TW202028490A - Steel pipe, steel pipe for bearing, and method for producing steel pipe - Google Patents

Abstract

A high-carbon steel pipe with which it is possible to shorten the annealing time of a weld is realized. The steel pipe is formed by welding the edges of a steel plate or steel strip containing 0.4-1.5 mass% of C, 0.03 mass% or less of P, and 0.3 mass% or less of Cu. The edges are welded at a welding temperature of 1200-1700 DEG C, and the width of a bond part is 0.2-1.2% with respect to the thickness of the steel plate or steel strip.

Description

Steel pipes, bearing steel pipes and steel pipe manufacturing methods

The present invention relates to steel pipes, bearing steel pipes, and steel pipe manufacturing methods.

Welded steel pipes are generally manufactured by welding steel plates or steel strips. For example, Patent Document 1 discloses an electric seam welded steel pipe as one of the welded steel pipes, which is a low-carbon steel plate with a carbon content of 0.15 mass% or more and 0.4 mass% or less after high-frequency welding, and then reduced diameter The rolling is mechanically narrowed so that the joint width of the electric seam welded portion is 25 μm or less.

[Prior Technical Literature] [Patent Literature] [Patent Document 1] Japanese Patent Publication "JP 2013-147751 A"

[The problem is solved by the invention] In high carbon steel pipes, annealing treatment is performed in order to improve the workability of the welded part. Patent Document 1 discloses that the joint width of the welded portion in the low carbon steel pipe is narrowed to prevent the strength of the welded portion from decreasing. However, Patent Document 1 does not disclose the effect of the bonding width of the welded portion in the high carbon steel pipe on the annealing time, and the annealing time is used to improve the workability of the welded portion.

The purpose of one aspect of the present invention is to realize a high carbon steel pipe capable of shortening the annealing time of the welded part.

[To solve the problem] In order to solve the aforementioned problems, a steel pipe of one aspect of the present invention includes: the aforementioned steel pipe is formed by welding the ends of a steel plate or steel strip, and the aforementioned steel plate or steel strip includes: carbon (C): 0.4 mass % Or more and 1.5% by mass or less; Phosphorus (P): 0.03% by mass or less; Copper (Cu): 0.3% by mass or less; the aforementioned ends are mutually welded at a welding temperature of 1200°C or more and 1700°C or less; The width of the joint formed by melting the ends by heating is 0.2% or more and 1.2% or less of the thickness of the steel sheet or steel strip.

In order to solve the aforementioned problems, a method of manufacturing a steel pipe according to one aspect of the present invention includes: the manufacturing method of the steel pipe is a method of manufacturing a steel pipe in which the ends of a steel plate or a steel strip are welded, and the steel plate or a steel strip includes: Carbon (C): 0.4% by mass or more and 1.5% by mass or less; Phosphorus (P): 0.03% by mass or less; Copper (Cu): 0.3% by mass or less; The aforementioned steel pipe manufacturing method includes: a contacting step, which combines the above The steel plate or steel strip is bent, and the aforementioned ends are brought into contact with each other; the welding step is to perform welding at a welding temperature of 1200°C or more and 1700°C under the condition that the aforementioned ends are pressed to make the aforementioned The width of the joint formed by melting the ends becomes 0.2% or more and 1.2% or less of the thickness of the aforementioned steel plate or steel strip.

[Invention Effect] According to one aspect of the present invention, a high-carbon steel pipe capable of shortening the annealing time of the welded part can be realized.

Hereinafter, an embodiment of the present invention will be described in detail with reference to FIGS. 1 and 2. Hereinafter, although electric seam welding is given as an example of the welding method of the present invention, the welding method of the present invention is not limited to electric seam welding, and may be a method of welding by heating the joint part of a steel pipe.

＜Steel pipe 1＞ Fig. 1 is a schematic view showing a welded portion 3 of a steel pipe 1 according to an embodiment of the present invention. The steel pipe 1 is formed by welding the ends of a steel plate or a steel strip, and the aforementioned steel plate or steel strip includes: carbon (C): 0.4% by mass or more and 1.5% by mass or less; phosphorus (P): 0.03% by mass or less ; Copper (Cu): 0.3% by mass or less. The aforementioned ends are welded to each other at a welding temperature of 1200°C or more and 1700°C or less, and the width of the joint 4 formed by melting the aforementioned ends by heating is 0.2% or more of the thickness of the aforementioned steel plate or strip and 1.2% or less.

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

The bonding part 4 and the heat-affected part 5 are formed in the welding part 3. In the welded part 3, the joint 4 is formed between the two heat-affected parts 5. During the electric seam welding, the electric seam welding part is heated to the solid-liquid coexisting area. Thereby, the carbon (C) contained outside the electric seam welded portion concentrates in the liquid phase and decreases in the solid phase. By pressurizing the ends of the steel strip 2 with each other (upset), the above-mentioned liquid phase concentrated in carbon (C) is discharged out of the electric seam welding part, and a weld bead is formed. On the other hand, the solid phase with reduced carbon (C) remains in the welded portion 3, and the bonded portion 4 is formed by the solid phase. In addition, the heat-affected zone 5 is a region where the metal structure of the end portion is changed by the heat during electric seam welding, and it is a region other than the joint 4.

In order to improve workability, it is preferable that the welding part 3 is subjected to the spheroidizing annealing treatment described later. In the welded part 3, the heat-affected zone 5 becomes a structure in which undissolved carbides and martensite are mixed by heating during welding. On the other hand, due to the heating during welding, the temperature of the junction 4 is higher than the temperature of the heat-affected zone 5, so the junction 4 has only a martensitic structure containing no undissolved carbides. Therefore, when spheroidizing annealing is applied to the welded part 3, although the carbide grows with the above-mentioned undissolved carbide as a nucleus, the heat-affected zone 5 completes the spheroidizing annealing in a short time, but the bond The part 4 does not contain the above-mentioned undissolved carbides. Therefore, in the joint 4, carbides are first precipitated in the joint 4, and then the carbides grow.

As a result, compared to the heat-affected portion 5, the joint portion 4 needs to be subjected to spheroidization annealing for a long time. In addition, the wider the width of the joint 4, the more long-term spheroidization annealing is required. That is, by narrowing the width of the bonding portion 4, the time required for the spheroidizing annealing of the bonding portion 3 can be shortened.

Fig. 2 is a diagram showing a photograph of the welded portion 3 of the steel pipe 1 according to an embodiment of the present invention. As shown in FIG. 2, the steel pipe 1 is formed so that the width of the joint 4 becomes 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 joint 4, the welding temperature is preferably above 1200°C and below 1700°C. In this way, as long as the width of the joint 4 is 0.2% or more and 1.2% or less of the thickness of the steel strip 2, the spheroidizing annealing time after welding can be relatively shortened. Moreover, when the width of the joint 4 is 0 (Zero), it becomes impossible to determine whether or not proper welding is performed. Therefore, the width of the joint 4 is preferably 0.2% or more of the aforementioned plate thickness.

In addition, as described above, the steel pipe 1 contains a specific amount of specific components, so that the rolling fatigue life is excellent. Here, the "rotation 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 rotational movement of the bearing of the steel pipe 1.

In addition, "excellent rolling fatigue life" means that the above-mentioned period is as long as conventional seamless steel pipes that are frequently used as high-carbon steel pipes. The rolling fatigue life can be obtained, for example, by the following method: after the welded steel pipe is cut and processed into a plate shape, it is measured by a thrust type (Thrust type) rolling fatigue test until it is in the base material and the welded part 3. The rolling fatigue life was obtained during the period until surface peeling occurred.

In addition, the steel pipe 1 may contain non-metallic inclusions such as sulfide and oxygen. However, in non-metallic inclusions, because sulfide (especially manganese sulfide (MnS)) will accumulate and precipitate on the surface of the steel pipe, and it becomes the cause of cracks and surface defects caused by non-metallic inclusions, so it can reduce rotation The risk of fatigue life.

In addition, when sulfides such as MnS are present on the surface portion that is in contact with the rotating body of the rotary bearing, the portion needs to be turned substantially, 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. By making the particle size of the non-metallic inclusions in the steel pipe 1 small within the above-mentioned preferred range, especially when the non-metallic inclusions are sulfides, it is possible to prevent reduction in rolling fatigue life and increase in manufacturing costs.

In addition, 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 above-mentioned preferred range, a steel pipe 1 with high cleanliness can be obtained. As a result, it is possible to obtain the steel pipe 1 having a better rolling fatigue life.

Regarding the diameter of the steel pipe 1, it is preferably 15 mm or more and 300 mm or less in diameter. In addition, 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 the steel strip in the above-mentioned preferable range, the steel pipe 1 can be manufactured without requiring special manufacturing conditions.

(A) to (d) of FIG. 3 are cross-sectional views each showing the cross-sectional shape of the steel pipe 1. As shown in FIG. 3(a), the steel pipe 1 may be a round pipe, and the cross-sectional shape of the aforementioned round pipe in a cross section perpendicular to the extending direction of the steel pipe 1 is approximately circular. Moreover, as shown in FIG. 3(b), the steel pipe 1 may be a square pipe whose cross-sectional shape is substantially rectangular. In addition, the steel pipe 1 may be a deformed pipe whose cross-sectional shape is other than the substantially circular or substantially rectangular shape. In addition, the cross-sectional shape of the deformed tube may include a semicircular shape shown in FIG. 3(c), or a drum shape shown in FIG. 3(d).

[Steel Belt 2] The steel belt 2 is suitable for the original material of the steel pipe 1. The steel plate 2 is formed into a tubular shape by roll forming, and is formed into a steel pipe 1 by performing welding, quenching treatment, tempering treatment, and spheroidizing annealing.

Here, as a seamless steel pipe that does not have a welded part 3, it is due to the influence of the center segregation of the steel bar as the base material, and the amount of non-metallic inclusions between the outer surface side and the inner surface side of the steel pipe And the size makes a big difference. Although the non-metallic inclusions on the outer surface side are fine and minute, the amount of the non-metallic inclusions on the inner surface side is large and the inner surface is exposed.

Therefore, when a seamless steel tube is used as a bearing, although the inner surface of the seamless steel tube is used for the outer gear train of the rotary bearing, a large number of non-metallic inclusions existing on the inner surface seriously affect the rolling fatigue life. Therefore, in order to prolong the rolling fatigue life, the part of the inner surface of the bearing seamless steel pipe that is in contact with the rotating body must be substantially turned, which increases the processing cost.

In contrast, the steel pipe 1, that is, the welded steel pipe such as the electric seam steel pipe welded with the steel strip 2 is different from the seamless steel pipe in that a large number of non-metallic inclusions are present inside the steel strip, and hardly exist on both the front and back sides. Therefore, compared with seamless steel pipes, the cleanliness of the inner surface side of the welded steel pipe using steel strip as the original material is higher, and the difference in cleanliness between the inner surface and the outer surface of the steel pipe can be reduced.

From this, it can be seen that since the inner surface of the steel pipe 1 has a high degree of cleanliness, the amount of cutting when processed into a part shape can be reduced, and an excellent rolling fatigue life equivalent to that of a seamless steel pipe can be obtained.

In addition, compared to the case of perforating a rod to produce a steel pipe, the steel pipe 1 is obtained by welding the steel strip 2 so that the steel pipe can be easily mass-produced.

Also, for example, the steel strip 2 refers to a coiled steel strip with a thickness of 10 mm or less in the steel plate. In this embodiment, although both the steel strip 2 and the steel plate can be used as the original material of this embodiment, it is preferable to use the steel strip 2 to manufacture the steel pipe 1. The productivity can be improved by using the steel strip 2 which is thinner than the steel plate, that is, has excellent roll formability as described later. Thereby, the steel pipe 1 can be manufactured more efficiently. In addition, the steel strip 2 can be obtained, for example, by hot rolling steel.

(Roll forming) In the roll forming of this embodiment, the steel strip 2 is formed into a tube shape by passing the steel strip 2 between rollers. Here, compared to the above-mentioned steel sheet, it is easier to roll forming the steel strip 2 as the original material. Therefore, it is preferable to apply hot rolling or the like to the steel before roll forming to make it into a coil shape. The steel strip 2. In addition, before the steel strip 2 is roll-formed, it may be cleaned with acid; or it may be annealed under the conditions of 600° C. or more and 800° C. or less, 1 hour or more and 50 hours or less. This makes roll forming easier.

(crush) In order to adjust the shape and strength of the welded portion 3, each end portion of the steel strip 2 that is formed into a tube shape by the above-mentioned roll forming is formed by a crushing roller to be the corner portion inside the tube at the end portion The portion is crushed (hereinafter referred to as "crushing"). By appropriately setting the width and angle of the crushing, it is possible to obtain a good welded portion 3 with less strength reduction in butt welding described later.

Fig. 4 is a schematic diagram showing the crushing width C and crushing angle θ of the steel pipe 1 according to an embodiment of the present invention. As shown in FIG. 4, the above-mentioned crushing width (crushing width C) is preferably a width of 5% or more and 40% or less of the plate thickness of the steel strip 2. In addition, the crushing width C refers to the length of the portion where the angle 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 taken as the long side direction of the steel strip 2.

The crushing angle θ refers to the angle formed by the plane of the bottom surface (or top surface) 2A of the steel strip 2 and the surface 2B formed by the aforementioned crushing, whichever is smaller. The crushing angle θ is preferably at least 20 degrees and within 60 degrees, more preferably 45 degrees.

When the crushing width C is small or the crushing angle θ is small, the molten metal generated during welding is discharged from the boundary portion of the welded ends of the steel strip 2 (the discharge amount of molten metal )Fewer. Therefore, a wide portion is formed in the joint 4, and a long-term annealing treatment is required in the wide portion. In addition, when the crushing width C is wide or the crushing angle θ is large, the area of the welded end of the steel strip 2 decreases, and therefore the strength of the welded portion 3 decreases.

On the other hand, when the crushing width C is a width of 5% or more and 40% or less of the above-mentioned plate thickness, the temperature of the welding part 3 will not be uneven, and the width of the formed joint 4 can be fixed . Therefore, it is not necessary to perform a long-term annealing treatment because a part of the joint portion 4 has a large width.

(welding) In the welding of the present embodiment, the ends of the steel strip 2 after the above-mentioned crushing are subjected to butt welding. In this way, the steel pipe 1 is obtained.

(A) and (b) of FIG. 5 are schematic diagrams showing the upset amount of the steel pipe 1 of this embodiment. In the above-mentioned butt welding, the butt joint is performed by pressing each of the above-mentioned ends against each other. At this time, as shown in Fig. 5(a), from the end of one side of the steel strip 2 before welding to the point away from the specific distance X in the plate width direction as P1, from the end of the other side of the steel strip 2 before welding The point to the point far away from the specific distance Y in the width direction of the board is set as P2. At this time, as shown in Figure 5(b), if the ends are pressed against each other and butt welding is performed, the distance Z between P1 and P2 after welding is longer than the length obtained by adding the distance X and the distance Y short. In this way, in the direction in which the ends are pressed against each other (in other words, the plate width direction), the length (X+Y-Z) of the ends shortened by pressing is called the end pressure.

The end pressure of each end is preferably 20% or more and 30% or less of the thickness of the steel strip 2. If the pressing amount is less than 20% of the above-mentioned plate thickness, the discharge amount of molten metal decreases. Therefore, the amount of the above-mentioned molten metal remaining in the joint 4 increases, and as a result, the width of the joint 4 becomes wider. In addition, when the end pressure is greater than 30% of the plate thickness, the end portion is excessively crushed.

As described above, if the end pressure is 20% or more and 30% or less of the plate thickness of the steel strip 2, the width of the joint 4 becomes narrow, and a sufficient molten metal discharge amount can be obtained.

As for the welding method of this embodiment, although high-density energy welding such as resistance welding, laser beam welding, and electron beam welding can be mentioned, resistance welding is preferred; and in resistance welding, high-frequency welding is used. (High frequency resistance welding) is better. High-frequency welding refers to resistance welding in which resistance heat caused by high-frequency current is applied while pressure is applied 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 using high frequency welding to weld the steel strip 2, the steel strip 2 can be welded efficiently and at low cost.

In addition, the welding temperature is preferably 1200°C or higher and 1700°C or lower for welding. If the welding temperature is too high, since the amount of molten metal generated during welding increases, the width of the joint 4 becomes wider. In addition, when the welding temperature is 1200° C. or lower, the amount of the molten metal described above decreases, and an unwelded portion is generated in a part of the welded portion 3. As long as the welding temperature is as described above, welding can be performed so that the width of the joint 4 is 0.2% or more and 1.2% or less of the thickness of the steel strip 2.

Furthermore, the above-mentioned welding may be a method of welding the ends of one steel strip 2 or steel plate to each other to form a steel pipe, or may connect the ends of two or more steel strips 2 or steel plates to each other.

(Quenching, tempering) The steel pipe 1 is quenched and tempered after welding. Thereby, welding cracks in the welding part 3 can be prevented.

(Spheroidizing annealing treatment) As described above, the metal structure of the joint 4 becomes hard martensite. Therefore, the workability of the welded portion 3 is reduced. Regarding the annealing treatment performed to improve the aforementioned reduction in workability, it is preferable to perform, for example, a spheroidizing annealing treatment. The spheroidizing annealing treatment refers to annealing high carbon steel at a temperature close to the aluminum (A1) transformation point so that carbides contained in the high carbon steel are spheroidized and precipitated. Through the spheroidizing annealing treatment, the high carbon steel is softened and the workability is improved.

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

According to the above method, the diameter reduction rolling step is not required, and a steel pipe 1 can be manufactured, which is provided with a welded portion 3 whose joint 4 has a width of 0.2% to 1.2% of the steel strip 2.

[Ingredients in steel pipe 1] The steel pipe 1 is made of the following components: Carbon (C): 0.4% by mass or more and 1.5% by mass or less; Phosphorus (P): 0.03% by mass or less; and Copper (Cu): 0.3% by mass or less, with the remainder being iron (Fe) and unavoidable impurities. Because the steel pipe 1 contains a specific amount of specific components, has a low content of impurities, and has high hardness and cleanliness, it has an excellent rolling fatigue life.

Carbon (C) The steel pipe 1 contains 0.4% by mass or more and 1.5% by mass or less of carbon (C). That is, the steel pipe 1 is a high-carbon welded steel pipe. Carbon (C) is the most basic element in carbon steel. Based on its content, the hardness of the steel pipe and the amount of carbides will vary greatly. By making the carbon (C) content 0.4% by mass or more, it is possible to obtain the necessary strength when used as a mechanical part such as a bearing. In addition, by setting the carbon (C) content to 0.6% by mass or more, the effect of obtaining the necessary strength when used as a mechanical part such as a bearing becomes remarkable.

In addition, by making the content of carbon (C) 1.5% by mass or less, it is possible to suppress the precipitation of carbides that do not form spherical carbides by the spheroidizing annealing treatment. Therefore, according to the spheroidizing annealing treatment, sufficient workability can be imparted to the welded portion 3.

Phosphorus (P) Phosphorus (P) is an element that reduces the ductility and toughness of steel pipes. The content of phosphorus (P) in the steel pipe 1 is preferably 0.03% by mass or less, more preferably 0.02% by mass or less, and particularly preferably 0.01% by mass or less. The steel pipe 1 may not contain phosphorus (P). By setting the phosphorus (P) content to 0.03% by mass or less, the toughness of the old Austenite grain boundary in the steel pipe 1 after quenching is improved, thereby preventing the rotation fatigue of the steel pipe 1 from decreasing after heat treatment .

Copper (Cu) Copper (Cu) is an element that improves the peelability of the oxide scale generated in the steel strip during hot rolling, thereby improving the surface characteristics of the steel strip and the steel pipe obtained from the steel strip. The content of copper (Cu) in the steel pipe 1 is preferably 0.3% by mass or less. The steel pipe 1 may not contain copper (Cu). By setting the content of copper (Cu) to 0.3% by mass or less, it is difficult to produce fine cracks on the surface of the steel strip 2 and the steel pipe 1 obtained from the steel strip 2.

[Other ingredients that can be contained in the steel pipe] In addition, the steel pipe 1 may further contain silicon (Si), manganese (Mn), chromium (Cr), sulfur (S), aluminum ( At least any one of Al), chromium (Cr), molybdenum (Mo), titanium (Ti), niobium (Nb), vanadium (V), and boron (B). Here, when at least any one of phosphorus (P), molybdenum (Mo), sulfur (S) and aluminum (Al) is contained, the total content of these components is preferably 7.2 mass relative to the steel pipe 1. % Or less, more preferably 6.0 mass% or less, particularly preferably 4.0 mass% or less. With the above preferred range, the impurities contained in the steel pipe 1 can be reduced, and the cleanliness of the steel pipe 1 can be further improved. As a result, a steel pipe 1 having a better rolling fatigue life can be obtained.

Silicon (Si) Silicon (Si) is an element that delays the precipitation of carbides during the spheroidizing annealing treatment. The silicon (Si) content in the steel pipe 1 is preferably 2.0% by mass or less. If the content of silicon (Si) is not excessive, the precipitation of carbides in the spheroidizing annealing treatment is not hindered, and the spheroidizing annealing treatment can be efficiently performed.

In addition, it is possible to prevent the hardening of the ferrous iron caused by the solid solution strengthening effect of silicon (Si), thereby preventing the steel pipe 1 from cracking during the forming process. In addition, it is possible to prevent the occurrence of scale defects on the surface of the steel strip 2 during the manufacturing process, and to prevent the reduction of rolling fatigue life due to grain boundary oxidation during the quenching and heating of the steel pipe 1.

Manganese (Mn) Manganese (Mn) is an element that is used in the quenching and heating of steel pipes to suppress the transformation of the ferrous iron in the steel of the steel pipe during the cooling process after quenching, so that it is still effective even at a relatively slow cooling rate. The main structure of Asada scattered iron can improve the hardenability of steel pipe.

The manganese (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. In this way, by making the content of manganese (Mn) 0.1% by mass or more, the hardenability of the steel pipe 1 can be prevented from decreasing, and the steel of the steel pipe 1 under cooling can be prevented from forming high temperatures such as corrugated iron and upper toughened iron. product. Thereby, the required hardness can be obtained when the steel pipe of this embodiment is used as a bearing. In addition, by making the content of manganese (Mn) 2.0% by mass or less, it is possible to prevent the hardening of the ferrous iron, which would hinder roll forming during pipe making.

Sulfur (S) Sulfur (S, sulfur) is an element that affects the workability and rolling fatigue life of steel pipes. The content of sulfur (S) in the steel pipe 1 is preferably 0.02% by mass or less. Sulfur (S) produces manganese sulfide (MnS) non-metallic inclusions. Since the generation of manganese sulfide (MnS)-based non-metallic inclusions will become the cause of fatigue failure due to stress concentration, there is a risk of reduced rolling fatigue life. In contrast, by making the content of sulfur (S) 0.02% by mass or less, the generation of manganese sulfide (MnS)-based non-metallic inclusions can be suppressed, and the reduction in rolling fatigue life can be prevented. In addition, by setting the sulfur (S) content to 0.02% by mass or less, it is possible to suppress the generation of secondary shear planes and tongue-like parts in the end face shape of the slit coil before pipemaking, and to form an appropriate welded part.

Aluminum (Al) Aluminum (Al) is an element that is used as an oxygen scavenger for molten steel and acts to fix nitrogen (N). The content of aluminum (Al) in the steel pipe is preferably 0.2% by mass or less, more preferably 0.005% by mass or more and 0.05% by mass or less. By making the content of aluminum (Al) 0.005 mass% or more, the effect of fixing nitrogen (N) becomes more significant. By making the content of aluminum (Al) below 0.2% by mass, it is possible to prevent the cleanliness of the steel from being damaged, and to prevent fatigue damage from reducing the rolling fatigue life. In addition, the surface quality of the steel strip 2 can be prevented from degrading.

Chromium (Cr) Chromium (Cr) is an element that can effectively improve hardenability. The chromium (Cr) content in the steel pipe 1 is preferably 5.0% by mass or less, more preferably 2.0% by mass or less, particularly preferably 0.5% by mass or more and 1.6% by mass or less, and most preferably 0.8% by mass or more and 1.5% by mass or less . By making the content of chromium (Cr) 0.2% by mass or more, the hardenability of the steel pipe 1 can be more improved. By setting the content of chromium (Cr) to 5.0% by mass or less, since the content of chromium (Cr) is not excessive, it is possible to prevent reduction in workability.

Molybdenum (Mo) Molybdenum (Mo) is an element that can be added in a small amount to impart the same improvement as chromium (Cr) to the hardenability and temper softening resistance of steel pipes. The content of molybdenum (Mo) in the steel pipe 1 is preferably 0.5% by mass or less. By setting the content of molybdenum (Mo) to 0.5% by mass or less, molybdenum (Mo) is not excessive, so that the steel strip 2 is easily softened when softening and tempering treatment is applied, and the roll formability is prevented from being reduced during pipe making.

Niobium (Nb) Niobium (Nb) is an element that forms very hard niobium (Nb)‧Titanium (Ti) carbide particles in the steel during the cooling process after the steel is cast, and imparts the function of improving wear resistance . However, if too much niobium (Nb) is added, the amount of niobium (Nb)·titanium (Ti) carbide particles produced is too large, which becomes an important cause of impairing toughness. Therefore, the content of niobium (Nb) in the steel pipe 1 is preferably 0.5% by mass or less.

Titanium (Ti) Titanium (Ti) is an element, which is the same as niobium (Nb). During the cooling process after casting of steel, very hard niobium (Nb)‧titanium (Ti) carbide particles are formed in the steel, and are given The function of improving abrasion resistance. However, if too much titanium (Ti) is added, it becomes an important cause of impairing toughness. Therefore, the titanium (Ti) content in the steel pipe 1 is preferably 0.3% by mass or less.

Vanadium (V) Vanadium (V) is an element that can effectively improve the toughness of steel. However, even if vanadium (V) is added excessively, the effect of improving toughness corresponding to the cost cannot be expected. Therefore, the content of vanadium (V) in the steel pipe 1 is preferably 1.5% by mass or less.

Boron (B) Boron (B) is an element that improves the hot workability of steel pipes and can effectively prevent cracks during hot rolling. However, if the steel pipe contains excessive boron (B), the hot workability is reduced. Therefore, the content of boron (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 of this embodiment includes a forming step of the steel strip 2 and a welding step of welding the ends of the steel strip 2 to each other. These steps are the same as the above-mentioned roll forming process and welding process. In other words, the steel pipe 1 can be manufactured by a manufacturing method that includes the following steps: a contacting step, which is a step of bending the steel plate or steel strip 2 and bringing the ends of the steel plate or steel strip 2 into contact with each other; a welding step, which In the state where the ends are pressed against each other, welding is performed at a welding temperature of 1200°C or more and 1700°C or less, so that the width of the joint 4 formed by melting the ends by heating becomes the steel plate or steel strip 2 The thickness of the plate is more than 0.2% and less than 1.2%.

＜Steel pipe for bearing＞ The steel pipe for bearing in this embodiment includes the steel pipe 1 of this embodiment mentioned above. In other words, the steel pipe 1 contains a specific amount of specific components and has a small content of impurities, so both the hardness and cleanliness are high, and it can be suitably applied to the bearing steel pipe.

[summary] A steel pipe according to an aspect of the present invention includes: the steel pipe is formed by welding the ends of a steel plate or a steel strip, and the steel plate or the steel strip includes: carbon (C): 0.4% by mass or more and 1.5% by mass % Or less; Phosphorus (P): 0.03% by mass or less; Copper (Cu): 0.3% by mass or less; the aforementioned ends are welded to each other at a welding temperature of 1200°C or more and 1700°C or less; wherein, the aforementioned ends are heated The width of the joint formed by melting the part is 0.2% or more and 1.2% or less of the thickness of the aforementioned steel plate or steel strip.

In the steel pipe according to one aspect of the present invention, the content of the carbon (C) in the steel pipe may be 0.6% by mass or more and 1.2% by mass or less.

The steel pipe according to one aspect of the present invention, wherein the aforementioned steel pipe may further satisfy at least one of the following conditions: silicon (Si): 2.0% by mass or less; manganese (Mn): 0.1% by mass or more and 2.0% by mass or less; sulfur (S) : 0.02% by mass or less; Aluminum (Al): 0.2% by mass or less.

In the steel pipe according to an aspect of the present invention, the aforementioned steel pipe may further satisfy at least one of the following conditions: chromium (Cr): 5.0 mass% or less; molybdenum (Mo): 0.5 mass% or less.

The steel pipe according to an aspect of the present invention may further satisfy at least one of the following conditions: titanium (Ti): 0.3 mass% or less; niobium (Nb): 0.5 mass% or less; vanadium (V): 1.5 mass% or less; boron (B): 0.01% by mass or less.

In the steel pipe of one aspect of the present invention, the aforementioned welding may be high frequency welding.

In the steel pipe according to one aspect of the present invention, the steel pipe system can be formed by heating the ends and pressing each other so that the amount of crushing is 20% or more and 30% or less with respect to the plate thickness.

The steel pipe according to one aspect of the present invention, wherein the corners of the end portions can be crushed, and the length of the crushed portion corresponding to the width direction of the steel plate or steel strip is 5% or more and 40% of the thickness of the steel plate. Below; Also, the angle formed by the smaller of the angle formed by the plane containing the top or bottom surface of the aforementioned steel plate or strip and the surface formed by the aforementioned crushing can be 20 degrees or more and 60 degrees or less .

In one aspect of the steel pipe of the present invention, the aforementioned steel pipe system may be a round pipe, a square pipe or a deformed pipe.

The steel pipe for a bearing according to one aspect of the present invention is characterized by the aforementioned steel pipe.

A method of manufacturing a steel pipe according to one aspect of the present invention includes: the manufacturing method of the steel pipe is a method of manufacturing a steel pipe in which the ends of a steel plate or a steel strip are welded, and the steel plate or steel strip includes: carbon (C): 0.4% by mass or more and 1.5% by mass or less; Phosphorus (P): 0.03% by mass or less; Copper (Cu): 0.3% by mass or less; The method for manufacturing the steel pipe includes: a contacting step, which is to bend the steel plate or strip , And bring the aforementioned ends into contact with each other; the welding step is to perform welding at a welding temperature of 1200°C or more and 1700°C or less with the aforementioned ends pressed against each other to melt the aforementioned ends by heating. The width of the joint is 0.2% or more and 1.2% or less of the thickness of the aforementioned steel plate or strip.

[Additional matters] The present invention is not limited to the above-mentioned embodiments, and various modifications can be made within the scope shown in the claims. Embodiments obtained by appropriately combining the technical means disclosed in the embodiments are also included in the technical scope of the present invention Inside.

[Example] <Examples and Comparative Examples> [Steel Manufacturing] First, steel with the composition shown in Table 1 was produced. In the remarks column, the steel pipe of one embodiment of the present invention is referred to as the "invention example", and the steel pipe of the comparative example is referred to as the "comparative example". In addition, in Table 1, the numerical values outside the numerical range included in the one embodiment of the present invention and the steel grades of the comparative examples are marked with underlines. In addition, the unit of the numerical value in Table 1 is mass %.

[Table 1] Steel grade C Si Mn P S Cr Cu T. Al Remarks A 1.49 0.09 1.02 0.006 0.008 1.42 0.15 0.006 Example of the invention B 1.01 1.92 0.60 0.018 0.001 0.92 0.19 0.003 Example of the invention C 1.11 2.05 0.74 0.006 0.008 1.21 0.26 0.002 Comparative example D 0.41 0.45 1.95 0.006 0.003 1.16 0.08 0.002 Example of the invention E 0.62 0.29 0.70 0.018 0.005 4.95 0.01 0.003 Example of the invention F 1.60 2.10 0.40 0.010 0.013 0.92 0.26 0.002 Comparative example G 1.00 0.01 1.75 0.003 0.018 0.69 0.13 0.014 Example of the invention H 1.55 0.02 0.50 0.005 0.014 0.14 0.15 0.009 Comparative example I 0.87 0.31 1.72 0.019 0.010 1.96 0.03 0.011 Example of the invention J 0.35 0.07 0.25 0.001 0.012 1.45 0.04 0.001 Comparative example K 0.82 0.24 1.74 0.002 0.017 0.62 0.18 0.011 Example of the invention L 0.69 0.35 0.79 0.004 0.006 1.00 0.17 0.008 Example of the invention M 0.72 0.33 1.20 0.009 0.011 0.58 0.16 0.007 Example of the invention N 1.09 0.47 1.26 0.015 0.002 1.56 0.23 0.011 Example of the invention O 1.05 0.72 1.32 0.009 0.005 1.32 0.13 0.015 Example of the invention P 1.02 0.57 1.48 0.007 0.004 1.42 0.20 0.008 Example of the invention Q 0.63 0.22 0.89 0.002 0.002 1.94 0.18 0.014 Example of the invention R 0.86 0.10 0.93 0.006 0.019 1.34 0.18 0.004 Example of the invention S 1.12 0.06 0.80 0.010 0.011 1.45 0.15 0.002 Example of the invention

[Manufacture of welded steel pipe] The various steel billets in Table 1 were heated to 1250 to 1300°C and hot rolled to produce hot rolled steel coils (steel strips) with a thickness of 6.0 mm. The hot-rolled steel coils obtained were pickled, and all steel types were annealed at 750°C for 10 hours. After that, the hot-rolled steel coil is slit in the longitudinal direction and roll-formed. After roll forming, under various welding temperature (heating temperature), crushing amount and end pressure, the end faces of the opposite hot-rolled steel coils are welded with each other at high frequency to produce steel pipes with a diameter of 34mm and a thickness of 6.0mm. Anneal the welded steel pipe. The annealing treatment system keeps soaking at 700°C and then performs air cooling.

[Evaluation of Steel Pipe] (Annealing test) When all the carbon in the steel precipitates 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 equation.

f = 15.3c (Equation 1) The precipitation amount of spherical carbides was evaluated as follows: After the steel pipe after welding was soaked and held at 700°C for 50 hours, the area ratio of spherical carbides (precipitation rate of spherical carbides) during air cooling was When 90% or more of f is regarded as ○, when less than 90% of f is regarded as ×.

The joint width of the joint width is measured at five points per 100 mm in the joint part, and the widest part is defined as the maximum joint width.

In order to obtain the area ratio of spherical carbides, a scanning electron microscope (SEM) was used to take pictures at a magnification of 5000 times for 20 positions of the maximum bonding width. By measuring the area of the spherical carbide of the joint in the photograph, and taking the average value of the measured value of the area of the spherical carbide, the area ratio of the spherical carbide is obtained.

(result) Table 2 shows the evaluation results of the amount of precipitation of spherical carbides in steel pipes manufactured under various welding conditions in each steel type. In Table 2, the steel grade of the comparative example is marked with the bottom line.

[Table 2] Test No. Steel grade Spherical carbide area ratio when all carbon of steel is precipitated% Heating temperature℃ Crushed amount mm End pressure mm Maximum bonding width μm The ratio of combined width to plate thickness% Spherical carbide precipitation rate% Evaluation of precipitation of spherical carbides Remarks 1 A 22.2 1600 1.0 1.6 60 1.0 92 ○ Example of the invention 2 1750 0.6 1.7 78 1.3 85 × Comparative example 3 B 15.5 1540 1.4 1.5 37 0.6 98 ○ Example of the invention 4 1630 0.0 1.4 112 1.9 56 × Comparative example 5 C 17.0 1340 1.8 1.6 36 0.6 80 × Comparative example 6 D 7.3 1340 1.7 1.5 20 0.3 91 ○ Example of the invention 7 1800 0.1 1.4 85 1.4 78 × Comparative example 8 E 9.5 1520 2.3 1.5 36 0.6 99 ○ Example of the invention 9 1720 2.8 1.7 77 1.3 76 × Comparative example 10 F 27.5 1540 0.1 1.3 40 0.7 82 × Comparative example 11 G 15.3 1290 1.4 1.4 40 0.7 93 ○ Example of the invention 12 1490 0.1 0.9 79 1.3 75 × Comparative example 13 H 27.4 1530 1.0 1.3 51 0.9 83 × Comparative example 14 I 13.3 1290 0.6 1.6 twenty two 0.4 94 ○ Example of the invention 15 1230 2.7 1 75 1.3 77 × Comparative example 16 J 5.4 1410 2.2 1.3 65 1.1 67 × Comparative example 17 1730 0.4 0.8 85 1.4 52 × Comparative example 18 K 12.5 1600 1.8 1.6 37 0.6 97 ○ Example of the invention 19 1850 1.5 1.7 76 1.3 70 × Comparative example 20 L 10.6 1580 1.4 1.7 40 0.7 98 ○ Example of the invention twenty one 1480 2.6 1.7 37 0.6 73 × Comparative example twenty two M 11.0 1410 1.2 1.3 62 1.0 95 ○ Example of the invention twenty three 1760 0.1 0.9 91 1.5 63 × Comparative example twenty four N 16.7 1290 0.8 1.7 59 1.0 94 ○ Example of the invention 25 1410 1.2 0.5 78 1.3 71 × Comparative example 26 O 16.1 1570 0.8 1.4 58 1.0 98 ○ Example of the invention 27 1800 2.1 1.3 87 1.5 74 × Comparative example 28 P 15.6 1540 2.1 1.3 33 0.6 91 ○ Example of the invention 29 1550 0.1 1.6 29 0.5 84 × Comparative example 30 Q 9.6 1570 2.0 1.4 35 0.6 91 ○ Example of the invention 31 1770 3.0 0.8 78 1.3 66 × Comparative example 32 R 13.2 1390 2.7 1.6 36 0.6 98 ○ Example of the invention 33 1620 2.2 0 260 4.3 57 × Comparative example 34 S 17.1 1520 1.2 1.6 26 0.4 96 ○ Example of the invention 35 1730 0.1 0.2 125 2.1 52 × Comparative example

In this way, in the example of the present invention, the area ratio of the spherical carbide is 90% or more of f under any conditions, and a good precipitation amount of the spherical carbide can be obtained. On the other hand, in the comparative example, a good precipitation amount of spherical carbides as in the example of the present invention could not be obtained. The reason for this is discussed 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 welded portion increases and the width of the joint portion becomes wider. In addition, "the width of the joint part becomes wider" means that the width of the joint part is wider than 1.2% of the 6 mm thickness of the steel strip, that is, 72 μm.

When the crushing amount is less than 0.3% of the steel strip thickness (No. 4, 7, 10, 12, 23, 29, 35), or when the crushing amount is greater than 2.4% of the steel strip thickness (No. 21), the temperature of the welding part is uneven during heating, and a wide part of the joint is formed in the joint.

When the end pressure is less than 1.2% of the above-mentioned plate thickness (No. 12, 15, 17, 23, 25, 31, 33, 35), the discharge amount of molten metal in the welded portion becomes smaller and the joint width becomes wider.

When the joint width is widened for the above-mentioned various reasons, the precipitation rate of spherical carbides is less than 90%. Therefore, the precipitation amount of spherical carbides was evaluated as ×.

(to sum up) As described above, in the examples of the present invention, the area ratios of spherical carbides are all 90% or more of f, and a good precipitation amount of spherical carbides can be obtained. In other words, through 50 hours of annealing treatment, a joint with good workability can be obtained. On the other hand, in the comparative example, the area ratio of the spherical carbides is all less than 90% of f, and sufficient precipitation of the spherical carbides cannot be obtained under the annealing treatment for 50 hours. In other words, we can know that in the steel pipe of the comparative example, in order to obtain a joint with good workability, it is necessary to perform an annealing treatment for a longer time.

1: Steel pipe 2: Steel belt 2A: bottom surface 2B: Surface formed by crushing 3: Welding department 4: Joint 5: Heat Affected Department C: crush width P1: A point away from a specific distance X in the width direction of the board P2: A point away from a specific distance Y in the width direction of the board W: width X, Y, Z: distance θ: crush angle

Fig. 1 is a schematic view showing a welded part of a steel pipe according to an embodiment of the present invention. Fig. 2 is a diagram showing a photograph of a welded part of a steel pipe according to an embodiment of the present invention. [Fig. 3] (a) to (d) are cross-sectional views each showing the cross-sectional shape of the steel pipe 1. Fig. 4 is a schematic diagram showing the crushing width and crushing angle of a steel pipe according to an embodiment of the present invention. [Fig. 5] (a) and (b) are schematic diagrams showing the upset amount of a steel pipe according to an embodiment of the present invention.

no.

Claims (11)

A kind of steel pipe, which contains: The aforementioned steel pipe is formed by welding the ends of a steel plate or a steel strip, and the aforementioned steel plate or steel strip includes: carbon (C): 0.4% by mass or more and 1.5% by mass or less; phosphorus (P): 0.03% by mass or less ; Copper (Cu): 0.3% by mass or less; The aforementioned ends are welded to each other at a welding temperature above 1200°C and below 1700°C; among them, The width of the joint formed by melting the end portion by heating is 0.2% or more and 1.2% or less of the thickness of the steel plate or steel strip. The steel pipe according to claim 1, wherein the content of the carbon (C) in the steel pipe is 0.6% by mass or more and 1.2% by mass or less. The steel pipe according to claim 1 or 2, wherein the aforementioned steel pipe also satisfies at least one of the following conditions: Silicon (Si): 2.0 mass% or less; manganese (Mn): 0.1 mass% or more and 2.0 mass% or less; sulfur (S): 0.02 mass% or less; aluminum (Al): 0.2 mass% or less. The steel pipe according to claim 3, wherein the aforementioned steel pipe also satisfies at least one of the following conditions: Chromium (Cr): 5.0% by mass or less; Molybdenum (Mo): 0.5% by mass or less. The steel pipe described in claim 4, wherein at least one of the following conditions is also met: Titanium (Ti): 0.3% by mass or less; Niobium (Nb): 0.5% by mass or less; Vanadium (V): 1.5% by mass or less; Boron (B): 0.01% by mass or less. The steel pipe according to any one of claims 1 to 5, wherein the aforementioned welding is high frequency welding. The steel pipe according to any one of claims 1 to 6, wherein the steel pipe is formed by heating the ends and pressing each other so that the amount of crushing is 20% or more and 30% or less relative to the thickness of the plate . The steel pipe according to any one of claims 1 to 7, wherein the corners of the aforementioned end portions are crushed, and the length of the crushed portion corresponding to the width direction of the steel plate or steel strip is the aforementioned plate thickness 5% or more and 40% or less; and, The smaller of the angle formed by the plane including the top or bottom 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 claims 1 to 8, wherein the steel pipe is a round pipe, a square pipe or a deformed pipe. A steel pipe for a bearing, which contains the steel pipe described in any one of Claims 1 to 9. A method for manufacturing steel pipes, which includes: The manufacturing method of the aforementioned steel pipe is a method of manufacturing a steel pipe by welding the ends of a steel plate or a steel strip, and the aforementioned steel plate or steel strip includes: carbon (C): 0.4% by mass or more and 1.5% by mass or less; phosphorus (P): 0.03% by mass or less; Copper (Cu): 0.3% by mass or less; The manufacturing method of the aforementioned steel pipe includes: The contacting step is to bend the aforementioned steel plate or steel strip and make the aforementioned ends contact each other; The welding step is to perform welding at a welding temperature of 1200°C or higher and 1700°C or lower with the ends pressed against each other, so that the width of the joint formed by melting the ends by heating becomes the aforementioned steel plate Or the thickness of the steel strip is more than 0.2% and less than 1.2%.

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