the corresponding final dimensions can thus be removed from the forming tool after hardening and cooling.

Since motor vehicle bodies usually have an anti-corrosion coating, the anti-corrosion coating is embodied in the form of a metal coating closest to the metal material, in particular steel, which forms the body. Therefore, past efforts and developments have focused on corrosion protective coatings for hardened parts.

However, the corrosion protection coating of the component to be hardened must meet different requirements than the corrosion protection coating of the unhardened component. The corrosion protection coating must be able to withstand the high temperatures generated during the hardening process. Since it has long been known that hot dip aluminized coatings can also withstand high temperatures, press hardened steels with protective layers of aluminum were first developed. Such coatings are able to withstand not only high temperatures but also hot-state forming. However, it is disadvantageous that, in motor vehicles in general, the conventional steel grades used are not hot dip aluminized but hot dip galvanised, and the use of different corrosion protection systems is fundamentally problematic, in particular when there is a risk of contact corrosion.

For this reason, the applicant has developed a method capable of providing a zinc coating that is also resistant to such high temperatures.

Basically, zinc coatings are much more complex to shape than aluminum coatings, because aluminum coatings tend to flake or crack at conventional forming temperatures. This does not occur with zinc.

However, it was originally expected that the zinc coating could not withstand high temperatures. However, special zinc coatings containing certain amounts of oxophilic elements can actually be processed at high temperatures, since the oxophilic elements rapidly diffuse to the air-side surface, where they oxidize and form a protective film like a glass, such as a zinc coating. Since then, such zinc coatings have been widely used, in particular for form hardening. Such zinc coatings have also enjoyed great success in press hardening.

In order to ensure optimum paint adhesion and optimum weldability, it is known to clean the finally formed and hardened parts by means of a smoothing or abrasion of the glass hard protective film layer.

DE102010037077B4 discloses a method for treating the surface of hardened corrosion-protected components made of steel sheet. Among them, the steel sheet is a steel sheet having a metal coating, heated for hardening and then press-hardened. After hardening, the oxides present on the corrosion protection coating as a result of heating are removed. Wherein the components are subjected to slide grinding in order to treat the surface of the metal coating, i.e. the corrosion protection layer, and wherein the corrosion protection coating is a zinc-based coating, and the surface treatment is carried out in such a way that oxides present on or adhering to the corrosion protection layer are abraded away, in particular micropores are exposed.

DE102007022174B3 discloses a method for producing and removing a temporary protective layer of a cathode coating. Wherein a steel sheet consisting of a hardenable steel alloy is provided with a zinc coating in a hot dip process. Wherein the aluminum content in the zinc bath is adjusted such that a surface scale of aluminum oxide is formed during melt hardening. Wherein after hardening such a pellicle is blasted by blasting the sheet metal part with dry ice particles.

Such a protective layer is usually only present in zinc coatings, whereas aluminum coatings usually do not require any cleaning or require less laborious cleaning.

WO2018/126471A1 discloses a sol-gel pretreatment of layers for reducing oxide layer formation and improving solderability. The aim of this is to produce an oxidation-protective coating for press-hardened steel, which is based on a silane-and titanium-containing binder and oxidation pigments, which are deposited in a distinct manner in a sol-gel process. In particular, a solvent such as methanol is used, and the solvent cannot be used in a steel production line. After press quenching, the coating should peel off by itself and tests were performed for titanium-based and silicon-based coatings in 2015/2016, but neither thick nor thin wet films were successful. The coating layer can not fall off automatically, and the weldability is not suitable for industrial application.

EP2536857B1 discloses a ceramic-based coating with a thickness of less than or equal to 25 μm, which coating should consist essentially of SiO₂、Al₂O₂And MgO, and if necessary, metal fibers made of tin. In this case, it has been found that such a coating results in the panels no longer being weldable and also paint delamination occurs.

Disclosure of Invention

The object of the invention is to create a method for the electrolytic galvanization of steel strips. Wherein the high temperature zinc alloy corrosion resistant protective layer is treated in such a way that after the high temperature treatment a sand blast cleaning can be omitted.

This object is achieved by a method having the features of claim 1.

Advantageous modifications are disclosed in the dependent claims.

It is another object of the invention to create an electrolytically galvanized steel strip which is constructed in such a way that cleaning of the oxide skin after the high-temperature treatment can be dispensed with.

This object is achieved by a galvanized metal strip having the features of claim 14.

Advantageous modifications are disclosed in the dependent claims.

The invention is based on the recognition that in some cases cleaning of the surface of a metal strip which has been formed with a zinc alloy coating which is resistant to high temperatures by means of electrolytic treatment and has been subjected to a temperature increase in order to produce structural changes can be dispensed with. In particular, mechanical cleaning of the galvanized steel sheet and the hardened parts made thereof can be dispensed with.

Post-cleaning treatment is indeed a controlled and mature process, but it does generate a lot of work. Furthermore, there is a risk of additional surface defects, which may result in higher overall costs. It has been shown that in some cases, the dimensional accuracy is reduced for very thin components.

If there is a correlated process sequence that requires scheduling of these cleaning steps throughout the production process, then cycle times may need to be adjusted.

According to the invention, it has been demonstrated that phosphatability, paintability and weldability can be successfully adjusted by treating the galvanized surface before the heating step for modifying the structure.

According to the invention, the oxide growth during hardening can be embodied without subsequent mechanical surface treatment, such as centrifugal blasting, slide grinding, or dry ice blasting.

According to the invention, it has surprisingly been demonstrated that the electrolytic deposition or co-deposition of tin on a zinc alloy coating, or the application of in particular a stannous salt solution such as a stannate salt solution after electrolytic galvanization, clearly modifies the surface without any type of cleaning being carried out.

Particularly surprisingly, metallic tin or salt solutions such as stannates have proven particularly effective in this respect.

For the electrolytic deposition of tin, the tin is preferably deposited or co-deposited in a final electrolytic step to deposit it on the surface of the zinc layer or to deposit it together with the zinc in the vicinity of the surface.

If the coating is applied after the electrolytic deposition of the zinc layer or zinc-aluminum layer, it is advantageous to carry out the coating in a PVD or CVD process with metallic tin or by means of a salt solution, in particular by means of a roll coating process.

Surprisingly, the presence of tin according to the invention does not have a negative effect on the phosphating. It is generally accepted by experts that tin generally has a negative effect on phosphatability, i.e. phosphate crystals are formed during the immersion phosphating.

Stannates have proven to be suitable salts. The term "stannate" includes both divalent stannates and tetravalent stannates.

The tetravalent stannates include in particular:

ammonium chlorotanninate hydrate H₈N₂Cl₆Sn

Barium stannate BaSnO₃

Bismuth stannate BiSn₂O₇

Lead stannate dihydrate PbSnO₃*2H₂O

Dihydrate lead stannate cadmium stannate CdSn₂O₄

Calcium stannate CaSnO₃

Cobalt stannate dihydrate CoSnO₃*2H₂O

Potassium stannate trihydrate

Copper stannate CuSnO₃

Lithium hexafluorostannate Li₂[SnF₆]

Sodium stannate Na₂SnO₃(acid anhydride)

Trihydrate and hexahydrate

Strontium stannate SrSnO₃

Zinc hexahydroxystannate Zn [ Sn (OH)₆]

Zinc stannate ZnSnO₃。

The divalent stannates may include, for example:

sodium stannate Na₂SnO₂

Calcium stannate CaSnO₂。

According to the invention, after electrodeposition or temper rolling, the salt solution in the form of an aqueous alkaline solution is applied to the galvanized surface by means of a roll coater before the cold-forming or annealing and hardening treatment. In this case, the layer thickness is very thin, 1 to 5 μm thick in aqueous form and 50 to 150nm thick when dry. When stannates are used, the tin coating is 30 to 90mg of tin per square meter in the form of K₂[SnO₃]。

According to the invention, it has been demonstrated that the surface resistance is very low for metal sheets to be hardened at conventional annealing times and that only a very low tendency to paint penetration is observed even when paint penetration tests are carried out. The optically detectable oxides are much less, which can be revealed by annealing the metallic luster on the plate. Generally, this silver color presents a problem because it indicates a lack of complete reaction. Tests have shown that the zinc-iron crystals of the zinc layer have reacted completely. It was also observed that good phosphate crystals were formed during the phosphating process. This is not to be expected in this form, since tin phosphating, according to the general opinion of the expert, has a negative effect.

For reasons that are not entirely clear, although silver color generally results in a reduction in emissivity, there is even a tendency to achieve slightly higher heating rates than if the zinc surface were not tin or stannate treated. It is not yet fully explained what this may be.

In general, the working principle of tin solutions cannot be explained in detail at present, but the effect is surprising and absolutely clear.

The invention therefore relates to a method for treating the surface of a steel strip which is coated with an electrolytically deposited corrosion protection layer of a zinc alloy and which is subjected to a temperature increase in order to modify the structure of the steel. Wherein tin is deposited on the steel strip or together with zinc during the electrolytic coating in such a way that tin is present at or near the surface or that tin is applied in metallic form or ionic form after the electrolytic coating.

In one embodiment, the tin is applied in ionic or metallic form after electroplating with the zinc alloy. Wherein in the ionic form the tin is applied from a salt solution and in the metallic form the tin is applied using a CVD or PVD process.

In one embodiment, the tin is applied from an alkaline or acidic solution.

In one embodiment, an aqueous stannate solution is used, which is adjusted to be basic or acidic.

In one embodiment, the tin in the solution is complexed with citric acid.

In one embodiment, the aqueous solution is applied in a layer thickness of 1 to 5 μm, in particular 1 to 3 μm, wherein the layer thickness on drying is 50 to 150nm, in particular 75 to 125nm, in particular 80 to 100nm.

In one embodiment, the tin coating is 30 to 90mg tin per square meter, in particular 40 to 80mg tin per square meter, in particular 50 to 60mg tin per square meter.

In one embodiment, a solution of 150-250 g/l K is used₂SnO₃*3H₂A solution of O.

In one embodiment, K is used in an amount of 150 to 250g/l₂SnO₃*3H₂O and 15 to 25g/l KOH.

In one embodiment, a solution having a pH of 12.5 to 13.5 is used.

In one embodiment, a solution having a pH of 4 to 5.5 is used and in this solution, tin is complexed with citric acid.

In one embodiment, the citric acid is 35-40g/l for complexing tin, wherein the pH value is 4-5.5.

In one embodiment, the solution has a concentration of 200g/l K₂SnO₃*3H₂O and 20g/l KOH.

In another aspect, the invention relates to an electrolytically galvanized steel strip coated with from 40 to 80mg of tin per square meter.

In one embodiment, the tin is deposited in metallic or ionic form.

In an embodiment, the tin is deposited from a salt solution, in particular a stannate solution, or by a PVD or CVD process.

In another aspect, the invention relates to the use of a steel strip as described above produced by a process as described above, in which a steel sheet is heated to austenitize it and then formed and press quenched, or formed, austenitized and press quenched.

Drawings

Embodiments of the invention are further described below with reference to the accompanying drawings, in which:

FIG. 1 is a flow chart of a conventional form of hardening process;

FIG. 2 is a conventional press quenching process;

FIG. 3 is a graph of an annealed untreated steel sheet and an annealed coated steel sheet according to the invention.

FIG. 4 is an electron microscope image of the annealed surface of the present invention;

FIG. 5 is an element distribution for four different measurement points;

fig. 6 shows the surfaces of the galvanized steel sheets at annealing times of 45 seconds and 200 seconds.

FIG. 7 shows the surface of the steel sheet in which the annealing time was 45 seconds and 200 seconds according to the present invention.

FIG. 8 is the electrical resistance of the sheet surface in the untreated and treated surfaces;

figure 9 is the paint penetration after six weeks for a surface not treated according to the invention and a surface treated according to the invention according to the VDA test.

Detailed Description

According to the invention, the surface of a galvanized metal sheet, in particular a steel sheet, is treated with tin or stannate, said galvanized metal sheet being formed and hardened in one step in a press hardening process or first being cold-formed in several steps of a press hardening process, reheated to an assembly blank and transferred to a forming tool where it is hardened.

For electrolytically galvanized cold rolled wide steel strip, tin is deposited in a final electrolytic step on the already formed zinc layer or co-deposited together with the zinc, so that tin is present at or near the surface. The tin content should be 30-90mg/m²。

In an advantageous embodiment, after the galvanization and any cleaning process, the electrolytically galvanized steel strip is subjected to a CVD or PVD treatment, wherein the above-mentioned amounts of tin are deposited on the surface of the galvanization layer.

In another advantageous embodiment, the tin may be applied by a salt solution after electroplating and any required cleaning steps, in particular in a roll coating process. In particular, a stannate solution is used.

Stannates that can be used have been listed above; potassium stannate solutions are particularly suitable. One of the basic methods is to apply stannate or tin to a surface in an ionic form.

In this connection, either an alkaline solution or an acidic solution, particularly a solution in which tin is complexed, may be used.

In particular, the aim is to produce an aqueous layer having a thickness of from 1 to 5 μm, a dry layer having a thickness of from 50 to 150nm, and in K₂[SnO₃]A tin coating in the form of 30 to 90mg of tin per square meter.

Fig. 1 and 2 are conventional methods in which a zinc layer of a galvanized steel sheet contains an element having an affinity for oxygen, such as aluminum, is austenitized before forming or austenitized after forming, and is quench-hardened in a press, respectively. After hardening, the surfaces of the two sheets have a glassy, in particular alumina-based, hard layer, which is preferably cleaned.

According to the invention, it has been found that treating the surface with a very small amount of tin obviously has such a strong effect on the formation of a glassy or hard layer that it either does not occur in this form or is treated in this form to such an extent that no cleaning is required.

The surface of conventionally produced hardened steel plate bars has a green-beige appearance, which is caused by oxides.

The sheet exhibited a silvery surface upon treatment with the stannate solution (see fig. 3).

Whereas for the conventional process, a silvery surface indicates that the zinc layer is not fully reacted with the underlying steel, which is not the case with the present invention. Measurements showed that the zinc layer was fully reacted in the same manner. However, a small amount of oxide is formed on the surface, where the surface resistance, which is a measure of spot weldability and paint penetration, is very low.

Fig. 2 is an electron microscope image of a surface treated or rendered according to the present invention. Wherein an alkaline solution of potassium stannate and potassium hydroxide is applied with a roll coater prior to heat treatment.

At different measurement points, elemental measurements were performed (see fig. 5), indicating the presence of a tin coating.

The concentration of the solution used for the treatment by roll coating was chosen so as to deposit 50 to 60mg of tin per square meter on a1 μm wet film. During the annealing, the layer applied thereon produces an oxide layer modification, so that no mechanical cleaning by means of a spinning wheel or other mechanical methods is necessary.

The solution resulting in the treatment according to the invention is K in a concentration of 180 to 220g/l₂SnO₃*3H₂And (4) O solution.

To increase the alkali capacity, 15 to 25g/l KOH may be added to the solution to a pH of about 13, e.g., 12.5 to 13.5.

Since in practice acidic solutions are usually easy to use and since stannate solutions tend to form precipitates during acidification, tin can be complexed properly to such an extent that a clear precipitate-free solution is obtained by adding citric acid in a concentration of 30-50 g/l, giving a pH of about 4.8.

Figure 6 again shows the surfaces of two conventional sheets not treated according to the invention after annealing at 870 ℃ for 45 seconds and 200 seconds. Both sheets exhibited the aforementioned beige color.

Fig. 7 shows the surfaces of two conventional sheets treated according to the present invention after annealing at 870 ℃ for 45 seconds and 200 seconds. The difference in surface color is clearly visible.

Fig. 8 is the corresponding resistance results, which show that very low surface resistance is achieved by the surface treatment according to the invention, which results in very good solderability.

Also in terms of corrosion, the surface treatment according to the invention achieves an advantage in terms of paint penetration, as shown by the results in fig. 13, since the paint penetration effect is so good that it penetrates only a little bit without mechanical cleaning, and not more than the other sheets.

The treatment according to the invention has been proposed in particular with stannates. Titanates, oxalates and zirconates also chemically react in substantially the same manner. Thus, it can be assumed that they are effective in the same way, in particular the corresponding tin compounds.

Tin is particularly effective, which is why surface treatments if tin is in metallic form are also successful. However, the advantage of depositing tin on the surface by means of stannate (i.e. in ionic form) is that the application can be carried out in a relatively simple manner using roll coating.

Naturally, all other methods by which the liquid ionic solution can be applied to the surface are also suitable.

The invention can be carried out in-line before the strip is cut into individual sheet blanks. The sheet metal blank cut from the strip can also be coated in a corresponding manner.

The sheet metal blank is then formed into a component blank by, inter alia, a multi-step process. It is also conceivable to first coat the component blank with a tin compound or tin. However, it has been found that tin or tin salt coatings are also well resistant to the forming process.

The component blank obtained in this way is then heated to a temperature at which a structural change in austenite results. The austenitized component blank is then conveyed into a form hardening tool where the component blank is hardened in a single stroke by contact with upper and lower tools that substantially have or correspond to the component blank. As the material of the component blank rests against the (in particular cooled) tool, heat is rapidly removed from the steel material, so that martensitic hardening takes place.

After being cut into sheet blanks, they may also be fed to a press quenching process in which the sheet blanks are austenitized and then formed and quench hardened by a single pressure stroke.

The advantage of the invention is that by means of the invention the surface of the sheet blank for form hardening or press hardening is successfully treated, so that mechanical final cleaning for removing the oxidized surface layer can be dispensed with, so that such a steel sheet can be processed in the same way as hot-dip aluminized sheets, for example, but with the advantage that a very high cathodic corrosion protection effect can be achieved compared to hot-dip aluminized sheets.

Claims

1. A method of surface treatment of a steel strip coated with an electrolytically deposited corrosion protection layer of a zinc alloy, the steel strip being subjected to elevated temperatures to alter the structure of the steel strip, characterized in that during the electrolytic deposition of the steel strip tin is deposited or tin is deposited together with zinc in such a way that after the electrolytic deposition tin is formed at or near the surface or tin is applied in metallic form or in ionogenic form.

2. Method according to claim 1, characterized in that after the electroplating with the zinc alloy, the tin is applied in ionic form, in which the tin is applied from a salt solution, or in metallic form, in which the tin is applied using a CVD or PVD process.

3. A method according to claim 1 or 2, characterized in that the tin is applied from an alkaline solution or an acidic solution.

4. A method according to any of the preceding claims, characterized in that an aqueous stannate solution is applied, which is adjusted to be alkaline or acidic.

5. The method of any of the preceding claims, wherein the tin in the solution is complexed with citric acid.

6. The method according to any one of the preceding claims, characterized in that the aqueous solution is applied in a layer thickness of 1 to 5 μm, in particular 1 to 3 μm, wherein, when dry, the layer thickness is 50 to 150nm, in particular 75 to 125nm, in particular 80 to 100nm.

7. The method according to any of the preceding claims, the tin coating being 30-90mg tin/m²In particular 40 to 80mg tin/m²In particular 50 to 60mg tin/m²。

8. The method according to any of the preceding claims, characterized in that a solution containing K in a concentration of 150 to 250g/l is used₂SnO₃*3H₂A solution of O.

9. The method according to any of the preceding claims, characterized in that a solution containing K in a concentration of 150 to 250g/l is used₂SnO₃*3H₂O and KOH with a concentration of 15 to 25 g/l.

10. The method according to any of the preceding claims, characterized in that a solution with a pH value of 12.5-13.5 is used.

11. The method according to any of the preceding claims, characterized in that a solution with a pH value of 4-5.5 is used, in which solution tin is complexed with citric acid.

12. The method according to any of the preceding claims, characterized in that the citric acid content is 35-40g/l for complexing tin, wherein the pH is 4-5.5.

13. The method according to any of the preceding claims, characterized in that the solution has a concentration of 200g/l of K₂SnO₃*3H₂O and 20g/l KOH.

14. Coated with 40-80 mg tin/m²The electrolytically galvanized steel strip of (1).

15. The electrolytically galvanized steel strip according to claim 14, characterized in that the tin is deposited in metallic or ionic form.

16. The electrolytically galvanized steel strip according to claim 14 or 15, characterized in that the tin is deposited from a salt solution, in particular a stannate solution, or by a PVD or CVD process.

17. Use of a steel strip according to claim 15 or 16, which steel strip has been produced by a method according to any one of claims 1-12, in which method the steel sheet is heated to achieve austenitization, forming and quench hardening, or the steel sheet is formed, austenitized and quench hardened.