Introduction
Super duplex steel 03Cr23Ni6Mo4Cu3NbN (SDS) is a new development of JSC “NPO ‘TSNIITMASH’, which has high corrosion and strength characteristics which are in demand in the oil and gas industries, as well as in chemical industry and nuclear power engineering. The combination of these characteristics is ensured by uniform distribution of austenite and ferrite. Special attention is paid to the technology of ingots and forgings manufacturing, prevention of formation and growth of σ-phase, and other secondary phases which negatively affect the mechanical and operational properties of steel. The influence of chemical composition and technological parameters, including ingot solidification and forgings cooling rates, on structure formation is investigated using computer modeling and experimental studies.

Methods
- Modeling of phase transformations of 1000 chemical compositions is carried out using the program complex Thermo-Calc version 2022a (with PRISMA) using databases TCFE11, MOBFE6.
- Production of 03Cr23Ni6Mo4Cu3NbN steel samples takes place using electroslag remelting (ESR) with the provision of increased crystallization rates. The cooling rates from 2.5 to 42 ℃/s are realized when the forgings are quenched.
- The chemical composition, microstructure, and impact toughness of steel are determined.

Results
The influence of alloying elements and cooling rate of forgings on the kinetics of σ-phase formation is described on the basis of computer modeling. Parametric models approximating the results of thermodynamic and kinetic calculations are developed. The models allow us to calculate the fraction of σ-phase at given cooling rates to estimate the value of impact toughness of steel.

Conclusions
On the basis of regression analysis, the influence of nitrogen, molybdenum, nickel, copper and carbon on the value of critical cooling rate has been established.
The equation for calculation of impact toughness depending on the volume fraction of σ-phase has been proposed.
The possibility of preventing the growth of complex nitrides of CrNbN system, negatively affecting the impact toughness, by controlling the conditions of solidification at ESR has been established.