Dynamic transformation of austenite to ferrite in low carbon steel.

摘要

The design of thermomechanical processing schedules to control microstructures requires the knowledge of the austenite-to-ferrite transformation start temperature (Ar{dollar}sb3{dollar}). Further, in curved mold continuous casting, transverse cracking occurs during the unbending procedure to straighten the solidified curved strand, and this can also strongly depend on the austenite-to-ferrite transformation. In both of these industrial processes, during deformation, the temperature usually decreases continuously. Thus, two new methods to determine the Ar{dollar}sb3{dollar}, based on continuous cooling compression (CCC) and continuous cooling torsion (CCT), have been developed. While the latter is applicable for low and high strains, the former can be only used for low strains.; The aim of this investigation was to determine the effect of deformation in the single phase austenite and two phase austenite plus ferrite region on the transformation and dynamic transformation behaviour of austenite-to-ferrite. CCC tests were carried out on a low carbon steel and the influence of strain, strain rate, cooling rate and austenite grain size, was examined.; During the application of strain, the generated dislocations cause an increase in stored strain energy. This energy adds to the driving force for austenite-to-ferrite transformation, increasing the kinetics of this transformation, raising the Ar{dollar}sb3{dollar} in this way. The faster kinetics leads to a finer polygonal ferrite grain size after transformation. In contrast to the effect of increasing strain, accelerated cooling rates decrease the transformation start temperature, but can still lead to grain refinement via high nucleation rates. By increasing the cooling rate, fine acicular ferrite with a high aspect ratio could be obtained. Since, the ferrite grain size is directly related to austenite grain size, by varying austenite grain size, a wide range of ferrite grain sizes could be obtained. Finally, deforming close to the Ar{dollar}sb3{dollar} maximizes the strain effect on dynamically transformed ferrite.