EP3591081A1 - Use of a steel for producing a steel component, namely for a gearwheel, a shaft, an axle or a tool holder to a thermochemically cured edge layer and such steel component having a thermochemically cured edge layer - Google Patents

Abstract

The invention relates to the use of a steel for producing a steel component, namely a gear wheel, a shaft, an axle or a tool holder, with a thermochemically hardened surface layer, the steel component having a structure consisting of at least 80% by volume of bainite in its core area and the steel used consists of (in% by weight) C: 0.1-0.30%, Si: 0-0.80%, Mn: 0.20-2.00%, Cr: 0-4.00 %, Mo: 0.5 - 1.80%, N: 0.004 - 0.020%, S: 0 - 0.40%, AI: 0.004 - 0.020%, B: 0 - 0.0025%, Nb: 0 - 0 , 20%, Ti: 0-0.02%, V: 0-0.40%, Ni: 0-0.5%, Cu: 0-0.3%, Co: 0-1.5%, remainder Iron and unavoidable impurities, the following condition applies to the Al content% Al, the Nb content% Nb, the Ti content% Ti, the V content% V and the N content% N of the steel:% Al / 27 +% Nb / 45 +% Ti / 48 +% V / 25>% N / 3.5. The invention also relates to a steel component made in accordance with its use.

Description

The invention relates to the use of a steel for producing a steel component, which is a gear, a shaft, an axis or a tool holder and which has a thermochemically hardened surface layer, and to such a steel component which has an edge layer produced by a thermochemical diffusion treatment has.

If "%" information on alloys or steel compositions is given below, then these refer to the weight, unless expressly stated otherwise.

Unless otherwise stated, all of the mechanical properties of the steel to be used according to the invention and of the steels used for comparison, as specified in the present text, have been determined in accordance with DIN EN ISO 6892-1.

The steel components considered here are typically components which, in practice, come into metallic contact with other components in a rolling or rolling motion and are therefore exposed to high mechanical loads in the area of their contact surface. Typical examples of such components are gears, shafts or axes. Comparable loads can occur with tool holders, For example, cutting tools and the like, come in the area of the contact surfaces between the holder and the respective tool.

The particular challenge here is that such steel components are generally complex in shape and can only be manufactured by complex machining. Such machining can be carried out particularly economically if the components consist of steels with low carbon contents, that is to say they have a low hardness. At the same time, a comparably low hardness and the associated high toughness of the steel, of which such components are made, prove to be favorable with regard to their fatigue strength, in particular under operating conditions in which the loads to be absorbed by the respective component occur dynamically.

For example, case hardening steels are typically used today for gear manufacturing, for example the steels with the 16MnCr5 / 16MnCrS5 (material numbers 1.7131 / 1.7139) and 18CrNiMo7-6 (material number 1.6587).

Tool holders, such as holders for cutting elements produced by powder metallurgy, are often produced from relatively expensive tool steels, such as steels with the material numbers 1.2311, 1.2312, 1.2738, 1.2343 or 1.2343.

Various heat treatment processes are known with which the service life of workpieces and tools made from such comparatively soft steels can be improved. These methods are based on the fact that a hardness is generated in an edge layer that bears the contact surface that is loaded during use than in the core area of the component that carries the relevant edge layer, in which a high level of toughness is still present even after the heat treatment.

As in leaflets 452 "Case hardening", edition 2008, and 477 "Heat treatment of steel - nitriding and nitrocarburizing", edition 2005, both published by the steel information center, PO Box 10 48 42, 40039 Düsseldorf, Germany, and at URL http : //www.stahl-online.de/index.php/service/publikationen/stahlanendung-merkblaetter/ available for download, explained in detail, the heat treatment processes aimed at forming a hardened surface layer zone on steel components work without and with chemical changes in the surface layer , The processes based on a chemical change, in which the hardening of the component surface layer is brought about by thermochemical diffusion processes, also differ again in whether or not an additional heat treatment (hardening) is carried out after the heat treatment.

Common processes that can be used to provide gearwheels and components that are subjected to comparable loads with a hardened surface layer include case hardening (see leaflet 452), in which the surface layer of the steel component is first subjected to carburizing or carbonitriding treatment to increase the carbon content and then the component undergoes hardening in order to achieve maximum hardness in the hardened surface layer, and nitriding or nitrocarburizing (see leaflet 477), in which the increase in hardness of the surface layer is achieved essentially by nitrogen diffused in, with an additional increase in hardness by can be achieved in combination with the carbon diffused in nitrogen.

Against the background of the prior art explained above, the object of the invention was to name a steel, the use of which is an optimal combination of properties, particularly in those hardened by a thermochemical diffusion treatment Steel components results that are in rolling or rolling contact with another component in use.

An outer layer hardened steel component should also be mentioned, which has an optimal combination of hardness in its outer layer and toughness in its core area supporting the outer layer with regard to its fatigue strength.

For the production of surface-hardened steel components which have an optimally tough core area and at the same time are well suited for surface hardening by a thermochemical process, the invention proposes the steel to be used according to claim 1.

A steel component which achieves the above-mentioned object accordingly has the features specified in claim 8 according to the invention.

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

The steel to be used according to the invention opens up a robust and cost-effective production route for the production of steel components to be hardened by a thermochemical diffusion treatment, such as gear wheels, axles, shafts or tools with special application conditions. The components produced from steel used in accordance with the invention, after the heat treatment carried out for their thermochemical surface hardening, have a higher toughness in their core area, also called "matrix", than is the case with steels commonly used today for this purpose.

The invention is based on the knowledge that a modification of a steel which forms a bainitic structure is basically the result of the publication EP 3 168 312 A1 a European patent application is already known for the forging production of components, is also particularly suitable as a material for the production of steel components with a thermochemically hardened surface layer. It has surprisingly been found that the alloy concept, which is intended for forging applications, has considerable advantages in thermochemical surface hardening due to the high tempering resistance of the bainitic structure of the steel proposed for use, particularly with regard to the toughness of the steel component in its core area.

In this regard, it proves to be particularly advantageous that the steel known per se from the publication of the aforementioned European patent application, as in the EP 3 168 312 A1 explained in detail, has a wide bainite window in the time-temperature diagram ("ZTU diagram"), ie reliably forms a bainitic structure dominated by bainite to at least 80% by volume over a large range of cooling rates. Surprisingly, it has been shown here that the known alloy specification ensures these properties of the steel even if the steel is not cooled from the forging heat as originally intended, but is subjected to a thermochemical diffusion treatment. This also applies if the respective steel component is subjected to hardening after the diffusion treatment, as is customary in case hardening.

Steel components produced from steel used according to the invention, that is to say gear wheels, shafts, axes or tool holders, are distinguished by a particularly homogeneous structure with a low variance in hardness. This optimally uniform distribution of the structural properties is also included Different dimensions of the steel components to be produced from steel to be used according to the invention and in the cooling conditions which vary over a wide range and which are caused by these dimensional differences. The homogeneous structural state that arises when the steel is used according to the invention also causes low internal stresses in the component. Accordingly, the steel components produced from the steel used in accordance with the invention tend to warp only slightly in the course of the thermochemical surface layer hardening and to the development of cracks or other stress-related damage.

According to the invention, a steel is therefore used to produce a steel component, which is a gear wheel, a shaft, an axis or a tool holder, with a thermochemically hardened surface layer, which consists of (in% by weight) 0.1-0. 30% C, up to 0.80% Si, 0.20 - 2.00% Mn, up to 4.00% Cr, 0.5 - 1.80% Mo, 0.004 - 0.020% N, up to 0, 40% S, 0.004 - 0.020% Al, up to 0.0025% B, up to 0.20% Nb, up to 0.02% Ti, up to 0.40% V, up to 0.5% Ni, 0.3% Cu, up to 1.5% Co and the balance iron and unavoidable impurities, where the Al content% Al, the Nb content% Nb, the Ti content% Ti, the V content % V and the N content% N of the steel meet the following condition: $$\%\mathrm{al}/\mathrm{27}+\%\mathrm{Nb}/\mathrm{45}+\%\mathrm{Ti}/\mathrm{48}+\%\mathrm{V}/\mathrm{25}>\%\mathrm{N}/\mathrm{3}.\mathrm{5th}$$

The steel to be used according to the invention is alloyed and can be processed in such a way that the steel component that is made from it has a structure consisting of at least 80% by volume of bainite in its core area. The production-related unavoidable impurities of the steel to be used according to the invention include all elements which are present in quantities which are ineffective in alloying technology with regard to the properties of interest here and on account of the the selected route for producing the steel powder or the respectively selected starting material (scrap) get into the steel. In particular, the inevitable impurities also include P contents of up to 0.0035% by weight.

A steel component produced from steel to be used according to the invention is thus characterized in that it has a structure consisting of at least 80% by volume of bainite. The remaining structure of a total of at most 20 vol .-% of the total structure is occupied by residual austenite, ferrite, pearlite and / or martensite. Typically, however, the contents of non-bainitic structural components of a steel component consisting of steel to be used according to the invention are minimized to such an extent that there is a completely bainitic structure in the technical sense.

The alloy concept on which the steel to be used according to the invention is based avoids expensive alloy constituents, such as are usually required today in the case and tool steels used for the production of steel components in question, in order to set the required hardness. This is achieved by selecting the alloy elements and their contents in the steel to be used according to the invention as follows:
Carbon ("C") is contained in the steel to be used according to the invention in a content of 0.1-0.3% by weight in order to contribute to increasing the strength of the material by carbide formation. For example, the addition of 0.01% by weight can increase the strength by approx. 70 MPa in each case. This effect occurs in particular from a content of at least 0.09% by weight of C, in particular at least 0.12% by weight of C. By limiting the C content to at most 0.30% by weight, in particular at most 0.25% by weight, it is achieved that the steel has good stretch and toughness properties despite its maximized strength. At the same time, the comparatively low C content in a steel to be used according to the invention also contributes to the acceleration of the bainite conversion, so that the formation of undesired structural components is avoided. An optimized effect of the presence of C in the steel to be used according to the invention can be achieved by setting the C content to 0.12-0.25% by weight.

Silicon ("Si") suppresses the formation of cementite in the steel to be used according to the invention and shifts the formation of ferrite at shorter times. The Si content of a steel to be used according to the invention is therefore limited to 0.80% by weight in order to allow the bainite conversion to take place as early as possible. At the same time, Si contents up to this upper limit contribute to increasing the strength through solid solution strengthening. In order to be able to use the advantageous effects of Si in the steel to be used according to the invention particularly reliably, the Si content is therefore preferably at least 0.2% by weight, in particular more than 0.45% by weight, such as at least 0.46% by weight .-%, set.

Manganese ("Mn") is present in the steel to be used according to the invention in a content of 0.20-2.00% by weight in order to adjust the tensile strength and yield strength by means of mixed crystal formation. A minimum content of 0.20% by weight of Mn is required in order to increase the strength. If this effect is to be achieved particularly reliably, a Mn content of at least 0.4% by weight can be provided. However, too high Mn contents would delay the bainite conversion and thus lead to a predominantly martensitic conversion. The Mn content is therefore limited to at most 2.00% by weight, in particular at most 1.5% by weight. The presence of Mn can be negatively influenced Avoid particularly safely by limiting the Mn content of the steel to be used according to the invention to a maximum of 1.2% by weight.

Optional chromium ("Cr") contents of up to 4.00% by weight contribute to the hardenability and corrosion resistance of the steel to be used according to the invention by the formation of special carbides and chromium nitrides in one of the nitriding treatments carried out according to the invention. For this purpose, for example at least 0.5% by weight or at least 0.8% by weight of Cr can be provided. The optimal effect of the presence of Cr is obtained with a Cr content of at least 1.00% by weight. Cr contents above 4.00% by weight would favor undesirable martensite formation in the structure of the steel to be used according to the invention. In order to reliably avoid this, the Cr content can be limited to up to 3% by weight or up to 2.5% by weight.

Molybdenum ("Mo") is present in the steel to be used according to the invention in contents of 0.5-1.8% by weight in order to delay the transformation of the structure into ferrite or pearlite and to enlarge the window for the bainite transformation. This effect occurs in particular if there is at least 0.6% by weight in the steel. At levels of more than 1.8% by weight, based on the use of the steel to be used according to the invention, which is the focus here, there is no longer an economically justifiable further increase in the positive effect of Mo. By limiting the Mo content to 1.8% by weight, the formation of a molybdenum-rich carbide phase which would have a negative effect on the toughness properties is reliably ruled out. Optimal effects of Mo in the steel to be used according to the invention can be expected if the Mo content is at least 0.7% by weight. Mo contents of at most 1.5% by weight or at most 1.0% by weight have proven to be particularly effective.

The presence of N in the contents of 0.004-0.020% by weight provided according to the invention enables the formation of nitrides and carbonitrides to increase the strength and increase the resistance to fine grains, without causing embrittlement. So Al forms with N aluminum nitride, which contributes to fine grain stability.

The sulfur ("S") content in the steel to be used according to the invention can be up to 0.4% by weight, in particular at most 0.1% by weight, in order to support the machinability of the steel. For this purpose, an S content of at least 0.001% by weight can be provided. If the S content is above 0.4% by weight, there is a risk of developing red brittleness. Optimal effects of the presence of S in the steel to be used according to the invention can be achieved at contents of 0.003-0.1% by weight.

The presence of B in contents of up to 0.0025% by weight, in particular at least 0.0001% by weight or at least 0.0005% by weight, in the steel to be used according to the invention delays the formation of ferrite or pearlite and ensures it thus the formation of the desired bainitic structure in the steel to be used according to the invention. B contents above 0.0025% by weight would entail the risk of embrittlement. The optionally available microalloying elements Nb, Ti and V form carbonitrides and can thus make a significant contribution to optimizing the fine grain stability and strength of the steel to be used according to the invention.

The alloy-technical fine adjustment with regard to the mechanical properties and the texture of a steel used according to the invention is carried out according to the alloy concept used according to the invention by means of a combined micro-alloy from the elements Boron ("B") in optional contents of up to 0.0025% by weight, in particular in contents of 0.0001 - 0.0025% by weight B or 0.0005 - 0.0025% by weight B, Nitrogen ("N") in contents of 0.004 - 0.020% by weight, in particular at least 0.006% by weight N or up to 0.0150% by weight N, aluminum ("AI") in contents of 0.004 - 0.020% by weight .-% and niobium ("Nb") in optional contents of up to 0.020% by weight, in particular up to 0.015% by weight and in particular at least 0.003% by weight or at least 0.005% by weight of Nb, titanium (" Ti ") in optional contents of up to 0.02% by weight or up to 0.015% by weight, in particular at least 0.001% by weight or at least 0.005% by weight of Ti, and vanadium (" V ") in optional Contained up to 0.40% by weight, in particular up to 0.3% by weight and in particular at least 0.01% by weight or at least 0.02% by weight of V.

In order to safely take advantage of the presence of the microalloying elements and of aluminum, it may be expedient to reduce the Al content to at least 0.005% by weight, the Ti content to at least 0.001% by weight and the V content to at least 0 , 02% by weight or the Nb content to at least 0.003% by weight. The microalloying elements V, Ti, Nb on the one hand and Al on the other hand can each be present in combination with one or more elements from the group "Al, V, Ti, Nb" or alone in amounts above the minimum contents mentioned. With effects of up to 0.01 wt.% Ti, up to 0.1 wt.% Nb, up to 0.075 wt.% V or up to 0.020 wt Use elements in the steel used according to the invention particularly effectively. Here, too, the cited upper limits of the contents of Ti, Nb, V or Al can be complied with individually or in combination with one another in order to achieve the optimum effect of the alloy element in question.

so miteinander verknüpft, dass der im erfindungsgemäß zu verwendenden Stahl enthaltene Stickstoff über die jeweils vorhandenen Gehalte an AI sowie die gegebenenfalls zusätzlich zugegebenen Gehalte an Nb, Ti und V vollständig abgebunden ist und Bor somit umwandlungsverzögernd wirken kann. Die erfindungsgemäße Abbindung von N ermöglicht darüber hinaus, dass das optional vorhandene Bor als gelöstes Element in der Matrix des Stahls wirksam wird und die Bildung von Ferrit und/oder Perlit unterdrückt.The contents of% Al,% Nb,% Ti,% V and% N of Al, Nb, Ti, V and N are in the steel to be used according to the invention via the condition $$\%\mathrm{al}/\mathrm{27}+\%\mathrm{Nb}/\mathrm{45}+\%\mathrm{Ti}/\mathrm{48}+\%\mathrm{V}/\mathrm{25}>\%\mathrm{N}/\mathrm{3}.\mathrm{5}$$

linked with one another in such a way that the nitrogen contained in the steel to be used according to the invention is completely bound via the respectively present contents of Al and the optionally additionally added contents of Nb, Ti and V, and boron can thus delay the conversion. The setting of N according to the invention also enables the optionally present boron to act as a dissolved element in the matrix of the steel and to suppress the formation of ferrite and / or pearlite.

Also optionally present Ni contents of up to 0.5% by weight improve the toughness of the steel to be used according to the invention. If this effect is to be used, it occurs from a Ni content of at least 0.1% by weight, in particular at least 0.15% by weight.

Cu is also one of the alloying elements that reaches or is added to the steel to be used according to the invention via the starting material, the content of which to avoid negative influences in the steel to be used according to the invention is max. 0.3% by weight is limited.

Cobalt ("Co") optionally present in the steel to be used according to the invention causes a shift in the formation of bainite at shorter times in contents of up to 1.5% by weight. The positive influence of Co can be used in particular in the case of Co contents of at least 0.25% by weight, in particular at least 0.5% by weight, with Co contents of up to 1.0% by weight. have proven to be particularly effective.

A steel alloy which is particularly suitable for the purposes according to the invention therefore consists, according to the above explanations, of (in% by weight) 0.12-0.25% C, 0.20-0.80% Si, 0.40-1.20% Mn, 1.0 - 3.0% Cr, 0.5-1.8% Mo, 0.004-0.020% N, up to 0.40% S, 0.004-0.020% Al, 0.0005-0.0025% B, up to 0.10% Nb, up to 0.015% Ti, up to 0.20% V, up to 0.5% Ni, and / or up to 1.5% Co, balance iron and unavoidable impurities, to which the explanations given above in this regard also apply.

In principle, the steel to be used according to the invention for the production of steel components is suitable for all of the thermochemical diffusion processes “carburizing” (carburizing), “carbonitriding”, “nitriding” or “nitrocarburizing” described in the aforementioned leaflets 452 and 477.

If case hardening is to be carried out, carburizing or carbonitriding is first carried out as thermochemical diffusion treatment, as explained in detail in leaflet 452. After the carburization (carburizing, carbonitriding) of the surface layer caused by thermochemical diffusion, conventional case hardening is followed by hardening according to the hardening processes "direct hardening (type A)", "single hardening (type B)", "hardening after isothermal hardening", which are also described in detail in leaflet 452 Convert (Type C) "or" Double Hardness (Type D) ". In direct hardening (type A), the steel component is quenched directly from the heat of the previous carburization or carbonitriding treatment. In single hardening (type B), the steel component is first cooled to room temperature after the previous carburization or carbonitriding treatment and then reheated to an austenitizing temperature above the Ac1 and below the Ac3 temperature of the steel and then quenched. When hardening after isothermal conversion (type C), the steel component is first cooled from the heat of the previous carburization or carbonitriding treatment to a temperature range in which certain carbide precipitates form, and then, starting from this temperature range, heated again to an austenitizing temperature above the Ac1 and below the Ac3 temperature of the steel, in order then to be quenched. In double hardening (type D), after the steel component has been cooled down to room temperature from the heat of the previous carburizing or carbonitriding treatment, as in single hardening type A, it undergoes two hardening processes, as is done only once in single hardening type A.

Regardless of which of the four conventional hardening processes mentioned here is used, in the case of thermochemical diffusion treatment and the subsequent hardening of the steel components consisting of steel to be used according to the invention, the cooling to be carried out must be set in such a way that carburizing or carbonitriding occurs on the one hand carburized outer layer hardness-increasing precipitates and in the non-carburized core area of the component, a structure consisting of at least 80% by volume of bainite in accordance with the above-mentioned requirement. For this purpose, the temperature range of 800-500 ° C. is to be run through in each case in a time t8 / 5 of at least 6 s, in particular at least 10 s, and at most 1000 s, in particular at most 600 s.

If, on the other hand, the thermochemical diffusion treatment is to be carried out as nitriding or nitrocarburizing in order to form the hardened surface layer, the procedure described in detail in leaflet 477 can be selected. After heating to an austenitizing temperature above the Ac3 temperature of the steel from which the steel component is made, the steel component is continuously cooled such that the temperature range from 800-500 ° C. in a time t8 / 5 of at least 6 s, in particular at least 10 s, and at most 1000 s, in particular at most 200 s, in order to form a structure consisting of at least 80% by volume of bainite in the component according to the above-mentioned requirement. This is followed by the nitriding or nitrocarburizing step, in which the steel component in each case in accordance with the instructions and instructions contained in leaflet 477 under an atmosphere containing nitrogen or nitrogen and carbon at a temperature below the Ac1 temperature of the steel from which the steel component is made exists, the temperature is maintained and then cooled.

In particular when the steel to be used according to the invention for the purposes of the invention has the composition already mentioned above as particularly preferred with (in% by weight) 0.12-0.25% C, 0.20-0.80% Si0 , 40-1.20% Mn, 1.0-3.0% Cr, 0.5-1.8% Mo, 0.004-0.020% N, up to 0.40% S, 0.004-0.020% AI, 0 , 0001 - 0.0025% B, up to 0.10% Nb, up to 0.01% Ti, up to 0.20% V or up to 0.5% Ni, and up to ?? % Co, remainder iron and unavoidable impurities, can be achieved by heat treatments adapted to the material, an optimized toughness without loss of strength property.

• a) aus dem erfindungsgemäß zu verwendenden Stahl in konventioneller Weise ein Stahlbauteil, bei dem es sich beispielsweise um ein Zahnrad, eine Welle, eine Achse oder einen Werkzeughalter und desgleichen handeln kann, geformt.
  Anschließend wird in einem Arbeitsschritt
• b) das betreffende Stahlbauteil dann einsatzgehärtet, indem
• b1) das Stahlbauteil zunächst in einem Aufkohlungsschritt über eine Dauer von 150 min bis 250 Stunden bei einer Temperatur von 900 -1050 °C unter einem Medium gehalten wird, das Kohlenstoff und optional zusätzlich Stickstoff enthält, um an dem Stahlbauteil eine carburierte oder carbonitrierte Randschicht mit einer Dicke von 0,3 - 15 µm zu erzeugen, und anschließend an den Aufkohlungsschritt dann so schnell auf Raumtemperatur abgekühlt wird, dass bei der Abkühlung der Temperaturbereich von 800 - 500 °C innerhalb von 6 - 600 s durchlaufen wird. Hierzu geeignete Abkühlgeschwindigkeiten betragen typischerweise bis zu 5 K/s, insbesondere mindestens 0,5 K/s, wobei die Abkühlung im Temperaturbereich von 800 - 500 °C insbesondere mit mehr als 1,5 K/s absolviert wird.

If the steel component is to be case hardened, this is done in one step

• a) formed from the steel to be used according to the invention in a conventional manner, a steel component, which can be, for example, a gear, a shaft, an axis or a tool holder and the like.
  Then in one step
• b) the steel component in question is then case hardened by
• b1) the steel component is first kept in a carburizing step over a period of 150 minutes to 250 hours at a temperature of 900-1050 ° C. under a medium which contains carbon and optionally also nitrogen in order to have a carburized or carbonitrided surface layer on the steel component to produce a thickness of 0.3 - 15 µm, and then, after the carburizing step, it is cooled to room temperature so quickly that the temperature range of 800 - 500 ° C is run within 6 - 600 s during cooling. Cooling speeds suitable for this purpose are typically up to 5 K / s, in particular at least 0.5 K / s, with cooling in the temperature range from 800-500 ° C. being carried out in particular at more than 1.5 K / s.

The duration over which the steel component is held under the carbon-containing medium during the carburizing step is determined in a manner known per se depending on the size of the component and taking into account the carbon-containing medium used in each case and the temperature at which the carburizing is carried out chosen that a carburized edge layer with a thickness lying within the specifications according to the invention is achieved. The shortest duration may be indicated, for example, for smaller components, such as gear parts, in particular gear wheels, shafts and axles, of automobile transmissions and the like, whereas the longest duration may be appropriate for large components, such as gear parts, in particular gear wheels, shafts and axles, of large gear units can, which are intended for slewing bearings, such as those used in wind turbines or ship propulsion systems.

In practice, the temperature at which the steel component is held during the carburizing step (step b.1) is typically up to 950 ° C. By choosing higher temperatures, the carburizing process can be accelerated and the time required for the required carburizing can be shortened accordingly.

• b2) auf eine Austenitisierungstemperatur erwärmt, die mindestens 20 °C oberhalb der Ac1-Temperatur und unterhalb der Ac3-Temperatur des Stahls liegt, aus dem das Stahlbauteil besteht, und ausgehend von der
  Austenitisierungstemperatur mit einer Abkühlgeschwindigkeit von 0,5 - 50 K/s, insbesondere mindestens 1,5 K/s oder mehr als 1,5 K/s, auf Raumtemperatur abgekühlt.

After step b1), the steel component is hardened

• b2) heated to an austenitizing temperature which is at least 20 ° C above the Ac1 temperature and below the Ac3 temperature of the steel from which the steel component is made, and starting from the
  Austenitizing temperature is cooled to room temperature with a cooling rate of 0.5-50 K / s, in particular at least 1.5 K / s or more than 1.5 K / s.

In order to reduce any stresses that may be present in the component after the thermochemical diffusion treatment (step b1), the steel component made of steel used according to the invention can optionally be subjected to a stress relief annealing between steps b1) and b2), during which it lasts for a period of 15-120 min Range of 150 - 680 ° C is kept.

The steel component can also optionally optionally be subjected to a tempering treatment in a manner known per se, after which it is held at a temperature of 150-275 ° C. for a period of 30-180 min and then cooled uncontrolled to room temperature becomes. Tempering like this can further reduce the risk of cracking.

In particular by using the method explained above, a case-hardened steel component according to the invention can be produced that from the steel to be used according to the invention, which consists of (in% by weight) 0.12-0.25% C, 0.20-0.80% Si, 0.40-1.20% Mn, 1.0-3.0% Cr, 0.5-1.8% Mo, 0.004-0.020% N, up to 0.40% S, 0.004-0.020% AI, 0.0001 - 0.0025% B, up to 0.10% Nb, up to 0.01% Ti, up to 0.20% V, up to 0.5% Ni, up to 1.0% Co and as the rest of iron and unavoidable impurities, is produced and an edge layer with a hardness of 500 - 800 HV and in its core area consists of at least 80 vol .-% of bainite, which consists of high tempered bainite, which comes from the structure that the steel component after insertion (step b.1) and before hardening (step b.2) had, and newly formed bainite and at most 20 vol .-% of residual austenite, ferrite, pearlite or martensite is composed.

This structure is created by hardening the components according to the invention in the two-phase area. The existing bainitic microstructure parts resulting from "old", that is to say before hardening (step b.2), can be separated from the new, high-tempered bainitic microstructure parts resulting from hardening by a slight brown coloring of the new bainite from the old, A distinction is made between high tempered bainite, which has a greyish color and an indicated granular structure.

The structure of a component according to the invention, which has undergone the case-hardening process described above and modified in accordance with the invention, is characterized in that it has a Charpy-V impact energy of more than 40 J, in particular more, determined in accordance with DIN EN 10045 in the core area of the steel component than 60 J.

• A) Aus dem Stahl wird ein Stahlbauteil geformt.
• B) Das Stahlbauteil wird einer Nitrier- oder Nitrocarburier-Behandlung unterzogen, bei der
• B.1) das Stahlbauteil zunächst über eine Austenitisierungsdauer von 15 - 120 min auf eine Austenitsierungsstemperatur, die mindestens 20 °C, insbesondere 20 - 100 °C oder 30 - 50 °C, oberhalb der Ac3-Temperatur des Stahls liegt, aus dem das Stahlbauteil besteht, durcherwärmt und anschließend so schnell auf Raumtemperatur abgekühlt wird, dass bei der Abkühlung der Temperaturbereich von 800 - 500 °C innerhalb von weniger als 200 s durchlaufen wird, um ein zu mindestens 80 Vol.-% bestehendes Gefüge im Bauteil zu erzeugen,
  und
• B.2) das Stahlbauteil anschließend für das Nitrieren oder Nitrocarburieren über eine Dauer von 60 min bis 100 Stunden unter einer stickstoff- oder stickstoff- und kohlenstoffhaltigen Atmosphäre, bei einer unterhalb der Ac1-Temperatur des Stahls, aus dem das Stahlbauteil besteht, liegenden, typischerweise 440 - 580 °C betragenden Temperatur gehalten und anschließend abgekühlt wird, um an dem Stahlbauteil eine gehärtete Randschicht mit einer Dicke von 1 - 1200 µm zu erzeugen.

If, according to the invention, the hardened surface layer is to be produced by nitriding or nitrocarburizing, the thermochemical diffusion treatment required for this, in particular based on the optimized composition of the steel to be used according to the invention, can be (in% by weight) 0.12-0.25% C, 0.1 20 - 0.80% Si, 0.40 - 1.20% Mn, 1.0 - 3.0% Cr, 0.5 - 1.8% Mo, 0.004 - 0.020% N, up to 0.40% S, 0.004 - 0.020% Al, 0.0005 - 0.0025% B, up to 0.10% Nb, up to 0.01% Ti, up to 0.20% V or up to 0.5% Ni and up to 1.5% Co, balance iron and unavoidable impurities, are carried out as follows:

• A) A steel component is formed from the steel.
• B) The steel component is subjected to a nitriding or nitrocarburizing treatment, in which
• B.1) the steel component first over an austenitizing period of 15-120 min to an austenitizing temperature which is at least 20 ° C, in particular 20-100 ° C or 30-50 ° C, above the Ac3 temperature of the steel from which the Steel component exists, heated through and then cooled to room temperature so quickly that the temperature range of 800 - 500 ° C is passed within less than 200 s during cooling in order to create an existing structure in the component with at least 80 vol.%,
  and
• B.2) the steel component is then subjected to nitriding or nitrocarburizing for a period of 60 minutes to 100 hours under an atmosphere containing nitrogen or nitrogen and carbon, at a temperature below the Ac1 temperature of the steel from which the steel component is made, typically 440-580 ° C temperature is maintained and then cooled to produce a hardened surface layer with a thickness of 1 - 1200 microns on the steel component.

In the nitriding or nitrocarburizing carried out in the manner indicated above, the Cr, V, Nb or Ti contents present in the steel used according to the invention in accordance with the invention result from the formation of nitrides for a high surface hardness. The bainitic core area (matrix) experiences a hardness increase of approx. 100 - 150 MPa through the formation of nitriding or nitrocarboration Special carbides, in particular from the Mo content contained in the steel (molybdenum-rich carbide).

The respectively set parameters "duration" and "temperature" of the nitriding or nitrocarburizing treatment are adjusted in a manner known per se depending on the component size in such a way that a hardened surface layer with a thickness lying within the specifications according to the invention is achieved.

If machining of the component is to be carried out, for example to optimize its dimensional accuracy, this is advantageously carried out between work steps B1) and B2) on the steel component, which is still relatively soft after work step B1), in order to prevent tool wear from machining in the finally hardened state Reduce.

The steel to be used according to the invention is particularly suitable for the production of gear wheels, axles, shafts or tool holders hardened by surface layers for cutting tools produced by powder metallurgy.

The invention is explained below using exemplary embodiments.

Two melts S1, S2, S3 to be used according to the invention have been melted, the composition of which is given in Table 1.

In a first experiment, a gearwheel was formed from steel S1. The gearwheel was then subjected in a conventional manner to carburizing at 920 ° C. for a period of 300 minutes under a carbon-containing atmosphere known per se for this purpose in a conventional manner in accordance with the procedure described in leaflet 452. This way the gear is through thermochemical diffusion created a carburized (carburized) surface layer with a thickness of 520 µm. The gearwheel was then cooled to room temperature, the cooling rate being 2 K / s and the critical temperature range of 800-500 ° C. being run through in a t8 / 5 time of 10 minutes.

The gear obtained was then heated to an austenitizing temperature of 920 ° C. and held at this temperature for 30 minutes. The gearwheel was then quenched at a cooling rate of 2 K / s. The critical temperature range of 800 - 500 ° C was run through in a t8 / 5 time of 600 s.

The case-hardened gear had a hardness of 750 HV on the surface of its hardened outer layer and a completely bainitic structure in its core area (matrix) carrying the hardened outer layer. The Charpy-V notched bar impact work of the unhardened core area of the gear was 106 J. on average from three samples.

In a second experiment, a gearwheel was again formed from steel S2. The gearwheel was then first subjected to carburizing at 920 ° C. for a period of 30 minutes under a carbon-containing atmosphere customary for this purpose in the prior art. In this way, a carburized (carburized) surface layer with a thickness of 535 µm was created on the gear by thermochemical diffusion.

The gear was then quenched in oil to room temperature. The critical temperature range of 800 - 500 ° C was run through in a t8 / 5 time of 17 s.

The gear then underwent stress relieving, in which it was held at 650 ° C. for one hour in order to relieve the stresses that had arisen during the carburizing treatment that had previously been carried out.

After the stress relief annealing, the component was heated in a hardening step to an austenitizing temperature and held at this temperature for one hour, which was 40 ° C. below the Ac3 temperature of the steel S2, the Ac3 temperature of the steel S2 previously being known per se Way has been determined by means of a dilatometer experiment. Then the gear was again quenched in oil so that the t8 / 5 time was 17 s.

After hardening, the gear was subjected to conventional tempering, in which it was held at 180 ° C for over an hour.

The case-hardened gearwheel on the surface of its hardened surface layer had a hardness of 750 HV and in its core area (matrix) supporting the hardened surface layer a completely bainitic structure consisting of newly formed and old highly tempered bainite. The Charpy-V notched bar impact work averaged 62 J for three samples.

In a third experiment, a gearwheel with a diameter of less than 40 mm was formed from the steel S3. The gearwheel was then subjected to carburizing at 920 ° C. for a period of 30 minutes under a carbon-containing atmosphere that is usually used for this purpose. In this way, a carburized (carburized) surface layer with a thickness of 530 µm was created on the gear by thermochemical diffusion. The gearwheel was then quenched in water at a cooling rate of 3 K / s to room temperature. The critical temperature range of 800 - 500 ° C was run through in a t8 / 5 time of 300 s.

After this carburizing treatment, the component was heated to an austenitizing temperature in a hardening step and held at this temperature for an hour, which was 920 ° C. The gear wheel was then quenched in water, the t8 / 5 time here being 300 s.

The case-hardened gear wheel had a hardness of 760 HV on the surface of its hardened outer layer and a completely bainitic structure in its core area (matrix) carrying the hardened outer layer. The Charpy-V notched bar impact work averaged 78 J for three samples.

With the third experiment it was thus possible to show that the addition of effective contents of Co can counter the risk that, in the case of steel components alloyed in accordance with the invention with small diameters of generally less than 40 mm and a rugged cooling in water, even in principle bainitic transforming steels leads to an undesirable martensitic transformation of the outer shell. The zone of martensitic transformation can be several millimeters thick without suitable countermeasures and is particularly troublesome during mechanical processing. The addition of cobalt can accelerate the start of the bainitic transformation, as can be seen from the in Fig. 1 reproduced ZTU diagram for steel S3 understandable. Table 1 element S1 S2 S3 C 0.19 0.17 0.18 Si 0.29 0.62 0.65 Mn 0.79 1.37 1.42 Cr 2.0 0.83 0.87 Mo 0.70 0.75 0.82 V 0.097 0.12 0.12 al 0,020 0,018 0,010 N 0,007 0,007 0,008 B 0.0001 0.0010 0.0001 Nb 0,020 0,002 0,002 Co 0.001 0.001 0.890 Ti 0.001 0.01 0.01 S 0.0016 0,003 0,003 Ni 0.22 0.12 0.10 Cu 0.03 0.03 0.04 Figures in% by weight, remainder iron and impurities

Claims (9)

Use of a steel consisting of (in% by weight) C: 0.1 - 0.30%, Si: 0 - 0.80%, Mn: 0.20 - 2.00%, Cr: 0 - 4.00%, Mo: 0.5 - 1.80%, N: 0,004 - 0.020%, S: 0 - 0.40%, al: 0,004 - 0.020%, B: 0 - 0.0025%, Nb: 0 - 0.20%, Ti: 0 - 0.02%, V: 0 - 0.40%, Ni: 0 - 0.5%, Cu: 0 - 0.3%, Co: 0 - 1.5% Rest of iron and unavoidable impurities
consists,
the Al content% Al, the Nb content% Nb, the Ti content% Ti, the V content% V and the N content% N of the steel meet the following condition:% Al / 27 +% Nb / 45 +% Ti / 48 +% V / 25>% N / 3.5 for the production of a steel component, namely a gear, a shaft, an axis or a tool holder, with a thermochemically hardened surface layer, the steel component in its core area at least one Has 80 vol .-% structure consisting of bainite. Use according to claim 1, characterized in that the steel (in% by weight) 0.12-0.25% C, 0.20-0.80% Si, 0.40-1.20% Mn, 1, 0-3.0% Cr, 0.5-1.8% Mo, 0.004-0.020% N, up to 0.40% S, 0.004-0.020% Al, 0.0001-0.0025% B, up to Contains 0.10% Nb, up to 0.015% Ti, up to 0.20% V, up to 0.5% Ni and / or up to 1.0% Co. Use according to one of the preceding claims, characterized in that a) a steel component is formed from the steel
and b) the steel component is case hardened by b1) the steel component is held in a carburizing step over a period of 150 minutes to 250 hours at a temperature of 900-1050 ° C. under a medium which contains carbon and optionally also nitrogen, in order to form a carburized or carbonitrided surface layer on the steel component To produce a thickness of 0.3 - 15 µm and, after the carburizing step, it is cooled to room temperature so quickly that the temperature range of 800 - 500 ° C is passed within 6 to 600 s during cooling,
and b2) the steel component is heated in a hardening step carried out after the carburizing step (work step b1) to an austenitizing temperature which is at least 20 ° C. above the Ac1 temperature and below the Ac3 temperature of the steel from which the steel component is made, and starting from the austenitizing temperature with a Cooling rate of 0.5 - 50 K / s is cooled to room temperature. Use according to claim 3, characterized in that between the steps b1) and b2) , the steel component is optionally subjected to a stress relief annealing at a temperature of 150-680 ° C for a period of 15-120 min. Use according to one of claims 3 or 4, characterized in that the steel component after hardening (step b2) is optionally subjected to a tempering treatment in which it is held at a temperature of 150-275 ° C for a period of 30-180 min is then cooled uncontrolled to room temperature. Use according to claim 1 or 2, characterized in that A) a steel component is formed from the steel
and B) the steel component is subjected to a nitriding or nitrocarburizing treatment, in which B.1) the steel component is initially heated over an austenitizing period of 15-120 min to an austenitizing temperature that is at least 20 ° C above the Ac3 temperature of the steel from which the steel component is made and then cooled to room temperature so quickly that when cooling, the temperature range of 800 - 500 ° C is run within less than 200 s in order to create an existing structure in the component with at least 80% by volume,
and B.2) the steel component is then held for a period of 60 minutes to 100 hours under a nitrogen or nitrogen and carbon-containing atmosphere for a nitriding or nitrocarburizing at a temperature below the Ac1 temperature of the steel of which the steel component is made and then cooled to produce a hardened surface layer with a thickness of 1 - 1200 microns on the steel component. Use according to claim 6, characterized in that the steel component is subjected to machining between work steps B.1) and B.2). Case hardened steel component, namely gear, shaft, axle or tool holder, consisting of a steel consisting of (in% by weight) 0.12 - 0.25% C, 0.20 - 0.80% Si, 0.40 - 1.20% Mn, 1.0 - 3.0% Cr, 0.5 - 1.8% Mo, 0.004 - 0.020% N, up to 0.40% S, 0.004 - 0.020% Al, 0.0001 - 0.0025% B, up to 0.10% Nb, up to 0.015% Ti, up to 0.20% V, up to 0.5% Ni, up to 1.0% Co and as from the rest iron and unavoidable Contamination exists, whereby the steel component has an edge layer with a hardness of 500 - 800 HV and in its core area consists of at least 80 vol.% Of bainite, which consists of highly tempered bainite, which comes from the structure that the steel component has after insertion (Work step b.1) and before hardening (work step b.2), and newly formed bainite and at most 20% by volume is composed of residual austenite, ferrite, pearlite or martensite. Steel component according to claim 8, characterized in that it is produced by applying the method according to one of claims 3 to 5 and the structure in the core area of the steel component has a Charpy-V impact energy of more than 40 J.

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