Method of making a metallic simifinished product

REF: LHG / WGW / JK / 2590?

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corresponding to the request for a

PATENT OF INVENTION

Applicant: OLSSON INTERNATIONAL INC.

Residence: One Oliver Plaza, PITTSBURG, ALLEGHENY

COUNTY, Pennsylvania 15222, United States.

Statement: METHOD TO CONTINUOUSLY TRANSFORM METAL FOUNDED IN A SOLIDIFIED PRODUCT.

Priority: of US Patent Application No. 337,931 of March 5, 1,973

The invention relates to the manufacture of metal products, in particular, especially, but not exclusively ferric metal products and ferric alloys, and even more particularly to the transformation of the metal from the molten state into a semi-finished or finished product. .

When the liquid metal is poured into a mold, solidification progresses from the solidified layer that forms almost immediately where the metal is in contact with the mold, inwardly in the direction of the center. The speed at which solidification progresses depends on three factors that are the following:

(1) the coefficient of thermal transfer through the walls of the mold;

(2) the excess heat in the molten metal, above the melting heat that must be evacuated to reach the solidification point; and (3) the transmission of heat through the metal layers already solidified in the mold, to the walls of the mold.

To determine the rate of solidification, a simplified formula that is used in foundry technology and that is suitable for the process described here is frequently used. This formula is as follows:

s = k

In this equation, s is the thickness of the wall of the cast piece in millimeters; T is the time, in minutes, that has elapsed since the solidification stage; 1/2 factor k for steel varies from 22 to 33 nm per minute ', and can be calculated from different data derived from

- 3 1/2 practical tests. Naturally, the exponent means square root. The influence of overheating is not indicated in this formula, but for the needs of this description, it should not be taken into account.

This equation indicates that the speed of growth of the wall of the piece that solidifies from the outside »

inwards, it decreases rapidly from the beginning of the formation of the wall and the thickness of the same. For example, in the beginning, hardening or solidification up to a thickness of 5 mm requires only 0.028 minutes approximately if k is equal to 30, but to obtain a thickness increase of 5 millimeters after 4 minutes when the wall thickness is 60 mm, approximately 4.7 minutes were needed.

The solidification rate has a considerable effect on the structure and composition of the metal.

The metal layer that solidifies rapidly near the wall of the mold ingest small randomly oriented oristals that constitute a rapid cooling zone. This area will vary, depending on the composition of the steel or other metal, between 5 mm and 10 mm or even 12 mm thick. The calm steel forms an abrupt cooling zone of approximately 10 mm thick. In the zone of abrupt cooling, the chemical composition will adapt intimately to the composition of the molten steel and its mechanical properties are also superior to those of the steel that solidifies then at least in most steels. As the cooling rate decreases inwards from the abrupt cooling zone, large crystals known as dendrites are formed, giving

- 4 place to formation, of a fragile structure at the lamination temperature and which may have internal cracks unless only small reduction passes are used in the cross section during the initial lamination passes. Towards the center of the casting, the crystalline structure usually changes again and presents dendritic crystals oriented more randomly and of quite important size,

In addition to the increase in the size and arrangement of dendritic crystals in the ingot, the composition thereof changes. In the abrupt cooling zone, as mentioned above, the composition of the metal is almost similar to that of molten metal, while the constituent elements rejected in the crystals that are formed first, are increasingly concentrated towards the center.

Therefore, the last solidifying material contains the highest concentration of these elements that have been continuously rejected during the crystallization process. This tendency to segregation increases with solidification time so that the most important ingots have a stronger segregation than smaller ones. Likewise, in mold ingots in molds, the chemical composition of the metal will rarely be uniform in cross-sectional areas examined at intervals along the entire length of the cast. Continuous casting largely eliminates this variation of the chemical composition in different longitudinal sections of a piece, but it is not avoided, particularly in the most important sections, a difference in chemical composition and structure, from the outside to the center .

- 5 To overcome the effect of d these variations and develop the necessary mechanical properties, the cast must be reduced in its section, laminated or forged. The original cast structure with large crystals must be broken by rolling or forged in a range that depends on the size of the ingot and the composition, which can vary between 1: 5 and 1: 30. To achieve equalization of the composition by diffusion it may be necessary or convenient to apply a heat treatment.

These procedures for the transformation of molten metal into a reasonable annual tonnage of finished products or even semi-finished rolled products require a high investment cost in smelting plants, rolling mills, reheating furnaces as well as high labor costs and with Most operations high fuel costs. Although continuous casting has greatly reduced costs, not only because of reduced labor costs and increased performance, but also due to the fact that castings of smaller cross-section can be economically manufactured, eliminating thus the operation of rolling of ingots of great dimensions up to the sizes and lengths of the billet, the costs of investment and labor, however, They remain high for every ton of finished product.

According to the invention, the product is improved and costs are reduced by lamination and fusion or joint welding of metal parts consisting of the rapidly solidified metal of the thickness of the rough cooling zone to produce a semi-finished body intended to be laminated or forged, a procedure that differs from the one that consists of molding a semi-finished body of important transverse dimensions, working it hot to provide it with good mechanical properties. In other words, molten metal is transformed into sections of reduced thickness whose surfaces are fused or welded together to form a tight unitary body, endowed to its full extent with good mechanical and chemical properties.

• By way of illustration of the invention, molten metal can be hardened by contact with a cold surface in the form of a thin sheet that is combined with one or more strips or sheets formed in the same way at a certain temperature and pressure in an atmosphere does not oxidize or in a vacuum, where they merge together and are integrated into a single thicker strip or sheet, whose thickness is slightly less than the sum of the thicknesses of the individual strips or sheets, but that has the mechanical consistency and chemical uniformity of the individual sheets. Said integrated unit can then be laminated with laminators of relatively reduced power and in a few passes, obtaining a finished product such as, for example, a metal sheet of determined thickness. The work of molding a thick section is eliminated, cooling it to solidify and then working it by means of powerful laminators in numerous passes until the final thickness is obtained. Other layering methods can be used to obtain the necessary composite thickness, for example by bending on itself or cold rolling a sheet in the longitudinal direction or in the transverse direction, or jointly joining several individual layers with rapidly cooled flat sides, to Get a bar-like shape.

The invention may be more clearly understood by referring to the accompanying schematic drawings, in.

which:

Figure 1 represents a longitudinal section through a form of apparatus for carrying out the invention;

• Figure 2 is a perspective view representing another procedure, in which most of the apparatus has been indicated only schematically; '

Figures 3 to 6 are schematic cross-sectional representations depicting the development of the thickened section by folding a flat sheet on itself, FIG. 3 depicting the flat cast piece of reduced thickness; Figures 4 and 5 representing the progressive stages of bending, and Figure 6 indicating how the layers ae finally overlap each other just before their final fusion; Y

Fig. 7 is a perspective view indicating how a section from a continuously formed low-thickness cast piece can be cut to a predetermined length and can be firmly wound on itself to be cast in a single semi-finished body. of circular section, and can be cast or welded into rolls provided with peripheral grooves.

Referring first to Figure 1, the apparatus it represents includes a container 2 provided with a refractory lining. A spoon 3 is provided with a controlled outlet orifice 4 to discharge the molten metal into the container 2 in order to maintain a

- 8 uniform depth of metal in the container 2. A plurality of water-cooled hollow rollers 5 extend transversely to the container 2, three of said rollers being represented here. Each roller is positioned so that its axis and its bearings, not shown, but indicated by circles 6 and 7 respectively, are located above the level of the liquefied metal 8 inside the container 2. The lower portion of the periphery of each roller 5 penetrates the molten metal surface and extends below it.

The rollers are driven at the same speed by any well known means (not shown). Above the container 2 is a bell 9 by means of which a non-oxidizing atmosphere of an inert gas or a vacuum can be maintained above the container. When each roller rotates, a metal layer whose thickness can typically vary between 5 mm and 10 or 12 mm, or even more, is continuously formed on each 'surface of the roller so that the composition of this layer generally corresponds to that of the metal in the area of ​​abrupt cooling described above. The thickness of the layer thus formed depends on the length of the curved surface of the roller that is submerged in the molten metal, the thickness of the rollers, the speed of rotation thereof, the degree of overheating of the metal as well as the type and the quality of the metal.

When the roller to the left rotates, the hardened layer 10 thus formed is carried out of the mass of liquefied metal contained in the container. In separator 11 ensures the separation of this layer from the top of the roller and supports the still fragile molded part while

- 9 moves towards the upper part of the central roller where it is superimposed on the similar layer 12 leaving the central roller, its lower surface being cooler and firmer on the hotter upper surface newly formed of the layer 12 while a roller Pressure 13 applies a slight uniform pressure to result in the melting or welding of the two layers in a single thicker piece. This thicker piece that moves on a support and intermediate separator 14 is placed on top of the hardened layer 15 in the upper part of the right roller or last roller of this figure and again the coldest lower surface of the first two layers now combined , they come into contact with the newly formed surface of layer 15. Again the fusion occurs under the uniform pressure applied by the pressure roller 16. The pressure roller 13 and the pressure roller 16 can be driven by a motor or can only rotate by contact with the upper part of the casting. The cast 17, now constituted by three separate formed opals, joined together or after leaving the mass of molten metal from which they were obtained, moves on a separator 18 and exits through a guide 19 in the discharge tip of the apparatus, cons. the guide replacing a bell extension. The guide, with the cast piece that travels through it, serves to limit the entry of air into the space under the hood or the exit of the non-oxidizing inert gas. A duct 20 is called in the hood through which air can be drawn from the hood or through which inert gas can be introduced. Each pressure roller 13 and 16 has a pressure adjustment device indicated schematically at 13 'and 16' respectively.

Each casting roller can have means indicated at 21 to apply a separation coating on the face of the casting roller along which it extends on the descending surface of the roller. The new metal that penetrates the container 2 rises through · _ the holes 22 formed in a false bottom 23 under&gt; the rollers where refractory baffles 24 serve to keep the slags or surface impurities away from the location where the respective rollers They penetrate the molten metal.

When the molten metal leaves the guide 19 and at a distance sufficiently far from the casting unit so that the metal may have cooled to a normal rolling temperature, it is conveniently passed through a reduction rolling stand 25. Á From this rolling stand, it can be cooled to obtain a semi-finished product or it can be laminated directly into a finished product.

Although three rollers have been illustrated, their number can be higher and in fact numerous narrow pieces can be formed separately next to each other in one or more rollers and can be cast together to produce bars or levers. Likewise, the rollers have been described in particular since they are given by way of simple illustration for an apparatus that provides the surface cooled abruptly with continuous movement, but, depending on a certain degree of the product, endless belts or a continuous succession can be used of mold elements such as a tractor tread. Nevertheless,

As indicated, the layers or castings are formed separately, and then joined together and secured above the level of the molten liquid contained in the container, or after they have been separated from the mass of molten metal from from which they have been formed. 3e has indicated that the area of ​​rough cooling can vary between 5 and 12 mm for different steels, but it has been proven that it was convenient to form layers or pieces smaller than 5 mm and consolidate them without exceeding 12 mm. If the maximum value of 12 mm is exceeded, the invention may lose some of the advantages related to the quality of the product, but since it is not practical to continuously melt pieces less than 100 mm thick, an improved product can however be obtained, and The expense corresponding to the initiation of work with a much thicker cast piece can be eliminated. For example, this expense would be abnormal, for example, by starting the work with a continuous casting consisting of conventional casting, rolling it until the finished product is obtained. In other words, the formation of a single piece starting from layers whose thickness is greater than abrupt cooling zone or it can be more economical and provide a superior quality product, in comparison with the conventional practice that consists of making the initial casting of a piece of important section, working it next until obtaining a product of reduced thickness.

Figure 2 illustrates a procedure for transforming the liquid metal into a billet section. A single water-cooled hollow roller 30 is used which rotates in a container 31 provided with a refractory lining and containing a constant level of molten metal provided by a supply source. Said source of supply, not shown, may be similar, for example, to the siphoning ladle 3 of Figure 1. The lower part of the periphery of the roller is immersed in the molten metal.

As in Figure 1, this roller is actuated in a conventional manner, the drive device not being represented, and the roller is arranged inside a bell that surrounds it (not shown) to maintain a controlled inert atmosphere or a vacuum above the molten metal.

The surface of the roller 30 is provided with a series of ribs and grooves, such that the metal solidifies on the roller forming a cast 32 provided with a series of adjacent flat surfaces 33, connected with each other on the loins. and in the south 15 years of the cast. In other words, in this view, instead of each layer being formed completely separately from the others, as in Figure 1, the cast includes partially separated multiple layers connected in the manner indicated above in the ribs. and in the grooves. Upon exiting the molding roller, the melt moves towards a compaction interval formed between vertical rollers 34, where the various flat surfaces 33 are agglomerated and pressed against each other and fused together to form a section of. 25 billet 35 which can be further reduced in a finished product, or which may constitute a semi-barbed section for further transformation into a finished product. A separator (not shown) but similar to the separators shown in Figure 1 can be used to support the cast as it separates from the forming roller. Likewise, it is possible to apply a mold separation fluid to the roller as in Figure 1. In both methods described herein, the casting is formed continuously.

In Figures 3 to 6, another procedure is indicated in which, as in Figure 2, the folded folds or portions of a single cast piece are joined together. In this modification, a flat cast is formed as in Figures 1 or 2, forming it in a water-cooled roller or other cooled mobile element, then separating it from the roller. In FIG. 3, said flat cast piece is shown in 40. As indicated in Figure 4, the center 41 of the cast piece remains flat while the side margins 42 of the same width as the center are bent up on each side of the cast and bends one side over the center and then, as depicted in Figure 6, the other side is bent over the first one with a uniform pressure to cause the contact surfaces to melt. This procedure can be carried out or in ocular lengths or by dividing the continuously formed cast into pieces of predetermined length. When the consolidated part 43 thus formed has cooled to the normal lamination temperature, it can be laminated by means of a reducing laminator as shown in Figure 1.

In Figure 7, a flat cast of the type depicted in Figure 6, is cut transversely or longitudinally to the desired dimension and then rolled firmly on itself as indicated in 50 and the resulting round body is passed through rollers 51 provided with grooves to add sufficient pressure.

-14-while the metal is still hot to melt the turns with each other and form a solid tubular section, which, after cooling, can be rolled or worked.

It is impossible to empirically indicate dimensions, temperatures and pressures and the experts in the field should establish the parameters of the procedure after reasonable experimentation. In some cases it may even be necessary to apply supplementary heat while compacting, bending or rolling to produce the fusion of separate castings or layers or folds or the rolled-up areas of the same cast. In all cases, the composite product, by way of illustration, presents the lines where the contact surfaces meet, but, in actual practice, said laminated appearance is not visible.

However, it should be noted that while the metal solidifies from the liquid state to the solid state, some metals develop plastic properties at a temperature slightly less than the solidification temperature, while in others there may be a range of 100 ° C or more. and even up to 200 ° C or more between the solid state and the development of good plastic qualities. The welding or melting or consolidation of the layers of the castings can be carried out in the solidification phase or approximately in this phase, that is to say that it manifests itself in the form of a solid although there are indications of what, because of a Mediocre plasticity, it is possible that some liquid metal is still present between the dendrite formations. In other words, welding, or fusion or

- 15 consolidation will usually be done at a temperature higher than the temperature for which the metal has sufficient plasticity for normal lamination. As a general rule, the pressure applied by the compaction rollers that initially press the layers together will result in a reduction in the thickness of the compound of two or more layers in a range of $ 0.2 to $ 2. Steel with a reduced carbon content of approximately $ 0.10 quickly develops good plastic properties below the solidification point and can in many cases undergo a compaction or consolidation pressure of the metal in a single-piece body such that it produces a reduction of section of $ 20, while steels of important impurity content such as S, P, Cu, Sn, etc., they will have a defective plasticity at least 150 ° C below the solidification temperature and cannot tolerate a reduction of more than $ 0.3. Therefore, the pressure destined to produce the fusion must be such that the reduction in thickness is never greater than that necessary to produce a good quality joint, that is sometimes of the order of - 2 $ based on the tolerance of the metal. 3i the pressure exceeds these critical limits, the agglomerated product will present several weak points or internal defects. After reaching the normal rolling temperature, the agglomerated cast piece may be subjected to a thickness reduction of up to $ 20 or more in a single rolling pass.

It is not difficult to understand for example that if three layers of rough cooling zone thickness of approximately 10 mm each are combined into a single body of 30 mm, and reduced by $ 20 to obtain a semi-finished product,

- 16 much less work is needed to transform it into a 10 or 20 gauge metal sheet than by rolling a molten ingot several cm thick to the same thickness and extensive hot work is not needed with this procedure because the metal is constituted totally or almost totally by a metal whose thickness corresponds to the zone of rapid cooling with crystalline structure, as explained above and that has the metal composition of the rapid cooling zone.

In summary, the Invention Patent requested must fall on the following: