The deviation from complete flatness is the direct result of the workpiece relaxation after hot or cold rolling, due to the internal stress pattern caused by the non-uniform transversal compressive action of the rolls and the uneven geometrical properties of the entry material. Due to the strict relationship between shape and flatness, these terms can be used in an interchangeable manner.
In the case of metal strips and sheets, the flatness reflects the differential fiber elongation across the width of the workpiece. This property must be subject to an accurate feedback-based control in order to guarantee the machinability of the metal sheets in the final transformation processes.
Some technological details about the feedback control of flatness are given in. Profile is made up of the measurements of crown and wedge. Crown is the thickness in the center as compared to the average thickness at the edges of the workpiece. Wedge is a measure of the thickness at one edge as opposed to the other edge. Both may be expressed as absolute measurements or as relative measurements. It is typically desirable to have some crown in the workpiece as this will cause the workpiece to tend to pull to the center of the mill, and thus will run with higher stability.
Maintaining a uniform gap between the rolls is difficult because the rolls deflect under the load required to deform the workpiece. The deflection causes the workpiece to be thinner on the edges and thicker in the middle. This can be overcome by using a crowned roller parabolic crown , however the crowned roller will only compensate for one set of conditions, specifically the material, temperature, and amount of deformation.
Other methods of compensating for roll deformation include continual varying crown CVC , pair cross rolling, and work roll bending. CVC was developed by SMS-Siemag AG and involves grinding a third order polynomial curve into the work rolls and then shifting the work rolls laterally, equally, and opposite to each other. The effect is that the rolls will have a gap between them that is parabolic in shape, and will vary with lateral shift, thus allowing for control of the crown of the rolls dynamically.
Pair cross rolling involves using either flat or parabolically crowned rolls, but shifting the ends at an angle so that the gap between the edges of the rolls will increase or decrease, thus allowing for dynamic crown control. Work roll bending involves using hydraulic cylinders at the ends of the rolls to counteract roll deflection. Another way to overcome deflection issues is by decreasing the load on the rolls, which can be done by applying a longitudinal force; this is essentially drawing.
Other method of decreasing roll deflection include increasing the elastic modulus of the roll material and adding back-up supports to the rolls. It is important to note that one could have a flatness defect even with the workpiece having the same thickness across the width. Also, one could have fairly high crown or wedge, but still produce material that is flat.
In order to produce flat material, the material must be reduced by the same percentage across the width. This is important because mass flow of the material must be preserved, and the more a material is reduced, the more it is elongated. If a material is elongated in the same manner across the width, then the flatness coming into the mill will be preserved at the exit of the mill.
The difference between the thickness of initial and rolled metal piece is called Draught. This is the case when the frictional force on the metal from inlet contact matches the negative force from the exit contact. There are six types of surface defects: . Many surface defects can be scarfed off the surface of semi-finished rolled products before further rolling.
Methods of scarfing have included hand-chipping with chisels 18th and 19th centuries ; powered chipping and grinding with air chisels and grinders; burning with an oxy-fuel torch, whose gas pressure blows away the metal or slag melted by the flame;  and laser scarfing. From Wikipedia, the free encyclopedia. Metal forming process.
For mills that use rollers to crush grain or stone, see roller mill. This section needs expansion. You can help by adding to it. December See also: Hot working. See also: Cold-formed steel. Main article: Roll bender.
Main article: Roll forming. Main article: Structural shape rolling. For the factory, see steel mill. Archived from the original on 5 October Retrieved 15 February Mott ed. Archived from the original on 29 July Retrieved 29 April Archived from the original on 7 April Retrieved 31 March Journal of Materials Engineering and Performance. Journal of Materials Processing Technology.
Archived from the original on 29 April Archived from the original on 10 August Archived 7 April at the Wayback Machine 2 September Journal of Process Control. Forming , fabrication , and finishing. Fabrication Piece work Sheet metal. Brazing Crimping Riveting Soldering Welding. Categories : Rolling mills Metal forming. Hidden categories: CS1 maint: archived copy as title Webarchive template wayback links Articles with short description Short description is different from Wikidata Use dmy dates from October Articles to be expanded from December All articles to be expanded Articles using small message boxes All articles with unsourced statements Articles with unsourced statements from April Articles with unsourced statements from March Wikipedia articles with BNF identifiers Wikipedia articles with LCCN identifiers.
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