EP1743103A1

EP1743103A1 - Ball element for two-part ball pivot and corresponding method of production

Info

Publication number: EP1743103A1
Application number: EP05747383A
Authority: EP
Inventors: Jochen Kruse; Dirk Adamczyk; Reinhard STÖTERAU; Jean-Paul Castanet
Original assignee: ZF Friedrichshafen AG
Current assignee: ZF Friedrichshafen AG
Priority date: 2004-05-04
Filing date: 2005-05-02
Publication date: 2007-01-17
Also published as: DE102004022248A1; MXPA06012713A; JP2007538203A; CN1950619A; BRPI0510575A; KR20070012819A; CN1950619B; WO2005106263A1; BRPI0510575B1; US20070211972A1; DE102004022248B4; KR101157685B1

Abstract

The invention relates to a method for producing balls, especially for ball pivots, and to a ball element for two-part ball pivots. The balls according to the invention are produced by cold extrusion and subsequent grinding, using a micro-alloy carbon-manganese steel. The use of micro-alloy carbon-manganese steel allows to obtain balls having excellent strength and hardness already due to the cold shaping. It is not necessary to use a tempering step as required for the production of generic balls, which allows the use of less expensive materials, thereby considerably reducing production costs. The invention allows for a simple and inexpensive method for producing balls for two-part ball pivots while maintaining and/or increasing surface and material quality as well as strength and wear resistance. As a result, the complexity of the method is reduced and the problem of dents which can occur on the ball surfaces during tempering is removed.

Description

Ball element for two-part ball stud and manufacturing process

description

The invention relates to a method for producing balls or ball segments, in particular for ball joints, according to claim 1. The invention further relates to a ball element for two-part ball pins according to the preamble of claim 7.

Two-part ball pins usually comprise a pin element and a separate ball perforated to receive the pin element. It is known here to produce balls for two-part ball pins, more precisely, ball elements such as perforated balls or ball segments by cold extrusion. In the prior art, for the production of the balls for two-part ball pins, tempering steel is usually used. After cold extrusion of the balls, the balls are first tempered. In connection with the nerging process, the balls are quenched by pouring the balls from the nerging furnace into the quenching medium in the hot or soft state.

When pouring into the quenching medium, however, the still soft balls collide with one another or against the walls of the quenching container, as a result of which undesirable impact points occur on the ball surfaces. These striking points have to be removed again in later steps, for example by grinding the spherical surfaces. However, essentially as much material has to be removed from the entire spherical surface as it corresponds to the depth of the impact points. Here, a considerable volume of material has to be removed, which on the one hand considerably extends the grinding time and on the other hand leads to rapid wear of the Grinding tools leads. In addition, the volume of material to be ground must be taken into account beforehand in the form of an oversize when producing the balls, which results in additional material costs.

Another disadvantage of known manufacturing methods for balls of this type is that, according to the prior art, tempering steel has to be used for this. This quenched and tempered steel is, however, more expensive than other steels, which among other things is related to the fact that the quenched and tempered steel has to be drawn in a drawing shop to achieve the desired material structure and annealed on spherical cementite (GKZ).

In addition, the balls made from tempering steel must of course be subjected to the corresponding tempering process after the cold pressing, so that the balls made from the tempering steel achieve the desired, intended hardness values and strength properties of the tempering steel. However, all of this is complex and therefore leads to high manufacturing costs for the balls.

With this background, it is an object of the present invention to provide balls, in particular for two-part ball journals, or a method for producing balls, with which the disadvantages of the prior art can be overcome.

The balls should in particular be simple and inexpensive to manufacture. In particular, the problem of the occurrence of the impact points on the ball surface is to be overcome and the need to subsequently eliminate the impact points is eliminated. At the same time, however, the high material and surface quality of the balls achieved with the known methods and the desired high strength of the balls should also be achieved or maintained.

This object is achieved by a method with the features of claim 1 or by a ball element with the features of claim 7. Preferred embodiments are the subject of the dependent claims. The method according to the invention for the production of spheres comprises the method steps shown below.

First, in a manner known per se, a rod section or wire section is produced from a semifinished product in a first method step. However, a semi-finished product is used, which consists of micro-alloyed carbon-manganese steel. In principle, any carbon-manganese steel with microalloying elements that has been hot-rolled after melting and has a fine-grained ferritic-pearlitic structure is suitable in principle.

The section is then pickled, in particular in order to remove oxidic coatings and to obtain a metallically clean surface of the section for the subsequent operations.

In a further process step, the rod section or wire section is then shaped by cold extrusion in such a way that the desired spherical shape is produced.

Finally, in a further process step, the spherical surface is ground to the intended dimension and shape.

The method according to the invention is extremely advantageous in several respects. First of all, instead of the tempering steel known from the prior art, a micro-alloyed carbon-manganese steel is used to produce the balls. The micro-alloyed carbon-manganese steel in particular does not have to be tempered, but, as has been shown, achieves excellent strength and hardness by means of the cold deformation which takes place in the process step of cold extrusion of the ball from the rod or wire section.

As a result, since the process step of tempering which is always necessary according to the prior art for producing the balls can be dispensed with, the effort associated with the tempering and the corresponding costs are also initially eliminated. In particular, however, this also relates to the state of the art Completely eliminates the problem of undesirable striking marks on the surfaces of the balls that occur when the hot or soft balls are poured from the tempering furnace into the quenching medium.

In other words, this also means that the balls can already be dimensioned considerably closer to the final dimensions during cold extrusion, since, as in the prior art, the considerable material removal during grinding of the balls, which is there to eliminate the impact points, no longer has to be taken into account was necessary. In this way, on the one hand, the semi-finished product used is more fully utilized, which already saves material costs. On the other hand, the time required for subsequent grinding is considerably reduced, since significantly less material has to be removed. Last but not least, the wear of the grinding tools and the amount of grinding sludge are significantly reduced, which also saves further costs and is environmentally friendly of the manufacturing process.

As has been shown, the balls, which are cold-pressed from microalloyed carbon-manganese steel, have even a considerably higher hardness than the tempered balls known from the prior art after the pressing, due to the cold working and due to the described special properties of the microalloyed steel.

On the one hand, this higher hardness improves the grindability of the balls and shortens the necessary grinding time. On the other hand, when handling the balls during the entire manufacturing process, especially after grinding, there are even fewer impact points on the ball surfaces. This is advantageous since a spherical shape that approximates the ideal spherical surface as far as possible without striking points leads to particularly smooth-running and low-wear spherical joints which show the lowest possible stick-slip effects when the ball moves in the bearing shell.

According to a preferred embodiment of the invention, the sections are subjected to a drawing process in a further method step after the pickling, or it is carried out after pickling, annealing and drawing the sections on spherical cementite (GKZ treatment). In this way, work hardening of the material is achieved even before the final cold extrusion, which further increases the strength of the balls subsequently obtained.

According to a further, likewise preferred embodiment of the invention, the wire or rod sections are phosphated and / or coated with a dry lubricant before the drawing or before the GKZ treatment. Since high compressive stresses occur between the workpiece and the tool during cold extrusion, measures usually have to be taken to prevent cold welding between the tool and the workpiece. This is done here by applying a carrier or phosphate layer to the wire or rod sections. In turn, a dry lubricant layer is arranged on the carrier layer, which has sufficient pressure resistance during cold extrusion and thus prevents metallic contact between the workpiece and the tool. For example, graphite, molybdenum disulfide, special soaps or waxes can be used as pressure-resistant solid lubricants.

According to a preferred embodiment of the invention, after grinding the ball surface, the balls are nitrocarburized in a further process step.

Nitrocarburizing leads to improvements in corrosion resistance and wear resistance, especially in the case of surface adhesion between the ball and the bearing shell. Furthermore, a nitrocarburized surface has a reduced coefficient of friction. The reason for this is the so-called connection layer that is produced on the surface of the sphere during nitrocarburizing and has a thickness of only a few hundredths of a millimeter. Nitrocarburizing is also a comparatively environmentally friendly process and is an advantageous alternative to galvanically deposited layers. Nitrocarburization is preferably carried out in a salt bath.

According to a further preferred embodiment of the invention, the balls are after after grinding or after nitrocarburizing, polished or re-ground in a further process step and then polished. This further increases the corrosion resistance and wear resistance of the ball surface, and the coefficient of friction is further reduced.

According to a further, likewise preferred embodiment of the invention, the carbon-manganese steel has a microalloying element for accelerating the nitrogen uptake during nitriding or nitrocarburizing. The microalloying element is particularly preferably vanadium.

The use of vanadium in particular as a microalloying element accelerates the nitrogen uptake during nitriding. In this way, higher hardness values and greater hardening depths of the connecting layer can be achieved with unchanged nitriding times, which also further improves the corrosion behavior. As an alternative, the same advantageous properties of the connecting layer as with tempered steel can be achieved with shorter process or nitriding times. Experiments have shown, for example, that the salt bath process time can be reduced by 33% from 90 minutes to 60 minutes in this way.

Overall, the optimized nitriding process or the shortening of the nitriding times results in a further cost advantage of the method according to the invention compared to the manufacturing methods known from the prior art for balls using tempered steels.

The invention also relates to a ball element, in particular for two-part ball pins. A two-part ball pin is composed essentially of a pin element and a perforated ball element in a manner known per se. According to the invention, however, the spherical element is distinguished by the fact that it consists of tempering-free carbon-manganese steel with micro-alloy elements.

The micro-alloyed carbon-manganese steel does not require a tempering process, but has excellent strength and hardness due to the cold forming Extrusion on. Thus, as already mentioned at the beginning, the remuneration necessary for the production of the balls according to the prior art can be omitted, as a result of which the corresponding outlay and the associated costs are also eliminated. In addition, the problem of undesired striking points on the spherical surfaces is solved, since the problematic pouring of the hot or soft spheres from the tempering furnace into the quenching medium is eliminated without replacement. According to preferred embodiments of the invention, the microalloyed carbon-manganese steel is drawn, GKZ-treated or coated, in particular phosphated.

According to a preferred embodiment of the invention, the spherical element is nitrocarburized. This improves corrosion resistance and wear resistance as well as the friction behavior of the ball element, in particular with regard to the adhesion between ball and bearing shell that occurs in ball joints due to the low angular velocities.

According to further, preferred embodiments of the invention, the ball element is ground and / or polished, whereby balls are obtained for particularly high-quality, durable and low-friction ball joints.

According to a further, likewise preferred embodiment of the invention, the microalloying elements comprise vanadium.

This gives the nitrided or nitrocarburized balls a particularly hard or particularly thick connecting layer, which in particular improves the corrosion behavior.

In the following, the invention is explained in more detail with the aid of drawings showing only one embodiment.

It shows:

1 shows the micrograph of the microstructure of a tempering steel for balls according to the prior art; FIG. 2 shows the structure of a microalloyed carbon-manganese steel for balls according to the present invention in a representation corresponding to FIG. 1; 3 shows a logarithmic plot of the cumulative fracture probability P against the tensile strength σ in MPa according to Weibull; 4 shows in a linear bar representation a comparison of the strengths of balls produced according to the invention with tempered balls according to the prior art; 5 shows the properties of the connecting layer produced by nitrocarburization in balls produced according to the invention in comparison with tempered balls according to the prior art; and FIG. 6 shows a ball produced according to the invention for a two-part ball pin in two different views.

1 shows the greatly enlarged micrograph of the ferritic-pearlitic structure of a tempering steel for balls according to the prior art. Specifically, this is the structure of a hot-rolled standard tempering steel with the designation 41Cr4.

FIG. 2 shows the micrograph of the likewise ferritic-pearlitic microstructure of a micro-alloyed carbon-manganese steel for balls according to the present invention in the same magnification as for the micrograph of the heat-treatable steel according to FIG. 1.

This is the micro-alloyed steel with the designation 35V1 or C-Mn-V, which is also hot-rolled during manufacture.

This steel has the following alloying elements (all information in

Weight percent):

0.35% C

0.20% Si

0.75% Mn

0.02% P

0.02% S 0.20% Cr 0.15% Ni 0.20% Cu 0.10% V 0.02% AI 0.01% N

1 and 2, one can see that the structure of the micro-alloyed steel according to FIG. 2 is much finer than that of the conventional tempering steel according to FIG. 1. The fine structure of the micro-alloyed steel according to FIG. 2 leads in particular to particularly good cold formability of the micro-alloyed steel, which advantageously accommodates the production of the balls according to the invention by cold pressing.

3 shows the strength of various cold-pressed balls calculated from hardness measurements. The representation is the cumulative fracture probability P in the form of a Weibull distribution, plotted logarithmically on the vertical axis, against the tensile strength σ in MPa plotted on the right axis. The tensile strength was calculated in accordance with DLN 50150 from measured hardness values, the hardness values being measured at different points on the balls.

3 contains measured values from three different types of spheres. The diamond-shaped measuring points designated with the letter A in the legend relate to the balls made of microalloyed carbon-manganese steel produced by cold pressing according to the invention. The square measuring points designated with the letter B in the legend in FIG. 3 relate to balls made from a tempered steel according to the prior art. Specifically, this is a common tempering steel with the designation 38MnB5. The triangular measuring points designated by the letter C in the legend in FIG. 3 in turn relate to the spheres according to the invention made of microalloyed carbon-manganese steel, the triangular measuring points relating to the spheres according to the invention after nitrocarburizing. It can be seen in FIG. 3 that the strength of the balls of microalloyed carbon-manganese steel (diamonds) according to the invention is considerably higher than the strength of the heat-treated steel according to the prior art (squares). This higher hardness is advantageous, among other things, when machining the balls by grinding, since in this way the grinding time can be significantly reduced, which saves costs.

On the other hand, due to the higher hardness when handling the balls during and after the manufacturing process, there are particularly few impact points on the ball surfaces. Balls for ball joints without impact points are particularly advantageous, since this enables particularly smooth-running, long-life and low-wear ball joints, which show a particularly low tendency to stick-slip effects when the ball moves in the bearing shell.

Finally, the greater hardness of the balls of microalloyed carbon-manganese steel according to the invention is also advantageous in that it also improves the corrosion resistance and the friction behavior when the balls are used in ball joints.

FIG. 3 also shows the strength of the spheres according to the invention made of microalloyed carbon-manganese steel, applied in the form of triangular measuring points, after the spheres according to the invention have been subjected to nitrocarburization. From the intersection of the imaginary Weibull straight line (the straight line defined by a group of measuring points in each case) with the y-axis at zero, it can be seen that the spheres according to the invention made of microalloyed carbon-manganese steel still have strength values even after nitrocarburization (triangular measuring points ), which are as high as those of the balls made of tempered steel (square measuring points).

Although it should actually be expected that due to the temperatures used in nitrocarburizing up to close to 600 ° C, there would be a recovery in the structure of the balls, which was cold-hardened during extrusion, and a corresponding decrease in the high strengths achieved by extrusion, but it has happened Surprisingly shown that the high strength of the Balls according to the invention are retained almost completely even after nitrocarburizing in an advantageous manner. The reason for this is to be seen in the fact that, because of the microalloying elements contained in the material of the balls according to the invention, there is no complete recovery of the work-hardened structure under the conditions of the nitrocarburizing process.

The recognizable in Fig. 3, in comparison to the tempering steel lower slopes of the Weibull straight line of the balls according to the invention made of microalloyed carbon-manganese steel (triangles or squares) merely indicate that due to the different degrees of deformation at different points of the ball a different there is a strong strain hardening of the material because the measured values shown were determined over the entire spherical cross-section. As tests have shown, this does not result in any negative effects with regard to the outstanding suitability of the balls according to the invention for use in ball joints.

FIG. 4 again shows the tensile strength, determined according to DLN 50150, from the hardness of various balls according to the invention made of a further microalloyed carbon-manganese steel with the designation 10MnSi7 (dotted vertical bars on the right in each case), and the tensile strength of the wires from which the respective balls are made (left hatched vertical bars). In addition, the representation of FIG. 4 again contains the strength values of a heat-treatable steel according to the prior art (horizontal bar) for comparison. The percentages on the right axis indicate the extent to which the wire from which the balls were pressed was pulled off before pressing. The wire was removed after hot rolling and before the balls were pressed.

It can be seen that the unrefined balls made from the microalloyed carbon-manganese steel (right dotted beams in each case) consistently have a higher strength than the balls made from the tempered steel (horizontal bar), and to a large extent independently of the degree of wire pulling and the resulting connected strength of the wire or starting material (left-hand hatched bars). 5 shows the hardness profile according to the invention of a compensation-free, Microalloyed carbon-manganese steel (35V1) produced ball after nitrocarburization, the hardness measurements being plotted against the depth below the ball surface.

According to the legend contained in FIG. 5, letter C again stands for the measured values of the carbon-manganese steel (triangular measuring points). For comparison, the corresponding hardness measured values of a ball made of conventional tempering steel according to the prior art are also plotted in the diagram according to FIG. 5, see again letter B in the legend in FIG. 5 (square measuring points).

It can be seen that the balls according to the invention made of microalloyed carbon-manganese steel (triangular measuring points) have a higher hardness than corresponding balls made of tempered steel according to the prior art (square measuring points) even after nitrocarburizing. The higher hardness is, as already explained above, i.a. advantageous for the particularly good wear resistance of the balls according to the invention and for a time and cost-saving, improved machinability of the balls during grinding.

For comparison, FIG. 5 also shows the target values for the hardness on the surface or at 0.2 mm depth that are structurally predetermined for balls for ball joints, see the two horizontal bars in the diagram according to FIG. 5. It can be seen that the connecting layer of the balls according to the invention (triangular measuring points) complies with or even exceeds the required hardness target values.

Finally, FIG. 6 shows a ball made according to the invention from tempering-free, micro-alloyed carbon-manganese steel for a two-part ball pin, which is perforated to receive the pin element, in two different views. It can be seen that the balls can be produced using the method according to the invention without problems, in particular without cracks and with a perfect surface quality.

The result clearly shows that it is now possible, thanks to the invention, to make balls easier and cheaper than before, in particular for two-part ball pins to produce, but at the same time the surface and material quality as well as the required strength and wear resistance of the balls can be maintained or even increased. On the one hand, due to the elimination of the previously required tempering, considerable savings are made on the one hand, and on the other hand the problem of the impact points on the spherical surfaces that often occur during tempering is eliminated.

The invention thus makes an important contribution to the particularly economical production of high-quality balls, in particular for ball joints, wheel suspensions, stabilizers and for comparable purposes.

Claims

Spherical element for two-part ball spigot and manufacturing process

1. Process for the production of balls or ball segments, in particular for ball joints, with the process steps: a) producing a rod section or wire section starting from a hot-rolled semi-finished product made of micro-alloyed carbon-manganese steel; b) pickling c) cold extrusion of the section to the ball or to the ball segment; and d) grinding the ball surface.

2. The method according to claim 1, characterized in that after method step b (pickling), in a further method step b 'at least one drawing process takes place.

3. The method according to claim 1, characterized in that after step b (pickling), in a further step b ", the section is annealed and drawn onto spherical cementite (GKZ).

4. The method according to claim 2 or 3, characterized in that the section before step b '(drawing), or during step b "(GKZ) phosphated and / or coated with a dry lubricant.

5. The method according to any one of claims 1 to 4, characterized in that after step d (grinding) in a further step e, a nitrocarburization of the balls or ball segments takes place.

6. The method according to claim 5, characterized in that the nitrocarburization in process step e takes place in a salt bath.

7. The method according to any one of claims 1 to 6, characterized in that the balls or ball segments after step d (grinding) or e (nitrocarburization) are ground and / or polished in a further step f.

8. The method according to any one of claims 1 to 7, characterized in that the carbon-manganese steel has a microalloying element for accelerating the nitrogen uptake during nitriding or nitrocarburizing.

9. The method according to claim 8, characterized in that the further microalloying element is vanadium.

10. Ball element, in particular for two-part ball pin, the pin comprising ball element and pin element, characterized in that the ball element consists of tempering-free carbon-manganese steel with micro-alloying elements.

11. Ball element according spoke 10, characterized in that the ball element is made of a drawn wire.

12. Ball element according to claim 10 or 11- characterized in that the ball element consists of a wire annealed on spherical cementite (GKZ).

13. Ball element according to one of claims 10 to 12, characterized in that the ball element consists of a coated, in particular phosphated wire.

14. Ball element according to one of claims 10 to 13, characterized in that the ball element is nitrocarburized.

15. Ball element according to one of claims 10 to 14, characterized in that the ball element is ground.

lό.Kugelelement according to one of claims 10 to 15, characterized in that the spherical element is polished.

17. Ball element according to one of claims 10 to 16, characterized in that the micro alloying elements comprise vanadium.