JPH0938711A - Method for controlling drawing-rolling of steel pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a pipe stock for finish-rolling having the objective reduced thickness pattern without any thickness deviation in a circu ferential direction at the time of performing a thinning rolling at the both pipe end parts by a final rolling reduction stand and a rolling reduction stand just before the final stand in a mandrel mill. SOLUTION: At the time of controlling a rolling reduction position so as to reduce the thickness at the front and rear end parts of the pipe by the final rolling reduction stand and the rolling reduction stand just before the final stand of the mandrel mill, caliver rolls in these stands are arranged so that a circular-arc center 30 of this caliver groove 29 is matched with the center axis 24A of the mandrel bar 24 after rolling reduction to offset the position of the circular-arc center 30 of the caliver groove 29 onto the minus side in the direction crossed orthogonally to the rolling center 32 to the center axis 24A of the mandrel bar 24.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of producing a seamless steel pipe from a hollow shell through a drawing and rolling machine and a drawing and rolling machine, in which the pipe direction at the tip and rear ends of the drawing and rolling machine is set in the circumferential direction. The present invention relates to a method for controlling the stretching and rolling of a steel pipe, which prevents uneven thickness and suppresses thickening of front and rear ends in a reduction rolling mill.

[0002]

2. Description of the Related Art Generally, a mandrel mill method, a plug mill method, or a Eugene Sejournet method, an Erhart push bench method, or another hot extrusion method is used to manufacture a seamless steel pipe. For small diameter size, there are many mandrel mill rolling methods that are more excellent in productivity and dimensional accuracy.

First, a general seamless steel pipe manufacturing process by the mandrel mill rolling method will be described with reference to FIG.

The manufacturing equipment includes a heating furnace 1, a piercing and rolling mill (Mannesmann piercer) 2, and a drawing and rolling mill (mandrel mill).
3, a reheating furnace 4 and a reduction rolling machine (stretch reducer) 5. A continuous casting machine 6 for hollow tube production may be used instead of the heating furnace 1 and the piercing and rolling machine 2.

In the heating furnace 1, a predetermined temperature (generally 11
The material billet 7 heated to 00 ° C to 1300 ° C is
A hollow shell 7A is obtained by piercing and rolling with a plug 9 between the piercer rolls 8 of the piercing and rolling machine 2.

Next, in the hollow shell 7A, a mandrel bar 10 having a surface coated with a lubricant for hot rolling is inserted in the drawing and rolling mill 3, and a plurality of mandrel mill rolls (hole type rolls) 11 are provided. The thickness of the reducer element pipe 1
It is set to 2.

The reducer element tube 12 is reheated in the reheating furnace 4 and then subjected to drawing rolling in the drawing rolling mill 5 to determine its outer diameter or to reduce the outer diameter to a small diameter.

In this final drawing rolling mill 5, tension between stands is applied to control the metal flow in the longitudinal direction, but tension between stands cannot be applied to the front and rear ends of the reducer element tube 12. Unsteady rolling,
The wall thickness of the front and rear end portions becomes thicker than that of a steady portion (intermediate portion) to which tension between stands is applied.

If the wall thickness at the front and rear ends is out of the wall thickness tolerance required for the product, it is cut as a crop, which is a major cause of lowering the product yield.

As a countermeasure against this, there has been proposed a rolling method in which the thickness of the front and rear end portions of the reducer element tube 12 is thinned in a stretch rolling machine before reduction rolling as compared with a steady portion. .

[0011] For example, in the hydraulic pressure reduction control method for a mandrel mill disclosed in Japanese Patent Publication No. 51-43825, in two specific stands of the mandrel mill,
A hydraulic pressure reduction mechanism that has a quick response compared to the conventional roll reduction device using a motor is provided, and the wall thickness at the rear end of the pipe tip has a pattern that offsets the thickening that occurs in the drawing and rolling mill in the next process. The reduction is controlled to reduce the wall thickness.

However, in this conventional method, in the stand of the mandrel mill, both ends of the pipe are strongly pressed from two directions orthogonal to each other for thinning, and as a result, rolling is performed in the vicinity of the edge of the arc portion of the caliber groove in the caliber roll. There is a problem that the thickened portion becomes thicker and uneven thickness occurs in the circumferential direction.

For example, as shown in FIG. 6, since two final reduction stands (odd number and even number stands) of the mandrel mill are thick finish stands, the mandrel bar loaded in the hollow shell 7A is used. 10 central axis 10A,
The arc center 11B of the arc portion of the roll groove bottom 11A of the caliber roll (mandrel mill roll) 11 is aligned, and the hollow shell 7A is finished to have a uniform thickness.

The above-mentioned Japanese Patent Publication No.
Assembling a hydraulic pressure reducing device as in Japanese Patent Publication No. 43825,
When the pipe end thinning rolling is performed, the caliber roll 11 moves so that the roll gap becomes small. Therefore, as shown in FIG. 6, the arc center 11B of the arc portion of the roll groove bottom 11A with respect to the central axis 10A of the mandrel bar. Roll roll bottom 1
The reference numeral 11B 'is shifted in the direction away from 1A.

Specifically, the wall thickness at the position BO = 45 ° in FIG. 6 is t × (1-cos) as compared with the position at BO = 0 °.
It becomes thicker only by 45 ° (t is the amount of thinning).

For this reason, uneven thickness occurs in which the wall thickness increases within the arc range of the roll groove bottom 11A, particularly in the vicinity of the end of the arc, and if the uneven thickness is large, problems such as rolling errors and product defects may occur. Occurs.

On the other hand, as in the tube rolling control method disclosed in Japanese Examined Patent Publication No. 3-33405, in the last stand, the uneven thickness of the end portion of the raw pipe tube caused by the thinning rolling occurs. In order to solve the problem, there is a method in which the thinning rolling is performed only on the intermediate portion M where the wall thickness becomes large (see FIG. 7).

[0018]

However, this rolling control method for a pipe has a problem that the inner surface shape of the raw pipe may be deteriorated or the uneven thickness may be deteriorated due to the deviation of the rolling timing. Have.

The present invention has been made in view of the above-mentioned problems of the prior art, and is designed so that the inner wall shape of the raw pipe is not deteriorated and the uneven thickness is not increased even if the reduction timing is deviated. An object of the present invention is to provide a method for controlling the stretching and rolling of a steel pipe that prevents uneven thickness in edge thinning rolling.

[0020]

Means for Solving the Problems The present invention, when a hollow shell containing a mandrel bar is stretch-rolled by a continuous stretch-rolling machine composed of a plurality of stands equipped with caliber rolls, In order to suppress the thickening of the pipe tip rear end portion in the post-drawing rolling mill, the front and rear end portions of the pipe are preliminarily formed from the central portion side in the pipe longitudinal direction by the final reduction stand of the draw rolling mill and the stand immediately before it. In the method of controlling the stretching and rolling of the steel pipe to control the roll reduction so that the thickness gradually becomes thin, a hydraulic pressure reduction device is provided in the final reduction stand and the stand immediately before it, thereby performing the roll reduction control that becomes the thin wall, Inside the caliber groove arc so that the arc center of the caliber groove on these stands coincides with the center axis of the mandrel bar after being pressed. Position with respect to the central axis of the mandrel bar, be provided with offset to the negative side of the direction perpendicular to the rolling center, is to achieve the above object.

That is, by preliminarily offsetting the center position of the caliber groove arc in the caliber roll to the minus side, when the caliber roll is pressed down, the arc center substantially coincides with the central axis of the mandrel bar, The occurrence of uneven thickness in the circumferential direction is prevented.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A more specific example of an embodiment of the present invention will be described in detail below with reference to the drawings.

As shown in FIG. 1, an example of this embodiment is a drawing and rolling mill 20 composed of a plurality of caliber rolls of first to eighth stands 20A to 20H, a hollow shell 22 having a mandrel bar. When 24 is charged and stretch-rolled, when the thinning of the front and rear ends of the hollow shell 22 is performed by the sixth stand 20F which is the final reduction stand and the fifth stand 20E which is the reduction stand immediately before that, these 5th and 6th
The hydraulic pressure reduction devices 26E and 26F having excellent responsiveness to the reduction control positions are used for the stands 20E and 20F, respectively,
Also, caliber rolls 28E and 28F of these stands
As shown in FIG. 2, the position of the roll groove arc center 30 of the caliber groove 29 is offset to the negative side in the direction orthogonal to the rolling center 32 with respect to the central axis 24A of the mandrel bar 24. It is arranged at the position to control the rolling position.

The offset amount x is such that the center of arc 30 of the caliber groove 29 is aligned with the center axis of the mandrel bar 24 after reduction.

In the final rolling stand 20F and the rolling stand 20E immediately before the final rolling stand 20F, rolling control is performed by the control device 34 as follows by using hydraulic rolling down devices excellent in response to the rolling position control.

Predicting or empirically grasping the degree of thickening formed at the front and rear ends of the finish pipe finished and rolled by the stretch reducer, and pre- and post-finishing of the finishing rolling pipe to offset the thickened portion. Calculate the thickness distribution in the longitudinal direction of the end portion theoretically or empirically, and use this thickness distribution as a target value so that the front and rear ends are thinner than the center portion in the longitudinal direction of the finishing rolling pipe. The pressing positions of the caliber rolls 28G and 28H are controlled.

Caliber rolls 28F, 2F in the final rolling stand 20F and the rolling stand 20E immediately before it.
8E is designed and set as shown in FIG.

Here, these caliber rolls 28E, 2
The radius R1 'of the 8F caliber groove arc portion is as shown by the equation (1), and as a result, the reduction amount y in the center of the roll groove bottom is as shown by the equation (2). .

R1 ′ = √ {OH ² + (PH−x) ² } (1) y = x + R1′−R1 (2)

Here, OH = R1 × sinBO, PH =
R1 x cosBO, x: Offset center fluctuation amount, y: Decrease in wall thickness reduction amount, P: Point of BO degree on the caliber arc from the roll groove bottom, R1: Conventional caliber groove arc radius, R1 '; Modified caliber groove Arc radius, T: Steady part wall thickness, t: Thinning amount by thin pipe end rolling.

Therefore, as shown in the figure, the caliber rolls 28E, 2E are compared with the conventional caliber rolls.
The roll groove bottom spacing of 8F is widened by 2 × y, and when the roll groove bottom and the mandrel bar 24 are set to have a constant portion wall thickness T, θ degrees from the roll groove bottom (0 ≦
The thickness T ′ at an arbitrary point Q of θ ≦ 45) is smaller than T as shown in the expression (3), and the thickness at the point P is as shown in the expression (4).

T−T ′ = (R1 + y−RB) − [√ {R1 ′ × cos θ + x) ² + (R′1 × sin θ) ² } −RB] = (x + R1 ′) − √ {(R1 ′ × cos θ + x) ^{2 + (R1 '× sinθ)} 2} = (x + R1') - √ thickness at ^{(R1 '2 + 2 × x} × R1' × cosθ + x 2) ≧ 0 point P = (T-y) ... (4)

From this state, when the pipe tip rear end thinning rolling is carried out with a thickness reduction amount tmm, the wall thickness of the caliber groove bottom is T-.
The wall thickness at point t or point P is as shown in equation (5).

(T−y) −t × cos BO (5)

Since the uneven thickness ratio is defined by the expression (6), the uneven thickness ratio when the offset amount is 0 is obtained by the expression (7), whereas according to the present invention, the expression (8) is obtained. And the uneven thickness ratio is reduced.

Uneven thickness ratio (%) = (maximum wall thickness−minimum wall thickness) / (T−t) × 100 (6) {(T−t × cosBO) − (T−t)} / (T− t) * 100 = t * (1-cosBO) / (T-t) * 100 ... (7) [{(Ty) -t * cosBO}-(T-t)] / (T-t) *. 100 = | {t × (1-cosBO) −y} | / (T−t) × 100 (8)

Specifically, for example, a raw tube having a diameter of 175 mm and a wall thickness of 17.5 mm is replaced with a diameter of 158 mm and a wall thickness of 6 mm.
Rolled to a length of 1 m as shown in Fig. 3
As a result of performing thinning rolling in a linear taper shape of 2.5 mm / m over that time, the uneven thickness ratio is as shown in FIG.

When the product allowable range of the uneven thickness ratio is set to be 10% or less, as shown in FIG. 5, in the past, even if the target thinning rolling was performed, if the uneven thickness ratio was large, it was cut off as a crop. On the other hand, according to the present invention, the uneven thickness ratio is small even if the target thinning rolling is performed, and most of the products are within the product allowable range.

[0039]

EFFECTS OF THE INVENTION Since the present invention is configured as described above, when a hollow shell containing a mandrel bar is stretch-rolled by a stretch-rolling machine, both the hollow shell and the final shell stand at the last stand and immediately before the stand. When performing pipe end thinning rolling, it is possible to obtain a finish rolling stock pipe with a target thickness reduction pattern without uneven thickness in the circumferential direction at both pipe ends, thus reducing the crop after finish rolling. The yield can be improved.

[Brief description of drawings]

FIG. 1 is a schematic plan view including a partial block diagram showing a drawing and rolling mill for carrying out the method of the present invention.

FIG. 2 is a cross-sectional view showing a caliber roll for thinning and rolling a pipe tip rear end by the method of the present invention.

FIG. 3 is a cross-sectional view showing a part of a pipe tip rear end that is subjected to pipe tip rear end thinning rolling by the method of the present invention.

FIG. 4 is a diagram showing the uneven thickness ratio of the pipe end portion subjected to the pipe end thinning rolling by the method of the present invention in comparison with the conventional method.

FIG. 5 is a process diagram showing a manufacturing process of a seamless steel pipe.

FIG. 6 is an enlarged cross-sectional view showing a tube end uneven thickness state in the case of performing tube end rear end thinning rolling with a conventional caliber roll.

FIG. 7 is a cross-sectional view showing a pipe end and a caliber roll in the case where the pipe tip rear end rolling is performed so that the pipe end uneven thickness is reduced by a conventional method.

[Explanation of symbols]

 20 ... Stretching rolling machine 20A-20H ... 1st-8th stand 22 ... Hollow element pipe 24 ... Mandrel bar 24A ... Center axis 26E, 26F ... Hydraulic reduction apparatus 28E, 28F ... Caliber roll 29 ... Caliber groove 30 ... Roll groove arc Center 32 ... Rolling center

Claims (1)

[Claims] 1. When a hollow shell having a mandrel bar inserted therein is stretch-rolled by a continuous stretch-rolling machine constituted by a plurality of stands each equipped with a caliber roll, a pipe tip in a post-drawing mill is used. In order to suppress the thickening of the end portion, the roll reduction is performed in advance by the final reduction stand of the drawing rolling machine and the stand immediately before that so that the front and rear ends of the pipe are gradually thinned from the center side in the longitudinal direction of the pipe to the pipe end. In the method of controlling the stretching and rolling of the steel pipe to control, a hydraulic pressure reduction device is provided in the final reduction stand and the stand immediately before the final reduction stand, thereby performing roll reduction control that becomes the thin wall, and the caliber rolls in these stands, The center of the caliber groove arc is aligned with the center of the mandrel bar so that the center of the arc of the caliber groove is aligned with the center axis of the mandrel bar after rolling. A method for controlling stretching and rolling of a steel pipe, which is provided by offsetting it to a minus side in a direction orthogonal to a rolling center with respect to an axis.