Method of tension leveling nonhomogeneous metal sheet

Abstract

A method of tension leveling a continuous strip of nonhomogeneous metal sheet in which the yield strength of the strip edges is less than the yield strength of the center. The method includes the steps of unwinding the strip from a coil supported on an uncoiler, passing the strip through tension-generating bridle rolls and, while under tension is passed, in a serpentine path between upper and lower sets of nested work rolls and through tension relaxing bridle rolls, and rewinding the strip into a coil on a recoiler. The improvement consists of the additional step of, prior to passing the strip between the work rolls, heating the strip edges to a temperature above that of the center of the sheet to create thermal expansion in the edges sufficient to produce a thermal strain such that, when added to an elastic strain at yield in the edges, total strain is equal to an elastic strain at yield in the center of the sheet. The result is that on relaxation and cooling, the total elastic and thermal contraction of the heated area is the same as the elastic contraction of the balance of the sheet. In a preferred embodiment of the invention, the heating step includes the step of continuously passing the edges of the strip adjacent to a heating element to produce the required temperature elevation.