| Based on the development and application of V micro-alloyed steel, the effects of V and its mechanism in the steel have been explored. In this paper, the effects of V on the hot deformation behavior during the process of high temperature and the V, N contents in the subsequent cooling process as well as the effect of their precipitation on the phase transformation behaviors are mainly studied. A quantitative method analyzing the V dissolution/precipitation is found. And on this basis, combined with the high speed wire rod production process, this paper discusses the effects of different process parameters on the microstructure and mechanical properties of V micro-alloyed steel.Effect of C and Si on the dynamic recrystallization(DRX) behavior in V micro-alloyed steel is studied by hot compression process on a Gleeble- 3500 thermal simulation testing machine. Results show that the change in C content almost has no effect on the stress strain curve and affects slightly the deformation activation energy of the material itself. However, Si addition enhances the solid solution strengthening and solute drag effect, which increase the deformation activation energy, peak stress and strain.Effect of C, Si and V on the static recrystallization(SRX) behavior in V micro-alloyed steel is investigated by adopting stress relaxation method. The solute drag effects of Si and V are also recalculated. Results show that Si can obviously increase the static recrystallization activation energy. The solute drag parameters SRP is 5, which means that the solute drag effect caused by adding 0.1 wt% Si is identical to that by adding 0.038 wt% V. A model of 50% static recrystallization time of V micro-alloyed steel considering the influence of V, Si simultaneously is established based on the parameters. In addition, the precipitation during the process of recrystallization results in lower Avrami index.The cooling behavior of V micro-alloyed steel is studied by using Gleeble-3500 thermal simulation testing machine combined with the microstructure observation. The results show that increasing the content of N in V steel can promote the precipitation of V(C, N), thus forming the intragrannular ferrite and consequently refine the cooling microstructure. In the meanwhile, it also can make the ferrite area towards left obviously and expand ferrite area significantly. Isothermal experiment is designed for quantitatively analyzing the refinement capacity of intragrannular ferrite caused by the precipitation of V(C, N). In terms of the experimental steels, the grain size in steel decreases by 8μm and theoretical yield strength increases by 24 MPa as the V(C, N) precipitation occurs in full.Quantitative research on the big particle precipitation(>0.2 μm) effect on strain induced dynamic phase transformation is performed. Results show that high N content can promote the precipitation of V on the surface of the large Ti precipitates. The big precipitates containing V can induce the formation of the intragrannular ferrite and obviously accelerate the dynamic recrystallization process. Then continuous dynamic recrystallization(CDRX) in the ferrite occurs and eventually the grains are refined. On this basis, a quantitative method of analyzing dissolved/undissolved V and Nb content is developed. The method includes the chemical dissolution, centrifugal sedimentation and inductively coupled plasma atomic emission spectroscopic(ICP-AES) analysis. This method greatly simplifies the experimental operation and shortens the test cycle. With the main ideology of centrifugal sedimentation, not only precipitation morphologies can be observed directly, solid solution contents can be measured also by using ICP-AES, which has a high precision of the measurement with the error of ±0.0038%.The high speed wire rod production process is simulated by using Gleeble- 3500 thermal simulation testing machine. Combined with the microstructure observation and microhardness measurement, four parameters of deformation temperature, spinning temperature, cooling rate and coiling temperature are studied. Results show that under the same condition of deformation degree and rate, the resulting microstructure becomes finer with decreasing deformation temperature, spinning temperature and coiling temperature and increasing cooling rate. The refinement capacity of intragrannular ferrite enhances with a lower deformation temperature, higher spinning temperature and lower cooling rate. More VC precipitates can be obtained with lowering deformation temperature, spinning temperature and cooling rate when the coiling temperature is close to 650 oC. There exists a certain measurement error of the volume fraction of precipitations during the calculation of fine-grain strengthening and precipitation strengthening. However, the calculation results can be optimized by adjusting the formula parameters. The combination effect of grain refinement strengthening and precipitation strengthening should be considered in the material composition design. When C and V content are higher, the precipitation strengthening effect can be more pronounced, even exceed grain refinement strengthening. The reasonable partitioning of grain refinement strengthening and precipitation strengthening effect can be obtained by adjusting the process parameters. |
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