NO339589B1 - High-strength seamless steel pipe with excellent resistance to hydrogen-induced cracks, as well as manufacturing process - Google Patents

Description

Field of the invention

The present invention relates to a seamless steel pipe having an excellent resistance to hydrogen induced cracking (hereinafter referred to as "HIC resistance" ("hydrogen induced cracking resistance")), which is used as a conduit pipe having a strength level of grade 5L-X70 or higher according to the American Petroleum Institute (API) standard.

Related technique

In recent years, well conditions in an oil well for crude oil and a gas well for natural gas (hereinafter generally referred to only as "oil well and the like") have become stricter, and the transportation of crude oil and natural gas has been carried out under a stricter environment. As the depth of the water is increased, the well conditions in the oil well and the like have a tendency to contain CO2, H2S, Cl" and the like in the surroundings, and H2S is often found in the crude oil and natural gas.

When the oil well and the like are in the seabed, as the depth of the water is increased, it is required that an offshore pipeline has high strength and thick wall thickness to withstand the water pressure on the seabed. Seamless steel pipes are usually used as the offshore pipeline in such deep seas.

In a pipeline used for the transportation of crude oil and natural gas containing a lot of H2S, not only does corrosion develop in a surface of a steel material due to H2S, but also a cracking phenomenon in the steel material, such as hydrogen-induced cracking or hydrogen-induced blistering or the like (hereinafter generally referred to as "HIC") due to absorption of hydrogen caused by the corrosion of the steel. This HIC is different from sulphide stress corrosion cracking, which is commonly known in a high-strength steel, and does not depend on external stress, so the occurrence of HIC is recognized without external stress.

When such an HIC occurs in a transport pipeline, it can lead to a rupture accident in the pipeline. As a result of this, large-scale environmental destruction may occur due to the leakage of crude oil or natural gas. Accordingly, in crude oil and natural gas transport pipelines, it is an important matter to prevent the occurrence of HIC.

The above-mentioned HIC is a cracking phenomenon in steel materials where inclusions such as MnS, AI2O3, CaO, CaS and the like that exist in steel change, during the rolling of a steel material, into elongated inclusions in the rolling direction or crushed cluster-like inclusions, hydrogen is absorbed into the interfaces between these the inclusions, and matrix steel accumulates and gasses, cracks are formed by the gas pressure in the accumulated hydrogen, and these cracks propagate in the steel.

To prevent HIC, which exhibits such behavior in steel, various steel materials for a conduit pipe have been proposed. For example, Laid-open Japanese Patent Application No. S50-97515 (GB-1491729) proposes steel for a conduit pipe where Cu: 0.2 - 0.8% is added to steel having strength according to grade X42 - X80 of the API standard, for to form an anti-corrosive film, which prevents hydrogen from being absorbed into the matrix steel.

Furthermore, published Japanese patent application No. S53-106318 proposes a steel material for a conduit where Ca: more than 0.005% - 0.020% or less, which is a relatively large amount, is added to steel, and inclusions (MnS) in the steel are sphero-iodized by with the help of a form control during Ca treatment, which reduces the sensitivity to crack formation. Even nowadays, HIC-resistant steels have been produced based on these proposed technologies.

Furthermore, since the most important application area for the HIC-resistant steel is a transportation pipeline for crude oil and natural gas, weldability is important. A low-carbon steel is thus used as the HIC-resistant steel, but high-strength steel is difficult to achieve due to the low C content in the steel. On the other hand, as mentioned above, consumers demand high strength materials. Accordingly, in order to meet this requirement, the following steps are often carried out: after finishing rolling a steel pipe by hot rolling, the steel pipe is heated and quenched, and then annealed.

Such treatment with quenching and tempering of a rolled steel pipe is effective in avoiding a ribbon-shaped microstructure with ferrite and pearlite where HIC is prone to occur.

As mentioned above, in the steel material lining the pipeline, weldability is important and high strength is required. Thus, after hot rolling, a rolled steel tube is often subjected to quenching and tarnishing. Furthermore, in the production of seamless steel pipe, from the point of view of preventing an increase in equipment costs and production efficiency, it has been considered to adopt a treatment that uses quenching and tempering after temperature equalization, without cooling a pre-rolled steel pipe to point Ar3, by directly connecting a tube rolling line to a heat treatment line

(hereinafter sometimes only referred to as "inline quenching/tempering (QT)" ("quench-ing/tempering")).

Accordingly, in order to improve the HIC resistance of a high-strength steel material for a conduit pipe, a seamless steel pipe of a high-strength material was produced by quenching and tempering after temperature equalization without cooling the rolled steel pipe to point Ar3 after hot rolling using a previously proposed steel where inclusions (MnS) is shape-controlled by means of Ca treatment. However, the occurrence of HIC showing some form of intercrystalline fracture was observed. Accordingly, although the HIC-resistant steel proposed in the above-described Laid-open Japanese Patent Application No. S53-106318 and the like was used as a high-strength steel, the HIC resistance is not necessarily improved.

Japanese patent application JPS575819 describes a seamless tube formed from steel containing 0.05-0.20% C, 0.1-1.0% Si, 0.5-2.0% Mn, 0.03% or less P, 0.006% or less S, 0.005% or less O, 0.01-0.07% Al, 0.05-0.1% of one or more of the elements Nb and V, and possibly one or more of the elements: 0.10-0.40% Cu, 0.05- 0.20% Ni, 0.05-0.8% of one or more of Cr and Mo and 0.0005-0.005% B. The tube is quenched through the outside and inside from a temperature of 900°C or higher with cooling water that has a temperature of 100°C - 50°C. It is then tempered at a temperature in the range of 550°C to AC-i.

Summary of the invention

The present invention was made with a view to the production of a seamless steel pipe that has high strength and HIC resistance, and one purpose of the present invention is to provide a high strength seamless steel pipe, which can exhibit excellent HIC resistance, and a method to its production.

The present inventors have compiled the knowledge of behaviors of HIC, which occurs in a conduit, to solve the above problem.

As explained above, HIC is a fracture in steel due to hydrogen-induced cracking or hydrogen-induced blistering, which occurs due to the fact that hydrogen formed by corrosion is absorbed into the steel and accumulates at the interface between the inclusions in the steel and the matrix steel and is gasified , and that the gas pressure is increased more than the steel's yield strength, so that cracks are formed, which propagate in the steel.

Therefore, in a conventional technology, for example, a control of the shape of the enclosure and the like was carried out so that the absorbed hydrogen is almost not gasified. However, for the high-strength steel having grade 5L-X70 or higher according to API, not all the starting points of HIC are at inclusions, and an HIC fracture exhibits a fracture-like sulfide stress corrosion cracking and may exhibit a form of intercrystalline fracture.

Accordingly, the relationships between HIC resistance of steel and a quenched microstructure therein were further investigated. As a result, it has recently been found that even in a quenched microstructure of bainite and martensite, the brittleness of a grain boundary is prevented by the precipitation of ferrite on the grain boundaries, and even if a very small crack appears in the steel, the propagation of the crack can be suppressed, thereby a seamless steel pipe can be obtained which has excellent HIC resistance.

The present invention has been carried out based on the above-mentioned knowledge, and the core of the present invention is the following high-strength seamless steel pipe and the following method for producing the high-strength seamless steel pipe.

The present invention provides a high-strength seamless steel pipe with excellent resistance to hydrogen-induced cracking, the steel pipe consisting of, in mass%, C: 0.03 - 0.11%, Si: 0.05 - 0.5%, Mn: 0 .8 - 1.6%, P: 0.025% or less, S: 0.003% or less, Ti: 0.002 - 0.017%, Al: 0.001 - 0.10%, Cr: 0.05 - 0.5%, Mo : 0.02 - 0.3%, V: 0.02 - 0.20%, Ca: 0.0005 - 0.005%, N: 0.008% or less and O (oxygen): 0.004% or less, optionally Cu: 0.05 - 0.5% and Ni: 0.05 - 0.5%. and Nb: 0.1 or less and the rest Fe and impurities, in which the microstructure of the steel is bainite and martensite, ferrite is precipitated on the grain boundaries, and the yield strength is 483 MPa or more.

Furthermore, the present invention provides a method for the production of a high-strength seamless steel pipe with excellent resistance to hydrogen-induced cracking, in which after rolling a fine blank having a composition according to claim 1 or claim 2 into a seamless steel pipe by hot rolling, the seamless steel pipe is immediately temperature equalized and then cooled at an exit temperature for quenching of (point Ar3+ 50°C) to 1100°C and at a cooling rate of 5°C/sec or more, and that the seamless steel pipe is then tempered at 550°C to points Aci, whereby a seamless steel pipe is produced where the microstructure of the steel is bainite and martensite, ferrite is precipitated at grain boundaries and the yield strength is 483 MPa or more.

Brief description of the drawings

Fig. 1 is a drawing showing a microstructure photograph of a seamless steel pipe having low HIC resistance; and

fig. 2 is a drawing showing a microstructure photograph of a seamless steel pipe having excellent HIC resistance.

Detailed description of the preferred embodiment

For the reason that it is desired to define a chemical composition, a microstructure for a steel tube and a method for its production, such as those described above for the present invention, will be explained. First, the introduction to defining a chemical composition of a seamless steel pipe according to the present invention will be described. In the following descriptions, the chemical composition is given in % by mass.

1. Chemical composition of steel

C: 0.03-0.11%

C (carbon) is an element that is necessary to increase the hardenability and to increase the strength of the steel. When the content of C is less than 0.03%, hardenability is lowered, and high strength is difficult to ensure. On the other hand, when the content of C exceeds 0.11%, in a case where QT is used, the steel tends to have a completely quenched microstructure, such as bainite and/or martensite or the like, whereby not only the HIC resistance of the steel is lowered, but also the weldability is lowered.

Say: 0.05-0.5%

Si (silicon) is added to steel for the purpose of deoxidizing the steel, and contributes to an increase in strength and an increase in resistance to softening during tempering of the steel. To achieve these effects, it is necessary to add 0.05% or more Si. However, since too much addition of Si reduces the toughness of the steel, the Si content was set at 0.5% or less.

Mn: 0.8-1.6%

Mn (manganese) is an effective element to increase the hardenability of the steel, to increase its strength and to increase the steel's hot workability. More specifically, to increase the hot workability of steel, 0.8% or more Mn is required. However, since too much addition of Mn reduces the toughness and weldability of the steel, the Mn content was set at 1.6% or less.

P: 0.025% or less

P (phosphorus) exists in the steel as impurities. Since the precipitation of P in grain boundaries deteriorates the toughness of the steel, the content of P was set to 0.025% or less. The content of P is preferably 0.015 or less, and more preferably 0.009% or less.

S: 0.003% or less

S (sulphur) exists in the steel as impurities. Since S generates sulfides such as MnS and the like and reduces HIC resistance, the content of S was set to 0.003% or less. The content of S is preferably 0.002% or less and more preferably 0.001% or less.

Ti: 0.002-0.017%

Ti (titanium) is an element that is effective in preventing the formation of cracks in the fine material. To demonstrate the effect, a Ti content of 0.002% or more is required. On the other hand, since too much addition of Ti reduces the toughness of the steel, the content of Ti was set to 0.017% or less, and preferably 0.010% or less.

Al: 0.001 -0.10%

Al (aluminium) is an indispensable element for deoxidizing the steel. When the content of Al is too low, deoxidation becomes insufficient and surface defects form on the fines, which deteriorates the steel's properties. Accordingly, the content of Al was set at 0.001% or more. On the other hand, excessive addition of Al produces cracks in the fines, which leads to deterioration of the steel's properties. Accordingly, the content of Al was set at 0.10% or less, and preferably 0.040% or less.

Cr: 0.05-0.5%

Cr (chromium) is an element to increase the steel's strength. The significant effect can be achieved by adding 0.05% or more Cr. However, since too much addition of Cr also saturates the effect, the content of Cr was set to 0.5% or less.

Mo: 0.02-0.3%

Mo (molybdenum) is an element to increase the steel's strength. The significant effect can be achieved by adding 0.02% or more Mo. However, since too much addition of Mo also saturates the effect, the content of Mo was set to 0.3% or less.

W: 0.02-0.20%

V (vanadium) is an element to increase the steel's strength. The significant effect can be obtained by adding 0.02% or more V. However, since too much addition of V saturates the effect, the content of V was set to 0.20% or less, and preferably 0.09% or less.

Approx: 0.0005-0.005%

Ca (calcium) is used for shape control of inclusions. To increase the HIC resistance by making the MnS inclusions spherical, a Ca content of 0.0005% or more is required. On the other hand, when the content of Ca exceeds 0.005%, the effect saturates, and further effects cannot be detected. In addition, Ca inclusions tend to be clustered so that HIC resistance is lowered. The upper limit for Ca content was therefore set at 0.005%.

N: 0.008% or less

N (nitrogen) is found in the steel as impurities. When the content of N is increased, cracks form in the fines, so that the properties of the steel deteriorate. The content of N was accordingly set at 0.008% or less. The content of N is preferably 0.006% or less.

O (oxygen): 0.004% or less

The content of O means a total content of soluble oxygen in the steel and the oxygen in oxide inclusions. This content of O is essentially the same as the content of O in oxide inclusions in the sufficiently deoxidized steel. Therefore, as the content of O is increased, there are increased oxide inclusions in the steel, which reduces the HIC resistance. A lower content of O is consequently better, and the content of O was set at 0.004% or less.

Cu (copper): 0.05 - 0.5%, Ni (nickel): 0.05 - 0.5%

These elements are to increase the strength of the steel. Consequently, when ensuring the strength of the steel, one of the elements or both elements can be included. The effect becomes significant for every content of Cu, Ni of 0.05% or more. However, since too much addition of one of the elements saturates the effect, the content of each element was set to 0.5% or less.

Nb: The content of Nb (niobium) does not affect the HIC resistance and the strength of the steel. The element Nb can therefore be thought of as an element that is an impurity, and its content is not defined in the present invention. However, when the content of Nb exceeds 0.1%, undesirable effects, such as deterioration of the steel's toughness, become significant. The range for Nb content is therefore 0.1% or less.

2. The microstructure of the steel pipe and the method of its production

In the seamless steel pipe according to the present invention, the microstructure of the steel pipe must be a quenched microstructure, such as bainite and/or martensite, to ensure the strength according to grade 5L-X70 or more when using a steel with a relatively low C content , as shown by the above chemical compositions. In order to achieve the microstructure, inline QT is preferably used.

However, since only a completely quenched microstructure with bainite and/or martensite is prone to generate HIC, which exhibits some form of intercrystalline fracture, such as sulphide stress corrosion cracking, it is important to precipitate ferrite at the grain boundary.

In the present invention, the precipitation of ferrite on the grain boundary of bainite or martensite has the effect of preventing the formation of HIC, which exhibits a form of intercrystalline fracture, such as sulphide stress corrosion cracking, while providing strength according to grade 5L- X70 or more.

Fig. 1 is a drawing showing a microstructure photograph of a seamless steel pipe having low HIC resistance. The microstructure of fig. 1 is a structure that has been etched with a nital and exhibits a completely quenched microstructure with bainite and/or martensite, where former austenite grain boundaries can be clearly recognized. In the case of such a microstructure, there is a tendency to form an HIC, which exhibits a form of intercrystalline fracture, such as sulfide stress corrosion cracking.

In contrast, fig. 2 is a drawing showing a microstructure photograph of a seamless steel pipe having excellent HIC resistance, which is related to the present invention. Fig. 2 shows a microstructure which has been etched with a nital, as in fig. 1. Because a ferrite phase has formed in the grain boundary, the former austenite grain boundaries are not clear in the microstructure. In the case of such a microstructure, HIC does not appear, showing a form of intercrystalline fracture.

In the present invention, by defining the above-described microstructure using a fine blank containing the chemical composition defined by the present invention as a material, a seamless steel pipe excellent in intended performance can be obtained, the i.e. HIC resistance. A preferred manufacturing method for producing a seamless steel tube, which simultaneously meets the microstructure and the high strength, is shown as follows.

That is to say, after heating a fine blank and finishing rolling it into the shape of a steel pipe by hot working, the obtained steel pipe is immediately temperature equalized to a temperature of (point Ar3+ 50°C) or more using an equalization pit, without it cools to point Ar3, and it is quenched.

When the exit temperature for the quenching is less than (point Ar3+ 50°C), variation in firmness appears. On the other hand, when the exit temperature of the quenching is increased, the toughness of the steel pipe is significantly lowered. The output temperature for the quench must therefore be 1100°C or less. The output temperature for the quenching is therefore set (point Ar3+ 50°C) to 1100°C.

The quenching of the pre-rolled steel tube is carried out by cooling it, for example, to room temperature, while the cooling rate is kept at 5°C/sec. When the cooling rate during this quenching is less than 5°C/sec, one cannot be sure of a microstructure including martensite and bainite, which is required to achieve the required strength. Accordingly, the cooling rate of 5°C/sec or more should be maintained.

To prevent reduction of strength in a heat-affected zone during welding, a tempering temperature of 550°C or more is required. However, when the tempering temperature exceeds point Aci, the strength of the steel pipe decreases. Annealing must therefore be carried out under a temperature condition from 550°C to point Aci.

The present invention does not limit production steps up to the finished rolling of a steel pipe from a fine billet, which is a starting material. Alternatively, wood

adopting, for example, a Mannesmann mandrel rolling process, a fine blank cast in a continuous casting machine or a fine blank obtained by rolling in a block rolling mill after casting is heated, and a hollow mantle blank is obtained by means of a hollow rolling mill, such as a inclined rolling mill. After a mandrel is inserted into the tube to roll it, a finishing roll is carried out using a sizing device or reduction device.

It should be pointed out that even in a different method of production than the methods of production described in said (3) according to the present invention, a seamless steel pipe having the chemical compositions and the microstructure defined in said (1) ) or (2) according to the present invention achieve the HIC resistance according to the present invention.

Example 1

Certain types of steel, having chemical compositions as shown in Table 1, were melted with a converter. Fine blanks produced by continuous casting were heated to 1100°C or more, and hollow shell blanks were obtained using a tilt-hole rolling mill. These hollow casing blanks were pre-rolled into steel tubes using a mandrel roll and a dimensioning device. Then, without cooling the steel tubes to point Ar3 or less, they were temperature equalized at 950°C and subjected to quenching and tempering treatment to produce seamless steel tubes. The steel tubes' size and heat treatment conditions are shown in table 2. In this case, the cooling rate was set to 30°C/sec.

Tensile test pieces according to JIS 12 were taken from the obtained steel pipes as tensile tests, and tensile strength ("tensile strength" TS) and yield strength ("yield strength" YS) were measured. It is pointed out that the tensile tests were carried out in accordance with JIS Z2241.

Further, test pieces having thickness from 12 to 20 mm, width of 20 mm and length of 100 mm were taken for HIC resistance tests. The test pieces were immersed in a hhS-saturated 0.5% ChbCOOH - 5% NaCI water solution (temperature of 25°C, pH = 2.7 - 4.0, so-called NACE environment) for 96 hours, and crack area ratio ratios" CAR (%)) were measured. These results are shown in Table 2.

Furthermore, after the HIC resistance tests, cross-sections of the HlC test sample pieces were cut and their microstructure was observed with an optical microscope. The resulting observation results are shown in table 2.

As can be seen from Table 2, all the steels with Nos. 1 to 14 according to Example 1 of the present invention meet the strength according to grade 5L-X70 and have an excellent condition where CAR = 0%.

On the other hand, steel No. 15 in comparative examples has contents of C and O which are outside their definition according to the present invention, and ferrite is not precipitated at the interface, whereby a worse result was obtained with CAR = 12.6%. Furthermore, the content of C in steel No. 16 is outside the specified values for the present invention, and ferrite does not appear at the grain boundary, which resulted in a worse result with CAR = 7.9%.

Furthermore, steel No. 17 in the comparative examples has an O content that is outside the specified values according to the present invention, and a worse result with CAR = 6.2% was obtained due to inclusions. Steel No. 18 had a content of Ca outside the definition for the present invention, and due to inclusions a worse result was obtained with CAR = 3.6%.

In the comparison examples, the steel in No. 19 has a content of Mn that is outside the specified values for the present invention, and ferrite does not exist at the interface, whereby a worse result was obtained for CAR = 10.8%. Furthermore, the steel in No. 20 has a content of C which is outside the specified values for the present invention, and consequently cannot meet the strength according to grade 5L-X70, even though the result for CAR = 0% was excellent.

Furthermore, in the comparative examples, the steel in No. 21 has a content of Ca outside the specified values for the present invention, and a worse result for CAR = 9.4% appeared due to inclusions.

Example 2

To confirm effects of heat treatment conditions, steel No. 3 in Table 1 was melted with a converter, and a fine blank produced by continuous casting was heated to 1100°C more, and a hollow jacket blank was produced using an inclined rolling mill. The hollow casing blank was pre-rolled into a steel tube using a mandrel rolling mill and a dimensioning device. Then the steel tube was cooled in a range from 920°C to 20°C, and seamless steel tubes were produced by changing the cooling output temperature, the cooling rate and the annealing temperature. The sizes of the steel tubes produced and the heat treatment conditions are shown in Table 3. In this case, point Ar3 for the No. 3 steel tested was 768°C, and point Aci for this was 745°C.

As in Example 1, tensile test pieces were taken according to JIS 12, and as tensile tests, tensile strength (TS) and yield strength (YS) were measured. Furthermore, HIC resistance tests were performed under the same conditions as in Example 1, and the crack area ratio (CAR (%)) was measured. Furthermore, after HlC resistance testing, cross-sections of HIC test specimens were cut, and observation of the microstructure was performed with an optical microscope. These results are shown in Table 3.

As can be seen from the results in Table 3, the steels in test No. 22 to 28 according to the examples of the present invention meet the heat treatment conditions specified for the present invention, and all these steels meet the strength according to grade 5L-X70, and has an excellent condition where CAR = 0%.

On the other hand, the steel in test No. 29 in the comparative examples uses a quenching temperature that is outside the specified values for the present invention, and ferrite does not precipitate at the grain boundaries, whereby a worse result was obtained for CAR = 7.4%. Furthermore, the steel in test no. 30 uses a tempering temperature which is outside the specified values for the present invention, and the strength could not satisfy grade 5L-X70.

Furthermore, in the comparative examples, the steel in test No. 31 uses a cooling rate that is outside the specified values for the present invention, and the microstructure of the steel is a ferrite-pearlite microstructure, whereby the strength of the steel could not meet grade 5L-X70.

Furthermore, since the exit temperature of quenching for the steel in Test No. 32 was less than (point Ar3+ 50°C), the strength of the steel could not meet grade 5L-X70.

Furthermore, in the comparative examples, the steel in Test No. 33 could not provide a tempering temperature of 550°C or more, a further welding test was conducted, and it was found that the strength was reduced in a zone that was heat affected by welding.

Industrial applicability

In the seamless steel pipe and the method of its production according to the present invention, the chemical compositions of the steels, the microstructure of the steel and the precipitation of ferrite at the grain boundaries in the steels are specified. Consequently, the steel can achieve high strength and stable, excellent HIC resistance. Furthermore, by specifying the conditions in a case where an inline QT is used, a pipeline having excellent HIC resistance and high yield strength of 483 MPa or more can be provided, without inhibiting the cost down or cost saving of the heat treatment process and the improvement in productivity. The seamless steel pipe and the method for its production according to the present invention can therefore be used to a large extent within technical fields that require a seamless steel pipe with high strength and which has excellent HIC resistance.

Claims (3)

1. High-strength seamless steel pipe with excellent resistance to hydrogen-induced cracking, the steel pipe consisting of, in mass%, C: 0.03 - 0.11%, Si: 0.05 - 0.5%, Mn: 0.8 - 1.6%, P: 0.025% or less, S: 0.003% or less, Ti: 0.002 - 0.017%, Al: 0.001 - 0.10%, Cr: 0.05 - 0.5%, Mo: 0 .02 - 0.3%, V: 0.02 - 0.20%, Ca: 0.0005 - 0.005%, N: 0.008% or less and O (oxygen): 0.004% or less, optionally Cu: 0, 05 - 0.5% and Ni: 0.05 - 0.5%. and Nb: 0.1 or less and the rest Fe and impurities, characterized in that the microstructure of the steel is bainite and martensite, ferrite is precipitated on the grain boundaries, and the yield strength is 483 MPa or more. 2. High-strength seamless steel pipe with excellent resistance to hydrogen-induced cracking as stated in claim 1, characterized in that it further contains, in % by mass, at least one of Cu: 0.05 - 0.5% and Ni: 0.05 - 0.5%. 3. Process for the production of a high-strength seamless steel pipe with excellent resistance to hydrogen-induced cracking, characterized in that after rolling a fine blank having a composition according to claim 1 or claim 2 into a seamless steel tube by hot rolling, the seamless steel tube is immediately temperature equalized and then cooled at an exit temperature for quenching of (point Ar3+ 50°C) to 1100°C and at a cooling rate of 5°C/sec or more, and that the seamless steel tube is then tempered at 550°C to points Aci, whereby a seamless steel tube is produced where the microstructure of the steel is bainite and martensite, ferrite is precipitated at grain boundaries and yield point is 483 MPa or more.