Effect of solutes on austenite processing in ultralow carbon steels

The effects of solutes in austenite in ultra-low carbon steel has to date not been sufficiently investigated. In order to gain an understanding of this, a series of steels were designed which feature the effects of particular elements in solution in austenite: boron, niobium, molybdenum, sulfur, phosphorus, and carbon. The influence of these solutes on austenite recrystallization kinetics, austenite flow stress, austenite to ferrite transformation, hot band texture development, and grain size evolution were studied.;It was found that the elements in solution retarded the austenite recrystallization kinetics, with the largest influence coming from the boron addition. The solutes also tended to increase the activation energy for recrystallization, and raised the recrystallization stop temperature. Austenite flow stress was increased by the solutes, especially at lower temperatures. The solute elements had little effect on the transformation temperatures, but some elements such as boron, niobium, and molybdenum, caused the formation of non-polygonal ferrite when deformed material was quenched from austenite, leading to a microstructure with higher hardness. It was also found that in these steels the austenite grain size of hot band reached a stable value when re-heated in austenite, with the stable grain size varying for the different solutes.;The influences of the solutes were related to the action of atoms which are segregated to the grain boundaries in austenite.