Please choose your delivery country and your customer group
A new scries of corrosion resistant C+N TWIP steels were developed, produced and studied. The base composition is Fe-(20-30)Mn-12Cr-(0.56-0.7)CN. The alloy concept was supported by the CALPHAD method. The production of the developed steels was carried out in conventional furnaces at atmospheric pressure. Defect-free castings were produced. The materials were diffusion annealed, hot worked, solution annealed and water quenched. Fully austenitic materials were obtained, with 80-100% elongation, 400-470 MPa yield strength and 1500-1650 MPa true fracture strength. The ISO-V impact toughness is around 300 J at room temperature, and the impact wear resistance of the developed steels is as good as that of Hadfield steel. The mechanical properties are directly related to the austenitic structure of the material. C+N alloying enhances the short range atomic ordering, stabilizing the structure. A theoretical relation between ordering and entropy was used analyzing the influence of the carbon to nitrogen ratio on the austenite stability. Another property related to the mechanical response of the developed steels is the stacking fault energy, which is influenced by the element contents, mainly Mn, N, and C and depends on the electronic and magnetic properties. The SFE of the developed steels was in the order of 31-41 mJ/m2. The microscopic characterization evidenced twin induced plasticity, being responsible for the cold work hardening of the materials. The uniform and pitting corrosion resistance of the developed steels is superior to the usual FeMnC and FeMnAlSi TWIP steels in acidic media and in chlorine containing solutions. Additionally to the alloy development, austenite decomposition was studied. It comes with the formation of M23 C6 and M2N precipitates. Thermodynamic calculations based on homogeneous nucleation were used to determine the driving force for precipitation, the composition of the precipitate nucleus and the critical radius at different temperatures. Simulations of isothermal holding of a precipitate nucleus at temperatures below 900 °C allow to distinguish that the growth of the carbides has three different stages, namely latent period, exponential growth and equilibrium, reflected by the size of the particle and the activities of C and N.