PRETEX® – the surface structure for highest demands in car body manufacturing
PRETEX® stands for PREcision TEXturing, the innovative surface structure for coldrolled and galvanised steel sheets, newly developed by Salzgitter AG.
PRETEX® is a registered trade mark and is protected internationally.
PRETEX® is used especially by the automotive industry for interior components and exterior panels of car bodies placing highest demands on forming properties and paintability.
The surface morphology of the sheet is a major quality feature of the product and significantly influences forming behaviour, adhesion and optical characteristics of the car body painting. Salzgitter AG developed the PRETEX®-process to fully meet the rising standards of complicated transfer press processes as well as painting characteristics of large surface body parts now and in the future.
What is PRETEX®
The defined surface structure of the sheet, matching customers’ specifications, is created by rerolling on a skin pass rolling mill. The surface structure of the textured working roll is thus transferred to the surface of the cold-rolled or galvanised steel sheet.
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Schematische Darstellung des Vorganges des Aufbringens
The special feature of the PREcision-TEXturing-process
PRETEX® - Surface structure
Conventional roll texturing processes deform the roll surface. This is either a plastic deformation caused by propelling a fine grain metallic blasting shot against the surface or is melted in part. Depending on the process, roughness structures with random distribution (stochastic) or with regular distribution (deterministic) are created.
Unlike conventional texturing processes the PRETEX® process hard chrome plates the working roll surface electrolytically according to the patented TOPOCROMprocess.
The reactor in which the entire working roll is plated is equipped with an anode cage and filled with a chrome electrolyte for the plating process. The chrome ions of the electrolyte are reduced during the plating process and deposited metallically on the roll surface.
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Precision and reproducibility covering the entire coil-length and coil-width
Roll texturing in the
The PRETEX®-process provides an absolutely uniform stochastic distribution of the hard chrome hemispheres, deposited on the roll surface. Their size and number per unit of area are controllable and reproducible by adjusting the computercontrolled coating parameters according to customer specifications. Thus tailormade surfaces for all applications can be produced.
Excellent forming properties
Friction between steel sheet
The surface morphology holds the lubricant and supports a hydrostatic and hydrodynamic lubrication towards reducing friction. A higher mean roughness value and lubrication pockets isolated from each other, serving as lubrication reservoirs, improve the friction and lubrication situation considerably. An uniform distribution of the lubrication pockets and an uniform lubrication transport in the contact area between the tool and the sheet is just as decisive.
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Tests of various surface structures have proven that PRETEX® texturised steel sheets reduce friction and transfer highest blank holder forces without appearance of cold fusing between tool and sheet surface. The deep drawing working area suitable for producing finished parts without wrinkles and fractures is thus extended.
The appearance of the car body painting forms the subjective impression of the customer in respect of the product quality
of the vehicle. This is why the surface morphology of the steel sheet is of major importance, not only in technical but also in economical respect. In view of the optical painting characteristics of the car series painting highest standards are set for the sheet surface.
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The permanent improvement of paintability together with simultaneous reduction of the paint-system coating thicknesses, eg for exterior car body parts, require high peak counts and small long wave and short wave structure features. Long wave structure features may possibly create the so-called ÃÂÃÂorange peel’ in the coating of painted autobody parts. This is a processlinked development during texturing of the working rolls by conventional processes, such as SBT, of chain sequences of roughness peaks on a roll surface. This is caused by the traversing process of propelling metallic blasting shot against the surface of the working rolls in longitudinal roll direction.
PRETEX®-surfaces have significantly fewer long wave structure feature compared to conventional sheets with SBT-texturing, as the chrome plating process takes place simultaneously over the entire roll surface. Depending on the natural spread of the paint, roughness structures may show a print of the base profile on the surface of the paint film. E. g. Solvent evaporation may result in a shrinking of the paint film which may cause a print of the base profile on the surface of the paint film. This base profile print may have a major influence on the structure of the paint coating, especially when the paints have a good natural spread.
The assessment of the paint is mostly carried out visually or by readings with optical processes. Optical readings for assessments of structure parts of the paint finishes vary between short wave and long wave parts. Compared with conventional structures, this is where PRETEX®-painted surfaces show significantly fewer long wave structure parts in a more favourable ratio to the short wave ones, thus creating the precondition for optimal paint finish.
Advantages of the PRETEX® structure in brief
PRETEX® offers significant proven advantages in comparison to conventional roughness structures:
- Uniform, defined roughness values over the entire length and width of the sheet as a result of the homogenous roughness
- Improved forming characteristics based on the excellent tribological properties during deep drawing resulting from the
finely distributed hydrostatic lubrication pockets which are isolated from one another.
- Excellent paintability even for vertical application resulting from extremely high peak counts and the stochastic distribution of the roughness structure.
- Cost savings by eliminating die marks on forming tools and reduction of the oiling or additional oiling even on hardto-
- Highly ecological through the use of thinner paint systems including waterbased paints as well as the resulting cost advantages for the customer.
- Environmentally sound coating process for rolls resulting from the closed reactor process developed specifically for
this application. Elimination of the rinsing water other-wise required according to the principle of 'avoidance instead of disposal'.
- Exact reproducibility of the roughness values through use of state-of-the-art electronic process control.
- Last, but not least, consistent integration of all processes into the Total Quality Management System certified according to the highest demands of the automotive industry and the principle of continuous improvement.
Production equipment and facilities
Cold strip mill – PRETEX®-plant
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