HK1184510B - Process for producing an iron-tin layer on a packaging steel substrate - Google Patents

Description

Method for producing an iron-tin layer on a packaging steel substrate

Technical Field

The present invention relates to a method for producing an iron-tin alloy layer on a packaging steel (packaging steel) substrate and to a substrate provided with said layer.

Background

Tin mill products include tinplate, electrolytically chrome plated steel (ECCS, also known as tin free steel or TFS), and black plate, uncoated steel. The packaging steel is usually provided in the form of tinplate or TFS, on which an organic coating may be applied. Due to the ever-increasing cost of raw materials and depletion of resources and reduction of carbon footprint, there is an increasing incentive to reduce the amount of tin used for tinplate in the field of packaging steel. TFS production currently involves the use of hexavalent chromium, a hazardous substance that is potentially harmful to the environment and poses a safety risk to workers. There is therefore a need to develop alternative metal coatings that can replace the traditional tinplate and TFS without having to resort to hexavalent chromium and minimize or even eliminate the use of tin.

Packaging steels are usually provided in the form of primary and secondary cold-rolled tin-plated rolled stock. The primary cold rolled (SR) product is cold rolled directly to finished gauge dimensions, followed by recrystallization annealing. The recrystallization is performed by continuously annealing or batch annealing the cold rolled material. After annealing, the material is typically cold-hardened, typically with a 1-2% reduction in thickness applied to improve material properties. The second cold rolled (DR) product is first cold rolled to achieve an intermediate product gauge, recrystallization annealed, and then second cold rolled to a final gauge. The resulting DR product is stiffer, harder and stronger than SR, allowing consumers to use lighter gauge steel in their applications. These uncoated, cold rolled, recrystallisation annealed SR and DR packaging steels are called black plates.

Tinplate is characterized by its excellent corrosion resistance and weldability. TFS is generally advantageous for adhesion to organic coatings and maintenance of coating integrity at temperatures above the melting point of tin. Tinplate is typically provided in a coating weight range of 1.0 to 11.2 grams per square meter, which is typically applied by electrolytic deposition. Electrolytic chromium-plated steel (ECCS) or tin-free steel (TFS) consists of a black steel sheet product that has been coated with a metallic chromium layer covered with a thin film of chromium oxide, both applied by electrolytic deposition. TFS is typically provided at coating weights of 20-110 and 2-20 mg/m for metal and chromium oxide coatings, respectively. Both tinplate and TFS can be provided with the same coating specification on both sides of the steel strip or with different coating weights on each side, the latter being referred to as differential thickness coated strip. Alternative metal coatings based on small amounts of tin that replace the traditional tinplate and TFS should be able to match the performance properties of the characteristic product required for each replacement.

Reducing the tin coating weight of conventionally manufactured tinplate (including reflow of an electrodeposited tin coating) to less than about 1 gram per square meter results in product performance degradation in corrosion resistance and compression of the soldering range. This observation has led to alternative product compositions and processing routes that can retain the properties of the tin product while reducing the weight of the applied tin coating. Examples include applying a thin nickel coating (e.g., a nickel coating thickness of 10-20 mg/m) prior to tin plating to ensure corrosion resistance and solder latitude are maintained at tin coating weights below 1 g/m. However, due to the presence of a passive, non-alloyed, tin-free layer near the outer surface of the product, these materials are not suitable for replacing TFS because of insufficient adhesion to the organic coating and retention of coating integrity at temperatures above the melting point of tin.

Object of the Invention

It is an object of the present invention to provide a method for producing a coating on a packaging steel substrate requiring a very small amount of tin, which can potentially be used as a substitute for TFS in combination with an additionally applied conversion layer.

It is another object of the present invention to provide a method for producing a coating on a packaging steel substrate requiring very low amounts of tin, which can be used as a sustainable alternative to conventional tinplate.

It is another object of the present invention to provide a substrate having good adhesion to organic coatings.

It is another object of the present invention to provide a substrate having improved mechanical properties.

The invention

In a first aspect of the invention, there is provided a coated substrate for packaging applications comprising 1. a recrystallisation annealed primary cold rolled steel substrate (SR black sheet) or 2. a secondary cold rolled substrate (DR black sheet) subjected to a recrystallisation annealing between the first and second cold rolling treatments, wherein one or both sides of the SR or DR black sheet substrate is coated with an iron-tin alloy layer containing at least 80 weight percent (wt. -%)%) of FeSn (50 atomic% tin and 50 atomic% iron), and wherein the iron-tin alloy layer is formed by: at a temperature T of at least 513 DEG C_aDiffusion annealing the electrodeposited tin layer for an annealing time t sufficient to convert the tin layer to an iron-tin alloy layer_aFollowed by rapid cooling in a non-oxidizing cooling medium while maintaining the coated substrate in a reducing or inert atmosphere prior to cooling, thereby obtaining a strong, stable surface oxide.

FeSn is a compound having 50 atomic percent (at%) tin and 50 at% iron. The inventors intend that the iron-tin layer consists essentially or entirely of FeSn. The adhesion and corrosion performance of the coated substrate can be improved by applying a conversion layer to the iron-tin alloy outer surface, particularly excluding the use of hexavalent chromium or chromate. The substrate can be used in place of TFS, allowing for great similarity in product performance due to its adhesion to organic coatings, corrosion resistance, and retention of coating integrity at temperatures exceeding the melting point of tin. The latter is particularly important when the polymer coating is applied, for example, by extrusion coating or lamination, since the surface temperature of the metal substrate in these processes can far exceed the melting point of tin (about 232 ℃).

US3285790 discloses the continuous annealing of tin coated fully hard steel which has been tin coated between two cold rolling steps to provide a more economical route to produce thin tinplate with steel gauge dimensions of 0.025-0.22 mm (0.001 "-0.0088"). The main purpose of the continuous annealing process described in US3285790 is to achieve recrystallization of bulk steel and involves heating the steel strip to a temperature of 649 to 982 ℃ (1200 to 1800 ° f). Such high annealing temperatures do not result in the FeSn layer of the invention but in a mixed iron-tin alloy layer which is reported to be very hard and brittle and which does not withstand bending well.

In US3174917 a tin layer is provided on an all-hard substrate. This requires annealing the steel at a combination of temperature and time sufficient to initiate recrystallization. Such treatments thus significantly affect the properties of the substrate. According to our invention, optimal recrystallization can be combined with optimal diffusion annealing, since recrystallization annealing and diffusion annealing are separate. This provides the opportunity to select the recrystallization annealing conditions most suitable for the fully hard substrate (e.g. similar to batch or continuous annealing, using different time-temperature profiles) to produce the best mechanical properties of the bulk steel (called blackplate after annealing) while individually optimizing the process conditions of the diffusion annealing of the tin-coated blackplate substrate to produce the best iron-tin alloy coating. Furthermore, unlike US3174917, our invention is able to produce packaging steel products of DR type. According to US3174917, the iron-tin alloy coated substrate obtained after continuous annealing should be cold rolled a second time to produce a product having DR-type properties of bulk steel. However, due to the large deformation applied during the second cold rolling step, the iron-tin alloy coating will crack severely, severely degrading the coating performance properties. US4487663 describes the manufacture of iron-tin alloy coated steel which specifically requires the use of a cathodic dichromate treatment to form an oxide film on the iron-tin alloy layer. The present invention specifically excludes the use of such dichromate treatments. The inventors have succeeded in producing alternative metal coatings to TFS or tinplate for packaging steel by providing a dense and uniform layer of iron-tin alloy on SR or DR ferrous plate products, regardless of whether the recrystallization annealing is batch or continuous before the diffusion annealing (see fig. 4). Such an iron-tin alloy layer may be made by using less than 1 gram per square meter of tin metal. This iron-tin layer is excellent in corrosion resistance when compared to TFS and tinplate if the latter is made with similar initial tin coating weights. Furthermore, unlike conventional tinplate, the iron-tin alloy coating does not subsequently undergo a physical or chemical change when exposed to temperatures of 200 to 600 ℃. It is particularly advantageous if the substrate has an additional organic coating, which can be applied, for example, by thermal lamination involving substrate temperatures in excess of 200 ℃. The organic coating was found to have excellent dry adhesion to the iron-tin alloy coating. This is due to the composition of the very thin mixed iron-tin oxide layer present on the outer surface. This mixed oxide layer was found to be very stable in thickness and composition (robust), even after exposure to high humidity and elevated temperature environments in accelerated storage tests. It was even found that after active (activery) removal of the native oxide layer by cathodic reduction treatment in a sodium carbonate solution, the native oxide layer spontaneously returns to its pre-treatment state after only a very short time and then remains stable. These results indicate that no passivation treatment needs to be applied, for example by applying the hexavalent chromium-based dichromate method to stop the growth of the mixed oxide layer. To obtain these properties, it is important that the iron-tin alloy consists mainly and preferably only of FeSn. Unlike TFS, the coated substrate of the present invention allows for thermal resistance welding and thus can be used to make three-piece welded cans.

The inventors have found that it is necessary to have a temperature (T) of at least 513 deg.C_a) And performing down-diffusion annealing on the tin-coated black steel plate substrate to obtain the coating layer of the invention. Selective diffusion annealing temperature T_aLower diffusion annealing time (t)_a) To obtain the conversion of the tin layer into the iron-tin layer. The major and preferably only iron-tin alloy component in the iron-tin layer is FeSn (i.e., 50 atomic% tin and 50 atomic% iron). It should be noted that the combination of diffusion annealing time and temperature may be interchanged to some extent. High T_aAnd short t_aWill result in a lower T_aAnd a longer t_aThe same iron-tin alloy layer. A minimum T of 513 ℃ is required_aSince the desired (50:50) FeSn layer cannot be formed at lower temperatures. And the diffusion annealing need not be at a constant temperature, but the temperature profile may also be such that the peak temperature is reached. It is important that the minimum temperature of 513 c is maintained for a sufficient time to achieve the desired amount of FeSn in the iron-tin diffusion layer. The diffusion annealing can thus be carried out at a constant temperature T_aNext for a specified time, or the diffusion anneal may, for example, include T_aThe peak metal temperature of (a). In this case, the diffusion annealing temperature is not constant. It has been found to be preferable to use a diffusion annealing temperature T of between 513 and 645 deg.C, preferably between 513 and 625 deg.C_a. Lower T_aLimiting the risk of affecting the bulk mechanical properties of the substrate during the diffusion annealing.

A significant advantage is that the annealing treatment to form the FeSn layer is not intended to result in recrystallization of the steel substrate. In the present invention, the substrate has been subjected to recrystallization prior to tin plating. Therefore, the method of the present invention can also be applied to a batch-recrystallization-annealed (BA) black steel sheet since recrystallization annealing is separated from alloying treatment by diffusion annealing. This makes the present method attractive for use in making a double cold rolled (DR) grade or a heavy temper cold rolled grade because the recrystallization annealing process occurs between two passes and diffusion annealing occurs only after tin plating after the second pass. The SR substrate has been recrystallized before providing a tin layer on one or both surfaces. The substrate may be of the low carbon, ultra low carbon or ultra low carbon steel grade conventionally used for tin-plated mill stock.

The substrate did not undergo further large scale reduction in thickness after formation of the FeSn layer. Further reduction in thickness can lead to cracking of the FeSn layer. The reduction in thickness caused by the temper rolling and the reduction in thickness to which the material is subjected during manufacture of packaging applications does not result in the formation of these cracks, or if they do, in adversely affecting the properties of the coated substrate. The reduction of the cold hardening rolling is usually 0 to 3%.

In a preferred embodiment, the iron-tin diffusion layer contains at least 85 wt% FeSn, preferably at least 90 wt%, more preferably at least 95 wt%. This is an embodiment where the iron-tin alloy is a single phase alloy consisting essentially or entirely of FeSn. Obviously, due to fluctuations in processing conditions, there may be other Fe and Sn compounds formed somewhat unintentionally, but the aim is to achieve as high a weight percentage of the single phase FeSn alloy phase as possible in the iron-tin alloy coating layer.

In addition to allowing the surface alloying process by diffusion annealing to take place, the heat treatment also affects the mechanical properties of the bulk steel substrate as a result of a combination of material ageing and recovery effects. The effect on the mechanical properties of a bulk steel substrate varies with the steel composition (e.g. carbon content of the steel) and the mechanical processing history of the material (e.g. cold rolling reduction, batch or continuous annealing). It has been found that the microstructure of the steel substrate does not change after brief exposure to the elevated temperatures (i.e. 513 to 625 ℃) required to produce the iron-tin surface alloy. In the case of mild steel (typically 0.05 to 0.15 wt% C) or very low carbon steel (typically 0.02 to 0.05 wt% C), yield and ultimate strength can be affected due to carbon going into solution. Also, different degrees of yield point elongation were observed after this heat treatment for CA and BA carbon steel grades. The yield point elongation effect can be suppressed by the cold hardening rolling. Interestingly, the formability of DR steel grades is significantly improved by this heat treatment. This effect is due to the recovery of the deformed steel, which is usually not annealed after the second cold rolling operation, and this results in improved elongation. This recovery effect becomes more pronounced with the increased reduction applied in the second cold rolling operation.

In one embodiment of the invention, the substrate is comprised of interstitial free ultra low carbon steel, such as titanium stabilized or titanium-niobium stabilized interstitial free steel. By using an ultra low carbon Interstitial Free (IF) atomic steel, such as a titanium or titanium-niobium stabilized ultra low carbon steel, the beneficial aspects of the mechanical properties of the bulk steel substrate, including the recovery effect of the DR substrate, can be retained during annealing without the potential drawbacks of carbon or nitrogen aging. This is due to the fact that: in the case of IF steels, all interstitial carbon and nitrogen present in the bulk steel are chemically bonded, preventing them from entering the solution during annealing. During the diffusion annealing test, no ageing effect of the IF steel was observed.

In one embodiment of the invention, the coated substrate is further provided with a conversion coating to reduce the pitting sensitivity of the material and to improve the adhesion to the organic coating, preferably wherein the coated substrate is first pre-treated to increase the surface tension of the outer surface prior to application of the conversion coating.

It has been found that the wet adhesion of an organic coating to the iron-tin alloy coating can be further improved by applying a conversion layer on the mixed surface oxide. This wet adhesion property has been achieved, for example, by exposing the organic coating material to various disinfecting media (cFor simulating the processing conditions applied when filling human or pet food into cans). Wet adhesion of lacquer and thermoplastic polymer (e.g. PET) when organically coated samples are exposed to a sterilization treatment comprising acetic acid or cysteine, by applying commercially available compounds, such as Granodine^TM(Henkel) or(Chemetall) type products. The wet adhesion may also be at 10mM KCr (SO)₄)₂×12H₂The improvement in O-electrolyte was obtained after cathodic treatment of the activated surface at 20 ℃ (pH adjusted to 2.3 by addition of sulfuric acid solution), whereby a current density of 2.5 amps/square decimeter was applied for 5 seconds. Other embodiments of the conversion treatment are also possible, for example using another salt as a source of trivalent chromium ions in the electrolyte, a different current density or a different treatment time.

It is surprisingly noted that the iron-tin alloy coating of the present invention is also very resistant to sulphur contamination. One well-known problem with conventional tinplate is surface contamination due to the formation of tin sulfide during the sterilization of sulfur-containing foods (e.g., during sterilization involving cysteine). Clearly, such iron-tin alloy coatings are not affected by sulfur contamination, whether or not an additional conversion layer is present, even at elevated sterilization temperatures such as 131 ℃ for pet food processing.

Although the mechanism is not fully understood, it is believed that the effect of the pretreatment may not be a passivation effect, but a shielding effect, and it also results in better adhesion.

To apply the conversion coating, dipping, spraying, or electrochemically assisted deposition may be used, while certain conversion coating chemistries require drying after application. It has been found that uniform application of the various conversion layers can be improved by pre-treating the iron-tin alloy coating to increase the surface tension level of the outer surface. The pretreatment may be carried out in a variety of ways, such as immersion in an acidic etching fluid (e.g. a sulfuric acid solution) followed by rinsing in water, or application of flame, corona (corona) or plasma treatment, the choice of the method used depending on the type of conversion layer used. The inventors have found that an effective pretreatment consists of immersing the substrate with the iron-tin alloy coating in a sodium carbonate solution for a short period of time (typically 1 second) followed by a cathodic current through the substrate at a current density of 0.8 amps per square decimeter.

An additional benefit of applying the conversion layer on top of the iron-tin alloy is that it inhibits pitting and cathode delamination.

In another preferred embodiment of the present invention, there is provided a coated substrate for packaging applications, wherein the coated substrate further has an organic coating consisting of a thermoset (i.e., lacquer) or thermoplastic single or multi-layer polymeric coating. The high melting point of the iron-tin alloy makes the coated substrate extremely suitable for coating with a polymer layer by direct extrusion, extrusion followed by lamination or film lamination, since the temperature required for the polymer to adhere to the substrate can easily exceed the melting point of conventional tin layers, as in conventional methods when applying PET. This is a clear advantage of the coated substrate of the present invention.

In a preferred embodiment of the invention, the coated substrate has a second tin layer, which is optionally reflowed, and optionally subjected to a passivation treatment free of hexavalent chromium. An additional tin layer is preferably applied by electrodeposition on top of the alloy layer, which is subsequently reflowed and subjected to a passivation treatment, in particular excluding the use of hexavalent chromium or chromates to prevent further oxidation of the tin surface. The substrate can be used as a more sustainable alternative to traditional tinplate because it requires significantly less tin to achieve similar product properties and excludes the use of hexavalent chromium or chromates.

In a preferred embodiment of the invention, the initial tin coating weight at which the iron-tin alloy layer is formed prior to annealing is at most 1000 mg per square meter of substrate, preferably 100 to 600 mg per square meter of substrate. This is at least 3 times lower than conventional tinplate, thus saving significant power (carbon emissions) and tin usage.

In a preferred embodiment of the present invention, there is provided a coated substrate for packaging applications, wherein the thermoplastic polymer coating is a polymer coating system comprising one or more layers, preferably comprising a polyester, a polyolefin, a polyimide or copolymers thereof, or blends thereof. Achieving excellent adhesion of the thermoplastic coating to the steel substrate is very important to obtain good can properties (e.g., good corrosion resistance). Up to now, ECCS was the substrate of choice for use with thermoplastic coatings due to its excellent adhesion properties. However, as mentioned previously, a problem with ECCS is that its production requires the use of hexavalent chromium. Despite the intensive research efforts, no alternative production route has yet been found which leads to economically, technically and environmentally satisfactory alternative solutions. The iron-tin diffusion layer of the present invention does provide excellent adhesion to polymer coatings without any treatment involving the aforementioned hazardous chemicals.

In a preferred embodiment of the present invention, a coated substrate for packaging applications is provided, wherein the thermoplastic polymer coating is a polymer coating system comprising one or more layers comprising a polyester, such as PET and/or PBT, or a polyolefin, such as PE or PP, or copolymers thereof, or blends thereof. The use of these known polymer coating systems on the novel substrates provides an excellent combination of properties. The fact that the iron-tin alloy layer has a very high melting point results in easy processing at the elevated temperatures required for laminating certain polymer systems, such as polyimides and polyesters.

In a preferred embodiment of the invention, a coated substrate for packaging applications is provided, wherein the steel substrate is coated on both sides with an iron-tin diffusion layer. The substrate may also be applied on both sides of the package, i.e. the side that becomes the inside of the package as well as the outside have the iron-tin alloy layer. This means that the amount of tin on both sides can be minimized and corrosion or deterioration of the appearance of the outside of the package is prevented by the good corrosion properties of the iron tin layer, optionally also with an organic top coat, such as a polymer coating or lacquer. The iron-tin alloy layer may also be part of a coating system on a differential thickness coated strip.

In a second aspect, a method of manufacturing a coated substrate for packaging applications by manufacturing an iron-tin alloy layer on a black steel substrate, comprising the steps of:

providing an SR or DR black steel sheet steel substrate suitable for electrolytic tinning;

providing a first tin layer on one or both sides of the black steel sheet substrate in a first electroplating step, preferably wherein the tin coating weight is at most 1000 mg per square meter of substrate surface, preferably 100 to 600 mg per square meter of substrate surface;

diffusion annealing the blackplate substrate with the tin layer in a reducing atmosphere to an annealing temperature (T513 ℃ C.) of at least_a) For a time sufficient to convert the first tin layer to an iron-tin alloy layer, so as to obtain an iron-tin alloy layer containing at least 80 weight percent (wt%) FeSn (50 at% tin and 50 at% iron);

the substrate with the iron-tin alloy layer is rapidly cooled in an inert, non-oxidizing cooling medium while the coated substrate is kept in a reducing or inert atmosphere before cooling, so that a strong, stable surface oxide is obtained.

The method of the invention can be incorporated into a modified standard tin plating production line. A temperature T of at least 513 DEG C_aEnsuring the rapid formation of the iron-tin alloy phase. The amount of tin required to form a dense and closed layer of iron-tin alloy on each surface is preferably up to 1000 mg/m, and the inventors have found that it is preferred to use from 100 to 600 mg/m of the substrate surface. It was found to be preferable to use a diffusion annealing temperature T of 513 to 645 deg.C, preferably 513 to 625 deg.C_a. Lower T_aLimiting the risk of affecting the bulk mechanical properties of the substrate during diffusion annealing.

In a preferred embodiment, a method of making a coated substrate for packaging applications is provided, wherein the ratio of iron to tin in the iron-tin alloy is about 1. As previously described herein, formation of FeSn at a 1:1 atomic% ratio is preferred because it results in a dense and closed layer that is free of cracks, resistant to deformation, and provides excellent adhesion.

In a preferred embodiment, a method of manufacturing a coated substrate for packaging purposes is provided, wherein the diffusion annealing is performed immediately upon termination of the first tin plating step, and/or wherein the diffusion annealing comprises very rapid heating to a temperature of 550 to 625 ℃ in a hydrogen-containing atmosphere (such as HNX) exceeding 300 ℃/sec, and/or wherein rapid cooling is performed after the diffusion annealing at a cooling rate of at least 100 ℃/sec, and/or wherein the cooling is preferably performed in a reducing or inert atmosphere, such as a helium, nitrogen or HNX atmosphere. Mixed cooling, such as initial cooling with nitrogen, e.g., from maximum temperature to 300 ℃, followed by water quenching, provides good surface quality. It was found that cooling in air leads to extensive and unwanted oxide growth on the FeSn layer, which leads to poor adhesion properties.

In one embodiment of the invention, the rapid cooling is achieved by water quenching, wherein the water used for quenching has a temperature between room temperature and its boiling temperature, preferably wherein the temperature of the water used for quenching has a temperature between 80 ℃ and the boiling temperature, preferably between 85 ℃ and the boiling temperature. The dissolved oxygen content in the water should be as low as possible.

In one embodiment of the present invention, there is provided a method, wherein:

diffusion annealing immediately after the first tin plating step is finished, and/or

The diffusion annealing process uses a heating rate of more than 300 ℃/s, preferably in a hydrogen-containing atmosphere, such as HNX containing 5% by weight of hydrogen, preferably to a temperature of 550 to 625 ℃, and/or

Rapid cooling directly after diffusion annealing at a cooling rate of at least 100 ℃/s, preferably at least 300 ℃/s, and/or

Cooling is preferably carried out in a reducing or inert medium, such as HNX or a nitrogen atmosphere, and/or

Cooling is preferably carried out by hot water quenching, with water temperature of 85 ℃, while keeping the substrate with the layer of iron-tin alloy isolated from oxygen by maintaining an inert or reducing atmosphere, such as HNX gas, before quenching.

It was found that cooling in air leads to extensive and unwanted oxide growth on top of the FeSn layer, which leads to poor adhesion properties. Furthermore, it was found that quenching in water after cooling in air not only leads to a large growth of surface oxides but also to material deformation by forming so-called cooling buckling. Mixed cooling such as initial cooling with nitrogen, e.g., from an annealing temperature to 300 c followed by water quenching to ambient temperature also achieves good results by providing a FeSn alloy layer with a small amount of surface oxide. The inventors have found that a very effective cooling method is to quench the heated and coated substrate directly in a water bath after diffusion annealing, while ensuring that the substrate is not in contact with oxygen prior to quenching, for example by maintaining the substrate in an inert or reducing atmosphere prior to quenching in water. It is important that the water contains as low a dissolved oxygen content as possible to avoid oxidizing the surface. The inventors have found that it is beneficial to use water with an elevated temperature for quenching the coated substrate. If the water temperature is too low, e.g., at room temperature, the cooling of the substrate becomes significantly uneven, resulting in buckling of the substrate. By using elevated temperatures, such as 80 or 85 ℃, buckling of the substrate due to uneven cooling can be prevented. It was found that the process contemplates the ability to cool a hot substrate at extremely high cooling rates in excess of 300 or 350 ℃/sec without adversely affecting the surface oxide properties or substrate shape. Another option is to use forced convection methods during quenching, such as using an optimized array of nozzles directed towards the strip surface to spray cooling water onto the strip to achieve a more uniform cooling rate on the strip surface. This method allows the use of lower water temperatures without increasing the risk of buckling of the strip. The water temperature in this option is preferably below 80 c, more preferably 30 to 70 c. Another option is to use indirect cooling by using cooling rolls. The advantage of this method is that the cooling medium is prevented from coming into direct contact with the strip, which significantly simplifies the problem of having to maintain a non-oxidizing atmosphere while cooling the substrate. In a preferred embodiment, the maximum annealing temperature is limited to 615 ℃. The inventors have found that the highest FeSn content is achieved in the iron-tin alloy layer for annealing temperatures of 550 c to just above 600 c.

In a preferred embodiment, there is provided a method of making a coated substrate for packaging, wherein at T_aThe time at (A) is at most 4 seconds, preferably at most 2 seconds, more preferably wherein at T_aThere is no residence time in the next. In the latter case, the substrate is heated to T_aAnd then cooling the substrate to perform diffusion annealing. At T_aThe short residence time of the lower part allows the production of the iron-tin alloy layer in a suitably modified conventional tin plating production line, and moreover, minimizes the risk of adversely affecting the bulk mechanical properties of the substrate.

In one embodiment, a method is provided for manufacturing a coated substrate for packaging purposes, wherein the iron-tin alloy layer is coated with a second tin layer in a second tin plating step on one or both sides of the substrate, optionally followed by a reflow step and/or a passivation treatment of said second tin layer. This process produces nearly traditional tinplate products, but with a significantly lower tin weight per unit surface. The optional passivation treatment is a hexavalent chromium-free passivation treatment.

The application of an additional tin layer on top of the iron-tin alloy may be achieved by a second tin plating step, before which the iron-tin alloy coated strip is immersed in an acidic solution (e.g. a sulfuric acid solution) in order to activate the surface prior to electrodeposition. The resulting product can be used directly in can manufacture, but requires passivation to be applied to prevent excessive tin oxide growth on the surface. As previously mentioned, a hexavalent chromium-free passivation treatment may be employed for this purpose. In fact, alternative passivation for conventional tin plates can also be used in combination with low-tin products based on the use of iron-tin alloy coatings with an iron content of 50 atomic%. Instead of applying the passivation directly, the second tin metal layer can also be reflowed using standard processing methods for conventional tinplate, for example melting and subsequent reflow using resistance heating or induction heating. After reflow, the surface needs to be passivated as previously described.

In a preferred embodiment, a method is provided for manufacturing a coated substrate for packaging applications, wherein one or both iron-tin alloy layers are coated with a conversion layer and/or wherein the coated substrate has an organic coating consisting of a thermosetting (i.e. lacquer) or thermoplastic single-or multi-layer polymer coating.

It was found to be advantageous in certain cases to pre-treat the iron-tin diffusion layer before applying the polymer coating layer as described above. It is believed that the effect of the pretreatment may not be a passivation effect, but a shielding effect, and it also results in better adhesion. It is again noted that the manufacture of the product and the process of the invention are achieved without chromate compounds or chromizing treatment at any stage.

In one embodiment of the invention, the diffusion annealing treatment to form the iron-tin alloyed layer is adapted to promote aging and/or recovery of the DR substrate in the SR or DR substrate.

In a third aspect, an apparatus for producing a coated substrate strip for packaging purposes by producing an iron-tin alloy layer on a packaging steel substrate, comprising:

one or more tin baths for providing a strip having a first tin layer on one or both sides thereof, optionally followed by one or more rinsing baths for removing excess electrolyte;

followed by a temperature T for_aThe first tin layer is diffusion annealed for an annealing time t sufficient to convert the first tin layer to an iron-tin alloy layer_aFollowed by a rapid cooling zone, preferably wherein the heating rate of the heating zone is at least 300 ℃/sec and/or wherein the atmosphere in the heating zone is a hydrogen-containing atmosphere, such as HNX;

optionally followed by one or more further tin baths, optionally preceded by a pretreatment zone to activate the iron-tin alloy surface, said further tin baths being intended to provide the strip with a second tin layer on one or both sides thereof, optionally followed by one or more rinsing baths for removing excess electrolyte;

optionally followed by a melting zone for melting and reflowing the second tin layer;

followed by rapid cooling, wherein the cooling rate after heating is preferably at least 100 ℃/sec;

optionally followed by a passivation zone, for example to apply a hexavalent chromium-free passivation layer.

It should be noted that the method is very compact and can be relatively easily installed in existing electrolytic tin plating lines, providing significant advantages in terms of ease of construction and cost.

It is noted that the production of the product and the process of the invention do not involve chromate compounds or chromate treatments, such as chromate passivation, at any stage.

In a preferred embodiment, a method of manufacturing a coated substrate for packaging applications is provided, wherein the heat treatment forming the diffusion layer is adapted to promote aging and/or recovery of the DR substrate in the SR or DR substrate. It was found that the ageing treatment (temperature, annealing atmosphere and annealing time are the main parameters) can be adjusted such that a significant increase in yield strength is obtained in the SR base material. The evaluation of the magnitude of these effects and the selection of relevant parameters is within the ability of the person skilled in the art.

To explain the differences between the SR and DR routes, reference is made to fig. 4, where the SR route is shown in comparison to the DR route. It is important to note that according to the invention, the recrystallization annealing takes place before tin plating in the SR route and before the second cold deformation step in the DR route. The cold rolling steps are represented in the figure by two circles at the respective top ends and represent the rolling process. The SR route has only one cold rolling step, which may consist of multiple passes (typically 4 or 5 roll stands), and the DR route has two cold rolling steps, which consist of multiple passes alone. After tin plating, the tin plate is diffusion annealed to produce the FeSn iron-tin alloy layer. The product thus obtained may have a second tin layer and optionally reflow, and/or have a conversion coating, a passivation layer or an organic coating. The so-called cold-rolling step of the SR base material (i.e. after recrystallization annealing and before tin plating) is not shown in fig. 4.

Examples

Samples of the packaging steel panels (grade TH 340) were thoroughly rinsed in a commercially available alkaline cleaner (Chela Clean KC-25 supplied by Foster Chemicals), rinsed in deionized water, immersed in a 50 gram/liter solution of sulfuric acid for 5 seconds at room temperature, and rinsed again. The sample was then plated with a 600 mg/m tin coating from the MSA bath typically used for producing tinplate in a continuous strip plating line. A current density of 10 amps/square decimeter was applied for 1 second.

After tin plating, the alloy is used with 5% H₂The sample was annealed in a reducing atmosphere of (gaseous) HNX. The sample was heated from room temperature to 600 ℃ at a heating rate of 100 ℃/sec. Immediately after the samples reached a peak temperature of 600 ℃, one sample was cooled by vigorous blowing with helium and the other sample was cooled by water quenching. When the sample was quenched in cold water, numerous dents were generated in the sample, a phenomenon known as "cooling buckling". However, when the water of the quenching tank is heated to 80 ℃ or more, the cooling buckling does not occur any more. In the case of cooling with helium, the cooling rate was 100 ℃/sec. Cooling by hot water quenching is much faster. Within about 1 second, the sample was cooled from 600 ℃ to 80 ℃, the water temperature in the quench tank.

The phases formed during this diffusion annealing step were analyzed by X-ray diffraction (see FIG. 1, which shows T at 600 deg.C)_aThe alloy phase formed by diffusion annealing, and the effect of the cooling rate after annealing). In both cases, iron is formedA tin alloy layer containing more than 90% of the desired FeSn alloy phase (96.6 and 93.8, respectively). Other examples show values for FeSn of 85.0 to 97.8% for annealing temperatures of 550 to 625 ℃, where annealing at annealing temperatures above 550 ℃ and below 615 ℃ results in a range of 92.2% to 97.8%.

The morphology of the coating was analyzed by scanning electron microscopy. SE (secondary electron) images of the two samples are given in fig. 2 and 3, which show SEM SE images of the samples cooled with helium (fig. 2) and with water (fig. 3). In both cases, a very dense and compact structure is formed, which is typical for FeSn alloy phases. On top of the water-cooled sample, very tiny triangular crystallites were also formed.

Fig. 5 shows the effect of the cold-rolling reduction of hardening (fig. 5 b) and the large cold-rolling deformation (fig. 5 c) on the formation of cracks in the FeSn layer, showing why this FeSn layer should be formed after the second cold-rolling step as described in fig. 4 in the method of manufacturing DR products. The FeSn layer formed between the two rolling steps will be subjected to too high deformation and cracking. The cold-rolling reduction by hardening did not lead to cracking (see fig. 5 b).

Fig. 6 shows a schematic illustration of a coating system made according to the present invention. Figure 6a shows a known tinplate remelted and passivated with chromate (cr (vi) treatment) and figure 6b shows a known ECCS substrate (TFS).

Fig. 6c to e are embodiments of the present invention. Fig. c shows a FeSn alloy on a substrate. The tin layer is completely converted into a FeSn iron tin alloy layer and a conversion layer and/or a passivation layer (indicated with "(c or p)" in fig. 6c-6 e) is provided on top. Fig. 6d shows a steel substrate with an FeSn alloy between the second tin layer and the substrate, the outer tin layer may be applied with a conversion layer and/or a passivation layer. Fig. 6e shows tinplate where the second tin layer reflows and subsequently passivates. The conversion and/or passivation treatment is a hexavalent chromium free process.

Claims (26)

1. A process for manufacturing a coated substrate for packaging applications by manufacturing an iron-tin alloy layer on an SR or DR ferrous steel substrate, the process comprising the steps of:

provide

Recrystallization annealed single-pass cold rolled steel substrate, i.e. SR black plate steel substrate, or

-a secondary cold rolled steel substrate, i.e. a DR black plate steel substrate, subjected to a recrystallization annealing between the first and second cold rolling treatments;

providing a first tin layer in a first tin plating step on one or both sides of the SR black steel sheet substrate or on one or both sides of the DR black steel sheet substrate, wherein the tin coating weight is at most 1000 mg per square meter SR or DR black steel sheet substrate surface;

heating the SR or DR steel blackplate substrate having said first tin layer to an annealing temperature T of 513 ℃ to 645 ℃ in a reducing atmosphere by heating at a heating rate of at least 300 ℃/s_aFor a time t sufficient to convert the first tin layer into one or more iron-tin alloy layers_aTo diffusion anneal it to obtain one or more iron-tin alloy layers containing at least 80 weight percent (wt%) of FeSn consisting of 50 at% tin and 50 at% iron;

rapidly cooling the coated substrate with the one or more iron-tin alloy layers in an inert, non-oxidizing cooling medium at a cooling rate of at least 100 ℃/s while maintaining the coated substrate in a reducing or inert atmosphere prior to cooling, thereby obtaining a strong, stable surface oxide.

2. The process for manufacturing a coated substrate for packaging applications as claimed in claim 1, wherein in the first tin plating step, the tin coating weight therein is 100 to 600 mg/m SR or DR steel black substrate surface.

3. A method of manufacturing a coated substrate for packaging applications as claimed in claim 1 or 2, wherein the ratio of iron to tin in the layer of iron-tin alloy contains at least 85 wt% FeSn.

4. A method of manufacturing a coated substrate for packaging applications as claimed in claim 1 or 2, wherein the ratio of iron to tin in the layer of iron-tin alloy contains at least 90 wt% FeSn.

5. A method of manufacturing a coated substrate for packaging applications as claimed in claim 1 or 2, wherein the ratio of iron to tin in the layer of iron-tin alloy contains at least 95 wt% FeSn.

6. A method of manufacturing a coated substrate for packaging purposes as claimed in claim 1 or 2, wherein rapid cooling is achieved by water quenching, wherein the water used for quenching has a temperature between room temperature and its boiling temperature.

7. A method of manufacturing a coated substrate for packaging purposes as claimed in claim 1 or 2, wherein rapid cooling is achieved by water quenching, wherein the water used for quenching has a temperature of 80 ℃ to its boiling temperature.

8. A method of manufacturing a coated substrate for packaging applications as claimed in claim 1 or 2, wherein

Diffusion annealing process immediately after the first tin plating step is finished, and/or

The diffusion annealing process is heated to a temperature of 550 to 625 ℃ in a hydrogen-containing atmosphere with a heating rate of more than 300 ℃/sec, and/or

Rapid cooling directly after diffusion annealing, and/or

Cooling is carried out in a reducing atmosphere, and/or

Applying a hot water quench, cooling with minimum dissolved oxygen content and water temperature of 85 ℃, while keeping the coated substrate with the layer or layers of iron-tin alloy isolated from oxygen by maintaining an inert or reducing atmosphere before quenching.

9. A method of manufacturing a coated substrate for packaging applications as claimed in claim 1 or 2, wherein

Diffusion annealing process immediately after the first tin plating step is finished, and/or

The diffusion annealing process is heated to a temperature of 550 to 625 ℃ in an HNX atmosphere with a heating rate of more than 300 ℃/s, and/or

Rapid cooling directly after diffusion annealing, and/or

Cooling is carried out in a nitrogen atmosphere, and/or

Applying a hot water quench, cooling with minimum dissolved oxygen content and water temperature of 85 ℃, while keeping the coated substrate with one or more layers of iron-tin alloy isolated from oxygen by maintaining HNX atmosphere before quenching.

10. A method of manufacturing a coated substrate for packaging applications as claimed in claim 1 or 2, wherein at T_aThe time for next is at most 4 seconds.

11. A method of manufacturing a coated substrate for packaging applications as claimed in claim 1 or 2, wherein at T_aThere is no residence time in the next.

12. The method of manufacturing a coated substrate for packaging purposes as claimed in claim 1 or 2, wherein the iron-tin alloy layer is coated with a second tin layer on one or both sides of the coated substrate in a second tin plating step.

13. A method of manufacturing a coated substrate for packaging purposes as claimed in claim 1 or 2, wherein the iron-tin alloy layer is coated with a second tin layer on one or both sides of the coated substrate in a second tin plating step, followed by a reflow step and/or passivation treatment.

14. A method of manufacturing a coated substrate for packaging applications as claimed in claim 1 or 2, wherein one or both iron-tin alloy layers are coated with a conversion layer.

15. The method of manufacturing a coated substrate for packaging applications as claimed in claim 14, wherein the coated substrate is first pre-treated to increase the surface tension of the outer surface before applying the conversion layer.

16. A method of manufacturing a coated substrate for packaging applications as claimed in claim 13, wherein the passivation treatment is a hexavalent chromium free passivation treatment.

17. A method of manufacturing a coated substrate for packaging applications as claimed in claim 1 or 2, wherein the coated substrate has an organic coating consisting of a thermosetting lacquer or a thermoplastic single or multi-layer polymer coating.

18. A method of manufacturing a coated substrate for packaging applications as in claim 17, wherein the thermoplastic single or multi-layer polymeric coating is a polymeric coating system comprising one or more layers comprising:

a polyester;

and/or a polyolefin;

and/or copolymers of polyesters and polyolefins;

and/or a blend of polyester and polyolefin.

19. A method of manufacturing a coated substrate for packaging applications as claimed in claim 18, wherein the polyester is PET and/or PBT and the polyolefin is PE or PP.

20. The method of manufacturing a coated substrate for packaging applications as claimed in claim 1 or 2, wherein the diffusion annealing treatment forming the iron-tin alloy layer is adapted to promote ageing and/or recovery of a DR ferrous steel substrate in the SR or DR ferrous steel substrate.

21. The method of manufacturing a coated substrate for packaging applications as claimed in claim 1 or 2, wherein the SR or DR ferrous steel substrate consists of interstitial free ultra low carbon steel.

22. The method of manufacturing a coated substrate for packaging applications as claimed in claim 1 or 2, wherein the SR or DR ferrous steel substrate consists of a titanium stabilized or titanium-niobium stabilized interstitial free steel.

23. A coated substrate for packaging applications manufactured by the method of any one of the preceding claims, wherein the initial tin coating weight before diffusion annealing to form the iron-tin alloy layer is at most 1000 mg/m SR or DR steel black sheet substrate surface.

24. A coated substrate for packaging applications as claimed in claim 23, wherein the initial tin coating weight before diffusion annealing to form the iron-tin alloy layer is from 100 to 600 mg/m SR or DR ferrous steel substrate surface.

25. An apparatus for producing an iron-tin alloy layer on an SR or DR ferrous steel substrate by a process as claimed in any one of claims 1 to 22 for producing a coated substrate strip for packaging applications comprising:

one or more tin baths for providing a strip having a first tin layer on one or both sides;

followed by a temperature T for at least 513 DEG C_aThe first tin layer is diffusion annealed for an annealing time t sufficient to convert the first tin layer to one or more iron-tin alloy layers_aFollowed by a rapid cooling zone comprising a non-oxidizing cooling medium, wherein the heating rate of the heating zone is at least 300 ℃/sec, wherein the atmosphere in the heating zone is a hydrogen-containing atmosphere, wherein the cooling rate after heating is at least 100 ℃/sec.

26. An apparatus for producing an iron-tin alloy layer on an SR or DR ferrous steel substrate by a process as claimed in any one of claims 1 to 22 for producing a coated substrate strip for packaging applications comprising:

one or more tin baths for providing a strip having a first tin layer on one or both sides, followed by one or more rinsing baths for removing excess electrolyte;

followed by a temperature T for at least 513 DEG C_aThe first tin layer is diffusion annealed for an annealing time t sufficient to convert the first tin layer to one or more iron-tin alloy layers_aFollowed by a rapid cooling zone comprising a non-oxidizing cooling medium, wherein the heating rate of the heating zone is at least 300 ℃/sec, wherein the atmosphere in the heating zone is an HNX atmosphere;

followed by one or more further tin baths, preceded by a pretreatment zone to activate the iron-tin alloy surface, for providing the strip with a second tin layer on one or both sides, followed by one or more further rinsing baths for removing excess electrolyte;

followed by a melting zone for melting and reflowing the second tin layer;

followed by rapid cooling in the rapid cooling zone, wherein the cooling rate after heating is at least 100 ℃/sec;

followed by a passivation zone to apply a passivation layer free of hexavalent chromium.