Production of steel sheets
The steel used to manufacture tinplate is low gauge and low carbon. There are a number of important steps in their production that affect the properties of tinplate.
Hot rolling: Cast steel plates are used continuously as they produce a cleaner steel. These slabs are scraped or cleaned to remove surface defects and then heated to 1200 to 13,000 degrees Celsius and rolled into thin gauge sheets.
Pickling: These plates are cooled and cleaned of any oxides that have formed during the pickling process. Earlier acids were used; however, innovations have made this step acid and oil free and more environmentally friendly.
Cold reduction: the hot steel plates are cooled with water and lubricants and re-rolled.
Cleaning: The coolant is removed using alkaline phosphates, silicates and sodium hydroxide and finally cleaned with hot air.
Annealing: This step softens the steel and achieves the desired grain structure. Batch annealing produces a deep-drawn, hard steel that can withstand vacuum and pressure, making it the best choice for making cans and bottles. Continuous annealing is a shorter process and results in smaller grains of steel.
Tempered rolling: annealed mild steel is lightly rolled to produce the desired final thickness of plate.
Double Reduction: Sometimes, instead of rolling by quenching, the steel is rolled using a second reduction. This produces thinner and stronger sheets. The sheets produced can be as thin as 0.14 to 0.24 mm, which makes them more attractive as a packaging material as it saves on transport costs and reduces carbon emissions.
Tinplate – production
Electroplating
Tin has been electroplated on steel sheets since the 1940s. Prior to this, tin was produced by the hot-dip method. Tinplate production is the main use of tin, using 30% of all tin produced. By 2018, 30% of all tin production comes from recycled sources.
The advantage of the electrolysis method is that thin tin coatings can be achieved and the thickness of the coating can be controlled. This is important because the coating thickness can vary on both sides of the sheet, resulting in a “differential coated tinplate”. Modern tinplate has a tin coating that is only 0.1 to 1.5 microns thick.
Electroplating: The steel sheet is electroplated by passing it through a plating bath filled with an electrolyte into which a 99.9% pure tin anode is dipped. Finally, any excess tin in the aluminium foil is removed and recovered.
Flow melting: In some cases, the tin coating is heated to melt it and form a layer of iron-tin alloy, thus improving the corrosion resistance of tin.
Passivation: The tin coating may be oxidised to form a thin film. To prevent the growth of this layer, a layer of chromium is added and then a layer of food grade oil is added to stabilise the coating during the passivation process.
Tinplate range
Tinplate is available in thicknesses ranging from 0.13mm to 0.50mm. Steel sheets make up the majority of the thickness as the tin coating is measured in microns (1 micron = 0.001 mm). There are thousands of possible combinations of steel and tin, so the customer must inform the manufacturer of the tinplate application in order to obtain the correct quality.
A number of other factors, such as the tempering, finish and grade of the tinplate also determine its suitability for different applications.
Tinplate temperature: Tinplate can be available in different temperatures depending on the quality of the steel and the different production steps such as hot rolling, annealing and tempering. The quality of the steel and tin is in turn regulated by various national and international standards, such as ASTM (American Society for Testing Materials) or ISO (International Standards Organisation).
Surface finishes: Tinplate is available in different finishes such as bright, clear stone, stone, matt and silver.
Grades: Tinplate is available in two grades. Standard grade tinplate has the best properties and can be painted and printed. In contrast, Grade II tinplate is a waste product of Standard Grade tinplate.
Environmental considerations
Tinplate is one of the most recycled materials in Europe. Previously, production methods were mainly based on chemicals.
Taking into account consumer preferences and the concern to protect the environment