Development of Horizontal Continuous Caster for Steel Billets*

By Shun-ichi TANAKA,* * Kiyomi TAG UCHI,* * * Akira HONDA,****

Takaho KA WA WA,* * * * * Eihachiro S UNAMI * * * * * and Shigeki KOMORI * * *

Synopsis For the purpose of achieving further development of the horizontal con-tinuous casting process established in the manufacture of small and medium section billets of carbon steel, the authors have developed techniques for horizontal continuous casting of stainless steel and high alloy steel grades and large-section billets. For the application of high-cycle intermittent withdrawal indispensable in horizontal continuous casting to very heavy cast strands of large-section billets, innovations have been made in equip-ment and control techniques of withdrawal. Casting of stainless steel and high alloy steel grades is now practicable through optimization of the mold conicity and use of break ring made of a new material. For further im-

provement of HCC billet quality, on the other hand, Hot Shot Blast to eliminate cold shut cracks on the billet surface and a new implementation of EMS to improve the billet center quality have been developed. These efforts are bringing about excellent results in application tests of HCC billets to such various products as rolled wires and rods, and seamless tubes and pipes.

I. Introduction

As has already been reported,1 Nippon Kokan K.K. has completed commercial development of the horizontal continuous casting process for carbon steel billet jointly with Davy-Loewy Ltd. Horizontal-type continuous casting facilities and operating tech-niques are being applied to the production of carbon steel billets for rolling into wires and rods in steel plants at home and abroad under the name of HORI-CAST Process. Some other groups of horizontal continuous casters have also started commercial production of carbon steel billets during the past few years,2,3~ and in-dustrialization of the horizontal continuous casting

process is thus underway in the manufacture of small and medium-section billets of carbon steel and low-alloy steel. Having this present status in view, the next step in progress of the horizontal continuous casting process would be casting of billets with a larger section and commercial casting of stainless steel and high-alloy steel billets. For this purpose, Nippon Kokan K.K. installed in the Keihin Works a large-capacity HORICAST in 1983 to develop horizontal continuous casting techniques of stainless steel and high-alloy steel round billets (Photo. 1). As compared with the conventional continuous casting process, the horizontal continuous casting

process is characterized by the following features. A horizontal continuous casting mold, which is

directly connected with the tundish, does not oscil-late. A cast strand is accordingly cast through inter-mittent withdrawal based on cyclic movements each consisting of a pull and a push. In order to achieve a satisfactory billet surface quality, the intermittent withdrawal must be effected at such a high cycle as at least 120 cpm. Since the inner walls of the mold are in direct contact with the outer surface of the cast strand, it is essential to assure smooth progress of solidification in the mold as well as to reduce the withdrawal resistance in the mold. This is particu-larly important in casting stainless steel which has higher linear expansion and high-temperature de-formation resistance. Through development efforts of large-capacity HORICAST for industrialization at Nippon Kokan K.K., these problems have been studied and solved as follows :

(1) Casting facilities and technology and know-how of withdrawal control permitting withdrawal of the cast strand at a high cycle have been completed to cope with a larger mass of the cast strand with a larger section.

Photo. 1. View of HORICAST (Keihin Works).

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Based on the paper published in Tetsu-to-Hagane, 67 (1981), No. 8, Special Issue on Continuous Casting of Steel, 1387, in

Japanese; and presented to the 107th ISIJ Meeting, April 1984, 5227 to 5230, at Chiba Institute of Technology in Narashino. Manuscript received April 13, 1984. © 1984 ISIJ Fukuyama Works, Nippon Kokan K.K., Kokan-cho, Fukuyama 721. Keihin Works, Nippon Kokan K.K., Minamiwatarida-cho, Kawasaki-ku, Kawasaki 210. Heavy Industry Group, Nippon Kokan K. K., Marunouehi, Chiyoda-ku, Tokyo 100. Technical Research Center, Nippon Kokan K.K., Minamiwatarida-cho, Kawasaki-ku, Kawasaki 210.

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(2) For various grades of stainless steel and nickel-base alloy steel, it is now possible to conduct stable casting operations and manufacture billets of a satis-factory surface quality by selecting an appropriate mold conicity and using a break ring of a further improved material.

(3) It is now possible to almost completely elimi-nate surface defects through application of shot blasting to high-temperature strand surface cast by the high-cycle withdrawal (named Hot Shot Blast) within the caster line. This report describes the above-mentioned de-velopment and the quality of billets manufactured by HORICAST.

II, HORICAST Facilities and Operations

1. Outline of Facilities

An outline of the horizontal continuous casters installed at Nippon Kokan K.K. is shown in Fig. 1 and Table 1. The casting facility in the Keihin Works is a large-capacity HORICAST, brought into operation in April 1983 for the purpose of principally casting alloy steel, stainless steel and super-alloy

grades tapped from the 50-t electric furnace, VAD, VOD and the 5-t VIF to produce 170330 mm round billets. Development efforts for the expansion of square billet size range are also being made by this large-capacity HORICAST: a casting test of 250 mm square carbon steel billets is now underway. Configuration of the large-capacity HORICAST is essentially the same as that at the Fukuyama Works, whereas all the facilities are larger in capacity, and in addition, the following improvements have been made :

(1) A liquid core length of 40 m of the cast strand is possible to cope with large-section cast strands.

(2) To reduce the time required for altering the casting size, the mold and a shortened secondary cooling zone have been incorporated into a single unit to permit size changing in 15 to 20 min per strand. Accordingly, a shape of the driven and table rollers capable of simultaneously coping with all sizes of round and square billets has been adopted.

(3) To accommodate the increased inertia of the cast strand, four driven rollers have been provided

per strand and braking means has been added. (4) A Hot Shot Blast equipment has been added

to further improve the billet surface quality (Photo. 2). (5) Four electromagnetic stirrers have been in-

stalled (Table 2).

Table 1. Specifications of HORICAST.

Fig. 1. Layout of HORICAST (elevation).

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2. Withdrawal Control of Large-section Cast Strand In the large-capacity HORICAST, the larger with-

drawal equipment and the larger section of cast strand have resulted in large withdrawal resistance during withdrawal of the cast strand and in a larger mass of the cast strand as shown in Fig. 2 as compared with a small-capacity HORICAST. More specifically, the inertia for pull-and-push motion, which can be expressed in terms of the mass of cast strand, has in-creased to eight times (19 t for 260 mm round) as large as 2.5 t (115 mm square) and the withdrawal resistance to three times. For the improvement of the billet surface quality, it is necessary to adopt intermittent withdrawal of the cast strand at a high cycle of at least 120 cpm. With a view to maintaining a high accuracy and a high cycle of intermittent withdrawal without being affected by the increase in the mass of cast strand and the withdrawal resistance in the large-capacity HORI-CAST, the following techniques have been developed:

(1) A braking unit has been added to the with-drawal equipment to cope with the inerita of the cast strand acting upon transition from pull to push in the intermittent withdrawal cycle, thus keeping a decelerating performance almost equal to that in small-capacity HORICAST.

(2) Accuracy and response speed of the with-

drawal equipment and the withdrawal controller have been improved.

(3) Withdrawal control has been improved to follow up the inertia increasing with the increase in the casting length.

As a result of additional application of these new techniques, 260 and 290 mm round cast strands are now cast at a stable casting rate of 1 m/min through intermittent withdrawal at a high cycle of 120 to 150 cpm. Casting of 310 and 330 mm round billets will be started in the near future.

3. Casting of Stainless Steel and High-alloy Steel

Casting of stainless steel and high-alloy steel billets by this process involves the following difficulties :

(1) Because of the features of this process, mold powder is not applicable for lubrication within the mold, so that these steel grades having a high hot strength do not allow stable withdrawal under the effect of a very high withdrawal resistance in the mold.

(2) One of the difficulties common to this pro-cess and ordinary continuous casting is the facts that casting of round billets tends to cause external dis-tortion of cast strands, often resulting in defective roundness of billets, longitudinal surface cracks or other defects.

(3) Has the material for the break ring used for connecting the mold and the tundish in this process sufficient erosion resistance and wear resistance to

permit satisfactory casting of these grades? Problems (1) and (2) above have been solved only through optimization of the mold conicity without a special plating giving lubricating function of the mold interior. Figure 3 shows the relationship between the mold conicity and the hydraulic pressure necessary for pull

'Table

Photo. 2. Hot shot blast unit.

2. Specifications of electromagnetic stirrers. Fig, 2. Withdrawal

(comparison

resistance and mass of cast strand. of large and small HORICAST)

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for SUS 304 260 mm round billets : optimization of the mold conicity reduces withdrawal resistance to a stable level with smaller variations. When casting various stainless steel grades with the optimum mold conicity, changes in withdrawal resistance are small and stable as shown in Fig. 4: it is now possible to cast these grades at a casting rate equal to that for carbon steel. A new material was developed for the break ring.

Boron nitride is commonly known as material for the break ring. In the HORICAST process, how-ever, a composite containing silicon nitride and a small amount of boron nitride was adopted for carbon steel grades and has been used in commercial pro-duction with excellent results superior to those of boron nitride in terms of costs and performance. For stainless steel grades, the above-mentioned

problems had been anticipated, and various ma-terials having SIALON composition (Si6-zAlzOzN$_z) as shown in Fig. 5 were provided. From among these materials, those with a SIALON coefficient of z=1 were selected in terms of the balance between erosion resistance and thermal shock resistance, and a com-posite with a slight addition of boron nitride is now applied.

With the use of break rings made of this new ceramic material, casting of large-section billets of carbon steel as well as of stainless steel and high-alloy steel grades as shown in Table 3 is satisfactorily carried on with an excellent surface quality of billets. This new break ring is exhibiting satisfactory per-formance as expected also in casting of SUS 321

(Ti-containing austenitic stainless steel) and NCF 825 (42 %Ni-21 %Cr-3 %Mo-AI-Ti-containing steel) shown in Table 3. Billets of these steel grades are applied at the Keihin Works for the manufacture of high-quality seamless tubes and pipes by the hot extrusion process

or the Mannesmann rolling process and are giving

excellent results equal or even superior to those manufactured by the conventional ingot-billet proc-

ess. With this achievement in view, switching from the conventional process to HORICAST billets is now

underway.

III. Quality of Cast Strand

1. Surface Quality 1. Withdrawal Cycle and Cold Shut Crack Depth

Initial solidification in the mold in this process is

quite different from that in the conventional continu-ous caster. A schematic profile of initially solidified shell as formed in a cycle of withdrawal is represented

in Fig. 6. Heat transfer occurs in three directions including not only the mold side, but also the break

Fig. 3. Relationship between mold conicity and hydraulic requirement for pull, (SUS304, 260 mm round)

Fig. 4. Change of hydraulic pressure requirement for pull.

Fig. 5. Erosion and

materials.

thermal shock resistance of break ring

Table 3. Test steel grades.

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ring side and the previously solidified shell side. Solidification, therefore, begins on these three sides. In the next pull, molten steel comes into the gap between the break ring and the solidified shell being separated from the break ring, and solidification is started again on the three sides.

This process of solidification results in a discon-tinuous structure at the portion separated from the break ring, which is known as " cold shut ". Ex-ternal view and a microstructure of cold shut are shown in Photo. 3. Cold shuts are observed as linear marks appearing on the cast strand surface at intervals corresponding to pull of the cast strand. Dendrites growing in parallel with the withdrawing direction of the cast strand face each other with a cold shut in-between. When there is a long time interval between two consecutive pulls, microcavities may discontinuously occur along the cold shut, which are called cold shut cracks (Photo. 4).

A cold shut crack is attributable to insufficient welding caused by overcooling of solidified shell beginning to solidify at a position in contact with the break ring near the triple point formed by molten

steel, the break ring and the mold. Temperature at this portion of solidified shell was calculated by finite

difference method for a typical case of carbon steel,

as shown in Fig. 7. Improvement of weldability is expected from the increase in temperature according as the withdrawing cycle time is reduced. With

reference to this consideration, the withdrawing cycle was gradually increased on a commercial caster casting

carbon steel, and the results show, as in Fig. 8, that a higher withdrawing cycle leads to a shallower depth

of cold shut crack. The cold shut depth demon-strates a tendency similar to that of cold shut crack,

due to the fact that a higher withdrawing cycle cor-responds to a thinner shell solidifying at the portion

in contact with the break ring. The effect of steel grade on the depth of cold shut

Fig .6. Schema of initially solidifi ed shell profile.

Photo. 3. Outlook (upper) shut. (SUS304)

and microstructure (lower) of cold

Fig. 7. Effect

triple

of cycle

point.

time on the temperature at the

Photo. 4. Microstructure of cold shut crack. (SUS304)

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and cold shut crack is illustrated in Fig. 9. As is clear from this figure, the carbon content exerts almost no effect on this depth, and the difference in depth is very slight between carbon steel and aus-tenitic stainless steel. 2. Improvement of Cold Shut Crack by Hot Shot Blast Fine lateral openings on billet surface and sub-surface micro-cavities of cold shut cracks can be pressure-welded through shooting of small metal shots onto a high-temperature cast strand, to the extent that these openings and cavities are indiscernible even by microscopic observation. To make full use of this effect of Hot Shot Blast, it is important that the cold shut crack has a small depth, being free of air oxidation and the cast strand has a sufficiently high surface temperature to ensure effective shooting of metal shots. Effect of Hot Shot Blast was ascertained through microscopic observation of cold shut crack depth on a carbon steel billet as shown in Fig. 10. In a billet cast at a low cycle of 80 cpm, cold shut cracks over 0.6 mm in depth still remain at ends, whereas ones shallower than 0.6 mm have been eliminated, thus clearly showing the effect of Hot Shot Blast. By the application of Hot Shot Blast with a smaller depth of cold shut cracks brought about by a higher-cycle casting, cold shut cracks have been eliminated. As shown in Fig. 11, samples cut from a square billet were subjected to a rolling test on a mini-rolling mill in the laboratory. Samples were rolled from

55 mm square to 25 X 55 mm after reheating at 1 100 °C for 1 hr (reduction=2.2). As observed in Photo. 5 giving the results of this rolling test, shallow openings resulting from cold shut cracks are present at portions corresponding to the billet corners in the case without the application of Hot Shot Blast, but no flaws are observed in the case with Hot Shot Blast.

Fig. 8. Effect of withdrawal

shut crack.

cycle on the depth of cold

Fig. 9. Effect of steel grade on the depth of cold shut and

cold shut crack.

Fig. 10. Effect of HSB on cold shut crack.

Fig. 11. Sample for test roll ing.

Photo. 5. Outlook of rolled billets. (reduction ratio: 2.2)

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As mentioned above, it is now possible to prevent occurrence of cold shut cracks which are surface de-fects intrinsic to the horizontal continuous casting

process through high-cycle withdrawal of cast strands and use of Hot Shot Blast.

2. Inner Quality

1. Macrostructure of Billets

This section describes first the features of cast structure of the portion near the surface layer and the

effect of this structure on the quality of billet. This

process is characterized by complete absence of mold powder in the mold and a higher ferrostatic pressure than in ordinary continuous casting in the initial stage of solidification. This results in a greater heat

transfer in the mold than in the ordinary continuous caster. This affects also the cast structure ; as shown

in Fig. 12, the amount of o-ferrite produced in an austenitic steel grade such as SUS 304 is very slight in the surface layer and is easily susceptible to dis-

solution into austenite in the next heating step. This

prevents surface defects caused by o-ferrite during rolling.4~ Another metallurgical feature of this process is observed in the macrostructure of billets. Photo-

graph 6 gives microstructures of cross-sections of a 260 mm high-alloy steel round billet, the analysis of which is shown in Table 4. As is clear from Photo. 6, the final solidified position of a horizontally cast strand is located 5 to 10 mm from the geometrical center toward the upper side, presenting an asymmetrical structure relative to the center. Growth of columnar

grains also deviates toward the upper side. This is attributable to the temperature difference between the upper and lower sides caused by natural convection within the liquid core and cooling of the lower side because of the support rollers arranged only on the lower surface side. SUS 329J1 shown in Photo. 6 is a grade which

crystallizes 1-phase during cooling after the comple-tion of solidification only in a-phase, to present a dual

phase of a+r at the ambient temperature. In this grade, the solidification structure is in columnar grains

Fig. 12. Distribution of o-ferrite. (SUS304)

Table 4. High alloy steel.

Photo. 6. Macrostructure of HORICAST billets. (260 mm round, cross-section)

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on the upper surface side and in equiaxed grains on the lower surface side-quite different between the upper and lower sides. This asymmetrical macro-structure is attributable to the sedimentation of crystals rather than to the temperature difference between the upper and lower sides. More particu-larly, there are many crystals floating in the liquid core, and when these crystals are sedimented, those on the lower side tend to become equiaxed grains. 2. Quality at Billet Center Also when equiaxed crystals are produced by electromagnetic strirring to give equiaxed crystal structure extending even to the upper side of the billet center in an attempt to prevent center porosity and segregation, the above-mentioned sedimentation of equiaxed crystals spoils these efforts. A higher stirring speed of molten steel in EMS tends to increase the amount of produced equiaxed crystals and this may make it possible to achieve the amount of equiaxed crystals necessary for surrounding the center portion. However, the degree of negative segregation in the white band portion caused by EMS would be increased. In the case of SUS 304, negative segregation of Ni,

in addition to that of carbon, in the white band

portion causes a change in Ni balance which would in turn cause increase in the amount of o-ferrite in this portion. Intensive stirring by EMS is not there-fore desirable. To cope with the above mentioned difficulties, multiple stage EMS has been successfully applied. As indicated in Fig. 13, two stirrers set at a distance of 1.5 m generate enough amount of equiaxed crystals to cover the center of billet, even if stirring intensity be reduced to 40 % to avoid intensive white band. Even with a single EMS equipment, these diffi-culties have been solved also. A single stirrer is located at a position where the liquid core size is

between 20 mm square and 40 mm square (measured from white band in the 114 mm square billet). As shown in Fig. 14, it is possible to make equiaxed crystals available in the billet center portion with relatively weak negative segregation at the white band. 3. Billet Cleanliness

In the horizontal continuous casting process in which the tundish is directly connected with the mold, a billet with a higher cleanliness is obtainable only if oxidation of molten steel flow between the ladle and the tundish is perfectly prevented. This feature of horizontal continuous casting leads to another advan-tage of stably casting small-section billets and Al or Ti containing grades which are believed to be difficult to cast because of easy occurrence of immersion nozzle clogging in the conventional continuous casting proc-ess. On the other hand, however, casting of lower-cleanliness molten steel results in localization of non-metallic inclusions in the upper side on the billet cross-section as in the one cast by the conventional curved-mold continuous caster.

Fig. 13. Effect of the distance between stirring intensity of Rl on He at the

(Carbon steel, 115 mm square)

2 stirrers and

double stirring.

Fig. 14. Effect of liquid core size at the position of stirring by R2 on He and negative segregation ratio at the single stirring. (Carbon steel, 115 mm square)

Fig. 15. Distribution of total oxygen content on the cross- section. (SUS304, 260 mm round)

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The total oxygen distribution on the cross-section of a 260 mm round billet of SUS 304 is illustrated in the height direction in Fig. 15. When molten steel has a low cleanliness, non-metallic inclusions float up in the liquid core and gather on the upper side of the billet, whereas, with clean molten steel, the billet contains less non-metallic inclusions which are uniformly distributed.

Iv. Results of Product Quality Test

Results of application tests of HCC billets are comprehensively arranged in Tables 5 and 6. Ap-

plications to wires and rods included steel for rein-forced concrete and section steel as well as cold-finished bar steel, machine structural carbon steel,

carbon steel for cold heading and cold forging, mild steel rods and hard steel wires. Billets were also

applied as materials for seamless tubes and pipes for such uses as machine structures, boiler heat exchangers

and oil country tubular goods. Trial hot forging was applied also with satisfactory results.

Though the various improvements achieved in equipment and operations as described above, it is now possible to produce high-quality billets even with

Table 5. Results of application tests of carbon steel.

Table 6. Results of a pplication tests of alloy steel and super alloy.

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alloy steel, stainless steel and high alloy steel grades and these billets are giving a high quality equal or even superior to that of conventional ingot-cast prod-ucts in various forms of product such as structural alloy steel round bars, forgings, springs, boiler heat exchangers of alloy steel, and corrosion resistant and heat resistant stainless steel tubes and pipes.

V. Conclusion

While commercial production of small and medium section billets of carbon steel by horizontal con-tinuous casting has already been started, the horizontal continuous casting process has made further step forward through new innovations achieved in the large-capacity HORICAST installed at Nippon Ko-kan K.K. :

(1) It is now possible to prevent occurrence of cold shut cracks which are intrinsic surface defects

in the horizontal continuously cast billet and the inner quality of billet has further been improved by the use of electromagnetic stirring.

(2) Techniques in equipment as well as in opera-tions for stably casting large-section billets of stainless steel and high-alloy steel grades have been established. Production of stainless steel and high-alloy steel billets has thus been started on this large-capacity HORICAST.

REFERENCES 1) M. Ishikawa, A. Honda and T. Anzai : Trans. ISIJ, 20

(1980), 570. 2) H. A. Krall and H. Huber: Het. Plant and Tech., 5 (1983), 44.

3) P. Voss-Spilker and W. Reichelt: Iron Steel Eng., (1983), June 32.

4) Y. Kinoshita, S. Takeda and N. Yoshiwara: Tetsu-to- Hagane, 65 (1979), 1176.

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