Influence And Application Of Additive Light Elements On Microstructure Evolution In Ultra Low Carbon Steel By Severe Plastic Deformation

Severe plastic deformation is a kind of technology for preparing ultra-fine grain and nanocrystalline material,and its application prospect is very wide.Highpressure torsion is one of the deformation technologies with industrialized prospect in the severe plastic deformation.This paper systematically studied the performance of ultra-low carbon steel under high-pressure torsion condition.The effects of phosphorus,sulfur and hydrogen on the microstructure and properties were studied in high pressure torsion.The main contents of this paper are as follows:(1)Based on the DEFORM finite element software,the three-dimensional deformation process of high pressure torsion was simulated,and the law of metal flow in the process of high pressure torsion was analyzed.The influence of different factors such as pressure and friction coefficient on the distribution of equivalent strain of high-pressure torsion specimen was studied.According to the finite element model,the high pressure torsion parameters were seted in prectise.Based on the traditional high-pressure torsion process,the mold of the ring sample was designed and the high pressure torsion of the ring was simulated and verified.(2)The effect of different torsion turns and rotational speeds on the micro structure evolution and mechanical properties of high-pressure torsion was studied.The experimental results show that high-pressure torsion can significantly improve the mechanical properties of ultra-low carbon steel and improve the yield strength of ultra-low carbon steel.The hardness of the ultra-low carbon steel reaches saturation after 10 cycles.The hardness of the unsaturated sample is unevenly distributed in the radial direction.Hardness of the center region is low,while at the edge region hardness is high.High-pressure torsion can significantly refine grains,and the dislocation density increases with the number of turns and reaches saturation after 10 turns.The microstructure of unsaturated sample showed the phenomenon of coarse grain in the center and fine grain in the edge rigion.By analyzing the mathematical model of strengthening mechanism,it is proved that the strengthening mechanism of ultra-low carbon steel in high-pressure torsion deformation is dominated by dislocation strengthening.(3)The effects of phosphorus and sulfur under high pressure torsion on the microstructure and mechanical properties of ultra-low carbon steel were studied.It is found that in addition to solid solution strengthening,phosphorus can also play a role of microstructure refinement during high pressure torsion,which improves the hardness of phosphorus addition ultra-low carbon steel.(4)The effect of hydrogen on the micro structure and mechanical properties of ultra-low carbon steel in the high pressure torsion process was studied.Hardness of hydrogen charged ultra-low carbon steel is rate depended.It is proved that hydrogen can improve dislocation density during high-pressure torsion at a suitable rotational speed.This effect is considered as a result of the interaction between hydrogen and dislocations.