cr0264_05 - 5 Deoxidation of Liquid Steel Steelmaking is a...

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5 ©2001 CRC Press LLC Deoxidation of Liquid Steel Steelmaking is a process of selective oxidation of impurities in molten iron. During this, however, the molten steel also dissolves some oxygen. Solubility of oxygen in solid steel is negligibly small. Therefore, during solidification of steel in ingot or continuous casting, the excess oxygen is rejected by the solidifying metal. This excess oxygen causes defects such as blowholes and nonmetallic inclusions in castings. It also has significant influence on the structure of the cast metal. Therefore, it is necessary to control the oxygen content in molten steel before it is teemed. Actually, the oxygen content of the bath in the furnace is high, and it is necessary to bring it down by carrying out deoxidation after primary steelmaking and before teeming the molten metal into an ingot or continuous casting mold. This chapter is concerned with thermodynamics and kinetics of deoxidation, and finally on industrial deoxidation. 5.1 THERMODYNAMICS OF DEOXIDATION OF MOLTEN STEEL The dissolution of oxygen in molten steel may be represented by the equation (5.1) where [O] denotes oxygen dissolved in the metal as atomic oxygen. For the above reaction, (5.2) where K O is equilibrium constant for Reaction (5.1), denotes partial pressure of oxygen in the gas phase in atmosphere, and h O is the activity of dissolved oxygen in liquid steel with reference to the 1 wt.% standard state. K O is related to temperature as 1 (5.3) Again, (5.4) where W O denotes the concentration of dissolved oxygen in weight percent, and f O is the activity coefficient of dissolved oxygen in steel in 1 wt.% standard state. In pure liquid iron, 1 2 --O 2 g () O [] = K O h O p O 2 12 ---------    equilibrium = p O 2 log K O 6120 T ----------- 0.15 + = h O f O = W O
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©2001 CRC Press LLC (5.5) The above relations would allow us to estimate W O in liquid iron at any value of with which the molten iron would be brought to equilibrium. This value of W O is nothing but solubility of [O] at that . However, oxygen tends to form stable oxides with iron. Therefore, molten iron becomes saturated with [O] when the oxide starts forming, i.e., when liquid iron and oxide are at equilibrium. This oxide, in its pure form, is denoted as Fe x O, where x is approximately 0.985 at 1600°C. For the sake of simplicity we shall take x equal to 1 often and designate this compound as FeO. For the reaction Fe x O(1) = x Fe(1) + [O] wt.%. , (5.6) where (5.7) Here, a Fe = the activity of Fe in the metal phase in the Raoultian scale (approximately 1), and denotes the activity of Fe x O in oxide phase. If the FeO is not pure and is present in an oxide slag, then a FeO < 1, and h (i.e., solubility of [O] in equilibrium with the slag) would be less. Example 5.1 Calculate the concentration of oxygen in molten iron at 1600°C in equilibrium with (a) pure Fe x O, and (b) a liquid slag of FeO-SiO 2 containing 40 mol.% SiO 2 .
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cr0264_05 - 5 Deoxidation of Liquid Steel Steelmaking is a...

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