ES2878652T3 - Procedure for the fabrication of a case-hardened steel construction part - Google Patents

Abstract

Method for manufacturing a case-hardened steel construction part, namely a gear wheel, a shaft, an axle or a tool holder, in which the steel construction part a) is molded from of a steel, which is constituted, in % by weight, by C: 0.1 - 0.30%, Si: 0 - 0.80%, Mn: 0.20 - 2.00%, Cr: 0 - 4 0.00%, 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% el The rest are iron and unavoidable impurities, including in particular P contents of up to 0.0035% by weight, the Al content % AI, the Nb content % Nb, the Ti content % Ti, the V content complying. % V and the content of N % N of the steel the following condition: % Al/27 + % Nb/45 + % Ti/48 + % V/25 > % N/3.5, and in which b) the piece of steel construction is case hardened, insofar as b1) the part steel construction is kept in a carburizing stage for a period of 150 min to 250 hours at a temperature of 900 - 1050 °C in a medium containing carbon and optionally additionally nitrogen, to generate in the steel construction part a carburized or carbonitrided edge layer with a thickness of 0.3 - 15 μm, and following the carburizing step it is rapidly cooled to room temperature so that during cooling the temperature range of 800 - 500 °C is covered in the from 6 to 600 s, and b2) the steel component is heated in a hardening step carried out after the carburizing step (working step b1) to an austenitization temperature that is at least 20 °C above temperature Ac1 and below the temperature Ac3 of the steel of which the steel construction part is made, and is cooled from the austenitization temperature with a cooling rate or 0.5 - 50 K/s up to room temperature, so that the resulting steel component has a thermochemically hardened edge layer and in its core region has a structure that is composed of at least 80% by volume bainite, which is made up of highly annealed bainite, which comes from the structure that the steel construction piece had after carburizing (work stage b.1) and before hardening (work stage b.2), and bainite newly formed, as well as a maximum of 20% by volume of residual austenite, ferrite, pearlite or martensite.

Description

DESCRIPTION

Procedure for the fabrication of a case-hardened steel construction part

The invention relates to a process for the manufacture of a case-hardened steel construction part. If indications are given in "%" below with respect to alloys or steel compositions, then these refer in each case to weight, as long as the contrary is not expressly stated.

All the mechanical properties indicated in the present text of the steel to be used according to the invention and of the steels mentioned if necessary for comparison have been determined, unless otherwise indicated, according to DIN EN ISO 6892-1.

In the case of the steel construction parts considered in this document, these are normally construction elements that in practice come into metallic contact with other construction parts in a rolling motion and are therefore exposed in the area of their contact surface at high mechanical loads. Typical examples of such building parts are sprockets, shafts, or shafts. Comparable loads can occur in the case of tool holders, for example cutting tools and the like, in the area of the contact surfaces between the holder and the respective tool.

There is a particular challenge in this regard that such steel workpieces have generally been complex molded and can only be produced by costly chip-removal machining. Chipping machining of this type can be carried out in a particularly cost-effective manner when the structural parts consist of steels with a low carbon content, that is, they have a low hardness. At the same time, the comparatively low hardness and accompanying this high toughness of the steel, from which such construction parts are made, is favorable in terms of their resistance to breaking load, in particular under operating conditions, in which they occur. dynamically the loads to be absorbed by the respective construction part.

For example, hardened steels are currently used for the production of sprockets, for which steels with 16MnCr5 / 16MnCrS5 (material numbers 1.7131 /1.7139) and 18CrNiMo7-6 (material number 1.6587) can be mentioned by way of example.

Tool holders, such as holders for powder metallurgically generated cutting bodies, are often made from relatively expensive tool steels, such as steels with 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 typically comparatively soft steels can be improved. These processes are based on the fact that a higher hardness is generated in a marginal layer bearing the contact surface subjected to load in use than in the core area of the construction part bearing the corresponding marginal layer, in which the It also finds a high toughness after heat treatment.

As explained in detail in data sheets 452 "Einsatzharten", 2008 edition, and 477 "Warmebehandlung von Stahl - Nitrieren und Nitrocarburieren", 2005 edition, both published by Stahl-Informations-Zentrum, Postfach 1048 42, 40039 Düsseldorf, Germany , and at URL http: //www.stah! -online.de/index.php/service/publikationen/stahlanwendung-merkblaetter/ provided for download, the heat treatment procedures aimed at the formation of a hardened marginal layer zone they work without and with chemical modification of the marginal layer on steel structural parts. Procedures that are based on a chemical modification, in which the hardening of the marginal layer of the construction part is caused by thermochemical diffusion processes, also differing again due to this if after the heat treatment an additional heat treatment is carried out (hardening) or not.

Common processes, with which toggle wheels and comparatively loaded construction parts can be provided in particular with a hardened marginal layer, include case hardening (see data sheet 452), in which first the layer of The edge of the steel construction part undergoes a carburization or carbonitration treatment, an increase in the carbon content and then the construction part undergoes a hardening, in order to achieve maximum hardness in the hardened marginal layer, and the nitration o or nitrocarburization (see data sheet 477), in which the increase in hardness of the marginal layer is achieved essentially by means of nitrogen introduced by diffusion, it being possible to achieve an additional increase in hardness by means of carbon introduced by diffusion in combination with nitrogen.

In the context of the state of the art explained above, the object of the invention was to mention a process for the manufacture of a steel construction part, which results in an optimal combination of properties in such coated steel construction parts. marginal hardened by thermal diffusion treatment, which are in rolling contact in use with another construction part.

Likewise, a piece of steel construction with a hardened marginal layer has been disclosed, which has an optimal combination, in terms of its resistance to breaking load, of hardness in its marginal layer and toughness in its core area bearing the marginal layer .

For the production of steel structural parts with hardened edge layer, which have an optimally tough core region, the invention proposes the method according to claim 1.

Advantageous embodiments of the invention are indicated in the dependent claims and explained in detail below as the general idea of the invention.

The steel to be used according to the invention opens up a robust and economical manufacturing path for the generation of steel construction parts with marginal layer to be hardened by a thermochemical diffusion treatment, specifically sprockets, shafts, shafts or tool holders with special application conditions. In this respect, the building parts generated from the steel used according to the invention have, after the heat treatment carried out in each case for their thermochemical marginal layer hardening, a higher toughness in their core region, also called " matrix ", than what is this the case in steels currently used on a regular basis for this purpose.

The invention is based on the knowledge that a modification of a steel that forms a bainitic structure is suitable, which is basically known from the publication EP 3168312 A1 of a European patent application already for the technical generation of forging of construction parts, in measurement Also special as a material for the production of structural steel parts with a thermochemically hardened marginal layer. Thus it has surprisingly been shown that the alloy concept envisaged per se for technical forging applications due to the high temper stability of the bainitic structure of the steel proposed for use according to the invention also presents considerable advantages during thermochemical hardening. of the marginal layer, in particular as regards the toughness of the steel construction part in its core region.

In this respect it is particularly advantageous that the steel itself known from the publication of the European patent application mentioned above, as explained in detail in EP 3168312 A1, has in the time-temperature diagram ("ZTU diagram") a wide bainite window, that is, through a wide range of cooling rates, reliably forms a bainite structure dominated by bainite by at least 80% by volume. Surprisingly, it has been shown in this case that the known alloying regulation guarantees these properties of the steel then also when the steel is not cooled, as originally envisaged, from the forging heat, but undergoes a thermochemical diffusion treatment. This also applies when the respective steel construction part, as is customary in case hardening, is hardened after diffusion treatment.

EP 2357262 A1 discloses a crankshaft and a manufacturing method for the same. JP 2006 169637 A discloses a process for the manufacture of a highly resistant, carburized building part. Document EP 1 070 760 to 2 discloses a high pressure resistant construction part and its manufacturing method.

The steel construction parts generated from the steel used according to the invention, namely sprockets, shafts, shafts or tool holders, are characterized by a particularly homogeneous structure with a low variance in hardness. This optimally uniform distribution of structural properties is also found in the case of the most diverse dimensions of steel construction parts to be manufactured from the steel to be used according to the invention and in the case of cooling conditions varying over a large interval, caused by these dimension differences. In addition, the homogeneous structural state that conforms to the use according to the invention of the steel causes low inherent stresses in the construction part. Correspondingly to this, the steel structural parts produced from the steel used according to the invention tend in any case negligibly during the thermochemical hardening of the marginal layer to deformation and to the production of cracks or other damage caused by stress.

According to the invention it is therefore used for the production of a steel structural part, in which it is a gear wheel, shaft, shaft or tool holder, with a hardened marginal layer of thermochemically a steel that is constituted by (in% by weight) 0.1 - 0.30% C, up to 0.80% Si, 0.20 - 2.00% Mn, up to 4 0.00% 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 as the remainder for iron and unavoidable impurities, complying with the content of Al% Al, the content of Nb% Nb, the content of Ti% Ti, the content of V% V and the content of N% N of the steel the following condition:

% AI / 27% Nb / 45% Ti / 48% V / 25>% N / 3.5.

The steel to be used according to the invention is alloyed in this regard and can be processed so that the steel construction part, which is made from this, has in its core zone a structure which is made up of at least 80% by volume per bainite. In this regard, the unavoidable impurities caused by the manufacture of the steel to be used according to the invention belong to all the elements which are present with respect to the properties of interest in this case in technically ineffective amounts of alloying and Due to the route selected in each case the steel powder or the starting material selected in each case (scrap) reach the steel. In particular, unavoidable impurities also include P contents of up to 0.0035% by weight.

A steel construction part generated from the steel to be used according to the invention is therefore characterized in that it has a structure that is made up of at least 80% by volume per bainite. The remaining structure of a total of at most 20% by volume of the total structure is taken up by residual austenite, ferrite, pearlite and / or martensite. Normally, however, the contents in non-bainitic structural constituent parts of a steel construction part which is constituted by steel to be used according to the invention are strongly minimized so that a completely bainitic structure.

The alloying concept on which the steel to be used in accordance with the invention is based avoids expensive alloying constituent parts, such as are currently commonly required in carburizing and tool steels used for the manufacture of metal parts. steel construction, discussed in this document, to adjust the required hardness. This is achieved because the alloying elements and their contents in the steel to be used according to the invention are selected as follows:

Carbon ("C") is contained in the steel to be used according to the invention in contents of 0.1-0.3% by weight, to contribute through the formation of carbides to increase the strength of the material. Thus, by adding 0.01% by weight in each case, an increase in strength of approx. 70 MPa. This effect begins 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 content of C to at most 0, 30% by weight, in particular not more than 0.25% by weight, it is achieved in this connection that the steel has, despite its maximized strength, good expansion and toughness properties. 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 bainite conversion, so that the production of undesired structural constituent parts is avoided. An optimized action of the presence of C in the steel to be used according to the invention can be achieved because the content of C is adjusted 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 displaces the formation of ferrite to 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 transformation to develop as soon as possible. At the same time, Si contents up to this upper limit contribute to the increase in strength by solidification of mixed crystal. In order to be able to exploit the advantageous actions of Si in the steel to be used according to the invention in a particularly safe manner, the Si content is therefore preferably set to at least 0.2% by weight, in particular more 0.45% by weight, such as at least 0.46% by weight.

Manganese ("Mn") is present in contents of 0.20 - 2.00% by weight in the steel to be used according to the invention, to adjust the tensile strength and the limit of necking through the formation of mixed glass. A minimum content of 0.20% by weight of Mn is necessary for an increase in strength to occur. If this effect is to be achieved particularly safely, then a Mn content of at least 0.4% by weight can be envisaged. Too high Mn contents would, however, lead to retardation of the bainite transformation and thus to a predominantly martensitic transformation. Therefore, the Mn content is limited to a maximum of 2.00% by weight, in particular a maximum of 1.5% by weight. The negative influences of the presence of Mn can be particularly safely avoided by limiting the Mn content in the steel to be used according to the invention to a maximum of 1.2% by weight.

The optionally available chromium ("Cr") contents of up to 4.00% by weight contribute, through the formation of special carbides and chromium nitrides in the case of a nitration treatment carried out according to the invention, to the hardenability and stability against corrosion of the steel to be used according to the invention. For this, for example, at least 0.5% by weight or at least 0.8% by weight of Cr can be provided. An optimal action of the presence of Cr results in a Cr content of at least 1.00 % in weigh. Cr contents that are above 4.00% by weight would favor an undesired martensite formation in the structure of the steel to be used according to the invention. To avoid this safely, the Cr content can be limited 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, to delay the transformation of the structure into ferrite or pearlite and increase the interval for the transformation of bainite. This action occurs in particular when at least 0.6% by weight is present in the steel. With contents of more than 1.8% by weight, no other economically justifiable increase in positive action is produced, with respect to the use of the steel to be used according to the invention, which is the central point in the present document. of Mo. By limiting the Mo content to 1.8% in By weight, the formation of a molybdenum-rich carbide phase, which would negatively influence toughness properties, is safely excluded. The optimal actions of Mo in the steel to be used according to the invention can be expected when the content of Mo amounts to at least 0.7% by weight. In this connection, Mo contents of at most 1.5% by weight or at most 1.0% by weight have proven particularly effective.

The presence of N in the contents provided according to the invention of 0.004-0.020% by weight allows the formation of nitrides and carbonitrides to increase the strength and increase the stability of fine grain, without reaching brittleness. Thus, Al with N forms aluminum nitride, which contributes to fine-grain stability.

The sulfur content ("S") in the steel to be used according to the invention may be up to 0.4% by weight, in particular at most 0.1% by weight, to promote capacity chip detachment from steel. For this purpose, an S content of at least 0.001% by weight can be provided. In the case of S contents that are above 0.4% by weight, there is a risk of the development of red embrittlement. Optimal actions of the presence of S in the steel to be used according to the invention can be achieved with 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 it retards the production of ferrite or pearlite and thus ensures the production of the intended bainitic structure in the steel to be used according to the invention. Contents in B that are above 0.0025% by weight would carry the risk of brittleness. The optionally existing micro-alloy elements Nb, Ti and V each form carbonitrides and can thus make an essential contribution to optimizing the fine-grain stability and strength of the steel to be used according to the invention.

The technical fine tuning of the alloy with respect to the mechanical properties and the quality of the structure of a steel used according to the invention is carried out according to the alloy concept used according to the invention through a combined microalloying of 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 of B or 0.0005 - 0.0025% by weight of B, nitrogen ("N") in contents of 0.004 - 0.020% by weight, in particular at least 0.006% by weight of N or up to 0.0150% by weight of N, aluminum ("AI") in contents of 0.004 - 0.020% by weight as well as 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 0.00 5% by weight of Ti, and vanadium ("V") in optional contents of 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.

To safely take advantage of the advantages of the presence of the microalloyed elements and aluminum, it may be convenient to adjust the Al content by at least 0.005% by weight, the Ti content by at least 0.001% by weight, the V content of at least 0.02% by weight or Nb content of at least 0.003% by weight. In this connection, the micro-alloying elements V, Ti, Nb on the one hand and Al on the other hand can be present in each case in combination with one or more elements of the group "Al, V, Ti, Nb" or alone in amounts that vary. found above the minimum contents mentioned. With contents of up to 0.01% by weight of Ti, up to 0.1% by weight of Nb, up to 0.075% by weight of V or up to 0.020% by weight of Al, it is possible to use in particular effective the actions of these elements in the steel used according to the invention. Also in this case, the mentioned upper limits of the Ti, Nb, V or Al contents can be met in each case alone or in combination with each other, in order to achieve the optimum action of the respective alloying element in each case.

The contents of% AI,% Nb,% Ti,% V and% N in AI, Nb, Ti, V and N are linked to each other in this respect in the steel to be used according to the invention through the condition

% AI / 27% Nb / 45% Ti / 48% V / 25>% N / 3.5

in such a way that the nitrogen content of the steel to be used according to the invention through the existing contents in each case in Al as well as the possibly additionally added contents in Nb, Ti and V has been completely isolated and the Boron can therefore act as a retarder of transformation. The isolation according to the invention of the N further enables the boron optionally existing as a dissolved element in the steel matrix to become effective and suppress the formation of ferrite and / or pearlite.

Likewise, the optionally existing 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 exploited, it occurs from a Ni content of at least 0.1% by weight, in particular at least 0.15% by weight.

The alloying elements which are added in a targeted manner or which enter the steel to be used according to the invention through the starting material also include Cu, the content of which is limited in order to avoid negative influences on the steel to be used. according to the invention by a maximum of 0.3% by weight.

Cobalt ("Co") optionally existing in the steel to be used according to the invention causes at contents of up to 1.5% by weight a displacement of the bainite formation at shorter times. The positive influence of Co can be used in this connection in particular with 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.

A steel alloy particularly suitable for the purposes according to the invention is thus constituted in a manner corresponding to the above explanations by (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, the remainder iron and unavoidable impurities, for which the explanations already given above in this regard also serve in this case.

Basically the steel to be used for the manufacture of steel construction parts is suitable for all thermochemical diffusion processes, described in the technical sheets 452 and 477 already mentioned above, "carburization" (carburization), "carbonitriding", " nitriding "or" nitrocarburizing ".

As long as a hardening by carburizing is to be carried out, a carburization or carbonitriding is carried out first as a thermochemical diffusion treatment, as explained in data sheet 452 in particular. After the carburization produced in this sense by thermochemical diffusion (carburization, carbonitriding) of the marginal layer, a hardening is carried out during the conventional carburization hardening according to the hardening procedure also described in detail in data sheet 452 "direct hardening (type A) "," simple hardening (type B) "," hardening after isothermal transformation (type C) "or" simple curing (type D) ". In the case of direct hardening (type A), the steel construction part is quenched directly from the heat of the previous carburization or carbonitriding treatment. In the case of simple hardening (type B), the steel construction after the previous carburization or carbonitriding treatment is cooled first to room temperature and then again heated to an austenitizing temperature which is above the Ac1 temperature and below the Ac3 temperature of the steel and then abruptly cooled. In the case of hardening after isothermal transformation (type C) the steel construction part is cooled from the heat of the previous carburization or carbonitriding treatment first to a temperature range in which certain carbide deposits are formed, and then, starting from this temperature range, it is heated again to an austenitizing temperature that is above the Ac1 temperature and below the Ac3 temperature of the steel, to then abruptly cool. In the case of double hardening (type D), the steel construction, after it has been cooled from the heat of the previous carburization or carbonitriding treatment, such as in the case of single type A hardening, to room temperature , goes through a hardening process twice, as is done in the case of the simple type A hardening only once.

Regardless of which of the four conventional hardening procedures mentioned in the present document is used, they have to be adjusted, in the thermochemical diffusion treatment and the subsequent hardening of the steel construction parts which are constituted by steel to be used according to With the invention, the cooling to be carried out in each case so as to adjust, on the one hand, the deposits that increase hardness in the carburized marginal layer by carburizing or carbonitriding and in the non-carburized core area of the construction part. a structure that after the measure explained above is made up of at least 80% by volume per bainite. For this, the temperature range of 800-500 ° C must be traversed in each case during cooling in a time t8 / 5 of at least 6 s, in particular at least 10 s, and at most 600 s.

If, on the other hand, for the purpose of the formation of the hardened marginal layer, the thermochemical diffusion treatment such as nitriding or nitrocarburization has to be carried out, which is not found in the current invention, then the process mode described in detail can be selected for this. in data sheet 477. In this sense, the steel construction part is cooled after heating to an austenitizing temperature that is above the Ac3 temperature of the steel, from which the steel construction part is constituted, of continuously so that the temperature range 800 - 500 ° C is traversed 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, for forming in the construction part a structure which after the measure explained above is constituted by at least 80% by volume per bainite. Then the nitriding or nitrocarburizing stage is then carried out, in which the steel construction part in each case corresponding to the indications and measures contained in data sheet 477 under an atmosphere containing nitrogen or an atmosphere containing nitrogen and carbon is kept at a temperature below the Ac1 temperature of the steel, of which the steel construction part is made, and is then cooled.

According to the invention, the steel structural part is subjected to a case hardening and is molded for this in one working step.

a) from the steel to be used according to the invention in a conventional way to give a steel construction part, in the case of which it is a sprocket, a shaft, a shaft or a tool holder . Then in a working stage

b) the respective steel construction part is then hardened by carburization, while b1) the steel construction part is first kept in a carburizing stage for a period of 150 min to 250 hours at a temperature of 900 - 1050 ° C with a medium containing carbon and optionally additionally nitrogen, to generate in the steel construction part a carburized or carbonitrided marginal layer with a thickness of 0.3 -15 | jm, and following the step of Carburization is then rapidly cooled to room temperature so that during cooling the temperature range of 800-500 ° C is traversed within 6-600 s. The cooling rates suitable for this are normally up to 5 K / s, in particular at least 0.5 K / s, the cooling taking place in the temperature range 800 - 500 ° C in particular with more than 1.5 K / s. s.

The duration, during which the steel construction part is kept during the carburization stage under the carbon-containing medium, is selected in a manner known per se depending on the size of the construction part as well as with consideration of the carbon-containing medium. in each case used and of the temperature, at which the carburization is carried out, so that a carburized marginal layer is achieved with a thickness that is within the specifications according to the invention. The shorter duration can be indicated in this respect for example for smaller construction parts, such as gear components, in particular gear wheels, shafts and shafts, of automobile gears and the like, while the longest duration can be indicated in Large construction parts, such as gear components, in particular sprockets, shafts and shafts, of large gears, which are intended for slewing rings, as used in wind power plants or ship propulsion.

In practice, the temperature at which the steel construction part is maintained during the carburization stage (working stage b.1) is found, normally up to 950 ° C. By choosing higher temperatures, the carburization process can be accelerated and correspondingly the time required for the required carburization can be limited.

After the working stage b1), the steel construction is heated in a hardening stage b2) to an austenitizing temperature, which is at least 20 ° C above the Ac1 temperature and below the Ac3 temperature of the steel of which the steel construction part is made, and starting from the austenitizing temperature it is cooled 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, up to room temperature.

In order to reduce stresses possibly existing in the building part after the thermochemical diffusion treatment (work step b1), the steel construction part consisting of steel used according to the invention can be subjected between work steps b1) and b2) optionally to a stress relief anneal, in which it is maintained for a period of 15-120 min in the range of 150-680 ° C.

Also optionally after hardening (working step b2), the steel construction can be subjected to a tempering treatment, in a manner known per se, in which it is maintained for a period of 30 - 180 min at a temperature of 150-275 ° C and then uncontrolled cooling to room temperature. By such a tempering, the risk of cracking can be further reduced.

In particular by applying the method explained above, a case-hardened steel construction part can be generated according to the invention, which has been manufactured 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 , from 0.5-1.8% of Mo, from 0.004-0.020% of N, up to 0.40% of S, from 0.004 - 0.020% of AI, from 0.0001 - 0.0025% of B, up to 0.10% of Nb, up to 0.01% of Ti, up to 0.20% of V, up to 0.5% of Ni, up to 1.0% of Co and as the balance by iron and unavoidable impurities, and has a marginal layer with a hardness of 500-800 HV as well as is constituted according to the invention in its core zone in at least 80% by volume per bainite, which is constituted by highly annealed bainite, which comes from the structure that featured the tr as well as cementation (working stage b.1) and before hardening (working stage b.2), and newly formed bainite as well as a maximum of 20% by volume by residual austenite, ferrite, pearlite or martensite.

This structural composition is produced by hardening of the building parts according to the invention in the two-phase field. In this respect, those with "old" bainitic structure proportions, that is, produced before hardening (working stage b.2), can be distinguished from those with "new" bainitic structure proportions, produced during the hardening and highly tempered by means of a slight brown color of the new bainite from the old highly tempered bainite, which has a gray coloration and an indicated granular structure.

In this respect, the structure of a construction part is characterized, which has gone through the process of hardening by carburizing explained above, modified according to the measure of the invention, because the latter has in the core area of the steel construction part an energy absorbed during the Charpy-V impact determined according to DIN EN 10045 of more than 40 J, in particular more than 60 J.

If the hardened marginal layer is to be generated by nitriding or nitrocarburizing, which is not found in the current invention, then the thermochemical diffusion treatment necessary for this can be carried out in particular starting from the optimized composition of the steel to be used according to the invention with (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, 0.0005 - 0 0.0025% B, up to 0.10% Nb, up to 0.01% Ti, up to 0.20% V or up to 0.5% Ni, as well as up to 1.5% of Co, the remainder iron and unavoidable impurities, as follows:

A) A steel construction part is cast from steel.

B) The steel construction part undergoes a nitriding or nitrocarburizing treatment, in which

B.1) the steel construction part is first heated during 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, of which the steel construction part is made, and then rapidly cooled to room temperature so that during cooling the temperature range of 800 - 500 ° C is traversed within less than 200 s, to generate a structure that is made up of at least 80% by volume in the building part, and

B.2) The steel construction part below for nitriding or nitrocarburising is kept for a period of 60 min to 100 hours under an atmosphere containing nitrogen or containing nitrogen and carbon, at a temperature that is below the Ac1 temperature of the steel, of which the steel construction part is made, which is normally 440 - 580 ° C and is subsequently cooled, in order to generate a hardened marginal layer with a thickness of 1 on the steel construction part - 1200 | jm.

During nitriding or nitrocarburization carried out in the manner indicated above, existing contents of Cr, V, Nb or Ti are produced from the steel used after the measurement of the invention by means of the formation of nitrides for a high surface hardness. The bainitic core zone (matrix) undergoes an increase in hardness during nitriding or nitrocarburizing by approx. 100 - 150 MPa through the production of special carbides in particular from the Mo content of the steel (carbide rich in molybdenum).

The individually adjusted parameters "duration" and "temperature" of the nitriding or nitrocarburizing treatment are here adjusted in a manner known per se depending on the size of the structural part so that a marginal layer is achieved. hardened to a thickness within specification according to the invention.

If chip-free machining of the construction part is to be carried out, for example to optimize its dimensional accuracy, then this is advantageously carried out between work steps B1) and B2) on the relatively soft steel construction part after step B1), to avoid tool wear compared to chip shedding in the final hardened state.

The steel to be used according to the invention is especially suitable for the manufacture of sprockets, shafts, shafts or tool holders with hardened marginal layer for cutting tools manufactured in powder metallurgy manner.

The invention is explained below by means of exemplary embodiments.

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

In a first test, a toothed wheel was molded from S1 steel. The sprocket was then subjected in a conventional manner according to the measurement of the procedure described in data sheet 452, first of all, to a carburization at 920 ° C for a period of 300 min under an atmosphere containing carbon composed of carbon dioxide. yes known for this purpose. In this way, a cemented (carburized) marginal layer with a thickness of 520 µm has been produced in the gear by thermochemical diffusion. The gear wheel was then cooled to room temperature, the cooling rate rising to 2 K / s and the critical temperature range of 800-500 ° C having been covered in a time t8 / 5 of 10 min.

The gear wheel obtained was then heated to an austenitizing temperature of 920 ° C and kept at this temperature for 30 min. The gear wheel was then quenched with a cooling rate of 2 K / s. In this respect, the critical temperature range of 800-500 ° C has been covered in a time t8 / 5 of 600 s.

The gear wheel hardened by carburizing in this way exhibited a hardness of 750 HV on the surface of its hardened marginal layer and a completely bainitic structure in its core area (matrix) bearing the hardened marginal layer. The energy absorbed during the Charpy-V impact of the unhardened core region of the gear wheel averaged 106 J.

In a second test, a toothed wheel was molded again from the S2 steel. The gear wheel was then first subjected to a carburization at 920 ° C for a period of 30 min under a carbon-containing atmosphere customary for this purpose in the state of the art. In this way, a cemented (carburized) marginal layer with a thickness of 535 pm has been produced on the gear wheel by thermochemical diffusion.

The gear wheel was then quenched in oil to room temperature. The critical temperature range of 800-500 ° C has been covered in this respect in a time t8 / 5 of 17 s.

Subsequently, the sprocket has gone through a stress relief anneal, in which it has been held for one hour at 650 ° C, to reduce stresses produced in the carburization treatment carried out previously.

After the stress relief annealing, the building part was heated in a hardening stage to an austenitizing temperature and kept at this temperature for one hour, which was 40 ° C below the Ac3 temperature of S2 steel. , the Ac3 temperature of the S2 steel having been determined above in a manner known per se by means of a dilatometer test. The gear wheel was then quenched again in oil, so that in this case too the time t8 / 5 was 17 s.

After hardening, the sprocket was subjected to a conventional tempering, in which it was kept for one hour at 180 ° C.

The gear wheel hardened by carburizing in this way exhibited a hardness of 750 HV on the surface of its hardened marginal layer and in its core area (matrix) bearing the hardened marginal layer a completely bainitic structure, which consisted of newly formed bainite. and old highly tempered. The energy absorbed during the Charpy-V shock was 62 J. on average in three samples.

In a third test, a sprocket with a diameter of less than 40 mm was molded from S3 steel. The sprocket was then first subjected to a carburization at 920 ° C for a period of 30 min under a carbon-containing atmosphere commonly used for this purpose. In this way, a cemented (carburized) marginal layer with a thickness of 530 pm has been produced on the gear wheel by thermochemical diffusion. The gear wheel was then quenched with a cooling rate of 3 K / s to room temperature in water. The critical temperature range of 800-500 ° C has been traversed in this regard in a time t8 / 5 of 300 s.

After this carburization treatment, the building part was heated in a hardening stage to an austenitizing temperature and was kept at this temperature for one hour, which amounted to 920 ° C. The gear wheel was then quenched in water, in this case the time t8 / 5 was 300 s.

The gear wheel hardened by carburizing in this way exhibited a hardness of 760 HV on the surface of its hardened marginal layer and a completely bainitic structure in its core area (matrix) bearing the hardened marginal layer. The energy absorbed during the Charpy-V shock amounted to 78 J. on average in three samples.

With the third test it was therefore possible to show that by adding effective Co contents the risk may be found that in steel structural parts alloyed according to the invention with small diameters generally less than 40 mm and a cooling abrupt in water, also with steels that are going to be transformed basically in a bainitic way, an undesired martensitic transformation of the outer shell is reached. The martensitic transformation zone can be several millimeters thick without suitable countermeasures and is especially altering in the case of mechanical machining. By the addition of cobalt the initiation of the bainitic transformation can be accelerated, as can be assumed by means of the ZTU diagram reproduced in Figure 1 for the S3 steel.

Table 1

Claims (5)

1. Process for the manufacture of a case-hardened steel construction part, specifically a sprocket, shaft, shaft or tool holder, in which the steel construction part a) It is molded from a steel, which is constituted, in% by weight, by C: 0.1 -0.30%, Yes: 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% the remainder are iron and unavoidable impurities, comprising in particular P contents of up to 0.0035% by weight, fulfilling the content of Al% AI, the content of Nb% Nb, the content of Ti% Ti, the content of V% V and the content of N% N of the steel the following condition:% Al / 27% Nb / 45% Ti / 48% V / 25>% N / 3.5, and in which b) the steel construction part is hardened by case hardening, insofar as b1) the steel construction part is kept in a carburizing stage for a period of 150 min to 250 hours at a temperature of 900 -1050 ° C in a medium containing carbon and optionally additionally nitrogen, to generate in the steel construction part a carburized or carbonitrided marginal layer with a thickness of 0.3 - 15 | jm, and following the carburization step it is rapidly cooled to room temperature so that during cooling the temperature range of 800 is traversed - 500 ° C within 6 to 600 s, Y b2) the steel construction part is heated in a hardening stage carried out after the carburization stage (working stage 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 construction part is made, and is cooled starting from the austenitizing temperature with a cooling rate of 0.5 - 50 K / s to room temperature, so that the steel construction piece obtained presents a thermochemically hardened marginal layer and in its core zone it presents a structure that is composed of at least 80% by volume of bainite, which is constituted by highly annealed bainite, which comes from the structure that the steel construction piece presented after cementation (work stage b.1) and before hardening (work stage b.2), and newly formed bainite, as well as a maximum of 20% by volume of residual austenite, ferrite, pearlite or martensite. 2. Process according to claim 1, characterized in that the steel contains (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 and / or up to 1 , 0% of Co. Method according to claim 1 or 2, characterized in that the steel construction part between the working steps b1) and b2) is optionally subjected to a stress relief anneal at a temperature of 150 - 680 ° C for a period of period of 15 -120 min. Method according to one of the preceding claims, characterized in that the steel structural part is optionally subjected after hardening (working step b2) to a tempering treatment, in which it is held for a period of 30 - 180 min. at a temperature of 150 - 275 ° C and then cooled to uncontrolled way up to room temperature. Method according to one of the preceding claims, characterized in that the structure in the core region of the steel component has an energy absorbed during the Charpy-V impact of more than 40 J, which is determined according to the DIN EN 10045 standard.