Operational Aspects of Experiences in Vacuum Technology by Production of High Quality Stainless and Alloyed Steels
Praktične izkušnje pri uporabi vakuumske tehnologije pri izdelavi visoko kvalitetnih nerjavnih in legiranih jekel
Koroušič B'., IMT, Ljubljana, A. Rozman, Metal d.o.o. Ravne, J. Triplat, Acroni d.o.o Jesenice, J. Lamut, Met & Mat. Department, University Ljubljana
Highlights of the technological scheme of manufacturing quality steels as stainless steels in the vacuum by the duplex EAF (electrical are furnace) + VOD (vacuum oxygen decar-burization) process Is presented here. Speclal attentlon is given to the stage of vacuum oxidation of carbon and chromium and to the use stage of degassing in vacuum. In short feature the VOD computer program is expalained. vvhich enables the effective prepara-tion of the complete technology for VOD - process from 15 to 100 tonnes.
Key vvords: production of high quality stainless steels by VOD process
Predstavitev tehnološke sheme izdelave kvalitetnih jekel v vakuumu kot so nerjavna jekla po duplex postopku EOP (elektro-obločna peč) + VOD (vakuumsko kisikovo žilavenje). Poseben poudarek je podan na procesno stopnjo vakuumske oksidacije ogljika in kroma ter izkušnje pri uporabi vakuuma. Na kratko bo predstavljen tudi računalniški program oziroma rezultati modeliranja, ki omogočajo hitro in učinkovito pripravo kompletne tehnologije VOD procesa za peči kapacitete 15 do 100 ton.
Ključne besede: izdelava kvalitetnih, nerjavnih jekel po VOD postopku
Introduction
The steel industry faces a number of problems related to an inereasing demand for clean steel im-posed by the consumer industry as vvell as inereasing costs pressures. For these reasons, the steel industry is in the past reasoning its steelmaking tech-niques vvhich resulted in a subdivision into tvvo phas-es, namely:
•	metallurgy in the melter and,
•	metallurgy In the ladle.
Accordingly, most of the metallurgical vvork shifted from the melter to the ladle vvhich became a metallurgical reactor. The great number of techniques de-veloped in the secondary metallurgy is based on the application of complex vacuum systems. Before choosing among these techniques, the steelmaker must be perfectly avvare of the objeetives to be achieved and the requirements to be satisfied by the steel product and reasonable in priče for the cus-tomer, vvhile stili generating a profit for the steelmaker.
The technology routes for secondary metallurgy are in no way a standardized task, even vvhere the
' Prof. Dr. Blaženko KOROUŠIČ, IMT. Lepi pot 11, 61000 Ljubljana
underlying basic processes are identical. The condi-tions differ too much from shop to shop. The state of technology by production of high quality stainless steels in Slovenia steelvvorks vvill be reported in this lexture, highlighting the vacuum processes using a typ čase - stainless steel production as an example.
General overvievv of vacuum processes
The principles of secondary metallurgical processes used the vacuum can be brierfly deseribed as fol-lovving remarks:
1.	Early in the 1950s, the vacuum degassing of molten steel vvas developed. The purpose was to re-duce the hydrogen content in steel and particulary in forging ingots. The best results vvere achieved by stream degassing during ladle - to - ladle or ladle -to - ingot teeming1.
2.	In 1955 appeared almost simultaneously the RH and DH processes vvhich led to the development of degassing facilities vvith much higher throughputs.
3.	In 1956 the first argon injeetion trials vvere carried out - the method of stirring and heating vvhilst degassing - to enable large reaction volume necessary and the comparatively high temperature losses in-curred led to the development of nevv advanced metallurgical techniques as: ASEA-SKF, FINKL, VAD, VOD combining nevv induetive stirring, nevv refraeto-
®J
Figure 1: Processing units in the secondary metallurgy Slika 1: Procesne naprave v ponovčni metalurgiji
ry lining materials, a sliding gate valve and oxygen lancing into the vacuum degassing unit29.
With the industrial application of the above processes, especially vacuum steelmaking equipment passed through several stages of development, see (Fig. 1).
The metallurgical possibilities of vacuum steelmaking have dramatically changed the role of the melting furnace. No longer is it necessary to use a particular type of furnace for the production of partic-ular grades of steel. In other vvords, ali furnace can produce nearly ali steels if equiped vvith a secondary steelmaking facility.
Therefore, the demand for a high availability of the vacuum units is one of the essential factors and the modular concept has proved to be useful in the se-lection of suitable equipment for each application.
Steel production in Slovenia steelvvorks vvas based for more than 120 years on melting in hearth fur-naces. During the 1960's and 1970's, the production moved progressively from open heart to electric are furnace melting.
Vacuum refining began in Steelvvork Ravne in 1970 vvith the installation of the vacuum degassing unit for the removal of hydrogen, particularly for the forging shop. The process did not find a vvide application.
Vacuum refining in Ravne began in 1984 vvith the installation of two VAD/VOD units (20 and 50 tonnes)
and some years later also in Jesenice vvith tvvo 90 tonnes VOD-units vvith the aim to manufacture more sophisticated steel grades (very lovv S, O, H, C con-tents and higher contents in alloys - first of ali stainless, dynamo, tool and other high alloyed steels)10.
1 Some fundamental aspects of metallurgical processes in vacuum
An atmosphere at reduced pressure is one of the purest environments possible. Most of the metallurgical processes are made up of heterogeneous reac-tions, and the operation in vacuum inereases the dri-ving force for interphase mass transfer vvhich is thereby accelerated.
Vacuum processing, compared to air handling, re-moves the difficulties due to the pick up of oxygen, ni-trogen and hydrogen contained in the atmosphere. On the other hand, vacuum processes are concerned vvith the removal of hydrogen, oxygen, carbon and unvvanted non-metallic inclusions from the melt. These benefits are often obtained at the priče of same side effects as, for example, the difficulties caused by interaetion betvveen melt and refractory lin-ings. Furthermore, just the development of new re-fractory lining led tovvards a successful application of vacuum treatment of the molten steel. With introduction of basic linings (magnesite or dolomite) into vacuum ladles began the era of vacuum metallurgical processes.
Activity of oxygen (ppm)
-	p(CO) = l bar	--P<c0)=100	mB - p(C	O) = 10 mB
i				
			x\	
		\		
				. . ......
1.000E-03
0.01 0.1 Percentage of carbon
pr95blf2
Figure 2: Deoxidation equilibria in liquid iron (1600 C) Slika 2: Dezoksidacijsko ravnotežje v tekočem železu pri (1600 C)
2 Reactions vvith carbon and alloying elements
Carbon, probably the most important element in steel, under reduced pressure has a very high affini-ty tovvards oxygen, as demonstrated by the follovving considerations:
For instance, a CO pressure less then 100 Pa is a very effective reaction partner. The principal reaction governing the removal of oxygen by carbon is that producing carbon monoxide:
c + o = cog
vvhere:
C, O mean elements C and O are in the melt.
It shows, for instance, that at Pc0 pressure of 1000 Pa and based strictly on thermodynamic considera-tions, equilibrium oxygen activity is very low (for the C-content about 0,01 % an oxygen activity of about 20 ppm can be expected).
In the practice this level will be never reached because of the reactions betvveen melt and refractory surface. As will be shown later, the vacuum way of producing stainless steel in VOD-process utilizes low partial pressure of carbon monoxide to the control se-lective carbon oxydation to prevent the imminent risk of the simultaneously chromium oxidation according to the reaction:
2Cr + 3 O = (Cr203)	(2)
where:
() means in the slag.
The formation of chromium oxide causes the slag to get crusty. Stirring with argon via the porous plug in the ladle bottom to break the slag layer and keep the metal exposed to vacuum is required. The selective oxidation of carbon in the molten metal depends on the effective pressure Pc0. The fraction of oxygen reacting with carbon appears as CO and C02 and the rest as metal oxides (Si02, MnO, Cr203, FeO).
3 Use vacuum by production of the stainless steels
The production of stainless steel represents the specific problem of achieving a very low carbon con-tent together with a high chromium content at the lovv-est cost. VOD-process (Vacuum Oxygen Decarbur-ization) is nowdays the most favourable process for stainless steel production. The VOD - refining is car-ried out in the teeming ladle vvhich has a basic vvear lining and is sufficiently insulated to vvithstand the ag-gressive slag at high temperature (over 1700 C) and long heat times (some hours). The conventional way
Figure 3: Vacuum oxygen decarburization process (VOD) Slika 3: Vakuumsko kisikovo žilavenje v VOD procesu
is that the ladle itself with a refractory lined cover con-stitutes the vacuum chamber or tank degasser of the type VOD-unit (see Fig. 3).
Oxygen is supplied at a controlled rate via a consumable refractory lance contained in a vacuum-tight housing on the top of the cover. A vvater-cooled lance vvith preferably a laval tuyere is also used. By using the laval tuyere the gas stream is kept concentrated and the distance from the lance tip to the steel surface can be increased to 1200 mm. The process is closely monitored using TV camera mounted on the vacuum cover.
4 Vacuum pumps
For creating vacuum mainly two different types of pumps are in use in the steel industry especially for large units. The most common type is a set of steam ejectors or alternatively a combination of steam ejec-tors and vvater ring pumps to save steam consumption, if the cooling vvater temperature is not too high
Figure 4: The suction capacity and the chamber pressure for the 30 t VAD/VOD unit in Metal d.o.o. Ravne Slika 4: Kapaciteta vleka in pritisk v ponovci 30 t.
VAD/VOD naprave v Metal d.o.o Ravne
(max. 32°C). The suction capacity at lovv pressure, i.e. the shape of the pump curve is of special interest for the production engineer. Fig. 4 shovvs the pump curve for a 30 tonnes ladle-lid system in Steelvvork Metal Ravne. The shape of the curve is very impor-tant and had to be flat in the lovver pressure regions to prevent the influence of small leakages on the vvorking pressure. To prevent or minimize splashing, the chamber pressure must be kept at constant value for example 90 mbar.
For the heats at Metal Ravne, a 15 tonnes VOD-unit vvith the oxygen blovving rate about 6 Nm3 02/min, the gas evolution of CO is about 900 kg/h if ali oxy-gen is used for CO-production. For a oxygen yield of 50 % at "end-point" (begin of intensive Cr oxidation) and taking into account the presence of Ar, the CO + Ar production will be about 500 kg/h, giving a chamber pressure of 1500 Pa. From practical experiences it is knovvn, that the chamber pressure is a very good
Figure 5: Arrangement of vacuum pump system for the
30 t. VAD/VOD unit Slika 5: Sistem vakuumskih naprav za 301. VAD/VOD napravo
indicator of the boiling reactions and the control of "overboiling" melt vvhich occurr normally at the begin-ning of the oxygen blovving. To obtain the lovv vvork-ing pressures already mentioned the steam ejectors combined vvith vvater ring pumps (see example on the Fig. 5). As is seen, betvveen the vacuum vessel and the vacuum pump, a gas cleaner/cooler is installed. The cooling is of special importance during the VOD treatment vvhile the gas cleaning always is important for the increased availability of the pumps. The nec-essary cleaning of the ejectors must be frequent and is difficult.
5 Process control by the production of stainless steels
The vacuum refining process, especially production of stainless steel, present the specific problem of achieving a very high quality at the lovvest priče. Hovvever, from cost and productivity reasons, VOD-process is nowdays a standard way of producing stainless steels in the combination vvith EAF furnace. The costs of refining treatment, used in the econom-ic assessment depends of the capacity of a unit and use rate, and many other variations of the input prices (the refractory linning, alloys, actual practice, steel grades). In Table 1 are presented the average total costs for three different capacities of the VOD-units for the production of the steel grade AISI 316L (charge materials, energy consumption, refractory costs, the the vacuum refining treatment, alloys and oxygen + argon).
Table 1: Cost estimate in DM/MELT for the production of the stainless-steel AISI 316L by EAF + VOD route Tabela 1: Ocena stroškov v DM/talino pri izdelavi ner-javnega jekla AISI 316L po postopku EOP + VOD
VOD-unit	15 tonnes	40 tonnes	90 tonnes
Total (DM)	35.700	90.600	193.950
Comparing the estimated costs reported in Table 1. it may be seen that by the production of the stainless-steels, the process control in the plant is of great importance. Technological decisions have to be tacken quickly and exactly since in many cases, for example, the temperature predictions by the smaller VOD-units is very short for manual calculations. The only way to arrive at a result vvithin the tirne available is to use a computer model. The control of the VOD-process can be divided into two levels:
•	Equipment control (LEVEL 1): Including measur-ing and control of for example the vacuum station, the vvater-cooled oxygen lance position, alloy-system. etc..
•	Process control (LEVEL 2): Including model pre-diction for oxygen rate, melt temperature and chemical composition during VOD, VCD, slag reduction and finally calculations of alloy and slag additions. Hovvever, from cost and production reasons, very important function has also a third level (LEVEL 3) for administration and production planning. A computer model simulation of the level 2 vvill be shortly ex-plained.
5.1	General vlew
The VOD-model for plant process control is developed to serve as an operator guide through the process, assuring an optimum and uniform refining of steel melt in VOD-unit. It vvill guide and advise the operator through ali the different process stages by production of the routine steel grades or operate as an expert system by the introduction of nevv steel quali-ties or improvement of the know-how (for example: change oxygen consumable lance vvith an vvater cooled oxygen lance).
5.2	Data bank
The data bank contains ali the information re-quired for the model simulation for each steel grade according to plant specifications. It contains also ali potentially useful data as physical-chemical data for the thermodynamic and kinetic calculations, al-loy material characteristics, limited values and so on.
5.3	Process control system
Each steel grade is individually dedicated to a VOD-cycle composed of a number of process steps:
•	Oxygen blovving during the VOD-stage,
•	Vacuum decarburization,
•	Slag reduction,
•	Calculation of addition of alloying materials.
MODEL VOD	MODEL VOD
AISI 316L, 93.7ton	AISI 316L , 93.7ton
[%CrJ
dC/dT-(\/mln)
0011 0 016 O.OU 0 013 0 01 0.00» 0.006 0 004 0.002 0
40 50 60 70 80 90 100 110 120
Tlme(min)
Tlme(min)
MODEL VOD AISI 316L 93.7tons
0.8
0.6
0.4
0,2
0
0 10 20 30 40 50 60 70 80 90 100 110 120
Time(min)
Figure 6: Influence of the initial melt temperature on VOD, VCD and slag reduction processes by the production
AISI 316 stainless steel
Slika 6: Vpliv začetne temperature taline na VOD, VCD in redukcijo žlindre pri izdelavi nerjavnega jekla AISI 316
(%C)
For each process step a number of the influenced input parameters vvas previously defined, such as: initial temperature and chemical composition, process tirne prediction, thermal losses and energy absorbtion during the oxydation period and so on. Fig. 6 shovvs an example the complexity of the influ-
ence initial melt temperature on VOD, VCD and slag reduction by the production of the stainless steel AISI 316 in 901. VOD-unit.
During the oxygen blovving in VOD, carbon content in melt decreased linearly vvith the reaction tirne to about 0.1 % C. Belovv this carbon content, the oxi-
MODEL VOD AISI 304, 16 to 93 ton
Tempora1uro(°C)
02-rate (Nm3/h)
									
							v		-
			93	t	\	N	N \		93t
		43t		I		\ 16t	\	N	
	16	:		I				43t	-
			\	ll					
0 10 20 30 40 50 60 70 80 90 100
Time(min)
MODEL VOD AISI 304, 16 to 93 ton
0.03 0.025 0.02 0.015
0.01 0.005 0
dC/dt (%C/min)
		A						
/			I61					
	//	-V"		43t				
	/ /		V		3t			
						V-		
						A		
20	30	40
Time(min)
MODEL VOD AISI 304, 16 to 93 ton
sym3ccr
40 50 60
Time(min)
1 8
1 7,5
1 7
1 6.5
16
1 5,5
15
Figure 7: Carbon - Cr oxidation, temperature profile and decarburization rate in austenitic steel AISI 321 for 14.0 ton
VOD-unit
Slika 7: Ogljik - krom oksidacija, temperaturni profil in hitrost razogljičenja pri izdelavi nerjavnega jekla AISI 316
dation of chromium was accelerated causes the for-	analysis of a simulating model calculation taking in-
mation of chromium oxide and slag getting in crusty.	• to account the thermal losses in the VOD-ladle. an
A varying starting temperature may have to be com-	indicative calculation of the VOD-process is done ac-
pensated by a change in starting analysis in order to	cording to principles reported from author10 " (see
reach a correct "carbon end point". Based on the	Fig. 7).
MODEL VOD	MODEL VOD
AISI 321, 14,3ton	AISI 321, 14.3ton
(%Cr)
30 40	50
Time(min)
30 40 50 Time(mln)
MODEL VOD	MODEL VOD
AISI 321, 14.3ton	AISI 321, 14.3ton
30 40 50
Tlme(mln)
dC/dI(ZC/mln)
0.025 0.02 0.015 0.01 0 005 0
0 10 ac02tldc/dt
20	30	40
Tlme(mln)
Figure 8: Influence of the unit capacity on metallurgical processes for the same input parameters as in Fig. 7 Slika 8: Vpliv velikosti peči na metalurške procese za enake vhodne parametre kot na sliki 7
Let us consider the typical example, presented in Fig. 8 to show influence of the unit size on the metallurgical processes of VOD-technology, for the same input process parameters.
Presently the making of stainless steels and other alloyed steels with a short process tirne betvveen the process steps is of great importance. As we learnt from this examples, the process modelling, combined vvith quick-reacting measuring devices helps to improve the vacuum process technology and it is the main purpose of our present and future vvork. The combination optimal of slag metallurgy and vacuum treatment is not yet accomplished.
References
1Tix, A.: Degassing of steel under vacuum-practice for large forging steel (ingots). Stahl und Eisen, 76, 1956, 61-68
2Eggenhofer, A., Kaiser, G., Bauer, F.: Operating expe-riences vvith a vacuum heating plant for 50 tonne heats, Berg - und Huttenmanische Monatshefte, 119, 1974, 338-343
3Baitey, W. H.: Finki VAD steelmaking at BSC River Don VVorks, Ironmaking & Steelmaking, 4,1977, 209-215
4Tiberg, M., Buhre, T., Herlitz, H.: The ASEA-SKF process for the refining of steel, ASEA-Z., 11,1966, 3, 47-50
5Schmidt, M., Etterich, O., Bauer, H., Fleischer, H.J.: Production of high alloy special steels in the basic oxygen
converter (I), Plants and process (II), Metallurgical basis for the oxidation of high-chromium-iron-carbon allos, Stahl und Eisen, 88, 1968, 153-168
6Bauer, H., Etterich, O.. Fleischer, H. J., Otto, J.: The re-ducing and oxidising vacuum treatment of alloy steel heats, Stahl und Eisen, 90, 1970, 725-735
'Baum, R., Zorcher, H.: Vacuum oxygen process, Metals Society Proceedings, London, May 5-6, 1977, 69-72
8Baum, R., Hentrich, R., Yun, Z., Zorcher, H.: The refin-ing of stainless steel in a vacuum-oxygen refining unit, Stahl und Eisen, 95, 1975, 8-11
9Suzuki, Y., Kuvvabara, T.: Secondary steelmaking: re-view of recent situation in Japan, Proceedings ofthe Iron and Steelmaking Conterence, London, May, 1977, 4-13
10	Koroušič, B.: Use of vacuum in modern metallurgical processes (Uporaba vakuuma v sodobnih jeklarskih procesih), XI. jugoslovanski vakuumski kongres, Gozd Martuljek, 17-20. april 1990
11	Koroušič, B.: VODMET - a general computer program for the simulation of the VOD process for production stainless steel and Ni-based alloys, The ninth inter. Vacuum Metallurgy Conterence on Special Melting, 1989, April 11-15., San Diego, California, USA