GB1577780A - Method of and apparatus for converting molten metals into solidified products - Google Patents

Description

PATENT SPECIFICATION

( 21) Application No 28215/79 ( 22) Filed 20 June 1977 ( 62) Divided out of No 1577779 ( 31) Convention Application No 700080 ( 32) Filed 28 June 1976 in ( 33) United States of America (US) ( 44) Complete Specification published 29 Oct 1980 ( 51) INT CL 3 B 21 C 37/00 B 22 D 11/06 11/12//B 23 K 20/00 ( 52) Index at acceptance B 3 A 124 182 B 3 F IG 2 B IG 2 C 5IG 2 Q Xl IG 3 G 2 GIG 3 G 2 X 1 G 3 WI IG 3 WX IG 4 A IG 4 M IG 4 VI B 3 R 10 18 X 14 61 6 ( 54) METHOD OF, AND APPARATUS FOR, CONVERTING MOLTEN METALS INTO SOLIDIFIED PRODUCTS ( 71) I, ERIK ALLAN OLSSON, a Subject of Sweden, of Rotfluhstrasse 15, CH-8702 Zollikon/ZH, Switzerland, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in

and by the following statement:

This invention relates to metal processing and more particularly to a method of and an apparatus for converting molten metal into a semi-finished product for conversion into a finished product.

In the usual process of casting molten metal, and particularly steel for the production of solid sections to be subsequently converted into finished products, such as, for example, the process of continuous casting, the molten metal is charged into an open-ended mould where the molten metal next to the cold mould walls solidifies to a skin which at first rapidly thickens as initial solidification proceeds.

However, the rate of solidification progressively decreases as the solidification toward the centre of the casting increases.

The solidification time, "T", of a billet, depending on whether it is round, square or rectangular can be roughly approximated as proportional to the square of the diameter or thickness of the billet and, is usually expressed by the formula T=k D 2 where k is a factor depending on cooling conditions and D is the diameter or thickness from one surface to the other.

It is, or course well known that when molten metal particularly steel, solidifies rapidly, the casting has a fine grain structure and the quick solidification prevents or minimises segregation of some elements, such as, for example, the alloying elements in steel On the other hand slower solidification leads to larger or coarser and less desirable grain structure with accompanying rejection by the crystals as they form of some of the alloying elements, as well as "impurities", (which, in the case of steel may include S, P, As, Zn, Sn, etc) and their resulting concentration in the area of the casting last to solidify As a result, the outer portion of the casting, often referred to as "the chill zone layer" is superior from the standpoint of its fine grain structure, and also because it most nearly corresponds to the composition of the melt from which it was produced To more nearly approach a uniformity of section across a conventionally cast ingot, from either a mould or continuous casting, heat-treating, rolling and forging operations are necessary which would not be necessary if a chill zone composition and structure prevailed across the entire section of the ingot.

A method has been proposed wherein several continuously formed thin strands are continuously brought together in face-toface contact at a temperature desirably above a usual hot rolling temperature but at which liguid metal is not visible When light pressure less than that required for deforming the solidified metal is applied to the strands which are in face-to-face contact, fusing or welding at the contacting surfaces occurs by intercrystalline diffusion which takes place under these conditions This may be referred to as "flowless welding" or "pressure welding".

Thus, when forming a billet for example of diameter D according to this method of combining several individual layers or strands, the solidification time, being based on the thickness of the individual layers, is accelerated, so that the formula, instead of being expressed as T=k D 2 will be expressed as (D)2 T=K(S) where S designates the number of layers and 01 kr so us ( 11) 1 577 780 2 1,577780 9 K is a factor depending on the cooling conditions In pressing together a number of strands to effect welding, there is, at least in most cases, a reduction in thickness of the order of no more than about 27/ so that to secure the dimension D this reduction of thickness must be taken into consideration in determining the dimension D of the finished casting In other words, a tone of thin metal solidifying in separate layers from a molten condition solidifies much more rapidly than a ton of metal cast as a single casting into a billet or slab of the dimension D.

I 5 With perhaps two or three separate layers or strands being combined into a semifinished product, the formation of each separate layer using separate casting rolls for each layer is commercially practical, even with a separate pressure roll means for each additional layer over two, but with perhaps four, five or even ten or more layers being combined into a single slab or billet, the complication and space requirements for a plant having a separate casting unit for each layer together with pressure roll passes for each layer above two layers, and the investment involved in such a plant rapidly offsets the economy and advantages of the process of my earlier application.

According to one aspect of this invention there is provided a method of converting molten metal into a semi-finished product for subsequent conversion into a finished product comprising the steps of forming a thin strand of metal on a primary casting unit having a moving chill surface on which molten metal from a molten metal source congeals, delivering said thin strand of metal to an assembly unit, folding said thin strand back upon itself a plurality of times in said assembly unit to form a body of predetermined thickness by sequentially placing layers formed by said strand in faceto-face relation with each layer at a temperature where it will pressure weld to the contacting surface of an adjacent such layer, and effecting pressure welding to form a semi-finished product.

According to another aspect of this invention there is provided an apparatus for converting molten metal into a semifinished product for subsequent conversion into a finished product comprising a primary casting unit comprising a single moving chill surface and means for retaining a pool of molten metal against said chill surface whereby a thin strand of rapidly congealed metal is formed and removed by the chill surface away from the pool, means for removing the thin layer of metal so formed by the chill surface and delivering said layer to an assembly unit, said assembly unit comprising assembly means for folding said strand back upon itself a plurality of times to form sequentially superimposed layers in face-to-face relation, and a common consolidating means for applying pressure to effect pressure welding of said layers of metal into a unitary body.

The invention also relates to semifinished products made by such a method.

A preferred embodiment of the present invention comprises a method of and apparatus for converting molten metal, especially steel, into a finished product where the molten metal is cast into thin layers by contacting a moving chill surface with molten metal against which surface a thin layer of the metal solidifies.

Conveniently the metal is cast as a continuous wide flat strand.

The grain structure and composition of a thin strand cast in the manner herein disclosed is determined at the time a chilled layer forms on a moving chill surface and the combining of several layers into a common product, such as a billet of slab, slowing down the rate of heat transfer from the thicker product, is of little, if any, consequence at this time.

In the following description, the term "strand" is used to designate the emerging casting, whether continuously or intermittently produced on a cold surface moving in contact with a body of molten metal The resulting solidified layer or strand is thereafter stripped from the moving cold or chill surface and is desirably about 3 mm thick and has the fine grain structure and uniform composition, for all practical purposes, of the melt from which it is formed Such a strand is often referred to as being of chill zone thickness The output of a complete unit, whatever the shape or size, is generally designated "product", "unitary product" or "semi-finished product".

Where several strands are being simultaneously cast and combined as in prior proposed apparatus, a higher rate of production can be achieved than with the herein disclosed improvement, assuming dimensions to be the same, because several strands are being simultaneously produced and combined into a semi-finished product.

However, the improvement herein disclosed will nevertheless result in a rate of conversion of liquid metal into a billet or slab faster than the same tonnage could be converted by conventional continuous or semicontinuous casting This results from the increase in solidification speed with decreased thickness of the solidifying product.

Taking as an example, if 1 metre wide and mm thick steel slabs are being made by fusing together 3 mm thick layers, this layer thickness of 3 mm will be achieved in approximately I second Using a travelling 1,577,780 1,577,780 belt conveyor as a chilling wall which, over a length of 1 metre is brought into contact with liquid steel, the belt speed can be I metre per second for with-drawing said 3 mm thick solidified layer This corresponds to a production of approximately 1400 kg per minute To achieve this figure by conventional continuous casting in one strand, a rather sophisticated and expensive continuous casting machine would be required, considering that it would take about 6 minutes to get the section completely solidified at a minimum withdrawing speed of 1 24 metres per minute from an interior liquid pool of at least 7 5 metres length During this time the strand has to be properly cooled and supported over this length.

An even higher production rate can be obtained if the strand as cast is kept thinner while the length over which the belt is in contact with liquid steel is again only I metre Theoretically, a 1 mm thick layer would enable a withdrawal speed of approximately 540 metres per minute (due to the increase of solidification rate with decreased thickness) corresponding to approximately 4 tons per minute This productivity is hardly achievable using present continuous casting techniques where a liquid pool of approximately 21 metres would be required and a casting speed of approximately 3 5 metres per minute would be necessary If the width of the layer or strand were to be split longitudinally into mm wide strips to be fused together into mm sq billets, an output of the order of about 54 metre billets per minute would be theoretically achieved.

In order that the invention may be more readily understood and so that further features thereof may be appreciated the invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 is a view showing a casting unit for forming a strand of metal; Figure 2 shows schematically an apparatus in which layers of metal are laid, end-over-end, to form a slab from a continuously cast length of chill cast metal; Figure 3 is an end view of the apparatus shown in Figure I; and Figure 4 is a fragmentary longitudinal section on a larger scale showing the electrically heated inlay of thermal insulation in the top of the reciprocating table of Figures 2 and 3 on which the slab is formed.

Referring to Figure 1 a casting unit comprises a casting ladle 2 has a discharge tube 3 that extends into an intermediate container 4 in which molten metal is normally maintained at a depth sufficient to immerse the discharge end of the tube 3 A flow control valve is indicated in the tube 3 at 5 For assuring that the molten metal in the container 4 will in operation of the apparatus immerse the lower end of the tube 3, the container 4 has an internal weir 6 Under normal operating conditions molten metal flows over the weir 6 which also serves to assure floatation and removal of slag from the metal From the outflow side of the weir the metal flows through a connecting passage 7 of generally U-shape into a vessel 8.

By means of a continuously moving endless cooling wall, here constituted by the surface of an internally cooled rotatable drum 9 dipping into the vessel 8 and rotating at a uniform speed, a thin continuous layer of solidifying metal is deposited on the drum and withdrawn by the drum from the vessel.

This layer, designated S is stripped from the upper surface of the drum at or close to the temperature where it is substantially entirely solidified and is moved horizontally.

Figures 2, 3 and 4 show an apparatus in which the continuously formed strands of metal typically about 3 mm in thickness i e.

of chill zone thickness, is delivered to a layering device wherein the strand is folded back upon itself a number of times and the resulting layers are pressure welded The strand S is severed after a length, corresponding to a multiple of the length of each folded back layer and sufficient to produce a product of the desired thickness, has been formed and most of it has been supplied to the folding unit.

In Figures 2 and 3, a reciprocable table 10 has a reversing drive, schematically indicated at 11 The table has a recess in the top filled with a heat insulating material 12 of a length and width greater than the corresponding demensions of the product to be formed, and, when desired, there may be provided in the heat insulating material 12, embedded electric resistances or inductive heating conductors (see Figure 4) 13 connected with an energising current source, not shown.

There is a tiltable frame 14 with trunnions at opposite sides carried in fixed supports 16, shown fragmentarily in Figure 3 only, about which the frame 14 may rock in a vertical arc At equal distances from the axis of the trunnions about which the frame 14 tilts there are pressure rollers 17 and 17 a, the said axis of the trunnions desirably being centred on the line of centres between the rollers 17 and 17 a Extensions 14 a at each side of the frame 14 are pivotally connected to the upper ends of piston rods 18 a with pistons, not shown, in fluid pressure cylinders 18, one such piston and cylinder being located at each side of the frame.

Each of the piston and cylinder units is pivoted at 19 to a supporting frame (not 1,577,780 shown), in a manner well known in the art to accommodate the arcuate movement of the extensions 14 a as the piston rods move up and down.

The heated layer or strand S of metal, is directed downwardly through a heat insulating and, if necessary, an electrically heated enclosure 20 into the bight or nip between rolls 17 and 17 a There is a shear 21 at the exit end of the enclosure 20.

Assuming that the table is initially moving toward the right as indicated by the full line arrow in Figure 1, the roll 17 is depressed to bear on the hot strand S descending between the rolls 17 and 17 a to press the strand S against the top of the partially formed product 22 thereby to pressure roll and weld the strand of metal S to the underlying layer of metal of the product 22 being formed on the table 10 When the table 10 reaches its right limit of travel, the pressure cylinders 18 will be operated to lift the roll 17 and lower the left roll 17 a and the table 10 will then reverse to fold the metal the other way to be flattened down the roll 17 a and pressure welded to the previously completed layer, the dotted arrow indicating this reverse travel of the table.

When the product 22 approaches the desired final thickness, the shear 21 will be operated by a counter, or a thickness detecting switch means of any well known or preferred type so that the last layer will be complete and the product will comprise a billet or slab which is a single length of metal, layered and fused, the length of metal in the strand being a multiple of the length of the individual layers of which the finished product is comprised.

No arrangement for removing the slab or casting when finished has been shown, but one table may be provided to be run into place to replace the one to be unloaded or the casting may be slid endwise from the table onto a receiving conveyor or carriage.

As indicated by broken lines in Figure 1 the apparatus is preferably contained within an enclosure to which an inert, i e nonoxidising, gas is supplied and from which air is removed to avoid oxidation of the metal.

The embodiment shown in Figure 2 may be located in a corresponding enclosure.

In the operation of the described apparatus, a continuous layer of metal, typically about 3 mm, in thickness, i e of chill zone thickness, is formed, folded into uniform lengths, which are stacked or layered in the manner described to form a slab or billet of the required thickness Each layer, after the first one, is placed on the hot layer beneath it and because of its high temperature and clean surface, and because of the elimination or substantial elimination of air from the enclosure and the resultant prevention of any appreciable oxidation of the metal, a fusion welding of the metal between the contacting surfaces occurs under application of appropriate pressure.

The appropriate pressure depends upon the physical properties of the layers It must be sufficient immediately to cause intimate contact to be established between the confronting surfaces When slabs or billets are being formed from metal with poor plastic properties, it may be preferable to apply a relatively high welding pressure, the metal being at a temperature where it can stand a relatively high pressure without rupturing In the case of other grades of metal, e g, low carbon steel, which quickly reach good plasticity (hot working properties), relatively high pressure can be applied immediately since the material quickly becomes stiff Absolute parameters for particular grades of carbon and alloy steels may have to be determined according to the size and shape of the product and the composition of the metal.

As an example a 150 mm thick slab with a length of 3 metres will be made by fusing together at least 50 layers of a continuously formed strip, which is each 3 mm thick it leaves the solidifying apparatus With the strip S advancing at a speed of 1 metre per second and with the piston and table 10 making a full back-and-forth stroke every 6 seconds a slab will be completed about every 150 seconds However, a certain degree of height reduction, due to the pressure to which the hot metal is subjected, must be taken into consideration so that some additional layers will be needed for achieving the desired thickness of 150 mm of the finished piece However, the reduction of thickness results in a corresponding extension of the length.

Thus, the stroke cycle of the table 10 may be somewhat shorter and the production rate per unit of time remains the same That is, the finished weight per time unit is the same or approximately the same.

It may be noted that the terms "billet" and "slab" as used herein to designate the composite body produced as herein described are not strictly limited to bodies intended to be converted to a finished product by reheating, but is intended primarily to indicate shape, as a billet primarily indicates a shape which is thicker and more nearly of square section while slab refers to a product the width of which is much greater than the thickness Each term, however, is used in the sense of a semifinished product intended to be subsequently converted to a finished product by further working, hot or cold.

Typically, each cast layer in a billet or slab will be of the same thickness and this thickness will be in the range of perhaps 2 1,577,780 mm to about 5 mm, but may be thinner or slightly thicker.

It is also well known to metallurgists that, in processes such as this where intercrystalline diffusion takes place, time and temperature are significant variables, so that in some cases it may be desirable to retain the pressure welded product at elevated temperature in the non-oxidising atmosphere to take advantage of these factors before exposing the product to ambient temperature and atmosphere.

Whilst the invention has been described with reference to an embodiment in which metal is solidified on a cooled roll it is to be appreciated that the metal could be solidified on any appropriate cooled surface such as the surface of an endless belt.

Reference is made to co-pending Application No 25764/77 (Serial No.

1577779) from which the present application is divided This parent application relates to a method of converting molten metal into a semi-finished product for subsequent conversion into a finished product comprising the steps of forming a thin layer of metal on a primary casting unit having a single moving chill surface on which molten metal from a molten metal source congeals, cutting or severing said layer into sections and delivering said sections of said thin layer of metal to an assembly unit, assembling a plurality of said thin sections of metal in said assembly unit into bodies of pre-determined thickness by placing such sections in face-to-face relation which each other at a temperature at which each section will pressure weld to the contacting surface of an adjacent such section, and defecting the pressure welding to form a semi-finished product.

Claims (1)

1. WHAT I CLAIM IS:

   1 A method of converting molten metal into a semi-finished product for subsequent conversion into a finished product comprising the steps of forming a thin strand of metal on a primary casting unit having a moving chill surface on which molten metal from molten metal source congeals, delivering said thin strand of metal to an assembly unit, folding said thin strand back upon itself a plurality of times in said assembly unit to form a body of predetermined thickness by sequentially placing layers formed by said strand in faceto-face relation with each layer at a temperature where it will pressure weld to the contacting surface of an adjacent such layer, and effecting pressure welding to form a semi-finished product.

   2 A method as claimed in claim I further comprising the step of supplying heat to the strand between its initial formation and its assembly into a semi-finished product.

   3 A method as claimed in claim 1 or 2 in which heat loss from the strand is retarded and the metal forming each layer is retained at a temperature below a melting temperature but at which it will pressure weld to the layer against which it will be placed.

   4 A method as claimed in any one of the preceding claims in which said sections of metal are kept in a non-oxidising atmosphere at least until they have been pressure welded to form a semi-finished product.

   An apparatus for converting molten metal into a semi-finished product for subsequent conversion into a finished product comprising a primary casting unit comprising a single moving chill surface and means for retaining a pool of molten metal against said chill surface whereby a thin strand of rapidly congealed metal is formed and removed by the chill surface away from the pool, means for removing the thin layer of metal so formed by the chill surface and delivering said layer to an assembly unit said assembly unit comprising assembly means for folding said strand back upon itself a plurality of times to form sequentially superimposed layers in face-to-face relation, and a common consolidating means for applying pressure to effect pressure welding of said layers of metal into a unitary body.

   6 An apparatus according to claim 5 further comprising means for removing bodies so formed after pressure welding has been completed.

   7 An apparatus as claimed in claim 5 or 6 wherein the primary casting unit is a continuously operating unit in which molten metal is continuously solidified into a thin, wide casting for delivery to said assembly unit.

   8 An apparatus as claimed in any one of claims 5, 6, or 7, wherein means is provided for maintaining the stand at a temperature at which pressure welding may be effected by the consolidating means between one said layer and an adjacent said layer.

   9 An apparatus as claimed in any one of claims 5 to 8 wherein the casting unit, removing and delivering means and the assembly unit are contained in a common enclosure in which a substantially nonoxidising environment is maintained.

   An apparatus as claimed in any one of claims 5 to 9 in which the assembly unit comprises at least a support for the layers which are being assembled and wherein the consolidating means comprise one pressure roll arranged to apply pressure to the layers which are being assembled, means being provided for effecting relative longitudinal reciprocable travel of said layers on said support relative to the pressure roll.

   1,577780 11 An apparatus as claimed in any one of claims 5 to 10 wherein the primary casting unit comprises a continuously driven chill surface arranged constantly to contact the pool of metal and to remove therefrom a thin layer of metal congealed to its surface as a continuous layer.

   12 An apparatus as claimed in any one of claims 5 to 11 in which the assembly means comprises a reciprocable bed-plate and cooperating rollers which fold the strand back upon itself as the bedplate reciprocates.

   13 An apparatus as claimed in claim 12 in which the reciprocable bedplate has a heat insulating insert on which the strand is layered.

   14 An apparatus as claimed in claim 13 in which means are provided for heating said insert.

   15 A semi-finished product made by a method according to any one of claims I to 4.

   16 A method substantially as herein described with reference to Figures 1, 2, 3 and 4 of the accompanying drawings.

   17 A semi-finished product made by a method according to claim 16.

   18 An apparatus substantially as herein described with reference to and as illustrated in Figures 1, 2, 3 and 4 of the accompanying drawings.

   FORRESTER KETLEY & CO.

   Chartered Patent Agents Forrester House, 52 Bounds Green Road, London NI 1 2 EY and Rutland House 148 Edmund Street Birmingham B 3 2 LD also at Scottish Provident Bldg, 29, St Vincent Place Glasgow Gl 2 DT Agents for the Applicant.

   Printed for Her Majesty's Stationery Office, by the Courier Press, Leamington Spa, 1980 Published by The Patent Office, 25 Southampton Buildings, London, WC 2 A l AY, from which copies may be obtained.