No steel pipes

Abstract

To provide a seamless steel pipe which has high strength and excellent low temperature toughness.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 (Mn, Mg)-containing particles, which satisfy, by mass%, 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 ND pieces / mm2, CE defined by Expression (1) and the ND satisfy Expression (2). Expression (1): CE=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5. Expression (2): 20.0×CE-ND×10-5≤7.0. Here, for an element symbol in Expression (1), the content of the corresponding element is substituted by mass%. When the corresponding element is not contained, "0" is substituted for the element symbol.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. Furthermore, in recent years, pipelines are sometimes laid in harsh environments, such as cryogenic environments. Therefore, the steel pipes (line pipes) that make up pipelines are required to have high strength and excellent low-temperature toughness.

[0003] To date, techniques for increasing the strength and low-temperature toughness of steel materials for line pipes have been proposed in Japanese Patent Laid-Open No. 2010-174343 (Patent Document 1) and Japanese Patent Laid-Open No. 2015-190042 (Patent Document 2).

[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. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-174343 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-190042 Summary of the Invention [Problem to be solved by the invention]

[0007] The above Patent Documents 1 and 2 propose techniques for achieving both strength and low-temperature toughness for steel materials intended for use as line pipes. However, steel materials achieving both strength and low-temperature toughness may be obtained by techniques other than those described in the above Patent Documents 1 and 2. Furthermore, the above Patent Documents 1 and 2 discuss steel plates for welded steel pipes, but do not discuss seamless steel pipes.

[0008] An object of the present disclosure is to provide a seamless steel pipe having high strength and excellent low-temperature toughness. [Means for solving the problem]

[0009] 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: 0.0007~0.0100%, Cu: 0.01 to 1.00%, Ni: 0.01 to 1.00%, Cr: 0.01 to 0.50%, Al: 0.030~0.100%, Ca: 0.0015% or less, Mg: 0.0010 to 0.0050%, and B: Contains 0.0005% or less, Mo: 0.01 to 0.30%, Nb: 0.01 to 0.10%, V: 0.01 to 0.10%, and Contains one or more elements selected from the group consisting of Ti: 0.001 to 0.010%, the balance being Fe and impurities, The yield strength is 450 MPa or more, In the seamless steel pipe, 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 ND particles / mm 2 When we define CE defined by formula (1) and the ND satisfy formula (2). CE=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5 (1) 20.0×CE-ND×10 -5 ≦7.0 (2) 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]

[0010] The seamless steel pipe according to the present disclosure has high strength and excellent low-temperature toughness. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram showing the relationship between FnA (=20.0×CE−ND×10 −5 ) and the CTOD value at −20° C., which is an index of low temperature toughness, in this example. DETAILED DESCRIPTION OF THE INVENTION

[0012] First, the present inventors have investigated, from the viewpoint of chemical composition, how to improve the strength and low-temperature toughness of seamless steel pipes intended for use as line pipes. As a result, the present inventors have found that the composition of seamless steel pipes is, 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: 0.0007 to 0.0100%, Cu: 0.01 to 1.00%, Ni: 0.01 to 1.00%, Cr: 0.01 to 0.50%, Al: 0.030 to 0.100%, Ca: 0.0015% or less, Mg: 0.0010 to 0.0007%. It was thought that a seamless steel pipe containing 0.050% or less of Mo, 0.0005% or less of B, and one or more elements selected from the group consisting of Mo: 0.01-0.30%, Nb: 0.01-0.10%, V: 0.01-0.10%, and Ti: 0.001-0.010%, with the balance being Fe and impurities, would have a high yield strength of 450 MPa or more and may have excellent low-temperature toughness.

[0013] On the other hand, it has been thought that increasing the strength of a steel material reduces its low-temperature toughness. 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 low-temperature toughness. Therefore, the present inventors conducted detailed studies on methods for improving the low-temperature toughness of a steel material while maintaining its yield strength at 450 MPa or more. As a result, the present inventors concluded that if a large number of fine inclusions or precipitates (hereinafter, inclusions or precipitates in a steel material may be simply referred to as "particles") are precipitated in the steel material, it may be possible to suppress the coarsening of crystal grains due to heat treatment during the manufacturing process, thereby achieving excellent low-temperature toughness while maintaining a yield strength of 450 MPa or more.

[0014] Specifically, the inventors focused on (Mn, Mg)-containing particles having an equivalent circle diameter of 0.10 μm or less. Hereinafter, (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, "(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. In addition, in seamless steel pipes 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, formed by the concentration of Mn in Mg sulfides.

[0015] Up until now, Mn sulfides have been known as inclusions that reduce low-temperature toughness in seamless steel pipes having the above-mentioned chemical composition. Mn sulfides are easily elongated and coarsened by hot working. Coarsened Mn sulfides are likely to become crack initiation sites at low temperatures. Therefore, Mn sulfides have been thought to reduce the low-temperature toughness of steel. From this perspective, it has been thought that reducing the number of Mn sulfides is preferable to improve the low-temperature toughness of seamless steel pipes having the above-mentioned chemical composition.

[0016] On the other hand, the present inventors considered that if the shape of Mn sulfides can be controlled using Mg and fine Mn sulfides can be dispersed, the low-temperature toughness of seamless steel pipes may be improved. In other words, by deliberately utilizing Mn sulfides, which have previously been thought to reduce the low-temperature toughness of steel, it may be possible to improve the low-temperature toughness of steel while maintaining a yield strength of 450 MPa or more. (Mn,Mg) sulfides are less likely to be elongated and more likely to become finer than Mn sulfides. Therefore, the present inventors considered that the low-temperature toughness of seamless steel pipes may be improved by increasing the number density of fine (Mn,Mg) sulfides (fine (Mn,Mg)-containing particles).

[0017] Specifically, the inventors manufactured various seamless steel pipes having the above-mentioned chemical composition and conducted a detailed study on the relationship between the number density of fine (Mn, Mg)-containing particles, yield strength, and low-temperature toughness. As a result, in seamless steel pipes having the above-mentioned chemical composition, it was found that the relationship between CE, defined by the following formula (1), and the number density ND (particles / mm 2 ) satisfies the following formula (2), the low temperature toughness of seamless steel pipes is significantly improved. CE=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5 (1) 20.0×CE-ND×10 -5 ≦7.0 (2) 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.

[0018] FnA=20.0×CE-ND×10 -5The FnA is defined as an index of low-temperature toughness in a seamless steel pipe having the above-mentioned chemical composition. The relationship between FnA and the low-temperature toughness of a seamless steel pipe 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 -20°C, which is an index of low-temperature toughness of a seamless steel pipe, 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.

[0019] Referring to Figure 1, when FnA exceeds 7.0, the CTOD value drops sharply and becomes less than 0.25 mm. On the other hand, when FnA is 7.0 or less, the CTOD value remains stable at 0.25 mm or more. In other words, Figure 1 proves that when FnA is 7.0 or less, the low-temperature toughness of seamless steel pipes can be stably improved.

[0020] As described above, the seamless steel pipe according to this embodiment has the above-mentioned chemical composition, and also has CE defined by the formula (1) and the number density ND (particles / mm 2 ) satisfies the formula (2). As a result, the seamless steel pipe according to this embodiment has a yield strength of 450 MPa or more and excellent low-temperature toughness.

[0021] The details of why the low-temperature toughness of a seamless steel pipe is significantly improved by setting FnA to 7.0 or less are not clear. However, the present inventors speculate as follows: CE, defined by formula (1), is an index of the strength of a seamless steel pipe. In other words, the larger CE, the higher the strength of the seamless steel pipe tends to be. On the other hand, the higher the strength of the seamless steel pipe, the lower the low-temperature toughness of the seamless steel pipe tends to be. Therefore, it is thought that the larger CE and the higher the strength of the seamless steel pipe, the more fine (Mn, Mg)-containing particles (with a circle equivalent diameter of 0.10 μm or less) need to be dispersed in order to improve low-temperature toughness. In this way, the present inventors believe that by adjusting the number density ND of the fine (Mn, Mg)-containing particles according to CE and setting FnA to 7.0 or less, the low-temperature toughness can be significantly improved even in seamless steel pipes with a high yield strength of 450 MPa or more. It is possible that the low-temperature toughness of a seamless steel pipe is significantly improved by setting the FnA to 7.0 or less through a mechanism different from the above. However, the fact that the low-temperature toughness of a seamless steel pipe is significantly improved by setting the FnA to 7.0 or less is proven by the examples described below.

[0022] The seamless steel pipe according to this embodiment, which was completed based on the above findings, has the following features.

[0023] [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: 0.0007~0.0100%, Cu: 0.01 to 1.00%, Ni: 0.01 to 1.00%, Cr: 0.01 to 0.50%, Al: 0.030~0.100%, Ca: 0.0015% or less, Mg: 0.0010 to 0.0050%, and B: Contains 0.0005% or less, Mo: 0.01 to 0.30%, Nb: 0.01 to 0.10%, V: 0.01 to 0.10%, and Contains one or more elements selected from the group consisting of Ti: 0.001 to 0.010%, the balance being Fe and impurities, The yield strength is 450 MPa or more, In the seamless steel pipe, 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 ND particles / mm 2 When we define CE defined by formula (1) and the ND satisfy formula (2), Seamless steel pipe. CE=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5 (1) 20.0×CE-ND×10 -5 ≦7.0 (2) 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.

[0024] [2] [1] A seamless steel pipe according to the present invention, The seamless steel pipe is a seamless steel pipe for line pipe. Seamless steel pipe.

[0025] The seamless steel pipe according to this embodiment will be described in detail below. "%" for elements means mass % unless otherwise specified.

[0026] [Chemical composition] The seamless steel pipe according to this embodiment contains the following elements.

[0027] C: 0.030~0.080% Carbon (C) improves the hardenability of steel and increases its 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 becomes too high, and the low-temperature toughness of the steel decreases, even if the contents of other elements are within the ranges of this embodiment. 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%.

[0028] 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 low-temperature toughness of the steel material decreases 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%, and more preferably 0.40%. 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%.

[0029] Mn: 1.00 to 2.50% Manganese (Mn) forms (Mn, Mg) sulfides together with Mg and S, increasing the number density (ND) of fine (Mn, Mg)-containing particles. As a result, the low-temperature toughness of the steel material is improved. Mn also improves the hardenability of the steel material and increases 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, and the low-temperature toughness of the steel material decreases, even if the contents of other elements are within the ranges of this embodiment. 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%.

[0030] P:0.050% or less Phosphorus (P) is an impurity. That is, the lower limit of the P content is more 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 low-temperature toughness of the steel material. 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%.

[0031] S: 0.0007 to 0.0100% Sulfur (S) forms (Mn, Mg) sulfides together with Mn and Mg, increasing the number density ND of fine (Mn, Mg)-containing particles. As a result, the low-temperature toughness of the steel material 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 MnS is formed in the steel material, even if the contents of other elements are within the ranges of this embodiment, and the low-temperature toughness of the steel material is reduced. Therefore, the S content is 0.0007 to 0.0100%. The lower limit of the S content is preferably 0.0008%, more preferably 0.0010%, and even more preferably 0.0012%. The upper limit of the S content is preferably 0.0090%, and even more preferably 0.0080%.

[0032] Cu: 0.01 to 1.00% Copper (Cu) improves the hardenability of steel materials and increases their strength. If the Cu 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 Cu content is too high, the hardenability of the steel material becomes too high, and the low-temperature toughness of the steel material decreases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cu content is 0.01 to 1.00%. The preferred lower limit of the Cu content is 0.02%, more preferably 0.03%, even more preferably 0.05%, and even more preferably 0.08%. The preferred upper limit of the Cu content is 0.80%, even more preferably 0.60%.

[0033] 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, and the low-temperature toughness of the steel material decreases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ni content is 0.01 to 1.00%. The lower limit of the Ni content is preferably 0.05%, and more preferably 0.10%. The upper limit of the Ni content is preferably 0.90%, more preferably 0.80%, and even more preferably 0.60%.

[0034] 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, and the low-temperature toughness of the steel material decreases, even if the contents of other elements are within the ranges of this embodiment. Therefore, when Cr is contained, the Cr content is 0.01 to 0.50%. To more effectively obtain the above effects, the lower limit of the Cr content is preferably 0.02%, more preferably 0.05%, and even more preferably 0.10%. The upper limit of the Cr content is preferably 0.45%, more preferably 0.40%, and even more preferably 0.30%.

[0035] 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 low-temperature toughness of the steel material 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 lower limit of the Al content is 0.035%, more preferably 0.040%. The preferred upper limit of the Al content is 0.090%, more preferably 0.080%. In this specification, the "Al" content refers to the content of "acid-soluble Al", i.e., "sol.Al".

[0036] Ca: 0.0015% or less Calcium (Ca) is unavoidably contained. That is, the lower limit of the Ca content is more than 0%. Ca deoxidizes and desulfurizes steel. If the Ca content is too high, even if the contents of other elements are within the ranges of this embodiment, Ca sulfides are formed, inhibiting the formation of (Mn, Mg) sulfides. As a result, the number density ND of fine (Mn, Mg)-containing particles decreases, and the low-temperature toughness of the steel material decreases. Therefore, the Ca content is 0.0015% or less. The preferred upper limit of the Ca content is 0.0013%, more preferably 0.0012%. To more effectively obtain the above effects, the preferred lower limit of the Ca content is 0.0001%, more preferably 0.0002%.

[0037] Mg: 0.0010 to 0.0050% Magnesium (Mg) forms (Mn, Mg) sulfides together with Mn and S, increasing the number density (ND) of fine (Mn, Mg)-containing particles. As a result, the low-temperature toughness of the steel material 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, reducing the low-temperature toughness of the steel material, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mg content is 0.0010 to 0.0050%. The preferred lower limit of the Mg content is 0.0012%, more preferably 0.0014%, and even more preferably 0.0015%. The preferred upper limit of the Mg content is 0.0048%, more preferably 0.0045%, and even more preferably 0.0040%.

[0038] B: 0.0005% or less Boron (B) is unavoidably contained. That is, the lower limit of the B content is more than 0%. B dissolves in steel to improve the hardenability of the steel material and increase its strength. On the other hand, if the B content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse nitrides are formed, and the low-temperature toughness of the steel material decreases. Therefore, the B content is 0.0005% or less. The preferred upper limit of the B content is 0.0004%, more preferably 0.0003%, and even more preferably 0.0002%. The preferred lower limit of the B content is 0.0001% to more effectively obtain the above effects.

[0039] The seamless steel pipe according to this embodiment contains one or more elements selected from the group consisting of Mo, Nb, V, and Ti. That is, only one of Mo, Nb, V, and Ti 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.

[0040] 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 low-temperature toughness of the steel material 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 0.28%, more preferably 0.25%, and even more preferably 0.18%.

[0041] 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 low-temperature toughness of the steel material 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 0.08%, more preferably 0.06%, and even more preferably 0.05%.

[0042] 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 low-temperature toughness of the steel material 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%.

[0043] Ti: 0.001 to 0.010% Titanium (Ti) improves the hardenability of steel materials and increases their strength. On the other hand, if the Ti content is too high, even if the contents of other elements are within the ranges of this embodiment, coarse nitrides are formed, reducing the low-temperature toughness of the steel material. Therefore, the Ti content is 0.001 to 0.010%. To more effectively obtain the above effects, the lower limit of the Ti content is preferably 0.002%, and more preferably 0.003%. The upper limit of the Ti content is preferably 0.009%, more preferably 0.007%, even more preferably 0.005%, even more preferably 0.004%, and even more preferably 0.003%.

[0044] 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.

[0045] [Yield strength] The seamless steel pipe according to this embodiment has the above-mentioned chemical composition and an FnA of 7.0 or less, and as a result, the seamless steel pipe according to this embodiment has a yield strength of 450 MPa or more and excellent low-temperature toughness.

[0046] 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.

[0047] 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, a tensile test was carried out in 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).

[0048] [Fine (Mn, Mg) containing particles] The seamless steel pipe according to this embodiment has the above-mentioned chemical composition, and further has CE defined by the formula (1) and a number density ND (particles / mm ) of (Mn, Mg)-containing particles having an equivalent circle diameter of 0.10 μm or less. 2 ) satisfies equation (2). CE=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5 (1) 20.0×CE-ND×10 -5 ≦7.0 (2) 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.

[0049] As described above, in this specification, "(Mn, Mg)-containing particles" refers to particles that satisfy the following conditions: Mg content is 10% or more and Mn content is 30% or more, by mass %. Furthermore, in this specification, (Mn, Mg)-containing particles having an equivalent circle diameter of 0.10 μm or less are also referred to as "fine (Mn, Mg)-containing particles."

[0050] FnA(=20.0×CE-ND×10-5 ) is an index of low-temperature toughness in a seamless steel pipe having the above-mentioned chemical composition. If FnA is too high, the CTOD value, which is an index of low-temperature toughness of a seamless steel pipe, drops sharply. Therefore, in this embodiment, FnA is set to 7.0 or less. In this embodiment, the upper limit of FnA is preferably 6.9, more preferably 6.8, and even more preferably 6.7. The lower limit of FnA is not particularly limited. The lower limit of FnA may be, for example, -3.0, -2.0, or -1.0.

[0051] The CE defined by formula (1) is an index showing the hardness of a seamless steel pipe. In this embodiment, the CE and the number density ND of the fine (Mn, Mg)-containing particles are not particularly limited as long as they satisfy the above-mentioned formula (2). The CE may be, for example, 0.300 to 0.600. By making the CE 0.360 or more, the yield strength and low-temperature toughness of the seamless steel pipe can be stably increased. Therefore, in this embodiment, the lower limit of the CE is preferably 0.360, more preferably 0.365, and even more preferably 0.370. The upper limit of the CE is preferably 0.580, more preferably 0.570, and even more preferably 0.560.

[0052] The number density ND of the fine (Mn, Mg)-containing particles is, for example, 0.50 × 10 5 ~10.00×10 5 (pcs / mm 2 ) The number density ND of the fine (Mn, Mg)-containing particles may be 1.00 × 10 5 (pcs / mm 2 ) or more, the yield strength and low-temperature toughness of the seamless steel pipe can be stably increased. Therefore, in this embodiment, the preferable lower limit of the number density ND of the fine (Mn, Mg)-containing particles is 1.00 × 10 5 (pcs / mm 2 ) The preferred upper limit of the number density ND of the fine (Mn, Mg)-containing particles is 9.00 × 10 5 (pcs / mm 2 ), and more preferably 8.50 × 105 (pcs / mm 2 )

[0053] In this embodiment, the number density ND of the fine (Mn, Mg)-containing particles can be determined by the following method. A test piece for microstructure observation is prepared from the central part 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 electrolytic solution (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.

[0054] 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. 2 That's all.

[0055] 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.

[0056] The identified particles are subjected to point analysis using energy dispersive X-ray spectrometry (EDS). The EDS point analysis determines the content of elements contained in each particle. In the EDS point analysis, an acceleration voltage is set to 200 kV, 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 a circle equivalent diameter of 0.10 μm or less, particles with a Mg content of 10% or more and a Mn content of 30% or more, by mass, are identified as "fine (Mn, Mg)-containing particles."

[0057] The total number of fine (Mn, Mg)-containing particles identified in the 10 visual fields is calculated. Based on the total number of fine (Mn, Mg)-containing particles and the total area of ​​the 10 visual fields, the number density ND (number / mm 2 In this embodiment, the number density ND (particles / mm 2 ) when calculating the unit is 10 5 pieces / mm 2 The number density is calculated by using exponential notation and the mantissa is rounded off to two decimal places.

[0058] Here, in EDS point analysis of steel material having the above-mentioned chemical composition, accurate quantification of the sulfur (S) content is often difficult with current technology. Therefore, 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 other than sulfides (oxides, etc.). 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 ND (particles / 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

[0059] [Low temperature toughness] The seamless steel pipe according to this embodiment has the above-mentioned chemical composition and an FnA of 7.0 or less. As a result, the seamless steel pipe according to this embodiment has a yield strength of 450 MPa or more and excellent low-temperature toughness. Here, excellent low-temperature toughness is defined as follows.

[0060] A CTOD test is performed on the seamless steel pipe according to this embodiment using a method in accordance with ISO 12135 (2021) to determine the CTOD value at -20°C. Specifically, a three-point bending CTOD test specimen is prepared from the seamless steel pipe according to this embodiment using 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°.

[0061] 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 -20°C in accordance with ISO 12135 (2021). The CTOD value (mm) is calculated based on the load at fracture in the load-opening curve and the plastic component of the clip gauge opening displacement obtained from the CTOD test, in accordance with ISO 12135 (2021). 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 -20°C. In this embodiment, a CTOD value of 0.25 mm or greater at -20°C determined by the above method is considered to exhibit excellent low-temperature toughness.

[0062] [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.

[0063] 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.

[0064] [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 further preferably 5 to 60 mm. In particular, even if the wall thickness is 20 mm or more, it has high strength and excellent low-temperature toughness.

[0065] [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.

[0066] [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.

[0067] 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.

[0068] If the average cooling rate of the billet is too slow, the (Mn, Mg) sulfides may coarsen in the molten steel. As a result, in the manufactured seamless steel pipe, the number density ND of the fine (Mn, Mg)-containing particles may decrease, and FnA may become too large. On the other hand, if the average cooling rate of the billet is too fast, the amount of crystallization and / or precipitation of the (Mn, Mg) sulfides may decrease. As a result, in the manufactured seamless steel pipe, the number density ND of the fine (Mn, Mg)-containing particles may decrease, 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.

[0069] 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.

[0070] [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.

[0071] 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.

[0072] 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)

[0073] 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 ND of fine (Mn, Mg)-containing particles may decrease in the manufactured seamless steel pipe. In this case, the seamless steel pipe will not have sufficient low-temperature toughness. 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 will increase 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 preferred manufacturing methods of this embodiment are satisfied, the FnA of the manufactured seamless steel pipe can be made 7.0 or less.

[0074] In this embodiment, the residence time t1 in the preparatory zone is preferably set to 70 to 200 minutes. If the residence time t1 in the preparatory 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 preparatory zone is too long, the (Mn, Mg) sulfides in the mother pipe may be excessively dissolved. In this case, the number density ND of fine (Mn, Mg)-containing particles in the produced seamless steel pipe decreases, and the seamless steel pipe may not have sufficient low-temperature toughness. Therefore, by setting the residence time t1 in the preparatory 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 7.0 or less.

[0075] 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)

[0076] 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 ND of fine (Mn, Mg)-containing particles may decrease in the manufactured seamless steel pipe. In this case, the seamless steel pipe may not have sufficient low-temperature toughness. Therefore, by setting LMP1 to 28,000 or more, provided that the other preferred manufacturing methods of this embodiment are met, the FnA of the manufactured seamless steel pipe can be set to 7.0 or less. Note that there is no particular upper limit to LMP1. The upper limit of LMP1 may be, for example, 30,233 or 30,000.

[0077] 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, at least 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 7.0 or less, provided that the other manufacturing methods satisfy the preferred ranges of this embodiment.

[0078] 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

[0079] 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.

[0080] In this embodiment, the residence time in the heating zone is preferably 40 to 100 minutes. Furthermore, in this embodiment, the residence time in the soaking zone is preferably 30 to 70 minutes. If these residence times are too short, the center of the mother pipe will not be heated sufficiently, which may make it difficult to stably perform the hot working described below. On the other hand, if these residence times are too long, the heating effect will saturate.

[0081] 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 7.0 or less, the seamless steel pipe according to this embodiment can have a yield strength of 450 MPa or more and excellent low-temperature toughness. Hereinafter, the hot working after heating will be described.

[0082] 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.

[0083] 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.

[0084] [Quenching process] In the quenching process, the prepared mother pipe is quenched. In this specification, "quenching" means rapidly cooling the mother pipe at or above the A3 point. 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 low-temperature toughness of the manufactured seamless steel pipe. Therefore, the quenching temperature is preferably 800 to 1000°C.

[0085] 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.

[0086] [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.

[0087] 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.

[0088] 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]

[0089] 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 Mo content and Nb content of test number 2 were rounded to two decimal places to mean 0%. The Ti content of test number 4 was rounded to two decimal places to mean 0%. The V content of test number 5 was rounded to two decimal places to mean 0%. Table 1 also shows the element contents of each test number and the CE calculated from the above formula (1).

[0090] [Table 1]

[0091] 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).

[0092] [Table 2]

[0093] 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.

[0094] [Evaluation test] The seamless steel pipes having the respective test numbers after tempering were subjected to a tensile test, a fine particle (Mn, Mg)-containing particle number density measurement test, and a low-temperature toughness evaluation test, which will be described below.

[0095] [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.

[0096] [Fine particle (Mn, Mg) containing particle number density measurement test] A fine (Mn, Mg) particle number density measurement test was conducted on the seamless steel pipe with each test number, and the number density ND (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 by the above-mentioned method using the prepared test pieces of each test number. TEM observation was performed on the prepared replica films by the above-mentioned method, 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 an Mg content of 10 mass% or more and an Mn content of 30 mass% or more were identified. Based on the total number of identified fine (Mn, Mg)-containing particles and the total area of ​​the observation field, the number density ND (particles / mm 2 ) was obtained. Furthermore, the number density ND (number / mm 2) and the above CE, FnA was calculated. The number density ND (number / mm 2 ) are shown in Table 2. The FnA obtained for each test number is shown in Table 2.

[0097] [Low temperature toughness evaluation test] A low-temperature toughness evaluation test was conducted on the seamless steel pipe of each test number. Specifically, a three-point bending CTOD test specimen was prepared from the seamless steel pipe of each test number using the method described above. The thickness B (mm) of the CTOD test specimen of each test number was as shown in Table 2. A CTOD test was conducted on the CTOD test specimen of each test number using the method described above, and the CTOD value (mm) at -20°C was obtained. The obtained CTOD value (mm) at -20°C for each test number is shown in the "CTOD value (-20°C) (mm)" column in Table 2.

[0098] [Test Results] Referring to Tables 1 and 2, the chemical compositions of the seamless steel pipes of test numbers 1 to 12 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 7.0 or less. As a result, in a low-temperature toughness evaluation test, these seamless steel pipes had CTOD values ​​of 0.25 mm or more at -20°C, demonstrating excellent low-temperature toughness. Furthermore, all of these seamless steel pipes had a microstructure mainly composed of tempered bainite.

[0099] On the other hand, the seamless steel pipe of test number 13 had an average billet cooling rate that was too slow. As a result, the FnA exceeded 7.0. As a result, in the low-temperature toughness evaluation test, the CTOD value of this seamless steel pipe at -20°C was less than 0.25 mm, and it did not exhibit excellent low-temperature toughness.

[0100] The seamless steel pipe of test number 14 had an average billet cooling rate that was too fast. As a result, the FnA exceeded 7.0. As a result, in the low-temperature toughness evaluation test, the CTOD value of this seamless steel pipe at -20°C was less than 0.25 mm, and it did not exhibit excellent low-temperature toughness.

[0101] For the seamless steel pipe of test number 15, the furnace temperature in the preheating zone was too low. Furthermore, the LMP1 in the preheating zone was too low. As a result, the FnA exceeded 7.0. As a result, in the low-temperature toughness evaluation test, the CTOD value of this seamless steel pipe at -20°C was less than 0.25 mm, and it did not exhibit excellent low-temperature toughness.

[0102] For the seamless steel pipe of test number 16, the furnace temperature in the preheating zone was too high. As a result, the FnA exceeded 7.0. As a result, in the low-temperature toughness evaluation test, the CTOD value of this seamless steel pipe at -20°C was less than 0.25 mm, and it did not exhibit excellent low-temperature toughness.

[0103] The seamless steel pipe of test number 17 had a preheating time that was too short. As a result, the FnA exceeded 7.0. As a result, in the low-temperature toughness evaluation test, the CTOD value of this seamless steel pipe at -20°C was less than 0.25 mm, and it did not exhibit excellent low-temperature toughness.

[0104] The seamless steel pipe of test number 18 had a low LMP1 in the preheating zone. As a result, the FnA exceeded 7.0. As a result, in the low-temperature toughness evaluation test, the CTOD value of this seamless steel pipe at -20°C was less than 0.25 mm, and it did not exhibit excellent low-temperature toughness.

[0105] The seamless steel pipe of test number 19 had an excessively low Mg content. As a result, the FnA exceeded 7.0. As a result, in the low-temperature toughness evaluation test, the CTOD value of this seamless steel pipe at -20°C was less than 0.25 mm, and it did not exhibit excellent low-temperature toughness.

[0106] The seamless steel pipe of test number 20 had an excessively low S content. As a result, the FnA exceeded 7.0. As a result, in the low-temperature toughness evaluation test, the CTOD value of this seamless steel pipe at -20°C was less than 0.25 mm, and it did not exhibit excellent low-temperature toughness.

[0107] The seamless steel pipe of test number 21 had an excessively high S content. As a result, in the low-temperature toughness evaluation test, this seamless steel pipe had a CTOD value of less than 0.25 mm at -20°C, and did not exhibit excellent low-temperature toughness.

[0108] The seamless steel pipe of test number 22 had an excessively low Al content. As a result, the FnA exceeded 7.0. As a result, in the low-temperature toughness evaluation test, the CTOD value of this seamless steel pipe at -20°C was less than 0.25 mm, and it did not exhibit excellent low-temperature toughness.

[0109] The seamless steel pipe of test number 23 had an excessively high Ca content. As a result, the FnA exceeded 7.0. As a result, in the low-temperature toughness evaluation test, the CTOD value of this seamless steel pipe at -20°C was less than 0.25 mm, and it did not exhibit excellent low-temperature toughness.

[0110] The seamless steel pipe of test number 24 had an excessively high B content. As a result, in the low-temperature toughness evaluation test, this seamless steel pipe had a CTOD value of less than 0.25 mm at -20°C, and did not exhibit excellent low-temperature toughness.

[0111] 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.

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: 0.0007-0.0100%, Cu: 0.01-1.00%, Ni: 0.01 to 1.00%, Cr: 0.01-0.50%, Al: 0.030-0.100%, Ca: 0.0015% or less, Mg: 0.0010 to 0.0050%, and B: Contains 0.0005% or less, Mo: 0.01-0.30%, Nb: 0.01 to 0.10%, V: 0.01 to 0.10%, and Contains one or more elements selected from the group consisting of Ti: 0.001 to 0.010%, the balance being Fe and impurities; The yield strength is 450 MPa or more, In the seamless steel pipe, 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 ND particles / mm 2 When we define CE defined by formula (1) and the ND satisfy formula (2), Seamless steel pipe. CE=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5 (1) 20.0×CE-ND×10 -5 ≦7.0 (2) 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, The seamless steel pipe is a seamless steel pipe for line pipe. Seamless steel pipe.

Images