No steel pipe

Abstract

To provide a seamless steel pipe that can achieve superior low-temperature toughness.SOLUTION: A seamless steel pipe has a specific chemical composition and has a cumulative 90% effective crystal grain size of 150 μm or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a seamless steel pipe. [Background technology]

[0002] Seamless steel pipes (particularly seamless steel pipes for line pipes) are required to have improved low-temperature toughness.

[0003] Grain refinement is one way to improve the low-temperature toughness of seamless steel pipes for linepipes. One method for refining grains is to utilize the pinning effect, which suppresses the growth of prior austenite grains with fine particles such as TiN. Another method for refining grains is to use intragranular ferrite transformation to refine grains, starting from inclusions present within prior austenite grains.

[0004] For example, Patent Document 1 discloses a method for optimizing the steel composition and using one or two types of Mg-containing oxide particles with a particle size of 0.002 to 0.1 μm and composite particles with a particle size of 0.005 to 2 μm, which are made of Mg-containing oxide particles and carbonitrides precipitated using the Mg-containing oxide particles as nuclei, in a total amount of 1×10 4 ~1×10 8 pieces / mm 2 The present invention discloses a high-toughness, high-tensile steel with excellent weld toughness, characterized in that it contains a martensite structure with an average prior austenite grain size of 50 μm or less, or a structure in which a mixed structure of martensite and lower bainite accounts for 70% or more. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-123245 Summary of the Invention [Problem to be solved by the invention]

[0006] However, Patent Document 1 is primarily directed to steel plates, and it is not easy to apply the technology of Patent Document 1 to seamless steel pipes. In steel plates, the average grain size can be refined by low-temperature rolling using TMCP (Thermo Mechanical Control Process: thermomechanical treatment or thermo-mechanical control). In contrast, seamless steel pipes are manufactured at higher temperatures than steel plates to maintain workability during hot pipe making. Recrystallization occurs during the manufacture of seamless steel pipes, and the average effective grain size of seamless steel pipes becomes larger than that of steel plates, resulting in reduced low-temperature toughness. Therefore, it is difficult to improve the low-temperature toughness of seamless steel pipes using the method of Patent Document 1.

[0007] The present invention has been made in view of the above problems, and has an object to provide a seamless steel pipe that can provide excellent low-temperature toughness. [Means for solving the problem]

[0008] As a result of extensive research, the present inventors have found that excellent low-temperature toughness can be obtained in a seamless steel pipe by limiting the contents of Mo and Nb to predetermined amounts or less and adding a predetermined amount of Mg. <1> A seamless steel pipe according to one embodiment of the present invention contains, by mass%, C: 0.03 to 0.08%, Si: 0.05 to 0.25%, Mn: 1.0 to 2.5%, P: 0.05% or less, S: 0.005% or less, Mo: 0.3% or less, Nb: 0.01% or less, N: 0.0150% or less, O: 0.0010 to 0.0050%, Ti: 0.005 to 0.050%, Mg: 0.0005 to 0.0050%, and the balance being Fe and impurities; In place of a portion of the Fe, Al: 0.007% or less is contained in mass%, The cumulative 90% effective grain size is 150 μm or less. < 2 > Above <1> The seamless steel pipe described in the above may have, in mass %, Si: 0.05 to 0.15%. < 3 > Above <1> or <2>The seamless steel pipe described in may contain, in mass %, one or more elements selected from the group consisting of Cu: 0.50% or less, Ni: 0.5% or less, Cr: 0.5% or less, V: 0.08% or less, B: 0.010% or less, and Ca: 0.0050% or less, in place of a portion of the Fe. < 4 > Above <1> ~< 3 The seamless steel pipe described in any one of the above items has a number density of Ti-Mg oxides having an equivalent circle diameter of 0.05 μm or more and less than 0.5 μm of 5000 particles / mm 2 It may be more than that. < 5 > Above <1> ~< 4 The seamless steel pipe according to any one of the above items <1> to <3> may have a tensile strength of 758 MPa or less. < 6 > Above <1> ~< 5 The seamless steel pipe according to any one of the above items <1> to <5> may have a yield ratio YR of 0.75 or more. < 7 > Above <1> ~< 6 The seamless steel pipe described in any one of the above items 1 to 5 may have a wall thickness of less than 50 mm. < 8 > Above <1> ~< 7 The seamless steel pipe according to any one of the above items <1> to <3> may be used for line pipes. [Effects of the Invention]

[0009] According to the above-described aspect of the present invention, a seamless steel pipe that can achieve excellent low-temperature toughness can be provided. DETAILED DESCRIPTION OF THE INVENTION

[0010] The seamless steel pipe according to this embodiment will be described below.

[0011] <Seamless steel pipe> As a result of extensive research into improving the low temperature toughness of seamless steel pipes, the following was discovered. (A) If the Nb content, which is used to refine the base metal structure, is more than a certain amount, the Nb-based inclusions do not precipitate uniformly because the temperature during hot pipe making is higher than the rolling temperature of the steel plate, which reduces the low-temperature toughness of the seamless steel pipe. (B) If the content of Mo is more than a certain amount, an MA structure (Martensite-Austenite constituent) is likely to be formed during quenching, which reduces the low-temperature toughness of the seamless steel pipe. (C) By including a predetermined amount of Mg, Ti-Mg oxides are formed, improving the low-temperature toughness of the seamless steel pipe. (D) When the cumulative 90% effective grain size is 150 μm or less, the low temperature toughness of the seamless steel pipe is improved.

[0012] The seamless steel pipe according to this embodiment has a configuration determined based on the above findings. The seamless steel pipe according to this embodiment has excellent low-temperature toughness due to the synergistic effect of each component. Each component of the seamless steel pipe according to this embodiment will be described below. In this specification, a numerical range expressed using "to" means a range that includes the numerical values before and after "to" as the lower and upper limits. Numerical values indicated as "less than" and "more than" do not fall within the numerical range. All % in chemical compositions indicate mass %.

[0013] (chemical composition) The chemical composition of the seamless steel pipe according to this embodiment contains, in mass%, C: 0.03-0.08%, Si: 0.05-0.25%, Mn: 1.0-2.5%, P: 0.05% or less, S: 0.005% or less, Mo: 0.3% or less, Nb: 0.01% or less, N: 0.0150% or less, O: 0.0010-0.0050%, Ti: 0.005-0.050%, Mg: 0.0005-0.0050%, and the balance being Fe and impurities.

[0014] "C: 0.03~0.08%" Carbon (C) is an element that improves the hardenability of steel and increases its strength. If the C content is less than 0.03%, the hardenability will be insufficient, making it difficult to ensure high strength. Therefore, the C content is 0.03% or more. On the other hand, if the C content exceeds 0.08%, the toughness of the steel will decrease. Therefore, the C content is 0.08% or less. The C content is preferably more than 0.03%, more preferably 0.04% or more. The C content is preferably less than 0.08%, more preferably 0.07% or less, and even more preferably 0.06% or less.

[0015] "Si: 0.05~0.25%" Silicon (Si) is an element that deoxidizes steel. If the Si content is less than 0.05%, the above-mentioned deoxidizing effect cannot be obtained. Therefore, the Si content is 0.05% or more. If the Si content exceeds 0.25%, the toughness of the steel decreases. Therefore, the Si content is 0.25% or less. The Si content is preferably less than 0.25%, more preferably 0.20% or less, and even more preferably 0.15% or less. Furthermore, since Si is an MA generating element like Mo, which will be described later, the Si content is preferably 0.05 to 0.15%.

[0016] "Mn: 1.0~2.5%" Manganese (Mn) is an element that improves the hardenability and strength of steel. Mn also improves the hot workability of steel. If the Mn content is less than 1.0%, the above effects cannot be sufficiently obtained. Therefore, the Mn content is 1.0% or more. On the other hand, if the Mn content exceeds 2.5%, Mn segregates in the steel, reducing toughness. Therefore, the Mn content is 2.5% or less. The Mn content is preferably 1.1% or more, and more preferably 1.3% or more. The Mn content is preferably 2.4% or less, more preferably 2.0% or less, and even more preferably 1.8% or less.

[0017] "P: 0.05% or less" Phosphorus (P) is an impurity and an element that reduces the toughness of steel. Therefore, the P content is preferably as low as possible. Therefore, the P content is limited to 0.05% or less. The P content is preferably less than 0.05%, more preferably 0.04% or less, and even more preferably 0.02% or less.

[0018] "S: 0.005% or less" Sulfur (S) is an element that combines with Mn to form MnS. Excessive S content causes the precipitation of coarse simple MnS, reducing the toughness of the steel. Therefore, the S content is set to 0.005% or less. The S content is preferably 0.004% or less, and more preferably 0.003% or less. From the viewpoint of complex precipitation of MnS and ensuring the low-temperature toughness of the base material and HAZ (heat affected zone), the S content is preferably 0.001% or more, and more preferably 0.002% or more.

[0019] "Mo: 0.3% or less" Mo is an element that promotes the formation of an MA structure during quenching. If the Mo content exceeds 0.3%, an excessive MA structure is formed during quenching, reducing the low-temperature toughness of the seamless steel pipe. Therefore, the Mo content is limited to 0.3% or less. A more preferable Mo content is 0.2% or less. Since Mo does not need to be added, the lower limit of the Mo content is 0%. Note that the MA structure is decomposed by tempering, and the decomposed MA structure is also a cause of the deterioration of toughness. Mo also promotes the formation of MA structures in the HAZ. HAZ toughness is also an issue when circumferentially welding seamless steel pipes, but reducing the content of MA-forming elements such as Mo and Si can also prevent a decrease in HAZ toughness. Seamless steel pipes are particularly well-suited for welding because they are constructed with lower heat input than thick plates, resulting in a faster cooling rate that promotes MA formation, making it effective to reduce the content of MA-forming elements.

[0020] "Nb: 0.01% or less" Nb is an element used in steel sheets to suppress the recrystallization of gamma grains during rolling and refine the structure. However, the temperature during hot pipe production of seamless steel pipes is higher than that during steel sheet rolling to facilitate processing, making it difficult to achieve a non-recrystallization temperature range. Therefore, it is difficult to uniformly precipitate Nb-based inclusions during hot pipe production. Specifically, if the Nb content exceeds 0.01%, it is difficult to uniformly precipitate Nb-based inclusions. As a result, the toughness and tensile strength vary greatly and the low-temperature toughness decreases. Therefore, the Nb content is limited to 0.01% or less. A more preferable Nb content is 0.005% or less. Since Nb is not necessary, the lower limit of the Nb content is 0%.

[0021] "N: 0.0150% or less" Nitrogen (N) is an element that affects the refinement of crystal grains. To obtain the above effect, the N content is preferably 0.0020% or more. If the N content is higher than 0.0150%, cracks may occur in the billet. Therefore, the N content is 0.0150% or less. The N content is preferably less than 0.0150%, more preferably 0.0120% or less, and even more preferably 0.0100% or less.

[0022] "O: 0.0010~0.0050%" O is an element that affects the formation of Ti oxides. To obtain a sufficient inclusion density, the O content is set to 0.0010% or more. On the other hand, excessive O content tends to form coarse oxides that can become fracture initiation points. Therefore, the O content is set to 0.0050% or less. The O content is preferably 0.0015% or more, more preferably 0.0020% or more. The O content is preferably 0.0045% or less, more preferably 0.0040% or less.

[0023] "Ti: 0.005~0.050%" Ti is an element that affects the formation of Ti oxides. To obtain a sufficient inclusion density, the Ti content is set to 0.005% or more. On the other hand, excessive Ti content makes it easier to form carbides such as TiC, which reduces the toughness of the base material and HAZ. Therefore, the Ti content is set to 0.050% or less. From the viewpoint of ensuring a sufficient inclusion density and ensuring the toughness of the HAZ, the Ti content is preferably 0.009% or more and 0.020% or less.

[0024] "Mg: 0.0005~0.0050%" Mg is an element that affects the formation of Ti-Mg oxides. If the Mg content is less than 0.0005%, the formation of Ti-Mg oxides is insufficient, and excellent low-temperature toughness cannot be obtained. Therefore, the Mg content is 0.0005% or more. A more preferable Mg content is 0.0012% or more. An even more preferable Mg content is 0.0020% or more. Excessive Mg content has a pinning effect, but also causes the Ti-Mg oxides to become extremely coarse. In this case, the coarse Ti-Mg oxides may become the starting point and deteriorate the low-temperature toughness. In addition, Mg is expensive. Therefore, the Mg content is 0.0050% or less. A more preferable Mg content is 0.0040% or less.

[0025] "Remain" The balance of the chemical composition of the seamless steel pipe according to this embodiment is Fe and impurities. The impurities referred to here refer to elements that are mixed in from ores or scraps used as raw materials for steel, or elements that are mixed in from the environment during the manufacturing process, etc.

[0026] The chemical composition of the seamless steel pipe according to this embodiment may contain, in mass %, 0.007% or less of Al in place of a portion of Fe.

[0027] "Al: 0.007% or less" Aluminum (Al) is an element that deoxidizes steel. If the Al content exceeds 0.007%, cracks may occur in the billet. Therefore, the Al content is preferably 0.007% or less. To obtain the above-mentioned deoxidizing effect, the Al content is preferably 0.001% or more, and more preferably 0.002% or more.

[0028] Furthermore, Al has a higher oxide-forming ability than Ti, and a high Al content suppresses the formation of Ti oxides. The Al content is preferably less than 0.007%, more preferably 0.006% or less, and even more preferably 0.005% or less. In this specification, the Al content refers to the content of acid-soluble Al (so-called sol. Al).

[0029] In addition, the chemical composition of the seamless steel pipe according to this embodiment may contain, in mass %, one or more elements selected from the group consisting of Cu: 0.50% or less, Ni: 0.5% or less, Cr: 0.5% or less, V: 0.08% or less, B: 0.010% or less, and Ca: 0.0050% or less, in place of part of Fe.

[0030] "Cu: 0.50% or less" Cu is an element that has the effect of increasing strength, so it may be contained. However, excessive Cu content causes hot embrittlement, leading to a decrease in the quality of the billet surface. Therefore, the Cu content is set to 0.50% or less. In order to increase strength, the Cu content is preferably 0.01% or more. Since Cu does not necessarily need to be contained, the lower limit of the Cu content is 0%. The C content is preferably 0.30% or less.

[0031] "Ni: 0.5% or less" Ni is an element that increases strength without reducing toughness, and may be contained. However, since Ni is an expensive element, the Ni content is set to 0.5% or less. The Ni content is preferably 0.4% or less, more preferably 0.3% or less, and even more preferably 0.2% or less. Since Ni does not need to be contained, the lower limit of the Ni content is 0%. The Ni content is preferably 0.01% or more.

[0032] "Cr: 0.5% or less" Chromium (Cr) is an element that improves the hardenability and strength of steel, and may be added. If the Cr content exceeds 0.5%, the toughness of the steel decreases. Therefore, the Cr content is 0.5% or less. The Cr content is preferably 0.4% or less, and more preferably 0.3% or less. Since Cr does not need to be contained, the lower limit of the Cr content is 0%. The Cr content is preferably 0.05% or more, and more preferably 0.1% or more.

[0033] "V: 0.08% or less" Vanadium (V) may be contained because it is an element that combines with C in steel to form V carbide and increase the strength of the steel. V also dissolves in Mo carbide to form carbide. By including V, the carbides are less likely to coarsen. If the V content is higher than 0.08%, the carbides will coarsen. Therefore, the V content is 0.08% or less. The V content is preferably less than 0.08%, more preferably 0.07% or less. Since V does not need to be contained, the lower limit of the V content is 0%. The V content is preferably 0.02% or more, more preferably 0.04% or more.

[0034] "B: 0.010% or less" Boron (B) is an element that improves hardenability with the addition of a small amount, so it may be contained. However, even if a large amount of B is contained, the above effect saturates and coarse borides such as Mo2B and Fe2B are formed. Therefore, the B content is set to 0.010% or less. The B content is preferably 0.009% or less, and more preferably 0.008% or less. Since B does not need to be contained, the lower limit of the B content is 0%. The B content is preferably 0.001% or more, and more preferably 0.002% or more.

[0035] "Ca: 0.0050% or less" Calcium (Ca) may be added because it is an element that spheroidizes MnS and improves toughness. If the Ca content is higher than 0.0050%, the cleanliness of the steel decreases, and the toughness of the steel decreases. Therefore, the Ca content is 0.0050% or less. The Ca content is preferably less than 0.0050%, more preferably 0.0040% or less, and even more preferably 0.0030% or less. Since Ca does not need to be added, the lower limit of the Ca content is 0%. The Ca content is preferably 0.0004% or more, and more preferably 0.0008% or more.

[0036] The chemical composition of the seamless steel pipe described above may be measured by a common analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S may be measured using the combustion-infrared absorption method, and N may be measured using the inert gas fusion-thermal conductivity method. Al may be measured by ICP-AES using the filtrate obtained by thermal decomposition of a sample with acid.

[0037] (Cumulative 90% effective grain size) The 90% cumulative effective grain size in the cross section of the seamless steel pipe according to this embodiment is 150 μm or less. Here, "90% cumulative effective grain size" refers to the grain size of the grain at which the area ratio is 90% when the areas of the grains are accumulated from the side with the smaller grain size. Here, the grain size of the grain is the circle-equivalent diameter of the region surrounded by grain boundaries with an orientation difference of 15° or more in an orientation map obtained by electron backscatter diffraction (EBSD). A more preferable 90% cumulative effective grain size is 130 μm or less. There is no particular lower limit to the 90% cumulative effective grain size, but it is, for example, 50 μm or more from the viewpoint of manufacturing conditions.

[0038] The cumulative 90% effective grain size may be measured as follows. A seamless steel pipe is cut, for example, perpendicular to the longitudinal direction to obtain a cross section (C-section). Next, the cross section at approximately half the wall thickness is observed using electron backscatter diffraction (EBSD). The measurement is performed in a 1000 μm × 1000 μm field of view with a measurement step of 2.0 μm, and the number of fields is, for example, 3. From the obtained orientation map, the circle-equivalent diameter (grain size) of the region surrounded by grain boundaries with an orientation difference of 15° or more is determined, and the area of the corresponding grain is also calculated. Next, the areas of the grains are accumulated from the side with the smaller grain size, and the grain size at which the area ratio is 90% is determined. The grain size at which this area ratio is 90% is defined as the cumulative 90% effective grain size.

[0039] (Ti-Mg oxides with a circle equivalent diameter of 0.05 μm or more and less than 0.5 μm are 5000 particles / mm 2 (End) In order to improve the low-temperature toughness of the seamless steel pipe, it is preferable that Ti-Mg-based oxides having a predetermined circle-equivalent diameter are present at a predetermined density. Specifically, in a cross section obtained by cutting the seamless steel pipe according to this embodiment in an arbitrary direction (for example, the thickness direction), Ti-Mg-based oxides having a circle-equivalent diameter of 0.05 μm or more and less than 0.5 μm are present at a density of 5000 particles / mm 2 The number density of Ti-Mg-based oxides having an equivalent circle diameter of 0.05 μm or more and less than 0.5 μm is preferably 6000 particles / mm 2 That's all.

[0040] In order to measure the number density of Ti-Mg-based oxides having an equivalent circle diameter of 0.05 μm or more and less than 0.5 μm in a cross section obtained by cutting the seamless steel pipe according to this embodiment in a direction perpendicular to the longitudinal direction, for example, it is preferable to measure over a wide field of view using a transmission electron microscope (TEM).

[0041] More specifically, for example, a seamless steel pipe is cut perpendicular to its longitudinal direction, and a section of the seamless steel pipe is observed using a TEM from the outer surface to approximately halfway through the wall thickness. Observation is performed at a magnification of 10,000 to 30,000 times, and the composition is analyzed using an EDS (Energy Dispersive X-ray Spectrometer) included in the TEM to identify Ti-Mg-based oxides. Furthermore, the Ti-Mg-based oxides are photographed, and the number of Ti-Mg-based oxides satisfying the circle-equivalent diameter is counted using image analysis to determine the number density of Ti-Mg-based oxides having a circle-equivalent diameter of 0.05 μm or more and less than 0.5 μm. Measurement of Ti-Mg-based oxides having a circle-equivalent diameter of 0.05 μm or more and less than 0.5 μm may be performed using software attached to the TEM that enables automated analysis. Ti-Mg-based oxides can be identified as those whose main composition is Ti, Mg, and O, and whose Mg content in the oxide is 5% by mass or more. The main composition being Ti, Mg, and O means, for example, that the total content of Ti, Mg, and O is 30% or more by mass. Ten or more visual fields are measured, and the number density is calculated from the number of Ti-Mg-based oxides obtained and the area of the measured visual fields.

[0042] (Tensile strength: 758 MPa or less) The tensile strength of the seamless steel pipe according to this embodiment is preferably 758 MPa or less. The tensile strength is more preferably 745 MPa or less. The tensile strength of the seamless steel pipe is preferably 531 MPa or more. More preferably, it is 545 MPa or more. The tensile strength of the seamless steel pipe is measured in accordance with JIS Z 2241:2011. Test specimens are taken from the steel pipe so that the longitudinal direction of the test specimen is the L direction.

[0043] (Yield ratio YR: 0.75 or more) It is preferable that the strength of the girth weld of a seamless steel pipe is higher than the strength of the base metal of the seamless steel pipe. Therefore, it is better to keep the tensile strength of the base metal of the seamless steel pipe lower than that of the weld metal. Therefore, it is preferable to control the yield ratio of the seamless steel pipe relative to the weld metal. The yield ratio YR of the seamless steel pipe according to this embodiment is preferably 0.75 or more. The yield ratio YR of the seamless steel pipe is more preferably 0.80 or more. The yield ratio of the seamless steel pipe is measured in accordance with JIS Z 2241 (2011).

[0044] (wall thickness) The wall thickness of the seamless steel pipe according to this embodiment is preferably less than 50 mm. If the wall thickness of the seamless steel pipe is less than 50 mm, it is preferable as a seamless steel pipe for line pipe. The wall thickness of the seamless steel pipe is preferably 25 mm or more.

[0045] <Manufacturing method> An example of the method for producing a seamless steel pipe according to this embodiment will be described below, but the method for producing a seamless steel pipe according to this embodiment is not limited to this.

[0046] Steel having the above-mentioned chemical composition is melted and refined. Subsequently, billets are produced from the molten steel by continuous casting. Alternatively, slabs or blooms may be produced from the molten steel, and the slabs or blooms may be hot-worked to produce billets.

[0047] Next, the billet is hot-rolled to produce a seamless steel pipe. Specifically, the billet is heated in a heating furnace, and the billet removed from the heating furnace is hot-worked to produce a seamless steel pipe. The surface temperature of the billet during hot-working is, for example, 900°C to 1250°C to maintain workability. A mother pipe is produced by piercing-rolling based on the Mannesmann process. The produced mother pipe is then elongated and sizing-rolled using a mandrel mill, reducer, sizing mill, or the like to produce a seamless steel pipe. The rolling temperature range for a typical thick plate is, for example, 900°C at the start and 650°C at the end, which is lower than that for hot-rolling.

[0048] The manufactured seamless steel pipe is subjected to quenching and tempering. For quenching and tempering, either in-line quenching or off-line quenching, which will be described below, may be employed.

[0049] [Inline QT] Inline quenching is a process in which seamless steel pipes are quenched and tempered immediately after hot-forming, or in which seamless steel pipes are quenched and tempered after being reheated in a reheating furnace. Compared to offline quenching (described below), inline quenching is advantageous in terms of energy efficiency because quenching can be performed using the heat from the hot-forming process.

[0050] The hot-formed seamless steel pipe is cooled from a temperature of (Ar3 point + 50°C) to 1100°C at a cooling rate of 5°C / second or more. At this time, a reheating furnace may be used to heat the pipe to the quenching start temperature before cooling. If the quenching start temperature is lower than (Ar3 point + 50°C), variations in strength will occur. On the other hand, if the quenching start temperature is increased, toughness will deteriorate, so it must be kept below 1100°C.

[0051] [Offline QT] Off-line QT is a process in which seamless steel pipes are cooled once after hot pipe making, and then reheated to the Ac3 point or higher, after which they are quenched and tempered.

[0052] After hot-forming, the seamless steel pipe is cooled to room temperature. The cooled seamless steel pipe is then heated to a temperature above the Ac3 point. The heated seamless steel pipe is then cooled from a temperature of (Ar3 point + 50°C) to 1100°C at a cooling rate of 5°C / second or faster. If the quenching start temperature is lower than (Ar3 point + 50°C), variations in strength will occur. On the other hand, if the quenching start temperature is increased, toughness will deteriorate, so it must be kept below 1100°C. [Example]

[0053] Next, examples of the present invention will be described, but the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0054] <Seamless steel pipe manufacturing> A number of molten steels having the chemical compositions shown in Tables 1A and 1B were produced. The symbol "-" in Tables 1A and 1B indicates that an element was not intentionally added. Billets were produced from the molten steel by continuous casting. Each produced billet was heated in a heating furnace and then pierced and rolled using a piercing mill to produce a mother pipe. The hot-working temperature of the heated billet was 900°C to 1250°C. Each mother pipe was then elongated using a mandrel mill. Each mother pipe was then sizing-rolled using a sizer to produce seamless steel pipes Nos. 1 to 17, 19, and 20 with wall thicknesses shown in Table 2. Furthermore, slabs were produced from the molten steel having the chemical compositions shown in Tables 1A and 1B by continuous casting, and No. 18 plate was then produced by hot rolling. The hot-rolling temperature was 650°C to 900°C.

[0055] <Cumulative 90% effective grain size> The specimens used for the cumulative 90% effective grain size analysis were taken from the 1 / 2t section of the seamless steel pipe and thick plate manufactured as described above, where t is the wall thickness. The seamless steel pipe was cut perpendicular to the longitudinal direction to obtain a cross section. Next, the cross section at approximately 1 / 2 of the wall thickness was observed using electron backscatter diffraction (EBSD). Measurements were performed in a 1000 μm × 1000 μm field of view with a measurement step of 2.0 μm, with three fields of view. From the obtained orientation map, the circle-equivalent diameter of the region surrounded by grain boundaries with a misorientation of 15° or more was determined, and the area was calculated accordingly. Next, the area of the crystal grains was accumulated from the smallest grain size side, and the grain size at which the area ratio reached 90% (cumulative 90% effective grain size) was determined. The results are shown in Table 2.

[0056] <Calculation of the number density of Ti-Mg oxides with an equivalent circle diameter of 0.05 μm or more and less than 0.5 μm> In the cross section obtained by cutting seamless steel pipes or steel plates in a direction perpendicular to the longitudinal direction, the vicinity of 1 / 2 of the wall thickness in the thickness direction from the outer surface of the seamless steel pipe was observed by TEM. Observation was carried out at a magnification of 10,000 times, and the composition was analyzed by EDS included in the TEM to identify Ti-Mg-based oxides. Further, the Ti-Mg-based oxides were photographed, and the number density of Ti-Mg-based oxides satisfying the equivalent circle diameter was obtained by counting the number of Ti-Mg-based oxides with an equivalent circle diameter of 0.05 μm or more and less than 0.5 μm through image analysis. Here, the Ti-Mg-based oxides were defined as those having a main composition of Ti, Mg, and O and a Mg content in the oxide of 5% or more by mass%. The number of fields of view was measured for 10 or more fields of view, and the number density was calculated from the number of obtained Ti-Mg-based oxides and the area of the measurement field of view. The obtained results are shown in Table 2.

[0057]

Table 1A

[0058]

Table 1B

[0059]

Table 2

[0060] <Tensile test> No. 12 test pieces (width 2 5 mm, gauge length 50 mm) specified in JIS Z 2241:2011 were taken from each seamless steel pipe in the longitudinal direction (L direction) of the seamless steel pipe or steel plate. Using the taken test pieces, a tensile test in accordance with JIS Z 2241:2011 was carried out in the atmosphere at normal temperature (25 °C), and the yield stress and tensile strength were determined. The yield stress was determined by the 0.5% total elongation method. Further, the yield ratio YR was determined from the yield stress and tensile strength.The results are shown in Table 2.

[0061] <CTOD test> A CTOD (Crack Tip Opening Displacement) test was conducted to evaluate low-temperature toughness. The test method is explained below. Three test pieces for the CTOD test were taken from each seamless steel pipe. The taken test pieces were notched and subjected to a CTOD test at a test temperature of -20°C in accordance with the BS7448 standard. A CTOD value of 0.50 mm or more was considered to be passing. The results are shown in Table 2.

[0062] Seamless steel pipes Nos. 1 to 9, 11, 13, 14, 16 and 19 satisfied all of the conditions stipulated in the present invention and therefore passed the CTOD test.

[0063] The seamless steel pipe No. 10 had a Si content exceeding the upper limit specified in the present invention, and therefore failed the CTOD test.

[0064] The seamless steel pipe No. 12 had a Mo content exceeding the upper limit specified in the present invention, and therefore had an excessive amount of MA structure formed, causing it to fail the CTOD test.

[0065] The seamless steel pipe No. 15 had a Mg content below the lower limit of the present invention and a cumulative 90% effective grain size exceeding 150 μm, so the seamless steel pipe No. 15 failed the CTOD test.

[0066] The seamless steel pipe No. 17 had an Nb content exceeding the upper limit specified in the present invention, and therefore, the seamless steel pipe No. 17 did not pass the CTOD test because the precipitation of NbC was not uniform.

[0067] No. 18 steel plate is a reference example of a conventional thick plate. In the case of thick plates, they are hot-rolled at a lower temperature (non-recrystallized region) than the manufacturing conditions for seamless steel pipes. Therefore, unlike No. 17 seamless steel pipe, which has the same chemical composition, the precipitation of NbC is uniform, and the plate passed the CTOD test.

[0068] The seamless steel pipe No. 20 had a Mg content exceeding the upper limit specified in the present invention. As a result, coarse Ti-Mg oxides were formed in the seamless steel pipe No. 20. The coarse Ti-Mg oxides formed became the origin of fracture, and the seamless steel pipe No. 20 failed the CTOD test.

[0069] As described above, the seamless steel pipe according to this embodiment has excellent low-temperature toughness, and is therefore suitable for use as a line pipe. [Industrial Applicability]

[0070] The seamless steel pipe of the present invention has excellent H low temperature toughness and is therefore highly industrially applicable.

Claims

1. In mass%, C: 0.03-0.08%, Si: 0.05-0.25%, Mn: 1.0 to 2.5%, P: 0.05% or less, S: 0.005% or less, Mo: 0.3% or less, Nb: 0.01% or less, N: 0.0150% or less, O: 0.0010 to 0.0050%, Ti: 0.005 to 0.050%, Mg: 0.0005-0.0050% and the balance being Fe and impurities, In place of a portion of the Fe, Al: 0.007% or less is contained in mass%, The cumulative 90% effective grain size is 150 μm or less. A seamless steel pipe characterized by:

2. In mass%, Si: 0.05-0.15% The seamless steel pipe according to claim 1, characterized in that

3. 3. The seamless steel pipe according to claim 1 or 2, characterized in that it contains, in mass %, one or more elements selected from the group consisting of Cu: 0.50% or less, Ni: 0.5% or less, Cr: 0.5% or less, V: 0.08% or less, B: 0.010% or less, and Ca: 0.0050% or less, in place of a portion of the Fe.

4. The number density of Ti-Mg oxides having a circle equivalent diameter of 0.05 μm or more and less than 0.5 μm is 5000 pieces / mm ２ The seamless steel pipe according to any one of claims 1 to 3, characterized in that:

5. 5. The seamless steel pipe according to claim 1, wherein the seamless steel pipe has a tensile strength of 758 MPa or less.

6. 6. The seamless steel pipe according to claim 1, wherein the yield ratio YR is 0.75 or more.

7. 7. The seamless steel pipe according to claim 1, wherein the wall thickness is less than 50 mm.

8. The seamless steel pipe according to any one of claims 1 to 7, which is for use as a line pipe.

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