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cr0264_07 - 7 Desulfurization in Secondary Steelmaking 7.1...

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7 ©2001 CRC Press LLC Desulfurization in Secondary Steelmaking 7.1 INTRODUCTION Except in free-cutting steels, sulfur is considered to be a harmful impurity, since it causes hot shortness in steels. Some decades back, for common grades of steel cast through the ingot route, the maximum permissible sulfur content was 0.04%. In the continuous casting route, it should be 0.02%. In special steel plates, the normal specification for sulfur is 0.005% these days, but there is a demand for ultra-low-sulfur (ULS) steel with as low as 10 ppm (0.001%), e.g., in line pipe, HIC resistive steels, and alloyed steel forgings. 1 Sulfur comes into iron principally through coke ash. It is effectively removed from molten iron by slag in a reducing environment only. Hence, traditionally, sulfur control used to be done during ironmaking in a blast furnace. Very little sulfur removal is possible in primary steelmaking due to the oxidizing environment. An exception is the electric arc furnace (EAF), where low-sulfur steels are produced through two-stage refining. In view of the consistent demand for lower-sulfur steel and the incapability of the blast furnace to achieve it, external desulfurization of liquid iron in a ladle during transfer to the steelmaking shop was developed. The process is capable of lowering sulfur content to 0.01% or so and is an essential feature of a modern integrated steel plant. Content below 0.01% must be accomplished in secondary steelmaking. There are now pro- cesses, such as the MPE process of Mannesman and the EXOSLAG process of U.S. Steel, 2 where desulfurization is achieved to some extent during tapping by using synthetic slag and utilizing the kinetic energy of the tapping stream. Desulfurization by treatment with synthetic slag on top of molten steel and gas stirring (either in an ordinary ladle, in a ladle furnace or VAD, or during vacuum degassing) are also being practiced. However, only the injection of a powder such as calcium silicide into the melt is capable of producing ULS steel. ULS can be achieved only if the dissolved oxygen is also very low. Gas stirring is required, so deep desulfurization is associated with deep deoxidation. The use of alumi- num in combination with calcium or rare earth (RE) metals achieves both. In addition, injection processes are capable of inclusion modification for further improvement of the properties of steel. Section 6.4.5 of this book has already stated that oxygen and sulfur dissolved in liquid steel retard the nitrogen desorption rate from steel in vacuum degassing. A low nitrogen level has been achieved in low-sulfur and low-oxygen steels. This is an additional benefit if deep desulfurization is done before or during vacuum treatment. Furnace slags contain oxides such as FeO, SiO 2 , P 2 O 5 , and MnO. These oxides are unstable in the presence of a deoxidized steel, especially when the slag and steel are intimately mixed. As a result, some reversion of phosphorus into the steel occurs. This slag also partly consumes added deoxidizers, so it does not allow proper utilization of them for steel deoxidation. The slag also causes wear on the ladle lining.
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