cleans steel production technology for ultra low carbon steel
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Cleans Steel Production Technology for Ultra Low Carbon Steel
Synopsis; By HY£ON- SOO CHOIII SUNGMO SEO'I JU HAN CHOpl SEOK-Y()I.ING PARK'I JUNG-HYUNG PARK51 KWANG-CHUN KIM'"
Steel cleanness of ultra low carbon steel is affected by many factors such as oxygen b lOwing In RH, Impurities 01 Fe alloys and slag deoxidation J'lr;tct lce . Oxygen btowing In RH is one of the detrlmel\\<ll lactors thaI deteriorale steel cleanness. For minimization of oxygen blowing in RH , following countermeasures have been applied . Selting up a standard of end point temperature and Iree oxygen conlents in molten steel from a BOF, conlrolling of temperature in the bubbling slation and slag deoxidation after bubbling are applied 10 control steel temperature and free oJlygen contents of molten steel in RH arrival and these countermeasures result in the reduction of oxygen btown ratio under 5% . The erlect of impurities in alloys such as oxides in Ti sponge was Identified and countermeasure has been applied to solve nozzle clogginC . In addition, the secondary slags kill ing pract ices was done to improve steel cleanness and above measures were confirmed for improving nozzle clogging and defects in cold rolled products that caused by inclus ions from stee!m aklng process . Key words: double slag practice , slag deoxidation , nozzle clogging, ultra
tow carbon steel
1. I ntroduction
Reoxidation of steel during secondary refining or after finishing refining causes problem for the production of clean steels . AlorTi is oxidized by the reducible oxides such as FeO, MnO and SiOz in slag , fluxes or refractories in the ladle and tundish . The major oxygen sources for reoxidation of molten steel are FeO, MnO, and Si02 in slag and oxygen in the atmosphere. Reoxidation phenomena of AI are investigated by many authors with different slag com position. 1-5)
The addition of slag deoxidanl during BOF tapping has been a com m only adopted practice to reduce the FeO in teeming ladle slag and to improve steel cleanness in ultra low carbon(ULC) steel production . However. the addition of slag deoxidant during tapping causes operational problems such as phosphorus reversion or insufficient free oxygen in molten steel for decarburization in RH . To cope with this problem, double slag practice was adopted to solve phosphorus reversion problem. Steel cleanness of ultra low carbon steel is deteriorated by oxygen blowing in RH that caused by insufficient free oxygen or inaccurate temperature control. For minimization of oxygen blowing in RH, following counter measures have been applied, Setting up a standard of end point temperature and free
oxygen contents in molten steel from the BOF, controlling of temperature in the bubbling station and slag deoxidation after bubbling are applied to control steel1emperature and free oxygen con1ents of m allen steel at RH arrival. In present trials, part of slag deoxidant was added just after bubbling in bubbling station and add itional slag deoxidant was added after decarburization in RH to reduce T. Fe in ladle slag.
2. Phosphorus control in the BOF
Minimization of operational variations is impoFtant to achieve consistent qual ity of ULC steels. In order to reduce variations. operational improvement such as double slag practice in BOF has been applied.
Figure 1 shows the steelmaking process forthe production of ULC steels in Gwangyang Works. POSCO. Molten steel after tapping from the BOF is transferred to a bubbling station . Molten steel temperature is adjusted in the bubbling station by the addition of coolant or bubbling considering present steel temperature, RH arrival time and temperature. Slag deoxidation practice is applied after bubbling and additional slag deoxidation is done after decarburizatlon in RH,
Ii Principal researcher, Sleeimaking Research Group, POSLAB, POSCO, Pohang, Korea 11 Senior researcher, Steelmaking Research Group, POSLAB, POSCO, Gwangyang, Korea .11 Junior researcher, Steelmaking Research Group, POSLAS, POSCO, Gwangyang, Korea
J } ,16' Steelmaking Department. Gwanayang works, POSCO, Gwangyang Korea
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cc
EJ:I ~~~ ~~,. q WI Figure 1. Steelmaking process for Ihe product ion of ULC steel.
O .... P SI80 discharging Oe-C T8Pl>lng SI8g c0811ng
u Figure 2. Operational procedure of the double slag practice .
700 - ------- .. - . .
600 - . • •• •• { 500 'r-.. :-•• _ • _ • . • - • i _. .- .. -.-._~
• ... --- ... ~ 3GQ
•
• t ... e;:
• 100 r-
,0<1 r-
2000 2200 2400 2600 2100 3000 1200 34Q0
O.YIl_n Con$umptlon(N7 ) during 1st blowl"g
Figure 3 . The relationship between oxygen consumption and [P] after 1" blowino .
80
70 ....•....•..•.•• •••.•. .. .••••. . .• .. ! ..... ........ ....... ............. .. ... : 60 •......• . • . •.•.... . •.... . ..• .••• .. ... . ..• .!\ ............................. j
.:::::::::::::::::::::~·:~~~·:::~~~~~:s~:::::: : :::~:::::::~::.:: !
...... .. . .. ................... .. .. .... .. ...... ........... ..... ....... .. ..
20 ..... • .•.•..•..• ...•..•.• .........••.• y ·;:i·4·ti.;;,·>?' ~· 'i, ·5:1'7;':·j.j ~';g'i ":
10 .. ........... . .. . . . ... . . . . . ... . .. . ............... R.~ .~.Q,g?.o.~ ........ .... . :
1.00 1.50 2.00 2.50
Obs . S(C.o/sI021
Figure 4 . The changes of De- P ralio after 1" blowing w ith observed bas icity .
;) .00
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Figure 2 shows the operational procedure of the double slag practice in the BOF . Fluxes such as burnt lime . iron are and recycled ladle slag are added before ,SI
blowing in the BOF. The am ount of iron ore is adjusted depend on Si contents and temperature of molten steel. The oxygen blOWing was slopped when molten slag over tlowform the mouth occurred. Afterthe 1,1 blowing end . slag was discharged by tilting the converter until molten iron is obseNed . The second oxygen blowing was done after slag discharging .
Figure 3 shows the relationship between oxygen consumption and [P] after 1" blowing . P content in molten iron is lower as the amount of oxygen gas used increases. Observed basicity of slag depends upon Si contents and burnt lime added . Figure 4 and 5 show the changes of De-P ratio and P contents after 1£1 blowing with observed basicity respectively . The P content in molten steel is lowered when the observed basiCity of slag is increased as expected.
Figure 6 shows the changes of [P J at 2''£1 blowing end with total Fe in slag. The phosphorus content after 2nd blowing ranges from 0.006to 0.015% and that made easy application of slag deoxidation practice after tapping from the BOF. On the contrary, the phosphorus content rises over 0.0015%when total Fe content in slag is low and that made slag deoxidation practice not possible especially for the steel grades requiring P lower than 0.015% .
In addition to the application of double slag practice to reduce operational variations, optimum campaign life of the converter has been set up considering cost and operational variations especially during the last stage of the converter campaign. Figure 7 shows the changes of [0] at blow end with the converter life and there is sharp increase [01 at blow end toward the end of the campaign life . It is caused by improper m ixing characteristics of molten steel when the ratio of height/diam eter of steet bath is relative small that occurs during the end of the campaign life . The campaign life was controlled between 5500 and 6000 heats to minimize large variations of molten steel at blow end .
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GOO •••• 0 •• ______ • _ .. _' - __ - _: . _ •• _. 0 ••• _____ " _______ • 0 •• ••••• _0 _______ 0)
... ..... ....... :: :+::: :!~ .. ~·--:t : ... ;'.:.: ':::.~ ':. ~ ~<!: : ::::: ~:: .. ... ! ~500 ~ ~ t;; 400
• • ... .. .. ..... .. .... ..• .... .. ..... ........ ...... ...... .. .............. ; . * 300 e &:
•
£200 .................. .......... .. .. ;'~'~~'.~~~~ ~' ;;~ :~~~. :~.;; :~~"-I
100 .. ........... .. ..... . .... R! .' .O.t2X2 . . ...••... . ..
o L-____________ ~ __ ~ ______ ~ __ ~~
1.2. 1 ..... 1.6 1 .8 Z ,O 2..2 ~A 2 .$ 2 .8 ~.O
Obs. BtC3iO 'SI02)
Figure 5. The changes of [P 1 after ", blowing with observed basicity.
o.o:zo ,...---·---------;:=:====~I
I. Doubi<t slag
~ ~
0.015
c. 0.010
f • •• 0
•• c
I 0 Conventional
0.005 -.-----------------------------1
0.000
14 16 18 20 22 24 26 2B Total Fe In slag ('/0)
Figure 6 . The changes of [P 1 at blow end after 2nd
blowing with total Fe in slag .
700
E 650 0. 0.
ci 600 > 0
550
500 a 2000 4000 6000 8000 10000
The converter life
Figure 7. The changes of [0] at blow end with the converter life .
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3. Slag deoxidation practice in the bubbling station and RH
In present trials , slag deoxidation was carried out at the bubb ling station and RH . The slag deoxidation procedures of conventional method and improved are summarized in Table1 . In the bubbling station , molten steel temperature was measured when a teeming ladle arrived . The bubbling time and amount of coolant were determined considering operational parameters such as arrival temperature, required RH arrival temperature and expected RH starting time . While bubbling, burnt lime added during tapping reacts with slag carried over from the BOF and forms homogeneous liquid slag. Even though steel temperature is not high enough bubbling, at least 30 seconds bubbling was done to homogenize BOF slag and burnt lime . Just after bubbling finished, slag deoxldant was added to promote homogeneous slag deoxidation. Table2 shows the compositions of slag deoxidant.
Table 1 . Comparison o( slag deoxidation me1hod
Process Bubbling sl8110n Anc r AH Irealmcnl
Convenllonal - Burnllime 1000 ~g (eOF)
- Slag deoxidanl 0-'50 kO - Slag deoxrdanl 400- 600 kg
Improved - Burnl lime BOO kO IBOF) - Slag daoxidan' 0-250 kg - Slag dee,idan' 400 - 600 kg - AI can chip: 0- 2 SO ka
Table 2 . Compositions or slag de oxidant (wt%)
size
=5 5- 30mm in dia.
After RH treatment, AI can chip and slag de oxidant was added sim ultaneously to im prove slag deoxidation efficiency. The amount of AI can chip and slag deoxidant was determined by the RH arrival conditions such as total Fe in slag . AI can chip showed better slag deoxidation efficiency com pared to that of AI pellets because it is light and does not penetrate into molten steel.
Table 3 shows the comparisons of the index of rejection ratio depend on slag deoxidation practice and steel grades. Com paring steel grade A and B those have the same Ti range. the rejection ratio was reduced by 80% by the application of slag deoxidation at the bubbling station and RH . Index of rejection ratio is also affected by the steel composition such as Ti .
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Table 3. Comparison of the index of rejection ratio depend on different slag deoxidation and steel grades.
SI •• ll/rade A Sleel grade B 51.el grade C
Ti largot (Range). (ppm) 160(100-200) 150(100-200) JOO(200-400)
GI Charged amount (Ton) 5.500 3.000 18.000
AJ Index of rejection rallo ('4) base 20% olbase 65% oIb.so
Slag deoxidation No VAS Yas
(after bubbling III SAP)
RH arrival T. Fe(%) 17 B B
Slag deoxidation No Yes Ye.
(aRer RH Irealmont)
Figure 8 shows the changes of total Fe in slag from RH to the end of casting . Approximately1 .6% of total Fe was decreased by the addition of 100kg deoxidant. Total Fe content increases during RH treatment. On the contrary, it decreases between after slag deoxidation in RH and the end of casting especially when total Fe content is high after slag deoxidation in RH .
:l ~ 6 ~1: ~L
: r-.-_--------------------1----~~.-_,._---1
AH arrival Alter RH treatment
Alter SiBil Ladle slag aller deoxidation casting
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ss , ,
.. ~ .' .. .
To investigate reoxidation of molten steel by ladle slag, a leemlng ladle after RH treatmen1 has been waited for 50 minules and slag and metal samples are taken. In this experimen1, total Fe content of slag at RH arrival was3 .78% . AfterRH treatment. 200kg of slag deoxidantwas added. There was no distinguishable change of soluble aluminum content (Figure 10) and that means reoxidation by slag is very little when T. Fe content is relatively low.
0 .055 -----_. __ . _ .. ..
1 • Sol. AI
I ..... -. --. ... ~ -
A T.AI
? 0.050 - . . --~ ... :.:. ~ _ . •• . ~. A • I ! •
I i:i • • • • • c • • :3 • 0.045 ;;: RH final
I 0.040 1._._ . . -0 10 20 30 40 50
TillIe Ancr RH Trcallncnl (milll
Figure 10. The changes 01 AI in mollen sleel according to time after RH Ireatmenl
Figure 11 shows the changes of total Fe in slag according to time after RH treatment. Total Fe content in slag maintained constant 20 minutes after RH treatment and T. Fe increased during initial stage that is caused by the slag entrapment that resides in the vacuum vessel .
Figure 8 . The changes of lolal Fe in slag from AH to 5.0 r-----------------------------------,
the end 01 casting
Figure 9 shows the changes of Alz0 3 in slag from RH to the end of casting and there Is a tendency at AIP3 content increase. It implies that there is reoxidation of slag and increase of AI20 J
content when total Fe content in slag is higher than a certain limit.
50
45
40 ~ ",.
~ 35
~ 30
25
20
15 After RH iraalrnenl After slag deoxidation Ladle slag allo r
casting
Figure g . The changes 0 r AI 20J
in slag from RH to the
end of casting
4.0
~ 3 0 ~ .
4) ~ r-: 2.0
1.0 T.Fe RH arrival 3.76%
~ I
I
0.0 '--------'------"'-------'-------'-------'----'
o 10 20 30 40 50
Time After RH treatment (min)
Figure 11 . The changes 01 T . Fe in slag according 10
time afler RH treatment
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4. Effects of alloys on steel Quality
Ti is widely used element for the ULC steels . Table 4 shows total Fe contents measured in different Ti sponges. Brand A Ti sponge consist of sintered type and sponge and showed much higher total Fe content than that of Brand B. Usually Ti is added during the last stage of RH treatment. therefore . its quality will affect on quality of molten steel . Figure 12 shows the changes nozzle clogging index by the use of different Ti sponge brand. The nozzle clogging index with Ti sponge brand A resulted in higher clogging index com pare to that of brand B as expected. The rejection ratio due to inclusion originated defects with Brand A was higher than that of Brand B and it agreed whit the trend of nozzle clogging index. To minimize quality problem that caused by alloys, the quality of alloys should be supervised properly.
Table 4. Total Fe content in Ti sponges
Brand Total Fe. ppm
Ti Sponge (Brand A) Sintered type : 390 Sponge : 450
TI Sponge (Brand 8) 100
Figure 12. The chanoes nozzlecioogino index by the useof dilferentTi sponge brand.
5. Conclusions
Double slag practice in the BOF has been developed to meet the requirement of high quality of ULC steels and its practice was optim ized considering P content at blow end for proper slag deoxidation and productivity,
• In addition to the application of double slag practice to reduce operational variations. optim um campaign life of the converter (5500 to 6000 heats) has been set up considering cost and operational variations.
• Slag deoxidation practice at both bubbling station and RH has been optim ized to minimize reoxidation caused by ladle slag. In bubbling
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station . slag deoxidation was done considering lower total Fe in slag and optimum free oxygen contents in molten steel to prevent oxygen blowing in RH due to insufficient free oxygen for decarburization . For slag deoxidation after RH . slag deoxidant and aluminum can chip was used to reduce total Fe content in slag Fe alloys or additives such as Ti also affects on the quality of ULC steels . In the case of Ti . particular brand of Ti showed higher nozzle clogging and resulted in high rejection ratio from a rolling mill and that is caused by higher oxygen content in Ti sponge .
6. References
1) K. Yoshida, I. Yamazaki . Y. Tozaki , N. Aoki ,J . Yoshiyam a and K. Arai : T etsu-to-Haaane. 76 (1990),1817 .
2) Y. Ogawa. M. Yano. M. MatsuoandY. Dems>te: CAMP-ISIJ,6(1993),1074 .
3) H. Suito and R. Inoue: Trans . Iron Steellnst . Jpn. , 24 (1984) , 40.
4) T. B. WinklerandJ. Chipman:Trans.AIME, 167 (1946),111 .
5) H. SUito, R. Inoue and M. Takada : Trans . Iron Steel/nst. Jpn ., 21 (1981) , 250 .