JPH0333405B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention is applicable to tube rolling in a stretching mill such as a mandrel mill, which rolls a blank tube with a cored rod inserted therein to control the wall thickness and outer diameter. This relates to a control method.

[Prior art]

Seamless pipes are usually made by drilling a hole in the center of a heated round piece of steel using a Mannesmann-Piercer or similar drilling mill.
A core rod is inserted inside the raw pipe obtained in this way, and it is rolled with an elongation rolling machine such as a mandrel mill.
Furthermore, it is finished into a product having a predetermined wall thickness and outer diameter by passing through a reducing mill such as a stretch reducer. By the way, the main purpose of the stretch reducer is to reduce the wall thickness and outer diameter to the target dimensions by subjecting the raw pipe that has been stretched and rolled using a mandrel mill, etc., to reduce the wall thickness and outer diameter to the target dimensions. However, due to the structure, it is not possible to apply a large tensile force to the top and bottom parts of the raw pipe. A phenomenon occurs in which the wall becomes extremely thick. Most of these thick-walled parts are off-gauge and are cut off, posing a major obstacle to improving yield.

For this reason, in the past, the degree of thickening formed at both ends of the tube exiting the stretch reducer was predicted or empirically grasped, and the pre-process of the stretch reducer, in other words, was carried out in order to offset this. Therefore, in the rolling process using a mandrel mill, a method has been adopted in which both ends are made thinner than the center (Japanese Patent Publication No. 43825/1983). To explain this specifically, when elongating a raw tube using a mandrel mill consisting of 8 stands, for example, in two specific stands such as the 5th and 6th stands (or the 6th and 7th stands), the bite of the tube is The following pressure-down control is performed using a hydraulic pressure-down device with excellent responsiveness during loading and tail-off.

That is, the roll gaps of the fifth and sixth stands are initially closed to a predetermined amount before the pipe passes through, and when the top of the pipe has finished passing, the roll gaps of both stands are enlarged to roll the middle part of the pipe. Before the bottom portion passes, the roll gaps of both stands are closed again to a predetermined amount, and rolling control is performed so that the top and bottom portions of the tube are thinner than the middle portion of the tube.

As a result, the thickness at both ends of the tube that has been thinned by the mandrel mill during the process of passing through the stretch reducer is offset, forming a tube with a uniform wall thickness distribution in the tube axis direction. The effect of improving yield is recognized.

However, in this conventional method, the two ends of the tube are perpendicular to each other on the stand in the mandrel mill.
As a result of being strongly pressed in order to thin the wall from the direction, the rolled portion near the groove edge of the elliptical hole shape of the caliber roll becomes thicker (four equally spaced in the circumferential direction) as shown in Figure 1 A, B, and C. This thick part is not resolved even after passing through the final stand of the mandrel mill or the stretch reducer, and the phenomenon of uneven thickness in the circumferential direction occurs, and this part becomes off-gauge. There were many problems, and there was a limit to the amount of cropping at the end of the tube.

Figure 1A is a schematic diagram showing the state in which the tube end is being rolled in the 5th stand of the mandrel mill by the conventional method; Figure 1B is a schematic diagram showing the state in which the tube end is being rolled in the 6th stand of the mandrel mill; C is a schematic sectional view of the tube end on the exit side of the mandrel mill, and in the figure, 5 and 6 are caliber rolls, 10 is a core bar, and 9 is a blank tube. As is clear from this figure,
As a result of strong pressure on the tube end from the top and bottom in the fifth stand, and from the left and right in the sixth stand,
On the exit side of the mandrel mill, there are thick walled parts 9a, 9b, 9c,
9d is formed in a protruding shape in the direction of the tube axis, and becomes an off-gauge crop length.

[Purpose] The present invention has been made in view of the above circumstances, and its purpose is to provide the front stand of the three stands including the last stand that performs the actual rolling of the elongation rolling mill, and the stands before and after it. At the last stand, thinning rolling is performed on the end of the raw tube to offset the thickening that occurs at the tube end on the outlet side of the reducing rolling machine, and at the last stand, the uneven thickness due to the thinning rolling is removed. It is an object of the present invention to provide a tube rolling control method that can greatly improve the yield by performing rolling that eliminates the problem.

[Structure of the invention]

In the pipe rolling control method according to the present invention, a raw pipe with a core rod inserted therein is passed between caliber rolls of a plurality of stands and stretched and rolled by alternately rolling it from two directions, and then in the process of performing reduction rolling. Thickness generated at the tube end on the exit side of the reducing rolling mill at the final stand that reduces the wall thickness of the rolling mill, and the other two stands excluding the last stand among the three consecutive stands including the stands before and after the final stand. In order to offset the thinning, thinning rolling is performed on the end of the raw pipe so that the wall thickness of each part corresponding to each rolling direction is the same, and the thinning rolling is performed at the rearmost stand. The method is characterized in that in order to eliminate uneven thickness at the end portions of the blank tube, thinning rolling is performed only on the intermediate portions of the respective portions.

〔Example〕

The present invention will be specifically described below based on drawings showing its implementation state. FIG. 2 is a schematic diagram showing the implementation state of the method of the present invention, and in the figure 1, 2...6, 7,
Reference numeral 8 indicates a roll stand, 9 indicates a raw pipe, and 10 indicates a core bar. A blank tube 9 formed by drilling a hole in the center of a round piece of steel with a piercer is passed through a mandrel mill with a core rod 10 inserted into the core hole, and a caliber is driven based on a control signal from a control unit 11. Rolls 1a, 2a...6a, 7a, 8a are used to elongate and roll the pipe into a tube having the target wall thickness and outer diameter. For example, a hydraulic rolling device with excellent responsiveness is used for each of the three consecutive stands, for example, the sixth and seventh stands 6 and 7, and the post-sizing stand that performs sizing rolling to adjust the tube shape, for example, the eighth stand 8, and a caliber roll is used. 6a, 7a, and 8a are shaped like holes as shown in FIGS. 4 and 5, and the following rolling control is performed.

In the sixth and seventh stands, the roll gap between the caliber rolls 7a and 8a is controlled so as to compensate for the thickening of the end portions of the raw tube relative to the middle portion that occurs in the stretch reducer. That is, when a raw pipe with a uniform wall thickness is passed through a stretch reducer, the average wall thickness distribution in the pipe axis direction at the top and bottom parts of the pipe as shown in Fig. 3 is determined, and the stretching method to eliminate this is determined. The wall thickness distribution for the top and bottom parts of the raw pipe on the reducer entry side is determined empirically or theoretically, and using this thickness distribution as a target value, the top of the raw pipe 9 is calculated for the 6th and 7th stands of the mandrel mill. The control pattern of the roll gap for the upper and lower parts is determined, and the hydraulic lowering device is controlled according to this pattern. To be more specific, the roll gap of the 6th and 7th stands is initially set to the value relative to the top end of the raw pipe 9, and when the top part is bitten by the 6th stand, the raw pipe is adjusted based on this point. 9, the roll gap of the sixth stand is increased at a predetermined acceleration so as to follow a predetermined roll gap control pattern for the top part, and at the same time as the top part finishes passing,
Open up to the roll gap defined for the middle part of the raw pipe. The same applies to the seventh stand.

When the bottom part of the raw pipe reaches the sixth stand, a predetermined acceleration is applied so that the roll gap of the sixth stand follows the roll gap control pattern defined from the roll gap for the middle part of the raw pipe 9 to the bottom part. Make it smaller. The same applies to the seventh stand.

On the other hand, in the eighth stand, no wall thickness reduction is applied to the middle part of the raw pipe 9, and a gap is formed between the raw pipe 9 and the core bar 10 by making the raw pipe 9 perfectly round. It functions as a so-called sizing pass to facilitate the removal of the core bar 10 from the raw pipe at the exit side of the mandrel mill, and also serves as a sizing pass when the raw pipe 9 passes through the top and bottom parts. In addition to the above functions, as shown in Figure 1 C,
At four locations in the circumferential direction of the raw pipe 9, for the strip-shaped thick-walled parts 9a to 9d formed in the pipe axial direction,
To solve this problem, roll gap control is performed using a caliber roll of a predetermined shape.

This roll gap control pattern is determined by taking into account the hole shape of the caliber roll used, the dimensions of the raw pipe, the rolling speed, etc.

Next, an example of the caliber rolls 6a, 7a, and 8a used in the sixth, seventh, and eighth stands will be shown. Figure 4 shows the 6th stage in the mandrel mill.
7 stand 6, 7 caliber roll 6a, 7a
FIG. 5 is a schematic cross-sectional view showing the relationship between the caliber roll 8a of the stand 8 in FIG. The hole shapes of the caliber rolls 6a and 7a of the sixth and seventh stands 6 and 7 are almost the same, and when looking at the cross section in the axial direction of the caliber rolls, as shown in FIG.
It is symmetrical to the left and right with respect to the X-X line that is orthogonal to the caliber roll axis, and is sandwiched between the X ₁ -X 1 line that passes through the X-X line and the pass center O and intersects the X- _X line at an angle α. The area a is a circular arc with a radius R ₁ centered on the path center O, and the other parts, in other words, b ₁ and b ₂ are
It is formed into a circular arc having a center P on the line X ₁ -X ₁ and a radius R ₂ .

On the other hand, when looking at the cross section in the axial direction of the caliber roll 8 in the eighth stand 8a, as shown in FIG. The area C is symmetrical on the left and right, and is sandwiched between the Y-Y line and the _Y1 - _Y1 line that passes through the path center O and intersects the Y-Y line at an angle β, and has a center P on the Y-Y line. radius
A region d ₁ that should face the arc of R ₃ and each of the four thick-walled portions 9a to 9d in the circumferential direction of the raw pipe 9,
d ₂ is an area sandwiched between the Y ₁ - _{Y 1} line and the Y ₂ - _{Y 2} line that passes through the path center O and intersects the Y ₁ - Y ₁ line at an angle θ, with the path center O as the center and an arc with a radius of R ₄ . , furthermore, the areas e _{1 and e 2 from} the flange part to the d ₁ and d ₂ areas have radii with the center P on the extension line of the Y ₂ - Y ₂ line, respectively.
It is formed into an arc of R ₅ . .

By the way, the material is carbon steel and the outer diameter is 158mm.
A raw pipe with a wall thickness of 6.5 mm is rolled using a core metal rod with a diameter of 139 mm, and then passed through a stretch reducer to reduce the outer diameter.
In the case of obtaining a tube of 73 mm and wall thickness of 5.51 mm, an example of the dimensions of the hole shapes of the caliber rolls 6a, 7a, 8a in the mandrel mill No. 6, 7, and 8 stands 6, 7, and 8 is shown below. In the figure, the 6th and 7th
The hole shapes of the caliber rolls 6a and 7a of the stands 6 and 7 are R ₁ : 76 mm, R ₂ : 150 mm, α=50°, and the hole shape of the caliber roll 8a of the 8th stand 8 is R ₃ : 36
mm, R ₄ : 250 mm, θ is 30°.

According to the method of the present invention as described above, the wall thickness reduction amount (mm) and wall thickness unevenness rate (%) of the top part of a tube (nominal wall thickness 7.0 mm) obtained by rolling with a mandrel mill and passing through a stretch reducer were determined. ). The results are shown in FIG. 6th
The figure shows the amount of wall thickness reduction (mm) on the horizontal axis and the thickness unevenness rate (%) on the vertical axis. This is the result when As is clear from this graph, it can be seen that when the method of the present invention is used, the thickness unevenness can be significantly suppressed compared to the conventional method in a region where the amount of wall thickness reduction is above a certain level.

In addition, in the above example, the sixth,
Although the case where the method of the present invention is implemented in stands 7 and 8 has been described, it is not limited to this.
If the final stand that performs the actual elongation rolling, that is, the stand that does not simply facilitate the drawing out of the core bar from the raw tube and performs rolling to adjust the tube shape, the applicable stand will change accordingly. Needless to say.

As described above, in the method of the present invention, stretching and
Not only can the thickening of the tube end that occurs in the reducer be improved, but also the thickness unevenness that occurs in the middle part of the part corresponding to each rolling direction can be reduced by thinning rolling to eliminate this thickening. In particular, the present invention has excellent effects, such as making it possible to significantly reduce uneven wall thickness around the entire circumference of the pipe, shortening the crop length and significantly improving yield, and being able to be applied as is to conventional equipment. It is something that plays.

[Brief explanation of the drawing]

Fig. 1 A, B, and C are schematic diagrams showing the manner in which striped thickness unevenness occurs at the end of the raw pipe, Fig. 2 is a schematic diagram showing the state of the method of the present invention, and Fig. 3 is a schematic diagram showing the state of the method of the present invention.
A graph showing the wall thickness distribution in the tube axis direction at the tube end on the reducer exit side. Figure 4 is an explanatory diagram showing the hole shape of the caliber roll used in the 6th and 7th stands. Figure 5 is the graph showing the hole shape of the caliber roll used in the 8th stand. An explanatory diagram showing the hole shape of the caliber roll, and FIG. 6 is a graph showing the test results. 1, 2, 3-8...stand, 1a, 2a, 3
a~8a...Calibur roll, 9...Main pipe, 9
a, 9b, 9c, 9d... uneven thickness portion, 10... core bar.

Claims (1)

[Claims] 1. A raw tube with a cored rod inserted inside is passed between caliber rolls of multiple stands and drawn down alternately from two directions for elongation rolling. In the process of reduction rolling, a final step is taken to reduce the wall thickness of the elongation rolling mill. In order to offset the thickening that occurs at the end of the pipe on the exit side of the reducing rolling mill, the base pipe is Thinning rolling is performed on the end portion so that the wall thickness of each part corresponding to each rolling direction is the same, and the thickness unevenness of the end portion of the mother pipe that occurs due to thinning rolling is performed at the rearmost stand. A pipe rolling control method characterized in that thinning rolling is performed only on intermediate portions of the respective portions in order to eliminate the problem.