No steel pipes

Abstract

To provide a seamless steel pipe which has high strength and excellent HAZ toughness after circumference welding.SOLUTION: A seamless steel pipe has a chemical composition described in the specification, and has a yield strength of 450 MPa or more. In the seamless steel pipe, when number density of Ti-containing particles, which satisfy a Ti content of 70 mass% or more and have an equivalent circle diameter of 0.10 μm or less, is defined by NDT pieces / mm2, and number density of (Mn, Mg)-containing particles, which satisfy a Mg content of 10% or more and a Mn content of 30% or more and have an equivalent circle diameter of 0.10 μm or less, is defined by NDM pieces / mm2, CE defied by Expression (1) and NDA defined by Expression (2) satisfy Expression (3). Expression (1): CE=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5. Expression (2): NDA=(0.1×NDT+NDM)×10-5. Expression (3): 20.0×CE-NDA≤5.0.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a seamless steel pipe, and more particularly to a seamless steel pipe suitable for use as a line pipe. [Background technology]

[0002] A pipeline is a system installed on land or the seabed that transports natural gas, crude oil, etc. Pipelines laid on the seabed are made up of multiple steel pipes (line pipes). Pipelines laid on the seabed are further subjected to high pressure from the production fluids that flow through them. Pipelines are also subjected to repeated strain from waves and seawater pressure from the outside. Therefore, the steel pipes (line pipes) that make up pipelines are required to have high strength and excellent toughness.

[0003] To date, technologies for increasing the strength and toughness of steel materials for line pipes have been proposed in Japanese Patent Laid-Open No. 2010-174343 (Patent Document 1), Japanese Patent Laid-Open No. 2015-190042 (Patent Document 2), and International Publication No. 2016 / 056216 (Patent Document 3).

[0004] The steel material proposed in Patent Document 1 is a thick, high-tensile, hot-rolled steel sheet containing, by mass%, 0.02 to 0.25% C, 1.0% or less Si, 0.3 to 2.3% Mn, 0.03% or less P, 0.03% or less S, 0.1% or less Al, 0.03 to 0.25% Nb, and 0.001 to 0.10% Ti, with Nb, Ti, and C contained so as to satisfy the formula ((Ti + Nb / 2) / C<4), with the balance consisting of Fe and impurities. This steel material further has a structure in which the microstructure at a position 1 mm from the surface in the sheet thickness direction is a single phase consisting of a bainite phase or a bainitic ferrite phase, and the ratio of the length of grain boundary cementite to the total grain boundary length is 10% or less. Patent Document 1 discloses that this steel material can ensure high strength and excellent low-temperature toughness.

[0005] The steel material proposed in Patent Document 2 is a steel plate for high-strength line pipe, and contains, in mass%, C: 0.02 to 0.20%, Si: 0.02 to 0.50%, Mn: 0.6 to 2.0%, P: 0.02% or less, S: 0.01% or less, Al: 0.010 to 0.080%, Nb: 0.002 to 0.060%, Ti: 0.003 to 0.030%, Ca: 0.0003 to 0.0060%, N: 0.0010 to 0.010%, REM: 0.0001 to 0.0300%, and Zr: 0.0001 to 0.0200%, with the balance being Fe and impurities. This steel further has an average grain size of 10 μm or less at the t / 4 (t: plate thickness) position, and a separation index (SI) of 0.30 mm / mm measured from the fracture surface of a Charpy test piece at a specified temperature. 2 The following is the description: Patent Document 2 discloses that this steel material has high strength and can ensure a high limit CTOD value even when separation occurs.

[0006] The steel material proposed in Patent Document 3 is a steel plate for line pipe, which contains, in mass%, C: 0.02 to 0.10%, Si: 0.01 to 0.50%, Mn: 0.10 to 1.0%, P: 0.015% or less, S: 0.0020% or less, Ca: 0.0002 to 0.0050%, Nb: 0.03 to 0.15%, Ti: 0.002 to 0.070%, Al: 0.002 to 0.080%, and N: 0.001 to 0.008%, in a range in which the Cp (= 4.46 [C] + 2.37 [Mn] / 6 + 22.36 [P]) value is 0.85 or less and the formula (0.8 ≦ [Mn] / [Nb] ≦ 25) is satisfied, with the balance being Fe and impurities. This steel further has a structure mainly composed of bainite. Patent Document 3 discloses that this steel has high strength, high toughness, and excellent HIC resistance. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-174343 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-190042 [Patent Document 3] International Publication No. 2016 / 056216 Summary of the Invention [Problem to be solved by the invention]

[0008] In recent years, development of oil and gas wells in harsher environments than ever before has progressed. Pipelines used in such harsh environments are required to have higher strength than ever before. Furthermore, seamless steel pipes without welds have been used instead of welded steel pipes for pipelines laid in such harsh environments. Here, seamless steel pipes used in pipelines have their pipe ends joined by circumferential welding. In this way, multiple seamless steel pipes (line pipes) are connected to form part of a pipeline.

[0009] On the other hand, when multiple seamless steel pipes are circumferentially welded together, the toughness of the heat affected zone (hereinafter referred to as HAZ (Heat Affected Zone)) of the weld is likely to decrease. As such, seamless steel pipes intended for use as line pipes are required to exhibit not only high strength but also excellent toughness in the HAZ after welding. On the other hand, Patent Documents 1 to 3 discuss steel plates for welded steel pipes, but do not discuss seamless steel pipes that are circumferentially welded. Patent Documents 1 to 3 also discuss the toughness of the steel material, but do not discuss HAZ toughness.

[0010] An object of the present disclosure is to provide a seamless steel pipe having high strength and excellent HAZ toughness after circumferential welding. [Means for solving the problem]

[0011] The seamless steel pipe according to the present disclosure has In mass%, C: 0.030~0.080%, Si: 0.50% or less, Mn: 1.00~2.50%, P: 0.050% or less, S: 0.0010~0.0100%, Cu: 1.00% or less, Ni: 0.01 to 1.00%, Cr: 0.01 to 0.50%, Ti: 0.003 to 0.010%, Al: 0.030~0.100%, N: 0.0020~0.0070%, Mg: 0.0005 to 0.0050%, Ca: 0 to 0.0015%, and B: Contains 0 to 0.0005% Mo: 0.01 to 0.30%, V: 0.01 to 0.10%, and Nb: Contains one or more elements selected from the group consisting of 0.01 to 0.10%; the balance being Fe and impurities, The yield strength is 450 MPa or more, In the seamless steel pipe, The number density of Ti-containing particles with a Ti content of 70 mass% or more and a circle equivalent diameter of 0.10 μm or less is defined as NDT particles / mm 2 and The number density of (Mn, Mg)-containing particles with an equivalent circle diameter of 0.10 μm or less and an Mg content of 10 mass% or more and an Mn content of 30 mass% or more is set to NDM particles / mm 2 When we define CE defined by formula (1) and NDA defined by formula (2) satisfy formula (3). CE=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5 (1) NDA = (0.1 × NDT + NDM) × 10 -5 (2) 20.0×CE-NDA≦5.0 (3) Here, the element symbols in formula (1) are substituted with the content of the corresponding element in mass %. If the corresponding element is not contained, "0" is substituted for the element symbol. [Effects of the Invention]

[0012] The seamless steel pipe according to the present disclosure has high strength and excellent HAZ toughness after circumferential welding. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a diagram showing the relationship between FnA (=20.0×CE−NDA) in this example and the CTOD value (mm) at −40° C., which is an index of HAZ toughness after welding. [Figure 2] FIG. 2 is a schematic diagram showing an enlarged view of a part of the observation field area when the microstructure of the seamless steel pipe according to this embodiment is observed with a microscope by the method according to this embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0014] First, the present inventors investigated, from the viewpoint of chemical composition, how to increase the strength of seamless steel pipes intended for use as line pipes and improve the HAZ toughness after welding. As a result, the present inventors found that the composition contains, in mass%, C: 0.030 to 0.080%, Si: 0.50% or less, Mn: 1.00 to 2.50%, P: 0.050% or less, S: 0.0010 to 0.0100%, Cu: 1.00% or less, Ni: 0.01 to 1.00%, Cr: 0.01 to 0.50%, Ti: 0.003 to 0.010%, Al: 0.030 to 0.100%, N: 0.0020 to 0.0070%, Mg: 0.0 It was believed that a seamless steel pipe containing Cr: 0.005-0.0050%, Ca: 0-0.0015%, and B: 0-0.0005%, and containing one or more elements selected from the group consisting of Mo: 0.01-0.30%, V: 0.01-0.10%, and Nb: 0.01-0.10%, with the balance being Fe and impurities, would have a high yield strength of 450 MPa or more and may be able to obtain excellent HAZ toughness after welding.

[0015] On the other hand, as mentioned above, if the strength of a steel material increases, the HAZ after welding tends to become hard, and the HAZ toughness tends to decrease. That is, in a seamless steel pipe having the above-mentioned chemical composition, increasing the yield strength to 450 MPa or more may result in insufficient HAZ toughness after welding. Therefore, the present inventors have conducted detailed studies on a method for increasing the HAZ toughness after welding while maintaining the yield strength of the steel material at 450 MPa or more.

[0016] The present inventors thought that if a large number of fine inclusions or precipitates (hereinafter, inclusions or precipitates in a steel material will also be simply referred to as "particles") are precipitated in a steel material, it would be possible to suppress the coarsening of crystal grains due to the heat during welding, and to obtain excellent HAZ toughness after welding while maintaining a yield strength of 450 MPa or more. Specifically, the present inventors focused on Ti-containing particles and (Mn, Mg)-containing particles having an equivalent circle diameter of 0.10 μm or less, and conducted detailed studies on improving HAZ toughness.

[0017] Hereinafter, Ti-containing particles having an equivalent circle diameter of 0.10 μm or less will also be referred to as "fine Ti-containing particles." Similarly, (Mn, Mg)-containing particles having an equivalent circle diameter of 0.10 μm or less will also be referred to as "fine (Mn, Mg)-containing particles." In this specification, "Ti-containing particles" refers to particles having a Ti content of 70% or more by mass, as determined by the method described below. Furthermore, in this specification, "(Mn, Mg)-containing particles" refers to particles having a Mg content of 10% or more and a Mn content of 30% or more by mass, as determined by the method described below.

[0018] The present inventors have found that in a seamless steel pipe having the above-mentioned chemical composition, most of the fine Ti-containing particles are fine Ti nitrides. The present inventors have also found that in a seamless steel pipe having the above-mentioned chemical composition, most of the fine (Mn, Mg)-containing particles are fine (Mn, Mg) sulfides. In this specification, (Mn, Mg) sulfides refer to sulfides containing Mn and Mg that are formed by the concentration of Mn in Mg sulfides.

[0019] Taking the above findings into consideration, the present inventors have further studied in detail the relationship between the number density of fine Ti-containing particles, the number density of fine (Mn, Mg)-containing particles in a seamless steel pipe having the above-mentioned chemical composition, and the HAZ toughness after welding. As a result, it has been found that in a seamless steel pipe having the above-mentioned chemical composition, the relationship between the CE defined by the following formula (1) and the number density NDT of fine Ti-containing particles (particles / mm 2 ), the number density of fine (Mn, Mg)-containing particles (NDM) (pieces / mm 2 ), and the NDA defined by the following formula (2), satisfying the following formula (3), the HAZ toughness after welding is significantly improved. CE=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5 (1) NDA = (0.1 × NDT + NDM) × 10 -5 (2) 20.0×CE-NDA≦5.0 (3) Here, the element symbols in formula (1) are substituted with the content of the corresponding element in mass %. If the corresponding element is not contained, "0" is substituted for the element symbol.

[0020] CE defined by formula (1) is an index of strength. The larger the CE, the stronger the seamless steel pipe and the harder the HAZ after welding. NDA defined by formula (2) is an index related to the suppression of coarsening of prior austenite grains in the HAZ by fine Ti-containing particles and fine (Mn, Mg)-containing particles. Fine (Mn, Mg) sulfides, which are the main constituents of the fine (Mn, Mg)-containing particles, have a higher melting temperature than fine Ti nitrides, which are the main constituents of the fine Ti-containing particles. In other words, fine (Mn, Mg)-containing particles are less likely to melt due to the heat during welding than fine Ti-containing particles. As a result, there is a possibility that a small number of fine (Mn, Mg)-containing particles can more effectively suppress coarsening of prior austenite grains in the HAZ. Taking the above findings into consideration, the inventors have calculated the number density NDT (particles / mm 2 ) and the number density NDM (number / mm 2 ) and obtained an NDA.

[0021] Formula (3) is defined as FnA = 20.0 × CE-NDA. FnA is an index of post-weld HAZ toughness in a seamless steel pipe having the above-mentioned chemical composition. The relationship between FnA and post-weld HAZ toughness will be explained in detail below with reference to the drawings. Figure 1 is a diagram showing the relationship between the FnA value and the CTOD value (mm) at -40°C, which is an index of post-weld HAZ toughness, for examples having the above-mentioned chemical composition among the examples described below. The FnA value and the CTOD value were determined by the method described below. In addition, in all of the examples shown in Figure 1, the yield strength was 450 MPa or more.

[0022] Referring to Figure 1, when FnA exceeds 5.0, the CTOD value drops sharply and becomes less than 0.25 mm. On the other hand, when FnA is 5.0 or less, the CTOD value remains stable at 0.25 mm or more. In other words, Figure 1 proves that when FnA is 5.0 or less, HAZ toughness can be stably increased.

[0023] The details of why the HAZ toughness of a seamless steel pipe after welding is significantly improved by setting the FnA to 5.0 or less are not clear. However, the present inventors speculate as follows: As described above, CE is an index of strength, and the larger the CE, the stronger the seamless steel pipe. On the other hand, as the strength of a seamless steel pipe increases, the strength of the HAZ after welding also increases, making the HAZ toughness more likely to decrease. Therefore, in order to obtain excellent HAZ toughness even when the strength of a seamless steel pipe is increased, it is thought that it is necessary to disperse more fine Ti-containing particles and / or fine (Mn, Mg)-containing particles. In this way, the present inventors believe that by adjusting the NDA according to the CE and setting the FnA to 5.0 or less, the HAZ toughness after welding can be significantly improved, even for seamless steel pipes with a high yield strength of 450 MPa or more. It is possible that the HAZ toughness of a seamless steel pipe after welding is significantly improved by setting the FnA to 5.0 or less through a mechanism different from the above. However, it has been proven by the examples described below that by making the FnA equal to or less than 5.0, the HAZ toughness after welding of a seamless steel pipe is significantly improved.

[0024] The seamless steel pipe according to this embodiment, which was completed based on the above findings, has the following features.

[0025] [1] A seamless steel pipe, In mass%, C: 0.030~0.080%, Si: 0.50% or less, Mn: 1.00~2.50%, P: 0.050% or less, S: 0.0010~0.0100%, Cu: 1.00% or less, Ni: 0.01 to 1.00%, Cr: 0.01 to 0.50%, Ti: 0.003 to 0.010%, Al: 0.030~0.100%, N: 0.0020~0.0070%, Mg: 0.0005 to 0.0050%, Ca: 0 to 0.0015%, and B: Contains 0 to 0.0005% Mo: 0.01 to 0.30%, V: 0.01 to 0.10%, and Nb: Contains one or more elements selected from the group consisting of 0.01 to 0.10%; the balance being Fe and impurities, The yield strength is 450 MPa or more, In the seamless steel pipe, The number density of Ti-containing particles with a Ti content of 70 mass% or more and a circle equivalent diameter of 0.10 μm or less is defined as NDT particles / mm 2 and The number density of (Mn, Mg)-containing particles with an equivalent circle diameter of 0.10 μm or less and an Mg content of 10 mass% or more and an Mn content of 30 mass% or more is set to NDM particles / mm 2 When we define CE defined by formula (1) and NDA defined by formula (2) satisfy formula (3). Seamless steel pipe. CE=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5 (1) NDA = (0.1 × NDT + NDM) × 10 -5 (2) 20.0×CE-NDA≦5.0 (3) Here, the element symbols in formula (1) are substituted with the content of the corresponding element in mass %. If the corresponding element is not contained, "0" is substituted for the element symbol.

[0026] [2] [1] A seamless steel pipe according to the present invention, Ca: 0.0001 to 0.0015%, and B: Contains one or more elements selected from the group consisting of 0.0001 to 0.0005% Seamless steel pipe.

[0027] [3] [1] or [2], wherein the seamless steel pipe is The seamless steel pipe is a seamless steel pipe for line pipe. Seamless steel pipe.

[0028] The seamless steel pipe according to this embodiment will be described in detail below. "%" for elements means mass % unless otherwise specified.

[0029] [Chemical composition] The seamless steel pipe according to this embodiment contains the following elements.

[0030] C: 0.030~0.080% Carbon (C) improves the hardenability of steel materials and increases their strength. If the C content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content is too high, the strength of the steel material becomes too high, even if the contents of other elements are within the ranges of this embodiment, and the HAZ toughness after welding decreases. Therefore, the C content is 0.030 to 0.080%. The preferred lower limit of the C content is 0.035%, more preferably 0.040%, even more preferably 0.045%, and even more preferably 0.050%. The preferred upper limit of the C content is 0.075%, even more preferably 0.070%.

[0031] Si:0.50% or less Silicon (Si) is unavoidably contained. That is, the lower limit of the Si content is more than 0%. Si deoxidizes steel. On the other hand, if the Si content is too high, the HAZ toughness after welding will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 0.50% or less. A preferred upper limit of the Si content is 0.45%, more preferably 0.40%, and even more preferably 0.30%. A preferred lower limit of the Si content to more effectively obtain the above effects is 0.01%, more preferably 0.03%, and even more preferably 0.05%.

[0032] Mn: 1.00 to 2.50% Manganese (Mn) forms (Mn, Mg) sulfides together with Mg and S, increasing the number density (NDM) of fine (Mn, Mg)-containing particles. As a result, HAZ toughness after welding is improved. Mn also improves the hardenability of the steel material, thereby increasing its strength. If the Mn content is too low, the above effects cannot be sufficiently obtained, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content is too high, the hardenability of the steel material becomes too high, even if the contents of other elements are within the ranges of this embodiment, and the HAZ toughness after welding is reduced. Therefore, the Mn content is 1.00 to 2.50%. The lower limit of the Mn content is preferably 1.10%, more preferably 1.20%, and even more preferably 1.30%. The upper limit of the Mn content is preferably 2.30%, more preferably 2.00%, and even more preferably 1.80%.

[0033] P:0.050% or less Phosphorus (P) is an impurity. That is, the lower limit of the P content is greater than 0%. If the P content is too high, even if the contents of other elements are within the ranges of this embodiment, P segregates at grain boundaries, reducing the HAZ toughness after welding. Therefore, the P content is 0.050% or less. A preferred upper limit of the P content is 0.040%, more preferably 0.030%, and even more preferably 0.020%. The P content is preferably as low as possible. However, an extreme reduction in the P content significantly increases manufacturing costs. Therefore, considering industrial production, a preferred lower limit of the P content is 0.001%, more preferably 0.003%, and even more preferably 0.005%.

[0034] S: 0.0010 to 0.0100% Sulfur (S) forms (Mn, Mg) sulfides together with Mn and Mg, increasing the number density NDM of fine (Mn, Mg)-containing particles. As a result, HAZ toughness after welding is improved. If the S content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the S content is too high, coarse Mn sulfides are formed in the steel material, reducing the number density NDM of fine (Mn, Mg)-containing particles, even if the contents of other elements are within the ranges of this embodiment. As a result, HAZ toughness after welding is reduced. Therefore, the S content is 0.0010 to 0.0100%. The lower limit of the S content is preferably 0.0012%, more preferably 0.0015%, and even more preferably 0.0020%. The upper limit of the S content is preferably 0.0090%, and even more preferably 0.0080%.

[0035] Cu: 1.00% or less Copper (Cu) is unavoidably contained. That is, the lower limit of the Cu content is more than 0%. Cu improves the hardenability of the steel material and increases its strength. On the other hand, if the Cu content is too high, even if the contents of other elements are within the ranges of this embodiment, the hardenability of the steel material becomes too high, and the HAZ toughness after welding decreases. Therefore, the Cu content is 1.00% or less. A preferred upper limit of the Cu content is 0.80%, more preferably 0.60%, and even more preferably 0.50%. To more effectively obtain the above effects, a preferred lower limit of the Cu content is 0.01%, more preferably 0.03%, even more preferably 0.05%, and even more preferably 0.10%.

[0036] Ni: 0.01 to 1.00% Nickel (Ni) improves the hardenability of steel materials and increases their strength. If the Ni content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ni content is too high, the hardenability of the steel material becomes too high, even if the contents of other elements are within the ranges of this embodiment, and the HAZ toughness after welding decreases. Therefore, the Ni content is 0.01 to 1.00%. The lower limit of the Ni content is preferably 0.02%, more preferably 0.03%, and even more preferably 0.05%. The upper limit of the Ni content is preferably 0.90%, more preferably 0.80%, and even more preferably 0.60%.

[0037] Cr: 0.01 to 0.50% Chromium (Cr) improves the hardenability of steel materials and increases their strength. If the Cr content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cr content is too high, the hardenability becomes too high, even if the contents of other elements are within the ranges of this embodiment, and the HAZ toughness after welding decreases. Therefore, the Cr content is 0.01 to 0.50%. The preferred lower limit of the Cr content is 0.02%, more preferably 0.05%, and even more preferably 0.10%. The preferred upper limit of the Cr content is 0.45%, more preferably 0.40%, and even more preferably 0.30%.

[0038] Ti: 0.003 to 0.010% Titanium (Ti) combines with N to form fine Ti nitrides, increasing the number density NDT of fine Ti-containing particles. As a result, HAZ toughness after welding is improved. If the Ti content is too low, the above effects cannot be sufficiently achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ti content is too high, Ti nitrides become coarse, reducing the number density NDT of fine Ti-containing particles, even if the contents of other elements are within the ranges of this embodiment. As a result, HAZ toughness after welding is actually reduced. Therefore, the Ti content is 0.003 to 0.010%. The lower limit of the Ti content is preferably more than 0.003%, more preferably 0.004%, and even more preferably 0.005%. The upper limit of the Ti content is preferably 0.009%, and even more preferably 0.008%.

[0039] Al: 0.030 to 0.100% Aluminum (Al) deoxidizes steel and inhibits Mg from forming oxides. If the Al content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Al content is too high, coarse oxide-based inclusions are formed, reducing the HAZ toughness after welding, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Al content is 0.030 to 0.100%. The preferred upper limit of the Al content is 0.090%, more preferably 0.080%. The preferred lower limit of the Al content is 0.035%, more preferably 0.040%. In this specification, the "Al" content refers to the content of "acid-soluble Al," i.e., "sol. Al."

[0040] N: 0.0020~0.0070% Nitrogen (N) combines with Ti to form fine Ti nitrides, increasing the number density NDT of fine Ti-containing particles. As a result, HAZ toughness after welding is improved. If the N content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the N content is too high, Ti nitrides become coarse, reducing the number density NDT of fine Ti-containing particles, even if the contents of other elements are within the ranges of this embodiment. As a result, HAZ toughness after welding is actually reduced. Therefore, the N content is 0.0020 to 0.0070%. The lower limit of the N content is preferably 0.0025%, more preferably 0.0028%, and even more preferably 0.0030%. The upper limit of the N content is preferably 0.0065%, more preferably 0.0060%, and even more preferably 0.0055%.

[0041] Mg: 0.0005 to 0.0050% Magnesium (Mg) forms (Mn, Mg) sulfides together with Mn and S, increasing the number density (NDM) of fine (Mn, Mg)-containing particles. As a result, HAZ toughness after welding is improved. If the Mg content is too low, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mg content is too high, coarse oxides are formed, even if the contents of other elements are within the ranges of this embodiment, and this actually reduces HAZ toughness after welding. Therefore, the Mg content is 0.0005 to 0.0050%. The preferred lower limit of the Mg content is 0.0006%, more preferably 0.0008%, and even more preferably 0.0010%. The preferred upper limit of the Mg content is 0.0048%, more preferably 0.0045%, and even more preferably 0.0040%.

[0042] The seamless steel pipe according to this embodiment contains one or more elements selected from the group consisting of Mo, V, and Nb. That is, only one of Mo, V, and Nb may be contained, and the contents of the other elements may be 0%. All of these elements improve the hardenability and strength of the steel material.

[0043] Mo: 0.01 to 0.30% Molybdenum (Mo) improves the hardenability of steel materials and increases their strength. On the other hand, if the Mo content is too high, even if the contents of other elements are within the ranges of this embodiment, the hardenability becomes too high, and the HAZ toughness after welding decreases. Therefore, when Mo is contained, the Mo content is 0.01 to 0.30%. To more effectively obtain the above effects, the lower limit of the Mo content is preferably 0.02%, more preferably 0.03%, and even more preferably 0.04%. The upper limit of the Mo content is preferably 0.25%, more preferably 0.20%, and even more preferably 0.18%.

[0044] V: 0.01 to 0.10% Vanadium (V) improves the hardenability of steel materials and increases their strength. On the other hand, if the V content is too high, even if the contents of other elements are within the ranges of this embodiment, the hardenability becomes too high, and the HAZ toughness after welding decreases. Therefore, when V is contained, the V content is 0.01 to 0.10%. To more effectively obtain the above effects, the lower limit of the V content is preferably 0.02%, and more preferably 0.03%. The upper limit of the V content is preferably 0.08%, more preferably 0.07%, and even more preferably 0.06%.

[0045] Nb: 0.01 to 0.10% Niobium (Nb) improves the hardenability of steel materials and increases their strength. On the other hand, if the Nb content is too high, even if the contents of other elements are within the ranges of this embodiment, the hardenability becomes too high, and the HAZ toughness after welding decreases. Therefore, when Nb is contained, the Nb content is 0.01 to 0.10%. To more effectively obtain the above effects, the lower limit of the Nb content is preferably 0.02%. The upper limit of the Nb content is preferably 0.09%, and more preferably 0.08%.

[0046] The balance of the seamless steel pipe according to this embodiment is composed of Fe and impurities. Here, the impurities refer to substances that are mixed in from raw materials such as ore and scrap or the manufacturing environment during industrial production of the seamless steel pipe according to this embodiment, and are acceptable within a range that does not adversely affect the seamless steel pipe according to this embodiment.

[0047] [Optional element] The seamless steel pipe according to this embodiment may contain Ca in place of a portion of Fe.

[0048] Ca: 0 to 0.0015% Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When contained, Ca controls the shape of Ti nitrides, refines the Ti nitrides, and increases the number density (NDT) of fine Ti-containing particles. Even if even a small amount of Ca is contained, the above effects can be obtained to some extent. On the other hand, if the Ca content is too high, even if the contents of other elements are within the range of this embodiment, Ca sulfides are formed, which inhibits the formation of (Mn, Mg) sulfides. As a result, the number density (NDM) of fine (Mn, Mg)-containing particles decreases, and the HAZ toughness after welding decreases. Therefore, the Ca content is 0 to 0.0015%. The preferred lower limit of the Ca content is more than 0%, more preferably 0.0001%, and even more preferably 0.0002%. The preferred upper limit of the Ca content is 0.0013%, and even more preferably 0.0010%.

[0049] The seamless steel pipe according to this embodiment may contain B in place of a portion of Fe.

[0050] B: 0 to 0.0005% Boron (B) is an optional element and does not necessarily need to be contained. That is, the B content may be 0%. When contained, B dissolves in steel to improve the hardenability of the steel material and increase its strength. Even if even a small amount of B is contained, the above effects can be obtained to some extent. On the other hand, if the B content is too high, coarse nitrides are formed, even if the contents of other elements are within the ranges of this embodiment, and the HAZ toughness after welding decreases. Therefore, the B content is 0 to 0.0005%. The preferred lower limit of the B content is more than 0%, and more preferably 0.0001%. The preferred upper limit of the B content is 0.0004%, and more preferably 0.0003%.

[0051] [Yield strength] The seamless steel pipe according to this embodiment has the above-mentioned chemical composition and an FnA of 5.0 or less, and as a result, the seamless steel pipe according to this embodiment has a yield strength of 450 MPa or more and exhibits excellent HAZ toughness after welding.

[0052] The lower limit of the yield strength of the seamless steel pipe according to this embodiment is preferably 455 MPa, and more preferably 460 MPa. The upper limit of the yield strength of the seamless steel pipe according to this embodiment is not particularly limited. The upper limit of the yield strength may be, for example, 650 MPa or 600 MPa.

[0053] The yield strength of the seamless steel pipe according to this embodiment can be determined by the following method. Specifically, a tensile test is performed by a method in accordance with JIS Z 2241 (2011). First, a tensile test specimen is prepared from the seamless steel pipe according to this embodiment. Here, when the wall thickness of the seamless steel pipe is 20 mm or more, a No. 4 test specimen (round bar test specimen) specified in JIS Z 2241 (2011) is prepared from the center of the wall thickness as the tensile test specimen. When the wall thickness of the seamless steel pipe is less than 20 mm, the thickness is taken as the entire wall thickness, and one of No. 12A, 12B, and 12C test specimens (arcuate test specimens) specified in JIS Z 2241 (2011) is prepared as the tensile test specimen depending on the outer diameter of the steel pipe. The longitudinal direction of the tensile test specimen is the pipe axis direction of the steel pipe. Using the prepared tensile test specimens, tensile tests were carried out in the air at room temperature (25°C), and the obtained 0.2% offset yield strength was defined as the yield strength (MPa). The maximum stress during uniform elongation was also defined as the tensile strength (MPa).

[0054] [Fine Ti-containing particles and fine (Mn, Mg)-containing particles] On the premise that the seamless steel pipe according to this embodiment has the above-mentioned chemical composition, CE defined by formula (1) and NDA defined by formula (2) satisfy formula (3). CE=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5 (1) NDA = (0.1 × NDT + NDM) × 10 -5 (2) 20.0×CE-NDA≦5.0 (3) Here, the element symbol in formula (1) is substituted with the content of the corresponding element in mass %. If the corresponding element is not contained, "0" is substituted for the element symbol. Furthermore, NDT in formula (2) represents the number density of Ti-containing particles with a Ti content of 70 mass % or more and a circle-equivalent diameter of 0.10 μm or less, expressed in units of particles / mm 2 The NDM in formula (2) is the number density of (Mn, Mg)-containing particles with an Mg content of 10 mass% or more, an Mn content of 30 mass% or more, and a circle-equivalent diameter of 0.10 μm or less, expressed in units of particles / mm 2 is assigned.

[0055] As mentioned above, CE defined by formula (1) is an index of strength. The larger the CE, the stronger the seamless steel pipe and the harder the HAZ after welding is likely to be. NDA defined by formula (2) is an index that indicates the degree to which fine Ti-containing particles and fine (Mn, Mg)-containing particles suppress the coarsening of prior austenite grains after welding. The larger the NDA, the greater the effect of suppressing the coarsening of prior austenite grains in the HAZ.

[0056] As described above, the NDA defined by formula (2) is an index relating to the suppression of coarsening of prior austenite grains in the HAZ by fine Ti-containing particles and fine (Mn, Mg)-containing particles. In a seamless steel pipe having the chemical composition according to this embodiment, most of the fine Ti-containing particles are fine Ti nitrides. Furthermore, in a seamless steel pipe having the chemical composition according to this embodiment, most of the fine (Mn, Mg)-containing particles are fine (Mn, Mg) sulfides. The (Mn, Mg) sulfides are particles formed when Mn is concentrated in Mg sulfides.

[0057] (Mn,Mg) sulfides are less likely to elongate than Mn sulfides and are more likely to become fine. In addition, (Mn,Mg) sulfides have a higher melting point than Ti nitrides. That is, fine (Mn,Mg)-containing particles mainly composed of fine (Mn,Mg) sulfides may be more effective in suppressing the coarsening of prior austenite grains in the HAZ than fine Ti-containing particles mainly composed of fine Ti nitrides. In consideration of the above findings, the inventors have determined the number density NDT (number / mm 2 ) and the number density NDM (number / mm 2 ) was adjusted to obtain the NDA defined by equation (2).

[0058] Here, the fine Ti-containing particles and the fine (Mn, Mg)-containing particles may exist alone or in a composite state. The composite state of the fine Ti-containing particles and the fine (Mn, Mg)-containing particles means that the fine Ti-containing particles and the fine (Mn, Mg)-containing particles are adjacent to each other and present in the steel material. This will be specifically described with reference to FIG. 2. FIG. 2 is a schematic diagram showing an enlarged view of a part of the observation field when the microstructure of the steel material according to this embodiment is observed with a microscope using the method according to this embodiment. In the observation field region 1 in FIG. 2, the fine Ti-containing particles 10 and the fine (Mn, Mg)-containing particles 20 are adjacent to each other.

[0059] Referring to Fig. 2, in the seamless steel pipe according to this embodiment, the fine Ti-containing particle 10 and the fine (Mn, Mg)-containing particle 20 may be observed as a single particle in combination. However, a person skilled in the art would naturally be able to determine whether the particles are single or combined based on the particle shape, contrast, and analysis results of the chemical composition. Therefore, in this embodiment, microscopic observation is performed using the method described below, and the particles shown in Fig. 2 are counted as one fine Ti-containing particle 10 and one fine (Mn, Mg)-containing particle 20.

[0060] In formula (3), FnA (= 20.0 × CE - NDA) is an index of the HAZ toughness after welding in a seamless steel pipe having the above-mentioned chemical composition. If FnA is too high, the HAZ toughness of the seamless steel pipe after welding will decrease. Therefore, in this embodiment, assuming that the seamless steel pipe has the above-mentioned chemical composition, FnA is set to 5.0 or less. The preferred upper limit of FnA is 4.9, more preferably 4.8, and even more preferably 4.7. The lower limit of FnA is not particularly limited. The lower limit of FnA may be, for example, -5.0, -4.0, or -3.0.

[0061] In this embodiment, the number density NDT of the fine Ti-containing particles and the number density NDM of the fine (Mn, Mg)-containing particles are not particularly limited as long as they satisfy the formula (3). The number density NDT of the fine Ti-containing particles is, for example, 0.5×10 6 ~15.0×10 6 (pcs / mm 2 The number density NDM of the fine (Mn, Mg)-containing particles may be, for example, 0.5 × 10 5 ~15.0×10 5 (pcs / mm 2 Furthermore, in this embodiment, the NDA defined by formula (2) is also not particularly limited as long as it satisfies formula (3). The NDA may be, for example, 1.0 to 30.0.

[0062] In this embodiment, the number density NDT of fine Ti-containing particles, the number density NDM of fine (Mn, Mg)-containing particles, and the NDA defined by formula (2) can be determined by the following method. A test piece for microstructure observation is prepared from the center of the wall thickness of a seamless steel pipe according to this embodiment. The surface of the test piece is mirror-polished and then ultrasonically cleaned in methanol. The cleaned surface is covered with a carbon vapor deposition film. The test piece whose surface is covered with the vapor deposition film is immersed in an electrolyte (10% acetylacetone-1% tetramethylammonium chloride-methanol solution) at 20°C to perform electrolysis. The electrolysis conditions are voltage: 100 mV, electrolysis amount: 10 C / cm 2 The deposited film is peeled off from the electrolyzed test piece, washed with ethanol, and then scooped up with a sheet mesh and dried.

[0063] This vapor-deposited film (replica film) is observed under a transmission electron microscope (TEM). Specifically, an arbitrary position is identified from the vapor-deposited film, and the observation is performed at a magnification of 20,000 times and an acceleration voltage of 200 kV. The size and number of observation fields are not particularly limited, but the total area of ​​the observation fields is preferably 1000 μm. 2 For example, if the observation field is 4.0 μm × 5.0 μm, the number of observation fields must be 50 or more, and the total area of ​​the observation fields must be 1000 μm. 2That's all.

[0064] In each observation field, particles having an equivalent circle diameter of 0.10 μm or less are identified. The particles can be identified from contrast. The equivalent circle diameter of the particles can be determined by image analysis of the image observed in TEM observation. In this embodiment, the lower limit of the equivalent circle diameter of the identified particles having an equivalent circle diameter of 0.10 μm or less is not particularly limited, but is, for example, 0.01 μm. That is, in this embodiment, particles having an equivalent circle diameter of 0.01 to 0.10 μm are identified.

[0065] The identified particles are subjected to point analysis using energy dispersive X-ray spectrometry (EDS). The EDS point analysis determines the elemental content of each particle. In the EDS point analysis, an acceleration voltage of 200 kV is used, and the target elements are quantified as Mg, Ti, V, Cr, Mn, Fe, Ni, Mo, and Nb. Based on the EDS analysis results for each particle with an equivalent circle diameter of 0.10 μm or less, particles with a Ti content of 70% or more by mass are identified as "fine Ti-containing particles." Furthermore, based on the EDS analysis results for each particle with an equivalent circle diameter of 0.10 μm or less, particles with an Mg content of 10% or more and an Mn content of 30% or more by mass are identified as "fine (Mn, Mg)-containing particles."

[0066] The total number of fine Ti-containing particles and the total number of fine (Mn, Mg)-containing particles identified in the 10 visual fields are counted to determine the density. As mentioned above, when fine Ti-containing particles and fine (Mn, Mg)-containing particles are combined, they are counted separately. Based on the total number of fine Ti-containing particles and the total area of ​​the 10 visual fields, the number density NDT (particles / mm 2 Based on the total number of fine (Mn, Mg)-containing particles and the total area of ​​the 10 fields of view, the number density NDM (number / mm 2 In this embodiment, as shown in Table 2 of the examples to be described later, the number density NDT (particles / mm 2 ) when calculating the unit of 106 pieces / mm 2 The number density is calculated by exponential notation and the mantissa is rounded to the nearest tenth. The number density NDM (number / mm 2 ) when calculating the unit of 10 5 pieces / mm 2 The number density is calculated by using exponential notation and the mantissa is rounded off to the nearest tenth.

[0067] Here, in EDS point analysis of steel material having the above-mentioned chemical composition, accurate quantification of the nitrogen (N) content and sulfur (S) content is often difficult with current technology. Therefore, in this embodiment, particles having a circle equivalent diameter of 0.10 μm or less and a Ti content of 70 mass % or more are specified as fine Ti-containing particles. In other words, the fine Ti-containing particles according to this embodiment may also include Ti compounds other than Ti nitrides. However, in a seamless steel pipe having the above-mentioned chemical composition, the amount of Ti compounds other than Ti nitrides (Ti carbides, Ti oxides, etc.) in the fine Ti-containing particles is negligibly small. That is, in this embodiment, the number density NDT (particles / mm 2 ) is essentially the number density (pieces / mm 2 ) is equivalent to

[0068] Furthermore, in this embodiment, among particles having an equivalent circle diameter of 0.10 μm or less, particles having an Mg content of 10 mass % or more and an Mn content of 30 mass % or more are specified as fine (Mn, Mg)-containing particles. That is, the fine (Mn, Mg)-containing particles according to this embodiment may also include compounds (oxides, etc.) other than (Mn, Mg) sulfides. However, in a seamless steel pipe having the above-mentioned chemical composition, the amount of compounds other than (Mn, Mg) sulfides in the fine (Mn, Mg)-containing particles is negligibly small. That is, in this embodiment, the number density NDM (numbers / mm 2 ) is essentially the number density (pieces / mm ) of (Mn, Mg) sulfides with a circle equivalent diameter of 0.10 μm or less. 2 ) is equivalent to

[0069] [HAZ toughness] The seamless steel pipe according to this embodiment has the above-mentioned chemical composition and an FnA of 5.0 or less. As a result, the seamless steel pipe according to this embodiment has a yield strength of 450 MPa or more and exhibits excellent HAZ toughness after welding. Here, excellent HAZ toughness is defined as follows.

[0070] A CTOD test is performed on the HAZ formed in the seamless steel pipe according to this embodiment using a method in accordance with ISO 12135 (2021), and the CTOD value at -40°C is determined. Specifically, first, a welded joint is produced from the seamless steel pipe according to this embodiment. The groove shape is a single-bevel, and the welding method is gas metal arc welding. A gas containing 80% Ar and 20% CO2 is used as the shielding gas for welding. The welding heat input is 2.0 kJ / mm, and the preheat and interlaminar temperatures are 125 to 150°C. The welding wire used is G69A2UCN1M2T as specified in JIS Z 3312 (2009), and the welding wire diameter is 1.2 mm.

[0071] In accordance with API RP 2Z (2005), an arbitrary position is identified from the HAZ near the fusion line (hereinafter also referred to as the "FL" (Fusion Line)) on the straight side of the square groove of the welded joint. Using the identified position as the notch position, a three-point bend CTOD test specimen is prepared by a method in accordance with ISO 15653 (2018). In this embodiment, a single edge notched bend (SENB) test specimen is used as the CTOD test specimen. The CTOD test specimen has a thickness of B, a width W of 2B, and a length L of 10B. The CTOD test specimen is also prepared so that the thickness B is as large as possible. The notch in the CTOD test specimen has a width of 2 mm and a tip shape of 60°.

[0072] A fatigue test to introduce a pre-crack is performed on the prepared CTOD test specimen. In this embodiment, the initial relative crack length a0 / W is set to 0.50. Specifically, the fatigue test is performed at room temperature (25°C), and a 2 mm-long fatigue pre-crack is introduced at the tip of the notch, with the initial crack length a0 set to B. The CTOD test specimen with the pre-crack introduced is then subjected to a CTOD test at -40°C in accordance with ISO 12135 (2021). The CTOD value (mm) is calculated based on ISO 12135 (2021) from the load at fracture in the load-opening curve and the plastic component of the clip gauge opening displacement obtained from the CTOD test. The same test is performed three times, and the smallest CTOD value (mm) is defined as the CTOD value (mm) of the seamless steel pipe at -40°C.

[0073] The validity of the CTOD test is evaluated in accordance with API RP 2Z (2005). Specifically, the test results are deemed appropriate if the fatigue pre-crack passes through 15% or more of the coarse-grain HAZ adjacent to the FL, which is the target structure, in the central region of the thickness B of the test piece, covering two-thirds of the total length of the fatigue pre-crack. If the test results are deemed inappropriate, the CTOD test is conducted again, starting with the preparation of a CTOD test piece. In this embodiment, if the CTOD value at −40°C obtained by the above method is 0.25 mm or more, it is deemed to indicate excellent HAZ toughness after welding.

[0074] [Microstructure] Preferably, the microstructure of the seamless steel pipe according to this embodiment is mainly composed of tempered bainite. More specifically, the microstructure of the seamless steel pipe according to this embodiment has a volume fraction of tempered bainite of 90% or more. The remainder of the microstructure is, for example, ferrite or pearlite. Note that the microstructure of the seamless steel pipe according to this embodiment may contain minute amounts of precipitates, inclusions, etc. in addition to tempered bainite, ferrite, and pearlite. However, in the microstructure of the seamless steel pipe according to this embodiment, the volume fraction of precipitates, inclusions, etc. is negligibly small compared to the volume fractions of tempered bainite, ferrite, and pearlite.

[0075] The volume fraction of tempered bainite can be determined by observation using the following method. First, a test piece is prepared from the center of the wall thickness of a seamless steel pipe according to this embodiment, with the observation surface being a plane including the pipe axial direction and the pipe radial direction. The observation surface of the test piece is polished to a mirror finish, and then immersed in a nital etching solution for about 10 seconds to reveal the structure by etching. The etched observation surface is observed using a scanning electron microscope (SEM) in 10 fields of view as secondary electron images. The field area is, for example, 0.01 mm 2 (Magnification: 1000x). In each field of view, tempered bainite is identified from the contrast. The area fraction of the identified tempered bainite is determined. The method for determining the area fraction is not particularly limited, and any well-known method may be used. For example, the area fraction of tempered bainite can be determined by image analysis. In this embodiment, the arithmetic mean value of the area fractions of tempered bainite determined in all fields of view is defined as the volume fraction of tempered bainite.

[0076] [Seamless steel pipe shape] The shape of the seamless steel pipe according to this embodiment is not particularly limited as long as it is a seamless steel pipe. That is, there are no particular limitations on the outer diameter, wall thickness, and length. When the seamless steel pipe according to this embodiment is a seamless steel pipe for line pipe, the outer diameter is preferably 50 to 600 mm. Furthermore, when the seamless steel pipe according to this embodiment is a seamless steel pipe for line pipe, the wall thickness is more preferably 5 to 60 mm. In particular, even if the wall thickness is 20 mm or more, the seamless steel pipe has high strength and excellent HAZ toughness after circumferential welding.

[0077] [Manufacturing method] A method for manufacturing a seamless steel pipe according to this embodiment will be described below. The method for manufacturing a seamless steel pipe described below is one example of a method for manufacturing a seamless steel pipe according to this embodiment. That is, the seamless steel pipe according to this embodiment may be manufactured by a manufacturing method other than the manufacturing method described below. One example of a method for manufacturing a seamless steel pipe according to this embodiment includes a steelmaking process in which molten steel is cast to manufacture a raw material, a hot working process in which the raw material is hot worked to manufacture a mother pipe, a quenching process in which the mother pipe is quenched, and a tempering process in which the quenched mother pipe is tempered.

[0078] [Steelmaking process] In the steelmaking process, first, molten steel satisfying the above-mentioned chemical composition is produced. The method for producing molten steel is not particularly limited, and any known method may be used. In other words, the production method is not limited as long as molten steel satisfying the above-mentioned chemical composition can be produced. Next, the prepared molten steel is cast to produce a raw material. The casting method is not particularly limited, but is, for example, a continuous casting method. When producing a raw material by continuous casting, it is preferably carried out by the following method.

[0079] When a material is produced by continuous casting, the material produced is preferably a billet with a circular cross section (round billet). Preferably, the cast round billet is cooled to room temperature at an average cooling rate of 10.0 to 13.0°C / min from 1400°C to 1000°C. In this specification, the average cooling rate of the billet from 1400°C to 1000°C is also referred to as the "average billet cooling rate." The average billet cooling rate can be determined by measuring the temperature of the billet surface with a non-contact thermometer.

[0080] If the average cooling rate of the billet is too slow, Ti nitrides and / or (Mn, Mg) sulfides may coarsen in the molten steel. As a result, the number density of fine Ti-containing particles and / or fine (Mn, Mg)-containing particles may decrease in the manufactured seamless steel pipe, and FnA may become too large. On the other hand, if the average cooling rate of the billet is too fast, the crystallization amount and / or precipitation amount of Ti nitrides and / or (Mn, Mg) sulfides may decrease. As a result, the number density of fine Ti-containing particles and / or fine (Mn, Mg)-containing particles may decrease in the manufactured seamless steel pipe, and FnA may become too large. Therefore, in this embodiment, the average cooling rate of the billet is preferably set to 10.0 to 13.0°C / min.

[0081] By the above-described method, molten steel is cast to produce a material. As described above, the material is preferably a billet with a circular cross section (round billet). In this case, the molten steel may be cast to produce a billet with a rectangular cross section, or a bloom. In these cases, it is preferable to perform blooming to produce a round billet from the billet with a rectangular cross section or the bloom.

[0082] [Hot processing process] In the hot working step, a prepared material is hot worked to produce a mother pipe. First, a billet is heated in a heating furnace. Preferably, the billet is heated by being charged into a continuous heating furnace. The following describes the case where the billet is charged into a continuous heating furnace. In this case, the heating furnace may be a rotary hearth type or a walking beam type.

[0083] In a continuous heating furnace, billets are charged through the charging port and heated as they move through the furnace. From the charging port to the discharge port, the heating furnace is divided into a preheating zone, a heating zone, and a soaking zone. The preheating zone is the zone with the charging port and has the lowest furnace temperature of the three zones. The heating zone is located between the preheating zone and the soaking zone. The soaking zone is the zone following the heating zone and has a discharge port at its rear.

[0084] In this embodiment, the heating conditions in the preheating zone are preferably as follows. Preheating furnace temperature T1: 1050-1200℃ Preheating time t1: 70 to 200 minutes Furthermore, the furnace temperature T1 (°C) in the preheating zone and the furnace time t1 (minutes) satisfy the following formula (A). (273.15+T1)×(20+Log(t1 / 60))≧28000 (A)

[0085] In this embodiment, the furnace temperature T1 in the preheating zone is preferably set to 1050 to 1200°C. If the furnace temperature T1 in the preheating zone is too high, the number density of fine Ti-containing particles and / or fine (Mn, Mg)-containing particles may decrease in the manufactured seamless steel pipe. In this case, sufficient HAZ toughness cannot be obtained after welding. On the other hand, if the furnace temperature T1 in the preheating zone is too low, the mother pipe will not be heated sufficiently, which increases the equipment load in the hot working described below. Therefore, by setting the furnace temperature T1 in the preheating zone to 1050 to 1200°C, provided that the other preferable manufacturing method of this embodiment is satisfied, the FnA of the manufactured seamless steel pipe can be made 5.0 or less.

[0086] In this embodiment, the residence time t1 in the preheating zone is preferably set to 70 to 200 minutes. If the residence time t1 in the preheating zone is too short, the center of the mother pipe may not be heated sufficiently, which may make it difficult to stably perform the hot working described below. On the other hand, if the residence time t1 in the preheating zone is too long, Ti nitrides and / or (Mn, Mg) sulfides in the mother pipe may become too dissolved. In this case, the number density of fine Ti-containing particles and / or fine (Mn, Mg)-containing particles in the produced seamless steel pipe decreases, making it difficult to obtain sufficient HAZ toughness after welding. Therefore, by setting the residence time t1 in the preheating zone to 70 to 200 minutes, provided that the other preferred manufacturing methods of this embodiment are satisfied, the FnA of the produced seamless steel pipe can be made 5.0 or less.

[0087] In this embodiment, it is preferable that the furnace temperature T1 (° C.) in the preheating zone and the furnace time t1 (minutes) satisfy the following formula (A). (273.15+T1)×(20+Log(t1 / 60))≧28000 (A)

[0088] LMP1 is defined as (273.15 + T1) × (20 + Log(t1 / 60)). LMP1 is the Larson-Miller parameter in the preheating zone. If LMP1 is too low, the number density of fine Ti-containing particles and / or fine (Mn, Mg)-containing particles may decrease in the manufactured seamless steel pipe. In this case, sufficient HAZ toughness cannot be obtained after welding. Therefore, by setting LMP1 to 28,000 or more, the FnA of the manufactured seamless steel pipe can be set to 5.0 or less, provided that the other preferable manufacturing methods of this embodiment are satisfied. Note that there is no particular upper limit to LMP1. The upper limit of LMP1 may be, for example, 30,233 or 30,000.

[0089] The detailed reasons why the FnA of a seamless steel pipe produced using a continuous heating furnace can be adjusted by adjusting the heating conditions in the preheating zone have not yet been clarified. However, it has been proven by the examples described below that, for a mother pipe having the above-mentioned chemical composition, if the heating conditions in the preheating zone satisfy the ranges of this embodiment, the FnA will be 5.0 or less, provided that the other manufacturing methods also satisfy the preferred ranges of this embodiment.

[0090] In this embodiment, it is further preferable that the heating conditions in the heating zone and the soaking zone be as follows. Furnace temperature in the heating zone: 1200-1320℃ Time in the heating zone: 40 to 100 minutes Furnace temperature in the soaking zone: 1200~1320℃ Soaking zone time: 30 to 70 minutes

[0091] In this embodiment, the furnace temperatures in the heating zone and the soaking zone are preferably set to 1200 to 1320°C. If the furnace temperatures in the heating zone and the soaking zone are too high, the crystal grain size in the manufactured seamless steel pipe may become too large, which may reduce the toughness of the manufactured seamless steel pipe. On the other hand, if the furnace temperatures in the heating zone and the soaking zone are too low, the mother pipe will not be heated enough, which will increase the equipment load in the hot working described below.

[0092] In this embodiment, the residence time in the heating zone is preferably 40 to 100 minutes. In this embodiment, the residence time in the soaking zone is further preferably 30 to 70 minutes. If these residence times are too short, the number density of fine Ti-containing particles and / or fine (Mn, Mg)-containing particles may decrease in the manufactured seamless steel pipe. In this case, sufficient HAZ toughness after welding cannot be obtained. In this case, the center of the mother pipe may not be heated sufficiently, making it difficult to stably perform the hot working described below. On the other hand, if these residence times are too long, the number density of fine Ti-containing particles and / or fine (Mn, Mg)-containing particles may decrease in the manufactured seamless steel pipe. In this case, sufficient HAZ toughness after welding cannot be obtained.

[0093] As described above, when a continuous heating furnace is used, it is preferable to carry out heating under the above-mentioned conditions. However, the seamless steel pipe according to this embodiment can also be produced without using a continuous heating furnace. In short, if the produced seamless steel pipe has the above-mentioned chemical composition and an FnA of 5.0 or less, the seamless steel pipe according to this embodiment can exhibit a yield strength of 450 MPa or more and excellent HAZ toughness after welding. The hot working after heating will be explained below.

[0094] A billet removed from the heating furnace is hot worked to produce a mother pipe (seamless steel pipe). In this embodiment, the hot working method is not particularly limited and may be a well-known method. For example, a mother pipe may be produced by carrying out the Mannesmann process as hot working. In this case, the round billet is pierced and rolled using a piercing mill. In piercing and rolling, the piercing ratio is not particularly limited, but is, for example, 1.0 to 4.0. The piercing-rolled round billet is further hot rolled using a mandrel mill, a reducer, a sizing mill, or the like to produce a mother pipe. The cumulative area reduction rate in the hot working process is, for example, 20 to 70%. Alternatively, a mother pipe may be produced from the billet using other hot working methods. A mother pipe may also be produced by forging, such as the Erhardt process. A mother pipe is produced through the above steps.

[0095] The mother pipe produced by hot working may be air-cooled (as-rolled). The mother pipe produced by hot working may be quenched directly after the hot working without being cooled to room temperature, or may be quenched after being reheated after the hot working. The quenching process will be described in detail below.

[0096] [Quenching process] In the quenching process, the prepared mother pipe is quenched. In this specification, "quenching" means rapidly cooling the mother pipe at the A3 point or above. The preferred quenching temperature is 800 to 1000°C. If the quenching temperature is too high, the prior γ grains may become coarse, which may reduce the HAZ toughness of the manufactured seamless steel pipe after welding. Therefore, the quenching temperature is preferably 800 to 1000°C.

[0097] The quenching method involves, for example, continuously cooling the mother tube from the quenching start temperature to continuously lower the surface temperature of the mother tube. The method of continuous cooling is not particularly limited, and any well-known method may be used. Examples of continuous cooling methods include a method of cooling the mother tube by immersing it in a water bath, or a method of accelerated cooling of the mother tube by shower water cooling or mist cooling. The tempering step will be described in detail below.

[0098] [Tempering process] In the tempering process, the mother pipe that has been subjected to the above-mentioned quenching is tempered. In this specification, "tempering" refers to the process of tempering the mother pipe after quenching. c1 The tempering temperature corresponds to the furnace temperature at which the quenched mother pipe is heated and held. The tempering time refers to the time from when the temperature of the mother pipe reaches the predetermined tempering temperature until the mother pipe is removed from the heat treatment furnace.

[0099] The tempering temperature is adjusted appropriately depending on the chemical composition of the seamless steel pipe and the yield strength to be obtained. That is, the tempering temperature is adjusted for a mother pipe having the chemical composition of this embodiment to adjust the yield strength of the seamless steel pipe to 450 MPa or more. In the tempering process of this embodiment, the tempering temperature is preferably 500 to 700°C. Furthermore, in the tempering process of this embodiment, the tempering time is preferably 5 to 240 minutes.

[0100] The seamless steel pipe according to this embodiment can be manufactured by the above-described manufacturing method. However, as mentioned above, the above-described manufacturing method is only an example, and the seamless steel pipe may be manufactured by other manufacturing methods. The present invention will be described in more detail below with reference to examples. [Example]

[0101] Molten steels having the chemical compositions shown in Table 1 were produced. In Table 1, "-" indicates that the content of each element was at the impurity level. Specifically, the V content of Test No. 1 was rounded to two decimal places to mean 0%. The Ca content and B content of Test No. 1 were rounded to five decimal places to mean 0%. The Mo content and Nb content of Test No. 5 were rounded to two decimal places to mean 0%. Table 1 also shows the element contents of each Test No. and the CE calculated from the above formula (1).

[0102] [Table 1]

[0103] A round billet was produced by continuous casting using the molten steel of each test number. After continuous casting, the round billet of each test number was cooled from 1400°C to 1000°C at the average billet cooling rate (°C / min) shown in Table 2, and then further cooled to room temperature. The cooled round billet of each test number was heated in a continuous heating furnace and hot worked. The continuous heating furnace had a preheating zone, a heating zone, and a soaking zone. Table 2 shows the furnace temperature T1 (°C) and residence time t1 (min) of the preheating zone for the heating of the round billet of each test number, as well as LMP1 calculated from T1, t1, and formula (A). Table 2 also shows the furnace temperature (°C) and residence time (min) of the heating zone, the furnace temperature (°C) of the soaking zone, and the residence time (min).

[0104] [Table 2]

[0105] The obtained mother pipes of each test number were subjected to quenching and tempering. Specifically, the mother pipes of each test number were quenched by holding them at 850 to 1000°C for 5 to 90 minutes and then water-cooling. The quenched mother pipes of each test number were further tempered by holding them at 500 to 700°C for 5 to 240 minutes. Seamless steel pipes of each test number were obtained through the above manufacturing process. The outer diameter (mm) and wall thickness (mm) of the seamless steel pipes of each test number are shown in Table 2.

[0106] [Evaluation test] The seamless steel pipes having the respective test numbers after tempering were subjected to the following tensile tests, fine Ti-containing particle and fine (Mn, Mg)-containing particle number density measurement tests, and HAZ toughness evaluation tests.

[0107] [Tensile test] Tensile tests were conducted on the seamless steel pipes of each test number using a method conforming to JIS Z 2241 (2011). Specifically, No. 4 test pieces, as specified in JIS Z 2241 (2011), were prepared from the center of the wall thickness of each seamless steel pipe. The longitudinal direction of the tensile test piece was the axial direction of the steel pipe. Using the prepared tensile test pieces, tensile tests conforming to JIS Z 2241 (2011) were conducted in air at room temperature (25°C), and the obtained 0.2% offset proof stress was defined as the yield strength (MPa). The maximum stress during uniform elongation obtained in the same tensile test was defined as the tensile strength (MPa). For the seamless steel pipes of each test number, the obtained yield strength (MPa) is shown as "YS (MPa)" and the tensile strength is shown as "TS (MPa)" in Table 2.

[0108] [Measurement test for number density of fine Ti-containing particles and fine (Mn, Mg)-containing particles] For seamless steel pipes with each test number, a test for measuring the number density of fine Ti-containing particles and fine (Mn, Mg)-containing particles was carried out, and the number density NDT (number / mm 2 ) and the number density NDM (number / mm 2 ) was obtained. Specifically, test pieces for microstructure observation were prepared from the center of the wall thickness of the seamless steel pipe of each test number. Replica films were prepared using the prepared test pieces of each test number by the method described above. The prepared replica films were subjected to TEM observation by the method described above, and particles with an equivalent circle diameter of 0.10 μm or less were identified. EDS point analysis was performed on each identified particle, and particles with a Ti content of 70 mass% or more were identified as fine Ti-containing particles. Similarly, EDS point analysis was performed on each identified particle, and particles with an Mg content of 10 mass% or more and an Mn content of 30 mass% or more were identified as fine (Mn, Mg)-containing particles.

[0109] Based on the total number of identified fine Ti-containing particles and the total area of ​​the observation field, the number density NDT of fine Ti-containing particles (particles / mm 2 Based on the total number of identified fine (Mn, Mg)-containing particles and the total area of ​​the observation field, the number density NDM (number / mm2 The number density NDT (particles / mm 2 ) and the number density NDM (number / mm 2 ) and the above equation (2), NDA (= (0.1 × NDT + NDM) × 10 -5 ) was obtained. Furthermore, FnA was calculated from the CE of each test number, the NDA of each test number obtained, and the above-mentioned formula (3). The number density NDT (number / mm 2 ) and the number density NDM (number / mm 2 ), NDA, and FnA are shown in Table 2.

[0110] [HAZ toughness evaluation test] A HAZ toughness evaluation test was conducted on the seamless steel pipe of each test number to evaluate the toughness of the HAZ after welding. Specifically, a welded joint was prepared using the seamless steel pipe of each test number by the method described above. Three-point bending CTOD test specimens were prepared from the prepared welded joints of each test number by the method described above. The thickness B (mm) of the CTOD test specimens of each test number was as shown in Table 2. A CTOD test was conducted on the CTOD test specimens of each test number by the method described above, and the CTOD value (mm) at -40°C was determined. The obtained CTOD value (mm) at -40°C for each test number is shown in the "CTOD value (-40°C) (mm)" column of Table 2.

[0111] [Test Results] Referring to Tables 1 and 2, the chemical compositions of the seamless steel pipes of test numbers 1 to 13 were appropriate. Furthermore, the manufacturing methods also satisfied the above-mentioned preferred conditions. As a result, these seamless steel pipes had yield strengths of 450 MPa or more and FnA values ​​of 5.0 or less. As a result, in a HAZ toughness evaluation test, these seamless steel pipes had CTOD values ​​of 0.25 mm or more at -40°C, demonstrating excellent HAZ toughness. Furthermore, all of these seamless steel pipes had a microstructure mainly composed of tempered bainite.

[0112] The seamless steel pipe of test number 14 had an average billet cooling rate that was too slow. As a result, the FnA exceeded 5.0. As a result, in the HAZ toughness evaluation test, the CTOD value of this seamless steel pipe at -40°C was less than 0.25 mm, and it did not exhibit excellent HAZ toughness.

[0113] The seamless steel pipe of test number 15 had an average billet cooling rate that was too fast. As a result, the FnA exceeded 5.0. As a result, in the HAZ toughness evaluation test, the CTOD value of this seamless steel pipe at -40°C was less than 0.25 mm, and it did not exhibit excellent HAZ toughness.

[0114] The seamless steel pipe of test number 16 had an excessively high furnace temperature in the preheating zone. As a result, the FnA exceeded 5.0. As a result, in the HAZ toughness evaluation test, the CTOD value of this seamless steel pipe at -40°C was less than 0.25 mm, and it did not exhibit excellent HAZ toughness.

[0115] The seamless steel pipe of test number 17 had a preheating time that was too short. As a result, the FnA exceeded 5.0. As a result, in the HAZ toughness evaluation test, the CTOD value of this seamless steel pipe at -40°C was less than 0.25 mm, and it did not exhibit excellent HAZ toughness.

[0116] The seamless steel pipe of test number 18 had a low LMP1 in the preheating zone. As a result, the FnA exceeded 5.0. As a result, in the HAZ toughness evaluation test, the CTOD value of this seamless steel pipe at -40°C was less than 0.25 mm, and it did not exhibit excellent HAZ toughness.

[0117] The seamless steel pipe of test number 19 had a billet average cooling rate that was too slow, and the LMP1 in the preheating zone was too low. As a result, the FnA exceeded 5.0. As a result, in the HAZ toughness evaluation test, the CTOD value of this seamless steel pipe at -40°C was less than 0.25 mm, and it did not exhibit excellent HAZ toughness.

[0118] The seamless steel pipe of test number 20 had a furnace time that was too short in the heating zone. As a result, the FnA exceeded 5.0. As a result, in the HAZ toughness evaluation test, the CTOD value of this seamless steel pipe at -40°C was less than 0.25 mm, and it did not exhibit excellent HAZ toughness.

[0119] The seamless steel pipe of test number 21 had been in the soaking zone for too long. As a result, the FnA exceeded 5.0. As a result, in the HAZ toughness evaluation test, the CTOD value of this seamless steel pipe at -40°C was less than 0.25 mm, and it did not exhibit excellent HAZ toughness.

[0120] The seamless steel pipe of test number 22 had an excessively low Mg content. As a result, the FnA exceeded 5.0. As a result, in the HAZ toughness evaluation test, the CTOD value of this seamless steel pipe at -40°C was less than 0.25 mm, and it did not exhibit excellent HAZ toughness.

[0121] The seamless steel pipe of test number 23 had an excessively low S content. As a result, the FnA exceeded 5.0. As a result, in the HAZ toughness evaluation test, the CTOD value of this seamless steel pipe at -40°C was less than 0.25 mm, and it did not exhibit excellent HAZ toughness.

[0122] The seamless steel pipe of test number 24 had an excessively high S content. As a result, in the HAZ toughness evaluation test, this seamless steel pipe had a CTOD value of less than 0.25 mm at -40°C, and did not exhibit excellent HAZ toughness.

[0123] The seamless steel pipe of test number 25 had an excessively low N content. As a result, the FnA exceeded 5.0. As a result, in the HAZ toughness evaluation test, the CTOD value of this seamless steel pipe at -40°C was less than 0.25 mm, and it did not exhibit excellent HAZ toughness.

[0124] The seamless steel pipe of test number 26 had an excessively high N content. As a result, the FnA exceeded 5.0. As a result, in the HAZ toughness evaluation test, the CTOD value of this seamless steel pipe at -40°C was less than 0.25 mm, and it did not exhibit excellent HAZ toughness.

[0125] The seamless steel pipe of test number 27 had an excessively low Ti content. As a result, the FnA exceeded 5.0. As a result, in the HAZ toughness evaluation test, the CTOD value of this seamless steel pipe at -40°C was less than 0.25 mm, and it did not exhibit excellent HAZ toughness.

[0126] The seamless steel pipe of test number 28 had an excessively high Ti content. As a result, the FnA exceeded 5.0. As a result, in the HAZ toughness evaluation test, the CTOD value of this seamless steel pipe at -40°C was less than 0.25 mm, and it did not exhibit excellent HAZ toughness.

[0127] The seamless steel pipe of test number 29 had an excessively low Al content. As a result, the FnA exceeded 5.0. As a result, in the HAZ toughness evaluation test, the CTOD value of this seamless steel pipe at -40°C was less than 0.25 mm, and it did not exhibit excellent HAZ toughness.

[0128] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure. [Explanation of symbols]

[0129] 10 Fine Ti-containing particles 20 Fine (Mn, Mg) containing particles

Claims

1. A seamless steel pipe, In mass%, C: 0.030-0.080%, Si: 0.50% or less, Mn: 1.00-2.50%, P: 0.050% or less, S: 0.0010-0.0100%, Cu: 1.00% or less, Ni: 0.01-1.00%, Cr: 0.01-0.50%, Ti: 0.003 to 0.010%, Al: 0.030-0.100%, N: 0.0020-0.0070%, Mg: 0.0005-0.0050%, Ca: 0 to 0.0015%, and B: 0 to 0.0005%; Mo: 0.01-0.30%, V: 0.01 to 0.10%, and Nb: Contains one or more elements selected from the group consisting of 0.01 to 0.10%; the balance being Fe and impurities; The yield strength is 450 MPa or more, In the seamless steel pipe, The number density of Ti-containing particles having a Ti content of 70 mass% or more and a circle equivalent diameter of 0.10 μm or less is set to NDT particles / mm 2 and The number density of (Mn, Mg)-containing particles having an Mg content of 10% by mass or more and an Mn content of 30% by mass or more and an equivalent circle diameter of 0.10 μm or less is set to NDM particles / mm 2 When we define CE defined by formula (1) and NDA defined by formula (2) satisfy formula (3). Seamless steel pipe. CE=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5 (1) 0.11 × 844 + 847) × 100 -5 (2) 20.0 × CE − NDA ≦ 5.0 (3) Here, the element symbols in formula (1) are substituted with the content of the corresponding element in mass %. If the corresponding element is not contained, "0" is substituted for the element symbol.

2. The seamless steel pipe according to claim 1, Ca: 0.0001 to 0.0015%, and B: Contains one or more elements selected from the group consisting of 0.0001 to 0.0005%; Seamless steel pipe.

3. The seamless steel pipe according to claim 1 or 2, The seamless steel pipe is a seamless steel pipe for line pipe. Seamless steel pipe.

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