JP5915818B2 - Seamless steel pipe for line pipe used in sour environment - Google Patents

Description

The present invention relates to a seamless steel pipe. More preferably, the present invention relates to a seamless steel pipe for a line pipe used in a sour environment containing hydrogen sulfide (H ₂ S) which is a corrosive gas.

  Crude oil and natural gas contain hydrogen sulfide and moisture. Such a wet hydrogen sulfide environment is called a sour environment. Line pipes are used as pipelines for transporting crude oil and natural gas produced from oil and gas wells. Therefore, the line pipe is used in a sour environment. In line pipes used in sour environments, hydrogen embrittlement due to hydrogen that has penetrated into steel becomes a problem due to corrosion in an environment containing hydrogen sulfide.

  Hydrogen embrittlement includes sulfide stress cracking that occurs in steel under static external stress, and hydrogen-induced cracking (hereinafter referred to as HIC) that occurs in the steel without external stress. In line pipes, HIC is often a problem. Therefore, the steel pipe for line pipes is particularly required to have HIC resistance.

  Steel pipes for line pipes include welded steel pipes and seamless steel pipes. The welded steel pipe has a seam portion (welded portion) extending in the axial direction or spiral shape. A steel plate used for a welded steel pipe has a central segregation portion generated at the time of continuous casting in the center of the plate thickness, and the central segregation portion has high HIC sensitivity. Therefore, it is preferable to use a seamless steel pipe for a steel pipe for a line pipe that particularly requires HIC resistance.

  In general, it is known that the higher the strength of steel, the easier it is to generate HIC. International Publication No. WO 2005/075694 (Patent Document 1) proposes a seamless steel pipe having high strength and excellent HIC resistance.

  Specifically, the steel product for line pipes disclosed in Patent Document 1 has a composition of mass%, C: 0.03 to 0.15%, Si: 0.05 to 1.0%, Mn: 0 5-1.8%, P: 0.015% or less, S: 0.04% or less, O: 0.01% or less, N: 0.007% or less, sol. Al: 0.01-0.1%, Ti: 0.024% or less, Ca: 0.0003-0.02% is contained, and the remainder consists of Fe and an impurity. Furthermore, in the steel material for line pipes, the size of TiN in the steel is 30 μm or less. Patent Document 1 describes that excellent HIC resistance can be obtained because TiN is fine.

International Publication No. 2005/075694 JP 2002-60893 A International Publication No. 2011/152240

  When manufacturing a high-strength seamless steel pipe, the strength of the seamless steel pipe is usually increased by performing quenching and tempering treatment after the hot working process. On the other hand, there is also a demand for a low-strength seamless steel pipe for line pipes that does not require high strength and has a yield strength of less than 450 MPa. Such a low-strength seamless steel pipe is usually not subjected to quenching and tempering and is omitted.

  Conventionally, as described above, it has been considered that if the strength is low, HIC hardly occurs. However, as a result of the investigation by the inventors of the present application, it has been newly found that not only when the strength is high, but also when the strength is low, a large number of blisters which are a kind of HIC and minute internal cracks may occur. did.

  A blister is a bulge that occurs near the surface of a steel material and extends in the axial direction of the steel material. In a HIC resistance test (NACE TM0284, etc.) defined by NACE, the occurrence of blisters may be confirmed even in high-strength seamless steel pipes exhibiting excellent HIC resistance. However, when the HIC (blister) stays cracked near the surface, it does not lead to leakage of the transported fluid and the like, and the blister is not particularly a problem in conventional high-strength seamless steel pipes.

  However, in a low-strength seamless steel pipe, when a tensile stress is applied, a plurality of blisters and minute internal cracks in the steel are connected in the thickness direction of the seamless steel pipe to form a SOHIC (Stress Oriented Hydrogen Inducted Cracking). May occur.

  Therefore, it is desirable to suppress the occurrence of blisters and minute internal cracks in a low-strength seamless steel pipe that is not quenched and tempered. In a low-strength steel material, minute internal cracks are generated for the same cause as blisters.

  An object of the present invention is to provide a seamless steel pipe that can suppress the generation of blisters and minute internal cracks when it is used for a line pipe used in a sour environment without quenching and tempering. is there.

  The seamless steel pipe according to this embodiment is a line pipe application used in a sour environment. The above-mentioned seamless steel pipe is mass%, C: 0.08 to 0.24%, Si: 0.10 to 0.50%, Mn: 0.3 to 2.5%, P: 0.02% or less S: 0.006% or less, Nb: 0.02 to 0.12%, Al: 0.005 to 0.100%, Ca: 0.0003 to 0.0050%, N: 0.0100% or less, O: 0.0050% or less, Ti: 0-0.1%, V: 0-0.03%, Cr: 0-0.6%, Mo: 0-0.3%, Ni: 0-0. 4%, Cu: 0 to 0.3%, and B: 0 to 0.005%, the balance comprising a chemical composition composed of Fe and impurities, and a structure composed of ferrite and pearlite, It has a yield strength of less than 450 MPa.

  The seamless steel pipe of the present embodiment does not perform quenching and tempering and can suppress the occurrence of blisters and minute internal cracks even if the strength is low.

FIG. 1 is a diagram showing the relationship between the yield strength of a seamless steel pipe and the number of blisters generated (pieces / 20 cm ² ). FIG. 2 is a photographic image of two surfaces (corresponding to the outer surface and the inner surface of a seamless steel pipe) after the blister number measurement test of the inventive example (steel A4, wall thickness 20 mm) in the examples. FIG. 3 is a photographic image of two surfaces (corresponding to the outer surface and the inner surface of a seamless steel pipe) of the coupon test piece after the blister number measurement test of the comparative example (steel B3, thickness 20 mm) in the examples.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  The present inventors investigated and examined the occurrence of blisters in a low-strength seamless steel pipe not subjected to quenching and tempering, and obtained the following knowledge.

  Blister occurs by the following mechanism. Hydrogen accumulates around the inclusions in the steel and forms the starting point for hydrogen blisters. When the steel material yields by increasing the starting hydrogen pressure, a crack is generated. When a crack is generated, dislocations and hydrogen are further accumulated at the crack tip, and the crack progresses. This produces a blister.

  In low-strength seamless steel pipes that are not quenched and tempered, there are many ferrites with low yield strength. Therefore, it is considered that ferrite yields and blisters are generated. Therefore, in order to suppress the generation of blisters, it is effective to increase the strength of the steel by strengthening the ferrite itself or increasing the ratio of pearlite in the steel.

FIG. 1 is a diagram showing the relationship between the yield strength of a seamless steel pipe and the number of blisters generated (pieces / 20 cm ² ). FIG. 1 was obtained by the following method. Seamless steel pipes with various chemical compositions were produced. At this time, the seamless steel pipe after hot working was allowed to cool or cooled at a cooling rate of less than 5 ° C./s, and quenching and tempering were not performed.

A yield strength test described below was performed on each manufactured seamless steel pipe to determine the yield strength. Further, a blister number measurement test described later was performed to determine the number of blisters (pieces / 20 cm ² ) generated in each seamless steel pipe, and FIG. 1 was created.

  Referring to FIG. 1, in the seamless steel pipe, the number of blisters was remarkably reduced as the yield strength increased until the yield strength reached 350 MPa. On the other hand, when the yield strength was 350 MPa or more, the number of blisters did not change so much even if the yield strength increased.

  In short, the curve of FIG. 1 has an inflection point where the yield strength is around 350 MPa. Therefore, if the yield strength is 350 MPa or more, the number of blisters can be kept low.

  Increasing the C content increases the pearlite ratio in the steel and increases the yield strength of the steel. However, if the C content increases, weldability decreases. Seamless steel pipes for line pipes are circumferentially welded at the site where the line pipes are installed. If the C content is increased, the toughness of the circumferentially welded joint portion is reduced, and sulfide stress cracking (SSC) is likely to occur. Therefore, it is difficult to increase the C content excessively.

  Moreover, the strength of the seamless steel pipe can be increased by carrying out quenching and tempering. However, in a low-strength seamless steel pipe, if the quenching and tempering are performed, the manufacturing cost is increased.

  In the case of a welded steel pipe such as a UOE steel pipe, cold working such as pipe making or pipe expansion is performed. Since the strength of the welded steel pipe is increased by cold working, there is a possibility that the number of blisters generated can be suppressed. However, as described above, seamless steel pipes are suitable for line pipes used in severe sour environments. Therefore, it is difficult to increase the strength by cold working or the like, which is not preferable in consideration of manufacturing costs.

  Therefore, in this embodiment, the C content is increased, and further the Nb content is increased. Specifically, the C content is 0.08 to 0.24%, and the Nb content is 0.02 to 0.12%. In this case, even a seamless steel pipe that is not subjected to quenching and tempering (quenching and tempering is omitted) can increase the strength and suppress the generation of blisters.

Preferably, the numerical value of the Nb content (mass%) is set to be equal to or higher than the F1 value defined by the formula (1).
F1 = 0.02 + (t−15) × 0.001 (1)
Here, a numerical value excluding the unit of the thickness (unit: mm) of the seamless steel pipe is substituted for t.

  The thickness of the seamless steel pipe used for the sour environment for line pipe is, for example, 10 to 50 mm. As the wall thickness increases, the cooling conditions of the seamless steel pipe after hot working also change. That is, the cooling rate becomes slow and the strength of the steel tends to decrease. If the Nb content is equal to or greater than the F1 value of the formula (1), the strength of the steel becomes 350 MPa or more, and the generation of blisters can be suppressed.

  The seamless steel pipe of this embodiment completed based on the above knowledge is as follows.

  The seamless steel pipe according to this embodiment is a line pipe application used in a sour environment. The above-mentioned seamless steel pipe is mass%, C: 0.08 to 0.24%, Si: 0.10 to 0.50%, Mn: 0.3 to 2.5%, P: 0.02% or less S: 0.006% or less, Nb: 0.02 to 0.12%, Al: 0.005 to 0.100%, Ca: 0.0003 to 0.0050%, N: 0.0100% or less, O: 0.0050% or less, Ti: 0-0.1%, V: 0-0.03%, Cr: 0-0.6%, Mo: 0-0.3%, Ni: 0-0. 4%, Cu: 0 to 0.3%, and B: 0 to 0.005%, the balance comprising a chemical composition composed of Fe and impurities, and a structure composed of ferrite and pearlite, It has a yield strength of less than 450 MPa.

Preferably, the content (% by mass) of Nb is not less than the F1 value defined by the formula (1).
F1 = 0.02 + (t−15) × 0.001 (1)
Here, a numerical value excluding the unit of the thickness (unit: mm) of the seamless steel pipe is substituted for t.

  Hereinafter, the seamless steel pipe of this embodiment is explained in full detail.

[Chemical composition]
The seamless steel pipe according to this embodiment has the following chemical composition.

C: 0.08 to 0.24%
Carbon (C) increases the hardenability and increases the strength of the steel. When heat treatment such as quenching and tempering is not performed after pipe making as in the seamless steel pipe of this embodiment, if the C content is too low, the strength of the steel becomes too low. If the C content is too low, it is difficult to obtain excellent HIC resistance. When the C content is 0.08% or more, high-strength pearlite is dispersed and precipitated in the steel. Therefore, the yield of ferrite is suppressed. Therefore, excellent HIC resistance is obtained, and the generation of blisters is suppressed. On the other hand, the seamless steel pipe of this embodiment is circumferentially welded as a line pipe on site. Therefore, if the C content is too high, the heat-affected zone (HAZ) of circumferential welding is cured and the SSC resistance is lowered. Therefore, the C content is 0.08 to 0.24%. The minimum with preferable C content is higher than 0.08%, More preferably, it is 0.10%. The upper limit with preferable C content is less than 0.24%, More preferably, it is 0.15%.

Si: 0.10 to 0.50%
Silicon (Si) deoxidizes steel. If the Si content is too low, this effect cannot be obtained. On the other hand, if the Si content is too high, the toughness of the weld heat affected zone decreases. If the Si content is too high, the precipitation of ferrite, which is a softening phase, is further promoted. For this reason, the HIC resistance is lowered, and blisters are easily generated. Therefore, the Si content is 0.10 to 0.50%. The minimum with preferable Si content is higher than 0.10%, More preferably, it is 0.15%, More preferably, it is 0.20%. The upper limit with preferable Si content is less than 0.50%, More preferably, it is 0.35%, More preferably, it is 0.30%.

Mn: 0.3 to 2.5%
Manganese (Mn) increases the hardenability of the steel and increases the strength of the steel. Mn further increases the toughness of the steel. If the Mn content is too low, this effect cannot be obtained. On the other hand, if the Mn content is too high, HIC tends to occur due to hardening of the steel by Mn segregation and formation of MnS. Therefore, the Mn content is 0.3 to 2.5%. The minimum with preferable Mn content is higher than 0.3%, More preferably, it is 0.5%, More preferably, it is 0.8%. The upper limit with preferable Mn content is less than 2.5%, More preferably, it is 2.0%, More preferably, it is 1.8%.

P: 0.02% or less Phosphorus (P) is an impurity. P decreases the toughness of the steel. Therefore, the P content is 0.02% or less. The preferable P content is less than 0.02%, more preferably 0.01% or less. The P content is preferably as low as possible.

S: 0.006% or less Sulfur (S) is an impurity. S forms MnS. MnS becomes the starting point of the blister. Accordingly, a lower S content is preferable. However, reducing the S content is costly. In the seamless steel pipe of the present embodiment, the S content may be 0.006% or less in order to reduce the manufacturing cost. In the seamless steel pipe of this embodiment, even if the S content is 0.005% or more, if the C content and the Nb content are appropriate, excellent HIC resistance is exhibited, and blistering is suppressed. Is done. However, a lower S content is preferred. A preferable S content is 0.003% or less.

Nb: 0.02 to 0.12%
Niobium (Nb) is dissolved in ferrite to increase the strength of the steel. Nb further combines with C and N to form carbonitrides and refines the steel by pinning hardening. Refinement improves the HIC resistance of the steel. Refinement further increases the toughness of the steel. When a steel material containing C in the above range and Mn in the above range and containing no Nb is made into a seamless steel pipe and then heat treatment is not performed (that is, an as-roll in which quenching and tempering are omitted) When the material is manufactured), the yield strength of the manufactured seamless steel pipe is about 250 MPa. However, if the Nb content is in the above range, the yield strength of the seamless steel pipe increases to 350 MPa or more. Therefore, the generation of blisters is suppressed. If the Nb content is too low, the above effect cannot be obtained. On the other hand, if the Nb content is too high, coarse Nb carbonitride is formed. Coarse Nb carbonitride serves as a starting point for blisters and further reduces HIC resistance. Therefore, the Nb content is 0.02 to 0.12%.

As described above, the thickness of a seamless steel pipe for use in a line pipe in a sour environment is 10 to 50 mm. The greater the thickness of the seamless steel pipe, the slower the cooling rate of the seamless steel pipe and the coarser the ferrite grains. As a result, the yield strength of the steel decreases. Therefore, Preferably the minimum of Nb content is more than F1 value (%) defined by the following formula (1).
F1 = 0.02 + (t−15) × 0.001 (1)
Here, a numerical value excluding the unit of the thickness (mm) of the seamless steel pipe is substituted for t in the formula (1).

  When the above-mentioned seamless steel pipe satisfies the formula (1), sufficient yield strength can be ensured not only in the base material but also in the weld heat affected zone formed by circumferential welding between the seamless steel pipes, and blisters are generated. Is suppressed. In the weld heat affected zone, there are a hardened region where the cooling rate after heating is fast and hardened, and a softened region where the cooling rate is slow and softened under repeated heat influences. When the above formula (1) is satisfied, sufficient yield strength is ensured in the softened region.

  The minimum with preferable Nb content is higher than 0.02%, More preferably, it is 0.03%, More preferably, it is 0.04%. The upper limit with preferable Nb content is less than 0.12%, More preferably, it is 0.10%, More preferably, it is 0.08%.

Al: 0.005 to 0.100%
Aluminum (Al) deoxidizes steel. If the Al content is too low, this effect cannot be obtained. On the other hand, if the Al content is too high, coarse cluster-like alumina inclusion particles are formed during circumferential welding, and the toughness at the weld heat affected zone (HAZ) decreases. Therefore, the Al content is 0.005 to 0.100%. The minimum with preferable Al content is higher than 0.005%, More preferably, it is 0.010%, More preferably, it is 0.020%. The upper limit with preferable Al content is less than 0.100%, More preferably, it is 0.060%, More preferably, it is 0.040%. In this specification, Al content means content of acid-soluble Al (sol.Al).

Ca: 0.0003 to 0.0050%
Calcium (Ca) suppresses clogging of the tundish nozzle during casting. Further, Ca suppresses the generation of HIC, blisters, and MnS that are the starting points of fine internal cracks. Therefore, Ca suppresses the generation of blisters and fine internal cracks. If the Ca content is too low, this effect will be insufficient. On the other hand, if the Ca content is too high, inclusions form clusters, and the toughness and HIC resistance of the steel decrease. Therefore, the Ca content is 0.0003 to 0.0050%. The minimum with preferable Ca content is higher than 0.0003%, More preferably, it is 0.0010%, More preferably, it is 0.0015%. The upper limit with preferable Ca content is less than 0.0050%, More preferably, it is 0.0040%, More preferably, it is 0.0030%.

N: 0.0100% or less Nitrogen (N) is an impurity. N forms coarse nitrides and lowers the toughness and SSC resistance of the steel. Therefore, a lower N content is preferable. Therefore, the N content is 0.0100% or less. The preferable N content is 0.0080% or less, and more preferably 0.0060% or less.

O: 0.0050% or less Oxygen (O) is an impurity. O forms coarse oxides or oxide clusters to lower the toughness and HIC resistance of the steel. Therefore, it is preferable that the O content is as low as possible. Therefore, the O content is 0.0050% or less. The O content is preferably 0.0040% or less, and more preferably 0.0030% or less.

  The balance of the chemical composition of the seamless steel pipe of this embodiment is composed of Fe and impurities. Impurities here refer to ores and scraps used as raw materials for steel, or elements mixed from the environment of the manufacturing process.

[Selected elements]
The seamless steel pipe of this embodiment may further contain one or more selected from the group consisting of Ti, V, Cr, Mo, Ni, Cu and B. All of these elements increase the strength of the steel.

Ti: 0 to 0.1%
Titanium (Ti) is a selective element. Ti, like Nb, combines with C and N to form carbonitrides and refines the steel by pinning hardening. On the other hand, if the Ti content is too high, the effect is saturated. Therefore, the Ti content is 0 to 0.1%. The minimum with preferable Ti content is 0.002%, More preferably, it is 0.005%. The upper limit with preferable Ti content is less than 0.1%, More preferably, it is 0.05%.

V: 0 to 0.03%
Vanadium (V) is a selective element. V forms carbides and strengthens the steel. On the other hand, if the V content is too high, coarse carbides are formed and SSC tends to occur. Therefore, the V content is 0 to 0.03%. The minimum with preferable V content is 0.01%, More preferably, it is 0.015%. The upper limit with preferable V content is less than 0.03%, More preferably, it is 0.025%.

Cr: 0 to 0.6%
Mo: 0 to 0.3%
Ni: 0 to 0.4%
Cu: 0 to 0.3%
Chromium (Cr), molybdenum (Mo), nickel (Ni), and copper (Cu) are all selective elements. All of these elements enhance the hardenability of the steel and strengthen the steel, and increase the HIC resistance in low-strength steel. On the other hand, if the content of these elements is too high, a hardened structure is generated locally, or uneven corrosion of the steel surface may be caused. Therefore, the Cr content is 0 to 0.6%, the Mo content is 0 to 0.3%, the Ni content is 0 to 0.4%, and the Cu content is 0 to 0.3%. %. The minimum with preferable Cr content is 0.01%, More preferably, it is 0.05%. The minimum with preferable Mo content is 0.01%, More preferably, it is 0.05%. The minimum with preferable Ni content is 0.01%, More preferably, it is 0.05%. The minimum with preferable Cu content is 0.01%, More preferably, it is 0.05%. The upper limit with preferable Cr content is less than 0.6%, More preferably, it is 0.5%. The upper limit with preferable Mo content is less than 0.3%, More preferably, it is 0.25%. The upper limit with preferable Ni content is less than 0.4%, More preferably, it is 0.3%, More preferably, it is 0.25%. The upper limit with preferable Cu content is less than 0.3%, More preferably, it is 0.25%.

Preferably, the total content of Cr, Mo, Ni, and Cu satisfies the following formula (2).
(Cr + Mo) / 5 + (Cu + Ni) / 15 <0.10 (2)
The content (mass%) of the corresponding element is substituted for each element symbol in the formula (2).
If Cr, Mo, Ni, and Cu satisfy Expression (2), the yield strength is less than 450 MPa even for a thick-walled seamless steel pipe.

B: 0 to 0.005%,
Boron (B) is a selective element. B enhances the hardenability of steel in a low-strength seamless steel pipe, and enhances HIC resistance in a low-strength steel. On the other hand, if the B content is too high, the SSC resistance of the steel decreases. Therefore, the B content is 0 to 0.005%. The minimum with preferable B content is 0.0001% or more, More preferably, it is 0.0003%. The upper limit with preferable B content is less than 0.005%, More preferably, it is 0.003%.

[Organization and strength]
The seamless steel pipe of this embodiment is not quenched and tempered after pipe making. That is, the seamless steel pipe of this embodiment is a so-called as-roll material in which quenching and tempering are omitted. As will be described later, the seamless steel pipe after pipe making is allowed to cool or cooled at a cooling rate of less than 2 ° C./s. Therefore, the structure of the seamless steel pipe of this embodiment is composed of ferrite and pearlite. Most of the structure is ferrite and the rest is pearlite. The structure here means a matrix structure that does not contain inclusions and precipitates.

  Even when cooled at a slow cooling rate as described above, the seamless steel pipe of the present embodiment has a yield strength of 350 MPa or more. In this specification, the yield strength means 0.2% proof stress. The preferred yield strength of the seamless steel pipe is 400 MPa or more. In the seamless steel pipe of this embodiment, the yield strength is less than 450 MPa.

[Production method]
An example of the manufacturing method of the seamless pipe for line pipes used in the sour environment by this embodiment is demonstrated.

  The steel having the above chemical composition is melted and refined by a well-known method. Subsequently, the molten steel is made into a continuous cast material by a continuous casting method. The continuous cast material is, for example, a slab, bloom or billet. Moreover, you may make molten steel into an ingot by an ingot-making method.

  Billets are manufactured by hot-working slabs, blooms, and ingots of continuous cast materials. For example, billets are manufactured by rolling a slab, a bloom, or an ingot.

  Subsequently, the manufactured billet is hot-made to produce a seamless steel pipe. Specifically, the billet is heated in a heating furnace. If hot pipe making is performed with coarse Nb inclusions remaining in the heated billet, Nb strengthening cannot be sufficiently obtained during cooling after hot pipe making. Therefore, in this embodiment, it heats even higher temperature compared with the time of manufacture of a normal seamless steel pipe. Specifically, the billet is heated to 1250 ° C. or higher during the heating.

  The billet extracted from the heating furnace is hot-worked to produce a seamless steel pipe. Specifically, piercing and rolling based on the Mannesmann method is performed to manufacture a raw pipe. Further, the produced raw pipe is further subjected to stretching rolling and constant diameter rolling by a mandrel mill, a reducer, a sizing mill or the like to produce a seamless steel pipe.

  The seamless steel pipe is cooled. At this time, it is preferable that the cooling rate in a high temperature region of 500 ° C. or higher where Nb carbonitride precipitates is high. Therefore, the seamless steel pipe is cooled at a cooling rate of 0.5 to 5 ° C./s until the temperature of the seamless steel pipe reaches 500 ° C., and then cooled at a cooling rate of less than 2 ° C. The cooling rate of less than 2 ° C./s includes cooling.

  The cooling rate can be adjusted, for example, by adjusting the interval between adjacent seamless steel pipes during cooling. For example, until the seamless steel pipe reaches 500 ° C., the distance between the adjacent seamless steel pipes is set as the distance D1, and at 500 ° C. or less, the distance is adjusted to the distance D2 shorter than the distance D1. Thereby, a gentle two-stage cooling rate can be realized.

  In the said manufacturing method, quenching and a tempering process are not implemented with respect to the seamless steel pipe after hot processing.

[Number of blisters]
In the seamless steel pipe manufactured by the above manufacturing method, generation | occurrence | production of a blister can be suppressed. In particular, when the Nb content (%) is equal to or greater than the F1 value defined by the formula (1), the number of blisters on the surface is less than 10/20 cm ² . Here, the number of blisters can be determined by the following blister number measurement test.

[Blister count measurement test]
An HIC test using a wet hydrogen sulfide environment (sour environment) is performed based on NACE TM0284-2011 defined by NACE (National Association of Corrosion Engineers) International. Specifically, a coupon test piece having a plate thickness × 20 mm width × 100 mm length (a length in the axial direction of the seamless steel pipe) is collected from the seamless steel pipe. The coupon test piece has a pair of surfaces corresponding to an outer surface and an inner surface of a seamless steel pipe.

In accordance with NACE TM0284, a 25 ° C. test bath is prepared in which 5% NaCl + 0.5% CH ₃ COOH aqueous solution is saturated with 100% H ₂ S gas at atmospheric pressure. The coupon specimen is immersed in the test bath for 96 hours. After immersing for 96 hours, the surface of the coupon test piece (20 mm width × 100 mm length corresponding to the inner and outer surfaces of the seamless steel pipe) is visually observed. Then, the total number of blisters generated on the surface is counted to obtain the number of blisters (pieces / 20 cm ² ).

  As described above, in the seamless steel pipe according to the present embodiment, the occurrence of blister can be suppressed by increasing the yield strength to 350 MPa or more with C and Nb. Therefore, it is excellent in HIC resistance, and SOHIC hardly occurs when a tensile stress is applied.

  Ingots of steels A1 to A15 and B1 to B6 shown in Table 1 were produced.

“-” In Table 1 indicates that it was substantially “0”% (impurity level). F2 in Table 1 is defined as follows.
F2 = (Cr + Mo) / 5 + (Cu + Ni) / 15
In short, F2 is the left side of Equation (2).

  With reference to Table 1, the chemical composition of steel A1-steel A15 was in the range of the chemical composition of the seamless steel pipe of this embodiment. On the other hand, steel B1 and steel B3 did not contain Nb, and the Nb content of steel B2 was less than the lower limit of the Nb content of this embodiment. The C content of steel B4 and steel B5 was less than the lower limit of the C content of the seamless steel pipe of this embodiment. F2 of steel type B6 did not satisfy the formula (2).

  Each steel ingot was hot forged to produce a plurality of billets for each steel. The billet was heated at the heating temperature shown in Table 2, and then the billet was pierced and rolled using a piercing machine (piercer) to produce a seamless steel pipe. At this time, three types of seamless steel pipes having a thickness = 12.7 mm, 25.4 mm, and 38.1 mm were manufactured for each steel. The manufactured seamless steel pipe was cooled at the first cooling rate shown in Table 2 until the temperature of the seamless steel pipe reached 500 ° C., and thereafter cooled at the second cooling rate.

[Microstructure observation test]
A microstructure observation test was carried out on three types of thick seamless steel pipes manufactured for each steel. In the cross section of each seamless steel pipe (surface perpendicular to the axial direction of the seamless steel pipe), the central thickness portion was etched with nital or the like. One arbitrary visual field (visual field area 40000 μm ² ) of the etched thickness central portion was observed. A 500 × optical microscope was used for observation.

  As a result of the microstructure observation test, any seamless steel pipe had a structure composed of ferrite and pearlite.

[Yield strength test]
A round bar tensile test piece having a parallel portion with an outer diameter of 6 mm and a length of 40 mm was collected from each of the three types of seamless steel pipes of each steel. The parallel part was parallel to the axial direction of the seamless steel pipe. Using the collected round bar tensile test pieces, a tensile test was performed at room temperature (25 ° C.) to obtain a yield strength YS (0.2% proof stress) (MPa).

[Blister count measurement test]
The blister number measurement test described above was performed on each of the three types of seamless steel pipes for each steel to determine the number of blisters.

[Test results]
Table 2 shows the test results. Further, FIG. 2 is a photographic image of two surfaces (corresponding to the outer and inner surfaces of a seamless steel pipe) of a coupon test piece after a blister count measurement test of steel A4 (thickness 20 mm), and FIG. It is a photograph image of two surfaces of a coupon test piece after a blister number measurement test of a wall thickness of 20 mm). 2 and 3, the upper surface corresponds to the outer surface of the seamless steel pipe, and the lower surface corresponds to the inner surface of the seamless steel pipe.

Referring to Table 2, the chemical compositions of steels A1 to A11, A14 and A15 were appropriate. Therefore, in a 12.7 mm seamless steel pipe having a wall thickness of 15 mm or less, the yield strength YS was less than 350 to 450 MPa. Therefore, as shown in FIG. 2, the generation of blisters on the surface was suppressed, and the number of blisters was less than 10/20 cm ² .

Further, the Nb contents of the steels A1 to A9 and A11, A14 and A15 were not less than the F1 value defined by the formula (1) in the seamless steel pipe having a thickness of 25.4 mm. Therefore, even in a seamless steel pipe having a wall thickness exceeding 15 mm, a yield strength of 350 to less than 450 MPa was obtained, and the number of blisters was less than 10 pieces / 20 cm ² .

Further, the Nb contents of the steels A2 to A9, A14, and A15 were F1 values or more in the seamless steel pipe having a wall thickness of 38.1 mm. Therefore, even in a seamless steel pipe having a wall thickness exceeding 35 mm, a yield strength of 350 to less than 450 MPa was obtained, and the number of blisters was less than 10 pieces / 20 cm ² .

On the other hand, although the chemical composition of steel A12 and A13 was appropriate, the heating temperature was too low for steel A12, and the first cooling rate was too slow for steel A13. Therefore, the yield strength YS was less than 350 MPa, and the number of blisters was 10/20 cm ² or more in any thickness seamless steel pipe.

On the other hand, the Nb content of steels B1 to B3 was too low. Therefore, even in a seamless steel pipe having a wall thickness of less than 20 mm, the yield strength was less than 350 MPa. As a result, as shown in FIG. 3, a large number of blisters were generated on the surface, and the number of blisters was 10/20 cm ² or more.

Moreover, C content of steel B4 and B5 was too low. Therefore, even in a seamless steel pipe having a wall thickness of less than 20 mm, the yield strength was less than 350 MPa, and the number of blisters was 10/20 cm ² or more.

  The F2 value of steel B6 did not satisfy the formula (2). Therefore, the yield strength of steel B6 exceeded 450 MPa.

  While the embodiments of the present invention have been described above, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit thereof.

Claims (2)

% By mass
C: 0.08 to 0.24%,
Si: 0.10 to 0.50%,
Mn: 0.3 to 2.5%
P: 0.02% or less,
S: 0.006% or less,
Nb: 0.02 to 0.12%,
Al: 0.005 to 0.100%,
Ca: 0.0003 to 0.0050%,
N: 0.0100% or less,
O: 0.0050% or less,
Ti: 0 to 0.1%,
V: 0 to 0.03%,
Cr: 0 to 0.6%,
Mo: 0 to 0.3%,
Ni: 0 to 0.4%,
Cu: 0 to 0.3%, and
B: 0 to 0.005%,
And the balance is a chemical composition composed of Fe and impurities,
With a structure consisting of ferrite and pearlite,
A seamless steel pipe for line pipe used in a sour environment having a yield strength of 350 to less than 450 MPa. The seamless steel pipe according to claim 1,
The Nb content (% by mass) is a seamless steel pipe having an F1 value or more defined by the formula (1).
F1 = 0.02 + (t−15) × 0.001 (1)
Here, a numerical value excluding the unit of the thickness (unit: mm) of the seamless steel pipe is substituted for t.

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