EP0144769B1

EP0144769B1 - Matrix coating flexible casting belts, method & apparatus for making matrix coatings

Info

Publication number: EP0144769B1
Application number: EP84113365A
Authority: EP
Inventors: Norman J. Bergeron; Wojtek S. Szczypiorski; James G. Villa; S. Richard Hazelett; R. William Hazelett; Dean A. Boozan; Thomas E. Brennan
Original assignee: Hazelett Strip Casting Corp
Current assignee: Hazelett Strip Casting Corp
Priority date: 1983-11-07
Filing date: 1984-11-06
Publication date: 1991-01-30
Anticipated expiration: 2004-11-06
Also published as: CA1255875C; FI844352L; DE144769T1; CA1235565A; AU572907B2; BR8405652A; EP0304607B1; DE3486201D1; ZM6984A1; KR850004030A; EP0304607A2; EP0144769A1; NO844412L; PL250296A1; NO168995B; KR930002544B1; FI844352A0; AU3487084A; EP0304607A3; ES547015A0

Description

This invention relates primarily to the flexible belts used in continuous casting machines for the casting of ferrous and non-ferrous metals. More particularly, this invention is directed to protective and thermally insulating matrix coatings, the methods of forming such coatings, the composition of the coatings, and the coated belts so produced. The casting belts are usually made of mild steel. Secondarily, the invention applies to the coating of other molten-metal-contacting surfaces in continuous casting machines, such as the coating of edge-dam blocks.
Numerous combinations of oils, graphite, soot, diatomaceous earth, silica, organic binders, etc., have been used to protect metallic casting belts or to insulate them and/or to act as parting agents to prevent adherence to the belts in continuous casting machines for casting molten metal. Such prior coatings are temporary or transitory in nature and may be continually applied and replenished during casting. The continual application of such coatings while casting requires precise maintenance and control in view of the need for consistent thermal conductivity. This continual application and replenishing of temporary insulative coatings is a difficult and imprecise art. For example, excess liquid or solvent or binder in the insulating coating material is likely to emanate gas in such quantity as to disturb the soundness of the cast product, resulting in porosity. Some of the gas thus liberated is at times hydrogen, which can detrimentally alter the metallurgical qualities of the cast metal. Also excess amounts of the temporary insulative coating material itself may accumulate near the edges of the cast product and usurp part of the continuously moving mold space, causing defects in the cast product.
A two-layer belt coating, including thermosetting resin and solvent, for use in continuously casting relatively low melting-point metals, such as aluminum, zinc and lead is described in U.S. Patent No. 3,871,905. Coatings containing resins are generally unsuitable for use for continuously casting metals having melting-point temperature significantly higher than aluminum.
A casting belt made of mild killed steel containing 0.2% to 0.8% by weight of titanium has been multiple-layer coated, as described in U.S. Patent No. 4,298,053. The surface of the belt is first coated by a "primer" layer of a nickel-aluminum alloy (80% by wt. of Ni and 20% by wt. of Al) stated to be 0.005 mm thick in the specification but claimed to be 0.05 mm thick in the only claim. This primer layer is coated by another layer between 0.01 and 0.5 mm thick made of chromium, or of an alloy of chromium, or of nickel, or of an alloy of nickel or of a stainless steel. Then, a third layer of colloidal graphite anti-adhesion agent is applied over the second layer. However, in our experience more thermal insulation and additional non-wettability are required than can be obtained by following the teaching of that patent.
Canadian Patent No. 1,062,877 of Thym and Gyongyos describes the coating of endless casting belts by several thin layers (80-100 micrometers, preferably 50-70 micrometers) on the endless casting belts until the desired thickness of ceramic layers is achieved to give the requisite thermal resistance. Such a build-up of multiple ceramic layers is laborious, time-consuming and expensive. The resulting built-up coating is machined mechanically, e.g. by grinding, in order to achieve the desired uniform surface finish and wetting behavior between this multiple-layer ceramic coating and the aluminum being cast. This built-up ceramic coating consists of AI₂0₃. CaZr0₃, AI₂0₃. MgO, ZrSi0₄ or AI₂0₃. Ti0₂. It is built up in thickness until it provides thermal resistance in the range of 10-⁴ to 10-³ _M ²- h - °C/kcal.
Such built-up ceramic coatings are usually relatively thick and relatively fragile and brittle. They have insufficient durability to withstand thermal shock, or to withstand the mechanical stretching and relaxing, the flexing and abrading which are inherent in continuous casting employing one or more moving belts as molten-metal-contacting-cooling surfaces.
Durability to withstand such mechanical and thermal stresses are important, as otherwise bits of the ceramic coating become loose and spall during the demanding service imposed upon them in continuous casting of molten metals. The loosened bits inevitably becomes inclusions in the cast metal product. Such inclusions can become a serious problem, as for instance in the case of copper destined for drawing into fine wire. Such inclusions cause the wire to break in the dies, resulting in significant productivity losses as the wire is restrung. Ceramic coatings are generally not flexible and tend to be fragile.
Problems associated with brittleness, ceramic flake-off and contamination of the cast product by ceramic particles are highlighted in German patent 24 11 448 of Theobald, in which patent an attempt was made to solve this problem when casting aluminum by applying over the relatively thick ceramic a second and protective abrasion resistive metal layer which has a higher temperature point of fusion than the metal to be cast.
The above problems are solved by the features of the independent claims.
A unitary-layer partially metallic, suitably adherent, mechanically and thermally durable, non-wetting, fusion-bonded matrix coating on endless, flexible metallic casting belts for continuous casting machines is described. This fusion-bonded matrix coating is also advantageous for coating other molten metal-contacting surfaces in continuous casting machines, such as edge-dam blocks that define moving side walls of a mold cavity. The fusion-bonded matrix (or reticulum) coating provides advantageous accessible porosity throughout the coating and comprises a nonmetallic refractory material interspersed substantially uniformly throughout a matrix of heat-resistant metal or metal alloy, for example, nickel or nickel alloy, such metal or metal alloy being fusion-bonded to a grit-blasted surface of the belt and serving to anchor and hold the nonmetallic material. The coating is applied by thermally spraying a powdered mixture directly on the roughened surface. The result is to insulate and protect the underlying belt from intimate molten metal contact, from heat stress and consequent distortion and from chemical or stress-corrosive action by the molten metal or its oxides or slags. The nonmetallic material may be present, at least partly, in the form of isolated particles encased within the metallic reticulum and/or in the form of a second reticulum intertwined with the metallic reticulum. The life of the coated belts is dramatically increased, and the surface quality and properties of the cast product are significantly improved. The coating controls and renders more uniform the rate of freezing of the metal being cast, resulting in improved metallurgical properties.
Formulations and a method of forming such coatings by thermal spraying are described.

Fig. 1 illustrates a side view of the casting zone, the casting belts and pulleys, and one of the casting side dams of a twin-belt continuous casting machine;
Fig. 2 is an enlarged cross-sectional view of the casting space and its surrounding parts, taken along the line 2-2 of Fig. 1;

With reference to Figs. 1 and 2, there is illustrated the casting zone and nearby components of a twin-belt casting machine which includes a lower casting belt 10 revolved around pulleys 12 and 14, which are parts associated with a lower carriage L. Pulley 12 is located at the input or upstream end of the machine, and pulley 14 is at the output or downstream end of the machine. A continuous moving casting mold C is defined by and between the lower casting belt 10 cooperating with a pair of spaced casting side dams 16 and 18 (Fig. 2) and with an upper casting belt 20, as they move together along the casting zone C. The side dams are guided by rollers 22. They each comprise a multiplicity of slotted dam blocks 24 strung on straps 25. Seals 26 keep water from entering between the belts so as to isolate the casting region C from water. Stationary guides 27 serve to guide the moving side dams. Upper casting belt 20 revolves around pulleys 28 and 30, which are parts of an upper carriage U. Finned backup rollers 32 define the position of the belts in casting zone C and permit fast-moving liquid coolant to travel along the reverse surface of each belt. Molten metal is introduced into the machine at its upstream end as indicated by the arrow 31 in Fig. 1. The cast product P issues from the downstream end.
In accordance with the present invention each of the belts 10 and 20 is coated before being installed on the respective belt carriages L and U. It will be understood from Figs. 1 and 2 that the molten-metal-contacting surface of each belt is its outer surface, sometimes called its front surface, while its inner surface is sometimes called the reverse surface. Such flexible casting belts 10 and 20 are usually made from low carbon steel rolled to be moderately hard and usually have a thickness in the range from 0,889 mm (0.035 of an inch) up to 1,651 mm (0.065 of an inch), but thinner or thicker belts may be used. Occasionally, for more demanding service, the belts are made from a titanium-containing steel, as described in Dompas U.S. Patent No. 4,092,155, which is work-hardened by rolling sufficiently to become full hard.
To coat a casting belt, such as belt 10 or 20, in accordance with the invention, any oily residue on the outer surface of the belt must first be thoroughly removed, as by alkali-detergent cleaning followed by wiping with a clean solvent.
Next, the outer surface of the belt is roughened by grit-blasting. For example, this grit-blasting is carried out with 20-grit aluminum oxide, applied at an air pressure (between about 40 and 100 psi) between about 300 and 700 kilopascals. The size 20-grit means particles of aluminum oxide which have passed through a screen having 20 wires per 25,4 mm (inch). Air pressure within the lower portion of this range is used when grit-blasting thinner belts in the lower portion of the belt thickness range described above, since the impacts of the grit may otherwise cause roughness on the reverse belt surface. Air pressure within the lower portion of the range may also be advisable when the belt is not intended to be subsequently roller-stretcher levelled. Usually, the belt will be roller-stretcher levelled after grit-blasting in order to control distortion within acceptable limits, as described below. Roughness of the blasted surface is normally in a preferred range from 0,0508 mm (0.002 of an inch) up to 0,0762 mm (0.003 of an inch) (2000 to 3000 microinches or 52 to 76 micrometers), which range is readily obtained, though the useful range of roughness may occasionally extend from about 0,0254 mm (0.001 of an inch) up to about 0,127 mm (0.005 of an inch).
Surface roughness figures as stated above are determined as measured by our preferred method, that of the method of surface grinding. In this preferred method, the thickness of a blasted belt sample is first measured by means of an ordinary machinist's micrometer caliper. The sample is then placed on the magnetic chuck of a surface grinder, and the roughness is carefully ground off to just that level at which the resulting ground surface appears smooth. The belt sample is then again measured with the micrometer caliper, and the difference in readings is taken as the roughness. By comparison, the extremes of roughness of a given grit-blasted surface as measured by a vertically measuring microscope at 400x are, in our experience, on the order of 150% of the measured values obtained by the surface grinding and micrometer method.
The grit-blasting process ordinarily distorts the belt, and roller-stretcher belt levelling will usually be required. Levelling is done by passing the belt with reversals in bending and ironing action through multiple closely spaced rollers, for example, as shown and described in U.S. Patent 2,904,860 of C. W. Hazelett.
Thermal spraying is then utilized to apply the one-coat fusion-bonded matrix protective insulative coating directly to the grit-blasted roughened belt surface. A successful method is to thermally spray the coating materials by means of a combustion flame-an oxyacetylene flame-at a standoff distance of at least 5 inches (127 mm), and at a traverse speed in the range of 30 to 50 feet (9 to 15 meters) per minute.
Oxyacetylene-sprayed coatings are successful if the material being sprayed does not burn up excessively in the flame.
Oversize nonmetallic particles may not entirely melt. Moreover, oxyacetylene flame may not be sufficient to retain nonmetallic particles molten for the time required to fuse them to other particles of the same species as finally deposited on the belt surface. If there is a preponderance of metallic particles intermixed with nonmetallics, the environment is not conducive for interfusion of the nonmetallic constituents. Thus, in such cases, the nonmetallic material may be present, at least partly, in the form of isolated particles encased within or surrounded by the metallic reticulum.
Plasma spraying is an alternative method of thermal spraying that uses electricity. Combustion (oxyacetylene) spraying is often called flame spraying. Such usage is apt to be confusing in that the plasma spray is often said to utilize a plasma flame. Both kinds of spraying may be said to utilize a flame. It is our terminology to use the phrase "thermal spraying" as being inclusive of both oxyacetylene flame spraying and electrically energized plasma spraying. Plasma spraying as ordinarily used runs hotter than oxyacetylene spraying and so results in less porosity.
It is our present-belief that the higher temperatures provided by electrically energized (plasma) spraying may enable the rapid fusing of metallic and nonmetallic materials supplied in coarse forms, such as sticks, rods or wires (as distinct from powdered form) and therefore may enable such coarse forms of metallic and nonmetallic materials to be employed. But regardless of whether this belief proves true in practice, the use of mixtures of appropriate metallic and nonmagnetic constituents as described further below is dramatically successful in providing fusion-bonded matrix coatings with suitable percentages of "accessible" porosity as described hereinafter.
In most prior applications of thermal spraying, porosity is avoided so far as possible. In the present invention we have found the opposite to be true. Controlled porosity characteristics in the fusion-bonded matrix coat are desirable and important. An appropriate level of controlled porosity contributes substantially to the insulative value of the matrix coating, while at the same time an appropriate level of porosity enhances the desired characteristic of non-wettability by molten metal. We believe that this non- wetting enhancement is due in large part to the air retained in the pores of the porous coating. When molten metal is introduced adjacent to the coated belt the air in the pores is heated and expands out of the pores and so supplies a gaseous film between the molten metal and the belt coating, thereby preventing the molten metal from wetting the coated belt, during the critical initial time when a skin of solidified metal is being formed on the product being cast in the continuous casting process.
Equally important is the fact that controlled porosity within the matrix coating has the virtue of acting as a blotter or disperser for moisture picked up on the surface of a casting belt, caused by condensation or by stray droplets of coolant. This blotting or dispersing of moisture prevents blowholes, rosettes, or needles that would otherwise appear in the surface of the cast product P adjacent to the location of a liquid contaminant. This feature of blotting dispersion of moisture is important, for example, in the casting of aluminum sheet product P with a high quality surface suitable for anodization, as opposed to lower surface quality which is acceptable for painting.
In addition, there are two more reasons why controlled porosity is desirable. One is its improvement of thermal shock resistance. The other is its increasing of resistance to spalling under mechanical rough handling. Both of these characteristics are important in a coating consisting, on a volume basis, largely of ceramic material or brittle material generally. Under thermal shock, the porosity appears to allow internal adjustments to occur without relatively massive dislocations appearing, there being already countless tiny dislocations present as pores, each of which we now believe contributes minutely to a myriad of needed internal mechanical adjustments for accommodating thermal shocks and mechanical flexings and stretchings. Thus, controlled porosity, far from detracting from effective strength of the matrix coating, actually increases it.
The desired porosity appears to extend throughout the unitary-layer, fusion-bonded matrix coating. That this porosity extends omnipresently throughout the matrix coating is evidenced by the fact that a steel belt so coated will rust if left moist.
In sum, substantial but controlled porosity within the unitary layer, fusion-bonded matrix coatings on belts of continuous casting machines in accordance with this invention has four advantages that are important to the present invention. There are upper limits to the desired range of such omnipresent porosity. The upper limit in a given formulation is reached when the integrity of the coating becomes impaired. In those matrix coatings where the metallic constituents are predominant (as determined by weight), this upper limit is at least about 35 percent "accessible" porosity by volume. In those matrix coatings where the nonmetallic constituents are predominant (as determined by weight) this upper limit is about 12 to 20% "accessible" porosity by volume.
There is a lower limit to the desired range of "accessible" porosity by volume in the matrix coating, because insufficient porosity will not yield the four advantages described above. This lower limit is about 4 to 8%.
As described below, tests and measurements were made of "accessible" void space, i.e. effective porosity, as a percentage of the volume of the matrix coating. These tests and measurements were conducted to give a better understanding of the parameters contributing to the desired porosity. Samples, usually of about 14 square inches of mild steel belt stock, were thermally sprayed to a thickness usually of about 0.050 inch (1.3 mm). They were thermally dried and then weighed. Then they were soaked briefly in water with detergent (Kodak Photo-Flo) added; then they were withdrawn and all unabsorbed water was wiped off. The specimen was weighed again, the increase in grams noted and divided by the coating volume in cubic centimeters to obtain the percentage of void space that was accessible to water, which had become blotted or absorbed within the coating. In a given sample there may be other voids that are closed and so not measurable by this water-absorption method, but we believe that those "accessible" voids which emit gas on heating and which absorb stray water are the more important voids with respect to overall advantageous performance of the matrix coating during casting. Hence, a method of measuring effective porosity which takes into account only fluid-accessible or, specifically, water-accessible porosity is especially suitable to our purposes.
Table A below lists the water-accessible porosities as a percentage of the total volume of the matrix- coating which were observed by measuring various test samples thermal spray coated with powdered mixtures of the listed formulations under the conditions stated.

Coating composition

The preferred unitary-layer, fusion-bonded, protective matrix coating is of the same composition throughout its thickness. This matrix coating comprises a nonmetallic refractory material interspersed substantially uniformly throughout a matrix of heat-resistant metallic component or constituent. This metallic constituent is a metal or a metal alloy, and it must exhibit five critical properties, as follows:

1) The metallic constituent must have heat resistance and resistance to thermal cycling. In other words, the metallic constituent must have a sufficiently high melting point relative to the temperature of the molten metal being cast that the metallic constituent resists undue degradation during the lifetime of the belt in continuous casting and also must resist undue deterioration due to the extreme and repeated thermal cycling which occurs during continuous casting. The melting point of the metallic constituent must be at least close to, but not necessarily above, the temperature at which the molten metal enters the continuous casting machine.
2) The metallic constituent must have thermal fusion bonding compatibility with the flexible steel casting belts normally used to which the matrix coating is fusion-bonded.
3) The metallic constituent must have at least a modicum of ductility in order to withstand the mechanical rough handling to which the matrix-coated belt is subjected during continuous casting. The moving belt is repeatedly flexed around pulley rolls and straightened out, and in addition the moving belt is subjected to a relatively high tension stress during use.
4) The metallic constituent must have thermal expansion rates that are not too far different from the thermal expansion rates of the nonmetallic constituents included in the matrix coating to withstand repeated extreme thermal cycling occurring during continuous casting without flakes spalling off.
5) The metallic constituent must have sufficient resistance to oxidation under the conditions of thermal spraying and also under the conditions of continuous casting so as to avoid undue deleterious oxidation.

We have found that nickel and nickel alloys are especially suitable for forming the metallic constituents of the matrix coatings of this invention. Cobalt, iron and titanium would also appear to have the hereinbefore described five critical properties so as to be useful as the metal or metal alloy for forming the metallic constituents of the matrix coatings. Those skilled in the art may find that other metals or metal alloys are also suitably operable.
The matrix coating of this invention is formed by thermal spraying of the metallic and nonmetallic constituents mixed in formulations within the following ranges:

Metallic constituents:
- about 38 to about 90 percent by weight,
Nonmetallic constituents:
- about 62 to about 10 percent by weight.

Our observations have led us to conclude that there must be a sufficient volume of the metallic constituents present in the matrix coating relative to the nonmetallic constituents so as to form an integral, fused network, reticulum or matrix of the metallic constituents for suitably holding or anchoring the nonmetallic constituents to the belt. Nonmetallic constituents, when present in the upper portion of the above range, may also form a network or reticulum entwined (intertwined) throughout the metallic reticulum. Nonmetallic constituents, when present in the lower portion of the above range, may be present, at least partly, in the form of isolated particles encased within or surrounded by the metallic reticulum. Thus, the metallic component forms the anchoring und holding matrix or reticulum, while the nonmetallic component is distributed uniformly throughout this reticulum either as a second reticulum or as discrete particles. The metallic constituent generally has a specific gravity averaging about one and one-half to about four times that of the nonmetallic. Thus, when both constituents are present in the coating at 50% by weight, the volume ratio of nonmetallic particles to metallic particles is about 2.5 to 1 in our usual formulations. On the other hand, when the metallic constituent comprises 85% by weight of the coating composition, then the volume ratio of nonmetallics to metallics is about 1 to 2.5.
Presently preferred compositions utilize at least as part of the metallic and nonmetallic constituents a composite nickel and graphite powder in which grains of nickel encapsulate graphite powder, the graphite comprising either about 15 or about 25 percent of the combined weight. Such composite nickel and graphite powders are available commercially, for example from Bay State Abrasives of Westborough, Massachusetts.
Presently preferred compositions utilize at least as part of the metallic and nonmetallic constituents a composite nickel and graphite powder in which grains of nickel encapsulate graphite powder, the graphite comprising either about 15 or about 25 percent of the combined weight. Such composite nickel and graphite powders are available commercially, for example from Bay State Abrasives of Westborough, Massachusetts.
Preliminary tests in the pouring of mild (1010) steel melting at about 1530°C (2786°F) onto steel casting belt samples having fusion-bonded matrix coatings in accord with the present invention have shown that commercially available, predominantly nickel alloy containing about 8 percent of aluminum and about 5 percent of molybdenum is a suitable metallic alloy for use with powdered zirconia or graphite as suitable nonmetallic constituents for forming a durable matrix coating.
Other metals, alloys, or nonmetallic refractories for example, silica or alumina could be suitable as constituents in the fusion-bonded, accessible-porosity, matrix coating provided by the present invention. The critical properties to be looked for in metals and alloys are set forth explicitly in greater detail above. They have suitable heat resistance and resistance to thermal cycling, bonding compatibility with low-carbon steel belts, a modicum of ductility, thermal expansion rates that are not too far different from those of the nonmetallic constituents included, and oxidation resistance if oxyacetylene flame spraying is to be the method of application.
A presently preferred insulative material for use at least as part of the nonmetallic constituent is zirconium oxide, Zr0₂, also called zirconia, which is used in powdered form, preferably of particle size running from 0.0005 to 0.0014 of an inch (12 to 36 micrometers). This zirconia nonmetallic constituent has the advantage that its coefficient of expansion more closely approximates that of steel and nickel than some other available metallic oxides which have a lower coefficient of expansion.
Yttria (yttrium oxide, Y₂0₃) added in any of various amounts up to about 20 percent may be helpful in stabilizing the structure of the zirconia crystals exposed to high temperatures, thus preventing premature loosening of the crystalline particles due to subtle changes in mechanical proportions during thermal cycling. Other metallic oxides may also be used for this heat-stabilizing purpose, notably magnesia (MgO) and lime (CaO). The latter is economical and has afforded acceptable results in our experience. Thus, economical lime (calcium oxide) is presently preferred as a heat-stabilizing compound. It is normally an ingredient in purchased zirconia, comprising about 4 to 5 percent by weight of the zirconia.
The particles or powder of the nonmetallic component are thoroughly mixed and blended with the powdered metallic component, and the resulting mixture is thermal-sprayed directly onto the grit-blasted surface of the belt. Segregation of the mixed powders during application must be avoided.
As discussed earlier, coatings of zirconia alone or of other nonmetallic substances alone may under certain adverse conditions lose adhesion and release bits of the nonmetallic substance into the freezing metal product. This flake-off problem has been minimized or avoided in the matrix coatings of this invention by attention to the following factors. The zirconia powder is preferably of fine particle size, sufficiently fine to pass through a screen having 300 or more wires per inch. There should be enough metallic constituents in the powder mix to form on the belt an integral, fused-together network or reticulum that will securely anchor and hold the zirconia particles in a relatively diskrete and discontinuous array and/ or in a second reticulum which is intertwined with the metallic reticulum as descirbed above.
Additionally, the finished unitary-layer, fusion-bonded, matrix coating should be brushed and dusted or vacuum cleaned before use.
Graphite is a highly heat-resistant separating agent which sublimes at about 3700°C. without melting. It is a useful nonmetallic constituent for the reason that it is non-wetting with respect to nearly all molten metals. Moreover, should particles of graphite get into the metallic product, its softness, friability, lubricity, and inertness forestall most of the problems associated with the incidental inclusion of foreign substances. Under the pressure of rolling or drawing, graphite particles break or divide into progressively finer particles.
Our experience has shown that when suitable powdered metallics and powdered nonmetallics are thoroughly mixed and blended together, some of the resulting mixtures (particularly those containing very fine particles) are apt not to flow freely and uniformly through the passages of a thermal-spray gun. The result is uneven coating. For producing a free-flowing powder blend in many cases, an addition to the powder blend of at least about 0.25 percent by weight of spherical fumed silica (Si0₂) particles as a lubricant has substantially enhanced flowing of the powder mixture and uniformity of thermal spray coating. The amount of this fumed silica lubricant is not critical, and good results have been obtained with most powder mixtures. A grade of 0.014 micro meter (14 millimicrons) fumed silica particles has been successful for producing a free-flowing powder blend. This size of 0.014 micro meter is less than a millionth of an inch and is a nominal size.
Examples of suitable formulations for forming the matrix coatings of this invention are set forth below.

Example I

Example II

Example III

Example IV

Example V-VIII

Similar formulations for forming matrix coatings of this invention may be obtained by substituting cobalt partially or fully for a corresponding weight percent of nickel in the foregoing four Examples.

Example IX

In this Example IX, the weight percent of aluminum is shown in the range 0 to 35, but the upper end of this range is subject to the limitation that the ratio of aluminum to nickel does not significantly exceed a one-to-one atomic ratio. Since the ratio of the atomic weight of aluminum to that of nickel is about 41 %, the weight percent of aluminum in the above Example does not significantly exceed about 41 % of the weight percent of nickel in this formulation.

Examples X-XVII

Magnesium zirconate can be substituted partially or fully for both a corresponding weight percent of zirconia and its proportionate weight percent of the heat-stabilizing agent Calcium Oxide in each of the foregoing Examples I-IX.

Examples XIX-XXII

Formulations a, d, e and f of Table A above, each modified to include at least about 0.25% by weight of spherical fumed silica as a lubricant, are further Examples suitable for forming fusion-bonded matrix coatings on flexible casting belts.
The preferred minimum deposited thickness of the fusion-bonded matrix protective insulating coating for use on flexible metal continuous casting belts 10, 20 (Fig. 2) is about 0.002 inch (0.05 mm), said minimum measurement being the thickness over the generality of the peaks of the underlying grit-blasted belt surface, which is the way a magnetic thickness gauge normally measures. However, advantages may be obtained by using matrix coatings as thin as about 0.0015 inch (0.038 mm).
Thermally sprayed coatings even thinner than 0.002 of an inch (0.05 mm) appear to be useful in some applications where nonwetting is more important than thermal insulation. Thus, a lower practical limit to thickness is not readily apparent. For extra insulation, thicknesses of several times this amount of 0.002 of an inch will on occasion be useful, since the coating which is the subject of the present invention is rugged and can withstand much flexing around the pulleys (rolls) of a continuous casting machine. But, depending on the casting application, more thickness is not necessarily better, not on flexible belts and especially not in uses where coating-loss impurities could seriously interfere with the quality of the cast product, as in the continuous casting of copper wire bar intended for fine wire drawing. Thicknesses as great as 0.015 of an inch (0.4 mm) are readily produced and are rugged. However, the expense of such thick coatings is also a limiting factor.
The accuracy with which insulation can be applied and controlled with these thermally sprayed fusion-bonded matrix belt coatings is not only a desirable feature in itself but, further, it enables planned proportioning of insulation between belts 10, 20 and edge dams 16, 18. That is, it enables the attainment of optimum comparative heat flux density through the belts 10, 20 as compared to heat flux into the edge dams 16, 18. The accurate proportioning of the density of heat flux between the broad belt surfaces on the one hand, and the relatively narrower moving edge dams on the other, is of importance in producing cast slab of first-class metallurgical quality where the thickness is greater than 1/4 of an inch (6 mm); see U.S Patent Application, Serial No. 493,359, filed May 10, 1983, the disclosure of which is incorporated herein by reference. The theory therein may explain the importance of proportioning the density of heat flux between the wide belt mold surfaces and the narrow edge dam mold surfaces.
To achieve such relative proportioning of heat extraction (heat flux), one may adjust the thickness of coatings on the belts as compared to that same coating composition on the blocks of the edge dams. Metals are usually better thermal conductors than non-metals; hence the ratio of metal to non-metal in the fusion-bonded matrix coating may be adjusted to control conductivity. For example, the thermal conductivity of nichrome (80% Ni, 20% Cr by wt.) is on the order of about ten times that of zirconia. Again, the metallic constituents themselves in the matrix coatings can be selected according to thermal conductivity or insulative value, and adjusting the content of metals of relatively low thermal conductivity to the content of metals of higher thermal conductivity. The conductivity of nichrome is on the order of about one-fourth that of nickel or of some low alloys of nickel.
The present invention may be applied to edge-dam blocks in themselves, in order to achieve advantages generally similar to those attained with belts. However, in accordance with the above-noted patent application relating to the insulation of edge-dam blocks, more insulation will generally be required on the edge-dam blocks than on the adjacent casting belts. This difference will ordinarily be achieved through applying a greater thickness of thermally-sprayed, fusion-bonded matrix coating insulating material, though composition ratios for adjusting and proportioning heat flux may be used.
A machine for employing the method for applying the coatings is described in the divisional application EP-A2-304 607.
In the following some of the results of the invention are summarized.
The present invention of thermally spraying a unitary-coat fusion-bonded matrix protective coating of powder mixtures of heat-resistant metallic and refractory non-metallic components is capable of meeting all of the following essential or desirable conditions. The fusion-bonded matrix coating (1) is adherent to the flexible base metal of the belt or to edge-dam blocks; (2) provides adequate thermal insulation; (3) is resistant to mechanical damage,-i.e., spalling flake-off or abrasion; (4) is resistant to thermal shock; (5) affords an acceptable often attractive surface finish on the cast product; (6) is acceptably non-wetting with respect to molten metal cast; (8) affords accurate proportioning of insulation between the belts and the edge dams; (9) has desirable accessible porosity throughout the matrix coating; (10) is compatible, because of surface characteristics, with additional minimally applied temporary top-coatings, such as oil or graphite or a combination; and (11) can be applied practically by means of a readily constructed and readily operated machine as described.
In accordance with customary practice in using belt casting machines, the user may find it desirable or may wish to apply a temporary top coating over the fusion-bonded matrix coated belts. For example, a temporary coating of colloidal graphite applied and dried from an aqueous or solvent solution has been found suitable for use on such matrix coated belts for casting copper product P.
Judging from previous experience, we believe that amorphous carbon or soot, applied for instance as a colloidal suspension, may be substituted for the graphite top-coat.
In the case of casting aluminum slab as the product P, diatomaceous silica may be included in this temporary top-coating. In the casting of copper, a trace of oil appears to be desirable and may be sprayed onto the fusion-bonded matrix coating of a new belt in minute quantities, however not enough to appear wet or to result in any decomposition of the oil.
In the casting of copper bar to be used for drawing into wire, belt life top and bottom was increased by a margin of nearly 2 to 1, when the belts had been fusion-bonded matrix coated in accordance with this invention. Surface quality was remarkably improved, owing in part to the ability to use much less oil or top-coating than conventional practice, thus reducing its attendant hydrogen-related porosity in the cast product. Improved metallurgy of the copper rod indicated that improved drawability was present also.
In an early test of casting of copper bar, the matrix coating of Example I was used on a top belt 20 only. The thickness was around 0.0508 mm (0.002 of an inch) on a hard-rolled, low-carbon titanium steel belt 1.12 mm (0.044 of an inch) thick. This cast was stopped after three hours, for reasons not related to the belt coating, which was still in excellent condition. No precoat of graphite was used at first, and a little pickup of copper was experienced. The next cast on this top belt ran 24 hours with two interruptions not related to the belt coating. The quantity of oil applied onto the belts was reduced as compared with conventional practice in casting copper bar in a twin-belt machine, with good results. The test was terminated after 24 hours due to reasons not related to the belt coating.
The above copper bar casting test was repeated with an Example I matrix coated low-carbon-steel upper belt 20 of No. 2 temper, no titanium content. The results were just as good as with the titanium-steel belt, and such good results were not expected, because such good results were contrary to previous experience in attempting to cast copper bar on such a non-titanium-containing steel belt. Prior experience had been that hairline cracks might be expected to occur in such a non-titanium-containing belt after 8 to 10 hours of repeated cyclic contact with molten copper and cyclic flexing. Such cracks did not appear in the matrix coated non-titanium-containing belt that was tested for eight to ten hours.
A further copper bar casting test was conducted with a fusion-bonded matrix coating according to Example III. This coating was applied onto low-carbon, hard-rolled titanium steel belts of 1.12 mm (0.044 inch) thickness. This time, such fusion-bonded matrix coated belts were used both as the top and bottom belts 20 and 10. Oil was lightly sprayed onto the bottom belt. After an initial light application of oil on the top belt, it was only necessary to wipe the top belt perhaps three times an hour, in order to dislodge slight pickup. Results were the best ever, including the longest belt life which we have seen for casting copper. Belt life, top and bottom, was increased by a margin of nearly 2 to 1.
An example of the benefits of the subject invention has been the experimental casting of aluminum alloys. Surface improvement of the metal being cast was remarkable. Rosettes and streaks formerly observable during the casting process were eliminated, on both the top and the bottom of the cast slab. Rejectable material was greatly reduced. The fusion-bonded matrix coated belts were still in good condition well beyond the useful life of conventional belts. The edges of the cast slabs were excellent, owing to the proportioned heat transfer between edges and belts by use of the insulative coatings.
In our experience, in order to operate advantageously in use, an endless flexible casting belt having a fusion-bonded matrix coating thereon in accordance with this invention will be capable of repeatedly flexing around a pulley roll having a diameter of 20 inches (508 mm) without occurrence of flaking or spalling of said coating.
Although the examples and observations stated herein have been the results of experimental field trials of belts matrix-coated, as described, on which were cast molten copper or molten aluminium and aluminum alloys, and tests with molten steel poured onto stationary sections of coated belt, allowing a vertical fall of fourteen inches before the molten steel impacted against the coated belt, this invention appears applicable to the continuous casting of any metal or alloy having a melting temperature equal to or less than steel.
Although specific presently preferred embodiments of the invention have been disclosed herein in detail, it is to be understood that these examples of the invention have been described for purposes of illustration. This disclosure is not to be construed as limiting the scope of the invention.

Claims

1. The method of providing a protective, insulative coating on a metal surface of a continuous casting machine or a clean roughened surface of an endless flexible metallic casting belt for use on a continuous casting machine, such surface being intended to be subject to contact with molten metal during casting, said method being one comprising: providing in readily heat fusible form metallic material having the properties of: a) heat resistance relative to the temperature of the molten metal being cast and resistance to thermal cycling, b) thermal fusion bonding compatibility with such metal surface, c) a modicum of ductility for withstanding repeated flexing around a pulley roll, d) sufficient resistance to oxidation under the conditions of thermal spraying and also under the conditions of continuous casting for avoiding undue oxidation, and e) thermal expansion rates compatible with predetermined non-metallic refractory material, providing in readily heat fusible form such non-metallic refractory material, and thermally fusing to said metal surface a coating comprising said metallic material substantially uniformly intermixed with said non- metallic refractory material, and thereby creating a desirable accessible porosity of at least 4% of the total volume of the coating.

2. The method of claim 1 comprising providing a powder mixture containing (1) heat-resisting metallic material, and (2) insulative, non-metallic refractory material, said metallic material constituting such a weight percent of said powder that subsequent thermal spraying of said powder onto said surface results in a continuous matrix of said metallic material with said non-metallic refractory material dispersed throughout said matrix and with said matrix holding said non-metallic refractory material and securing said non-metallic refractory material to the surface of an endless flexible metallic casting belt.

3. The method of claim 1 or 2, wherein said metallic material and said non-metallic refractory material are reduced to powder and are substantially uniformly intermixed prior to fusing to said surface and said fusing step comprises thermal spraying and wherein said metallic material comprises about 38 to about 90 percent by weight of said coating and said metallic material so selected from the group consisting of nickel, cobalt, iron and titanium and said non-metallic refractory material comprises from about 10 to about 62 percent by weight of said coating and said non-metallic refractory material is selected from the group consisting of graphite, zirconia, magnesia, zirconate, silica and alumina.

4. The method of claim 3 wherein the metallic material includes nickel as the predominent constituent and the non-metallic refractory material comprises powdered zirconia as the predominent constituent and the thermal spraying is carried out at a standoff distance of at least 3 inches (76 mm) and at a transverse speed in the range of 30 to 50 feet per minute (9 to 15 meters per minute).

5. A method according to any one of claims 1, 2, 3 or 4 wherein said coating has a matrix structure including a continuous reticulum of the metallic material, and the non-metallic refractory material is interspersed throughout this matrix, said coating having a thickness in the range of about 0.0015 of an inch (0.04 mm) to about 0.15 of an inch (4 mm).

6. A method according to claim 5 wherein the ratio of the specific gravity of metallic material to non- metallic refractory material is in the range of about 1 1/2:1 to about 4:1.

7. A method according to claim 5 or 6, wherein the substantially uniform mixture of metallic material and non-metallic refractory material has present therein a heat stabilizing amount of a heat stabilizing agent selected from the group consisting of yttria, magnesia and lime.

8. A method according to any of claims 5 to 7, wherein the substantially uniform mixture of metallic material and non-metallic refractory material has present therein a flow enhancing amount of spherical fumed silica as a flow enhancing lubricant.

9. A method according to any one of claims 1 to 8, wherein the powder mixture comprises in weight percent

4 to 5 aluminum,

2 to 3 molybdenum,

55 to 57.5 nickel, plus trace impurities, zirconia and

1.5 to 2 calcium oxide, in the zirconia.

10. A method according to any one of claims 1 to 8, wherein the powder mixture comprises in weight percent

6 aluminum,

4 molybdenum,

52 nickel, plus trace impurities,

22 to 23 zirconia,

1 to 1.5 calcium oxide, in the zirconia

13 to 14.8 graphite and

0.2 to 0.5 spherical fumed silica.

11. A method according to any one of claims 1 to 8, wherein the powder mixture comprises in weight percent

57 to 60 nickel, plus trace impurities,

25 to 28.8 zirconia,

1 to 1.5 calcium oxide, in the zirconia,

13 to 15 graphite and

0.2 to 0.5 spherical fumed silica.

12. A method according to any one of claims 1 to 8 wherein the powder mixture comprises in weight percent

14 chromium,

54 nickel, plus trace impurities,

29.8 to 30.4 zirconia,

1.4 to 1.6 calcium oxide, in the zirconia and

0.2 to 0.6 Spherical fumed silica.

13. A method according to any one of claims 1 to 8, wherein the powder mixture comprises in weight percent

38 to 90 metallic component including 0 to 35 aluminum and balance nickel, plus trace impurities and

62 to 10 nonmetallic component including 0 to 40 graphite, 0.3 to 0.8 spherical fumed silica, 4 to 20 lime of the sum of zirconia plus lime and balance zirconia.

14. The method for coating a metal surface according to Claim 1 with the further step of relative proportioning of the density of heat flux between belt surfaces on the one hand and edge dams on the other hand in a machine for continuously casting metal product directly from molten metal, wherein the molten metal is introduced into a moving mold of said machine, said moving mold being defined above and below by upper and lower matrix coated endless flexible metallic belts and being laterally defined by first and second matrix coated edge dams mainly metallic, characterized in that said method is a method comprising: coating said endless flexible belts and edge dams with a matrix coating, said proportioning comprising: determining the density of heat flux through the said belts and proportioning said heat flux in relation to the density of heat flux into the said edge dams by at least one of the following steps: a) adjusting the relative thickness of the matrix coatings applied on the belts as compared to the thickness of the matrix coatings applied on the edge dams, b) adjusting within at least one of the matrix coatings the ratio of metallic content of non-metallic content, and c) adjusting within at least one of the matrix coatings the content of at least one metal of relatively low thermal conductivity relative to the content of at least one metal of higher thermal conductivity.

15. An endless flexible casting belt for use in a continuous metal casting machine for continuously casting molten metal, said belt having fusion-bonded to a surface thereof a protective, insulative coating, characterized in that said coating is one comprising: a metallic material; a non-metallic refractory material substantially uniformly interspersed throughout said metallic material, and said metallic material being in the form of a matrix holding, supporting and anchoring said non-metallic refractory material on the belt surface, said coating having a desired accessible porosity of at least 4% of the total volume of the coating.

16. An endless flexible casting belt according to Claim 15 characterized in that said coating is one comprising: a metallic material having the properties of: a) heat resistance relative to the temperature of the metal to be cast and resistance to thermal cycling, b) thermal fusion bonding compatibility with the belt surface, c) a modicum of ductility for withstanding repeated flexing around a pulley roll, d) sufficient resistance to oxidation under the conditions of thermal spraying and also under the conditions to be encountered in continuous casting for avoiding undue oxidation, and e) thermal expansion rates compatible with predetermined non-metallic refractory material, said predetermined non-metallic refractory material being substantially uniformly dispersed throughout said metallic material, and said metallic material being in the form of a matrix holding, supporting and anchoring said non-metallic material on the belt.

17. An endless flexible casting belt of claim 15 or 16 wherein said metallic material and said non- metallic refractory material are reduced to powder and are substantially uniformly intermixed prior to fusing to said surface and said fusing step comprises thermal spraying and wherein said metallic material comprises about 38 to about 90 percent by weight of said coating and said metallic material so selected from the group consisting of nickel, cobalt, iron and titanium and said non-metallic refractory material comprises from about 10 to about 62 percent by weight of said coating and said non-metallic refractory material is selected from the group consisting of graphite, zirconia, magnesia, zirconate, silica and alumina.

18. An endless flexible casting belt of claim 17 wherein the metallic material includes nickel as the predominent constituent and the non-metallic refractory material comprises powdered zirconia as the predominent constituent and the thermal spraying is carried out at a standoff distance of at least 3 inches (76 mm) and at a transverse speed in the range of 30 to 50 feet per minute (9 to 15 meters per minute).

19. An endless flexible casting belt of any one of claims 15, 16, 17 or 18 wherein said coating has a matrix structure including a continuous reticulum of the metallic material, and the non-metallic refractory material is interspersed throughout this matrix, said coating having a thickness in the range of about 0.0015 of an inch (0.04 mm) to about 0.15 of an inch (4 mm).

20. An endless flexible casting belt of claim 19 wherein the ratio of the specific gravity of metallic material to non-metallic refractory material is in the range of about 1 1/2:1 to about 4:1.

21. An endless flexible casting belt of claim 19 or 20 wherein the substantially uniform mixture of metallic material and non-metallic refractory material has present therein a heat stabilizing amount of a heat stabilizing agent selected from the group consisting of yttria, magnesia and lime.

22. An endless flexible casting belt of any of claims 19 to 21 wherein the substantially uniform mixture of metallic material and non-metallic refractory material has present therein a flow enhancing amount of spherical fumed silica as a flow enhancing lubricant.

23. An endless flexible casting belt according to any one of claims 15 to 22, wherein the powder mixture comprises in weight percent

4 to 5 aluminum,

2 to 3 molybdenum,

55 to 57.5 nickel, plus trace impurities,

35 zirconia and

1.5 to 2 calcium oxide, in the zirconia.

24. An endless flexible casting belt according to any one of claims 15 to 22, wherein the powder mixture comprises in weight percent

6 aluminum,

4 molybdenum,

52 nickel, plus trace impurities,

22 to 23 zirconia,

1 to 1.5 calcium oxide, in the zirconia,

13 to 14.8 graphite and

0.2 to 0.5 spherical fumed silica.

25. An endless flexible casting belt according to any one of claims 15 to 22, wherein the powder mixture comprises in weight percent

57 to 60 nickel, plus trace impurities,

25 to 28.8 zirconia,

1 to 1.5 calcium oxide, in the zirconia,

13 to 15 graphite and

0.2 to 0.5 spherical fumed silica.

26. An endless flexible casting belt according to any one of claims 15 to 22, wherein the powder mixture comprises in weight percent

14 chromium,

54 nickel, plus trace impurities,

29.8 to 30.4 zirconia,

1.4 to 1.6 calcium oxide, in the zirconia and

0.2 to 0.6 spherical fumed silica.

27. An endless flexible casting belt according to any one of claims 15 to 22, wherein the powder mixture comprises in weight percent

28. A method of casting molten metal in a continuous casting machine having endless flexible metal casting belts, characterized in that said molten metal is cast between coated surfaces of endless flexible metal casting belts in said continuous casting machine, which endless flexible casting belts are belts according to any of claims 15 to 27.

29. A method according to claim 28 wherein the molten metal is selected from the group consisting of mild steel, copper and aluminum.

30. A flowable formulation for thermal fusion comprising in weight percent

4 to 5 aluminum,

2 to 3 molybdenum,

55 to 57.5 nickel, plus trace impurities,

35 zirconia and

1.5 to 2 calcium oxide, in the zirconia.

31. A flowable formulation for thermal fusion comprising in weight percent

6 aluminum,

4 molybdenum,

52 nickel, plus trace impurities,

22 to 23 zirconia,

1 to 1.5 calcium oxide, in the zirconia,

13 to 14.8 graphite and

0.2 to 0.5 spherical fumed silica.

32. A flowable formulation for thermal fusion comprising in weight percent

57 to 60 nickel, plus trace impurities,

25 to 28.8 zirconia,

1 to 1.5 calcium oxide, in the zirconia,

13 to 15 graphite and

0.2 to 0.5 spherical fumed silica.

33. A flowable formulation for thermal fusion comprising in weight percent

14 chromium,

54 nickel, plus trace impurities,

29.8 to 30.4 zirconia,

1.4 to 1.6 calcium oxide, in the zirconia and

0.2 to 0.6 spherical fumed silica.

34. A flowable formulation for thermal fusion comprising in weight percent

38 to 90 metallic component, including 0 to 35 aluminum and balance Nickel, plus trace impurities and

62 to 10 nonmetallic component, including 0 to 40 graphite, 0.3 to 0.8 spherical fumed silica, 4 to 20 lime of the sum of zirconia plus pime and balance zirconia.