OBRABOTKAMETALLOV Vol. 26 No. 3 2024 technology Research results of various authors and discussion Chemical composition of microalloyed steels In 1936, it was discovered that microalloying additions of metal niobium could strengthen “soft” steels, although the main strengthening mechanism could not be identified at that time. Steels containing a small amount of vanadium or titanium have been available for a long time, the rapid development and use of microalloyed steels was initiated by the recognition of the advantages of adding a small amount of niobium in C-Mn steels [13–21]. This happened in 1958 with the first successful production of niobium-treated steel by Great Lakes Steel Corporation in the USA [22–28, 46]. Various factors contributed to this development, including the relatively affordable availability of ferroniobium in the late 1950s and the discovery of very large deposits of ores containing niobium in Brazil and Canada at that time, which guaranteed the stability of future supplies and prices. The advantage of grain refinement effects due to the release of microalloyed elements in the presence of N and C was well known [13–16, 29–39, 46]. Steels used up to 1980 are characterized by the use of air cooling of the sheet and high strip winding temperatures. As already noted [1–3, 43–46], these are steels with a ferrite-pearlite structure with a strength of up to ~420 MPa. The most obvious factor affecting strength was grain refinement. The release of carbides and nitrides occurs at 3 stages of processing of micro-alloyed steels. Type 1 particles are formed in the liquid phase and during or after solidification at the liquid/solid interface and in δ-ferrite [14]. Type 2 particles are deposited in austenite during hot deformation, such as controlled rolling, as the temperature decreases [14]. Type 3 particles are formed during or after the phase transformation of austenite into ferrite, originating at the austenite/ferrite interface and in ferrite [13–17]. Electron microscope image of the films (Figure 2) showed that these particles, with sizes up to 150 nm, are mainly located in the form of ribbons along the boundaries of austenitic grains or in former inter-dendritic regions [47–48]. In [49], various methods were used to track the release of microalloys after modeling various conditions of thermomechanical controlled rolling (TMCR) of austenite in the Gleeble thermomechanical simulator. Atom probe tomography (APT), scanning transmission electron microscopy using focused ion beam (STEM-on-FIB), and electrical resistivity measurements provided additional information about the state of the particles and were correlated with each other. It has been demonstrated that accurate measurements a b Fig. 2. TEM micrographs of extraction of predominantly TiN particles: a – probably particles formed during solidification; b – particles of NbC grown on the TiN core [48]
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