JPH06182601A - Outer cutting method for mandrel bar - Google Patents

Abstract

PURPOSE:To make the jogs on the on the surface of the mandrel bar smooth to such a level not to cause any trouble by executing at least once outer cutting operation in such a manner as to satisfy a specified condition while cutting to the final outside diameter is repeated at the time of outer cutting the bar having a spiral jogs on the surface thereof. CONSTITUTION:A holder 2 where a cutter 1 is mounted is installed on a tool rest 3 and forced to make a rotary motion. A feed roller 4 disposed on the inlet side of the tool rest 3 feeds a work piece 5 in the axial direction and checks the rotation in the circumferential direction. A guide roller 6 disposed on the outlet side of the tool rest 3 positions the shaft center of the work piece 5 in such a manner as to be aligned with the center of rotation of the cutter 1. The work piece 5, that is, the mandrel bar having the spiral jogs on the surface thereof is outer-cut by the outer cutting device. At this time, while cutting to the final finished outside diameter is repeated, at least one cutting operation is executed in such a manner as to satisfy the following specified condition: t<3h (wherein, t: depth of cut, h: depth of jogs before outer cutting).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for cutting a mandrel bar used for manufacturing a mandrel bar used in a hot seamless tube mandrel mill.

[0002]

2. Description of the Related Art One of the processes for producing hot seamless pipes
In the mandrel mill, which is one of the two types, rolling is performed by a multi-stand hole type roll that feeds in the axial direction while restraining the outer surface of the raw pipe and a mandrel bar that restrains the inner surface of the raw pipe. The mandrel bar is an important tool for determining the inner surface quality of the rolled raw tube, and is usually manufactured by the following process.

The material of the mandrel bar is usually SK
Hot work tool steels such as D6 and SKD61 are used. An ingot melted with this component is slab-rolled and then subjected to a predetermined heat treatment to obtain a bar material. During the heat treatment, bending occurs in the bar material, so the bending is corrected by a rotary straightener, and then the mandrel bar is cut to a predetermined outer diameter by an external cutting device. Finally, the surface roughness and outer diameter dimension are finished by belt grinding, and further surface treatment such as induction hardening and tempering is applied to use for a mandrel mill.

It is the mandrel bar trimming process that largely determines the dimensional accuracy of the mandrel bar manufactured by such a process. However, since the mandrel bar is a long material that normally extends from 15 to 30 m, it is so-called. It is difficult to cut with a lathe, and it is common to use an external cutting device having a special structure different from that of a lathe.

FIG. 1 shows the structure of an external cutting device for a mandrel bar.
Is schematically shown in. A plurality of holders 2 to which the blades 1 are attached are attached to an annular tool rest 3 on concentric circles around the axis, and the blades 1 attached to the plurality of holders 2 together with the tool rest 3. Make a rotational movement. The feed rollers 4 installed on the entrance side of the tool rest 3 are one or more pairs of hole type rolls whose surfaces are knurled,
The long workpiece 5 such as a mandrel bar is fed in the axial direction, and the surface of the workpiece 5 is knurled to prevent the workpiece 5 from rotating in the circumferential direction. Disk-shaped guide rollers 6 are arranged on the entrance side and the exit side of the tool rest 3 so as to approach the tool rest 3. A plurality of guide rollers 6 are provided on each side in the circumferential direction (normally, four guide rollers 6 are equally arranged in the circumferential direction), and the work material 5 is placed so that the axis of the work material 5 coincides with the rotation center of the cutting tool 1. To position.

The cutting conditions are generally 0.7 to 2 m / min for feeding the material to be processed, while the tool rest is rotated at 100 to 300 rpm while a cut of about 3 mm is made by one external cutting, The outer cutting is repeated up to a predetermined outer diameter.

[0007]

With the use of the external cutting device having such a structure, a long material such as a mandrel bar can be externally cut with high efficiency. According to the research, when a mandrel bar processed with such an external cutting device is used for rolling, spiral irregularities are generated on the inner surface of the pipe after rolling, and the cause is the spiral shape existing on the bar surface after external cutting. It has become clear that the irregularities on the bar surface are those in which the spiral irregularities existing on the surface of the bar material before the external cutting remain as they are after the external cutting.

The cause of spiral irregularities on the surface of the bar material before external cutting is as follows.

Since the bar material is a large diameter long material having an outer diameter of 100 to 450 mm and a length of 15 to 30 m, it is difficult to avoid uneven heat generation in the circumferential direction and the longitudinal direction during heat treatment, which is an extreme case. Bends of up to tens of mm occur. When such a mandrel bar is used for actual rolling, bending occurs in the rolled raw pipe and a transport trouble of the mandrel bar occurs, which is a serious problem in terms of product quality and pipe manufacturing efficiency. Therefore, the heat-treated bar material is a facility (rotary straightener) in which a pair of rolls, one of which has a convex axial profile and the other of which has a concave profile, are inclined with respect to each other so that the axial direction is in a twisted position. The bend is corrected by. At that time, since the bar material moves in the axial direction while rotating, the locus of the contact point with the roll on the bar material becomes a spiral shape, and if the bar material has a large bend as described above, the bar material rolls. It is pressed down tightly at certain points above, resulting in a spiral asperity on the bar material surface.

According to the investigation by the present inventors, when the hardness of the bar material is Hs35 or more and the depth is 0.1 mm or more, although the cutting is carried out with the above-mentioned cutting depth of about 3 mm, Most of the unevenness remains after the external cutting. In general, the bar material needs strength that does not cause elongation or deformation within a range that does not cause breakage during rolling or transportation, and thus is heat-treated so that Hs is 35 or more. Therefore, the surface often has spiral irregularities even after the external cutting.

Heretofore, it has been relatively rare that the spiral unevenness remaining after the external cutting becomes a problem. This is because the depth of the unevenness is as shallow as 0.1 to 0.2 mm. However, in recent years, the demand for dimensional accuracy of products has become strict, so that the outer diameter dimensional accuracy is high in both the circumferential direction and the axial direction with respect to the surface of the mandrel bar, that is, the distance from the straight axis to the surface is It is now required to be the same in all positions. For this reason, the spiral irregularities remaining on the surface of the mandrel bar adversely affect the inner diameter distribution and the wall thickness distribution of the product.

The spiral irregularities remaining on the surface of the mandrel bar also adversely affect the life of the bar. That is, when a mandrel bar having irregularities on the surface is used for rolling, the protrusions have a remarkable increase in surface temperature during rolling or stripping. Therefore, when it is rapidly cooled by bar cooling after use or when this is repeated, the concave portion is subject to a large thermal shock and thermal fatigue, and cracks are likely to occur and propagate. If the crack becomes deep, breakage will occur during rolling or during transportation, which will cause a great trouble, so that it must be recut or scrapped. Therefore, when the surface of the mandrel bar has irregularities, the life is usually shortened on average.

Therefore, it is important to remove the spiral irregularities from the surface of the bar after the external cutting in order to secure the product quality and to extend the life of the bar.

An object of the present invention is to provide a method of cutting a mandrel bar which can reduce the spiral irregularities on the surface of the bar to a level at which there is no problem.

[0015]

In order to achieve the above object, the inventors of the present invention have made a mandrel bar a spiral concavo-convex existing on the surface of a bar material before being trimmed which is straightened by a rotary straightener. It was planned to clarify the conditions that remain after the external cutting by the external cutting device, and prepared bar materials with different hardness and different depths of unevenness, and conducted comparative tests.

As a result, when the bar material having a hardness of less than Hs35 is used, the spiral unevenness is almost eliminated by ordinary external cutting, and the flatness is obtained to the extent that the dimensional accuracy of the product is not adversely affected, and the hardness is high. Even if Hs is 35 or more, the depth of spiral irregularities is 0.
If the thickness is less than 1 mm, the unevenness depth is significantly reduced, which is a level that does not affect the product quality, but the hardness is Hs.
It was found that those having 35 or more and spiral irregularities having a depth of 0.1 mm or more on the surface hardly lost the spiral irregularities.

On the other hand, the feed rate of the mandrel bar and the rotational speed of the blade in the cutting device are within the general conditions, that is, 0.7.
It was confirmed that at ~ 2 m / min and 100 to 300 rpm, the disappearance of the spiral irregularities was not affected.

The reason why the irregularities having a depth of about 0.1 to 0.2 mm are not removed while securing a cutting depth of 3 mm is that the external cutting device is different from the so-called lathe, and the axial center of the workpiece is Thought that is not fixed. That is, it is considered that the blade escapes due to surface hardening or surface irregularities during straightening, or the surface irregularities are picked up by a guide roller and the material shakes at the blade portion.

On the basis of the above test results and consideration, a method for flatly cutting a bar material having spiral irregularities was examined. As a result, the cutting depth and the depth of spiral irregularities of the bar material before external cutting were examined. It was found that the depth of the spiral concavo-convex after ablation drastically changes depending on the relationship, and that if this relationship is specified, it is possible to perform ablation almost flatly.

The present invention has been made based on the above findings, and a plurality of blades arranged on concentric circles around the axis are rotated, and the workpiece is moved in the axial direction by the center of the blade to move the workpiece. With an external cutting device that cuts the outer surface of the processed material,
When mandrel bar having spiral irregularities on the surface is externally cut, at least one external cutting is performed during repeated cutting to the final outer diameter t ≦ 3h, where t: depth of cut h: before cutting The gist is a method of cutting a mandrel bar, which is characterized in that the mandrel bar is formed under the condition of the unevenness of the surface.

[0021]

FIG. 2 shows the relationship between the depth of the spiral irregularities and the depth of cut in the bar material before external cutting, which was investigated by the present inventors. The abscissa represents the ratio t / h of the cutting depth t to the spiral concavo-convex depth h of the bar material, and the ordinate represents the concavo-convex depth of the mandrel bar after external cutting. As is clear from this figure, the uneven depth after external cutting changes rapidly at the boundary of t / h = 3.
When / h> 3, deep unevenness remains, whereas t / h ≦ 3
In the case of, the unevenness becomes shallow and the surface of the mandrel bar is almost smoothed. The reason is considered as follows.

The reason why the spiral unevenness remains during external cutting with the above-described cutting device is that the variation of the cutting resistance is large, and this variation of the cutting resistance is also affected by the size of the cutting depth. However, it is also closely related to the depth of the unevenness itself. However, as described above, when the ratio (t / h) of the cutting depth t to the concave-convex depth h is larger than 3, the fluctuation of the cutting resistance does not decrease, whereas when t / h ≦ 3, the fluctuation of the cutting resistance is small. It is presumed that, as a result of the critical reduction of the surface roughness, smooth cutting with no remaining irregularities can be achieved.

The lower limit of the cutting depth t is not particularly limited, but with a commonly used cutting tool, when the cutting depth is less than 0.2 mm, the cutting edge is slippery without cutting the work material and cuts. Since it often becomes impossible, the thickness is preferably 0.2 mm or more.

When the bar surface is smoothed by the external cutting with the cutting depth limited, the depth of the spiral unevenness does not extremely increase again by the subsequent external cutting. Therefore, the cutting for limiting the cutting depth may be the first cutting, the last cutting, or the middle cutting to the final dimension.

The number of times of external cutting for limiting the cutting depth may be at least once, including the finishing step of finishing to the final dimension.
That is, as described above, since the spiral unevenness is not re-formed on the bar surface once smoothed by the external cutting by the subsequent external cutting, the effect is saturated even if this is repeated, and Repeating this external cutting with the depth of cut restricted to a small value causes an increase in man-hours and cost, and is uneconomical. Therefore, it is necessary to perform at least once.

Further, the mandrel bar is often used as a mandrel bar having a smaller diameter after the life of the mandrel bar is reached by using the actual machine, and after the heat treatment, the mandrel bar is often used as a mandrel bar having a smaller cutting depth. It goes without saying that shaving is equally effective.

The external cutting method of the present invention is particularly effective for a bar material having a material hardness of Hs35 or more and an unevenness depth of 0.1 mm or more, which cannot be expected to remove unevenness by the conventional method, but less than Hs35. And the depth of unevenness is 0.1 mm or more or H
It can be applied to those having a s35 or more and a depth of unevenness of less than 0.1 mm, and when applied to these, the effect of removing unevenness which greatly exceeds the conventional method can be provided.

[0028]

EXAMPLES Examples and comparative examples of the present invention will be described below.

[0029] An ingot made of hot work tool steel (SKD6) is rolled into a bar shape, and further hardened to Hs33 by heat treatment.
After adjusting to -40, the bar material having an outer diameter of 185 mm, which is straightened by a rotary straightener, is used to make an outer diameter of 176 to 1 by using an external cutting device for a mandrel bar shown in FIG.
It was cut to 79 mm. At this time, the cut amount was adjusted variously, and the influence of the cut amount on the elimination of the spiral irregularities on the material surface was investigated. The material feed rate was 1 m / min, and the blade rotation speed was 180 rpm. The survey results are shown in Table 1.

[0030]

[Table 1]

Nos. 1 to 7 and Nos. 10 to 14 are examples in which a bar material having a hardness of Hs 35 or more and an uneven depth of 0.1 mm or more was cut out. In the processing including the external cutting of t ≦ 3h (Invention example No. 1 to 7), the uneven depth which was about 0.1 to 0.3 mm before the external cutting could be reduced to 0.04 mm or less after the external cutting. . However, machining not including external cutting for t ≦ 3h (Comparative Example No. 10
Nos. 14 to 14) could reduce the depth of the unevenness to a minimum of 0.07 mm.

Nos. 8 and 15 are examples for a bar material having a hardness of Hs35 or more and an uneven depth of less than 0.1 mm, No. 9 and
16 is an example for a bar material having a hardness of less than Hs35 and a depth of unevenness of 0.1 mm or more. For such a bar material, machining that does not include external cutting for t ≦ 3h (Comparative Example No. 1
5, 16), the uneven depth can be made considerably smaller, but t ≦
The depth can be further reduced in the machining including the external cutting of 3 h (Invention Example Nos. 8 and 9).

6 mandrel bars externally cut by the method of the present invention shown in No. 1 and 6 mandrel bars prepared by the comparative external cutting method shown in No. 14 were used as an actual set under the same conditions, and their life was examined. It was As a result, the mandrel bar according to the comparative method had an average life of about 1500 passes due to cracks caused by the spiral irregularities on the surface, whereas the mandrel bar cut by the method of the present invention was rolled up to an average of about 2500 passes. Could be used. Further, the cause of the life of the mandrel bar according to the method of the present invention was rough surface, not deep cracks.

[0034]

As is clear from the above description, the mandrel bar external cutting method of the present invention removes the spiral irregularities remaining on the bar surface after external cutting to a level that does not cause a problem, and heats it with a mandrel mill. It has a great effect on improving the inner quality of seamless pipes and improving the life of mandrel bars. Also,
Although the external cutting is performed with a limited depth of cut, since the high-efficiency tool turning type external cutting device as in the past is used, the efficiency decrease due to the limited depth of cut is slight. Therefore, a high quality mandrel bar can be efficiently manufactured.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an external cutting device for a mandrel bar.

FIG. 2 is a chart showing the influence of the depth of cut on the unevenness depth of the bar surface.

[Explanation of symbols]

 1 Cutlery 3 Turret 4 Feed roller 5 Work material (mandrel bar) 6 Guide roller

Claims (1)

[Claims] 1. An external cutting device for cutting an outer surface of a workpiece by rotating a plurality of blades arranged on a concentric circle around the axis and moving the workpiece in the axial direction at the center thereof. When externally cutting a mandrel bar having spiral irregularities on its surface, at least one external cutting is performed while repeating cutting up to the final finishing outer diameter, t ≦ 3h, where t: depth of cut h: External cutting method for mandrel bars, which is performed under the condition that the unevenness of the surface of the mandrel bar before external cutting is performed.