Formation of fine ferrite grains during hot deformation of Cu bearing steel

In order to refine the microstructure of low carbon steels, effect of copper addition was investigated in terms of super-cooling of austenite, dynamic austenite to ferrite transformation, and dynamic recrystallization of ferrite. Large decrease in transformation starting temperature was observed by copper addition. This is caused by the segregation of copper to austenite grain boundaries during prolonged austenitizing treatment. Fine ferrite grain structure in size of 2.1 μm was obtained by the deformation of super-cooled austenite at the reduction of 50% at 1043K in 2 mass% Cu bearing steel whose Ar₃ temperature was decreased to 1008K by austenitizing for 14.4 ks at 1223K. This grain refining process has two stages. In the first stage, the austenite was transformed to ferrite through a deformation induced dynamic transformation (DIDT). The DIDT ferrite was characterized by fine cementite particles which cannot be observed in usual ferrite formed by thermal transformation. Volume fraction of the DIDT ferrite increases with increasing the degree of deformation. In the next stage, the ferrite grains were further refined and equiaxed with increasing the degree of deformation. A possibility that a dynamic recrystallization of DIDT ferrite occurs during the deformation was shown and this results in the marked grain refinement of ferrite.