Influence of composition and thermomechanical processing on microstructure evolution in AISI 430 ferritic stainless steel(FSS)

This thesis examines the influence of austenite potential and hot roll finish temperature on the evolution of microstructure in the ferritic stainless steel grade AISI 430. In particular, it focuses on the influence of these variables on the hot band annealing behaviour of this steel. The material employed was obtained from laboratory and commercial heats. Two hot roll finish temperatures, viz. 600°C and 800°C for the commercial heats and two alloy compositions of austenite potential 11 and 61% for the laboratory heats were studied. Electron channelling contrast (ECC) obtained in scanning electron microscopy was used to follow the evolution of microstructure. Limited micro texture measurements were made using electron backscattered techniques. It was found that a low finish temperature produced a hot rolled microstructure that showed limited softening and a fully recrystallised microstructure after annealing while a high hot roll finish temperature produced a completely softened as-hot-rolled microstructure and only partial recrystallisation after annealing. A high austenite potential encouraged the ferrite phase to undergo extensive continuous recrystallisation during hot band annealing. However, the affinity for the precipitation of carbo-nitrides tended to play a role in slowing down the process. On the other hand, the ferrite phase deformed in the presence of a low austenite content mostly underwent extended recovery during hot band annealing. The softening here was affected by a low driving force. The end microstructures after annealing were however similar in both cases in as much as they consisted of elongated structures. The martensite phase was found to behave similarly regardless of the austenite content, where both recovery to produce subgrains and occasional recrystallisation occurred. During final recrystallisation after cold rolling, the high austenite potential coupled with a short hot band anneal time resulted in incomplete recrystallisation. This caused sharper alpha fibre texture components in the final sheet. A long hot band anneal time however resulted in sharper gamma-fibre texture components. In the case of low austenite potential, a long hot band anneal produced a random texture in the final texture. Ridging was observed in all cases but a high austenite content was found to lessen its severity. Also, it (ridging) was reduced by the random texture produced by long hot band annealing in the case where the austenite potential was low.

Grantor dc:publisher.institution

Centre for Materials Engineering

Year dc:date.issued

1998

Author dc:creator

• Machio, Nyongesa

Advisor dc:contributor.advisor

• Knutsen, Robert D

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/11427/17931

OAI identifier oai:identifier

oai:open.uct.ac.za:11427/17931