JPH1094806A - Method for rolling seamless steel tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent an occurrence of fin flaws of a material in an outside diameter contracting stand by measuring an outside diameter in each position in the axial length direction of a hollow tube stock on the inlet side of the outside diameter contracting stand, and adjusting the roll gap of the outside diameter contracting stand according to measured result. SOLUTION: The outside diameter in each position in the axial length direction of the hollow tube stock obtained by piercing with a piercer 1, is continuously measured by an outside diameter measuring equipment 2, and measured results are inputted to a computing element 4. A draft in the outside diameter contracting stand 3 in each position in the axial length direction of the hollow tube stock, is calculated by the computing element 4 according to the inputted outside diameters and a finish target outside diameter in the outside diameter contracting stand 3. Also, the computed result of the draft and a critical draft by which the fin flaws of the material, which have been obtained preliminarily by an experiment, do not occur, are compared by the computing element 4, and a roll gap adjustment quantity is obtained. A diameter is contracted while adjusting and controlling to the critical draft or below by which the fin flaws of the material do not occur. Consequently, the draft does not become excessive, and the fin flaws of the material to the gap of rolls are prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for rolling a seamless steel pipe, and more particularly, to a method for rolling a seamless steel pipe by a mandrel mill for elongating and rolling a hollow shell into which a mandrel bar is inserted.

[0002]

2. Description of the Related Art As one of methods for producing a seamless steel pipe, there is a method called a Mannesmann-mandrel mill method.

FIG. 2 is a view showing the steps of the method.
As shown in FIG. 2, in this method, a solid round slab 11 is heated in a heating furnace 12 and then pierced and rolled by a piercing machine 13 as a rough rolling mill to form a hollow shell. Subsequently, the hollow shell is stretched and rolled by a subsequent mandrel mill 14 to finish it to a predetermined thickness, and is heated as it is or reheated, and then reduced in diameter by a stretch reducer 15 to finish the outer diameter to a predetermined size. , And the product is a seamless steel pipe.

[0004] The above-mentioned mandrel mill 14 is generally constituted by continuously arranging four to eight two-roll stands having a pair of hole-shaped rolls in the direction of the pass line. Adjacent roll stands are alternately arranged with a phase shift of 90 degrees so that the roll-down directions of the hole type rolls in a plane perpendicular to the pass line are different. Further, the roll hole type is made smaller as the roll stand at a later stage. And in this mandrel mill 14, the mandrel bar 14
Rolling is performed by passing the hollow shell into which a is inserted through the roll hole mold of each roll stand.

[0005] As the mandrel mill 14, there is a mandrel mill 14 in which four to eight three-roll stands provided with a pair of three hole-shaped rolls are arranged in series.

In such a method for manufacturing a seamless steel pipe, the dimensional accuracy of a hollow shell obtained by piercing and rolling with a piercing machine as a rough rolling machine is inferior, and the outer diameter thereof varies in the axial direction. I do. When the ratio of the wall thickness t to the outer diameter D of the hollow shell to be obtained, that is, the wall thickness outer diameter ratio (t / D) is small, the fluctuation of the outer diameter in the axial direction is small during the piercing and rolling by the drilling machine. Because the phenomenon that the outer diameter of the rear end of the rolling becomes large occurs,
Become noticeable.

[0007] When the outer diameter variation of the hollow shell in the axial direction is small, it is usually absorbed by elongation rolling using a mandrel mill. However, in the case of a hollow shell having a small (t / D) and a large variation in the outer diameter in the axial direction, or in the case of using a small number of roll stands of a mandrel mill,
It is not completely absorbed by the elongation rolling by the mandrel mill, and causes a variation in the tube thickness in the axial direction at the outlet side of the mandrel mill.

[0008] As a method for preventing a change in the wall thickness in the axial direction on the exit side of the mandrel mill due to a change in the outer diameter of the hollow shell in the axial direction, for example, "International Tube A"
ssociation: Conference papers in Italy (1993.5.10
-14) ”Revamping of Seamless Tube Plant by Mini-MP
The method indicated in M Technology "" is known.

That is, in the method, a pair of four hole-shaped rolls are provided on the entrance side of the mandrel mill, and a driving source for the four rolls is arranged close to one outside diameter reducing stand which is separate from the mandrel mill. In this method, the outer diameter of the hollow shell is reduced to the same outer diameter in the axial direction by the outer diameter reducing stand and supplied to the mandrel mill.

[0010]

However, as described above, when the outer diameter of the hollow shell is reduced to the same outer diameter in the axial direction by the outer diameter reducing stand, the outer diameter varies greatly in the axial direction. In the case of a raw tube, there has been a problem that the material starts to bite into the gap between the rolls to cause seizure flaws. For this reason, there has been a demand for the development of a rolling method capable of preventing the material from being entangled at the time of reduction rolling of the outer diameter of the raw tube at the outer diameter reducing stand.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object thereof is to provide a seam capable of reducing the outer diameter of a hollow shell with an outer diameter reducing stand without causing material bulging. An object of the present invention is to provide a method for rolling a steel-free pipe.

[0012]

The gist of the present invention resides in the following method for rolling a seamless steel pipe.

A method for rolling a seamless steel pipe by a mandrel mill having an outer diameter reducing stand for reducing the outer diameter of a hollow shell drilled by a drilling machine, which is disposed in close proximity to an entrance of the stand. A method for rolling a seamless steel pipe, comprising: measuring an outer diameter at each position in an axial direction of a hollow shell at an entrance side of a stand; and adjusting a roll gap of the outer diameter reducing stand based on the measurement result. .

The present invention has been made based on the following findings as a result of many experiments conducted by the present inventors.

That is, when the material is caught in the roll gap, the outer diameter of the hollow shell is too large compared to the outer diameter to be finished by the outer diameter reducing stand, and the outer diameter working ratio ｛(the outer diameter before diameter reduction) -Outer diameter after diameter reduction / outer diameter before diameter reduction) x 100 (%)} was found to occur only in large portions.

Therefore, the outer diameter working degree is changed variously,
Investigation into the presence or absence of material biting revealed that there was a critical working rate at which material biting occurred, and it was necessary to reduce the outer working rate to a critical working rate or less according to the outer diameter of the hollow shell. did.

For this purpose, the outer diameter at each position in the axial direction of the hollow shell, which has been pierced and rolled by the piercing machine, is continuously measured on the entrance side of the outer diameter reduction stand. Based on this, it was found that it was necessary to adjust the roll gap so that the finished outer diameter at the outer diameter reduced diameter stand was equal to or less than the critical working degree.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The rolling method of the present invention will be described below in detail with reference to FIG.

FIG. 1 shows a control flow chart for carrying out the rolling method of the present invention, and shows a drilling machine 1 and an outer diameter reducing stand 3.
The outer diameter measuring device 2 is disposed between the first and second measuring devices. Further, a mandrel mill main body 6 is disposed downstream of the outer diameter reducing stand 3.

Then, the outer diameter Di at each position in the axial direction of the hollow shell obtained by piercing and rolling by the piercing machine 1 is continuously measured by the outer diameter measuring device 2, and the measurement result is calculated by a computer. Enter 4

Based on the input outer diameter Di and the target outer diameter D to be finished by the outer diameter reducing stand 3, the computer 4 to which the outer diameter Di has been input is used to calculate each of the axial lengths of the hollow shell by the following formula. The degree of processing Ri at the outer diameter reducing stand 3 at the position is calculated. Further, the arithmetic unit 4 compares the calculation result of the working degree Ri with a critical working degree Rr which is determined in advance by experiment and does not cause material biting out, and when Ri> Rr, the working degree is Ri. = Rr is determined by the following equation, and the roll gap adjustment amount △ Rgi is determined by the following equation.

Ri = {(Di-D) / Di} × 100 (%) Dg = Di × (1-100 × Ri) ΔRgi = D−Dg The roll gap adjustment amount △ Rgi obtained by the above-described formula by the arithmetic unit 4 is output to the roll gap driving device 5a of the control device 5 of the outer diameter reducing stand 3. Then, the roll gap is adjusted to a value larger than the reference roll gap Rg.

On the other hand, the processing degree R obtained by the above equation
When i satisfies Ri ≦ Rr, the calculation of the above and the formula is omitted, and the roll gap of the outer diameter reducing stand 3 is kept at the reference roll gap Rg.

As described above, the roll gap of the outer diameter reduction stand 3 is previously determined by an experiment based on the outer diameter measurement results at each position in the axial direction of the hollow shell obtained by piercing and rolling by the piercing machine 1. Critical working degree R at which no material bulge occurs
When the diameter is reduced while being adjusted and controlled to be equal to or less than r, the working degree does not become excessive. As a result, the material is prevented from getting into the gap between the rolls, and seizure flaws do not occur.

The critical working degree at which the material does not bite into the roll gap varies depending on the roll diameter and the roll hole shape of the hole-shaped roll, but the roll hole shape has four substantially round holes. The critical working ratio of a four-roll stand provided with a mold roll is usually 5 to 10%.

The outer diameter reducing stand is not limited to the four-roll stand having four hole rolls disclosed in the above-mentioned document, but may be a two-roll stand or a three-roll stand having two or three hole rolls. It may be something.
Further, a multi-roll stand having five or more hole-shaped rolls may be used.

However, the biting of the material into the roll gap is
There is a characteristic that the smaller the number of the rolls provided, the more easily the roll stand is generated, and the lower the critical working degree. Therefore, when an outer diameter reducing stand provided with a small number of hole-shaped rolls is used, a hollow shell in which the above-mentioned thickness outer diameter ratio (t / D) is small and the outer diameter of the rear end of the rolling is significantly large is used. When the diameter is reduced, the outer diameter fluctuation in the axial direction becomes as large as possible, and the pipe wall thickness after mandrel mill rolling may increase in the axial direction. For this reason, it is preferable to use a roll stand having four, more desirably five or more hole-shaped rolls as the outer diameter reducing stand. However, when the number of provided rolls is increased, the structure becomes complicated, the maintenance is troublesome, the equipment cost increases, and the cost is not economical. In reality, a roll stand having up to four rolls is required. It is preferably used.

[0028]

EXAMPLE When five two-roll stands are arranged in a row, and the distance between the groove bottoms is Dm and the distance between the groove edges is Dh, the ratio (Dh / Dm) is 1.02. A mandrel mill was prepared in which an outer diameter reducing stand composed of four hole-shaped rolls to be formed was arranged close to the entrance side.

Then, the outer diameter is changed in the range of 184 to 192 mm at a pitch of 2 mm, and the thickness is 15 mm (t / d =
(0.082 to 0.078) is pierced and rolled by a piercing machine, a mandrel bar having an outer diameter of 143 mm is inserted into these hollow shells, and the roll gap is changed by the outer diameter reducing stand described above. 175mm outside diameter, wall thickness 1 without adjustment
When the diameter was reduced to 5 mm, the occurrence of material biting was examined.

As a result, as shown in Table 1, no material biting occurred in a hollow shell having an outer diameter of 188 mm or less before processing. On the other hand, in a hollow shell having an outer diameter of 190 mm or more before processing, material biting occurred, and the critical processing degree was 7.0%.

[0031]

[Table 1]

Therefore, the target dimension of the drilling machine is 1
86 mm, 15 mm thick hollow shell (t / d = 0.08)
When 1) is reduced to an outer diameter of 175 mm and a wall thickness of 15 mm with the above outer diameter reducing stand, the outer diameter at each position in the axial direction of the outer diameter reducing stand is continuously measured on the entrance side of the outer diameter reducing stand. The rolling method of the present invention for increasing the roll gap of the outer diameter reducing stand at the time of rolling a portion where the degree of work required based on the outer diameter measurement result exceeds 7%, and the roll gap without performing the outer diameter measurement In this case, the outer diameter reduction rolling was performed for each of 200 rolls by the rolling method of the conventional method without changing the rolling method. In each case, a mandrel bar having an outer diameter of 143 mm was inserted into the hollow shell.

Then, the number of pipes in which material bulging occurred was examined, and the results are shown in Table 2.

[0034]

[Table 2]

As is evident from the results shown in Table 2, when the conventional rolling method was used, 10% of the pipes began to be clogged. On the other hand, when rolling was performed according to the method of the present invention, no material bulge occurred at all.

[0036]

According to the present invention, the occurrence of material biting at the outer diameter reducing stand can be reliably prevented. As a result, there is no occurrence of seizure flaws, so that the number of maintenance steps is unnecessary. Further, there is no reduction in the material yield when the seizure flaw is unrepairable.

[Brief description of the drawings]

FIG. 1 is a diagram showing a control flow for implementing a method of the present invention.

FIG. 2 is a view showing a manufacturing process of a seamless steel pipe by a Mannesmann-mandrel mill method.

[Explanation of symbols]

 1: punching machine, 2: outer diameter measuring device, 3: outer diameter reducing stand, 4: arithmetic unit, 5: control device, 5a: roll gap driving device, 6: mandrel mill body.

Claims (1)

[Claims] 1. A method of rolling a seamless steel pipe by a mandrel mill having an outer diameter reducing stand for reducing the outer diameter of a hollow shell drilled by a drilling machine, which is disposed in close proximity to an entrance side thereof. The outer diameter of the hollow shell is measured at each position in the axial direction of the hollow shell at the entrance side of the diameter reducing stand, and the roll gap of the outer diameter reducing diameter stand is adjusted based on the measurement result. Rolling method.