EP3489386A1 - Coated steel substrate and method for producing a hardened component from a coated steel substrate - Google Patents

Abstract

The invention relates to a coated steel substrate for hot forming, comprising: a first coating (4) containing at least 85% by weight of aluminum and a second coating (11) overlying the first coating (4), the second coating (11) comprising a first coating (4) Copper-containing coating is. The invention further relates to a method for producing a hardened molded article comprising the steps of: coating a steel substrate (2) with an aluminum coating (4), applying a copper-containing coating (11); Working out a board from the steel substrate (2); and hot working the board to produce a cured molding.

Description

The invention relates to a coated steel substrate and a method for producing a cured component from a coated substrate.

Coating systems for corrosion protection of metallic components as well as coated components hot-formed molded parts are well known from the prior art. For safety-related body parts of motor vehicles, for example, aluminum-silicon-coated high-strength and ultra-high-strength tempering steels, in particular manganese-boron-containing tempering steels such as 22MnB5 or 34MnB5 are used. However, aluminum-silicon coatings have a tendency to hydrogen charge during hot reduction, which can lead to hydrogen-induced cracking (hydrogen embrittlement). To avoid this, it is known to apply a further coating to the aluminum coating. This additional coating is intended to reduce unwanted hydrogen uptake of the aluminum coating during the thermoforming process.

From the EP 2 270 257 A1 , according to the US 2012/0073351 A1 , a coated steel sheet and a method for hot working a coated steel sheet is known. The steel sheet has an aluminum coating and an overlying surface coating. The surface coating contains a compound having a wurtzite crystal structure. As compounds having a wurtzite crystal structure, in particular ZnO, but also AlN, GaN, InN, TiN, TlN, MnS, MnSe, ZnO, ZnS, CdS, CdSe are mentioned.

From the WO2016132194A1 For example, a steel sheet having a first aluminum-based coating and a second zinc coating and a method for producing press-hardened components is known. The zinc coating has a thickness less than or equal to 1.1 microns.

It is an object of the present invention to propose a hot-formable coated steel sheet which, as part of the hot-forming process, is subject to a particularly low hydrogen absorption and whose surface enables simple and good further processing.

For solution, a coated steel substrate for hot working is proposed comprising: a first coating containing at least 85% by weight of aluminum and a second coating overlying the first coating; wherein the second coating is a copper-containing coating.

An advantage of the coated steel substrate is that the copper-containing coating acts as a barrier against undesirable uptake of diffusible hydrogen during subsequent processing. For the purposes of this disclosure, copper-containing coating shall include any coating containing copper or a copper alloy. The copper-containing coating may also contain other alloying elements, especially zinc. Preferably, a zinc-copper coating is used which, when applied, contains at least 60 weight percent zinc and at least one weight percent copper. The copper-containing coating is in particular a nanocrystalline coating or has a nanocrystalline structure. Within the scope of the present disclosure, nanocrystalline is understood as meaning a crystalline substance whose mean particle size is in the range of nanometers, for example less than 400 nanometers, optionally also less than 20 nanometers, in particular less than 10 nanometers.

The steel substrate can be, for example, a hardenable, in particular manganese-containing steel material. This can include other micro-alloying elements in addition to manganese, such as niobium and / or titanium with a mass fraction of preferably at most 1000 ppm, and / or further micro-alloying elements in small proportions by weight, such as boron and / or vanadium. Examples of a usable steel material are 17MnB5, 22MnB5, 26MnB5 or 34MnB5. The starting material (strip material) may have a tensile strength of, for example, at least 450 MPa. A molded part produced from the coated steel substrate may have a ultimate tensile strength of, for example, at least 1100 MPa, preferably at least 1300 MPa, in particular at least 1500 MPa, more preferably even at or above 1900 MPa. It is also possible that other subregions have a lower tensile strength of less than 1100 MPa and therefore higher ductility.

As the starting material for the steel substrate, a constant thickness material or variable sheet thickness material can be used. Examples of different sheet thicknesses are, for example, sheets welded together from several sheets, so-called tailor welded blanks (TWB) or one-piece blanks with different thicknesses, which are produced by flexible rolling, so-called tailor rolled blanks (TRB). The steel substrate can be present both as a strip material (coil) and as a rectangular plate or shaped section.

The first coating contains at least 85% by weight of aluminum, including the possibility of using a pure aluminum coating (100% by weight of Al), as well as the use of an alloy containing at least 85% by weight of aluminum as the main alloying constituent and optionally further alloying constituents. For example, silicon with, for example, between 5 and 15 weight percent and / or iron with up to 5 weight percent and / or other alloying elements in lower proportions. In the context of the present disclosure, due to the main constituent aluminum, the term aluminum coating or aluminum-based coating is generally also used for the first coating, with the said possibilities of other alloy compositions being intended to be encompassed conceptually. The aluminum coating, for example, in the hot dip process in a molten bath with at least 85 percent by weight of aluminum and optionally other alloying ingredients or other common coating methods are applied to the steel substrate. An exemplary composition of the molten bath or coating may contain up to 3% by weight of iron, 9 to 12% by weight of silicon, and the balance of aluminum. The thickness of the first coating may for example be between 5 and 50 micrometers, preferably between 10 and 35 micrometers.

The second coating may have a minimum thickness of, for example, at least 10 nanometers (10 nm), in particular at least 200 nanometers (200 nm). The maximum thickness of the nanocrystalline coating can be, for example, up to two micrometers (2 μm), in particular up to 1500 nanometers (1500 nm), in particular up to 800 nanometers (800 nm). The zinc-nickel coating alloy compositions (e.g., at least 60 wt% Zn and at least 5 wt% Cu) mentioned in the present disclosure particularly refer to an upper and / or middle depth region of the zinc-nickel coating.

The application of the particular nano-crystalline zinc-copper coating can take place, for example, by means of electroless and / or electrolytic deposition on the aluminum-coated steel substrate. Alternatively, the second coating may also be applied by a dipping method, PVD method, CVD method or hot dip galvanizing method.

In particular, the second coating, when applied to the steel substrate, may contain 80 to 90% by weight of zinc and / or 10 to 20% by weight of copper. According to a first possibility, it can be provided that the second coating contains only zinc and copper. Alternatively, the second coating may also include other alloying ingredients, in particular up to 5 weight percent iron and / or up to 10 weight percent nickel, or others.

The object is further achieved by a method of producing a cured molded article comprising the steps of: coating a steel substrate with an aluminum coating, wherein the aluminum coating contains at least 85 weight percent aluminum in a state applied to the steel substrate; Rolls of Steel substrate; Applying a copper-containing coating, the copper-containing coating being applied after coating the steel substrate with the aluminum coating; Working out a board from the steel substrate; Hot forming the board to produce a cured molding.

Substantially the same advantages are achieved with said method as with the coated steel substrate, so that reference is made to the above description. It is understood that all of the abovementioned inventive and preferred features of the coated steel substrate can also be transferred to the process, and conversely, that all features mentioned in connection with the process can also be applied to the product. The copper-containing coating forms a barrier to undesirable uptake of diffusible hydrogen, especially during subsequent hot working. It has thus been found that the proposed method can achieve a reduction in the absorption of diffusible hydrogen of more than 50%. In this way, unwanted hydrogen embrittlement of the steel material can be avoided or at least reduced. Overall, therefore, a particularly high mechanical strength or wear resistance of the cured molded part is achieved. In particular, the copper-containing coating can be applied as a zinc-copper coating which, when applied, contains at least 60% by weight of zinc and at least one, preferably at least 5% by weight of copper. The second coating is preferably applied to the precoated steel substrate to form a crystalline or nano-crystalline structure. First coating particles deposit on the surface of the precoated steel substrate and then grow to larger crystals over time. The shape of the grown nanocrystals is arbitrary and can be globular to stem-shaped.

The preferably nano-crystalline zinc-copper coating can be applied, for example, by means of electroless deposition, electrolytic deposition, electrochemical (galvanic) deposition, PVD processes, CVD processes and / or fire-coating processes, in particular hot-dip galvanizing processes. A particularly efficient process is electroless plating, or a electroless metal deposition, which is understood in particular coating methods that proceed without the use of an external power source. According to one possible embodiment, the nano-crystalline zinc-copper coating can be applied by means of an alloyed zincate stain. In particular, the zincate stain may contain at least sodium hydroxide, zinc oxide, complexing agents and metal salts such as iron and copper. The pH of the alloyed zinc stain may be, for example, 11-14. The coating can be carried out, for example, at temperatures between 20 ° C and 98 ° C. After coating, the coated steel substrate can be dried, for example by means of hot-air drying.

The period of time for applying the zinc-copper coating to the aluminum-coated steel substrate may be, for example, at least 0.5 minutes and / or up to 5.0 minutes. The longer the coating time, the thicker the nano-crystalline zinc-copper coating becomes. Preferably, the time period is selected so that the second coating has a thickness of 10 nanometers (10 nm) to two micrometers (2 μm), in particular from 200 nanometers (200 nm) up to 800 nanometers (800 nm).

For good adhesion, it is advantageous if the surface of the aluminum coating is activated before applying the nano-crystalline zinc-copper coating, which can be done, for example, by means of pickling. For this purpose, an aluminum-containing stain can be used. By activating the outer oxide layer of the aluminum coating is removed, so that favorable conditions for connecting the aluminum coating to be applied with the zinc-copper coating can be achieved. Before activation, the steel substrate can optionally be degreased, depending on the surface condition. After activation, the steel substrate can optionally be dekapiert.

As part of the production of the cured molded part, the substrate is rolled as a strip material to a desired thickness over the length. The rolling can be carried out as conventional or as flexible rolling. In conventional rolling, a uniform thickness over the length is produced, while in flexible rolling a variable thickness is produced over a length of the strip material. The The inventive method is particularly well suited for flexibly rolled strip material, since even the thinner rolled substrate sections after application of the nano-crystalline zinc-copper coating have an increased resistance to the diffusion of hydrogen, so that even in these thinner areas, a reduced microcracking tendency given is. The steel substrate may be provided with the aluminum coating before or after rolling.

At a suitable point of the process, blanks are produced from the rolled strip material. This process step is also referred to as singulation. The separation can be carried out by mechanical cutting or by laser cutting. The term boards in the context of the present disclosure, both rectangular metal sheets that have been cut out of the strip material, as well as form cuts to be included. Molding cuts are sheet metal elements that have been worked out of the strip material and whose outer contours have already been adapted to the shape of the final product. The production of boards from the steel substrate can be carried out before or after the coating of the steel substrate with the nano-crystalline zinc-copper coating. In the first case, singulation before coating, the coating is carried out piece by piece. In the second case, singulation after coating, the coating can be carried out in a continuous process of the strip material by a dipping bath, which is particularly efficient.

The sheet metal blanks are hot-worked after application of the nano-crystalline zinc-copper coating, it being possible for further process steps to be interposed. As part of the hot forming, the board is heated to Austenitisierungstemperatur at least in a partial area; then placed in a thermoforming mold and formed in the thermoforming mold and cooled, so that a hardened molded part is formed. The heating is carried out in a suitable heating device, for example in an oven. The rapid cooling of the molding in the forming tool produces a hardened, at least partially martensitic microstructure. This process of hot forming and rapid cooling in a forming tool is also referred to as press hardening.

Due to the zinc-copper coating, there is a significant reduction in the uptake of hydrogen in the heating device, since the zinc-copper coating prevents or at least inhibits a reaction of the water vapor with the aluminum coating. In addition, the abrasion in the tool due to the zinc-copper coating is reduced, which reduces wear and extends the life. Another advantage is that the residence time in the heating device can be reduced due to the dark surface and thus the higher degree of absorption of the zinc-copper coating. Finally, another advantage is that the zinc-copper coating can be phosphated in a later step, or that a phosphate layer adheres to the zinc-copper coating, which is not the case with an aluminum-coated surface.

The hot working may be carried out as an indirect process according to a first possibility, comprising the substeps of cold preforming, then heating the cold preformed component to austenitizing temperature, followed by hot working to produce the final contour of the product. The hot forming can also be carried out as a direct process according to a second possibility, which is characterized in that the component is heated directly to Austenitisierungstemperatur and then hot-formed to the desired final contour in one step. A previous (cold) preforming does not take place here.

Since the hydrogen loading in the hot forming process according to the present method is particularly low, can be dispensed with a dew point control in the oven. Another advantage of the zinc-copper coating is that after press hardening surface cleaning may be dispensed with and that the surface of the zinc-copper coating allows phosphating, which in turn leads to better corrosion protection.

Figur 1

ein erfindungsgemäßes Verfahren schematisch in einer ersten Ausführungsform;

Figur 2

ein erfindungsgemäßes Verfahren schematisch in einer zweiten Ausführungsform;

Figur 3A

ein erfindungsgemäßes beschichtetes Stahlsubstrat schematisch im Querschnitt;

Figur 3B

eine vergrößerte Schnittdarstellung (Rasterelektronenmikroskop) eines erfindungsgemäßen beschichteten Stahlsubstrats;

Figur 4A

eine vergrößerte Draufsicht auf die Oberfläche eines Aluminium-beschichteten Stahlsubstrats nach dem Walzen;

Figur 4B

eine vergrößerte Draufsicht auf die Oberfläche eines Aluminium-beschichteten Stahlsubstrats nach dem Aufbringen einer nanokristallinen Zink-Kupfer-Beschichtung;

Figur 4C

eine vergrößerte Draufsicht auf die Oberfläche eines erfindungsgemäß beschichteten Stahlsubstrats nach dem Warmumformen;

Figur 5A

eine grafische Darstellung des Schichtaufbaus (GDOES-Analyse) eines erfindungsgemäß beschichteten Stahlsubstrats vor dem Warmumformen;

Figur 5B

eine grafische Darstellung des Schichtaufbaus (GDOES-Analyse) eines erfindungsgemäß beschichteten Stahlsubstrats nach dem Warmumformen;

Figur 6A

eine vergrößerte Draufsicht auf die Oberfläche eines Aluminium-beschichteten Stahlsubstrats (ohne Zink-Kupfer-Beschichtung) nach dem Warmumformen;

Figur 6B

eine vergrößerte Draufsicht auf die Oberfläche eines erfindungsgemäßen beschichteten Stahlsubstrats nach dem Warmumformen;

Figur 7

eine grafische Gegenüberstellung des in das beschichtete Blech eindiffundierten Wasserstoffs bei Verwendung eines Aluminium-beschichteten Stahlsubstrats (ohne Zink-Kupfer-Beschichtung) verglichen mit der Verwendung eines erfindungsgemäßen beschichteten Stahlsubstrats;

Figur 8

eine grafische Darstellung der Verweilzeit bei Verwendung eines Aluminium-beschichteten Stahlsubstrats (ohne Zink-Kupfer-Beschichtung) verglichen mit der Verwendung eines erfindungsgemäßen beschichteten Stahlsubstrats, jeweils nach einem Walzen.

Preferred embodiments will be explained below with reference to the drawing figures. Herein shows

FIG. 1

a method according to the invention schematically in a first embodiment;

FIG. 2

a method according to the invention schematically in a second embodiment;

FIG. 3A

a coated steel substrate according to the invention schematically in cross section;

FIG. 3B

an enlarged sectional view (scanning electron microscope) of a coated steel substrate according to the invention;

FIG. 4A

an enlarged plan view of the surface of an aluminum-coated steel substrate after rolling;

FIG. 4B

an enlarged plan view of the surface of an aluminum-coated steel substrate after the application of a nanocrystalline zinc-copper coating;

FIG. 4C

an enlarged plan view of the surface of a steel substrate coated according to the invention after hot working;

FIG. 5A

a graphical representation of the layer structure (GDOES analysis) of a steel substrate according to the invention coated before hot working;

FIG. 5B

a graphical representation of the layer structure (GDOES analysis) of a steel substrate coated according to the invention after hot working;

FIG. 6A

an enlarged plan view of the surface of an aluminum-coated steel substrate (without zinc-copper coating) after hot working;

FIG. 6B

an enlarged plan view of the surface of a coated steel substrate according to the invention after hot working;

FIG. 7

a graphical comparison of the diffused into the coated sheet hydrogen using an aluminum-coated steel substrate (without zinc-copper coating) compared with the use of a coated steel substrate according to the invention;

FIG. 8

a graph of residence time using an aluminum-coated steel substrate (without zinc-copper coating) compared with the use of a coated steel substrate according to the invention, each after rolling.

The FIGS. 1 to 8 will be described together below.

FIG. 1 shows a process according to the invention for producing a cured product from a coated steel substrate 2 after a first process. As a steel substrate in the present disclosure, a hardenable flat steel product is included, which is produced from a steel material, which may have a yield strength of 150 to 1100 MPa and a tensile strength of 300 to 1200 MPa. The steel material may have a carbon content of at least 0.1% by mass, especially at least 0.2% by mass, up to 0.35% by mass. For example, 22MnB5 may be used as steel substrate 2, although other hardenable, especially manganese-containing, steel materials may be used, such as 17MnB5, 26MnB5 or 34MnB5. The flat steel product can be present as strip material or boards produced therefrom.

In method step S1, the steel substrate 2, which is wound on a coil 3 in the initial state and can also be referred to as steel strip or strip material, is provided with a first coating 4. The first coating, when applied to the steel substrate, contains at least 85 weight percent aluminum. In the present embodiment, it is further contemplated that the first coating further comprises up to 15 weight percent silicon. It is understood that other alloying elements may be included at the expense of the silicon content, for example iron and / or others with a total of up to 5 weight percent. The aluminum coating is applied to the steel substrate 2 in the hot dip process in a molten bath 5 of a coating device 6, wherein other conventional coating methods such as electrolytic coating are also conceivable.

After the application of the first coating 4, the coated steel substrate 2 is rolled. In FIG. 4A For example, the surface of the aluminum-coated steel substrate 2 after rolling is shown in a greatly enlarged view by a scanning electron microscope. The rolling tracks or the rolling direction can be seen, which is shown as arrow R.

The coated steel substrate 2 is rolled in the present case by means of flexible rolling, without being limited thereto. For this purpose, the steel strip 2, which has a largely constant sheet thickness over the length before the flexible rolling, rolled by means of rollers 7, 8 such that it receives along the rolling direction a variable sheet thickness. During rolling, the process is monitored and controlled, using the data determined by a sheet thickness measurement 9 as an input signal for controlling the rolls 7, 8. After flexible rolling, the strip material 2 has different thicknesses in the rolling direction. The strip material 2 is rewound to the coil 3 after the flexible rolling, so that it can be fed to the next process step.

According to a modified embodiment, the steel strip 2 can also be rolled over the length by means of a conventional rolling method, that is to say with a uniform sheet thickness. According to a further modification, which applies to both process guides flexible rolling and conventional rolling, the application of the first coating (S1) can also take place after rolling (S2), that is to say by reversing the sequence of steps S1 and S2.

After the rolling process, the steel strip 2 can be smoothed in a subsequent process step, which takes place in a belt straightening device. The smoothing step is optional and may be omitted.

After rolling (S1) or smoothing (if provided), the aluminum-coated steel substrate 2 is separated in process step S3. Depending on the shape of the sheet metal blanks to be produced, these can be punched out of the strip material 2 as a shaped cut, wherein a not further used edge is omitted as scrap , or the strip material 2 can be easily cut into sections.

After separation, the aluminum-coated steel substrate 2, which is now in the form of blanks, is coated with a second coating 11. The second coating is applied as a nano-crystalline zinc-copper coating 11 on the steel substrate 2, wherein the second coating in the applied state, at least 60 weight percent zinc and at least 5 weight percent copper and optionally other alloying ingredients such as up to 5 weight percent iron and / or up to 10 weight percent nickel. The structure of the steel substrate 2 provided with aluminum coating 4 and zinc-copper coating 11 is shown in FIG FIG. 3A shown schematically and in FIG. 3B as a scanning electron microscope, enlarged view of section III off FIG. 3A shown. It can be seen that the nano-crystalline zinc-copper coating 11 is substantially thinner than the aluminum coating 4. The aluminum coating 4 may for example have a thickness D4 of between 5 and 50 micrometers, preferably between 10 and 35 micrometers. The thickness D11 of the layer structure of the zinc-copper coating 11 depends on the type and duration of the coating process and may for example be between 10 nanometers and 2 micrometers. A greatly enlarged plan view of the surface of the nano-crystalline zinc-copper coating 11 provided steel substrate is in FIG. 4B shown. It is the fine-grained crystalline structure of the zinc-copper coating 11 can be seen and the rolling tracks generated by rolling, which are shown as the rolling direction R. In this case, the grown crystals may on average have a longest extent of, for example, less than 400 nm. The shape of the crystals is arbitrary according to growth and may be globular to stem-shaped.

In the present case, the zinc-copper coating 11 is applied to the aluminum-coated steel substrate 2 by means of electroless and / or electrolytic deposition. For this purpose, the boards 2 are immersed in the present process guide individually or in groups in a filled with electrolytic liquid 12 immersion tank 13 of an electrolytic coating apparatus 14. The coating is preferably carried out without current at temperatures of, in particular, between 20 ° C. and 98 ° C. As the electrolytic liquid, an alloyed zinc stain may be used, which may particularly contain sodium hydroxide, zinc oxide, complexing agents and metal salts such as iron and copper. For example, the pH of the alloyed zincate stains may be 11-14. In an exemplary embodiment, the alloyed zinc stain may contain at least one or more of the following ingredients: sodium hydroxide 10 to 345 g / l, zinc carbonate to 87 g / l, ferrous chloride to 23 g / l, zinc oxide to 63 g / l, potassium sodium tartrate of 45 to 100 g / l, sodium gluconate to 7 g / l, salicylic acid to 4 g / l, zinc sulfate to 24 g / l, nickel sulfate to 20 g / l, sodium cyanide to 25 g / l and / or sodium nitrate to 1 g / l.

After immersing the aluminum-coated board 2 in the Zinkatbeize 12, a Zinkatschicht deposited on the surface of the board 2 from. The deposition of the zincate layer is based on a redox reaction with the following partial reactions. The zinc deposition takes place in three steps by the formation of aluminum hydroxide with oxidation of aluminum (reaction 1).

(1) Al + 3 OH- => Al (OH) 3 + 3 e-

Here, the elemental aluminum binds with the release of three electrons to three hydroxide groups. Likewise, the aluminate is formed by reaction of the sodium hydroxide solution with aluminum hydroxide (reaction 2).

(2) Al (OH) 3 + NaOH => NaAl (OH) 4

At the same time, the reduction of zinc bound in zincate to elemental zinc occurs (reaction 3). The zincate dissociates to divalent zinc and 4 hydroxide groups. The Attachment of 2 electrons to the divalent zinc completes the formation of elemental zinc. The reduction of zinc and its deposition on the aluminum-silicon surface is as follows:

(3) Zn (OH) ₄ ^2- => Zn ²⁺ + 4 OH ^- Zn ²⁺ + 2 e ^- => Zn.

The entire reaction (4) of the zinc deposition is summarized in the following step, wherein the layer thickness is time-dependent.

(4) 3 Na 2 Zn (OH) 4 + 2 Al => 2 NaAl (OH) 4 + 3 Zn + 4 NaOH

Copper and zinc are deposited together. The thin, conductive and externally stromlosen deposited zincate layer contains predominantly zinc, and copper, but may also contain other metals such as nickel or iron. The zincate process is influenced by the temperature of the bath, the duration of the process, the alloy type of aluminum (Al / AlSi) and by added ions. The shape of the zinc crystals changes with the solution concentration of the zincate stain.

The layer thickness of the deposit 11 can be varied over the residence time of the board 2 in the Zinkatbeize 12. Preferably, the time period is selected such that the second coating 4 has a thickness of 10 nanometers (10 nm) to two micrometers (2 μm), in particular from 200 nanometers (200 nm) to 800 nanometers (800 nm). For example, the time to apply the zinc-copper coating 11 to the aluminum-coated steel substrate 2 may be between 0.5 minutes and up to 5.0 minutes. In this case, in particular, with increasing coating time, more and more copper and zinc are deposited, so that correspondingly the coating crystals grow up or more and more coating crystals are formed, so that the thickness of the nanocrystalline coating 4 increases overall over time. It is understood that other coating methods are possible for applying the nano-crystalline zinc-copper coating 11 on the aluminum-coated steel substrate, such as electrochemical (galvanic) deposition, PVD method, CVD method or Hot dip galvanizing.

Before applying the nano-crystalline zinc-copper coating 11, the surface of the aluminum coating 4 may optionally be activated in an intermediate step. The activation can be carried out, for example, by means of a pickling containing aluminum. By activating the outer oxide layer of the aluminum coating 4 is removed, so that favorable conditions for connecting the aluminum coating 4 are achieved with the zinc-copper coating 11 to be applied.

For example, the aluminum oxide layer can be removed wet-chemically using a zincate process. For this purpose, a pickling solution adapted to the substrate 2 can be used. When applying the mainly consisting of aqueous, NaOH-alkaline zinc oxide solution pickling forms Natriumzinkat (reaction 1). The zincate stain is therefore a sodium zincate solution with excess NaOH and usually strong basic pH.

(1) ZnO + 2NaOH + H2O => Na2Zn (OH) 4.

In the reaction of a pickling solution with a technical aluminum surface, the existing oxide layer dissolves to form sodium aluminate (reaction 2).

(2) Al 2 O 3 + 2 NaOH + 3 H 2 O => 2NaAl (OH) 4.

The upstream activation particularly good conditions for a high adhesion of the deposited zinc-copper coating 11 is achieved on the aluminum coating 4, so that an unwanted reoxidation of the aluminum is prevented. Instead of the pickling process described here by way of example for activating the surface, chemical processes or mechanical processes (grinding) are also possible.

After applying the nano-crystalline zinc-copper coating 11 in step S4, the boards 2 are hot-worked in step S5. During hot forming the semi-finished product is heated to a temperature which is usually above the AC1 or AC3 temperature of the material, for example between 750 ° C to 1000 ° C. The heating can be carried out selectively by suitable methods, such as, for example, by means of inductive heating, conductive heating, heating in the roller hearth furnace, contact heating by hot plates, infrared, or other known methods. After heating to Austenitisierungstemperatur the semi-finished product is then placed in a thermoforming mold 15 and formed therein and cooled or quenched so fast that at least partially a martensitic hardness structure formed in the molded part thus produced.

The hot forming (S5) can be carried out as a direct process after a first possibility. The board 2 is heated directly to Austenitisierungstemperatur and then hot-formed into the desired final contour in one step. A previous (cold) preforming does not take place here. The hot forming can also be carried out as an indirect process according to a second possibility, which comprises the substeps cold preforming, subsequent heating of the cold preformed component to Austenitisierungstemperatur and subsequent hot forming to produce the final contour of the molded part.

In FIG. 4C the surface of the hot-formed molding is shown greatly enlarged by means of a scanning electron microscope. In comparison with FIG. 4B It can be seen that the fine zinc-copper crystals present before hot-working ( FIG. 4B ) have bonded together after hot forming to larger grain structures ( FIG. 4C ).

In the FIGS. 5A and 5B is the layer structure of a steel substrate 2 coated according to the invention before ( FIG. 5A ) and after ( FIG. 5B ) shown hot working. These representations are the results of analysis by glow discharge spectroscopy (GDOES). Glow discharge spectroscopy refers to a spectroscopic method for the quantitative analysis of metals and other non-metallic solids. In the FIGS. 5A and 5B is the depth (d) of the workpiece on the x-axis indicated by the surface in micrometers (μm). On the y-axis, the mass concentration (k) of the respective alloy components is shown in percent. The mass concentration (k) is scaled with the values in the right boxes. The absolute proportions of the respective alloying elements (Fe, Si, O, Al, Cu, Zn) are given by the percentages readable on the y-axis multiplied by the respective scale value indicated in the box. For copper (Cu), for example, before hot-forming ( FIG. 5A ) on the surface (depth d = 0 .mu.m) has a value of about 76% (mass concentration k read on the y-axis) of the scale value of 10% given in the box, that is to say a total of about 7.6%. There is also a corresponding value for zinc of about 54%, for aluminum of about 22%, for silicon of about 13.7% (= 76% x 18%) and for oxygen of less than 1%, each on the surface of the workpiece (depth = 0 μm). At a depth of 1 μm, there is a typical distribution of aluminum-coated material with mass fractions of about 90% aluminum, 9% silicon and a balance of about 1% containing iron, copper, zinc and oxygen. From a depth of about 20 microns is the mass distribution of the steel substrate, here 22MnB5, in the present case about 98% iron and the rest distributed to the other alloying components.

After hot forming ( FIG. 5B ), the proportions of aluminum and silicon, starting from a depth between about 1 .mu.m and 20 .mu.m, have shifted into greater depths of more than 30 .mu.m into the steel substrate due to diffusion processes, and reduced accordingly. Thus, the proportion of aluminum after hot working at a depth of 1 .mu.m is about 40% and that of silicon is about 4%, whereas the proportion of iron has increased to about 50%. Also on the surface, that is to say in the area of the nano-crystalline zinc-copper coating 11, in particular at a depth of up to 1 μm, diffusion processes have led to a change in the mass concentration. For example, the proportion of zinc on the surface has been reduced from about 54% to about 50% and that of copper from about 7.6% before hot forming to 2% after hot working.

In the Figures 6A and 6B are scanning electronical recordings of the surfaces of a only with aluminum coating 4 (but not with zinc-copper coating) coated workpiece ( FIG. 6A ) and a workpiece coated according to the invention with aluminum coating 4 and zinc-copper coating 11 applied thereon ( FIG. 6B ) faced each other. For the substrate with aluminum coating (without further coating above) according to FIG. 6A is a relatively rough surface structure recognizable. In contrast, the substrate 2 according to the invention has applied to the aluminum coating 4 zinc-copper coating 11 according to FIG. 6B a very fine-grained or fine-crystalline surface structure.

A significant advantage of the present method is that the zinc-copper coating 11 in the heating device prevents or at least inhibits a reaction of water vapor with the aluminum coating 4. Consequently, the risk of hydrogen embrittlement is minimized, which has an overall positive effect on the strength and life of the molded part.

Experiments have shown that the uptake of diffusible hydrogen (Hdiff) in the production of moldings according to the inventive method compared to the production of moldings according to a conventional method in which no zinc-copper coating is applied to the aluminum coating, significantly reduced can be. This is due to the nanocrystalline zinc-copper coating 11, which forms a barrier against diffusible hydrogen.

In FIG. 7 shows the comparison results mentioned, wherein the amount of absorbed diffusible hydrogen (Hdiff) is indicated on the y-axis. The column to the left of the dashed line indicates the absorbed hydrogen for workpieces that have only been provided with aluminum coating 4 (but not zinc-copper coating). The three columns on the right of the dashed line indicate the absorbed hydrogen for workpieces that have been provided according to the invention with aluminum coating 4 and applied thereon zinc-copper coating 11, for different coating periods of one minute (left column), 3rd Minutes (middle column) and 5 minutes (right column). It can be seen that the Absorption of diffusible hydrogen (Hdiff) decreases with increasing duration or increasing thickness of the zinc-copper coating 11. Furthermore, it can be seen that the uptake of diffusible hydrogen (Hdiff) is significantly reduced in the samples according to the invention with zinc-copper coating 11 compared to the samples without zinc-copper coating 11 (Hdiff = 100%), namely by 54%. for samples with one minute coating time, 62% for a 3-minute coating period, and 60% for a 5-minute coating period.

A further advantage of the steel substrate coated according to the invention or of the method according to the invention is that the abrasion in the thermoforming tool 15 is reduced due to the zinc-copper coating 11.

Experiments in connection with the heating of the boards in the course of hot working (S5) have further shown that the coated steel substrate 2 according to the invention with aluminum coating and nano-crystalline zinc-copper coating compared to conventional steel substrate, the only aluminum coating but has no zinc-copper coating, heats up much faster. This effect is also due to the nanocrystalline zinc-copper coating 11, which has a dark surface, which consequently has a higher degree of absorption of heat.

FIG. 8 FIG. 12 shows the heating rates of steel substrate 2 coated according to the invention, shown in solid line, and conventional aluminum-coated steel substrate, shown as a dot-dash line. On the y-axis the temperature is given in degrees Celsius T (° C); on the x-axis the time in seconds t (s). It can be seen that the Ac3 temperature, which is about 870 ° C. for 22MnB5, is reached after about 100 seconds in the case of the steel substrate 2 with aluminum coating 4 and zinc-copper coating 11 coated according to the invention, while the aluminum coated substrate with about 130 seconds takes much longer. Thus, the residence time of the steel substrate 2 according to the invention can be reduced in the heating device, resulting in faster cycle times.

Finally, another advantage is that the zinc-copper coating 11 in a later step may be phosphated, or that a phosphate layer adheres to the zinc-copper coating 11, which is not the case with an aluminum-coated surface. The phosphate layer improves the adhesion of a cathodic dip coating to be applied later to the molded part, which in turn leads to improved corrosion protection.

It is understood that the process control according to the invention can also be modified. For example, intermediate steps which are not separately shown here can also be provided between the mentioned steps. For example, after the application of the nanocrystalline zinc-copper coating 11, the coated steel substrate 2 can be provided with a protective layer, for example a lacquer, which protects the coating until hot-forming.

FIG. 2 shows a method according to the invention for producing a molded part from a steel substrate 2 according to a second process procedure. This largely corresponds to the method according to FIG. 1 , so that reference is made to the above description in terms of similarities. The same or modified components or steps are provided with the same reference numerals, as in FIG. 1 , In the following, the differences between the present method will be discussed.

The process steps S1 (application of the aluminum coating 4), S2 (rolling) and S5 (hot working) correspond to the process steps S1, S2 and S5 according to FIG FIG. 1 ,

The only difference of the present process management consists in the order of the steps of applying the nano-crystalline coating 11 (step S3) and singulating (step S4), which compared to the method according to FIG. 1 are turned over. That is, according to the present method FIG. 2 After the rolling (S2), the band-shaped aluminum-coated steel substrate 2 is provided with the nano-crystalline zinc-copper coating 11 in step S3. This is done in a continuous process by a filled with electrolytic liquid 12 immersion tank 13. Dabei the application of the nano-crystalline zinc-copper coating 11 takes place as in the embodiment according to FIG. 1 with a zincate stain. With regard to all other details, reference is made to the above description to avoid repetition.

After the application of the nano-crystalline coating 11, the separation into boards takes place in step S4. This is done as in the method according to FIG. 1 , to the description of which reference is made. After separation, there is a board provided with aluminum coating 4 and with nano-crystalline zinc-copper coating 11, which is subsequently supplied to the hot forming. Hot forming takes place as in the process FIG. 1 , to the description of which reference is again made. All details according to the FIGS. 3 to 8 apply to the present method FIG. 2 alike.

LIST OF REFERENCE NUMBERS

2

steel substrate

3

coil

4

first coating

5

melting bath

6

coater

7

roll

8th

roll

9

thickness control

10

cutter

11

second coating

12

electrolytic liquid

13

plunge pool

14

coater

15

Thermoforming tool

D

thickness

d

depth

Hdiff

diffusible hydrogen

k

mass concentration

R

direction

S1-S6

steps

T

temperature

t

Time

Claims (15)

A coated steel substrate for hot working comprising: a first coating (4) containing at least 85 weight percent aluminum, and a second coating (11) overlying the first coating (4), characterized in that the second coating (11) is a nano-crystalline zinc-copper coating which, when applied, contains at least 60% by weight of zinc and at least 5% by weight of copper. Coated steel substrate according to claim 1,
characterized,
that the second coating (11) in the applied state 80 to 90 weight percent zinc and 10 to 20 weight percent copper. Coated steel substrate according to claim 1 or 2,
characterized,
that the second coating (11) contains up to 5 percent by weight iron and / or up to 10 weight percent nickel. Coated steel substrate according to claim 1 or 2,
characterized,
that the second coating (11) contains only zinc and copper. Coated steel substrate according to any one of claims 1 to 4,
characterized,
that the second coating (11) has a thickness (D11) of up to two microns (2 microns), in particular of up to 800 nanometers (800 nm), and / or
that the second coating (11) of at least 10 nanometers has a thickness (D11) (10 nm), especially of at least 200 nanometers (200 nm). Coated steel substrate according to any one of claims 1 to 5,
characterized,
in that the second coating (11) is applied by means of electroless deposition, electrodeposition, by a galvanic process, PVD process, CVD process or fire-coating process. Method for producing a hardened molded part with the steps: Coating a steel substrate (2) with an aluminum coating (4), wherein the aluminum coating (4) contains at least 85 percent by weight aluminum in a state applied to the steel substrate (2), Applying a nano-crystalline zinc-copper coating (11) which, when applied, contains at least 60% by weight of zinc and 5% by weight of copper, wherein the application of the nano-crystalline zinc-copper coating (11) after coating (S3) of the Steel substrate (2) with the aluminum coating (4) takes place; Working out a board from the steel substrate (2); Hot forming the board to produce a cured molding. Method according to claim 7,
characterized,
that the nano-crystalline zinc-copper coating (11) by particular electroless deposition is applied to the steel substrate (2). Method according to claim 7 or 8,
characterized,
in that the nano-crystalline zinc-copper coating (11) is applied by means of an alloyed zincate stain which contains in particular at least sodium hydroxide, zinc oxide, complexing agents and metal salts such as iron and copper. Method according to one of claims 7 to 9,
characterized,
that the nano-crystalline zinc-copper coating (11) is applied to the steel substrate (2) within a time of 0.5 to 5.0 minutes. Method according to one of claims 7 to 10,
characterized,
in that the surface of the aluminum coating (4) is activated prior to the application of the nano-crystalline zinc-copper coating, in particular by means of pickling in an aluminum-containing pickle, wherein an outer oxide layer of the aluminum coating is removed. Method according to one of claims 7 to 11,
characterized,
that is provided as a further step:
Rolling, in particular flexible rolling, wherein the rolling is carried out before or after the coating of the steel substrate (2) with the aluminum coating (4). Method according to one of claims 7 to 12,
characterized,
that the working out of the board from the steel substrate (2) before or after the coating of the steel substrate (2) with the nano-crystalline zinc-copper coating (11). Method according to one of claims 7 to 13,
characterized,
that the hot forming of the board includes the following steps: Heating at least a portion of the board to austenitizing temperature; Inserting the board into a thermoforming tool (15); Hot forming and cooling of the board in the thermoforming mold (15) to produce a cured molding. Method according to one of claims 7 to 14,
characterized,
that is provided as a further step:
Phosphating the cured molding.

Images