Hot working tool steel

Abstract

The present invention relates to a matrix-type hot working tool steel which has improved wear resistance in demanding applications during use. It is particularly suitable for applications in hot forging, die casting or hot extrusion. It should also be suitable for press hardening, in particular for press hardening of advanced high strength steels (AHSS), and have a high hot wear resistance. The hot working tool steel according to the invention has a composition as defined by the independent claim.

Description

Technical Field

[0001] The present invention relates to matrix type hot work tool steel. Background Art

[0002] Vanadium alloy matrix tool steels have been on the market for decades and have attracted considerable interest due to the fact that they combine high wear resistance with excellent dimensional stability and good toughness. Matrix tool steels are steels that do not contain any primary carbides or only contain very low amounts of small primary carbides and have a matrix consisting of tempered martensite.

[0003] US 3117863 is probably the first patent for a matrix steel. The basic idea of ​​US 3117863 is to produce a steel with the composition of a known high speed steel (HSS) matrix. The structure of this type of steel was developed to improve the toughness and fatigue strength of the steel by refining the microstructure.

[0004] WO 03 / 106727 A1 of the present applicant discloses a hot working base steel having excellent toughness and ductility as well as good hot strength and wear resistance. This material is marketed under the name And well-known.

[0005] EP 1 300 482 A1 discloses another base steel having high hardness and wear resistance and very high toughness and is therefore particularly suitable for tools subjected to stress at high temperatures, such as tools for hot and warm forming. This steel is marketed under the name W360 is well known and has a nominal composition of 0.50%C, 0.20%Si, 0.25%Mn, 4.5%Cr, 3.00%Mo and 0.60%V.

[0006] The base steel is usually produced by vacuum arc remelting (VAR) or electroslag remelting (ESR) to improve chemical homogeneity and microscopic cleanliness.

[0007] Other examples of hot working tool base steels are given in JP2003226939A, EP3050986A1, US2004 / 0187972A1 and US2005 / 0161125A1.

[0008] Modern matrix steels are being developed with the aid of software for calculating phase diagrams and equilibrium phase equilibria as a function of temperature. (TC) is a user-friendly and frequently used software for this purpose, in order to find the composition that leads to a large austenite single phase area at the soaking temperature because of the fact that the dissolution of possible MC carbides formed by segregation during casting is of paramount importance.

[0009] Hot working base steels have a wide range of applications, such as die casting and forging. Steels are usually produced by conventional metallurgy followed by electroslag remelting (ESR). However, a disadvantage of known steels is limited wear resistance. In particular, wear resistance can limit the life of known steels in harsh hot working operations such as hot forging, extrusion and press hardening. These tools are expensive and often require welding for repair. Therefore, weldability is important. However, the weldability of tool steels with high carbon content is generally considered to be poor and requires special measures, such as high preheating temperatures. Therefore, it would be useful if the steel could be welded with standard welding consumables (preferably without preheating). Summary of the invention

[0010] The object of the present invention is to provide a matrix type hot working tool steel which has improved wear resistance in demanding applications during use. In particular, the steel should be suitable for applications in hot forging, die casting or hot extrusion. It should also be suitable for press hardening, in particular for press hardening of advanced high strength steels (AHSS). For these applications, the hot wear resistance must be high.

[0011] Tempering resistance is an important property because in use, the steel may be subjected to high temperatures for long periods of time. Therefore, it is preferred that the steel not only has a high hardness after hardening, but also has a small hardness reduction. Other important properties include high ductility and toughness, which means that the steel should have a high cleanliness with respect to micro-slag, a complete absence of grain boundary carbides, and a uniform hardness for thicknesses up to 300 mm.

[0012] It should be possible to adjust the hardness over large intervals in order to optimize the steel for the intended use. It should also be possible to obtain high tensile and yield strengths combined with sufficient ductility.

[0013] The foregoing objects as well as further advantages are achieved to a large extent by providing a hot working tool steel having a composition as set forth in the claims.

[0014] The invention is defined in the claims. DETAILED DESCRIPTION

[0015] The importance of the individual elements and their interaction with each other as well as the limitations of the chemical composition of the claimed alloys are briefly explained below. All percentages of the chemical composition of the steel are given throughout the specification in weight % (wt.%). The amounts of hard phases are given in volume % (vol.%). The upper and lower limits of the individual elements can be freely combined within the limits set out in the claims. The arithmetic precision of the values ​​can be increased by one or two digits. Thus, a value given as, for example, 0.1% can also be expressed as 0.10% or 0.100%.

[0016] Carbon (0.5–0.9%)

[0017] It is present at a minimum content of 0.5%, preferably at least 0.55, 0.60, 0.66, 0.67 or 0.68%. The upper limit of carbon is 0.9%, and can be set at 0.85, 0.80, 0.75, 0.74, 0.73 or 0.72%. The preferred range is 0.6-0.8% and 0.65-0.75%. In any case, the amount of carbon should be controlled so as to limit the M content in the steel. 23 Amounts of primary carbides of the C6, M7C3 and M6C types, preferably the steel contains no such primary carbides.

[0018] Silicon (0.03–0.8%)

[0019] Silicon is used for deoxidation. Si is present in steel in dissolved form. Si is a strong ferrite former and increases carbon activity and therefore increases the risk of forming undesirable carbides, which adversely affects impact strength. Silicon also tends to segregate at the interface, which may lead to reduced toughness and thermal fatigue resistance. Therefore, Si is limited to 0.8%. The upper limit can be 0.7, 0.6, 0.5, 0.40, 0.35, 0.30, 0.28, 0.27, 0.26, 0.25, 0.24, 0.23 and 0.22%. The lower limit can be 0.05, 0.10, 0.11, 0.12, 0.13, 0.14 or 0.15%.

[0020] Manganese (0.1–1.8%)

[0021] Manganese helps improve the hardenability of the steel and helps improve machinability by forming manganese sulfides together with sulfur manganese. Therefore, manganese should be present in a minimum content of 0.1%, preferably at least 0.2, 0.3, 0.35 or 0.4%. At higher sulfur contents, manganese prevents red brittleness in the steel. Mn also causes undesirable microsegregation, resulting in a banded structure. The steel should contain a maximum of 1.8%, preferably a maximum of 0.8, 0.75, 0.7, 0.6, 0.55 or 0.5%.

[0022] Chromium (4.0–6.6%)

[0023] Chromium will be present in an amount of at least 4% to provide good hardenability in larger sections during heat treatment. If the chromium content is too high, this can lead to the formation of high temperature ferrites, which reduce hot workability. The lower limit may be 4.5, 4.6, 4.7, 4.8 or 4.9%. The upper limit may be 6.0, 5.9, 5.8, 5.7, 5.6, 5.5, 5.4, 5.3, 5.2 or 5.1%.

[0024] Molybdenum (1.8–3.5%)

[0025] Mo is known to have a very favorable effect on hardenability. Mo is essential for achieving a good secondary hardening response. The minimum content is 1.8% and can be set to 1.9, 2.0, 2.1, 2.15 or 2.2%. Mo is a strong carbide former and is also a strong ferrite former. Therefore, the maximum content of Mo is 3.5%. Mo can be limited to 2.9, 2.7, 2.6, 2.5, 2.4 or 2.3%.

[0026] Tungsten (W≤0.5%)

[0027] Tungsten is not an essential element in the present invention. The upper limit is 0.5%, and can be set to 0.4, 0.3, 0.2 or 0.1%.

[0028] Nickel (≤1%)

[0029] Nickel is not an essential element in the present invention. The upper limit may be set to 0.5, 0.4, 0.3 or 0.25%.

[0030] Vanadium (1.3–2.3%)

[0031] In the matrix of steel, vanadium forms uniformly distributed primary precipitated carbides and carbonitrides of VC and V type (C, N). These carbides and carbonitrides can also be expressed as MX, where M is mainly V, but Cr and Mo may be present, and X is one or more of C, N and B. However, hereinafter, only VC is used in the same sense as MX. Vanadium is used to form a relatively large amount of VC in a controlled amount, and therefore should be present in an amount of 1.3%-2.3%. The lower limit can be set to 1.35, 1.4, 1.45, 1.5 or 1.55%. The upper limit can be set to 2.2, 2.1, 2.0, 1.9, 1.8, 1.7 or 1.65%.

[0032] Aluminum (≤0.1%)

[0033] Aluminum can be used in combination with Si and Mn for deoxidation. The lower limit is set to 0.001, 0.003, 0.005 or 0.007% to ensure good deoxidation. The upper limit is limited to 0.1% to avoid precipitation of undesirable phases such as AlN. The upper limit can be 0.05, 0.04 or 0.3%.

[0034] Nitrogen (≤0.12%)

[0035] Nitrogen is an optional element. N is limited to 0.12% to avoid too high a quantity of hard phases, especially V(C,N). However, the nitrogen content can be balanced with the vanadium content to form primary precipitated vanadium-rich carbonitrides. These will partially dissolve during the austenitizing step and then precipitate as nano-sized particles during the tempering step. The thermal stability of vanadium carbonitrides is considered to be better than that of vanadium carbides, so the tempering resistance of tool steels can be improved and the resistance to grain growth at high austenitizing temperatures can be enhanced. If the nitrogen content is deliberately controlled for the reasons mentioned above, the lower limit can be set to 0.006, 0.007, 0.08, 0.09, 0.01, 0.012, 0.013, 0.014 or 0.015%. The upper limit may be 0.11, 0.10, 0.09, 0.08, 0.07, 0.06, 0.05, 0.04 or 0.03%.

[0036] Copper (≤1%)

[0037] Cu is an optional element that can help increase the hardness and corrosion resistance of the steel. However, once copper is added, it cannot be extracted from the steel. This greatly makes waste disposal more difficult. For this reason, copper is not usually added intentionally. The upper limit can be limited to 0.5, 0.4, 0.3, 0.2 or 0.15%.

[0038] Cobalt (≤5%)

[0039] Co is an optional element. Co causes the solidus temperature to rise, and therefore provides an opportunity to increase the hardening temperature. Therefore, during austenitization, a larger portion of carbides can be dissolved, thereby enhancing hardenability. However, Co is expensive, and a large amount of Co can also lead to reduced toughness and wear resistance. Therefore, the maximum amount is 5%. However, deliberate addition of Co is usually not performed. The maximum content can be set to 2, 1, 0.5 or 0.2%.

[0040] Niobium (≤0.1%)

[0041] Niobium is similar to vanadium in that it forms carbonitrides of the M(N,C) type. However, Nb results in a more angular shape of the M(N,C) and can reduce hardenability at high levels. Therefore, the maximum amount is 0.1%, preferably 0.05%. Nb precipitates are more stable than V precipitates and can therefore be used for grain refinement, since the fine dispersion of NbC acts to pin the grain boundaries, resulting in grain refinement and improved toughness and improved resistance to softening at high temperatures. For this reason, Nb is an optional element and can be present in an amount of ≤0.1%. The upper limit can be set to 0.06, 0.05, 0.04, 0.03, 0.01 or 0.005%. The lower limit can be set to 0.005, 0.006, 0.007, 0.008, 0.009 or 0.01%.

[0042] Ti, Zr and Ta

[0043] These elements are carbide formers and may be present in the alloy in the claimed range to modify the composition of the hard phase. However, these elements are not usually added. The amount of each element is preferably ≤ 0.5%, 0.1% or ≤ 0.05%, preferably 0.01% or 0.005%.

[0044] Boron (≤0.01%)

[0045] B may be used to further increase the hardness of the steel. The amount is limited to 0.01%, preferably ≤ 0.006%, more preferably 0.005%.

[0046] Ca, Mg and REM (rare earth metals)

[0047] These elements can be added to the steel in the claimed amounts for modifying non-metallic inclusions and / or to further improve machinability, hot workability and / or weldability. The amount of Ca and Mg is preferably ≤ 0.01%, preferably ≤ 0.005%. The amount of REM is preferably ≤ 0.2%, preferably ≤ 0.1%, or even 0.05%.

[0048] Impurity Elements

[0049] Trash elements cannot be avoided during the manufacture of steel. Therefore, tramp elements are included in the balance, the levels of which are not essential to the definition of the present invention.

[0050] P, S and O are the main impurities, which generally have a negative impact on the mechanical properties of steel. These elements are unavoidable and can be present in steel at common impurity levels. However, since these elements can have a negative impact on the properties of steel, their impurity content can be further limited. The preferred limits are as follows. P can be limited to 0.1%, 0.05 or 0.03%. S can be limited to 0.5, 0.1, 0.05, 0.0015, 0.0010, 0.0008, 0.0005, or even 0.0001%. O can be limited to 0.01, 0.003, 0.0015, 0.0012, 0.0010, 0.0008, 0.0006 or 0.0005%.

[0051] Steel production

[0052] Tool steels having the claimed chemical composition may be produced by conventional metallurgy including smelting (melting) in an electric arc furnace (EAF) and further refining in a ladle, optionally followed by vacuum treatment before casting. The ingot may also be subjected to electroslag remelting (ESR) to further improve the cleanliness and microstructural homogeneity of the ingot. In addition, the steel may also be subjected to vacuum induction melting (VIM) and / or vacuum arc remelting (VAR). An alternative processing route for the claimed steel is gas atomization followed by hot isostatic pressing (HIP) to form a hot isostatically pressed (HIPed) ingot, which may also be used in the as-HIPed condition. The ingot may be subjected to further hot working to final size and soft annealing to a Brinell hardness of ≤360 HBW, preferably ≤300 HBW. Brinell hardness is measured with a 10mm diameter tungsten carbide ball and a load of 3000kgf (29400N) and can also be expressed as HBW 10 / 3000 Before use, the steel may undergo hardening and tempering.

[0053] The steel is usually delivered to the customer in a soft annealed condition which leaves the ferrite matrix with carbides uniformly distributed therein. For large sizes and according to a preferred embodiment, the soft annealed steel also has uniform properties, the uniformity of hardness should have an average hardness of ≤360 HBW, and for a thickness of at least 100 mm, and the maximum deviation from the average Brinell hardness value in the thickness direction measured according to ASTM E10-01 is less than 10%, preferably less than 5%, and wherein the minimum distance of the center of an indentation from the edge of the sample or the edge of another indentation should be at least 2.5 times the diameter of the indentation, and the maximum distance should not exceed 4 times the diameter of the indentation.

[0054] Atomized powders can also be used in additive manufacturing.

[0055] Herein, the present invention will be described in more detail.

[0056] The hot working steel according to the present invention consists of the following in weight % (wt. %):

[0057]

[0058]

[0059] The balance is iron and impurities.

[0060] Preferably, the hot working tool steel meets at least one of the following requirements:

[0061] More preferably, the composition of the steel meets one or more of the following requirements:

[0062]

[0063] Preferably, the steel meets at least one of the following requirements:

[0064]

[0065] In particularly preferred embodiments, all of these requirements are met.

[0066] To enhance wear resistance, the composition can be adjusted so that the steel in the hardened and tempered condition contains a small and controlled amount of vanadium carbides with a size greater than or equal to 1 μm. The size is given in terms of the equivalent circular diameter (ECD), which is calculated from the image area (A) obtained in the image analysis. The ECD has the same projected area as the particle and is equal to 2√(A / π).

[0067] The steel should preferably contain 0.2 - 4 vol. %, preferably 0.5 - 3 vol. % and more preferably 1.5 - 2.3 vol. % VC.

[0068] The amount of M6C and M7C3 should each be limited to 2 vol%, preferably 0.5 vol%, and more preferably 0.1 vol%.

[0069] By choosing the austenitizing time and temperature, the cooling rate (t 5 / 8 ) and the appropriate combination of tempering temperature, the hardness of the steel can be adjusted. Typically, the steel is tempered twice for two hours (2x2h) in order to reduce the amount of retained austenite to less than 2 volume %.

[0070] After hardening and tempering to a hardness of 55-57HRC, the mechanical properties of the steel should preferably meet at least one of the following requirements:

[0071] Yield strength (Rp0.2): ≥1700 MPa, preferably ≥1725 MPa, more preferably ≥1750 MPa.

[0072] Tensile strength (Rm): ≥1950 MPa, preferably ≥2050 MPa, more preferably ≥2050 MPa, most preferably ≥2100 MPa.

[0073] Elongation (A5): ≥3%, preferably ≥4, more preferably ≥5%, most preferably ≥6%.

[0074] Reduction of area (Z): ≥5%, preferably ≥10%, more preferably ≥15%, most preferably ≥20%.

[0075] Example 1

[0076] Table 1 discloses the hardness in Rockwell C (HRC) as a function of the hardening parameters austenitizing time and temperature. It can be seen that the hardness can be easily adjusted in the range of 49 to 61 HRC. The composition of the ESR ingot is as follows: C 0.71%, Si 0.22%, Mn 0.46%, Cr 5.01%, Mo 2.24%, V 1.62%, Al 0.007%.

[0077]

[0078] Table 1. Hardness (HRC) in the hardened and tempered condition. For all samples, cooling was carried out in vacuum at t8 / 5 = 300 s and tempering was carried out for 2 x 2 h.

[0079] The tempering resistance of steels austenitized at 1130°C and tempered at 580°C and 600°C, respectively, was examined. The steel samples were subjected to heating at 600°C for 10 hours. In the first case, the hardness decreased from 58.4HRC to 53.6HRC, and for the second sample, the hardness decreased from 55.9HRC to 52.8HRC. The hardness losses were therefore 4.8HRC and 3.1HRC, respectively.

[0080] These values ​​can be compared with the steel A sample of the steel having a nominal composition of C 0.5%, Si 0.2%, Mn 0.5%, Cr 5.0%, Mo 2.3% and V 0.5% was prepared. The steel was austenitized at 1050°C for 30 minutes with a temperature of t 8 / 5= 300s and tempering at 540°C for 2 x 2h, the steel was hardened to 57.8 HRC. The initial hardness was 57.8 HRC and the hardness after 10 hours at 600°C was 49.4 HRC. Therefore, the hardness loss of the known steel was 8.4 HRC. Therefore, it can be concluded that the steel according to the invention has a better tempering resistance than the known steel.

[0081] The cleanliness of the steel according to the invention with respect to fine slag was tested according to ASTM E45-97, Method A, Plate Ir and the results are given in Table 2.

[0082] A A B B C C D D T H T H T H T H 0.5 0 0.5 1.0 0 0 0.5 1.0

[0083] Table 2. Cleanliness of Plate Ir according to ASTM E45-97, Method A.

[0084] Example 2

[0085] The ESR ingot of Example 1 was hot rolled to a diameter of 196 mm, from which three samples were taken in the LC2 direction and the mechanical properties were examined. 8 / 5 = 300 seconds cooling in vacuum followed by tempering twice at 560°C for 2 hours hardened the steel sample to a hardness of 56 HRC. The following average values ​​of the examination are given below:

[0086] Yield strength (Rp0.2): 1761MPa

[0087] Tensile strength (Rm): 2117MPa

[0088] Elongation (A5): 7%

[0089] Sectional reduction (Z): 26%

[0090] Example 3

[0091] In this example, the steel according to the invention is compared with a standard base steel or forging tool used.

[0092] The alloy has the following composition (in weight %):

[0093]

[0094]

[0095] The balance is iron and impurities.

[0096] The alloy was subjected to standard heat treatment, forging and soft annealing to a hardness of about 300 HBW. Both steels were hardened and tempered by heating to 1100°C for 30 minutes, quenched and tempered twice (2x2h) at 540°C during two hours. The hardness of the invention steel was 57HRC and the hardness of the comparison steel was 56HRC. The wear resistance of the steel was checked by the Pin on Disk method using 800 mesh alumina paper from the same batch. It was found that the wear loss of the invention steel was 178 mg / min and the wear loss of the comparison steel was 219 mg / min.

[0097] Other samples of the inventive steel were prepared to obtain the same hardness as the comparative steel. This was achieved by heating to 1100°C for 30 minutes and tempering at 540°C for 2 x 2h. The hardness was 56HRC. As expected, the wear loss of this sample was slightly higher (189 mg / min) compared to the steel with a hardness of 57HRC, but significantly lower than the comparative steel with the same hardness.

[0098] Example 4

[0099] Steel samples of the same composition as in Example 1 were prepared for welding tests. Solid blocks of steel were milled to sharp 90° internal angles and the samples were subjected to two different hardening treatments. The first heat treatment consisted of austenitizing at 1050°C for 30 minutes and then 8 / 5 = 300 seconds cooling in vacuum followed by tempering twice at 560° C. for 2 hours. The second heat treatment differs in that austenitization is carried out at 1130° C. for 10 minutes.

[0100] The samples were then TIG welded using 1.6 mm diameter rods with three different standard welding consumables at room temperature (RT), 80° C., 225° C. and 325° C. The applicant has Caldie TIG and QRO 90 TIG and UTP A 696 TIG from Schweissmaterial GmbH.

[0101] At all temperatures, the consumable Caldie TIG experienced cracking. However, surprisingly, it was found that the other two consumables could be used to produce crack-free welds also at room temperature without cracking. Thus, the steel of the present invention has surprisingly good weldability.

[0102] Industrial Applicability

[0103] The steel of the present invention is useful for hot working applications where the tool is subject to wear. In particular, the steel is suitable as a tool for hot forging, press hardening, die casting, high pressure die casting or hot extrusion.

Claims

1. Hot working tool steel for hot forging, press hardening, die casting or hot extrusion, consisting of the following by weight (wt.%): The balance of iron and impurities, The steel contains carbides with a size of ≥1 μm and the amount of the carbides in volume % meets the following requirements: VC 0.2–4, Where the cleanliness meets the following maximum requirements with respect to micro-slag according to ASTM E45-97, Method A, Plate Ir:

2. The hot working tool steel according to claim 1, wherein the steel meets at least one of the following requirements:

3. The hot working tool steel according to claim 1 or 2, wherein the steel meets at least one of the following requirements:

4. The hot working tool steel according to claim 1 or 2, wherein the steel contains carbides with a size of ≥ 1 μm and satisfies at least one of the following requirements with respect to the amount of carbides in volume %: VC 0.5–4 M6C≤2 M7C3≤2.

5. The hot working tool steel according to claim 4, wherein the steel contains carbides with a size of ≥ 1 μm and satisfies at least one of the following requirements with respect to the amount of carbides in volume %: VC 0.5–3 M6C≤0.5 M7C3≤0.

5.

6. Hot working tool steel according to claim 5, wherein the steel contains carbides with a size of ≥ 1 μm and the amount of the carbides in volume % satisfies at least one of the following requirements: VC 1.5–2.3 M6C≤0.1 M7C3≤0.

1.

7. The hot working tool steel according to claim 1 or 2, wherein the steel after hardening and tempering has a hardness of 55-57 HRC, and wherein the steel meets at least one of the following requirements: Cleanliness meets the following maximum requirements for micro-residue according to ASTM E45-97, Method A, Plate Ir: 。 8. The hot working tool steel according to claim 1 or 2, wherein the steel meets at least one of the following requirements:

9. The hot working tool steel according to claim 1 or 2, wherein the steel meets at least one of the following requirements:

10. The steel according to claim 1 or 2, having an average hardness of ≤ 360 HBW, wherein the steel has a thickness of at least 100 mm and the maximum deviation from the average Brinell hardness value in the thickness direction measured according to ASTM E10-01 is less than 10%, preferably less than 5%, and wherein the minimum distance of the center of the notch from the edge of the sample or the edge of another notch should be at least 2.5 times the diameter of the notch, and the maximum distance should not exceed 4 times the diameter of the notch.

11. Use of the steel according to any of the preceding claims as a tool for hot forging, press hardening, die casting, high pressure die casting or hot extrusion.

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