3.1.1 Temperature analysis
At different velocity and temperatures of rolling the temperature changes of bar immediately in deformation zone is of interest to study. As shown in  these changes can lead to different combination of structure over bar cross-section during dynamic recrystallization. In this case, the temperature was controlled at three points: P1 - bar center; P2 - half of bar radius; P3 - bar surface. The graphs of temperature changes at points P1-P3 obtained by tracing of points along their movement trajectory are presented in Fig. 2. It makes possible to see the dynamic of temperature changes in zones typical for RSR.
As can be seen, the greatest temperature fluctuations (the change reaches 20-50°C per deformation cycle) occur in the surface layer of bar where strains and stresses are localized. Since the deformation zone is open, the sharp change in temperature in contact zone occurs with each single reduction by roll. In the moment of point P3 passing between work rolls (non-contact area of deformation) the surface temperature can sharply change again.
At maximum heating temperature T0 = 450°C the surface temperature in the deformation zone decreases while the temperature at points P1 and P2 smoothly increases in the reduction zone. At decrease in heating temperature of workpiece to 350-400°C the tendency to decrease in surface temperature of bar reduces, and the temperature at point varies in temperature range of P1 and P2. (Fig. 2b, c). In the calibration zone of bar in all three cases the temperature fluctuations of surface reduce since the reductions are insignificant, and the temperature of inner layers almost do not change.
The rolling velocity also significantly impacts on the changes in temperature field of workpiece in the deformation zone. At the rotary velocity of 90 rpm the surface temperature in reduction zone intensively increases, the graph is stepped. (Fig. 2d). Since the time of one cycle decreases the temperature in the contact zone with colder roll have no time to change and remain the constant. It should be noticed in this case that the temperature gradient over section of bar ΔТ sharply increases with the growth of rolling velocity. Difference between temperatures at points P1 and P3 is more 100°C, it can additionally effect on structure formation processes.
Since the rolling in an industry is mostly done in several passes, it is effectually to analyze the temperature change after each pass. The graphs of temperature change of bar after each pass ΔT relatively heating temperature before rolling T0 for five simulated regimes are presented in Fig. 3. The change in temperature ΔT was determined as the difference between T0 and average temperature at points P1-P3 on the exit of deformation zone.
For regimes 2, 3, and 4 with constant temperature before each pass and the same rolling velocity it should be noticed that ΔT decreases with reducing of bar diameter. For higher heating temperatures of 400 and 450°C, the temperature increase is 50-70°C in the first pass, and there is no increase in the last one. For regime 1 with gradual decrease in temperature the deformation heating with each subsequent pass, on the contrary, increases. For the first pass, it is minimal and equals 25°C, and in the last pass it reaches approximately 100°C.
At the heating temperature 300°C and the increase in rotary velocity to 90 rpm the temperature increase after RSR is the largest and it is 90-125°C, for first four passes, and it decreases to 60°C in last pass.
Based on data obtained the following general conclusions can be made:
- at the same heating temperature before rolling T0 the deformation heating of bars decreases with reduction of bar diameter;
- reducing the heating temperature by 50°C leads to a decrease in deformation heating by approximately the same amount;
- increase in the rotary velocity of rolls has a significant effect on the deformation heating of bar after RSR (mainly in surface layer of bar).
As can be seen, the combination of rolling temperature-velocity conditions at selection of deformation regime has complex effect that cannot be easily predicted or calculated analytically. Since the temperature is one of the main parameters effected on structure and properties formation the simulation can be useful tool for preliminary analysis and forecasting of product properties formation.
3.1.2 Deformation parameters analysis
The equivalent strain parameter can be used for the complex characteristic of technological process conditions influence on material properties. During the severe plastic deformation processes the large values of equivalent strain (4-6 or more) are achieved with relatively small changes in the overall dimensions of workpiece. It makes possible to obtain the materials with ultrafine-grain or nanostructure and significantly increase the strength properties. [24, 25].
During RSR process the field of equivalent strain is formed with pronounced gradient of distribution over section of rolled product. The equivalent strain ε reaches the maximum values in peripheral zone of workpiece where the greatest shear strains occur locally. The values of ε are minimal in the central zone and, mainly, determined by reducing of cross-section area of bar.
The temperature and rotary velocity of rolls significantly effect on the gradient of equivalent strain field. With decrease in the initial temperature and increase in the rotary velocity of rolls the growth of maximum values of ε at insignificant change in level of minimum values in the center is observed. In these dependencies the influence of rheological component on the strain state of workpiece is appeared. As known the deformation heating is due to dissipation of plastic deformation power which is proportional to flow stress and strain rate. Disregarding some conventions, it can be argued that the following self-activating scheme is implemented. The initial nonuniform distribution of ε creates nonuniform deformation heating. In the most heated near surface layers, the resistance to deformation of metal (flow stress) decreases, which contributes to an even greater localization of the accumulated strain on these layers with an increase in its maximum values. This, in turn, further enhances the deformation-temperature gradient. It is obvious that when implementing such scheme, the force of metal on roll decreases due to a decrease in the level of contact stresses during local heating. Data obtained at simulation (Fig. 4) appear this fact. For all studied regimes the growth of maximum values of ε and the increase in the gradient with the decrease in the rolling force are observed.
For a comparative analysis the development of severe plastic deformation area (SPD area) expressed in relative fraction of bar cross-section area with the value ε ≥ 8 was determined for each regime (Fig. 5).