Effect of Pre- and Post-Heat Treatment on Severely Plastic Deformed Low-Carbon Steels

Severe plastic deformation (SPD) is a top-down process for fabrication of nanostructures through which a con siderable increase in the materials strength is achieved. However, some drawbacks like low ductility and decreased formability have limited its capabilities. A solution to such disadvantagesis implementation of suitable heat treatment procedures in order to improve the materials formability while preserving or even enhancing their strength. In this work, low-carbon steel sheets were processed via SPD to the strain of ۱.۱۶. Samples were subjected to heat treatment before and afterSPD. Two temperatures in the range of inter-critical (۸۵۰ °C) and upper-critical (۱۰۵۰ °C) annealing were selected for conventional annealing of the sheets followed by water-quenching. Microstructureof samples was characterized by optical microscopy, and their mechanical behavior was investigated via tensile and Vickers hardness tests. It is revealed that both heat treatment procedures could improve the ductility and formability of SPDed samples, as well as their ultimate tensile strength (UTS). Although post-heat treatment typically improved the samples ductility and workhardenability, pre-heat treatment could lead to significant enhancement in their UTS. A balanced improvement in mechanical properties was concluded for the sample subjected to pre-heat treatment at ۸۵۰ °C. In this sample, UTS increased about ۷۸% and ۳۸% in comparison with that of as-received and SPDed samples, respectively, while its ductility was marginally increased in comparison to thelatter. The observations would be justified by considering the microstructure modification, e.g., decrease in grain size, increase in high-angle grain boundaries, and elimination of Widmanstatten structure.

Keywords:

Extra-low carbon steel , Severe plastic deformation , Heat treatment , Work hardenability , Strength improvement , Microstructure