OBRABOTKAMETALLOV technology Vol. 26 No. 3 2024 of electrical resistivity in steel can control the total consumption of dissolved microalloys (Nb) during hot treatment. The obtained results were supplemented with APT measurements of the steel matrix. Particles that formed during cooling or isothermal exposure could be distinguished from particles caused by deformation by confirming STEM measurements with APT results, since APT specifically provided detailed information about the chemical composition of particles, as well as the distribution of elements (Figure 3, 4). а b c d e f Fig. 3. High-angle diffraction in a dark field (HADDF)-STEM images containing particles present after quenching from 1,200 °C and 950 °C: a – grain boundaries with TiN particles; b – large (approximately 80 nm) cuboid particles, which EDS identified as TiN enriched with Nb; c – smaller TiN particles (<15 nm); d – TiN with Nb nuclei; e – TiN with larger Nb nuclei are uniformly distributed inside the grain; f – particles enriched with Nb or TiNb (or Ti) (C, N) [49] Later it was discovered that Nb could slow down the recrystallization rate of austenite [48], which was associated with the introduction of controlled rolling and thermomechanical controlled treatment [49, 50]. The effect of the microalloying content (wt. %) on the recrystallization of austenite is shown in Figure 5. The C content used in HSLA steels before 1980 was 0.07–0.12 %. Meanwhile, up to 2 % Mn content was usually used together with various additives and combinations of V, Nb and Ti (max. 0.1 %) [46, 52–55]. Reducing the C content could improve weldability while maintaining strength, and it was the same as that of soft steels, but the main problem with reducing the C content was that ductility and toughness were not as good as those of quenched and tempered steels [14]. By adding microalloying elements into the steel composition, the critical temperatures of austenite transformation can be controlled in order to achieve the final mechanical properties [1, 2, 13, 14, 46, 47, 53–55]. These critical temperatures are: the grain coarsening temperature during reheating, the recrystallization temperature during hot rolling, and the transformation temperature during cooling [1, 2, 14, 46, 53, 54]. The main effects of micro-alloying elements are shown in Table [53].
RkJQdWJsaXNoZXIy MTk0ODM1