OBRABOTKAMETALLOV Vol. 26 No. 3 2024 technology The above analysis of the literature shows that careful selection of the chemical composition of steel in combination with an appropriate thermomechanical scheme makes it possible to obtain a wide range of microstructures, from classic combinations of ferrite and pearlite to more advanced bainite phases with an optimal balance of mechanical properties. The accumulation of strain in the austenite is enhanced and consequently the grain sizes in the final microstructures are reduced. The presence of Mo promotes the presence of non-polygonal phases, and this constituent modification causes an increase in strength due to the formation of substructure, as well as due to an increase in dislocation density [76–80]. Research in the field of microalloyed steels has expanded over the decades, and the focus has been on improving its strength and environmental resistance through microstructure control. Advances in desulfurization are important because it helps control microstructure. Over the years, the sulfur content in microalloyed steels has been reduced, allowing the toughness of the steels to continually improve (Figure 8). Sulfide control is believed to improve the toughness of micro-alloyed steels [77]. In [52], the authors believe that 1980 represents the starting point for the strength of microalloyed steels. From the early 1960s until about 1980, microalloyed steels were lowhardenability steels with a ferrite + pearlite microstructure and a tensile strength ≤ 420 MPa. The obvious choice for solving this problem was the products of low-temperature transformation: matrices consisting of bainite and martensite. This was achieved in the mid-1980s for treating steels using the Intermittent Accelerated Cooling (IAC) and Intermittent Direct Quenching Fig. 7. The role of niobium in HSLA steels in the thermomechanical processing process Fig. 8. The effect of sulfur content on the impact strength of pipes [53]
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