A flat machine and supporting precision leveler can produce completely flat and unchanged sheet metal
In tension leveling, the coil is under significant tension between the traction and the traction bridle placed before and after the roller leveling device (see Figure 5). In tension alignment, all parts of the metal are pulled over the yield point, from top to bottom and from side to side. All previous trap stress histories should be deleted. The material should be completely flat and relatively free of internal stress.
Tension leveling is usually limited by the thickness of the metal. Due to the large amount of equipment and horsepower involved, full tension leveling is usually not practical for thick hot-rolled coils. However, more and more tension leveling lines are being found processing cold-rolled steel and aluminum products at Toll processing plants and service centers.
A flat machine and supporting precision leveler can produce completely flat and unchanged sheet metal.
Trapped stress and stability
The flater and accompanying precision leveler (see Figure 6) can produce a completely flat sheet and keep it that way. This is an important problem for handling heavy materials.
Manufacturers who use laser burners or turret punches want to make sure their parts will stay flat. If cutting or heating releases the trapped stress, the material will not remain flat. In the cutting line, leveling machine mainly for the surface effect, but with the use of roller leveling, will affect the stability and surface quality of the workpiece.
A flattener can be used to flatten materials, although not as effectively as a straightener. The flattener mainly hardens the surface, raising its yield point rather than lengthwise, resulting in a better, smoother surface of the hot-rolled plate. It also increases the amount of elongation required to exceed the elastic limit of the surface.
When the plate is bent on the leveler, the surface is stretched or compressed in proportion to the distance from the neutral center line. When the plate is not bent, the stretching and compression are in reverse, also proportional to the distance from the center line. The problem with this reversal process is that when the metal surface is forced past the yield point, it does not want to go back to zero, meaning that trapped internal stress has been added. The random trapping stress may be exchanged for a more consistent trapping stress in the received mill material, but it is not eliminated.
On the other hand, the flattener improves the surface hardness of the metal before the roll flattening, without longitudinal extension. Now, when the plate coil is bent on the leveling roller, the need to extend beyond the yield point and permanently change the length of the surface relative to the core is more demanding than below the surface. Thus, there is no problem with the surface returning to its original half-plane state, and the residual trapping stress is minimal. The result is flat and stable.
The trapping stress from the mill is variable and random. They are inconsistent from one edge of the coil to the other, or from one end of the coil to the other.
Unsupported flatteners are mainly used to control coil groups or longbows, an external surface effect. The trapped stress is in the middle, so the metal is unstable. Using a precision level, the yield point exceeds 80% of the distance from the top and bottom surfaces to the center. Only the middle 20% of the thickness does not exceed the yield point, so it is more stable.