A hot forming tool for press hardening

Abstract

The invention relates to a hot forming tool for press hardening, wherein the tool comprises a main body and, partly or fully of a laser metal deposited surface layer, the main body consist of a tool steel alloy comprising 0.25 – 0.45 C, 0.10 - 1.50 Si, 0.2 – 1.0 Mn, 2 - 6 Cr, 1 – 3 Mo, 0.4 – 1.0 V, 0.001– 0.06 Al, ≤ 0.12 N and the laser metal deposited surface layer comprising 0.27 - 0.40 C, 0.10 - 0.35 Si, 0.2 - 0.8 Mn, 4.0-6.0 Cr, 2.0-3.0 Mo, 0.4-1.0 V, 0.001-0.06 Al, 0.003-0.12 N. The laser metal deposited surface layer has a hardness of 53-59 HRC.

Description

[0001] A HOT FORMING TOOL FOR PRESS HARDENING

[0002] TECHNICAL FIELD

[0003] The invention relates to a hot forming tool for press hardening.

[0004] BACKGROUND OF THE INVENTION

[0005] Press hardening or hot stamping is a hot forming method in which blanks are both hot formed and quenched in one step in a water-cooled tool to achieve high strength. Use of press hardening allows producing lightweight components with very high mechanical properties and complex shapes. Press hardening of Advanced High Strength Steel (AHSS) and Ultra High Strength Steels (UHSS) with tensile strengths of up to 2000 MPa is used for producing high strength car body parts in order to reduce the weight of the automobile body, while at the same time maintaining or improving the crashworthiness.

[0006] However, press hardening of AHSS and UHSS sheet material may result in large abrasive and adhesive wear of the press forming tool. In the past, many attempts have been made to reduce the wear by using wear resistant tool materials and / or by applying surface treatments such as laser hardening, nitriding and different types of PVD-coatings. In addition, the hot forming tool is subjected to severe thermal cycling, which may lead to thermal fatigue.

[0007] DISCLOSURE OF THE INVENTION

[0008] The object of the present invention is to enhance the performance of the hot forming tool and thereby increase the tool life time.

[0009] This object is achieved to significant measure by providing a surface layer of a particular vanadium alloyed matrix tool steel having a high toughness and hardness by laser metal deposition (LMD) on a main body consisting of a hot work tool steel.

[0010] The invention is defined in the claims. DETAILED DESCRIPTION

[0011] Vanadium alloyed matrix 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.

[0012] The inventive hot forming tool comprises a main body and a surface layer, wherein the main body has a composition within the following limits (wt. %):

[0013] C 0.25 - 0.45

[0014] Si 0.10 - 1.50

[0015] Mn 0.2 - 1.0

[0016] Cr 2 - 6

[0017] Mo 1 - 3

[0018] V 0.4 - 1.0

[0019] Al 0.001 - 0.06

[0020] N < 0.12 optionally one or more of Ni, Cu, Co, W and Nb in an amount of up to 1 % each, and optionally one or more of Zr, Ta, Ti, and B in an amount of up to 0.1 % each, preferably 0.05 % each, balance Fe apart from impurities.

[0021] The main body of the hot forming tool can be produced by any technique known in the art. The main body is then subjected to conventional hardening and tempering in order to give the main body the desired hardness. The hardness may be in the range of 40 - 55 HRC.

[0022] The main body is thereafter, partly or fully, provided with a surface layer by use of Laser Metal Deposition (LMD), wherein a laser beam is focused onto the surface of the main body, where a melt pool is created. Pre-alloyed metal powder having the desired composition is simultaneously injected into the melt pool under a protective atmosphere, thereby forming a strong metallurgical bond between the added material and the main body. The rapid solidification results in a high hardness of the deposited material, while the mechanical properties of the main body remain largely unaffected. The Laser Metal Deposition (LMD) may be performed by the extreme speed laser cladding technology (EHLC) also known as Ultra High Speed Laser Cladding (UHSLC), wherein the coating thickness is 10 to 500 pm, preferably 10 to 250 p which result s in a low dilution rate and a very fine and dense microstructure, which only requires grinding and / or polishing as post treatment.

[0023] The vanadium alloyed matrix tool steel used for the laser metal deposition (LMD) in the present invention has the following composition in weight % (wt. %):

[0024] C 0.27 - 0.40

[0025] Si 0.10 - 0.35

[0026] Mn 0.2 - 0.8

[0027] Cr 4.0 - 6.0

[0028] Mo 2.0 - 3.0

[0029] V 0.4 - 1.0

[0030] Al 0.001 - 0.06

[0031] N 0.003 - 0.12 optionally one or more of Ni, Cu, Co, W and Nb in an amount of up to 1 % each and optionally one or more of Zr, Ta, Ti and B in an amount of up to 0.1 % each, balance Fe apart from impurities.

[0032] The composition of the main body and the surface layer may according to one embodiment have the same nominal composition within the limits set out above. Preferably, the amounts of C, Cr, Mo and V in the laser metal deposited surface layer and in the main body do not differ from each other with not more than 15 %.

[0033] According to another embodiment, the main body is a low chromium steel containing only 2.3 to 2.9 % Cr in order to obtain a thermal conductivity of more than 30 W / mK at 600 °C. A preferred composition also fulfils at least one of the following limits (wt. %):

[0034] C 0.34 - 0.42

[0035] Si 0.10 - 0.50

[0036] Mn 0.5 - 1.0

[0037] Cr 2.3 - 2.9

[0038] Mo 2.0 - 2.5

[0039] V 0.7 - 1.0

[0040] Al 0.001 - 0.06

[0041] N < 0.02

[0042] B 0.001 - 0.005 optionally one or more of Ni, Cu, Co, W and Nb in an amount of up to 1 % each, and optionally one or more of Zr, Ta, and Ti in an amount of up to 0.05 % each, balance Fe apart from impurities.

[0043] The particle size distribution (PSD) of the feedstock for conventional LMD is such that at least 95 vol. % of all particles has a size of 20 - 150 pm, preferably have more than 80 vol. % of all particles have a size of 50 - 150 pm. In a preferred embodiment, the pre-alloyed powder fulfils at least one of the following requirements:

[0044] D10 < 50 pm D50 70 +15 pm D90 < 130 |l

[0045] However, the particle size distribution (PSD) of the feedstock for (EHLC) or (UHSLC) may be much finer.

[0046] The particle size distribution was analysed using Camsizer XT equipment.

[0047] The deposition is repeated layer by layer until the desired thickness has been reached. The hot forming tool may thereafter be tempered twice at a temperature of 540 °C for 2h, in order to reduce the amount of retained austenite to below the X-ray detection limit of 2 vol. %.

[0048] 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. %). Upper and lower limits of the individual elements can be freely combined within the limits set out in the claims.

[0049] COMPOSITION OF THE LMD-POWDER

[0050] The LMD-powder used fulfils the following requirements:

[0051] Carbon (0.27 - 0.40 %)

[0052] Carbon is to be present in a minimum content of 0.27 %, preferably at least 0.28, 0.29, 0.30, 0.31, 0.32, 0.33 or 0.34 %. The upper limit for carbon is 0.40% and may be set to 0.39, 0.38, 0.37, 0.36 or 0.35 %. Preferred ranges are 0.30 - 0.38 % and 0.33 - 0. 37 %. In any case, the amount of carbon should be controlled such that the amount of primary carbides of the type M23C6, M7C3 and MeC in the steel is limited, preferably the steel is free from such primary carbides.

[0053] Silicon (0.10 - 0.35 %)

[0054] Silicon is used for deoxidation. Si is present in the steel in a dissolved form. Si is a strong ferrite former and increases the carbon activity and therefore the risk for the formation of undesired carbides, which negatively affect the impact strength. Silicon is also prone to interfacial segregation, which may result in decreased toughness and thermal fatigue resistance. Si is therefore limited to 0.35%. The upper limit may be 0.34, 0.32, 0.30, 0.28, 0.26, 0.24 and 0.22 %. The lower limit may be 0.12, 0.14, 0.16, 0.18 and 0.20 %. Preferred ranges are 0.10 - 0.25 % and 0.15 - 0.24 %.

[0055] Manganese (0.2 - 0.8 %)

[0056] Manganese contributes to improving the hardenability of the steel and together with sulphur manganese contributes to improving the machinability by forming manganese sulphides.

[0057] Manganese shall therefore be present in a minimum content of 0.2 %, preferably at least 0.3, 0.35, 0.4, 0.45 or 0.5 %. At higher sulphur contents manganese prevents red brittleness in the steel. The steel shall contain maximum 0.8 %, preferably maximum 0.7, 0.6, 0.55 or 0.5 %.

[0058] Chromium (4.0 - 6.0 %)

[0059] Chromium is to be present in a content of at least 4.0 % 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 lower limit may be 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8 or 4.9 %. The upper limit may be 6.0, 5.8, 5.6, 5.5, 5.4, 5.2 or 5.1 %.

[0060] Molybdenum (2.0 - 3.0 %)

[0061] 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 2.0 %, and may be set to 2.1, 2.2, 2.25 or 2.3 %. Molybdenum is a strong carbide forming element and also a strong ferrite former. The maximum content of molybdenum is therefore 3.0 %. Preferably Mo is limited to 2.9, 2.8, 2.7, 2.6, 2.5, 2.4 or 2.35 %. Vanadium (0.4 - 1.0 %)

[0062] Vanadium forms evenly distributed primary precipitated carbides and carbonitrides of the type V(N,C) in the matrix of the steel. This hard phase may also be denoted MX, wherein M is mainly V but Cr and Mo may be present and X is one or more of C, N and B. Vanadium shall therefore be present in an amount of 0.4 - 1.0 %. The upper limit may be set to 0.9, 0.8, 0.7, 0.6, 0.58, 0.56 or 0.55 %. The lower limit may be 0.42, 0.44, 0.46, 0.48, 0.50 or 0.52 %. A preferred range is 0.4 - 0.6.

[0063] Aluminium (0.001 - 0.06 %)

[0064] Aluminium is used for deoxidation in combination with Si and Mn. The lower limit is set to 0.001, 0.003, 0.005 or 0.007 % in order to ensure a good deoxidation. The upper limit is restricted to 0.06 % for avoiding precipitation of undesired phases such as AIN. The upper limit may be set to 0.05, 0.04, 0.03, 0.02 or 0.015 %.

[0065] Nitrogen (0.003 - 0.12 %)

[0066] Nitrogen is restricted to 0.003 - 0.08 % in order to obtain the desired type and amount of hard phases, in particular V(C,N). When the nitrogen content is properly balanced against the vanadium content, vanadium rich carbonitrides V(C,N) will form. These will be partly dissolved during the austenitizing step and then precipitated during the tempering step as particles of nanometer size. The thermal stability of vanadium carbonitrides is considered to be better than that of vanadium carbides, hence the tempering resistance of the tool steel may be improved and the resistance against grain growth at high austenitizing temperatures is enhanced. The lower limit may be 0.003, 0.004, 0.007, 0.011, 0.012, 0.013, 0.014, 0.015, 0.016 or 0.017 %. The upper limit may be 0.12, 0.10, 0.08, 0.07, 0.06, 0.05, 0.04 or 0.03 %.

[0067] Nickel (< 1 %)

[0068] Nickel may be present in an amount up to 1 %. It gives the steel a good hardenability and toughness. However, because of the expense, the nickel content of the steel should be limited. The upper limit may therefore be set to 0.8, 0.5 or 0.3 %. However, Ni is normally not deliberately added.

[0069] Copper (< 1.0%)

[0070] Cu is an optional element, which may contribute to increasing the hardness and the corrosion resistance of the steel. If used, the preferred range is 0.02 - 1 %. However, it is not possible to extract copper from the steel once it has been added. This drastically makes the scrap handling more difficult. For this reason, copper is normally not deliberately added.

[0071] Cobalt (< 1 %)

[0072] Co is an optional element. Co causes the solidus temperature to increase and therefore provides an opportunity to raises the hardening temperature, which may be 15 - 30 °C higher than without Co. During austenitization it is therefore possible to dissolve larger fraction of carbides and thereby enhance the hardenability. Co also increases the Mstemperature. However, large amount of Co may result in a decreased toughness and wear resistance. The maximum amount is 1. However, for practical reasons, such as scrap handling, deliberate additions of Co is not made. The maximum impurity content may then be set to 0.3 %, 0.2 % or 0.1 %.

[0073] Tungsten (< 1 %)

[0074] In principle, molybdenum may be replaced by twice as much with tungsten because of their chemical similarities. However, tungsten is expensive and it also complicates the handling of scrap metal. The maximum amount is therefore limited to 1 %, preferably 0.5 %, more preferably 0.3 % and most preferably no deliberate additions are made.

[0075] Niobium (< 1 %)

[0076] Niobium is similar to vanadium in that it forms carbonitrides of the type M(N,C) and may in principle be used to replace part of the vanadium but that requires the double amount of niobium as compared to vanadium. However, Nb results in a more angular shape of the M(N,C). The maximum amount is 1%, preferably 0.1 %, more preferably 0.05 %, and most preferably no deliberate additions are made.

[0077] Ti, Zr and Ta

[0078] 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. The maximum amount is therefore 0.1 %, preferably 0.05 % and most preferably no deliberate additions are made.

[0079] Boron (< 0.1 %)

[0080] B may be used in order to further increase the hardness of the steel. The amount is limited to. 0.1%, preferably 0.05, more preferably 0.01 %, even more preferably 0.005 %, most preferably < 0.0035 %. If used, a preferred range for the addition of B is 0.001 - 0.004 %.

[0081] Impurity elements

[0082] P, S and O are the main impurities, which have a negative effect on the mechanical properties of the steel. P may therefore be limited to 0.03%, preferably to 0.01%. S may be limited to 0.03, 0.01, 0.003, 0.001, 0.0008, 0.0005 or even 0.0001%. O may be limited to 0.015, 0.012, 0.010 or 0.008 %.

[0083] COMPOSITION OF THE MAIN BODY

[0084] The main body is a hot work tool steel and the influence of the alloying elements are basically the same as for the LMD-powder. However, broader limits may be tolerated for the main body, fulfils the requirements:

[0085] Carbon (0.25 - 0.45 %)

[0086] Carbon is to be present in a minimum content of 0.25 %, preferably at least, 0.26, 0.27, 0.28, 0.29, 0.30, 0.31, 0.32, 0.33 or 0.34 %. The upper limit for carbon is 0.45 % and may be set to 0.44, 0.43, 0.42, 041, 040, 0.39, 0.38, 0.37, 0.36 or 0.35%. Preferred ranges are 0.30 - 0.38 % and 0.33 - 0. 37 %. In any case, the amount of carbon should be controlled such that the amount of primary carbides of the type M23C6, M7C3 and MeC in the steel is limited, preferably the steel is free from such primary carbides.

[0087] Silicon (0.10 - 1.5 %)

[0088] Silicon is used for deoxidation and provide solid solution strengthening. Si is present in the steel in a dissolved form. Si is therefore limited to 1.5 %. The upper limit may be 1.4, 1.3, 1.1, 1.0, 0.8, 0.50, 0.28, 0.26, 0.25, 0.24 and 0.22 %.

[0089] Manganese (0.2 - 1.0 %)

[0090] 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 0.3, 0.35, 0.4, 0.45 or 0.5 %. At higher sulphur contents manganese prevents red brittleness in the steel. The steel shall contain maximum 1.0 %, preferably maximum 0.9, 0.8, 0.7, 0.6, 0.55 or 0.5 %.

[0091] Chromium (2 - 6 %)

[0092] Chromium is to be present in a content of at least 2 % in order to provide a sufficient hardenability in combination with al high thermal conductivity. If the chromium content is too high, this may lead to the formation of high-temperature ferrite, which reduces the hot- workability. The lower limit may be 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4.0, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8 or 4.9 %. The upper limit may be 6.0, 5.8, 5.6, 5.5, 5.4, 5.2 or 5.1 %. For high thermal conductivity, the upper limit may be further reduced to 3 %, preferably 2.9 %.

[0093] Molybdenum (1 - 3 %)

[0094] 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 1 %, and may be set to 1.5, 2.0, 2.1, 2.2, 2.25 or 2.3 %. Molybdenum is a strong carbide forming element and also a strong ferrite former. The maximum content of molybdenum is therefore 3.0 %. Preferably Mo is limited to 2.9, 2.8, 2.7, 2.6, 2.5, 2.4 or 2.35 %.

[0095] Vanadium (0.4 - 1.0 %)

[0096] Vanadium forms evenly distributed primary precipitated carbides and carbonitrides of the type V(N,C) in the matrix of the steel. The upper limit may be set to 0.9, 0.8, 0.7, 0.6, 0.58, 0.56 or 0.55 %. The lower limit may be 0.42, 0.44, 0.46, 0.48, 0.50 or 0.52 %.

[0097] Aluminium (0.001 - 0.06 %)

[0098] Aluminium is used for deoxidation in combination with Si and Mn. The lower limit is set to 0.001, 0.003, 0.005 or 0.007 % in order to ensure a good deoxidation. The upper limit is restricted to 0.06 % for avoiding precipitation of undesired phases such as AIN. The upper limit may be 0.05, 0.04, 0.03, 0.02 or 0.015 %.

[0099] Nitrogen (< 0.12 %)

[0100] Nitrogen is restricted to 0.12 % in order to obtain a low amount of hard phases, in particular V(C,N) in the base material. The upper limit may be set to 0.12, 0.10, 0.07, 0.05, 0.04, 0.03, 0.02, 0.015 or 0.010 %. Nickel (< 1%)

[0101] Nickel may be present in an amount up to 1 %. It gives the steel a good hardenability and toughness. However, because of the expense, the nickel content of the steel should be limited. The upper limit may therefore be set to 0.8, 0.5 or 0.3 %. However, Ni is normally not deliberately added.

[0102] Copper (< 1.0 %)

[0103] Cu is an optional element, which may contribute to increasing the hardness and the corrosion resistance of the steel. If used, the preferred range is 0.02 - 1%. However, it is not possible to extract copper from the steel once it has been added. This drastically makes the scrap handling more difficult. For this reason, copper is normally not deliberately added. The maximum impurity content may then be set to 0.8, 0.7, 0.6, 0.6, 0.5, 0.4, 0.3, 0.2, or 0.1 %.

[0104] Cobalt (< 1 %)

[0105] Co is an optional element. Co causes the solidus temperature to increase and therefore provides an opportunity to raises the hardening temperature, which may be 15 - 30 °C higher than without Co. During austenitization it is therefore possible to dissolve larger fraction of carbides and thereby enhance the hardenability. Co also increases the Mstemperature. However, large amount of Co may result in a decreased toughness and wear resistance. The maximum amount is 1. However, for practical reasons, such as scrap handling, deliberate additions of Co are not made. The maximum impurity content may then be set to 0.3, 0.2, or 0.1 %.

[0106] Tungsten (< 1 %)

[0107] In principle, molybdenum may be replaced by twice as much with tungsten because of their chemical similarities. However, tungsten is expensive and it also complicates the handling of scrap metal. The maximum amount is therefore limited to 1 %, preferably 0.5 %, more preferably 0.3 %, and most preferably no deliberate additions are made.

[0108] Niobium (< 1 %)

[0109] Niobium is similar to vanadium in that it forms carbonitrides of the type M(N,C) and may in principle be used to replace part of the vanadium but that requires the double amount of niobium as compared to vanadium. However, Nb results in a more angular shape of the M(N,C). The maximum amount is 1%, preferably 0.1 %, more preferably 0.05 %, and most preferably no deliberate additions are made.

[0110] Ti, Zr and Ta

[0111] 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. The maximum amount is therefore 0.1 %, preferably 0.05 % and most preferably no deliberate additions are made.

[0112] Boron (< 0.1 %)

[0113] B may be used in order to further increase the hardness of the steel. The amount is limited to.

[0114] O.1%, preferably 0.05, more preferably 0.01 %, even more preferably 0.005 %, most preferably < 0.0035 %. If used, a preferred range for the addition of B is 0.001 - 0.004 %.

[0115] Impurity elements

[0116] P, S and O are the main impurities, which have a negative effect on the mechanical properties of the steel. P may therefore be limited to 0.03 %, preferably to 0.01 %. S may be limited to 0.003, 0.001, 0.0008, 0.0005 or even 0.0001 %. O may be limited to 0.015, 0.012, 0.010 or 0.008 %.

[0117] EXAMPLE 1

[0118] A hot forming tool for press hardening tool as defined in claim 1 was produced in accordance with the method of claim 7.

[0119] The main body was heat treated to a hardness of 52 HRC and had the following composition:

[0120] C 0.36

[0121] Si 0.19

[0122] Mn 0.46

[0123] Cr 5.14

[0124] Mo 2.27

[0125] V 0.54

[0126] Al 0.029 N 0.006

[0127] The pre-alloyed powder used had a mean size of 67 pm and a tap density of 4.2 g / cm3. The LMD -powder had the following composition:

[0128] C 0.35

[0129] Si 0.18

[0130] Mn 0.46

[0131] Cr 5.09

[0132] Mo 2.26

[0133] V 0.53

[0134] Al 0.008

[0135] N 0.01

[0136] The main body was preheated to 400 °C, before applying the pre-alloyed powder by a conventional LMD-process to a thickness of 2.4 mm. The thus coated main body was tempered twice at a temperature of 540 °C (2x2h). The LMD-coated surface was then milled to a final thickness of 1 mm. The hardness of the machined surface layer was 57 HRC.

[0137] The amount of retained austenite in the surface layer was found to be below the detection limit of the X-ray diffraction, which was measured in accordance with the standard practice method set out in ASTM E975-13.

[0138] The combination of a main body having a hardness of 52 HRC and an outermost surface layer having a hardness of 57 HRC resulted in a hot forming tool having a unique combination of toughness and hardness and thus being very suitable for press hardening of high strength steels such as AHSS and UHSS.

[0139] EXAMPLE 2

[0140] A hot forming tool for press hardening tool as defined in claim 1 was produced in accordance with the method of claim 5.

[0141] The main body had the following composition in weight %: C 0.38

[0142] Si 0.28

[0143] Mn 0.75

[0144] Cr 2.6

[0145] Mo 2.25

[0146] V 0.84

[0147] Al 0.021

[0148] B 0.0025

[0149] N 0.005

[0150] The main body was austenitized at 1020 °C for 30 minutes in a vacuum furnace and quenched by high pressure gas which resulted in a hardness of 51 HRC. The steel was then tempered twice at 645 °C to a hardness of 45 HRC. The thermal conductivity of the steel of the main body was measured at temperatures of 400 °C and 600 °C and it was found to be to be 33 W / mK at both temperatures.

[0151] The pre-alloyed powder for the LMD used had a mean size of 67 pm and a tap density of 4.2 g / cm3. The LMD-powder had the following composition:

[0152] C 0.35

[0153] Si 0.18

[0154] Mn 0.46

[0155] Cr 5.09

[0156] Mo 2.26

[0157] V 0.53

[0158] Al 0.008

[0159] N 0.01

[0160] The main body was preheated to 400 °C, before applying the pre-alloyed powder by a conventional LMD-process to a thickness of 2.4 mm. The thus coated main body was tempered twice at a temperature of 540 °C (2x2h). The LMD-coated surface was then milled to a final thickness of 1 mm. The hardness of the machined surface layer was 57 HRC. The amount of retained austenite in the surface layer was found to be below the detection limit of the X-ray diffraction, which was measured in accordance with the standard practice method set out in ASTM E975-13. The combination of a main body having a hardness of 45 HRC and a high thermal conductivity at working temperatures and an outermost surface layer having a hardness of 57 HRC resulted in a hot forming tool having a unique combination of toughness and hardness and thus being very suitable for press hardening of high strength steels such as AHSS and UHSS.

[0161] INDUSTRIAL APPLICABILITY

[0162] The hot forming tool for press hardening of the present invention is particular useful for stamping high strength steels.

Claims

CLAIMS1. A hot forming tool for press hardening, wherein the tool comprises a main body and, partly or fully of a laser metal deposited surface layer, the main body consist of a tool steel alloy having the following composition in weight %:C 0.25-0.45Si 0.10-1.50Mn 0.2 -1.0Cr 2-6Mo 1-3V 0.4 -1.0Al 0.001-0.06N <0.12 optionally one or more of Ni, Cu, Co, W, and Nb in an amount of up to 1 % each, and optionally one or more of Zr, Ta, Ti, and B in an amount of up to 0.1 % each, balance Fe apart from impurities, wherein the laser metal deposited surface layer has the following composition in weight %:C 0.27 - 0.40Si 0.10-0.35Mn 0.2 -0.8Cr 4.0 -6.0Mo 2.0 -3.0V 0.4 -1.0Al 0.001-0.06N 0.003-0.12 optionally one or more of Ni, Cu, Co, W and Nb in an amount of up to 1 % each, and optionally one or more of Zr, Ta, Ti and B in an amount of up to 0.1 % each, balance Fe apart from impurities,and wherein the laser metal deposited surface layer has a hardness of 53-59HRC.

2. A hot forming tool for press hardening as defined in claim 1, wherein the surface layer has a thickness of 0.3 - 29 mm, preferably 1 - 3 mm.

3. A hot forming tool for press hardening as defined in claim 1 or 2, wherein the laser metal deposited surface layer contains no more than 0.8 volume % carbides larger than 1 pm, preferably less than 0.5 volume % carbides larger than 0.8 pm.

4. A hot forming tool for press hardening as defined in any of the preceding claims wherein composition of the main body fulfils at least one of the following requirements:C 0.27 - 0.40Si 0.10 - 0.35Mn 0.2 - 0.8Cr 4.0 - 6.0Ni < 0.8Mo 2.0 - 3.0V 0.4 - 0.6Al 0.001 - 0.06N 0.003 - 0.12 and / or the main body has a hardness of 44 - 54 HRC, preferably 51 - 53 HRC.

5. A hot forming tool for press hardening as defined in any of claims 1 to 3, wherein composition of the main body fulfils at least one of the following requirements:C 0.34 - 0.42Si 0.10- 0.50Mn 0.5 - 1.0Cr 2.3 - 2.9Mo 2.0 - 2.5V 0.7 - 1.0Al 0.001 - 0.06N < 0.02B 0.001 - 0.0056. A hot forming tool for press hardening as defined in claim 5, wherein the main body has a hardness of 42 - 47 HRC and / or a thermal conductivity at 600 °C of at least 32 W / mK.

7. A method of producing a hot forming tool for press hardening as defined in claim 1, comprising the steps of: providing a main body having a composition as defined in claim 1, 4 or 5, providing a pre-alloyed powder having a grain size of 20 - 150 pm with the following composition in weight %:C 0.27 - 0.40Si 0.10 - 0.35Mn 0.2 - 0.8Cr 4.0 - 6.0Mo 2.0 - 3.0V 0.4 - 1.0Al 0.001 - 0.06N 0.003 - 0.12 optionally one or more of Ni, Cu, Co, W and Nb in an amount of up to 1 % each, and optionally one or more of Zr, Ta, Ti and B in an amount of up to 0.1 % each, balance Fe apart from impurities, applying the pre-alloyed powder to the surface of the main body by laser metal deposition to a pre-determined layer thickness, tempering the thus obtained hot forming tool twice at a temperature of 500 - 650°C for at least Ih in order to reduce or eliminate any retained austenite, andmachining the tempered surface to a final thickness thereby obtaining a hot forming tool having a hardness of 53 - 59 HRC in the surface layer.

8. A method of producing a hot forming tool for press hardening as defined in claim 7, wherein the method further comprising pre-heating the main body to a temperature of 300 °C to 500 °C before the laser metal deposition.

9. A method of producing a hot forming tool for press hardening as defined in claim 7 or8, wherein the pre-alloyed powder fulfils at least one of the following requirements:DIO < 50 p D50 70 ±15 pm D90 < 130 p10. A method of producing a hot forming tool for press hardening as defined in any of claims 7 to 9, wherein the as-deposited surface layer has a thickness of 1 - 30 mm and wherein the thickness of the machined surface layer is 0.3 - 29 mm.