Investigation into the dephosphorization of ferromanganese alloys for production of advanced high strength steels

Abstract

The focus of the current study was to investigate dephosphorization of ferromanganese to produce a low P containing alloy that could effectively be used for the production of AHSS. The study involved conducting laboratory scale testwork to investigate dephosphorization of of FeMn (HCFeMn and MCFeMn) alloys by CaO-based slag systems. Addition of Na2O, BaO, and CaF2 to MnO-CaO-SiO2 slag was investigated to study the influence on dephosphorization. The effect of MnO-BaO-BaF2 slag system without CaO was also investigated in a preliminary way. The testwork was carried out in a 25 kVA induction furnace at temperatures of 1350°C, 1400°C, and 1450°C at different retention times of 5 minutes, 30 minutes and 60 minutes. The analytical results of the product samples were used to evaluate the P-partition ratio.

In general, analysing the low P contents of the slags and alloys proved to be a challenge and at the lowest concentrations the uncertainty was large. The analytical results of the slags generally reported higher MnO-contents than was anticipated. This resulted in dilution of the other major slag components, i.e. SiO2 and CaO. Low P2O5 contents were found in the slags, suggesting that the removal of P from the alloys was minimal. The alloy results reported C-pickup after the tests due to the dissolution of the graphite from the crucible. Loss of Mn from the alloys was also observed. The P-content of the alloys were generally higher than in the feed alloy.

The results generally showed the 〖 L〗_p remained small at <1 which is an indication that dephosphorization had not been achieved. The baseline slag comprising of 40%CaO-40%SiO2-20%MnO reported relatively higher 〖 L〗_p values. Addition of Na2O and CaF2 did not show any added benefit. Substituting half of CaO by BaO, resulted in similar L_p values to those of the baseline slag under a few conditions, namely 1350°C and 1450°C at 30 minutes. The baseline slag was however not outperformed by the BaO-bearing slag under any of the other conditions. Increasing the temperature generally resulted in lower 〖 L〗_p values. This may be attributed to the exothermic nature of the phosphorus removal reaction which should be favourable at lower temperatures. Increasing the basicity (%CaO/%SiO2 ratio) of the baseline slag showed an initial increase in 〖 L〗_p value for basicities of 0.7 to 0.9, thereafter a slight reduction in the 〖 L〗_p value was obtained. The latter results were not expected as higher basicity is anticipated to improve the P capacity of the slag. Increasing the basicity resulted in increased slag liquidus temperature of about 1500°C which negatively affected dephosphorization. In summary, based on the 〖 L〗_p obtained, the conditions investigated with the CaO-based slags appeared to have been unfavourable for dephosphorization of FeMn alloys, as most of this impurity element remained in the alloy. The BaO-BaF2-MnO slag showed potential to dephosphorise HCFeMn alloy, however the slag posed numerous challenges and the slag should be investigated further