EP3872231A1 - Method for conditioning the surface of a metal strip coated with a zinc alloy corrosion protection layer - Google Patents

Abstract

The invention relates to a method for conditioning the surface of a metal strip coated with a zinc alloy corrosion protection layer, which is subjected to a temperature increase to change the structure of the metal, characterized in that after galvanizing the metal strip and before the temperature increase for the purpose of changing the structure of the metal strip, tin is applied to the surface is applied.

Description

The invention relates to a method for conditioning the surface of a metal strip coated with a zinc alloy anti-corrosion layer according to the preamble of claim 1.

It has long been known to provide metallic sheets, in particular metallic strips, which could corrode under normal application conditions, with protective layers.

In general, anti-corrosion layers on metal strips can be organic coatings, for example paints, although these paints can also contain anti-corrosive agents.

In addition, it is known to protect metal strips by means of metal coatings. Such metal coatings can consist of an electrochemically more noble metal or of an electrochemically less noble metal.

In the case of a coating made of an electrochemically more noble metal or a metal that passivates itself, such as aluminum, one speaks of a barrier protective layer, whereby, for example, when aluminum is applied to steel, the steel material suffers corrosion if this barrier protective layer is no longer present in places is, for example due to mechanical damage. A common barrier protective layer of steel is the aforementioned aluminum layer, which is usually applied by hot-dip coating.

If an electrochemically less noble metal is applied as a protective layer, one speaks of a cathodic corrosion coating, because if the corrosion protection coating is mechanically damaged down to the steel material, the electrochemically less noble metal is first corroded before the steel material itself is exposed to corrosion.

The most commonly used cathodic protective coating on steel is a zinc coating or an alloy based on zinc.

Various galvanizing processes are known. A common galvanizing process is so-called hot-dip galvanizing (also known as hot-dip galvanizing). Steel is immersed continuously (e.g. strip or wire) or piece by piece (e.g. components or circuit boards) at temperatures of around 450 ° C to 600 ° C in a melt of liquid zinc (the melting point of zinc is 419.5 ° C). The zinc melt conventionally has a zinc content of at least 98.0% by weight in accordance with DIN EN ISO 1461. A resistant alloy layer of iron and zinc is formed on the steel surface and above it is a firmly adhering pure zinc layer, the composition of which corresponds to the zinc melt. In the case of a continuously galvanized strip, the zinc layer has a thickness of 5 µm to 40 µm. In the case of a piece-wise galvanized component, the zinc layer can have a thickness of 50 µm to 150 µm.

In the case of electrolytic galvanizing (galvanic galvanizing), steel strips or steel plates are not immersed in a zinc melt, but in a zinc electrolyte. The steel to be galvanized is introduced into the solution as a cathode and an electrode made from the purest possible zinc or an electrolyte with a high amount of dissolved zinc is used as the anode. Electricity is passed through the electrolyte solution. The zinc present in ionic form (oxidation level + II) is reduced to metallic zinc and deposited on the steel surface. Compared to hot-dip galvanizing, electrolytic galvanizing can apply thinner layers of zinc. The zinc layer thickness is proportional to the strength and duration of the current flow, whereby - depending on the workpiece and anode geometry - a layer thickness distribution is created over the entire workpiece.

Careful surface pretreatment is required to ensure adhesion to the zinc layer. This can be, for example, degreasing, alkaline cleaning, rinsing and / or pickling. After galvanizing, one or more subsequent treatments can be carried out, such as phosphating, oiling, application of organic coatings (e.g. cataphoretic dip painting - KTL).

In this case, not only pure metal layers are usually deposited. There are also a large number of known alloys that are deposited; in addition to pure aluminum coatings, there are also coatings that contain aluminum and zinc and coatings that contain small amounts of aluminum in addition to a predominant zinc content, which can also contain other elements, such as Example zinc, nickel, chromium and magnesium and other elements, as well as mixtures thereof. If zinc corrosion protection layers or galvanized steel strip are mentioned in the course of the registration, alloys based on zinc are also included.

In addition, it has long been known to design at least parts of the vehicle bodies to be high-strength, in particular to reduce the weight of vehicle bodies, in order to ensure sufficient strength in the event of a crash. The weight saving arises from the fact that high-strength steel grades with comparably low wall thicknesses can be used and are therefore low in weight.

Even when using high-strength steel types, there are different approaches and the most varied types of steel which can be used.

Steel grades are particularly often used which, through quench hardening, which means that the cooling takes place above the critical cooling rate, become high-strength. A common type of steel that can be hardened by quench hardening are the so-called boron-manganese steels, such as the most commonly used 22MnB5, but also variants of this steel, such as 20MnB8, 30MnB8. Non-hardenable steels such as micro-alloyed steel can also be hot-formed in a direct or indirect process.

Such steel grades can be easily deformed and cut to size in the unhardened state.

In order to bring such steel grades into the desired shape and harden them, especially in bodywork, there are essentially two different methods.

The first and somewhat older method is the so-called press hardening. During press hardening, a flat plate is cut out of a steel strip made of a quench-hardenable steel alloy, for example a 22MnB5 or a similar manganese-boron steel. This flat blank is then heated to such an extent that the steel structure has the appearance of gamma iron or austenite. In order to achieve this structure, the so-called austenitizing temperature Ac ₃ must be exceeded, at least if complete austenitizing is desired.

Depending on the steel, this temperature can be between 820 ° C. and 900 ° C., such steel blanks, for example, being heated to about 900 ° C. to 930 ° C. and held at this temperature until the structure changes completely.
Then such a steel blank is transferred in the hot state in a press, in which the hot steel blank is brought into the desired shape with a single press stroke by means of an upper tool and a lower tool, each of which is correspondingly shaped. As a result of the contact of the hot steel material with the comparatively cool, in particular cooled, press tools, that is to say forming tools, energy is withdrawn from the steel very quickly. In particular, the heat must be extracted so quickly that the so-called critical hardening speed is exceeded, which is usually between 20 ° and 25 ° Kelvin per second. Depending on the alloy composition, this can also be as low as 10 ° Kelvin per second, which means that this cooling is also caused by ex. Air can be reached.

If the cooling is carried out so quickly, the structure of the austenite does not change back into a ferritic initial structure, but a martensitic structure is achieved. Due to the fact that austenite can dissolve considerably more carbon in its lattice than martensite, the carbon precipitates cause lattice distortion, which leads to the high hardness of the end product. As a result of the rapid cooling, the martensitic state is stabilized, so to speak. As a result, hardnesses or tensile strengths R _m of more than 1500 MPa can be achieved. Hardness profiles can also be set by means of suitable measures, which will not be discussed in more detail, such as complete or partial rewarming.

Another, somewhat more recent way of producing hardened steel components, especially for bodywork, is press hardening developed by the applicant. In press hardening, a flat steel plate is cut out of a steel strip and this flat steel plate is then formed in the cold state. In particular, this reshaping does not take place with a single press stroke, but rather, as is customary in conventional press lines, for example in a five-stage process. This process allows considerably more complex shapes, so that in the end a complex-shaped component, such as the B-pillar or a longitudinal member of a motor vehicle, can be produced.

In order to subsequently harden such a fully formed component, this component is also austenitized in a furnace and transferred in the austenitized state to a molding tool, the molding tool having the contour of the final component. The preformed component is preferably shaped before heating in such a way that after heating and thus even a thermal expansion of this component already largely corresponds to the final dimensions of the hardened component. This austenitized blank is inserted into the mold in the austenitized state and the mold is closed. The component is preferably touched on all sides by the molding tool and held in a clamping manner, and the contact with the molding tool also removes the heat in such a way that a martensitic structure is generated.
In the clamped state, shrinkage cannot take place, so that the hardened end component with the corresponding final dimensions can be removed from the mold after hardening and cooling.

Since motor vehicle bodies usually have an anti-corrosion coating, the anti-corrosion layer closest to the metal material forming the body, in particular steel, being a metallic coating, anti-corrosion coatings for hardened components have also been sought and developed in the past.

Corrosion protection coatings for components to be hardened are, however, exposed to different requirements than corrosion protection coatings for components that are not hardened. The high temperatures that arise during hardening must be withstood by the anti-corrosion coatings. Since it has been known for a long time that hot-dip aluminized coatings can withstand high temperatures, press hardening steels were first developed, which have a protective layer made of aluminum. Such coatings are able to withstand not only the high temperatures, but also the deformation in the hot state. The disadvantage, however, is that hot-dip galvanizing is not usually used in motor vehicles, but hot-dip galvanizing, and it is fundamentally problematic to use different corrosion protection systems, especially when there is a risk of contact corrosion.

Processes have therefore been developed by the applicant which make it possible to provide zinc coatings which also withstand such high temperatures.

Basically, zinc coatings are considerably less complicated than aluminum coatings when it comes to forming, since aluminum coatings tend to peel off or crack at conventional forming temperatures. This does not happen with zinc.

However, it was initially expected that zinc coatings would not be able to withstand the high temperatures. Special zinc coatings, which, however, have a certain proportion of elements with an affinity for oxygen, are able to be processed even at high temperatures because the oxygen-affine elements quickly diffuse to the air-side surface and oxidize there and form an oxide layer on the zinc coating. In the meantime, zinc coatings of this kind have become established, particularly for hot-stamping. Such zinc coatings can also be used with great success in press hardening.

In order to ensure optimal paint adhesion and optimal weldability, it is known to clean the reshaped and hardened components in such a way that the protective oxide layer is leveled or removed.

From the DE 10 2010 037 077 B4 a method for conditioning the surface of hardened corrosion-protected components made of sheet steel is known, the sheet steel being a sheet steel coated with a metallic coating and being heated and quenched for hardening. After hardening, the oxides present on the anti-corrosion coating due to the heating are removed, the component being subjected to vibratory grinding to condition the surface of the metallic coating, i.e. the anti-corrosion layer, the anti-corrosion coating being a zinc-based coating and the surface conditioning so it is carried out that oxides lying or adhering to the corrosion protection layer are ground off and, in particular, a microporosity is exposed.

From the DE 10 2007 022 174 B3 a method for producing and removing a temporary protective layer for a cathodic coating is known, a steel sheet made of a hardenable steel alloy being provided with a zinc coating in the hot-dip process, the aluminum content in the zinc bath being adjusted so that a superficial oxide skin of aluminum oxide is formed during the melt hardening forms, this thin skin being blown off after hardening by irradiating the sheet metal component with dry ice particles.

Another alternative for removing or conditioning the oxide layer is what is known as wheel blasting, in which abrasive particles are blasted onto the belt, the oxide layer being blasted off or leveled by the particles. An example of this is the EP 1 630 244 B1 or EP 2 233 508 B1 .

Such protective layers usually only appear in zinc coatings, while aluminum coatings often do not have to be cleaned or only need to be cleaned less laboriously.

From the WO 2018/126471 A1 a sol-gel preconditioning of the layer to reduce oxide layer formation and increase weldability is known. The aim is to create an anti-oxidation coating for press-hardening steel materials on the basis of binders containing silane and titanium and oxidic pigments, which are apparently applied in the sol-gel process. In particular, solvents such as methanol are used here, which cannot be used on steel production plants. The coating should fall off by itself after press hardening, although tests with titanium and silicon-based coatings took place in 2015/16 and were not successful with either a thick or a thin wet film. The coating does not fall off by itself, nor is it suitable for industrial applications.

From the EP 2 536 857 B1 a ceramic-based coating with a thickness of 25 µm is known, which should essentially consist of SiO ₂ , Al ₂ O ₃ and MgO, with metallic fibers made of tin if necessary. However, it is known from the literature that a high concentration of SiO ₂ can lead to poor weldability. The object of the invention is to create a method for conditioning the surface of a metal strip coated with a zinc alloy corrosion protection layer, so that a blast cleaning (conditioning of the component surface using blasting material, vibratory grinding or the like) after hardening can be dispensed with.

The object is achieved with a method having the features of claim 1.

Advantageous further developments are characterized in the dependent claims.

Another object is to create a galvanized metal strip which is designed in such a way that the oxide layer does not need to be cleaned off.

The object is achieved with an alloy-galvanized metal strip with the features of claim 16.

Advantageous further developments are characterized in the dependent claims.

According to the invention, it was recognized that under certain circumstances it is possible to dispense with cleaning the surface of a metal strip which is galvanized and has been subjected to a temperature increase for the purpose of changing the structure. especially the mechanical cleaning of an alloy-galvanized steel sheet and a hardened component produced from it can be dispensed with.

A cleaning post-treatment is a manageable and well-established process, but a higher workload is generated. In addition, there is a risk of additional surface defects, which also entails higher costs overall. In the case of very thin components, it has been found that, under certain circumstances, the dimensional accuracy of the components can be restricted.

If there are interconnected process sequences which provide that these cleaning steps are arranged inline within an entire production process, the cycle time may have to be adapted.

According to the invention, it has been found that to adjust the phosphatability, paintability and weldability, a surface treatment of the galvanized surface before the hot forming process or other methods for changing the structure of steel strip is successful. According to the invention, the oxide growth during the hardening process can be designed in such a way that subsequent mechanical surface conditioning, such as, for example, centrifugal blasting, vibratory grinding or dry ice blasting, is unnecessary.

According to the invention, it has been found that, surprisingly, certain aqueous salt solutions, such as salt solutions, the preferred stannates, but also oxalates, zirconates and titanates, apparently modify such that any type of cleaning is unnecessary.

In particular and surprisingly it has been shown that stannates or tin develop a particular effectiveness here.

This is all the more surprising because sheet metal coated with zinc alloy can usually be insufficiently phosphatizable in the annealed state.

The term stannate comprises the salts of tin acids (II) and - (IV).

• Ammoniumhexachlorostannat H₈N₂Cl₆Sn,
• Bariumstannat BaSnO₃
• Wismutstannat BiSn₂O₇
• Bleistannatdihydrat PbSnO₃*2H₂O
• Cadmiumstannat CdSn₂O₄
• Calciumstannat CaSnO₃
• Cobalt(II)-stannatdihydrat CoSnO₃*2H₂O
• Kaliumstannattrihydrat K₂SnO₃*3H₂O
• Kupfer(II)-stannat CuSnO₃
• Lithiumhexafluorstannat Li₂[SnF₆]
• Natriumstannat Na₂SnO₃ (Anhydrid)
• Trihydrat und Hexahydroxid
• Strontiumstannat SrSnO₃
• Zinkhexahydroxostannat Zn[Sn(OH)₆]
• Zinkstannat ZnSnO₃.

Stannates (IV) are in particular:

• Ammonium hexachlorostannate H ₈ N ₂ Cl ₆ Sn,
• Barium stannate BaSnO ₃
• Bismuth stannate BiSn ₂ O ₇
• Lead stannate dihydrate PbSnO ₃ * 2H ₂ O
• Cadmium stannate CdSn ₂ O ₄
• Calcium stannate CaSnO ₃
• Cobalt (II) stannate dihydrate CoSnO ₃ * 2H ₂ O
• Potassium stannate trihydrate K ₂ SnO ₃ * 3H ₂ O
• Copper (II) stannate CuSnO ₃
• Lithium hexafluorostannate Li ₂ [SnF ₆ ]
• Sodium stannate Na ₂ SnO ₃ (anhydride)
• Trihydrate and hexahydroxide
• Strontium stannate SrSnO ₃
• Zinc hexahydroxostannate Zn [Sn (OH) ₆ ]
• Zinc stannate ZnSnO ₃ .

• Natriumstannat Na₂SnO₂
• Calciumstannat(II) CaSnO₂.

Stannates (II) are for example:

• Sodium stannate Na ₂ SnO ₂
• Calcium stannate (II) CaSnO ₂ .

According to the invention, in particular, an aqueous alkaline solution is bsp. applied by means of a roll coater or by a spray-squeeze treatment or another treatment to a galvanized surface after the optional skin-pass, preferably carried out in hot-dip galvanizing, and before the cold forming or the annealing and hardening process or microstructure change process. In this case, very thin layers are used, which are 1-5 µm in water and 50-250 nm thick when dried. The tin coverage when using stannates is 30-90 mg tin per m ² in the form of K ₂ [SnO ₃ ].

According to the invention, it has been found that with a normal annealing time of sheets that are to be subjected to hardening, the surface resistance is very low and even in a cyclic corrosion test according to VDA 233-102 climate change test, only a very low tendency to infiltration of paint could be determined. Significantly fewer oxides can be seen visually, which is revealed by the silvery color of the annealed sheet. Such silveryness usually poses a problem, since it _{indicates a lack of thorough reaction or a stable Al 2} O ₃ layer. Investigations have shown that the zinc-iron crystals of the zinc layer have reacted through. In addition, a good formation of the phosphate crystals during the phosphating could be determined. This was not to be expected in this form.

Although this cannot be explained, despite the silvery appearance, which usually produces a reduction in emissivity, a somewhat higher heating rate tends to be achieved than without tin or stannate treatment of the zinc surface. The reason for this has not yet been fully clarified.

Overall, it cannot yet be said how the tin solution works in detail, but the effect is surprising and absolutely clear.

Figur 1

der Herstellungsweg beim Formhärteprozess bzw. phs-ultraform Prozess nach dem Stand der Technik;

Figur 2

den Herstellungsweg beim Warmumformprozess bzw. Presshärten bzw. phs-directform Prozess nach dem Stand der Technik;

Figur 3

den Herstellungsweg einer Variante des mehrstufigen Warmumformprozess bzw. mehrstufigen Presshärten bzw. phs-multiform Prozess nach dem Stand der Technik

Figur 4

ein Anlagenschema einer Feuerverzinkungsanlage nach dem Stand der Technik

Figur 5

ein Anlagenschema einer elektrolytischen Verzinkungsanlage nach dem Stand der Technik

Figur 6

eine elektronenmikroskopische Aufnahme der Oberfläche nach dem Glühen ohne Konditionierung (Stand der Technik);

Figur 7

eine elektronenmikroskopische Aufnahme der erfindungsgemäß konditionierten Oberfläche nach dem Glühen;

Figur 8

den Vergleich von zwei Stahlblechen nach der Glühung links ohne Konditionierung und rechts entsprechend erfindungsgemäß konditioniert;

Figur 9

ein Schliff des erfindungsgemäß konditionierten Stahlblechs mit der Elementverteilung an vier verschiedenen Messpunkten;

Figur 10

die Oberfläche eines verzinkten Stahlblechs nach dem Glühen bei einer Glühzeit von 45 Sekunden und 200 Sekunden;

Figur 11

die Oberfläche des Stahlblechs nach dem Glühen mit einer erfindungsgemäßen Oberflächenkonditionierung nach 45 Sekunden und 200 Sekunden;

Figur 12

der elektrische Widerstand der Blechoberfläche bei erfindungsgemäß behandelten Oberflächen;

Figur 13

die Lackunterwanderung bei erfindungsgemäß konditionierten Oberflächen nach sechs Wochen gemäß VDA-Test.

The invention is explained by way of example with the aid of a drawing. It shows:

Figure 1

the manufacturing route in the form hardening process or phs-ultraform process according to the state of the art;

Figure 2

the manufacturing route in the hot forming process or press hardening or phs-directform process according to the state of the art;

Figure 3

the manufacturing route of a variant of the multi-stage hot forming process or multi-stage press hardening or phs-multiform process according to the state of the art

Figure 4

a system diagram of a hot-dip galvanizing line according to the state of the art

Figure 5

a system diagram of an electrolytic galvanizing system according to the state of the art

Figure 6

an electron microscope image of the surface after annealing without conditioning (prior art);

Figure 7

an electron micrograph of the surface conditioned according to the invention after annealing;

Figure 8

the comparison of two steel sheets after annealing on the left without conditioning and on the right conditioned accordingly according to the invention;

Figure 9

a section of the steel sheet conditioned according to the invention with the element distribution at four different measuring points;

Figure 10

the surface of a galvanized steel sheet after annealing at an annealing time of 45 seconds and 200 seconds;

Figure 11

the surface of the steel sheet after annealing with a surface conditioning according to the invention after 45 seconds and 200 seconds;

Figure 12

the electrical resistance of the sheet metal surface in the case of surfaces treated according to the invention;

Figure 13

the infiltration of paint on surfaces conditioned according to the invention after six weeks according to the VDA test.

According to the invention, the surface of a galvanized metal sheet, in particular steel sheet, which is to be reshaped and hardened in a heating process or reshaped and then hardened is conditioned with tin or stannates, the conditioning with stannates being discussed below.

The stannates that can be used have already been listed; a potassium stannate solution is particularly suitable, with the application of stannate or tin in ionic form to the surface being one way in principle.

Both basic and acidic solutions can be used here and, in particular, solutions in which the tin is complexed can be used.

In particular, an aqueous layer thickness of 1-5 μm is aimed for, with a dried layer thickness of 50-250 nm, preferably 50-150 nm and a tin coating of 30-90 mg tin / m ² in the form of K ₂ [SnO ₃ ].

the Figures 4 and 5 show a hot-dip galvanizing plant or an electrolytic galvanizing plant. The stannate can preferably be applied in the area of chemical passivation (in Figure 4 ) or the "Passivation" station (in Figure 5 ) can be made.

In the Figures 1 to 3 one sees conventional processes in which a galvanized steel sheet, in which the zinc layer contains an element with an affinity for oxygen, for example aluminum, is either austenitized before forming or austenitized after forming and is quench hardened in a press. This corresponds to the phs-ultraform process ( Figure 1 ) whereby, after cold forming, the formed part is austenitized via Ac3 and then form hardened. In Figure 2 the phs-directform process is shown, in which the blank is first austenitized and formed while it is warm, followed by trimming. Figure 3 shows a variant for this, the so-called phs-multiform process, in which after austenitization and an optional pre-cooling to 450 ° C to 650 ° C in particular, a multi-stage process with several forming steps or cutting and punching processes are subsumed under the term "hot forming steps, After hardening, the sheets heat-treated in this way have a layer on the surface, in particular of aluminum oxide and zinc oxide, which is preferably cleaned off.

According to the invention, it was found that the conditioning of the surface with very small amounts of tin obviously affects the formation of the oxide layer to such an extent that it does not arise in this form or is conditioned to such an extent that it does not have to be cleaned off.

A conventionally produced hardened steel plate shows a greenish-beige appearance on the surface, which is caused by the increased formation of zinc and manganese oxides. This is in Figure 6 shown.

When conditioned with a stannate solution, the sheet shows a silvery surface ( Figure 7 ) consisting mainly of zinc oxides or tin oxides.

While in conventional processes silvery surfaces indicate a lack of reaction of the zinc layer with the underlying steel, this is not the case with the invention. Measurements have shown that the zinc layer reacted in the same way. However, there are few aluminum oxides formed on the surface, the surface resistance as a measure of the suitability for spot welding and the infiltration of paint being very low.

Figure 8 shows again a comparison of a hardened galvanized steel sheet according to the prior art with one treated according to the invention. Both sheets of quality 22MnB5 with a zinc layer of 140 g / m ² (on both sides) were annealed for 45 seconds at a temperature above Ac3. The appearance of the prior art sheet is much darker.

In Figure 9 a surface formed and conditioned according to the invention can be seen in an electron microscopic sectional view, a basic solution of potassium stannate with potassium hydroxide being applied with a roll coater before the heat treatment. Here the steel grade 340LAD with a zinc coating of 180 g / m ^{2 was} annealed at 870 ° C. for 200 seconds. The layers above measuring point 7 (MP7) are preparation-related CSP redeposits and are therefore of no significance. It can be clearly seen that the lighter layer in the plane of the MP7 represents the Sn / Zn oxide; this is also confirmed by the components of the MP7, which show significantly high values for Sn. The layer is very thin and almost completely over the entire surface of the belt. Underneath there is a darker layer of Al oxide (MP6) which is also present over the entire surface of the strip. Again underneath is the reacted Zn / Fe layer, which partially slightly oxidized areas (but not in MP4 in Figure 9 shown).

Element measurements were carried out at different measuring points, which show the tin coating described above.

The concentration of the solution, which is used for conditioning by means of roll coating, is chosen so that with a wet film of 1 µm 50-60 mg tin / m ^{2 are} deposited. A layer applied in this way causes a modification of the oxide layer that forms during annealing, so that mechanical cleaning by means of a centrifugal wheel or other mechanical processes is no longer necessary.

A solution which effects conditioning according to the invention has a solution concentration of 180-220 g / l K ₂ SnO ₃ * 3H ₂ O.

In order to increase the base capacity, 15-25 g / l KOH can be added to the solution so that a pH value of approx. 13, i.e. 12.5-13.5 is established.

Since acidic solutions are usually used in practical operation and stannate solutions often tend to form precipitates during acidification, the tin can be suitably complexed as an alternative to KOH to such an extent that a clear, precipitate-free solution is obtained by adding citric acid in an amount of 30-50 g / l is added, resulting in a pH of about 4.8.

In Figure 10 the surface of a conventional sheet metal not conditioned according to the invention for another type of steel (22MnB5 with zinc coating Z140 - 140 g / m ² ) after 45 seconds and 200 seconds of annealing at 870 ° C can be seen again. Both sheets show the beige-green color already mentioned.

In Figure 11 in two sheets (again 22MnB5 with zinc coating Z140-140 g / m ² ) that were conditioned according to the invention, the surfaces can be seen after 45 seconds and 200 seconds annealing time at 870 ° C (i.e. above Ac3). The differences in the surface color are clearly visible.

In Figure 12 one can see the corresponding resistance results for different steel grades and annealing durations, each at 870 ° C., which show that the surface conditioning according to the invention achieves a very low surface resistance, which can be expected to be very weldable. The third grade, 20MnB8, was coated with a zinc-iron layer, ie a so-called galvannealed layer of 180 g / m ² .

With regard to corrosion, the surface conditioning according to the invention also provides an advantage in terms of the infiltration of the paint, because as the results in Figure 13 show, the paint infiltration results are so good that obviously a cathodic dip paint applied to the metal sheets without mechanical cleaning is only infiltrated very slightly and not to a greater extent than with other metal sheets. For this purpose, the VDA 233-102 climate change test was carried out and the paint infiltration in mm as well as the respective cross-cut value in the cross section according to DIN EN ISO 16276-2 before and after the aforementioned corrosion test according to VDA 233-102 was determined. The scale ranges from 0 (very good) to 5 (total release). You can see that mostly the value before and after the test 0 was excellent. Sometimes small areas have flaked off which led to values of 1 and sometimes 2.

The conditioning according to the invention was presented in particular on the basis of the stannates; the titanates, oxalates and zirconates can also have an essentially identical reaction. Accordingly, it can be assumed that these are equally effective, in particular the corresponding tin compounds.

However, the tin seems to be particularly effective, which is why the surface conditioning is also successful when the tin is metallic. The deposition of the tin on the surface with the help of the stannates, i.e. in ionic form, has the advantage, however, that the application can be carried out in a comparatively simple manner using a roll-coating or dip-squeeze process.

Of course, all other methods with which liquid ionic solutions can be applied to a surface are also suitable.

The deposition of metallic tin is nevertheless conceivable and possible, for example, using a CVD or PVD process.

The advantage of the invention is that it is possible to condition the surface of a steel sheet provided for form hardening or press hardening in such a way that mechanical final cleaning to remove oxidic surface layers can be dispensed with, so that such sheets can be produced in the same way as, for example, hot-dip aluminized sheets , can be processed, but with the advantage that a high cathodic corrosion protection effect is achieved compared to hot-dip aluminized sheets.

Claims (19)

Process for conditioning the surface of a metal strip coated with a zinc alloy corrosion protection layer, which is subjected to a temperature increase to change the structure of the metal,
characterized in that
after galvanizing the metal strip and before the temperature increase, tin is applied to the surface for the purpose of changing the structure of the metal strip. Method according to claim 1,
characterized in that
the metal strip is a steel strip, in particular boron-manganese steel strip, preferably the quality 22MnB5 or 20MnB8. Method according to claim 1,
characterized in that
the zinc alloy layer is applied by hot-dip galvanizing or ELO galvanizing or by means of a PVD process. Method according to claim 1,
characterized in that
the tin is applied in ionic form or in metallic form, the tin being applied in ionic form from an aqueous salt solution and in metallic form using a CVD or PVD process. Method according to claim 1 or 2,
characterized in that
the tin is applied from an alkaline or acidic solution. Method according to one of the preceding claims,
characterized in that
an aqueous stannate solution is applied, which is adjusted to be basic or acidic. Method according to one of the preceding claims,
characterized in that
the tin is complexed in the solution. Method according to one of the preceding claims,
characterized in that
the aqueous solution is applied with a layer thickness of 1-5 μm, in particular 1-3 μm, the dried layer thickness being 50-250 nm, preferably 50-150 nm, in particular 75-125 nm, in particular 80-100 nm. Method according to one of the preceding claims,
characterized in that
the tin coating is 30-90 mg tin / m ² , in particular 40-80 mg tin / m 2, and in particular ^{50-60 mg tin / m 2} . Method according to one of the preceding claims,
characterized in that
an aqueous solution with a solution concentration of 150-250 g / l K ₂ SnO ₃ * 3H ₂ O is used. Method according to one of the preceding claims,
characterized in that
an aqueous solution with 150-250 g / l K ₂ SnO ₃ * 3H ₂ O and 15-25 g / l KOH is used. Method according to one of the preceding claims,
characterized in that a solution is used which has a pH of 12.5-13.5. Method according to one of the preceding claims,
characterized in that a solution is used which has a pH value of 4 - 5.5 and in which the tin is complexed. Method according to claim 9,
characterized in that
to complex the tin, citric acid is contained in an amount of 35 - 40 g / l, the pH value being 4 - 5.5. Method according to one of the preceding claims,
characterized in that
the solution concentration is 200 g / l K ₂ SnO ₃ * 3H ₂ O with 20 g / l KOH. Galvanized metal strip, in particular galvanized steel strip, coated with 40-80 mg tin / m ² . Metal strip according to claim 14,
characterized in that
the tin is deposited in metallic or ionic form. Metal strip according to claim 14 or 15,
characterized in that
the tin is deposited from a stannate solution or by means of PVD or CVD processes. Use of a metal strip according to one of claims 15 to 16, produced by a method according to one of claims 1 to 12, in a method in which a steel sheet is heated for the purpose of austenitizing and then reshaped or is reshaped first and then for the purpose of austenitizing heated and after austenitizing the sheet or the formed sheet are cooled at a rate above the critical cooling rate.

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