Method for producing a sheet steel product protected against corrosion

Abstract

A cost-favorable process for production of corrosion-resistant sheet steel products, having good characteristics of use for certain application purposes includes applying a zinc-containing coating by electro-galvanizing a flat steel product, finally cleaning mechanically and/or chemically the flat steel product, applying a magnesium-based coating to the finally cleaned zinc-containing coating by means of vapour deposition, and heat treating the coated flat steel product to form a diffusion or convention layer between the zinc-containing coating and the magnesium-based coating at a temperature of 320 .degree. C. to 335 .degree. C. under normal atmosphere.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a National Phase Application of International Application No. PCT/EP2006/066632, filed on Sep. 22, 2006, which claims the benefit of and priority to German patent application no. DE 10 2005 045 780.0, filed Sep. 23, 2005, which is owned by the assignee of the instant application. The disclosure of each of the above applications is incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

[0002] The invention relates to a process for manufacturing corrosion-resistant flat steel products, which are provided at least with a first zinc-containing coating, and a second coating lying thereon, which is based on pure magnesium or a magnesium alloy. Such processes are used for example to produce sheet steel, which due to its optimized corrosion resistance is particularly suitable for use in the construction, domestic appliance or motor vehicle industries.

BACKGROUND

[0003] Coatings, which in the predominant number of applications, consist of zinc or zinc alloys are applied to sheet steel in order to improve its corrosion resistance. Such zinc or zinc alloy coatings, due to their barrier and cathodic protection effect, ensure very good corrosion resistance of the coated sheet steel. However despite the quality already achieved up till now, higher and higher requirements in the corrosion resistance and general characteristics of coated sheet steel are demanded by the processors.

[0004] At the same time apart from the heavy cost pressure there is a need for better workability of coated sheet steel. In particular surface qualities optimized in relation to the respective intended purpose are demanded.

[0005] These demands in practice cannot be met alone by increasing the coating thickness, since on the one hand this is countermanded by economic and ecological reasons and on the other hand increasing the coating thickness involves a general degradation in the formability of sheet steel galvanized in this way.

[0006] Galvanized sheet steel is usually converted to consumer articles by forming, joining, organic coating (for example painting) or similar processes. Particularly in the field of motor vehicle body construction the bonding together of preformed steel parts is gaining acceptance. A further important factor is the formability of the coatings, that is to say their ability to withstand even greater transforming stresses, as they occur for example in the case of deep-drawing, without serious damage. None of these demands can be met to the same degree with conventional pure-galvanized products. Rather, conventionally coated sheet steel usually has particularly good characteristics as regards a certain requirement feature, while shortcomings must be accepted as regards other requirement features.

[0007] Thus for example hot-dip galvanized sheet steel is characterized by high corrosion resistance in both the unpainted as well as in the painted state. Although electro-galvanized sheet steel, in comparison to hot-dip galvanized sheet steel, generally has a further improved surface quality and equally improved bonderizing-ability in preparation for paint finishing, it must be considered that the production of electro-galvanized sheet steel is more cost-intensive than hot-dip galvanizing due to higher energy consumption and the waste disposal requirements, which the wet chemical process entails.

[0008] An improvement in the performance characteristics of galvanized sheet steel can be obtained by applying a second layer, which is based on pure magnesium or a magnesium alloy, to the first protective layer formed by galvanizing. A characteristic combination is achieved by application of this second magnesium-containing layer, wherein the characteristics of the first zinc-containing layer and the second magnesium-based layer are optimally enhanced.

[0009] In order to be able to utilize this optimum characteristic combination of the different layers to the full, the coating process is preferably carried out in such a way that breakdown of the layers is avoided. For this reason a diffusion or convection layer is formed between the zinc-containing and the magnesium-based layer, which ensures the magnesium-containing layer adheres firmly to the zinc layer.

[0010] A process, which permits a second layer to be applied to a sheet steel previously coated with a corrosion-protective coating, is for example disclosed by the German Patent DE 195 27 515 C1 or the corresponding European Patent EP 0 756 022 B1. The corrosion-resistant sheet steel manufactured by this process has enhanced forming and spot weld ability. For this reason the sheet steel provided with the zinc layer by hot-dip galvanizing or electro-galvanizing is firstly cleaned mechanically or chemically. By means of a suitable PVD (physical vapor deposition) process, a top layer is then deposited on the previously zinc-coated steel substrate. Afterwards the strip coated in this way undergoes heat treatment, which is carried out for at least ten seconds within a temperature range of 300.degree. C.-400.degree. C. in an inert gas or oxygen-lean atmosphere. As the result of this heat treatment the metal of the coating diffuses into the first zinc-containing corrosion protective layer lying on the steel substrate.

[0011] In order to be able to precisely control the diffusion process and to achieve good uniformity of the top layer, the sheet steel in the course of the prior art process, before the vacuum coating, undergoes vacuum pre-treatment by ion bombardment or plasma treatment. The galvanized steel substrate to be plated with the second layer of metal is fine-cleaned and conditioned by this pre-treatment so that the metal, deposited in the subsequent PVD process, is distributed widely and densely as a thin layer over the entire zinc coating. Corresponding fine cleaning is necessary, according to the statements of the professional world, particularly if a magnesium-based coating is applied as an external layer to galvanized sheet steel in order to improve its bonding and painting performance.

[0012] Despite the characteristic improvements attainable by using the method described in DE 195 27 515 C1 or EP 0 756 022 B1, this process has not become generally accepted in practice. This is due inter alia to the high construction and operating costs, which are incurred when setting up and maintaining a production line designed for executing the prior art process. These are caused inter alia because a large part of the stages in the prior art process must be carried out under vacuum, in order to manufacture flat steel products plated with at least a zinc coating and a surface layer applied thereon, which meet the strict requirements of the users. Furthermore on an industrial scale it has proven difficult, in the case of economic continuous production, within the narrow time window prescribed in DE 195 27 515 C1, to heat the strip to 300-400.degree. C. with homogeneous temperature distribution over the strip profile.

SUMMARY OF THE INVENTION

[0013] In one aspect, the invention is directed to a process, which permits economical production of corrosion-resistant sheet steel with good performance characteristics for certain application purposes.

[0014] In one aspect, the invention includes a process for manufacturing a flat steel product made from corrosion-resistant steel, wherein a zinc-containing coating is applied by electro-galvanizing to a flat steel product, wherein the flat steel product if required is finally cleaned mechanically and/or chemically, wherein a second magnesium-based coating is applied directly to the finally cleaned zinc-containing coating by means of physical vapor deposition and wherein under normal atmosphere after application of the second coating, post heat treatment of the coated flat steel product is carried out for forming a diffusion or convection layer between the zinc-containing and the magnesium-based coating, at a heat treatment temperature of 320.degree. C. to 335.degree. C.

[0015] In accordance with the invention, the steel substrate, which is a flat product such as strip or sheet, made from low carbon steel, is firstly galvanized in a conventional way and then cleaned mechanically or chemically in a way, which is equally conventional. Mechanical or chemical cleaning in this case can take place alternatively or in combination, in order to ensure the surface of the zinc coating is as free as possible of grease and loosely adhering zinc material or other residues.

[0016] For the invention it is essential that the galvanized flat steel product is completely clean at the end of this cleaning. Thus deviating from the notion, prevailing up until now in the professional world that such an intermediate step is indispensable, with the process according to the invention no further fine cleaning takes place before the magnesium-containing coating is deposited on the zinc-layer. Instead according to the invention the flat steel product, plated with the zinc layer, is fed in the purely mechanically or chemically final cleaned state into the physical vapor deposition chamber/module, where it is provided with the magnesium-containing external layer.

[0017] Surprisingly it has also been shown that a previously galvanized steel sheet or strip, provided in such a way with a magnesium layer, while dispensing with prior reactive plasma cleaning, apart from a surface quality optimized in relation to its optical appearance possesses a bonding performance, which meets all requirements arising in the practical use of such sheet steel.

[0018] A test for evaluating bonding performance of coated sheet steel, used in the motor vehicle and steel-making industry, is the so-called "adhesive bead test".

[0019] In this test a commercially available structural adhesive, suitable for bonding body components, is applied to the previously degreased surface to be examined. The adhesive is applied in the form of two parallel adhesive beads with a height of 4-5 mm and a width of about 10 mm. In order to ensure standard conditions, the geometry of the bead is then adjusted by means of a template. After the adhesive has hardened, possibly assisted by heat, the sheet steel is bent at an angle of approx. 100.degree.. Due to tension between the adhesive and the coating surface, produced by bending, in this case the adhesive bead usually firstly breaks vertically to the specimen surface and then peels away along the specimen surface.

[0020] In the case of coated sheet steel with poor bonding performance peeling away takes place in the transient area between the individual coatings or between the lowest coating and the steel substrate. With the method of production according to the invention on the other hand the peeling action, if it occurs at all, is limited to the border between the free surface of the outer lying coating or to the area of the adhesive bead itself. That is to say, despite simplification of the process achieved by the invention, in the case of sheet steel provided according to the invention with a zinc-magnesium plating system, the applied coatings adhere so firmly amongst themselves and to the steel substrate, that in the adhesive bead bending test, the adhesive does not peel away in the coatings or between the coatings and the steel substrate, but at most between the adhesive and the coating or only in the adhesive itself. The quality of an adhesive bond produced with a flat product according to the invention thus only depends on the bonding performance of the adhesive on the surface of the coating. Chipping or lifting of the plating system applied to the steel substrate is reliably prevented, despite fine cleaning being dispensed with according to the invention before vapor deposition of the magnesium layer, due to the heat treatment carried out according to the invention, following application of the magnesium coating.

[0021] Apart from the particularly good bonding performance, the stone chip resistance of flat steel products coated according to the invention also meets the requirements demanded in practice. Thus stone chip resistance, which corresponds to that of sheet steel coated in the conventional way, can be ensured for sheet steel coated according to the invention, particularly while maintaining the temperature windows of the heat treatment, indicated below as preferable dependent on the type of zinc coating, despite reactive plasma cleaning being dispensed with before physical vapor deposition plating.

[0022] Accordingly flat products manufactured according to the invention are particularly suitable for producing motor vehicle body components, which are formed by bonding individual components with one another.

[0023] A pre-condition for the good bonding performance achieved according to the invention is that the steel strip, vapor deposition plated according to the invention with the magnesium layer while dispensing with fine cleaning, undergoes heat treatment following vapor deposition, during which time it is held within the temperature range of 320.degree. C. to 335.degree. C., in order to form the diffusion or convection layer between the zinc coating and the magnesium layer. The temperatures of the heat treatment are preferably purposefully selected with regard to as good as possible bonding performance of the finished flat steel product, so that in each case they lie in the upper spectrum of the optimum temperature range for the respective application.

[0024] With respect to suitability of the process according to the invention for economic industrial use, it is of prime importance that the post heat treatment according to the invention can be carried out in air. This also contributes to reducing the capital expense and the costs generally linked with carrying out the process according to the invention to a minimum.

[0025] The post heat treatment is preferably carried out so that the coated strip in each case is held for a duration of up to 15 seconds, in particular 5-10 seconds, in the range of the optimum heat treatment temperature specified by the invention, so that its surface when leaving the heat-treatment furnace is at the correct heat treatment temperature.

[0026] Normal measuring instruments, such as temperature sensors placed on the strip surface can be used for measuring the respective treatment temperature; said measuring instruments are positioned for example in the discharge region of the furnace at a place, where on the one hand their signals and function are no longer disturbed by the operation of the furnace and on the other hand it is ensured that no substantial cooling of the strip takes place on leaving the furnace. Suitable positioning of the measuring instrument is particularly important if an induction furnace with correspondingly straying electromagnetic fields is used for post heat treatment.

[0027] The zinc is applied by electro-galvanizing, thus optimized characteristic combinations arise in the case of the flat products manufactured according to the invention, if the heat treatment temperature selected during the post heat treatment is 320.degree. C. to 335.degree. C. When this temperature is maintained, it is possible to ensure in an especially reliable way that no Fe--Zn rich phases are formed in the plating layer, as a result of which the bonding characteristics of sheet steel coated according to the invention might be impaired.

[0028] Any PVD process, which is already proven in practice for this purpose, can be used for physical vapor deposition of the magnesium or the magnesium alloy on the galvanized steel substrate.

[0029] Practical trials have shown that the working results achieved with the process according to the invention can be further improved if the sheet steel provided with the zinc-containing coating, in the course of its final cleaning, is chemically pre-conditioned by rinsing with a suitable pre-conditioning agent. For this purpose the galvanized steel strip can be rinsed with an alkaline solution in the course of chemical final cleaning.

[0030] Likewise with regard to an optimized plating result, it may be advantageous if the chemical final cleaning for example comprises pickling the steel substrate by rinsing with an acid, in particular hydrochloric acid. Following pickling, rinsing with de-mineralized water can ensue in order to remove residues, still remaining on the zinc coated sheet after pickling, as completely as possible.

[0031] Further optimization of the coating result can be achieved if the steel substrate provided with the zinc-containing coating has a roughness Ra on its free surface of at least 1.4 .mu.m, in particular 1.4-1.6 .mu.m, when entering the physical vapor deposition, with roughness levels of more than 1.4 .mu.m being advantageous. Likewise it is advantageous for optimum adhesion of the magnesium coat to the zinc coating, if the zinc-coated flat steel product has a nib rate RPC of at least 60 per cm when entering the physical vapor deposition. The nib rate RPC and average roughness Ra are calculated by the contact stylus procedure, wherein when determining average roughness Ra the methods used are those indicated in DIN EN ISO 4287:1998 and when determining the nib rate RPC the methods are those indicated in the Iron and Steel Test Sheet September 1940.

[0032] Furthermore it has proven advantageous for the result of the physical vapor deposition if the flat steel product, provided with the zinc-containing coating, before entering the physical vapor deposition, is heated to above ambient temperature, however to a temperature below the alloy temperature or held at this. Practical trials have shown that the temperatures particularly suitable for this purpose lie in the range of 230.degree. C.-250.degree. C., in particular approx. 240.degree. C.

[0033] The invention therefore makes available a process, which can be carried out particularly economically in a continuously running operation and provides a product that due to its surface quality and bonding performance is particularly suitable for producing components of motor vehicle bodies with application of joining techniques, such as inter-bonding.

BRIEF DESCRIPTION OF THE DRAWING

[0034] FIG. 1 is an inverted FE-SEM photograph of a cross slice specimen of a steel strip coated in accordance with an embodiment of the invention and heat-treated at a temperature of 332.degree. C.

DESCRIPTION

[0035] The invention is described in detail below on the basis of two exemplary embodiments.

Exemplary Embodiment 1

[0036] A module for PVD plating and post heat treatment has been integrated into an existing conventional plant for continuous steel strip electro-galvanizing behind the conventional lines used for galvanizing and in front of the plant for final treatment of the finish-coated steel strip.

[0037] The steel strip firstly electro-galvanized in the known way in the conventional galvanizing lines of the plant, converted in this manner, after the galvanizing process and final cleaning likewise carried out in the conventional plant, is fed into the module for PVD plating and post heat treatment, where it is PVD plated and post heat treated. Afterwards the steel strip is returned to the conventional plant, in which for example it is phosphatized and oiled within the context of final treatment.

[0038] Steel qualities, which are typical of motor vehicle manufacture, are considered as material for the steel strip, processed in this plant and having normal dimensions. It has proven particularly advantageous if the average roughness of the cold rolled steel used for the electro-galvanized sheet lies at the upper limit of the motor vehicle-standard Ra specification for external parts of 1.1-1.6 .mu.m. A further increase in the Ra value above 2 .mu.m would be advantageous as regards the adhesive power of the coating and the bonding performance associated therewith, but under economic criteria at present it does not appear expedient since today such a product would not comply with the specifications of the motor vehicle customers.

[0039] A nib rate value RPC of>60 per cm is preferred. Both values can also be positively influenced during the electro-galvanizing process. A further possibility of controlling these values consists of a cementation process as the ultimate stage of final cleaning.

[0040] At strip speeds of 20-180 metres per minute the steel strip is firstly provided conventionally by way of electrolysis on either side with a zinc deposit of 3 .mu.m in vertically arranged electrolysis cells by means of soluble anodes. After rinsing and drying the now galvanized steel strip, the galvanized substrate is thoroughly finally cleaned and prepared for application of the magnesium-containing coating.

[0041] In order to optimize the result of subsequent physical vapor deposition however, it may be advantageous, as part of the final cleaning, to include pickling of the galvanized steel strip, wherein the steel strip is kept in each case for 5 seconds in a 0.5% hydrochloric acid bath heated to 20.degree. C. In order to neutralize the acid, the steel strip was then rinsed with de-mineralized water.

[0042] The steel strip cleaned this way, after passing through several compression phases, enters a vacuum chamber, in which without any further treatment stage magnesium physical vapor deposition is carried out by means of a PVD process using a commercial JET evaporator. In order to ensure a constant magnesium thickness of 300 nm at varying strip speeds, the JET evaporator by suitable heat or mechanical means is able to supply evaporation rates of between 6 .mu.m.times.meter per minute and 54 .mu.m.times.meter per minute. Via a further number of compression phases the steel strip, now also plated with a magnesium layer, is then again conveyed to normal atmosphere.

[0043] Treatment by means of NIR emitters is used in this case for post heat treatment. The heating-up time here depends on the strip speed, but can be varied by switching off individual modules. The peak temperature of the heat treatment according to the invention is 327.degree. C..+-.7 K. In order to reliably maintain this narrow temperature window under the conditions of industrial application, a special image-rendering pyrometric process is used, which makes it possible to accurately control the temperature heat treatment according to the invention locally and with respect to time. Different steel substrates and coating conditions in this case may cause deviating emissivities, so that extensive calibration is necessary.

[0044] After a free strip run of 10 metres, the steel strip is cooled down by means of water. The residual heat in the strip is controlled so that the strip dries independently.

[0045] FIG. 1 as an inverted illustration shows an FE-SEM photograph of a cross slice specimen of steel strip coated according to the invention and heat-treated at a temperature of 332.degree. C. The advantageous layered structure, with the steel substrate S, the zinc layer Z applied thereon by electro-galvanizing and the magnesium-containing Zn--Mg coating M lying on the zinc layer Z, is clearly recognizable there. The layer, to be seen above the coating M, is bedding-in compound E, which was required for preparing the cross slice.

Exemplary Embodiment 2

[0046] Under the same process conditions at a strip speed of 36 meters per minute as well as with an evaporation rate, increased to 96 .mu.m.times.meter per minute by suitable constructional means, of the evaporator at a strip speed of 64 metres per minute, magnesium deposits of 1500 nm were achieved and thermally alloyed according to the invention. The advantageous forming of the zinc-magnesium alloy coating was also demonstrated in these tests.

Claims

1. A process for manufacturing corrosion-resistant flat steel products, comprising: applying a zinc-containing coating by electro-galvanizing a flat steel product, mechanically and/or chemically finally cleaning the flat steel products, directly applying to the finally cleaned zinc-containing coating by means of vapor deposition a second coating, the second coating being magnesium based, and under normal atmosphere heat treating the flat steel product after applying the second coating to form a diffusion or convection layer between the zinc-containing coating and the magnesium-based coating, at a heat treatment temperature of 320.degree. C. to 335.degree. C.

2. The process according to claim 1, wherein the flat steel product provided with the zinc-containing coating, in the course of its final cleaning, is chemically pre-conditioned by rinsing with an alkaline pre-conditioning agent.

3. The process according to claim 1 wherein the flat steel product, provided with the zinc-containing coating, in the course of its final cleaning, is pickled by rinsing with an acid.

4. The process according to claim 3, wherein after pickling the flat steel product is rinsed with de-mineralized water.

5. The process according to claim 1 wherein heat treating the flat steel product after applying the second coating is carried out within a duration of 15 seconds at most.

6. The process according to claim 1, wherein the flat steel product, provided with the zinc-containing coating has on its free surface a roughness Ra of at least 1.4 .mu.m prior to vapor deposition.

7. The process according to claim 1, wherein a nib rate RPC of the flat steel product, provided with the zinc-containing coating, prior to vapor deposition, is at least 60 per cm.

8. The process according to claim 1, wherein the flat steel product, provided with the zinc-containing coating, before entering the vapor deposition, is heated to a temperature above ambient temperature but below an alloying temperature of the magnesium coating.

9. A process for manufacturing corrosion-resistant flat steel products, comprising: applying a first coating including zinc by electro-galvanizing a flat steel product, applying a second coating comprising magnesium by vapor deposition to the first coating without fine cleaning the first coating, and heat treating the flat steel product with first and second coatings under normal atmosphere to form a diffusion or convection layer between the first and second coatings, at a heat treatment temperature of 320.degree. C. to 335.degree. C.

10. The process according to claim 9 wherein heat treating the flat steel product after applying the second coating is carried out within a duration of 15 seconds at most.

11. The process according to claim 9, wherein the flat steel product, provided with the first coating has on its free surface a roughness Ra of at least 1.4 .mu.m prior to applying the second coating.

12. The process according to claim 9, wherein a nib rate RPC of the flat steel product including the first coating, but prior to vapor deposition of the second coating, is at least 60 per cm.

13. The process according to claim 9, wherein the flat steel product, provided with the first coating, before application of the second coating, is heated to a temperature above ambient temperature but below an alloying temperature of the second coating.