The Continuous Casting of Stainless Steels

Home , Continuous casting

BRIMACOMBE, J.K., KUMAR, S.. HLADY. CO., and SAMARASEKERA. I.V. The continuous casting of stainless steels. INFACON 6. Proceedings olllle 1st 1"lenwtiofllll Chromium Steel and Alloys Congress, Cape TOIVII. Volume 2. Johannesburg, SAIMM, 1992. pp. 7-23.

The Continuous Casting of Stainless Steels

J.K. BRIMACOMBE, S. KUMAR, C.O. HLADY, and LV. SAMARASEKERA

Centre jar Metallurgical Process Engineering, University ofBritish Columbia, Canada

The continuous casting of stainless steels is now an established technology, although newer processes like thin-strip casting and horizontal casting are the harbingers of an emerging era. This review examines the various aspects of the continuous casting of stainless steels. The different processes employed to manufacture stainless-steel billets, blooms, slabs, and strip are described. Fundamental knowledge pertaining to the casting of stainless steels is examined to provide a basic understanding of the important events that occur during the process. Quality problems in the cast products are discussed in terms of the mechanisms and operating design parameters that influence them.

Introduction The different grades of stainless steel that are continuously Stainless steel has been continuously cast for more than cast can be classified in terms of their microstructure as ferritic, martensitic, and austenitic3, 8-1 I; these can be four decades I,The first major installation in North America for the continuous casting of stainless-steel slabs was represented on a Schaeffler diagramJ , Figure 2. As shown in 12 established in the early 1950's at Atlas Steel in Canada'. Table 1 , the stainless-steel family covers a wide range of Over the years, the continuous-casting process for the alloy content, which makes its processing more complex production of steel billets, blooms, and slabs has emerged than that of plain carbon gradesJ,IJ. The chemical reactivity, as one of the most important technologies in the steel physical properties, and solidification modes differ widely industry worldwide2, particularly in the stainless-steel for the various grades of stainless steels. Although stainless sector. A revolutionary development in refining in the steels have been continuously cast for about four decades, 1970's, namely the invention of the AOO, followed by the the semifinished products are not entirely free from defects. development of similar novel processes such as the VOD, Problems with respect to cleanliness, the depth and paved the way for the production of high-quality stainless unifonnity of oscillation marks, segregation and centreline steel at reduced cost. Stainless steel is now manufactured porosity, surface and internal cracks, and shape defects, such by the process route shown in Figure 1, where, following as longitudinal and transverse depressions, continue to melting in an electric furnace, the molten steel is refined in plague the industry, Although considerable progress has an AOO or VOO furnace and then stirred to homogenize been made in improving the quality of semifil1ished sections the temperature before continuous casting3- 7. through technological developments such as new tundish designs, and EMS in the mould and sub-mould regions, to name a few, the goal of producing defect-free products on a STEELMAKING consistent basis has not been fully realized. ( Elltctrlc Arc """-I

+ " ~ A+M "f 10 Z + u• , ~ A' F ',I i• ,Ql.l

"" FIGURE I. Process route for the manufacture of continuously cast C'C

THE CONTINUOUS CASTING OF STAINLESS STEELS 7 TABLE I Tables II to IV provide a summary of conventional and CHEMICAL COMPOSmON OF STAINLESS STEELS 12 horizontal casters that process stainless steel and alloy steels in North America l4. 15 , Japan 15 • and Europe15, Grade Type Composition. % respectively. The advent of horizontal cas ring and strip C Mn 51 Cr N1 casting marks the beginning of a period of unprecedented change with respect to the production of continuously cast AusteniLic 201 0,15 5,5-7,5 1,0 16,~18,0 3,5-5,5 semi-finished sections. These developments, which are 202 0,15 7,5-10,0 1,0 17,~19,0 4,~,0 being driven by the need to significantly reduce costs without compromising quality. will continue well into the 304 0,08 2,0 1,0 18,~20,0 8,~10,5 next century and have heralded a new era in continuous 309 0,20 2,0 1,0 22,~24,0 12,~15,0 casting. The horizontal continuous-casting process has been adopted extensively for the production of stainless-steel 316 0,08 2,0 1,0 16,~18,0 10,~14,0 billets, as is evident from Tables II to IV. The direct Fcrritic 405 0,08 1,0 1,0 11,5-14,5 coupling of the tundish to the mould, and the elimination of strand bending and unbending, make the horizontal-casting 409 0,08 1,0 \,0 10,5-11,7 0,50 process particularly attractive for stainless steels containing

430 0,12 1,0 1,0 16,~18,0 reactjve elements, and also for crack-susceptible grades. The single-wheel casting process developed at Allegheny Mancnsitic 403 0,15 1,0 0,5 11,5-13,0 Ludlum for the production of stainless-steel strip is an impressive advance in continuous-casting technologyl6-19. 410 0,15 1,0 1,0 11,5-13,5 Twin-roll processes for the casting of stainless-steel strip 420 0,15 1,0 1,0 12,~14,0 have also been piloted worldwide2o-23 , and commercial production by these methods is likely within the next 431 0,20 1,0 1,0 15,~17,0 1,25-2,50 decade.

TABLE II CONVENTIONAL AND HORlZONTALCONTlNUOUS CASTERS PRODUCING STAINLESS AND ALLOY STEELS IN NORTH AMERICAI4, 15

Companies Caster Ladle Annual Product Steel type capacity capacity sizes grades t let mm AI Tech Watervliet NY Billet 32 40 115 sq Stainless Allegheny Ludlum Slab 125 500 205 X Stainless Brackenridge, PA 660-1320 Armco Inc. Baltimore MD HCC billet 50 60 100 sq to 302, 303, 304, 150 X200 305, 306, 316, 410,430, Nirtonic 32 Butler PA Slab 165 900 150-205 x Stainless 660 - 1320 Atlas Steel, Tracy Quebec Slab 70 100 125 x 1320 300 and 400 125 x 990 series Weiland Ontario Bloom billet 70 N.A. 155 sq, 145 Stainless, sq and 200 alloy, and X250 carbon Carpenter Tech. Reading PA Billet 35 N.A, 125 sq to Stainless, 305 x 355 alloy, and carbon Eastern Stainless Slab 60 150 125 x 865 Stainless Baltimore MD 125 X 1270 J & L Speciality Slab 125 420 125-255 x Stainless Products Midland PA 610-1320 Lukens Steels Slab 165 660 230 x 2160 Ni-Cr-Mo Coatesville PA grades Washington Steel Slab 55 200 140 x Stainless Houston PA 675-1320

8 INCSAC I TABLE III CONVENTIONAL AND HORIZONTAL CONTINUOUS CASTERS PRODUCING STAINLESS AND ALLOY STEELS IN JAPANIS

Companies Caster Ladle Annual Product Steel type capacity capacity sizes grades t kt mm Aichi Steel Billet 20 N.A. 135 sq Stainless 185 sq 185 X 320 Kawasaki Steel Slab N.A. N.A. 200 X 304, 31 OS,430, Chiba Works 800-1280 410, R410DH, Caster I 420J I, 420J2, 631, HCSI6, 409, RU09Sr Caster 2 Slab N.A. 410 200,260 X 1300-1700 Nippon Kokan Slab 40 N.A. 50-120 sq 304,316,321, Fukuyama 120-220 rna. 347,405, 329Jl, NCF825 Keihin Bloom 50 N.A. 115-250 sq Billet 170-330 dia Nisshin Steel Billet N.A. 360 155 X 1030 StainJess, Shunan Works alloy, and carbon Sumitomo Steel Slab 20 156 183-336 dia 304,316,321, 310,347, UNS8800, S31803

TABLE IV CONVENTIONAL AND HORIZONTAL CONTINUOUS CASTERS PRODUCING STAINLESS AND ALLOY STEELS IN EUROPEIS

Companies Caster Ladle Annual Product Steel type capacity capacity sizes grades t kt mrn Avesta AB Thin 65 400 80x 304L,304, Degerfors slab 1560-2080 316,316Ti, Sweden 410, super austenitic, duplex Boschgott HCC N.A. 200 180-350 dia. Stainless Hardsgutte Germany ImphySA Billet 30 N.A. 130-180 dia. 304L,305, France 316L Krupp StaW Billet 50 N.A. 135-200 sq Ferritic, Siegen anddia. austenitic, Germany duplex Bochum Slab N.A. N.A. 220-126 X Stainless Germany 865-1650 Temi Steel Slab 180 N.A. 160x Ferritic, Terni Italy 720-1330 austenitic Thyssen Slab 75 264 150-280 X 305,309,310, Krefeld 800-1600 309S,316L, Germany 317

THE CONTINUOUS CASTING OF STAINLESS STEELS 9 This paper reviews the state-of-the-art in the production spinels and oxides of Ca, AI, Si, Ti, and MnJO,Jl. However, of stainless steels by continuous casting. with special the benefits of this technology have not been assessed attention to new developments and product quality. The completely~ the filters are expensive to utilize and also tend solidification structure and high-temperature mechanical to clog during the operation. behaviour of a variety of stainless-steel grades are Another interesting development is the electromagnetic examined and linked to quality problems such as brake that has been employed to decelerate the molten segregation and crack formation. The mechanisms involved streams issuing from the submerged nozzle. This reduces in the fonnation of surface and internal cracks, as weU as in the severity of impingement of the liquid metal against the longitudinal and transverse depressions, are also described; solidified shell formed at the narrow face of the mould, as and the influence of machine design and operating well as the velocity of the molten metal following the parameters on these defects is delineated. impingements. A beneficial effect on fluctuations in metal levels and on the internal quality of slabs (inclusions and Continuous Casting Processes for Stainless pinholes) has been observed'. Steels Horizontal Continuous Casting Conventional Continuous Casting Horizontal continuous casters have been utilized for the The conventional continuous-casting process for billets, production of square and round billets. In this process, blooms, and slabs has been the subject of detailed analyses liquid steel flows from a ladle into a tundish, which is for the past three decades. The knowledge generated is connected directly to a water-cooled copper mould via a based on the results of mathematical modelling, physical refractory assembly, Figure 432. The refractory joint, or modelling, in-plant measurements, and laboratory breakring, is a critical component of the horiwntal-casting experiments, and provides important linkages among machine. Numerous breakring materials have been tested to quality problems and process (operating and design) reveal that boron nitride is preferable to alumina, while parameters2. Details concerning optimization of the process sialon material has also been tried successfully33. The fit of parameters from the viewpoint of quality products are well the boron nitride breakring inside the copper mould must documented in the literature24- 27 . Numerous new be sufficiently tight to prevent the penetration of liquid technologies are being evaluated in tbis area to further metal and also to minimize any ring motion during the enhance the quality of continuously cast products. casting process34 . Control of steel temperature during the casting operation The mould is stationary while the strand is withdrawn is very critical because of its influence on the final product intermittently, through a sequential pull-push-pause quality. For example, at the start and end of casting, as well motion, to prevent sticking of the solid shell to the mould". as during ladle changes, the temperature of the molten steel In another design, the mould-tundish assembly is oscillated falls and the removal of inclusioos becomes difficult',2'. A and the strand is withdrawn continuously, in a way very low-frequency channel type of induction-heating system, similar to the conventional casting process32•35 . This shown in Figure 3, has been installed on a trial basis to help technique has been tried successfuUy for low-alloy steels maintain steel temperature~ its beneficial effect has been and austenitic stainless steels, and also for crack-sensitive seen on the quality of slabs cast at the start and end of and segregation-prone alloys such as ferritic stainless heats'·2'. grades, Cr-Ni valve steels, and others35. The frictional Inclusions smaller than about 80 J.lrn cannot be removed force between the mould and the solid shell of stainless easily in the tundish by flotation29. Therefore, ceramic steel is larger than in the casting of plain carbon steeP4. foam filters are being evaluated for removing i.Delusions Thus, tbe casting of stainless steels is difficult without and for improving the cleanliness of steePO.31. The lubrication, and has led to the development of a self mechanism for removing inclusions with these filters is lubricating mould coated with nickel-containing dispersed based on a simple screening action, which involves solid solid lubricant (e.g. polycarbon monofluoride); then state sintering30,Jl. The inclusions absorbed are AI2MgO. stainless steels have been cast smoothly34.

Molten steel

Inductor

Tundish Refractory Nozzle

Cooling Jacket

HGVRE 3. Schematic sketch of a channellype of induction-heating FIGURE 4. Schematic diagram of a horizontal cOnljnuous~cas(ing system28 machine32

10 INCSAC I The horizontal-casting process offers numerous Reaction Products advantages over the conventional casting process. The In plain carbon steels that are aluminium-killed, alumina direct, air-tight mould-tundish connection is important in inclusions are the primary deoxidation products in the the continuous casting of stainless steels because it prevents molten metal lJ. However, in stainless steels containing the oxidation of reactive elements such as Ti or AJ36-41. In aluminium and titanium, it is common to find complex addition. mould flux is not required and the entrapment of corundum-type inclusions such as (Cr-A1hOJ or spinel mould slag is therefore eliminated. In horizontal-casting type oxides such as MnTi20 4 or MnCr204' Occasionally. machines, the strand is not subjected to bending and A1,O, (alumina) and FeO-Al,O, (hercynite) inclusions are unbending35•J1- 39, so that cracking problems related to these also observed13. In grades containing high concentrations events are eliminated. Machine designers claim that the of titanium, carbide or carbonitride inclusions such as cost of installation of horizontal casters is lower than that of Ti(C,N) are present in the molten metal as microscopic conventional machines owing to their smaller space and suspensions9. 13 . Since a wide variety of complex height requirements35-38,42; the operating and maintenance compounds are encountered in the casting of stainless costs are also reported to be lower. The lower height and steels. it is almost impossible to have a general-purpose the absence of an open tundisb-mould stream offer mould flux that can bandle all the different grades and increased operator safety39A2. temperatures efficiently. Notwithstanding the advantages offered by the horizontal continuous-casting process over its conventional Shrouding counterpart. the application of this process has been restricted largely because of technological limitations. An Ladle-to-lundish and tundish-to-rnould shrouding is important constraint is the working life of the refractory necessary to minimize chemical interaction between the breakring that connects the mould to the tundish, although molten metal and the atmosphere. Shrouding facilitates a new materials are being evaluated to overcome this belter surface quality in the final product, and also reduces problem. Moreover, the cost of the withdrawal system for the severity of operating problems such as tundish skulling the horizontal caster is higher than for conventional and nozzle c1ogging7.13,44 The introduction of an inert gas continuous-casting machines. Finally, the witness marks on such as argon through tundish stopper rods and ladle-to horizontally cast billets may be sites of cracking during hot tundish shrouds further enhances the quality of the casting working32. operation by minimizing the buildup of solid inclusions in the nozzles l3 . However, precise control of the gas flow (and Strip Casting pressure) is required since excessive flow promotes Strip casting has attracted the attention of steel makers agitation in the mould and causes the entrapment of mould because it allows the elimination of at least part of the flux in the final product; on the other hand, inadequate flow rolling operation, which leads to a reduction in energy (or pressure) leads to poor flushing of the non-metallics, consumption and capital expenditure20-22 . There are two which causes nozzle clogging l3 . kinds of strip-casting machines that are being evaluated: single-roll and twin-roll. Allegheny Ludlum is involved in Chemical Interaction between Mould Flux and Molten the development of the single-roll casting process for the Steel production of stainless-steel bands I6- 19 . Research During casting, the properties of the mould flux change conducted by them has examined process parameters and owing to the absorption of non-metallic inclusions and their relationships with product-quality concerns such as direct chemical reaction between the solutes in the steel and dimensional control. edge quality, surface roughness, and the mould flux, as indicated earlier45. In a study by internal soundness. Twin-roll casters for the production of Lindenberg and Loh, a number of mould fluxes were tested 2 23 stainless-steel strip are also being developed 0- . Various for their crystallization behaviour and chemical interaction aspects of the casting process, such as the castability of with various grades of molten high-alloy steels". The alloys, solidification behaviour, microstructure, and product extent of interaction between the mould flux and the molten quality. have been examined to optimize the operating and metal was found to depend on the difference in chemical 2 23 design parameters 0- ,4J. potential between the two phases. In the case of aluminium killed or titanium-containing steel grades. the difference Fundamental Aspects ofStainless-Steel between the oxygen potential of the mould flux and the Casting molten steel is large, and results in the reduction of less· Chemical Reactivity stable oxides, reoxidation of the meh, and transfer of the Stainless steels contain a relatively large proportion of oxides of AI, Cr, and Ti to the flux phase.· l3. Chemical reactive elements that have a high affinity for oxygen and analysis of the metal adjacent to the mould flux revealed an nitrogen. Thus, the role played by the shrouding of the increased carbon content due to the absorption of lhc molten metal in the continuous casting of stainless steels is element from the mould flux45. Such interaction and mass critical to minimize operating problems such as nozzle exchange depend not only on the thermochemical blockage and breakouts, and to enhance product quality. properties of the mould flux but also on the operating Moreover, the casting of stainless steel requires a superior parameters. For example, deep oscillation marks that form mould-flux technology since a wide variety of complex with a long mould oscillation stroke are usually compounds are formed between the reactive solutes in the accompanied by a high local concentration of carbon. stainless steel and the constituents of the mould powder. sulphur, and non-metallic inclusions45 .

THE CONTINUOUS CASTING OF STAINLESS STEELS II Solidification and Structure of Stainless Steel been studied by Hasegawa e/ (/1.'. The REZ is an important Stainless steels fall in the range of the &--y phase fields in quality parameter since it is directly linked to the severity the Fe-Cr-Ni tcrnary system, very close to the transition of ridging, a surface problem that occurs during the press ~orming of cold-rolled stainless-steel sheets7.4ii.4\). Ridging from the peritectic to the eutectic reaction9A6. As illustrated in Figure 5, these phase fields include the formation of IS frequently observed in type 430 stainless steels as narrow 5o primary ferrite, the formation of primary ferrile followed raised areas running parallel to the rolling direclion . The by a three-phase (L, 8, y) reaction, the formation of primary ridging problem is more severe in stainless-steel sheet ferrite and austenite, and the formation of primary rolled from continuously cast slabs than in that produced from ingots. Studjes have indicated that ridging is related austenite46 . It should be emphasized that the structure of the final product, and the structures present during and strongly to the presence in hal-rolled sheet of elongated immediately after solidification, may be different from one columnar grains, which appear as a band-like structure another. For example, some austenitic stainless steels oriented parallel to the rolling direction. Increasing the contain a large proportion of ferrite during and after REZ reduces the severity of the ridging problem. Figure 8 solidification. The composition of the steel, a balance shows a schematic diagram of the solidified structure of an between ferrite stabilizers (Cr, Si, Mo, Nb, Ti, AI) and 18 per cent chromium stainless-steel slab cast with austenite stabilizers (Ni, C, N, Mn, Cu, Moj, governs the secondary EMS and superheat levels greater than 20°C. solidification behaviour. In continuous casting, the critical The EMS resulted in the formation of banded equiaxed grains that prevented the growth of columnar grains? EMS ratio of CreqINieq at which the solidification mode changes from primary austenitic to primary ferritic solidification is was not effective in the case of superheats lower than 20°C, around 1,5 for austenitic stainless steels47 . at which point the cast structures were similar for both the 7 It has been reported that fully ferritic stainless steels, as stirred and the unstirred slabs . Figure 9 summarizes the in the case of plain carbon grades, solidify initially to form effect of EMS and tundish superheat on the REZ. the classic chill zone, which then gives way to a columnar-equiaxed pattern with increasing distance from the surface of the cast sectjon, Figure 6 13 . On the other hand, with fully austenitic grades, the structure is essentially columnar to the centre of the cast section 13 ; a typical macrostructure of an austenitic stainless steel is shown in Figure 7 13 . Owing to the large size and directionality of the columnar grains, the fully austenitic grades tend to be hot short l3. Small controlled amounts. about 2 to 6 per cent, of delta ferrite in the fully austenitic structure refines the size of the columnar grains and improves the hot-working characteristics considerably13.44. The effect of secondary EMS (below the mould) on the ratio of the equiaxed zone (REZ) in the cast structure has

Temperature. "c

1500

FIGURE 6. Macrostructure of tyflC 430 fcrritic stainless sleel 13

1400

'300

1200

5 10 15 20 25 30 Weight-"/" Ni I I , 30 25 20 15 10 5 0 Weighl-"1o Cr

FIGURE 5. Phase diagram of the Fe-Cr-Ni systcm at 70 per ccnt Fe46 FIGURE 7. Macrostructure of type 304 austenitic slllinlcss laeel B

12 INCSAC 1 0 w Hasegawa el al.7 have attempted to explain the formation 0 of the banded equiaxed structure with the help of the !low ( ,I' pattern developed during the intermittent application of , EMS, Figure 10. The banded grains form at a place where 'it~ ,/ ) I the molten pool is stagnant with respect to the transverse j and longitudinal directions? The crystal fragments flow R into this stagnant region from the lower part of the molten t\ '..... -- ' .. - 2 1 metal pool and collect as a band of equiaxed grains? E O~ Ill> - '~ - Ill> Ill> " JP... ~ However, this explanation has not been validated by other ~ ~ I 1/ ~ ~ workers. ~ '\ Research undertaken to assess the welding properties of I 3 Ill>'--..... fully austenitic slainless grades has revealed that steels ~ mill> ~ Ill> having residual delta-ferrite contents of less than 2 per cent (, are highly sensitive to hot cracking, whereas those j .. r-;'\ containing more than 6 per cent have poor hot a workability44. The fraction of delta ferrite in a cast section \ can be predicted by use of a mathematical model lhat is , \ --- \J based on heat transfer and the diffusion-controlled '6-y transformation reaction5o. Brittleness in the fully ferritic " grades is related to large cast grains and a relatively high OBIT (ductile-brittle transition temperature)13. 6

Upper Side FIGURE 10. Schcmalic represenlation of lhe flow pattern developed Juring the application of EMS7

Cast Structure and Hot Workability 3 Austenitic stainless steels tend to have poor hot workability --f---'--~------'-'-'-'-'-'- as compared with the ferritic grades because the cast - structure is columnar to the centre of the section l3 . The large columnar grains give rise to anisotropy in the material 5 properties and calise hot shortness in this grade of steeP3. The presence of a small amounl of delta ferrite in fully austenitic stainless steel has been observed to be beneficial since it refines the austenite grain size and reduces the Lower Side directionality associated with the columnar structure, thereby improving the hot workability I3,44. In the ferritic FIGURE 8. A schematic diagram? showing the banded equiaxed cast grades, since the cast structure consists of a chill zone structure after the appliciltion of EMS wilh a superheat grealer lhun 20°C together with the columnar--equiaxed pattern, the problem I Chill and columnar ZOlle of hot shortness is less severe13 . However, the ferritic 2 Banded cquiaxcd zone grades are inherently brittle owing to their large grain size 3 Columnar or coexisting colurnnur--equiaxcJ zone and relatively high DBIT", 4 Equiax.cd zone with fine grains Lindenberg el al.44 related the cracking susceptibility of 5 Equiax.ed zone with some (;olumnar grains continuously cast high-alloy steel slabs to the difference between the temperatures for zero ductility and zero

Nozrle 1)PIll strength (t1TZDS), based on the resulls of a series of hot oof 0 0 • 0: I tearing tests conducted on samples solidified ill situ. These ..: I 0 "- authors modified the conventional Schaeffler diagram ~ \ (valid for room temperature) to make it applicable at high ~ \ temperature, as shown in Figure I )44. '" 0 "0 N 0 Hot Ductility 0 0 0 \ 0 0 ... '" \ 00 The ductility of stainless steel at elevated temperatures is ..'" \ 0>,2 o g 0""'0 0 O? important with respect to cracking susceptibility during the oS \ 0 ..0 ~ " g" t"eo.,"'tiIe: 0 cr \ W 0 0 0 0 continuous-casting process. In stainless steels, two zones of 0 "" \ 0 low ductility have been identified. The first zone lies in the ....0 \ "" 30 SO°C 9, 0 temperature zone within to of the solidus while .. the second zone occurs3 at temperatures between 800 and Wlfhout ~ ....r\\ )ISO°C. In the conventional continuous-casting of steel " ..... billets, only the former is important from the viewpoint of 0 "- crack formation; in conventional slab casting, however, lO 20 30 0 "" both zones are critical. Superheat ('C) The low-ductility zone encountered at temperatures close FIGURE 9. Effect of EMS and lundish superheat on the REZ in the casl to the solidus arises from the presence of a liquid film, rich 51 struclure of an 18 per CCIlI chromium sleeP in solute elements, between the dendrites • Cracking in

THE CONTINUOUS CASTING OF STAlNLESS STEELS 13 , u 25 "ustcnitic_ SUS 32' + lIU:.lLen I ti...:: z 100 -0- 0.00) S

90 -fr (J.(J(J4 S ~ 10 "c ~ • '0 -o-U.OU75 • + > u ferritlc- 70 . , . U813 austeni.tic ~ 15 , ~ U um 60 Z I. 1.'(01 Y' 50 z I mo.•••.•..!-U04 IIJlII 4U ;:; h:n llic <; 30

I' 25 3D 2U L-'-~-=~~=--~:-7:::-'-:-=:~--:-'-:c:---' " Cc-cquiV/lII:"l " 900 1UOU 1 100 1 200 1 300 ~ l,~ ~i \c.'c t 1,)7 Ho -+ Ii -+ 2' Ilb t ) .. Ti Testing lempcrature rC)

AGURE 11. Schacfflcr diagram for stainless steels at high tcmpcraturcs44 FIGURE 13. Effect of sulphur on the hot ductility of stainless steels] (the numbers denote the Wcrkstoff Numbers in the DIN specification) this zone of low ductility occurs due to hot tearing. The effect of phosphorus on ductility at temperatures between 1200 and 14000 C has been studied for a high-alloy steel, 100 and the results are shown in Figure 12. The poor ductility is o SUS 631 • SUS 310 attributed to segregation of phosphorus near the austenite 0.. 304 UD ~ SUS 304 grain boundaries JO . Sulphur also has a deleterious effect on g 80 the hot ductility of stainless steel, Figure 133, The ductility of various stainless-steel grades, . '\ characterized in terms of the reduction in the diameter of "~ 60 fractured tensile specimens, has been studied by Ogura ef '0 '0 lIf.3 in the intermediate temperature range; the results are .§ ·10 presented in Figure 14. Thus, it is clear that stainless steels U, have reduced ductility at temperatures between 800 and 'l: II50°C and, therefore, are susceptible to cracking; " 20 moreover the SUS 631 (AISI 631) grade is more susceptihle to cracking than the SUS 304 (AISI304) grade. This may be due to the presence of I per cent aluminium in 1I'-----'-_-J-_.L...._.L...._.L--_.L--~'-:-___J__' the type 631 grade, which leads to embrittlement from the 60U 800 1 000 1 200 precipitation of Al N in this temperature range; this Testins temperature ("C) mechanism was reported by Thomas et af.5 l . Similar results have been published by Unger ef aU and Kinoshita ef al. lO . FIGURE 14. Hot ductjlity of stainless-slee! grades3

Quality Aspects of Continuously Cast Stainless Steels 100 Cleanliness Owing to the presence of reactive elements in stainless 80 steels, a variety of inclusions, such as oxides, nitrides, and i! Q. -O''''~ carbonitrides of AI, Ti, and Mn, form7.9. 11 ,44. Oxide 0 60 "t>- , cleanliness is related directly to the oxygen content of the ~• .. 0030[%pl~ steel. The chemical composition of the oxides in stainless 40 steel grades AISI 304 and AISI 316 reveals a strong .~ ~ dependence on the titanium level and the total oxygen , , ~ , content, as shown in Figure IS. At higher levels of oxygen, '" 20 , conditions become favourable for the formation of ~ chromium-manganese spinels, which have a deleterious 0 effect on surface quality. Hasegawa et aU have classified 600 800 1000 1200 1400 the surface defects arising from inclusions in titanium Temperolure l~/.l containing stainless steels based on their morphologies, Table v. The effect of titanium and nitrogen on the FIGURE 12. Effect of phosphorus on the reduction of area of an as-cast formation of type A defects is shown in Figure 16, where 13 per cellI IllClllganese slee1 tO the product of nitrogen and titanium is seen to be criticaP,

14 INCSAC I rr .;onlon!: 0,019/0,021 " oxides of chromium, manganese, silicon, and aluminium, 52 "'0,-----",-----,,----, with lengths up to 50 mm In horizontal casting, the problem of inclusions is less severe because of the direct mould-tundish coupling. The distribution of inclusions in the cast product is interesting because, owing to their low density, more inclusions are found in the upper portions than in the lower part of the cast billet section53 .54 . Water-model studies on horizontal moulds have revealed forward flow in the lower regions and reverse flow in the upper zone of the liquid pool in the s4 °'--'---'-''--'-+-~~---' mould . The reverse flow apparently entrains inclusions of ° 0.1 0,2 0,3 0," 0,5 alumina and titanium nitrides, which become entrapped by the advancing solidification frontS4 .

FIGURE [5. Oxide composition as a function of the titanium and tolal OsciUation and Witness Marks oxygen contents of steel9 Deep oscillation marks are not desirable because they may lead to cracks, local segregation, and the entrapment of TABLEY inclusions8. In addition, non-uniformity in the depth of INCLUSION -RELATED SURFACE DEFECTS IN TITANIUM-CONTAINING GRADES? oscillation marks worsens with increasing depth of the marks8. Hence, it is essential to minimize the depth of Type Morphology oscillation marks. Figure 17 shows the beneficial role of high oscillation frequency and short stroke length in A Rough surface with clusters of subsurface producing shtillow oscillation rnarksS5 • However, as can be blowholes filled with mould slag seen in Figure 18, these conditions also lead to a reduction B Clusters of alumina and titanium nitride in the consumption of mould flux, which could result in a sticking problem55 . This happens because, at a higher C Blowholes (not clustered) filled with mould slag oscillation frequency, the amount of molten slag entering D Entrapped scum the mould-shell air gap is reduced. The inflow can be increased by the application of a less-viscous slag with a lower freezing point56. Witness marks on the surface of a cast section are characteristic of the horizontal-casting process, where the strand is withdrawn intermittently. The mechanism of witness-mark formation has been described by Lima et ai. 32. The depth of the witness marks, which corresponds to the thickness of the solid shell formed in the vicinity of the breakring during strand withdrawals3 , is important, and needs to be minimized since each mark is a plane of weakness that may cause surface cracking during hot working32. The depth of witness marks on stainless and carbon steels has been reported for various operating horizontal-casting machines, Table VP2. Witness marks are found to be

FIGURE 16. Effect of titanium and nitrogen on the frequency of type A Stroke defects in stainless stee[1 • Large T 1.4 ~ a Mlddla • In addition, the temperature of the molten steel in the • )( Small mould influences the formation of this defect. Based on 8 water-model experiments, it was found that the inclusions -£ 1.2 ~ accumulated at the meniscus are dependent on the shape ~ of the SEN (submerged entry nozzle)'. Temperature measurements in the mould revealed that the meniscus ui 1.0 region closest to the SEN had the lowest temperature, o" which assisted in the accumulation of titanium nitride 7 inclusions . The accumulated inclusions further reduced the ~ 0.8 temperature of the meniscus, causing freezing of the liquid E steel and the formation of a solid crust7. Skin laminations and seams are other inclusion-related ~ surface-quality problemsS2 • They tend to be more severe in 80 100 120 the first slabs of a heat52. The skin laminations are related Oscillation cycle (c/min) to the entrapment of refractories, flux, or any exogenous FIGURE 17. Impact of oscillation frequency and stroke length on the materials52. The seams are fine, straight lines consisting of depth of oscillation marks in lype 430 stainless steel55

THE CONTINUOUS CASTING OF STAINLESS STEELS 15 .~ § Since the depth of witness marks is related directly to the •o < • thickness of the solid shell formed against the breakring, ~ "-0 "1 ":;: :2. ~ operating parameters that influence shell growth are t 1.4 32 e. • 1.0 a.' important , Many investigators have reported the b a.' a.' beneficial effects of a higher superheat and an increased b' a.' 1.1 withdrawal frequency in reducing the depth of witness , 0.8 32 ~ 1.2 a.' marks .42,53, An increase in the withdrawal frequency • 0.8 1.1 reduces the time available for solidification of the primary ::;" "

~ Steel Casting Carbon 100 0.3 a None --O-Q400--o-__/ --

FIGURE 19. The effect of tundish superheat on the generation of *Cold shut index - no absolute value given depressions on the billet surfaceS7

16 INCSAC 1 Macrosegregation in continuously cast billets and blooms has been related to operating parameters that influence the -- Casting direction cast structure, such as superheat, strand dimensions, and .~. steel composition49. It has been found that the severity of centreline segregation and porosity can be reduced by increasing the REZ49. In continuously cast slabs, the problem is linked not only to the cast structure but also to bulging of the slab close to the bollom of the liquid poo149. During bulging, solute-rich liquid moves into the strand core and generates macrosegregation. Thus, maintenance problems, such as poor roll alignment. inadequate roll pitch, improper hydraulic roll pressure, and roll eccentricity, are critical49. The application of soft reductioo at the pool bottom is also Before improvement After improvement beneficial in minimizing the segregation problem. ( Non-stirred) (Stirred, M+F) Segregation is also associated with oscillation marks4s.s8 . The segregation of carbon and sulphur has beeo observed 1cm near the surface of slabs4s . Takeuchi and Brimacombe reported positive segregation of phosphorus and manganese FIGURE 20. Macroslructures of a round section of AISI 304 grade (with along subsurface hooks adjacent to oscillation marks in and without EMS)SJ austenitic stainless-steel slabs". They also observed positive segregation of nickel and silicon at the overflow region of hooks adjacent to oscillation marks, as well as at S8 the bottom of oscillation marks without hooks . The '" 50)~------. segregation is caused by the transport of solute-rich liquid 8 to the surface of the slab by overflow or by bleeding'S. The N severity of the segregation problem increases with deeper (; 40 • • oscillation marks; an increase in oscillation frequency will ~ obviously reduce the problem'S. b 30 ~ The beneficial effect of EMS in minimizing segregation c has been studied by numerous authors7.44. EMS in the 5 mould modifies the structure primarily by dissipating the af superheat49. Beyond the mould, the EMS affects the cast structure by reducing the thermal and concentration gradients ahead of the solidification front49. The beneficial effects of EMS on segregation and central porosity in horizontally cast billets has been reported by many workers37.38.53. Figure 20 shows the macrostructure of a roond section of a 304 stainless-steel grade both with FIGURE 21. Effecl of inlensily of EMS on Ihe REZSJ and without mould stirring. In the non-stirred case. the columnar grains grow towards the centre, forming bridges concentration of solutes such as sulphur and phosphorus in and large cavities53 . Stirring results in finer equiaxed 53 steel is critical to crack formation. Table VU summarizes grains and a reduced central segregation . The effect of the the important points related to the generation of internal intensity of EMS on the REZ oext to the lower and upper 53 cracks in sections produced by the conventional sides is shown in Figure 2 I for the A1SI 304 grade . The continuous-casting process. REZ increases steadily with stirring intensity, but then 53 In slab casting, the important internal cracks are bulging reaches a plateau , possibly owing to the complete related cracks, straightening or bending cracks, and cracks dissipation of superheat the mould. The EMS improves in under depressions and comers. Bulging of the broad faces the central segregation but also creates negative segregation s3 of a slab gives rise to triple-point cracks, radial streaks, and zones in the cast section . Mould stirring gives a wider centreline cracks; their detailed mechanisms have been REZ with a lower negative segregation than with secondary published elsewhere60. Straightening or bending cracks stirring. A combination of mould and secondary stirring form due to excessive tensile strains near the solidification gives the best result. lshizaka el al. employed a three-stage front during straightening or bending on a liquid core. In EMS system and found a remarkable effect on the REZ and 33 billet casting, the main internal cracks include midway also on the minimization of intemaJ CrdCks . cracks, off-corner cracks, straightening or bending cracks, Cracks centreline cracks, and diagonal cracks; the mechanisms of formation of these problems have already been Infernal cracks published59.60. AU internal cracks in continuously cast steel products form Lindenberg et al.44 have correlated the internal cracking in the high-temperature zone of low ductility within about susceptibility of ferritic stainless-steel slabs with their high 50°C of the solidus temperature, where the temperature mechanical properties. Figure 22 shows the microsegregation of solutes like sulphur and phosphorus relationship between the frequency of internal cracks (due creates liquid films between dendrites; therefore, the to bulging and cracks under comers and depressions) and

THE CONTINUOUS CASTING OF STAINLESS STEELS 17 TABLE VII INTERNAL CRACKS IN CONVENTIONAL CONTINUOUS CASTING

Type Causes Influencing factors Midway cracks Surface reheating in or Spray design; mould parameters (if billet has below the sprays60 dark or bright patches at mould exit); steel composition (Mn/S ratio and P content) and superheat Diagonal cracks Asymmetrical cooling in Thermo-mechanical behaviour of the mould; the sprays or the mould60 adverse mould-shell interaction; deep and non-uniform oscillation marks; spray design and maintenance; steel composition Pinch-roll cracks Squeezing on a strand with Excessive pinch-roll pressure; steel composition liquid core6O and superheat Straightening or bending cracks Large deformation near the Excessive bending or straightening strains; solidification front due to steel composition and superheat straightening or bending Off-corner cracks In billets, bulging of the Thermo-mechanical behaviour of the mould; solid shell and hinging at adverse mould-shell interaction; deep, off comers59 non-unifonn oscillation marks; steel composition In slabs, buckling of the Heat extraction at the meniscus; copper-plate solid shell due to thickness; mould-coating thickness or squeezing of end plates conductivity; cooling-water velocity; cooling- against the narrow faces62• 63 channel configuration; mould-flux composition Centreline cracks In billets, due to sudden Spray cooling near the point of complete (ghost lines in slabs) decrease in the centreline solidification; steel composition and superheat temperature at the point of complete solidification60 In slabs, bulging of the Roll gap; spray cooling; steel composition wide face60 Triple-point cracks [n slabs, bulging of the Roll gap, steel composition wide face60 the temperatures for zero ductility and zero strength, !:J.TZDS; a higher !J.TZDS increases the frequency of internal cracking44. !J.TZDS was calculated from the results of hot JO > tearing tests carried out at different temperatures on steel ~ z 44 . -•c samples solidified in situ Figure 23 shows the results of ",;. the tests for a ferritic stainless steel containing 13 per cent ~• ~ " ~ E '" chromium. Lowering the surface temperature of the slabs 0 ~ ~ E from 1100 to 900°C improves the strength of the strand and ~" --., 44 " • reduces the severity of bulging-related cracks . ~• -- iJ-.... o ",------, Inlemal cn.c1a under <:omen '''' ~ -.nd ~prcSliof1l C Inll:lIlai CriCks due to bulging ,0 .. ~ / Inlernal cncks (loul) I D • V ~ ,0' • ./ ~ A·CratleXSCr1i12 "• U " , " V ;; a·O"uk-X8Cr17 ~ .-- ~ C·OndcX8CrNb17 c ~ .~ " 0 " ~ 0 0 f "0 10 • A -" I -~- --- '" -- o 1000 1100 /}Oo I Joe 1(00 1500 D "'" Testing tempet"ature, ·C 0 " " " t:.Tws("C) FIGURE 23. Hot ductility and ma",imum tensile strength of a 13 per cent FIGURE 22. Effect of!J.Tws on the frequency of internal cracking+4 chromium steel solidified ill sitlt«

18 INCSAC 1 With martensltlc grades, susceptibility to internal Measurement of strength and ductility cracking increases with increasing carbon content44• As at elevated temperature with low-alloy steels, cracking susceptibility increases sharply when the carbon content exceeds the value for the peritectic reaction44. This is explained by the fact that the 44 Calculation of slab surface !i.TZDS increases significantly beyond the peritectic point . temperature with different In austenitic stainless steels, susceptibility to hot cracking secondary cooling paltern is related to the cast structure and the amount of delta ferrite present, as mentioned earl.ier. Sluface cracks Mapping of The main surface cracks observed in continuously cast slab ductility steels include longitudinal, transverse, and star cracks. The important points related to surface cracks in shapes produced by the conventional continuous-casting process NO are summarized in Table YIlI. Stainless-steel slabs of AISI 310 grade are more susceptible to transverse surface cracking than AISI 304 grade because of the lower ductility of the former9. Slabs of AISI 631, a martensilic grade, are also prone to transverse cracks9 . In both cases, the poor ductility is attributed to segregation of sulphur at the grain boundaries9; a reduction in the sulphur level minimizes Ihe generation of surface cracks. In addition, the AISI 631 grade has a high NO aluminium content, which aggravates its low-ductility problem because of the precipitation of AIN at the grain boundaries. The cracking tendency can also be reduced by YES straightening of the cast section at surface temperatures above 950°C or below 750°C so that the low-ductility zone Decision of secondary is avoided completely. A flow chart, shown in Figure 24, cooling pattern outlines the methodology for determining the secondary cooling pattern to minimize the cracking problem (surface FIGURE 24. A methodology ror the determination of slab cooling to cracks and internal cracks due to bulgingp. minimize cracking]

TABLE Vtrl SURFACE C!lACKS IN CONVENTIONAL CONTLNUOUS CASTING

Type Cause Influencing factor Longitudinalmidface cracks Tn slabs, due to tensile Mould disorders - alignment, taper, oscillation, strains generated in the mould lubrication; overcooling in the upper sprays; and upper sprays60 steel temperature and composition In billets, stream impingement Stream alignment; scratch or gouge marks on a face due to cocked stream, on the inner surface of the mould wall localized reheating of the strand surface in the mould64 Longitudinal corner cracks Tn slabs, situated just off the Mould support conditions corners, caused by bulging of the narrow face in the mould60 In billets, non-uniform Corner radius; presence of corner key-holes; cooling in the mould or the mould alignment; thermo-mechanical behaviour sprays60 of the mould; adverse mould-shell interaction; decp. non-uniform oscillation marks; spray design and maintenance; steel composition Transverse midface and In slabs, large surface Spray cooling; steel composition comer cracks gradients in the spray zone (precipitation of complex intermetallic or at the straightener within compounds of AI, Nb, Ti) the temperature range 700 to 900°C60 Star cracks Scrapi ng of copper from the Chromium or nickel plating; machine alignment mould wall60,61

THE CONTINUOUS CASTING OF STAINLESS STEELS 19 In conventionally cast slabs, the transverse cracks are solid shell at the off-corner of the broad faces generates a generated in the secondary-cooling zone, or beyond it, at tensile strain close to the solidification front and a the straightener owing to embrinlement at lower subsurface crack; therefore, it is common to find a crack temperatures caused by precipitation of compounds such as below or at the base of a surface depression. A similar 3 60 AIN, Ti(C,N) etc. . . In conventionally cast billets, the mechanism has been proposed for the generation of off transverse cracks are related to sticking or binding in the corner internal cracks in billelsS9. 61 mould . In horizontally cast steel billets, transverse cracks The formation of longitudinal depressions in slabs bas are observed in the vicinity of the witness marks and are been related to the behaviour of the slag rim at the 34 related to the pull time ; the mechanism in the generation meniscus; proper heat-flow conditions near the meniscus of transverse cracks has been described by Nakai el al., will belp minimize this quality problem6'. Mahapatra et who propose that there is a critical shell thickness below a1.62 have found that variables important for controlling the which transverse cracks form 34 • A reduction in the pull heat eXlrdction at the meniscus are copper-plate thickness, time or the introduction of a 'push-back' step in the cycle mould-coating thickness or conductivity, cooling-water leads to the formation of a thicker sbell and therefore velocity, cooling-channel configuration, and mould-flux minimizes the problem of transverse cracks34 . composition. Longitudinal cracks in conventionally cast slabs are related to the nature of the mould-sbell interaction, which Transverse depressions depends on mould cooling, taper, and lubrication. The role Transverse depressions have been observed in AISI 304, of mould lubrication in the generation of this quality 316, and 321 grades, being more severe in the 304 steel65 • problem is significant. In stainless steels, particularly those On the other hand, in resulphurized type 304 and 316 containing aluminium and titanium, the composition of the grades, transverse depressions are not a problem; simUar mould flux changes significantly during the casting of a behaviour is exhibited by 0, I per cent carbon steels heat3. For example, during the casting of the AISI 631 containing high levels of sulphurM. grade, which contains about I per cent aluminium, the Wolf has attributed the formation of transverse softening temperature of the molten flux increases with depressions in austenitic stainless steels and plain carbon increasing slag basicity because of chemical interaction steels to local overcooling of the strand during initial between the mould flux and the molten steel3. Aluminium solidification that is followed by contraction due to 'plastic in the molten steel reduces tbe SiO, content of the mould hinge effect' and rebending6S . Austenitic stainless steels flux and therefore increases its basicity3. The high with Creq/Ni cq around 0,55 tend to be sensitive to the frequency of longitudinal cracks has also been linked to the formation of depressions because of a large local shrinkage melting rate of the mould flux, which is related to the associated with the 8-y transformation at the end of morphology of the carbon in it45. solidification; this phenomenon is very similar to that In conventionally cast billets, longitudinal comer cracks observed in 0.1 per cent plain carbon steels6s . However, are generated because of non-unifonn cooling in the corner this theory may not adequately explain the generation of regions of the mould, which are often caused by an surface or subsurface cracks bclow these depressions. excessive corner radius. Longitudinal surface cracks have Samarasekera and Brimacombe66 postulate that transverse been observed in rounds cast by the horizontal-casting depressions in billet casting are relaled to sticking or processS3 . These cracks originate from non-uniform shell binding in the mould as a result of which the withdrawal growth in the mould53. Mould stirring has a beneficial system pulls the strand and generates longitudinal tensile effect since it enhances the formation of a uniform solid stresses and slrains. Depending on lhe magnitude of the sheIls3; increasing the intensity of mould stirring reduces stress, the solid shell may flow plasticly and form a the frequency of the cracking problem53. depression 011 the billet surface (similar to necking in a Star cracks in conventionally cast slabs arise due to the tensile test)66. scraping of copper from the mould walls60,63. The copper Examination of the shell below a transverse depression in penetrates the slab surface and causes hot shortness in austenitic stainless steel revealed a zone of locally reduced localized regions. These cracks are roughly 1,5 to 3,0 mm heat flow and retarded shell growth65. A local enrichment deep and appear in clusters in the pattern of a star60. in ferrite at the depression site was also observed65. As Chromium plating of the copper mould prevents tbe mentioned earlier, excessive ferrite is not desirable in penetration of copper into the slabs and therefore eliminates austenitic steel since it lowers its hot workability. star crack.s60. Nakano el al. have found that precipitation In billet casting, transverse depressions can be minimized hardened copper moulds baving a layer of electroforrned by operating with proper taper, lubrication, and oscillation nickel (about 3 mm thick) increase the grinding yield of the characteristics; the thermomechanical behaviour of the slabs and improve the mould life; this happens because the mould, i.e. excessive distortion, may also cause adverse nickel layer reduces mould wear63. mould-shell interaction6? According 10 Wolf, Ibe frequency of depressions can be reduced by ensuring a low Depressions heat flux near the meniscus, which produces a thinner initial shell; he has suggested that this condition can be Longitudinal depressions achieved by operating wirh higher casting speeds, higher Longitudinal depressions, 2 to 5 mm deep, have been superheats, and an optimum slag-film stability, observed on the broad face of conventionally cast type 304 corresponding to minimum heat flux and friction6s . The stainless-steel slabs at off-corner locations61 • They are application of soft mould cooling with the belp of a hot top associated with bulging of the. narrow face or are caused by mould has also been proposed by the same author as a buckling of the solid shell resulting from squeezing of the potential method to reduce depressions in austenitic end plates against the narrow faces6 1. 62• Buckling of the steels6s .

20 INCSAC I Acknowledgments 14. Continuous Caster Roundup - United States and The authors are grateful to the Canadian steel industry and Caoada. (1991). Iroll and SteelmakeI', 18, (I I), pp. the Natural Sciences and Engineering Research Council of 16-31. Canada for supporting work on continuous casting. 15. Hlady, e.O. (1991). Continuous casting of stainless steel. Internal Report. Centre for Metallurgical Process References Engineering, University of British Columbia, Vancouver. Canada. I. Harabuchi, T.B. and Phclke, R.D. (1988). Design and operations. Continuous castillg. vol. 4, ISS-AIME, 16. Grubb, J. F., Love, D. B., Murphy, A. and Nauman, J. Warrendale, PA. D. (1986). Casting speciality steel strip. 69th Steelmaking COllferellce Proceedillgs, ISS, Warrendale, 2. Brimacombe, J.K. and Samarasckera, LV. (1988). PA, pp. 841--S47. Fundamental analysis of the continuous casting process for quality improvements. Proceedings of II/do-US 17. Love: D. B. and Nauman, J. D. (1988) Controlling the phySical and mechanical properties of cast stainless Workshop 011 Principles of Solidification alld Materials Processillg, Hyderabad, Illdia, Oxford and IBH steel band. Casting of near-net shape products, Publishing Company, New Delhi, India, pp. 179-222. Metallurgical Society, Warrendale, PA, pp. 597--{j11. 3. Ogura, S., Kawaharada. A.. Matsuzaki, M., Kityaoka, 18. Dean, D. e. (1988). The physical metallurgy of strip H. and Ohtsubo, H. (1986). Production of high grade cast type 304 stainless steel. Iroll alld SteelmakeI' 15 (3), pp. 12-15. ', stainless steel, Kawasaki Technical Report, 14, pp. 23-30. 19. Nauman, J. D. and Love, D. B. (1986). The mechanical 4. Spaccarotella, A .• Morelli, R., Dischino, G. and processing of cast stainless steel strip. Mechanical Provantini, G. (1985). Influence of mould oscillation working and steel processing, XXN. ISS, Warrendale, and powder lubrication on surface quality of austenitic PA, pp. 2I3-220. stainless steel slabs at Terni Works. Continuous Castillg 20. Yoshida, C., Yasunaka, H., Nakagawa, T., Koyama, S. '85. Institute of Metals, London, pp. 42.1-42.1 I. and Nozaki, S. (1988). Development of twin-roll strip 5. Van Wijngaarden, MJ.U.T., Van den Heever, P.M. and caster and metallurgical aspects of stainless steel. 4th Louw, T.A. (1990). The production of high-quality steel International Conference on Continuous Casting. 2, Belgium. in a continuous billel casting operation. 73rd Steelmaking Conference Proceedings. ISS. Warrendale, 21. Yamauchi, T., Nakanori, T., Hasegawa, M., Yabuki, T. PA, pp. 213-219. and Ohnishi, N. (1988). Strip casting of stainless steels by twin-rolls. 71st Steelmaking COllference 6. Shimizu, S., Imada, Y.• Ishikawa, S., Kihara, K. and Proceedings, ISS. Warrendale, PA. pp. 161-165. Futamura, T. (1990). 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Kawasaki Steel Technical Report, (1984). Heat flow, solidification and crack formation. 14, pp. 37-44. COlltilluous castillg, vol. 2, ISS-AI ME, Warrendale, PA. 25. Continuous casting of steel blooms and slabs, voJ. I, 9. Unger, K.D., Biesterfeld. W., Berentzen, F. and course notes, (1990). Centre for Metallurgical Process Thielmann, R. (1985). Metallurgical problems Engineering. University of British Columbia, encountered in stainless steel continuous casting. Vancouver, Canada. Continllous Casting '85, Institute of Metals, London, pp. 61.l--{j I.7. 26. Ibid., (1990), vol. 2. 10. Kinoshita, K., Yoshii, Y., Kilaoka, H. and Kawaharada, 27. Ibid., vols. I and 2. A. (1984). Continuous casting of high-alloy steels. 28. Mabuchi, M., Nozaki, T., Habu, Y. and Yoshii, Y. Journal ofMetals, 36, (3), pp. 38-43. (1986). Development of tundish heating system and its II. Vonesh, F. A. and Schmel, R. F. (1987). Inert gas application to stainless steel casting. 69th Steelmakillg Conference Proceedings, l55, Warrendale, PA pp. shrouding of molten metal streams. Iron and Steel 737-742. • Engineer. 64. (7), pp. 35-43. 29. Martinez, E., Maeda, M., Heaslip, L. 1., Rodriguez, G. 12. Metals Haudbook, vol. I, Properties and selection of and McLean, A. (1986). Effects of fluid flow on the irons, steels, and high performance alloys (1990). ASM inclusion separation in continuous casting tundish. [ntemational, Mctals Park, Ohio, USA, pp. 841-930. Transactions ISIJ, 26, pp. 724-731. 13. Ardito, V.P. and Walsh, R.A. (1981). Problems in the 30. Yamada, K., Watanabe, T., Fukuda, K., Kawaragi, T. casting of stainless and high alloy steels. 2nd Process and Tashlfo, T. (1987). Removal of non-metallic Technology Conference, COll1inUOliS Casting of Steel, inclusion by ceramic filter. Transactions ISIJ, 27 pp. ISS-AlME. Warrendale, PA. pp. 18D-187. 873-877. '

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Production of billets for stainless Calltilluous Castillg of Steel, ISS-AIME, Warrendale, steel seamless tubes by a horizontal continuous casting PA, pp. 166-174. process. 6th International Iron and Steel Congress, Nagoya, ISIJ, Japan, 3, pp. 676-683. 49. Lait, J. E. and Brimacombe, J. K. (1982). Solidification during continuous casting of steel. ISS Transactions, I, 34. Nakai, K., Umeda, Y., Sugitani, Y. and Miura, M. pp. 1-13. (1984). Effects of factors on the stability of casting in horizontal continuous casting. Transactions ISIJ, 24, 50.Laki, R. S., Beech, J. and Davies, G. J. (1985). pp. 983-988. Prediction of dendritic arm spacings and delta-ferrite fractions in continuously cast stainless steel slabs. 35. Hentrich, R., Shanma, D. L., Dittert, D. and Roller, E. (1986). The horizontal casting of high-alloy steels using lronmakin8 and Steelmaking, 12, pp. 233-241. the Krupp oscillating mold process. Iron and 51. Thomas, B. G., Brimacombe, J. K. and Samarasekera, I. 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