Removal of nitrogen from steel using novel fluxes
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I.
INTRODUCTION
IN order
to produce a high-quality steel, considerable effort has been given to removing various elements from liquid iron. As a result, it is now possible to reduce the concentration of many of the impurities, such as carbon, sulfur, phosphorus, oxygen, and hydrogen, in steel below the desired level. However, other than limited denitrogenization during vacuum degassing, no process exists for removing nitrogen from steel, and nitrogen picked up from the air during steelmaking processes aggravates this problem. No commercial practice using a flux or slag from steel in the ladle exists. Thermodynamically, it is possible to remove the nitrogen by using special slags that have a strong affinity for nitrogen, as shown by Suito and Inoue. tl] Previous work by Sasagawa et al. r2] indicated that a BaO-A1203-TiO2 slag could remove a considerable amount of nitrogen from steel. However, these slags would be expensive due to their high BaO content, and there also may be excessive Ti pickup in the metal. Therefore, it is the objective of this work to study the feasibility of denitrogenization of steel using lime-based slags, including additives to increase the basicity (BaO) or which have a strong affinity for nitrogen (TiO2). The nitride capacity for CaO-AI203-TiO2 slags and CaO-BaOAIzO3-TiO z slags and the activity o f TiO2 in the slag w e r e
measured. The activity of TiOz was determined in order to estimate the amount of titanium pickup in an aluminumkilled steel treated with the slag. The kinetics of the nitrogen reaction between the metal and slag were also investigated. II. T H E R M O D Y N A M I C CONSIDERATION FOR NITROGEN REMOVAL The reaction of nitrogen between slag and metal can be written as
3 3 [N] + 2 (02-) = (N3-) + 2 [O]
where the square brackets indicate species dissolved in the metal and the parentheses indicate species dissolved in the slag. In Reaction [1], it is assumed that nitrogen in the slag is present as a nitride ion. Similar reactions also can be written if the nitrogen in the slag is incorporated in the network, as descussed in detail by Ito and Fruehan, r3j Min and Fruehan, [41 and Ma~Jnez and Sano, [~j and first postulated by Mulfinger. J61 The equilibrium constant for Eq. [1] is given as fN3 (pct N)a3o/2 K[il - fN[pct N]a~/~?
METALLURGICALTRANSACTIONSB
[2]
In previous studies, as an analogy to other capacities (sulfide, phosphate, carbonate, etc.), [71 the nit_ride capacity was defined by Eq. [3] based on the ionic gasslag reaction expressed by Eq. [4]. The nitride capacity is a convenient method of expressing the ability of a slag to remove nitrogen and can be defmed by Eqs. [3] and [4]: p3/4 a 3o/22 02 CN3- = ( p c t N 3-) " ~ = K[4 ] - [3] --N2 fN 31 3 --N2 + (O 2-) = (N 3-)
+
3
0 2
[4]
where K[4], ao 2-, fN3-, and Cs3- are the equilibrium constant of Eq. [4], the oxygen ion activity, the activity coefficient of nitride ion, and the nitride capacity for the slag-metal reaction, respectively. It should be noted that the definition given by Eq. [3] assumes nitride
