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Hot rolling mills get the relationship between crown or profile and shape

Jime

In warmwalzwerk

On a hot mill, it is possible to change the crown or thickness profile without changing shape or flatness, and vice versa.

Hot rolling mills try to get the relationship between crown or profile and shape, but achieving perfect shape and perfect crown control is almost impossible. With the introduction of new automatic thickness control (AGC) technology, conditions at the plant have improved, but perfection remains an elusive goal.

The abrasion

In a cold rolling mill, every time we change the thickness profile, we also change the flatness. The roll clearance of the cold mill shall begin to be set to the same profile as the crown of the incoming hot mill product. Then, as the cold mill gap is adjusted to reduce the convexity, the coil will also come out flat -- if the hot mill gets the relationship between convexity and flatness right to start.

If the roll of a cold or hot mill is too large, the mill will roll out to the center, creating a center catch in the process.

Center buckles, another type of edge-to-edge length differential defect, are created when the center is longer than the edge.

If the crown of cold or hot rolling coil is too small, the mill will roll the edge and produce edge wave. This is quite common in the main roll of the mill.

Mill work and support rolls are bent and compressed under vertical loads to reduce the thickness of the plate or roll. In theory, if the surface of the work roll is absolutely parallel, the top and bottom surfaces of the rolled product will be parallel -- no convexity. But the fact is, under rolling loads, everything is deflecting, so it doesn't work that way.

The mill deliberately makes the center convexity of the work roll slightly larger to allow deformation and compression under load. The amount of crown roll added is a compromise, a best guess. In addition, the mill can bend these huge rolls to control the clearance profile, or hydraulically inflate them like a giant steel balloon.

Surface is an edge-to-edge length differential defect.

You have probably seen pictures of all the controls and computer monitors on the mill control podium. Each time the operator pulls a lever, he changes the movement of the mill frame, thus changing the degree of deformation.

The hot rolling mill strategy is to correctly deal with the relationship between crown and shape, so that they can get the correct results in cold rolling. If this sounds difficult, it is!

The roll tension through the mill also affects the roll thinning of the mill. It should not surprise us then if the rolling mill rolls the head and tail thicker than the middle because they do not have sufficient tension.

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