A method for improving the wear resistance of vanadium alloyed powder metallurgy produced cold work tool steels and a vanadium alloyed cold work steel

Abstract

The invention is directed to a method for improving the wear resistance of a vanadium alloyed powder metallurgy produced cold work tool steel comprising 1-20 %V, 1 – 5 %Mo and ≤ 1 %W by HIPing the steel and subjecting the HIPed steel to a heat treatment at a temperature of at least 5 °C higher than the solidus temperature of the tool steel for a time of at least 30 minutes in order to increase the size of the VC-particles, wherein the cold work tool steel after hardening and tempering does not contain noticeable amounts of other carbides than VC.

Description

[0001] A METHOD FOR IMPROVING THE WEAR RESISTANCE OF VANADIUM ALLOYED POWDER METALLURGY PRODUCED COLD WORK TOOL STEELS

[0002] TECHNICAL FIELD

[0003] The invention relates to a method for improving the wear resistance of vanadium alloyed powder metallurgy (PM) produced cold work tool steel.

[0004] BACKGROUND OF THE INVENTION

[0005] Vanadium alloyed cold work tool steels are commonly used for wear applications. Typical applications are blanking, forming, fine blanking, powder pressing and a number of different types of knives. Tool failure is related to several different mechanisms such as plastic deformation, which occur when the compression stress exceeds the compressive yield stress. Failures due to chipping and cracking occurs as a result of too large carbides and low ductility / toughness. Abrasive wear is the result of sliding contact between the substrate and the surface of the cold work tool steel. Important parameters influencing the abrasive wear are linked to the carbides in the cold work tool steel and to the hardness of the steel. In particular, the type, the geometrical form, the volume fraction and the hardness of the carbides have a decisive influence on the abrasive wear. Galling and adhesive wear is caused by sliding contact and heavy friction, which basically is influenced by the friction coefficient and the hardness of the cold work tool steel. Accordingly, it is a delicate problem to find an optimal composition and structure of a cold work tool steel because an increase in the volume fraction of carbides will result in a low ductility / toughness.

[0006] These cold work tool steels can be produced by conventional metallurgy, spray forming or powder metallurgy.

[0007] Conventionally produced highly alloyed tool steels suffer from segregation leading to coarse carbides and an uneven distribution leading to banding and un-isotropy. These problems can to a certain extent be mitigated by Electro Slag Refining (ESR). Although the wear resistance is high, the toughness is relatively low perpendicular to the rolling direction. Vanadium alloyed tool steels produced by Powder Metallurgy (PM) are characterized by a very fine metallurgical structure and a uniform distribution of minute carbides. This leads to a steel having a high toughness but a lower wear resistance due to the small size of the carbides.

[0008] Spray Formed (SF) vanadium alloyed tool steels contain carbides, which are coarser than those in PM-steels and finer than those present in conventionally produced tool steels.

[0009] However, these steels are not isotropic and suffer from an uneven carbide size distribution over the cross section. They also have a relatively low toughness because of a high content of oxygen and a high number of large non-metallic inclusions.

[0010] Typical carbide size distributions for PM-produced tool steels are 0.1-1.7 pm, for SF-produced tool steel 0.3 - 8 pm and for conventionally cast tool steels 1-25 pm. The carbides are uniformly distributed in PM-tool steel. In SF-tool steels typical sizes in the surface are 0.3 -2.9 pm, in the mid-radius 0.4 - 5.8 pm and in the core of the ingot 0.4 - 8 pm.

[0011] In the past, attempts have been made to produce PM-tool steels with larger carbides.

[0012] US 2013 / 0343944 Al and US 9855603 B2 both disclose a method of increasing the wear resistance of PM ledeburitic steels. The method includes high temperature annealing of HIPed steels at a temperature of 1100 °C to 1180 °C for a time of 12 - 24 hours, wherein the annealing temperature should be at least 10 °C below the fusing temperature of the lowest melting structure phase. The average carbide phase diameter could be increased to > 3.2 pm or even to approximately 4 pm by the known method.

[0013] WO 86 / 04360 Al discloses a method for making the carbides in High Speed Steels (HSS) or cold work tool steel produced from powder by compaction larger by subjecting the material during consolidation or thereafter to a temperature exceeding 1150 °C for a sufficient time to let the carbides grow. By this method it is possible to obtain MeC-carbides with an effective maximum size of between 4 and 15 pm. The content of MC is minimized and the content of MeC is preferred in order to obtain a good grindability. US 2009 / 0257903 Al discloses a method for improving the hardness and edge strength of a High Speed Steel (HSS) by restricting the content of Si to 0.7 - 2 % and consolidating the steel by HIP without the presence of liquid phase.

[0014] JP 6345945B2 discloses aHSS consisting of C:1.0 to 1.8 %, Si:0.1 to 1.0 %, Mn:0.1 to 1.0 %, Cr:2.0 to 7.0 %, Mo:2.0 to 7.0 %, W:3.0 to 15.0 %, V:2.0 to 5.0 % and the balance Fe with inevitable impurities, wherein the elements Mo and W were added to form carbides. The HSS steel is produced by PM and conventional HIP -treatment including heat treatment at a temperature of 1200 - 1240 °C followed by forging or rolling, hardening and tempering. The area ratio of coarse carbides having a carbide diameter (ECD) of 2-10 pm is 2 -10 vol. % and the area ratio of fine carbides of less than 2 pm (ECD) is 3-10 vol. %.

[0015] However, the above methods are very time and energy consuming and have only resulted in a moderate increase of the carbide particle size.

[0016] Accordingly, it is a general object of the present invention to provide a manufacturing method for improving the wear resistance of a vanadium alloyed powder metallurgy produced cold work tool steel and at the same time maintaining the isotropy of the tool steel.

[0017] Another object is to provide a manufacturing method for improving the compressive strength of a vanadium alloyed powder metallurgy produced tool steel.

[0018] A further object is to provide a vanadium alloyed powder metallurgy produced tool steel having an improved compressive strength and / or an improved wear resistance.

[0019] DISCLOSURE OF THE INVENTION

[0020] In order to accomplish the above general object of the present invention, the applicant has developed a method, wherein the HIPed material remains in the container and is subjected to a heat treatment at a temperature of at least 5 °C higher than the solidus temperature of the tool steel for a time of at least 30 minutes in order to increase the size of the vanadium carbide particles. The inventive method for improving the wear resistance of a vanadium alloyed powder metallurgy produced tool steel comprises the steps of:

[0021] • providing a steel melt comprising in weight % (wt. %):

[0022] V 1 -20

[0023] Mo 1 - 5

[0024] W < 1

[0025] • atomizing the steel melt to form a powder, which comprises 1- 30 volume % vanadium carbides, wherein at least 80 % of the number of the vanadium carbide particles has a size of < 1.5 pm,

[0026] • filling the powder into a container for hot isostatic pressing, degassing and sealing the container,

[0027] • subjecting the container to hot isostatic pressing at a temperature below the solidus temperature of the cold work tool steel, thereby forming a solid body within the hot isostatic pressed container,

[0028] • subjecting the solid body within the hot isostatic pressed container to a heat treatment at a temperature of at least 5 °C higher than the solidus temperature of the tool steel for a time of at least 30 minutes in order to increase the size of the vanadium carbide particles,

[0029] • removing the container from the solid body,

[0030] • optionally forging or rolling the solid body,

[0031] • subjecting the cold work tool steel thus obtained to hardening and tempering, wherein the cold work tool steel after hardening and tempering does not contain noticeable amounts of other carbides than VC. The invention is limited to a cold work tool steel produced by powder metallurgy, wherein the tool steels are relatively isotropic and wherein the amount of other carbides than VC, which after hardening and tempering are larger than 1 pm, is limited to 1 vol. %. Other carbides are in particular carbides of the type MeC and M7C3. The amount of Mo is therefore restricted.

[0032] Tungsten is likely to form large undesired carbides of the type MeC and / or M7C3 and should therefore be carefully controlled. The maximum amount is limited to 1 %, preferably 0.5 %, more preferably 0.3 %, By not adding W and restricting Mo, as set out above, it is possible to completely avoid the formation of large primary MeC-carbides, which remain in the microstructure after hardening and tempering and give rise to a decreased ductility and toughness.

[0033] The heat treatment in step v) of claim 1 preferably is performed in a separate furnace in order not to block the production in the HIP-apparatus. However, the heat treatment in step v) may very well be performed in the HIP-apparatus and would benefit from the fact, that the heat transfer is very high during HIP-conditions, which may lead to shorter treatment times.

[0034] The invention is defined in the claims.

[0035] DETAILED DESCRIPTION

[0036] The present invention provides a manufacturing method for improving the wear resistance of a vanadium alloyed powder metallurgy produced tool steel and at the same time maintaining most of the isotropy of the tool steel.

[0037] The method of the present invention can be applied to all types of cold work tool steels produced by PM provided that the steels contain 1 - 20 % V and comprise 1- 30 volume % vanadium carbides (VC) and fulfil the compositional requirements of 1-5 % Mo and < 1 % W such that the cold work tool steel after hardening and tempering does not contain noticeable amounts of other carbides than VC.

[0038] The vanadium carbides in the vanadium alloyed powder metallurgy produced tool steels are for simplicity denoted VC, although they may deviate somewhat from stoichiometry and may also contain smaller amounts of other alloying components such as Cr, Mo, W and Nb as well as minor contents of N and B.

[0039] It is important, that the steel does not contain noticeable amounts of other carbides than VC, which are larger than 1 pm after hardening and tempering. Accordingly, the amount of other carbides such as MeC and M7C3 larger than 1 pm after hardening and tempering is restricted to 1 vol. %. The reason for the limitation of other carbides than VC in the hardened and tempered cold work tool steel is that such carbides tend to be very much larger than VC, in particular after a super-solidus heat treatment. In addition, the large carbides MeC and M7C3 may also be elongated or rod shaped, which is detrimental to the isotropy, ductility and toughness of the steel alloy. The carbides MeC and M7C3 also have a substantially lower hardness than carbides of the type MC such as VC.

[0040] In order to maintain uniform properties of the steel it is therefore necessary to carefully control the super-solidus heat treatment in order to obtain carbides having a low aspect ratio (AR). The AR used here is the conventional aspect ratio, wherein the longest axis is divided by the maximum orthogonal width of the orthogonal axis. The vanadium carbides are near spherical and at least 55 %, preferably at least 60 %, more preferably at least 65 % of the number of the vanadium carbides has an aspect ratio < 1.5. By controlling the amounts of Mo and W within the claimed ranges it is possible to avoid or eliminate other carbides than vanadium carbides in the microstructure. Accordingly, the steel is practically free from large rod like phases having an aspect ratio (AR) of >3, which phases result in an embrittlement of the alloy.

[0041] The size of the carbides should be controlled in order to obtain an improved wear resistance without suffering to much in ductility and toughness. At least 25 % of the number of the vanadium carbides has a size of > 3 pm. At least 4 %, preferably at least 5 %, more preferably at least 6 % of the number of the vanadium carbides has a size of > 8 pm, wherein less than 5 % of the number of the vanadium carbides has a size of > 15 pm. The size of the carbides is the Equivalent Circular Diameter (ECD), which refers to the diameter of a circle, that has the same area as the particle being measured. The broadest aspect of the invention is set out in claim 1. The inventive idea is valid for the whole scope of claim 1

[0042] All percentages of the chemical composition of the steels are given in weight % (wt. %) throughout the description. Upper and lower limits of the individual elements can freely be combined within the limits set out in the application. Accordingly, a lower limit set out in the description may be combined with an upper limit set out in a claim or in the description and vice versa. The same applies to the content of vanadium carbides. The amount of the phases is given in volume % (vol. %). The arithmetic precision of the numerical values can be increased by one or two digits for all values given in the present application. Hence, a value reported as e.g. 0.1 % can also be expressed as 0.10 % or 0.100 %.

[0043] The importance of the separate elements and their interaction with each other as well as the limitations of the chemical ingredients of the claimed alloy are briefly explained in the following.

[0044] Molybdenum (1 - 5 %)

[0045] Mo is known to have a very favourable effect on the hardenability. Molybdenum is essential for attaining a good secondary hardening response. Molybdenum is a strong carbide forming element and also a strong ferrite former. Mo needs to be restricted also for the reason of limiting the amount of other hard phases than MX. In particular the amount of MeC-carbides larger than 1 pm after hardening should be limited, preferably to < 1 vol. %. Most preferably no MeC-carbides should be present in the microstructure. The maximum content of molybdenum may therefore be limited to 4.5 %. Preferably Mo is limited to 4.2 %, 3.9 % or even 3.7 %. However, Mo may be added for increasing the hardenability and for attaining a good secondary hardening after hardening and tempering. The minimum content is 1 % and may be set to 1.5, 2.0, 2.5, 3.0, 3.1, 3.2, 3.3, 3.4 or 3.5 %.

[0046] Tungsten (< 1 %)

[0047] Tungsten is likely to form large undesired carbides of the type MeC and M7C3 and should be carefully controlled. The maximum amount is therefore limited to 1 %, preferably 0.5 %, more preferably 0.3 % and most preferably no deliberate additions are made. By not adding W and restricting Mo, as set out above, it is possible to completely avoid the formation of large primary MeC-carbides, which remain in the microstructure after hardening and tempering giving rise to a decreased ductility and toughness.

[0048] Vanadium (1 - 20 %)

[0049] Vanadium forms evenly distributed primary precipitated carbides and carbonitrides of the type M(C,N) in the matrix of the steel. In the present steels M is mainly vanadium but minor amounts of Cr and Mo may be present. Vanadium should preferably be present in an amount of 2, 3, 4, 5, 6 or 7 %. The upper limit may be set to any integer below 20 %. However, the toughness decreases with increasing amount of VC such that it is preferred to limit the amount of V to 10, preferably 9, 8.5 or 8 %. A preferred range is 7.2 - 8.5 %. The upper limit may be set to 8.4 %, 8.3 %, or 8.25 %. The lower limit may be 7.3 %, 7.4 %, 7.5 %, 7.6 %, 7.7 %, 7,75 %, and 7.8 %. The upper and lower limits may be freely combined within the limits set out in claim 1. Preferred ranges include 7.7 - 8.3 %.

[0050] Carbon

[0051] The amount of carbon should be controlled such that the amount of carbides of the type M23C6, M7C3 and MeC in the steel is limited to less than 1 vol. % each or in total, preferably the steel is free from said carbides after hardening and tempering. The amount can easily be calculated in relation to the selected contents of Mo, W and V by use of the software Themo-Calc (version 2023B) and the database TCFE7. Carbon is preferably present in a minimum content of 1.2 %, more preferably at least 1.4. 1.6, 1.8, 2.0 or 2.2 %. The upper limit for carbon may be set to 3.4, 3.2, 3.0, 2.8, 2.6, or 2.4 %.

[0052] The oxygen content of the scold work tool steel is preferably restricted to 0.015 % (150 ppm) in order to obtain a clean steel with a low number of non-metallic inclusions. In particular, it is desirable that the cleanliness of the steel fulfils the following maximum requirements with respect to micro-slag according to ASTM E45-97, Method A; Plate 1-r:

[0053]

[0054] The method of the present invention has the advantage in that the carbides grow very rapidly, because of the presence of a liquid phase in which the diffusion is extremely much faster than in solid austenite. As a result thereof, there will be a very rapid growth of the vanadium carbides such that the treatment time can be relatively short. However, since the inventive method is performed at a high temperature and with the presence of a liquid phase it is a requisite, that the HIPed tool steel remains in the container during the heat treatment for carbide growth.

[0055] The treatment time for carbide growth is limited by the longevity of the capsule, which depends on factors such as the treatment temperature, the amount of the liquid phase, the composition of the alloy and the composition and thickness of the steel container. In most cases conventionally used carbon steel is employed. However, it is conceivable to use stainless steel, in particular FECRAL-alloys, Alumina Forming Austenitic Stainless Steel (AFA) and Alumina Forming Martensitic Stainless Steel (AFM), which all have a surface layer of alumina. The minimum treatment time for obtaining a sufficient carbide growth is 30 minutes. The treatment time for carbide growth is solely limited by the longevity of the capsule. However, since the diffusion in the liquid phase is very rapid, the treatment time will be much shorter than for a treatment without the presence of liquid phase. The treatment time may be restricted to 10 hours or 9, 8, 7, 6, 5, 4, 3, 2 or 1 hour. The upper time limit may be set taking the container material, treatment temperature and the desired carbide growth into consideration.

[0056] The treatment temperature is selected according to the desired amount of liquid phase but is at least 5 °C higher than the solidus temperature (Ts). The upper temperature limit is influenced by the composition of the tool steel and the properties and thickness of the material used for the container. The lowest temperature is 5 °C above Ts and may be set to 10°C, 15 °C or 20 °C above Ts. The highest temperature above Ts may be set such that the amount of a liquid phase does not exceed 50 %, or 40 % or 30 % or 20 %. The upper limit may thus be 100, 90, 80, 70, 60, 50,40, 30 or 20 °C above Ts. The amount of liquid phase as calculated by ThermoCalc (version 2023B) using the database TCFE7 should preferably be 20 - 40 %, more preferably 25 - 35 % and most preferably about 30 %.

[0057] EXAMPLE 1 In this example the wear resistance of the proprietary cold work tool steel Vanadis® 8 Super Clean in its original state is compared to the same steel, which has been subjected to the inventive method. The material of the container was conventionally used carbon steel.

[0058] The composition of the investigated steels was as follows (in weight %):

[0059] C 2.31

[0060] Si 0.37

[0061] Mn 0.50

[0062] Cr 4.64

[0063] Mo 3.60

[0064] V 8.33

[0065] Fe bal.

[0066] In order to increase the size of the vanadium carbide particles, the tool steel body produced according to the claimed method was subjected to a heat treatment at a temperature of 30 °C higher than the solidus temperature for a time of 2 hours in a separate furnace after HIPing. The amount of liquid phase at this temperature was estimated by calculation in Thermo-Calc (version 2023B) using the database TCFE7 to be around 30 %.

[0067] The container material was removed and the materials were forged to a dimension of 150x100 mm. Identical specimens were prepared for examination and subjected to hardening at 1100 °C for 30 minutes in a vacuum furnace, cooled by rapid gas quenching with a pressure of 5 bar which resulted in a cooling rate T8 / 5 = 100 s and natural cooling to room temperature. Tempering was performed at 550 °C for 1 hour repeated three times (3xlh).

[0068] This resulted in a hardness of 63 HRC for the steel not subjected to the inventive method and a hardness of 65 HRC for the treated steel having the coarser carbide distribution. The wear resistance of the material obtained by the inventive method was found to be more than 2.5 better than that of the comparative material.

[0069] The microstructure was examined in a scanning electron microscope. The number of the vanadium carbide particles having a size of at least 0.15 pm was examined in an area of 1.71E+4 pm2, in which area 3848 particles were detected. The size of all the vanadium carbide particles was <2.5 pm before the treatment, wherein the majority of the particles (46 %) had a size in the range of 0.15 - 0.5 pm and 0.2 % of the particles had a size of 1.5 pm or more.

[0070] The maximum size of the particles after the heat treatment increased to somewhat more than 20 pm, wherein more than 1600 particles had a size > 2.5 pm, more than 700 particles were > 5 pm and more than 200 particles were > 10 pm.

[0071] EXAMPLE 2

[0072] A capsule of the same type as for example 1, which contained the steel Vanadis® 8 Super Clean was subjected to HIPing and to a heat treatment at a temperature of 30 °C higher than the solidus temperature for different times in a separate furnace. The capsule was found to be intact after 2 and 5 hours. However, leakage of liquid metal was detected after 19 hours.

[0073] Although the claimed method is described in relation to PM-steels containing 1 -20 weight % V and 1- 30 vol. % VC it is conceivable, that the claimed method also can be used for improving the wear resistance of nitrogen alloyed steels containing 1- 30 vol. % VN. It is also conceivable, that the claimed method can be used to increase the size of other carbides, nitrides and carbonitrides of the elements Ti, Zr, Hf, Nb and Ta in corresponding fractions.

[0074] INDUSTRIAL APPLICABILITY

[0075] The method of the present invention can be applied to all types of cold work tool steels produced by PM provided that the steels contain 1 - 20 % V and comprise 1- 30 volume % vanadium carbides (VC) and fulfil the compositional requirements of 1-5 % Mo and < 1 % W, wherein the cold work tool steel after hardening and tempering does not contain noticeable amounts of other carbides than VC.

Claims

CLAIMS1. A method for improving the wear resistance of a vanadium alloyed powder metallurgy produced cold work tool steel, comprising the steps of:i) providing a melt of a cold work tool steel comprising in weight % (wt. %):V 1 -20Mo 1 - 5W < 1ii) atomizing the steel melt to form a powder, which comprises 1- 30 volume % vanadium carbides, wherein at least 80 % of the number of the vanadium carbide particles has a size of < 1.5 pm,iii) filling the powder into a container for hot isostatic pressing, degassing and sealing the container,iv) subjecting the container to hot isostatic pressing at a temperature below the solidus temperature of the cold work tool steel, thereby forming a solid body within the hot isostatic pressed container,v) subjecting the solid body within the hot isostatic pressed container to a heat treatment at a temperature of at least 5 °C higher than the solidus temperature of the cold work tool steel for a time of at least 30 minutes in order to increase the size of the vanadium carbide particles,vi) removing the container from the solid body,vii) optionally forging or rolling the solid body,viii) subjecting the cold work tool steel obtained in step vi) or vii) to hardening and tempering, wherein the cold work tool steel after hardening and tempering does not contain noticeable amounts of other carbides than VC.

2. The method of claim 1, wherein the cold work tool steel does not contain more than 1 volume % of carbides larger than 1 m other carbides than vanadium carbides.

3. The method of claim 1 or 2, wherein the treatment in step v) is performed in the HIP furnace used in step iv) or in a separate furnace not being the said HIP furnace optionally with intermediate cooling before heating in the separate furnace.

4. The method of any of the preceding claims, wherein the cold work tool steel fulfils at least one of the following requirements:the amount of liquid phase calculated by Thermo-Calc version 2023B using the database TCFE7 is 20 - 40 vol. %, more preferably 25 - 35 vol. % and most preferably 28 - 32 vol. %,the tool steel comprises vanadium carbide particles having a size of 10 - 20 pm,at least 25 % of the number of vanadium carbide particles has a size of > 3 pm,the tool steel comprises vanadium carbides in each of the following size classes: 1 -2 pm, 3 - 5 pm, 6- 8 pm, 9 - 12 pm and > 12 pm,at least 4 %, preferably at least 5 %, more preferably at least 6 % of the number of the vanadium carbides has a size of > 10 pm.the volume fraction of vanadium carbides is 5 - 25 % or 10 - 20 % or 12- 18 %the carbon content in the matrix is 0.4 - 0.7 %,the matrix comprises 4.3 - 5.3 % Cr and / or 1.8 - 2.5 % Mo and / or 0.2 - 0.7 %V.

5. The method of any of the preceding claims, wherein the vanadium carbides are near spherical and at least 55 %, preferably at least 60 %, more preferably at least 65 %, of the number of the vanadium carbides has an aspect ratio < 1.5 and / or wherein thevanadium carbides are near spherical and less than 6%, less than 5 %, or less than 4% or less than 3 % of the number of the vanadium carbide having a size of 2 - 30 pm has an aspect ratio < 3.0.

6. A vanadium alloyed powder metallurgy produced cold work tool steel obtainable with the method of claim 1, wherein the cold work tool steel comprises:V 1 -20Mo 1 - 5W < 1wherein the cold work tool steel comprises 1- 30 volume % vanadium carbides and does not contain noticeable amounts of other carbides than VC.

7. The vanadium alloyed powder metallurgy produced tool steel as defined in claim 6, wherein the cold work tool steel after hardening and tempering does not contain more than 1 volume % of carbides larger than 1 pm other than vanadium carbides and / or wherein the oxygen content is not more than 150 ppm and / or wherein the cleanliness of the steel fulfilled the following maximum requirements with respect to micro-slag according to ASTM E45-97, Method A; Plate 1-r:

8. The vanadium alloyed powder metallurgy produced tool steel as defined in claim 6 or 7, wherein the cold work tool steel fulfils at least one of the following requirements:the cold work tool steel comprises vanadium carbide particles having a size of 10 - 20 pm,at least 25 % of the number of the vanadium carbide particles has a size of > 3 pm,the tool steel comprises vanadium carbides in each of the following size classes: 1 -2 pm, 3 - 5 pm, 6- 8 pm, 9 - 12 pm and > 12 pm,at least 4 %, preferably at least 5 %, more preferably at least 6 % of the number of the vanadium carbides has a size of > 10 pm,the volume fraction of vanadium carbides is 5 - 25 % or 10 - 20 % or 12 - 18 %,9. The vanadium alloyed powder metallurgy produced tool steel as defined in any of the claims 6 to 8, wherein the vanadium carbides are near spherical and at least 55 %, preferably at least 60 %, more preferably at least 65 %, of the number of the vanadium carbides has an aspect ratio < 1.5 and / or wherein the vanadium carbides are near spherical and less than 6% or less than 5 % or less than 4 % or less than 3 % of the number of the vanadium carbides has a size of 2 - 30 pm has an aspect ratio < 3.0.

10. The vanadium alloyed powder metallurgy produced tool steel as defined in any of claims 6 to 9, wherein the cold work tool steel comprises vanadium carbide particles having a size of 10 - 20 pm, and wherein the number of the vanadium carbide has a size of > 10 pm is > 4 %, preferably > 5 %, more preferably > 6 % of the number of the vanadium carbide has a size of > 1 pm.

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