A method of producing cold work tool steel with improved resistance to chipping and cold work tool steel and steel produced by said method

Abstract

The present invention relates to a method of improving the resistance to chipping of a PM cold work tool steel, wherein the steel comprises: 1.2 - 3.5 C; 0.1 - 2.2 Si; 0.1 - 1.0 Mn; 2 - 20 Cr; ≤ 5 Mo; ≤1 W; 1 - 20 V, balance Fe, optional components and impurities. The steel is subjected to conventional gas atomizing and HIP followed by a super-solidus treatment of the HIPed container, which results in an improved compressive yield strength and an improved resistance to chipping. The invention also relates to a steel produced by said method.

Description

[0001] A METHOD OF PRODUCING COLD WORK TOOL STEEL WITH IMPROVED RESISTANCE TO CHIPPING AND COLD WORK TOOL STEEL

[0002] TECHNICAL FIELD

[0003] The invention relates to a method of manufacturing a cold work tool steel having an improved resistance to chipping and a cold work steel thus obtained.

[0004] BACKGROUND OF THE INVENTION

[0005] Vanadium alloyed powder metallurgy (PM) tool steels have been on the market for decades and attained a considerable interest, because of the fact that they combine a high wear resistance with an excellent dimensional stability and because they have a good toughness. These steels have a wide range of applications such as for knives, punches and dies for blanking, piercing and cold extrusion. The steels are produced by powder metallurgy. The basic steel composition is firstly atomized and thereafter the powder is filled into a capsule and subjected to hot isostatic pressing (HIP) in order to produce an isotropic steel. The performance of the steels tends to increase with increasing content of vanadium. A high performance steel produced in this way is CPM®10V. It has high carbon and vanadium contents as described in US 4,249,945. Uddeholm Vanadis® 8 Super Clean is a cold work steel. Its high vanadium content and balanced composition result in a tool steel with a fine and uniform carbide distribution. Uddeholm Vanadis® 8 Super Clean offers a combination of extremely high wear resistance and good toughness and is therefore well suited for applications such as blanking and forming, fine blanking and powder pressing, since it has a high resistance to chipping. It has high carbon and vanadium contents as described in the patent EP 3132066 B 1.

[0006] However, the increased use of Advanced High Strength Steels (AHSS) and Ultra High Strength Steels (UHSS) for severe cold work applications imposes higher demands on the tooling materials due to the higher stresses, which are required to penetrate the AHSS or UHSS materials as compared to softer materials. The high contact pressures during the forming operations may result in plastic deformation of the tool steel, which changes the tool geometry and precision, resulting in poor quality parts and increased maintenance costs. The high strength steels therefore require additional cutting clearances between the punch and the die. However, excessive cutting clearances may lead to cracking problems due to the high tensile strength of AHSS and UHSS.

[0007] The operations blanking, cutting, stamping, punching and forming present unique tooling challenges due to the high strength and hardness, leading to increased wear, chipping and plastic deformation, which results in reduced tool life, decreased precision and production interruptions. Chipping generally represents the most common failure mechanism, because of the intense mechanical load.

[0008] Although the known (PM) steel has a higher toughness than conventionally produced tool steels, there is a need for further improvements in order to reduce the risk for tool failure, such as chipping and fracture. Until now the standard measure to counteract chipping is to reduce the hardness of the tool.

[0009] DISCLOSURE OF THE INVENTION

[0010] The present invention was made to mitigate the problems inherent in the prior art.

[0011] The invention is limited to a cold work tool steel produced by powder metallurgy, wherein the tool steels are isotropic and comprise 1 - 20 weight % V and 1- 30 volume % vanadium carbides (VC), wherein the amount of other carbides larger than 1 pm than VC after hardening and tempering is limited to 1 vol. %. Other carbides are in particular carbides of the type MeC and M7C3.

[0012] Accordingly, it is a general object of the present invention to provide a manufacturing method for cold work tool steels, wherein the risk for tool failure such as chipping is reduced.

[0013] In order to accomplish the above general object of the present invention, the applicant has researched in detail the reasons for tool failure and found that it is possible to improve the performance of the tool steel by an increase in the compressive strength, which is obtained by implementing a super-solidus treatment of the cold work tool steel produced by gas atomization and hot isostatic pressing. The above general object is solved by the method set out in claim 1.

[0014] A further object is to provide a cold work tool steel produced by powder metallurgy having an improved resistance to chipping and plastic deformation.

[0015] The invention is defined in the claims.

[0016] DETAILED DESCRIPTION

[0017] The invention is limited to cold work tool steels produced by powder metallurgy, wherein the tool steels are relatively isotropic and comprise 1 -20 weight % V and 1- 30 volume % vanadium carbides (VC) and do 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 is restricted to < 1 vol. %, preferably < 0.5 % or 0.3 %. Most preferably the microstructure is free from such carbides after hardening and tempering.

[0018] The inventor of the present invention has surprisingly found, that it is possible to reduce the problems mentioned above by increasing the compressive yield strength of the cold work tool steel by a super-solidus treatment of the cold work tool steel. The cold work tool steel is produced by gas atomization and hot isostatic pressing. The cold work tool steel is thereafter remained in the HIP-capsule and subjected to a super-solidus treatment, which surprisingly results in an increased compressive yield strength (RC02) as well as an increased size of the vanadium carbides.

[0019] 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 for the chemical composition of the steel are given in weight % (wt. %) throughout the description. The amounts of hard phases are given in volume % (vol. %)•

[0020] 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.

[0021] 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 %.

[0022] 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.

[0023] Carbon (1.2 - 3.5 %)

[0024] Carbon is to be present in a minimum content of 1.2 %, 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 %. A preferred range is 2.2 -2-4 %. In any case, 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.

[0025] Chromium (2 - 20 %)

[0026] Chromium is to be present in a content of at least 2 %, preferably 3, 4 or 5 % in order to provide a good hardenability in larger cross sections during heat treatment. The upper limit may be set to any integer between 5 and 20 %. The chromium content should be at least 10, preferably 11, 11, 12, 13 or 14 % for applications in corrosive environments.

[0027] Molybdenum (< 5 %)

[0028] 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 may therefore be set to 3.0, 3.1, 3.2, 3.3, 3.4 or 3.5 %.

[0029] Tungsten (< 1 %)

[0030] 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.

[0031] Vanadium (1 - 20 %)

[0032] 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 %.

[0033] Nitrogen (< 0.3 %)

[0034] 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. Niobium (< 20 %)

[0035] 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. The maximum addition of Nb is 20%. However, Nb results in a more angular shape of the M(C,N). The preferred maximum amount is therefore 2, 1, 0.5 or even 0.3 %. Preferably, no niobium is deliberately added.

[0036] Silicon (0.1 -2.2 %)

[0037] 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 - 2 %. For a good deoxidation, it is preferred to adjust the Si content to at least 0.2 %. Si is a strong ferrite former. Si may also deteriorate the toughness of the steel and should preferably be limited to 1, 0. 7 or 0.5 %.

[0038] Manganese (0.1 - 1 %)

[0039] 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 0.1 %, preferably at least 0.2 %. At higher sulphur contents manganese prevents red brittleness in the steel. The steel shall contain maximum 1 %, preferably maximum 0.8 or 0.6 %. Preferred ranges are 0.22 - 0.52 %, 0.3 - 0.4 and 0.30 - 0.45 %.

[0040] Nickel (< 2 %)

[0041] Nickel is optional and may be present in an amount of up to 2 %. 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 0.5 or 0.3%. Most preferably, no deliberate nickel additions are made.

[0042] Cobalt (< 15 %)

[0043] Co is an optional element. It contributes to increase the hardness of the martensite. The maximum amount is 15 % 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 %. Sulphur (< 0.3 %)

[0044] 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 < 0.3 %. If sulphur is not deliberately added, then the impurity content of S may be limited to 0.05, 0.04, 0.003, 0.001, 0.0008, 0.0005 or even 0.0001%.

[0045] Boron (< 0.1 %)

[0046] 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.

[0047] Al (< 0.1 %)

[0048] 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 0.05 %.

[0049] Ti, Zr and Ta

[0050] 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.

[0051] The alloys of the present invention can be produced by any suitable method of Powder Metallurgy (PM) such as by conventional gas- or water- atomization of pre-alloyed steel melts followed by conventional hot isostatic pressing (HIP). Gas-atomization is the preferred atomization method. In particular, the close-coupled gas atomization method such as VIGA can be used for this purpose.

[0052] The super-solidus heat treatment may be performed in the HIP or in a separate furnace after the HIPing, wherein the HIPed material remains in the container and is subjected 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. It has surprisingly been found that the claimed method results in an improved compressive strength, which, in turn, results in an improved resistance against chipping. In addition, the steel material has a high toughness and ductility, because of a high amount of near spherical morphology of the vanadium carbides and the absence of detrimental rod-shaped carbides. Near spherical morphology carbides have an aspect ratio (AR) of < 2.0. The AR used is the conventional aspect ratio, wherein the longest axis is divided by the maximum orthogonal width of the shortest axis. 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 after hardening and tempering. Accordingly, the inventive material is practically free from large rod like phases, which would result in the embrittlement of the alloy.

[0053] EXAMPLE 1

[0054] In this example the compressive yield strength 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.

[0055] The composition of the investigated steel was as follows (in weight %):

[0056] C 2.31

[0057] Si 0.37

[0058] Mn 0.50

[0059] Cr 4.64

[0060] Mo 3.60

[0061] V 8.33

[0062] Fe bal.

[0063] The steel powder was produced by Vacuum Induction melting Gas Atomization (VIGA) and two identical HIP containers were filled with the powder.

[0064] The material of the container was conventionally used carbon steel. The containers were subjected to HIPing at 1140 °C for 240 minutes. One of the containers was subjected to the inventive method after being removed from the HIP by heating in a furnace at a temperature of 1280 °C, which is about 30 °C higher than the calculated solidus temperature, resulting in an amount of about 30 % liquid phase as calculated by Thermo-Calc (TC version 2023B) using the database TCFE7. The container material was removed and the materials were forged to a dimension of 150x100 mm. Specimens for compressive yield strength testing were prepared 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).

[0065] 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:

[0066] CONVENTIONAL INVENTIVE

[0067] Rc02 (MPa) 3031 3160

[0068] Rc002 (MPa) 2369 2600

[0069] HRC 65.2 66.1

[0070] The compressive yield strength testing reveals that the inventive material has a higher resistance against plastic deformation, but more important a higher resistance to permanent deformation. In addition, the inventive material has a higher hardness. Accordingly, the inventive material will also have a higher resistance to chipping, because of the higher compressive yield strength.

[0071] 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. The size reported 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 maximum size of the particles of the inventive material after the super-solidus treatment was slightly higher 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.

[0072] 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:

[0073]

[0074] EXAMPLE 2

[0075] In this example a cold work tool steel was treated by the inventive method in order to examine the growth of the carbides in this type of steel.

[0076] The composition of the investigated steel had the following nominal composition (in weight %):

[0077] C 2.1

[0078] Si 1.0

[0079] Mn 0.4

[0080] Cr 6.8

[0081] Mo 1.5

[0082] V 5.4

[0083] Fe bal.

[0084] The steel powder was produced by Vacuum Induction melting Gas Atomization (VIGA) and two identical HIP containers were filled with the powder.

[0085] The material of the container was conventionally used carbon steel. The containers were subjected to HIPing at 1140 °C for 240 minutes. One of the containers was subjected to the inventive method after being removed from the HIP by heating in a furnace at a temperature of 1280 °C, which is about 30 °C higher than the calculated solidus temperature, resulting in an amount of about 30 % liquid phase as calculated by Thermo-Calc (TC Version 2023B) using the database TCFE7.

[0086] The container material was removed and the materials were forged to a dimension of 150x100 mm. Specimens for compressive yield strength testing were prepared and subjected to hardening at 1180 °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 560 °C for 1 hour repeated three times (3xlh).

[0087] The microstructure was examined in a scanning electron microscope in the same way as for example 1. The size of all the vanadium carbide particles was < 1.9 pm before the supersolidus treatment and < 0.1 % of the particles had a size of 1.5 pm or more.

[0088] The maximum size of the particles of the material after the super-solidus treatment was less than 20 pm, wherein less than 10 % of the particles had an ECD of > 8 pm and more than 60 % of the particles had an ECD of 1.5 - 10 pm. The vanadium carbide particles were found to be near spherical, wherein more than 70 % of the particles had an aspect ratio (AR) of < 1.5. The material was free from large detrimental rod like particles having an ECD of > 8 pm and an AR of > 3.

[0089] 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:

[0090]

[0091] INDUSTRIAL APPLICABILITY

[0092] The claimed method can be successfully applied to all types of cold work tool steels produced by PM provided that the steels contain 1 - 20 weight % V in order to improve the compressive strength of the material and thereby increasing the resistance to plastic deformation and chipping.

Claims

CLAIMS1. A method for improving the resistance to chipping of vanadium alloyed powder metallurgy produced cold work tool steel, comprising the steps of:i) providing a melt of a cold work tool steel consisting of in weight %C 1.2- 3.5Si 0.1 -2.2Mn 0.1 - 1.0Cr 2 - 20Mo < 5W < 1V 1 - 20Nb <20Ni <2Co < 15N < 0.3B < 0.1S < 0.3optionally < 0.1 % of one or more of the elements Al, Ti, Zr, Ta, Be, Bi, Se, Ca, Mg, REM each or in total,Fe and impurities balance,ii) atomizing the cold work tool steel melt to form a powder, which comprises 1- 30 volume % vanadium carbides, wherein at least 80 % of the carbide particles have 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 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 cold work tool steel for a time of at least 30 minutes in order to increase the compressive yield strength and 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 bodviii) 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 30 % of the vanadium carbides have a size of > 3 pm.

2. The method of claim 1 fulfilling at least one of the following requirements:C 1.3 - 3.2Si 0.1 - 1.2Mn 0.2 - 0.8Cr 4 - 13Mo 0.7 - 4W < 0.5V 3 - 10Ni 0.1 - 1.2Nb < 0.5<0.15<0.05at least 20 % of the number of the vanadium carbides in the size interval 1 - 30 pm obtained in step v) has a size of > 3 pm,and / or wherein the vanadium carbides are near spherical and at least 80 %, preferably at least 90 %, more preferably at least 95 %, of the number of the vanadium carbides obtained in step v) having a size of 2 - 30 pm has an aspect ratio <2.0and / or 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.

3. The method of claim 1 or 2 fulfilling at least one of the following requirements:1.3 -2.40.2 - 0.80.3 - 0.6Cr 4 - 8Mo 2.5 - 3.8W < 0.3V 6- 9.5Ni 0.2 - 0.8Co 8 -11N 0.05 - 0.15B 0.001 - 0.

034. The method of any of the preceding claims, wherein the cold work tool steel obtained in steps v) and viii) fulfils one or more of the requirements:at least 25 % of of the number of the vanadium carbides in the size interval 1 - 30 pm obtained in step v) has a size of > 3 pm,the cold work tool steel comprises vanadium carbides having a size of 10 - 20 pm,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,the carbon content in the matrix of the cold work tool steel is 0.4 - 0.7 % after hardening.

5. A cold work tool steel produced by powder metallurgy and having an improved resistance to chipping, wherein the tool steel comprises 1 - 20 weight % V and 1- 30 volume % vanadium carbides, and wherein the composition of the cold work tool steel consists of in weight %:C 1.2 - 3.5Si 0.1 - 2.2Mn 0.1 - 1.0Cr 2 - 20Mo < 5W < 1V 1 - 20Nb < 20Ni < 2Co < 15N < 0.3B < 0.1S < 0.3optionally < 0.1 % of one or more of the elements Al, Ti, Zr, Ta, Be, Bi, Se, Ca, Mg, REM each or in total,Fe and impurities balance,and 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 80 %, preferably at least 90 %, more preferably at least 95 %, of the number of the vanadium carbides having a size of 2 - 30 pm has an aspect ratio < 2.0.

6. The cold work tool steel according to claim 5, wherein the steel fulfils at least one of the following requirements:C 1.3 - 3.2Si 0.1 - 1.2Mn 0.2 - 0.8Cr 4 - 130.7 - 4< 0.5V 3 - 10Nb < 0.5Ni 0.1 - 1.2Co 4 -12N < 0.15B <0.05at least 30 % of the number of the vanadium carbides in the size interval 1 - 30 pm has a size of > 3 pm,the cold work tool steel comprises vanadium carbides having a size of 10 - 20 pm,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,the carbon content in the matrix of the cold work tool steel is 0.4 - 0.7 % after hardening.

7. The cold work tool steel according to claim 5 or 6, wherein the steel fulfils at least one of the following requirements:C 1.3 -2.4Si 0.2 - 0.8Mn 0.3 - 0.6Cr 4 - 8Mo 2.5 - 3.8W < 0.3V 6- 9.5Ni 0.2 - 0.8Nb < 0.1Co 8 -11N 0.05 - 0.15B 0.001 - 0.

038. The cold work tool steel of any of the preceding claims, wherein the vanadium carbides are near spherical and at least 60 %, preferably 65 %, more preferably at least 70 %, of the number of the vanadium carbides having a size of 2 - 30 pm has an aspect ratio < 1.5.

9. The cold work tool steel of any of the preceding claims, wherein the vanadium 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 and / or 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:

10. The cold work tool steel according to claim 5, wherein the matrix of the cold work tool steel after hardening comprises 4.3 - 5.3 % Cr, and / or 1.8 - 2.5 % Mo and / or 0.2

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