Argon solubility in liquid steel
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I. INTRODUCTION
ARGON is an inert gas used to homogenize liquid steel and to shield alloyed steel from air. Argon prevents clogging in refractory nozzles, removes inclusions from steel by flotation action, and prevents reoxidation of liquid steel streams. Despite the beneficial properties of the inert gas, argon is also believed to be the root cause of surface defects (known as blowholes or pencil pipe blisters) in critical steel products for automotive applications. Gases such as nitrogen and hydrogen show a certain solubility in liquid steel that is larger than their respective solubility in solid steel. As a result, gas bubbles can be formed at solidification and trapped in the solid skin, with detrimental effects to the final products. The users of active argon gas in steelmaking implicitly assume that this inert gas does not dissolve at all, neither in liquid nor in solid steel. Is this assumption valid? In this article, fundamental studies of the solubility behavior of argon in steel are reported. By means of an ultrahigh vacuum (UHV) desorption technique, the solubility has been experimentally determined at the Interfaculty Reactor Institute (IRI) of the Delft University of Technology. The IRI equipment is capable of measuring very low concentrations of gas in steel. The experimental values have been compared to calculated values using thermodynamics and empirical values for the formation energy of solute/solvent interfaces. A comparison with literature values is made for the system argon-iron as well as for other noble gas-metal systems. Conclusions regarding the role of inert argon as active gas are formulated.
II. EXPERIMENTAL A. Sample Preparation Samples used for measuring argon solubility in liquid steel have been taken from steel melts produced at Hoogovens Research & Development. Ultralow carbon steel is melted in a vacuum induction furnace and exposed to an argon atmosphere of 0.9 3 105 Pa for at least 2 hours to achieve gas/liquid equilibrium. As an alternative, saturation of liquid steel with argon can also be achieved by gas bubbling through molten steel for a certain period of time. In this case, however, very small bubbles with negligible flotation potential might be dispersed in the steel, which might cause larger gas mounts to be entrapped in the samples. In such a case, the amount of argon found does not reflect the argon solubility in liquid steel. In order to investigate the influence of the steel grade on the argon solubility, two steel melts have been made, one with titanium and the other with titanium/ niobium alloy additions. A vacuum pipette (i.d. 6 mm) is used for taking samples from the steel melt. The advantage of using this type of pipette is the quick filling and instantaneous solidification of the steel, thus obtaining a sample that is considered to be representative of the melt composition. At IRI, about 15 to 25 mg of material is cut from the sample and subsequently etched for 5 minutes in 10 wt pct HCl acid to remove impurities from the surface. The etching step is necessar
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