| Ultra fine-grained process is one of important methods to improve steel materials properties. It improves the strength of steel without sacrificing the toughness. Recently, researchers have developed different thermo-mechanical processes to produce ultrafine ferrite (UFF). Among these approaches, deformation induced ferrite transformation (DIFT) is a simple way to produce UFF(<4 μ m). If the steel is deformed at non-recrystallization temperature, so that the ferrite transformation temperature will increase to deformation temperature (above Ar3 or Ae3), then the ferrite will be dynamically formed during deformation and produce UFF finally. Meanwhile in micro alloyed steel ,the effects of Nb on the DIFT are described as follows; (i )the ferrite transformation was retarded by the consumption of strain energy for the dynamic precipitation of Nb(CN).(ii) Nb plays a great role for promoting DIFT through it's effective retarding on recrystallization of deformed austenite. The purpose of the present study is to investigate the interaction between Nb precipitation and deformation induced ferrite transformation during deformation. The steel was made with a chemistry of 0.094%C-1.45%Mn-0.045%Nb. Solution treatment was carried out at 1250℃ for 30 minutes followed by water quenching. Using Gleeble-2000, the steel was reheated to 1200℃ for 300s, then cooled to various deformation temperature at a rate of 10℃/s and then deformed with the amount of deformation, reduction up to 70% were applied at a strain rate 1s-1. The volume fraction of deformation induced ferrite was measured. The micrograph of austenite and Nb(CN) was investigated with OM and TEM respectively. The main results are as follows:(1)Deformed at 850 ℃ or 880 ℃ (above Ae3), deformation induced ferrite can be dynamically formed during deformation; at the early stage of deformation between 10%-50%,the volume of ferrite increase slowly and then increase faster.(2)Deformed at 790 ℃ or 820 ℃ (above Ae3), with heavy deformation 60% and 70%, with a large amount ferrite existed, the NbC precipitated dynamically. (3)Deformed at 850 ℃ or 880 ℃ (above Ae3), with moderate deformation 40%, a little amountslowly, but ferrite volume increase significantly. Through theoretical analysis, it is considered that perform heavy reduction above Ae3 will increase significantly the amount of strain stored energy and the effective nucleasites of ferrite in austenite and induced massive fine ferrite. During deformation, Nb(CN) precipitated and the retardation of C and Nb in austenite was reduced. When heavy reduction performed below Ae3, DIFT was accelerated cause strain stored energy. When massive ferrite formed, the untransformed austenite solute more C, so decrease the solid solubility of Nb, induced dynamic precipitation of Nb(CN).
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