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Continuous Casting I1 Billets and Blooms

TUESDAY AFTERNOON, APRIL 7,1970

The second session on Continuous Casting convened a t 2:00 p.m. The chairmen were J. Macnamara, division super- intendent, primary production, The Algoma Steel Corp., Ltd., Sault Ste. Marie, Ontario, Canada and J. T. Black, general manager, Connors Steel Co., Birmingham, Ala.

Continuous Casting Il-Billets and Blooms 145

Continuous Casting: I ts Metallurgical Aspects Relative to High-Grade Alloy and

Carbon Steels by L. Backer and P. Gosselin

lNT RODUCT ION The spectacular development of the automobile in-

dustry, to which SAFE is closely tied, has, since 1950, created serious problems due to the lack of modern and appropriate equipment for finishing steel produced in the steel works.

Because two 30-ton electric arc furnaces did not justify the installation at SAFE of a modern blooming mill, the alternative was to turn towards new and different techniques. A forging press for ingots put into service in 1953 was only an interim solution since the installation of a new continuous bar and wire mill necessitated the economic production of a larger quantity of 5- by 5-inch billets.

Our studies were directed towards a new process, continuous casting. It has considerable advantages but, at the same time, presented certain risks due to lack of practical experience and knowledge about its application to our particular methods of production.

Following extensive technological and metallurgical studies, continuous casting operations started in 1960. Since that time, the casting plant has operated nonstop and has produced, to date, approximately 600,000 tons of high-grade alloy and carbon steels for mechanical engineering applications; the greatest part was manufactured for the automobile industry.

This presentation outlines the major conclusions of numerous trials and studies. It deals primarily with a review of metallurgical aspects of continuous casting; in fact, this technique has introduced many new ideas into the standard methods of teeming ingot steel.

TECHNOLOGICAL SPECIFICATIONS OF THE INSTALLATION

Major Features of the Continuous Casting Machine Our continuous casting machine was constructed by

Demag according to the patents and ideas of Mannes- mann-Bohler & Junghans Associates. It is a four-strand vertical machine producing 5- by 5-inch billets and 8- by 8-inch blooms from a 40-45-ton ladle. A more de- tailed description of the installation, a diagram of which is shown in Fig. 1, has been the subject of previous d i s c ~ s s i o n s . ~ ~ ~

Casting Conditions Steelmaking. Liquid steel for continuous casting is

produced exclusively in two 40-tons electric arc fur-

L. BACKER is the principal engineer and head of the metallurgical dept. and P. GOSSELIN is the chief engineer of the steelworks dept., SociBtC des Aciers Fins de I'Est (SAFE), Hagondange, France.

naces. These furnaces are both equipped with Asea magnetic stirrers. Recently, one of the furnaces was equipped with a uhf transformer; as a result, our overall production from the two furnaces is expected to increase to better than 500 tons per day. It must be understood that the quality problems, which we will dis- cuss in detail, necessitate using two slags in nearly all our production.

Temperature Control. In order to work under optimum conditions, the temperature of the steel must be precisely controlled throughout casting. The casting time used in our normal operating conditions runs from 50 minutes for 8- by 8-inch blooms, to 60 minutes for 5- by 5-inch billets. The aim temperature in the tundish is 36°F greater than the liquidus temperature. The aim

VERTICAL' C, C . Mb,CH!NE , SAFE,

Fig. I-Schematic plan of the SAFE continuous casting plant.

146 Open Hearth Proceedings, 1970

Fig. 2-Schematic plan of the tundish.

temperature of the metal in the furnace is 145°F greater than the temperature in the tundish.

Tundish. The role of the tundish is of particular importance, both from thermal and quality points of view. It is imperative that the tundish protect the metal from possible reoxidation and favor the separation of inclusions. In a sense, we consider the tundish as the "hot top" of a normal ingot and therefore attribute a role of primary importance to it.

While awaiting the installation of specially designed tundishes, we have been using, for approximately 2 years, a type of tundish as shown in Fig. 2. Among the advantages of this design, note the possibility of sep- arating the inclusions in the "antichamber," and feed- ing metal with very little turbulence into the strands.

Immersed Nozzle. The oxidation of metal and, in particular, the aluminum content are the principal sources of bad surface defects. A research study to protect the casting streams by the use of an immersed tube and a synthetic layer of slag in the mold was begun in 1965.

Our attempts were unsuccessful until a fused silica refractory was used. It resists thermal shock very well, has a low thermal conductivity, good mechanical resistance, low porosity, and good chemical resistance even at casting t e m p e r a t ~ r e . ~

As can be seen in Fig. 3, the nozzle provides a perfect fit. The internal dimension of the nozzle permits a discharge four to five times greater than required. The inside diameter of the nozzle is @ 2 inches; the discharge is controlled by means of a stopper.

From the start, it became evident that a covering layer of slag had to be used. The perfecting of this slag layer resulted in the use of synthetic products which, among other things, contain SO2, AlrOa, CaO, and Na2C0,, with well-defined melting points and size consist. The effects are thermal insulation, lubrication of the mold walls, and a cleaning effect due to the trapping of impurities.

Molds. Studies undertaken with regard to corner cracks forced our attention to the perfection of solid molds, cast in alloyed copper chromium-plated, of which the underformability and perfect geometry are. the principal characteristics. By this means, corner cracks have been eliminated.

Cooling. The primary cooling determines the equality and thickness of the external zone of original solidification, which plays a primary role in the definition of the quality of the cast products and, in particular, their tendency to deformation.

The operating conditions of the secondary cooling are closely allied to withdrawal rates and to the type of the steel to be cast. These coilditions intervene in the appearance of external and internal cracks, and alter the depth of the cone of solidification and, as a result, the quality of the "equiaxis zone" of billets and blooms.

Casting Speed. The withdrawal rate has an important influence on the solidification phenomenon and conse-

Fig. 3-Schematic plan of the process of casting in continuous casting with immersed tube and powder.

quently on the quality of the cast products; we have had to search for the optimum conditions in accordance with the operating demands of our continuous casting installation. With the exception of a few special variations, we now operate with a casting speed of 67 inches per minute for 5- by 5-inch billets, and 28 inches per minute for 8- by 8-inch blooms. I t must be understood that the search for correlation between withdrawal rates and cooling conditions is imperative in all cases.

Cooling of Billets and Blooms after Casting. To obtain optimum quality and to avoid the possible formation of shrinkage cracks, we practice controlled cooling of product from continuous casting. With our present equipment, the aim exit temperature is from 160" to 85OoF, reached after approximately 16 hours.

Transformation of Continuous Casting Products After inspection and conditioning, our 5- by 5-inch

billets are rolled to rounds, hexagons, and squares of diameters between 3'4 and 2-inches. Specifically, in the case of rounds, the reduction ratio varies from 6 to 600.

The conditioned 8- by 8-inch blooms are rolled di- rectly into products from 2 to 4 inches in diameter on our 22-inch mill (reduction ratio from 6.3 to 25). In certain cases, they may be rolled down to 5-by 5-inch billets for re-rolling on our continuous mill.

Yield Yield calculations are always very complex. The few

figures cited relate only to results obtained from starting with our 4-ton ingots. The percentage of average yield of liquid steel and conditioned billets is:

Rolled ingots to 5-by 5-inch 81.3% Continuous casting 5- by 5-inch billets 94.6% Continuous casting 8- by 8-inch blooms 94.1% Continuous casting: 8- by ?-inch re-rolled to

5- by 5-inch billets 89.8%

Production The current production of our steel works is about

500 tons per day, of which 85% goes to continuous casting. The breakdown for the two sections is: 70%, 5- by 5-inch billets; 30%, 8- by 8-inch blooms.


Table I. Variation of the Chemical Composition and of the Mechanical Characteristics in 5- by 5-in. Billets Coming from the Same Strand

(Grade 38 CD4)

Mechanical Characterlstlcs Chemical Analysfs Oil Quenchlng 8500-Temperlng 4500C

C Si M n 8 P N1 Cr Mo R E A % Z KIM KtM

Start 0.39 0.24 0.68 0.010 0.019 0.34 0.96 0.22 146.0 136.7 7.0 30.5 5.5 3.4 0.38 0.26 0.68 0.011 0.018 0.33. 0.96 0.23 145.6 136.1 6.5 30.5 5.7 3.4 0.39 0.25 0.69 0.010 0.019 0.35 0.94 0.22 145.7 136.3 6.5 30.5 5.2 3.2 0.38 0.24 0.69 0.011 0.020 0.35 0.95 0.21 145.0 135.6 7.5 30.5 5.5 3.2 1 8::: 0.24 0.70 0.009 0.020 0.35 0.95 0.23 145.8 135.4 7.5 30.5 5.7 3.2

0.24 0.68 0.011 0.018 0.35 0.94 0.21 146.8 137.5 7.0 30.5 5.5 3.2 End 0.39 0.26 0.69 0.012 0.020 0.34 0.95 0.22 1457 136.4 6.5 30.5 5.7 3.4;;

Table II. Variation of the Impact Resistance (Mesnager) in Relation to Cast,Method and Rolling Rate

0 1 1 Quenching Impact Resistance, Kgf/cma 85WC Transversal

Grade Temperfng at Cast Method , Rolling Rate Langltudlnal Transversal Longitudinal

Ingot 4 T.200 1:56 35 CD4 250°C C.C. 8" x 8" 1:10.56

C.C. 5" x 5" 1:3.56 Ingot 4 T.200" 1 :56

38 C4 650°C C.C. 8" X 8 1: 10.56 C.C. 5" x 5" 1:3.66 Ingot 4 T.200 1:56

16 NC6 - C.C. 8" x 8" 1:10.56 C.C. 5" x 5" 1:3.66

QUALITY LEVEL OF CDNTINUOUS CASTING STEEL The first metallurgical studies undertaken on contin-

uous casting products revealed that this technique is characterized by a certain number of peculiarities which may be explained, for the major part, by the very special kinetics of solidification of liquid steel.

In effect-and with reference to the normal technique of casting many tons in strands-the extremely rapid solidification of the steel in continuous casting strands, the modification of the mechanism for the separation of inclusions, the shrinkage of the steel at the bottom of the cone of solidification, the distinctly low reduction ratio of finished products, and so forth, necessarily in- duce differences and appearances which might surprise metallurgists who are not acquainted with these products.

Metallurgical Quality Advantage of Continuously Cast Steel Continuous casting is considered an interesting tech-

nique for the production of high-grade alloy and carbon steels. The homogeneity of the products gives the steel a very close response to heat treatment.

At the present time, judiciously selected and carefully executed heat treatments are replacing the action of alloying elements more and more. The homogeneity factor plays a very important role. Tests carried out through chemical analysis, through hardening tests (Jominy tests, for example), through hardening and tempering, and through case-hardening, of different samples (top, bottom, and center of strands 1, 2, 3, or 4) of a 40-ton cast confirm this fact perfectly (Table I ) .

In addition, numerous studies have confirmed that the use of the continuous casting technique leads to the improvement of two characteristics: fatigue and impact strength (Table 11). In the case of impact strength, there is an improvement in the ratio between the transverse and longitudinal properties. This phenomenon is particularly interesting in the case of improved machinability, where the introduction of elements facilitating machining generally brings about a lowering of transverse impact strength. This conclusion is valid especially in the case of steels containing 0.030 to 0.040% sulfur, which the automobile industry uses more and more.

Surface Defects Since the production of the first casts, we have estab;

lished that the surface of billets and blooms presented defects whose nature and importance could cast some doubts on the quality of the finished, rolled products. This fact obligated us to employ a very careful inspection and conditioning. These methods prohibited us from direct rolling of as-cast billets and blooms for the manufacture of high-grade alloy steels.

Pinholes. The pinholes are created at the moment of solidification by the release of gas whose escape is made impossible during casting conditions. These pinholes appear in the form of round cavities of varying dimens- sions on the surface of rough cast products; they appear as dark colored lines on rolled bars. The causes of this phenomenon are hydrogen and carbon monoxide. We control the amount of hydrogen in the liquid steel ankl obtain satisfactory results with less than 5 ppm. The replacement of lubricating oil, with a completely de- hydrated synthetic slag, has also contributed to tlie reduction of these defects.

From our experience, the possible role of carbon monoxide is probably greater than that of hydrogen in the creation of pinholes. It is a question of a deoxidation problem which is easily resolved by using :about 0.015% Al. Under this amount, the results may vary; they depend on the amount of carbon in the metal and the kinetics of deoxidation. With a complete deoxidation with calcium silicon. in the ladle,. we have been able, for example, to obtain excellent results with carbon or low-alloy steels (0.25-0.30% carbon) using amounts of aluminum ranging from 0.008-0.012%. Silicon alone, in normal amounts, is not able to stop pinholes.

Surface Inclusions. The, frequency and importance of surface defects arising from inclusions are undeniably greater than those of 'pinholes, and their suppression necessitates greater efforts. These inclusions are generally a Ca-aluminate base, encrusted in the metal skin. There are two possibilities for their origin: descent of inclusion floating on the surface of the meniscus between the liquid steel and the mold wall and/or the formation of inclusions during solidification.


Fig. k l n f l u e n c e of casting temperature on the surface inclusions index.

The exact manner for the formation of these "refractory" impurities is very complex. It is important to em- phasize the variations which exist between grades of steel: The manganese-silicon steels systematically give excellent results, whereas "refractory" impurities are frequent in chrome-manganese steels with 0.20% carbon.

After a few years of operation in "direct casting," we ended by using the technique of an immersed tube and a covering layer of slag. The improvement obtained was extremely spectacular, thus confirming the neces- sity to closely control the following factors: protection of the liquid steel against re-oxidation during casting; a steady flow of steel; and maintenance of an insulating layer a t the surface of the steel.

The introduction of this method produced a radical improvement in the surface quality which we measure by the variations in condition yield (average for all grades): blooms (8 by 8 inches), from 92% to 98%; and billets (5 by 5 inches), from 96% to 98%. The production rate in the conditioning department has similarly improved. We are currently examining the effect of casting temperature and amount of aluminum.

Fig. 4 is a histogram relating to the influence of temperature. In the case of "good" casts (rollable without complete conditioning of the surface after a visual inspection and the spot repair of descaled product), we note a greater tendency towards the higher temperatures, whereas the majority of the "bad" casts, which necessitate complete conditioning because of numerous "refractory" impurities, are cold.

Fig. 5 shows the probability of having surface defects is much greater with amounts of aluminum in excess of 0.020%; this confirms the possibility of re-oxidation of this element and the difficulties of separation when casting.

External Cracks. The longitudinal external cracks are very serious and generally result in the scrapping of the billet or bloom having this defect. This phenomenon is increasedi in the case of steels containing amounts of sulfur. in excess of approximately 0.020%. The pre-

Fig. 5-Influence of aluminum content of the steel on the surface quality.

cipitation of interdendritic sulfides results in brittleness and explains their formation.

At the time of the studies we also found a correlation between these defects and mold quality: we have been able to resolve the problem by using solid cast molds, which are practically impossible to deform and which have a perfect geometry.

Internal Defects Contrary to surface defects, the possible internal de-

fects which may be found in continuous casting products are difficult to remove during the conditioning process. We must avoid their information at the moment of solidification, that is, a t the actual time of casting.

In the following sections, we shall try to summarize these internal defects, their nature and origin, and give possibilities for their elimination or their reduction.

Internal Cracks. The peculiar solidification of continuous cast steel and especially the interdendritic segregation of sulfides are the principal causes of internal cracks. These cracks are situated either in the zone under the thin solidification layer or in zones fur- ther away from the surface (Fig: 6). From our experience, the following factors are the cause of these defects: chemical composition of the steel and amount of sulfur > 0.015%; fast casting speed and high casting temperature; excessive secondary cooling; and align- ment of support rolls and pinch rolls.

We have obtained very good results in the production of steels with 0.020-0.040 sulfur by decreasing the casting speed and the quantity of water in the secondary cooling, particularly in the first zone. At the same time, i t is very important to have a strict control of the mechanical withdrawal system (rolls, etc.), to avoid the deformation of products which are not completely solidified. This is why, in our opinion, a vertical installation presents an advantage over "curved" withdrawal or "curved molds."

Central Porosity. It is well known that the shape of the solidification cone and the shrinkage of the steel at the moment of complete solidification can produce


A. Sulfur point. 6. Acid etch~ng

Fig. &Internal crocks on a continuous casting sample slide.

a certain central "porosity" whose naturc and importance depend upon the operating conditions, particularly on the withdrawal rate and the casting temperature. Besides these two factors, the castability and the vis- cosity of the metal affect the formation of "bridges of solidification." We have found, for example, relatively important accidental "central holes" in steel with a high content of aluminum and cast at a very low tempcra- ture and high withdrawal rate. In this case, the bottom of the solidification zone extends itself considerably, creating "bridges" whose supply of liquid steel becomes difficult, if not impossible, and necessarily produces shrinkage at the moment of solidification.

In practice we recognize two types of defects (Figs. 7 and 8) :

"Cloud": These are numerous microcavities dispersed in a diameter of approximately 1 inch which came from the steady shrinkage of the metal at the moment of solidification. This type of defect welds perfectly when the metal is rolled with a reduction ratio in excess of 6 and it does not present any difficulties in the final use of the steel.

"Hole": This is a single central hole of variable diameter whose existence is due more to the "bridges" of solidification rather than to the shrinkage of the steel. This defect practically disappears with reduction ratios around 10.

We must state that it is always a question of nonoxi- dized defects, the elimination of which, rolled or forged, is perfectIy possible because of the fact that a layer of oxides does not oppose the perfect weld of crystals between them. We note, in this connection that tests are being carried out with IRSID, using an electromagnetic stirrer device. This device ought to permit the reduction and even the suppression of axial porosity.

Segregation. Continuous casting assures an advantage with regard to segregation between "top and bottom" and between the different strands.

A possible segregation in the section apparently worries some metallurgists who are not accustomed to the structure of continuous casting products. In ccr- tain instances, a phenomenon of segregation can be

created at the bottom of the solidification cone at the time of precipitation of crystals in the "equiaxed" zone. Trials undertaken show that it is a question of "positive" segregation with greater amounts of elements such as sulfur, phosphorus, and manganese, but the degree of segregation does not exceed the level which would cause difficulties in the use of the steel. We have never had a problem with the users of our steel for this reason.

Based on our experience, trouble with the withdrawal system and, in particular, the mechanical stresses on steel which is not completely solidified, can produce an exaggerated segregation; this is illustrated in Figs. 9, 10, and 11. This factor must therefore be watched very carefully in order to avoid possible difficulties.

We must talk about the macroexamination taken lengthwise and crosswise on continuous cast products. Examination of these structures often induces discussions among metallurgists and brings about reactions from technicians who are only acquainted with the normal appearance of products rolled from ingots. The difference is related to the solidification of continuous casting steel. The mechanics of solidification, which occur in an ingot of several tons, can just as well lead to the formation of segregated zones. However, in this case, the greater amount of rolling necessarily required smooths out these imperfections even more. In any case, we do not think-and we are speaking about our own experience-that these imperfections may be a t the beginning of possible difficulties for heat treatment or from the point of view of resistance to fatigue.

Inclusions. The problem of nonmetallic inclusions in continuous casting steel requires much more extensive study, because of the fact that, in comparison with the process of teeming into normal ingots, the separation of inclusions follows a different route. As has been demonstrated in a study by IRSID on our installation," with the aid of radioactive tracers, convection currents oppose themselves and lead to the entrapment of inclusions in the body of the steel (Fig. 12).

We have undertaken numerous studies to examine this question; its full description is not possible within the general framework of this paper. Therefore, we limit ourselves to the presentation of general conclusions.


A. Sulfur point. A. Sulfur point.

:4. , ., - ,

<% u. k:: : :- , . I . . P&;k2:,::- - A.

$<.+ ,q . 2. . . .I. .. &*:;.&;,:,;:;.r il, - .. .. I . : , : . Y : ' ' , q&#;$if;;Gi ; ,; 2 . . : ..*: r -

6. Acid etching. B. Acid etchi'ng.

Fig. 7-The "cloud" a t the center o f a continuous casting billet. Fig. &The "hole" a t the center of a continuous casting billet.

Obtaining a very pure steel from the point of view of oxide inclusions requires a painstaking manufacturing process with the use of deoxidizing additions, such as carbon, silicon, manganese, aluminum and calcium, in order to obtain a level of oxygen in the order of 40 ppm in the liquid steel in the ladle. Even when insisting on painstaking production methods, we feel that the main problem is at the cast, between the ladle and the tundish, on the one hand, and the tundish and the mold on the other. I t is at these locations that the protection of the steel against reoxidation must be assured as well as the separation of inclusions formed during the cooling of the steel. These points are so important that, without precautions during the casting cycle, we cannot see the use in the application of a more complex refining process, such as ladle degassing or manufacturing under vacuum.

Studies, in collaboration with IRSID,' by means of radioactive elements (cerium) which we used to iden- tify the aluminum additive in the steel, have shown that the quantity and the nature of the inclusions are the same (Fig. 13) in continuous casting as in the case of steel teemed in ingots and that it is only the reduction ratio that has any effect on the size of the inclusion in the finished product (Fig. 14).

We have confirmed the formation of aluminum-based inclusions on the walls of the nozzle, which acts as a trap for inclusions formed by the reoxidation of steel and turbulence of the metal.

Chemical and radiocrystallographic analyses show that these deposits are composed of magnesium-aluminate (Spinel Also,. MgO) or lime aluminate ( 2 ALO,. CaO and 6 AJO,. CaO); in a few cases we have also found small amounts of pure alumina.


A. Sulfur point. A. Sulfur point.

8. Acid etching. 8. Acid etching.

Fig. 9-Appearance of products obtained with too much pressure on Fig. l&Appearance of products obtained with too much pressure on tho withdrawal rolls. Transverse section. the withdrawal rolls. Longitudinal section.

Fig. 11-Appearance of a + 1% in, rolled from the billet shown in Figs. 9 and 10. 6.


Fig. 12-Schematic pla,n of the solidification cone and the convec- t ion currents i n a 5- by 5-in. bil~let.

- -

Fig. 13-A. 50X.

Fig. l&Micrograph of a line of alurninaus i~nclusions. B. 500X.

Fig. l&Autoradiograph of the axial region of the longitudinal section of a + 1% in. MM round produced from the same cast but submitted to a different rolling rate. Left: Reduction ratio 7 (continuous casting). Right: Reduction ratio 150 (ingot).

Continuous Casting 11-Billets and Blooms 153

As has been noted in other studies,' these deposits cause blocking of the nozzle and stopping of the flow. There is also a risk of these deposits becoming loose and being drawn into the flow of the liquid steel; the separation of these macroscopic inclusions is difficult. One often finds them again in the interdendritic spaces.

Aluminum presents the greatest risk as a deoxidizer because of its affinity for oxygen and its natural disposi- tion towards the formation of inclusions of considerable size. On the other hand, this element presents certain advantages; it is efficient for the time of deoxidization, and it produces a fine-grained steel under economic conditions. In addition, it has a protective effect on the lining of the ladle and tundish refractories. Under these conditions, the suppression or replacement of aluminum presents all kinds of difficulties; the metallurgists is obliged to live with this element. In our opinion, this is possible by maintaining a level of 0.025-0.030% aluminum in the solid steel, by avoiding reoxidation during the casting time, and by facilitating, as much as possible, the separation of inclusions coming either from the accumulation of very fine inclusions or from in situ formations.

Continuous casting steel does not present problems of microscopic inclusions. The steel matrix remains very clean and can comply to the most exacting requirements.

MANUFACTURE OF HIGH-GRADE ALLOY AND CARBON STEELS BY CONTINUOUS CASTING

After considering all the factors outlined previously, we think that continuous casting can be applied to the manufacture of high-grade alloy and carbon steels. At the present time we produce the following grades:

1. High-grade alloy steels with 0.30 to 0.80% carbon content 2. Steels for mechanical engineering uses:

Chrome (AISI 5131-5140) Chrome-molybdenum (AISI 4118-4150) Nickel-chrome (AFNOR 16 NC6-20 NC6) Chrome-manganese (AFNOR 16 MC5-20 MC5) Silicon-manganese (AISI 9255-9262) Nickel-chrome- (AISI 8617-8640)

molybdenum etc.

3. Certain special grades whose total amount of alloying additives do not exceed 4 to 5%.

We carry out a rigid inspection of the products before they are put to manufacturing uses. We believe, at the present time, that we have not yet reached the stage offering total guarantee of surface quality; in particular, we believe that the "direct" rolling of rough continuous '

casting products, without preliminary inspection and appropriate conditioning is not possible.

Inspection of Products Before Conditioning In practice, we take two trial billets (or blooms) per

casting strand and we scarf them very rapidly (loss of approximately 3% on 5- by 5-inch billets). The goal of this scarfing is to bring surface defects into evidence. The products are carefully inspected; ruptures and "refractory" impurities of different sizes are counted separately to establish an index. If the index obtained in this manner is inferior to defined standards, we un- dertake a unique inspection operation after shot blasting with spot repairs of possible defects. If the index is too high, we recommend the necessary scarfing of the total production of the strand, followed by visual inspection and repair of defects still present. The control department follows the execution of the conditioning closely; products showing anomalies are either removed or ap- proved with restrictions (for use in lower quality products, compulsory scarfing, reconditioning after rolling, etc.). It must be noted that, no matter what the cleanliness index, for certain critical uses (cold forging, for instance) the products are entirely conditioned.

In addition to surface inspection, the internal quality of the cast products is also inspected. We cut off two

small sample plates from each strand; these samples are submitted to an acid bath and subsequently ex- amined. One then determines the possible ,presence of internal cracks, abnormal axial po;osity, etc. Unfavor- able results cause com~ulsorv restrictions. the reiection of the billet or of the eit ire &and productibn.

The final inspection deals with both the surface aspects of the rolled products and the search for possible internal defects by lathe tests, acid tests, crushing tests, etc.

In a general way, the inspection categorizes each cast whose development through continuous casting has given rise to difficulties (breakouts, frozen nozzles, incorrect temperatures, etc.) or whose inspection results if the semi-finished products have not been entirely satisfactory. It is our opinion that these :precautions are necessary in order to achieve a quality that com- plies with the exacting requirements of the users of steel for mechanical engineering purposes. -

USES OF STEELS MANUFACTURED BY SAFE'S CONTINUOUS CASTING

We do not sell as-cast continuous casting billet products. The finished rolled products undergo, in all cases, a reduction ratio greater than 5 and up to approximately 600. Ninety percent of our steels are applied'in automo- tive parts. In the following discussion, a few examples are given of the most recent applications. .:

Hot Forging For a rolling rate in excess of 5, the largest sections

produced are rounds from @ 2 inches to @ 2% inches rolled from a 5- by 5-inch billet and rounds of @ 3% inches from 8- by 8-inch blooms. As many examples show, the use of continuous casting products; partially rolled or not at all, is possible in certain particular cases (forgings, for instance) where one replaces rolling by forging. On the other hand, for the majority of forged parts, it is possible to assure a certain homogeneity throughout the part by a preliminary rolling. In effect, the mechanical characteristics vary greatly under relatively low reductions, as shown in Figs. 15 and 16, pub- lished by Engelmann and his associates.'

L I I I I I I I 0 4 ' h / I / . 41. 41. '1. '1.

UWCTIOII RATE

Fig. I s v a r i a t i o n of the mechanical characteristics in relation' to rolling rate.'

Starting, therefore, with a product which is partiaily rolled or not at all, for a part of which certain pieces are only slightly forged, one risks obtaining some heterogeneity between different points. On the other hand, if the forging assures a reduction greater than 5, using rough products or products only partially rolled can be considered, providing that the stamping of the blanks can be done under good conditions. We have noticed, in fact, that cold shearing is easier with products adequately rolled.


Fig. I b V a r i a t i o n of the impact resistance in relation to rolling rate?

Fig. 17-Photo and macrograph of a spindle and the corresponding hot-forged blank. Chrome-manganese steel rolled 6 2% in. (from 8- by 8-in. blooms).

Among the numerous hot-forged parts manufactured, we shall cite a few examples by showing a photo, the macroexamination of the blank, and the parts forged (Figs. 17, 18, and 19) .

Cold Heading and Forging The spectacular development of the cold-heading

and cold-forging technique is fully justified by the

Fig. 18-Photo and macrograph of a hot-forged fork joint from chrome steel (37 C4) @ 1% in. (from 5- by 5-in. billet).

Fig. 19-Photo and mocrograph of a hot-forged primary axle chrome- molybdenum steel @ 3Y2 in. (from 8- by 5-in. bloom).

Continuous Casting It-Billets and Blooms 155

KT-

Fig. 20-Photo and macrograph of a cold-forged part. Carbon steel (from 5- by 5-in. bloom).

Fig. 22-Parts obtained by cold heading from continuous casting steel.

For cold forging, we first cite the finest example of a three-cusped joint which is produced from a "tripod" shape, of special section, having a maximum diagonal of 3 inches. These shapes are rolled from 8- by 8-inch blooms of carbon steel. The outline of the rough shape and the finished piece, as well as the macrographs, are presented in Fig. 20. This forging, which has been produced approximately two years, has never given difficulties.

Another part forged from 1%-inch rounds, produced from 5- by 5-inch billets of nickel-chrome steel, is a pinion (Fig. 21).

In cold heading, our experience is related to the use of carbon steel wire, chrome molybdenum and chrome- nickel having undergone reduction ranging from 40 to 600 from 5- by 5-inch billets. Fig. 22 shows a few examples. Keeping the reduction ratio in mind, the "central porosity" possibly existing in the rough products is completely eliminated and we have not run across any difficulties for this reason.

On the other hand, it is important to assure ourselves of the good condition of the surface conditioning, in order to eliminate all defects which might produce burst-

Fig. 21-Photo and macrogroph of o cold-forged pinion chrome- manganese steel + 1 % in. (from 5- by 5-in. billet).

economic savings it produces, and it is normal that steelmakers follow this trend. Paralleling this development, the requirements in matters of quality of steels become greater; it requires a structure which is perfectly spheroid, very ductile, and without superficial or internal defects.


ing during cold stamping. It is for this reason that we have declined to produce Al-killed low carbon (C 764 0.15) wire steel in continuous casting up to now; this grade presents undeniable risks in matters of surface and subsurface nonmetallic inclusions.

I

1 Machinability 'A portion of our continuous cast steel is always ear-

marked for machining applications either from the rough stock or after cold working. Generally, there is no problem with surface conditions, except when draw- ing. In fact, this method of cold working aggravates the surface defects and needs the use of good surface quality steel.

Besides the low steels with higher S-content which we have outlined, we also produce leaded steel for use in the manufacture of high-speed, mass-produced ma- chined parts.

I Other Products In the manufacture of all kinds of springs, manganese-

silicon steels lend themselves to continuous casting very well.

We also manufacture steels with high limits of elas- ticity which are used for manufacturing chains; these .products demand surface conditions without seam defects in order to avoid any cracking at the time of bending and treatment of the links.

Tool steels ranging from 0.7 to 1.2% carbon have been produced.

With regard to steels for ball bearings, we have obtained very good results in producing steels in which the toughness and wearing performances are generally higher than for steels produced under normal operating processes. On the other hand, the special macroexamination appearance of the central zone on rolled products prohibits the manufacture of ball bearings in the final inspection. These show, in fact, polar zones of different

I complexions.

GENERAL CONCLUSIONS After 10 years of experience and research, we can

now say that the objectives of SAFE have been reached. We produce industrial steels by means of the continuous casting process and we can assure a level of quality equal to that of steel produced by other processes.

Metallurgically, the studies and experiences have shown that continuous casting steel, thanks principally to its homogeneity and its particular structure, has three pfincipal advantages: (1) good machinability, (2) pre- cise response to heat treatment and very low deformation in hardening, and (3) improvement in the resistance to fatigue of parts.

'.As we have outlined in this paper, numerous difficulties have been mastered in order to meet the demands of our customers regarding quality. We are aware of the problems which still exist.

Table Ill. Possible Defects in Continuous Products

In summarizing the factors which may affect obtaining optimum quality (Table 111) , one must understand that "ideal" operating conditions include many compromises. The addition of aluminum is, for instance, necessary to suppress pinholes; but, over a certain limit, it induces the formation of surface and even internal inclusions in the metal. A high casting temperature is desired to en- sure a steel free of oxide inclusions; on the other hand, it is disastrous from the point of view of cracks, segre- gations, breakouts, etc. Taking into account the fact that the realization of these optimum operating conditions depends upon the combination of the parameters and the grade of steel in question, it would be necessary to assure perfect and appropriate controls, which are not realizable in all cases, due to industrial operating conditions.

Under these conditions, the use of a continuous casting plant, within the framework of producing a range of high-grade alloy steels, forces on us:

(1) A team of highly qualified technicians to maintain the parameters of the plant and to promote the unceas- ing improvements which must still be made in the continuous casting technique.

(2) Appropriate and original inspection methods and methods of conditioning adaptable to particular and dif- f erent cases.

(3 ) Permanent study and research towards constantly improving the overall results.

It also seems to us that it is of prime importance to clearly define all the possible end uses of continuous casting this necessitates having a thorough knowledge of the use of the products and a close and active asso- ciation with our customers.

Among the ideas guiding our present research, we must mention perfecting production methods, greater protection of liquid steel against oxidation, obtaining optimum casting conditions, complete elimination of axial porosity, and finally reduction of completely solidified products with or without heating. The casting of sections greater than 8 by 8 inches presents advantages both in manufacturing and in the matter of quality, whereas the reduction permits rapid and economic casting by the simplification of the conditioning of products having been reduced.

In conclusion, we must again take up the problem of the "peculiarity" of continuous casting. It is, in fact, a metal whose appearance, studied by macroexamination, tends to be different relative to products produced by normal processes. This is perfectly normal when taking into consideration the solidification and the res~ective reduction ratios. However, to our knowledge, this peculiar "Appearance" has never been the cause of any difficulties and we have yet to see any laboratory trials showing any possible disastrous influences on the mechanical characteristics. On the other hand, the advantages of continuous casting products-their chemical homogeneity, their susceptibility to heat treatment and impact tests-have been sufficiently demonstrated for this method to be taken into consideration in the estab- lishment of overall plans.

We do not ignore the fact that the end use of continuous casting steels often starts discussions among metallurgists, but we feel that the final answer can only come from its practical industrial results.

REFERENCES 1 Misson, M., Ingdnieurs de I'dutomobile, Vol. 36, No. 6, 1962, pp.

319-327. S ~ i s s o n , M . and Pomey, J . , Rev. Met., Vol. 61, 1964, pp. 303-309. :' Gosselin, P., C. I. T . , No. 11, 1961. ' Poppmeier, W . and Tarmann, B., Rev. Met., Vol . 65, 1968, pp.

113-119. - -

J K o h n , A. , Wanin, M. and Arnoult, J . , Rev. Met., Vol . 66. 1969. "Kohn . A. , Wanin, M.. Arnoult, J . , Thomas. R . and Backer. L.,

Rev. Met., Vol. 66, 1969, pp. 325-339. 'Duderstadt, G . C., Iyengar. R. K . and Matesa, J . M., Electric FUT-

noce Proceedings, TMS-AIME, Vol . 25, 1961, pp. 61-66. "ngelmann, W . , Voss, H . and Kolb, R. , Stahl und Eisen, Vol . 81.

1961. pp. 1020-1030.

Continuous C a s t i n g Il-Billets a n d Blooms 157

DISCUSSYON

by N. L. Samways

It is a great pleasure to have the opportunity of dis- cussing Messieurs Backer and Gosselin's paper on the metallurgy of strand cast steels.

.First, however, I would like to express, publicly, the appreciation of many of us who have visited your plant throughout the last ten years. We thank you for all your help, advice, and patience. Our visits were invariably most interesting and enjoyable. We have benefited im- measurably from your hard work and perseverance in developing techniques and practices for the billet casting of quality steels. I think it is true to say that you have pioneered the way for casting aluminum-killed steels through the use of stopper-rod-controlled oversize nozzles with shrouds and mold fluxes. This technique .is in wide use throughout the world today.

There are three items in your excellent review on which I would like to comment: (1) temperature control, (2) segregation, and (3) induction stirring.

Temperature Control Your aim temperature in the tundish is stated to be

20°C (36°F) above the liquidus; I would be interested

N. L. SAMWAYS is assistant works chief metallurgist, Jones & Laughlin Steel Corp., Aliquippa Works Div., Aliquippa, Pa.

in your comments on (a) the major metallurgical benefits obtained from low casting temperatures; (b) your success in meeting this aim, both from cast-to-cast and within a cast; and (c) the major control factor in- fluencing temper~ture control, apart from tap temperature.

At J&L we are currently making a study of factors affecting temperature in the tundish and there appear to be three major significant factors: (a) slag thickness; (b) number of heats on the ladle, which is a measure of refractory thickness; and (c) ladle preheat.

Segregation In your paper you imply that segregation has not been

a problem, provided that precautions are taken to avoid mechanical stresses on the steel during solidification. I would welcome your comments in more detail on the types of problem that you encountered with the withdrawal system.

Electromagnetic Stirring You refer in your paper to cooperative trials with

IRSID involving the use of an electromagnetic stirrer. This is a very intriguing development both for minimiz-is ing axial porosity and possibly also for minimizing segregation. Would you be kind enough to give us a brief description of the results to date?

Thank you again for your excellent review.

A UTHORS' REPL Y

Temperature Control (a) The advantage of relatively low casting tempera-

tures is the improvement of the internal quality of steel (central porosity, internal cracks, sulfides segregation), the reduction of the breakout risk, and a larger safety about internal cracks. In addition, it is true that low temperatures are less favorable to obtaining a surface aspect without inclusions.

(b) The realization of "ideal" casting temperatures for 1969 was the following (%of cast) :

( 10% 1 25% 1 42% 1 16% 1 5% 1 -2O'C -1O'C Ideal tern- + 10'C + 20'C - 66'F - 50'5' perature of + 50'F + 68'F

casting

During a continuous casting the temperature reaches its maximum value after about 10 minutes and then decreases of 10°C max. until the end of the casting (60 minutes).

(c) The fine precision of the casting temperature is obtained due to the precautions taken during every operation: first in the furnace, where the electromagnetic stirring gives an homogeneous temperature of the bath in its whole mass; then at the continuous casting where the compact form of the tundish permits a big reduction of the losses. Also, the shape of the ladle, its lining with silico-aluminous bricks, and the important layer of slag which covers it, contribute to decrease the loss of temperature.

Segregation In our paper we cited the problem of external stresses

which risk an increase in the internal segregation if they occur before the complete solidification of the liquid heart.

Figs. 9, 10, and 11 correspond to research experiments and not to an usual industrial running. In fact, we have crushed, with a pair of pinch rolls at about 15 feet under the mold, on a liquid heart of 2 to 2 23/64 inches of diameter.

In the usual running, such accidents cannot occur, but a bad adjustment of the withdrawal system and particularly of the guide-rolls under the mold, can form less important but nevertheless visible defects on rough or rolled products.

For that reason, we try to eliminate all the possible causes of deformation of the bloom or billet walls before complete solidification of the liquid heart. We have, therefore, suppressed almost all the guide rolls. The vertical continuous casting permits and elimination of guiding.

Electromagnetic Stirring We are in the first step of our experiments with an

electromagnetic stirring which will soon be described in conjunction with the IRSID engineers. From the first results, the electromagnetic stirring seems to decrease the central porosity and the importance of the inter- mediate dendritic zone, while it increases the central equiaxial zone. Our actual tests consist of the exact fixing of the stirring position, the determination of the frequency and the length of the stirring, and the deeper study of the obtained structure.

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