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Thermodynamics of Titanium Oxide in Ladle Slags

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Thermodynamics of Titanium Oxide in Ladle Slags ISIJ International, Vol. 41 (2001), No. 12, pp. 1447–1453 Thermodynamics of Titanium Oxide in Ladle Slags Sung-Mo JUNG and R. J. FRUEHAN1) Formerly Postdoctoral research associate, at Department of Materials Science and Engineering, Carnegie Mellon University, now at Graduate School of Iron and Steel Technology, Pohang University of Science and Technology, Pohang 790-784, Korea. 1) Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213 USA. (Received on May 17, 2001; accepted in final form on August 27, 2001) It is necessary to have information on the thermodynamic behavior of titanium oxide in ladle type slags in order to control the titanium content in several grades of steel. In the present study, the thermodynamics was determined from the equilibrium between Fe–Csat–Ti and CaO–SiO2-–30%Al2O3–MgO–TiOx slags , in equilibrium with CO and from the equilibrium between Fe–Csat–(16 18)%Cr–Si–Ti and CaO–SiO2– 20%Al2O3–MgO–TiOx slags in equilibrium with CO. From the experiment with Fe–Csat–Ti alloy, the activity coefficients of TiO1.5 and TiO2 vary with basicity from 0.3 to 1.5 and from 0.5 to 2.3, respectively. And from , the experiment with Fe–Csat–(16 18)%Cr–Si–Ti alloy, the activity coefficients of TiO1.5 and TiO2 vary with ba- sicity from 0.4 to 1.4 and from 0.6 to 3.5, respectively. The results obtained from the equilibrium between , Fe–Csat–(16 18)%Cr–Si–Ti and CaO-SiO2–20%Al2O3–MgO–TiOx slags were used to estimate the titanium content of silicon-added stainless steel. Experiments were also conducted using Fe–Al–Ti melts in equilibri- um with CaO–SiO2–Al2O3–TiOx slags saturated with MgO. In this case most of the titanium in the slag is 41 present as TiO2(Ti ). The present results were used to predict the titanium content of aluminum-killed steel in equilibrium with ladle slags containing titanium oxide and the predictions agreed with plant data. KEY WORDS: ladle slags, titanium oxide, activity, silicon-added stainless steel, aluminum-killed steel. the formation of titanium carbonitride using the thermody- 1. Introduction namic data obtained. But these slags are not relevant to the Titanium is one of the most important alloying elements ladle processes. for the production of several grades of steels. As examples, No information on the thermodynamic behavior of titani- titanium content should be controlled in the refining of Ti- um oxide in ladle type slags is available. The objective of microalloyed HSLA steels, which are being produced for the present research was to obtain thermodynamic informa- line pipes, pressure vessels, and offshore oil constructions.1) tion on titanium oxide in ladle type slags for the control of Titanium is also used for the production of less expensive titanium in carbon steel, aluminum deoxidized steel, and ferritic grades of stainless steel which are suitable for auto- stainless steel. The composition ranges are given in Table motive exhaust application by replacing the use of expen- 1. This was achieved by investigating the equilibrium of Ti sive alloying elements such as nickel. Therefore, the addi- with carbon-saturated iron, Fe–Al alloy, or stainless steel tion of titanium to plain carbon steel and stainless steel in and the ladle type slags. the ladle for improving properties has increased in recent years. In order to predict the inclusions formed and the re- 2. Experimental lationship between titanium in the metal and slag, the ther- modynamic activity of titanium oxides in ladle type slag 2.1. Experimental Equipment and Procedure must be known. A MoSi2-resistor furnace with an alumina tube was used The number of previous investigations dealing with the for the equilibrium experiments. The master slag was pre- equilibrium between metals and slags containing titanium pared by melting mixtures of reagent grade CaO, SiO2, oxide is limited. Benesch et al.2,3) studied the distribution of Al2O3, and MgO in a MgO crucible using an induction fur- titanium between CaO–SiO2–MgO–Al2O3–TiO2 slags and nace in open air and the master alloy, carbon-saturated iron, carbon saturated iron. However, the slag composition they was melted in a graphite crucible under a carbon monoxide investigated was in a relatively narrow range, i.e., the MgO content of 5 or 7% and the lime to silica ratio of about one. Table 1. The composition range of slags investigated at In addition, they assumed only TiO2 in the molten slags. 1 873 K. 4) Kishi et al. measured both TiO1.5 and TiO2 contents in their studies on the equilibrium between CaO–Al2O3–TiOx slags and iron–chromium alloys. Recently Morizane et al.5) measured the activities of TiO1.5 and TiO2 in CaO–SiO2– MgO–Al2O3–TiO2 blast furnace type slags and predicted 1447 © 2001 ISIJ ISIJ International, Vol. 41 (2001), No. 12 atmosphere at 1 923 K. By chemical analysis, it was found monoxide atmosphere at 1 873 K. In this case, the oxygen that the slag composition was 47.97%CaO–13.18%SiO2– partial pressure is determined by C/CO equilibrium as 30.97%Al2O3–7.88%MgO and the master alloy was Fe– shown in Eq. (2). C –0.20%Ti. And for the experiment study on ladle type sat 1 slags of stainless steel, another master slag and alloy with C( s)15 O (g) CO(g) ........................(2) 2 2 54.3%CaO–13.0%SiO2–19.9%Al2O3–7.6%MgO–4.5%TiO2 and Fe–C –17.8%Cr–0.64%Ti were prepared by melting, 52 2 8) sat DG2° 111 700 87.65T (J/mol) repectively. In each experiment, proper amount of reagent grade chemicals were mixed with the master slag to adjust In the third set of experiments, where the ladle type slags were in equilibrium with Fe–Al–Ti alloys, the oxygen po- the initial TiO2 content and the slag basicity, i.e. the lime to tential is determined by Eq. (3). Therefore, it is necessary silica ratio. The initial slag compositions except for TiO2 to estimate the activity of Al O in the slag in order to cal- were chosen to be in the liquid region of CaO–SiO2–MgO– 2 3 6) culate the oxygen partial pressure. Al2O3 system at 1 873 K. For the equilibrium experiments between ladle type slags 3 and Fe–C –Ti alloys or Fe–C –18%Cr–Ti, 5.0 g of slag 2Al15O (g) (Al O ) .......................(3) sat sat 2 223 was made by mixing the master slag and TiO2, and 10 g of 52 1 9) the master alloys were equilibrated in a graphite crucible at DG3° 1 503 630 379.63T (J/mol) 1 873 K for 24 hr under a CO atmosphere. And for the equi- In this study, the activity of Al2O3 was based on the values librium experiments between ladle type slags and Fe–Al–Ti 10) alloys, 8.0 g of slags uniformly mixed and 15–20 g of alloys calculated by Rein and Chipman. They computed the ac- were melted in a MgO crucible under a purified Ar atmo- tivity of Al2O3 by Gibbs–Duhem equation, regarding the sphere. Since the Fe–Al–Ti experiments were conducted in slag as consisting of the four major oxide components, MgO crucibles, the slags were saturated with MgO and CaO, SiO2, Al2O3, and MgO along with minor proportions contained significantly more MgO. Preliminary experi- of other oxides and of sulfur. ments indicated for these experiments that 4 hr was suffi- The following relations are derived by using the equilib- cient to achieve equilibrium. The crucible containing a rium constant K3 in the Reaction (3). sample was then pulled out of the furnace and rapidly aAl O D GRT°52 ln 23 ......................(4) quenched in an Ar gas stream. The ground slag samples 3 2 ⋅ 32/ aPAl O and the metals were chemically analyzed for CaO, SiO2, 2 Al O , MgO, TiO and TiO and for C, Si and Ti, respec- 2 3 1.5 2 aAl O tively. K 5 23 ..............................(5) 3 2 ⋅ 32/ aPAl O 2.2. Chemical Analysis 2 5 O 1 Al 1 Ti The SiO2 content in the slag and silicon in metal was de- log fAl e Al[%O] e Al[%Al] e Al[%Ti]...........(6) termined gravimetrically by dehydration. The contents of CaO and MgO were determined by titration with KMnO In Eq. (6), the value fAl can be obtained by using the con- 4 centration of Al, O, and Ti, along with the related interac- and with EDTA, respectively. The Al2O3 content was deter- 11) mined by atomic emission spectrometry. The titanium in tion parameters. The computed oxygen potential in this metal and slag was analyzed by diantipyryl-methane ab- case is considerably less certain than that for the C–CO sorptiometry. The carbon in metal was analyzed by com- case. In particular, there are uncertainties in the activities of 1 bustion infrared spectroscopy. The contents of the Ti3 and Al and Al2O3. Ti41 ions in the slag were determined by the following 3.2. Activity and Activity Coefficient of TiO1.5 and method.7) A slag sample was dissolved under an Ar atmo- TiO 1 1 2 sphere into a mixture of hot HCl(1 1), HF(1 1), and the The experimental results for the first and second sets of 31 ferric ammonium sulfate solution. The content of Ti was experiments are given in Table 2 and Table 3, respectively. 21 obtained by titrating Fe ion, which was produced by the The activity and activity coefficient of one component in following ion exchange reaction, with potassium dichro- slags are usually determined by other oxides at a given tem- mate.
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