Description of the manufacturing process and characteristics of this type of tinplate. Thin tinplate with high temper that reduces costs.
Tinplate – or reduced TFS – double, supplied in coils or packets of sheets, is a product obtained from a steel with a low carbon content, subjected after annealing to a second reduction of thickness in cold and then coated by electrolysis in its two tin faces – or chromium-.
This second cold reduction is carried out on a SKIN-PASS train of two bodies or boxes, specially studied for this work. The reduction percentages range between 25 and 45% depending on the case. In figure 1, the second reduction of the base steel starting from a thickness of 0.23 is shown in order to obtain 0.17.
Figure nº 1: Outline of a SKIN-PASS train
This elongation depends on the product to be obtained (DR 520 to DR 620) and on the characteristics of the base steel (analysis – thickness – casting – coil). The percentage of this elongation at the 2nd reduction, after annealing, is a function of the characteristics of the DR type requested. For example, extensions tend to be:
– Approx. 30% to obtain a type DR 520
– Approx. 35 to 40% to obtain a type DR 550
– Approx. 40 to 45% to obtain a DR 580 type
The surface of the working cylinders of the second box of the SKIN-PASS train are suitably treated to obtain the desired surface finish of the material. The most usual are shiny finish and stone. The latter is the most used.
During the process it is necessary to carry out a cooling and subsequent air drying of the belt surface. Figure 2 details this operation.
Figure 2: Lubrication of the band during the 2nd reduction
The rest of the manufacturing process of the reduced double tin plate is common to that of the reduced simple type. A brief description of it is given in the article of this Web:
The reduced double material is delivered in sheets or coils in thicknesses that normally vary between 0.20 and 0.14 mm both included. As to coil widths or sheet dimensions, they move in values similar to those used for reduced simple tin, depending ultimately on the characteristics of the manufacturing line.
The reduced double tinplate has a high rigidity, characterized by a hardness measured on the Rockwell 30T scale between 70 to 80 degrees. This measurement is made on a Rockwell 15T equipment for thicknesses less than 0.20 mm and later converted into Rockwell 30T scale values. Its breaking load is of the order of 60 Kgr / mm2. The following table reflects the characteristics of the three main types of tinplate according to European and American standards.
The most common of these is the DR 550. The harder varieties can be delivered in thicknesses greater than 0.20 mm and are obtained by lamination of renitrurado steel. Type DR 660 can have breaking loads greater than 70 Kgr / mm2. All base steels whose elastic limit (longitudinal) is> 500 n / mm2 fall into the DR category.
The mechanical properties of the reduced double tinplate require special care in the direction of the grain. The container bodies should be wound in the direction of rolling, ie the base of the container a to be parallel to the direction of rolling. Failure to do so can result in problems of lashes cracked in the bodies.
In the cutting – or shearing – of the bodies, it is very important to watch the sharpening of the blades, since the cuts that are not made in a “clean” way can also cause cracks in the eyelashes. When it is used for packaging bodies, it is necessary to use “defibrating boxes” or “flexor”, a set of pairs of rollers that partially break the rolling fibers, to improve the conformation of the tinplate.
As a consequence of its greater hardness, it allows reducing the thickness used, producing lighter and cheaper packages than those made with reduced simple tinplate.
Typical uses of this tinplate are: bodies of cylindrical containers type 3 pieces for different kinds of preserves, closing capsules, and even caps and funds for containers. In the latter case the thickness must be greater.