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 <111> plane normals are parallel to the compression direction with the <110> 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.
Highlights
Quenched and tempered (QT) steels are characterized by a good toughness with a high hardness and strength
The formation mechanism, chemical composition, and crystallographic orientation of Dynamic strain-induced transformation (DSIT) ferrite were investigated in a micro-alloyed steel and compared to pre-eutectoid ferrite
In order to create the basic requirement for the formation of DSIT ferrite, the final experiment temperature was chosen in a way that it was between Ae3 (809 ◦C) and Ar3 (690 ◦C for λ = −3.0 ◦Cs−1)
Summary
Quenched and tempered (QT) steels are characterized by a good toughness with a high hardness and strength. In order to ensure good cold formability, the microstructure of the semi-finished parts must meet certain properties before quenching and tempering. The optimal cold forming properties of semi-finished QT steels are achieved after a soft annealing process prior to the forming process. A specific form of TMC processing is the formation of ferrite by dynamic strain-induced transformation (DSIT) [2,3,4]. This ferrite formation takes place dynamically during the deformation between Ae3 (equilibrium temperature of the austenite to ferrite transformation) and Ar3 (transformation temperature from austenite to ferrite, depending on the cooling rate) and differs from the dynamic transformation (DT) above Ae3 that
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