Seamless steel pipe and method for producing same

Abstract

Provided is a seamless steel pipe comprising 0.05-0.20% C, 0.05-0.50% Si, 0.20-1.00% Mn, 0.030% or less P, 0.015% or less S, 0.05-1.00% Cr, 0.010-0.080% Al, 0.0020-0.0100% N, 0.0005-0.0050% B, and 0.01-0.05% Ti. The seamless steel pipe may further comprise, as optional elements, 0.30% or less Mo, 0.01-0.05% Nb, 0.01-0.20% V, 0.001-0.200% Te, and 0.0005-0.0050% Ca. The area ratio of a tempered martensite phase is 90% or greater, the prior γ (austenite) grain size number is 9.0 or greater, the tensile strength of the side wall is 1,000 MPa or greater, and the transition temperature VTrs is -60°C or lower.

Description

Seamless Steel Pipe and Method for Manufacturing the Same

[0001] The present invention relates to a seamless steel pipe and a method for manufacturing the same.

[0002] For example, an airbag mounted on an automobile includes an inflator (gas generator), and seamless steel pipes are used as members constituting the inflator in side airbags, knee airbags and the like. Seamless steel pipes for inflators are not only required to have high dimensional accuracy, but also required to have high strength and high toughness to withstand high pressure, and are further required to be excellent in workability and weldability. In addition, for high toughness, low-temperature toughness premised on use in cold regions is also required.

[0003] Japanese Unexamined Patent Publication No. 10-140250, Japanese Unexamined Patent Publication No. 2004-76034

[0004] As a method for manufacturing such a seamless steel pipe, methods including the Mannesmann-mandrel mill process (see Patent Documents 1 and 2) are widely used. While these methods are suitable for mass production, they require repeated high-temperature heating, straightening and cold drawing, and have the problem of relatively high energy consumption.

[0005] Therefore, as a method suitable for manufacturing relatively small-sized seamless steel pipes such as those for inflators described above, development of a manufacturing method with low energy consumption and a relatively small number of processes is demanded. Further, development of a seamless steel pipe that can be manufactured by such a manufacturing method and can be applied to inflators is also demanded.

[0006] The present invention has been made in view of such a background, and aims to provide a seamless steel pipe applicable to inflators and a manufacturing method capable of efficiently manufacturing the same.

[0007] A first aspect of the present invention contains, in mass%, essential elements such as C: 0.05 to 0.20%, Si: 0.05 to 0.50%, Mn: 0.20 to 1.00%, P: 0.030% or less, S: 0.015% or less, Cr: 0.05 to 1.00%, Al: 0.010 to 0.080%, N: 0.0020 to 0.0100%, B: 0.0005 to 0.0050%, and Ti: 0.01 to 0.05%, and optional elements such as Mo: 0.30% or less, Nb: 0.01 to 0.05%, V: 0.01 to 0.20%, Te: 0.001 to 0.200%, and Ca: 0.0005 to 0.0050%. The seamless steel pipe has a chemical composition consisting of the remainder Fe and unavoidable impurities, a microstructure in which the area ratio of the tempered martensite phase is 90% or more, a prior γ (austenite) grain size of 9.0 or larger, a tensile strength of the sidewall of 1000 MPa or more, and a transition temperature VTrs of -60°C or lower.

[0008] A second aspect of the present invention is a method for manufacturing the above-mentioned seamless steel pipe, comprising: heating a steel ingot having the above-mentioned chemical composition to 1200 to 1350°C and performing bloc rolling to produce a steel billet; heating the steel billet to 850 to 1100°C and performing product rolling to produce a material; cold forging and / or machining the material to produce an intermediate pipe material; cold working the intermediate pipe material to produce a formed pipe material with a reduction ratio of 20% or more; quenching the formed pipe material by high-frequency induction heating at a temperature of Ac3 point or higher and 1050°C or lower, with an average heating rate of 200°C / second or higher, followed by rapid cooling to 200°C with an average cooling rate of 200°C / second or higher; and then tempering at a heating temperature of over 400°C and 650°C or between 100°C and 250°C.

[0009] The seamless steel pipe of the first embodiment not only possesses the above-mentioned specific chemical components, but also has excellent properties such as strength and toughness, making it usable as a seamless steel pipe that can be applied to inflators. Furthermore, by employing the manufacturing method of the second embodiment, this superior seamless steel pipe can be manufactured in an efficient method with relatively low energy consumption and a relatively small number of steps.

[0010] An explanatory diagram illustrating the manufacturing method of seamless steel pipes in the example.

[0011] First, I will explain the reason for limiting the chemical composition of the seamless steel pipes mentioned above.

[0012] C: 0.05–0.20%; Carbon (C) contributes to increasing the strength of steel through martensitic transformation after quenching, so it is included at a concentration of 0.05–0.20%. If the C content is too low, there is a problem in that it is difficult to obtain sufficient tensile strength. On the other hand, if the C content is too high, the workability and weldability may decrease. A more preferable range for the C content is 0.07–0.18%.

[0013] Si: 0.05-0.50%; Since Si (silicon) is essential as a deoxidizing agent during steelmaking, it should be included at a concentration of 0.05% or more. On the other hand, excessive addition of Si may lead to a decrease in workability and weldability, so the upper limit of Si content is set at 0.50%.

[0014] Mn: 0.20–1.00%; Manganese (Mn) is an effective element for improving the hardenability of steel and for stably obtaining a martensitic structure during quenching, so it should be included in a concentration of 0.20–1.00%. If the Mn content is too low, sufficient hardenability may not be obtained. On the other hand, if the Mn content is too high, it may lead to a decrease in toughness and weldability.

[0015] P: 0.030% or less; Although phosphorus (P) is contained as an impurity, if it is present in excess, it tends to segregate at grain boundaries, and segregation at grain boundaries causes a significant decrease in toughness. Therefore, the upper limit of the P content should be 0.030% or less. Preferably, it should be 0.020% or less.

[0016] S: 0.015% or less; S (sulfur) is present as an impurity, but if present in excess, it increases sulfide-based nonmetallic inclusions, causing a significant decrease in toughness. Therefore, the upper limit of the S content is set at 0.015%. Furthermore, since it also leads to a decrease in ductility in the direction perpendicular to the rolling direction (longitudinal direction), it is more preferable to set the S content to 0.008% or less.

[0017] Cr: 0.05-1.00%; Cr (chromium) is an element that enhances the hardenability and tempering resistance of steel, and is effective in improving strength and toughness, so it should be included at a concentration of 0.05% or more. On the other hand, excessive addition of Cr may lead to increased material costs and decreased workability, so the upper limit of the Cr content is set at 1.00%.

[0018] Al: 0.010–0.080%; Aluminum (Al) is essential as a deoxidizing agent during steelmaking, so it should be included at a concentration of 0.010% or more. On the other hand, excessive addition of Al may lead to a decrease in toughness due to an increase in oxide-based nonmetallic inclusions, so the upper limit of the Al content is set at 0.080%.

[0019] N: 0.0020 to 0.0100%; N (nitrogen) is present as an impurity in the steelmaking process at a concentration of 0.0020% or more. On the other hand, if N is added in excess, it can lead to a decrease in workability due to solid-solution N and a deterioration of strength and toughness due to the formation of coarse nitrides with nitride-forming elements such as Al, Ti, Nb, and V. Therefore, the upper limit of the N content is set at 0.0100%.

[0020] B: 0.0005 to 0.0050%; Adding a small amount of B (boron) causes grain boundary segregation in steel, significantly improving its hardenability. However, excessive B content can lead to coarse precipitation of borides at the grain boundaries, potentially reducing toughness. This precipitation can be suppressed by adding approximately 3.4 times the mass of N (nitrogen) in Ti. Therefore, to achieve both improved hardenability and suppression of toughness reduction, it is preferable to include B together with Ti.

[0021] Ti: 0.01-0.05%; Titanium (Ti) is an element that is finely dispersed in steel as carbonitrides, strongly pinning grain boundaries and suppressing grain growth, thereby refining the grains and effectively improving toughness. It is also an element that suppresses the formation of B nitrides, thereby enhancing the aforementioned effect of B. To obtain these effects, Ti should be included at a concentration of 0.01% or more. Furthermore, to enhance the effect of B, it is preferable to include Ti / N such that Ti / N ≥ 3.4 (Ti and N each represent mass percent). On the other hand, excessive addition of Ti promotes the formation of coarse Ti nitrides and reduces toughness, so the upper limit should be 0.05%.

[0022] <Optional Elements> The chemical composition of the seamless steel pipe described above contains the essential elements within the above ranges, and may also contain optional elements such as Mo: 0.30% or less, Nb: 0.01 to 0.05%, V: 0.01 to 0.20%, Te: 0.001 to 0.200%, and Ca: 0.0005 to 0.0050%. All of these optional elements may be included within the above ranges, only some of them may be included within the above ranges, or none of these optional elements may be included at all.

[0023] Mo: 0.30% or less; Mo (molybdenum) is an optional element and does not need to be included, but it may be present up to about 0.06% as an unavoidable impurity in production by electric furnace melting. Since Mo is an effective element for improving hardenability, it can be actively added as needed. On the other hand, excessive addition of Mo leads to increased material costs and decreased workability, so the Mo content is limited to 0.30% or less.

[0024] Nb: 0.01-0.05%; Niobium (Nb) is an optional element and does not need to be included, but if included, it is preferable to keep it in the range of 0.01-0.05%. Nb is an element that is finely dispersed in steel as carbonitrides, strongly pinning the grain boundaries and suppressing grain growth, thereby refining the grains and effectively improving toughness. To obtain this effect, it should be included at a concentration of 0.01% or more. On the other hand, adding too much Nb will coarseen the carbonitrides, making it difficult to obtain the aforementioned effect and also leading to increased material costs, so the upper limit of the Nb content is set at 0.05%.

[0025] V: 0.01-0.20%; V (vanadium) is an optional element and does not need to be included, but if included, it is preferable to keep it in the range of 0.01-0.20%. V combines with C and is finely dispersed as carbides in steel, contributing to increased strength. To obtain this effect, it should be added at a concentration of 0.01% or more. On the other hand, if the V content is too high, it can lead to a decrease in toughness and an increase in material costs, so the upper limit of the V content is set at 0.20%.

[0026] Te: 0.001–0.200%; Te (tellurium) is an optional element and does not need to be included, but it may inevitably be included as an impurity in production by electric furnace melting. Te has the effect of improving the anisotropy of sulfide-based nonmetallic inclusions present in steel, thereby increasing ductility in the direction perpendicular to the rolling direction (longitudinal direction). To obtain this effect, Te can be added at a concentration of 0.001% or more. On the other hand, excessive addition of Te leads to saturation of the effect and increases material costs, so the upper limit of the Te content is set at 0.200%.

[0027] Ca: 0.0005–0.0050%; Calcium (Ca) is an optional element and does not need to be included, but it may inevitably be included as an impurity in production by electric furnace melting. Ca fixes sulfur, which is an unavoidable impurity in steel, as sulfides, improving the anisotropy of toughness and increasing toughness in the direction perpendicular to the rolling direction (longitudinal direction). To obtain this effect, Ca can be added at a concentration of 0.0005% or more. On the other hand, excessive addition of Ca saturates the effect, increases inclusions, and actually reduces toughness, so the upper limit of the Ca content is set at 0.0050%.

[0028] Next, the seamless steel pipe described above has the chemical composition described above, and further possesses a specific metallic state and strength characteristics.

[0029] The metallographic state of the above-mentioned seamless steel pipe is, firstly, that it has a microstructure in which the area ratio of the tempered martensite phase is 90% or more. This makes it possible to ensure the desired strength properties, toughness, and workability. If the area ratio of the tempered martensite phase is less than 90%, it becomes difficult to ensure these properties. The microstructure is confirmed by observation with an optical microscope, as described in the examples below.

[0030] Furthermore, the seamless steel pipes described above have a prior γ (austenite) grain size of 9.0 or higher in their metallographic structure. This ensures the desired low-temperature toughness. On the other hand, if the grain size is less than 9.0, it becomes difficult to ensure low-temperature toughness. The grain size is measured in accordance with JIS G0551:2020.

[0031] Furthermore, the seamless steel pipe described above has a tensile strength of 1000 MPa or more in its sidewall. This tensile strength is necessary when the steel pipe is used for various applications, and is also necessary when applied to an inflator. The tensile strength of the sidewall is measured by taking a test piece so that the pipe axis is in the tensile direction, as described in the examples below.

[0032] Furthermore, the seamless steel pipe described above has a transition temperature VTrs of -60°C or lower. By possessing this characteristic, the desired low-temperature toughness can be reliably ensured. This measurement is performed by the method described in the examples below.

[0033] Next, the seamless steel pipes described above can be used for airbag inflators. For seamless steel pipes used for inflators, the dimensions are typically within the range of an outer diameter of 25 to 40 mm, an inner diameter of 20 to 35 mm, and a length of 100 to 450 mm.

[0034] Next, the seamless steel pipe described above can be manufactured by the manufacturing method of the second embodiment described above.

[0035] In this manufacturing method, first, a steel ingot having the above chemical composition is heated to 1200 to 1350°C and rolled to produce a steel billet. Then, the steel billet is heated to 850 to 1100°C and rolled to produce a raw material.

[0036] By heating the fractional rolling process at a temperature of 1200°C or higher, AlN and Ti carbonitrides can be sufficiently dissolved, allowing for fine precipitation during subsequent product rolling and quenching / tempering processes after cold working, thereby achieving grain growth suppression and grain refinement effects. On the other hand, raising the fractional rolling temperature to 1350°C or higher saturates these effects and leads to decreased productivity and increased energy loss; therefore, the upper limit is set at 1350°C.

[0037] If the heating temperature for product rolling exceeds 1100°C, AlN and Ti carbonitrides that were dissolved during bloc rolling may precipitate as coarse particles, potentially preventing sufficient suppression of abnormal grain growth and grain refinement. On the other hand, if the heating temperature for product rolling falls below 850°C, the processing load increases, not only increasing the load on the rolling mill but also potentially leading to a finer and flatter structure after rolling, resulting in poor cold forgeability.

[0038] Next, the intermediate tube material is manufactured by cold forging and / or machining the material. For example, as shown in the embodiment described later, holes can be drilled from the tip and rear ends of a rod-shaped material by forging, leaving a partition plate made of excess material in the axial center of the circular tube. Then, the partition plate portion can be punched out to obtain a circular intermediate tube material with a through hole. The inner surface of this intermediate tube material can also be shaped by machining to facilitate subsequent cold working. Before forming the steel material into an intermediate tube material, the material can be subjected to a heat treatment (material annealing). In this case, the annealing conditions are preferably such that the material is held at 600°C to 750°C for 60 minutes or more and then allowed to cool, or held at 730°C to 900°C or higher for 60 minutes or more and then cooled at a cooling rate slower than 100°C / hour.

[0039] Furthermore, it is possible to obtain a cylindrical intermediate pipe material by applying only mechanical processing to the raw material.

[0040] Next, the intermediate pipe material is subjected to cold working to reduce the surface area by 20% or more to produce a formed pipe material. In this case, if necessary, the intermediate pipe material may be subjected to heat treatment (intermediate annealing) before cold working. Intermediate annealing is preferably carried out under conditions of holding at 600°C to 750°C for 60 minutes or more and then allowing to cool, or holding at 730°C to 900°C or higher for 60 minutes or more and then cooling at a cooling rate slower than 100°C / hour.

[0041] As for the type of cold working, any method or other methods are applicable as long as they are plastic working methods capable of controlling the outer diameter and inner diameter of a circular tubular material such as drawing, extrusion, and forging, and capable of producing seamless steel pipes. This cold working is performed under a condition that the area reduction rate is 20% or more. Even when cold working is repeated a plurality of times, it is sufficient that the total area reduction rate is 20% or more. More preferably, 30% to 80% is more preferable. The area reduction rate can be calculated by the following formula based on the relationship between the cross-sectional area A of the cross section orthogonal to the axial direction of the intermediate pipe material and the cross-sectional area B at the same position of the formed pipe material: Area reduction rate (%) = (A-B) / A × 100.

[0042] Next, the formed pipe material is heated by high-frequency induction heating at a temperature between Ac3 point and 1050°C inclusive under the condition that the average heating rate is 200°C / sec or more, then quenched by rapid cooling under the condition that the average cooling rate to 200°C is 200°C / sec or more, and then tempered at a heating temperature of more than 400°C and 650°C or less, or 100°C or more and less than 250°C.

[0043] In order to obtain a desired structure after quenching, the heating by high-frequency induction heating needs to be performed at a temperature of Ac3 point or higher, which can reliably achieve γ (austenite) transformation. On the other hand, when the heating temperature exceeds 1050°C, AlN and Ti carbonitrides dissolve into solid solution or coarsen, which makes it impossible to exert sufficient effects of suppressing abnormal grain growth and refining crystal grains, and it becomes difficult to obtain a desired grain size.

[0044] In addition, the heating by this high-frequency induction heating needs to be performed under the condition that the average heating rate is 200°C / sec or more. When the average heating rate is less than 200°C / sec, heating takes time, γ (austenite) crystal grains coarsen, the prior austenite grain size coarsens in the final metal structure, and there is a risk that a sufficiently low transition temperature VTrs cannot be obtained.

[0045] As mentioned above, the quenching after the above heating needs to be performed by rapid cooling under the condition that the average cooling rate to 200°C is 200°C / sec or more. If the cooling rate to 200°C is lower than 200°C / sec, sufficient quenching cannot be achieved, the fraction of ferrite and bainite structures in the structure after quenching increases, and it becomes difficult to obtain a desired structure.

[0046] As described above, the tempering treatment after quenching is performed at 650°C or lower. When the temperature exceeds 650°C, the martensite structure after tempering is greatly softened, and there is a possibility that desired mechanical properties (tensile strength) cannot be obtained. In addition, considering low-temperature temper brittleness, tempering by heating at 250 to 400°C is not preferable. Therefore, the heating temperature for the tempering treatment needs to be more than 400°C and 650°C or lower, or 100°C or more and less than 250°C. Note that it is difficult to obtain a tempering effect with heating below 100°C.

[0047] (Experimental Example 1) The seamless steel pipe of the present application and the manufacturing method thereof will be described with reference to working examples. In this example, as shown in Table 1, 23 types of steel materials (Steel Nos. 1 to 23) were prepared. Using these steel materials, seamless steel pipes were produced, and various properties were evaluated. Appropriate manufacturing conditions were also evaluated.

[0048]

[0049] <Production of Seamless Steel Pipe> A slab having the above-mentioned chemical component composition was prepared, and blooming and product rolling were performed to produce a raw material. Before producing the intermediate tube, the raw material is annealed as necessary. The raw material annealing conditions were set as holding at 750°C for 300 minutes, followed by furnace cooling. The cooling rate at this time was about 20°C / hour. The heating temperatures for blooming and product rolling and the presence or absence of raw material annealing are shown in Table 2 and Table 3, respectively.

[0050] The intermediate tube is produced by subjecting the raw material to cold forging or machining. When producing an intermediate tube by cold forging, a prismatic steel billet with a side length of 160 mm is produced in the above-mentioned blooming, and then processed into a rod shape with a diameter of φ35 mm by product rolling to obtain a raw material. On the other hand, when producing an intermediate tube by machining, a prismatic steel billet with a side length of 160 mm is produced in the above-mentioned blooming, and then processed into a rod shape with a diameter of φ42 mm by product rolling to obtain a raw material.

[0051] When manufacturing an intermediate pipe material by cold forging, as shown in Figure 1, a process (B) is performed to form a hole 810 from one end (tip) 81 to near the axial center of a rod-shaped material 8 (A), and a process (C) is performed to form a hole 820 from the other end (rear end) 82 to near the axial center, so that a partition plate 83 made of excess material remains in the axial center portion of the circular pipe. Subsequently, the partition plate 83 is punched out to obtain a circular intermediate pipe material 85 having a through hole 850 (D). The inner surface of this intermediate pipe material 85 is shaped by machining (cutting) to allow for smooth subsequent cold working. In this example, a cylindrical intermediate pipe material with an outer diameter of φ40.2 mm and an inner diameter of φ30.1 mm was formed.

[0052] When manufacturing the intermediate pipe material by machining, as described above, a rod-shaped material with a diameter of φ42 mm was used, and an intermediate pipe material of the same dimensions as in the case of cold forging was formed by cutting.

[0053] A portion of the obtained intermediate pipe material was subjected to annealing (intermediate annealing). The intermediate annealing conditions were heating at 700°C for 120 minutes followed by cooling. The manufacturing methods of the intermediate pipe material (cold forging or machining) and whether or not intermediate annealing was performed are shown in Tables 2 and 3, respectively.

[0054] Next, as shown in Figure 1, a formed pipe 88 was manufactured by cold working the intermediate pipe material 85 (E). In this example, the final dimensions were adjusted to change the reduction ratio from 20% to 60%. The reduction ratios are shown in Tables 2 and 3.

[0055] Next, the molded pipe material 85 was heated by high-frequency induction heating, then quenched by water cooling, and subsequently tempered. The conditions for the high-frequency heating rate (°C / sec), quenching temperature (°C), water cooling rate (°C / sec), and tempering temperature (°C) are shown in Tables 2 and 3. The tempering treatment was performed by holding the material at the heating temperature shown in the tables for 2 hours, followed by air cooling.

[0056] The seamless steel pipes obtained after this quenching and tempering treatment (hereinafter referred to simply as "steel pipes" as appropriate) were evaluated for their properties as follows. The evaluation results are shown in Tables 2 and 3.

[0057] <Microstructure> A steel pipe was cut in a cross section perpendicular to the axial direction, embedded in resin, and polished. The polished surface of the sample was etched with Nital, and five fields of view were observed using an optical microscope at a field of view of 0.4 mm × 0.3 mm (magnification: 400x). The area percentage of the tempered martensite phase (M phase %) was determined by image analysis.

[0058] <Previous γ (austenite) grain size> Samples were prepared using the same method as above, etched with picric acid aqueous solution, and observed using an optical microscope for five fields of view at 0.4 mm × 0.3 mm (magnification: 400x). The previous γ (austenite) grain size was determined according to the line segment method (JIS G0551:2020).

[0059] <Tensile Strength> In accordance with the provisions of JIS Z 2241:2011, a JIS No. 12A tensile test specimen (gauge length: 50 mm) was taken with the tensile direction oriented in the axial direction of the pipe, and a tensile test was conducted in accordance with the above JIS provisions to measure the tensile strength.

[0060] <Transition Temperature VTrs> A test specimen with a width of 10 mm, a length of 55 mm, and the same thickness as the steel pipe was cut parallel to the pipe axis from the fabricated steel pipe, and a notch was machined into it. The notch shape was a V-notch (notch angle 45°), with a depth of 2 mm and a notch bottom shape of R0.25 mm. The depth direction of the notch was in the circumferential direction of the steel pipe. These specimens were subjected to Charpy impact tests at arbitrary temperatures from 0 to -120°C, and the fracture surface was observed using SEM. A 127 μm × 95 μm area (magnification: 1000x) in the center of the fracture surface was observed, and the area ratio of grain boundary fracture surfaces and pseudo-cleavage fracture surfaces was measured to determine the brittle fracture surface ratio. A tendency for the brittle fracture surface ratio to increase with decreasing test temperature was observed. The test temperature at which the brittle fracture surface ratio reached 50% was defined as the transition temperature VTrs.

[0061]

[0062]

[0063] As shown in Table 2, for steels No. 1 to 15, good results were obtained in all aspects, including microstructure, prior γ grain size, tensile strength, and transition temperature VTrs, because the chemical composition and manufacturing conditions were appropriate.

[0064] On the other hand, as shown in Table 3, steel No. 16, lacking B and Ti in its chemical composition, had a lower area ratio of the tempered martensite phase in its microstructure, coarser prior γ grain size, reduced tensile strength, and an inability to obtain a sufficiently low transition temperature VTrs.

[0065] Steel No. 17 suffered from an excessively low bloc rolling temperature during manufacturing, which resulted in coarsening of the prior γ grain size in the final microstructure, preventing the achievement of a sufficiently low transition temperature (VTrs).

[0066] Steel No. 18 was heated to an excessively high temperature during manufacturing, which resulted in coarsening of the prior γ grain size in the final metal structure, preventing the achievement of a sufficiently low transition temperature (VTrs).

[0067] Steel No. 19 suffered from an excessively low reduction ratio during cold working from intermediate tube material to formed tube material. This resulted in coarsening of the prior γ grain size in the final microstructure, preventing the achievement of a sufficiently low transition temperature (VTrs).

[0068] In the heat treatment of the formed pipe material of steel No. 20, the heating rate during quenching was too low, resulting in coarsening of the prior γ grain size in the final metal structure, and preventing the achievement of a sufficiently low transition temperature VTrs.

[0069] In the heat treatment of the formed pipe material of steel No. 21, the quenching temperature was too high, which resulted in coarsening of the prior γ grain size in the final metal structure, and a sufficiently low transition temperature VTrs could not be obtained.

[0070] In the heat treatment of the formed tube material of steel No. 22, the cooling rate during quenching was too slow, resulting in a low area ratio of the tempered martensite phase in the microstructure, and thus a decrease in tensile strength.

[0071] In the heat treatment of the formed pipe material of steel No. 23, the tempering temperature was in the range of 250°C to 400°C, which resulted in low-temperature tempering brittleness, and a sufficiently low transition temperature VTrs could not be obtained.

Claims

1. In mass%, the essential elements are C: 0.05-0.20%, Si: 0.05-0.50%, Mn: 0.20-1.00%, P: 0.030% or less, S: 0.015% or less, Cr: 0.05-1.00%, Al: 0.010-0.080%, N: 0.0020-0.0100%, B: 0.0005-0.0050%, Ti: 0.01-0.05%, and the optional elements may be Mo: 0.30% or less, Nb: 0.01-0.05%, V: 0.01-0.20%, Te: 0.001-0.200%, Ca: 0.0005-0.0050%. A seamless steel pipe having a chemical composition consisting of the remainder Fe and unavoidable impurities, a microstructure in which the tempered martensite phase accounts for 90% or more of the area, a prior γ (austenite) grain size of 9.0 or higher, a tensile strength of the sidewall of 1000 MPa or higher, and a transition temperature VTrs of -60°C or lower.

2. The seamless steel pipe described in claim 1, wherein the seamless steel pipe is for use as an airbag inflator, has an outer diameter of 25 to 40 mmφ, an inner diameter of 20 to 35 mmφ, and a length of 100 to 450 mm.

3. A method for manufacturing a seamless steel pipe according to claim 1 or 2, comprising: heating a steel ingot having the above chemical composition to 1200 to 1350°C and performing bloc rolling to produce a steel billet; heating the steel billet to 850 to 1100°C and performing product rolling to produce a material; cold forging and / or machining the material to produce an intermediate pipe material; cold working the intermediate pipe material to produce a formed pipe material with a reduction ratio of 20% or more; quenching the formed pipe material by high-frequency induction heating at a temperature of Ac3 point or higher and 1050°C or lower, with an average heating rate of 200°C / second or higher, followed by rapid cooling to 200°C with an average cooling rate of 200°C / second or higher; and then tempering at a heating temperature of over 400°C and 650°C or 100°C and 250°C or higher.

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