Effects of Temperature and Holding Time on the ...

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when the steel was solid (Run 5, 1773K), silica phase is found in the pellet. The Al2O3 content increases substan- tially in the chromite phase (spinel, referred as ...
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Effects of Temperature and Holding Time on the Sintering of Ladle Filler Sand with Liquid Steel  Bombeck, and Du Sichen Zhiyin Deng, Björn Glaser, Marc Andre

In the present work the effects of temperature and holding time on the sintering of ladle filler sand are studied. Laboratory experiments are carried out using pellets made of chromite based filler sand and two steel grades containing different contents of Mn and Al. It is found that the liquid steel plays a major role in the sintering behavior. The results also show that the amount of liquid phase in the sintered sand pellets increases with the increase of temperature and holding time. The Al2O3 content increases substantially in the chromite phase (spinel), especially in the region close to the liquid phase, when the temperature is high enough or when the holding time is long enough. Higher content of dissolved Al would accelerate the formation of the alumina-rich chromite.

1. Introduction During casting process, the blocking of the ladle well sometimes occurs due to the sintering of the well filler sand. In that case, oxygen lancing is usually used to remove the sand plug. The lancing procedure would seriously affect the steel cleanliness.[1–3] Therefore, improving the ladle freeopening rate is a very important task for the metallurgists. The sintering behavior of the sand was investigated by a number of researchers.[4–10] Some of them studied the sintering of sand in the absence of steel,[4–6] while some studied the interaction behavior between sand and liquid steel.[7–10] In a previous study by the present authors,[11] a comparison between the sintering behaviors with and without steel was carried out at 1873 K. It was found that the presence of liquid steel could enhance the sintering of the sand and that dissolved elements, e.g., Mn have great impact on the sintering behavior. Several metallurgists also proposed that the properties of the filler sand, such as composition and size distribution should be considered when in contact with liquid steel.[2,11] The industrial operation factors, such as temperature and ladle holding time, were also discussed in literatures.[1,4,6,12] However, most of those discussions were mainly based on statistics of industrial data. Little attention was paid to the

[] Z. Y. Deng School of Materials and Metallurgy, Northeastern University, Shenyang 110819 China Z. Y. Deng, Dr. B. Glaser, Prof. D. Sichen Department of Materials Science and Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden Email: [email protected] M. A. Bombeck PURMETALL GmbH & Co. KG, 46049 Oberhausen, Germany DOI: 10.1002/srin.201500277

ß 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

mechanism of the sintering and to explaining systemically the effects of temperature and ladle holding time. As a continuation of the previous study,[11] the focus of the present work is on the effects of temperature and holding time on the ladle well blocking when chromite based filler sand is used. The emphasis is given to the mechanism of the sintering and the impact of the presence of liquid steel.

2. Experimental 2.1. Materials and Pellet Preparation Two different steel grades (Steel-I and Steel-II) and a chromite based filler sand (provided by PURMETALL in Germany) were used in the laboratory experiments. The main compositions of the steel grades are listed in Table 1. Steel-I is a low alloyed steel, while Steel-II is basically pure iron with very low concentrations of different elements. The sand contains two phases, namely chromite (Fe, Mg) O  (Cr, Al)2O3 and silica. Most of the sand grains have the size range of 0–1 mm. The silica content of the sand is lower than 20 mass%, and the composition of chromite is listed in Table 2. Note that EDS was employed to analyze the composition in Table 2. In fact, the Cr2O3 content measured by XRF method[13] is nearly 6–8 mass% lower. In the experiments, some small sand pellets were used. The detailed description regarding the preparation of sand pellet can be found in a previous publication.[11]

2.2. Experimental Procedure The detailed description of the experimental setup can also be found in the previous publication.[11] As shown in

steel research int. 86 (2015) No. 9999

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Steel

C

Si

Mn

P

S

Cr

Al

I

0.23

0.30

II

0.005 0.003 0.06 0.004 0.004 0.01 0.001

1.05 0.010 0.005 1.20 0.025

Table 1. Compositions of experimental steels (mass%).

Figure 1a, the experimental setup mainly consists of a furnace (with alumina reaction tube) and a water-cooled quenching chamber. The quenching chamber is internally connected to the alumina reaction tube using O-ring sealing. Both molybdenum crucibles and alumina crucibles were used. The molybdenum crucible was only used for the sand pellets (see Figure 1b) to avoid the direct contact between sand and any oxide. The alumina crucible was employed to study the interaction between sand and steel as shown in Figure 1c. The experimental conditions for each run are given in Table 3. To enable better discussion, the results of two experiments from previous study[11] are also included in the table (marked). Before the start of a typical experiment, a graphite sample holder containing the experimental crucible was put in the quenching chamber. Afterward, the whole system was vacuum sealed. The system was evacuated and filled with high-purity argon gas for three times, before the furnace was heated to the target temperature. When it reached the temperature, the sample holder was lowered into the alumina reaction tube to a position of 1573 K, and kept there for 15 min to preheat the sample. Thereafter, the sample holder was lowered to the even temperature zone for reaction. The time was defined as time zero (t ¼ 0 min) at this moment. After the experimental time, the sample holder was immediately lifted into the quenching chamber. A high flow of argon gas was injected onto the sample to obtain an efficient cooling.

2.3. Analysis After the experiments the sintered sand pellets in contact with steel were cut and prepared for analysis. Both the morphology and phases of the sand were studied by a scanning electron microscope (SEM, HITACHI S-3700N) with energy dispersive spectrometer (EDS). Special attention was paid to the area close to the steel–sand interface in the microscopic study. The fractions of the liquid phase were evaluated using image analysis.

3. Results 3.1. Phases in Sintered Sand Table 3 presents the phases found in different sand pellets obtained under different experimental conditions. It can be seen that liquid phase (L) and chromite phase (C) are detected in all sand pellets. Note that the liquid phase is confirmed by the composition shown in Table 4 based on the phase diagrams of the MnO–SiO2–Al2O3 system and FeO–SiO2–Al2O3 system.[14] The amount of the liquid phase (in area percentage) is also given in this table. It is necessary to mention that the area percentage of the liquid phase is associate with certainties introduced by the uncertainty of image analysis. Although the values should be used with precaution, they reveal clearly that the amount of liquid phase increases with the increase of temperature and holding time. In the absence of steel, silica phase (S) is found in all the samples studied at different temperatures (1773–1923 K). Even in the case when the steel was solid (Run 5, 1773 K), silica phase is found in the pellet. The Al2O3 content increases substantially in the chromite phase (spinel, referred as aluminarich chromite in the following text) in the region having direct contact with liquid in the samples obtained at 1923 K (Runs 8 and 13) and the samples with longer holding time (180 min, Run 10). Note that FeO  Al2O3 and FeO  Cr2O3 spinel form solid solution over the whole range along the FeO  Al2O3–FeO  Cr2O3 join. It also shows that steel composition has substantial effect on the sintering behavior. For example, at 1923 K, a large amount of alumina-rich chromite grains are detected with Steel-I, while only a small amount is found in the case of Steel-II.

3.2. Effect of Temperature

3.2.1. Sand Without Steel The SEM images of the sintered sand pellets without steel are given in Figure 2. Note that the sample at 1773 K has many pores. Only trace of liquid phase (L) is seen in this sample. With increasing temperature, the amount of liquid phase increases and the pores in the sand pellets become less and less. This result is also in line with the increasing area percentage of liquid phase (see Table 3). In fact, even at 1923 K, the amount of the liquid phase (Figure 2d) in the sand is very small compared with the solid phases. The figures show evidently that silica grains are still found in the pellets after 60 min of reaction time.

3.2.2. Sand in Contact with Steel-I MgO

Al2O3

SiO2

Cr2O3

FeO

7–8

12–13