Powder-Metallurgically Produced Steel Material Containing Hard Material Particles, Method for Producing a Component from Such a Steel Material, and Component Produced from the Steel Material
A steel material which has a minimised density, a good wear resistance and a concomitantly high service life with maximised resistance to extreme temperature changes and likewise optimised corrosion resistance. The steel material is produced by powder metallurgy and 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%, optionally one or more elements from the group Nb, Ni, Co, and 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 separately added hard material particles in contents of 2.5 to 30 wt. % are embedded in the steel matrix. From steel alloy powder alloyed in this way, a solid semifinished product is formed by a sintering process or an additive process, which undergoes a heat treatment and is then finished to form the respective component.
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 language as metal matrix composites.
Likewise, the invention relates to a method for producing such a steel material.
Finally, the invention also relates to components which are made from a steel material of the type according to the invention.
Specifically, the goal of invention is a steel material which is suitable for the production of components which are subjected to very high surface loads in practical use and at the same time are moved quickly. An example of such components is rolling guide rollers which are used in machines (rolling mills) for wire rolling. On these rollers, the wire to be rolled and moved at a high conveying speed is conducted while hot at temperatures of more than 1000° C. Due to its high temperature, a scale layer forms on the wire. In addition to the high temperature and the high dynamic loads, which the rollers are exposed to because of their high rotational speeds associated with the high conveying speed of the wire, the rolling guide rollers are therefore also exposed to high abrasive loads with their surfaces coming into contact with the wire.
In order to be able to withstand this combination of loads, the wear resistance, in particular the resistance to abrasive wear, the corrosion resistance, the resistance to thermal shock stress and the weight of steels, from which rolling guide rollers and other components subjected to comparable stress in practical use are made, are subject to high demands.
Various attempts at meeting this requirement profile are known. Thus, EP 0 773 305 B1 describes a wear and corrosion resistant powder metallurgical tool steel intended for the manufacture of components used for processing reinforced plastics and other abrasive and corrosive materials. In addition to iron (in wt. %), the steel should have an Mn content of 0.2-2.0%, a P content of max. 0.1%, an S content of max. 0.1%, an Si content of max. 2.0%, a Cr content of 11.5-14.5%, an 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 of 1.47-3.77%. The contents of C, Cr, Mo, V and N are linked together by two formulas in such a way that, on the one hand, the formation of ferrite in the structure of the component made of the steel is avoided. On the other hand, the formation of excessive amounts of residual austenite during the heat treatment, which the component undergoes in the course of its production, should be prevented. Likewise, an optimised combination of metal wear, abrasion and corrosion resistance is to be obtained through the composition determined by the formulas. In the course of the production of the tool steel, the alloying elements of the steel form by precipitation M7C3 and MC carbides, which can make up a proportion of 16-35% of the volume of the steel. The maximum hardness of the precipitation-hardened steel after hot working, annealing and hardening is 58 HRC.
Another group of powder metallurgically produced steel materials for the production of components of the type in question is described for example in U.S. Pat. No. 4,249,945 A. In a preferred embodiment, these steels have a steel matrix which comprises 0.1-1 wt. % of Mn, up to 2 wt. % of Si, 4.5-5.5 wt. % of Cr, 0.8-1.7 wt. % Mo, up to 0.14 wt. % S, 8-10.5 wt. % V, 2.2-2.6 wt. % C, with the remainder being iron and unavoidable impurities, and contain 13.3-17.3 vol. % vanadium carbides. The steel achieves a hardness of up to 63 HRC.
U.S. Pat. No. 4,880,461 A finally discloses a process for the powder metallurgical production of a steel material in which a matrix of steel with high Mo and/or W contents is used and in which additionally 2 to 12% hard materials are embedded in the matrix. The hard materials may be nitrides, carbides or carbonitrides. The matrix material contains contents of Mo and W which satisfy 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 such that the matrix material itself can develop a high hardness by precipitation of carbides. The material thus produced has a maximum hardness of more than 70 HRC.
Against the background of the prior art explained above, the object was to provide a steel material which offers a further optimised combination of properties for the production of components which are exposed in practice to high mechanical, corrosive, thermal and abrasive loads.
Likewise, a method for the production of components made of such a steel will be given.
Finally, components should be specified, for whose manufacture the steel according to the invention is particularly suitable.
With respect to the steel, the invention has achieved this object by steel obtained according to Claim 1.
The solution according to the invention of the object set out above with regard to the method consists in that during the production of components from a steel according to the invention, at least the working steps mentioned in claim 12 are undergone.
Finally, steel according to the invention is particularly suitable for the production of components which, in practical use, perform movements with high acceleration or speed and in particular are exposed to high surface and temperature stresses.
Examples of such components are rolling guides for rolling mills for wire production, but also different tools and other components, which are required to have not only a high stability under mechanical stress and wear resistance, but also an optimised behaviour under the action of high dynamic forces. Piston pins and push rods for internal combustion engines can also be mentioned in this context.
Advantageous embodiments of the invention are defined in the dependent claims and, like the general concept of the invention, are explained in detail in the following.
The steel material according to the invention is produced by powder metallurgy and has the following composition (in wt. %):
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- 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 separately added hard material particles in contents of 2.5-30 wt. % are embedded in the steel matrix.
To maximise the mechanical properties of a steel material according to the invention, in the composite steel matrix according to the invention 2.5 to 30 wt. % separately added hard material particles are present. The hard material particles in question may in particular be titanium carbide TiC particles.
The steel according to the invention is thus composed such that it has a maximised resistance to extreme temperature changes and also optimised corrosion resistance, in addition to a good wear resistance and a concomitantly high lifespan, at a minimised density.
If statements are made in this text regarding the alloy contents of steels and steel materials, these are in each case by weight, unless expressly stated otherwise.
In a steel material according to the invention, the alloy spans are chosen so that a wider area is available for vanadium alloyed, high-strength and wear-resistant materials, said area being more useful for the use of hard material particles referred to in technical language also as metal matrix composites (“MMCs”). The two most important alloying elements in this alloy system are carbon and vanadium.
Carbon is responsible for martensitic hardening as well as for the formation of hard vanadium carbide, resulting in optimised wear resistance in combination with high hardness and high strength. C is therefore present in the steel according to the invention in contents of 1.5-5.0 wt. %. The carbon here has two main tasks: On the one hand C is needed 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 existing alloying elements, 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 take place; if C is too high, residual austenite is stabilised. Both effects can reduce hardness and wear resistance. The ratio of carbon to the carbide-forming elements is therefore always important.
On the one hand, silicon is used for the deoxidation during the melting of the starting materials, which are part of the steel alloy powder alloyed according to the invention 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 wt. % of Si, in particular at least 0.7 wt. % of Si, higher C contents would be necessary. The lowered melting point in turn facilitates the atomisation process. Silicon also reduces the viscosity of the molten metal, which also contributes to the simplification of the powder atomisation process. At the same time, silicon increases the through-hardenability of the steel material, since the conversion projections in the time-temperature diagram are shifted to longer times. The strength of the austenite to hardening temperature is increased by the dissolved amount of Si, which explains the higher stability of the austenite and enables longer cooling periods. These effects are achieved at Si contents of up to 2.0 wt. %, in particular up to 1.5 wt. %. Too high Si contents would lead to a stabilisation of the ferrite, which would reduce the amount of martensite present in the structure of the steel after hardening and thus also reduce the hardness and wear resistance of the steel material according to the invention.
Manganese is present in the steel material according to the invention, in order to optimise the hardness of the steel and its atomisability during the production of the steel powder. Thus, by the presence of sufficient contents of Mn, similar to the presence of Si, the melting point of the steel is lowered and the viscosity of the molten metal lowered, so that the targeted addition of Mn also contributes to the simplification of the atomisation process. At the same time, manganese also increases the through-hardenability of the steel material. Likewise, the dissolved portion of Mn contributes to the stabilisation of austenite. In addition, Mn binds sulphur by forming MnS, reducing the risk of hot cracking and improving machinability. These effects are reliably achieved at Mn contents of at least 0.3 wt. %, in particular at least 0.7 wt. %, and Mn contents of up to 2.0 wt. %, in particular up to 1.5 wt. %. Excessive contents of manganese could on the one hand stabilise the austenitic phase to the extent that the soft annealing time would be significantly increased. On the other hand, the austenitic phase could also be stabilised to such an extent by excessively high Mn contents that residual austenite remains in the microstructure after hardening. This microstructure would be significantly softer than martensite, reducing hardness and wear resistance. Mn contents of a steel material according to the invention of approximately 1.2 wt. % prove to be particularly practical.
Chromium is used in the steel according to the present invention in combination with Mo and V to adjust the tempering resistance, corrosion resistance and hardenability. Consequently, by varying the Cr content, these three properties can be adapted according to the respective requirements. At low Cr contents of 3.0-8.0 wt. %, Cr has a positive influence on the tempering resistance and the through-hardenability in particular. With increasing Cr contents, the corrosion resistance and the contribution of Cr to carbide formation increase. Average Cr contents of more than 8.0 wt. % to less than 11.0 wt. % therefore constitute a transitional region. For increased demands on corrosion resistance, the Cr content here is not sufficient. However, a higher hardness of the steel matrix arises as a result of increasing Cr carbide formation. At contents of at least 11.0 wt. % Cr, in particular at least 12.0 wt. %, in the steel material according to the invention tempering and corrosion resistance are achieved with maximum hardness and strength, which withstand the highest demands. In this case, the advantageous effects of Cr can be particularly reliably used, in that the Cr content is set to at least 12.5 wt. %. Too high Cr contents would cause more Cr carbides to form. However, the formation of Cr carbides would bind C, which would reduce the formation of martensite, so that the desired high hardness of martensite could no longer be achieved. Moreover, if the Cr contents were significantly increased beyond the upper limit prescribed by the invention, the ferritic phase would be stabilised, which would also not achieve the required hardness and wear resistance. Therefore, according to the invention, the maximum content of Cr is limited to 15.0 wt. %, in particular at most 14.0 wt. %, with Cr contents of up to 13.5 wt. % having been found to be particularly suitable in practice.
An optimised effect of the carbon content of the steel matrix of a steel material according to the invention with respect to the formation of vanadium carbides VC can be ensured at low Cr contents of up to 8 wt. %, in that the C content % C of the steel matrix is a target content % CTarget, calculated as follows:
% CTarget=0.2×% V+0.4
where % V is the respective V content of the alloy of the steel matrix.
If, on the other hand, Cr is used in the range of 11.0-15.0 wt. %, the C content % C should be approximately 30% higher than the target content % CTarget determined according to the formula given above. In this case, the C content of the steel matrix is thus optimally adjusted to correspond to a target content % CTarget, which is calculated as follows:
% CTarget=(0.2×% V+0.4)×1.3
where again % V signifies the respective V content of the alloy of the steel matrix.
Accordingly, in the case of the average Cr contents of >8.0 wt. % to <11.0 wt. %, it is advantageous to choose a C content which lies between the C minimum contents which, according to the two preceding formulas, can be determined for the low Cr and high Cr contents.
The content % CTarget is in each case a target quantity which should be optimally sought for the C content in the production of the alloy powder. It goes without saying that this target content is considered to have been reached when the actual C content % C is the same as the target content % CTarget of the respective steel material according to the invention within the tolerances specified by the conventional or predetermined alloying technology. A practical value of the deviation of the actual C content % C from the target content % CTarget, which is still permitted in this respect, amounts to 0.2 wt. %. For the actual C content % C of the steel matrix, the following should then apply: % C=% CTarget±0.2 wt. %.
The C content being adjusted in accordance with the above-described proviso compensates for carbon being bound by Cr due to the Cr carbide formation. In this way it can be ensured that sufficient C is always available for the formation of martensite and an optimised hardness and wear resistance is achieved, which are sufficient for most applications.
Accordingly, depending on the respective V content % V at Cr contents of up to 8 wt. % for the target content % CTarget, for example, the following values result (data in wt. %):
For the steel material V15 with up to 8 wt. % Cr and a nominal V content of 15 wt. %, a tolerance range of the V content of, for example, +/−0.5 wt. % is permitted, so that its actual V content may vary between 14.5-15.5 wt. %. At the same time, a tolerance of +/−0.2 wt. % for the actual C content is allowed by the target value % CTarget. The actual C content of the steel material V15 can thus be 3.2-3.6 wt. %.
Molybdenum, like chromium, increases the corrosion resistance, hardenability and tempering resistance of components made from steel according to the invention when Mo contents of at least 0.5 wt. %, in particular at least 0.9 wt. %, are present. Too high contents of Mo, however, worsen the formability of the steel, since the high-temperature strength is significantly increased. In addition, high contents of Mo would also stabilise the ferritic phase. Therefore, the maximum content of Mo in steel according to the invention is limited to 2.0 wt. %, in particular max. 1.5 wt. %. The Mo content of a steel according to the invention, which is particularly suitable for the purposes of the invention, is accordingly in the region of 1.2 wt. %.
Vanadium is present in the steel according to the invention at contents from 6.0% to 18.0 wt. % to achieve optimised wear resistance through the formation of vanadium-rich carbides or carbonitrides. In addition, vanadium increasingly participates 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. Maximised positive effects of the presence of V can be achieved when at least 14.5 wt. % V is present in the steel according to the invention. High V contents of at least 16 wt. % 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 rolling guide rollers, which are exposed to maximum loads during use. On the other hand, by restricting the V content to 17.4 wt. % or 17.0 wt. %, to 16.0 wt. % or more preferably at most 15.5 wt. %, it can be reliably avoided that too much carbon is bound by carbide formation. With low V contents and correspondingly reduced C contents tending toward the lower edge of the content range indicated according to the invention for V, the steel material according to the invention can be processed by machining more easily than at the higher V and C contents. A simplified machinability results accordingly when the V content is limited to max. 12 wt. %, in particular max. 10 wt. %, and thus also the C content dependent on the V content is limited in the manner described above.
Niobium is optionally present at contents of up to 2.0 wt. % in the steel according to the present invention. Nb has a very similar mode of action to vanadium. It mainly participates in the formation of hard and wear-resistant monocarbides. Therefore, in each case based on their contents in at. %, Nb and V can be exchanged alternately in a ratio of 1:1, if this proves to be expedient, for example, with regard to the availability of these alloying elements.
Nickel may optionally be present in the steel material according to the invention at contents up to 1.0 wt. % in order to stabilise the austenite portion similarly to Mn and thus improve hardenability. Thus, the presence of Ni ensures that austenite is actually formed at the respective hardening temperature and that no unwanted 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 contents should not be too high, since there is the risk that residual austenite will be present in the microstructure after hardening. If Ni is to be added, the Ni content is then preferably at least 0.2 wt. %, wherein optimised effects of the presence of Ni are regulated with adjusted Ni contents of up to 0.4 wt. %.
Cobalt may also optionally be present at contents of up to 1.0 wt. % in the steel material according to the present invention. Similar to nickel, Co has a stabilising effect on austenite formation and hardening temperature. However, unlike nickel or manganese, Co does not lower the final temperature of the martensite, so its presence is less critical with respect to the formation of residual austenite. In addition, cobalt increases the heat resistance. If these positive effects are to be utilised by the addition of Co, contents of at least 0.3 wt. % of Co prove to be particularly expedient, with optimised effects occurring at Co contents of up to 0.5 wt. %.
Tungsten, like Co and Ni, can optionally be added to the steel in contents of up to 1.0 wt. %. Above all, tungsten increases the tempering resistance and, above all, participates in carbide formation in the secondary hardness maximum during tempering. The presence of W shifts the tempering temperatures to higher temperatures. In addition, the heat resistance is increased by W, similar to cobalt. However, excessive W contents would also stabilise the ferritic phase. If the positive effects of W are to be utilised, contents of at least 0.3 wt. % W are therefore found to be particularly expedient, with optimised effects occurring at W contents of up to 0.5 wt. %.
The remainder of the steel respectively comprises iron and unavoidable impurities which enter the steel due to the manufacturing process or raw materials, from which the constituents of the steel alloy powder are recovered, but have no effect in terms of properties.
Sulphur may be present in contents up to 0.35 wt. % in the steel material to improve machinability. At higher S contents, however, the properties of the composite steel material according to the invention are deteriorated. In order to be able to safely take advantage of the favourable effect of the presence of S, at least 0.035 wt. % may be present in the steel material according to the invention. If, on the other hand, the machinability is not improved by the targeted addition of S, the S content can accordingly be limited to less than 0.035 wt. %.
The unavoidable impurities also include contents of P of up to 0.035 wt. % and, for example, in total up to 0.2 wt. % oxygen.
Nitrogen is also not alloyed with the steel material according to the invention in a targeted manner, but due to the nitrogen affinity of the alloy constituents passes into the steel material during the atomisation process. In order to avoid negative effects of N on the properties of the steel material, the content of N should be less than 0.12 wt. %, in particular limited to a maximum of 0.1 wt. %.
The density of steel material according to the invention is typically in the range of 6.4-7.6 g/cm3, the density of the pure steel matrix material typically being 7.0-7.6 g/cm3.
Its minimised density and its resulting low weight makes steel material according to the invention particularly suitable for the production of components which are repeatedly exposed to rapid acceleration in practical use and in which consequently a lower mass inertia has a particularly favourable effect.
The powder metallurgical production allows further optimisation of the density and wear resistance of steel according to the invention by targeted addition of hard phases with low density in accordance with the respective application, if this is desired with regard to the particular intended property. Here it has been shown that the performance characteristics of steel material according to the invention are increased, in that the steel material contains 2.5 to 30 wt. % hard material particles, which are embedded in the finished steel its steel matrix composed in the manner described above.
The hard materials exist as powder in the initial state, like the steel alloy powder forming the steel matrix.
Hard materials, also known as “hard phases” in technical language, can be carbides, nitrides, oxides or borides. Accordingly, the group of suitable hard materials includes Al2O3, BaC, SiC, ZrC, VC, NbC, TiC, WC, W2C, Mo2C, V2C, BN, Si3N4, NbN or TiN.
Titanium carbide TiC has been found 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. Likewise, the low density of TiC has an advantageous effect.
At hard material contents alloyed with the steel material of less than 2.5 wt. %, 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 wt. % of alloyed hard material particles in the steel material according to the invention, wherein contents of at least 7.5 wt. % are found to be particularly effective. In order to reliably avoid excessive embrittlement of the material as a consequence of the presence of the hard material particles, the content of alloyed hard material particles can be limited to not more than 25 wt. % in the material according to the invention. The contents of hard material particles mentioned here in a steel material according to the invention prove to be particularly expedient when the alloyed hard material is titanium carbide TiC.
Steel according to the present invention, after hardening and tempering, achieves hardness values typically in the range of 58-70 HRC.
After a soft annealing, which is generally 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.
In the production of components according to the invention from a steel according to the invention, at least the following working steps are performed:
- a) A steel alloy powder is provided comprising (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 being iron and unavoidable impurities.
- b) The steel alloy powder is mixed with hard material particles with the proviso that the content of hard material particles in the obtained steel alloy powder-hard material particle mixture is 2.5-30 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.
With regard to the practical implementation and the embodiments of steps a) to e) of the method according to the invention, the following information applies:
Step a)
Powder production may be accomplished in a conventional manner, for example by gas atomisation or any other suitable method. For this purpose, the alloy powder can be produced, for example, by gas or water atomisation or a combination of these two atomisation methods. An atomisation of a melt alloyed according to the invention for the alloy powder is conceivable.
Alternatively, however, it is also possible initially to provide the alloying elements of the steel alloy powder individually in powder form in quantities corresponding to the content proportions provided for the respective alloying element and then to mix these amounts of powder into the steel alloying powder composed according to the invention.
If necessary, those which have an average diameter of less than 500 μm are selected by screening from the powder particles for the further processing according to the invention, with powders having average particle sizes of less than 250 μm, in particular less than 180 μm, having proven particularly suitable.
Regardless of the manner of its production, 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).
Step b)
The steel alloy powder provided in step a) is mixed with the respectively selected hard material powder. The amount of admixed hard material particles is determined, taking into account the information given above with regard to the optimised selection of the content of hard materials, in such a way that the content of the hard material particles in the finished mixture is in the range of 2.5-30 wt. %,
Step c)
If necessary, the alloy powder prepared in step a) or step b) may be dried in a conventional manner to remove residues of liquids and other volatiles which could hinder the subsequent shaping process.
Step d)
From the alloy powder comprising hard material particles, a blank (semifinished product) is then formed. For this purpose, the alloy powder can be brought into the respective form in a manner known per se by means of a suitable sintering process, in particular by hot isostatic pressing (“HIPing”). In general, HIPing will be performed. Typical pressures during HIPing are in the range of 900-1500, in particular 1000 bar, at a temperature of 1050-1250° C., in particular 1080-1200° C. In the course of hardening, austenite, VC and Cr carbide form in the microstructure of the steel material.
Alternatively, the respective component can also be produced from the alloy powder prepared and provided according to the invention in an additive process. The term “additive” summarises all manufacturing processes in which a material is added to produce a component, wherein this addition generally takes place in layers. “Additive manufacturing processes”, which are often referred to as “generative processes” in technical language, are contrasted with the conventional subtractive production processes, such as machining processes (e.g. milling, drilling and turning), in which material is removed, in order to give form to the component respectively to be manufactured. The additive design principle makes it possible to produce geometrically complex structures that cannot be realised or can only be realised with great difficulty using conventional manufacturing processes, such as the aforementioned machining processes or primary moulding processes (casting, forging) (see VDI Status report “Additive Manufacturing”, September 2014, published by the Association of German Engineers e.V., Production Technology and Manufacturing Methods, www.vdi.de/statusadditiv). Further definitions of the methods, which are summarised under the generic term “additive methods”, can be found, for example, in VDI Guidelines 3404 and 3405.
Step e)
The semifinished product obtained after step d) still requires finishing in order to give it on the one hand the desired performance characteristics and on the other hand the required final shape. Finishing includes, for example, mechanical, in particular material-removing machining of the semifinished product, and heat treatment, which may comprise hardening and tempering.
The invention will be explained in more detail below on the basis of exemplary embodiments:
In the manner explained above, alloy powders composed according to the invention are shaped into a blank (semifinished product), for example by hot isostatic pressing or another suitable sintering process. For this purpose, the respective alloy powder can be filled into in a suitable mould, for example a cylindrical capsule, and then held 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., for a sufficient time until a solid body is formed. Typically, in hot isostatic pressing, the pressure is in the range of 102-106.7 MPa and the heating is typically at the target temperature of 1150-1153° C., which is maintained for a duration of typically 200-300 minutes, particularly 245 minutes, also typically at a heating rate of 3 K/min-10 K/min.
The production of the semifinished product is followed by the heat treatment. In this case, the respective semifinished product is heated at a heating rate of typically 5 K/min to a hardening temperature (austenitising temperature) of 1050-1200° C., at which it is held until it is completely warmed through. Typically, this will take 30 to 60 minutes. Subsequently, the semifinished products, thus heated, are quenched. They are cooled with a suitable quenching medium, for example with water, oil, a polymer bath, moving or static air or, if the cooling is carried out in a vacuum oven, with gaseous nitrogen, in the range of 5-30 min to room temperature. In particular for large semifinished products, it may be expedient to allow 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., to ensure uniform heating.
In order to avoid reactions with the ambient atmosphere, curing in a vacuum oven can also be carried out in a manner known per se. However, this is not a prerequisite for the success of the method according to the invention.
After hardening, tempering may be carried out in which the semifinished product is held for a period of, for example, 90 minutes at the respective tempering temperature, which is typically 450-550° C. The tempering conditions are determined in a manner known per se depending on the respective hardening temperature and the desired level of hardness, i.e. the desired strength selected. The heating and cooling rates are usually in the order of 10 K/min for tempering. In contrast to curing, the heating and cooling rates during tempering are not critical. By tempering, the brittle martensite relaxes through diffusion of carbon. Together with e.g. V, Cr and Mo, this forms what is known as “temper carbide”. 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 carbide formation.
Since there is usually a narrow temperature range (approx. 50° C. roughly between 450 and 650° C.) in such alloy systems, this is referred to as a secondary hardness maximum, since temperatures below or above this result in a lower hardness.
Using the above-described general procedure in the practical production of steel materials according to the invention and components produced therefrom, cylindrical semifinished 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 respectively contained (in wt. %) 2.5% C, 0.9% Si, 0.9% Mn, 4.5% Cr, 1.2% Mo and 10.0% V, residual iron and unavoidable impurities. In addition, 5 wt. % of TiC was alloyed with the steel material V10a, 10.0 wt. % of TiC with the steel material V10b, 15 wt. % of TiC with the steel material V10c and 20 wt. % of TiC with the steel material V10d.
The following are provided in Table 1: the austenitising temperature AT, the hardness HRC (“HRC_v”) existing before the subsequent heat treatment step, either, if tempering has been performed, the tempering temperature ST and tempering time St, or, if soft annealing has been performed, the soaking temperature WT and the soaking annealing time Wt, the hardness HRC (“HRC_n”) after the previous heat treatment step and the density p of the samples V1-V8.
The heating to the respective austenitising temperature AT was carried out in a vacuum oven. There, the samples V1-V8 were kept at the austenitising temperature AT for an austenitising time At. Subsequently, the samples were cooled to room temperature in the vacuum oven by exposure to gaseous nitrogen applied at a pressure of 3.5 bar.
After curing, samples 1-8 were subjected to either tempering or annealing treatment. In the tempering treatment, the samples 1, 3, 5, 7 were kept at the tempering temperature ST for the tempering period St. This tempering treatment was carried out twice to obtain an optimal tempering result.
During the soft annealing, the samples 2, 4, 6, 8 were kept at the annealing temperature WT for a duration Wt. After the end of the annealing time, the oven was switched off and the samples 2, 4, 6, 8 were cooled slowly to room temperature in the switched-off oven.
Claims
1. A steel material produced by powder metallurgy and having a steel matrix comprising (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%,
- optionally one or more elements selected from the group consisting of Nb, Ni, Co, and 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
- residual iron and unavoidable impurities,
- wherein 2.5-30 wt. % of separately added hard material particles are embedded in the steel matrix.
2. The steel material according to claim 1, wherein, when the Cr content of the steel matrix is 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, where % CTarget=0.2×% V+0.4 wt. % and % V denotes the respective V content of the steel matrix.
3. The steel material according to claim 1, wherein, when the Cr content of the steel matrix is 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, where % CTarget=(0.2×% V+0.4 wt. %)×1.3 and % V denotes the respective V content of the steel matrix.
4. The steel material according to claim 1, wherein, when the Cr content of the steel matrix is more than 8 wt. % and less than 11 wt. %, the C content of the steel matrix is between % CTarget1 and % CTarget2 where % CTarget1=0.2×% V+0.4 wt., % CTarget2 (0.2×% V+0.4 wt. %)×1.3, and % V denotes the respective V content of the steel matrix.
5. The steel material according to claim 1, wherein the Si content of the steel matrix is at least 0.7 wt. % and at most 1.5 wt. %.
6. The steel material according to claim 1, wherein the Mn content of the steel matrix is at least 0.7 wt. % and at most 1.5 wt. %.
7. The steel material according to claim 1, wherein the S content of the steel matrix is at least 0.035 wt. %.
8. The steel material according to claim 1, wherein the Mo content of the steel matrix is at least 0.9 wt. % and at most 1.5 wt. %.
9. The steel material according to claim 1, wherein the steel matrix further comprises one or more elements selected from the group consisting of (in wt. %): Ni: 0.2-0.4%, Co: 0.3-0.5%, and W: 0.3-0.5%.
10. The steel material according to claim 1, wherein the hard material particles are TiC particles.
11. The steel material according to claim 1, wherein the hard material particles are present in a D50 particle size of at most 50 μm.
12. A method for producing a component which comprises a steel material according to claim 1, comprising:
- a) preparing a steel alloy powder comprising (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, optionally one or more elements from the group consisting of Nb, Ni, Co, and 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 iron and unavoidable impurities,
- b) mixing the steel alloy powder with hard material particles, wherein the content of hard material particles in the steel alloy powder and hard material particle mixture is 2.5 to 30 wt. %,
- c) optionally, drying the steel alloy powder or the steel alloy powder and hard material mixture,
- d) forming a solid semifinished product from the steel alloy powder or the steel alloy powder and hard material particle mixture by a sintering process,
- c) processing the resulting semifinished product into the component.
13. The method according to claim 12, wherein the alloy constituents of the steel alloy powder are respectively provided in powder form and mixed into the steel alloy powder.
14. The method according to claim 12, wherein the processing of the resulting semifinished product comprises a material-removing machining of the semifinished product.
15. A component which performs movements involving high acceleration or velocity comprising a steel material according to claim 1.
16. The method according to claim 12, wherein the sintering process is one of hot isostatic pressing and an additive process.
Type: Application
Filed: Nov 21, 2017
Publication Date: Jun 18, 2020
Inventors: Horst Hill (Grefrath), André van Bennekom (Ainring)
Application Number: 16/349,463
