Manufacturing method for seamless clad steel pipes

Abstract

The objective is to provide a method for manufacturing seamless clad steel pipes that exhibits excellent bonding quality between the outer and inner layers, and in which the wall thickness of the inner layer remains constant throughout the entire length. [Solution] A composite billet is formed by interlocking an inner layer material inside an outer layer material to form a composite billet, and a perforation and rolling step of heating and perforating the composite billet, wherein in the composite billet forming step, an inner layer material having an outer layer material comprising a hollow round bar portion having an inner circumferential surface and a solid round bar portion having an inner surface that closes the end of the hollow round bar portion is interlocked with an inner layer material having an outer circumferential surface of a solid round bar and a rear end surface provided at one end of the solid round bar in the axial direction, such that the inner circumferential surface of the hollow round bar portion of the outer layer material and the outer circumferential surface of the solid round bar of the inner layer material, and the inner surface of the solid round bar portion of the outer layer material and the rear end surface of the solid round bar of the inner layer material face each other, thereby forming a composite billet that satisfies formulas (1) and (2).

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a seamless clad steel pipe by hot piercing rolling of a composite billet.

Background Art

[0002] In the conventional method for manufacturing a seamless clad steel pipe, a hollow round bar made of a metal material for forming an outer layer and a solid round bar having the same outer diameter as the base material hole diameter made of a metal material for forming an inner layer are fitted together. Next, the composite billet in this fitted state is hot extrusion rolled to form a composite billet, and then hot piercing and stretching rolling is performed.

[0003] In Patent Document 1, when manufacturing a clad pipe made of two or more types of metals, a round billet made of a metal for forming an inner layer or a pipe made of a metal for forming an intermediate layer is fitted into a pipe body made of a metal for forming an outer layer, and further, a round billet made of a metal for forming an inner layer is fitted into this pipe body. After heating the laminated billet in this fitted state to a predetermined temperature, it is subjected to extrusion rolling by an inclined rolling mill to form a laminated billet, and then a method for manufacturing a clad pipe in which the laminated billet is pierced and stretched is disclosed.

[0004] In addition, regarding the relationship between the metal material for forming the outer layer and the metal material for forming the inner layer, there are cases where the metal material for forming the outer layer is a difficult-to-machine material and cases where the metal material for forming the inner layer is a difficult-to-machine material. In Patent Document 2, a method for manufacturing a rolled clad seamless pipe that performs piercing rolling based on a rolling relational expression that focuses on differences in metal flow, etc. between a difficult-to-machine steel material and a steel material with good workability is disclosed, from a rolled material bar in which a difficult-to-machine steel material is arranged on the outside and a steel material with good workability is arranged at the center.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

[0006] In the technology disclosed in Patent Document 2, since the metals are not metallurgically joined at the interface between the inner and outer layers, the movement of the inner layer forming metal material in the rolling direction during drilling and rolling is hindered by the plug. As a result, the inner layer forming metal material in the center is pushed toward the rear end, causing displacement of the inner layer material within the outer layer material, and resulting in a seamless steel pipe with a single layer of outer layer forming metal material in a portion of the leading edge or along its entire length after drilling and rolling, instead of becoming a clad steel pipe.

[0007] In the technology disclosed in Patent Document 1 to solve this problem, the base material for forming the outer layer and the asphalt material for forming the inner layer are fitted together, and hot kneading is performed before drilling and rolling. As a result, the adhesion between the base material for forming the outer layer and the asphalt material for forming the inner layer is improved, the behavior of the metal material for forming the inner layer during rolling is improved, and a seamless clad steel pipe is manufactured. However, in this technology, oxidation occurs in both the base material for forming the outer layer and the composite material for forming the inner layer during heating and rolling when hot-rolling is performed. This can occur not only on the surface of the outer layer but also at the interface between the inner and outer layers, and if oxides are generated at the interface, it can lead to a decrease in the quality of the joint between the inner and outer layers of seamless clad steel pipes.

[0008] Therefore, the present invention has been made in view of these circumstances, and aims to provide a method for manufacturing seamless clad steel pipes that has excellent joining quality between the outer layer material and the inner layer material, and in which the wall thickness of the inner layer material is constant along the entire length, thereby solving the aforementioned problems. Here, "constant wall thickness of the inner layer material" means that the wall thickness of the inner layer material on the inner surface of the raw pipe is within ±20% of the target wall thickness. [Means for solving the problem]

[0009] The inventors found that in order to suppress oxidation that leads to a deterioration in the bonding quality between the outer and inner layers of clad steel pipes, it is effective to reduce the heating process performed prior to the drilling and rolling process, and in order to create a clad steel pipe in which the wall thickness of the inner layer is constant along its entire length, it is effective to provide a wall that pushes the inner layer backward during drilling and rolling, thereby suppressing the displacement (movement) of the inner layer. Furthermore, we found that it is effective to have a specific size relationship between the outer and inner layers of a composite billet with walls.

[0010] This invention is based on the above findings, and its gist is as follows.

[0011] [1] A method for manufacturing a seamless clad steel pipe consisting of two or more layers, comprising: a composite billet forming step of embedding an inner layer material inside an outer layer material to form a composite billet; and a perforation and rolling step of heating the composite billet and perforating and rolling it, wherein in the composite billet forming step, a hollow round bar portion having a cylindrical shape extending along the axial direction and having an inner circumferential surface; and a solid round bar portion provided at one end of the hollow round bar portion in the axial direction and having an inner surface that closes the end of the hollow round bar portion, A method for manufacturing a seamless clad steel pipe, comprising an outer layer material having a round bar shape extending along the axial direction, and having an outer circumferential surface of a solid round bar and a rear end surface provided at one end of the solid round bar in the axial direction, and a composite billet formed by interlocking the inner circumferential surface of the hollow round bar portion of the outer layer material with the outer circumferential surface of the solid round bar of the inner layer material, and the inner surface of the solid round bar portion of the outer layer material with the rear end surface of the solid round bar of the inner layer material, so as to face each other, wherein the composite billet satisfies the following formulas (1) and (2). 0.20 ≤ Di / Do ≤ 0.80 ···(1) 0.60 ≤ Li / Lo ≤ 0.90 ···(2) Here, in equations (1) and (2), Di(mm): Outer diameter of the solid round bar of the inner layer material. Do (mm): Outer diameter of the solid round bar section of the outer layer material. Li (mm): Total length in the axial direction of the solid round bar of the inner layer material. Lo (mm): This is the total length in the axial direction of the solid round bar section and the hollow round bar section of the outer layer material. [2] A method for manufacturing a seamless clad steel pipe consisting of two or more layers, comprising: a composite billet forming step of fitting an inner layer material inside an outer layer material to form a composite billet; and a perforation and rolling step of heating the composite billet and perforating and rolling it, wherein in the composite billet forming step, the outer layer material comprises a hollow round bar portion having an inner circumferential surface and being cylindrical in shape extending along the axial direction, and a solid round bar portion provided at one end of the hollow round bar portion in the axial direction and having an inner surface that closes the end of the hollow round bar portion, wherein the inner circumferential surface of the hollow round bar portion of the outer layer material is tapered so that the inner diameter decreases as it is directed from the tip of the outer layer material toward one side in the axial direction, and extends along the axial direction A method for manufacturing a seamless clad steel pipe, wherein the inner layer material is rod-shaped and has an outer circumferential surface of a solid round bar and a rear end surface provided at one end of the solid round bar in the axial direction, the outer circumferential surface of the solid round bar of the inner layer material is tapered so that the outer diameter decreases from the tip of the inner layer material toward one side in the axial direction, and the inner layer material is fitted into the outer layer material to form a composite billet such that the inner circumferential surface of the hollow round bar portion of the outer layer material and the outer circumferential surface of the solid round bar of the inner layer material, and the inner surface of the solid round bar portion of the outer layer material and the rear end surface of the solid round bar of the inner layer material face each other, and the composite billet satisfies the following formulas (3) and (4). 0.20 ≤ Dimax / Do ≤ 0.80 ···(3) 0.60 ≤ Li / Lo ≤ 0.90 ···(4) Here, in equations (3) and (4), Dimax (mm): Maximum outer diameter of the tapered round bar of the inner layer material. Do (mm): Outer diameter of the solid round bar section of the outer layer material. Li (mm): Total length in the axial direction of the tapered round bar of the inner layer material. Lo (mm): This is the total length in the axial direction of the solid round bar section and the hollow round bar section of the outer layer material. [3] A method for manufacturing a seamless clad steel pipe according to [1] or [2], wherein the drilling and rolling of the composite billet is performed such that the reduction ratio during drilling and rolling satisfies the following formula (5). 0.5 ≤ (r - t) / r ≤ 0.9 ··· (5) Here, in formula (5), r (mm) is the radius of the solid round bar portion of the outer layer material before piercing rolling, and t (mm) is the wall thickness of the plain tube after piercing rolling.

Advantages of the Invention

[0012] According to the present invention, since the hot rolling process before piercing rolling is unnecessary, oxidation can be reduced at the interface between the inner layer and the outer layer by fitting in the composite billet formation, and a seamless clad steel excellent in the bonding quality between the inner layer material and the outer layer material and having a constant wall thickness of the inner layer material can be obtained.

Brief Description of the Drawings

[0013] [Figure 1] The piercing rolling according to an embodiment of the present invention is an explanatory view of the positional relationship of the composite billet, the plug, and the rolling roll during (a) piercing rolling and (b) before piercing rolling. [Figure 2] In the piercing rolling according to an embodiment of the present invention, it is a plan view when the inner layer material of the composite billet is a solid round bar. [Figure 3] In the piercing rolling according to an embodiment of the present invention, it is a plan view when the inner layer material of the composite billet is a tapered round bar. [Figure 4] In the piercing rolling according to an embodiment of the present invention, it is a plan view of the plain tube that has become a clad steel pipe after piercing rolling.

Mode for Carrying Out the Invention

[0014] The method for manufacturing a seamless clad steel pipe according to the present embodiment will be described while referring to the drawings. Note that the present invention is not limited by this embodiment.

[0015] The method for manufacturing a seamless clad steel pipe according to the present embodiment is a method for manufacturing a seamless clad steel pipe composed of two or more layers, including a composite billet forming step of fitting an inner layer material inside an outer layer material to form a composite billet, and a piercing rolling step of heating and piercing rolling the composite billet. In the composite billet forming process, a composite billet is formed by fitting the inner layer material inside the outer layer material, such that the inner surface of the hollow round bar portion of the outer layer material and the outer surface of the solid round bar portion of the inner layer material, as well as the inner surface of the solid round bar portion of the outer layer material and the rear end surface of the solid round bar portion of the inner layer material, face each other, forming a mating portion. The inner layer material is fitted inside the outer layer material such that the inner surface of the hollow round bar portion of the outer layer material and the outer surface of the solid round bar portion of the inner layer material, as well as the inner surface of the solid round bar portion of the outer layer material and the rear end surface of the solid round bar portion of the inner layer material face each other.

[0016] In another embodiment, in the composite billet forming process, the outer layer material comprises a hollow round bar portion having an inner circumferential surface and being cylindrical in shape extending along the axial direction, and a solid round bar portion provided at one end of the hollow round bar portion in the axial direction and having an inner surface that closes the end of the hollow round bar portion, wherein the inner circumferential surface of the hollow round bar portion of the outer layer material is tapered so that the inner diameter decreases as it moves from the tip of the outer layer material toward one side in the axial direction, and the inner layer material having an outer circumferential surface of a solid round bar and a rear end surface provided at one end of the solid round bar in the axial direction, wherein the outer circumferential surface of the solid round bar of the inner layer material is tapered so that the outer diameter decreases as it moves from the tip of the inner layer material toward one side in the axial direction.

[0017] <Perforated Rolling> Referring to Figure 1, we will first explain the perforation and rolling method of this embodiment. Figure 1 is a schematic diagram illustrating an example of rolling a composite billet using a perforation and rolling mill. The perforating rolling mill has a pair of rolling rolls 4a and 4b whose rotating shafts are inclined toward each other at an angle FA (also called the roll inclination angle FA) and which face each other for clamping the material to be rolled (composite billet 1), and a pair of guide shoes 7a and 7b positioned perpendicular to the rolls to prevent the axis of the composite billet 1 from moving during rolling. Furthermore, the mill (perforating rolling mill) has a plug 5 for perforating the composite billet 1 as it advances in the rolling direction RD and a plug bar 6 that supports it, with the composite billet 1 positioned between them.

[0018] Figure 1(a) shows a cross-sectional view of a composite billet being drilled and rolled, and the drilling and rolling mill. The composite billet 1 is reduced in diameter by being gripped by the rolling rolls 4a and 4b, which are barrel-shaped or cone-shaped rolls, and advances in the rolling direction RD while twisting in the circumferential direction, and is perforated and rolled by the rolling rolls 4a and 4b and the plug 5.

[0019] <Composite billet formation process> Next, the process for forming the rolled material (composite billet 1) in this embodiment will be described. Figure 2 shows a longitudinal cross-sectional view of the composite billet according to this embodiment. The outer layer material 2 of the composite billet 1 is cylindrical in shape extending along the axial direction and comprises a hollow round bar portion 2a having an inner circumferential surface 2d, and a solid round bar portion 2b provided at one end of the hollow round bar portion 2a in the axial direction and having an inner surface 2c that closes the end of the hollow round bar portion 2a. The inner layer material 3 of the composite billet 1 extends along the axial direction and is in the shape of a round bar, having an outer circumferential surface 3b of a solid round bar and a rear end surface 3a provided at one end of the solid round bar in the axial direction.

[0020] The inner layer material 3 is fitted inside the outer layer material 2 so that the inner circumferential surface 2d of the hollow round bar portion 2a of the outer layer material 2 and the outer circumferential surface 3b of the solid round bar of the inner layer material 3, and the inner surface 2c of the solid round bar portion 2b of the outer layer material 2 and the rear end surface 3a of the solid round bar of the inner layer material 3 face each other, thereby forming a composite billet 1 having a fitted portion T. Here, the solid round bar portion 2b of the outer layer material 2 acts as a wall to prevent the inner layer material from shifting during drilling and rolling. Since hot rolling is not performed prior to drilling and rolling, oxidation at the interface between the inner and outer layers in the mating portion T can be reduced, and a seamless clad steel pipe can be manufactured that ensures higher joining quality between the inner and outer layers. In other words, the method for manufacturing a seamless clad steel pipe according to this embodiment includes a composite billet forming step and a perforation rolling step without performing hot kneading rolling.

[0021] Embodiment 1 Next, a specific example of the composite billet will be described. FIG. 2 shows a cross-sectional view of the composite billet 1 when the fitting portion T of the composite billet has a cylindrical shape, which is composed of an outer layer material 2 having a hollow round bar portion 2a and a solid round bar portion 2b, and a solid round bar inner layer material 3. In this composite billet 1, the dimensions of the outer layer material 2 and the inner layer material 3 satisfy the ranges specified by the following formulas (1) and (2). 0.20 ≦ Di / Do ≦ 0.80 ··· (1) 0.60 ≦ Li / Lo ≦ 0.90 ··· (2) Here, Di (mm): outer diameter of the solid round bar of the inner layer material, Do (mm): outer diameter of the solid round bar portion of the outer layer material, Li (mm): total axial length of the solid round bar of the inner layer material, Lo (mm): total axial length of the solid round bar portion and the hollow round bar portion of the outer layer material.

[0022] First, in formula (1), when Di / Do < 0.20, there is a high possibility that the wall thickness of the inner layer material of the plain tube becomes non-uniform in the circumferential direction after piercing rolling. On the other hand, when 0.80 < Di / Do, the wall thickness of the outer layer material becomes too thin, and there is a high possibility that the clad steel pipe cannot be manufactured because it breaks approximately parallel to the longitudinal direction from the tip of the billet from the initial stage of piercing. Therefore, 0.20 ≦ Di / Do ≦ 0.80, preferably 0.30 ≦ Di / Do ≦ 0.70, and more preferably 0.40 ≦ Di / Do ≦ 0.60.

[0023] Next, in formula (2), when Li / Lo < 0.60, there is a high possibility that the wall thickness of the inner layer material of the plain tube after piercing becomes too thin or disappears at the rear end of the plain tube, so that the function as a clad steel pipe cannot be maintained. On the other hand, when 0.90 < Li / Lo, the outer layer material near the axial center on the rear end side of the billet, which plays a role in preventing the displacement of the inner layer material, that is, the cylindrical portion of the outer layer material, cannot withstand the reaction force in the rolling direction from the plug during piercing and breaks, and there is a high possibility that the clad steel pipe cannot be manufactured. Therefore, 0.60 ≦ Li / Lo ≦ 0.90, preferably 0.65 ≦ Li / Lo ≦ 0.85, and more preferably 0.70 ≦ Li / Lo ≦ 0.80.

[0024] Embodiment 2 FIG. 3 shows a cross-sectional view of the composite billet 1 in the case where the fitting portion T of the composite billet has a tapered shape in which the inner diameter of the hollow round bar portion of the outer layer material or the outer diameter of the solid round bar of the inner layer material decreases as it goes from the tip (8, 10) at one end face of the outer layer material or the inner layer material toward one side in the axial direction. Let the maximum outer diameter of the tapered round bar of the inner layer material be Dimax, the minimum outer diameter of the tapered round bar of the inner layer material be Dimin, and the taper angle of the tapered round bar of the inner layer material be θi.

[0025] In the composite billet 1 having such a tapered fitting portion, the dimensions of the inner layer material and the outer layer material are basically controlled within the ranges specified by the following formulas (3) and (4) in the same concept as the case where the fitting portion has a cylindrical shape. 0.20 ≦ Dimax / Do ≦ 0.80 ··· (3) 0.60 ≦ Li / Lo ≦ 0.90 ··· (4) Here, Dimax (mm): the maximum outer diameter of the tapered round bar of the inner layer material, Do (mm): the outer diameter of the solid round bar portion of the outer layer material, Li (mm): the total axial length of the tapered round bar of the inner layer material, Lo (mm): the total axial length of the solid round bar portion and the hollow round bar portion of the outer layer material.

[0026] First, in formula (3), when Dimax / Do < 0.20, there is a high possibility that the wall thickness of the inner layer material of the perforated raw pipe will not be uniform in the circumferential direction. On the other hand, when 0.80 < Dimax / Do, the wall thickness of the outer layer material becomes too thin, and from the initial stage of perforation, there is a high possibility that the outer layer material will break and a clad steel pipe cannot be manufactured, approximately parallel to the longitudinal direction from the tip of the billet. Therefore, 0.20 ≦ Dimax / Do ≦ 0.80, 0.30 ≦ Dimax / Do ≦ 0.70 is preferable, and 0.40 ≦ Dimax / Do ≦ 0.60 is more preferable.

[0027] Next, in formula (4), when Li / Lo < 0.60, the thickness of the inner layer material of the perforated plain pipe becomes too thin or disappears at the rear end of the plain pipe, so there is a high possibility that the function as a clad steel pipe cannot be maintained. On the other hand, when 0.90 < Li / Lo, the outer layer material near the axial center on the rear end side of the billet, which plays a role in preventing the displacement of the inner layer material, that is, the solid round bar part of the outer layer material cannot withstand the reaction force in the rolling direction from the plug during perforation and is likely to break, making it impossible to manufacture the clad steel pipe. Therefore, 0.60 ≤ Li / Lo ≤ 0.90, preferably 0.70 ≤ Li / Lo ≤ 0.90, and more preferably 0.80 ≤ Li / Lo ≤ 0.90. In addition, if the taper angle θi (°) of the tapered round bar is excessively large, the thickness ratio of the outer layer material and the inner layer material in the longitudinal direction of the plain pipe after perforation rolling becomes uneven. Therefore, the taper angle of the tapered round bar is set to 0 < θi ≤ 6°. The taper angle is more preferably 0 < θi ≤ 5°, and even more preferably 0 < θi ≤ 4°. In addition, assuming that the minimum outer diameter of the tapered round bar of the inner layer forming material is Dimin, in order to suppress the unevenness of the thickness ratio of the outer layer material and the inner layer material in the longitudinal direction of the plain pipe after perforation rolling, it is preferable that 0.20 ≤ Dimin / Do ≤ 0.60.

[0028] The composite billet of this embodiment is not only composed of two layers of the outer layer and the inner layer with two different metals, but also, depending on the application of the steel pipe, such as three layers with three different metals, two or more metals that fulfill the purposes and functions of each layer can be applied. In the two-layer clad steel pipe, for example, the inner layer is made of stainless steel with corrosion resistance, and the outer layer is made of high-strength steel with high strength.

[0029] <Perforation rolling process> As shown in FIG. 1, perforation rolling is performed with the surface where the inner layer material 3 is exposed (the surface of the tip 10 of the inner layer material) as the front in the rolling direction (the side that first contacts the plug 5). In the perforation rolling of the above composite billet, it is preferable to perform perforation rolling so that the reduction ratio during perforation rolling satisfies the following formula (5). FIG. 4 shows a plan view (outer layer 11, inner layer 12) of the plain pipe of the clad steel pipe after perforation rolling. The front end 8 and rear end 9 of the outer layer material of the raw pipe correspond to the front end 8 and rear end 9 of the outer layer material of the composite billet. 0.5 ≤ (rt) / r ≤ 0.9 ···(5) Here, r (mm) is the radius of the solid round bar section of the outer layer material before drilling and rolling, and t (mm) is the wall thickness of the raw tube after drilling and rolling.

[0030] In equation (5), when (rt) / r < 0.5, that is, to increase the wall thickness t of the raw tube after drilling and rolling, it is necessary to use a small plug outer diameter for drilling and rolling. Consequently, the plug bar outer diameter must be reduced, which makes it highly likely that the plug bar will not be able to withstand the load during drilling and therefore drilling and rolling will not be possible.

[0031] When 0.9 < (rt) / r, that is, when the wall thickness t of the raw tube after perforation and rolling is small, there is a high probability that either or both of the inner and outer layers will rupture, mainly at the leading and trailing ends of the raw tube after perforation. Therefore, 0.5 ≤ (rt) / r ≤ 0.9 is preferred, 0.6 ≤ (rt) / r ≤ 0.85 is more preferred, and 0.7 ≤ (rt) / r ≤ 0.8 is even more preferred.

[0032] The heating and other drilling conditions when drilling and rolling composite billets should be the same as those used when drilling and rolling ordinary billets. The heating conditions are preferably 1100-1300°C, maintained for a time sufficient to evenly heat the billet. For the perforation and rolling conditions, it is preferable to set the roll inclination angle FA of the rolling rolls 4a and 4b to a range of 6 to 11°, and to set the roll spacing, shoe spacing, and plug advance distance according to the billet size.

[0033] Although the perforation rolling process in this embodiment was described for a two-layer clad steel pipe, the same effects can be obtained for three or more layers using the perforation rolling conditions of the composite billet according to this embodiment. In other words, it is effective in suppressing oxide formation that leads to a decrease in the bonding quality of three-layer materials, and in preventing the inner layer material from being pushed to the rear during perforation rolling, making it possible to manufacture high-quality clad steel pipes with three or more layers in which the wall thickness of the inner layer material is constant throughout the entire length. [Examples]

[0034] The present invention will be described in more detail below based on examples. It should be noted that the present invention is not limited to the following embodiments, and modifications can be made as appropriate within the scope of the spirit of the present invention, and all such modifications fall within the technical scope of the present invention.

[0035] To confirm the effects of the present invention, two-layer composite billets No. 1 to 21 shown in Table 1 were prepared, and the cladding condition of the raw pipes after drilling and rolling was investigated. In Table 1, billets No. 1-10 and 20 are composite billets where the mating portion is cylindrical as shown in Figure 2, while billets No. 18 and 19 are composite billets where the outer layer material is a hollow cylindrical shape without a solid round bar portion 2b and the lengths of the outer and inner layers are equal (Li / Lo = 1.0). Also, billets No. 11-17 and 21 are composite billets where the mating portion is tapered as shown in Figure 3.

[0036] For the composite billets used in the evaluation experiment of this embodiment, the materials of the inner and outer layers were S15C for the outer layer and SUS430 for the inner layer, respectively. The billet dimensions were a diameter (outer diameter of the cylindrical part of the outer layer) of ΦDo 58 mm and a total length (total length in the axial direction of the outer layer) of Lo 250 mm. For each composite billet, the dimensions and shape of the inner layer were varied as shown in Table 1 and investigated.

[0037] The plug material was SKD61 hot work tool steel, with a maximum outer diameter of Φ49 mm and a total length of 130 mm. Before drilling and rolling, the plug material was held in an atmospheric furnace at 950°C for 4 hours, then cooled to room temperature, and subsequently heated to create an oxide scale 0.4 to 0.7 mm thick on its surface before being used for drilling and rolling.

[0038] For billets No. 1-18, hot rolling of the composite billets was not performed prior to drilling and rolling. For billets No. 19-21, hot rolling was performed prior to drilling and rolling as a comparative example. The heating conditions for the composite billet during perforation rolling were 1250°C at an ambient temperature for 1 hour. The perforation conditions were 52.5 mm for the roll gap (distance between the bottom dead center of the upper roll and the top dead center of the lower roll in the upper and lower roll arrangement), 90 rpm for the roll rotation speed, 8° for the roll inclination angle FA, 65 mm for the shoe spacing, 1 rpm for the shoe rotation speed, and 15 mm for the lead. The rolling conditions during hot kneading rolling were the same as the perforation conditions, except that plugs 5 and plug bars 6 in Figure 1 were not used.

[0039] Table 1 shows the evaluation results of various composite billets after drilling and rolling. The evaluation of the raw pipes after drilling and rolling shown in Table 1 was performed based on the wall thickness of the inner layer material and the bonding quality between the inner and outer layers.

[0040] In the evaluation of clad steel pipes based on wall thickness, the pipe was cut perpendicular to the axis at the center, tip (30 mm from the end, near the center), and rear end (30 mm from the end, near the center) along the longitudinal direction of the pipe, and the wall thickness was measured at four locations at 90-degree intervals on the circumference of each cross-section. A good quality clad steel pipe was marked with ○ if the total thickness of the steel pipe, including the inner and outer layers, and the wall thickness of the inner layer were both within ±20% of the target wall thickness, while a condition where the clad structure (two-layer structure of inner and outer layers) was formed along the entire length of the steel pipe, but outside the range of the target wall thickness was marked with △. Conditions where the end or end of the pipe was torn, or where the inner layer material was missing over a wide area of ​​the end (more than half the total length of the pipe), resulting in a pipe that did not become a clad steel pipe (and instead became a seamless steel pipe with a single outer layer in some parts), were marked with an "X".

[0041] In evaluating the bonding quality between the inner and outer layers, the interface between the inner and outer layers was observed using an optical microscope at a total of eight fields / sections, spaced 45 degrees circumferentially, in five cross-sections perpendicular to the longitudinal direction of the clad steel pipe. A score of ○ was given if the ratio of the total length of oxides with a thickness exceeding 20 μm to the total length of the interface between the inner and outer layers in the entire observed field of view was 10% or less, and a score of × was given if it exceeded 10%. The oxides referred to here are oxide scales (wustite (FeO), magnetite (Fe3O4), and hematite (Fe2O3)). While the magnification for observation with an optical microscope is not specified, 50 to 100x is preferred.

[0042] In cases where the inner layer material was cylindrical, Nos. 4, 8, and 9 were high-quality clad steel pipes with good joint quality, as the inner layer material on the inner surface of the raw pipe was within the target wall thickness range. Furthermore, the actual values ​​of (rt) / r were between 0.5 and 0.9. In No. 3, the inner layer material was missing at the very end, but the joint quality was good in the areas where the inner layer material was present. In Nos. 1, 2, and 6, the inner layer material was missing at the rear end for more than half of the total length of the raw pipe, and oxides were observed at the joints in the areas where the inner layer material was present. In No. 5, a tear occurred in the outer layer material (hollow round bar section and cylindrical section of the outer layer material) at the rear end axis of the composite billet in the early stages of drilling, causing the inner layer material to move towards the rear end of the billet, so the tip of the raw pipe did not become a clad steel pipe, but the joint quality was good in the areas where it did become a clad steel pipe. In No. 7, the inner layer material appeared to have buckled during drilling and rolling, resulting in no inner layer material at the tip of the raw pipe, and in the central part of the raw pipe, there was a large variation in the thickness of the inner layer material, which was joined to the outer layer material, and in some longitudinal positions, only a portion of the inner circumference of the raw pipe was joined with the inner layer material. In No. 10, the outer layer material at the tip of the composite billet tore in the longitudinal direction from the early stages of drilling, so it did not become a clad steel pipe. In No. 18, the outer layer material was a hollow round bar (hollow cylinder) shape without the solid round bar section 2b of the outer layer material. As a result, the metal material for forming the inner layer in the center was pushed towards the rear end by the plug 5, causing displacement of the inner layer material within the outer layer material. As a result, the tip side of the raw pipe did not become a clad steel pipe, but the joining quality in the part that did become a clad steel pipe was good. In No. 19, the same billet as No. 18 was subjected to hot kneading before drilling and rolling. Although not as severe as in No. 18, the metal material for forming the inner layer in the center was pushed towards the rear end by the plug 5, resulting in the tip side of the raw pipe not becoming a clad steel pipe. The joining quality was also poor due to the hot kneading process. In No. 20, the same billet as No. 4 was subjected to hot kneading before drilling and rolling. Although the inner layer material on the inner surface of the raw pipe was within the target wall thickness range, the joining quality was poor due to the hot kneading process.

[0043] In cases where the inner layer material was tapered, Nos. 13, 14, and 16 were high-quality clad steel pipes with good joint quality, as the inner layer material was within the target wall thickness range on the inner surface of the raw pipe. Furthermore, the actual values ​​of (rt) / r were between 0.5 and 0.9. In No. 12, the inner layer material was lost at the very end, but the joint quality was good in the areas where the inner layer material was present. In Nos. 11 and 17, the inner layer material was lost at the rear end for more than half of the total length of the raw pipe. In No. 15, a tear occurred in the outer layer material (hollow round bar section and cylindrical section of the outer layer material) at the rear end axis of the composite billet during the initial drilling stage, resulting in a clad steel pipe with a defective tip shape. However, the joint quality was good in the areas where there were no defects and the pipe was clad steel pipe. In No. 21, a billet similar to No. 13 was subjected to hot kneading before drilling and rolling. The inner layer material on the inner surface of the raw pipe was within the target wall thickness range, but the joint quality was poor due to the hot kneading process.

[0044] [Table 1] [Explanation of symbols]

[0045] T mating part RD (Rolling Direction) Lo: The entire length in the axial direction of the solid round bar section and the hollow round bar section of the outer layer material. Li: The entire length in the axial direction of the solid round bar of the inner layer material. Outer diameter of the solid round bar section of the outer layer material Di (Outer diameter of solid round bar for inner layer material) Dimax inner layer material tapered round bar maximum outer diameter Dimin Minimum outer diameter of tapered round bar for inner layer material θi Taper angle 1. Composite billet 2 Outer layer material 2a Hollow round bar section of the outer layer material 2b Solid round bar section of the outer layer material 2c Inner surface of the solid round bar section of the outer layer material 2d Inner circumferential surface of the hollow round bar portion of the outer layer material 3. Inner layer material 3a Rear end face of the solid round bar of the inner layer material 3b Outer surface of the solid round bar of the inner layer material 4a, 4b Rolling rolls 5 plugs 6 Plug Bars 7a, 7b Guide shoe 8. Tip of the outer layer material 9. Rear end of outer layer material 10. Tip of the inner layer material 11. Outer layer of the raw tube after perforation and rolling 12. Inner layer of the raw tube after perforation and rolling.

Claims

1. A method for manufacturing a seamless clad steel pipe consisting of two or more layers, A composite billet forming process in which an inner layer material is embedded inside an outer layer material to form a composite billet, The aforementioned composite billet is heated and then perforated and rolled in a perforation and rolling process, Includes, In the composite billet forming process, A hollow round bar portion having a cylindrical shape extending along the axial direction and an inner circumferential surface, A solid round bar portion is provided at one end of the hollow round bar portion in the axial direction, and has an inner surface that closes the end of the hollow round bar portion, The outer layer material includes, It extends along the axial direction and forms a round bar shape, and the outer surface of the solid round bar and The rear end surface provided at one end of the solid round bar in the axial direction, An inner layer material having, The inner circumferential surface of the hollow round bar portion of the outer layer material and the outer circumferential surface of the solid round bar of the inner layer material, and the inner surface of the solid round bar portion of the outer layer material and the rear end surface of the solid round bar of the inner layer material face each other. By interlocking them, a composite billet is formed to form an interlocking portion. The composite billet is a method for manufacturing seamless clad steel pipes that satisfies the following formulas (1) and (2). 0.20 ≤ Di / Do ≤ 0.80 ... (1) 0.60 ≤ Li / Lo ≤ 0.90 ... (2) Here, in equations (1) and (2), Di (mm): Outer diameter of the solid round bar of the inner layer material. Do (mm): Outer diameter of the solid round bar section of the outer layer material. Li (mm): Total length in the axial direction of the solid round bar of the inner layer material. Lo (mm): This is the total length in the axial direction of the solid round bar section and the hollow round bar section of the outer layer material.

2. A method for manufacturing a seamless clad steel pipe consisting of two or more layers, A composite billet forming process in which an inner layer material is embedded inside an outer layer material to form a composite billet, The aforementioned composite billet is heated and then perforated and rolled in a perforation and rolling process, Includes, In the composite billet forming process, A hollow round bar portion having a cylindrical shape extending along the axial direction and an inner circumferential surface, An outer layer material comprising a solid round bar portion provided at one end of the hollow round bar portion in the axial direction and having an inner surface that closes the end of the hollow round bar portion, wherein the inner circumferential surface of the hollow round bar portion of the outer layer material is tapered, with the inner diameter decreasing as it moves from the tip of the outer layer material toward one side in the axial direction, It extends along the axial direction and forms a round bar shape, and the outer surface of the solid round bar and The inner layer material having a rear end surface provided at one end in the axial direction of the solid round bar, has an outer circumferential surface of the solid round bar of the inner layer material tapered so that the outer diameter decreases as you move from the tip of the inner layer material toward one side in the axial direction, A composite billet is formed by fitting the inner layer material to the outer layer material so that the inner circumferential surface of the hollow round bar portion of the outer layer material and the outer circumferential surface of the solid round bar of the inner layer material, and the inner surface of the solid round bar portion of the outer layer material and the rear end surface of the solid round bar of the inner layer material face each other, thereby forming a mating portion. The composite billet is a seamless clad steel pipe manufacturing method in which the taper angle of the outer layer material or the inner layer material is 6° or less, and the following formulas (3) and (4) are satisfied. 0.20 ≤ Dimax / Do ≤ 0.80 ... (3) 0.60 ≤ Li / Lo ≤ 0.90 ... (4) Here, in equations (3) and (4), Dimax (mm): Maximum outer diameter of the tapered round bar of the inner layer material. Do (mm): Outer diameter of the solid round bar section of the outer layer material. Li (mm): Total length in the axial direction of the tapered round bar of the inner layer material. Lo (mm): This is the total length in the axial direction of the solid round bar section and the hollow round bar section of the outer layer material.

3. A method for manufacturing a seamless clad steel pipe according to claim 1 or 2, wherein in the perforation and rolling step of the composite billet, the perforation and rolling is performed such that the reduction ratio during perforation and rolling satisfies the following formula (5). 0.5≦(rt) / r≦0.9...(5) Here, in equation (5) r (mm): Radius of the solid round bar section of the outer layer material before drilling and rolling. t (mm): This is the wall thickness of the raw tube after drilling and rolling.

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