OBRABOTKAMETALLOV technology Vol. 26 No. 3 2024 Operating principle of microalloying elements [53] Element wt. % Effect С <0.25 Strengthening Mn 0.5–2.0 Slows down the decay of austenite during accelerated cooling Reduces the transition temperature from viscous to brittle fracture. Si 0.1–0.5 Deoxidizer in molten steel. Solid-solution strengthening. Stabilizes austenite Аl <0.02 Deoxidizer Nb 0.02–0.06 It strengthens ferrite very strongly in the form of niobium carbides/nitrides Тi 0–0.06 Austenite grain control by titanium nitrides. Strong ferrite strengthener V 0–0.10 Strong ferrite strengthener with vanadium carbonitrides N <0.012 Detrimental impurity Мо 0–0.3 Promotes the formation of bainite. Increases the strength of ferrite Ni 0–0.5 Increases the fracture toughness Cu 0–0.55 Improves corrosion resistance Cr 0–1.25 Increases resistance to atmospheric corrosion in the presence of copper B 0.0005 Increases hardenability S <0.05 Detrimental impurity P <0.012 Detrimental impurity Carbon in steel is necessary to obtain the required strength of steel, while strengthening is ensured by the formation of perlite in the steel structure. Manganese is used to strengthen a solid solution and obtain the required steel strength. It is known that the presence of manganese in steel leads to a shift of γ→α transformation to a region of lower temperatures, which leads to grain refinement and the formation of ferrite with an increased dislocation density and, as a result, to an increase in the yield strength of steel. To limit the values of the yield strength in rolled products and ensure that the required yield strength values are obtained in the base metal of pipes after pipe processing, as well as to ensure satisfactory weldability, the manganese content is limited to 1.0–1.5 %. Silicon is used to harden a solid solution and ensure the required strength of steel. Besides, the addition of silicon is necessary for deoxidation of steel during smelting. In accordance with this, the minimum silicon content in steel should be at least 0.15 %. With high silicon content, the number of silicate inclusions increases, which leads to a deterioration in impact strength, thus, the maximum silicon content is limited to 0.80 % to prevent embrittlement of steel. The addition of aluminum is necessary for deoxidation and modification of steel. At the same time, the minimum sufficient aluminum content is 0.02 %. When the aluminum content is more than 0.06 %, the impact energy decreases. Chromium, nickel, and copper are added into steel to increase strength properties, as well as to stabilize the structure when heating the metal for rolling and reducing the grain size during rough rolling. In addition, at maximum concentrations of no more than 0.08 %, these elements in steel have a positive effect on the corrosion resistance of pipes. Titanium in steel is necessary for the binding of nitrogen into TiN nitrides, which inhibit grain growth when steel is heated, which contributes to grain grinding. The nitrogen content in steel is limited to 0.012 %, since the presence of free nitrogen in a solid solution of the α-phase has a negative effect on the operation of the impact during the impact-bending test.
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