WO2007021243A9

WO2007021243A9 - Powder metallurgically manufactured steel, a tool comprising the steel and a method for manufacturing the tool

Info

Publication number: WO2007021243A9
Application number: PCT/SE2006/050290
Authority: WO
Inventors: Stefan Sundin
Original assignee: Erasteel Kloster Ab; Stefan Sundin
Priority date: 2005-08-18
Filing date: 2006-08-18
Publication date: 2007-06-14
Also published as: KR101333740B1; EP1917376A4; SE529041C2; EP1917376B1; CN101243199B; RU2415961C2; WO2007021243A1; CN101243199A; EP1917376A1; JP5225843B2; SE0501827L; BRPI0614983A2; KR20080038130A; JP2009504922A; WO2007021243B1; RU2008104934A

Abstract

A steel that has been manufactured powder metallurgically and that is characterised by having a chemical composition containing, in % by weight, 1.1-2.3 C+N, 0.1-2.0 Si, 0.1-3.0 Mn, max 20 Cr, 5-20 (Mo+W/2), 0-20 Co, where the total contents of niobium and vanadium (Nb + V) is balanced in relation to the ratio between the contents of niobium and vanadium (Nb/V), such that the contents of these elements and the ratio them between lie within an area that is defined by the coordinates A, B, C in the system of coordinates in Fig. 1, where A: [4.0; 0.55], B: [4.0; 4.0], C: [7.0; 0.55], and no more than 1 % in total of Cu, Ni, Sn, Pb, Ti, Zr, and Al, balance iron and unavoidable impurities from the manufacturing of the steel. The invention also relates to tools for hot working or chip removal or cold working, or an advanced machine element manufactured from the steel, as well as a method for the manufacturing of such.

Description

POWDER METALLURGICALLY MANUFACTURED STEEL, A TOOL COMPRISING THE STEEL AND A METHOD FOR THE MANUFACTURING OF THE TOOL

TECHNICAL FIELD

The invention relates to a new steel, preferably a powder metallurgically manufactured high speed steel having improved grindability and being suitable for tools for chip removal, preferably coated tools such as gear cutting tools, taps and end-cutters with shaving separators for which a large toughness is required in combination with a good hardness, especially hot hardness. Yet another field of application is tools the use of which requires a large toughness in combination with a hardness and a strength suitable for the application. Among the applications can be mentioned tools for hot-working, such as dies for extrusion of aluminium profiles and rollers for hot-rolling, advanced machine elements and press rollers, i.e. tools for the stamping of patterns or profiles in metals etc. Yet another field of application can be cold-working tools for which a good grindability and a good hardness are important properties.

For steels to be used for example in tools for the extrusion of aluminium profiles, one of the most important properties is that the steel has a high tempering resistance, which means that it should be able to be exposed to high temperatures for a long time without losing the hardness that the steel has gained from hardening and tempering. On the other hand, this hardness does not need to be extremely high, suitably in the magnitude of 50- 55 HRC.

If the steel is to be used instead in advanced machine elements, the primary properties are a higher hardness and strength in combination with a high toughness and there are also strict requirements on homogeneous properties, hi this case, the hardness after tempering may typically lie in the range of 55-60 HRC.

Even higher demands on hardness, 60-70 HRC, still in combination with a large toughness however, are put on steels for tools for the stamping of patterns or profiles in metals etc., as well as on steels for chip removal, such as gear cutting tools, taps and end-cutters with shaving separators. Taps should have a hardness in the range of 60-67 HRC, while end-cutters should have a hardness in the range of 62-70 HRC. Similar demands are put on the steel if it is to be used in tools for cold- working. The invention also relates to tools for hot working or chip removal or cold working, or an advanced machine element manufactured from the steel, as well as a method for the manufacturing of such.

PRIOR ART

One type of steel that is used for cutting operations is the high speed steel that is marketed under the trade name ASP® 2052 and that is characterised by the following nominal composition in % by weight: 1.6 C, 4.8 Cr, 2.0 Mo, 10.5 W, 8.0 Co, 5.0 V, balance iron and unavoidable impurities. Another high speed steel is ASP® 2030 having the nominal composition 1.28 C, 4.2 Cr, 5.0 Mo, 6.4 W, 3.1 V, 8.5 Co, balance iron and unavoidable impurities, Yet another high speed steel is ASP® 2060 having the nominal composition 2.3 C, 4.2 Cr, 7.0 Mo, 6.5 W, 6.5 V, 10.5 Co, balance iron and unavoidable impurities, All contents are in % by weight.

BRIEF ACCOUNT OF THE INVENTION

It is desirable for steels to be used in tools for chip removal to improve grindability as grindability is a time-consuming operation in the manufacturing of such tools. Accordingly, the object of the invention is to provide a novel steel, preferably a high speed steel, having the same beneficial properties as the above mentioned prior art steels but for which the grindability of the material has been improved. More specifically, the steel should have the following properties:

• a good grindability in the hardened and tempered condition,

• a good toughness in the hardened and tempered condition,

• a good hardness in the hardened and tempered condition, • a high yield point,

• a high fatigue strength,

• a high flexural strength, and

• a good resistance to wear.

These and other prerequisites can be accomplished by a steel that has been manufactured powder metallurgically and that is characterised by having a chemical composition containing, in % by weight, 1.1-2.3 C+N, 0.1 - 2,0 Si, 0.1 - 3,0 Mn, max 20 Cr, 5 - 20 (Mo+W/2), 0-20 Co, where the total content of niobium and vanadium (Nb + V) is balanced in relation to the ratio between the content of niobium and vanadium (Nb/V), such that the content of these elements and the ratio them between lie within an area that is defined by the coordinates A, B, C in the system of coordinates in Fig. 1, where A: [4.0; 0.55], B: [4.0; 4.0], C: [7.0; 0.55], and no more than 1 % in total of Cu, Ni, Sn, Pb, Ti, Zr, and Al, balance iron and unavoidable impurities from the manufacturing of the steel.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in greater detail in the following description of performed tests and with reference to the appended drawings, of which:

Fig. 1 shows the relation between on the one hand the total contents of Nb and V

(Nb+ V) and on the other hand the relation between the contents of Nb and V (Nb/V), for the steel according to the invention in the form of a system of coordinates, Fig. 2 shows a graph over the size of MX-carbides as a function of the volume portion of MX-carbides,

Fig. 3 shows a graph over the size of M₆X-carbides as a function of the volume portion of M₆X-carbides,

Fig. 4 shows a graph over the distribution in carbide size for various heat treatments and Nb/V ratios, Fig. 5 shows a chart over the lattice spacing in the plane d^w₎ for d₍m₎MX- and d(₃₃i). o.₅AM₆X-carbides as a function of the Nb/V ratio, Fig. 6 shows a photo of the microstructure of the steel F according to the invention, after heat treatment no. 6,

Fig. 7 shows a graph over grindability, G ratio, as a function of the size of MX- carbides, and

Fig. 8 shows a graph over energy consumption during grinding in relation to chip excavation rate.

DETAILED DESCRIPTION OF THE INVENTION

Without binding the invention to any particular theory, the importance of the various alloying materials as well as of the various structural elements in achieving the desired property profile, will be explained in more detail. In case of alloying contents, percentages are always given in % by weight, and in case of structural elements percentages are always given in % by volume, unless otherwise stated.

Together with nitrogen, carbon should exist at a content of at least 1.1 % and 2.3 % at the most, preferably at least 1.4 % and 2.0 % at the most, and even more preferred between 1.60 and 1.90 %, in order to, when dissolved in the martensite, give the material a hardness in the hardened and tempered condition that is suitable for its purposes. Carbon and nitrogen should furthermore, in combination with niobium and vanadium, contribute to an adequate amount of primary precipitated MX-carbides, - nitrides, -carbonitrides of the type (Nb, V)X, and, in combination with tungsten, molybdenum and chromium to contribute to the achievement of an adequate amount of primary precipitated M₆X-carbides, -nitrides, -carbonitrides in the matrix. To simplify, such hard phase particles are mentioned as carbides in the continued description, but it should be understood that if the steel contains nitrogen the term carbides also relates to nitrides and/or carbonitrides. The purpose of such carbides is to give the material its desirable resistance to wear. Furthermore, they contribute in giving the steel a fine- grained structure as the carbides may function to limit the grain growth. In a preferred embodiment, the steel contains between 1.65 and 1.80 % carbon and nitrogen, which, in combination with a balanced amount of other alloying elements, in particular silicon, chromium, vanadium and niobium, will give the steel a property profile well suitable for its purpose, which can be achieved by a standard manufacturing process, i.e. the manufacturing does not require any extraordinary efforts but proceeds in accordance with standard methods.

Normally, the nitrogen content is not more than 0.1 %, but by the powder metallurgical manufacturing technique it is possible to dissolve much higher contents of nitrogen in the steel. One embodiment of the steel is accordingly characterised by the steel containing a large amount of nitrogen, 2,3 % at the most, which can be obtained by solid phase nitration of the manufactured powder. Hereby, nitrogen can replace carbon in the hard materials that are to be part of the steel of the final tool. By replacing carbon by nitrogen the advantage is attained that the resistance to adhesive wear decreases, which is an advantage particularly when the tool operates on tacky materials such as aluminium and certain stainless steels. The steel will also be easier to temper, which means that the tempering temperature can be lowered, which can be advantageous. Contents lower than 1.1 % carbon + nitrogen will not result in adequate hardness and resistance to wear, while contents of more than 2.3 % may lead to brittleness problems.

Silicon is added to the steel at a content of at least 0.1 % in order to improve the steel's fluidity, which is important in the melt metallurgical process. By increased addition of silicon the steel melt will be more fluid, which is important in order to avoid clogging in connection with granulation, hi order to avoid clogging during granulation the silicon content should be at least 0.2 % and even more preferred at least 0.4 %. Silicon also contributes to increased carbon activity an in a silicon alloyed embodiment it can be present in amounts of up to about 2 %. Problems with brittleness will arise at contents above 2 % and accordingly the steel should suitably not contain more than 1.2 % Si as the risk of formation of large M₆X-carbides and impaired hardness in the hardened condition will be larger at contents there above, which means that it is even more preferred to limit the silicon content to not more than 1.0 %. In a preferred embodiment, the silicon content is between 0.55 % and 0.70 %, which, in addition to the above mentioned advantages, has proven to result in a steel that in combination with the carbon content preferred for the steel is easy to heat treat. By that it is meant that the steel can be heat treated within a broad temperature range while retaining its property profile, which gives advantages in manufacturing.

Manganese can also be present primarily as a residual product from the metallurgical melt process in which manganese has the known effect of putting sulphuric impurities out of action by the formation of manganese sulphides and for this purpose it should be present in the steel at a content of at least 0.1 %. The maximum content of manganese in the steel is 3.0 % but preferably the content of manganese is limited to a maximum of 0.5 %. In a preferred embodiment, the steel contains 0.2 to 0.4 % Mn.

Sulphur may be present in the steel as a residual product from the manufacturing of the steel, at contents of up to 800 ppm, without affecting the mechanical properties of the steel. Sulphur can be deliberately added as an alloying element, up to 1 % at the most, thus contributing to improved machinability and workability. In one embodiment of the invention, having sulphur deliberately added for this purpose, the sulphur content should be between 0.1 and 0.3 % and the content of manganese should then be chosen to be somewhat higher than in the non-sulphur alloyed embodiment, suitably from 0.5 % to a maximum of 1.0 %.

Also phosphorus may be present in the steel as a residual product from the manufacturing of the steel, at contents of up to 800 ppm, without affecting the mechanical properties of the steel.

Chromium should exist in the steel at a content of at least 3 %, preferably at least 3.5 %, in order to, when dissolved in the matrix of the steel, contribute to the steel achieving adequate hardness and toughness after hardening and tempering. Chromium can also contribute to the resistance to wear of the steel by being included in primarily precipitated hard phase particles, mainly M₆X-carbides. Also other primarily precipitated carbides contain chromium, however not to the same extent. Too much chromium will however result in a risk of residual austenite that can be hard to convert. By deep freezing of the material, the residual austenite content can be eliminated or at least minimized. For this reason the steel can be allowed a content of chromium of up to about 20 % but preferably the content of chromium is limited to a maximum of 12 %. hi the fields of application intended for the steel the steel need not contain more than 6 % in order to achieve the desired property profile, hi a preferred embodiment the steel contains between 3.5 and 4.5 % Cr and most preferred between 3.8 and 4.2 % Cr.

Molybdenum and tungsten will, just like chromium contribute to the matrix of the steel getting adequate hardness and toughness after hardening and tempering. Molybdenum and tungsten can also be included in primarily precipitated carbides of the M₆X-type of carbides and as such it will contribute to the resistance to wear of the steel. Also other primarily precipitated carbides contain molybdenum and tungsten, however not to the same extent. The limits are chosen in order to, by adaptation to other alloying elements, result in suitable properties, hi principle, molybdenum and tungsten can partially or completely replace each other, which means that tungsten can be replaced by half the amount of molybdenum, or molybdenum can be replaced by double the amount of tungsten. By experience, it is however known that about equal amounts of molybdenum and tungsten are to be preferred since this results in certain advantages in production technology or more specifically in heat treatment technology. The total content of molybdenum + tungsten should be in the range of 5 to 20 %, more preferred not more than 15 %. Properties suitable for the purpose will be achieved in combination with other alloying elements at a content of between 9 and 12 % (Mo + W/2). Within these ranges the content of molybdenum should, in a preferred embodiment, be chosen in the range of 4.0 to 5.1 % and the content of tungsten should suitably be chosen in the range of 5.0 to 7.0. The nominal content of molybdenum is 4.6 % and for tungsten it is 6.3 %.

The optional presence of cobalt in the steel depends on the intended use of the steel. For applications in which the steel is normally used at room temperature or is normally not heated to particularly high temperatures in use, the steel should not contain deliberately added cobalt, since cobalt reduces the toughness of the steel and the risk of chipping in use of the tool. Moreover the hardness in a soft annealed condition will increase with an increased content of cobalt and at contents above about 14 % the tools become markedly difficult to machine, i.e. to turn, mill, drill, saw etc. If the steel is to be used in chip cutting tools, for which hot hardness is of prominence, it is however suitable for it to contain considerable amounts of cobalt, which in that case can be allowed at contents of up to 20%, but a desired hot hardness can be achieved at a content of cobalt in the range of 7 to 14 %. When used in chip cutting tools the steel according to the invention should even more preferred contain between 8.0 and 10.0 % Co and even more preferred between 8.8 and 9.3 % Co.

Niobium is an element that plays an important role in the steel according to the invention. It is previously known that small additions of niobium, of up to 1 %, can contribute in keeping down the size of carbides, which is positive inter alia for the toughness and hardness of the material. According to previously known arguments, niobium may replace vanadium. This will however affect the resistance to wear and the material will also be hard to grind, especially if the steel contains niobium and/or vanadium at contents of about 4 % or more.

Something that is not previously known, at least not to the knowledge of the present applicant, is that there is a relation between on the one hand the total content of vanadium and niobium and on the other hand the ratio between vanadium and niobium, where the steel, despite a high content of such carbide formers is nevertheless surprisingly easy to grind. This relation forms the basis for the inventive idea and has become clear to the present applicant by extensive tests that are described further below.

According to the inventive idea, the total content of niobium and vanadium on the one hand should be balanced in relation to the ratio between the content of niobium and vanadium (Nb/V) on the other hand, such that the content of those elements as well as the ratio them between will lie within an area that is defined by the coordinates A, B, C in the system of coordinates in Fig. 1. More preferably, the total content of these elements (Nb+ V) and the ratio them between (Nb/V) is balanced within an area that is defined by the coordinates D, E, F, and even more preferred within an area that is defined by the coordinates G, H, I, where:

[(Nb+V); (Nb/V)]

A: [4.0; 0.55]

B: [4.0; 4.0] C: [7.0; 0.55]

D: [4.25; 0.55]

E: [4.25; 3.5]

F: [6.7; 0.55]

G: [4.5; 0.55] H: [4.5; 3.0]

I: [6.4; 0.55] It has been shown within the scope of the invention that despite a high alloying content of niobium and vanadium, the size of primarily MX-carbides can be limited, which contributes to the improved grindability.

It has further been shown that a steel according to the invention will get less growth of MX-carbides in the various hot-working operations that the steel undergoes during manufacturing, such as HIP:ing, forging, rolling, the higher the ratio of Nb/V of the steel.

It was also found in the investigation that there is a relation between the size of the formed carbides and their total content in the steel, the size of the carbides increasing the higher the carbide content is in the steel. This relation is valid for both M₆X- and MX-carbides. The investigation has furthermore shown that at fixed volume portions and process parameters, the M₆X-carbides are larger than the MX-carbides. This means that if a steel is desired that has a given largest size of the carbides, the alloying composition can be balanced in order to give the steel a content of MX-carbides that is between 1.5 and 2 times as large as the content of M₆X-carbides.

It has furthermore surprisingly been found that steel that is alloyed by niobium has a stronger relation between the increase in size of MX-carbides and the content of MX- carbides, than steel without addition of niobium. This result indicates that an addition of niobium is only advantageous up to a certain maximum content of MX-carbides, but not there above.

According to the inventive idea, a steel can be provided that fulfils the highly put demands on toughness and hardness in combination with a high yield point, a high fatigue strength, a high flexural strength and a relatively good resistance to wear, and that also has improved grinding properties. This is achieved if the steel is given a composition according to present claim 1, where the composition has been balanced in respect of the total content of niobium and vanadium in combination with a certain ratio between niobium and vanadium. Hence, the total content of niobium and vanadium should fulfil the condition 4.0 < Nb + V < 7.0, preferably 4.25 < Nb + V < 6.7 and even more preferred 4.5 < Nb + V < 6.4, at the same time as the ratio between niobium and vanadium should fulfil the condition 0.55 < Nb/V < 4.0, preferably 0.55 < Nb/V < 3.5 and even more preferred 0.55 < Nb/V < 3.0. In the most preferred embodiment the steel should contain 2.0 to 2.3 % Nb and 3.1 to 3.4 % V. In addition, the steel should have a content of MX-carbides of not more than 15 % by volume, preferably not more than 13 % by volume, and even more preferred not more than 11 % by volume, where at least 80 %, preferably at least 90 %, and even more preferred at least 95 % of the MX- carbides have a carbide size in the longest extension of the carbide of not more than 3 μm, preferably not more than 2.2 μm, and even more preferred not more than 1.8 μm. The composition of the steel should also be balanced in respect of the M₆X-carbide- forming elements chromium, molybdenum and tungsten, such that the content in the steel of M₆X-carbides will be not more than 15 % by volume, preferably not more than 13 % by volume and even more preferred not more than 12 % by volume, where at least 80 %, preferably 90 %, and even more preferred at least 95 % of the M₆X-C arbides have a carbide size in the longest extension of the carbide of not more than 4 μm, preferably not more than 3 μm, and even more preferred not more than 2.5 μm.

Besides that, the steel according to the invention should not contain any deliberately added additional alloying elements. Copper, nickel, tin and lead and carbide-formers such as titanium, zirconium and aluminium may be allowed at a total content of not more than 1 %. Besides these and the above mentioned elements, the steel contains no other elements than unavoidable impurities and other residual products from the metallurgical melt treatment of the steel.

LABORATORY SCALE EXPERIMENTS

Nine test materials were manufactured in total. The chemical composition of these materials is presented in Table 1 below.

Table 1 : Chemical composition in % by weight for the examined steels; balance iron and im urities at normal contents

Powder was manufacture from the steels by gas atomizing. The respective steel powders were consolidated by fast hot isostatic pressing, so called HIP/QIH, in small test capsules on top of larger production capsules. Samples were taken out from the small test capsules, which samples were heat treated in several ways in order to simulate typical conditions for production, according to Table 2 below:

Table 2: Heat treatment for simulation of typical conditions for production in the ASP rocess

Carbide content and size

The contents, as well as the size, of MX-carbides in the examined steels vary depending on which of the heat treatments in Table 2 that the steel has been exposed to. This is clear from Table 3 below.

Table 3: The contents of MX-carbides in the steel, the size of these in dependence of heat treatment

* relates to average carbide size for all MX-carbides

** relates to average size of the 100 largest carbides on an area of about 20,000 μm Fig. 1 shows a graph over the size of MX-carbides for heat treatment no. 6. In the figure, steels with an addition of niobium have been marked by solid black dots, while steel without addition of niobium have been marked by rings. It can be seen in the figure that the MX-carbides for Nb-containing steels are considerably smaller in size than they are in steels without addition of Nb.

A corresponding investigation in respect of the examined steels' content and size of M₆X-carbides in dependence of which heat treatment according to Table 2 above that the steels have been exposed to, is presented in Table 4 below.

The maximum content at which an addition of niobium has a positive effect on the size of MX-carbides varies in dependence of dwell time and temperature during processes such as HIP:ing, rolling and forging, at temperatures that are typical to high speed steels. One conclusion from the investigation is that for a steel having a content of MX- carbides of not more than 15 % by volume, preferably not more than 13 % by volume, and even more preferred not more than 11 % by volume, the addition of niobium seems to be advantageous, while the addition of niobium on the contrary seems to result in larger MX-carbides for steels having a larger portion of MX-carbides.

Table 4: The contents of M_δX-carbides in the steel, the size of these in dependence of heat treatment

Fig. 3 shows a graph over the size of M₆X-carbides for heat treatment no. 6, for the steels in Table 4. hi the figure, steels with an addition of niobium have been marked by solid black dots, while steel without addition of niobium have been marked by rings. From the figure, it can be seen that the addition of Nb does not have any measurable effect on the size of the M₆X-carbides.

It has further been shown that a steel according to the invention will be less affected in respect of the size of the MX-carbides in the various hot- working operations that the steel undergoes during manufacturing, such as HIP:ing, forging, rolling, the higher the ratio of Nb/V of the steel, as is clear from Fig. 4. Fig. 4 shows that the hot- working operations have little effect on the size of the MX-carbides in steels having a Nb/V ratio of about 0.6 or more.

Fig. 5 shows a chart over the lattice spacing in the plane d_(hki₎ for MX- and M₆X- carbides as a function of the Nb/V ratio. For MX-carbides the (11 l)-spacing was measured, and for M₆C-carbides the (331)-spacing was measured. Here, it is clear that the addition of niobium seem to have no effect on the spacing between the lattices in the M₆C-carbides, indicating that an addition of niobium has no effect on the composition of the M₆C-carbides. There seems to be a linear relation between the lattice spacing and the increase in Nb/V ratio, for MX-carbides, indicating that niobium is dissolved in the MX-carbides. Steel G however deviates from this, which is likely because large MX- carbides (> 20 μm) are formed in the melt before granulation takes place, which means that a smaller amount of Nb is available for MX-carbides formed during or after granulation.

Microstructure

The steel according to the invention has a microstructure that in the hardened and tempered condition consists of a structure of tempered martensite containing MX- carbides and M₆X-carbides that are evenly distributed in the martensite, obtainable by hardening of the product from an austenitizing temperature of between 950 and 1250 °C, cooling to room temperature and tempering at 480-650 ⁰C. The steel according to the invention should have a content of MX-carbides of not more than 15 % by volume, preferably not more than 13 % by volume, and even more preferred not more than 11 % by volume, where at least 80 %, preferably at least 90 %, and even more preferred at least 95 % of the MX-carbides have a carbide size in the longest extension of the carbide of not more than 3 μm, preferably not more than 2.2 μm, and even more preferred not more than 1.8 μm. The composition of the steel should also be balanced in respect of the M₆X-carbide-forming elements chromium, molybdenum and tungsten, such that the content in the steel of M₆X-carbides will be not more than 15 % by volume, preferably not more than 13 % by volume and even more preferred not more than 12 % by volume, where at least 80 %, preferably 90 %, and even more preferred at least 95 % of the M₆X-carbides have a carbide size in the longest extension of the carbide of not more than 4 μm, preferably not more than 3 μm, and even more preferred not more than 2.5 μm.

Fig. 6 is a photograph of the microstructure of a steel according to the invention, namely alloy F in Table 2. The figure shows the evenly distributed'MX-carbides as black/dark grey, and the somewhat larger M₆X-carbides are white/light grey. The steel contains 5.5 % by volume of MX-carbides having an average size of 0.5 μm, where the 100 largest MX-carbides within an area of about 20,000 μm have an average size of 1.1 μm, and 11.8 % by volume of M₆X-carbides having an average size of 1.2 μm, were the 100 largest M₆X-carbides within an area of about 20,000 μm have an average size of 2.2 μm. The light areas that surround the MX-carbides come from the etching and there is nothing corresponding to this in the material in reality.

Grindability According to one aspect of the invention, the steel should have a good grindability. The size of above all the MX-carbides affects the grindability of a steel such that the grindability gets impaired the larger the carbides in the steel. The grindability of a steel can be given as its G ratio, and it is a measurement on how hard the material is to grind. The G ratio of the steel was measured in the hardened and annealed condition by surface grinding a test piece of 7x7x 150 mm by commercial discs of alumina, so called white discs, down to a size of 2x7x150 mm. The G ratio is usually given as the volume of steel material that is ground off in relation to the volume of grinding disc that is consumed. An easily ground material has a high G ratio, while a material that is difficult to grind is characterised by a low value of the G ratio. Fig. 7 shows the grindability as a function of the size of the MX-carbides. It is clear that a steel having MX-carbides of small size is considerably improved in grindability as compared to other steels having a content of MX-carbides in the same volume range.

By comparing energy consumption during grinding, values of the highest chip excavation rate could be compared for the steel according to the invention, called PUD 169 and having the following composition 1.69 % (C +N), 0.65 % Si, 0.3 % Mn, 4.0 % Cr, 4.6 % Mo, 6.3 % W, 9.0 % Co, 3.2 % V and 2.1 Nb, balance iron and impurities, and a reference steel having the following composition: 1.6 C, 4.8 Cr, 2.0 Mo, 10.5 W, 8.0 Co, 5.0 V, balance iron and unavoidable impurities, called ASP 2052. The result is shown in Fig. 8 and it is clear there from that the steel according to the invention can be milled at a chip excavation rate that is about 60 % higher than for the reference material at the same energy consumption, which is a considerable advantage from a manufacturing point of view.

From the steel according to the invention and from the reference material a number of cutting tool inserts were made which were coated by TiAlN, so called Futura coating. The steel plates were used in a test in which the cutting speed that corresponds to one hour's (1 h) life span was determined for the two materials. In the tests the following parameters were used: radial cutting depth = 10 mm, axial cutting depth = 3 mm, feed = 0.1 mm/tooth, dry machining, working material = Impax.

In the test, a cutting speed of 83 m/min was measured for the steel according to the invention while the cutting speed for the reference material was measured to be 77 m/min, which accordingly means that the steel according to the invention has considerably much better performance than the reference material.

PILOT SCALE EXPERIMENTS

Hardness in the hardened and tempered condition, Two variants of about 200 kg each were made from the steel according to the invention by gas atomizing and HIP:ing. Pilot capsules of about 10 kg were made from this powder, and test pieces were taken from the capsules in order to evaluate hardness after hardening and tempering. These variants of the steel according to the invention are intended for applications with high demands on hardness, still in combination with a large toughness however, such as for tools for the stamping of patterns or profiles in metals etc., as well as steels for tools for chip removal, such as taps and end-cutters with shaving separators. Similar demands are put on the steel if it is to be used in tools for cold- working. The chemical composition of these steels is given in Table 5. The results are shown in Table 6. Table 5: Chemical composition in % by weight for two variants of the steel according to the invention; balance iron and impurities at normal contents

Table 6: Hardness of the steel accordin to the invention for various heat treatments, HRC

c

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Depending on the field of application for which the steel is intended, an optimum hardness is chosen in the hardness range of 50-70 HRC. For fields of application in which a lower hardness is desired, 50-55 HRC, but a higher toughness is preferred, the content of primarily C is being limited, as well as any existing N and at least some of W, V, Nb, Mo and Co, such that the contents are at about the lower limits for the steel, and the austenitizing temperature during hardening is chosen to be lower than 1100 °C.

For steels to be used for hot- working tools, such as for the extrusion of aluminium profiles, one of the most important properties is that the steel has a high tempering resistance, which means that it should be able to be exposed to high temperatures for a long time without losing the hardness that the steel has gained from hardening and tempering. On the other hand, this hardness does not need to be extremely high, suitably in the magnitude of 50-55 HRC. If the steel is to be used instead in advanced machine elements, the primary properties are a higher hardness and strength in combination with a large toughness. In this case, the hardness after tempering may typically lie in the range of 55-60 HRC. For these two fields of application the steel is heat treated suitably at an austenitizing temperature of 1000-1250 ⁰C, typically 1150-1200 ⁰C, and is tempered at a tempering temperature of 550-600 ⁰C, 3x1 h.

Even higher demands on hardness, 60-70 HRC, still in combination with a large toughness however, are put on steels for tools for the stamping of patterns or profiles in metals etc., as well as on steels for chip removal, such as gear cutting tools, taps and end-cutters with shaving separators. Taps should have a hardness in the range of 60-67 HRC, while end-cutters should have a hardness in the range of 62-70 HRC. Similar demands are put on the steel if it is to be used in tools for cold- working. For these two fields of application the steel is heat treated suitably at an austenitizing temperature of 1000-1250 °C, typically 1150-1200 °C for tools for chip removal and 1000-1200 ⁰C for tools for cold working, and is tempered at a tempering temperature of 480-580 °C, typically 550-570 °C, 3x1 h, and has a hardness in the range of 50-55 HRC. In case the steel contains nitrogen, the tempering temperature can be lowered according to the above reasoning.

In a preferred embodiment the steel has a nominal composition according to the following: 1.69 % (C +N), 0.65 % Si, 0.3 % Mn, 4.0 % Cr, 4.6 % Mo, 6.3 % W, 9.0 % Co, 3.2 % V and 2.1 % Nb, balance iron and impurities. Such as steel is particularly well suited for tools for cutting for which a considerably improved grindability has been noted compared to the materials mentioned in the introduction, other properties being comparable. The steel has also been shown to have improved machinability as compared primarily to ASP 2052.

Claims

1. A tool steel for hot working or a tool steel for cold working or a tool steel for chip cutting machining, or a steel for advanced machine elements, which has been manufactured by powder metallurgy and which is characterised in that it has a chemical composition in % by weight, containing:

1.1-2.3 C+N

0.1 - 2,0 Si

0.1 - 3,0 Mn max 20 Cr

5 - 20 (Mo+W/2),

0-20 Co, where the total contents of niobium and vanadium (Nb + V) is balanced in relation to the ratio between the contents of niobium and vanadium (Nb/V), such that the contents of these elements as well as the ratio them between lie within an area that is defined by the coordinates A, B, C in the system of coordinates in Fig. 1, where

A: [4.0; 0.55]

B: [4.0; 4.0]

C: [7.0; 0.55] and no more than 1 % in total of Cu, Ni, Sn, Pb, Ti, Zr, and Al, balance iron and unavoidable impurities from the manufacturing of the steel.

2. A steel according to claim 1, charac t eri s ed in that the total contents of niobium and vanadium (Nb + V) is balanced in relation to the ratio between the contents of niobium and vanadium (Nb/V), such that the contents of these elements as well as the ratio them between lie within an area that is defined by the coordinates D, E, F, where:

D: [4.25; 0.55]

E: [4.25; 3.5]

F: [6.7; 0.55]

3. A steel according to claim 2, characteri s ed in that the total contents of niobium and vanadium (Nb + V) is balanced in relation to the ratio between the contents of niobium and vanadium (Nb/V), such that the contents of these elements as well as the ratio them between lie within an area that is defined by the coordinates G, H, I, where: G: [4.5; 0.55] H: [4.5; 3.0] I: [6.4; 0.55]

4. A steel according to claim 3, characterised in that the total content of carbon and nitrogen in the steel is between 1.4 and 2.0 %, preferably between 1.60 and 1.90 %.

5. A steel according to claim 4,characterised in that the total content of carbon and nitrogen in the steel is between 1.65 and 1.80 %.

6. A steel according to claim 1, characterised in that the steel contains 0.2- 1.2 % Si, preferably 0.4-0.8 % Si.

7. A steel according to claim 6, characterised in that the steel contains 0.55-0.70 % Si.

8. A steel according to claim ^characterised in that the steel contains 0.1-0.5% Mn, preferably 0.2-0.4 % Mn.

9. A steel according to claim ^characterised in that the steel contains 3-6 % Cr, preferably 3.5-4.5 % Cr.

10. A steel according to claim 9, characterised in that the steel contains 3.8-4.2 % Cr.

11. A steel according to claim ^characterised in that the steel contains 5-15% (Mo + W/2), preferably 9-12 % (Mo + W/2).

12. A steel according to claim 11, characterised in that the steel contains 4.0-5.1 Mo and 5.0 -7.0% W.

13. A steel according to claim 11, characterised in that the steel contains 4.4 - 4.9 Mo and 6.1 -6.7% W.

14. A steel according to claim 1, characterised in that the steel contains 5.0-14.0 % Co, preferably 8.0-10.0 % Co and even more preferred 8.8-9.3 Co.

14. A steel according to claim 1, characterised in that the steel contains 2.0-2.3 % Nb and 3.1-3.4% V.

15. A steel according to claim 1, characteri s ed in that it is hardened at an austenitizing temperature of 950-1250 °C and is tempered at a tempering temperature of 480-650 °C, 3x1 h, and that is has a hardness in the range of 50-70 HRC.

16. A steel according to claim 15, charact eri s e d in that it has a micro-structure composed of tempered martensite with a content of MX-carbides of not more than 15 % by volume, where at least 80 % of the MX-carbides have a carbide size in the longest extension of the carbide of not more than 3 μm, and a content of M₆X-carbides of not more than 15 % by volume, where at least 80 % of the MβX-carbides have a carbide size in the longest extension of the carbide of not more than 4 μm.

17. A steel according to claim 16, characteri sed in that the content of MX- carbides is not more than 13 % by volume and even more preferred not more than 11 % by volume, where at least 90 % and even more preferred at least 95 % of the MX- carbides have a carbide size in the longest extension of the carbide of not more than 2.2 μm and even more preferred not more than 1.8 μm, and that the content of M₆X- carbides is not more than 13 % by volume, preferably not more than 12 % by volume, where at least 90 % and even more preferred at least 95 % of the M₆X-carbides have a carbide size in the longest extension of the carbide of not more than 3 μm and preferably not more than 2.5 μm.

18. A tool for hot working or chip cutting machining or cold working, or an advanced machine element, ch aract eri s ed in that it comprises a steel according to any one of the preceding claims.

19. A tool for hot working or an advanced machine element according to claim 18, characterised in that the steel is hardened at an austenitizing temperature of 950- 1050 °C and is tempered at a tempering temperature of 550-600 ⁰C, 3x1 h, and that is has a hardness in the range of 50-55 HRC.

20. A tool for chip cutting machining or for cold working according to claim 18, characteri s ed in that the steel is hardened at an austenitizing temperature of 1000- 1250 °C and is tempered at a tempering temperature of 480-580 °C, 3x1 h, and that is has a hardness in the range of 60-70 HRC.

21. A method for the manufacturing of a tool for hot working or chip cutting machining or cold working, or of an advanced machine element, charac t eri s ed in that it comprises the manufacturing of a steel melt and gas atomisation of said steel melt to form a steel powder, consolidation of said steel powder by hot isostatic pressing, so called HIP, to form a steel blank or a tool blank having nearly the final shape of the tool, with a chemical composition according to any one of claims 1-14, which is being hardened at an austenitizing temperature of 950-1250 °C and is tempered at a tempering temperature of 480-650 °C, 3x1 h, giving the steel a hardness in the range of 50-70 HRC, and a micro-structure composed of tempered martensite with a content of MX- carbides of not more than 15 % by volume, where at least 80 % of the MX-carbides have a carbide size in the longest extension of the carbide of not more than 3 μm, and a content of M_όX-carbides of not more than 15 % by volume, where at least 80 % of the M₆X-carbides have a carbide size in the longest extension of the carbide of not more than 4 μm, and grinding the tool blank to final dimensions.

22. A method for the manufacturing of a tool for hot working or chip cutting machining or cold working, or of an advanced machine element, according to claim 21, characteri s e d in that the steel blank undergoes hot working and/or cold working of the steel blank to form a tool blank before hardening and tempering.

23. A method for the manufacturing of a tool for hot working or chip cutting machining or cold working, or of an advanced machine element, according to claim 21, ch arac t eri s e d in that the tool is being surface coated by e.g. PVD or CVD.