Alex McLean

The temperature coefficient of the surface tension of pure liquid metals

The surface tensions of liquid Fe, Co, Ni, Cu, Ag, Zn, Pb, Cd, and Sn have been measured by the sessile droplet method and/or the levitated droplet method over wide ranges of temperature. The values of surface tension obtained by the levitated droplet method have always been found to be higher than those measured by the sessile droplet method, a result which is attributed to decreased droplet contamination with the containerless levitation method. Negative temperature coefficients of surface tension have been obtained for all of the metals investigated in this work. Based on a literature survey of the available experimental data, it is shown that the values of both surface tension and the temperature coefficient are influenced in a systematic manner by the presence of surface active impurities. On this basis, positive values of the temperature coefficient of surface tension for liquid Zn and Cd found in the literature may be explained in terms of impurity effects which tend to be particularly pronounced in volatile metals.

Comparison of surface tension measurements using the levitated droplet method

Measurements of the surface tension of some metals by the levitating drop technique have been carried out by two independent laboratories in order to establish the levels of agreement. Results obtained for pure iron, cobalt, and copper showed reasonable agreement, but in the case of stainless steel, significant differences were apparent, which were shown to be a consequence of the different hydrogen concentrations in the respective environmental gases. Surface tension values for pure gold obtained by this method showed good agreement with the data obtained by established methods and show that the levitating drop technique is capable of yielding reliable results.

Influence of rare earth metals on the nucleation and solidification behavior of iron and 1045 steel

Two series of experiments have been conducted to determine the influence of rare earth additions on the nucleation and crystallization behavior of pure iron and 1045 steel. In the first series, additions of rare earth suicide or cerium dioxide powder to two-Kg 1045 steel ingots indicated that rare earth suicide can refine the as-cast structure of such ingots. However, if the holding time after rare earth silicide addition is over two minutes, the grain refinement decreases. With cerium dioxide additions, a relatively large columnar zone was obtained. In the second series, the effects of cerium metal or cerium dioxide powder additions on the degree of undercooling obtainable in pure iron and 1045 steel were examined by the lévitation melting method. Surface tension measurements of the levitated droplets were carried out at the same time to investigate the possible effects of surface tension variations on the nucleation and crystallization behavior of the metals. The experimental data show that rare earth inclusions can greatly reduce the degree of undercooling of iron and steel, and that a small amount of dissolved cerium can further reduce the degree of undercooling of levitated droplets. The structure and reaction products obtained with Fe-Ce levitated droplets were examined with both optical and scanning electron microscopy as well as X-ray diffraction analysis. The experimental results clearly indicated that cerium solute redistribution during solidification is the dominant factor in refining the as-cast structure. A nucleation and solidification model for the Fe-Ce levitated droplets has been developed, which can successfully explain the experimental results.

Sulfide shape control in high strength low alloy steels

Directionality of mechanical properties—such as toughness and bend formability—is typical of hot rolled steels processed on modern, hot strip mills. In aluminum killed steels, directionality results mainly from elongated (type II) manganese sulfide inclusions. Directionality can be reduced by retaining the original globular shape of the precipitated sulfides. This can be accomplished by promoting the formation of sulfides which are more stable and have a higher melting point than that of manganese sulfide. Thermodynamic considerations indicate that additions of Ti, Zr, Ca, Mg, and rare earths are suitable for this purpose. Experimental work on laboratory heats containing 0.020 to 0.25 pct S involved mainly additions of rare earths (mischmetal or silicides) to a V−Al−N high strength, low alloy steel. Other strong sulfide formers were not utilized either because of too high vapor pressure at steelmaking temperatures or because of their strong interaction with nitrogen. For cerium contents of 0.03 to 0.04 pct, the shape of inclusions, identified as rare earth sulfides, was globular. Control of sulfide shape contributed to a marked improvement in toughness and formability of steel in the direction transverse to the rolling direction. The results have been verified in full scale plant trials.

Determination and Prediction of Water Vapor Solubilities in CaO-MgO-SiO2 Slags

A thermogravimetric technique has been used to obtain accurate measurements of water vapor dissolution in molten CaO-MgO-SiO2 slags. These measurements indicate that the solubility of water vapor is independent of temperature over a 200 °C range (1375 to 1575 °C) and varies directly with the square root of the water vapor partial pressure. Water vapor was observed to be amphoteric in nature with respect to dissolution in liquid silicates. A minimum in solubility was observed at approximately the metasilicate composition. Results obtained during the present study indicate that the empirically defined basicity index(NCao + NMgo)/Nsio2 is inadequate to describe the dissolution of water vapor in silicate melts. The activity of silica has been identified as a more accurate indicator. Based on this approach, ambiguous results from previous studies in the CaO-MgO-SiO2 slag system have been rationalized. From a statistical evaluation of the data, a relationship in terms of the activity of silica has been obtained which permits calculation of water vapor contents in CaO-MgO-SiO2 slags.

Solubilities of carbon dioxide in sodium silicate melts

Carbonate solubilities in Na2O-SiO2 melts were measured over the composition range XNa2O/ (XNa2O + XSiO2) = 0.5 to 1.0 and the temperature range 1100 to 1300 ‡C. The solubility increased with increasing XNa2O and decreased with increasing temperature. Carbonate capacities calculated from the experimental results compared favorably with values for sulfide and phosphate capacities obtained from the literature. In addition, an excellent correlation was obtained between carbonate capacity and the activity of sodium oxide. The carbonate capacity, which is an easier parameter to obtain, is a good measure of the basicity of sodium silicate melts. It would appear that carbonate capacity could be an excellent basicity index for iron and steelmaking slags as well as for fluxes used in other high temperature technologies.

Electromagnetic Levitation of Silicon and Silicon-Iron Alloy Droplets

Abstract In this paper, the design of an electromagnetic levitation system and a technique for non-conductive silicon heating and conductive silicon levitation is described. The aim of the work is to describe the various parameters including coil design, applied power and specimen weight that govern the temperature of levitated silicon and silicon-iron alloy droplets.

Distribution of macro-inclusions in low carbon aluminium-killed steel slabs

ABSTRACT Macro-inclusions in low carbon, aluminium-killed steel slabs were characterised by step-machining within a 10 mm zone from the slab surface using an ASPEX automatic inclusion analyzer. Dendritic structures within the cross-section of slabs were examined. The results show that alumina clusters and alumina associated with bubbles are the dominant macro-inclusions. Along the slab width direction, macro-inclusions were mostly found at the slab centre because of the deeper hooks and freezing meniscus surrounding the submerged entry nozzle. In terms of slab thickness, inclusions were mainly concentrated within the zone 3.5–6 mm from the top of the slab surface, where the columnar dendrites showed a relatively small inclination angle, indicating small cross-flow velocities at the solidification front. The number density of macro-inclusions were strongly dependent on the washing effect produced by the flow velocity. High speed casting promotes this behaviour and improves the surface quality of the slabs.

Formation and evolution of macro inclusions in IF steels during continuous casting

The evolution of macro inclusions during continuous casting was investigated by large-area inclusion characterization using ASPEX and analysis of nozzle blockage deposits. Six kinds of inclusions over 5 μm were observed in samples taken from the tundish and the slabs: single alumina particles, alumina dendrites, refractory-related alumina, alumina associated with bubbles, alumina clusters and Al–Ti complex oxides. By examining the morphology of the nozzle blockage deposit, it was concluded that the refractory-related alumina in the slabs came from the decarburization layer washed away by the steel stream. Some of the alumina clusters that came from the nozzle blockage deposit dislodged by the steel flow, were formed by the agglomeration and sintering of 5–20 μm inclusions that were carried over from the tundish. Two kinds of Al–Ti oxides were found in the tundish, and their evolution mechanisms during the casting process were proposed.

The Science and Technology of Slags for Iron and Steelmaking

In the quest to generate information pertaining to the characterization, properties and performance of molten slags, measurements and models are two interdependent requirements. Without measurements, our models are incomplete and unsatisfactory. Without models, we fail to realize, or perhaps even comprehend, the potential significance of our measurements. In this context, the concept of optical basicity will be revisited and examples presented of how optical basicity can be used to design slags with appropriate characteristics for specific applications in iron and steelmaking.