Selected Refractory Advances in Steel-Handling Systems
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n the late 1970s, combined blowing methods were introduced to exploit the adyantages and disadvantages of the top- and bottom-blown process. In the combined blowing process, most of the oxygen is blown through the top oxygen lance; however, part of the oxygen and other gases, such as Ar, CÔ 2 , N 2 , and hydrocarbons are inserted through the rather sophisticated multiple-hole tuyères in the furnace bottom. Ail thèse processes are shown in Figure 1. Refractory Requirements for BOF Vessels Refractories for BOF applications hâve
TOP BLOWN PROCESS
developed in parallel with BOF steelmaking technology. In the 1950s when the BOF process was introduced, mainly pitch-bonded bricks were used in European BQF linings. Ever increasirig operating températures and more efficient stirring of the steel bath required continuously improved BOF refractories. The improvements hâve been accomplished by using higher purity and higher density magnesia sinters and by introducing natural flake graphite which had to be protected by rrietaÛic powders against oxidation. AU materials hâve to be held together with a good organic binder. The rôles of thèse four raw materials will be discussed below. Présent BOF linings çontain the refractories listed in Table I. Most of the presently used BOF refractories are made from synthetic MgO sinter rather than from naturel magnesite. The types ôf refractories used in BOF linings vary from country to country, depending on operating parameters and the cost structure of raw materials in various countries. Doloma and magnesiadoloma refractories are used frequentiy in Europe, less in Japan, and practically not at ail in the United States. Today, high-performance BOF vessels in the United States use a high percentage (at least 75%) of magnesia-graphite refractories, with the remaining 25% divided between low-carbon-containing resin and/or pitch-bonded and pitchimpregnated, burned magnesia brick.
BOTTOM BLOWN PROCESS
COMBINED PROCESS
^P Figure 1. Diagrarhs of the top-blown, bottom-blown and combined BOF processes.
MRS BULLETIN/NOVEMBER1989
Selected Refractory Advances in Steel-Handling Systems
Rôle of Magnesia Grains. Most magnesia sinters used to produce highperformance refractories are made from sea water and/or from u n d e r g r o u n d brine. High-quality magnesia sinters contain 96-99% MgO. The main différence between the sinters made from the sea water and from the brine is the boron content. B 2 0 3 is usually lower in sinters made from underground brine. The higher boron content is associated with formation of the liquid phase at high températures and consequently lowers the refractoriness of magnesia refractories. The typical properties of some available sinters and fused magnesia are shown in Table II. The high purity and high density of magnesia is important for good résistance to steelmaking slag and for improving chemical stability of MgO in contact with carbon at high températures. The Rôle of Carbon and Graphite in MgO Refractories. Carbon is always présent in the matrix of magnesia refra
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