Test Results
The results were organized into part length flatness, thickness flatness, part width flatness, and overall flatness by material thickness and grade. Length flatness was defined as deviation from flat measured in inches per foot of rolling direction. Similarly, width flatness was the deviation measured across the rolling direction, again expressed in inches per foot. Overall grade flatness was an amalgam of these numbers.
Figure 4
Length flatness showed the most marked improvement using TP-CTL (see Figure 4). With TP-CTL, flatness was improved 1.5 times in C1010 and 5.5 times in grade 80 material. In other words, as the yield strength of the material increased, the flatter the material became when TP-CTL was used. This illustrated that higher-strength materials, which had a much higher surface yield strength after passing through the temper mill, were much more inclined to return to their original as-rolled conditions.
This length flatness measurement demonstrated the process well because the leveler did not have enough force to exceed the surface yield strength of the material yet coil set was removed effectively.
Figure 5
The second significant improvement was better thickness flatness with TP-CTL. It was found that thinner materials were improved more by TP-CTL than thicker materials (see Figure 5). At thicknesses of 1/4 in. and less, TP-CTL decreased the flatness deviation more than 80 percent, but at 1¼2 in. only 75 percent. This result occurred because thicker material requires more force both to reduce and level it.
Figure 6
Figure 6 shows the results of measuring width flatness. It should be noted that it is difficult to squeeze the edges of the material as much as the center, because when the steel is originally rolled, the center of the material is thicker. This is because the rolling mill uses that crown to aid tracking in the mill. Under the huge forces applied, the temper mill rolls deflected slightly because of the crown. This contributed to the marginally out-of-flat numbers observed. Parts cut closer to the sheet edges exhibited more out-of-flat conditions than those cut from the middle two-thirds of the material.
Figure 7 combines the results of the three measurements. The most improvement was in the grade 80 material, in which yield strength was highest.
Figure 7
The application of heat into the cut parts was another area investigated. Prevailing industry thought has been the more heat that is applied to the material, the more it is subject to movement. This was not the case in the study. In all of the materials tested, the difference in flatness between laser-cut and plasma-cut material was negligible, and no reliable trends were discovered.
