JP2014166649A - Method for manufacturing seamless steel pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method which sufficiently prevents the generation of thickness deviation and scratches in a cross section during seamless steel pipe draw rolling using a plug mill.SOLUTION: In a method for manufacturing a seamless steel pipe, a hollow tube stock with a circular or an elliptical cross section is used as a starting material to carry out hot draw rolling of a plurality of paths with a pair of hole type rolls 1 with almost circular arc-shaped caliber that works the outer surface side to be rolled and a plug mill 2 having a plug with a circular cross section that works the inner surface side; and at this time, press down position 90 degrees change is performed in which contact positions of the caliber bottoms of tubes to be rolled are shifted by 90 degrees in each path and the next path. Such a draw rolling is performed to satisfy the following formula (1) in each path. Dp≤2×R1-2×t+0.05×t(1), wherein Dp: a plug diameter (mm) at a bottom dead point of a roll caliber tool, R1: a hole radius (mm) at a caliber bottom, and t: an outlet-side target wall thickness (mm).

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, a seamless steel pipe is made of a cylindrical steel slab and is made into a hollow shell by Mannesmann drilling or the like, and after appropriately adjusting the thickness and outer diameter through an intermediate rolling process called an elongator, a plug mill or a mandrel mill, etc. The film is stretched and rolled at, and further subjected to intermediate heating, and finally is subjected to constant diameter rolling with a sizer or stretch reducer to obtain a final product. Since seamless steel pipes have a certain material and strength over the entire length of the pipe, they are used in oil wells and the like where the usage environment is severe, or are applied to structural steel pipes for mechanical structures.

  Structural steel pipes and some oil well steel pipes require high dimensional accuracy in construction, but seamless steel pipes are difficult to dramatically increase in wall thickness accuracy due to manufacturing technology. For example, in ERW welded pipes, the material is obtained by rolling thin plates, so the thickness of the wall is accurate to the micron order except for ERW welds. Things that should be easy to eccentric. Furthermore, in a plug mill, a pair of perforated rolls and an internal tool (plug) are used, and rolling is usually performed in two or more passes, and the rolled tube is brought into contact with the caliber bottom in each pass and subsequent passes. Since it is necessary to perform rolling by shifting the circumferential direction position of 90 degrees (this is referred to as changing the rolling position to 90 degrees), it is difficult to achieve a uniform thickness at the opposite portion, and uneven thickness may occur within the tube cross section. There is sex.

Eccentric eccentric thickness due to heating can be reduced by controlling the furnace heating pattern, etc. However, the uneven thickness in plug mill rolling is necessary to achieve the final target thickness by changing the reduction position by 90 degrees. Difficult to mitigate.
On the other hand, for example, in a mandrel mill in which a rolling stand composed of two or more rolls is continuously stretched and rolled with an odd-numbered stand and an even-numbered stand facing each other, it is a multi-stand, so the lower part of the main pressure is relatively lower than the previous stand A roll with a narrow arc shape range, a so-called open caliber, is used, and the rear stand that determines the wall thickness and shape uses a roll with a closed caliber that increases the range to suppress uneven thickness (non-patent literature) 1).

  However, the plug mill rolling method applied in the production of large-diameter steel pipes as a rolling device for reducing the wall thickness usually rolls with a pair of caliber rolls and a single inner surface tool (plug). A hollow shell manufactured by an elongator or the like can be rolled only with a predetermined combination of tools. In addition, due to the positional relationship between the plug and the roll caliber, a region of strong local reduction is formed in the cross section of the tube, and wrinkles may be generated there. From the viewpoint of preventing such uneven thickness and wrinkles, some plug mills produce a plurality of hole molds in a roll having a long trunk length, and with this, it is possible to perform extension rolling as if it were multi-stand rolling. (Non-Patent Document 2).

  In addition, regarding the reduction setting by the plug mill, a method has been proposed in which the reduction setting is performed based on the characteristic value indicating the relative positional relationship in consideration of the geometrical effect on the assumption that there is only one type of plug diameter ( Patent Document 1). Furthermore, in this setting method, changes due to the thermal expansion of rolls and plugs are not considered, and it cannot be set optimally.When changing the lot such as the size of the material to be rolled, the roll gap value of the previous material is used as a reference. In addition to correction by the geometric dimension difference determined by the rolling tool diameter and the target wall thickness, the deformation resistance difference due to the material change between the previous and next materials, the temperature difference between the previous and next materials, or the previous material In consideration of correction due to the difference in rolling tool diameter of the next material, there is a method for determining the initial rolling reduction value at which the first thickness or length of the material to be rolled becomes the target value using the rolling information of the previous material. It has been proposed (Patent Document 2).

  In addition, when the steel slab drilled by the Piercer mill is expanded with an elongator and subsequently stretch-rolled with a plug mill consisting of a pair of upper and lower main rolls and a return roll, A method of manufacturing a seamless steel pipe is proposed in which the thickness deviation in the longitudinal direction of the pipe is reduced by changing at a predetermined speed based on the outer diameter of the plug used in the pipe, the outer diameter of the pipe that has passed through the elongator, and the temperature. (Patent Document 3).

  Further, in a mandrel mill, a technique is generally used in which inner surface properties are ensured by attaching and rolling a graphite-based lubricant to a bar which is an inner surface tool (Patent Document 4). In the mandrel mill, the bar moves in the rolling direction at a constant speed or close to that of the rolled material, regardless of the presence or absence of control, so there is little increase in the relative speed difference between the inner surface tool (bar) and the tube to be rolled. The effect of preventing wrinkles is obtained.

JP-A-57-9521 Japanese Patent Laid-Open No. 2-80110 JP 2004-202573 A Japanese Patent Laid-Open No. 50-144868

Edited by Japan Iron and Steel Institute, “Third Edition Steel Handbook”, Maruzen Co., Ltd., issued on November 20, 1980, p979 Edited by Japan Iron and Steel Institute, “Third Edition Steel Handbook”, Maruzen Co., Ltd., issued on November 20, 1980, p961

However, the method of producing and rolling a plurality of perforations in the roll of the plug mill requires a large roll having a large diameter and a long trunk length in order to produce a large diameter steel pipe, and also has dimensions (especially outer diameter). However, in order to manufacture a wide range of steel pipes, a large number of large rolls must be held, which is not realistic and cannot be employed.
In addition, the reduction setting method as shown in Patent Document 1 or Patent Document 2 has a cross section perpendicular to the rolling direction even if the total thickness is close to the set value and the average wall thickness can be adjusted. No improvement can be expected for the thickness distribution, especially the rolling distribution based on the relationship between the hole shape and the plug shape. Further, in the method of manufacturing a seamless steel pipe shown in Patent Document 3, it is possible to suppress a thickness deviation in the longitudinal direction of the pipe, but in a plug mill, usually, a plurality of rolling operations are performed once. Every time the rolled tube is rotated 90 degrees in the circumferential direction, it is difficult to suppress uneven thickness in the cross section perpendicular to the longitudinal direction of the tube.

  In addition, the inner surface tool (plug) is fixed in the plug mill, and the relative speed difference between the inner surface tool and the pipe to be rolled is large in the plug mill, and from the outlet side to the inlet side immediately after one pass rolling. It is difficult to form a strong lubricating film because it is a unique rolling method in which the tube to be rolled is returned and the second pass rolling is performed without any gaps, and the plug temperature before rolling is generally low. Is limited. In particular, in the case of rolling a steel pipe material in which an oxide scale that reduces seizure flaws or the like hardly occurs, or rolling under low temperature conditions, it is very difficult to prevent the occurrence of flaws on the inner surface.

  As described above, in seamless steel pipe drawing and rolling using a plug mill, it is difficult to apply any means for changing the shape of the hole or using a lubricant. There was a problem that could not be prevented.

In order to solve the above-mentioned problems, the inventors focused on the contact relationship between a rolled tube during drawing and rolling with a plug mill, a hole roll, and a plug, and examined them by FEA (finite element analysis) and rolling experiments. As a result, the invention having the following gist was completed.
[1] A hollow raw tube 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 rolled tube is a substantially arc shape, and a cross section for processing the inner surface side Multiple passes are hot-rolled in a plug mill having a circular plug, and at that time, the reduction position 90 degrees is changed by shifting the caliber bottom contact position of the rolled tube by 90 degrees between each pass and the next pass. In the method for producing a seamless steel pipe, a method for producing a seamless steel pipe is characterized in that in each pass, stretching and rolling are performed so as to satisfy the following formula (1).
Dp ≦ 2 × R1-2 × t + 0.05 × t (1)
Here, Dp: plug diameter [mm] at the bottom dead center of roll bite, R1: caliber bottom hole radius [mm], t: outlet side target thickness [mm]

[2] Using a single plug mill, the reduction position is changed by 90 degrees 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 used. It replaces | exchanges for what satisfy | fills said Formula (1), The manufacturing method of the seamless steel pipe as described in [1] characterized by the above-mentioned.

  ADVANTAGE OF THE INVENTION According to this invention, the internal surface flaw and uneven thickness of a to-be-rolled pipe can be reduced significantly in the extending | stretching rolling of the steel pipe 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 the local strong pressure lower part does not arise in the plug mill rolling 1st pass which satisfies Formula (1). It is the schematic of the FEA result which illustrates the contact form of a to-be-rolled tube, a roll, and a plug when a plug with an excessively large diameter is used in the first pass of plug mill rolling and a locally strong lower portion is generated. It is the schematic of the FEA result which illustrates the contact form of the to-be-rolled tube of the 2nd rolling using the plug of the diameter which satisfy | fills Formula (1), a roll, and a plug. 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 uneven thickness and an inner surface flaw arise in the rolling 2nd pass which uses a plug with an excessive diameter.

In the drawing and rolling of seamless steel pipes using a plug mill, as described above, it is difficult to change the hole shape. Therefore, the inventors studied FEA (finite element analysis) and rolling experiments on rolling conditions in which the rolled tube is rolled in contact with the hole-type roll and the plug under the optimum conditions during the drawing and rolling in the plug mill.
In the plug mill rolling, usually, a hollow shell having a substantially circular cross-section reduced by the elongator rolling prior to this is used as a starting material, after the first pass rolling, it is returned to the entry side and the second pass rolling is performed. At this time, the tube cross-sectional shape becomes substantially elliptical by rolling in each pass, but the inventors have found that the contact between the rolled tube and the plug and roll is characteristic by FEA and rolling experiments.

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 0 (specifically, any two points in the pass line center line). Caliber side surfaces forming arcs BC and DE each having a radius of curvature R2 (R2> R1) are connected to both sides of a caliber bottom portion forming an arc CD having a radius of curvature R1 centered at Z, and reversely bent to the outside thereof. The flanges forming the arcs AB and EF having the curvature radius R3 (R3 << R2) are connected to each other.

The caliber shape used in the present invention has a substantially arc-shaped hole-shaped roll in FIG. 1 in which the angle range θ of the arc CD of the radius of curvature R1 of the caliber bottom (that is, the caliber bottom hole radius R1) is 90 to 150 degrees. Say.
On the other hand, the plug 2 has a circular cross-sectional shape.
In the above plug mill, when the plugs having different diameters are used, the first round of rolling and the second round of rolling performed by changing the rolling position by 90 degrees are performed on the hollow shell having a substantially circular cross section. FIGS. 2 to 5 show examples of results obtained by FEA for the contact state between the material to be rolled, the roll, and the plug in each rolling. 2 and 4 are examples of the first pass and the second pass when the plug diameter is appropriate, wrinkles are not generated, and the uneven thickness is small. FIGS. 3 and 5 are respectively the plug diameters being excessive and the inner surface wrinkles. This is an example of the first pass and the second pass when many occurrences and uneven thickness are large. In these figures, 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, P1 is a plug contact start line on the inner surface of the pipe, 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.

  When FIG. 2 and FIG. 3 are compared, when the plug diameter is large (FIG. 3), it can be seen that the contact with the roll starts early near the flange side, and the contact length is also long. This phenomenon indicates that the reduction is locally increased in the vicinity of the contact start point, which leads to inducing uneven thickness and inner surface flaws in the first pass rolling. Furthermore, as shown in FIG. 4 and FIG. 5, in the second pass rolling, the contact state varies greatly depending on the plug diameter. That is, when the plug diameter is small and appropriate (FIG. 4), the bottom of the caliber that will be the unrolled part is intensively contacted in the first pass, whereas when the plug diameter is excessive (FIG. 5). In addition to the caliber bottom, the flange side is also in great contact. Therefore, in the case where the plug diameter is excessive, in addition to the vicinity that was strongly reduced in the first pass, the portion that was the bottom of the caliber in the first pass rolling will be reduced again, and uneven thickness and inner surface flaws will be reduced. Will trigger.

From the above, it is considered that it is possible to suppress the occurrence of uneven thickness and inner surface flaws by optimizing the contact state between the rolled tube and the rolls and plugs, and the rolling conditions therefor are determined through experiments or FEA. We studied diligently. As a result, it is found that when the plug diameter Dp is rolled using a plug satisfying the roll caliber bottom hole radius R1, the outlet side target thickness t and the following formula (1), the occurrence of uneven thickness and inner surface flaws can be suppressed. It was.
Dp ≦ 2 × R1-2 × t + 0.05 × t (1)

  The geometrically appropriate range is Dp ≦ 2 × R1-2 × t, but the further added term (+ 0.05 × t) is the deformation behavior of the rolled tube in actual tube rolling. Is corrected in consideration of the above. That is, when the thickness of the tube to be rolled is thin, deformation of the tube to be rolled occurs outside the rolling mill, and the contact between the tube to be rolled and the plug becomes loose on the flange side in the tube cross section, and the wall on the flange side is reduced. In contrast to the reduced thickness reduction, in the case of a thick-walled tube, the tube is rolled in a roll bite while maintaining the shape before rolling without deforming the rolled tube outside the rolling mill because of the high rigidity of the tube. Therefore, the correction is made in consideration of the tendency that the entire circumference is easily reduced.

Thereby, as shown in FIG. 2 and FIG. 4, the contact between the tube to be rolled, the roll, and the plug becomes appropriate, and the occurrence of uneven thickness and inner surface flaws can be suppressed.
If the plug diameter is too small, the wall thickness distribution in the circumferential direction of the pipe cross section becomes extremely non-uniform. This adverse effect can be avoided by always setting the plug diameter to be used as a suitable diameter for calculation, but in order to do so, it is necessary to provide a large number of plugs having slightly different diameters. However, it is not realistic from the standpoint of cost increase to excessively subdivide the pitch of the plug diameter possessed. Therefore, in consideration of the plug diameter pitch that can be industrially held, it is preferable to set the lower limit value to a value obtained by subtracting 0.2 × t from the right side of the formula (1).

  Further, in the case where a plurality of passes are stretched and rolled using one plug mill, the reduction of the rolling position by 90 degrees is performed by rotating the rolled tube by 90 degrees 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, if it is found at the rolling scheduling stage that the next pass does not satisfy the formula (1), it is time consuming to replace the hole-type roll and the rolling efficiency is reduced. To do. That is, it is preferable to replace the plug for the next path with one that satisfies the formula (1).

A seamless steel pipe of general carbon steel (JIS G3101 SS400 applicable 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. → drilling (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 = 95.5mm of plug mill hole type roll
The plug mill plug used was a Dp = 162 mm plug in the present invention, and a Dp = 164 mm plug was used in the comparative example.
-Number of rolling N: 30 in both the present invention example and the comparative example The effect of suppressing the thickness deviation is the thickness deviation ratio in the cross section at the longitudinal center (divided marks in the circumferential direction into 16 points, and the measurement point is an ultrasonic thickness gauge) And (the maximum thickness−minimum thickness) / average thickness was determined). Further, the effect of preventing the generation of inner surface flaws was evaluated by the presence or absence of wrinkles exceeding a depth of 0.3 mm in the entire length excluding 0.5 m each of the unsteady portion at the longitudinal end of the tube.

In the above rolling schedule, the plug diameter Dp suitable for the present invention in the first pass of the plug mill rolling is Dp ≦ 2 × R1-2 × t + 0.05 × t
= 2 x 95.5-2 x 14.5 + 0.05 x 14.5 = 162.725
However, if the plug diameter is 162.725 mm or less, it is within the scope of the present invention. However, if the plug diameter is too small, the wall thickness distribution in the circumferential direction of the tube cross section becomes extremely nonuniform. The value obtained by subtracting 2 × t is set as the lower limit. Therefore, the plug diameter of 162 mm is within the scope of the present invention, and the plug diameter of 164 mm is excessive. On the other hand, in the second pass, a plug diameter of 164 mm is an appropriate range.

  As a result of rolling, the uneven thickness ratio of the total number in the comparative example was 10.7%, whereas in the example of the present invention, it was greatly improved to 8.2%. Further, in the comparative example, internal flaws occurred in 13 out of 30, but in the inventive example, no internal flaws occurred.

A seamless steel pipe made of general carbon steel (JIS G4053 SCM435 applicable 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: 300 mm, heating temperature: 1250 ° C. → drilling (piercer) → elongator outlet side target dimensions: outer diameter 329 mm × thickness 11.5 mm (= starting material for plug mill rolling)
・ Plug mill rolling: Target dimension on the first pass (thickness 9.3 mm) → Target dimension on the second pass (thickness 8.5 mm)
・ R1 = 160mm for plug mill hole type roll
In the comparative example, the same plug mill plug was used in the first pass and the second pass, and in the present invention example, plugs having different Dp were used in the first pass and the second pass.
(Example of the present invention) Dp of plug for the first pass = 300 mm (≦ 2 × R1-2 × t + 0.05 × t = 2 × 160-2 × 9.3 + 0.05 × 9.3 = 301.865), 2 Dp of the plug for the pass = 302 mm (≦ 2 × R1-2 × t + 0.05 × t = 2 × 160-2 × 8.5 + 0.05 × 8.5 = 303.425)
(Comparative example) Dp of plug for first pass = 302 mm (> 2 × R1-2 × t + 0.05 × t = 2 × 160-2 × 9.3 + 0.05 × 9.3 = 301.865), 2 passes Eye plug Dp = 302 mm (≦ 2 × R1-2 × t + 0.05 × t = 2 × 160-2 × 8.5 + 0.05 × 8.5 = 303.425)
-Rolling number N: 30 for both the inventive example and the comparative example After rolling, the uneven thickness and the inner surface flaw were evaluated in the same manner as described above. As a result, in the comparative example in which a plug having a diameter of 302 mm was continuously used, 11 out of 30 Although wrinkles were generated in the books and the uneven thickness ratio was 9.7%, no wrinkles were generated in the examples of the present invention, and the uneven thickness ratio was greatly improved to 8.3%.

1 Hole type 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 blank tube having a circular or elliptical cross section is used as a starting material, a pair of perforated rolls whose caliber shape for processing the outer surface side between the rolled parts is a substantially arc shape, and a cross section for processing the inner surface side is circular. A plurality of passes are hot-rolled in a plug mill having a plug, 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 seamless steel pipe, the rolling process is performed so that it may satisfy | fill following formula (1) by each pass, The manufacturing method of the seamless steel pipe characterized by the above-mentioned.
Dp ≦ 2 × R1-2 × t + 0.05 × t (1)
Here, Dp: plug diameter [mm] at the bottom dead center of the roll bite,
R1: Caliber bottom hole radius [mm], t: Delivery side target plate thickness [mm]   Using one plug mill, the reduction position of 90 degrees is changed by rotating the rolled tube 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 ( It replaces | exchanges for what satisfy | fills 1), The manufacturing method of the seamless steel pipe of Claim 1 characterized by the above-mentioned.

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