Atmosphere-Controlled Tundishes: Their Design and Operation
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- Laura Collins
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1 Continuous Casting II Billets and Blooms 137 Atmosphere-Controlled Tundishes: Their Design and Operation by D. I. Brown and G. Harry, Jr. There are at least two contemporary philosophies regarding the tundish: One is to give it short measure from the standpoints of design, planning, and expense and to expect operating trouble. The second is to expand large amounts of time and funds in planning, testing, engineering, and equipment and to expect that it wiil work. J&L chose the latter approach in its trials and we are confident that our system will be satisfactory when placed in operation, although we realize some modifications may be necessary. The tundish, basically. distributes the metal from the ladle to each of the strands of the machine. It appears to be a simple device which should not be difficult to build or operate. However, for those who have experience with the tundish, the opposite is most often true. First-generation tundishes were usually open boxes lined with fireclay brick and had the nozzles tucked toward the ends in rather shallow wells. Sometimes a roof was swung in over the box to protect the ladle bottom from overheating and to cut down loss of temperature. Usually, natural gas or propane was burning over the box. These open shallow tundishes were significant contributors to poor operations and to poor steel quality with some of the first casting machines. One of the big operating difficulties was that the stream from the ladle had a tendency to drill a hole in the bottom of the box. Therefore, the floor of the tundish was built up so that at the point of stream impingement the refractories were twice as thick as elsewhere. Another difficulty was that most early tundishes did not provide an offset D. I. BROWN and G. HARRY, JR. are, respectively, asst. to works manager and general foreman BOF and continuous casting, Aliquippa Works Div., Jones and Laughlin Steel Corp., Aliquippo, Pa pouring box. The stream entered directly into the box between strands and the exit stream from these adjacent strands was affected each time metal was introduced. The streams from the nozzle wavered as the ladle stream entered the box before settling down after ladle shut-off. This was not an overly serious operating problem when casting large sections, b~11 on 4 ft x 4 ft and smaller sections it was a substantial problem. One way to solve this problem on small sections was to move the tundish right down on top of the mold. When this was done, the meniscus could not be seen, slag could not be picked (which really should not be necessary) and, where copper plugs were used, there was difficulty in shutting off the stream in case of a breakout. It was next to impossible to service the nozzle, burn out, trim up, etc., with the tundish 6-9 in. from the top of the mold. Another big problem was that scum and slag formed on the surface of the metal in tundish. These oxides had a tendency to build up at the slag line and had to be burned out of the tundish after the heat was cast. In fact, provision was always made so that the hot tundish, immediately after pouring, could be swung over the dumping box, inverted at least 110" and hosed down with oxygen lances. This operation took time, added immensely to refractory costs, and caused major tume problems on the casting floor. One was never able to get the box really clean even with the best lancing practice. The residue iron oxides fused on to the refractories of the tundish and formed new compounds, which had lower softening temperatures. These complex oxides would melt, run down the walls, and plug the nozzle if the preheating temperature was too high. The only real cure to this problem was to heat the tundish upside down, but only one two-strand machine in the United States is so designed. What did these inadequacies of the past teach us? How could we improve the performance of this all important part of the casting system-the tundish? When J&L decided on a six-strand billet machine our process development engineers were authorized funds to design, build, and test a tundish. Fig. 1 shows the J&L experlmental tundish pilot plant setup. Our objectives in the tundish trials were: 1. To lose as little temperature across the tundish (i.e., from ladle to mold) as possible. 2. To prevent reoxidation of the metal and thus eliminate the scum and slag forming in the tundish. 3. To improve the system of introducing metal into the box and into the molds. 4. To ensure safe operation in case of a full running stopper in the ladle. Thus the overflow had to be designed to handle the whole ladle of metal easily and safely in an emergency. Fig. 1
2 138 Open Hearth Proceedings, 1969 Fig. 2 Fig. 3 Fig. 4! Let us consider the parameters in reverse order. The old system, an overflow at the top of the box, seldom worked satisfactorily. The notch was not kept hot and when metal tried to flow out it froze. As soon as this overflow plugged, the whole top of the machine was in jeopardy because the metal just flowed over the sides of the tundish and down onto the mold tables. Our tundish design with an inside overflow system is shown in Fig. 2. Here the overflow chamber and the discharge chute are always hot as this compartment serves as an exhaust for burner combustion gases. Notice the outlet or hole is big enough to accommodate the full.flow of a 2Y4 in. or 2% in. ladle nozzle. The metal is diverted via steep angled runners into a ladle away from the machine. These provisions added in. to the length of the tundish, but our trials showed the penalty associated with longer tundishes is minimal so that we could discard the double tundlish system with its more significant disadvantages. Double tundishes required double-stoppered ladles and the proportion of these difficulties sometimes necessitated shutdown and complete redesign of the metal introduction system; We are not yet sure what the maximum length of a tight tundish can be, but 20-ft tundjshes work well and we believe 30-ft tundishes will work well also if they are designed properly. Parameter 3 called for a better method of metal introduction into the tundish. The pouring spout or box, off to one side and away from the nozzles, is certainly an improvement (Fig. 31, although not perfect. In cases the metal flowing in from the spout will cause the center stream to waver but not to the extent formerly experienced. The Fig. 5 t pouring box plus the slide gate ladle valve will certainly correct the nozzle stream problem. We plan to incorporate the new flow control system into our plant. Once we get rid of stopper rods in our ladles, the ladle failures will decrease and argon bubbling and degassing of ladles bound for the casting machine will be much easier and safer to handle. We also hope to eliminate the stopper rod in the tundish through extension of the slide gate to this unit, but presently (see Fig. 4) we must employ the stopper rod in accomplishing the following: A. Start-up. First, the positive shut-off of a stopper is a great comfort during the early start-up period when strand reliability can be erratic. Having stoppers permits selective start-up of any strand or strands. For instance, on our machine built to 3-ft centers, it is impossible to see the middle strands at the straightener when the outside strands are casting. To overcome the visual handicaps we may start the middle strands first and give them a 3-4-ft head start. This permits the tundish to fill to the right depth prior to opening the other strands, and it permits us to observe the center strands without obstruction as they enter the straighteners and shears. B. Shrouding. When we shroud a stream from the tundish to the mold, there must be a good shut-off device in the tundish in case of breakouts or other sudden malfunctions of the strand. C. Metered metal flow. In some cases it may be desirable to cast aluminum-killed steels. By using an oversized tundish nozzle and shrouded or submerged teeming, we can maintain proper metal flow and prevent reoxidation. The stopper rod is necessary in this case to serve as a metering device to deliver metal at a constant rate to the mold. The stoppers must be air cooled and this piping adds a few more connections. D. Reoxidation. Much has been said regarding reoxidation of killed steel poured thru air into an open tundish. We know that, even if the oxygen content of steel in ladle is pulled down to below 100 ppm via degassing or special slag practices, pouring through air increases the oxygen content toward former levels. Thus, in the J&L tundish, we have made a provision to control the atmosphere by constructing a tight cover with positive sealing joints (Fig. 5). Such construction then permits us to fire the premix burners in the tundish roof in such a way that the combustibles in the oven run over 1.0% and stay at this level throughout the cast. Some of our early test data are shown in Table I. Under these conditions we should minimize reoxidation of the metal in the tundish. Over the two campaigns of this tundish design,
3 Continuous Casting II Billets and Blooms 139 Fig. 7 Fig. 6 we experienced no oxide scum or slag buildup in the tundish as long as we kept the burners on and firing in the manner prescribed. When we deliberately cut back on the burners and lost control of the combustibles in the oven, slag formed. In a very short time it showed considerable volume and appeared similar to that formed in the old open tundishes. When the atmosphere is controlled, we get no slag on the metal or buildup at the metal level of the tundish; hence, the former chore of burning out the tundish is eliminated. All we get is a glass-like coating which is easily drained through the nozzles, and the most skull we ever experienced was a small plate on the floor of the pouring box. Of course, we agree that reoxidation of the metal starts as soon as it leaves the ladle and that to prevent any reoxidation this stream should be shrouded. In fact, our tundish car is built to raise and lower the whole tundish. This will permit us to come in over the machine with ceramic tubes and then lower them into the molds prior to starting the cast. We believe the combination of a well-deoxidized heat in the ladle, shrouded ladle streams, atmospherecontrolled tundishes, and shrouded or submerged teeming to the molds will upgrade the quality of continuously cast steel and maximize operational efficiency. If it is necessary or desirable to degas for any reason, the above system will maintain the oxygen content of the metal to that achieved in the degasser and possibly only then will we really begin to ascertain the full benefits of degassed steel. Do not feel that our system is a simple solution; the reverse is true. Fig. 6 shows the plumbing for air and gas lines to the tundish station over our machine, a complex duplicated in all five of our preheat stations. But we believe the overall system promises quality and production incentives sufficient to offset the heavy investment in the machine's building and maintenance. Parameter one involved temperature loss which is a complex problem. Because of the metal heat losses in the tundish, it is usually necessary to super-heat the metal in the furnace by 50"-100" and this increases the oxygen problems, cuts down on furnace life, causes more stopper failures, and decreases ladle lining life. In order to keep heat losses to a minimum the J&L tundish is fitted with a tight roof, arched to maintain shape as is the tundish bottom. We intend to use the five burners mounted in the roof as preheat burners. These premixed gas-fired burners have a half million Btu capacity each and we fire them during casting. Fig. 7 is a photograph of the roof minus the stoppers. The roof moves with the tundish and is plugged in at any of the five preheat stations on the casting floor of our shop. By constantly firing during casting, our tundish is really a furnace and is hot when the first metal is introduced. Therefore, we expect to minimize the super-heating of the steel in the ladle. In fact, we hope to approach normal BOF tap temperatures unless we are to argon bubble or degas. Having shown some of the J&L trial tundishes and discussed the operating procedures, we will now present some of the test data from the actual tundish trials. Our first experimental tundish was designed with 3 ft. 6 in. from nozzle-to-nozzle with a side pouring box. It had a tight roof and six highenergy burners mounted over each nozzle. Normal trial procedures were to preheat, using the six roof burners, to at least 2200 F as read at the hottest spot which could be sighted looking into the box from the pouring spout. We took an immersion temperature reading on the ladle 2-3 min. after tap, but no other ladle temperatures were measured during casting. From finish tap to open up over the tundish ranged from 3 to 11 min., but 6 min. became a good average as the campaign progressed., Our first tundish was fitted with eight thermocouples, two of which protruded 2 in. into the metal bath (see Fig. 8). The two protruding couples are numbered 7 and 8. The other couples imbedded in the lining are numbered 1-6. To check our Table 11. Preheat Tests: Firing Rate vs Temperature Table I. 0, and Combustibles* in Tundish.. -_ , POSITION, TlME % 0, % COMBUSTIBLES CENTER 1: NORTH L NORTH CENTER SOUTH CENTER 1: SOUTH * HB, CH4, CO. Magnified Sectson 01 Tundlrh W~II shoring omtall 01 Therrnmouple Potilions Fig. 8 TEMPERATURE TlME TO AnAlN AT LINING 'F MID.POINT FIRING RATE I\T SURFACE AT TlME TRIAL taruhr) lo(min.)! e:
4 140 Open Hearth Proceedings, 1969 I Table Ill. Summary of Operating Data on Heats Successfully Cast PREHEAT TRIAL TIME FIRING RATE TEMP. (max.) NO. HRS. MM BTU HR. "F :: / / / / I / I continuous probe couples, dip or immersion temperatures were also measured in the spout and near the outer nozzle right next to the overflow. We used the %-in. diameter Zjrcon nozzles on all trials and the maximum metal height in the tundish was 15% in. To determine the effect of wreheat time on temperature gradients in the lining, five dry runs were conducted. In the first four trials the burners were fired at prespecified rates until a surface temperature of 2350 F was attained as measured by the optical. Gas flow was then stopped and a short soak period allowed. These data are shown in Table 11. In the last test the burners were set at a 500,000 Btu until we reached LADLE SUPPL. HEATING -1-1 l-llmllg- HOLD FIRING TEMP. TlME RATE TlME "F MIN. MM BTU.HR. MIN..11-1, Total Total Total First First Total ' Total Total Total Last ' Total Total Total Total Total Total 2350, but in this case we reduced the firing rate to hold the surface temperature constant over 3 hr. A graph of the fifth preheat trial is seen in Fig. 9. Tables I11 and IV summarize some of the tests on which we felt we had reliable data. The range of variables cover: Preheat times Preheat temp. Ladle temp. Supplementary heating Supplementary time Casting rate Considering crepancy from hr 2120"-2350 F 2880"-2980 F 0.4 to 0.6 million Btu/hr 0 to entire cast approx. 540 lb/min. all cases, a diszero to 40 F exists between our probe couple and immersion couple taken at the same spot in the tundish. A temperature difference of up to 25 F was found between the inner and outer nozzles of this six-nozzle box. Tables I11 and IV show, in summary: 1. The temperature drop, ladle to tundish, averaged 69 F; range 45" to 85"F, taken 3 min. after start to cast. Thus, total time for these temperatures is 9 min. on Trial 7, Table Temperature gradient inner to outer nozzle: The average near the beginning is 32 F (20-45") range; the average near the end is 19 F (0-45") range. 3. Over the short period of casting (usually 16 min.), refractory temperature at the midpoint of lining increased no more than 100 F. During the same period, shell temperature increased 20 F. In the longe? casting time, trial 15, the working lining midpoint temperature increased 230 F and the steel temperature at probe 6 increased 40 F. 4. A preheat temperature of 2350 F can be reached in 45 min. at a firing rate of 900,000 Btu/hr. However, the temperature gradients are so steep that we feel the practice would decrease lining life. The heat loss of the metal in the tundish is increased also. 5. For every 100,000 Btu/hr increase in firing rate, the inner to outer casting nozzle temperature at 13 min. casting time deer-eases by 5 F. Our regression analysis of these data indicate that 68% of the variation in this temperature difference can be attributed to firing rate. 6. Ladle temperature did not have a significant effect on both temperature gradients over the range studied (2890" to 2980dF). All steels were AISI 1020 type. Twenty-five trials were run on the Table IV. Temperature Differences "F SPOUT TO OUTER NOZZLE LADLE TO TUNDISH TRAIL AT 3 MIN AT 13 MIN AT 3 MIN. _--- A_T_iQ.IA --.!EL samaul:a_y LWLWIM--PE, GQPIWWLS-E EQWUYUUS-RR I I I Z l 1 1 : I THERMOCOUPLE POSITION Fig. 9
5 first tundish. After examining the data we decided that we were on the right road to achieving our initial goals. We felt, however, that we must provide for stopper rods and that the overflow system could be improved. Inasmuch as we had to freeze the tundish design, we made these changes and had one built and delivered in time for us to test at the Aliquippa bessemer before we shut that facility down. The last five heats shown on Tables 111 and IV are the comparable information of four of the heats run through the new tundish. The main differences between the two tundishes are that the new one has nozzles on 3-ft. centers and employs only five burners. A cross section of this tundish is shown in Fig. 5. A photograph of this box without the top is shown in Fig. 10. We have taken pains to see that the walls are built to receive the arched roof. Notice, in Fig. 10, the slope of the front walls, the angle of the top of the wall, and the nozzle wells. The overflow dam can be seen in the very forefront of the picture. Generally, a satisfactory refractory practice has been developed using a 50% Al,O, working lining, 4% in. thick, and a fireclay safety lining, 2% in. thick. Both a square and a round bottom tundish configuration have been used and we are prepared to recommend a round bottom design because of: (1) good mold visibility, (2) tighter lining with minimum joint penetration, (3) better drainage, and (4) the possibility of unusually good flow characteristics. Because the colder section of the lining is the pouring spout (during preheat), a refractory weakness develops at the junction between the pouring box and tundish proper The key to overcoming his weakness, at least to a tolerable degree, is the ability to "tie-in" these refractory corners. However, the trial tundish as well as the current concept of the billet machine tundish has the tundish wall sloping toward the pouring trough. This results in a sloped wall meeting a vertical wall and prevents "tying-in" brick from both walls. A castable corner has been considered, but this would be excessively costly. The reversing of the tundish slope to the opposite wall IS considered to be the most likely soiution to the problem. An issue is raised concerning this situation because at full production, patching each tundish in this area could require two men per turn. In the new design the burners fire between the nozzles because we have stoppers working over the nozzles. Fig. 11 shows the roof looking up at the firing block burners and one can see the holes to the stoppers. We feel that, by using a standard arch brick in this roof and by using the high Alumina burner blocks, this roof should give us good life if we can lick the old problem of distortion. Distortion was not a problem during our trials, but we recognize that the short cast times and the limited campaign was not a good gauge of tundish life and/or distortion. The lined tundish minus the roof appears in Fig. 12. The overflow is at the right-hand side of the box and the hot gases exhaust down through the emergency hole into the emergency chute, thus keeping the spill-over system hot at all times and constantly ready to handle metal if need be. So far we have not said anything about atmosphere control. Early in our program, our research people checked out the effect of shielded or tight tundishes on the casting of Al-killed steels.' Table V is a summary of some of their work. Notice how the cast speed is affected by open or shielded practice. Shielding did not solve the A1 problem, but in a11 cases it retarded reoxidation enough to permit longer casting times at higher casting speeds. We attempt to maintain at least 1% combustibles in our tundish atmosphere and oxygen as low as possible. This is done through adjusting the premixing of gas and air in the Bloom burner. Care has been taken to supply constant or as nearly as possible constant air and gas in a way which permits some control over the exhaust gases, which are really what we depend on for atmosphere control. Some users of atmosphere-controlled tundishes introduce inert gases into the tundish. We may have to do this also, but results on our tundish trials indicate it is possible to hold close enough to our target to prevent reoxidation of the steel, which causes a buildup of scum and slag in the tundish which has to be hosed out. The oxygen hosing practice is the cause of premature failure and wear on the lining. The burners have pilot lights and the header to each pilot is secured right to the tundish roof as was shown in Fig. 7. The other gases, burner air, burner gas. and stopper rod cooling air are all on flexible coupled hoses which make each tundish station resemble a metallized octopus. However, the flexibility of being able to set the roof on a tundish and place both together on the tundish car and keep them as an operating unit at preheat or at the machines is a great operating advantage. We will always have at least one tundish and roof, hot and on wheels. ready to roll in over the machine at any and all times. This is necessary for successful series casting which we plan to do. Our tundish can be tilted 100" on the car if need be. We also have provided for raising and lowering the tundish on the tundish car to permit us to use ceramic tubes or other Continuous Casting II Billets and Blooms Fig. 10 Fig. 11 Fig. 12 Table V. Comparison of Experimental Data for Shielded and Open Tundish MAX. CASTING TUNDISH TIME. STEEL TEMP... castcb- 2- I~~~-~~~~"~~NIH~J.Z.LEil ISHIELDED TUNDISH, %.IN. NOZZLES) (OPEN TUNDISH. %.IN. NOZZLE] ISHIELDED TUNDISH, %,IN. NOZZL9 AVG. CASTING SPEED. LBLM'N shrouding devices. We believe that we have a tight design in which we can control the atmosphere and thus upgrade the quality of the steel. We believe our tundish is safe and that the emergency metal system is so good as to instill greater confidence in the casting crew than might otherwise be the case.
6 142 Open Hearth Proceedings, 1969 We have experienced very good metal flow in our tundish trials. This was true even with relatively low temperature (below 2800 F) in the tundish and also with relatively high A1 content (0.010% acid soluble). Nozzle blockage did not occur in either case. It should be mentioned that we used acid practice bessemer steels in our trials. The a year from now we will know how effects of higher phos and N, may good the tundish really is. have had deceiving effects. The silicon contents of the steel in our case were also kept on the high side, REFERENCE 0.25/0.35. Thus we do not claim that our tundish is superior to all others 'G. C. Duderstadt, R. K. Iyenger, and J. M. Matesa, "Tundish Nozzle Blockage in because we recognize some of the Continuous Casting." JOURNAL OF limitations of our trials. However, April METALS. DISCUSSION by A. B. Glossbrenner Mr. Brown has done a very thorough and concise job of presenting J&L's intelligent approach to tundish design and testing. Timken also found that we could have wasted much money and time without a great deal of planning, engineering, and actual mock-up testing. The tundish for a continuous cast machine is anything but a "simple device." In both cases we profited from the earlier work and learning in- $olved in the first tundishes used on the various machines in operation. J&L's solution to the overflow problem is rather ingenious and is certainly one that could not have been accomplished once the machine had been built and started up. We certainly agree that a slide 'A. B. GLOSSBRENNER is asst. general supt., Timken Roller Bearing Co., Canton, Ohio. gate ladle valve has much to offer and could solve some sticky problems, especially in the case of aluminum-killed steels. As to the offset pouring box, we have found that the extended nozzle controls the strand feeding such that the ladle stream has little effect. However, we question whether the slide gate valve on the tundish is compatible with the extended nozzle. ' We cannot agree that pouring through air will cause oxygen contents to climb back to levels that were reached before degassing. Our standard practice-teeming into conventional molds after degassing -consistently produces grades of steel with oxygen under 15 ppm. We do not expect this to change when these same types are continuously cast. We agree that tundish sealing and atmosphere control are very necessary. Where J&L has used gas with close combustion control, Timken uses electric glo bars in a nitrogen atmosphere. At this point we would be interested to learn how constant the metal temperature in the tundish can be maintained throughout the cast? Also, what type and how reliable are the protruding couples? We also agree that quality continuously cast steel is only possible when the conditions that Mr. Brown has listed are provided. And if you think that air and gas lines begin to make things complex, please consider what copper busses and watercooled bar holders require. This paper is most interesting to us at Timken. Our research and experimentation on tundishes have been completely independent, and yet the results and conclusions are almost identical to those at J&L. We thank the Committee for this opportunity to discuss this fine and informative paper.