JP7196549B2 - Steel pipe manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a steel pipe, a steel pipe for bearings, and a method for manufacturing a steel pipe.

Welded steel pipes are generally manufactured by welding steel plates or steel strips. For example, in Patent Document 1, after high-frequency welding of a low-carbon steel plate having a carbon content of 0.15% by mass or more and 0.4% by mass or less, diameter reduction rolling is performed so that the bond width of the electric resistance welded portion is 25 μm or less. Disclosed is an electric resistance welded steel pipe, which is a type of welded steel pipe that is mechanically narrowed by

JP 2013-147751 A

Annealing treatment is applied to high-carbon steel pipes to improve the workability of welds. Patent Literature 1 discloses that reduction in strength of the welded portion is prevented by narrowing the bond width of the welded portion in the low-carbon steel pipe. However, there is no disclosure about the influence of the bond width of the weld zone in the high-carbon steel pipe on the annealing time for improving the workability of the weld zone.

An object of one aspect of the present invention is to realize a high-carbon steel pipe capable of shortening the annealing time of a welded portion.

In order to solve the above problems, a steel pipe according to an aspect of the present invention has C: 0.4% by mass or more and 1.5% by mass or less, P: 0.03% by mass or less, and Cu: 0.3% by mass. % or less, the ends are welded at a welding temperature of 1200 ° C or higher and 1700 ° C or lower, and the ends are melted by heating The width of the bond portion to be formed is characterized by being 0.2% or more and 1.2% or less of the plate thickness of the steel plate or steel strip.

In order to solve the above problems, a steel pipe according to an aspect of the present invention may have a C content of 0.6% by mass or more and 1.2% by mass or less.

In order to solve the above problems, a steel pipe according to an aspect of the present invention has Si: 2.0% by mass or less, Mn: 0.1% by mass or more and 2.0% by mass or less, and S: 0.02% by mass. or less, and Al: 0.2% by mass or less.

In order to solve the above problems, the steel pipe according to one aspect of the present invention may further satisfy at least one of Cr: 5.0 mass % or less and Mo: 0.5 mass % or less.

In order to solve the above problems, a steel pipe according to an aspect of the present invention has Ti: 0.3 mass% or less, Nb: 0.5 mass% or less, V: 1.5 mass% or less, and B: 0 At least one of the conditions of .01% by mass or less may be further satisfied.

In order to solve the above problems, in the steel pipe according to one aspect of the present invention, the welding may be high frequency welding.

In order to solve the above problems, the steel pipe according to an aspect of the present invention is heated at the ends and pressed against each other so that the crushing amount is 20% or more and 30% or less with respect to the plate thickness. may be formed by

In order to solve the above problems, in the steel pipe according to one aspect of the present invention, the corners of the ends are crushed, and correspond to the crushed portions in the width direction of the steel plate or steel strip. The length is 5% or more and 40% or less of the plate thickness, and the smaller of the angles formed by the plane including the upper surface or the lower surface of the steel plate or steel strip and the surface formed by the crushing may be 20 degrees or more and 60 degrees or less.

In order to solve the above problems, the steel pipe according to one aspect of the present invention may be a round pipe, a square pipe, or a deformed pipe.

In order to solve the above problems, a steel pipe for bearings according to one aspect of the present invention includes any one of the steel pipes described above.

In order to solve the above problems, a steel pipe manufacturing method according to an aspect of the present invention comprises C: 0.4% by mass or more and 1.5% by mass or less, P: 0.03% by mass or less, and A method for manufacturing a steel pipe in which the ends of a steel plate or steel strip are welded together, comprising: bending the steel plate or steel strip to bring the ends into contact; The ends are pressed against each other so that the width of the bond formed by melting the ends is 0.2% or more and 1.2% or less with respect to the thickness of the steel plate or steel strip. , welding at a temperature of 1200° C. or higher and 1700° C. or lower.

ADVANTAGE OF THE INVENTION According to one aspect of the present invention, it is possible to realize a high-carbon steel pipe capable of shortening the annealing time of the welded portion.

1 is a schematic diagram showing a welded portion of a steel pipe according to one embodiment of the present invention; FIG. FIG. 3 is a photograph showing a welded portion of a steel pipe according to one embodiment of the present invention; (a) to (d) are cross-sectional views each showing a cross-sectional shape of the steel pipe 1. FIG. FIG. 2 is a schematic diagram showing the crushing width and crushing angle of a steel pipe according to one embodiment of the present invention; (a) and (b) are schematic diagrams showing the amount of upset of a steel pipe according to one embodiment of the present invention.

An embodiment of the present invention will be described in detail below with reference to FIGS. 1 and 2. FIG. Although electric resistance welding will be described below as an example of the welding method of the present invention, the welding method of the present invention is not limited to electric resistance welding, and is a method of welding and joining by heating the seam portions of steel pipes. I wish I had.

<Steel pipe 1>
FIG. 1 is a schematic diagram showing a welded portion 3 of a steel pipe 1 according to this embodiment. The steel pipe 1 is made by welding the ends of a steel plate or steel strip containing C: 0.4% by mass or more and 1.5% by mass or less, P: 0.03% by mass or less, and Cu: 0.3% by mass or less. formed by The ends are welded together at a welding temperature of 1200° C. or higher and 1700° C. or lower, and the width of the bond portion 4 formed by melting the ends by heating is the thickness of the steel plate or steel strip. 0.2% or more and 1.2% or less.

As shown in FIG. 1, the welded portion 3 is a portion where the steel strip 2 is welded, for example, a weld bead portion. In addition, the steel strip 2 may be a steel plate.

A bond portion 4 and a heat affected zone 5 are formed in the welded portion 3 . A bond 4 is formed between two heat affected zones 5 at the weld 3 . During electric resistance welding, the electric resistance welded portion is heated to a solid-liquid coexistence zone. As a result, C contained outside the electric resistance welded portion is concentrated in the liquid phase and decreased in the solid phase. The C-enriched liquid phase is discharged to the outside of the electric resistance weld by pressing (upsetting) the ends of the steel strip 2 to form a bead. On the other hand, the solid phase with reduced C remains in the welded portion 3, and the bond portion 4 is formed by the solid phase. The heat-affected zone 5 is a region other than the bond portion 4 where the metallographic structure of the end portion is changed by the heat during electric resistance welding.

The welded portion 3 is preferably subjected to a spheroidizing annealing treatment, which will be described later, in order to improve workability. In the weld zone 3, the heat affected zone 5 becomes a structure in which undissolved carbide and martensite are mixed due to heating during welding. On the other hand, the bonding portion 4 becomes hotter than the heat-affected zone 5 due to heating during welding, and thus has a structure of only martensite that does not contain the undissolved carbide. Therefore, when the weld zone 3 is subjected to spheroidizing annealing, the heat-affected zone 5 grows carbide with the undissolved carbide as a nucleus, so the spheroidizing annealing is completed in a short time, but the bond part 4 is undissolved. Contains no carbides. Therefore, in the bond portion 4, after the carbide precipitates inside the bond portion 4, the carbide grows.

As a result, the bond 4 needs to undergo a longer spheroidizing anneal than the heat affected zone 5 . Further, the wider the bond portion 4, the longer the spheroidizing annealing is required. That is, by narrowing the width of the bond portion 4, the time required for the spheroidizing annealing of the weld portion 3 can be shortened.

FIG. 2 is a photograph showing the welded portion 3 of the steel pipe 1. As shown in FIG. As shown in FIG. 2 , the steel pipe 1 is formed such that the width of the bond portion 4 is 0.2% or more and 1.2% or less of the plate thickness of the steel strip 2 . In order to achieve such a width of the bond portion 4, the welding temperature is preferably 1200° C. or higher and 1700° C. or lower. Thus, if the width of the bond portion 4 is 1.2% or less of the plate thickness, the spheroidizing annealing time after welding can be made relatively short. In addition, if the width of the bond portion 4 is 0 (zero), it becomes impossible to determine whether the welding was performed properly. preferable.

In addition, the steel pipe 1 is also excellent in rolling contact fatigue life by containing specific components in specific amounts as described above. Here, the term "rolling fatigue life" refers to the period until surface peeling occurs in the base material of the steel pipe 1 and the welded portion 3 due to the rolling motion of the bearing using the steel pipe 1 .

In addition, the term "excellent in rolling contact fatigue life" means that the above period is as long as that of seamless steel pipes, which have been frequently used as high-carbon steel pipes. The rolling contact fatigue life is obtained, for example, by cutting a welded steel pipe open and processing it into a plate, and then measuring the period until surface peeling occurs in the base metal and the weld zone 3 by a thrust-type rolling contact fatigue test. be able to.

Moreover, the steel pipe 1 may contain non-metallic inclusions such as sulfides and oxygen. However, among non-metallic inclusions, sulfides, especially MnS, agglomerate and precipitate on the steel pipe surface, causing cracks and surface flaws originating from the non-metallic inclusions, resulting in a reduction in rolling contact fatigue life. There is a risk that it will decrease.

In addition, if sulfides such as MnS are present on the surface portion of the rolling bearing that contacts the rolling elements, extensive turning work is required for that portion, which may increase the manufacturing cost. Therefore, the grain size of non-metallic inclusions such as sulfides is preferably 20 μm or less, more preferably 10 μm or less. Since the grain size of the non-metallic inclusions in the steel pipe 1 is as small as the preferred range described above, it is possible to prevent a reduction in rolling contact fatigue life and an increase in manufacturing costs, especially when the non-metallic inclusions are sulfides. can do.

Moreover, when the steel pipe 1 contains oxygen as non-metallic inclusions, the oxygen content in the steel pipe 1 is preferably 20 ppm or less, more preferably 15 ppm or less. By setting the oxygen content in the steel pipe 1 within the preferred range described above, the steel pipe 1 with a higher degree of cleanliness can be obtained. As a result, it is possible to obtain the steel pipe 1 having a better rolling contact fatigue life.

The steel pipe 1 preferably has a diameter of 15 mm or more and 300 mm or less. Moreover, the thickness of the steel plate or steel strip used to form the steel pipe 1 is preferably 2 mm or more and 10 mm or less. By setting the diameter of the steel pipe 1 and the thickness of the steel plate or steel strip within the preferred ranges described above, the steel pipe 1 can be manufactured without requiring special manufacturing conditions.

(a) to (d) of FIG. 3 are cross-sectional views showing cross-sectional shapes of the steel pipe 1, respectively. As shown in (a) of FIG. 3, the steel pipe 1 may be a round pipe having a substantially circular cross-sectional shape perpendicular to the extending direction of the steel pipe 1 . Further, as shown in FIG. 3(b), the steel pipe 1 may be a square pipe having a substantially rectangular cross-sectional shape. Moreover, the above-mentioned cross-sectional shape may be a deformed tube having a cross-sectional shape other than substantially circular or substantially rectangular. In addition, examples of the cross-sectional shape of the deformed tube include a semicircular shape as shown in FIG. 3(c) and an hourglass shape as shown in FIG. 3(d).

[Steel strip 2]
The steel strip 2 is suitably used as a material for forming the steel pipe 1 . The steel strip 2 is roll-formed into a tubular shape, and welded, quenched, tempered, and spheroidized to form the steel pipe 1 .

Here, in the seamless steel pipe, which is a steel pipe without the welded portion 3, the amount and size of nonmetallic inclusions differ between the outer surface side and the inner surface side of the steel pipe due to the influence of the center segregation of the steel bar that is the base material. The non-metallic inclusions on the outer surface side are fine and in a very small amount, but on the inner surface side there are a large amount of non-metallic inclusions and they are exposed on the inner surface.

Therefore, when a seamless steel pipe is used for a bearing, the inner surface of the seamless steel pipe is used for the outer ring of the rolling bearing, and the large amount of non-metallic inclusions present on the inner surface greatly affects the rolling contact fatigue life. . For this reason, in order to lengthen the rolling contact fatigue life, it is necessary to extensively turn the parts that come into contact with the rolling elements, which increases the machining cost.

On the other hand, in a welded steel pipe such as an electric resistance welded steel pipe to which steel strip 2 is welded, unlike a seamless steel pipe, most of the non-metallic inclusions are present inside the steel strip, and are mostly present on the front and back surfaces. do not. Therefore, a welded steel pipe using a steel strip as a raw material has a higher degree of cleanliness on the inner surface side than a seamless steel pipe, and the difference in cleanliness between the inner surface and the outer surface of the steel pipe can be reduced.

As described above, since the steel pipe 1 has a high degree of cleanliness on the inner surface side, it is possible to obtain an excellent rolling contact fatigue life comparable to that of a seamless steel pipe while reducing the cutting amount when working into a component shape.

In addition, since the steel pipe 1 is obtained by welding the steel strips 2, the steel pipe can be mass-produced more easily than when steel pipes are manufactured by piercing bars.

In addition, the steel strip 2 is, for example, a coil-shaped steel sheet having a thickness of 10 mm or less. In the present embodiment, both the steel strip 2 and the steel plate can be used as the forming material of the present embodiment, but it is preferable to manufacture the steel pipe 1 using the steel strip 2 . Productivity is improved by using the steel strip 2 that is thinner than the steel plate and has excellent roll formability, which will be described later. Thereby, the steel pipe 1 can be manufactured more efficiently. The steel strip 2 can be obtained, for example, by hot rolling steel.

(Roll forming)
In the roll forming in this embodiment, the steel strip 2 is formed into a tubular shape by passing the steel strip 2 between rollers. Here, since it is easier to roll-form the steel strip 2 than the steel plate described above, it is preferable to hot-roll the steel before roll-forming to form the coil-shaped steel strip 2. . Moreover, before the steel strip 2 is roll-formed, it may be washed with an acid, or annealed under the conditions of 600° C. or more and 800° C. or less and 1 hour or more and 50 hours or less. This makes roll forming easier.

(crushing)
In order to adjust the shape and strength of the welded portion 3, the ends of the steel strip 2 formed into a tubular shape by the roll forming are crushed by crushing rolls at the corners located inside the pipe. Being crushed (hereinafter, "crushing"). By appropriately setting the width and angle of the crushing, it is possible to obtain a sound welded portion 3 with little decrease in strength in butt welding, which will be described later.

FIG. 4 is a schematic diagram showing the crushing width C and the crushing angle θ of the steel pipe 1 according to this embodiment. As shown in FIG. 4 , the width of the crushing (crushing width C) is preferably 5% or more and 40% or less of the thickness of the steel strip 2 . The crushing width C is the portion where the corner of the steel strip 2 is crushed in the width direction of the steel strip 2 when the direction along the longitudinal direction of the steel pipe 1 is the longitudinal direction of the steel strip 2. means the length corresponding to

The crushing angle θ means the smaller angle between the plane including the lower surface 2A (or the upper surface) of the steel strip 2 and the surface 2B formed by crushing. The crushing angle θ is preferably 20 degrees or more and 60 degrees or less, and most preferably 45 degrees.

When the crushing width C is small or the crushing angle θ is small, the amount of molten metal generated during welding discharged from the boundary between the welded ends of the steel strip 2 (molten metal discharge amount) is small. As a result, a wide portion is formed in the bond portion 4, and a long-time annealing process is required for the wide portion. Further, when the crushing width C is large or the crushing angle θ is large, the area of the welded end portion of the steel strip 2 is reduced, so the strength of the welded portion 3 is reduced.

On the other hand, when the crushing width C is 5% or more and 40% or less of the plate thickness, the temperature of the welded portion 3 does not vary, and the width of the formed bond portion 4 can be kept constant. can. Therefore, long-time annealing treatment due to the wide part of the bond portion 4 is not required.

(welding)
In welding in this embodiment, the ends of the steel strips 2 subjected to the crushing are butt-welded. Thereby, the steel pipe 1 is obtained.

(a) and (b) of FIG. 5 are schematic diagrams showing the amount of upset of the steel pipe 1 according to the present embodiment. In the butt welding, the ends are pressed together and butted together. At this time, as shown in (a) of FIG. When the point P2 is a predetermined distance Y in the plate width direction from the end portion, as shown in FIG. The distance Z between P1 and P2 of is shorter than the sum of distance X and distance Y. Thus, the length (X+Y−Z) of the end portion shortened by the pressing in the direction in which the end portions are pressed against each other (in other words, the plate width direction) is called an upset amount.

The amount of upset at each end is preferably 20% or more and 30% or less with respect to the plate thickness of the steel strip 2 . If the upset amount is less than 20% of the plate thickness, the molten metal discharge amount is reduced. Therefore, the amount of the molten metal remaining in the bonding portion 4 increases, and as a result, the width of the bonding portion 4 increases. Also, if the amount of upset is greater than 30% of the plate thickness, the ends are excessively crushed.

As described above, if the amount of upset is 20% or more and 30% or less with respect to the thickness of the steel strip 2, a sufficient molten metal discharge amount for narrowing the width of the bond portion 4 can be obtained.

Examples of the welding method in this embodiment include high-density energy welding such as resistance welding, laser beam welding and electron beam welding. Resistance welding is preferable, and among resistance welding, high frequency welding (high frequency resistance welding) is preferred. High-frequency welding is resistance welding in which joining is performed by resistance heat due to high-frequency current while applying pressure to the welded joint. In this embodiment, high frequency contact resistance welding may be used, or high frequency induction resistance welding may be used. By welding the steel strip 2 by high-frequency welding, the steel strip 2 can be welded efficiently and at low cost.

Also, the welding temperature is preferably 1200° C. or higher and 1700° C. or lower. If the welding temperature is too high, the amount of molten metal generated during welding increases, so the width of the bond portion 4 increases. Also, when the welding temperature is 1200° C. or less, the amount of the molten metal decreases, and a part of the welded portion 3 is not welded. If the welding temperature is as described above, welding can be performed so that the width of the bond portion 4 is 0.2% or more and 1.2% or less of the plate thickness of the steel strip 2 .

In addition, the welding may be one in which the ends of one steel strip 2 or steel plate are welded to form a steel pipe, or the ends of two or more steel strips 2 or steel plates are spliced together. may be

(Quenching/Tempering)
The steel pipe 1 is subjected to quenching treatment and tempering treatment after welding. Thereby, the weld crack of the welded part 3 can be prevented.

(Spheroidizing annealing treatment)
As described above, the metal structure of the bond portion 4 may be hard martensite. Therefore, workability of the welded portion 3 is deteriorated. As the annealing treatment performed to improve the deterioration of workability, for example, spheroidizing annealing treatment is preferably performed. The spheroidizing annealing treatment is to anneal the high-carbon steel at a temperature near the A1 transformation point so that the carbides contained in the high-carbon steel are spheroidized and precipitated. Spheroidizing annealing softens the high carbon steel and improves workability.

The bond portion 4 can transform the martensite structure into a ferrite structure in which globular carbides are dispersed by the spheroidizing annealing treatment. As a result, the workability of the bond portion 4 is improved.

According to the above-described method, the steel pipe having the welded portion 3 in which the width of the bond portion 4 is 0.2% or more and 1.2% or less of the plate thickness of the steel strip 2 without the need for a diameter-reducing rolling process. 1 can be manufactured.

[Components contained in Steel Pipe 1]
The steel pipe 1 contains C (carbon): 0.4% by mass or more and 1.5% by mass or less, P: 0.03% by mass or less, and Cu: 0.3% by mass or less, and the balance is Fe and unavoidable impurities. consists of The steel pipe 1 contains a specific component in a specific amount and has a low content of impurities. Therefore, the steel pipe 1 has high hardness and cleanliness, and is excellent in rolling contact fatigue life.

(C)
The steel pipe 1 contains 0.4% by mass or more and 1.5% by mass or less of C. That is, the steel pipe 1 is a high carbon welded steel pipe. C is the most basic element in carbon steel, and the hardness and carbide content of steel pipes vary greatly depending on the content. When the C content is 0.4% by mass or more, the strength required for use as mechanical parts such as bearings can be obtained. Moreover, when the C content is 0.6% by mass or more, the effect of obtaining the strength required for use as a mechanical component such as a bearing becomes remarkable.

In addition, when the C content is 1.5% by mass or less, precipitation of carbides that do not form spherical carbides due to the spheroidizing annealing treatment can be suppressed. Therefore, sufficient workability can be imparted to the welded portion 3 by the spheroidizing annealing treatment.

(P)
P (phosphorus) is an element that reduces the ductility and toughness of steel pipes. The P content in the steel pipe 1 is preferably 0.03% by mass or less, more preferably 0.02% by mass or less, and even more 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 1 increases after quenching, and it is possible to prevent the rolling contact fatigue resistance of the steel pipe 1 after heat treatment from deteriorating. .

(Cu)
Cu (copper) is an element that improves the surface properties of a steel strip and a steel pipe obtained from the steel strip by improving the releasability of oxide scale formed on the steel strip during hot rolling. The Cu content in the steel pipe 1 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 surface of the steel strip 2 and the steel pipe 1 obtained from the steel strip 2 .

[Other components that may be contained in the steel pipe]
In addition, the steel pipe 1 contains Si, Mn, Cr, S, Al, Cr, Mo, Ti, Nb, V, and B. Here, when at least one of P, Mo, 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 1, and 6.0% by mass. It is more preferably 4.0% by mass or less, more preferably 4.0% by mass or less. By being within the preferred range described above, impurities contained in the steel pipe 1 can be reduced, and the cleanliness of the steel pipe 1 can be further enhanced. As a result, it is possible to obtain the steel pipe 1 having a better rolling contact fatigue life.

(Si)
Si (silicon) is an element that retards precipitation of carbides in spheroidizing annealing. The Si content in the steel pipe 1 is preferably 2.0% by mass or less. Since the Si content is not too high, precipitation of spheroidal carbides is not hindered in the spheroidizing annealing treatment, and the spheroidizing annealing treatment proceeds efficiently.

Moreover, hardening of ferrite due to the solid-solution strengthening action of Si can be prevented, thereby preventing cracks from occurring in the steel pipe 1 during forming. In addition, it is possible to prevent the occurrence of scale defects on the surface of the steel strip 2 in the manufacturing process, and to prevent the rolling contact fatigue life from decreasing due to intergranular oxidation during the quenching heating of the steel pipe 1. can.

(Mn)
When a steel pipe is heated for quenching, Mn (manganese) suppresses the ferrite transformation of the steel in the steel pipe during the cooling process after the quenching. It is an element that enhances hardenability.

The content of Mn in the steel pipe 1 is preferably 0.1% 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. In this way, the Mn content of 0.1% by mass or more prevents the hardenability of the steel pipe 1 from deteriorating, and produces high temperatures such as pearlite and upper bainite in the steel of the steel pipe 1 during cooling. Objects can be prevented from forming. As a result, when the steel pipe 1 is used as a bearing, the hardness required for the bearing can be obtained. In addition, when the Mn content is 2.0% by mass or less, it is possible to prevent ferrite from hardening and hindering roll forming during pipe making.

(S)
S (sulfur) is an element that affects the workability and rolling contact fatigue life of steel pipes. The S content in the steel pipe 1 is preferably 0.02% by mass or less. S generates MnS-based nonmetallic inclusions. The formation of MnS-based non-metallic inclusions may cause fatigue fracture due to stress concentration, and may reduce the rolling contact fatigue life. On the other hand, when the S content is 0.02% by mass or less, it is possible to suppress the formation of MnS-based non-metallic inclusions and prevent the rolling contact fatigue life from being shortened. In addition, since the S content is 0.02% by mass or less, it is possible to suppress the formation of secondary sheared surfaces and tongues in the slit coil end face shape before pipe making, and to form a suitable welded portion.

(Al)
Al (aluminum) is an element that is used as a deoxidizing agent for molten steel and also exhibits the action of fixing N (nitrogen). The Al content in the steel pipe 1 is preferably 0.2% 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 pronounced. When the Al content is 0.2% by mass or less, it is possible to prevent the cleanliness of the steel from being impaired, and as a result, it is possible to prevent the rolling contact fatigue life from being reduced due to fatigue fracture. Moreover, deterioration of the surface quality of the steel strip 2 can be prevented.

(Cr)
Cr (chromium) is an element effective in improving hardenability. The Cr content in the steel pipe 1 is preferably 5.0% by mass or less, more preferably 2.0% by mass or less, and more preferably 0.5% by mass or more and 1.6% by mass or less. More preferably, it is 0.8% by mass or more and 1.5% by mass or less. When the Cr content is 0.2% by mass or more, the hardenability of the steel pipe 1 can be further improved. In addition, when the Cr content is 5.0% by mass or less, the Cr content is not too large, so it is possible to prevent the workability from deteriorating.

(Mo)
Mo (molybdenum) is an element that, like Cr, contributes to improving the hardenability and temper softening resistance of steel pipes when added in small amounts. The Mo content in the steel pipe 1 is preferably 0.5% by mass or less. When the Mo content is 0.5% by mass or less, which is not too large, the steel strip 2 is easily softened when it is annealed, and it is possible to prevent the roll formability from deteriorating during pipe making.

(Nb)
Nb (niobium) is an element that contributes to the improvement of wear resistance by forming very hard Nb-Ti-based carbide particles in steel in the cooling process after casting the steel. However, if a large amount of Nb is added, the amount of Nb/Ti-based carbide particles produced becomes excessive, which is a factor in impairing the toughness. Therefore, the content of Nb in the steel pipe 1 is preferably 0.5% by mass or less.

(Ti)
Ti (titanium), like Nb, is an element that forms very hard Nb/Ti-based carbide particles in steel during the cooling process after casting the steel and contributes to the improvement of wear resistance. However, if a large amount of Ti is added, it becomes a factor that impairs the toughness. Therefore, the Ti content in the steel pipe 1 is preferably 0.3% by mass or less.

(V)
V (vanadium) is an element effective in improving the toughness of steel. However, even if V is added excessively, the effect of improving toughness commensurate with the cost cannot be expected. Therefore, the content of V in the steel pipe 1 is preferably 1.5% by mass or less.

(B)
B (boron) is an element that improves the hot workability of steel pipes and is effective in preventing cracks during hot rolling. However, if the steel pipe contains an excessive amount of B, the hot workability rather deteriorates. Therefore, the content of B in the steel pipe 1 is preferably 0.01% by mass or less.

<Manufacturing method of steel pipe 1>
The method of manufacturing the steel pipe 1 in this embodiment includes a step of forming the steel strip 2 and a step of welding the ends of the steel strip 2 together. These steps are similar to the roll forming and welding processes described above, respectively. In other words, the steel pipe 1 is formed by bending the steel plate or steel strip 2 to bring the ends of the steel plate or steel strip 2 into contact with each other, and by heating and melting the ends to form the bond portion 4. Welding at a temperature of 1200 ° C. or higher and 1700 ° C. or lower with the ends pressed against each other so that the width of the plate is 0.2% or more and 1.2% or less with respect to the plate thickness. Manufactured by a manufacturing method including:

<Steel pipes for bearings>
The bearing steel pipe according to the present embodiment includes the steel pipe 1 according to the present embodiment described above. In other words, the steel pipe 1 contains a specific component in a specific amount and has a low content of impurities, so that it has high hardness and cleanliness, and can be suitably used as a steel pipe for bearings.

[Additional notes]
The present invention is not limited to the above-described embodiments, but can be modified in various ways within the scope of the claims, and can be obtained by appropriately combining technical means disclosed in different embodiments. is also included in the technical scope of the present invention.

<Examples and Comparative Examples>
[Manufacturing of steel]
First, steels having chemical compositions shown in Table 1 were produced. In the remarks column, steel pipes according to one embodiment of the present invention are referred to as "present invention examples" and steel pipes according to comparative examples are referred to as "comparative examples."

[Manufacturing of welded steel pipes]
By heating slabs of various steels shown in Table 1 to 1250 to 1300° C. and hot rolling, hot-rolled coils (steel strips) having a thickness of 6.0 mm were produced. The obtained hot-rolled coils were pickled and annealed at 750° C. for 10 hours for all steel grades. After that, 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 various welding temperatures (heating temperatures), crushing amounts, and upset amounts to produce steel pipes with a diameter of 34 mm and a thickness of 6.0 mm. Annealing treatment was applied to the welded steel pipe. Annealing was carried out by air cooling after soaking at 700°C.

[Evaluation of steel pipe]
(annealing test)
When all the carbon in the steel is precipitated as globular carbides, the carbon content c (mass%) of the steel and the area ratio f of the globular carbides are expressed by the following equations.

f=15.3c (Formula 1)
Regarding the amount of precipitated spheroidal carbide, when the steel pipe after welding is soaked at 700°C for 50 hours and then air-cooled, the area ratio of spheroidal carbide (precipitation rate of spheroidal carbide) is 90% or more of f, ◯, 90%. The case of less than was evaluated as x.

The bond width, which is the width of the bond portion, was measured by measuring 5 points on the bond portion every 100 mm, and the widest point was taken as the maximum bond width.

In order to determine the area ratio of spherical carbide, 20 positions at the maximum bond width were photographed using a scanning electron microscope (SEM) at a magnification of 5000 times. The area ratio of the spherical carbide was determined by measuring the area of the spherical carbide in the bond portion in the photograph and averaging the measured values of the area of the spherical carbide.

(result)
Table 2 shows the results of evaluating the amount of spheroidal carbide precipitates in steel pipes manufactured under various welding conditions for each steel type.

Thus, in the present invention examples, the area ratio of spherical carbide was 90% or more of f under any conditions, and a good amount of precipitated spherical carbide was obtained. On the other hand, in the comparative examples, the amount of precipitated spherical carbides as good as in the examples of the present invention was not obtained. The reason for this is considered below.

When the welding temperature is higher than 1700°C (No. 2, 7, 9, 17, 19, 23, 27, 31, 35), the amount of molten metal generated in the weld increases and the bond width becomes wider. . In addition, "the bond width is wide" refers to the case where the bond width is wider than 72 μm, which is 1.2% of the steel strip thickness of 6 mm.

If the amount of crushing is less than 0.3 mm, that is, less than 5 % of the thickness of the steel strip (No. 4, 7, 10, 12, 23, 29, 35), or more than 2.4 mm, that is, In the case of more than 40% of the plate thickness (No. 21), unevenness occurred in the temperature of the welded portion during heating, and a portion with a wide bond width was formed in the bonded portion.

When the upset amount is less than 1.2 mm, that is, less than 20 % of the plate thickness (Nos. 12, 15, 17, 23, 25, 31, 33, and 35), the amount of molten metal discharged from the weld is small. and the bond width became wider.

For the various reasons described above, when the bond width was widened, the spherical carbide precipitation rate was less than 90%. Therefore, the precipitation amount of spherical carbide was evaluated as x.

(summary)
As described above, in all the examples of the present invention, the area ratio of spherical carbide was 90% or more of f, and a good amount of precipitation of spherical carbide was obtained. That is, the bond part with good workability was obtained by the annealing treatment for 50 hours. On the other hand, in all of the comparative examples, the area ratio of globular carbide was less than 90% of f, and sufficient precipitation of globular carbide could not be obtained by the annealing treatment for 50 hours. In other words, it was suggested that the steel pipe according to the comparative example requires a longer annealing treatment in order to obtain a bond portion with good workability.

REFERENCE SIGNS LIST 1 steel pipe 2 steel strip 2A lower surface 2B surface formed by crushing 3 weld zone 4 bond zone 5 heat affected zone

Claims (7)

C: 0.4% by mass or more and 1.5% by mass or less , P: 0.03% by mass or less, Cu: 0.3% by mass or less , Si: 2.0% by mass or less, Mn: 0.1% by mass The ends of steel plates or steel strips containing 2.0% by mass or less, S: 0.02% by mass or less, Al: 0.2% by mass or less, and Cr: 5.0% by mass or less are welded together A method for manufacturing a steel pipe,
The steel plate or steel strip is bent , the corners of the ends are crushed, and the length corresponding to the crushed portion in the plate width direction of the steel plate or steel strip is set to the thickness of the steel plate or steel strip. A step of contacting the ends of 5% or more and 40% or less with respect to each other;
The ends are heated so that the width of the bond formed by melting the ends by heating is 0.2% or more and 1.2% or less of the thickness of the steel plate or steel strip. and welding at a temperature of 1200° C. or more and 1700° C. or less in a state of being pressed against each other so that the amount of crushing is 20% or more and 30% or less of the thickness of the steel pipe. Production method. 2. The method for manufacturing a steel pipe according to claim 1, wherein the content of C in the steel pipe is 0.6% by mass or more and 1.2% by mass or less. 3. The method for manufacturing a steel pipe according to claim 1, further satisfying the condition that Mo : 0.5% by mass or less . It further satisfies at least one of Ti: 0.3% by mass or less, Nb: 0.5% by mass or less, V: 1.5% by mass or less, and B: 0.01% by mass or less. The method for manufacturing the steel pipe according to claim 3 . The method for manufacturing a steel pipe according to any one of claims 1 to 4 , wherein the welding is high frequency welding. The end portion is such that the smaller angle between the plane including the upper surface or the lower surface of the steel plate or steel strip and the surface formed by the crushing is 20 degrees or more and 60 degrees or less. 6. The method for manufacturing a steel pipe according to any one of claims 1 to 5 , wherein the corners of the steel pipe are crushed . The method for manufacturing a steel pipe according to any one of claims 1 to 6 , wherein the steel pipe is a round pipe, a square pipe, or a deformed pipe.

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