Seamless pipe manufacturing method

Abstract

To provide a method for efficiently manufacturing seamless pipes.SOLUTION: There is provided a method for manufacturing seamless pipes from a first steel material by carrying out the following steps: casting the first steel material into an ingot, using, as a core 31, a bar material 110 from a second steel material whose solidus temperature is higher than the liquidus temperature of the first steel material so as to obtain a steel ingot 11 through which the core 31 passes; forming the steel ingot 11 into a rod shape to obtain a rod-shaped clad material 14 in which an outer periphery of a cross section perpendicular to a longitudinal direction of the rod is made of the first steel material and a central portion of the cross section is made of the second steel material; and removing the second steel material by forming a through hole in the clad material 14 along the longitudinal direction of the rod.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a seamless pipe made of steel. [Background technology]

[0002] Conventionally, a method for producing seamless pipes of low-phosphorus 13Cr steel, which are used for oil country tubular goods and the like, from a rough pipe obtained by hot rolling such as the Mannesmann process has been known (see, for example, Patent Document 1). In this method, a round bar-shaped raw material (billet) with a diameter of approximately 200 mm is used as the base material, and tilt rolling is performed using, for example, a Mannesmann rolling mill, in which the raw material is rolled not only in the longitudinal direction but also in the circumferential direction. This tilt rolling concentrates plastic deformation at the center of the raw material, and a rolling shaft called a piercer is inserted along the centerline of the round bar, thereby obtaining a rough pipe with a diameter of approximately 250 mm. The rough pipe is then shaped into the product shape in the subsequent hot rolling process, resulting in a seamless pipe.

[0003] Furthermore, a method for manufacturing a seamless pipe by drilling a through hole in a solid bar stock is known. For example, Patent Document 2 listed below exemplifies a workpiece rotation device for efficiently performing gun drilling, a machining process for drilling through holes. In gun drilling, holes are drilled from both ends of the bar stock using a gun drill, and the through hole is formed by connecting the holes in the middle. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-122137 [Patent Document 2] Patent No. 4227551 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the method of manufacturing seamless pipes from rough pipes obtained by hot rolling has the following problem. Because the rough pipes obtained by hot rolling have a large diameter, when attempting to obtain steel pipes of the desired dimensions by subsequent hot rolling or the like, the number of steps required for diameter reduction increases, which can lead to high costs. While gun drilling can efficiently produce small-diameter rough pipes by appropriately selecting the diameter of the solid bar used and reduce the number of subsequent steps, gun drilling can sometimes cause misalignment between the centers of two holes drilled from both ends of the bar. For example, when the base material is stainless steel, such as SUS316 or SUS316L, which is a difficult-to-cut material, due to its poor machinability, it is very difficult to machine. Furthermore, the misalignment is particularly likely to occur due to gun drill runout, making it difficult to efficiently machine a highly accurate pipe shape.

[0006] The present invention has been made in view of the above-mentioned problems in the prior art, and aims to provide a method for efficiently manufacturing seamless pipes. [Means for solving the problem]

[0007] The present invention provides a method for producing a seamless pipe made of a first steel material, comprising at least a step of casting the first steel material into an ingot using a bar material made of a second steel material as a core to obtain a steel ingot through which the core passes; forming the steel ingot into a rod shape to obtain a rod-shaped clad material in which an outer periphery of a cross section perpendicular to the longitudinal direction of the rod is made of the first steel material and a central portion of the cross section is made of the second steel material; and removing the second steel material by providing a through hole in the clad material along the longitudinal direction of the rod, The present invention relates to a method for manufacturing a seamless pipe, wherein the second steel material is a steel material having a solidus temperature higher than the liquidus temperature of the first steel material. [Effects of the Invention]

[0008] In the method for manufacturing a seamless pipe of the present invention, a steel ingot penetrated by a core is cast and then formed into a rod to obtain a rod-shaped clad material whose central cross section is made of the steel material of the core. By drilling a through hole in this rod-shaped clad material and removing the steel material of the core, a seamless pipe itself or a rough pipe to be processed into a seamless pipe can be efficiently produced. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 2 is a perspective view of a seamless pipe in the first embodiment. [Figure 2] 1 is a flow chart showing the flow of a manufacturing process for a seamless pipe in Example 1. FIG. [Figure 3] FIG. 2 is an explanatory diagram of a casting stand in the first embodiment. [Figure 4] 1 is an explanatory diagram of a manufacturing process of a seamless pipe in Example 1. FIG. [Figure 5] FIG. 10 is a diagram showing a state in which a support member for supporting an end of a core is combined with a mold in Example 1. [Figure 6] FIG. 10 is a perspective view showing another square bar forming a core in the first embodiment. [Figure 7] FIG. 11 is a perspective view showing a square bar forming a core in Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0010] The embodiments of the present invention will be specifically described using the following examples. Example 1 This example relates to a method for efficiently manufacturing austenitic stainless steel piping 1 for high-pressure hydrogen, which will be described with reference to FIGS. The pipe 1 (FIG. 1) of this example is a steel pipe that is an example of a seamless pipe, having an outer diameter of 6.35 mm, a length of 2500 mm, and a through hole 10 with a diameter of 2.11 mm. The steel material (an example of a first steel material) that is the material for the pipe 1 is SUS316L, an austenitic stainless steel. SUS316L is a steel material that has excellent resistance to hydrogen embrittlement. The seamless pipe 1 made of SUS316L is suitable, for example, as a high-pressure hydrogen pipe for supplying hydrogen, which serves as fuel, to a fuel cell.

[0011] SUS316L is a difficult-to-cut material, and the processing conditions for machining such as cutting are strictly restricted, making it a material that is difficult to process. The manufacturing method of this example is effective for efficiently producing piping 1, which is a seamless pipe made of SUS316L, which is difficult to process. This manufacturing method can reduce the manufacturing cost of piping 1 made of austenitic stainless steel such as SUS316L.

[0012] The flow of the manufacturing method of the pipe 1 of this example will be described with reference to the flow diagram of Figure 2. In this manufacturing method, the following steps are carried out in order: a casting step P1 for obtaining a rectangular columnar steel ingot 11, a hot working step P2 for forming the steel ingot 11 into a round bar 13, a heat treatment step P3 for solution heat treatment, a gun drilling step P4 for drilling through holes in the round bar 13 to obtain a rough pipe 15, and a cold working step P5 for processing the rough pipe 15 into the product shape.

[0013] The casting process P1 is a process for casting a steel ingot 11 that will be the material for the pipe 1. The casting process P1 is an example of a process for casting a steel ingot 11 through which the core 31 passes by casting SUS316L into a block using a square bar 110 of S10C, a low-carbon steel, as the core 31. The steel ingot 11 is a clad material in which the outer periphery of the S10C square bar 110 is surrounded and integrated with SUS316L. SUS316L is an example of a first steel material. S10C is an example of a second steel material. The square bar 110 is an example of a bar made of the second steel material.

[0014] As shown in Figure 3, the casting process P1 is carried out using a casting mold 3 and a casting table 2 that holds four molds 3. The mold 3 is a mold for molding a steel ingot 11 inside. In this example, the mold 3 has an internal shape that corresponds to a rectangular steel ingot 11 that is 450 mm square and 1700 mm high. The mold 3 has a cylindrical shape that tapers slightly at the end. If the mold is cylindrical, the steel ingot 11 can be removed from the mold by pulling it in the opposite direction of the taper.

[0015] A pair of notches 301 are arranged facing each other at the end of mold 3. The pair of notches 301 are used to hold the sides of steel ingot 11 from both sides when demolding. Steel ingot 11 can be demolded from mold 3 by gripping it and pulling it in the opposite direction of the tapered end.

[0016] During casting, a heat-insulating board 32 is fitted into the notch 301. A heat-insulating board 34 is fitted into the opening 303 above the mold 3. The heat-insulating boards 32 and 34 are heat-insulating boards containing a heat-generating layer. The heat-generating layer is a layer that generates heat, for example, through a thermite reaction between the heat-generating material Met.Al and the reduced material iron oxide. The heat-insulating boards 32 and 34 can prevent a drop in the molten metal temperature in areas where a drop in temperature is likely to occur, such as the notch 301 and opening 303 of the mold 3.

[0017] A square hole 340 is formed in the heat insulating board 34 that is fitted into the opening 303 of the mold 3. The square hole 340 formed in the center of the heat insulating board 34 is a hole for inserting the end of the square bar 110 that will become the core 31. The square hole 340 allows the end of the square bar 110 to be positioned with high precision.

[0018] The casting table 2 is a device that holds four casting molds 3 in an upright position. Four rectangular recesses 20 are provided on the top surface of the casting table 2 for setting up the molds 3. The bottom of the recesses 20 is provided with a sprue 22 for supplying high-temperature molten metal into the inside of the molds 3, and a square hole 24 for setting up a square bar 110. The sprue 22 is the opening of a runner (not shown), which is a flow path inside the casting table 2. The other end of the runner is connected to a melting pipe 200 for supplying molten metal to the casting table 2.

[0019] The square hole 24 is a hole for inserting one end of the square bar 110. As described above, the other end of the square bar 110 is inserted into the square hole 340 of the heat insulation board 34, which is fitted into the opening 303 of the mold 3. The square bar 110, with both ends inserted into the square hole 24 of the casting table 2 and the square hole 340 of the heat insulation board 34, is aligned along the central axis of the rectangular column-shaped internal space of the mold 3. The square bar 110 held in this manner in the internal space of the mold 3 functions as a core 31 during casting.

[0020] The casting process P1 is performed with four molds 3 set on the casting table 2. SUS316L melted in an electric furnace is supplied to the casting table 2 using the melting pipe 200 and poured into the molds 3 through the gate 22. This casting process P1 produces a steel ingot 11 having an S10C square bar 110 (core 31) passing through it along its central axis. The square bar 110 protrudes slightly from the top and bottom surfaces of the steel ingot 11. The amount of protrusion of the square bar 110 (core 31) corresponds to the amount of insertion of the square bar 110 (core 31) into the square hole 340 of the casting table 2 or the heat-retaining board 34.

[0021] In this example, the clad material is obtained by utilizing the relationship between the liquidus temperature of SUS316L, an example of the first steel material, and the solidus temperature of S10C, an example of the second steel material. Here, the solidus temperature is, simply put, the temperature at which a material begins to melt. The liquidus temperature is, simply put, the temperature at which a material completely melts. Since it is essential that the second steel material used as the core 31 does not melt during casting, it is necessary to select a steel material with a higher solidus temperature than the liquidus temperature of the first steel material.

[0022] The solidus temperature of S10C, selected for the square bar 110 in this example, is 1491°C, which is higher than the liquidus temperature of 1432°C of the SUS316L used as the casting material. Therefore, when pouring molten SUS316L into the mold 3, adjusting the temperature of the molten metal so that the temperature is below the solidus temperature of S10C prevents the square bar 110 forming the core 31 from melting. Therefore, as shown in Figure 4, the steel ingot 11 has a columnar region occupied by S10C in the core 31, which is formed along the central axis, and the region occupied by S10C is surrounded by SUS316L. The steel ingot 11 is a clad material obtained by the melt casting method. Note that Figure 4 illustrates a cross section of the steel ingot 11 including the central axis to clearly show the internal structure.

[0023] The hot-working process P2 is a process for forming the steel ingot 11 into a rod. In this hot-working process P2, a round bar 13 is produced in which the outer periphery of a cross section perpendicular to the longitudinal direction of the rod is made of SUS316L (an example of a first steel material) and the center of the cross section is made of S10C (an example of a second steel material). In the hot-working process P2, the steel ingot 11 together with the core 31 is first rolled in the longitudinal direction to form a billet 12 ( FIG. 4 ) with a rectangular cross section. The billet 12 is 160 mm square and 12 m long. The billet 12 is a clad material in which a region 120 along the central axis is made of S10C. Note that FIG. 4 illustrates a cross section including the central axis of the billet 12 so that the internal structure can be seen.

[0024] The outer surface of the billet 12 is smoothed by surface processing such as polishing as appropriate. Thereafter, the billet 12 is formed into a round bar 13 by hot processing such as hot rolling, as shown in FIG. 4. The round bar 13 has a diameter of 18 mm and a length of 5 m. Note that instead of forming the round bar 13 by hot rolling, the round bar 13 may be formed by hot forging. The round bar 13 is a solid bar-shaped clad material in which the outer periphery of the cross section perpendicular to the longitudinal direction is made of SUS316L and the center portion 130 of the cross section is made of S10C. Note that FIG. 4 illustrates a cross section including the central axis of the round bar 13 so that the internal structure can be seen.

[0025] The round bar 13 is supplied to a heat treatment process P3. In this heat treatment process P3, the round bar 13 undergoes a solution treatment, which is a heat treatment typically performed on SUS316L. In this example, the solution treatment involves holding the round bar 13 at 1040°C for 30 minutes, followed by water cooling. The round bar 13 after the solution treatment is optionally subjected to a peeling process to remove the outer surface. Peeling and centerless machining are desirable to improve the corrosion resistance and shape accuracy of the outer diameter of the round bar 13. The round bar 13 is then cut into round bars 14 (FIG. 4) of a length suitable for gun drilling. This round bar 14 is an example of a solid rod-shaped clad material, with a center 140 of a cross section perpendicular to the longitudinal direction made of S10C and an outer periphery made of SUS316L. Note that FIG. 4 illustrates a cross section including the center axis of the round bar 14 so as to show the internal structure.

[0026] The subsequent gundrilling process P4 is a process for producing a rough pipe 15 having a through hole 150 as shown in FIG. 14 by drilling a through hole along the central axis in the round bar 14. Note that FIG. 4 shows the cross-sectional structure of the rough pipe 15 including the central axis. Gundrilling is a mechanical process for drilling holes from both end faces of a solid bar material and connecting them at the center to form a pipe. In this example, the gundrilling process P4 is an example of a process for removing the S10C (an example of a second steel material) core 31 from the round bar 14 by forming a through hole in the round bar 14. S10C (an example of a second steel material), a low-carbon steel, has excellent machinability and is therefore much easier to cut than SUS316L (an example of a first steel material) used to form the pipe 1. Therefore, gundrilling is significantly easier for the round bar 14, which is a rod-shaped clad material. It is possible to perform gun drilling before heat treatment, but as will be described later, there is a problem of oxide scale formation, so it is preferable to perform heat treatment first.

[0027] In the configuration of this example, the cross-sectional area of ​​the core 31 during casting is adjusted so that the diameter of the region occupied by S10C in the center of the cross section of the round bar 14 is slightly smaller than the diameter of the hole to be drilled by gun drilling. Therefore, the gun drilling process P4 makes it possible to nearly completely remove S10C from the round bar 14, and to produce a rough pipe 15 in which SUS316L is exposed over the entire inner surface of the hole, thereby making it possible to obtain a steel pipe having the excellent properties of SUS316L.

[0028] The rough pipe 15 is supplied by the cold working process P5. In the cold working process P5, cold working such as cold rolling and cold drawing is carried out, and heat treatment is carried out as necessary to reduce the diameter of the rough pipe 15. This makes it possible to produce the piping 1, which is a seamless pipe processed to meet specifications for dimensions such as the outer diameter, inner diameter, and length.

[0029] The piping 1 of this example manufactured as described above is a seamless pipe with no welds and has excellent strength. Furthermore, the austenitic stainless steel (SUS316L) from which this piping 1 is made has excellent resistance to hydrogen embrittlement. Therefore, the piping 1 of this example is suitable as a piping for high-pressure hydrogen, etc. The piping 1 can be used with high reliability as a piping for high-pressure hydrogen in, for example, hydrogen stations, fuel cell vehicles, etc.

[0030] The manufacturing method for the pipe 1 in this example involves a relatively simple process: a rod-shaped clad material (round bar 14) is obtained by hot working a cast steel ingot 11, and then gun drilling is performed on the rod-shaped clad material (round bar 14) to obtain a rough pipe 15, which is then processed into the pipe 1. In this manufacturing method, the round bar 14 for gun drilling is obtained from the steel ingot 11, which uses a square bar 110 of low-carbon steel S10C as the core 31. This round bar 14 is a rod-shaped clad material, with the center of the cross section made of low-carbon steel (S10C) and the outer periphery made of austenitic stainless steel (SUS316L). The low-carbon steel in the center has better machinability than the austenitic stainless steel from which the pipe 1 is made, and because it contains a low content of alloying elements other than Fe, it does not harden due to quenching during solution heat treatment. Therefore, the clad material round bar 14 is a material suitable for gun drilling. By performing gun drilling on this round bar 14, it is possible to manufacture a highly accurate pipe 1 with little misalignment of the inner diameter.

[0031] Gun drilling involves machining an inner diameter that is slightly larger than the size of the low-carbon steel region in the center of the cross section of the round bar 14. Therefore, the low-carbon steel in the center of the cross section of the round bar 14 is removed by gun drilling. The piping 1, which is made from the rough pipe 15 obtained by gun drilling, is not a clad pipe with low-carbon steel, but a pipe made of austenitic stainless steel.

[0032] As described above, the manufacturing method of the pipe 1 of this example significantly reduces the manufacturing costs of seamless pipes with outer diameters of approximately 6 to 30 mm, which has been difficult to reduce due to the complicated processes involved. For example, in the method of manufacturing steel pipes from crude pipes processed by the Mannesmann process shown as the prior art, it is difficult to manufacture small-diameter steel pipes, for example, with outer diameters of 6 to 30 mm, at low cost. This is because manufacturing methods using the Mannesmann process, which are difficult to manufacture small-diameter crude pipes, are likely to incur excessive processing costs for reducing the diameter of large-diameter crude pipes. In contrast, the manufacturing method of the pipe 1 of this example easily produces small-diameter crude pipes by gun drilling small-diameter solid bar material obtained by hot rolling. By appropriately selecting the diameter of the solid bar material after hot rolling, small-diameter crude pipes can be efficiently manufactured, and the processing costs for reducing the diameter can be reduced. The manufacturing method of the pipe 1 of this example is particularly effective for efficiently manufacturing small diameter steel pipes, and this manufacturing method makes it possible to manufacture steel pipes with outer diameters of, for example, 6 to 30 mm at low cost.

[0033] In this example, the core material is S10C, a low-carbon steel with a low content of alloying elements other than Fe. Instead of low-carbon steels such as S10C and SS steel, pure iron or low-alloy steels such as low-carbon Cr steel with relatively low alloying elements may also be used. The standard for low carbon is a carbon content of 0.20% or less. Low carbon ensures a high melting point and excellent machinability. In this example, the austenitic stainless steel SUS316L is used as the steel material for the pipe 1. However, pipes made of other austenitic stainless steels, such as SUS316, can also be manufactured without problems using the method of the present invention. Furthermore, pipes made of not only austenitic stainless steels but also precipitation-hardened stainless steels, such as SUS630, can also be manufactured without problems because, like SUS316, they have a lower melting point than pure iron or low-carbon steels.

[0034] In the manufacturing method of the pipe 1 of this example, the heat treatment step P3 for solution heat treatment is performed before the gun drilling step P4. In other words, in this manufacturing method, after performing the necessary heat treatment on the steel material that will become the pipe 1, the low-carbon steel portion of the clad material round bar 14, which originates from the core 31, is removed. If the heat treatment were performed after removing the portion originating from the core 31, the heat treatment would generate oxide scale on the inner surface of the pipe, which would then need to be removed. Performing the heat treatment prior to gun drilling prevents oxide scale from forming on the inner surface of the pipe. Note that if the center of the round bar 14 is quenched during heat treatment, the material may harden, resulting in a deterioration in machinability. In this example, the core 31 made of low-carbon steel is used, and because low-carbon steel has poor hardenability, there is little risk of material hardening due to heat treatment and a deterioration in machinability. Low-carbon alloy steels with a relatively low alloying element content can also be used because they have a high melting point and poor hardenability.

[0035] In this example, a casting table 2 is used, in which molten metal is poured from the bottom side of the mold 3. Alternatively, molten metal may be poured from the ceiling side of the mold. In this example, a heat-insulating board 34 is used to regulate the position of the end of the square bar 110 that will become the core 31. Alternatively, a support member 35 (see FIG. 5) that supports the end of the square bar 110 may be used in addition to the heat-insulating board 34. The support member 35 has four arms 353 extending radially from a ring-shaped portion 351 having a hole 350 for inserting the end of the square bar 110. The tips of the arms 353 are fixed to the corners of the edge that forms the opening of the mold, thereby positioning the hole 350 on the central axis of the interior space of the mold. Note that if sufficient positional accuracy can be ensured when the end of the square bar 110 is inserted into the square hole 24 of the casting table 2, the structure for fixing or supporting the other end may be omitted.

[0036] In this example, a rectangular steel ingot 11 is cast, but the steel ingot may be cylindrical or polygonal. In this example, a cylindrical mold is used, but molds of various structures can be used. A split mold or a mold combining a cylindrical portion and a split mold may also be used.

[0037] As a square bar for the core, a square bar 110 shown in FIG. 6, which has an irregular outer surface, may be used. The outer surface of this square bar 110 has multiple grooves 110T extending perpendicular to the axial direction. In a steel ingot cast using the square bar 110 shown in FIG. 6 as a core, the grooves 110T are filled with SUS316L, the first steel material. Therefore, the grooves 110T, which are perpendicular to the rolling direction, are useful for preventing interfacial delamination that can occur between the S10C core and the SUS316L outer periphery when the steel ingot is subjected to hot working or other processes.

[0038] Furthermore, it is also possible to create minute irregularities on the outer peripheral surface of the square bar by shot blasting, or to create jagged or other irregularities on the outer peripheral surface of the square bar by knurling. Similar to the groove 110T in Figure 6, these irregularities are useful in preventing interfacial peeling that can occur between the S10C core and the SUS316L outer peripheral portion.

[0039] Example 2 This example is an example in which the processing accuracy of the pipes manufactured by the manufacturing method of Example 1 is compared. The details of this example will be explained with reference to Table 1. In this example, three types of rough pipes (samples 1 to 3) were produced by the manufacturing method of the present invention by changing the combination of the piping material and the material of the round bar that would become the core, as well as a comparative rough pipe (sample 4) that did not use a core, as shown in Table 1. The rough pipes of samples 1 to 4 were then evaluated for the degree of misalignment of the inner diameter due to gun drilling.

[0040] [Table 1]

[0041] In this example, austenitic stainless steel (SUS316, SUS305) and precipitation hardening stainless steel (SUS630) are used as the piping materials, as shown in Table 1. Pure iron and low-carbon steel (S10C) are used as the core materials (samples 1 to 3). Table 1 also shows the liquidus temperature or solidus temperature and Vickers hardness (HV) for each material.

[0042] The steel ingot cast in this example has a diameter of approximately 130 mm and a length of approximately 300 mm. This steel ingot is then formed into a round bar with an outer diameter of 25 mm and a length of 850 mm by hot forging. After undergoing the same solution heat treatment as in Example 1, this round bar is cut at both ends and machined to an outer diameter of 21 mm by peeling using a lathe. A through hole with an inner diameter of 7 mm is then drilled along the center axis of the round bar using a gun drill to produce a rough pipe. This rough pipe is an intermediate material that is then processed into a product by cold rolling, cold drawing, or other processes.

[0043] A core is used when casting the steel ingots that will be the basis for the rough pipes of Samples 1 to 3. The core is a round bar with a diameter of 20 mm and a length slightly longer than the height of the casting mold (not shown). The steel ingots that will be the basis for the rough pipes of Samples 1 to 3 can be cast by pouring molten metal into a cylindrical casting mold with the core positioned along its central axis. This steel ingot is a clad material made of the steel material of the core and an oiled steel material. On the other hand, the steel ingot that will be the basis for the rough pipe of Sample 4 is a steel ingot cast without using a core. This steel ingot is not a clad material made of two types of steel, but a steel ingot made of a single type of steel.

[0044] Gun drilling, which is used to obtain rough pipe, is a mechanical process that creates a through hole by connecting two holes drilled at both ends of a round bar in the middle. Therefore, after gun drilling, the rough pipe may have a misalignment, which is a step in the inner diameter, at the center of its length. Naturally, a large misalignment can cause uneven wall thickness in the pipe, so a small misalignment is desirable.

[0045] The rough pipes of Samples 1 to 3, which correspond to examples of the present invention, achieved a misalignment of 0.2 mm or less and were rated as ◯. On the other hand, the rough pipe of Sample 4, which was produced without using a core and serves as a comparative example, had a misalignment of 0.9 mm and was rated as ×. The liquidus temperatures of stainless steels in the table are calculated values ​​using phase diagram calculation software. The solidus temperatures of pure iron are literature values. The solidus temperature of S10C is a calculated value using phase diagram calculation software. The other configurations and effects are the same as those of the first embodiment.

[0046] Example 3 This example is based on the configuration of Example 1, but is an example in which the square bar 110 forming the core 31 in the casting step P1 is changed. This will be described with reference to FIGS.

[0047] The square bar 110 (FIG. 7) in this example is an accumulation of scrap material produced by cutting a steel material (not shown) made of low-carbon steel S10C, which is compressed and formed into a rod shape. The scrap material in this example is scrap material called chips produced during cutting, and is made of S10C, an example of a second steel material. The shape of the chips produced during cutting varies depending on the processing conditions, etc., and can be spiral, ribbon, chip, or other shapes. In this example, a spiral chip with a length of 20 mm or more is used.

[0048] The square bar 110 made of spiral chips in this example is a porous body in which the spiral chips are intertwined, forming three-dimensional gaps inside. In particular, spiral chips with a length of 20 mm or more are suitable for compression molding into a predetermined shape because they are prone to plastic deformation due to the intertwining of the chips.

[0049] In this example, the square bar 110 has many irregularities formed on its outer peripheral surface. In the steel ingot 11 (see FIG. 4 ) obtained by casting SUS316L, an example of the first steel material, into an ingot using this square bar 110 as the core 31, scrap SUS316L has penetrated into the irregularities on the outer peripheral surface of the square bar 110 and has also penetrated into the three-dimensional gaps inside.

[0050] In this steel ingot 11, the interface between the S10C core and the SUS316L outer periphery has a complex three-dimensional shape. This steel ingot 11 is less likely to experience interfacial delamination between the S10C and SUS316L when hot-rolled into a rod, thereby improving design flexibility when selecting hot-rolling conditions.

[0051] Chips, which are scrap material produced during cutting, are distributed in large quantities on the market at low cost as scrap. Chips produced during cutting are particularly inexpensive among scrap. If such chips are used as the material for the square bar 110 that forms the core, the manufacturing cost of seamless pipes can be reduced, resulting in a cost-effective product. Chips produced during the gun drilling process of Example 1 can also be used.

[0052] In this example, the square bar 110 is obtained by compression molding a spiral chip. Alternatively, the spiral chip may be compressed to obtain a block-shaped compact, and then an electric current is passed through the compact to weld the chips together, thereby obtaining the square bar 110. In this example, taking into consideration the fact that plastic deformation due to entanglement is likely to occur, spiral chips with a length of 20 mm or more are used as the material for the square bar 110. If a large current is passed through the compact after compression to weld the chips together, plastic deformation due to entanglement is not necessary. Spiral chips shorter than 20 mm, ribbon-shaped chips, chip-shaped chips, etc. can easily be used as the material for the square bar 110.

[0053] It is also possible to use a square bar obtained by compression molding scrap material generated by punching press processing as the core. Furthermore, it is also possible to use a square bar obtained by compression molding an empty iron can as the core. By using such a core, the manufacturing cost of the seamless pipe can be reduced.

[0054] A square bar obtained by compression molding granular metal powder made of the second steel material may also be used as the core. Alternatively, a square bar obtained by sintering metal powder made of the second steel material may also be used as the core. These square bars are porous, with numerous pores opening on the outer surface. A steel ingot cast using such a square bar as the core is less likely to experience interfacial peeling between the second steel material making up the core and the first steel material on the outer periphery. The other configurations and effects are the same as those of the first embodiment.

[0055] Although specific examples of the present invention have been described in detail as examples, these examples merely disclose examples of the technology encompassed by the claims. Needless to say, the scope of the claims should not be interpreted as being limited by the configurations, numerical values, etc. of the specific examples. The claims encompass technologies that are obtained by variously modifying, changing, or appropriately combining the specific examples using publicly known technology and the knowledge of those skilled in the art. [Explanation of symbols]

[0056] 1 Piping (seamless pipe) 11 Steel ingot 110 square bar 12 Steel billet 13 Round bar 14 Round bar (rod-shaped clad material) 15 Coarse tube 150 through holes 2 Casting Table 22 Spout 24 square hole 3. Mold 301 Notch 303 Opening 31 Core 32, 34 Heat insulation board 340 square hole

Claims

1. When manufacturing a seamless pipe made of the first steel material, at least a step of casting the first steel material into an ingot using a bar material made of a second steel material as a core to obtain a steel ingot through which the core passes; forming the steel ingot into a rod shape to obtain a rod-shaped clad material in which an outer periphery of a cross section perpendicular to the longitudinal direction of the rod is made of the first steel material and a central portion of the cross section is made of the second steel material; and removing the second steel material by providing a through hole in the clad material along the longitudinal direction of the rod; A method for manufacturing a seamless pipe, wherein the second steel material is a steel material having a solidus temperature higher than the liquidus temperature of the first steel material.

2. 2. The method for manufacturing a seamless pipe according to claim 1, wherein the second steel material is one of pure iron, low-carbon steel, and low-carbon alloy steel.

3. 2. The method for manufacturing a seamless pipe according to claim 1, wherein the step of removing the second steel material is a step of removing the second steel material by forming the through-hole by drilling.

4. 4. The method for manufacturing a seamless pipe according to claim 3, further comprising the step of subjecting the clad material to a heat treatment prior to the step of removing the second steel material.

5. 5. The method for manufacturing a seamless pipe according to claim 1, wherein the first steel material is an austenitic stainless steel or a precipitation hardening stainless steel.

6. 5. The method for manufacturing a seamless pipe according to claim 1, wherein the rod material made of the second steel material forming the core is a rod material having irregularities on its outer circumferential surface.

7. 7. A method for manufacturing a seamless pipe according to claim 6, wherein the rod material made of the second steel material that forms the core is an accumulation of metal powder or metal pieces made of the second steel material, and the unevenness on the outer peripheral surface is formed by the metal powder or metal pieces being exposed on the outer peripheral surface.

8. 8. A method for manufacturing a seamless pipe according to claim 7, wherein the rod material made of the second steel material forming the core is an accumulation of scrap material generated by cutting a member made of the second steel material.

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