JP7180586B2 - Method for manufacturing seamless steel pipe - Google Patents

Description

TECHNICAL FIELD The present invention relates to a seamless steel pipe manufacturing method, and more particularly to a seamless steel pipe manufacturing method capable of manufacturing a seamless steel pipe having excellent outer diameter dimensional accuracy and circularity.

A seamless steel pipe is generally produced by heating a billet to a predetermined temperature in a heating furnace, then piercing-rolling it into a hollow shell, stretching the hollow shell, and further stretching and rolling the hollow shell. Manufactured by diameter rolling. A piercing-rolling mill such as a piercer is used for the piercing-rolling, and an elongating-rolling mill such as a mandrel mill or a plug mill is used for the elongation-rolling. Further, the sizing rolling is a process for obtaining the final finished dimensions of the steel pipe, and is performed using a sizing mill such as a reducer or a sizer (also referred to as a sizing mill).

A sizer is a tandem rolling mill provided with a plurality of stands arranged in series in the line direction, and the number of stands is generally about 5 to 28. Each stand has a plurality of grooved rolls.

For example, FIG. 1 is a schematic diagram showing the structure of a general two-roll stand type sizer 900, FIG. 1(a) is a side view of the line, and FIG. A view from the front (that is, in the direction of the tube axis) is shown.

The sizer 900 includes a plurality of rolling stands 910, and the rolling stands 910 are arranged in series in the conveying direction (line direction) of steel pipes 920, which are the materials to be rolled, indicated by arrows in FIG. 1(a). . Although FIG. 1(a) only shows four stands for convenience, the actual sizer can have more stands.

Each rolling stand 910 has two caliber rolls 930 facing each other. Furthermore, the rotation axes of the grooved rolls 930 of each stand are alternately shifted by 90 degrees as shown in FIG. That is, the stand in which the rotation axis of the grooved roll is arranged vertically and the stand in which the axis of rotation of the grooved roll is arranged horizontally are alternately arranged. FIG. 1 shows a two-roll stand type sizer as an example, but in the case of a three-roll stand type sizer, one stand has three grooved rolls, and each stand rotates the roll rotation axis by 60 degrees. placed staggered.

In sizing rolling, such a sizing mill is used to perform rolling so that the steel pipe has the final finished dimensions. Therefore, since the dimensions and shape of the finally obtained seamless steel pipe are determined by the sizing conditions, various methods have been proposed to improve the roundness of the seamless steel pipe.

For example, in Patent Document 1, in sizing rolling using a 3-roll stand type or 4-roll stand type sizing mill with the number of stands N, depending on the ratio t / D of the thickness t and the outer diameter D of the product, Techniques for controlling the gap of grooved rolls in the N-1th stand have been proposed. According to the technique, it is possible to prevent the occurrence of outer surface scratches and to manufacture a seamless steel pipe having excellent roundness.

Further, in Patent Document 2, when performing sizing rolling on a plurality of mother pipes, based on information such as load and torque when a certain mother pipe is rolled, Techniques for adjusting the pressing position of grooved rolls during rolling have been proposed. According to the technique, it is possible to prevent the occurrence of bite marks and to manufacture seamless steel pipes with excellent roundness.

JP-A-10-337719 Japanese Patent Application Laid-Open No. 2002-102912

However, the present inventors' studies have revealed that the conventional sizing rolling methods as described in Patent Documents 1 and 2 have the following problems.

First, in conventional sizing rolling, the outer diameter of the tube is gradually brought closer to the target outer diameter by reduction at each stand of the sizing mill. Rolling is carried out so that In order to ensure the roundness of the finally obtained seamless steel pipe, the cross-sectional shape of the pipe is adjusted by filling the grooved roll with the steel pipe in the final stand.

The grooved rolls used in sizing rolls are not in a state in which the opposing surfaces of the grooved rolls are in contact with each other, but the roll surfaces are separated by a predetermined distance (gap g) as shown by the arrow g in FIG. 1(b). are separated from each other. The grooved roll is generally designed so that the grooved shape becomes a perfect circle when the gap is set to a predetermined distance, and the gap at this time is called a reference gap.

Therefore, as described above, in the conventional method of making the cross-sectional shape of the pipe a perfect circle by filling the grooved rolls in the final stand with steel pipes, the gap between the grooved rolls in the final stand is controlled to be the reference gap. There is a need to. At this time, if the gap is determined, the outer diameter d of the bottom portion of the grooved roll is inevitably determined, so the final outer diameter of the seamless steel pipe is the outer diameter d of the bottom portion of the grooved roll.

Therefore, in conventional constant-diameter rolling, in order to obtain the desired product outer diameter while ensuring roundness, the bottom portion outer diameter d is set to the desired product outer diameter when the gap is arranged so as to be the reference gap. Therefore, it was necessary to replace the grooved roll of the final stand every time the outer diameter of the seamless steel pipe to be manufactured changed. Therefore, in addition to low productivity, there is a problem that it is necessary to prepare a variety of grooved rolls for each outer diameter of the product.

Even in the conventional sizing rolling method, it is possible to change the outer diameter of the finally obtained seamless steel pipe by changing the gap in the final stand without exchanging grooved rolls. However, in that case, since the operation is performed with a gap that deviates from the standard gap, the shape of the caliber in the final stand deviates from a perfect circle, resulting in a decrease in roundness.

Therefore, in conventional sizing rolling, it was not possible to produce a seamless steel pipe having both excellent outer diameter dimensional accuracy and roundness without using grooved rolls suited to individual products.

In addition, the method proposed in Patent Document 2 is to perform rolling once to acquire information such as load and torque, and to control transition rolling based on this information. However, there is also the problem that it is difficult to obtain sufficient roundness and outer diameter dimensional accuracy.

An object of the present invention is to solve the above-mentioned problems, and to produce a seamless steel pipe having both excellent outer diameter dimensional accuracy and roundness without using grooved rolls suitable for individual products. An object of the present invention is to provide a method for manufacturing a seamless steel pipe.

The inventors have found that the above problem can be solved by an approach that is completely different from the conventional technique of making the cross-sectional shape of the pipe a perfect circle by filling the grooved roll with the steel pipe in the final stand.

An example of an experiment conducted to obtain the above findings will be described.

In this experiment, for comparison, sizing rolling was performed under two conditions, the conventional sizing rolling conditions (comparative example) and the conditions of the present invention (invention example), and the outer diameter of the obtained seamless steel pipe was measured. evaluated.

For the sizing rolling, a sizing mill equipped with eight stands arranged in series was used to sizing the draw-rolled hollow shell having an outer diameter of 337 mm to produce a seamless steel pipe. The target outer diameter (target outer diameter) of the final seamless steel pipe was set to 320 mm.

FIG. 2 is a graph showing the groove circumference in each rolling stand of the sizing mill used. In the figure, a square (□) indicates the outer diameter of the comparative example, and a white circle (○) indicates the outer diameter of the invention example. The comparative example is a conventional constant-diameter rolling condition. It is a condition for rolling so that the outer diameter is Note that the caliber circumference of the fifth stand is larger than that of the fourth stand. This is because Also, the grooved roll in the final stand of the comparative example has a gap that deviates from the standard gap so that the outer diameter of the bottom portion is the target outer diameter of the tube.

On the other hand, in the invention example, unlike the conventional case, the condition was that the caliber circumference at the final stand was larger than the caliber circumference at the seventh stand. In the seventh stand, instead of controlling the outer diameter of the bottom portion, the perimeter of the grooved roll of the grooved roll was controlled to match the target perimeter of the seamless steel pipe. In the case of this experiment, since the target outer diameter is 320 mm, the target outer peripheral length is 320 mm×π, that is, about 1005.3 mm.

FIG. 3 shows the outer diameters of the seamless steel pipes obtained under the above two conditions (the outer diameter at the delivery side of the sizing mill). indicates the diameter. Here, the outer diameter of the bottom portion of the seamless steel pipe refers to the outer diameter of the portion corresponding to the bottom portion of the final stand. Similarly, the outer diameter of the flange portion of the seamless steel pipe refers to the outer diameter of the portion corresponding to the flange portion of the final stand. Also, the average outer diameter refers to the average value of the outer diameter of the bottom portion and the outer diameter of the flange portion.

As can be seen from the results shown in FIG. 3, in the comparative example, the outer diameter of the bottom portion controlled by the final stand is close to the target outer diameter, but the flange portion and average outer diameter are different from the target outer diameter. far off. This is because, in the conventional method, the cross-sectional shape of the pipe is adjusted by filling the grooved rolls with the steel pipe in the final stand, and the pipe expands in the flange direction, which cannot be directly controlled. Therefore, in the conventional constant diameter rolling method, in order to avoid such a decrease in roundness, as described above, the bottom outer diameter d is desired when the gap is arranged to be the reference gap. Therefore, it is necessary to prepare a caliber roll for the final stand for each outer diameter of the seamless steel pipe to be manufactured. On the other hand, in the invention examples, both the outer diameter of the bottom portion and the outer diameter of the flange portion are close to the target outer diameter, and a seamless steel pipe having excellent roundness is obtained.

The present invention is based on the above findings, and the gist thereof is as follows.

1. A billet is pierced and rolled to form a hollow shell,
stretching and rolling the hollow shell;
A method for manufacturing a seamless steel pipe, wherein the stretch-rolled hollow shell is sizing-rolled by a sizing mill having N stands arranged in series to obtain a seamless steel pipe, the method comprising:
Each stand of the sizing mill is equipped with a plurality of grooved rolls,
(a) the groove circumference L _N at the N-th stand of the sizing mill is equal to or greater than the groove circumference L _N-1 at the N-1th stand;
(b) In the N−1th stand of the sizing mill, the N−1th control the grooved roll of the second stand,
(c) In the N-th stand of the sizing mill, the caliber of the N-th stand so that ΔD defined by the following formula (2) is −0.20 or more and 0.20 or less A method for manufacturing seamless steel pipes by controlling rolls.
ΔL = (L _N-1 -C)/C x 100 (%) (1)
ΔD=( _DN -D)/D×100(%) (2)
[here,
L _N-1 : Perimeter length (mm) of the grooved roll of the N-1th stand of the sizing mill,
C: Target peripheral length of the seamless steel pipe (mm)
D _N : Outer diameter (mm) of the bottom portion of the grooved roll of the N-th stand of the sizing mill,
D: Target outer diameter (mm) of the seamless steel pipe]

According to the present invention, it is possible to manufacture a seamless steel pipe having both excellent outer diameter dimensional accuracy and roundness without using different grooved rolls according to the outer diameter of the product.

FIG. 2 is a schematic diagram showing the structure of a general two-roll stand type sizer. 4 is a graph showing the groove circumference in each rolling stand of the sizing mill used in the experiment. 3 is a diagram showing the outer diameter of a seamless steel pipe obtained by sizing rolling under the conditions shown in FIG. 2. FIG. FIG. 4 is a schematic diagram showing a calculation method of the groove circumference in a single R type grooved roll. It is a schematic diagram which shows the calculation method of the grooved circumference length in a double-R type grooved roll.

Embodiments of the present invention will be described below. The following description shows a preferred embodiment of the present invention, and the present invention is not limited by the following description.

In the present invention, a billet is pierced and rolled to form a hollow shell, the hollow shell is stretch-rolled, and the stretch-rolled hollow shell is sizing-rolled into a seamless steel pipe. The billet is not particularly limited, and any billet made of steel can be used. Moreover, the piercing-rolling and elongation-rolling are not particularly limited and can be performed under arbitrary conditions. After the elongation-rolling and before the shaping-rolling, the hollow shell can optionally be rolled. A reeler can be used for the rolling tube.

The sizing rolling is performed using a sizing mill having N stands arranged in series. The number of stands N of the sizing mill is not particularly limited and can be any number, but in general, it is preferably 5 to 28, more preferably 6 to 12. .

Each stand of the sizing mill comprises a plurality of grooved rolls. Although the number of grooved rolls per stand is not particularly limited, it is generally preferable to set it to 2 or 3. Moreover, the present invention can be applied not only when the caliber of the grooved roll is formed of a single arc, but also when the caliber of the grooved roll is formed by connecting a plurality of arcs.

(a) _LN ≧LN _-1
In the present invention, it is important that the groove circumference L _N at the N-th stand of the sizing mill is greater than or equal to the groove circumference L _N-1 at the (N−1)-th stand. In conventional sizing rolling, as described above, steel pipes are filled in grooved rolls in the final stand to make the cross-sectional shape of the pipes perfectly circular. Therefore, the groove circumference L _N at the Nth stand is set to be less than the groove circumference L N-1 at the (N− ₁ )th stand (L _N <L _N-1 ). On the other hand, in the present invention, since the control of the outer circumference of the steel pipe is performed at the (N-1)th stand, there is no need to fill the grooved roll with the steel pipe at the final stand. Therefore, L _N is set to be equal to or greater than L _N-1 .

In the present invention, the groove circumference refers to the circumference of the groove in the groove rolls set up in the gap during actual rolling.

(b) ΔL: -0.24 to 0.24%
Also, in the N-1th stand of the sizing mill, the N-1th to control the caliber roll of the stand.
ΔL = (L _N-1 -C)/C x 100 (%) (1)
here,
L _N-1 : Perimeter length (mm) of the grooved roll of the N-1th stand of the sizing mill,
C: Target peripheral length of seamless steel pipe (mm)

In the present invention, the grooved rolls of the (N−1)th stand are controlled with the intention of making the outer peripheral length of the steel pipe the target for the (N−1)th stand. For this purpose, it is desirable to set the groove circumference L _N-1 of the groove roll of the N-1th stand to the target circumference C of the seamless steel pipe, but in actual control, a certain width is required. Permissible. Therefore, specifically, the value of ΔL defined by the above formula (1) is set to -0.24 to 0.24%. If ΔL is out of the above range, at least one of outer diameter dimensional accuracy and circularity of the finally obtained seamless steel pipe will be insufficient.

The target outer circumference C refers to the outer circumference of the finally manufactured seamless steel pipe. For example, when the target outer diameter D is determined, Dπ can be set as the target outer diameter.

Moreover, the grooved circumference L of the grooved roll can be determined according to a conventional method. For example, in the case of a single R tarp grooved roll having a single caliber curvature, the grooved roll circumferential length L of the grooved roll can be obtained from the following equation.

Here, the definition of each symbol in the formula is as follows (see FIG. 4).
L: Perimeter length of groove R: Radius of curvature of groove S: Offset

The offset refers to the offset of the pass center _CP , which is the actual center of the groove, with respect to the center of curvature C of the groove. As shown in FIG. Defined as taking a positive value. θ is an angle defined by the following formula.

Further, in the case of a double-R type grooved roll having two caliber curvatures, the grooved roll circumferential length L of the grooved roll can be obtained from the following equation.

Here, the definition of each symbol in the formula is as follows (see FIG. 5).
L: Perimeter of the groove R1: First radius of curvature of the groove R2: Second radius of curvature of the groove S1: Offset of R1 with respect to path center _CP S2: Offset of R2 with respect to path center _CP

Also, θ1 and θ2 are angles defined by the following equations.

Therefore, the groove circumference L _N-1 of the grooved roll of the N-1th stand of the sizing mill is calculated from the curvature radius and the offset value of the grooved roll of the N-1th stand. be able to.

(c) ΔD: -0.20 to 0.20%
Furthermore, in the Nth stand, the grooved roll of the Nth stand is controlled so that ΔD defined by the following formula (2) is −0.20 or more and 0.20 or less,
ΔD=( _DN -D)/D×100(%) (2)
here,
D _N : Outer diameter (mm) of the bottom portion of the grooved roll of the N-th stand of the sizing mill,
D: Target outer diameter of the seamless steel pipe (mm)

The “bottom part” is the bottom part of the caliber (groove) provided on the outer peripheral surface of the grooved roll, as shown by point B in FIGS. is the point. The "bottom part" can also be defined as a point where the depth of the caliber is maximum in the direction of the rotational axis of the grooved roll and minimum in the direction of the tube axis (pass line direction). The term "outer diameter of the bottom portion" refers to the diameter of the caliber at the bottom when the caliber rolls are set up in the gap during actual rolling. As shown in FIGS. 4 and 5, the bottom portion B is normally positioned at the center in the width direction of the caliber roll, and the outer diameter of the bottom portion It is the distance between the bottoms of the roll.

In the present invention, the caliber roll of the Nth stand is controlled with the intention of making the outer diameter of the steel pipe the target outer diameter at the Nth stand. For this purpose, it is desirable to set the bottom outer diameter D _N of the grooved roll of the Nth stand to the target outer diameter D of the seamless steel pipe, but a certain width is allowed in actual control. Therefore, specifically, the value of ΔD defined by the above formula (2) is set to −0.20 to 0.20%. If ΔD is out of the range, at least one of outer diameter dimensional accuracy and circularity of the finally obtained seamless steel pipe becomes insufficient.

The method for manufacturing a seamless steel pipe according to the present invention can be used to manufacture seamless steel pipes of any size without particular limitation. However, the rigidity of the pipe is correlated with the ratio (t/D) of the wall thickness t to the outer diameter D of the pipe. If t/D is 1.9% or more, the rigidity is improved, and as a result, it is possible to prevent the inner and outer surfaces of the pipe from being damaged due to buckling during sizing rolling. Therefore, from the viewpoint of scratch prevention, t/D is preferably 1.9% or more, more preferably 2.1% or more, and even more preferably 2.5% or more. On the other hand, if t/D is 25% or less, it is possible to prevent sizing defects caused by excessive rigidity, and as a result, it is possible to control the outer peripheral length more reliably. Therefore, t/D is preferably 25% or less, more preferably 24% or less, and even more preferably 21% or less. In the present invention, the target outer diameter may be used as D for obtaining t/D.

EXAMPLES Next, the present invention will be described more specifically based on examples. However, the present invention is not limited by the following examples, and can be modified as appropriate within the scope of the gist of the present invention, and any of these are included in the technical scope of the present invention. be

The billet was pierced and rolled by a piercer to form a hollow shell, and then the hollow shell was stretch-rolled by an elongator and a plug mill, and rolled by a reeler. After that, the hollow shell was rolled by a sizer to produce a seamless steel pipe.

For the sizing rolling, a two-roll stand type sizing mill having two grooved rolls in each stand was used. Table 1 shows the outer diameter and wall thickness of the hollow shell subjected to sizing rolling (that is, the hollow shell after stretching and rolling) and the sizing conditions. As the sizing conditions, the target outer diameter D, wall thickness, ΔL, and ΔD are described. Since the wall thickness of the pipe does not change during sizing rolling with a sizer, the wall thickness of the seamless steel pipe after sizing is equal to the wall thickness of the hollow shell used. The dimensions of the seamless steel pipe to be manufactured ranged from t/D: 2.4%, which is relatively easy to process, to t/D: 22%, which is relatively difficult to process. The rolling temperature (the temperature at the outer surface of the hollow shell to be subjected to sizing rolling) was uniformly set at 900° C. in order to prevent the motor of the rolling mill from tripping.

Next, the outer diameter dimensional accuracy and circularity of the obtained seamless steel pipes were evaluated by the following procedures. First, the maximum outer diameter _Dmax (mm) and the minimum outer diameter _Dmin (mm) of the finally obtained seamless steel pipe were measured on the delivery side of the sizing mill, and An average outer diameter D _ave was obtained. Using the obtained average outer diameter D _ave (mm) and the target outer diameter D (mm), the dimensional accuracy of the outer diameter was determined by the following equation (4). In addition, the roundness was determined by the following formula (5).
D _ave = (D _max + D _min )/2 (3)
Outer diameter dimensional accuracy (%) = (D _ave - D) / D x 100 (4)
Roundness (%) = ( _Dmax - _Dmin )/ _Dave x 100 (5)

Table 1 also shows the evaluation results of outer diameter dimensional accuracy and roundness. As can be seen from the results shown in Table 1, in the examples that meet the requirements of the present invention, seamless rolls with both excellent outer diameter dimensional accuracy and roundness can be obtained without properly using grooved rolls according to the outer diameter of the product. A steel pipe was obtained. In particular,
According to the manufacturing method of the present invention, a seamless steel pipe having an outer diameter dimensional accuracy of 0.2% or less and a roundness of 0.5% or less could be manufactured. On the other hand, in the comparative examples that did not satisfy the conditions of the present invention, one or both of the outer diameter dimensional accuracy and the roundness were inferior.

900 sizing mill 910 stand 920 steel pipe 930 grooved roll

Claims (1)

A billet is pierced and rolled to form a hollow shell,
stretching and rolling the hollow shell;
A method for manufacturing a seamless steel pipe, wherein the stretch-rolled hollow shell is sizing-rolled by a sizing mill having N stands arranged in series to obtain a seamless steel pipe, the method comprising:
Each stand of the sizing mill is equipped with a plurality of grooved rolls,
(a) the groove circumference L _N at the N-th stand of the sizing mill is equal to or greater than the groove circumference L _N-1 at the N-1th stand;
(b) In the N−1th stand of the sizing mill, the N−1th control the grooved roll of the second stand,
(c) In the N-th stand of the sizing mill, the caliber of the N-th stand so that ΔD defined by the following formula (2) is −0.20 or more and 0.20 or less A method for manufacturing seamless steel pipes by controlling rolls.
ΔL = (L _N-1 -C)/C x 100 (%) (1)
ΔD=( _DN -D)/D×100(%) (2)
[here,
L _N-1 : Perimeter length (mm) of the grooved roll of the N-1th stand of the sizing mill,
C: Target peripheral length of the seamless steel pipe (mm)
D _N : Outer diameter (mm) of the bottom portion of the grooved roll of the N-th stand of the sizing mill,
D: Target outer diameter (mm) of the seamless steel pipe]

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