JP2020079434A - Steel pipe, steel pipe for bearing, and method for producing steel pipe - Google Patents

Abstract

To achieve a high carbon steel pipe (1) that can shorten the annealing time of a weld (3).SOLUTION: A steel pipe (1) is formed by welding edges of steel plates or steel strips (2) containing C: 0.4 mass% or more and 1.5 mass% or less, P: 0.03 mass% or less, and Cu: 0.3 mass% or less, the edges are welded at a weld temperature of 1,200°C or higher and 1,700°C or less, and the width of a bond part (4) is 0.2% or more and 1.2% or less of the plate thickness of the steel plate or steel strip (2).SELECTED DRAWING: None

Description

  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, high-frequency welding is performed on a low-carbon steel sheet having a carbon content of 0.15 mass% or more and 0.4 mass% or less, and then diameter reduction rolling is performed so that a bond width of an electric resistance welded portion is 25 μm or less. Discloses an electric resistance welded steel pipe which is a kind of welded steel pipe and is mechanically narrowed.

JP, 2013-147751, A

  The high carbon steel pipe is annealed to improve the workability of the weld. Patent Document 1 discloses that the reduction of the 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 welded portion in the high carbon steel pipe on the annealing time for improving the workability of the welded portion.

  One aspect of the present invention aims to realize a high carbon steel pipe capable of shortening the annealing time of a welded portion.

  In order to solve the above-mentioned subject, the steel pipe which concerns on one aspect of this invention has C:0.4 mass% or more and 1.5 mass% or less, P:0.03 mass% or less, and Cu:0.3 mass. % Steel plate or steel strip containing less than or equal to 1%, 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 formed bond portion is 0.2% or more and 1.2% or less with respect to the plate thickness of the steel plate or steel strip.

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

  In order to solve the above problems, the steel pipe according to one aspect of the present invention has Si: 2.0 mass% or less, Mn: 0.1 mass% or more and 2.0 mass% or less, S: 0.02 mass%. At least one of the following conditions and Al: 0.2% by mass or less may be further satisfied.

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

  In order to solve the above-mentioned subject, the steel pipe concerning one mode 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 condition out of 0.01 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, in the steel pipe according to an aspect of the present invention, the ends are heated and pressed against each other so that the crushed amount is 20% or more and 30% or less with respect to the plate thickness. It may be formed by

  In order to solve the above problems, in the steel pipe according to one aspect of the present invention, the end is crushed at a corner, and corresponds to a crushed portion in the plate width direction of the steel plate or the steel strip. The length is 5% or more and 40% or less with respect to the plate thickness, and the smaller one 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. The angle 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 bearing according to an aspect of the present invention is characterized by including the steel pipe according to any one of the above.

  In order to solve the above-mentioned subject, a manufacturing method of a steel pipe concerning one mode of the present invention is C: 0.4 mass% or more and 1.5 mass% or less, P: 0.03 mass% or less, and Cu:0. A method for producing a steel pipe, in which the ends of steel sheets or steel strips are welded to each other, containing 3 mass% or less, the step of bending the steel sheet or steel strips to bring the ends into contact with each other, and heating the ends. In the state where the end portions are pressed against each other such that the width of the bond portion formed by melting the portion is 0.2% or more and 1.2% or less of the plate thickness of the steel plate or the steel strip. And a step of welding at a temperature of 1200° C. or more and 1700° C. or less.

  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.

It is a schematic diagram which shows the welding part of the steel pipe which concerns on one Embodiment of this invention. It is a figure showing the photograph which shows the welding part of the steel pipe which concerns on one Embodiment of this invention. (A)-(d) is sectional drawing which shows the cross-sectional shape of the steel pipe 1, respectively. It is a schematic diagram which shows the crushing width and crushing angle of the steel pipe which concerns on one Embodiment of this invention. (A) And (b) is a schematic diagram which shows the upset amount of the steel pipe which concerns on one Embodiment of this invention.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to FIGS. 1 and 2. Hereinafter, although description will be given by taking electric resistance welding as an example of the welding method in the present invention, the welding method in the present invention is not limited to electric resistance welding and is a method of welding and joining by heating the seam portion of the steel pipe. I wish I had it.

<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 welds the edge part of the steel plate or steel strip containing C: 0.4 mass% or more and 1.5 mass% or less, P: 0.03 mass% or less, and Cu: 0.3 mass% or less. It is formed by. The ends are welded 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 equal to the plate thickness of the steel plate or the steel strip. On the other hand, it is 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. The steel strip 2 may be a steel plate.

  A bond 4 and a heat-affected zone 5 are formed in the weld 3. The bond part 4 is formed between the two heat-affected parts 5 in the weld part 3. During electric resistance welding, the electric resistance welded portion is heated to a solid-liquid coexisting region. As a result, C contained outside the electric resistance weld is concentrated in the liquid phase and decreased in the solid phase. The liquid phase enriched with C is discharged to the outside of the electric resistance welded portion 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 solid phase forms the bond portion 4. Further, the heat-affected zone 5 is an area in which the metallographic structure at the end portion is changed by heat during electric resistance welding, and is an area other than the bond portion 4.

  The welded portion 3 is preferably subjected to a spheroidizing annealing treatment described later in order to improve workability. In the welded portion 3, the heat-affected zone 5 has a structure in which undissolved carbide and martensite are mixed due to heating during welding. On the other hand, the bond portion 4 has a higher temperature than the heat-affected zone 5 due to the heating during welding, and thus has a structure of only martensite that does not contain the undissolved carbide. Therefore, when the spheroidizing annealing is applied to the welded portion 3, the spheroidizing annealing is completed in a short time because the carbide is grown in the heat-affected zone 5 with the undissolved carbide as a nucleus, but the bond portion 4 is not melted. Carbides are not included. Therefore, in the bond part 4, after the carbide is first deposited inside the bond part 4, the carbide grows.

  As a result, the bond portion 4 needs to be annealed for a longer period of time than the heat-affected zone 5. Further, the wider the bond portion 4 is, the longer the spheroidizing annealing needs to be performed. That is, by reducing the width of the bond portion 4, it is possible to shorten the time required for the spheroidizing annealing of the weld portion 3.

  FIG. 2 is a view showing a photograph showing the welded portion 3 of the steel pipe 1. 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 realize 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. Further, when the width of the bond portion 4 is 0 (zero), it is impossible to determine whether the welding is properly performed. Therefore, the width of the bond portion 4 is 0.2% or more of the plate thickness. preferable.

  Further, the steel pipe 1 is excellent in rolling contact fatigue life because it contains a specific component in a specific amount as described above. Here, the "rolling fatigue life" refers to a period until surface peeling occurs in the base material of the steel pipe 1 and the welded portion 3 due to rolling movement of the bearing using the steel pipe 1.

  In addition, "excellent in rolling contact fatigue life" means that the above period is as long as a seamless steel pipe that has been frequently used as a high carbon steel pipe. The rolling fatigue life is obtained, for example, by cutting a welded steel pipe and processing it into a plate shape, and then measuring the period until surface peeling occurs in the base material and the welded portion 3 by a thrust type rolling fatigue test. be able to.

  Further, the steel pipe 1 may contain non-metallic inclusions such as sulfide and oxygen. However, among the non-metallic inclusions, sulfides, especially MnS, aggregate and precipitate on the surface of the steel pipe, causing cracks and surface scratches originating from the non-metallic inclusions, resulting in a rolling fatigue life. It may decrease.

  Further, if sulfides such as MnS are present on the surface portion of the rolling bearing that comes into contact with the rolling elements, it is necessary to significantly turn the portion, which may increase the manufacturing cost. Therefore, the particle size of non-metallic inclusions such as sulfides is preferably 20 μm or less, and more preferably 10 μm or less. Since the particle size of the non-metallic inclusions in the steel pipe 1 is as small as the above-mentioned preferable range, reduction of rolling fatigue life and increase of manufacturing cost are prevented especially when the non-metallic inclusions are sulfides. can do.

  When the steel pipe 1 contains oxygen as a non-metallic inclusion, the oxygen content in the steel pipe 1 is preferably 20 ppm or less, and more preferably 15 ppm or less. When the oxygen content in the steel pipe 1 is within the above-mentioned preferable range, the steel pipe 1 having higher cleanliness can be obtained. As a result, the steel pipe 1 having a better rolling fatigue life can be obtained.

  The diameter of the steel pipe 1 is preferably 15 mm or more and 300 mm or less. 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. Since the diameter of the steel pipe 1 and the thickness of the steel plate or the steel strip are within the above-described preferable ranges, the steel pipe 1 can be manufactured without requiring special manufacturing conditions.

  3A to 3D are cross-sectional views showing the cross-sectional shape 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 in a cross section perpendicular to the drawing direction of the steel pipe 1. Further, as shown in FIG. 3B, the steel pipe 1 may be a rectangular pipe having the above-mentioned cross-sectional shape of a substantially rectangular shape. Further, it may be a deformed tube whose cross-sectional shape is other than substantially circular or rectangular. The cross-sectional shape of the deformed tube may be, for example, a semicircular shape as shown in FIG. 3C or a drum shape as shown in FIG. 3D.

[Steel strip 2]
The steel strip 2 is preferably used as a raw material for the steel pipe 1. The steel strip 2 is formed into a tubular shape by being subjected to roll forming, and becomes the steel pipe 1 by being subjected to welding, quenching treatment, tempering treatment and spheroidizing annealing.

  Here, in the seamless steel pipe, which is a steel pipe having no welded portion 3, the amount and size of nonmetallic inclusions on the outer surface side and the inner surface side of the steel pipe differ due to the influence of the center segregation of the steel bar as the base material. The non-metallic inclusions on the outer surface side are fine and minute, but a large amount of the non-metallic inclusions are present on the inner surface and are 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 the large amount of non-metallic inclusions on the inner surface has a great influence on the rolling fatigue life. .. From this, in order to prolong the rolling fatigue life, it is necessary to largely perform the turning process on the portion that comes into contact with the rolling elements, which increases the processing cost.

  On the other hand, the steel pipe 1, that is, a welded steel pipe such as an electric resistance welded steel pipe to which the steel strip 2 is welded, unlike the seamless steel pipe, has many non-metallic inclusions inside the steel strip, and is mostly present on the front and back surfaces. do not do. Therefore, the welded steel pipe using a steel strip as a base material has a higher cleanness on the inner surface side than the 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, since the steel pipe 1 has a high degree of cleanliness on the inner surface side, it is possible to obtain a rolling fatigue life as good as that of the seamless steel pipe while reducing the amount of cutting when processing into a component shape.

  Further, since the steel pipe 1 is obtained by welding the steel strip 2, the steel pipe can be easily mass-produced as compared with the case of manufacturing a steel pipe by punching a bar.

  In addition, the steel strip 2 refers to, for example, a coil-shaped steel sheet having a thickness of 10 mm or less among steel sheets. In the present embodiment, either the steel strip 2 or the steel plate can be used as the raw material of the present embodiment, but it is preferable to manufacture the steel pipe 1 using the steel strip 2. The productivity is improved by using the steel strip 2 which is thinner than the steel plate and has excellent roll formability 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 the rollers. Here, since the roll forming is easier when the steel strip 2 is used as the base material than the above steel plate, it is preferable to form the coiled steel strip 2 by hot rolling the steel before the roll forming. .. Further, before the steel strip 2 is roll-formed, it may be washed with an acid or annealed under the conditions of 600° C. or higher and 800° C. or lower and 1 hour or longer and 50 hours or shorter. This facilitates roll forming.

(Crushing)
In order to adjust the shape and strength of the welded portion 3 between the ends of the steel strip 2 formed into a tubular shape by the roll forming, the corner portions located inside the pipe at the ends are formed by a crushing roll. It is crushed (hereinafter, "crushing"). By properly setting the width and angle of the crushing, it is possible to obtain a sound welded portion 3 with little strength reduction in butt welding 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 plate thickness of the steel strip 2. In addition, the crushing width C is a portion in which the corner portion of the steel strip 2 is crushed in the plate width direction of the steel strip 2 when 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 one of the angles formed by the plane including the lower surface 2A (or the upper surface) of the steel strip 2 and the surface 2B formed by the 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 when the crushing angle θ is small, the amount of molten metal generated during welding is discharged from the boundary between the welded ends of the steel strip 2 (molten metal discharge amount) is small. Therefore, a wide portion is formed in the bond portion 4, and a long-time annealing process is required in the wide portion. Further, when the crushing width C is large or the crushing angle θ is large, the area of the welded end of the steel strip 2 is reduced, so that 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 bond portion 4 to be formed may be constant. it can. Therefore, long-time annealing treatment is not required due to the large part of the bond portion 4.

(welding)
In the welding of the present embodiment, the ends of the crushed steel strip 2 are butt-welded together. Thereby, the steel pipe 1 is obtained.

  5A and 5B are schematic diagrams showing the upset amount of the steel pipe 1 according to the present embodiment. In the butt welding, the ends are pressed against each other and abutted against each other. At this time, as shown in (a) of FIG. 5, a point separated by a predetermined distance X in the plate width direction from one end of the steel strip 2 before being welded is P1, and the other end of the steel strip 2 is When the point separated from the end portion by a predetermined distance Y in the plate width direction is P2, as shown in FIG. 5B, when the end portions are pressed against each other and welded to each other, after welding, The distance Z between P1 and P2 is shorter than the total length of the distance X and the distance Y. In this way, 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 referred to as the upset amount.

  The upset amount at each end is preferably 20% or more and 30% or less with respect to the plate thickness of the steel strip 2. When 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 bond portion 4 increases, and as a result, the width of the bond portion 4 increases. Moreover, when the upset amount is larger than 30% of the plate thickness, the end portion is excessively crushed.

  As described above, when the upset amount is 20% or more and 30% or less with respect to the plate 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 the present embodiment include high-density energy welding such as resistance welding, laser beam welding, and electron beam welding. However, resistance welding is preferable, and high-frequency welding (high-frequency resistance) among resistance welding is preferable. Welding) is preferred. High-frequency welding is resistance welding in which welding is performed by resistance heat generated by high-frequency current while applying pressure to the welded joint. In the present embodiment, high frequency contact resistance welding 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.

  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 produced during welding increases, and the width of the bond portion 4 becomes wider. Further, when the welding temperature is 1200° C. or lower, the amount of the molten metal becomes small, and a part of the welded portion 3 which is not welded is generated. With the welding temperature as described above, welding can be performed 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.

  Moreover, the said welding may weld the edge part of one steel strip 2 or a steel plate, and may form a steel pipe, and may join the edge parts of two or more steel strips 2 or steel plates. May be

(Quenching and tempering)
After welding, the steel pipe 1 is subjected to quenching treatment and tempering treatment. Thereby, weld cracking of the welded portion 3 can be prevented.

(Spheroidizing annealing treatment)
As described above, the metal structure of the bond part 4 may be hard martensite. Therefore, the workability of the weld 3 is reduced. As the annealing treatment performed to improve the deterioration of the workability, for example, spheroidizing annealing treatment is preferably performed. The spheroidizing annealing treatment means annealing the high carbon steel at a temperature near the A1 transformation point so that the carbide contained in the high carbon steel is spheroidized and precipitated. By performing the spheroidizing annealing treatment, the high carbon steel becomes soft and the workability is improved.

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

  According to the method as described above, the steel pipe provided with 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 having to perform the diameter reduction rolling step. 1 can be manufactured.

[Components contained in steel pipe 1]
The steel pipe 1 contains C (carbon): 0.4 mass% or more and 1.5 mass% or less, P: 0.03 mass% or less, and Cu: 0.3 mass% or less, with the balance being Fe and unavoidable impurities. Consists of. The steel pipe 1 contains a specific component in a specific amount, has a small amount of impurities, and has high hardness and cleanliness, and thus has excellent rolling fatigue life.

(C)
The steel pipe 1 contains 0.4 mass% or more and 1.5 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 the amount of carbide of the steel pipe greatly vary depending on the content. When the content of C is 0.4% by mass or more, the strength required for use as a mechanical component such as a bearing can be obtained. Further, 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.

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

(P)
P (phosphorus) is an element that reduces the ductility and toughness of a steel pipe. The P content in the steel pipe 1 is preferably 0.03 mass% or less, more preferably 0.02 mass% or less, and further preferably 0.01 mass% or less. When the P content is 0.03 mass% or less, it is possible to prevent the toughness of the former austenite grain boundary in the steel pipe 1 from increasing after quenching and to prevent the rolling fatigue resistance of the steel pipe 1 after heat treatment from decreasing. ..

(Cu)
Cu (copper) is an element that improves the surface properties of the steel strip and the steel pipe obtained from the steel strip by improving the peelability of the oxide scale produced in the steel strip during hot rolling. The Cu content in the steel pipe 1 is preferably 0.3 mass% or less. When the Cu content is 0.3 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 can be contained in the steel pipe]
Further, the steel pipe 1 is a range in which the problem of efficiently manufacturing a high carbon welded steel pipe can be solved, and in addition to the above-mentioned components, Si, Mn, Cr, S, Al, Cr, Mo, Ti, Nb, V, And B may be further included. Here, when at least one of P, Mo, S, and Al is contained, the total content of these components is preferably 7.2 mass% or less with respect to the steel pipe 1, and 6.0 mass%. The content is more preferably below, and even more preferably 4.0% by mass or less. By being in the above-described preferable range, 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, the steel pipe 1 having a better rolling fatigue life can be obtained.

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

  Further, it is possible to prevent the hardening of ferrite due to the solid solution strengthening action of Si, and thereby to prevent the steel pipe 1 from cracking during the forming process. In addition, it is possible to prevent scale flaws from being generated on the surface of the steel strip 2 in the manufacturing process, and to prevent rolling fatigue life from being shortened due to grain boundary oxidation occurring during quenching and heating of the steel pipe 1. it can.

(Mn)
Mn (manganese) suppresses the ferrite transformation of the steel in the steel pipe in the cooling process after quenching when the steel pipe is heated by quenching, 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 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. As described above, when the Mn content is 0.1% by mass or more, deterioration of the hardenability of the steel pipe 1 is prevented, and high-temperature generation of pearlite and upper bainite is generated in the steel of the steel pipe 1 during cooling. It is possible to prevent an object from being formed. Thereby, when the steel pipe 1 is used as a bearing, the hardness required for the bearing can be obtained. Further, when the Mn content is 2.0% by mass or less, it is possible to prevent the ferrite from being hardened and hindering the roll forming during pipe making.

(S)
S (sulfur) is an element that affects the workability and rolling fatigue life of steel pipes. The S content in the steel pipe 1 is preferably 0.02 mass% or less. S produces MnS-based non-metallic inclusions. The generation of MnS-based non-metallic inclusions may be the starting point of fatigue fracture due to stress concentration, which may reduce the rolling fatigue life. On the other hand, when the content of S is 0.02 mass% or less, the generation of MnS-based nonmetallic inclusions can be suppressed, and the reduction of rolling contact fatigue life can be prevented. Further, when the S content is 0.02 mass% or less, it is possible to suppress the generation of secondary shear planes 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 has an action of fixing N (nitrogen). The Al content in the steel pipe 1 is preferably 0.2 mass% or less, and more preferably 0.005 mass% or more and 0.05 mass% or less. When the content of Al is 0.005 mass% or more, the effect of fixing N becomes more remarkable. When the Al content is 0.2 mass% or less, the cleanliness of the steel can be prevented from being impaired, and as a result, the reduction in rolling contact fatigue life due to fatigue fracture can be prevented. Further, it is possible to prevent the deterioration of the surface quality of the steel strip 2.

(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 0.5% by mass or more and 1.6% by mass or less. It is more preferably 0.8% by mass or more and 1.5% by mass or less. When the Cr content is 0.2 mass% or more, the hardenability of the steel pipe 1 can be further improved. Further, when the content of Cr is 5.0 mass% or less, the content of Cr is not too large, and thus it is possible to prevent the workability from being deteriorated.

(Mo)
Mo (molybdenum) is an element that contributes to the improvement of the hardenability and the resistance to temper softening of steel pipes, like Cr, even when added in a small amount. The Mo content in the steel pipe 1 is preferably 0.5 mass% or less. When the content of Mo is not too large as 0.5% by mass or less, it is possible to prevent the steel strip 2 from being softened easily when the steel strip 2 is annealed and to prevent the roll formability at the time of pipe forming from being deteriorated.

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

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

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

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

<Method of manufacturing steel pipe 1>
The method for manufacturing the steel pipe 1 in the present embodiment includes a step of forming the steel strip 2 and a step of welding the ends of the steel strip 2 to each other. These steps are similar to the roll forming process and the welding process described above, respectively. In other words, the steel pipe 1 includes a step of bending the steel plate or the steel strip 2 to bring the ends of the steel plate or the steel strip 2 into contact with each other, and a bond portion 4 formed by melting the end portion by heating. Welding at a temperature of 1200° C. or more and 1700° C. or less in a state in which the end portions are pressed against each other so that the width becomes 0.2% or more and 1.2% or less with respect to the plate thickness. It is manufactured by a manufacturing method including.

<Bearing steel pipe>
The steel pipe for bearing 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 contains a small amount of impurities, and thus has high hardness and high cleanliness, and thus can be suitably used as a bearing steel pipe.

[Appendix]
The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the claims, and the embodiments obtained by appropriately combining the technical means disclosed in the different embodiments. Is also included in the technical scope of the present invention.

<Examples and Comparative Examples>
[Manufacturing of steel]
First, steels having the chemical compositions shown in Table 1 were manufactured. In the remarks column, the steel pipe according to one embodiment of the present invention is referred to as “invention example”, and the steel pipe according to comparative example is referred to as “comparative example”.

[Manufacture of welded steel pipe]
A slab of each of the steels in Table 1 was heated to 1250 to 1300° C. and hot-rolled to manufacture a hot-rolled coil (steel strip) having a thickness of 6.0 mm. The obtained hot-rolled coil was pickled, and all steel types were annealed at 750° C. for 10 hours. Then, 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 manufacture steel pipes having a diameter of 34 mm and a thickness of 6.0 mm. The steel pipe after welding was annealed. In the annealing treatment, after soaking at 700° C., air cooling was performed.

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

f=15.3c (Formula 1)
Regarding the amount of spherical carbide precipitation, when the steel pipe after the above-mentioned welding was soaked at 700° C. for 50 hours and then air-cooled, the spherical carbide area ratio (spherical carbide precipitation ratio) was 90% or more of f, ○, 90% When less than, it was evaluated as x.

  The bond width, which is the width of the bond portion, was measured at every 100 mm in the bond portion at five locations, and the widest portion was defined as the maximum bond width.

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

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

  As described above, in any of the examples of the present invention, the area ratio of the spherical carbides was 90% or more of f, and a good amount of the precipitated spherical carbides was obtained. On the other hand, in the comparative example, the good precipitation amount of the spherical carbide as in the example of the present invention was not obtained. The reason for this is discussed below.

  Under conditions where 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 widens. .. 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 plate thickness 6 mm of the steel strip.

  When the amount of crushing is less than 0.3% with respect to the plate thickness of the steel strip (No. 4, 7, 10, 12, 23, 29, 35) or more than 2.4% (No. 21), unevenness was generated in the temperature of the welded portion during heating, and a portion having a wide bond width was formed in the bonded portion.

  When the upset amount is less than 1.2% of the above plate thickness (No. 12, 15, 17, 23, 25, 31, 33, 35), the molten metal discharge amount in the weld is small, and the bond The width has become wider.

  When the bond width was widened for various reasons described above, 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 each of the examples of the present invention, the area ratio of the spherical carbide was 90% or more of f, and a good amount of the precipitated spherical carbide was obtained. That is, a bond part having good workability was obtained by the annealing treatment for 50 hours. On the other hand, in each of the comparative examples, the area ratio of the spherical carbides was less than 90% of f, and sufficient precipitation of the spherical carbides could not be obtained by the annealing treatment for 50 hours. That is, it was suggested that in the steel pipe according to the comparative example, a longer annealing treatment was required in order to obtain a bond part having good workability.

1 Steel pipe 2 Steel strip 2A Lower surface 2B Surface formed by crushing 3 Welded portion 4 Bonded portion 5 Heat affected zone

Claims (11)

C: 0.4 mass% or more and 1.5 mass% or less, P: 0.03 mass% or less, and Cu: formed by welding the ends of a steel strip or steel strip containing 0.3 mass% or less,
The ends are welded at a welding temperature of 1200° C. or higher and 1700° C. or lower,
A width of a bond portion formed by melting the end portion by heating is 0.2% or more and 1.2% or less with respect to the plate thickness of the steel plate or steel strip.   The 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.   At least one condition of Si: 2.0 mass% or less, Mn: 0.1 mass% or more and 2.0 mass% or less, S: 0.02 mass% or less, and Al: 0.2 mass% or less The steel pipe according to claim 1 or 2, wherein the steel pipe is further filled.   The steel pipe according to claim 3, further satisfying at least one condition of Cr: 5.0 mass% or less and Mo: 0.5 mass% or less.   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, further satisfying at least one condition. The steel pipe according to claim 4.   The said welding is high frequency welding, The steel pipe of any one of Claims 1-5 characterized by the above-mentioned.   The said edge part is heated, and it is formed by pressing mutually so that it may become a crushing amount of 20% or more and 30% or less with respect to the said plate thickness, It is formed. The steel pipe according to the item. The edge is crushed at the corner, and the length corresponding to the crushed portion in the plate width direction of the steel plate or steel strip is 5% or more and 40% or less with respect to the plate thickness,
The smaller one of the angles formed by the plane including the upper surface or the lower surface of the steel plate or the steel strip and the surface formed by the crushing is 20 degrees or more and 60 degrees or less. The steel pipe according to any one of 1 to 7.   The steel pipe according to any one of claims 1 to 8, which is a round pipe, a square pipe, or a deformed pipe.   A steel pipe for bearing, comprising the steel pipe according to any one of claims 1 to 9. 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, of a steel pipe in which end portions of steel plates or steel strips are welded to each other. A manufacturing method,
A step of bending the steel plate or steel strip to bring the ends into contact with each other,
The ends are joined to each other such 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 plate thickness of the steel plate or steel strip. And a step of welding at a temperature of 1200° C. or more and 1700° C. or less in a pressed state.