JP2007125588A - Seamless steel tube, and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a seamless steel tube having high durability, and a seamless steel tube manufacturing method capable of manufacturing the seamless steel tube with excellent production efficiency and high quality at low cost. <P>SOLUTION: The seamless steel tube 1 is the seamless steel tube 1 which is manufactured through a hot hydrostatic extrusion step (C) for manufacturing a seamless steel tube intermediate body 1a by performing the hot hydrostatic extrusion of at least a cylindrical steel billet 2, and the depth of a continuous flaw formed in an inner circumferential surface and an outer circumferential surface of the steel tube is ≤50 μm from each surface. The seamless steel tube manufacturing method includes a billet forming step (A), a first heating step (B), the hot hydrostatic extrusion step (C), a second heating step (D), a spreading step (F)/(G), a third heating step (H), and a pickling step (I). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a seamless steel pipe that uses a seamless steel pipe formed in a cylindrical shape as a material such as a coil spring, and a method for manufacturing the same.

In recent years, along with the wave of weight reduction of automobiles, panel materials have been made aluminum, chassis and main bodies have been made high tensile (high tensile strength), and the like. And the need for this weight reduction is further increased as global warming due to exhaust gas from automobiles and the like has been taken up internationally and reduction of CO ₂ contained in the exhaust gas has attracted attention as a major issue.

For example, in automobiles, means taken as a CO ₂ reduction method can be divided into development of a new power source such as a hybrid engine, a fuel cell motor system, and a battery motor system, or improvement of an existing engine.

  Means that are widely taken up in the improvement of the current engine include “improvement of combustion method” and “labor saving of power source by weight reduction”. In particular, in Europe, fuel regulations will be introduced in 2010, so automakers are working to reduce the weight of cars that have immediate effect by scraping the surface.

In order to meet such demands, in recent years, springs used in automobiles have been reduced in weight. For example, Patent Document 1 discloses a seamless steel pipe for a spring (the “seamless steel pipe” of the present invention) in which a material made of spring steel is heated to perform Mannesmann drilling, and then finish rolling a hollow shell obtained by mandrel mill rolling. Corresponding to the above-mentioned manufacturing method.
JP-A-1-247532

  However, when a seamless steel pipe is manufactured by Mannesmann drilling, the minimum rolling diameter is limited. When a seamless steel pipe of about 10 mm, for example, is to be manufactured from the hollow shell described above, a number of drawing processes and heating processes are performed. I had to.

In particular, when the material is spring steel, the hot deformation resistance is high, and the area reduction rate by one drawing is, for example, 20% (specifically, a 100 mm ² diameter pipe is 80 mm ² ). It must be made considerably smaller than mild steel. For this reason, the manufacturing method described in Patent Document 1 has a problem that the production efficiency is extremely poor.

  Further, since the decarburization and rolling pattern are generated by repeated hot working on the surface of the seamless steel pipe manufactured by the manufacturing method described in Patent Document 1, the outer peripheral surface and inner peripheral surface of the seamless steel pipe are peeled off. (Grinding) is forced. Grinding causes damage to the inner peripheral surface of the seamless steel pipe.

  Moreover, in the manufacturing method described in Patent Document 1, since a seamless steel pipe is manufactured by Mannesmann drilling, the tool may be damaged when a material having high hardness is used. Therefore, since only a material with low hardness can be used, there is a problem that only a seamless steel pipe with low durability can be manufactured.

  On the other hand, there is also a method for producing a seamless steel pipe by repeating heat treatment and pressing, but for example, producing a small diameter seamless steel pipe with a diameter of about 10 mm has a problem that processing efficiency is extremely poor and feasibility is poor. is there.

  Due to the above problems, it has been proposed in the past to manufacture seamless steel pipes in Patent Document 1 and the like, but in reality, it is not actually performed to manufacture a seamless steel pipe having a high hardness and a small diameter. is there.

  The present invention has been made in view of the above problems, and provides a highly durable seamless steel pipe, and the manufacture of a seamless steel pipe capable of producing such a seamless steel pipe with high production efficiency, high quality and low cost. It aims to provide a method.

  The seamless steel pipe according to the present invention that has solved the above problems is a seamless steel pipe that is formed from a billet of a cylindrical steel material through a hot isostatic pressing process, and is formed continuously on the inner and outer peripheral surfaces of the steel pipe. The depth of the ridge is 50 μm or less from each surface.

  Thus, since the depth of the continuous wrinkles formed on the outer peripheral surface and the inner peripheral surface at the time of extrusion is very small, such as 50 μm or less from the surface (inner peripheral surface and outer peripheral surface) of the steel pipe, it continues. Even if it is expanded and contracted, a seamless steel pipe can be realized that has few points that are likely to be the starting point of breakage or the like.

The seamless steel pipe of the present invention preferably contains non-metallic inclusions in the metal structure of the steel pipe, and the maximum thickness of the non-metallic inclusions in the direction orthogonal to the pipe axis is preferably 50 μm or less.
Thus, the seamless steel pipe of the present invention is a case where the maximum length of the non-metallic inclusions in the direction parallel to the pipe axis of the seamless steel pipe is very small, 50 μm or less, and is thus continuously extended and contracted. In addition, it is possible to further eliminate a portion that tends to be a starting point of destruction.
Note that the non-metallic inclusions in the present invention include, for example, it is assumed to refer to glassy inclusions such as _{CaO · SiO 2 · Al 2 O} 3 system.

In the seamless steel pipe of the present invention, the inner and outer peripheral surfaces of the steel pipe preferably have an average roughness Ra of 12.5 μm or less.
As described above, the seamless steel pipe of the present invention has an average roughness Ra of the inner and outer peripheral surfaces of 12.5 μm or less, which is very smooth. It is possible to further eliminate locations that are likely to be starting points.

In the seamless steel pipe of the present invention, the steel material is preferably spring steel.
Thus, the spring steel which was not able to be used as a steel material of a conventional seamless steel pipe can be made into a seamless steel pipe having high hardness by passing through a hot isostatic pressing process.

  A manufacturing method of a seamless steel pipe according to the present invention that has solved the above problems includes a billet forming process, a first heating process, a hot isostatic pressing process, a second heating process, an extension process, and a third process. A heating step and a pickling step are included.

As described above, in the seamless steel pipe manufacturing method of the present invention, the steel material is formed into a cylindrical billet in the billet forming step, the formed billet is heated in the first heating step, and the subsequent hot isostatic pressing is performed. A seamless steel pipe intermediate is produced by preparing for processing and performing hot isostatic extrusion in a hot isostatic extrusion process. At this time, since the hot isostatic pressing is performed, the seamless steel pipe intermediate can be manufactured with almost no continuous rod formed by sliding with the hot isostatic extruding device. Furthermore, in this manufacturing method, the seamless steel pipe intermediate is heated in the second heating step so as to be easily formed, and at least one of pilger mill rolling and drawing is performed on the seamless steel pipe intermediate in the extension step. Extend by that. Then, in order to prevent work hardening, the seamless steel pipe intermediate stretched by the third heating process is heated, and the seamless steel pipe intermediate stretched and heated in the pickling process is pickled (washed), thereby smoothing. A seamless steel pipe having an outer peripheral surface and an inner peripheral surface can be manufactured.
That is, for example, when manufacturing a relatively long and slender seamless steel pipe, since it is not necessary to perform Mannesmann drilling or mandrel mill rolling, the manufacturing cost is lower than in the past, and the manufacturing efficiency is high and the manufacturing cost is low. be able to. Moreover, since it manufactures by the hot isostatic pressing which can obtain a smooth outer peripheral surface and inner peripheral surface, a high-quality seamless steel pipe can be manufactured.

In the seamless steel pipe manufacturing method of the present invention, it is preferable to further include a straightening step of bending and straightening the pickled seamless steel pipe after the pickling step.
In this way, by performing bending correction, it is possible to prevent local abnormal stress from being generated when, for example, a coil spring or the like is manufactured using the seamless steel pipe. Product can be manufactured.

The seamless steel pipe manufacturing method of the present invention may further include a grinding step of grinding the inner peripheral surface of the heated seamless steel pipe intermediate after the second heating step.
Moreover, in the manufacturing method of the seamless steel pipe of this invention, it is good to grind the internal peripheral surface of the said seamless steel pipe after the said correction process.

  In this way, it is possible to produce a seamless steel pipe with excellent durability and high reliability by eliminating the portion that is not quenched by grinding the decarburized layer generated by the heat treatment and making the surface hardness uniform. it can.

In the method for producing a seamless steel pipe of the present invention, the hot isostatic pressing temperature is preferably 1050 ° C. or more and less than 1300 ° C.
In this way, by defining the hot isostatic pressing temperature within a specific range, the maximum size of non-metallic inclusions and the number per unit area can be appropriately controlled and have excellent durability. Seamless steel pipe can be manufactured.

In the seamless steel pipe manufacturing method of the present invention, instead of the second heating step, a slow cooling step of slowly cooling the seamless steel pipe intermediate subjected to hot isostatic pressing may be used.
Thus, instead of the second heating step, it is possible to spheroidize the carbide in the metal structure by gradually cooling the seamless steel pipe intermediate subjected to hot isostatic pressing. In addition to the excellent durability as described above, a seamless steel pipe with improved cold workability, grindability (machinability), toughness and the like can be produced.

Since the seamless steel pipe of the present invention has a smooth outer peripheral surface and inner peripheral surface by being manufactured by hot isostatic pressing, even if it is continuously expanded and contracted, it is the starting point for fracture, etc. As a result of the absence of easy-to-be-formed parts, durability such as fatigue life is excellent.
Moreover, the seamless steel pipe of this invention can aim at the weight reduction of about 30 to 40% compared with the solid steel material of the same diameter.

According to the seamless steel pipe manufacturing method of the present invention, since the seamless steel pipe is easily manufactured by hot isostatic pressing without drilling or the like, the number of steps can be reduced. As a result, production efficiency can be increased and it can be manufactured at low cost.
Moreover, since the manufacturing method of the seamless steel pipe of the present invention performs hot isostatic pressing, the outer peripheral surface and the inner peripheral surface are smooth, and a high-quality seamless steel pipe can be manufactured.
Furthermore, since the seamless steel pipe manufacturing method of the present invention does not perform Mannesmann drilling or mandrel mill rolling, the seamless steel pipe can be easily manufactured.

  Hereinafter, a seamless steel pipe and a manufacturing method thereof according to the present invention will be described in detail with reference to the drawings as appropriate.

[1. Seamless steel pipe]
First, with reference to FIG. 1, the seamless steel pipe which concerns on this invention is demonstrated in detail.
FIG. 1 is an enlarged cross-sectional view of a main part of a hot isostatic pressing apparatus used for producing a seamless steel pipe according to the present invention.

  As shown in FIG. 1, the seamless steel pipe according to the present invention is subjected to at least a hot isostatic pressing process for producing a seamless steel pipe intermediate 1a by hot isostatic extrusion of a cylindrical steel billet 2. Manufactured.

The seamless steel pipe 1 can have a very smooth surface (outer peripheral surface and inner peripheral surface) from the hot working stage by performing hot isostatic pressing in the hot isostatic pressing process.
This hot isostatic extrusion process uses a viscoplastic pressure medium 17 when extruding the seamless steel pipe intermediate 1a with the hot isostatic extrusion apparatus 10, and thus, for example, a large pressure around 1 GPa (10,000 atm). It is possible to greatly reduce the friction between the hot isostatic extrusion apparatus 10 and the seamless steel pipe intermediate 1a even when pressure is applied, and the seamless steel pipe 1 having no surface defects or rough surface and excellent surface properties. Can be manufactured.
In addition, as the pressure medium 17 used here, it is suitable to use the lubricant which added graphite to fats and oils.

The seamless steel pipe 1 has a depth of continuous wrinkles formed on its inner and outer peripheral surfaces of 50 μm or less from the surface.
In addition, the continuous rod means that the outer peripheral surface and the inner side of the seamless steel pipe 1 are moved from one end side to the other end side by contacting with a part of the hot isostatic pressing device when the seamless steel pipe 1 is manufactured. A sliding flaw formed continuously on the peripheral surface for a long time.

  Here, for example, when the coil spring is continuously expanded and contracted to cause breakage or the like, it is usually caused by surface abnormalities such as continuous flaws on the surface of the coil spring or breakage or cracks from non-metallic inclusions. Often.

Therefore, in the present invention, the depth of the continuous wrinkles is 50 μm or less, preferably 45 μm or less, more preferably 40 μm or less, even more preferably 38 μm or less, and even more preferably 35 μm or less from the outer peripheral surface and the inner peripheral surface. As a result, it is possible to eliminate almost all the points that are likely to be the starting point of destruction.
On the other hand, when the depth of the continuous wrinkles exceeds 50 μm from the outer peripheral surface and the inner peripheral surface, it is not preferable because the continuous wrinkles may be broken or cracked when continuously extended.

Moreover, when the hot isostatic pressing apparatus 10 is used, it is possible to reduce the surface roughness of the outer peripheral surface and the inner peripheral surface of the seamless steel pipe 1 such that the average roughness Ra is 12.5 μm or less. .
If the average roughness Ra exceeds 12.5 μm, the skin is so rough that many portions that are likely to start from breaking or cracking when continuously stretched are included, which is not preferable. In addition, it is so preferable that average roughness Ra is low. For example, the average roughness Ra is preferably 12 μm or less, more preferably 10 μm or less, still more preferably 8 μm or less, and even more preferably 4 μm or less, but there is no lower limit.
In addition, it is preferable to prescribe | regulate surface roughness Ra by measuring roughness in the pipe-axis direction in the internal peripheral surface and outer peripheral surface of a seamless steel pipe.

As shown in FIG. 1, the hot isostatic extrusion apparatus 10 includes a container 13 having a die 11 provided at a front portion thereof via a support member 12, a seal piston 14 disposed in the container 13, and the seal piston. 14 and a mandrel 16 that penetrates the seal piston 14 and the stem 15 and forms the inner diameter of the seamless steel pipe intermediate 1a.
In addition, this seamless steel pipe intermediate body 1a becomes the seamless steel pipe 1 through various processes performed later.

  This hot isostatic extrusion apparatus 10 arranges the billet 2 which has been softened by heating to a high temperature (for example, 1050 ° C. or more and less than 1300 ° C.) in advance in the container 13, and the billet is moved by the seal piston 14 via the pressure medium 17. By pressing 2, the billet 2 is discharged from between the die 11 and the mandrel 16 to produce the seamless steel pipe intermediate 1 a.

Specifically, for example, the billet 2 of a hollow steel material having an inner diameter of 50 to 60 mm and an outer diameter of 140 to 160 mm is hot isostatically extruded using the hot isostatic extrusion apparatus 10. For example, the seamless steel pipe 1 having an outer diameter of 30 to 60 mm and a thickness of 4 to 7 mm can be easily manufactured.
Needless to say, the inner diameter and outer diameter of the billet 2 and the outer diameter and thickness of the seamless steel pipe 1 can be changed as appropriate, and are not limited to the above ranges.

  Here, as a steel material used for manufacturing the seamless steel pipe 1 of the present invention, for example, so-called spring steel such as SiCr steel or higher strength SiCrV steel can be used. Specifically, JIS G 3560, JIS G 3561 and the like can be suitably used. More specifically, KHV12N, KHV10N, KHV6N, KHV7, CRV, HRS6, SRS600, UHS1900, UHS2000 manufactured by Kobe Steel, Ltd. Etc. can be used particularly preferably.

  As such a steel material, the composition thereof is C: 0.3 to 1.0 mass%, more preferably 0.5 to less than 0.7 mass%, Si: 0.1 to 3.0 mass%, and more. Preferably 1.0 to 3.0 mass%, Mn: 0.05 to 1.5 mass%, more preferably 0.5 to 1.5 mass%, Ni: 0 to 2.5 mass%, more preferably 0.05 to 0.5% by mass, Cr: 0 to 2.0% by mass, more preferably 0.05 to 1.5% by mass, Mo: 0 to 0.8% by mass, V: 0 to 0.8% Preferably containing 0.05% by mass, more preferably 0.05-0.3% by mass, Ti, Nb, Co, W: 0-0.5% by mass with the balance being Fe and inevitable impurities Can do.

(C: 0.3-1.0% by mass)
C is an element useful for increasing the tensile strength of a wire drawing material, that is, a seamless steel pipe (including a seamless steel pipe intermediate), and ensuring fatigue characteristics and sag resistance.
If the C content exceeds 1.0% by mass, the susceptibility to defects is increased, cracks from surface defects and non-metallic inclusions are generated, and the fatigue life is deteriorated.
However, if the C content is less than 0.3% by mass, not only the tensile strength required for a high stress spring can be secured, but the amount of proeutectoid ferrite that promotes fatigue cracking increases and the fatigue characteristics deteriorate. Let
Therefore, the C content in the present invention is defined as 0.3 to 1.0 mass%. In addition, as more preferable content, it is less than 0.5-0.7 mass%.

(Si: 0.1-3.0% by mass)
Si is an element that contributes to improving fatigue characteristics and sag resistance by increasing the tensile strength of seamless steel pipes by solid solution strengthening.
When the Si content is less than 0.1% by mass, the above-described effects cannot be obtained.
However, if the Si content exceeds 3.0% by mass, surface deoxidation, wrinkles, and the like increase, resulting in poor fatigue resistance.
Therefore, the Si content in the present invention is defined as 0.1 to 3.0% by mass. In addition, more preferable content is 1.0-3.0 mass%.
In addition, it is necessary to contain Si as much as the C content is lowered.

(Mn: 0.05 to 1.5% by mass)
Mn is an element that contributes to the improvement of fatigue properties by densifying and ordering the pearlite structure.
If the Mn content is less than 0.05% by mass, the effects described above cannot be obtained.
However, if the Mn content exceeds 1.5% by mass, a bainite structure is likely to be formed during hot isostatic pressing and the fatigue characteristics are deteriorated.
Therefore, the Mn content in the present invention is specified to be 0.05 to 1.5 mass%. In addition, more preferable content is 0.5-1.5 mass%.

(Ni: 0 to 2.5% by mass)
Ni may not be contained, but it is preferable to contain Ni because it has the effect of reducing notch sensitivity and increasing toughness. In addition, Ni has an effect of suppressing breakage trouble and improving the fatigue life when, for example, spring winding is performed.
In order to acquire such an effect, it is good to contain 0.05 mass% or more of Ni.
However, if the Ni content exceeds 2.5% by mass and becomes excessive, a bainite structure is likely to be formed at the time of hot isostatic pressing or the like, which is counterproductive.
Therefore, the content of Ni in the present invention is defined as 0 to 2.5% by mass. In addition, More preferably, it is 0.05-0.5 mass%.

(Cr: 0 to 2.0% by mass)
Cr may not be included, but the pearlite lamella spacing is reduced to increase the strength after hot isostatic pressing or heat treatment and improve sag resistance. Is preferred.
In order to acquire such an effect, it is good to contain 0.05 mass% or more of Cr.
However, if the Cr content exceeds 2.0% by mass, the patenting time becomes too long, and the toughness and ductility deteriorate.
Therefore, the Cr content in the present invention is defined as 0 to 2.0 mass%. In addition, More preferably, it is 0.05-1.5 mass%.

(Mo: 0 to 0.8 mass% or less)
Mo may not be contained, but may be contained because it contributes to increasing the strength of the spring steel by increasing the hardenability.
In order to acquire such an effect, it is good to contain 0.2 mass% or more of Mo.
However, if the Mo content is too large, the toughness is extremely deteriorated, so it is necessary to keep the content to 0.8% by mass or less.
Therefore, the Mo content in the present invention is specified to be 0 to 0.8% by mass or less.

(V: 0 to 0.8% by mass)
V can be excluded, but it is useful to improve the workability of the seamless steel pipe by making the pearlite nodule size fine, so it is preferable to contain V. Further, V is useful for improving the toughness and sag resistance of the spring material, for example, when the spring material is used.
In order to acquire such an effect, it is preferable to contain V 0.05 mass% or more.
If the V content exceeds 0.8% by mass and excessively contained, a bainite structure is likely to be formed during hot isostatic pressing, and the fatigue life is deteriorated.
Therefore, the content of V in the present invention is defined as 0 to 0.8 mass%. In addition, More preferably, it is 0.05-0.3 mass%.

(Inevitable impurities)
The basic component composition in the seamless steel pipe of the present invention is as described above, and the balance is substantially made of Fe, but does not hinder the characteristics of the seamless steel pipe using the steel material other than the various components described above. The seamless steel pipe containing a trace component is also included in the scope of the present invention. Examples of the trace component include impurities, particularly unavoidable impurities such as P, S, As, Sb, and Sn.

Thus, since the seamless steel pipe 1 of the present invention is manufactured using spring steel having high hardness, even if it is a hollow material, it is used for general structural rolled steel (JIS G 3101) or mechanical structure as in the conventional product. Hardness comparable to that of a solid material manufactured using carbon steel (JIS G 4051) or the like can be provided.
Moreover, despite using such a hard steel material, it can be manufactured into a seamless steel pipe intermediate by hot isostatic pressing, to produce a seamless steel pipe, that is, without being drilled, etc. Easy to manufacture and inexpensive to manufacture. It is also possible to provide further desired properties by performing appropriate cold working, heat treatment, etc. after hot isostatic pressing.
Therefore, the seamless steel pipe 1 of the present invention can be provided with a high hardness that could not be achieved conventionally, such as a Vickers hardness of Hv500 or higher. Needless to say, the seamless steel pipe 1 of the present invention can have a higher hardness than this, for example, a Vickers hardness of Hv550 or higher, a higher hardness of Hv600 or higher, and a higher hardness. Can be Hv650 or higher.

And the seamless steel pipe 1 of this invention contains a nonmetallic inclusion in metal structure, and the maximum thickness of the nonmetallic inclusion in the direction orthogonal to the tube axis of the said seamless steel pipe shall be 50 micrometers or less. Good. In addition, in a member that requires a life exceeding 10 million times (stretching operation), the non-metallic inclusions contained therein have a maximum thickness in the direction perpendicular to the tube axis of the seamless steel pipe of less than 30 μm, and More preferably, the number per 100 mm ² is less than 30.

If the maximum thickness of the nonmetallic inclusions in the direction orthogonal to the tube axis exceeds 50 μm, for example, in repeated fatigue parts, stress may concentrate and fatigue failure may occur. On the other hand, since it is preferable not to contain nonmetallic inclusions from the viewpoint of improving durability, there is no lower limit.
The maximum thickness of non-metallic inclusions and the number per 100 mm ² can be measured by a conventionally known means such as observing the cross section with a scanning electron microscope.

As described above, the seamless steel pipe 1 of the present invention has a depth of continuous wrinkles, a maximum length of nonmetallic inclusions contained in the metal structure, a maximum thickness, the number per unit area, a range of composition components, In addition, since the average roughness Ra between the inner peripheral surface and the outer peripheral surface is appropriately controlled, even if it is continuously expanded and contracted, there is no breakage and the durability is excellent. Moreover, the seamless steel pipe 1 of this invention can have hardness higher than the conventional seamless steel pipe.
Moreover, since it is a hollow material, 30-40% of weight reduction can be achieved compared with a solid material. This realizes a weight reduction substantially equal to that of a β-titanium alloy when used as a vehicle part.

[2. Manufacturing method of seamless steel pipe]
Next, the manufacturing method of the seamless steel pipe which concerns on this invention is demonstrated.
In addition, FIG. 2 is explanatory drawing explaining the flow of the preferable manufacturing method of the seamless steel pipe based on this invention.

As a method of manufacturing the seamless steel pipe 1 according to the present invention, as shown in FIG. 2, for example, a billet forming step (FIG. (A)), a first heating step (FIG. (B)), heat Interhydrostatic extrusion process (FIG. (C)), second heating process (FIG. (D)), extension process ((F) and / or (G)), and third heating process (Fig. (H)) and pickling step (Fig. (I)), and by performing the process contents of these steps, the seamless steel pipe 1 as a product can be manufactured. .
In addition, as shown in FIG. 2, the method for manufacturing a seamless steel pipe according to the present invention more preferably includes a bending straightening step (FIG. (J)) after the pickling step (FIG. (I)). More preferably, a grinding step (FIG. (E)) is included after the heating step (FIG. (D)).
Below, the content of each process of the manufacturing method of the seamless steel pipe which concerns on this invention of the most preferable aspect is demonstrated in order.

  In the billet forming step (A), the steel material 2a is formed into a cylindrical billet 2 (hollow billet). Such molding can be performed by forging or the like.

  And in a 1st heating process (B), this billet 2 is put into a heating furnace, and heat processing are performed at 1050 degreeC or more and less than 1300 degreeC, and it softens. As the heating furnace, an electric furnace, a gas furnace, or the like can be used. The heating time at this time is not particularly limited as long as the billet 2 can be sufficiently softened. In addition, the heating time can be appropriately changed in consideration of the size of the billet 2 and the like.

In the hot isostatic extrusion process (C), the billet 2 heated to 1050 ° C. or more and less than 1300 ° C. is subjected to hot isostatic extrusion using the hot isostatic extrusion device 10 described above, thereby seamless steel pipe intermediate The body 1a is manufactured.
Note that the extrusion temperature of the billet 2 in the hot isostatic extrusion step (C) is preferably the heating temperature of the first heating step (B), that is, 1050 ° C. or more and less than 1300 ° C. From the viewpoint of the number and size of inclusions in the steel that affect the durability of the spring, the extrusion temperature is most preferably between 1100 ° C and less than 1280 ° C. It is possible to obtain the merit that the damage to the jigs can be reduced. Therefore, the recommended range of the extrusion temperature should be set at 1050 ° C. or more and less than 1200 ° C. by slightly lowering the lower limit range. However, if more importance is attached to quality, it should be set at 1100 ° C. or more and less than 1200 ° C.

  The reason why the extrusion temperature is set from the viewpoint of the number and size (maximum thickness) per unit area of nonmetallic inclusions in the metal structure is as follows. When the extrusion temperature rises, the non-metallic inclusions are softened, and are deformed and divided by the extrusion, so that miniaturization proceeds. However, since crystallization and accompanying crystal growth proceed simultaneously, if the extrusion temperature is too high, there is a concern that the nonmetallic inclusions become coarse due to crystallization. On the other hand, the softening temperature of the non-metallic inclusion is 1100 ° C. or higher and lower than 1280 ° C. in the case of a steel material (for example, spring steel), and it is recommended to set an extrusion temperature equal to or higher than the softening temperature. Further, the deformation / splitting effect of the nonmetallic inclusions due to the deformation of the metal structure depends on the relative strength difference between the nonmetallic inclusions and the metal structure. Therefore, if the strength of the metal structure is lowered more than the nonmetallic inclusions softened due to the extrusion temperature being too high, the effect of refining is impaired simultaneously with the progress of crystallization. Therefore, it is preferable that the extrusion temperature be equal to or higher than the softening temperature of the nonmetallic inclusions and as low as possible.

  On the other hand, the life of the mold during hot isostatic pressing becomes shorter as the extrusion temperature becomes higher. Therefore, when considering the total cost, the extrusion temperature should be lower. Therefore, the extrusion temperature is set low by 50 ° C., and 1050 ° C. is set as the lower limit of the extrusion temperature. If the extrusion temperature is lower than this, the adverse effects of non-metallic inclusions cannot be ignored, and the pressing force becomes too large, causing problems such as poor quality and increased load on the equipment. .

  Moreover, the feature of hot isostatic pressing is that when the hot isostatic extrusion is performed in the specific temperature range described above, the friction between the steel material and the tool of the hot isostatic extrusion device 10 is normal extrusion. Since it is smaller than the apparatus, the surface state of the seamless steel pipe intermediate 1a can be made smooth. As a result, for example, the surface of the seamless steel pipe intermediate 1a is beautiful, the average roughness Ra can be 50 μm or less, and the crystal grains on the surface tend to be small. Therefore, when the seamless steel pipe 1 manufactured thereby is processed into a product called a coil spring, for example, since the stress is maximized at the outermost surface of the hollow spring, it has fine crystal grains and the surface. Low roughness and few surface defects (such as wrinkles and wrinkles) lead to longer spring life.

  In the second heating step (D), the seamless steel pipe intermediate 1a is heated in a heating furnace. The heating temperature at this time is preferably 650 to 750 ° C. The heating time is not particularly limited as long as the entire seamless steel pipe intermediate 1a is heated. For example, the heating time may be 0.1 to 1 hour. If the heating temperature is less than 650 ° C. or the heating time is less than 0.1 hour, the seamless steel pipe intermediate 1a is not sufficiently heated, so that the next process cannot be smoothly ground or extended. . On the other hand, when the heating temperature exceeds 750 ° C., the quenching temperature is reached, which is not preferable. Moreover, even if heating time exceeds 1 hour, the effect of a heating will be saturated and it is economically unpreferable.

  In a grinding process (E), the grinding process for removing the decarburization layer formed in the internal peripheral surface of the heated seamless steel pipe intermediate body 1a is performed. In addition, since a grinding dimension changes with the kind of steel materials to be used, a 1st heating process, and a 2nd heating process, it is preferable to perform an experiment etc. beforehand and to set conditions suitably.

In the extension process, the heated seamless steel pipe intermediate 1a is extended. The seamless steel pipe intermediate 1a can be extended by at least one of pilger mill rolling (F) and drawing (G).
For example, after performing moderate rolling by drawing (G), drawing (drawing) using the die 11 may be performed by pilger mill rolling (F).

  Pilger mill rolling (F) is a process in which the seamless steel pipe intermediate body 1a is pressed and flattened from both sides. By this, the seamless steel pipe intermediate body 1a is elongated, so that the drawing process in the next step can be reduced. Here, the pilger mill rolling (F) is not performed by simply pressing the seamless steel pipe intermediate 1a flatly from two directions, but for example, pressing uniformly from three directions, four directions or more. By rolling and rolling, it is possible to extend the seamless steel pipe intermediate 1a with higher roundness, and more actively save labor in the drawing process (G).

  In addition, if this pilger mill rolling (F) is performed excessively, for example, when it is drawn, a variation in thickness occurs in the circumferential direction of the seamless steel pipe intermediate 1a, and therefore it can be omitted. In the case where the drawing process (G) is repeatedly performed, a pilger mill rolling (F) rolling process can be used instead. However, even in this case, it is preferable that the roundness of the seamless steel pipe intermediate body 1a subjected to the surface-reducing process is finally ensured by performing a drawing process by a drawing process (G).

  Next, since the stretched (drawn) seamless steel pipe intermediate 1a is work-hardened, in the third heating step (H), the seamless steel pipe intermediate 1a is annealed by heating to 650-750 ° C. If the third heat treatment temperature is less than 650 ° C., the annealing is insufficient, which is not preferable. On the other hand, if the third heat treatment temperature exceeds 750 ° C., the effect of annealing is no longer saturated, which is economically undesirable.

In the pickling step (I), the annealed seamless steel pipe intermediate 1a is pickled to remove oils and fats (pressure medium 17) and scales adhering to the seamless steel pipe intermediate 1a.
Since the drawing process (G) can only perform a diameter reduction process of 25% or less per time, the above-described drawing process (G), the third heating process (H), and the pickling process (I) are performed a predetermined number of times. By repeating, it is preferable to manufacture the seamless steel pipe 1 having a desired diameter.

  In addition, it is more preferable to finally perform the bending correction in the bending correction step (J) to manufacture the seamless steel pipe intermediate 1a as a product.

In addition, in the manufacturing method of the seamless steel pipe of this invention, it may replace with the said 2nd heating process (D), and it is good also as a slow cooling process (not shown) which anneals the heated seamless steel pipe intermediate body 1a.
Normally, the seamless steel pipe intermediate 1a subjected to hot isostatic pressing and reduced in diameter is exposed to the atmosphere, and thus is rapidly cooled from the temperature of 1050 ° C. or more and less than 1300 ° C. Therefore, since the seamless steel pipe intermediate 1a is in a state where it has been quenched, the hardness becomes very high. In such a state, subsequent processes such as a grinding process (E) and an extension process (Pilger mill rolling (F) and / or drawing (G)) cannot be performed.

Therefore, after heating in the 1st heating process (C) and making the state of the metal structure into an austenite structure once, annealing is performed by gradually cooling the seamless steel pipe intermediate 1a in the slow cooling process (not shown). Is what you do.
As a result, the residual stress can be removed, the seamless steel pipe intermediate 1a can be softened, the machinability can be improved, and the cold workability can be improved. For example, the extension process (drawing process) can be performed without performing the second heating process. (G) and / or Pilger mill rolling (F)) can be performed. That is, there is an advantage in that the number of manufacturing steps can be simplified.

Slow cooling can be performed by using a heat retaining device capable of temperature control and slow cooling rate control so that the seamless steel pipe intermediate 1a subjected to hot isostatic pressing does not rapidly cool.
The slow cooling rate at this time is preferably 0.1 to 0.3 ° C./second. When the slow cooling rate is faster than 0.3 ° C./second, the annealing effect by slow cooling cannot be obtained. On the other hand, if the slow cooling rate is slower than 0.1 ° C./second, the time required for proceeding to the extension process becomes longer, and the production efficiency becomes worse.

  As described above, according to the seamless steel pipe manufacturing method of the present invention, a high-quality seamless steel pipe having a smooth outer peripheral surface and inner peripheral surface can be manufactured. Moreover, since this can be manufactured without performing Mannesmann drilling or mandrel mill rolling as in the prior art, labor to complete the product can be saved and simplified, and the manufacturing can be performed at low cost.

[3. Hollow spring]
The seamless steel pipe 1 of the present invention described above can be suitably used as a material for hollow springs such as compression coil hollow springs, tension coil hollow springs, and torsion hollow springs. In particular, as described above, if the seamless steel pipe 1 is manufactured with a steel material having a high hardness (for example, spring steel) and a hollow spring is produced using this, a compression coil hollow spring having a large spring constant can be obtained. .
More specifically, for example, a hollow valve spring, a hollow suspension spring, and the like can be given as practical examples. In this way, it is possible to reduce the weight by 30 to 40% as compared with a conventional solid valve spring or suspension spring.

The use of such a hollow valve spring has the following advantages. That is, in order to improve the engine performance, it is necessary to improve the output characteristics and the like by a method of improving the output by increasing the limit rotational speed of the engine or a method of improving the shape of the intake / exhaust port.
Here, as a factor that determines the engine speed limit, the movement of the valve train is a major limiting factor. The rotational movement of the crankshaft transmitted via the cam leads to the vertical movement of the valve, and the valve spring serves to press the valve that moves up and down and push it back. Therefore, the time when the vertical movement of the valve spring itself cannot follow the rotational movement of the cam is the engine limit speed. Therefore, in order to improve the performance of the engine, it is essential to increase the spring load or reduce the inertia weight of a reciprocating portion such as a valve including the spring. In order to satisfy these requirements, it is desirable to reduce the weight of the spring itself, reduce the size, and maintain and improve the spring load. In this way, it is possible to increase the engine limit rotational speed as the valve spring natural frequency increases due to weight reduction.
The fuel efficiency improvement effect by reducing the inertia weight of the valve spring is large, and generally a fuel efficiency improvement of about 0.5 to 1.0% can be achieved with respect to the 20% reduction of the spring weight, and a great effect is expected. .

  In addition, the use of a hollow suspension spring has the following advantages. In other words, a car's suspension spring has a great influence on driving performance such as ride comfort and cornering, and is a major factor in determining the merchandise of the car. This suspension performance largely depends on the basic structure of the automobile, the characteristics of the suspension itself including the spring, and the unsprung weight of the tire portion. Suspension springs used for suspension support high loads because they support the entire weight of the vehicle, and the weight is therefore increased. Therefore, if the weight of the same part can be reduced as in the case of the engine valve spring, the unsprung weight can be reduced. For example, the lightening effect of the unsprung load can be 10 times as much as the body lightening effect. It also leads to downsizing of the entire structure, improving not only the ride comfort but also the product power of adopting a new structure.

  In addition, current motorsport is positioned as a powerful tool to improve the brands of car companies, where building a major position is an important strategy for car companies. In particular, adopting the technology used in motor sports leads to a great added value for automobiles and enhances merchantability. At present, WRC (World Rally Championship) and JGTC (All Japan GT Championship; SUPER GT), which are the top motor sports brands, must use the same materials as mass-produced commercial vehicles for springs and other important parts. Etc. are obligatory. Therefore, a hollow spring manufactured using the seamless steel pipe 1 of the present invention whose function can be improved by changing the structure is very easy to understand in terms of functional added value and has a high commercial value.

Example 1
Next, the seamless steel pipe of the present invention and the manufacturing method thereof will be specifically described by comparing an example that satisfies the requirements of the present invention with a comparative example that does not satisfy the requirements of the present invention.

First, test No. using KHV10N (made by Kobe Steel) as a steel material. Cylindrical billets according to 1 to 9 were produced. These billets were prepared to have an outer diameter of 143 mm and an inner diameter of 52 mm. And these billets were heated at each temperature of 950-1400 degreeC as shown in Table 1. And the heated test No. The billet of 1-9 was hot isostatically extruded by the hot isostatic extrusion apparatus at each temperature (extrusion temperature), and the seamless steel pipe was manufactured. Thereafter, test no. The seamless steel pipes 1 to 9 were subjected to spheroidizing annealing treatment at 680 ° C. for 16 hours, followed by pilger mill rolling and drawing to extend to an outer diameter of 10.6 mm and an inner diameter of 5.9 mm. And test no. After the seamless steel pipes 1 to 9 were heated at 700 ° C. for 0.5 hours, they were pickled and tested. It was set as the seamless steel pipe of 1-9.
In addition, as a pressure medium at the time of hot isostatic extrusion, a pressure medium in which graphite was mixed with a lubricating oil containing molybdenum disulfide and a synthetic oil as a matrix was used.

Test no. For the seamless steel pipes 1 to 9, the maximum thickness (μm) of nonmetallic inclusions and the number per unit area (number / 100 mm ² ) were measured, and the extrudability and rough skin were evaluated.

The maximum thickness (μm) of non-metallic inclusions and the number per unit area (number / 100 mm ² ) were measured by observing the cross section with a scanning electron microscope.

  The evaluation of extrudability was evaluated as “possible” when the hot isostatic extrusion process was performed without problems, and the hot isostatic extrusion process was smoothed because the extrusion resistance was too strong. Those that could not be carried out were evaluated as “NG”.

  The evaluation of rough skin was conducted according to Test No. 1 to 9 seamless steel pipes were visually inspected. Those without rough skin were evaluated as “None”, and those with rough skin were evaluated as “Generated”.

  Table 1 shows heating temperature conditions, measurement results, and evaluation results.

As shown in Table 1, test no. When it was 1 (950 ° C.), the extrusion resistance was strong, and hot isostatic extrusion could not be performed (NG).
Test No. 2 (1000 ° C.), the maximum thickness of the non-metallic inclusions in the seamless steel pipe was as large as 52 μm, and the number was as large as 32 pieces / 100 mm ² .
On the other hand, test No. whose extrusion temperature is 1300 degreeC or more is shown. In Nos. 7 to 9, the maximum thickness of the non-metallic inclusions in the seamless steel pipe was as large as 58 μm or more, or the number of non-metallic inclusions was as large as 42 or more / 100 mm ² . Moreover, rough skin was also generated. Test No. 1, test no. 2 and test no. 7 to 9 were comparative examples.

On the other hand, test No. which is extrusion temperature 1050 degreeC or more and less than 1300 degreeC. In 3-6, the maximum thickness of the nonmetallic inclusions was as small as 38 μm or less, and the number was as small as 20 or less / 100 mm ² . Moreover, rough skin was not generated. Test No. 3 to 6 were examples.

Therefore, in order to improve the smoothness of the surface, it has been found that the extrusion temperature of the billet in the hot isostatic extrusion process described above should be set to 1050 ° C. or higher and lower than 1300 ° C. In particular, for example, in the case of a coil spring, the extrusion temperature is more preferably 1050 to 1250 ° C. from the viewpoint of the number and size of non-metallic inclusions in the metal structure that affect the durability of the coil spring. It was found that it is preferable to set to. Further, considering the damage of jigs and tools, it is advantageous to extrude on the low temperature side, and the extrusion temperature should be set at 1050 ° C. or more and less than 1200 ° C. If the quality is more important. It was found that the temperature is preferably set to 1100 ° C. or higher and lower than 1200 ° C.
By setting such an extrusion temperature, the surface of the seamless steel pipe can be smoothed.

<< Example 2 >>
Next, in << Example 2 >>, a coil spring was manufactured using a seamless steel pipe satisfying the requirements of the present invention, a seamless steel pipe not satisfying the requirements of the present invention, and a solid steel material, and a fatigue test was performed. In addition, as for these steel materials, all used the same thing as << Example 1 >>.
First, a seamless steel pipe was produced by hot isostatic extrusion at an extrusion temperature of 1150 ° C., and then a spring size having an outer diameter of 76.0 mm, an inner diameter of 54.8 mm, a height of 153.5 mm, and a number of turns of 6.76. Test No. 10 and 11 coil springs (both hollow) were produced.

In addition, Test No. 11 coil springs are formed by forming surface wrinkles (pseudo wrinkles corresponding to continuous wrinkles) on the outer peripheral surface and inner peripheral surface thereof.
Test No. The coil spring of No. 11 After producing a seamless steel pipe under the same conditions as No. 10, a surface flaw whose depth exceeds 50 μm is formed by applying a filer to the outer peripheral surface and sliding a rougher surface on the inner peripheral surface. The coil spring of the above-mentioned conditions was produced using the seamless steel pipe.

Test No. The 12 coil springs are formed by forming non-metallic inclusions whose maximum thickness in the direction orthogonal to the tube axis exceeds 50 μm in the metal structure.
Test No. The coil spring No. 12 was manufactured at a temperature of 1350 ° C. in hot isostatic pressing to produce a seamless steel pipe, and a coil spring (hollow product) having the above-described conditions was produced using the seamless steel pipe.

In addition, Test No. The coil spring 13 is a solid coil spring which is a conventional product.
Test No. The coil spring No. 13 has a test no. Use a solid billet of the same material as in Nos. 10 to 12, and do not use a mandrel. A solid steel material is produced by hot isostatic pressing under the same conditions as 10 to 12, and using this steel material, the outer diameter is 76.0 mm, the height is 153.5 mm, and the number of turns is 6.76. A coil spring (solid product) was prepared.

These test Nos. A fatigue test was conducted using 10 to 13 coil springs.
The fatigue test was performed by extending and contracting a predetermined number of times (400,000 times) in a single operation fatigue test.
In addition, Test No. The depth of surface defects, the size of non-metallic inclusions, and the surface roughness (average roughness Ra) in 10 to 13 coil springs were measured using a scanning electron microscope.
The results are shown in Table 2.

As shown in Table 2, test no. The ten coil springs satisfied the requirements of the present invention, so that even if the expansion and contraction operation was performed 400,000 times, no breakage or the like occurred and good results could be obtained.
That is, the hollow coil spring satisfying the requirements of the present invention was able to obtain the same results as the solid coil spring (Test No. 13).

  On the other hand, test no. Since Nos. 11 and 12 do not satisfy any of the requirements of the present invention, breakage occurred from the inner surface flaw or the inside by 18 to 200,000 expansion / contraction operations, and good results could not be obtained.

  As mentioned above, although the seamless steel pipe of the present invention and the manufacturing method thereof have been described in detail with reference to the best mode and examples, the contents of the present invention are not limited to the above description, and the present invention Should be construed according to the claims. Moreover, it cannot be overemphasized that it can change widely and modify | change within the range which does not deviate from the meaning of this invention.

  For example, hot glass lubrication extrusion may be performed by using glass powder instead of the pressure medium 17 used for hot isostatic pressing.

  Further, as a preferred embodiment using the seamless steel pipe of the present invention, a coil spring or the like has been described, but a substantially U-shaped hollow spring material such as a stabilizer, a hollow bar material such as a drive shaft, or an architecture It can also be used as a frame material for objects and bicycles.

It is a principal part expanded sectional view of the hot isostatic extrusion apparatus used in order to manufacture the seamless steel pipe which concerns on this invention. It is explanatory drawing explaining the flow of the preferable manufacturing method of the seamless steel pipe which concerns on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Seamless steel pipe 1a Seamless steel pipe intermediate body 2 Billet 10 Hot isostatic extrusion apparatus 11 Die 12 Support member 13 Container 14 Seal piston 15 Stem 16 Mandrel 17 Pressure medium

Claims (10)

A seamless steel pipe formed from a billet of cylindrical steel through a hot isostatic extrusion process,
A seamless steel pipe characterized in that the depth of continuous ridges formed on the inner and outer peripheral surfaces of the steel pipe is 50 μm or less from each surface. Containing non-metallic inclusions in the metal structure of the steel pipe,
The seamless steel pipe according to claim 1, wherein a maximum thickness of the non-metallic inclusions in a direction orthogonal to the pipe axis is 50 µm or less.   The seamless steel pipe according to claim 1 or 2, wherein the surface roughness of the inner peripheral surface and the outer peripheral surface of the steel pipe has an average roughness Ra of 12.5 µm or less.   The seamless steel pipe according to any one of claims 1 to 3, wherein the steel material is spring steel. A billet forming step of forming a steel material into a cylindrical billet;
A first heating step for heating the billet;
A hot isostatic extrusion process for producing a seamless steel pipe intermediate by hot isostatic extrusion of the heated billet;
A second heating step for heating the seamless steel pipe intermediate;
An extending step of extending the seamless steel pipe intermediate after heating by performing at least one of pilger mill rolling and drawing; and
A third heating step for heating the extended seamless steel pipe intermediate;
Pickling step of pickling the stretched and heated seamless steel pipe intermediate; and
The manufacturing method of the seamless steel pipe characterized by including.   The method for producing a seamless steel pipe according to claim 5, further comprising a straightening step of bending and straightening the pickled seamless steel pipe after the pickling step.   The method for manufacturing a seamless steel pipe according to claim 5 or 6, further comprising a grinding step of grinding an inner peripheral surface of the heated seamless steel pipe intermediate after the second heating step.   The method for manufacturing a seamless steel pipe according to claim 6 or 7, wherein an inner peripheral surface of the seamless steel pipe is ground after the straightening step.   The method for producing a seamless steel pipe according to any one of claims 5 to 8, wherein the hot isostatic pressing temperature is 1050 ° C or higher and lower than 1300 ° C.   10. The method according to claim 5, wherein, instead of the second heating step, a slow cooling step of gradually cooling the seamless steel pipe intermediate subjected to hot isostatic pressing is performed. The manufacturing method of the seamless steel pipe as described.

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