Patent Publication Number: US-2015072167-A1

Title: Tinplate, coated with a polymer coating, and methods for its production

Description:
FIELD OF THE DISCLOSURE 
     The disclosure concerns a tinplate, coated with a polymer coating, and methods for its production and an apparatus for carrying out the methods. 
     BACKGROUND OF THE DISCLOSURE 
     Tinplate is a thin, cold-rolled steel sheet, whose surface is coated with tin. The application of the tin coating on the steel sheet is, as a rule, carried out electrolytically. Tinplate is mainly used for the production of packagings, in particular, cans for food products for human consumption and pet food, packagings for chemical-technical goods, aerosol cans, beverage cans, and for the production of parts for such packagings, such as closures, lashing belts, valve plates, can lids, and lid rings. 
     Tinplate is characterized by a high corrosion resistance and stability with respect to acids and by a good formability. For certain applications, for example, for the production of packagings for food products for human consumption and beverage cans, the tinplate surface is also provided with a lacquer or a polymer coating so as to guarantee an additional protection, in addition to the protection from corrosion provided by the tin coating. For this purpose, a plastic surface, made of polyethylene terephthalate (PET) or polypropylene (PP), for example, is applied on the tinplate. Film-coated tinplate is suitable, in particular, for the production of valve plates, bottoms of aerosol cans, tear-off lids for cans, and deep-drawn containers and vacuum closures. 
     It has become evident, however, that during the forming of tinplates equipped with PET coatings, stress cracks are formed in the PET coating that, upon contact with aggressive and in particular acid-containing goods, lead to an attack of the tin coating of the tinplate lying underneath. Also, the emulsions of lubricants, which are applied for a better processability on the tinplate surface, are also responsible for the crack initiation in the PET and can moreover enter into the cracks formed in the PET coating and attack the tin surface of the tinplate, which can lead to an adhesion loss and to a detachment of the PET coating from the tinplate surface. 
     From publications DE 40 09 839 A1, DE 34 36 412 C2, and EP 664 209 A1, tin-plated steel sheets are known, which are coated with a polyester resin film, in particular made of polyethylene terephthalate (PET). The coating of the tinplate surface with the polyester resin film is thereby carried out by laminating a polyester film, in particular a PET film, on the surface of the tinplate. In order to guarantee a sufficient adhesion of the polyester resin film on the tinplate surface, a chromium-containing adhesive layer is applied on the tinplate surface before laminating on the polyester resin film; it is formed, for example, by a monolayer of hydrated chromium oxide or by a double layer of metal chromium with an overlying layer of hydrated chromium oxide. Without this adhesive layer between the tinplate surface and the polyester resin film, the polyester resin film, in particular, a PET film, would be detached from the tinplate, in particular during the formations in the methods for the production of packagings or during sterilizing operations with the packages or when filling them with hot goods. The chromium packagings used for the production of the chromium-containing adhesive layer are, however, toxic and dangerous for the environment. 
     As an alternative material for tinplate, electrolytically chromium-plated steel sheets are known from the state of the art (Electrolytic Chromium Coated Steel, ECCS). This material, which is also designated as “Tin-Free Steel (TFS),” is found as cold-rolled steel sheets that have been electrolytically equipped with a coating of chromium and chromium oxide. The surface of this material has a good adhesion for polymer materials, such as for polyethylene terephthalate or polypropylene, and thus can be coated with this polymer, for example, by laminating a polymer film so as to allow an additional corrosion protection. The adhesion of the polymer coating on the chromium surface of the ECCS or the TFS withstands even strong deformations, such as, for example, during the production of packaging containers and during sterilization processes. ECCS sheets equipped with polymer coatings are therefore used, in particular, in production methods for containers in which strong deformations of the sheets are necessary, such as, for example, during the production of valve plates for aerosol cans, wherein the organic coating with the ECCS takes place before the formation process, because otherwise, strong wear and tear of the tools occurs. 
     From EP 848 664 B1, for example, a chromium coating against a corrosion-protected steel strip (ECCS or TFS) is known, on which a film made of polyethylene terephthalate was laminated. 
     Such steel sheets, which are protected against corrosion with a chromium coating, also prove to be disadvantageous, however, because of the toxicity of the chromium compounds used in the production process, in particular the liquid chromic acid (chromium VI) of the refinement bath. 
     From WO 97/03823-A is known a corrosion-resistant steel sheet that has a metal corrosion protection layer, which may be, by way of example, an electrolytically applied tin- or chromium oxide layer on which a transparent polymer film is applied, on one or both sides, by laminating on a polymer film. The polymer film consists thereby of polyethylene terephthalate (PET), polyvinyl chloride (PVC), or polypropylene (PP). 
     An adhesion promoter, in particular an adhesive layer, is thereby provided between the metal corrosion protection layer of the steel sheet and the laminated polymer film. For the production of the corrosion-resistant steel sheet, a steel sheet, which is galvanically coated with a metal corrosion protection layer and passivated, with a thickness between 0.05 mm and 0.5 mm, is used and heated to temperatures of approximately 160° C. The polymer film is laminated on the heated steel sheet by means of rotating rollers. The thickness of the laminated polymer film is between 5 and 100 μm. The polymer film thereby has, preferably on one side, an adhesive layer that has a lower melting point than the polymer material of the polymer film. The polymer film is laminated on, oriented with the adhesive layer toward the surface of the metal corrosion protection layer of the steel sheet. 
     In this method to laminate a polymer film on the metal corrosion protection layer of a steel sheet, a special polymer film with an adhesive layer is used to laminate the polymer film on the surface of the corrosion protection layer of the steel sheet. Such polymer films with an adhesive layer are very expensive in their production. Furthermore, the handling of such polymer films with an adhesive layer is more cumbersome and the method parameters must be maintained within the specified limiting values during the lamination, in particular the temperatures, which are determined by the melting temperatures of the polymer film and the adhesive layer. In particular with tin-plated steel sheets, it has become evident that an adhesive layer cannot be dispensed with if a sufficiently good adhesion of the polymer film on the tin-plated surface of the steel sheet is to be guaranteed. The polymer films, on the other hand, adhere better on the chromium surfaces of ECCS or TFS; however, the production of ECCS yields toxic and environmentally detrimental wastes because of the chromium-containing substances used in the coating of the steel sheet. 
     SUMMARY OF THE DISCLOSURE 
     Proceeding from this, embodiments of the disclosure provide as complete as possible a chromium-free method for the production of a highly corrosion-resistant steel sheet. The highly corrosion-resistant steel sheet produced with the method should be suitable especially for the production of packagings and should not suffer any damage with regard to corrosion resistance even with strong deformations during the production process and during a sterilization operation of the produced packaging. 
     The disclosure also provides an apparatus for carrying out the disclosed methods as well as a tinplate made according to the disclosed methods. Preferred embodiments of the methods, apparatus, and tinplate are also disclosed. 
     In the method in accordance with the disclosure for the coating of a chromium-free surface of a tin-plated steel sheet (tinplate) with a polymer coating, the chromium-free tin surface of the tin-plated steel sheet is first electrochemically oxidized in a first step and, in a second step, a polymer coating is applied on the oxidized tin surface. By the electrochemical oxidation of the tin surface, a chromium-free passivation of the tin surface is guaranteed, which prevents an unhindered increase of tin oxide on the tinplate surface. In contrast to the known methods for the passivation of tinplate against the increase of tin oxide on the tinplate surface, the passivation of the tinplate surface in the method in accordance with the disclosure takes place without a use of chromium-containing substances, in particular without the use of toxic and environmentally detrimental chromium oxides. Surprisingly, it was determined that this chromium-free passivation of the tinplate surface by an electrochemical oxidation not only prevents an unhindered increase of tin oxide on the tinplate surface, but at the same time also forms a good adhesion basis for polymers. In this way, in the second step of the method in accordance with the disclosure, a polymer coating can be applied without any problems on the oxidized tin surface of the tinplate, wherein the oxidized tin surface allows a very good adhesion of the polymer coating. It has become evident that the adhesion between the oxidized tin surface and the polymer coating withstands even strong deformations, as occur, for example, in methods for the production of cans with multiple deep-drawing or in the production of valve plates. The adhesion between the oxidized tin surface and the polymer coating also readily withstands a sterilization operation without a detachment of the polymer coating from the tinplate surface occurring during the sterilization. 
     In the method in accordance with the disclosure for the production of a tinplate coated with a polymer coating, in a first method step, a tin coating is first electrolytically deposited on one or both sides of a steel sheet. In a second step, an electrochemical oxidation of the surface of the tin coating takes place, and finally, a polymer coating is applied on the oxidized surface of the tin coating. The electrochemical oxidation of the tin surface is thereby preferably carried out immediately and, in particular within a few seconds after the deposition of the tin coating on the steel sheet. The electrochemical oxidation of the tin surface hereby preferably takes place also without additional intermediate steps, in particular without an intermediate cleaning or a temperature treatment of the tinplate surface. 
     The electrochemical oxidation of the tin surface can, in particular, take place by anodic polarization of the tin-plated steel sheet in an aqueous and chromium-free electrolyte. For example, the electrochemical oxidation of the tin surface can be carried out by immersion of the tinplate into a soda solution (sodium carbonate solution). A thin tin oxide layer, which essentially consists of tetravalent tin oxide (SnO 2 ), thereby forms on the (chromium-free) tin surface of the tinplate. In contrast to divalent tin oxide (SnO), which forms on the tinplate surface during storage of tinplate in an oxygen-containing atmosphere, this tetravalent tin oxide is substantially more inert and prevents an unhindered increase of a (divalent) tin oxide on the tinplate surface upon contact with oxygen. The thickness of the oxide layer formed during the electrochemical oxidation of the tinplate surface, consisting essentially of tetravalent tin oxide, is appropriately in the nm range and is preferably thinner than  100  nm. With the formation of this passivating tin oxide layer on the tinplate surface, a charge density is preferably produced on the tin surface, which is at most 40 C/m 2 , in the electrochemical oxidation step. 
     After the electrochemical oxidation of the tinplate surface, it is provided with a polymer coating, wherein the polymer coating appropriately has a thickness in the range of 10 to 100 μm and is preferably applied by the lamination of a polymer film on the oxidized tin surface. It is particularly appropriate for this to use a coextruded plastic film with a polymer layer and an adhesion promoter layer, which is laminated on the oxidized tin surface of the steel sheet in that the adhesion promoter layer of the plastic film is laid on the oxidized tin surface and is laminated on with the effect of heat, by means of lamination rolls or lamination rollers. The adhesion promoter layer further increases the adhesion of the already effective adhesion of the polymer coating on the oxidized tin surface of the tinplate. 
     It has proved to be particularly appropriate if the tin-plated steel sheet is heated to temperatures above the melting temperature of the tin coating (232° C.) during the polymer film lamination. In this way, the tin coating of the tinplate is melted, so that at least the areas of the tin coating of the tinplate close to the surface are present in the molten state during the polymer film lamination. In this way, the adhesion between the (molten) tin surface of the steel sheet and the laminated polymer coating is additionally improved. In particular, in this way, a sufficient adhesion can be guaranteed between the polymer coating and the tin surface of the tinplate even when using polymer materials of the polymer coating with a melting point higher than 232° C. or when using coextruded plastic films with an adhesion promoter layer whose melting point is higher than 232° C. It is particularly preferred, however, if the steel sheet is heated to temperatures that are both higher than the melting temperature of the tin and higher than the melting temperature of the polymer material that is used for the formation of the polymer coating, or of any adhesion promoter present, while the polymer film is being laminated on the tin-plated surface. 
     The application of a polymer coating without an additional adhesion promoter on the oxidized tin surface of the tinplate is also possible. Only if very strong formation operations are undertaken in the following processing operations during the processing of the tinplate produced in accordance with the disclosure is the use of an adhesion promoter between the oxidized tin surface of the tinplate and the polymer coating required, so as to prevent a detachment of the polymer coating during the formation steps. 
     For special uses of the tinplate produced in accordance with the disclosure, in which high formation rates with deep-drawing rates of at least D/d=β=1.7 (D=round-blank diameter; d=cup diameter) are required, the use of an adhesion promoter between the polymer coating and the tin surface of the tinplate has proved to be appropriate. Intermediate layers that contain glycol-modified polyethylene terephthalate (PETG), glycol-modified polycyclohexylenedimethylene terephthalate (PCTG), and/or isophthalic acid (IPA), or mixtures thereof have proved to be suitable adhesion promoters. 
     The polymer material of the polymer coating is appropriately a thermoplastic polyester, in particular polyethylene terephthalate (PET). The melting point of polyethylene terephthalate is in the range of 260-270° C. In order to guarantee as good as possible an adhesion with the application of the polymer coating on the oxidized tin surface, it is expedient to heat the tin-plated steel sheet to temperatures above the melting point of polyethylene terephthalate when applying the polymer coating, so that during the application of the polymer coating, both the tin surface of the tinplate as well as at least the areas of the polymer coating that are close to the surface facing the tinplate are present in the molten state and, in this way, can enter into a close material connection. It has proved to be particularly appropriate if the tin-plated steel sheet is kept in the temperature range between 270° C. and 290° C., and preferably at approximately 280° C., when the polymer coating is applied. 
     In order to prevent a sticking of the polymer film to the heated lamination rolls, for example, when laminating a polymer film on the oxidized tin surface of the steel sheet by means of lamination rolls, it is expedient to use a multilayer plastic film that has an antiblock layer on its upper side for the formation of the polymer coating. Such an antiblock layer can, for example, be formed by a silicon oxide layer on the upper side of the polymer film. 
     The method in accordance with the disclosure can be carried out in strip tin-plating units, wherein a steel strip is moved, by means of a transporting device, at a strip speed of preferably more than 200 m/min, and with particular preference more than 500 m/min, through a tin-plating device. The following electrochemical oxidation of the tin surface is carried out in an oxidation device, preferably by conducting the tin-plated steel strip at the strip speed through an electrolysis bath with an aqueous electrolyte, wherein the steel strip is thereby appropriately connected as an anode so as to oxidize the tin surface electrochemically (anodically). 
     Subsequently, the polymer coating is applied on the oxidized tin surface of the moving steel strip in a plastic coating device, for the purpose of laminating a polymer film on the oxidized tin surface, on one or both sides, preferably by means of laminating rolls. The tin-plating device and the oxidation device are thereby arranged one behind the other as seen in the moving direction of the strip, and preferably so close to one another that at the typical strip speeds of more than  200  m/min, within a very short time, and preferably within a few seconds after the tin coating, the tin-plated surface of the steel strip can be electrochemically oxidized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other advantages of the method in accordance with the disclosure and of the tinplate in accordance with the disclosure can be deduced from the embodiment examples described below, which are explained with reference to the accompanying drawings. The figures of the drawings show the following: 
         FIG. 1 : Schematic representation of an apparatus for carrying out the method in accordance with the disclosure for the production of tinplate coated with a polymer coating; 
         FIG. 2 : schematic representation of a tinplate in accordance with the disclosure with a polymer coating without an adhesion promoter layer; 
         FIG. 3 : schematic representation of a tinplate in accordance with the disclosure with a polymer coating with an adhesion promoter layer. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     The starting material for the method in accordance with the disclosure for the production of a tinplate coated with a polymer coating is preferably a hot-rolled and unalloyed or low-alloy steel sheet in the form of a strip (steel strip) with a low carbon content of, for example, 20 to 900 ppm. The alloy components of the steel appropriately fulfill the specifications of the International Standard ASTM A 623-11 (Standard Specification for Tin Mill Products), wherein a use of the tinplates produced in accordance with the disclosure for the production of packagings for food for human consumption is ensured. Basically, all steel types that have a composition suitable for the production of fine or very fine sheets can be used for the method in accordance with the disclosure. The hot-rolled steel strip is first pickled in a pickling unit (not shown), subsequently rinsed and dried, and then cold-rolled in a cold rolling device. The steel strip is thereby rolled to a thickness of less than 1.0 mm (fine sheet) and preferably to thicknesses of 0.1 to 0.5 mm (very fine sheet). After the cold rolling, the steel strip is first conducted through a continuous annealing furnace, in which the steel strip is heated to temperatures of 550° C. to 700° C., for the recrystallizing annealing of the steel. The formability of the cold-rolled steel strip is, once again, produced by the recrystallizing annealing. After the recrystallization annealing, the steel strip can be finished or temper-rolled in a temper-rolling mill, if required for the production of the forming characteristics needed for the planned processing purposes. During the temper-rolling, a required further thickness reduction of the steel strip can also be attained under certain circumstances. After the finishing or temper-rolling, a cleaning of the steel strip is carried out by means of an alkaline electrolytic treatment and by pickling with a subsequent rinsing. 
     Subsequently, the steel strip  10 , as shown schematically in  FIG. 1 , is conducted through a tin-plating device  7 . The steel strip  10  is thereby unwound from a roll  12  as a continuous strip and moved through a transporting device  6  at a strip speed of preferably more than 200 m/min and up to 750 m/min, through a tank  7   a  with a tin-containing electrolyte, and conducted, as a cathode, between tin anodes. In this way, the tin of the anodes is dissolved and deposited on the steel strip as a tin coating. The tin can thereby be deposited in any thickness and, if required, on both sides of the steel strip  10 . The thickness of the applied tin layer is regularly between 0.5 g/m 2  and 12 g/m 2 . However, a coating of the steel strip with thinner or with thicker tin layers is also possible. 
     Immediately after the coating of the steel strip with the tin coating and, in particular without additional intermediate steps, the tin surface of the tin-plated strip  10  is oxidized electrochemically in an oxidation device  8 . For this purpose, the freshly tin-plated steel strip  10  is conducted, for example, into an electrolyte bath with an acidic, chromium-free and aqueous electrolyte and connected as an anode. In this way, the fresh tin surface of the tin-plated steel strip  10  is anodically polarized. A thin oxide layer is thereby formed, with a layer thickness in the nm range, on the tin surface of the tin-plated steel strip, which essentially consists of tetravalent tin oxide (SnO 2 ). This tetravalent tin oxide is substantially more inert than divalent tin oxide (SnO), which is formed during the storage of tin-plated steel sheets in an oxygen atmosphere. With this (essentially tetravalent and inert) tin oxide layer that is formed during the electrochemical oxidation of the freshly tin-plated surface, a high resistance of the tin-plated steel strip surface against corrosion and reaction with sulfur is guaranteed. The thin tin oxide layer, essentially consisting of tetravalent tin oxide (SnO 2 ), particularly prevents an unhindered increase of additional (divalent) tin oxide upon contact of the tin-plated surface with air oxygen. 
     The electrochemical oxidation of the tin surface is carried out, for example, as an anodic oxidation of the tin-plated steel strip  10  in soda solution, that is, in an aqueous sodium carbonate solution. For the purpose, the tin-plated steel strip is moved at the strip speed and conducted through an electrolysis bath  8   a  with a soda solution. The concentration of sodium carbonate in the soda solution is preferably 1 wt % to 10 wt %, primarily 2 wt % to 8 wt %, preferably 3 wt % to 7 wt %, above all 4 wt % to 6 wt %, in particular approximately 5%. 
     The oxidation device  8  for the electrochemical oxidation of the surface of the tin coating appropriately comprises an electrolysis bath  8   a  with a vertical tank, which is filled with the electrolyte. In the vicinity of the bottom, within the vertical tank, there is a deflection roller, via which the tin-plated steel strip  10  is deflected. A potential is placed between the tin-plated steel strip  10  and the counter-electrode (for example, a steel cathode) in the vertical tank. The charge quantity Q, transferred during the electrochemical oxidation, is thereby preferably below 40 C/m 2 . The current density that prevails in the electrolysis bath is preferably in the range of 1.0 A/dm 2  to 3 A/dm 2 . The density of the tin oxide layer that is thereby being formed is preferably lower than 100 nm and is, with particular preference, in the order of magnitude of 10 nm. 
     The anodization time corresponds to the residence time of the tin-plated steel strip in the electrochemical oxidation bath (electrolyte bath). This is specified by the length of the electrolyte bath or its filling level and the anode length and the strip speed and, at the typical strip speeds, is expediently in the range of 0.1 sec to 1 sec, in particular between 0.1 sec and 0.7 sec, preferably in the range of 0.15 sec to 0.5 sec, and ideally around 0.2 sec. Via the filling level, it is possible to adjust the anodization time to suitable values as a function of the strip speed, so as to form the preferred layer thickness of the electrochemically produced tin oxide layer. 
     The distance between the steel strip  10  and the counter-electrode in the electrolysis bath  8   a  is adjusted with respect to the unit conditions. It is, for example, in the range of 3 to 15 cm, preferably in the range of 5 to 10 cm, and in particular around 10 cm. The temperature of the electrolyte is preferably in the range of 30 to 60° C., in particular in the range of 35 to 50° C. 
     The current density in the electrolysis bath is set, for example, in the range of 1.0 to 3 A/dm 2 , preferably 1.3 to 2.8 A/dm 2 , and in particular around 2.4 A/dm 2 . The entire charge quantity thereby fluctuates in the range between 0.2 C and 0.4 C and is preferably, for example, 0.3 C. The corresponding charge densities (with regard to the area of the oxidized tinplate strip) thereby lie in the range from 0.2 C/dm 2  to 0.4 C/dm 2 . 
     The tin-plated steel strip  10  is conducted into a plastic coating device  9  after the electrochemical oxidation of the tin surface, at a strip speed of a maximum 200 m/min. Since the steel strip cannot be conducted through the plastic coating device at the high strip speeds of approximately 750 m/min that are used during the tin plating of the steel strip in the tin-plating device, it is expedient to carry out the method step of the polymer coating separately, that is, with a prior winding up of the tin-plated steel strip into a coil and the intermediate storage of the coil. This is possible without a problem, since the tin surface is resistant to a (further) unhindered increase of a (divalent) tin oxide layer because of the electrochemical oxidation. However, it is also possible to apply the polymer coating immediately after the tin-plating and the oxidizing of the tin surface in the plastic coating device  9  without an intermediate storage and with a continuously running steel strip. There, a polymer coating is applied on one or both sides of the tin-plated steel strip. For this purpose, the steel strip is first heated in a heating device  11 , which can be designed, for example, as an induction heating or also as an infrared or microwave heating, to temperatures which are at least above the melting temperature of the tin (232° C.). Appropriately, the temperature of the steel strip  10  is also above the melting temperature of the polymer material during the polymer coating. Preferably, the polymer material is polyethylene terephthalate (PET with a melting temperature between approximately 235 and 260° C., dependent on the degree of crystallization and on the degree of polymerization) or polypropylene (PP with a melting temperature of approximately 160° C.) or also PE (with a melting temperature of approximately 130-145° C.). 
     During the heating of the tin-plated steel strip to temperatures above the tin melting point, a thin and very dense alloy layer is formed between the steel strip surface and the tin layer, which consists of iron atoms of the steel and tin atoms of the tin coating. This alloy layer leads to a very good adhesion of the tin coating on the steel strip surface and represents, moreover, a very effective corrosion barrier. With a complete melting of the tin coating, moreover, a shiny surface of the tin layer is also produced. 
     A film  16  made of a polymer material is supplied to the heated steel strip  10 , and by means of (appropriately heated) lamination rolls  9   a  is pressed onto the surface of the tin coating, on one or both sides, in the plastic coating device  9 . The polymer film  16  can be a film made of a polyester, such as polyethylene terephthalate and in particular a biaxially oriented or amorphous polyester film or a film made of polypropylene or also a film made of a polymer laminate consisting of polyethylene terephthalate and polypropylene and polyethylene. If necessary, a polymer film with an adhesion promoter layer is used, which will be described below. As a result of the temperature of the heated steel strip  10 , at least the area of the tin coating close to the surface melts thereby and (depending on the selected temperature of the steel strip) perhaps also at least the area of the polymer film  16  facing the tin-plated steel strip  10 , which will then adhere to the oxidized surface of the tin coating upon being pressed by the lamination rolls  9   a.    
     In order to prevent a cementing of the polymer film on the perhaps heated lamination rolls during the lamination of the polymer film  16  on the oxidized tin surface of the steel sheet  10  by means of the lamination rolls  9   a,  a multilayer polymer film  16  that has an antiblock layer on its upper side is appropriately used for the formation of the polymer coating. Such an antiblock layer can be formed on the upper side of the polymer film, for example, by a silicon oxide layer. 
     After laminating the polymer film, the tin- and polymer-coated steel strip  10  undergoes a cooling to approximately 20° C. Afterward, optionally, the polymer coating can still be completely melted and then quenched to a temperature below the glass transition point in a cooling device  15  (for example, a water bath). In this way, for example, an amorphous structure is formed in the polyethylene terephthalate or a minimal crystalline structure in the polypropylene when using PET or PP as the polymer material. The melting of the polymer coating is thereby carried out in a particularly appropriate manner by another heating of the steel strip  10  to temperatures above the melting point of the used polymer material in a melting device  14 . The melting of the polymer coating is appropriately carried out in the melting device  14  by an inductive heating of the steel strip  10  in an induction coil  14   a.  By this post-heating, inherent stresses in the polymer coating are relieved by relaxation, which leads to an increase in the adhesion between the tin coating and the polymer coating and, in this way, to a stabilization of the bonding of these layers. With the use of PET as the polymer material, for example, the relaxation time is less than 0.5 sec, so that a brief heating of the polymer coating to temperatures above the PET melting temperature (approximately 260°) is sufficient to bring about the desired relaxation. At typical strip speeds of more than 200 m/min, an induction coil  14   a,  for example, which extends over less than 1 meter along the direction of movement of the strip in the melting device, is sufficient for the purpose, so as to thus heat the steel strip  10  inductively in this section and, in this way, to melt the polymer coating. 
     The subsequent quenching of the melted polymer coating in the cooling device  15  can take place, for example, by an air cooling or by immersing the steel strip into a tank with a cooling liquid. Finally, the coated steel strip  10  is wound up on a roll  13  by the transporting device  6 . 
       FIG. 2  shows a correspondingly produced tinplate. This comprises the layers steel sheet  1 , tin coating  2 , tin oxide layer  3 , and the polymer coating  4  (for example, made of PET). 
     The tin plates produced in accordance with the disclosure are characterized by a high corrosion resistance, which is attained by the metal corrosion protection layer made of tin and the polymer coating. The thin iron-tin alloy layer also contributes thereby to the corrosion resistance; it is formed between the steel strip surface and the tin layer during the heating of the tin-plated steel strips to temperatures above the tin melting point. The combination of these corrosion protection layers is thereby particularly advantageous, because with the polymer coating, the release of tin ions from the tin coating due to the effect of air is avoided. The tinplates produced in accordance with the disclosure are also inert with respect to aggressive and, in particular, acid-containing goods because of the polymer coating and are therefore very suitable for the production of packagings for such goods. In comparison to matt-gray ECCS (TFS), the tinplates in accordance with the disclosure exhibit a high degree of luster because of the shiny surface of the tin coating that forms during a complete melting of the tin coating. This is advantageous, in particular, when using transparent or translucent polymer coatings, because the tinplate thus has an optically very attractive shiny surface. In comparison to known methods for the production of steel sheets that are equipped with a metal corrosion protection layer and a polymer coating, the methods in accordance with the disclosure are also characterized in that they are completely chromium-free-—that is, no chromium-containing substances are used. 
     The steel strips produced in accordance with the disclosure are also characterized by a very good adhesion of the polymer coating on the tin coating, which, as a result of the oxidized tin surface, is also already attained without an adhesion promoter or additional adhesion layers. The additional use of adhesion promoter layers between the tin coating and the polymer coating is necessary only for special applications in which very high deformation rates appear. 
     With small deformation rates, which, for example, appear in the production of round lids or bottoms for cans and that can be defined by a drawing ratio β=D/d (with D=round-blank diameter and d=can diameter) of β&lt;1.2, the use of an adhesion promoter layer is not necessary. With larger deformation rates, as appear, for example, with greater deep drawing steps (for example, in the production of valve plates) with β&gt;1.7, it is, on the other hand, expedient to use an adhesion promoter, and with even larger deformation rates of β&gt;2 (which, for example, occur with singly and multiply deep-drawn cans and DWI cans), an adhesion promoter appears to be necessary to reliably prevent a detachment of the polymer coating from the tin surface. 
     Glycol-modified polyethylene terephthalate (PETG, wherein less than 50% of the diol component is made of cyclohexanedimethanol), glycol-modified polycyclohexylenedimethylene terephthalate (PCTG, wherein more than 50% of the diol component is made of cyclohexanedimethanol), and/or isophthalic acid (IPA) have proved to be suitable adhesion promoters. Adhesion promoters that have a fraction of PETG and 5 to 25 vol % IPA or PCTG have proved to be particularly preferred. For the formation of an adhesion promoter layer between the oxidized tin surface of the tinplate and the polymer coating, a multilayer polymer film is appropriately used that contains a polymer layer (for example, made of PET) and an adhesion promoter layer made of one of the materials mentioned above. Such polymer films are available as co-extruded films, wherein the thickness of the adhesion promoter layer is in the range of 3 to 6 μm with a total thickness of the polymer film of 10 to 40 μm. This multilayer polymer film is oriented toward the tin surface for the application of the polymer coating with the adhesion promoter layer and is thus laminated on the oxidized tin surface.  FIG. 3  shows a correspondingly produced tin plate in a sectional view. This comprises the layers steel sheet  1 , tin coating  2 , tin oxide layer  3 , and the laminated polymer coating with the adhesion promoter layer  5  and the polymer layer  4  (for example, made of PET). 
     The tinplates produced in accordance with the disclosure are suitable for the production of packaging containers, in particular for food and for technical goods, such as two-part cans (deep-drawn and stretched, DWI cans), and aerosol cans. Also, can bodies of three-part cans can be taken into consideration, if the polymer coating in the welding area is removed before the body welding. Also, parts of such packaging containers can be made from the steel strips produced in accordance with the disclosure, such as lashing belts, valve plates, can lids, and lid rings. In addition, the method in accordance with the disclosure can also be used for the production of steel sheets for use in other areas, such as for the production of sheets for the construction area or for the production of household appliances. 
     The disclosure is not limited to the described embodiment examples. Thus, for example, it is possible within the scope of the disclosure to wind the steel strip  10  on a roll (coil) after the electrochemical oxidation of the tin surface and to supply it in this form to the next method step (application of the polymer coating). This is not taken into consideration in the schematic representation of the apparatus in accordance with the disclosure of  FIG. 1 . 
     The polymer coating can also be applied on the tin coating by coating methods other than lamination. Thus, after the electrochemical oxidation of the tin surface, for example, by means of direct extrusion also, a molten polymer material can be applied on the oxidized tin coating as is described, for example, in Patent DE 197 30 893 C1. 
     In the application of the polymer coating, combinations of different polymer materials are also possible. Thus, for example, a polymer coating made of PET can be applied on the upper side of the tin-plated steel strip, and a polymer coating made of PP can be applied on the underside of the strip. A polymer coating (PP or PET) can also be thereby replaced by a lacquering.