Research On Effect Of Rolling Process For Microstructural Property,Oxidation And Decarburizaiton Of 20CrMnTi

The "Thirteenth Five-Year Plan" period is the key five years for China’s special steel industry to change from a manufacturing power to a creating power.The key indicator reflecting the strength of alloyed steel production and product manufacturing is that the product quality has reached the international advanced level.At present,the common technical problems of hardness and oxidation and decraburization control after hot rolling of gear steel in the actual production process for China still need to be solved.In this paper,the research is based on the gear steel 20CrMnTi.The hot deformation behavior,the continuous cooling behavior of supercooled austenite,the influence of rolling process parameters on microstructure evolution,macro-hardness,and oxidation and decarburization are studied.The main work of the thesis is as follows:(1)The effects of deformation temperature and deformation rate on the prior austenitic morphology and deformation resistance of the experimental steel are investigated by a single pass compression test.Based on the flow stress curve and combined with the hyperbolic sinusoidal model and linear fitting,the constitutive equation for experimental steel is established,and the stress-strain curve of the experimental steel is accurately predicted.Moreover,the stored deformation energy calculated by the flow stress curve is added to the ferrite ortho-equilibrium calculation equation to predict the ferrite equilibrium transition temperature after different deformation processes.(2)The continuous cooling transformation curves of supercooled austenite for experimental steel under static and dynamic conditions are established using methods of expansion and metallographic test.The effets of deformation and cooling rate on microstructural transformation is analyzed.The results indicate that deformation can promote the ferrite and pearlite transformation during high temperature region,and inhibit the bainite and martensite transformation during middle and low temperature region.However,due to the addition of alloying elements such as Cr and Mn,the critical cooling rates of bainite for 20CrMnTi under dynamic and static conditions are still less than 0.5℃/s.(3)Thermal simulator is used to simulate different thermal deformation and cooling processes,and the effects of various process parameters on microstructure evolution and hardness are studied.It’s shown that higher volume fractions of pearlite and ferrite and significantly decreased hardness of the experimental steel can be caused by decreasing deformation temperature and cooling rate.In order to improve production efficiency,rapid cooling technology at the high temperature region is applied,and the deformation process is optimized.The experimental scheme of deformation-rapid cooling-deformation is proposed.The hardness of gear steel 20CrMnTi successfully reaches the requirements for alloyed structural steels of GB/T 3077-2015.(4)The morphology of the oxide layer of the experimental steel at different deformation temperature and cooling process is the inner alloying element oxide layer+the outer Fe1-yO+Fe3O4+Fe2O3 oxide layer.The thickness of the oxide layer and the decarburization layer increases with the deformation temperature increases and the cooling rate decreases.When rapid cooling during high temperature region is applied,resident time of high temperature region is greatly shorten,and the oxidation and decarburization are simultaneously inhibited.