Steel strips (width 400-1800mm and thickness 1-8mm) produced by a hot strip mill or a cold rolling mill, will have inherent defects due to unbalanced residual stresses in the strip. The residual stresses pronounce in the form of warp, bow, twist or bend when the strip is cut into small pieces using the laser, plasma or oxy-fuel cutting. Industrial users of hot rolled steel strip prefer flat and leveled sheet. Defects in the form of coil set, camber, edge wave, twist or buckle are also not acceptable to such users/industries. The bowing & warpage in the sheet and other defects should be restricted to a minimum extent.
Steel service centers use roll-leveler to flatten the sheet by reducing or balancing the residual stresses. Correct knowledge and mapping of residual stress across the length and width of the strip can be an important input for roll-leveler to achieve correct setting of the rolls to give improved flatness, which is desired by the end users.
In light of the above requirements, we are looking for a non-destructive method to accurately determine or measure the inherent residual stress in a steel strip surface, before it is processed through the leveler. The residual stress measured, thus, will enable the leveler to correctly flatten the sheet. The stress-measuring unit can be mounted on a cut-to-length machine just before the leveler (as shown in figure 1) and its output would be fed to the control system of the leveler so that leveler rolls are adjusted automatically to give flat and stress-free strip. The speed of the line is approximately about 60 meters per minute for HSM Line (Hot Strip Line) and 100 meters per minute for CR Line (Cold Rolled Line). The device should be able to measure the stress on the running line before the shearing and leveling operation. There is also a requirement to measure residual stresses after the leveling operation to see the stress after leveling.