WO2017133725A1

WO2017133725A1 - Method for making a rolling bearing component from austenitic steel

Info

Publication number: WO2017133725A1
Application number: PCT/DE2017/100041
Authority: WO
Inventors: Werner Trojahn; Christian SCHULTE-NÖLLE
Original assignee: Schaeffler Technologies AG & Co. KG
Priority date: 2016-02-05
Filing date: 2017-01-24
Publication date: 2017-08-10
Also published as: DE102016201753B3

Abstract

Method for making a rolling bearing component from austenitic steel, said method involving the following steps: cutting a blank off a non-solutionized semifinished product; solutionizing the blank by heating same to a temperature >1000°C; hot-forming the heated blank; quenching the blank to a temperature ≤500°C; cold-forming the blank; cooling the blank in a controlled manner to ambient temperature; finish-machining the blank to complete the rolling bearing component.

Description

Method for producing a rolling bearing component from an austenitic

stole

The invention relates to a method for producing a rolling bearing component from an austenitic steel.

Rolling bearing components are preferably made of a martensitic steel, but sometimes also made of an austenitic steel. Examples of such steels can be found in DE 10 2004 043 134 A1, DE 10 2009 013 506 A1 or DE 10 2012 212 426 B3. Also known are so-called TWIP or TRIP steels (TWIP = Twinning Induced Plasticity; TRIP = Transformation Induced Plasticity). The production of a rolling bearing component, for example in the form of a ring or a rolling element from such a steel via a plurality of individual process steps. Usually, a solution-annealed semifinished product is used as the starting material, from which a blank is cut to length, followed by a series of further processing steps in which the blank is machined, heat-treated and formed. Solution annealed Mn

However, carbon and nitrogen austenites are difficult to machine, and sawing and drilling operations are difficult to accomplish. On the other hand, solution annealing is indispensable in order to achieve the desired properties, not only strength and hardness but also toughness and, depending on the alloy, the necessary corrosion resistance on the finished rolling bearing component.

The invention is based on the problem of specifying a simplified method of producing a rolling bearing component from an austenitic steel requiring little process steps.

To solve this problem, a method for producing a rolling bearing component from an austenitic steel is provided according to the invention, with the following steps: Separating a blank from a non-solution-annealed semi-finished product,

Solution annealing of the blank by heating to a temperature> 1000 ° C, hot working of the heated blank,

Quenching the blank to a temperature <500 ° C,

Cold forming of the blank,

controlled cooling of the blank to room temperature,

machining finishing of the blank to complete the rolling bearing component. In the method according to the invention, the solution annealing step is not carried out on the semifinished product, but is performed only on the blank separated from the semifinished product. This means that the solution annealing and the required quenching are shifted from the semifinished product to the blank, the so-called "blank." Due to the smaller wall thickness and the associated higher quenching rate, the microstructure remains precipitate-free, resulting in increased toughness and notched-bar bending work on the solution-annealed blank This also results in increased toughness and notched-bar bending work on the finished part, ie after cold forming (> 20 joules) .The structure can be adjusted to eliminate precipitation and martensite.

By the semifinished product has been stored after the forming and optionally after the last hot forming pass, respectively the respective blanks are separated from the non-solution-heat-treated semi-finished product, usually by sawing. Since the structure of the semifinished steel is approximately in the thermodynamic see equilibrium, is a large part of the interstate atoms bound in the form of iron or chromium carbides or nitrides before. Since the non-solution annealed material has a relatively low toughness, the separation of the blanks, for. B. in the form of forged bolts, in this state relatively little effort feasible.

Subsequently, the blank is heated to a temperature> 1000 ° C in order to solution-anneal the blank. This heating, which is preferably carried out in a protective gas atmosphere, causes the carbide, nitride and carbonitride precipitates in solution can go so that forms a purely austenitic structure. So it comes to the almost complete solution of existing excretions.

Then, at this high temperature level, the hot forming of the heated blank sets in. This means that the blank is hot formed directly at this temperature level. After completion of the hot working, the blank is quenched, so that its temperature is lowered to a temperature <500 ° C. The quenching is preferably carried out in water. The hot forming leads to the formation of a fine grain especially in the strongly formed areas. By means of a correspondingly fine grain, in the later race or rolling area of the rolling bearing component, for example a bearing ring or a rolling element, the yield strength can be raised in accordance with the degree of deformation (> 1500 MPa), whereby the critical degree of deformation for achieving the application-specific inertia also decreases (in the range φ = 0.5 - 2). Since the structure is completely austenitic due to the previous solution annealing, the structure is so tough that, despite the abrupt quenching and a possible thermochemical influence of the surface layer, no crack formation occurs down to the depth.

Subsequently, the blank is immediately further formed. This is followed by cold forming, which may be one-stage or multi-stage. As part of this cold forming, there is a strong strain hardening, depending on the material used, if necessary, with twin formation. Depending on the material, a high hardness, usually of at least 500 HV, is achieved in the strongly formed area. During cold working, the blank gives off heat to the forming tool but gains heat of transformation so that after cold working it has approximately the same temperature as before.

The strength and hardness-increasing cold forming is then followed by a controlled cooling of the blank to room temperature. Within the framework of this process, which is controlled by the cooling rate, dislocations present in the microstructure are blocked or pinned by the formation of precipitates of interstitial atoms. By means of this dislocation pinning, the hardness of the austenitic microstructure in the forming area can be increased even further. simultaneously Part of the residual stresses is reduced, which has a positive effect on the dimensional accuracy of the finished part.

In the final step, the final machining of the blank takes place for the completion of the rolling bearing component. The type of processing depends on the dimensional requirements that are placed on the rolling bearing component.

Overall, the method allows for a simplified production process of a rolling bearing component made of an austenitic steel, after the solution heat treatment step has been shifted within the process chain and not the solution is annealed, but only the blank itself. At the same time it is ensured that a pure Austenitic structure results, which has the appropriate strength and hardness parameters. Preferably, a semifinished product with a hardness of 200-400 HV is used. The hardness is thus sufficiently low to allow easy separation of the blank, usually as described by sawing.

The heating for solution annealing should be at a temperature of at least 125 ° C, in particular at least 150 ° C. The solution annealing preferably takes place under protective gas.

As described, hot forming with grain remodeling results in grain refining, that is, a finer grain is formed as compared with rolling bearing components made from a solution annealed semifinished product. As a result, improved mechanical properties, as described in the introduction, can be achieved. The quenching process is preferably carried out in such a way that the blank has a particle size of> 7 in the one or more formed regions after quenching. The higher the grain size number, the smaller the average section length in mm and the smaller the average grain diameter. Conversely, the number of grains per mm ^{2 increases} accordingly. The cold forming after quenching should preferably be carried out at a temperature of 300-500 ° C. Thus, after cold forming occurs immediately after quenching, the cold working temperature approximates the temperature that the blank has immediately after quenching. The blank itself shows after cold working in the or the deformed areas preferably a hardness of 500 - 720 HV, ultimately depending on the steel used and in particular the respective degree of deformation.

As described, cooling to room temperature is controlled, ie with a defined cooling ramp. The cooling rate should be <150 K / h, which means that it is cooled relatively slowly. As already described, there is a further increase in strength or increase in hardness after interstitial atoms form precipitates. These precipitates form pinning centers for dislocations, which consequently can not migrate. This can be used to increase the hardness. The cooling should be controlled so that the on

Room temperature cooled blank has a hardness in the reformed area (s) of 520 - 750 HV.

As already described, a steel is used as the starting material, ie as a semi-finished product, which forms a purely austenitic microstructure through the corresponding process route.

A steel of the following composition can be used:

- 16 - 21% Cr

- 16 - 21% Mn

- 0.5 - 5% Mo and / or 0.25 - 2% Cu

a total of 0.8 - 1, 1% C and N, wherein the C / N ratio is> 0.5 and <1, 2, and a balance of Fe and a maximum of 2.5% impurities caused by melting.

Such a steel is known by way of example from DE 10 2004 043 134 A1 or DE 10 2009 013 506 A1. These two publications are incorporated with their disclosure content fully in the disclosure of the present application. The means that the disclosure content of the present invention also includes the complete disclosure content of these two documents. All of the steel compositions disclosed in these publications are consequently also disclosed as being relevant to the invention.

For steels with> 12% chromium, the austenitic steels are usually corrosion-resistant grades. Depending on the Mo content, it is also possible to add Cu as an alloying element, in which case Cu is added in particular if the Mo content is> 2%. Thus, either Mo or Cu can be alloyed or a combined alloying of Mo and Cu can take place. If only Mo is alloyed, the Mo content should be between 2.5 and 5%. If only Cu is alloyed, the Cu content should be between 0.5 and 2%. If both alloying partners are used, Mo should be added to at least 2% and Cu to at least 0.25%.

Alternatively to the use of such a steel, the use of a TWIP steel or a TRIP steel is also conceivable. These steel grades are also austenitic steels or steels in which the described processing ensures that a purely austenitic, precipitation-free microstructure is given before the hard-hardening cold forming.

In addition to the method itself, the invention further relates to a rolling bearing component, produced by the described method. Such a rolling bearing component may for example be a ring of a rolling bearing or a rolling element, ie in any case a component that undergoes a rolling load during operation. The application can be carried out under oil or grease lubricated conditions or in media lubricated operation. Depending on which condition or lubrication is present, the maximum surface pressure is designed. Thus, the maximum surface pressure experienced by the rolling bearing component according to the invention should be <2800 MPa in the case of EHD conditions, whereas in the case of mixed friction <2000 MPa and with media lubrication, for example when used under water, <1500 MPa. Overall, the invention allows a better machining of the blanks and better formability in the context of cold forming. The end product, ie the finished rolling bearing component, has a higher strength, a higher yield strength, a higher hardness and a high-precision final geometry. The properties of the manufactured rolling bearing component are therefore excellent, resulting in particular improvements in carrying capacity and life.

The invention will be explained below with reference to embodiments with reference to the drawings. The drawings are schematic representations and show: a micrograph of a usable, non-solution-annealed semifinished product, a micrograph of a blank prepared from the semifinished product of FIG. 1 after solution annealing and hot working, wherein the core structure is shown, a micrograph of the blank after solution annealing and hot working showing the highly deformed edge structure, a blank produced from the semifinished product according to FIG. 1 after solution annealing and hot forming, quenching and cold forming, and a microsection of a previously annealed, semi-finished product.

For better comparability, first of all, FIG. 5 will be discussed. FIG. 5 shows a micrograph of a semifinished product of X35CrMnN18-18 (trade name "CARNIT").

The micrograph of the solution annealed semifinished product, which is not formed, apparently shows relatively large grains. Unlike the semifinished product to be used according to the invention, of which a cut is shown in FIG. Again, this is a steel X35CrMnN18-18 (trade name "CARNIT"), but this semi-finished product is not solution annealed, and the micrograph clearly shows massive grain boundary precipitates of carbides, nitrides, and carbonitrides.

FIG. 2 shows, starting from the semifinished product according to FIG. 1, a micrograph of the microstructure in the region of the only slightly deformed core structure after solution annealing and hot forming. Apparently, relatively fine grains are formed in the core structure, compared with the large grains according to FIG. 5. Occasionally recrystallization occurs.

FIG. 3 shows a micrograph of the blank produced from the semifinished product according to FIG. 1 after the solution heat treatment and the hot forming in the severely deformed edge structure. As can be seen, compared with FIG. 2, which shows the core structure, there are strongly elongated grains, as well as recrystallization in this area.

Finally, FIG. 4 shows a micrograph of the additionally quenched and cold-formed blank in the region of the edge structure. Sliding lines and twins show up in a fine-grained austenitic structure.

Claims

claims

Method for producing a rolling bearing component from an austenitic steel, comprising the following steps:

Separating a blank from a non-solution-heat-treated semi-finished product,

Solution heat treatment of the blank by heating to a temperature

> 1000 ° C,

Hot working of the heated blank,

Quenching the blank to a temperature <500 ° C,

Cold working of the blank,

controlled cooling of the blank to room temperature,

- Machining finishing of the blank to complete the rolling bearing component.

A method according to claim 1, characterized in that a semi-finished product with a hardness of 200 - 400 HV is used.

A method according to claim 1 or 2, characterized in that the heating to a temperature of at least 1 125 ° C, in particular of 1 150 ° C takes place.

4. The method according to any one of the preceding claims, characterized marked that the cold forming takes place at a temperature of 300 - 500 ° C.

5. The method according to any one of the preceding claims, characterized in that the blank has a hardness of 500 - 720 HV after the cold forming in the one or more formed areas.

6. The method according to any one of the preceding claims, characterized in that the controlled cooling takes place at a cooling rate <150K / h.

7. The method according to any one of the preceding claims, characterized in that the cooled to room temperature blank has a hardness in the or the transformed areas of 520 - 750 HV.

8. The method according to any one of the preceding claims, characterized in that either a steel of the following composition is used:

- 16 - 21% Cr

- 16 - 21% Mn

- 0.5 - 5% Mo and / or 0.25 - 2% Cu

in total 0.8-1.1% C and N, the C / N ratio being> 0.5 and <1, 2,

- as well as a balance of Fe and a maximum of 2.5% impurities caused by melting,

or that a TWIP or TRIP steel is used as steel.

9. Rolling bearing component, produced by the method according to one of the preceding claims.