Operational Experience of the ASEA-SKF Ladle Furnace ~rocdss at Bofors Steelworks

Transcription

1 Modern Refining Techniques 57 Operational Experience of the ASEASKF Ladle Furnace ~rocdss at Bofors Steelworks by N. Grevillius, P. Geete, and T. Krey Some features of the ASEASKF refining plant at the Bofors Steelworks are presented. The treatment in the ladle furnace is described, and an account is given of a method for desulfurization. The flexibility of the process is briefly discussed. Operational results regarding contents of hydrogen, oxygen, aluminum, and alloying elements as well as temperature, nonmetallic inclusions and refractory life are given. The economics of the refining process are reported. : IN'TRODUCTION The ASEASKF process is a method for refining and alloying steel, inductively stirred in a ladle while heating and degassing are carried out. The first plant built for continuous production was put into operation in May 1967, by the Bofors'Company, Sweden. A schematic view of this installation is shown in Fig. 1. PLANT FEATURES The ladle furnace containing the molten steel to be treated is transported from the melting furnace, lowered into a low frequency stirrer coil, and positioned on a carriage that can be moved between the heating and the degassing positions. In the heating position, the ladle is provided with a roof through which three graphite electrodes are inserted, similar to a conventional electric arc furnace. The degassing station consists of a vacuumtight lid connected to a fourstep steam ejector. The jacket of the ladle furnace is made of austenitic stainless steel and has a watercooled top. Before the ladle is preheated, the nozzle is inserted and the stopper rod is set in the proper position. After this, the stopper rod and the horizontal part of the stopper set are taken away. These parts are put back after the treatment, just before teeming. During the refining, the stopper rod has been preheated under vacuum in an electric furnace. The plant is supplied with equipment for charging alloys, consisting of an automatic weighing system and nine pockets for different alloying elements. A scale car runs along the pockets, and can be charged from up to three pockets for each addition to the ladle. The alloys are transported from the scale car on a belt conveyor to the ladle furnace, where they can be added through the roof at the heating station, as well as at the degassing station. According to the Bofors practice, there is no need to add alloys during the degassing; consequently, this possibility is not utilized. N. GREVILLIUS, P. GEETE, and T. KREY are, respectively, Assistant to the Superintendent of the Steel Melting Dept., Superintendent of the Steel Melting Dept., and Director of Steel Production and Development, AB Bofors Steel Division, Bofors, Sweden. Table I. Data for the 50ton ASEASKF Unit at the Bofors Steelworks, Sweden Total area 3,900 sh ft (360 mzl Ladle weight, incl. molten stecl 85 tons Ladle height 13.0 ft (3.97 m) Ladle diameter Distance from heat to ladle top 8.5 ft 3.5 it m) (1.05 m) Electrode diameter 10 in. (250 mmj Electrode circle diameter 24 in. (600 mm) Transformer rating 6.0 Mva Induction stirrer 2 x 350 kva Stirring frequency 1.3 c'/s Final pressure 0.5 mbars (0.4 torr) Steam consumption at 8 atg and 375'F (190 C) 12,100 lb/hr (5,500 kg/hr) The boil is observed in the control room by means of television equipment, which makes it possible to switcl~, on the different ejector stages at the right moments, to avoid excessive boil. Some data for the ASEASKF installation at Bofors are given in Table I. STEELMAKING OPERATIONS Steel for treatment in the ASEASKF plant is first melted in a conventional 17foot arc furnace with a heat weight of 50 tons. If necessary, the heat is decarburized and dephosphorized and the temperature is raised to about 2950 F (1620 C) before taphing into the ladle furnace. A careful slagoff is made before tapping. The production program is mixed, comprising carbon steels, constructional steels, stainless and tool steels. Heats requiring a desulfurization, are refined under a high basic slag in a furnace ladle lined with basic bricks (96% MgO) in the slag attack zone and neutral (75% AlzOs) bricks in the rest of the ladle. When there is no need for a desulfurization, the treatment is carried out either under a neutral slag in a ladle lined with 85% A1,0, bricks in the slag attack zone and 75% A1,0;, bricks in the rest of the ladle or under basic slag in a ladle of the first mentioned type, as convenient. For the time being, three ladles with alumina bricks and two ladles with a basic slag attack zone are in oper,ation..' Treatment in an Entirely ~lumina~ined Ladle,. Furnace.. The main operations for normal 'prac'tice in a neutral ladle furnace are shown in Fig. 2., :'' The treatment starts with a heating of approximately 30 minutes, raising the teniperature from about 2820 F (1550 C) to 2880 F (1580 C). The rise in temperature is rather slow in the beginning, as the lining is cold, but reaches about 4 F (2 C). a minute at the end of the heating period. Alloys are added to reach the lower limit of the analysis. Silicon,, an exception to this,

2 58 Electric Furnace Proceedings, 1970 Temperature "F "C $[per lnsertlng Sam hng Al oddttlon %%28rt Heattnq All0 adddlon Heot!nq Tra Deaosslna 2940l~~O f T All0 T ~ D Heotlna o d d ~ v ~ ~ ~ 2goo Fig. 2Practice in neutral furnace ladle. Treatment time, minutes is kept at % in order to attain a boil of suitable intensity during the following degassing. Although heating can take place during the addition of the alloys, this is avoided in order to eliminate splashing on the electrodes. The temperature drop due to the additions averages about 55 F (30 C). Continued heating raises the temperature to 2880 F (1580 C) in an average time of 1015 minutes, after which degassing follows. The degassing creates a vigorous boil, bringing the oxygen and hydrogen contents down to low values. The standard time for degassing is 20 minutes, resulting in a temperature drop of about 80 F (45 C). The end pressure, 0.4 torr, is reached in about 10 minutes. The furnace ladle is brought back to the heating position, a sample for analysis is taken, and a small amount of aluminum is added to the bath in order to be sure of keeping a low oxygen content. After resumed heating to about teeming temperature, final analysis adjustments are made; this means that small amounts of alloys are added to bring the content of the different elements to

3 Modern defining Techniques 59 Sulphur in steel % Fig. 3Desulfurizing response. the middle of the analysis intervals. During the heating period, the temperature is checked a few times to ensure that the prescribed teeming temperature is reached rapidly after the final additions. Depending on the treatment time after degassing, it may be necessary, in certain cases, to add more aluminum in order to keep the oxygen activity low. The powerful inductive stirring is kept working during the whole treatment, giving a rapid distribution of added alloys, an increased degassing effect and removal of inclusions, as well as reliable samples and temperature measurements. The average time from tapping to teeming is 2%2% hours, which is well within the 3% hours required in the arc furnace from tap to tap. Treatment with Basic Slag in the Ladle Furnace One reason for choosing the ASEASKF process was, that it was assumed to be suitable for desulfurization as, in addition to degassing, it offered both stirring and heating facilities. Metallurgical reactions are thus promoted and the necessary time is obtained. A research project for desulfurization was started by Bofors in cooperation with ASEA, The Royal Institute of Technology in Stockholm, and the Metallurgical Research Plant in Lu1e"a In pilot tests it was found that al high degree of sulfur removal could be obtained in a well deoxidized bath under a molten, high basic slag with addition of strong desulfurizing agents to the bath if the heat was effectively stirred an'd if the ladle had a basic lining at least in the slag attack zone. Misch metal was by far the most efficient desulfurizer, as shown in Fig. 3. By the end of 1969, the process wis fully developed in the Bofors ASEASKF unit, and heats with a sulfur content of 0.001% and less are now being produced. The most common sequence of operatio,ns is the following (Fig. 4). 11 Approximately 300 pounds of lime are added to the heat after finished tapping. In a heating period, an addil tional quantity of 600 pounds of lime is added in portionspower offbringing the total addition of lime to 0.8% of the heat weight. Small additions of aluminum powder enhance melting of the lime and keep the slag well reduced. At temperatures above 2865 F (1575"C), a molten, high basic slag is created with a basicity index, CaO + MgO, between The temperature is raised SiOz from about 2805 F (1540 C) to 2910 F (1600 C) in approximately one hour, after which 2030 minutes are allowed for the addition of alloys, and to compensate for the temperature loss. Degassing follows during 20 minutes. The ladle furnace is moved back to the heating position. A limited quantity of aluminum is added to the bath, followed in a few minutes by an addition of misch metal, if required. The slag is still reduced by means of aluminum powder. As misch metal has an extremely high affinity to oxygen as well as to sulfur, it is important that it is added to a well deoxidized bath. Stable sulfides of rare earth metals are precipitated and almost entirely removed in 15 minutes by means of the stirring. During the removal of sulfides, final analysis adjustments are made and the correct teeming temperature is obtained. The total time from tapping to teeming is 23/43 hours. The sulfur content decreases from an average value of 0.025% at tap to an average content of 0.013% after I I90 Treatment time.minutes Fig. &Practice for desulfurizing in the furnace ladle.

4 60 Electric Furnace Proceedings, 1970 Table II. Oxygen Content for Steel SAE 4135 Treatment Stage Oxygen Con ' tent, ppm Before tipping Before degassing hter degassing Inserting stopper rod I ,8 1, ,6 Hydrogen con!mt. P P Fig. SHydrogen content of molten steel in mold. degassing. This is sufficient in most cases. Consequently, no misch metal is added in such heats. An addition of 0.2% misch metal brings the sulfur content to 0.002% or below, while 0.1% misch metal gives a sulfur content of %. PLANT FLEXIBILITY, The two practices mentioned are standard procedures in which the sequence of operations can often be changed considerably, if desired. For example, there is generally no inconvenience if degassing takes place before the heating. Degassing can be omitted in all heats where a hydrogen removal is not necessary. It has been found that a precipitation deoxidation by means of alu, minum gives as low oxygen contents as degassing and with the same certainty. This is considered to be due to the efficient stirring. In accordance with this, desulfurization shows as good results in heats with a strong aluminum deoxidation as in degassed heats. At Bofors there is no need to reduce the refining time in the ASEASKF plant, as the taptotap time in the arc furnace exceeds the refining time. Consequently, the process is run in what is believed to be the most economical way. However, it is not difficult to shorten the time considerably. By overheating in the arc furnace, for instance, the heating facility in the ladle furnace could be used exclusively for adjusting the teeming temperature. This, however, results in a somewhat lower production and possibly higher lining consumption in the arc furnace. The heating rate in the ladle furnace could be increased, but this would most likely result in a higher lining wear in the slag attack zone. Alloy additions can be made to any desired amount. The only limiting factor is the available time for melting the alloys and for heating. Normally, 2 tons is the maximum amount to be added in the Bofors ladle furnace, although substantially larger additions have been made. The heating facility makes it possible to postpone teeming, if this should be necessary. Teeming from a furnace ladle has been carried out without problems after 7 hours in the ASEASKF plant. PLANT PERFORMANCE Hydrogen As mentioned, hydrogen removal is the only reason for degassing. Hydrogen samples are taken in the molds during bottom pouring, by dipping an evacuated pyrex tube in the steel. The samples' are immediately placed in solid CO,. The average hydrogen content for all degassed heats hitherto made amounts to 1.7 ppm. The hydrogen con Fig. &Oxygen Oxygen content, PPm content after degassing for steel similar to SAE tent before degassing is generally about 4 ppm but shows great variations. A representative month (Nov. 1969) has been chosen to illustrate the distribution of the hydrogen values in the mold (Fig. 5). No heat was treated under basic slag during that month. Such heats have shown an approximately 0.7 ppm higher average value. This is assumed to be due to a less intense boil owing to a lower oxygen activity caused by the well reduced slag. Humidity from the lime may also contribute to increased hydrogen values. Practice has recently been changed to degassing before melting of the basic slag for steel qualities sensible to flakes. Oxygen In Table I1 the oxygen content at different stages of the treatment is shown for a constructional steel with 0.35% C, similar to SAE The values are an average of 50 heats, refined in a neutrally lined ladle furnace and thus without desulfurization. The distribution of oxygen contents immediately after degassing is shown in Fig. 6. Heats being desulfurized generally show a lower oxygen content before teeming than nondesulfurized heats. This can be explained by a somewhat greater aluminum addition after degassing and also by the strong deoxidizing effect of misch metal. Oxygen content below 10 ppm have been reached in heats with an addition of 0.2% misch metal and with an aluminum content in the bath below 0.010%. Aluminum Although the slag is kept reduced by the addition of aluminum powder and the oxygen activity of the' bath is maintained at low values by aluminum, the aluminum content in desulfurized heats is generally kept below 0.010%. A comparison of aluminum values between heats refined under basic and neutral slag is given in Fig. 7. Alloying Elements The precision of the contents of alloying elements at teeming has been examined for the latest 50 heats of..

5 8 6. Modern kefining Techniques 61 4 Temperature Teeming has taken place at the exact prescribed tem Neutral practice perature in 97% of all heats. No deviation greater than 2 9 F (5 C) was found. The teeming temperature is, within a few degrees, the same from the first to the last ingot, as the furnace ladle receives heat from the molten steel during at least two hours. The exact teeming tem O '10' '12' '14' perature gives a very high reproducibility from heat to A[ ~ontent,~/~~10~ heat. The surfaces of the ingots, for instance, are almost free from defects, which is of great importance, es pecially for heavy forgings. 2 Fig. 7Aluminum O ! content of steel similar to SAE 4135 in mold. Lint (a) Carbon 1 Carbon content,% Basic practice steel SAE Approximately one third of the heats have been refined under basic slag. The results are given in Fig. 8. In four heats, the carbon content has fallen outside the aimed analysis. As decoding could be made during the treatment in the ladle furnace, these heats are excluded in Fig. 8. In all heats, the contents of Si, Mn, and Cr are within the internal requirement. This is explained by the efficient stirring, wfiich makes the heat homogeneous and the sampling accurate. The yield of alloying additions is virtually 100 %. Nonmetallic Inclusions Macroinclusions are virtually completely eliminated by the ASEASKF process, which is also a result of the powerful stirring. Slag inclusions are extremely seldom found by blue fracture test. In a control method used at Bofors, ingots of approximately 16 inches square are rolled to bars of 4 inches square. Polished specimens from the bars are observed in a1 microscope in the longitudinal direction, and inclusions exceeding inch (0.075 mm) are recorded. The result is expressed as the total length of such inclusions per unit area (e.g., in/sq in) and a distinction is made between oxides and sulfides. While normal values for steel not refined in the I L 2 'I C) Manganese 0 ganese ent,% (c) Manganese L Internal reuulreni. nt (b) Silicon ~ L Internal requirement IChromium content,% (d) Chromium ' Fig. 8Analysis precision of steel similar to SAE 4135.

6 62 Electric Furnace Proceedings, 1970 ladle furnace amount to in/sq in for oxides, the value is zero for more than 95% of the heats treated in the ladle furnace. This is regardless of whether or not the steel has been degassed. The value for sulfide inclusions is lowered in proportion to the decrease in sulfur content. As the steel treated by the ASEASKF process is almost free from oxide macroinclusions, the content of oxide microinclusions must be proportional to the oxygen analysis. Oxide microinclusions in heats being treated with misch metal are oxides of cerium, lanthanum, and other rare earth metals, while aluminum has not been indicated. Their size is uniform, 36 pm, and they are evenly distributed, which is considered to be a highly favorable condition. Refractory Life The average refractory life is shown in Table 111. Slag attack zone Rest of ladle Vacuum lid Heating roof Table Ill. Refractory Life in ofo or; ASEASKF Unit Number of Heats Neutral Practice Basic Practice The high wear of the slag attack zone is due to the difficulty in obtaining a suitable slag composition in each heat. Slag from the arc furnace can be detrimental. Special attention will be given to this problem in the near future. The fact that the life of the rest of the'ladle is shorter in basic practice,.although the lining is of the same type as in neutral practice, is assumed to be due to a lower oxygen activity, causing enhanced dissolution of the lining, and to contact with the basic slag during teeming. ECONOMICS OF OPERATION The costs per ton for treatment in the ASEASKF plant are approximately as follows: Ladle furnace lining $3.20 Labor, 2 men, ($3.35 a manhour) 0.52 Maintenance and repairs 1.10 Electrical energy, (1.0 cent/kwhr) Heating, 3700 kwhr 0.74 Stirring, 700 kwhr 0.14 Electrodes, 1.3 lb/ton 0.40 Steam 0.10 Cooling water 0.0'5 ~otal: $6.25 The price of misch metal is $1.35 per pound, which means $3.00 per ton of steel when 0.1% is added. The total price of the ASEASKF installation was $1,300,000. From the above figures it is obvious th'at an increase in lining life is of great importance. Estimated savings per ton in the arc furnace operation follow. 'The calculation is valid for a mixture of 50% two sl'ag heats and 50% one slag heats. Lining in conventional ladle $0.62 Decrease inlabor cost 1.43 Decrease in furnace refractory consumption 0.30 Decrease in energy consumption 0.56 Decrease in electrode consumption 0.34 Decrease in maintenance and repair cost ~otal:$3.71 Net operating cost: $2.54 In addition to the savings in the arc furnace,.the following should be considered. After development of the Bofors desulfurizing process, it has become possible to do all the refining in the ladle furnace and to reach a taptotap time in the arc furnace of 3% hours, compared with an average of 5% hours for heats produced by two slag practice and 4% hours for heats under one slag without refining in the ASEASKF unit. A considerable increase in production has thus been obtained. This is of great value when there is a lack of furnace capacity, which has been the case at Bofors during the past two years. Large cost savings have been obtained due to fewer internal and external rejections, less hydrogen anneal, and improved ingot surfaces. The operational costs are high for the ASEASKF process. However, it has been proved that the method gives. great savings, and according to the Bofors experience the total economy is very favorable. DISCUSSION by Patrick E. Dempsey You gave an association of misch metal with a low sulfur level. 1. How much misch metal was added to give 0.002% sulfur? 2. Was the 0.2% misch metal based on metal weight? 3. How was it added? 4. Was the bath bare or slag covered? 5. At what stage of the heat was it added? PATRICK E. DEMPSEY is affiliated with H. M. Harper Co., Morton Grove, Illinois. 6. What was the slag condition? 7. What was the sulfur level before adding the misch metal? Autl~ors' Reply: 0.2% misch metal (220 pounds, as the metal heat weight was 50 tons) was added to reach L0.002% S. The misch metal was put in steel capsules that were plunged into the bath after degassing and after a few minutes of precipitation deoxidation by aluminum, i.e., in the final stage of treatment. The bath was covered by an approximately 2inch thick layer of a molten, high basic and wellreduced slag. The sulfur content of the bath was % before the addition.