Microstructural evolution of AISI304 stainless steel in the Steckel Mill rolling process

• Main Author: Parker, Sa-aadat
• Other Authors: Knutsen, Robert D
• Format: Dissertation
• Language: English
• Published: University of Cape Town 2014
• Subjects: Mechanical Engineering
• Online Access: http://hdl.handle.net/11427/8619

Includes bibliographical references (leaves [105]-110). === The microstructural evolution of AISI304 stainless steel in the Steckel mill rolling process is investigated. This study includes the analysis of mill logs, microstructural examination of the mill product, deformation simulations and post deformation heat treatments. The mill logs from industry contains information about various process variables such as temperature, roll speed, dimensions of the mill strip and forces applied to it during the hot mill rolling process. The strain, strain rates and stresses on the mill strip can be calculated from the mill logs. An understanding of the metallurgical changes during rolling process can be gained by analysing the mean flow stress trends that evolve during rolling. Microstructural examination of the strip in different regions allows us to evaluate the property variations in the strip. This was done with microhardness testing, conventional optical microscopy and electron backscatter diffraction. The middle section of the strip demonstrated full recrystallization whereas the head and tail sections demonstrated no signs of recrystallization. The property differences through thickness proved to be negligible. Laboratory simulation was done in uniaxial compression testing on a Cam Plastometer. It was found that the temperature has a profound influence on the flow stress and the microstructure. The strain rates experienced in hot rolling does not have a significant effect on the flow stress and no measurable effect on the hardness. Heat treatments were done on the deformed uniaxial compression samples. The results of these heat treatments were analysed by two different methods: to deform the sample again after the heat treatment and to compare the yield stress from the first and second deformation and to measure the changes in room temperature hardness with the heat treatment time. The latter led to the development of a time to 50% recrystallization equation that allows the prediction of a direct annealing time for complete softening at the conclusion of the hot rolling process.