Zirconium in Steels

Zirconium in Steels  Zirconium (Zr) (atomic number 40 and atomic weight 91.22) has density of 6.52 gm/cc. Melting point of Zr is 1855 deg C and boiling point is 4377 deg C. Zr  has a hexagonal close pack crystal structure. The phase diagram of the Fe-Zr binary system is given at Fig 1. Fig 1 Fe-Zr binary phase diagram Zr is being used as a alloying element in steels since the early 1920s, but has never been universally employed, as have niobium (Nb), titanium(Ti), and vanadium (V). Historically, the main use of additions of Zr to steel was for combination preferentially with sulphur, to avoid the formation of manganese sulphide (MnS), known to have a deleterious influence of the impact toughness of wrought and welded steel. These days there has been a renewed interest in the addition of Zr to the micro alloyed steels. Zr is highly reactive and has a strong affinity, in decreasing order, for oxygen (O), nitrogen (N), sulphur (S), and carbon (C). Its affinity for O, S, and N is the primary reason for its use in steelmaking. Due to this property it controls  the nonmetallic inclusions of sulphides and oxy-sulphides. it is also used for the fixation of N mainly in boron (B) steels. Zr also inhibits grain growth and prevents strain aging but its use for either of these reasons is limited. Because of its relatively high price and also due to the availability of cheaper replacements, general acceptance of Zr for use as an alloying element in steels is limited. Addition agents Te Zr addition agents in the liquid steel are iron-silicon-zirconium (FeSiZr) alloy, ferrozirconium (Fe-Zr) alloy, Zr alloy scrap and pure Zr sponge. Out of these the most popular addition agent is FeSiZr since it is...

Aluminum in Steels

Aluminum in Steels  Aluminum (Al) is used for deoxidizing and grain refining in steels. It is a strong deoxidizer. It is also used as nitride former and as an alloying agent. Its ability to scavenge nitrogen (N) from steel makes it a useful addition in drawing quality steels, especially for automotive applications. Aluminum (Al) is being used as a deoxidizing element in steels for more than 100 years. Deoxidation of steel with Al is common practice today. Al plays an important role in secondary metallurgy. It forms aluminum oxide or alumina (Al2O3) alumina and decreases the amount of oxygen in the steel during the production of killed steels. Metallic Al is the most common addition agent. It is usually done in the form of notched bars, shots, pigs, small ingots, chopped wire, briquettes and other convenient forms such as coiled machine fed wire. Purity of deoxidation grade of Al is normally over 95 % with the main tramp elements being zinc, tin, copper, magnesium, lead and manganese. Coiled aluminum wire is usually made to 99 % minimum specification. A wire feeding machine is shown in Fig 1. Fig 1 Wire feeding machine Al is also added as ferro-aluminum which is a dense and highly efficient form of aluminum addition. Ferro-aluminum contains 30 % to 40 % Al and normally added in lump form. Al may be added to the steel making furnace, teeming ladle, Ladle furnace, continuous casting tundish or ingot mould. Each type of addition has its specific purpose, and each addition produces its own characteristic results. Al is a very powerful deoxidizer, but has a disadvantage because of its low density. The density of liquid aluminum at steelmaking temperatures (1600 deg C) is about 2.0 tons/cum while the density of steel at...