The Research Of Vanadium Precipitation In Non Quenched And Tempered Microalloyed Oil Well Pipe Steel

In this paper, the V isothermal precipitation kinetics in a new type ofnon-quenched and tempered oil well pipe steel were studied. The chemicalcomposition in steel and hot working process all can affect the process of Vcarbonitride precipitation, thus affecting the performance of the pipe product, so westudy the influence of rolling deformation and deformation temperature on theprecipitation behavior of V in the tested steel, which is expected to provide guidancefor the actual production of controlled rolling and cooling process parameterssetting.The precipitation and dissolution in tested steel were studied through both thenumerical calculation and the stress relaxation method. The precipitation-time-temperature curve （PTT curve） was determined in austenite range of600℃⁹50℃using the stress relaxation method on Gleeble3500thermal/dynamic simulator. Theprecipitates formed in the sample during stress relaxation at different temperatureswere observed under transmission electron microscope （TEM）. The precipitationrule of micro alloying elements and the change of chemical composition on the ruleof influence in the tested steel was calculated. Comparative analysis of criticalnucleation size, critical nucleation work, relative nucleation rate （NrT curve） andPTT curve on homogeneous nucleation and dislocation nucleation modes.The results of the experiments show that the precipitation-temperature-timecurve of the tested steel is double C-shaped, with minimum incubation temperatureat875℃and650℃, respectively. With the increasing degree of deformation, Theprecipitation start time is an order of magnitude slower when the temperature isconstant. This show that precipitation is accelerated by the deformation. Themorphology of the precipitation phase in austenite is mainly small round particleswithin5to15nm size. These small circular dispersion particles are vanadiumcarbonitride precipitated from deformed austenite, which will lead to the stress relaxation.The solid solution temperature of the tested steel is about980℃. Thecalculations show that V precipitation is significantly affected by the content of C, N,Mn, V. With the increase of the content of C, N, Mn, V, it will conducive toprecipitate more V(C_xN_1-x) phase in austenite, but at the same time make the contentof solid solution state element in austenite increased, the result will affect γ→α phasetransition. VN phase was mainly precipitated in higher temperature stage in austenitedistrict, when the temperature continues down to under738℃, the precipitated phasemainly for VC at this time. The vanadium carbonitride mainly precipitated ondislocations. The stored energy can be effective in promoting the vanadiumcarbonitride precipitation.