The "Tempering Plastic" Behavior Of 700T And It’s Effect Of Residual Stress

As an important basic material,700 T high strength steel is widely used in energy,transportation,power and other industries.However,the residual stresses introduced in the manufacturing process of high strength steel 700 T will lead to problems such as straightness defects,reduced service life and stress corrosion.In the actual production,the steel mill generally improves the performance of the strip by tempering process,and eliminates the residual stress in the plate.However,for a long time,the regulation mechanism of residual stress in the tempering process of high strength steel has been rarely studied.The steel mill cannot determine the most suitable tempering process parameters of high strength steel 700 T to achieve the best effect of stress elimination.Therefore,in-depth study on the regulation mechanism of residual stresses in the tempering process of high strength steel is of great academic value and engineering significance for the reasonable formulation of 700 T processing technology and the guidance of field production.In this paper,700 T high strength steel was taken as the research object.Focusing on the evolution law and regulation mechanism of residual residual stress in tempering process,systematic research was carried out from the aspects of microstructure evolution law,residual stress evolution law and basic mechanism of eliminating residual stress in tempering process at different tempering temperatures and different tempering holding times by means of non-isothermal tempering expansion test,isothermal tempering test,TEM,SEM,EPMA and other tests.The main conclusions are as follows.(1)With the increase of tempering temperature,the internal microstructure changes of high strength steel 700 T can be divided into the following stages: carbon atom separation and segregation stage(50°C – 100°C),cementite I precipitation stage(100°C– 200°C),cementite II precipitation stage(300°C – 450°C),alloy carbide precipitation stage(500°C – 550°C),Mn partitioning stage(550°C – 600°C)and alloy carbide second precipitation stage(600°C – 650°C).(2)Effect of tempering holding time on microstructure of 700 T prolonging holding time can induce carbide nucleation,precipitation,coarsening and dissolution.After prolonging the holding time at 150°C,the microstructure change of 700 T is mainly the precipitation and growth of cementite I;at 400°C,the microstructure change of 700 T is mainly the precipitation,coarsening and dissolution of cementite II.After holding at600°C for a period of time,the microstructure change of 700 T mainly forms Mncontaining carbides by Mn replacement mechanism.Nb and Ti alloy carbides were dissolved at 650 °C,and Mn-containing carbides were precipitated.(3)In the tempering process,there are three process windows for the improvement of residual stages: 100°C,350 – 400 °C,550°C – 650°C.At the same time,there are three temperature points with extremely low stages: 100°C,400°C,650°C.The residual stress at the three temperature points is 46.37%,62.67%,19.29% of the original state,and the asymmetry of neutral surface torque is 0.96%,0.64%,1.47%,respectively.The improvement of residual stress at 650°C for 15 min is the most obvious.(4)When the tempering temperature is below 600°C and the holding time is prolonged,the residual stress in the material decreases first and then increases,and the longest tempering time should not exceed 45 min.When tempering at 650°C,the residual stress increases slightly with the increase of tempering time.The best tempering time is15 min.(5)The regulation mechanism of residual stress varies with tempering temperature.When tempered at 350°C – 400°C,the main mechanism of residual stress evolution is precipitation plasticity and coarsening induced plasticity of cementite.The best tempering time is 30 min,and the tempering residual stress changes to 15.25% of hot rolled state.The main mechanism of high temperature tempering at 550°C – 650°C affecting the evolution of residual stress is the precipitation plastic mechanism of Nb,Ti and other alloying elements in Mn replacement precipitates and the precipitation of Mn-containing carbides.The plastic behavior of Nb and Ti in Mn replacement alloy carbides is mainly produced at 550°C – 600°C.The precipitation plasticity at 600°C – 650°C is mainly caused by dissolution of Nb and Ti carbides and precipitation of Mn carbides.The plastic behavior generated by Mn carbide precipitation is more obvious.Tempering at 650°C for15 min can reduce the residual stress to 29.22% of the hot rolled state.Considering the material properties and internal residual stress level,if the residual stress is considered,the best tempering process of high strength steel 700 T is that the holding temperature is 600°C and the holding time is 45 min.If the performance of steel is taken as the priority,the optimal tempering process of high strength steel 700 T is at650°C for 15 min.It is found that the evolution mechanism of residual stress is different under different tempering temperatures,and the residual stress evolution is dominated by the plastic behavior caused by the coarsening of cementite at intermediate tempering temperature.The high temperature tempering stage is dominated by the plastic behavior generated by Mn replacement and Mn carbide precipitation,and the optimal tempering process at 700 T is pointed out,which provides a theoretical basis and technical preparation for the accurate regulation of residual stresses in actual production.