Seamless steel pipes and airbag inflator bottles

Abstract

Disclosed is a seamless steel pipe which has the chemical composition set forth in the description, wherein: the relationship between the chemical composition and the wall thickness WT (mm) satisfies WT / (5C + Mo + Cr) ≥ 1.00; and the relationship between the chemical composition and the prior γ grain size GN satisfies GN - 1.51 × (Mn + 85P - 30Ca) ≥ 8.50. This seamless steel pipe has a tensile strength of 1,000 MPa or more, an elongation at break of 8.0% or more, and a limiting hydrogen concentration of 2.5 ppm or more.

Description

[Technical field]

[0001] The present invention relates to a seamless steel pipe and an inflator bottle for an airbag. [Background technology]

[0002] The automobile industry is actively introducing devices that pursue safety. Among these, airbag systems are being installed in vehicles, which use gas or other materials to inflate an airbag between the occupant and the steering wheel or instrument panel before the occupant hits them during a collision, absorbing the kinetic energy of the occupant and reducing injury. Conventionally, airbag systems have used explosive chemicals, but from the perspective of environmental recyclability, systems that use high-pressure gas filling have been developed and are becoming more widely used.

[0003] The above system constantly keeps the gas, etc., that is blown into the airbag at high pressure during a collision, and then releases the gas all at once at the time of the collision. Therefore, the steel pipes for the airbag are subjected to stress at a high strain rate in an extremely short period of time. Therefore, the steel pipes used are required to have high strength and excellent burst resistance.

[0004] Recently, there has been an increasing demand for lighter automobiles. From this perspective, there is also a demand for thinner and lighter steel pipes for in-vehicle airbags. To ensure high burst pressure even with a thin wall, inflator bottles made from high-strength seamless steel pipes with a tensile strength of 900 MPa or more have come to be used in airbag systems.

[0005] Furthermore, for example, when manufacturing inflator bottles and the like, a diameter reduction process is performed, so that steel pipes for airbags are required to have excellent diameter reduction processability.

[0006] Against this background, for example, seamless steel pipes for airbags are disclosed in Patent Documents 1 and 2. According to Patent Documents 1 and 2, improvements in strength, toughness, and workability are investigated. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2004-27303 A [Patent Document 2] JP 2004-76034 A Summary of the Invention [Problem to be solved by the invention]

[0008] However, in order to ensure higher reliability as a steel pipe for airbags, it is necessary to suppress embrittlement caused by hydrogen that penetrates into the steel pipe during the manufacturing process and in the usage environment. Patent Documents 1 and 2 do not consider the hydrogen embrittlement resistance of the steel pipe at all.

[0009] An object of the present invention is to provide a seamless steel pipe and an inflator bottle for an airbag which have high strength, excellent diameter reduction workability, and excellent resistance to hydrogen embrittlement. [Means for solving the problem]

[0010] The present invention has been made to solve the above problems, and its gist is the seamless steel pipe and airbag inflator bottle described below.

[0011] (1) Chemical composition, in mass%, C: 0.05-0.20%, Si: 0.05 to 0.50%, Mn: 0.30-1.50%, P: 0.025% or less, S: 0.020% or less, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Cr: 0.01 to 1.20%, Mo: 0.01 to 0.50%, Ti: 0.001 to 0.050%, Nb: 0.001 to 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 to 0.0050%, REM: 0~0.0050%, Sn: 0~0.100%, As: 0~0.010%, The balance is Fe and impurities. Assuming that the content of each of the elements is within the above-mentioned range, The chemical composition satisfies the following formula (i) in relation to the wall thickness: Furthermore, the chemical composition satisfies the following formula (ii) in relation to the prior austenite grain size: The tensile strength is 1000 MPa or more, The elongation at break is 8.0% or more, The limit hydrogen concentration is 2.5 ppm or more. Seamless steel pipe. WT / (5C+Mo+Cr)≧1.00 (i) GN-1.51×(Mn+85P-30Ca)≧8.50 (ii) In the above formula, the element symbols indicate the content (mass%) of each element in the steel, and when the element is not contained, it is set to zero. In addition, WT indicates the wall thickness (mm) of the seamless steel pipe, and GN indicates the prior austenite grain size.

[0012] (2) The chemical composition is, in mass%, V: 0.001 to 0.100%, B: 0.0001~0.0050%, Mg: 0.0001 to 0.0100%, REM: 0.0001~0.0100%, Sn: 0.001 to 0.100%, and As: 0.001 to 0.010%, Contains one or more selected from A seamless steel pipe as described in (1) above.

[0013] (3) 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, in mass%, C: 0.05-0.20%, Si: 0.05 to 0.50%, Mn: 0.30-1.50%, P: 0.025% or less, S: 0.020% or less, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Cr: 0.01 to 1.20%, Mo: 0.01 to 0.50%, Ti: 0.001 to 0.050%, Nb: 0.001 to 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 to 0.0050%, REM: 0~0.0050%, Sn: 0~0.100%, As: 0~0.010%, The balance is Fe and impurities. Assuming that the content of each of the elements is within the above-mentioned range, The chemical composition of the cylindrical portion satisfies the following formula (i) in relation to the wall thickness of the cylindrical portion: Furthermore, the chemical composition of the cylindrical portion satisfies the following formula (ii) in relation to the prior austenite grain size of the cylindrical portion: The tensile strength of the cylindrical portion is 1000 MPa or more, The breaking elongation of the cylindrical portion is 8.0% or more, The limit hydrogen concentration of the cylindrical portion is 2.5 ppm or more. Airbag inflator bottle. WT / (5C+Mo+Cr)≧1.00 (i) GN-1.51×(Mn+85P-30Ca)≧8.50 (ii) In the above formula, the element symbols indicate the content (mass%) of each element in the steel in the cylindrical portion, and when the element is not contained, it is set to zero. In addition, WT indicates the wall thickness (mm) of the cylindrical portion, and GN indicates the prior austenite grain size in the cylindrical portion.

[0014] (4) The chemical composition of the cylindrical portion is, in mass%, V: 0.001 to 0.100%, B: 0.0001~0.0050%, Mg: 0.0001 to 0.0100%, REM: 0.0001~0.0100%, Sn: 0.001 to 0.100%, and As: 0.001 to 0.010%, Contains one or more selected from The airbag inflator bottle according to (3) above. Effect of the Invention

[0015] According to the present invention, it is possible to obtain a seamless steel pipe and an inflator bottle for an air bag which have high strength, excellent diameter reduction workability, and excellent resistance to hydrogen embrittlement. [Brief description of the drawings]

[0016] [Figure 1] FIG. 2 is a diagram for explaining the shape of an arc-shaped tensile test specimen used in measuring the limiting hydrogen concentration. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] The present inventors have conducted extensive research into a method for achieving both strength and diameter reduction workability of a seamless steel pipe while also ensuring hydrogen embrittlement resistance. As a result, they have obtained the following findings.

[0018] (a) In order to realize high strength seamless steel pipes, it is necessary to increase the content of elements that improve hardenability. In particular, it is effective to ensure sufficient contents of C, Mo, and Cr. However, if the contents of these elements are excessive relative to the wall thickness of the seamless steel pipe, it becomes difficult to ensure diameter reduction workability. From this perspective, it is important to control the balance between the chemical composition and wall thickness of the seamless steel pipe, and specifically, the following formula (i) is satisfied. WT / (5C+Mo+Cr)≧1.00 (i)

[0019] (b) In addition, in order to ensure the diameter reduction processability, it is also important to improve the ductility, and specifically, the breaking elongation needs to be 8.0% or more. In addition to controlling the chemical composition, it is possible to achieve the above breaking elongation by manufacturing under appropriate conditions.

[0020] (c) Excessive Mn content not only reduces the hydrogen diffusion rate, causing localized concentration, but also generates MnS, which leads to deterioration of hydrogen embrittlement resistance. P also segregates at grain boundaries, which deteriorates hydrogen embrittlement resistance. On the other hand, Ca has the effect of suppressing the generation of MnS, improving hydrogen embrittlement resistance.

[0021] (d) The inventors' investigation revealed that the degree of deterioration of hydrogen embrittlement resistance varies depending on the prior austenite grain size. As a result of evaluating the effects of the contents of Mn, P, and Ca and the prior austenite grain size GN on hydrogen embrittlement resistance, it was found that excellent hydrogen embrittlement resistance can be obtained by adjusting the contents of each element within a predetermined range and satisfying the following formula (ii). GN-1.51×(Mn+85P-30Ca)≧8.50 (ii)

[0022] (e) Furthermore, in order to improve hydrogen embrittlement resistance, it is necessary to preheat the steel during the tempering process. Although the mechanism by which preheating improves hydrogen embrittlement resistance is not clear, it is believed that this is because the temperature distribution in the thickness direction is eliminated and the metal structure becomes uniform.

[0023] The present invention has been made based on the above findings. Each of the features of the present invention will be described in detail below.

[0024] (A) Chemical composition The reasons for limiting the chemical composition of the seamless steel pipe according to one embodiment of the present invention are as follows: In the following description, "%" for the content of each element means "mass %".

[0025] C: 0.05 to 0.20% C is an effective element for increasing the strength of steel at low cost. If the C content is less than 0.05%, it is difficult to obtain the desired tensile strength, and if it exceeds 0.20%, workability and weldability decrease. Therefore, the C content is set to 0.05 to 0.20%. The C content is preferably in the range of 0.06% to 0.18%, more preferably 0.07% to 0.17%. When diameter reduction workability is particularly important, the C content is more preferably less than 0.17%.

[0026] Silicon: 0.05 to 0.50% Silicon is an element that not only has a deoxidizing effect, but also improves the hardenability of steel and increases its strength. For this purpose, the silicon content is set to 0.05% or more. However, if the silicon content exceeds 0.50%, the toughness decreases, so the silicon content is set to 0.50% or less. The preferred range of silicon content is 0.10% or more and 0.40% or less, and the more preferred range is 0.15% or more and 0.30% or less.

[0027] Mn: 0.30-1.50% Mn has a deoxidizing effect and is an effective element for improving the hardenability of steel and improving strength and toughness. However, if the Mn content is less than 0.30%, sufficient strength and toughness cannot be obtained. On the other hand, if the Mn content exceeds 1.50%, coarsening of MnS occurs, which is elongated during hot rolling, and the toughness and hydrogen embrittlement resistance properties decrease. In addition, excessive Mn reduces the hydrogen diffusion rate and causes localized concentration, resulting in a decrease in hydrogen embrittlement resistance properties. For this reason, the Mn content is set to 0.30 to 1.50%. The Mn content is preferably 0.40% or more and 1.20% or less, and more preferably 0.50% or more and 1.00% or less.

[0028] P:0.025% or less P is contained in steel as an impurity, and causes a decrease in toughness and hydrogen embrittlement resistance due to grain boundary segregation. In particular, when 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.

[0029] S: 0.020% or less S is also contained in steel as an impurity, and reduces the toughness of the steel pipe, particularly in the T direction (the direction perpendicular to the axial direction of the steel pipe). If the S content exceeds 0.020%, the toughness of the steel pipe in the T direction decreases significantly, so the S content is set to 0.020% or less. The preferred S content is 0.010% or less.

[0030] Cu: 0.01 to 0.50% Cu improves the strength and toughness by increasing the hardenability of steel. This effect is manifested if the Cu content is 0.01% or more. However, if the Cu content exceeds 0.50%, the alloy cost increases. Therefore, the Cu content is set to 0.01 to 0.50%. The Cu content is preferably 0.05% or more, more preferably 0.10% or more, and further preferably 0.20% or more. The Cu content is preferably 0.40% or less, and more preferably 0.35% or less.

[0031] Ni: 0.01 to 0.50% Ni enhances the hardenability of steel, thereby improving strength and toughness. This effect is manifested when the Ni content is 0.01% or more. However, if the Ni content exceeds 0.50%, the alloy cost increases. Therefore, the Ni content is set to 0.01 to 0.50%. The Ni content is preferably 0.05% or more, more preferably 0.10% or more, and further preferably 0.20% or more. The Ni content is preferably 0.45% or less, and more preferably 0.40% or less.

[0032] Cr: 0.01~1.20% Cr improves the hardenability of steel and also improves temper softening resistance, thereby improving strength and toughness. This effect is manifested if the Cr content is 0.01% or more. However, if the Cr content exceeds 1.20%, the alloy cost increases. Therefore, the Cr content is set to 0.01 to 1.20%. The Cr content is preferably 0.05% or more, more preferably 0.10% or more, and further preferably 0.20% or more. The Cr content is preferably 1.00% or less, and more preferably 0.90% or less.

[0033] Mo: 0.01 to 0.50% Mo improves the hardenability of steel and also improves temper softening resistance, thereby improving strength and toughness. This effect is manifested if the Mo content is 0.01% or more. However, if the Mo content exceeds 0.50%, the alloy cost increases. In addition, if the Mo content is excessively high, the strength tends to increase even in air cooling after hot pipe making of seamless steel pipe, and softening heat treatment is required before cold drawing, which increases the manufacturing cost. Therefore, the Mo content is set to 0.01 to 0.50%. The Mo content is preferably 0.05% or more, more preferably 0.10% or more, and further preferably 0.20% or more. The Mo content is preferably 0.45% or less, more preferably 0.40% or less.

[0034] Ti: 0.001 to 0.050% Ti fixes N in steel and improves toughness. Finely dispersed Ti nitrides strongly pin grain boundaries, refine grains, and improve the toughness of steel. To obtain this effect, it is necessary to contain 0.001% or more. However, if it is contained in excess of 0.050%, the nitrides become coarse and the toughness decreases. Therefore, the Ti content is set to 0.001 to 0.050%. The Ti content is preferably 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.

[0035] Nb: 0.001 to 0.100% Nb is finely dispersed as carbides in steel and strongly pins the grain boundaries. This has the effect of refining the grains and improving the toughness of steel. In order to obtain this effect, it is necessary to contain 0.001% or more, but if it is contained in excess of 0.100%, the carbides become coarse and the toughness decreases. Therefore, the Nb content is set to 0.001 to 0.100%. The Nb content is preferably 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, more preferably 0.060% or less.

[0036] Ca: 0.0005~0.0025% Ca fixes S, which is present as an inevitable impurity in steel, as sulfides, improves the anisotropy of toughness, and increases the toughness of the steel pipe in the T-direction, thereby improving burst resistance. In addition, it also contributes to improving hydrogen embrittlement resistance by suppressing the generation of MnS. This effect is manifested if the Ca content is 0.0005% or more. However, if it is contained in excess of 0.0025%, the inclusions increase, and the toughness decreases. Therefore, the Ca content is set to 0.0005 to 0.0025%. In order to reliably obtain the effect of improving hydrogen embrittlement resistance, the Ca content is preferably 0.0010% or more, more preferably more than 0.0010%, even more preferably 0.0012% or more, and even more preferably 0.0015% or more.

[0037] Al: 0.080% or less Al has a deoxidizing effect and is an effective element for improving toughness and workability. However, if the content exceeds 0.080%, the occurrence of defects becomes significant. Therefore, the Al content is set to 0.080% or less. The Al content is preferably 0.060% or less, and more preferably 0.040% or less. Since the Al content may be at the impurity level, the lower limit is not particularly set, but it is preferably 0.005% or more. The Al content in the present invention refers to the content of acid-soluble Al (so-called "sol. Al").

[0038] N: 0.0100% or less N forms fine nitrides, which strongly pin the grain boundaries, refine the grains, and improve the toughness of the steel. However, if the content exceeds 0.0100%, the nitrides become coarse, and the toughness decreases. 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. Since the N content may be at the impurity level, the lower limit is not particularly set, but it is preferably 0.0005% or more, and more preferably 0.0010% or more.

[0039] V: 0~0.100% V is an element that ensures toughness and increases strength by precipitation strengthening, so it may be contained as necessary. However, if it is contained in an amount exceeding 0.100%, it will cause a decrease in toughness. Therefore, if it is contained, 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 effect of V can be observed even in small amounts, but to obtain a sufficient effect, it is preferable to contain 0.001% or more.

[0040] B: 0~0.0050% B is an element that segregates at grain boundaries in steel when added in small amounts and significantly improves the hardenability of steel, so it may be included as necessary. However, if more than 0.0050% of B is included, boride tends to precipitate coarsely at grain boundaries, decreasing toughness. Therefore, the B content, if included, is set to 0.0050% or less. The B content is preferably 0.0030% or less, more preferably 0.0020% or less. Although the effect of B can be observed even in small amounts, it is preferable to include 0.0001% or more, and more preferably 0.0005% or more to ensure a sufficient effect.

[0041] Magnesium: 0 to 0.0050% Like Ca, Mg is an element that fixes S, which is present as an inevitable impurity in steel, as sulfides, improves the anisotropy of toughness, and increases the toughness in the T-direction of the steel pipe, thereby increasing the burst resistance, so it may be contained as necessary. However, if it is contained in an amount exceeding 0.0050%, the inclusions increase, and the toughness decreases. Therefore, the Mg content when contained is set to 0.0050% or less. The Mg content is preferably 0.0040% or less, and more preferably 0.0030% or less. The effect of Mg is observed even in a small amount, but to ensure a sufficient effect, it is preferable to contain 0.0001% or more, and more preferably 0.0005% or more.

[0042] REM: 0 to 0.0050% Like Ca, REM is an element that fixes S, which is present as an inevitable impurity in steel, as sulfide, improves the anisotropy of toughness, increases the toughness in the T-direction of the steel pipe, and thereby increases the burst resistance, so it may be contained as necessary. However, if it is contained in an amount exceeding 0.0050%, the inclusions increase and the toughness decreases. Therefore, when REM is contained, the content is set to 0.0050% or less. The REM content is preferably 0.0040% or less, more preferably 0.0030% or less. The action of REM is observed even in a small amount, but to ensure a sufficient effect, it is preferable to contain 0.0001% or more, and more preferably 0.0005% or more.

[0043] In this embodiment, "REM" refers to Sc, Y, and lanthanoids, a total of 17 elements, and "REM content" refers to the content of one type of REM when there is one type, and the total content of two or more types when there are two or more types. REM is also generally supplied as misch metal, which is an alloy of multiple types of REM. For this reason, one or more individual elements may be added and contained, or they may be added in the form of misch metal, for example.

[0044] Sn: 0 to 0.100% Sn has the effect of improving corrosion resistance, so it may be contained as necessary. However, if Sn is contained in excess, it will lead to a decrease in toughness. Therefore, the Sn content is 0.100% or less. The preferred Sn content is 0.080% or less, and more preferably 0.060% or less. Although the effect of Sn is observed even in a small amount, in order to ensure a sufficient effect, it is preferable to contain 0.001% or more, and more preferably 0.003% or more.

[0045] As: 0 to 0.010% As has the effect of improving corrosion resistance, so it may be contained as necessary. However, if As is contained in excess, it will cause a decrease in hot workability. Therefore, the As content is 0.010% or less. The As content is preferably 0.008% or less, and more preferably 0.006% or less. Although the effect of As is observed even in small amounts, it is preferable to contain 0.001% or more, and more preferably 0.002% or more to ensure a sufficient effect.

[0046] The seamless steel pipe according to this embodiment contains the above-mentioned elements, with the balance being Fe and impurities. Here, the term "impurities" refers to components that are mixed in due to various factors in raw materials such as ores and scraps and manufacturing processes during the industrial production of steel materials, and are permissible within a range that does not adversely affect the present invention.

[0047] The chemical composition of the seamless steel pipe according to this embodiment satisfies the following formula (i) in relation to the wall thickness, on the premise that the content of each element is within the above-mentioned range. As described above, by ensuring a sufficient content of C, Mo and Cr, the hardenability is improved, and it is possible to realize high strength of the seamless steel pipe. However, from the viewpoint of diameter reduction workability, it is necessary to adjust the balance between the chemical composition and the wall thickness of the seamless steel pipe. By satisfying the following formula (i), it is possible to ensure diameter reduction workability. The value of the left side of the following formula (i) is preferably 1.20 or more, more preferably 1.50 or more, and even more preferably 2.00 or more. WT / (5C+Mo+Cr)≧1.00 (i) In the above formula, the element symbols indicate the content (mass%) of each element in the steel, and when no element is contained, it is set to zero. Also, WT indicates the wall thickness (mm) of the seamless steel pipe.

[0048] Furthermore, the chemical composition of the seamless steel pipe according to this embodiment satisfies the following formula (ii) in relation to the prior austenite grain size, provided that the contents of each element are within the above-mentioned ranges. By adjusting the contents of Mn and P, which deteriorate hydrogen embrittlement resistance, and Ca, which improves hydrogen embrittlement resistance, in accordance with the prior austenite grain size, it is possible to obtain excellent hydrogen embrittlement resistance. The value of the left side of the following formula (ii) is preferably 9.00 or more, more preferably 9.50 or more, and even more preferably 10.00 or more. GN-1.51×(Mn+85P-30Ca)≧8.50 (ii) In the above formula, the element symbols represent the content (mass%) of each element in the steel, and when no element is contained, the value is set to zero. GN represents the prior austenite grain size.

[0049] The prior austenite grain size is measured in accordance with ASTM E112 (2013). Specifically, a test piece is taken including the entire thickness of the seamless steel pipe so that the surface including the pipe axis direction and the wall thickness direction (hereinafter also referred to as the "longitudinal section") becomes the test surface (hereinafter referred to as the "observation surface"), and the observation surface is mirror-polished. After polishing, a picral etching solution is used to reveal the prior austenite grain boundaries in the observation surface.

[0050] Then, using an optical microscope, five visual fields are observed so that the 1 / 4 position of the wall thickness from the outer surface of the seamless steel pipe is the center of the visual field. Then, the prior austenite grain size of each visual field is obtained by the comparison method specified in ASTM E112 (2013), and the average value is the prior austenite grain size of the seamless steel pipe. In this case, the standard observation magnification is 100 times, and 200 times or 400 times is used depending on the grain size. When the observation magnification is 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). Q=6.64log 10 (M / 100) (I) In the above formula, M is the observation magnification.

[0051] The prior austenite grain size is not particularly limited as long as it satisfies the above formula (ii), and can be, for example, 10.0 or more, or 11.0 or more.

[0052] (B) Thickness The wall thickness of the seamless steel pipe according to this embodiment is not particularly limited as long as it satisfies the above formula (i), and can be, for example, 1.00 mm or more or 1.50 mm or more. On the other hand, from the viewpoint of weight reduction, a thinner wall is preferable, and the wall thickness is preferably 2.60 mm or less, more preferably less than 2.50 mm, and even more preferably 2.40 mm or less. In general, the thinner the wall, the more difficult the diameter reduction process is. 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 the diameter reduction processability even in the case of a thin wall.

[0053] (C)Characteristics The seamless steel pipe according to the present embodiment has high strength, specifically, a tensile strength of 1000 MPa or more. If the tensile strength is 1000 MPa or more, the pipe exhibits excellent burst resistance even when used as an inflator bottle for an airbag, which is subjected to stress at a high strain rate in an extremely short period of time.

[0054] As described above, in order to increase the strength, it is necessary to increase the content of elements that improve hardenability, which increases the risk of reducing the diameter. When the diameter reduction is important, the tensile strength is preferably less than 1200 MPa.

[0055] Moreover, the seamless steel pipe according to this embodiment has excellent ductility in order to ensure the diameter reduction workability, specifically, the breaking elongation is 8.0% or more, preferably 9.0% or more, and more preferably 10.0% or more.

[0056] The tensile strength and breaking elongation are measured in accordance with JIS Z 2241:2011. Specifically, a tubular test piece of a certain length is cut out from a seamless steel pipe to prepare a No. 11 test piece in accordance with JIS Z 2241:2011. The No. 11 test piece is then used to perform a tubular tensile test as specified in JIS Z 2241:2011 to measure the tensile strength and breaking elongation.

[0057] Furthermore, the seamless steel pipe according to this embodiment has excellent resistance to hydrogen embrittlement, specifically, the limit hydrogen concentration is 2.5 ppm or more. This makes it possible to ensure high reliability when used as a steel pipe for airbags, etc. It is more preferable that the limit hydrogen concentration is 2.7 ppm or more. In this embodiment, the limit hydrogen concentration is specifically determined by the following method.

[0058] A plurality of arc-shaped tensile test pieces having the shape shown in FIG. 1 are taken from a seamless steel pipe. The arc-shaped tensile test pieces are cut out from the seamless steel pipe to have 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. The arc-shaped tensile test pieces are produced by cutting out 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 the seamless steel pipe, and then providing a reduced width portion in the center in the longitudinal direction while leaving a gripped portion at each end in the longitudinal direction, and further providing a U-notch in the center in the longitudinal direction of the reduced width portion. The gripped portions are each 45 mm long and 9.0 mm wide, and the reduced width portion is 30 mm long and 2.0 mm wide. In addition, both ends of the reduced width portion are curved with a curvature radius of 5.0 mm and connected to the gripped portions. Furthermore, the U-notch has a notch width of 0.20 mm, a notch depth of 0.35 mm, and a notch bottom radius of 0.10 mm.

[0059] Next, a cathodic charge constant load test is performed on multiple arc-shaped tensile test pieces while immersing them in various aqueous solutions containing 3% NaCl and 0 to 30 g / L of ammonium thiocyanate at a potential in the range of -0.9 to -1.2 V. At this time, a stress of 90% of the tensile strength of each seamless steel pipe is applied.

[0060] After that, only the arc-shaped tensile test specimens that have a durability of more than 200 hours are stored in liquid nitrogen, and the parallel part of the reduced width is cut to prepare test specimens for hydrogen concentration measurement, and the hydrogen concentration is measured by thermal desorption hydrogen analysis. In thermal desorption hydrogen analysis, the test specimens for hydrogen concentration measurement are heated from room temperature to 200°C at a heating rate of 100°C / hour, and the amount of hydrogen released is measured to determine the hydrogen concentration in the test specimen. The highest hydrogen concentration obtained is then regarded as the limiting hydrogen concentration.

[0061] (D) Airbag inflator bottle The airbag inflator bottle according to one embodiment of the present invention comprises a cylindrical portion extending in one direction and a reduced diameter portion formed on at least one end side in the one direction of the cylindrical portion. The reduced diameter portion may be formed on both ends in the one direction of the cylindrical portion.

[0062] The airbag inflator bottle according to this embodiment is manufactured by cutting the seamless steel pipe described above to a predetermined length, and then subjecting one or both ends to diameter reduction processing. Therefore, the chemical composition, prior austenite grain size, wall thickness, and properties of the cylindrical portion are the same as those of the seamless steel pipe from which it is made, and therefore a description thereof will be omitted.

[0063] In addition, the reduced diameter portion has the same or greater strength and thickness as the raw seamless steel pipe as a result of the reduced diameter processing. In other words, if the raw seamless steel pipe has high strength and excellent hydrogen embrittlement resistance, the airbag inflator bottle manufactured from the seamless steel pipe will also have high strength and excellent hydrogen embrittlement resistance.

[0064] (E) Manufacturing method A seamless steel pipe according to one embodiment of the present invention can be produced by the following method.

[0065] The steel having the chemical composition described in the above item (A) is melted by a conventional method and then cast into an ingot or a slab. Alternatively, the slab may be cast into a circular billet shape for pipe production by the so-called "round CC" method.

[0066] In the next step, the cast ingot or slab is subjected to blooming or hot forging. This step is to obtain a material to be used for the final hot pipe production (for example, pipe production by hot piercing, rolling and drawing processes, or pipe production by hot extrusion press). Note that the slab shaped like a circular billet by the above-mentioned "Round CC" method can be directly used to finish a seamless steel pipe, so blooming or hot forging is not necessarily required.

[0067] The material to be used for the final hot pipe production produced by the above-mentioned blooming or hot forging, or a cast piece in the shape of a circular billet (hereinafter collectively referred to as "steel billet"), is subjected to a hot pipe production process, a cold working process, a quenching process, and a tempering process in that order to produce the seamless steel pipe of this embodiment.

[0068] <Hot pipe making process> After the above-mentioned steel slab is heated, hot pipe making is performed to produce a mother pipe having a predetermined shape. A general method may be used for hot pipe making, and for example, the Mandrel-Mannesmann process may be adopted. The heating temperature of the steel slab may be, for example, 1000 to 1300°C.

[0069] <Cold working process> The mother pipe obtained by the above method is subjected to cold working for the purpose of improving the dimensional accuracy. The cold working method is not particularly limited as long as it can uniformly work the mother pipe, and for example, it is industrially advantageous to use a so-called cold drawing machine using a perforated die and a plug or a cold rolling machine called a cold pilger mill.

[0070] <Quenching process> The cold-worked blank pipe is then subjected to induction hardening, which involves induction heating to a temperature of 900 to 1050°C and then quenching. If the heating temperature is less than 900°C, austenitization is not completed, and high strength may not be achieved. On the other hand, if the heating temperature exceeds 1050°C, austenite grains grow rapidly and become coarse, making it impossible to achieve excellent toughness.

[0071] Furthermore, rapid heating by high-frequency heating suppresses the growth of austenite grains, resulting in a fine metal structure. From the viewpoint of suppressing the growth of austenite grains, the holding time at the above heating temperature is preferably 10 seconds or less, depending on the size of the mother tube. The heating temperature refers to the temperature at the outer surface of the mother tube. For rapid cooling, an appropriate method such as water cooling or oil cooling may be used as long as a sufficient hardened structure can be obtained.

[0072] <Tempering process> The induction-hardened blank pipe is subjected to a tempering treatment in which it is heated to 370 to 410°C and then cooled to room temperature. If the tempering temperature is less than 370°C, the strength can be ensured but the ductility and low-temperature toughness are reduced. In particular, if the 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, the strength is reduced and it becomes impossible to obtain a tensile strength of 1000 MPa or more even if excellent ductility and low-temperature toughness are obtained.

[0073] The holding time at the above heating temperature is preferably 10 to 30 minutes, depending on the size of the mother pipe. This heating temperature refers to the temperature at the outer surface of the mother pipe. There are no particular limitations on the cooling rate during tempering. Therefore, cooling may be performed according to the equipment, such as natural cooling in the air, forced air cooling, mist cooling, oil cooling, water cooling, etc.

[0074] In order to obtain excellent hydrogen embrittlement resistance, it is necessary to preheat the steel before heating it to the above heating temperature. Specifically, the steel is preheated so that the retention time in the temperature range of 250 to 350°C is 5 minutes or more. As mentioned above, it is believed that preheating eliminates the temperature distribution in the thickness direction and makes the metal structure uniform.

[0075] As described above, the airbag inflator bottle according to this embodiment is manufactured by cutting the seamless steel pipe manufactured by the above-mentioned method to a predetermined length, and then subjecting one or both ends of the pipe to diameter reduction. The cutting and diameter reduction may be performed by any known method.

[0076] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. EXAMPLES

[0077] Steels having the chemical compositions shown in Tables 1-1 and 1-2 were melted and cast into rectangular billets by a converter-continuous casting process. The rectangular billets were then formed into circular billets by hot forging and cooled to room temperature.

[0078] [Table 1-1]

[0079] [Table 1-2]

[0080] The circular billet was heated, and a blank pipe was produced by the Mannesmann mandrel method, and cooled to room temperature. The blank pipe obtained was cold worked using a cold drawing machine so as to have the outer diameter and wall thickness shown in Table 2. Then, quenching and tempering were performed under the conditions shown in Table 2 to produce seamless steel pipes. The preheating time in Table 2 means the residence time in the temperature range of 250 to 350°C. All quenching was performed by high-frequency heating followed by water quenching, and the cooling rate was adjusted to be 150°C / sec. All cooling during tempering was performed by natural cooling in the atmosphere.

[0081] [Table 2]

[0082] First, the prior austenite grain size of each seamless steel pipe obtained was measured. The prior austenite grain size was measured in accordance with ASTM E112 (2013). Specifically, a test piece including the entire wall thickness was taken so that the longitudinal section of the seamless steel pipe was the observation surface, and the observation surface was mirror-polished. After polishing, the prior austenite grain boundaries in the observation surface were revealed using a picral etching solution. Then, using an optical microscope, observation was performed in five fields so that the position of 1 / 4 of the wall thickness from the outer surface of the seamless steel pipe was the center of the field of view. Then, the prior austenite grain size of each field was obtained by the comparison method specified in ASTM E112 (2013), and the average value was taken as the prior austenite grain size of each seamless steel pipe. In this case, the standard observation magnification was 100 times, and depending on the grain size, it was 200 times or 400 times. In addition, when the observation magnification was 200x or 400x, correction was performed in accordance with ASTM E112 (2013) using the correction value Q defined by the following formula (I). Q=6.64log 10 (M / 100) (I) In the above formula, M is the observation magnification.

[0083] Next, the mechanical properties, diameter reduction workability, and hydrogen embrittlement resistance of each seamless steel pipe were evaluated by the following methods.

[0084] <Mechanical properties> A tubular test piece of a certain length was cut out from each seamless steel pipe to prepare a No. 11 test piece conforming to JIS Z 2241:2011. The No. 11 test piece was then used to carry out a tubular tensile test as specified in JIS Z 2241:2011 to measure the tensile strength TS, yield stress YS, and breaking elongation EL.

[0085] <Diameter reduction workability> Two tubular test pieces with a length of 300 mm were cut out from each seamless steel pipe, and one end of each tubular test piece was subjected to diameter reduction processing under conditions where the reduction ratio was 0.60 and 0.50, forming a reduced diameter section with a length of 30 mm. Here, the reduction ratio in the diameter reduction processing is the value obtained by dividing the outer diameter of the reduced diameter section by the outer diameter of the seamless steel pipe before the diameter reduction processing. The presence or absence of cracks in the reduced diameter section was then observed.

[0086] If no cracks occurred under both the working ratios of 0.60 and 0.50, the diameter reduction workability was judged to be extremely excellent (EX). If cracks occurred under the working ratio of 0.50 but not under the working ratio of 0.60, the diameter reduction workability was judged to be excellent (G). On the other hand, if cracks occurred under both the working ratios of 0.60 and 0.50, the diameter reduction workability was judged to be poor (NA).

[0087] <Hydrogen embrittlement resistance> From each seamless steel pipe, an arc-shaped tensile test specimen having the shape shown in Fig. 1 was taken, and a cathodic charge constant load test was carried out. Specifically, a cathodic charge constant load test was carried out at a potential in the range of -0.9 to -1.2 V while immersing multiple arc-shaped tensile test specimens having a gripped portion and a reduced width portion in various aqueous solutions containing 3% NaCl and ammonium thiocyanate in the range of 0 to 30 g / L. At this time, a stress of 90% of the tensile strength of each seamless steel pipe was applied.

[0088] Then, only for the test pieces whose durability exceeded 200 hours, the parallel portion of the reduced width was cut after storage in liquid nitrogen, and the hydrogen concentration was measured by thermal desorption hydrogen analysis. In the thermal 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 highest value of the hydrogen concentrations obtained was taken as the limit hydrogen concentration (Hc) and was used as an index of hydrogen embrittlement resistance. In this embodiment, when Hc was 2.5 ppm or more, it was determined that the hydrogen embrittlement resistance was excellent.

[0089] Table 3 shows the results of each of the above evaluations.

[0090] [Table 3]

[0091] As shown in Table 3, test numbers 1 to 22, which satisfy all the requirements of the present invention, had high tensile strength and excellent diameter reduction workability, and also had excellent hydrogen embrittlement resistance. In contrast, test numbers 23 to 44, which are comparative examples that do not satisfy the requirements of the present invention, showed poor diameter reduction workability or hydrogen embrittlement resistance. [Industrial Applicability]

[0092] According to the present invention, it is possible to obtain a seamless steel pipe having high strength, excellent diameter reduction workability, and further excellent resistance to hydrogen embrittlement. Therefore, the seamless steel pipe according to the present invention is suitable as a material for an inflator bottle for an airbag.

Claims

1. The chemical composition, in mass%, is C: 0.05-0.20%, Si: 0.05-0.50%, Mn: 0.30 to 1.50%, P: 0.025% or less, S: 0.020% or less, Cu: 0.01 to 0.50%, Ni: 0.01-0.50%, Cr: 0.01-1.20%, Mo: 0.01-0.50%, Ti: 0.001 to 0.050%, Nb: 0.001 to 0.100%, Ca: 0.0005-0.0025%, Al: 0.080% or less, N: 0.0100% or less, V: 0-0.100%, B: 0 to 0.0050%, Mg: 0 to 0.0050%, REM: 0 to 0.0050%, Sn: 0-0.100%, As: 0 to 0.010%, The balance is Fe and impurities. Assuming that the content of each of the elements is within the above-mentioned range, The chemical composition satisfies the following formula (i) in relation to the wall thickness: Furthermore, the chemical composition satisfies the following formula (ii) in relation to the prior austenite grain size: The tensile strength is 1000 MPa or more, The breaking elongation is 8.0% or more, The limit hydrogen concentration is 2.5 ppm or more. Seamless steel pipe. WT / (5C+Mo+Cr)≧1.00...(i) GN-1.51×(Mn+85P-30Ca)≧8.50...(ii) In the above formula, the element symbols indicate the content (mass%) of each element in the steel, and when the element is not contained, it is set to zero. In addition, WT indicates the wall thickness (mm) of the seamless steel pipe, and GN indicates the prior austenite grain size.

2. The chemical composition, in mass%, V: 0.001-0.100%, B: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050%, REM: 0.0001-0.0050%, Sn: 0.001 to 0.100%, and As: 0.001 to 0.010%, Contains one or more selected from The seamless steel pipe according to claim 1.

3. 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, in mass%, C: 0.05-0.20%, Si: 0.05-0.50%, Mn: 0.30 to 1.50%, P: 0.025% or less, S: 0.020% or less, Cu: 0.01 to 0.50%, Ni: 0.01-0.50%, Cr: 0.01-1.20%, Mo: 0.01-0.50%, Ti: 0.001 to 0.050%, Nb: 0.001 to 0.100%, Ca: 0.0005-0.0025%, Al: 0.080% or less, N: 0.0100% or less, V: 0-0.100%, B: 0 to 0.0050%, Mg: 0 to 0.0050%, REM: 0 to 0.0050%, Sn: 0-0.100%, As: 0 to 0.010%, The balance is Fe and impurities. Assuming that the content of each of the elements is within the above-mentioned range, The chemical composition of the cylindrical portion satisfies the following formula (i) in relation to the wall thickness of the cylindrical portion: Furthermore, the chemical composition of the cylindrical portion satisfies the following formula (ii) in relation to the prior austenite grain size of the cylindrical portion: The tensile strength of the cylindrical portion is 1000 MPa or more, The breaking elongation of the cylindrical portion is 8.0% or more, The limit hydrogen concentration of the cylindrical portion is 2.5 ppm or more. Airbag inflator bottle. WT / (5C+Mo+Cr)≧1.00...(i) GN-1.51×(Mn+85P-30Ca)≧8.50...(ii) In the above formula, the element symbols indicate the content (mass%) of each element in the steel in the cylindrical portion, and when the element is not contained, the content is set to zero. In addition, WT indicates the wall thickness (mm) of the cylindrical portion, and GN indicates the prior austenite grain size in the cylindrical portion.

4. The chemical composition of the cylindrical portion is, in mass%, V: 0.001-0.100%, B: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050%, REM: 0.0001-0.0050%, Sn: 0.001 to 0.100%, and As: 0.001 to 0.010%, Contains one or more selected from 4. The air bag inflator bottle according to claim 3.

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