Effect of Annealing Cycle on Drawability and Microstructure of Cold Rolled Steel Sheet

Abstract

Abstract The effect of the annealing cycle on microstructure, mechanical properties and draw ability of aluminium killed steel sheets at three different heating rates and holding times were investigated. Tensile test, hardness test, optical microscope and transmission electron microscope (TEM) techniques were utilized. The results of samples for different times in laboratory experiments were compared with the production samples. The results for these experiments showed that lower heating rates (12 °C/h) occurred at a lower annealing temperature, increase draw ability. This effect is attributable to the longer time available to initiate nucleation at the lowest heating rate. However, for all three heating rates (12 °C/h, 24 °C/h and 36 °C/h), the aluminium and nitrogen combine to form atmospheres or pre-precipitation clusters at polygonised sub-grains and as rolled boundaries, modifying the development of the recrystallized structure. Different amounts of AlN were precipitated prior to recrystallization for each heating rate resulting in different final recrystallized grain sizes, mechanical properties and drawabilities. The heating rate strongly influences recrystallization and drawability with a lower heating rate reducing both the temperature of the start of recrystallization and the recrystallized grain size. It is inferred that the lower rate promotes nucleation of recrystallization overgrowth of recrystallized. The effect of holding time on mechanical properties was to: (i) decrease tensile strength, yield strength and hardness and (ii) increase drawability, elongation and grain size. Tensile strength and hardness results from laboratory experiments were lower than those for production samples, but the trends were similar.