METAL GUIDING SEPARATION RINGS FOR HORIZONTAL CONTINUOUS CASTING MACHINES Yu. G. Zel'tser, V. M. Shchukin, V. G. Babiev, and Yu. A. Smirnov
UDC 666.762.93:621.746.047
The share of continuously cast steels in the total volume of steel production is steadily increasing. The presence of old steel melt shops in domestic industry is very large and therefore one of the most important problems is conversion of steel melt shops from teeming into molds into production of billets in continuous casting machines. Especially desirable appears conversion of such shops into casting in horizontal continuous casting machines, which are distinguished by compactness and small size, which is very pressing as the result of the lack of area. Billets produced on machines of this type have significantly better quality than billets from ingots teemed in molds and are no poorer than billets produced on other types of continuous casting machines. The low capital investment for these machines provides profitable operation of them with a small production program. Two types of horizontal continuous casting machines are produced, with withdrawal of the billet (ingot) in one direction and with withdrawal of the billets in two directions. In the first case high effectiveness of the machine must be provided by casting of billets close in shape to the small cross section finished product (round or square) or by a highly productive method in casting of large heats and also of series of heats, which requires the use of high-quality refractories in separation rings. In the second case it is effective to use machines for casting of high-quality steels and also of a flat billet but reliable service is completely impossible without high-quality separation rings made of nontraditional refractory materials. The method of connection of the metal reservoir and the casting mold by a chamottegraphite metal guide used at present on the four-strand horizontal continuous casting machine with one-sided withdrawal at Karaganda Metallurgical Combine was applicable for casting 1520 tons of steel in each strand. There are cases of casting up to 25-30 tons per strand. In this case the wear of the refractories reaches 40-90%. A further increase in effectiveness of use of the machine in production requires casting of heats of up to 130 tons, that is, 30-40 tons per strand, which not rarely leads to 100% wear of the metal guide and early shutdown of one or more strands. The increased wear of the face of the chamotte-graphite metal guide also causes poorer conditions of formation of the shell, somtimes leading to hang-up of the ingot on the initial portion of the casting mold surface. In such a situation resumption of casting is possible only after a pause and the portion of the ingot in the area of the hang-up is scrapped. To eliminate repeated hang-ups of the shell, casting is resumed at a lower rate with a longer pause in each cycle of withdrawal, which increases the depth of surface defects. Wear of the face of the metal guide is caused by formation of the billet in the portion of the liner with precision machining for installation of the chamotte--graphite metal guide. This leads to damage of the sleeve and poorer conditions of matching of the casting mold with the refractory in subsequent pourings. In individual cases repair of the feeding hole of the sleeve is necessary because of progressing coarse defects on its surface. In foreign practice recently there has been a tendency toward decreasing the number of strands on horizontal machines, which significantly reduces the operating costs but increases the weight of metal cast per strand and the time of the process. This method is based on unconditional use of separation rings of refractories distinguished by long life. In the USSR horizontal continuous casting machines with two sided withdrawal of the billets have been developed, providing the capability of casting billets of higher quality. A feature of this method is delivery of the metal to the center of the casting mold through All-Union Scientific-Research, Planning, and Design Institute for Metallurgical Machinery Building. Karaganda Metallurgical Combine. Translated from Ogneupory, No. 3, pp. 2729, March, 1991. 146
0034-3102/91/0304-0146 $12.50
9 1991 Plenum Publishing Corporation
TABLE i. Characteristics of the Refractory Materials for Preparation of the Separation Rings AppaMaterial rent density, g/cm s
BGP RS-N
i'Thermal Ult. strength, Contact icoeffi- N/sin angle cient of . of wetring, Iinear tombend deg Iexpan- pression, (35G IO-6K-1 sive steel )
2.|--2,2 1.5--2.0 100--180 60--90 2,2--2,4 Not det. [20--[60 50--80
Elastic modulus, I0 s N /mm 0
141
70--80
136
73,5
a hole in one of its walls. Into the hole is pressed a feeding refractory ring, one face of which is a portion of the working surface of the casting mold. Solidification of the metal has already started in the separation ring in connection with which a basic requirement here is minimum wear of the refractory. Experience has shown the possibility of reliable casting with a wear of the ring working face at the level of several millimeters. More wear leads to hanging up of the ingot and also to significant damage of the casting mold. To a large degree reliable and long operation of the refractory in casting machines is determined by its thermal strength, resistance to mechanical erosion, and absence of interaction with the carbon-containing molten material (steel). In the latter case increased wear of the refractory, passage of it into the molten material, and, as the result, complete failure may be observed. Interaction of the refractory with the molten material is basically characterized by the contact angle of wetting 8, having determined which it is possible to calculate the work of adhesion W. Determination of % is by the "sessile drop" method. On the basis of a combination of work done by the All-Union Scientific-Research, Planning, and Design Institute for Metallurgical Machinery Building together with the Central Scientific-Research Institute for Bridges (Leningrad) the refractory materials - hot-pressed borosil (BGP) based on BN and SiO 2 and reaction sintered material (RS-N) based on BN, AIN, and crystalline silicon* - were first selected and then thoroughly tested (Table i). The experimental separation rings of BGP and RS-N were prepared in the Central Scientific-Research Institute for Bridges and tested on the experimental horizontal continuous casting machine with two-sided withdrawal of the billets of the All-Union Scientific-Research, Planning, and Design Institute for Metallurgical Machinery Building, in which the arc furnace provided delivery to the machine of individual heats of up to 2 tons. Investigations of the rings (about 100 tested) showed reliable casting for each of them of three to seven heats. During the tests the material, design, and method of installation of the ring in the casting mold were determined more accurately. For production tests it was necessary to prepare rings under production conditions with use of standard methods of production. The borosil rings were produced at Bogdanovich Refractory Plant% to TU 14-203-72--85, which were developed by the central Scientific-Research Institute for Bridges, the All-Union Scientific-Research, Planning, and Design Institute for Metallurgical Machinery Building, and Bogdanovich Refractory Plant. The rings were tested on the horizontal continuous casting machine of Karaganda Metallurgical Combine. More than 15 rings were produced and an average of four heats of 35GS steel were cast through each ring. For example, the quantity of steel cast through one ring was 48.3 tons (wear of the ring 30%) and through another 62.2 tons (wear 25%). One of the used rings was subjected to phase analysis. An insignificant change in composition may be judged from the results of this analysis. The original material was primarily BN and SiO 2 in an amorphous phase with traces of SigN 4, Fe203, and AI203 while in the
*The authors' developments of the materials were made in the Central Scientific-Research Institute for Bridges (under the supervision of V. A. Ryabov) with participation of the AllUnion Scientific-Research, Planning, and Design Institute for Metallurgical Machinery Building. The wettability was determined in Ural Polytechnic Institute under the supervision of V. N. Kozhurkov, V. P. Manov, and S. I. Popel' %G. I. Bad'in, A. A. Vyatkin, and G. S. Shlyagina participated in production of the rings. 147
used material there were also observed cristobalite and B203. material was 36 HRA and of the material after service 27 HRA.
The hardness of the original
In addition to tests in the continuous casting machine rings were repeatedly thermal cycled for the purpose of determination of the possibility of their repeated use in the casting mold. Their density and hardness were measured and they changed as follows: Density, g/cm ~ Original . . . . . . . . . . After 2 cycles . . . . . . . After 5 cycles . . . . . . .
2.18 2.18 2.19
Hardness, HRA 36 33 26
Based on the results obtained it is possible to confidently recommend separation rings of BGP for use in horizontal continuous casting machines with one sided withdrawal of the billets for casting of large heats and also for the "heat on heat" method. The use of borosil separation rings is being delayed by their high cost. This is the result of the short life and high cost of production equipment of silicided graphite and also the high energy consumption and low productivity in preparation of the rings. The comparatively high specific costs in use of borosil separation rings was responsible for searching for new refractory materials with similar service properties but lower cost. RS-N material based on BN, AIN, and crystalline silicon, which during reaction sintering in a nitrogen atmosphere changes into SisN~, answered such requirements. The RS-N experimental rings were produced at the Central Scientific-Research Institute for Bridges. The basis of their production method was cold forming in a metal die and sintering in a nitrogen atmosphere. The cost of these rings in full-scale production presumably must be four to six times less than for borosil rings. The RS-N rings were tested in the horizontal continuous casting machine of Karaganda Metallurgical Combine in casting 35GS steel. About 10 rings, through which were cast an average of three heats, were tested. For example, the quantity of metal cast through one ring was 62.8 tons (wear of the ring 20%) and through another 47.8 tons (wear 10%). The results obtained show that the service properties of RS-N separation rings are at least as good as those of borosil rings and they may be recommended for use on horizontal continuous casting machines. The same as for borosil, all of the properties and especially the thermal strength of RS-N material significantly exceed those of traditional refractory materials. Borosil rings were also used on the horizontal continuous casting machine with twosided withdrawal of the billets of Kramatorsk Metallurgical Plant. In this case the separation rings play a decisive role in casting of full-weight heats and production of highquality billets. Solidification of the ingot occurs on the working surface of the ring located in the upper wall of the casting mold. The temperature differential on the working surface of the ring is 1000~ On the edge of the ring in contact with the mold the temperature is 350-400~ while on the inner surface it reaches 1300-1400~ The ring is installed in the continuous casting machine mold using a tapered or cylindrical fit. Significant differences in life were not noted. Borosil rings also successfully passed production tests on this machine. In casting St6 steel with heat weights of 30 and 35 tons the wear of the rings was less than 5%. Therefore borosil separation rings made it possible to solve the problem of introduction of horizontal continuous casting machines. Without them use of these machines, especially with two-sided withdrawal of the billets, would be impossible. Apparently borosil may be used not just for production of separation rings. The high properties of this new refractory material must provide it a deserving place among ceramic parts for steel production. The primary disadvantage of it is high cost. This disadvantage may be eliminated by RS-N reaction-sintered material. With basic properties little poorer than borosil, it is several times less expensive. In our opinion these materials must find wide use not only in steel production but also as constructional materials in machine building.
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