Plug for rolling seamless steel pipes, method for manufacturing the same, plug mill for rolling seamless steel pipes, and method for manufacturing seamless steel pipes

Abstract

To provide: a seamless steel pipe rolling plug that has improved adhesion and wear resistance and that has realized sufficient deformation suppression without being subjected to multiple heat treatments; a manufacturing method thereof; a seamless steel pipe rolling plug mill; and a seamless steel pipe manufacturing method.SOLUTION: A seamless steel pipe rolling plug 1 is characterized in that arithmetic mean roughness Ra of an outer surface of a plug base material is 0.04 μm or more, and a surface of the plug base material has an oxide layer.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a mandrel (hereinafter referred to as "plug") used in Mannesmann plug mill rolling of seamless steel pipes. In particular, it relates to a plug for seamless steel pipe rolling in which a hard oxide layer excellent in adhesion to the base material and wear resistance is formed on the surface to improve the plug life, a method for manufacturing the same, a plug mill for seamless steel pipe rolling, and a method for manufacturing seamless steel pipes.

Background Art

[0002] As a plug used in plug mill rolling in the conventional method for manufacturing seamless steel pipes, a high-alloy steel of a high Cr type containing, in mass%, C: 1.0 to 2.0%, Cr: 12 to 20%, Ni: 1.0 to 2.5%, one or two of Mo and W in (Mo + W): 1.0 to 2.5%, and the balance mainly consisting of Fe is used. Since a plug made of such a high Cr type high-alloy steel has excellent high-temperature strength and high-temperature oxidation resistance, it is particularly suitable as a plug used under severe conditions such as plug mill rolling.

[0003] However, even with a plug using such a material, in actual operation, due to high pressure at high temperatures, there is a problem that it often becomes unusable due to plug damage such as surface wear, local gouging, and seizure at a stage where the number of rolled pieces is small. Since these damages cause defects on the inner surface of the product and poor dimensional accuracy, development of a plug with excellent wear resistance is desired to improve the plug life and the like.

[0004] Generally, in order to improve wear resistance, it is necessary to increase the strength of the plug surface. As a technique for solving this problem, for example, Patent Document 1 discloses a plug in which a large amount of Mo or W for high-strengthening is added to have a hardness of 900 HV or more.

[0005] Furthermore, for example, Patent Document 2 discloses a method in which an oxide layer is generated on the surface of the plug material by heating it in an oxidizing atmosphere, followed by a first heat treatment in which the material is cooled at a rate faster than air cooling over a temperature range of at least 350°C, and then a second heat treatment in which the plug material is heated again in an oxidizing atmosphere to generate another oxide layer on its surface. As a result, in the oxide layer formed by the first heat treatment, the portion where the generation of Ni-enriched iron particles is insufficient and the adhesion to the iron is poor is peeled off during the cooling process at a rate faster than air cooling after the first heat treatment, and then, in the process of forming a new oxide layer by the subsequent heat treatment, the amount of Ni-enriched iron particles increases in the portion with poor adhesion, and the Ni is selectively oxidized, forming an oxide layer that is nailed into the interface between the oxide layer and the iron (anchor effect). This anchor effect obtained by the second heat treatment improves the adhesion between the oxide layer and the iron, and the oxide layer protects the plug and extends the plug life. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 57-198243 [Patent Document 2] Japanese Patent Application Publication No. 61-281819 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] In recent years, there has been a trend towards higher alloy content in the rolled materials used for manufacturing seamless steel pipes, which has led to increased rolling loads and a tendency to increase rolling speeds in order to improve productivity. This has resulted in problems such as increased hardness due to the addition of large amounts of high alloys in the rolled materials and increased rolling speeds, which leads to increased unit costs for plugs. Patent Document 1 discloses a technology that incorporates a large amount of alloying elements for the purpose of increasing strength, but there is also a need to establish a technology that can improve wear resistance without incorporating large amounts of alloying elements.

[0008] Furthermore, as described in Patent Document 2, performing heat treatment twice in the manufacturing of plugs to improve adhesion between the oxide layer and the base metal is time-consuming and increases manufacturing costs.

[0009] Furthermore, as described in Patent Document 2, when improving the adhesion of the oxide layer by heat treatment, if segregation occurs near the surface of the base material, the anchoring effect may not be sufficiently obtained, and when it comes into contact with the rolled material, the oxide layer may partially peel off, causing seizing and gouging. This gouging is then transferred to the rolled material, causing scratches on the inner surface of the rolled material. Since the plug mill plug is on the entire circumferential rolling surface, it needs to be replaced even if only a partial peeling of the oxide layer occurs.

[0010] Furthermore, even if the adhesion of the oxide layer is improved, it is difficult to improve the wear resistance of the oxide layer. Under harsh rolling conditions, even if seizing can be prevented by the oxide layer wearing down and becoming thinner, if the high-temperature strength of the plug body is low, it becomes difficult to suppress the deformation of the plug. Conversely, even if it has high high-temperature strength, if the oxide layer has low adhesion to the base material, and the base material is mainly composed of iron, it is difficult to suppress seizing, resulting in premature peeling, seizing, and gouging. Thus, it was difficult to obtain a long-lasting plug that combined excellent adhesion and wear resistance of the oxide layer, suitable for use under harsh rolling conditions.

[0011] The present invention has been made in view of the above circumstances, and aims to provide a plug for rolling seamless steel pipes that improves adhesion and wear resistance and achieves sufficient deformation suppression without requiring multiple heat treatments, a method for manufacturing the same, a plug mill for rolling seamless steel pipes, and a method for manufacturing seamless steel pipes. [Means for solving the problem]

[0012] To solve the above problems, the present invention makes it possible to form a highly adhesive oxide layer that is not affected by the heat treatment method by roughening the outer surface of the plug base material. Therefore, even if the plug material is heat treated only once, a highly adhesive oxide layer is formed, and even if the oxide layer is worn or peeled off during the rolling process, sufficient time can be ensured for the plug base metal to be exposed on the surface and for the rolled material to come into metal-to-metal contact.

[0013] The present invention is completed based on the above findings and provides the following [1] to [7]. [1] A plug for rolling seamless steel pipes, characterized in that the arithmetic mean roughness Ra of the outer surface of the plug base material is 0.04 μm or more, and the plug base material has an oxide layer on its surface. [2] The plug for rolling seamless steel pipes according to [1], characterized in that the outer surface of the plug base material has polished marks perpendicular to the rolling direction of the plug, and in a test in which the roughness is measured in the rolling direction of the plug, the arithmetic mean roughness Ra is 0.04 μm or more. [3] The plug base material has a compositional composition in mass%, containing Si: 0.3-1.5%, Mn: 0.3-1.5%, Ni: 0.01-5.0%, Cr: 12-20%, and a total of one or two of Mo and W: 0.1-3.0%, with the remainder being Fe and unavoidable impurities, as described in [1] or [2]. [4] The plug for rolling seamless steel pipes according to [3], characterized in that the component composition further contains, by mass%, C: 1.0-2.0%, Al: 0.01-0.1%, and V: 0.01-0.1%. [5] A method for manufacturing a seamless steel pipe rolling plug according to any of the above [1] to [4], A method for manufacturing plugs for rolling seamless steel pipes, characterized by processing the plug base material by polishing, grinding, or cutting. [6] A plug mill for rolling seamless steel pipes, characterized by comprising a plug for rolling seamless steel pipes as described in any of [1] to [4] above. The method for manufacturing seamless steel pipe, characterized by using the plug for seamless steel pipe rolling according to any one of [1] to [4] above.

Advantages of the Invention

[0014] According to the present invention, there is provided a plug for seamless steel pipe rolling that improves adhesion and wear resistance and realizes deformation suppression without performing multiple heat treatments. Thereby, a plug with a longer life can be obtained compared to conventional plugs.

Brief Description of the Drawings

[0015] [Figure 1] It is a diagram showing an overview of plug mill rolling. [Figure 2] It is a diagram showing an overview of a hot sliding tester. [Figure 3] It is a diagram showing a test piece after a typical hot sliding test.

Embodiments for Carrying Out the Invention

[0016] The present invention will be described with reference to the drawings. Note that the present invention is not limited by this embodiment.

[0017] The plug for seamless steel pipe rolling of the present invention has an Ra of 0.04 μm or more on the outer surface of the plug base material and has an oxide layer on the surface of the plug base material.

[0018] FIG. 1 is a diagram showing an overview of plug mill rolling of the present invention, and the plug for seamless steel pipe rolling will be described below. As shown in FIG. 1, the plug 1 of the present invention is provided in a rolling mill 101 and the rear end is supported by a bar 3. The plug 1 of the present invention is used to perform a wall thickness reduction rolling for reducing a hollow S fed between a pair of rolling arc-shaped (caliber) rolls 2a and 2b to a predetermined wall thickness. After subsequent processes, it becomes a seamless steel pipe.

[0019] The shapes of the plug 1 and the plug base material of the present invention are not particularly limited. For example, as shown in FIG. 1, they can have a cylindrical shape or a substantially conical shape in which the cross-sectional shape gradually becomes smaller in the rolling direction.

[0020] The shape of the plug 1 of the present invention is not particularly limited. As for the size, if the outer diameter of the plug is less than φ40 mm, when the oxide scale wears, it is likely to affect the dimensional accuracy of the bare tube. Therefore, the outer diameter of the plug (the maximum outer diameter of the plug in a cross-sectional view perpendicular to the rolling direction) is preferably φ40 mm or more. The upper limit is not particularly limited, but it is preferably φ500 mm or less.

[0021] <Outer surface roughness of the plug base material> Next, the reason for limiting the arithmetic mean roughness Ra of the outer surface of the plug base material of the present invention will be explained.

[0022] Ra: 0.04 μm or more In order to improve the adhesion between the oxide layer formed on the plug base material and the plug base material, the arithmetic mean roughness Ra of the outer surface of the plug base material is set to 0.04 μm or more. In order to further suppress the peeling of the oxide, the outer surface roughness Ra of the plug base material is preferably 0.50 μm or more. Considering the ease of processing, it is more preferably 1.00 μm or more so that the effect can be obtained even with cutting. Further, it is preferably 5.00 μm or more. Also, although the upper limit value is not particularly limited, if the base material is too rough, transfer defects or partial seizure may occur due to the unevenness along the roughness formed on the surface of the test piece after heat treatment. Therefore, the outer surface roughness Ra of the plug base material is preferably less than 100.00 μm. The outer surface roughness Ra of the plug base material is more preferably less than 10.00 μm. The arithmetic mean roughness Ra is measured in accordance with the provisions of JIS B 0601:2013. Also, the above-mentioned roughness measurement direction is not limited to a specific direction.

[0023] Furthermore, particularly because the presence of polishing marks perpendicular to the rolling direction of the plug improves the adhesion of the oxide layer, it is preferable that the arithmetic mean roughness Ra of the outer surface of the plug base material is 0.04 μm or more in a test in which the roughness is measured in the rolling direction (axial direction) of the plug. It is more preferable that the arithmetic mean roughness Ra is 0.50 μm or more in a test in which the roughness is measured in the rolling direction (axial direction) of the plug. It is even more preferable that it is 1.00 μm or more. It is most preferable that it is 5.00 μm or more. In addition, although there is no particular upper limit, it is preferable that the arithmetic mean roughness Ra of the outer surface of the plug base material is less than 100.00 μm in a test in which the roughness is measured in the rolling direction (axial direction) of the plug. It is more preferable that the outer surface roughness Ra of the plug base material is less than 10.00 μm. Furthermore, measuring the roughness in the rolling direction (axial direction) of the plug means running the contact terminals of the roughness meter parallel to the rolling direction of the plug and measuring in accordance with the provisions of JIS B 0601:2013 mentioned above.

[0024] The reason why the adhesion of the oxide layer is improved by having abrasive marks perpendicular to the rolling direction is that, although the detailed mechanism is unknown, the following can be considered: Because the roll shape of a plug mill is caliber-shaped, when the central plug comes into contact with the pipe, the material flows in the circumferential direction. Therefore, tensile stress acts on the plug in the circumferential direction. By providing roughness perpendicular to the rolling direction, the metal flow becomes smoother and the load on the plug is reduced. On the other hand, if roughness is provided parallel to the rolling direction, irregularities are created in the circumferential direction, making it difficult for the material to flow, and thus the metal flow tends to stagnate. Therefore, it is thought that peeling of the oxide layer is less likely to occur when the abrasive marks are perpendicular to the rolling direction compared to when they are parallel.

[0025] Polishing marks refer to the linear patterns that appear on the surface of a plug when it is ground or polished while rotating. In other words, polishing marks are linear patterns formed on the plug surface perpendicular to the rolling direction of the plug.

[0026] Furthermore, it is preferable that the outer surface of the plug base material has polishing marks perpendicular to the rolling direction (axial direction) of the plug, for the reason that the adhesion of the oxide layer is improved by having polishing marks perpendicular to the rolling direction (axial direction) of the plug.

[0027] Furthermore, while there are no particular limitations on the inner surface roughness of the plug base material, it is preferable that the inner surface roughness Ra of the plug base material be 0.05 μm or more, because the inner diameter changes (increases) due to the peeling of the oxide layer on the inner diameter of the plug, which loosens the fit between the bar and the plug and makes it easier for play to occur. It is preferable that the inner surface roughness Ra of the plug base material be less than 10.00 μm.

[0028] <Oxide layer> In this invention, it is necessary to have an oxide layer on the surface of the plug base material. Details are described below. If there is no oxide layer on the plug base material, the adhesion and wear resistance of the plug base material will decrease, so it is necessary to have an oxide layer on the surface of the plug base material. In order to obtain thermal insulation properties that greatly contribute to adhesion, wear resistance, and deformation suppression, the thickness of the oxide layer is preferably 0.10 mm or more. More preferably 0.40 mm or more. On the other hand, if the thickness of the oxide layer is too thick, the oxide layer may peel off all at once, so the thickness of the oxide layer is preferably 3.00 mm or less. More preferably 2.00 mm or less. Furthermore, since it is thought that the spinel type oxide layer of iron has better high-temperature ductility than the corundum type oxide layer, a spinel type structure is preferred for the oxide layer, for example, a layer composed of Fe and O containing Cr. The spinel type is a crystal structure that shows a face-centered cubic arrangement. Also, the spinel type structure shown here includes both the normal spinel type and the reverse spinel type. Other examples include oxide layers that are normally formed on the surface of hot tool steels such as hematite, magnetite, and wustite. To obtain the above oxides, it is preferable to control the oxygen concentration, water vapor content, heating temperature, and holding time in the furnace.

[0029] <Component composition of plug base material (material)> If the amount of alloying elements added is small and the high-temperature strength is low, the plug will be prone to overheating and deformation during rolling. However, if a large amount of alloy is added to increase the high-temperature strength, the oxide layer will become thinner or its adhesion will decrease, which may cause the plug to overheat easily. Taking these points into consideration, the preferred range of component content in the alloy composition of the plug base material of the present invention and the reasons for this will be explained next. The percentages indicating the alloy components below are in mass percentages.

[0030] The alloy of the plug base material of the present invention preferably has a composition in mass% of Si: 0.3-1.5%, Mn: 0.3-1.5%, Ni: 0.01-5.0%, Cr: 12-20%, and a total of one or two of Mo and W: 0.1-3.0%, with the remainder being Fe and unavoidable impurities.

[0031] Si: 0.3~1.5% Si is included to deoxidize the molten metal and improve its flowability. Furthermore, the presence of Si reduces the amount of wustite in the oxide layer and increases the amount of magnetite, a high-spinel oxide known for its hardness and high wear resistance. This effect is not achieved if the Si content is less than 0.3%. Therefore, the Si content should be 0.3% or higher, preferably 0.4% or higher, more preferably 0.5% or higher, and even more preferably 0.6% or higher. On the other hand, if the Si content exceeds 1.5%, it leads to a decrease in toughness. Also, excessive Si content causes a layer of SiO2 to form at the interface between the oxide and the base metal, reducing impact resistance. Therefore, the Si content should be 1.5% or less, preferably 1.4% or less, more preferably 1.2% or less, even more preferably 1.0% or less, and most preferably 0.7% or less.

[0032] Mn: 0.3~1.5% Mn is added along with Si for deoxidation. If the Mn content is less than 0.3%, a sufficient deoxidation effect cannot be obtained. Therefore, the Mn content should be 0.3% or more. Preferably, it should be 0.4% or more. More preferably, it should be 0.5% or more. Even more preferably, it should be 0.6% or more. On the other hand, if the Mn content is greater than 1.5%, the toughness decreases. Therefore, the Mn content should be 1.5% or less. Furthermore, considering the range in which the spinel-type oxide layer increases, it is preferable that the Mn content be 1.4% or less. It is more preferable that the Mn content be 1.3% or less, and even more preferable that be 1.2% or less.

[0033] Ni: 0.01~5.0% Ni improves the hardenability of the base material, and adding Ni has no effect if the Ni content is less than 0.01%. Therefore, the Ni content should be 0.01% or more. Preferably, it should be 0.6% or more. More preferably, it should be 1.0% or more, and even more preferably, 1.2% or more. On the other hand, this hardenability improvement effect saturates when the Ni content exceeds 5.0%. Furthermore, Ni contributes to improving the adhesion of the oxide layer through the mixing of Ni-enriched base iron particles in the oxide layer, but if Ni is added in excess, it can lead to a decrease in machinability due to insufficient softening and deterioration of heat treatability due to the formation of retained austenite. Therefore, the Ni content should be 5.0% or less. Also, the Ni content should preferably be 4.5% or less. More preferably, the Ni content should be 3.5% or less. Even more preferably, it should be 3.0% or less.

[0034] Cr: 12-20% Cr provides high-temperature strength to plugs used in plug mill rolling under high temperature and high pressure, and also provides corrosion resistance to plugs often used in corrosive environments, such as those containing lubricants. To obtain these effects, it is necessary to have a Cr content of 12% or more. Therefore, the Cr content should be 12% or more, preferably 13% or more, more preferably 14% or more, and even more preferably 15% or more. On the other hand, if the Cr content exceeds 20%, it becomes difficult to make the oxide layer sufficiently thick when heat treatment is applied, and a ferrite phase is formed, impairing the heat treatability. Therefore, the Cr content should be 20% or less. Furthermore, in order to reduce the corundum-type oxide layer and include more spinel-type oxides, it is preferable to have a Cr content of 19% or less, more preferably 18% or less.

[0035] Total of one or two types of Mo and W (Mo+W): 0.1-3.0% Mo and W dissolve in the base material to improve high-temperature properties such as resistance to high-temperature softening. Furthermore, they combine with C during tempering to form fine composite carbides, improving wear resistance. In addition, because Mo and W have a stronger bonding affinity to O than Fe, selective oxidation occurs when forming the oxide layer. This results in a nail-like interface between the oxide layer and the base metal, providing high adhesion through an anchoring effect. These effects can be achieved by setting the total content of one or both Mo and W (Mo+W) to 0.1% or more. Therefore, the total content of one or both Mo and W (Mo+W) should be 0.1% or more. Preferably, it should be 0.2% or more, more preferably 0.3% or more, and even more preferably 0.4% or more. On the other hand, since Mo and W are ferrite-forming elements, increasing their content narrows the composition range in which austenite can be produced. Furthermore, even if the amount of C and Ni is increased to compensate for this narrow composition range, there are the aforementioned limitations on the content of each element. For this reason, the total content of Mo and W (Mo+W) should be 3.0% or less. Preferably, it should be 2.8% or less, more preferably 2.6% or less, and even more preferably 2.4% or less. For the reasons above, and from the viewpoint of economy, one or two types of Mo and W are included, and the total content of Mo and W (Mo+W) should be 0.1 to 3.0%.

[0036] In this invention, the above-mentioned components form the basic composition. In this invention, the following components may be added as optional elements as needed.

[0037] C: 1.0~2.0% Carbon (C) combines with Cr, Mo, V, W, etc., to form high-hardness composite carbides, thereby enhancing wear resistance. It also partially dissolves into the austenite matrix, contributing to the increased strength of martensite formed during quenching. In this way, carbon (C) improves wear resistance as well as hardness and strength from room temperature to high temperatures. If the carbon content is less than 1.0%, the amount of carbide crystallization is small, and therefore high wear resistance cannot be obtained. For this reason, when carbon is included, the carbon content should be 1.0% or more. Furthermore, from the viewpoint of achieving a better balance between the amount of carbide crystallization and thermal shock resistance, it is preferably 1.2% or more. More preferably 1.3% or more. On the other hand, if the carbon content exceeds 2.0%, the amount of carbide crystallization becomes excessive, making it extremely sensitive to thermal shock and causing cracking to occur prematurely. For this reason, when carbon is included, the carbon content should be 2.0% or less. Furthermore, it is preferably 1.8% or less. From the viewpoint of balancing the amount of carbide crystallization and thermal shock resistance, it is more preferably 1.5% or less.

[0038] Al: 0.01~0.1% Al is added along with Si and Mn for deoxidation. If the Al content is less than 0.01%, a sufficient deoxidation effect cannot be obtained. Therefore, if Al is included, the Al content should be 0.01% or more. Preferably, it should be 0.03% or more. On the other hand, if the Al content is greater than 0.1%, the toughness decreases. Therefore, if Al is included, the Al content should be 0.1% or less. Preferably, it should be 0.09% or less. More preferably, it should be 0.08% or less.

[0039] V: 0.01~0.1% V, when added in small amounts, forms carbides and nitrides, contributing to grain refinement and improving toughness. This effect can be obtained when the V content is 0.01% or more. Therefore, when V is included, the V content should be 0.01% or more, preferably 0.03% or more. On the other hand, if the V content exceeds 0.1%, the above effect saturates, and toughness decreases as a hard phase is formed. Therefore, when V is included, the V content should be 0.1% or less, preferably 0.08% or less, and more preferably 0.05% or less.

[0040] The remainder, excluding the above, consists of Fe and unavoidable impurities.

[0041] <How to manufacture a plug> Next, the method for manufacturing the plug for rolling seamless steel pipes according to the present invention will be described.

[0042] Plugs may be manufactured using common, known manufacturing methods. For example, they can be manufactured by casting, in which molten steel is poured into a mold made using a wooden pattern or the lost-wax casting method. In the as-cast state, the surface roughness of the base material exceeds 100.00 μm. Furthermore, when observing the surface of the base material, the cast surface is formed in a shape that is convex relative to the reference plane. As the oxide layer wears away, the convex parts of the cast surface become exposed, which can cause seizing, gouging the plug surface and accelerating deformation. Therefore, from the standpoint of suppressing seizing, it is undesirable for the surface of the plug base material to be convex. In order to suppress seizing, it is important to minimize contact between the metal surface of the plug and the rolled material, so it is preferable for the surface of the plug base material to be concave due to processing.

[0043] The surface roughness of the plug base material of the present invention can be achieved by processing the plug base material by polishing, grinding, or cutting. For polishing, polishing with abrasive cloth or paper having a grit size of 1000 or less is preferred. The grit number for polishing indicates that the smaller the number, the coarser the surface. For polishing with abrasive cloth or paper, emery paper, grinding with abrasive wheels, lapping, polishing (buffing), barrel polishing, and other methods such as electrolytic polishing can be used. The grit number of the abrasive cloth represents the grit size measured by the method specified in JIS R 6011 or JIS R 6012, relative to the grit size specified in JIS R 6010. In the JIS standard, it is denoted by P, but in this application, it is denoted by the commonly called grit number (# or no).

[0044] The reason for using a grit size of 1000 or less for polishing and grinding is to obtain a roughness of Ra of 0.04 μm or more on the outer surface. Preferably, it is 240 or less. On the other hand, regarding the lower limit, the lower the grit size, the easier it is to remove material from the base material, so if the force is not applied evenly, the corners, which have a smaller surface area, will be removed too much, and so-called burrs are likely to occur. If burrs occur, it will be necessary to grind the entire surface to correct it, which may cause the dimensions to deviate from the specified dimensions. Because it is difficult to achieve parallelism, it is preferable to use a grit size of 80 or higher.

[0045] Polishing and grinding using abrasive cloth and paper, polishing and grinding using electrolytic polishing, and cutting are preferred because they allow for uniform control of surface roughness. Furthermore, cutting methods such as NC lathe machining, where the pitch and depth of roughness can be controlled by a program, are even more preferred. For example, a cutting speed of 5 to 500 mm / sec and a cutting amount of 0 to 30 μm per pass are preferred, and a cutting speed of 100 to 300 mm / sec and a cutting amount of 0 to 10 μm per pass are more preferred.

[0046] Another method for creating surface roughness on the outer surface is shot blasting. However, shot blasting involves using air to push out metal fragments or sand and impact the base material. Because the surface is processed by impact, there is a possibility that the base material surface may become convex. Furthermore, the shape of the shot material changes with each use, making it difficult to achieve uniform processing. Due to variations in adhesion in areas with different roughness, some oxide layers may peel off, potentially causing gouges due to burning. For these reasons, shot blasting is not a desirable processing method.

[0047] As described above, in this invention, by applying a predetermined roughness to the surface of the plug base material and, in some cases, manufacturing the plug with predetermined components, an oxide layer with excellent adhesion and wear resistance is formed even with a single heat treatment (without repeating heat treatment multiple times), making it difficult for the base metal to be exposed on the surface even after a harsh rolling process. This improves the lifespan of the plug. Furthermore, because the unit cost is high and the desired effect can be obtained even with a reduced amount of alloying elements intended for high strength, the raw cost of the plug can be reduced. Furthermore, by improving the plug's lifespan, the amount of pipe that can be rolled per plug increases, reducing the cost of the tool as a percentage of the product's manufacturing cost, thus lowering overall manufacturing costs.

[0048] Furthermore, the present invention also provides a plug mill for rolling seamless steel pipes equipped with the aforementioned plug for rolling seamless steel pipes, and a method for manufacturing seamless steel pipes using the aforementioned plug for rolling seamless steel pipes.

[0049] Furthermore, the present invention can also be applied to plugs used in equipment other than plug mills, such as piercers and elongators. [Examples]

[0050] The present invention will be further described below based on the following examples.

[0051] Example 1 To evaluate the adhesion of the oxide layer, a simulated plug mill rolling experiment was conducted using a hot roll wear and thermal shock testing apparatus. The relationship between the remaining thickness of the oxide layer on a test piece simulating a plug and the peeling and scorching state of the oxide layer, which are indicators of adhesion, was evaluated.

[0052] Figure 2 is a schematic diagram of the above-mentioned simulated test (hot sliding test machine). Specifically, a rotating body called a mating piece R and a test piece 4 of a sample coated with an oxide layer were used. The mating piece R could be heated by placing a coil 5 around it, and was set to maintain a constant temperature throughout the test. The dimensions of test piece 4 were 30 mm in length x 5 mm in width x 15 mm in height, and the dimensions of mating piece R were φ190 mm x 5L mm. The mating piece R, which had reached the specified temperature, was brought into contact with test piece 4 at 350 rpm under a load of 150 kg for 1 minute.

[0053] Test specimens for adhesion evaluation were prepared by processing the surface of a base material having the component composition shown in Table 1 above, to the roughness shown in Table 2. The processing was carried out using the methods described in Table 2. Levels 1 to 4 were performed by surface machining using a milling machine or surface grinding using a surface grinding machine. Levels 5 and 6 were polished using a grinding wheel method with 1000 and 3000 grit sandpaper, respectively. Each test specimen was held at 1185°C for 2 hours in an atmospheric furnace to form an oxide layer on the surface of the test specimen. The evaluation criteria are as follows, and the evaluation results are shown in Table 2. (judgment criteria) A (Pass, better): No areas of oxide layer peeling were observed, and no gouges due to baking were found. B (Pass, Excellent): There was no deformation (gouging) due to seizing, and although some peeling of the oxide layer was observed, it was less than 1.0% of the total, and the adhesion was judged to be excellent. C (Pass): There is localized burning, transfer defects are observed on the mating piece, and some areas of oxide layer delamination are observed, but this accounts for less than 1.0% of the total, and adhesion is judged to be good. D (Fail): Peeling of the oxide layer was observed throughout, and the adhesion was judged to be poor.

[0054] Figure 3 shows a front view (top) and a side view (bottom) of a typical test specimen 4 after a hot sliding test. For the side view, the test specimen 4 was cut in half after the test, and processed so that the center of contact with the bottom R portion of the mating piece R was visible. The contact area 13 with the mating piece was observed from the side using a digital microscope (Keyence VHX-5000) at a magnification of 30x.

[0055] Figure 3(a) shows the results for Level 4 in Table 2, and Figure 3(b) shows the results for Level 6. As shown in Figure 3(a), for Level 4 (Ra 0.640 μm) in Table 2, the oxide layer 11 remained in the mating contact area 13 (contact area with mating part R) even after the test, and no gouging 20 occurred. However, as shown in Figure 3(b), for Level 6 (Ra 0.025 μm), the oxide layer 11 did not remain on the base material 10 in the mating contact area 13 (contact area with mating part R), gouging 20 occurred, and adhesion was poor. For Level 5, some peeling of the oxide layer 11 was observed on the base material 10, but the peeled area was less than 1.0% of the total, and no gouging 20 due to burning occurred.

[0056] In Level 1, which is an unprocessed cast surface, some peeling of the oxide layer 11 was observed on the base material 10, but the peeled area was less than 1.0% of the total, and no gouges 20 occurred on the plug, but localized galling occurred, and transfer defects were observed on the mating piece. In Level 1 (Ra121.290μm), the thickness of the oxide layer before testing was the same as in Level 3 (Ra1.180μm), but by leaving the cast surface as is, it had a convex shape, and furthermore, by increasing the roughness of the base material, it was located on the outer surface side of the base material, and as the oxide layer wore away, the metal part of the base material was exposed more quickly, causing galling in that area. In addition, defects were observed on the mating piece R. This is thought to be due to transfer defects or localized galling caused by the formation of irregularities along the roughness of the surface of the test piece after heat treatment due to the base material being made too rough. In Levels 2 to 4, no peeling of the oxide layer was observed throughout the test piece, and no deformation (gouges) due to galling was observed. Based on the above results, the ranking of the evaluation results is (Excellent) A > B > C > D. Levels where A, B, or C were obtained were judged to have good adhesion, and those with a remaining oxide layer thickness of 0.10 mm or more after the test, resulting in evaluation results of A, B, or C, were judged to have reached an acceptable level of adhesion.

[0057] Example 2 Table 3 shows the component composition of the plug base material used in this embodiment. Plug base materials with steel No. A to W as their component composition were subjected to a heat treatment in which an oxide layer was formed on the surface of the plug base material by holding it at 1185°C for 2 hours in the atmosphere (using an atmospheric furnace), thereby obtaining plugs No. 1 to 35 as shown in Table 4. For plugs No. 1 to 21, the base material surface was machined using a milling process, and the target value for the arithmetic mean roughness Ra of the base material was set to 10.00 μm. For plugs No. 22 to 23, the plug base material was left unprocessed, resulting in the as-cast surface (Ra of 100.00 μm or higher). For plugs No. 24-27 using steel No. B, the target value of the arithmetic mean roughness Ra of the base material was varied from 100.00 to 0.01 μm. For plugs No. 28-31 using steels No. H, K, V, and W, the target value for the arithmetic mean roughness Ra of the base material was set to 1.00 μm. The actual Ra values ​​for the above plugs are shown in Table 4.

[0058] [evaluation] Using each of the plugs described above, hot plug mill rolling was performed on the same rolled material until the plug's lifespan was reached. The rolled material used had an outer diameter of φ223 mm, a wall thickness of 16.99 mm, and a length of 8 m, with a material specification of 0.2 mass%C-13 mass%Cr. The rolling was performed under conditions that resulted in a reduction of 30%. The temperature of the rolled material was set to 950-1000°C as measured by an infrared thermometer. After each rolling cycle, the plugs were water-cooled, and after the outer surface temperature of the plugs had dropped below 50°C, they were water-cooled for 10 minutes before being reused.

[0059] The lifespan of the spark plugs was judged based on the following criteria, which evaluate whether good wear resistance and deformation suppression were achieved.

[0060] (Evaluation of adhesion and wear resistance) After rolling, the plug surface was visually inspected to check for any burning. If no burning was observed, a non-contact thickness measuring instrument using an ultrasonic sensor (Panametrics Japan Co., Ltd. 37DL PLUS) was used to measure the oxide layer thickness over a 30mm x 30mm area. If the thickness was less than 0.10mm, it was determined, based on the above results, that the oxide layer of the plug had peeled off and wear had occurred. On the other hand, if the thickness of the oxide layer was 0.10mm or more across the entire plug surface, it was determined that the oxide layer remained on the plug and no wear had occurred. Furthermore, even after 10 or more rolling cycles, plugs with an oxide layer thickness of 0.10 mm or more across the entire surface were judged as ◎, plugs with an oxide layer thickness of 0.10 mm or more across the entire surface were judged as ○ most of the surface were judged as △, plugs with an oxide layer thickness of less than 0.10 mm in some areas (less than 10% of the measurement range) were judged as △, and plugs with an oxide layer thickness of less than 0.10 mm in areas exceeding 10% of the measurement range after less than 5 rolling cycles were judged as ×. The results are shown in Table 4.

[0061] (Deformation evaluation) After rolling, dimensional measurements were taken using a 3D scanner. If a volume change of 10% or more was observed, the plug was considered to have deformed; if the volume change was less than 10%, the plug was considered not to have deformed. In this evaluation, a contact thermometer was used to confirm that the surface temperature of the plug had returned to room temperature (25°C or below) before determining whether or not deformation had occurred. ◎ indicates that the volume change was less than 10% throughout the entire plug even after 10 or more rolling cycles; ○ indicates that the volume change was less than 10% throughout the entire plug even after 5 to 10 rolling cycles; △ indicates that the volume change was less than 10% in most parts of the plug, but locally (less than 5% of the measurement range) showed a volume change of 10% or more; and × indicates that the volume change was 10% or more throughout the entire plug after fewer than 5 rolling cycles. The results are shown in Table 4.

[0062] (Overall evaluation criteria) ◎ (Pass (Superior)): Even after more than 10 rolling cycles, no wear or deformation occurred on the plug (adhesion and wear resistance evaluation ◎ and deformation characteristics ◎). ○ (Pass): Even after 5 to 10 rolling cycles, no wear occurred on the plug, and no deformation occurred on the plug (adhesion and wear resistance evaluation ○ and deformation characteristics ○, or adhesion and wear resistance evaluation ◎ and deformation characteristics ○, or adhesion and wear resistance evaluation ○ and deformation characteristics ◎). △ (Pass): Even after performing rolling 5 to 10 times, it was determined that the adhesion / wear resistance evaluation was △ and the deformation characteristics were △, or adhesion / wear resistance evaluation was △ and the deformation characteristics were ○, or adhesion / wear resistance evaluation was △ and the deformation characteristics were ◎, or adhesion / wear resistance evaluation was ○ and the deformation characteristics were △, or adhesion / wear resistance evaluation was ◎ and the deformation characteristics were △. × (Failure): It was determined that at least one of wear or deformation occurred in the plug after fewer than 5 rolling cycles (including either an adhesion / wear resistance evaluation failure or a deformation characteristic failure).

[0063] The results are shown in Table 4.

[0064] In the examples of the present invention, the Ra value is 0.04 μm or higher. The plugs of the present invention exhibit excellent adhesion and wear resistance of the oxide layer, and sufficient deformation suppression was achieved. As a result, it was found that these plugs are cost-effective and have a long lifespan. Among the examples of the present invention, plugs No. 1, 2, 5, and 25-26, in particular, which are thought to have a thick spinel-type oxide layer with a suitable roughness range and high wear resistance, showed even better results. In addition, plugs No. 28-29 also have high wear resistance because they are thought to have originally formed a thick spinel-type oxide layer, and further improvement in the adhesion of the oxide layer by setting the Ra value to around 1.00 μm resulted in even better results. Furthermore, plugs No. 30-31 showed excellent results when their Ra was reduced to 0.81-0.97 μm by surface grinding from the cast surface. Plugs No. 22 and 23 remained in their cast surface state with an Ra exceeding 100.00 μm. Even after 5 to 10 rolling cycles, the oxide layer thickness was less than 0.10 mm in some parts of the plug surface (less than 10% of the measurement range), resulting in localized galling and transfer defects to the mating piece.

[0065] On the other hand, comparative examples that fell outside the scope of the present invention either failed to secure an Ra of 0.04 μm or more, or did not have an oxide layer. As a result, the plugs of the comparative examples failed to achieve the desired effect of at least one of the following: high adhesion of the oxide layer, wear resistance, or deformation suppression.

[0066] Example 3 To evaluate the effect of the polishing direction on the adhesion of the oxide layer, under the conditions of Level 3 in Table 2, samples were prepared in which the polishing direction of the base material was parallel to the rotation direction of the mating piece (corresponding to the outer surface of the plug base material having polishing marks parallel to the rolling direction of the plug) and perpendicular to the rotation direction (corresponding to the outer surface of the plug base material having polishing marks perpendicular to the rolling direction of the plug). The hot sliding test (Figure 2) used in Example 1 was performed using the samples with the changed polishing direction. The time until the oxide layer peeled off completely was measured and the adhesion of the oxide layer was evaluated. Specifically, after a 1-minute test, the adhesion, including the peeling state of the oxide layer, was evaluated in the same way as in Example 1. If the result was A to C, the same sample was used and the same test was performed again for 1 minute. This was repeated until a D result was obtained. The evaluation results are as follows. When the polishing direction of the base material was aligned parallel to the rotation direction of the mating piece, the result was 3 minutes (a D grade in the third test). The result was 5 minutes (a D grade in the 5th test) when the direction of the polished surface of the base material was perpendicular to the direction of rotation of the mating piece. Based on the above, by making the direction (roughness) of the polished surface of the base material perpendicular to the rotation direction of the mating piece, the time required to reach a D rating was increased, meaning that the oxide layer was more tightly bonded.

[0067] [Table 1]

[0068] [Table 2]

[0069] [Table 3]

[0070] [Table 4] [Explanation of Symbols]

[0071] 1 plug 2a Rolling arc type (caliber) roll 2b Rolling arc type (caliber) roll 3 bars S Hollow 4 Test specimens 5 coils 6. Rolling direction 7. Direction of rotation R Opponent 10 Base material 11. Oxide layer 13. Contact range of the opposing party 20 Egret 101 Rolling mill 201 Hot Roll Wear and Thermal Shock Testing Apparatus

Claims

1. The arithmetic mean roughness Ra of the outer surface of the plug base material is 0.04 μm or more, and the plug base material Having an oxide layer on the surface, The outer surface of the plug base material has polishing marks perpendicular to the rolling direction of the plug, and in a test in which roughness measurement was performed in the rolling direction of the plug, the arithmetic mean roughness Ra was 0. It is 0.4 μm or larger, The plug base material is characterized by having a component composition in mass%, containing Si: 0.3-1.5%, Mn: 0.3-1.5%, Ni: 0.01-5.0%, Cr: 12-20%, and a total of one or two of Mo and W: 0.1-3.0%, with the remainder being Fe and unavoidable impurities, for a plug for rolling seamless steel pipes.

2. The above component composition further comprises, in mass%, C: 1.0-2.0%, Al: 0.01-0.1%. The seamless steel pipe rolling plug according to claim 1, characterized in that it contains V: 0.01 to 0.1%.

3. A method for manufacturing a plug for rolling seamless steel pipes according to claim 1 or 2, A method for manufacturing plugs for rolling seamless steel pipes, characterized by processing the plug base material by polishing, grinding, or cutting.

4. A plug mill for rolling seamless steel pipes, characterized by comprising the plug for rolling seamless steel pipes described in claim 1 or 2.

5. A method for manufacturing a seamless steel pipe, characterized by using a plug for rolling seamless steel pipes as described in claim 1 or 2.

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