CN100420758C - High-strength seamless steel pipe with excellent hydrogen-induced cracking resistance and preparation method thereof - Google Patents

Abstract

High-strength seamless steel pipe with excellent resistance to hydrogen-induced cracking, consisting of the following elements (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, the balance being Fe and impurities, also characterized in that the microstructure of the steel is bainite and/or martensite, iron The body precipitates onto the grain boundaries and the yield stress is 483 MPa or higher. In addition, in order to ensure high strength of the steel, the seamless steel pipe preferably contains (in mass %) at least one of 0.05-0.5% of Cu and 0.05-0.5% of Ni. In order to produce the above-mentioned steel pipes, it is necessary to limit the quenching start temperature, cooling rate and tempering temperature after rolling. With this configuration, there can be provided a seamless steel pipe having a yield stress of 483 MPa or more and excellent resistance to hydrogen-induced cracking, which is suitable for use as a pipe.

Description

High-strength seamless steel pipe with excellent hydrogen-induced cracking resistance and preparation method thereof

technical field

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

Background technique

In recent years, well conditions in oil wells for crude oil and gas wells for natural gas (hereinafter collectively referred to as "oil wells, etc.") have become increasingly severe and transportation of crude oil and natural gas is performed under harsh environments. With increasing water depth, well conditions in oil wells etc. tend to contain _CO2 , _H2S , ^Cl- and other substances in the environment, and crude oil and natural gas often contain _H2S .

When the oil well is on the seabed, as the water depth increases, the offshore pipeline steel pipe requires high strength and thick wall thickness to support the water pressure on the seabed. Since offshore pipeline steel pipes are in such deep sea, seamless stainless steel pipes are usually used.

In pipeline steel pipes used to transport crude oil or natural gas containing a large amount of H ₂ S, not only due to the corrosion of H ₂ S on the surface of steel materials, but also due to the adsorption of hydrogen generated by corrosion into the steel to cause cracks in steel materials such as hydrogen induced Cracking or hydrogen-induced foaming, etc. (hereinafter collectively referred to as "HIC"). This HIC is different from sulfide stress corrosion cracking, which usually exists in high-strength steel, while the former does not depend on external stress, so it is believed that HIC occurs in the absence of external stress.

When this kind of HIC appears in the transportation pipeline, it will lead to the rupture accident of the pipeline. As a result, large-scale environmental damage occurs due to leakage of crude oil or natural gas. Therefore, in a transportation pipeline for crude oil or natural gas, it is important to prevent the occurrence of HIC.

The HIC mentioned above is the phenomenon of steel material cracking, that is, the inclusions such as MnS, Al ₂ O ₃ , CaO, CaS present in the steel during the rolling of the steel material become elongated or crushed into clusters in the rolling direction , the hydrogen adsorbed into the interface between these inclusions and the matrix steel (matrix steel) is accumulated and gasified, the gas pressure of the accumulated hydrogen produces cracks, and these cracks propagate in the steel.

In order to prevent HIC exhibiting such behavior in steel, various steel materials for use in pipeline steel pipes have been proposed. For example, Japanese Patent Application Laid-Open No. S50-97515 proposes steel for pipeline steel pipes in which 0.2-0.8% of Cu is added to steel having a strength of grade X42-X80 in API standards to form a corrosion-resistant film, whereby Prevents hydrogen from being absorbed into the base steel.

Furthermore, Japanese Patent Application Laid-Open S53-106318 proposes a steel material for pipeline steel pipes in which more than 0.005-0.020% or less of Ca (which is a relatively large amount) is added to the steel, and the Ca-treated Shape control spheroidizes inclusions (MnS) in the steel, thereby reducing crack susceptibility. Even current HIC-resistant steels are prepared on the basis of these proposed techniques.

In addition, because HIC-resistant steels are mainly used in crude oil and natural gas transportation pipelines, weldability is important. Therefore, low-carbon steel is used in HIC-resistant steel, but it is difficult to obtain high-strength steel due to the low carbon content of the steel. On the other hand, as mentioned above, users require high-strength materials. Therefore, in order to meet the demand, a step of heating and quenching the steel pipe after finish rolling the steel pipe by hot rolling, followed by tempering is often carried out.

This quenching and tempering treatment of rolled steel pipes is effective in avoiding the banded microstructure of ferrite and pearlite in which HIC tends to occur.

As described above, among steel materials used for pipeline steel pipes, weldability is important, and high strength is required. Therefore, after hot rolling, the rolled steel pipe is usually quenched and tempered. In addition, in the production of seamless steel pipes, from the standpoint of suppressing the increase in equipment cost and production efficiency, it is considered to connect the pipe rolling line directly to the heat treatment line without cooling the finish-rolled steel pipe to the Ar ₃ point (hereinafter Sometimes referred to simply as "in-line quenching/tempering (QT)"), a treatment that applies quenching and tempering is employed after soaking.

Therefore, in order to improve the HIC resistance of high-strength steel materials used for pipeline steel pipes, the rolled steel pipes were not cooled to In the case of Ar ₃ , seamless steel pipes made of high-strength materials are produced by quenching and tempering after soaking. However, HIC in the form of intergranular fractures was observed. Therefore, even if the HIC-resistant steel suggested in the above-mentioned Japanese Patent Application Laid-Open S53-106318 and the like is applied to high-strength steel, the HIC resistance does not necessarily improve.

Contents of the invention

The present invention was obtained after searching for the production of a seamless steel pipe having high strength and HIC resistance, and therefore an object of the present invention is to provide a high-strength seamless steel pipe which can exhibit excellent HIC resistance and a production method thereof.

In order to solve the above-mentioned problems, the present inventors compiled knowledge about HIC behavior occurring in pipeline steel pipes.

As explained above, HIC is cracking of steel caused by hydrogen-induced cracking or hydrogen-induced blistering, which is produced by the fact that the hydrogen generated by corrosion absorbs into the steel and the inclusions in the steel are separated from the matrix. Gas builds up and vaporizes at the interface between the steels, and the pressure of this gas increases beyond the yield strength of the steel to create cracks, which in turn propagate through the steel.

Therefore, in conventional techniques, for example, shape control of inclusions and the like are performed so that absorbed hydrogen is hardly gasified. However, for high-strength steels with API grade 5L-X70 or higher, all HIC initiation points are not on the inclusions, but rather, HIC cracks exhibit ruptures similar to sulfide stress corrosion cracking and can exhibit form of intergranular fracture.

Therefore, the relationship between the HIC resistance of steel and its quenched microstructure is further studied. As a result, it has recently been found that even in the quenched microstructure of bainite and/or martensite, grain boundary brittleness is prevented by precipitating ferrite to grain boundaries, and even if fine cracks occur in steel, the propagation of cracks can be suppressed, thereby obtaining a seamless steel pipe having excellent HIC resistance.

The present invention has been accomplished based on the knowledge described above, and the gist of the present invention is the following high-strength seamless steel pipes (1) and (2) and the following production method (3) of the high-strength seamless steel pipes.

(1) A high-strength seamless steel pipe with excellent HIC resistance, characterized in that it consists of the following (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: 0.001-0.1%, 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, the balance being Fe and impurities, also characterized in that the microstructure of the steel is bainite and/or Martensite, ferrite precipitates to the grain boundary, and the yield stress is 483MPa or higher.

(2) A high-strength seamless steel pipe other than the above-mentioned seamless steel pipe (1), which preferably also contains (by mass %) at least one of 0.05-0.5% of Cu and 0.05-0.5% of Ni.

(3) A method for producing a high-strength seamless steel pipe excellent in HIC resistance, characterized in that after the billet having the composition described in (1) and (2) above is hot-rolled into a seamless steel pipe, the seamless The steel pipe is immediately uniformly heated, and then cooled at a quenching start temperature of (Ar ₃ point + 50°C) to 1100°C and at a cooling rate of 5°C/s or higher, and then the seamless steel pipe is cooled at 550°C to Ac ₁ Point tempering, thus producing such a seamless steel pipe: the microstructure of steel is bainite and/or martensite, ferrite precipitates on the grain boundary, and the yield stress is 483MPa or higher.

Description of drawings

Figure 1 shows a photograph of the microstructure of a seamless steel pipe with poor HIC resistance; and

Fig. 2 is a photograph of the microstructure of a seamless steel pipe with excellent HIC resistance.

Detailed ways

The reasons for limiting the chemical composition, the microstructure of the steel pipe, and the production method as those described above in the present invention will be explained below. First, the reason for limiting the chemical composition of the seamless steel pipe of the present invention will be described. In the following description, the chemical composition is represented by mass %.

1. Chemical composition of steel

C: 0.03-0.11%

C (carbon) is an essential element for improving hardenability and increasing strength of steel. When the C content is less than 0.03%, hardenability is reduced, and high strength is difficult to ensure. On the other hand, when the C content exceeds 0.11%, in the case of applying QT, the steel tends to have a fully quenched microstructure such as bainite and/or martensite, etc., so that not only the HIC resistance of the steel decreases, but also Solderability is also reduced.

Si: 0.05-0.5%

Si (silicon) is added to the steel for the purpose of deoxidizing the steel, and it contributes to increasing the strength and improving the softening resistance of the steel during tempering. In order to obtain these effects, it is necessary to add 0.05% or more of Si. However, the Si content is set at 0.5% or less because adding an excessive amount of Si reduces the flexibility of the steel.

Mn: 0.8-1.6%

Mn (manganese) is an element effective for increasing the hardenability of steel to increase its strength and improving the hot workability of steel. In particular, in order to improve the hot workability of steel, it is necessary to add 0.8% or more of Mn. However, the content of Mn is set at 1.6% or less because the addition of an excessive amount of Mn reduces the flexibility and weldability of the steel.

P: 0.025% or less

P (phosphorus) exists as an impurity in steel. The P content is set at 0.025% or less because the segregation of P on grain boundaries reduces the flexibility of the steel. The P content is preferably 0.015% or less, more preferably 0.009% or less.

S: 0.003% or less

S (sulfur) exists as an impurity in steel. Since S generates sulfides such as MnS and the like and lowers the HIC resistance, the S content is set to 0.003% or less. The S content is preferably 0.002% or less, more preferably 0.001% or less.

Ti: 0.002-0.017%

Ti (titanium) is an element effective in preventing slab cracks. In order to exhibit this effect, the content of Ti needs to be 0.002% or more. On the other hand, the Ti content is set at 0.017% or less, and preferably 0.010% or less, because adding an excessive amount of Ti reduces the flexibility of the steel.

Al: 0.001-0.10%

Al (aluminum) is an essential element for deoxidation of steel. When the Al content is too small, deoxidation becomes insufficient, and surface defects are generated on the slab, deteriorating the properties of the steel. Therefore, the Al content is set to 0.001% or more. On the other hand, since excessive addition of Al will cause cracks in the steel slab, this will lead to deterioration of the properties of the steel. Therefore the Al content is set at 0.10% or less, and preferably 0.040% or less.

Cr: 0.05-0.5%

Cr (chromium) is an element used to increase the strength of steel. A remarkable effect can be obtained by adding 0.05% or more of Cr. However, since the effect can only be saturated to a certain level even if Cr is added in excess, the Cr content is set at 0.5% or less.

Mo: 0.02-0.3%

Mo (molybdenum) is an element for increasing the strength of steel. Adding 0.02% or more of Mo can achieve a remarkable effect. However, since the effect is saturated only to a certain level even if Mo is added in excess, the Mo content is set at 0.3% or less.

V: 0.02-0.20%

V (vanadium) is an element for increasing the strength of steel. A remarkable effect can be obtained by adding 0.02% or more of V. However, since the effect is saturated only to a certain level even if V is added in excess, the V content is set at 0.20% or less, preferably 0.09% or less.

Ca: 0.0005-0.005%

Ca (calcium) is used for inclusion shape control. To improve HIC resistance by rounding MnS inclusions, Ca content needs to be 0.0005% or more. On the other hand, when the Ca content exceeds 0.005%, the effect is saturated and no further effect is possible. In addition, Ca inclusions tend to cluster so that HIC resistance decreases. Therefore, the upper limit of the Ca content is set to 0.005%.

N: 0.008% or less

N (nitrogen) exists as an impurity in steel. When the N content increases, cracks are generated on the slab so that the properties of the steel deteriorate. Therefore the N content is set at 0.008% or less. The preferred N content is 0.006% or less.

O (oxygen): 0.004% or less

The O content means the total content of oxygen soluble in steel and oxygen in oxide inclusions. This oxygen content is essentially the same as in oxide inclusions in fully deoxidized steel. Therefore, when the O content increases, the oxide inclusions in the steel will increase, thereby reducing the HIC resistance. Therefore, the smaller the O content, the better, so the O content is set at 0.004% or less.

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

These elements are elements for increasing the strength of steel. Therefore, one or both of these two elements may be contained when the steel strength should be ensured. This effect becomes remarkable when both Cu and Ni contents are 0.05% or more. However, even if either of the two elements is excessively added, the effect is saturated, so the content of each element is set to 0.5% or less.

Nb: Nb (niobium) content has no effect on the HIC resistance and strength of the steel. Therefore, Nb element can be considered as an impurity element, and its content is not limited in the present invention. However, when the Nb content exceeds 0.1%, some undesired effects such as deterioration of flexibility of the steel become apparent. Therefore, the Nb content range is preferably 0.1% or less.

2. Steel pipe microstructure and its production method

In the seamless steel pipe of the present invention, by using relatively low C steel as indicated by the chemical composition mentioned above, the microstructure of the steel pipe must be a quenched microstructure such as bainite and/or martensite to ensure 5L -X70 strength or higher. In order to obtain this microstructure, online QT is preferably applied.

However, since fully quenched microstructures with only bainite and/or martensite tend to produce HIC (which manifests as a form of intergranular fracture such as sulfide stress corrosion cracking), precipitation of ferrite at grain boundaries Body matters.

In the present invention, the precipitation of ferrite on the grain boundaries of bainite and/or martensite has the effect of preventing the generation of HIC, which is manifested as a form of intergranular fracture such as sulfide stress corrosion cracking, and at the same time 5L-X70 or higher strength is guaranteed.

Figure 1 shows a photo of the microstructure of seamless steel pipes with poor HIC resistance. The microstructure in Figure 1 is a structure etched by Nital etchant, which exhibits a fully quenched microstructure of bainite and/or martensite in which prior austenite grain boundaries can be clearly identified. In the case of this microstructure, HIC tends to develop in the form of intergranular fractures such as sulfide stress corrosion cracking.

In contrast, FIG. 2 is a photograph showing the microstructure of a seamless steel pipe having excellent HIC resistance related to the present invention. Like Fig. 1, Fig. 2 shows the microstructure etched by Nital etching solution. Because the ferrite phase is produced at the grain boundaries, the original austenite grain boundaries are not clear in the microstructure. In the case of this microstructure, HIC in the form of intergranular fractures does not occur.

In the present invention, a seamless steel pipe excellent in target performance, ie, HIC resistance, can be obtained by defining the above microstructure while using a steel slab containing the chemical composition defined in the present invention as a material. A preferred production method for obtaining a seamless steel pipe satisfying microstructure and high strength simultaneously is shown below.

That is, after heating the billet by hot working and finishing rolling it into the shape of a steel pipe, the obtained steel pipe is immediately homogenized to (Ar ₃ point + 50°C) using a soaking furnace without cooling it to Ar ₃ point ) temperature or higher, then quenched.

When the starting temperature of quenching is less than (Ar ₃ point + 50°C), the strength changes. On the other hand, when the starting temperature of quenching increases, the flexibility of the steel pipe decreases significantly. Therefore, the starting temperature of quenching must be 1100°C or lower, and therefore, the starting temperature of quenching is set from (Ar ₃ point + 50°C) to 1100°C.

Quenching of the finish-rolled steel pipe is performed by cooling it to, for example, room temperature while maintaining a cooling rate of 5°C/sec. When the cooling rate during quenching is less than 5°C/sec, the microstructure including martensite and bainite required for required strength cannot be ensured. Therefore, a cooling rate of 5°C/sec or higher should be maintained.

A tempering temperature of 550° C. or higher is required in order to prevent the strength reduction of the welded area affected by heat. However, when the tempering temperature exceeds Ac ₁ point, the strength of the steel pipe decreases. Therefore, tempering must be done at a temperature of 550°C to Ac ₁ point.

The present invention is not limited to the production steps up to finish rolling of a steel billet from a raw material into a steel pipe. As an alternative, for example, by processing a slab cast by a continuous caster using, for example, a Mannesmann-mandrel mill or by heating a slab obtained by rolling in a blooming mill after casting , and hollow tube blanks obtained by piercing machines such as skew rolling mills. After the mandrel is inserted into the tube for rolling, a sizing mill or a reducing mill is used for finish rolling.

Note that even in production methods other than the production method described in (3) described in the present invention, seamless steel pipes having the chemical composition and microstructure defined in (1) and (2) described in the present invention can be obtained HIC Resistance of the Invention.

(Example 1)

Some kinds of steels having the chemical composition shown in Table 1 were melted by a converter. The billet produced by continuous casting is heated to 1100°C or higher, and then a hollow billet is obtained by using a turning roll piercer. These hollow tube blanks are finished rolled into steel pipes through a mandrel mill and a sizing mill. Then, without cooling the steel pipes to the Ar ₃ point or lower, these steel pipes were uniformly heated at 950°C, and subjected to quenching and tempering treatments, thereby producing seamless steel pipes. Steel pipe dimensions and heat treatment conditions are shown in Table 2. In this case, the cooling rate was set at 30°C/sec.

A tensile test sample of JIS 12 was prepared from the obtained steel pipe for tensile testing, and tensile strength (TS) and yield strength (YS) were determined. Note Tensile testing is performed according to JIS Z 2241.

In addition, samples with a thickness of 12-20 mm, a width of 20 mm, and a length of 100 mm were taken for the HIC resistance test. The sample was soaked for about 96 hours in an aqueous solution of 0.5% CH ₃ COOH-5% NaCl saturated with H ₂ S (temperature is 25° C., pH=2.7-4.0, commonly referred to as NACE environment), and the fracture area ratio ( CAR (%)). These results are shown in Table 2.

In addition, after the HIC resistance test, cross-sections of the HIC test samples were cut out and their microstructures were observed by an optical microscope. Observations obtained are shown in Table 2.

It can be seen from Table 2 that all the steels numbered 1-14 according to the embodiment of the present invention meet the strength of 5L-X70 grade, and have an excellent state of CAR=0%.

On the other hand, steel No. 15 in Comparative Example had C and O contents outside the range defined by the present invention, and ferrite was not precipitated on the interface, thereby obtaining a deterioration result of CAR = 12.6%. Moreover, the C content in steel No. 16 also exceeds the specified value of the present invention, and ferrite does not exist on the grain boundary, so the deterioration result of CAR=7.9% is obtained.

In addition, the steel No. 17 in the comparative example had an O content exceeding the specified value of the present invention, and a deterioration result of CAR=6.2% was obtained due to inclusions. Steel No. 18 had a Ca content outside the specified value range of the present invention, and resulted in deterioration of CAR = 3.6% due to inclusions.

In the comparative example, the Mn content in steel No. 19 was out of the specified value range of the present invention, and ferrite did not exist at the interface, thereby obtaining a deterioration result of CAR = 10.8%. In addition, the C content in the No. 20 steel is beyond the specified value range of the present invention, so even if there is an excellent result of CAR=0%, it cannot meet the strength of 5L-X70 grade.

Furthermore, the Ca content of the steel No. 21 in the comparative example exceeded the prescribed value of the present invention, and a deterioration result of CAR = 9.4% was obtained due to inclusions.

(Example 2)

In order to confirm the effect of heat treatment conditions, steel No. 3 in Table 1 was melted by a converter, and the slab produced by continuous casting was heated to 1100° C. or higher, and then a hollow billet was obtained by using a skew rolling mill. The hollow tube blank is finished rolled into a steel tube through a mandrel type seamless tube rolling mill and a sizing mill. Then the steel pipe is cooled in the range of 920°C to 20°C, and a seamless steel pipe is produced by changing the cooling start temperature, cooling rate and tempering temperature. The dimensions and heat treatment conditions of the produced steel pipes are shown in Table 3. In this case, the Ar ₃ point of test steel No. 3 was 768°C, while its Ac ₁ point was 745°C.

Like embodiment 1, the tensile test sample of preparation JIS 12 is used for tensile test, measures tensile strength (TS) and yield strength (YS).

In addition, the HIC resistance test was performed under the same conditions as in Example 1, and the crack area ratio (CAR (%)) was measured. In addition, after the HIC resistance test, the cross-section of the HIC test sample was cut out, and then its microstructure was observed through an optical microscope. These results are shown in Table 3.

Find out from table 3 result, according to the steel of test number 22～28 of the embodiment of the present invention satisfy the heat treatment condition that the present invention stipulates, and all steels of embodiment all satisfy the intensity of 5L-X70 grade, and have CAR=0% excellent condition.

On the other hand, the steel of Test No. 29 in the comparative example was subjected to a quenching temperature outside the specified range of the present invention, and there was no ferrite precipitation at the grain boundary, thereby obtaining a deterioration result of CAR=7.4%. The steel of test number 30 also adopts a tempering temperature exceeding the specified value range of the present invention, and its strength cannot meet the 5L-X70 grade.

In addition, in the comparative example, the steel of test number 31 adopts a cooling rate exceeding the specified value range of the present invention, and the microstructure of the steel is a ferrite-pearlite microstructure, thus, the steel strength cannot satisfy the 5L-X70 grade .

Furthermore, since the quenching initiation temperature in the steel of Test No. 32 was less than (Ar ₃ point + 50° C.), the steel strength could not satisfy the 5L-X70 grade.

Furthermore, in Comparative Example, the steel of Test No. 33 could not secure a tempering temperature of 550° C. or higher, and thus an additional welding test was performed, and it was found that the strength decreased in a region affected by welding heat.

Industrial applicability

In the seamless steel pipe and its production method according to the present invention, the chemical composition of the steel, the microstructure of the steel, and the precipitation of ferrite at grain boundaries in the steel are specified. Therefore, the steel can obtain high strength and stability, excellent HIC resistance. In addition, by specifying the conditions in the case of applying in-line QT, it is possible to provide a pipe having excellent HIC resistance and a high yield stress of 483 MPa or more with cost reduction or expense saving in the heat treatment process and improvement in productivity. Therefore, the seamless steel pipe and its production method of the present invention can be widely used in technical fields requiring high-strength seamless steel pipe excellent in HIC resistance.

Claims (4)

1. A high-strength seamless steel pipe with excellent resistance to hydrogen-induced cracking, characterized in that it consists of the following elements: by 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, the balance is Fe and impurities, and it is also characterized in that the microstructure of the steel is bainite and martensitic Tensite, ferrite precipitates to the grain boundary, and the yield stress is 483MPa or higher. 2. The high-strength seamless steel pipe with excellent hydrogen-induced cracking resistance according to claim 1, characterized in that it also includes, in mass %, at least one of 0.05-0.5% Cu and 0.05-0.5% Ni kind. 3. A production method of a high-strength seamless steel pipe with excellent resistance to hydrogen-induced cracking, characterized in that after the steel billet having the composition described in claim 1 is rolled into a seamless steel pipe by hot rolling, the seamless steel pipe is immediately The seam steel pipe is soaked, then cooled at a quenching start temperature of (Ar ₃ point + 50°C) to 1100°C at a cooling rate of 5°C/s or higher, and then the seamless steel pipe is cooled at 550°C to Ac ₁ point Tempering at a temperature of 100°C, thereby producing a seamless steel pipe in which the microstructure of the steel is bainite and martensite, ferrite precipitates on the grain boundaries, and the yield stress is 483MPa or higher . 4. A method for producing a high-strength seamless steel pipe with excellent resistance to hydrogen-induced cracking, characterized in that after the billet having the composition described in claim 2 is made into a seamless steel pipe by hot rolling, the The seamless steel pipe is soaked, then cooled at a quenching start temperature of (Ar ₃ point + 50°C) to 1100°C at a cooling rate of 5°C/s or higher, and then the seamless steel pipe is cooled at 550°C to Ac Tempering at a temperature of ₁ point, thus producing such a seamless steel pipe: the microstructure of the steel is bainite and martensite, ferrite precipitates on the grain boundary, and the yield stress is 483MPa or more high.

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