Distribution of Nitrogen between Molten Steel and Slags Containing Titanium Monoxide
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Ox >CeO2 >ZrO2 >SiO2 >Al2 O3 > B2 O3 >MgO>CaO>BaO TiOx (x = 1.5 to 2) is currently recognized as the most effective constituent in a slag for removing nitrogen. It has also been reported that no considerable amount of nitrogen can be removed from molten steel by slag without applying very low oxygen potential. Despite progress made by previous studies, further efforts are required to find more effective slag systems. The purpose of this study is to investigate the removal of nitrogen from steel by newly proposed slag systems: TiO slag and Ti2O3 slag. These slags are expected to be much more effective than TiO2 slags. The nitrogen distribution ratio between the proposed slag systems and molten steel is measured by using two new slag-metal equilibration techniques. For the equilibrium of nitrogen between molten steel and slag, the reaction may be written as ½N þ
3 O2 ¼ 2
CN ¼
PENG FAN, Research Associate, and W.D. CHO, Research Associate Professor, are with the Department of Metallurgical Engineering, University of Utah, Salt Lake City, UT 84112. Contact e-mail: [email protected] Manuscript submitted June 2, 2005. Article published online June 27, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS B
½1
where the elements in square brackets are dissolved in the metal and the parentheses indicate the species dissolved in the slag. Using 1 wt pct standard state for nitrogen, the nitride capacity for slag-metal reaction, CN , can be expressed by Eq. [2]: Ka
The removal of nitrogen has become extremely important in view of its adverse effects on the chemical, mechanical, and physical properties of many steels, and thus, its content should be reduced to a minimum. Two methods have been attempted to remove nitrogen from molten steel: vacuum degassing and flux treatment.[1] After discovering the inefficiency of the degassing process,[2] the investigations have focused on the various slag systems. Extensive research has been conducted for dozens of binary, ternary, and multicomponent slag systems containing CaO, Al2O3, SiO2, B2O3, and TiO2 and many other oxides to find effective slag systems for nitrogen removal from molten steel.[2–17] Hereafter, TiOx (x = 1.5 to 2) will be used to reflect the coexistence of TiO1.5 (i.e., Ti2O3) and TiO2 in slag, because some previous studies showed that a part of TiO2 initially added in a slag was reduced to Ti2O3 under the strongly reducing atmosphere or by metal employed in the experiments. Most of the studies attempted to measure nitride capacity as one of the indexes to express the ability of a slag to remove nitrogen from molten steel. The comprehensive review of nitride capacity data reported in the literature indicates that the comparative nitride capacity of an oxide is as follows:
3 N3 þ ½O 2
3=2 ðO2 Þ
fðN3 Þ
3=2
¼
ðpct NÞ aðOÞ ½pct N f½N
½2
where K is the equilibrium constant for Eq. [1]; [pct N], a½O , and f½N represent the nitrogen content, oxygen potential, and nitrogen activity coefficient in the metal phase, respectively. Though nitride capacity is usually used to
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