| With the exploitation of oil and gas resources with high temperature,high pressure and high corrosion,the traditional high strength iron-based and nickelbased oil country tubular goods(OCTG)are difficult to meet the rigorous requirements,and it is urgent to use OCTG with high strength,high toughness and high corrosion resistance.Titanium alloy has great potential for oil and gas industry because of a low density,good medium-high temperature specific strength and outstanding corrosion resistance.However,it is difficult to produce high strengthα+β titanium alloy seamless tube due to the severe oxidation during the production process,as well as the structural inheritance and obvious influence of temperature on the deformation resistance.In this work,the deformation characteristics and micro structure evolution mechanism of TC4 titanium alloy seamless tube were systematically studied by combining with physical experiment simulation,finite element simulation and factory preparation,which provided a theoretical basis for the control of microstructure and properties.The high temperature oxidation behavior and β grain-growth behaviors were studied to clarify the effect of high temperature heating on the microstructure evolution.Through the growth kinetics analysis of oxidation weight gain and αcase,the formation mechanism of oxide layer and α-case was illustrated.It is suggested that the oxidation process is mainly controlled by two mechanisms:the outward diffusion of titanium and aluminum and the inward diffusion of oxygen through the oxide scales,and the oxidation rate is accelerated obviously above Tβ.The difference in the morphology and the formation mechanism of α-case at different temperature ranges is mainly owing to the participation of the grain boundary and grain orientation of the nucleation site.Prior β grain size increases with an increase in heat-treatment temperature and time,which is described by the modified Sellars model,and β grain-growth activation energy(Qg=161.0 kJ/mol)during β treatment is deduced.In order to avoid the accelerated growth of α-case caused by coarsening of β grains,the long-time heat preservation of blank should be avoided in production process of titanium alloy tube.The effect of rotary piercing and subsequent holding time in β region on the βgrain evolution by hot compression simulation was evaluated.Under high temperature and high strain rate,the deformation mechanism is dominated by dynamic recovery(DRV)and supplemented by dynamic recrystallization(DRX)of grain boundary remotion,and post-dynamic recrystallization(Post-DRX)can effectively refine β grain size and improve the uniformity of microstructure.The Post-DRX model and its grain-growth model is established,and the Post-DRX grain-growth activation energy is obtained as QPC=117.5 kJ/mol,which is lower than Qg.The results show that the microstructure of Post-DRX is easier to coarsen.The main characteristic of the lamellar micro structure during continuous cooling transformation process is that the variation tendency of the colony a size and the αplate thickness is positively correlated with the prior β grain size.Lamellar structure parameters significantly affect strength,ductility and impact toughness of TC4 titanium alloy,which obeys the Hall-Petch relationship.The crack propagation path of the impact toughness is ascribed to the effects of boundaries and crack deflection,which are mainly determined by the colony α morphology.With the increase of temperature and the decrease of strain rate,the hot deformation softening mechanism in α+β region changes from DRX to DRV.Combined cooling phase transformation,the deformation mechanism near Tβ(>900℃)is DRX of β grain and separation of partial precipitated a phase.At relatively lower deformation temperature(<900℃),the spheroidization mechanism is CDRX(Continuous dynamic recrystallization)of the colony α.Under higher strain rate,the adiabatic shear deformation and the flow localization easily occurred,resulting in the occurrence of flow instability.A strain-compensated Arrhenius-type constitutive model(SCA)and a physically-based constitutive model were calculated.Based on the processing map,the optimized deformation parameters are found at the temperature of 850-920℃ and the strain rate of 0.01-1 s-1.The rationality of the optimized temperature parameters is verified according to the finite element simulation analysis of continuous rolling and reducing-sizing process.The influence of thermomechanical processes(TMP)on the microstructure evolution and properties of titanium alloy in α+β region was studied through designing rolling routes to simulate the hot rolling technology of seamless tube.A mixture microstructure with colony α,interlace a and globular a is obtained by a new process "near Tβ deformation+on-line solution(950℃)+small deformation",which significantly improves the strength and impact properties.The hot-rolled TC4 titanium alloy seamless tube produced by on-line solution process can obtain high strength and toughness without complex off-line heat treatment,whose tensile strength and impact energy are respectively 8.0%and 6.4%higher than those of the hot-rolled tube produced by the conventional process.The Ni and Nb microalloying renders the microstructure of TC4 titanium alloy tube with a very high ultimate tensile strength of 1021 MPa,tensile yield strength of 948 MPa,elongation of 14.1%and 0℃ impact energy of 44.9 J. |
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