CH645046A5 - METHOD FOR CONTINUOUSLY POURING STEEL. - Google Patents

Description

The invention relates to a method according to the preamble of claim 1 and a steel strand according to the preamble of claim 5.

It is known that a desirable property of steel in its cast state is that its components and impurities, which are normally found in the steel, are distributed as uniformly as possible within the steel product. The term "component" is used here in the usual sense, i.e. a “component” is to be understood as a component of a chemical system or a phase or a combination of phases that form a chemical system that occur in a characteristic configuration in an alloy microstructure, whereas “impurities” are elements or compounds, whose presence in a material or product is undesirable. The concept of components thus includes substances or materials that are combined to form a chemical system to produce a special steel made by casting. However, the term components does not include the impurities or undesirable elements or compounds that are present in the cast metal. In any case, segregation or separation of the components in cast steel makes them less suitable for subsequent further processing such as forging or hot forming or rolling out into a rod and warping into a wire. The term "segregation" or "excretion" used here is also used in the usual sense, i.e. the term "segregation" or "excretion" is a term used with reference to the non-uniform distribution or concentration of components (or contaminants) caused during the solidification of the metal. A concentration or accumulation of impurities in various positions or layers within the metal is referred to in the literature as excretion, for example. The excretion that occurs between the arms of dendrites is referred to as low excretion or micro-excretion, which is why the composition within a single crystal can vary. Macro-excretion occurs around primary or secondary blowholes or shrinkage cavities, such as blowholes and pores, as well as in similar districts or areas, and is often seen in the form of protruding lines that have a clearly erect or hanging conical shape that are clearly evident when the ingots or blocks are divided and etched. Elimination zones tend to occur in the central areas of the product obtained by casting and normally within the part that is mainly occupied by coaxial crystals. Micro-excretion can occasionally be overcome by tempering or freshening, whereas macro-excretion persists in the subsequent heating and processing stages. So-called blowout excretions occur around the so-called

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called blowhol cavities. During normal precipitation in steel, the components or constituents (solutes) in the iron (solvent) which are excreted from the solidifying liquid accumulate at the progressive solid-liquid contact surface, so that the components or constituents of the lowest melting point accumulate Concentrate in the final solidifying portions, whereas reversed or inverse excretion reverses, and the liquid with a high concentration of solute is trapped between the dendrites, causing a decrease in solute concentration from the bar or block surface towards the center is caused. An inverse excretion then consists in the concentration of constituents or impurities to a higher degree near the outer surfaces (compared to the inside) of an ingot or block or cast body.

Methods for casting steel are known in which steel products with a comparatively high degree of separation of impurities and alloy components are obtained within the cast steel. Due to the high concentration of components and impurities in the steel, an inverse or reverse excretion normally occurs. Such a non-uniform distribution of impurities and / or components within the cast steel makes it desirable that the total amount thereof within the steel be reduced, so that subsequent processing of the cast steel does not lead to unacceptable internal characteristics and surface characteristics in the product to be produced from the cast steel. However, reducing the total amount of impurities usually requires expensive additional refining or refining of the steel before casting and is sometimes not feasible or impractical from a commercial point of view, while occasionally adding or adding special components or components (including alloying elements) is desirable or desirable necessary is.

Among the contaminants and alloying elements within steel that tend to separate out in the case of prior art products are sulfur, oxygen, phosphorus, manganese, silicon and carbon. Any significant elimination of these elements makes the products less suitable from a commercial point of view. For example, a significant elimination can result in a non-uniform tensile or breaking strength within the steel, so that it is less suitable for further processing into wire. Elimination of gaseous contaminants can lead to porous areas near the surface of the cast product, which, among other disadvantages, leads to an inadequate surface quality of sheets. In this context, reference is made to B. C. Whitmore and J. W. Hlinka "Continuous Casting of Low-Carbon Steel Slabs by the Hazelett Strip-Casting Process", Open Hearth Proceedings, 1969.

A high level of microprecipitation of manganese leads to problems in many end products which have been produced from ingots obtained by continuous casting, due to the large influence on the austenite to pearlite / bainite-martensite conversions. For example, ingots obtained by continuous casting, which have been rolled into wire rods, often have high concentrations of manganese in the core, which favors the local formation of martensite during the casting, which leads to frequent breaks during the subsequent wire drawing process.

This problem has been known for a long time, but only a few publications have been found that communicate the production data obtained in practice. A study by Hans Van Vuuren from African Iron and Steel Industriai Corporation, Ltd. (A copy is included on pages 306-334 of the Steelmaking Proceedings, volume 61, Chicago Meeting, April 16-20, 1978), describes an attempt to control microprecipitation and the effects of the method in the case of a rod-shaped one End product.

Van Vuuren concluded that the occurrence of central martensite can usually be avoided in some steel wire rods by limiting the total concentration of manganese to a maximum of 0.75%, phosphorus to a maximum of 0.020% and monitoring the cooling during the cooling stage after the rolling process. No microprecipitation values of the continuous casting blocks are reported (ISCOR is believed to start continuously casting a 205mm x 315mm block on a Concast Bloom machine), but the rod end product was analyzed and revealed that the manganese micro-excretion values lie between 101.5 and 139.0% of the basic analysis. Due to the pronounced thermal diffusion between the time of casting and the time of rolling out, it is assumed that the microprecipitation values of the manganese in the cast ingot were considerably larger than in the rod-shaped end product.

To date, the known methods of the prior art, which deal with solving problems due to an excretion (e.g. wire break), include the repair (e.g. homogenization) of the intermediate products (e.g. rods or bars) instead of Avoidance of excretion during the casting process. One reason for this is obviously due to the fact that it is considerably more difficult to control the continuous casting process carried out in practice.

In the known ingot or ingot casting processes, precipitation occurs when the molten steel slowly solidifies, the contaminants entering the upper part of the ingot or ingot by gravity separation. In the case of processes leading to higher quality products, an occasional concentrated layer of impurities and / or solidification voids in the upper part of an ingot or block is physically removed, skinned or chamfered before the cast rod is further processed. Reference is made, for example, to the work “Recent Developments in Machine Scarfing of Continuous Cast and Roiled Steel”, published in the magazine “Iron and Steel Engineer”, January 1978, pages 68-71 and US Pat. No. 4,155,399, column 1, Lines 61 through 68.

Methods for continuously casting steel are also known which have been developed to avoid the machining of a large number of ingots or blocks and the need to remove the top layer. In the case of what is considered to be the most essential and most common commercial method of continuously casting steel from the applicant, the molten metal is poured into a vertically arranged mold with an open end made of a highly conductive material such as copper which is circulated for cooling water. If the outer surface of the metal adjoins to create a skin or shell of solidified or solidified steel

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Solidified mold wall, the strand or strand of steel is slowly pulled off the bottom of the mold, while molten metal is continuously poured into the upper part of the mold. This type of process is sometimes referred to as the Junghans or Junghans-Rossi type of the continuous casting system and has been successfully carried out in practice, for example by Concast AG, Zurich, Switzerland, and Koppers Co., Inc., USA. (Reference is made, for example, to US Pat. No. 2,135,183.) Even here, however, it may be necessary to peel off a surface for certain applications. In this connection, reference is made to US Pat. No. 4,155,399.

In the case of a Junghans-type process, the mold can oscillate or oscillate vertically along a comparatively short path, so that the mold moves with the beach or strand during each downward vibration, increasing the heat transfer during the periods, where there is no relative movement between the beach or strand and the shape. Such oscillations or oscillations increase the possible casting speed, but often cause undesirable oscillation or oscillation points or rings which extend around the cast body on its surface.

When the strands leave the mold, water jets are usually directed onto the surface of the semi-solidified strands to complete their solidification. In order to reduce the vertical height of the building that houses the Junghans type casting machine, it is known to use guide rollers to deflect the strand through an angle of approximately 90 ° over a radius of, for example, 12.20 m and then the strand redirect even further so that it occupies a horizontal position for cutting or for subsequent processing. In order to avoid twisting the strand twice in this way and to be able to install a caster in a smaller building, curved shapes have been developed, in which case the strands emerge from the shapes in accordance with the desired curved path and then the strands can be aligned in a deflection or bending stage for a horizontal orientation for machining production.

A remarkable representation of the traditional continuous steel casting process can be found in the December 1963 issue of Scientific American Magazine under the title "The Continuous Casting of Steel" by LV Gallagher and BS Old, Volume 209, No. 6, pages 74-88.

It follows that in a prior art casting process using a vertical mold, which normally requires a five or more storey building, there is no rapid change in the orientation of the hardening steel and that in this process the melted Metal can temporarily remain in a horizontal position in the center of the strand, as is evident, for example, from US Pat. No. 3,542,115. Contaminants thus have the possibility to float upwards as the solidification progresses, and in general excretion occurs, which occasionally occurs in a (long) transverse profile of a cast rod of the prior art with a size of 10.16 x 10.16 cm in Form of a line of excretion can be observed, which occurs approximately 2.54 cm from the inner radius of the strand.

On a research scale, steel has also been continuously cast into relatively horizontal shapes, using a double-belt casting machine that

In principle worked in a similar way to the earlier Hazelett Strip-Casting Corp. machine of the type known from US Pat. No. 2,640,235 (mentioned in the Whitmore and Hlinka publication). These two authors stated that due to the influence of gravity and the approximately 20 ° from the horizontal position of the steel strand during hardening in the case of these research attempts, the impurities within the steel tended to rise to near a wide zone of segregated material to form the surface of the cast material. These two authors stated that when coupled to the surface, an oxide attack state was an internal oxide precipitation that was found in cross-profile macro-etch tests. Although the level of oxide precipitation is variable, the profile was similar in the case of any plate or slab that was cast, and the authors recognized that from this data it is evident that the oxide precipitation was strong enough to cause poor sheet surface quality to effect. A number of possible solutions to the problem have been examined, e.g. concentration in the elimination of molded pool slag, the use of stationary, water-cooled copper edge dams, the use of submerged supply lines and the like. However, the authors found that all of these attempts to solve the problem remained fruitless. It has been suggested that larger concentrations of precipitating oxides were trapped in the solidifying plates or raw slabs at a time when the top skin was about 1.25 to 1.90 cm thick. The authors recognized that casting at an angle of 20 ° leads to a product that is unacceptable from a metallurgical point of view for the production of sheet metal, made of either aluminum-based or vacuum-treated steel, since no way could be determined, to remove these oxides and because the inclusions segregated towards the top of the cast block. The authors suggested reverting to the prior art practice in which the form (Junghans type) is arranged in a vertical position as a way to overcome the excretion problem.

It is believed that the off-center distribution of the constituents or components and contaminants also leads to unpredictable changes in subsequent tests during further processing, for example hot rolling, of the material.

It is an object of the invention to enable the production of a cast steel strand, which is characterized by a new lack of excretion of manganese, oxygen, sulfur and carbon compared to a rod or a rod of the prior art. The inventive method is characterized by the features stated in the characterizing part of claim 1. A steel strand cast according to this method can have steel which, if one examines a (long) cross-sectional profile for a macro-excretion, a maximum deviation in the oxygen content of less than 20 ppm (0.0020%) and an oxygen excretion standard deviation of less than 8 ppm (0.008%) in the case of samples with about 0.01% oxygen, further shows a maximum difference in sulfur content of less than about 0.004% (40 ppm) and a standard sulfur excretion deviation of less than 0.001% (10 ppm) in the case of a sample with about 0.02% sulfur and a maximum deviation in the carbon content of less than 0.01% (100 ppm) and a carbon excretion standard deviation of less than 0.004% (40 ppm) in the case of a sample with about 0.185% carbon and an improved

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Tensile strength and elongation in the case of the cast product. The term standard deviation is equivalent to the term mean square deviation.

Correspondingly good results can be expected in the case of Si, P, Cr and other alloying elements which are normally used in steel production. A microsegregation analysis was carried out for C, Mn, S, Cr and Si by an electron microsample. The results obtained showed a much lower microprecipitation of manganese compared to samples obtained by the known continuous method. For example, batches of molten steel containing approximately 0.46% carbon and approximately 0.98% manganese were cast using both the process described here and the known continuous process of the prior art. Samples of the cast strands were cut in the vertical direction and small samples were taken from the same locations, approximately half the distance from the edge towards the center. As a result, neither the best nor the worst districts were selected for the comparison. The small samples were then used for electron microscopic examination and analyzed for the purpose of determining the concentration of manganese along an arbitrary line of approximately 1800 microns.

This method is well known and is believed to be the easiest method of determining microprecipitation in metals. The known method is essentially based on the bombardment of a test specimen with a beam of high-energy electrons of small diameter, which causes the sample to emit characteristic X-rays, corresponding to the concentration of the elements present. The X-rays are then analyzed by diffraction with a crystal (to select an element) at a time, their intensity being determined by means of a suitable detector. The concentrations of a special element can be determined by comparing the relative intensities of the X-rays which are generated by an element present in the test specimen with a known standard. In order to achieve maximum accuracy, approximately Vi of one percent, the ratio can be corrected taking into account the absorption and fluorescence of the emitted X rays. Alternatively, it can be assumed that the average degree of intensity represents the base concentration which is obtained by a normal chemical analysis and that the deviations in intensity directly represent deviations from the base concentration.

If this latter method is used to compare a number of test objects, it has proven to be expedient to assign a value to each test object which indicates the average intensity of the pronounced deviations and thus the average concentration deviation from the base level. This value can be referred to as the microprecipitation value and is expressed as a percentage of the base concentration.

In the case of the present comparison, the basic analysis of the test specimen examined showed approximately 0.98% manganese. These results indicated that the steel rod made in the prior art had a manganese micro-excretion value expressed as a percentage of the basic analysis of about 300%, whereas the manganese micro-excretion value of the cast strand produced by the method of the present invention was less than 175%.

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An important advantage of the present invention is that the new product can be manufactured using a known casting device of the prior art, using process methods which are within the experimental experience of those who have a normal experience in this field.

The preferred practice of the invention is to form a moving domed shape by rotating a casting wheel with a peripheral recess on its central axis and moving a belt along its length in contact with the peripheral recess on the upper part of the casting wheel, moving the Belt and casting wheel in connection with the lower part of the casting wheel and moving the belt away from the casting wheel, pouring molten steel into the arched or arched shape, cooling the mold, causing the molten steel to solidify in the arched shape, pulling off the cast strand from the domed shape, usually additional cooling of the cast strand using an aftercooler after the cast material has exited the closed portion of the mold and progressive alignment of the cast strand as it moves away from the domed shape. Reference is made, for example, to US Pat. No. 3,623,535 and US Pat. No. 359,348.

The steel strand according to the invention is characterized by the characterizing part of patent claim 5.

The invention is described below with reference to the drawing, for example. Show it:

Fig. 1 is a schematic representation of an example of a system of the prior art, which is suitable for producing a cast steel product according to the invention, with a casting machine with a rotatable casting wheel, defined by a peripheral recess and with an endless belt that is part of the length covered the recess to create a closed shape in this section,

2 is a diagram illustrating the sulfur distribution in a steel strand cast according to the method of the invention.

3 is a diagram illustrating the oxygen distribution in a steel strand cast according to the method of the invention.

4 is a diagram illustrating the oxygen distribution in a steel strand which was cast according to the known Hazelett double-belt process,

Fig. 5 is a graph illustrating the oxygen distribution in a steel strand cast by another prior art method, i.e. the so-called Junghans type process and very specifically using a Concast machine of the type that is successful in practice,

6 is a graph illustrating carbon distribution in a steel strand cast according to the present invention.

7 is a graphical representation in which the tensile strength of a new cast steel product according to the invention is compared with another cast steel product produced by a method of the prior art,

8 is a graph illustrating the X-ray intensity deviation due to microsegregation of manganese in a steel strand cast by the method of the invention.

Fig. 9 is a graph showing the X-ray intensity deviation due to microprecipitation of manganese in a steel strand cast by the known continuous process of the prior art.

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The casting device shown in FIG. 1 is one with a casting wheel 10 for the production of the product according to the invention. The device is similar to the device known for example from US Pat. No. 3,623,535. The casting wheel 10 defines a peripheral recess G within the wheel, which is covered over part of its surface by an endless flexible band or belt 11 to produce a closed shape M. The flexible belt 11 is held in engagement with part of the periphery of the casting wheel by carrier rollers 12, 13 and 14 and moves with the casting wheel 10 when it rotates. In the vicinity of the belt support roller 12, where the closed mold M begins, molten steel is introduced into the mold M from a casting head or funnel 16 through an opening 16a. According to a particularly advantageous embodiment of the invention, all outer surfaces of the casting wheel and belt are continuously cooled by spraying a cooling liquid, the outer part of the recess and the belt being cooled by cooling jets from spray nozzles, not shown, which are located in head pieces or manifolds S2, S3 and S4, and wherein the inner portion of the peripheral recess E is cooled by irradiation with a cooling liquid from nozzles in the header S1. The coolant sprayed from each nozzle or the coolant sprayed from groups of nozzles along the inside of the peripheral recess can be individually adjusted so that the volume of the coolant that is sprayed out can be changed, which in turn increases the speed at which the steel inside the form M is cooled, can be changed. The supply of cooling liquid to the nozzles or groups of nozzles can also be controlled by controllable valves, whereby the spraying can be started and the supply of cooling liquid can be interrupted and the total volume of the cooling flow can be changed. Reference is made, for example, to the cooling arrangement described in US Pat. No. 3,279,000.

Beyond and above the belt support roller 14 there is an extended bending or deflection section 18. This section 18 serves to align the cast steel strand B, which is pulled off the peripheral splitting of the casting wheel 10 after the strand leaves the closed section of the form M. The bending section 18 has a plurality of carrier guide rollers 19 which are mounted on a frame, not shown. Lateral guide rollers, also not shown, can also be used in the bending section 18 to limit the cast steel strand to an approximate vertical plane. Although the carrier guide rollers 19 may either be driven or may not be driven, it has been found to be advantageous to drive at least a portion of the carrier rollers 19 to aid in the alignment of the cast strand.

In the case of a particularly advantageous embodiment of the invention, a post-cooling device 21 is arranged above and adjacent to the belt support roller 14 in order to direct a direct flow of cooling liquid onto the cast steel strand, which emerges from the curved shape M.

When operating the casting device according to an embodiment of the method of the invention, the casting wheel 10 is set in motion counterclockwise, and molten steel is poured from the funnel 16 through the opening 16a into the closed mold M, which is between the peripheral split E of the casting wheel and the flexible band 11 is formed. Molten steel is poured into mold M in a controlled manner using a method known for the casting of ferrous and non-ferrous metals, and at a speed that the rotating movement of the casting wheel moves the rod in the M form away from the opening 16a passes through the opening as fast as the molten steel flows to maintain the surface of the molten steel pool at a constant level at the M-shaped insertion point. The exit end of the snout 16a is as close as possible to the mold M entry opening to allow the molten steel to flow directly from the snout into the pool of molten steel in the mold.

When the molten steel is passed around the casting wheel 10 inside the mold M, cooling liquid is directed directly against the mold from the nozzles in the device S1, and the nozzles of the other devices SI, S3 and S4, as well as the amount of cooling liquid that the Band and the casting wheel is set in the desired manner to control the cooling rate of the molten steel.

The preferred embodiment of the invention has a state of very uniform cooling above and along the longitudinal axis of the cast strand. Reference is made in this connection, for example, to US Pat. No. 3,279,000. At the beginning there is rapid cooling and solidification of the molten steel on the surfaces of the casting wheel and belt, as a result of which the formation of a skin or shell of hardened steel with an equiaxed grain structure is caused. Continuous extraction or removal of heat from the partially solidified strand causes the steel to solidify progressively and uniformly (including a uniform manner at every point around the periphery) within the molten core to produce a dendritic or substantially coaxial structure, depending on the Overheating of the steel from the shell or shell in the direction of the center of the solidified steel strand B.

The steel that has melted into the M form in the upper portion of the casting wheel moves in a downward direction around the lower portion of the casting wheel and then in an upward direction until it exits the closed portion of the M form near the belt support roller 14 , passes through the after-cooling jets from the nozzles in connection with the device 21 and reaches a guide roller 15, whereupon the strand is continued by the casting wheel.

According to a preferred embodiment of the invention, the temperature of the outer surface of the peripheral skin of the solidified steel when it emerges from the closed portion of the form M does not exceed a temperature of about 1370 ° C, but the temperature does not exceed less than about 1037 or 1049 ° C.

The strand leaving the casting wheel has a shape which corresponds to the curvature of the shape M, which is why the strand is progressively aligned by progressively increasing the radius of the strand B as it moves through the extended bending section 18. The guide rollers 19 support the strand and guide it through the detour or alignment section above the casting wheel 10, at least a few of the guide rollers 19 preferably being driven to pull the strand B out of the casting wheel 10 over its length. The melted core of strand B is fully solidified by the time it passes at least the last spray from the last nozzle on device 21 to ensure that the strand is fully solidified before it reaches a point that on a level

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with the level of the pool of molten steel when entering form M. As a result, the molten steel in the core of the strand does not flow in the opposite direction to the shape movement through the unconsolidated strand center, creating a blow hole or pore in the center of the strand. The strand is thereby also solid and has a metallurgical effect before entering the bend section 18, and its temperature just before the bend can also be adjusted by adjusting the volume of cooling liquid supplied by the device 21 to the internal stresses of the strand to control during alignment.

In the illustrated embodiment of the casting wheel 10, the shape M has an approximately trapezoidal shape with a small extension in the inner portion of the peripheral recess and a larger extension adjacent to the band 11. This means that a steel strand is cast using a typical casting device 10 has a maximum width of approximately 66.67 mm, a minimum width of 53.97 mm and a depth of 47.62 mm with an approximate radius of 0.63 cm, which is the smaller width with the two sides of the strand connects. However, other strand sizes and strand shapes can be cast if desired. For example, 12.19 cm 2 strands can advantageously be cast at a speed of approximately 13.4 m per minute and approximately 10.6 cm strands at a speed of approximately 10.6 m per minute.

It is believed that the new cast steel strands of the present invention can be produced at a comparatively high linear speed, since the comparatively large length of the curved or curved shape M, cooled by rapidly quenching cooling jets, enables solidification despite the comparatively high rotational speed 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 the continuous cast steel manufacturing processes previously carried out, in which the hardening steel is in a horizontal direction or approximately horizontal orientation remains for a substantial period of time, allowing contaminants to float upward during the solidification process. It is believed that when a strand B having a comparatively small cross-section is cast according to the method of the present invention, the strand is rapidly sprayed through the coolant that is sprayed out of the nozzles in connection with devices S1, S2, S3, S4 and 21 is solidified quickly before a substantial elimination of constituents or impurities can take place.

The method described above, in which a relatively quickly rotating casting wheel with a relatively small diameter is cooled sufficiently to solidify contaminants quickly before separation and / or reverse separation can take place, thus produces the new casting steel product of the invention with properties, which differ significantly from the properties of a cast steel which was produced in continuously operating casting devices or casting machines of the prior art.

It is assumed that the design or design of the wheel shape, the design or design of cooling water spray zones and the comparatively small cross section of the cast strands make it possible to achieve a higher heat transfer in comparison to the previously known continuous cast steel production processes.

Some ideas of the high cooling rate or solidification result from the metallurgical sizes of the casting systems. The metallurgical size is defined as the distance between the upper part of the liquid melt in the mold and the point, the complete solidification or solidification.

In the case of the method according to the invention, a metallurgical size of about 4.57 m or less was used to produce a 12.19 cm 2 strand at 10.67 to 13.41 m per minute and in the case of a 20.57 cm 2 strand at a speed from 7.62 to 10.67 m per minute. In the case of conventional continuous casting systems of the Junghans type mentioned at the outset, metallurgical sizes of approximately 15.24 m to 21.336 m are generally used when casting ingots with a size of 10.16 x 10.16 cm at a speed of 254 to 305 cm reported per minute. It has been found that, in the case of the method according to the invention, the cast strands produced according to the invention are completely solidified in about 25 to 30 seconds, it being assumed that in the case of a strand of the Junghans type it takes about 6 minutes to complete solidification.

It is believed that the high rate of cooling reduces the flow of liquid from higher concentrations of detached material into the interdendritic channels and thereby reduces inverse excretion while the non-metallic contaminants present in the liquid steel are unevenly distributed with the liquid freeze or freeze.

It is further believed that the rapid change of orientation of the molten steel in moving wheel shapes reduces the chances of impurities being removed in undesired positions within the cast strand. That is, to give an example, the cast strand, with its initially comparatively thin solidified shell or shell and a large melted center, is moved counterclockwise from approximately the distributor S2 (see FIG. 1) at the distributors S3, S4 and then 21 to a point where the solidified shell or shell has become quite large with respect to the melted center, which means that the cast strand has moved over an arc of more than 90 ° and preferably more than 180 ° assumed that such a change of orientation of the casting body over the course of the solidification tends to eliminate the formation of large concentrations of excreted constituents or components or impurities which would otherwise float in the upper part of the interior of the solidifying shell or casing, since essentially the "upper part" of the solidifying shell or shell always changed during the rotation of the wheel.

Measurements were made to determine the level of sulfur and oxygen contamination and the level of carbon in new cast steel strands made by the process of the present invention. In order to produce an excretion profile from the wheel side of cast strands to the strip side of the cast strands, three sets of samples for analysis purposes were punched out of (long) cross profiles of the cast strand, a sample from the center of the section, a 20 mm to the left of the center and a 20 mm to the right of the center. The values obtained were then averaged to obtain an average profile for each strand. The results of such investigations are shown in FIGS. 2, 3 and 6. It is assumed that in the case of the status s

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the technology “(long) cross profiles” and even “cross profiles” are interchangeable if there is no likelihood of confusion with a short cross profile (cf. ASTM provision E399-74, gap-level orientation identification code, e.g. B.).

Fig. 2 is a graph showing the average sulfur percentage depending on the position between the wheel side and the strand (Omm) and the band side of the strand (44 mm in this case), the steel having a composition of approximately 0.45 wt% carbon, 0.02 wt% sulfur, 0.99 wt% manganese, 0.02 wt% phosphorus and 0.21 wt% silicon (Sample No. 45) . The maximum deviation from the average sulfur content over the strand was 0.0013% (13 ppm), in the case of the measurements shown in FIG. 2, and the mean square deviation, also called standard deviation, was 0.000498%. This represents an unexpectedly high, uniform distribution of sulfur without significant deleterious excretion. Studies of other specimens of the new product showed that maximum sulfur deviations were 0.00114% to 0.004% (11.4 ppm to 40 ppm) and that standard sulfur deviations were 0.000483% to 0.00138% in the case of Test specimens with 0.01755% and 0.02993% sulfur as shown below.

Average of 3 sulfur measurements in each specified position in ppm

Sample No.

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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

_ -

By

cut -

175.5

299.3

231.2

236.0

320.4

maximum

Deviation =

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40.0

11.7

13.0

12.0

Default-

Deviation =

4.83

13.8

4.88

4.98

5.08

The diagram shown in FIG. 3 shows the average oxygen content (ppm) as a function of the position between the wheel and the belt sides for the cast rod No. 45, as cast, as measured in FIG. 2. The oxygen content was approximately 70 ppm (0.007%) and the maximum deviation from the average oxygen content over the strand, as shown in Fig. 3, was 5 ppm (0.0005%) and the standard deviation was 1.651 ppm. This in turn also represents an unexpectedly favorable result, namely a very high degree of uniformity of components in the structure of the new cast steel strand. It should also be noted that the center porosity, which occasionally occurs in a steel strand produced by continuous casting (even in those according to the invention), can contribute to the measured oxygen content, in the particular position in the strand. It is believed that such porosity is not a proper excretion to the professional world and is generally cured during subsequent hot processing.

The improved oxygen excretion properties which can be achieved according to the invention result from a comparison of FIG. 3 with FIGS. 4 and 5. In the case of the diagram shown in FIG. 4, the percentage of

Oxygen content depending on the position between the bottom and the upper part of the cast strand, which was cast using a Hazelett strip casting machine with a substantially horizontal shape. The diagram was taken from page 43, Fig. 6, of a work by B.C. Whitmore and J.W. Hlin-ka, "Continuous Casting of Low-Carbon Steel Slabs by the Hazelett Strip-Casting Process", Open Hearth Proceedings, 1969. By conversion on a ppm basis, FIG. 4 shows a maximum deviation of approximately 100 ppm (0.01%) or more and a standard deviation of approximately 29.88 ppm. This approximately 20 ° from the horizontal Hazelett experiment procedure thus produced a cast rod with a significant oxygen elimination problem, even when the average oxygen content was relatively low, i.e. was about 0.004%. The worst excretion occurred near the top surface of the strand, as can be seen in Figure 4 and the Whitmore and Hlinka publication.

Fig. 5 shows the oxygen content depending on the position in the case of a cast steel strand which was produced using a continuously operating casting device with a vertical shape of the continuous type, including an arched, oscillating shape. The graph shows an average of five test specimens, which were taken from a (long) cross-sectional profile from the bottom to the upper part of the strand, the 0.46% by weight carbon, 0.94% by weight manganese, 0.021% by weight Phosphorus, 0.016 wt .-% sulfur and 0.22 wt .-% silicon contained. The maximum deviation shown in Figure 5 is approximately 26.5 ppm and the standard deviation is 10.6 ppm. The average oxygen content is around 0.006%. Another test specimen with an average oxygen content of approximately 0.009% showed a maximum deviation of 29 ppm.

A strand cast according to the method of the invention further has an unexpectedly uniform carbon distribution, as can be seen in FIG. 6, which consists of a graph representing the average profile of the carbon content of such a cast strand (Sample No. 48). This particular strand had approximately 0.185 wt% carbon, 0.59 wt% manganese, 0.01 wt% phosphorus, 0.032 wt% sulfur and 0.17 wt% silicon. The points entered in the diagram of FIG. 6 are average values of three measurements for each position over the cast strand between the wheel and the belt sides, which was also the case in FIGS. 2 and 3. From Fig. 6 there is a maximum carbon deviation over the strand of approximately 0.009% (90 ppm) and a standard deviation of 0.00305%. The molten steel is preferably produced from a chemical system which contains carbon as a component in a concentration of approximately 0.04% by weight to 1.4% by weight. The maximum deviation in the case of test objects in this area with a degree of freedom (variance) in the case of test objects produced according to the invention is expected to be proportional.

It has been found that particularly advantageous results are achieved when the carbon content of the steel is between approximately 0.06% by weight and 0.80% by weight.

Further measurements were made to determine the tensile strength of the new cast steel strand of the invention and to compare it with the tensile strength of a steel rod obtained by a conventional casting method of the prior art, using one in practice usual continuous casting machine with a vibrating or oszil8

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645 046

gently curved shape, whereby steel strands were cast from the same molten steel. The measurements were carried out at a loading or loading speed of 0.001 / second using a 2t, 54 cm extensometer.

7 shows the tensile strength of a test specimen of a strand cast according to the method of the invention of approximately 7522 to 7733 kg / cm 2, compared to the strength of a strand cast according to the prior art of approximately 6541 to 6611 kg / cm 2. The composition of the steel in the case of this melt was 0.45% by weight

Carbon, 0.97% by weight of manganese, 0.019% by weight of phosphorus, 0.017% by weight of sulfur and 0.21% by weight of silicon.

It is believed that an increase of approximately 10 to 15% or more strength in the strands made in accordance with the present invention is consistent with the unusually uniform distribution of the constituents or components and contaminants found in the new product of the present invention. In the case of these tests, it was also found that the new cast strands had a greater percentage elongation and a larger proportional limit in kg / cm 2 than the strands produced according to the prior art. Reference is made to the following results:

Test specimen tensile strength% elongation proportional

in kg / cm2

Limit kg / cm:

1

7522

10th

4501

2nd

7733

13

5019

3rd

7592

16

5040

4th

6611

8th

4340

5

6541

8th

4046

New strand

State of the art strand

8 shows the intensity of the characteristic manganese X-rays along an arbitrary line of approximately 800 microns in length within a test piece of a steel strand cast according to the method of the present invention. It follows that the degree of intensity is relatively constant over the line marked with 100%, which corresponds to a base concentration of 0.98% manganese and which is also an absolute value

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of approximately 11 units in the graphical rejection of the electron micro sample analyzer. There is only a remarkable change, measured at approximately 173% of the base level, i.e. equivalent to a local concentration of about 1.69% manganese.

9 shows the manganese X-ray intensity of a test specimen which was cast using the continuous method of the prior art. Note that the level of intensity has many peaks that correspond to small areas of manganese excreted. Here, an absolute value of about 12 units was found on the streak map from the electron micro-sample unit and used for the 100% line. The average value of the particularly important peaks is about 320% of the base level, and the maximum deviation of the manganese content observed was over 400%, as can be seen from the following values:

top

% Base level

number 1

2nd

3rd

4th

5

6

396% 227% 292% 404% 262% 335%

30th

35

40

45

It has been found that particularly advantageous results are achieved when the manganese content of the steel is approximately 0.30 to 1.20% by weight.

The invention has been described in more detail with reference to preferred configurations. It goes without saying that deviations and modifications can be carried out within the scope of the invention. For example, only a few representative samples of an unlimited number of steel compositions that can be cast by the method of the present invention have been given above. This means that the method of the invention can also be used to produce steel strands with concentrations and constituents and impurities other than those specified, with the achievement of corresponding, proportionally improved results.

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4 sheets of drawings

Claims (13)

645 046 PATENT CLAIMS 1. Process for the continuous casting of steel, with the following process steps: a) pouring molten steel into a moving closed mold »which is formed by means of at least one moving band which seals the mold over part of its length, b) cooling the mold, whereby the molten steel on the mold walls begins to solidify, producing a skin of solid steel around a molten core, c) pulling off the at least partially solidified strand from the outlet to the closed part of the mold and d) cooling the strand by impacting the cooling liquid supplied by nozzles, characterized in that an endless moving circumferential shape of a casting wheel is used for the continuous casting of the strand, that the strand is bent during solidification over an arc of more than 90 °, which reduces the formation of concentrations of separated constituents and impurities. 2. The method according to claim 1, characterized in that step a) includes the use of a closed mold which is formed by a peripheral recess in the rotating casting wheel and the band which seals the recess over part of its length. 3. The method according to claim 1 or 2, characterized in that the rotational speed of the casting wheel for changing the direction of movement of the molten steel in the form and for controlling the flotation and excretion of impurities in the steel is changed. 4. The method according to claim 1, characterized in that the strand is bent over an arc greater than 180 °. 5. Steel strand, produced by the method according to claim 1, characterized in that it has a maximum deviation from the average oxygen content of less than 25 ppm in the cross-sectional profile. 6. Steel strand according to claim 5, characterized in that it has a standard deviation from the average oxygen content of less than 10 ppm in the cross-sectional profile. 7. Steel strand according to claim 5, characterized in that it has a standard deviation from the average oxygen content of less than 5 ppm in the cross-sectional profile. 8. Steel strand according to claim 5, characterized in that it has a maximum deviation of less than 275% of the average manganese content in the cross-sectional profile. 9. Steel strand according to one of claims 5 to 8, characterized in that it has a maximum deviation from the average sulfur content of less than 0.004% in the cross-sectional profile. 10. Steel strand according to one of claims 5 to 8, characterized in that it has a standard deviation from the average sulfur content of less than 0.0015% in the cross-sectional profile. 11. Steel strand according to one of claims 5 to 10, characterized in that it has a maximum deviation from the average carbon content of less than 0.01% in the cross-sectional profile. 12. Steel strand according to one of claims 5 to 11, characterized in that it has a standard deviation of the average carbon content of less than 0.004% in the cross-sectional profile. 13. Steel strand according to claim 5, characterized in that it has a maximum deviation from the average sulfur content of less than 40 ppm in the cross-sectional profile and a maximum deviation from the average carbon content of less than 100 ppm.