MX2008003506A - Method for producing a sheet steel product protected against corrosion - Google Patents

Abstract

The invention relates to a cost-saving production of steel sheets protected against corrosion and exhibiting good use-related properties for determined applications. For this purpose, the inventive method for producing a sheet steel product protected against corrosion consists in electrolytically applying a coating zinc-containing layer to the sheet steel product, if necessary, in mechanically and/or chemically cleaning said product, in immediately applying a second magnesium-based coating layer to the cleaned coating zinc-containing layer by vapour-phase deposition and, after applying said second layer, in subsequently heat-treating the coated sheet steel product at a normal atmosphere and a processing temperature ranging from 320 to 335°C in such a way that a diffusion or convection layer is formed between the coating zinc-containing and magnesium-based layers.

Description

METHOD TO PRODUCE A PRODUCT OF PROTECTED STEEL FILM AGAINST CORROSION DESCRIPTION OF THE INVENTION The invention relates to a method for producing steel sheet products protected against corrosion that are provided with at least a first coating layer containing zinc and a second coating layer based on pure magnesium or a Magnesium alloy that is on top of the previous one. These methods are used, for example, to produce thin sheets of steel which by virtue of optimizing their corrosion resistance are particularly suitable for use in the field of construction industry, household appliances or household appliances. the automotive industry. To improve its protection against corrosion, coatings are applied to the steel sheets, which in most cases consist of zinc or zinc alloys. By virtue of its barrier and cathodic protective effect, this type of zinc or zinc alloy coatings ensure a very good protection against corrosion of coated steel sheets. However, despite the quality already achieved so far, the demands of users are increasingly high with regard to the protection against corrosion and the general properties of coated sheets. REF. : 190717 In this aspect there is at the same time, in addition to a strong economic pressure, the requirement that the coated steel sheets can be processed better. In particular, optimized surface characteristics are required in relation to the respective application purpose. These requirements can not be satisfied in practice only by increasing the thickness of the coating, since on the one hand this is not good for economic and ecological reasons, and on the other hand the increase in the thickness of the coating is accompanied by a general worsening of the ability of these galvanized steel sheets for further processing. Further processing of the galvanized steel sheets to obtain objects of use is usually carried out by deformation, bonding, organic coating (for example, lacquering) or similar processes. It is gaining in importance, in particular in the field of the construction of car bodies, the gluing of preformed sheet parts to obtain complete construction groups of the bodywork. Another important characteristic is the deformation capacity of the coatings, that is to say, their capacity to withstand also higher deformation stresses as they occur, for example, with the deep drawing without suffering serious damage. Each one of these requirements can not be satisfied in the same way with the conventional products galvanized with zinc only. Rather, conventionally coated steel sheets generally have particularly good properties in the area of a certain required characteristic, while in the area of the other required characteristics it is necessary to accept less. Thus, for example, steel sheets galvanized to fire by melt immersion coating are characterized by a high protection against corrosion in the unlacquered state as well as in the lacquering. It is true that in comparison with fire-galvanized steel sheets, galvanized steel sheets in an electrolytic form usually further improve the surface quality and also improve their capacity for phosphating intended to prepare a lacquering. However, in this regard we must accept the fact that the production of galvanized steel sheets in an electrolytic form is more cost-intensive than fire galvanizing due to the greater introduction of energy and the confinement measures involved in the process. wet chemical. An improvement in the properties of use of the galvanized steel sheets can be obtained by applying a second layer based on pure magnesium or a magnesium alloy on the first tuning layer formed by the galvanizing. By applying this second layer containing magnesium a combination of properties is obtained in which the properties of the first layer containing zinc and the second layer based on magnesium are optimally complemented. In order to take advantage of this optimal combination of properties of the different layers, the coating process is preferably carried out in such a way that a continuous alloying of the layers is avoided. For this purpose, a diffusion or convection layer is formed between the zinc-containing layer and the magnesium-based layer, which guarantees the bonding of the magnesium base layer to the zinc layer. A method that allows the application of a second layer on a steel sheet previously provided with a protective coating against corrosion is known, for example, from DE 195 27 515 Cl or the corresponding EP 0 756 022 Bl. The stainless steel sheets protected against corrosion that are produced according to this method have better deformation and spot welding capabilities. For this purpose, the steel sheet provided with the zinc layer by fire galvanizing or electrolytic galvanizing is first cleaned mechanically or chemically. A coating layer is then deposited on the steel substrate previously coated with zinc by means of a suitable method of physical deposition in vaporized phase (PVD = Physical Vapor Deposition as it is known in English). Then, the tape thus coated runs for at least ten seconds a heat treatment which is carried out in an inert gas or oxygen deficient atmosphere in the temperature range of 300 ° C to 400 ° C. As a result of this heat treatment, the metal of the coating diffuses into the first zinc-containing corrosion protection layer on the steel substrate. In order to be able to precisely control the diffusion process and obtain a high uniformity of the cover layer, in carrying out the known method the stainless steel sheet is subjected to a vacuum pretreatment by ion bombardment or a plasma treatment prior to vacuum coating. By this pretreatment the galvanized steel substrate that must be covered with the second metal layer is finely cleaned and conditioned so that the metal deposited during the subsequent PVD coating is distributed over the zinc layer as a thin layer covering the full and dense all over the surface. According to the opinion of experts in the field, a deep cleaning is required in particular if a magnesium-based layer is applied as an external layer on a thin sheet of galvanized steel to improve its suitability for bonding and lacquering.

Despite the property improvements that can be obtained using the method described in DE 195 27 515 Cl and respectively EP 0 756 022 Bl, this process was not imposed in practice. This is due, among other things, to the high costs of installation and operation that are generated in the installation and maintenance of an installed production line to carry out the known method. These are caused inter alia because a large part of the operating steps of the known method must be performed in a vacuum in order to produce steel sheet products coated with at least one layer of zinc and a cover layer applied thereon which satisfies the high demands of users. Additionally, at an industrial technical level it is difficult to achieve a heating of the belt at 300-400 ° C with homogeneous distribution of the temperature over the cross section of the belt within the narrow window of time established in DE 195 27 515 Cl in an economic processing that is carried out continuously. The object of the invention is to create a method that allows the economic production of steel sheets protected against corrosion with good properties of use for certain application purposes. From the state of the art explained above, this problem is solved by a method for producing a product of steel sheet protected against corrosion in which, according to the invention, on a steel sheet product is applied by electrolytic deposition a coating layer containing zinc, in which the steel sheet product is subjected, if necessary, to a conclusive cleaning in mechanical and / or chemical form, in which directly on the coating layer containing zinc conclusively cleaned a second layer of magnesium-based coating is applied by vapor deposition, and in which after the second coating layer is applied a thermal after-treatment is carried out at normal atmospheric pressure of the coated steel sheet product to form a diffusion or convection layer between the zinc-containing coating layer and the magnesium-based coating layer a treatment temperature that is 320 ° C to 325 ° C. According to the invention, the steel substrate, of which it is a flat product such as a carbon-deficient steel strip or sheet, is first galvanized in a conventional manner and is also cleaned in a conventional manner by the mechanical or chemical means. Mechanical or chemical cleaning can be used alternatively or in combination to ensure a zinc coating surface that is substantially free of grease and free of loose zinc material and other debris. It is essential for the invention that the cleaning of the galvanized steel sheet product is conclusive at the end of this cleaning. Accordingly, unlike the idea that hitherto exists in the specialized field that it is not possible to forego such an intermediate stage, in the method according to the invention no additional fine cleaning prior to deposition no longer takes place. of the coating layer containing magnesium on the Zn layer. In contrast, according to the invention the steel sheet product provided with the zinc layer only enters the state conclusively cleaned mechanically and / or chemically in the vaporized deposition, where it is coated with the outer layer containing magnesium . It was unexpectedly discovered that also a sheet or strip of steel previously galvanized, provided with a layer of magnesium without intercalating a fine cleaning with plasma has, in addition to a surface quality optimized in regard to its optical appearance, a tendency to adhesion that satisfies all the requirements that are presented in the practical application for this type of sheets. A test that was introduced in the automotive industry and the steel production industry to dictate the adequacy of the adhesiveness of a coated steel sheet is the so-called "crawler track test". In this test it is applied on the surface to be tested, previously defatted, a commercial structured glue suitable for gluing body components. The glue is applied in the form of two glue tracks arranged parallel, whose width is approximately 10 mm with a height of 4-5 mm. To guarantee standardized conditions the geometry of the track is then adjusted by means of a template. After curing the glue, if necessary assisted by application of heat, the sheet is bent at an angle of approximately 100 °. By virtue of the stresses generated by bending between the glue and the coating surface, the glue caterpillar usually breaks first perpendicular to the test surface and then detaches along the test surface. In the case of coated sheets insufficiently suitable for bonding, the release takes place in the transition zone between the individual coating layers or between the lower coating layer and the steel substrate. However, with the production form according to the invention, the detachment process, and this if it happens, is limited only to the limit between the free surface of the outer covering layer or the area of the glue caterpillar. same That is to say that despite the simplification of the process achieved with the invention, in a steel sheet provided with a zinc-magnesium coating system in the form according to the invention, the applied coating layers adhere so strongly to each other. another and on the steel substrate that in the glue crawler test the release of the glue does not take place in the coating layers or between the coating layers and the steel substrate, but in any case between the glue and the coating or only on the glue itself. Accordingly, the quality of a bonded joint produced with the flat product according to the invention already only depends on the adhesion capacity of the glue on the surface of the coating. A detachment or cracking of the coating system applied on the steel substrate is safely avoided by the heat treatment which according to the invention is carried out following the application of the Mg coating, although according to the invention a fine cleaning prior to vapor deposition of the magnesium layer is dispensed with. In addition to the particularly good adhesive ability, also the stone impact resistance of the coated steel sheet products according to the invention satisfies the requirements that are required in practice. Thus, in particular if the temperature windows of the heat treatment which are indicated below are preferred as a function of the type of the zinc coating, it is possible to guarantee for stone sheets coated in accordance with the invention impact strengths of stone which are analogous. to those of the sheets coated in a conventional manner, in spite of dispensing with the fine cleaning with plasma prior to coating by vapor deposition. Therefore, the flat products produced according to the invention are particularly suitable for producing vehicle body components which are formed by individual sheet parts adhered to one another. The prerequisite for the good adhesive capacity obtained according to the invention is that the steel strip which, according to the invention, is coated vaporized with the magnesium layer, dispensing with the fine cleaning, passes a thermal treatment after the vaporized deposition. wherein it is maintained in a temperature range of 320 ° C-335 ° C to form the diffusion or convection layer between the zinc coating and the magnesium layer. In attention to a good adhesive ability as possible of the finished steel sheet product to process, the temperatures of the heat treat are selected objectively so that in each case they are in the upper range of the optimum temperature range for the respective application case. For the suitability of the method according to the invention for economic use on an industrial scale, it is of particular importance that the thermal after-treat according to the invention can be carried out in the air. This also contributes to reducing to a minimum the expense in apparatuses and consequently the costs associated with the execution of the method according to the invention. The thermal after-treat is preferably carried out in such a way that the coated tape is maintained in each case for a period of up to 15 seconds, in particular 5-10 seconds in the optimum treat temperature range specified in each case by the invention, so that when leaving the heat treat furnace its surface has the respective treat temperature. For the measure of the respective treat temperature it is possible to use the usual measuring devices as temperature sensors that are applied to rub on the surface of the tape, which, for example, in the exit area of the oven are placed in a site in which on the one hand its signals and function are no longer disturbed by the operation of the oven and on the other hand it is ensured that a substantial cooling of the belt leaving the oven has not yet taken place. Proper positioning of the measuring device is of particular importance if an induction furnace with correspondingly dispersed electromagnetic fields is used for thermal after-treat. The application of zinc takes place by electrolytic galvanization, thus, in the flat products processed according to the invention, optimum combinations of properties result if the treat temperature selected during the thermal after-treat is from 320 ° C to 335 ° C. If this temperature range is observed, it can be ensured with particular certainty that Fe-Zn-rich phases are not formed in the coating layer by means of which the adhesive qualities of a coated sheet according to the invention could worsen. For the vapor deposition of magnesium or magnesium alloy on the galvanized steel substrate it is possible to use all the PVD processes that have already been proven in practice for this purpose. Practical tests showed that the work results obtained by the method according to the invention can be further improved if the steel sheet provided with the zinc-containing coating is preconditioned chemically wet in the course of its conclusive cleaning by washing with a suitable preconditioning agent. For this purpose the galvanized steel tape can be washed with an alkaline solution in the course of chemical conclusive cleaning. It can also be favorable in view of an optimized coating result if this chemical conclusive cleaning comprises, for example, a pickling of the steel substrate by washing with an acid, in particular hydrochloric acid. After the descaling can be followed by a wash with completely desalinated water in order to completely eliminate the remaining acid residues on the galvanized sheet after pickling. It is possible to obtain further optimization of the coating result if the steel substrate provided with the zinc-containing coating has a roughness Ra of at least 1.4 μm, in particular 1.4-1.6 μm on its free surface upon entering the vaporized deposition, being that roughness values greater than 1.4 μm are favorable. It is also favorable for an optimized adhesion of the Mg coating on the zinc coating that the zinc coated steel sheet product has a RPC peak number of at least 60 / cm upon entering the vapor deposition. The number of RPC peaks and the average roughness Ra are determined in the process of cutting of palpation, being that when determining the average roughness Ra the ways of proceeding indicated in the DIN EN ISO 4287: 1998 standard are used and when determining the number of RPC peaks indicated in the iron / steel test sheet SEP 1940. It was additionally checked that it is favorable for the result of the vaporized deposition that prior to its entry into the vaporized deposition, the steel sheet product provided with the zinc-containing coating is heated or maintained at a temperature that is above ambient temperature but nevertheless below the alloy temperature. Practical tests have shown that temperatures particularly suitable for this purpose are in the range of 230 ° C to 250 ° C, in particular at about 240 ° C. Accordingly, with the invention there is provided a method that can be carried out particularly economically in a continuous line work sequence and that yields a product that by virtue of the quality of its surface and its adhesive ability is particularly suitable for the production of components for vehicle bodies with the use of modern joining techniques, such as glue. In the following the invention is explained in more detail by means of two exemplary embodiments.

Embodiment example 1 In a conventional installation already existing for the continuous electrolytic galvanization of steel tape, a PVD vapor deposition and thermal after-treatment module was integrated downstream of the conventional units used for the galvanizing and upstream of the devices for the final finishing treatment of the finished coating tape. The steel strip that is first electrolytically galvanized in a known manner in the conventional galvanizing units of the installation reconfigured in this way passes, after the galvanizing process and an optional final cleaning that is still absolved in the conventional installation, to the module of PVD deposition and thermal after-treatment in which it is vaporized and subjected to thermal after-treatment. The steel strip is then guided back to the conventional installation, where phosphates and oil, for example, are included in the context of the final treatment. As raw material for the steel belts that have the usual dimensions that are processed in this installation, the typical steel qualities in the automotive industry come into consideration. It was found to be particularly favorable that the average roughness value of the cold thin film used for the electrolytically galvanized thin sheet lies in the upper limit of the typical specification of 1.1-1.6 μm Ra of the automotive industry for parts external. An additional increase in the Ra value by 2 μm would be favorable in view of the adhesion capacity of the coating and the adhesive ability added to it, but for the time being it is not reasonable from an economic point of view since a product of this nature does not currently correspond. to the customer specifications for cars. For the number of peaks an RPC value is preferred > 60 / cm. It is also possible to positively influence both values during the electrolytic galvanization process. Another possibility of adjusting these values consists of a cementing process as the last step of the conclusive cleaning. At belt speeds of 20 to 180 m / min, the steel strip is first provided with a bilateral zinc layer of 3 μm by the electrolytic path, by means of soluble anodes, in conventional manner in electrolysis cells which are arranged perpendicularly. After washing and drying the steel strip now galvanized, the galvanized substrate is essentially conclusively clean and prepared for the application of the coating containing magnesium. However, in order to optimize the result of the subsequent vapor deposition it may be favorable to further carry out a pickling of the galvanized steel strip in the course of conclusive cleaning, in which the steel strip is kept in a hot bath at 20 ° C. ° containing 0.5% hydrochloric acid in each case 5 seconds. To neutralize the acid, it is then washed with completely desalinated water. The steel strip conclusively cleaned in this way passes through several pressure stages to a vacuum chamber in which without another treatment step the magnesium vaporization is carried out by a PVD method using a commercial JET vaporizer. To ensure a constant magnesium layer of 300 nm at variable band speeds, the JET vaporizer is adjusted by suitable thermal and mechanical measurements to provide vaporization rates between 6 μm * m / min and 54 μm * m / min. Through another series of pressure stages, the steel strip now also provided with an Mg layer is then transferred back to the normal atmosphere. For thermal after-treatment, a treatment using NIR radiators (near infrared) is used in this case. The heating time depends on the speed of the band, but can be adapted by disconnecting individual modules. The peak temperature of the heat treatment is 327 ° C ± 7K according to the invention. In order to safely maintain this narrow temperature window under the conditions of use on an industrial scale, a special pyrometric image-producing method is used, which makes it possible to locally and temporarily control the thermal treatment according to the invention. Steel substrates and different coating conditions can have discrepant emission effects, so it is necessary to perform a precise calibration. After a 10 m free run of the belt, the steel belt is cooled with water. The remaining heat in the tape is adjusted so that the tape dries alone. In FIG. 1, an FE-REM shot of a preparation of a transverse polish of a steel strip coated according to the invention and thermally treated at a temperature of 332 ° C is reproduced in inverted representation. The favorable layer structure with the steel substrate S, the zinc layer Z applied to it by the electrolytic coating and the magnesium-containing coating M of ZnMg lying on the Z-layer of zinc is clearly seen there. In the case of the layer that is recognized above the coating M, it is an incorporation mass E that was needed to prepare the transverse polish. EXAMPLE 2 With the same process conditions, Mg layers of 1500 mm were made and thermally alloyed according to the invention at a belt speed of 36 m / min as well as an expanded vaporization regime by suitable construction measures of the evaporator up to 96 μm * m / min at a belt speed of 64 m / min. Also in these analyzes a favorable formation of the alloy coating of Zn-Mg was proved. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (8)

CLAIMS Having described the invention as above, the content of the following claims is claimed as property: 1. Method for producing products of steel sheet protected against corrosion, characterized in that a coating layer is applied to a steel sheet product. containing zinc by means of electrolytic deposition, - where the steel sheet product is subjected if necessary to mechanical and / or chemical conclusive cleaning, - where the coating layer containing zinc in a conclusively clean form is directly applied by vapor deposition a second coating layer based on magnesium, and - where after applying the second coating layer a thermal after-treatment of the steel sheet product is carried out at normal atmospheric pressure to form a diffusion layer or convection between the zinc-containing coating layer and the coating layer magnesium based at a treatment temperature that is 320 ° C to 335 ° C. Method according to claim 1, characterized in that the steel sheet product provided with the zinc-containing coating is preconditioned in chemically wet form during its final cleaning by washing with an alkaline preconditioning medium. Method according to claim 1 or 2, characterized in that the steel sheet product provided with the zinc-containing coating is etched during its conclusive cleaning by washing with an acid, in particular hydrochloric acid. 4. Method according to claim 3, characterized in that after the pickling the steel sheet product is washed with completely desalinated water. Method according to any of the preceding claims, characterized in that the thermal after-treatment is completed within a maximum of 15 seconds. Method according to any of the preceding claims, characterized in that the steel sheet product provided with the zinc-containing coating has a roughness of at least

1.4 μm on its free surface upon entering the vaporized deposition. Method according to any of the preceding claims, characterized in that the number of peaks RPC of the steel sheet product provided with the zinc-containing coating is at least 60 / cm on entering the vaporized deposition. Method according to any of the preceding claims, characterized in that the steel sheet product provided with the zinc-containing coating is heated or maintained at a temperature higher than room temperature but lower than the alloy temperature of the coating. magnesium prior to its entrance to the vapor deposition.