EP3585917B1 - Method for coating steel sheets or steel strips and method for producing press-hardened components therefrom - Google Patents

Description

The invention relates to a method for coating a steel sheet or steel strip, to which an aluminum-based coating is applied in the hot-dip process and the surface of the coating is freed from a native aluminum oxide layer. The invention also relates to a method for producing press-hardened components from these steel sheets or steel strips with an aluminum-based coating.

In the following, aluminum-based coatings are understood to mean metallic coatings in which aluminum is the main component in percent by mass. Examples of possible aluminum-based coatings are aluminum, aluminum-silicon (AS), aluminum-zinc-silicon (AZ), as well as the same coatings with admixtures of additional elements such as magnesium, manganese, titanium and rare earths.

It is known that hot-formed steel sheets are being used more and more, especially in automobile construction. The process, also known as press hardening, can be used to produce high-strength components that are primarily used in the bodywork area. Press hardening can in principle be carried out using two different method variants, namely using the direct or indirect method. While the process steps of forming and hardening run separately in indirect processes, they take place together in one tool in the direct process. In the following, however, only the direct method is considered.

In the direct process, a sheet steel blank is heated to the so-called austenitizing temperature (Ac3), then the heated blank is transferred to a molding tool and formed in a one-step forming step to form the finished component, and the cooled molding tool at the same time at a speed that is above the critical cooling rate of the steel is cooled, so that a hardened component is produced.

Well-known hot-formable steels for this area of application are, for example Manganese-boron steel "22MnB5" and recently also air-hardenable steels according to the European patent EP 2 449 138 B1 .

In addition to uncoated steel sheets, steel sheets with anti-scaling protection are also used in the automotive industry for press hardening. In addition to the increased corrosion resistance of the finished component, the advantages here are that the blanks or components do not scale in the furnace, which reduces wear on the press tools due to flaked scale and the components do not have to be blasted before further processing.

For press hardening, the following (alloy) coatings applied by hot dipping are currently known: aluminum-silicon (AS), zinc-aluminum (Z), zinc-aluminum-iron (ZF / galvannealed), zinc-magnesium-aluminum-iron (ZM), as well as electrolytically deposited coatings made of zinc-nickel or zinc, the latter being converted into an iron-zinc alloy layer before hot forming. These anti-corrosion coatings are usually applied to the hot or cold strip in a continuous process.

The production of components by quenching preliminary products made of press-hardenable steels by hot forming in a forming tool is out of the German patent DE 601 19 826 T2 known. Here, a sheet metal blank previously heated above the austenitizing temperature to 800 - 1200 ° C and possibly provided with a metallic coating made of zinc or based on zinc is formed into a component in a tool that is cooled in some cases by hot forming, with rapid heat extraction during the forming process The sheet metal or component in the forming tool undergoes quench hardening (press hardening) and the required strength properties are achieved through the resulting martensitic hardening structure.

The production of components by quenching precursors made of press-hardenable steels coated with an aluminum alloy by hot forming in a forming tool is out of the German patent DE 699 33 751 T2 known. Here, a sheet metal coated with an aluminum alloy is heated to over 700 ° C before forming, with an intermetallic alloyed compound based on iron, aluminum and silicon being formed on the surface, followed by the Sheet metal formed and cooled at a rate above the critical cooling rate.

From the German Offenlegungsschrift DE 10 2016 102 504 A1 an aluminum-based coating for steel sheets and strips and a method for their production is known. The coating comprises an aluminum-based coating that is applied in a hot-dip process. Subsequently, a randomly formed layer created by atmospheric oxidation is removed in an upstream alkaline pretreatment with subsequent acidic pickling in some cases. A cover layer, which contains aluminum oxide and / or hydroxide and is produced by means of anodic oxidation, plasma oxidation or hot water treatment, is in turn applied to the coating freed from the randomly formed layer. The mean thickness of the top layer is less than 4 µm and more than 0.1 µm.

The disclosure document EP 2 045 360 A1 discloses a method for producing a steel component which is coated with an aluminum coating, which is then provided with a zinc coating. The aluminum coating contains at least 85% by weight Al and optionally up to 15% by weight Si; the zinc coating is at least 90% by weight Zn. Between the aluminum and zinc coating, the flat steel product provided with the aluminum coating can advantageously be masked in order to improve the surface roughness of the aluminum coating.

Also in the German patent application DE 10 2009 007 909 A1 a method for producing a steel component is disclosed which is provided with an aluminum coating and then with an aluminum coating. The flat steel product provided with the aluminum coating and the aluminum coating is additionally coated with a top layer which contains at least one metallic salt of phosphoric acid as its main component. Possible metals for metal phosphate formation include Fe, Mn, Ti, Co and V, from which group only Mn is described as being particularly advantageous. The layer to be coated or the flat steel product can be cleaned between the individual coating steps.

The advantage of aluminum-based coatings is that, in addition to a larger process window (e.g. with regard to the heating parameters), the finished components do not have to be blasted before further processing. In addition, there is no risk of liquid metal embrittlement with aluminum-based coatings compared to zinc-based coatings and no microcracks can form in the near-surface substrate area at the former austenite grain boundaries, which can have a negative effect on the fatigue strength at depths of more than 10 µm.

The disadvantage of using aluminum-based coatings, e.g. made of aluminum-silicon (AS), is, however, the inadequate paintability of the formed component in the cathodic dip painting (KTL) typical of automobiles if too short a heating time was used for press hardening. In the case of short heating times, the KT-lacquered substrate shows insufficient corrosion resistance.

In contrast to the zinc-based coatings, aluminum-based coatings cannot be phosphated or can only be phosphated insufficiently and thus no improvement in the corrosion resistance can be achieved by the phosphating step. For these reasons, so far, when processing boards with aluminum-based coatings by means of press hardening, minimum heating times of the board have to be observed, as a result of which the coating is alloyed with iron and a surface is formed that causes the painted component to be sufficiently resistant to corrosion.

The alloying of the coating with iron and the formation of a corrosion-resistant surface, however, require a correspondingly long dwell time in the roller hearth furnace which is usually used, which means that long furnaces are necessary in order to enable sufficient cycle times. The profitability of press mold hardening is thus reduced. Longer ovens are more expensive to buy and operate, and they also take up a lot of space. The minimum dwell time is thus determined by the coating and not by the base material, for which only the required austenitizing temperature would be required. In addition, the corrosion resistance is reduced by the stronger alloying with iron, since the aluminum content in the Alloy layer decreases with furnace dwell time and the iron content increases.

Another disadvantage of known AS coatings is that with very short annealing times, that is, when the coating is not alloyed with the base material, the resistance spot welding process (WP welding) of the press-hardened component is extremely poor. This is expressed, for example, in only a very small welding area. One of the reasons for this is a very low contact resistance with short glow times.

From the published patent application DE 10 2015 210 459 A1 a method for hot forming a steel component is known, which is heated in a heat treatment step to an area of complete or partial austenitization, and the heated steel component is both hot formed and quench hardened in a forming step, the heat treatment step being preceded by a first pretreatment step in which in one the steel component is provided with a corrosion-resistant protective layer to protect against scaling in the heat treatment step. Before the heat treatment step is carried out, a surface oxidation takes place in a second pretreatment step, in which a non-reactive, corrosion-resistant oxidation layer is formed on the scale protection layer, by means of which abrasive tool wear is reduced in the forming step. In terms of process technology, surface oxidation can take place, for example, by pickling passivation.

One of the disadvantages of the prior art described there is that the aluminum-silicon coating results in a rough, hard surface structure of the steel component, which leads to severe tool wear during press hardening. The roughness of the metal surface of the steel component should be reduced by means of the additional oxidation layer, which should reduce the abrasive tool wear in the forming step.

The disadvantage here, however, is that a surface oxidation before the heat treatment due to the reduction in the surface roughness does not improve the paint adhesion on the press-molded component and the weldability. In addition, the additional step of surface oxidation is time-consuming and energy-consuming and thus increases manufacturing costs considerably.

The object of the invention is therefore to provide a cost-effective method for coating steel sheets or steel strips which provides excellent suitability of the steel sheets or steel strips for the production of components by means of press hardening and their further processing. In particular, the dwell time in the furnace should be reduced and, nevertheless, good WP weldability and corrosion resistance on the press-hardened component should be guaranteed after painting. Furthermore, a method for the production of press-hardened components from such steel sheets or steel strips is to be specified.

The teaching of the invention comprises the coating of a steel sheet or steel strip to which an aluminum-based coating is applied in the hot-dip process and the removal of a native aluminum oxide layer from the surface of the coating, characterized in that transition metals or transition metal compounds are then formed on the liberated surface of the coating be deposited in an edition.

The previously used term exempt is to be understood in the sense of, as far as technically possible, exempt from the native aluminum oxide layer.

In this case, the overlay is preferably an areal deposit. Accordingly, there can be an overlay over the entire surface or an overlay that is not necessarily covering. The covering overlay can be network-like with an ordered or disordered structure or distribution, which is then a layer of point-like overlay and imperfections.

A coating with a layer weight - based on iron - is deposited ^{in the range from 7 to 25 mg / m 2} , preferably 10 to 15 mg / m ^2.

Furthermore, the teaching of the invention comprises a method for the production of press-hardened components from steel sheets or steel strips with an aluminum-based coating, the steel sheets or steel strips treated according to the invention being heated to a temperature above Ac3 at least in some areas with the aim of hardening, then at this temperature reshaped and then cooled with the aim of hardening at a rate that is at least partially above the critical cooling rate.

It is known that pure Al ₂ O _{3 has} an almost optimal Pilling-Bedworth ratio, which promotes the formation of effective passive layers. Extensive investigations have shown that the aluminum oxide layers formed during the heat treatment in the course of the press mold hardening of untreated AS coatings, usually less than 10 nm, remain extremely thin and are therefore ineffective with regard to the required improvement in resistance spot weldability and corrosion resistance.

An aluminum oxide layer with mixed oxides of the metals and / or their compounds is advantageously formed on the coating with the applied metals and / or their compounds under an atmosphere with oxygen or under water vapor. Surprisingly, the investigations found that by removing the native oxide layer of an AS coating, followed by the deposition of certain metals or their compounds (preferably Fe and its compounds), which _{can form mixed oxides with Al 2} O ₃ (e.g. corundum, eskolaite , Hematite, karelianite, tistarite, ilmenite, perovskite and / or spinel), the renewed formation of a thin aluminum oxide layer is prevented before and during the heat treatment. The aluminum oxide layer with the mixed oxides is preferably formed in a furnace with a temperature> 750 ° C., preferably from 850 to 950 ° C., and an oven dwell time> 90 s, preferably 120 to 180 s.

Instead, an aluminum-rich oxide layer is formed, which is doped with cations from the previously deposited substances. These cations suppress the self-limitation of the oxide layer growth described above and thus enable the growth of significantly thicker aluminum oxide layers during the heat treatment, whereby oxide layer thicknesses of over 80 nm can be achieved, which, compared to thinner aluminum oxide layers, result in significantly better resistance spot weldability and better corrosion behavior in the KT-coated state .

The essence of the invention is that the Al-based metallic coating is chemically treated, especially before the heat treatment, so that it is freed from its native oxide layer and certain metals or their compounds that can form mixed oxides _{with Al 2} O ₃ deposited on the surface of the coating. These prevent the formation of a pure aluminum oxide layer during the heat treatment prior to press hardening. Instead, the deposited substances are partially or completely incorporated into the newly formed oxide layer.

As a result of this doping with metal or transition metal cations, the oxide layer grows in the course of the heat treatment to a much greater thickness (> 80 nm) than with untreated Al-based coatings (<10 nm). A self-limitation of the aluminum oxide growth is avoided.

Unlike in the Offenlegungsschrift DE 10 2015 210 459 A1 described, the core property-improving modification of the AS surface, namely the creation or formation of a thick aluminum oxide layer, is not carried out before the heat treatment, but in-situ, in the course of the heat treatment for press hardening. The thick aluminum oxide layer that determines the properties only grows in the course of the heat treatment in the furnace.

The technical advantage is that the in-situ generation of the oxide layer saves resources and energy and can be implemented highly efficiently with simple and existing system technology.

In the process according to the invention, very thick oxide layers of up to 250 nm are formed under the furnace dwell times described in Table 1 at an furnace temperature of 950 ° C. Components produced according to the invention have the large welding areas in resistance spot welding described in Table 2 and very good corrosion resistance in the KT-lacquered state 3, if they are tested in the alternating corrosion test in accordance with Volkswagen PV1210.

The treatment according to the invention consists of the application of transition metals or transition metal compounds from the group consisting of titanium, vanadium, chromium, iron and manganese and / or their compounds, almost completely iron and / or its compounds on the Al-based metallic coating by means of chemical deposition, preferably in a wet chemical process. This consists at least of the application of a solution of compounds of the elements listed above, which react in an external current-free reaction with the Al-based metallic coating. The term no external current is used in the sense of not electrolytic. Chemical deposition is preferably carried out by means of a spray, dip, or roller application.

Provision is also preferably made for the removal of the native oxide layer formed in the atmosphere and the chemical deposition to take place in a single process step. For this purpose, the two treatment steps can be carried out in a continuously operating coating system that is connected downstream of a hot dip coating system or is separate from the hot dip coating system.

This treatment is preferably carried out in the presence of compounds of other metals, for example from the group consisting of cobalt, molybdenum and tungsten and / or their compounds. For example, molybates, tungstates or cobalt nitrate noticeably accelerate the deposition of iron, but are only deposited to a small extent themselves, which makes the method according to the invention even more efficient. However, iron or its compounds are preferably deposited because iron or iron compounds are readily available, inexpensive and non-toxic. In addition, iron is already contained in the base material.

The removal of the native oxide layer and deposition of the substances according to the invention can advantageously also be carried out simultaneously in a single wet-chemical step when using alkaline media. Such deposition processes can be carried out in continuously operating systems at belt speeds of up to 120 m / min or more. The amount of active ingredient required can be less than 100 mg / m ² .

According to the invention, the metals and their chemical compounds can also be applied by electrolytic deposition. For this purpose, the native oxide layer of the Al-based coating (eg AS) is removed with alkaline pickling, rinsed and the metal or the chemical compound is electrochemically deposited from an electrolyte. In the case of the electrochemical Aftertreatment in aqueous media is advantageously maintained at an electrolyte temperature of 20 ° C. to 85 ° C. and current densities between 0.05 and 150 A / dm ^{2 are used} . When using ionic liquids for metal deposition, electrolyte temperatures greater than or equal to 85 ° C. can also be used. The treatment of the metal strip can be carried out in a continuous strip line with process speeds of up to 120 m / min or more.

The inventive treatment of the aluminum-based coating, consisting of the removal of the native oxide layer and subsequent treatment of the AS surface with metal-containing solutions, can also shorten the minimum dwell time in the furnace during the subsequent further processing of the steel sheet by hot forming or press hardening which significantly increases productivity. In the case of untreated AS coatings, the minimum dwell time in the furnace for the oxide layer to grow is determined by the weldability requirements in resistance spot welding and the corrosion resistance in the KT-lacquered state.

During the investigations it was found that from a layer weight of approx. 10 mg / m ² the active ingredient applied to the AS surface, based on the guiding element iron, shows a significant shortening of the minimum holding time in the heat treatment. Specifically, a 1.2 mm thick substrate of a steel alloy suitable for press form hardening (22MnB5) with an AS coating (150g / m ² ) with an iron layer of approx. 15 mg / m ^{2 exhibited} after an oven dwell time of 3 minutes at 950 ° C Oven temperature properties which, in the case of untreated specimens of the same sheet thickness, are only reached after 6 minutes in the oven. The necessary oven dwell time could thus be halved compared to the standard process.

the Figures 1 and 2 show the depth profile for the elements Al, Fe and O after press hardening of sheet metal with an AS coating with a treatment according to the invention with an iron-containing solution ( Figure 2 ) compared to an untreated sheet ( Figure 1 ) with an oven dwell time of 6 minutes and an oven temperature of 950 ° C in an air atmosphere. In Figure 2 The deeper introduction of oxygen in the sample treated according to the invention is clearly recognizable, which indicates a significantly thicker oxide layer compared to the untreated sample. In addition, the accumulation of iron in the oxide layer can be clearly seen.

The inventive treatment of the surface of the coated steel strip can advantageously be carried out in a treatment part downstream of the process part of a continuously producing hot dip coating system or in a separate system, for example via spray bars with nozzles, in a dipping process and by means of electrolytic deposition or spray electrolysis, in each case also in combination. The separate system can be, for example, a coil coating or an electrolytic coil finishing system. An alkaline cleaning prior to the treatment according to the invention and subsequent rinsing of the steel sheet or steel strip provided with an aluminum-based coating advantageously removes the (native) oxide layer created by atmospheric oxidation and thus creates a defined initial state for the deposition of metallic species according to the invention.

According to the invention, the surface can be treated over the entire surface of the belt or only partially or on one or both sides. In the case of the electroless treatment, the concentration of the feed solution, its temperature, the spray pressure, the shear of the sprayed solution relative to the surface of the metal strip to be treated and the volume brought into contact with the surface can change the molar amount of the deposited metal species. In the case of electrolytic deposition, the molar amount of metal species deposited is determined by the composition of the electrolyte, flow conditions, temperature, current density and treatment time.

Embodiments:

Pre-treatments of the samples according to the invention are, for example, as follows: The AS-coated sheet is subjected to an immersion treatment in an alkaline solution containing metal cations at a temperature of 50 ° C. for a few seconds. The native oxide layer is removed and the iron-containing layer is applied.

Alternatively, the AS-coated sheet metal is subjected to an immersion treatment in a 20% sodium hydroxide solution at room temperature for 30 s to remove the native oxide layer. This is followed by rinsing with fully demineralized water Water. This is followed by the electrolytic deposition of an iron-containing layer at an electrolyte temperature of 50 ° C. The deposition takes place for 1 or 10 s at a current density of 23 A / dm ² .

• Ofentemperatur für die Wärmebehandlung: 950 °C
• Atmosphäre: Umgebungsluft
• Ofenverweildauer (bei Blechdicke bis 1,5 mm): 2, 3, 4, 6 min
• danach Abkühlen im gekühlten Flachwerkzeug auf <200°C

Test parameters for press hardening

• Oven temperature for heat treatment: 950 ° C
• Atmosphere: ambient air
• Oven dwell time (for sheet thicknesses up to 1.5 mm): 2, 3, 4, 6 min
• then cooling in the cooled flat tool to <200 ° C

Table 1 shows for the purely wet-chemical pretreatment of the samples that the thickness of the aluminum oxide layers increases significantly with increasing active ingredient coverage (Fe) and the length of time in the furnace. Without treatment according to the invention, the layer thickness of the oxide layer is less than 10 nm. With an iron ^{deposition of about 7 mg / m 2} and a residence time of 2, 3 or 4 minutes, no significant layer formation is achieved. This also applies to an iron application of approx. 11 mg / m ² and a residence time of 2 minutes. Table 1: Layer formation on the sample surface depending on the iron layer and the duration of the furnace Iron level / mg / m ² Oven dwell time / min 2 3 4th 6th Layer thickness of the top layer / nm approx. 7 no significant stratification 170 approx. 11 140 200 230 approx. 15 150 220 230 250

Table 2 shows that the pretreated and press-hardened AS specimens with an iron-containing coating already show a pronounced weld area even after short annealing times. Without treatment according to the invention, there is no measurable welding area with short annealing times. Table 2: Welding area according to SEP1220-2 depending on the iron layer and annealing time Iron level / mg / m ² Oven dwell time / min 2 3 4th 6th Welding area / n / a approx. 7 2.2 2.1 2.1 1.2 approx. 11 2.2 2 1.7 1.7 approx. 15 2.5 2.1 1.7 1.6

The infiltration of the scratch after 12 weeks in the Volkswagen PV 1210 corrosion test is lower on samples with the treatment according to the invention than on untreated samples, as shown in Table 3. Table 3: Infiltration after 12 weeks of Volkswagen PV 1210 on KT-lacquered samples depending on the iron coating and annealing time Oven dwell time / min Iron coverage / mg / m ² Infiltration (UW) at the scratch / mm after 12 weeks VW PV1210 2 approx. 11 UW <1 approx. 15 UW <1 3 approx. 7 UW <1 approx. 11 UW <1 approx. 15 UW <1 4th approx. 7 UW <1 approx. 11 UW <1 approx. 15 UW <1 6th approx. 7 UW <1 approx. 11 UW <1.5 approx. 15 UW <1.5 without treatment according to the invention 2.5 0 UW> 2 or severe filiform corrosion 6th 0 1.5 <UW <2

Figure 3 shows an example of a cross-section on a sheet metal section with AS coating and treatment ^{according to the invention, deposited without external current, with an iron coating of approx. 15 mg / m 2} after press hardening. The oven dwell time was 3 minutes at an oven temperature of 950 ° C. under an air atmosphere.

Here A denotes the base material; B the diffusion zone consisting of a Matrix of the base material into which Al and Si have diffused from the coating; C a layer rich in Fe-Al phases; D the alloy zone consisting of different Al-Fe, Al-Fe-Si phases; E the oxide layer made of aluminum and iron oxide; F the investment.

Claims (14)

1. Method for coating a steel sheet or steel strip, to which an aluminum-based coat is applied in a hot-dip process, the surface of the coat being freed from a naturally occurring aluminum oxide layer and then transition metals or transition metal compounds being deposited on the freed surface of the coat in order to form a top layer, characterized in that the top layer is deposited as a planar deposit and having a layer weight-based on iron-in the range of 7 to 25 mg/m², preferably 10 to 15 mg/m², the transition metals or the transition metal compounds comprising at least one chemical element from the group of titanium, vanadium, chromium, manganese, or iron and/or the compounds thereof and the transition metals or the transition metal compounds predominantly or almost completely comprising iron or the compounds thereof.
2. Method according to claim 1, characterized in that the transition metals or the transition metal compounds are deposited in the presence of at least one further chemical element from the group of cobalt, molybdenum, tungsten, and/or the compounds thereof.
3. Method according to either claim 1 or claim 2, characterized in that the transition metals or the transition metal compounds are deposited by a chemical deposition, in particular by means of a spraying, dipping, or rolling application.
4. Method according to at least one of claims 1 to 3, characterized in that the removal of the atmospherically occurring, natural oxide layer and the chemical deposition are performed in a single method step.
5. Method according to claim 4, characterized in that the two treatment steps are performed in a continuously operating coating installation which is located downstream of a hot-dip coating installation or is separate from the hot-dip coating installation.
6. Method according to at least one of claims 1 to 3, characterized in that the transition metals or the transition metal compounds are deposited electrolytically.
7. Method according to claim 6, characterized in that the transition metals or transition metal compounds are applied electrolytically in an aqueous medium as the electrolyte at an electrolyte temperature of 25°C to 85°C and current densities between 0.05 and 150 A/dm².
8. Method according to at least one of claims 1 to 7, characterized in that an aluminum oxide layer having mixed oxides from the top layer is formed on the coat with the top layer when exposed to an oxygen atmosphere or when exposed to steam.
9. Method according to claim 8, characterized in that the aluminum oxide layer with the mixed oxides is formed in a furnace at a temperature of >750°C, preferably from 850 to 950°C, and a furnace dwell time of >90 s, preferably 120 to 180 s.
10. Method according to at least one of claims 1 to 9, characterized in that the formation of the mixed oxides prevents self-limitation of the layer growth of the aluminum oxide.
11. Method according to at least one of claims 8 to 10, characterized in that corundum, eskolaite, hematite, karelianite, tistarite, ilmenite, perovskite, and/or spinel are formed as mixed oxides.
12. Method according to at least one of claims 1 to 11, characterized in that aluminum, aluminum-silicon (AS), or aluminum-zinc-silicon (AZ) with optional admixtures of additional elements, such as magnesium, manganese, titanium, and rare earths, is applied to the sheet steel or steel strip as an aluminum-based coat.
13. Method for producing press-hardened components consisting of steel sheets or steel strips having an aluminum-based coat produced according to at least one of claims 1 to 12, characterized in that the steel sheets or steel strips are heated at least in regions to a temperature above Ac3, are subsequently formed at this temperature and thereafter are cooled, with the aim of hardening, at a rate which is above the critical cooling rate at least in regions.
14. Method according to any of claims 1 to 13, characterized in that a steel that can be hardened by heat treatment, preferably a heat-treatable steel alloyed with manganese and boron, particularly preferably 22MnB5, is used for the steel sheets or steel strips.

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