ES2989905T3 - Procedure for the production of a steel construction part with a coating from a flat steel product - Google Patents

Abstract

The present invention relates to a method for manufacturing a steel component by hot forming a steel flat coated with an aluminium-based anti-corrosion coating, to a steel component manufactured from a steel flat coated with an aluminium-based anti-corrosion coating by hot forming and to a steel flat for manufacturing such a steel component. The method comprises at least the following steps: a) providing a steel flat coated with an aluminium-based anti-corrosion coating; b) heating the coated steel flat in a hot forming furnace whose furnace atmosphere contains at least 16% by volume of oxygen and has a dew point of -20°C to +15°C; c) optionally maintaining the coated steel flat in the hot forming furnace; d) transporting the heated steel flat from the heating furnace to a forming die and placing the steel flat in the forming die; e) simultaneously forming and cooling the steel blank in the forming die. The steel component obtained after forming has, on the surface of the anti-corrosion coating, an aluminium oxide layer with an average thickness of at least 50 nm, with no more than 10 AlN rods per 1000 μm2 being present on the aluminium oxide layer. (Automatic translation with Google Translate, without legal value)

Description

DESCRIPTION

Procedure for the production of a steel construction part with a coating from a flat steel product

The invention relates to a method for producing a steel component by hot forming a flat steel product coated with an aluminium-based anticorrosive coating. Furthermore, the invention relates to a steel component produced by hot forming a flat steel product coated with an aluminium-based anticorrosive coating and to a flat steel product for producing such a steel component.

Materials with good corrosion resistance are required for automotive and other industrial applications. Products made of steel that are provided with a corrosion protection coating are suitable for this purpose. Such coatings consist, for example, of aluminium or aluminium alloys or zinc or zinc alloys and protect the underlying steel substrate from corrosion, on the one hand, due to their mechanical barrier effect and, on the other, due to their cathodic protection effect.

When we talk about flat steel products, this is understood to mean, for example, a steel strip or a steel sheet or a cut-out made from a steel sheet, such as a flat bar. A flat bar is understood to mean sheet metal boards that generally have more complex contours than the steel strips or steel sheets from which they are derived.

When we speak of a flat steel product coated with an aluminium-based anti-corrosion coating, we are referring to a flat steel product coated at least on one side with an aluminium-based anti-corrosion coating. A coated flat steel product thus consists of a steel substrate, which can be, for example, a steel strip or a steel sheet or a blank produced from a steel sheet, such as a flat bar, and an anti-corrosion coating present on at least one side of the steel substrate.

During hot forming, flat steel products are heated and cooled in a mould. In principle, two types of hot forming are distinguished. In so-called direct hot forming, the flat steel product to be formed is first heated and immediately after heating is placed in a forming tool, in which it is simultaneously formed and cooled. In indirect hot forming, the flat steel product to be formed is first cold formed in a first forming tool, then heated and, after heating, cooled in a second forming tool, in which, however, it is only slightly or not at all formed. Compared to direct hot forming, indirect hot forming offers the advantage of lower tool wear when using uncoated flat steel products. The scale that forms on uncoated flat steel products during heating rubs against the surfaces of the forming tool, which therefore wears out more quickly. Scale formation and thus tool wear can be reduced by coating the flat steel product with an anti-corrosion coating.

WO 2008/053273 A1 discloses a method for producing a steel component with an aluminium-based anticorrosive coating. The aluminium or aluminium alloy-coated sheet metal to be formed is heated at a rate of 4 to 12 °C/s, held at a temperature of 880 - 940 °C for 3 to 13 minutes, then transferred to a forming tool, formed and cooled at a rate of at least 30 °C/s. After forming, the coating on the component has a multilayer structure with an interdiffusion layer, an intermediate layer, an intermetallic layer and a surface layer. The interdiffusion layer consists of 86-95% Fe, 4-10% Al and up to 5% Si. The intermediate layer consists of 39-47% Fe, 53-61% Al and up to 2% Si. The intermetallic layer consists of 62-67% Fe, 30-34% Al and 2-6% Si. The surface layer consists of 39-47% Fe, 53-61% Al and up to 2% Si. The construction part will have good weldability and crack stability.

However, even when using coated flat steel products, tool wear remains a problem that increases maintenance and repair costs. This applies in particular to the forming of flat steel products that are provided with an aluminium-based anti-corrosion coating.

In addition to the prior art described above, a flat steel product for hot-press forming is known from EP 2993 248 A1, which consists of a steel substrate produced from a steel containing 0.1-3% by weight of Mn and up to 0.01% by weight of B and has an Al-based protective coating applied to the steel substrate, which optionally contains a total of up to 20% by weight of other alloying elements. Since the protective coating contains a total of 0.005-0.7% by weight of at least one alkaline earth or transition metal as an additional alloying component, the absorption of hydrogen during the heating required for hot forming should be minimized. For hot-press forming, the thus coated steel flat product is annealed in a standard atmosphere, which by definition contains 20.9% by volume of oxygen.

Finally, EP 1 767 286 A1 discloses a method for producing a steel component by hot compression moulding from hot-rolled or cold-rolled steel sheet, which is provided with a protective coating based on Al or Zn and has a strength of at least 1200 MPa after forming. This known method also requires that hydrogen embrittlement be counteracted. The steel sheet, the steel substrate of which contains 0.05% to 0.5% by weight of carbon, is heated to an annealing temperature prior to pressing, which is limited downwards by the Ac3 temperature of the steel substrate and upwards by 1100 °C. Heating takes place in a furnace atmosphere with a hydrogen concentration of no more than 6% by volume and a dew point of no more than 10 °C. When processing aluminium-clad flat steel products, the oxygen content of the furnace atmosphere is simultaneously adjusted in each case to at least 21% by volume or at most 1% by volume.

The invention is based on the objective of providing a method that minimises the wear of a forming tool in direct hot forming for the production of a steel construction part provided with an aluminium-based anti-corrosion coating, which has optimised suitability for painting, bonding and welding.

With regard to the method, this objective is solved by a method with the characteristics indicated in claim 1. Advantageous and preferred configurations of the method according to the invention are indicated in the claims dependent on claim 1.

A process according to the invention for producing a steel component provided with an aluminium-based anticorrosive coating comprises at least the following process steps:

a) to provide a flat steel product with an aluminium-based anti-corrosion coating;

b) heating the coated steel flat product in a hot forming furnace, the furnace atmosphere of which contains 16% by volume to 20% by volume of oxygen and has a dew point of -20 °C to 15 °C;

c) optionally clamping the coated steel flat product in the hot forming furnace;

d) transporting the heated flat steel product from the heating furnace to a forming tool and placing the flat steel product into the forming tool;

e) simultaneously forming and cooling the flat steel product in the forming tool.

In a process according to the invention, a flat steel product is provided in process step a), which is provided with an aluminium-based anti-corrosion coating. The steel used for the steel substrate of the flat steel product is preferably a steel consisting of 0.1 - 0.4% by weight of C, preferably 0.15 - 0.3% by weight of C, particularly preferably 0.19 - 0.25% by weight of C, 0.5 - 3.0% by weight of Mn, preferably 0.9 - 1.6% by weight of Mn, 0.05 - 0.5% by weight of Si, preferably 0.15 - 0.4% by weight of Si, 0.01 - 0.2% by weight of Al, preferably 0.01 - 0.1% by weight of Al, 0.005 - 1.0% by weight of Cr, preferably 0.1 - 0.4% by weight of Cr, 0.0005 - 0.01% by weight of Al, of B, 0.001 - 0.2% by weight of V, preferably 0.001 - 0.010% by weight of V, particularly preferably 0.002 - 0.006% by weight of V, 0.001 - 0.1% by weight of Ti, 0.001 - 0.1% by weight of Nb, optionally 0.01 - 0.4% by weight of Ni, optionally 0.01 - 0.8% by weight of Cu, optionally 0.002 - 1.0% by weight of Mo, optionally 0.001 - 1.0% by weight of W, up to 0.01% by weight of N, and the remainder consisting of iron and unavoidable impurities, whereby the unavoidable impurities can account for up to 0.1% by weight of P and up to 0.05% by weight of S.

Typically, steel substrates particularly suitable for the present invention have a thickness of 0.4 to 3 mm.

Where statements are made concerning alloy contents and compositions of liquid or solid phases, these relate to weight or mass, unless otherwise expressly stated. Where statements are made concerning atmospheric compositions, these relate to volume, unless otherwise expressly stated.

The anti-corrosion coating present on the steel substrate is made of pure aluminium or an aluminium alloy. Suitable aluminium alloys typically consist of 3-15% by weight of Si, preferably 7-12% by weight of Si, particularly preferably 9-10% by weight of Si and optionally one or more elements selected from the group consisting of iron, transition metals other than iron, alkaline earth metals or mixtures thereof, in the following contents: 2-3.5% by weight of Fe, 0.05-2% by weight of transition metals other than Fe, preferably 0.1-0.5% by weight of transition metals other than Fe, particularly preferably 0.15-0.4% by weight of transition metals other than Fe, 0.05-2% by weight of alkaline earth metals, preferably 0.1-0.5% by weight of alkaline earth metals ... 0.4% by weight of alkaline earth metals, with aluminium and unavoidable impurities as the remainder. In the case of transition metals, a distinction is made between iron and other transition metals, because iron may be present in higher amounts than other transition metals. Iron, other transition metals and also elements from the group of alkaline earth metals form a compact, thin and opaque oxide layer which reduces the penetration of diffusible hydrogen. The following alkaline earth or transition metals have proven to be particularly suitable in this respect: Mg, Ca, Sr, Ba, Zr and Ti.

The anti-corrosion coating can be applied to the steel substrate in a conventional manner, for example by means of a hot dip coating process. Other application methods are also conceivable which enable the application of an anti-corrosion layer. A suitable anti-corrosion coating normally has a thickness of at most 30 µm per side, in particular a thickness of 10 to 30 µm per side.

In process step b), the coated steel flat product is heated for hot forming, which is carried out in process step e). The invention is based on the knowledge that the wear of the forming tool is influenced by the composition of the atmosphere of the heating furnace. The heating furnace is also referred to as a hot forming furnace, because the coated steel substrate is heated there before the hot forming process and for the hot forming process. A preferred example of a hot forming furnace is a roller hearth furnace. Starting from an oxygen content of 6% by volume in the furnace atmosphere, an aluminum oxide layer is formed on the aluminum-based anticorrosive coating. In the presence of nitrogen, rod- or needle-shaped aluminum nitride (AIN) particles are formed on the surface of the coated steel flat product in addition to the aluminum oxide layer. AIN generally has a very high hardness. The hardness of AIN is typically around 1230 HV1. If these AIN rods or needles grow beyond the aluminium oxide layer, they damage the surface of the forming tool during the forming process and cause increased wear of the forming tool. Particularly high wear can be observed at oxygen content of up to 13% by volume. Tool wear decreases as the oxygen content increases.

The invention is based on the knowledge that at oxygen contents in the atmosphere of the hot forming furnace of at least 16% by volume, the formation of the AIN rods is impeded to such an extent that significantly fewer AIN rods are present grown beyond the aluminium oxide layer than at oxygen contents of less than 16% by volume. The heating of the coated steel flat product in process step b) therefore takes place in a hot forming furnace whose atmosphere contains at least 16% by volume, preferably at least 18% by volume, particularly preferably at least 19% by volume of oxygen.

In atmospheres with at least 16% by volume of oxygen, aluminium oxide layers with an average thickness of 50 nm and more are regularly formed. Since the thickness of the aluminium oxide layer can vary over the surface covered by the aluminium oxide layer, the average thickness of the aluminium oxide layer is understood in question as the arithmetic mean value of at least three thickness values determined at different points in the aluminium oxide layer. Aluminium oxide layers with an average thickness of at least 50 nm have proven to be favourable for reducing tool wear. Aluminium oxide layers with an average thickness of at least 50 nm are assumed to have a levelling effect. This means that AIN rods do not grow beyond this layer thickness. Furthermore, aluminium oxide layers of at least 50 nm average thickness, preferably at least 100 nm average thickness, have proven to be beneficial in preventing tool caking on the moulded part as well as in reducing tool scratching, since the aluminium oxide layer acts as a separating agent between the forming tool and the surface of the steel flat product to be formed and separates the liquid phase constituent parts from the tool surface in the case of insufficiently annealed steel flat products. Aluminium-based anticorrosive coatings in which aluminium oxide layers of at least 50 nm average thickness have been formed typically contain Fe(AI1-xSix), t1 Fe3(AI,Si)5, Fe2Al5 and Fe3(AI1-xSix).

Opaque aluminium oxide layers have proven to be particularly effective in reducing tool wear. Opaque aluminium oxide layers are aluminium oxide layers that completely cover the surface of the anti-corrosion coating. In contrast, at oxygen contents below 16% by volume, oxide layers with an average thickness of less than 50 nm are formed. These are often not opaque, which has a negative impact on tool wear.

In principle, a furnace atmosphere of up to 100% by volume oxygen would be possible to reduce the formation of AIN. However, pure oxygen is very explosive. To improve the suitability for welding and the suitability for painting and bonding, the oxygen content of the furnace atmosphere is limited to a maximum of 20% by volume.

In a preferred embodiment, the hot forming furnace atmosphere contains at least 50% by volume of nitrogen in addition to oxygen in order to reduce the explosiveness of the furnace atmosphere. In accordance with the minimum proportion of oxygen according to the invention, the proportion of nitrogen in the furnace atmosphere is preferably less than 84% by volume, particularly preferably less than 82% by volume, very particularly preferably less than 81% by volume. In this preferred embodiment, it is possible to provide a method for producing a steel component provided with an aluminium-based corrosion protection coating, in which the wear of the forming tool can be minimised despite the presence of nitrogen in the furnace atmosphere.

In another preferred embodiment, the atmosphere of the hot forming furnace consists of at least 16% by volume of oxygen, particularly preferably at least 18% by volume of oxygen, very particularly preferably at least 19% by volume of oxygen, and the remainder in each case of nitrogen and unavoidable impurities.

In another preferred embodiment, one or more noble gases can also be added to the atmosphere of the hot forming furnace in addition to oxygen and nitrogen in proportions of up to 1.5% by volume, so that the atmosphere of the hot forming furnace consists of at least 16% by volume of oxygen, particularly preferably at least 18% by volume of oxygen, very particularly preferably at least 19% by volume of oxygen, up to 1.5% by volume of one or more noble gases, and in each case the remainder of nitrogen and unavoidable impurities.

In another preferred embodiment, one or more noble gases can be added to the atmosphere of the hot forming furnace without adding appreciable proportions of nitrogen, so that the atmosphere of the hot forming furnace consists of at least 16% by volume of oxygen, particularly preferably at least 18% by volume of oxygen, very particularly preferably at least 19% by volume of oxygen, and the remainder in each case of noble gases and unavoidable impurities.

In all embodiments, the furnace atmosphere may be contaminated with other gases in proportions of up to 1.1% by volume. In addition, the hot forming furnace atmosphere may also contain water vapour with a water content of 0.5 to 15 g/m3.

For safety reasons, furnace atmospheres whose composition is neither explosive nor harmful to health are preferred.

In order to ensure oxygen proportions according to the invention in the hot forming furnace, the dew point<t>P of the furnace atmosphere is set to values between -20 °C and 15 °C, preferably to values between -15 °C and 15 °C. A dew point below -20 °C leads to a reduction in the oxygen proportion in the hot forming furnace, while values above 15 °C can cause an increase in hydrogen absorption.

In a preferred embodiment, a further reduction in tool wear, but also an improvement in welding suitability as well as in painting and adhesion suitability can be achieved if the ratio of the volume proportion of nitrogen, V(N2), to the volume proportion of oxygen, V(O2), is 3.80 to 5.30 during heating in step b) in the hot forming furnace:

3.80 < V(N2) / V(O2) < 5.30

with V(N2) = volume of nitrogen in the hot forming furnace in vol. %, V(O2) = volume of oxygen in the hot forming furnace in vol. Nitrogen to oxygen ratios of 4.0 to 5.2 have proven to be particularly effective in this respect.

In another preferred embodiment, the flat steel product is heated to a temperature T1 of between 830 and 950 °C, preferably between 900 and 950 °C, in a hot forming furnace with furnace temperatures of between 830 and 950 °C in working step b). Heating preferably takes place in a continuous furnace, particularly preferably in a roller hearth furnace. The furnace temperature corresponds in this respect to the temperature to which the flat steel product is to be heated. Typically, the flat steel product is introduced into the hot forming furnace at room temperature. Residence times of 2 to 15 minutes for the flat steel product in the hot forming furnace have proven to be particularly favourable.

Optionally, the flat steel product can be held at this temperature in the hot forming furnace in stage c) after reaching the temperature T1. In this case, the residence time of the flat steel product in the hot forming furnace is also 2 to 15 minutes and includes the time for heating up to the furnace temperature as well as the time for optionally holding the flat steel product at the furnace temperature. The furnace atmosphere during holding is the same as in stage b) during heating.

The flat steel product obtained in process step b) after heating or, if appropriate, in process step c) after optional maintenance and provided with a corrosion-resistant coating has an aluminium oxide layer with an average thickness of at least 50 nm, preferably at least 100 nm, on the surface of the corrosion-resistant coating. Lower average thicknesses are usually not opaque, which has a negative effect on tool wear. As regards the influence of the aluminium oxide layer thickness on tool wear, there is no upper limit for the average thickness of the aluminium oxide layer. Thinner aluminium oxide layers with an average thickness of at most 150 nm have proven to be favourable for the targeted production of flat steel products with improved welding suitability and improved paintability and adhesion.

AIN rods may be present in the aluminium oxide layer after heating or, if appropriate, after optional maintenance. After heating or, if appropriate, after optional maintenance, a maximum of 10 AIN rods per 1000 pm2 are present in the aluminium oxide layer on the surface of a flat steel product according to the invention. A higher number of AIN rods per 1000 pm2 has a negative effect on the inspection and cleaning intervals within a forming campaign. A forming campaign is understood to mean the set of all the bars to be formed in a sequence under the same hot forming conditions. A forming campaign typically comprises up to 1,500 bars. The forming of a bar in the forming tool is also referred to as a stroke. While with a density of 11 or more AIN rods per 1000 pm2 of surface, hot forming tools must be checked and cleaned after 40 - 60 % of the component surface in a forming campaign has been applied, with a density of at most 10 AIN rods per 1000 pm2 of surface, hot forming tools must, on the other hand, only be checked and cleaned after more than 60 % of the component surface in a forming campaign has been applied, in particular after 70 - 80 % of the component surface in a forming campaign has been applied. Tool wear can be further reduced by checking and cleaning the hot forming tools after more than 80 % of the component surface has been coated in a forming campaign if the density of AIN rods per 1000 pm2 of surface is limited to a maximum of 5. Particularly low wear can be achieved with a rod density of a maximum of 2, in particular zero AIN rods per 1000 pm2 of surface. In this case, checking and cleaning the hot forming tools is only necessary after 100 % of the component surface has been coated.

In the present case, the term "applied component surface of a forming campaign" refers to the surface of the bars that have been hot formed in the mould in total within a forming campaign. For forming campaigns with, for example, 1000 bars, this means that inspection and cleaning of the mould with a density of 11 or more AIN rods per 1000 pm2 of surface is only necessary after 400-600 strokes, with at most 10 AIN rods per 1000 pm2 of surface only after more than 600, in particular after 700-800 strokes, with at most 5 AIN rods per 1000 pm2 of surface only after more than 800 strokes and with at most 2 AIN rods per 1000 pm2 of surface only after the end of the forming campaign.

The flat steel product obtained in process step b) after heating or, if applicable, in process step c) after optional maintenance has a corrosion protection coating, the thickness of which is typically 30 to 130 g/m2 or 12 - 60 µm per side. Beneath the aluminium oxide layer, the corrosion protection coating typically contains four intermetallic phases: Fe(Al1-xSix), t1 Fe3(AI,Si)5, Fe2Al5 and Fe3(AI1-xSix).

In a preferred embodiment, tool wear can be further reduced if the AIN rods extend beyond the aluminum oxide layer by at most 6 µm, preferably by at most 2 µm, i.e. they protrude from the aluminum oxide layer by at most 6 µm, preferably by at most 2 µm, particularly preferably by at most 1.0 µm. If AIN rods are produced, their height h or the length to which the AIN rods protrude from the aluminum oxide layer is typically 0.5 - 6 µm. The height h of the AIN rods here refers to their length protruding from the aluminum oxide layer. The height of an AIN rod is understood here to be the perpendicular vertical line from the tip of the AIN rod to the surface of the aluminum oxide layer.

In another preferred embodiment, the AIN rods have a diameter D of at most 2 µm, preferably at most 1.1 µm, which also has a favourable effect on tool wear. Typically, their diameter is 1 - 2 µm. The diameter of an AIN rod is understood to mean the width of the AIN rod running halfway along the height of the AIN rod and parallel to the surface of the aluminium oxide layer.

The negative effects on tool wear can be reduced particularly effectively if the AIN rods protrude from the aluminium oxide layer after heating by a maximum of 6 pm, preferably a maximum of 2 pm, and also a maximum of 2 pm in thickness.

The flat steel product obtained in process step b) after heating or, if necessary, in process step c) after optional maintenance and provided with a corrosion-resistant coating is transported from the hot-forming furnace to a forming tool in process step d) and placed in the latter. The transport from the hot-forming furnace to the forming tool and the placement of the flat steel product in the tool are carried out in a known manner. Transfer times of 3 to 15 s have proven to be particularly favourable in this respect. The transfer time in question is understood to mean the time interval between discharge of the furnace and closing of the mould.

In process step e), the flat steel product is formed into a component in the forming tool in a conventional manner and is cooled at the same time. Cooling is also carried out in a conventional manner. In this case, shaping is preferably carried out immediately after the flat steel product has been placed in the mould. Cooling rates of 20 to 1000 K/s, preferably 25 to 500 K/s, have proven to be particularly favourable. These cooling rates have proven to be particularly suitable for curing the flat steel product.

In a preferred embodiment, the process for producing a steel component provided with an aluminium-based anti-corrosion coating consists solely of process steps a) to e), i.e. it does not comprise any further process steps in addition to process steps a) to e).

The steel component obtained by the method according to the invention after carrying out the working step e) is provided with an aluminium-based anti-corrosion coating, in particular it has an aluminium-based anti-corrosion coating. During heating in working step b) and, if necessary, optional maintenance in process step c), iron atoms can diffuse from the steel substrate into the anti-corrosion coating, which can increase the thickness of the anti-corrosion coating. The thickness of the anti-corrosion coating of the steel component after hot forming is therefore typically 30 to 130 g/m2 or 12 - 60 µm per side. On the surface of the anti-corrosion coating, the steel component has an aluminium oxide layer with an average thickness of at least 50 nm, preferably at least 100 nm. The average thickness of the aluminium oxide layer is at most 150 nm. Per 1000 pm2 of surface area, at most 10, preferably at most 5, particularly preferably at most 2, and most preferably no AIN rods protrude from the aluminium oxide layer. In particular, at most 10 AIN rods per 1000 pm2 are therefore present in the aluminium oxide layer, preferably at most 5, particularly preferably at most 2, and most preferably no AIN rods.

In another preferred embodiment, the AIN rods protruding from the aluminium oxide layer of the steel component have a height of at most 6 µm, preferably at most 2 µm, particularly preferably at most 1.0 µm and have a diameter of at most 2 µm, preferably at most 1.1 µm. Beneath the aluminium oxide layer, the anti-corrosion coating typically has four intermetallic phases: Fe(AI1-xSix),<t>1 Fe3(AI,Si)5, Fe2Al5 and Fe3(AI1-xSix).

A steel component can be produced by a method according to one of claims 1 to 7.

The invention is explained in detail below by means of drawings and exemplary embodiments. They show

Fig. 1: Schematic representation of the growth of AIN rods and the aluminum oxide layer

Fig. 2: Schematic representation of the aluminium oxide layer and the AIN rods to explain the abbreviations used

The growth of the AIN rods is shown schematically in Figure 1. An aluminium-based anti-corrosion coating 2 has been applied to a steel substrate 1. First, a thin aluminium oxide layer 3 forms on the aluminium-based anti-corrosion coating 2 (Figure 1a)). The aluminium oxide layer is very brittle. When the coated flat steel product is heated, the anti-corrosion coating expands beneath the aluminium oxide layer and melts, at least partially. The volume expansion cracks the brittle aluminium oxide layer 3 in places. In the presence of nitrogen, AIN nitrides can germinate in the voids of the aluminium oxide layer 3, which initially grow into small AIN rods 4 (Figure 1 b)). The rods can also be called AIN needles due to their elongated shape. AIN 4 rods typically have a round cross section with an almost constant diameter D along their length (Figure 2). The AIN rods continue to grow in length and width as the heating process progresses, while the thickness of the aluminium oxide layer increases (Figure 1c) and Figure 1d)). At the end of the heating process, the AIN rods protrude from the aluminium oxide layer with a height h (Figure 2).

For the tests, four forming campaigns were carried out with 1,000 flat bars in each case. For this purpose, steel strips with the compositions indicated in Table 1 were provided and coated on both sides in a conventional manner by means of hot dip coating with aluminium-based anticorrosive coatings with the compositions indicated in Table 2. Flat bars were punched from the coated steel strips in each case in a conventional manner and were placed in a roller-hearth furnace for heating. The flat bars were heat-treated under the conditions indicated in Table 3.

The flats were subjected to heat treatment in hot forming furnaces with furnace atmospheres with oxygen contents of 15 to 20% by volume and dew points of -15 °C to 10 °C. In principle, the furnace atmospheres can be adjusted in various ways. One possibility is to allow the hot forming furnace to cool down before introducing the furnace atmosphere according to the invention, which can typically take up to 20 h, and to heat it up again after having introduced the furnace atmosphere. Another possibility is to introduce the furnace atmosphere according to the invention into a tempered hot forming furnace, which can have a favourable effect on reducing the heating time. Another possibility, which contributes to further reducing the heating times, is to heat the furnace atmosphere according to the invention before introducing it into the hot forming furnace. In tests 1 to 4, the furnace atmospheres were adjusted in each case because the respective gases had been introduced into the furnace via a gas pipe.

The 1.5 mm thick strips were heated to T1 flat product temperatures of 925 °C (test 1), 910 °C (tests 2, 4) and 930 °C (test 3). The residence times t of the strips in the hot forming furnace, which include the time from heating to exiting the furnace, were between 4 and 6 minutes. The heated strips were removed from the hot forming furnace in each case, samples were taken and a portion of the strips were placed in a forming press within the range of 3 to 10 s, shaped into a component and then sampled again. Several strips were produced for each test condition until tool wear made it necessary to stop the test for cleaning and, if necessary, tool maintenance. For each test, samples were taken from a formed plate or component before and after forming, in each case at the beginning, after 250 strokes and after 500 strokes. Sampling of the formed plates was carried out in flat areas of the components, where the plates were not subjected to any forming or only to a slight degree of forming.

The area number, diameter and height of the AIN rods were determined using a scanning electron microscope at a magnification of V = 2000x in each case on 3 flat grindings per plate with a size of 10*15 mm2 in each case. In each case, 3 surfaces of 100 mm2 in size were measured and the determined values were arithmetically averaged. The thickness of the oxide layer was determined on 3 samples per plate by X-ray photoelectron spectroscopy (XPS). The samples were degreased with n-heptane, rinsed with propanol and blown with air. The samples were each fixed on a sample holder, introduced into the measuring chamber of the X-ray photoelectron spectroscope and analysed under high vacuum. The vessel pressure was < 5 * Í07 kPa (5*108 mbar). Argon was used as the sputtering gas. The radiation was excited as Al Ka with a bombardment voltage of 2 or 4 kV. At least one thickness measurement of the aluminium oxide layer was carried out on each sample. The thickness measurement results of all samples of a plate were arithmetically averaged in each case and are referred to as the average aluminium oxide layer thickness or mean thickness. A height was determined for each AIN rod. For AIN rods inclined towards the surface, the vertical perpendicular from the tip of the AIN rod to the surface was measured as the height h. In each case, a width value was measured at half the height of each AIN rod, which is referred to as the diameter of the AIN rod. For AIN rods inclined towards the surface, the width was measured along a section running parallel to the surface.

The condition of the forming tools was visually inspected on site by experienced personnel in each case. The forming tools were visually inspected after 100 strokes in each case.

In test 1, a maximum of 2 AIN rods per 1000 pm2 with a height of maximum 0.7 pm and a diameter of maximum 1.1 pm were determined in each case in the samples taken before and after hot forming, both before and after hot forming. The oxide layer had an average thickness of 120 nm in each case before and after hot forming. Under the conditions of test 1, 700 strokes could be performed before the forming tool had to be cleaned. The forming tool had only minimal striations, so that it did not need to be overhauled prematurely.

In test 2, no AIN rods were found in the samples taken before and after hot forming, both before and after hot forming; after hot forming, a maximum of 2 AIN rods per 1000 pm2 with a height of maximum 0.5 pm and a diameter of maximum 1 pm were determined in each case. The oxide layer had an average thickness of 103 nm before hot forming and an average thickness of 135 nm after hot forming. Under the conditions of test 2, 1000 strokes could be carried out before the forming tool had to be cleaned. In addition, the forming tool had only minimal striations, so that premature overhaul was not necessary in this case either.

In test 3 (not according to the invention), the furnace atmosphere contained only 15% by volume of oxygen not according to the invention. An average of 13 AlN rods per 1000 |jm2 were measured in each case before and after hot forming in the samples taken before and after hot forming. The height of the AlN rods was between 3 and 5.5 µm and the diameter was between 1.5 µm and 2 µm. The oxide layer had an average thickness of 45 nm after hot forming. Under the conditions of test 3, massive dust formation on the tool and abrasion damage to the radii occurred, which is why the tool had to be cleaned after 500 strokes and the radii checked after 1000 strokes.

In test 4 (not according to the invention), the furnace atmosphere contained 21% by volume oxygen, 78% nitrogen, the remainder noble gases and unavoidable impurities. In the samples taken before and after hot forming, a maximum of 1 AIN rod per 1000 jm2 with a height of maximum 0.6 jm and a diameter of maximum 1.1 jm was determined in each case. The oxide layer had an average thickness of 2100 nm. Under the conditions of test 1, 800 strokes were possible before the forming tool had to be cleaned. The forming tool had only minimal scratches, so that it did not have to be checked prematurely. However, the increased formation of oxide layers led to a deterioration in the suitability for welding, suitability for painting and adhesion.

T l 1. In i i n n n n r hi rr im r z in vi l

Table 2. In i i n n n r l minium

Tl. In i i n n n r l mini

Claims (7)

1. Process for the production of a steel construction part provided with an aluminium-based anticorrosive coating, comprising the following process steps: a) to provide a flat steel product with an aluminium-based anti-corrosion coating; b) heating the coated steel flat product in a hot forming furnace, the furnace atmosphere of which contains 16% by volume to 20% by volume of oxygen and has a dew point of -20 °C to 15 °C; c) optionally clamping the coated steel flat product in the hot forming furnace; d) transport the heated flat steel product to a forming tool; e) simultaneously forming and cooling the flat steel product in the forming tool. 2. Method according to claim 1, characterized in that the steel used for the steel substrate of the flat steel product consists of 0.1 - 0.4% by weight of C, 0.5 - 3.0% by weight of Mn, 0.05 - 0.5% by weight of Si, 0.01 - 0.2% by weight of Al, 0.005 - 1.0% by weight of Cr, 0.0005 - 0.01% by weight of B, 0.001 - 0.2% by weight of V, 0.001 - 0.1% by weight of Ti, 0.001 - 0.1% by weight of Nb, optionally 0.01 - 0.4% by weight of Ni, optionally 0.01 - 0.8% by weight of Cu, optionally 0.002 - 0.5% by weight of Mn, 0.001 ... 1.0% by weight of Mo, optionally 0.001-1.0% by weight of W, up to 0.01% by weight of N, and the balance consisting of iron and unavoidable impurities, where up to 0.1% by weight of P and up to 0.05% by weight of S can be attributed to the unavoidable impurities. 3. Method according to claim 1 or 2, characterized in that the anticorrosive coating present on the steel substrate consists of 3 - 15% by weight of Si as well as optionally one or more elements selected from the group consisting of iron, transition metals other than iron, alkaline earth metals and mixtures thereof, in the following contents: 2 - 3.5% by weight of Fe, 0.05 - 2% by weight of transition metals other than Fe, 0.05 - 2% by weight of alkaline earth metals, and the remainder consisting of aluminium and unavoidable impurities. 4. Method according to one of the preceding claims, characterized in that the atmosphere of the hot forming furnace consists of at least 16% by volume of oxygen, up to 1.5% by volume of one or more noble gases, and the remainder in each case of nitrogen and unavoidable impurities. 5. Method according to one of the preceding claims, characterized in that the ratio of the volume proportion of nitrogen to the volume proportion of oxygen in the hot forming furnace is between 3.80 and 5.30. 6. Method according to one of the preceding claims, characterized in that the coated steel flat product is heated to a temperature between 830 and 950 °C. 7. Method according to one of the preceding claims, characterized in that the residence time of the flat steel product in the hot forming furnace is 2 to 15 minutes.