EP3585917A1

EP3585917A1 - Method for coating steel sheets or steel strips and method for producing press-hardened components therefrom

Info

Publication number: EP3585917A1
Application number: EP18714124.7A
Authority: EP
Inventors: Frank Beier; Kerstin Körner; Marc Debeaux
Original assignee: Salzgitter Flachstahl GmbH
Current assignee: Salzgitter Flachstahl GmbH
Priority date: 2017-02-21
Filing date: 2018-02-14
Publication date: 2020-01-01
Anticipated expiration: 2038-02-14
Also published as: WO2018153755A1; EP3585917B1; KR102285532B1; KR20190115001A; RU2729674C1; US20200232057A1; US11613791B2

Abstract

The invention relates to a method for coating a steel sheet or steel strip to which an aluminium-based coating is applied in a dip-coating process and the surface of the coating is freed of a naturally occurring aluminium oxide layer. In order to provide a low-cost method for coating steel sheets or steel strips that makes the steel sheets or steel strips outstandingly suitable for the production of components by means of press hardening and for the further processing thereof, it is proposed that transition metals or transition metal compounds are subsequently deposited on the freed surface of the coating to form a top layer. The invention also relates to a method for producing press-hardened components from the aforementioned steel sheets or steel strips with an aluminium-based coating.

Description

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

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

As aluminum-based coatings are hereinafter understood metallic coatings in which aluminum is the main component in mass percent.

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, e.g. Magnesium, manganese, titanium and rare earths.

It is known that hot-formed steel sheets are used more and more frequently, especially in the automotive industry. The process, also known as press hardening, enables the production of high-strength components, which are mainly used in the bodywork area. The press-hardening can basically be carried out by means of two different process variants, namely by means of the direct or indirect process. While in indirect processes, the process steps of forming and hardening run separately from each other, they take place together in direct process in a tool. In the following, however, only the direct method is considered.

In the direct process, a sheet steel plate on the so-called

Austenitisierungstemperatur (Ac3) heated, then the so-heated board is transferred to a mold and formed in a one-step forming step to the finished component and thereby cooled by the cooled mold simultaneously with a speed that is above the critical cooling rate of the steel, so that a hardened component is generated. Known thermoformable steels for this application are, for example, the Manganese-boron steel "22MnB5" and recently also air-temperable steels according to the European patent EP 2 449 138 B1.

In addition to uncoated steel sheets are also steel sheets with a

Scaling protection for press hardening used by the automotive industry. The advantages here are in addition to the increased corrosion resistance of the finished component in that the boards or components do not scale in the oven, whereby the wear of the press tools is reduced by chipped scale and the components must not be laboriously blasted before further processing.

For press hardening, the following hot-dip (alloy) coatings are currently known: aluminum-silicon (AS), zinc-aluminum (Z), zinc-aluminum-iron (ZF / galvannealed), zinc-magnesium-aluminum-iron (ZM), as well as electrodeposited coatings of zinc-nickel or zinc, the latter being converted into an iron-zinc alloy layer before hot-forming. These anticorrosion coatings are usually applied to the hot or cold strip in continuous flow processes.

The manufacture of components by quenching precursors

Press-hardenable steels by hot forming in a forming tool is known from German patent DE 601 19 826 T2. Here is a previously heated above the Austenitisierungstemperatur to 800 - 1200 ° C and possibly provided with a metallic coating of zinc or based on zinc sheet metal blank formed in a case by case cooled tool by hot forming into a component, during the forming by rapid heat removal the Sheet metal or component undergoes a quench hardening (press hardening) in the forming tool and the required martensitic hardness structure

Strength properties achieved. The production of components by quenching of aluminum alloy-coated precursors of press-hardenable steels by hot forming in a forming tool is known from German Patent DE 699 33 751 T2. Here, a coated with an aluminum alloy sheet is heated to above 700 ° C before forming, with an intermetallic alloyed compound based on iron, aluminum and silicon is formed on the surface and subsequently the Sheet formed and at a speed above the critical

Cooling rate cools down.

From the German patent application DE 10 2016 102 504 A1 is a

Aluminum-based coating for steel sheets and strips and a method for their production known. The coating comprises an aluminum-based coating applied in a hot-dip process. Subsequently, a randomly formed by atmospheric oxidation, arbitrarily formed layer is removed in a preceding alkaline pretreatment with case wise subsequent acid pickling. On the coating freed from the arbitrarily formed layer, a covering layer is again applied which contains aluminum oxide and / or hydroxide and is produced by means of anodic oxidation, plasma oxidation or hot water treatment. The average thickness of the cover layer is less than 4 μηη and more than 0.1 μηη.

The published patent application EP 2 045 360 A1 discloses a method for the production of a steel component which is coated with an aluminum coating which is subsequently provided with a zinc coating. The aluminum coating contains at least 85% by weight of Al and optionally up to 15% by weight of Si; the zinc coating at least 90 wt .-% Zn. Between aluminum and zinc coating can advantageously a Dekapieren provided with the aluminum coating

Flat steel products are made to the surface roughness of the

Aluminum coating to improve. Also in the German patent application DE 10 2009 007 909 A1 is a

Discloses a method for producing a steel component, which is provided as it were with an aluminum coating and then with an aluminum coating. The provided with the aluminum coating and the aluminum coating

Flat steel product is additionally coated with a topcoat, which as

Main component contains at least one metallic salt of phosphoric acid.

Possible metals for the formation of metal phosphate include Fe, Mn, Ti, Co and V, with only Mn being described as particularly advantageous from this group. In each case, a cleaning of the layer to be coated or of the flat steel product can take place between the individual coating steps. The advantage of the aluminum-based coatings is that in addition to a larger process window (eg with regard to the heating parameters), the finished components do not have to be blasted prior to further processing. In addition, with aluminum-based coatings over zinc-based coatings there is no risk of liquid metal embrittlement and no microcracks in the near-surface substrate area can form on the former Austenitkorngrenzen, which may have a negative effect on the fatigue strength at depths over 10 μηη. A disadvantage of the use of aluminum-based coatings, for example

Aluminum-silicon (AS), however, is the inadequate paintability of the formed component in the automotive-typical cathodic dip coating (KTL), when too short a heating time was used in press hardening. In short

Heating times are insufficient for the KT-coated substrate

Corrosion resistance on.

In contrast to the zinc-based coatings, aluminum-based coatings can not or only insufficiently phosphating and thus can not be achieved by the phosphating step improvement in corrosion resistance. For these reasons, so far in the processing of boards with

aluminum-based coatings by means of press hardening minimum heat-up times of the board are maintained, whereby the coating is ironed with iron and forms a surface that causes sufficient corrosion resistance of the painted component.

The alloying of the coating with iron and the formation of a

corrosion-resistant surface, however, require a correspondingly long residence time in the roller hearth furnace commonly used, whereby long furnaces are necessary to allow sufficient cycle times. The efficiency of the press molding is thus reduced. Longer ovens are more expensive in the

Acquisition and operation and also have a very large footprint. The minimum length of stay is thus determined by the coating and not by the basic material, for which only the achievement of the necessary

Austenitizing temperature would be necessary. In addition, the corrosion resistance is reduced by the stronger alloying with iron, since the aluminum content in the Alloy layer decreases with the Ofenverweilzeit and the iron content increases.

A further disadvantage of known AS coatings is that, with very short annealing times, that is to say when no alloying of the coating with the base material has taken place, the weldability in the resistance spot welding (WP) method of the press-formed component is extremely poor. This is expressed, e.g. in a very small area of sweat. One of the reasons for this is a very low contact resistance with short annealing times. The published patent application DE 10 2015 210 459 A1 discloses a method for

Hot forming of a steel component known which in a

Heat treatment step in a range of complete or partial

Austenitisierung is heated, and the heated steel component is both hot-formed and quench-hardened in a forming step, wherein the

Heat treatment step is a first pre-treatment step process technology upstream, in which the steel component is provided to protect against scaling in the heat treatment step with a corrosion-resistant protective layer. In this case, before the heat treatment step is carried out in a second pretreatment step, a surface oxidation takes place in which an inert, corrosion-resistant oxidation layer is formed on the scale protection layer by means of which an abrasive tool wear in the forming step is reduced. The

Surface oxidation can process technically, for example by a

Beizpassivierung done. A disadvantage of the prior art described therein, inter alia, 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. By means of the additional oxidation layer, the roughness of the metal surface of the steel component is to be reduced, whereby the abrasive tool wear should be reduced in the forming step.

The disadvantage here, however, that is not improved by a surface oxidation before the heat treatment due to the reduction of the surface roughness, the paint adhesion to the press-form hardened component and the weldability. In addition, the additional step of surface oxidation is time- and energy-consuming and thus increases the production costs considerably.

The object of the invention is therefore an inexpensive method for

Specify coating of steel sheets or steel strips, which provides an 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 Ofenverweingauer should be reduced while still ensuring good WP weldability and corrosion resistance on pressformhardened component after painting. Furthermore, a method for the production of press-hardened components from such steel sheets or steel strips

be specified.

The teaching of the invention comprises coating a steel sheet or

Steel strip on which an aluminum-based coating is applied by the hot dip method and liberating the surface of the coating of a natively formed aluminum oxide layer, characterized in that subsequently on the liberated surface of the coating transition metals or

Transition metal compounds are deposited to form a support. The term used above is exempted in the sense of being technically possible to be exempt from the native aluminum oxide layer.

Preferably, in this case, the support is a flat precipitate. Accordingly, there may be a full-surface edition or not necessarily covering edition. The opaque overlay may be net-like with ordered or disordered structure or distribution, which is then a layer of punctiform overlays and voids.

Preferably, a support with a coating weight - based on iron - in the range of 7 to 25 mg / m ² , preferably 10 to 15 mg / m ² deposited.

Furthermore, the teaching of the invention comprises a method for producing press-hardened components from steel sheets or steel strips with a

aluminum-based coating, wherein the treated according to the invention

Steel sheets or steel strips with the aim of curing at least partially heated to a temperature above Ac3, then at this temperature be formed and then cooled with the aim of curing at a speed that at least partially above the critical

Cooling speed is. 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 this means that the aluminum oxide layers formed, in particular, during the heat treatment in the course of the press-form hardening of untreated AS coatings remain extremely thin, generally below 10 nm, and thus with respect to the required improvement of the

Resistance point weldability and corrosion resistance are ineffective.

Advantageously, on the coating with the deposited metals and / or their compounds under an atmosphere with oxygen or water vapor, an aluminum oxide layer with mixed oxides of the metals and / or their

Formed connections. Surprisingly, it was found in the investigations that by removing the natively formed 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 (eg corundum, eskolait, hematite, Karelianite, tistarite, ilmenite, perovskites and / or spinels), the reformation of a thin alumina layer before and during the heat treatment is prevented. Preferably, the aluminum oxide layer is formed with the mixed oxides in an oven with a temperature> 750 ° C, preferably from 850 to 950 ° C, and a furnace residence time> 90 s, preferably 120 to 180 s.

Instead, an aluminum-rich oxide layer is formed, which is doped with cations of the previously deposited substances. These cations suppress the above

described self-limiting oxide layer growth and thus allow the growth of much thicker alumina layers during the

Heat treatment, wherein oxide layer thicknesses of over 80 nm can be achieved, which cause a much better resistance spot weldability and better corrosion behavior in the KT-coated state compared to thinner aluminum oxide layers. The essence of the invention is therefore that the Al-based metallic coating is chemically treated in particular before the heat treatment so that it freed from its native oxide layer and certain metals or their compounds which can form mixed oxides with Al2O3, on the surface of the Coating be deposited. These prevent the formation of a pure

Aluminum oxide layer during heat treatment before press hardening.

Instead, the deposited substances are partially or completely incorporated into the newly forming oxide layer. By this doping with metal or transition metal cations grows the

Oxide layer in the course of heat treatment to much larger thicknesses (> 80 nm) than untreated Al-based coatings (<10 nm). Self-limiting alumina growth is avoided. Unlike in the published patent application DE 10 2015 210 459 A1, the modification of the AS surface improving the core, namely the formation or formation of a thick aluminum oxide layer, is not carried out before the heat treatment, but in-situ during the heat treatment achieved for press hardening. Here, the property-determining, thick grows

Aluminum oxide layer only in the course of heat treatment in the oven.

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 systems engineering.

In the process according to the invention, very thick oxide layers of up to 250 nm are formed under the furnace residence times described in Table 1 at 950 ° C. oven temperature. Components produced according to the invention have the large welding ranges described in Table 2 in resistance spot welding and very good corrosion resistance in the KT-coated state on the table 3, when in the

Corrosion change test to be tested in accordance with Volkswagen PV1210.

The treatment according to the invention consists of the application of

Transition metals or transition metal compounds, for example, from the group titanium, vanadium, chromium, iron, and manganese and / or their compounds, preferably almost completely iron and / or its compounds, on the Al-based metallic coating by means of a chemical deposition, preferably in a wet-chemical process. This consists at least of the application of a solution of compounds of the above-mentioned elements, which in

electroless reaction with the Al-based metallic coating. The term without external power is used in the sense of non-electrolytic. Preferably, chemical deposition takes place by means of an injection, dipping or rolling application. It is also preferably provided that the removal of the atmospheric native oxide layer and the chemical deposition take place in a single process step. For this, the two treatment steps in one at a

Hot dip coating system downstream or to the

Hot dip coating plant separate continuous

Coating system to be performed. Preferably, this treatment is carried out in the presence of compounds of other metals, for example from the group cobalt, molybdenum and tungsten and / or their compounds. For example, molybates, tungstates or cobalt nitrate significantly accelerate the deposition of iron, but are self-deposited only to a small extent, thereby making the process of the invention even more efficient. However, preference is given to iron or its compounds

deposited because iron or iron compounds are readily available, inexpensive and non-toxic. In addition, iron is already included in the base material.

The removal of the natively formed 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 used in continuous systems

Belt speeds of up to 120 m / min or more are performed. The required drug cost 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. To this end, the natively-formed oxide layer of the Al-based coating (e.g., AS) with alkaline

Pickling removed, rinsed and electrochemically deposited the metal or chemical compound from an electrolyte. In the electrochemical Aftertreatment in aqueous media is advantageously maintained at an electrolyte temperature of 20 ° C to 85 ° C and worked at current densities between 0.05 and 150 A / dm ² . When using ionic liquids for metal deposition also electrolyte temperatures greater than or equal to 85 ° C can be applied. The treatment of the metal strip can be carried out in a continuous belt 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 initially formed native oxide layer and subsequent treatment of the AS surface with metal-containing solutions, can also in the subsequent further processing of the steel sheet by

Hot reduction or press hardening, a reduction of the minimum residence time can be achieved in the furnace, which significantly increases the productivity. For untreated AS coatings, the minimum residence time in the furnace for the growth of the oxide layer is determined by the requirement for weldability in resistance spot welding and the corrosion resistance in the KT-coated state.

In the investigations, it was found that from a layer weight of about 10 mg / m ² applied to the AS surface active ingredient, based on the guide element iron, shows a significant reduction in the minimum holding time in the heat treatment. Specifically, a 1.2 mm thick substrate of a mild steel alloy (22MnB5) with AS coating (150 g / m ² ) suitable for press-forming had one

Iron deposit of about 15 mg / m ² already after a furnace residence time of 3 min at 950 ° C oven temperature properties that can be achieved in untreated samples of the same sheet thickness after 6 min oven residence time. The necessary

Oven residence time was halved compared to the standard process.

Figures 1 and 2 show the depth profile for the elements AI, Fe and O after the press-hardening of sheets 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) in a Oven residence time of 6 min and a furnace temperature of 950 ° C in an air atmosphere. In Figure 2 is clearly visible the deeper oxygen input in the inventive treated sample, indicating a significantly thicker oxide layer compared to the untreated sample. In addition, the accumulation of iron in the oxide layer is clearly visible. The treatment according to the invention of the surface of the coated steel strip can advantageously be carried out in a process part of a continuously producing

Hot dip coating system downstream treatment part or a separate system, for example via spray bars with nozzles, in a dipping process and by means of an electrolytic deposition or spray electrolysis, in each case also in combination. For the separate plant, it may be e.g. one

Band coating or an electrolytic strip finishing plant act. 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 eliminates the (native) oxide layer formed by atmospheric oxidation and thereby creates a defined initial state for the deposition of metallic species according to the invention. The treatment of the surface can according to the invention over the entire

Ribbon surface or even partially or on one or both sides done. In the case of electroless treatment, by concentrating the feed solution, its temperature, the injection pressure, the shear of the sprayed solution relative to the surface of the metal strip to be treated, and the volume contacted with the surface, the molar amount of the deposited metal species can be varied. In electrolytic deposition, the deposited molar amount of the metal species is determined by electrolyte composition, flow conditions, temperature, current density and treatment time. EXAMPLES

Examples of pretreatments of the samples according to the invention are as follows:

The AS-coated sheet is immersed in a metal cation-containing alkaline solution at a temperature of 50 ° C for a few seconds. The natively formed oxide layer is removed and the iron-containing layer is applied.

Alternatively, the AS-coated sheet to remove the natively formed oxide layer in a 20% sodium hydroxide solution for 30 seconds at room temperature one

Subjected to immersion treatment. This is followed by rinsing with demineralized 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 .

Test parameters for press hardening

• Oven temperature for heat treatment: 950 ° C

• Atmosphere: ambient air

• oven residence time (with sheet thickness up to 1.5 mm): 2, 3, 4, 6 min

• Then cool down to <200 ° C in the cooled flat tool

Table 1 shows, for the purely wet-chemical pretreatment of the samples, that the thickness of the aluminum oxide layers increases significantly with increasing drug occupancy (Fe) and residence time in the oven. Without treatment according to the invention is the

Layer thickness of the oxide layer less than 10 nm. With an iron support of about 7 mg / m ² and residence time of 2, 3 or 4 min. no significant stratification is achieved yet. This also applies to an iron coating of about 1 1 mg / m ² and a residence time of 2 min.

Table 1: Layer formation on the sample surface as a function of

Iron pad and oven dwell time

Table 2 illustrates that the pretreated and in air atmosphere

Press-hardened AS samples with an iron-containing coating have a pronounced weld area even after short annealing times. Without

Treatment according to the invention is not measurable with short annealing times

Welding area available.

Table 2: Welding range according to SEP1220-2 depending on the iron support and annealing

The infiltration of the Ritz after 12 weeks in the corrosion test Volkswagen PV 1210 is less than on samples with treatment according to the invention

untreated samples as shown in Table 3.

Table 3: Infiltration after 12 weeks Volkswagen PV 1210 on KT-coated samples depending on iron deposit and annealing time

FIG. 3 shows, by way of example, a cross-section on a sheet-metal section with AS coating and treatment according to the invention, deposited with no external current, with an iron deposit of about 15 mg / m ² after press-hardening. The oven residence time was 3 minutes at a furnace temperature of 950 ° C under air atmosphere.

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

Claims

claims

A process for coating a steel sheet or steel strip to which an aluminum-based hot dip coating is applied and the surface of the coating is freed of a natively formed aluminum oxide layer, characterized in that subsequently on the liberated surface of the coating transition metals or transition metal compounds to form a support be deposited.

2. The method according to claim 1, characterized in that the support is deposited as a surface precipitate.

3. The method according to claim 1 or 2, characterized in that the

Transition metals or the transition metal compounds at least one

Chemical element selected from the group titanium, vanadium, chromium, manganese or iron and / or its compounds.

4. The method according to claim 3, characterized in that the transition metals or transition metal compounds predominantly or almost completely comprise iron or its compounds.

5. The method according to at least one of claims 1 to 4, characterized in that a support with a layer weight - based on iron - in the range of 7 to 25 mg / m ² , preferably 10 to 15 mg / m ² , is deposited.

6. The method according to at least one of claims 1 to 5, characterized in that the deposition of the transition metals or the transition metal compounds in the presence of at least one further chemical element from the group cobalt, molybdenum, tungsten and / or their compounds.

7. The method according to at least one of claims 1 to 6, characterized in that deposition of the transition metals or the transition metal compounds takes place by a chemical deposition.

8. The method according to claim 7, characterized in that the chemical Separation by means of a spray, dipping, or rolling application takes place.

9. The method according to at least one of claims 1 to 8, characterized in that the removal of the atmospheric native oxide layer and the chemical deposition take place in a single process step.

10. The method according to claim 9, characterized in that the two

Treatment steps in one of a hot dip coating plant

downstream or to the hot dip coating plant separate continuous coating system can be performed.

1 1. Method according to at least one of claims 1 to 7, characterized

characterized in that the deposition of the transition metals or the

Transition metal compounds takes place electrolytically.

12. The method according to claim 1 1, characterized in that the electrolytic application of the transition metals or transition metal compounds in an aqueous medium as the electrolyte is carried out with an electrolyte temperature of 25 ° C to 85 ° C, at current densities between 0.05 and 150 A / dm ^second ,

13. The method according to at least one of claims 1 to 12, characterized

characterized in that an aluminum oxide layer with mixed oxides is formed from the support on the coating with the support under an atmosphere with oxygen or under steam.

14. The method according to claim 13, characterized in that the

Aluminum oxide layer with the mixed oxides in an oven with a temperature> 750 ° C, preferably 850 to 950 ° C, and a furnace residence time> 90 s,

preferably 120 to 180 s, is formed.

15. The method according to at least one of claims 1 to 14, characterized

characterized in that a formation of the mixed oxides, a self-limiting the layer growth of the alumina is avoided.

16. The method according to at least one of claims 13 to 15, characterized in that corundum, eskolait, hematite, karelianite, tistarite, ilmenite, perovskite and / or spinel are formed as mixed oxides.

17. The method according to at least one of claims 1 to 16, characterized

characterized in that the aluminum-based coating aluminum, aluminum-silicon (AS) or aluminum-zinc-silicon (AZ) with optional admixtures of additional elements, such as magnesium, manganese, titanium and rare earth, is applied to the steel sheet or steel strip.

18. A method for producing press-hardened components from steel sheets or steel strips with an aluminum-based coating prepared according to at least one of claims 1 to 17, characterized in that the steel sheets or steel strips are at least partially heated to a temperature above Ac3, then formed at this temperature and are then cooled with the aim of curing at a rate that is at least partially above the critical cooling rate.

19. The method according to any one of claims 1 to 18, characterized in that a hardenable by heat treatment steel is used for the steel sheets or steel strips.

20. The method according to claim 19, characterized in that a manganese and boron alloyed tempering steel is used.

21. A method according to claim 20, characterized in that a 22MnB5 is used as the quenching steel.