Ironmaking & Steelmaking: Processes,Products and Applications

Hard inclusions with high melting temperatures such as Al₂O₃ (2054°C) and MgO·Al₂O₃ (2135°C) generate nozzle blockage problems during continuous casting of Al-killed valve spring steel and are very detrimental with respect to fatigue properties. In the present paper, inclusion modification in Al-killed valve spring steel by Na₂CO₃ addition was investigated in the laboratory using a graphite tube resistance furnace. The results show that inclusions with high melting temperature can be successfully modified into Na₂O-containing inclusions with lower melting temperatures by the addition of Na₂CO₃. The effectiveness of inclusion modification can be enhanced by increasing the Na₂CO₃ addition and/or decreasing the amount of Al. This suggests that Na₂CO₃ addition could possibly be a substitute for Ca treatment as a method for preventing nozzle blockage during continuous casting of Al-killed steel.

Substitution of pulverised coal injection (PCI) by solid biocarbon fuel has the potential to achieve substantial reduction in GHG emissions associated with blast furnace ironmaking. A systematic evaluation was conducted on the performance of solid biocarbons produced from a single raw biomass source using different pyrolysis technologies. A techno-economic model was developed to evaluate the value-in-use (VIU) of the prepared solid biocarbon in blast furnace ironmaking. The VIU of solid biocarbon is strongly influent by its O/C (oxygen to carbon) mass ratio which is determined by the pyrolysis technology and conditions employed. It also dictates the cost of raw materials required to support the blast furnace ironmaking process and the potential GHG emissions achievable. In order to balance all factors that may affect the VIU of solid biocarbon, close collaboration between steelmakers and solid biocarbon producers is critical for producing suitable solid biocarbon fuel to replace PCI.

The decarburisation process is studied with the help of mathematical modelling of RH degasser with reaction interface area approach, considering multi-component mixed phase mass transport theory. An algorithm is developed by considering Ar gas, bath surface and CO gas bubbles as the reaction sites for decarburisation process. On the basis of this, the model is developed using MATLAB. The model is tested with five sets of data which are obtained from JSW Steel Plant Ltd. The results obtained from the model have been compared with the industrial data as well as the data obtained from literature survey. It is shown that the nitrogen and hydrogen removal are triggered more for higher CO evolution rate. The relations between carbon removal and factors like area of interface, time and vacuum pressure are proposed.

A full-scale finite-element stress model of a slab and its mould was developed to analyse the non-uniform slab shrinkage of special steel in a vertical caster during slab continuous casting. The stress model was based on the heat transfer model which was validated by comparing the calculated temperature with the measured ones. According to the shrinkage calculation, the single linear taper is not suitable for the continuous casting of this special steel, and the narrow face taper need to be calculated and optimised. It will provide a helpful tool for further improving the casting parameters and operations for special steel.

The effect of CaO on the reduction behaviour of iron ore–coal composite pellets has been studied in a laboratory scale multi-layer bed rotary hearth furnace at 1250°C for 20 min. Reduced pellets have been characterised through weight loss, porosity measurement, phase analysis by XRD, and morphology study by SEM. The addition of CaO to the composite pellets showed different effects at different carbon levels. For higher carbon-containing pellets (C/Fe₂O₃ molar ratio at the upper stoichiometric level of 3), the addition of CaO increased the extent of reduction for all three layers significantly up to a certain limit (4 wt-%); and thereafter the degree of reduction is decreased with a further increase in CaO percentage in the pellets. For low carbon-containing pellets (C/Fe₂O₃ molar ratio of 1.66), the addition of CaO to the pellets did not show any beneficial effect.

Flue gas circulation is an important method for energy conservation and pollutant emission reduction in iron ore sintering. In this paper the effects of flue gas recirculation ratio on sintering of different iron ores including haematite, magnetite and limonite were studied by illustrating the variation of sinter bed temperature, atmosphere and mineralisation characteristics of different types of iron ores induced by the circulation. It shows that the proper flue gas circulation ratios for haematite, magnetite and limonite are 37, 30 and 25%, respectively. For magnetite ore, preheating and high consumption of oxygen in combustion zone caused more silicate minerals and less acicular calcium ferrite, thereby lowering sinter tumbler strength. As for haematite ore, the rapid change of temperatures of combustion, melting and solidification zones leads to elevated combustion efficiency and increased formation of acicular calcium ferrite, which enhances the sinter strength. When using limonite ore as the main raw material, high oxygen consumption, lower maximum temperature of sintering bed, higher cooling rate and larger porosity of sinter are observed.

The ‘Petrurgic process’ has become a prominent process to address the energy saving issue in the glass ceramic-making process. If the basic oxygen furnace (BOF) slag could be utilised by this process, the environmental problem caused by dumping the waste without recycling might also be resolved effectively. However, it is difficult to directly use BOF slag itself as a valuable material because of high melting point and large amount of iron oxide. Therefore, the current research was focused on elucidating crystallisation behaviour of the iron oxide-devoid BOF slag melt during cooling process with the intention of using the BOF slag to the Petrurgic process. The slag melt showed two crystallisation processes during cooling: the eutectic reaction at the early stage and the diffusion process at the later stage. In addition, the crystallisation mechanism was similar throughout the cooling rate, although the cooling rate influenced the morphology of the crystallisation product.

The inclusion evolutions after calcium treatment in Al-killed steel with different sulphur content were determined by chemical experiments and thermodynamics calculations. The results show that the inclusions are calcium aluminate with a little amount of calcium sulphide in low content sulphur steel after calcium treatment. As the sulphur content in steel increases, the typical inclusions generated in steel are layered or homogeneous irregular oxysulphides. A number of pure sulphides can be observed in higher content sulphur steel, and the morphology of sulphides with different CaS/MnS mass ratio has different shape. Thermodynamics calculations of Al–Ca–O–S–Mn system inclusions in steel at 1873 K and during solidification process were comprehensively conducted, considering all types of inclusions. The experimental results are in good agreement with thermodynamic calculations, which can predict the formation of the inclusions in Al-killed steel with different sulphur content.

The edge shape problem of strip has great influence on the shape control precision, so it is extremely necessary to establish the high-precision strip edge shape model. Based on the theoretical analysis and the practical engineering, a new high-precision shape detection model, which consists of wide channels in the middle of detection roll and narrow channels on both sides, is established to analyse the problem of strip edge shape. Instance data showed that the strip lateral thickness difference was directly related with the shape state of strip, and the strip edge coverage rate and the strip deviation amount had important influences on the shape detection precision, the pattern recognition accuracy and the shape control effect, so the strip edge shape model has become one of the key factors to improve industrial application effect of the shape closed-loop control system in the present situation.

Pre-oxidation of fines of magnetite containing materials is usually carried out to get better yield of metals. Titaniferous magnetite ore (TMO) is one kind of low-grade iron ore (around 45–50% of total Fe) with a significant amount of TiO₂ (23.23%) and V₂O₅ (0.403%). TMO fines have been pre-oxidised at 973 K (700°C) for 9 h under air atmosphere. The effect of reduction of raw TMO fines as well as the pre-oxidised TMO fines using boiler grade coal in the form of cylindrical briquettes has been studied in the temperature range of 1273 K (1000°C) to 1473 K (1200°C) for periods of 10, 20, 30, 40 and 60 min to estimate the relative yield of iron. The influence of temperature and time on reduction experiments has also been investigated with XRD, FESEM analyses along with chemical analysis of the reduced samples. The most novel result is that the yield of Fe by direct reduction of raw TMO (92.42%) is even marginally better than that of reduction of pre-oxidised TMO (90.89%) at 1473 K (1200°C) for 60 min. Thus the single-step reduction of raw TMO is techno-economically more viable than the pre-oxidation followed by reduction technique.

Based on an inverse algorithm and a full-scale heat transfer model of slab/mould, the non-uniform temperature fields of mould and slab are obtained. Considering the formation mechanism and heat transfer characteristics of the slag films and air gap, a mathematical model of the heat transfer between the liquid/solid slag films and air gap is further developed. According to the model, the non-uniform distributions and evolution of liquid/solid slag films and air gap are comprehensively revealed, which proposes a theoretical foundation for exploring the complex heat transfer of mould/slab and provides a helpful tool for further improving the casting parameters and operations.