GB1571059A - Pierced metal tube blanks and methods of making such blanks - Google Patents

Description

(54) PIERCED METAL TUBE BLANKS AND METHODS OF MAKING SUCH BLANKS (71) We, EISENWERK-GESELL SCHAFT MAXIMILIANSHUTTE MBH, a German Company, of 8458 Sulzbach Rosenberg, West Germany do hereby declare the invention, for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to pierced metal blanks having an annular cross-section and a stepped internal diameter for use in the manu facture of seamless tubes. It also relates to methods of making such blanks. The invention is especially applicable to the manufacture of precision tubes such as those from which the bearing rings of ball bearings are made.

Seamless tubes can be produced by diagonal rolling, Pilger rolling, rolling in two-high rolling mills or on a push bench. These various methods all have the common feature that a substantially cylindrical pierced metal blank is fitted over a mandrel and the pierced blank is stretched and deformed to a tube bloom by rolling it over the mandrel. The necessary pierced blanks can be produced by diagonal rolling or by punching of billets in a press. The formation of the blank by piercing by diagonal rolling calls for cylindrical billets and is therefore not suitable for the direct use of continuously cast billets. Having regard to the great economic advantages of continuous casting, piercing in a punching press has therefore now regained importance in the production of pierced blanks from continuously cast billets of square cross-section.

In piercing in a punching press, a foursided billet, previously heated to the required deformation temperature, is usually placed in a circular mould and is brought to the required shape of the pierced blank which is closed at one end, by driving a cylindrical piercing mandrel into the billet in the mould.

The billet cross-section is so selected that the longitudinal corners of the billet rest suib- stantially in contact with the internal face of the wall of the mould and, when the piercing mandrel is driven in, the four cavities, of segment-shaped cross-section, between the longitudinal faces of the billet and the wall of the mould are filled out with the material of the billet. During piercing, the objective is as far as possible to limit the flow of material to the lateral filling out of the mould cavity and as far as possible to avoid a flow of material counter to the pressing direction.

When a flow of material counter to the pressing direction occurs, known as ascending piercing, this is associated with higher pressing forces and in particular with increased wear of the mould.

To ensure filling of the mould and the production of a pierced blank of circular cross-section, the diameters of the mould and of the piercing mandrel should be in a ratio of approximately 1.8:1. A further limitation arises from the fact that the ratio of billet length to tube diameter and mandrel diameter should not exceed 5:1 and 7:1 since otherwise there is a risk of the piercing mandrel running laterally out of line as it is driven into the billet, resulting in eccentric piercing. This is a serious disadvantage, in that it is almost impossible to produce tubes of uniform wall thickness from eccentrically pierced blanks.

The aforementioned dimensional relationships also set a limit to the length of the finished tube produced from the blank. A further disadvantage lies in rhe fact that, for each different tube diameter, the use of a correspondingly dimensioned square billet or a mould corresponding to the billet crosssection is necessary.

To enable seamless tubes of greater length or wall thickness to be manufactured, in spite of the aforementioned dimensional relationships, it is proposed in German Patent Specification No. 1 298 494, that a pierced blank having a stepped internal diameter be used. In a pierced blank of this type which has a uniform wall thickness throughout, the bore camprises, starting from the closed end, a cylindrical head portion having a diameter equal to the internal diameter of the finished tube, an adjoining conical transition, and a cylindrical main portion having a larger diameter than the internal diameter of the finished tube. Having regard to the uniformity of the wall thickness over its entire length, the pierced blank has an external diameter which varies according to the internal diameter, that is it tapers in a stepped manner from the open to the closed end.

In order to satisfy the requirement, hitherto considered essential, of a wall thickness which is uniform over the whole length of the pierced blank, a mould having a correspondingly stepped internal diameter is necessary.

Such a mould is understandably more expensive than a cylindrical mould. It furthermore suffers from the disadvantage that it is subjected in the conical transition to more intense wear, because the square billet introduced into the mould bears at the start of pressing with virtual point contact against the conical surfaces and consequently a considerably higher loading is applied in this region, when the piercing mandrel is driven in, than in the remainder of the mould. Moreover, in the lower, that is the narrowest part of the mould, pressing of the billet material in a direction parallel to the axis occurs together with undesired ascending piercing involving a flow of material counter to the pressing direction.

The object of the present invention is, while retaining the advantages achievable with a pierced blank having a stepped internal diameter, to avoid the aforementioned disadvantages and in particular to provide a pierced blank which can be manufactured with reduced wear of the mould and increased production efficiency, while using different piercing mandrels having different tube and head diameters, but using the same mould, the blanks being formed from billets of uniform square cross-section.

The invention is based on the discovery that, contrary to the hitherto held view, no great importance needs to be attached to a uniform wall hickness of the pierced blank over its entire length.

According to this invention, we provide a method of making a pierced metal blank having an annular cross-section, one end closed, the other end open and a stepped internal diameter for use in the manufacture of seamless tubes, the portion of smaller internal diameter being adjacent the closed end and being connected to the portion of larger internal diameter by a conical transistion and the wall thickness of the blank in the portion of larger internal diameter being less than in the portion of smaller internal diameter, the method being characterised in that a metal billet of square uniform cross-section is placed in a substantially cylindrical piercing mould and is pierced by pressing a stepped mandrel, which has an external shape the same as the internal shape of the pierced blank, axially into the billet in the mould, the material of the billet being pressed outwards into contact with the mould by the pressing of the mandrel into it.

Preferably the axial length I of the transition and the axial length L of the portion of smaller diameter are in ratio in which L : l is from 1 : 1 to 5 : 1; and the axial length L and the diameter d of the portion of smaller diameter are in a ratio in which L : d is from 0.2 : 1 to 5 : 1; and, the internal diameter D of the portion of larger diameter and the internal diameter d of the portion of smaller diameter are in a ratio in which D : d is from 1.02 : 1 to 2 : 1; Thus, in a pierced blank made by the method in accordance with this invention, the wall thickness consequently increases at the reduction in internal diameter, that is to say from the open towards the closed end of the blank.

Since the pierced blank made by the method in accordance with this invention, apart from a slight conicity to facilitate ejection from the mould, is externally cylindrical, the invention makes it possible to produce tubes of different internal diameters from square billets all of the same size without changing the mould.

When making blanks for the production of tubes of different internal diameters, it is therefore only necessary for identical square billets to be pierced by piercing mandrels having correspondingly differing diameters of the piercing mandrel head.

Preferably, the internal diameter ratio is not greater than 1.5:1. The larger internal diameter D is adapted to the diameter of the mould and ensures, in the corresponding portion of the mould, a piercing action which only forces the billet material outwards transversely to the direction of pressing, that is without any flow of material in the axial direction. In the region also of the conical transition and of the piercing mandrel head where the bore has a diameter d, no substantial axial flow of material occurs. This is in contrast to the punching out of a pierced blank having a uniform wall thickness over its entire length, since in the formation of a blank in accordance with the invention, the square billet rests, right from the start, upon the end of the mould and is substantially in contact over the entire length of its corners with the mould wall. Also, because of this, the risk of sideways deflection of the piercing mandrel is quite considerably reduced. To this may be added the fact that, not only the mould but also the piercing mandrel is subjected to considerably less wear in the region of the piercing mandrel head and of the transition, because the wear decreases by the extent to which the piercing action results in the flow of the material of the billet only transversely to the pressing direction.

The use of considerably higher deforming forces are possible when the above conditions are satisfied without risk of tearing off of the closed end of the pierced blank if the intermediate stretching or drawing of the blank is carried out with a stepped mandrel, which fits against the wall of the bore of the blank only in the portion of smaller diameter and/or of the transition. Since, during roughing, the mandrel bears predominantly with its conical transition against the pierced blank and consequently in this region the deforming forces are also transmitted, there is no longer a risk of tearing away of the closed end of the pierced blank.

As a further advantage, the thickness of the closed end can be reduced. This leads to smaller cropping losses. Normally, the thickness of the end must be sufficient to withstand the maximum deforming force.

This applies also to final ramming and drawing of the blank during subsequent tube production by means of a stepped mandrel, since the saving of material resulting from the thinner closed end of the pierced blank is greater than the loss of material associated with the reduction in diameter in the forward part of the mandrel and the accompanying larger wall thickness in this part of the blank.

When using a stepped mandrel, there is preferably an annular gap between the cylindrical main part of the mandrel, that is the part of larger diameter situated behind the conical transition and the corresponding cylindrical larger part of the bore, so that there is considerably less friction between the material of the pierced blank and the mandrel during roughing. This leads to a saving in drawing force of up to 30%.

An example of a blank and of a method of making a blank in accordance with the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic axial section of the blank; Figure 2 is a diagrammatic representation of a mould with a four-sided billet therein and a piercing mandrel in a starting position, before forming the blank; Figure 3 is a view similar to Figure 2, but with the piercing mandrel in its final position after piercing; and, Figure 4 is a diagrammatic axial section of the pierced blank shown entering a preliminary push bench.

A pierced blank 1 of annular cross-section comprises a central bore 2, which is cylindrically shaped in a main portion 3 of larger diameter and has a head portion 4 of smaller diameter and a conical transition 5. Consequently, the wall thickness is greater in the region of the head portion 4 and the transition 5 than in the region of the main portion 3.

Corresponding to the wall thickness, the piercing diameter decreases from the open end towards the closed end from D in the portion 3 to d in the head portion 4. The head portion 4 has an axial length L and the transition 5 an axial length l.

The pierced blank as illustrated in Figure 1 is made from a square hot metal billet 10, using a stepped piercing mandrel 11 in a conventional punching press, comprising a mould 7, an ejector 8 and a pressing plunger 9. The piercing mandrel 11 has a cylindrical main part 12, a transition 13 and a piercing head 14, corresponding the shape of the bore 2.

When piercing a conventional square billet 10, which has its longitudinal corners substantially in contact with the inner wall of the cylindrical mould 7 and its end face against the ejector 8, the piercing mandrel 11 penetrates, after a slight initial banking-up of material, into the billet and thus displaces the material of the billet transversely to the pressing direction and radially outwards into spaces 15 and 16, of segment-shaped crosssection, between the longitudinal faces of the billet and the wall of the mould.

The finished pierced blank can be shaped in a push bench by means of a mandrel 19, which is stepped in a manner similar to the piercing mandrel and is mounted at the forward end of a pressing plunger. The shaping or roughing can of course alternatively be carried out on an extrusion press. In either case, the pierced blank permits the use of high deformation forces, because the transference of force from the mandrel to the blank takes place essentially in the region of the conical transition 5 as well as through the end 6. This is especially the case when the diameter of the main part of the mandrel is smaller than the diameter D of the bore, so that there is an annular clearance. The push bench is equipped with a roll stand 18.

The pierced blank 1 is pushed into the roll stand 18 by means of the mandrel 19 and is drawn over the mandrel between the rolls with an accompanying reduction in cross-section.

A tube bloom of from two to five metres in length is thus produced, which can then be finally drawn if desired using a stepped mandrel rod, on a push bench to a length of, for example, from ten to thirty metres. The push bench then requires considerably fewer roll stands than is usual and/or it permits the production of longer tubes.

In an operating test, square billets of dimensions 200 X 200 X 1000 mm were pierced in a cylindrical mould having an internal diameter of 282 mm using various different stepped piercing mandrels having a diameter D of the main part of 165 mm and differing head diameters d of 110, 130 and 160 mm.

The length L of the cylindrical head part was in each case 100 mm and the axial length 1 of the transition in each case was 80 mm. By using the aforementioned piercing mandrels, it was possible for pierced blanks of 1000 mm length to be produced with a wall thickness of the main portion 3 of about 58 mm.

The pierced blanks were roughed in a preliminary push bench comprising a number of roll stands and mandrels which differed from the aforementioned piercing mandrels only in that the diameter of their head parts was 145 mm. Consequently, there was an annular gap of width 10 mm between the main portions of the mandrels and the inner wall of the pierced blank. It was possible with considerably reduced application of force to produce, on the preliminary push bench, roughed tube blooms of length approximately 2 m to approximately 4 m, and these were then finally drawn with a reduced application of force on a push bench.

WHAT WE CLAIM IS:- 1. A method of making a pierced metal blank having an annular cross-section, one end closed, the other end open and a stepped internal diameter, for use in the manufacture of seamless tubes, the portion of smaller internal diameter being adjacent the closed end and being connected to the portion of larger internal diameter by a conical transition and the wall thickness of the blank in the portion of larger internal diameter being less than in the portion of smaller internal diameter, the method being characterised in that a metal billet of square uniform cross-section is placed in a substantially cylindrical piercing mould and is pierced by pressing a stepped mandrel, which has an external shape the same as the internal shape of the pierced blank, axially into the billet in the mould, the material of the billet being pressed outwards into contact with the mould by the pressing of the mandrel into it.

2. A method according to Claim 1, in which the axial length l of the transition and the axial length L of the portion of smaller diameter are in a ratio in which L : lis from 1 : 1 to 5 : 1; and the axial length L and the diameter d of the portion of smaller diameter are in a ratio in which L : d is from 0.2: 1 to 5 : 1; and, the internal diameter D of the portion of larger diameter and the internal diameter d of the portion of smaller diameter are in a ratio in which D : jim from 1.02 : 1 to2 : 1; 3. A method according to Claim 2, in which the ratio D : d is not greater than 1.5 : 1.

4. A method according to Claim 2 or Claim 3, in which the external diameter decreases towards the closed end to provide a taper sufficient to facilitate release of the blank from a piercing mould.

5. A method according to any one of Claims 1 to 4, in which there are more than two portions of different internal diameters, the diameters of the portions decreasing from the open end to the closed end and there being a conical transition between each pair of adjacent portions.

6. A pierced blank made by a method in accordance with any one of Claims 1 to 5.

7. A method of making a seamless tube from a pierced blank in accordance with Claim 6, characterised in that the pierced blank is drawn using a stepped mandrel which fits against the wall of the bore of the blank only in the portion of smaller diameter and/or in the transition.

8. A method according to Claim 7, characterised in that the pierced blank is roughed or shaped before it is drawn.

9. A method according to any one of Claims 1 to 5, in which the corners of the billet are substantially in contact with the mould along the whole length of the billet before the billet is deformed by the mandrel.

10. A method according to Claim 1, substantially as described with reference to Figures 1 to 3 of the accompanying drawings.

11. A method according to Claim 7, substantially as described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (11)

**WARNING** start of CLMS field may overlap end of DESC **. head diameters d of 110, 130 and 160 mm. The length L of the cylindrical head part was in each case 100 mm and the axial length 1 of the transition in each case was 80 mm. By using the aforementioned piercing mandrels, it was possible for pierced blanks of 1000 mm length to be produced with a wall thickness of the main portion 3 of about 58 mm. The pierced blanks were roughed in a preliminary push bench comprising a number of roll stands and mandrels which differed from the aforementioned piercing mandrels only in that the diameter of their head parts was 145 mm. Consequently, there was an annular gap of width 10 mm between the main portions of the mandrels and the inner wall of the pierced blank. It was possible with considerably reduced application of force to produce, on the preliminary push bench, roughed tube blooms of length approximately 2 m to approximately 4 m, and these were then finally drawn with a reduced application of force on a push bench. WHAT WE CLAIM IS:-

1. A method of making a pierced metal blank having an annular cross-section, one end closed, the other end open and a stepped internal diameter, for use in the manufacture of seamless tubes, the portion of smaller internal diameter being adjacent the closed end and being connected to the portion of larger internal diameter by a conical transition and the wall thickness of the blank in the portion of larger internal diameter being less than in the portion of smaller internal diameter, the method being characterised in that a metal billet of square uniform cross-section is placed in a substantially cylindrical piercing mould and is pierced by pressing a stepped mandrel, which has an external shape the same as the internal shape of the pierced blank, axially into the billet in the mould, the material of the billet being pressed outwards into contact with the mould by the pressing of the mandrel into it.

2. A method according to Claim 1, in which the axial length l of the transition and the axial length L of the portion of smaller diameter are in a ratio in which L : lis from 1 : 1 to 5 : 1; and the axial length L and the diameter d of the portion of smaller diameter are in a ratio in which L : d is from 0.2: 1 to 5 : 1; and, the internal diameter D of the portion of larger diameter and the internal diameter d of the portion of smaller diameter are in a ratio in which D : jim from 1.02 : 1 to2 : 1;

3. A method according to Claim 2, in which the ratio D : d is not greater than 1.5 : 1.

4. A method according to Claim 2 or Claim 3, in which the external diameter decreases towards the closed end to provide a taper sufficient to facilitate release of the blank from a piercing mould.

5. A method according to any one of Claims 1 to 4, in which there are more than two portions of different internal diameters, the diameters of the portions decreasing from the open end to the closed end and there being a conical transition between each pair of adjacent portions.

6. A pierced blank made by a method in accordance with any one of Claims 1 to 5.

7. A method of making a seamless tube from a pierced blank in accordance with Claim 6, characterised in that the pierced blank is drawn using a stepped mandrel which fits against the wall of the bore of the blank only in the portion of smaller diameter and/or in the transition.

8. A method according to Claim 7, characterised in that the pierced blank is roughed or shaped before it is drawn.

9. A method according to any one of Claims 1 to 5, in which the corners of the billet are substantially in contact with the mould along the whole length of the billet before the billet is deformed by the mandrel.

10. A method according to Claim 1, substantially as described with reference to Figures 1 to 3 of the accompanying drawings.

11. A method according to Claim 7, substantially as described with reference to the accompanying drawings.