JPH05237532A - Thickness control method of stretch reducer - Google Patents

Abstract

PURPOSE:To control the thickness in the stretch reducing operation with high accuracy. CONSTITUTION:When the stretch reduction of a tube stoch 122 is executed by a continuous multiple reducer, the physically applicable maximums tensile factor is obtained for the respective stands based on the outer diameter and the thickness of the tube before' and after the stretch reduction in the respective stands and the target outer diameter and the target thickness of the finished tube. The actual tensile factor is obtained which is practically applied based on the outer diameter and the thickness of the tube before and after the stretch reduction for the respective stands when the first tube is stretch-reduced, the roll speed, and the actual outer diameter and the actual thickness of the finished tube. For the stands where the actual tensile factor exceeds the maximum tensile factor, the actual tensile factor is corrected so as not to exceed the maximum tensile factor, and the actual tensile factor for other respective stands are obtained and set so as to obtain the finished tube of the target thickness based on the corrected actual tensile factor.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to reduction rolling of steel pipes and the like, and more particularly to reduction rolling of steel pipes and the like manufactured in a drawing and rolling process of the preceding process to obtain an outer diameter and wall thickness of a target dimension. The present invention relates to a method for controlling the thickness of a reduction rolling mill, which is suitable for application when a finished tube is manufactured by the reduction rolling mill.

[0002]

2. Description of the Related Art A method for manufacturing a seamless steel pipe, which is one of the seamless pipes, will be briefly described with reference to FIG. 7 showing a typical example thereof.

First, a raw material 10 (for example, round steel) is prepared. The material 10 is heated to a required temperature by a heating furnace 12, then sent to a piercing and rolling machine 14 (for example, Mannesmann Piercer), pierced and rolled, and processed into a hollow rod-shaped material 16.

Since the hollow rod-shaped material 16 thus formed by piercing and rolling is still thick, a mandrel bar 18 is inserted through it in order to reduce the wall thickness by the next drawing and rolling machine 20 (for example, a mandrel mill). The raw material is stretch-rolled in the state and processed into the raw pipe 22 to be supplied to the following reduction rolling machine 26.

The raw tube 22 formed by drawing and rolling
Is heated again to the required temperature by the heating furnace 24, and then sent to the squeeze rolling machine 26 (for example, a stretch reducer) and squeezed and rolled to the required size to form the finished pipe 28.

The finished pipe 28 formed by the above-described reduction rolling machine 26 has a wall thickness distribution having a steady portion at the center in the longitudinal direction as shown in FIG. 8, for example. Rolling is performed so as to obtain the target wall thickness.

As a reduction rolling mill used for the above reduction rolling, a stretch reducer usually composed of 8 to 28 stands is most widely adopted.

In this stretch reducer, generally,
While applying an outer diameter reduction of 5 to 7% per stand, tension between stands is applied to squeeze and roll the raw pipe 22, thereby making the outer diameter of the product (finished pipe 28) constant and at the same time.
We are adjusting the wall thickness.

In this stretch reducer, the rolls of each stand are often rotated independently by, for example, a DC motor so that the roll rotation speed of each stand can be freely and accurately converted. ..

In the stretch reducer, the tension between the stands can be adjusted by giving the number of rotations of the roll in each stand, so that finished pipes of various wall thicknesses can be manufactured from the raw pipe of the same size. have. This characteristic is qualitatively shown in FIG.

As can be seen from FIG. 9, the gradient of the roll rotational speed from No. 1 to the final No. n stand is abruptly changed as shown in (c) → (b) → (a) in the figure. For example, the tension between the stands becomes stronger and the product thickness becomes thinner. Conversely, if the gradient is loosened as shown in (a) → (b) → (c) in the figure, the tension between the stands will decrease. It is known that the product thickness increases.

In other words, in general, it means that the wall thickness of the rolled finished pipe can be controlled by using the tensile coefficient, and the wall thickness of the rolled finished tube is increased by increasing the tension between the stands and increasing the value of this tensile coefficient. It can be made thin, and conversely, it can be made thicker by weakening the tension between stands and decreasing the value of this tensile coefficient.

Now, assuming that the tensile coefficient is Z, this tensile coefficient Z can be theoretically obtained by the following equation (1).
Here, σ1 is the stress in the pipe longitudinal direction, and Kf is the yield stress.

Z = σ1 / Kf (1)

On the other hand, in the stretch reducer, since no tool is used for the inner surface machining, the wall thickness of the rolled finished pipe is greatly dependent on the roll rotational speed gradient, that is, the roll rotational speed deviation between the stands, as described above. have.
This means that the wall thickness of the rolled finished pipe is greatly affected by the friction between the roll surface and the surface of the raw pipe, that is, the product thickness varies greatly depending on the surface condition of the roll or the surface temperature of the raw pipe. ing.

In the stretch reducer, the roll rotation speed of each stand is adjusted to obtain the target wall thickness for the rolled finished pipe, but the wall thickness control range is naturally limited. For example, as shown in FIG. 9, the wall thickness of the rolled finished pipe becomes smaller as the rotation speed deviation becomes larger, but there is a limit regardless of the type of mill, and the wall thickness cannot be made infinitely.

Therefore, the conventional wall thickness control method using a stretch reducer using a stretch reducer has a problem that it is very difficult to obtain a target wall thickness for a steady part of a rolled finished pipe due to the above-mentioned characteristics of the stretch reducer. is there.

Techniques for solving this problem include, for example, Japanese Patent Publication No. 44-24743 or Japanese Patent Publication No.
Japanese Patent Application Laid-Open No. 1-44369 discloses a method in which the wall thickness or length of a pipe before and after drawing and rolling is measured, and the roll speed is adjusted so that the wall thickness of the next material becomes a target wall thickness. According to this technique, it is possible to bring the second and subsequent element pipes of the lot to be drawn and rolled close to the target wall thickness while referring to the rolled state of the first lot of the same lot.

[0019]

However, as described above, the control range of the wall thickness is also limited in the second element pipe, and the problem that the target wall thickness is not always achieved is still left. It remains.

The present invention has been made to solve the above-mentioned problems of the conventional rolling mill, and is capable of controlling the rolling mill so that the thickness of the finished pipe can be accurately set to the target thickness. An object is to provide a method for controlling wall thickness.

[0021]

SUMMARY OF THE INVENTION The present invention, as shown in FIG. 1 in its outline, is carried out in a continuous multi-stage rolling mill by squeezing and rolling a raw pipe. And, the maximum tensile coefficient that can be physically added based on the target outer diameter and target wall thickness of the finished pipe is obtained for each stand, and the outer diameter and wall thickness of the pipe before and after drawing and rolling in each stand at the time of the previous drawing and rolling, The actual tension coefficient actually added is calculated for each stand based on the roll rotation speed and the actual outer diameter and actual wall thickness of the finished pipe, and for the stands where the actual tensile coefficient exceeds the maximum tensile coefficient, The above-mentioned problem is solved by correcting the tensile coefficient to be equal to or less than the maximum tensile coefficient, and determining and setting the actual tensile coefficient for each of the other stands based on the corrected actual tensile coefficient.

[0022]

As a result of a detailed study on the control of the wall thickness of the pipe by the reduction rolling machine, the present inventor found that the accuracy of the wall thickness of the finished pipe is closely related to the maximum tensile coefficient that can be physically added to the stand during rolling. We found that there is a relationship.

That is, in the conventional rolling wall thickness control method, the tensile coefficient actually added to the stand can be physically added when changing the roll speed in order to match the finished pipe with the target dimension. If the material has a wall thickness accuracy defect that is approximately equal to or larger than the maximum tensile coefficient, and the difference between the actually added tensile coefficient and the physically addable maximum tensile coefficient is large. There was a large tendency for materials with poor wall thickness accuracy to occur.

Conventionally, the wall thickness or length of the last rolled finished pipe is measured, and the number of rotations of the roll motor is changed based on the measurement result so that the next finished pipe has the target wall thickness. However, at that time, the tensile coefficient is set beyond the range of the maximum tensile coefficient that can be physically added, or in the vicinity of that range. As a result, due to the influence of disturbance (roll surface condition or raw pipe surface temperature, etc.), tension is not applied to the raw pipe from the rolling roll, and slippage may occur, resulting in a very unstable rolling state, It is conceivable that the poor thickness accuracy is caused by such an unstable rolling state.

The present invention has been made based on the above findings. In the present invention, the calculated maximum tensile coefficient that can be physically added and the actual tensile coefficient that is actually added based on the previous rolling results. For each stand, the actual tension coefficient obtained is larger than the maximum tension coefficient. For the stands, after correcting the actual tension coefficient to be less than or equal to the maximum tension coefficient, the finished pipe of the target dimension is obtained for each stand. Since the actual tensile coefficient is set again, it is possible to perform the rolling operation under a stable rolling condition.

Further, since the roll rotation speed of each stand can always be set back to the tension coefficient, the control effect is good, and the wall thickness can be controlled by slightly changing the roll rotation speed. It is possible to greatly improve the wall thickness accuracy of the finished pipe.

FIG. 2 shows an example of the results obtained when the present invention is applied to draw rolling.

In FIG. 2, the diameter is 89.1 mm and the wall thickness is 4.0.
By applying the method of the present invention to a mm tube, the diameter is 21.7 mm and the wall thickness is 3.2 mm (left in the figure), and the diameter is 38.1 mm and the wall thickness is 3.7.
It is the graph which showed the average ratio of the finished wall thickness with respect to the target wall thickness when rolling the finished pipe of mm (the right in the figure) with the result by the conventional method.

From FIG. 2, it is understood that according to the present invention, a finished pipe having extremely excellent wall thickness accuracy is obtained.

The present invention will be described in more detail.

The maximum physically addable tensile modulus can be determined by the method disclosed in "Iron and Steel", No. 51 (1965), p. 928 et seq. In this method, since the simple formula is used during the calculation and the roll caliber is assumed to be a perfect circle, the maximum tensile coefficient calculated is an approximate one, but the maximum tensile coefficient calculated by experiment is It agrees within the error range compared with the value of. The method of calculating the maximum tensile coefficient will be briefly described below.

FIG. 3 shows an example of a rolling state in the rolling stand of the reduction rolling mill, and reference numeral 30 is a rolling roll.

The balance of forces in the stand is as shown by the arrows in FIG. 3, and the neutral line where the forces are balanced between the material and the roll takes a complicated trajectory unlike in the case of plate rolling. This is because the peripheral speed of the roll changes in the roll axial direction. Sb is the area of the area where the backward slip occurs on the rolling-in side (the hatched portion in FIG. 3), and Sf is the area of the area where the forward slip on the exit side.
The force balance in the rolling direction of the tube is macroscopically given by the following equation (2).

Fb + Pr = μPc (Sb-Sf) / S + Ff (2) where Ff and Fb: front tension, rear tension (kg) Pc: roll surface pressure (for three rolls) (kg) Pr : Rolling surface pressure reaction force in rolling direction (kg) S: Total contact area between roll and tube (mm ² ) μ: Coefficient of friction between roll and material

In the above equation (2), (Sb-Sf) /
If S = γ (contact area ratio), this is the roll surface pressure Pc
, And the rolling direction reaction force Pr can be approximately calculated and substituted to obtain the following equation (3).

[0036] γ = [{(D0-t0 ) Zb - (D1-t1) Zf} ÷ {2 (1-Z) (D0-D1) 0.5} + (3/4) (D0 2 -D1 2) ÷ {(D0 + 2D1) (D0-D1) ^0.5 }] ÷ (μ · R ^0.5 ) ... (3) where D0, D1: stand outside diameter and outlet outside diameter (m
m) t0, t1: Stand stand-in side wall thickness and stand-out wall thickness (mm) R: Roll groove bottom radius (mm) Zf: Front tension / yield stress (hereinafter referred to as front tension coefficient) Zb: Backward tension / yield stress (Hereinafter, referred to as backward tensile coefficient)

In the above equation (3), the tensile coefficient Z is the average of the front tensile coefficient Zf and the rear tensile coefficient Zb, that is, (Zf
+ Zb) / 2.

Also, the outer diameter D1 on the outlet side of each stand is
The reduced tube ratio {(D0-D1) / D0} × 100 (%) can be applied to the entrance-side outer diameter D0 to be determined.

Further, the deformation of the pipe, that is, the outlet wall thickness t 1 at each stand is described in “Recent Progress of Steel Pipe Manufacturing Technology in Japan” (July 1974) edited and issued by the Japan Iron and Steel Institute, for example. It can be calculated using Neumann Hancke's theory of tube deformation during draw rolling described in.

That is, the following stress-strain relational expression (4),
The outlet wall thickness t 1 can be expressed by the following equation (7) from the tresca yield condition equation (5), the thin-walled pipe balance equation (6), and the like.

(Σr−σm) / εr = (σθ−δm) / εθ = (σl−δm) / εl (4) σl−σθ = Kf (5) σr = (t / D) σθ (6) )

Here, σr, σθ, and σl are the average stresses (kg / mm ² ) in the radius, circumference, and longitudinal directions of the pipe, and εr
, Εθ, εl are the average strains in each direction, and σm
Is the average of the average stress in each direction (σr + σθ + σl / 3)
Is. Kf is the yield stress of the material, t is the wall thickness of the pipe, and D is
Is the outer diameter of the tube.

T 1 = t 0 / exp (εr) (7)

Further, the average radial strain εr in the equation (7) is given by the following equation (8), and the radial radial strain εθ in the equation (8) is given by the following equation (9). Be done. However, C1 is the ratio of the inlet side wall thickness t0 to the inlet side outer diameter D0 (t0 /
D0), ln {} are natural logarithms.

Εr = [{2Z (C1-1) + (1-2C1)} / Z (1-C1)-(2-C1)] · εθ (8) εθ = ln {D0-t0) / (D1-t0)} (9)

Therefore, the outlet wall thickness t 1 at each stand
Can be calculated by applying the above equations (8) and (9) to the above equation (7).

As described above, the outlet side outer diameter D1 and the outlet side wall thickness t1
Then, the maximum tensile coefficient Zmax of each stand can be obtained as follows based on the equation (3).

In order to obtain the maximum tensile coefficient, it is necessary to set the tensile coefficient = 0 on the entrance side of the first stand and the exit side of the final stand, and simultaneously solve the equation (3) for the stands in the opposite direction from the stand on both ends. ..

Since the front stand has a full front slide, the tensile coefficients Zf and Zb are calculated so that the contact area ratio γ converges to -1.

First, in the first stand, the backward tension coefficient Z
The forward tension coefficient Zf is determined so that the contact area ratio γ converges to -1 with b = 0.

Next, in the second stand, the front tension coefficient Zb is determined to be equal to the front tension coefficient Zf of the first stand, and the front tension coefficient Zf is determined so that the contact area ratio γ converges to -1. Similarly, the tensile coefficients Zf and Zb after the next stand are determined.

On the other hand, in the latter stage stand, since the rear surface slides all over, the tensile coefficients Zf and Zb are calculated so that the contact area ratio γ converges to 1.

First, in the final stand, the forward tension coefficient Z
The backward tension coefficient Zb is determined so that the contact area ratio γ converges to 1 with f = 0.

Next, in the (final-1) th stand, the rear tensile coefficient Zf is determined so that the contact area ratio γ converges to 1 on the assumption that the front tensile coefficient Zf is equal to the rear tensile coefficient Zb of the final stand. Similarly, the tension coefficients Zf and Zb of the immediately preceding stand are sequentially determined.

In this way, the tensile coefficients Zf and Zb for each stand are determined. At that time, in a stand where the tensile coefficient Zf obtained as the entire front sliding and the tensile coefficient Zf obtained as the entire rear sliding intersect, the tensile coefficient due to the entire front sliding and the tensile coefficient due to the entire rear sliding are compared,
The one with the smaller tensile coefficient is adopted. It should be noted that, in this crossing stand, naturally, it is necessary to repeatedly calculate so that the wall thickness of the raw pipe obtained from the full front sliding and the wall thickness of the raw pipe obtained from the full rear sliding match.

As described above, the average value of the front tensile coefficient and the rear tensile coefficient in each stand: (Zf + Zb) / 2
As a result, the maximum tensile coefficient Zmax of each can be determined.

Next, the actual tensile coefficient will be described.

The actual tensile coefficient is the tensile coefficient actually added at the time of the previous rolling (for example, the first rolling).
In the present invention, the actual tensile coefficient at the previous rolling is calculated in order to set the tensile coefficient after the next rolling (for example, the second rolling) to an appropriate value. The calculation of the actual tension coefficient is performed based on the previous rolling results as shown in the flowchart of FIG. 4 described later, but the outline of the calculation is as follows:
The front tension of each stand is determined so that the volume velocity (volume x material velocity) on the stand entry side becomes equal to the volume velocity on the final stand exit side. The tension coefficient of each stand is finally calculated as (front tension + rear tension) / 2. However, the backward tension of the first stand is set to 0 because nothing is working, and similarly, the forward tension of the final stand is also set to 0.

[0059]

Embodiments of the present invention will now be described in detail with reference to the drawings.

FIGS. 4 and 5 are flow charts showing a wall thickness control method according to an embodiment of the present invention.

In this example, the number of roll revolutions (number of roll revolutions during squeeze rolling) in each stand, which is the actual result of the first rolling, the outer diameter and wall thickness of the pipe before and after squeeze rolling, and the outer diameter and wall thickness of the finished pipe. The thickness is used to find the actual tensile coefficient actually applied. The outer diameter of the rolled pipe in each stand is calculated by using the contraction rate, and the wall thickness is similarly calculated by using the formulas (7) to (9).

First, for each stand, the number of rotations of the first roll, the outer diameter and wall thickness of the pipe before and after the reduction rolling, the shrinkage ratio, etc. are set as initial values (step 1).

Next, the front tension coefficient Zf of the first stand
(1) is assumed to be a predetermined value and its rearward tension coefficient Z
The tensile coefficient Z (1) is obtained for the first stand with b (1) set to 0 (step 2). The tension coefficient Z (1) of this first stand is given by {Zb (1) + Zf (1)} / 2.

In step 3, if Zf (1) is 1.0 or more, it cannot be rolled by definition (there is a risk that the raw pipe may be broken during rolling). To determine. When Zf (1) is less than 1.0, the above-mentioned tensile constant Z (1) is used to calculate the deformation of the tube in the first stand in accordance with the above equations (7) to (9), and in the first stand, Obtain the outlet wall thickness (step 4).

From the number of rotations of the first rolling roll, the first
The exit side material velocity VI (1) at the stand is determined (step 5).

From the delivery side material velocity VI (1) and the delivery side wall thickness, the delivery side mass flow (volume velocity) in the first stand
Ask for.

In order to check whether or not the mass flow is constant on the entrance side and the exit side, the exit side mass flow of the first stand is obtained in advance (step 6).

Thereafter, the forward tension coefficient Zf (2) is assumed for the second stand (step 7).

If the forward tension coefficient Zf (2) is 1.0 or more in step 8, Zf (1) is corrected and step 2
Return to (step 9).

When Zf (2) is less than 1.0, the delivery side wall thickness and delivery side material speed VI (2) in the second stand are obtained in the same manner as in the first stand, and the second stand delivery is performed. The mass flow on the side is obtained (steps 10 to 12).

Next, in step 13, the mass flow of the first stand exit side (the second stand entrance side) and the mass flow of the second stand exit side are compared, and the front tension coefficient Zf (2) is set so that the two match. After correction (step 14), the front tension coefficient Zf (2) of the second stand is repeatedly calculated.

When the mass flows on the inlet side and the outlet side of the second stand match in step 13, the forward tension coefficient Zf is reached up to the (final -1) stand according to the same procedure as described above.
(II-1) and backward tensile coefficient Zb (II-1) are calculated (step 15).

Further, for the last stand, the tension coefficient Z (II) is calculated by setting the forward tension constant Zf (II) = 0 (step 16) and the tension coefficient Z (II) is used to determine the case of other stands. The delivery side wall thickness and delivery side material velocity VI (II) are obtained in accordance with the same procedure as above, and the delivery side mass flow is obtained from both of them (steps 17 to 19).

At step 20, it is judged whether or not the inlet side mass flow of the final stand and the outlet side mass flow match, and the outlet side plate thickness matches the wall thickness after rolling. The front tension coefficient Zf (1) of the stand is corrected, and the process returns to step 2 to make another assumption of the front tension coefficient (step 21).

By repeating the above series of operations, the calculated value of the actual tension coefficient actually added to each stand is obtained.

After that, the maximum tensile coefficient that can be physically added is calculated for each stand by the method using the above-mentioned equation (3) (step 22), and the maximum tensile coefficient and the actual tensile coefficient are added. Compared with the calculated value of the actual tensile coefficient, the actual tensile coefficient of the stand for which the actual tensile coefficient is larger than the maximum tensile coefficient is lowered to the maximum tensile coefficient or less, and the actual tensile coefficient of each stand is set as the target. Reset to obtain the wall thickness (step 23).
After resetting the actual tensile factors for all stands, drawing rolling is performed.

Table 1 shows an example of the result when the thickness control method according to the present embodiment described in detail above is actually applied to perform reduction rolling.

This specific example is an example in which a blank tube of a predetermined size is rolled into a finished tube of a target size by using a drawing and rolling mill consisting of 19 stands. The blank tube used before rolling has an outer diameter of 89 mm. The finished pipe has a target dimension of an outer diameter of 34.0 mm and a wall thickness of 5.0 mm. In addition, the number of raw pipes used was 800, the temperature of the raw pipe on the first stand entry side was set to 920 to 930 ° C, and the material (raw pipe) speed on the first stand entry side was set to 1.94 m / sec. Applied the example.

The wall thickness difference shown in Table 1 above is the average value of the difference between the measured wall thickness and the target wall thickness of the finished pipe obtained by rolling, and the defect rate is 5 mm for the finished pipe. This is the ratio of the measured wall thickness of the cut end when cut to 0.5 m that did not fall within ± 4% of the target wall thickness. For comparison, Table 1 also shows the results of the conventional method in which drawing rolling was performed under the same conditions as in this example except that the actual tensile modulus was not taken into consideration.

FIG. 3 is an actual tensile coefficient for each stand when this embodiment was carried out (graph A shown by a black circle in the figure).
Is shown in comparison with the maximum tensile coefficient that can be physically added (graph B shown as a white square in the figure), and the actual tensile coefficient (black square It is the diagram which also showed the graph C) shown by.

The above graph C corresponds to the conventional example shown in Table 1, and the maximum tensile coefficient is exceeded up to the fourth stand in the preceding stage.

Table 2 shows the shrinkage ratio and the rotation speed of each stand used in this embodiment.

[0083]

[Table 1]

[0084]

[Table 2]

As shown in Table 1 above, the average value of the difference between the target wall thickness and the measured wall thickness is -0.02 mm according to the method of the present invention.
On the other hand, according to the conventional method, it is +0.05 mm,
Also, the defect rate was 0.02% according to the present invention, while it was 0.08% according to the conventional method.

Therefore, it can be seen that a finished pipe having an extremely high wall thickness precision can be obtained by performing the reduction rolling by adopting the present invention.

[0087]

As described above, according to the present invention,
It is possible to control the wall thickness of the finished pipe manufactured by drawing and rolling to the target wall thickness with extremely high accuracy.

Therefore, the present invention is extremely effective when applied to the thickness control of the reduction rolling mill in the reduction rolling process, and has an excellent effect that it is industrially highly useful.

[Brief description of drawings]

FIG. 1 is a flowchart showing an outline of the present invention.

FIG. 2 is a diagram showing an average ratio of finished wall thickness to target wall thickness according to the present invention in comparison with a conventional method.

FIG. 3 is a diagram for explaining the principle of the present invention.

FIG. 4 is a flowchart showing a part of the flow of one embodiment of the present invention.

FIG. 5 is a flowchart showing another part of the flow of the one embodiment.

FIG. 6 is a diagram showing a relationship between a tensile coefficient set for each stand and a maximum tensile coefficient.

FIG. 7 is a diagram showing an example of a manufacturing process of a steel pipe.

FIG. 8 is a diagram showing a wall thickness distribution in the longitudinal direction.

FIG. 9 is a diagram showing the relationship between roll rotation speed and wall thickness set for each stand.

[Explanation of symbols]

 10 ... Material, 12, 24 ... Heating furnace, 14 ... Perforation rolling machine, 16 ... Hollow rod-shaped material, 18 ... Mandrel bar, 20 ... Stretching rolling machine, 22 ... Element tube, 26 ... Drawing rolling machine, 28 ... Finished tube.

Claims (1)

[Claims] Claims: 1. When a raw tube is drawn and rolled by a continuous multi-stage rolling mill, physically, based on the outside diameter and wall thickness of the tube before and after drawing and rolling at each stand, and the target outside diameter and target wall thickness of the finished tube. The maximum addable tensile coefficient is calculated for each stand, and based on the outer diameter and wall thickness of the pipe before and after drawing and rolling at each stand at the time of the last rolling, the rotation speed of the roll, and the actual outer diameter and wall thickness of the finished pipe. The actual tension coefficient actually added is calculated for each stand, and for the stands where the actual tension coefficient exceeds the maximum tension coefficient, the actual tension coefficient is corrected to the maximum tension coefficient or less, and the actual tension after correction is adjusted. A method for controlling the wall thickness of a reduction rolling mill, which is characterized in that an actual tensile coefficient for each of the other stands is obtained and set based on the coefficient.