Year

1995

Degree Name

Master of Engineering (Hons.)

Department

Department of Materials Engineering

Abstract

The effect of annealing cycle on the microstructure, mechanical properties and drawability of aluminium killed steel sheets at three different heating rates and different holding times were studied using tensile test, hardness test, optical microscope and transmission electron microscope techniques. The results of sample for different times in laboratory experiments were compared with production samples. The results for these experiments showed that at lower heating rates (12 °C/h ) recrystallization occurred at a lower annealing temperature. 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 polygonized subgrains and as rolled boundaries, modiflying the development of the recrystallized structure. Different amounts of AIN 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 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 over growth of recrystallized. The effect of holding time on mechanical properties was to : (i) decrease tensile strength, yield strength and hardness and (ii) increase elongation and grain size. Tensile strength and hardness results from laboratory experiments were lower than those for production samples, but the trends were similar. The curve of yield strength from the production samples was however sharper than for the laboratory samples. Holding times that produced the coarsest grain size also resulted in the lowest yield strength,tensile strength and hardness.

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