Rob Boom

Effect of MgO and Al2O3 variations in oxygen steelmaking (BOF) slag on slag morphology and phosphorus distribution

Abstract Operational data of BOF and the slag samples for different starting conditions of phosphorus (0.06–0.26%P) and silicon content (0.3–1.2%Si) of hot metal have been analysed. The contribution of parameters which are well known to affect phosphorus distribution at tap, such as basicity, temperature, FeO content of slag, slag mass etc., is investigated through models of the ionic theory of slag, optical basicity, regular solution approach, and molecular theory of slag. The best overall results are obtained by the model based on the molecular theory of slag in which several operational parameters are also incorporated. Investigations of different slag samples, based on optical, SEM, EPMA and X-ray studies, reveal the effect of MgO and Al2 O3 on slag morphology and phosphorus distribution in different phases. It is important to consider the phosphorus distribution ratio in the solid and liquid part of the slag. The solid part of the slag, which is mostly dicalciumsilicate, can contain up to 5% phosphorus. The phosphorus content of the liquid part of the slag may depend upon the phosphorus content of the hot metal or the phosphorus load of the slag. It is found that incorporation of the effect of dicalciumsilicate in the model improves the accuracy of prediction. For better process control the addition of iron ore towards the end of the blow must be avoided while treating high phosphorus hot metal or during the production of ultralow phosphorus steels.

High-Temperature Mechanical Behavior and Fracture Analysis of a Low-Carbon Steel Related to Cracking

Cracking in continuously cast steel slabs has been one of the main problems in casting for decades. In recent years, the use of computational models has led to a significant improvement in caster performance and product quality. However, these models require accurate thermomechanical properties as input data, which are either unreliable or nonexistent for many alloys of commercial interest. A major reason for this lack of reliable data is that high-temperature mechanical properties are difficult to measure. Several methods have been developed to assess the material strength during solidification, especially for light alloys. The tensile strength during solidification of a low carbon aluminum-killed (LCAK; obtained from Tata Steel Mainland Europe cast at the DSP plant in IJmuiden, the Netherlands) has been studied by a technique for high-temperature tensile testing, which was developed at Sumitomo Metal Industries in Japan. The experimental technique enables a sample to melt and solidify without a crucible, making possible the accurate measurement of load over a small solidification temperature range. In the current study, the tensile test results are analyzed and the characteristic zero-ductility and zero-strength temperatures are determined for this particular LCAK steel grade. The fracture surfaces are investigated following tensile testing, which provides an invaluable insight into the fracture mechanism and a better understanding with respect to the behavior of the steel during solidification. The role of minor alloying elements, like sulfur, in hot cracking susceptibility is also discussed.

Slags and fluxes entering the new millennium, an analysis of recent trends in research and development

Abstract This paper details the recent trends in slag related research from 1999 to 2003. The trends in the following are outlined: • Annual publication rate • NGeographical origin of the work • Nature of the research (e.g.thermodynamics, kinetics) • Nature of the slag studied. This study has revealed that slag research in the USA and Europe has decreased to about a half over the last 20 years and that there has been a significant decrease in Japanese research in this area in recent years. However, these decreases have been offset by significant increases in Chinese and Korean research activity. There has been a significant increase in the amount of work carried out on recycling and environmental aspects of slags in recent years.

Challenge to control mould heat transfer during thin slab casting

Abstract At the thin slab caster of Tata Steel, IJmuiden, mild cooling mould powders were introduced with the aim to control the mould heat transfer during casting. These mild cooling mould powders are characterised by specific values of basicity, solidification point and chemical composition. Application of these mould powders resulted in a redistribution of mould heat transfer during casting, i.e. a reduced and more stable mould heat transfer in the critical upper part of the mould and an increased mould heat transfer in the lower part of the mould. The average mould heat transfer and hence the shell thickness at mould exit are comparable to the standard powder. The application of mild cooling mould powders also resulted in improved solidification behaviour of the steel shell. A thinner chill zone with smaller thickness variations was observed. Furthermore, it was found that the mould taper required optimisation to match the changes in shrinkage behaviour to ensure uniform solidification. The use of mild cooling powders was observed to give an increase in mould friction. Mould thermal monitoring indicated that the solid slag films fractured (sheeting) in the upper part of the mould. However, no operational problems were reported, which indicate that the first 200 mm under the steel meniscus is essential for initial solidification and for the formation of a homogeneous steel shell. All these findings can be understood by considering the crystallisation properties of the mould slag, which include the cooling rate. Mild cooling has been shown to provide uniform heat transfer and adequate lubrication for high speed thin slab casting.

Development of Mould Flux for High Speed Thin Slab Casting

Mould powders impact the stability of the continuous casting process for steel at all casting speeds. The main functions of mould powder are to provide sufficient lubrication and to control the mould heat transfer between the solidifying steel shell and the copper mould. At higher casting speeds associated with thin slab casting, the role of the mould powder is even more important. Actual casting speeds for the thin slab caster at Corus IJmuiden are between 5.4 and 6 m/min; the production level is around 1.3 Mt/year (coils). It has been decided to increase the production of this caster to a level of 1.8 Mt/year (coils). In order to meet this demand, the steel in mould time has to be increased to approximately 85% and the maximum casting speed will be increased to 8 m/min. A collaborative project between Sumitomo Metal Industries (SMI) and Corus IJmuiden was initiated to develop mould powders which facilitate casting speeds up to 8 m/min at the thin slab caster. Main subjects of this project are: mould powder design, characterisation of mould powder and mould slag, trials at the pilot caster of Sumitomo and finally plant trials at the thin slab caster of Corus. A special point of attention is the condition to use mould powder as a granulated material at the thin slab caster. As a consequence, the characterisation work focussed on the choice of raw materials and on the corresponding phase relations at elevated temperatures. Typical of the developed mould powders are so-called mild cooling properties which will result in a controlled mould heat transfer during casting. In this paper, several aspects of this joint project between Sumitomo and Corus will be described.

Mould powder investigations for high speed casting

Abstract Mould powders significantly determine the stability of the continuous casting process of steel. The processes leading to melting of mould powder and solidification of mould slag were studied in situ using high temperature X-ray diffraction with additional powder X-ray diffraction and microscopic techniques. It was shown that during heating, a powder shows a specific sequence of phase relations before melting takes place. During cooling and solidification, one or more crystalline phases can be formed. The findings on high temperature properties were confirmed by the analyses of slag rims and slag films, obtained from the Corus thin slab caster. Finally, the in depth characterisation, together with some calculations on mould slag was compared with plant data of the thin slab caster. Results from this work give a better understanding of the mould powder functions at the thin slab caster and are being used to guide mould powder design for the current and even higher casting speeds.

Some aspects of alumina created by deoxidation and reoxidation in steel

Abstract The morphology of alumina was studied in laboratory experiments and in samples from the steel plant. Varying morphologies of alumina were observed after deoxidation and reoxidation but a clear discrimination between alumina formed by deoxidation or reoxidation was not observed. Local concentration gradients during the addition and dissolution of aluminium can explain different growth mechanisms which lead to the observed varying morphologies of alumina.

Sulphur removal in ironmaking and oxygen steelmaking

Sulphur removal in the ironmaking and oxygen steelmaking process is reviewed. A sulphur balance is made for the steelmaking process of Tata Steel IJmuiden, the Netherlands. There are four stages where sulphur can be removed: in the blast furnace (BF), during hot metal (HM) pretreatment, in the converter and during the secondary metallurgy (SM) treatment. For sulphur removal a low oxygen activity and a basic slag are required. In the BF typically 90% of the sulphur is removed; still, the HM contains about 0.03% of sulphur. Different HM desulphurisation processes are used worldwide. With co-injection or the Kanbara reactor, sulphur concentrations below 0.001% are reached. Basic slag helps desulphurisation in the converter. However, sulphur increase is not uncommon in the converter due to high oxygen activity and sulphur input via scrap and additions. For low sulphur concentrations SM desulphurisation, with a decreased oxygen activity and a basic slag, is always required.

Kinetic study on gas molten particle reduction of iron ore fines at high temperature

A kinetic study of gas molten particle reduction has been carried out using a high temperature drop tube furnace at the typical conditions of the smelting cyclone of the HIsarna ironmaking process. The results demonstrated that particle size has a significant effect on the reduction rate of fine iron ore particles and that the reduction ability of H2 is two to three times higher than that of CO at high temperature. It was found that almost all of the Fe2O3 in the iron ore particles was reduced to Fe3O4 and FeO in the first 210 ms. The morphology images of the partially reduced spherical particles showed that a liquid FeO product layer was formed outside the solid Fe3O4 unreacted core. The kinetic analysis revealed that the rate controlling step of the gas molten particle reduction was the diffusion of Fe3+ in the liquid product layer and that the activation energy was ∼156 kJ mol − 1.

Microstructure of metallurgical grade silicon and its acid leaching behaviour by alloying with calcium

Abstract The efficiency of acid leaching as a low-cost refining method of metallurgical grade silicon (MG-Si) is determined by the microstructure of silicon. In the present work the influence of microstructure of MG-Si on its leaching behaviour was studied in order to explore the potential of this method in achieving solar grade purity of silicon. The microstructure of silicon was modified by alloying with 3–10 wt-% of calcium, and the influence of the concentration of alloying elements and the solidification conditions on the leaching efficiency was evaluated. It was shown that the major impurity and its location determine the preferred leaching agent. A mechanism of removal of metallic and non-metallic impurities was proposed. Control of the alloying process resulted in enhanced removal of the major part of the impurities, including phosphorus.