Seamless steel pipe and inflation bottle for airbag

Abstract

A seamless steel pipe having a chemical composition described in the description, the chemical composition satisfying [WT / (5C + Mo + Cr) > = 1.00] in relation to the thickness WT (mm) and [GN-1.51 * (Mn + 85P-30Ca) > = 8.50] in relation to the original gamma grain size GN, the tensile strength being at least 1000 MPa, the elongation at break being at least 8.0%, and the limit hydrogen concentration being at least 2.5 ppm.

Description

Technical Field

[0001] The invention relates to a seamless steel pipe and an inflator bottle for a safety air bag. Background Art

[0002] The automotive industry is actively introducing safety-focused devices. Among these are airbag systems, which deploy between the passenger and the steering wheel, dashboard, and other components in a collision, using gas or other means to absorb the passenger's kinetic energy and mitigate injuries. While conventional airbag systems used explosives, environmentally friendly systems have led to the development and widespread adoption of systems using high-pressure gas fillers.

[0003] This system maintains high pressure on the gas, etc., that is blown into the airbag during a collision, and then releases the gas all at once during a collision. This stresses the airbag steel tube at a high strain rate in a very short period of time. Therefore, the steel tube used must have high strength and excellent burst resistance.

[0004] The demand for lighter vehicles has recently intensified. In line with this, thinner and lighter steel tubes for automotive airbags are also desired. To ensure high burst pressure even with thin walls, airbag systems use inflators made from high-strength seamless steel tubes with a tensile strength of 900 MPa or more.

[0005] Furthermore, since diameter reduction is performed during the manufacture of, for example, an inflator cylinder, steel tubes for air bags are required to have excellent diameter reduction properties.

[0006] Against this background, seamless steel tubes for air bags are disclosed in, for example, Patent Documents 1 and 2. According to Patent Documents 1 and 2, studies have been conducted to increase strength and improve toughness and workability.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent Application Laid-Open No. 2004-27303

[0010] Patent Document 2: Japanese Patent Application Laid-Open No. 2004-76034 Summary of the Invention

[0011] Problems to be solved by the invention

[0012] However, to ensure higher reliability, steel tubes for airbags require suppression of embrittlement caused by hydrogen that penetrates the steel tubes during the manufacturing process and in the usage environment. Patent Documents 1 and 2 do not investigate the hydrogen embrittlement resistance of steel tubes at all.

[0013] An object of the present invention is to provide a seamless steel pipe and an airbag inflator having high strength, excellent diameter reduction workability, and excellent hydrogen embrittlement resistance.

[0014] Solutions for solving problems

[0015] The present invention has been made to solve the above-mentioned technical problems, and its gist is a seamless steel pipe and an airbag inflator cylinder as described below.

[0016] (1) A seamless steel pipe,

[0017] Its chemical composition is expressed in mass %.

[0018] C: 0.05~0.20%,

[0019] Si: 0.05-0.50%,

[0020] Mn: 0.30-1.50%,

[0021] P: 0.025% or less,

[0022] S: 0.020% or less,

[0023] Cu: 0.01~0.50%,

[0024] Ni: 0.01~0.50%,

[0025] Cr: 0.01~1.20%,

[0026] Mo: 0.01~0.50%,

[0027] Ti: 0.001~0.050%,

[0028] Nb: 0.001~0.100%,

[0029] Ca: 0.0005~0.0025%,

[0030] Al: 0.080% or less,

[0031] N: 0.0100% or less,

[0032] V: 0~0.100%

[0033] B: 0~0.0050%,

[0034] Mg: 0~0.0050%,

[0035] REM: 0~0.0050%

[0036] Sn: 0~0.100%,

[0037] As: 0~0.010%,

[0038] Balance: Fe and impurities,

[0039] Assuming that the content of each element is within the above range,

[0040] The relationship between the chemical composition and the wall thickness satisfies the following formula (i):

[0041] Furthermore, the relationship between the chemical composition and the prior austenite grain size satisfies the following formula (ii):

[0042] Tensile strength is above 1000MPa,

[0043] Elongation at break is more than 8.0%,

[0044] The limit hydrogen concentration is 2.5 ppm or more.

[0045] WT / (5C+Mo+Cr)≥1.00···(i)

[0046] GN-1.51×(Mn+85P-30Ca)≥8.50···(ii)

[0047] The element symbols in the above formula represent the content of each element in the steel (mass %), and are expressed as zero when not contained. WT represents the wall thickness (mm) of the seamless steel pipe, and GN represents the prior austenite grain size.

[0048] (2) The seamless steel pipe according to (1) above, wherein

[0049] The above chemical composition contains, in mass%,

[0050] V: 0.001~0.100%,

[0051] B: 0.0001~0.0050%,

[0052] Mg: 0.0001~0.0100%,

[0053] REM: 0.0001~0.0100%

[0054] Sn: 0.001~0.100%, and

[0055] As: one or more kinds of 0.001 to 0.010%.

[0056] (3) An airbag inflator cylinder comprising:

[0057] A cylindrical portion extending in one direction, and a reduced diameter portion formed on at least one end side of the cylindrical portion in the aforementioned one direction,

[0058] The chemical composition of the cylindrical portion is expressed in mass %.

[0059] C: 0.05~0.20%,

[0060] Si: 0.05-0.50%,

[0061] Mn: 0.30-1.50%,

[0062] P: 0.025% or less,

[0063] S: 0.020% or less,

[0064] Cu: 0.01~0.50%,

[0065] Ni: 0.01~0.50%,

[0066] Cr: 0.01~1.20%,

[0067] Mo: 0.01~0.50%,

[0068] Ti: 0.001~0.050%,

[0069] Nb: 0.001~0.100%,

[0070] Ca: 0.0005~0.0025%,

[0071] Al: 0.080% or less,

[0072] N: 0.0100% or less,

[0073] V: 0~0.100%

[0074] B: 0~0.0050%,

[0075] Mg: 0~0.0050%,

[0076] REM: 0~0.0050%

[0077] Sn: 0~0.100%,

[0078] As: 0~0.010%,

[0079] Balance: Fe and impurities,

[0080] Assuming that the content of each element is within the above range,

[0081] The relationship between the chemical composition of the cylindrical portion and the wall thickness of the cylindrical portion satisfies the following formula (i):

[0082] Furthermore, the relationship between the chemical composition of the cylindrical portion and the prior austenite grain size of the cylindrical portion satisfies the following formula (ii):

[0083] The tensile strength of the cylindrical portion is 1000 MPa or more.

[0084] The elongation at break of the cylindrical portion is 8.0% or more,

[0085] The limit hydrogen concentration of the cylindrical portion is 2.5 ppm or more.

[0086] WT / (5C+Mo+Cr)≥1.00···(i)

[0087] GN-1.51×(Mn+85P-30Ca)≥8.50···(ii)

[0088] The element symbols in the above formula represent the content (mass %) of each element in the steel of the cylindrical portion, and are expressed as zero when not contained. In addition, WT represents the wall thickness (mm) of the cylindrical portion, and GN represents the prior austenite grain size of the cylindrical portion.

[0089] (4) The air bag inflator according to (3) above, wherein:

[0090] The chemical composition of the cylindrical portion contains, in mass %,

[0091] V: 0.001~0.100%,

[0092] B: 0.0001~0.0050%,

[0093] Mg: 0.0001~0.0100%,

[0094] REM: 0.0001~0.0100%

[0095] Sn: 0.001~0.100%, and

[0096] As: one or more kinds of 0.001 to 0.010%.

[0097] Effects of the Invention

[0098] According to the present invention, a seamless steel pipe and an airbag inflator having high strength, excellent diameter reduction workability, and excellent hydrogen embrittlement resistance can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0099] Figure 1This is a diagram for explaining the shape of an arc-shaped tensile test piece used for measuring the limiting hydrogen concentration. DETAILED DESCRIPTION

[0100] The present inventors have conducted intensive research on a method for achieving a balance between strength and diameter reduction workability of seamless steel pipes while ensuring hydrogen embrittlement resistance. As a result, they have obtained the following findings.

[0101] (a) To achieve high strength in seamless steel pipes, it is necessary to increase the content of elements that improve hardenability. Adequate levels of C, Mo, and Cr are effective. However, if the content of these elements is excessive relative to the wall thickness of the seamless steel pipe, diameter reduction workability cannot be ensured. From this perspective, it is important to balance the chemical composition and wall thickness of the seamless steel pipe, specifically to ensure that it satisfies the following formula (i).

[0102] WT / (5C+Mo+Cr)≥1.00···(i)

[0103] (b) Furthermore, in order to ensure diameter reduction workability, it is also important to improve ductility. Specifically, the elongation at break needs to be 8.0% or higher. This elongation at break can be achieved by controlling the chemical composition and manufacturing under appropriate conditions.

[0104] (c) Excessive Mn content not only reduces the hydrogen diffusion rate, causing localized hydrogen enrichment, but also generates MnS, which degrades hydrogen embrittlement resistance. Furthermore, P segregates at grain boundaries, further degrading hydrogen embrittlement resistance. On the other hand, Ca suppresses the formation of MnS, thereby improving hydrogen embrittlement resistance.

[0105] (d) The present inventors have studied and found that the degree of deterioration in hydrogen embrittlement resistance varies depending on the prior austenite grain size. Furthermore, the effects of the Mn, P, and Ca contents and the prior austenite grain size GN on hydrogen embrittlement resistance were evaluated. It was found that excellent hydrogen embrittlement resistance can be achieved by adjusting the contents of each element within a specified range and satisfying the following formula (ii).

[0106] GN-1.51×(Mn+85P-30Ca)≥8.50···(ii)

[0107] (e) To further improve hydrogen embrittlement resistance, preheating is required during the tempering process. Although the mechanism by which preheating further improves hydrogen embrittlement resistance is not yet clear, it is believed that this is because the temperature distribution in the wall thickness direction is eliminated, making the metallographic structure more uniform.

[0108] The present invention has been made based on the above findings. Hereinafter, each configuration of the present invention will be described in detail.

[0109] (A) Chemical composition

[0110] The chemical composition of the seamless steel pipe according to one embodiment of the present invention is limited for the following reasons: In the following description, "%" for the content of each element means "mass %".

[0111] C: 0.05~0.20%

[0112] C is an effective element for increasing steel strength at low cost. If its content is less than 0.05%, it is difficult to achieve the desired tensile strength, while if it exceeds 0.20%, workability and weldability are reduced. Therefore, the C content is set to 0.05-0.20%. The preferred range for the C content is 0.06% to 0.18%, and the more preferred range is 0.07% to 0.17%. When reducing workability is particularly important, the C content is more preferably less than 0.17%.

[0113] Si: 0.05-0.50%

[0114] Si is an element that not only has a deoxidizing effect but also enhances the hardenability of steel, thereby increasing its strength. For this purpose, the Si content is set to 0.05% or higher. However, if the content exceeds 0.50%, toughness decreases, so the Si content is set to 0.50% or lower. The preferred range for the Si content is 0.10% or higher and 0.40% or lower, and the more preferred range is 0.15% or higher and 0.30% or lower.

[0115] Mn: 0.30~1.50%

[0116] Mn is an effective element that not only has a deoxidizing effect but also enhances the hardenability of steel and improves its strength and toughness. However, if its content is less than 0.30%, sufficient strength and toughness cannot be obtained. On the other hand, if the Mn content exceeds 1.50%, MnS coarsens, which elongates during hot rolling, and the toughness and hydrogen embrittlement resistance are reduced. In addition, excess Mn reduces the diffusion rate of hydrogen, causing local enrichment, thereby reducing the hydrogen embrittlement resistance. Therefore, the Mn content is set to 0.30-1.50%. The preferred Mn content is 0.40% or more and 1.20% or less, and the more preferred range is 0.50% or more and 1.00% or less.

[0117] P: 0.025% or less

[0118] P, contained in steel as an impurity, can cause a decrease in toughness and hydrogen embrittlement resistance due to grain boundary segregation. In particular, if the P content exceeds 0.025%, the decrease in toughness and hydrogen embrittlement resistance becomes significant. Therefore, the P content is set to 0.025% or less. The P content is preferably 0.020% or less, and more preferably 0.015% or less.

[0119] S: 0.020% or less

[0120] S is also contained in steel as an impurity and particularly reduces the toughness of the steel pipe in the T direction (a direction perpendicular to the pipe axis). If the S content exceeds 0.020%, the reduction in toughness in the T direction of the steel pipe becomes significant. Therefore, the S content is set to 0.020% or less. The preferred S content is 0.010% or less.

[0121] Cu: 0.01~0.50%

[0122] Cu improves the strength and toughness of steel by enhancing its hardenability. This effect is achieved when the content is 0.01% or more. However, if the content exceeds 0.50%, the alloy cost will increase. Therefore, the Cu content is set to 0.01-0.50%. The preferred Cu content is 0.05% or more, more preferably 0.10% or more, and even more preferably 0.20% or more. In addition, the Cu content is preferably 0.40% or less, and more preferably 0.35% or less.

[0123] Ni: 0.01~0.50%

[0124] Ni enhances the hardenability of steel, thereby improving strength and toughness. This effect is achieved when Ni is present at a content of 0.01% or more. However, if Ni is present in an amount exceeding 0.50%, the alloy cost increases, so the Ni content is set to 0.01-0.50%. The preferred Ni content is 0.05% or more, more preferably 0.10% or more, and even more preferably 0.20% or more. The Ni content is preferably 0.45% or less, and more preferably 0.40% or less.

[0125] Cr: 0.01~1.20%

[0126] Cr enhances the hardenability of steel and its resistance to temper softening, improving strength and toughness. This effect is achieved when the content is 0.01% or more. However, if the content exceeds 1.20%, the alloy cost increases. Therefore, the Cr content is set to 0.01-1.20%. The preferred Cr content is 0.05% or more, more preferably 0.10% or more, and even more preferably 0.20% or more. The Cr content is preferably 1.00% or less, and more preferably 0.90% or less.

[0127] Mo: 0.01~0.50%

[0128] Mo enhances the hardenability of steel and increases the resistance to temper softening, thereby improving strength and toughness. This effect can be achieved when the content is 0.01% or more. However, if the content exceeds 0.50%, the alloy cost will increase. In addition, if the Mo content is too high, the strength of the seamless steel pipe will tend to increase during air cooling after hot working and pipe making, and softening heat treatment will be required before cold drawing, which will increase the manufacturing cost. Therefore, the Mo content is set to 0.01-0.50%. The preferred Mo content is 0.05% or more, more preferably 0.10% or more, and even more preferably 0.20% or more. The Mo content is preferably 0.45% or less, more preferably 0.40% or less.

[0129] Ti: 0.001~0.050%

[0130] Ti fixes nitrogen in the steel, improving toughness. Furthermore, finely dispersed Ti nitrides strongly pin grain boundaries, refining the grains and improving the toughness of the steel. To achieve this effect, a Ti content of 0.001% or more is necessary, but if it exceeds 0.050%, the nitrides coarsen and the toughness decreases. Therefore, the Ti content is set to 0.001-0.050%. The preferred Ti content is 0.003% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. The Ti content is preferably 0.045% or less, more preferably 0.040% or less, and even more preferably 0.030% or less.

[0131] Nb: 0.001~0.100%

[0132] Nb is finely dispersed in the steel as carbides, strongly pinning grain boundaries. This has the effect of refining the grains and improving the toughness of the steel. To achieve this effect, a Nb content of 0.001% or more is necessary. However, if the content exceeds 0.100%, the carbides coarsen and the toughness decreases. Therefore, the Nb content is set to 0.001-0.100%. The preferred Nb content is 0.005% or more, more preferably 0.010% or more, and even more preferably 0.015% or more. The Nb content is preferably 0.080% or less, and more preferably 0.060% or less.

[0133] Ca: 0.0005~0.0025%

[0134] Ca fixes S, which exists as an inevitable impurity in steel, into sulfides, improving toughness anisotropy and increasing the toughness in the T direction of the steel pipe, thereby enhancing burst resistance. Furthermore, by suppressing the formation of MnS, it also contributes to improved hydrogen embrittlement resistance. This effect is achieved with a Ca content of 0.0005% or more. However, a content exceeding 0.0025% increases inclusions and reduces toughness. Therefore, the Ca content is set to 0.0005-0.0025%. To reliably achieve the effect of improving hydrogen embrittlement resistance, the Ca content is preferably 0.0010% or more, more preferably more than 0.0010%, further preferably 0.0012% or more, and even more preferably 0.0015% or more.

[0135] Al: 0.080% or less

[0136] Al is an effective element that has a deoxidizing effect and enhances toughness and workability. However, if it contains more than 0.080%, the generation of macro-streak-flaw will become significant. Therefore, the Al content is set to 0.080% or less. The Al content is preferably less than 0.060%, more preferably less than 0.040%. It should be noted that the Al content can be an impurity level, so its lower limit is not particularly limited, but it is preferably set to 0.005% or more. It should be noted that the Al content referred to in the present invention refers to the content of acid-soluble Al (so-called "sol.Al").

[0137] N: 0.0100% or less

[0138] Nitrogen forms fine nitrides, which strongly pin grain boundaries, refine grains, and improve the toughness of the steel. However, if the content exceeds 0.0100%, the nitrides coarsen, which in turn reduces toughness. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0080% or less, and more preferably 0.0050% or less. It should be noted that the N content can be an impurity level, so its lower limit is not particularly limited, but is preferably 0.0005% or more, and more preferably 0.0010% or more.

[0139] V: 0~0.100%

[0140] V is an element that ensures toughness and enhances strength through precipitation strengthening, so it can be included as needed. However, a content exceeding 0.100% can lead to a decrease in toughness. Therefore, when V is included, the V content is set to 0.100% or less. The V content is preferably 0.050% or less, and more preferably 0.010% or less. The effects of V can be confirmed even in trace amounts, but to achieve a sufficient effect, a content of 0.001% or more is preferably used.

[0141] B: 0~0.0050%

[0142] B is an element that segregates at the grain boundaries in steel and significantly improves the hardenability of steel by adding a trace amount, so it can be contained as needed. However, if B is contained in an amount exceeding 0.0050%, it can be confirmed that borides are coarsely precipitated at the grain boundaries, and the tendency of toughness reduction can be confirmed. Therefore, when B is contained, the B content is set to 0.0050% or less. The B content is preferably 0.0030% or less, more preferably 0.0020% or less. Even a trace amount can confirm the effect of B, but in order to ensure a sufficient effect, it is preferably contained at least 0.0001%, more preferably at least 0.0005%.

[0143] Mg: 0~0.0050%.

[0144] Like Ca, Mg is an element that fixes S, an inevitable impurity in steel, into sulfides, improving toughness anisotropy and enhancing the toughness in the T-direction of the steel pipe, thereby enhancing burst resistance. Therefore, Mg may be included as needed. However, if it exceeds 0.0050%, inclusions increase, and toughness decreases. Therefore, when Mg is included, the Mg content should be kept to 0.0050% or less. The Mg content is preferably 0.0040% or less, and more preferably 0.0030% or less. The effects of Mg can be observed even in trace amounts, but to ensure a sufficient effect, the content is preferably 0.0001% or more, and more preferably 0.0005% or more.

[0145] REM: 0~0.0050%

[0146] Like Ca, REM is an element that fixes S, an inevitable impurity in steel, as sulfides, improving toughness anisotropy and enhancing the toughness in the T-direction of the steel pipe, thereby enhancing burst resistance. Therefore, it can be included as needed. However, if it exceeds 0.0050%, inclusions increase, and toughness decreases. Therefore, when included, the REM content should be set to 0.0050% or less. The REM content is preferably 0.0040% or less, and more preferably 0.0030% or less. The effects of REM can be observed even in trace amounts, but to ensure a sufficient effect, the content is preferably 0.0001% or more, and more preferably 0.0005% or more.

[0147] In this embodiment, "REM" refers to a total of 17 elements, including Sc, Y, and lanthanides. "REM content" refers to the content of a single REM element, and to the total content of these elements when two or more REM elements are present. Furthermore, REM is typically supplied as an alloy of multiple REM elements, i.e., misch metal. Therefore, one or more individual elements may be added and contained, or, for example, as a misch metal.

[0148] Sn: 0~0.100%

[0149] Sn improves corrosion resistance and may be included as needed. However, excessive Sn content can reduce toughness. Therefore, the Sn content is 0.100% or less. A preferred Sn content is 0.080% or less, and more preferably 0.060% or less. Even trace amounts of Sn can demonstrate its effects, but to ensure a sufficient effect, a content of 0.001% or more, and more preferably 0.003% or more, is preferred.

[0150] As: 0~0.010%

[0151] As improves corrosion resistance and may be included as needed. However, excessive As content can reduce hot workability. Therefore, the As content is preferably 0.010% or less. A preferred As content is 0.008% or less, and more preferably 0.006% or less. Even trace amounts of As can demonstrate its effects, but to ensure a sufficient effect, an As content of 0.001% or more, and more preferably 0.002% or more, is preferred.

[0152] The seamless steel pipe of this embodiment contains the aforementioned elements, with the balance being Fe and impurities. Here, "impurities" refer to components that are introduced into steel materials during industrial production due to various factors, such as raw materials such as ore and scrap, and during the manufacturing process. These impurities are permitted as long as they do not adversely affect the present invention.

[0153] The chemical composition of the seamless steel pipe of this embodiment, provided that the content of each element is within the above-mentioned range, satisfies the relationship between the chemical composition and the wall thickness of the seamless steel pipe as follows (i). As described above, by sufficiently ensuring the content of C, Mo, and Cr, the hardenability is improved, and high strength of the seamless steel pipe can be achieved. However, from the perspective of diameter reduction workability, it is necessary to adjust the balance between the chemical composition and the wall thickness of the seamless steel pipe. Diameter reduction workability can be ensured by satisfying the following formula (i). The left side value of the following formula (i) is preferably greater than 1.20, more preferably greater than 1.50, and even more preferably greater than 2.00.

[0154] WT / (5C+Mo+Cr)≥1.00···(i)

[0155] The element symbols in the above formula represent the content (mass %) of each element in the steel, and when not contained, they are expressed as zero. In addition, WT represents the wall thickness (mm) of the seamless steel pipe.

[0156] Furthermore, the chemical composition of the seamless steel pipe of this embodiment, provided that the content of each element is within the above-described range, satisfies the relationship between the chemical composition and the prior austenite grain size as shown in the following equation (ii). By adjusting the contents of Mn and P, which degrade hydrogen embrittlement resistance, and Ca, which improves hydrogen embrittlement resistance, according to the prior austenite grain size, excellent hydrogen embrittlement resistance can be achieved. The value on the left side of equation (ii) is preferably 9.00 or greater, more preferably 9.50 or greater, and even more preferably 10.00 or greater.

[0157] GN-1.51×(Mn+85P-30Ca)≥8.50···(ii)

[0158] The element symbols in the above formula represent the content (mass %) of each element in the steel, and when not contained, they are expressed as zero. GN represents the prior austenite grain size.

[0159] Prior austenite grain size was measured in accordance with ASTM E112 (2013). Specifically, a test piece encompassing the entire wall thickness was collected from a seamless steel pipe, using the surface encompassing both the pipe axis and the wall thickness (hereinafter referred to as the "longitudinal section") as the test surface (hereinafter referred to as the "observation surface"). The observation surface was then mirror-polished. After polishing, picric acid etching was used to visualize the prior austenite grain boundaries within the observation surface.

[0160] Then, using an optical microscope, 5 visual fields are observed in the form of a position at a wall thickness of 1 / 4 from the outer surface of the seamless steel pipe as the center of the visual field. Then, by the comparative method specified in ASTM E112 (2013), the original austenite grain size of each visual field is obtained, and its average value is used as the original austenite grain size of the seamless steel pipe. At this time, 100 times is used as a benchmark observation magnification, and is set to 200 times or 400 times according to grain size. In addition, when the observation magnification is set to 200 times or 400 times, the correction value Q defined by the following (1) formula is used, and is corrected according to ASTM E112 (2013).

[0161] Q=6.64log 10 (M / 100)···(I)

[0162] Wherein, M in the above formula is the observation magnification.

[0163] It should be noted that the prior austenite grain size is not particularly limited as long as the above formula (ii) is satisfied, and can be, for example, 10.0 or more or 11.0 or more.

[0164] (B) Wall thickness

[0165] The wall thickness of the seamless steel pipe of this embodiment is not particularly limited as long as it satisfies the above formula (i). For example, it can be set to 1.00 mm or more or 1.50 mm or more. On the other hand, from the perspective of lightweighting, thin walls are preferred, with a wall thickness of preferably 2.60 mm or less, more preferably less than 2.50 mm, and even more preferably 2.40 mm or less. Generally speaking, the thinner the wall, the more difficult it is to reduce the diameter. However, in the present invention, by adjusting the balance between the chemical composition and wall thickness of the seamless steel pipe, it is possible to ensure diameter reduction even with thin walls.

[0166] (C) Characteristics

[0167] The seamless steel pipe of this embodiment has high strength, specifically, a tensile strength of 1000 MPa or greater. A tensile strength of 1000 MPa or greater allows excellent burst resistance even when used as an airbag inflator that is subjected to stress at a high strain rate in an extremely short period of time.

[0168] It should be noted that, as described above, in order to increase the strength, it is necessary to further increase the content of elements that improve hardenability, which results in an increased risk of reduced diameter workability. When prioritizing diameter workability, the tensile strength is preferably less than 1200 MPa.

[0169] Furthermore, the seamless steel pipe of this embodiment has excellent ductility due to its secured diameter reduction workability. Specifically, the elongation at break is 8.0% or more, preferably 9.0% or more, and more preferably 10.0% or more.

[0170] Tensile strength and elongation at break were measured in accordance with JIS Z 2241:2011. Specifically, a tubular test piece of a certain length was cut from a seamless steel pipe to produce a No. 11 test piece in accordance with JIS Z 2241:2011. This No. 11 test piece was then subjected to the tubular tensile test specified in JIS Z 2241:2011 to measure tensile strength and elongation at break.

[0171] Furthermore, the seamless steel pipe of this embodiment exhibits excellent resistance to hydrogen embrittlement. Specifically, its limiting hydrogen concentration is 2.5 ppm or greater. This ensures high reliability when used as a steel pipe for airbags, etc. The limiting hydrogen concentration is more preferably 2.7 ppm or greater. In this embodiment, the limiting hydrogen concentration is determined specifically by the following method.

[0172] Collect multiple seamless steel pipes Figure 1An arc-shaped tensile test piece of the shape shown. The arc-shaped tensile test piece is made as follows: After cutting an arc-shaped test piece with a length of 120 mm, a width of 9.0 mm, and a thickness equal to the original wall thickness d of the steel pipe from a seamless steel pipe, a narrowing portion is provided in the central portion in the longitudinal direction while leaving a holding portion at each end in the longitudinal direction, and a U-cut is further provided in the central portion in the longitudinal direction of the narrowing portion. The holding portions are 45 mm long and 9.0 mm wide, and the narrowing portion is 30 mm long and 2.0 mm wide. In addition, a curved surface with a curvature radius of 5.0 mm is formed at both ends of the narrowing portion and is connected to the holding portion. In addition, the U-cut has a cut width of 0.20 mm, a cut depth of 0.35 mm, and a cut bottom radius of 0.10 mm.

[0173] Next, multiple arc-shaped tensile test specimens were immersed in various aqueous solutions containing 3% NaCl and ammonium thiocyanate in a concentration range of 0 to 30 g / L, and subjected to a cathodic charge constant load test at a potential ranging from -0.9 to -1.2 V. A stress equal to 90% of the tensile strength of each seamless steel pipe was applied.

[0174] Only the arc-shaped tensile test specimens with a durability time exceeding 200 hours were stored in liquid nitrogen. The parallel sections of the tapered portion were then cut to create hydrogen concentration test specimens. The hydrogen concentration was then measured using temperature-programmed desorption hydrogen analysis. In temperature-programmed desorption hydrogen analysis, the hydrogen concentration in the specimen was determined by heating the specimen from room temperature to 200°C at a rate of 100°C / hour. The amount of hydrogen released was measured, and the maximum of the obtained hydrogen concentrations was taken as the limiting hydrogen concentration.

[0175] (D) Airbag inflation cylinder

[0176] An airbag inflator according to one embodiment of the present invention includes a cylindrical portion extending in one direction and a reduced diameter portion formed at at least one end of the cylindrical portion in the one direction. The reduced diameter portion may also be formed at both ends of the cylindrical portion in the one direction.

[0177] The airbag inflator cylinder of this embodiment is manufactured by cutting the aforementioned seamless steel tube into predetermined lengths and then reducing the diameter of one or both ends. Therefore, the chemical composition, prior austenite grain size, wall thickness, and properties of the cylindrical portion are identical to those of the raw seamless steel tube. Therefore, a detailed description thereof is omitted.

[0178] It should be noted that the reduced diameter portion, as a result of the diameter reduction process, has the same or higher strength and wall thickness as the seamless steel pipe used as the raw material. In other words, if the seamless steel pipe used as the raw material achieves high strength and excellent hydrogen embrittlement resistance, the airbag inflator manufactured from this seamless steel pipe can also achieve high strength and excellent hydrogen embrittlement resistance.

[0179] (E) Manufacturing method

[0180] The seamless steel pipe according to one embodiment of the present invention can be produced by the following method.

[0181] Steel having the chemical composition described in Section (A) above is melted by conventional methods and then cast into ingots or billets. Note that billets in the shape of round bars for pipe making can also be produced using the so-called "round continuous casting" (CC) method.

[0182] The next step is to subject the cast ingot or billet to initial rolling or hot forging. This step is used to obtain the billet used in the final hot-working pipemaking process (e.g., pipemaking using hot piercing, rolling, and stretching, or pipemaking using hot extrusion). It should be noted that the billet produced in the round billet shape by the "round billet CC" method can be directly processed into seamless steel pipe, so initial rolling or hot forging is not necessarily required.

[0183] The seamless steel pipe of this embodiment is produced by sequentially subjecting the billet produced by the initial rolling or hot forging and used in the final hot-working pipemaking process, or the cast billet produced in the shape of a round bar (hereinafter collectively referred to as "billet") to a hot-working pipemaking process, a cold working process, a quenching process, and a tempering process.

[0184] <Hot working pipe making process>

[0185] The billet is heated and then hot-worked to produce a billet having a predetermined shape. Conventional methods, such as the Mannesmann mandrel rolling method, can be used for hot-working pipe production. The billet heating temperature can be set to, for example, 1000-1300°C.

[0186] Cold working process

[0187] To improve dimensional accuracy, the blank tube obtained by the above method is subjected to cold working. The cold working method is not particularly limited as long as it can uniformly process the blank tube. For example, industrially, it is advantageous to use a so-called cold drawing mill using a piercing die and a plug, or a cold rolling mill known as a cold pilger.

[0188] <Quenching process>

[0189] Next, the cold-worked shell is subjected to induction hardening, which involves induction heating to a temperature of 900-1050°C followed by rapid cooling. Heating temperatures below 900°C prevent complete austenitization, sometimes resulting in a lack of high strength. On the other hand, heating temperatures exceeding 1050°C cause austenite grains to grow rapidly and coarsen, preventing excellent toughness.

[0190] Furthermore, rapid heating using high-frequency heating can suppress austenite grain growth, resulting in a fine metallographic structure. While the holding time at the heating temperature depends on the size of the tube, it is preferably 10 seconds or less to suppress austenite grain growth. The heating temperature refers to the temperature of the outer surface of the tube. During rapid cooling, appropriate methods such as water cooling or oil cooling may be used as long as a sufficient quenched structure is achieved.

[0191] Tempering process

[0192] After induction quenching, the billet is subjected to a tempering treatment, heating to 370-410°C and then cooling to room temperature. If the tempering temperature is below 370°C, while strength can be maintained, ductility and low-temperature toughness are reduced. In particular, if ductility is reduced, sufficient diameter reduction workability cannot be ensured even if the above formula (i) is satisfied. On the other hand, if the tempering temperature exceeds 410°C, although excellent ductility and low-temperature toughness can be achieved, strength is reduced, making it impossible to achieve a tensile strength of 1000 MPa or more.

[0193] The holding time at the above heating temperature depends on the size of the tube, but is preferably 10 to 30 minutes. The heating temperature refers to the temperature of the outer surface of the tube. There are no particular restrictions on the cooling rate during tempering. Therefore, cooling methods such as natural cooling in the atmosphere, forced air cooling, spray cooling, oil cooling, and water cooling, as appropriate for the equipment, can be used.

[0194] Furthermore, to achieve excellent hydrogen embrittlement resistance, preheating is required before reaching the aforementioned heating temperature. Specifically, preheating should be performed so that the temperature remains within the 250-350°C temperature range for at least 5 minutes. As described above, preheating is believed to eliminate temperature distribution in the wall thickness direction, resulting in a more uniform metallurgical structure.

[0195] As described above, the airbag inflator of this embodiment is manufactured by cutting the seamless steel tube manufactured by the above method into a predetermined length and then reducing the diameter of one or both ends thereof.

[0196] Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

[0197] Example

[0198] Steels having the chemical compositions shown in Tables 1-1 and 1-2 were melted and cast into rectangular bars using a converter-continuous casting process. The rectangular bars were further formed into round bars by hot forging and cooled to room temperature.

[0199] [Table 1-1]

[0200] Table 1-1

[0201]

[0202] [Table 1-2]

[0203] Table 1-2

[0204]

[0205] The round billet is heated, and a tube billet is produced by the Mannesmann mandrel rolling method, and then cooled to room temperature. The obtained tube billet is cold worked using a cold drawing machine to achieve the outer diameter and wall thickness shown in Table 2. Then, quenching and tempering are carried out under the conditions shown in Table 2 to produce a seamless steel pipe. The preheating time in Table 2 refers to the residence time in the temperature range of 250 to 350°C. It should be noted that all quenching is carried out by water quenching after high-frequency heating, and the cooling rate is adjusted to 150°C / second. In addition, all cooling during tempering is set to natural cooling in the atmosphere.

[0206] [Table 2]

[0207] Table 2

[0208]

[0209] For each seamless steel pipe obtained, first, the original austenite grain size is measured. The original austenite grain size is measured according to ASTM E112 (2013). Specifically, a test piece containing the entire wall thickness is collected with the longitudinal section of the seamless steel pipe as the observation surface, and the observation surface is mirror-polished. After grinding, picric acid etching solution is used to make the original austenite grain boundaries in the observation surface appear. Then, using an optical microscope, 5 fields of view are observed with a position of 1 / 4 of the wall thickness from the outer surface of the seamless steel pipe as the center of the field of view. In addition, the original austenite grain size of each field of view is obtained by the comparison method specified in ASTM E112 (2013), and its average value is used as the original austenite grain size of each seamless steel pipe. At this time, 100 times is used as the reference observation magnification, and 200 times or 400 times is set according to the grain size. When the observation magnification is set to 200 times or 400 times, correction is performed in accordance with ASTM E112 (2013) using the correction value Q defined by the following formula (I).

[0210] Q = 6.64 log 10 (M / 100)···(I)

[0211] Wherein, M in the above formula is the observation magnification.

[0212] Next, the mechanical properties, diameter reduction workability, and hydrogen embrittlement resistance of each seamless steel pipe were evaluated by the following methods.

[0213] Mechanical properties

[0214] A tubular test piece of a certain length was cut from each seamless steel pipe to produce a No. 11 test piece in accordance with JIS Z 2241: 2011. The No. 11 test piece was then subjected to a tubular tensile test specified in JIS Z 2241: 2011 to measure tensile strength TS, yield stress YS, and elongation at break EL.

[0215] <Diameter reduction workability>

[0216] Two 300 mm long tubular test pieces were cut from each seamless steel pipe. One end of each tubular test piece was then subjected to diameter reduction processing at a reduction ratio of 0.60 or 0.50, forming a 30 mm long reduced diameter portion. The reduction ratio in diameter reduction processing refers to the value obtained by dividing the outer diameter of the reduced diameter portion by the outer diameter of the seamless steel pipe before diameter reduction processing. The reduced diameter portion was then inspected for cracks.

[0217] Furthermore, if no cracks occurred under both working ratios of 0.60 and 0.50, the diameter reduction workability was judged to be extremely excellent (EX). Furthermore, if cracks occurred under a working ratio of 0.50 but not under a working ratio of 0.60, the diameter reduction workability was judged to be excellent (G). On the other hand, if cracks occurred under both working ratios of 0.60 and 0.50, the diameter reduction workability was judged to be poor (NA).

[0218] Hydrogen embrittlement resistance

[0219] Collected from various seamless steel pipes Figure 1 The arc-shaped tensile test pieces of the shape shown were subjected to a cathodic charge constant load test. Specifically, the cathodic charge constant load test was conducted at a potential ranging from -0.9 to -1.2 V while immersing the arc-shaped tensile test pieces, each with a gripping portion and a tapered portion, in various aqueous solutions containing 3% NaCl and ammonium thiocyanate in a range of 0 to 30 g / L. A stress of 90% of the tensile strength of each seamless steel pipe was applied.

[0220] Then, only the test pieces with a durability time exceeding 200 hours were stored in liquid nitrogen. The parallel sections of the tapered portion were then cut and the hydrogen concentration was measured using temperature-programmed desorption hydrogen analysis. In temperature-programmed desorption hydrogen analysis, the test piece was heated from room temperature to 200°C at a heating rate of 100°C / hour, and the amount of hydrogen released was measured to determine the hydrogen concentration in the test piece. The maximum value of the obtained hydrogen concentrations was designated as the limiting hydrogen concentration (Hc), which was used as an indicator of hydrogen embrittlement resistance. In this example, when Hc was 2.5 ppm or higher, it was judged that the hydrogen embrittlement resistance was excellent.

[0221] Table 3 summarizes the above evaluation results.

[0222] [Table 3]

[0223] Table 3

[0224]

[0225] As shown in Table 3, Test Nos. 1 to 22, which met all the requirements of the present invention, showed high tensile strength, excellent diameter reduction workability, and excellent hydrogen embrittlement resistance. In contrast, Test Nos. 23 to 44, comparative examples that did not meet the requirements of the present invention, showed deteriorated diameter reduction workability and hydrogen embrittlement resistance.

[0226] Industrial applicability

[0227] According to the present invention, a seamless steel pipe having high strength, excellent diameter reduction workability, and excellent hydrogen embrittlement resistance can be obtained. Therefore, the seamless steel pipe of the present invention is suitable as a material for airbag inflators.

Claims

1. A seamless steel pipe, Its chemical composition is calculated by mass% as C: 0.05-0.20%, Si: 0.05-0.50%, Mn: 0.30-1.50%, P: 0.025% or less, S: 0.020% or less, Cu: 0.01~0.50%, Ni: 0.01~0.50%, Cr：0.01～1.20％、 Mo: 0.01~0.50%, Ti: 0.001~0.050%, Nb: 0.001~0.100%, Ca: 0.0005~0.0025%, Al: 0.080% or less, N: 0.0100% or less, V：0～0.100％、 B：0～0.0050％、 Mg: 0~0.0050%, REM: 0~0.0050% Sn: 0~0.100%, As: 0~0.010%, Balance: Fe and impurities, Assuming that the content of each element is within the above range, The relationship between the chemical composition and the wall thickness satisfies the following formula (i): Furthermore, the relationship between the chemical composition and the prior austenite grain size satisfies the following formula (ii), the tensile strength is 1000 MPa or more, Elongation at break is more than 8.0%, The limit hydrogen concentration is 2.5ppm or above. WT / (5C+Mo+Cr)≥1.00···(i) GN-1.51×(Mn+85P-30Ca)≥8.50···(ii) in, The element symbols in the above formula represent the content of each element in steel, in mass %, and are recorded as zero when not contained. In addition, WT represents the wall thickness of the seamless steel pipe, in mm, and GN represents the prior austenite grain size.

2. The seamless steel pipe according to claim 1, wherein The chemical composition contains, in mass%, V：0.001～0.100％、 B：0.0001～0.0050％、 Mg: 0.0001~0.0100%, REM: 0.0001~0.0100% Sn: 0.001~0.100%, and As: one or more kinds of 0.001 to 0.010%.

3. An airbag inflator cylinder comprising: a cylindrical portion extending in one direction, and a reduced diameter portion formed on at least one end side of the cylindrical portion in the one direction, The chemical composition of the cylindrical portion is C: 0.05-0.20% by mass, Si: 0.05-0.50%, Mn: 0.30-1.50%, P: 0.025% or less, S: 0.020% or less, Cu: 0.01~0.50%, Ni: 0.01~0.50%, Cr：0.01～1.20％、 Mo: 0.01~0.50%, Ti: 0.001~0.050%, Nb: 0.001~0.100%, Ca: 0.0005~0.0025%, Al: 0.080% or less, N: 0.0100% or less, V：0～0.100％、 B：0～0.0050％、 Mg: 0~0.0050%, REM: 0~0.0050% Sn: 0~0.100%, As: 0~0.010%, Balance: Fe and impurities, Assuming that the content of each element is within the above range, The relationship between the chemical composition of the cylindrical portion and the wall thickness of the cylindrical portion satisfies the following formula (i): Furthermore, the relationship between the chemical composition of the cylindrical portion and the prior austenite grain size of the cylindrical portion satisfies the following formula (ii): The tensile strength of the cylindrical portion is 1000 MPa or more. The elongation at break of the cylindrical portion is 8.0% or more, The limit hydrogen concentration of the cylindrical portion is 2.5 ppm or more, WT / (5C+Mo+Cr)≥1.00···(i) GN-1.51×(Mn+85P-30Ca)≥8.50···(ii) in, The element symbols in the above formula represent the content of each element in the steel of the cylindrical portion, and the unit is mass %, and when not contained, it is recorded as zero; In addition, WT represents the wall thickness of the cylindrical portion, and the unit is mm, and GN represents the prior austenite grain size of the cylindrical portion.

4. The airbag inflator according to claim 3, wherein: The chemical composition of the cylindrical portion contains, in mass %, a V：0.001～0.100％、 One or more of B: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0100%, REM: 0.0001 to 0.0100%, Sn: 0.001 to 0.100%, and As: 0.001 to 0.010%.

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