JP2707519B2 - Surface treatment method of mandrel bar for seamless steel tube rolling - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for rolling a seamless steel pipe, and more particularly to a method for treating a surface of a mandrel bar used in mandrel mill rolling.

[0002]

2. Description of the Related Art In general, a process of manufacturing a seamless steel pipe by the Mannesmann mandrel method includes a drilling step of converting a heated billet into a hollow shell, a stretching rolling step of reducing the thickness of the drilled hollow shell, and It comprises three steps of a finish rolling step of reducing the drawn and rolled tube to a predetermined outer diameter. Mandrel rolling is a process that occupies a major part of the elongation rolling process.In this process, a mandrel bar is inserted into a hollow shell, and the shell is reduced in outer diameter by a hollow roll and simultaneously with a hollow roll. The wall thickness is reduced between the mandrel bars.

[0003] In order to reduce the frictional force generated between the surface of the mandrel bar and the material to be rolled, for example, a graphite-based lubricant is usually applied before rolling. However, the mandrel bar is always in contact with the hot tube during rolling, and because of the high pressure, its surface is in a very severe friction state.
This causes a reduction in the life of the mandrel bar and a reduction in product quality. Such a phenomenon is remarkably exhibited in a mandrel bar used for an alloy steel having a high rolling load or a thin-walled tube.

[0004] By the way, the manufacturing process of a commonly used mandrel bar is, as shown in FIG.
After softening and straightening the material of the system, etc., it is subjected to a peeling process, and then processed and straightened at the front and rear ends, the outer diameter, and polished to improve the surface roughness, and heat treatment of quenching and tempering. To obtain a finish hardness of about 55-60 Hs (for example, see the 3rd Edition Iron and Steel Handbook III (2), edited by the Iron and Steel Institute of Japan, p. 979). In this case, the finishing accuracy of the surface of the mandrel bar is determined by polishing before quenching. As for an example of this polishing process, as shown in FIG. 4, the mandrel bar 1 advances in the direction of arrow F while being rotated by the turning roller 2, and its circumferential direction is polished by a polishing device 3 such as a polishing grindstone. It is common.

[0005]

However, in the circumferential polishing as described above, a polishing line 4 is inevitably generated as shown in FIG. 5, and a defect S is generated from the polishing line 4 as a starting point. There is a problem that the life of the mandrel bar 1 is reduced. SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has as its object to provide a surface treatment method for a mandrel bar for rolling a seamless steel pipe, in which a conventional polishing process is improved.

[0006]

The present invention is characterized in that after polishing the surface of the mandrel bar body in the circumferential direction, the surface is subjected to finish polishing in the longitudinal direction of the shaft to a surface roughness Ra of 4 to 12 μm. This is a surface treatment method for a mandrel bar for steelless tube rolling.

[0007]

[Operation] According to the present invention, the polished line generated when the circumferential direction is polished is subjected to finish polishing in the longitudinal direction of the shaft to finish the surface roughness Ra to 4 to 12 μm. The frictional force generated between the surface of the mandrel bar and the material to be rolled can be reduced, thereby improving the life and quality of the mandrel bar.
Here, the reason for limiting the surface roughness Ra to 4 to 12 μm is as follows.
If the surface roughness is less than 4 μm, the adhesion of the scale may be impaired. Therefore, it is appropriate to set the lower limit to 4 μm.
If the thickness exceeds 12 μm, there is a possibility that the effect of the polishing grain may occur. Therefore, it is necessary to limit the upper limit to 12 μm.

[0008]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a side view showing an embodiment according to the present invention. As shown in the figure, a longitudinal polishing device 5 such as a flexible grindstone for polishing the axial longitudinal direction is disposed adjacent to a circumferential polishing device 3 for polishing the surface of the mandrel bar 1 in the circumferential direction. Then, the surface of the mandrel bar 1 traveling in the direction indicated by the arrow F while being rotated by the turning roller 2 is first polished circumferentially to a surface roughness Ra of 10 to 25 .mu.m by a circumferential polisher 3, and then a longitudinal polisher. 5, the longitudinal direction of the shaft is polished, and finished so that the surface roughness Ra is 4 to 12 μm.

Rolling was performed using A and B mandrel bars manufactured under the conditions shown in Table 1 using the present invention, and the results of use are shown in Table 2. For comparison, Table 2 also shows the results of use of two conventional mandrel bars A and B polished only in the circumferential direction using the same outer diameter and material as described above. As is clear from Table 2, all the mandrel bars of the present invention have a smaller wear amount than the conventional mandrel bar, so that their life is greatly improved. Further, this reduces the frictional force between the mandrel bar surface and the material to be rolled, so that the rolling reaction force applied to the rolling roll also decreases as shown in FIG. 2, and inner surface defects and bulging defects which cause quality defects are reduced. It also has the side effect of decreasing.

[0010]

[Table 1]

[0011]

[Table 2]

[0012]

As described above, according to the present invention, after the surface of the mandrel bar main body is polished in the circumferential direction, finish polishing is performed so that the surface roughness Ra is 4 to 12 μm in the longitudinal direction of the shaft. As a result, not only the life of the mandrel bar is extended, but also the product quality is greatly improved.

[Brief description of the drawings]

FIG. 1 is a side view showing an embodiment according to the present invention.

FIG. 2 is a characteristic diagram for comparing rolling loads applied to rolling rolls.

FIG. 3 is an explanatory view of a manufacturing process of a conventional mandrel bar.

FIG. 4 is a side view showing a configuration of a surface polishing process of a conventional mandrel bar.

FIG. 5 is an explanatory view of a problem at the time of polishing the surface of a conventional mandrel bar.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Mandrel bar 2 Turning roller 3 Circumferential polishing device 4 Polishing item 5 Longitudinal polishing device

Claims (1)

(57) [Claims] After polishing the surface of the mandrel bar body in the circumferential direction, the surface roughness Ra is 4 to 12 μm in the longitudinal direction of the shaft.
m. Finishing and polishing the surface of a mandrel bar for seamless steel pipe rolling.