Research On Intragranular Ferrite Of Vanadium Microalloyed Steels

In this paper, the nucleation of intragranular ferrite of two tested steels with differentvanadium and nitrogen content in the process of isothermal transformation issystematically researched on the basis of studying hot deformation behavior and phasetransformation behavior for the purposes of the application and development in vanadiummicroalloyed steel.Double pass compression test was carried out on a Gleeble-3500thermo-mechanicalsimulator to study static recrystallization and strain-induced precipitation. PTT curve wasdrawed. Also, the phase transformation behaviors of tested steels were carried out and themetallographic was observed. Finally, the continuous cooling transformation(CCT) curveswere constructed. The results show that, the precipitation of vanadium has a stronginhibitory effect on the static recrystallization, which makes the softening curve appear aplatform. Also increasing the V content in the steel lengthens the precipitation time. In theprocess of phase transformation, the start points and the end points drop as a function ofthe increment of cooling speeds, which result in the reduction of ferrite grain size andbainitic laths. The deformation of austenite zone can fine the microstructure, promote thephase transformation temperature and increase ferrite regin. In addition, increasing the Vcontent in the steel makes the bainite regin moves to the right, resulting in ferrite regindecreases.Thermal simulation, optical microscopy (OM), scanning electron microscopy (SEM),transmission electron microscopy (TEM) and electron back-scattered diffraction (EBSD)were employed to investigate the effect of isothermal treatment on formation ofintragranular ferrite in vanadium microalloyed steel and also precipitates or inclusionswere revealed by means of energy spectrum analysis and area scanning. The findingsindicate that the ferrite is in the form of isolated idiomorphs when it forms at hightransformation temperatures, whereas lower temperatures are associated with the multiplenucleation of laths or plates. The precipitates are complex (Ti,V)(C,N). The intragranularidiomorphs consist of several sub-grains with low-angle grain boundaries between them.The intragranular acicular ferrite laths or plates growing from the same inclusion keep the same orientation relationship to the opposite directions, but the remaining ones arediferently oriented. The formation of intragranular ferrite laths or plates can partitionaustenite grains into compartments and finally facilitate the attainment of fine-grainedmicrostructures.The paper also analysed the effect of intragranular ferrite on grain refinement byusing the platform after double pass compression on the basis of above researchs. It isfound that there exists a competitive relationship between static recrystallization andprecipitation during the isothermal period. Ferrite grain size doesn’t change regularly as afunction of isothermal time. Finally ferrite grain size decreases by about20%and the yieldstrength increases about18.23MPa after isothermal period.