JP2010504420A - Method for producing corrosion-resistant flat steel products - Google Patents

Abstract

  The present invention relates to a manufacturing method for cost reduction of a steel sheet having corrosion resistance and good use relevance for a certain application. To that end, the method of the present invention for producing a corrosion resistant sheet steel product electrolytically applies a zinc-containing layer coating to the sheet steel product and, if necessary, mechanically attaches the product. And / or chemically cleaned and immediately applied to the cleaned zinc-containing layer coating by vapor deposition, and after the second layer is applied, subsequently coated The sheet steel product consists of heat treating the standard atmosphere at a processing temperature of 320 ° C. to 335 ° C. to form a diffusion or convection layer between the zinc-containing coating and the magnesium-based layer.

Description

Detailed Description of the Invention

  The present invention relates to a method for producing a corrosion-resistant flat steel product, provided at least with a first zinc-containing coating and an overlying second coating (based on pure magnesium or magnesium alloy). The method is used, for example, in the manufacture of sheet steel (Stahlfeinblechen), which is particularly suitable for use in construction, indoor appliances or the automotive industry due to its optimized corrosion resistance.

  In many applications, coatings made of zinc or zinc alloys are applied to sheet steel to improve its corrosion resistance. Said zinc or zinc alloy coating guarantees a very good corrosion resistance of the coated sheet steel due to their barrier and cathodic protection effect. However, despite the quality achieved so far, there is a high demand by manufacturers for the corrosion resistance and overall properties of coated sheet steel.

  At the same time, apart from the great cost pressure, there is a demand for higher workability of coated sheet steel. In particular, there is a need for surface quality that is optimized for each application.

  In fact, simply increasing the coating thickness cannot meet these requirements. Because, on the one hand, it is disadvantageous for economic and environmental reasons, and on the other hand, increasing the coating thickness results in an overall deterioration in the formability of the sheet steel galvanized in the manner described above. It is because it makes it.

  Galvanized sheet steel is typically converted into consumer goods by molding, bonding, organic coating (eg, painting), or similar processing. In particular, in the field of automobile body structures, bonding with pre-formed steel members is permitted. A further important factor is the formability of the coating, ie the ability to withstand a higher deformation pressure without serious damage, for example when deep drawing. For conventional pure-galvanized products, the requirements cannot be met to the same extent. Rather, by conventional methods, coated sheet steel usually has good properties for some requirements, while the disadvantages for other requirements must be tolerated.

  Thus, for example, soaked galvanized sheet steel is characterized by high corrosion resistance both in the uncoated layer and after the coated layer. Compared to doubly pickled galvanized sheet steel, electrolytic galvanized sheet steel usually has better improved surface quality and improved bonderizing ability (Phosphatierbarkeit) in preparation for paint finish, but electrolytic galvanized The manufacture of sheet steel is more cost intensive than galvanizing because of higher energy consumption and waste disposal requirements with thermal chemical methods.

  The performance characteristics of the galvanized sheet steel can be improved by applying a second layer based on pure magnesium or magnesium alloy to the first protective layer formed by galvanizing. By providing the second magnesium-containing layer, a combination of characteristics can be obtained, and the characteristics of the first zinc-containing layer and the second magnesium-based layer are optimally enhanced.

  In order to make full use of the optimal feature combinations of the different layers, the coating method can preferably be carried out in a manner that avoids layer collapse. For the above reasons, a diffusion or convection layer (Konvektionsschicht) is formed between the zinc-containing layer and the magnesium-based layer, where it is ensured that the magnesium-containing layer is firmly bonded to the zinc layer. .

  A method for applying a second layer to a sheet steel that is pre-coated with a corrosion-resistant coating is disclosed, for example, by German Patent DE 195 275 515 C1 or the corresponding European Patent EP 0756022 B1. The corrosion-resistant sheet steel produced by the above method has high formability and spot weldability. For this reason, the sheet steel to which the zinc layer is applied by dipping galvanizing or electrolytic galvanizing is first mechanically or chemically cleaned. The top layer is deposited on a pre-zinc-coated steel substrate by a suitable PVD (physical vapor deposition) method. Thereafter, the strip to be coated by the above method is subjected to a heat treatment carried out in an atmosphere inclined to oxygen or an inert gas within a temperature range of 300 ° C. to 400 ° C. for at least 10 seconds. As a result of the heat treatment, the coating metal diffuses into the first zinc-containing corrosion-resistant layer on the steel substrate.

  In order to accurately control the diffusion process and achieve good uniformity of the top layer, in the course of prior art methods, in the course of vacuum coating, in vacuum, pretreatment by ion irradiation or plasma treatment is performed in a sheet steel. To apply. The galvanized sheet substrate to be plated with the second layer of metal is fine-cleaned and conditioned by pretreatment so that the metal deposited in subsequent PVD processes is widely spread throughout the zinc coating and Distributed as a dense thin layer. According to the industry, a corresponding fine cleaning is necessary, especially when applying a magnesium-based coating as an outer layer to galvanized sheet steel to improve its bonding and paintability.

  Despite the improvements in characteristics achieved using the methods described in DE19527515C1 or EP0756022B1, the method was in fact generally not accepted. This is because of the particularly high construction and operating costs when setting up and maintaining a production line designed to implement prior art methods. They are, among other things, plated with at least a zinc coating and a layer applied thereon, and most steps in prior art methods are carried out under vacuum to produce flat steel products that meet the strict requirements of users. Because it must be done. Furthermore, also for industrial scale, in the case of economical continuous production, the strip can be heated to 300-400 ° C. with a homogeneous temperature distribution across the strip profile within the narrow time window described in DE 19527515 C1. I know it ’s difficult.

  The object of the present invention is to create a method that allows the economical production of corrosion-resistant sheet steel with good performance characteristics for a particular application.

The object is according to the invention,
Applying zinc-containing coating to flat steel products by electrolytic galvanizing,
If necessary, the steel product is finally cleaned mechanically and / or chemically,
A second magnesium-based coating is applied directly to the zinc-containing coating that has been finally cleaned by physical vapor deposition, and after application of the second coating, post-heat treatment of the coated flat steel product is performed at 320 ° C. under standard atmosphere. Carried out at a heat treatment temperature of ~ 335 ° C to form a diffusion or convection layer between the zinc-containing coating and the magnesium-based coating;
It is achieved on the basis of the above prior art through a method for producing a corrosion-resistant steel flat bar product.

  In accordance with the present invention, a steel substrate that is a flat steel product (eg, a low carbon steel strip or sheet) is first galvanized in a conventional manner and then mechanical or chemical (as well as conventional). Method). In this case, one or a combination of mechanical or chemical cleaning is performed to ensure that grease and lightly deposited zinc material or other residues are as little as possible from the surface of the zinc coating. To do.

  For the present invention, it is important that the galvanized flat steel product is thoroughly cleaned at the end of the cleaning. Thus, deviating from the widespread use in the industry to date that the intermediate step is essential, the method of the present invention further performs fine cleaning prior to the magnesium-containing coating deposited on the zinc layer. There is no need to do. Instead, according to the method of the present invention, a physical vapor deposition that provides a magnesium-containing outer layer is applied to a flat steel product that is plated with a zinc layer, with a final mechanically or chemically final cleaning.

  Surprisingly, the pre-galvanized sheet steel or strip, provided with a magnesium layer in the manner described above, has a quality against its visual appearance versus that of prior art reactive plasma cleaning. Apart from the surface to be improved, it has a bonding performance (Klebeignung) that meets all requirements arising from the actual use of the sheet steel.

  The test used to evaluate the bonding performance of coated sheet steel used in the automotive and steel manufacturing industries is the so-called “adhesive bead test”.

  In the test, a commercially available structural adhesive suitable for bonded car body parts is applied to a pre-degreased test surface. The adhesive is applied in the form of two parallel adhesive beads 4-5 mm high and about 10 mm wide. The bead placement is adjusted by the template to ensure a standard state. After the adhesive is cured (optionally with the aid of heat), the steel plate is bent to an angle of about 100 °. Due to the tension between the adhesive and the coating surface caused by the pending, in this case the adhesive bead usually breaks perpendicular to the specimen surface and then peels along the test surface.

  In the case of coated sheet steel with poor bonding performance, there is an Abschaelen in the transitional region (Uebergangsbereich) between individual coatings or between the lowest coating and the steel substrate. On the other hand, when using the production method according to the present invention, even if a peeling action occurs, it is limited to the boundary between the free surfaces of the outer coating or to the area of the adhesive bead itself. That is, in the case of the sheet steel of the present invention provided with a zinc-magnesium plating system, despite the simplification of the method achieved by the present invention, the applied coating adheres firmly between them. And adheres firmly to the steel substrate, so that in the adhesive bead bending test, even if there is at most between the adhesive and the coating, or only the adhesive itself may peel off, The adhesive does not peel between or between the coating and the steel substrate. The quality of the adhesive bond produced by the flat steel product of the present invention is therefore dependent on the bond performance of the adhesive on the coating surface. In the present invention, since the heat cleaning is performed after the magnesium coating, fine cleaning before the magnesium layer deposition can be omitted, so that it is possible to reliably prevent chipping or lifting of the plating system applied to the steel substrate. it can.

  Apart from particularly good bonding performance, the stone chip resistance of the flat steel product coated according to the invention also meets the requirements in practice. Thus, despite the elimination of reactive plasma cleaning prior to physical vapor deposition, a stone equivalent to that of sheet steel coated by conventional methods while maintaining the heat treatment temperature frame determined by the type of zinc coating. Chip resistance can be ensured for the coated sheet steel of the present invention.

  Accordingly, the flat steel product produced according to the present invention is particularly suitable for the production of automotive body parts formed by joining individual parts together.

  The preconditions for good bonding performance achieved by the present invention are the heat treatment in the temperature range of 320 ° C. to 335 ° C. after the deposition, without fine cleaning, on the steel strip vapor deposited with the magnesium layer according to the present invention. To form a diffusion or convection layer between the zinc coating and the magnesium layer. Since the temperature of the heat treatment is specifically deliberately selected for the best possible bonding performance of the finished flat steel product, in each case the temperature is in the upper temperature range of the optimum temperature range for the individual application. It is in.

  With regard to the suitability of the method of the invention for economic and industrial applications, it is of utmost importance that the post-heat treatment of the invention can be carried out in air. This leads to a minimal reduction in capital costs and costs normally associated with the implementation of the method of the present invention.

  Hold the coated strip for 15 seconds (especially 5-10 seconds) within the optimum heat treatment temperature range specified by the present invention so that the surface when leaving the heat treatment furnace is at the correct heat treatment temperature. In addition, it is preferable to carry out post heat treatment.

  Temperature detectors installed on the strip surface can be used for the measurement of individual process temperatures; on the one hand, where the operation and operation of the furnace does not disturb their signal and function, while the furnace The measuring instrument is installed, for example, in the discharge area of the furnace, where it is ensured that no substantial cooling of the strip occurs when leaving. In particular, when an introduction furnace having a corresponding stray electromagnetic field is used for post-heat treatment, proper installation of the measuring device is important.

  Since zinc is applied by electrolytic galvanizing, an optimal combination of features occurs in the flat steel product produced according to the present invention when the heat treatment temperature selected during post heat treatment is between 320 ° C and 335 ° C. . If the temperature is maintained, it can be ensured in a reliable manner that no Fe-Zn rich layer is formed in the plating layer, as a result of the bonding characteristics of the sheet steel coated according to the invention. May decrease.

  Any PVD method that has already been demonstrated for that purpose can be used for physical vapor deposition of magnesium or magnesium alloys on galvanized steel substrates.

  When chemically preconditioning a sheet steel to which a zinc-containing coating is applied by rinsing with a suitable preconditioning agent in its final cleaning process, the work results achieved with the method according to the invention can be further improved. It has been found by actual tests. For this purpose, the galvanized steel strip can be rinsed with an alkaline solution in the course of a chemical final wash.

  Similarly, for optimized plating results, it is advantageous if the final chemical cleaning involves pickling of the steel substrate by rinsing with acid (especially hydrochloric acid). By rinsing with demineralized water after pickling, it is ensured that the residue remaining in the zinc-coated sheet after pickling is removed as completely as possible.

  When entering physical vapor deposition at a roughness level above 1.4 μm, which is advantageous, a steel substrate to which a zinc-containing coating is applied has at least 1.4 μm (especially 1.4-1.6 μm) on its free surface. ), The coating result can be further optimized. Similarly, when entering physical vapor deposition, it is advantageous for optimum adhesion of the magnesium coating to the zinc coating that the zinc-coated flat steel product has at least 60 nibrate RPC per cm. is there. The nibrate RPC and the average roughness Ra are calculated by the contact needle method, where the method shown in DINENISO 4287: 1998 is used in determining the average roughness Ra and in determining the nibrate RPC. Uses the method shown in Iron and Steel Test Sheet September 1940.

  Furthermore, the flat steel product to which the zinc-containing coating is applied can be heated to or kept at a temperature below the alloy temperature and above the ambient temperature prior to entering physical vapor deposition. It has been found advantageous for physical vapor deposition results. It has been found by actual tests that a particularly suitable temperature for this purpose is in the range 230 ° C. to 250 ° C. (especially about 240 ° C.).

  Therefore, it is a method that can be carried out particularly economically in continuous work operations, and due to its surface quality and bonding performance, the application of bonding technology (for example to products particularly suitable for the production of automotive body parts) The present invention can provide such a method for providing interbonding (verkleben).

  The invention is described in detail below on the basis of two embodiments.

Embodiment 1
The module for PVD plating and post heat treatment is a continuous steel strip electrolytic galvanizer after the conventional line used for galvanizing and before the plant for final treatment of the finish coated steel strip. Integrated into a conventional plant for Ising.

  After the galvanizing process and the final cleaning carried out in the same way in a conventional plant, the steel strip processed in the above-mentioned manner that is electrolytic galvanized in a known manner in the conventional galvanizing line of the plant, Supplied in modules for PVD plating and post heat treatment (steel strips are PVD plated and post heat treated). The steel strip is then returned to the conventional plant where, for example, phosphating and beoelen in the range of the final treatment.

  Steel quality, the standard for automobile manufacturing, is processed in the plant and is considered as a material for steel strips with standard dimensions. It has been found to be particularly advantageous when the average roughness of the cold-rolled sheet used for the electrolytically galvanized sheet is at the upper limit of the automotive standard Ra standard for the outer part of 1.1 to 1.6 μm. . Further increases in Ra values above 2 μm are advantageous with respect to coating adhesion and associated bonding performance, but under current economic standards the product cannot meet the standards of automotive customers This is disadvantageous.

  Nibrate values RPC greater than 60 per cm are preferred. Both values can also act positively between electrolytic galvanizing processes. A further possibility to control these values consists of a cementation process as the final stage of the final cleaning.

  At a strip speed of 20-180 m / min, first a 3 μm zinc deposit on either side of the steel strip is provided by conventional electrolysis in an electrolytic cell arranged vertically by a soluble cathode. After rinsing and drying the galvanized steel strip, the galvanized substrate is thoroughly final cleaned and ready for application of the magnesium-containing coating.

  However, to optimize the results of subsequent physical vapor deposition, it is advantageous to include a galvanized steel strip pickling as part of the final wash, where a 0.5% hydrochloric acid bath heated to 20 ° C. In, hold the steel strip in each case for 5 seconds. The steel strip was then rinsed with demineralized water to neutralize the acid.

  After passing through several compression stages, the steel strip cleaned by the above method enters a vacuum chamber where it does not require any additional processing steps, by PVD method using a commercial JET evaporator. Conduct magnesium physical vapor deposition. To ensure a constant 300 nm magnesium thickness even under varying strip speeds, a JET evaporator with appropriate heating or mechanical means can be used between 6 nm × m / min and 54 nm × m / min. It can be provided at an evaporation rate. Through a number of further compression stages, the steel strip plated with the magnesium layer is then transported again into the standard atmosphere.

  In this case, treatment with an NIR emitter is used for post-heat treatment. Here, the heating time depends on the strip speed, but can be changed by cutting off the individual modules. The peak temperature of the heating time according to the present invention is 327 ° C. ± 7K. In order to ensure that the narrow temperature range is maintained under the conditions of industrial application, a special image rendering pyrometry (spezielles bildgebendes pyrometrisches Verfahren) is used to carry out the thermal heat treatment of the present invention over time. It can be accurately and locally controlled. In this case, since the emissivity deviates due to the difference in the steel substrate and coating conditions, a wide range of calibration is required.

  After running the strip freely for 10 m, the steel strip is cooled with water. The residual heat in the strip is controlled to dry the strip individually. FIG. 1 as an inverted view shows an FE-SEM photograph of a cross-section specimen of a steel strip coated according to the present invention and heat treated at a temperature of 332 ° C. An advantageous layered structure with a steel substrate S, a zinc layer Z applied thereon by electrolytic galvanizing, and a magnesium-containing Zn-Mg coating M on the zinc layer Z is clearly recognized. be able to. The layer visible on the coating M is the embedding compound E needed to prepare the cross section.

<< Embodiment 2 >>
According to the present invention, under the same process conditions with an evaporation rate increased to 96 nm × m per minute and a strip speed of 36 m per minute by appropriate structural means of the evaporator at a strip speed of 64 m per minute 1500 nm magnesium deposition was achieved and alloyed by heat. An advantageous formation of a zinc-magnesium alloy coating has also been demonstrated in the test.

Claims (8)

A method for producing a corrosion-resistant flat steel product comprising:
Applying zinc-containing coatings to flat steel products by electrolytic galvanizing;
If necessary, the steel flat product is mechanically and / or chemically final cleaned;
Applying a second magnesium-based coating directly to the final cleaned zinc-containing coating by vapor deposition; and
After the application of the second coating, a post-heat treatment of the coated flat steel product is carried out at a heat treatment temperature of 320 ° C. to 335 ° C. under a standard atmosphere to form a diffusion layer between the zinc-containing coating and the magnesium-based coating. Or form a convection layer;
Said method.   The method according to claim 1, characterized in that the flat steel product to which the zinc-containing coating is applied is chemically preconditioned by rinsing with an alkaline preconditioning agent in the course of the final cleaning.   3. A method according to claim 1 or 2, characterized in that the flat steel product to which the zinc-containing coating is applied is picked by rinsing with acid (especially hydrochloric acid) in the course of final cleaning.   4. A method according to claim 3, characterized in that after pickling, the flat steel product is rinsed with demineralized water.   The method according to claim 1, wherein the post-heat treatment is carried out within a maximum duration of 15 seconds.   6. A flat steel product to which a zinc-containing coating is applied has a roughness Ra of at least 1.4 [mu] m on its free surface when entering the vapor deposition. The method described in 1.   7. A method according to any one of the preceding claims, characterized in that the nibrate RPC of the flat steel product to be provided with a zinc-containing coating is at least 60 per cm, especially entering the deposition.   The flat steel product to which a zinc-containing coating is applied is heated or maintained at a temperature above ambient temperature and below the alloying temperature of the magnesium coating before entering the deposition. Item 8. The method according to any one of Items 1 to 7.

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