TECHNICAL FIELD The present invention relates to a new family of wear resisant or machinability improved steel material, the use of these steel materials and a method for the manufacture of the steel materials.

PRIOR ART

In many cases there are high demands for wear resistance with components of steel. As examples of fields, where a high wear resistance is required, tools for forming and cutting of various working materials may be mentioned, such as sheet steel, aluminium, textile, paper, ceramic- coated working materials, etc., i.e. conventional cold work applications. Further examples are at the moulding/cutting of plastic granulates or in mould inserts, screws, nozzles, tubes at plastic manufacture according for instance to the principle extrusion, injection moulding, pressure moulding. High wear resistance is also required for engineering parts such as for instance pump parts, valve parts, hammers, counter parts, fragmentation knives for tyres, paper, wood, metal, etc., wear components or knives within the packing industry, food industry, pulp industry, mining and mineral industry or other processing industry, as well as parts exposed to wear in transmissions and engines in the vehicle industry. Besides high wear resistance, it is also desirable that the steel materials have a high hardness. In certain cases, also good ductility and good corrosion resistance are required. To achieve the different property profiles the man skilled in the art may, besides giving the steel a specific composition of alloy elements, use various manufacturing processes. Among the rational manufacturing processes for the manufacturing of steels in large quantities, you find melt-metallurgical processes comprising conventional conventionally casting, electro slag refining (ESR), spray forming and powder metallurgical manufacture. In the term powder metallurgical manufacture, the manufacture of steel powders is included, on one hand through atomizing of a steel melt with inert gas, and on the other hand through atomizing of a steel with the processes included in the designation PREP, Plasma Rotating Electrode Process. Each one of these processes contributes in its way to obtaining a desirable combination of properties.

As to the wear resistance, one may simply say that conventionally cast cold work steels are characterized of relatively coarse carbides, up to 200 μm or larger in their longest extension, which gives the steel very good abrasive wear properties. Because alloyed steels, which are conventionally cast, show segregations, a characteristic carbide distribution is obtained, wherein long carbides accumulated in stripes in the matrix result in inhomogeneous properties in the longitudinal and transversal directions. Further, high-alloy steels cannot be manufactured with a good yield within the whole range of the desired dimensional program for manufacturing technical reasons.

The spray formed cold work steels have smaller and rounder carbides than the ones conventionally manufactured, normally between 1 and 20 μm, which are evenly distributed in the matrix of the steel. This manufacturing technique enables manufacturing of very high-alloyed steels, for instance high-speed steels, e.g. Tl 5 and even higher alloyed high-speed steels, and tool steels, e.g. Weartec, with up to 15% vanadium. With a finer carbide structure it is normally so that the wear resistance decreases but through the spray forming technique this effect may be compensated as it makes it possible to alloy with up to 15% vanadium, whereby some of the comparatively softer chromium carbides of M ₇ C ₃ -type are replaced by harder MX-carbides. In this way, one has managed to manufacture steels with extremely good wear resistance, like the steel called Weartec mentioned above.

The steels manufactured through spray forming also have a better ductility and more homogenous properties irrespective of direction than the steels conventionally manufactured. As the solidification speed of the spray formed materials varies between the surface and the centre a size gradient of the carbides from surface to centre is obtained, where the finest carbides are found in the surface as the solidification speed there is somewhat higher. There are also defects in the material caused by hot cavities and hot cracks which cannot always be welded together during the heat treatment. The resaon is that the spray forming has proved to be a process which is comparatively difficult to control because of many process parameters.

The powder metallurgically manufactured steels have, thanks to the sophisticated manufacturing technique, a microstructure which is entirely homogenous in all directions. Thanks to this fact and that the steels get very fine carbide particles, normally between 1 and 3 μm, an optimal combination of comparatively good wear resistance, hardness and ductility is obtained. By balancing the alloy composition, in the first place by adding still higher amounts of vanadium and possibly niobium, the forming of very hard carbides may be promoted at the expense of less hard carbides. In this way, also the powder metallurgical materials may obtain extremely good wear resistance despite comparatively very small carbides in general.

Among the materials conventionally manufactured we find the standardized steels AISI D2, D6, AND D7, which are today used for cold work applications with more or less abrasive wear. The nominal compositions of these known steels are shown in Table 1. Table 1 - Conventional cold work steels - nominal compositions, weight-%

The US patent publication US 6,348,109 describes a steel material which may be manufactured through spray forming. The material has proved to have a better combination of wear resistance and toughness than conventional ledeburitic cold work steels of the types AISI D2, D6, and D7

Further, in the US patent application US 2004/0094239 Al a spray formed steel material is described, which has excellent wear resistance, good corrosion resistance, hardenability and tempering response as well as adequate toughness. The steel material is in the first place intended to be used in plastic moulding equipment and there be used as engineering parts to feed and direct plastic masses into machines for the manufacture of plastic components and also in moulding tools and tool parts for injection moulding of plastics. However, the known steel material is not limited to these application fields but may be utilized also for a variety of other applications, where said properties are necessary or desirable, e.g. for parts in pumps to feed wearing media and for wear metal components in machines and other pieces of equipment.

Among the powder metallurgically manufactured materials we find the steels which are known under the trade names Vanadis 4 and Vanadis 10. The nominal compositions of these steels are shown in Table 2.

Table 2 - Powder metallurgically manufactured cold work steels - nominal compositions, weight-%, balance Fe and impurities

The above powder metallurgically manufactured steels offer extremely good combinations of wear resistance and toughness but the powder metallurgical process is very timeconsuming and advanced.

In the document WO 03/069004 Al another powder metallurgically manufactured steel material is described, which is in the first place used for the manufacture of tools for encapsulation of electronic components in plastics. The steel material has a good corrosion resistance, including good resistance against pitting at spark machining and very good wear resistance. It may be hardened and tempered to a hardness of 61 to 64 HRC, preferably 62 to 63 HRC, and has very high compression strength in hardened and tempered condition as well as good polishability and good dimensional stability also during long use of the tool made of the steel. This known steel material contains in weight-% 2.2 to 3.3 % (C + N), but min. 0.3 % C and min. 0.06 % N, 0.1 to 2.0 % Si, 0.1 to 2.0 % Mn, 19 to 23 % Cr, max 2.0 % Ni, max 2.0 % Co, 0.5 to 3.0 % (Mo + W/2), but max. 1.0 % W, 4.2 to 7.5 % (V + Nb/2), but max. 0.1 % Nb, max. 0.2 % S, and balance essentially only iron and impurities.

The manufacture of powder for the powder metallurgy is to be distinguished from the manufacture of metal granulates, where, as described in e.g. US 3,888,956 and US 5,017,218, a steel melt is allowed to fall down on a plate and to bounce aside in order to fall down in a water bath, where the granulate of a size of 2 to 25 mm is quenched. Granulates manufactured in this way have up to now been used as cooling metal scrap, i.e. granulate has been added to cool the melt during the steel manufacturing process. Further, the granulate is manufactured with various chemical compositions, which is then used at the manufacturing of investment casting tools, so called metallic moulding tools, i.e. a smaller amount of the granulate is re-melted and cast to a tool.

However, it has proved that there are requirements for a possibility to improve certain of the known steel materials further. Furthermore, it has proved that there are requirements for a more rational, sturdier, simpler, more reliable and/or cheaper method for the manufacture of steel materials than today's known techniques with spray forming and powder metallurgical manufacturing. Above all, there are requirements for being able to mass produce large volumes of steel material which may be used at a mass manufacture of larger components for which the advanced manufacturing methods with spray forming and powder metallurgy are not conceivable alternatives because of the expense.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a method for the manufacture of a new family of steel materials with a more homogenous hard phase distribution alternatively with a more homogenous distribution of the particles improving the machinability than ingot cast and hot worked material, where the manufacturing process is more rational, sturdier, simpler, more reliable and/or cheaper than today's potential technique with spray forming or powder metallurgical manufacturing. This object is achieved by the method according to the present invention comprising the following steps:

- manufacturing of a steel melt;

- granulation of the melt by making a jet of the melt hit a refractory target and be disintegrated into droplets, which are allowed to fall down into a container containing a cooling medium, so that they are quenched to a granulate with a lobate to round shape and a size of 0.5 mm to 30 mm, preferably a size of 1 mm to 10 mm;

- filling of a capsule with the granulate formed by quenching; and

- hot isostatic pressing or hot extrusion of the capsule with the granulate to a body that is at least nearly completely dense.

It is also an object of the invention to provide a wear resistant steel material with a more homogenous hard phase distribution than conventionally cast and hot worked material, which may be manufactured with a more rational, sturdier, simpler, more reliable and/or cheaper manufacturing progress than what is possible with today's potential technique with spray forming or powder metallurgical manufacturing.

This object is achieved by the wear resistant steel material of the present invention having a composition including the following alloy elements in weight-%:

C: 0.1 to 5 %

C+N: 0.1 to8 %

Si: 0.01 to 4 %

Mn: 0.01 to 15 %

Cr: 0.01 to 40 %

Mo: 0.01 to 15 %

V: 0.01 to 20 %, and pos: sibly additionally

Ni: < 40 %

W: < 15 %

Nb: < 15 %

Co: < 20 %

Ti: < 5 %

Zr: < 5 %

Cu: < 5 %

Al: < 1 %

S: < 1 % balance essentially only Fe and possible impurities;

- that it is manufactured by hot isostatic pressing or hot extrusion of granules of a size of 0.5 to 30 mm, preferably 1 to 10 mm, obtained through quenching of a split melt with the above composition; and

- that the steel after the hot isostatic pressing or hot extrusion contains an even distribution of carbides, nitrides and/or carbonitrides of a size up to 50 μm, preferably up to 10 μm, of different shapes but also with a round shape, which carbides are at least one of the types MX- , M ₇ C ₃ -, and M ₆ C-carbides, wherein the majority of these carbides, nitrides and/or carbonitrides has a size which is larger than 0.1 μm, preferably larger than 0.2 μm.

The term "MX-carbides" relates here to carbides, nitrides and carbonitrides, wherein M substantially is vanadium and X is carbon and/or nitrogen. As a rule, a minor portion of the vanadium content may be replaced by twice as much niobium. In the term "MX-carbides" also carbides, nitrides and carbonitrides are included, wherein M essentially is titanium and/or zirconium. The term "M ₇ C ₃ -carbides" here refers to carbides, wherein M essentially is chromium and C is carbon. The term "M ₆ C-carbides" here refers to carbides, wherein M essentially is molybdenum and C is carbon. The term "accessory elements" relates to deliberately added elements which may be added to the steel melt without detrimentally influencing the properties of the steel.

In the description there are a number of designations, the meaning of which is shown below:

HRC = hardness according to Rockwell

R _A - the amount of retained austenite in the matrix after hardening and tempering, vol.-% t _8-5 = cooling rate expressed in seconds for cooling from 800°C to 500°C

T _A = austenitizing temperature, °C h = hour

M ₇ C ₃ (lamella eutectics) = eutectic precipitation of M ₇ C ₃ -carbides in austenite wherein the carbides substantially are lamella shaped

With the method described above for the manufacturing of a steel melt, the process becomes more rational, sturdier, simpler, more reliable and cheaper than today's potential technique with spray forming or powder metallurgical manufacturing. The steel material mentioned above obtains a more homogeneous hard phase distribution than conventionally cast and hot worked materials in case a wear resistant steel is manufactured, and obtains consequently a very good wear resistance which makes the material suitable to use for tools for forming and cutting of various working materials such as sheet steel, aluminium, textile, paper, ceramic coated working !*b'-1Q- 2009

material etc., i.e. conventional cold work applications. Further, as tools for moulding/cutting of plastic granulates or in mould inserts, screws, nozzles, tubes in plastic manufacture according for instance to the principle extrusion, injection moulding, pressure moulding. High wear resistance is also required for engineering parts such as for instance pump parts, valve parts, hammers, counter parts, fragmentation knives for tyres, paper, wood, metal, etc., wear parts or knives within the packing industry, food industry, pulp industry, mining and mineral industry or other processing industry, as well as parts exposed to wear in transmissions and engines in the vehicle industry. In addition, it is an advantage if the wear resistant steel material according to the invention also fulfils one or several of the conditions below: • Adequate cleanness

• High fraction, 3 to 40 vol.-% of larger, about 0.1 to 100 μm, round to irregular hard phase particles formed during the solidification during the granulation. The size being little influenced at the subsequent hot isostatic pressing process.

• Good heat treatment properties: austenitizing temperature T _A = 950 to 1150°C. • Good hardenability.

• Dimensional stability at heat treatment.

• Dimensional stability at use, during operation - low aging tendency, low content of retained austenite R _A after high temperature tempering.

• Secondary hardening with a hardness of 50 to 66 HRC. • Good surface coating properties with PVD/CVD/nitration.

• Good spark machining properties.

• Abrasive wear resistance better than or equal to powder metallurgically manufactured steel materials.

• Toughness preferably equal to or better than AISI d2. • High compression strength.

• Comparatively good fatigue properties.

• Acceptable machinability.

• Acceptable grindability.

• Good corrosion resistance for chosen alloys. \ _r ^; #

The quenching and granulation of a melt with the above composition gives a high fraction, 3 to 40 vol.-%, of larger, up to 50 μm, preferably up to 10 μm, round to irregular hard phase particles, wherein the majority of these carbides, nitrides and/or carbonitrides have a size larger than 0.1 μm, preferably larger than 0.2 μm, which are formed at the solidification reaction after the granulation. The alloy compositions indicated are balanced so that you obtain an even distribution of the above mentioned hard phase particles, usually consisting of MX, M ₇ C ₃ and/or M ₆ C in a matrix. At the granulation, which most easily takes place in water, surface oxides may form on the granules. Any surface oxides are suitably reduced before the hot isostatic pressing (HIP) in order to give the steel material an adequate cleanness; so that the hot isostatic compacted steel material is void of or essentially void of oxide inclusions. This may be done by pickling the granulate or by reducing the granulate in a reducing atmosphere, e.g. hydrogen gas. Of course, also other methods may be used, which are known to the man skilled in the art.

A preferred temperature at the performance of the hot isostatic pressing is 1000 to 1350°C, preferably 1150°C. Due to the shape of the granules, hot working of the hot isostatic pressed capsule may be required in order to obtain a completely dense body. The particle size is influenced only to a minor extent at the subsequent HIP process and gives a good wear resistance. If it is desirable to increase the particle size, it can be achieved, at least to some extent, by extending the retention time up to 10 or 20 h and/or increase the HIP temperature somewhat for some alloys.

It is conceivable that the steel material may be used in a hot isostatic pressed condition but often the completely dense body is hot worked by rolling or forging to blanks (e.g. bars, rods) with dimensions up to 01000 mm or somewhat there above. These may then be soft annealed to a hardness of about 150 to 350 HB. Subsequently, the completely dense body is machined to one or several steel products, which are hardened and tempered in order to obtain a preferably martensitic microstructure, which gives a property profile suitable for applications exposed to wear also in a corrosive environment, hi addition, an adequate ductility/toughness is obtained.

An alternative to the HIP is a compaction of the granules by hot extrusion. The capsule filled with granules and emptied of gas is hot extruded in one or several steps, wherein the granules are pressed together to an at least almost completely dense body. A preferred temperature at the performance of the hot extrusion is 1000 to 1350°C, preferably 1150°C. It is also concievable to directly after the hot extrusion proceed to a hot work step in the form of forging or hot rolling to obtain a blank of the desired size. As to the sulphide sizes described in the present application, these refer to sizes after compaction to an at least almost completely dense body, i.e. before possible additional heat treatment.

At the hardening, the material is austenitized at a temperature between 950 and 1150°C, and subsequently the material is quenched, preferably to room temperature or close thereto. The cooling medium is adapted to dimension and alloy content. At the quenching, the austenite is converted to martensite but in some case a certain content of retained austenite may be present at room temperature, and therefore a deep freezing down to -40°C or in liquid nitrogen at about - 196°C may be required. The tempering may also be performed one or several times at a temperature of 150 to 650°C. Typical for the steel material of the invention is therefore that it after hardening and tempering has a microstructure which at room temperature consists of a matrix substantially consisting of martensite, and in this matrix an even distribution of irregular but also round hard phase particles of the type mentioned above, and besides that secondarily precipitated hard phase particles with sub-microscopic size, may occur.

Further, it is also an object of the invention to provide a machinability improved steel material with an improved ductility, which has a more even distribution of machinability improving particles, which may be manufactured in a more rational, sturdier, simpler, more reliable and/or cheaper manufacturing progress than what is possible with today's potential technique with spray forming or powder metallurgical manufactured. This object is achieved by the machinability improved steel material of the present invention having the following composition in weight-%:

C: 0.1 to 2.0% C+N: 0.1 to 2.2%

Si: 0.01 to 2 %

Mn: 0.01 to 15 %

Cr: 0.01 to 18 %

Mo: 0.01 to 5 % V: 0.01 to 2 %

S: 0.01 to 1 %, and possibly additionally one or more of the following accessory alloy elements:

Ni: < 10 %

W: < 4 % Nb: < 1 %

Co: < 5 %

Ti: < 0,5 %

Zr: < 0,5 %

Cu: < 2 % Al: < 1 %

Ca: 5-75 ppm

O: 50-100 ppm balance essentially only Fe and possible impurities;

- that it is manufactured by hot isostatic pressing or hot extrusion of granules of a size of 0.5 to 30 mm, obtained through quenching of a split melt with the above composition; and - that the steel after the hot isostatic pressing or hot extrusion has a microstructure containing an even distribution of manganese sulphides, MnS, and/or calcium manganese sulphides with a size of 0.1 to 30 μm and an irregular but also essentially rounded shape, wherein the majority of these sulphides preferably has a size of 0.1 to 10 μm, even more preferred 0.1 to 3 μm.

By producing granules according to the method described above, a steel material may be obtained, which instead of a very good abrasive wear resistance show a very good machinability and essentially improved ductility in transverse direction as compared to conventially manufactured, machinability improved steel materials which are known on the market. These improvements are achieved by the steel material obtaining a more homogenous distribution of machinability improving sulphide particles than steel materials conventionally manufactured. More exactly, the steel has a microstructure which after compaction contains an even distribution of essentially sperodized manganese sulphides, MnS and/or calcium manganese oxysulphides with a size of 0.1 to 50 μm. The majority of these sulphides preferably has a size of max. 10 μm, even more preferred 0.1 to 3 μm.

Among today's known steel materials, which are conventionally manufactured with an admixture of up to 0.15% sulphur, as an example of a hot work steel is Hl 3, and AISI 420 is an example of a corrosion resistant plastic forming steel. The machinability improved steel material of the invention normally has a hardness in hardened and tempered condition of max. 55 HRC, preferably max. 45 HRC, and even more preferred the hardness ranges between 30 and 40 HRC in order to be machined with cutting tools to a final shape with the tool makers.

Additional features characterizing the invention and what is achieved with it will be apparent from the following detailed description.

BRIEF DESCRIPTION OF THE ENCLOSED DRAWINGS

Below, the invention will be described more in detail with reference to preferred embodiments and to the enclosed drawings.

Fig. 1 shows the microstructure of an ingot cast steel material in hot worked, hardened and tempered condition;

Fig. 2 shows the microstructure of a powder metallurgical steel material in hot worked, hardened and tempered condition; Fig. 3 shows the microstructure of a spray formed metallurgical steel material in hot worked, hardened and tempered condition;

Fig. 4 shows the microstructure of a steel material manufactured according to the method of the invention in hot worked, hardened and tempered condition;

Figs. 5 and 6 shows the microstructure of still a steel material manufactured according to the method of the invention in hot worked, hardened and tempered condition;

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

According to the present invention a steel melt is first manufactured having the following composition in weight-%:

and possibly additionally one or more of the following accessory alloy elements:

balance essentially only Fe and possible impurities.

At the granulation of the steel melt, a preferably vertical jet of the melt is allowed to fall down on to a refractory target located above a water surface in a container in a granulation device, so that the jet is disintegrated to droplets by the impact force, which droplets are spread radially from the refractory target and fall down into the water, where they are subjected to quenching so that the granules formed have solidified at least at the surface when they have reached the bottom of the container, at the latest. The fall height of the melt is adapted so that the granules formed obtain a typical size of 0.5 to 30 mm, preferably 1 to 10 mm and a lobate to round shape. After the discharge from the granulation device, the granules are dried, which to some extent may take place by means of possibly remaining intrinsic heat in the granules.

The quenching of a melt with the above composition gives a high fraction, 3 to 40 vol.-%, of, round to irregular hard phase particles, with a size of up to 50 μm, preferably up to 10 μm, wherein the majority of these carbides, nitrides and/or carbonitrides have a size above 0.1 μm, preferably above 0.2 μm, which are formed during the solidification during the granulation process. By adding a high content of vanadium and a corresponding stoichiometric content of carbon to an alloy system where Fe-C and at least one carbide forming element is included in the alloy system, C-Cr-Mo- W-V-Ti-Nb-Zr-Co-Ni-Fe, a precipitation of carbides in the melt and in the retained melt regions is obtained thanks to the quenching, or a primary precipitation of carbides, nitrides and carbonitrides, which are more difficult to dissolve, such as e.g.V(C,N) and Nb(C ₅ N), Ti(C ₅ N) and Zr(C ₅ N), may take place directly in the melt. This precipitation sequence may also be found in granulated steel fragments with a size of 1 to 30 mm, which is shown in Fig. 4 as a typical case for the first described case and in Fig. 5 for the latter described case.

After possible reduction of the surface oxides of the dry, quenched granulate, the granulate is filled into capsules, which are then hot isostatic pressed, alternative hot extruded, to an at least close to completely dense body. The hot isostatic pressing is suitably performed at a temperature of 1000 to 1350 ⁰ C ₅ preferably 1150 ⁰ C.

The reduction of surface oxides gives the steel material manufactured an adequate cleanness, so that the hot isostatically pressed steel material has a low content of or is more or less void of oxide inclusions. In this way, the occurrence of surface oxides in the granule boundaries may be avoided while possible oxide inclusions inside the very granules are not influenced by such a reduction process. Possible occurrence of such oxides may be avoided or minimized by another process, which is known to the man skilled in the art, e.g. by melting and refining the steel melt in vacuum and by using clean metal raw materials as insert material. In certain applications with high requirements for abrasive wear resistance, it is, however, not necessary to reduce the oxides as these instead may contribute to the wear resistance of the steel material. By hot working of the hot isostatic pressed body any surface oxides may be broken, and therefore their possible negative effect on the ductility of the material may be reduced.

The hot isostatically pressed capsule is then hot worked, first to obtain a completely dense body, if necessary, and then to be hot worked further to blanks of a size up to 01000 mm or somewhat larger, or to other products. Then the product is soft annealed to a hardness between about 150 and 350 HB, and thereafter it is machined to the desired shape by cutting operations.

The heat treatment is performed in the following way. The steel material is hardened by austenitizing at 950 to 1150°C, wherein the material, after through heating to the desired temperature, is retained at the temperature in question during about 30 min to 2 h, and subsequently it is quenched to room temperature. During the austenitizing, a structure conversion of the matrix to austenite is obtained and a certain amount of the carbides may be entirely or partly dissolved while the alloy elements therein diffuse into the austenite. At the quenching, the austenite is converted to martensite. A certain amount of retained austenite may occur after the high temperature tempering in some of the preferred embodiments, and in those cases it may be suitable to deep freeze the material. The tempering of the steel material may be performed one or several times at a temperature of 150 to 650°C, typically during a retention time of 2 h at the time, hi the upper region of the temperature range, typically from 500 to 650°C, a characteristic secondary hardening of the material is obtained as a secondary precipitation of hard phase particles with sub-microscopic size. Typical for the steel material of the invention is thus that it after hardening and tempering has a microstructure which at room temperature consists of a matrix substantially consisting of martensite, and in this matrix in liquid phase precipitated hard substances of MX type, which typically has a rounded to round shape, in addition to which secondarily precipitated hard particles of sub-microscopic size, may occur, substantially in the form of MX-, M ₇ C ₃ - and M ₆ C -carbides, nitrides and/or carbonitrides.

Preferably, the completely dense body is worked to one or several steel products, which is/are hardened and tempered to obtain a microstructure consisting predominantly of martensite and having an adequate ductility/toughness.

With the above method for the manufacture of the steel material including the presented features, the manufacturing progress will be more rational, sturdier, simpler, more reliable and cheaper than today's potential technique with spray forming or power metallurgical manufacturing, and a new family of steel material is obtained.

The effect of the alloy elements on the properties of the steel

Carbon, and where appropriate also nitrogen, are included in the matrix of the steel and contribute to giving the steel the necessary hardness. Together with vanadium or other carbide forming elements such as titanium, niobium and zirconium, carbon and nitrogen are included in primarily and secondarily precipitated MX-carbides, which in a very efficient manner contribute to giving the steel a very good abrasive wear resistance. Carbon also combines with chromium and molybdenum and forms M ₇ C ₃ - och M ₆ C-carbides, which also contribute to the wear resistance.

Silicon may be used for desoxidation of the steel. Silicon also contributes to the tempering resistance of the steel and thereby contributes to higher strength. Manganese is added in connection with the desoxidation of the steel so that the slags formed will become easier to separate from the melt. Manganese is also added to contribute to the hardenability of the steel and may in some cases alone stand for this effect. Manganese may also contribute to giving the steel good machinability by the formation of manganese sulphides together with sulphur. By also adding calcium and oxygen, calcium manganese oxysulphides are formed, which besides contributing to good machinability may result in the steel obtaining a better ductility than if pure manganese sulphides are formed.

Chromium contributes to giving the steel good hardenability and contributes to the wear resistance and hardness of the steel through precipitation of M ₇ C ₃ -carbides. hi high contents, chromium also contributes to giving the steel increased corrosion resistance.

Also molybdenum contributes to the hardenability of the steel. Molybdenum may at least partly be replaced by twice as much tungsten amount. Preferably, the steel does not, however, contain tungsten above impurity amount.

In addition to the above elements, the steel may also contain other elements which in various ways contribute to giving the desired properties.

Nickel is an element which increases the hardenability and the toughness in contents above 1%. As nickel is austenite stabilizing, it is in many cases desirable to limit the nickel content in the material. Nickel also has a solution hardening effect.

Cobalt is added in the first place to increase the hot hardness of the material but also gives some solution hardening effect.

Copper is an element which gives some solution hardening effect. In contents above 0.6% a precipitation hardening effect may be obtained by tempering.

Below, a more detailed description of the different embodiments of the invention follows.

According to a preferred embodiment of the invention, the steel melt has a composition which comprises the following alloy elements, in weight-%:

In addition to the elements mentioned above, the steel melt may also contain one or several of the accessory elements indicated in Table 3 in the description above, balance essentially only Fe and possible impurities.

In this embodiment, carbon and possibly also nitrogen shall be present in adequate amounts so that the steel, after hot isostatic pressing or hot extrusion and hot working, in the hardened and tempered condition of the steel, on the one hand will be able to form, partly together with vanadium, up to 15 vol.-% of MX-carbides, wherein M substantially is vanadium, and on the other hand will be able to form, partly together with chromium, up to 30 vol.-% OfM ₇ C ₃ - carbides, wherein M substantially is chromium. The total content of MX-carbides and M ₇ C ₃ - carbides shall amount to 3-40 vol.-%. Carbon and any nitrogen shall further be included in solid solution in the martensitic matrix of the steel in its hardened condition in order in this manner to contribute to the hardness of the steel.

The steel material according to this first preferred embodiment is in the first place intended to be used in plastic moulding equipment, and there be used as engineering parts, e.g. screws and barrels to feed and guide plastic masses in machines for the manufacture of plastic components, e.g. elements in aggregates for injection moulding and extrusion, and also in moulding tools and tool parts for injection moulding of plastics. Further, the steel material is suitable for the manufacture of tools for encapsulation of electronic components in plastics. Further, the steel material has a good corrosion resistance, including a good resistance against pitting at spark machining and a very good wear resistance. It may be hardened and tempered to a hardness of 58 to 64 HRC, preferably 59 to 62 HRC. However, the steel material is not limited to these application fields but may be utilized also for a variety of other applications, where said properties are necessary or desirable, e.g. for parts in pumps to feed wearing media, wear metal parts in machines and other pieces of equipment and for knives being used in more or less corrosive environment, e.g. in the food industry and plastic industry.

Preferably, the steel melt according to this first preferred embodiment has the following composition in weight-%:

balance essentially only Fe and possible impurities. In a first conceivable variant, the steel melt according to the first preferred embodiment has the following composition in weight-%:

balance essentially only Fe and possible impurities.

In a second conceivable variant, the steel melt according to the first preferred embodiment has the following composition in weight-%:

balance essentially only Fe and possible impurities.

According to a second preferred embodiment of the invention, the steel melt has the following com osition in weight-%:

balance essentially only Fe and possible impurities.

After the hot isostatic pressing or hot extrusion and hot working to blanks, the wear resistant steel material has a microstructure containing up to 30 vol.-% of evenly distributed MX-, M ₇ C ₃ - and/or M ₆ C-carbides having a size of 1 to 50 μm, preferably 1 to 10 μm and having an irregular but also round/rounded shape.

Within the concept of this second preferred embodiment of the invention it is conceivable that a third variant of the steel melt has the following composition in weight-%:

balance essentially only Fe and possible impurities.

After the hot isostatic pressing or hot extrusion and hot working to blanks, the wear resistant steel material has a microstructure containing 5 to 20 vol.-% of evenly distributed MX-, M ₇ C ₃ - and/or M ₆ C-carbides having a size of 1 to 50 μm, preferably 1 to 10 μm and having an irregular but also round/rounded shape. _t

balance essentially only Fe and possible impurities.

After the hot isostatic pressing or hot extrusion and hot working to blanks, the wear resistant steel material has a microstructure containing 10 to 15 vol.-% of evenly distributed M ₇ C ₃ - carbides, with a certain amount of MX-carbides, having a size of 1 to 50 μm, preferably 1 to 10 μm and an irregular but also round/rounded shape.

Further, within the concept of this second preferred embodiment of the invention it is conceivable that a fourth variant of the steel melt has the following composition in weight-%:

balance essentially only Fe and possible impurities.

After the hot isostatic pressing or hot extrusion and hot working to blanks, the wear resistant steel material has a microstructure containing 10 to 30 vol.-% of evenly distributed MX-, M ₇ C ₃ - and/or M ₆ C -carbides having a size of 1 to 50 μm, preferably 1 to 10 μm with an irregular but also round/rounded shape.

The application fields for the steel material according to this second embodiment of the invention may be anyone among wear metal products, e.g. in the mining industry and other process industries, to tools in the conventional cold work field for the manufacture of tools for cutting and punching, cold extrusion, powder compacting, deep drawing, etc., i.e. components which are exposed to heavy abrasive wear.

Vanadium, carbon and nitrogen shall be present in necessary amounts so that the material after the hot isostatic pressing or hot extrusion and hot working in hardened and tempered condition will contain 10 to 30 vol.-% of MX- and M ₇ C ₃ -carbides in the matrix, of which up to 8 vol.-% consist of M ₇ C ₃ -carbides. Suitably, the steel contains 1.5 to 4% carbon and 0.1 to 3% nitrogen, som that the total content of C+N amounts to 1.6 to 7%. The vanadium content has to be min. 3% and max. 12%, wherein vanadium at least partly may be replaced by twice as much niobium amount up to max. 1.0% Nb.

Preferably this fourth variant has the following composition in weight-%:

balance essentially only Fe and possible impurities.

After the hot isostatic pressing or hot extrusion and hot working to blanks, the wear resistant steel material has a microstructure containing 15 to 25 vol.-% of evenly distributed MX-, M ₇ C ₃ - and/or M ₆ C-carbides having a size of 1 to 50 μm, preferably 1 to 10 μm, and an irregular but also round/rounded shape.

Even more preferred, this fourth variant has the following composition in weight-%:

balance essentially only Fe and possible impurities.

After the hot isostatic pressing or hot extrusion and hot working to blanks, the wear resistant steel material has a microstructure containing 16 to 24 vol.-% of evenly distributed MX and M ₇ C ₃ -carbides with a size of 1 to 50 μm, preferably 1 to 10 μm and an irregular but also round shape, of which up to 8% are evenly distributed M ₇ C ₃ -carbides with a size of 1 to 50 μm, preferably 1 to 10 μm, and an irregular but also round/rounded shape.

According to a third embodiment of the invention the steel melt has a composition which instead is balanced in order to obtain a machinability improved material with good ductility, toughness and/or hot hardness. In this third embodiment the steel melt has the following composition in weight-%:

and possibly also 3 to 75 ppm calcium and 10 to 100 ppm oxygen, balance essentially only Fe and possible impurities. After the hot isostatic pressing or hot extrusion, the steel material has a microstructure containing an even distribution of manganese sulphides, MnS, having a size of 0.1 to 30 μm with an irregular but also essentially rounded shape, wherein the majority of these sulphides preferably has a size of 0.1 to 10 μm, even more preferred 0.1 to 3 μm. By adding 3 to 75 ppm calcium and 10 to 100 ppm oxygen, the manganese sulphides may be modified as previously mentioned so that the manganese sulphides entirely or partly are replaced by calcium manganese oxysulphides. There may be a certain amount of carbides also in the steel material according to this third embodiment, hi analogy with the inventive idea, these carbides have a distribution, shape and size as described above.

Within the concept of this third preferred embodiment of the invention it is conceivable that a fifth variant of the steel melt has the following composition in weight-%:

and preferably also 3 to 75 ppm calcium and 10 to 100 ppm oxygen, balance essentially only Fe and possible impurities.

In this way, the steel obtains in the first place a very good machinability in combination with good toughness and ductility. In tough hardened condition of the steel the microstructure has a martensitic matrix with up to 30 vol.-% of ferrite and the steel material has a hardness between 290 and 360 HB. The good machinability is achieved by alloying with 0.01 to 1% sulphur, preferably 0.03 to 0.5%, and even more preferred 0.1-0.2%, wherein manganese combines with sulphur and forms the manganese sulphides mentioned above. Preferably, also calcium and oxygen are added, which cause a globulizing of the manganese sulphides and instead the formation of calcium manganese oxysulphides. If also calcium and oxygen are added, the sulphur content may be reduced to 0.035 to 0.25 S in combination with 3 to 100 ppm Ca, preferably 5 to 75 ppm Ca, suitably max. 40 ppm Ca, and 10 to 100 ppm O. These calcium manganese oxysulphides are not as elongated as the manganese sulphides and consequently an improvement of the ductility may be obtained. Manganese may also entirely or partly replace chromium as an element increasing the hardenability.

It shall be possible to deliver the steel material according to this third embodiment in a tough hardened condition to enable the manufacture of holders for plastic moulding tools and the very plastic moulding tool in very heavy sizes. Despite the fact that the content of the hardenability increasing elements, nickel and molybdenum, has been reduced, the steel has a hardenability that enables hardening in air, also in very heavy sizes. Air hardening is advantageous, as retained stresses may be avoid or at least reduced. Retained stresses in the material may result in deformations in connection with cutting machining to finished shape. The hardening is performed by austenitizing at a temperature of 900 to HOO ⁰ C, preferably at 950 to 1025°C, or about 1000°C, followed by quenching in oil, polymer bath, in gas in a vacuum furnace or most preferred in air. The tempering is performed as high temperature tempering at 510 to 650°C, preferably at 540 to 620 ⁰ C, during at least one hour, preferably through double tempering during two hours at each time.

Besides the elements mentioned above, the steel according to this third embodiment may also contain up to 2% copper, preferably 0.40%, and even more preferred max. 0.25% copper to increase the corrosion resistance and/or the hardness. However, already in small contents, copper impairs the hot ductility. Moreover, copper cannot be extracted from the steel, and therefore the scrap metal handling and the recovery of the material is more difficult.

Strong carbide formers such as niobium, titanium, and zirconium shall normally not be present in the material according to this third embodiment, as possible carbides would detrimentally influence the toughness and the ductility.

In a sixth variant according to the third embodiment of the invention the steel melt has the following composition in weight-%:

balance iron and unavoidable impurities.

Also this steel material shows a very good machinability in combination with good toughness and ductility. The good machinability is achieved by alloying with 0.01 to 1% sulphur, preferably 0.03 to 0.5%, and even more preferred 0.1-0.2%, wherein manganese combines with sulphur and forms manganese sulphides as mentioned above. Preferably, also calcium and oxygen are added, which cause a globulizing of the manganese sulphides and formation of calcium manganese oxysulphides. If calcium and oxygen are also added, the sulphur content may be reduced to 0.035 to 0.25% S in combination with 3 to 100 ppm Ca, preferably 5 to 75 ppm Ca, suitably max. 40 ppm Ca, and 10 to 100 ppm O. These calcium manganese oxysulphides are not as elongated as the manganese sulphides and consequently an improvement of the ductility may be obtained. In addition, this steel material has a high hot wear resistance and a good combination of other properties, such as tempering resistance, thermal conductivity, hardenability and strength.

The steel material is intended to be used for machining of working material in hot condition. Typical examples of the use of the steel are tools for extrusion and die casting of light metals, especially aluminium. Another type of use is forging tools. Vanadium shall also be present in the steel in an amount of min. 0.4% and max. 1.3%. Vanadium contributes to giving the steel a good tempering resistance, a good wear resistance and contributes to a good strength by forming vanadium carbides which contribute to the formation of a comparatively fine crystalline structure.

DISCLOSURE OF PERFORMED EXPERIMENTS The performed experiments will now be described with reference to the figures of the enclosed drawings

In Table 4 below the compositions are shown for a number of test alloys. Steels Nos. 1 to 3 and 7 constitute comparison material while steels Nos. 4, 5 and 6 are examples of steel materials manufactured according to the method of the invention. Steels Nos. 1 and 7 are commercial steel materials manufactured in a conventional manner through ingot casting. Steel No. 2 is a commercial steel material manufactured through powder metallurgical manufacturing. Steel No. 3 is a commercial steel material manufactured through spray forming. Steels Nos. 4, 5 and 6 are commercial compositions, with which experiments have been performed with a manufacturing process according to the method of the invention. Steel No. 5 has substantially the same composition as steel No. 3 and steel No 6 has substantially the same composition as steel No. 7 . The reference materials, steels Nos. 1, 2, 6 and 7 have not been analyzed concerning all elements present, but it shall be realized that the steels besides the elements indicated contain unavoidable impurities in normal contents, which emanate from the manufacturing of the steel.

Table 4 - Chemical com osition of the test alloys

balance essentially only iron.

From two steel melts of 4 ton each having a composition according to steel No. 4 and 5 in Table 4 above, granules were manufactured according to the method initially described. Capsules were filled with 25 kg each of the granulate, and thereafter the capsules were sealed, evacuated from gas/air and compacted by hot isostatic pressing. Thereafter, the material was hot worked by forging at 1150°C to blanks with a size in the transversal direction of 70 x 50 mm. The forged blanks were hardened through austenitizing at 1025°C during 30 min in a vacuum furnace, and subsequently they were cooled at a cooling rate tg _-5 of about 100 s. Thereafter, the blanks were tempered through heating twice to 525 ⁰ C, retention time 2 h, with intermediate cooling to room temperature.

Micros tructu re and hardness

The microstructure and hardness of the blanks obtained were analyzed and compared with the reference materials in hardened and tempered condition. Heat treatment data and measured hardness are shown in Table 5. Table 5 - Heat treatment data and hardness in hardened and tempered condition

The microstructure of steel No. 1 is shown in the light optical photo in Fig. 1. The steel shows the inhomogeneous microstructure which is typical for ingot cast materials with comparatively coarse and in the forging direction elongated carbides which form elongated streaks in the material. The majority of the carbides are between 10 and 20 μm, but the steel contains also carbides of a length of up to 50 μm. The steel contains about 13% M ₇ C ₃ -carbides in a matrix which consists of martensite.

The microstructure of steel No. 2 is shown in Fig. 2. In a matrix of martensite there is an even distribution of comparatively round MX- and M ₇ C ₃ -carbides with a size of 1 to 3 μm in its longest extension. The carbide content amounts to about 15 vol.-% of MX-carbides and 7 vol.-% of M ₇ C ₃ -carbides. As may be seen from the figure, the MX-carbides are somewhat smaller than the M ₇ C ₃ -carbides, the number of which is fewer but on the contrary has a somewhat larger size.

Fig. 3 shows the microstructure of steel No. 3, which is a spray formed steel material. In a matrix of martensite there is an even distribution of comparatively round MX- and M ₇ C ₃ -carbides with a size of 1 to 7 μm. A comparatively great portion of MX-carbides has sizes about 5 μm while the majority of the M ₇ C ₃ -carbides have sizes below 5 μm. The abundant occurrence of larger MX-carbides is a reason for the steel having very good abrasive wear resistance which is shown below.

Fig. 4 shows the microstructure of steel No. 4, which is manufactured according to the method of the invention in hot worked, hardened and tempered condition, hi a matrix of martensite there is an even distribution of comparatively round, very fine chromium rich M ₇ C ₃ -carbides having a size in the longest extension of about 1 to 2 μm, which carbides have been precipitated in the retained melt areas in the matrix during the solidification of the melt droplets during the granulation. The microstructure resembles to a great extent the microstructure obtained in powder metallurgically manufactured materials, which is surprising with regard to the comparatively very simple manufacturing method. The carbide content amounts to about 13 vol.- % and through hot isostatic pressing, hot working and subsequent hardening and tempering the majority of the carbides are found as discrete particles or as small carbide aggregates. Continuous large networks of carbides are missing. A certain occurrence of very small, rounded surface oxides may also be found.

Figs. 5 and 6 show the microstructure of steel No. 5, which is manufactured according to the method of the invention in heat treated, hardened and tempered condition. In Fig. 5 there is an even distribution of somewhat irregular but comparatively rounded MX- and M ₇ C ₃ -carbides having a size of 1 to 10 μm in a matrix of martensite. The total carbide content of steel No. 5 amounts to 21 vol.-%, of which 16 vol.-% are MX-carbides and 5 vol.-% are M ₇ C ₃ -carbides. As compared to steel No. 3, which has substantially the same composition as steel No. 5 but which has been manufactured with a spray forming technique, steel No. 5 shows a more finely dispersed distribution of the carbides, more as the carbide structure found in powder metallurgically manufactured materials, e.g. steel No. 2. There is a certain occurrence of large, irregular MX-carbides with a size of up to 10 μm in the matrix. These large carbides may in a very positive manner contribute to giving the steel a very good abrasive wear resistance. In the figure there is also a certain occurence of broken surface oxides.

Fig. 6 shows an additional example of the microstructure of steel No. 5. As may be seen, the steel has an analogous carbide distribution with even somewhat more rounded carbides than shown in Fig. 5.

Abrasive wear resistance

The wear resistance of the steel of the invention was compared with that of the reference materials through pin-to-disc-tests with Al ₂ O ₃ as grinding medium. The result is shown in Table 6 below, wherein the abrasive wear resistance of the steels is shown as the amount of worn off material per time unit (mg/min). Steel No. 3 has the best wear resistance while steel No. 5, which had been manufactured according to the method of the invention, has a comparatively very good wear resistance, considerably much better than steel No. 2, which is a powder metallurgically manufactured material. Steel No. 4, which had been manufactured according to the method of the invention, has a wear resistance on a level with the conventionally manufactured steel No. 1.

The hardness also influences on the result in such a way that harder materials generally have better wear resistance. This is a contributing reason for the somewhat lower result for steel No. 4. The type of carbides and their size in the material influence the result. Through a higher content of vanadium in the steel material a larger portion of hard MX-carbides is obtained at the expense of a smaller portion of less hard M ₇ C ₃ - or M ₆ C-carbides, which gives an increased wear resistance. Steel No. 1 and steel No. 4 with about 13 vol.-% of M ₇ C ₃ -carbides have a comparatively low wear resistance as compared with e.g. steel No. 5 containing about 21 vol.-% of carbides distributed on 16 vol.-% of MX-carbides and 5 vol.-% of M ₇ C ₃ -carbides. As to steel No. 2, it is in the first place the smaller size of the carbides which results in the fact that the abrasive wear resistance is not as good as with steels Nos. 3 and 5.

Ductility

Investigations of the ductility were performed by Charpy impact testing of unnotched test specimens of steels Nos. 6 and 7. Steel No. 6 was manufactured by hot isostatic pressing of granules in accordance with the invention and steel No. 7 was conventionally cast. Before HIP the granules were processed in a reduction step in order to reduce any surface oxides. Microscopic studies of the materials however showed that the microstructure contained some small broken oxides in the regions between the prior granules. By further research it will be possible to optimise the reduction step for more complete reduction of the oxides. The steels were then hot worked by forging using various area reduction rates. The forging break the remaining surface oxides into smaller particles and as is known to skilled person in this field a higher area reduction rate will result in smaller oxide particles and also in a reduction of oxide films in the regions between the prior granules. Steel No. 6 were then hardened by austenitizing at 1025 °C/30min and tempering at 525 °C /2x2h and steel No. 7 were austenitized at 1030 °C/30min and tempering at 540 °C /2x2h. The result is shown in table 7. When the result is analysed it shall be considered that Steel No. 7 has a higher reduction rate than the two examples of steel No. 6 and it shall also be considered that both examples of Steel No. 6 has higher hardness (approx. 1,2-1,4 HRC units). Having this in mind it can be seen that Steel No. 6 according to the invention shows a ductility that is significantly improved in the transverse and longitudinal directions. In the short transverse direction the result is in the same range as Steel 7 but here, the lower area reduction rate of steel no 6 (45%) has a significant influence on the result. At a higher reduction rate, the ductility in short transverse direction is likely to exceed that of Steel No. 7

INDUSTRIAL APPLICATIONS The method of the invention is intended to be used where powder metallurgical manufacturing or spray forming cannot be performed because of the expense. With the method a manufacturing process is obtained which also is more rational, simpler and more reliable than today's potential technique with spray forming. The steel material of the invention is intended to be used in applications with high requirements for wear resistance with components for forming and cutting of different working materials such as sheet steel, aluminium, textile, paper, ceramic coated work material, etc., e.g. conventional cold work applications. Additional examples are at the moulding or cutting of plastic granulates or in mould inserts, screws, nozzles, tubes at plastic manufacture according for instance to the principle extrusion, injection moulding, pressure moulding. High wear resistance is also required for engineering parts such as for instance pump parts, valve parts, hammers, counter parts, fragmentation knives for tyres, paper, wood, metal, etc., wear parts or knives within the packing industry, food industry, pulp industry, mining and mineral industry or other processing industry, as well as parts exposed to wear in transmissions and engines in the vehicle industry.

DISCUSSION

Through the method of the invention, where the manufacturing progress is more rational, sturdier, simpler, more reliable and cheaper than today's potential technique with spray forming or powder metallurgic manufacturing, the steel material may be manufactured with a more homogeneous hard phase distribution than in conventionally cast and hot worked material. With the method of the invention it is possible to manufacture steel materials, also high alloy steels, which after hot isostatic pressing and hot working has a microstructure having an even distribution of irregular but also round or rounded MX-, M ₇ C ₃ - och M ₆ C-carbides with a size of about 1 to 50 μm, preferably 1 to 10 μm. As, thanks to the quenching, a considerably small dendrite size is obtained for steels being granulated a carbide size is obtained which neither is too fine to give good wear resistance, as is the case in powder metallurgically manufactured materials, nor is segregated as in conventionally ingot cast or with electro slag refined materials. With this new manufacturing method an even distribution of adequately large carbides OfM ₇ C ₃ - or M _ό C-type is obtained also for materials without any, at least to some extent, primarily precipitated carbides containing V or Nb. Besides an equal or better abrasive wear resistance, the products from this new manufacturing method have a conceivably lower ductility than powder metallurgically manufactured materials. Otherwise, the response to heat treatment is equal, which also the other properties are. Also when compared with powder metallurgically manufactured materials an improved abrasive wear resistance is noticed while the ductility is better with PM-steels.

Thanks to the method of the invention, steel materials, which today are conventionally cast and which are not sophisticated enough to be manufactured through the expensive spray forming or powder metallurgical manufacturing methods, are given considerably improved properties at reasonable costs. Further, the typical lamellar solidification structure OfM ₇ C ₃ which arises in conventionally or with electro slag refined manufactured alloys of AISI D2-type may be avoided, which at the hot working gives streaks which imply anisotropic properties in the longitudinal and the transversal directions. For conventionally ingot cast steels, the method of the invention thus offers a possibility to manufacture a steel material with improved and considerably more homogeneous properties in the longitudical and transversal directions.

ALTERNATIVE EMBODIMENTS

It is conceivable to manufacture steel materials, wherein granules of different compositions are mixed in order to combine the various properties of the materials in such a way. It may suitably be performed in such a way that granules of a certain fraction size are mixed with granules of another, smaller fraction size. By suitable methods or means in connection with the positioning of the granules in the container for the hot isostatic pressing, it is possible to make sure that the smaller granules are distributed in the gaps between the larger granules. By choosing suitable fraction sizes it will in this manner also be possible to evenly distribute a certain amount of the smaller granules among the larger ones, in a determined manner. In an analogous manner, it is also conceivable to mix granules and powder metallurgically manufactured metal powder in the same container for hot isostatic pressing. The man skilled in the art will realize that the technique also may be utilized to manufacture the hot isostatic pressed blanks, wherein certain areas have been given an admixture of granules/powder of another material while the remaining areas only consist of the first material. Further, it is realized that more than two materials may be mixed in this way. It is realized that other cooling media than water may be used at the granulation process. For example, it is conceivable to use a hydrocarbon such as photogene or oil. Further, it is conceivable to use liquid nitrogen, which through adaption of the pressure in the container would be allowed to form a strongly cooling gas above the cooling bath. In addition, it is conceivable to spray a mist of cooling medium cooling the granules, and in such a case no cooling bath is used. An additional conceivable way is to provide the cooling container with a fluidized bed at the bottom with an intensified supply of cooling gas. It is realized that the use of another cooling medium than water and oxygen-containing gas such as air implies that the granules do not oxidize. This would imply that the oxide reduction step which is now required in certain cases need not be performed. Further, it is realized that various cooling media cool the granules at different cooling rates, and therefore it will be possible to influence the carbide structure and the size of the carbides by a suitable choice of cooling medium.