WO2019171624A1

WO2019171624A1 - Steel pipe and production method for steel pipe

Info

Publication number: WO2019171624A1
Application number: PCT/JP2018/031922
Authority: WO
Inventors: 秋月　誠; 翔平三町
Original assignee: 日新製鋼株式会社
Priority date: 2018-03-09
Filing date: 2018-08-29
Publication date: 2019-09-12
Also published as: JP6630870B1; TW201938813A; WO2019172314A1; JPWO2019172314A1

Abstract

A steel pipe that includes 0.2–1.2 mass% of C, no more than 0.03 mass% of P, and no more than 0.3 mass% of Cu and has a weld part that has a metal structure that includes carbides and reheated ferrite.

Description

Steel pipe and method for manufacturing steel pipe

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

A welded steel pipe is generally manufactured by welding a steel plate or a steel strip. For example, Patent Document 1 discloses an electric-welded steel pipe that is a kind of welded steel pipe after high-frequency welding is performed on a high-carbon steel sheet having a carbon content of 0.6% by mass, followed by cold drawing and hot reduction rolling. A method of manufacturing is disclosed.

Japanese Patent Publication “Japanese Unexamined Patent Publication No. 2015-062920”

By the way, when a high carbon steel plate or a high carbon steel strip as described in Patent Document 1 is welded, a weld crack occurs in a welded portion or the like. Therefore, a high carbon welded steel pipe as described in Patent Document 1 usually requires further manufacturing processes such as cold drawing rolling and hot reduction rolling in order to crush weld cracks. Thus, there is a problem that high-carbon welded steel pipes cannot be produced efficiently.

Therefore, an object of the present invention is to provide a high carbon welded steel pipe that can be efficiently manufactured in order to solve such a problem.

In order to solve the above-described problem, a steel pipe according to one embodiment of the present invention includes C: 0.2 mass% or more and 1.2 mass% or less, P: 0.03 mass% or less, and Cu: 0.3 mass%. %, And the metal structure of the welded portion is a metal structure containing reheated ferrite and carbide.

Moreover, the manufacturing method of the steel pipe which concerns on 1 aspect of this invention contains C: 0.2 mass% or more and 1.2 mass% or less, P: 0.03 mass% or less, and Cu: 0.3 mass% or less. A steel pipe manufacturing method in which the metal structure of the welded portion is a metal structure containing reheated ferrite and carbide, the steel plate or steel strip being formed into a tubular shape by roll forming, and after the forming step A welding process for manufacturing steel pipes by welding end faces of the steel plates facing each other or end faces of the steel strips facing each other, a quenching process for quenching the steel pipe after the welding process, and the quenching A tempering step of performing a tempering treatment on the steel pipe after the entering step.

According to one aspect of the present invention, it is possible to provide a welded steel pipe that does not require many manufacturing processes.

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

<Steel pipe>
The steel pipe according to the present embodiment includes C: 0.2 mass% or more and 1.2 mass% or less, P: 0.03 mass% or less, and Cu: 0.3 mass% or less, and the metal structure of the welded portion is , A metal structure containing reheated ferrite and carbides. Here, examples of the reheating process include a tempering process described later. The metal structure containing ferrite and carbide referred to herein refers to, for example, tempered martensite, bainite, and pearlite. Note that the high-carbon welded steel pipe in which the metal structure of the weld is a metal structure containing ferrite and carbide that has been reheated, for example, after the steel sheet or steel strip is formed into a tubular shape by roll forming, A welded part is formed by welding the end faces or the end faces of the steel strips facing each other, and is manufactured by subjecting the welded part to a quenching process and a tempering process. According to such a manufacturing method, although the steel pipe which concerns on this embodiment is a high carbon welded steel pipe with much C content, a weld crack does not generate | occur | produce. Therefore, in order to crush the weld crack, a manufacturing process such as cold drawing rolling and hot diameter reduction rolling is not required, and the manufacturing can be efficiently performed. Moreover, the steel pipe which concerns on this embodiment is excellent also in rolling fatigue life by including a specific component only a specific quantity as mentioned above.

In addition, the “welded part” here refers to a part where a steel plate or a steel strip is welded, for example, a weld bead part. The “rolling fatigue life” refers to a period until surface peeling occurs in the base material and welded portion of the steel pipe due to the rolling motion of the bearing using the steel pipe according to the present embodiment. Further, “excelling in rolling fatigue life” means that the period is as long as a seamless steel pipe that has been frequently used as a high carbon steel pipe. The rolling fatigue life can be obtained, for example, by measuring a period until surface peeling occurs in the base metal and the welded portion by a thrust type rolling fatigue test after cutting a welded steel pipe into a plate shape. .

The diameter of the steel pipe is preferably 15 mm or more and 300 mm or less. Moreover, it is preferable that the thickness of a steel pipe is 2 mm or more and 10 mm or less. When the diameter and thickness of the steel pipe are in the above-described preferable ranges, the steel pipe according to the present embodiment can be manufactured without requiring special manufacturing conditions.

Further, the steel pipe according to the present embodiment may include non-metallic inclusions such as sulfide and oxygen. Among non-metallic inclusions, sulfides, especially MnS, aggregate and precipitate on the surface of the steel pipe, causing cracks and surface flaws originating from the non-metallic inclusions, resulting in reduced rolling fatigue life. There is a fear. Further, when a sulfide such as MnS is present on the surface portion of the rolling bearing that comes into contact with the rolling elements, a large turning process is required for that portion, which may increase the manufacturing cost. Therefore, the particle size of non-metallic inclusions such as sulfide is preferably 20 μm or less, and more preferably 10 μm or less. The particle size of the nonmetallic inclusions in the steel pipe is as small as the above-mentioned preferable range, and particularly when the nonmetallic inclusions are sulfides, the rolling fatigue life is reduced and the manufacturing cost is prevented from increasing. be able to. Moreover, when the steel pipe which concerns on this embodiment contains oxygen as a nonmetallic inclusion, it is preferable that the oxygen content in a steel pipe is 20 ppm or less, and it is more preferable that it is 15 ppm or less. When the oxygen content in the steel pipe is within the above-described preferable range, a steel pipe with higher cleanliness can be obtained. As a result, a steel pipe having a better rolling fatigue life can be obtained.

[Steel and steel strip]
A steel plate and a steel strip are suitably used as a shape material of the steel pipe according to the present embodiment. A steel plate and a steel strip are formed into a tubular shape by roll forming, and a steel pipe according to this embodiment is formed by performing welding, quenching treatment, and tempering treatment.

Here, a seamless steel pipe, which is a steel pipe having no weld, has a difference in the amount and size of non-metallic inclusions on the outer surface side and the inner surface side of the steel pipe due to the influence of central segregation of the steel bar, which is a base material. The non-metallic inclusions on the outer surface side are fine and minute, but on the inner surface side, a large amount of non-metallic inclusions are present and exposed on the inner surface. Therefore, when a seamless steel pipe is used for the bearing, the outer ring of the rolling bearing uses the inner surface of the seamless steel pipe, but non-metallic inclusions present in large amounts on the inner surface have a significant effect on the rolling fatigue life. . For this reason, in order to lengthen the rolling fatigue life, it is necessary to significantly turn the portion in contact with the rolling element, which increases the processing cost.

On the other hand, the steel pipe according to the present embodiment, that is, a welded steel pipe such as an ERW steel pipe welded to a steel plate or a steel strip, differs from a seamless steel pipe in that many non-metallic inclusions exist inside the steel plate or steel strip. However, it hardly exists on the front and back. Therefore, a welded steel pipe using a steel plate or a steel strip as a raw material has a higher cleanliness on the inner surface side than a seamless steel pipe, and can reduce the difference in cleanliness between the inner surface and the outer surface of the steel pipe. From the above, the steel pipe according to the present embodiment has a high cleanliness on the inner surface side, so that it has an excellent rolling fatigue life comparable to that of a seamless steel pipe while reducing the amount of cutting when machining into a part shape. be able to.

Further, since the steel pipe according to the present embodiment is obtained by welding a steel plate or a steel strip, the steel pipe can be mass-produced as compared with a case where a steel pipe is manufactured by welding a bar.

In addition, the above-mentioned steel strip means a coil-shaped thing with a thickness of 10 mm or less, for example among steel plates. In this embodiment, although both a steel plate and a steel strip can be used as the shape material of this embodiment, it is preferable to manufacture a steel pipe using a steel strip. By being a steel strip thinner than a steel plate, it is more productive. Thereby, the steel pipe concerning this embodiment can be manufactured more efficiently. The steel strip can be obtained, for example, by hot rolling steel.

(Roll molding)
In roll forming in the present embodiment, a steel plate or steel strip is formed into a tubular shape by passing the steel plate or steel strip between rollers. Here, since it becomes easier to roll-form when the steel strip is used as a shape member, it is preferable to form a coiled steel strip by subjecting the steel to hot rolling before roll forming. Moreover, before roll-forming a steel plate or a steel strip, it may be washed with an acid or annealed under conditions of 600 ° C. or higher and 800 ° C. or lower and 1 hour or longer and 50 hours or shorter. Thereby, it becomes easier to roll-form.

(welding)
In the welding in this embodiment, the end surfaces of steel plates deformed into a tubular shape or the end surfaces of steel strips are butt welded. Thereby, the steel pipe concerning this embodiment is obtained. Examples of the welding method in the present embodiment include high-density energy welding such as resistance welding, laser beam welding, and electron beam welding. Resistance welding is preferable, and among the resistance welding, high-frequency welding is preferable. By welding a steel plate or steel strip by high frequency welding, the steel plate or steel strip can be welded efficiently and at low cost. Moreover, it is preferable to perform welding at 1300 degreeC or more and 1600 degrees C or less.

(Quenching)
The steel pipe according to the present embodiment is subjected to a quenching process after welding. In particular, by performing quenching immediately after welding, weld cracks in the welded portion can be suitably prevented. In the quenching treatment, the temperature of the steel pipe is 50 ° C. or higher and 200 ° C. relative to the Ms point (martensitic transformation start temperature) from a temperature higher by 50 ° C. or higher than the A3 transformation point or Acm transformation point of each obtained steel pipe. Cooling is preferably performed so that the temperature is lower than that below, and cooling is more preferably performed so that the temperature is lower by 100 ° C. or more and 200 ° C. or less than the Ms point. In this case, for example, it is preferable to cool by cooling with water or oil from the outer surface of the steel pipe. When the temperature of the steel pipe subjected to the cooling treatment is within the above-described preferable range, the metal structure in the welded portion of the steel pipe becomes a metal structure centered on martensite. Thereby, when the steel pipe is tempered, the tensile stress generated along with the martensitic transformation in the welded portion can be suitably reduced. As a result, the tempering effect of preventing weld cracking can be maximized.

(Tempering)
Moreover, in this embodiment, the tempering process is performed with respect to the steel pipe which performed the quenching process. The temperature of the tempering treatment is preferably 500 ° C. or more and 50 ° C. or less higher than the A1 transformation point, more preferably 600 ° C. or more and 750 ° C. or less, and further preferably 700 ° C. or more and 730 ° C. or less. Further, the tempering time is preferably 5 seconds or more and 5 minutes or less, and more preferably 10 seconds or more and 1 minute or less. Thus, welding cracks can be prevented because the tempering time is short. Moreover, it is preferable that a tempering process is performed with respect to a steel pipe promptly after quenching. For example, the tempering treatment is preferably performed within 5 minutes, more preferably within 1 minute after quenching.

The steel pipe according to the present embodiment is a high carbon welded steel pipe with a high C content in the steel pipe. Therefore, martensitic transformation occurs in the metal structure of the welded portion that is rapidly heated by welding, and the metal structure of the welded portion may become hard martensite. There is a possibility that weld cracking may occur in the weld due to the tensile stress generated along with the martensitic transformation and the processing strain (tensile stress) remaining in the steel by roll forming. On the other hand, as described above, the quenching treatment and the tempering treatment are performed after the welding, so that the tensile stress generated due to the martensitic transformation in the metal structure of the welded portion can be reduced. Thereby, since the toughness can be increased at the welded portion of the steel pipe, it is possible to prevent the occurrence of weld cracks.

[Components contained in steel pipe]
The steel pipe according to the present embodiment includes C (carbon): 0.2 mass% or more and 1.2 mass% or less, P: 0.03 mass% or less, and Cu: 0.3 mass% or less, with the balance being Fe. And inevitable impurities. Since the steel pipe which concerns on this embodiment contains a specific component only by a specific quantity and there is little content of an impurity, hardness and cleanliness are high. As described above, the steel pipe according to the present embodiment is excellent in rolling fatigue life because it has no weld cracks and has high hardness and cleanliness. Even when the steel pipe according to this embodiment substantially contains only the above-mentioned contents of C, P and Cu in addition to Fe and unavoidable impurities, the metal structure of the welded portion is regenerated. A metal structure containing heat-treated ferrite and carbide can solve the problem of efficiently producing a high-carbon welded steel pipe.

(C)
The steel pipe concerning this embodiment contains 0.2 mass% or more and 1.2 mass% or less C. That is, the steel pipe according to the present embodiment is a high carbon welded steel pipe. C is the most basic element in carbon steel, and the hardness and the amount of carbide vary greatly depending on the content in the steel pipe. When the C content is 0.2% by mass or more, undissolved carbides remain during quenching and heating, and excellent wear resistance can be ensured. Particularly, when the C content is 0.6% by mass or more, the wear resistance becomes remarkable. Moreover, since the toughness after hot rolling does not fall because content of C is 1.2 mass% or less, it becomes a steel pipe excellent in manufacturability and handleability. As a result, when the steel pipe concerning this embodiment is utilized as a bearing, it can prevent that the rolling element which comprises a bearing wears. Moreover, in order to improve the productivity of a steel pipe, when manufacturing a steel pipe after performing a softening annealing process to a steel plate or a steel strip, sufficient ductility can be provided to the said steel plate or a steel strip.

(P)
P (phosphorus) is an element that decreases the ductility and toughness of a steel pipe. The P content in the steel pipe is preferably 0.03% by mass or less, more preferably 0.02% by mass or less, and further preferably 0.01% by mass or less. When the P content is 0.03% by mass or less, the toughness of the prior austenite grain boundaries in the steel pipe is increased after quenching, and the rolling fatigue resistance of the steel pipe after heat treatment can be prevented from being lowered.

(Cu)
Cu (copper) is an element that improves the surface properties of steel plates and steel strips and steel pipes obtained from steel plates or steel strips by improving the peelability of the oxide scale formed on the steel plates or steel strips during hot rolling. It is. The Cu content in the steel pipe is preferably 0.3% by mass or less. When the Cu content is 0.3% by mass or less, fine cracks are less likely to occur on the steel plate and steel strip, and the surface of the steel pipe obtained from the steel plate or steel strip.

[Other components that can be contained in steel pipes]
Moreover, the steel pipe which concerns on this embodiment is a range which can solve the subject of manufacturing a high carbon welded steel pipe efficiently, In addition to the above-mentioned component, it is Si, Mn, Cr, S, Cr, Al, Ni, and Mo. At least one of them may be further included. Here, when at least one of P, Mo, Ni, S, and Al is included, the total content of these components is preferably 7.2% by mass or less with respect to the steel pipe, and 6.0% by mass. % Or less, more preferably 4.0% by mass or less. By being in the above-mentioned preferable range, the impurities contained in the steel pipe can be reduced, and the cleanliness of the steel pipe can be further increased. As a result, a steel pipe having a better rolling fatigue life can be obtained.

(Si)
Si (silicon) is one of the elements having a great influence on the ductility of a steel pipe. The Si content in the steel pipe is preferably 0.8% by mass or less, more preferably 0.5% by mass or less, and further preferably 0.3% by mass or less. As described above, since the Si content is not too high, hardening of the ferrite due to the solid solution strengthening effect of Si can be prevented, thereby preventing the steel pipe from cracking during the forming process. In addition, it can prevent the generation of scale flaws on the surface of the steel sheet or steel strip in the manufacturing process, and can prevent the rolling fatigue life from decreasing due to the occurrence of grain boundary oxidation during quenching and heating of the steel pipe. it can.

(Mn)
Mn (manganese) suppresses the ferrite transformation of steel in the steel pipe in the cooling process after quenching when the steel pipe is quenched and heated, and becomes a martensite-centered structure even at a relatively slow cooling rate. It is an element that enhances hardenability. The Mn content in the steel pipe is preferably 0.2% by mass or more and 2.0% by mass or less, and more preferably 0.5% by mass or more and 1.5% by mass or less. Thus, when the content of Mn is 0.2% by mass or more, deterioration of the hardenability of the steel pipe is prevented, and high-temperature products such as pearlite and upper bainite are added to the steel of the steel pipe during cooling. The formation can be prevented. Thereby, when the steel pipe concerning this embodiment is used as a bearing, the hardness required for a bearing can be obtained. Moreover, it can prevent that a ferrite hardens | cures and roll formation at the time of pipe making is inhibited because content of Mn is 2.0 mass% or less.

(S)
S (sulfur) is an element that affects the rolling fatigue life. The S content in the steel pipe is preferably 0.03% by mass or less, and more preferably 0.02% by mass or less. S generates MnS-based nonmetallic inclusions. The generation of MnS-based non-metallic inclusions may be a starting point for fatigue failure due to stress concentration, which may reduce the rolling fatigue life. On the other hand, when the S content is 0.03% by mass or less, generation of MnS-based nonmetallic inclusions can be suppressed, and reduction in rolling fatigue life can be prevented. Moreover, when the S content is 0.03% by mass or less, generation of secondary shear surfaces and tongues in the slit coil end face shape before pipe making can be suppressed, and a suitable welded portion can be formed.

(Cr)
Cr (chromium) is an element effective for improving hardenability. The Cr content in the steel pipe is preferably 2.0% by mass or less, more preferably 0.5% by mass or more and 1.6% by mass or less, and 0.8% by mass or more and 1.5% by mass or less. More preferably, it is as follows. When the Cr content is 0.2 mass% or more, the hardenability of the steel pipe can be further improved. When the content of Cr is 2.0% by mass or less, the content of Cr is not too much, so that the workability can be prevented from being lowered.

(Al)
Al (aluminum) is an element that is used as a deoxidizer for molten steel and also exhibits an action of fixing N (nitrogen). The content of Al in the steel pipe is preferably 0.1% by mass or less, and more preferably 0.005% by mass or more and 0.05% by mass or less. When the Al content is 0.005% by mass or more, the effect of fixing N becomes more remarkable. When the Al content is 0.1% by mass or less, the cleanliness of the steel can be prevented from being impaired, and as a result, reduction in rolling fatigue life due to fatigue failure can be prevented. Moreover, the fall of the surface quality of a steel plate and a steel strip can be prevented.

(Ni)
Ni (nickel) is an element that improves the hardenability of the steel pipe and prevents low temperature embrittlement. In addition, since Ni has an action to counteract the adverse effects of molten metal embrittlement caused by the inclusion of Cu in the steel pipe, especially when adding 0.2 mass% or more of Cu, the content of Ni in the steel pipe is set to Cu. The same amount is extremely effective. The Ni content in the steel pipe is preferably 2.0% by mass or less. Because the Ni content is not too high at 2.0% by mass or less, the steel plate or steel strip is difficult to soften even if annealing is performed to soften the steel plate or steel strip, and roll forming during pipe making It can prevent a decline in sex.

(Mo)
Mo (molybdenum) is an element that contributes to the improvement of the hardenability and temper softening resistance of steel pipes in the same way as Cr when added in a small amount. The Mo content in the steel pipe is preferably 0.3% by mass or less. When the Mo content is not too high at 0.3% by mass or less, it is easy to soften the steel sheet or steel strip when it is subjected to the softening annealing treatment, and it is possible to prevent the roll formability from being lowered during pipe making. it can.

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

<Bearing steel pipe>
The steel pipe for bearing according to the present embodiment includes the steel pipe according to the above-described embodiment. In other words, the steel pipe according to the present embodiment includes a specific component in a specific amount and has a low impurity content, so that the hardness and cleanliness are high, and the metal structure of the weld is reheated. In addition, since it has a metal structure containing ferrite and carbide, there is no weld cracking, so it can be suitably used for a steel pipe for bearings.

[Summary]
In order to solve the above-described problem, a steel pipe according to one embodiment of the present invention includes C: 0.2 mass% or more and 1.2 mass% or less, P: 0.03 mass% or less, and Cu: 0.3 mass%. %, And the metal structure of the welded portion is a metal structure containing reheated ferrite and carbide.

Moreover, the steel pipe which concerns on 1 aspect of this invention WHEREIN: Content of said C in the said steel pipe is 0.00.
It is preferable that it is 6 mass% or more and 1.2 mass% or less.

Moreover, in the steel pipe which concerns on 1 aspect of this invention, the said steel pipe is Si: 0.8 mass% or less, Mn: 2.0 mass% or less, S: 0.03 mass% or less, Cr: 2.0 mass% It is preferable to further include at least one of the following and Al: 0.1% by mass or less.

Moreover, in the steel pipe according to one aspect of the present invention, it is preferable that the steel pipe further includes at least one of Ni: 2.0 mass% or less and Mo: 0.3 mass% or less.

Further, in the steel pipe according to one aspect of the present invention, the welded portion of the steel pipe is formed by welding end surfaces facing each other in a tubular shape or facing end surfaces of steel strips. It is preferable that a quenching process and a tempering process are performed later.

Further, in the steel pipe according to one aspect of the present invention, the steel pipe has a temperature of 50 ° C. or more higher than the A3 transformation point or the Acm transformation point of the steel of the steel pipe, and the temperature of the steel pipe is 50 with respect to the Ms point. It is preferable that the quenching process which cools so that it may become temperature lower than 200 degreeC and less than 200 degreeC and the tempering process which temper at the temperature below 50 degreeC higher than 500 degreeC or more and A1 transformation point are performed.

In the steel pipe according to one aspect of the present invention, the welding is preferably high-frequency welding.

In one embodiment of the present invention, the steel pipe preferably contains non-metallic inclusions, and the particle diameter of the non-metallic inclusions is preferably 20 μm or less.

In one embodiment of the present invention, the non-metallic inclusion is preferably a sulfide.

Further, the steel pipe for bearing according to one aspect of the present invention includes the above steel pipe.

Furthermore, the manufacturing method of the steel pipe which concerns on 1 aspect of this invention contains C: 0.2 mass% or more and 1.2 mass% or less, P: 0.03 mass% or less, and Cu: 0.3 mass% or less. A steel pipe manufacturing method in which the metal structure of the welded portion is a metal structure containing reheated ferrite and carbide, the steel plate or steel strip being formed into a tubular shape by roll forming, and after the forming step A welding process for manufacturing steel pipes by welding end faces of the steel plates facing each other or end faces of the steel strips facing each other, a quenching process for quenching the steel pipe after the welding process, and the quenching A tempering step of performing a tempering treatment on the steel pipe after the entering step.

<Examples and Comparative Examples>
[Manufacture of steel]
First, steels having the composition shown in Table 1 were manufactured.

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

Also, as shown in Table 2, in Examples 1 to 17, the steel pipe was further subjected to quenching and tempering after welding. The tempering process was performed under conditions of 680 ° C. for 1 minute.

[Evaluation of steel pipe]
About the above-mentioned steel pipe, the roll formability, the presence or absence of weld cracks, and the surface skin were confirmed and evaluated as follows.

(Roll formability)
As shown in Table 2, when roll forming was possible at the time of pipe making, it was evaluated as “possible”, and when roll forming was not possible, it was evaluated as “impossible”. The following evaluation was performed only about what was roll-formed.

(Weld crack)
The steel pipes of the examples and comparative examples were examined for the presence or absence of weld cracks in the weld bead portion formed by high frequency welding. As shown in Table 2, when there was a weld crack in the weld bead portion, it was evaluated as “present”, and when there was no weld crack, it was evaluated as “none”. In addition, as shown in Table 2, the steel types A, B, C, F, H, J, K, N, O, P, R, S, U, V, and X contain a specific amount of specific components. Among steel pipes of Y, Z and Z, steel pipes that can be roll-formed during pipe making and have no weld cracks are described as examples. Moreover, the other steel pipe was described as a comparative example.

(Surface)
The surface of the steel pipe of each Example and Comparative Example was examined for the presence of scale wrinkles and fine cracks, and the surface skin was inspected. As shown in Table 2, when there was a scale flaw on the surface of the steel pipe, “Yes” was evaluated, and when there was no scale flaw, “No” was evaluated. Similarly, when there was a fine crack on the surface of the steel pipe, it was evaluated as “Yes”, and when there was no fine crack, it was evaluated as “No”.

(Rolling fatigue test)
A rolling fatigue test was performed on the steel pipes of Examples 1 to 17 having no weld cracks in the weld bead portion. The steel pipe was cut into a length of 70 mm, and the opposite side of the weld bead portion was cut in the longitudinal direction to obtain an open pipe test piece. About the particle size of the sulfide (MnS) which is a nonmetallic inclusion of the rolling fatigue test piece which exists in the surface which hits the inner side of a steel pipe among the said test pieces, it calculated | required as follows. The inner surface of the steel pipe was observed using an optical microscope with a magnification of 100 times. Among the non-metallic inclusions in an area of 1.44 mm ² in one field of view, the equivalent circle diameter of the sulfide having the largest particle diameter is obtained using image processing, and the equivalent circle diameter is determined by the particle diameter of the non-metallic inclusions. It was. This was measured for 60 visual fields, and the maximum inclusion particle size at 30000 mm ² was predicted by extreme value statistics. The test piece is flattened by press correction, a disk-shaped test piece having a diameter of 60 mm is cut out from the test piece by electric discharge machining, and heat-treated so as to have a hardness of 680 HV or more and 770 HV or less, followed by surface polishing. Thus, a rolling fatigue test piece was obtained. The surface of the rolling fatigue test piece, that is, the surface corresponding to the outside and inside of the steel pipe was cut at a depth of 0.1 mm.

Next, three SUJ2 3/8 inch steel balls are provided on the rolling fatigue test piece using a thrust type rolling fatigue tester, and the rigid balls are rotated at a rotational speed of 3000 rpm while supplying lubricating oil. Thus, rolling fatigue was applied to the rolling fatigue test piece. One test was performed until surface peeling occurred from the rolling fatigue test piece, and the test was repeated 20 times. Rolling fatigue was given to the rolling fatigue test piece, and the number of rotations until surface peeling occurred was plotted on a Weibull plot paper to determine the rolling fatigue life (L10). The results are shown in Table 3.

As shown in Tables 1 to 3, including: C: 0.2% by mass or more and 1.2% by mass or less, P: 0.03% by mass or less, and Cu: 0.3% by mass or less; However, the high carbon welded steel pipes according to Examples 1 to 17 of tempered martensite, which is a metal structure containing tempered ferrite and carbide, had no weld cracks. Further, among Examples 1 to 17, the steel pipes of Examples 1 to 3, 5 to 7 and 10 to 17 having particularly low impurities have not only weld cracks but also high cleanliness. It had excellent rolling fatigue life even when it was shallow. From these facts, it was confirmed that the steel pipes of Examples 1 to 17, particularly the steel pipes of Examples 1 to 3, 5 to 7, and 10 to 17 can be suitably used for bearings that require excellent rolling fatigue life. It was.

<Reference example>
[Manufacture of seamless steel pipes]
Among the steel types shown in Table 1, slabs of steel types A, B, M and P were heated to 1300 ° C. and a round bar having a diameter of 70 mm was produced by hot rolling. A seamless steel pipe having a diameter of 60.5 mm and a thickness of 6.0 mm or 6.5 mm was manufactured by pushing a mandrel into the center of the round bar.

(Rolling fatigue test)
A rolling fatigue test was performed using the obtained seamless steel pipe. Measurement of the particle size of sulfide (MnS), which is a non-metallic inclusion in the seamless steel pipe, production of a rolling fatigue test piece, and evaluation of the rolling fatigue life were performed in the same manner as in the above-described Examples and Comparative Examples. However, for Reference Examples 3, 5, 8, and 11, cutting was performed from the inner surface of the seamless steel pipe to 0.6 mm, and the length was defined as the cutting length of the rolling fatigue test surface. The results are shown in Table 4.

From Table 4, the reference examples 3 and 5 having a deep cutting depth of 0.6 mm have excellent rolling fatigue life, but the reference examples 2, 4 and 6 having a shallow cutting depth are obtained on the inner surface side of the seamless steel pipe. Non-metallic inclusions aggregated in the steel, and the rolling fatigue life was extremely low.

In general, seamless steel pipes containing more S than high carbon welded steel pipes in order to improve machinability are more non-clear than high carbon welded steel pipes, as is clear from the comparison with the above-described examples. The particle size of sulfide (MnS) which is a metal inclusion is large. Therefore, as in the seamless steel pipe shown in the reference example, in order to ensure an excellent rolling fatigue life, it is necessary to make the cutting depth deeper than that of the high carbon welded steel pipe. In contrast, high carbon welded steel pipes welded with steel plates or steel strips, like the high carbon welded steel pipes of Examples 1 to 3, 5 to 7, and 10 to 17, differ from the seamless steel pipes in that they are sulfided on the inner surface side of the steel pipe. The product is difficult to segregate, and the particle size of the sulfide can be reduced only by reducing the S content. Therefore, the high carbon welded steel pipes of Examples 1 to 3, 5 to 7, and 10 to 17 can ensure an excellent rolling fatigue life while reducing the cost with a small amount of cutting.

Claims

C: 0.2 mass% or more and 1.2 mass% or less, P: 0.03 mass% or less, and Cu: 0.3 mass% or less, and the metal structure of the welded portion is a reheat-treated ferrite and A steel pipe characterized by a metal structure containing carbide.
The steel pipe according to claim 1, wherein the C content in the steel pipe is 0.6 mass% or more and 1.2 mass% or less.
Si: 0.8 mass% or less, Mn: 2.0 mass% or less, S: 0.03 mass% or less, Cr: 2.0 mass% or less, and Al: 0.1 mass% or less The steel pipe according to claim 1, further comprising a steel pipe.
The steel pipe according to claim 3, further comprising at least one of Ni: 2.0 mass% or less and Mo: 0.3 mass% or less.
The welded portion of the steel pipe is formed by welding the end faces facing each other of steel plates formed into a tubular shape or the end faces facing each other of the steel strip, and is subjected to a quenching process and a tempering process after formation. The steel pipe according to any one of claims 1 to 4, wherein the steel pipe is provided.
In the steel pipe,
Quenching treatment for cooling so that the temperature of the steel pipe is 50 ° C. or more and 200 ° C. or less lower than the Ms point from a temperature that is 50 ° C. or more higher than the A3 transformation point or Acm transformation point of the steel of the steel pipe. ,and,
The steel pipe according to claim 5, wherein a tempering treatment is performed to temper at a temperature not lower than 500 ° C and not higher than 50 ° C with respect to the A1 transformation point.
The steel pipe according to claim 5 or 6, wherein the welding is high-frequency welding.
The steel pipe according to any one of claims 1 to 7, wherein the steel pipe contains non-metallic inclusions, and the particle size of the non-metallic inclusions is 20 µm or less.
The steel pipe according to claim 8, wherein the non-metallic inclusion is a sulfide.
A bearing steel pipe comprising the steel pipe according to any one of claims 1 to 9.
C: 0.2 mass% or more and 1.2 mass% or less, P: 0.03 mass% or less, and Cu: 0.3 mass% or less, and the metal structure of the welded portion is a reheat-treated ferrite and A method of manufacturing a steel pipe, which is a metal structure containing carbide,
A forming step of forming a steel sheet or steel strip into a tubular shape by roll forming;
After the forming step, a welding step of manufacturing a steel pipe by welding the end faces of the steel plates facing each other or the end faces of the steel strips facing each other;
A quenching process for quenching the steel pipe after the welding process;
And a tempering step of tempering the steel pipe after the quenching step.