Effect of two-phase zone deformation process parameters on deformation resistance of ultra-low carbon steel

To address the issue that the excessive strength of hot-rolled ultra-low carbon steel sheets is detrimental to subsequent cold rolling and forming, deformation in the austenite-ferrite two-phase zone can be applied to reduce strength, but this significantly increases deformation resistance during hot rolling. Therefore, compression deformation of low-carbon steel in the austenite-ferrite two-phase region (773-845 ℃) was conducted using a Gleeble-1500 thermomechanical simulator. Optical microscopy (OM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) were employed to investigate the effects of different deformation temperatures and strains on peak stress and microstructure, aiming to obtain process parameters that reduce deformation resistance in the two-phase zone and to analyze the corresponding softening mechanisms. The results show that the microstructure of the steel after two-phase zone deformation consists of ferrite and pearlite. Under a constant strain rate of 1 s^-1, strains of 30% or 60%, and deformation temperatures ranging from 775 ℃ to 825 ℃, dynamic recrystallization is difficult to occur when the specimen is deformed at a low temperature of 775 ℃ with a small strain of 30%. During deformation, the microstructure is dominated by coarse grains that have only undergone recovery and growth. Under these conditions, the ferrite grain size reaches a maximum of 55.4 μm, and the deformation resistance is minimized at 103 MPa. The primary mechanism for the reduction in deformation resistance in the two-phase zone under these process parameters is grain coarsening. Investigating the influence of deformation process parameters in the two-phase zone on the deformation resistance of low-carbon steel is of significant importance for achieving precise control of deformation resistance in industrial production and effectively reducing the strength of hot-rolled plates.