Microstructural, Chemical, and Crystallographic Investigations of Dynamic Strain-Induced Ferrite in a Microalloyed QT Steel

Abstract

Dynamic strain-induced transformation (DSIT) enables the formation of fine-grained ferritic microstructures, which are well suited for cold forming processes in the as-rolled condition. In this work, the formation mechanism, chemical composition, and crystallographic orientation of DSIT ferrite were investigated in a micro-alloyed steel and compared to pre-eutectoid ferrite. High-resolution techniques, such as scanning transmission electron microscopy and atom probe tomography (APT), were used for the investigations. To generate DSIT ferrite and pre-eutectoid ferrite, different experimental routes were applied using a compression deformation dilatometer. The results show a large number of NbC precipitates within DSIT ferrite, and show that the formation of DSIT ferrite is accompanied with C diffusion and the formation of retained austenite. APT measurements revealed that the C- and Mn concentration in DSIT ferrite is higher compared to pre-eutectoid ferrite. The crystallographic orientation of DSIT ferrite was examined using electron backscatter diffraction. The crystallographic orientation of DSIT ferrite after the deformation route revealed that the plane normals are parallel to the compression direction with the directions pointing towards the radial direction of the compressed sample. The results suggest that the formation of DSIT ferrite is a displacive mechanism, accompanied by C diffusion.

Keywords

• dynamic strain-induced ferrite
• thermomechanical processing
• micro-alloyed steel
• atom probe tomography
• electron backscatter diffraction