Elimination of yield point in steel sheets by rapid temperature change

As shown by Koistinen,^1,2 rapid induction heating and quenching can be used to eliminate the yield point and stretcher strains that appear during forming operations on low-carbon steel sheets. Here it is pointed out that such rapid temperature change can produce inhomogeneous plastic deformation, and is then the thermal equivalent of temper rolling or flex-leveling. Theoretical heating (cooling) requirements for eliminating discontinuous yielding on this basis are derived from existing thermoelastic solutions, and experimental results on aged temper-rolled sheet specimens are presented in support of the calculations. The necessary rate of temperature change is more easily reached by quenching than by rapid heating, but rapid heating is essential to control the carbon and nitrogen taken into solution and for high product output. The required rate of temperature change is easily obtainable for sheets of 14 gage (0.19 cm) and heavier, but as sheets become thinner the minimum rate increases rapidly. Because of an increased rate of aging after rapid heating and cooling, the process is of little interest to steel producers as a substitute for temper rolling, but may be attractive to industries which consume large quantities of rimmed steel sheet.