EFFECTS OF STEEL - APPLIED FOR LARGE-DIMENSION CASTINGS FOR THE POWER ENGINEERING - REFINING IN THE LADLE-FURNACE

The changes of a sulphur content during refining in melting low-alloy and high-alloy steels (G17CrMoV5-10; GX12CrMoNiVNbN9-1) applied for large-dimension castings for the power engineering are presented in the hereby paper. The investigated steel was melted in the oxygen-recovery melting technology with an application of maximum 70% of the process scrap. In addition, after steel melting in the electric arc furnace (EAF), the secondary metallurgy was performed in the ladle furnace (LF). It was shown that the application of the secondary metallurgy by a synthetic slag in the ladle furnace and argon bubbling of a metal bath leads to obtaining in the final analysis: 0.0043-0.0046% of sulphur (a decrease of S content during refining in LF reached 40%). Current measurements of FeO in the slag and maintaining its content below 0.8%, support obtaining such low sulphur content in steel. So low level of the slag oxidizing is one of the necessary conditions for a deep desulphurisation of the metal bath. Without the secondary metallurgy the sulphur content in low-alloy cast steel was 0.007%, while 0.01% in high-alloy cast steel. Controlling of the gas (oxygen, nitrogen) content during steel melting and correcting the amount of additions (e. g. deoxidants), allowed to obtain the low oxygen content (below 45 ppm for two investigated steel grades) and nitrogen content (88 ppm for low-alloy steel and 330 ppm for high-alloy steel), which warrants a good combination of strength and plastic properties.