Influence of Pre-strain on the Cementite Spheroidization of 22MnB5 Steel and Its Effect on Mechanical Properties

Spheroidization is a common technique to improve the formability and machinability of material. Multiple combinations of deformation level and annealing time were employed to optimize the microstructure in 22MnB5 steel, aiming to achieve maximum spheroidization and keeping the process most economical. The critical annealing temperature for spheroidization was decided through differential scanning calorimetry measurement. Spheroidization was achieved by annealing of as-received and three different amounts of cold-rolled material, namely 10, 20, and 30% reduction in thickness. Annealing was done at 720 °C for four different times, 10, 20, 40, and 60 h, at all deformation levels. Convoluted multiple whole profile fitting technique of x-ray diffraction pattern was used to estimate the stored dislocation density in the material after different deformation levels. The microstructural investigation was performed using optical microscopy, scanning electron microscopy and EBSD techniques. The extent of spheroidization was measured through the average aspect ratio of cementite precipitate. Mechanical properties were measured through surface and core micro-hardness of initial and spheroidized samples. Microstructural characterization showed that deformation produced sub-grains and low angle grain boundaries, facilitating an easy path for diffusion of carbon. However, it was also found that a very high amount of pre-strain is also detrimental to the spheroidization process because it results in the formation of very long and sharp cementite lamella, which required more annealing time for spheroidization. The best processing parameter for spheroidization of 22MnB5 steel, considering the optimum spheroidization and minimum cost of the process, was found as 20% cold rolling reduction followed by 20 h annealing.