OBRABOTKAMETALLOV technology Vol. 26 No. 3 2024 ture Tnr [53]), austenite grains are sintered and deformation structures are introduced inside the grains [53, 84–98]. Deformation at this stage significantly increases the rate of nucleation at the boundaries of austenite grains and within austenite grains. This intragranular nucleation of ferrite is one of the most important aspects of controlled rolling. Microalloying elements such as niobium and titanium can increase the austenite recrystallization stopping temperature, thereby facilitating the use of this route. Deformation in the intercritical region leads to further strengthening of austenite and the formation of a substructure in ferrite. Accelerated cooling in the austenite-toferrite range reduces the ferrite grain size and increases both the strength and toughness of the steel. These routes are shown schematically in Figure 12. The synergistic effect from the interaction of the processes of austenite recrystallization and deformation precipitation of microalloying elements is one of the important technological issues [84–99]. In order to study the above processes, recrystallization-precipitationtemperature-time (RPTT) diagrams have been developed based on recrystallization and precipitation curves [53], where an example of such a diagram is shown in Figure 13. Below the precipitate solubility temperature (T0), three interaction modes are possible [53 , 86]. In mode 1, recrystallization is completed (based on the curves for the beginning of recrystallization Rs and the end of recrystallization Rf) before particle precipitation begins, and thus the stop of recrystallization is not achieved. Accordingly, recrystallized austenite eventually precipitates particles along the Ps curve. In mode 2, particle separation begins along the PDs curve, where the formation of new and powerful nucleation centers occurs due to plastic deformation. In mode 3, particle precipitation occurs before recrystallization, and now both the beginning and the end of recrystallization are stopped [86]. There are different groups of recrystallization processes (see Figures 14–16), many of it are interconnected and the boundaries between them are often unclear. The term recrystallization is commonly used to describe the replacement of the strain microstructure by new grains during annealing; this is called static recrystallization (SRX) [100]. SRX occurs when strain-hardened metals are heated above about half the melting temperature, i.e., 0.5 Tmelting. The temperature at which this can be achieved is usually called the TREX recrystallization temperature. The latter depends on the type of lattice, the concentration of alloying elements and the size distribution of secondary phases [100]. During annealing, the microstructure is characterized by a mixture of an increasing number of recrystallized grains and a decreasing number of deformed grains. This process is sometimes called discontinuous static recrystallization (dSRX) [100, 101]. Fig. 12. Schematic diagram of grain refinement in steels during thermomechanical processing [98] Fig. 13. Schematic representation of the RPTT diagram [86]
RkJQdWJsaXNoZXIy MTk0ODM1