EP2004870A1

EP2004870A1 - Cold-working steel

Info

Publication number: EP2004870A1
Application number: EP07748400A
Authority: EP
Inventors: Magnus Tidesten; Lennart JÖNSON; Odd Sandberg
Original assignee: Uddeholms AB
Current assignee: Uddeholms AB
Priority date: 2006-04-13
Filing date: 2007-04-12
Publication date: 2008-12-24
Anticipated expiration: 2027-04-12
Also published as: CA2644363A1; EP2004870B1; TWI365916B; JP5323679B2; CN101421430A; US20090010795A1; KR20080110674A; SE0600841L; AU2007239111A1; EP2004870A4; WO2007120110A1; TW200745352A; BRPI0709944A2; RU2437951C2; AU2007239111B2; CN101421430B; RU2008136562A; MX2008012947A; JP2009533554A

Abstract

11905 22 ABSTRACT The invention relates to a cold-working steel having the following chemical composition in % by weight: 1.3 - 2.4 (C+N), whereof at least 0.5 C, 0.1 - 1.5 Si, 5 0.1 - 1.5 Mn, 4.0 - 5.5 Cr, 1.5 - 3.6 (Mo+W/2), but max 0.5 W, 4.8 - 6.3 (V+Nb/2), but max 2 Nb, and max 0.3 S, 10 in which the content of (C+N) on the one hand and of (V+Nb/2) on the other hand, are balanced in relation to each other such that the contents of these elements are within an area that is defined by the coordinates A, B, C, D, A in the system of coordinates in Fig. 11, where the coordinates of [(C+N), (V+Nb/2)] for these points are: A: [1.38, 4.8] 15 B: [1.78, 4.8] C: [2.32, 6.3] D: [1.92, 6.3], balance essentially only iron and impurities at normal contents.

Description

COLD-WORKING STEEL

TECHNICAL FIELD The invention relates to a cold-working steel, i.e. a steel intended to be used in the working in cold condition of a working material. Typical examples of the use of the steel are tools for cutting and punching, thread cutting, such as threading dies and thread taps, cold extrusion, the pressing of powder, deep drawing, cold forging. The invention also relates to a method of working a metal working material or the pressing of powder by a tool comprising the steel, as well as a method of producing the steel.

BACKGROUND OF THE INVENTION

There are many requirements on high quality cold-working steels, such adequate hardness for the application, and good resistance to wear and high toughness/ductility. It is important for optimal tool performance that these properties are satisfied.

VANADIS ^R 4 is a powder metallurgically manufactured cold- working steel that is manufactured and marketed by the present applicant and that has a combination of resistance to wear and toughness/ductility for high performance tools that is considered to be excellent. The nominal composition of the steel is, in % by weight: 1.5 C, 1.0 Si, 0.4 Mn, 8.0 Cr, 1.5 Mo, 4.0 V, balance iron and unavoidable impurities. The steel is particularly suitable for applications in which adhesive/abrasive wear or chipping are the dominant problems, i.e. for soft/tacky working materials such as austenite stainless steels, simple carbon steels, aluminium, copper etc., as well as for thick working materials. Typical examples of cold- working tools for which the steel can be used are mentioned in the above introduction. Generally, it can be stated that VANADIS^® 4, which is the object of Swedish patent no. 457,356, is characterised by good resistance to wear, high compressive strength, good hardenability, excellent toughness, excellent dimensional stability in connection with heat treatment and good resistance to tempering; all properties that are important to a high performance cold-working steel.

The applicant manufactures and markets another powder metallurgically manufactured cold-working steel VANADIS^® 6, that is characterised by excellent resistance to wear and relatively good toughness, whereby the steel is suitable for applications in which abrasive wear is a dominant feature and in which manufacturing takes place in long series of manufacture. The nominal composition of the steel is, in % by weight: 2.1 C, 1.0 Si, 0.4 Mn, 6.8 Cr, 1.5 Mo, 5.4 V, balance iron and unavoidable impurities. The resistance to chipping, the machinability and the grindability are not as good as for VANADIS^® 4.

A follow-up to the above mentioned VANADIS^® 4 is marketed under the name VANADIS ^R 4 Extra and is characterised by a toughness that is even better than that of VANADIS ^R 4, its other performance features being maintained or improved as compared to this material and having in principle the same field of application. The steel has had a huge commercial success and it has the following chemical composition, in % by weight: 1.38% C, 0.4% Si, 0.4% Mn, 4.7% Cr, 3.5% Mo, 3.7% V.

Several commercial steels are known that fall within the wide composition range specified in US patent no. 4,249,945. The steel having the chemical composition 2.45 C, 0.50 Mn, 0.90 Si, 5.25 Cr, 9.75 V, 1.30 Mo and 0.07 S is available on the market, and a steel is also comprised that contains 1.80 C, 0.50 Mn, 0.90 Si, 5.25 Cr, 1.30 Mo and 9.00 V. The steels are powder metallurgically manufactured and marketed for use in applications that require good resistance to wear and adequate toughness.

Due to excellent properties, the above mentioned VANADIS^® steels have gained a leading market position among high performance cold-working steels. Also the above mentioned competitive steels have been successful on the same market. VANADIS^® 4 Extra in particular has proven to have excellent properties.

Hence, the present applicant has the ambition of providing yet another high performance cold-working steel having a property profile that is considerably much better than that of the above mentioned steels. According to one aspect of the invention, the steel should have properties for the application that in general are improved, particularly in relation to VANADIS ^R 6. According to another aspect it has been a desire to provide a steel having good resistance to wear, beneficially at the same level as VANADIS^® 6 and VANADIS^® 10, but having considerably improved toughness/ductility in relation to these steels. According to yet another aspect, the steel is characterised by good machinability and improved resistance to wear. According to yet another aspect of the invention, it is also an object to be able to provide a steel having high hardness, preferably in combination with good hardenability. The fields of application of the steel are in principle the same as for VANADIS ^R 4. ACCOUNT OF THE INVENTION

It is an object of the invention to provide a steel that fulfils at least some of the above mentioned high demands on a high performance cold-working steel. This is achieved by a cold-working steel with the following chemical composition in % by weight: 1.3-2.4 (C + N), whereof at least 0.5 C, 0.1-1.5 Si, 0.1-1.5 Mn, 4.0-5.5 Cr, 1.5-3.6 (Mo + W/2), but max 0.5 W, 4.8-6.3 (V + Nb/2), but max 2 Nb, and max 0.3 S, the content of (C + N) on one hand and of (V + Nb/2) on the other hand being balanced in relation to each other such that the contents of these elements are within an area that is defined by the coordinates A, B, C, D, A in the system of coordinates in Fig. 11, where the coordinates [(C + N), (V + Nb/2)] for these points are: A: [1.38, 4.8], B: [1.78, 4.8], C: [2.32, 6.3], D: [1.92, 6.3], balance essentially only iron and impurities at normal contents. It is also an object to provide a method for cutting, shearing, punching and/or forming working in cold condition of a metal working material, by a tool comprising a steel according to the invention, a method of pressing a metal powder by a tool comprising a steel according to the invention, and a method of manufacturing a steel according to the invention.

The steel according to the invention is powder metallurgically manufactured, which is a prerequisite for the steel to be to a high degree free from oxide inclusions. Preferably, the powder metallurgical manufacturing comprises gas atomizing of a steel melt by nitrogen as an atomizing gas, whereby the steel alloy will achieve a minimum content of nitrogen. If desired, the steel powder can be nitrided in solid phase in order to further increase the content of nitrogen in the steel. Thereafter consolidation takes place by hot isostatic pressing. The steel can be used in this condition or after forging/rolling to a final dimension.

When nothing else is stated the present description always refers to percent by weight in respect of the chemical composition of the steel and percent by volume in respect of the structural components of the steel. By the denotation MX-carbides, MyX3-carbides or just carbides is always intended carbides as well as nitrides and/or carbonitrides, if nothing else is stated. By MόC-carbides is always meant nothing but carbides.

The following is true for individual alloying materials and their mutual relations and for the structure and heat treatment of the steel.

Carbon, and where appropriate also a certain amount of nitrogen, should be present at an amount in the steel that, in the hardened and tempered condition of the steel, typically from an austenitizing temperature T_A of 1050⁰C, is adequate together with vanadium and where appropriate niobium to form 8-13 % by weight of MX-carbides, where M is essentially vanadium and X is carbon and nitrogen, preferably predominantly carbon, of which carbides at least 90 % by volume have an equivalent diameter of max 2.5 μm, preferably max 2.0 μm. Such MX-carbides contribute in a manner that is known per se to the person skilled in the art, to give the steel a desirable resistance to wear and also they have a certain effect of giving finer grains, and also a certain amount of secondary hardening. By an adapted heat treatment, i.e. choice of austenitizing temperature and tempering temperature, the steel's content of MX-carbides can be varied within the above range such that a microstructure is obtained that is suitable for the purpose, which is described in greater detail in the description of the performed experiments and in the description of the enclosed figures. In addition to these MX-carbides, the steel should be essentially free from other primary precipitated carbides such as M₇X₃- and M_όC-carbider.

Preferably, the steel does not contain more nitrogen than what is inevitably and naturally comprised due to uptake from the surroundings and/or from added raw materials, i.e. max about 0.12%, preferably max about 0.10%. In a conceivable embodiment, the steel may however contain a larger, deliberately added amount of nitrogen, that can be supplied by solid phase nitriding of the steel powder that is used in the manufacturing of the steel. In this case, the major part of (C + N) may be nitrogen, which means that in this case said M is mainly vanadium carbonitrides in which nitrogen is the main ingredient together with vanadium, or even is pure vanadium nitrides, while carbon exists essentially only as dissolved in the matrix of the steel in its hardened and tempered condition.

Vanadium should be present in the steel at a content of at least 4.8 % but max 6.3% in order, together with carbon and any nitrogen present, to form the above mentioned MX- carbides at a total content of 8-13 % by volume, in the hardened and tempered condition of use of the steel. Vanadium may in principle be replaced by niobium but that requires the double amount of niobium as compared with vanadium, which is a drawback. Niobium also results in a more angular shape of the MX-carbides and they become larger than pure vanadium carbides, whereby fractures or chippings can be initiated, hence lowering the toughness of the material, which is a drawback. Accordingly, niobium must not be present at a content above 2%, preferably max 1% and suitably max 0.1%. It is most preferred that the steel does not contain any deliberately added niobium and it should not be tolerated at contents above impurity contents in the form of residual elements originating from raw materials included in the manufacturing of the steel.

According to one aspect of the invention, the contents in the steel of (C + N) on the one hand and of (V+Nb/2) on the other hand, should be balanced in relation to each other such that the contents of these elements are within an area that is defined by the coordinates A, B, C, D, A in the system of coordinates in Fig. 11, where the coordinates of [(C + N), (V + Nb/2)] for these points are: A: [1.38, 4.8], B: , 1.78, 4.8], C: [2.32, 6.3], D: [1.92, 6.3]. Within these ranges, a steel with a very beneficial property profile can be provided. An adapted combination of hardness, resistance to wear, ductility and machinability can be obtained by an adapted heat treatment. Within this widest range of composition it is generally true that the hardness and the resistance to wear will increase the higher the total amount of (C + N) and (V+Nb/2) in the steel, while ductility is favoured the lower the total amount of these elements.

According to a more preferred embodiment, the content of these elements should be within an area defined by the coordinates E, F, G, H, E in the system of coordinates in Fig. 11 , where the coordinates of [(C + N), (V + Nb/2)] for these points are: E: [1.48, 4.8], F: [1.68, 4.8], G: [2.22, 6.3], H: [2.02, 6.3].

According to an even more preferred embodiment, the contents of (C + N) on the one hand and of (V+Nb/2) on the other hand, should be balanced in relation to each other such that the contents of these elements are within an area that is defined by the coordinates K, L, M, N, K in the system of coordinates in Fig. 11, where the coordinates of [(C + N), (V + Nb/2)] for these points are: K: [1.62, 5.2], L: [1.82, 5.2], M: [2.05, 5.8], N: [1.85, 5.8].

According to yet another aspect of the invention, the contents of (C + N) on the one hand and of (V+Nb/2) on the other hand, should be balanced in relation to each other such that the contents of these elements fulfil the requirement 0.32 < (C + N)/(V + Nb/2) < 0.35.

According to yet another aspect of the invention, the contents of (C + N) on the one hand and of (V+Nb/2) on the other hand, should be balanced in relation to each other such that the contents of these elements are within an area that is defined by the coordinates A', B', C, D', A' in the system of coordinates in Fig. 11, where the coordinates of [(C + N), (V + Nb/2)] for these points are: A': [1.52, 5.2], B: [1.93, 5.2], C: [2.18, 5.9], D: [1.77, 5.9].

Carbon also contributes to the hardness by being present in solid solution in the matrix of the steel in its hardened and tempered condition, at a content of 0.4-0.6 % by weight at an austenitizing temperature T_A of 980 - 1050 C.

Silicon is present as a residual element from the manufacturing of the steel, at a content of at least 0.1%, normally at least 0.2%. Silicon increases the carbon activity in the steel and hence it contributes to give the steel an adequate hardness. Contents that are too high may lead to brittleness problems due to solution hardening and hence the maximum content of silicon in the steel is 1.5%, preferably max 1.2%, suitably max 0.9%. A content of Si that is beneficial for the steel is 0.2-0.5 Si. The steel has a nominal content of 0.4% Si.

Manganese is added to the steel at a content of at least 0.1%, in order to bind the amount of sulphur that may be present in the steel, by forming manganese sulphides. Manganese, as well as the elements chromium and molybdenum, also contributes to give the steel an adequate hardenability, which means that a manganese content of 0.1% can be tolerated without any negative effects on the steel properties. At high contents, manganese can cause an undesirable stabilisation of residual austenite, which leads to impaired hardness. Residual austenite will also make the steel less dimensional stable which is a major drawback. Hence, the manganese content should not exceed 1.2% Mn and a beneficial manganese content for the steel is in the range of 0.1-0.9% Mn. The steel has a nominal content of 0.4% Mn.

As mentioned above, chromium contributes to the hardenability of the steel and for that reason it should be present at a content of at least 4.0%, preferably at least 4.5%. Chromium is also a carbide forming element and in many steels it is used to contribute to the steel's resistance to wear by the formation of MyX3-carbides. Such carbides can be dissolved at various extent by choice of a suitable austenitizing temperature at the hardening, and chromium and carbon that have been dissolved in the austenite in this manner can then be precipitated at various extent to form very small secondary precipitated carbides that will efficiently contribute to give the steel a desired hardness in connection with the tempering. The steel according to the invention should among other things exhibit very good resistance to wear and it should be able to be hardened to a comparatively high hardness. It has now been shown that this can be achieved at the same time as the steel is given a surprisingly good ductility, superior to some of the applicant's own steels that are marketed for similar applications. By limiting the content of chromium, it has been possible to avoid or at least minimize the formation of MyX3-carbides in favour of the formation of primary precipitated MX-carbides. In order to achieve such a beneficial carbide composition, the chromium content should therefore be limited to max 5.5% and even more preferred max 5.1%. A content of chromium that is beneficial for the steel is 4.8%.

The major part of the chromium that is added to the steel will be dissolved in the steel in order that way to contribute to the hardenability of the steel. According to the concept of the invention, the steel should have a requisite hardenability in order for varying dimensions to be hardened all the way through and if the steel is to be used in coarse dimensions hardenability is a particularly important aspect. Therefore, molybdenum should be present in the steel at a content of at least 1.5%. Without risking precipitation of non-desirable MόC-carbides, the content of molybdenum can be tolerated at up to 3.6% Mo. Preferably, the steel contains between 1.5 and 2.6% Mo and even more preferred between 1.6 and 2.0% Mo.

To a certain extent molybdenum can be replaced by tungsten but this requires double the amount of tungsten as compared to molybdenum, which is a drawback. It also makes scarp handling more difficult. Hence, tungsten should not exist at a content of more than max 0.5%, preferably max 0.3% and suitably max 0.1%. It is most preferred that the steel does not contain any deliberately added tungsten and in the most preferred embodiment it should not be tolerated at contents above impurity level in the form of residual elements originating from raw materials included in the manufacturing of the steel.

Sulphur is present in the steel primarily as an impurity at a content of max 0.03%. It is however conceivable according to one embodiment, in order to improve the machinability of the steel, that the steel contains deliberately added sulphur at a content of up to max 0.3%, preferably max 0.15%.

A nominal composition of the steel according to the invention is 1.77% C, 0.4% Si, 0.4% Mn, 4.8% Cr, 2.5% Mo and 5.5% V, balance essentially iron. The following composition is an example of a conceivable variant of the steel, within the scope of the invention: 1.9% C, 0.4% Si, 0.4% Mn, 4.8% Cr, 3.5% Mo, 5.8% V, balance essentially iron.

The following composition is yet another example of a conceivable variant of the steel: 1.67% C, 0.4% Si, 0.4% Mn, 4.8% Cr, 2.3% Mo, 5.2% V, balance essentially iron.

The following composition is yet another example of a conceivable variant of the steel: 1.80% C, 0.4% Si, 0.4% Mn, 4.8% Cr, 1.8% Mo, 5.8% V, balance essentially iron.

The variants above have been optimized to achieve somewhat differing property profiles, such that the steel with an increased content of the carbide formers molybdenum and vanadium will get a better resistance to wear at the expense of a somewhat lower ductility. The steel having a decreased content of these two elements will get a higher ductility at the expense of a somewhat lower resistance to wear.

In the manufacturing of the steel, a steel melt is first prepared containing the intended amounts of carbon, silicon, manganese, chromium, molybdenum, possibly tungsten, vanadium, possibly niobium, possibly sulphur beyond impurity content, nitrogen at unavoidable content, balance iron and impurities. A powder is produced from this melt, by nitrogen gas atomization. The droplets formed in gas atomization are quench cooled, such that the formed vanadium carbides and/or mixed carbides of vanadium and niobium do not have time to grow but become extremely thin - having a thickness of no more than a fraction of a micrometre - and get a pronounced irregular shape that comes from the carbides being precipitated in areas of residual melt in the dendrite network in the rapidly solidifying small droplets, before the droplets solidify to form powder grains. In the case that the steel is to contain nitrogen beyond an unavoidable impurity content, this is achieved by nitriding of the powder, e.g. such as is described in SE 462,837.

After sieving, which if the powder is to be nitrided is suitably performed before the nitriding, the powder is charged in capsules that are then evacuated and sealed and are exposed to hot isostatic pressing, HIP:ing, as is known per se, at high temperature and high pressure; 950-1200°C and 90-150MPa; typically at about 115O⁰C and 100 MPa, such that the powder is consolidated to form a completely dense body. By the HIP: ing, the carbides will get a much more regular shape than they have in the powder. The predominant volume part has a size of max about 1.5 μm and a rounded shape. Occasional particles are still elongated and somewhat longer, max about 2.5 μm. The transformation is most likely due to a combination of a breaking up of the very thin particles in the powder and coalescence.

The steel can be used in the HIP:ed condition. Normally, the steel is however hot- worked after HIP:ing, by forging and/or hot rolling. This is performed at an initial temperature of between 1050 and 1150⁰C, preferably about 1100⁰C. Hereby, an additional coalescence and in particular a spheroidisation of the carbides take place. After forging and/or rolling, at least 90% by volume of the carbides have a size of max 2.5 μm, preferably max 2.0 μm.

In order to be able to work the steel by cutting tools, it must first be soft annealed. This takes place at a temperature of below 950⁰C, preferably about 900⁰C. When the tool, by cutting, has been given its final shape, it is hardened and tempered. In the austenitization, the MX-carbides are to some extent dissolved in order to be secondary precipitated in the annealing. Besides these MX-carbides, the steel should not contain any other carbides. The hardening can take place from a considerably much lower austenitizing temperature than what is conventional for steels with a corresponding resistance to wear, normally between 980 and 1150⁰C, preferably below 1100⁰C, in order thereby to avoid undesirably extensive dissolving of MX-carbides. A suitable austenitizing temperature is 1000-1050⁰C. This is a decisive advantage for the tool manufacturer, since the steel may then be heat treated together with the major part of other tool steels on the market. In the hardened condition of the steel, T_A 980 - 1050⁰C, the matrix consists essentially of martensite only that contains 0.4-0.6% carbon in solid solution.

The subsequent tempering can be performed at a temperature of between 200 and 600⁰C, preferably at a temperature of between 500 and 560⁰C. The final result is the microstructure that is typical for the invention and that consists of tempered martensite and in the tempered martensite 8-13 % by volume of MX-carbides, where M is essentially vanadium and X is carbon and nitrogen, preferably in the main carbon, of which carbides at least 90 % by volume have an equivalent diameter of max 2.5 μm, preferably max 2.0 μm. The carbides have a predominantly round or rounded shape but occasional longer carbides may exist. In this description, the equivalent diameter D_ekv is defined as 2Λ/ A/π, where A is the area of the carbide particle in the studied section. Typically, at least 96 % by volume of the MX-carbides, -nitrides and/or carbonitrides have a D_ek_v < 3.0 μm. Normally, the carbides are also spheroidised to such an extent that no carbides have an actual length above 3.0 μm in the viewed section. After hardening and tempering, the steel has a hardness of 58-66 HRC.

Other characteristics and aspects of the invention are clear from the appended claims, and from the following account of experiments that have been made.

BRIEF DESCRIPTION OF THE DRAWINGS In the following description of experiments made, reference will be made to the enclosed drawings, of which

Fig. 1 shows the microstructure of a steel according to the invention, after hardening and tempering, Fig. 2 shows the microstructure of a commercial comparative material, after hardening and tempering, Fig. 3 shows the microstructure of a yet a commercial comparative material, after hardening and tempering,

Fig. 4 is a graph that shows the hardness of a steel according to the invention, as a function of the austenitizing temperature,

Fig. 5 is a graph that shows the hardness of a steel according to the invention at different austenitizing temperatures and as a function of tempering temperature, Fig. 6 is a graph showing the ductility of a high temperature tempered steel according to the invention, as well as a number of comparative materials, Fig. 7 is a graph showing the machinability the steel according to the invention, as well as a number of comparative materials, Fig. 8 is yet a graph showing the machinability of a steel according to the invention, as well as a comparative material,

Fig. 9 shows the combination of un-notched impact energy and resistance to wear for a steel according to the invention as well as for a number of comparative materials, Fig. 10 shows wear rate in wear tests of the steel according to the invention as well as a number of comparative materials,

Fig. 11 shows a graph over the relation between the content of carbon and any existing nitrogen in relation to the content of vanadium and any existing niobium,

Fig. 12 shows a graph over the edge wear on the upper and lower knife after cutting tests, Fig. 13a, b shows the side face of the upper knife after cutting tests, Fig. 14a, b shows the front face of the upper knife after cutting tests, and Fig. 15a, b shows the front face of the lower knife after cutting tests.

ACCOUNT OF CONDUCTED EXPERIMENTS

The chemical composition of the examined steels is given in Table 1. In the table, the sulphur shown for some of the steels is an impurity. Other impurities have not been accounted for but do not exceed normal impurity levels. Balance is iron. In table 1, steel

7 has a chemical composition according to the invention. Steels 1-5 are reference materials.

Table 1 - Chemical composition for examined steels, in % by weight

Steels 1-5 are commercial steels of which all but steel no. 1 are the applicant's steels. Material samples of these steels were ordered and analysed in respect of chemical composition. All of these steels are powder metallurgically manufactured and were ordered in the soft-annealed condition. A melt of 6 ton was produced from steel no. 7 according to a conventional melt metallurgical technique. Metal powder was manufactured from the melt by nitrogen gas atomisation of a melt jet. The small droplets formed were quench cooled.

Blanks of 2 ton each were produced from the powder of steel no. 7, having the chemical composition according to Table 1. The steel powder was filled into capsules of sheet metal, which were then sealed, evacuated, heated to about 1150⁰C and thereafter subjected to hot isostatic pressing (HIP) at about 1150⁰C and a pressure of 100 MPa. The originally obtained carbide structure of the powder was broken down in the HIP:ing at the same time as the carbides coalesced. In the HIP:ed condition of the steel, the carbides have obtained a more regular shape, approaching a spheroidised shape. They are still very small. The predominant part, more than 90 % by volume, has an equivalent diameter of max 2.5 μm, preferably max about 2.0 μm. Thereafter, the blanks were forged at a temperature of 1100⁰C to the dimension 100 mm round bar. Steel no. 7 was soft-annealed at 900⁰C and its microstructure was examined and hardness testing was performed. The carbides are present in the material in the form of very small, still max about 2.0 μm large, in terms of equivalent diameter, essentially spheroidised MX-carbides. After soft-annealing, test samples were taken from steel no. 7 for continued examinations. The same type of test samples were taken from the reference materials 1-5 that had been ordered in the soft-annealed condition.

The heat treatment in connection with hardening and tempering of the various steels is presented in Table 2. The microstructure in the hardened and annealed condition was examined for three of the steels, more specifically steel no. 7 according to the invention, shown in Fig. 1, and reference steels no. 4 and 1, shown in Figs. 2 and 3, respectively. The steel according to the invention, Fig. 1, contained 11.7 % by volume of MX- carbides in the matrix that consisted of tempered martensite. No carbides besides MX- carbides could be detected. Occasional carbides having an equivalent diameter of more than 3.0 μm could be found in the steel according to the invention in the hardened and tempered condition.

Reference steel no. 4, Fig. 2, contained, in the hardened and tempered condition, a total of about 14.4 % by volume of carbides, whereof about 9.2 % by volume were MC- carbides and about 5.2 % by volume were MyC3-carbides. As is clear from the figure, the MyC3-carbides are relatively large, in general larger than the MC-carbides, and this has a negative effect primarily on ductility. Reference steel no. 1, Fig. 3, contained, in the hardened and tempered condition, about 15.7 % by volume of MC-carbides. No other carbides were detected. The high content of carbides results in a relatively good resistance to wear but a lower ductility for the steel.

Hardness after the heat treatment as defined in Table 2 is also given in Table 2. Following high temperature tempering, steel 7 according to the invention obtained a hardness comparable with the high alloy reference material no. 5, and the hardness was about 1 HRC unit higher than the examined reference materials no. 2-4.

The impact strength of the above materials was also examined and the results are shown in Fig. 6. The impact energy (J) absorbed in both the LC2 and the CR2 directions, was measured and for steel no. 7 according to the invention a dramatic improvement was measured as compared primarily with reference material no. 4 that is the material intended for further development. The best value for steel no. 7 according to the invention was 37 J in the cross direction (CR2), which was measured after high temperature tempering. This corresponds to an improvement of about 60% as compared to reference material no. 4.

Even when hardness is taken into account it is clear that steel no. 7 according to the invention has a unique combination of high hardness and very good ductility, closest in relation to reference material no. 5 that has comparable hardness, which is shown in Fig. 9. The sample rods were cut and ground, un-notched sample rods with the dimension 7 x 10 mm and the length 55 mm, hardened to hardnesses according to Table 2.

The hardness of steel no. 7 according to the invention was also examined after various austenitizing temperatures and tempering temperatures. The results are shown in the graphs in Figs. 4 and 5. Already at a relatively low austenitizing temperature of 1030⁰C steel no. 7 exhibited a hardness maximum, which must be seen as very advantageous from a heat treatment point of view as the major part of tool steels on the market are heat treated at about that temperature. The major part of the reference steels must be heated to about 1060-1070⁰C in order to obtain a maximum hardness. For reference steel no. 1 a maximum hardness is not achieved until a temperature of 1100-1150⁰C.

As is clear from Fig. 5, a pronounced secondary hardening is achieved by tempering at a temperature of between 500 and 550⁰C. The steel also provides the possibility of low temperature tempering at about 200-250⁰C. It is also clear from the figure that residual austenite can be eliminated by high temperature tempering.

The resistance to wear of the steel according to the invention was also compared with a number of reference materials and the results are given in Fig. 10. In the wear test, sample rods were used having the dimension 0 15 mm and the length 20 mm. The examination was conducted as a pin-on-disc test with SiO₂ as the abrasive wearing agent. Before the wear tests, reference steels no. 2-5 and steel no. 7 according to the invention had been high temperature tempered to a hardness of 62.5 HRC. Reference steel no. 1 had somewhat higher hardness, 62.7 HRC, obtained by hardening from 1120°C/30 min and tempering at 540°C/3x2h. Wear rate in mg/min is also shown in Table 2. Steel no. 7 was shown to have about the same good resistance to wear as reference steel no. 4 and was superior to reference steels no. 2 and 3. Reference steel no. 5 has somewhat better resistance to wear as compared to steel no. 7. Reference steel no. 1 had the best resistance to wear of all steels. In two different experiments, the machinability of steel no. 7 according to the invention was compared with reference steels 2-5 and the result is shown in Table 2 and also in Fig. 7 and Fig. 8. Fig. 7 shows the result when testing the machinability by turning of soft-annealed test samples with a hard metal cutting edge and in Fig. 8 shows drilling tests for the materials with uncoated drills. The results of these tests show that steel no. 7 according to the invention has a very good machinability, i.e. high values of V30 and VlOOO, practically the double as compared to reference material 4.

In application tests, the resistance to edge wear was examined by cutting tests. Cutting knives were manufactured from steel no. 4 and steel no. 7. The knives were hardened and tempered to a hardness of 60.5 HRC and 60.0 HRC respectively. The cutting tests were performed in an ESSA eccentric press with a maximum cutting load capacity of 15 tons and a cutting speed of 200 cuts per minute. The cutting was performed on high strength steel strips in the steel grade Docol 1400M, width 50 mm, thickness 1 mm. The cutting clearance was 0.05 mm.

The edge wear on both upper and lower knives were measured and the result is shown in Fig. 12. In Fig. 12 a graph shows the edge wear after 100 000 cuts and after the test has finished. For the knife manufactured from steel no. 5, the test had to be stopped after 150 000 cuts because of the chipping of the edge. The knife manufactured from steel no. 7 showed no tendency to chipping after 315 000 cuts when the test was finished. It is evident that steel no. 7 showed a much better resistance to edge wear than steel no. 5.

In Fig. 13a, b, the side face of the upper knife of steel No. 5 after 150 000 cuts and steel No. 7 after 315 000 cuts is shown after the finished tests, i.e. the face of the cutting tool that is parallel to the cutting direction. It can be seen from the figures that steel No. 5 shows significantly more abrasive wear after 150 000 cuts compared to steel No. 7 after more than twice as many cuts.

Fig. 14a, b, shows the front face of the upper knife of steel No. 5 and steel No. 7, and Fig. 15a,b, shows the front face of the lower knife of steel No. 5 and steel No. 7 i.e. the face of the cutting tool that is perpendicular to the cutting direction of the steel plate, after 150 000 cuts and 315 000 cuts respectively,. It can be seen that both the upper and lower knife manufactured from steel No. 5 shows chipping of the edge while the edge of steel No. 7 shows no tendency to chipping. The application tests indicate that the inventive steel have better toughness and better resistance to wear than the reference steel No. 5. Particularly the resistance to chipping is advantageous.

According to the concept of the invention, the steel should have a good hardenability. With a steel according to the invention, it has proven possible to let the hardenability vary within the wide ranges of the steel composition. This can be done by varying the content of molybdenum within the given limits, such that a steel according to the invention having a content of molybdenum of or close to the lower limit of the range will obtain a hardenability that is relatively low in comparison with a steel according to the invention that has a content of molybdenum of or close to the upper limit of the range, but in the entire range of molybdenum content a hardenability is obtained that exceeds the hardenability of reference materials no. 1 and 4. On a relative scale of between 1-10, where 1 = the most poor hardenability and 10 = the best hardenability, steel no. 7 according to the invention gets the rating 10. A variant of the steel according to the invention having a content of 2.3% molybdenum will obtain a rating of 4. These ratings and ratings for some reference materials are shown in Table 2.

Table 2

By calculations by known theoretical calculations, i.e. in Thermo CaIc, the carbide content and the amount of molybdenum in solid solution in the matrix at equilibrium were calculated for a variant of the inventive steel denoted steel No. 6 and compared to steels No. 4 and 7. Steel No. 6 has a composition containing 1.8% C, 0.4% Si, 0.4% Mn, 4.8% Cr, 1.8 % Mo and 5.8% V and is designed in order to be able to reduce the cost of alloying elements even further. The result is shown in Table 3 below.

Table 3

Compared to steel no. 7, steel No. 6 has a lower amount of molybdenum in solid solution in the matrix which results in a lower hardenability. However, the hardenability is in order of steel No. 4 and is sufficient for hardening and tempering of round bars with 0250 mm or square bars with a dimension up to 400 x 200 mm, which covers the dimensions of tools for the intended application area. Because the lower amount of MC- carbides in the matrix, steel No. 6 will have higher ductility than steel No. 7 on behalf of lesser resistance to abrasive wear. Compared to steel No. 4, both steel No. 6 and 7 of the invention will have higher ductility and better resistance to abrasive wear.

As a conclusion, it can be said that with a steel according to the invention a material is obtained that has high hardness and very good resistance to wear, which makes the steel suitable to use in cold- working tools for cutting and punching, thread cutting, such as threading dies and thread taps, cold extrusion, the pressing of powder, deep drawing, as well as in machine knives. By the steel also exhibiting surprisingly good ductility, relatively good machinability and by the steel in its most preferred embodiment also exhibiting very good hardenability, allowing the steel to become hardened all the way through with good results for even very coarse dimensions, a steel can be provided which has a property profile that is very suitable and unusually good for the application. A steel can also be provided within the scope of the invention, which steel has not quite as good hardenability but for the rest has the same good properties, which is yet an advantage as seen from a cost point of view in case tools of thinner dimensions are to be manufactured.

Claims

1. A cold- working steel, charac teri s e d in that it has the following chemical composition in % by weight:

1.3 - 2.4 (C+N), whereof at least 0.5 C, 0.1 - 1.5 Si,

0.1 - 1.5 Mn,

4.0 - 5.5 Cr,

1.5 - 3.6 (Mo+W/2), but max 0.5 W,

4.8 - 6.3 (V+Nb/2), but max 2 Nb, and max 0.3 S, in which the content of (C+N) on the one hand and of (V+Nb/2) on the other hand, are balanced in relation to each other such that the contents of these elements are within an area that is defined by the coordinates A, B, C, D, A in the system of coordinates in Fig. 11, where the coordinates of [(C+N), (V+Nb/2)] for these points are:

A: [1.38, 4.8]

B: [1.78, 4.8]

C: [2.32, 6.3]

D: [1.92, 6.3], balance essentially only iron and impurities at normal contents.

2. A cold- working steel according to claim 1, c haracteri s e d in that the content of (C+N) on the one hand and of (V+Nb/2) on the other hand, are balanced in relation to each other such that the contents of these elements are within an area that is defined by the coordinates E, F, G, H, E in the system of coordinates in Fig. 11, where the coordinates of [(C+N), (V+Nb/2)] for these points are:

E: [1.48, 4.8]

F: [1.68, 4.8]

G: [2.22, 6.3] H: [2.02, 6.3]

3. A cold- working steel according to claim 2, c haracteri s e d in that the content of (C+N) on the one hand and of (V+Nb/2) on the other hand, are balanced in relation to each other such that the contents of these elements are within an area that is defined by the coordinates K, L, M, N, K in the system of coordinates in Fig. 11, where the coordinates of [(C+N), (V+Nb/2)] for these points are: K: [1.62,5.2] L: [1.82, 5.2] M: [2.05, 5.8] N: [1.85,5.8]

4. A cold- working steel according to any one of the preceding claims, characterised in that the content of (C +N) on the one hand and of (V+Nb/2) on the other hand, are balanced in relation to each other such that the contents of these elements fulfil the condition 0.32 < (C+N)/(V+Nb/2) < 0.35.

5. A steel according to any one of the preceding claims, characterised in that it contains 0.1-1.2 % Si, preferably 0.2-0.9 % Si.

6. A steel according to claim 5, characterised in that it contains 0.4% Si.

7. A steel according to any one of the preceding claims, characterised in that it contains 0.1-1.3% Mn, preferably 0.1-0.9% Mn.

8. A steel according to claim 7, characterised in that it contains 0.4% Mn.

9. A steel according to any one of the preceding claims, characterised in that it contains 4.5-5.1% Cr.

10. A steel according to claim 9, characterised in that it contains 4.8% Cr.

11. A steel according to any one of the preceding claims, characterised in that it contains 1.5-2.6% (Mo+W/2).

12. A steel according to any one of the preceding claims, characterised in that it contains 1.6-2.0% (Mo+W/2).

13. A steel according to claim 12, characterised in that it contains 1.8%

(Mo+W/2).

14. A steel according to any one of the preceding claims, characterised in that it contains max 0.3% W, preferably max 0.1% W.

15. A steel according to any one of the preceding claims, c harac teri s e d in that it contains max 0.3% Nb, preferably max 0.1% Nb.

16. A steel according to any one of the preceding claims, c harac teri s e d in that it contains max 0.15 % S .

17. A steel according to any one of the preceding claims, c harac teri s e d in that the steel has a hardness in the range 58-63 HRC achieved after hardening from a temperature of between 980 and 1050⁰C, preferably 59-62 HRC achieved after hardening from a temperature of between 980 and 1020⁰C, and tempering at a temperature of between 500 - 560°C/2x2 h.

18. A steel according to any one of the preceding claims, c harac teri s e d in that the steel has a microstructure after hardening from 1050⁰C and tempering that contains 8-13 % by volume of MX-carbides, -nitrides and/or -carbonitrides that are evenly distributed in the matrix of the steel, where M is essentially vanadium and X is carbon and/or nitrogen, of which carbides, nitrides and/or carbonitrides at least 90 % by volume have an equivalent diameter, D_ekv, of less than 3.0 μm, and is essentially free from MyC₃-carbider, -nitrides and/or carbonitrides.

19. A steel according to claim 18, c harac teri s e d in that at least 90% by volume of said MX-carbides have a maximum extension of 2.0 μm.

20. A method for cutting, shearing, punching and/or forming working of a metal working material in the cold condition, by a tool comprising a steel according to any one of claims 1-19.

21. A method for the pressing of a metal powder by a tool comprising a steel according to any one of claims 1-19.

22. A method of manufacturing a steel, comprising the following production steps: a) production of a metal powder from a metal melt, b) hot isostatic pressing of the powder at a temperature of between 950 and 1200⁰C and a pressure of between 90 and 150 MPa to form a consolidated body, c) hot working of said consolidated body at a temperature of initially between 1050 and 1150⁰C, d) soft-annealing at about 900⁰C, e) hardening from a temperature of between 980 and 1050⁰C and tempering at a temperature of between 500 and 560⁰C to a hardness in the range of 58-66 HRC, preferably 61-63 HRC, ch aracteri s e d in that the metal powder has a composition according to claim 1.