Roughening process
The various current roughening or skin pass or re-rolling processes apply above all to the technical treatment of steel strip , which is to be given certain desired texture characteristics . These texture features help the steel strip to achieve more favorable deep-drawing properties.
General
Structure in the as-rolled and annealed condition
After the structure has been deformed by up to 75% in the rolling process , it is strongly stretched and consolidated and has to be annealed for further processing. The annealing is carried out above the recrystallization temperature . The recrystallization temperature is the temperature at which the solidification introduced into the material by the cold rolling process breaks down again.
Why dressing?
Passage has several functions:
- Elimination of the strip waviness, which resulted from the release of the internal stresses during the annealing process. The roll roughness increases the friction in the gap, so that the strip is only slightly elongated (skin-pass degree).
- In addition, after annealing there is usually a more or less pronounced yield point in the material, which can be very damaging for further manufacturing steps. If thin sheets with a pronounced yield point are deep-drawn, local flow of the material can occur as soon as the forming stress reaches the yield point. Lines suddenly appear that run transversely or at an angle of 45 ° to 60 ° to the direction of pull , so-called " flow figures " or Lüdersche lines . This pronounced yield point can be eliminated by rolling with a deformation between 0.5 and 2%.
- Setting the desired roughness / surface structure by using appropriately structured rollers. So can u. a. the formation of lubrication pockets for the forming oil improves the deep drawing or stretch drawing of the strip.
Optimal skin pass level
When skin pass or re-rolling, the product is cold-rolled with a degree of rolling of about 0.4 to 2.5%. The mechanical properties of the cold strip must not be viewed separately from the history of the strip in the manufacturing process. The optimal skin pass degree at which the yield point reaches its lowest value depends on the steel analysis as well as the hot rolling, cold rolling, annealing and re-rolling conditions. For deep-drawing qualities that do not have a pronounced yield point elongation, skin pass grades of 0.5 to 0.8% are generally sufficient. As already described above, a good flatness and a reliable achievement of the roughness requirements and number of points are decisive for these materials .
In the stress-strain diagram, the optimal skin-pass grade describes the point at which the pronounced yield point disappears and a minimum is present at the yield point. If the degree of skin pass is increased further, the yield strength also increases again and the material solidifies.
For the skin pass process in the cold rolling mill, the specifications for the skin pass degree come from TQ. In the case of new steel grades, they can be determined in the laboratory and are sent to the system with the production order via the work schedule, depending on the material.
Determination of the degree of skin pass in the framework
A simple degree of skin pass is determined by comparing the circumferential speeds of the deflection rollers in the inlet and outlet. The pulleys are provided with position sensors. The skin pass degree corresponds to a distance difference between the deflection roller in the inlet and outlet.
Inlet / outlet :
,
Specification in%.
Texture overview
SBT
The SBT process (Shot Blasting Texturing) is the most widely used process. With the help of a centrifugal wheel, a defined blasting grain is thrown onto the roller surface and creates a crater there. The shape and form of the crater can be varied via the grain size and the impact speed.
Advantage: The texture is very easy to produce and can hide small defects in the surface (important when producing outdoor parts). The texture can be used for rework without any disadvantages.
Disadvantage: The rollers manufactured using this process often do not have a high level of durability. Due to the process, the number of peaks is lower than the number of peaks in EBT or EDT textures.
EDT
In the EDT (Electro Discharge Texturing) process , the roller surface is roughened by spark erosion . This is done by moving the work roll past an electrode in a tank that contains a dielectric (= insulating medium, here oil) . Small craters are made in the roll surface by sparking.
Advantage: The surface has similar properties to the SBT surface, but with a higher number of tips.
Disadvantage: Due to the higher proportion of peaks, this texture tends to pick up dirt or foreign particles (tinsel). However, this can be alleviated by lightly brushing the texture with the roller brush machine in the grinding shop. By brushing over the roughness peaks are ground off a little.
EBT
With the EBT process (electron beam texturing) , a defined crater is shot into the roll surface with the help of an electron beam in a vacuum. The electron beam can apply a defined texture in a targeted manner.
Advantage: The roughness is very homogeneous and thus represents a very good surface for later painting processes, which enables a high paint gloss.
Disadvantage: Overlaying effects (moire) can very easily arise if the skin is passed twice. Reworking of EBT-treated material is only possible with a specially developed anti-moire texture.
See also
literature
- Lutz Löffler: Investigations into the roughening of steel strip. Verlag Stahleisen, Düsseldorf 1988, ISBN 3-514-00415-3 .