Plug mill rolling method and plug mill

Abstract

To provide a plug mill rolling method advantageously avoiding pit-shaped defect residue of a hollow tube inner surface by keeping rolling pressure in a plug mill by giving a method that allows prevention of buckling even in the case of a bar of a small outer diameter.SOLUTION: According to a plug mill rolling method, a plug to be fitted at one end of a cylindrical bar disposed on a central axis of a hollow tube is inserted inside a punched hollow tube, pressing is given between at least a pair of pressure rolls arranged outside the plug and the hollow tube to extend the hollow tube. A middle position in a longitudinal direction of the bar away from the other end of the bar fixed and supported is supported according to shift of a bar radial direction generated to the bar caused by rolling load given in the direction of the bar shaft.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a plug mill rolling method in which a cylindrical steel billet is formed into a hollow blank by Mannesmann piercing or the like in the production of a seamless steel pipe, and the hollow blank is then elongated by a plug mill, and to a plug mill used in this method. [Background technology]

[0002] The manufacturing process for seamless steel pipes involves first heating a cylindrical steel billet to a high temperature in a heating furnace and forming it into a hollow blank using a Mannesmann piercing machine called a piercer. This hollow blank is then expanded and elongated in a second piercing machine called an elongator. It is then elongated in a plug mill, and expanded and polished in, for example, a reeler mill. It is then heated again to a high temperature in a reheating furnace and formed into a seamless steel pipe of the desired dimensions using a sizer or stretch reducer.

[0003] 1, the plug mill includes a cylindrical (mandrel) bar 2 arranged on a pass line S of a hollow blank 1, a plug 3 attached to one end (front end) of the bar 2, a pair of rolling rolls 4 that roll down the hollow blank 1 between the plug 3, and a return roll 5 that feeds the hollow blank 1 in the reverse direction after rolling. The other end (rear end) of the bar 2 is held by a rear end fitting 6 and a bar clamp 7, and the axial position of the bar 2 is adjustable.

[0004] Elongation rolling by this plug mill is a rolling technique in which a reduction is applied to the hollow blank 1 using rolls 4 located on the outside of the hollow blank 1 and a plug 3 inserted inside the hollow blank 1, thereby elongating the hollow blank 1. Incidentally, Fig. 1 shows the state in which elongation rolling of the hollow blank 1 has been completed, after which the rolls 4 are released and the plug 3 is removed from the bar 2, and the elongated hollow blank 1 is then reversed by the return rolls 3 and transported to the inlet side of the plug mill.

[0005] Before being elongated by the plug mill, the hollow blank has a highly rough inner surface due to rolling in the piercer and elongator. Therefore, elongation rolling by the plug mill also serves to smooth the surface of the hollow blank. To smooth the surface of the hollow blank, a certain level of rolling pressure must be applied to the hollow blank. The pressure applied to the work rolls 4 and bar 2 in the plug mill increases proportionally. Because axial pressure acts on the bar 2 through the plug 3, applying a rolling pressure above a certain level can cause buckling of the bar 2 depending on its section modulus. Therefore, in the case of a thick-walled hollow blank, which must be elongated by a thin bar 2 with a small outer diameter 8 and low buckling stress, it may be impossible to apply a large rolling pressure while preventing buckling of the bar.

[0006] Each standard specifies a length range for steel pipe products. For example, the API 5CT oil country tubular goods standard is a representative example of a seamless steel pipe product. The main length range in this standard, Range 2, is approximately 8 m, while Range 3 is a maximum of approximately 14 m. Furthermore, when a sizer is used in the forming process, the length of the hollow blank pipe produced by the plug mill is roughly equivalent to the length of the final product, so the bar length of the plug mill must necessarily be 14 m or longer. Therefore, when manufacturing this type of product using plug mill rolling, it is extremely difficult to prevent buckling of the long bar.

[0007] In the case of a hollow blank that is not sufficiently reduced when rolled by a plug mill, the highly rough inner surface cannot be completely smoothed, and the possibility of pit-like defects occurring on the inner surface of a steel pipe product increases.

[0008] The mechanism by which pit-like defects occur on the inner surface of a steel pipe is shown in Figure 2. That is, the inner surface of the hollow blank 1 before plug rolling has a highly rough surface texture, as described above. When the hollow blank 1 is reduced and elongated between the roll 4 and the plug 3 in the plug mill, the surface texture of the inner surface of the pipe improves, as in the hollow blank 1a after elongation. The degree of improvement in surface texture increases as the difference between the wall thickness t1 of the hollow blank 1 and the wall thickness t2 of the hollow blank 1a increases. To increase this difference, a larger rolling pressure R is required. In other words, if it is difficult to apply a large rolling pressure R in order to prevent the buckling of the bar 2 described above, pit-like defects 20 will remain on the inner surface of the hollow blank 1a.

[0009] Regarding these inner surface defects of hollow blanks, Patent Document 1 proposes a technology in which a lubricant is injected onto the inner surface of the hollow blank to reduce friction of the plug and thereby suppress the occurrence of defects. Furthermore, Patent Document 1 also proposes devising a lubricant flow path built into the bar in order to suppress deformation of the bar. However, because the bar structure described in Patent Document 1 is a structure that allows expansion and bending, a different approach is required to resolve the above-mentioned problem related to bar buckling. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Publication No. 2017-217683 Summary of the Invention [Problem to be solved by the invention]

[0011] Generally, plugs used in plug mills are replaced with ones with an appropriate diameter depending on the thickness of the hollow blank, and then plug rolling is performed. That is, the outer diameter of the bar must be smaller than the outer diameter of the plug. Therefore, when rolling a thick-walled pipe using a plug mill, for example, it is necessary to use a bar with a small outer diameter (low anti-buckling pressure). In order to avoid buckling of a bar with a small outer diameter, it is necessary to reduce the rolling pressure, which increases the possibility of pit-like defects remaining on the inner surface of the steel pipe product, as described above.

[0012] Therefore, an object of the present invention is to provide a plug mill rolling method that prevents buckling of the bar, particularly a bar with a small outer diameter, thereby maintaining the rolling pressure in the plug mill at a constant level or higher and advantageously avoiding the formation of pit-like defects on the inner surface of a hollow shell. Another object of the present invention is to provide a plug mill that is suitable for use in this plug mill rolling method. [Means for solving the problem]

[0013] The inventors conducted various studies to find a way to solve the above-mentioned problems. First, FIG. 3 shows an existing theory (Euler's formula) for calculating the magnitude of the external force P that will cause a cylindrical member, such as a bar, to buckle when an axial external force P is applied to the member. According to this theory, the shorter the axial length L of the member, the greater the external force P that will cause the member to buckle, making the member less likely to buckle. Therefore, assuming a constant rolling pressure, shortening the length from the front end of the bar to the fixed point at the rear end—specifically, the effective length L from the front end of the bar to the fitting 6 on the rear end side of the bar in FIG. 1—is effective in preventing bar buckling. In the plug mill shown in FIG. 1, shortening the effective length L by moving the rear end fitting 6 and the existing bar clamp 7 of the bar 2 closer to the plug 3 of the bar 2 would theoretically prevent bar buckling.

[0014] However, existing bar clamps 7 and the like are fixed equipment, and making them movable would require large-scale equipment modifications, making this not a realistic solution. In other words, it was not possible to suppress the remaining pit-like defects on the inner surface of thick-walled pipes with the existing equipment limitations.

[0015] Therefore, a method for shortening the above-mentioned effective length L without making major modifications to existing equipment was explored. As a result, the inventors discovered that by providing a new gripping device for gripping the bar at a position near the tip of the bar that does not interfere with the hollow shell after rolling and by devising a gripping method, it is possible to prevent buckling, particularly in bars with small outer diameters. The present invention is based on the above-mentioned findings and has the following gist.

[0016] 1. A plug mill rolling method comprising inserting a plug into a pierced hollow blank, the plug being attached to one end of a cylindrical bar arranged on the central axis of the hollow blank, and applying a rolling reduction between the plug and at least one pair of rolling rolls arranged outside the hollow blank to elongate the hollow blank, a plug mill rolling method, wherein a longitudinal intermediate position of the bar, which is spaced from the other fixedly supported end of the bar, is supported in accordance with a radial displacement of the bar caused in the bar due to a rolling load applied in the axial direction of the bar.

[0017] 2. The plug mill rolling method according to 1 above, wherein the intermediate position is gripped via at least three contact points spaced apart from one another at intervals of 120° or less in the circumferential direction of the bar, at least two contact points in a region below a horizontal line passing through the central axis of the bar are brought into contact with the bar, and at least one contact point in an region above the horizontal line is provided with a clearance (gap) between itself and the bar.

[0018] 3. The plug mill rolling method according to 1 or 2 above, wherein the clearance is 20 mm or more and 30 mm or less.

[0019] 4. A plug mill for elongating a hollow shell, a bar disposed on a pass line of the hollow blank; a plug attached to one end of the bar; a pair of rolling rolls for rolling down the hollow blank between the plug and the pair of rolling rolls; and a gripping device for gripping a portion of the bar at an intermediate position spaced from the other end of the bar that is fixedly supported, The gripping device has a function of supporting the bar in accordance with radial displacement of the bar caused by a rolling load applied in the axial direction of the bar.

[0020] 5. A plug mill as described in 4 above, wherein the gripping device comprises at least one pair of a lower metal fitting which contacts the bar in a lower region of a horizontal line passing through the pass line and has at least two contact points spaced apart at intervals of 120° or less from each other, and an upper metal fitting which contacts the bar in an upper region of the same line and has at least one contact point spaced apart at an interval of 120° or less from an adjacent contact point of the lower bearing metal fitting in the circumferential direction of the bar, and the upper metal fitting has means for changing its position in the vertical direction.

[0021] 6. A plug mill as described in 4 or 5 above, wherein the gripping device consists of two or more pairs of the lower metal fittings and the upper metal fittings.

[0022] 7. A plug mill according to any one of claims 4 to 6, wherein the bar has a cylindrical shape. [Effects of the Invention]

[0023] According to the present invention, in particular in plug rolling using a small-diameter bar, it is possible to prevent buckling of the bar and maintain the rolling pressure in the plug mill at a certain level or higher, thereby reliably suppressing the formation of pit-like defects on the inner surface of a hollow shell. Furthermore, according to the present invention, it is possible to prevent the size of a device for gripping the bar at an axially intermediate position in order to prevent buckling of the bar from increasing. [Brief explanation of the drawings]

[0024] [Figure 1] FIG. 1 is a side view showing an outline of a plug mill facility. [Figure 2] FIG. 1 is a view showing a state in which a hollow blank is elongated by a plug mill. [Figure 3] 1 is a diagram showing an existing theory for calculating the magnitude of an external force P that will cause a cylindrical member to buckle when the external force P is applied to the member in the axial direction. [Figure 4] FIG. 1 is a side view showing an outline of a plug mill according to the present invention. [Figure 5] 1 shows details of a gripping device according to the present invention; [Figure 6] 1A to 1C are diagrams showing various modes of contact points between a metal fitting and a bar in a gripping device according to the present invention. [Figure 7] 10A to 10C are diagrams showing various modes of contact points between a metal fitting and a bar in a gripping device. [Figure 8] FIG. 5 is a diagram schematically illustrating the measurement position of the bar 2, the force applied, and the like in the plug mill of FIG. DETAILED DESCRIPTION OF THE INVENTION

[0025] The present invention will be described in detail below. First, the plug mill used in the plug mill rolling method of the present invention will be described with reference to FIGS. The plug mill shown in Fig. 4 has the same basic configuration as the plug mill shown in Fig. 1, and the same components are given the same reference numerals and will not be described again. That is, the plug mill shown in Fig. 4 is characterized in that a gripping device 10 is installed at an intermediate position spaced from the rear end (other end) of the bar 2 in the plug mill shown in Fig. 1, specifically, at an intermediate position on the front end side of the bar 2 in region A that does not interfere with the hollow shell after rolling.

[0026] This intermediate position needs to be a position that does not interfere with the rolled hollow blank, and furthermore, according to the existing theory shown in Fig. 3, it is preferable to position the gripping device 10 as close to the tip of the bar as possible. More specifically, it is preferable to install the gripping device 10 at a distance in the tube axial direction from the axial center of the rolling rolls 4 that is the same as the length of the hollow blank 1 that produces the maximum fluctuation in production, that is, the length obtained by adding fluctuation in length that occurs during plug mill rolling due to dimensional errors in the tools and differences in blank temperature. Each standard for steel pipe products requires a certain length range, and in addition to the API5CT oil country tubular goods standard, boiler pipe products, such as those regulated by the ASTM / ASME standard, also require a length of 20 m. In the present invention, the gripping device 10 may be installed at a distance equal to the maximum length within the required length range of each steel pipe standard to be manufactured.

[0027] As shown in FIG. 5, the gripping device 10 is a pair of a lower metal member 11 having a V-shaped cross-section receiving surface 11a and an upper metal member 12 having an inverted V-shaped cross-section receiving surface 12a. The upper metal member 12 is arranged to be movable up and down relative to the lower metal member 11. That is, as shown in FIG. 5(a), the gripping device 10 can fix the bar 2 through contact with the receiving surfaces 11a and 12a of the lower metal member 11 and the upper metal member 12, respectively. However, in the method of the present invention, as shown in FIG. 5(b), the upper metal member 12 is moved upward to place the bar 2 on the receiving surface 11a of the lower metal member 11, while the middle point of the bar 2 is gripped via a clearance 13 between the upper metal member 12 and the receiving surface 12a of the upper metal member 12, thereby preventing buckling of the bar 2. This point will be explained in detail below.

[0028] Here, the clearance (gap) in the gripping device 10 is the minimum distance between one of a pair of contacts (opposing each other across the bar) in the gripping device supporting the bar and the outer surface of the bar when the other contact is in contact with the bar, in a cross section perpendicular to the tube axis. If the bar does not contact both contacts, the clearance refers to the minimum distance between the bar and the contact in the gap that occurs when the bar is moved and installed so that it contacts one of the contacts in the cross section. Note that the position of the clearance indicated by 13 in Figure 5(b) will vary depending on the size of angle 14 and the relative position of the bar and the metal fittings. In other words, the clearance of the present invention is not limited to that shown in Figure 5(b).

[0029] Now, a small-diameter bar must be used when the hollow blank has a thick wall, and the length of the hollow blank after plug rolling is relatively short. This is because the volume of the cylindrical steel billet that serves as the raw material for the hollow blank is limited, and the outer diameter of the hollow blank in the plug mill is the same for each rolling schedule (unit of production using rolling rolls of the same size). Therefore, the thicker the wall, the shorter the length of the hollow blank during plug rolling. Furthermore, in order to withstand the pressure in the radial direction of the bar that tends to buckle the bar, the gripping device 10 needs to be a high-strength facility that can withstand a large reaction force.

[0030] However, adding such high-strength equipment to an existing plug mill is difficult to secure installation space for and requires a great deal of cost, making it an unrealistic approach.

[0031] Therefore, in the present invention, as shown in Figure 5(b), when using a bar with a particularly small diameter, a clearance 13 is set between the bar 2 and the upper metal fitting 12, thereby reducing the reaction force applied to the gripping device 10 while keeping the bar within a range where it will not buckle. Note that the clearance 13 indicates the minimum distance between the outer circumferential surface of the bar 2 and the upper metal fitting 12.

[0032] In other words, the region where the bar does not buckle is the amount of radial displacement (deflection) of the bar due to rolling pressure that does not lead to yielding of the bar material.From the perspective of preventing bar buckling, a displacement amount up to the point where yielding is not reached is acceptable, and the allowable displacement amount is set as the clearance.

[0033] Here, the above-mentioned clearance 13 is preferably set to 20 mm or more and 30 mm or less. This is because a clearance less than 20 mm may increase the reaction force applied to the gripping device, requiring a large-scale, high-strength device configuration. In other words, ensuring a clearance of 20 mm or more makes it possible to suppress radial displacement of the bar 2, which may lead to buckling of the bar 2 due to rolling pressure, and to suppress the reaction force applied by the bar 2 to the gripping device 10. If the clearance 13 is set to 20 mm or more, the increase in radial displacement of the bar can be suppressed to 22% or less compared to the case without clearance shown in FIG. 5(a), for example. Similarly, it is possible to suppress the reaction force applied by the bar 2 to 1 / 10 or less.

[0034] The above-mentioned radial displacement of the bar and the reaction force exerted by the bar 2 on the gripping device 10 were calculated by elastic deformation analysis using the finite element method under actual rolling conditions.

[0035] On the other hand, if the clearance exceeds 30 mm, there is a risk that the effect of preventing buckling of the bar 2 due to the rolling pressure will be reduced. That is, in a plug bar with a small outer diameter, buckling will occur even if the displacement in the bar radial direction is 30 mm or less, and therefore, with a clearance exceeding 30 mm, the gripping effect of the gripping device cannot be obtained.

[0036] The gripping device according to the present invention regulates the geometric relationship of at least three contact points that are assumed to come into contact with the bar via a clearance, thereby achieving a gripping state in which the difference between the inscribed circle formed by the gripping device through which the mother tube passes and the diameter of the mother tube after processing is 20 mm to 30 mm. This gripping state allows for steady displacement of the bar (in the bar radial direction) during mother tube rolling, while also ensuring that the bar is supported by the gripping device when it deforms significantly.

[0037] Next, the shape of the lower metal fitting 11 and upper metal fitting 12 of the gripping device 10 is advantageously one that has a V-shaped cross section, as shown in Fig. 5. That is, it is preferable that the outer circumferential surface of the bar 2 contacts the upper metal fitting and the outer circumferential surface of the bar and the lower metal fitting at two or more points or surfaces, respectively, so that horizontal movement of the bar 2 can be prevented when an external force is applied to the bar 2. Furthermore, in order to hold the bar at the center of the cross section of the workpiece (on the pass line S), it is preferable that the cross-sectional dimensions of each receiving surface 11a and 12a be symmetrical.

[0038] The gripping device 10 has been described above as a typical example of a configuration consisting of a pair of metal fittings with a V-shaped cross section, as shown in Figure 5. The following describes in detail the structural requirements that the gripping device 10 must have. Specifically, the gripping device 10 is composed of at least one pair of lower fittings having at least two contact points that contact the bar 2 below a horizontal line passing through the center axis (on the pass line S), and an upper fitting having at least one contact point that contacts the bar above the horizontal line and is spaced 120° or less from an adjacent contact point of the lower fitting around the periphery of the bar. The gripping device 10 must have at least one contact point on the lower fitting and at least two contact points on the upper fitting, for a total of three contact points. Furthermore, these at least three contact points must be spaced 120° or less from each other around the periphery of the bar.

[0039] 6 and 7 show the relative positions of the bar 2 and the contacts on each metal fitting. In these figures, the contacts belonging to the lower fitting (hereinafter also referred to as lower contacts) are indicated by open circles (○), and the contacts belonging to the upper fitting (hereinafter also referred to as upper contacts) are indicated by filled circles (●).

[0040] Figure 6 shows three examples of contacts spaced circumferentially around the bar at intervals of 120° or less according to the present invention. In Figure 6(a), there are two lower contacts and one upper contact; in Figure 6(b), there are three lower contacts and one upper contact; and in Figure 6(c), there are two lower contacts and two upper contacts. In all cases, the contacts are spaced 120° or less apart, the lower contacts are in contact with the bar, and the upper contacts have a clearance 13 between them. The bar is supported horizontally by at least two lower contacts, and the clearance 13 allows for the absorption of radial displacement that does not result in buckling of the bar, while the upper contacts prevent displacement that would result in buckling.

[0041] On the other hand, Figure 7 shows three cases that do not satisfy the conditions of the present invention. In Figure 7(a), the lower contact point is 1 and the upper contact point is 1; in Figure 7(b), the lower contact point is 2 and the upper contact point is 2; and in Figure 7(c), the lower contact point is 1 and the upper contact point is 4. Figure 7(a) shows a case where the distance between the lower and upper contact points exceeds 120°, making it impossible to fix the bar on the correct pass line and preventing buckling of the bar. Figure 7(b) also shows a case where the distance between the lower and upper contact points exceeds 120°, making it impossible to prevent buckling of the bar, as shown by the two-dot chain line. Figure 7(c) shows a case where the lower contact point is 1, making it impossible to fix the bar on the correct pass line.

[0042] To realize the contact arrangement described above, the cross-sectional shapes of the lower metal fittings 11 and upper metal fittings 12 of the gripping device 10 can be changed as appropriate. For example, the V-shaped lower metal fittings 11 and upper metal fittings 12 shown in FIG. 5 can be made into the form shown in FIG. 6(c) by adjusting the opening angle 14 of the V shape. Also, the form shown in FIG. 6(a) can be made into a V-shaped lower metal fitting and a rectangular cross-sectional upper metal fitting. Similarly, the form shown in FIG. 6(b) can be made into a V-shaped lower metal fitting and a rectangular cross-sectional upper metal fitting. This can be achieved with a single upper metal fitting with a rectangular cross section. [Example]

[0043] As shown in FIG. 4, a bar 2 in a plug mill was used with the following specifications. A gripping device 10 shown in FIG. 5 was attached to the bar 2, and plug rolling was performed under the conditions shown in Table 1. The angle 14 shown in FIG. 5 was 120°, and the gripping device was positioned in the bar axial direction horizontally from the axial center of the roll 4 shown in FIG. 4 at a distance equal to the length of the hollow blank 1 at which the maximum amplitude was produced. The gripping device was moved from Case 1, where it was installed 14,300 mm from the bar tip, toward the bar rear end to Case 2, where it was moved 16,400 mm from the bar tip. The various conditions related to the bar 2 in FIG. 4 are schematically shown in FIG. 8. The load acting on the bar 2 is 21, the distance from the point of application of this load 21 (the axial center of the roll 4) to the gripping device 10 is 22, the displacement of the bar 2 is 23, and the reaction force at the gripping device 10 is 24. The measurement results are shown in Table 1. Table 1 also shows the clearance 13 in the gripping device 10 shown in FIG.

[0044] The displacement 23 of the bar 2 and the reaction force 24 at the gripping device 10 were calculated by elastic deformation analysis using the finite element method under actual rolling conditions.

[0045] [Plug mill bar 2 specifications] Dimensions: Outer diameter φ120.0mm, pipe thickness 22.0mm, length 22000mm Young's modulus: 21000kgf / mm 2 ·Yield strength: 80kgf / mm 2

[0046] [Table 1]

[0047] The plug mill used here had an effective plug bar length of 22,000 mm. First, the position of the gripping device 10 was set in Case 1 within a range where it would not interfere with the hollow blank being rolled by the plug mill, that is, in order to produce a hollow blank of 14 m in length within Range 3, which is a typical length range for seamless steel pipe products as defined in the API 5CT oil country tubular goods standard. This was assumed to be 14,300 mm from the load application point on the delivery side. Next, Case 2 was set, in which the gripping device 10 was set at a position 16,300 mm from the load application point on the delivery side. The load application point is synonymous with the tip position of the plug bar.

[0048] Next, the gripping device 10 is V-shaped as shown in Figure 5, with an opening angle 14 of 140°. Furthermore, a bar with an outer diameter of φ120.0 mm and a thickness of 22.0 mm was used, which previously required a low rolling pressure to prevent internal defects because buckling would occur if the rolling pressure was applied. The clearance 13 of the upper metal fitting 12 of the gripping device 10 was tested under two conditions: 0 mm and 20 mm.

[0049] Under all of the above four conditions, a rolling load 21 of up to 80 tons, which is necessary to prevent the occurrence of inner surface defects, was applied, but no buckling was observed in the bars of the above sizes. Furthermore, in Case 1 where the distance 22 is 14,300 mm, when the upper metal clearance 13 is 20 mm, the reaction force 24 of the gripping device is reduced to approximately 1 / 17 compared to when the clearance 13 is 0 mm. Incidentally, when the distance 22 is less than 14,300 mm, this does not satisfy the maximum length of the seamless steel pipe to be manufactured, so this is not implemented.

[0050] Furthermore, in Case 2, where the distance 22 is 16,300 mm and the upper metal clearance 13 is 20 mm, the reaction force 24 of the gripping device is reduced to approximately 1 / 14 of that when the clearance 13 is 0 mm, resulting in a reduction in the equipment requirements for the gripping device. If the distance 22 exceeds 16,300 mm, there is a possibility of bar buckling because the displacement will reach the yield strength of the bar. However, this does not apply when the clearance 13 is less than 20 mm.

[0051] In the above embodiment, an example was shown in which the cross-sectional dimensions of the plug mill bar were an outer diameter of 120.0 mm and a pipe thickness of 22.0 mm, but the application of the present invention is not limited to this.

[0052] As described above, by installing a gripping device according to the present invention, it became possible to manufacture a steel pipe with an appropriate rolling pressure. As a result, in the manufacture of a steel pipe using the plug bar given as an example above, the incidence rate of inner surface defects, which had conventionally occurred at 2.5%, was reduced to 1.6%. [Explanation of symbols]

[0053] 1. Hollow tube 2 Bar 3 Plug 4. Rolling mill 5 Return roll 6 Rear end bracket 7 Bar Clamp 8 outer diameter 10 Gripping device 11 Lower hardware 12 Upper hardware 13 Clearance 14 Opening angle 20 Defects S Pass Line R Rolling pressure

Claims

1. A plug mill rolling method comprising: inserting a plug into a pierced hollow blank; the plug being attached to one end of a cylindrical bar arranged on a central axis of the hollow blank; and elongating the hollow blank by applying a rolling reduction between the plug and at least one pair of rolling rolls arranged outside the hollow blank, the method comprising: a plug mill rolling method comprising: gripping an intermediate position of the bar in the longitudinal direction, which is spaced from the other end of the bar that is fixedly supported, via at least three contact points spaced from one another at intervals of 120° or less in the circumferential direction of the bar; bringing at least two contact points in a region below a horizontal line passing through the central axis of the bar into contact with the bar; and providing a clearance (gap) between at least one contact point in an upper region of the horizontal line passing through the central axis of the bar and the bar; and supporting the intermediate position in accordance with radial displacement of the bar that occurs in the bar due to a rolling load applied in the axial direction of the bar.

2. 2. The plug mill rolling method according to claim 1, wherein the clearance is 20 mm or more and 30 mm or less.

3. 1. A plug mill that applies elongation rolling to a hollow shell, a bar disposed on a pass line of the hollow blank; a plug attached to one end of the bar; a pair of rolling rolls for rolling down the hollow blank between the plug and the pair of rolling rolls; and a gripping device for gripping a portion of the bar at an intermediate position spaced from the other end of the bar that is fixedly supported, The gripping device comprises at least one pair of lower metal fittings that contact the bar in a lower region of a horizontal line passing through the pass line and have at least two contact points spaced apart by an angle of 120° or less from each other, and upper metal fittings that contact the bar in an upper region of the same line and have at least one contact point that is spaced apart from an adjacent contact point of the lower bearing metal fitting in the circumferential direction of the bar by an angle of 120° or less, the upper metal fittings having means for changing their position in the vertical direction, and the gripping device has the function of supporting the bar in accordance with radial displacement of the bar that occurs in the bar due to the rolling load applied in the axial direction of the bar.

4. 4. The plug mill according to claim 3, wherein the holding device comprises two or more pairs of the lower metal member and the upper metal member.

5. 5. The plug mill according to claim 3, wherein the bar has a cylindrical shape.

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