OBRABOTKAMETALLOV technology Vol. 26 No. 3 2024 The use of controlled cooling reduces the amount of alloying elements required, and steels can reach strengths of about 600 MPa [60–64]. Scientists continue to make efforts to develop high-strength low-alloy (HSLA) steels with a combination of high strength and high toughness. To obtain good toughness and weldability, the carbon content is reduced. The decrease in strength due to lower carbon content is compensated by the addition of Si and Mn. Further increases in strength are achieved through precipitation hardening and grain size reduction by microalloying Nb, V and Ti individually or in combination [65, 66]. It has been established that vanadium atoms in solution delay the bainite reaction at lower transformation temperatures (by 30–40 °C) within the cooling rate range of 1–50 °C/s [64–71]. Nano-sized carbonitrides are formed during prolonged soaking at 450–650 °C, which significantly increases the yield strength, preventing the movement of dislocations. Grain refinement is realized when TiN particles fix the austenite grain boundary during heating for rolling, and Nb and NbCN atoms slow down the recrystallization of deformed austenite [54]. Compared to Nb-Ti microalloyed steels, V-N steels exhibit grain refinement due to intragranular nucleation of ferrite on VN particles, partly due to the similar lattice size of VN with ferrite [14, 46]. The introduction of N into microalloyed V steel stimulates the release of V carbonitrides and increases their volume fraction. Paper [72] used the CALculation of Phase Diagrams (CALPHAD) approach to study the precipitation of nitrides and carbonitrides in pipe steels in response to new developments in complex chemical compositions and thermomechanical processing of high-strength low-alloy (HSLA) steels. This software package is based on minimizing the Gibbs free energy of individual phases in an equilibrium state. The calculation results showed that the nitride release temperature in Ti-Nb microalloyed steels increased depending on the titanium concentration, while the niobium concentration significantly increased the niobium carbonitride release temperature. Carbonitride particles form at much lower temperatures in low carbon steels (< 0.03 wt. %) than in medium carbon steels (> 0.1 wt. %). This is in good agreement with independent experimental data from the literature, where the growth of austenite grains in steels of similar compositions was studied. Although dissolution and particle growth are controlled by process kinetics, these results proved that thermodynamic calculations can effectively predict the composition and sequence of particle formation in chemically complex systems, allowing for more accurate design of experiments to determine critical temperatures for grain coarsening during reheating, heating recrystallization, rolling, and transformation upon cooling. This can minimize the amount of testing required to obtain optimal chemical compositions and heat treatment procedures where austenite grain growth in steel of similar composition has been studied. In [73, 74], to study the microsegregation phenomena and behavior of complex particles (Ti,Nb)(C,N) during continuous casting, a unidirectional solidification setup was used to simulate the crystallization process. In the specimens studied by the authors, a dendritic structure was detected along the direction of solidification. This shows that the addition of titanium, niobium to high strength low alloy (HSLA) steel results in unwanted (Ti,Nb) (C,N) precipitates due to microsegregation. The effect of cooling rate on the formation of (Ti,Nb) (C,N) was investigated. The composition of large particles was determined using FE-SEM with EDS. Large particles (Ti,Nb) (C,N) can be divided into three types based on composition and morphology. As the cooling rate increases, Ti (Ti,Nb) (C,N) particles transform into Nb (Ti,Nb) (C,N) particles. It is noted in [75] that if small Nb(C,N) and NbC particles containing niobium have a diameter of the order of several nanometers, usually ≤ 50 nm, then large particles containing niobium can have a length from submicron to hundreds of micrometers. The formationmechanismof ≤ 50 nmniobiumcarbide or carbonitride particles is well known, and its beneficial effects on strength and toughness are well documented. Whereas, large Nb particles worsen the characteristics of steel. Despite numerous studies of large particles with a high Nb content, no experimental evidence has been proposed for the alleged mechanisms of its formation. Defects associated with large Nb-rich particles include cracking of slabs during reheating, failures in tensile tests, hydrogen cracking problems, and increased screening under ultrasonic testing. Thework [54] schematically shows the roleof niobiumfor HSLA steels in theprocess of thermomechanical processing (Figure 7).
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