OBRABOTKAMETALLOV technology Vol. 26 No. 3 2024 (IDQ) methods (Figure 9). The benefits of higher cooling rates and lower coiling temperatures were used to achieve higher strength later with lower carbon steels. Numerous studies carried out during the period 1956–1980 related to the production of microalloyed steel and hot rolling indicated that the latter was to become the main route of steel production. In a short time this led to the introduction of computer control and modeling to produce small, less than 10 µm, uniformly distributed ferrite grains (Figure 9). Fig. 9. Schematic diagram of thermomechanically controlled processing (TMCP) and microstructures resulting from this process [14] Figure 10 schematically shows how the microstructure and properties of steels have changed over time due to advances in steel development and processing. The chemical composition of the steel and the temperature at which rolling is carried out are critical factors affecting the microstructure, phase composition and texture development of steels. The thermomechanical control process is an effective method that combines controlled rolling and controlled cooling technology to obtain superior comprehensive mechanical properties by controlling the formation of microstructure during deformation [1–3, 12–22]. Due to the low carbon content and micro-alloying combined with various thermomechanical controlled rolling (TMCR) conditions, the microstructure of high-strength pipe steel generally contains various microstructural components such as polygonal ferrite (PF), quasipolygonal ferrite (QF), acicular ferrite (AF), bainite and martensitic-austenitic (MA) components, forming a complex mixed microstructure with different characteristics. The addition of micro-alloying elements such as Mn, Mo, Cr, Ni, V, Nb and Ti can help obtain ideal microstructure and mechanical properties [45, 46, 52–56]. Fig. 10. Development of pipe steels using the example of HSLA steel research: TMCP – thermomechanical controlled rolling; ACC – accelerated cooling; DQ: direct quenching [14]
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