JP2587348B2 - Rolling method of seamless 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 method for rolling a seamless steel pipe, and more particularly to a method for rolling a seamless steel pipe having an excellent inner surface property and a uniform wall thickness all around the steel pipe.

[0002]

2. Description of the Related Art Japanese Patent Laid-Open No. 33107/1988 discloses a method of rolling a seamless steel pipe by using a compact facility. The invention in this publication is to realize a stretching function equivalent to a plug mill by a mandrel mill,
The equipment is constituted by a rolling mill row comprising four or two stands. That is, the case where rolling is performed by a mandrel mill of a rolling mill row of four stands is shown.

[0003]

A mandrel mill, which has conventionally been composed of seven to eight stands, is now composed of four stands, and when a seamless steel pipe is manufactured by rolling from a raw tube, the number of stands still remains. It turned out that there was such a problem.
One of them is the problem of the inner surface roughness of the product. At the time of rolling of the second stand of the mandrel mill, the scale on the inner surface of the raw pipe during the rolling at the roll opening side is peeled off, so that irregularities remain on the inner surface and the inner surface roughness deteriorates. Another is the problem of uneven product wall thickness. One of the causes is a ridge generated during the second stand rolling. The raw tube being rolled comes into contact with the mandrel bar and the roll die, but is in a non-contact state on the roll opening side. For this reason, the metal flow becomes unbalanced on the roll opening side, and the swelling occurs from the contact portion with the mandrel bar of the raw tube during rolling to the non-contact portion, and ridges are generated, and the thickness of the product is reduced. Causes non-uniformity.

[0004] Regarding the inner surface roughness, there was no knowledge about the products rolled by the conventional mandrel mill of 7 to 8 stands, and as a result of pursuing the reason, as a result of pursuing the 7th to 8th stands, It has been found that the irregularities on the inner surface generated in the elongation rolling of the two stands have already been eliminated in the third stand, so that the problem has been solved without straddling the subsequent rolling of the rolling mill train. As for the ridges, as shown in FIG. 3 for explaining the reduction distribution of each stand for each mandrel mill stand, the amount of thinning in the first and second stands of the conventional eight stand mandrel mill is shown in FIG. In the first and second stands of the four-stand mandrel mill, the conventional mill is reduced by half by halving it.
Although up to three stands under reduced pressure are required, it has been found that the ridge is almost eliminated by one third stand in the present invention.

The present invention solves the above-mentioned problem by using a compact mandrel mill having four stands, and reduces the inner surface roughness (C-direction Rmax) of the pipe at the mandrel mill exit side to 20.
The purpose of the present invention is to provide a method for rolling a seamless steel pipe in which the inner surface roughness and ridges of the product are at least equal to or higher than those of a mandrel mill of 7 to 8 stands. And

[0006]

In order to achieve the above object, the present invention provides a method for rolling a seamless steel pipe with a mandrel mill, wherein a rolling mill train comprises four stands, a first stand, It is constructed by a rolling method of a seamless steel pipe characterized in that the raw pipe is elongated and rolled by the second stand, the wall thickness is reduced by 0.1 mm or more by the third stand, and round-up rolling is performed by the fourth stand. .
The stretching of the conventional mandrel mill is 5 or less, but in the present invention, the stretching of the mandrel mill is preferably limited to 3 or less.

The present invention will be described with reference to the drawings. FIG.
(A) to (D) show the arrangement of the rolling rolls 1 of the mandrel mill including the four rolling mill rows of the first stand to the fourth stand. Roll 1 of the first and third stands
Is a horizontal type, and that of the second stand and the fourth stand is a vertical type. The arrangement of the rolling rolls 1 may be such that each of these rolling rolls is rotated by 45 °, and an X-shaped arrangement or four units of the front stage or the rear stage of 7 to 8 mandrel mills are used. 1a is a hole type of the rolling roll 1, and 2 is a mandrel mill bar. The raw tube 3 is rolled by the die 1 a of the rolling roll 1 and the mandrel mill bar 2. FIG. 1 shows a rolled state of the raw tube 3 from the first stand to the fourth stand. At the time of rolling of the second stand in FIG. 1B, the scale adhering to the inner surface 3a of the rolling tube 3 corresponding to the roll opening 4 of the rolling roll 1 peeled off, and as a result, the inner surface of the tube 3 there. Becomes uneven. This unevenness
When the crude tube rolled between the first stand and the second stand is rounded up and rolled by a total of three stands, the product is not erased and remains in the product. Therefore, in order to remove the unevenness, it is necessary to roll the thickness of the first stand by 0.1 mm or more at the third stand. The scale that causes this unevenness is the scale that is generated on the inner surface of the pipe during transport of the raw pipe from the pipe rolling in the previous process to the mandrel mill rolling. And the depth of irregularities after the scale is peeled is small. Next, ridges generated in the second stand and their removal will be described with reference to FIG. FIG.
As shown in (a), the metal flow 7a of the contact portion 5 of the raw tube 3 and the metal flow of the non-contact portion 6 of the raw tube 3 in the second stand being rolled with respect to the rolling roll 1 and the mandrel bar 2 are shown. As a result, a ridge 8 having an uneven thickness is generated in the raw tube 3. The ridge is
As shown in (b), the metal flow 7d of the contact portion of the raw tube 3 and the metal flow 7c of the non-contact portion of the raw tube 3 in the third stand are being rolled with respect to the rolling roll 1 and the mandrel bar 2. The ridge 8 generated in the second stand is erased by reducing the thickness of the third stand in order to reduce the imbalance between the two.

In the third stand of the present invention, the thickness of the reduced thickness is 0.3.
1 mm or more is required. If it is 0.1 mm or more, if it is smaller than this, the inner surface of the tube cannot be sufficiently smoothed, and the ridge cannot be sufficiently removed, so that it may remain. on the other hand,
The maximum wall thickness reduction in the third stand of the present invention is not problematic for smoothing the inner surface of the tube (securing the inner surface roughness), but the limit is about 1.3 mm in order to sufficiently erase the ridge. In addition, in order to satisfy the use of the product by the user, the inner surface roughness on the exit side of the mandrel mill needs to be 20 μm or less in consideration of the change in the inner surface roughness in the standard machine and the drawing machine after the mandrel mill. The thickness reduction of the first stand of the mandrel mill is in the range of 1.0 to 4.1 mm, and the thickness reduction of the second stand is in the range of 2.3 to 4.2 mm. In the fourth stand, since the round-up rolling is performed, the thickness reduction is zero.

[0009]

According to the present invention, in a method of rolling a seamless steel pipe with a mandrel mill, a rolling mill row is constituted by four stands, and a raw pipe is stretched and rolled by first and second stands.
The thickness of the reduced thickness is rolled by 0.1 mm or more by the third stand,
Roundup rolling is performed by the fourth stand.
The stand smoothes the unevenness of the inner surface of the tube generated in the second stand and almost eliminates the ridge.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. The present invention was implemented by a rolling mill train shown in FIG. [Example 1] Outer diameter of base tube 185 mm x thickness 10.5 mm
Was placed in a 156 mm diameter mandrel bar and rolled. At that time, the thickness reduction of the first stand and the second stand is 2.9 mm and 3.4 mm, respectively. The thickness reduction of the third stand is 0.5 mm. Further, the thickness reduction in the fourth stand is 0 mm. As a result, the inner surface roughness (Rmax in the C direction) of the pipe at the outlet side of the mandrel mill was 20 μm
The following and the ridge can be almost eliminated, and the product outer diameter 1
At 39.8 mm x thickness 6.6 mm, the conventional 7-8
The same or better quality as the stand mandrel mill was obtained.

Example 2 A base tube having an outer diameter of 158 mm and a thickness of 8.4 mm was loaded into a 132 mm diameter mandrel bar and rolled. At that time, the thickness reduction of the first stand and the second stand is 2.5 mm and 3.0 mm, respectively. The thickness reduction of the third stand is 0.5 mm. Further, the thickness reduction in the fourth stand is 0 mm. As a result, the inner surface roughness of the pipe at the mandrel mill exit side (Rma in the C direction)
x) was 20 μm or less and ridges could be almost eliminated, and in the final step product outer diameter of 60.5 mm × thickness of 3.9 mm, a quality equal to or higher than that of a conventional 7 to 8 stand mandrel mill was obtained.

Example 3 A tube with an outer diameter of 132 mm and a thickness of 8.6 mm was charged into a 107 mm diameter mandrel bar and rolled. At that time, the thickness reduction of the first stand and the second stand is 2.9 mm and 3.4 mm, respectively. The thickness reduction of the third stand is 0.5 mm. Further, the thickness reduction in the fourth stand is 0 mm. As a result, the inner surface roughness of the pipe at the mandrel mill exit side (Rma in the C direction)
x) was 20 μm or less, and the ridge was almost eliminated, and in the final product outer diameter of 34.0 mm × thickness of 3.4 mm, a quality equivalent to or higher than that of the conventional 7 to 8 stand mandrel mills was obtained.

In the above embodiment, the third stand was rolled with the thickness reduced in the range of 0.1 to 1.3 mm, and similar results were obtained.

[0014]

The present invention is configured as described above.
The raw tube is rolled by a mandrel mill consisting of four rolling mill rows, and a product with good surface roughness and uniform thickness can be obtained, and a quality equal to or higher than that of a 7 to 8 mandrel mill. I was able to secure.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a roll arrangement and the like of a mandrel mill including four rolling mill rows in the present invention.

FIG. 2 is a diagram illustrating generation of a ridge and erasure thereof.

FIG. 3 is a view for explaining a comparison of the thickness reduction of each stand between a four-stand mandrel mill of the present invention and a conventional eight-stand mandrel mill.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Roll roll 1a Hole type 2 Mandrel bar 3 Raw tube 4 Roll opening 5 Contact part 6 Non-contact part 7a-7d Metal flow 8 Ridge

Claims (2)

(57) [Claims] In a method of rolling a seamless steel pipe by a mandrel mill, a rolling mill row is composed of four stands, a raw pipe is stretched and rolled by a first stand and a second stand, and a wall thickness is reduced by a third stand. A method for rolling a seamless steel pipe, comprising: rolling by 0.1 mm or more in amount and round-up rolling by a fourth stand. 2. The method of rolling a seamless steel pipe according to claim 1, wherein in the method of rolling a seamless steel pipe with a mandrel mill, the inner surface roughness (Rmax in the C direction) at the exit side of the mandrel mill is maintained at 20 μ or less. .