| Abstract: | With the objective of studying the effect of vanadium and nitrogen microalloying on microstructure and strength of low carbon steels with different manganese contents, three series of low carbon steels (0.1% C) with manganese content (between 0.8 and 3.5%), vanadium content (up to 0.17%) and nitrogen content (up to 0.025%) have been designed and investigated in the hot forging condition using a preheating and finish forging temperatures of 1200 and 950°C, respectively. Steels with a manganese content up to 2.3% revealed ferrite-pearlite structures, whereas higher manganese contents from 2.7 to 3.5% resulted in the formation of bainitic structures. A pronounced effect of manganese on the mechanical properties of steels was detected at lower manganese contents < 1.5%, due to solid solution and grain refining effects, and higher manganese contents > 2.3, due to bainite formation. Manganese content in the range of 1.5-2.3% had less pronounced effect due to solely solid solution hardening. Vanadium microalloying effectively increased the strength of steels through solely precipitation strengthening or both precipitation strengthening and grain refining effect. The effectiveness of vanadium was greatly enhanced by increasing the nitrogen content. The grain refinement of vanadium-nitrogen microalloying seems to be due to inhibition of austenite grain growth as a result of precipitation of vanadium nitride in austenite during forging. Precipitation strengthening of these steels is achieved by precipitation of vanadium carbide and nitride in ferrite or bainite. Nitrogen enhanced the precipitation strengthening of vanadium microalloyed steels which could be attributed to the finer vanadium nitride dispersion precipitates compared with vanadium carbide. Up to 70% of the total nitrogen content of steel precipitates as vanadium nitride which could be achieved with V/N ratio of about 6-7. Microalloying of low carbon-manganese steels (0.1% C and 1.8% Mn) with 0.15% vanadium and 0.025% nitrogen was found to be effective in attaining high levels of yield and ultimate tensile strengths of 835 and 940 N/mm2, respectively in the forging condition. |
