Problematika výroby vysoce legovaných ocelí určených pro energetiku a petrochemický průmysl

Abstract

High alloyed steels are widely used in energy and petrochemical industry. The requirements put on these steels for conventional and nuclear power engineering include in particular achieving of strength properties at increased temperatures, a sufficient level of fracture toughness, good weldability, sufficient corrosion resistance, structural stability and resistance to radiation damage. Materials used for the petrochemical industry must resist for corrosive impact of oil, but also the influence of salty seawater. Further, these steels must have a higher strength, high resistance to intergranular corrosion and reduce tendency to stress corrosion cracking. In both cases, nitrogen is a very important alloying element. Higher nitrogen content supports the precipitation of VN particles, which leads to increase creep resistance of steel containing 9 – 12% wt. chromium. In case of two-phase, duplex stainless steel, nitrogen influences the phase composition of steel, mechanical properties and increases the value of pitting resistance equivalent. This PhD thesis concerns the problems of increasing the nitrogen content in the steel during the production of high alloyed steels. Nitrogen alloying of steel using only nitrogen gas is described and evaluated. Numerical simulations of casting and solidification of steel ingots as well as determination of temperature of liquidus and solidus by using differential thermal analysis of selected high alloyed steels were used to improve the internal quality of ingots. Finally, the development of the production technology of a new super duplex stainless steel grade in VÍTKOVICE HEAVY MACHINERY a.s. is described in experimental part of this thesis. Very high content of nitrogen, 2 450 ppm, was achieved only by using nitrogen gas. Furthermore, temperature of liquidus and solidus of super duplex stainless steel were experimentally determined and computer model for monitoring and control the steel production process was developed.