cr0264_01 - 1 2001 CRC Press LLC Introduction 1.1 HISTORY...

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Unformatted text preview: 1 2001 CRC Press LLC Introduction 1.1 HISTORY OF SECONDARY STEELMAKING Prior to 1950 or so, after steel was made in furnaces such as open hearths, converters, and electric furnaces, its treatment in a ladle was limited in scope and consisted of deoxidation, carburization by addition of coke or ferrocoke as required, and some minor alloying. However, more stringent demands on steel quality and consistency in its properties require controls that are beyond the capability of the steelmaking furnaces. This is especially true for superior-quality steel products in sophisticated applications. This requirement has led to the development of various kinds of treat- ments of liquid steel in ladles, besides deoxidation. These have witnessed massive growth and, as a result, have come to be variously known as secondary steelmaking, ladle metallurgy, secondary processing of liquid steel, or secondary refining of liquid steel. However, the name secondary steelmaking has more or less received widest acceptance and hence has been adopted here. Secondary steelmaking has become an integral feature of modern steel plants. The advent of the continuous casting process, which requires more stringent quality control, is an added reason for the growth of secondary steelmaking. Steelmaking in furnaces, also redesignated now as primary steelmaking , is therefore increasingly employed only for speedy scrap melting and gross refining, leaving further refining and control to secondary steelmaking. There are processes, such as vacuum arc refining (VAR) and electroslag remelting (ESR), that also perform some secondary refining. However, they start with solidified steel and remelt it. Hence, by convention, these are not included in secondary steelmaking. Harmful impurities in steel are sulfur, phosphorus, oxygen, hydrogen, and nitrogen. They occupy interstitial sites in an iron lattice and hence are known as interstitials . The principal effects of these impurities in steel are loss of ductility, impact strength, and corrosion resistance. When it comes to detailed consideration, each element has its own characteristic influence on steel proper- ties. These will be briefly mentioned in subsequent chapters associated with them. Oxygen and sulfur are also constituents of nonmetallic particles in steel, known as inclusions . These particles are also harmful to properties of steel and should be removed as much as possible. Carbon is also present as interstitial in iron lattice. However, unlike the other interstitials, it is generally not considered to be harmful impurity and should be present in steel as per specification. But, today, there are grades of steel in which carbon also should be as low as possible....
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cr0264_01 - 1 2001 CRC Press LLC Introduction 1.1 HISTORY...

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