TECHNICAL FIELD OF THE INVENTION

The present invention relates to a steel alloy suitable for cutting applications and to a tool comprising such a steel alloy. The steel alloy is preferably manufactured using powder metallurgy.

The steel alloy is suitable for use in applications that require a high toughness in combination with hardness and strength, in particular hot hardness and thermal stability. Such applications include cutting tools for chip removing machining, such as end mills, gear cutting tools or milling tools formed for hobbing of workpieces, thread-cutting taps, boring tools, drilling tools, turning tools, etc. The steel alloy is also suitable for hot-working tools, such as extrusion dies, rollers for hot rolling, press rollers for stamping of patterns in metal, etc. The tools may be provided with a coating applied using physical vapour deposition (PVD) or chemical vapour deposition (CVD).

BACKGROUND AND PRIOR ART

A steel alloy suitable for cutting and hot-working applications is known from WO9302818. The steel alloy is a high speed steel alloy manufactured using powder metallurgy. It typically comprises, in weight percent (wt. %), 0.8 wt. % C, 4 wt. % Cr, 8 wt. % Co, 3 wt. % Mo, 3 wt. % W, 1 wt. % Nb, 1 wt. % V, 0.5 wt. % Si, 0.3 wt. % Mn, balance Fe and unavoidable impurities. This steel alloy has a high toughness and an excellent grindability. However, the hot hardness, i.e. the hardness at elevated temperature, and the thermal stability, i.e. the ability of the alloy to maintain its properties and microstructure over time at elevated temperature, show potential for improvement for the above mentioned applications. This should preferably be achieved while maintaining a good thermal conductivity at high temperatures, since a good thermal conductivity is desirable for cutting tools in order to conduct heat away from the cutting edge via the cutting tool. Moreover, it is desired that the steel alloy has an adequate machinability prior to hardening.

SUMMARY OF THE INVENTION

It is a primary objective of the present invention to provide a steel alloy which has improved thermal stability and hot hardness in comparison with the above discussed prior art steel alloy, in combination with an improved or at least similar thermal conductivity. It is a secondary objective to provide a tool which has excellent thermal stability and hot hardness in combination with a good thermal conductivity.

According to a first aspect of the present invention, the primary objective is achieved by means of a steel alloy according to claim1. The steel alloy comprises:

C: 0.40-1.2 wt. %,

Si: 0.30-2.0 wt. %,

Mn: max 1.0 wt. %,

Cr: 3.0-6.0 wt. %,

Mo: 0-4.0 wt. %,

W: 0-8.0 wt. %, wherein (Mo+W/2)≥3.5 wt. %,

Nb: 0-4.0 wt. %,

V: 0-4.0 wt. %, wherein 1.0 wt. %≤(Nb+V)≤4.0 wt. %,

Co: 25-40 wt. %,

S: max 0.30 wt. %,

N: max 0.30 wt. %,

the balance being Fe and unavoidable impurities.

With the steel alloy according to the present invention, an improved hot hardness and thermal stability can be achieved in comparison with a similar steel alloy with a lower amount of cobalt, such as the one described above. Although the steel alloy according to the invention comprises a limited amount of expensive alloying elements such as molybdenum and tungsten, it is still possible to achieve the desired properties of the steel alloy at hot-working conditions after hardening and tempering. The steel alloy is therefore suitable for cutting machining and hot-working applications, wherein e.g. a good thermal stability is crucial. The steel alloy according to the invention has also proved to have adequate machinability in soft annealed condition, i.e. the condition in which the steel alloy is subjected to machining for forming a tool. The steel alloy also has a relatively high thermal conductivity, thus being suitable for cutting applications in which it is desired to conduct generated heat away from the cutting edge.

According to one embodiment, the steel alloy comprises 27-33 wt. % Co. This helps achieving a good hot hardness and thermal stability without having problems with hardening the steel alloy.

According to another embodiment, the steel alloy comprises 28-30 wt. % Co. Within this interval, the hot hardness and thermal stability are optimised.

According to another embodiment, the steel alloy comprises 0.60-0.90 wt. % C. Within this range, a fine grain structure and a good wear resistance can be achieved without causing brittleness.

According to another embodiment, the steel alloy comprises 0.30-1.1 wt. % Si. This reduces the risk of forming large M6C carbides and impaired hardness, while still maintaining the fluidity of the steel alloy during the melt metallurgical process.

According to another embodiment, the steel alloy comprises 3.5-5.0 wt. % Cr. In this range, Cr will contribute to a sufficient hardness and toughness after hardening and tempering, without risking retained austenite in the steel matrix.

According to another embodiment, the steel alloy comprises 0.10-0.50 wt. % Mn. At these levels, Mn can put sulfuric impurities out of action by the formation of manganese sulfides, improving the machinability of the steel alloy.

According to another embodiment, the steel alloy comprises 2.0-4.0 wt. % Mo and 2.0-4.0 wt. % W. In these amounts, Mo and W contribute to an adequate hardness and toughness of the steel matrix after hardening and tempering.

According to another embodiment, the steel alloy comprises 0.90-1.3 wt. % Nb and 0.90-1.3 wt. % V. The grindability of the steel alloy can thereby be optimised.

According to another embodiment, the steel alloy comprises max 0.080 wt. % S. In this embodiment, the steel alloy is not intentionally alloyed with sulfur, but S may be present as an impurity without effect on the mechanical properties of the steel alloy.

According to another embodiment, the steel alloy comprises less than 1.0 wt. % unavoidable impurities, preferably less than 0.75 wt. % unavoidable impurities, and more preferably less than 0.50 wt. % unavoidable impurities. Below these levels, the impurities have very little effect on the properties of the steel alloy.

According to another embodiment, the steel alloy is a powder metallurgy steel alloy. Preferably, the steel alloy is in the form of a powder metallurgy steel alloy produced by gas atomisation. Using gas atomisation, it is possible to obtain a powder metallurgy steel alloy with high purity, low level of inclusions and very fine dispersed carbides. Gas atomised powder is spherical and may be densified into a homogeneous material using for example hot isostatic pressing (HIP).

According to another aspect of the present invention, the above mentioned secondary objective is achieved by means of a tool comprising the proposed steel alloy. Such a tool has a good thermal stability, hot hardness and thermal conductivity and is therefore suitable for hot-working and cutting applications.

According to one embodiment of this aspect of the invention, the tool is a cutting tool configured for chip removing machining.

According to one embodiment of this aspect of the invention, the tool is provided with a coating applied using physical vapour deposition or chemical vapour deposition. The PVD or CVD coating forms a wear resistant outer layer.

Further advantages and advantageous features of the invention will appear from the following description of the invention and embodiments thereof.

