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Electroslag technology of production of large forging ingots.

The task of producing high-quality large ingots from high-alloy steels and alloys has not as yet been completely fulfilled. With every increase of the requirements of the users and the properties of thick plates and forgings and the appropriate increase of the complexity of the alloying systems of steel and alloys this problem becomes more acute, in particular in the production of certain types of the so-called nickel-based superalloys it is now necessary to use three-stage (vacuum induction melting + electroslag remelting + vacuum ARC remelting) technology of producing ingot weighing only up to 20 t and with the diameter of approximately 1000 mm.

Recently, as a result of the development of power engineering there has been new interest in the possibilities of electroslag technology of increasing the quality of forging ingots. For example, until recently, only three electroslag furnaces producing ingots weighing 100 t and more (Shanghai Heavy Machinery 200 t; Saarschmidte 165 t; Japan Steel Works 110) operated anywhere in the world.

There are reports on the construction in the world of at least seven electroslag melting furnaces for producing ingots weighing from 100 to 450 (!)t (1)-(3). It is justified to assume that in near future the problem of producing high-quality heavy forging ingots will also become important in the power engineering of the CIS countries, especially Russia and Ukraine. Work is therefore being carried out to explore new possibilities of using different electroslag technologies for producing high-quality forging ingots of creep-resisting alloys and high-alloy steels, primary for power engineering.

Figure 1 shows electroslag technologies combine on the basis of specific features used for the production of large forging ingots. In addition to the varieties of classic electroslag remelting and, only electroslag feeding is used now in the industry for producing forging ingots. This technology increases the density and reduces the chemical heterogeneity of the steel in the axial volumes of the ingot whilst minimising the amount of material cut from the top of the ingot. This process, improved by experts of the Austrian company Bohler, is referred to as BEST, and the process developed by the Italian company Terni, is referred to as TREST. The principal difference between these two processes is that the format uses a watercooled head attachment, the second one a lined attachment. The authors assume that in addition to these verified methods, technologies of electroslag remelting by the two-circuit system (ESR TC) in electroslag surfacing with liquid metal (ESS LM) are also promising.

[FIGURE 1 OMITTED]

Figure 2 shows the principal diagram of ESR TC, and Figure 3 shows the diagram of ESS LM (4), (5).

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

In ESR TC use is made of the principal variation of one of the main characteristics of the classic remelting processes in special electrometallurgy--the presence of two independent power sources makes it possible to break the rigid connection between the power supplied to the remelting electrode and the productivity of remelting. This makes it possible to vary the rate of remelting in a wide range without impairing the metallurgical quality of the ingot. Consequently, in the industrial conditions it is possible to prevent segregation phenomena by reducing the remelting rate without impairing the quality of the surface of the ingot.

Figure 4 compares the macrostructures of the ingots produced by electroslag remelting and ESR TC with a diameter of 500 mm, melted from the same electrodes in a single ESR furnace and at the same type of slag 1/3-1/3-1/3 and at the same power, generated in the slag pool. In ESR TC it is possible to more than halve (in comparison with standard ESR) the depth of the liquid metal pool without reducing the total remelting power. This results was obtained at the ratio of the electric power supplied through the consumable electrode to the power supply through the current-conducting section of the solidification mould, of approximately 65:35 of the total remelting power (550-570 kVA).

[FIGURE 4 OMITTED]

In the experiments carried out in a pilot plant ESR furnace, fitted with two transformers with a part of 720 kV*A each, this ratio was varied in a wide range so that it was possible to change not only the total depth of the liquid metal pool but also its profile, making it almost flat.

It should be mentioned that the projects of gigantic ESR furnaces abroad include projects for ESR TC (3).

It is assumed that the promising results, obtained at the present time using ESR TC in pilot plant ingots of high-alloy austenitic and ledeburitic steels, and also Inconel 718 type alloys indicate the high promise of this ESR technology.

Evidently, the need for reducing the remelting rate in order to increase the quality of the ingot may restrict the application of ESR TC2 ingots of a relatively small weight (possibly 50--60 t). However, this restriction is not very important. It is quite clear that the main area of application of ESR TC is the remelting of creep-resisting alloys, susceptible to spotty liquation. The ingots of these alloys have a considerably smaller weight. In addition to this, it is quite possible that the application of ESR TC will make it possible to avoid three-stage remelting and it would be sufficient to use two-stage processes, for example, vacuum ARC remelting + ESR TC or AOD + ESR TC.

A completely new approach to improving the quality is applied when increasing the weight of the ingot. The attempts to produce large ingots from several small ingots have been made in metallurgy for a long period of time now, including application in industry of two electroslag technologies--electroslag welding and melting of the central part of the ingots, the so-called MXKV process (6), (7).

The concept of ESR TC developed recently for producing heavy forging ingots is based on three principal special features, determining the important advantages of this technology (8)--(10): firstly, it is electroslag technology without consumable electrodes; secondly, ESR TC can be used to produce hollow ingots; thirdly, ingots of variable chemical composition can be produced by ESR TC.

Under the supervision of Academician B.I. Medovar the technology of ESR LM was developed for the production of composite rolling in rolls (11), (12). In the general form, the ESR LM can be described as follows. A separate steel melting system which may be represented by, for example, an induction furnace for ASR, is used for melting metal for the formation of the working layer of the roll with the required chemical composition.

At the same time, a slag of type ANF-29, ANF-32, ANF-94, or some other composition is melted in a slag-melting furnace. Here, the liquid slag is transferred to the equipment and poured into the gap between the forming section of the current-conducting solidification mould and the deposited blank of the rolling roll. This blank maybe in the form of a used roll in reconditioning surfacing, cast or forged bars with the dimensions similar to those of the rolling roll in surfacing. This blank should be machined mechanically to the finish stage.

The blanks in the cast or forged state can also be used without special treatment of the surface but with preliminary centring. As soon as the liquid slag makes contact with the current-conducting section of the solidification mould, the power source--deposited blank-liquid slag--current conducting solidification mould--power source electrical circuit is closed, and the electroslag process starts in the slag pool. Subsequently, the pre-melted metal may be supplied. In the industrial conditions, the pouring device may be represented by induction furnaces of special design, for example, induction furnace supplying metal portions, and an angle, or induction furnace of the channel type, etc.

The liquid metal, poured into the solidification mould, should not make contact with the current-conducting section because this would cause short-circuiting. Therefore, the control system of the process takes into account, controls and regulates not only the electrical and mass-geometrical parameters of the process and of the deposited blank with time but also constantly corrects the level of the slag-metal interface. This is carried out using a special inductance level sensor measuring the difference of the electrical parameters of the slag and the metal.

If this level increases, the signal from the sensor produces a command for withdrawing the deposited blank from the solidification mould, and if the level drops to the minimum determined by the technological parameters they, and a command is issued for pouring in the next portion of the metal.

It is very important that the application of the solidification mould as the non-consumable electrode makes it possible to maximise the symmetry of current supply to the slag and, consequently, avoid problems with the nonuniform melting of the deposited blank. In practice, melting is controlled with the accuracy up to 5 mm.

ESR LM may be carried out using different combinations of the materials. The deposited metal can be more or less refractory in comparison with the axis on which surfacing is carried out, or have the same melting point. In production of ingots this means that the axis can be in the form of cast iron, steel, and also spent rolls.

The technology of ESR LM has been introduced at the Novokramatorsk Engineering Plant which operates two specialised systems for melting blanks of the working and support rolls for continuous wide-strip rolling mills with a diameter of more than 1500 mm and weighing up to 50 t (Figure 5).

[FIGURE 5 OMITTED]

We have proposed a concept of a multi-profile furnace for producing blanks of especially large rolling in rolls of thick plate mills 5000 and 5500 whose weight reaches 240--250 t, and also of solid and hollow forging ingot weighing up to 300 t.

The ESS/ESR furnace ZhM-300 has two independent melting positions connected by the general working platform and differing by productivity and mass of products. The first position is designed for melting ingots and blanks with a diameter of up to 1800 mm (weighing up to 80 t) the second position for ingots and blanks with a diameter from 1400 to 3200 mm (weight up to 300 t).

The furnace is fitted with a set of:-conducting solidification mould and also mandrels (for melting hollow ingots) and the flux supply device used for both positions.

The current-conducting solidification mould for melting solid ingots has an additional cooling section which approximately doubles the length of the mould in comparison with the solidification mould for melting rolls and hollow ingots.

The metal is supplied into the solidification mould using pouring systems with the useful volume of liquid metal of up to 10 t, with two systems for each position. When the liquid start is used, the furnace contains a separately standing slag-melting equipment with the set of crucibles-ladles of different capacity.

In order to correct the composition of the slag and deoxidise it, it is necessary to use a system for dosed supply of fresh slag, pure fluorite and deoxidation agents into the slag pool.

To maintain and balance the temperature of the heavy ingots and blanks slowly withdrawn from the solidification mould, it is necessary to use a system of heating during this process.

The electric power system for ESR LM provides for the application of special three-phase low-frequency power sources (a transformer and a thyristor converter) 50 Hz, 10 000 V, approximately 0.1--10 Hz, 120 V. To prevent the resonance of equipment, the converters regulate the frequency under load. The low-frequency power sources were selected in order to reduce the effect on the external mains and the reactive losses in the short-circuits of the furnace. The power of the main power sources is 5 and 10 MV*A respectively for the first and second position.

In addition to the main source, each melting position is fitted with the power source for organising the control of rotation of the slag in the form of a three-phase transformer with a frequency converter. The frequency must be strictly synchronised with the main current source. This low-voltage and high-current transformer for rotating the slag is connected to the slit in the separation section of the current-conducting solidification mould and produces circular current in it.

The rotation of the slag pool leads to the disappearance of visible electric arcs at the slag--solidification mould boundary, prevents rapid local wear of the latter, and also equalises the temperature throughout the entire perimeter of the current-conducting section and results in the distribution of the liquid metal of every portion in the cross-section of the ingot (solid, hollow or surfacing of a roll) which in turn ensures the uniform temperature field of the liquid metal pool.
Table 1. Technical characteristics of ESS/ESR ZhM-300 furnace

Technical characteristics of EShN/EShP ZhM-300 furnace

 First melting Second melting
 position position

Maximum ingot weight, t 80 300

Productivity of equipment, 20 000 45 000
t/year to

Coefficient of replacement of 2.7 2.7
equipment, minimum

Working regime of the position, 24 24
h/day

Thickness of the lay of
deposited metal, mm:

in enlarging the ingot - to 500

in cladding the roll to 200 to 200

Vertical displacement of the to 9000 to 9500
lifting table, mm

The power of the sources, MV A 5 10

Number of power sources of the 1 1 (from two
device for rotating the slag blocks)
pool

Working speed in withdrawal of 1...100 1...100
the ingot (blank), mm/min

Capacity of the lifting table, 80 000 300 000
kg

Maximum calculated temperature 850 850
resulting from heating during
operation, [degrees]C


To ensure efficient operation of equipment, each melting position of the system is powered from two independent inputs (working and backup) with automatic connection of the backup.

The ESS/ESR furnace ZhM-300 operates using the advanced system of control in the form of the three-level distribution computerised system of collecting data and controlling the technological parameters of the ESR process, with the levels described as follows:

0--the sensors and actuating elements,

1--Programmable Automation Controller (PAC), the panels for local control of devices, the local panel of manual control of the parcels, and the local panel for manual control of the ESR process on the platform of the solidification mould;

2--SCADA working station.

The control system includes several subsystems fulfilling the following functions:

-- centralised visualisation and control of the technological parameters;

-- analysis of the production situation, tracking of the boundary values of the process parameters, treatment of failure situation;

-- calculation and transfer of recommendations to the operator;

-- direct control of the ESR process in the automatic regime;

-- the calculation and issue of the values of settings (tasks) to the regulators of the local automation systems.

A special feature of the control system is extensive backup and the presence of several protection systems which is essential for 'saving' a large ingot. Melting can also be carried out in the manual control regime.

The control system is fitted with the engineering calculation program ESSR LModcal[c] for modelling of the solidification of the ingot (blank). The control system is self-teaching and uses the actual values of measurements of the temperature field, electrical parameters, and other data.

The possibilities of the furnace system are characterised by the range of the products planned for realisation:

-- forging ingots, produced by the two main technological processes ESS LM and ESR LM. The maximum weight of the forging ingot should be 300 t, minimum 60 t (four solid section ingots). The maximum diameter of the forging ingots, produced by enlarging of the melted solid ingots, is approximately 2600 mm, the minimum diameter 1400 mm;

-- the blanks for support rolling rolls of thick-plate mills with the diameter and length of the barrel of up to 2400, 6400 mm, respectively, the total length of up to 11 500 mm, and the support the rolls of the strip rolling mills with the diameter and length of the barrel of more than 1820 and 2400 mm, respectively, the total length of more than 7000 mm, melted by ESS LM with the thickness of the deposit of up to 200 mm with all of the materials, used for the rolling rolls:

-- hollow ingots, produced using the main technological process ESR LM. The overall dimensions and the weight of the hollow ingots/blanks are restricted by the following parameters: maximum weight 300 t, minimum weight 20 t, maximum external diameter 3200 mm, minimum external diameter 1400 mm, the maximum wall thickness of the hollow ingot 700 mm.

References

(1.) Holzgruber H., et al., New plant concepts and further developments for the production of large sized ESR ingots, Proc. of the China International Forgemasters meeting (May 24-27, 2010. Chengdu, China), Cheng du, 2010. 94--102.

(2.) Prof. Zhouhua Jiang, private communication, 2010.

(3.) Aoya Exhibition Co., Ltd. //http://www.forgingexpo.cn/english/project6/index.asp

(4.) Petrenko V., et al., ESR with two power sources and process control, Proc. LMPC-2005, TMS, Santa-Fe, New Mexico, Sept. 11--14, 2005 (electronic).

(5.) Vdovin K.N., Rolling rolls MGTU, Magnitogorsk, 2005.

(6.) Paton B.E., A new method of welding blanks of super-large sections; electroslag welding with a stationary electrode and additions of pieces of materials, in: Prob-lemy Elektroshlakovoi Tekhnologii, Naukova Dumka, Kiev, 1978.

(7.) Austel W., et al.,Elektroshlakovyi pereplav, 1983, No. 6, 301--306.

(8.) Paton B.E., et al., Sovremen. Elektrometallurgiya,2007, No. 1, 3--7.

(9.) Medovar L.B., et al., New possibility to utilize ESR for the ingot segregation reduction, A collection of theses for communication, Third Baosteel biennial academic conf. Shanghai, 2008, vol. 2, P. 33--38.

(10.) Makhnenko V., et al., New method of low segregation ESR forging ingots production (computer simulation of the ESR ingot enlargement), Proc. of the Intern. Forgemasters Meeting, IFM-17, 2008, Santander, Spain, 3--7 Nov., 2008 (electronic).

(11.) Medovar B. I., et al., Electroslag technologies in the XXI century, Proc. of the ASIA Steel international conference, Beijing, 2000, 40--43.

(12.) Paton B.E., et al., Stal', 1998, No. 11, 24--28.

L.B. Medovar, V.Ya. Saenko, A.P. Stovpchenko, A.K. Tsykulenko, N.T. Shevchenko, V.M Zhuravel, A.A. Polishko, B.B. Fedorovskii, G.V. Noshchenko and V.A. Lebed'

E.O. Paton Electric Welding Institute, Kiev Elmet Rol Engineering Company, Kiev
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Title Annotation:ELECTROSLAG TECHNOLOGY
Author:Medovar, L.B.; Saenko, V.Ya.; Stovpchenko, A.P.; Tsykulenko, A.K.; Shevchenko, N.T.; Zhuravel, V.M.;
Publication:Advances in Electrometallurgy
Date:Jul 1, 2010
Words:3025
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