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- July 6, 2022
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- sheet and plate steel flatness,
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Steel has the tendency to cling to its original shape and molecular composition. Why? Because it was given a unique configuration when it was rolled, cooled, and coiled at the steel mill.
In its natural state, steel is not at all flat. In fact, a hot-rolled sheet coil generally reveals some or all symptoms of poor flatness (see Figure 1).
For example, 72-inch-wide material with a 100,000-PSI yield strength can have a crossbow of 2 to 12 in., wavy edges, and excessive crown. These shape imperfections must be removed to produce a high-quality steel sheet that is not only flat, but will remain flat during such subsequent manufacturing operations as laser, plasma, and punch cutting.
Temper passing, in conjunction with a roller leveler, cut-to-length (TP-CTL) line, can be an alternative for improving these conditions.
What Is Temper Passing?
Temper passing is compressing flat steel under very high pressure between two work rolls to flatten and elongate it (see
). A cold reducing process, temper passing (also called temper rolling or temper pass) strives to decrease the material thickness between 1.5 and 2 percent (alternatively, elongating the coil 1.5 to 2 percent) without appreciable spreading, or widthwise elongation. At this percent reduction (or lengthwise elongation), temper passing increases the material surface’s yield strength and improves its finish.
Both integrated steel mills and coil processors have used temper-pass processing extensively. Initially, it was used in coil-to-coil processing as a way to improve surface finish, equalize surface stresses, and help reshape the incoming raw material.
What Is Leveling?
When material is leveled, it is compressed or stretched about its neutral centerline (see
). As the fibers of the steel pass between two alternating, opposing rolls, they are bent in opposite directions in direct proportion to their distance from the neutral centerline.
The center of the material is in an elastic condition and wants to remain in its original condition, while the surface of the material has been stressed beyond its initial yield point. Because the reverse-bending actions have stressed the surface, the material is unable to return to its initial near-flat shape, and opposing forces now are present in the material.
This results in material that is flat but contains large amounts of internal stress. These stresses are released when the sheet is processed into parts using mechanical or thermal processes.
Combining Temper Pass and Leveling in One Line
When temper passing and leveling are combined, the increased surface yield strength created by temper passing allows the subsequent leveling to increase the extension of the material through the neutral axis without exceeding the surface yield point. This allows the surface of the sheet or plate to revert to its original condition while minimizing the internal trapped stresses.