Features of the crystal structure of cast СrMnCN stainless steel before and after low-temperature deformation

Abstract: Problem. Cold-resistant chromium-nickel stainless steels possess low strength properties. Their replacement requires using steels with enhanced strength characteristics and lower cost. Steels alloyed with nitrogen and carbon meet these requirements; however, their crystal structure at low temperatures has been insufficiently studied. Aim. To determine the influence of low temperatures and low-temperature deformation on the crystal structure parameters of a nickel-free austenitic steel with a high content of interstitial elements. Methods. The crystal structure of cast CrMnCN steel with a (С+N) content of 1.27 wt. % was investigated by X-ray diffraction methods in the temperature range from −190 to 20 °C. The steel was preliminarily homogenised at 1000 °С, then heated to 1150 °С and quenched in water. Tensile tests were carried out at temperatures from −196 to 20 °С. After testing, the crystal structure was analysed. Results. After quenching, the dendritic structure of the steel consists of austenite with a lattice parameter а=3.6405 Å and clusters of dispersed particles. Upon cooling of the steel, the FCC lattice parameter decreased, while in specimens tested in tension, it increased as the test temperature decreased. At cryogenic test temperatures, the crystal lattice exhibits auxetic behaviour. During cooling and low-temperature deformation, stacking faults (SFs) form in the steel. After testing at −196 °С, the SF concentration is twice as high as in steel cooled to the same temperature. The combination of low ductility of the steel at −196 °С, a small amount of plastic deformation work, and a high dispersion of coherent scattering regions (CSRs) indicates the onset of martensitic transformations. Conclusions. Upon cooling of the steel, the lattice parameter decreases. Conversely, during low-temperature deformation, the FCC lattice parameter increases, and at −196 °С the steel exhibits auxetic behaviour. A feature of the response of the crystal structure to low-temperature deformation is an increase in the SF concentration and a small CSR size.