Calcination of Slags for Electroslag Remelting

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JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2010, 17(8) : 01-05

Calcination of Slags for Electroslag Remelting DONG Yan-wu'*2,

JIANG Zhou-hua'

,

XIAO Zhi-xin' ,

LI Zheng-bang'

(1. School of Materials and Metallurgy, Northeastern University, Shenyang 110004, Liaoning , China; 2. Central Iron and Steel Research Institute, Beijing 100081, China) Abstract: The hydrides in industrial lime, alumina, magnesia, and calcium fluoride were investigated through differ-

ential thermal analysis and X-ray diffraction, and their mass losses during heating up were studied by thermogravimetric analysis method. The results indicate that the industrial alumina, lime, and magnesia, which have more moisture or hydride and mainly include y-AlzO3. Ca(OH)Z, and M g ( O H ) 2 , lose more mass during thermogravimetric analysis process. However, the mass of premelted slag consisting of lime, fluorite, alumina, and magnesia has almost no change, which means no hydride in it. Some relationships for calculating the mass loss were established according to the results of thermogravimetric analysis. These results will be in favor of setting up the rational calcination criterion for slag used in electroslag remelting process. Key words: electroslag ; calcination; differential scanning calorimeter; X-ray; thermogravimetric analysis

It is well known that just a few parts per million of hydrogen dissolved in steel can cause cracking and decrease the corrosion r e s i ~ t a n c e ~ ' - ~ Hydrides '. in slag can induce the hydrogen increase in steel, especially for slag without special Industrial lime, calcium fluoride, alumina, and magnesia are the common slag component used in electroslag remelting processr6'. A t present, the calcination at a certain temperature is used to get rid of hydration in slag; however, the calcination temperature and calcination time are usually determined by experience, which is lack of academic support^[^-^^. In this study, experiments have been designed to investigate the characteristics of industrial lime, fluorite, alumina, and magnesia through differential thermal analysis, X-ray diffraction, and thermogravimetric analysis.

1

Differential Thermal Analysis

The experiments were conducted at 100 - 1100 'C using the apparatus of DSC851e differential thermal system. Fig. 1 shows the curves of DSC (differential scanning calorimeters and T G ( thermogravimetric

analyzers) of industrial lime, alumina, calcium fluoride, and magnesia, respectively. These figures show that endothermic peaks appear in the curve of lime, alumina, and magnesia, and no obvious peaks emerge for calcium fluoride. It can be seen from Fig. 1 ( a ) that two endothermic peaks appear for lime; one is at the range of 400- 530 "C, and the other is at 600-780 'C. It should be the decomposition temperature of Ca(OH), in the range of 400-530 "C and CaC03 at 600-780 'C on the basis of the theoretical calculationC8]. T o decrease the hydride in lime, roasting temperature should be at least over 530 "C. For the industrial alumina and magnesia, the only endothermic peaks appear at 260-310 "C and 350 - 430 "C, respectively. T o identify the substance of phase transformation, further experiment must be done.

2

Experiment of X-Ray

T h e apparatus of X' Pert Pro MPD made in Holland was used to investigate the main compositions in Al(OH), and Mg(OH),. Fig. 2 shows the

Foundation Item: Item Sponsored by National Natural Science Foundation of China (50904015) ; Postdoctoral Science Foundation of China (20090450020) ; Fundamental Research Funds for Central Universities of China (N090402012) Biopraphy:DONG Yan-wu(1978-), Male, Post-Doctor, Lectureship; E-mail: yanwu_dong@163. com; Received Date: August 3, 2009

Journal of Iron and Steel Research, International

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(a) Lime; (b) Industrial alumina; (c> Calcium fluoride; (d) Magnesia. Fig. 1 DSC and TG curves for different materials at a temperature rise rate of 10 'Ic /s

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Fig. 2 X-ray diffraction pattern of industrial alumina (a) and magnesia (b)

results of Al(OH), , a-Alz03, and y-AlZO3in industrial alumina. Al(OH), is the hydride in aluminaCg3 and the y-Alz03 is prone to absorb water owing to higher surface area"']. The higher the y-A1203 and Al(OH), contents in alumina are, the higher the water or hydride is. However, the a-Alz03 is stable and there is almost no change at moist atmosphere. The thermal decomposition temperature of Al(OH), is over 950 CCr1 ; therefore, the phase transformation in the differential thermal analysis of alumina is due to the volatilization of water in y-Al2O3. The substance of phase transformation in Fig. 1 (d) should be Mg(OHI2 according to Fig. 2 (b).

3 Themogravimetry Two kinds of mixtures were investigated in following experiments. One was composed of 70% of CaFz and 30% of AlzQ , named slag LO in this study, which was the conventional slag used in electroslag remelting process, and the other consisting of CaF2CaO-Al2O3-SiO2-MgOwas named slag L1. Isothermal mass loss of slag An electrical resistance furnace was used in thermogravimetric experiment, as shown in Fig. 3. In this experiment, slag is put into the furnace when the furnace temperature reaches 800 'C , and then

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Issue 8

Calcination of Slags for Electroslag Remelting

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Effect of thickness on mass loss of slag A parameter f must be defined to investigate the mass loss ratio of materials, as follows :

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mass loss is measured continually by an electronic balance combined with computer. Fresh slag without any treatment and premelted slag were investigated in the experiment. Fig. 4 ( a > and ( b > show the process curves of experiments. The mass of premelted slag almost keep unchanged during the whole process, which indicate that hydrides have almost been removed and the premelted slag are favorable for decreasing hydrogen levels in steel. However, the mass loss is large for fresh slag especially for slag L1 because it contains much lime and CaO is the main composition in lime, which is prone to absorb water vapor in moist atmosphere to form Ca(OHI2[ll'.

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where, W,is the initial mass of slag; W, is the instantaneous mass of slag; and W, is the final mass of slag after roasting. In this experiment, the isothermal mass loss of slag L1 was investigated. The experiment was carried out at 800 'C using the thermogravimetric apparatus. Various curves could be obtained in the process when different thicknesses of slag were used. Then, the relationship of time and mass loss ratio is obtained for different thicknesses of slag. It can be seen from Fig. 5 that the slag with a larger thickness needs a longer time for complete mass loss. The relationship of baking time and thermal decomposition ratio is almost linear before 80% of mass loss. The experiments are coincident with the thermal decomposition theory of solid phaseC''', and the process can be described as follows. The kinetics of thermal decomposition process was determined by the nucleation rate and growth of nucleus crystal and their expansion rate at the beginning and the intermediate stages. The process is rapid and reaches 80% rapidly. The kinetics of thermal decomposition process is controlled by diffusion. The thin film forms with the reaction; furthermore, the thickness increases gradually, which limits the rate of the process. Fig. 6 shows the relationship of baking time and complete decomposition for different thicknesses of slag. It seems that both thermal transmission and diffusion may be restrictive step when the nucleation rate, growth, and expansion of crystal are almost the

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TG curves of fresh and premelted slag at 800 'c

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Relationship of mass loss ratio to baking time for different thicknesses of slag L1 at 800 'c

Vol. 17

Journal of Iron and Steel Research, International

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Relationship of thickness to baking time for slag L1 at 800 “c

same. However, the amount of slag used in present experiment is small and the temperature of slag will reach the furnace temperature quickly when slag is put into furnace. Therefore, diffusion should be the main controlling factor in this process. FENG Yang-jieC131 found that kinetic parameters had some change when the mass of the specimens was different. Activity energy decreased when the mass increased because alterative diffusion path and resistance affected the decomposition mechanism owing to the increase of mass. ‘. The birth of a large number of nucleus crystal owing to the increase in slag mass results in the increase in the product film thickness. The diffusion rate became gradually the restrictive factor with the increase in the thickness of product film at the final period. Therefore, the difficult diffusion of the slag results in a large thickness of the slag. A fitted relationship of baking time to complete decomposition is obtained in terms of the experimental results. r = l O . 52le(h/38.833)-5.657 (2) where, r is the baking time; and h is the slag thickness.

Effect of baking temperature on baking time Further experiments were conducted at different temperatures, and the purpose is to investigate the relationship of baking temperature and the baking time. The result is illustrated in Fig. 7. The thickness of slag is 31 mm in the experiment. It can be observed that the baking time decreases with the increase in the baking temperature at a same thickness because the more activated molecules are nucleated at a higher temperature.

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Fig. 7 Relationship of baking temperatureto baking time for slag L1

Similarly, a fitted relationship of baking time to baking temperature was obtained in terms of the experiment results. r=286. 7e(-t/314.83) +2.46 (3) where, t is the baking temperature.

4

Conclusions

1) The substance of CaCQ , y-A1203, Al(OH), , and Mg(OHI2 is present in lime, alumina, and magnesia, which is the main hydride in them. No hydride is observed in calcium fluoride. 2) The lowest baking temperature of slag used for electroslag remelting process should be over 530 ‘C according to the differential therinal analysis and X-ray. 3) Thermogravity experiments indicate that more hydrides are present in fresh slag; however, small mass loss is observed for premelted slag, 4) The results obtained in the present study are favorable for setting the baking criterion. References:

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Toribio J , Ovejero E. Microstructure-Based Modeling of Hydrogen Assisted Cracking in Pearlitic Steels [J]. Materials Science and Engineering, 2001, 319-321A: 540. Pishko R, McKimpson M, Shewmon P G. The Effect of Steelmaking on the Hydrogen Attack of Carbon Steel [J]. Metallur gical Transactions, 1979, lOA(7) : 887. Takagi S, Terasaki S, Tsuzaki K, et al. A New Evaluation Method of Hydrogen Embrittlement Fracture for High Strength Steel by Local Approach [J]. ISU International, 2005, 45(2): 263. Jauch R , Choudhury A, Tince F, et al. Electroslag Remelting Process at Rtjchling Burbach for Heavy Forging Ingots of 2 300 mm Diameter [J]. Iron and Steelmaking, 1979, 6(2) : 75. Nakamura Y, Harashima R. Hydrogen Contents of Slag and Ingot in the Electroslag Remelting Process (ESR) CJ]. Journal of the Iron and Steel Institute of Japan, 1977, 63(8): 1235.

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Kiyoshi Okada, Yoshitoshi Saito, Masanori Hiroki, et al. Water Vapor Sorption on Mesoporous Gamma-Alumina P r e pared by the Selective Leaching Method [J]. Journal of P o r ous Materials, 1997, 4(4): 253. SHI Hui-sheng, ZHAO Yu-jing, LI Wen-wen. Effects of Temperature on the Hydration Characteristics of Free Lime [J]. Cement and Concrete Research, 2002, 32(5): 789. S U Yi-zeng. Introduction of Solid Chemistry [MI. Beijing: Beijing University Press, 1986 (in Chinese). FENG Yang-jie, CHEN Wei, ZOU Wen-qiao. Kinetic Mechanism of Solid Thermal Decomposition With Non-Isothermal Gravimetric Analysis [J]. Journal of East China University of Science and Technology, 1991, 17(5) : 591.