EP1249511B1 - High speed steel with good high temperature strength manufactured by powder metallurgy - Google Patents

Abstract

Steel object produced by decomposing a liquid metal stream of an alloy with nitrogen to form a metal powder and compacting the powder with optional deformation comprises a chemical composition containing (in weight %) 1.51-2.5 carbon, up to 0.8 silicon, up to 1.5 manganese, 3.5-4.5 chromium, 13.3-15.3 tungsten, 2.0-3.0 molybdenum, 4.5-6.9 vanadium, 10.05-12.0 cobalt, up to 0.52 sulfur, up to 0.2 nitrogen, maximum 100 ppm oxygen, and a balance of iron. The amount of manganese minus sulfur is at least 0.19. The ratio of concentrations of tungsten to molybdenum is 52.-6.5. Preferred Features: The steel has the following composition (in weight %) 1.75-2.38 C, 0.35-0.75 Si, 0.28-0.54 Mn, 3.56-4.25 Cr, 13.90-14.95 W, 2.10-2.89 Mo, 4.65-5.95 V, 10.55-11.64 Co and 0.018-0.195 N.

Description

The invention relates to a high speed and high ductility toughened steel article produced by powder metallurgy by dicing a liquid metal stream of an alloy with nitrogen to metal powder and compacting the powder at high temperature under full pressure and optionally thermoformed.

High speed steels include alloys having from about 0.8 to 1.0 weight percent carbon, from 14 to 18 weight percent tungsten, from about 4.5 weight percent chromium, up to 2 weight percent molybdenum, at least 1.2 to 1.5% by weight of molybdenum, at least 1.2 to 1.5% by weight of vanadium and from 3 to 20% by weight of cobalt, the remainder being iron. The cause of the achievable with these high-speed steels high performance is the interaction of the carbide-forming elements vanadium, tungsten, molybdenum and chromium and acting on the matrix or matrix element cobalt. In addition to tungsten and molybdenum, vanadium in particular is suitable for imparting a high tempering resistance to the alloy up to a temperature of about 600 ° C. At the same time high carbon and high vanadium content, a large amount of vanadium carbides are formed, which causes a special wear resistance of the material. In particular sizing tools are therefore made with high-speed steels which have an increased carbon and vanadium content. Melt metallurgical or melting technology with a solidification in molds, however, appears with an alloy having the chemical composition in wt .-% 1.3 to 1.5 C, about 13% W, 4% Cr, 1% Mo, 8 to 12% Co and about 4.5% V, remainder of iron to be achieved economic manufacturability, even this material of the high carbide content and the solidification structure due to difficult and with lowered narrow forging temperature is deformed and only low toughness values, in particular low impact resistance in the thermally tempered state, having.

On the one hand to be able to further increase the carbon content and the concentration of the carbide-forming elements in view of an increase of the carbide content and thus the wear resistance of the material, but on the other hand to achieve sufficient processability and homogeneity of the article made thereof, powder metallurgy production is such alloyed parts advantageous.

Powder metallurgy production essentially involves atomizing a molten steel to metal powder, introducing and compacting the metal powder into a capsule, sealing the capsule, and heating and hot isostatically pressing the powder in the capsule to a dense homogeneous material.

This PM material can be used directly after appropriate heat treatment for the manufacture of articles or previously subjected to hot working, for example by forging.

Highly stressed high-speed steel objects, in particular cutting tools with a long service life, for economical machining of parts require a multi-layered high profile of properties.

It is an object of the present invention to provide a high speed steel tooling, preferably a high performance cutting tool, which has a high degree of oxide purity, low crack initiation potential, and increased cutting edge severity, high hardness with adequate toughness, and high thermal wear resistance Condition of the material and has improved hot hardness and high heat resistance.

Another object of the invention is to provide a high speed steel article for use as a tool for high speed machining of materials without the addition of lubricants, particularly for machining light metals and such alloys.

According to the invention, the specification is achieved in a high-speed steel article of the type mentioned above in that the article has a high degree of purity with a content and configuration of non-metallic inclusions corresponding to a value K0 of at most 3 according to DIN 50 602 testing and the following chemical composition in terms of weight. % Carbon (C) 1.51 to 2.5 Silicon (Si) to 0.8 Manganese (Mn) to 1.5 Chrome (Cr) 3.5 to 4.5 Tungsten (W) 13.3 to 15.3 Molybdenum (Mo) 2.0 to 3.0 Vanadin (V) 4.5 to 6.9 Cobalt (Co) 10.05 to 12.0 Sulfur (S) to 0.52 Nitrogen (N) 0.018 to 0.195 Oxygen (N) max 100 ppm with a value of manganese minus sulfur (Mn - S) of at least 0.19, iron and impurities and by-products accompanying the remainder, provided that the ratio of the concentrations of tungsten to molybdenum is between 5.2 and 6.5 and that the content of cobalt is at most 70% of the value of tungsten + molybdenum.

The advantages achieved with the article according to the invention are to be seen as a cumulative effect with regard to the improvement of the material properties, as pictorially a chain has only the carrying capacity of its weakest link. Oxide inclusions are defects with mostly edged structure and represent, as was found, starting from a critical size, the starting point of cracks in the material tempered to high hardness at an optionally changing stress state in this. Because in a matrix with high hot hardness or high temperature crack initiation was disproportionately increased by coarse oxides in the material, however, as shown, inclusions with a small diameter and low longitudinal extent are not very effective, a cumulative value of not more than 3 in the test for non-metallic inclusions according to DIN 50 602 method K0 was recognized as important according to the invention.

The excellent property profile of the alloy according to the invention results synergistically from the interaction of the elements in their respective activities. It is essential that in the high-speed steel, the concentration values elements carbon, chromium, tungsten, molybdenum, vanadium and cobalt are present within narrow limits and that the oxygen content does not exceed a maximum value. The content of carbon is in the light of the high affinity of the elements tungsten, Molybdenum and Vanadin to see this. The above alloy metals form stable primary carbides and secondary hard carbides, however, are also intercalated after interaction and respective activity in the matrix mixed crystals.

If the carbon concentration exceeds 2.5% by weight, a pronounced embrittlement of the high-speed steel material occurs, which can lead to the unusability of the article, for example a cutting tool. Lower contents than 1.51% by weight reduce the carbide content and decisively the wear resistance of the material. According to the invention, the carbon content of the alloy is 1.51 to 2.5% by weight.

The maximum chromium concentration of 4.5% by weight is due to the fact that higher contents lead to a chromium content in the matrix which has a stabilizing effect on the retained austenite content during curing. Up to a minimum value of 3.5 wt .-% chromium is carried out by the incorporation of the alloy atoms in the solid solution a desired solidification thereof, so that according to the invention a content range of 3.5 to 4.5 wt .-% is provided in the material.

Tungsten and molybdenum have a high carbon affinity, form almost identical carbides and, according to widely-held opinion of the experts, due to the respective atomic weight in terms of mass, 2 to 1 interchangeable. It has surprisingly been found that this interchangeability is not completely given, but by the respective activity of these alloying elements Mischkarbidbildung and the proportion of elements in the solid solution are controllable, which will be discussed in more detail in the discussion of the high temperature strength of high-speed steel.

Vanadin is one of the strongest monocarbide formers, whose carbides are characterized by high hardness and substantiate the special wear resistance of the material. The wear resistance is promoted by the fine formation and a substantially homogeneous distribution of monocarbides, as this is created by a powder metallurgical production of the material. In particular, vanadium, but also the elements tungsten and molybdenum are included To bring high temperatures partially in solution, which after a forced cooling of the article has a significant secondary hardness potential by excretion of finely distributed vanadinreicher secondary carbides by tempering treatments and is beneficial to the heat resistance of the material advantageous. Higher contents than 6.9% by weight of vanadium either cause higher carbon contents of the alloy, causing it to become brittle or depleting and reducing the strength, in particular reducing the heat resistance of the matrix. Vanadium concentrations below 4.5 wt .-% lead to a significant deterioration of the wear behavior of the coated part.

Cobalt is not a carbide-forming element in high speed steel, but it solidifies the matrix and significantly enhances the thermal resistance of the article. High cobalt contents of more than 12.0 wt .-% act in the given high-speed steel embrittling to the basic mass of the material, whereas lower concentrations than 10.05 wt .-% cause a significant reduction in matrix hardness at elevated temperature.

Cobalt in the inventively provided limits of 10.05 to 12.0 wt .-% causes due to the high diffusion coefficient that when starting the hardened part of the increased nucleation because of facilitates the diffusion processes and thus the secondary carbide precipitates in large numbers and large amount fine be formed distributed, also coarsen only slowly and advantageous to the matrix strength, especially at high temperature, are effective.

The fine secondary carbides, which give the material in the tempered state high hardness and strength, are increased by diffusion processes at high application temperatures or there is coagulation. Due to a high tungsten content in the alloy and consequently in the secondary carbides, the size of the tungsten atoms is smaller due to a smaller diffusion coefficient than the elements molybdenum and vanadium, so that a significantly slower coarsening and stabilization of the system at high Temperature, as was found, also occurs in mixed carbides. The tungsten content of 13.3 to 15.3 wt .-% of the invention ensures at the predetermined levels of other strong carbide-forming elements a low tendency to coarsening of the secondary hard carbides at elevated temperatures and thus a small carbide particles long-term, which blocks the dislocations in the matrix lattice and a Softening of the material dilated. The material remains longer, even at high thermal loads hard, so has an increased heat resistance.

Molybdenum plays an important role in reaction kinetics or mixed carbide formation, with a content of from 2.0 to 3.0 being determined to be effective according to the invention.

A maximum level of 100 ppm oxygen is provided in terms of the number of non-metallic inclusions and the property profile of the material under the stresses.

Of essential importance for a high heat resistance of the tempered material is the ratio of the concentrations of tungsten and molybdenum and the cobalt concentration matched to these elements. At values of tungsten to molybdenum contents of 5.2 to 6.5, the rate of secondary carbide particle coarsening, and thus, hardening of the material at high temperatures, is minimized, with less than 70% cobalt, as measured by tungsten + molybdenum concentration, an increase causing the nucleation sites to form secondary carbides and thereby promote a finely dispersed distribution thereof, which generally ensures high hot strength of the high speed steel tool article.

Although silicon in the alloy acts as a solidifying agent and deoxidizer, it should not exceed a content of 0.8% by weight for reasons of hardenability of the material.

Although manganese can influence the hardening behavior of the material, it is primarily to be considered together with the sulfur content, sulfur and manganese elements to be considered as the workability of the steel due to sulfide inclusion. In the case of preferably low manganese contents in the steel, the value: manganese minus sulfur should not fall below 0.19, because this can lead to hot forming problems and lowered material properties at high use temperatures.

Nitrogen may have a favorable effect on improving the heat resistance due to the formation of carbonitrides which are difficult to dissolve at high temperatures in the material according to the invention, but should only be alloyed to a content of 0.2% by weight in order to avoid manufacturing problems.

In embodiments of the invention for further improving the service properties of the high-speed steel, it may have one or more elements (s) with the following concentration value (s) in% by weight, based on the above composition. C 1.75 to 2.38 Si 0.35 to 0.75 Mn 0.28 to 0.54 Cr 3.56 to 4.25 W 13.90 to 14.95 Not a word 2.10 to 2.89 V 4.65 to 5.95 Co 10.55 to 11.64 N 0.018 to 0.195 With such an element-specific restriction of the chemical composition, individual properties of the material can be particularly promoted.

A further narrowing of the concentration range of alloy components can be advantageously used for specific material orientation for particular applications, wherein the article based on the first-mentioned composition one or more element (s) with the following (s) Concentration value (s) in wt .-% has C 1.69 to 2.29 Si 0.20 to 0.60 Mn 0.20 to 0.40 Cr 3.59 to 4.19 W 13.60 to 14.60 Not a word 2.01 to 2.80 V 4.55 to 5.45 Co 10.40 to 11.50 N 0.02 to 0.1 (O) max 90 ppm

The further object of the invention is achieved by the use of a high speed and high speed toughened steel cutting tool which is powder metallurgically produced and optionally thermoformed by dividing a liquid metal stream of an alloy with nitrogen into metal powder and compacting the powder at high temperature having a high degree of purity with a content and configuration of non-metallic inclusions corresponding to a value K0 of at most 3 according to the test according to DIN 50 602 and the following chemical composition in% by weight C 1.51 to 2.5 Si to 0.8 Mn to 1.5 Cr 3.5 to 4.5 W 13.3 to 15.3 Not a word 2.0 to 3.0 V 4.5 to 6.9 Co 10.05 to 12.0 S to 0.52 N 0.018 to 0.195 O max 100 ppm with a value: manganese minus sulfur (Mn-S) of not less than 0,19, iron and impurities and by-products as balance, with The proviso has that the ratio of the concentrations of tungsten to molybdenum between 5.2 and 6.5 and that the content of cobalt is at most 70% of the value of tungsten + molybdenum, for a high-speed cutting without lubricant of material parts, in particular of light metals, and such alloys. In such requirements, it has been shown that particularly large increases in service life can be achieved under difficult conditions by the use of tools according to the invention, which can provide particular economic advantages in a machining.

Based on comparative testing, the invention will be explained in more detail.

Table 1 shows the chemical composition of a high speed steel article according to the present invention and those of comparative materials.

• Probengeometrie: Halbscheiben Rd 30 x 10 mm
• Austenitisierung im Vakuum bei 1210 °C
• Abschreckung im Stickstoffstrom
• Anlassen: 3 x 2H

In Fig. 1, the starting curves of the materials are shown. The sample geometry and the heat treatment conditions were as follows:

• Sample geometry: half discs Rd 30 x 10 mm
• Austenitization in vacuum at 1210 ° C
• Deterrence in the nitrogen stream
• Starting: 3 x 2H

• Rundprobe Rd 5,0 mm
• Gehärtet im Vakuum bei 1210 °C
• Anlassen: 3 x 2h

2 comparatively shows the bending strength of the materials in the 4-point bending process for the following sample data.
The testing was carried out according to the conditions shown in Fig. 2a and given below.
Sample geometry:

• Round sample Rd 5.0 mm
• Hardened in vacuum at 1210 ° C
• Starting: 3 x 2h

In Fig. 3, the curve of the hot hardness of the materials at 650 ° C is shown in logarithmic dependence of the time, all samples having approximately the same initial hardness of 67 to 68 HRC. The hot hardness test was carried out by means of a dynamic method developed by the Material Competence Center Leoben (Zeitschrift für Metallkunde 90 (1999) 8, 637)

From a comparison of the test results, it can be seen that the hardness starting curves (Figure 1) of the various materials are close together and that at a tempering temperature above 570 ° C the alloy 1 gives the highest hardness values.

Although the material according to the invention has the highest bending fracture toughness (FIG. 2), the differences from the comparative materials are not significantly pronounced.

When comparing the hot hardness of the high-speed steel materials (FIG. 3), a clear superiority of the article composed according to the invention can be seen.

This high hardness and the special oxidic purity of the material caused that in practical use in a
High-speed dry machining with intermittent cutting of castings from an aluminum-silicon alloy 38% improved tool life of the cutting tool was found, the wear was mainly due to increased accumulation of silicon in the Al-Si alloys.

Claims (5)

1. A high-speed-steel article, with high heat resistance and toughness, which is manufactured by powder metallurgy by dispersal of a liquid metal flow of an alloy with nitrogen to form metal powder and by compacting the powder at high temperature under all-round pressure and which is optionally hot-worked, which article has a high degree of purity with a content and configuration of non-metallic inclusions corresponding to a K0 value of at most 3 when tested according to DIN 50 602, and has the following chemical composition in wt.%: carbon (C) 1.51 to 2.5 silicon (Si) up to 0.8 manganese (Mn) up to 1.5 chromium (Cr) 3.5 to 4.5 tungsten (W) 13.3 to 15.3 molybdenum (Mo) 2.0 to 3.0 vanadium (V) 4.5 to 6.9 cobalt (Co) 10.05 to 12.0 sulphur (S) up to 0.52 nitrogen (N) 0.018 to 0.195 oxygen (O) max 100 ppm with a value: manganese minus sulphur (Mn-S) of at least 0.19, remainder iron and manufacturing-induced impurities and accompanying elements, with the proviso that the ratio of the concentrations of tungsten to molybdenum lies between 5.2 and 6.5 and that the content of cobalt is at most 70% of the value of tungsten + molybdenum.
2. A high-speed-steel article according to claim 1, containing one or more elements with the following concentration value(s) in wt.%: C 1.75 to 2.38 Si 0.35 to 0.75 Mn 0.28 to 0.54 Cr 3.56 to 4.25 W 13.90 to 14.95 Mo 2.10 to 2.89 V 4.65 to 5.95 Co 10.55 to 11.64 N 0.018 to 0.195
3. A high-speed-steel article according to claim 1, containing one or more elements with the following concentration value(s) in wt.%: C 1.69 to 2.29 Si 0.20 to 0.60 Mn 0.20 to 0.40 Cr 3.59 to 4.19 W 13.60 to 14.60 Mo 2.01 to 2.80 V 4.55 to 5.45 Co 10.40 to 11.50 N 0.02 to 0.1 (O) max 90 ppm
4. A high-speed-steel article according to any one of claims 1 to 3, wherein the article is a cutting tool.
5. Use of a high-speed-steel cutting tool, with high heat resistance and toughness, which is manufactured by powder metallurgy by dispersion of a liquid metal flow of an alloy with nitrogen to form metal powder and by compacting the powder at high temperature under all-round pressure, and which is optionally hot-worked, has a high degree of purity with a content and configuration of non-metallic inclusions corresponding to a K0 value of at most 3 when tested according to DIN 50 602, and has the following chemical composition in wt.%: C 1.51 to 2.5 Si up to 0.8 Mn up to 1.5 Cr 3.5 to 4.5 W 13.3 to 15.3 Mo 2.0 to 3.0 V 4.5 to 6.9 Co 10.05 to 12.0 S up to 0.52 N 0.018 to 0.195 O max 100 ppm with a value: manganese minus sulphur (Mn-S) of at least 0.19, remainder iron and manufacturing-induced impurities and accompanying elements, with the proviso that the ratio of the concentrations of tungsten to molybdenum lies between 5.2 and 6.5 and that the content of cobalt is at most 70% of the value of tungsten + molybdenum, for high-speed cutting, without lubricants, of material parts, in particular made of light metals and similar alloys.

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