Abstract
Advanced high-strength steels (AHSS) are sophisticated materials being developed by the steel industry to mitigate challenges related to the performance of motor vehicles. To meet the requirements of AHSS, the ferromanganese alloys (FeMn) utilized in the production of the steel are required to contain acceptable levels of unwanted impurities, i.e. P, S, N, H, and C. The focus of the current study was to investigate dephosphorization of ferromanganese to produce a low-P alloy that could be effectively utilized in the production of AHSS. The study involved conducting laboratory-scale testwork to study the efficiency of CaO-based slag systems to dephosphorize FeMn alloys. The addition of Na2O, CaF2, and BaO to MnO-CaO-SiO2 slag was considered. The test work was carried out in a 25 kW induction furnace at temperatures of 1350°C, 1400°C, and 1450°C. The P partition coefficient (Lp) remained small at <1, which is an indication that dephosphorization had not been achieved. The baseline slag, comprising 40%CaO-40%SiO2-20%MnO, reported higher Lpvalues. Addition of Na2O and CaF2 did not show any further benefit. Substituting half of the CaO by BaO, resulted in similar Lpvalues to those of the baseline slag under conditions of 1350°C and 1450°C at 30 minutes. In summary, based on the Lpvalues 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.
Highlights
In the automotive industry the drive towards lightweight, high-strength steel grades to mitigate the challenges around the escalating energy crisis and environmental problems is a priority
The study involved conducting preliminary thermochemical FactSage calculations followed by experimental laboratory tests to investigate the dephosphorization of South African FeMn alloys by different synthetic CaO-based slag systems, similar to the systems applied in dephosphorization of iron
The calculations generally predict that dephosphorization of both alloys will not be possible with the CaO slags as the Lp values obtained were well below unity
Summary
In the automotive industry the drive towards lightweight, high-strength steel grades to mitigate the challenges around the escalating energy crisis and environmental problems is a priority. It has been estimated that a 10% weight reduction in automobiles would reduce fuel consumption by between 3% and 7% (Demiri, 2013). This has led to the development of advanced high-strength steels (AHSS) (Baluch, Udin, and Abdullah, 2014). Based on the above expression, it can be deduced that the removal of P from the alloy can be aided by the following (Chaudary and Goel, 1994):. ➤ Higher oxygen activity in the alloy ➤ High activity of basic oxide (aO2–) in the slag ➤ Low activity coefficient of phosphate (γPO43–) in the slag ➤ High K values, which can be achieved at low temperatures since the reaction is of exothermic nature. As indicated by Equation [1], high concentrations of O2–, and basic slags, are efficient for dephosphorization (Wagner, 1975)
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