Lisova L. Physic-chemical processes at the ESR and slags development to improve the efficiency of remelting of the high-strength steels

The thesis is devoted to the study of the peculiarities of physicochemical processes in the gas-slag-metal system inherited to the electroslag remelting (ESR). On this basis, the new slags have developed, which improve both the energy efficiency of the process and quality of the ingots from high-strength steel grades. The physicochemical interactions were analysed, and the analytic procedure (physicochemical model) of the ESR process in an inert atmosphere has formulated. According to this model the mass of metal and entire gas-slag-metal reacting system increase while remelting of the consumable electrode, and the masses of slag and gas phases remain constant that allows us to calculate the dynamics of the ESR process in a protective atmosphere. It was shown that the slag oxidises the active elements from a steel that is especially visible at the early stage (first third) of the remelting. When 20 % wt. of metal (steel 316L) remelted under the slag (oxide-fluoride composition), the MnO (0,05-0,07 %wt.) and FeO (up to 0.01 %wt.) both appear, and their contents rise till the end of the remelting. Under fluoride slag (≥ 97 % CaF2) only the content of these oxides decreases while remelting (after 30 min of melting). Based on the results of physicochemical modelling, experimental research of the slag systems properties and pilot tests the following new slags have proposed: Slag ANF-39 (% wt.) 29 – 35 CaF2 / 30 – 36 Al2O3 / 27 – 32 CaO / 2 – 4 MgO / 1 – 3 SiO2 is recommended to replace the ANF-6 slag for ESR of tool steels in stationary moulds. In comparing with ANF-6, the new ANF-39 slag provides a reduction of electricity consumption while ESR (to 15%) and does not significantly oxidise a metal (the content of oxide non-metallic inclusions is 0.5 points lower). Slag ANF-37 (wt.%): 35 – 40 CaF2 / 18 – 25 Al2O3 / 30 – 35 CaO / 10 – 16 SiO2 due to the sufficient amount of dicalcium silicate (2CaO·SiO2 - 34-37%) provides the self-disintegration of the slag skull from both the external and internal surfaces of hollow ingots. This slag is proposed for the ESR of heavyweight pipes and shells, in particular for power generation. The technical specifications for new slags were made: TU U 20.5-05416923-112: 2015 (ANF-39) and TU U 20.5-05416923-109: 2014 (ANF-37).