JP2013027899A - Method of manufacturing seamless steel pipe - Google Patents

Abstract

The present invention provides a method of manufacturing a seamless steel pipe capable of greatly reducing torsion of a pipe to be rolled in drawing rolling using a plug mill, which has been difficult to apply to either means of guide installation or hole shape change. .
A hollow steel pipe having a circular or elliptical cross section as a starting material, a pair of perforated rolls 1 whose caliber shape for processing the outer surface side of the rolled tube is a substantially arc shape, and a cross section for processing the inner surface side. In a plug mill having a circular plug 2, a plurality of passes are hot-rolled, and at that time, a reduction position 90 that shifts the caliber bottom contact position of the rolled tube by 90 degrees between each pass and the next pass. In the seamless steel pipe manufacturing method in which the degree is changed, rolling is performed so as to satisfy Expression (1) in each pass. Hc = 2 × t + Dp> 2 × R1 (1) Hc: Caliber bottom hole type height [mm], t: Delivery side target wall thickness [mm], Dp: Plug diameter [mm] at the bottom dead center of roll bite, R1: Caliber bottom hole radius [Selection] Figure 1

Description

  The present invention relates to a method of manufacturing a seamless steel pipe, and more particularly to a method of manufacturing a seamless steel pipe in which a cylindrical steel slab is formed into a hollow shell by Mannesmann drilling or the like, and this is rolled by a plug mill.

  In general, seamless steel pipes are made of cylindrical steel slabs, made into hollow shells by Mannesmann drilling, etc., drawn and rolled with a plug mill or mandrel mill, etc., further subjected to intermediate heating, etc., and finally sizer and stretch It is made into a final product by rolling with constant diameter by reducer. Since seamless steel pipes have a certain material and strength over the entire length of the pipe, they are used in places where the usage environment is severe, or are applied to structural steel pipes for mechanical structures.

  Structural steel pipes require high dimensional accuracy in construction, but seamless steel pipes are difficult to increase dramatically in wall thickness accuracy due to manufacturing technology. For example, in ERW steel pipes, the material is obtained by sheet rolling, so the thickness of the wall is accurate to the micron order, except for ERW welds. What is supposed to be easy to be eccentric. Furthermore, in the plug mill, a pair of perforated rolls and an inner surface tool (plug) are used, and rolling is usually performed in two passes or more, and each roll and the next pass are brought into contact with the caliber bottom. Since it is necessary to carry out rolling by shifting the circumferential direction position of 90 degrees (this is referred to as a 90-degree reduction of the reduction position), there is a possibility of uneven thickness due to twisting. In particular, in plug mill rolling under non-tension or extremely low tension conditions, the rolled tube is relatively free to move during rolling, so that twisting is likely to occur.

Eccentric unevenness due to heating can be reduced by controlling the furnace heating pattern, etc., but unevenness due to twisting in plug mill rolling is the final target wall thickness that is caused by the 90-degree change in the reduction position that induces the torsion leading to the twisting. Difficult to mitigate because it is necessary to achieve.
On the other hand, in the rolling of bar wire, a technique for preventing twisting by installing a guide between stands is known (Patent Document 1), where the twist in bar wire rolling is effectively prevented by equipment. ing.

  In addition, regarding the rolling method for optimal operation of the hole mold and material, various types of hole molds are used in steel bar rolling, etc., so it is necessary to grasp the characteristics of individual hole rolling and to adopt a reduction pattern suitable for it (hole mold) There is also a report that twist can be reduced by changing the shape (non-patent document 1).

JP-A 61-266114

Iron and Steel, Vol.89 (2003), p.758-764

However, if the above guide is applied to the drawing and rolling of steel pipes with a wide variety of types and dimensions (outer diameter, wall thickness) compared to bar wires, the guide settings must be revised each time the type and dimensions are changed. Therefore, application of guides to steel pipe rolling must be limited.
Further, in steel pipe drawing and rolling, it is necessary to adopt a so-called oval round system, that is, a caliber shape in which a caliber shape is limited to a substantially arc shape in order to ensure a round shape after rolling, and there is a range in which the hole shape can be changed. Since it is narrow, it is difficult to adopt the twist preventing method by changing the hole shape suggested in Non-Patent Document 1.

  As described above, in the conventional steel pipe drawing and rolling, it is difficult to apply any means of guide installation and hole shape change, and there is a problem in that the occurrence of twisting cannot be sufficiently prevented.

In order to solve the above-mentioned problems, the inventors studied the rolling conditions in which the rolled tube is difficult to rotate during the drawing and rolling in the plug mill by FEA (finite element analysis), and as a result, obtained the following knowledge.
(a) Plug mill rolling usually starts with a hollow element tube having a substantially circular cross section reduced by an elongator rolling prior to this, and then returns to the entry side after rolling for the first pass and rolling for the second pass. Do. The tube cross-sectional shape becomes a substantially elliptical shape by rolling each pass.
(b) Between the first pass and the second pass, the rolled tube is changed 90 degrees as described above, but the rolling of the second pass causes the rolled tube to fall in the roll bite. In many cases, twisting occurs over the entire length of the tube.
(c) FIG. 2 shows an example of the contact form between the material to be rolled, the roll, and the plug obtained by FEA in the case where no fall occurs in the first pass, and FIG. 3 shows the case where the fall occurs in the second pass. In FIGS. 2 and 3, 10 is a rolled pipe (however, 1/4 part of the entire circumference of the pipe), K1 is a roll contact start line on the outer surface of the pipe, K2 is a contact end line, and P1 is a plug contact start on the inner surface of the pipe. Line P2 is the contact end line. The tube outer surface region from the roll contact start line K1 to the roll contact end line K2 is a contact portion with the roll, and the tube inner surface region from the plug contact start line P1 to the plug contact end line P2 is a contact portion with the plug.

In the case where no collapse occurs, as shown in FIG. 2, the rolled tube 10 starts contact with the roll and plug from the caliber bottom side to the flange side almost simultaneously or near the flange side first, and rolling of the contact portion with the roll and plug begins. The contact length, which is the direction length, is substantially uniform in the pipe circumferential direction. On the other hand, when twisting occurs, as shown in FIG. 3, the rolled tube 10 starts preferentially contact with the roll and plug from the caliber bottom side, and the flange side remains in a non-contact state. The contact length of A is rapidly reduced from the caliber bottom side toward the flange side.
(d) From the above, if the flange side can be brought into contact with the roll and plug in the same contact form as the caliber bottom side, it is considered possible to make it difficult to twist the rolled tube, and the rolling conditions therefor Were studied through experiments and FEA. As a result, the caliber bottom hole height Hc determined by the formula Hc = 2 × t + Dp from the plug diameter (specifically plug diameter) Dp and the delivery target wall thickness t at the roll bite bottom dead center becomes the caliber bottom hole radius R1. On the other hand, when rolling was performed using a plug with Hc> 2 × R1, it was found that collapse did not easily occur. That is, rolling is performed using a plug satisfying the following formula (1).
Hc = 2 × t + Dp> 2 × R1 (1)
Hc: Caliber bottom hole height [mm], t: Delivery target wall thickness [mm], Dp: Plug diameter [mm] at the bottom dead center of roll bite, R1: Caliber bottom hole radius Figure 4 shows formula (1) 2 is an example of the result of FEA determining the contact form between the rolled pipe and rolls and plugs in the second pass of rolling, and the rolled pipe 10 has rolls and plugs in the entire area from the caliber bottom side to the flange side. You can see that there is enough contact.

This invention is made | formed based on the said knowledge, The summary is as follows.
(1)
A hollow steel pipe having a circular or elliptical cross section as a starting material, a pair of perforated rolls whose caliber shape for processing the outer surface side of the tube to be rolled is a substantially arc shape, and a plug having a circular cross section for processing the inner surface side A plurality of passes are hot-rolled in a plug mill having the following, and at that time, the rolling position is changed by 90 degrees to shift the caliber bottom contact position of the rolled tube by 90 degrees between each pass and the next pass. In the manufacturing method of a steelless pipe, it rolls so that following formula (1) may be satisfy | filled by each pass, The manufacturing method of the seamless steel pipe characterized by the above-mentioned.
Hc = 2 × t + Dp> 2 × R1 (1)
Hc: Caliber bottom hole height [mm], t: Delivery target thickness [mm], Dp: Plug diameter [mm] at bottom dead center of roll bite, R1: Caliber bottom hole radius (2)
Using one plug mill, the reduction position 90 degrees is changed by rotating the rolled pipe 90 degrees in the circumferential direction between each pass and the next pass, and the plug for the next pass is expressed by the above formula ( The method for producing a seamless steel pipe according to the above (1), wherein the steel pipe is replaced with one satisfying 1).

  ADVANTAGE OF THE INVENTION According to this invention, the twist of a to-be-rolled tube can be reduced significantly in the extending | stretching rolling using a plug mill.

1 is a cross-sectional view showing an embodiment of the present invention (specifically, a cross-sectional view perpendicular to the rolling direction and passing through the bottom dead center of a roll bite). It is the schematic of the FEA result which illustrates the contact form of a to-be-rolled tube, a roll, and a plug in case a fall does not arise in the plug mill rolling 1st pass. It is the schematic of the FEA result which illustrates the contact form of a to-be-rolled tube, a roll, and a plug in case a fall occurs in the 2nd pass of plug mill rolling. It is the schematic of the FEA result which illustrates the contact form of the to-be-rolled tube of the 2nd rolling which satisfy | fills Formula (1), a roll, and a plug.

FIG. 1 is a cross-sectional view showing an embodiment of the present invention (specifically, a cross-sectional view perpendicular to the rolling direction and passing through the bottom dead center of a roll bite). 1 is a pair of perforated rolls, and 2 is a plug.
The caliber of the plug mill hole roll 1 is generally symmetrical with respect to a plane passing through the caliber bottom point Z and the pass line center point O (specifically, any two points within the bus line center line). The sides of the caliber forming the arcs BC and DE with the radius of curvature R2 (R2> R1) are connected to both sides of the bottom of the caliber forming the arc CD with the radius of curvature R1 having the center of curvature R1, and the curvature of the reverse curve is respectively connected to the outside. The flanges forming the arcs AB and EF having the radius R3 (R3 << R1) are connected to each other.

The caliber-shaped hole roll used in the present invention has a substantially arc-shaped hole roll having an arc CD angle range θ of 90 ° to 150 ° in the radius of curvature R1 of the caliber bottom (ie, caliber bottom hole radius R1) in FIG. Say.
On the other hand, the plug 2 has a circular cross-sectional shape.
And in this invention, as above-mentioned, in extending | stretching rolling, it rolls so that Formula (1) may be satisfy | filled with respect to the delivery target thickness t in each pass.
Hc = 2 × t + Dp> 2 × R1 (1)
Hc: Caliber bottom hole type height [mm], t: Delivery target wall thickness [mm], Dp: Plug diameter [mm] at the bottom dead center of roll bite, R1: Caliber bottom hole radius To satisfy equation (1) In order to perform rolling, a perforated roll and / or a plug may be selected so that a combination of Dp and R1 satisfying the formula (1) is obtained.

As a result, as shown in FIG. 4, in any pass, the rolled tube 10 is in sufficient contact with the rolls and plugs in the entire area from the caliber bottom side to the flange side, making it difficult to collapse, so that twisting is effectively prevented. Can do.
When Hc ≦ 2 × R1, the fall prevention effect is poor. However, if the caliber bottom hole type height Hc is excessive, there is a possibility of causing defects and uneven thickness due to strong pressure on the flange, and therefore, Hc is preferably 2 × R1 + 4 [mm] or less.

  In addition, when a plurality of buses are drawn and rolled using one plug mill, the reduction of the rolling position by 90 degrees is performed by rotating the rolled tube by 90 ° in the pipe circumferential direction between each pass and the next pass. . Instead of this, it is disadvantageous to rotate the perforated roll 90 degrees around the center line of the pass line because the equipment becomes complicated. Also, when it is found that the next pass does not satisfy the formula (1) at the rolling scheduling stage, it is time-consuming to cope with the hole roll exchange, and the rolling efficiency is lowered. Correspond. That is, it is preferable to replace the plug for the next path with a plug satisfying the formula (1).

Seamless carbon steel pipe of general carbon steel (JIS G3101 SS400 steel) is drawn and rolled in two passes using one plug mill (the rolled tube is rotated 90 degrees in the pipe circumferential direction between the first pass and the second pass) According to the following rolling schedule.
Billet diameter: 170 mm, heating temperature: 1250 ° C. → piercing (piercer) → elongator outlet side target dimensions: outer diameter 198 mm × thickness 17.0 mm (= starting material for plug mill rolling)
-Plug mill rolling: target dimension on the first pass (wall thickness 14.5 mm) → target dimension on the second pass (side diameter 193 mm x wall thickness 13.0 mm)
・ R1 of plug roll hole roll = 95.5 mm (θ = 130 degrees in Fig. 1)
・ In the conventional example, the same plug mill plug is used in the first pass and the second pass, and in the present invention example, plugs having different Dp are used in the first pass and the second pass (the first pass is the conventional example). The same plug was used, and the second pass was replaced with a plug with a different Dp).
(Invention Example) Dp of plug for first pass = 164 mm ((Hc = 2 × t + Dp = 2 × 14.5 + 164 = 193)> (2 × R1 = 2 × 95.5 = 191)) Dp of plug for second pass = 166 mm ((Hc = 2 × t + Dp = 2 × 13.0 + 166 = 192)> (2 × R1 = 2 × 95.5 = 191))
(Conventional example) Dp of the plug for the first pass = 164 mm ((Hc = 2 × t + Dp = 2 × 14.5 + 164 = 193)> (2 × R1 = 2 × 95.5 = 191)) Dp of the plug for the second pass = 164 mm ((Hc = 2 × t + Dp = 2 × 13.0 + 164 = 190) ≦ (2 × R1 = 2 × 95.5 = 191))
-Number of rolling N: 20 in both the present invention and the conventional example The twist prevention effect according to the present invention is to mark the leading end and the trailing end of the inlet side tube, and to mark the leading end and the trailing end of the outlet side tube. Taking a photograph, measuring the amount of marking position (pipe circumferential position) deviation from the entry side, and identifying the case where the twist angle obtained from the measured value is 15 degrees or more as the occurrence of twist, the twist occurrence rate (= The number of rolled rolls / N (× 100%)) was evaluated.

  As a result, the twist occurrence rate in the conventional example was 20%, whereas in the example of the present invention, it was greatly improved to 5%.

A seamless steel pipe made of general carbon steel (JIS G4053 SCM435 applicable steel) is rolled and rolled in two passes using one plug mill (the rolled tube is rotated 90 degrees in the pipe circumferential direction between the first and second passes). According to the following rolling schedule.
Billet diameter: 300 mm, heating temperature: 1250 ° C. → drilling (piercer) → elongator outlet side target dimensions: outer diameter 329 mm × wall thickness 11.0 mm (= starting material for plug mill rolling)
-Plug mill rolling: target dimension on the first pass (wall thickness 8.5 mm) → target dimension on the second pass (thickness 7.7 mm)
・ R1 of plug mill hole roll = 160 mm (θ = 110 degrees in Fig. 1)
・ In the conventional example, the same plug mill plug is used in the first pass and the second pass, and in the present invention example, plugs having different Dp are used in the first pass and the second pass (the first pass is the conventional example). The same plug was used, and the second pass was replaced with a plug with a different Dp).
(Invention Example) Dp of plug for first pass = 304 mm ((Hc = 2 × t + Dp = 2 × 8.5 + 304 = 321)> (2 × R1 = 2 × 160 = 320)) Dp of plug for second pass = 306 mm ((Hc = 2 × t + Dp = 2 × 7.7 + 306 = 321.4)> (2 × R1 = 2 × 160 = 320))
(Conventional example) Dp of the plug for the first pass = 304 mm ((Hc = 2 × t + Dp = 2 × 8.5 + 304 = 321)> (2 × R1 = 2 × 160 = 320)) Dp of the plug for the second pass = 304 mm ((Hc = 2 × t + Dp = 2 × 7.7 + 304 = 319.4) ≦ (2 × R1 = 2 × 160 = 320))
-Number of rolling N: 20 in both the present invention and the conventional example The twist prevention effect according to the present invention is to mark the leading end and the trailing end of the inlet side tube, and to mark the leading end and the trailing end of the outlet side tube. Taking a photograph, measuring the amount of marking position (pipe circumferential position) deviation from the entry side, and identifying the case where the twist angle obtained from the measured value is 15 degrees or more as the occurrence of twist, the twist occurrence rate (= The number of rolled rolls / N (× 100%)) was evaluated.

  As a result, in the conventional example, the twist occurrence rate was 5%, whereas in the example of the present invention, it was greatly improved to 0% (no twist occurred).

1 Hole roll 2 Plug
10 Rolled tube
K1: Roll contact start line
K2: Roll contact end line
P1: Plug contact start line
P2: Plug contact end line

Claims (2)

A hollow steel pipe having a circular or elliptical cross section as a starting material, a pair of perforated rolls whose caliber shape for processing the outer surface side of the tube to be rolled is a substantially arc shape, and a plug having a circular cross section for processing the inner surface side A plurality of passes are hot-rolled in a plug mill having the following, and at that time, the rolling position is changed by 90 degrees to shift the caliber bottom contact position of the rolled tube by 90 degrees between each pass and the next pass. In the manufacturing method of a steelless pipe, it rolls so that following formula (1) may be satisfy | filled by each pass, The manufacturing method of the seamless steel pipe characterized by the above-mentioned.
Hc = 2 × t + Dp> 2 × R1 (1)
Hc: Caliber bottom hole height [mm], t: Delivery target thickness [mm], Dp: Plug diameter [mm] at bottom dead center of roll bite, R1: Caliber bottom hole radius   Using one plug mill, the reduction position 90 degrees is changed by rotating the rolled pipe 90 degrees in the circumferential direction between each pass and the next pass, and the plug for the next pass is expressed by the above formula ( The method for producing a seamless steel pipe according to claim 1, wherein the steel pipe is replaced with one satisfying 1).

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