A wear resistant vanadium alloyed powder metallurgy produced cold work tool steel and method of producing said steel, use of said steel

Abstract

The invention is directed to a wear resistant vanadium alloyed powder metallurgy produced cold work tool steel for knives consisting of in weight % (wt.%): 2.2 - 2.4 C; 0.1 - 0.55 Si; 0.2 - 0.8 Mn; 4.1 - 5.1 Cr; 3.1 - 4.5 Mo; 7.2 - 8.5 V optional components and impurities balance Fe, wherein the cold work tool steel after hardening does not contain more than 1 volume % of carbides larger than 1 pm other than vanadium carbides and wherein at least 25 % of the vanadium carbides have a size of 3 pm and / or wherein the vanadium carbides are near spherical and at least 55 of the number of the vanadium carbides has an aspect ratio ≤ 1.5. The invention is also directed to a method of producing said steel and the use of said steel.

Description

[0001] A WEAR RESISTANT VANADIUM ALLOYED POWDER METALLURGY PRODUCED COLD WORK TOOL STEEL

[0002] TECHNICAL FIELD

[0003] The invention relates to a wear resistant vanadium alloyed powder metallurgy (PM) produced cold work tool steel. In particular, the invention relates to a cold work tool steel PM for knives.

[0004] BACKGROUND OF THE INVENTION

[0005] The abrasive wear of a tool steel is known to be governed by a structure parameter, A, that equals the primary carbide size, multiplied by the volume fraction of carbides multiplied by carbide hardness: A = (carbide size)x(carbide volume fraction)x(carbide hardness) as shown in ASM Specialty Handbook, Tool Materials, ASM 1995, p. 134, Fig. 13; (from Budinski, K.G., Wear of Tool Steels in Wear of Materials, American Society of Mechanical Engineers, 1977, pp. 100-109).

[0006] The hardness and volume fraction of carbides depend on the steel chemistry. Vanadium carbides VC have a high degree of covalent bonding and therefore a high hardness of about 2800 HV. Molybdenum / Tungsten carbides of the type MeC have a hardness of 1400 HV, carbides of the type M7C3 have a hardness of 1500 HV and M23C7 have a hardness of 1200 HV. Accordingly, vanadium alloyed steels are best suited for cold work applications.

[0007] High Speed Steels (HSS), on the other hand, are used at high temperatures and therefore alloyed with high amounts of Mo and W resulting in high amounts of primary carbides of the type MeC, M7C3 and possibly also M23C7, which are much softer than VC.

[0008] Vanadium alloyed cold work tool steels can be produced by conventional metallurgy, spray forming or powder metallurgy.

[0009] Conventionally produced highly alloyed tool steels suffer from segregation leading to coarse carbides, generally 1-25 pm and an uneven distribution leading to banding of carbides in the size range of 30 - 60 pm or even larger and, consequently, to an 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.

[0010] 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 an isotropic steel having a high toughness but a lower wear resistance due to the small size of the carbides.

[0011] 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.

[0012] 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.

[0013] 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.

[0014] 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.

[0015] W02015 / 160302 Al discloses a high-performance vanadium alloyed powder metallurgy (PM) produced cold work tool steel, which can be used for knives. The known steel contains about 8 %V and has a hardness of 60 - 64 HRC and is therefore resistant to wear. Its high vanadium content and balanced composition result in a tool steel with a fine and uniform carbide distribution, wherein the steel is free from Cr-carbides after hardening and tempering. This steel, known as Vanadis® 8 Super Clean has been found to a outperform the steel CPM-10V as a knife steel

[0016]

[0017] Although the known steel has a very good toughness and edge retention, it would be of great interest to improve the edge stability and the edge retention further in order to optimize the steel for demanding applications, such as for survival knives and recycling knives, were a high wear resistance and edge retention is important.

[0018] Edge retention is the ability of a knife to maintain its sharpness as it is used for cutting.

[0019] Different tests exist to rank the edge retention of different steels such as cutting ropes (5 / 8” Manila rope), and those described in WO2014079889 Al and US20100281965A1 and the standardized CATRA test according to ISO8442-5:2004(E).

[0020] The edge stability refers to the ability of the edge to withstand rolling, which is caused plastic deformation of the edge when the stress applied to the edge exceeds the compressive yield stress of material, resulting in a permanent deformation of the edge. When the edge is rolled it is no longer aligned with the material being cut, which leads to a decreased ability to cut.

[0021] DISCLOSURE OF THE INVENTION

[0022] The present invention was made to mitigate the problems inherent in the prior art.

[0023] Accordingly, it is a general object of the present invention to provide a vanadium alloyed powder metallurgy (PM) produced cold work tool steel having an improved edge stability and edge retention, when used for demanding knife applications and at the same time maintaining a high toughness of the cold work tool steel. A further object is to provide a method for producing the cold work tool steel.

[0024] The invention is limited to wear resistant cold work tool steel produced by powder metallurgy, wherein the amount of other carbides than VC after hardening is limited to 1 vol. %. Other carbides are in particular carbides of the type MeC and M7C3.

[0025] The inventive wear resistant vanadium alloyed powder metallurgy produced cold work tool steel for knives consists of in weight % (wt.%):

[0026] C 2.2 - 2.4

[0027] Si 0.1 - 0.55

[0028] Mn 0.2 - 0.8

[0029] Cr 4.1 - 5.1 Mo 3.1 -4.5

[0030] V 7.2 - 8.5

[0031] Co <5

[0032] Ni < 3

[0033] W < 1

[0034] Nb <2

[0035] N < 0.3

[0036] S < 0.5

[0037] optionally <0.1 % of one or more of the elements Al, B, Ti, Zr, Ta, Be, Bi, Se, Ca, Mg, REM each or in total, Fe and impurities balance,

[0038] 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 wherein at least 25 % of the number of vanadium carbides has a size of > 3 pm,

[0039] and / or 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.

[0040] 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 super-solidus temperature of the tool steel for a time of at least 30 minutes in order to perform a controlled increase of the size of the vanadium carbide particles.

[0041] The above general object is solved by the method set out in claim 1 and in the independent method claim.

[0042] The invention is defined in the claims.

[0043] DETAILED DESCRIPTION

[0044] The present invention provides a wear resistant vanadium alloyed powder metallurgy produced cold work tool steel for knives having an improved edge retention as a result of a carefully controlled composition and a specific super-solidus heat treatment method for increasing the size of the vanadium carbides (VC) in the powder metallurgy produced tool steel. The super-solidus heat treatment is controlled to obtain the desired growth of the vanadium carbides.

[0045] It is important that the steel does not contain noticeable amounts of other carbides than V, 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 two-fold: firstly, in that MeC and M7C3 are much softer than VC and, secondly in that the 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.

[0046] 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 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.

[0047] The size of the carbides should be controlled in order to obtain an improved edge retention 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.

[0048] The size is the Equivalent Circular Diameter (ECD), which refers to the diameter of a circle that has the same area as the particle being measured.

[0049] 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 results in an embrittlement of the alloy.

[0050] The preset invention is therefore limited to a cold work tool steel having the following basic composition in weight % (wt.%):

[0051] C 2.2 - 2.4

[0052] Si 0.1 - 0.55

[0053] Mn 0.2 - 0.8

[0054] Cr 4.1 - 5.1

[0055] Mo 3.1 -4.5

[0056] V 7.2 - 8.5

[0057] 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. All percentages of the chemical composition of the steel are given in weight % (wt. %) throughout the description. The amounts of hard phases are given in volume % (vol. %).

[0058] The broadest aspect of the invention is set out in claim 1. The inventive idea is valid for the whole scope of claim 1. Upper and lower limits for one or more of the chemical elements may be freely combined within the limits set out in claim 1, in order to form a more limited range for one or more of the elements. This may be necessary, in order to delimit the invention over prior art not known to the applicant. Accordingly, multiple restrictions are expressly allowable for all elements defined in claim 1 and a pointer for a certain combination is not needed, because multiple restriction of elements solely leads to a limitation of the scope of protection and not to a new invention. It is also allowable to form a new range for an element by the combination of two different values of an upper range or by the combination of two different values of a lower range.

[0059] 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 %. The steel may also contain conventional amounts of undesirable residual elements or impurities, which are not intentionally added but originate from the raw materials, the refractories or from the atmosphere during steelmaking. The upper limits of the impurities can be specified but need not be specified as long as they do not deviate from conventional impurity amounts.

[0060] Carbon (2.2 - 2.4 %)

[0061] Carbon is to be present in a minimum content of 2.2 %, preferably at least 2.25 %. The upper limit for carbon may be set to 2.4 % or 2.35 %. Preferred ranges are 2.25 - 2.35 % and 2.26 -2. 34 %. In any case, the amount of carbon should be controlled such that the amount of carbides of the type M23C6, M7C3 and MeC larger than 1 pm in the steel after hardening and tempering is limited to 1 vol. %, preferably the steel is free from said carbides.

[0062] Chromium (4.1 - 5.1 %)

[0063] Chromium is to be present in a content of at least 4.1 % in order to provide a good hardenability in larger cross sections during heat treatment. If the chromium content is too high, this may lead to the formation of high-temperature ferrite, which reduces the hot-workability. The chromium content is therefore preferably 4.5 - 5.0 %. The lower limit may be 4.2 %, 4.3 %, 4.4 % or 4.5 %. The upper limit may be 5.1%, 5.0 %, 4.9 % or 4.8 %.

[0064] Molybdenum (3.1 - 4.5 %)

[0065] Mo is known to have a very favourable effect on the hardenability. Molybdenum is essential for attaining a good secondary hardening response. The minimum content is 3.1 %, and may be set to 3.2 %, 3.3 %, 3.4 % or 3.5 %. 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 to < 1 vol. %, preferably to < 0.5 vol. %. Most preferably no MeC-carbides should be present in the microstructure. The maximum content of molybdenum is therefore 4.5 %. Preferably Mo is limited to 4.2 %, 3.9 % or even 3.7 %.

[0066] Tungsten (< 1 %)

[0067] 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 therefore limited to 1 %, preferably 0.5 %, more preferably 0.3 % or even 0.1 % 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 and give rise to a decreased ductility and toughness.

[0068] Vanadium (7.2 - 8.5 %)

[0069] 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 significant amounts of Cr and Mo may be present. Vanadium shall therefore be present in an amount of 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 %.

[0070] Nitrogen (< 0.3 %)

[0071] Nitrogen may optionally be introduced in the steel in an amount of up to 0.3 %. If the atomization is made by the use of nitrogen gas, then the content of N may be in the range of 0.02 - 0.15 % or 0.02 - 0.08 % or 0.03 - 0.06 %. Higher amounts may be obtained by alloying the melt with N.

[0072] Niobium (< 2 %)

[0073] Niobium is similar to vanadium in that it forms carbonitrides of the type M(C,N) and may in principle be used to replace vanadium but that requires double the amount of niobium as compared to vanadium. Hence, the maximum addition of Nb is 2.0%. The combined amount of (V + Nb / 2) should be 7.2 - 8.5 %. However, NbC has a more angular shape than VC and tends to precipitate in the melt before the atomization, which leads to clogging of the nozzle. The preferred maximum amount is therefore 0.5 %. Preferably, no niobium is added.

[0074] Silicon (0.1 - 0.55 %)

[0075] Silicon is used for deoxidation. Si is present in the steel in a dissolved form. Si increases the carbon activity and is beneficial for the machinability. Si is therefore present in an amount of 0.1 - 0.55 %. For a good deoxidation, it is preferred to adjust the Si content to at least 0.2 %. Si is a strong ferrite former and should preferably be limited to < 0.5 %. Manganese (0.2 - 0.8 %)

[0076] Manganese contributes to improving the hardenability of the steel and together with sulphur manganese contributes to improving the machinability by forming manganese sulphides. Manganese shall therefore be present in a minimum content of 0.2 %, preferably at least 0.22 %. At higher sulphur contents manganese prevents red brittleness in the steel. The steel shall contain maximum 0.8 %, preferably maximum 0.6 %. Preferred ranges are 0.22 - 0.52 %, 0.3 - 0.4 and 0.30 - 0.45%.

[0077] Nickel (< 2 %)

[0078] Nickel is optional and may be present in an amount of up to 3 %. It gives the steel a good hardenability and toughness. Because of the expense, the nickel content of the steel should be limited as far as possible. Accordingly, the Ni content is limited to 1%, preferably 0.3%. Most preferably, no nickel additions are made.

[0079] Cobalt (< 5 %)

[0080] Co is an optional element. It contributes to increase the hardness of the martensite. The maximum amount is 5 % and, if added, an effective amount is about 4 to 5 %. However, for practical reasons such as scrap handling there is no deliberate addition of Co. A preferred maximum content is 1 %. CO is most preferably reduced to impurity contents.

[0081] Sulphur (< 0.5%)

[0082] S contributes to improving the machinability of the steel. At higher sulphur contents there is a risk for red brittleness. Moreover, a high sulphur content may have a negative effect on the fatigue properties of the steel. The steel shall therefore contain < 0.5 %, preferably < 0.03 %.

[0083] Al (< 0.1 %)

[0084] Al may be used for deoxidation and may then be present in an amount of up to 0.1 %. A preferred range is 0.005 to 0.08 % more preferred 0.01 to 0.06 %.

[0085] Be, Bi, Se, Ca, Mg, O and REM (Rare Earth Metals)

[0086] These elements may be added to the steel in the claimed amounts in order to further improve the machinability, hot workability and / or weldability. However, it is preferred to limit the oxygen content to not more than 0.015 % (150 ppm). In addition, the content of Ar may be limited to 0.05 ppm. Boron (< 0.1 %)

[0087] Substantial amounts of boron may optionally be used to assist in the formation of the hard phase MX. Lower amounts of B may be used in order to increase the hardness of the steel. The amount is then limited to 0.01%, preferably <0.004%. Generally, no boron additions are made.

[0088] Ti, Zr and Ta

[0089] These elements are carbide formers and may be present in the alloy in the claimed ranges for altering the composition of the hard phases. However, normally none of these elements are added.

[0090] 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.

[0091] 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 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.

[0092] 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 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 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 a 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.

[0093] 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 (TC Version 2023B) using the database TCFE7 should preferably be 20 - 40 %, more preferably 25 - 35 % and most preferably about 30 %.

[0094] EXAMPLE 1

[0095] 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.

[0096] The composition of the investigated steels was as follows (in weight %):

[0097] C 2.31

[0098] Si 0.37

[0099] Mn 0.50

[0100] Cr 4.64

[0101] Mo 3.60

[0102] V 8.33

[0103] Fe bal.

[0104] The steel powder was produced by Vacuum Induction melting Gas Atomization (VIGA) and two identical HIP containers were filled with the powder. The material of the container was conventionally used carbon steel. The containers were subjected to HIPing at 1140 °C for 240 minutes. 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:

[0105]

[0106] 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 being removed from the HIP-fumace. The amount of liquid phase at this temperature was estimated to be around 30 % according to a calculation in Thermo-Calc version 2023B using the database TCFE7.

[0107] 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).

[0108] 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 sizes of all the vanadium carbide particles were < 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.

[0109] 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.

[0110] 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. The specimens were subjected to compressive yield strength testing at room temperature in accordance with the standard test method ASTM E9. The values for the start of the plastic deformation Rc002 and the compressive yield strength Rc02 were measured in an universal testing machine. The reported values are mean values of five measurements. In addition, the hardness of the specimens was examined. The examination revealed the following results:

[0111] CONVENTIONAL INVENTIVE

[0112] Rc02 (MPa) 3031 3160

[0113] Rc002 (MPa) 2369 2600

[0114] HRC 65.2 66.1

[0115] The compressive yield strength testing reveals that the inventive material has a higher resistance against permanent plastic deformation (Rc02). Accordingly, the inventive material will also have a higher resistance deformation of the edge, so called rolling.

[0116] The two steels were soft annealed and forged, followed by conventional knife making in order to make two identical knives, which were subjected to the same hardening and tempering treatment. The finished knives having identical edge angels were subject to different edge retention tests, such as the rope cutting test, and it was found that the inventive steel outperformed the comparative steel.

[0117] It is conceivable that the claimed steel can be used for many other demanding applications such as blanking, fine blanking, powder compaction tooling and blanking of electrical sheets.

[0118] INDUSTRIAL APPLICABILITY

[0119] The claimed steel has an improved edge retention and can be used in knives for demanding applications.

Claims

CLAIMS1. A wear resistant vanadium alloyed powder metallurgy produced cold work tool steel for knives consisting of in weight % (wt.%):C 2.2 - 2.4Si 0.1 - 0.55Mn 0.2 - 0.8Cr 4.1 - 5.1Mo 3.1 -4.5V 7.2 - 8.5Co <5Ni < 3W < 1Nb <2N < 0.3S < 0.5optionally <0.1 % of one or more of the elements Al, B, Ti, Zr, Ta, Be, Bi, Se, Ca, Mg, REM each or in total,Fe and impurities balance,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 wherein at least 25 % of the number of vanadium carbides has a size of > 3 pm,and / or 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.

2. The wear resistant vanadium alloyed powder metallurgy produced cold work tool steel of claim 1, fulfilling at least one of the following requirements:C 2.25 - 2.35Si 0.2 - 0.5Mn 0.2 - 0.6Cr 4.5 - 5.0Mo 3.5 - 3.7V 7.7 - 8.3Nb < 1N 0.02- 0.08Co < 1Ni < 1W < 0.3O < 0.0153. The wear resistant vanadium alloyed powder metallurgy produced cold work tool steel according to claims 1 or 2, wherein > 4 %, preferably > 5 %, more preferably > 6 % of the number of the vanadium carbides has a size of > 10 pm and / or wherein 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:

4. The wear resistant vanadium alloyed powder metallurgy produced cold work tool steel according to any of the preceding claims, wherein the cold work 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.

5. A method of producing a wear resistant vanadium alloyed powder metallurgy produced cold work tool steel for knives according to claim 1 or 2, by the steps of:i) providing a melt of steel having a composition as defined in claim 1 or 2,ii) atomizing the steel melt in order to form a powder, which comprises 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 conventional 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 5 - 65 °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 of the cold work tool steel,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 more than 1 volume % of carbides larger than 1 pm other than vanadium carbides and wherein at least 25 % of the number of the vanadium carbides has a size of > 3 pm,and / or 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.

6. The method of claim 5, wherein the amount of liquid phase calculated by ThermoCalc version 2023B using the database TCFE7 is 20 - 40 vol. %, more preferably 25 - 35 vol. % and most preferably 28 - 32 vol. %.

7. The method of claim 5 or 6, wherein the powder is produced by use of vacuum induction gas atomisation (VIGA) and wherein 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:

8. Use of the wear resistant vanadium alloyed powder metallurgy produced cold work tool steel as defined in any of claims 1-4 for knives, hand knives, granulator knives, recycling knives, circular knives, extruder components, rolls, dies, anvils and powder compaction components.

9. Use of the wear resistant vanadium alloyed powder metallurgy produced cold work tool steel as defined in any of claims 1-4 for cutting, forming, punching, blanking fine blanking, powder compression and / or extrusion.

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