Study On The Segregation Of C Atom Under Stress Annealing Of Low Carbon Steel

Grain ultra-refinement is a basic subject of super steel project.The main methods of this subject are alloying and deformation and recrystallization.In this paper,low-carbon steel under cold rolling was chosen as the study project,and the compressive stress was applied during the annealing process.Then the micro-Vickers hardness of the samples were tested.Meanwhile,the segregation behavior of solute atoms and precipitation of carbides in the stress field were studied using metallographic microscope,X-ray diffraction,transmission electron microscope,high-resolution scanning transmission electron microscopy,and three-dimensional atom probe.The effects of stress and annealing temperature on the microstructure evolution of deformed martensite were comprehensively analyzed and the strengthening mechanism was explored.After conventional annealing at 200?,the C atoms in steel were segregated at the grain boundary,and after stress annealing at 20 MPa-200?,the C atoms were segregated at dislocation and the grain boundary,which proves that the stress has a significant effect on the solute atoms.Applying stress during the annealing process can promote the segregation of solute atoms and change the diffusion and precipitation behavior of solute atoms,which was an effective method to improve the comprehensive mechanical properties.After annealing at 200?,fine,unstable Cohenite and?carbide precipitated in the microstructure,and with the increase of temperature,the carbide gradually grew and changed to more stable Fe₃C phase.The stress can increase the number of carbides particles,reduce the size,and make the carbides distributed orderly along the weak interface of the deformation lath.In the quenched-cold rolled microstructure,the martensite laths were distributed in parallel along the rolling direction,and high density dislocations were present in the laths.During the annealing process,as the temperature increases,the dislocations recovery,the martensite laths widen,and the laths were outwardly bowed to form a bamboo-like structure.The dislocations appear in the high-density and low-density areas,and recrystallization occurs at 550?,ferrite equiaxed grains were formed.Then at 650?,the recrystallization process was complete.After complete recrystallization at 650?,the grain size is larger in conventional annealed samples,with an average grain size of 4.66?m and a hardness of 118 HV.The application of stress can increase the position of the recrystallized nucleation,inhibit the recrystallization grain growth and refine the ferrite grains.At 20 MPa-650?,the refinement effect was most obvious,the average grain size was 1.36?m,the hardness value was 312 HV,which was 2.64 times of the conventional annealed samples,which effectively strengthens the structure.