No steel pipes

Abstract

To provide a seamless steel tube having a high strength, while having excellent low-temperature toughness at an extremely low temperature environment.SOLUTION: The seamless steel tube has a chemical constitution as described in the specification, with a yield strength of 690 MPa or higher. In the seamless steel tube, when, by mass %, a Mg content of 10% or more and a Mn content of 30% or more are satisfied and a number density of (Mn, Mg)-containing particles with an equivalent circle diameter of 0.10 μm or less is defined as ND particles / mm2, a CE defined by an expression (1) and the ND satisfy an expression (2): CE=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5 (1); and 30.0×CE-ND×10-5≤12.0 (2), in which a content of a corresponding chemical element is assigned by mass% to a symbol of element in the expression (1). However, when the corresponding chemical element is not included, "0" is assigned to the symbol of element.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This disclosure relates to steel materials, and more specifically to seamless steel pipes. [Background technology]

[0002] With the increasing demand for oil and natural gas resources, the development of offshore oil fields is becoming more active. In this development, offshore structures such as platforms and jack-up rigs are used. Furthermore, as offshore structures become larger, their weight is also increasing. Therefore, seamless steel pipes are sometimes used as components of offshore structures (hereinafter simply referred to as "offshore structural components"). By using hollow, seamless steel pipes as offshore structural components, the weight of the offshore structure can be reduced. On the other hand, such seamless steel pipes for offshore structural components require high strength.

[0003] International Publication No. 2005 / 052205 (Patent Document 1), International Publication No. 2013 / 051231 (Patent Document 2), and Japanese Patent Application Publication No. 2019-127620 (Patent Document 3) propose high-strength steel materials for use as members of offshore structures.

[0004] The steel material described in Patent Document 1 is a high-tensile steel containing, by mass%, C: 0.01-0.10%, Si: 0.5% or less, Mn: 0.8-1.8%, P: 0.020% or less, S: 0.01% or less, Cu: 0.8-1.5%, Ni: 0.2-1.5%, Al: 0.001-0.05%, N: 0.003-0.008%, O: 0.0005-0.0035%, with the remainder being Fe and impurities, and with an N / Al ratio of 0.3-3.0. Patent Document 1 discloses that this steel material has a yield strength of 420 MPa or more and excellent weld toughness.

[0005] The steel material described in Patent Document 2 is a high-tensile steel plate containing, by mass%, C: 0.05~0.14%, Si: 0.01~0.30% or less, Mn: 0.3~2.3%, P: 0.008% or less, S: 0.005% or less, Al: 0.005~0.1%, Ni: 0.5~4%, B: 0.0003~0.003%, and N: 0.001~0.008%, with Ceq(=[C]+[Mn] / 6+[Cu+Ni] / 15+[Cr+Mo+V] / 5)≦0.80, and the central segregation hardness index HCS satisfying the formula (HCS=5.5[C]4 / 3+15[P]+0.90[Mn]+0.12[Ni]+0.53[Mo]≦2.5), with the remainder being Fe and impurities. Furthermore, the hardness of the central segregation portion of the steel plate satisfies the formula (HVmax / HVave ≤ 1.35 + 0.006 / Ct / 750, where HVmax is the maximum Vickers hardness of the central segregation portion, HVave is the average Vickers hardness of the portion excluding the central segregation portion and 1 / 4 of the plate thickness from the front and back surfaces, C is the carbon content (mass%), and t is the plate thickness of the steel plate (mm)). Patent Document 2 discloses that this high-tensile steel plate has a yield strength of 620 MPa or higher and excellent toughness of the heat-affected zone of multi-layer welds with low to medium heat input.

[0006] The steel material described in Patent Document 3 is a high-strength seamless steel pipe, with the following composition by mass: C: 0.10-0.18%, Si: 0.03-1.0%, Mn: 0.5-2.0%, P: 0.020% or less, S: 0.0025% or less, Cu: 0.1-1.0%, Cr: 0.10-0.60%, Ni: 0.2-1.0%, Mo: 0.10-0.40%, Ti: 0.004-0.02% The composition is 0%, V: 0.02~0.40%, B: 0.0005~0.005%, Al: 0.045% or less, N: 0.008% or less, Ca: 0.0004~0.0040%, Nb: 0~0.05%, the remainder being Fe and impurities, and the chemical composition satisfies the formula (C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5×B≦0.28). This high-strength seamless steel pipe has a structure in which the size of the prior austenite grains is 7.0 or larger according to the grain size number in accordance with ASTME112-13, and the total number of carbonitride inclusions, sulfide inclusions, and oxide inclusions with a particle size of 5 μm or larger is 100 grains / cm 2as described below. Patent Document 3 discloses that this high-strength seamless steel pipe has a yield strength of 625 MPa or more and a tensile strength of 695 MPa or more, and the absorbed energy obtained by a Charpy impact test at -40°C is 135 J or more. [Prior Art Document] [Patent Document]

[0007] [Patent Document 1] International Publication No. WO 2005 / 052205 [Patent Document 2] International Publication No. WO 2013 / 051231 [Patent Document 3] Japanese Unexamined Patent Publication No. 2019-127620 [Summary of Invention] [Problem to be Solved by Invention]

[0008] In recent years, with further increase in size of marine structures, higher strength has been required for seamless steel pipes for marine structural members. Specifically, seamless steel pipes having a yield strength of 690 MPa or more have been demanded. On the other hand, Patent Documents 1 to 3 above do not discuss seamless steel pipes having a yield strength of 690 MPa or more.

[0009] Furthermore, in recent years, there has been a growing demand for marine structural members that are expected to be used particularly in regions such as the North Sea, the Arctic coast, and Siberia. Such marine structural members are required to have excellent low-temperature toughness even in an extremely low-temperature environment of, for example, -60°C or lower. On the other hand, Patent Documents 1 to 3 above do not discuss seamless steel pipes having excellent low-temperature toughness in an extremely low-temperature environment of -60°C or lower.

[0010] An object of the present disclosure is to provide a seamless steel pipe having high strength and excellent low-temperature toughness in an extremely low-temperature environment. [Means for Solving the Problem]

[0011] The seamless steel pipe according to the present disclosure, in mass%, C: more than 0.080% to 0.180%, Si: 0.50% or less, Mn: 0.50% to 2.50%, P: 0.050% or less, S: 0.0010% to 0.0100%, Cu: 0.10% to 1.00%, Ni: 0.10% to 1.00%, Cr: 0.10% to 0.60%, Mo: 0.01% to 0.60%, Al: 0.030% to 0.100%, Ca: 0.0015% or less, Mg: 0.0010% to 0.0050%, B: 0.0005% to 0.0050%, and N: 0.0150% or less, and further contains Nb: 0.01% to 0.10%, V: 0.01% to 0.10%, and Ti: 0.001% to 0.050%, one or more elements selected from the group consisting of the foregoing,[[$END]] the balance being Fe and impurities,[[$END]] yield strength is 690 MPa or higher,[[$END]] in the seamless steel pipe,[[$END]] the number density of (Mn, Mg)-containing particles satisfying a Mg content of 10 mass% or more, a Mn content of 30 mass% or more and having an equivalent circle diameter of 0.10 µm or less is defined as ND particles / mm 2 , when defined as,[[$END]] CE defined by formula (1) and said ND satisfy formula (2). CE = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1) 30.0×CE - ND×10 -5 ≤ 12.0 (2) Here, for the element symbols in formula (1), the content of the corresponding element in mass% is substituted. When the corresponding element is not contained, "0" is substituted for the element symbol.

Effect of the Invention

[0012] The seamless steel pipe according to this disclosure has high strength and excellent low-temperature toughness in cryogenic environments. [Brief explanation of the drawing]

[0013] [Figure 1] Figure 1 shows the relationship between the value of FnA in this embodiment and the absorbed energy (J) at -60°C, which is an indicator of the low-temperature toughness of seamless steel pipes in an extremely low-temperature environment. [Modes for carrying out the invention]

[0014] First, the inventors investigated how to improve the strength and low-temperature toughness in cryogenic environments of seamless steel pipes intended for use in marine structural members, from the perspective of chemical composition. As a result, the inventors found that the following composition ratios (by mass%) were optimal: C: greater than 0.080 to 0.180%, Si: 0.50% or less, Mn: 0.50 to 2.50%, P: 0.050% or less, S: 0.0010 to 0.0100%, Cu: 0.10 to 1.00%, Ni: 0.10 to 1.00%, Cr: 0.10 to 0.60%, Mo: 0.01 to 0.60%, Al: 0.030 to 0.100%, Ca: 0.0015% or less, Mg: 0.0010 to 0.005% We considered that a seamless steel pipe containing 0%, B: 0.0005~0.0050%, and N: 0.0150% or less, and further containing one or more elements selected from the group consisting of Nb: 0.01~0.10%, V: 0.01~0.10%, and Ti: 0.001~0.050%, with the remainder being Fe and impurities, would have a high yield strength of 690 MPa or more and potentially excellent low-temperature toughness in cryogenic environments.

[0015] On the other hand, it has been believed that as the strength of steel increases, its low-temperature toughness decreases. In other words, with seamless steel pipes having the above-mentioned chemical composition, increasing the yield strength to 690 MPa or higher may result in insufficient low-temperature toughness in cryogenic environments. Therefore, the inventors have investigated in detail a method to improve the low-temperature toughness in cryogenic environments while maintaining the yield strength of seamless steel pipes at 690 MPa or higher. As a result, the inventors hypothesized that by precipitating a large number of fine inclusions or precipitates (hereinafter, inclusions or precipitates in steel are also simply referred to as "particles") in the steel, it may be possible to suppress the coarsening of crystal grains due to heat treatment during the manufacturing process, thereby obtaining excellent low-temperature toughness in cryogenic environments while maintaining a yield strength of 690 MPa or higher.

[0016] Specifically, the inventors focused on (Mn,Mg)-containing particles with an equivalent circular diameter of 0.10 μm or less. Hereinafter, (Mn,Mg)-containing particles with an equivalent circular 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" means particles that satisfy the following mass percentages: Mg content of 10% or more and Mn content of 30% or more. Furthermore, in the seamless steel pipe having the above-described chemical composition, the fine (Mn,Mg)-containing particles are mostly fine (Mn,Mg) sulfides. In this specification, (Mn,Mg) sulfide means a sulfide containing Mn and Mg, formed by the concentration of Mn into Mg sulfide.

[0017] To date, manganese sulfides have been known as inclusions that reduce the low-temperature toughness of seamless steel pipes having the above-mentioned chemical composition in cryogenic environments. Mn sulfides are easily stretched and coarseened by hot working. Coarseened Mn sulfides tend to become the starting point for cracks at low temperatures. Therefore, it has been thought that Mn sulfides reduce the low-temperature toughness of steel materials. From this perspective, it has been considered preferable to reduce the number of Mn sulfides in order to improve the low-temperature toughness of seamless steel pipes having the above-mentioned chemical composition.

[0018] On the other hand, the inventors considered that if the shape of Mn sulfide could be controlled by Mg and fine Mn sulfide could be dispersed, it might be possible to improve the low-temperature toughness of seamless steel pipes in cryogenic environments. In other words, by deliberately utilizing Mn sulfide, which has been thought to reduce the low-temperature toughness of steel materials until now, it may be possible to improve the low-temperature toughness of seamless steel pipes in cryogenic environments while maintaining a yield strength of 690 MPa or higher. (Mn,Mg) sulfide is less stretchable and tends to become finer compared to Mn sulfide. Therefore, the inventors considered that increasing the number density of fine (Mn,Mg) sulfide (fine (Mn,Mg)-containing particles) might improve the low-temperature toughness of seamless steel pipes in cryogenic environments.

[0019] Specifically, the inventors manufactured various seamless steel pipes having the above-mentioned chemical composition and investigated in detail the relationship between the number density of fine (Mn,Mg)-containing particles, the yield strength, and the low-temperature toughness in an extremely low-temperature environment. As a result, in the seamless steel pipe having the above-mentioned chemical composition, the CE defined by the following formula (1) and the number density ND (particles / mm²) of (Mn,Mg)-containing particles with an equivalent circular diameter of 0.10 μm or less 2 It was revealed that the low-temperature toughness of seamless steel pipes in cryogenic environments is significantly increased when the following equation (2) is satisfied. CE=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5 (1) 30.0 × CE-ND × 10 -5 ≤12.0 (2) Here, the elemental symbols in equation (1) are substituted with the content of the corresponding element in mass percent. If the corresponding element is not present, "0" is substituted for that elemental symbol.

[0020] FnA = 30.0 × CE - ND × 10 -5This is defined as follows. FnA is an index of low-temperature toughness in a seamless steel pipe having the above-described chemical composition. The relationship between FnA and the low-temperature toughness of the seamless steel pipe will be explained in detail below using the drawings. Figure 1 shows the relationship between the value of FnA and the absorbed energy (J) at -60°C, which is an index of low-temperature toughness of the seamless steel pipe in an extremely low-temperature environment, for an example of the above-described chemical composition among the examples described later. The value of FnA and the absorbed energy at -60°C were determined by the method described later. In addition, in all the examples shown in Figure 1, the yield strength was 690 MPa or higher.

[0021] Referring to Figure 1, when FnA exceeds 12.0, the absorbed energy at -60°C drops sharply to less than 75J. On the other hand, if FnA is 12.0 or less, the absorbed energy at -60°C remains stable at 75J or more. In other words, Figure 1 demonstrates that if FnA is 12.0 or less, the low-temperature toughness of seamless steel pipes can be stably increased even in extremely low-temperature environments.

[0022] As described above, the seamless steel pipe according to this embodiment has the above-mentioned chemical composition, and contains CE as defined by formula (1), and the number density ND (particles / mm²) of (Mn,Mg)-containing particles with an equivalent circular diameter of 0.10 μm or less. 2 ) satisfies equation (2). As a result, the seamless steel pipe according to this embodiment has a yield strength of 690 MPa or more and excellent low-temperature toughness in cryogenic environments.

[0023] The detailed reason why reducing FnA to 12.0 or less significantly increases the low-temperature toughness of seamless steel pipes in cryogenic environments remains unclear. However, the inventors speculate as follows: CE, defined in equation (1), is an indicator of the strength of seamless steel pipes. In other words, the larger the CE, the stronger the seamless steel pipe tends to be. On the other hand, as the strength of the seamless steel pipe increases, its low-temperature toughness tends to decrease. Therefore, it is thought that as CE increases and the strength of the seamless steel pipe increases, it may be necessary to disperse more fine (Mn,Mg)-containing particles (with an equivalent circle diameter of 0.10 μm or less) to improve low-temperature toughness in cryogenic environments. In this way, the inventors believe that by adjusting the number density ND of fine (Mn,Mg)-containing particles according to CE and reducing FnA to 12.0 or less, it may be possible to significantly improve the low-temperature toughness in cryogenic environments even for seamless steel pipes with a high yield strength of 690 MPa or more. It is also possible that reducing FnA to 12.0 or less significantly increases the low-temperature toughness of seamless steel pipes in cryogenic environments through a mechanism different from the one described above. However, the fact that reducing FnA to 12.0 or less significantly increases the low-temperature toughness of seamless steel pipes in cryogenic environments has been demonstrated by the examples described later.

[0024] Based on the above findings, the gist of the seamless steel pipe according to this embodiment is as follows:

[0025] [1] It is a seamless steel pipe, In mass%, C: more than 0.080~0.180%, Si: 0.50% or less, Mn: 0.50~2.50%, P: 0.050% or less, S: 0.0010~0.0100%, Cu: 0.10~1.00%, Ni: 0.10~1.00%, Cr: 0.10~0.60%, Mo: 0.01~0.60%, Al: 0.030~0.100%, Ca: 0.0015% or less, Mg: 0.0010~0.0050%, B: 0.0005~0.0050%, and N: 0.0150% or less, and further comprises Nb: 0.01~0.10%, V: 0.01~0.10%, and Ti: 0.001~0.050%, one or more elements selected from the group consisting of the balance being Fe and impurities, a yield strength of 690 MPa or more, in the seamless steel pipe, when the number density of (Mn,Mg)-containing particles satisfying a Mg content of 10 mass% or more and a Mn content of 30 mass% or more and having an equivalent circle diameter of 0.10 µm or less is defined as ND particles / mm 2 , CE defined by formula (1) and said ND satisfy formula (2), a seamless steel pipe. CE=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5 (1) 30.0×CE-ND×10 -5 ≦12.0 (2) Here, for the element symbols in formula (1), the content of the corresponding element in mass% is substituted. When the corresponding element is not contained, "0" is substituted for the element symbol.

[0026] [2] The seamless steel pipe according to [1], wherein the seamless steel pipe is a seamless steel pipe for marine structural members, a seamless steel pipe.

[0027] Hereinafter, the seamless steel pipe according to the present embodiment will be described in detail. Unless otherwise specified, "%" related to elements means mass%. In the following description, a seamless steel pipe is also simply referred to as "steel material".

[0028] [Chemical Composition] The seamless steel pipe according to this embodiment contains the following elements:

[0029] C: More than 0.080~0.180% Carbon (C) enhances the hardenability and strength of steel. If the C content is too low, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the C content is too high, the weldability of the steel decreases, even if the content of other elements is within the range of this embodiment. In other words, when steel is welded, the heat-affected zone (HAZ) of the weld hardens, reducing the cold crack resistance of the steel. Therefore, the C content is greater than 0.080% to 0.180%. The preferred lower limit of the C content is 0.085%, more preferably 0.090%, more preferably 0.095%, and still more preferably 0.100%. The preferred upper limit of the C content is 0.170%, and more preferably 0.160%.

[0030] Si:0.50% or less Silicon (Si) is inevitably present. That is, the lower limit of the Si content is greater than 0%. Si deoxidizes steel. On the other hand, if the Si content is too high, the low-temperature toughness of the steel decreases, even if the content of other elements is within the range of this embodiment. Therefore, the Si content is 0.50% or less. The preferred upper limit of the Si content is 0.45%, and more preferably 0.40%. The preferred lower limit of the Si content to more effectively obtain the above effects is 0.01%, more preferably 0.03%, more preferably 0.05%, and still more preferably 0.10%.

[0031] Mn: 0.50~2.50% Manganese (Mn) forms (Mn,Mg) sulfides 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 is increased. Mn further improves the hardenability of the steel and increases its strength. If the Mn content is too low, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mn content is too high, even if the content of other elements is within the range of this embodiment, the hardenability of the steel becomes too high and the low-temperature toughness of the steel decreases. Therefore, the Mn content is 0.50 to 2.50%. The preferred lower limit of the Mn content is 0.60%, more preferably 0.80%, and still more preferably 1.00%. The preferred upper limit of the Mn content is 2.30%, more preferably 2.00%, and still more preferably 1.80%.

[0032] 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 content of other elements is within the range of this embodiment, P will segregate at the grain boundaries, reducing the low-temperature toughness of the steel. Therefore, the P content is 0.050% or less. The preferred upper limit of the P content is 0.040%, more preferably 0.030%, and still more preferably 0.020%. It is preferable to have as low a P content as possible. However, an extreme reduction in the P content will significantly increase manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the P content is 0.001%, more preferably 0.003%, and still more preferably 0.005%.

[0033] S: 0.0010~0.0100% Sulfur (S) forms (Mn,Mg) sulfides 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 is increased. If the S content is too low, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the S content is too high, even if the content of other elements is within the range of this embodiment, coarse MnS is formed in the steel, and the low-temperature toughness of the steel decreases. Therefore, the S content is 0.0010 to 0.0100%. The preferred lower limit of the S content is 0.0012%, more preferably 0.0013%, more preferably 0.0015%, and still more preferably 0.0020%. The preferred upper limit of the S content is 0.0090%, and more preferably 0.0080%.

[0034] Cu: 0.10~1.00% Copper (Cu) enhances the hardenability and strength of steel. If the Cu content is too low, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Cu content is too high, even if the content of other elements is within the range of this embodiment, the hardenability of the steel becomes too high, and the low-temperature toughness of the steel decreases. Therefore, the Cu content is 0.10 to 1.00% or less. The preferred lower limit of the Cu content is 0.12%, more preferably 0.15%, still more preferably 0.20%, and still more preferably 0.25%. The preferred upper limit of the Cu content is 0.80%, and still more preferably 0.60%.

[0035] Ni: 0.10~1.00% Nickel (Ni) enhances the hardenability and strength of steel. If the Ni content is too low, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Ni content is too high, even if the content of other elements is within the range of this embodiment, the hardenability of the steel becomes too high and the low-temperature toughness of the steel decreases. Therefore, the Ni content is 0.10 to 1.00%. The preferred lower limit of the Ni content is 0.15%, and more preferably 0.20%. The preferred upper limit of the Ni content is 0.90%, more preferably 0.80%, and still more preferably 0.60%.

[0036] Cr: 0.10~0.60% Chromium (Cr) enhances the hardenability and strength of steel. If the Cr content is too low, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Cr content is too high, even if the content of other elements is within the range of this embodiment, the hardenability of the steel becomes too high and the low-temperature toughness of the steel decreases. Therefore, when included, the Cr content is 0.10 to 0.60%. A preferred lower limit for the Cr content to more effectively obtain the above effects is 0.12%, more preferably 0.15%, and even more preferably 0.20%. A preferred upper limit for the Cr content is 0.55%, more preferably 0.50%, and even more preferably 0.45%.

[0037] Mo: 0.01~0.60% Molybdenum (Mo) enhances the hardenability and strength of steel. If the Mo content is too low, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mo content is too high, even if the content of other elements is within the range of this embodiment, the hardenability of the steel becomes too high and the low-temperature toughness of the steel decreases. Therefore, when included, the Mo content is 0.01 to 0.60%. A preferred lower limit for the Mo content to more effectively obtain the above effects is 0.02%, more preferably 0.03%, and even more preferably 0.05%. A preferred upper limit for the Mo content is 0.58%, more preferably 0.55%, and even more preferably 0.50%.

[0038] Al: 0.030~0.100% Aluminum (Al) deoxidizes steel and inhibits Mg from forming oxides. If the Al content is too low, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Al content is too high, even if the content of other elements is within the range of this embodiment, coarse oxide inclusions are formed, and the low-temperature toughness of the steel decreases. Therefore, the Al content is 0.030 to 0.100%. The preferred lower limit of the Al content is 0.035%, and more preferably 0.040%. The preferred upper limit of the Al content is 0.090%, and more preferably 0.080%. As used herein, "Al" content refers to "acid-soluble Al," that is, the content of "sol.Al."

[0039] Ca: 0.0015% or less Calcium (Ca) is inevitably present. That is, the lower limit of the Ca content is greater than 0%. Ca deoxidizes and desulfurizes steel. If the Ca content is too high, even if the content of other elements is within the range of this embodiment, Ca sulfides will be 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 decreases. Therefore, the Ca content is 0.0015% or less. The preferred upper limit of the Ca content is 0.0013%, and more preferably 0.0012%. The preferred lower limit of the Ca content to more effectively obtain the above effects is 0.0001%, and more preferably 0.0002%.

[0040] Mg: 0.0010~0.0050% 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 is increased. If the Mg content is too low, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mg content is too high, even if the content of other elements is within the range of this embodiment, coarse oxides are formed, and the low-temperature toughness of the steel decreases. 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%.

[0041] B: 0.0005~0.0050% Boron (B) dissolves in steel, improving its hardenability and increasing its strength. If the B content is too low, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the B content is too high, even if the content of other elements is within the range of this embodiment, coarse nitrides are formed, reducing the low-temperature toughness of the steel. Therefore, the B content is 0.0005 to 0.0050%. The preferred lower limit of the B content is 0.0006%, more preferably 0.0008%, and even more preferably 0.0010%. The preferred upper limit of the B content is 0.0045%, more preferably 0.0040%, even more preferably 0.0030%, and even more preferably 0.0020%.

[0042] N: 0.0150% or less Nitrogen (N) is inevitably present. That is, the lower limit of the N content is greater than 0%. N forms nitrides, and the pinning effect refines the crystal grains of the steel. As a result, the low-temperature toughness of the steel is increased. On the other hand, if the N content is too high, even if the content of other elements is within the range of this embodiment, coarse nitrides will be formed, and the low-temperature toughness of the steel will actually decrease. Therefore, the N content is 0.0150% or less. The preferred upper limit of the N content is 0.0140%, more preferably 0.0120%, still more preferably 0.0100%, still more preferably 0.0080%, and still more preferably 0.0070%. The preferred lower limit of the N content to more effectively obtain the above effects is 0.0005%, more preferably 0.0010%, still more preferably 0.0015%, and still more preferably 0.0020%.

[0043] The seamless steel pipe according to this embodiment contains one or more elements selected from the group consisting of Nb, V, and Ti. That is, it may contain only one of Nb, V, and Ti, and the content of other elements may be 0%. All of these elements enhance the hardenability and strength of the steel material.

[0044] Nb: 0.01~0.10% Niobium (Nb) combines with carbon and / or nitrogen in the steel to form fine precipitates, thereby increasing the strength of the steel. On the other hand, if the Nb content is too high, even if the content of other elements is within the range of this embodiment, coarse precipitates will be formed, reducing the low-temperature toughness of the steel. Therefore, when included, the Nb content is 0.01 to 0.10%. A preferred lower limit for the Nb content to more effectively obtain the above effect is 0.02%. A preferred upper limit for the Nb content is 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0045] V: 0.01~0.10% Vanadium (V) combines with carbon (C) in steel to form carbides, thereby increasing the strength of the steel. On the other hand, if the V content is too high, even if the content of other elements is within the range of this embodiment, coarse carbides will be formed, reducing the low-temperature toughness of the steel. Therefore, when included, the V content is 0.01 to 0.10%. A preferred lower limit for the V content to more effectively obtain the above effect is 0.02%, and more preferably 0.03%. A preferred upper limit for the V content is 0.09%, more preferably 0.08%, and still more preferably 0.07%.

[0046] Ti: 0.001~0.050% Titanium (Ti) enhances the hardenability and strength of steel. However, if the Ti content is too high, even if the content of other elements is within the range of this embodiment, coarse nitrides will form, reducing the low-temperature toughness of the steel. Therefore, the Ti content is 0.001 to 0.050%. A preferred lower limit for the Ti content to more effectively obtain the above effects is 0.002%, and more preferably 0.003%. A preferred upper limit for the Ti content is 0.045%, more preferably 0.040%, more preferably 0.035%, more preferably 0.030%, and still more preferably 0.025%.

[0047] The remainder of the seamless steel pipe according to this embodiment consists of Fe and impurities. Here, impurities refer to substances that are mixed in from the raw materials such as ore, scrap, or the manufacturing environment during the 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.

[0048] [Yield strength] The seamless steel pipe according to this embodiment has the above-described chemical composition and an FnA content of 12.0 or less. As a result, the seamless steel pipe according to this embodiment has excellent low-temperature toughness even when it has a high strength of 690 MPa or more in yield strength.

[0049] The preferred lower limit of the yield strength of the seamless steel pipe according to this embodiment is 695 MPa, and more preferably 700 MPa. On the other hand, if the yield strength is too high, it may be difficult to stably ensure low-temperature toughness. Therefore, from the viewpoint of stabilizing low-temperature toughness, the preferred upper limit of the yield strength is 840 MPa, and more preferably 800 MPa.

[0050] In the seamless steel pipe according to this embodiment, the tensile strength is not particularly limited. The tensile strength of the seamless steel pipe according to this embodiment may be, for example, 760 MPa or more. Furthermore, in this embodiment, the upper limit of the tensile strength is not particularly limited. The upper limit of the tensile strength may be, for example, 900 MPa or 890 MPa.

[0051] 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 in accordance with JIS Z 2241 (2011). First, a tensile test specimen is prepared from the seamless steel pipe according to this embodiment. If the wall thickness of the seamless steel pipe is 20 mm or more, a No. 4 test specimen (round bar test specimen) as specified in JIS Z 2241 (2011) is prepared as a tensile test specimen from the center of the wall thickness. If the wall thickness of the seamless steel pipe is less than 20 mm, the thickness is taken as the total wall thickness, and one of the No. 12A, 12B, or 12C test specimens (arc-shaped test specimens) as specified in JIS Z 2241 (2011) is prepared as a tensile test specimen according to the outer diameter of the steel pipe. The longitudinal direction of the tensile test specimen is the pipe axis direction of the steel pipe. Tensile tests are performed on the prepared tensile specimens at room temperature (25°C) in air, and the obtained 0.2% offset proof stress is defined as the yield strength (MPa). The maximum stress during uniform elongation is defined as the tensile strength (MPa). In this embodiment, the yield strength (MPa) and tensile strength (MPa) are obtained by rounding the obtained values ​​to the first decimal place.

[0052] [Fine (Mn, Mg) containing particles] The seamless steel pipe according to this embodiment, assuming it has the above-described chemical composition, has CE defined by formula (1) and the number density ND (particles / mm²) of (Mn,Mg)-containing particles with an equivalent circular diameter of 0.10 μm or less. 2 ) and satisfy equation (2). CE=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5 (1) 30.0 × CE-ND × 10 -5 ≤12.0 (2) Here, the elemental symbols in equation (1) are substituted with the content of the corresponding element in mass percent. If the corresponding element is not present, "0" is substituted for that elemental symbol.

[0053] As stated above, in this specification, "(Mn,Mg)-containing particles" means particles that satisfy the following conditions in terms of mass percent: Mg content of 10% or more and Mn content of 30% or more. Furthermore, in this specification, (Mn,Mg)-containing particles with an equivalent circle diameter of 0.10 μm or less are also referred to as "fine (Mn,Mg)-containing particles".

[0054] FnA(=30.0×CE-ND×10 -5 FnA is an index of low-temperature toughness in a seamless steel pipe having the above-described chemical composition. If FnA is too high, the absorbed energy at -60°C, which is an index of low-temperature toughness of a seamless steel pipe in an extremely low-temperature environment, will decrease sharply. Therefore, in this embodiment, assuming the above-described chemical composition, FnA is set to 12.0 or less. In this embodiment, the preferred upper limit of FnA is 11.9, more preferably 11.8, and even more preferably 11.7. The lower limit of FnA is not particularly limited. The lower limit of FnA may be, for example, 5.0, 6.0, or 7.0.

[0055] The CE defined in formula (1) is an index indicating the strength of the seamless steel pipe. In this embodiment, the CE and the number density ND of fine (Mn,Mg)-containing particles are not particularly limited as long as they satisfy formula (2) described above. The CE may be, for example, 0.300 to 0.700. A more preferred lower limit for the CE is 0.350, more preferably 0.375, more preferably 0.400, and still more preferably 0.450. A more preferred upper limit for the CE is 0.650, more preferably 0.625, more preferably 0.600, and still more preferably 0.550.

[0056] Furthermore, the number density ND of fine (Mn,Mg)-containing particles is, for example, 0.5 × 10⁻⁶. 5 ~10.0×10 5 (pcs / mm 2 ) may also be used. Furthermore, the number density ND of fine (Mn,Mg) containing particles is 2.0 × 10 5 (pcs / mm 2 By doing so, the yield strength and low-temperature toughness of the seamless steel pipe can be stably increased. Therefore, in this embodiment, the preferred lower limit of the number density ND of fine (Mn,Mg)-containing particles is 2.0 × 10 5 (pcs / mm 2 ) Furthermore, the preferred upper limit of the number density ND of fine (Mn,Mg)-containing particles is 9.0 × 10⁻⁶.5 (pcs / mm 2 ) and more preferably 8.0 × 10 5 (pcs / mm 2 ) and more preferably 7.0 × 10 5 (pcs / mm 2 )

[0057] In this embodiment, the number density ND of fine (Mn,Mg)-containing particles can be determined by the following method. A test specimen for microstructure observation is prepared from the center of the wall thickness of the seamless steel pipe according to this embodiment. After mirror polishing the surface of the test specimen, it is ultrasonically cleaned in methanol. The cleaned surface is covered with a carbon vapor deposition film. The test specimen with the vapor deposition film covering the surface is immersed in an electrolyte solution (10% acetylacetone - 1% tetramethylammonium chloride - methanol solution) at 20°C and electrolysis is performed. The electrolysis conditions are: voltage: 100mV, electrolysis rate: 10C / cm 2 The following procedure is followed: The vapor-deposited film is peeled off from the electrolyzed test specimen. The obtained vapor-deposited film is washed with ethanol, then scooped up with a sheet mesh and dried.

[0058] This deposited film (replica film) is observed using a transmission electron microscope (TEM). Specifically, an arbitrary position is selected on the deposited film, and it is observed at a magnification of 20,000x and an acceleration voltage of 200kV. While there are no particular limitations on the size or number of observation fields, the total area of ​​the observation fields is 1000 μm². 2 The above is sufficient. For example, if the field of view is 4.0 μm × 5.0 μm, the number of fields of view should be 50 or more, and the total area of ​​the fields of view should be 1000 μm. 2 That concludes this section.

[0059] In each observation field, particles with an equivalent circle diameter of 0.10 μm or less are identified. These particles can be identified based on their contrast. The equivalent circle diameter of a particle can also be determined by image analysis of the observed image during TEM observation. In this embodiment, the lower limit of the equivalent circle diameter for particles with an equivalent circle diameter of 0.10 μm or less is not particularly limited, but for example, it is 0.01 μm. That is, in this embodiment, particles with an equivalent circle diameter of 0.01 to 0.10 μm are identified.

[0060] Point analysis is performed on the identified particles using energy dispersive X-ray spectrometry (EDS). The elemental content in each particle is determined by the EDS point analysis. In the EDS point analysis, the acceleration voltage is set to 200kV, 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 mass percent Mg content of 10% or more and a Mn content of 30% or more are identified as "fine (Mn,Mg) containing particles".

[0061] Determine the total number of fine (Mn,Mg) particles identified in 10 fields of view. Based on the total number of fine (Mn,Mg) particles and the total area of ​​the 10 fields of view, calculate the number density ND (particles / mm²) of the fine (Mn,Mg) particles. 2 ) is determined. In this embodiment, as described in Table 2 of the Examples below, the number density ND (particles / mm²) of (Mn,Mg) containing particles with an equivalent circle diameter of 0.10 μm or less is determined. 2 When calculating ), the unit is 10 5 pieces / mm 2 Then, the mantissa obtained by expressing the number density in exponential notation is rounded to the second decimal place.

[0062] Here, in point analysis of steel materials having the above-described chemical composition using EDS, accurate quantification of sulfur (S) content is often difficult with current technology. Therefore, in this embodiment, particles with an equivalent circular diameter of 0.10 μm or less, containing 10% by mass or more of Mg and 30% by mass or more of Mn are identified as fine (Mn,Mg)-containing particles. That is, the fine (Mn,Mg)-containing particles in this embodiment may also include compounds other than sulfides (such as oxides). However, in seamless steel pipes having the above-described chemical composition, compounds other than (Mn,Mg) sulfides are negligibly small in the fine (Mn,Mg)-containing particles. That is, in this embodiment, the number density of fine (Mn,Mg)-containing particles is ND (particles / mm²). 2 This essentially refers to the number density (particles / mm³) of (Mn,Mg) sulfides with an equivalent circular diameter of 0.10 μm or less. 2 This corresponds to ).

[0063] [Low temperature toughness] The seamless steel pipe according to this embodiment has the above-described chemical composition and an FnA content of 12.0 or less. As a result, the seamless steel pipe according to this embodiment has a yield strength of 690 MPa or more and excellent low-temperature toughness. Here, excellent low-temperature toughness is defined as follows.

[0064] A Charpy impact test will be performed on the seamless steel pipe according to this embodiment using a method compliant with ASTM A370 (2021) to determine the absorbed energy at -60°C. Specifically, a full-size V-notch test specimen will be prepared from the center of the wall thickness of the seamless steel pipe according to this embodiment. If a full-size test specimen cannot be prepared, a sub-size V-notch test specimen will be prepared from the center of the wall thickness of the seamless steel pipe. The width direction of the V-notch test specimen will be parallel to the diameter direction (wall thickness direction) of the seamless steel pipe.

[0065] A Charpy impact test is performed on the fabricated V-notch specimens in accordance with ASTM A370 (2021) to determine the absorbed energy (J) at -60°C. If a sub-sized V-notch specimen is used, the obtained absorbed energy is divided by the reduction factor described in API 5CT (2021) to convert it to the absorbed energy of a full-sized V-notch specimen. The absorbed energy (J) at -60°C is rounded to the nearest tenth. In this embodiment, if the absorbed energy at -60°C determined by the above method is 75J or higher, it is judged to exhibit excellent low-temperature toughness.

[0066] [Microorganisms] Preferably, the microstructure of the seamless steel pipe according to this embodiment consists mainly of tempered martensite. More specifically, the microstructure of the seamless steel pipe according to this embodiment has a volume ratio of tempered martensite of 90% or more. The remainder of the microstructure is, for example, tempered bainite, ferrite, or pearlite. In addition to tempered martensite, tempered bainite, ferrite, and pearlite, the microstructure of the seamless steel pipe according to this embodiment may also contain trace amounts of precipitates, inclusions, etc. However, in the microstructure of the seamless steel pipe according to this embodiment, the volume ratio of precipitates, inclusions, etc. is negligibly small compared to tempered martensite, tempered bainite, ferrite, and pearlite.

[0067] Furthermore, when determining the volume fraction of tempered martensite by observation, it can be determined by the following method. First, a test specimen is prepared from the center of the wall thickness of the seamless steel pipe according to this embodiment, with the observation surface being a plane that includes both the axial direction and the radial direction of the pipe. After polishing the observation surface of the test specimen to a mirror finish, it is immersed in a Nital etching solution for about 10 seconds to reveal the microstructure by etching. The etched observation surface is observed using a scanning electron microscope (SEM) to obtain 10 fields of view as secondary electron images. The field of view area is, for example, 0.01 mm². 2(Magnification 1000x). In each field of view, tempered martensite is identified from the contrast.

[0068] The area ratio of the identified tempered martensite is determined. The method for determining the area ratio is not particularly limited and any well-known method may be used. For example, the area ratio of tempered martensite can be determined by image analysis. In this embodiment, the arithmetic mean of the area ratios of tempered martensite obtained in all fields of view is defined as the volume ratio of tempered martensite. It should be noted that it is difficult to distinguish between tempered martensite and tempered bainite using the method described above. However, it can be assumed that the microstructure of a seamless steel pipe having the above chemical composition and a yield strength of 690 MPa or more contains almost no tempered bainite. That is, in a seamless steel pipe having the above chemical composition and a yield strength of 690 MPa or more, regions that can be identified as tempered martensite or tempered bainite by observation are identified as tempered martensite.

[0069] [Shape of seamless steel pipes] 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, the outer diameter, wall thickness, and length are not particularly limited. When the seamless steel pipe according to this embodiment is a seamless steel pipe for marine structural members, the preferred outer diameter is 50 to 600 mm. Furthermore, when the seamless steel pipe according to this embodiment is a seamless steel pipe for marine structural members, the preferred wall thickness is 5 to 60 mm. In particular, even with a wall thickness of 20 mm or more, it has high strength and excellent low-temperature toughness.

[0070] [Manufacturing method] The method for manufacturing seamless steel pipes according to this embodiment will be described below. The method for manufacturing seamless steel pipes described below is just one example of a method for manufacturing seamless steel pipes according to this embodiment. In other words, seamless steel pipes according to this embodiment may be manufactured by methods other than the method described below. An example of the method for manufacturing seamless steel pipes according to this embodiment comprises a steelmaking step of producing a base material by casting molten steel, a hot working step of producing a raw pipe by hot working the base material, a quenching step of performing hardening on the raw pipe, and a tempering step of performing tempering on the hardened raw pipe.

[0071] [Steelmaking process] In the steelmaking process, first, molten steel satisfying the above-mentioned chemical composition is produced. The method of producing the molten steel is not particularly limited and any well-known method is acceptable. In other words, as long as molten steel satisfying the above-mentioned chemical composition can be produced, the production method is not limited. Next, the prepared molten steel is cast to produce the raw material. The casting method is not particularly limited, but for example, continuous casting is used. When producing the raw material by continuous casting, it is preferable to carry it out in the following manner.

[0072] When manufacturing materials by continuous casting, it is preferable that the manufactured material be a billet with a circular cross-section (round billet). Preferably, the cast round billet is cooled to room temperature with 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.

[0073] If the average billet cooling rate is too slow, (Mn,Mg) sulfides may coarseen in the molten steel. As a result, the number density ND of 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 billet cooling rate is too fast, the amount of (Mn,Mg) sulfides crystallized and / or precipitated may decrease. As a result, the number density ND of fine (Mn,Mg)-containing particles may decrease in the manufactured seamless steel pipe, and FnA may become too large. Therefore, in this embodiment, it is preferable to set the average billet cooling rate to 10.0 to 13.0 °C / min.

[0074] The material is manufactured by casting molten steel using the method described above. As mentioned 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 may be produced. In these cases, it is preferable to perform bloc rolling to produce a round billet from a billet with a rectangular cross-section or from a bloom.

[0075] [Hot working process] In the hot working process, the prepared material is hot-worked to produce the raw pipe. First, the billet is heated in a heating furnace. Preferably, the billet is heated by being placed in a continuous heating furnace. The following describes the case where the billet is heated by being placed in a continuous heating furnace. In this case, the heating furnace may be a rotary hearth type heating furnace or a walking beam type heating furnace.

[0076] In a continuous heating furnace, the billet, charged into the furnace through the charging port, is heated as it moves through the furnace. The furnace is divided into three sections, from the charging port to the outlet: a preheating zone, a heating zone, and a uniform heating zone. The preheating zone is the section containing the charging port and has the lowest furnace temperature among the three sections. The heating zone is located between the preheating zone and the uniform heating zone. The uniform heating zone follows the heating zone and has an outlet at its rear end.

[0077] In this embodiment, it is preferable to set the heating conditions in the preheating zone as follows. Preheating zone furnace temperature T1: 1050~1200℃ Pre-heating zone in-furnace time t1:70~200 minutes Furthermore, the furnace temperature T1 (°C) in the preheating zone and the time spent in the furnace t1 (minutes) satisfy the following equation (A). (273.15+T1)×(20+Log(t1 / 60))≧28000 (A)

[0078] In this embodiment, it is preferable to set the furnace temperature T1 in the preheating zone 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 in the manufactured seamless steel pipe may decrease. In this case, sufficient low-temperature toughness of the seamless steel pipe cannot be obtained. On the other hand, if the furnace temperature T1 in the preheating zone is too low, the raw pipe will not be heated sufficiently, and the equipment load in the hot working process described later will increase. Therefore, provided that the other preferred manufacturing methods of this embodiment are satisfied, setting the furnace temperature T1 in the preheating zone to 1050 to 1200°C makes it possible to stably achieve an FnA of 12.0 or less in the manufactured seamless steel pipe.

[0079] In this embodiment, it is preferable to set the furnace time t1 in the preheating zone to 70 to 200 minutes. If the furnace time t1 in the preheating zone is too short, the center of the raw tube may not be sufficiently heated, making it difficult to stably perform the hot working process described later. On the other hand, if the furnace time t1 in the preheating zone is too long, the (Mn,Mg) sulfides in the raw tube may dissolve too much. In this case, the number density ND of fine (Mn,Mg)-containing particles in the manufactured seamless steel tube decreases, and the low-temperature toughness of the seamless steel tube cannot be sufficiently obtained. Therefore, provided that the other preferred manufacturing methods of this embodiment are satisfied, setting the furnace time t1 in the preheating zone to 70 to 200 minutes makes it possible to stably achieve an FnA of 12.0 or less in the manufactured seamless steel tube.

[0080] In this embodiment, it is preferable that the furnace temperature T1 (°C) and the furnace time t1 (minutes) in the preheating zone satisfy the following equation (A). (273.15+T1)×(20+Log(t1 / 60))≧28000 (A)

[0081] LMP1 is defined as (273.15 + T1) × (20 + Log(t1 / 60)). LMP1 is the Larson-Miller parameter in the pre-heating 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 low-temperature toughness of the seamless steel pipe may not be sufficient. Therefore, provided that the other preferred manufacturing methods of this embodiment are satisfied, setting LMP1 to 28000 or higher can stably keep the FnA of the manufactured seamless steel pipe at 12.0 or lower. Note that there is no particular upper limit to LMP1. The upper limit of LMP1 may be, for example, 30233 or 30000.

[0082] Furthermore, the detailed reasons why the FnA of the manufactured seamless steel pipe can be adjusted by adjusting the heating conditions in the preheating zone when using a continuous heating furnace are not yet clear. However, it has been proven by the examples described later that, at least for a raw pipe having the above-mentioned chemical composition, if the heating conditions in the preheating zone meet the scope of this embodiment, the FnA will be 12.0 or less, provided that the other manufacturing methods meet the preferred scope of this embodiment.

[0083] In this embodiment, it is preferable to further set the heating conditions in the heated zone and the homogenized zone as follows. Furnace temperature in the heating zone: 1200~1320℃ In-furnace time in the heating zone: 40-100 minutes Furnace temperature in a homogeneous zone: 1200~1320℃ In-fuel time in a uniform climate: 30-70 minutes

[0084] In this embodiment, it is preferable to set the furnace temperature in the heating zone and the uniform zone to 1200 to 1320°C. If the furnace temperature in the heating zone and the uniform zone is too high, the grain size of the manufactured seamless steel pipe may become too large, and the toughness of the manufactured seamless steel pipe may decrease. On the other hand, if the furnace temperature in the heating zone and the uniform zone is too low, the raw pipe will not be heated sufficiently, and the equipment load in the hot working process described later will increase.

[0085] In this embodiment, it is preferable to keep the heating zone in the furnace for 40 to 100 minutes. Furthermore, in this embodiment, it is preferable to keep the soaking zone in the furnace for 30 to 70 minutes. If these furnace times are too short, the center of the raw tube may not be sufficiently heated, making it difficult to stably perform the hot working process described later. On the other hand, if these furnace times are too long, the heating effect will saturate.

[0086] As described above, when using a continuous heating furnace, it is preferable to perform heating under the conditions described above. However, the seamless steel pipe according to this embodiment can also be manufactured without using a continuous heating furnace. In short, if the manufactured seamless steel pipe has the above-described chemical composition and FnA is 12.0 or less, the seamless steel pipe according to this embodiment can have a yield strength of 690 MPa or more and excellent low-temperature toughness. The hot working process after heating will be described below.

[0087] A raw pipe (seamless steel pipe) is manufactured by hot working on a billet extracted from a heating furnace. In this embodiment, the method of hot working is not particularly limited and any well-known method may be used. For example, the raw pipe may be manufactured by the Mannesmann process as the hot working method. In this case, the round billet is perforated and rolled using a perforating machine. When perforating and rolling, the perforation ratio is not particularly limited, but for example, it is 1.0 to 4.0. The perforated and rolled round billet is further hot-rolled using a mandrel mill, reducer, sizing mill, etc., to make a raw pipe. The cumulative reduction ratio in the hot working process is for example 20 to 70%. Alternatively, the raw pipe may be manufactured from the billet by other hot working methods. The raw pipe may also be manufactured by forging such as the Erhardt process. The raw pipe is manufactured by the above process.

[0088] Raw tubes manufactured by hot working may be air-cooled (as-rolled). Raw tubes manufactured by hot working may be quenched directly after hot working without cooling to room temperature, or they may be reheated after hot working before quenching. The quenching process will be described in detail below.

[0089] [Heat treatment process] In the quenching process, the prepared raw tubes are subjected to quenching. In this specification, "quenching" means rapidly cooling raw tubes with an A3 point or higher. The preferred quenching temperature is 800 to 1000°C. If the quenching temperature is too high, the crystal grains of the prior γ grains may become coarse, which may reduce the low-temperature toughness of the manufactured seamless steel tube. Therefore, a quenching temperature of 800 to 1000°C is preferred. The holding time at the quenching temperature is not particularly limited, but for example, it is 5 to 90 minutes.

[0090] The quenching method involves continuously cooling the raw tube from the quenching start temperature, thereby continuously lowering the surface temperature of the raw tube. The method of continuous cooling is not particularly limited and any well-known method may be used. Examples of continuous cooling methods include immersing the raw tube in a water bath or accelerating the cooling of the raw tube by shower water cooling or mist cooling. The tempering process will be described in detail below.

[0091] [Tempering process] In the tempering process, the raw tube that has undergone the above-described quenching is subjected to tempering. In this specification, "tempering" refers to the process of tempering the raw tube after quenching. c1 This means reheating to a temperature below 1 / 20 and holding it there. Here, the tempering temperature corresponds to the furnace temperature at which the raw tube is heated and held after quenching. The tempering time refers to the time from when the temperature of the raw tube reaches the predetermined tempering temperature until it is removed from the heat treatment furnace.

[0092] The tempering temperature is adjusted as appropriate according to the chemical composition of the seamless steel pipe and the yield strength to be obtained. In other words, the tempering temperature is adjusted for a raw pipe having the chemical composition of this embodiment to adjust the yield strength of the seamless steel pipe to 690 MPa or higher. In the tempering process according to this embodiment, the preferred tempering temperature is 500 to 700°C. In the tempering process according to this embodiment, the preferred tempering time is 5 to 240 minutes.

[0093] The seamless steel pipe according to this embodiment can be manufactured by the manufacturing method described above. However, as stated above, the above manufacturing method is just one example, and it may be manufactured by other manufacturing methods. The present invention will be described in more detail below with reference to examples. [Examples]

[0094] Molten steel having the chemical composition shown in Table 1 was produced. In Table 1, "-" indicates that the content of each element is at the impurity level. Specifically, the Nb content in test number 1 was 0%, rounded to the third decimal place. The V content in test number 2 was 0%, rounded to the third decimal place. The Ti content in test number 3 was 0%, rounded to the fourth decimal place. Table 1 also shows the elemental content for each test number and the CE calculated from formula (1) above.

[0095] [Table 1]

[0096] Round billets were manufactured using the molten steel of each test number by continuous casting. After continuous casting, the round billets of each test number were 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. After cooling, the round billets of each test number were heated in a continuous heating furnace to perform hot working. The continuous heating furnace had a preheating zone, a heating zone, and a uniform zone. For the heating of the round billets of each test number, the furnace temperature T1 (°C), time in the furnace t1 (minutes), and LMP1 calculated from T1, t1 and equation (A) are shown in Table 2. Furthermore, the furnace temperature (°C), time in the furnace (minutes), furnace temperature (°C), and time in the furnace (minutes) of the uniform zone are also shown in Table 2.

[0097] [Table 2]

[0098] The raw tubes for each test number obtained were subjected to quenching and tempering. Specifically, each raw tube for each test number was quenched by holding it at 850-1000°C for 5-90 minutes, followed by water cooling. The quenched raw tubes for each test number were then tempered by holding them at 500-700°C for 5-240 minutes. Through the above manufacturing process, seamless steel pipes for each test number were obtained. The outer diameter (mm) and wall thickness (mm) of the seamless steel pipes for each test number are shown in Table 2.

[0099] [Evaluation Test] For the seamless steel pipes of each test number after tempering as described above, the following tensile tests, fine (Mn,Mg) particle number density measurements, and low-temperature toughness evaluation tests were performed.

[0100] [Tensile test] Tensile tests were conducted on each seamless steel pipe with a test number in accordance with JIS Z 2241 (2011). Specifically, a No. 4 test specimen, as specified in JIS Z 2241 (2011), was prepared from the center of the wall thickness of each seamless steel pipe with a test number. The longitudinal direction of the tensile test specimen was the same as the axial direction of the steel pipe. Using the prepared tensile test specimen, a tensile test was conducted at room temperature (25°C) in air in accordance with JIS Z 2241 (2011), and the obtained 0.2% offset proof strength was defined as the yield strength (MPa). The maximum stress in uniform elongation obtained in a similar tensile test was defined as the tensile strength (MPa). For each seamless steel pipe with a test number, the obtained yield strength (MPa) is shown as "YS (MPa)" and the tensile strength as "TS (MPa)," and these are shown in Table 2.

[0101] [Number density measurement test of fine (Mn,Mg) contained particles] For each test number of seamless steel pipe, a number density measurement test of fine (Mn,Mg)-containing particles was performed, and the number density ND (particles / mm³) of fine (Mn,Mg)-containing particles was determined. 2The number density ND (particles / mm²) of the fine (Mn,Mg) particles was determined. Specifically, test specimens for microstructural observation were prepared from the center of the wall thickness of the seamless steel pipe for each test number. Replica films were prepared using the test specimens for each test number using the method described above. TEM observation was performed on the prepared replica films using the method described above, and particles with an equivalent circle diameter of 0.10 μm or less were identified. Point analysis by EDS was performed on each identified particle using the method described above, and particles with a Mg content of 10% by mass or more and a Mn content of 30% by mass or more were identified. Based on the total number of identified fine (Mn,Mg) particles and the total area of ​​the observation field, the number density ND (particles / mm²) of fine (Mn,Mg) particles was determined. 2 Furthermore, the number density ND (particles / mm³) of fine (Mn,Mg)-containing particles was determined. 2 Using the above CE, FnA was determined. The number density ND (particles / mm³) of fine (Mn,Mg)-containing particles for each test number obtained was calculated. 2 The results are shown in Table 2. The FnA values ​​for each test number obtained are shown in Table 2.

[0102] [Low-temperature toughness evaluation test] Charpy impact tests were performed on each test number of seamless steel pipe according to the method compliant with ASTM A370 (2021). Specifically, full-size V-notch specimens were prepared from the center of the wall thickness of each test number's seamless steel pipe. The dimensions of the full-size V-notch specimens were 10 mm in width and 2 mm in notch depth. The longitudinal direction of the V-notch specimen was perpendicular to the axial direction of the seamless steel pipe. The width direction of the V-notch specimen was parallel to the radial direction of the seamless steel pipe. Charpy impact tests were performed on the prepared V-notch specimens according to ASTM A370 (2021) to determine the absorbed energy at -60°C. For each test number, the arithmetic mean of three V-notch specimens was calculated and defined as the absorbed energy (J) at -60°C. The obtained absorbed energy (J) at -60°C for each test number is shown in Table 2.

[0103] [Test Results] Referring to Tables 1 and 2, the chemical composition of the seamless steel pipes for test numbers 1 to 17 was appropriate. Furthermore, the manufacturing method met the preferred conditions described above. As a result, these seamless steel pipes had a yield strength of 690 MPa or higher and an FnA of 12.0 or lower. Consequently, in the low-temperature toughness evaluation test, these seamless steel pipes showed excellent low-temperature toughness, with an absorbed energy of 75 J or higher at -60°C. All of these seamless steel pipes had a microstructure mainly composed of tempered martensite.

[0104] On the other hand, the seamless steel pipe of test number 18 had an excessively slow average billet cooling rate. As a result, its FnA value exceeded 12.0. Consequently, in the low-temperature toughness evaluation test, this seamless steel pipe had an absorbed energy of less than 75 J at -60°C, failing to demonstrate excellent low-temperature toughness.

[0105] The seamless steel pipe in test number 19 had an excessively fast billet average cooling rate. As a result, its FnA value exceeded 12.0. Consequently, in the low-temperature toughness evaluation test, this seamless steel pipe had an absorbed energy of less than 75 J at -60°C, failing to demonstrate excellent low-temperature toughness.

[0106] In test number 20, 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 12.0. Consequently, in the low-temperature toughness evaluation test, this seamless steel pipe had an absorbed energy of less than 75 J at -60°C, and did not exhibit excellent low-temperature toughness.

[0107] In test number 21, the furnace temperature in the preheating zone was too high. As a result, the FnA value exceeded 12.0. Consequently, in the low-temperature toughness evaluation test, this seamless steel pipe had an absorbed energy of less than 75 J at -60°C, and did not exhibit excellent low-temperature toughness.

[0108] The seamless steel pipe in test number 22 had an in-furnace time in the preheating zone that was too short. As a result, its FnA value exceeded 12.0. Consequently, in the low-temperature toughness evaluation test, this seamless steel pipe had an absorbed energy of less than 75 J at -60°C, and did not exhibit excellent low-temperature toughness.

[0109] The seamless steel pipes in test numbers 23 and 24 had an LMP1 in the pre-heating zone that was too low. As a result, the FnA exceeded 12.0. Consequently, these seamless steel pipes did not exhibit good low-temperature toughness in the low-temperature toughness evaluation test, with an absorbed energy of less than 75 J at -60°C.

[0110] The seamless steel pipe in test number 25 had too low a magnesium content. As a result, its FnA value exceeded 12.0. Consequently, in the low-temperature toughness evaluation test, this seamless steel pipe showed an absorbed energy of less than 75 J at -60°C, failing to demonstrate excellent low-temperature toughness.

[0111] The seamless steel pipe in test number 26 had too low an Al content. As a result, its FnA value exceeded 12.0. Consequently, in the low-temperature toughness evaluation test, this seamless steel pipe showed an absorbed energy of less than 75 J at -60°C, failing to demonstrate excellent low-temperature toughness.

[0112] The seamless steel pipe in test number 27 had too low a sulfur content. As a result, its FnA value exceeded 12.0. Consequently, in the low-temperature toughness evaluation test, this seamless steel pipe had an absorbed energy of less than 75 J at -60°C, and did not exhibit excellent low-temperature toughness.

[0113] The seamless steel pipe in test number 28 had too high a sulfur content. Furthermore, the LMP1 of the pre-tropical zone was too low. As a result, in the low-temperature toughness evaluation test, this seamless steel pipe had an absorbed energy of less than 75 J at -60°C, and did not exhibit good low-temperature toughness.

[0114] The seamless steel pipe in test number 29 had too high a Ca content. Furthermore, the LMP1 of the pre-tropical zone was too low. As a result, the FnA exceeded 12.0. Consequently, in the low-temperature toughness evaluation test, this seamless steel pipe showed an absorbed energy of less than 75 J at -60°C, and did not exhibit excellent low-temperature toughness.

[0115] The seamless steel pipe in test number 30 had too low an Al content. As a result, its FnA value exceeded 12.0. Consequently, in the low-temperature toughness evaluation test, this seamless steel pipe showed an absorbed energy of less than 75 J at -60°C, failing to demonstrate excellent low-temperature toughness.

[0116] The seamless steel pipe in test number 31 had too low a carbon content. As a result, its yield strength was less than 690 MPa, meaning it did not exhibit the desired high strength.

[0117] The seamless steel pipe in test number 32 had too high a sulfur content. As a result, in the low-temperature toughness evaluation test, this seamless steel pipe had an absorbed energy of less than 75 J at -60°C, and did not exhibit excellent low-temperature toughness.

[0118] The seamless steel pipe in test number 33 had too high a Ca content. As a result, its FnA value exceeded 12.0. Consequently, in the low-temperature toughness evaluation test, this seamless steel pipe showed an absorbed energy of less than 75 J at -60°C, failing to demonstrate excellent low-temperature toughness.

[0119] The embodiments of this disclosure have been described above. However, the embodiments described above are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of this disclosure.

Claims

1. It is a seamless steel pipe, In mass percent, C: more than 0.080 to 0.180%, Si: 0.50% or less, Mn: 0.50 to 2.50%, P: 0.050% or less, S: 0.0010-0.0100%, Cu: 0.10-1.00%, Ni: 0.10-1.00%, Cr: 0.10-0.60%, Mo: 0.01 to 0.60%, Al: 0.030-0.100%, Ca: 0.0015% or less, Mg: 0.0010-0.0050%, B: 0.0005 to 0.0050%, and, N: Contains 0.0150% or less, and further, Nb: 0.01 to 0.10%, V: 0.01–0.10%, and, It contains one or more elements selected from the group consisting of Ti: 0.001 to 0.050%, The remainder consists of Fe and impurities. The yield strength is 690 MPa or higher. In the aforementioned seamless steel pipe, The number density of (Mn,Mg)-containing particles that satisfy the following conditions: Mg content of 10% by mass or more, Mn content of 30% by mass or more, and equivalent circle diameter of 0.10 μm or less is ND particles / mm². 2 When defined as, The CE defined in equation (1) and the ND satisfy equation (2), Seamless steel pipe. CE=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5 (1) 30.0×CE-ND×10 -5 ≦12.0 (2) Here, the elemental symbols in equation (1) are substituted with the content of the corresponding element in mass percent. If the corresponding element is not present, "0" is substituted for that elemental symbol.

2. A seamless steel pipe according to claim 1, The aforementioned seamless steel pipe is a seamless steel pipe for marine structural members. Seamless steel pipe.

Images