GB2040197A - Continuous cast steel product having reduced microsegregation - Google Patents

Abstract

Steel is continuously cast in a mould comprising a grooved wheel (10) and an endless flexible belt (11) with cooling by sprays (S1 to S4). The casting and withdrawing of the bar and the cooling of the mould and bar are controlled to eliminate microsegregation in the bar and to provide a very uniform distribution of constituents and impurities, in particular oxygen, carbon, sulphur and manganese, in transverse section. The bar has improved properties for subsequent processing, in particular higher tensile strength. <IMAGE>

Description

SPECIFICATION Continuous cast steel product having reduced microsegregation The present invention relates to a novel cast steel product, and particularly to a novel continuously cast steel product having a uniform distribution of constituents, and the method of producing said product.

A desirable property of steel in its as-cast condition is a uniform distribution within the steel product of the constituents and impurities normally found in steel. As used herein, what applicants believe to be the meaning of these terms is their standard meanings in the art, that is, "constituent" means one of the ingredients which make up a chemical system or a phase or combination of phases which occur in a charactertistic configuration in an alloy microstructure, while "impurities" means elements or compounds whose presence in any material is undesired. Constituents, as used herein, then, would include the materials combined into a chemical system to produce the particular type of steel being cast but would not include the impurities, or undesired elements or compounds present in the cast metal.In any case, segregation of the components in the cast steel makes it less suitable for subsequent processing such as forging or rolling into rod and then drawing into wire. As used herein, the term "segregation" also has what applicants believe to be its normal meaning in the art, that is, segregation is a term used in reference to the non-uniform distribution or concentration of constituents (or impurities) which arises during the solidification of the metal. A concentration or accumulation of impurities in various positions within the metal is, for example, referred to in the art as segregation. The segregation that occurs between the arms of dendrites is referred to as minor or microsegregation and thus the composition may vary within a single crystal.Macrosegregation occurs around primary or secondary shrinkage cavities, such as pipe and in similar regions, and is often revealed as marked lines, having a pronounced erect or inverted cone shape, which are made evident when the ingots are sectioned and etched. Zones of segregation tend to occur in the middle regions of the casting and usually within that part mainly occupied by equiaxed crystals. Microsegregation may sometimes be overcome by annealing, but macrosegregation persists through subsequent heating and working operations. So-called pipe segregates occur around the pipe cavity.In normal segregation in steel, the constituents (solute) in the iron (solvent) rejected from the freezing liquid accumulate at the advancing solid/liquid interface so that the constituents of lowest melting point concentrate in the last portions to solidify, but in inverse segregation this is reversed, for the liquid with high solute concentration becomes trapped in between the dendrites thereby causing a decrease in concentration of solutes from the ingot surface toward the center. Inverse segregation, then, is a concentration of constituents or impurities to a higher degree near the outer surfaces (as compared to the interior) of an ingot or casting.

Prior art methods of casting steel have provided cast steel products having a relatively high degree of segregation of impurities and alloying materials within the cast steel. Because of the high level of constituents and impurities in steel, inverse segregation normally occurs. Such uneven distribution of impurities and/or constituents within the cast steel makes it desirable that the total amount of same within the steel be reduced so that subsequent processing of the cast steel does not result in unacceptable internal and surface characteristics in the product manufactured from the cast steel.A reduction in the total amount of impurities, however, usually requires expensive additional refining of the steel prior to casting and is sometimes commercially unfeasible or impractical altogether while sometimes the addition of particular constituents (including alloying elements) is desirable or necessary.

Among the impurities and alloying elements within the steel likely to become segregated in prior art products are sulphur, oxygen, phosphorus, manganese, silicon and carbon. Any significant segregation of same will make it less commercially useful. For example, significant segregation may cause non-uniformity of tensile strength within the steel and make it less suitable for subsequent drawing into wire.Segregation of gaseous impurities may result in areas of porosity near the top surface of the cast product, which, among other drawbacks, causes inferior sheet surface quality. (See Whitmore, B.C. and Hlinka, J.W., "Continuous Casting of Low-Carbon Steel Slabs by the Hazelett Strip-Casting Process", Open Hearth Proceedings, 1969.) Severe microsegregation of manganese will cause problems in many end products made from the continuously cast billet due to its great effect on the austenite to pearlite/bainite/martensite transformations.

For example continuously cast billets rolled into wire rod often have high concentrations of manganese in the core which will promote the local formation of martensite during casting, thus causing frequent breakage during the subsequent wire drawing process.

This problem has long been known in the art but there have been few publications found by the inventors which disclose actual production data. A related study by Hans Van Vuuren of the South African Iron and Steel Industrial Corporation, Ltd. (copy contained on pages 306 to 334 of Steelmaking Proceedings, Vol. 61, Chicago Meeting, April 16-20, 1978.) illustrates one approach to controlling microsegregation and its effects in a final rod product.

Van Vuuren concluded that central martensite could usually be avoided in some steel wire rod by limiting the total amount of manganese to 0.75% maximum, phosphorus to 0.020% maximum, and then controlling the cooling in the cooling line subsequent to the rolling process. There was no mention of the microsegregation values of the continuous cast blooms (ISCOR is believed to start by continuously casting a 205 mm x 315 mm bloom on a Concast Bloom Machine) but the final rod product was analyzed and showed manganese microsegregation values ranging from 101.5% to 139.0% of the base analysis. Because of the extensive thermal diffusion between the time of casting and the time of rolling, it is believed that the microsegregation values of manganese in the cast bloom would have been much greater than in the final rod product.

Up until now the prior art methods of solving problems due to segregation (e.g., wire breakage) involved repairing (e.g., homogenizing) the intermediate products (e.g., rod) instead of avoiding the initial segregation during casting. One reason for this is believed to be that it is much more difficult to control the commercial high volume continuous casting process.

In prior art ingot casting methods, segregation occurs as the molten steel slowly solidifies, the impurities being allowed to float by gravity separation to the top of the ingot. In higher quality applications, a resulting concentrated layer of impurities and/or solidification cavity at the top of the ingot sometimes had to be physically removed (scalped or scarfed) before the cast steel could be further processed (see, for example, "Recent Developments in Machine Scarfing of Continuous Cast and Rolled Steel", Iron and Steel Engineer, January, 1978, pgs. 68-71 and U.S. Patent No.4,155,399, col. 1, lines 61-68.

Methods of continuous casting of steel have been developed to avoid the handling of a large number of ingots and the necessity of removing the top surface layer. In what the applicants consider to be the most material and most commercially accepted prior art method of continuous casting of steel, molten metal is poured into an open ended vertically-disposed mold constructed of a highly conductive material such as copper, within which water is circulated for cooling purposes. As the periphery of the metal solidifies to form a shell of solidified steel adjacent to the mold wall, the strand of steel is slowly withdrawn from the bottom of the mold while molten metal is continuously poured into the top of the mold.This type of process is sometimes referred to as the Jung hans-type or Junghans-Rossi-type of continuous casting system and has been successfully commercialized by Concast A.G. of Zurich, Switzerland and Koppers Co., Inc. of the United States, for example. An early Junghans patent is U.S. Patent No. 2,135,183 (U.S. Class 164-83). Even here, however, a surface may need to be scarfed for certain applications -- see U.S. Patent No. 4,155,399 (U.S.

Class 164-82).

In the Junghans-type process the mold may be vertically oscillated along a short path so that the mold moves with the strand during each downward oscillation to increase the heat transfer during the times when there is no relative movement between the strand and the mold. Such oscillation increases the possible speed of casting but often creates undesirable oscillation marks or rings extending around the casting on the surface thereof.

As the strand leaves the mold, water sprays are normally directed onto the surface of the semi-solid strand to complete solidification thereof. In order to reduce the vertical height requirement of the building containing the Junghans-type casting machine, guide rollers have been utilized to bend the strand through an arc of approximately 90" about a radius of, for example, forty feet, and then to rebend the strand so that it extends horizontally for cutting or subsequent processing. To avoid bending the strand twice in this fashion, and to be able to install the caster in a smaller building, curved molds have been developed so that the strand emerges from the mold conforming to the desired curved path and then is straightened in one bending step to a horizontal orientation for cutting.

A very readable exposition of traditional continuous steel casting is provided in the December 1963 issue of Scientific American magazine, "The Continuous Casting of Steel," by L. V. Gallagher and B. S. Old, Vol.

209, No. 6, pp. 74-88.

It will thus be seen that casting according to prior art vertical mold processes does not rapidly (it usually takes a 5 story building or more) change the orientation of the solidifying steel, and allows molten metal in the center of the strand to, at times, remain in a horizontal attitude as shown in U.S. Patent No. 3,542,115 (U.S. Class 164-82) assigned to Concast Incorporated, for example. Thus, impurities have an opportunity to float upwardly during the progress of solidification and in general, segregation occurs which sometimes may be observed ina (long) transverse section of a prior art 4 inch x 4 inch square bar as a line of segregation occurring about 1 inch in from the inner radius of the strand.

On a research basis, steel has also been continuously cast in relatively horizontal molds, this being performed on twin-belt casting machines similar in principle to the early Hazelett Strip-Casting Corp.

machine as shown in U.S. Patent No. 2,640,235 (mentioned in the Whitmore and Hlinka publication). These two authors reasoned that because of the influence of gravity and the approximately 20 from horizontal attitude of the steel strand during solidification in these research projects, the impurities within the steel tended to rise and form a substantial zone of segregated material near the top surface of the casting. These two authors state that coupled with the top-surface oxide pit condition was an internal oxide segregation noted in macroetch tests of transverse sections. Although the oxide segregation varied in degree, the profile was similar in all slabs cast and the authors concluded that from this data it was apparent that the oxide segregation was severe enough to cause inferior sheet surface quality. A number of possible solutions were tried, such as concentrating on the elimination of mold-pool slag, use of stationary, water-cooled copper edge-dams, use of submerged feed tubes, etc., but the authors admitted that all these attempts at a solution proved fruitless. It was reasoned that heavy concentrations of segregating oxides were trapped in the solidifying slab at the time when the top skin was between 1/2 to 3/4 inch thick. The authors concluded that casting at a 20 angle resulted in a metallurgically unacceptable product for sheet application produced from either Al-killed or vacuum-treated steel because no way to remove these oxides was found and the inclusions segregated toward the upper part of the cast billet.They suggested going back to operating the mold in the prior art (Junghans-type) vertical position as a possibility for overcoming the segregation problem. It is believed by applicants that the off-center and segregated distribution of the constituents and impurities also caused unpredictable variations in subsequent attempts at processing such as hot-rolling of the material.

Generally described, the present invention provides an as-cast steel bar characterized by, when compared to the prior art, a novel lack of segregation of manganese, oxygen, sulphur and carbon. On the contrary, the present invention provides an as-cast steel bar characterized by a particularly uniform distribution of manganese, oxygen, sulphur and carbon.The cast steel bar of the invention comprises steel that, when viewed in (long) transverse cross-section for macrosegregation, displays a maximum average variation in oxygen content less than about 20 ppm (0.0020%) and an oxygen segregation standard deviation less than about 8 ppm (0.0008%) on samples containing about 0.01% oxygen, a maximum average variation in sulphur content less than about 0.004% (40 ppm) and a sulphur segregation standard deviation less than about 0.001% (10 ppm) on a sample containing about 0.02% sulphur, and a maximum average variation in carbon content less than about 0.01% (100 ppm) and a carbon segregation standard deviation of less than about 0.004% (40 ppm) on a sample containing about 0.185% carbon, and an improved as-cast tensile strength and elongation.Similar good results are expected for Si, P, Cr and other alloying elements normally used in steel. Microsegregation analysis has been carried out for C, Mn, S, Cr, and Si by electron microprobe.

The results indicate much less microsegregation of manganese compared to prior art Concast samples. For example, heats of molten steel, containing about .46% carbon and about .98% manganese, were cast both by the method disclosed herein and by the well-known prior art Concast process. Samples of the as-cast bars were sectioned transversely and small specimens were cut from equivalent locations about one-half the distance from the edge toward the center. Thus neither the best nor the worst areas were selectd for comparison. The small specimens were mounted for electron micro-probe examination and analyzed to determine the concentration of manganese along a randomline, about 1800 microns long.

This procedure is generally well-known in the art and is believed to be the easiest method for detecting microsegregation in metals. Basically the process involves bombarding the specimen with a small diameter beam of high energy electrons which cause the speciment to give off characteristic x-rays corresponding to the concentration of the elements present. The x-rays are analyzed by diffracting them with a crystal (to select one element at a time) then measuring their intensity with any appropriate detector. Concentrations of a specific element may be determined by comparing the relative intensities of the x-rays generated by the element in the specimen to a known standard. To obtain maximum accuracy, about one-half of one percent, the ratio should be corrected for absorption and fluorescence of the emitted x-rays.Alternately, the average intensity level can be assumed to represent the base concentration obtained by a normal chemical analysis and then the variations in intensity directly represent variations from the base concentration.

When this latter procedure is used to compare a number of specimens, it is useful to assign one value to each specimen which indicates the average intensity of the significant variations and thus the average variation in concentration from the base level. This value may be called the microsegregation value and is expressed as a percentage of the base concentration.

In our comparison the base analysis of our sample was about 0.98% manganese. The results indicated that the prior art as-cast steel bar has a manganese microsegregation value, expressed as a percentage of base analysis, of about 300% while the manganese microsegregation value of the as-cast bar produced by the method of our invention was less than about 175%.

One important feature of the invention is that our novel product can be made on a known, prior art casting machine using methods of operation within the experimental skills of those having ordinary skill in our art.

Our preferred method of practicing our invention consists of forming a moving arcuate mold by rotating a casting wheel, having a peripheral groove, on its central axis and moving a band along its length into contact with the peripheral groove at the upper part of the casting wheel, moving the band and wheel in conjunction about the lower portion of the wheel, and moving the band away from the wheel (thusly forming one type of endless moving surface-type mold), pouring molten steel into the arcuate mold, cooling the mold to cause the molten steel to solidify in the arcuate mold, withdrawing the cast bar from the arcuate mold, normally additionally cooling the cast bar by the use of an after-cooler after the cast bar has exited the closed portion of the mold, and progressively straightening the cast bar as it moves away from the arcuate mold. See for example U.S.Patent No.3,623,535 (U.S. Class 164-87) and U.S. Patent No.359,348 (U.S. Class 164-263).

Thus, it is an object of the present invention to provide an improved as-cast steel product.

It is a further object of the invention to provide a novel continuous cast steel bar characterized by a lack, to an unusual degree, of segregation or inverse segregation of impurities or constituents within the bar.

It is a further object of the invention to provide a cast steel bar more suitable than prior art bars for subsequent processing such as rolling into rod and drawing into wire or hot-forming as by forging.

It is a further object of the invention to provide a method for continuously casting a steel product having improved internal properties.

Other objects, features and advantages of the present invention will become apparent upon reading the following specification when taken in conjunction with the accompanying drawings.

Figure 1 is a schematic diagram illustrating one example of prior art apparatus suitable to produce the cast steel product of the invention, the apparatus including a casting machine having a rotatable casting wheel defined by a peripheral groove therein and an endless band covering a portion of the length of the groove so as to form a closed mold over that portion.

Figure 2 is a graph showing sulphur distribution in a steel bar cast according to our invention.

Figure 3 is a graph showing oxygen distribution in a steel bar cast according to our invention.

Figure 4 is a graph showing oxygen distribution in a steel bar cast according to the prior art Hazelett twin-belt process.

Figure 5 is a graph showing oxygen distribution in a steel bar cast according to another prior art process, the Junghans-type process, and in particular, a Concast machine of the commercially successful type.

Figure 6 is a graph showing carbon distribution in a steel bar cast according to our invention.

Figure 7 is a histograph comparing the tensile strength of the novel cast steel product of our invention to another cast steel product produced by a prior art process.

Figure 8 is a graph showing the x-ray intensity variation due to microsegregation of manganese in a steel bar cast according to our invention.

Figure 9 is a graph showing the x-ray intensity variation due to microsegregation of manganese in a steel bar cast according to the prior art Concast process.

Referring now in more detail to the drawing, in which like numerals of reference illustrate like parts throughout the several views, Figure 1 shows a casting wheel apparatus 10 for producing the product of the present invention. This apparatus is similar to that disclosed in U.S. Patent No.3,623,535 (U.S. Class 164-87) for example. A casting wheel 10 defines a peripheral groove G therein which is covered, for a portion of the periphery of the casting wheel, by an endless flexible band or belt 11 to form a closed mold M. The flexible band 11 is held against a portion of the periphery of the casting wheel by band support wheels 12, 13 and 14 and moves with the wheel 10 as it rotates. Near the band support wheel 12 where the closed mold M begins, molten steel is discharged from a pouring pot ortundish 16 into the mold M through a spout 16a.In our preferred embodiment, all exterior surfaces of the casting wheel and band are continuously cooled by a spray of coolant fluid, the outer portion of the groove and band being cooled by cooling sprays from nozzles (not shown) from headers or manifolds S2, S3, S4, and the inner portion of the peripheral groove G being cooled by sprays from the nozzles on header S1. The spray of each nozzle (or groups of nozzles) along the inner side of the peripheral groove may be individually adjusted to vary the volume of cooling fluid sprayed therefrom, and thus vary the rate at which the metal within the mold M is cooled. A supply of coolant fluid to the nozzles (or groups of nozzles) may likewise be controlled by adjustable valves to allow starting and stopping of the coolant flow and to permit variation in the total volume of coolant flow.See, for example, the cooling arrangement suggested by Southwire Company's U.S. Patent No.3,279,000 (U.S. Class 164-433).

An extended bending section 18 is positioned beyond and above the band support wheel 14. The bending section 18 serves as a means for straightening the cast steel bar B withdrawn from the peripheral groove of the casting wheel 10 after exiting the closed portion of mold M. The bending section 18 includes a plurality of support guide rolls 19 mounted on a frame (not shown). Side guide rolls (not shown) may also be utilized in the bending section 18 to confine the cast steel bar approximately to a vertical plane. Although the support guide rolls 19 may be either driven or non-driven, we find it is preferable that at least some of the support rolls 19 be driven to assist in the straightening of the cast bar.

In our preferred embodiment an after-cooling header 21 is located above and adjacent the band support wheel 14 to apply a direct spray of coolant fluid onto the cast steel bar emerging from the arcuate mold M.

In the operation of the system according to the method of the invention, the casting wheel 10 is rotated in a counterclockwise direction and molten steel is poured from the tundish 16 through the spout 16a into the closed mold M formed between the peripheral groove G of the casting wheel and the flexible band 11.

Molten steel is poured in a controlled manner well known in the ferrous and non-ferrous casting arts into the mold M at a rate so that the rotation of the casting wheel moves the steel in the mold M away from the spout 1 6a as fast as the molten steel flows through the spout to maintain the surface of the pool of molten steel at a constant level at the entrance of the mold M. The exit end of the spout 1 6a is located as closely as possible to the entrance to the mold M to allow the molten steel to flow directly from the spout into the pool of molten metal in the mold.

As the molten steel is carried around the casting wheel 10 within the mold M, coolant fluid is directed against the mold from the nozzles in header S1 and the nozzles of the other headers S2, S3 and S4, and the amount of coolant applied to the band and casting wheel is adjusted as desired to control the rate of cooling of the molten metal. Our preferred embodiment has a condition of very uniform cooling about and along the longitudinal axis of the cast bar, see for example Southwire Company's-U.S. Patent No.3,279,000 to Cofer (U.S. Class 164-433). Initial rapid cooling and solidification of the molten metal occurs at the surfaces of the casting wheel and band, causing the formation of a skin or shell of solidified steel having an equiaxed grain structure.Continued extraction of heat from the partially solidified bar then causes solidification of the metal within the molten core in a progressive and uniform (including uniform at each point around the periphery) manner two form a dendritic or substantially equiaxed structure, depending upon the superheat of the steel, from the shell toward the center of a solid steel bar B.

The steel which entered the mold M as molten metal at an upper portion of the casting wheel moves in a downward direction about the lower portion of the casting wheel and then in an upward direction until it leaves the closed portion of the mold M near the band support wheel 14, passes through the after-coolant spray from the nozzles associated with header 21 and reaches a guide wheel 15, whereupon it is guided away from the casting wheel. In our preferred embodiment, the temperature of the exterior surface of the peripheral skin of the solidified steel as it emerges from the closed portion of the mold M does not exceed about 2500"F. but is not less than about 1900 or 2000"F.The cast bar leaving the casting wheel has a shape conforming to the curvature of the arcuate mold M and therefore is progressively straightened by progressively increasing the radius of the bar B as the bar moves through the extended bending section 18.

The guide rolls 19 support the bar and guide it through its unbending or straightening path above the casting wheel 10, at least one pair of the guide wheels 19 preferably being driven to pull the bar B along its length from the casting wheel 10. The molten core of the bar B is completely solidified by the time it passes at least the last coolant spray from the last nozzle on header 21 to assure that the bar is completely solid before reaching a point which is on a level with the level of the pool of molten metal at the entrance to the mold M.

Thusly, the molten metal in the core of the bar will not flow opposite to mold movement through the unsolidified bar center thereby creating a void in the center of the bar. The bar is thereby also solid and sound metallurgically before entering the bending section 18, and its temperature just prior to bending may also be adjusted by adjusting the volume of coolant supplied by the header 21 in order to control the internal stresses of the bar during straightening.

In the embodiment of the casting wheel 10 disclosed herein, the mold M is approximately trapezoidal in shape with a small dimension located at the inner portion of the peripheral groove and a large dimension located adjacent the band 11. Thus the steel bar cast by a typical casting machine 10 may be approximately 2-5/8 inches wide at its largest width, 2-1/8 inches wide at its smaller width and 1-7/8 inches deep, with an approximately 1/4 inch radius joining the smaller width with the two sides of the bar. Other bar sizes and shapes may be cast as desired. To date, for example, applicants have been successful in casting an approximately 4.8 square inch bar at a speed of approximately 44 feet per minute (528 inches per minute) and an approximate 8.1 square inch bar at a speed of approximately 35 feet per minute (420 inches per minute).

Applicants believe that the novel cast steel bar of the present invention may be produced at a relatively high linear speed because the relatively long length of the arcuate mold M cooled by quick chilling coolant sprays allows solidification to be achieved in spite of the relatively high rotational velocities of the casting wheel 10. Furthermore, the relatively small radius of the casting wheel 10 causes the orientation of the molten steel to change rapidly as the wheel rotates, in contrast to prior art commercially proven steel continuous casting techniques wherein the solidifying steel remains in a horizontal or approximately horizontal orientation for a substantial period, allowing impurities to float upwardly during the process of solidification.Applicants believe that when casting a bar B with a relatively small cross-sectional area according to the present invention, the bar B may be quickly solidified by the coolant spray from the nozzles associated with headers S1, S2, S3, S4 and 21 before any substantial segregation of constituents or impurities may occur. Thus, the method of the invention, whereby a relatively rapidly rotating casting wheel having a relatively small radius is cooled sufficiently to quick freeze impurities before segregation and/or inverse segregation can occur, produces the novel cast steel product of our invention, having properties significantly different from cast steel produced in prior art continuous casting machines.

Applicants believe that the design of the wheel mold, cooling water spray zones, and the relatively smaller cross-section of the cast bar makes it possible to achieve a higher rate of heat transfer compared to the prior art continuous steel casting methods.

Some idea of the high rate of cooling or solidification can be obtained from the metallurgical heights of the casting systems. The metallurgical height is defined as the distance between the top of the liquid pool in the mold to the point of complete solidification. In our method, we have worked with a metallurgical height of about 15 feet or less when manufacturing a 4.8 sq. in. bar at 35 to 44 FPM (420 to 528 IPM) and for 8.1 sq. in.

bar at 25 to 35 FPM (300-420 IPM). In the conventional continuous casting systems of the Junghans-type the metallurgical heights are generally reported to be about 50 to 70 feet for casting of 4" by 4" billets at a speed of 100 to 120 inches per minute (8.33 to 10.0 FPM). We have found that our cast bar becomes completely solid in about 25 to 30 seconds whereas we understand it requires about 6 minutes for complete solidification of the Junghans-type bar.

We believe that our fast rate of cooling reduces the flow of liquid of higher solute concentration into the interdendritic channels and thereby reduces the inverse segregation while the non-metallic impurities present in the liquid steel freeze with the liquid with a random distribution.

We also believe that the quick orientation change of molten steel in the moving wheel mold reduces the chances of segregation of impurities at an undesired position within the cast bar. That is, offering an example, as the cast bar with its initial relatively thin frozen shell and large molten center moves counter-clockwise from opposite approximately manifold S2 (see Figure 1), past manifolds S3, S4, and then 21 to a point where the frozen shell has become quite large with respect to the molten center, which means that the cast bar has moved through an arc of more than 90" and preferably more than 180", applicants believe that such orientation change of the casting throughout the course of solidification tends to eliminate the formation of heavy concentrations of segregated constituents or impurities which would otherwise normally float to the upper portion of the interior of the solidifying shell, because, essentially, the "upper portion" of the solidifying shell is always changing throughout the rotation of the wheel.

Measurements have been made to determine the degree of segregation of sulphur and oxygen impurities, and the degree of segregation of carbon, in novel cast steel bars that were cast according to the method disclosed. To establish a segregation profile from the wheel side of the cast bars to the band side of the cast bars, three sets of samples for analysis were punched from (long) transverse sections of the as-cast bar, one at the center of the section, one 20 mm to the left of the center and one 20mm' to the right of the center. The values obtained were then averaged to obtain an averge profile for each bar. Such results are shown in Figures 2,3 and 6.Applicants believe that those in the art use "(long) transverse" and just "transverse" interchangeably if there is no likelihood of confusion with a short transverse section (see ASTM Designation E399-74, Crack Plane Orientation Identification Code, for example).

Figure 2 is a graph of average percentage sulphur composition vs. position between the wheel side of the bar (Omm) and the band side of the bar (44mm in this case), the steel having a composition by weight of approximately 0.45% carbon, 0.02% sulphur, 0.99% manganese, 0.02% phosphorus and 0.21% silicon (Specimen #45). The maximum variation in average sulphur content across the bar was 0.0013% (13 ppm), for the measurements shown in Figure 2, and the standard deviation was 0.000498%. Applicants believe this represents an unexpectedly high uniform distribution of sulphur with no significant deleterious segregation.Tests of other specimens of the novel product indicated maximum average sulphur content variations ranging from 0.00114% to 0.004% (11.4 ppm to 40 ppm), and sulphur standard deviations varying from 0.000483% to 0.00138% in samples having 0.01755% and 0.02993% sulphur, respectively, as shown below.

Average of3 sulphur measurements at each position listed in PPM Specimen # = 26 41 43 45 48 5 mm = 170.3 308.0 234.3 226.7 316 15 mm = 180.7 310.6 233.0 240.0 328 25 mm = 174.3 271.0 223.7 235.0 316 35 mm = 170.7 297.6 227.0 239.7 317 40 mm = 181.7 297.6 231.0 238.7 325 47 mm = 311.0 238.7 -- -- Average = 175.5 299.3 231.2 236.0 320.4 Range = 11.4 40.0 11.7 13.0 12.0 Std. Deviation = 4.83 13.8 4.88 4.98 5.08 Figure 3 is a graph of average oxygen content (in ppm) vs. position between wheel and band sides for the same cast bar, &num;45, measured in Figure 2. The oxygen content was approximately 70 ppm (0.007%) and the maximum variation in average oxygen content across the bar, as shown in Figure 3, was 5 ppm (0.005%), and the standard deviation was 1.651 ppm.This again represents an unexpectedly good result ~ a very high degree of uniformity of constituents in the structure of the novel cast steel bar. It should also be noted that center porosity, sometimes present in a continuously cast steel bar (even ours), may contribute to the measured oxygen content at that particular location in the bar. Applicants believe that such porosity does not represent true segregation to those in the art and is generally healed during subsequent hot processing.

The improved oxygen segregation properties of the present invention may be seen by comparing Figure 3 to Figures 4 and 5. Figure 4 is a graph of percentage of oxygen content vs. position between the bottom and the top of a cast bar cast using a Hazelett Strip-Casting machine having a substantially horizontal mold. The graph is taken from page 43, Figure 6 of Whitmore, B.C. and Hlinka, J.W., "Continuous Casting of Low-Carbon Steel Slabs by the Hazelett Strip-Casting Process," Open Hearth Proceedings, 1969. Converting to a ppm basis, Figure 4 shows a maximum variation of approximately 100 ppm (0.01%) or more and a standard deviation of approximately 29.88 ppm. This approximately 20" from horizontal Hazelett experimental mold process thus produced a cast bar with a significant oxygen segregation problem even when the average oxygen content was relatively low, about 0.004%.The worst segregation was located near the top surface of the bar as is evident from Figure 4 herein and the Whitmore and Hlinka publication.

Figure 5 is a graph of oxygen content vs. position for a cast steel bar produced by a Concast vertical mold continuous casting machine including an arcuate oscillating mold. The graph represents an average of five samplings taken in (long) transverse section from the bottom to the top of the bar, which had a composition by weight of 0.46% carbon, 0.94% manganese, 0.021% phosphorus, 0.016% sulphur and 0.22% silicon. The maximum variation shown in Figure 5 is approximately 26.5 ppm, and the standard deviation is 10.6 ppm.

The average oxygen content is about 0.006%. Another specimen, having an average oxygen content of about 0.009%, showed a variation of 29 ppm.

A cast bar produced according to the method of the invention also has an unexpectedly uniform carbon distribution, as shown in Figure 6 which is a graph showing an average profile of the carbon content of such a cast bar (Specimen &num;48). This particular bar had a composition by weight of approximately 0.185% carbon, 0.59% manganese, 0.01% phosphorus, 0.032% sulphur, and 0.17% silicon. The points plotted in Figure 6 are averages of three measurements for each position across the cast bar between wheel and band sides, as was the case for Figures 2 and 3. Figure 6 shows a maximum average carbon content variation across the bar of approximately 0.009% (90 ppm), and a standard deviation of 0.00305%.In accordance with this invention, the steel melt is preferably prepared from a chemical system which has carbon present as one of the constituents in a range of approximately 0.04% by weight to 1.4% by weight. The maximum variation for samples in this range at variance with our specimens are expected to be proportional. Applicants have found that particularly superior results are achieved when the carbon content of the steel is between about 0.06% and 0.80% by weight.

Still further measurements have been made to determine the as-cast tensile strength of the novel cast steel bar of the invention, and to compare it to the as-cast tensile strength of a steel bar cast using a prior art commercially proven Concast machine including an oscillating, arcuate mold, both steel bars having been cast from the same steel melt. The measurements were carried out at a strain rate of 0.001/second using an one inch Extensometer. Figure 7 is a histograph showing the tensile strength of a sample of the novel cast bar to be approximately 107-110 ksi (107,000-110,000 pounds per square inch) as compared to that of the prior art Concast bar which had a tensile strength of approximately 93-94 ksi.The composition of the steel on this melt was 0.45% carbon, 0.97% manganese, 0.019% phosphorus, 0.017% sulphur, and 0.21% silicon.

Applicants believe that the approxmate 10%-15% or more increase in tensile strength of their novel product is consistent with the unusually uniform distribution of constituents and impurities observed in the novel product, as described above. In these same tests it was also observed that the novel as-cast bar had a greater percent elongation and a greater proportional limit in ksi than the prior art Concast bar, see below: Proportional Test Sample Tensile Strength % Elongation Limit Novel 1 107 ksi 10 64.3 ksi Bar 2 110ksi 13 71.7ksi 3 108 ksi 16 72.0 ksi Prior 4 94 ksi 8 62.0 ksi Art Bar 5 93 ksi 8 57.8 ksi Figure 8 graphically shows the intensity of characteristic manganese x-rays, along a random line of about 1800 microns in length, within a specimen of steel bar cast by the present invention.It can be seen that the intensity level is relatively constant about the line marked 100% which corresponded to the base concentration of 0.98% manganese and which also corresponded to an absolute reading of approximately 11 units on the graphical readout from the electron microprobe analyzer. There is only one significant variation which measures about 173% of the base level; i.e., equivalent to a local concentration of about 1.69% manganese.

Figure 9 graphically shows the manganese x-ray intensity in a specimen cast by the prior art Concast process. It should be noted that the intensity level contains many peaks which correspond to small areas of segregated manganese. Here, an absolute reading of approximately 12 units was observed on the strip chart readout from the electron microprobe unit and used for the 100% line. The average value of the most significant peaks is about 320% of the base level and the maximum variation in manganese content observed was over 400%, see below: Peak % of Base Level &num;1 396% 2 227% 3 292% 4 404% 5 262% 6 335% Applicants have found that particularly superior results are achieved when the manganese content of the steel is between about 0.30% and 1.20% by weight.

While this invention has been described in detail with particular reference to preferred embodiments thereof, it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described herein before and as defined in the appended claims. For example, applicants have reported herein only a representative sampling of the infinite number of steel compositions which may be cast according to the present invention. For other compositions in which the concentrations of the constituents and impurities are at variance with the exact concentrations in the specimens analyzed, applicants expect proportionally improved results.

Claims (25)

1. A continuous casting process comprised of the steps of: (a) casting molten metal into an advancing closed mold formed by at least one moving band which seals said mold over a portion of its length, (b) cooling the mold thereby causing the molten metal to begin to solidify on the mold walls forming a skin of solid metal about a molten core, (c) withdrawing the at least partially solidified cast bar from the exit to the closed portion of the mold, and (d) cooling the cast bar by direct and/or indirect impingement of coolant sprays thereon; wherein the new process comprises a continuous steel casting process characterized by; (e) controlling said steps (a) through (d) so as to give a continuous length of cast steel bar having lack of microsegregation, and especially uniform distribution of constituents and impurities, when measured in transverse section.

2. The process of Claim 1 wherein step (e) is further characterized as providing a maximum variation in average sulfur content of less than about 0.004% (40 ppm) when measured in transverse section.

3. The process of Claim 1 wherein step (e) is further characterized by said average sulphur content being calculated from random empirical data the standard deviation of which is less than about 0.0015%.

4. The process of Claims 1 or 2 wherein step (e) is further characterized as providing a maximum variation in average oxygen content of less than about 0.002% (20 ppm) when measured in transverse section.

5. The process according to any of the preceding Claims wherein step (e) is further characterized as providing a maximum variation in average carbon content of less than about 0.01% (100 ppm) when measured in transverse section.

6. The process according to Claims 1-4 wherein step (e) is further characterized as providing an average carbon content being calculated from random empirical data the standard deviation of which is less than about 0.004%.

7. The process according to any of Claims 1,4-6, wherein step (e) is further characterized as providing a maximum variation in average sulphur content of less than about 20 ppm.

8. The process according to Claim 1,wherein step (e) is further characterized as providing a maximum variation in manganese content of less than about 400% of the average manganese content.

9. The process according to any of the preceding Claims further characterized by the fact that step (a) includes a closed mold formed by peripheral groove in a rotating casting wheel and band which seals said groove over a portion of its length.

10. The process according to any of Claims 1-8 further characterized by the fact that step (a) includes a continuously advancing closed mold formed by at least one endless moving surface in conjunction with other sealing surfaces so as to provide a closed mold.

11. The process according to any of Claims 1, 2, 5-10, wherein step (e) is further characterized in that the maximum variation in average oxygen content is less than about 10 ppm.

12. The process according to any of the preceding Claims further characterized by the fact that there is provided a wheel-belt type endless moving surface mold for continuously casting steel bar suitable for commercial use wherein said cast bar is rotated during solidification throughout a radial arc of substantially more than about 90" and the uniformity of the formation of concentrations of segregated constituents and impurities is controlled to such an extent that a commercially acceptable steel bar is formed.

13. The method according to any of the preceding Claims, wherein said rotation of said casting wheel changes the orientation of said molten steel in said mold sufficiently rapidly to prevent any substantial flotation and segregation of impurities in said steel.

14. The process according to any of the preceding Claims, wherein step (e) is further characterized by said continuous length of cast steel bar having, for a given steel melt, a tensile strength at least 10% greater than a steel bar cast on a Junghans type caster from the same melt.

15. The process according to Claim 12, further characterized by the fact said are is more than about 180".

16. A continuous cast steel bar comprising in transverse cross-section a maximum average variation in oxygen content less than about 25 ppm.

17. A continuous cast steel bar comprising in transverse cross-section an oxygen segregation standard deviation less than about 10 ppm.

18. A continuous cast steel bar comprising in transverse cross-section an oxygen segregation standard deviation less than about 5 ppm.

19. A continuous cast steel bar comprising in transverse cross-section a maximum average variation in manganese content of less than about 275% of the average manganese content.

20. The continuous cast steel bar according to Claims 16-19 further comprising in transverse cross-section a maximum average variation in sulfur content less than about 0.004%.

21. The continuous cast steel bar according to Claims 16-19 further characterized as comprising in transverse cross-section a sulfur segregation standard deviation less than about 0.0015%.

22. The continuous cast steel bar according to Claims 16-21 further characterized as comprising in transverse cross-section a maximum average variation in carbon content less than about 0.01%.

23. The continuous cast steel bar according to Claims 16-21 further characterized as comprising in transverse cross-section a carbon segregation standard deviation of less than about 0.004%.

24. The continuous cast steel bar according to any of Claims 16-23 further characterized by: having a tensile strength at least 10% greater and an elongation at least 10% greater than a steel bar cast on a Junghans-type caster from the same melt.

25. A continuous cast steel bar having, in transverse cross-section a maximum average variation in oxygen content less than 25 ppm, a maximum variation in sulfur content less than 40 ppm, and a maximum average variation in carbon content less than 100 ppm.