EP3323902B1 - Steel material containing hard particles prepared by powder metallurgy, method for producing a component from such a steel material and component produced from the steel material - Google Patents

Description

The invention relates to a steel material which is produced by powder metallurgy and contains hard material particles. Such steel materials are also referred to in technical terms as metal matrix composites.

The invention also relates to a method for producing such a steel material.

Finally, the invention also relates to components made from a steel material of the type according to the invention.

In particular, the invention aims at a steel material which is suitable for the production of components that are exposed to the highest surface loads in practical use and are simultaneously moved quickly. An example of such components are roll guide rollers that are used in machines (roll stands) for wire rolling. The wire to be rolled and moved at a high conveying speed is guided on these rollers in the hot state at temperatures of more than 1000 ° C. Due to its high temperature, a layer of scale forms on the wire. In addition to the high temperature and the high dynamic loads to which they are exposed due to their high rotational speeds associated with the high conveying speed of the wire, the roller guide rollers are therefore also exposed to high abrasive loads on their surfaces that come into contact with the wire.

In order to be able to withstand this collective load, high demands are placed on the wear resistance, in particular the resistance to abrasive wear, the corrosion resistance, the resistance to thermal shock stress and the weight of the steels from which roller guide rollers and other components that are subjected to comparable loads in practical use are made posed.

Various attempts are known to meet this requirement profile. So is in the EP 0 773 305 B1 describes a wear-resistant and corrosion-resistant, powder-metallurgical tool steel intended for the manufacture of components that are used for processing reinforced plastics and other abrasive and corrosive materials. In addition to iron (in% by weight), the steel has an Mn content of 0.2 - 2.0%, a P content of max. 0.1%, an S content of max. 0.1%, a Si content of max. 2.0%, a Cr content of 11.5 - 14.5%, a Mo content of max. 3.0%, a V content of 8.0 - 15.0 %, an N content of 0.03-0.46% and a C content that should be 1.47-3.77%. The contents of C, Cr, Mo, V and N are linked by two formulas in such a way that on the one hand the formation of ferrite in the structure of the component made of steel is avoided and on the other hand the formation of excessive amounts of residual austenite during the To prevent heat treatment that the component goes through in the course of its manufacture. An optimized combination of metal wear, abrasion and corrosion resistance should also be obtained via the composition determined by the formulas.

Another group of steel materials produced by powder metallurgy for the production of components of the type in question is, for example, in U.S. 4,249,945 A described. In a preferred embodiment, these steels have a steel matrix consisting of 0.1-1% by weight Mn, up to 2% by weight Si, 4.5-5.5% by weight Cr, 0.8-1 , 7% by weight Mo, up to 0.14% by weight S, 8-10.5% by weight V, 2.2-2.6% by weight C, the remainder being iron and unavoidable impurities , and contain 13.3 - 17.3% by volume of vanadium carbides. The steel reaches a hardness of up to 63 HRC.

From the EP 0 515 018 A1 Furthermore, pre-alloyed, vanadium-rich particles of a cold work steel and processes for their production are known. The particles consist of a cold work steel alloy with a vanadium carbide dispersion of the MC type with a carbide particle size which is essentially completely below 6 μm, the content of the vanadium dispersion in the particles being 18.5-34.0% by volume. The particles are made by atomizing a molten tool steel alloy at a temperature above 2910 ° F and rapidly cooling the atomized alloy.

From the U.S. 4,880,461 A Finally, a method for the powder-metallurgical production of a steel material is known in which a matrix made of steel with high Mo and / or W contents is used and in which an additional 2 - 12% hard materials are embedded in the matrix. The hard materials can be nitrides, carbides or carbonitrides. The matrix material contains Mo and W that meet the condition 18% ≤ W + 2Mo ≤ 40%. At the same time, the C content of the matrix material is matched to the high Mo and W contents so that the matrix material can develop a high level of hardness even through the precipitation of carbides. The hardness is further increased by adding large amounts of Co. The material produced in this way has a maximum hardness of more than 70 HRC.

Against the background of the prior art explained above, the task arose of creating a steel material which offers a combination of properties that is further optimized for the production of components that are exposed to high mechanical, corrosive, thermal and abrasive loads in practical use.

A process for the production of components from such a steel should also be mentioned.

Finally, components should be specified for the production of which the steel according to the invention is particularly suitable.

With regard to steel, the invention has achieved this object by means of the steel provided according to claim 1.

The solution according to the invention to the object set above in relation to the method consists in that at least the work steps mentioned in claim 11 are run through in the production of components from a steel according to the invention.

Finally, steel according to the invention is particularly suitable for the production of components which, in practical use, execute movements with high acceleration or speed and are in particular exposed to high surface and temperature loads.

Examples of such components are roll guides for roll stands for wire production, but also other tools and other components that require not only high stability under mechanical stress and wear resistance, but also optimized behavior under the effect of high dynamic forces. Piston pins and pushrods for internal combustion engines should also be mentioned here.

Advantageous embodiments of the invention are specified in the dependent claims and are explained in detail below, like the general inventive concept.

• jeweils optional ein Element oder mehrere Elemente aus der Gruppe "Nb, Ni, Co, W", wobei der Gehalt an Ni, Co und W jeweils höchstens 1,0 % und der Gehalt an Nb höchstens 2,0 % beträgt,
• Rest Eisen und unvermeidbare Verunreinigungen,
• wobei in der Stahlmatrix TiC-Hartstoffpartikel in Gehalten von 2,5 - 30 Gew.-% eingebettet sind.

The steel material according to the invention is produced by powder metallurgy and has the following composition (in% by weight): C: 1.5 - 5.0%, Si: 0.3 - 2.0%, Mn: 0.3 - 2.0%, P: 0 - <0.035%, S: 0 - <0.35%, N: 0 - <0.1%, Cr: 3.0 - 15.0%, Mon: 0.5 - 2.0%, V: 6.0 - 18.0%,

• each optionally one or more elements from the group "Nb, Ni, Co, W", the content of Ni, Co and W being at most 1.0% and the content of Nb at most 2.0%,
• Remainder iron and unavoidable impurities,
• TiC hard material particles are embedded in the steel matrix in contents of 2.5-30% by weight.

In order to maximize the mechanical properties of a steel material according to the invention, 2.5-30% by weight of separately added hard material particles are present in the steel matrix composed according to the invention. The hard material particles in question are titanium carbide particles TiC.

The steel according to the invention is thus composed in such a way that, with a minimized density, in addition to good wear resistance and an associated long service life, it has a maximized resistance to extreme temperature changes and an equally optimized corrosion resistance.

If information on the alloy content of steels and steel materials is given in this text, these refer to the weight, unless expressly stated otherwise.

In a steel material according to the invention, the alloy ranges are selected so that a broad and, for the use of hard material particles, also called metal matrix composites ("MMCs"), meaningful range for vanadium-alloyed, high-strength and wear-resistant materials are available. The two most important alloying elements in this alloy system are carbon and vanadium.

Carbon is responsible for both martensitic hardening and the formation of hard vanadium carbide, which in combination with high hardness and the associated high strength results in optimized wear resistance. C is therefore present in the steel according to the invention in contents of 1.5-5.0% by weight. The carbon has two main tasks: On the one hand, C is required for the martensitic hardening of the metal matrix. On the other hand, the presence of sufficient amounts of C leads to the formation of hard carbides with the alloying elements present, in particular with V, Cr and, if present, Nb. If there is too little C in the alloy of the steel matrix, the formation of martensite does not occur, if too much C residual austenite is stabilized. Both effects can reduce hardness and wear resistance. So the ratio of carbon to the carbide-forming elements is always important.

On the one hand, silicon is used for deoxidation in the melting of the primary materials which are part of the steel alloy powder alloyed according to the invention and provided for the production of components according to the invention. In addition, the presence of silicon increases the carbon activity and thus leads to a lowering of the melting temperature. Without the targeted addition of at least 0.3% by weight of Si, in particular at least 0.7% by weight of Si, higher C contents would be necessary. The atomization process is in turn facilitated by the lowered melting point. Silicon also reduces the viscosity of the metal melt, which also helps to simplify the powder atomization process. At the same time, silicon increases the hardenability of the steel material, since the transformation noses in the ZTU diagram are shifted for longer times. The strength of the austenite at the hardening temperature is increased by the dissolved proportion of Si, which explains the greater stability of the austenite and enables longer cooling times. These effects are achieved with Si contents of up to 2.0% by weight, in particular up to 1.5% by weight. Excessive Si contents would lead to a stabilization of the ferrite, as a result of which the proportion of martensite present in the structure of the steel after hardening would be reduced and thus the hardness and wear resistance of the steel material according to the invention would also decrease.

Manganese is present in the steel material according to the invention in order to optimize the atomizability of the steel during the production of the steel powder and its hardness. The presence of sufficient Mn contents, similar to the presence of Si, lowers the melting point of the steel and lowers the viscosity of the metal melt, so that the targeted addition of Mn contributes to the simplification of the atomization process. At the same time, manganese also increases the hardenability of the steel material. The dissolved Mn also contributes to the stabilization of the austenite. In addition, Mn binds sulfur by forming MnS, which reduces the risk of hot cracks and improves machinability. These effects are achieved with Mn contents of at least 0.3% by weight, in particular at least 0.7% by weight, and Mn contents of up to 2.0% by weight, in particular up to 1.5% by weight. -%, reliably achieved. Too high a manganese content could, on the one hand, stabilize the austenitic phase to such an extent that the soft annealing time would be significantly increased. On the other hand, if the Mn content is too high, the austenitic phase could also be stabilized to such an extent that residual austenite would remain in the structure after hardening. This microstructure would be significantly softer than martensite, which would reduce hardness and wear resistance. Mn contents of a steel material according to the invention of about 1.2% by weight have proven to be particularly practical.

In the steel according to the invention, chromium is used in combination with Mo and V to adjust the tempering resistance, corrosion resistance and hardenability. By varying the Cr content, these three properties can consequently be adapted according to the respective requirements. At low Cr contents of 3.0 - 8.0% by weight, Cr primarily has a positive influence on the tempering resistance and hardenability. With increasing Cr contents, the corrosion resistance and the contribution of Cr to carbide formation increase. Average Cr contents of more than 8.0% by weight to less than 11.0% by weight represent a transition range in this respect. The Cr content here is not yet sufficient for increased demands on corrosion resistance. However, the steel matrix is already harder as a result of increasing Cr carbide formation. With contents of at least 11.0% by weight of Cr, in particular at least 12.0% by weight, in the steel material according to the invention, with maximized hardness and strength, tempering and corrosion resistance are achieved which also meet the highest requirements. The advantageous effects of Cr can be used particularly reliably in that the Cr content is set to at least 12.5% by weight. Too high a Cr content would cause more Cr carbides to form. However, the formation of Cr carbides would set C, which would reduce the martensite formation, so that the desired high hardness of the martensite could no longer be achieved. If the Cr content is significantly higher than the upper limit specified in accordance with the invention, the ferritic phase would also be stabilized, whereby the required hardness and wear resistance would likewise not be achieved. Therefore, according to the invention, the maximum content of Cr is limited to 15.0% by weight, in particular at most 14.0% by weight, with Cr contents of up to 13.5% by weight being particularly suitable in practice have turned out.

wobei mit %V der jeweilige V-Gehalt der Legierung der Stahlmatrix bezeichnet ist.An optimized effect of the C content of the steel matrix of a steel material according to the invention in relation to the formation of vanadium carbides VC can be ensured at low Cr contents of up to 8 wt Target salary% C target, which is calculated as follows: $$\%\mathrm{C\; goal}=0.2\times \%\mathrm{V}+0.4$$

where% V denotes the respective V content of the alloy of the steel matrix.

wobei auch hier mit %V der jeweilige V-Gehalt der Legierung der Stahlmatrix bezeichnet ist.If, on the other hand, Cr is used in the range of 11.0-15.0% by weight, the C content% C should be around 30% higher than the target content% C target determined using the above formula. In this case, the C content of the steel matrix is optimally adjusted in such a way that it corresponds to a target content% C target, which is calculated as follows: $$\%\mathrm{C\; goal}=\left(0.2\times \%\mathrm{V}+0.4\right)\times 1.3$$

Here too,% V denotes the respective V content of the alloy of the steel matrix.

In the case of the average Cr contents of> 8.0% by weight to <11.0% by weight, a C content is accordingly advantageously chosen which lies between the minimum C contents, which according to the two above formulas for the low Cr and high Cr contents can be determined.

The% C target content is in each case a target value that should be aimed for in the production of the alloy powder in an optimal way for the C content. It goes without saying that this target content is considered to have been achieved when the actual C content% C, within the tolerances specified for alloy engineering or customary, corresponds to the target content% C target of the respective steel material according to the invention. A practical value of the deviation in amount of the actual C content% C from the target content% C target, which is still permitted in this regard, is 0.2% by weight. For the actual C content% C of the steel matrix,% C =% C target ± 0.2% by weight should then apply.

The C content set in accordance with the stipulation explained above compensates for the fact that carbon is bound by Cr as a result of the Cr carbide formation. In this way it can be ensured that sufficient C is always available for the formation of martensite and that an optimized hardness and wear resistance are achieved, which are sufficient for most applications.

Accordingly, depending on the respective V content% V with Cr contents of up to 8% by weight for the target content% C target, for example, the following values (data in% by weight) result: Designation % V % C goal V8 8th 2.0 V10 10 2.4 V12 12th 2.8 V15 15th 3.4 V17 17th 3.8

In the case of the steel material V15 with up to 8% by weight Cr and a nominal V content of 15% by weight, a tolerance range of the V content of, for example, +/- 0.5% by weight is permitted, so that its actual V content can vary between 14.5-15.5% by weight. At the same time, a tolerance of +/- 0.2% by weight around the target value% C target is permitted for the actual C content. The actual C content of the steel material V15 can thus be 3.2-3.6% by weight.

Like chromium, molybdenum increases the corrosion resistance, hardenability and tempering resistance of components made from steel according to the invention if Mo contents of at least 0.5% by weight, in particular at least 0.9% by weight, are present. However, excessively high Mo contents impair the formability of the steel, since the high-temperature strength is significantly increased. In addition, high contents of Mo would also stabilize the ferritic phase. The maximum Mo content in the steel according to the invention is therefore limited to 2.0% by weight, in particular a maximum of 1.5% by weight. The Mo content of a steel according to the invention, which is particularly suitable for the purposes according to the invention, is accordingly in the range of 1.2% by weight.

Vanadium is present in the steel according to the invention in contents of 6.0% by weight to 18.0% by weight in order to achieve optimized wear resistance through the formation of vanadium-rich carbides or carbonitrides. In addition, vanadium is increasingly involved in the formation of carbides during tempering in the secondary hardness maximum. These effects increase with increasing V contents, so that the property profile of the steel material according to the invention can also be adapted to the respective requirements by varying the V contents. Maximized positive effects of the presence of V can be achieved if at least 14.5% by weight of V are present in the steel according to the invention. High V contents of at least 16% by weight lead to particularly high wear resistance, so that steel materials according to the invention with such high V contents are particularly suitable for use as a material for roller guide rollers, the maximum in use Are exposed to loads. On the other hand, the fact that the V content is limited to 17.4% by weight or 17.0% by weight, to 16.0% by weight or in particular at most 15.5% by weight, can be reliably avoided that too much carbon is bound by carbide formation. With low V contents tending towards the lower edge of the content range given according to the invention for V and correspondingly reduced C contents, the steel material according to the invention can be machined more easily than with the higher V and C contents. Simplified machinability is accordingly achieved when the V content is reduced to a maximum of 12% by weight, in particular a maximum of 10% by weight, and thus also the C content determined as a function of the V content in the manner described above is limited.

Niobium is optionally present in contents of up to 2.0% by weight in the steel according to the invention. Nb works very similarly to vanadium. It mainly participates in the formation of hard and wear-resistant monocarbides. Therefore, based in each case on their contents in atomic%, Nb and V in a ratio of 1: 1 can be exchanged alternately if this proves to be expedient, for example with regard to the availability of these alloying elements.

Nickel can optionally be present in contents of up to 1.0% by weight in the steel material according to the invention in order to stabilize the austenite content in a similar way to Mn and thus improve the hardenability. The presence of Ni ensures that austenite is actually formed at the respective hardening temperature and that no undesired ferrite is formed in the structure of the steel. However, an excessively high Ni content increases the cooling time required for martensite formation. At the same time, the Ni content should not be too high, as there is a risk that retained austenite will be present in the structure after hardening. If Ni is to be added, the Ni content is therefore preferably at least 0.2% by weight, with Ni contents of up to 0.4% by weight resulting in optimized effects of the presence of Ni.

Cobalt can also optionally be present in contents of up to 1.0% by weight in the steel material according to the invention. Similar to nickel, Co has a stabilizing effect on austenite formation and the hardening temperature. In contrast to nickel or manganese, Co does not lower the final temperature of the martensite, which is why its presence is less critical with regard to the formation of retained austenite. In addition, cobalt increases the heat resistance. If these positive influences are to be used through the addition of Co, contents of at least 0.3% by weight of Co prove to be particularly expedient, with optimized effects occurring with Co contents of up to 0.5% by weight.

Like Co and Ni, tungsten can optionally be added to the steel in contents of up to 1.0% by weight. Above all, tungsten increases the tempering resistance and participates in the formation of carbide, especially during tempering in the secondary hardness maximum. The tempering temperatures are shifted to higher temperatures due to the presence of W. Similar to cobalt, the heat resistance is also increased by W. However, too high a W content would also stabilize the ferritic phase. If the positive influences of W are to be used, contents of at least 0.3% by weight of W therefore prove to be particularly expedient, with optimized effects occurring at W contents of up to 0.5% by weight.

The remainder of the steel consists of iron and unavoidable impurities that get into the steel due to the manufacturing process or the raw materials from which the constituents of the steel alloy powder are obtained, but have no effect there in terms of properties.

Sulfur can be present in the steel material in contents of up to 0.35% by weight in order to improve the machinability. With higher S contents, on the other hand, the properties of the steel material composed according to the invention are worsened. In order to be able to safely use the beneficial effect of the presence of S, at least 0.035% by weight can be present in the steel material according to the invention. If, on the other hand, the machinability is not to be improved by the targeted addition of S, the S content can accordingly be limited to less than 0.035% by weight.

The unavoidably present impurities also include P contents of up to 0.035% by weight and, for example, a total of up to 0.2% by weight of oxygen.

Nitrogen is also not added to the steel material according to the invention in a targeted manner, but gets into the steel material during the atomization process due to the nitrogen affinity of the alloy components. In order to avoid negative influences of N on the properties of the steel material, the N content should be less than 0.12% by weight, in particular limited to a maximum of 0.1% by weight.

The density of steel material according to the invention is typically in the range of 6.4-7.6 g / cm ³ , the density of the pure steel matrix material typically being 7.0-7.6 g / cm ³ .

Its minimized density and its resulting low weight makes steel material according to the invention particularly suitable for the production of such components that in practice Repetitive use are exposed to rapid acceleration and in which, as a result, a lower mass inertia has a particularly beneficial effect.

Powder metallurgical production allows the density and wear resistance of steel according to the invention to be further optimized by the targeted addition of hard phases with low density, optionally in the sense of the respective application, if this is desired with regard to the desired property. It has been shown here that the use properties of steel material according to the invention are increased in that it contains 2.5 to 30% by weight of hard material particles which, in the finished steel, are embedded in its steel matrix composed in the manner explained above.

Like the steel alloy powder forming the steel matrix, the hard materials are in the initial state as a powder.

The hard materials, also called "hard phases" in technical terminology, can be carbides, nitrides, oxides or borides. The group of suitable hard materials accordingly includes Al ₂ O ₃ , B ₄ C, SiC, ZrC, VC, NbC, TiC, WC, W ₂ C, Mo ₂ C, V ₂ C, BN, Si ₃ N ₄ , NbN or TiN .

Titanium carbide TiC has proven to be particularly suitable for the purposes according to the invention. Titanium carbide has a hardness of 3200 HV and thus increases the hardness and wear resistance of the steel particularly effectively. At the same time, TiC is chemically resistant and has no negative impact on corrosion resistance. The low density of TiC also has a beneficial effect.

With hard material contents of less than 2.5% by weight added to the steel material, there is no improvement in the wear resistance. In order to be able to use the effect of the hard materials particularly reliably, it is therefore advantageous to provide at least 5% by weight of alloyed hard material particles in the steel material according to the invention, with contents of at least 7.5% by weight having proven to be particularly effective. In order to reliably avoid excessive embrittlement of the material as a result of the presence of the hard material particles, the content of alloyed hard material particles in the material according to the invention can be limited to a maximum of 25% by weight. The contents of hard material particles mentioned here in a steel material according to the invention prove to be particularly expedient in the case of the alloyed hard material around titanium carbide TiC.

After hardening and tempering, steel according to the invention achieves hardness values which are typically in the range of 58-70 HRC.

After soft annealing, which is usually carried out for mechanical processing, the typical soft annealing hardness of steel material according to the invention is typically up to 65 HRC due to the presence of the hard material particles provided according to the invention.

1. a) Es wird ein Stahllegierungspulver bereitgestellt, das aus (in Gew.-%) 1,5 - 5,0 % C, 0,3 - 2,0 % Si, 0,3 - 2,0 % Mn, < 0,035 % P, <0,35 % S, < 0,1 % N, 3,0 - 15,0 % Cr, 0,5 - 2,0 % Mo, 6,0 - 18,0 % V, jeweils optional einem Element oder mehreren Elementen aus der Gruppe "Nb, Ni, Co, W", wobei der Gehalt an Ni, Co und W jeweils höchstens 1,0 % und der Gehalt an Nb höchstens 2,0 % beträgt, und als Rest aus Eisen und unvermeidbaren Verunreinigungen besteht.
2. b) Das Stahllegierungspulver wird mit TiC-Hartstoffpartikeln mit der Maßgabe vermischt, dass der Gehalt an TiC-Hartstoffpartikeln an der erhaltenen Stahllegierungspulver-Hartstoffpartikel-Mischung 2,5 - 30 Gew.-% beträgt.
3. c) Optional wird das Stahllegierungspulver oder die Stahllegierungspulver-Hartstoff-Mischung getrocknet.
4. d) Aus dem Stahllegierungspulver oder der Stahllegierungspulver-Hartstoff-Mischung wird durch ein Sinterverfahren, insbesondere durch Heiß-Isostatisches-Pressen, oder durch ein additives Verfahren ein festes Halbzeug gebildet.
5. e) Das erhaltene Halbzeug wird zu dem Bauteil fertig bearbeitet.

When producing components according to the invention from a steel according to the invention, at least the following work steps are carried out:

1. a) A steel alloy powder is provided which consists of (in% by weight) 1.5-5.0% C, 0.3-2.0% Si, 0.3-2.0% Mn, <0.035% P, <0.35% S, <0.1% N, 3.0 - 15.0% Cr, 0.5 - 2.0% Mo, 6.0 - 18.0% V, each optionally with one element or more elements from the group "Nb, Ni, Co, W", the content of Ni, Co and W in each case being at most 1.0% and the content of Nb being at most 2.0%, and the remainder of iron and the unavoidable Impurities.
2. b) The steel alloy powder is mixed with TiC hard material particles with the proviso that the content of TiC hard material particles in the steel alloy powder / hard material particle mixture obtained is 2.5-30% by weight.
3. c) Optionally, the steel alloy powder or the steel alloy powder / hard material mixture is dried.
4. d) A solid semi-finished product is formed from the steel alloy powder or the steel alloy powder / hard material mixture by a sintering process, in particular by hot isostatic pressing, or by an additive process.
5. e) The semi-finished product obtained is finished to form the component.

With regard to the practical implementation and the configurations of work steps a) to e) of the method according to the invention, the following information applies:

Work step a)

The powder can be produced in a conventional manner, for example by gas atomization or any other suitable method. For this purpose, the alloy powder can be produced, for example, by gas or water atomization or a combination of these two atomization processes. Atomization of a melt alloyed in accordance with the invention to form the alloy powder is conceivable.

As an alternative, however, it is also possible to initially provide the alloying elements of the steel alloy powder individually in powder form in quantities that are appropriate for the respective Alloy element correspond to intended content proportions and then to mix these amounts of powder to form the steel alloy powder composed according to the invention.

If necessary, those with an average diameter of less than 500 μm are selected from the powder particles for further processing according to the invention by sieving, powders with average particle sizes of less than 250 μm, in particular less than 180 μm, having proven particularly suitable.

Regardless of how it is produced, the alloy powder provided according to the invention optimally has a bulk density of 2-6 g / cm3 (determined according to DIN EN ISO 3923-1) and a tap density of 3-8 g / cm3 (determined according to DIN EN ISO 3953).

Work step b)

The steel alloy powder provided in step a) is mixed with the hard material powder selected in each case. The amount of admixed hard material particles is determined taking into account the information given above with regard to the optimized selection of the hard material content so that the hard material particles content of the finished mixture is in the range of 2.5-30% by weight.

Work step c)

If necessary, the alloy powder produced in step a) or step b) can be dried in a conventional manner in order to remove residues of liquids and other volatile components that could hinder the subsequent shaping process.

Work step d)

A raw part (semi-finished product) is now formed from the alloy powder containing hard material particles. For this purpose, the alloy powder can be brought into the respective shape in a manner known per se by a suitable sintering process, in particular by hot isostatic pressing ("HIPen"). Usually the HIPing will be carried out. Typical pressures during HIPing are in the range from 900 to 1500, in particular 1000 bar, at a temperature of 1050 to 1250 ° C., in particular 1080 to 1200 ° C. In the course of hardening, austenite, VC and Cr carbide are formed in the structure of the steel material.

Alternatively, the respective component can also be produced in an additive process from the alloy powder procured and provided according to the invention. The term “additive” encompasses all manufacturing processes in which a material is added to produce a component, this addition usually taking place in layers. "Additive manufacturing processes", which are often referred to as "generative processes" in technical terms, are in contrast to the classic subtractive manufacturing processes, such as machining processes (e.g. milling, drilling and turning), in which material is removed in order to remove the material to give each component to be produced its shape. The additive construction principle makes it possible to manufacture geometrically complex structures that cannot be realized or can only be realized at great expense using conventional manufacturing processes, such as the machining processes or primary forming processes (casting, forging) mentioned above (see VDI status report "Additive Manufacturing Processes", September 2014) from the Association of German Engineers, Department of Production Technology and Manufacturing Processes, www.vdi.de/statusadditiv). More detailed definitions of the processes, which are summarized under the generic term "additive processes", can be found in VDI guidelines 3404 and 3405, for example.

Work step e)

The semifinished product obtained after step d) still requires finishing in order to give it the desired properties on the one hand and the required final shape on the other. Finishing includes, for example, mechanical, in particular machining, processing of the semifinished product, and heat treatment, which can consist of hardening and tempering.

The invention is explained in more detail below using exemplary embodiments:
Alloy powders composed according to the invention in the manner explained above are formed into a raw part (semi-finished product), for example by hot isostatic pressing or another suitable sintering process. For this purpose, the respective alloy powder can be filled into a suitable form, for example a cylindrical capsule, and then at typical pressures of 900-1500 bar (90-150 MPa), in particular 1000 bar (100 MPa), at a temperature of 1050-1250 ° C , in particular 1150 ° C, be held for a sufficient period of time until a solid body is formed. Typically, the pressure in hot isostatic pressing is in the range of 102-106.7 MPa and the heating to the target temperature, typically 1150-1153 ° C, which is above a A duration of typically 200-300 min, in particular 245 min, is also typically carried out at a heating rate of 3 K / min - 10 K / min.

Heat treatment followed the production of the semi-finished product. The respective semi-finished product is heated at a heating rate of typically 5 K / min to a hardening temperature (austenitizing temperature) of 1050-1200 ° C, at which it is kept until it is completely warmed through. Typically 30 - 60 minutes are required for this. The semi-finished products heated in this way are then quenched. They are cooled to room temperature within 5-30 minutes with a suitable quenching medium, for example with water, oil, a polymer bath, moving or still air or, if the cooling is carried out in a vacuum furnace, with gaseous nitrogen. In the case of large semi-finished products in particular, it can be useful to carry out the heating to the hardening temperature in several preheating stages, e.g. 400 ° C, 600 ° C and 800 ° C or a preheating temperature in the range of 600 - 800 ° C, in order to ensure uniform heating.

In order to avoid reactions with the ambient atmosphere, hardening can be carried out in a vacuum furnace in a manner which is also known per se. However, this is not a prerequisite for the success of the procedure according to the invention.

After hardening, tempering can be carried out in which the semifinished product is kept at the respective tempering temperature, which is typically 450-550 ° C., for a period of, for example, 90 minutes. The tempering conditions are selected in a manner known per se, depending on the respective hardening temperature and the desired level of hardness, i.e. the desired strength. The heating and cooling rates during tempering are generally of the order of 10 K / min. In contrast to hardening, the heating and cooling speeds during tempering are not critical. Tempering causes the brittle martensite to relax through diffusion of carbon. Together with e.g. V, Cr and Mo this forms the so-called "tempered carbides". This increases the toughness. At the same time, the strength and hardness of the steel material decrease only slightly, since these properties are increased again by the formation of carbide.

Since such alloy systems usually have a narrow temperature range (approx. 50 ° C roughly between 450 and 650 ° C), one speaks of secondary hardness maximum, since temperatures below or above this mean a lower hardness.

Using the general procedure explained above in the practical production of steel materials according to the invention and components made therefrom, cylindrical semi-finished products have been produced from four steel materials V10a-V10d according to the invention.

The steel matrix of the steel materials V10a, V10b, V10c and V10d each contained (in% by weight) 2.5% C, 0.9% Si, 0.9% Mn, 4.5% Cr, 1.2% Mo and 10.0% V, the remainder iron and unavoidable impurities. In addition, the steel material V10a was alloyed with 5% by weight TiC, the steel material V10b with 10.0% by weight TiC, the steel material V10c with 15% by weight TiC and the steel material V10d with 20% by weight TiC.

The austenitizing temperature AT, the hardness HRC ("HRC_v") present before the subsequent heat treatment step, either the tempering temperature ST and the tempering duration St, if a tempering has been carried out, or the soft annealing temperature WT and the soft annealing duration Wt, if a soft annealing has been carried out , the hardness HRC ("HRC_n") after the previous heat treatment step and the density ρ of the samples V1 - V8 are given in Table 1.

The heating to the respective austenitizing temperature AT took place in a vacuum furnace. There, the samples V1-V8 were kept at the austenitizing temperature AT for an austenitizing time At. This was followed by cooling to room temperature in a vacuum furnace by applying gaseous nitrogen applied at a pressure of 3.5 bar.

After hardening, samples 1-8 were subjected to either a tempering or a soft annealing treatment. During the tempering treatment, samples 1, 3, 5, 7 were held at the tempering temperature ST for the tempering duration St. This tempering treatment was carried out twice in order to obtain an optimal starting result.

During the soft annealing, samples 2, 4, 6, 8 were held at the soft annealing temperature WT for a period Wt. After the annealing time had elapsed, the furnace was switched off and samples 2, 4, 6, 8 were slowly cooled to room temperature in the switched off furnace. Table 1 sample material added TiC content AT At HRC_v ST St. WT Wt HRC_n ρ [% By weight] [° C] [min] middle [° C] [min] [° C] [H] middle [g / cm ³ ] 1 V10a 5 1800 60 64.0 500 90 - - 62.5 7.19 2 V10a 5 - - 900 8th 39.0 3 V10b 10 69.0 500 90 - - 65.0 7.05 4th V10b 10 - - 900 8th 44.0 5 V10b 15th 69.0 500 90 - - 65.0 6.88 6th V10b 15th - - 900 8th 46.0 7th V10b 20th 69.0 500 90 - - 66.0 6.72 8th V10b 20th - - 900 8th 50.0

Claims (14)

1. Steel material produced by powder metallurgy and having a steel matrix constituted as follows (in wt.%): C: 1.5 - 5.0 % Si: 0.3 - 2.0 %, Mn: 0.3 - 2.0 %, P: 0 - <0.035% S: 0 - <0.35%, N: 0 - <0.1%, Cr: 3.0 - 15.0%, Mo: 0.5 - 2.0 %, V: 6.0 - 18.0 %,

   respectively optionally one or more elements from the group "Nb, Ni, Co, W", wherein the content of Ni, Co and W is respectively at most 1.0% and the content of Nb is at most 2.0%, residual iron and unavoidable impurities,

   wherein TiC hard material particles in contents of 2.5 - 30 wt.% are embedded in the steel matrix.
2. Steel material according to claim 1, characterised in that, at Cr contents of up to 8.0 wt.%, the C content of the steel matrix with a maximum deviation of at most 0.2 wt.% corresponds to a target quantity %CTarget, for which %CTarget = 0.2 x %V + 0.4 wt.% applies, wherein %V denotes the respective V content of the steel matrix.
3. Steel material according to claim 1, characterised in that, at Cr contents of at least 11.0 wt.%, the C content of the steel matrix with a maximum deviation of at most 0.2 wt.% corresponds to a target quantity %CTarget, for which: %CTarget = (0.2 x %V + 0.4 wt.%) x 1.3 applies, wherein %V denotes the respective V content of the steel matrix.
4. Steel material according to claim 1, characterised in that, at Cr contents of more than 8 wt.% and less than 11 wt.%, the C content of the steel matrix is between the %CTarget contents determined according to claims 2 and 3.
5. Steel material according to any one of the preceding claims, characterised in that its Si content is at least 0.7 wt.% or at most 1.5 wt.%.
6. Steel material according to any one of the preceding claims, characterised in that its Mn content is at least 0.7 wt.% or at most 1.5 wt.%.
7. Steel material according to any one of the preceding claims, characterised in that its S content is at least 0.035 wt.%.
8. Steel material according to any one of the preceding claims, characterised in that its Mo content is at least 0.9 wt.% or at most 1.5 wt.%.
9. Steel material according to any one of the preceding claims, characterised in that in the presence of one or more elements from the group "Ni, Co, W", the following applies for the contents of the respective element Ni, Co or W (in wt.%): Ni: 0.2 - 0.4 %, Co: 0.3 - 0.5 %, W: 0.3 - 0.5 %.
10. Steel material according to any one of the preceding claims, characterised in that the hard material particles are present in a D50 particle size of at most 50 µm.
11. Method for producing a component which comprises a steel obtained according to any one of preceding claims, comprising the following steps:

    a) A steel alloy powder is prepared which comprises (in wt.%) 1.5 - 5.0 % C, 0.3 - 2.0 % Si, 0.3 - 2.0 % Mn, < 0.035 % P, < 0.35 % S, < 0.1 % N, 3.0 - 15.0 % Cr, 0.5 - 2.0 % Mo, 6.0 - 18.0 % V, respectively optionally one or more elements from the group "Nb, Ni, Co, W", wherein the content of Ni, Co and W is respectively at most 1.0 % and the content of Nb is at most 2.0 %, and the remainder is iron and unavoidable impurities.

    b) The steel alloy powder is mixed with TiC hard material particles with the proviso that the content of TiC hard material particles in the obtained steel alloy powder and hard material particle mixture is 2.5 to 30 wt.%.

    c) Optionally, the steel alloy powder or the steel alloy powder and hard material mixture is dried.

    d) From the steel alloy powder or the steel alloy powder and hard material mixture, a solid semifinished product is formed by a sintering process, in particular by hot isostatic pressing, or by an additive process.

    e) The resulting semifinished product is processed into the component.
12. Method according to claim 11, characterised in that for the working step a) the alloy constituents of the steel alloy powder are respectively provided in powder form and mixed into the steel alloy powders.
13. Method according to any one of claims 11 or 12, characterised in that the finishing (step e)) comprises a material-removing machining of the semifinished product.
14. Component which, in practical use, performs movements involving high acceleration or velocity, made from a steel material obtained according to any one of claims 1 to 10.