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Behaviors of Denitrogenation in RH-MFB
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JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2013, 20(7) : 40-44
Behaviors of Denitrogenation in RH-MFB ZHOU Jian 1 ,
QIN Zhe 1 ,
ZHANG Bo 2 ,
PENG Qi-chun 2 ,
QIU Sheng-tao 1 ,
GAN Yong1
(Γ. National Engineering Research Center of Continuous Casting Technology, Central Iron and Steel Research Institute, Beijing 100081, China; 2. Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Wuhan University of Science and Technology, Wuhan 430081, Hubei, China) Abstract: According to the analysis related to kinetic mechanism of vacuum denitrogenation and combining with the actual production of R H - M F B (a combination of Ruhstahl-Hausen vacuum degassing process with a multifunctional oxygen lance) at Liansteel, the limit step and model equation of vacuum denitrogenation are determined. Meanwhile, the influencing factors of nitrogen removal from liquid steel in vacuum of R H - M F B are analyzed. T h e results show that the limit step of vacuum denitrogenation in R H - M F B is the mass transfer of nitrogen in liquid boundary layer and the reaction follows first order kinetics. Keeping the necessary circulation time under the working pressure (67 Pa) is helpful to nitrogen removal from steel. T h e oxygen content in molten steel has little influence on the removal of ni trogen after deep deoxidation, while the sulphur content in liquid steel is always relatively low and has little effect on denitrogenation. T h e sharp decrease of carbon content in steel drives the process of denitrogenation reaction so as to exhibit a faster denitrogenation rate. T h e interfacial chemical reaction and argon blowing play a major role in the ni trogen removal when the carbon content in liquid steel is stable. Key words: R H - M F B ; denitrogenation; kinetics; influencing factor
Nitrogen is an important factor in influencing the quality of products. It not only reduces the me chanical properties of the steels, but also is the main cause of formation of cracks, skin blowhole, central porosity and so on. Using vacuum treatment device can decline the content of nitrogen to lower level in some condition. Therefore, Liansteel introduces a set of RH-MFB (a combination of Ruhstahl-Hausen vacuum degassing process with a multifunctional ox ygen lance) vacuum treatment equipment in early 2006 for improving the quality of the steel and im proving market competitiveness, and yet the equip ment has been put into trial production in November at the same year. This paper focuses on the behav iors of denitrogenation in the vacuum of RH-MFB.
1
from interior liquid steel to liquid boundary layer; the diffusion of [N] from liquid boundary layer to liquid/at mosphere interface; the chemical reaction of [ N ] oc curring in liquid/atmosphere interface, 2 [ N ] = N 2 ; N2 diffusion through the gas boundary layer; and the mass transfer of N2 from gas boundary layer to gas phase. The schematic diagram of the kinetic mass transfer of denitrogenation is shown in Fig. 1. Force of interface reaction 5?
fr.
vNsSSNS \XV\V\N> V\\ Kss
Gas boundary layer Reaction interface Liquid boundary layer
Kinetics of Vacuum Denitrogenation
1.1
Analyzing limit step of denitrogenation Because of avoiding the top slag, RH denitrogena tion process is relatively slightly affected by ladle slag. So the denitrogenation process in vacuum chamber can be regarded as five steps: the mass transfer of [ N ]
Fig. 1
Schematic diagram of kinetic mass transfer of denitrogenation
Biography:ZHOUJian(1968—), Male, Doctor, Senior Engineer! E-mail: zj@lysteel, comi Received Date: April 29, 2012 Corresponding Author: QIN Zhe(1977—), Male, Doctor, Engineers E-mail: [email protected]
Behaviors of Denitrogenation in RH-MFB
Issue 7
Denitrogenation velocity is decided by t h e slo west step. T h e convection of interior liquid steel and the pumping action of vacuum s y s t e m m a k e the rates of the first step and t h e fifth step fast, which are impossible to become the restrictive step of denitro genation process. C o n s e q u e n t l y , t h e restrictive step may be t h e second s t e p , the third s t e p , t h e fourth step or their coupling. A s s u m i n g t h e restrictive step respectively is the second s t e p , the t h i r d step and the fourth s t e p , yet the denitrogenation velocity can be expressed as follows1-1-1 : k\
T ? ( W [ N]
i«
-W[N]
r(W[N]—W[N]
(1)
)
(2)
)
^* RT
z
^
N
(3)
2e^
A m o n g t h e m , Eqn. ( 3 ) can be r e w r i t t e n as dwi[N] _ F 100M -km (pf. ^N2e)di RT pV F 100M W[N] ) (4) (W[N] - * . VpRTK'l, where,
kg. H o w e v e r , W[N] is usually less t h a n 0. 0 1 % . T h a t is, kg is t w o orders of magnitude higher t h a n kc. T h e r e f o r e , t h e diffusion of N 2 in gas boundary layer ( n a m e l y t h e fourth step) is impossible to become the restrictive step. 2) If t h e second step and t h e third step are the restrictive steps V a c u u m chamber is p u m p i n g constantly during R H - M F B refining process and the equilibrium con centration of nitrogen in gas phase is considered as W[N] « » 0 . T h e r e u p o n , Eqn. ( 1 ) and Eqn. ( 2 ) can become dw[ N ] _ . di
,
F
yW[N]
(7)
U
F
(8)
~k\
dW[N]
F
at
. 41 .
In
k\
W[N]
1
1
W[N]
W[N] 0
F
Ψ
[ N ] in m o l t e n steel at equilibrium; F is surface area of molten s t e e l ; V is volume of m o l t e n s t e e l , WN is mole a m o u n t of N 2 ; pu2 is partial p r e s s u r e of N 2 in gas p h a s e ; pN
e
is equilibrium partial p r e s s u r e of N 2 ;
M is relative molecular m a s s of N 2 , M = 2 8 g / m o l ; p is density of m o l t e n s t e e l , jO=7 000 k g / m 3 ; R is gas c o n s t a n t , R = 8. 314 J / ( m o l · K ) ; T is t h e r m o d y namic t e m p e r a t u r e ; K N
is reaction equilibrium con
stant of 2 [ N ] = N 2 , and under t h e t e m p e r a t u r e of 1600 "C, K N ' = 0 . 0 0 4 5 ; £ , , kc, kg are m a s s transfer coefficients; and i is denitrogenation time. S u b s t i t u t e the c o n s t a n t s into Eqn. ( 4 ) , and it can be expressed as Eqn. ( 5 ) . dwiI N ] .F (TO[N]—W[N] = - 1 . 269£, V at
)
(5)
Discussion of limit step of denitrogenation T h e second s t e p , the third step and the fourth step regarded as t h e restrictive steps are discussed, respectively. 1) If the fourth step is the restrictive step, compa ring Eqn. ( 2 ) and Eqn. ( 5 ) , t h e following equation can be obtained.
'
W[N]
w h e r e W[N]
F
L
vY' ! - i In W [ N : < 5 TO[N; 1
Y',-
(9) (10)
-fc,
d u t N] is denitrogenation velocity; W[N] is mass di
percent of [ N ] in molten steel; W[N] is mass percent of
2
di I n t e g r a t i n g Eqn. ( 7 ) and Eqn. ( 8 ) can obtain Eqn. ( 9 ) and Eqn. ( 1 0 ) .
F
·
1V —F
W[N] 0
(11)
kit
kct
(12)
is m a s s percent of initial nitrogen in
m o l t e n steel. P l o t t i n g Y'i and i , if the relationship is linear, t h e m a s s transfer in liquid boundary layer is the r e strictive step and the reaction is first order kinetics, and t h e linear slope is k\. P l o t t i n g Y 2 and i , if the relationship is linear, interfacial chemical reaction is t h e restrictive step and the reaction is a second order reaction, and also t h e linear slope is kc. W h e n the relationship of Y\ , Y' 2 and time are unlikely to be a linear relationship, the denitrogenation process is neither first order reaction nor second order reaction, but is controlled by b o t h interfacial chemical reaction and m a s s transfer in liquid b o u n d a r y layer [1 ~, 2] .
1. 2
(6) kc = l. 269/fe. T h e dimension of kc multiplied by W[N] is t h e same as kg from analyzing t h e dimension of kc and
1. 3 Determination of limit step and denitrogenation equation 1. 3. 1 Field sampling T h e equipment capacity of R H in Liansteel is 100 t and the average t r e a t m e n t cycle is 33 min. T h e steel samples are sampled from inlet, refining 3 — 5 , 10 — 1 5 , 20 — 25 min and t h e end during production p r o c e s s , and t h e n t h e s e samples are cut in order to m a k e into small specimen of about 0. 5 g. T h e n i t r o -
gen content in steel is measured by the TC-500 oxy gen-nitrogen analyzer made by LECO company of USA. The sample steel is SPHE (a kind of cold rolled automobile surface steel) and the typical ele ments before the treatment are shown in Table 1. The sampling heats in test process are random and the corresponding sampling time of per heat is recor ded during sampling process. Table 1 Pretreatment typical elements of SPHE (mass percent, % ) S
P
N
0.08
0.008
0.014
0.003 6
0.09
0.007
0.014
0. 0038
0.01
0.04
0.007
0.009
0.003 2
0.01
0.07
0.007
0.012
0. 003 5
C
Si
Mn
7230575
0.036
0.01
7230576
0.030
0.01
7230580
0.031
Average
0.032
0.7
▲ 7230575
1. 3. 2 Test results and determination of limit step The test results of nitrogen content in steel of two heats during RH-MFB refining process are shown in Table 2. Meanwhile, these data are put in to Eqn. (11) and Eqn. (12) for calculation. The ratio of F/V is taken as a constant for calculation. Table 2 Changes of nitrogen content and related calculation result during RH refining process
7230575
7230576
Treatment time/min
W[N]/10
6
Y\
y.
0
35
0
0
3
31
0.12
36.87
14
29
0.19
59.11
25
22
0.46
168. 83
32
20
0.56
214.29
0
38
0
0
15
32
0.17
49.34
20
28
0.31
93.98
32
25
0.42
136. 84
40
20.3
0.63
229. 45
According to the calculation data from Table 2, plotting Y' and t and the regression line is obtained as shown in Fig. 2 and Fig. 3 , respectively. By com paring the regression line of two heats, J?2 (the cor relation coefficient square) values of Y i are both higher than the values of Y 2 . If the value is higher, it is reflected that the curve equation is closer to the curves which is determined by the data points above and the reliability of the equation is also stronger. Therefore, the regression line of Y\ is closer to the real situation. T h u s , it can be confirmed that the re strictive step of nitrogen removal is the mass trans fer of nitrogen in liquid boundary layer and the reac-
|
0.6 - ■ 7230576 0.5 0.4
-
0.3
-
0.2
-
ri =0.0146? fi2=0.969 3
10 Fig. 2
30
20 //min
"
150
-
1
■ 7230576
▲
r2'=6.481 21 / ^=0.958 1 y
100
1
▲
40
Relationship of F', to time
▲ 7230575
200
50
7 y/m
Π =0.017 5ί Ä^O.963 6 .
À
0.1
250
Heats
Vol. 20
Journal of Iron and Steel Research, International
• 42 ·
/y± 10
20 //min
JT
m
ri =4.978 9/ Ä2=0.931 7
30
40
Fig. 3 Relationship of V'2 to time tion follows first order kinetics. 1. 3. 3 Model equation of denitrogenation Based on these analysis and calculation, the mass transfer of nitrogen in liquid boundary layer is used as the restrictive step of denitrogenation and the denitrogenation rate of the restrictive step can be characterized by the rate of the whole process, yet the expression is Eqn. (7). Fig. 2 can determine the F_ _ 0 . 0175 + 0. 0146 slope of straight line, k = jrki: 0. 016, and the mass transfer coefficient of k\ is equal V to 0. 016 X F' Fig. 4 shows the relationship of theoretical nitrogen content in liquid steel to time when the initial nitro gen content is 40X 10~6 (mass percent) and 30 X 10"6 (mass percent) , respectively. The scattered points in Fig. 4 are the actual test nitrogen contents. It can be seen that the nitrogen content in liquid steel is gradually decreased with the prolonged vacuum treatment time from the curves and scattered points in
Behaviors of Denitrogenation in RH-MFB
Issue 7
♦ Theoretical calculation . 7230575
A 7230576 * Lowest theoretical calculation • Other heats
10
20
30
40
50
initial rapid depressurization and maintain high vacu um degree of R H - K T B which is beneficial to n i t r o gen removal in Ref. [ 3 ] . T h e vacuum influences de nitrogenation kinetics of liquid steel under laborato ry conditions and t h e results s h o w t h a t t h e nitrogen content ( m a s s percent ) in liquid steel reaches 0. 002 0 % in 30 min under 67 Pa w h e n the initial ni trogen content (mass percent) is 0. 0 0 4 8 % [ 1 ] . T h a t is in accordance w i t h R H - M F B practical t r e a t m e n t effect of Liansteel.
f/min
Fig. 4 Relation of nitrogen content to time under theoretical calculation and actual conditions
Fig. 4. T h e s e scattered points are distributed above the curve of the inlet nitrogen content ( m a s s per cent) as 30 X 10~ 6 and a r o u n d the curve of t h e inlet nitrogen content ( m a s s p e r c e n t ) as 40 X 1 0 ~ 6 , and yet the denitrogenation equation can be regarded as the above curve in Fig. 4.
2 Analysis of Influencing Factors 2.1
Vacuum degree Fig. 5 s h o w s t h e relation of v a c u u m degree and nitrogen content in steel to t r e a t m e n t time during R H t r e a t m e n t process of Liansteel. F r o m Fig. 5 it can be concluded t h a t the p r e s s u r e in vacuum cham ber is sharply decreased to 4 k P a in first 3 min of r e fining process and the denitrogenation rate is the fastest in this p r o c e s s , so t h a t t h e curve slope exhib its negative in figure and t h e negative value is t h e maximum. Following w i t h t h e pressure further de creased to the operating vacuum degree of 67 P a , ni trogen content in steel is also decreased. After the pressure keeps s t a b l e , nitrogen content in steel is decreased continuously and denitrogenation rate is also relatively stable with the operation of alumin ium-deoxidizing and alloying further.
2. 2
Surface active elements ( O , S) O x y g e n and sulfur are regarded as the surface active elements which are enriched on t h e surface layer of liquid steel and have great influence on ni t r o g e n removal in m o l t e n steel. Sulfur content ( m a s s p e r c e n t ) in steel less than 0. 0 1 5 % can increase rapidly t h e denitrogenation rate in comparison with t h e higher sulfur content1-4-1. T h e sulfur content ( m a s s p e r c e n t ) in m o l t e n steel of Liansteel after t r e a t m e n t by R H is about 0. 0 0 8 % and changes little during refining p r o c e s s , t h u s it has little effect on nitrogen removal from liquid steel. Fig. 6 is the relationship of oxygen content to nitrogen content in steel. It s h o w s t h a t oxygen con t e n t in liquid steel drops sharply and t h e n the n i t r o gen content is also decreased. T h e oxygen content in steel has little influence on nitrogen removal after deep deoxidation, and at this time vacuum degree plays a major role in nitrogen removal. T h e denitrogena tion rate is large under t h e higher oxygen content and sulfur content by t h e contrast test w i t h or w i t h out carbon-oxygen reaction, so t h a t carbon-oxygen reaction has significant effect on denitrogenation process and accelerates nitrogen removal from liquid steel C 5 ] . O x y g e n mainly reacts w i t h carbon and gen erates a lot of CO gas bubbles when the oxygen con-
T h e above results are well fit for the conclusion of 110
40
32
*~^^W|Ni
1 80, 6
i 4
1 >
2 0 0
Fig. 5
28 o "■^Vacuum degree
5
"
15 25 Treatment time/min
m
35
content to treatment time
3
32 "
24
20 18
20 18
\
7230576
-X v
7230575\
^
1 28
24
Relation of vacuum degree and nitrogen
\
36
36
100
1 9°
• 43 ·
1
350
Fig. 6
250
_,
^
!-,>
150 100 8 w[O)/10-e
:
1
V1/
1
1
Relationship of oxygen content to nitrogen content in steel
Vol. 20
Journal of Iron and Steel Research, International
• 44 ·
tent is h i g h e r , while t h e low oxygen content at CO gas p h a s e / m e t a l interface of t h e process of d e n i t r o genation reaction is beneficial to nitrogen remov al1-6-1. T h e s e views also i n t e r p r e t t h e reason of good denitrogenation effect w h e n oxygen content is higher in Fig. 6. 2. 3
Carbon content and treatment temperature T h e basic t h e o r y of nitrogen removal in decarbonization process is t h a t t h e CO gas b u b b l e s gener ated from decarbonization form a lot of small vacuum c h a m b e r s and these b u b b l e s during u p w a r d m o v e m e n t process will carry nitrogen out of liquid steel so as to achieve the goal of nitrogen removal. T h e study of t h e nitrogen removal of V O D ( v a c u u m oxy gen decarburization) process s u g g e s t s t h a t the n i t r o gen desorption reaction occurs at the C O gas bubbles formed inside steel ( 5 1 % , volume percent) , at b a t h surface ( 3 6 % , volume p e r c e n t ) and injected A r gas b u b b l e ( 1 3 % , volume p e r c e n t ) [ 7 ] . T h e r e f o r e , car bon-oxygen reaction in vacuum plays a leading role in nitrogen removal. T h e initial carbon content is relatively high dur ing R H - M F B refining and the a m o u n t of decarburi zation is l a r g e r , and a lot of CO gas b u b b l e s are gen erated at t h e stage of drop lance blowing oxygen. It creates superior kinetic conditions for nitrogen r e m o v a l , t h u s removing of nitrogen in liquid steel ef fectively. Fig. 7 shows the relationship of carbon content to nitrogen content during R H process. From Fig. 7 , it can be seen t h a t w h e n t h e carbon content drops sharply the nitrogen content decreases accordingly. W h e n the carbon content tends to be s t a b l e , the de cline of nitrogen content mainly depends on b a t h surface denitrogenation and injected A r gas bubble denitrogenation.
During R H - M F B treatment process, the variation range of molten steel temperature in vacuum chamber is within 20—50 °C and the average t e m p e r a t u r e is over 1 600 °C , so t h e t r e a t m e n t process can be regarded as an isothermal p r o c e s s , the t e m p e r a t u r e variation has little influence on denitrogenation process.
3
1 ) T h e limit step of denitrogenation in R H M F B of Liansteel is the m a s s transfer of nitrogen in liquid b o u n d a r y layer and the reaction follows first order kinetics. T h e differential equation of denitro dw[ N] : genation can be regarded as and - * i di ~' VWm the m a s s transfer coefficient of nitrogen in liquid b o u n d a r y layer is ^=0.
3) W h e n oxygen content in steel drops sharply t h e nitrogen content is also decreased, but t h e oxy gen content in steel has little influence on nitrogen removal after deep deoxidation. T h e relatively low s u l p h u r content has little effect on denitrogenation. 4) T h e sharp decrease of carbon content in steel drives t h e process of denitrogenation reaction so as to exhibit a faster denitrogenation r a t e , and the in terfacial chemical reaction and argon blowing play a major role in t h e nitrogen removal w h e n t h e carbon content in liquid steel is stable. References: [1]
[3] 7230576 [4]
[5]
/ 500
400
100
60
[6] 20 0
Wie/10-6
Fig. 7
Relationship of carbon content to nitrogen content in steel
016X
F" 2) T h e denitrogenation rate is very fast in the case of t h e p r e s s u r e decreasing s h a r p l y , and keeping the necessary circulation time under the working p r e s s u r e is helpful to denitrogenation.
[2]
-w>300 140
Conclusions
[7]
FU Jie. Process Kinetics of the Metallurgy of Steel [ M ] . Bei jing: Metallurgical Industry Press, 2001 (in Chinese). GUO Han-jie. Metallurgical Physical Chemistry [M], Beijing: Metallurgical Industry Press, 2004 (in Chinese). ZHU Wan-jun. Investigation of Deep Denitrogenation Method During RH-KTB [ D ] , Wuhan: Wuhan University of Science and Technology, 2006 (in Chinese). CHENG Guo-guang, ZHAO Pei, XU Xue-lu, et al. Process of Vacuum Denitrogenation of Steel [ J ] . Iron and Steel, 1999, 34 (1) : 16 (in Chinese). FU Jie, CHANG He-ming, DI Lin, et al. Study on Kinetics of Nitrogen Removal From Liquid Steel Under Vacuum [ J ] . Iron and Steel, 2000, 35(10): 24 (in Chinese). Shinme K, Matsuo T, Morishige M. Acceleration of Nitrogen Removal in Stainless Steel Under Reduced Pressure [ J ] . Trans ISIJ, 1988, 28(4): 297. Kitamura T, Miyamoto K, Tsujino R, et al. Mathematical Re action Model for Nitrogen in Vacuum Degasser Desorption and Decarburization [ J ] . ISIJ Inter, 1996, 36(4): 395.
%
%# ScienceDirect
JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2013, 20(7) : 40-44
Behaviors of Denitrogenation in RH-MFB ZHOU Jian 1 ,
QIN Zhe 1 ,
ZHANG Bo 2 ,
PENG Qi-chun 2 ,
QIU Sheng-tao 1 ,
GAN Yong1
(Γ. National Engineering Research Center of Continuous Casting Technology, Central Iron and Steel Research Institute, Beijing 100081, China; 2. Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Wuhan University of Science and Technology, Wuhan 430081, Hubei, China) Abstract: According to the analysis related to kinetic mechanism of vacuum denitrogenation and combining with the actual production of R H - M F B (a combination of Ruhstahl-Hausen vacuum degassing process with a multifunctional oxygen lance) at Liansteel, the limit step and model equation of vacuum denitrogenation are determined. Meanwhile, the influencing factors of nitrogen removal from liquid steel in vacuum of R H - M F B are analyzed. T h e results show that the limit step of vacuum denitrogenation in R H - M F B is the mass transfer of nitrogen in liquid boundary layer and the reaction follows first order kinetics. Keeping the necessary circulation time under the working pressure (67 Pa) is helpful to nitrogen removal from steel. T h e oxygen content in molten steel has little influence on the removal of ni trogen after deep deoxidation, while the sulphur content in liquid steel is always relatively low and has little effect on denitrogenation. T h e sharp decrease of carbon content in steel drives the process of denitrogenation reaction so as to exhibit a faster denitrogenation rate. T h e interfacial chemical reaction and argon blowing play a major role in the ni trogen removal when the carbon content in liquid steel is stable. Key words: R H - M F B ; denitrogenation; kinetics; influencing factor
Nitrogen is an important factor in influencing the quality of products. It not only reduces the me chanical properties of the steels, but also is the main cause of formation of cracks, skin blowhole, central porosity and so on. Using vacuum treatment device can decline the content of nitrogen to lower level in some condition. Therefore, Liansteel introduces a set of RH-MFB (a combination of Ruhstahl-Hausen vacuum degassing process with a multifunctional ox ygen lance) vacuum treatment equipment in early 2006 for improving the quality of the steel and im proving market competitiveness, and yet the equip ment has been put into trial production in November at the same year. This paper focuses on the behav iors of denitrogenation in the vacuum of RH-MFB.
1
from interior liquid steel to liquid boundary layer; the diffusion of [N] from liquid boundary layer to liquid/at mosphere interface; the chemical reaction of [ N ] oc curring in liquid/atmosphere interface, 2 [ N ] = N 2 ; N2 diffusion through the gas boundary layer; and the mass transfer of N2 from gas boundary layer to gas phase. The schematic diagram of the kinetic mass transfer of denitrogenation is shown in Fig. 1. Force of interface reaction 5?
fr.
vNsSSNS \XV\V\N> V\\ Kss
Gas boundary layer Reaction interface Liquid boundary layer
Kinetics of Vacuum Denitrogenation
1.1
Analyzing limit step of denitrogenation Because of avoiding the top slag, RH denitrogena tion process is relatively slightly affected by ladle slag. So the denitrogenation process in vacuum chamber can be regarded as five steps: the mass transfer of [ N ]
Fig. 1
Schematic diagram of kinetic mass transfer of denitrogenation
Biography:ZHOUJian(1968—), Male, Doctor, Senior Engineer! E-mail: zj@lysteel, comi Received Date: April 29, 2012 Corresponding Author: QIN Zhe(1977—), Male, Doctor, Engineers E-mail: [email protected]
Behaviors of Denitrogenation in RH-MFB
Issue 7
Denitrogenation velocity is decided by t h e slo west step. T h e convection of interior liquid steel and the pumping action of vacuum s y s t e m m a k e the rates of the first step and t h e fifth step fast, which are impossible to become the restrictive step of denitro genation process. C o n s e q u e n t l y , t h e restrictive step may be t h e second s t e p , the third s t e p , t h e fourth step or their coupling. A s s u m i n g t h e restrictive step respectively is the second s t e p , the t h i r d step and the fourth s t e p , yet the denitrogenation velocity can be expressed as follows1-1-1 : k\
T ? ( W [ N]
i«
-W[N]
r(W[N]—W[N]
(1)
)
(2)
)
^* RT
z
^
N
(3)
2e^
A m o n g t h e m , Eqn. ( 3 ) can be r e w r i t t e n as dwi[N] _ F 100M -km (pf. ^N2e)di RT pV F 100M W[N] ) (4) (W[N] - * . VpRTK'l, where,
kg. H o w e v e r , W[N] is usually less t h a n 0. 0 1 % . T h a t is, kg is t w o orders of magnitude higher t h a n kc. T h e r e f o r e , t h e diffusion of N 2 in gas boundary layer ( n a m e l y t h e fourth step) is impossible to become the restrictive step. 2) If t h e second step and t h e third step are the restrictive steps V a c u u m chamber is p u m p i n g constantly during R H - M F B refining process and the equilibrium con centration of nitrogen in gas phase is considered as W[N] « » 0 . T h e r e u p o n , Eqn. ( 1 ) and Eqn. ( 2 ) can become dw[ N ] _ . di
,
F
yW[N]
(7)
U
F
(8)
~k\
dW[N]
F
at
. 41 .
In
k\
W[N]
1
1
W[N]
W[N] 0
F
Ψ
[ N ] in m o l t e n steel at equilibrium; F is surface area of molten s t e e l ; V is volume of m o l t e n s t e e l , WN is mole a m o u n t of N 2 ; pu2 is partial p r e s s u r e of N 2 in gas p h a s e ; pN
e
is equilibrium partial p r e s s u r e of N 2 ;
M is relative molecular m a s s of N 2 , M = 2 8 g / m o l ; p is density of m o l t e n s t e e l , jO=7 000 k g / m 3 ; R is gas c o n s t a n t , R = 8. 314 J / ( m o l · K ) ; T is t h e r m o d y namic t e m p e r a t u r e ; K N
is reaction equilibrium con
stant of 2 [ N ] = N 2 , and under t h e t e m p e r a t u r e of 1600 "C, K N ' = 0 . 0 0 4 5 ; £ , , kc, kg are m a s s transfer coefficients; and i is denitrogenation time. S u b s t i t u t e the c o n s t a n t s into Eqn. ( 4 ) , and it can be expressed as Eqn. ( 5 ) . dwiI N ] .F (TO[N]—W[N] = - 1 . 269£, V at
)
(5)
Discussion of limit step of denitrogenation T h e second s t e p , the third step and the fourth step regarded as t h e restrictive steps are discussed, respectively. 1) If the fourth step is the restrictive step, compa ring Eqn. ( 2 ) and Eqn. ( 5 ) , t h e following equation can be obtained.
'
W[N]
w h e r e W[N]
F
L
vY' ! - i In W [ N : < 5 TO[N; 1
Y',-
(9) (10)
-fc,
d u t N] is denitrogenation velocity; W[N] is mass di
percent of [ N ] in molten steel; W[N] is mass percent of
2
di I n t e g r a t i n g Eqn. ( 7 ) and Eqn. ( 8 ) can obtain Eqn. ( 9 ) and Eqn. ( 1 0 ) .
F
·
1V —F
W[N] 0
(11)
kit
kct
(12)
is m a s s percent of initial nitrogen in
m o l t e n steel. P l o t t i n g Y'i and i , if the relationship is linear, t h e m a s s transfer in liquid boundary layer is the r e strictive step and the reaction is first order kinetics, and t h e linear slope is k\. P l o t t i n g Y 2 and i , if the relationship is linear, interfacial chemical reaction is t h e restrictive step and the reaction is a second order reaction, and also t h e linear slope is kc. W h e n the relationship of Y\ , Y' 2 and time are unlikely to be a linear relationship, the denitrogenation process is neither first order reaction nor second order reaction, but is controlled by b o t h interfacial chemical reaction and m a s s transfer in liquid b o u n d a r y layer [1 ~, 2] .
1. 2
(6) kc = l. 269/fe. T h e dimension of kc multiplied by W[N] is t h e same as kg from analyzing t h e dimension of kc and
1. 3 Determination of limit step and denitrogenation equation 1. 3. 1 Field sampling T h e equipment capacity of R H in Liansteel is 100 t and the average t r e a t m e n t cycle is 33 min. T h e steel samples are sampled from inlet, refining 3 — 5 , 10 — 1 5 , 20 — 25 min and t h e end during production p r o c e s s , and t h e n t h e s e samples are cut in order to m a k e into small specimen of about 0. 5 g. T h e n i t r o -
gen content in steel is measured by the TC-500 oxy gen-nitrogen analyzer made by LECO company of USA. The sample steel is SPHE (a kind of cold rolled automobile surface steel) and the typical ele ments before the treatment are shown in Table 1. The sampling heats in test process are random and the corresponding sampling time of per heat is recor ded during sampling process. Table 1 Pretreatment typical elements of SPHE (mass percent, % ) S
P
N
0.08
0.008
0.014
0.003 6
0.09
0.007
0.014
0. 0038
0.01
0.04
0.007
0.009
0.003 2
0.01
0.07
0.007
0.012
0. 003 5
C
Si
Mn
7230575
0.036
0.01
7230576
0.030
0.01
7230580
0.031
Average
0.032
0.7
▲ 7230575
1. 3. 2 Test results and determination of limit step The test results of nitrogen content in steel of two heats during RH-MFB refining process are shown in Table 2. Meanwhile, these data are put in to Eqn. (11) and Eqn. (12) for calculation. The ratio of F/V is taken as a constant for calculation. Table 2 Changes of nitrogen content and related calculation result during RH refining process
7230575
7230576
Treatment time/min
W[N]/10
6
Y\
y.
0
35
0
0
3
31
0.12
36.87
14
29
0.19
59.11
25
22
0.46
168. 83
32
20
0.56
214.29
0
38
0
0
15
32
0.17
49.34
20
28
0.31
93.98
32
25
0.42
136. 84
40
20.3
0.63
229. 45
According to the calculation data from Table 2, plotting Y' and t and the regression line is obtained as shown in Fig. 2 and Fig. 3 , respectively. By com paring the regression line of two heats, J?2 (the cor relation coefficient square) values of Y i are both higher than the values of Y 2 . If the value is higher, it is reflected that the curve equation is closer to the curves which is determined by the data points above and the reliability of the equation is also stronger. Therefore, the regression line of Y\ is closer to the real situation. T h u s , it can be confirmed that the re strictive step of nitrogen removal is the mass trans fer of nitrogen in liquid boundary layer and the reac-
|
0.6 - ■ 7230576 0.5 0.4
-
0.3
-
0.2
-
ri =0.0146? fi2=0.969 3
10 Fig. 2
30
20 //min
"
150
-
1
■ 7230576
▲
r2'=6.481 21 / ^=0.958 1 y
100
1
▲
40
Relationship of F', to time
▲ 7230575
200
50
7 y/m
Π =0.017 5ί Ä^O.963 6 .
À
0.1
250
Heats
Vol. 20
Journal of Iron and Steel Research, International
• 42 ·
/y± 10
20 //min
JT
m
ri =4.978 9/ Ä2=0.931 7
30
40
Fig. 3 Relationship of V'2 to time tion follows first order kinetics. 1. 3. 3 Model equation of denitrogenation Based on these analysis and calculation, the mass transfer of nitrogen in liquid boundary layer is used as the restrictive step of denitrogenation and the denitrogenation rate of the restrictive step can be characterized by the rate of the whole process, yet the expression is Eqn. (7). Fig. 2 can determine the F_ _ 0 . 0175 + 0. 0146 slope of straight line, k = jrki: 0. 016, and the mass transfer coefficient of k\ is equal V to 0. 016 X F' Fig. 4 shows the relationship of theoretical nitrogen content in liquid steel to time when the initial nitro gen content is 40X 10~6 (mass percent) and 30 X 10"6 (mass percent) , respectively. The scattered points in Fig. 4 are the actual test nitrogen contents. It can be seen that the nitrogen content in liquid steel is gradually decreased with the prolonged vacuum treatment time from the curves and scattered points in
Behaviors of Denitrogenation in RH-MFB
Issue 7
♦ Theoretical calculation . 7230575
A 7230576 * Lowest theoretical calculation • Other heats
10
20
30
40
50
initial rapid depressurization and maintain high vacu um degree of R H - K T B which is beneficial to n i t r o gen removal in Ref. [ 3 ] . T h e vacuum influences de nitrogenation kinetics of liquid steel under laborato ry conditions and t h e results s h o w t h a t t h e nitrogen content ( m a s s percent ) in liquid steel reaches 0. 002 0 % in 30 min under 67 Pa w h e n the initial ni trogen content (mass percent) is 0. 0 0 4 8 % [ 1 ] . T h a t is in accordance w i t h R H - M F B practical t r e a t m e n t effect of Liansteel.
f/min
Fig. 4 Relation of nitrogen content to time under theoretical calculation and actual conditions
Fig. 4. T h e s e scattered points are distributed above the curve of the inlet nitrogen content ( m a s s per cent) as 30 X 10~ 6 and a r o u n d the curve of t h e inlet nitrogen content ( m a s s p e r c e n t ) as 40 X 1 0 ~ 6 , and yet the denitrogenation equation can be regarded as the above curve in Fig. 4.
2 Analysis of Influencing Factors 2.1
Vacuum degree Fig. 5 s h o w s t h e relation of v a c u u m degree and nitrogen content in steel to t r e a t m e n t time during R H t r e a t m e n t process of Liansteel. F r o m Fig. 5 it can be concluded t h a t the p r e s s u r e in vacuum cham ber is sharply decreased to 4 k P a in first 3 min of r e fining process and the denitrogenation rate is the fastest in this p r o c e s s , so t h a t t h e curve slope exhib its negative in figure and t h e negative value is t h e maximum. Following w i t h t h e pressure further de creased to the operating vacuum degree of 67 P a , ni trogen content in steel is also decreased. After the pressure keeps s t a b l e , nitrogen content in steel is decreased continuously and denitrogenation rate is also relatively stable with the operation of alumin ium-deoxidizing and alloying further.
2. 2
Surface active elements ( O , S) O x y g e n and sulfur are regarded as the surface active elements which are enriched on t h e surface layer of liquid steel and have great influence on ni t r o g e n removal in m o l t e n steel. Sulfur content ( m a s s p e r c e n t ) in steel less than 0. 0 1 5 % can increase rapidly t h e denitrogenation rate in comparison with t h e higher sulfur content1-4-1. T h e sulfur content ( m a s s p e r c e n t ) in m o l t e n steel of Liansteel after t r e a t m e n t by R H is about 0. 0 0 8 % and changes little during refining p r o c e s s , t h u s it has little effect on nitrogen removal from liquid steel. Fig. 6 is the relationship of oxygen content to nitrogen content in steel. It s h o w s t h a t oxygen con t e n t in liquid steel drops sharply and t h e n the n i t r o gen content is also decreased. T h e oxygen content in steel has little influence on nitrogen removal after deep deoxidation, and at this time vacuum degree plays a major role in nitrogen removal. T h e denitrogena tion rate is large under t h e higher oxygen content and sulfur content by t h e contrast test w i t h or w i t h out carbon-oxygen reaction, so t h a t carbon-oxygen reaction has significant effect on denitrogenation process and accelerates nitrogen removal from liquid steel C 5 ] . O x y g e n mainly reacts w i t h carbon and gen erates a lot of CO gas bubbles when the oxygen con-
T h e above results are well fit for the conclusion of 110
40
32
*~^^W|Ni
1 80, 6
i 4
1 >
2 0 0
Fig. 5
28 o "■^Vacuum degree
5
"
15 25 Treatment time/min
m
35
content to treatment time
3
32 "
24
20 18
20 18
\
7230576
-X v
7230575\
^
1 28
24
Relation of vacuum degree and nitrogen
\
36
36
100
1 9°
• 43 ·
1
350
Fig. 6
250
_,
^
!-,>
150 100 8 w[O)/10-e
:
1
V1/
1
1
Relationship of oxygen content to nitrogen content in steel
Vol. 20
Journal of Iron and Steel Research, International
• 44 ·
tent is h i g h e r , while t h e low oxygen content at CO gas p h a s e / m e t a l interface of t h e process of d e n i t r o genation reaction is beneficial to nitrogen remov al1-6-1. T h e s e views also i n t e r p r e t t h e reason of good denitrogenation effect w h e n oxygen content is higher in Fig. 6. 2. 3
Carbon content and treatment temperature T h e basic t h e o r y of nitrogen removal in decarbonization process is t h a t t h e CO gas b u b b l e s gener ated from decarbonization form a lot of small vacuum c h a m b e r s and these b u b b l e s during u p w a r d m o v e m e n t process will carry nitrogen out of liquid steel so as to achieve the goal of nitrogen removal. T h e study of t h e nitrogen removal of V O D ( v a c u u m oxy gen decarburization) process s u g g e s t s t h a t the n i t r o gen desorption reaction occurs at the C O gas bubbles formed inside steel ( 5 1 % , volume percent) , at b a t h surface ( 3 6 % , volume p e r c e n t ) and injected A r gas b u b b l e ( 1 3 % , volume p e r c e n t ) [ 7 ] . T h e r e f o r e , car bon-oxygen reaction in vacuum plays a leading role in nitrogen removal. T h e initial carbon content is relatively high dur ing R H - M F B refining and the a m o u n t of decarburi zation is l a r g e r , and a lot of CO gas b u b b l e s are gen erated at t h e stage of drop lance blowing oxygen. It creates superior kinetic conditions for nitrogen r e m o v a l , t h u s removing of nitrogen in liquid steel ef fectively. Fig. 7 shows the relationship of carbon content to nitrogen content during R H process. From Fig. 7 , it can be seen t h a t w h e n t h e carbon content drops sharply the nitrogen content decreases accordingly. W h e n the carbon content tends to be s t a b l e , the de cline of nitrogen content mainly depends on b a t h surface denitrogenation and injected A r gas bubble denitrogenation.
During R H - M F B treatment process, the variation range of molten steel temperature in vacuum chamber is within 20—50 °C and the average t e m p e r a t u r e is over 1 600 °C , so t h e t r e a t m e n t process can be regarded as an isothermal p r o c e s s , the t e m p e r a t u r e variation has little influence on denitrogenation process.
3
1 ) T h e limit step of denitrogenation in R H M F B of Liansteel is the m a s s transfer of nitrogen in liquid b o u n d a r y layer and the reaction follows first order kinetics. T h e differential equation of denitro dw[ N] : genation can be regarded as and - * i di ~' VWm the m a s s transfer coefficient of nitrogen in liquid b o u n d a r y layer is ^=0.
3) W h e n oxygen content in steel drops sharply t h e nitrogen content is also decreased, but t h e oxy gen content in steel has little influence on nitrogen removal after deep deoxidation. T h e relatively low s u l p h u r content has little effect on denitrogenation. 4) T h e sharp decrease of carbon content in steel drives t h e process of denitrogenation reaction so as to exhibit a faster denitrogenation r a t e , and the in terfacial chemical reaction and argon blowing play a major role in t h e nitrogen removal w h e n t h e carbon content in liquid steel is stable. References: [1]
[3] 7230576 [4]
[5]
/ 500
400
100
60
[6] 20 0
Wie/10-6
Fig. 7
Relationship of carbon content to nitrogen content in steel
016X
F" 2) T h e denitrogenation rate is very fast in the case of t h e p r e s s u r e decreasing s h a r p l y , and keeping the necessary circulation time under the working p r e s s u r e is helpful to denitrogenation.
[2]
-w>300 140
Conclusions
[7]
FU Jie. Process Kinetics of the Metallurgy of Steel [ M ] . Bei jing: Metallurgical Industry Press, 2001 (in Chinese). GUO Han-jie. Metallurgical Physical Chemistry [M], Beijing: Metallurgical Industry Press, 2004 (in Chinese). ZHU Wan-jun. Investigation of Deep Denitrogenation Method During RH-KTB [ D ] , Wuhan: Wuhan University of Science and Technology, 2006 (in Chinese). CHENG Guo-guang, ZHAO Pei, XU Xue-lu, et al. Process of Vacuum Denitrogenation of Steel [ J ] . Iron and Steel, 1999, 34 (1) : 16 (in Chinese). FU Jie, CHANG He-ming, DI Lin, et al. Study on Kinetics of Nitrogen Removal From Liquid Steel Under Vacuum [ J ] . Iron and Steel, 2000, 35(10): 24 (in Chinese). Shinme K, Matsuo T, Morishige M. Acceleration of Nitrogen Removal in Stainless Steel Under Reduced Pressure [ J ] . Trans ISIJ, 1988, 28(4): 297. Kitamura T, Miyamoto K, Tsujino R, et al. Mathematical Re action Model for Nitrogen in Vacuum Degasser Desorption and Decarburization [ J ] . ISIJ Inter, 1996, 36(4): 395.