Evaluation of Mechanical Properties and Microstructures of Ultrafine Grain Low-Carbon Steel Processed by Cryorolling and Annealing

In the present study, low-carbon steel was first evenly cryorolled by 50% reduction at liquid nitrogen temperature (LNT) and then annealed at 400-550 degrees C for 1800s. A transmission electron microscope was employed to analyze the microstructures and the tangled dislocations in the processed steel. The presence of Fe3C particles in the steel was confirmed by X-ray diffraction method, and the mechanical properties were measured by an electronic universal tensile machine. It was found that cryorolling at LNT significantly improved the potentials of refined ferrite grains. Grain refinement at LNT occurred due to the suppression of dynamic recovery during cryorolling, thus resulting in high defect density and abundant nucleation sites for ferrite grains. An average ferrite grain size of 133nm was observed in the specimen annealed at 450 degrees C for 1800s, and its strength increased to 970.2MPa with a reasonable ductility of 12.34%. The work extended the cryorolling from alloys and austenitic stainless steels to the low-carbon steels and provided a technical support for the fabrication of ultrafine grained low-carbon steel.