Ferrite Grain Refinement, Grain Size Distribution, and Texture After Thermomechanical Processing and Continuous Cooling of Low-C Steel

Single-pass and multi-pass deformation schedules were applied over 973 K to 1323 K (700 °C to 1050 °C) using a Gleeble® thermomechanical simulator to understand the effect of the number of deformation passes, strain per pass, and interpass interval on ferrite grain refinement, particularly for the formation of an ultrafine ferrite grain structure (ULFG) with favorable high-angle grain boundaries and texture components. The microstructure, texture, and grain boundaries were characterized using SEM and EBSD. In the single-pass schedule, a decrease in deformation temperature from 1323 K to 1073 K (1050 °C to 800 °C) refined the ferrite grain size from 9.5 to 2.0 μm and intensified the beneficial γ-fiber (〈111〉//ND) texture. The ferrite grain size remained unchanged, whereas Cube and Goss texture strengthened with a further decrease in deformation temperature to 973 K (700 °C). For the multi-pass schedule at 1073 K (800 °C), where the total deformation was applied in three successive passes with 10-second interpass time, an ULFG structure was also developed with 2.5 μm grain size. Grain growth during the interpass intervals weakened the texture in the multi-pass deformed samples compared to that of the single-pass deformed samples. The evolution of microstructure and texture has been explained considering that restoration mechanisms (recrystallization, grain growth, and phase transformation) acted during and after deformation. Finally, ferrite grain refinement during different processing schedules was predicted using a simple mathematical approach based on the experimental data, with suggestions for future improvements.