A powder metallurgical tool steel

Abstract

A powder metallurgical steel, suitable for cold working, having a chemical composition comprising (in percent by weight): C 0.9 – 1.3, Si 0.2 – 0.8, Mn 0.1 – 0.6, S equal to or less than 0.2, Cr 3.0 – 5.5, Mo 1.8 – 3.5, W 1.8 – 4.0, V 1.3 – 2.5, Nb 1.3 – 2.5, N 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, balance Fe and any impurities equal to or less than 1.5 wt.-% in total.

Description

[0001] A POWDER METALLURGICAL TOOL STEEL

[0002] TECHNICAL FIELD

[0003] The present disclosure relates in general to a powder metallurgical tool steel suitable for cold working applications. The present disclosure further relates to a method for manufacturing a powder metallurgical tool steel. The present disclosure also relates to a tool comprising the powder metallurgical steel.

[0004] BACKGROUND

[0005] Cold work tool steels are steels used to produce tools that are exposed to surface temperatures up to about 200 °C, i.e. cold working applications. Examples of such tools include for example tools for stamping, blanking, powder compaction or cold extrusion. A cold work tool steel should have an adequate hardness, toughness and wear resistance for the intended cold working application.

[0006] ASP® 2005 is a powder metallurgical high speed steel, offered by Erasteel, suitable for cold work applications. Said 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 condition, the steel has a hardness of about 60-64 HRC as well as good toughness and wear resistance. Although the steel works very well in many cold work applications, it would be advantageous to further improve the toughness at a given hardness.

[0007] Another example of a powder metallurgical high speed steel suitable for cold work applications is ASP® 2012, also offered by Erasteel. Said 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. ASP®2012 has a higher toughness, but a lower hardness, compared to ASP® 2005, typically about 56-59 HRC. Furthermore, the wear resistance is somewhat lower compared to the wear resistance of ASP® 2005.

[0008] Yet another example of a powder metallurgical high speed steel, offered by Erasteel and suitable for cold work applications, is ASP® 2023. Said steel has a nominal composition comprising 1.28% C, 4.1% Cr, 5.0% Mo, 6.4% W and 3.1% V.

[0009] WO 03 / 000944 Al discloses a cold work steel with 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, max 0.5 % W and max 0.5 % Nb. It is described that the steel has a hardness of 54-66 HRC after hardening and tempering.

[0010] SUMMARY

[0011] The object of the present invention is to provide a steel, suitable for cold work applications, having a good balance between hardness and toughness, and preferably also wear resistance.

[0012] The object is achieved by the subject-matter of the appended independent claim(s). Various embodiments are defined by the dependent claims.

[0013] In accordance with the present disclosure, a powder metallurgical tool steel is provided. The powder metallurgical steel has a chemical composition comprising (in percent by weight):

[0014] C 0.9 - 1.3,

[0015] Si 0.2 - 0.8,

[0016] Mn 0.1 - 0.6,

[0017] S equal to or less than 0.2,

[0018] Cr 3.0 - 5.5,

[0019] Mo 1.8 - 3.5,

[0020] W 1.8 - 4.0,

[0021] V 1.3 - 2.5,

[0022] Nb 1.3 - 2.5,

[0023] N equal to or less than 0.2,

[0024] Co equal to or less than 3.0,

[0025] Ni equal to or less than 1.0,

[0026] Cu equal to or less than 1.0, balance Fe and any impurities equal to or less than 1.5 wt.-% in total.

[0027] The herein described powder metallurgical tool steel has a very good balance between hardness and toughness in the hardened and tempered condition, which is a result of the chemical composition. More specifically, the toughness, at a given hardness, is improved by a reduction of the size and amount of carbides other than of MC type. Furthermore, the toughness is further improved by the composition resulting in a small size of the MC carbides. Compared to previously known powder metallurgical tool steels, this is primarily achieved by the combination of a medium content of carbon, relatively low amount of molybdenum and tungsten, and a relatively high amount of niobium. The fact that the carbides are of small size also enables a good edge strength, i.e. chipping resistance, and good resistance to galling and adhesive wear.

[0028] The herein described powder metallurgical tool steel may be hardened to a hardness above 62 HRC while still achieving a good toughness.

[0029] The present disclosure also relates to the use of the powder metallurgical steel described herein for producing a tool adapted for cold working applications.

[0030] The present disclosure also provides a method for manufacturing a steel, the method comprising: a) producing a steel powder having the following chemical composition, in wt.-%:

[0031] C 0.9 - 1.3,

[0032] Si 0.2 - 0.8,

[0033] Mn 0.1 - 0.6,

[0034] S equal to or less than 0.2,

[0035] Cr 3.0 - 5.5,

[0036] Mo 1.8 - 3.5,

[0037] W 1.8 - 4.0,

[0038] V 1.3 - 2.5,

[0039] Nb 1.3 - 2.5,

[0040] N equal to or less than 0.2,

[0041] Co equal to or less than 3.0,

[0042] Ni equal to or less than 1.0,

[0043] Cu equal to or less than 1.0, balance Fe and any impurities equal to or less than 1.5 wt.-% in total; b) compacting the steel powder, preferably wherein said compacting comprises hot isostatic pressing; c) optionally soft annealing and / or stress relieving the compacted steel; d) hardening by subjecting the steel to an austenitization temperature of equal to or above 1000 °C, followed by quenching; e) tempering by subjecting the hardened steel to a temperature of 520 to 600 °C. Furthermore, the present disclosure provides a cold work tool comprising the above described powder metallurgical tool steel. The cold work tool may for example be a stamping tool, a blanking tool, a powder compaction tool, an extrusion tool, a roll or a knife, but is not limited thereto.

[0044] BRIEF DESCRIPTION OF DRAWINGS

[0045] Fig. 1 schematically illustrates a round bar having a longitudinal axis,

[0046] Fig. 2 represents a SEM image of steel Fl when hardened at 1150°C and tempered at 560°C,

[0047] Fig. 3 represents a SEM image of steel Fl when hardened at 1180°C and tempered at 560°C,

[0048] Fig. 4 represents a SEM image of steel F3 when hardened at 1150°C and tempered at 560°C,

[0049] Fig. 5 schematically illustrates the principle of the tribological test performed, and

[0050] Fig. 6 represents a graph showing test results of coefficient of friction vs. normal load for steels F3, ASP®2005 and ASP®2023 during sliding against a counter body of 304 steel.

[0051] DETAILED DESCRIPTION

[0052] The invention will be described in more detail below with reference to exemplifying embodiments. The invention is however not limited to the exemplifying embodiments discussed, but may be varied within the scope of the appended claims.

[0053] When ranges are disclosed in the present disclosure, such ranges include the respective end values of the range, unless explicitly disclosed otherwise. Similarly, when an open range is disclosed, the open range also includes the single end value of the open range, unless explicitly disclosed otherwise.

[0054] The present disclosure provides a powder metallurgical tool steel having a chemical composition consisting of, in percent by weight:

[0055] C 0.9 - 1.3,

[0056] Si 0.2 - 0.8, Mn 0.1 - 0.6,

[0057] S equal to or less than 0.2,

[0058] Cr 3.0 - 5.5,

[0059] Mo 1.8 - 3.5,

[0060] W 1.8 - 4.0,

[0061] V 1.3 - 2.5,

[0062] Nb 1.3 - 2.5,

[0063] N equal to or less than 0.2,

[0064] Co equal to or less than 3.0,

[0065] Ni equal to or less than 1.0,

[0066] Cu equal to or less than 1.0, balance Fe and any impurities equal to or less than 1.5 wt.-% in total.

[0067] The herein described powder metallurgical tool steel has primarily been developed for use in cold work applications. Cold work applications are here intended to mean such applications wherein the temperature of the steel, during use as or in a tool, is up to about 200 °C. It should however be noted that the powder metallurgical tool steel described herein is also suitable for use in tool applications at medium temperatures, typically up to about 550 °C. Examples of cold work tools for which the herein described steel may suitably be 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 knives (including industrial knives). It may also be used in for example plastic injection moulders or extrusion tools. Moreover, it may be used in certain cutting tools where lower temperatures and adhesive wear may be expected, such as taps or other cutting tools for working of aluminum based materials or some stainless steels.

[0068] The herein described powder metallurgical tool steel may also be used in applications other than tools, for example in components where a combination of good fatigue resistance and high hardness is needed. Examples of such components include, but are not limited to, bearings, gears or engine parts (for example camshafts or parts of injectors).

[0069] In the following, the importance of the different alloying elements of the powder metallurgical steel will be briefly discussed. All percentages for the chemical composition are given in weight-% (wt.-%), unless explicitly disclosed otherwise. Upper and lower limits, described as preferred or suitable, of the individual elements of the composition can be freely combined within the broadest limits described, unless explicitly disclosed otherwise. As already mentioned above, any range specified herein includes the respective end values of the range, unless explicitly disclosed otherwise.

[0070] Carbon (C): 0.9 - 1.3 wt.-%

[0071] Carbon is an essential element of the composition and contributes both to hardness and the wear resistance. Carbon, when dissolved in the martensite, contributes to the intended hardness in the hardened and tempered condition of the herein described steel. Furthermore, carbon forms primary precipitated V / Nb rich MC carbides, as well as primary precipitated Mo / W / Cr rich MgC carbides. The primary precipitated carbides contribute to the wear resistance and may also have the advantageous effect of limit grain growth. Therefore, the herein described steel comprises at least 0.9% C.

[0072] Preferably, carbon is present in an amount of at least 0.95%. Suitably, carbon is present in an amount of at least 1.00%.

[0073] Excessive amounts of carbon may however lead to a too high content of MgC carbides. Furthermore, too high content of carbon increases the risk of coarse carbides in the steel, which may reduce the toughness. Furthermore, a too high content of carbon may lower the solidus temperature and thereby increase the risk of (partial) melting during hardening, which in turn may significantly reduce toughness. Therefore, the steel comprises equal to or less than 1.3% C. Preferably, carbon is present in an amount of equal to or less than 1.20%. Suitably, carbon may be present in an amount of equal to or less than 1.15%.

[0074] Silicon (Si): 0.2 - 0.8 wt.-%

[0075] Silicon is an effective deoxidizing element in the steel production. Silicon may also contribute to improved hardenability, toughness and wear resistance. Therefore, the steel according to the present disclosure comprises 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%.

[0076] However, a too high content of silicon may lead to a risk of formation of large carbides and / or an excessive amount of carbides and should therefore be avoided. Moreover, high amounts of silicon may negatively affect hardness after tempering and may in some cases lead to problems relating to embrittlement. Therefore, the herein described steel comprises equal to or less than 0.8% Si. Preferably, silicon is present in an amount of equal to or less than 0.7%. Suitably, silicon may be present in an amount of equal to or less than 0.6%. nese n): 0.1 - 0.6 wt.-%

[0077] Manganese is an element frequently used during steel production, both in conventional processes and powder metallurgical processes, for the purpose of deoxidation and immobilization of sulfur by formation of manganese sulfides. Manganese may also have an influence on hardenability because it helps to reduce the risk of formation of carbides in grain boundaries during quenching from hardening temperature. A reduced risk of formation of carbides in grain boundaries by alloying with manganese enables lower quenching speeds. This may in practice mean that tools with higher thickness can be hardened. Therefore, the herein described steel comprises 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%.

[0078] However, manganese is also an austenite stabilizing element and may therefore, when present in high amounts, lead to an increased amount of retained austenite after hardening. Retained austenite may lead to impaired hardness and problems with dimensional stability. Although the amount of retained austenite can be reduced by conversion to martensite during tempering, higher amounts of retained austenite could make the tempering more difficult and therefore increase the manufacturing costs. Therefore, the steel comprises equal to or less than 0.6% Mn. Preferably, manganese is present in an amount of equal to or less than 0.5%. Suitably, manganese may be present in an amount of equal to or less than 0.4%.

[0079] Sulfur (S): equal to or less than 0.2 wt.-%

[0080] Sulfur is an element that may typically be present as an impurity in the powder metallurgical steel according to the present disclosure. However, sulfur may also, if desired, be used in small amounts for the purpose of improving soft machinability of the steel through formation of manganese sulfides, MnS. Therefore, the herein described steel may comprise sulfur in an amount of equal to or less than 0.2%. Preferably, sulfur is present in an amount of equal to or less than 0.15%.

[0081] Suitably, sulfur may be present in an amount of equal to or less than 0.07%, or even equal to or less than 0.05%. In amounts equal to or less than 0.05%, sulfur is considered to be an inevitable impurity, and considered to have no effect on the desired properties of steel. However, in case the herein described steel comprises at least 0.07% S, the steel suitably also comprises at least 0.2 % Mn in order to ensure a sufficient amount of manganese that is not bound in manganese sulfides and therefore present to contribute to increased hardness.

[0082] Chromium (Cr): 3.0 - 5.5 wt.-%

[0083] Chromium is an important element in the herein described steel since it, when dissolved into the matrix of the steel, contributes to achieving desired hardness and toughness after hardening and tempering. Chromium is also a carbide forming element, and may as such contribute to increased hardness by being a part of secondary carbides. Such carbides may also contribute to the wear resistance. Therefore, the herein described steel comprises at least 3.0% Cr. Preferably, the chromium is present in an amount of at least 3.4%. Suitably, the steel comprises at least 3.7% Cr.

[0084] However, too high amounts of chromium may result in too much residual austenite that may be difficult to convert to martensite during tempering. Furthermore, higher amounts of chromium may lead to a risk of a too high amount of and / or coarse primary MgC carbides, which may be difficult to dissolve during austenitization. Therefore, the steel according to the present disclosure comprises equal to or less than 5.5% Cr. Chromium may preferably be present in an amount of equal to or less than 5.2%. Suitably, chromium may be present in an amount of equal to or less than 4.6%.

[0085] Molybdenum (Mo): 1.8 - 3.5 wt.-%

[0086] Molybdenum is an important element for achieving desired hardness and toughness. Similarly to chromium, it is dissolved into the matrix, thereby enabling desired hardness and toughness after hardening and tempering. Moreover, it may contribute to increased hardness and wear resistance by formation of secondary carbides. Furthermore, molybdenum is a frequently used alloying element in tool steels, and may therefore typically be present in scrap that may be used as a raw material for the production of the herein described steel. Replacing such scrap with other lower alloyed raw materials may unduly increase the costs of the steel. Furthermore, a too low amount of molybdenum in the steel would lead to future problems in the scrap handling. Therefore, the herein described steel comprises 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%.

[0087] The herein described steel comprises at most 3.5% Mo. Higher amounts of molybdenum may increase the risk of formation of coarse MgC carbides, which in turn may reduce the toughness. Moreover, higher amounts of molybdenum may unduly increase the alloying costs. Preferably, the steel comprises equal to or less than 3.2% Mo. Suitably, the molybdenum content may be equal to or less than 2.8%.

[0088] Tungsten (W): 1.8 - 4.0 wt.-%

[0089] Tungsten has essentially the same effects as molybdenum, and may in principle be used to partly or fully replace molybdenum in steels of the type according to the herein described powder metallurgical tool steel. It is commonly known that a certain amount of molybdenum may be replaced by double the amount of tungsten and vice versa. Furthermore, tungsten is a frequently used alloying element in tool steels, and may therefore typically be present in scrap that may be used as a raw material for the production of the herein described steel. Replacing such scrap with other lower alloyed raw materials may unduly increase the costs of the steel. Furthermore, a too low amount of tungsten in the steel would lead to future problems in the scrap handling. Therefore, the herein described steel comprises at least 1.8%. Preferably, the steel may comprise at least 2.0% W. Suitably, tungsten may be present in an amount of at least 2.2%.

[0090] The herein described steel comprises at most 4.0% W. Higher amounts of tungsten may increase the risk of formation of coarse primary MgC carbides, may reduce toughness and would unduly increase the alloying costs. Preferably, tungsten is present in an amount of equal to or less than 3.5%. Suitably, the steel may comprise equal to or less than 3.2 % W.

[0091] As evident from the above, the herein described steel as a Mo equivalent ([Mo]eq) of at least 2.7%, wherein [Mo]eq = [wt.-% Mo] + 0.5*[wt.-% W], Suitably, molybdenum and tungsten may be present in such amounts that the [Mo]eq of the steel is equal to or greater than 3.2%. Furthermore, molybdenum and tungsten may suitably be present in such amounts that the [Mo]eq of the steel is equal to or lower than 4.6%.

[0092] By experience, it has been found that about equal amounts of molybdenum and tungsten are to be preferred since this results in advantages in the production. Therefore, the ratio [wt.-% W] / [wt.-% Mo] may suitably be between 0.8 and 1.2, preferably between 0.9 and 1.1 (including the end values). Vanadium (V): 1.3 - 2.5 wt.-%

[0093] Vanadium is an element frequently used in powder metallurgical tool steels since it is an efficient carbide forming element. Vanadium forms hard primary precipitated MC carbides together with carbon, which carbides are evenly distributed in the matrix. These carbides limit the grain growth in the steel, which in turn contributes to increased toughness. Furthermore, these carbides are beneficial for the adhesive wear resistance of the steel. The herein described powder metallurgical tool steel comprises at least 1.3% V. Preferably, vanadium is present in an amount of at least 1.5%. Suitably, vanadium may be present in an amount of at least 1.7%.

[0094] However, too high amounts of vanadium may risk formation of an unduly high amount of carbides and / or carbides having a large size. A too high amount or too large carbides may reduce the toughness and is therefore not desired. In view of also comprising niobium, the herein described steel need not comprise more than 2.5%V. Preferably, vanadium may be present in an amount of equal to or less than 2.2%. Suitably, the vanadium content may be equal to or less than 2.0%.

[0095] Niobium (Nb): 1.3 - 2.5 wt.-%

[0096] Niobium is an element that plays an important role in the herein described steel. Like vanadium, niobium is a strong carbide former and may be used for limiting grain growth, which is beneficial for the toughness. Niobium also has the advantage, compared to vanadium, of forming more stable carbides. It is previously known that niobium may be used to replace vanadium, and it has been proposed that a certain amount of vanadium may be replaced by the double amount of niobium in tool steels. However, niobium tends to produce larger MC carbides than vanadium, and the niobium carbides typically have a different shape than vanadium carbides, said shape not being beneficial for toughness. Therefore, it is not suitable to completely replace vanadium with niobium. Moreover, since vanadium is the most frequently used carbide former in tool steels, it is not suitable to completely replace vanadium in order not to disturb scrap handling. Furthermore, vanadium has a considerably higher solubility in austenite than niobium. Therefore, vanadium contributes to the hardness (in the hardened and tempered condition) in a way similar to that of Mo, W and Cr, whereas the contribution of niobium is much less than that of vanadium. Also for this reason, it is not suitable to completely replace vanadium by niobium.

[0097] It has been found that, when alloying with both vanadium and niobium, the resulting MC carbides will be smaller compared to if only one of vanadium and niobium would be added. In particular, the growth of the MC carbides during various steps of the manufacturing process, such as compacting and any subsequent hot working, will be lower compared to if only one of vanadium and niobium would be added. A small carbide size is advantageous for the toughness of the steel, as well as for the adhesive wear resistance. A small carbide size is also advantageous for grindability of the steel, which is typically an important property when producing cold work tools, and it helps to make sharp edges in e.g. knives. Moreover, it also reduces the risk for micro chipping at the edges of tools. Two different types of MC carbides are expected to be formed in the steel due to the presence of both vanadium and niobium. Both types of MC carbides comprise vanadium and niobium but the first type is higher in V and the other type is higher in Nb. In other words, the steel is expected to comprise both V-rich MC carbides and Nb-rich MC carbides.

[0098] Therefore, the present powder metallurgical tool steel comprises at least 1.3% Nb. Preferably, the steel comprises at least 1.5% Nb. Suitably, niobium may be present in an amount of at least 1.7%.

[0099] However, too high contents of niobium may lead to difficulties in production as there may be an increased risk of forming niobium carbides already in the melt, which in turn may lead to problems during atomization. Furthermore, if the niobium carbides are formed too early in the production process (as a result of a too high content of niobium in the composition), they may risk growing too large. Therefore, the herein described steel comprises equal to or less than 2.5% Nb. Preferably, niobium may be present in an amount of equal to or less than 2.2%. Suitably, niobium may be present in an amount of equal to or less than 2.0%.

[0100] Suitably, the herein described powder metallurgical tool steel comprises vanadium and niobium in such amounts 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).

[0101] Nitrogen (N): equal to or less than 0.2 wt.-%

[0102] Nitrogen is an element that may be included in precipitated particles, more specifically carbonitrides or nitrides, and which in principle may be used to partly replace carbon. In the present powder metallurgical tool steel, nitrogen is not an element that is particularly aimed for. However, the powder metallurgical steel may typically comprise a certain amount of nitrogen. For example, nitrogen may be present as a result of the atomization if nitrogen is used as the medium for atomization and / or in the protective atmosphere. Nitrogen may also be present as a result of the raw material used for producing the powder metallurgical tool steel. Therefore, the powder metallurgical steel may comprise equal to or less than 0.2%. Preferably, the steel may comprise equal to or less than 0.12% N. Suitably, nitrogen may be present in an amount of equal to or less than 0.07%.

[0103] Cobalt (Co): equal to or less than 3.0 wt.-%

[0104] Cobalt is not an essential element to the chemical composition of the herein described steel, and deliberate additions of cobalt may unduly increase the alloying costs. However, if desired, cobalt may be added in amounts equal to or less than 3.0% for the purpose of increasing hardness. Preferably, the steel comprises equal to or less than 1.5% Co. Suitably, the steel comprises equal to or less than 1.0% Co.

[0105] If not purposively added, cobalt may typically be present as a result of the scrap used during production. In such a case, cobalt is typically present in amounts of less than 1.0%. Completely avoiding cobalt from the chemical composition of the steel may be difficult and would unduly increase the manufacturing cost of the steel due to requiring alternative raw material. It may however be reasonable from a cost perspective to limit the amount of Co to equal to or less than 0.6%.

[0106] Nickel (Ni): equal to or less than 1.0 wt.-%

[0107] Nickel is not an essential element to the chemical composition of the herein described steel but may typically be present as a result of the scrap used for producing the steel. Nickel is an austenite stabilizing element, and a too high content thereof may increase the risk of an unduly high amount of retained austenite after hardening that may be difficult to convert to martensite during subsequent tempering. Therefore, in the herein described powder metallurgical tool steel, nickel may be present in the amounts of equal to or less than 1.0%. Preferably, nickel is present in an amount of equal to or less than 0.5%. Suitably, nickel may be present in an amount of equal to or less than 0.3%. to or less than 1.0 wt.-%

[0108] Copper is not an essential, or even desired, element to the herein described steel but may be present as a result of the scrap used for producing the steel. More specifically, in certain steels copper may be added for the purpose of increasing corrosion resistance and / or hardness by precipitation.

[0109] Removing copper from steels is not possible once it is present, and if such steels are used as scrap in the production of the herein described steel, copper will inevitably be present. It should however be noted that the presence of copper in the herein described steel will not lead to an increased hardness in view of the intended hardening and tempering conditions. Copper is an austenite stabilizing element, and a too high content thereof may increase the risk of an unduly high amount of retained austenite that may be difficult to convert to martensite during tempering.

[0110] In the present powder metallurgical tool steel, copper may be present in amounts of equal to or less than 1.0% without substantially negatively affect the desired properties. However, the copper content should preferably be limited to maximally 0.5% for the reasons described above. Suitably, the copper content of the steel is equal to or less than 0.3%.

[0111] Impurities equal to or less than 1.5 wt.-% in total

[0112] Any steel may typically comprise elements that are not purposively added for achieving a desired property and therefore constituting impurities. Impurities may be present due to the raw material used and / or as a result of the manufacturing process.

[0113] The powder metallurgical tool steel according to the present disclosure may comprise equal to or less than 1.5 % of such impurities in total. It should here be noted that, as discussed above, the elements S, N, Co, Ni and Cu need not be purposively added and therefore present as impurities in the herein described steel. However, the content of these elements shall not be considered to be included in the total sum of equal to or less than 1.5% of impurities, even if not deliberately added as alloying elements. Preferably, the powder metallurgical tool steel comprises equal to or less than 1.0% in total, or even equal to or less than 0.6% in total, of impurities (not including S, N, Co, Ni and Cu).

[0114] One example of a normally occurring impurity in the type of steels to which the herein described powder metallurgical tool steel belongs is phosphorus. Phosphorus is an inevitable impurity element which is extremely difficult to completely avoid. It may typically be allowed in amounts of up to 0.05% without negatively affecting the desired properties. Preferably, the powder metallurgical tool steel comprises equal to or less than 0.030% P.

[0115] Another example of a normally occurring impurity in steels is aluminum, which may for example result from the manufacturing process both in case of conventionally produced steels and powder metallurgical steels. Aluminum may typically be allowed in amounts of up to 0.1%. 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 may be equal to or less than 0.2% or equal to or less than 0.1%. Moreover, the allowable content of magnesium and calcium, respectively, may suitably be equal to or less than 0.02% each. Furthermore, the allowable content of REM may suitably be equal to or less than 0.2%. The allowable content of tin may suitably be equal to or less than 0.1%. Moreover, the allowable content of oxygen may suitably be equal to or less than 200 ppm.

[0116] Method of production

[0117] The powder metallurgical tool steel described herein may be produced by atomization of a melt using a suitable atomization medium. Preferably, gas atomization is used. A steel powder is obtained by said atomization. The steel powder could for example have a maximum particle size of equal to or less than 1000 pm, and D50 could for example be about 100-200 pm.

[0118] The steel powder is thereafter compacted. Compacting could suitably be made through hot isostatic pressing (HIP), optionally preceded by cold isostatic pressing (CIP). These processes are as such previously known and will therefore not be further discussed in the present disclosure. The compacted steel powder may thereafter be processed into an intermediate product form, such as a rod, bar or blank, if not already in such a form. The processing into such forms may involve any previously known process therefore, such as forging and / or rolling.

[0119] The compacted steel powder, optionally further processed to desired intermediate product form, may thereafter suitably be soft annealed to enable soft machining to the intended geometrical configuration of the final product, such as a tool. Soft annealing may for example be made in a protective atmosphere at 850-900 °C for 1-4 hours, although other temperatures and durations are also plausible. Cooling from the soft annealing temperature is preferably conducted fairly slowly, for example at a rate of about 10 °C / h down to about 700 °C followed by air cooling, for example to avoid distortion.

[0120] If desired, the compacted steel powder or intermediate product may thereafter be stress-relieved.

[0121] This could for example be made at a temperature of about 600-700 °C for about 1-3 h. Suitably, cooling from the temperature of the stress-relieving step should be slow at least down to 500 °C. The compacted steel powder or intermediate product may thereafter be machined to the geometrical shape of the final product, if needed. This may be made in accordance with any previously known method therefore.

[0122] Thereafter, the powder metallurgical tool steel may be hardened and tempered. Hardening may suitably be made by subjecting the powder metallurgical tool steel to an austenitization temperature equal to or above 1000 °C, preferably equal to or above 1100 °C, preferably in a protective atmosphere or in vacuum. According to one embodiment, the steel is subjected to an austenitization temperature of equal to or above 1140 °C. The hardness, after hardening and tempering, typically increases with the austenitization temperature and austenitization temperatures up to at least 1200 °C are suitable. The duration at austenitization temperature depends on the austenitization temperature used, and may be shorter at higher austenitization temperatures. The duration is suitably selected so as to ensure an austenitization throughout the steel, and may thus depend on the dimension of the steel product at this stage of production. The powder metallurgical tool steel is thereafter quenched from the austenitization temperature, preferably to a temperature of equal to or below 100 °C. Quenching should preferably be performed with as high quenching rate as possible without risking causing distortions or the like. Quenching should therefore be made by forced cooling using an appropriate quenching medium. By way of example, the quenching medium may be nitrogen gas, although other quenching media are also possible. Suitably, quenching is made at a quenching rate of at least 7 °C / s.

[0123] In case of larger dimensions (for example in case of large tools) it may be appropriate to temporarily interrupt the quenching at about 540-560 °C for homogenization in order to reduce the risk of cooling cracks. In such a case, the steel is quenched from the austenitization temperature to about 540-560 °C (preferably using a quenching rate of at least 7 °C / s) followed by holding at about 540- 560 °C for a suitable duration to achieve a homogenization, and thereafter quenched to a temperature of equal to or less than 100 °C.

[0124] After hardening, the powder metallurgical tool steel is subjected to tempering. The purpose of tempering is to convert residual austenite that, in addition to martensite, may be present in the microstructure of the powder metallurgical steel after hardening, and ultimately obtain a matrix which essentially consists of tempered martensite. Tempering may suitably be made at a temperature of 520-600 °C, preferably 530-580 °C. The duration of tempering may suitably be at least 1 hour or at least 2 hours, although shorter durations are also plausible. Tempering may, if needed or desired, be performed in a plurality of consecutive steps with intermediate cooling to a temperature of about 50 °C or less, preferably equal to or less than about room temperature.

[0125] Microstructure

[0126] After hardening and tempering, the powder metallurgical tool steel has a microstructure wherein the matrix essentially consists of tempered martensite. The term "essentially consists of" shall here be considered to mean that at least 95 vol.-% of the matrix consists of tempered martensite. It should here be noted that a certain volume percentage is generally, within this technical field, determined by considering an area percentage of the relevant constituent component (such as a phase) in a sample, said area percentage considered to correspond to the volume percentage.

[0127] The microstructure further comprises carbides in a total amount of equal to or less than 10 vol.-%, typically equal to or less than 8 vol.-%. The total amount of carbides, as well as the types thereof, depend on the composition, as well as the austenitization temperature used during hardening in view of the fact the different carbides are dissolved at different temperatures. However, in the hardened and tempered condition, the steel is expected to comprise both vanadium rich MC carbides (herein called MC-V) and niobium rich carbides (herein called MC-Nb). The steel may also comprise MgC carbides, but the amount thereof may be low by appropriate selection of austenitization temperature. Preferably, the steel comprises, in the hardened and tempered condition, less than 2 vol.-% of MgC carbides; more preferably equal to or less than 1 vol.-% of MgC carbides. In general, MgC carbides are softer and tend to grow larger than MC carbides and may therefore lead to a lower toughness. A low amount of MgC carbides is therefore desired. In case MgC carbides cannot be avoided, at least a reduction of the carbide size is desired. This may be achieved by increasing the austenitization temperature of the herein described steel during hardening, leading to at least a partial dissolution of said carbides.

[0128] The average maximum size of the carbides should preferably be equal to or less than 4 pm. The average maximum size is here 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 magnitude, for example lOOOx, the size of said 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 pm.

[0129] The amount of carbides and their size are achieved by the composition of the powder metallurgical tool steel and the fact that it is produced by powder metallurgy in contrast to conventional process comprising casting (such as ingot casting or continuous casting). Moreover, it is affected by the temperature used during hardening, as described above.

[0130] The fact that the herein described steel is produced according to a powder metallurgical route also enables a good cleanliness, which is important in order to achieve a good toughness. The cleanliness, KO, should be equal to or lower than 40 when measured according to DIN50602. Preferably, K0<10 or even K0<5, when measured according to DIN50602. A desired cleanliness may be achieved through appropriate control of the powder metallurgical process.

[0131] Properties

[0132] The herein described powder metallurgical tool steel enables a very good balance between the conflicting properties hardness and toughness. This means that for a certain hardness, the herein described steel has an improved toughness; or for a certain toughness, the herein described steel has a higher hardness, compared to conventionally used cold work steels. This is achieved without an unduly high alloying cost, and / or without risking causing problems in future scrap handling.

[0133] The powder metallurgical tool steel may be hardened such that it, in the hardened and tempered condition, has a hardness of at least 62HRC, although a lower hardness is also possible if desired. Typically, a hardness of about 62-66 HRC may be achieved in the hardened and tempered condition. This is comparable with the possible hardness of ASP®2005 and considerably higher than the hardness of ASP®2012.

[0134] Furthermore, the herein described powder metallurgical steel has a very good toughness in the hardened and tempered condition. The achievable toughness is to some extent dependent of the reduction ratio of hot working (if applicable) before hardening, as well as the hardness after hardening and tempering. In the present disclosure, the term "reduction ratio" is intended to mean the reduction ratio RR as calculated according to Eq. 1 below, wherein A0 represents the cross sectional area before reduction (i.e. cross sectional area before hot working) and Al represents the cross sectional area after reduction (i.e. cross sectional area after hot working). In general, the toughness increases with reduction ratio during hot working. Furthermore, toughness may typically be increased by lowering the hardness after hardening and tempering, which in turn may be made by using a lower austenitization temperature during hardening. In practice, this means that to some extent a compromise between hardness and toughness needs to be made, depending on the intended use of the herein described steel, in the same way as for previously known cold work steels.

[0135] Moreover, it should be recognized that a measured toughness will be dependent of the orientation of the sample taken in relation to the product of the herein described power metallurgical steel. For the purpose of defining said orientations, Figure 1 schematically illustrates a round bar 1 as an example of a product obtained after hot working of the herein described powder metallurgical tool steel. The round bar has a longitudinal axis A. In the present disclosure, a toughness in the longitudinal direction shall be considered to mean a toughness measured on a sample taken in a plane coinciding with or parallel with the longitudinal axis A. A toughness in the transverse direction shall be considered to mean a toughness measured on a sample taken in a plane perpendicular to the longitudinal axis A. In other words, a toughness in a certain direction is herein described in relation to the orientation of the sample taken from the product (which will inherently be perpendicular to the direction of impact during testing).

[0136] However, for the purpose of comparison, the herein described powder metallurgical tool steel may for example when being in the hardened and tempered condition, and in case of having been subjected to hot working with a reduction degree of 96% before hardening, have a toughness in the transversal direction of at least 40 J while having a hardness of at least 62 HRC. The toughness is here intended to mean the toughness when determined in accordance with SEP 1413, which is an unnotched impact test. The toughness of the herein described powder metallurgical tool steel is considerably better than the toughness of ASP® 2005 at similar hardness in the hardened and tempered condition.

[0137] Moreover, the herein described powder metallurgical tool steel also has a good adhesive wear resistance in view of the even distribution and small size of the MC carbides. These small carbides are also expected to give good edge strength in terms of chipping resistance. These are important wear mechanisms of tools, and the herein described steel may therefore improve the quality of cold work tools and thereby has the ability to prolong the service life thereof. Experimental Results

[0138] Experimental test 1

[0139] Four different steels, produced on a laboratory scale, were tested. The compositions of the different steels are specified in Table 1, in which the content of any possible non-specified impurity element was less than 0.03%. The steels were produced by gas atomization of melts to obtain steel powders having the composition specified below, except for V4 which was obtained by mixing of two steel powders having different compositions to thereby obtain 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. Hardening was performed at two different austenitization temperatures, 1150 °C and 1180 °C, for the purpose of investigating obtainable hardness. The steels were quenched to room temperature from the austenitization temperature. Thereafter, the steels were each tempered at 560 °C in three steps for about lh each.

[0140] Table 1

[0141] The hardness, in the hardened and tempered condition, was measured and the results are specified in Table 2. As can be seen from the test results, the different steels may be hardened to a hardness well above 62 HRC. The highest hardness was achieved for steel V2, which also comprises the highest amount of carbon. Furthermore, steel V4 could be hardened to a hardness above 64 HRC, which is a very good hardness. Table 2

[0142] Furthermore, for the purpose of estimating the amount of carbides in the different steels, calculations for the respective compositions were made using the software ThermoCalc, database TCFE10. The calculations made were based on a hardening temperature (austenitization temperature) of 1180 °C. The tempering is not performed at a sufficiently high temperature to affect the size and amounts of carbides, and was therefore not considered in the calculations. The results are presented in Table 3. As can be seen from the results, the estimated total amount of carbides is well below 10 vol.-% and the estimated amount of MgC carbides is well below 2 vol.-%, which is advantageous for toughness. In fact, the calculations demonstrate that it may be possible to avoid any presence of MgC carbides in steel V3 when using a hardening temperature of 1180 °C.

[0143] Table 3 Experimental test 2

[0144] Three full-scale industrial melts were produced and each gas atomized to obtain steel powders. The compositions of the resulting steels are specified in Table 4, in which the content of any possible non-specified impurity element was less than 0.03%. The steel powders were compacted by hot isostatic pressing at 1150 °C and 1000 bar. The compacted steels were hot worked (by forging followed by rolling) to round bars, using different reduction ratios, and thereafter soft annealed at 880 °C. Hardening was performed at different austenitization temperatures, followed by quenching to room temperature. Thereafter, the steels were tempered at 560 °C in three steps of about lh each. The reduction ratios (calculated in accordance with Eq. 1 above), and the austenitization temperatures during hardening, are specified in Table 5 below.

[0145] Table 4

[0146] Samples for measurement of hardness, in the hardened and tempered condition, were taken transversal to the longitudinal direction of the bars (i.e. perpendicular to the longitudinal axis A of the round bar 1 schematically illustrated in Fig. 1). Furthermore, ten samples for measurement of toughness, in the hardened and tempered condition, were taken both in the transversal and longitudinal direction of the rods. Toughness was determined according to SEP1314 (un-notched impact toughness) and as a mean value of the toughness of the ten samples. The results are presented in Table 5, wherein transverse toughness corresponds to toughness measured on a sample taken in a transversal direction (i.e. perpendicular to the longitudinal axis A of the round bar 1 schematically illustrated in Fig. 1).

[0147] For sake of comparison, corresponding test results for the previously known ASP® 2005 and ASP® 2023 are also included in Table 5. It should however be noted that the ASP® 2005 and ASP® 2023 results relate to somewhat different reduction ratios (93% and 94%, respectively), intermediate of the ones tested for the steels Fl and F2, and lower than the one tested for the steel F3. The difference in reduction ratios should therefore be taken into account when comparing the results. Table 5

[0148] In general, toughness typically decreases with increasing hardness for powder metallurgical tool steels. This was also observed from the test results presented in Table 5.

[0149] The test results demonstrate that the steels F1-F3 may be hardened to a hardness well above 62 HRC, and similar to or higher than ASP® 2005. Furthermore, each of the steels F1-F3 demonstrates a higher toughness than ASP® 2005. For example, it can be seen that steel F2, at reduction ratio 96% and a hardness of 64.2 HRC, has a toughness in the transversal direction which is 37.5% higher than the toughness of ASP® 2005 at the somewhat lower hardness 64 HRC. Furthermore, it can be seen that steel F3, at reduction ratio 96% and hardness 63.9 HRC, has a toughness in the transversal direction with is 65% higher than the toughness of ASP® 2005 at the similar hardness 64 HRC. This is despite the fact that the comparison is made with steels F2 and F3 having a higher reduction ratio than ASP® 2005. Steel Fl has both a higher hardness and transversal toughness than ASP® 2005 when hardened at 1150 °C or 1180 °C.

[0150] For the purpose of estimating the amount of carbides in the different steels, calculations for the respective compositions were made using the software ThermoCalc, database TCFE10. The calculations made were made for four different hardening temperatures and the results are presented in Table 6. The results indicate that the total amount of carbides is well below 8 vol.-% for each of the steels for all hardening temperatures. Furthermore, it can be seen that the amount of MsC carbides decreases with increasing hardening temperature. Moreover, it can be seen that, for steel F3, it may be possible to avoid presence of MgC carbides if hardening at 1150 °C or higher.

[0151] Table 6 cont. of Table 6

[0152] In order to examine the accuracy of the above described calculations, samples from the respective steels after hardening and tempering were examined in scanning electron microscope (SEM). Figure 2 illustrates a SEM image of steel Fl when hardened at 1150 °C, showing that the microstructure comprises fine MC carbides as well as some MgC carbides. Figure 3 illustrates a SEM image of steel Fl when hardened at 1180 °C, showing that the microstructure still comprises some MgC carbides despite the higher hardening temperature. Steel F2 also showed presence of MgC carbides at the different hardening temperatures tested. Steel F3 showed only a few very small MgC carbides in the sample hardened at 1100 °C, but contained no MgC carbides when hardened at temperatures of 1150 °C or higher. Figure 4 illustrates a SEM image of steel F3 when hardened at 1150 °C. Moreover, the carbide size in steel F3 was determined and the result is presented in Table 7. The determination of carbide size was performed by measuring the three largest carbides in 10 different fields of view, at a magnitude of lOOOx, and determining an average value thereof. Thus, the carbide size presented in Table 7 represents an average of the largest carbides. As evident from the results, the carbides in steel F3 are small for all hardening temperatures.

[0153] Table 7

[0154] Experimental test 3

[0155] To further investigate the obtainable properties of steel F3, described above, the experimental test 2 was repeated with the difference of tempering being conducted at 540 °C in three steps of about lh each. The results are presented in Table 8.

[0156] Table 8

[0157] As can be seen from the result, when comparing with the results given in Table 5, a lower temperature during tempering may increase the hardness of steel F3 further while still achieving a good toughness.

[0158] Experimental test 4

[0159] A tribological test was performed to investigate the adhesive wear resistance of the herein described power metallurgical steel compared to the previously known ASP®2005 and ASP®2023. Figure 5 schematically illustrates the principle of the tribological test performed. A cylindrical test specimen 3 slides against a cylindrical counter body 4 while a continuously increasing normal load is applied. More specifically, the normal load is continuously increased along the sliding path, resulting in increasing plastic deformation of the test specimen 3 and counter body 4 and thereby sliding distance, to pass across the widening track 5.

[0160] A tribological load scanner was used to perform the test in accordance with the principle described above. A description of said load scanner may for example be found 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 here be noted that the set-up is unique and results in that each point along the contact path of the test specimen has only met one single point on the counter body, and only experience one specific normal load.

[0161] Cylindrical test specimens with a diameter of 10 mm and a length of 100 mm were produced from steel F3, described above, as well as ASP®2005 and ASP®2023. The tests specimens of each of the three steels were heat treated by hardening at 1150°C followed by tempering at 560 °C in three steps of about lh each. The counter body, having the same dimensions as the test specimens, consisted of conventionally produced 304 steel. The test was performed in dry contact, i.e. without the use of any lubricant, and at room temperature. The studied parameters were coefficient of friction vs normal load. The load range studied was 150-850 N (normal load). Five strokes were performed for each of the test specimens, each stroke being along the same track and with the same normal load increase along the sliding path.

[0162] The result is shown in Figure 6, representing an average of the five strokes. It can be clearly seen from the results that steel F3 has a significantly lower coefficient of friction over the whole load range. A lower friction indicates less adhering to the steel of the counter body. Thus, it can be seen that steel F3 has better resistance against galling / adhesive wear. Galling is the adhesive wear caused by microscopic transfer of material between metallic surfaces during transverse motion (sliding). Without being bound by theory, it is believed that the improved adhesive wear resistance is (at least partly) a result of the very fine MC carbides of the herein described powder metallurgical steel.

Claims

CLAIMS1. A powder metallurgical tool steel having a chemical composition comprising, in wt.-%:C 0.9 - 1.3,Si 0.2 - 0.8,Mn 0.1 - 0.6,S equal to or less than 0.2,Cr 3.0 - 5.5,Mo 1.8 - 3.5,W 1.8 - 4.0,V 1.3 - 2.5,Nb 1.3 - 2.5,N 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, balance Fe and any impurities equal to or less than 1.5 wt.-% in total.

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

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

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

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

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

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

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

9. The steel according to any one of the preceding claims, wherein the steel comprises 1.5 - 2.2 wt.-% Nb, preferably 1.7 - 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 lower than 4.6%.

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

12. The steel according to any one of the preceding claims, wherein the steel, in a hardened and tempered condition, comprises less than 2 vol.-% of MgC carbides.

13. The steel according to any one of the preceding claims, wherein the carbides of the steel, in a hardened and tempered condition of the steel, have an average maximum size of 4 pm; wherein the average maximum size is determined based on the size of the three largest carbides identified by microscopy, the size of said three largest carbides measured in 10 different fields of view.

14. The steel according to any one of the preceding claims, wherein the steel, in the hardened and tempered condition and when having been hot worked with a reduction 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 transversal direction according to SEP 1413.

15. Use of the powder metallurgical tool steel according to any one of the preceding claims for producing of a tool adapted for cold working applications.

16. A method for manufacturing a powder metallurgical tool steel, the method comprising: a) producing a steel powder having the following chemical composition, in wt.-%:C 0.9 - 1.3,Si 0.2 - 0.8,Mn 0.1 - 0.6,S equal to or less than 0.2,Cr 3.0 - 5.5,Mo 1.8 - 3.5,W 1.8 - 4.0,V 1.3 - 2.5,Nb 1.3 - 2.5,N 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, balance Fe and any impurities equal to or less than 1.5 wt.-% in total; b) compacting the steel powder, preferably wherein said compacting comprises hot isostatic pressing; c) optionally soft annealing and / or stress relieving the compacted steel; d) hardening by subjecting the steel to an austenitization temperature of equal to or above 1000 °C, followed by quenching; e) tempering by subjecting the hardened steel to a temperature of 520 to 600 °C. A cold work tool comprising the powder metallurgical tool steel according to any one of claims 1 -14.

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