OBRABOTKAMETALLOV Vol. 26 No. 3 2024 technology From the analysis of the literature above, it is clear that by adding microalloying elements such as Ti, Nb and V, grain growth, recrystallization and particle precipitation kinetics can be controlled. TiN is thermodynamically quite stable and helps prevent grain growth at high temperatures, especially in the heataffected zone of welded joints. Nb is less stable compared to Ti and forms carbon nitrides Nb(C,N) at approximately 900 °C, which nucleate under the influence of deformation and slow down the recrystallization of deformed austenite [14–16]. After rolling, the deformed austenite transforms into ferrite and pearlite, and V(C,N) is released in the ferrite phase, which also gives some increase in strength. The temperature-time dynamics changed from conventional high-temperature rolling to recrystallization-controlled rolling to thermomechanical controlled rolling or TM rolling plus rapid cooling after rolling, as shown in Figure 18. As shown, TMCR consists of two successive stages: controlled rolling and the subsequent process accelerated cooling. During finish rolling in a hot strip mill, the austenite grains are pulled into a pancake-shaped shape with a high dislocation density. Until the 1980s, air, air/water mist, oil or water quenching were selected to achieve proper mechanical properties based on CCT charts and sheet thickness. Cooling has now become much more flexible, allowing new strategies with new resulting microstructures, as shown in Figure 19 [103]. For plates, accelerated cooling (ACC), direct quenching (DQ) and direct quenching and self-tempering (DQST) were introduced [103]. To provide high cooling rates with stringent requirements for uniformity and controllability, equipment manufacturers were forced to develop advanced cooling systems. Comprehensive control of residual stresses and deformations, increasing the heat transfer coefficient took a lot of time and is currently still the subject of research and development. In other cases, quenching and tempering was replaced by quenching, rapid cooling, isothermal exposure in the field of the bainite phase, and many other possibilities [103]. New cooling strategies have also led to the emergence of new steel grades such as dual-phase (DP), complexphase (CP) and transition ductility (TRIP) steels. Fig. 17. Typical difference in the contributions of hardening mechanisms in the case of recrystallization controlled rolling at a high temperature of finishing rolling and thermomechanical controlled rolling at a finishing temperature of about 900 °C [103]
RkJQdWJsaXNoZXIy MTk0ODM1