No steel pipes

Abstract

To provide a seamless steel tube having a high strength, while having excellent HAZ toughness at an extremely low temperature environment after welding.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 a Ti content satisfies 70 mass% or more, a number density of Ti-containing particles with an equivalent circle diameter of 0.10 μm or less is defined as NDT particles / mm2, a Mg content of 10 mass% or higher and a Mn content of 30 mass% or higher 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 NDM particles / mm2, a CE defined by an expression (1) and NDA defined by an expression (2) satisfy an expression (3): CE=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5 (1); NDA=(0.1×NDT+NDM)×10-5 (2); and 30.0×CE-NDA≤10.0 (3).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) and International Publication No. 2013 / 051231 (Patent Document 2) 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. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] International Publication No. 2005 / 052205 [Patent Document 2] International Publication No. 2013 / 051231 [Overview of the project] [Problems that the invention aims to solve]

[0007] In recent years, with the increasing size of offshore structures, seamless steel pipes for offshore structural members are required to have even higher strength. Specifically, there is a demand for seamless steel pipes with a yield strength of 690 MPa or higher. On the other hand, the above-mentioned Patent Documents 1 and 2 do not consider seamless steel pipes with a yield strength of 690 MPa or higher.

[0008] By the way, when constructing an offshore structure, seamless steel pipes for offshore structural members are joined together by welding. On the other hand, when multiple seamless steel pipes are welded together, the toughness of the heat-affected zone of the welding (hereinafter referred to as HAZ (Heat Affected Zone)) tends to decrease. In recent years, furthermore, there has been a growing demand for offshore structural members that are assumed to be used in regions such as the North Sea, the Arctic coast, and Siberia. In such offshore structural members, for example, excellent toughness in the HAZ after welding is required in an extremely low temperature environment of -60°C or lower. On the other hand, in Patent Documents 1 and 2 above, seamless steel pipes having excellent HAZ toughness in an extremely low temperature environment of -60°C or lower have not been studied.

[0009] An object of the present disclosure is to provide a seamless steel pipe having high strength and excellent HAZ toughness in an extremely low temperature environment after welding.

Means for Solving the Problems

[0010] The seamless steel pipe according to the present disclosure is 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%, Ti: 0.003 to 0.050%, Al: 0.030 to 0.100%, Ca: 0.0015% or less, Mg: 0.0005 to 0.0050%, N: 0.0020 to 0.0070%, and, B: 0.0005 to 0.0050%, contains, and further, Nb: 0.01 to 0.10%, and, contains at least one element selected from the group consisting of V: 0.01 to 0.10%. 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 Ti-containing particles that meet the requirements of having a Ti content of 70% by mass or more and an equivalent circle diameter of 0.10 μm or less is NDT particles / mm². 2 Defined as, 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 NDM particles / mm². 2 When defined as, The CE defined in equation (1) and the NDA defined in equation (2) satisfy equation (3). CE=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5 (1) NDA = (0.1 × NDT + NDM) × 10 -5 (2) 30.0 × CE - NDA ≤ 10.0 (3) 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. [Effects of the Invention]

[0011] The seamless steel pipes described herein possess high strength and excellent HAZ toughness in cryogenic environments after welding. [Brief explanation of the drawing]

[0012] [Figure 1] Figure 1 shows the relationship between FnA (=30.0 × CE-NDA) in this embodiment and the absorbed energy (J) at -60°C, which is an indicator of HAZ toughness in a cryogenic environment for seamless steel pipes after welding. [Figure 2] Figure 2 is a schematic diagram showing a magnified portion of the observation field when the microstructure of a seamless steel pipe according to this embodiment was observed under a microscope using the method according to this embodiment. [Modes for carrying out the invention]

[0013] First, the inventors investigated, from the perspective of chemical composition, how to increase the strength of seamless steel pipes intended for use in marine structural members and how to improve HAZ toughness in cryogenic environments after welding. As a result, the inventors found that the following chemical composition ratios (by mass%) are: 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%, Ti: 0.003 to 0.050%, Al: 0.030 to 0.100%, Ca: 0.0015% or less, Mg: 0 We considered that a seamless steel pipe containing 0.0005-0.0050% of N, 0.0020-0.0070% of B, and further containing one or more elements selected from the group consisting of 0.01-0.10% of Nb and 0.01-0.10% of V, with the remainder being Fe and impurities, would have a high yield strength of 690 MPa or more and potentially obtain excellent HAZ toughness even in cryogenic environments after welding.

[0014] On the other hand, seamless steel pipes having the above-mentioned chemical composition tend to have a hardened heat-affected zone (HAZ) after welding, resulting in reduced HAZ toughness. In other words, with seamless steel pipes having the above-mentioned chemical composition, if the yield strength is increased to 690 MPa or higher, sufficient HAZ toughness may not be obtained in cryogenic environments. Therefore, the inventors of this invention have investigated in detail a method to improve HAZ toughness even in cryogenic environments after welding while maintaining the yield strength of the steel material at 690 MPa or higher.

[0015] The inventors hypothesized that by precipitating a large number of fine inclusions or precipitates (hereinafter, inclusions or precipitates in the steel material are also simply referred to as "particles") in the steel material, it would be possible to suppress grain coarsening due to heat during welding, and thus obtain excellent heat-affected zone (HAZ) toughness even in an extremely low-temperature environment while maintaining a yield strength of 690 MPa or higher. Specifically, the inventors focused on Ti-containing particles with an equivalent circle diameter of 0.10 μm or less, and (Mn,Mg)-containing particles, and conducted detailed studies to improve HAZ toughness.

[0016] Hereinafter, Ti-containing particles with an equivalent circle diameter of 0.10 μm or less will also be referred to as "fine Ti-containing particles." Similarly, (Mn,Mg)-containing particles with an equivalent circle diameter of 0.10 μm or less will also be referred to as "fine (Mn,Mg)-containing particles." In this specification, "Ti-containing particles" means particles that, as determined by the method described later, have a Ti content of 70% or more by mass. Furthermore, in this specification, "(Mn,Mg)-containing particles" means particles that, as determined by the method described later, satisfy the requirements of having a Mg content of 10% or more and a Mn content of 30% or more by mass.

[0017] The inventors have found that in a seamless steel pipe having the above-described chemical composition, most of the fine Ti-containing particles are fine Ti nitrides. Furthermore, the inventors have found that in a seamless steel pipe having the above-described chemical composition, most of the fine (Mn,Mg)-containing particles are 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.

[0018] Taking the above findings into consideration, the inventors further investigated the relationship between the number density of fine Ti-containing particles, the number density of fine (Mn,Mg)-containing particles, and the HAZ toughness after welding for seamless steel pipes having the above-mentioned chemical composition. As a result, in seamless steel pipes having the above-mentioned chemical composition, the relationship between CE, defined by the following formula (1), and the number density of fine Ti-containing particles NDT (particles / mm³) 2 ), number density of fine (Mn,Mg) particles NDM (particles / mm³) 2 It was revealed that when the NDA defined by the following equation (2) satisfies the following equation (3), the HAZ toughness after welding is significantly increased even in an extremely low-temperature environment. CE=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5 (1) NDA = (0.1 × NDT + NDM) × 10 -5 (2) 30.0 × CE - NDA ≤ 10.0 (3) 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.

[0019] CE, defined by equation (1), is an index of strength. A higher CE indicates higher strength of the seamless steel pipe, but the HAZ after welding tends to become harder. NDA, defined by equation (2), is an index of the suppression of coarsening of prior austenite grains in the HAZ by fine Ti-containing particles and fine (Mn,Mg)-containing particles. Fine (Mn,Mg) sulfides, which are the main component of fine (Mn,Mg)-containing particles, have a higher melting point than fine Ti nitrides, which are the main component of fine Ti-containing particles. In other words, fine (Mn,Mg)-containing particles are less likely to melt due to the heat during welding compared to fine Ti-containing particles. As a result, fine (Mn,Mg)-containing particles may be able to more effectively suppress the coarsening of prior austenite grains in the HAZ with a smaller number of particles. Considering the above findings, the inventors have determined the number density NDT (particles / mm²) of fine Ti-containing particles. 2 ) and the number density of fine (Mn,Mg) particles NDM (particles / mm³) 2 We negotiated with them and obtained an NDA.

[0020] Equation (3) is defined as FnA = 30.0 × CE - NDA. FnA is an index of the HAZ toughness after welding in a cryogenic environment for seamless steel pipes having the above-described chemical composition. Below, the relationship between the FnA of a seamless steel pipe and the HAZ toughness of the welded seamless steel pipe in a cryogenic environment will be explained in detail with reference to 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 the HAZ toughness of the welded seamless steel pipe in a cryogenic environment, for an example of the example described later that has the above-described chemical composition. The value of FnA and the absorbed energy were determined by the method described later. In addition, in all examples shown in Figure 1, the yield strength was 690 MPa or higher.

[0021] Referring to Figure 1, when FnA exceeds 10.0, the absorbed energy drops sharply to less than 75 J. On the other hand, if FnA is 10.0 or less, the absorbed energy remains stable at 75 J or more. In other words, Figure 1 demonstrates that if FnA is 10.0 or less, the HAZ toughness can be stably increased.

[0022] The detailed reasons why reducing FnA to 10.0 or less significantly improves the post-weld HAZ toughness of seamless steel pipes are not yet clear. However, the inventors speculate as follows: As mentioned above, CE is an indicator of strength, and the higher the CE, the higher the strength of the seamless steel pipe. On the other hand, if CE is high, the strength of the post-weld HAZ also increases, and the HAZ toughness in cryogenic environments tends to decrease. Therefore, even if the strength of the seamless steel pipe is increased, it is thought that it may be necessary to disperse more fine Ti-containing particles and / or fine (Mn,Mg)-containing particles in order to obtain excellent HAZ toughness in cryogenic environments. In this way, the inventors believe that by adjusting NDA according to CE and reducing FnA to 10.0 or less, it may be possible to significantly improve the HAZ 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 10.0 or less significantly increases the HAZ toughness of seamless steel pipes in cryogenic environments after welding through a mechanism different from the one described above. However, the fact that reducing FnA to 10.0 or less significantly increases the HAZ toughness of seamless steel pipes in cryogenic environments after welding has been demonstrated by the examples described later.

[0023] Based on the above findings, the gist of the seamless steel pipe according to this embodiment is as follows:

[0024] [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: less than 0.050%, 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%, Ti: 0.003 - 0.050%, Al: 0.030 - 0.100%, Ca: less than 0.0015%, Mg: 0.0005 - 0.0050%, N: 0.0020 - 0.0070%, and, B: 0.0005 - 0.0050%, contains, and further, Nb: 0.01 - 0.10%, and, V: contains one or more elements selected from the group consisting of 0.01 - 0.10%, the balance consists of Fe and impurities, the yield strength is 690 MPa or more, in the seamless steel pipe, when the Ti content satisfies 70 mass% or more, and the number density of Ti-containing particles with an equivalent circle diameter of 0.10 μm or less is defined as NDT per mm 2 and, when the Mg content satisfies 10 mass% or more, and the Mn content satisfies 30 mass% or more, and the number density of (Mn, Mg)-containing particles with an equivalent circle diameter of 0.10 μm or less is defined as NDM per mm 2 it is defined that, CE defined by formula (1) and NDA defined by formula (2) satisfy formula (3), Seamless steel pipe. CE = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1) NDA = (0.1 × NDT + NDM) × 10 -5 (2) 30.0 × CE - NDA ≤ 10.0 (3) <000C0209>Here, in the element symbols in formula (1), the content of the corresponding element is substituted in mass%. When the corresponding element is not contained, "0" is substituted in the element symbol.

[0025] [2] [1] A seamless steel pipe as described above, The aforementioned seamless steel pipe is a seamless steel pipe for marine structural members. Seamless steel pipe.

[0026] The seamless steel pipe according to this embodiment will be described in detail below. Unless otherwise specified, the "%" in relation to elements refers to mass percent. Also, in the following description, the seamless steel pipe will be simply referred to as "steel material".

[0027] [Chemical composition] The seamless steel pipe according to this embodiment contains the following elements:

[0028] 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, even if the content of other elements is within the range of this embodiment, the HAZ after welding hardens, and the HAZ toughness after welding decreases. 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 0.175%, and more preferably 0.170%.

[0029] 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 HAZ toughness after welding will decrease, 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%, more preferably 0.40%, and still more preferably 0.30%. The preferred lower limit of the Si content to more effectively obtain the above effects is 0.01%, more preferably 0.03%, still more preferably 0.05%, and still more preferably 0.10%.

[0030] Mn: 0.50~2.50% Manganese (Mn) forms (Mn,Mg) sulfides with Mg and S, increasing the number density NDM of fine (Mn,Mg)-containing particles. As a result, the HAZ toughness after welding 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 HAZ toughness after welding decreases. Therefore, the Mn content is 0.50 to 2.50%. The preferred lower limit of the Mn content is 0.60%, more preferably 0.70%, and still more preferably 0.80%. The preferred upper limit of the Mn content is 2.30%, more preferably 2.00%, and still more preferably 1.80%.

[0031] 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 HAZ toughness after welding. 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%.

[0032] S: 0.0010~0.0100% Sulfur (S) forms (Mn,Mg) sulfides with Mn and Mg, increasing the number density (NDM) of fine (Mn,Mg)-containing particles. As a result, the HAZ toughness after welding 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 Mn sulfides are formed in the steel, and the number density (NDM) of fine (Mn,Mg)-containing particles decreases. As a result, the HAZ toughness after welding decreases. Therefore, the S content is 0.0010 to 0.0100%. The preferred lower limit of the S content is 0.0012%, more preferably 0.0015%, and even more preferably 0.0020%. The preferred upper limit of the S content is 0.0090%, and even more preferably 0.0080%.

[0033] 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 HAZ toughness after welding decreases. Therefore, the Cu content is 0.10 to 1.00%. The preferred lower limit of the Cu content is 0.11%, more preferably 0.13%, and even more preferably 0.15%. The preferred upper limit of the Cu content is 0.80%, more preferably 0.60%, and even more preferably 0.50%.

[0034] 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 HAZ toughness after welding decreases. Therefore, the Ni content is 0.10 to 1.00%. The preferred lower limit of the Ni content is 0.11%, more preferably 0.13%, and even more preferably 0.15%. The preferred upper limit of the Ni content is 0.90%, more preferably 0.80%, and even more preferably 0.60%.

[0035] 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 becomes too high, and the HAZ toughness after welding decreases. Therefore, the Cr content is 0.10 to 0.60%. The preferred lower limit of the Cr content is 0.11%, more preferably 0.13%, and even more preferably 0.15%. The preferred upper limit of the Cr content is 0.55%, more preferably 0.50%, and even more preferably 0.45%.

[0036] Mo: 0.01~0.60% Molybdenum (Mo) enhances the hardenability and strength of steel. However, if the Mo content is too high, even if the content of other elements is within the range of this embodiment, the hardenability will become too high, and the HAZ toughness after welding will decrease. Therefore, when Mo is included, the Mo content is 0.01 to 0.60%. The preferred lower limit of the Mo content to more effectively obtain the above effects is 0.02%, more preferably 0.03%, and even more preferably 0.04%. The preferred upper limit of the Mo content is 0.55%, more preferably 0.50%, and even more preferably 0.45%.

[0037] Ti: 0.003~0.050% Titanium (Ti) combines with nitrogen to form fine Ti nitrides, increasing the number density (NDT) of fine Ti-containing particles. As a result, the HAZ toughness after welding is improved. If the Ti 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 Ti content is too high, even if the content of other elements is within the range of this embodiment, the Ti nitrides become coarser, and the number density (NDT) of fine Ti-containing particles decreases. As a result, the HAZ toughness after welding actually decreases. Therefore, the Ti content is 0.003 to 0.050%. The preferred lower limit of the Ti content is greater than 0.003%, more preferably 0.004%, and even more preferably 0.005%. The preferred upper limit of the Ti content is 0.045%, more preferably 0.040%, and even more preferably 0.030%.

[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 HAZ toughness after welding decreases. Therefore, the Al content is 0.030 to 0.100%. The preferred upper limit of the Al content is 0.090%, and more preferably 0.080%. The preferred lower limit of the Al content is 0.035%, and more preferably 0.040%. 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 controls the shape of the Ti nitride, refining the Ti nitride and increasing the number density (NDT) of fine Ti-containing particles. On the other hand, if the Ca content is too high, even if the content of other elements is within the range of this embodiment, Ca sulfide will be formed, inhibiting the formation of (Mn,Mg) sulfide. As a result, the number density (NDM) of fine (Mn,Mg)-containing particles decreases, and the HAZ toughness after welding decreases. Therefore, the Ca content is 0.0015% or less. The preferred upper limit of the Ca content is 0.0013%, and more preferably 0.0010%. 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: 0.0005~0.0050% Magnesium (Mg), together with Mn and S, forms (Mn,Mg) sulfides, increasing the number density (NDM) of fine (Mn,Mg)-containing particles. As a result, the HAZ toughness after welding 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 HAZ toughness after welding is actually reduced. Therefore, the Mg content is 0.0005 to 0.0050%. The preferred lower limit of the Mg content is 0.0006%, more preferably 0.0008%, and even more preferably 0.0010%. The preferred upper limit of the Mg content is 0.0048%, more preferably 0.0045%, and even more preferably 0.0040%.

[0041] N: 0.0020~0.0070% Nitrogen (N) combines with Ti to form fine Ti nitrides, increasing the number density (NDT) of fine Ti-containing particles. As a result, the HAZ toughness after welding is improved. If the N 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 N content is too high, even if the content of other elements is within the range of this embodiment, the Ti nitrides become coarser, and the number density (NDT) of fine Ti-containing particles decreases. As a result, the HAZ toughness after welding actually decreases. Therefore, the N content is 0.0020 to 0.0070%. The preferred lower limit of the N content is 0.0025%, more preferably 0.0028%, and even more preferably 0.0030%. The preferred upper limit of the N content is 0.0065%, more preferably 0.0060%, and even more preferably 0.0055%.

[0042] B: 0.0005~0.0050% Boron (B) dissolves in steel to improve its hardenability and increase 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, and the HAZ toughness after welding decreases. Therefore, the B content is 0.0005 to 0.0050%. The preferred lower limit of the B content is 0.0006%, and more preferably 0.0007%. The preferred upper limit of the B content is 0.0045%, more preferably 0.0040%, more preferably 0.0030%, 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 and V. That is, it may contain only one of the elements Nb or V, and the content of the other element may be 0%. All of these elements enhance the hardenability of the steel and increase its strength.

[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 HAZ toughness after welding. 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%, and more preferably 0.08%.

[0045] V: 0.01~0.10% Vanadium (V) combines with carbon (C) in the 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 HAZ toughness after welding. 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%, more preferably 0.07%, and still more preferably 0.06%.

[0046] 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.

[0047] [Yield strength] The seamless steel pipe according to this embodiment has the above-described chemical composition and an FnA content of 10.0 or less. As a result, the seamless steel pipe according to this embodiment has a yield strength of 690 MPa or more and exhibits excellent HAZ toughness after welding.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] [Fine Ti-containing particles and fine (Mn, Mg)-containing particles] Assuming that the seamless steel pipe according to this embodiment has the above-described chemical composition, the CE defined by formula (1) and the NDA defined by formula (2) satisfy formula (3). CE=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5 (1) NDA = (0.1 × NDT + NDM) × 10 -5 (2) 30.0 × CE - NDA ≤ 10.0 (3) Here, the elemental symbol in equation (1) is 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. In equation (2), NDT is the number density of Ti-containing particles that satisfy the Ti content of 70 mass percent or more and have an equivalent circle diameter of 0.10 μm or less, in units of particles / mm². 2 Substituting this, in equation (2), NDM is defined as 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, in units of particles / mm 2 It is substituted.

[0052] As described above, CE, as defined by formula (1), is an index of strength. The larger the CE, the higher the strength of the seamless steel pipe, but the HAZ after welding tends to become harder. In this embodiment, CE is not particularly limited as long as formula (3) is satisfied. CE may be, for example, 0.300 to 0.700. A more preferred lower limit for CE is 0.350, more preferably 0.375, more preferably 0.400, and still more preferably 0.450. A more preferred upper limit for CE is 0.650, more preferably 0.625, more preferably 0.600, and still more preferably 0.550.

[0053] The NDA defined by equation (2) is an index indicating the degree to which fine Ti-containing particles and fine (Mn,Mg)-containing particles suppress the coarsening of prior austenite grains after welding. The larger the NDA, the greater the effect of suppressing the coarsening of prior austenite grains in the HAZ. In this embodiment, the number density NDT of fine Ti-containing particles and the number density NDM of fine (Mn,Mg)-containing particles are not particularly limited as long as they satisfy equation (3). For example, the number density NDT of fine Ti-containing particles is 0.5 × 10⁻⁶. 6 ~10.0×106 (pcs / mm 2 ) may also be the case. The number density NDM of fine (Mn,Mg)-containing particles is, for example, 0.5 × 10⁻⁶. 5 ~10.0×10 5 (pcs / mm 2 ) may also be. In this embodiment, the NDA defined by formula (2) is not particularly limited as long as it satisfies formula (3). The NDA may be, for example, 1.0 to 20.0.

[0054] In this embodiment, the seamless steel pipe having the chemical composition is composed of mostly fine Ti-containing particles, which are fine Ti nitrides. Furthermore, in this embodiment, the seamless steel pipe having the chemical composition is composed of mostly fine (Mn,Mg)-containing particles, which are fine (Mn,Mg) sulfides. (Mn,Mg) sulfides are particles formed when Mn is concentrated into Mg sulfides.

[0055] (Mn,Mg) sulfides are less stretchable and more easily formed into fine particles compared to Mn sulfides. Furthermore, (Mn,Mg) sulfides have a higher melting point compared to Ti nitrides. That is, fine (Mn,Mg)-containing particles, mainly composed of fine (Mn,Mg) sulfides, may be more effective in suppressing the coarsening of prior austenite grains in the HAZ compared to fine Ti-containing particles, mainly composed of fine Ti nitrides. Considering these findings, the inventors have determined the number density NDT (particles / mm²) of fine Ti-containing particles. 2 ) and the number density of fine (Mn,Mg) particles NDM (particles / mm³) 2 ) was adjusted to obtain the NDA defined in equation (2).

[0056] Here, the fine Ti-containing particles and the fine (Mn,Mg)-containing particles may exist individually or in combination. Note that the combination of fine Ti-containing particles and fine (Mn,Mg)-containing particles means that the fine Ti-containing particles and the fine (Mn,Mg)-containing particles are adjacent to each other and exist within the steel material. This will be explained with reference to Figure 2. Figure 2 is a schematic diagram showing a magnified portion of the observation field when the microstructure of a seamless steel pipe according to this embodiment is observed under a microscope using the method according to this embodiment. In observation field 1 in Figure 2, the fine Ti-containing particles 10 and the fine (Mn,Mg)-containing particles 20 are adjacent to each other.

[0057] Referring to Figure 2, in the seamless steel pipe according to this embodiment, the fine Ti-containing particles 10 and the fine (Mn,Mg)-containing particles 20 may appear to be combined and appear as a single entity. However, a person skilled in the art can naturally determine whether they are individual or combined from the particle shape, contrast, and chemical composition analysis results. Therefore, in this embodiment, microscopic observation is performed using the method described later, and the particles shown in Figure 2 are counted as one fine Ti-containing particle 10 and one fine (Mn,Mg)-containing particle 20.

[0058] In equation (3), FnA (=30.0 × CE-NDA) is an index of the HAZ toughness after welding in a seamless steel pipe having the above-described chemical composition. If FnA is too high, the HAZ toughness of the seamless steel pipe in a cryogenic environment after welding will decrease. Therefore, in this embodiment, assuming the above-described chemical composition, FnA is set to 10.0 or less. The preferred upper limit of FnA is 9.9, more preferably 9.8, and even more preferably 9.7. The lower limit of FnA is not particularly limited. The lower limit of FnA may be, for example, -3.0, -1.5, or 0.0.

[0059] In this embodiment, the number density NDT of fine Ti-containing particles, the number density NDM of fine (Mn,Mg)-containing particles, and the NDA defined by formula (2) can be determined by the following method. A test 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-deposited film. The test specimen with the surface covered with the vapor-deposited film 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.

[0060] 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 observation field is 4.0 μm × 5.0 μm, the number of observation fields should be 50 or more, and the total area of ​​the observation fields should be 1000 μm. 2 That concludes this section.

[0061] 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.

[0062] 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 Ti content of 70% or more by mass are identified as "fine Ti-containing particles". Furthermore, based on the EDS analysis results for each particle with an equivalent circle diameter of 0.10 μm or less, particles with 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".

[0063] The total number of fine Ti-containing particles and the total number of fine (Mn,Mg)-containing particles identified in 10 fields of view are counted to determine the number density. As mentioned above, if fine Ti-containing particles and fine (Mn,Mg)-containing particles are present in combination, they are counted separately. Based on the total number of fine Ti-containing particles and the total area of ​​the 10 fields of view, the number density NDT (particles / mm²) of the fine Ti-containing particles is calculated. 2 The number density of fine (Mn,Mg) particles is calculated based on the total number of fine (Mn,Mg) particles and the total area of ​​the 10 fields of view. 2 ) is determined. In this embodiment, as described in Table 2 of the Examples below, the number density NDT (particles / mm) of fine Ti-containing particles is determined. 2 When calculating ), the unit is 10 6 pieces / mm 2 The number density is obtained by rounding the mantissa to the second decimal place. Similarly, as shown in Table 2 of the examples described later, the number density NDM (particles / mm²) of fine (Mn,Mg)-containing particles is calculated. 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.

[0064] In this embodiment, accurate quantitative determination of nitrogen (N) and sulfur (S) content in EDS point analysis of steel materials having the above-mentioned chemical composition is often difficult with current technology. Therefore, in this embodiment, particles with an equivalent circular diameter of 0.10 μm or less and a Ti content of 70 mass% or more are identified as fine Ti-containing particles. That is, fine Ti-containing particles in this embodiment may also include Ti compounds other than Ti nitrides. However, in seamless steel pipes having the above-mentioned chemical composition, the amount of Ti compounds other than Ti nitrides (such as Ti carbides and Ti oxides) in the fine Ti-containing particles is negligibly small. That is, in this embodiment, the number density NDT of fine Ti-containing particles (particles / mm²) 2 ) essentially refers to the number density (particles / mm³) of Ti nitrides with an equivalent circular diameter of 0.10 μm or less. 2 This corresponds to ).

[0065] In this embodiment, particles with an equivalent circular diameter of 0.10 μm or less, having a Mg content of 10% by mass or more and a Mn content of 30% by mass or more are further identified as fine (Mn,Mg)-containing particles. That is, the fine (Mn,Mg)-containing particles according to this embodiment may also include compounds other than (Mn,Mg) sulfides (such as oxides). However, in the seamless steel pipe having the above-described 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 of fine (Mn,Mg)-containing particles is NDM (particles / mm²). 2 This essentially refers to the number density (particles / mm³) of (Mn,Mg) sulfides with an equivalent circle diameter of 0.10 μm or less. 2 This corresponds to ).

[0066] [HAZ toughness] The seamless steel pipe according to this embodiment has the above-described chemical composition and an FnA content of 10.0 or less. As a result, the seamless steel pipe according to this embodiment has a yield strength of 690 MPa or more and exhibits excellent HAZ toughness in an cryogenic environment after welding. Here, excellent HAZ toughness is defined as follows.

[0067] A Charpy impact test will be performed on the HAZ formed on the seamless steel pipe according to this embodiment, in accordance with ASTM A370 (2021), to determine the absorbed energy at -60°C. Specifically, first, a welded joint will be fabricated from the seamless steel pipe according to this embodiment. The groove shape will be single-bevel, and the welding method will be submerged arc welding. The welding heat input will be 50 kJ / cm, and the preheating and interlayer temperatures will be 100 to 250°C. The welding wire will be S80J4-H4 as specified in JIS Z 3183 (2012), and the diameter of the welding wire will be 4.0 mm.

[0068] A full-size V-notch specimen is prepared from the fabricated welded joint. If a full-size specimen cannot be prepared, a sub-size V-notch specimen is prepared from the center of the wall thickness of the seamless steel pipe. The length direction of the V-notch specimen is parallel to the pipe axis direction of the seamless steel pipe. The width direction of the V-notch specimen is parallel to the pipe diameter direction (wall thickness direction) of the seamless steel pipe. The notch position of the V-notch specimen is within 0.5 mm of the melting line on the straight side of the V-groove of the welded joint. More specifically, the central 60% of the thickness of the V-notch specimen is divided into 9 equal parts in the direction of that thickness to identify 10 verification points. At the identified 10 verification points, the distance between the notch position and the melting line on the straight side of the V-groove of the welded joint is measured. The V-notch specimen is prepared so that the arithmetic mean of the 10 measurement results is 0.5 mm or less.

[0069] A Charpy impact test is performed on the fabricated V-notch specimen 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 determined that the material exhibits excellent HAZ toughness in an extremely low-temperature environment after welding.

[0070] [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.

[0071] 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.

[0072] 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.

[0073] [Shape of seamless steel pipe] 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 HAZ toughness after welding.

[0074] [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.

[0075] [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.

[0076] 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.

[0077] If the average billet cooling rate is too slow, Ti nitrides and / or (Mn,Mg) sulfides may coarseen in the molten steel. As a result, the number density of fine Ti-containing particles and / or fine (Mn,Mg)-containing particles may decrease in the manufactured seamless steel pipe, and the FnA may become too large. On the other hand, if the average billet cooling rate is too fast, the amount of crystallized and / or precipitated Ti nitrides and / or (Mn,Mg) sulfides may decrease. As a result, the number density of fine Ti-containing particles and / or fine (Mn,Mg)-containing particles may decrease in the manufactured seamless steel pipe, and the 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.

[0078] 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.

[0079] [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.

[0080] 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.

[0081] 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)

[0082] 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 of fine Ti-containing particles and / or fine (Mn,Mg)-containing particles in the manufactured seamless steel pipe may decrease. In this case, sufficient HAZ toughness after welding 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 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 reduce the FnA of the manufactured seamless steel pipe to 10.0 or less.

[0083] 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 pipe may not be sufficiently heated, making it difficult to stably perform the hot working described later. On the other hand, if the furnace time t1 in the preheating zone is too long, the Ti nitride and / or (Mn,Mg) sulfide in the raw pipe may dissolve too much. In this case, the number density of fine Ti-containing particles and / or fine (Mn,Mg)-containing particles in the manufactured seamless steel pipe decreases, and sufficient HAZ toughness after welding cannot be 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 reduce the FnA of the manufactured seamless steel pipe to 10.0 or less.

[0084] 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)

[0085] 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 of fine Ti-containing particles and / or fine (Mn,Mg)-containing particles in the manufactured seamless steel pipe may decrease. In this case, sufficient HAZ toughness after welding cannot be obtained. Therefore, provided that the other preferred manufacturing methods of this embodiment are satisfied, setting LMP1 to 28000 or higher can make the FnA of the manufactured seamless steel pipe 10.0 or lower. Note that there is no particular upper limit to LMP1. The upper limit of LMP1 may be, for example, 30233 or 30000.

[0086] 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 range of this embodiment, the FnA will be 10.0 or less, provided that the other manufacturing methods meet the preferred range of this embodiment.

[0087] 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

[0088] 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.

[0089] In this embodiment, it is preferable to set the furnace time in the heating zone to 40 to 100 minutes. In this embodiment, it is further preferable to set the furnace time in the uniform zone to 30 to 70 minutes. If these furnace times are too short, the number density of fine Ti-containing particles and / or fine (Mn,Mg)-containing particles in the manufactured seamless steel pipe may decrease. In this case, sufficient HAZ toughness after welding cannot be obtained. Furthermore, in this case, the center of the raw pipe may not be sufficiently heated, making it difficult to stably perform the hot working described later. On the other hand, if these furnace times are too long, the number density of fine Ti-containing particles and / or fine (Mn,Mg)-containing particles in the manufactured seamless steel pipe may decrease. In this case, sufficient HAZ toughness after welding cannot be obtained.

[0090] 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 10.0 or less, the seamless steel pipe according to this embodiment can exhibit a yield strength of 690 MPa or more and excellent HAZ toughness after welding. The hot working process after heating will be described below.

[0091] 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.

[0092] 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.

[0093] [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 become coarse, which may reduce the HAZ toughness of the manufactured seamless steel tube after welding. 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.

[0094] 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.

[0095] [Tempering process] In the tempering process, the raw tube that has undergone the above-described quenching is tempered. In this specification, "tempering" refers to the process of tempering the raw tube after quenching. c1 This means reheating to a temperature below 1.5°C 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.

[0096] 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.

[0097] 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]

[0098] Molten steel having the chemical composition shown in Table 1 was produced. In Table 1, "-" indicates that the content of each element is at an 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 also 0%, rounded to the third decimal place. Table 1 also shows the elemental content for each test number and the CE calculated from formula (1) above.

[0099] [Table 1]

[0100] 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.

[0101] [Table 2]

[0102] 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.

[0103] [Evaluation Test] For the seamless steel pipes of each test number after tempering as described above, the following tests were performed: tensile tests, number density measurements of fine Ti-containing particles and fine (Mn,Mg)-containing particles, and HAZ toughness evaluation tests.

[0104] [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.

[0105] [Number density measurement test of fine Ti-containing particles and fine (Mn,Mg)-containing particles] For each test number of seamless steel pipe, a number density measurement test was conducted on fine Ti-containing particles and fine (Mn,Mg)-containing particles to determine the number density of fine Ti-containing particles (NDT) (particles / mm³). 2 ) and the number density of fine (Mn,Mg) particles NDM (particles / mm³) 2 The following was determined. Specifically, test specimens for microstructure 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 prepared 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 Ti content of 70 mass% or more were identified as fine Ti-containing particles. Similarly, point analysis by EDS was performed on each identified particle, and particles with a Mg content of 10 mass% or more and a Mn content of 30 mass% or more were identified as fine (Mn,Mg)-containing particles.

[0106] Based on the total number of identified fine Ti-containing particles and the total area of ​​the observation field, the number density NDT (particles / mm³) of the fine Ti-containing particles is calculated. 2The number density (NDM) of the fine (Mn,Mg) particles was determined based on the total number of identified fine (Mn,Mg) particles and the total area of ​​the observation field. 2 The number density NDT (particles / mm³) of fine Ti-containing particles for each test number was calculated. 2 ) and the number density of fine (Mn,Mg) particles NDM (particles / mm³) 2 ) and the above equation (2) give NDA(=(0.1×NDT+NDM)×10 -5 The number density NDT (particles / mm³) of the fine Ti-containing particles for each test number was calculated. Furthermore, FnA was calculated from the CE of each test number, the obtained NDA of each test number, and the above formula (3). 2 ) and the number density of fine (Mn,Mg) particles NDM (particles / mm³) 2 Table 2 shows the ) NDA and FnA.

[0107] [HAZ Toughness Evaluation Test] For each test number, a HAZ toughness evaluation test was performed to assess the toughness of the HAZ after welding. Specifically, welded joints were fabricated using the seamless steel pipes of each test number using the method described above. Full-size V-notch test specimens were fabricated from the fabricated welded joints of each test number using the method described above. A Charpy impact test was performed on each V-notch test specimen using the method described above to determine the absorbed energy (J) at -60°C. For each test number, the arithmetic mean of three V-notch test 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.

[0108] [Test Results] Referring to Tables 1 and 2, the chemical composition of the seamless steel pipes for test numbers 1 to 18 was appropriate. Furthermore, the manufacturing method also 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 10.0 or lower. Consequently, in the HAZ toughness evaluation test, these seamless steel pipes showed excellent HAZ toughness even in an extremely low-temperature environment, 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.

[0109] The seamless steel pipe in test number 19 had an excessively slow billet average cooling rate. As a result, its FnA value exceeded 10.0. Consequently, in the HAZ toughness evaluation test, this seamless steel pipe had an absorbed energy of less than 75 J at -60°C, failing to demonstrate excellent HAZ toughness in cryogenic environments.

[0110] The seamless steel pipe in test number 20 had an excessively fast billet average cooling rate. As a result, its FnA exceeded 10.0. Consequently, in the HAZ toughness evaluation test, this seamless steel pipe had an absorbed energy of less than 75 J at -60°C, failing to demonstrate excellent HAZ toughness in cryogenic environments.

[0111] In test number 21, 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 10.0. Consequently, in the HAZ toughness evaluation test, this seamless steel pipe absorbed less than 75 J at -60°C, failing to demonstrate excellent HAZ toughness in cryogenic environments.

[0112] In test number 22, the furnace temperature in the preheating zone was too high. As a result, the FnA value exceeded 10.0. Consequently, in the HAZ toughness evaluation test, this seamless steel pipe absorbed less than 75 J of energy at -60°C, failing to demonstrate excellent HAZ toughness in cryogenic environments.

[0113] The seamless steel pipe in test number 23 had an in-furnace time in the preheating zone that was too short. As a result, its FnA value exceeded 10.0. Consequently, in the HAZ toughness evaluation test, this seamless steel pipe absorbed less than 75 J of energy at -60°C, failing to demonstrate excellent HAZ toughness in cryogenic environments.

[0114] In test number 24, the LMP1 of the pre-tropical zone was too low. As a result, the FnA exceeded 10.0. Consequently, in the HAZ toughness evaluation test, this seamless steel pipe had an absorbed energy of less than 75 J at -60°C, and did not exhibit excellent HAZ toughness in an extremely low-temperature environment.

[0115] The seamless steel pipe in test number 25 had an in-furnace time in the heating zone that was too short. As a result, the FnA value exceeded 10.0. Consequently, in the HAZ toughness evaluation test, this seamless steel pipe had an absorbed energy of less than 75 J at -60°C, and did not exhibit excellent HAZ toughness in an extremely low-temperature environment.

[0116] The seamless steel pipes in test numbers 26 and 27 were kept in the furnace for too long in the uniform zone. As a result, the FnA value exceeded 10.0. Consequently, in the HAZ toughness evaluation test, these seamless steel pipes had an absorbed energy of less than 75 J at -60°C, and did not exhibit excellent HAZ toughness in the cryogenic environment.

[0117] The seamless steel pipe in test number 28 had too low a magnesium content. As a result, its FnA value exceeded 10.0. Consequently, in the HAZ toughness evaluation test, this seamless steel pipe had an absorbed energy of less than 75 J at -60°C, and did not exhibit excellent HAZ toughness in an extremely low-temperature environment.

[0118] The seamless steel pipe in test number 29 had too low an Al content. As a result, its FnA value exceeded 10.0. Consequently, in the HAZ toughness evaluation test, this seamless steel pipe had an absorbed energy of less than 75 J at -60°C, and did not exhibit excellent HAZ toughness in an extremely low-temperature environment.

[0119] The seamless steel pipe in test number 30 had too low a sulfur content. As a result, its FnA value exceeded 10.0. Consequently, in the HAZ toughness evaluation test, this seamless steel pipe had an absorbed energy of less than 75 J at -60°C, and did not exhibit excellent HAZ toughness in an extremely low-temperature environment.

[0120] The seamless steel pipe in test number 31 had an excessively high sulfur content. As a result, its FnA value exceeded 10.0. Consequently, in the HAZ toughness evaluation test, this seamless steel pipe exhibited an absorbed energy of less than 75 J at -60°C, failing to demonstrate excellent HAZ toughness in cryogenic environments.

[0121] The seamless steel pipe in test number 32 had an excessively high Ca content. As a result, its FnA value exceeded 10.0. Consequently, in the HAZ toughness evaluation test, this seamless steel pipe exhibited an absorbed energy of less than 75 J at -60°C, failing to demonstrate excellent HAZ toughness in cryogenic environments.

[0122] The seamless steel pipe in test number 33 had too low a Ti content. As a result, its FnA value exceeded 10.0. Consequently, in the HAZ toughness evaluation test, this seamless steel pipe had an absorbed energy of less than 75 J at -60°C, and did not exhibit excellent HAZ toughness in an extremely low-temperature environment.

[0123] The seamless steel pipe in test number 34 had too high a Ti content. As a result, its FnA value exceeded 10.0. Consequently, in the HAZ toughness evaluation test, this seamless steel pipe had an absorbed energy of less than 75 J at -60°C, and did not exhibit excellent HAZ toughness in an extremely low-temperature environment.

[0124] The seamless steel pipe in test number 35 had too low a nitrogen content. As a result, its FnA value exceeded 10.0. Consequently, in the HAZ toughness evaluation test, this seamless steel pipe had an absorbed energy of less than 75 J at -60°C, and did not exhibit excellent HAZ toughness in an extremely low-temperature environment.

[0125] The seamless steel pipe in test number 36 had too high a nitrogen content. As a result, its FnA value exceeded 10.0. Consequently, in the HAZ toughness evaluation test, this seamless steel pipe had an absorbed energy of less than 75 J at -60°C, and did not exhibit excellent HAZ toughness in an extremely low-temperature environment.

[0126] 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. [Explanation of Symbols]

[0127] 10 Fine Ti-containing particles 20 Fine (Mn, Mg) containing particles

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%, Ti: 0.003 to 0.050%, Al: 0.030-0.100%, Ca: 0.0015% or less, Mg: 0.0005-0.0050%, N: 0.0020–0.0070%, and, B: Contains 0.0005 to 0.0050%, and further, Nb: 0.01–0.10%, and, V: Contains one or more elements selected from the group consisting of 0.01 to 0.10%, 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 Ti-containing particles that satisfy the requirement of having a Ti content of 70% by mass or more and an equivalent circle diameter of 0.10 μm or less is NDT particles / mm². 2 Defined as, 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 NDM particles / mm². 2 When defined as, The CE defined by equation (1) and the NDA defined by equation (2) satisfy equation (3), Seamless steel pipe. CE=C+Mn / 6+(Cu+Ni) / 15+(Cr+Mo+V) / 5 (1) 0.11×844+000)×10 -5 (2) 30.0 × CE - NDA ≤ 10.0 (3) 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.

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