Recrystallization Behavior of Deep Drawing Low Carbon Steel Sheets Produced by Mobarakeh Steel Plant

In the present work the recrystallization behavior of deep drawing grade Al-killed low carbon steel sheets produced by Mobarakeh Steel Plant (MSP) has been studied. Salt bath isothermal annealing has been applied to cold rolled samples and the resulted microstrucrures have been compared with the microstructure obtained from the box annealing process in MSP. It is found that at 635 and 700ºC the isothermal annealing produces equiaxed grains, while box annealing process presents an elongated grain structure. The recrystallization fractions have been measured using the point counting method. The linear intercept method has been employed to measure the grain size. It is revealed that at 550ºC, in salt bath annealing, recrystallization is stopped after 50% recrystallization with elongated new grains. Constant grain sizes upon holding after recrystallization at 635 and 700ºC during isothermal annealing show that the grain growth is stopped. Further holding at 700ºC results an abnormal grain growth in the recrystalized microstructure. The hardness variations during annealing, with a decrease at the beginning of annealing followed by an increase trend at all temperatures, show a good consistency with microstructural observations. Investigation of the results leads to the conclusion that a precipitation process occurs during annealing which greatly affects both restoration processes and resulted microstructure as well as the properties. The effect of the precipitation process depends on the time of its occurrence with respect to each restoration process. A Relatively high value of 315 KJ/mole for recrystallization activation energy has been related to the simultaneous occurrence of recrystallization and precipitation processes.