Effect of the Slag Composition and a Protective Atmosphere on Chemical Reactions and Non‐Metallic Inclusions during Electro‐Slag Remelting of a Hot‐Work Tool Steel

Abstract

The remelting behavior of the hot-work tool steel X37CrMoV5-1 is investigated with several experimental melts on a lab-scale ESR-plant. The investigated parameters comprise a variation of the slag compositions and the use of a protective nitrogen atmosphere. Variations of the slag composition include slags with different contents of CaF 2 , CaO, and Al 2 O 3 as well as a variation of the SiO 2content in the slag. The remelted ingots are forged and analyzed regarding their chemical composition. The distribution and composition of the non-metallic inclusions (NMI) is studied by an automated SEM-EDX method. Additionally, the chemical composition of the slag after remelting is analyzed. The results show clearly an equilibrium reaction between Si and Al in the steel with SiO 2 and Al 2 O 3 in the slag as well as the effect of oxygen in open ESR operation. A protective atmosphere reduces the Si-losses during remelting, but has no major effect on the number or composition of NMI compared to open remelting. The content of NMI, especially the larger ones, is reduced significantly in all remelting experiments. The majority of the NMI are MA-spinel type except for the CaO-free slag, where alumina inclusions prevail. In general, remelting leads to an almost complete removal of sulfides, a reduction of oxisulfides, and a slight increase of oxides.

Deoxidation of Electroslag Remelting (ESR) – A Review

“…However, there are controversial findings on whether the oxygen content in the remelted ingot is dependent on the original oxygen content of the steel electrode or not. 1,14,16,18,20,[38][39][40][41][42][43][44] Plöckinger 1) claimed that the oxygen content level was dependent only on the reactions between slag and liquid steel, and not concerned with the initial oxygen content of steel electrode. In recent decades, the oxygen content in the steel used as the electrode for ESR could be reduced to a low level because of the improvement of steelmaking technologies.…”

“…A low initial oxygen content of the steel electrode (13 ppm) was reduced to 8 ppm-12 ppm after open air atmosphere ESR in the case where the FeO content in the slag was at a low level (0.1-0.2 mass%). 43) It is really a tough case to lower the oxygen content to a lower level after ESR of low oxygen steel. In order to further lower the oxygen during ESR of the steel with low oxygen content, the strategic point is to prevent the oxygen pickup during the ESR process, in which the oxygen potential of slag is the key source.…”

“…In general, a majority of the original oxide inclusions in the steel electrode are removed during the ESR process, especially a preferential elimination of large inclusions. 6,43,44) Apart from some types of original oxide inclusions in the steel electrode (for example, MnO-SiO 2 -Al 2 O 3 inclusions 45) ), other types of the original oxide inclusions could partially survive from the steel electrode to ESR ingot. 46,47) The factors, which affect the oxide inclusion evolution during ESR, greatly influence the oxygen content of ESR ingot.…”

“…The ESR practice conducted in open air atmosphere normally leads to higher FeO content in the slag than the remelting in protective atmosphere, 43) which results in a higher oxygen content of liquid steel. For protective atmosphere ESR, inert gas is introduced into the gas protective cap of protective atmosphere ESR equipment to protect the remelting atmosphere against the surrounding air.…”

“…The laboratory-scale ESR trials with a frequency of 4.5 Hz show that even though the ESR trials of the low oxygen steel electrode are performed in open air atmosphere, the oxygen content is reduced, instead increased, from 13 ppm in the steel electrode to 8 ppm-12 ppm in the ingots of 165 mm in diameter because of a low FeO content in the slag (0.1-0.2 mass%). 43) Continuous or periodic addition of deoxidizing agents into the slag pool is widely used for deoxidation of ESR. 4,5,7,8,12,38,[74][75][76] The main function of deoxidizing agents addition during the ESR process is to reduce the concentration of FeO in the slag.…”