Nb Solid Solution, Precipitation And Deformation Induced Ferrite Phase Transformation Coupling Relations

Deformation induced ferrite transformation is one of the most promising technology for developing New Generation Steel with ultrafine grain microstructure. A great deal of research works have show that the thermodynamic and kinetics of DIFT could be effected obviously by a lot of factors, such as strain, strain rate, strain temperature and the chemical composition of the steel etc. For microalloyed element Nb, there are two contrary viewpoints for it's effects on DIFT. One is Nb promotes DIFT, but another one is that it would restrain DIFT. In this paper, hot deformation processing was designed for research on the relationship between Nb and DIFT. Different isothermal time was given following a pre-strain at different temperature in the austenite region for a 0.094%C-1.45%Mn-0.045%Nb steel, and then a secondary heavy deformation at 780 C was given for inducing ferrite transformation. The volume fraction and grain size of deformation induced ferrite (DIF), obtained under different conditions of original austenite and precipitation volume fraction, were investigated. As a result, three experimental phenomena were found.1. Pre-strain 780 C: The static recrystallization of deformed austenite was restrained totally within 120s. DIP changed smoothly with holding time prolonging but increased abruptly at the beginning time of deformation induced precipitate occurring.2. Pre-strain 850-880 C: At the early period, DIF increased with holding time prolonging, but decreased when static recrystallization begin to occur.3. Pre-strain 900-950C C: DIP decreased normally because of recrystallization of deformed austenite having occurred at a very early time.Obviously, precipitation of Nb is a positive factor for promoting DEFT, but the recrystallization of deformed austenite is a negative factor for restraining DIFT. Nb plays a great role for promoting DIFT through it's effective retarding on recrystallization of deformed austenite. Furthermore, precipitate of Nb in deformed austenite is more helpful for DEFT.