DE69330035T2 - IRON METAL CASTING MATERIALS, ESPECIALLY FOR ROLLING REELS - Google Patents

Abstract

A method of making an engineering ferrous metal comprising the steps of adding to liquid engineering ferrous metal solid alloy carbide particles and thereafter permitting the ferrous metal to solidify. The alloy carbide particles are coated with iron or an iron alloy to allow wetting to occur between the powder and the liquid ferrous metal and the particles have a density which matches that of the ferrous metal to provide a uniform distribution of the carbide particles in the ferrous metal. A roll may be made having at least a shell made of metal by such a method by centrifugal casting or electroslag remelting. <IMAGE>

Description

This invention relates to cast iron or steel, defined herein as "engineering ferrous metals". More particularly, this invention relates to a rolling mill roll comprising cast iron or steel.

Mechanical properties of engineering ferrous metals, and hence of a rolling mill roll made from engineering ferrous metals, are influenced by the presence of carbides, the distribution and formation of which depends on carbon content, heat treatment and the addition of other alloying elements. Carbon is added to engineering ferrous metals while the cast iron or steel is molten, and the carbon is subsequently precipitated as metal carbides during solidification and during phase transformation in the solid state. Such carbides are referred to herein as "transformation carbides". The type and amount of transformation carbide present in engineering ferrous metals is thus limited by the thermodynamics and chemistry of a particular ferrous metal and is represented by the relevant phase diagram.

There has been a long-standing desire to provide carbides of different composition and volume fraction in engineering ferrous metals than is possible with the previously known techniques described above, which rely on phase transformation as the metal cools from the liquid state.

US-A-5,023,145 discloses an alloy with a substantially uniform distribution of three-carbide aggregates in which a uniform distribution of the three carbides is achieved by Using a relatively heavier binder material with a relatively lighter carbide, making the resulting aggregates relatively similar in density.

EP-A-0 386 515 discloses a process for producing a metallic body using an electroslag purification process in which solid alloy carbide particles in the form of tungsten carbide are added to the melt.

WO 91/16466 discloses an iron-based alloy with tungsten carbide particles dispersed therein.

WO 93/03192 discloses the addition of iron-coated alloy carbide particles (WTiC) of appropriate density (7 to 7.9 g/cm³) to an iron-based melt.

An object of the invention is to provide a new and improved process for producing an engineering ferrous metal which overcomes or reduces the above problems and a further object of the invention is to provide a new and improved engineering ferrous metal product having a desired carbide content which is not limited by thermodynamic considerations. According to one aspect of the invention we provide a process according to claim 1.

By "wetting" we mean the ability of the liquid engineering ferrous metal to wet the coating metal. In particular, for example, where the interfacial tension between the liquid engineering ferrous metal and the solid coating metal is such that the contact angle there is between 0º and 90º.

By "a suitable density" we mean a density that is in the range of 6-8 g per cc. This compares to a typical density of 7 g per cc for cast iron and steel. More preferably, the alloy carbide particles have a density of ± 5% of the density of the engineering ferrous metal to which they are added.

The wettability of the coated particles and the density of the alloy carbide particles each promote a uniform distribution of the carbide particles in the liquid engineering ferrous metal, which is maintained as the metal solidifies.

By "uniform distribution" we mean a uniform distribution over the entire cross-section of a cast workpiece made of engineering ferrous metal, without significant segregation. The solid carbide particles are not oriented in any direction and are distributed throughout all phases of the microstructure.

The coating metal preferably comprises iron or an iron-carbon alloy, but may also comprise an alloy of two or more elements selected from the group comprising iron, nickel, copper, titanium and carbon, or may be nickel or copper and common minor materials.

The coating metal may comprise nitrogen, e.g. up to about 0.1% nitrogen.

If the coating is iron, then because iron has a higher melting point than the engineering ferrous metal to which it is to be added, which would inhibit wettability, it is preferred to add an appropriate amount of at least one alloying element, such as carbon, nickel, copper or titanium, to the iron to produce an alloy having a melting point that matches the operating temperature of the ferrous metal. By "match" we mean that the melting point of the coating and said operating temperature are preferably within 20-30°C of each other.

The term "operating temperature" is defined herein as the temperature of the engineering ferrous metal at which the coated carbide particles are added.

The iron coating can contain up to 3.5% carbon.

When the alloy carbide particles are coated with iron or with an iron alloy having a lower carbon content than that of the engineering ferrous metal to which they are added, the coated alloy carbide particles can be added to the engineering ferrous metal and allowed to dwell therein for a sufficient time to allow the carbon from the engineering ferrous metal to diffuse into the coating, thus producing a composition having a melting point that matches the operating temperature of the engineering ferrous metal.

It is particularly preferred that the carbide particles have a suitable density, as described above, when the particles are left in the molten engineering ferrous metal for a residence time long enough to allow carbon to diffuse into the coating to cause the melting points to match.

If desired, the composition of the coating metal may be such as to provide a melting point which is more than 30°C below the operating temperature of the engineering ferrous metal to which the coated particles are added. The coated alloy carbide particles may be added to the liquid engineering ferrous metal either in the furnace or in a ladle into which the metal has been poured from the furnace, or in the metal stream poured from the furnace into the ladle, or in the metal stream poured from the ladle into a mold.

It is preferred to add the coated alloy carbide particles in the melting furnace to maximize the residence time of the alloy carbide particles in the ferrous metal before the onset of solidification.

It is particularly preferred that the melting furnace is an induction furnace, since an induction furnace ensures good stirring of the melt.

We have found that the titanium in the carbide oxidizes to form titanium oxide, which can react with silicon from the furnace lining to form a hard crust on the metal surface that can trap the carbide particles and reduce their dispersion in the molten metal.

The coated alloy carbide particles may be added to the melt of the engineering ferrous metal in the form of a powder comprising powder particles having a particle size of up to 2 mm and preferably about 500 micrometers (microns) and containing alloy carbide particles having a particle size of up to 10 micrometers (microns) and preferably in the range 1-5 micrometers (microns) and more preferably 2-5 micrometers (microns).

The powder particles may include:

25% - Coating metal

30% - T

35% - W

Balance - Carbon (including up to 3.5% free carbon) and usual minor components

The coating metal may comprise iron such as alpha iron and may comprise entirely or substantially entirely alpha iron and up to 3.5% free carbon.

Such a coating metal has a melting point of about 1,520ºC and the powder is therefore suitable for addition to engineering ferrous metal with an operating temperature of about 1,500ºC or above.

Alternatively, the coating metal may comprise an alloy of iron, nickel and carbon.

In this case, the powder and the alloy carbide particles may have the same size as mentioned above, and the powder particles may comprise:

27% - Coating metal

30% - T

35% - W

Balance - Carbon (including up to 0.5% free carbon) and usual minor components

The coating material may comprise an alloy of iron, nickel and carbon:

59% - Nickel

41% - iron and up to 0.5% free carbon and usual minor components

Such a coating metal has a melting point of about 1,420ºC and is particularly suitable for addition to technical ferrous metals with an operating temperature of below about 1,500ºC.

When powder with an alpha iron matrix is added to metal with such a relatively low operating temperature, it is found that the powder cannot break down sufficiently at the temperature of the melt to achieve the best distribution of the alloy carbide particles in the melt. With such engineering ferrous metal compositions, it is not possible to increase the temperature of the melt because the iron tends to become a "white" iron as a result of overheating which inhibits the formation of graphite. White iron is unsuitable for use as a roll material.

If desired, the coating metal may be an alloy of either iron, nickel and copper or iron and copper, in each case with or without carbon, to provide a lower melting point.

When the coating metal contains significant amounts of carbon, e.g. 2-3.5%, it has been found that the product to which the powder has been added is softer than expected because more graphite is transformed in the metal. This is believed to be due to the free carbon in the relatively fine powder particles acting as an inoculant. Accordingly, the carbon content of the powder is preferably reduced to less than about 0.5% to minimize the softening effect.

However, if the product is a roll used in an application where surface chipping due to cycles of reverse thermal stress is likely, it may be desirable to increase the amount of graphite in the product, because graphite provides better thermal resistance and therefore resistance to surface chipping.

If the plating metal contains nickel, a similar softening effect can occur even if the plating metal has relatively low carbon, e.g. 0.5%. This softening effect can be overcome or reduced by adjusting the chemical composition of the engineering ferrous metal.

Alternatively, the coated alloy carbide particles can be added during an electroslag remelting process.

According to a second aspect of the present invention we provide an engineered ferrous metal product according to claim 22.

The microstructure may comprise matrix and carbide resulting from phase transformation of the engineering ferrous metal; said carbide is referred to herein as "transformation carbide", as previously mentioned.

The discrete alloy carbide particles preferably have a higher hardness than the transformation carbide present in the microstructure.

The discrete alloy carbide particles can increase the hardness of the engineering ferrous metal due to the law of mixtures and/or the phase transformation of the engineering ferrous metal can be modified and enhanced by the presence of the discrete alloy carbide particles.

The discrete alloy carbide particles may be distributed in the matrix and/or the transformation carbide.

The discrete alloy carbide particles can be uniformly distributed in the microstructure.

The following applies to both the first and second aspects of the invention:

The alloy carbide particles can have a hardness of about 3,000 vpn.

The alloy carbide is preferably selected from the group comprising chromium, molybdenum, titanium, tungsten, niobium, vanadium or mixed carbides thereof, such as Cr₇C₃, (CrMo)₇C₃ or mixed carbonitrides.

The carbide can be, for example, a mixed tungsten titanium carbide of the type (TiW)C, in which the ratio of titanium to tungsten is approximately 1:1 by weight.

The alloy carbide can contain Ti and W in the ratio range:

1 : 1 to 1 : 1.17

include.

The alloy carbide may contain nitrogen, e.g. up to about 0.1% nitrogen. The alloy carbide preferably comprises:

42% - T

49% - W

9% - C and usual residues

The alloy carbide can contain up to 0.1% nitrogen.

The alloy carbide particles have a very low thermal expansion coefficient compared to the thermal expansion coefficient of engineering ferrous metals. Accordingly, if relatively large alloy carbide particles were present in the engineering ferrous metals, this would lead to high stresses during cooling, causing dislocations for the shape and leading to thermal fatigue.

The alloy carbide particles preferably have a maximum dimension of up to 10 microns, and preferably 1-5 microns, and more preferably 2-5 microns.

The amount of alloy carbide particles added is such that up to 20 vol.% alloy carbide particles in the solid metal is achieved. In general, the alloy carbide content may be in the range of 0.1 to 20 vol.% and may be in the range of 1 to 20% and more preferably 3 to 10% if a hardening effect based on the law of mixtures is provided. However, the alloy carbide content may be lower, e.g. down from about 1% to about 0.5% or 0.1% or less if a hardening effect based on a modification of the transformation of the microstructure is provided.

The technical ferrous metal preferably has a carbon content in the range of 0.3 to 3.8%.

The technical ferrous metals may contain chromium and may have a chromium content greater than or equal to 1%.

The technical iron metal may contain nitrogen, e.g. up to 0.1% nitrogen.

If the coating metal and/or the alloy carbide and/or the engineering ferrous metal contains nitrogen, it is considered that titanium carbonitrides may be precipitated in the microstructure of the engineering ferrous metal. The total nitrogen content of all components may be limited to about 0.1% nitrogen.

The engineering ferrous metal may have a composition that falls within any of the following ranges: Undefined chilled cast iron such as: Chromium iron such as: High alloy steels such as: Tool steels such as:

or

Such steel can be used for the finishing stands of a hot strip steel rolling mill. Medium alloy steels such as: Low alloy steels such as: SG irons such as:

In all cases, iron and common minor components make up the remainder.

Nitrogen may also be present up to 0.1% if appropriate.

According to a third aspect of the invention we provide a rolling mill roll having at least an outer part comprising an engineering ferrous metal according to the second aspect of the invention or an engineering ferrous metal manufactured according to the first aspect of the invention.

According to a fourth aspect of the invention we provide a method of making a rolling mill roll comprising casting a metal made according to the first aspect of the invention or according to the second aspect of the invention to make at least an outer part of the rolling mill roll.

In a first alternative, the mill roll may be a composite mill roll of the type having a core and an outer shell, optionally with at least one intermediate layer, the shell comprising said outer part.

The alloy carbide particles can be introduced into a molten engineering ferrous metal from which the shell is to be made, and then the engineering ferrous metal with the alloy carbide particles therein can be cast into a mold.

The carbide particles can be introduced into the mold under a protective atmosphere of inert gas, such as argon, or under a vacuum.

The composite roller can be manufactured by centrifugal casting.

In a second alternative, a roll may be manufactured by performing an electroslag remelting process on a consumable electrode comprising an inner body provided with an outer cladding comprising a hollow member containing said coated alloy carbide particles.

The inner body can be tubular or solid.

The hollow member may comprise a sleeve or a plurality of discrete hollow members such as tubes.

The material within the or each hollow member may also comprise powder alloy component(s).

The alloying element powder may have a mesh size of less than 3 mm, but may have a mesh size of up to 5 mm and may be in the range 2-5 mm. Any hollow element or sleeve may comprise steel, such as mild steel or stainless steel.

When the plating comprises discrete elements, at least one of the elements may have a different composition from the other elements.

The inner body may have a constant composition over its entire cross-section or it may have a varying composition over at least a portion of its cross-section.

The droplets of molten coated alloy carbide or molten coated alloy carbide and alloy component from the hollow elements will solidify relatively quickly on or near the mold wall due to their proximity in space and temperature to the mold wall, whereby the area of the molten casting near the mold wall has a relatively high and uniform distribution of alloy carbide or carbide and alloy component. If the alloy carbide particles do not melt completely, they will naturally be trapped in the melt near the mold wall as it solidifies relatively quickly.

There may be a relatively small uniform distribution of alloy carbide particles or carbide particles and alloy constituent in or near the center of the ingot.

The melting point of the alloying component and/or alloy carbides may be higher than the melting point of the slag and the inner part of the consumable electrode.

The outer part of the cast ingot can be hardened by a dispersion hardening effect of the alloy carbides.

Alternatively, the outer part of the rolled ingot may be additionally or alternatively hardened by a secondary hardening heat treatment process.

The electroslag remelting process can be carried out to provide a roll comprising an inner portion having a first composition and a surface portion having a second composition different from the first composition, and the metal of the roll between the inner portion and the surface portion having a composition that changes from the first composition to the second composition without discontinuity. This can be achieved by matching the voltage and current and the slag composition of the electroslag remelting process so that the molten pool is relatively flat.

Instead or in addition, the cooling rate of the mold can be adjusted as well as the movement rate of the mold.

Alternatively, the electroslag remelting process may be performed to provide a roll with substantially uniform composition across the entire cross-section of the roll.

By "electroslag remelting" (ESR) we mean a process in which a consumable electrode is melted beneath an electrically conductive slag in a moving mold from the lower end of which a continuously poured ingot emerges; the electrode is heated by heat transfer from the slag and the slag is heated by electrical conduction from the electrode through the slag to a counter electrode supplied by the ingot.

In third and fourth alternatives, the roll may be a composite roll made by ESR plating or spray plating. In ESR plating, a shaft is plated using the electroslag remelting process, using either a wire feed as the starting material or a hollow electrode. In the former case, the coated particles may be introduced into the molten pool by a powder feeder. In the latter case, either a powder feeder may be used or the coated particles may preferably be introduced into the hollow electrode during the primary melting and casting process in a similar manner to the preparation of the shell metal described above.

In spray plating, the coated particles can be incorporated into a spray deposited layer. It is possible to produce a spray deposited layer hardened by the coated carbide particles with sufficient depth to provide the full service life of a mill roll.

In a fifth alternative, a roll can be manufactured by monobloc static casting, where a roll is manufactured simply by filling a stationary mold with a single engineering ferrous metal.

In a sixth alternative, a roll may be manufactured by a double cast static process in which a roll is manufactured by first filling a mold partially to the top of a cylinder portion of the roll with an engineering ferrous metal of a desired composition for an outer shell portion of the roll, manufactured by the method of the first aspect of the invention, or according to the second aspect of the invention, provided with carbide particles, and when an outer portion of the shell metal has solidified, a metal of a desired core composition is fed into the bottom of the mold to dilute the first metal until a predetermined amount of metal has been displaced from a spillway between the ends and generally about halfway, an upper journal portion of the mold, the spillway is then closed and the filling of the mold with the second portion is completed.

In a seventh alternative, a roll is manufactured by casting a metal ingot manufactured according to the first aspect of the invention, or according to the second aspect of the invention, and then forging the ingot to provide a forged roll, followed by heat treating the forged roll.

A roll according to the present invention meets a market need that has not been satisfactorily met heretofore by providing a roll with higher fracture toughness and resistance to surface chipping as well as improved wear resistance.

A roll embodying the present invention is intended for use in the finishing stands of hot and cold strip rolling mills. It can be used for roughing stands in hot strip rolling mills and for other flat rolling purposes.

In particular, a roller with said outer part made of the above-listed technical ferrous metals is intended for use in the specified corresponding applications:

Undefined chilled cast iron All stands in hot strip rolling mills

Chromium iron roughing stand in hot strip rolling mills

Chromium iron cold strip rolling mills

Alloy steel hot strip rolling mills

Tool steel finishing stands in cold or hot strip rolling mills

The core may be flaky, compact/vermicular or nodular cast iron, steel or other suitable material.

By providing particles having a density matching that of the engineering ferrous metal into which they are incorporated, segregation as a result of centrifugal casting is avoided.

The spinning process tends to segregate less dense particles toward the bore of the shell. Alloy carbide particles at a density that matches the parent metal result in alloy carbide particles being evenly distributed throughout the cut of the shell and not centrifuged toward the bore.

Engineering ferrous metals produced by a process embodying the first aspect of the invention, or according to the second aspect of the invention, may be used with benefit in other centrifugal casting processes, for example for the manufacture of liners of diesel engines and pipes such as for gas or oil, or other centrifugal cast articles such as hollow sleeves for use in jet mills and other sections.

According to a fifth aspect of the invention we provide a method of making a centrifugally cast product comprising pouring a metal made according to the first aspect of the invention or according to the second aspect of the invention into a centrifugal casting mold and performing a centrifugal casting process thereon.

However, the invention is not limited to products made by centrifugal or spin casting. The invention can be applied to a wide range of engineering ferrous metals for many purposes.

The engineering ferrous metals can be cast into conventional ingot molds and subsequently forged into components, e.g. rolling mill rolls.

Rolls made in this manner would typically be hardened to a martensite or bainite structure and would be used in cold rolling steel. For these applications, a uniform structure is required to give the cold rolled product a uniform finish.

It is surprising that the carbide particles are uniformly distributed throughout the matrix and within the transformation carbide microstructure because one would have expected that during solidification the solidifying phase would push the alloy carbide particles in front of the interface of the solidifying liquid metal, so one would expect the alloy carbide particles to be segregated at the grain boundaries.

An example of the invention will now be described with reference to the accompanying drawings in which:

Fig. 1 is a photomicrograph showing an interior portion of a jacket embodying the invention at a magnification of 500,

Fig. 2 is an enlargement of a portion of Fig. 1, at a magnification of 1,500,

Fig. 3 and 4 are similar to Fig. 2 and 3, but from a central region of the shell,

Fig. 5 and 6 are similar to Fig. 1 and 2, but from an outer region of the mantle,

Fig. 7 is a photomicrograph of a shell of a roller of the same basic composition but not embodying the invention, at a magnification of x500,

Fig. 8 is a photomicrograph of a forged test piece at a magnification of x400.

Fig. 9 is a photomicrograph showing a sample of a second example of the invention at a magnification of x300,

Fig. 10 is a scanning electron micrograph showing a sample of a third example of the invention, at a magnification of x350,

Fig. 11 is a scanning electron micrograph showing a sample of the third example of the invention, at a magnification of x3,000,

Fig. 12 is a schematic representation of a centrifugal casting plant used in Examples I-III,

Fig. 13 is a schematic side view of another embodiment showing an electroslag remelting apparatus when in use in a process embodying the present invention, and

Fig. 14 is a cross-section through a consumable electrode for use in the apparatus of Fig. 13.

EXAMPLE I

In a first example, a technical ferrous metal melt comprising an undefined chilled cast iron with the following composition was prepared in a conventional manner in an induction furnace.

Carbon 3.3%

Silicon 0.85%

Manganese 0.50%

Nickel 2.3%

Chromium 1.1%

Molybdenum 0.1%

Sulphur 0.050%

Phosphorus 0.060%

Iron and usual minor components Rest

After the iron had been brought to operating temperature, i.e. approximately 1500ºC, a powder comprising alloy carbide particles coated with an iron coating having a carbon content of 3.00% was added to the melt in the furnace in an amount equal to 10% by weight. The powder particles had a particle size of up to 500 microns and comprised:

25% - Alpha Iron

30% - T

35% - W

Rest - C (of which 3% was free carbon) and usual minor components

The alloy carbide particles had a mean particle size of up to 10 microns and comprising a solid solution of 30% Ti and 35% W carbides and had a density of about 7 to match that of iron.

The above operating temperature is higher than the usual temperature to which metal of the above composition is heated prior to tapping because the coating metal of the powder has a melting temperature of about 1520ºC and it is desirable that the metal be as hot as possible. A temperature higher than about 1500ºC is not possible because of the tendency to "whiten" the undefined chilled iron.

The melt is then allowed to remain in the furnace for about 15 minutes to reach its operating temperature of about 1500°C. The molten metal was then poured into a ladle and then transferred in the ladle from the furnace to a conventional centrifugal casting plant shown in Figure 1 where the metal from the ladle was poured into the centrifugal casting plant to form a shell 5 of the metal described above in a conventional manner. To produce a composite mill roll, metal of suitable composition, such as nodular cast iron, would then be cast to form a core C. However, the present example was concerned with providing a shell, i.e. a tubular product, and so the core metal was not cast.

The residence time of the metal in the furnace after the addition of the carbide particles was about 15 minutes to allow the temperature to reach, and the residence time in the ladle before the Pouring into the centrifugal casting machine was about 15 minutes. Thus, in the present example, about 30 minutes elapsed after the addition of the carbide particles and before pouring. If desired, the residence time could be longer, e.g. 1 hour or 1.5 hours or more, or could be shorter.

After the mantle had solidified, samples were taken from areas adjacent to the inner, middle and outer parts of the mantle and appropriate samples were prepared in a conventional manner to obtain the photomicrographs shown in Figs. 1-6.

From these micrographs it will be seen that the added carbide particles are relatively uniformly distributed on a macroscopic scale throughout the thickness of the shell. It is particularly noted that the carbide particles are not segregated at grain boundaries but are relatively uniformly distributed throughout the microstructure, regardless of the phases present as a result of phase transformation during solidification and cooling. It can also be seen from Fig. 7 that the microstructure is of the same type as in a control base material of the same composition but without alloy carbide particle addition.

Chemical analysis of a sample from several locations was performed which shows that only about half of the added carbide had been taken up to yield 5% by weight of carbide. By extra carbide we mean discrete carbide particles in addition to the carbide that is naturally present as a result of a phase change upon cooling.

An Edax (energy dispersive X-ray) analysis showed a ratio of Ti:W in the carbide of about 4:1, but this is on an atomic % basis, and on a corresponding wt % basis this gives:

Ti : W to 1 : 1

The dual nature of the carbides with a dark core suggests that the outer part of the carbide is W-rich and the inner part is Ti-rich.

As mentioned above, the alloy carbide dispersion is uniform over the entire sample cross-section and has an "area fraction" of about 5%, which corresponds to a "volume fraction" of 5%. Although there is some evidence of clustering of the particles, there are only a few particles in each cluster and the clustering is independent of the arrangement of the particles in the engineering ferrous metal.

In particular, there was no indication that alloy carbide particles had been displaced in front of the solidification boundary layer, i.e. the alloy carbide particle distribution was independent of the underlying microstructure.

The carbide particle size was in the range of 2-S microns.

The hardness of the sample was tested and found to be VHN₃₀ 810, and the hardness of a sample cut from the control roll was found to be VHN₃₀ 650.

This increase in hardness of about 160 is slightly greater than would be expected from the law of mixtures. It is believed that this is due to a hardening effect due to small particle size (Orowan hardening).

EXAMPLE II EXAMPLE II

A composite roll comprising a shell made of undefined chilled cast iron and a core of nodular cast iron was manufactured using a centrifugal casting technique similar to that of Example I, except that in this case a finished roll was manufactured by casting the nodular cast iron core C after completion of the shell forming step.

In this example, the shell metal had the following composition:

T.C. - 3.40

Silicon - 0.85

Magnesium - 0.51

Nickel - 4.13

Chromium - 1.63

Molybdenum - 0.46

Sulfur - 0.13

Phosphorus - 0.35

Copper - 0.06

Lead - 0.0013

Vanadium - 0.021

Titanium - 0.175

Iron and usual minor components - rest

This metal was again produced in an induction furnace and the metal in the induction furnace was heated above the melting point of the metal to the normal operating temperature, which is, for example, approximately 200ºC above the liquidus temperature. In the present example, the liquidus temperature was 1,215ºC and the operating temperature was 1,460ºC.

The induction furnace was covered and argon was fed into the interior of the cover, but can also be injected into the melt. The induction coil was turned off. The powder of coated alloy carbide particles was then injected into the melt by blowing through a lance with argon or in any other desired manner. If desired, any other inert gas can be used instead of argon, or the enclosed space above the melt in the induction furnace can be connected to a vacuum and particles can be added in any desired manner, e.g. by gravity feed, especially if the vacuum is provided. The induction coil was then turned on so that the melt was mixed. Other techniques to prevent oxidation can be used, such as using a flux or adding the coated alloy carbide powder as a core wire, although the flux may inhibit the dispersion of particles in the melt and a core wire would be expensive.

The powder of coated alloy carbide particles comprises:

27% - Coating metal

30% - T

35% - W

Balance - carbon (including 0.5% free carbon) and usual minor components).

The coating metal comprised 59% Ni and 41% iron (including up to 0.5% free carbon and common minor components).

After a suitable dwell time as mentioned above, the metal was poured into a ladle where it remained at a temperature corresponding to the liquidus temperature plus about 170-180ºC and then the metal was poured into the mold of the centrifugal casting machine poured at a temperature of the liquidus temperature plus about 80-110ºC. The exact temperature depends on the size, with the smaller molds being hotter and the larger ones colder.

After the roll had solidified, similar samples were taken as in Example I, and Figure 9 is an optical D.I.C. (differential interference contrast) photomicrograph at x300 magnification, unetched, of a sample taken from the center of the shell, showing a matrix with graphite flakes and small, uniformly and randomly distributed alloy carbide particles. The microstructure was similar to that of Example I, with the alloy coated particles distributed throughout the matrix and transformation carbide.

A U.T.S. (tensile stress) test was performed on another sample from the shell and showed that the sample had a U.T.S. (tensile stress limit) of 600-700 MPa. An undefined chilled iron of the same composition but without the alloy carbide particles has a U.T.S. (tensile stress limit) of 385-493 MPa.

EXAMPLE III

A composite roll comprising a high chromium iron shell and a nodular cast iron core was manufactured by the centrifugal casting technique of Example II using an argon atmosphere.

The shell metal had the following composition:

T.C. - 2.95

Si-1.63

S-0.035

P-0.040

Mn-0.95

Ni - 0.77

Cr - 17.28

Mon - 2.08

Approx - 0.93

Pb-0.0081.

V-0.031

Fe and usual residues - Rest

The powder coated alloy carbide was as described in Example I. The powder was added to the metal in amounts equal to 10 wt%. 5 wt% was present in the solid metal.

The metal has a liquidus temperature of 1255ºC and was heated to an operating temperature of 1560ºC. The temperatures in the ladle and in the centrifuge were related to the liquidus temperature in a similar manner as in Example II, with the temperature at addition to the centrifuge being 1345ºC. After the roll had solidified, samples were taken as in Example II.

Fig. 10 is a scanning electron micrograph at x350 magnification of a sample taken from the center of the shell. Fig. 11 is a similar micrograph but at x3,000 magnification. It will be seen that the discrete added alloy carbide particles are randomly and uniformly distributed and are present in both the matrix and the transformation carbide. The carbide has largely separated into dark angular titanium-rich particles of about 1-10 microns in size and reddish, rounded tungsten-rich particles generally less than about 5 microns in size. Edax (energy dispersive X-ray) analysis showed that the tungsten-rich particles also contain high levels of molybdenum, while the metal itself contains low levels of titanium and tungsten in solution, both in the matrix and in the transformation carbide. This indicates the dissolution of the (TiW)C alloy carbide particles in the melt or at least their tungsten-rich outer layers, followed by their reprecipitation after solidification.

A U.T.S. (tensile stress) test was carried out on a similar sample from the shell and this showed that the sample had a U.T.S. (tensile stress limit) of 800-850 MPa. A high chromium iron of the same composition but without the alloying carbide addition has a U.T.S. (tensile stress limit) of 740-830 MPa.

EXAMPLE IV

A technical ferrous metal melt with the following composition was prepared in a conventional manner.

After the steel had been brought to tapping temperature, carbide particles of the same type as described hereinbefore in connection with the first example were added to the melt in the melting furnace in an amount corresponding to 10 wt.%.

The metal was then left in the furnace as before to reach its tapping temperature, and then an ingot was cast.

After the usual skimmed end of the casting, the ingot was 5¹/₂" in diameter and 6¹/₂' long and weighed 20 kg.

This cast block was then step forged in an open die forge to form a round bar in three stages as follows:

2 : 1-4¹/₂' diameter

4 : 1-3¹/₂' diameter

10 : 1-2¹/₂' diameter

The resulting forging was heat treated to simulate a conventional forging roll heat treatment. Photomicrographs were taken at a magnification of x400 at different positions and at cross and longitudinal sections. These showed that the forging had a substantially uniform continuous microstructure. Fig. 8 is an example of one of these photomicrographs taken along a longitudinal section.

From these photomicrographs it can be seen that the basic matrix structure was martensitic and that the added carbide particles are again relatively uniformly distributed throughout the forging. Again it was found that about half of the added carbide had been recovered to give 5 wt% of the extra carbide.

EXAMPLE V

Referring now to Figures 13 and 14, a further embodiment is illustrated. An electroslag remelting apparatus is shown in Figure 13 and is of substantially conventional type comprising a cylindrical, water-cooled mold 10 which is movable vertically up or down. An electrode holder 11 holds, e.g. by being welded thereto, the upper end of a consumable electrode 12. Initially, the lower end of the consumable electrode is immersed in molten slag 30 contained between a bottom plate 13 and the wall of the mold 10. Electric current is then passed therethrough to cause the lower end of the electrode to melt and as droplets of metal fall from the lower end of the electrode 12, they pass through the slag and are purified and then solidified to form an ingot 14.

The basic operation of the electroslag remelting process is completely conventional, as is the equipment, and further discussion is therefore not necessary.

The consumable electrode 12 comprises an inner body 120 to whose outer surface 121 is welded a hollow cladding 122 comprising a plurality of discrete tubular elements 123 in the form of tubes 124 containing powder 125.

The inner body 120 is made of low alloy steel, cast iron or mild steel, such as steel with 0.2% carbon.

The tubes 124 are made of mild steel in the present example, but may be made of stainless steel, and if desired, the tubes may be made of high alloy steel or cast iron, or indeed any material of suitable composition, such as

Carbon 0.5%-3%

Aluminum 0.0%-1%

Molybdenum 0.5%-5%

Vanadium 0.0%-12%

Chromium 0.5%-15%

Tungsten 0.0%-8%

Silicon 0.0%-3%

Titanium 0.0%-8%

Nickel 0.0%-5%

Iron and usual residues.

Preferably, the tubes have a composition in the range:

Carbon 0.5%-3%

Aluminum 0.0%-0.1%

Molybdenum 0.5%-5.0%

Vanadium 0.1%-12%

Chromium 0.5%-15%

Tungsten 0.1%-8%

Silicon 0.0%-3%

Titanium 0.1%-8%

Nickel 0.1%-5%

Iron and usual residues.

The tube can have the following composition:

Carbon 1%

Molybdenum 2.5%

Vanadium 0.7%

Chromium 14%

Iron and usual residues.

The inner body may comprise a low alloy steel which may comprise:

Carbon 0.4%

Chromium 1%

Nitrogen up to 0.1%

Iron and usual residues.

If desired, the tubes may comprise other relatively high alloy metals, such as:

Carbon 3%

Molybdenum 5%

Vanadium 8%

Nickel 2%

Iron and usual residues.

Carbon 1%

Vanadium 12%

Iron and usual residues

Carbon 2%

Tungsten 5%

Titanium 5%

Iron and usual residues

Carbon 3%

Silicon 2%

Iron and usual residues

If desired, the inner body may comprise other relatively low alloy metal, such as:

Carbon 0.2%

Chromium 0.5

or

Carbon 3%

Silicon 2%

or pure cast iron or mild steel.

However, the tubes may be intended solely to hold the powder 125, thus could be made of metal of the same composition as the inner body, or could actually be made of suitable non-metallic material, such as silicon dioxide having a melting point such that it melts in the bath at a desired rate to release the alloying additive. The materials described above are applicable to a hollow plating of any suitable configuration. In the present example, the tubes have the following composition:

Carbon 0.2%

Iron and usual residues.

The powder comprises a mixture of coated alloy carbide particles as previously described in connection with Example I (but may be any of the other coated alloy carbide particles previously described) and particles of an alloy constituent or constituents. The powder may comprise, for example: Carbon up to 10% (with 5% as graphite)

Tungsten 35-40%

Titanium 30-40%

Iron, carbide and

usual residues rest.

Some of the carbon is combined with the tungsten and titanium as an intermetallic compound, but the tungsten and titanium are combined as a solid solution and some of the carbon present is present as free graphite.

In this example, the powder mixture has the following composition:

Chrome 2

Tungsten 40

Titanium 40

Molybdenum 1

Vanadium 1

Carbon 6%

Iron, carbide and

usual residues rest.

Alternatively, the powder mixture has the following composition:

Chrome 2

Tungsten 35

Titanium 30

Molybdenum 1

Vanadium 1

Carbon 6%

Iron, carbide and

usual residues rest.

In general, one or more of the alloying elements may be present in the powder mixture, either as elements or combined with carbon as an intermetallic compound or alloy.

carbon

molybdenum

chrome

tungsten

titanium

Vanadium

niobium

If desired, the powder may be made from a powdered alloy of any of the compositions or range of compositions described as suitable for the above-mentioned tubes, or may be made from alloys of other compositions or from ingredients providing such compositions.

The amount of carbide particles added is such that up to 1-20 vol.% of carbide particles are achieved in the surface part.

The tubes 124 in this example are 16 mm OD (outside diameter), 13 mm ID (inside diameter), but can range from 6 mm ID (inside diameter) to 28 mm ID with appropriate OD (outside diameter).

The tubes are provided with constrictions at intervals along their length to hold discrete amounts of carbide powder at positions along the length of the tubes, such constrictions may be provided e.g. every 10 mm.

In the present example, the constrictions are provided by folding the tubes over to close or substantially close their bores, but such constrictions may of course be provided in any other desired manner.

In the present example, the tubes 124 are attached to the inner body 120 so that they extend longitudinally thereof parallel to the longitudinal axis of the inner body, but, if desired, a single tube or tubes may be spirally wrapped around the inner body or one or more tubes may extend around the circumference of the body so that they lie in a plane perpendicular to the longitudinal axis of the inner body, so long as the tubes are the correct size to hold the correct amount of powder for release into the melt in the longitudinal position of the body.

Further alternatively, one or more tubes may be arranged to extend around the inner body in a plane or corresponding plane inclined to the longitudinal axis of the inner body at less than a right angle.

Although in this case the plating is in the form of discrete elements provided by the tubes 124 containing powder 125, if desired the plating may comprise a hollow sleeve, e.g. a sleeve comprising inner and outer generally cylindrical walls interconnected by generally annular shaped walls, the space between the walls containing powder as described hereinbefore, and the sleeve secured to the inner body, e.g. by welding or by a slide fit. In this case the hollow sleeve is provided with constrictions in its wall at appropriate positions along the length of the sleeve to divide the powder into discrete amounts for sequential release into the melt pool when the sleeve and inner body are melted.

In the present example, the melting point of an alloy to which the carbides are added, e.g., solid tungsten-titanium solution capable of at least partially melting in the molten bath, and the alloy carbides are higher than the melting point of the slag 30 and the inner body 120 of the consumable electrode 12. The melting point of the cladding tube/sleeve metal of the tubes and/or any alloying constituent may be 40°C below the melting point of the inner part 20 and 60°C above the melting point of the slag 30. The melting point of the tubes 22 in the present example is 1530°C.

The inner portion may have a melting point ranging from 1,160ºC to 1,600ºC and the cladding may have a melting point ranging from 1,160ºC to 1,600ºC.

The melting point of the inner and outer parts may differ by 20ºC to 60ºC. The slag may have a melting point that differs from the lower melting point of the inner part and the cladding by 20ºC to 60ºC.

The mold is cooled with water to a temperature ranging from 15ºC to 65ºC and the molten metal pool 31 has a temperature ranging from 1400ºC to 1600ºC, possibly from 1160ºC to 1600ºC.

The voltage and current used in the electroslag remelting process are adjusted to provide a relatively high voltage and a relatively low current or vice versa and in addition the composition of the slag is adjusted to provide a relatively viscous slag. As a result, turbulence in the slag and melt pool is minimized so that a desired composition gradient is achieved.

In addition, the voltage and current used are adjusted so that the bottom of the melt pool is relatively flat.

The metal droplets leaving the electrode plating, including the contents thereof, are denser than the metal droplets leaving the inner body of the electrode when they are more highly alloyed. This results in the droplets from the electrode plating falling downwards adjacent to the wall of the mold and, because the bottom of the melt pool is relatively flat, there is relatively little tendency for these droplets to run to the center of the melt pool before they have solidified as a result of their spatial and temperature proximity to the mold wall.

The composition of the slag is adjusted to reduce the ionic capacity of the slag by adjusting the balance of silicon, calcium and aluminum in the slag to reduce the tendency toward electromagnetic stirring, as well as adjusting the composition of the slag to provide a relatively higher viscosity.

Typically the slag has the following composition: 33¹/₃% CaO, 33¹/₃% CaF₂, 33¹/₃% Al₂O₃.

However, the slag may have a composition of 20% CaO, 80% CaF₂, 0% Al₂O₃.

The cooling rate provided by the water cooling to the mold can be adjusted by setting the mold raw temperature to be in the range of 15ºC to 65ºC. In addition, the mold movement rate can also be adjusted.

Typically, the electrode has a diameter that is about 0.9% of the diameter of the mold cavity.

The electrode is arranged so that approximately 20% by weight of the electrode comprises plating and the remainder is occupied by the inner body. This ratio can range from 1% to 40%.

Although the melting point of the plating has been described above as being lower than the melting point of the metal of the inner body, this can be reversed if desired.

As the electrode 12 melts, droplets of liquid metal are solid particles of the non-melting metal carbide from the tubes 124 and powder 125 and fall generally vertically downward, and because of the relatively close spatial and temperature proximity to the wall of the mold 110, the particles solidify relatively quickly at or adjacent to the mold wall, and the carbide particles are trapped by the solidifying melt adjacent the mold wall so that the metal at the surface of the ingot 14 has a relatively high uniform distribution of alloy and alloy carbides and a matrix composition substantially similar to the composition of the metal of the tubes 124.

The metal droplets falling from the center of the bottom of the inner part fall vertically downwards, and thus the metal solidified in the center of the ingot has a composition substantially similar to the composition of the inner part of the electrode.

This is facilitated when the metal from the plating has a greater density than the metal from the inner body, because this causes the droplets to be adjacent to the mold wall together and the precautions mentioned hereinbefore to avoid mixing result in low slag viscosity, low electromagnetic stirring of the slag and a relatively flat melt pool base.

Between these two extremes there is a limited mixing of the material from the two regions so that there is a change in composition between the two extremes without discontinuity. Accordingly, the ingot has a relatively uniform transition region between the interior portion and the surface portion. The ingot, and hence any resulting product such as a roll as described hereinbefore, has an elastic modulus which increases in direct proportion to the amount of distribution of the alloy/alloy carbide throughout the cross-section of the ingot.

In this example, the composition on the surface of the cast block is:

Carbon 0.5%

Molybdenum 0.2%

Vanadium 0.9%

Tungsten 1.2%

Chromium 0.5%

Titanium 0.15%

Iron, carbide and

usual residues rest.

The composition in the middle of the cast block is:

Carbon 0.5%

Molybdenum 0.1%

Tungsten 0.3

Titanium 0.03%

Vanadium 0.2%

Chromium 0.5%

Iron, carbide and

usual residues rest.

The variation in composition is believed to be due to the relatively higher diffusion of chromium and carbon, compared to the relatively lower diffusion and denser tungsten and titanium.

The composition gradient does not exceed 40% per 100 mm in the radial direction and this condition applies to any position along the longitudinal extension of the roll.

EXAMPLE VI

In this example, the electroslag remelting apparatus and operating procedure described in connection with Example V can also be used, but the operating conditions are modified to aim at providing a roll of uniform composition, e.g., the voltage and current used are adjusted so that the bottom of the melt pool is relatively deep. In general, conventional operating conditions and slag compositions were used.

An electrode comprising an inner portion with an outer cladding of tubular configuration, both made of low carbon steel, was prepared such that the space therebetween contained alloy carbide powder as described in connection with Example V. The amounts were calculated for a target addition of 1 wt.%.

A conventional electroslag remelting process was carried out.

Hardness tests were carried out on the remelting roll and this showed that a hardness of around Hv 250 was achieved over the majority of the cross-section of the roll.

This is considerably higher than that expected from the law of mixtures, where for a 1% addition of carbide (Hv approximately 3000) to a low carbon steel (Hv approximately 180) a value of around Hv 210 is expected.

Microstructure examination showed a good uniform and statistical dispersion of added carbide particles with a volume approaching 1 wt.%.

EDAX (Energy Dispersive X-ray) analysis of individual carbide particles shows a strong presence of titanium with only traces of tungsten. This suggests that dissolution of tungsten from the particles in the melt has occurred. This would retard pearlite transformation cooling, allowing the development of a bainitic structure, thus giving greater hardness due to modification of the phase transformations in this way.

Moreover, since the low carbon steel of this example contained about 0.1% nitrogen, it is contemplated that at least a portion of this nitrogen formed titanium carbonitride particles and therefore at least a portion of the above-mentioned carbide particles comprise such titanium carbonitride particles, which may also contain tungsten.

Unless otherwise stated, compositions herein are expressed in weight percent.

References to particle size refer to the largest dimensions of the largest particle, unless otherwise specified.

Claims (47)

1. A method of making a rolling mill roll comprising the steps of adding solid alloy carbide particles to liquid cast iron or steel and then allowing the cast iron or steel to solidify, the alloy carbide particles having a density of 6 to 8 g per cm3 matching that of the cast iron or steel, the solid carbide particles being coated with a metal which allows wetting to occur between the particles and the liquid cast iron or steel, and pouring said metal to provide at least an outer portion of said roll. 2. Process according to claim 1, characterized in that the coating metal comprises iron or an iron-carbon alloy or an alloy of two or more elements selected from the group comprising iron, nickel, copper, titanium and carbon, or nickel or copper and usual minor components, and/or nitrogen. 3. A method according to claim 1 or 2, characterized in that the coating metal has a melting point which is within about 20º-30ºC of the temperature of the technical ferrous metal at which said coated carbide particles are added. 4. A method according to any one of the preceding claims, characterized in that the alloy carbide particles are mixed with iron or with an iron alloy having a lower carbon content than that of the cast iron or steel to which they are added, and the coated alloy carbide particles are added to the cast iron or steel and maintained therein for a time sufficient to allow carbon from the cast iron or steel to diffuse into the coating to produce a composition having a melting point consistent with the working temperature of the cast iron or steel. 5. A process according to any one of the preceding claims, characterized in that the coated alloy carbide particles are added to the liquid cast iron or steel either in the melting furnace or in a ladle into which the metal has been poured from the melting furnace, or in the metal stream which is poured from the melting furnace into the ladle, or in the metal stream which is poured from the ladle into a mold. 6. Process according to claim 5, characterized in that the coated alloy carbide particles are added in the melting furnace. 7. A method according to any one of the preceding claims, characterized in that the coated alloy carbide particles are added to the cast iron or steel in an inert environment. 8. A process according to any one of the preceding claims, characterized in that the coated alloy carbide particles are added to the cast iron or steel melt in the form of a powder comprising powder particles having a particle size of up to 2 mm and preferably about 500 micrometers (500 microns) and containing alloy carbide particles having a particle size of up to 10 micrometers (10 microns) and preferably in the range 1-5 micrometers (1.5 microns) and more preferably 2.5 micrometers (2-5 microns). 9. A method according to any one of the preceding claims, characterized in that the coated alloy carbide particles are added to the cast iron or steel melt in the form of a powder, the powder particles comprising: 25% - Coating metal 30% - T 35% - W Balance - Carbon (including up to 3.5% free carbon) and usual minor components 10. A method according to any one of the preceding claims, characterized in that the coating metal comprises completely or substantially completely alpha iron and up to 3.5% free carbon. 11. A method according to any one of claims 1 to 8, characterized in that the coated alloy carbide particles are added to the cast iron or steel melt in the form of a powder, the powder particles comprising: 27% - Coating metal 30% - T 35% - W Balance - Carbon (including up to 0.5% free carbon) and usual minor components 12. A method according to any one of claims 1 to 8 or claim 11, characterized in that the coating metal comprises an alloy of iron, nickel and carbon. 13. A method according to claim 12, characterized in that the alloy of iron, nickel and carbon comprises: 59% - Nickel 41% - iron and up to 0.5% free carbon and usual minor components 14. A method according to any one of claims 1 to 4 or any one of claims 8 to 13, if dependent on any one of claims 1 to 4, characterized in that the coated alloy carbide particles are added during an electroslag remelting process. 15. A method according to any one of the preceding claims, characterized in that the alloy carbide is selected from the group consisting of chromium, molybdenum, titanium, tungsten, niobium, vanadium or mixed carbides thereof, such as Cr₇C₃, (CrMo)₇C₃ or mixed carbonitrides thereof. 16. Method according to one of the preceding claims, characterized in that the carbide is a mixed tungsten-titanium carbide of the type (TiW)C, the weight ratio of titanium to tungsten being approximately 1:1 and preferably in the ratio range: 1 : 1 to 1 : 1.17 17. A method according to any one of the preceding claims, characterized in that the alloy carbide particles have a maximum dimension of up to 10 micrometers (10 microns), and preferably 1-5 micrometers (1-5 microns), and more preferably 2-5 micrometers (2-5 microns). 18. Process according to one of the preceding claims, characterized in that the amount of alloy carbide particles added is such that up to 20%, preferably up to 5 to 20%, by volume, of alloy carbide particles in the solid metal are achieved. 19. A method according to claim 18, characterized in that the alloy carbide content is in the range 0.1 to 20 vol.% and more preferably 3 to 10% when providing a hardening effect based on the law of mixtures. 20. A method according to claim 18, characterized in that the alloy carbide content is in the range of 1 to about 0.5 vol.% or 1 to about 0.1 vol.% or less when a hardening effect based on a modification of the transformation of the microstructure is provided. 21. Process according to one of the preceding claims, characterized in that the cast iron or steel is selected from the group comprising iron, chromium iron, high alloy iron, tool steel, medium alloy steel, low alloy steel, S.G. iron, steel or cast iron, with a carbon content which is in the range 0.3 to 3.8% and which may contain nitrogen up to 0.1%. 22. A rolling mill roll having at least one outer part comprising a cast iron or steel product comprising a carbon alloy having a microstructure resulting from phase transformation upon cooling and having discrete alloy carbide particles dispersed therein, the alloy carbide having a composition such that the density of the alloy carbide, which is in the range of 6 to 8 g per cm3, matches that of the cast iron or steel. 23. Rolling mill roll according to claim 22, characterized in that the microstructure comprises a matrix and transformation carbide which have resulted from phase transformation of the technical iron metal. 24. Rolling mill roll according to claim 23, characterized in that the discrete alloy carbide particles are distributed in the matrix. 25. Rolling mill roll according to claim 23 or claim 24, characterized in that the discrete alloy carbide particles are distributed in the transformation carbide. 26. Rolling mill roll according to one of claims 22 to 25, characterized in that the discrete alloy carbide particles are uniformly distributed in the microstructure. 27. Rolling mill roll according to one of the preceding claims, characterized in that the alloy carbide is selected from the group comprising chromium, molybdenum, titanium, tungsten, niobium, vanadium or mixed carbides thereof, such as Cr₇C₃, (CrMo)₇C₃ or mixed carbonitrides thereof. 28. Rolling mill roll according to one of the preceding claims, characterized in that the carbide is a mixed tungsten-titanium carbide of the type (TiW)C, the weight ratio of titanium to tungsten being approximately 1:1 and preferably in the ratio range: 1 : 1 to 1 : 1.17 29. Rolling mill roll according to one of the preceding claims, characterized in that the alloy carbide particles have a maximum dimension of up to 10 micrometers (10 microns) and preferably 1-5 micrometers (1-5 microns) and more preferably 2-5 micrometers (2-5 microns). 30. Rolling mill roll according to one of the preceding claims, characterized in that the amount of alloy carbide particles added is such that up to 20%, preferably up to 5 to 20%, by volume, of alloy carbide particles in the solid metal are achieved. 31. A rolling mill roll according to claim 30, characterized in that the alloy carbide content is in the range 0.1 to 20 vol.% and more preferably 3 to 10%, when a hardening effect based on the law of mixtures is provided. 32. A rolling mill roll according to claim 30, characterized in that the alloy carbide content is in the range of from 1 to about 0.5 vol.% or from 1 to about 0.1 vol.% or less when a hardening effect based on a modification of the transformation of the microstructure is provided. 33. Rolling mill roll according to one of the preceding claims, characterized in that the cast iron or steel is selected from the group comprising iron, chromium iron, high alloy steel, tool steel, medium alloy steel, low alloy steel, S.G. iron, steel or cast iron, with a carbon content which is in the range 0.3-3.8 % and which may contain nitrogen up to 0.1%. 34. A method according to any one of claims 1 to 21, characterized in that the rolling mill roll is a composite rolling mill roll of the type having a core and an outer shell, optionally with at least one intermediate layer, the shell comprising said outer part. 35. Rolling mill roll according to one of claims 22 to 33, characterized in that the rolling mill roll is a composite rolling mill roll of the type having a core and an outer shell, optionally with at least one intermediate layer, the shell comprising said outer part. 36. A method according to claim 34, characterized in that the alloy carbide particles are introduced into a molten technical ferrous metal from which the shell is to be made, and then the technical ferrous metal with the alloy carbide particles therein is poured into a mold. 37. Method according to claim 34 or claim 36, characterized in that the composite roller is manufactured by centrifugal casting. 38. A method according to any one of claims 1 to 21, characterized in that the roll is manufactured by performing an electroslag remelting process on a consumable electrode comprising an inner body provided with an outer plating comprising a hollow member containing said coated alloy carbide particles. 39. A method according to claim 38, characterized in that the hollow element also contains a powder alloy component. 40. A method according to claim 38 or claim 39, characterized in that the electroslag remelting process is carried out to provide a roll comprising an inner part having a first composition and a surface part having a second composition different from the first composition, and the metal of the roll between the inner part and the surface part has a composition which changes from the first composition to the second composition without discontinuity. 41. A method according to claim 38 or claim 40, characterized in that the electroslag remelting process is carried out to provide a roll having a substantially uniform composition over the entire cross-section of the roll. 42. A method according to claim 34 or claim 36, characterized in that the roller is manufactured by electroslag remelt plating or spray plating. 43. Method according to one of claims 1 to 21, characterized in that the roller is manufactured by monoblock static casting by filling a stationary mold with a single cast iron or steel. 44. Method according to one of claims 1 to 21, characterized in that a roller is manufactured by a double-cast static casting process. 45. A method according to any one of claims 34, 36, 37 to 39 or claims 42 to 44, characterized in that the core comprises flaky, compact, vermicular or nodular cast iron or steel. 46. A method according to any one of claims 1 to 21, characterized in that it comprises casting an ingot of said metal and then forging the ingot to provide a forged roll, followed by heat treating the forged roll. 47. Rolling mill roll according to one of claims 22 to 33 or 35, characterized in that the rolling mill roll is a forged roll.