R E F R A C T O R I E S FOR THE CONSUMER

CORUNDUM-GRAPHITE STOPPER-MONOBLOCKS FOR CONTINUOUS CASTING MACHINES L. M. Aksel'rod

UDC 666.762.11+666.762.8]:621.746.329.047

Corundum-graphite stopper-monoblocks are produced according to TU 14-8-371-81 in a number of refractory plants and are used on the continuous casting machines of practically all electric steel melt shops of the country. As is known, stopper-monoblocks are used in the flow-control devices of continuous casting machine tundishes in place of the earlier used traditional plug stoppers. With only a rare exception 1150 mm long stopper-monoblocks are made and used at present with a head portion of two configurations, tapered and spherical. Stopper-monoblocks with a tapered head portion are used by the absolute majority of users although the production and use of stopper monoblocks with a spherical head portion were first introduced. Operating experiments with the continuous casting machine of the Electric Melt Shop of Uzbek Metallurgical Plant provided a basis for switching to the use of stopper monoblocks with a tapered head portion in a pair with tundish nozzles to TU 14-8-238-77. Of the stopper-monoblocks produced in refractory plants 96% are fastened with a metal pin (Fig. la). Experience has shown that reliable operation of a 1150 mm long stopper-monoblock is guaranteed with a level of metal in the tundish of not more than 700 mm. Taking into consideration that in contrast to plug stoppers stopper-monoblocks are not cooled during teeming with compressed air (more accurately their metal fastening is not cooled), the distance from the meniscus of the metal in the tundish to the pin must not be less than 200-250 mm. Otherwise warping of the metal fastening occurs and the stopper-monoblock moves to the side, that is, its head portion is displaced relative to the seat of the tundish nozzle and loses the capability of overlapping the nozzle channel. Another method of fastening stopper-monoblocks with a threaded metal rod has been developed (Fig. ib). This method of fastening is more rigid and required better preparation of the fastening is more rigid and required better preparation of the fastening assembly and better installation of the stopper-monoblock in the tundish. It is necessary to place the stopper-monoblock and the nozzle of the tundish on the same axis or otherwise leakage of metal or lack of closing of the stopper cannot be avoided, which risks an emergency situation. The warping of the metal fastening described above also occurs in this case. However, the threaded fastening has a definite advantage in blowing of the metal in the ladle with inert gas through the stopper-monoblock since in this case it is easier (with mortar) to provide sealing of the supply of argon to the stopper-monoblock and to avoid leakage. In domestic metallurgy there is not a common opinion on the question of the effectiveness of delivery of inert gas to the metal through the stopper-monoblock. Abroad a number of publications [1-3] present evidence of the positive influence of blowing of the metal with gas in the tundish and draw a conclusion of the high effectiveness of blowing of gas into the metal through gas-permeable plugs installed in the head portion of the stopper monoblock [3, 4]. From the point of view of reducing the rate of clogging of the submerged ladle channel and reducing the quantity of nonmetallic inclusions, primarily alumina ones but also sulfides, which influences the quality of the ingot itself and its surface condition, even eliminat ~ ing the necessity of conditinoing of the finished slabs, the effectiveness of blowing of the metal with argon is high. In the opinion of Japanese authors the service life of monoblocks may also be increased by blowing of gas through a gas-permeable plug [4]. In the domestic literature there is not clear proof of the necessity of blowing with inert gas. Attempts have been made to determine the effectiveness of blowing of inert gas through a through channel (Fig. 2a) in comparison with blowing through gas-permeable plugs (Fig. 2b) but the final word is for metallurgists and it still has not been spoken. All-Union Refractory Institute. ber, 1991.

Translated from Ogneupory, No. 9, pp. 31-34, Septem-

0034-3102/91/0910-0487512.50 9 1992 Plenum Publishing Corporation

487

~7

l

Z

J

a

b

Fig. i. Methods of fastening a stopper-monoblock: a) fastening with a pin; b) threaded connection; I) metal rod; 2) stopper-monoblock; 3) pin.

Fig. 2. Variations of delivery of gas to the metal through the stopper monoblock: a) through a through channel; b) through a gas-permeable plug; i) stopper monoblock; 2) gas-permeable plug. Table i gives the characteristics of corundum-graphite stopper-monoblocks and gaspermeable plugs domestically produced and also of the same parts used abroad. It should be noted that the foreign firms offer, in addition to corundum-graphite stopper-monoblocks, these same parts with a strengthened head portion designed for use in pouring special steels and for a longer period. Stopper-monoblocks are offered with the capability of blowing with argon through a 5-6 mm diam. through channel or through a gas-permeable plug [5-8]. The life of stopper-monoblocks may be expressed by the tons of steel cast or by the number of heats (steel teeming ladles) cast through a tundish with the stopper-monoblocks and then the life of each monoblock is determined in nominal heats, that is, the share of the heats proportional to the number of strands of the continuous casting machine. In our opinion the life of a stopper-monoblock must be expressed by the time of service and the length of contact with the failing factors. The question of life is in need of being made more precise since the evaluation of continuous casting machine refractories in tons of metal poured used in the USSR is convenient for a comparative evaluation of specific consumptions of refractories but as the result of the difference in rates of metal flow for different casting machines (different mold cross-sections and pouring rates) it is does not always provide a true picture. In the literature the life of stopper-monoblocks is given as different, normally 6-10 heats [3, 6, 9]. Variations in the life of stopper-monoblocks in domestic steel plants are regular and are related to a number of factors dependent both upon the producer and upon the user. It is desirable to clarify certain of them, starting with the metallurgical factors:

488

TABLE i.

Characteristics of Stopper-Monoblocks Vesuvius

Character-

istics

stopper- lhead0SSRportlon" gas-pe~eable " " J b , p l u g developed ~onohlock

[t o

ioevelopeu

y!

In~ All-

lbY the

TU '~ 9 Union Unlon Instl11.-8-371-87 [ for 9 .

All-

[Refractory Itute [Ref ractoriesllnst xtute

-

and Gas-Permeable Plugs OAMAGDJ

[5]

,topper-]t .e.d I ,aspe~.onoblock |

portion

able

stqppermonoblock

plug

Sirma

head

portion

monoblock

portion

52-- 54 29--3t

5--7

able

plug

92

91

25

27

X i Weighs %

AI:!O:~ C

65--68 15-- 16 8--12

SiL)~

ZrO~ MgO

-I0 8--12

.

70--72 . .

16--20

18--;]0

--

--

>90 ----

--

50--56 28--33 14--18

-15,3 15,0

--

--

.

>90 --

5 2 - - 70 20--22

-20

- -

--

--

--

--

70

1 3 - - 1 5

--

-

8 3 - 85

72.4

Open

porosity,

%

Gas perme2 ability, D m

28--31 >~0

16--20

17.5

--

--

26--32

17--18

17

13~- 15

10-- 12

--

1(}

9 There is not data for the gas-permeable plug.

~Ar 1

Fig. 3. Plan of erosion of the stopper monoblock 1 in the slag belt.

I 1 / 2 / #5

J Fig. 4. Plan of a portion of the joint of the stopper-monoblock with the tundish nozzle: i) stopper-monoblock; 2) glazed coating; 3) graphite-containing tundish nozzle; 4) piece of magnetite; 5) portion of the nozzle with burned off graphite; 6) piece of mortar. i. The product range of steel poured, which determines perature, and oxygen content in the steel.

its chemical composition,

tem-

The fact that the life of corundum-graphite stopper-monoblocks in pouring type ShKhl5 steel (temperature in the tundish 1480-1500~ may reach five to seven heats of 6-9 h of continuous operation while in pouring 32G2S, 25KhT, and other types (temperature in the tundish 1520-1545~ the guaranteed life does not exceed 6 h of pouring may be considered experimentally established. In this case the higher operating temperature in combination with the presence in the metal of components aggressive to the refractory such as IMnl and 489

ITi} Creates more severe service conditions for the stopper-monoblocks, which their head portion wears more intensely.

as the result of

Anos factor is known having a negative influence on the life of corundum-graphite stopper-monoblocks, the oxygen content in the steel, but it has not been reflected in domestic works. There is a report [9] that in steel containing, in addition to sulfur, up to 80 ppm 02 erosion of the head portion of stopper blocks is accelerated. Blowing with argon and production of stopper-monoblocks with a strengthened head portion, in the composition of which zirconium dioxide or periclase is used in place of the alumina-containing constituent (Table i), may be measures for increasing life. Correct selection of the geometric parameters of the tundish nozzle channel in accordance with the necessary rate of metal flow makes it possible to pour with a completely open stopper, avoiding operation of the stopper for slowing. Since the existing TU 14-8-238-77 provides for production of nozzles with channels of different diameters (intervals of 4-5 mm), the user has the capability of a quite broad choice. 2. The presence of an aggressive slag in the tundish, as the result of which local erosion of the stopper-monoblock occurs in the slag belt (Fig. 3). The reason for erosion is use of a thermal insulation slag mixture the components of which form the CaO-SiO2-CaF=-Na20 system. During fusion of these component at a temperature below 1400~ there is formed a low-viscosity aggressive slag which in combination with the reaction of oxidation of the carbon of the refractory by the oxygen of the air quite intensely attacks the stopper-monoblock material. In the tundish the slag acquires additional aggressiveness in the case of entry of the steel teeming ladle slag, which may occur at the end of pouring of a heat before replacement of the ladle, and since there are four to six such changes some slag accumulates in the tundish. In a number of cases the thickness of the slag layer by the fifth heat reaches 100-150 mm, which leads to freezing of a crust of vitrified slag on the stopper and an increase in load on the fastening assembly. Elimination of slag erosion is possible by several means including the use of thermal insulation mixtures such as rice husks not having in their composition aggressive components, prompt recording of the start of entry of slag into the tundish [i0], pouring of a series with maintenance of different levels of metal in the tundish such as 640 and 700 mm, skimming off of the slag if a large quantity of it accumulates, and production of stopper-monoblocks with strengthening in the slag belt either as the result of increasing the part thickness in this zone or as the result of use of a zirconium dioxide-base material with a reduced graphite content. 3. Absolute fulfillment of approved conditions of preparation of the stopper-monoblocks for service. The stopper-monoblock m u s t b e heated together with the tundish so that thermal conditions making it possible to avoid cracking of the parts are created. It is necessary that the moisture content of the stopper-monoblocks before hating be less than 0.5% and the heating time not exceed 2-2.5 h to avoid burning off of the graphite from the parts. The burners must be located in such a manner that there is not local overheating of the stopper-monoblocks. Otherwise heat cracking, intense erosion, and fusion of the glaze with adhesion of the head portion of the stopper-monoblock to the seat of the nozzle may occur. It is especially important to control the condition of the whole fastening assembly, avoiding curvature of the parts (rod, fork, etc.) and "breaking in" of the stopper to the seat of the nozzle by force impacts to avoid fracture of both the nozzle seat and the heat portion of the stopper-monoblock. 4. Freeing of the tundish prepared for pouring from foreign objects, residues of slag, mortar, etc. and elimination of the possibility of entry of foreign materials into the tundish during heating of it. These materials enter into the joint of the head portion of the stopper-monoblock with the tundish nozzle, which leads to adhesion of one refractory part to the other. In this case at the start of pouring it is impossible to raise the stopper and free the passage for entry of metal into the nozzle. An investigation of the reasons for adhesion of the stopper-monoblock to the nozzle seat made it posible to reveal foreign bodies, pieces of magnetite (Fe30 u) and mortar, in

490

Fig. 5. Microstructure of a portion of the joint of the stopper-monoblock with the tundish nozzle (zone of welding of the refractory to magnetite): I) corundum-graphite material of the stopper-monoblock; II) glaze; III) magnetite; i) corundum; 2) vitreous mul!ite phase; 3) spinellide (obviously corresponding to hercynite FeO'AI=O 3) in glassy phase. • Reflected light. the zone of contact. In addition burning off of the graphite from the nozzle to a depth of 5-7 mm is observed (Fig. 4). Inclusions of black color observed visually in the form of pieces up to 30 mm in size apparently enter into the tundish from the metal cover of the tundish, oxidizing from the surface during service. The composition of the growths on the inner surface of the tundish cover is identical to the composition of the pieces of black color adhering to the stoppermonoblock. The white particles up to i0 mm in size are refractory aluminosilicate material o f microcrystalline structure, obviously in the original composition of the mortar. The identification of the foreign bodies was made from the results of petrographic analysis* (Fig. 5), which indicates welding of the oxide inclusions (magnetite and mortar) to the glaze of the stopper-monoblock and the nozzle material. The link occurring prevents free opening of the stopper. The reasons for introduction of foreign matter into the zone of contact of the stopper with the nozzle are eliminatable but require appropriate constant inspection. A number of the factors influencing the quality of a stopper-monoblock depend upon preparation and normally determine the reliability of parts in service. As is known, the reliability of refractories in service is evaluated by the so-called risk of the user, which for stopper-blocks is close to unity, that is, the possibility of failure of a part must be eliminated. An analysis of deviations of the quality indices of corundum-graphite stopper-monoblocks the weight contents of AI203 and carbon and the open porosity,* makes it possible to judge the quite high reliability of parts from the point of view of the probability of supply of them with deviations from specification. The most frequently encountered deviation from specification is carbon content, which is an indication of burning off of graphite during firing of the parts. *Petrographic analysis made by G. G. Mel'nikovaya. %Analysis made by M. V. Fraifel'd. 491

The quality indices of the finished product are the result of observation of production discipline. For highly critical parts such as stopper-monoblocks observation of all parameters of the process from quality of the original materials to packing and shipping of the finished product is important. Below are given the characteristic and most frequently encountered in practice deviations from the method having a detrimental effect of the quality of parts and the possible consequences of them. Deviation from the method

Change in qualit 7

Use of graphite corresponding to types GE-3, GL-I, GL-2 Use of refractory clay with a coarse-grained composition

Addition of a large (>15%) quantity of scrap to the charge Nonobservance of the material composition of the charge and method of mixing Displacement of the metal rod forming the channel relative to the axis of the part in assembly of the die High residual pressure in the die in evacuation of the charge (>0.015 MPa) Low pressing pressure

Nonobservance of drying and firing conditions Nonobservance of glazing and firing method Nonobservance of packing and tranportation method

Visible and hidden cracks in pressing Nonuniform structure of the material, reduction in strength of the part, acceleration of wear in service Change in structure of the material, acceleration of wear in service Nonuniform structure of the part, acceleration of erosion in service, reduction in heat resistance Variation in wall thickness of the stopper-monoblock [ii], acceleration of wear in service Visible and hidden cracks in the part, failure in service Reduction in strength, increase in porosity, accelerated wear in service Formation of cracks, failure of part in service Burning off of graphite, accelerated wear in service Spalls, cracks, accelerated wear in service

Undoubtedly these deviations and other nonobservances of the method may be avoided by increasing the qualifications of those responsible and organization of more rigid control. However, at present it is difficult to control a number of nonobservances, especially those involved with the finished part. Determination of the uniformity of a refactory part such as a stopper monoblock with a weight of 36 kg and a length of 1150 mm and revealing the presence of cavities and hidden cracks and a difference in wall thickness visually is not possible. The only solution making it possible to reduce the risk of the user to a minimum may be organization of 100% x-ray inspection of production in combination with existing measures of inspection. This is confirmed by foreign experience [5, 12]. All-Union Institute for Refractories together with Spectrum Scientific and Production Union is adopting x-ray inspection of refractory parts for continuous casting machines in one of the refractory plants [Ii]. We assume transmission of the experiBnce obtained to other producing plants to be desirable. Increasing the reliability of stopper-monoblocks in service as the result of higher quality production and better qualified use is a significant reserve for decreasing their specific consumption and increasing the effectiveness of continuous casting of steel on continuous casting machines. LITERATURE CITED i~

2. 492

Fachber. Hutten. Metallweiter., 23, No. 8, 655-666 (1985). B. Sansby, Continuous Casting of Steel. Proceedings of the International Conference, London-Biarritz, 1976, London (1977), pp. 3-6.

3. 4. 5. 6. 7. 8. 9. i0. ii. 12.

N. Toshio, Kinzoku, 51, No. 7, 13-17 (1981). K. Oki, K. Klaida, T. Takahashi, and E. Ito, Taikabutsu. Refr., 206, No. 27 (3), 119121 (1975). Brochure of the Vesuvius Co., USA. Reference List of Isostatically Pressed Products: Brochure of the OAMAG company, Austria-Federal Republic of Germany. Special Refractories for Continuous Casting: Brochure of the Sirma company, Italy~ G. Y. Bayly, Industrial Heating, 51, No. 4, 18-22 (1984). W. Kronnert, 29 Int. Feuerfest-Kolloq: Feuerfest Werkst. Stranggu~bereich. Aachen, 9-10 Oct. 1986, Aachen (1987), pp. 47-79. Trans. Iron Steel Inst. Jpn., 26, No. 6, 590 (1986). V. G. Kras'ko, Yu. M. Rapoport, and L. M. Aksel'rod, Ogneupory, No. 6, 11-17 (1990). Alumina Graphite Refractories for Continuous Casting: Brochure of Tokyo Yogyo Co. Ltd., Japan.

REFRACTORIES FOR DELIVERY OF ARGON INTO A TREATMENT LADLE L. V. Uzberg, G. V. Efimova, E. P. Smirnov, V~ V. Pichugin, and V. V. Trofimov

UDC 666.762.32:[669.18o046o517982:621.746.329

To a large degree the effectiveness of development of steel melting production is related to ladle methods of treatment of steel. Refining of metal outside the steel melting furnace makes it possible to increase productivity in melting of steel and to significantly improve its quality. The basic direction in development of ladle treatment is development of ladles for secondary treatment of steel making it possible to accomplish various process of ladle refining. In our country ladles for secondary treatment of steel of domestic design are only being developed but imported equipment similar to ladles for secondary treatment of steel, particularly treatment ladles, is in operation. At the Belorussian Metallurgical Plant in 1987 an imported 100-ton treatment ladle unit was placed in service in the Electric Melt Shop. The treatment of steel in this unit includes electromagnetic stirring, blowing with argon, and electric arc heating. The unit for delivery of argon to the ladle includes a set of refractories produced in the Federal Republic of Germany by the Didier Werke company consisting of a blowing cone (plug) of Dipermal 80 in a metal housing with a gas delivery tube, a Resistal 052 sleeve, and a subinal VTT one-piece well block. Data on the properties of the imported refractories is shown in Table i. During the start-up period in the Electric Melt Shop of Belorussian Metallurgical Plant an analysis was made of the data on treatment of steel in the treatment ladle. The length of blowing of the metal with argon varied from 5-10 to 50-80 min and more than 50% of the heats were blown for 20-35 min. The total length of treatment of the metal in the treatment ladle, including electromagnetic stirring, was from 13 to 150 min. The temperature of the metal before treatment varied from 1530 to 1700~ with an average of 1560-1590~ As the result of treatment as a rule the metal temperature after arc heating increased by 20-60~ (an average of 35~ The total argon consumption in treatment of steel during a single campaign varied from 0.006 to 0.137 mS/ton (at normal conditions). It should be noted that the conditions of steel treatment used at the Belorussian Metallurgical Plant differ from those used in the steel plants of the country, particularly in the long time of blowing with argon. This creates more severe service conditions for the assembly for supplying argon to the ladle and particularly of the porous refractories. Therefore selection of the corresponding analogs is very important. Eastern Refractory Institute. Belorussian Metallurgical Plant. Ogneupory, No. 9, pp. 34-36, September, 1991.

Translated from

0034-3102/91/0910-0493512.50 9 1992 Plenum Publishing Corporation

493