Effects of inclusions on the quality of steel

Abstract

Mini steel of mills or plants in Bangladesh melt scraps in Induction Furnaces. Very little refining the liquid steels is possible in these induction furnaces. In many plants the molten steels are cast into what are usually known as pencil ingots. These ingots are cast in vertical moulds resulting in the formation of pipe at the top. Such steel plants that produce pencil ingots do not have refining (ladle refining) facilities. In absence of proper refining and quality control, these ingots usually contain slags, non-metallic inclusions and inhomogeneities and are usually of poor quality. As a result deformed bars produced from pencil ingot contain significant amount of slags and inclusions and show sev.ere segregation effect and inhomogeneity in the microstructure. These defects affect the properties ofthe reinforcing bars. Since these pencil ingots are produced in small lots and with little or no metallurgical control, the products obtained from different heats using apparently similar raw materials usually give different amount of inclusions and these give uncertain physical and mechanical properties. As a result the finished products of these pencil ingots are in general of inferior quality and give substandard reinforcing bars. Many of the steel plants do not use ladle refining furnace (LRF) for refining the liquid steel or do not do the ladle refining of the melt properly. The billets as welL as the . reinforcing bars produced from such unrefined or improperly refined melts contain slags, . inclusions and inhomogeneity in the microstructure .. The deformed bars thus produced show inferior properties. Proper refining in LRF of induction melted. assorted .scrap can give fairly clean and . refined liquid steels. The billets and the reinforcing bars produced from such refined melts are generally free from inclusions and slags. Still better quality is obtained if such refined steel can be cast continuously in billet form. Metallographic study of the reinforcing bars produced from properly refined continuously cast billet show uniform grain size, no heterogeneity in the microstructure, little or no . slag and little inclusions and thus have better properties. The following is summary of the results obtained through this investigation. .:. Pencil ingots contain higher and larger sized inclusions. During hot rolling some of the inclusions (>40~m) are plastically deformed or fractured and act as a critical flaw size to cause failure . •:. Reinforcing bars produced from such pencil ingots showed low yield strength, tensile strength, percentage of 'elongation and reduction in area and in general fail in bend tests . •:. Billets cast without any ~efining give poor physical properties but sometimes satisfactory results . •:. Billet processing through ladle refining fu.rDace ensures smaller sized «20 ~) and lower number of inclusions .The reinforcing bars from such properly refined continuously cast billets give superior mechanical properties such as yield strength, tensile strength, ductility, formability and micro.structural requirements.