Nitrogen in Fe–Mn–Al–C–based steels

Abstract

Nitrogen and AlN solubility in Fe-Mn-Al-N, Fe-Mn-Al-C-N and Fe-Mn-Al-Ni-C-N alloys is calculated using the methods of interaction parameters and up-to-date thermodynamic data. The obtained values of balanced nitrogen solubility in these alloys are rather close to the convergence values obtained for liquid metal according to Thermo-Calc data base. Polythermic sections of the phase diagrams of these alloys are calculated and balanced residual values of dissolved nitrogen are determined for the liquidus and solidus temperatures; these temperatures are lower than nitrogen purity limited values, achieved by the modern technologies (0.0030%) for the most investigated alloys. It is possible to obtain up to 0.010% of dissolved nitrogen in the melts Fe-20%Mn-5%Al and Fe-25%Mn-5%Al at the temperatures of steel melting and alloying by nitrogen (1550-1600 °C), while in presence of carbon and with aluminium content more than 5% this value becomes lower than nitrogen purity limited value. Nitrogen content in a solid solution depends strongly on Al and C content. If Al content is 5 %, the maximal nitrogen content in a solid solution Fe-20%Mn-Al can be equal to 9.3•10-4 % at the solidus temperature. This value does not correspond to possibilities of the up-todate industrial technologies. Thereby, AlN nitrides will form in liquid metal at micro-alloying of the steels of Fe-Mn-Al-Ni-C system by nitrogen, before and during crystallization. At the temperature lower 1100 °C, practically complete whole nitrogen is presented in the form of aluminium nitrides. It is displayed experimentally that complete nitrogen content in these alloys has effect on their strength (σ max) at hot deformation towards its elevation. In the case of warm and cold deformation, the tendency of lowering of alloys strength with increase of nitrogen content in these alloys is observed.