NO157808B - CONTINUOUS CASTED STEEL BAR AND PROCEDURE FOR ITS MANUFACTURING. - Google Patents

Description

The present invention relates to a new stopping steel product,

and in particular a new continuously stopped steel product with an even distribution of its constituents, as well as a method for producing such a product. A desirable property for steel in a freshly stopped state is the even distribution inside the steel product of the constituents and contaminants that are normally present in steel. These terms are used in the present text in the sense that is assumed to be the usual meaning of these expressions in the subject area in question. This means that the term "constituent" means one of the ingredients that make up a chemical system, a phase or combination of phases that appear in a characteristic context in an alloy's microstructure, while the term "impurities" means elements or compounds whose presence is undesirable in any material. Such components, in the sense used here, will then include the materials that are combined in a chemical system to produce the particular type of steel that is stopped, but will not include the impurities or unwanted elements or compounds present in the stopped metal. Segregation or hardening of the components in stopped steel, however, makes the steel less suitable in all cases for subsequent processing, such as forging or rolling into bars and then drawing into wire. In this connection, the term "hardening" is also used in the sense that is assumed to be common within the present field, namely an expression that is used in reference to the uneven distribution or concentration of constituents (or impurities) that occurs during the solidification of the metal. A concentration or accumulation of contaminants at different places inside the metal is referred to e.g. in the non-fiction literature as victory. The tempering that occurs between the branches of dendrites is referred to as fine tempering or micro-tempering, and the material composition can thus vary within a single crystal. Macrosegregation occurs around primary or secondary shrinkage cavities, such as rib cavities, as well as in similar areas, and is often characterized as marked lines with a distinct

upright or inverted cone shape, which is clearly visible when the ingot in question is cut and etched. Segregation zones tend to occur in the middle of the stope and usually in the part mainly occupied by equiaxed crystals. Micro-segregation can sometimes be overcome by heat treatment, but macro-segregation persists through subsequent heat and processing processes. So-called rbr seigering occurs around rudder cavities. During normal hardening in steel, the constituents (solvents) in the iron (solvent) that are ejected from the broken melt are collected at the moving interface between solid and liquid material, so that the components with the lowest melting point are concentrated in the parts that break last, but with reverse hardening the opposite applies, namely that melt with a high concentration of solvents is trapped between the dendrites, so that a decreasing concentration of solvents is produced from the ingot surface inwards towards the middle area. Reverse tempering is thus a concentration of constituents or contaminants to a higher degree near the outer surfaces (as opposed to the interior) of an ingot or a block. Previously known methods for stiffening steel have produced stiffening steel products with a relatively high degree of toughness of contaminants and alloy materials inside the stiffening steel. Due to the high content of components and impurities in steel, reverse tempering normally occurs. Such an uneven distribution of impurities and/or alloy components in the hardened steel makes it desirable that the total content of such components in the steel is reduced, so that a subsequent treatment of the hardened steel does not lead to internal properties and surface properties that cannot be accepted for the product produced from the base steel. A reduction of the overall contamination content, however, usually requires a costly further refining of the steel for stamping and is sometimes commercially inappropriate or altogether impractical, while sometimes the addition of special substances (including alloying elements) is desirable or necessary.

Among the impurities and alloying elements in steel

which are easily subject to corrosion in previously known steel products are sulphur, oxygen, phosphorus, manganese,

silicon and carbon. Any significant tempering in a steel product will make it less commercially usable.

Such significant victory can e.g. provide uneven tensile strength for the steel and make it less suitable for subsequent drawing into wire. Sequestration of contaminants in gaseous form can lead to porous areas near the top of the stopped product, which among other disadvantages results in poorer surface quality for steel slabs (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 micro-seizing of manganese will cause problems in many end products made from continuously stopped ingots because of the marked effect such seizing has on the transformation from austenite to pearlite/bainite/ martensite. Continuously stopped ingots that are rolled into wire rods have e.g. often high concentrations of manganese in the core, which will promote local formation of martensite during quenching, and thus cause frequent fractures during the subsequent wire drawing process.

This problem has long been known in this field, but it has only been possible to find a few publications that provide actual production data. One such investigation was carried out by Hans Van Vuuren of the South African Iron and Steel Industrial Corporation, Ltd. (copy on pages 306 to 334 of Steelmaking Proceedings, Volume 61, Chicago, April 16-20, 1978) describes a process for controlling micro-seizure and its effect on the final bar-shaped product.

Van Vuuren has concluded that central martensite can usually be avoided in certain wire rods of steel by limiting the total manganese content to roughly 0.75% and the phosphorus content to roughly 0.020%, after which the cooling is regulated after the rolling process. Nothing is mentioned here about micro-segregation values for the continuously stopped steel blanks (ISCOR is assumed to start with continuous stopping of a 205 x 315 mm steel blank in a Concast stoping machine), but the final bar product showed micro-segregation values for manganese from 101.5 to 139.0% of the basic analysis. due to the strong thermal diffusion in the time interval between stopping and rolling, it is assumed that the micro-toughness values for manganese in the stopped steel blank would have been much greater than in the final bar product.

until now, the previously known methods of solving problems in connection with tempering (e.g. wire breakage) have involved the remediation (e.g. homogenization) of intermediate products (e.g. rods) rather than trying to avoid it initial victory during the stbping. One of the reasons for this is believed to be that it is much more difficult to regulate a continuous industrial step process at high production rates.

In previously known ingot restaging methods, tempering occurs during the slow breaking of the steel melt, as the contaminants are allowed to move under gravity separation to the upper side of the ingot. In order to achieve higher steel quality, it has sometimes been necessary to remove a concentrated layer of impurities and/or steel fracture cavities on the upper side of the ingot so that the stopped steel could be subjected to further processing (see e.g. "Recent Developments in Machine Scarfing of Continuous Cast and Rolled Steel", iron and Steel Engineer, January 1978, pages 68-71, and US Patent No. 4,155,399, column 1, lines 61-68.

Methods for continuous stopping of steel have been developed to avoid handling a large number of ingots and the necessity of removing a layer on the top side of the ingot. In a method which appears to be the most suitable and commercially accepted previously known method for continuous stoping of steel, molten metal is poured into an open, vertically arranged stope mold, lined with a well-conducting material, such as copper, in which water is circulated for to achieve effective relaxation. As the outer regions of the metal break to form a shell of solid steel against the die wall, a steel strand is slowly pulled out from the bottom of the die, while molten metal is continuously poured in at the top of the die. Such a process is often called the Junghans process or a Junghans/Rossi type process for continuous stopping and has been successfully marketed e.g. by Concast A.G. in Zttrich, Switzerland as well as Koppers Co., Inc. in the USA. An early Junghans patent is

US Patent No. 2,135,183. Even in this connection, however, it may be necessary to remove a barre surface for certain applications, see US Patent No. 4,155,399.

In the Junghans-type process mentioned, the stop mold can be swung back and forth over a short distance in the vertical direction, so that the mold moves together with the stop string during each swing downwards to increase the heat transition in the time periods where there is no relative movement between the string and the stop shape. Such a swinging movement increases the possible stop speed, but often produces unwanted swing marks or rings that extend around the surface of the stop object.

When the strand leaves the stop mold, water is usually sprayed against the surface of the semi-solidified strand to complete its solidification. In order to reduce the vertical construction height necessary for a stopping machine of the Junghans type, lining rollers have been used to bend the stopping string in an arc of approximately 90° with a radius of e.g. 12 m, after which the string is bent again to continue horizontally for cutting or subsequent processing. In order to avoid bending the string twice in this way, but still install the stapling machine in a smaller building, curved stapling forms have been developed so that the stapling string can run out of the form along a bent curved path and then be straightened in a stapling step to a horizontal direction for cutting down.

A very enlightening study of ordinary continuous steel stbping is given in the December 1963 issue of Scientific

American magazine at the article "The Continuous Casting

of Steel", by L.V. Gallagher and B.S. Old, Volume 209,

No. 6, pages 74-88.

It will thus be appreciated that prior art stoping in vertical stop forms does not quickly change the direction of the solidified steel strand (typically taking place over five stories or more), as well as allowing molten metal in the interior of the strand to remain in a horizontal position for certain time intervals, such like for example. described in US Patent No. 3,542,115. Contaminants will thus have an opportunity to float up during the solidification process and hardening will thus usually take place, which can often be observed directly in a cross-section of a steel bar with dimensions 10 x 10 cm, as a hardening line occurs approximately 2.5 cm inward from the strand's inner radius.

On a research basis, steel has also been continuously stopped

in relatively horizontal stop shapes, and this is then carried out in stop machines with a double belt, which in principle works in a similar way to the original stop machine from Hazelett Strip-Casting Corp., as described in US patent document no. 2,640,235 (mentioned in the above cited publication by Whitmore and Hlinka). These two authors have then found that due to the influence of gravity and because the steel strand forms an angle of approximately 20° with the horizontal direction during solidification in these research works, the contaminants in the steel material will tend to rise and form a clear tempering zone near the upper side of the stop goods. These two authors state that together with oxide stop pits on the upper side of the stope, internal oxide segregation was found in cross-section macrotek samples. Although the degree of this oxide segregation could vary, the material profile was in principle the same for all stop pieces examined, and based on this data the authors have found that this oxide segregation was sufficiently severe to result in reduced surface quality for steel plates. A number of possible solutions were tested, such as oak concentration on the removal of stope slag, the use of

stationary, water-cooled copper dam edges, the use of submerged feeding pipes, etc., but the authors admit that all these attempts proved fruitless. It was concluded that strong concentrations of tempering oxides were enclosed in the solidified casting when the casting shell on the upper side was of a thickness between 12 and 19 mm. The authors found from this that casting at an angle of 20° led to a metallurgical product that was not acceptable for platemaking from Al-sealed or vacuum-treated steel, because it was not possible to find any way to remove these oxides and cure the entrapped material took place in the upper part of the cast ingot. It was proposed to return to operation of the mold according to known techniques (Junghans type) and in a vertical position as a possibility to overcome this buckling problem. It is also assumed that the hardening distribution of the material components and contaminants away from the center area of the ingot will also produce unpredictable variations in subsequent attempts at material treatment, such as hot-rolling of the material.

On this background of prior art, it is then an object of the present invention to overcome the above-mentioned disadvantages by producing a steel billet which, in the cast state and compared to prior art, is characterized by a high degree of absence of manganese tempering, oxygen, sulfur and carbon.

According to the invention, this is achieved primarily by a cast steel ingot, the distinctive feature of which according to the invention is that it has a sulfur content which, measured across a cross-section of the ingot and in relation to its mean value, has a variation range of less than 0.004% (40 ppm) , preferably less than 20 ppm, and/or a standard deviation less than 0.0015% calculated from arbitrary empirical data.

In order to enhance the beneficial effect, the ingot preferably has, as a further distinctive feature, that over a cross-section of the ingot and in relation to the mean value, it exhibits a range of variation of less than 25 ppm, preferably less than 20 ppm, for the ingot's oxygen content, a range of variation of less than 400 % and preferably less than 275% for the ingot manganese content and a range of variation for the ingot carbon content of less than 0.01% and/or a standard deviation less than 0.004%.

The invention also relates to a method for producing a continuously cast steel ingot, and where molten steel is poured from a crucible arranged on the upper side of a rotating casting wheel into a mold formed by a circumferential groove in the casting wheel as well as a cover band that closes the opening of the groove, and solidified melt is extracted from the casting mold as an endless cast ingot, the method having as a distinctive feature according to the invention that such molten steel is used for the casting and such casting conditions that the cast ingot over its cross-section shows a variation in the manganese content of less than 400% of the average manganese content, a variation of the sulfur content less than 0.004% of the average sulfur content, a variation of the oxygen content less than 0.002% of the average oxygen content and a variation of the carbon content less than 0.01% of the average carbon content.

Significantly improved tensile strength and elongation at break are then achieved in the untreated cast state than with Concast-cast test pieces according to known technology. In order to measure the distribution of the alloy components over the ingot cross-section, steel containing about 0.46% carbon and about 0.98% manganese was cast into ingots both by the method of the invention and by the well-known Concast process according to known technology. Test pieces of the newly cast ingots were cut crosswise and small pieces were cut out at corresponding locations at about half the distance from the outer edge towards the center of the ingot. Neither the best nor the worst areas were thus selected for comparison. The small pieces of metal were mounted for electron microprobe examination and analyzed to determine the concentration of the various alloy constituents along an arbitrary line of about 1800 µm length.

This method is well known in the present field and is believed to be the easiest way to detect microsegregation in metals. In principle, this process involves bombarding test pieces with an electron beam of high energy and small diameter, which causes the test piece in question to emit characteristic X-rays corresponding to the concentration of the elements present. These X-rays are analyzed by deflecting them with the help of a crystal to separate an element of the gang, after which the intensity of the individual separated beam components is measured using a suitable detector. The concentration of a particular element can be determined by comparing the relative intensity of the X-ray component produced by the element in the sample with a known standard. In order to achieve the greatest possible accuracy, which is around 1/2%, this ratio should be corrected for absorption and fluorescence in connection with the emitted X-rays. Alternatively, the average intensity level can be assumed to represent the general concentration of the element in question obtained by a chemical analysis, so that the intensity variations directly represent variations in relation to this average level.

When the latter method is used to compare a number of test pieces, it is appropriate to assign a value to each test piece that indicates the average concentration of the relevant alloy component, so that the concentration variation in relation to the average value can be determined. This variation can be termed the microsegregation value and is expressed as a certain percentage of the basic concentration.

Results of such analyzes will be indicated in connection with the subsequent figure description.

An important feature of the present invention is that the new product can be manufactured using a known casting machine using operating methods that are within the experimental area of expertise for ordinary professionals in the field. In practice, this means that a movable curved mold is formed by turning a casting wheel with a circumferential groove around its central axis, while a covering band is moved in its longitudinal direction into contact with the circumferential groove on the upper part of the casting wheel, as the band and the wheel are moved together around the lower part of the wheel, and the band on it is displaced away from the casting wheel. In this way, an endless moving surface-type mold is formed, and molten steel is poured into the curved mold which is cooled to cause the molten steel to solidify in the mold, after which the cast ingot is withdrawn from the mold and normally further cooled using post-cooling equipment of the cast ingot when it has emerged from its closed position in the mould. The ingot is gradually straightened as it is moved away from the curved mold. In this connection, refer to e.g. to US Patent No. 3,623,535. and No. 359,348.

The invention will now be explained in more detail by the following detailed description with reference to the attached drawings on which: Fig. 1 schematically shows an example of a previously known apparatus which is suitable for producing the casting product of steel according to the invention, as this apparatus comprises a casting machine with a rotatable casting wheel made with a circumferential track, as well as an endless belt covering eh part of the track length to form a closed mold over this part of the track. Fig. 2 is a graphical representation showing the sulfur distribution in a steel ingot which has been stopped according to the present invention. Fig. 3 is a graphic representation showing the oxygen distribution in a steel ingot which has been stopped according to the present invention. Fig. 4 is a graphical representation showing the oxygen distribution in a steel ingot that has been stopped according to known technique by the Hazélett double-belt process. Fig. 5 is a graphic representation showing the oxygen distribution in a steel ingot which has been stopped by means of another previously known process, namely a process of the Junghans type and particularly carried out in a Concast machine of the commercially successful design. Fig. 6 is a graphical representation showing the carbon distribution in a steel ingot stopped according to the present invention. Fig. 7 is a histographic representation which compares the tensile strength of the new stopped steel product according to the present invention with another stopped steel product produced by a previously known process. Fig. 8 is a graphic representation showing the intensity variation for X-rays due to microsegregation of manganese in a steel ingot stopped according to the present invention. Fig. 9 is a graphical representation showing the intensity variation for X-rays due to microsegregation of manganese in a steel ingot stopped according to the known technique of the Concast process.

Reference must now be made in more detail to the drawings, in which the same numerical references indicate corresponding parts in all figures. Fig. 1 shows a stopper wheel apparatus 10 for producing the stopper product according to the present invention. This device is of the same type as described e.g. in US Patent No. 3,623,535. Stop wheel 10

is extended with a circumferential groove G, which over part of the stope wheel's circumference is covered by an endless bendable band or belt to form a closed shape M. The bendable band 11 is held in contact with part of the stope wheel's circumference by means of carrier wheels 12, 13 and 14, and moves together with the wheel 10 when it is turned. Near the carrier wheel 12 for the belt and where the closed mold M begins, molten steel is fed from an inlet container or crucible 16 to the stop mold M through an inlet nozzle 16a. In the preferred embodiment, all outer surfaces of the stop wheel and the belt are continuously cooled by spraying a coating fluid, the outer parts of the track and the belt being cooled by sprayed coating fluid from nozzles (not shown), or from coating heads or manifolds S2, S3, S4, while the inside of the circumferential groove G is coated with spray liquid from nozzles on a nozzle head Sl. The ejected liquid from each nozzle (or group of nozzles) along the inside of the circumferential groove can be set individually to vary the amount of coating liquid that is ejected from the nozzle, as well as a corresponding variation of the rate of reduction of the metal located in the stop mold M. The supply of dressing fluid to the nozzles (or groups of nozzles) can also be regulated by means of adjustable valves to allow the start and stop of the coolant flow and allow appropriate variation of the total amount of coolant supplied. Reference is made in this connection, e.g. to the wiring arrangement proposed by Southwire Company in US Patent No. 3,279,000.

An extended straightening area 18 is arranged outside and above the band-carrying wheel 14. The straightening area 18 serves to straighten the stopped steel billet B which emerges from the circumferential groove in the stop wheel 10 after the bar has left the closed area of the mold M. The straightening area 18 comprises a number supporting lining rollers 19 which are mounted on a supporting frame (not shown). Side lining rollers (not shown) can also be used in the straightening area 18 to keep the stopped steel ingot approximately in one and the same vertical plane. Although the supporting liner rollers 19 can either be driven or free-running, it is preferable that at least some of the supporting rollers 19 are driven to help straighten the stop bar.

In the preferred embodiment, a dressing head 21 for post-dressing is arranged above and near the belt-carrying wheel 14 to spray dressing fluid directly onto the stopped steel billet emerging from the curved stop shape M.

When operating the stopper plant according to the method of the invention, the stopper wheel 10 is turned in a clockwise direction while molten steel is fed from the crucible 16

through the inlet spout 19a to the closed stop mold M which is formed between the circumferential groove G of the stop wheel and the bendable belt 11. Molten steel is supplied in a regulated manner in a well-known manner according to conventional casting technique for iron and other metals, to the stop mold M in such a quantity strdm that the rotation of the stop wheel guides the molten steel in the mold M away from the inlet 16a as fast as molten steel flows through the inlet, thereby maintaining the surface of the available supply of molten steel at a constant level on the inlet side of the mold M. The outlet end of the spout 16a is placed as close as possible entered into the mold M to allow the molten steel to flow directly from the spout into the molten metal in the stop mold.

As the molten steel is fed around the stop wheel

10 inside the ice stop mold M, coating fluid is directed towards the mold from the nozzles on the die holder Sl as well as the nozzles on the outer heads S2, S3 and S4, /.as the drum of die is added, which is supplied to the belt and the stop wheel is adjusted as desired to regulate the cooling of the molten metal . IN

according to the preferred embodiment, very uniform cooling is arranged around and along the longitudinal axis of the stop bar, such as e.g. disclosed in Southwire Company's US Patent No. 3,279,000. Initial rapid cooling and solidification of the metal melt takes place against the inner surfaces of the stopper wheel and the belt, so that a crust or shell of solidified steel with equiaxed grain structure is formed. Continued extraction of heat from the partially solidified ingot then produces solidification of the metal within the molten core in a gradual and uniform manner (uniform at every point around the circumference) to form a dendritic or substantially equiaxed material structure, depending on the superheating of the steel, in the direction of the outer crust towards the middle area of the solid steel ingot B.

The steel entering the mold M as molten metal at an upper part of the stop wheel moves in a downward direction around the lower part of the stop wheel and then in an upward direction until it leaves the closed part of the stop mold M near the band-carrying wheel 14, and then passes through the post-cooling of sprayed dressing agent from the nozzles assigned to the dressing head 21, and then reaches a dressing wheel 15, whereupon the bar is fed away from the stop wheel. In the preferred embodiment, the temperature on the surface of the solidified steel does not exceed approx. 1370°C when the ingot moves out of the enclosed position in the stop shape M, but is also not lower than 1040 to 090°C.

The stopped ingot leaving the stop wheel has a shape corresponding to the curvature of the curved stop shape M, and the ingot is therefore gradually straightened by gradually increasing its radius as the ingot B moves through the extended straightening area 18. The lining rollers 19 support the ingot 6g feed it through its straightening path on the upper side of the stop wheel 10, at least one pair of the guide wheels 18 being preferably driven to pull the ingot B longitudinally out from the stop wheel 10. The molten core of the ingot B is completely solidified at least at the time it passes the sprayed dressing liquid from the last nozzle on the dressing head 21, thereby ensuring that the ingot is completely solid for it reaches a point where it is level with the surface of molten metal at the entrance to the stop mold M. The molten metal in the core of the steel ingot will thus not be able to flow opposite the movement of the stop mold through the unsolidified central area of the ingot, so that it o voids appear in the middle area of the bar. The ingot will thus be solid and metallurgically whole before it enters the straightening area 18. The temperature of the ingot just prior to straightening can also be regulated by adjusting the quantity flow of dressing agent supplied from the dressing head 21, with the aim of regulating the internal stresses in the ingot during straightening .

In the embodiment of the stop wheel 10 described above, the stop shape M is approximately trapezoidal with its smallest cross-sectional dimension inside the circumferential groove and its largest dimension towards the belt 11. A steel ingot stopped by a typical stop machine 10 can thus have a maximum width of approximately 67 mm and a minimum width of 54 mm, as well as a height distance of 48 mm between the two wide sides, with a radius of approximately 6.4 mm connecting the smallest wide side with the bar's two slanted sides. Other bar sizes and shapes can be made on request. Until now, it has been e.g. successfully stopped a steel billet with a cross section of approximately 31 cm 2 at a stopping speed of approximately 13.5 cm per min., as well as a bar with a cross section of approximately 52 cm 2 with a speed of approximately 10.5 cm per my.

It is believed that the new stopped steel ingot according to the invention can be produced with a relatively high linear stopping speed because the relatively long longitudinal extent of the curved stopping shape M sprayed on with a fast-coiling dressing agent allows satisfactory solidification to take place despite a relatively high rotational speed of the stopping wheel 10. The small radius of the stop wheel 10 further causes a rapid change of direction of the molten steel when the wheel turns, in contrast to the commercially operated previously known stopping technique where the solidified steel is kept in a horizontal or nearly horizontal direction for a significant period of time, so that contaminants are allowed to flow up under the steel breaking process. It is further assumed that by stopping an ingot B with a relatively small cross-section, the ingot B according to the present invention can be quickly made to solidify with the help of sprayed coating agent from the nozzles on the coating heads S1, S2, S3, S4 and 21 for some significant curing of the constituents or contaminants is given time to take place. The method of the invention, which consists of relatively fast rotation of a stopper wheel with a relatively small radius by sufficiently fast lowering of the stopper mold so that quick-freezing of the contaminants for normal and/or reverse tempering can take place, thus gives the new stopper product of steel according to present invention. This stopping product has properties that are significantly different from stopped steel produced in continuous stopping machines according to known techniques.

It is believed that such an embodiment of the stopping wheel, the spraying zones for dressing water and the relatively small cross-section of the stopped ingot makes it possible to achieve faster heat transfer than with previously known continuous stbpe methods for steel.

A certain concept of the rapid reduction and solidification can be obtained from the metallurgical height of the stock plant. The metallurgical height is defined as the distance between the surface of the amount of liquid in the stope mold to the point where the stope fracture is complete. In the present method, the work is carried out with a metallurgical height of about 4.5 m or less when a bar with a cross section of 31 cm 2 is produced at a cutting speed of • 10.5 to 13.5 m per min., as well as when producing a bar with a cross-section of 52 cm 2 at a step speed of 7.5 to 10.5 m per my. In the case of the conventional Junghans-type continuous stope plants, the metallurgical height is usually stated to be from 15 to 21 m when stopping ingots with a cross-section of 10 x 10 cm at a stope speed of 2.5 to 3 m per minute. my. It has been found that the ingot according to the invention completely breaks in 25 to 30 seconds, while it is understood that it takes about 6 minutes. for complete solidification of a bar according to the Junghans process.

It is assumed that the rapid quenching according to the invention reduces the flow of liquid material with a higher solution concentration into the inner dendrite channels, so that the reverse tempering is reduced, while the non-metallic impurities present in the steel melt are frozen together with the liquid to an arbitrary distribution.

It is also believed that the rapid change of direction of the molten steel in the stopper form on the moving wheel reduces the possibilities for the contamination of the contaminants to undesired places in the ingot. When the ingot with its initially relatively thin solidified shell and large molten core moves in a clockwise direction from a point directly opposite the dress head S2 (see fig. 1) past the kjble heads S3, S4 and finally 21 to a point where the solidified shell is has become quite thick in relation to the melt core, which means that the ingot has moved over an arc of more than 90° and preferably more than 180°, it is believed that such a change in direction of the intrusion process during the intrusion helps to eliminate the formation of strong concentrations of defeated constituents or impurities, which would otherwise normally flow up to the upper part of the interior of the solidified shell, when in fact "the upper part" of the solidified shell is always in a change of direction during the rotation of the wheel.

Measurements have been made to determine the degree of hardening of sulfur and oxygen contaminants, as well as the degree of hardening of carbon in steel ingots stopped according to the method of the invention. In order to indicate a fatigue profile from the wheel side to the belt side of such stopped bars, three samples were taken for analysis from longitudinal sections of the stop bars, namely one sample in the middle of the taken section, one 20 mm to the left of the center and one 20 mm to the right of the center. The mean value was then calculated for these sample values to arrive at an average profile for each ingot. Such results are shown in fig. 2, 3 and 6. It is assumed that the terms "longitudinal cross-section" and simply "cross-section" are used interchangeably if there is no likelihood of confusion with a short cross-section (see e.g. in American standard ASTM Designation E399 -74, Crack Plane Orientation Identification Code).

Fig. 2 is a graphical representation of the average sulfur content in percent as a function of the measuring point between the wheel side of the stope ingot (0 mm) and the band side of the ingot (44 mm in this case), as the stope steel calculated in % by weight had a content of approximately 0, 45% carbon, 0.02% sulphur, 0.99% manganese, 0.02% phosphorus and 0.21% silicon (sample no. 45). The biggest variation in average sulfur content over the bar cross-section was 0.0013% (13 ppm) for the . measurements shown in fig. 2, while the standard deviation was 0.000498%.

This seems to represent, to an unexpected extent, a particularly even distribution of the sulfur content without any sign of significantly harmful weathering. Examination of other samples of the new product resulted in the largest variations in mean sulfur content from 0.00114% to 0.004% (11.4 ppm to 40 ppm), as well as standard deviations for the sulfur content varying from 0.000483% to 0.00138% in samples with a nominal sulfur content of 0.01755% and 0.02993%, as shown in the following table.

Mean value of 3 measurements of the sulfur content stated in ppm for each measurement point.

Fig. 3 is a graphical representation of average oxygen content (in ppm) as a function of the measurement location between the wheel side and the belt side of one and the same stop bar (bar no. 45 measured in Fig. 2). The oxygen content was approximately 70 ppm (0.007%) and the greatest variation in mean oxygen content across the bar cross-section was, as shown in fig. 3 5 ppm (0.0005%), while the standard deviation was 1.651 ppm. This again represents a surprisingly good result, namely a very high degree of uniformity with regard to the distribution of the components over the material structure in the stopped steel ingot according to the invention. It should also be noted that the porous middle area, which sometimes exists in continuously stopped steel ingots (also according to the invention), can contribute to the measured oxygen content at the particular place in question in the ingot. However, it is believed that such porosity is not considered true victory by those skilled in the art and is usually made to disappear by subsequent heat treatment.

The improved properties with regard to oxygen sequestration which have been achieved by the present invention will appear clearly when comparing fig. 3 with fig. 4 and 5. Fig. 4 is a graphical presentation which indicates the oxygen content in % as a function of the measuring location calculated from the underside to the top of a stop bar which is stopped in a Hazelett strip stop machine with a mainly horizontal stop shape. This graphic representation is taken from page 43, fig. 6 of the previously mentioned article by Whitmore, B.C. and Hlinka, J.W., "Continuous Casting of Low-Carbon Steel Slabs by the Hazelett Strip-Casting Process", Open Hearth Proceedings, 1969. Converted to ppm values indicate fig. 4 a maximum variation of approximately 100 ppm (0.01%) or more and a standard deviation of approximately 29.88 ppm. This stopping process, which was carried out using an experimental Hazelett machine with a stopping shape at an angle of approximately 20° in relation to the horizontal direction, thus produced a stopping bar with considerable oxygen separation even when the average oxygen content was relatively low, namely around 0.004%. The worst defeat was found near the top of the ingot, as will be clearly seen from the present fig. 4 as well as the aforementioned publication by Whitmore and Hlinka.

Fig. 5 is a graphical presentation that indicates the oxygen content as a function of the measuring point for a stopped steel ingot produced in a continuous vertical stoping machine of the Concast type and with a curvilinear stop shape. The curve shown represents an average of five test pieces taken in longitudinal cross-section from the underside to the top of the ingot, which calculated in weight% had a content of 0.46% carbon, 0.94% manganese, 0.021% phosphorus, 0.016% sulfur and 0.22% silicon. As will be seen from fig. 5, the range of variation is approximately 26.5 ppm, while the standard deviation is 10.6 ppm. The average oxygen content is around 0.006%. Another sample which had an average oxygen content of about 0.09% showed a range of variation of 29 ppm.

A stop bar produced according to the method of the invention also has an unexpectedly uniform carbon distribution, as will be seen from fig. 5, which is a graphical representation indicating an average profile for the carbon content in such a stopped ingot (sample no. 48). This particular ingot had an additive content by weight of approximately 0.184% carbon, 0.59% manganese, 0.01% phosphorus, 0.032% sulfur and 0.17% silicon. The points drawn in fig. 6 represents mean values of three measurements for each measurement point over the stopped ingot between its wheel side and belt side, as was also the case in fig. 2 and 3. Fig. 6 shows a greatest average variation of the carbon content over the bar cross-section of approximately 0.09% (90 ppm), as well as a standard deviation of 0.00305%. In accordance with the present invention, the steel melt is preferably produced from a chemical composition with carbon as one of the components and with a content calculated in weight% of approximately 0.04 to 1.4%. The largest range of variation is assumed to be proportional to the percentage carbon content for the respective samples within this percentage range. It has been found that particularly favorable results are obtained when the carbon content of the steel is between 0.06 and 0.8% by weight.

Further measurements have been carried out to determine the tensile strength of a stopped steel ingot according to the invention, as well as to compare it with the tensile strength of a steel ingot stopped in a commercially recognized Concast machine with a swinging curved stop shape, the compared steel ingots being stopped from one and the same steel melt. These measurements were carried out at a stretching rate of 0.001/sec. using a 1 inch (2.54 mm) extensometer. Fig.

7 is a histographic presentation which shows that the tensile strength of a test piece of the stbpe ingot according to

invention is approximately 75-77 kp/mm 2 , compared to the Concast ingot stopped according to the known technique and which achieved a tensile strength of approximately 65-66 kp/mm 2 . The material additions in this steel melt were 0.45% carbon, 0 .97% manganese, 0.019% phosphorus, 0.017% sulfur and 0.21% silicon. It is believed that it achieved a yield of 10-15% or

more in tensile strength for the new product according to the invention is written from the exceptionally even

distribution of the steel's alloy constituents and impurities observed in the new product, as described above. During these investigations, it was also observed that the stopped ingot according to the invention had a greater percentage tensile elongation and a higher proportionality limit under tension than the Concast ingot according to known technology, as will be seen from the following table:

Fig. 8 graphically shows the intensity of characteristic X-ray spectrum from manganese along an arbitrary line of length about 1800 pm inside a sample of a steel ingot stopped according to the present invention. It will be appreciated that the intensity level is relatively constant along the line marked 100% which corresponds to the nominal content of 0.98% manganese and also corresponds to an absolute reading of approximately 11 units on the graphic reading scale of the electronic microprobe analyzer. There is actually only a significant variation, which has been measured at around 173% of the base level, which corresponds to a local concentration of around 1.69% manganese. Fig. 9 graphically shows the X-ray intensity from manganese in a specimen which has been stopped in accordance with the known Concast process. It should be noted that the intensity level in this case includes many peaks which correspond to small areas of manganese sequestration. A mean absolute reading level of approximately 12 units was in this case observed on the reading strip from the electronic microprobe unit, and this level was used as the 100% line. The average level for the most pronounced peaks is around 320% of the base level and the largest observed variation in the manganese content was over 400%, as will be seen from the following table:

It was found that particularly favorable results were obtained when the manganese content of the steel was between about 0.30 and 1.20% by weight.

Although the present invention has been described in detail with particular reference to preferred embodiments, it will be understood that variations and modifications can be made within the basic principle and framework of the invention, as described above and defined in the following patent claims. In this description, it is thus e.g. discussed only a representative selection of the large number of steel alloys that can be stopped during the practice of the present invention. For other material compositions in which the content of alloy constituents and contaminants is different from the present concentrations in the analyzed samples, correspondingly improved results can obviously be expected.

Claims (5)

1. Continuously cast steel ingot, characterized in that it has a sulfur content as measured across a cross-section of the ingot and relative to its mean value, has a range of variation less than 0.004% (40 ppm), preferably less than 20 ppm, and/or a standard deviation less than 0.0015% calculated from arbitrary empirical data. 2. Continuously cast steel ingot as stated in claim 1, characterized in that over a cross-section of the ingot and in relation to the mean value it exhibits a range of variation of less than 25 ppm, preferably less than 20 ppm, for the oxygen content of the ingot. 3. Continuously cast steel ingot as specified in claim 1 or 2, characterized in that over a cross-section of the ingot in relation to the mean value, it exhibits a range of variation of less than 400% and preferably less than 275% for the ingot's manganese content. 4. Continuously cast steel ingot as specified in claims 1-3, characterized in that over a cross-section of the ingot and in relation to the mean value, it has a range of variation for the carbon content of the ingot of less than 0.01% and/or a standard deviation of less than 0.004%. 5. Method for producing a continuously cast steel ingot, and where molten steel is poured from a crucible (16) arranged on the upper side of a rotating casting wheel (10) and into a mold formed by a circumferential groove (G) in the casting wheel as well as a cover band (11) which closes the opening of the groove, and the solidified melt is withdrawn from the mold as an endless cast ingot (B), characterized by the fact that such a steel melt and such casting conditions are used for the casting that the cast ingot across its cross-section exhibits a manganese content variation of less than 400% of the average manganese content, a sulfur content variation of less than 0.004% of the average sulfur content, an oxygen content variation of less than 0.002% of the average oxygen content, and a carbon content variation less than 0.01% of the average carbon content.