Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21D11/00—Bending not restricted to forms of material mentioned in only one of groups B21D5/00, B21D7/00, B21D9/00; Bending not provided for in groups B21D5/00 - B21D9/00; Twisting
  • B21D11/06—Bending into helical or spiral form; Forming a succession of return bends, e.g. serpentine form
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/06—Continuous casting of metals, i.e. casting in indefinite lengths into moulds with travelling walls, e.g. with rolls, plates, belts, caterpillars
  • B22D11/0637—Accessories therefor
  • B22D11/0648—Casting surfaces

Definitions

• This invention relates to the casting of metal strip. It has particular but not exclusive application to the casting of ferrous metal strip.
• nip is used herein to refer to the general region at which the rolls are closest together.
• the molten metal may be poured from a ladle into a smaller vessel or series of vessels from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip.
• This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow, although alternative means such as electromagnetic barriers have also been proposed.
• Patent discloses that the application of vibratory movements of selected frequency and amplitude make it possible to achieve a totally new effect in the metal solidification process which dramatically improves the heat transfer from the solidifying molten metal, the improvement being such that the thickness of the metal being cast at a particular casting speed can be very significantly increased or alternatively the speed of casting can be substantially increased for a particular strip thickness.
• the improved heat transfer is associated with a very significant refinement of the surface structure of the cast metal.
• the Arithmetic Mean Roughness Value which is generally indicated by the symbol R a .
• This value is defined as the arithmetical average value of all absolute distances of the roughness profile from the centre line of the profile within the measuring length l m .
• the centre line of the profile is the line about which roughness is measured and is a line parallel to the general direction of the profile within the limits of the roughness-width cut-off such that sums of the areas contained between it and those parts of the profile which lie on either side of it are equal.
• the Arithmetic Mean Roughness Value may be defined as
• a light oxide deposit can be beneficial in ensuring a controlled even flux during metal solidification on to the casting roll surfaces.
• the oxide deposit melts as the roll surfaces enter the molten metal casting pool and assists in establishing a thin liquid interface layer between the casting surface and the molten metal of the casting pool to promote good heat flux.
• the oxides then resolidify with the result the heat flux decreases rapidly.
• the resulting heat flux variations in the solidifying shells produce the localised distortions leading to crocodile-skin surface defects.
• a method for continuously casting metal strip of the kind in which a casting pool of molten metal is formed in contact with a moving casting surface such that metal solidifies from the pool onto the moving casting surface, wherein the casting surface is provided by a solid coating on a heat conductive body and the coating is formed of a material such that the wetting angle of said molten metal on the casting surface is less than 40° and which has a melting temperature greater than the temperature of the casting surface during metal solidification.
• the coating is formed of a material such that the wetting angle of the molten metal on the casting surface is less than 20°.
• the coating surface has an Arithmetic
• the coating material should be chosen so as not to melt or to dissolve in said molten metal at the temperature of the casting surface during solidification.
• the coating material is at least partially amorphous. It may, for example, comprise an amorphous alloy of two metals. One of those metals may be phosphorous .
• the coating material may comprise an amorphous nickel-phosphorous alloy containing about 10% phosphorous.
• the heat conductive body may be a body of copper or a copper alloy.
• the molten metal may be a ferrous metal.
• the molten metal may be molten steel.
• the choice of a coating material on which the molten steel has a low wetting angle enables solidification of the steel into a single phase solid structure on the casting surface.
• the invention also provides a method for continuously casting steel strip of the kind in which a casting pool of molten steel is formed in contact with a moving casting surface such that steel solidifies from the casting pool onto the moving casting surfaces, wherein the casting surface is provided by a solid coating on a heat conductive body and the coating is formed of a material such that the wetting angle of the molten steel on the casting surface is less than 40° and which has a melting temperature greater than the temperature of the casting surface during metal solidification, and wherein the steel solidifies into a single phase solid structure on the casting surface which phase does not transform before the strip has left the casting surface.
• the method of the present invention may be carried out in a twin roll caster.
• the invention further provides a method of continuously casting metal strip of the kind in which molten metal is introduced into the nip between a pair of parallel casting rolls via a metal delivery nozzle disposed above the nip to create a casting pool of molten metal supported on casting surfaces of the rolls immediately above the nip and the casting rolls are rotated to deliver a solidified metal strip downwardly from the nip, wherein the casting surfaces of the rolls are provided by solid coatings on heat conductive roll bodies which coatings are formed of a material such that the wetting angle of said molten metal on the casting surfaces of the rolls is less than 40° and which material has a melting temperature greater than the temperature of the casting surfaces during metal solidification.
• Figure 1 illustrates experimental apparatus for determining metal solidification rates under conditions simulating those of a twin roll caster
• Figure 2 illustrates an immersion paddle incorporated in the experimental apparatus of Figure 1
• Figure 3 illustrates thermal resistance values obtained during solidification of a typical steel sample in the experimental apparatus
• Figure 4 illustrates the relationship between wettability of an interface layer and measured heat flux and interface resistance
• Figures 5 illustrates the effect of wettability on the resistance to nucleation
• Figure 6 illustrates shell surface temperatures occurring in steel shells deposited on chromium substrates
• Figure 7 plots the results of heat flux measurements on steel shells deposited on nickel- phosphorous substrate and on chromium substrates;
• Figure 8 plots K-value measurements on steel shells deposited during dip tests using nickel-phosphorous alloy and chromium substrates
• Figures 9 and 10 are photomicrographs of steel shells deposited in the dip tests to which Figure 8 refers;
• Figure 12 to 16 are photomicrographs of steel shells deposited during the dip test to which Figure 11 refers;
• Figure 17 is a plan view of a continuous strip caster which is operable in accordance with the invention.
• Figure 18 is a side elevation of the strip caster shown in Figure 17;
• Figure 19 is a vertical cross-section on the line 19-19 in Figure 17;
• Figure 20 is a vertical cross-section on the line
• Figure 21 is a vertical cross-section on the line 21-21 in Figure 17.
• FIGS. 1 and 2 illustrate a metal solidification test rig in which a 40 mm x 40 mm chilled block is advanced into a bath of molten steel at such a speed as to closely simulate the conditions at the casting surfaces of a twin roll caster. Steel solidifies onto the chilled block as it moves through the molten bath to produce a layer of solidified steel on the surface of the block. The thickness of this layer can be measured at points throughout its area to map variations in the solidification rate and therefore the effective rate of heat transfer at the various locations. It is thus possible to produce an overall solidification rate as well as total heat flux measurements. It is also possible to examine the microstructure of the strip surface to correlate changes in the solidification microstructure with the changes in observed solidification rates and heat transfer values.
• the experimental rig illustrated in Figures 1 and 2 comprises an induction furnace 1 containing a melt of molten metal 2 in an inert atmosphere which may for example be provided by argon or nitrogen gas.
• An immersion paddle denoted generally as 3 is mounted on a slider 4 which can be advanced into the melt 2 at a chosen speed and subsequently retracted by the operation of computer controlled motors 5.
• Immersion paddle 3 comprises a steel body 6 which contains a substrate 7 in the form of a chrome plated copper disc of 46 mm diameter and 18 mm thickness. It is instrumented with thermo-couples to monitor the temperature rise in the substrate which provides a measure of the heat flux.
• the total resistance to heat flow from the melt to the substrate is governed by the thermal resistances of the solidifying shell and the shell/substrate interface.
• the heat transfer resistance is dominated by the solidifying shell resistance.
• our experimental work has demonstrated that under thin strip casting conditions, where solidification is completed in less than a second, the heat transfer resistance is dominated by the interface thermal resistance at the surface of the substrate .
• the heat transfer resistance is defined as
• Figure 3 illustrates thermal resistance values obtained during solidification of a typical manganese killed low carbon steel sample in the test rig. This shows that the shell thermal resistance contributes only a small proportion of the total thermal resistance which is dominated by the interface thermal resistance.
• the interface resistance is initially determined by the melt/substrate interface resistance and later by the shell/substrate interface thermal resistance. Furthermore, it can be seen that the interface thermal resistance does not significantly change in time which indicates that it will be governed by the melt/substrate thermal resistance at the initial melt/substrate contact.
• melt/substrate interface resistance and heat flux are determined by the wettability of the melt on a particular substrate. This is illustrated in Figure 4 which shows how interface resistance increases and heat flux decreases with increasing wetting angle which corresponds with reducing wettability.
• the substrate it is necessary for the substrate to have an Arithmetic Mean Roughness Value (R a ) of less than 5 microns in order to obtain full wetting of the substrate, even with the application of vibration energy.
• R a Arithmetic Mean Roughness Value
• the casting surfaces of the rolls can be formed of a material which has a much higher affinity for steel melts so producing much better wetting than can be achieved with a chrome surface.
• the smoothness of the substrate is not particularly critical although it is desirable in practice that the casting surfaces have an Arithmetic Mean Roughness Value (R a ) of less than 10 microns in order to produce strip having a reasonably good surface finish and a refined microstructure.
• Wetting angles above 75° represent poor wetting above which there will be a significant energy barrier to metal solidification.
• Twin roll strip casters for casting ferrous metals have traditionally employed casting rolls with chromium or nickel casting surfaces generally produced by plating. Such surfaces are tough and are generally able to withstand the thermal stresses associated with strip casting. Moreover, steel melts have moderately good wettability on chromium and nickel surfaces enabling effective heat flux values to be achieved. We have also determined that the metal oxides deposited from typical steel melts used for strip casting have a high affinity for chromium and nickel and they therefore exhibit good wettability on such casting surfaces ie very low wetting angles. This means that there is a very strong tendency for an oxide coating to spread over the casting surfaces and to build up as casting proceeds .
• Figure 6 illustrates the measurement of shell surface temperatures occurring in steel shells deposited on chromium surfaces in the dip tested illustrated in Figures 1 and 2 for both a clean substrate surface and a surface with a heavy oxide deposit. It will be seen that for smooth substrate surface the surface of the solidifying shell decreases smoothly in temperature as solidification progresses. Where the substrate has a heavy oxide covering the shell initially proceeds with under-cooling down to a temperature approaching 1200°C at which stage there is a sudden reversal and the temperature of the shell increases. It is thought that the under-cooling proceeds while the oxides are in liquid form but as the temperature approaches 1200°C the oxides solidify to provide nucleation sites for metal solidification.
• Figure 7 plots heat flux measurements obtained on solidification of a carbon steel onto substrates of the nickel-phosphorous alloy compared with heat flux measurements on solidification of shells of the same steel onto chrome substrates.
• the carbon steel in these tests had the following composition which we designate an M06 steel:
• Figure 8 provides the results of measurements of K- value (an indication of heat flux) for multiple dip tests using a carbon steel melt of the above composition and with the nickel-phosphorous alloy and chromium substrates. It will be seen that the results in both Figures 7 and 8 demonstrate that the nickel-phosphorous alloy substrates resulted in much higher heat flux than the normal chrome substrates. In the case of the nickel-phosphorous alloy substrate there was a variation in heat flux over different tests and particularly a decreasing K-value throughout successive dip tests in the results shown in Figure 8. These variations were due to melting of the nickel- phosphorous alloy substrate surface as testing proceeded.
• Figures 9 and 10 are photomicrographs of M06 steel shells deposited onto substrates of nickel-phosphorous alloy in the case of Figure 9 and a traditionally chromium substrate in the case of Figure 10, both photomicrographs being shown to a magnification of xlOO. It will be seen that the shell deposited onto the nickel-phosphorous alloy substrate is almost twice as thick as the shell deposited on the chromium substrate, reflecting the much higher heat flux and more rapid solidification obtained with the nickel-phosphorous alloy substrate. This demonstrates that very much higher solidification rates are achievable and that accordingly the invention will permit strip casting to proceed at dramatically higher production rates than have hitherto been thought possible.
• the microstructure of the shell deposited on the nickel-phosphorous alloy substrate is significantly finer than that deposited on the traditional chrome substrate and is moreover remarkably even throughout the shell.
• This microstructure exhibits prior austenite grain boundaries that exactly follow dendritic grain boundaries demonstrating that the liquid carbon steel has solidified directly into austenite. With this solidification process there is no potential for the development of crocodile-skin defects since these defects arise only when both ⁇ and ⁇ phases are present in the solidifying steel shells. It has been found that the enhanced wettability of the nickel-phosphorous alloy is due to its amorphous structure.
• Liquids will in general have a high surface affinity for other liquids since they have no preferential orientation and we have now found that a similar effect can be produced when wetting an amorphous solid. Wettability of a liquid metal on a coating surface can accordingly be dramatically increased if the coating material is totally amorphous.
• the nickel-phosphorous alloy with a 10% phosphorous content is a substantially eutectic composition and can readily be deposited by an electroless process so as to have an effectively totally amorphous structure. If the phosphorous content is varied so as to depart significantly from the eutectic composition the deposited two-metal alloy coating will exhibit a partially crystalline structure rather than being totally amorphous.
• a crystalline structure can also be produced by annealing the coating at elevated temperature after deposition, a phenomenon that is used to increase hardness in some coating applications.
• Figure 11 provides the results of measurements of K-value for multiple dip tests. Dip tests 1-27 all used steel melts having the above described M06 composition. In tests 1-9 steel shells were deposited on both a smooth nickel substrate having an R a value of 5.6 and on a nickel- phosphorous alloy substrate having a 10% phosphorous content and a Mean Roughness Value (R a ) of 8.7.
• Figure 11 further shows the results of tests 16-23 in which M06 steel shells were deposited on smooth nickel- phosphorous alloy substrates which all had a 10% phosphorous content but in which one of the substrates had been subjected to annealing at 400°C for 1.5 hours after electroless deposition and the other substrate was not subjected to any annealing. It will be seen that the substrate not subjected to annealing produced the high K- values as previously experienced in tests 1-9 whereas the annealed substrate produced much lower K-values comparable with those achieved with the plain nickel substrate.
• Figures 12 is a photomicrograph of the shell deposited on the nickel substrate in test number 11
• Figure 13 is a photomicrograph of the shell deposited on the annealed substrate in dip test number 18
• Figure 14 is a photomicrograph of the shell deposited on the unannealed nickel phosphorous alloy substrate in the same dip test number 18. It will be seen that the microstruecures exhibited in the shells deposited on the nickel substrate in test 11 and the annealed substrate in test 18 are similar. In both cases the shells are relatively thin and have coarse microstructure exhibiting initial solidification into ferrite.
• the shell of Figure 14 as deposited onto the unannealed alloy substrate is very much thicker and exhibits the finer microstructure associated with the extremely high solidification rates achievable in accordance with the present invention which result in initial solidification directly into austenite.
• Figure 8 further provides the results of tests 24- 27 in which shells were deposited on a nickel-phosphorous alloy substrate having a 10% phosphorous content but only partially annealed at 400°C for 45 minutes as compared with a control substrate of nickel as previously used in tests 1-15. It will be seen that the partially annealed substrate resulted in K-values generally between those of the unannealed and annealed substrates of tests 16-23, further demonstrating the effect of an amorphous coating and the graded reduction of K-value and heat flux according to the degree to which a crystalline structure is present in the coating.
• Figure 11 also provides results obtained from tests 29-31 in which shells were deposited on a nickel- phosphorous alloy substrate of 10% phosphorous content from a peritectic steel composition having a carbon content of
• the peritectic steel composition produced a shell demonstrating the same microstructure as previously achieved with M06 steels on the nickel-phosphorous alloy substrates and with the same K-values, demonstrating a similar heat flux on solidification.
• FIG. 15 The solidification structure of the shell of peritectic steel produced in test number 30 is shown in Figure 15 and the solidification structure of a shell of the same steel deposited onto a textured chrome substrate is shown in Figure 16. It will be seen that the structure of Figure 15 is much the same as that exhibited in Figures 9 and 14 and exhibits prior austenite grain boundaries demonstrating that the liquid carbon steel has solidified directly into austenite. Moreover, there is no indication of any ferrite growth even after solidification has progressed to the stage where the cooling rate has decreased.
• the material of the casting surfaces of the rolls must have a melting temperature higher than the temperature of the casting surfaces during metal solidification.
• the temperature of the casting surfaces will be dependent on the wetting angle of the molten metal on the casting surfaces. More specifically, the temperature that the casting surface coating experiences will be higher as the wetting angle decreases.
• the coating material may therefore be chosen so as to provide a balance between high heat flux and rapid solidification and maintenance of a coating temperature which is safely below the melting temperature of the coating.
• the results illustrated in Figures 8 and 11 for the unannealed nickel-phosphorous substrates show a progressive loss of performance due to erosion of the coating.
• other two metal amorphous coatings may be used in accordance with the invention.
• Table 1 sets out the relevant criteria for a number of possible alloy coatings which might be used in accordance with the invention in the casting of thin strip steel. Table 1
• the interface temperature has been calculated on the assumption of perfect contact of a steel metal at 1650°C with a coating of 25°C.
• FIGS 17 to 21 illustrate a twin roll continuous strip caster which may be operated in accordance with the present invention.
• This caster comprises a main machine frame 11 which stands up from the factory floor 12.
• Frame 11 supports a casting roll carriage 13 which is horizontally movable between an assembly station 14 and a casting station 15.
• Carriage 13 carries a pair of parallel casting rolls 16 to which molten metal is supplied during a casting operation from a ladle 17 via a tundish 18 and delivery nozzle 19 to create a casting pool 30.
• Casting rolls 16 are water cooled so that shells solidify on the moving roll surfaces 16A and are brought together at the nip between them to produce a solidified strip product 20 at the roll outlet.
• This product is fed to a standard coiler 21 and may subsequently be transferred to a second coiler 22.
• a receptacle 23 is mounted on the machine frame adjacent the casting station and molten metal can be diverted into this receptacle via an overflow spout 24 on the tundish or by withdrawal of an emergency plug 25 at one side of the tundish if there is a severe malformation of product or other severe malfunction during a casting operation.
• Roll carriage 13 comprises a carriage frame 31 mounted by wheels 32 on rails 33 extending along part of the main machine frame 11 whereby roll carriage 13 as a whole is mounted for movement along the rails 33.
• Carriage frame 31 carries a pair of roll cradles 34 in which the rolls 16 are rotatably mounted.
• Roll cradles 34 are mounted on the carriage frame 31 by interengaging complementary slide members 35, 36 to allow the cradles to be moved on the carriage under the influence of hydraulic cylinder units 37, 38 to adjust the nip between the casting rolls 16 and to enable the rolls to be rapidly moved apart for a short time interval when it is required to form a transverse line of weakness across the strip as will be explained in more detail below.
• the carriage is movable as a whole along the rails 33 by actuation of a double acting hydraulic piston and cylinder unit 39, connected between a drive bracket 40 on the roll carriage and the main machine frame so as to be actuable to move the roll carriage between the assembly station 14 and casting station 15 and vice versa.
• Casting rolls 16 are contra rotated through drive shafts 41 from an electric motor and transmission mounted on carriage frame 31.
• Rolls 16 have copper peripheral walls formed with a series of longitudinally extending and circumferentially spaced water cooling passages supplied with cooling water through the roll ends from water supply ducts in the roll drive shafts 41 which are connected to water supply hoses 42 through rotary glands 43.
• the roll may typically be about 500 mm diameter and up to 2000 mm long in order to produce 2000 mm wide strip product.
• Ladle 17 is of entirely conventional construction and is supported via a yoke 45 on an overhead crane whence it can be brought into position from a hot metal receiving station.
• the ladle is fitted with a stopper rod 46 actuable by a servo cylinder to allow molten metal to flow from the ladle through an outlet nozzle 47 and refractory shroud 48 into tundish 18.
• Tundish 18 is also of conventional construction. It is formed as a wide dish made of a refractory material such as magnesium oxide (MgO) .
• MgO magnesium oxide
• One side of the tundish receives molten metal from the ladle and is provided with the aforesaid overflow 24 and emergency plug 25.
• the other side of the tundish is provided with a series of longitudinally spaced metal outlet openings 52.
• the lower part of the tundish carries mounting brackets 53 for mounting the tundish onto the roll carriage frame 31 and provided with apertures to receive indexing pegs 54 on the carriage frame so as to accurately locate the tundish.
• Delivery nozzle 19 is formed as an elongate body made of a refractory material such as alumina graphite.
• Nozzle 19 may have a series of horizontally spaced generally vertically extending flow passages to produce a suitably low velocity discharge of metal throughout the width of the rolls and to deliver the molten metal into the nip between the rolls without direct impingement on the roll surfaces at which initial solidification occurs.
• the nozzle may have a single continuous slot outlet to deliver a low velocity curtain of molten metal directly into the nip between the rolls and/or it may be immersed in the molten metal pool.
• the pool is confined at the ends of the rolls by a pair of side closure plates 56 which are held against stepped ends 57 of the rolls when the roll carriage is at the casting station.
• Side closure plates 56 are made of a strong refractory material, for example boron nitride, and have scalloped side edges 81 to match the curvature of the stepped ends 57 of the rolls.
• the side plates can be mounted in plate holders 82 which are movable at the casting station by actuation of a pair of hydraulic cylinder units 83 to bring the side plates into engagement with the stepped ends of the casting rolls to form end closures for the molten pool of metal formed on the casting rolls during a casting operation.
• the ladle stopper rod 46 is actuated to allow molten metal to pour from the ladle to the tundish through the metal delivery nozzle whence it flows to the casting rolls .
• the clean head end of the strip product 20 is guided by actuation of an apron table 96 to the jaws of the coiler 21.
• Apron table 96 hangs from pivot mountings 97 on the main frame and can be swung toward the coiler by actuation of an hydraulic cylinder unit 98 after the clean head end has been formed.
• Table 96 may operate against an upper strip guide flap 99 actuated by a piston and a cylinder unit 101 and the strip product 20 may be confined between a pair of vertical side rollers 102.
• the coiler is rotated to coil the strip product 20 and the apron table is allowed to swing back to its inoperative position where it simply hangs from the machine frame clear of the product which is taken directly onto the coiler 21.
• the resulting strip product 20 may be subsequently transferred to coiler 22 to produce a final coil for transport away from the caster.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Continuous Casting (AREA)
• Soft Magnetic Materials (AREA)
• Manufacture Of Alloys Or Alloy Compounds (AREA)
• Manufacture And Refinement Of Metals (AREA)

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