JPS5916847B2 - Method for controlling the elongation length of a pipe in a continuous elongation rolling mill - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for controlling the elongation length in a continuous elongation mill, particularly a mandrel mill, which mainly reduces the wall thickness of a tube. This invention relates to a method of controlling the circumferential speed of each stand and roll for each raw tube so that the length of the rolled tube matches a target value.

Mandrel mills are currently widely used in hot rolling lines for small-diameter seamless steel pipes, and a typical manufacturing method for small-diameter steel pipes including this will be briefly described below.

When the material (round steel billet) is heated to the required temperature in the heating furnace, it is processed by a piercing rolling machine (e.g. Mannesmann piercer).
Since the hollow rod-shaped material that has been punched and rolled in this way is still thick, it is then sent to a drawing mill (mandrel mill) to mainly reduce the wall thickness to form the blank tube for the reduction rolling mill. Ru.

This raw tube is sent to a reheating furnace, heated to a required temperature, and then reduced to a required size by a reducing mill (for example, a trench reducer).

By the way, not only in the above-mentioned pipe manufacturing process but also in the process of manufacturing seamless steel pipes, it is not possible to produce products with uniform length or uniform wall thickness by simply keeping the rolling conditions (mill centamp) constant for each rolling mill. It is well known that it cannot be obtained.

In order to make the length or thickness of the product uniform, manual or automatic adjustments must be made somewhere along the rolling line.

In the above-mentioned pipe manufacturing process, it has conventionally been common to adjust the length or wall thickness of the raw pipe in a reducing mill.

The reason for this is that the reducing rolling mill is a finishing rolling mill, and it is sufficient to keep the product length constant here. This is because, as shown in Japanese Patent No. 43469, it was believed that this could be achieved by changing the rotational speed of each stand depending on the length of the blank tube of the reducing mill measured at the mill entrance side.

However, there is a limit to the length adjustment using a reducing mill.

In a reducing rolling mill, as mentioned above, the number of roll rotations is changed according to the length of the raw pipe (or the average wall thickness), and the tension between the stands is maintained appropriately to increase the finished length or the average wall thickness. A method that matches the target value is generally used.

However, when using a rolling schedule with a large finished diameter (small diameter reduction ratio) in a reducing mill, the number of stands used decreases, and when rolling thin-walled products, the friction between the rolls and the tube increases. By reducing the force,
Because the tension that can be physically generated is limited to a narrow range, when the length of the tube rolled in the mandrel mill in the previous process is large, it is impossible to absorb all of it by the reducing mill. It becomes impossible.

Therefore, the purpose of the present invention is to provide a continuous elongation rolling machine. The object of the present invention is to obtain a method for appropriately controlling the length of the pipe or the average wall thickness so as to eliminate as much as possible.

Continuous stretching mills include mandrel mills, multi-stand plug mills, etc., and here we will explain how to control the elongation length of the filter tube in a mandrel mill.

A mandrel mill is a tandem rolling mill consisting of multiple roll stands with oval or circular grooves cut, and during rolling, the hollow blanks supplied from the piercing mill in the previous process are used. A mandrel bar coated with a lubricant is placed on the tube, and the cross-sectional area of the tube is gradually reduced using the bar and roll holes to obtain a blank tube for a reducing mill.

After rolling, the mandrel bar is pulled out from the pipe by a stripper, cooled in a cooling tank, and then coated with oil lubricant again and subjected to rolling.

Usually, a dozen or more of these mandrel bars are used in rotation.

In such a feed drill mill, for the reasons mentioned above, it is necessary to minimize the rolling length.
Various causes of rose tree were investigated.

As a result, the lengthening of the tube in the mandrel mill is
! ! It was found that the weight and temperature of the raw pipe, the friction characteristics of the mandrel bar used in circulation, the temperature of the tool, etc. affect the thickness of the pipe (or average wall thickness).

In particular, the influence of the weight of the raw pipe and the mandrel bar is significant.

We will discuss the influence of the weight of the raw pipe.

If other rolling conditions are constant, the thicker the raw pipe weight, the longer the elongated length after rolling, and the elongated length/unit weight (in other words, average wall thickness) will be approximately constant. There was found.

This result is important in that mandrel mills should be able to absorb loose wood in the shape of a raw tube (outer diameter, wall thickness).

Therefore, if the purpose is to keep the wall thickness of the finished product constant after reducing, there is no need to particularly consider the weight of the raw pipe.

Next, we will discuss the influence of the mandrel bar used during rolling.

The surface condition due to the usage history of the bar affects the rolling length, and generally, when a bar that has been used many times is used, the rolling length tends to be longer.

The influence of this bar is of a nature that gradually changes with each use.

Differences in surface conditions due to differences in the usage history of each bar appear as differences in lubricant adhesion, which changes the friction coefficient between the bar and the material. length or average wall thickness).

If the coefficient of friction between the bar and the raw pipe is high, stress that tends to roll occurs between the stands, and the rolled raw pipe will have a large wall thickness and a short length.

The opposite is true if the coefficient of friction becomes smaller.

Finally, we will discuss the effect of the temperature of the raw tube.

The lower the temperature, the longer the rolling length generally tends to be.

What should be noted here is the characteristic unique to mandrel mill rolling that the lower the temperature of the rolled material, the longer the rolling length, which is the opposite tendency to general rolling.

As mentioned above, bulges in rolled bars or average wall thickness during mandrel mill rolling are mainly caused by the inter-stand acting force caused by fluctuations in the coefficient of friction (particularly the friction between the inner surface of the tube and the mandrel bar). It can be said that it is a thing.

Furthermore, generally speaking, as a control method for the elongated length or average wall thickness, it is conceivable to perform either or both of roll/gamble control and roll rotation speed control at the same time. Therefore, in the present invention, roll rotation speed control is adopted, and the roughness of the rolled wall (or average wall thickness) due to the rolling characteristics of the mandrel mill is investigated in advance for each raw pipe. We adopted a method of changing the rotation speed.

A concrete implementation method of the present invention will be explained using an 8-stand mandrel mill as an example.

FIG. 1 shows the circumferential speed of each stand and roll used for rolling in a mandrel mill as a circumferential speed ratio with respect to the first stand.

As the tube reaches the rear stand, the cross-sectional area of the tube naturally decreases, and the tube speed increases accordingly, so the circumferential speed ratio is set to gradually increase accordingly.

In FIG. 2, curves A and C each indicate peripheral speed ratios with different rotational speed slopes.

If other conditions are constant, the raw pipe rolled with the rotational speed gradient curve A will have a larger elongated length/unit weight than the pipe with a curved mouth.

Hereinafter, for convenience of explanation, the roll circumferential speed ratio from No. 1 to Na8 will be represented by the roll circumferential speed ratio of the finishing stand, and this will be simply referred to as the roll circumferential speed ratio.

If the friction coefficient of the mandrel bar is large, a large roll peripheral speed ratio is required to obtain the same elongation length/unit weight.
On the other hand, if it is smaller, the same elongation length/unit weight can be obtained with a smaller roll circumferential speed ratio.

If the relationship between this roll peripheral speed ratio and the mandrel bar, material temperature, tool temperature, etc. is determined in advance through experiments, it is possible to select and use the appropriate rotation speed for each raw pipe, and the mandrel mill It is possible to minimize the length of the tree.

If we further organize these relationships, the mandrel
The relationship between elongation length/unit weight in a mill can be easily expressed as the difference (μb - μa) between the coefficient of friction μb between the outer surface of the tube and the roll and the coefficient of friction μa between the inner surface of the tube and the mandrel bar.

If this difference (μb - μa) is constant, then the same - extension length /
Unit weight can be obtained.

This relationship is shown in FIG. If the difference in friction coefficient is large,
The roll circumferential speed ratio that provides the same elongation length/unit weight becomes smaller.

In Figure 2, straight line C is the rotation speed ratio that gives a larger elongated length/unit weight than straight line 2, and straight line E is the roll peripheral speed that gives a smaller elongated length/unit weight compared to straight line 2. It is a ratio.

If the relationship shown in FIG. 2 is determined experimentally, an arbitrary elongated length/unit weight can be obtained if the outer surface friction coefficient and inner surface friction coefficient are given for each raw pipe.

The internal and external friction coefficients can be expressed in the following format for each pipe manufacturing schedule and material.

μa=f(μB, ΔTM, ΔTB) μb=g(μR1ΔTM, ΔTR) Where, μB: Frictional characteristic value of the bar at standard temperature ΔTM: Deviation of base tube temperature from standard value ΔTB: Standard value of bar temperature μR: Frictional characteristic value of the roll under standard conditions ΔTR: Deviation of roll temperature from the standard value If these relational expressions are determined experimentally, various disturbances in the pipe manufacturing line (fluctuations in rolling temperature, tool Control that can follow changes in temperature, etc.) is possible.

These calculation formulas are calculated by measuring the finished length of the product after rolling.
You can also make corrections online.

Next, an embodiment of the method based on the present invention will be described.

(1) Target schedule Mandrel/mill inlet dimensions: Outer diameter 190mm x wall thickness 16
mm x length 8.5 m Mandrel mill finish dimensions: outer diameter 15 F! m x wall thickness 6 mm x length 26 m (2) Mandrel bars used: 12 bars used in circulation (3
) Friction characteristic value of mandrel/bar (standard condition) Mandrel/mandrel/bar number lLB bar number “8 1 0.12 7 0.25
2 0.20 8 0.08
3 0.09 9 0.24
4 0.15 10 0.135
0.14 11 0.126
0.17 12 0.11 (4)
Relational expression 1 used: Coefficient of friction between the inner surface of the tube and the bar μa = f (μB, ΔTM, ΔTB) "μB11 Coefficient of friction between the outer surface of the tube and the roll μb = g (μR9ΔTM, ΔTR) = 0.3 = constant (5
) (μb-μa) vs. roll circumferential speed ratio Line 2 in Figure 2 was used.

(6) Results of implementation The rolling rotation speed was selected for each element t according to the frictional characteristic value of the mandrel bar, and compared with the case where no prongs were added.

. , Average stretched length Standard deviation of stretched length (scream) (6) Conventional method 380 25800 0.87% Invention method 2
52 26000 0.31% (Pilento weight was extremely small and could be ignored.

) By selecting and using the rolling rotation speed according to the friction characteristic value of the mandrel bar, it was possible to reduce the elongation length to about 1/3 of that of the conventional method.

Although a mandrel mill has been described above, the present invention is applicable not only to a mandrel mill but also to other continuous tube elongation mills (for example, multi-stand plug mills).

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between the stand number and the roll peripheral speed ratio of each stand in a mandrel mill. FIG. 2 is a graph showing the relationship between the difference in the friction coefficient between the inner and outer surfaces and the roll circumferential speed ratio of the final stand.

Claims (1)

[Claims] 1 Multiple stands with grooved rolls and mandrels
In a continuous elongation rolling mill that gradually reduces the outer diameter and wall thickness of a pipe using a bar, the friction +54 value of the mandrel bar is determined in advance through experiments, and the weight and temperature information of the raw pipe are measured or predicted online. A method for controlling the elongation length of a pipe in a continuous elongation mill, which is based on the above, and is characterized in that the finished pipe length after rolling is made to match a target value by changing the peripheral speed ratio of the rolls for each raw pipe.