The invention relates to a method for producing a friction-optimized zinc coating on a steel component.

For both visual and functional reasons, steel components are often galvanized in technical applications. Filigree bulk parts, in particular, such as screws and clamping sleeves made of steel, are usually coated with pure zinc in the galvanic deposition process and then passivated. The base properties of the zinc layer protect the underlying, more noble metal from corrosion. Even if a crack occurs, the base material is protected from attack by the anodic effect of the zinc, whereby the latter itself is oxidized and can then only ensure a protective effect for a limited time. Pure zinc already oxidizes in the air atmosphere and is therefore protected by passivation layers.

The deposited layer system consisting of the zinc and passivation layers is not only required to provide corrosion protection, but also to have high wear resistance and, as a result, constant coefficients of friction for repeated tightening. However, galvanized and passivated steel components of the prior art, for example bolts, exhibit a strong variation as well as a significant increase of the friction value with repeated tightening. This results from the layer properties and the adhesive strength of the zinc layer on the steel base material. In the case of electrical contact elements, however, an increasing friction value with repeated tightening of screws also leads to decreasing contact pressure forces of the conductor against the current bar, resulting in increasing electrical contact resistances, which impair the functionality of the contact elements.

As an alternative to electro-galvanizing, steel components are also frequently hot-dip galvanized or sherardized, but this is not economically feasible in some cases, for example for filigree bulk parts made of steel. In addition, hot-dip galvanizing leads to the formation of a zinc coating and, in particular, to the formation of intermetallic zinc-iron phases with significantly increased coefficients of friction, wherein the coefficients of friction for a hot-dip galvanized steel screw increase even more significantly with repeated tightening.

In addition, a method for producing a galvanized annealed steel strip is already known from DE 689 12 019 T2, in which the steel strip is first cleaned and then at least one side of the steel strip is electroplated with a zinc coating. Subsequently, the coated steel strip is conveyed through an induction coil, wherein the strip is heated to a temperature between 427° C. and 510° C., resulting in a complete conversion of the zinc coating into a zinc-iron alloy coating. Finally, the steel strip is cooled. However, such a method has the disadvantage that the melting temperature of the zinc is exceeded in several process steps, and therefore this method can be applied to strips and sheets, but not economically to bulk parts and smaller steel components such as screws and clamping sleeves. In addition, such a method leads to extensive transformation of the zinc coating into zinc-iron phases with an iron mass fraction above 10%, which also results in a deterioration of the friction value, especially in the case of repeated tightening, and also in a strong fluctuation of the coefficients of friction of the manufactured and coated steel parts.

The invention is therefore based on the object of providing a method for producing a friction optimized zinc coating on a steel component, which results in a coating having good corrosion protection properties, good adhesion and a stable, constantly low friction value, including in the case of repeated use of the steel component, in particular in repeated tightening, wherein the method can be carried out both easily and economically.

The object is solved according to the invention by a method for producing a friction-optimized zinc coating on a steel component according to claim 1. Advantageous further developments are indicated in the dependent claims.

In the method according to the invention for producing a friction optimized zinc coating on a steel component, in particular on a bulk material part, a zinc layer is first applied to the surface of the steel component by means of a galvanic deposition process and subsequently a heat treatment is carried out at a temperature below 420° C. for the targeted forming of intermetallic zinc-iron phases in the galvanically deposited zinc layer in order to optimize the friction value of the steel component.

The inventors have recognized that the properties of the zinc-iron phases between the surface of the steel component and the galvanically applied zinc coating can be specifically controlled and adjusted by means of heat treatment. This makes it possible to increase the adhesive strength of the zinc layers in a simple and easily repeatable manner and to optimize the zinc layer properties, in particular the layer hardness and the surface friction value. In addition, the heat treatment has the advantage that it can be carried out economically for any steel components, including bulk parts.

In addition, comparative tests have shown that, by means of the method according to the invention, a constantly low friction value can be achieved in a readily reproducible manner and without strong fluctuations between individual steel components manufactured in accordance with the invention, which friction value does not increase strongly even when the steel component is used several times, in particular when a screw is tightened several times, but ideally remains essentially unchanged over at least 10 tightening procedures and particularly preferably over at least 20 tightening procedures. The absolute value of the friction value is initially of secondary importance, although a low friction value is particularly preferred.

The steel components can initially be any component made from any iron alloy and preferably from an iron-carbon alloy, which particularly preferably has a carbon mass fraction of less than 2%. Each individual steel component is preferably formed in one piece. Furthermore, the method is preferably used to coat numerous steel components at the same time, wherein all the steel components coated at the same time are particularly preferably formed identically to one another. In general, the steel components are preferably each a steel component having at least one thread and, particularly preferably, exactly one thread. The steel components are preferably bulk parts, in particular filigree bulk parts, and preferably connecting means, such as, for example, screws, clamping sleeves, pockets, and/or electrical connection elements or, alternatively, parts thereof, such as, for example, components of a terminal block. Also preferred are steel components for screw-clamping-sleeve connections. The steel components can have any function, wherein the steel components are preferably provided for fixing an electrical conductor. The requirements imposed on the surface of the coated steel component, in particular a zinc and/or a passivation layer, are on the one hand good corrosion protection properties and on the other hand a constant coefficient of friction during repeated tightening.

For this purpose, a zinc coating is applied to at least part of the surface and preferably to the entire surface of the steel components. According to the invention, this application is carried out in a galvanic deposition process, i.e. by electroplating. Electroplating is understood to mean all methods for the electrochemical deposition of metals onto the metallic surface of steel components using an electrolyte, wherein the electrolyte is preferably an electrically conductive liquid, in particular an aqueous salt solution.

Preferably, a pure zinc layer is applied by the galvanic deposition process, i.e. a pure zinc coating, wherein the pure zinc layer particularly preferably contains no more than 1% of further metal atoms, except for any metal atoms diffused in from the surface of the steel component, while further substances, in particular polymers from the deposition process, may be incorporated in the zinc layer. In particular, the zinc layer is applied by means of a pure and/or an iron- and/or aluminum-free zinc electrolyte.

The galvanic application of the zinc layer can be carried out either by heating the steel component and/or the electroplating bath or without adjusting the temperature. Generally, however, the application is carried out without heating above 420° C., preferably without heating above 100° C., and particularly preferably without heating above 50° C. Also preferably, there is no heating of the steel components between the electroplating and the heat treatment to form the intermetallic zinc-iron phases.

The zinc coating is basically a full zinc layer on the surface of the steel component, wherein the zinc coating preferably completely covers the surface of the steel component and in particular preferably completely closing it so that no oxygen and/or no liquid can reach the surface of the steel component. A friction optimized zinc coating is understood to be a coating of zinc or a zinc alloy which has optimized properties with regard to the friction value or the friction coefficient and in particular has a particularly low friction value or friction coefficient, a friction value or a friction coefficient which changes particularly little during repeated tightening and/or a particularly constant friction value or friction coefficient. In particular, at least the outer surface or rather surface of the zinc layer is designed in such a way that an optimized friction value or friction coefficient is obtained.

The heat treatment can initially be in any form and in particular have any temperature profile. Preferably, the heat treatment is carried out by heating to a fixed temperature, holding at this temperature for a certain time and subsequently cooling. Such a heating cycle is preferably carried out only once, although it is also possible in principle to repeat it several times. Furthermore, heating is preferably performed uniformly and/or uninterruptedly up to the specified temperature. Cooling also preferably takes place uninterruptedly and in particular preferably down to the initial temperature before the heat treatment. Likewise, the heat treatment takes place in air or, alternatively, in a gaseous atmosphere, i.e. outside a liquid. Particularly preferably, the heat treatment is carried out inside a furnace, especially inside an electric furnace. For this purpose, the steel component to be treated is preferably placed in a corresponding furnace. In particular, the heat treatment is an annealing at a fixed temperature. Particularly preferably, the heat treatment is carried out starting from a temperature below 100° C., very particularly preferably below 50° C. and especially preferably starting from room temperature.

According to the invention, the heat treatment takes place on the solid, i.e. below the melting temperature of the zinc of about 420° C. This leads to diffusion processes in the solid, which result in the formation, stabilization and/or further development of the desired intermetallic zinc-iron phases. In general, for the temperature range of the heat treatment, which is preferably between 200° C. and 420° C., particularly preferably between 230° C. and 420° C. and very particularly preferably between 250° and 400° C., the higher the temperature, the faster the formation, stabilization and/or further developing of the desired intermetallic zinc-iron phases.

The formation of one or more different intermetallic zinc-iron phases according to the invention also includes the stabilization and/or the further developing and/or the modification of zinc-iron phases already formed during the galvanic deposition process. Preferably, however, at least one intermetallic zinc-iron phase is formed by the heat treatment which was not previously present or was present only in a very small or, alternatively, significantly smaller proportion. Particularly preferably, the heat treatment serves to optimize the layer structure of the zinc coating and in particular of the layers of intermetallic zinc-iron phases contained therein. Very particularly preferably, the heat treatment is carried out to form several, superimposed, merging layers and/or to stabilize and further develop these layers.

According to the invention, at least one intermetallic zinc-iron phase is formed by means of the heat treatment, wherein preferably a layer of pure zinc, i.e. an η-phase, is present at the same time, in particular on the surface of the zinc coating, which layer of pure zinc contains elements other than zinc only in the form of unavoidable impurities and necessary auxiliary materials. In addition, further intermetallic zinc-iron phases are usually present in small proportions alongside the specific intermetallic zinc-iron phase(s) formed in a targeted manner.

In general, intermetallic zinc-iron phases with different stoichiometries are formed depending on the temperature and the holding time of the heat treatment, wherein the stoichiometry has a direct influence on the properties and in particular the hardness of the deposited zinc layer and thus also directly influences the wear resistance and/or the friction value. There are numerous different intermetallic zinc-iron phases, but only some of them are of decisive importance with regard to corrosion protection and the friction value properties of an electroplated zinc coating on a steel component. First, for example, a face-centered cubic r-phase which is characterized by brittle material behavior is formed on galvanized steel components. In addition, a hexagonal δ-phase with a lower iron content can form, which is characterized by very ductile properties and, when occurring in a closed layer, by high corrosion resistance. The ζ-phase with a monoclinic crystal structure forms in the form of a brittle palisade layer. The iron content in this compound is lower than in the previously presented intermetallic phases. The pure zinc (η-phase) contains no iron atoms and has the lowest hardness. However, the iron mass fraction in the intermetallic zinc-iron phases within the zinc coating of the steel component is preferably not more than 10%, particularly preferably not more than 7.5% and very particularly preferably not more than 6%.

Optimization of the friction value is understood to mean, in particular, a reduction in the initial friction value, in particular during a first actuation or use of the steel component, and/or a reduction in the increase in the friction value during subsequent actuations or uses of the steel component. Alternatively or additionally, optimization may also involve keeping the friction value as low and/or constant as possible for as long as possible during repeated actuation or use of the steel component. In particular, however, it is also optimal to reduce the friction value during repeated actuation or use of the steel component.

In a preferred embodiment of the method for producing a friction-optimized zinc coating according to the invention, the application of a zinc layer and all subsequent method steps for producing the zinc-coated steel component are carried out at temperatures of below 420° C., whereby melting of the zinc and thus an undesirable phase transformation as well as unfavorable material movement on the surface of the steel component can be easily avoided in an advantageous manner. Particularly preferably, a temperature of 400° C. is not exceeded.

According to an advantageous further development of the method according to the invention for producing a zinc coating with an optimized zinc coating, the holding time of the heat treatment is between 10 minutes and 10 hours, preferably between 20 minutes and 6 hours and particularly preferably between 30 minutes and 4 hours, whereby a good and extensive formation of intermetallic zinc-iron phases can be achieved. Here, the holding time is the duration of the heat treatment during which the zinc-coated steel component is held at an elevated temperature, in particular at the maximum temperature of the heat treatment. Overall, the layer thickness and the iron content in the zinc layer can be controlled by varying the holding time and/or the temperature, wherein an increasing holding time leads to stronger diffusion of iron into the zinc layer and thus to an increased iron content. Preferably, the heat treatment is carried out in a continuous furnace, particularly preferably in the case of heat treatment of numerous steel components, in particular bulk parts, at the same time. Alternatively, however, the heat treatment can also be carried out in a chamber furnace.

The duration of the heat treatment also depends, among other things, on the number of steel components treated at the same time, wherein a longer duration of the heat treatment is preferred in particular when treating numerous parts at the same time, for example in a lattice box or crate, in order to ensure that internal parts have also been heated for a sufficient duration. Accordingly, heat treatment of individual steel components or rather individually arranged steel components can be carried out for a significantly shorter time. With regard to the duration of the heat treatment, it also applies that, with increasing duration, a better reproducibility of the desired result can be achieved, in particular over all steel components heated at the same time.

Preferably, the minimum holding time, especially at a temperature of 300° C., is at least 15 minutes and particularly preferably 20 minutes, since at a temperature of 300° C. and a duration of 10 minutes no measurable iron diffusion can yet be detected. The maximum holding time is in principle not limited, but with strongly increasing time no significant changes can be observed, and therefore a holding time of at most 4 hours is reasonable and, particularly preferably, the holding time is less than 3 hours and, very particularly preferably, less than 2 hours. In principle, however, a holding time of more than 3 hours, in particular of more than 4 hours, can be useful, especially at lower temperatures, such as at a temperature of between 220° C. and 330° C., preferably between 230° C. and 320° C., and particularly preferably between 250° C. and 310° C. Even at a holding time of 10 hours at 300° C., a maximum iron mass content of 6% could be measured in the zinc layer.

In order to specifically form, stabilize and/or further develop a ζ-phase (zeta phase) of the iron-zinc, due to which particularly good coefficients of friction of the zinc-coated steel component can be achieved, a possible embodiment of the method according to the invention provides that the heat treatment is carried out at a temperature of between 220° C. and 330° C., preferably between 230° C. and 320° C., particularly preferably between 250° C. and 310° C. and very particularly preferably at 300° C. and/or for a holding time of between 30 minutes and 2 hours, particularly preferably between 45 minutes and 1.5 hours and very particularly preferably 1 hour. In this context, the ζ-phase has a significant influence on the constancy of the friction value, especially when the steel component is tightened several times. Preferably, the heat treatment to form, stabilize and/or further develop the ζ-phase is carried out at a temperature of 300° C. over a holding time of between 30 minutes and 2 hours, particularly preferably between 45 minutes and 1.5 hours, and very particularly preferably 1 hour. At the same time, formation, stabilization and/or further developing of a δ-phase (delta phase) of the iron-zinc may occur in addition to the ζ-phase, which is even preferred in some cases.

However, the δ-phase, which is particularly ductile and leads to particularly good corrosion protection of the zinc-coated steel component, can alternatively or additionally also be specifically formed by heat treatment at a temperature of between 310° C. and 390° C., preferably between 330° C. and 370° C., particularly preferably between 340° C. and 360° C. and very particularly preferably at 350° C. In particular, a multistep heat treatment is also conceivable, especially first for forming, stabilizing and/or further developing the ζ-phase and subsequently at higher temperature for forming, stabilizing and/or further developing the δ-phase. Alternatively, a δ-phase can be formed first and then a heat treatment can be carried out to form the ζ-phase.

Ideally, the holding time for forming, stabilizing and/or further developing the δ-phase, in particular at a temperature of 350° C., is more than 30 minutes, more preferably more than 45 minutes and most preferably more than 1 hour or, alternatively, between 1 hour and 3 hours, more preferably between 1.5 hours and 2.5 hours and very particularly preferably 2 hours. Especially at lower temperatures, a longer holding time is also useful.

Preference is also given to an embodiment of the method according to the invention in which the heat treatment is carried out at a temperature of below 350° C., preferably below 340° C. and very particularly preferably at a maximum of 330° C. and/or at at least 200° C., preferably at least 220° C. and particularly preferably at least 230° C. Furthermore, the holding time is preferably less than 10 hours, particularly preferably less than 8 hours and very particularly preferably less than 5 hours and/or at least 10 minutes, preferably at least 20 minutes and particularly preferably at least 30 minutes.

If, on the other hand, heat treatment is carried out at significantly elevated temperatures, in particular just below the melting point of the zinc, a Γ-phase (gamma phase) of the iron-zinc is thereby formed, stabilized and/or further developed. Such a heat treatment is preferably carried out at at least 390° C., particularly preferably at at least 400° C. and very particularly preferably at at least 410° C., wherein the maximum temperature is again preferably 420° C. This heat treatment is further preferably carried out over a long period of at least 3 hours, particularly preferably at least 4 hours and very particularly preferably at least 5 hours.

The surface of the steel component can be pretreated as desired before applying the zinc layer. In this case, the pretreatment is particularly preferably carried out for conditioning the surface of the steel component. This may include, for example, cleaning, in particular degreasing, of the surface. Grinding or chemical removal of oxide layers is also conceivable. In addition, the surface of the steel component can also be pickled and/or pre-galvanized before the zinc layer is applied. The pickling can be carried out in any way, for example by immersion pickling, spray pickling, rotary pickling and/or electrochemical pickling.

In a preferred further development of the method according to the invention for producing a friction-optimized zinc coating on a steel component, a heat treatment to counteract possible hydrogen embrittlement is carried out before the zinc layer is applied to the surface of the steel component in the galvanic deposition process and/or after pickling and/or after pre-galvanizing, preferably at temperatures of between 200° C. and 250° C. In particular, this heat treatment is aging against hydrogen embrittlement, in which the hydrogen diffuses out of the steel component and/or is distributed more uniformly in the material, whereby the hydrogen embrittlement can be at least significantly reduced or even completely avoided or, alternatively, eliminated. It is also preferable to carry out the final galvanizing in the galvanic deposition process immediately after this heat treatment, in particular after the aging. Such a heat treatment, and in particular an aging process, is particularly useful for significantly strengthened steel base materials of the steel component.

In order to obtain even better corrosion protection and in particular protection of the zinc layer against wear and to protect the pure zinc on the surface of the zinc-coated steel component against oxidation, and in order to achieve even better optimization of the friction value, the zinc surface is preferably conditioned in the galvanic deposition process after the zinc layer has been applied to the surface of the steel component, in particular by passivation of the zinc surface, wherein the passivation is preferably effected by an organoceramic coating or by means of at least one, preferably several organoceramic layers. Passivation is preferably carried out by immersion in a passivating agent. The organoceramic layers are furthermore preferably formed predominantly from chromium and/or zinc oxides.

A preferred embodiment of the method for producing a friction-optimized zinc coating according to the invention provides that the heat treatment for forming the zinc-iron phases is carried out after passivation or rather following on from passivation when a temperature-resistant passivation layer is used. For this purpose, the passivation layer must be temperature-resistant at least up to the maximum temperature of the heat treatment, preferably up to at least 10° C. and particularly preferably at least 20° C. above the maximum temperature of the heat treatment. In particular, the heat treatment to form the zinc-iron phases is preferably carried out as the last production step of the zinc-coated steel components. Also preferably, passivation is performed immediately after applying the zinc layer to the surface of the steel component in the galvanic deposition process in order to protect the newly formed zinc layer from oxidation as quickly as possible. In the case of a non-temperature-resistant passivation layer, the heat treatment must be carried out accordingly before passivation.

According to a preferred further development of the method according to the invention for producing a friction-optimized zinc coating, the zinc layer is applied to the surface of the steel component in the galvanic deposition process by means of an alkaline zinc electrolyte which preferably contains nitrogen-containing polymers and/or is preferably cyanide-free. The selection of the zinc electrolyte used in the galvanic deposition process also has an influence on the friction value of the zinc-coated steel component, which results in particular from substances, especially organic substances, incorporated in the zinc layer. It was found that weakly acidic electrolytes and/or electrolytes with sulfur-containing surfactants are only suitable to a very limited extent since a strong increase in the friction value was observed there. Accordingly, the pH of the zinc electrolyte is preferably larger than 2.5, particularly preferably larger than 5.0, very particularly preferably larger than 7.0 and especially preferably larger than 8.0.

Several embodiments of the invention are subsequently described in more detail with reference to a drawing. The FIGURE shows:

FIG. 1 a schematic flow diagram of a method for producing a zinc coating with an optimized zinc coating on a steel component.

In a first embodiment of the method for producing a friction-optimized zinc coating on a steel component, a pure zinc coating is first applied to the surface of a steel screw for an electrical terminal block by means of a galvanic deposition process. A cyanide-free alkaline zinc electrolyte in aqueous solution is used, wherein the solution preferably also contains nitrogen-containing polymers. The use of this alkaline zinc electrolyte results in the nitrogen-containing polymers being at least partially incorporated in the zinc coating, as a result of which the steel screw has significantly more stable friction values.

Immediately after the coating, a heat treatment is carried out, for which the zinc-coated steel screw is heated to 300° C. for 30 minutes in a furnace in order to form and stabilize a ζ-zinc-iron phase between the surface pure zinc and the steel surface of the coated steel screw. This optimized layer structure is characterized in this case by the stabilization of the ζ-phase in the transition area from the zinc layer to the base material. The course of the friction value is characterized by an almost constant friction value over ten tightening cycles, and the scatter of the coefficients of friction could also be significantly reduced as a result.

Alternatively, the duration of the heat treatment can be between 20 minutes and 4 hours. In any case, only one temperature cycle is carried out with a single heating, holding at 300° C. and cooling. Such a heat treatment to form an intermetallic zinc-iron phase can be carried out, for example, in a continuous furnace, wherein the steel screws are heated up to 300° C. at a heating rate of about 10 K/min and held at this temperature for 30 min. This is followed by cooling in still air, in particular at a cooling rate of about 5 K/s.

In addition, however, heat treatment exclusively at a lower temperature and for a longer period is possible. For this purpose, for example, the steel screws are heated up to 250° C. in a chamber furnace at a heating rate of about 10 K/min to form an intermetallic zinc-iron phase and held at this temperature for about 6 hours up to 10 hours or longer. This is followed by cooling in still air, in particular at a cooling rate of about 5 K/s.

Particularly preferably, the heat treatment at 300 C is not followed by complete cooling, but the steel components are then kept at a temperature of 250° C. for a longer period, in particular 6 hours, as shown in FIG. 1. This achieves a more uniform result over all the steel components and a more undisturbed, uniform layer structure of each steel component.

In another embodiment, the electroplating of a steel component of an electrical contact element is carried out using a cyanide-free, alkaline zinc electrolyte. Passivation is carried out immediately thereafter, wherein organoceramic layers formed predominantly from chromium and/or zinc oxides are applied to the galvanized surface of the steel component.

Subsequently, as the last production step of the steel component, a heat treatment of the zinc-coated and passivated component is carried out in a furnace at a temperature of 300° C. for a period of 30 minutes.

Another embodiment proceeds from a component identically coated with zinc, wherein heat treatment takes place in two steps. First, the component is heated to 350° C. and held there for 30 minutes. Subsequently, the component is cooled to 300° C. and held there for a further hour.

After complete cooling, passivation is carried out, which can be done by means of a non-heat-resistant passivating agent.