OBRABOTKAMETALLOV technology Vol. 26 No. 3 2024 In contrast, normal dynamic recrystallization (i.e. dDRX) occurs during deformation as long as the temperature is above about 0.5 Tmelting. New grains appear during nucleation deformation and then completely replace the original microstructure at high deformations (Figure 16). As in the case of SRX (Figure 14), there is a gradual transformation of subgrains, formed mainly near the grain boundaries. These dynamic processes, which include a nucleation step, are similar to those that occur during dSRX and are sometimes referred to as discontinuous dynamic recrystallization (dDRX) [101]. It has been established that the dynamic mechanism differs significantly from the mechanism of static recrystallization. The latter leads to the formation of a homogeneous and dislocation-free grain structure. As a result, dDRX grains initially have wavy boundaries and contain dislocation substructures that vary from grain to grain [101]. Even after dDRX has fully developed, regions containing substructures continue to be present, unlike materials that have undergone dSRX. Dynamic recrystallization controlled rolling involves initiating dynamic recrystallization in one or more passes during the rolling process. It is typical for the rolling of wire and rods on continuous rolling lines, as well as the rolling of strips and seamless pipes [97]. This can be achieved by applying large strain in a single pass or by accumulating strain over several separate passes. In both methods, a critical strain is required to initiate dynamic recrystallization. The final grain size of ferrite can reach 1–2μm [1–3, 46, 47, 51–58, 84–99]. The analysis of literature sources shows that the traditional way (until the 1970s) to fine-grained structural steels with ferrite-perlite structure (FP) consisted in the inclusion of grain-refining elements such as aluminum, and then in the normalization of materials at a temperature of about 920 °C after rolling [1–3, 12–22, 45, 46, 52–56]. The author [14, 55] notes that “when steel treated with niobium was normalized to improve impact properties, the strength advantage was lost.” Thus, there was a need for an alternative route to fine grain structural steel sheet. One of the problems associated with high-strength low-alloy (HSLA) steels is the complex interaction of its strengthening mechanisms, which makes it difficult to optimize its manufacturing parameters. The chemical composition of the steel preliminarily determines the constituent phases in the microstructure. The matrix component can be austenitic, ferritic, pearlitic, martensitic or bainitic, which is a critical factor in the grain refinement process due to differences in crystal structure, microstructural configuration, defects, stacking fault energy (SFE), deformation and annealing. On the other hand, the TMCR temperature promotes the release of microalloying elements [48, 49, 50, 51, 52]. In 2016, the authors of [93, 94] reported on high-strength low-alloy (HSLA) steel.At a TMCR temperature of 579 °C, the reported yield strength (YS) was in the range of 701–728 MPa, the tensile strength was 996– 997 MPa, and the elongation was 21–23 %. At TMCR temperature of 621 °C, the yield strength, tensile strength and elongation were in the range of 749–821 MPa, 821–876 MPa and 19–25%, respectively. Since the mid-1960s, steel mills began to produce fine-grained structural steels by reducing the final rolling temperature [85–105]. The basic idea was to improve the strength and toughness characteristics of structural steels by grain refinement. Compared to conventional hot rolling at high rolling temperatures, new steels were rolled at a lower final rolling temperature. It has been established that repeated recrystallization of austenitic structures leads to a decrease in grain size, but there is a limit that is difficult to overcome. Deformation at temperatures at which recrystallization does not occur was effective in deforming austenite, which had a dense population of slip planes, a high dislocation density, and a high intrinsic energy, which provided a high nucleation density for the austenite transformation products. Initially, ferrite-pearlite microstructures were primarily considered, and then the role of rapid cooling became an additional opportunity to increase the level of strength. Higher cooling rates or greater undercooling increase the driving force, and with a lower diffusion coefficient, finer microstructures such as bainite and martensite can be achieved. A comparison of the contribution of the hardening mechanism in industrial hot-rolled structural steel with that in fine-grained structural steel is shown in Figure 17 [103].
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