In the production of plate rolling, due to the existence of a remaining straight unbent end, a pre-bending process is generally required before rolling, and in China workers often call it a pressure head.
The generation of the remaining unbent end is shown in Figure 1.
The theoretical remaining straight side is t1=t/2 when bending symmetrically, and t1<t/2 when bending asymmetrically. In order to prevent the metal sheet/plate from falling out of the rolls of the plate rolling machine, the actual remaining straight edge is often larger than the theoretical value.
Many factories use ordinary symmetrical three-roller plate rolling machines when rolling, and the remaining straight sides are relatively large. For example, the remaining straight sides of the 20×2000 plate rolling machine are more than 180mm when rolling, and the 30×3000 steel plate rolling machine can be above 300mm. Such a large remaining straight edge is difficult to eliminate during rounding, and appropriate technical measures must be taken to pre-bend the edge of the metal plate.
Commonly used pre-bending methods
- Press free bending (see Figure 2a);
- Use the upper roller of the plate rolling machine to pre-bend the arc backing plate while crimping it (see Figure 2h);
- The wedge iron is pre-bent with the upper roller of the plate rolling machine while crimping (see Figure 2c).
The first pre-bending method is suitable for plates of various thicknesses. As the thickness and width of the plate increase, the tonnage and width of the press will also increase accordingly, but the roundness after pressing is not good. If forming molding is used instead, the roundness can be improved, but the tonnage of the press needs to be large.
The latter two are suitable for occasions where δ0≥2δ, δ≤24mm, and the bending force does not exceed 60% of the capacity of the plate rolling machine, but the radius of the arc is difficult to control.
In addition, the four-roll plate rolling machine itself has a pre-bending function, but its pre-bending thickness can only reach about 75% of the maximum thickness of the plate.
New pre-bending process
The following introduces a process method of using a symmetrical three-roll steel plate rolling machine without pre-bending the circle.
The rolling process is shown in Figure 3. The main process is steel plate blank preparation → flat ring rolling → welding longitudinal seam → rounding.
First, the steel plate blank is rolled into a flat circle on the rolling machine, and the rolling method is as follows:
- Draw a line in the middle of the blank in the length direction as the control line.
- Roll the two halves of the billet into two semicircles respectively. It is not difficult to see that the part of the straight line segment of the flat circle is exactly the part of the remaining straight side.
When rolling oblate, the most important thing is to control the misalignment. For this reason, it is necessary to accurately center and maintain the parallelism between the busbar and the roll. When it is flat and round, it is also prone to over-rolling. At this time, it has to be pulled apart and spot welded.
The shape after rolling should generally make the butt joint slightly concave, not protruding.
Correction of the rolling circle process
Correction of the rolling circle is performed after rolling, and correction rolling is done after the butt joint is welded (longitudinal seam double-sided welding) and then rounded on the plate rolling machine. Correction rolling is more difficult and time-consuming than rolling. The specific method is as follows:
- First, use the upper roller to repeatedly roll the oblate straight section, and the two straight sections alternately.
- Use the upper roller to repeatedly roll the entire circumference, and the amount of pressing and lifting of the upper roller can be adjusted first.
Before correction rolling, the inner and outer welds of the butt joint should not exceed 1 mm in height, otherwise jumping will occur during correction rolling, which will cause great damage to the plate rolling machine.
The circularity deviation of the cylinder rolled out by this method is very small, and the ellipticity can be controlled within 3mm. However, due to the need to go through two rolls, rounding is time-consuming and productivity is relatively low. If there is no pre-bending equipment and tools, this method is a good method for single-piece or batch production of round and simple workpieces, and it greatly saves materials. Finally, I would like to remind the application of this method: the diameter of the semicircle after rolling the plate into an oblate circle should be greater than or equal to the minimum diameter of the rolling cylinder that the plate rolling machine can roll. Read More: Plate rolling machine