EP0322397B1 - High speed steel prepared by powder metallurgy, wear-resistant part prepared thereby and process for its manufacture - Google Patents

Description

The invention relates to a powder-metallurgically produced high-speed steel for wearing parts, in particular tools, containing C, Cr, Nb, V, W and / or Mo, optionally Co and / or Mn and / or Si and / or as, the rest iron and unavoidable impurities, e.g. P, S, O.

Such high-speed steels are used, among other things. for the production of tools for the machining of workpieces, e.g. Milling cutters, drills, reamers, but also for tools for non-cutting shaping such as Drawing nozzles, extrusion dies etc. used.

Very large niobium carbides from TYp MC occur in the melt metallurgical production of niobium alloyed high-speed steels, which can have a grain size of over 100 µm and impair the toughness and cutting edge durability of wear parts made from these high-speed steels. Since niobium also has only a very low solubility in the base alloy, only high-speed steels alloyed with niobium generally have no pronounced secondary hardness behavior.

The alloying element vanadium likewise forms carbides of the type MC, which, however, have a lower thermal stability than niobium carbides. For this reason, when using high hardness or Austenitizing temperatures, as are necessary in particular in the production of cutting tools, in order to achieve the required performance properties, namely hardness, to coarsen the austenite grain and the carbides precipitated with a reduction in toughness.

Attempts have been made to alloy high-speed steels with niobium, with higher niobium contents, in particular those above 1.5%, leading to the formation of coarse niobium carbides, which reduces the toughness properties of the tools were partially influenced and cutting edge parts broke out in practical use. JP-PA 144456/1983 has disclosed a powder metallurgical process for producing high-speed steel, an Nb concentration in the steel being limited to 0.1 to 1.5% by weight and high tungsten and / or molybdenum contents improving hardness values should provide after the heat treatment.

The aim of the invention is to produce high-speed steels which, in addition to having sufficient wear resistance and hardness, also have great thermal stability. In addition, the steels should have a uniformly fine carbide distribution in order to obtain appropriate toughness properties, especially on fine cutting edges, and hardness values of up to 70 HRC should also be possible.

und die obere Grenze des C-Gehaltes durch die Formel
$${\text{C}}_{\text{max}}\text{ = 1,0 + (\% Nb × 0,15) + (\% V × 0,24)}$$

gegeben ist.This goal is achieved in a powder-metallurgically produced high-speed steel of the type mentioned at the outset in that the steel has an Nb content of from 2% by weight to 15% by weight, preferably from 3% by weight to 10% by weight, in particular of more than 4% by weight to 10% by weight, and a vanadium content of 1 to 4% by weight, preferably 1.5 to 2.5% by weight, that the steel has 10 to 30 vol. -% preferably 10 to 22 vol .-%, contains metal carbides and that the lower limit of the C content by the formula
$${\text{C.}}_{\text{min}}\text{= 0.45 + (\% Nb × 0.1) + (\% V0.20)}$$

and the upper limit of the C content by the formula
$${\text{C.}}_{\text{Max}}\text{= 1.0 + (\% Nb × 0.15) + (\% V × 0.24)}$$

given is.

A process for the powder metallurgical production of Wear parts, in particular tools, made from high-speed steels with the proven composition according to the invention, the alloy components being melted and overheated by 100-600 ° C., preferably about 300 ° C., and the melt overheated in this way is atomized, particularly gas, by atomizing the powder, in the course of which Consolidation under the application of temperature and optionally pressure, in particular in a sintering process, is formed into a shaped body, which shaped body is optionally subjected to a soft annealing process after annealing and / or hot forging and is shaped into a wearing part by machining or non-cutting processing, whereupon the wearing part heats up above its austenitizing temperature or is subjected to high-speed steel hardening, from which temperature the wearing part is cooled, in particular quenched, and at least two tempering or. Secondary hardening is subjected.

According to the invention, it is advantageous if the curing or. Austenitization takes place at a temperature which is 50-100 ° C higher than that of a high-speed steel that is niobium-free or has a niobium content of less than 2 to 4% by weight and has the same carbide content after soft annealing and which depending on Composition between 1100 and 1260 ° C is set.

The indicated niobium content and vanadium content as well as the amount of metal carbides formed in the steel due to the regulation of the carbon content create a high-speed steel which has the desired advantageous properties. Because the superheated melt of the alloy components is powder atomized, a powder is obtained in which the niobium carbides which form during solidification are in finely divided form. These very finely divided niobium carbides hinder the grain growth at the high austenitizing temperatures provided according to the invention.

According to the invention, a wearing part manufactured by powder metallurgy, in particular a tool, consisting of a high-speed steel with the proven composition according to the invention is provided.

The carbon values given in the formulas for C _min and C _max result from the interaction of the carbide-forming elements in high-speed steel, which means that the metal carbides can have different carbon concentrations. The factors in the formulas result from the fact that NbC can bind 0.10 to 0.15% carbon and VC 0.20 to 0.24% carbon. The summands 0.45 and 1.0 in the formulas take into account the carbon content to form the basic hardness of the matrix and the Nb and V-free carbides. The MIN and MAX values are finally determined by the Cr, Mo, W contents.

According to the invention, the production of the powder metallurgical high-speed steel is carried out as follows:

The individual alloy components are melted together and the melt is overheated by approximately 100 to 600 ° C., preferably 300 ° C., as a result of which the alloy components niobium and carbon are distributed in the melt. After holding at this temperature for at least 20 to 30 seconds, the melt is atomized into a powder under protective gas. (In principle, water atomization is also possible). Due to the rapid cooling, small, well-distributed niobium carbides separate out. Shaped bodies are then produced from these powders using temperature and optionally pressure. For this purpose, the powders are filled into steel containers made of alloyed or unalloyed steel, sealed gas-tight and consolidated using pressure and temperature, for example by means of hipping, extrusion or forging. When consolidating, ensure that the temperature is selected so that no liquid phases occur. Consolidation temperatures are about 1,050 to 1,100 ° C, at a pressure of 1000 bar or if working without pressure, about 1,200 to 1,250 ° C. The consolidation can be followed by a glow.

In a subsequent hot forming, e.g. hot forging at 1,150 ° C, the strength can e.g. the bending strength of the molded body can be increased. The hot shaping which is carried out if necessary is followed by soft annealing at a temperature of about 700 to 850 ° C., preferably 800 ° C. The annealed workpiece is then formed into the desired wear part or tool by machining or non-machining. After the production of the tool body, the workpiece is hardened and at an austenitizing temperature of up to 1,350 ° C. During this hardening process, the niobium carbide inhibits grain growth and the undissolved vanadium carbide contributes to the formation of a very fine grain before quenching in air, water or oil. The higher austenitization temperature provided according to the invention enables a larger amount of the carbides present to disintegrate or to dissolve at this temperature, so that a fine and hard grain structure is achieved in the matrix during the subsequent cooling. After quenching, a first tempering takes place at a temperature of about 500 to 600 ° C, at which fine metal carbides (e.g. vanadium mixed carbide of the type MC) are precipitated. In the course of the second or a further tempering, the hardness properties of the workpiece can be increased even further.

The higher austenitizing temperature can be used without the occurrence of toughness-reducing phenomena or grain coarsening, melting and other disadvantageous processes. Because chrome is the excretion Influenced by carbides, the chromium content is limited to a range of 2 to 5% by weight. Any cobalt present should be in the range of 0-10% by weight.

In the steels or workpieces produced according to the invention, the metal carbides have a size of less than 6 μm. A further reduction in the grain size of the metal carbides can be achieved by increasing the melt temperature or the rate of solidification in the course of the production of the metal powder.

The invention is explained in more detail below with the aid of examples.

example 1

A high-speed steel alloy with the composition C = 1.81% by weight, Si = 0.3% by weight, Mn = 0.2% by weight, P = 0.02% by weight, S = 0.02% by weight. %, Cr = 4.3% by weight, Mo = 3.7% by weight, V = 1.5% by weight, W = 6.1% by weight and Nb = 6.3% by weight. %, Remainder of impurities and iron (workpiece analysis) were melted in an induction furnace and cast into a billet. The bloom was melted and the melt overheated by 300 ° C and atomized in a stream of nitrogen. The atomized powder was filled into a capsule made of structural steel St52, shaken, evacuated to 10⁻³ Torr and welded gas-tight. The powder consolidation was carried out at 1,150 ° C and a pressure of 1,070 bar. After the formation of a milling cutter, hardening or austenitizing was carried out at a temperature of 1,290 ° C. without coarsening of the grain or melting at the grain boundaries. This austenitizing temperature, which is about 50 ° C above the conventional hardening temperature, enabled higher carbide or carbon contents to be dissolved in the matrix and thus the hardness and wear resistance to be improved in the tempering processes.

The hardness measurement was 68.8 HRC. In the machining test, the milling cutters produced according to the invention showed an increase in performance of about 30 to 50% in the cutting of St52 and tempering steel of the type X38CrMoV51, compared to milling cutters of the alloy S6-5-2-5.

Example 2

A high-speed steel of the composition C = 2.49% by weight, Si = 0.35% by weight, Mn = 0.20% by weight, P = 0.025% by weight, S = 0.005% by weight %, Cr = 4.7% by weight, Mo = 4.01% by weight, V = 2.3% by weight, W = 1.82% by weight and niobium = 9.89% by weight %, Remainder of impurities and iron melted and poured into a block. The block was gas atomized at a temperature exceeding the liquidus temperature by 350 ° C. The powder was used in a sintering process to produce a shaving wheel, such as is used for the fine machining of gear wheels in the automotive industry. The hardening took place at an austenitizing temperature of 1,300 ° C, which was followed by a double tempering at 580 ° C. After starting twice, the shaving wheel was finished by grinding. The hardness measurement in the working area of the tool gave a value of 69.5 HRC.

Compared to a high-speed steel S6-5-3-8 (ASP 30) made from powder metallurgy, an increase in output of 40 to 50% was achieved in the production of externally toothed bevel gears.

Claims (11)

1. High-speed steel, produced by powder metallurgy, for wearing parts, in particular tools, containing C, Cr, W and/or Mo, optionally Co and/or Mn and/or Si and/or Al, and with an Nb content of 2-15% by weight and a vanadium content of 1 to 4% by weight, remainder iron and inevitable impurities, e.g. P, S and O, the steel containing 10 to 30% by volume metal carbides and the lower limit of the C content being given by the formula
$${\text{C}}_{\text{min}}\text{ = 0.45 + (\%Nb × 0.1) + (\% V · 0.20)}$$

and the upper limit of the C content being given by the formula
$${\text{C}}_{\text{max}}\text{ = 1.0 + (\% Nb × 0.15) + (\% V × 0.24).}$$

2. High-speed steel according to Claim 1, having an Nb content of 3 to 10% by weight.

3. High-speed steel according to Claim 1, having an Nb content of more than 4 to 10% by weight.

4. High-speed steel according to one of Claims 1 to 3, having a V content of 1.5 to 2.5% by weight.

5. High-speed steel according to one of Claims 1 to 4, with the content of metal carbides being 10-22% by volume.

6. Process for the production by powder metallurgy of wearing parts, in particular tools, from high-speed steels having a composition according to one of Claims 1-5, wherein the alloy constituents are melted and are superheated by 100 to 600°C, preferably about 300°C, and the molten metal thus superheated is atomised to powder, in particular gas-atomised, whereupon the powder is formed into a formed body in the course of a hot-shaping operation, under the action of temperature and optionally pressure, in particular in a sintering operation, which body, optionally after an annealing and/or hot-forging operation, is subjected to a soft annealing operation and is formed into the wearing part by cutting or non-cutting machining, whereupon the wearing part is heated beyond its austenitisation temperature or is subjected to a high-speed steel hardening operation, from which temperature the wearing part is cooled, in particular quenched, and is subjected to at least two tempering or secondary hardening operations.

7. Process according to Claim 6, characterised in that the hardening or austenitisation operation takes place at a temperature which is 50 to 100°C higher than for a high-speed steel which is niobium-free or has a niobium content of less than 2 to 4% by weight and has the same carbide content after the soft annealing has been performed, and which temperature is set between 1,100 and 1,260°C, depending on the composition.
8. Process according to Claim 6 or 7, characterised in that the soft annealing temperature is set to 700 to 850°C, preferably about 800°C.

9. Process according to one of Claims 6 to 8, characterised in that the hardening or austenitisation temperature is set to up to 1,350°C, in particular up to 1,290°C.

10. Process according to one of Claims 6 to 9, characterised in that a content of 10 to 30% by volume, preferably 10 to 22% by volume, of metal carbides in the formed body is set during the soft annealing operation.

11. Wearing part, in particular tool, produced by powder metallurgy, consisting of a high-speed steel having a composition according to one of Claims 1 to 5.