Pär Jönsson

The Effect of Different Non-Metallic Inclusions on the Machinability of Steels

Considerable research has been conducted over recent decades on the role of non-metallic inclusions and their link to the machinability of different steels. The present work reviews the mechanisms of steel fractures during different mechanical machining operations and the behavior of various non-metallic inclusions in a cutting zone. More specifically, the effects of composition, size, number and morphology of inclusions on machinability factors (such as cutting tool wear, power consumption, etc.) are discussed and summarized. Finally, some methods for modification of non-metallic inclusions in the liquid steel are considered to obtain a desired balance between mechanical properties and machinability of various steel grades.

Slag-metal reactions during ladle treatment with focus on desulphurisation

Abstract Within several cooperative projects, KTH (Royal Institute of Technology), Ovako Steel AB, and MEFOS have investigated the desulphurisation of bearing steel during vacuum degassing. The work includes thermodynamic calculations of the slag-metal equilibrium, CFD modelling of slag-metal reactions, and plant trials. Results from the various studies are presented and discussed in this paper. Models for predicting slag properties (sulphide capacity, viscosity, and oxide activities) in liquid slags as functions of slag composition and temperature have been used for the calculation of data which have been employed in static and dynamic modelling of sulphur refining. The results from static modelling show that the method allows fast and easy evaluation of the theoretical desulphurisation conditions during degassing at Ovako Steel AB, as well as theoretical determination of the parameters that have the greatest influence on the equilibrium sulphur distribution. The conclusion from dynamic modelling is that the vacuum degassing operation can be described dynamically with the present knowledge of sulphide capacity, sulphur distribution, viscosity, and oxide activities of ladle slags if this knowledge is combined with fluid flow modelling to derive the overall kinetics. The presented model approaches have been found useful in understanding the sulphur refining process at Ovako Steel AB. The dynamic modelling concept is also believed to have potential for dynamic descriptions of other slag-metal reactions in steelmaking.

Pilot plant study of nozzle clogging mechanisms during casting of REM treated stainless steels

Abstract A pilot scale bottom teemed high frequency furnace with a nominal capacity of 600 kg and an adjustable nozzle temperature was used to study clogging of rare earth metal (REM) treated stainless steels. The influence of the following variables on the clogging was studied: amount of REM, fraction of oxide clusters, total oxygen content in the steel, reoxidation, aluminium additions before REM additions and silicon additions during casting. Overall, the results show that, during the present experimental conditions, mainly two typical clogging rates could be identified, fast and slow clogging. Two main differences could be detected between the fast and slow clogging rates. Steel containing mainly small single inclusions clogged faster than steel containing mainly large inclusion clusters. The reason was believed to be that the small inclusions could stick to the nozzle wall at narrow passages where the steel flow velocity was high while the larger ones could not. The source of the small inclusions was believed to be reoxidation. It was also found that the main part of the nozzle accretion was caused by agglomeration of inclusions and inclusion clusters, while only a thin oxide film along the nozzle wall was caused by oxide precipitation at the nozzle wall.

Determination of macroinclusions during clean steel production

Abstract There are very few macroinclusions in clean steel, but the few that exist are very harmful for material properties of the final steel product. To date, very little information on large, so called macroinclusions in clean steel production has been presented. Therefore, the present study has focused on providing information on these inclusions during different stages of the steelmaking process, based on plant trials carried out at Uddeholm Tooling, Hagfors, Sweden. Macroinclusion size distributions have been determined using optical microscopy and classified according to a modified version of the Swedish standard SS 111116, as well as using immersed ultrasonic scanning. For the sake of completeness, the size distribution of microinclusions has also been determined using optical microscopy. Inclusion compositions were determined using a scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS). Two types of liquid steel sampler were used in the investigation: a rapid solidifying (RS) sampler and the LSHR (liquid sampling hot rolling) sampler, suitable for immersed ultrasonic scanning. The results are critically discussed with respect to process conditions, such as alloy additions that took place during the plant trials.

Optimisation of ladle slag composition by application of sulphide capacity model

Abstract The sulphur distribution ratio for ladle slags at Ovako Steel AB has been determined by calculating the sulphide capacity using the Royal Institute of Technology (KTH) model and the alumina activity via an empirical expression suggested by Ohta and Suito, and by using the well established theories for sulphur refining. A parameter study was carried out based on plant data from a previous study on desulphurisation. The effect of carbon and aluminium in the steel, the temperature of the steel, and alumina and lime in the slag on the sulphur distribution ratio was investigated. It was shown that the Al2 O3 /CaO ratio had the largest influence on the conditions studied, and that when it increased the sulphur distribution ratio decreased. Based on these results, new plant trials were carried out, in which the alumina content in the slag was changed. The calculated sulphur distribution ratios for these trials were found to be in good agreement with experimentally determined sulphur distribution ratios. It is concluded that the present approach can be used to optimise multicomponent ladle slags with respect to sulphur refining.

Pilot plant study of clogging rates in low carbon and stainless steel grades

Abstract In the present paper, the clogging tendency of aluminium or silicon killed low carbon and stainless steel grades has been investigated using pilot scale equipment. Steel was melted and deoxidised in a 600 Hz induction furnace, then teemed through a temperature controlled nozzle into a mould situated on a scale measuring the teemed mass. The steel and nozzle temperatures could be adjusted within ±1°C. It was found that the clogging rate, when teeming through a hot nozzle, increased with an increasing amount of alumina phase present in the steel melt. This, in combination with composition analysis and a detailed crystallographic investigation using electron backscatter diffraction, suggested that the clogging was created by deoxidation products from the steel melt. Furthermore, it was found that the freezing effect onto the nozzle wall when teeming non‐clogging steel through a cold nozzle was of the same order of magnitude as the clogging rate of the aluminium killed steels teemed through a hot nozzle. Successful trials were also performed where the nozzle accretion during teeming was eliminated by the addition of negative oxygen interacting elements such as aluminium or silicon to the melt. A theoretical assessment based on a mass balance and thermodynamic equilibrium predicted that the removal efficiency increased with a decreased total oxygen content of the steel melt.

Magnesia–carbon refractory dissolution in Al killed low carbon steel

Abstract The effects of rotation speed, steel temperature and steel composition on the rate of dissolution of MgO–C refractory into Al deoxidised molten steel were investigated using the rotating cylinder method. Cylinders or rods of MgO–C refractory material were immersed in an Al deoxidised molten steel. Experiments were performed for steel temperatures between 1873 and 1973 K and rotation speeds between 100 and 800 rev min−1 as well as for different immersion times. For each case, the dissolution rate of MgO–C material was determined from measurement of the decrease in the rod radius. The experimental results showed that the dissolution rate of the MgO–C refractory material increased with an increase in steel temperature and rotation speed. The findings strongly suggest the diffusion of magnesium through the slag layer formed around the refractory rods to be a rate determining step. This thin oxide layer at the steel/refractory interface was found to be owing to reaction between magnesium vapour and CO generated by the reaction between MgO and C in the refractory. Oxide inclusions were also found in the steel melt and they were shown to mainly consist of MgO and Al2O3 or a mixture of the two.

Visible light-driven g-C3N4/m-Ag2Mo2O7 composite photocatalysts: synthesis, enhanced activity and photocatalytic mechanism

The g-C3N4/m-Ag2Mo2O7 composite photocatalysts with well-aligned band structures are successfully fabricated by a simple two-step method with different mass contents of m-Ag2Mo2O7. The as-prepared samples are evaluated as photocatalysts toward rhodamine B (RhB) degradation in aqueous solution under visible light irradiation (λ > 420 nm). The results demonstrate that the photocatalytic activities of the composites are strongly influenced by the weight ratio of g-C3N4 to m-Ag2Mo2O7. When it is 6 : 1, the composite exhibits the highest photocatalytic efficiency. More specifically, this value is as high as 3.4 and 6.1 times that of pure g-C3N4 and P25 respectively. In order to investigate the mechanism causing the enhanced photocatalytic degradation, the band structures are determined by UV-vis diffuse reflection spectroscopy and the Mott–Schottky technique. Moreover, the reactive radicals involved in the photocatalytic process are examined in detail via active species trapping (AST) experiments. The improved photocatalytic activities can be attributed to the efficient separation of the photo-induced charge carriers and the strong redox capacities benefit from the synergetic effect between g-C3N4 and m-Ag2Mo2O7.

The Use of an Enhanced Eulerian Deposition Model to Investigate Nozzle Clogging During Continuous Casting of Steel

Nozzle clogging caused by the build-up of non-metallic inclusions on ceramic walls is a serious industrial problem during continuous casting of steel. The current theoretical study uses the extended Eulerian model to predict the inclusion deposition rate in a submerged entry nozzle (SEN). The model considers Brownian and turbulent diffusion, turbophoresis, and thermophoresis as transportation mechanisms. First, the steel flow in a tundish was simulated using a three-dimensional CFD model. The obtained flow parameter in a SEN was then put into the Eulerian deposition model to predict the deposition rate of non-metallic inclusions. Thereafter, the deposition rates of different-size inclusions in the SEN were predicted and compared. The result shows that the steel flow is non-uniform in the SEN of the tundish. This leads to an uneven distribution of the inclusion deposition rates at different locations of the inner wall of the SEN. In addition, large size inclusions among the size of inclusions considered show a large deposition rate, due to the strong effect of turbophoresis.

Inclusion characteristics in bearing steel before and during Ingot casting

Size, distribution, and compositions of inclusions were studied during secondary refining and casting in application to high-carbon chromium bearing steel production. Samples were collected in the ladle, tapping stream and mold. Total oxygen content, number of inclusions and composition of inclusions were then determined using melt extraction, optical microscopy and scanning electron microscopy, respectively. The results show that the inclusion characteristics do not change significantly during the final treatment in the ladle. A future change in stirring conditions may therefore be necessary. The analyses of the samples taken in the tapping stream and in the mold showed an increase in the oxide inclusion content This increase is most likely due to reoxidation during sampling.