| Duplex stainless steel is widely used in offshore oil refining,marine transportation industry and gas equipment manufacturing due to its good plasticity,toughness,corrosion resistance and welding performance.However,due to the coexistence of austenite phase and ferrite phase,the proportion of the two phases in the high temperature hot rolling process is changing constantly,and the uneven strain distribution leads to complex deformation behavior.The surface micro cracks,ears and other defects are easy to produce and difficult to accurately control during the hot processing of wire and bar products.Therefore,this topic aims at the above problems to carry out the research on the deformation behavior in the thermal processing process,and optimizes the processing parameters in combination with the actual production line.Through the high temperature compression experiment of the sample,the effects of different deformation temperature,strain rate and plastic deformation on the deformation behavior of 2209 duplex stainless steel were analyzed,and the hot deformation constitutive equation and the hot processing map under different strain were established.The results show that the softening degree of ferrite phase and austenite phase increases with the decrease of deformation rate and the increase of deformation temperature during high temperature deformation,and the processing performance is better in the high temperature and medium strain rate range.The microstructure of the samples under different parameters was characterized by SEM and EBSD techniques.The softening mechanism and grain structure of 2209 duplex stainless steel under different strain,deformation temperature and strain rate were analyzed.The results show that the softening mechanism of ferrite grains is dominated by dynamic recovery and dynamic recrystallization,and it is relatively complete at high temperature.Austenite is dominated by dynamic recrystallization,which contains a small amount of subgrains,and the strain rate has a significant effect on its dynamic recovery effect.The strain partitioning model is constructed based on the mixing rule,and the strain partitioning process in thermal deformation is simulated based on the real microstructure of stainless steel.The experimental results show that when the initial deformation temperature is low,the deformation comes from ferrite and the strain rate gradually decreases.At the same time,the austenite softening mechanism is not complete,the stress from ferrite is accepted and a large amount of strain energy is accumulated.The strain rate gradually increases,and the two corresponding strain rates are consistent when a certain amount of deformation is reached.The microscopic simulation shows that the strain distribution is not uniform during the deformation process.The stress of austenite phase is higher than that of ferrite phase,while the equivalent plastic strain is significantly smaller than that of ferrite phase.The transformation of ferrite phase and austenite phase was further analyzed by XRD technology.Under the influence of temperature,the austenite phase changed to ferrite phase.Based on the above research,the rolling process of 2209 duplex stainless steel bar was optimized and the finite element simulation was carried out.The results show that the maximum temperature drop of the bar is mainly affected by the sudden change of the hole shape,and the temperature gradient has a significant effect on the dual-phase structure and properties of the stainless steel.When the shape coefficient of the elliptical hole is 2,the filling degree of the elliptical hole is 93.94 %,and the filling degree is the best in the subsequent round hole,reaching 98.92 %.Combined with the fitting relationship between pass shape coefficient and fullness,a new rolling spread model is constructed,and the calculation accuracy is higher than 95 %,which can be used to guide the design of the opening pass of 2209 duplex stainless steel bar and the optimization of process parameters. |
PDF Full Text Request