Finishes:
Annealed and pickled finish - 2D
By means of further rolling of the steel in a cold state, both its surface smoothness and thickness accuracy are improved. Since stainless steel is cold hardened, that is, it is hardened by the cold-working process, it has to be annealed after the rolling so as to regain its softness and working characteristics. This is most often carried out in an ordinary air atmosphere, which results in a new coating of oxide that has to be removed by pickling in an acid bath. Steel sheet processed in this way has a semi-matt silver-grey surface, which is known as finish 2D (D=dull), and has an Ra value in the order of 0.2-1.0 µm. This value increases with greater sheet thicknesses. It is used mostly for industrial equipment and for the manufacture of pipes and tubes for the process industry.
Skinpass rolled finish - 2B
Cold-rolled and annealed sheet as described above may be given a smoother surface by means of a further light rolling between highly polished rollers. The result is a semi-bright grey surface which is termed skinpass-rolled or finish 2B (B=bright) and has an Ra value of between 0.1 and 0.5 µm. Skinpass-rolled steel sheet is often used for process equipment within the food industry when a surface which is easy to keep clean is required.
Bright annealed finish - BA
If, after cold rolling, the sheet coil is annealed in a protective gas that prevents oxidisation of the surface (for example, hydrogen and nitrogen from cracked ammonia), pickling is not needed since a bright finish is directly acquired. This surface quality is termed bright annealed finish or BA. Since the surface has not come into contact with corrosive acids, it attains a brighter finish than either of the above examples, and an Ra value of 0.03-0.1 µm. It is used for the manufacture of household equipment and appliances (dish washers, washing machines, etc.).
Polished
By polishing the cold rolled sheet steel with a grinding bell of a certain emery grain size (measured by mesh grit), a surface is obtained which has a silver-like metallic shine, which improves the decorative effect. The finer the grain of the grinding bell, the brighter and more even the finish. However, polishing often results in a surface finish that is poorer than that of the original material. Polishing with a coarse grinding bell of 80-100 mesh gives, for example, an Ra value of 1.0-2.0 µm. The roughness of the surface in this case is still not up to that of finish 2B. Using successively finer grit bells, an almost mirror-like surface can be produced. This mirror-finish has an Ra value of 0.02-0.05 µm.
Surface roughness and structure can also be affected by different polishing techniques, even if no differences are detectable with the naked eye. Wet polishing, for example, produces a much finer surface than polishing with a dry bell. The Ra value could be less than 0.4 µm. The mesh size of the grinding bell and the resultant surface roughness of the sheet (Ra), do not, therefore, have any fixed relation. Wet polishing also produces a cleaner surface than dry polishing, which can be compared to the ploughing of furrows in a field. This means that dirt, bacteria and corrosive substances do not attach as easily to wet polished surfaces.
The ground or polished surface has more or less clearly visible scoring, which also makes scratches from daily wear and tear less visible. A polished surface is also easy to re-create, for example after repairs or welding work, compared with a 2B or 20 surface. Polished sheet is used for purposes where quality of appearance and hygiene are very important, such as in commercial kitchens (work surfaces, cooking utensils, dishwashers, freezers) , in house- holds (cooking vessels), and in public places (sanitary facilities, hospital equipment, kick-plates).
Brushed
The use of brushing (with Scotch Brite, for example), usually on a 2B surface, provides a silky-matt finish without grinding scores. A brushed surface also has the advantage that it can easily be re-created. Brushed stainless steel sheet is used in situations where appearance is an important factor, such as in the building industry (sheet metal cladding and lift interiors), the vehicle industry (car hub caps, wall panels for express trains) and the home (sink units). The Ra value for brushed sheet can be around 0.2-0.4 µm.
Patterned
Patterned sheet is produced by means of extra rolling of cold-rolled brushed sheet coils between special imprinted rolls. In this way, a waffle-type pattern can be obtained on both sides of the sheet. At the same time, the rigidity and strength of the material is increased by almost 50%. This means that it is possible to use a thinner sheet, and thus obtain a lower weight.