Application of Al-free deoxidizer in rail steel manufacture

Application of Al-free deoxidizer in rail steel manufacture

Journal of University of Science and Technology Beijing Volume 15, Number 5, October 2008, Page 534 Metallurgy Application of Al-free deoxidizer in ...

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Journal of University of Science and Technology Beijing Volume 15, Number 5, October 2008, Page 534

Metallurgy

Application of Al-free deoxidizer in rail steel manufacture Wei Wu and Liu Liu Metallurgical Technology Research Department, Central Iron & Steel Research Institute, Beijing 100081, China (Received 2008-03-16)

Abstract: Because there are a lot of influences of alumina inclusions on the performance of rail steel, it is reasonable to adopt the deoxidization method with Al-free deoxidizer. According to the characteristics of Al-free deoxidization process, the control of aluminum content, the deoxidization with ladle furnace(LF) slag, the deoxidization process under vacuum by carbon, and the inclusions modification technology by calcium treatment were studied. All of them were applied to practical production. The results indicate that the adoption of Al-free deoxidization process leads to the total oxygen content in the steel below 20 ppm, which meets the requirement of clean rail steel. © 2008 University of Science and Technology Beijing. All rights reserved. Key words: rail steel; deoxidizer; refining; oxygen activity

Ca or Al.

1. Introduction Many domestic and oversea studies on the failure of steel rails [1-9] show that most fractures of the rail are caused by alumina inclusions in the steel after aluminum deoxidization. But a lot of plants adopt the process by Al-free deoxidizer, which includes the weak deoxidization of molten steel during tapping, and then the deep deoxidization with carbon under vacuum in RH treatment. The oxygen content in the molten steel after deoxidization decreases to 10-20 ppm. To develop the Al-free deoxidization technology Hiroyasu et al. [10] studied the thermodynamic data of equilibrium of Ca-O, Mg-O and Mg-Al-Ca-O. They obtained thermodynamic data of deoxidization by Ca, Mg and Mg-Al-Ca. Suito et al. [11-13] studied the deoxidization equilibrium by Ca, Al in the liquid iron, and obtained thermodynamic data of deoxidization by Table 1.

The characteristics of the deoxidization process with the Al-free deoxidizer for rail steels have also been studied to make an adjusting deoxidization technology and the low oxygen in the heavy duty rail steel.

2. Process control 2.1. Aluminum content control The comparison of inclusions in deoxidization processes of rail steel by aluminum and Al-free deoxidizer is shown in Table 1. It can be seen that after Al-free deoxidization process is adopted, B and C type inclusions in rail steel are greatly reduced. The inclusions grade for 90% heats of rail steel by Al-free deoxidization is lowed to the level of İ1.5; its quality can meet the standard requirement of the high speed rail steel.

Qualitative properties of rail steel with different deoxidization processes [1]

Steel mark

deoxidizer

T[O] /ppm

[Als] /%

PD3

Aluminum Al-free

15-43 8-20

U71Mn

Aluminum Al-free

17-38 12-20

Corresponding author: Wei Wu, E-mail: [email protected] © 2008 University of Science and Technology Beijing. All rights reserved.

Grade of nonmetallic inclusions B

C

D

0.010-0.012 0.002-0.005

2.0 1.3

2.5 1.2

1.0 0.83

0.007 0.002-0.004

1.71 1.17

2.0 1.3

1.0 0.6

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W. Wu et al., Application of Al-free deoxidizer in rail steel manufacture

It is important to control the aluminum content below 40 ppm to form the liquid deoxidization products. The ferroalloys with low aluminum content must be used to prevent the aluminum from entering into molten steel. The reaction between the alumina and the melts is: 2Al 2 O 3 (s) +3[Si]=3SiO 2 +4[Al]

'G Ƞ = 653205  126.4 T

(J ˜ mol 1 )

(1)

To reduce the aluminum content in steel, it is expected to decrease the activity of Al2O3 in the slag and to choose a better slag composition with suitable basicity. According to Eq. (1), the relation between [Als] and the oxygen activity in the slag of CaO-Al2O3-SiO2 can be calculated. The calculation conditions are 0.7% C, 0.6% Si, 0.9% Mn, 30%-60% (CaO), 0~20% (Al2O3), 4% (MgO), the other composition is below 3%. The temperature is 1550°C and R(CaO/SiO2) is 2-5. The activity interaction coefficients and the activity values of Al2O3 and SiO2 are selected from the published report [14]. The calculation results are shown in Fig. 1.

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Fig. 2 shows the relation between solvent aluminum and oxygen activity in the molten steel in equilibrium. It is seen that under the condition of [Als] exceeding 0.004%, the decrease of oxygen content by use of aluminum is not remarkable in the refining process. Generally the solvent aluminum content in steel is 0.001%-0.002% in the LF refining; its oxygen activity lies the range of 12-17.6 ppm. If the ratio between the total oxygen and the active oxygen is 3, the total oxygen content in the steel will be 36-53 ppm.

Fig. 2. Relation between solvent aluminum and oxygen activity in the molten steel under slag-steel equilibrium.

2.2. Deoxidization with slag in LF process To further reduce oxygen content in the molten steel, it is necessary to use the silicon deoxidization accordant with slag in LF. The reaction between silicon and oxygen is as follows. 1/2[Si]  [O]=1/2SiO 2 (s) ,

'G Ƞ =  290520  110.8 T (J ˜ mol 1 ) lg[%O] Fig. 1. Relation among solvent aluminum content, alumina content and slag basicity.

Fig. 1 shows that the content of solvent aluminum in the steel increases with increment of the alumina content in the molten slag. When the slag basicity (CaO/SiO2) changes from 2 to 5, the content of solvent aluminum in the molten steel increases. The heavy duty rail steel requires its total aluminum [Alt] content below 0.004%. If the ratio of [%Als]/[%Alt] is 0.9, the value of [Als] should be below 0.0036%. Under the condition of equilibrium between slag and steel, the value of [Als] below 0.0036% can be gained when the slag basicity is 3 and the content of (Al2O3) is below 5%, but the slag basicity (CaO/SiO2) is 2.5 even the content of (Al2O3) is upwards to 20%. So it is useful to decrease the LF slag basicity for the minor solvent aluminum content in the steel and the effective desulfurization at the same time.

1 1 15171.55 lg a sio 2  lg[%Si]  6.28  2 2 T

(2) (3)

According to Eq. (3), the deoxidization ability of silicon with different a SiO 2 in slag is calculated as shown in Fig. 3. The deoxidization ability of silicon can be intensified by adjusting the slag composition with lower activity of silicon dioxide. If the activity of silicon dioxide is 0.5, the oxygen activity value is 44.3 ppm, when the equilibrium silicon content is equal to 0.6%. While the activity of silicon dioxide becomes 0.1, the oxygen activity value will be 6.3 ppm on the same slag condition. The rational slag composition should be controlled as follows. (1) The slag basicity is 2.5-3.0; (2) The alumina content is below 10%; (3) The calcium oxide is 45%-50%. The silicon dioxide activity of above slag composi-

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tion is controlled in the range from 0.01 to 0.5. When the solvent aluminum content is below 0.004% and the silicon content is about 0.6%, the oxygen activity is controlled within 10 ppm, which meets the deoxidization requirement of rail steel.

J. Univ. Sci. Technol. Beijing, Vol.15, No.5, Oct 2008

cium treatment, which may deeply deoxidize the molten steel on one hand, and control the morphology and dimension of inclusions on the other hand. Calcium has the strong deoxidization ability, especially in existent Al2O3-containing slag. Calcium can reduce Al2O3 in the slag into calcium aluminate as the liquid deoxidization products. The reaction between calcium and aluminum is as follows. [Ca]+[O]=CaO (s) , 'G Ƞ =  645200  148.7 T (J ˜ mol 1 )

(4) 2[Al]  3[O] Al 2 O 3 (s) ,

'G Ƞ

1202000  386.3 T (J ˜ mol 1 )

(5)

Substituting Eq. (4) into Eq. (5), yields: 3[Ca]  A12 O 3(s) =3CaO (s)  2[Al] , Fig. 3. Relation between oxygen activity and silicon content with different silicon dioxide activities.

2.3. Deoxidization with carbon under vacuum Based on the thermodynamics, the equilibrium oxygen content with carbon will decrease with the decrease of vacuum pressure and the increase of carbon content as shown in Fig. 4. Under a vacuum pressure of 67 Pa, the equilibrium oxygen activity is far below 0.1 ppm for rail steel. But the actual oxygen content is controlled by the reaction kinetics. The critical bath depth of reaction procession keeps downwards with diminishing carbon or oxygen content. At that time, the restrictive procedure of [C]-[O] reaction moves from the mass transfer to the reaction at interface. The reaction rate becomes slower until it stops completely. But the lower actual oxygen content can be obtained for the rail steel of a higher carbon content. So the goal of deep deoxidization will be reached.

'G Ƞ

733600  59.8 T (J ˜ mol 1 )

(6)

On the CaO-Al2O3 phase diagram, the liquid phase 3CaO·A12O3 and 12CaO·7A12O3 appears at the steelmaking temperature. The 3CaO·A12O3 exists under the condition of a Al 2 O 3 =0.1 and a CaO =0.2˗but 12CaO·7A12O3 exists under the condition of a Al 2 O 3 =0.25 and a CaO =0.1. The predominance area graph of deoxidization products is calculated according to Eq. (6) as shown in Fig. 5. It is seen that when the aluminum content is 0.002% the liquid aluminates 12CaO·7A12O3 and 3CaO·A12O3 are obtained when the calcium content in molten steel is from 2.4 to 12 ppm.

Fig. 5. Relation among Ca, Al content and composition of deoxidization products for rail steel at 1873 K.

2.5. Application of Al-free deoxidizer Fig. 4. Relation between oxygen activity and vacuum pressure of RH degasser at 1600°C.

2.4. Calcium treatment and inclusions modification The technology with Al-free deoxidizer adopts cal-

The Al-free deoxidization process is applied in the production of rail steel. The results show that the oxygen content of steel is gradually decreased by refining steps. The oxygen content at the end of smelting is 1000 ppm, after deoxidization at tapping, the oxygen content is down to 175 ppm, and at LF and

W. Wu et al., Application of Al-free deoxidizer in rail steel manufacture

RH processing, the oxygen content becomes 98.2 ppm and 53 ppm individually; in the tundish it is 25.7 ppm, the oxygen content in the casting bloom is 22.8 ppm, and the oxygen content in the rail through the slow cooling is below 20 ppm. From observation on the different procedures, some considerations can be deduced. First, the end of point control of the combined blown converter should be improved to make a rather lower content of oxygen in the bath. It abates the secondary refining difficulty and the consumption of deoxidizing agent. Second, using proper slag composition and silicon deoxidization in tapping and LF, the oxygen content is rapidly decreased. Third, the deoxidization rate in RH is greater than 50%, which means that tundish metallurgy plays an important role for the rail steel. The quantities of inclusions in rail steel with Al-free deoxidizer apparently decrease. According to the results of electrolytic analysis of the nonmetallic inclusions in the production of rail steel, the total inclusions in rail steel by Al-free deoxidization process is less than that by Al deoxidization, especially the alumina inclusions decrease dramatically, from 33.5 ppm by aluminum deoxidization to 4.7 ppm by Al-free deoxidization. So the internal quality of heavy duty rail steel is good and the [Als] content meets the need of standard requirement of high speed rail steel.

3. Conclusions (1) To decrease the aluminum content of molten steel, the refining process should be processed under the condition of slag with a lower alumina content and a proper basicity for the prevention of aluminum reduction from it. (2) The rational LF slag composition (the basicity=2.5-3.0, alumina contentİ10% and calcium oxide content=45%-50%) can intensify the deoxidization by silicon and decrease the oxygen content of molten steel. (3) In RH treatment, the oxygen content of the steel will decrease with controlling the vacuum pressure in the degasser and improving the condition of carbon and oxygen reaction kinetics. (4) Calcium modification is adopted not only to deep deoxidization for molten steel but also to control

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the morphology and dimension of inclusions. (5) The deoxidization technology with Al-free deoxidizer is applied to the production of heavy duty rail steel and it obtains apparent effects of the oxygen content İ20 ppm in steel and the rails are qualified well.

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