MANUFACTURING AND FEATURES OF TINPLATE
The different stages of the process of tinplate manufacturing will be detailed thoughout this paper: from the base steel production and its different varieties to the several consecutive sequences of tinning.
TABLE OF CONTENTS
1.-What is tinplate and for what is it used?
3.-Base Steel manufacturing
5.-Base Steel composition
6.-Types of base steel
10.-Tin Free Steel (TFS)
In spite of being invented in ancient times, tinplate is a material which reached a huge development over the 20th century. The United States bet high on this industry. As a consequence, it achieved its greatest degree of activity in that country in the seventies decade. Afterwards, the huge deployment of beverage containers, the use of aluminium as raw material by North America, the alternative containers and the obsolescence of the American steelmaking industry, caused the decline of that market.
It did not happen the same in other areas of the world. Europe was able to update their steelmaking industry in time so as to maintain its competitiveness and, consequently, to make possible that the European tinplate was able to resist in its fight against aluminium. That fact did not avoid that a wide variety of canning had a negative effect somehow. However, the mentioned sector overcomed that situation by means of the merge of several metallurgical companies which allowed themselves to keep their technological excellence.
In the same way, the tinplate market and other similar products continued well positioned in that market in other geographical areas, such as South America, The Far East and Asia.
Nowadays, although this sector still has to survive in a competitive market, it continues being transcendental all over the world. For this reason, it is recommendable to get know a little bit more about how this material is made.
1.- WHAT IS TINPLATE? FOR WHAT IS IT USED?
Tinplate is a material which is made through a mix of steel sheets and catalytic covering or by a process of passivation of the tin sheet in both sides of this component.
This material is widely used in the food industry to protect and increase the life cycle of the canned products, for instance sardines, peppers, tomatoes, grains, condensed milk, chocolate, spices, coffee, among others.
In the same line, its use has spread to household items, such as household appliance, cleaning products, insecticide and many more. It could be also found in car parts, decoration materials or sealing caps.
2.- HOW IS TINPLATE MADE?
Tinplate contains a 99%, or even more, of steel, that is to say, it is a product which is made up basically with steel. The manufacturing process really begins in the blast furnace and in the tin foundry, but, in practice, it is considered to start from the steel manufacturing process. In fact, this is the moment when the composition and type of steel ingot will be selected in order to define its future use as rolled product. As a result, the basic raw materials required to manufacture tinplate are steel and refined tin.
2.1- BASE STEEL MANUFACTURING TO OBTAIN TINPLATE
There are two essential processes when obtaining raw steel in liquid state. The first one is from iron ore (a) and the second one is from scrap (b).
- From iron ore (foundry process): the iron ore is a rock which is composed of iron oxides and other different minerals, such as gangue. The minerals and metallic waste used in the elaboration process are initially full of stains. Iron is never in pure state in nature. For this reason, it can be found in the form of oxides and sulphides. In the industrial process which is carried out in the blast furnace, the main goal is to get a product as much rich in iron as possible. However, other components will be found in it. Among parasitic matters, a lot of them have a temperature in combustion and evaporation stages which is lower than the one of iron. That is why most of them will desappear when heating this material. Moreover, other components, whose density is lower than the one of iron, can be found and they will float on the molten metal. In this regard, it is highly convenient to take advantage of the easiness of carbon to react with oxygen so as to release these molecules of iron compounds in the form of iron oxides (FeO), magnetic oxides (Fe3O) and ferric oxides. (Fe2O3)
Carbon, in its combustion process, is a great devorer of oxygen which will take it from the air, but also from the oxide, to form the CO and CO2 compounds (carbon monoxide and carbon dioxide respectively). In this operation, the surplus part of carbon will leave traces in the latter, which will be combined with iron (from 3% to 6%). The resulting material in the blast furnace is called ‘smelting.’
If our purpose is to release carbon from the mentioned material, a supplementary operation will be required, which consist in adding oxygen to re-form CO and CO2, carbonic gases. Furthermore, the mentioned oxygen is able to react to other elements which are contained in the foundry, such as manganese and, thus, being able to form manganese oxide (MnO), silica (SiO2), alumina (Al2O3), among other residues.
All these operations are first carried out in the blast furnace and, then, in the refining process, as it will be detailed in the following paragraphs.
To sum up, it could be said that after the preparation stage of the mineral in the agglomeration workshop, the extraction of the iron is developed with the help of a fuel, like carbon (coque) in the blast furnace. In spite of this, pure iron is not obtained yet, but cast iron, which is the result of a liquid mix of iron (96%) plus carbon (3%), and the rest comes from the coke which has not been burnt, in combination with some residues (phosphorus, sulfur, etc) which come from the gangue. Figure number 1 shows the outline of a blast furnace.
Figure nº 1: Outline of a blast furnace
- From scrap (electrical procedure): the components which the furnace is fed on can be from raw materials (pieces of machinery carefully selected) to prepared scrap, which is selected, crushed, measured and with a minimum content of iron of 92%. This group of elements is melted in a electronic furnace.
Pig iron or iron coming from blast furnace, and also the scrap which has melted in an electronic furnace, are fragile iron-carbon alloys and with a high content in the latter element. In the same way, high sulfur and phosphorus content may be found.
The manufacturing of steel is withing the process of refinement to reduce and control the percentages of other elements which difer from iron, with the purpose of producing ingots from the purity features and malleability required.
In general terms, these elements and the spots are reduced by means of oxidation with iron oxide or oxygen, and they are removed by flotation with the addition of high melting point materials, for instance limestone.
Nowadays, four basic procedures are developed to obtain steel: Siemens-Martin process or open crucible, Bessemer or Thomas process, top-blown process and electric furnace. The latter is not used frequently in the manufacturing process of tinplate. The first and the second are likely the most widely used in the pneumatic procedures.
- The “open crucible” process. The crucible is usually loaded with molten iron, which comes from the blast furnace, scrap and cold cast iron. The spot are oxidised causing the formation of an oxidising slag, and the fuel used can be liquid or gaseous. The capacity of a blast furnace like this type, can be up to 400 tons, and through the use of oxygen in bulk, of relative economy, the purpose is to have a high performance, reaching 50 tones per hour. These blast furnaces can be fixed or tilting.
- Bessemer or Thomas process. In this system, the spots are reduced by the direct injection of air or oxygen, through the molten metal, by means of nozzles which are located at the bottom of the converter. (See figure number 2)
Figure nº 2: The manufacturing process of steel in Thomas converter
In order to burn the carbon from the foundry, air or a mixture of air and oxygen is blown though over the molten metal, which is poured into the converter, which, at the same time, constitutes a large steel retort with a 25-50 tons capacity. The refractory walls of the converter are made of basic dolomite, which is unassailable to the lime that must be introduced to eliminate the phosphorus of the foundry.
The productivity and quality can be improved, by means of controlling the composition of the injected oxidising gas, either air, air enriched with oxygen, pure oxygen, a mixture of oxygen and carbon dioxide, etc. For instance, the variant VLN (very low nitrogen) not only increases the production, but also reduce the incorporation of nitrogen to steel, which is normally undesirable.
Different stages can be identified in this process. In the first phase, many sparks are produced, which are caused by the combustion of silicon that lasts from two to three minutes. In the second, flames are generated by the carbon combustion, with a persistence which lasts from ten to twelve minutes. The main feature of the third process is the release of gases which is provoked by the subsequent blowing and the combustion of phosphorus, whose interval goes from three to five minutes.
Throughout this process, different solid elements are usually added: lime, scrap, ferromanganese or specular pig iron, according to needs.
In the field of the manufacturing process of tinplate steel, there is a system called Duple, which is the combination of the Bessemer acid process with the basic open crucible process one. The main purification is carried out in the converter whereas, in the second part of the process, the phosphorus reduction is developed.
- The upper oxygen injection process. The Bessemer process is widely used, mainly due to its low installation cost and operating flexibility. However, it has important limitations in terms of quality, since it requires a raw material which is high in phosphorus content and restrictions of the use of scrap material.
These difficulties affected strongly some manufactures. This fact originated the development of superior oxygen injection processes, through which tin steels with a high quality, with low nitrogen content and high or low phosphorus, according to the requests. There are three systems which are widely used in the production of laminated materials. They are called LD, DDAC and KALDO.
- LD process: This process was the first one which was based on the availability of oxygen in large quantities. It consist in replacing the injection of air from the bottom of a Bessemer converter, by the injection of a oxygen jet at high pressure at the top. This produces a strong agitation and therefore, a very quick oxidation of the spots. It is a fast and efficient process, with initial investments not much important that the one required in the Bessemer process, which reduces maintenance problems and produces a material low in nitrogent. One of its drawbacks is the need of using iron tools high or low in phosphorus. This system was developed by two companies which are located in Link and Donawitz. The name of ‘LD process’ comes from the initials of these cities.
- The DDAC process: it is like the previous one, except for the fact that lime is injected together with the oxygen. It is a type of tecnology whose origin is French.
- The KALDO process: it was developed in Sweeden. In this kind of procedure a crucible and a rotary and tilting kiln. In the latter, an injection of oxygen jet takes place in the upper part and also lime in the form of injections or in pieces is added to the previous element.
The oxygen processed are versatile. They allow us to use reasonable amounts of ore and scrap, so as to produce low phosphorus and nitrogen steel, all of them with good mechanical features.
- Steel casting: once the steel has been obtained by any of the procedures describes previously and when it is in liquid state in the converter, it must be transformed to solid state, by meand of performing its casting. There are two casting procedures from an industrial perspective. The one which is more classical, allows you to get lingots from the steel (‘cast in ingot molds’). The other one consists in following the procedure known as ‘continuous casting.’
- Ingot casting
It is the conventional procedure par exellence. By using a spoon, the molten steel is poured into molds, which, once they are cooled, gives rise to manageable ingots. The following operations are carried out with those ingots.
- Continuous casting
This one constitutes the most modern plate making process. The production of semi-finished productos of very high quality is one of its main advantages. Besides, it allows us to reduce the production costs, and increases the steel mill. Indeed, thanks to this system, the operations are less numerous and simpler than in casting molds. Nowadays, the use of ingots for steeel destined for laminates is being banishing. See figure number 3:
Figure number 3: Continuous casting
In the continuous casting, the content of the converter is poured, by means of a spoon, in a regular and uninterrupted manner, into a bottomless and cooled, of such section that corresponds to the one of the desired plate. Afterwards, is passes through a seried of rollers to be flattened, and finally, by means of an oxyfuel blowtorch, the plates are cut in the desired length. In this way, plates (slab) with a length between five to twenty meters, with a one meter or more width and about twenty cm thick are obtained.
2.2- THE MANUFACTURING SEQUENCE
Figure number 4 represents the sequence with the different operations or phases involved when processing steel plates, which will be the basic raw materials in the tinplate manufacturing plants.
Figure number 4: manufacturing of steel plates
When the ingot casting process has been developed, once the steel has been cast in ingots, the first operation that is carried out is the rolling of the ingot to turn it into a slab. This procedure is usually carried out on reversible, double-high rolling mills. The ones which are so-called Universal type have cylinders or lateral rollers that work simulataneously on the four sides of the ingot, eliminating the operation of turning the ingot on its axis during this stage.
The final product of this procedure, the plate or slab, has a thickness which is from 125 to 230 mm, the approximate width of the tin that is finally to be obtained, and a length that depends on the size of the original ingot.
The plates generated both by continuous casting and by rolling of ingots are prepared to be treated in hot rolling.
Hot rolling represents the next step. However, there is usually an intermediate stage, which consists in cooling and storing the plates, making a selection process, the preparation of the surface (steep) and the heating of the plate at the suitable temperature to develop the lamintating process. Omitting this intermediate stage requires a very exact planning, and a very competent technological capacity, which ensures the absence of defects in the plates or slabs. Figure number 5 shows the different phases of the hot rolling process.
Figure number 5: Hot rolling process
The surface preparation is done in the ‘husker,’ in which the iron plate is subjected to the release of iron oxides and spots, by means of a weak rolling pressure, releasing them, in this way, by the action of high pressure water. The plate is heated at 880 ºC. It is necessary to eliminate this layer of oxides as it damages the laminate, since it causes the erosion of the cylinders, provokes grooves on the metal, incrustations and other manufacturing defects (breaks, slippings, folds, etc.)
The hot rolling mil reduces the plate to a continuous band of about 2 mm thick. It is normally compounded by two sections, one devoted to roughing and another one about finishing. It can be continuous or reversible, depending on the capacity of the installation, among other factors.
The roughing train often involves from four to six boxes which reduce the initial thickness of the plate between 25% to 50% per box. The finishing train includes from four to seven boxes, reducing, one more time, that thickness between 25 % to 30% per box, except the latter which only reduces 10%.
The maximum speed of this operation could reach the 100 km/h. Once the material is out, its temperature it is of 850 ºC. Then, that material is cooled using a curtain of water at 590ºC, in order to form coils with it.
The oxidised layer of the coild must be removed and lubricated before proceeding with cold rolling. This procedure is normally carried out in a series of tanks which contain hot diluted sulfuric acid (whose temperature is about 100 ºC). The coild is washed, dried and lubricates with palm oil or another kind of lubricant which is suitable to develop the cold rolling process. The removing line is usually provided with a circular cutter, which cuts the edges, ensuring that these are proper for the cold reduction or rolling. Besides, it is in charge of fixing the maximum width of the sheet what will be obtained and also the one that will give the best economic results. See figure number 6
(Figure number 6: Cold rolled, annealed and skin-pass of the base steel)
The next operation is cold rolling, which can be done in continuous (tandems) or reversible trains. Lubricants and coolants are used during this operation, and the resulting thickness is very close to the desired finish. Therefore, the reduction is of 1.8 or 2 mm to a measurement between 0.15 to 0.3 mm approximately according to the final caliber of the tinplate to be produced.
The obtained coil is made of a very hard material and subject to high tautnesses This one needs an adequate treatment to give it to the necessary machinability, causing a recrystallization of the steel. This treatment is called ‘annealing’ and it is the heating of the metal in a reducing atmosphere to avoid all oxidation (mixture of nitrogen and hydrogen) and can be made continuous (continous annealing) or in hood furnaces (batch annealing).
Continuous annealing: The band circulates through an oven at 630ºC. In the same way. Each point of the material stays a minimum of time of 1,5 minutes in the oven. The continuous annealing presents several advantages and disadvantages, such as the following ones:
- Reduction of the ‘in progress’ manufacturing materials.
- Reduction of the manufacturing periods.
- Improvement of the quality of manufactured products:
- Constant and continuous heating along the coil, obtaining, in that way, more homogeneous mechanical qualities.
- Fast cycle, through which fine grains are got and, in consequence, an isotropic metal and superior mechanical qualities, such as the improvement of the elastic limit.
- Better resistance to corrosión. The rapid cycle does not allow elements, such as carbon, manganese or phosphorus to move on the material surface, as it happens in the annealing or hood process.
- It enables us to use the type MR steel composition, which is lower in hardening elements: carbon, manganese, etc (Continuous annealing = 0.08 % of carbon, Base annealing =0.10% / 0.13% of carbon) which favours operations, such as welding, speciality, among others.
- Difficult programming of the orders, since the maximum variation of width between consecutive coils is of 50 mm, and thickness of 10%.
- Delicate leading of the line. Risk of breaking the band in the oven. This implies a considerable stop. The elasticity limit of the band at the temperatura in the annealing furnace is low (30 N / mm2 approximately).
- Impossibility of line stops.
- Thin crystallography that generates a less ductile, of weak anisotropic metal, which affects in a negative manner the embossing process.
- Risk of the appearance of Lüder lines.
Hood annealing: Several stacked coils are covered with a hood in a reducing atmosphere at 680ºC for, at least, 85 hours, which are distributed in 32 hours of heating, 34 of cooling under hood up to 170º, and 19 hours of accelerated cooling outdoors. See figure number 7.
Figure number 7: Scheme of the hood annealing process
In both cases, the previous removal of the residues used in the lubricants and coolants which have been used initially, is essential. This method should be in concordance with the type of annealing which has been done. It is normally developed by electrochemical means, such as the degreasing bath heated to 95ºC, or mechanical, with brushing on both sides. After that, the tempering process, the surface treatment or finishing operation (temper rolling or skin-pass). It is carried out by means of a very light reduction or lamination, without lubricant, which normally does not exceed the 2% of thickness. For this purpose, a rolling train, which is integrated by two boxes, is used. The band, when passing between the cylinders under the effect of the established pressure and traction, undergoes a superficial modification of the structure achieving an increase in the hardness of the surface layer, but preserving the internal softness. In short, this operation provides the band with the desired hardness, a good leveling and a materials with a finishing surface, which depends on the cylinders roughness used in the boxes of the rollling train. By combining the surface finish of the boxes cylinders of this lamination, and the final remelting of the tin after the tinning operation, different surface finishes of the tin plate are achieved. The main types of finishes are:
- Brilliant finish: In order to achieve this finish, it is essential to obtain a mirror polish on the cylinders. It is also necessary to apply the remelting of tinning. This type of finish is, together with the one of the stone, the most requested.
- Stone finish: The cylinders are subjected to two grinding passes with a special grain wheel, which gives the product a striated appearance. As in the preceding case, the tin coating is refunded, which also ensures the brilliance.
- Matte finish: It presents a slightly reflexive surface. So as to do this, the cylinders are shot peened and the remelting operation after the tinning is eliminated.
When it comes to reduced double tinplate, the last gauge reduction is done by replacing the tempering operation with a new caliper or lamination reduction of approximately 33%, with the consequent elongation of the material in a similar percentage, this time using surface lubricants. Thus, high mechanical characteristics with a small thickness are transmitted to the tinplate. It is a common practice to prepare the coils in a previous way to the tinning operation. It consists mainly of cutting the edges and eliminating the sections of low quality or of caliber out of specifications, forming coils of optimal size for the tinning line.
In the electrolytic tinning, the material prepared passes continuously through the operations of electrolytic cleaning, electrolytic pickling, electro-deposition of tin, remelting tin (flow-melting), passivation treatment and lubrication. After this series of operations, the product can be cut into sheets at the ordered size (length, since the width was given when preparing the coils), inspected, selected and packed; or it can be coiled to be sent to the customer or to be cut, etc. in a separate cutting line. It is normal to make the selection of the material on the same line before packing. Later, we will deal with these points in more detail.
2.3- THE BASE STEEL COMPOSITION
The base steel used to make tinplate, is essentially a mild steel low in carbon, with a generic composition, such as the following type:
- Carbon Carbon 0.04 – 0.15%
- Silicon 0.08% máximum
- Sulfur 0.015 – 0.05%
- Phosphorus 0.01 – 0.14%
- Copper 0.02 – 0.20%
- Manganese 0.20 – 0.70%
- Nitrogen 0.001 – 0.025%
Due to the use of scrap in the steel manufacturing process, there may be other elements such as nickel, chromium and tin, but no other element is intentionally added. The only exceptions are phosphorus and nitrogen, which can provide special properties. The use of copper to increase the resistance to corrosion, and of some carbides to control aging has fallen into disuse.
The influence of some elements in the base steel of the tinplate is given in broad strokes in the following paragraphs:
- Carbon: This element increases the elastic limit, the tensile breakage limit and the hardness, decreases the elongation and the ductility. Within the usual limits of the carbon in relation to the tinplace, the variations in the content of this element are slightly important, since other elements have a higher influence.
- Silicon: It exists in tinplate as a residual element and it hardens lightly and, in some cases, it affects its resistant to corrosion adversely.
- Sulphur: In the steel which contains this element, the effects of sulphur are counteracted by manganese, which is always present. However, due to its consequences on ductility, surface quality and resistance to corrosion, it is done as much as possible, within the economic limits, to reduce it to the feasible minimum.
- Phosphorus: It is an element that has a strong effect on hardness and the resistance to corrosion. When it is likely that acid corrosion occurs, it must be kept within certain limits, and the content is increase when a more intense resistance of the steel in required and there is no danger of corrosion.
- Copper: Although the presence of copper in tinplate increment the resistance, it does not do it effectively within the usual ranks, since when these limits are exceeded, it grows the resistance to the atmospheric corrosion. However, it may reduce the resistance to internal corrosion. Therefore, a maximum of copper is specified in most types of tinplate.
- Manganese: It is used mainly to react to other elements, such as sulphur, to transform them into inoperative.
- Nitrogen: In Bessemer steels, it is a normal constituent element in percentages higher than 0.01%, and this was always considered as a disadvantage. Subsequently, it has been found that its intentional addition increments the steel strength without modifying the resistance to corrosion appreciably. But it could affect to aging. The development of oxygen processes (LD, among others) has allowed us to control the content of this elements accurately, so as to obtain an uniform and continuous quality.
- Nickel: This element in tinplate, within the allowed limits, does not affect widely the physical or mechanical features of the tinplate, but it does affect the chemical ones, specially the corrosion in certain types of packaging The same happens with chrome.
- Tin: It has a high solubility in iron or steel, and percentages higher than 10% are necessary for a second phase to be developed. In conventional levels, it has no effect on the metallography structure. Its presence increases the elastic limit and the limit of tensile rupture progressively, with a reduction in the module of elasticity. It is important to notice that small amounts of tin, in steels which contain copper, may cause serious difficulties in the heating work of steel.
In the steel industry, practical rules are sometimes used to calculate the effect of alloyed and residual elements on the mechanical properties of tinplate steel. One of the most common is the Strohmayer indez, which is calculated by multipluying the nitrogen content by 5, addind the phosphorus and multiplying the result by 1000.
2.4- TYPES OF BASE STEEL
Traditionally, there are three types of base steel used to obtain tinplate. However, with the current technologies, it is sometimes difficult to maintain the classic types. These are:
Type I: It is a cold rolled steel, which comes from open crucible furnaces (Siemmens Martin). It presents a low level in metalloids and residual elements, specially is limited in phosphorus, with the following composition:
- Carbon 0.05 – 0.13%
- Manganese 0.30 – 0.60%
- Sulphur 0.04 max.
- Phosphorus 0.015 max.
- Silicon 0.010 max.
- Copper 0.06 max.
- Nickel 0.04 max
- Chrome 0.06 max.
- Molybdenum 0.05 max.
- Arsenic 0.02 max.
- Nitrogen 0.02 max.
It is used when looking for a high resistance to very corrosive products.
Type MR: This type of steel is the most used. It is from the same source as the previous one, cold rolle and used in mildly corrosive products. Its composition includes the following elements:
- Carbon 0.05 – 0.15%
- Manganese 0.30 – 0.60%
- Sulphur 0.04 max.
- Phosphorus 0.020 máx.
- Silicon 0.010 max.
- Copper 0.20 max.
Type MC: It is a steel made in a Bessemer converter or Siemmens Martin furnace, used when the resistance and the content of the container is low in corrosivity. The analysis of its composition is:
- Carbon 0.05 – 0.15%
- Manganese 0.25 – 0.60%
- Sulphur 0.04 max.
- Phosphorus 0.03 – 0.15%
- Silicon 0.010 max.
- Copper 0.20 max.
There are also some special steels such as type D, calmed aluminum, used in specific cases of deep drawing.
In general, manufactures in Europe have been forced to try to eliminate the specialised tinplate productions so as to find economic and competitive procedures, and, in that way, they try to simplify this problem.
Taking into account the corrosion resistance, the steel surface is also transcendental. This one depends partly on the kind of atmosphere used in the annealing process. An atmosphere which contains nitrogen, hydrogen, carbon dioxide and water vapour produces a tinplate which is less resistant than a dry nitrogen one, which contains from 4% to 8% of hydrogen. In the same way, disolved tin ions, in general, have an inhibitory effect on the corrosion of tinplate by some products.
When this material is intended for the manufacture of tin, it is logical to think that it will be in prolonged contact with food products, and therefore it must accomplish certain requirements regarding spots. The American Society for Testing and Materials (ASTM) classifies in five groups, with miminimun tin contents, the following elements: grade AA = 99.98% tin, A = 99.80%, B = 99.7%, C-1 = 99.0% and C-2 = 99.0%. The minimum specified for the manufacturing of tinplate is grade A. Its analysis is:
- Tin (minimum) 99.80%
- Antimony 0.04%
- Arsenic 0.04%
- Bismuth 0.015%
- Copper 0.03%
- Iron 0.015%
- Lead 0.05%
- Nickel and Cobalt 0.015%
- Silver 0.01%
- Sulphur 0.003%
- Zinc 0.001%
- Cadmium 0.001%
- Aluminum 0.001%
3.2.- ELECTROLYTIC TINNING
The development of this tinning process was one of the most important steps in the industry of the sector. The electro-deposition of tin in a continuous narrow tape (strip), started in Germany in 1930, but it was during the 2nd World War when this technique was developed intensively, mainly in the USA, due to the shortage of tin.
There are a lot of technical and economic reasons that made electrolytic tinplate remove coke tinplate o hot tin from the market, which was the traditional means of obtaining it. One of the most important is the precise control of the amount of tin deposited and the uniformity in thickness. Another is the availability of differential electrolytic tinplate, which consists of applying different thicknesses of tin coating on each side of the sheet.
As it has been already mentioned, in the tempering operation of the steel base, the electrolytic tinplate can be produced in several finishes. However, three are the most usual. ‘Brilliant,’ which is the most common, ‘matte’ that consists of tin base steel with a rough surface and then not remelting the tin and, finally ‘stone’ which is the same as the previous finish but remelting the tin, which gives a bright finish, but not reflective.
The manufacturing methods are basically two: the acid and the alkaline or basic process. The latter has two variants which are widely used: horizontal halogen lines and alkaline or vertical lines. There is a fourth procedure, which used fluoroborates as an electrolyte, but which is very little used. The lines that the acid process use, are those that produce the highest percentage of electrolytic tinplate. They are frequently called Ferrostan, because this name was registered in their day by US Steel for its tinplate, and therefore there were later many licensees of its technology in the world. Electrolytes are the most delicate part in every process. For example, in acid lines, it is a solution of stannous salts in acid, but the good functioning as electrolyte depends on the various additives used, which have three main objectives: to avoid oxidation, to favour the formation of compact ad non-spongy deposits, and to improve wettability.
Some classic advantages of using alkaline electrolyte are having a simpler electrolyte, which is easy to handle and non-corrosive in relation to steel. This fact reduced the initial cost of the equipment. The preparatory section is also simpler.
The acid lines have the advantafe of using less current for tin-plating than the alkaline ones. Therefore, a smaller anode surface is necessary and the electrical efficiency is greater in the electro-deposition. However, the alkaline lines produce a tinplate with better characteristics of corrosion resistance.
In broad terms, the three types of lines are composed of the following sections:
- Cutting – coiling – packaging
The entrance or uncoiling section has the necessary equipment to develop the handling of the black sheet coils, and for the continuous feeding of the line, by cutting the beginning and end of each coil and the electric welding machine of the end of one with the principle of the next. It includes guide and feed rollers and power, voltage, etc.
Between the uncoiling and preparation section, there is a device to store black sheet band, intended to accumulate a certain amount of band, which feeds the line while the change and splicing of the roll is made in the previous uncoiling section. There are several types, predominating the well and ‘accordion’ ones.
The preparation section has two main objectives, cleaning (degreased) and pickling. The first is the elimination of all the residual elements on the surface of the strip, which come from lubricants, cooling agents, etc., whereas the second one tries to eliminate the iron oxide adhering to both sides of the strip.
The treatment is carried out in a succession of chemical or electro-chemical baths, with alternate washings with water jet and / or steam. The layout and type depends on the line; for example, the degreasing is normally done in a detergent or alkaline solution and it is eliminated in the line of this type or diminished because the degreasing is carried out automatically in the tinning section.
The tinning section is totally different in each process, being the one of the halogen process the one that occupies more surface, since the band moves flatly, normally in 3 levels, whereas in the other two systems it follows a sinusoidal or serpentine form.
Figure number 9: Tinning line, electrolytic cast and anodes
Before the tinning, the marks corresponding to the differential tin plate are made in the black sheet strip, when this type is the one which is manufactured. Therefore, these marks are on the steel base. Some manufacturers can mark the steel base so that, in case of later difficulties with tinplate, the manufacturer can be identified. These marks are not visible on the sheet as such, they must be found in the laboratory.
The finishing section includes different steps. The fundamental ones are: refusion, passivation and lubrication.
- Refusion: The purpose of this process is to provide the tinplate with a bright finish, since the electro-deposition of the tin produces a micro-rough surface with a matt appearance. However, this step is vital for the part of this material which is going to be in contact with corrosive elements. During its remelting process, an iron-tin alloy layer is formed, and due to the speed with which it is produced, the thickness is very low. For this reason, the matte tinplate, which has not been undergone in a reflow process, should not be used with those aims, that is to say, aims which require the presence of an iron-tin alloy layer.
- Passivation is a characteristic of electrolytic tinplate. In it, specially in low coatings, it is convenient to provide it with a protective layer that prevents oxidation, not only during the manufacture, but also in successive operations, such as varnishing. It is also necessary to provide a surface chemically more suitable for lithography and varnishing, and which presents certain protective hardness. Passivation aims to cover this aspect, and basically consists in forming a layer of chromic oxide. The methods used are several chemical or electro-chemical, designed each of them to obtain special characteristics. Normally they are known by the acronym USS and a three-digit number. The first one indicates the type of solution (1 = chromic acid, 2 = chromium phosphate, 3 = sodium dichromate, 4 = sodium carbonate), the second indicates the polarity of the tinplate in the solution (0 = not electrolytic, 1 = cathodic, 2 = cathodic / anodic) and the third figure refers approximately to the current level used.
There are three basic types of passivation, which are:
- Passivation 300: It is obtained by chemical process, by immersion in a sodium dichromate solution, to generate a layer of chromium oxide. It gives good results from the point of view of the varnish adherence. It offers a weak protection against sulfuration. This passivation is however unstable. Its effectiveness is reduced over time.
- Passivation 311: It is obtained through electrochemical process by electrolytic deposition in a bath of sodium dichromate of a chromium layer and chromium oxide. This type of passivation is the most used. It is recommendable since it offers efficient performances from the point of view of the adherence of varnishes, even though these ones are inferior to passivation 300. It is much more stable in time than 300. Therefore, a compromise between the stability over time and the quality of the adherence of lithographic systems can be considered.
- Passivation 312: It is achieved by the same system as 311. In fact, it is a passivation 311 which has been reinforced. It is used mainly for tinplate that must resist sulfur products, such as meat, soups, products for dogs and cats, etc. Its adhesion to inks and varnishes is inferior to 311.
The different passivation treatments, do not only affect the adherence of varnishes, welding, etc., but also produce various forms of attack or staining, when the tin is subjected to contact with corrosive products, or with sulfur compounds. Even from the aesthetic or presentation point of view, this detail is important, for example, with condensed milk.
- Finally, the finishing section carries out the lubrication operation. This lubricant has as its main purpose, no to protect the tinplate itself, but the passivation. So as not to counteract the properties of this, is very light, so the method of application most commonly used is by electro-deposition, even though it can also be done by sprinkling or immersion.
Three types of oil are normally used, cottonseed oil, dioctyl sebacate and dibutyl sebacate. The second is the most usual. The normal amount of lubricant is in the order of 0.005 g / m2. The lubricant is usually dried by means of steam coils and hot air.
The last part of the line depends on its formation of the final product and the productivity to be obtained, rather than the type of line (acid, among others). If the production is exclusively in coils, there will be no cut but there will be a strip storage device, to allow the change of coils. If the production is exclusively in cut sheets, the storage device is not necessary. In the cutting operation, the selection of surface defects is made, but this control is not very efficient due to the speed of the line, so the product obtained is classified as “unassorted”. If you want to separate ‘first’ from ‘second’, it is necessary to use the auxiliary services of a selection line. It is in this section where the leaves of caliber are separated out of specifications and perforated (“pin holes”). When coils are delivered, it is normal to not make this separation, which decreases the yield, leaving the customer to do this operation when cutting. ‘Unselected’ quality is defined as the normal product of an electrolytic tinning line.
A tin-plated electrolyte line carries a very complex auxiliary equipment, but, in fact, the line itself, something similar happens to the icebergs, the sunken part is not visible. The electrical and electronic equipment, controls, pumps, electrolyte tanks, purification equipment, water and steam systems, air, etc., are of great importance and are usually installed underground. The installed electric power is also very strong.
The speed of the strip during tinning is a function of the available electric current for electrolysis, the state of the electrolyte, of the size and thickness of the material, the deposition of tin that is desired and other factors.
As a consequence of the increase in the price of tin and being too risky the fact of seeing the supply sources in danger, during the last third of the last century a substitute product of tinplate, TFS or chrome plate, was developed. This material soon aroused an interest that has been increasing with the pass of time. Its advantage lies in the fact that it is a perfectly valid option for the manufacture of caps, funds, accessories and inlay containers, and slightly cheaper than tin.
The TFS is constituted by an identical support to the one of tinplate: steel. Although the protection is ensured, not by a light layer of tin applied by electrolytic deposition and by a passivation film, but by a mixed coating of chromium and chromium oxide.
In the steel industry it is common to manufacture the TFS on a mixed installation, which can produce tin plate or chrome plate with a series of not very complex changes, which are carried out in a moderate time. The application of chromium is also carried out by electrolytic system. Figure 10 shows the special part of a line devoted to the manufacture of chromed sheet.
It is not necessary to go into detail regarding the process of obtaining it, because as it has already indicated, the base steel is the same as for tinplate, and the coating line follows a sequence analogous to tinplate.
Finally, it should be mentioned the important participation of the large canmaker companies have had in the development in the steel manufacturing for tinplate, in the manufacture of tinplate itself, in other alternative materials such as TFS, and of course, in its usage and application.