cr0264_10 - 10 Clean Steel Technology 10.1 INTRODUCTION It...

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10 ©2001 CRC Press LLC Clean Steel Technology 10.1 INTRODUCTION It was mentioned in Chapter 1 that 1. The manufacture of cleaner steels is a thrust area in steelmaking in view of more stringent customer demands, especially for plates and sheets. 2. Achievement of good cleanliness is possible only if attention is paid to this goal at all stages of secondary steelmaking, from furnace to mold. The term clean steel should mean a steel free of inclusions. However, as Chapter 9 pointed out, no steel can be free from all inclusions. Macroinclusions are the primary harmful ones. Hence, a clean steel means a cleaner steel, i.e., one containing a much lower level of harmful macroinclusions. Chapter 9 also dealt with some details of inclusions, their origin, and modifications. Chapter 5 and Chapter 8 contain discussions related to the science and technology of clean steel. However, this information is somewhat scattered. Also, not all aspects of clean steel production have been touched upon. In view of the importance of the topic, this chapter looks at clean steel technology in an integrated manner and it contains • a summary of points discussed in earlier chapters • information about refractories in secondary steelmaking, with special emphasis on clean steel technology • a discussion of tundish metallurgy for clean steel 10.2 SUMMARY OF EARLIER CHAPTERS 10.2.1 D EOXIDATION P RACTICE With reference to Chapter 5, better cleanliness and removal of deoxidation products can be achieved if the following points are kept in mind: 1. Carryover slag from the furnace into the ladle does not directly cause “dirtiness” in steel. However, it should be deoxidized well. Otherwise, FeO and MnO, and to some extent SiO 2 present in it, will keep transferring oxygen into the melt. It is especially serious for aluminum-killed grades for the continuous casting route. Besides lowering aluminum yield, the slag also requires the addition of CaO and CaF 2 for proper desulfurization (see Chapter 7). Hence, in modern steelmaking practice, as much attention as possible is paid toward the prevention of slag carryover.
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©2001 CRC Press LLC 2. Deoxidation product should be chemically stable. Otherwise, it tends to decompose and transfer oxygen into liquid steel. Moreover, it should be liquid for its faster growth and removal by flotation. The use of more than one deoxidizer (i.e., complex deoxidation) helps to achieve both of these objectives satisfactorily. For ingot casting of rimming and semi-killed grades, deoxidation by Si+Mn is adequate. But, for killed grades, and for continuous casting, aluminum is the principal deoxidizer. The addition of some calcium as Ca-Si alloy in addition to the aluminum tends to improve cleanliness besides inclusion modification (see Chapters 7 and 9).
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cr0264_10 - 10 Clean Steel Technology 10.1 INTRODUCTION It...

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