JP3205118B2 - Automatic thickness control method for wrought steel pipe - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technology for preventing a change in the thickness of a forged steel pipe in the longitudinal direction due to a change in the thickness of a skeleton, which is a raw material, in the longitudinal direction in a process of manufacturing the forged steel pipe.

[0002]

2. Description of the Related Art A forged steel pipe is manufactured by heating a steel strip called a skelp, and thickly forming a rear end portion of the steel strip on the exit side of a heating furnace.
FIG. 4 shows a production drawing of a forged steel pipe. The skeleton, which is a material, is rewound by the uncoiler 1, straightened by the leveler 2, cut off the rear part of the skeleton and the front part of the next skeleton by the shear 3, and then is flushed with the rear end of the skeleton by the flash butt welder 4. The end of the next skeleton is welded to form an endless skeleton. The skelp 5 is further stored on the loop floor 8 via the pinch roll 6 and the magnet roll 7, and the pinch roll 11 provided on the front surface of the heating furnace 15 synchronized with the speed of the entrance of the mill stand 38 as a tube forming machine. After that, it enters the heating furnace 15.

[0003] The heated skeleton is placed on a mill stand 38.
Is formed by a forming roll 17 as a first stage, and is formed into a pipe shape by a forging roll 18 as a second stage. The skelp formed in a pipe shape is gradually reduced or thickened by squeezing rolls (19 to 30) arranged continuously and continuously in the third and subsequent stages. At the end of steel pipe with the desired wall thickness and outer diameter. The outer diameter is determined by the roll caliber of the last stand, and the wall thickness is controlled by changing the number of roll rotations of each stand.

[0004] In the above-described process of manufacturing a forged pipe, the thickness variation in the longitudinal direction of the skeleton, which is a raw material, directly affects the thickness variation in the longitudinal direction of a steel pipe as a product.
If left unchecked, not only the yield will deteriorate, but also defective products may sometimes be generated. As a method of solving such a problem, the thickness of the skelp is conventionally continuously measured by a thickness gauge installed on the side of the heating furnace,
A method of delaying the measured thickness signal from the measuring point to the mill stand and synchronizing it with the line speed and controlling the mill stand roll speed according to this thickness signal to control the thickness of the forged steel pipe is proposed. Have been.

As an example, there is an invention disclosed in Japanese Patent Publication No. 54-24394. The invention disclosed herein is to continuously measure the thickness of the skeleton at the front of the heating furnace, track the measured thickness signal to the control rolling stand, and control the control rolling stand according to the difference from the reference thickness of the skeleton. It shows a method of controlling and controlling the thickness variation of the skeleton due to the thickness variation in the longitudinal direction of the tube.

According to the document, the thickness of a skeleton is continuously measured on the front side of a heating furnace, and the measured thickness signal is tracked to a control rolling stand to control the control rolling stand in accordance with a difference from a reference thickness of the skeleton. Is supposed to. However, when performing high-precision thickness control,
The tracking error during the tracking cannot be ignored. That is, since the control is performed in accordance with the difference between the skeleton thickness variation and the skeleton reference thickness, as the tracking error increases, the deviation that should be controlled in order to perform the target thickness control is increased. As a result, the wall thickness cannot be controlled with high accuracy.

[0007] Another example is disclosed in Japanese Patent Application Laid-Open No. 64-4011.
There is an invention disclosed in Japanese Patent Publication No. According to the document, also before control in the control rolling stand, the tracking delay of the thickness variation of the skeleton measured in advance,
It is supposed to be controlled. Therefore, as the tracking error increases, the amount different from the deviation that should be controlled for controlling the target thickness is controlled, resulting in a problem that high-precision thickness control cannot be performed. Was.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in order to solve the above-mentioned problems, and by precisely correcting the amount to be tracked for each skelp, the precision of the forged steel pipe can be improved. Another object of the present invention is to provide a simple automatic thickness control method.

[0009]

In order to achieve the above-mentioned object, the present invention relates to a wrought steel pipe manufacturing process, wherein a skelp (a hot-rolled steel strip) which is a material of a forged steel pipe is measured by a thickness gauge installed in front of a heating furnace. ) Was continuously measured, and the thickness signal measured by the thickness gauge was synchronously delayed with the line speed to a controlled rolling stand continuously arranged in multiple stages on the rear surface of the heating furnace. The control rolling stand is controlled based on a thickness deviation signal which is a difference between a thickness signal and a preset reference thickness signal, and a thickness caused by a thickness variation of a skelp among thickness variations in a longitudinal direction of a pipe. In the method of controlling the fluctuation, the relay portion, which is a weld of the skeleton and the skeleton, detects the relay portion with the thickness meter each time it passes through the thickness gauge, and calculates the motor current value of the roll of the control rolling stand. Due to changes in Measuring the amount of tracking delay until the relay portion is detected, and correcting the tracking amount from the thickness gauge to the control rolling stand so that tracking calculation of the thickness deviation signal of the skeleton is performed accurately thereafter. An automatic wall thickness control method for a forged steel pipe, characterized in that the rotation speed of each stand is controlled in accordance with the principle of constant mass flow to suppress the wall thickness fluctuation in the longitudinal direction of the pipe.

[0010]

According to the present invention, when the skew thickness variation measured in advance before control by the control stand is controlled by delaying the tracking, even if the tracking error tends to increase, the amount to be tracked for each skelp is controlled. However, since it is accurately corrected based on the track record, it is possible to control the thickness of the steel pipe with high accuracy.

[0011]

An embodiment of the present invention will be described below with reference to the drawings. In FIG. 1, an X-ray thickness gauge 10 is installed on the front surface of a preheating furnace 12 to continuously measure the thickness of the skeleton 5 in the longitudinal direction. The measured thickness signal is transmitted together with a signal from a length measuring PLG 31 (pulse generator) attached to a pinch roll 11 installed on the rear surface of the X-ray thickness meter.
The signal is sent to the control computer 34, where the tracking is delayed to the mill stand 38, which is a tube forming machine, in synchronization with the line speed. The skeleton 5 is introduced into a mill stand 38 via a preheating furnace 12, a return drum 13, a turnaround 14, and a heating furnace 15. The skelp is controlled by a preform roll 16 arranged on the rear surface of the heating furnace so that both edges of the skelp follow the abutting position at the time of forging, guided to a mill stand 38, and formed into a first-stage forming roll 17. And formed into a pipe shape by the forging roll 18. The skeleton formed into a pipe shape is gradually reduced in thickness and reduced in outer diameter by a drawing roll (19 to 30) downstream from the forging roll 18 and has a desired thickness on the exit side of the drawing roll 30 in the final stage. Outer diameter steel pipe 37
Finished. The automatic thickness control of the present invention is performed by the control computer 34 for each of the stands 20 to 30 using the squeezing roll 20 as a pivot stand.

The control flow chart of the present invention will be described below with reference to FIG. The junction, which is a weld between the skeleton and the skeleton, has a large thickness variation and is usually several percent or more thicker than the other portions of the skeleton. Therefore, the relay portion is easily detected by the thickness gauge, and when the relay portion passes through the control rolling stand, a large current fluctuation appears from the motor for the control rolling stand. It can be easily determined. Therefore, from the relay detection by the thickness gauge, the pulse generator (PLG) until the relay detection at the third, fourth, or fifth control rolling roll at which a relatively large current fluctuation appears. Pulse number a
Is calculated, and the advance amount d ′ of the skelp per pulse is calculated by the following equation (1). d '= 1 / a (1) where l is the skelp from the thickness gauge to the third, fourth, or fifth controlled rolling roll. It is the distance converted to the length.

Next, when a unique value is accidentally detected in the advance amount d 'of the skeleton in one pulse, n times (usually 5 times) so as not to increase the tracking error.
Each time the relay passes the amount of advance of the skeleton per one pulse, the moving average is performed by the following equation (2). d = (one-time correction value d ′ + two-time correction value d ′ +... + n-th time correction value d ′) / n (2) Next, the skelp per pulse obtained by the equation (2) Based on the advance amount (tracking correction value) and the count amount of PLG, tracking calculation is performed as a distance moved from the thickness gauge as in the following equation. Tracking distance from thickness gauge = d x PLG count amount (3)

Next, a control method using the above-described tracking correction will be described with reference to FIG. First, a target stretch pattern as shown in FIG. 2 is preliminarily calculated for each roll in the upper process computer 35 shown in FIG. 1 based on the size and steel type to be formed. Initialize the number and perform pipe making. Next, as described above, when tracking of the thickness variation in the longitudinal direction of the skeleton is delayed by the X-ray thickness gauge 10 and the length measuring PLG 31, the thickness reduction at each stand is performed according to a predetermined thickness reduction rate at each stand. The control unit 34 calculates the meat amount Δti.

Further, in synchronism with the timing in which the thickness variation in the longitudinal direction of the skelp is delayed by tracking, the control computer 34 controls the pipe speed at each drawing roll downstream of the pivot stand so as to follow the thickness variation. Determined by the formula. ΔVi = h (Δti) (4) Here, the function h is determined for each size and product type with a constant mass flow.

Next, the pitch diagram PDi (Δt) corresponding to the skeleton thickness variation Δt is calculated in the control computer 34 by the following equation. PDi (Δt) = PDi ₀ × I (Δt) (5) where PDi ₀ is a pitch diagram at each control stand when there is no variation in skeleton thickness, and function I is skeleton This is a function indicating the relationship between the thickness variation and the rate of change of the pitch diagram, and the function I is determined in advance as follows.

FIG. 4 shows the amount actually reduced when the acceleration / deceleration command corresponding to the target thickness reduction is output to the control motor from the relation of the constant mass flow with the pitch diagram PDi being fixed. It should be noted that the negative wall thickness actually means a wall thickness increase. If the pitch diagram is constant even when the wall thinning amount changes, the pipe forming phenomenon becomes as shown by a broken line A in FIG. However, in the actual pipe forming phenomenon, the pitch diagram also changes due to the change in the number of revolutions of the control roll due to the change in the wall thickness reduction. 4 becomes a solid line B shown in FIG. Therefore, the solid line B is a function f (Δt) of the wall thickness reduction Δt.
Equation (5) for calculating the pitch diagram according to the skeleton thickness variation is expressed by the following equation. PDi (Δt) = PDi ₀ × I (Δt) = PDi ₀ × f (Δt) / Δt (6) The equation (5) is determined in advance as described above.

Next, a roll rotation speed acceleration / deceleration command downstream of the pivot stand is calculated in the control computer 34 by the following equation. ΔNi = ΔVi / (π × PDi (Δt)) (7) Next, the number of rotations of the stands downstream from the i-th stand is calculated for each stand, and the above equation ( 5)
The control computer 34 is added to the rotation speed calculated by
Then, the rotation speed control is performed by giving the speed command of each stand roll rotation drive motor to each stand roll drive main piping panel 33 to accurately produce a steel pipe having a target thickness and an outer diameter.

FIG. 5 shows the result when the present invention is not implemented, and FIG. 6 shows the result when the present invention is implemented. FIG. 5 shows that tracking deviation has occurred, and the part to be originally controlled arrives at the control rolling stand with a delay, resulting in poor control accuracy. On the other hand, FIG. Therefore, the control accuracy is extremely good.

[0020]

As described above, according to the present invention, when the tracking error is controlled by delaying the fluctuation of the thickness of the skeleton which is measured before the control by the control stand, even if the tracking error tends to be large, the skeleton does not increase. Since the amount to be tracked is accurately corrected for each time based on the track record, it is possible to control the thickness of the steel pipe with high accuracy.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a system configuration of the present invention;

FIG. 2 is a graph showing a target stretch coefficient and a target roll rotation speed for each stand of a representative size;

FIG. 3 is a control flowchart according to the present invention;

FIG. 4 is a diagram showing a relationship between a target thickness reduction allowance and an actual thickness reduction allowance of a forged steel pipe when a constant pitch is controlled at a constant mass flow with a constant pitch diagram at a typical size;

FIG. 5 is a view showing a tube forming result when the present invention is not carried out;

FIG. 6 is a view showing a pipe forming result when the present invention is carried out;

FIG. 7 is a view showing a result of manufacturing a forged steel pipe when the present invention is not implemented;

FIG. 8 is a view showing a result of manufacturing a forged steel pipe when the present invention is carried out;

FIG. 9 is a diagram showing a production line up to a mill stand for forged steel pipes.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Uncoiler 2 Leveler 3 Shear 4 Flash butt welder 5 Skelp 6 Pinch roll 7 Magnet roll 8 Loop floor 10 X-ray thickness gauge 11 Pinch roll 12 Preheating furnace 13 Return drum 14 Turnaround 15 Heating furnace 16 Preform roll 17 Forming roll 18 Forging Rolls 19 to 30 Squeezing rolls 18-a to 30-a Roll drive motor 31 PLG for length measurement 32 Rotation detector 33 Roll drive main board 34 Control computer 35 Process computer 36 Operation panel 37 Steel pipe 38 Mill stand

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) B21B 37/78 B21B 37/00 B21C 37/08

Claims (1)

(57) [Claims] In the process of manufacturing a wrought steel pipe, the thickness in the longitudinal direction of a skeleton (steel strip of a hot-rolled steel plate), which is a material of the wrought steel pipe, is continuously measured by a thickness gauge installed in front of a heating furnace. The thickness signal measured by the thickness gauge is synchronously delayed with the line speed up to the control rolling stand continuously arranged in multiple stages on the rear surface of the heating furnace, and the thickness is a difference between the delayed thickness signal and a preset reference thickness signal. A method for controlling the control rolling stand based on the deviation signal to control a thickness variation of a skeleton in a thickness variation of a skeleton in a thickness variation of a tube in a longitudinal direction, wherein the skeleton and a skeleton are welded. Each time the relay section passes through the thickness gauge, the relay section is detected by the thickness gauge, and a change in the motor current value of the roll of the control rolling stand causes a tracking delay amount until the relay section is detected. Total Measure, correct the tracking amount from the thickness gauge to the control rolling stand so that the tracking calculation of the subsequent thickness deviation signal of the skeleton is performed accurately, and according to the principle of constant mass flow, the rotation speed of each stand is corrected. An automatic thickness control method for a forged steel pipe, characterized in that a thickness change in a longitudinal direction of the pipe is controlled by controlling the thickness.