CA3172957A1 - Method of electrolytic galvanization of steel strip with a conditioned zinc layer - Google Patents

Abstract

The invention relates to a method of conditioning the surface of a steel strip coated with an electrolytically deposited zinc alloy anticorrosion layer, which is subjected to an increase in temperature to alter the steel microstructure, characterized in that tin is deposited or is codeposited with zinc during the electrolytic coating of the steel strip, such that tin is present at the surface or close to the surface, or tin is applied in metallic form or ionic form after the electrolytic coating.

Description

METHOD OF ELECTROLYTIC GALVANIZATION OF STEEL STRIP WITH A
CONDITIONED ZINC LAYER
The invention relates to a method for electrolytic galvanization of steel strip in which the zinc corrosion protection layer is conditioned.
It has long been known to provide protection layers for metallic sheets, in particular me-tallic strips, which could corrode under normal conditions of use.
In general, corrosion protection layers on metal strips can be organic coatings such as paints; these paints can easily also contain corrosion-inhibiting agents.
It is also known to protect metal strips by means of metal coatings. Such metal coatings can consist of an electrochemically more noble metal or consist of an electrochemically more base metal.
In the case of a coating composed of an electrochemically more noble metal or a metal that is self-passivating such as aluminum, one speaks of a barrier protection layer; for ex-ample when aluminum is applied to steel, the steel material then suffers from corrosion if this barrier protection layer is no longer present in some places, for example due to me-chanical damage. A common barrier protection layer for steel is the above-mentioned aluminum layer, which is usually applied by means of hot-dip coating.
If an electrochemically more base metal is applied as a protection layer, one speaks of a cathodic anti-corrosion coating because if the corrosion protection coating suffers a me-chanical injury down to the steel material, the electrochemically more base metal is cor-roded first before the steel material itself is subjected to the corrosion.
The most commonly used cathodic protection coating on steel is a zinc coating.
There are various known galvanization methods. A common galvanization method is the so-called hot-dip galvanization (also known as batch galvanization). In this case, steel is dipped continuously (e.g. strip and wire) or by the piece (e.g. components) at Date Recue/Date Received 2022-08-25

2 temperatures of about 450 C to 600 C into a bath of molten zinc (the melting point of zinc is 419.5 C). The zinc bath conventionally contains at least 98.0 wt%
zinc according to DIN EN ISO 1461. On the steel surface, a tough alloy layer of iron and zinc forms that is covered by a firmly adhering pure zinc layer whose composition corresponds to that of the zinc bath. In a continuously galvanized strip, the zinc layer has a thickness of 5 pm to 40 pm. In a component that is galvanized by the piece, the zinc layer can have thick-nesses of 50 pm to 150 pm.
With an electrolytic galvanization (galvanic zinc plating), steel strips or steel plates are immersed not in a zinc bath, but rather in a zinc electrolyte. In this case, the steel that is to be galvanized is introduced into the solution as a cathode and an electrode composed of the purest possible zinc is used as an anode. Electrical current is conducted through the electrolyte solution. In this case, the zinc that is present in ionic form (oxidation stage +II) is reduced to metallic zinc and is deposited onto the steel surface. In comparison to hot-dip galvanization, thinner zinc layers can be deposited with electrolytic galvanization.
The zinc layer thickness in this case is proportional to the intensity and duration of the current flow, wherein ¨ depending on the geometry of the workpiece and anode ¨
a layer thickness distribution across the entire workpiece is produced.
.. Insuring the adhesion and homogeneity of the zinc layer requires a careful pretreatment of the surface. For example, this can be degreasing, alkaline cleaning, pickling, flushing, and/or descaling. After the galvanization, one or more aftertreatments can be performed, for example phosphating, oiling, or application of organic coatings (CIP -cathodic immer-sion painting).
Usually, this involves the depositing of not just pure metal coatings. There are also nu-merous known alloys that are deposited; in addition to pure aluminum coatings there are also coatings that contain aluminum and zinc and coatings that, in addition to the zinc that they predominantly contain, also contain small quantities of aluminum;
other ele-ments can also be contained, for example zinc, nickel, chromium, magnesium, and other .. elements as well as mixtures thereof.
It has also long been known, particularly for purposes of reducing the weight of vehicle bodies, to embody at least parts of vehicle bodies with a high strength in order to ensure a sufficient strength in the event of a crash. The weight savings are achieved by virtue of Date Recue/Date Received 2022-08-25

3 the fact that high-strength steel grades can be used with comparatively thin wall thick-nesses and therefore have a low weight.
Even when using high-strength steel grades, there are different approaches and an ex-tremely wide variety of steel grades that can be used.
It is especially common to use steel grades that are high-strength due to quench harden-ing. Common steel grades that can be hardened by means of quench hardening are the so-called boron-manganese steels, for example 22MnB5 which is the most commonly used, but also derivatives of this steel such as 22MnB8 and 30MnB8.
Steel grades of this kind can be easily shaped and cut to size in the unhardened state.
There are essentially two different procedures, particularly in vehicle body construction, for bringing such steel grades into the desired shape and hardening them.
The first, somewhat older procedure is what is known as press hardening. In press hard-ening, a flat sheet bar is cut out from a sheet steel strip made of a quench-hardenable steel alloy such as a 22MnB5 or a similar manganese-boron steel. This flat sheet bar is then heated to such an extent that the steel structure is in the form of gamma iron or austenite. In order to achieve this structure, it is thus necessary to exceed the so-called austenitization temperature Ac3, at least if a complete austenitization is desired.
Depending on the steel, this temperature can be between 820 C and 900 C; for exam-pie, such steel sheet bars are heated to about 900 C to 930 C and are kept at this tem-perature until the structural change is complete.
Such a steel sheet bar is then transferred in the hot state to a press in which by means of an upper tool and a lower tool that are each correspondingly shaped, the hot steel sheet bar is brought into the desired shape with a single press stroke. Through the contact of the hot steel material with the comparatively cool, in particular cooled, press tools, i.e.
forming tools, energy is removed from the steel very quickly. In particular, the heat must be removed quickly enough that the so-called critical hardening speed is exceeded, which is usually between 20 and 25 Kelvin per second.
Date Recue/Date Received 2022-08-25

4 If cooling is carried out at such a speed, then the structure of the austenite does not change back into a ferritic initial structure; instead, a martensitic structure is achieved.
Due to the fact that austenite can dissolve significantly more carbon in its structure than martensite, carbon precipitation phenomena cause lattice distortion, which results in the high hardness of the end product. The rapid cooling stabilizes the martensitic state, so to speak. This makes it possible to achieve hardnesses and tensile strengths Rm of greater than 1500 MPa. It is also possible to establish hardness profiles by means of suitable measures that need not be discussed in greater detail, for example complete or partial reheating.
An additional, somewhat newer way to produce hardened steel components, particularly for vehicle body construction, is form hardening, which was developed by the applicant.
In form hardening, a fiat steel sheet bar is cut out from a steel strip and this fiat steel sheet bar is then formed in the cold state. In particular, this forming takes place not with a single press stroke, but rather ¨ as is customary in conventional press lines ¨ for exam-ple in a five-step process. This process enables production of significantly more complex shapes so that it is possible in the end to produce a complexly shaped component such as a B-pillar or a longitudinal member of a motor vehicle.
In order to then harden such a fully formed component, this component is likewise aus-tenitized in a furnace and in the austenitized state, is transferred into a forming tool, said forming tool having the contour of the final component. Preferably, the pre-formed com-ponent is shaped before the heating in such a way that after the heating and thus also after a thermal expansion has taken place, this component already corresponds as much as possible to the final dimensions of the hardened component. This austenitized blank is placed into the forming tool in the austenitized state and the forming tool is closed. In this case, the component is preferably touched by the forming tool on all sides and held in a clamped fashion and, by means of the contact with the forming tool, the heat is like-wise removed in such a way that a martensitic structure is produced.
In the clamped state, shrinkage cannot take place so that the hardened final component with the corresponding final dimensions can be removed from the forming tool after the hardening and cooling.
Date Recue/Date Received 2022-08-25 Since motor vehicle bodies customarily have a corrosion protection coating, with the cor-rosion protection layer the closest to the metal material of which the vehicle body is com-posed ¨ in particular steel ¨ being embodied in the form of a metallic coating, past efforts

5 and development have focused on corrosion protection coatings for hardened compo-nents.
Corrosion protection coatings for components that are to be hardened, however, have to satisfy different requirements than corrosion protection coatings of components that are not hardened. The corrosion protection coatings must be able to withstand the high tem-peratures that are produced during hardening. Since it has long been known that hot-dip aluminized coatings can also withstand high temperatures, press-hardening steels with a protection layer of aluminum were developed first. Such coatings are able to withstand not only the high temperatures, but also the forming in the hot state. It is disadvanta-geous, however, that usually in motor vehicles, conventional steel grades are used that undergo not hot-dip aluminizing procedures, but rather hot-dip galvanizing procedures and it is fundamentally problematic to use different corrosion-protection systems, particu-larly when there is a risk of contact corrosion.
For this reason, the applicant has developed methods that make it possible to provide zinc coatings, which likewise resist such high temperatures.
Basically, zinc coatings are much less complicated than aluminum coatings when it comes to forming since aluminum coatings tend to flake off or crack at conventional forming temperatures. This does not happen with zinc.
Initially, though, zinc coatings were not expected to be able to withstand the high tem-peratures. But special zinc coatings that contain a certain amount of elements with an af-finity for oxygen can in fact also be processed at high temperatures because the ele-.. ments with an affinity for oxygen diffuse quickly to the surface on the air side where they oxidize and form a glass-like protective film for the zinc coating. In the time since, such zinc coatings have come into widespread use, particularly for form hardening.
Zinc coat-ings of this kind have also been used with great success in press hardening.
Date Recue/Date Received 2022-08-25

6 In order to ensure optimal paint adhesion and optimal weldability, it is known to clean the finally formed and hardened components in such a way that the glass-hard protective film layer is evened out or abraded.
DE 10 2010 037 077 B4 has disclosed a method for conditioning the surface of hardened corrosion-protected components made of sheet steel in which the sheet steel is a sheet steel with a metallic coating that is heated for the hardening and then quench-hardened.
After the hardening, the oxides that are present on the corrosion protection coating due to the heating are removed, wherein for conditioning the surface of the metallic coating, i.e. the corrosion protection layer, the component undergoes a slide grinding, and wherein the corrosion protection coating is a zinc-based coating and the surface condi-tioning is carried out in such a way that oxides that are present on or adhering to the corrosion-protection layer are ground away and in particular, a micro-porosity is exposed.
DE 10 2007 022 174 B3 has disclosed a method for producing and removing a temporary protection layer for a cathodic coating, wherein a sheet steel composed of a hardenable steel alloy is provided with a zinc coating in the hot-dip immersion process, wherein the aluminum content in the zinc bath is adjusted so that during the melt hardening, a super-ficial oxide skin of aluminum oxide forms, wherein after the hardening, this thin skin is blasted away by blasting the sheet metal component with dry ice particles.
Protective layers of this kind usually occur only with zinc coatings, whereas aluminum coatings often do not require any cleaning or require only a less laborious cleaning.
WO 2018/126471 Al has disclosed a sol-gel preconditioning of the layer for reducing the oxide layer formation and increasing weldability. The intent of this is to produce an oxida-tion protection coating for press-hardened steel materials, based on silane-containing and titanium-containing bonding agents and oxidic pigments, which are clearly deposited in the sol-gel process. In particular, solvents such as methanol are used here, which cannot be used in steel production lines. After the press hardening, the coating is supposed to fall off on its own, but tests with titanium-based and silicon-based coatings were carried out in 2015/16 and were not successful with either a thick or thin wet film.
The coating does not fall off on its own and the weldability is also not suitable for industrial applica-tions.
Date Recue/Date Received 2022-08-25

7 EP 2 536 857 B1 has disclosed a ceramic-based coating with a thickness 25 pm, which should essentially consist of SiO2, A1202, and MgO, with metallic fibers made of tin being included where necessary. In this case, it has been discovered that such a coating results in the fact that the sheet is no longer weldable and paint delamination also occurs.
The object of the invention is to create a method for electrolytic galvanization of steel strip in which the high temperature-resistant zinc alloy corrosion protection layer is condi-tioned in such a way that it is possible to dispense with a blast cleaning after a high-tem-perature process.
The object is attained with a method having the features of claim 1.
Advantageous modifications are disclosed in the dependent claims thereof.
Another object is to create an electrolytically galvanized steel strip, which is constituted in such a way that it is possible to dispense with the cleaning of an oxide skin after a high-temperature process.
The object is attained with a galvanized metal strip having the features of claim 14.
Advantageous modifications are disclosed in the dependent claims thereof.
The invention is based on the realization that under certain circumstances, it is possible to dispense with a cleaning of the surface of a metal strip that is electrolytically coated with a high temperature-resistant zinc alloy coating and has been subjected to a temper-ature increase in order to produce a structural change. In particular, it is possible to dis-pense with the mechanical cleaning of a galvanized sheet steel and of a hardened com-ponent that is produced from it.
A cleaning aftertreatment is indeed a controllable and well-established process, but it does create a larger amount of work. In addition, there is a risk of additional surface de-fects, which can incur higher overall costs. With very thin components, it has turned out Date Recue/Date Received 2022-08-25

8 that under certain circumstances, the dimensional accuracy of the components can be re-duced.
If there are interconnected process sequences, which require these cleaning steps to be arranged inline within an overall production process, then it may be necessary to adjust the cycle time.
According to the invention, it has turned out that the phosphatability, paintability, and weldability can be successfully adjusted by means of a conditioning of the galvanized sur-face before the heating step for changing the structure.
According to the invention, the oxide growth during the hardening process can be em-bodied in such a way that it is unnecessary to perform a subsequent mechanical surface conditioning such as centrifugal blasting, slide grinding, or dry ice blasting.
According to the invention, it has surprisingly turned out that the electrolytic deposition or co-deposition of tin onto the zinc alloy coating or the application of in particular stan-nous salt solutions such as salt solutions of stannates after the electrolytic galvanization clearly modifies the surface in such a way that it is not necessary to perform a cleaning of any kind whatsoever.
In particular and surprisingly, it has turned out that metallic tin or salt solutions such as stannates are especially effective in this regard.
With an electrolytic deposition of tin, the tin is preferably deposited in a last electrolysis step or co-deposited in order to deposit it onto the surface of the zinc layer or to deposit it in the vicinity of the surface together with zinc.
If the coating is applied after the electrolytic deposition of the zinc layer or a zinc-alumi-num layer, then this is advantageously carried out with metallic tin in the PVD or DID
process or by means of a salt solution particularly by means of a roll coating process.
It is surprising that the presence of tin according to the invention does not negatively in-fluence the phosphating. The prevailing wisdom among experts actually assumes that Date Recue/Date Received 2022-08-25

9 normally, tin negatively influences the phosphatability, i.e. the formation of phosphate crystals in the dip phosphating.
Stannates have turned out to be suitable salts. The term "stannates" includes the salts of stannic acids (II) and (IV).
Stannates (IV) particularly include:
ammonium hexachlorostannate H8N2C16Sn barium stannate BaSnO3 bismuth stannate BiSn207 lead stannate dihydrate PbSn03*2H20 cadmium stannate CdSn204 calcium stannate CaSnO3 cobalt(II) stannate dihydrate CoSn03*2H20 potassium stannate trihydrate copper(II) stannate CuSnO3 lithium hexafluorostannate Li2[SnF6]
sodium stannate Na2Sn03(anhydride) trihydrate and hexahydroxide strontium stannate SrSnO3 zinc hexahydroxostannate Zn[Sn(OH)6]
zinc stannate ZnSn03.
Stannates (II) for example include:
sodium stannate Na2SnO2 calcium stannate(II) CaSn02.
According to the invention, a salt solution in the form of an aqueous alkaline solution is applied by means of a roll coater onto a galvanized surface after the electrolytic deposi-tion or skin pass rolling and before the cold forming or annealing and hardening process.
In this case, very thin layer thicknesses are used, which are 1-5 pm in the aqueous form Date Recue/Date Received 2022-08-25 and are 50-150 nm thick when dry. When stannates are used, the tin coating is 30-90 mg of tin per m2 in the form of K2[Sn03].
According to the invention, it has turned out that with a conventional annealing time for 5 sheet metals that are to undergo a hardening, the surface resistance is very low and even with a paint infiltration test, only a very low paint infiltration tendency could be ob-served. Significantly fewer oxides could be optically detected, which is revealed by a me-tallic sheen on the annealed sheet. Usually, such a silvery color poses a problem since it indicates a lack of a complete reaction. Tests showed that the zinc-iron crystals of the

10 zinc layer had completely reacted. A good formation of phosphate crystals in the phos-phating could also be observed. This was not to be expected in this form since according to the prevailing wisdom among experts, tin phosphating exerts a negative influence.
For reasons that are not entirely clear, despite the silvery color, which usually produces a reduction in emissivity, there is even a tendency for somewhat higher heating rates to be achieved than without tin or stannate treatment of the zinc surface. It has not yet been possible to fully explain what the reason for this might be.
By and large, it is not yet possible to say at this time how the tin solution works in detail, but the effect is surprising and absolutely clear.
The invention thus relates to a method for conditioning the surface of a steel strip, which is coated with an electrolytically deposited zinc alloy corrosion protection layer and is sub-jected to a temperature increase in order to change the structure of the steel, wherein during the electrolytic coating of the steel strip, tin is deposited or is deposited together with zinc, in such a way that tin is present at the surface or in the vicinity of the surface or tin in metallic form or ionic form is applied after the electrolytic coating.
In one embodiment, the tin is applied in ionic form or in metallic form after the galvanic coating with a zinc alloy, wherein in ionic form, the tin is applied from a salt solution and in metallic form, the tin is applied using a DID or PVD process.
In one embodiment, the tin is applied from an alkaline or acidic solution.
Date Recue/Date Received 2022-08-25

11 In one embodiment, an aqueous stannate solution is applied, which is adjusted to be al-kaline or acidic.
In one embodiment, the tin in the solution is complexed with citric acid.
In one embodiment, the aqueous solution is applied with a layer thickness of 1 ¨ 5 pm, in particular 1 ¨ 3 pm, wherein the layer thickness when dry is 50 ¨150 nm, particularly 75 ¨ 125 nm, especially 80 ¨ 100 nm.
In one embodiment, the tin coating is 30 ¨ 90 mg tin/m2, particularly 40 ¨ 80 mg tin/m2, and especially 50 ¨ 60 mg tin/m2.
In one embodiment, a solution with a solution concentration of 150 ¨ 250 g/I
K2Sn03*3H20 is used.
In one embodiment, a solution with 150 ¨ 250 g/I K2Sn03*3H20 and 15 ¨ 25 g/I
KOH is used.
In one embodiment, a solution is used, which has a pH value of 12.5 ¨ 13.5.
In one embodiment, a solution is used, which has a pH value of 4 ¨ 5.5, and in the solu-tion, the tin is complexed with citric acid.
In one embodiment, citric acid is contained in a quantity of 35 ¨ 40 g/I for complexing the tin, wherein the pH value is 4 ¨ 5.5.
In one embodiment, the solution concentration is 200 g/I K2Sn03*3H20 with 20 g/I KOH.
In another aspect, the invention relates to an electrolytically galvanized steel strip coated with 40 ¨ 80 mg tin/m2.
In one embodiment, the tin is deposited metallically or in ionic form.
Date Recue/Date Received 2022-08-25

12 In one embodiment, the tin is deposited from a salt solution, in particular a stannate so-lution, or by means of a PVD or DID process.
In another aspect, the invention relates to the use of an above-mentioned steel strip, which is produced with an above-mentioned method, in a method in which a steel sheet is heated to achieve the austenitization and then formed and quench-hardened or is formed, austenitized, and quench-hardened.
The invention will be explained by way of example based on the drawings. In the draw-ings:
Fig. 1 shows the production path in conventional form hardening;
Fig. 2 shows the production path in conventional press hardening;
Fig. 3 shows a sheet steel after the annealing without conditioning and a sheet steel after annealing with an annealing coating according to the invention;
Fig. 4 shows an electron microscope image of the surface that has been condi-tioned according to the invention after the annealing;
Fig. 5 shows the element distribution at four different measuring points;
Fig. 6 shows the surface of a galvanized sheet steel after the annealing with an annealing time of 45 seconds and 200 seconds;
Fig. 7 shows the surface of the sheet steel after the annealing with a surface conditioning according to the invention after 45 seconds and 200 seconds;
Fig. 8 shows the electrical resistance of the sheet surface in untreated and treated surfaces;
Date Recue/Date Received 2022-08-25

13 Fig. 9 shows the paint infiltration in surfaces not conditioned according to the in-vention and surfaces conditioned according to the invention after six weeks according to the VDA test.
According to the invention, the surface of a galvanized sheet metal, in particular sheet steel, which is formed and hardened in one step in a press hardening process or is first cold formed in several steps in a form hardening process and then heated as a compo-nent blank, transferred to a forming tool, and hardened therein, is conditioned with tin or stannates.
With electrolytically galvanized cold-rolled wide strip, tin is deposited in a last electrolysis step onto the already present zinc layer or is co-deposited together with zinc so that tin is present at the surface or in the vicinity of the surface. The tin content should be 30-90 mg/m2.
In an advantageous embodiment, after the galvanization and any cleaning processes, the electrolytically galvanized strip passes through a DID or PVD unit in which the above-mentioned quantity of tin is deposited onto galvanized surface.
In another advantageous embodiment, after the galvanization and any required cleaning steps, the tin is applied as a salt solution, particularly in a roll coating process. In particu-lar, a stannate solution is used.
The stannates that can be used have already been listed above; a potassium stannate so-lution is particularly suitable, wherein basically, one approach is to apply stannate or tin to the surface in ionic form.
In this connection, both alkaline and acidic solutions can be used and in particular, solu-tions in which the tin is complexed can be used.
In particular, the aim is to produce an aqueous layer thickness of 1 ¨ 5 pm, with a dry layer thickness of 50 ¨ 150 nm, and a tin coating of 30 ¨ 90 mg tin/m2 in the form of K2[Sn03].
Date Recue/Date Received 2022-08-25

14 Figs. 1 and 2 show conventional methods in which a galvanized sheet steel whose zinc layer contains an element with an affinity for oxygen, for example aluminum, either is austenitized before the forming or is austenitized after the forming and is respectively quench-hardened in a press. After the hardening, the surfaces of the two sheets have a glass-like, hard layer particularly composed of aluminum oxide, which is preferably cleaned.
According to the invention, it has been discovered that the conditioning of the surface with very small quantities of tin clearly has such a powerful influence on the formation of the glassy or hard layer that it either does not occur in this form or is conditioned to such a degree that it does not have to be cleaned.
A conventionally produced hardened steel sheet bar has a greenish-beige appearance on the surface, which is caused by oxides.
In a conditioning with a stannate solution, the sheet exhibits a silvery surface (Fig. 3).
Whereas with conventional methods, silvery surfaces indicate the lack of a complete re-action of the zinc layers with the underlying steel, this is not the case with the invention.
Measurements have shown that the zinc layer has completely reacted in the same way.
However, small amounts of oxides have formed on the surface, wherein the surface re-sistance as a measure for the spot-weldability and the paint infiltration is very low.
Fig. 2 shows a surface that is embodied and conditioned according to the invention in an electron microscope image, wherein an alkaline solution of potassium stannate with po-tassium hydroxide was applied with a roll coater before the heat treatment.
At different measuring points, element measurements were performed (Fig. 5), which in-dicate the presence of a tin coating.
The concentration of the solution that is used for the conditioning by means of roll coat-ing is selected so that with a wet film of 1 pm, from 50 ¨ 60 mg tin/m2 are deposited.
During the annealing, a layer applied to this produces a modification of the oxide layer Date Recue/Date Received 2022-08-25 that forms so that a mechanical cleaning by means of a centrifugal wheel or other me-chanical methods is no longer necessary.
A solution that produces a conditioning according to the invention has a solution concen-5 tration of 180 ¨ 220 g/I K2Sn03*3H20.
In order to increase the base capacity, the solution can have 15 ¨ 25 g/I KOH
added to it so that a pH value of approx. 13, i.e. 12.5 ¨ 13.5 is produced.
10 Since in practical operation, acidic solutions are usually used readily, and since stannate solutions often tend to form precipitates during acidification, the tin can be suitably com-plexed to such an extent that a clear precipitate-free solution is obtained by adding citric acid in a quantity of 30 ¨ 50 g/I, which results in a pH value of approx. 4.8.

15 Fig. 6 once again shows the surface of a conventional sheet that is not conditioned ac-cording to the invention after 45 seconds and 200 seconds of annealing time at 870 C.
Both sheets exhibit the above-mentioned beige-green color.
Fig. 7 shows the surfaces of two sheets, which were conditioned according to the inven-tion, after 45 seconds and 200 seconds of annealing time at 870 C. The differences in the surface color are clearly visible.
Fig. 8 shows the corresponding resistance results, which demonstrate that with the sur-face conditioning according to the invention, a very low surface resistance is achieved, which gives rise to the expectation of a very good weldability.
Also with regard to corrosion, the surface conditioning according to the invention achieves an advantage when it comes to paint infiltration because, as the results in Fig.
13 demonstrate, the paint infiltration results are so good that a cathodic immersion paint applied to the sheets without mechanical cleaning has infiltrated only slightly and not to a greater degree than in other sheets.
The conditioning according to the invention has been presented particularly in conjunc-tion with stannates. But titanates, oxalates, and zirconates also chemically react in Date Recue/Date Received 2022-08-25

16 essentially the same way. One can therefore assume that they are effective in the same way, particularly the corresponding tin compounds.
Tin appears to be particularly effective, which is why the surface conditioning is also suc-cessful if the tin is in metallic form. But the deposition of the tin onto the surface with the aid of stannates, i.e. in ionic form, has the advantage that the application can be carried out in a comparatively simple way using a roll coating method.
Naturally, all other methods with which liquid ionic solutions can be applied to a surface are also suitable.
The application can take place inline on the strip before it is cut into individual sheet bars.
The sheet bars cut out from the strip can also be coated in a corresponding way.
The sheet bars are then formed by means of an in particular multi-step process into a component blank. It is also conceivable to first coat the component blank with the tin compound or the tin. It has turned out, however, that the tin or tin salt coating also tol-erates the forming processes well.
Then a component blank that is obtained in this way is heated to a temperature that pro-duces a structural change to austenite. The austenitized component blank is then con-veyed to a form hardening tool in which the component blank is hardened in a single stroke by means of the contact with an upper tool and lower tool, which essentially have the shape of the blank or correspond to it. Due to the placement of the material of the component blank against the ¨ in particular cooled ¨ tools, the heat is removed from the steel material so quickly that a martensitic hardening occurs.
After being cut into sheet bars, they can also be fed to a press hardening process in which the sheet bar is austenitized and then formed and quench hardened with a single press stroke.
The invention has the advantage that by means of it, the surface of a sheet steel pro-vided for form hardening or press hardening is successfully conditioned so that it is possi-ble to dispense with a mechanical final cleaning for removing oxidic surface layers so that Date Recue/Date Received 2022-08-25

17 sheets of this kind 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 is achieved in comparison to hot-dip aluminized sheets.
Date Recue/Date Received 2022-08-25

Claims (17)

Claims

1. A method for conditioning the surface of a steel strip, which is coated with an electrolytically deposited zinc alloy corrosion protection layer and is subjected to a temperature increase in order to change the structure of the steel, characterized in that during the electrolytic coating of the steel strip, tin is deposited or is deposited to-gether with zinc, in such a way that tin is present at the surface or in the vicinity of the surface or tin in metallic form or ionic form is applied after the electrolytic coating.

2. The method according to claim 1, characterized in that after the galvanic coating with a zinc alloy, the tin is applied in ionic form or in metallic form, wherein in ionic form, the tin is applied from a salt solution and in metallic form, the tin is applied using a CVD or PVD process.

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

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

5. The method according to one of the preceding claims, characterized in that the tin in the solution is complexed with citric acid.

6. The method according to one of the preceding claims, characterized in that the aqueous solution is applied with a layer thickness of 1 ¨ 5 pm, in particular 1 ¨ 3 pm, wherein the layer thickness when dry is 50 ¨ 150 nm, particularly 75 ¨

125 nm, especially 80 ¨ 100 nm.

7. The method according to one of the preceding claims, characterized in that the tin coating is 30 ¨ 90 mg tin/m2, particularly 40 ¨ 80 mg tin/m2, and especially 50 ¨ 60 mg tin/m2.

8. The method according to one of the preceding claims, characterized in that a solution with a solution concentration of 150 ¨ 250 g/I K2SnO3*3H20 is used.

9. The method according to one of the preceding claims, characterized in that a solution with 150 ¨ 250 g/I K2SnO3*3H20 and 15 ¨ 25 g/I KOH is used.

10. The method according to one of the preceding claims, characterized in that a solution is used, which has a pH value of 12.5 ¨ 13.5.

11. The 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 the solution, the tin is complexed with citric acid.

12. The method according to claim 9, characterized in that citric acid is contained in a quantity of 35 ¨ 40 g/I for complexing the tin, wherein the pH value is 4 ¨ 5.5.

13. The method according to one of the preceding claims, characterized in that the solution concentration is 200 g/I K2SnO3*3H20 with 20 g/I KOH.

14. An electrolytically galvanized steel strip coated with 40 ¨ 80 mg tin/m2.

15. The electrolytically galvanized steel strip according to claim 14, characterized in that the tin is deposited metallically or in 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 means of a PVD or CVD process.

17. A use of a steel strip according to one of claims 15 ¨ 16, which is produced with a method according to one of claims 1 ¨ 12, in a method in which a steel sheet is heated to achieve austenitization and then formed and quench-hardened or is formed, austenitized, and quench-hardened.