TW201938813A - Steel pipe and production method for steel pipe - Google Patents

Abstract

A steel pipe that includes more than 0.70 but no more than 1.20 mass% of C, no more than 0.03 mass% of P, and no more than 0.30 mass% of Cu and has a weld part that has a metal structure that includes carbides and reheated ferrite.

Description

Steel pipe and manufacturing method of steel pipe

The present invention relates to a steel pipe and a method for manufacturing a steel pipe.

The welded steel pipe is generally manufactured by welding a steel plate or a steel strip. For example, Patent Document 1 discloses a method for manufacturing a straight seam welded steel pipe, which is one of the types of welded steel pipes. High-frequency welding of high-carbon steel plates is followed by cold-rolling and hot-rolling reduction.

[Prior technical literature]
[Patent Literature]
[Patent Document 1] Japanese Published Patent Gazette "Japanese Patent Laid-Open No. 2015-062920"

[Inventive problem solving]
However, when a high-carbon steel plate or a high-carbon steel strip as described in Patent Document 1 is welded, a welding crack occurs at a welded portion or the like. Therefore, in general, in the high-carbon welded steel pipe described in Patent Document 1, in order to remove welding cracks, it is necessary to further perform manufacturing steps such as cold rolling reduction and hot shrinkage reduction rolling. Therefore, there is a problem that high-carbon welded steel pipes cannot be manufactured efficiently.

The present invention has been made to solve the aforementioned problems, and an object thereof is to provide a high-carbon welded steel pipe that can be efficiently manufactured.

[Means for solving problems]
In order to solve the foregoing problem, a steel pipe according to one aspect of the present invention is characterized by including: carbon (C): 0.70 mass% or more and 1.20 mass% or less; phosphorus (P): 0.03 mass% or less; and copper (Cu): 0.30% by mass or less, wherein the metal structure of the welded portion is a metal structure containing ferrous iron and carbides subjected to reheating treatment.

In addition, the method for manufacturing a steel pipe according to one aspect of the present invention is characterized in that the steel pipe includes: carbon (C): 0.70 mass% or more and 1.20 mass% or less; phosphorus (P): 0.03 mass% or less; and copper (Cu ): 0.30% by mass or less, and the metal structure of the welded portion is a metal structure including ferrous iron and carbides subjected to reheating treatment; and the aforementioned manufacturing method includes a forming step of forming a steel plate or a steel strip by Roll forming into a tubular shape; a welding step, which is performed after the forming step, welding each end surface of the opposite steel plate or each end surface of the opposite steel strip to produce a steel tube; and a quenching step, which is performed after the foregoing The steel tube after the welding step is subjected to quenching treatment; the tempering step is performed after the aforementioned quenching step is performed on the steel tube.

[Effect of the invention]
According to one aspect of the present invention, the effect of providing a welded steel pipe that does not require complicated manufacturing steps can be achieved.

Hereinafter, one embodiment of the present invention will be described in detail.

＜ steel pipe ＞
The steel pipe system in this embodiment includes: carbon (C): 0.70 mass% or more and 1.20 mass% or less; phosphorus (P): 0.03 mass% or less; and copper (Cu): 0.30 mass% or less. It is a metal structure containing reheated ferrous iron and carbides. Here, the reheating treatment may be, for example, a tempering treatment described later. The metal structure containing ferrous iron and carbides herein refers to, for example, tempered Asada loose iron, toughened iron, and polyiron. The metal structure of the welded portion of the steel pipe is a high-carbon welded steel pipe including a metal structure of ferrous iron and carbides that have been reheated. The manufacturing method may be, for example, forming a steel pipe or a steel strip into a tube by roll forming. Thereafter, each end surface of the opposite steel plate or each end surface of the opposite steel strip is welded to form a welded portion. Next, after forming a welded part, it is manufactured by applying a quenching treatment and a tempering treatment. According to this manufacturing method, even if the steel pipe in this embodiment is a high-carbon welded steel pipe with a high carbon (C) content, welding cracks do not occur. Therefore, it is not necessary to perform manufacturing steps such as cold-rolling rolling and hot-rolling reduction rolling in order to remove welding cracks, and the manufacturing can be performed efficiently. In addition, the steel pipe in this embodiment contains only a specific amount of a specific component as described above, thereby obtaining a good rolling fatigue life.

The "welded portion" as used herein refers to a portion where a steel plate or a steel strip is welded, such as a bead portion. In addition, "rotational fatigue life" refers to a period during which the bearing using the steel pipe in this embodiment performs a rolling operation until the base material of the steel pipe and the welded portion are subjected to surface peeling. "Good rolling fatigue life" means that it is as long as a seamless steel pipe widely used as a high-carbon steel pipe. The rotational fatigue life can be determined by, for example, cutting a welded steel pipe into a plate shape, and measuring the period until the surface of the base metal and the welded part is peeled off by a thrust-type rotational fatigue test.

The diameter of the steel pipe is preferably 15 mm or more and 300 mm or less. The thickness of the steel pipe is preferably 2 mm or more and 10 mm or less. By making the diameter and thickness of the steel pipe into the above-mentioned preferred ranges, the steel pipe of this embodiment can be manufactured without requiring special manufacturing conditions.

In addition, the steel pipe in this embodiment may contain non-metallic inclusions such as sulfides and oxides. Among the non-metallic inclusions, sulfides, especially manganese sulfide (MnS), will condense and precipitate on the surface of the steel pipe, which will cause the fracture and surface damage caused by non-metallic inclusions. As a result, there is a risk of reducing the rolling fatigue life. In addition, when a sulfide such as manganese sulfide (MnS) is present on a surface portion that is in contact with the rotating body of the rotary bearing, the aforementioned portion needs to be subjected to a large turning process, so that the manufacturing cost may increase. Therefore, the particle size of non-metallic inclusions such as sulfides is preferably 10 μm or less. By making the particle size of the non-metallic inclusions in the steel pipe as small as the foregoing preferred range, particularly when the non-metallic inclusions are sulfides, it is possible to prevent a reduction in rotational fatigue life and an increase in manufacturing costs. When the steel pipe in this embodiment contains oxides as non-metallic inclusions, the oxide content in the steel pipe is preferably 20 ppm or less, more preferably 15 ppm or less, and particularly preferably 10 ppm or less. By setting the oxygen content in the steel pipe to the above-mentioned preferred range, a steel pipe with high definition can be obtained. As a result, a steel pipe having a good rolling fatigue life can be obtained.

[Steel plates and strips]
The steel plate and the steel strip are suitable as the original material of the steel pipe in the present embodiment. The steel sheet and the steel strip are formed into a tube shape by applying roll forming, and are subjected to welding, quenching, and tempering treatment, and further become a steel tube in this embodiment.

Here, as a seamless steel pipe without a welded steel pipe, the amount of non-metallic inclusions (e.g., vulcanized) on the outer and inner sides of the steel pipe is affected by the segregation of the center of the base steel bar. The amount of material) and size. Although the non-metallic inclusions on the outer surface side are fine and trace, the non-metallic inclusions on the inner side surface are not only present in large amounts, but also exposed on the inner surface. Therefore, when a seamless steel pipe is used for a bearing, although the outer surface of the rolling bearing uses the inner surface of the seamless steel pipe, a large amount of non-metallic inclusions existing on the inner surface bring a large effect on the rotational fatigue life. Therefore, in order to increase the rotational fatigue life, it is necessary to turn a portion that is in contact with the rotating body to a large extent, resulting in a high machining cost.

In contrast, the steel pipe in this embodiment, that is, a welded steel pipe such as a straight seam welded steel pipe welded from a steel plate or a steel strip, is different from a seamless steel pipe, and most of the non-metallic inclusions exist in the steel plate or the steel strip. In the interior, it hardly exists on the inner and outer surfaces. Therefore, compared with the seamless steel pipe, a welded steel pipe using a steel plate or a steel strip has higher cleanliness on the inner surface side and can reduce the difference in cleanliness between the inner and outer surfaces of the steel pipe. Based on the above, the steel pipe in this embodiment has a high degree of cleanliness on the inner surface side, so that the amount of cutting when processing the shape of a part can be reduced, and a good rolling fatigue life can be obtained as much as a seamless steel pipe.

In addition, the steel pipe in this embodiment is obtained by welding a steel plate or a steel strip, and therefore, the steel pipe can be produced in a large amount compared with a method of manufacturing a steel pipe by welding a bar.

In addition, the aforementioned steel strip or the steel sheet may be, for example, a rolled shape having a thickness of 10 mm or less. In this embodiment, although both a steel plate and a steel strip can be used as the original material of this embodiment, it is preferable to use a steel belt to manufacture a steel pipe. Since the steel strip is thinner than the steel plate, it can have high productivity. Thereby, the steel pipe in this embodiment can be manufactured more efficiently. In addition, the steel strip can be obtained, for example, by hot-rolling and rolling the steel.

(Roll forming)
In the roll forming of this embodiment, a steel plate or a steel strip is formed into a tube by passing a steel plate or a steel strip between rollers. Here, since it is relatively easy to perform roll forming using a steel strip as a raw material, it is preferable to apply a process such as hot rolling and calendering to the steel before the roll forming to make it into a rolled steel strip. In addition, before the steel sheet or steel strip is roll-formed, it can also be cleaned with acid; or it can be annealed at a temperature of 600 ° C to 800 ° C for 1 hour to 50 hours. This makes roll forming easier.

(welding)
In the welding of the present embodiment, butt welding is performed on each end surface of the steel plate or the end surface of the steel strip that is formed into a tubular shape. Thereby, the steel pipe of this embodiment can be obtained. Examples of the welding method of this embodiment include high-density energy welding such as resistance welding, laser beam welding, and electron beam welding, but resistance welding is preferred, and high-frequency welding is preferred in resistance welding. . By welding a steel plate or a steel strip with high frequency welding, the steel plate or the steel strip can be welded efficiently and at low cost. The welding is preferably performed at a temperature of 1300 ° C or higher and 1600 ° C or lower.

(Quenched)
After welding, the steel pipe in this embodiment is quenched. In particular, since quenching is performed immediately after welding, welding cracks in the welded portion can be appropriately prevented. In the quenching treatment, it is preferably calculated from the temperature at which the obtained steel pipe is 50 ° C or higher relative to the A3 transformation point or Acm transformation point, and it is preferable to apply cooling so that the temperature of the steel pipe becomes relative to the Ms point ( The temperature at the beginning of metamorphosis is as low as 50 ° C and 200 ° C; and more preferably, cooling is applied to a temperature 100 ° C and 200 ° C lower than the Ms point. In this case, it is preferable to apply cooling by, for example, water cooling or oil cooling outside the steel pipe. By making the temperature of the steel pipe subjected to the cooling treatment within the above-mentioned preferable temperature range, the metal structure in the welded portion of the steel pipe can be a metal structure mainly composed of loose iron. Thereby, when a tempering process is applied to a steel pipe, the tensile stress generated in the welded portion due to the transformation of loose iron in Asada can be appropriately reduced. As a result, the effect of tempering for preventing welding cracks can be maximized.

(Tempering)
In addition, in the present embodiment, a tempering treatment is applied to the steel pipe subjected to the quenching treatment. As the tempering temperature, tempering is preferably performed at a temperature of 500 ° C or higher and 50 ° C or lower relative to the A1 transformation point; more preferably, the tempering is performed at 600 ° C or higher and 750 ° C or lower; The best line is tempered above 700 ° C and below 730 ° C. The time for the tempering treatment is preferably 5 seconds or more and 5 minutes or less, and more preferably 10 seconds or more and 1 minute or less. In this way, the time for the tempering treatment is set to a short time, which can prevent welding cracking. The tempering treatment is preferably applied to the steel pipe quickly after quenching. For example, the tempering treatment is preferably performed within 5 minutes after quenching, and more preferably within 1 minute.

The steel pipe system in this embodiment is a high carbon welded steel pipe having a high carbon (C) content in the steel pipe. Therefore, the metal structure of the welded part that has been subjected to rapid heating during welding may cause the deformation of the loosened iron in Asada, and the metallic structure of the welded part may become hardened. The tensile stress caused by the distortion of the Asada scattered iron and the residual processing strain (tensile stress) caused by roll forming may cause welding cracks in the welded portion. In particular, when the carbon content in the steel pipe is higher than 0.70% by mass, welding cracks easily occur in the welded portion. On the other hand, by applying a quenching treatment and a tempering treatment after welding as described above, the tensile stress generated in the metal structure of the welded portion due to the deformation of the loose iron in Asada can be reduced. As a result, the toughness of the welded portion of the steel pipe can be improved, and even if the carbon content in the steel pipe is higher than 0.70% by mass, the occurrence of welding cracking can be prevented.

[Inclusions in steel pipes]
The steel pipe system in this embodiment includes: carbon (C): 0.70% by mass or more and 1.20% by mass or less; phosphorus (P): 0.03% by mass or less; and copper (Cu): 0.30% by mass or less, the rest is Iron (Fe) and unavoidable impurities. The steel pipe in this embodiment contains only a specific amount of a specific component and has a small amount of impurities, and therefore has high hardness and cleanliness. Therefore, since the steel pipe in this embodiment does not cause welding cracks, and has high hardness and cleanliness, it also has a good rolling fatigue life. In addition, in the case where the steel pipe in this embodiment contains substantially only the aforementioned content of carbon (C), phosphorus (P), and copper (Cu) in addition to iron (Fe) and inevitable impurities, as long as it is a weld The metal structure is a metal structure containing ferrous iron and carbides after reheating, which can solve the problem of efficiently manufacturing high-carbon welded steel pipes.

Carbon (C)
The steel pipe in this embodiment contains carbon (C) of 0.70 mass% or more and 1.20 mass% or less. That is, the steel pipe in this embodiment is a high carbon welded steel pipe. Carbon (C) is the most basic element in carbon steel, and the hardness and the amount of carbides vary greatly based on the content in the steel pipe. When the carbon (C) content is 0.70% by mass or more, undissolved carbides remain during quenching and heating, thereby ensuring good abrasion resistance. In addition, the carbon (C) content is made 1.20% by mass or less, so that the toughness after hot rolling and calendering is not reduced, so that it becomes a steel pipe having good manufacturability and handleability. As a result, when the steel pipe according to the present embodiment is used as a bearing, it is possible to prevent abrasion of the rotor constituting the bearing. In addition, in order to improve the manufacturability of the steel pipe, when the steel pipe or the steel strip is subjected to softening annealing treatment, the steel pipe or the steel strip can be provided with sufficient ductility. In order to ensure good abrasion resistance, the content of C is preferably large, and the lower limits of the content of C are preferably in the order of 0.90 mass%, 0.85 mass%, 0.80 mass%, and 0.75% by mass.

Phosphorus (P)
Phosphorus (P) is an element that reduces ductility and toughness of steel pipes. The phosphorus (P) content in the steel pipe is preferably 0.03% by mass or less, more preferably 0.02% by mass or less, and particularly preferably 0.01% by mass or less. By reducing the content of phosphorus (P) to 0.03 mass% or less, the toughness of the old Vosted iron grain boundaries in the steel pipe after quenching is improved, and the rotation fatigue resistance of the steel pipe after heat treatment is prevented from being reduced.

Copper (Cu)
Copper (Cu) is an element that improves the peelability of scales (rolled scales) produced on steel plates or steel strips during hot rolling and calendering, thereby improving the surface characteristics of steel plates, steel strips, and steel pipes obtained from steel plates or steel strips. The copper (Cu) content in the steel pipe is preferably 0.30% by mass or less. By setting the copper (Cu) content to 0.30% by mass or less, it is difficult to cause fine cracks on the surface of the steel plate, the steel strip, and the steel pipe obtained from the steel plate or the steel strip.

[Other ingredients that can be included in steel pipes]
In addition, the steel pipe according to the present embodiment may further contain silicon (Si), manganese (Mn), chromium (Cr), and sulfur (S) in addition to the above components within a range that can solve the problem of efficiently manufacturing a high-carbon welded steel pipe. ), At least one of aluminum (Al), nickel (Ni), and molybdenum (Mo). Here, when at least one of phosphorus (P), molybdenum (Mo), nickel (Ni), sulfur (S), and aluminum (Al) is contained, the total content of these components is preferably relative to The steel pipe is 7.20% by mass or less, more preferably 6.00% by mass or less, and particularly preferably 4.00% by mass or less. With the above-mentioned preferred range, impurities contained in the steel pipe can be reduced, and the cleanliness of the steel pipe can be further improved. As a result, it is possible to obtain a steel pipe having superior performance in rotational fatigue life.

Silicon (Si)
Silicon (Si) is one of the elements having a large influence on the ductility of steel pipes. The silicon (Si) content in the steel pipe is preferably 0.80% by mass or less, more preferably 0.50% by mass or less, and particularly preferably 0.30% by mass or less. As described above, when the content of silicon (Si) is not excessive, the hardening of the ferrous iron due to the solid solution strengthening effect of silicon (Si) can be prevented, thereby preventing the occurrence of steel pipe during the forming process. rupture. In addition, rust marks can be prevented from being produced on the surface of the steel plate or steel strip during the manufacturing process, and the reduction in rotation fatigue life caused by grain boundary oxidation during quenching and heating of the steel pipe can be prevented.

Manganese (Mn)
Manganese (Mn) is used in the case of quenching and heating steel pipes to suppress the transformation of ferrous iron in the steel during the cooling process after quenching, so that it is mainly based on Asada loose iron even at a relatively slow cooling rate. Element to improve the hardenability of steel pipes. The manganese (Mn) content in the steel pipe is preferably 0.20% by mass or more and 2.00% by mass or less, and more preferably 0.50% by mass or more and 1.50% by mass or less. In this way, by reducing the manganese (Mn) content to 0.20% by mass or more, the hardenability of the steel pipe can be prevented from being reduced, and the steel of the steel pipe being cooled can be prevented from forming high-temperature products such as boron iron and toughened iron. This makes it possible to obtain a desired hardness when using the steel pipe of this embodiment as a bearing. In addition, by controlling the content of manganese (Mn) to be 2.00% by mass or less, it is possible to prevent the iron particles from being hardened, which may hinder the roll forming during tube formation.

Sulfur (S)
Sulfur (S, sulfur) is an element that affects the fatigue life of rotation. The content of sulfur (S) in the steel pipe is preferably 0.03% by mass or less, and more preferably 0.02% by mass or less. Sulfur (S) generates manganese sulfide (MnS) -based non-metallic inclusions. Since the generation of manganese sulfide (MnS) -based non-metallic inclusions will become the starting point of fatigue failure caused by stress concentration, there is a risk that the rotational fatigue life will be reduced. In contrast, by setting the sulfur (S) content to 0.03% by mass or less, it is possible to suppress the generation of manganese sulfide (MnS) -based non-metallic inclusions, thereby preventing a reduction in rotational fatigue life. In addition, by setting the sulfur (S) content to 0.03% by mass or less, it is possible to suppress generation of a secondary shear plane and a tongue-like portion in the shape of the end face of the slit coil before the tube is formed, and thus an ideal welded portion can be formed.

Chromium (Cr)
Chromium (Cr) is an element effective for improving hardenability. The chromium (Cr) content in the steel pipe is preferably 2.00 mass% or less, more preferably 0.50 mass% or more and 1.60 mass% or less, and particularly preferably 0.80 mass% or more and 1.50 mass% or less. When the content of chromium (Cr) is 0.20% by mass or more, the hardenability of the steel pipe can be further improved. By reducing the content of chromium (Cr) to 2.00% by mass or less, the content of chromium (Cr) is prevented from being excessive, and thus it is possible to prevent a decrease in workability.

Aluminum (Al)
Aluminum (Al) is an element used as a deoxidizing agent for molten steel and exhibits an action of fixing nitrogen (N). The content of aluminum (Al) in the steel pipe is preferably 0.10% by mass or less, more preferably 0.005% by mass or more and 0.05% by mass or less. When the content of aluminum (Al) is 0.005% by mass or more, the effect of fixing nitrogen (N) becomes more significant. By setting the aluminum (Al) content to 0.10% by mass or less, the cleanliness of the steel can be prevented from being damaged, and furthermore, the result can be obtained that the fatigue failure can be prevented from reducing the rotation fatigue life. In addition, it is possible to prevent a reduction in the surface quality of the steel sheet and the steel strip.

Nickel (Ni)
Nickel (Ni) is an element that improves the hardenability of steel pipes and prevents embrittlement at low temperatures. In addition, nickel (Ni) exhibits the effect of eliminating the negative effect of embrittlement of molten metal caused by copper (Cu) contained in steel pipes, especially when copper (Cu) is added in an amount of 0.20% by mass or more. The effect of setting the content of nickel (Ni) in the same amount as that of copper (Cu) can be greatly achieved. The nickel (Ni) content in the steel pipe is preferably 2.00% by mass or less. By reducing the content of nickel (Ni) to 2.00% by mass or less, the amount of nickel is not excessive, and it is possible to prevent the steel sheet or steel strip from being softened to reduce the pipe making when the annealing treatment is applied for the purpose of softening the steel sheet or steel strip. The roll formability at the time.

Molybdenum (Mo)
Molybdenum (Mo) is added in a small amount and can impart the same elements as chromium (Cr) to improve the hardenability and tempering softening resistance of steel pipes. The content of molybdenum (Mo) in the steel pipe is preferably 0.30% by mass or less. By setting the content of molybdenum (Mo) to 0.30% by mass or less, molybdenum is not excessive, and can be easily softened when a softening and tempering treatment is applied to a steel plate or a steel strip, and the roll formability can be prevented from being lowered during pipe making.

＜ Manufacturing method of steel pipe ＞
The method for manufacturing a steel pipe in this embodiment includes a forming step, a welding step, a quenching step, and a tempering step. The forming step, welding step, quenching step, and tempering step are the same as the roll forming, welding, quenching, and tempering processes described above, respectively. Through these steps, a steel pipe of this embodiment is manufactured.

＜ bearing steel pipe ＞
The steel pipe for a bearing in this embodiment includes the steel pipe of this embodiment described above. In other words, the steel pipe of this embodiment contains only a specific amount of a specific component, and has a small amount of impurities, so the hardness and cleanliness are high, and the metal structure of the welded portion is a metal containing reheated ferrous iron and carbides. organization. Therefore, even if the content of carbon is higher than 0.70% by mass, welding cracking does not occur. Therefore, the steel pipe of this embodiment can be used suitably for a steel pipe for bearings.

[summary]
In order to solve the above problems, one aspect of the steel pipe of the present invention is characterized by including: carbon (C): 0.70 mass% or more and 1.20 mass% or less; phosphorus (P): 0.03 mass% or less; and copper (Cu): 0.30 mass % Or less, the metal structure of the welded part is a metal structure containing ferrous iron and carbides that have been reheated.

In addition, in one aspect of the steel pipe of the present invention, preferably, the aforementioned steel pipe further includes: silicon (Si): 0.80% by mass or less; manganese (Mn): 2.00% by mass or less; sulfur (S): 0.03% by mass The following; at least one of chromium (Cr): 2.00% by mass or less, and aluminum (Al): 0.10% by mass or less.

In addition, in one aspect of the steel pipe of the present invention, preferably, the steel pipe further includes at least one of nickel (Ni): 2.00% by mass or less and molybdenum (Mo): 0.30% by mass or less.

In addition, in one aspect of the steel pipe according to the present invention, it is preferable that the welding portion of the steel pipe is formed by welding opposite end surfaces of a tubular steel plate or opposite end surfaces of a steel strip by welding, and After that, a quenching treatment and a tempering treatment are applied.

In addition, in the steel pipe of one aspect of the present invention, it is preferable to apply a quenching treatment to the steel pipe, which is cooled so that the A3 transformation point or Acm transformation point of the steel relative to the steel pipe is 50 ° C higher. The above temperature becomes a temperature that is 50 ° C or more and 200 ° C or less below the Ms point of the steel pipe; and tempering, which is a temperature of 500 ° C or more and 50 ° C or less relative to the A1 transformation point. Tempering.

In addition, in one aspect of the steel pipe of the present invention, preferably, the aforementioned welding is high-frequency welding.

In the steel pipe according to one aspect of the present invention, preferably, the steel pipe includes non-metallic inclusions, and a particle diameter of the non-metallic inclusions is 10 μm or less.

In the steel pipe according to one aspect of the present invention, the non-metallic inclusions are preferably sulfides.

A bearing steel pipe according to an aspect of the present invention includes the steel pipe described above.

Moreover, in the manufacturing method of the steel pipe according to one aspect of the present invention, the steel pipe includes: carbon (C): 0.70 mass% or more and 1.20 mass% or less; phosphorus (P): 0.03 mass% or less; and copper (Cu): 0.30% by mass or less, and the metal structure of the welded portion is a metal structure including reheated ferrous iron and carbides; and the manufacturing method includes a forming step of forming a steel plate or a steel strip It is formed into a tubular shape by roll forming; a welding step, which is after the forming step, welds each end surface of the opposite steel plate or each end surface of the opposite steel strip to manufacture a steel pipe; and a quenching step, which is The steel tube after the welding step is subjected to a quenching treatment; the tempering step is performed after the quenching step is performed to the steel tube.

[Example]
<Examples and Comparative Examples>
[Manufacture of steel]
First, a steel having the composition shown in Table 1 was produced.

[Table 1]

[Manufacture of welded steel pipe]
The various steel billets in Table 1 were heated to 1250 to 1300 ° C. and hot-rolled to produce a hot-rolled steel coil (steel strip) having a thickness of 6.0 mm. The obtained hot-rolled steel coils were pickled, and steel grades E, H, M, N, O, P, Q, U, W, X, and Y were annealed at 700 ° C for 25 hours. B, C, D, F, G, I, J, K, L, R, S, T, V, and Z were annealed at 750 ° C for 10 hours. After that, the hot-rolled steel coil is slit in the longitudinal direction and roll-formed. After the roll forming, each end surface of the opposing hot-rolled steel coil was subjected to high-frequency welding under conditions of 1350 ° C or higher to produce a steel pipe having a diameter of 34 mm and a thickness of 6.0 mm.

In addition, as shown in Table 2, for Examples 1 to 12, a quenching treatment and a tempering treatment were further applied to the steel pipe after welding. Here, the temperature of the steel pipe during cooling in Table 2 refers to the temperature of the steel pipe during cooling during the quenching treatment, and the difference from the Ms point in Table 2 refers to the difference between the temperature of the Ms point and the temperature of the steel pipe during cooling during the quenching treatment. value. The tempering treatment is performed under the conditions of 680 ° C and 1 minute.

In Tables 1 and 2, the steel types corresponding to the comparative examples among the steel types are underlined. In addition, in the content (% by mass) of each component in the steel, those who deviate from the content in the preferable range and become important factors of the comparative example are underlined. Here, an important factor to be a comparative example means at least any one of a steel pipe which cannot be roll-formed at the time of pipe manufacturing and a steel pipe having a weld fracture.

[Evaluation of steel pipes]
About the said steel pipe, the roll formability, the presence or absence of welding cracks, and the surface texture were confirmed and evaluated as follows.

(Roll Formability)
As shown in Table 2, the case where the roll forming can be completed at the time of pipe manufacturing is evaluated as "OK", and the case where the roll forming cannot be completed is evaluated as "No". The following evaluations were made only for roll formable persons.

(Welding crack)
Regarding the steel pipes of Examples and Comparative Examples, it was investigated whether or not there was a welding crack in a bead portion formed by high-frequency welding. As shown in Table 2, when there is a welding crack of the bead portion, the evaluation is "Yes"; when there is no welding crack, the evaluation is "No". In addition, as shown in Table 2, the steel types include steel types A, B, C, F, H, J, K, N, O, P, R, S, U, V, X, and Y in a specific amount. Among Z steel pipes, a steel pipe having a carbon content of more than 0.70% by mass and being roll-formable at the time of pipe manufacturing without welding cracks is described as an example. In addition, other steel pipes are described as a comparative example.

(Surface texture)
The surface of the steel pipe of each of the Examples and Comparative Examples was investigated for rust residue marks and fine cracks, and the surface texture was checked. As shown in Table 2, the evaluation was "Yes" when there were rust marks on the surface of the steel pipe; the evaluation was "No" when there were no rust marks. Similarly, when there are micro cracks on the surface of the steel pipe, it is evaluated as "yes"; when there are no micro cracks, it is evaluated as "no".

[Table 2]

[Rotation fatigue test]
The steel pipes of Examples 1 to 12 and Reference Examples 1 to 5 in which welding cracks did not occur were subjected to a rotation fatigue test. A length of 70 mm was cut from the steel pipe, and the opposite side of the bead portion was cut in the longitudinal direction to obtain an open pipe test piece. The particle size of the sulfide (manganese sulfide MnS) of the non-metallic inclusions in the rotation fatigue test piece, which is present on the surface corresponding to the inner side surface of the steel pipe, among the aforementioned test pieces was obtained in the following manner. The inside surface of the steel pipe was observed using a 100-fold magnification optical microscope. The circle-equivalent diameter of the sulfide having the largest particle diameter among non-metallic inclusions having an area of 1.44 mm ² in one observation area was determined using image processing, and the circle-equivalent diameter was used as the particle diameter of the non-metallic inclusions. Based on this, 60 observation areas were tested, and the extreme particle statistics were used to predict the particle size of the largest inclusions among 30,000 mm ² . The test piece was pressed into a flat shape by a straightening press, and a circular plate-shaped test piece with a diameter of 60 mm was cut from the test piece by electric discharge processing, and heat treated to a hardness of 680 HV or more and 770 HV or less, and then the surface was applied. Grinded as a rotation fatigue test piece. The surface of the rotation fatigue test piece, that is, the surface corresponding to the outside and inside of the steel pipe, was cut to a depth of 0.1 mm.

Next, using a thrust-type rotation fatigue tester, three SUJ2 3 / 8-inch steel balls were set on the rotation fatigue test piece, and while the lubricant was supplied, the steel balls were rotated at a speed of 3000 rpm to give rotation fatigue. To the rotation fatigue test piece. One test was performed until surface peeling occurred from the rotation fatigue test piece, and the aforementioned test was repeated 20 times. The number of rotations from rotation fatigue to rotation fatigue test piece until surface peeling was plotted on a Weibull plot, and the rotation fatigue life (L10) was obtained. The results are shown in Table 3.

[table 3]

As shown in Tables 1 to 3, the steel pipe contains: carbon (C): 0.70% by mass or more and 1.20% by mass or less; phosphorus (P): 0.03% by mass or less; copper (Cu): 0.30% by mass or less, and the welded portion The metal structure is a metal structure containing tempered ferrous iron and carbides, that is, the high-carbon welded steel pipes in Examples 1 to 12 of tempered Mata loose iron did not cause welding cracks. In addition, in Examples 1 to 12, the steel pipes of Examples 1 to 5 and 7 to 12 with particularly few impurities not only did not have welding cracks, but also had a high degree of cleanliness, and had good rotation fatigue life even with a shallow cutting depth on the surface. . From this, it can be confirmed that the steel pipes of Examples 1 to 12, especially the steel pipes of Examples 1 to 5, and 7 to 12, can be ideally applied to bearings requiring good rolling fatigue life.

＜ Reference example ＞
[Manufacture of seamless steel pipe]
The steel billets of steel types A, B, M, and P among the steel types shown in Table 1 were heated to 1300 ° C, and round bars having a diameter of 70 mm were produced by hot rolling and rolling. By pushing the core rod into the center of the round rod, a seamless steel tube with a diameter of 60.5 mm and a thickness of 6.0 mm or 6.5 mm is produced.

[Rotation fatigue test]
The obtained seamless steel pipe was used to perform a rotation fatigue test. The methods of measuring the particle size of the sulfide (manganese sulfide MnS) of non-metallic inclusions in the seamless steel pipe, making a rotational fatigue test piece, and evaluating the rotational fatigue life were performed in the same manner as in the above Examples and Comparative Examples. Among them, Reference Examples 8, 10, 13 and 16 were cut from the inner surface of the seamless steel pipe to 0.6 mm, and the aforementioned length was taken as the cutting length of the rotation fatigue test surface. The results are shown in Table 4.

[Table 4]

It can be seen from Table 4 that although reference examples 8 and 10 with a deep cutting depth of 0.6 mm have good rotational fatigue life, the seamless steel pipes of reference examples 7, 9 and 11 with a shallow cutting depth have Non-metallic inclusions condense, and the rolling fatigue life is significantly reduced.

In general, in order to improve the cuttability, a seamless steel pipe containing more sulfur (S) than a high-carbon welded steel pipe can be clearly seen from the comparison with the above embodiment. The particle size of the metal inclusion sulfide (manganese sulfide Mns) is large. Therefore, like the seamless steel pipe shown in the reference example, in order to ensure a good rotational fatigue life, it is necessary to have a deeper cutting depth than a high carbon welded steel pipe. In contrast, like the high carbon welded steel pipes of Examples 1 to 5 and 7 to 12, the high carbon welded steel pipe welded from each end surface of a steel plate or a steel strip is different from a seamless steel pipe, and it is not easy for the inner surface of the steel pipe. Sulfide segregation occurs on the side, and only by reducing the sulfur content, the particle size of the sulfide can be reduced to 10 μm or less. Therefore, the high-carbon welded steel pipes of Examples 1 to 5 and 7 to 12 can maintain a good cutting fatigue life while keeping a small amount of cutting to reduce costs.

Claims (10)

A steel pipe characterized by comprising: Carbon (C): 0.70% by mass or more and 1.20% by mass or less; Phosphorus (P): 0.03 mass% or less; and Copper (Cu): 0.30% by mass or less, Among them, the metal structure of the welded portion is a metal structure including ferrous iron and carbides that have been reheated. The steel pipe according to claim 1, further comprising: Silicon (Si): 0.80 mass% or less; Manganese (Mn): 2.00 mass% or less; Sulfur (S): 0.03 mass% or less; Chromium (Cr): 2.00 mass% or less; and Aluminum (Al): 0.10 mass% or less At least one of them. The steel pipe according to claim 2, further comprising: At least one of nickel (Ni): 2.00% by mass or less and molybdenum (Mo): 0.30% by mass or less. The steel pipe according to any one of claims 1 to 3, wherein the welded portion of the steel pipe is formed by welding opposite end surfaces of a tubular steel plate or opposite end surfaces of a steel strip by welding, and After that, a quenching treatment and a tempering treatment are applied. The steel pipe according to claim 4, wherein: The quenching treatment is performed to cool the temperature from 50 ° C or higher to the A3 or Acm transformation point of the steel relative to the steel pipe, to 50 ° C or higher and 200 ° C or lower to the Ms point relative to the temperature of the steel pipe. Temperature; and Tempering is performed at a temperature of 500 ° C or higher and 50 ° C or lower relative to the A1 transformation point. The steel pipe according to claim 4 or 5, wherein the welding is high frequency welding. The steel pipe according to any one of claims 1 to 6, wherein the steel pipe includes non-metallic inclusions, and a particle size of the non-metallic inclusions is 10 μm or less. The steel pipe according to claim 7, wherein the non-metallic inclusion is a sulfide. A bearing steel pipe characterized by including the steel pipe according to any one of claims 1 to 8. A method for manufacturing a steel pipe, which is characterized by: The aforementioned steel pipe includes: Carbon (C): 0.70% by mass or more and 1.20% by mass or less; Phosphorus (P): 0.03 mass% or less; and Copper (Cu): 0.30% by mass or less, and The metal structure of the welded part is a metal structure containing reheated ferrous iron and carbides; The aforementioned manufacturing method includes: Forming step, which is forming a steel plate or a steel strip into a tube by roll forming; A welding step, which is after the forming step, welding each end surface of the opposite steel plate or each end surface of the opposite steel strip to manufacture a steel pipe; A quenching step, which is a quenching treatment applied to the steel pipe after the foregoing welding step; and The tempering step is performed by applying a tempering treatment to the steel pipe after the quenching step.