Powder metallurgy tool steel

Abstract

A powder metallurgy steel suitable for cold working, having a chemical composition comprising (in weight percent) the following: C, 0.9-1.3; 0.2 to 0.8 part of Si; 0.1 to 0.6 part of Mn; s, equal to or less than 0.2; 3.0 to 5.5 parts of Cr; 1.8 to 3.5 parts of Mo; 1.8 to 4.0 parts of W; v, the number of the components ranges from 1.3 to 2.5; 1.3 to 2.5 parts of Nb; n is equal to or less than 0.2; co equal to or less than 3.0; ni equal to or less than 1.0; cu equal to or less than 1.0; the balance being Fe and a total of equal to or less than 1.5 wt.-% of any impurities.

Description

Technical Field

[0001] The present disclosure generally relates to a powder metallurgy tool steel suitable for cold working applications. The present disclosure also relates to a method for manufacturing a powder metallurgy tool steel. The present disclosure also relates to tools comprising powder metallurgy steel.

[0002] Background

[0003] Cold working tool steels are steels used for producing tools that are exposed to surface temperatures of up to about 200 °C, i.e., for cold working applications. Examples of such tools include, for example, tools for stamping, blanking, powder compaction or cold extrusion. Cold working tool steels should have sufficient hardness, toughness and wear resistance for the intended cold working applications.

[0004] ASP 2005 is a powder metallurgy high speed steel suitable for cold working applications provided by Erasteel. The steel has a nominal composition comprising 1.5% C, 4.0% Cr, 2.5% Mo, 2.5% W and 4.0% V. In the hardened and tempered state, the steel has a hardness of about 60 - 64 HRC as well as good toughness and wear resistance. Although the steel machines very well in many cold working applications, it would be advantageous to further improve the toughness at a given hardness.

[0005] Another example of a powder metallurgy high speed steel suitable for cold working applications is also provided by Erasteel ASP 2012. The steel has a nominal composition comprising 0.60% C, 1.0% Si, 0.3% Mn, 4.0% Cr, 2.0% Mo, 2.1% W and 1.5% V. Compared with ASP 2005, ASP 2012 has higher toughness but lower hardness, typically about 56 - 59 HRC. In addition, the wear resistance is slightly lower compared to the wear resistance of ASP 2005.

[0006] Yet another example of a powder metallurgy high speed steel provided by Erasteel and suitable for cold working applications is ASP 2023. The steel has a nominal composition comprising 1.28% C, 4.1% Cr, 5.0% Mo, 6.4% W and 3.1% V.

[0007] WO 03 / 000944 A1 discloses a cold-work steel having a chemical composition comprising 1.25% - 1.75% (C + N), 0.1% - 1.5% Si, 0.1% - 1.5% Mn, 4.0% - 5.5% Cr, 2.5% - 4.5% (Mo + W / 2), 3.0% - 4.5% (V + Nb / 2), and wherein the steel comprises at least 0.5% C, a maximum of 0.5% W and a maximum of 0.5% Nb. The steel is described as having a hardness of 54 - 66 HRC after hardening and tempering.

[0008] Overview

[0009] The object of the present invention is to provide a steel suitable for cold-working applications, which has a good balance between hardness and toughness and preferably also has wear resistance.

[0010] This object is achieved by the subject matter of the appended independent claims. Various embodiments are defined by the dependent claims.

[0011] According to the present disclosure, a powder metallurgy tool steel is provided. The powder metallurgy steel has a chemical composition comprising (by weight percentage):

[0012]

[0013] the balance Fe and any impurities in total equal to or less than 1.5 wt.-%.

[0014] The powder metallurgy tool steel described herein has a very good balance between hardness and toughness in the hardened and tempered state, which is the result of the chemical composition. More specifically, the toughness at a given hardness is improved by reducing the size and amount of carbides other than MC type. In addition, the toughness is further improved by a composition that results in small-sized MC carbides. Compared with previously known powder metallurgy tool steels, this is mainly achieved by a combination of a medium carbon content, a relatively low amount of molybdenum and tungsten, and a relatively high amount of niobium. The fact that the carbides have a small size also enables good edge strength, i.e., chipping resistance, and good resistance to galling and adhesive wear.

[0015] The powder metallurgy tool steel described herein can be hardened to a hardness above 62 HRC while still achieving good toughness.

[0016] The present disclosure also relates to the use of the powder metallurgy steel described herein for the production of tools suitable for cold-working applications.

[0017] The present disclosure also provides a method for manufacturing a steel, the method comprising:

[0018] a) Produce steel powder having the following chemical composition in wt.-%:

[0019]

[0020]

[0021] the balance Fe and any impurities in total equal to or less than 1.5 wt.-%;

[0022] b) Compact the steel powder, preferably wherein the compaction includes hot isostatic pressing;

[0023] c) Optionally, perform soft annealing and / or stress relieving on the compacted steel;

[0024] d) Harden by subjecting the steel to an austenitizing temperature equal to or higher than 1000 °C, followed by quenching;

[0025] e) Temper the hardened steel by subjecting it to a temperature between 520 °C and 600 °C.

[0026] In addition, the present disclosure provides a cold working tool comprising the powder metallurgy tool steel described above. The cold working tool can be, for example, a stamping tool, a blanking tool, a powder compaction tool, an extrusion tool, a roll or a cutting tool, but is not limited thereto. Brief Description of the Drawings

[0028] Figure 1 Schematically shows a round bar having a longitudinal axis,

[0029] Figure 2 representing an SEM image of steel F1 when hardened at 1150 °C and tempered at 560 °C,

[0030] Figure 3 representing an SEM image of steel F1 when hardened at 1180 °C and tempered at 560 °C,

[0031] Figure 4 representing an SEM image of steel F3 when hardened at 1150 °C and tempered at 560 °C,

[0032] Figure 5 Schematically shows the principle of the tribological tests performed, and

[0033] Figure 6 representing a graph showing the test results of the coefficient of friction versus the normal load during sliding of steel F3, 2005 and 2023 against a counterbody relative to 304 steel.

[0034] Detailed description

[0035] The present invention will be described in more detail below with reference to exemplary embodiments. However, the present invention is not limited to the exemplary embodiments discussed, but may vary within the scope of the appended claims.

[0036] When ranges are disclosed in the present disclosure, such ranges include the corresponding end values of the range unless otherwise expressly disclosed. Similarly, when an open range is disclosed, the open range also includes the individual end values of the open range unless otherwise expressly disclosed.

[0037] The present disclosure provides a powder metallurgy tool steel having a chemical composition consisting of the following in weight percentages:

[0038]

[0039] the balance Fe and any impurities in total equal to or less than 1.5 wt.-%.

[0040] The powder metallurgy tool steel described herein has mainly been developed for cold working applications. Cold working applications are herein intended to mean such applications where, during use as a tool or in a tool, the temperature of the steel is up to about 200 °C. However, it should be noted that the powder metallurgy tool steel described herein is also suitable for tool applications at moderate temperatures, typically up to about 550 °C. Examples of cold working tools for which the steel described herein can be suitably used include, but are not limited to, tools for stamping, tools for blanking, tools for pressing, tools for powder compaction, tools for press hardening, rolls and cutting tools (including industrial cutting tools). It can also be used, for example, in plastic injection molding machines or extrusion tools. In addition, it can be used in certain cutting tools where lower temperatures and adhesive wear can be expected, such as taps or other cutting tools for machining aluminum-based materials or some stainless steels.

[0041] The powder metallurgy tool steel described herein can also be used in applications other than tools, for example in components where a combination of good fatigue resistance and high hardness is required. Examples of such components include, but are not limited to, bearings, gears or engine parts (such as camshafts or injector parts).

[0042] In the following, the importance of the different alloying elements of powder metallurgy steels will be briefly discussed. Unless otherwise explicitly stated, all percentages of chemical composition are given in weight-% (wt.-%). Unless otherwise explicitly stated, the description of the various elements of the composition as preferred or suitable upper and lower limits can be freely combined within the broadest limits described. As already mentioned above, unless otherwise explicitly stated, any range specified herein includes the corresponding end values of that range.

[0043] Carbon (C): 0.9 wt.-%-1.3 wt.-%

[0044] Carbon is an essential element of the composition and contributes to both hardness and wear resistance. When dissolved in martensite, carbon contributes to the expected hardness in the hardened and tempered states of the steels described herein. In addition, carbon forms primary precipitated V / Nb-rich MC carbides, as well as primary precipitated Mo / W / Cr-rich M6C carbides. The primary precipitated carbides contribute to wear resistance and can also have the beneficial effect of restricting grain growth. Therefore, the steels described herein contain at least 0.9% C. Preferably, carbon is present in an amount of at least 0.95%. Suitably, carbon is present in an amount of at least 1.00%.

[0045] However, excessive carbon may lead to too high a content of M6C carbides. In addition, too high a content of carbon increases the risk of coarse carbides in the steel, which may reduce toughness. In addition, too high a content of carbon can reduce the solidus temperature and thus increase the risk of (partial) melting during hardening, which in turn can significantly reduce toughness. Therefore, the steel contains equal to or less than 1.3% C. Preferably, carbon is present in an amount equal to or less than 1.20%. Suitably, carbon may be present in an amount equal to or less than 1.15%.

[0046] Silicon (Si): 0.2 wt.-%-0.8 wt.-%

[0047] Silicon is an effective deoxidizing element in steel production. Silicon can also contribute to improving hardenability, toughness, and wear resistance. Therefore, the steels according to the present disclosure contain at least 0.2% Si. Preferably, silicon may be present in an amount of at least 0.3%. Suitably, silicon may be present in an amount of at least 0.4%.

[0048] However, too high a content of silicon may lead to the risk of forming large carbides and / or excessive carbides and should therefore be avoided. In addition, a large amount of silicon may have a negative impact on the hardness after tempering and may cause problems related to embrittlement in some cases. Therefore, the steels described herein contain equal to or less than 0.8% Si. Preferably, silicon is present in an amount equal to or less than 0.7%. Suitably, silicon may be present in an amount equal to or less than 0.6%.

[0049] Manganese (Mn): 0.1 wt.-%-0.6 wt.-%

[0050] Manganese is an element that is frequently used during steel production and is used for the purpose of deoxidizing and fixing sulfur by forming manganese sulfide in both conventional processes and powder metallurgy processes. Manganese may also have an impact on hardenability as it helps to reduce the risk of carbide formation at grain boundaries during quenching from the hardening temperature. The reduced risk of carbide formation at grain boundaries by alloying with manganese enables a lower quenching rate to be achieved. This may mean in practice that tools with a higher thickness can be hardened. Therefore, the steel described herein contains at least 0.1% Mn. Preferably, manganese may be present in an amount of at least 0.15%. Suitably, manganese may be present in an amount of at least 0.2%.

[0051] However, manganese is also an austenite stabilizing element and thus, when present in high amounts, may cause an increase in the amount of retained austenite after hardening. Retained austenite may cause problems with impaired hardness and dimensional stability. Although the amount of retained austenite can be reduced by transformation to martensite during tempering, a higher amount of retained austenite can make tempering more difficult and thus increase manufacturing costs. Therefore, the steel contains Mn equal to or less than 0.6%. Preferably, manganese is present in an amount equal to or less than 0.5%. Suitably, manganese may be present in an amount equal to or less than 0.4%.

[0052] Sulfur (S): equal to or less than 0.2 wt.-%

[0053] Sulfur is an element that can generally be present as an impurity in the powder metallurgy steel according to the present disclosure. However, if desired, sulfur can also be used in small amounts for the purpose of improving the soft machinability of the steel by forming manganese sulfide MnS. Therefore, the steel described herein may contain sulfur in an amount equal to or less than 0.2%. Preferably, sulfur is present in an amount equal to or less than 0.15%.

[0054] Suitably, sulfur may be present in an amount equal to or less than 0.07%, or even equal to or less than 0.05%. At an amount equal to or less than 0.05%, sulfur is considered an unavoidable impurity and is considered to have no effect on the desired properties of the steel.

[0055] However, in the case where the steel described herein contains at least 0.07% S, the steel also suitably contains at least 0.2% Mn in order to ensure that there is a sufficient amount of manganese that is not bound in manganese sulfide and thus present to contribute to increased hardness.

[0056] Chromium (Cr): 3.0 wt.-%-5.5 wt.-%

[0057] Chromium is an important element in the steels described herein because, when dissolved into the matrix of the steel, chromium helps to achieve the desired hardness and toughness after hardening and tempering. Chromium is also a carbide-forming element and can thus contribute to increased hardness by being part of secondary carbides. Such carbides can also contribute to wear resistance. Accordingly, the steels described herein contain at least 3.0% Cr. Preferably, chromium is present in an amount of at least 3.4%. Suitably, the steel contains at least 3.7% Cr.

[0058] However, an excessive amount of chromium can lead to an excessive amount of retained austenite, which may be difficult to transform into martensite during tempering. In addition, higher amounts of chromium can lead to a risk of an excessive amount of and / or coarse primary M6C carbides, which may be difficult to dissolve during austenitization. Accordingly, the steels according to the present disclosure contain Cr equal to or less than 5.5%. Chromium may preferably be present in an amount equal to or less than 5.2%. Suitably, chromium may be present in an amount equal to or less than 4.6%.

[0059] Molybdenum (Mo): 1.8 wt.-%-3.5 wt.-%

[0060] Molybdenum is an important element for achieving the desired hardness and toughness. Similar to chromium, it dissolves into the matrix, enabling the desired hardness and toughness to be achieved after hardening and tempering. In addition, it can contribute to increased hardness and wear resistance by forming secondary carbides. Further, molybdenum is an alloying element frequently used in tool steels and can thus typically be present in scrap, which can be used as a raw material for producing the steels described herein. Replacing such scrap with other less alloyed raw materials may unduly increase the cost of the steel. In addition, too low an amount of molybdenum in the steel will lead to future problems in scrap disposal. Accordingly, the steels described herein contain at least 1.8% Mo. Preferably, molybdenum may be present in an amount of at least 2.0%. Suitably, molybdenum may be present in an amount of at least 2.2%.

[0061] The steels described herein contain at most 3.5% Mo. Higher amounts of molybdenum may increase the risk of forming coarse M6C carbides, which in turn can reduce toughness. In addition, higher amounts of molybdenum may unduly increase the alloying cost. Preferably, the steel contains Mo equal to or less than 3.2%. Suitably, the molybdenum content may be equal to or less than 2.8%.

[0062] Tungsten (W): 1.8 wt.-%-4.0 wt.-%

[0063] Tungsten has basically the same function as molybdenum and in principle can be used to partially or completely replace molybdenum in steels of the type of powder metallurgy tool steels described herein. It is generally known that a certain amount of molybdenum can be replaced by twice the amount of tungsten and vice versa. In addition, tungsten is an alloying element often used in tool steels and can thus usually be present in scrap, which can be used as a raw material for producing the steels described herein. Replacing such scrap with other less alloyed raw materials may unduly increase the cost of the steel. In addition, too low an amount of tungsten in the steel will lead to future problems in scrap disposal. Therefore, the steels described herein contain at least 1.8% of W. Preferably, the steel can contain at least 2.0% of W. Suitably, tungsten can be present in an amount of at least 2.2%.

[0064] The steels described herein contain at most 4.0% of W. Higher amounts of tungsten may increase the risk of forming coarse primary M6C carbides, may reduce toughness and will unduly increase the alloying cost. Preferably, tungsten is present in an amount equal to or less than 3.5%. Suitably, the steel can contain equal to or less than 3.2% of W.

[0065] As is apparent from the above, the steels described herein have at least 2.7% of Mo equivalent ([Mo]eq), where [Mo]eq = [wt.-% Mo] + 0.5 * [wt.-% W]. Suitably, molybdenum and tungsten can be present in such amounts that the [Mo]eq of the steel is equal to or greater than 3.2%. In addition, molybdenum and tungsten can suitably be present in such amounts that the [Mo]eq of the steel is equal to or less than 4.6%.

[0066] Empirically, it has been found that approximately equal amounts of molybdenum and tungsten will be preferred as this results in advantages in production. Therefore, the ratio [wt.-% W] / [wt.-% Mo] can suitably be between 0.8 and 1.2, preferably between 0.9 and 1.1 (including the end values).

[0067] Vanadium (V): 1.3 wt.-%-2.5 wt.-%

[0068] Vanadium is an element often used in powder metallurgy tool steels as it is an effective carbide-forming element. Vanadium forms hard primary precipitated MC carbides together with carbon, which are uniformly distributed in the matrix. These carbides limit grain growth in the steel, which in turn helps to increase toughness. In addition, these carbides are beneficial to the adhesive wear resistance of the steel. The powder metallurgy tool steels described herein contain at least 1.3% of V. Preferably, vanadium is present in an amount of at least 1.5%. Suitably, vanadium can be present in an amount of at least 1.7%.

[0069] However, an excessive amount of vanadium may risk forming an overly high amount of carbides and / or carbides with large sizes. An excessive amount or large size of carbides may reduce toughness and are thus undesirable. Considering that niobium is also included, the steels described herein do not need to contain more than 2.5% of V. Preferably, vanadium may be present in an amount equal to or less than 2.2%. Suitably, the vanadium content may be equal to or less than 2.0%.

[0070] Niobium (Nb): 1.3 wt.-%-2.5 wt.-%

[0071] Niobium is an element that plays an important role in the steels described herein. Like vanadium, niobium is a strong carbide former and can be used to limit grain growth, which is beneficial to toughness. Compared with vanadium, niobium also has the advantage of forming more stable carbides. It has been previously known that niobium can be used to replace vanadium, and it has been proposed that a certain amount of vanadium in tool steels can be replaced with twice the amount of niobium. However, niobium tends to produce larger MC carbides than vanadium, and niobium carbide generally has a different shape from vanadium carbide, and this shape is not conducive to toughness. Therefore, it is not appropriate to completely replace vanadium with niobium. In addition, since vanadium is the most commonly used carbide former in tool steels, in order not to interfere with scrap handling, it is not appropriate to completely replace vanadium. In addition, vanadium has a significantly higher solubility in austenite than niobium. Therefore, vanadium contributes to hardness (in the hardened and tempered state) in a manner similar to that of Mo, W, and Cr, while the contribution of niobium is much smaller than that of vanadium. Also for this reason, it is not appropriate to completely replace vanadium with niobium.

[0072] It has been found that when alloyed with both vanadium and niobium, the resulting MC carbides will be smaller compared to if only one of vanadium and niobium is added. In particular, the growth of MC carbides during various steps of the manufacturing process such as compaction and any subsequent hot working will be lower compared to if only one of vanadium and niobium is added. Small carbide size is beneficial for the toughness of the steel as well as the resistance to adhesive wear. Small carbide size is also beneficial for the grindability of the steel, which is usually an important property when producing cold working tools, and it helps to produce a sharp edge in, for example, cutting tools. In addition, it also reduces the risk of micro-cracking at the edge of the tool. Due to the presence of both vanadium and niobium, two different types of MC carbides are expected to form in the steel. Both types of MC carbides contain vanadium and niobium, but the first type has a higher V, while the other type has a higher Nb. In other words, the steel is expected to contain both V-rich MC carbides and Nb-rich MC carbides.

[0073] Therefore, the powder metallurgy tool steel of the present invention contains at least 1.3% of Nb. Preferably, the steel contains at least 1.5% of Nb. Suitably, niobium may be present in an amount of at least 1.7%.

[0074] However, too high a content of niobium may lead to production difficulties because there may be an increased risk of forming niobium carbide already in the melt, which in turn may lead to problems during atomization. Additionally, if niobium carbide forms prematurely during the production process (due to too high a content of niobium in the composition), they may have the risk of growing too large. Therefore, the steel described herein contains Nb equal to or less than 2.5%. Preferably, niobium may be present in an amount equal to or less than 2.2%. Suitably, niobium may be present in an amount equal to or less than 2.0%.

[0075] Suitably, the powder metallurgy tool steel described herein contains vanadium and niobium in such an amount that [wt.-%V] + 0.5*[wt.-%Nb] is equal to or higher than 2.5%. Additionally or alternatively, the ratio [wt.-%Nb] / [wt.-%V] may suitably be between 0.85 and 1.15 (including the end values).

[0076] Nitrogen (N): equal to or less than 0.2 wt.-%

[0077] Nitrogen is an element that can be included in the precipitate particles, which are more specifically carbonitrides or nitrides, and nitrogen can in principle be used to partially replace carbon. In the powder metallurgy tool steel of the present invention, nitrogen is not a specifically targeted element. However, powder metallurgy steels can generally contain a certain amount of nitrogen. For example, if nitrogen is used as the atomizing medium and / or for the protective atmosphere, nitrogen can be present as a result of atomization. Nitrogen can also be present due to the raw materials used for producing the powder metallurgy tool steel. Therefore, the powder metallurgy steel can contain N equal to or less than 0.2%. Preferably, the steel can contain N equal to or less than 0.12%. Suitably, nitrogen may be present in an amount equal to or less than 0.07%.

[0078] Cobalt (Co): equal to or less than 3.0 wt.-%

[0079] Cobalt is not an essential element of the chemical composition of the steel described herein, and intentionally adding cobalt may excessively increase the alloying cost. However, if necessary, for the purpose of increasing hardness, cobalt can be added in an amount equal to or less than 3.0%. Preferably, the steel contains Co equal to or less than 1.5%. Suitably, the steel contains Co equal to or less than 1.0%.

[0080] If not added purposefully, cobalt can generally be present due to the scrap used during production. In such cases, cobalt is usually present in an amount less than 1.0%. Completely avoiding cobalt from the chemical composition of the steel may be difficult and will excessively increase the manufacturing cost of the steel due to the need to replace raw materials. However, from a cost perspective, it may be reasonable to limit the amount of Co to equal to or less than 0.6%.

[0081] Nickel (Ni): equal to or less than 1.0 wt.-%

[0082] Nickel is not an essential element of the chemical composition of the steels described herein, but may generally be present due to the scrap used in the production of the steel. Nickel is an austenite stabilizing element, and its excessive content may increase the risk of an excessive amount of retained austenite after hardening, and the retained austenite may be difficult to transform into martensite during subsequent tempering. Therefore, in the powder metallurgy tool steels described herein, nickel may be present in an amount equal to or less than 1.0%. Preferably, nickel is present in an amount equal to or less than 0.5%. Suitably, nickel may be present in an amount equal to or less than 0.3%.

[0083] Copper (Cu): equal to or less than 1.0 wt.-%

[0084] Copper is not an essential or even desirable element for the steels described herein, but may be present due to the scrap used in the production of the steel. More specifically, in certain steels, copper may be added for the purpose of increasing corrosion resistance and / or hardness by precipitation. Once copper is present, it is impossible to remove copper from the steel, and if such steel is used as scrap in the production of the steels described herein, copper will inevitably be present. However, it should be noted that, considering the expected hardening and tempering states, the presence of copper in the steels described herein does not result in an increase in hardness. Copper is an austenite stabilizing element, and its excessive content may increase the risk of an excessive amount of retained austenite, and the retained austenite may be difficult to transform into martensite during tempering.

[0085] In the powder metallurgy tool steels of the present invention, copper may be present in an amount equal to or less than 1.0% without substantially negatively affecting the desired properties. However, for the reasons described above, the copper content should preferably be limited to a maximum of 0.5%. Suitably, the copper content of the steel is equal to or less than 0.3%.

[0086] Total impurities equal to or less than 1.5 wt.-%

[0087] Any steel may generally contain elements that are not intentionally added to achieve the desired properties, and thus such elements constitute impurities. Impurities may be present due to the raw materials used and / or as a result of the manufacturing process.

[0088] The powder metallurgy tool steels according to the present disclosure may contain such impurities in a total amount equal to or less than 1.5%. It should be noted here that, as discussed above, the elements S, N, Co, Ni, and Cu do not need to be intentionally added and thus are present as impurities in the steels described herein. However, the contents of these elements should not be considered included in the total amount of impurities equal to or less than 1.5%, even if not intentionally added as alloying elements. Preferably, the powder metallurgy tool steel contains impurities (excluding S, N, Co, Ni, and Cu) in a total amount equal to or less than 1.0%, or even in a total amount equal to or less than 0.6%.

[0089] An example of an impurity that is typically present in the type of steel to which the powder metallurgy tool steel described herein belongs is phosphorus. Phosphorus is an inevitable impurity element that is extremely difficult to completely avoid. It can generally be allowed in amounts up to 0.05% without having a negative impact on the desired properties. Preferably, the powder metallurgy tool steel contains P equal to or less than 0.030%.

[0090] Another example of an impurity that is typically present in steel is aluminum, which can be introduced by the manufacturing process in both the case of conventionally produced steel and powder metallurgy steel, for example. Aluminum can generally be allowed in amounts up to 0.1%.

[0091] Other examples of impurities include, but are not limited to, titanium, magnesium, calcium, rare earth metals (REM), tin, and oxygen. Suitably, the allowable content of Ti can be equal to or less than 0.2% or equal to or less than 0.1%. In addition, the allowable content of magnesium and calcium can each suitably be equal to or less than 0.02% respectively. In addition, the allowable content of REM can suitably be equal to or less than 0.2%. The allowable content of tin can suitably be equal to or less than 0.1%. In addition, the allowable content of oxygen can suitably be equal to or less than 200 ppm.

[0092] Production method

[0093] The powder metallurgy tool steel described herein can be produced by atomizing a melt using a suitable atomizing medium. Preferably, gas atomization is used. Steel powder is obtained by said atomization. The steel powder can, for example, have a maximum particle size equal to or less than 1000 μm, and the D50 can be, for example, about 100 μm - 200 μm.

[0094] Thereafter, the steel powder is compacted. Compaction can suitably be carried out by hot isostatic pressing (HIP), optionally preceded by cold isostatic pressing (CIP). These processes are themselves previously known and will therefore not be discussed further in the present disclosure. Thereafter, the compacted steel powder can be processed into an intermediate product form, such as a rod, bar, or blank, if not already in such a form. Thus, processing into such a form can involve any previously known processes, such as forging and / or rolling.

[0095] Thereafter, the compacted steel powder, optionally further processed into the desired intermediate product form, can suitably be soft annealed to enable soft machining into the expected geometric configuration of a final product such as a tool. The soft annealing can, for example, be carried out in a protective atmosphere at 850 °C - 900 °C for 1 - 4 hours, although other temperatures and durations are also feasible. Cooling starting from the soft annealing temperature is preferably carried out rather slowly, for example, at a rate of about 10 °C / h down to about 700 °C, followed by air cooling, for example, to avoid deformation.

[0096] If desired, the compacted steel powder or intermediate product can thereafter be stress relieved. This can be carried out, for example, at a temperature of about 600 °C - 700 °C for a duration of about 1 h - 3 h. Suitably, the cooling starting from the temperature of the stress relief step should be slowly reduced to at least 500 °C.

[0097] If desired, the compacted steel powder or intermediate product can thereafter be machined into the geometry of the final product. This can thus be carried out according to any previously known method.

[0098] Thereafter, the powder metallurgy tool steel can be hardened and tempered. Hardening can suitably be carried out by subjecting the powder metallurgy tool steel preferably in a protective atmosphere or in a vacuum to an austenitizing temperature equal to or higher than 1000 °C, preferably equal to or higher than 1100 °C. According to one embodiment, the steel is subjected to an austenitizing temperature equal to or higher than 1140 °C. After hardening and tempering, the hardness generally increases with the austenitizing temperature, and an austenitizing temperature up to at least 1200 °C is suitable. The duration at the austenitizing temperature depends on the austenitizing temperature used and can be shorter at higher austenitizing temperatures. The duration is suitably selected to ensure the austenitization of the entire steel and can thus depend on the size of the steel product at this production stage. Thereafter, the powder metallurgy tool steel is quenched from the austenitizing temperature, preferably to a temperature equal to or lower than 100 °C. The quenching should preferably be carried out at the highest possible quenching rate without the risk of causing deformation etc. Thus, the quenching should be carried out by forced cooling using a suitable quenching medium. By way of example, the quenching medium can be nitrogen, but other quenching media are also possible. Suitably, the quenching is carried out at a quenching rate of at least 7 °C / s.

[0099] In the case of larger dimensions (e.g. in the case of large tools), it may be appropriate to temporarily interrupt the quenching at about 540 °C - 560 °C for homogenization in order to reduce the risk of cooling cracks. In such a case, the steel is quenched from the austenitizing temperature to about 540 °C - 560 °C (preferably using a quenching rate of at least 7 °C / s), then held at about 540 °C - 560 °C for a suitable duration to achieve homogenization, and thereafter quenched to a temperature equal to or less than 100 °C.

[0100] After hardening, the powder metallurgy tool steel is subjected to tempering. The purpose of tempering is to transform the retained austenite that may be present in the microstructure of the hardened powder metallurgy steel in addition to martensite, and ultimately to obtain a matrix consisting essentially of tempered martensite. The tempering can suitably be carried out at a temperature of 520 °C - 600 °C, preferably 530 °C - 580 °C. The duration of tempering can suitably be at least 1 hour or at least 2 hours, although shorter durations are also feasible. If desired or required, the tempering can be carried out in more than one successive step, with an intermediate cooling to a temperature of about 50 °C or lower, preferably equal to or lower than about room temperature.

[0101] Microstructure

[0102] After hardening and tempering, the powder metallurgy tool steel has a microstructure in which the matrix consists essentially of tempered martensite. The term "consisting essentially of" should be considered herein to mean that at least 95 vol.-% of the matrix consists of tempered martensite. It should be noted here that in the art, a volume percentage is typically determined by considering the area percentage of the relevant constituent components (such as phases) in a sample, which is considered to correspond to the volume percentage.

[0103] The microstructure also contains carbides in a total amount equal to or less than 10 vol.-%, typically equal to or less than 8 vol.-%. Considering the fact that different carbides dissolve at different temperatures, the total amount of carbides and their type depend on the composition and the austenitizing temperature used during hardening. However, in the hardened and tempered state, the steel is expected to contain both vanadium-rich MC carbides (referred to herein as MC-V) and niobium-rich carbides (referred to herein as MC-Nb). The steel may also contain M6C carbides, but by appropriately selecting the austenitizing temperature, the amount of M6C carbides can be low. Preferably, the steel contains less than 2 vol.-% of M6C carbides in the hardened and tempered state; more preferably equal to or less than 1 vol.-% of M6C carbides. Generally, M6C carbides are softer than MC carbides and tend to grow larger, and thus can result in lower toughness. Therefore, a low amount of M6C carbides is required. In cases where M6C carbides cannot be avoided, at least the carbide size needs to be reduced. This can be achieved by increasing the austenitizing temperature of the steel described herein during hardening, resulting in at least partial dissolution of the carbides.

[0104] The average maximum size of the carbides should preferably be equal to or less than 4 μm. The average maximum size is herein intended to mean the average size of the three largest carbides identified in a sample using a microscope (such as LOM or SEM) at an appropriate magnification, for example 1000x, i.e., the sizes of the three largest carbides measured in 10 different fields of view. Suitably, the average maximum size of the carbides should be equal to or less than 2 μm.

[0105] The amount and their size of the carbides are achieved by the composition of the powder metallurgy tool steel and the fact that, compared to conventional processes including casting (such as ingot casting or continuous casting), the powder metallurgy tool steel is produced by powder metallurgy. In addition, as described above, it is affected by the temperature used during hardening.

[0106] The fact that the steel described herein is produced according to a powder metallurgy route also enables good cleanliness to be achieved, which is important for achieving good toughness. When measured according to DIN50602, the cleanliness K0 should be equal to or lower than 40. Preferably, when measured according to DIN50602, K0 < 10 or even K0 < 5. The desired cleanliness can be achieved by appropriately controlling the powder metallurgy process.

[0107] Properties

[0108] The powder metallurgy tool steel described herein enables a very good balance to be achieved between the conflicting properties of hardness and toughness. This means that for a given hardness, the steel described herein has improved toughness; or for a given toughness, the steel described herein has a higher hardness compared to conventionally used cold-worked steels. This is achieved without unduly high alloying costs and / or without the risk of causing problems in future scrap disposal.

[0109] The powder metallurgy tool steel can be hardened such that it has a hardness of at least 62 HRC in the hardened and tempered state, although lower hardnesses are possible if required. Generally, a hardness of approximately 62 - 66 HRC can be obtained in the hardened and tempered state. This is comparable to the possible hardness in 2005 and significantly higher than the hardness in 2012.

[0110] In addition, the powder metallurgy steels described herein have very good toughness in the hardened and tempered state. The achievable toughness depends to some extent on the reduction ratio of the hot working (if applicable) before hardening, and the hardness after hardening and tempering. In the present disclosure, the term "reduction ratio" is intended to mean the reduction ratio RR calculated according to Equation 1 below, where A0 represents the cross-sectional area before compression (i.e., the cross-sectional area before hot working), and A1 represents the cross-sectional area after compression (i.e., the cross-sectional area after hot working).

[0111]

[0112] Generally speaking, toughness increases with the increase of the reduction ratio during hot working. In addition, toughness can usually be increased by reducing the hardness after hardening and tempering, which in turn can be achieved by using a lower austenitizing temperature during hardening. In practice, this means that to some extent, a compromise between hardness and toughness needs to be made in the same way as for previously known cold-worked steels, depending on the intended use of the steels described herein.

[0113] In addition, it should be recognized that the measured toughness will depend on the orientation of the samples taken with respect to the products of the powder metallurgy steels described herein. For the purpose of defining said orientation, Figure 1 Schematically shows a round bar 1 as an example of a product obtained after hot working the powder metallurgy tool steel described herein. The round bar has a longitudinal axis A. In the present disclosure, the toughness in the longitudinal direction should be considered to mean the toughness measured on a sample taken in a plane coinciding with or parallel to the longitudinal axis A. The toughness in the transverse direction should be considered to mean the toughness measured on a sample taken in a plane perpendicular to the longitudinal axis A. In other words, the toughness in a certain direction related to the orientation of the samples taken from the product (which will inherently be perpendicular to the impact direction during testing) is described herein.

[0114] However, for comparison purposes, the powder metallurgy tool steels described herein can have, for example, a toughness of at least 40 J in the transverse direction and a hardness of at least 62 HRC when in the hardened and tempered state and having undergone hot working with a reduction ratio of 96% before hardening. The toughness herein is intended to mean the toughness when determined according to SEP 1413, which is a notchless impact test. The toughness of the powder metallurgy tool steels described herein in the hardened and tempered state is significantly better than the toughness of 2005 at a similar hardness.

[0115] Furthermore, considering the uniform distribution and small size of MC carbides, the powder metallurgy tool steel described herein also has good resistance to adhesive wear. These small carbides are also expected to give good edge strength in terms of chipping resistance. These are important wear mechanisms for tools, and thus the steel described herein can improve the quality of cold working tools and thereby has the ability to extend their service life.

[0116] Experimental results

[0117] Experimental test 1

[0118] Four different steels produced on a laboratory scale were tested. The compositions of the different steels are specified in Table 1, where the content of any possible non-specified impurity elements is less than 0.03%. The steels were produced by gas atomization of the melt to obtain steel powders with the compositions specified below. Except for V4, V4 was obtained by mixing two steel powders with different compositions, thereby obtaining the final composition specified below. The resulting steel powders were each compacted by hot isostatic pressing at a temperature of 1150 °C and a pressure of 1000 bar, and thereafter soft annealed at 880 °C. For the purpose of studying the achievable hardness, hardening was carried out at two different austenitizing temperatures of 1150 °C and 1180 °C. The steels were quenched from the austenitizing temperature to room temperature. Thereafter, the steels were each tempered in three steps at 560 °C, each step lasting approximately 1 h.

[0119] Table 1

[0120]

[0121] The hardness in the hardened and tempered states was measured, and the results are specified in Table 2. As can be seen from the test results, the different steels can be hardened to a hardness far above 62 HRC. For steel V2, the highest hardness was obtained, and steel V2 also contains the highest amount of carbon. In addition, steel V4 can be hardened to a hardness above 64 HRC, which is a very good hardness.

[0122] Table 2

[0123] Number Hardening temperature [°C] Hardness [HRC] V1 1150 62.1 V1 1180 62.8 V2 1150 64.7 V2 1180 65.5 V3 1150 62.8 V3 1180 63.2 V4 1150 63.6 V4 1180 64.6

[0124] Furthermore, for the purpose of estimating the amount of carbides in different steels, the software ThermoCalc and the database TCFE10 were used to perform the calculations for the corresponding compositions. The calculations performed were based on a hardening temperature (austenitizing temperature) of 1180 °C. Tempering was not carried out at a high temperature sufficient to affect the size and amount of carbides and was therefore not taken into account in the calculations. The results are presented in Table 3. As can be seen from the results, the total estimated amount of carbides is far below 10 vol.-%, and the estimated amount of M6C carbides is far below 2 vol.-%, which is beneficial for toughness. In fact, the calculations show that when using a hardening temperature of 1180 °C, it is possible to avoid the presence of any M6C carbides in steel V3.

[0125] Table 3

[0126]

[0127] Experimental test 2

[0128] Three full-scale industrial melts were produced, and each melt was gas atomized to obtain steel powder. The composition of the resulting steel is specified in Table 4, where the content of any possible non-specified impurity elements is less than 0.03%. The steel powder was compacted by hot isostatic pressing at 1150 °C and 1000 bar. The compacted steel was hot worked (by forging followed by rolling) into round bars using different compression ratios and was thereafter soft annealed at 880 °C. Hardening was carried out at different austenitizing temperatures, followed by quenching to room temperature. Thereafter, the steel was tempered in three steps at 560 °C, each step for approximately 1 h. The compression ratio (calculated according to Equation 1 above) and the austenitizing temperature during hardening are specified in Table 5 below.

[0129] Table 4

[0130] Number C Si Mn P S Cr Ni Mo W Co V Cu Nb F1 1.19 0.57 0.27 0.019 0.014 3.96 0.14 3.04 3.06 0.41 1.80 0.091 1.77 F2 1.09 0.53 0.28 0.019 0.019 3.96 0.14 3.05 3.12 0.46 1.81 0.084 1.75 F3 1.06 0.55 0.26 0.019 0.016 3.98 0.12 2.39 2.43 0.34 1.79 0.072 1.81

[0131] In the hardened and tempered states, samples for measuring hardness were taken transversely to the longitudinal direction of the bar (i.e., perpendicular to Figure 1 the longitudinal axis A of the round bar 1 schematically shown in Figure 1 ). Furthermore, in the hardened and tempered states, 10 samples for measuring toughness were taken both in the transverse and longitudinal directions of the bar. The toughness was determined according to SEP1314 (notched impact toughness) and as the average of the toughness of 10 samples. The results are presented in Table 5, where the transverse toughness corresponds to the toughness measured on samples taken in the transverse direction (i.e., perpendicular to

[0132] For ease of comparison, previously known 2005 and The corresponding test results for 2023 are also included in Table 5. However, it should be noted that 2005 and the 2023 results involve slightly different compression ratios (93% and 94% respectively), which are between the compression ratios tested for Steel F1 and Steel F2 and lower than the compression ratio tested for Steel F3. Therefore, when comparing the results, the differences in compression ratios should be taken into account.

[0133] Table 5

[0134]

[0135]

[0136] Generally speaking, for powder metallurgy tool steels, toughness usually decreases as hardness increases. This was also observed from the test results presented in Table 5.

[0137] The test results show that Steels F1 - F3 can be hardened to a hardness far higher than 62 HRC and are similar to or higher than 2005. In addition, each of Steels F1 - F3 exhibits higher toughness than 2005. For example, it can be seen that Steel F2 at a compression ratio of 96% and a hardness of 64.2 HRC has 37.5% higher toughness in the transverse direction than 2005 at a slightly lower hardness of 64 HRC. In addition, it can be seen that Steel F3 at a compression ratio of 96% and a hardness of 63.9 HRC has 65% higher toughness in the transverse direction than 2005 at a similar hardness of 64 HRC. This is the case despite the fact that comparisons were made with Steels F2 and F3 which have higher compression ratios than 2005. When hardened at 1150 °C or 1180 °C, Steel F1 has both higher hardness and transverse toughness than 2005.

[0138] For the purpose of estimating the amount of carbides in different steels, the software ThermoCalc, database TCFE10 was used to calculate the corresponding compositions. The calculations were carried out for four different hardening temperatures and the results are presented in Table 6. The results indicate that for all hardening temperatures, the total amount of carbides in each steel is far lower than 8 vol.-%. In addition, it can be seen that the amount of M6C carbides decreases as the hardening temperature increases. In addition, it can be seen that for Steel F3, if hardened at 1150 °C or higher, it is possible to avoid the presence of M6C carbides.

[0139] Table 6

[0140]

[0141]

[0142] Table 6 continued

[0143]

[0144] To check the accuracy of the calculations described above, samples from the corresponding steels after hardening and tempering were examined in a scanning electron microscope (SEM). Figure 2 The figure shows the SEM image of steel F1 when hardened at 1150 °C, showing a microstructure containing fine MC carbides and some M6C carbides. Figure 3 The figure shows the SEM image of steel F1 when hardened at 1180 °C, showing that despite the higher hardening temperature, the microstructure still contains some M6C carbides. The presence of M6C carbides was also shown for steel F2 at different hardening temperatures tested. The sample of steel F3 hardened at 1100 °C showed only a small amount of very small M6C carbides, but contained no M6C carbides when hardened at 1150 °C or higher temperatures. Figure 4 The figure shows the SEM image of steel F3 when hardened at 1150 °C.

[0145] In addition, the carbide sizes in steel F3 were determined and the results are presented in Table 7. The determination of the carbide sizes was carried out by measuring the three largest carbides in 10 different fields of view at a magnification of 1000x and determining their average. Thus, the carbide sizes presented in Table 7 represent the average of the largest carbides. As is evident from the results, the carbides in steel F3 are small for all hardening temperatures.

[0146] Table 7

[0147] Hardening temperature [°C] 1100 1150 1180 1200 Carbide size [μm] 1.3 1.2 1.3 1.3

[0148] Experimental test 3

[0149] To further investigate the obtainable properties of steel F3 described above, Experiment Test 2 was repeated, except that tempering was carried out in three steps at 540 °C, each step being about 1 h. The results are presented in Table 8.

[0150] Table 8

[0151]

[0152]

[0153] As can be seen from the results, when compared with the results given in Table 5, lower temperatures during tempering can further increase the hardness of steel F3 while still achieving good toughness.

[0154] Experimental test 4

[0155] Tribological tests were carried out to study the adhesive wear resistance of the powder metallurgy steels described herein compared to those previously known in 2005 and 2023.

[0156] Figure 5 The principle of the tribological tests carried out is schematically illustrated. While applying a continuously increasing normal load, the cylindrical test specimen 3 slides against the cylindrical counterbody 4. More specifically, the normal load continuously increases along the sliding path, resulting in an increase in the plastic deformation of the test specimen 3 and the counterbody 4 and thus an increase in the sliding distance to pass through the widened track 5.

[0157] According to the principle described above, the tests were carried out using a tribological load scanner. A description of the load scanner can be found, for example, in Magnus Heldin et al., “On the critical roles of initial plastic deformation and material transfer on the sliding friction between metals”, Wear 477 (2021) 203853. It should be noted here that the setup is unique and results in each part of the contact path of the test specimen only encountering a single part on the counterbody and only experiencing a specific normal load.

[0158] Cylindrical test specimens with a diameter of 10 mm and a length of 100 mm were produced from the steel F3 described above as well as in 2005 and 2023. The test specimens of each of the three steels were heat-treated by hardening at 1150 °C followed by tempering in three steps at 560 °C, each step for approximately 1 h. The counterbodies having the same dimensions as the test specimens were made of conventionally produced 304 steel. The tests were carried out under dry contact, i.e., without using any lubricant and at room temperature. The parameter studied was the coefficient of friction relative to the normal load. The load range studied was 150 N - 850 N (normal load). Five strokes were carried out for each test specimen, each stroke along the same track and with the same increase in normal load along the sliding path.

[0159] The results are presented in Figure 6Shown is the average over five strokes. It can be clearly seen from the results that steel F3 has a significantly lower coefficient of friction over the entire load range. The lower frictional force indicates less adhesion of the steel to the counterbody. Thus, it can be seen that steel F3 has better resistance to scoring / adhesive wear. Scoring is an adhesive wear caused by the microscopic transfer of material between metal surfaces during lateral movement (sliding). Without being bound by theory, it is believed that the improved resistance to adhesive wear is (at least in part) the result of the very fine MC carbides of the powder metallurgy steels described herein.

Claims

1. A powder metallurgy tool steel having a chemical composition comprising the following in wt.-%: The balance Fe and any impurities in total equal to or less than 1.5 wt.-%.

2. The steel according to claim 1, wherein the steel comprises 0.95 wt.-% - 1.20 wt.-% C, preferably 1.00 wt.-% - 1.15 wt.-% C.

3. The steel according to any one of claims 1 or 2, wherein the steel comprises 0.3 wt.-% - 0.7 wt.-% Si, preferably 0.4 wt.-% - 0.6 wt.-% Si.

4. The steel according to any one of the preceding claims, wherein the steel comprises 0.15 wt.-% - 0.5 wt.-% Mn, preferably 0.2 wt.-% - 0.4 wt.-% Mn.

5. The steel according to any one of the preceding claims, wherein the steel comprises 3.4 wt.-% - 5.2 wt.-% Cr, preferably 3.7 wt.-% - 4.6 wt.-% Cr.

6. The steel according to any one of the preceding claims, wherein the steel comprises 2.0 wt.-% - 3.2 wt.-% Mo, preferably 2.2 wt.-% - 2.8 wt.-% Mo.

7. The steel according to any one of the preceding claims, wherein the steel comprises 2.0 wt.-% - 3.5 wt.-% W, preferably 2.2 wt.-% - 3.2 wt.-% W.

8. The steel according to any one of the preceding claims, wherein the steel comprises 1.5 wt.-% - 2.2 wt.-% V, preferably 1.7 wt.-% - 2.0 wt.-% V.

9. The steel according to any one of the preceding claims, wherein the steel comprises 1.5 wt.-% - 2.2 wt.-% Nb, preferably 1.7 wt.-% - 2.0 wt.-% Nb.

10. The steel according to any one of the preceding claims, wherein Mo and W are present in such amounts that [wt.-% Mo] + 0.5 * [wt.-% W] is equal to or less than 4.6%.

11. The steel according to any one of the preceding claims, wherein the steel has a total carbide amount equal to or less than 10 vol.-%, preferably equal to or less than 8 vol.-% in the hardened and tempered state.

12. The steel according to any one of the preceding claims, wherein the steel comprises less than 2 vol.-% of M6C carbide in the hardened and tempered state.

13. The steel according to any one of the preceding claims, wherein the carbide of the steel has an average maximum size of 4 μm in the hardened and tempered state of the steel; wherein the average maximum size is determined based on the sizes of the three largest carbides identified by microscopy, and the sizes of the three largest carbides are measured in 10 different fields of view.

14. Steel according to any one of the preceding claims, wherein the steel in the hardened and tempered state and when hot-worked at a compression ratio of 96% before hardening has a hardness of at least 62 HRC and a toughness of at least 40 J when measured in the transverse direction according to SEP 1413.

15. Use of a powder metallurgy tool steel according to any one of the preceding claims for the production of tools suitable for cold working applications.

16. A method for manufacturing a powder metallurgy tool steel, the method comprising: a) producing a steel powder having the following chemical composition by wt.-%: the balance Fe and any impurities in total equal to or less than 1.5 wt.-%; b) compacting the steel powder, preferably wherein the compacting comprises hot isostatic pressing; c) optionally subjecting the compacted steel to soft annealing and / or stress relieving; d) hardening by subjecting the steel to an austenitizing temperature equal to or higher than 1000 °C followed by quenching; e) tempering the hardened steel by subjecting it to a temperature between 520 °C and 600 °C.

17. A cold working tool comprising a powder metallurgy tool steel according to any one of claims 1 to 14.

Images