WO2007114413A1

WO2007114413A1 - High-strength seamless steel pipe for mechanical structure which has excellent toughness and weldability, and method for manufacture thereof

Info

Publication number: WO2007114413A1
Application number: PCT/JP2007/057360
Authority: WO
Inventors: Yasuhiro Shinohara; Tetsuo Ishizuka; Kazuhiro Inoue; Bunshi Kato; Teruhisa Takamoto; Junichi Okamoto
Original assignee: Nippon Steel Corporation
Priority date: 2006-03-28
Filing date: 2007-03-27
Publication date: 2007-10-11
Also published as: CN101410542A; CN101410542B; JP2007262468A; EP2028284A4; EP2028284A1; EP2028284B1; JP4751224B2

Abstract

Disclosed is a high-strength seamless steel pipe for use in a mechanical structure, which has the following chemical composition (by mass): C: not less than 0.03 and less than 0.1%; Mn: 0.8 to 2.5%; Ti: 0.005 to 0.035%; Nb: 0.003 to 0.04%; B: 0.0003 to 0.003%; Si: not more than 0.5%; Al: not more than 0.05%; P: not more than 0.015%; S: not more than 0.008%; and N: not more than 0.008%; and at least one component selected from the following components: Ni: 0.1 to 1.5%; Cr: 0.1 to 1.5%; Cu: 0.1 to 1.0%; and Mo: 0.05 to 0.5%; with the remainder being Fe and unavoidable impurities. In the steel pipe, the metallographic structure is a single structure comprising self-tempered martensite or a mixed structure comprising self-tempered martensite and lower bainite. The high-strength seamless steel pipe has excellent toughness and weldability.

Description

High-strength seamless steel pipe for machine structures with excellent toughness and weldability and its manufacturing method

Technical field

The present invention relates to a machine structural member, in particular, a seamless steel pipe suitable for a structural member such as a cylinder, a bush, and a boom, and a mechanical member such as a shaft, and a manufacturing method thereof.

Background art

Machine parts used in automobiles and industrial machines are given predetermined mechanical properties by tempering and heat treatment after steel bars are forged and cut into a predetermined shape. In recent years, for hollow-shaped parts, steel pipes having the required mechanical properties are used as raw materials, and there are increasing cases of reducing manufacturing costs by shortening the forging process and omitting the heat treatment process.

However, in general, steel pipes are more expensive than bar steels, and since seamless steel pipes are particularly expensive to manufacture, even if steel pipes are used as the material for hollow-shaped parts, the cost reduction effect may not be sufficient. .

Therefore, the provision of inexpensive steel pipes with reduced production costs has been studied, and so-called non-tempered steel pipes for machine parts and structural steel pipes have been proposed that omit the heat treatment after hot pipe making. (For example, Japanese Patent Application Laid-Open No. 5-2 0 2 4 47, Japanese Patent Application Laid-Open No. 1 0-1 3 0 7 8 3, Japanese Patent Application Laid-Open No. 1 0-2 0 4 5 7 1, — 3 2 4 9 4 6, Japanese Patent Laid-Open No. 1 1 1 3 6 0 1 7, Japanese Patent Laid-Open No. 2 0 0 4-2 9 2 8 5 7, Japanese Patent Laid-Open No. 2 0 0 1 — 2 4 7 9 3 1 publication). However, Japanese Patent Application Laid-Open Nos. 5-2 0 2 4 4 7, Japanese Patent Application Laid-Open Nos. 1 0-1 30 7 8 3, Japanese Patent Application Laid-Open Nos. 10-2 0 4 5 7 1, and Japanese Patent Application Laid-Open Nos. 10 0-3 2 The steel pipes described in Japanese Patent No. 4 9 4 6, Japanese Patent Laid-Open No. 1 1 1 3 6 0 1 7, and Japanese Patent Laid-Open No. 2 0 4-2 9 2 8 5 7 all have high C content, A large amount of nitride-forming elements are added to improve the hardenability and precipitation strengthening ability and to obtain a predetermined strength.

As a result, the alloy costs increase, and preheating to prevent cracking during welding is necessary, which impairs productivity.

In the method described in Japanese Patent Laid-Open No. 2 0 0 1 — 2 4 7 9 3 1, the hot rolling is performed at a considerably low temperature of 60 to 75 ° C., thereby refining the metal structure and increasing the strength. Is to improve. Although low-temperature rolling is a common technology for thick plate rolling, there are problems such as easy burning due to contact with tools when seamless rolling is performed at low temperatures. The scope of application is greatly limited.

In addition, techniques have been proposed for improving the strength of a seamless steel pipe by hot cooling after hot working (for example, Japanese Patent No. 3 5 0 3 2 1 1 and Japanese Patent Laid-Open No. 7-4 1 8 5 No. 6 publication). In the method described in Japanese Patent No. 3 5 0 3 2 1 1, the inner surface of the steel pipe after final finish rolling is allowed to cool, and the outer surface is heated from the temperature of Ar3 point or higher to 500 to 400 ° C. It is cooled at 10 to 60 ° C / s and then allowed to cool. The method described in Japanese Patent Application Laid-Open No. 7-4 1 8 5 6 is a method of directly quenching or accelerating cooling while hot rolling.

However, these are oil well pipes, and since tempering is performed after accelerated cooling, the production cost is high, and it is not necessary to consider weldability. Therefore, these contain 0.1% or more of carbon. On the other hand, among steel pipes for machine structures, steel pipes used for cylinders and bushes have toughness and melting properties. In many cases, contact is required, and it is preferable to limit the carbon content to less than 0.1%. Disclosure of the invention

The present invention has been made in view of the current situation as described above, and is particularly suitable for structural members such as cylinders, bushes, booms, and mechanical members such as shafts, and has high strength, high toughness, and The present invention provides a seamless steel pipe for machine structural members that requires weldability, and provides a method for producing the steel pipe at low cost without tempering. The present inventors have found that an optimum structure capable of achieving both high strength and high toughness across the entire plate thickness direction even in an environment where the cooling rate differs between the outer surface and the inner surface due to accelerated cooling only from the outer surface. We examined the combination of chemical components that generate water and the cooling rate and stop temperature of accelerated cooling.

The present invention has been made based on the knowledge obtained by such studies, and the gist thereof is as follows.

(1) By mass%, C: 0.03 to less than 0.1%, η η: 0 · 8 to 2.5%, T i: 0.0 0 5 to 0.0 3 5%, N b : 0. 0 0 3 to 0.0 4%, B: 0. 0 0 0 3 to 0, 0 0 3%, S i: 0.5% or less, A 1: 0. 0 5% or less , P: 0.0 1 5% or less, S: 0.0 0 8% or less, N: 0.0 0 8% or less, N i: 0.1 to 1.5%, C r : 0.1 to 1.5%, Cu: 0.1 to 1.0%, Mo: 0.05 to 0.5%, 1 type or 2 or more types, balance Fe and inevitable For mechanical structures with excellent toughness and weldability, characterized by the fact that the metal structure is a single structure of self-tempered martensite or a mixed structure of self-tempered martensite and lower veinite. High strength seamless steel pipe.

(2) In the metal structure, a large-angle grain boundary with an orientation difference of 15 ° or more. The high-strength seam-less steel pipe for machine structures having excellent toughness and weldability as described in (1) above, wherein the average diameter of the region surrounded by 3 is 30 m or less.

(3) In the metal structure, the average particle size of cementite is 40 O nm or less, and the density is 2 XI 0 ⁵ particles / mm ² or more, (1) or (2) High-strength seamless steel pipe for machine structures with excellent toughness and weldability as described in 1.

(4) A method for producing a high strength steel pipe for mechanical structure having excellent toughness and weldability as described in any one of (1) to (3) above, which has a chemical composition as described in (1) above. A steel slab made from the above is pierced and rolled hot, and is formed by a drawing process. The obtained steel pipe is heated from 75 ° C or higher to a temperature T [ Toughness and weldability, characterized by accelerated cooling from the outer surface of the steel pipe and air cooling while rotating in the circumferential direction at a cooling rate V [° CZ s] of 5 to 50 ° CZ s For producing high-strength steel pipes for machine structures with excellent resistance.

1 5 0 <T <8 2 1. 3 4 XV-. . ^{3) 1 2} ... (1) Best mode for carrying out the invention

The inventors first examined the metal structure and cementite morphology of the seamless steel pipe for machine structures manufactured by the conventional quenching and tempering process, and the effects on strength and toughness, and obtained the following knowledge. When steel pipes are manufactured by quenching and tempering treatment, cementite precipitates in the matrix during quenching, and residual austenite 卜 decomposes into cementite and ferrite 際 during tempering. The resulting tempered martensite cocoon structure has an average particle size of cementite cocoons of 50 O nm or more, and a balance between strength and toughness (hereinafter referred to as strength-toughness balance). Also called. )

Next, the present inventors assume a process of producing a seamless steel pipe by accelerated cooling without performing tempering treatment, and a metal structure that improves both strength and toughness, and production for obtaining the same. The conditions were examined.

As a result, by optimizing the steel microstructure and the conditions of the steel composition and accelerated cooling, a structure in which cementite is finely precipitated in the grains, especially self-tempered martensite or lower bainite, It was found that the strength-toughness balance was improved by using self-tempered martensite.

Furthermore, as a result of investigating the relationship between cementite morphology, strength and toughness in detail, it is extremely good that the average particle size is 40 O nm or less and the density is 2 X 10 ⁵ pieces / mm ² or more. It was found that a strength-toughness balance can be obtained.

It was also found that particle size control is important from the viewpoint of balance between strength and toughness. In the present invention, a crystal orientation map of steel having a single structure of self-tempered martensite or a mixed structure of self-tempered martensite and a lower bainite is created by Electron Back Scattering Pattern (EBSP). The relationship between strength and toughness was investigated.

As a result, the average grain size of the region surrounded by the large-angle grain boundaries with a misorientation of 15 ° or more (hereinafter also referred to as the large-angle grain boundary average diameter) is 30.

It was found that the strength-toughness balance was improved in the following cases.

Hereinafter, the present invention will be described in detail.

The reason why the chemical composition of the steel pipe is limited in the present invention will be described. Note that “%” shown below means “% by mass” unless otherwise specified. c: c is an extremely effective element for improving the strength, and at least 0.03% is required to obtain the target strength. However, when 0.1% or more of C is contained, the low temperature toughness is remarkably lowered, and cracking during welding becomes a problem. Therefore, C is limited to 0.03 to less than 0.1%.

M n: M n is an essential element for improving the balance between strength and low temperature toughness, and its lower limit is 0.8%. However, if it exceeds 2.5%, the low temperature toughness deteriorates rather, so 2.5% was made the upper limit.

T i: T i forms fine T i N and not only refines the structure, but also increases hardenability and contributes to toughness. This effect is small at less than 0.05%, so the lower limit was set to 0.05%. However, if it exceeds 0.035%, coarse TiN and TiC precipitate and the low-temperature toughness deteriorates remarkably, so the upper limit was made 0.035%.

Nb: Nb not only suppresses recrystallization of austenite during rolling and refines the structure, but also increases hardenability and strengthens the steel. Since this effect is small at less than 0.03%, the lower limit is set to 0.03%. However, if it exceeds 0.04%, the toughness deteriorates due to the formation of coarse Nb precipitates, so the upper limit was made 0.04%.

B: B is an element that increases hardenability and strengthens, and the lower limit for obtaining the effect is 0.0 0 0 3%. On the other hand, if the content is more than 0.0 33%, the hardenability is lowered, and some of the ferrite is generated and the target strength cannot be satisfied. Therefore, the upper limit is set to 0.0 33%.

The upper limits of deoxidizing elements Si and A 1 and impurities P, S, and N are limited as follows.

S i: S i is a deoxidizing element, but if added in excess, the low temperature toughness is impaired, so the upper limit was made 0.5%. Add A 1 as deoxidizing element In this case, it is not necessary to add S i, and the lower limit may be 0%.

A 1: A 1 is a deoxidizing element, but if added excessively, coarse A 1 oxides are formed, resulting in low temperature toughness and poor weldability. Therefore, the upper limit is set to 0.05%. did. When Si is added as a deoxidizing element, it is not necessary to add A 1, and the lower limit may be 0%.

P: P is an impurity and lowers toughness, so the upper limit was made 0.015%. From the viewpoint of securing toughness, the P content is preferably 0.0 1% or less.

S: S is an impurity and lowers the toughness, so the upper limit was made 0.08%. From the viewpoint of securing toughness, the S content is preferably not more than 0.005%.

N: N is an impurity, and if it exceeds 0.08%, the toughness is impaired by the formation of coarse TiN and the formation of upper veinite, so the upper limit was made 0.08%. Note that N may contain 0.01% or more because it forms fine nitrides such as TiN and contributes to the refinement of the structure.

Furthermore, one or more of Ni, Cr, Cu, and Mo may be added.

Ni: Ni is an element that improves the strength, and is added at 0.1% or more. However, if it exceeds 1.5%, the prayer may be uneven and the structure may become uneven, and the toughness may deteriorate, so the upper limit is set to 1.5%.

C r: C r is an element that improves the strength, and is added at 0.1% or more. However, if it exceeds 1.5%, the toughness may deteriorate due to the formation of Cr precipitates, so the upper limit is set to 1.5%.

Cu: Cu is an element that improves the strength, and is added at 0.1% or more. However, if it exceeds 1.0%, the upper bainite may be generated, and the toughness may be impaired, and the weldability may be deteriorated. Therefore, the upper limit is set to 1.0%.

Mo: Mo is an element that contributes to increasing the strength. To obtain the effect of improving hardenability, 0.05% or more is added. However, if it exceeds 0.5%, weldability may be impaired, so the upper limit is made 0.5%. Next, the metal structure will be described. The metal structure of the steel of the present invention is a single structure of self-tempered martensite or a mixed structure of self-tempered martensite and lower bainite. Self-tempered martensite and lower bainite are structures obtained by accelerated cooling, and these structures can improve the balance between strength and toughness without tempering.

When the steel pipe is accelerated and cooled from the outer surface, the cooling speed is slower on the inner surface than on the outer surface, so that a high-temperature transformation phase such as bainite is easily generated. In addition, when the plate is thick, the steel pipe may be deformed if it is cooled so that the cooling speed of the inner surface is increased. Therefore, it is necessary to control the cooling speed to such an extent that the steel pipe does not deform.

In such a case, the bainitic transformation may occur on the inner surface of the steel pipe, but if it is the lower one, there is no particular problem because the balance of strength and toughness can be secured. However, in steel pipes for machine structures, in order to make the entire surface in the plate thickness direction lower, it is necessary to add a large amount of Mo, which may impair the economy.

Therefore, the steel microstructure must be a single structure of self-tempered martensite or a mixed structure of self-tempered martensite and lower bainite.

In the present invention, the self-tempered martensite is a structure in which the austenite phase has undergone martensite transformation during accelerated cooling, and is cooled after the accelerated cooling is stopped, and fine cementite precipitates in the lath. is there. The structure obtained by normal tempering is tempered martensi. Compared to this, the cementite of self-tempered martensite is extremely fine.

In the present invention, the lower veinite is defined as a structure in which a lath-shaped ferrite is generated during accelerated cooling and fine cementite is precipitated in the lath. Self-tempered martensite and lower bainite are common in that there is no coarse cementite at the grain boundaries and fine cementite in the matrix.

The self-tempered martensite and the lower vein are both lath-shaped, but they can be distinguished by the precipitation form of cementite in the lath. In other words, the self-tempering martensite has multiple long axis directions of cementite, and the lower bainite has one long axis direction of cementite.

Differences between the self-tempered martensite and lower veinite defined in the present invention and other structures will be described.

The upper veinite is a structure formed by a mixed structure of acicular cementite and martensite austenite at the lath boundary. Ferrite is not a lath like bainite but a lump. Pearlite is a plate-like cementite precipitated at grain boundaries.

The self-tempered martensite and the lower venite are 20 0 0 0 0 times using a scanning electron microscope (Scanning Electron Microscope, SEM) Can be determined by observing at a magnification of 5 0 0 0 0 0. For the sample, the observation surface should be mirror-polished and etched with nital.

In addition, the average grain size of the region surrounded by large-angle grain boundaries with an orientation difference of 15 ° or more affects the propagation of cracks when fracture occurs. Since the toughness decreases when the large-angle grain boundary average diameter is 30 m or more, the large-angle grain boundary average diameter is preferably 30 m or less from the viewpoint of strength-toughness balance. The smaller the large-angle grain boundary average diameter, the better the balance between strength and toughness.

With the current technology, it is difficult to make it 3 or less. The large-angle grain boundary average diameter can be obtained from the crystal orientation map measured by E B S P.

The average particle size of cementite is preferably 400 nm or less. This is because when the average particle size of cementite exceeds 400 nm, toughness decreases. Although the average particle size of cementite is preferably smaller, cementite finer than 30 nm is difficult to discriminate with SEM. Therefore, in the present invention, cementite particles with a particle size of 30 nm or more are difficult to distinguish. The upper limit of the average particle size of it is specified at 400 nm.

Further, if the density of cementite is 2 × 10 ⁵ pieces Z mm ² or more, there is almost no generation of coarse cementite, and a very good balance of strength and toughness can be obtained. The upper limit of the density of cementite candy is not particularly limited, but is inevitably determined by the amount of C added and the average particle size.

Next, a manufacturing method will be described. In the present invention, the conditions for accelerated cooling of a steel pipe having the above chemical components from a temperature of 75 ° C. or higher are important. The cooling rate is for the inner surface of the steel pipe.

The stop temperature of accelerated cooling is one of the basic technologies of the present invention. This is because it has a great influence on the precipitation behavior of cementite in the matrix, which is most effective in improving the balance of strength and toughness.

Accelerated cooling is performed at the cooling rate V [° C / s] described above, martensitic transformation is performed, and acceleration cooling is stopped at a temperature T [° C] shown by the following formula (1). Evening light precipitates in the matrix and self-tempered martensite can be obtained.

On the other hand, when the stop temperature of accelerated cooling is below 150 ° C, no cementite precipitates and no self-tempered martensite is obtained in the subsequent air cooling. Therefore, the lower limit was set to less than 1550 ° C. 1 5 0 <T <8 2 1. 3 4 χ v- ⁰ ^{3 1 1 2} (1)

Next, the range of the cooling rate in the accelerated cooling of the steel pipe will be described. If the cooling rate is less than 5 ° C // s, upper veins and ferrites are generated.On the other hand, if it exceeds 50 ° C / s, uniform cooling becomes difficult, and after cooling, the steel pipe deforms greatly. . Therefore, the accelerated cooling rate was limited to 5 to 50 ° C / s. Note that when the cooling rate is within the above range, it is easy to generate lower veins when the cooling rate is slow.

The reason why the temperature at which the accelerated cooling of the steel pipe is started is limited to 7500 ° C or more is that the metal structure at the start of the accelerated cooling is an austenite single phase. If the temperature of the steel pipe at the time of starting the accelerated cooling is too high, the austenite grains become coarse and the toughness may be lowered. Therefore, the accelerated cooling start temperature is preferably 9500 ° C. or lower.

The cooling start temperature and cooling stop temperature of the inner surface of the steel pipe can be measured before and after accelerated cooling, that is, by measuring the temperature of the inner surface of the steel pipe with a contact thermometer on the inlet side and outlet side of the cooling device. The cooling rate can be calculated from the passing speed of. The temperature of the outer surface of the steel pipe may be measured with a radiation thermometer, and the temperature of the inner surface of the steel pipe may be obtained by heat conduction calculation.

In addition, thermocouples are attached to the inner and outer surfaces of steel pipes having various outer diameters and wall thicknesses, and cooling curves corresponding to various heating temperatures, refrigerant ejection conditions, and cooling times are created. Can also be determined.

The steel pipe of the present invention is a seamless steel pipe, and the pipe making process is generally hot piercing, rolling, and drawing, but after cold piercing by machining, it is heated. It may be produced by a hot extrusion press. Further, it may be reduced in diameter.

Once the steel pipe manufacturing process is completed, the steel pipe may be heated by a heating furnace or induction heating, and the steel slab is hot pierced, rolled, and stretched. If the temperature of the steel pipe is 7500 ° C or higher after it is piped, it can be accelerated and cooled inline as it is.

The method of accelerated cooling was limited to the method of cooling only from the outer surface while rotating the steel pipe in the circumferential direction. Thereby, it is possible to cool uniformly in the circumferential direction and the longitudinal direction.

On the other hand, if the steel pipe is not rotated, there is a problem that the lower surface of the steel pipe is excessively cooled, and when cooling from the inner surface side, water is accumulated on the lower surface and the cooling rate is not uniform.

The cooling method can be selected arbitrarily, such as a method in which water is applied directly to the outer surface of the steel pipe, a method in which water is applied in the tangential direction of the outer circumference of the steel pipe, or mist cooling. In the present invention, the applicable steel pipe shape is preferably a shape whose length is at least five times the outer diameter. This is because, when the length is less than 5 times the outer diameter, when accelerated cooling from the outer surface is performed by water cooling, the water also penetrates into the inner surface of the steel pipe, resulting in uneven cooling and bending of the steel pipe. Because there is. In order to ensure uniform and accelerated cooling, the length of the steel pipe is preferably 10 times or more of the outer diameter.

(Example)

Steel with the chemical composition shown in Table 1 was melted, and a bloom with a diameter of 1700 mm was produced by a continuous forging process of the converter. In Table 1, a blank means that the analytical value of the component was below the detection limit.

These blooms were heated to 1240 ° C, pierced and rolled by the Mannesmann-plug mill method, then reheated to 9500 ° C, reduced in diameter, directly fed, and cooled by ring cooling. The water cooled.

For some steel pipes, seamless steel pipes with reduced diameter were cooled to room temperature, reheated to 9500C, and then water cooled from the outside by ring cooling. Steel pipe size after diameter reduction is 3 sizes, outer diameter 1 26 mm, wall thickness 1 2.2 mm (small), outer diameter 1 3 8 mm, wall thickness 16.4 mm (medium), and The outer diameter was 14 6 mm and the wall thickness was 2 0.6 mm (large). Both lengths were 6.5 m.

Specimens are taken from any position in the circumferential direction, longitudinal direction, and thickness direction of the manufactured steel pipe, embedded in resin, mirror-polished, etched, and the maximum magnification is 50 by SEM. The structure was observed as a 100-fold magnification, and the structure was classified into self-tempered martensite (M), lower bainite (LB), upper bainite (UB), and ferrite (F). In addition, image analysis was performed using 10 SEM micrographs with a magnification of 100 000 to 500 000, and the average value of the equivalent circle radius and unit area (mm ²⁾ ) The number of hits was obtained.

Furthermore, the metal structure was observed with an optical microscope, and Vickers hardness was measured at 10 kgf according to JISZ2244. In addition, the surface of the sample embedded in the resin was electropolished and the crystal orientation was measured using EBSP mounted on the SEM to identify grain boundaries with an orientation difference of 15 ° or more. The average value of the circle-equivalent radii in the enclosed area was determined by image analysis, and is shown in the column for the average diameter of large-angle grain boundaries in Table 2.

The tensile test was conducted according to JISZ 2 2 4 1 using No. 11 test piece of JISZ 2 2 0 1, and the yield strength and tensile strength were measured. The toughness was evaluated by performing a Charpy impact test in accordance with JISZ 2 2 4 2 using a 2 mmV notch full-size test piece at 40 ° C and measuring the absorbed energy (v E_ _{4 Q} [J ]).

Weldability is achieved by welding steel pipes to each other at room temperature using a welding wire having a strength of 7800 MPa class, carbon dioxide gas welding, 2 After 4 hours, the presence of cracks was inspected by visual inspection, and those without cracks were accepted.

After accelerated cooling, the shape (bending) of the steel pipe was measured at room temperature. A steel pipe with a length of 6.5 m is placed in contact with the flat plate at three ends, one end, a distance from the end of 0.5 m, and a distance of 1 m. The maximum amount of float at the opposite end of the steel pipe was measured while rotating the steel pipe.

The maximum float at the end of the steel pipe is the height from the bottom flat plate at the end of the steel pipe that has been lifted. Steel pipes with a maximum float of 10 mm or less at the end of the steel pipe were accepted as steel pipe shapes.

table 1

The results are shown in Table 2. The underline in Table 2 means outside the scope of the present invention or outside the preferred range. Nos. 1 to 13 as examples of the present invention were steel pipes manufactured under appropriate accelerated cooling conditions, and had an appropriate metal structure and strength and toughness required as a steel pipe for mechanical structures.

No. 14 is an example in which the amount of C, B, and Ni was high, and the accelerated cooling stop temperature was high, so that the upper vane microstructure was formed and the toughness was lowered and the weldability was also lowered. .

N o. 15 has a low C content, insufficient hardenability, and a high acceleration / cooling stop temperature, so that part of the upper vane structure is formed and the toughness is reduced. This is an example where the shape deteriorates due to the high cooling rate.

No. 16 is an example in which the amount of P is too high and the accelerated cooling start temperature is low, so that ferriting occurs and the toughness is lowered.

No. 17 is an example in which the Si content was too high, resulting in the formation of upper veinite and poor toughness. No. 18 is an example in which the N content, Cu content, and S content are too high, and the cooling rate is slow, so that the upper vane is formed and the toughness is impaired and the weldability is also reduced. .

No. 19 is too high in A1 and Nb, and the accelerated cooling stop temperature is high, so the upper bainite is generated and the toughness is impaired, and because A 1 is excessively contained, the weldability is also low. This is a bad example.

No. 20 is an example where the cooling rate was too slow, and No. 2 1 and 25 were examples where the upper cooling was generated and the toughness was reduced because the accelerated cooling stop temperature was too high. .

No. 2 2 and 2 4 are examples in which the cooling rate was too high and the accelerated cooling stop temperature was high, resulting in a mixed structure of tempered martensite and upper bainite, low toughness, and poor shape. is there.

N o. 2 3 has an accelerated cooling start temperature that is too low to produce Ferai This is an example of poor toughness.

Table 2

Industrial applicability

According to the present invention, high-strength seamless steel pipes for machine structures excellent in toughness and weldability, suitable for machine structural members, in particular, structural members such as cylinders, bushes, booms, and mechanical members such as shafts, And it becomes possible to provide the method of manufacturing this steel pipe cheaply. Therefore, the industrial contribution of the present invention is extremely remarkable.

Claims

The scope of the claims

1-% by mass, C: 0.0 3 to less than 0.1%, M n: 0.8 to 2.5%, T i: 0. 0 0 5 to 0.0 3 5%, N b: 0. 0 0 3 to 0.0. 4%, B: 0. 0 0 0 3 to 0.0. 0 3%, S i: 0.5% or less, A1: 0. 0 5% or less, P: 0.0 15% or less, S: 0.00 8% or less, N: 0.00 8% or less, and Ni: 0.1 to 1.5%, Cr : 0.1-: L. 5%, Cu: 0.1-1.0%, Mo: 0.05-0.5%, including one or more, the balance Fe and A mechanical structure with excellent toughness and weldability, which consists of inevitable impurities, and the metal structure is a single structure of self-tempered martensite or a mixed structure of self-tempered martensite and lower vane iron. High strength seamless steel pipe.

2. The toughness and weldability according to claim 1, wherein in the metal structure, an average diameter of a region surrounded by a large-angle grain boundary having an orientation difference of 15 ° or more is 30 / im or less. Excellent high strength seamless steel pipe for machine structure.

3. The toughness according to claim 1 or 2, wherein in the metal structure, the average particle size of cementite is 400 nm or less and the density is 2 × 10 ⁵ pieces / mm ² or more. High strength seamless steel pipe for machine structures with excellent weldability.

4. A method for producing a high-strength steel pipe for mechanical structure having excellent toughness and weldability according to any one of claims 1 to 3, comprising a steel having the chemical component according to claim 1. The steel slab is pierced and rolled hot, and formed by a drawing process. The obtained steel pipe is heated from 75 ° C or higher to a temperature T [° that satisfies the following equation (1): Accelerate from the outer surface of the steel pipe while rotating in the circumferential direction at a cooling rate V [° C / s] of 5 to 50 ° C / s until C] A method for producing high-strength steel pipes for machine structures with excellent toughness and weldability, characterized by cooling and air cooling.

1 5 0 <T 8 2 1. 3 4 XV ^{0 3 1 12} (1)