technological developments in near-net-shape casting for mini-steelmills

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ELSEVIER Technological Developments for Mini-Steelmills L.L. Teoh School of Mechanical and Production Engineering Nanyang Technological University J. Mater. Process. Technol. 44 (1994) 249-256 in Near-Net-Shape Casting Journal of Materials Processing Technology Abstract New continuous casting techniques from liquid steel into sections closer to the final profile of the fiat products -- sheets or strip -- are being developed. This near-net-shape casting technology could have an enormous economic impact on many small-scale steelworks (or mini-steelmills) and allows them to compete effectively against large integrated steelmills to produce flat rolled products. This paper focus on problems and solutions of heat extraction rate to meet the productivity requirements, feeding molten steel uniformly into a near net shape channel, lateral containment, solidification behaviour of steel shell growth, and process control so as to achieve adequate as-cast surface quality necessary to allow subsequent direct roiling to produce high quality sheets or strip. Various processes are compared and their potential merits and drawbacks of these processes are discussed. 1. INTRODUCTION Continuous casting of near-net-shape flat products, either as thin slab or strip, is a new emerging technology and encouraging results are available from numerous research and development activities world-wide. It is customary to classify the process routes, as shown in Fig. 1, according to the cast product thickness [1 ]: (i) Thin slab or skelp, ranging 20-60 mm in thickness and 1,500-2,000 mm in width. The as-cast thin slab or skelp could be directly fed, without conditioning, into a hot-strip finishing mill. (ii) Strip or thin sheet, with 1-6 mm thick and 1,200-1,500 mm wide. The aim is to make these as-cast sections suitable for direct cold rolling into strip. This route not only eliminates the need of costly conventional hot strip mill, but also offers the possibility of producing new alloy steels in strip form. (iii) Thin strip or foils, whose thickness is around 20 I.tm to 500 I.tm and up to 300 mm wide. It produces a final net-shape flat products without processing through the hot and cold strip mills. Many casting projects have been developed to the conversion of cast steel into its final shape with widely varying production rates. If commercially viable, the benefits of this technology would be enormous [2 ], including: a. Lower demands for energy, operational personnel and maintenance, by directly rolling thin sections into final products, thereby eliminating the intermediate steps such as cooling, scarfing and dressing, and roughing; b. Production in smaller tonnages of various steel grades and quality; c. Flexibility in producing varying coil width and weights to meet customer requirements; d. Lower operating costs and capital investment due to fewer plant components in a more compact arrangement than in the conventional facilities. It has been estimated that by employing thin slab caster and strip caster can cut production costs of sheet metal production by one-third and three quarters, respectively, compared to conventional continuous casting [3,4 ]; and 0924-0136/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved. SSDI 0924-0136(94)00120-K

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Page 1: Technological developments in near-net-shape casting for mini-steelmills

ELSEVIER

Technological Developments for Mini-Steelmills

L.L. Teoh

School of Mechanical and Production Engineering Nanyang Technological University

J. Mater. Process. Technol. 44 (1994) 249-256

in Near-Net-Shape Casting

Journal of Materials Processing Technology

Abstract New continuous casting techniques from liquid steel into sections closer to the final profile of

the fiat products -- sheets or strip -- are being developed. This near-net-shape casting technology could have an enormous economic impact on many small-scale steelworks (or mini-steelmills) and allows them to compete effectively against large integrated steelmills to produce flat rolled products. This paper focus on problems and solutions of heat extraction rate to meet the productivity requirements, feeding molten steel uniformly into a near net shape channel, lateral containment, solidification behaviour of steel shell growth, and process control so as to achieve adequate as-cast surface quality necessary to allow subsequent direct roiling to produce high quality sheets or strip. Various processes are compared and their potential merits and drawbacks of these processes are discussed.

1. I N T R O D U C T I O N

Continuous casting of near-net-shape flat products, either as thin slab or strip, is a new emerging technology and encouraging results are available from numerous research and development activities world-wide. It is customary to classify the process routes, as shown in Fig. 1, according to the cast product thickness [1 ]: (i) Thin slab or skelp, ranging 20-60 mm in thickness and 1,500-2,000 mm in width. The

as-cast thin slab or skelp could be directly fed, without conditioning, into a hot-strip finishing mill.

(ii) Strip or thin sheet, with 1-6 mm thick and 1,200-1,500 mm wide. The aim is to make these as-cast sections suitable for direct cold rolling into strip. This route not only eliminates the need of costly conventional hot strip mill, but also offers the possibility of producing new alloy steels in strip form.

(iii) Thin strip or foils, whose thickness is around 20 I.tm to 500 I.tm and up to 300 mm wide. It produces a final net-shape flat products without processing through the hot and cold strip mills.

Many casting projects have been developed to the conversion of cast steel into its final shape with widely varying production rates. If commercially viable, the benefits of this technology would be enormous [2 ], including: a. Lower demands for energy, operational personnel and maintenance, by directly rolling thin

sections into final products, thereby eliminating the intermediate steps such as cooling, scarfing and dressing, and roughing;

b. Production in smaller tonnages of various steel grades and quality; c. Flexibility in producing varying coil width and weights to meet customer requirements; d. Lower operating costs and capital investment due to fewer plant components in a more compact

arrangement than in the conventional facilities. It has been estimated that by employing thin slab caster and strip caster can cut production costs of sheet metal production by one-third and three quarters, respectively, compared to conventional continuous casting [3,4 ]; and

0924-0136/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved. SSDI 0924-0136(94)00120-K

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Conventional process route Near-net-shape casting techniques

Molten steel

I Slab caster [ /

Slab 1150-350 mm thick)

] ,Slab conditioning ]

i ] Reheatingfumace ]

[ Rougher mill ]

Thin slab (20-60 mm thick) i

[ n0t finishin~ mill [

Hot strip (1-6 mm thick) ~ ¥

I ol ollm I Cold strip (20-500 pro) ,,

Strip Caster I

Caster I I

Figure 1. Process routes for production of fiat-rolled products.

e. Less space requirement, due to the omission of intermediate storage, inspection and conditioning of intermediate products. In addition, the viability of thin section casting technology could have great implication in

today's steel industry structure. Traditionally, mini-steelmills have concentrated on various lower-value, lower-quality steel products. The main barriers that have prevented these mini-steelmills from moving into high-value flat-rolled products market are the requirements of huge economic scale of slab casting facilities (ie, > 2 million annual tonnes) and the quality limitations imposed by the residual contaminations of steels produced by the electric arc furnaces. Due to the advances in near-net-shape casting and secondary treatment of molten steel, the mini-steelmills can enter the fiat-product market with a lower conversion costs.

2. PROCESS REQUIREMENTS

The primary goal of thin section caster is to achieve metallurgical and chemical homogeneity in the thin as-cast section at rapid solidification rate. Several requirements have to be fulfilled in order for these new thin slab casters to compete successful with their high tonnage counterparts [5]: a. The available of appropriate process control system for furnace to supply molten steel with

the desired temperature and chemistry in a continuous fashion [6 ]. Tundish must also be able to control the cleanliness of the melt and also to streamline the flow on its way towards the caster. It is imperative that all the operating components from metal delivery system through the tundish to the caster must be functioned reliably and controlled consistently.

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b. An acceptance molten steel delivery system to maintain stable meniscus level in the mould. Liquid metal must be fed under conditions of controlled flow, pressure, temperature, and chemical environment to achieve meniscus stability and uniform shell growth [7 ]. At the exit of the mould, the thin section should be easily stripped away from the mould surface, without sticking to it. In addition, at the exit of a slab caster, the surface temperature over the entire strand should be kept high enough to allow direct hot rolling, since this is conducive to energy savings, reduced stock and shorter production time.

c. Commercialisation of thin section caster relies on the attainment of high casting capacity to match the performance of both the furnace cycle and the rolling mill, and to avoid the excessive multiplication of casting strands. This means that the withdrawal speed has to be increased in inverse proportion to the as-cast thickness. For a current single strand slab caster having a heat size of 120 tonnes and with casting capacity of 3 t min 1, withdrawal speed has to increase from 2.5 m min 1 to 19 m min ~ if the cast section thickness decreases from 150 mm to 20 mm [8 ]. The high speeds required for thin slab casters can be reduced by installing multi-strands, but only to a certain extent owing to cost penalties and complexity of operation.

d. To achieve high production rate, high casting speeds are necessary, hence high cooling rates, of the order of 102-104 °C sec "1. The caster must be capable of extracting the superheat (~25°C) and latent heat of solidification of steel (271 kJ kg") together with a fraction of its sensible heat (cooled to say, 12000C). This capability amounts to about 490 kJ kg ~ of cast steel. For a heat size of 60 tonnes cast in 80 min. (at the production rate of 0.75 t min~), the total heat-extraction requirement would be 5.5 MW [9 ].

e. Some degree of controlled reduction is required to break up solidification structures and to regenerate a more uniform and fine grained microstructure to obtain desired mechanical properties. It appears that hot reduction ratios in the range of 2.5-3, in contrast with 60 for the classical route, are still necessary to mitigate casting defects such as composition segregation, microshrinkage, and surface imperfection [10 ].

f. A data base for the evaluation of the effects of process parameters on product quality as well as appropriate technology to ensure that the cross-sections of thin section, i.e., its flatness, profile and width are kept under control. This is due to stringent requirements on the shape of the cast section if it is to replace conventionally rolled materials. Any non-uniformity in gauge tends to persist through the rolling operation to the final product. In addition, the surface of cast section has to be nearly perfect, being free from cracks, tears, folds and excessive scale, as high casting speed can result in large quantities of unacceptable strip in a short period of time. More significantly, the mechanical properties of the end products must be equal to or exceeded those produced traditionally. Major impetus for direct casting of strip is to produce material that can be rolled with a minimum

amount of cold reduction. Its shape and surface quality must meet stringent requirements set by the current standards for flat-rolled products. This presents a significant challenge in the development of strip casting [11 ]. If excessive surface conditioning were required, the process would loose its economic viability owing to the large surface area to be inspected and conditioned.

3. TYPES OF CASTERS

Currently, there are seven categories of thin section casters, as listed in Table 1. The oscillating mould processes have been developed by Germany's SMS Schloemann Siemag [12], Mannesmann Demag Huttentechnik [13], Italy's Danielli, and Austria's Voest-Alpine Induswieanlagenbau [14 ]. They use familiar technology of slag lubrication and molten steel feeding via a refractory shroud, developed by the conventional slab casting, to produce thin slabs. A typical caster displays a funnel-shaped oscillating mould, as shown in Fig. 2a, with the specially contoured submerged nozzle at the top region to prevent steel re-oxidation and to ensure solidification of the strand shell with low transverse stresses. The broad faces of cast shell is deformed during casting to the required shape within the mould, while the end thicknesses are held constant. With the addition of new low melting temperature mould powders, this caster is running at a moderate casting speeds of 4-5 m min "1 and with oscillation frequencies of 60 strokes rain "1 and stroke lengths of 3 mm.

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Table 1 Examples of potential near-net-shape casters

Process As-cast dimension (mm) Casting Capacity Ref.

classification Thickness Width Speed per strand

Oscillating mould 40-50 1200-1600 up to 6 m min~ 820 000 t y't [ 12-14]

Twin-belt casters 25-40 250-2000 up to 4 m rain 4 600 000 t y-I [ 15]

Open-mould caster 50-75 500-600 up to 20 m 5 t min -t [18] min-'

Twin-roll casters 5-25 up to 1000 up to 24 m up to 220 t h 1 [19,20] min 4

Melt-drag process 0.25-1.4 up to 600 9-72 m min 4 51 t h 4 [21]

Spray casting 12-18 up to 2000 3 m min 4 100 000 t y4 [22]

Inversion casting 1-10 200-1000 60 m min 4 1 Mt y-~ [23]

(a) Oscillating mould

(b) Twin-belt casters

(d) Twin-roll caster

I ' l - I

soos, ,o Ilitll

~ . Melt

Stream

(f) Spray casting

(c) Open-mould caster

(e) Melt-drag processes

Hot skin pass ~ - 7 \ mm

0.7 mm ~ ~r17-17~ I7-,,~, I

(g) Inversion casting

Figure 2. Near-net-shape casting processes

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The Hazelett twin-belt moving mould caster (Fig. 2b) has been used for casting non-ferrous metals thin slabs of up to 38 mm thick for the past 25 years [15 ]. The merit of the twin-belt system is that it is a frictionless mould and therefore does not impose the casting speed limitations inherent in the oscillating mould system. It also allows casting speed as high as 2 or 3 times the limits set for the oscillating mould caster to be reached, thus has enabled a direct linkage between the caster and the rolling mill. Currently, there are three configurations of belt casters under developments: horizontal, vertical and angled. The horizontal casters use an injection nozzle but face problems concerning side sealing and leakage at the belt-nozzle interface [16 ]. Angled designs employ some form of overflow device to introduce steel into the mould. While vertical option requires the developments of proper edge containment [ 17 ].

In a separate development, British Steel Corporation (BSC) adopted a radical different approach and pioneered a horizontal thin slab caster based on a open-mould (continuous mould car) system (Fig. 2c) [18 ]. The thin slab solidifies along three sides against a travelling mould, which is movable on a railway track. This process has already been industrially developed for the production of ferro-alloy under the name of the Frag-Casting process.

Numerous pilot-scale twin-roll casters (Fig. 2d) have been installed world-wide. Both thick strip (around 25 mm thick) that needs some hot working and strip (around 5 mm thick) which can be directly cold rolled are the kinds of semi-products targeted by this technology [19,20 ]. Process control has to ensure that solidification takes place at the narrowest point. This requires maintaining a constant temperature across the width of the section. Without a uniform feed and excellent temperature and composition control, it has been shown that a perfect cast surface cannot be obtained.

In the melt drag (using single roll or double roll casting) process [21 ], the metal was poured into a transfer tundish which directed the flow of metal into the casting tundish. Molten metal meniscus was "dragged" from an orifice onto a cooled, rotating drum (Fig. 2e). Cast steel strip of up to 600 mm wide and 0.25-1.4 mm thick can be readily produced by varying the rotation speed and the cooling intensity of the drum. Surface speeds of the casting roll can varied from 15 m min -1 to 60 m min 1.

There are two variations of spray casting (viz., spray rolling and spray deposition) which involves direct conversion of an atomized metal stream into a fully consolidated thin section are available. In the spray rolling [22 ], spray forming of the sheet stock is performed by directing the atomized spray evenly over the surface of a water cooled drum. It is accomplished by deposition at relatively high rate, 50-100 kg min 1, during which individual droplets collide with the substrate surface before the previous ones have completely solidified. In the spray deposition technology (Fig. 2f), liquid steel stream emerged from a refractory nozzle in the bottom of the tundish is being atomised with the aid of a gas in an atomising unit. The particles which are accelerated into a high velocity are cooled down during the flight to a temperature just above the solidus before they impinge onto a moving belt made of ceramic blocks. These particles are further compacted at high density after being rolled and are free from macro-segregation.

Germany's Mannesmann Demag Huttentechnik (MDH) has been pursuing the development of inversion casting since 1986 [23 ]. In this process (Fig. 2g), a 0.7 mm thick substrate plate at ambient temperature is led through a bath of molten steel with controlled temperature. During the predetermined contact time between the substrate plate and the bath, a layer of molten metal crystalises onto the substrate and is pulled out with the substrate plate as a composite body with a controlled final thickness. With this process, it would be feasible to produce rounds, sections and plates in sizes between 1-10 mm thick and 200-1000 mm wide, which could be subsequently processed in the cold rolling mill.

Table 2 sums up some of the merits and drawbacks of various near-net-shape casting processes. The advantage of the oscillating mould configuration is that only its mould system is new, while the rest of the technology is simply the equivalent conventional casting machine. However, it suffers from several drawbacks including difficult in hot charging of thin slabs and excessive usage of casting powder per tonne of steel produced, due to its increased surface area. Common problems associated with belt-type casters have been the difficulty of feeding liquid steel uniformly across a wide and thin section, lateral containment and no width adjustment during

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casting. The most serious impediment to the realization of twin-roll and single-roll casters are surface quality problems, followed by difficulty in strip shape control [24 ]. While the main disadvantage of spray deposition process is a comparatively lower yield as a result of"overspray".

Table 2 Comparison of various near-net-shape casting processes

Oscillating mould Merits o Familiar steel feeding & casting technology o Thermomechanical treatment possible o Easy to link with the existing rolling mill

Drawbacks o Direct hot charging more difficult due to handling problems o High friction between mould and strand o Limited range of section thickness o Use of casting powders

Twin-belt casters Merits o Higher casting speed o Technology proven for non-ferrous metals o No casting powder required o Thermomechanical treatment possible

Drawbacks o Direct hot charging more difficult due to handling problems o Uniform metal feeding more difficult o Difficult edge containment o Section width cannot be varied during casting

Twin-roll casters Merits o Simple machine design o Two chill surfaces & heavier thicknesses possible o Easier handling control of hot strip

Drawbacks o Precise process synchronization required o Difficulty in uniform metal feeding and edge containment o Section width not easily varied and surface quality problems o Constant temperature essential across section width

Melt-drag process Merits o Simple tundish system o Easy to vary section width o Simpler synchronization o Less porosity and segregation tendency in casting

Drawbacks o "Free" surface problems o Limited thickness range and strip shape control problem o Lower production rate o More difficult casting process control

Spray casting Merits o Higher cooling rates and minimum segregation in casting o Easy casting process control o Insensitive to alloy composition changes o Flexible process, simple technology

Drawbacks o Residual porosity and rough surface problems o Difficult to obtain uniform across width thickness o Lower casting yield due to overspray

Inversion casting Merits o Clads and composites casting possible o Casting rounds and sections possible o Higher production rate and simple machine design

Drawbacks o No width adjustment o Limited thickness range o More sensitive to hot strip handling

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4. FUTURE TRENDS

As a new process, thin section casting faces a number of problems and but also has many potential advantages. Over the years, considerable efforts have gone into the development of improved practices to produce clean steel with precise composition. However, the use of Ca-Si treatments in secondary steelmaking to prevent nozzle clogging has unfortunately produces steels not suitable for most applications. Alternative treatments are being developed to overcome this difficulty. New refractories are available to enable molten steel flow to be controlled with precision. Knowledge on the mathematical modeling to predict inclusion distribution in continuously cast thin slab [25 ] and to study the design of casting substrates capable of imparting uniform and controllable solidification are also available [26 ]. The quality limitations in making fiat products imposed by residual contamination of scrap for the electric arc furnace steelmaking could be overcome by increased efforts in scrap management and also by replacing scrap with the direct-reduced iron (DRI).

Attempts to develop thin slab caster with somewhat higher casting rate ( ie, > 6 m min 1 ) than is possible for the oscillating mould approach has not yet meet with commercial success, although Japan's Kawasaki KH caster reportedly showed remarkable results at the casting speeds of 10-12.5 m min t [27 ]. Formidable problems need to be resolved to ensure productivity of the existing thin slab casters to match the current output requirements. Considerable efforts have been made to increase casting speed by various means. These include eliminating the superheat between the tundish and the mould, and increasing the cooling rate at the exit of the mould.

While the feasibility to produce thin slab has been proven, much work remains in further improvements in efficiency, durability and reliability of the operating components to allow adequate level of continuous operation under commercial conditions, as well as improving the consistency of its cast products. Among these activities, several trends can be observed: a. The performance of the casting operation and the quality of thin strand could be improved

significantly if sensors capable of real-time chemical analysis as well as on-line detection of inclusion levels in flowing molten steel are developed. Work has continued in trying to search solutions to two main problems, viz., the difficulty of obtaining a truly representative reading of the bulk melt, and the high melt flow rates.

b. Development of thin slab casting depends critically on an understanding of fundamental knowledge of heat and fluid flow [28 ]. It also relies on formulation of process models which related process variables (temperature, pressure, composition, etc) to relevant product properties (microstructure, strength, density) or features (thickness, surface finish, dimensional uniformity). Currently, incomplete understanding of these functional relationships limits the degree of process control that can be achieved. Furthermore, suitable solution for feeding of liquid metal uniformly into a narrow channel and the extraction of heat at the rate which meets productivity requirements, without resulting surface cracks, are vital for new process under development.

c. The quality of product is of course a function of thin slab quality. As the thickness of cast section is close to the end product thickness and total rolling reduction is comparable low, the requirements for the surface quality and profile of the thin slabs are critical, with paramount influence on the quality and yield of the final products. It is thus necessary to understand during solidification the behaviour of steel shell growth up to a thickness of several millimetres in order to achieve a metallurgically and chemically homogeneous thin slab without segregation of non-metallic inclusions [29 ]. Further investigations aimed at the causes of casting defects such as surface irregularities, width and thickness variation, segregation and internal cracks are desirable.

d. Improved understanding and ability to predict the properties of as-cast sections in relation with their composition and structure must be actively pursued. It is important to understand more precisely the process variables for achieving high quality steel, as well as establishing appropriate casting techniques for high-quality slabs suitable for high-grade cold-rolled steel sheets.

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The greatest challenges for advancing the strip casting process lie in the potential for direct control of strip microstructure, and hence the material properties. A formidable barrier to the development of improved product properties and caster productivity is the lack of sufficient process understanding. In addition, strip casting involves melt-substrate interaction, high surface to volume ratio, steep thermal gradients, tremendous heat fluxes and high solid-liquid interracial growth velocities [30,31 ]. These characteristics create measurement difficulties that make the formulation of reliable process models a formidable task. Future research efforts on strip casting technology require employing a semi-empirical, multidisciplinary approach [32 ]. Such an effort would involve contributions from materials scientists, heat and fluid flow modeling experts, and electronics and computer software engineers to address the complex issues that must be understood to advance the process.

REFERENCES

1. P. Nilles and C. Marique, Metallurgical Plant & Technology, 12-5 (1989) 72. 2. B. Lindorfer, et al, Steel Times, 221 (1993) 304. 3. R.A. Gleixner, Iron & Steel Engineer, 65-11 (1988) 30. 4. G. Holleis, et al, Steel Times, 217 (1989) 605. 5. S.L. Wigman and M.D. Millett, Steel Times, 220 (1992) 516. 6. R. Gottardi, et al, Steel Times, 220 (1992) 524. 7. Y.H. Wang and I.G. Saucedo, Iron & Steelmaker, 17-1 (1990) 14. 8. M. Cygler and M. Wolf, Iron & Steelmaker, 13-8 (1986) 27. 9. J.K. Brimacombe and I.V. Samarasekera, Modem Steel Processing, The Iron & Steel

Society, Warrendale, PA, 1989, pp 104. 10. C. Dunand, et al, Hot & Cold-rolled Sheet Steel, Ed by R. Pradhan and G. Ludkovaky, The

Metallurgical Society, Warrendale, PA, 1988, pp 375. 11. J. Herbertson, et al, Steel Times, 220 (1992) 520. 12. E. Hoffken, Metallurgical Plant & Technol. Intl, 16-5 (1993) 56. 13. F.P. Plescbiutschnigg, et al, Metallurgical Plant & Technol. Intl, 16-4 (1993) 44. 14. A. Flick, et al, Metallurgical Plant & Technol. Intl, 16-5 (1993) 30. 15. P.C. Regan, et al, Iron & Steel Engineer, 64-2 (1987) 41. 16. S. Itoyama, et al, Transaction ISIJ, 28 (1988) 553. 17. R. Ozgu, et al, 1990 Steelmaking Conf. Proc., 1990, pp 221-233. 18. H.S. Mart, et al, Near Net Shape Casting, Iron & Steel Society, Warrendale, PA, USA,

1987, pp 21-30. 19. J.C. Grosjean, et al, Iron & Steelmaker, 20-8 (1993) 27. 20. J.A. Burgo, et al, Iron & Steel Engineer, 67 (1990) 51. 21. K. Sbibya and M. Ozawa, ISIJ International, 31 (1991) 661. 22. D. Apelian, et al, Rapidly Solidified Crystalline Alloys, The Metallurgical Society,

Warrendale, 1985, pp 93-109. 23. D. Kothe D, et al, Metallurgical Plant & Technology, 13-1 (1990) 12. 24. J.W. Hlinka, et al, 1989 Steelmaking Conf. Proc., 1989, pp 133-144. 25. T.E. Shelenberger, Casting of Near Net Shape Products, Ed by Y. Sahai, et al, The

Metallurgical Society, Warrendale, PA, 1988, pp 657-672. 26. J.W. Hlinka, et al, ibid, pp 115-132. 27. H. Tozawa, et al, Kawasaki Steel Technical Report, 22 (1990) 22. 28. A.W.D. Hill, Advance Materials & Processes, Ed by H.E. Exner and V. Schumacher,

Informationsgesellschaft, Verlag, 1990, pp 75-86. 29. Y. Fujita, et al, ISIJ Intemational, 29 (1989) 495. 30. I. Jimbo and A.W. Cramb, Iron & Steelmaker, 20-12 (1993) 51. 31. H. Yamane, et al, 10th Process Tech. Conf. Proc., 10 (1992) 343. 32. H. Murakami, et al, ibid, 10 (1992) 347.