Ironmaking & Steelmaking: Processes,Products and Applications

Inclusion removal from liquid steel by bubble flotation under turbulent conditions is analysed using a water model. Turbulence is realised by impeller stirring in a water containing vessel. First, the effects of variables such as filter pore size, gas flowrate, NaCl concentration, and stirring intensity on bubble size are investigated. Second, particle removal by bubble flotation is studied using the water containing vessel system. The results indicate that particle removal rate by bubble flotation is controlled by non-first order kinetics. The factors affecting the particle removal rate constant k ₁ are discussed and a final empirical equation is derived as follows: -dc/dt = k ₁ c ^1·3665 and k ₁ = A(d _p/d _B)^{2·65ϵ0·104} Q _g ^1·630, where c is particle number density, t is time, A is a constant parameter, d _p and d _B are the particle and bubble diameter respectively, ϵ is the turbulent energy dissipation rate, and Q _g is the gas flowrate.

Centreline shrinkage and segregation in continuously cast billets have an adverse effect on cast product quality. Nozzle electromagnetic stirring (N-EMS) is an effective method to refine the solidification structure of continuously cast billets and thereby improve the cast product quality. The effect of N-EMS on the solidification structure of continuously cast Sn-3·5Pb alloy has been investigated by means of experiments and numerical analysis. The results show that, upon imposing N-EMS, crystal grains within the billets are refined and continuous casting can be conducted at a lower pouring temperature; the solidification structure of the resulting obtained billets is much finer than without N-EMS. The mechanism by which the solidification structure of the billets is improved is considered to be a change in the flow and temperature distribution of molten metal in the nozzle.

The dynamic strain aging behaviour of a niobium microalloyed steel has been examined. Hot tensile testing was carried out on heat treated and as received specimens. Heat treated specimens were austenitised at 1000°C for 1 h, and then cooled in air or in a stainless steel cylinder to obtain various amounts of free or uncombined interstitial solutes in solid solution, to examine the effect on the dynamic strain aging behaviour of the steel. It was found that dynamic strain aging takes place in niobium microalloyed steel during tensile testing at temperatures ranging from ambient temperature to 450°C at a crosshead displacement rate of 2 mm min^-1. As a result, the ultimate tensile strength and initial work hardening rate exhibit maximum values at temperatures between 200 and 350°C. Also, load-extension graphs for tested specimens show serrated behaviour and yield points at 200, 250, and 300°C. It is believed that dynamic strain aging in niobium steel is caused by interaction between dislocations and interstitial solutes (nitrogen and carbon) or solute pairs consisting of one interstitial and one substitutional solute atom (for example Mn-C and Mn-N).

The important functions promoted by powdered flux added over the liquid steel surface in continuous casting moulds are strongly affected by the thickness of the liquid layer that forms as a result of the heat absorbed. The present work discusses the results of a three-dimensional steady state model, developed to represent the coupled fluid flow and heat transfer phenomena that determine thickness profiles of the liquid flux layer. Since the laminar flow of the liquid slag layer depends on the shearing imposed on it by the turbulent motion of the liquid steel beneath it, and since additionally this motion is strongly influenced by the flow characteristics of the steel stream poured into the mould through the submerged entry nozzle (SEN), separate turbulent flow models for the liquid steel in the SEN and the mould were also developed. The consistency among the models and their accuracy was judged by comparing thickness and temperature flux profiles measured in plant against predicted ones; the comparison showed good agreement. The effects of casting speed, mould width, and flux viscosity and heat of melting on the liquid layer thickness were investigated. The last variable was found to exert the most marked influence. Different from conventional casting moulds, where the liquid layer thickness increases with increasing casting speed, in compact strip process moulds the thickness remains almost constant with increasing casting speed. This difference is well accounted for by the model, which suggests that this behaviour stems from the different slag flow patterns generated in straight, wide moulds and in thin moulds having a central upper funnel shaped section.

A finite element thermal stress model to compute the thermomechanical state of the solidifying shell during continuous casting of steel in a square billet casting mould has been applied to investigate longitudinal cracks. A two-dimensional thermoelastoviscoplastic analysis was carried out within a horizontal slice of the solidifying strand which moves vertically within and just below the mould. The model calculates the temperature distributions, the stresses, the strains in the solidifying shell, and the intermittent air gap between the casting mould and the solidifying strand. Model predictions were verified with both an analytical solution and a plant trial. The model was then applied to study the effect of mould corner radius on longitudinal crack formation for casting in a typical 0·75%/m tapered mould with both oil and mould powder lubrication. With this inadequate linear taper, a gap forms between the shell and the mould in the corner region. As the corner radius of the billet increases from 4 to 15 mm, this gap spreads further around the corner towards the centre of the strand and becomes larger. This leads to more temperature non-uniformity around the billet perimeter as solidification proceeds. Longitudinal corner surface cracks are predicted to form only in the large corner radius billet, owing to tension in the hotter and thinner shell along the corner during solidification in the mould. Off corner internal cracks form more readily in the small corner radius billet. They are caused by bulging below the mould, which bends the thin, weak shell around the corner, creating tensile strain on the solidification front where these longitudinal cracks are ultimately observed.

An optimal single element flow control device (SEFCD) is proposed in the present study for a small capacity twin strand tundish, to produce steel billet with the desired cleanliness, designed by means of numerical experimentation. To achieve this goal, a mathematical model based on the SOLA-SURF technique and the k-ϵ (where k is the kinetic energy of the fluid and ϵ is its dissipation) two equation turbulence model has been developed to simulate the fluid flow phenomena of molten steel, as well as the mass transfer phenomena of tracer during steady state operation in the tundish. The accuracy of the mathematical model has also been verified by water model experiments, which indicate that the mathematical model is sufficiently accurate and suitable as a tool to develop an optimal SEFCD as required in the present study. Several SEFCD designs have then been evaluated using the mathematical model. The simulated results show that a billet tundish with a high, long, trough shaped pouring pad arrangement has the highest residence time, which promotes inclusion removal, and is thus considered the optimal SEFCD design for the billet tundish.

A potential source of inclusions in liquid steel is ladle glaze, the slag/inclusion coating on the refractory lining of the ladle which is always present after casting and before reuse of the ladle. The composition and quantity of this glaze can vary considerably, depending on the range of steel qualities being made and, in order to model the effect of glaze on inclusion/top slag formation, a combined fluid flow-thermodynamic model has been developed. The behaviour of ladle glaze for various different steels has been studied under pilot plant conditions and the results incorporated into the mathematical model. The implications of the pilot plant and model results are discussed in the context of the production of clean steel.

The erosion of hearth refractories is widely recognised as the main limitation for a long campaign blast furnace life. Distributions of liquid iron flow and refractory temperatures have a significant influence on hearth wear. In this investigation, a comprehensive computational fluid dynamics model is described which predicts the fluid flow and heat transfer in the hearth; specifically, the flow and temperature distributions in the liquid iron melt, and temperature distributions in the refractories. The accuracy and representativeness of the model was evaluated using plant data from BHP Steel's Port Kembla no. 5 blast furnace and OneSteel's Whyalla no. 2 blast furnace. Generally, there is good agreement between measured and calculated refractory temperature profiles. A series of sensitivity tests provided cause-effect relationships between operational and fluid flow parameters (floating deadman, different extent of refractory erosion, presence of embrittled layer).