The invention relates to a piercing mandrel for the production of seamless tubes having a piercing mandrel base body and a piercing mandrel lug, in particular a piercing mandrel for piercing heated round blocks of metal for the production of seamless tubes.

It is generally known that, in the course of the production of seamless tubes, a cylindrical starting material heated in a furnace, in particular round blocks heated in a rotary hearth furnace, is formed into a tubular hollow block in a skew rolling mill using an axially fixed internal tool consisting of a piercing mandrel and mandrel bar. In the skew rolling mill, the round block is advanced helically through the skew rolling mill, after being gripped by the skewed rolls of the skew rolling mill, and thus over the axially fixed piercing mandrel and in this way is formed into the seamless hollow block tube. The piercing mandrel itself has a diameter which increases continuously from the tip. In the further course of the method, the seamless hollow block tube is stretched into a seamless tube. The piercing mandrel, which typically consists of a highly heat resisting tool steel, has the task, in addition to piercing the round block, of re-welding any material tears, smoothing the inner surface of the resulting hollow block and bringing the wall thickness thereof as uniformly as possible to the desired dimension. Since the piercing mandrel has to perform its work under the influence of the rolling heat, it is subjected to extreme stress and has only a limited service life. If the service life of the piercing mandrel, also referred to as the rolling mandrel, is coming to an end, the piercing mandrel can lose its shape and a surface can become damaged or material layers can become welded on. At the same time, this also means a deterioration in the tube quality, in particular by reason of defects on the inner surface of the hollow block and a non-uniform wall thickness.

A particular problem in this case is posed by difficult-to-form, highly alloyed tube materials, such as chromium-containing materials with more than 5 wt.% chromium, in particular more than 13 wt.% chromium which place particular thermal and mechanical stress on the piercing mandrel and particularly the mandrel tip thereof, and so it has signs of wear even after only a few piercing procedures.

Over the course of time, many measures have been proposed which should reduce the heat input into the piercing mandrel and increase the service life. Examples of this are producing the piercing mandrel tips from particularly heat-resistant materials, such as e.g. technical ceramic, coating the piercing mandrel surface with additional materials, controlled oxidation of the surface, frequently replacing the mandrels in conjunction with water spray or immersion cooling, as well as internal cooling of the piercing mandrels with water through the mandrel bar.

US patent 5,778,714 discloses a coated piercing mandrel having a piercing mandrel base body and a piercing mandrel lug connected thereto, which is coated. The piercing mandrel base body and the piercing mandrel lug are made of molybdenum, a molybdenum alloy or a heat-resistant steel. The coating can be a ceramic which is obtained by adding tungsten carbide to hard alloys based upon cobalt-chromium.

European patent EP 2 944 387 B1 also discloses a piercing mandrel having a piercing mandrel base body with a build-up layer welded on in the region of the tip of the piercing mandrel base body. Moreover, a spray coating is applied to the piercing mandrel base body with the build-up layer. As a main component, the build-up layer comprises an alloy having a transition metal and can additionally have hard particles, wherein these hard particles can be carbides, oxides, nitrides, borides or a mixture thereof. Niobium carbide (NbC), tungsten carbide (WC), titanium carbide (TiC), vanadium carbide (VC) or chromium carbide (CraC2) are proposed as carbides.

Furthermore, in order to reduce the costs of replacing worn piercing mandrels, it has been proposed in German laid-open document DE 100 24246 A1 to design the piercing mandrel in two parts having a mandrel lug and a piercing mandrel base body which can be screwed thereto via a corresponding thread, and to produce the mandrel lug and base body from different materials in order to increase the service life in accordance with the thermal and mechanical stresses, and to replace only the mandrel lug if necessary. For this purpose, a blind hole having an internal thread is provided in the piercing mandrel base body and the mandrel lug is correspondingly designed having a shaft which has an external thread.

European patent specification EP 2 991 780 B1 discloses a piercing mandrel having a piercing mandrel lug and a piercing mandrel base body. The piercing mandrel lug is conically tapered in the outer diameter in the longitudinal extension towards the piercing mandrel base body. With this inverted conical geometry of the piercing mandrel lug, the service life could be increased and an improvement in the inner surface of the hollow block could be achieved. The piercing mandrel lug consists of a lug core and a lug cap which is releasably screwed thereto. The piercing mandrel lug can also be connected to the piercing mandrel base body via a screw connection or a bayonet closure.

The object of the invention is to increase the service life of the piercing mandrel for the production of seamless tubes, in particular when piercing round blocks of metal, in particular of more highly alloyed steel materials which are difficult to form.

The object is achieved by a piercing mandrel having the features of claim 1 and alternatively by a piercing mandrel having the features of claim 3. Advantageous embodiments of the invention are given in claims 2 and 4 to 16.

In accordance with the invention, in the case of a piercing mandrel for the production of seamless tubes having a piercing mandrel base body and a piercing mandrel lug, wherein the piercing mandrel lug is releasably connected to the piercing mandrel base body, the object set is achieved by virtue of the fact that the piercing mandrel lug is connected to the piercing mandrel base body via a snap ring securing system. This releasable connection makes it possible to provide a simple and cost-effective replacement of the piercing mandrel lug in the event of wear, as only the piercing mandrel lug most greatly affected by wear has to be replaced instead of the complete piercing mandrel together with the piercing mandrel base body. In an advantageous manner, this snap ring securing system offers a cost-efficient, heat-resistant and secure fastening of the piercing mandrel lug, in particular in the operating state, i.e. during the rolling procedure. Moreover, the releasable connection by means of a snap ring connection system ensures simple removal or insertion with a relatively low force requirement and replacement of the piercing mandrel lug. In addition, the releasable connection by means of a snap ring securing system ensures that the piercing mandrel lug can be processed to smaller diameters even after wear has occurred.

The snap ring securing system also ensures low manufacturing effort and thus low manufacturing costs. The snap ring securing system can be advantageously deployed independently of the selected materials for the piercing mandrel base body and piercing mandrel lug. It also offers the advantage that only the worn piercing mandrel lug has to be replaced and the piercing mandrel base body does not have to be scrapped.

Alternatively, the object set is achieved in accordance with the invention by means of a piercing mandrel for the production of seamless tubes having a piercing mandrel lug and a piercing mandrel base body by virtue of the fact that the piercing mandrel lug consists of a material based upon tungsten carbide, i.e. has more than 50 wt.% tungsten carbide. In particular, it is a so-called tungsten carbide-cobalt hard metal which is also referred to as WC-Co hard metal and consists predominantly of particles of tungsten carbide (WC) as well as cobalt (Co) as a binder phase in order to hold the WC particles together. WC gives the material its hardness and Co improves the toughness and strength. The strength and hardness are better the smaller the grain size and are also influenced by the Co content. Co makes up about 4 % to 30 % of the total mass.

Surprisingly, it has been shown in tests that tungsten carbide as a piercing mandrel lug material has both very good wear and temperature resistance, as well as very good ductility by reason of the Co binder phase or matrix, and so brittle fracture can be avoided. As a result, it is possible to achieve a significant increase in the service life of the piercing mandrel lug and also an improvement in the inner surface of the hollow block which is also improved by the existing geometry of the piercing mandrel base body and piercing mandrel lug. In tests, a large number of piercing procedures could be performed on particularly highly alloyed materials with a piercing mandrel lug consisting of tungsten carbide without registering any damaging changes to the geometry of the mandrel and thus internal defects and geometry deviations on the hollow block.

Therefore, tungsten carbide as a piercing mandrel lug material combines the advantageous properties of ceramics and steel which are typically used as piercing mandrel lug material, but have the disadvantages that steel is sufficiently ductile, but has only very limited resistance to high levels of wear and high temperatures, and technical ceramics have high wear and temperature resistance, but are very brittle.

Also, by using tungsten carbide as the piercing mandrel lug material, partly complex and cost-intensive internal cooling of the piercing mandrel lug can be dispensed with.

Within the scope of this invention, the wording "based upon" means that at least 50 % of the piercing mandrel lug material is tungsten carbide.

A piercing mandrel which combines the alternative solutions in relation to the snap ring securing system, on the one hand, and the material based upon tungsten carbide, on the other hand, proves to be particularly advantageous. However, this combination is not compulsory.

In a constructively simple manner, provision is made that the piercing mandrel lug is inserted into a piercing mandrel base body recess in an operating state of the piercing mandrel with a piercing mandrel lug shaft. In this case, provision is preferably made that the piercing mandrel lug shaft is held in the piercing mandrel base body recess via the snap ring securing system.

It is particularly advantageous that the snap ring securing system consists of at least one snap ring and at least one allocated groove in the piercing mandrel base body recess, in which the at least one snap ring rests. Therefore, the piercing mandrel lug or the piercing mandrel lug shaft, which is a wear part, can still maintain a smooth and cylindrical outer surface without grooves which is simple to produce.

The holding forces and redundancy of the snap ring securing system are improved by virtue of the fact that the snap ring securing system consists of two snap rings and two allocated grooves in the piercing mandrel base body recess, in which one of the two snap rings rests in each case, and the grooves are spaced apart from one another in the longitudinal direction of the piercing mandrel base body recess. Assembly is also facilitated because the clamping forces required for holding the piercing mandrel lug are distributed to two snap rings and the clamping force can be successively overcome during assembly and disassembly of the piercing mandrel lug.

In this case, provision is advantageously made that the at least one snap ring in its non-expanded resting state has an inner diameter which is smaller than the outer diameter of the piercing mandrel lug shaft. In a preferred embodiment, provision is made that the piercing mandrel lug is conically tapered in the outer diameter in the longitudinal extension towards the piercing mandrel base body.

In a preferred embodiment, the piercing mandrel lug is formed of at least 70 % tungsten carbide, preferably at least 85 % tungsten carbide. For the remainder, the piercing mandrel lug material comprises binding agents, such as e.g. cobalt.

In a particularly advantageous manner, provision is made that the piercing mandrel lug consists completely of a material based upon tungsten carbide.

In particular, the piercing mandrel lug does not have a coating, i.e. it is configured without a coating.

In a further advantageous embodiment of the invention, provision is made that the grain size of the tungsten carbide, which is used as the piercing mandrel lug material, is 5 to 15 pm, preferably 6 to 10 pm, particularly preferably 7 to 9 pm. In this case, the grain size is typically determined by means of metallography, in particular by means of microscopy and measuring.

In a further preferred embodiment, the piercing mandrel lug and the piercing mandrel base body are produced from different metallic materials.

An exemplified embodiment of the invention will be explained in greater detail with reference to the following description. In the drawing:

Figure 1 shows a schematic view of the piercing process in a skew rolling mill with a piercing mandrel, and

Figure 2 shows a sectional view of a front portion of a piercing mandrel base body having a piercing mandrel lug which is held thereon via a snap ring securing system.

Figure 1 schematically shows a skew rolling mill, in which a solid round block 2 of steel, in particular a difficult-to-form, highly alloyed material, such as a chromium- containing material with more than 5 wt.% chromium, in particular more than 13 wt.% chromium, is formed into a hollow block tube 3 with the aid of a piercing mandrel 1. The skew rolling mill comprises two work rollers 6 which form the round block 2 into a hollow block tube 3 via an internal tool consisting of the piercing mandrel 1 and a so- called mandrel bar 7. By means of the work rollers 6, the round block 2 or the hollow block tube 3 is driven helically in the rolling direction W via the axially fixed internal tool.

The piercing mandrel 1 consists of a piercing mandrel base body 4, to which a piercing mandrel lug 5 is connected at the front as seen in the direction opposite the rolling direction W. The diameter of the piercing mandrel base body 4 increases continuously as seen in the rolling direction W, starting from the piercing mandrel lugside end up to a maximum value which determines the inner diameter of the hollow block tube 3. The piercing mandrel lug 5 consists completely or exclusively of a material based upon tungsten carbide. The piercing mandrel base body 4 is produced from a highly heat resisting tool steel.

The mandrel bar 7, the piercing mandrel lug 5 and the piercing mandrel base body 4 have a common central longitudinal direction L which extends substantially in parallel with the rolling direction W. In the embodiment illustrated in figure 1 , the piercing mandrel lug 5 has a diameter which tapers conically towards the piercing mandrel base body 4 and as seen in the longitudinal direction L. In the transition region from the piercing mandrel base body 4 to the piercing mandrel lug 5, a space 9, i.e. a hollow space, is formed between the piercing mandrel base body 4 and the piercing mandrel lug 5 with respect to the resulting hollow block tube 3 during the piercing process, said space not coming into contact with the steel of the hollow block tube 3 during the piercing procedure.

Figure 2 shows a sectional view of a front portion of the piercing mandrel base body 4 with a piercing mandrel lug 5 arranged thereon at a front or lug-side piercing mandrel base body tip 4b of the piercing mandrel 1. The piercing mandrel base body 4 and the piercing mandrel lug 5 are releasably connected to one another.

It can be seen that the piercing mandrel lug 5 comprises a front, in particular conically tapering piercing mandrel lug tip 5a and an adjoining cylindrical piercing mandrel lug shaft 5b which accordingly has a constant diameter. The piercing mandrel lug shaft 5b is inserted into a piercing mandrel base body recess 4a of the piercing mandrel base body 4 almost completely, but at least with more than 75 % of its length, in an application state of the piercing mandrel 1, in which the piercing mandrel lug 5 is connected to the piercing mandrel base body 4. The piercing mandrel base body recess 4a is formed as a cylindrical blind hole, is oriented centrally in relation to the longitudinal direction L of the piercing mandrel base body 4 and extends with its longitudinal extension in the longitudinal direction L of the piercing mandrel base body 4. The diameter of the piercing mandrel lug shaft 5b and the inner diameter of the piercing mandrel base body recess 4a are designed as a fit in order to ensure a stable and form-fitting hold of the piercing mandrel lug 5 in the piercing mandrel base body recess 4a as well as secure transmission of the rolling forces which occur. The longitudinal direction L of the piercing mandrel lug shaft 5b inserted into the piercing mandrel base body 4 and the longitudinal direction L of the piercing mandrel base body 4 coincide or are concentric. The plug connection between the piercing mandrel lug shaft 5b and the piercing mandrel base body recess 4a advantageously permits a simple and cost-effective replacement of the piercing mandrel lug 5 which is subject to wear.

For simplified insertion of the piercing mandrel lug shaft 5b into the piercing mandrel base body recess 4a, the piercing mandrel lug shaft 5b has a piercing mandrel lug shaft chamfer 5c at the end opposite the piercing mandrel lug tip 5a, said chamfer typically having a diameter which tapers conically away from the piercing mandrel lug shaft 5b.

Moreover, the piercing mandrel lug 5 is held in the longitudinal direction L in the piercing mandrel base body recess 4a via a snap ring securing system 8.

In the embodiment shown in figure 2, the snap ring securing system 8 consists of two snap rings 8a and 8b which are arranged one behind the other at a spaced interval X in the axial longitudinal direction L in the piercing mandrel base body recess 4a. Of course, more than two snap rings 8a, 8b or only a single snap ring 8a can also be used for the snap ring securing system 8. The spaced interval X is about 25 % to 50 % of the depth of the piercing mandrel base body recess 4a.

The snap rings 8a, 8b have a rectangular cross-section and are each arranged in a rectangular groove 10a, 10b located in the outer wall of the piercing mandrel base body recess 4a and thus in the piercing mandrel base body 4. The exact configuration of the snap rings 8a, 8b and grooves 10a, 10b is dependent upon the respective diameter of the piercing mandrel lug shaft 5b and follows e.g. DIN EN 10270-1 or DIN 7993-B in relation to the snap rings 8a, 8b. The inner diameter of the snap rings 8a, 8b is smaller than the outer diameter of the piercing mandrel lug shaft 5b in order to hold it in a clamping manner. Accordingly, the inner diameter of the snap rings 8a, 8b in the expanded state and in the state resting on the piercing mandrel lug shaft 5b is equal to the outer diameter of the piercing mandrel lug shaft 5b. In this case, the snap rings 8a, 8b have also achieved a circular ring shape in relation to their inner diameter. In the relaxed resting state, the snap rings 8a, 8b protrude into the piercing mandrel base body recess 4a, even if they are oriented in a centred manner with respect to the longitudinal direction L.

During the course of inserting the piercing mandrel lug shaft 5b into the piercing mandrel base body recess 4a, the snap rings 8a, 8b rest in the grooves 10a, 10b in the piercing mandrel base body 4 and, when the piercing mandrel lug shaft chamfer 5c arrives at this location, are expanded by the piercing mandrel lug shaft chamfer 5c against the spring force of the snap rings 8a, 8b. The piercing mandrel lug shaft 5b with its smooth and cylindrical outer surface which is also without grooves is thus held in a clamping manner in relation to the longitudinal direction L by the two snap rings 8a, 8b which are resiliently tensioned in the direction of the piercing mandrel lug shaft 5b. As a result, a frictional connection is produced between the piercing mandrel lug shaft 5b and the piercing mandrel base body 4 via the snap rings 8a, 8b. At the same time, the snap rings 8a, 8b rest in the grooves 10a, 10b and thus hold the piercing mandrel lug shaft 5b in the longitudinal direction L in the piercing mandrel base body recess 4a.

For disassembly and also the preceding assembly, the snap rings 8a, 8b and the grooves 10a, 10b are dimensioned in such a way that, on the one hand, in response to insertion and withdrawal of the piercing mandrel lug shaft 5b, the snap rings 8a, 8b yield resiliently in order to permit a movement of the piercing mandrel lug shaft 5b in the longitudinal direction L, and, on the other hand, during operation of the piercing mandrel 1 , the piercing mandrel lug 5 is held securely on the piercing mandrel base body 4.

The snap rings 8a, 8b and the grooves 10a, 10 with a rectangular cross-section have been described above. Other cross-sections, such as semi-circular, round or square, would also be feasible. Overall, snap rings 8a, 8b with a round cross-section and correspondingly formed grooves 10a, 10 are preferable.

List of reference signs

1 piercing mandrel

2 round block

3 hollow block tube

4 piercing mandrel base body

4a piercing mandrel base body recess

4b piercing mandrel base body tip

5 piercing mandrel lug

5a piercing mandrel lug tip

5b piercing mandrel lug shaft

5c piercing mandrel lug shaft chamfer

6 work rollers

7 mandrel bar

8 snap ring securing system

8a first snap ring

8b second snap ring

9 space

10a first groove

10b second groove

L longitudinal direction

W rolling direction

X axial spaced interval