LU82107A1 - CONTINUOUSLY CAST STEEL PRODUCT HAVING REDUCED MICROSEGREGATION AND MANUFACTURING METHOD THEREOF - Google Patents

Description

δ Λ à-Λ "B GRAND-DUCHÉ DE LUXEMBOURG L-2481 ..... k ........ I ........ y. ij du ........ .........? ..?. Λ.?. ™. ^ ..? .. §.9 ....... Minister „,.„, Of the National Economy and of the Middle Classes

Title issued: ........................................ 3S, jrftJrgfr Property Service Industrial

vfObfë 'LUXEMBOURG

Invention Patent Application i * I. Request .... S.QUthw.ire .... C.Qmp.aay. ,, ...... 12.6 ..... Ee.rtilla ... .s.tr.e.et .., ...... Cax.r.Qlltan., ...... Georgia .......... α) 30117 / EUA / represented by Jean Waxweiler, 2Ï-25 Allée Scheffer, Luxembourg ^ acting as .................................... .................................................. .................................................. .................................................. ... · ..................... * ........................ ........................................... (2) agent ... .................................................. .................................................. .................................................. .................................................. ...................................

deposit ........ this twenty-three ...... j.an.vi.er .... mil .... neuf ..... cent ..... quatre- v ingt. (3) at ........ 1.5 .. * .. 0.Q .. hours, at the Ministry of National Economy and the Middle Classes, in Luxembourg: 1. this request for obtaining '' a patent for an invention concerning: .......................................... .................................................. .................................................. .................................................. .................................................. .............................— (4)

Continuously cast steel product having a reduced microsegregation and its manufacturing process.

declares, assuming responsibility for this declaration, that the inventor (s) is (are): ......................... .................................................. .................................................. .................................................. .................................................. .................................................. .... (5)

George Charles Ward, 258 Almon Road, Carrollton, GA 30117, USA

Thomas Noell Wilson, Route Ι, Βοχ 514, Carrollton, GA 30117, USA “üSäa ^ Ytiiäär” ^ nîîa'7 ë "Henson 'cïrcle"' Rt. 5, Car roi 1 tön, GA 3 Ö ï 17, EDA

2. the delegation of power, dated ......................................... .............................. the ................... .................................................. ...

3. description in language ................. French ......................... of the invention in two copies; 4 ................. 4 ............. drawing boards, in two copies; 5. the receipt of the taxes paid to the Luxembourg Registration Office on January 23, one thousand nine hundred and eighty ....................... ..........................................

claims for the above patent application the priority of one (or more) application (s) of (6) ___________________________ patent .......................... ............... filed in (7) ....................... E ... .HE HAS.............................................. .........................................

the twenty-fourth of january nineteen hundred and seventy-nine under n 006.148 and the twenty-nine august nineteen hundred and seventy-nine pennies ... ... on ..... ho ....... 7OV550 .................................................. .................................................. .................................................. .................................................. .......................

in the name of George -Char les · - ^ Wardy Th © mas-- -Neell ••• Wi-l-son7; Bnday --Kumar ..... SinW ^. elect domicile for him / her and, if designated, for his / her representative, in Luxembourg ................................ ........

.... J.ean .... Waxw.ei.ler..r..21.rr.2.5 .... Âllé.e Scheffer., Luxembourg ............. .................................................. .. (io>; requests the issue of a patent for the invention for the subject described and represented in the abovementioned appendices, - with postponement of this issue to ..............: ................................. months.

The agent.........................

.................. \ ........................

IL Minutes of Deposit \ •. The above application for a patent for invention has been filed with the Ministry of National Economy; 1 \ and of the Middle Classes, Industrial Property Service in Luxembourg, dated: V “^ 23,01.1980 15 OÛ. * Pr. The Minister at 3 * o'clock Ê: \ ïe National Economy and the Middle Classes, i si \ * \ p-d.

I S:: Jt \ i 3- Yk 3%. / ^ 1 \ & ^ PRIORITY CLAIM Filing of the patent application in a.u.a.

January 24, 1979 1® number 006.148 and August 29, 1979 under number 70.550

DESCRIPTIVE MEMORY FILED IN SUPPORT OF A PATENT INVENTION APPLICATION IN THE GRAND DUCHY OF LUXEMBOURG

by ; southwire company

STEEL CAST PRODUCT

w CONTINUES WITH MICROSEGREGATION

for REDUCED AND ITS MANUFACTURING PROCESS.

V

18,956

Said company: SOUTHWIRE COMPANY SW.253F (960)

Continuously cast steel product having reduced microsegregation and its manufacturing process.

The present invention relates to a new cast steel product and in particular to a new continuously cast steel product having a uniform distribution of the constituents and to a process for its production.

An advantageous property of steel in its raw state of casting is a uniform distribution in the steel product of the constituents and impurities normally present in the steel. The terms "constituent" and "impurities" as used herein have the following meanings, which the Applicant believes to be their normal meanings: The term "constituent means one of the ingredients which constitute a chemical system or a phase or a combination of phases which is in a characteristic configuration in an alloy microstructure, while the term "impurities" denotes elements or compounds the presence of which in any material is undesirable. The constituents, as this term is used here, will therefore include the materials combined in a chemical system to produce the particular type of steel which is cast, but will not include impurities, or undesirable elements or compounds present in the cast metal. In any case, the segregation of constituents in the cast steel makes it less usable for further processing, for example for forging or bar rolling and 4 then wire drawing. i, the term "segregation" also has what the Applicant believes to be its normal meaning in the art, that is to say, this term is used in connection with non-uniform distribution or concentration of constituents (or impurities) which occurs during the solidification of the metal. A concentration or accumulation of impurities in various positions in the metal, for example, is called segregation in the art. The segregation that occurs between the branches of dendrites is called minor segregation or microsegregation and thus the composition can vary within a single crystal.

Macro-aggregation occurs around primary and secondary withdrawal cavities, as in recess and in η ..... ^ 2 of similar regions, and is often revealed in the form of marked lines, having a pronounced shape of a right cone or overturned, which can be seen clearly when the ingots are cut and attacked. Segregation zones tend to be found in the middle regions of the casting and crystals usually in the part usually occupied by / equiaxes. The microsegregation can sometimes be remedied by annealing, but the macroseggregation remains during the subsequent heating and working operations. Segregated

StSUrQ

so-called y ions occur around the shrinkage cavity. In normal segregation in steel, the constituents (dissolved body) in iron (solvent) discharged out of the solidifying liquid accumulate at the advancing solid / liquid interface, so that the constituents having the most low melting point are concentrated in the last portions to solidify, but in reverse segregation it is the opposite, since the liquid having a high concentration of dissolved substances is trapped between the dendrites, which causes a decrease in the concentration dissolved substances from the surface of the ingots towards the center. Reverse segregation is therefore a concentration of constituents or impurities to a higher degree near the outer surfaces (relative to the interior) of an ingot or a molded part.

Previous steel casting processes have produced relatively high degree cast steel products

S

of segregation of impurities and alloying materials in cast steel. Due to the high content of constituents and impurities in steel, reverse segregation normally occurs. This irregular distribution of impurities and / or constituents in the cast steel makes it desirable that the total amount of these impurities and / or these constituents in the steel be reduced so that the subsequent treatment of the cast steel does not give no features; internal and surface unacceptable in the product made from cast steel. A reduction in the total amount of impurities, however, usually requires costly additional refining of the steel before casting and is sometimes impossible or inconvenient industrially, 3 while sometimes the addition of particular constituents (including elements of 'alloy) is desirable or necessary

Among the impurities and alloying elements in steel that tend to segregate in prior art products are sulfur, oxygen, phosphorus, manganese, silicon and carbon. Any significant segregation of these elements will make the product commercially less useful. For example, significant segregation can cause a lack of uniformity in tensile strength in steel and make it less usable for subsequent wire drawing. The segregation of gaseous impurities can give porosity zones near the upper surface of the cast product, which results, among other disadvantages, in a lower quality of the surface of the sheets. (See Whitmore, B.C., and Hlinka, J.W., "Continous Casting of Low-Carbon Steel Slabs by the Hazelett Strip-Casting Process", Qpen Hearth Proceedings, 1969). Severe microsegregation of manganese will cause difficulties in many end products formed from the continuously cast billet due to its significant effect on the transformation of austenite into perlite / bainite / martensi.te. For example, billets obtained by continuous casting and rolled into bars for wire format often have high concentrations of manganese in the central portion, cheerful will favor the local formation of martensite during casting, thus causing frequent ruptures during the subsequent wire drawing, This problem has been known in the art for a long time, but there have been few publications found by the inventors which describe actual production results. A study on this subject by Hans Van Vuuren, of the South African Iron and

-, ISQOR

Steel Industrral Corporation, LtcV'.V (reproduction contained on pages 306 to 334 of Steelmakmg Proceedings, Vol. 61, Chicago Meeting, April 16-20, 1978) illustrates one way of combating microsegregation and its effects in a final bar product .

Van Vuuren concluded that central martensite could usually be avoided in certain wire rods by limiting the total amount of manganese to a maximum of 0.75%, that of phosphorus to a maximum of 0.020%, and then regulating the cooling in the cooling chain 4 - * * t digital microsegregation concerning the blooms of continuous casting (it is believed that ISCOR begins with the continuous casting of a bloom of 205 mm x 315 mm on a Concast Bloom Machine) , but the final bar product was analyzed and manganese microsegregations ranging from 101.5% to 139.0% were found from the base analysis. Due to the large thermal diffusion between the time of casting and the time of rolling, it is believed that the microsegregation of manganese in the casting bloom must have been much stronger than in the final bar product.

Until now, the prior art methods for solving the problems due to segregation (for example the breaking of wires) have involved a refurbishment (for example a homogenization) of the intermediate products (for example bars) instead of 'a removal of the initial segregation during casting. One reason is believed to be that it is much more difficult to properly conduct the industrial process of large-scale continuous casting.

In prior methods of casting ingots, segregation occurs while the molten steel slowly solidifies, the impurities floating by gravity separation on the upper surface of the ingot. In higher quality applications, a concentrated layer resulting from impurities and / or a solidification cavity at the top of the ingot sometimes had to be physically removed (scalped or cut) before the cast steel was subjected to a further treatment (see , for example, "Recent Developments in 5 Machine Scarfing of Continous Cast and Rolled Steel", Iron des- and Steel Engineer, January 1978, pages 68-71 and patent / EUA No. 4 155 399, col. 1, lines 61 to 68.

Continuous steel casting methods have been developed to avoid the handling of a large number of ingots and the need to remove the upper surface layer. In what the Applicant considers to be ^ The most important and industrially used prior process for the continuous casting of steel, the molten metal is dropped into a mold with open ends arranged vertically formed of a very conductive material like copper, or water is circulated for cooling purposes. While

T

5 as the periphery of the metal solidifies to form a shell of solidified steel near the mold wall, the steel strand is slowly taken out from the bottom of the mold while molten metal is continuously poured into the top of the mold. This type of process is sometimes called the Junghans or Junghans-Rossi type continuous casting system and has been successfully marketed by Concast AG of Zurich, Switzerland, and by Koppers Co., Inc. of the United States of America , for example. An old Junghans patent is the U.S. patent. No. 2,135,183. Even in this case, however, it may be necessary to decouple a surface for certain applications - see the BreveïŸE.U.A.

No. 4,155,399.

In the Junghans type process, the mold can be oscillated vertically on a short path so that the mold moves with the strand during each oscillation in order to increase the heat transfer during periods when there is no relative movement between the strand and the mold. Such oscillation increases the possible rate of casting, but often creates unwanted oscillation marks or rings extending around the cast on its surface.

When the strand leaves the mold, water jets are normally directed towards the surface of the semi-solid strand so as to complete its solidification. To reduce the necessary vertical height of the building containing the Junghans type casting machine, guide rollers were used to bend the strand over an arc of approximately 90 ° to a radius of approximately 4.25 meters for example, and to then decenter it so that it extends horizontally to be cut or for further processing. To avoid having to bend and decenter the strand in this way and to be able to install the casting apparatus in a smaller building, curved molds have been made so that the strand comes out of the. mold following the desired curved path and it is then straightened in a single bending step to a horizontal orientation to be cut.

A very clear description of the "* -, 6 classical steel continuous" flow can be found in the December 1963 issue of the journal Scientific American, "The Continous Casting of Steel", by L. V. Gallagher and B. S. Old, Vol. 209, H ° 6, pages 74 to 88.

It will thus be seen that casting according to the vertical mold methods of the prior art does not rapidly change the orientation of the steel being solidified (a building of 5 or more storeys is usually required), and allows the molten metal in the central part of the strand to remain perfectly in a horizontal attitude, as indicated in the patent! EUA No. 3,542,115 assigned to Concast Incorporated, for example. Thus, the impurities have the possibility of floating while moving upwards during the course of solidification and, in general, a segregation occurs which can sometimes be observed in a cross section (long) of a bar with a square section. of 10.16 cm x 10.16 cm in the form of a segregation line located approximately 2.5 cm inside from the inside radius of the strand.

In the course of research, the steel was also continuously cast in relatively horizontal molds, this being done using two-belt casting machines similar in principle to the old machine of Hazel ^ tt Strip-Casting Corporation as described in the patent ”-11.JA

No. 2,640,235 (mentioned in the publication by Whitmore and Hlinka). These two authors indicated that due to the influence of gravity and the approximately 20 ° angle of the steel strand with the horizontal during solidification in these research projects, the impurities in the steel had tendency to rise and form a large area of material separated by pie segregation from the upper surface of the casting. These two authors indicate that in addition to this zone of oxide on the upper surface, there was an internal segregation of oxide noted in macroscopic attack tests of transverse sections. Despite variations in the degree of oxide segregation, the pattern was similar in all cast slabs and the authors concluded from these results that it was evident that the oxide segregation was severe enough to cause lower quality 7 of the surface of the sheets. We have tried a number of possible solutions, such as concentrating or eliminating the slag from the whole mold, the use of fixed lateral water-cooled copper dams, the use of submerged supply tubes, etc., but the authors acknowledged that all of these attempts did not solve the problem. It was explained that high concentrations of oxides during segregation were trapped in the slab during solidification when the upper crust was between 13 and 19 mm thick. The authors concluded that casting at an angle of 20 ° produced a metallurgically unacceptable product for the production of sheet metal from Al-quenched steel or vacuum-treated steel because no way of finding it was found. remove these oxides and that a segregation of inclusions occurred towards the upper part of the billet. They suggested returning to the use of the mold in the vertical position of the prior art (Junghans type) as a possibility for solving the problem of segregation.

The Applicant believes that the decentralized distribution with segregation of the constituents and impurities also caused unpredictable variations in subsequent treatment attempts such as by hot rolling of the material.

In general, the present invention relates to a crude steel casting bar characterized, compared to the prior art, by a new absence of segregation of manganese, oxygen, sulfur and carbon. Contrary to the prior art, the present invention provides a crude steel rod for casting characterized by a particularly uniform distribution of manganese, oxygen, sulfur and carbon. The cast steel bar according to the present invention comprises steel which, when viewed in cross section (lengthwise) with respect to macrosegregation, exhibits a maximum mean variation in oxygen content of less than about 20 ppm (0.0020%) and an oxygen segregation standard deviation of less than about 8 ppm (0.0008%) on samples containing about 0.01% oxygen, a maximum average variation in content sulfur less than about 0.004% (40 ppm) and a standard deviation of 8 ^ "...

sulfur segregation of less than about 0.001¾ (10 ppm) on a sample containing about 0.02% sulfur, and a maximum mean change in carbon content of less than about 0.01% (100 ppm) and a standard deviation of carbon segregation of less than about 0.004% (40 ppm) on a sample containing about 0.185% carbon, and improved tensile strength and elongation in the raw state. Similar good results are predicted for Si, P,

Cr and the other alloying elements normally used in steel. The microsegregation analysis was performed for C, Mn, S, Cr and Si by electron microprobe. The results indicate far less microsegregation of manganese than in prior art Concast samples. For example, molten metal charges, containing about 0.46% carbon and about 0.93% manganese, were poured both by the method described here and by the Concast method well known in the prior art. Samples of the raw casting bars were cut transversely and small samples were cut from places equivalent to about half the distance from the edge to the center. Thus, neither the best nor the worst areas were chosen for comparison, a small samples were mounted for examination by electron microprobe and they were o-analyzed to determine the concentration of manganese along a randomly chosen line , about 1800 microns in length.

This procedure is generally well known in the art and is believed to be the easiest method for detecting microsegregation in metals. Essentially, the method consists in bombarding the sample with a small diameter beam of high energy electrons with the result that the sample emits characteristic X-rays corresponding to the concentration of the elements present. X-rays are analyzed by diffracting them with a crystal (to choose only one element at a time) and then measuring their intensity with any suitable detector.

The concentrations of a particular element can be determined by comparing the relative intensities of the X-rays produced 9 ... · * »Uib.

η by the element in the sample to a known standard. To obtain the maximum accuracy, around 0.5%, the ratio must be corrected to take into account the absorption and fluorescence of the X-rays emitted. As a variant, it can be assumed that the average level of intensity represents the basic concentration obtained by normal chemical analysis and then the variations in intensity directly represent the variations with respect to the basic concentration.

When using this latter procedure to compare a certain number of samples, it is useful to assign to each sample a quantity which indicates the average intensity of significant variations, and thus the average variation of concentration compared to the basic level. . This quantity can be called the microsegregation index and is expressed as a percentage of the basic concentration.

In the present comparison, the manganese content of the sample according to the basic analysis was 0.98%. The results indicated that the prior art crude steel bar had a manganese microsegregation index, expressed as a percentage of the basic analysis, of approximately 300%, while the microsegregation index of the manganese of the raw casting bar produced by the process according to the present invention was less than about 175%.

An important feature of the present invention is that the new product can be prepared on a known casting machine, according to the prior art *, using procedures well within the reach of ordinary skill in the art.

The preferred embodiment of the present invention consists in forming a movable curved mold by rotating a casting wheel, comprising a peripheral groove, around its central axis and in advancing a strip along its length in contact with the peripheral groove at the top of the casting wheel, advancing the strip and the wheel together around the bottom of the wheel, and spreading the strip from the wheel (so as to form a type of surface mold endless mobile), to pour molten steel into the curved mold, to cool the mold <10 in order to cause solidification of the molten steel in the curved mold, to remove the casting bar from the curved mold, normally to further cool the casting bar by using an additional cooler after the casting bar has been removed from the closed part of the mold, and to progressively straighten the casting bar tj ^ as it leaves the curved mold. See for example the brief £ * E.U.A. N ° 3 623 535 des and the brevefVE.U.A. No. 359,348.

Thus, the object of the present invention is to provide an improved raw steel product for casting.

It also aims to provide a new continuous bar of cast steel characterized by an absence, to an exceptional degree, of segregation or reverse segregation of impurities or constituents in the bar.

It also aims to provide a process for the continuous casting of a steel product having improved internal properties.

Other objects, features and advantages of the invention will also result from the description below.

To the accompanying drawings ^

FIG. 1 schematically represents an example of an apparatus of the prior art which can be used for obtaining the cast steel product according to the invention, the apparatus comprising a casting machine comprising a rotating casting wheel defined by a periherical groove in this wheel and an endless band covering part of the length of the groove so as to form a closed mold on this part.

Figure 2 is a graph showing the distribution of sulfur in a cast steel bar according to the present invention.

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

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

11

FIG. 5 is a graph showing the distribution of oxygen in a steel bar cast according to another method of the prior art, the Junghans type method, and in particular a Concast machine of the industrially efficient type.

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

FIG. 7 is a histogram comparing the tensile strength of the new cast steel product according to the present invention with another cast steel product obtained by a process according to the prior art.

FIG. 8 is a graph showing the variation of the intensity of the X-rays due to the microsegregation of manganese in a steel bar cast according to the present invention.

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

In the drawings, where the same references designate identical elements, FIG. 1 represents a casting wheel apparatus 10 which can be used for obtaining the product according to the present invention. This apparatus is similar to that described in the patent. .AT. No. 3,623,535 for example. A casting wheel 10 defines a peripheral groove G which is covered, on a part of the periphery of the casting wheel, by an endless flexible strip or belt 11 so as to form a closed mold M. The flexible strip 11 is held against part of the periphery of the casting wheel by wheels 12, 13 and 14 for supporting the strip and it advances with the wheel 10 when the latter turns. Near the strip support wheel 12 where the closed mold M begins, the molten steel is discharged from a crucible or from a funnel 16 into the mold M by a chute 16a. In the preferred embodiment, all the external surfaces of the casting wheel and of the strip are continuously cooled by spraying with coolant, the external part of the groove and the strip being cooled by cooling jets from nozzles (not 12 shown) from manifolds S2, S3, S4, and the inner part of the peripheral groove G being cooled by jets from nozzles on the collector SI. The jet of each nozzle (or groups of nozzles) along the inner side of the peripheral groove can be adjusted individually so as to vary the volume of cooling fluid which it sends, and thus to vary the rate at which the metal in the mold M is cooled. The supply of the coolant to the nozzles (or to the nozzle groups) can also be regulated by adjustable valves so as to allow the establishment and stopping of the coolant stream and to allow a variation in the volume. total coolant flow. See, pageexample, the cooling system suggested by the breve ^ E.U.A. N ° 3,279,000 of

Southwire Company.

An extended bending zone 18 is placed above and above the band support wheel 14. The bending zone 18 serves as a means for straightening the cast steel bar B discharged from the peripheral groove of the wheel casting 10 after leaving the closed part of the mold M. The bending zone 18 comprises a multiplicity of support guide rollers 19 mounted on a frame (not shown). Side guide rollers (not shown) can also be used in the bending zone 18 to retain the cast steel bar approximately in a vertical plane. Although the support guide rollers 19 can be driven or not, the Applicant finds it preferable that at least some of the support rollers 19 are driven to help straighten the casting bar.

In the preferred embodiment, an additional cooling manifold 21 is located above and near the band support wheel 14 so as to send a direct spray of cooling fluid onto the cast steel bar leaving the curved mold Mr.

In operating the system according to the method of the present invention, the casting wheel 10 is rotated counterclockwise and the molten steel is dropped from the casting funnel 16 13 by the chute 16a in the closed mold M formed between the peripheral groove G of the casting wheel and the flexible strip 11. The molten steel is poured in a controlled manner well known in the technique of casting ferrous and non-ferrous metals ferrous in the mold M at a rate such that the rotation of the casting wheel advances the steel in the mold M from the chute 16a as fast as the molten steel flows through the chute so as to maintain the mass of molten steel at a constant level at the entrance to the mold M. The outlet end of the trough 16a is located as close as possible to the entrance to the mold M so as to allow the molten steel to '' flow directly from the chute into the mass of molten steel in the mold.

While the molten steel is entrained around the casting wheel 10 in the mold M, cooling fluid is sent against the mold by the nozzles of the manifold SI and the nozzles of the other collectors S2, S3 and S4 and the quantity of coolant sent to the strip and to the casting wheel is adjusted as necessary to adjust the cooling rate of the molten metal. According to the * preferred mode of implementation of the process, very uniform cooling is carried out around and along the longitudinal axis of the casting bar, see for example the patent of the E.ü.A. No. 3,279,000 in the name of Cofer.

Initial rapid cooling and molten dietary solidification occurs on the surfaces of the casting wheel and strip, causing the formation of a solidified steel crust or shell having an equiaxed grain structure. The continuation of the removal of heat from the partially solidified bar then causes the metal to solidify in the molten central zone in a progressive and uniform manner (also uniform at each point around the periphery) to form a dendritic or substantially equiaxed structure. , depending on the overheating of the steel, from the enclosure to the center of a solid steel bar B.

The steel which entered the mold M in the molten metal state at an upper part of the casting wheel advances in a descending direction around the lower part of the casting wheel and then in an ascending direction 14 up to that it leaves the closed part of the mold M near the band support wheel 14, passes through the sprayed mass of additional coolant coming from the nozzles associated with the manifold 21 and arrives at a guide wheel 15, after which it is guided so as to be distant from the casting wheel. In the preferred embodiment, the temperature of the outer surface of the peripheral crust of the steel solidifies while it leaves the closed part of the mold M does not exceed approximately 1370 ° C, but is not lower at around 1040-1090 ° C. The casting bar leaving the casting wheel has a shape corresponding to the curvature of the curved mold M and consequently is gradually straightened by gradual increase in the radius of the bar B while the bar advances in the extended bending zone 18. The rollers of guide 19 support the bar and guide it on its straightening path above the casting wheel 10, at least two of the guide wheels 19 being preferably driven to pull the bar B along its length out of the wheel casting 10. The central molten portion of the bar B is completely solidified when it passes through at least the last sprayed mass of coolant from the last nozzle on the manifold 21 to ensure that the bar is completely solid before reach a point located at the same level as the surface of the mass of molten metal at the entrance to the mold M. Thus, the molten metal in the central portion of the. bar will not flow in the opposite direction to the movement of the bar in the central non-solidified area of the bar to create a cavity in the central part of the bar.

The bar is thus also full and metallurgically sound before it enters the bending zone 18, and its temperature just before this zone can also be adjusted by adjusting the volume of cooling fluid supplied by the manifold 21 so as to limit the internal stresses. of the bar during its recovery.

In the embodiment of the casting wheel 10 described here, the mold M is of approximately trapezoidal shape with a small dimension located in the inner part 15 of the peripheral groove and a large dimension located near the strip 11. Thus, the steel bar cast by a typical casting machine 10 may have a width of about 66.7 mm at its largest width, a width of about 54.0 mm at its smallest width and a height of 47.6 mm, with a radius of approximately 6.35 mm connecting the smallest width to the two sides of the bar. Other dimensions and bar shapes can be cast if desired. So far, for example, demand will have had effective results in casting a bar of about 31 cm at a speed of about 2 13.4 meters per minute and a bar of about 52 cm at a speed of about 10.7 meters per minute.

Applicants believe that the new cast steel bar according to the present invention can be produced at a relatively high linear speed because the relatively large length of the curved mold M cooled by a cooling fluid ensuring rapid cooling makes it possible to obtain the solidification in spite of the relatively high rotational speeds of the casting wheel 10. In addition, the relatively small radius of the casting wheel 10 causes the orientation of the molten steel to change rapidly when the wheel turns, unlike techniques industrially proven continuous casting of steel according to the prior art in which the solidifying steel remains in a horizontal or approximately horizontal orientation for a significant period, allowing the impurities to float by moving upwards during the course of the solidification. The Applicant believes that when a bar B is poured with a relatively small cross-sectional area according to the present invention, the bar B can be rapidly solidified by spraying coolant from the nozzles associated with the manifolds SI, S2, S3, S4 and 21 before any significant segregation of constituents or impurities occurs. Thus, the process according to the present invention, in which a relatively rapidly rotating casting wheel and having a relatively small radius is cooled sufficiently to rapidly solidify the impurities before segregation and / or reverse segregation can occur gives the new product. steel '%' ......

16 cast according to the present invention, having properties significantly different from those of the cast steel produced in the continuous casting machines of the prior art.

The Applicant believes that the production of the mold in the form of a wheel, the spraying zones of cooling water and the relatively small section of the casting bar make it possible to obtain a higher speed of heat transfer than in processes for the continuous casting of steel according to the prior art.

We can get an idea of the high speed of cooling or solidification from the metallurgical heights of the casting systems. The metallurgical height is defined as the distance between the surface of the liquid mass in the mold and the point of complete solidification. In the present process, we worked with a metallurgical height of about 4.6 meters or less to make a bar 2 of 31 cm at 10.7 to 13.4 meters per minute and a bar of 252 cm at 7, 6 to 10.7 meters per minute. In conventional continuous casting systems of the Junghens type, metallurgical heights are generally reported to be approximately 15.2 to 21.3 meters for casting 10.2 cm x 10.2 cm billets at a speed of 2, About 5 to 3 meters per minute. It has been found that the casting bar according to the invention becomes completely solid in approximately 25 to 30 seconds, while it is understood that it takes approximately 6 minutes for the complete solidification of the Junghans type bar.

It is believed that the high cooling rate according to the invention reduces the flow of liquid of high relative concentration of dissolved substances in the metter-dendritic channels and thus reduces reverse segregation while the non-metallic impurities present in the liquid steel. freeze with liquid with a statistical distribution.

It is also believed that the rapid change in orientation of the molten steel in the advancing wheel-shaped mold reduces the risk of segregation of impurities to an undesirable position in the casting bar. That is to say, for example, that when the casting bar with its relatively thin initial solidified envelope and its large molten central zone advances η - '- 17 anticlockwise from the manifold to almost opposite S2 (see Figure 1), passes in front of collectors S3 and S4 and then 21 at a stage where the solidified envelope has become very large compared to the central molten zone, this mistletoe means that the bar poured has advanced by an arc of more than 90 ° and preferably more than 180 °, the Applicant thinks that this change of orientation of the casting during the whole course of solidification tends to eliminate the formation of high concentrations of separate constituents or impurities that would normally float to the top of the interior of the envelope being solidified, because essentially the "top" of the envelope being solidified is constantly changing throughout wheel rotation.

Measurements were made to determine the degree of segregation of sulfur and oxygen impurities and the degree of carbon segregation in new cast steel bars having been cast according to the method described. To establish a segregation profile from the wheel side of the casting bars to the strip side of the casting bars, three groups of samples were punched out for analysis from (long) cross sections of the raw casting bar. , one in the center of the section, one 20 mm to the left of the center and one to 20 mm to the right of the center. We then took the average of the values obtained to obtain an average profile for each bar. These results are represented in FIGS. 2, 3 and 6. The Applicant believes that the expressions interchangeably use the expressions "cross section (long) and simply" cross section "if there is no risk of confusion with a short cross-section (see ASTM Designation E399-74, Crack Plane Orientation Identification Code, for example).

FIG. 2 is a graph of the average percentage of sulfur present as a function of the position between the wheel side of the bar (0 mm) and the strip side of the bar (44 mm in this case), the steel having a composition in weight of about 0.45% carbon, 0.02% sulfur, 0.99% manganese, 0.02% phosphorus and 0.21% silicon (Sample No. 45). The maximum variation * 18 of average sulfur content in the bar is 0.0013% (13 ppm), for the measurements shown in Figure 2, and the standard deviation is 0.000498%. The Applicant believes that this represents a distribution of sulfur of unexpected uniformity without harmful segregation. Tests on other samples of the new product indicated maximum variations in average sulfur content ranging from 0.00114% to 0.004% (11.4 ppm to 40 ppm) and standard deviations for sulfur varying between 0, 000483% and 0.00138% in samples with sulfur contents of 0.01755% and 0.02993%, respectively, as shown below.

Average of 3 measurements of sulfur content at each position indicated, in ppm

Sample H ° 26 41_ 43 45 48 5 mm 170.3 308.4 234.3 226.7 316 15 mm 180.7 310.6 233.0 240.0 328 25 mm 174.3 271.0 223.7 235 .0 316 35 inm 170.7 297.6 227.0 239.7 317 40 mm 181.7 297.6 231.0 238.7 325 47 mm - 311.0 238.7

Average 175.5 299.3 231.2 236.0 320.4

Interval 11.4 40.0 11.7 13.0 12.0

Standard deviation 4.83 13.8 4.88 4.98 5.08

FIG. 3 is a graph of average oxygen content (in ppm) as a function of the position between the wheel and strip sides for the same casting bar, sample No. 45, as in FIG. 2. The oxygen content is d about 70 ppm (0.007%) and the maximum change in oxygen content in the bar, as shown in Figure 3, is 5 ppm (0.0005%) and the standard deviation is 1.651 ppm. This again represents a surprisingly good result - a very high degree of uniformity of the constituents in the structure of the new cast steel bar. It should also be noted that the porosity in the center, sometimes present in a continuously cast steel bar (even according to the invention), can contribute to the measured oxygen content at this particular location of the bar. The Applicant believes that such porosity does not represent true segregation for those skilled in the art and is generally eliminated by subsequent heat treatment.

The improved properties regarding oxygen segregation according to the present invention can be seen by comparing Figure 3 to Figures 4 and 5. Figure 4 is a graph of the percentage of oxygen content as a function of the position between bottom and bottom. top of a casting bar which was cast using a Hazelett Strip-Casting machine having a substantially horizontal mold. The graph is found on page 43, Figure 6, of Whitmore, BC, and Hlinka, JM, "Continous Casting of Low-Carbon Steel Slabs by the Hazelett Strip-Casting Process", Open Hearth Proceedings, 1969. conversion to ppm, Figure 4 indicates a maximum variation of approximately 100 ppm (0.01%) or more and a standard deviation of approximately 29.88 ppm. This Hazelett experimental mold process at approximately 20 ° relative to the horizontal thus produces a casting bar posing a notable problem of oxygen segregation even when the average oxygen content is relatively low, of about 0.004%. The strongest segregation is found near the top surface of the bar, as seen from Figure 4 and from the publication by Whitmore and Hlinka.

FIG. 5 is a graph of oxygen content as a function of the position for a casting bar produced by a Concast vertical mold continuous casting machine comprising a curved oscillating mold. The graph represents an average for five samples taken in a cross section (long) from the bottom to the top of the bar, the latter having a composition by weight of 0.46% of carbon, 0.94% of manganese, 0.021% of phosphorus , 0.016% sulfur and 0.22% silicon. The maximum variation shown in Figure 5 is about 26.5 ppm and the standard deviation is 10.6 ppm. The average oxygen content is approximately 0.006%. Another sample, having an average oxygen content of around 0.009%, shows a variation of 29 ppm.

A casting bar produced according to the process of the invention also has a surprisingly uniform distribution of carbon, as shown in FIG. 6 which is a graph representing an average profile of the carbon content of such a casting bar (Sample No. 48 ). This particular bar had a composition by weight of about 0.185% carbon, 0.59% manganese, 0.01% phosphorus, 0.032% sulfur and 0.17% silicon. The points plotted in FIG. 6 are averages of three measurements for each position in the casting bar between the wheel and belt sides, as was the case indicated for FIGS. 2 and 3. FIG. 6 / maximum variation of the average carbon content in the bar of approximately 0.009% (90 ppm) and a standard deviation of 0.00305%. According to the present invention, the steel melt is preferably prepared from a chemical system which contains carbon present as one of the constituents in the range of about 0.04% by weight to 1.4% in weight. The maximum variation for * samples in this range, unlike the present samples, should be proportional. The Applicant has found that particularly superior results are obtained when the carbon content of the steel is between 0.06% and about 0.80% by weight.

Still other measurements were made to determine the tensile strength in the raw state of the new cast steel bar according to the invention and to compare it with the tensile strength in the raw state of casting of a steel bar casting using a Concast machine industrially proven according to the prior art comprising an oscllant curved mold, the two steel bars having been cast from the same steel melt. Measurements were made at an elongation speed of 0.001 / second using a 2.54 cm extensometer. Figure 7 is a histogram showing that the tensile strength of a new cast bar is about 7,520-7,730 kg / cm, compared to that of the prior art Concast bar which had 2 strength tensile strength of approximately 6,540-6,610 kg / cm.

The steel composition of this melt was approximately 0.45% carbon, 0.97% manganese, 0.019% phosphorus, 0.017% sulfur and 0.21% silicon. Applicants believe that the increase of about 10% or more in tensile strength of the new product is consistent with the exceptionally uniform distribution of constituents and impurities observed in the new product, as described above.

21

In these same tests, it was also observed that the new raw casting bar had a higher percentage of elongation and a higher proportion in kg / cin than the Concast bar of the prior art, see below. ; Test tube Resistance% of allon- Limit of __ in traction gement proportionhalb

New 1 7 523 kg / cm2 10 4.52 kg / cm bar 2 7 734 kg / cm2 13 5.04 kg / cm2 3 7 593 kg / cm2 16 5.06 kg / cm2 7?

Bar according to 4 6 609 kg / cm 8 4.36 kg / cm ïî "! ^ Ante" 4 6 539 kg / cm2 8 4.06 kg / cm2 higher 3 3

FIG. 8 graphically represents the intensity of X-rays characteristic of manganese, along a line taken at random, of approximately 1800 microns in length, in a sample of steel bar cast according to the present invention.

We can see that the intensity level is relatively constant around the line marked 100% which corresponds to the basic concentration of 0.98% of manganese and which also corresponds to an absolute reading of approximately 11 units on the indicator. electronic microprobe analyzer graph. There is only one large variation which measures around 173% of the baseline, i.e. equivalent to a local concentration of around 1.69% of manganese.

FIG. 9 graphically represents the intensity of manganese X-rays in a sample cast according to the Concast method of the prior art. It should be noted that the intensity level contains many peaks which correspond to small areas of manganese segregation. Here, an absolute indication of about 12 units was observed on the recording tape indicator of the electron microprobe unit and used as the 100% line. The average value of the most important peaks is around 320% of the base level and the maximum variation in manganese content observed is more than 400%, see below:

Peak% of base level.

No. 1,396% 2,227% 3,292% 4,404% 5,262% 6,335%

Claims (25)

1. A continuous casting process, comprising the steps according to which ^ (jyQ a) is poured ^ ürTlTietal melted in a closed mold N which advances Jii) formed by at least one moving strip ") which closes the mold on part of ^ its J ^ ngjieur, b) coolitrTtT'moule so that the molten metal begins to solidify on the walls of the mold, forming a crust of metal around a central molten area, c) we bring out the cast bar at least partially solidified by the outlet from the closed part of the mold ^ and d) the casting bar is cooled by sending spraying directly or indirectly on it) coolant v, characterized in that, for continuous steel casting, e ) steps a) to d) are adjusted so as to obtain a continuous length of cast steel bar free of micro-segregation and having a particularly uniform distribution of the constituents and of the impurities, when the measurement is made in cross section. 2. A method according to claim 1, characterized in that in step e) a maximum variation in average sulfur content of less than about 0.004% (40 ppm) is obtained when the measurement is made in cross section. 23 3. A method according to claim 1, characterized in that in step e) the average sulfur content is calculated according to statistical empirical results whose standard deviation is less than about 0.0015%. 4. A method according to one of claims 1 and 2, characterized in that in step e) a maximum variation of the average oxygen content of less than about 0.002% (20 ppm) is obtained when the cross section measurement 5. A method according to any one of the preceding claims, characterized in that in step e) a maximum variation in average carbon content of less than about 0.01% (100 ppm) is obtained when the cross section measurement. 6. A method according to any one of claims 1 to 4, characterized in that in step e) one obtains an average carbon content calculated according to empirical statistical results whose standard deviation is less than 0. 004% approx. 7. A method according to any one of claims 1 and 4 to 6, characterized in that in step e) a maximum variation in average sulfur content of less than about 20 ppm is obtained. 8. A method according to claim 1, characterized in that in step e) there is obtained a maximum variation of average manganese content of less than about 400% of the average manganese content. 9. A method according to any one of the preceding claims, characterized in that one uses d | u ^ s step a) a closed mold for ^^ by a peripheral device ^ in a rotating casting wheel5 and a strip j closes this groove over part of its length. 10. A method according to any one of claims 1 to 8, characterized in that one uses in step a) a continuously advancing closed mold formed by at least one endless belt movable together with other surfaces sealing so as to constitute a closed mold. 11. A method according to any one of claims 1, 2 and 5 to 10, characterized in that in step e) the V · - ------..... 24 Maximum variation of the content average oxygen is less than about 10 ppm. 12. A method according to any one of claims 1 to 11 t characterized in that there is provided a mold with a moving surface of the endless wheel and belt type which can be used industrially in which the cast bar is turned during its solidification on a radial arc of substantially more than about 90 ° and the uniformity of the formation of concentrations of separate constituents and impurities is controlled to an extent such that a commercially acceptable steel bar results. 13. A method according to any one of claims * 1 to 12 / characterized in that the rotation of the casting wheel changes the orientation of the molten steel in the mold quickly enough to prevent any significant flotation and segregation of impurities in the steel. 14. A method according to any one of claims 1 to 13 / characterized in that in step e) the continuous length of cast steel bar has, for a given steel melt, a tensile strength at least 10% higher than that of a steel bar cast on a Junghans type casting machine from the same melt. 15. A method according to claim 12, characterized in that the arc is more than about 180 °. 16. A continuous cast steel bar with a maximum average oxygen content variation of less than about 25 ppm in cross-section. 17. A continuous cast steel bar having a cross-sectional standard deviation of oxygen segregation of less than about 10 ppm. 18. A continuous cast steel bar having a cross-sectional standard deviation of oxygen segregation of less than about 5 ppm. 19. A continuous cast steel bar having in cross section a maximum average variation in manganese content of less than approximately 275% of the average manganese content. „L: ' 20. A d1acierjcontinue bar according to one of the claims -% ~ --- ·· 25 cations 16 to 19, characterized in that it has in cross section a maximum average variation in sulfur content of less than about 0.004%. 21. A continuous casting steel bar according to one of claims 16 to 19, characterized in that it has in cross section a standard deviation of sulfur segregation. less than about 0.0015%. 22. A continuous casting steel bar according to one of claims 16 to 21, characterized in that it has in cross section a maximum average variation in carbon content of less than about 0.01%. 22 The Applicant has found that particularly superior results are obtained when the manganese content of the steel is between approximately 0.30% and 1.20% by weight. It is obvious that the invention is not limited to the embodiments described and that all variants can be made. For example, the Applicant has reported here only a sample representative of the unlimited number of steel compositions which can be cast according to the present invention. For other compositions in which the concentrations of the constituents and of the impurities differ from the exact concentrations in the samples analyzed, the applicant expects results to be improved in a proportional manner. 23. A continuous cast steel bar according to one of - claims 16 to 21, characterized in that it has in cross section a standard deviation of carbon segregation of less than about 0.004%. 24. A continuous casting steel bar according to any one of claims 16 to 23, characterized in that it has a tensile strength greater than at least 10% and an elongation greater than at least 10% ä those of a steel bar poured on a Junghans type casting machine from the same melt. 25. A continuous cast steel bar having, in cross section, a maximum average variation in oxygen content of less than 25 ppm, a maximum variation in sulfur content of less than 40 ppm and a maximum average variation in carbon content less than 100 ppm. <