JP6648647B2 - Low alloy steel material, low alloy steel pipe and container, and method of manufacturing the container - Google Patents

Description

  The present invention relates to a low alloy steel material, a low alloy steel pipe and a container, and a method for manufacturing the container. More specifically, the present invention relates to a low-alloy steel material, a low-alloy steel pipe and a container for high-pressure hydrogen having excellent properties in a high-pressure hydrogen environment, and a method for manufacturing the container.

  2. Description of the Related Art In recent years, development of a fuel cell vehicle that runs on hydrogen as a fuel and research on practical use of a hydrogen station that supplies hydrogen to the fuel cell vehicle has been advanced. Until now, low alloy steels have been mainly used in pressure accumulators (hereinafter, collectively referred to as "tanks" described below and also referred to as "containers") installed in hydrogen stations. The conventional 45 MPa class hydrogen station mainly uses JIS SCM435 steel containing Cr and Mo. In particular, in a large-sized thick-walled container, 1.6 to 2.0% by mass of Ni is added in addition to Cr and Mo. The contained SNCM439 steel is commonly used.

  As a low alloy steel having excellent mechanical properties and fatigue resistance properties in a high-pressure hydrogen gas environment, Patent Document 1 discloses a tensile alloy containing Cr, Mo, and V in addition to 0.10 to 0.20 mass% of C. A low alloy steel having a strength of 900 to 950 MPa is disclosed. Patent Literature 2 discloses a low alloy steel having a tensile strength of 900 to 950 MPa in which Ni is further added to the low alloy steel disclosed in Patent Literature 1. Patent Literature 3 discloses a low alloy steel containing 0.15 to 0.60 mass% of C, Mo and V, and having a tensile strength of 900 MPa or more. Patent Document 4 contains 0.05 to 0.12% by mass of C, and Patent Document 5 contains 0.05 to 0.15% by mass of C, mainly containing a bainite structure. A low alloy steel is disclosed. Patent Literature 6 discloses a low-alloy steel containing a specific amount of Mn, Cr, Mo, and V, having a tensile strength of 900 MPa or more, and suitable for a thick container exceeding 12 mm in thickness. Patent Document 7 discloses a steel material containing Mn, Ti, Nb, V, and the like, and having excellent fatigue crack propagation resistance in a high-pressure hydrogen environment.

JP 2009-46737 A JP 2009-275249 A JP 2009-74122 A JP 2012-107332 A JP 2012-107333 A JP 2014-173160 A International Publication No. WO 2014/156187

  As a fuel cell vehicle that is being commercialized, for example, a vehicle equipped with a tank that can be filled with 70 MPa of hydrogen at a higher pressure than 45 MPa in order to secure a cruising distance equivalent to a gasoline vehicle ( Hereinafter, "70 MPa class fuel cell vehicle" is required. A hydrogen station that supplies hydrogen to a 70 MPa class fuel cell vehicle is required to be able to store hydrogen at a higher pressure than the hydrogen tank.

By the way, high pressure vessels are classified into the following four types.
・ Type I: Metal container ・ Type II: Metal liner ・ Hoop-wrapped container ・ Type III: Metal liner ・ All-around container ・ Type IV: Non-metal liner ・ All-around container

  From the viewpoint of strength and durability against embrittlement due to high-pressure hydrogen gas (hereinafter referred to as “hydrogen gas embrittlement”), Type III is used for a 70 MPa class fuel cell vehicle and a container installed in a hydrogen station for the purpose. A composite container having an inner metal liner reinforced with carbon fibers or a Type IV composite container having an inner plastic liner reinforced with carbon fibers is used. Austenitic stainless steel (SUS316) or an aluminum alloy is used for the inner metal liner of the Type III composite container.

  However, composite containers of Type III and Type IV are very expensive. For this reason, as a container for high-pressure hydrogen of the 70 MPa class, a metal container of Type I, which is inexpensive as compared with the above-described composite container, particularly made of low alloy steel is desired.

  Patent Documents 1 to 7 disclose low alloy steels, low alloy steel materials excellent in hydrogen gas embrittlement resistance, and containers made of such low alloy steels. In particular, for a thick-walled container as disclosed in Patent Document 6, it is effective to use steel containing a small amount of B (boron), which is effective for improving hardenability. However, when these low-alloy steel containers are applied to hydrogen stations and fuel cell vehicles, it is necessary to design a life based on the fatigue life, so that good and stable fatigue resistance is required.

In normal atmospheric fatigue tests, each material has an upper limit of its own fatigue limit (stress showing infinite fatigue life (ie, stress that does not cause fatigue failure). (Stress amplitude), which is の of the difference between the two.) The symbols “σ _W ” and “σ _a ” are used for “fatigue limit” and “stress amplitude” in the fatigue test, respectively. In many cases, the working stress condition is set based on a “durability ratio” which is a quotient obtained by dividing the above-mentioned fatigue limit (σ _W ) by the tensile strength (σ _B ) of the material.

In the case of low alloy steel, the durability ratio (σ _W / σ _B ) is usually about 0.4 to 0.5. On the other hand, as will be described later, a detailed study by the present inventors has revealed for the first time that in a high-pressure hydrogen gas environment, the fatigue life may be extremely reduced even under a stress condition equal to or lower than the durability ratio. For this reason, when low alloy steel is used in a high-pressure hydrogen gas environment, it has been extremely difficult to design a life based on the durability ratio.

  The present invention enables a life design based on the durability ratio under a high-pressure hydrogen gas environment, has a high tensile strength of 850 MPa or more and a durability ratio of 0.45 or more, and can be used under a high-pressure hydrogen gas environment. High pressure hydrogen container having excellent fatigue resistance and good hydrogen gas embrittlement resistance, low alloy steel for high pressure hydrogen and low alloy steel pipe for high pressure hydrogen suitable for use as a material for the container, and production of the container The aim is to provide a method.

  The present invention has been completed in order to solve the above-mentioned problems, and the gist of the present invention is to provide a low-alloy steel material, a low-alloy steel pipe and a container, and a method for manufacturing the container, both of which are described below for high-pressure hydrogen. is there.

(1) In mass%,
C: 0.20 to 0.60%,
Si: 0.05 to 1.0%,
Mn: 0.35 to 3.0%,
P: 0.025% or less,
S: 0.010% or less,
Al: 0.005 to 0.10%,
B: 0.0003-0.01%,
Ti: 0 to 0.5%,
Zr: 0 to 1.0%,
Hf: 0 to 2.0%,
O: 0.005% or less,
N: 0.008% or less Cr: 0 to 5.0%,
Mo: 0 to 1.5%,
V: 0 to 1.0%,
W: 0 to 3.0%,
Nb: 0 to 0.1%,
Ta: 0 to 0.2%,
Ni: 0 to 5.0%,
Cu: 0 to 3.0%,
Co: 0 to 3.0%,
Ca: 0 to 0.01%,
Mg: 0 to 0.01%,
REM: 0 to 0.50%,
The balance: Fe and impurities, and
Fn1 represented by the following formula [1] is 0.008 to 0.05, and
Fn2 represented by the following formula [2] is 0.0003 or more;
Has a chemical composition,
The number of carbonitride-based inclusions having a particle size of 10 μm or more is 10/100 mm ² or less in cross-sectional observation,
The durability ratio is 0.45 or more;
Low alloy steel for high pressure hydrogen.
Fn1 = Ti + 0.5Zr + 0.25Hf [1]
Fn2 = B− (11/14) N + (11/48) Ti + (11/91) Zr + (11/178) Hf [2]
However, Ti, Zr, Hf, B and N in the above formula mean the contents (% by mass) of the respective elements in the steel, and Ti, Zr and Hf are zero when they are not contained.

(2) The chemical composition is represented by mass%
Cr: 0.1-5.0%,
Mo: 0.1-1.5%,
V: 0.01 to 1.0%,
W: 0.01 to 3.0%,
Nb: 0.001 to 0.1%,
Ta: 0.001 to 0.2%,
Ni: 0.1 to 5.0%,
Cu: 0.1 to 3.0%;
Co: 0.1-3.0%,
Ca: 0.0001-0.01%,
Mg: 0.0001 to 0.01%, and
REM: 0.0001-0.50%,
The low-alloy steel material for high-pressure hydrogen according to the above (1), comprising at least one member selected from the group consisting of:

(3) the prior austenite crystal grains have an ASTM particle size number of 9.0 or more;
The low-alloy steel material for high-pressure hydrogen according to (1) or (2).

(4) The low-alloy steel material for high-pressure hydrogen according to any one of (1) to (3) above,
Low alloy steel pipe for high pressure hydrogen.

(5) The low-alloy steel pipe for high-pressure hydrogen described in (4) above,
A tensile strength of 850 MPa or more;
High pressure hydrogen container.

(6) A method for producing the high-pressure hydrogen container according to the above (5),
After forming the low-alloy steel pipe for high-pressure hydrogen into a predetermined shape, after heating and holding at 880 to 950 ° C, quenching is performed at an average cooling rate of 2 ° C / sec or more in a temperature range of 800 to 500 ° C, Then, tempering,
Manufacturing method of high-pressure hydrogen container.

  According to the present invention, it is possible to design a life based on a durability ratio under a high-pressure hydrogen gas environment, have a high tensile strength of 850 MPa or more, and have excellent fatigue resistance characteristics under a high-pressure hydrogen gas environment. A container for high-pressure hydrogen having good hydrogen gas embrittlement resistance, a low-alloy steel material for high-pressure hydrogen and a low-alloy steel tube for high-pressure hydrogen suitable for use as a material of the container can be obtained. Further, according to the method of the present invention, such a high-pressure hydrogen container can be stably obtained.

Using the steels A and W described in the examples, a fatigue test in a normal temperature atmosphere and a fatigue test in a high-pressure hydrogen gas at a normal temperature of 90 MPa were performed, and the horizontal axis represents the fatigue life (unit: times), and the vertical axis. The ratio “σ _a / σ _B ” between the stress amplitude (σ _a ) (unit: MPa) and the tensile strength (σ _B ) (unit: MPa) is used to determine the durability ratio (σ _{W) in} a high-pressure hydrogen gas environment. / Σ _B ) is a diagram showing an example in which the fatigue life is extremely reduced under the stress condition of: Note that the data of steel A is a white circle and a white triangle, and the data of steel W is a black triangle.

Using various low alloy steel materials, the present inventors varied the stress amplitude (σ _a ) in various ways and found that the ratio “σ _a / σ _B ” to the tensile strength (σ _B ) at room temperature. The effect on the fatigue life (in the atmosphere and high-pressure hydrogen gas environment, the fatigue life (the symbol “N _f ” is used as the number of repetitions of stress until fatigue fracture occurs) was examined. As a result, it has been clarified that in a high-pressure hydrogen gas environment, the fatigue life (N _f ) may be extremely reduced even under stress conditions equal to or lower than the durability ratio (σ _W / σ _B ).

FIG. 1 shows an example of a case where the fatigue life (N _f ) is extremely reduced under a high pressure hydrogen gas environment under a stress condition that provides a durability ratio (σ _W / σ _B ). In FIG. 1, plots of “in the atmosphere” with white circles and “in hydrogen” with white triangles are detailed data of Test No. 1 using steel A in Examples described later. The plot of “in hydrogen (inclusions)” is the data of test No. 23 using steel W in the examples (see Tables 1 and 2 of the examples).

In FIG. 1, the horizontal axis is the fatigue life (N _f ) (unit: times), and the vertical axis is the ratio between the stress amplitude (σ _a ) (unit: MPa) and the tensile strength (σ _B ) (unit: MPa). “Σ _a / σ _B ”.

As shown by the white circles, in the fatigue test in the atmosphere, the fatigue life (N _f ) on the horizontal axis changes according to “σ _a / σ _B ” on the vertical axis, but the predetermined “σ _a / σ _B ” In the following, fatigue failure does not occur, resulting in an infinite fatigue life (hereinafter, referred to as “infinite life”). In the example of FIG. 1, it has been abort the test ¹⁰⁷ times, was interpreted as "infinite life" 10 ⁷ times endurance criteria. The upper limit of the stress amplitude (σ _a ) at which the infinite life is reached is the fatigue limit (σ _W ), and the ratio “σ _W / σ _B ” between the (σ _W ) and the tensile strength (σ _B ) is the durability ratio. become. The durability ratio of the low alloy steel is usually about 0.4 to 0.5, and is 0.5 in FIG.

On the other hand, as indicated by white triangles, in the fatigue test in high-pressure hydrogen gas, under the stress condition where the stress amplitude (σ _a ) is larger than the fatigue limit (σ _W ), the influence of hydrogen causes the fatigue test in the atmosphere. In comparison, the fatigue life (N _f ) decreases. However, there is almost no effect of hydrogen on the durability ratio, and in the case of FIG. 1, the durability ratio is 0.5 as in the atmosphere. Therefore, even in a high-pressure hydrogen gas environment, if the members are designed under stress conditions equal to or lower than the durability ratio, the same life design as in the atmosphere should be possible. However, as indicated by black triangles, in a fatigue test in high-pressure hydrogen gas, even under a stress condition equal to or lower than the durability ratio, fatigue failure may occur with an extremely short life.

The present inventors have investigated the cause of a decrease in fatigue life (N _f ) under stress conditions of a durability ratio or lower in a high-pressure hydrogen gas environment of a low-alloy steel, as indicated by black triangles in FIG. As a result, it became clear that the phenomenon was caused by fatigue fracture starting from carbonitride-based inclusions.

  Therefore, a further detailed investigation yielded the following important findings.

Carbonitride-based inclusions that act as starting points of fatigue fracture under stress conditions below the durability ratio and cause a significant reduction in fatigue life (N _f ) have a particle size of 10 μm or more. However, if the number of coarse carbonitride-based inclusions having a particle size of 10 μm or more is 10/100 mm ² or less in the cross-sectional observation, the fatigue life a shown in the black triangle in FIG. Since a regular decrease does not occur, a good fatigue life can be secured. The “particle size” of the above-mentioned inclusions means that the longest dimension of each inclusion is a major axis a (μm), the direction perpendicular to the major axis is the width, and the maximum value of the width is the minor axis b (μm) In this case, it indicates a value of (a × b) ^0.5 .

  It is widely known that, under high cycle conditions, which is a problem with bearing steel and the like, fatigue fracture starting from inclusions occurs. However, under the low cycle condition supposed to be used as a material for high-pressure hydrogen, the above-described fatigue fracture from the inclusion starting point does not occur in the atmosphere. Therefore, the fatigue fracture starting from the coarse carbonitride-based inclusions described above is a peculiar phenomenon in a high-pressure hydrogen gas environment, and the influence of hydrogen trapped around the inclusions and the size of the inclusions themselves This is considered to be a fracture phenomenon in which (stress concentration) is superimposed. In the case of ordinary steel, coarse carbonitride-based inclusions are rarely generated. However, in the case of steel containing B for improving hardenability, Ti, Zr or Hf having a strong nitride-forming ability is contained in order to suppress the formation of B nitride (BN) and secure solid solution B. It is necessary that Ti, Zr and Hf form coarse carbonitride-based inclusions (hereinafter, sometimes referred to as “Ti-Zr-Hf carbonitride-based inclusions”).

Further, by increasing the durability ratio (σ _W / σ _B ), in other words, by increasing the fatigue limit (σ _W ) as much as possible, there is an advantage that the member can be used under higher stress conditions. The durability ratio is strongly influenced by the homogeneity of the base tissue. From this viewpoint, even in the case of a material for high-pressure hydrogen, the refinement of the prior austenite crystal grains is effective for improving the durability ratio, and the desirable prior austenite crystal grains are preferably at least 9.0 in ASTM grain size number. found. Furthermore, in the case of production by quenching and tempering, the higher the fraction of the martensite phase (hereinafter, simply referred to as "martensite ratio"), the more homogeneous the structure becomes, and the higher the durability ratio. It was also confirmed that it was desirable to secure a site rate.

  The present invention has been completed based on the above contents. Hereinafter, each requirement of the present invention will be described in detail.

1. Chemical composition:
The reasons for limiting the chemical composition of the low-alloy steel material for high-pressure hydrogen, the low-alloy steel tube for high-pressure hydrogen, and the container for high-pressure hydrogen according to the present invention are as follows. In the following description, “%” for the content of each element means “% by mass”.

C: 0.20 to 0.60%
C is an element effective for improving the hardenability of steel, and needs to be contained at 0.20% or more. On the other hand, the effect is saturated even if C is excessively contained in excess of 0.60%. The risk of quenching during quenching increases. Therefore, the content of C is set to 0.20 to 0.60%. A preferred lower limit of the C content is 0.25%, and a more preferred lower limit is 0.30%. A preferred upper limit of the C content is 0.50%, and a more preferred upper limit is 0.40%.

Si: 0.05 to 1.0%
Si is an element effective for deoxidizing steel. To obtain a deoxidizing effect, the Si content needs to be 0.05% or more. On the other hand, when the Si content exceeds 1.0%, the precipitation of a ferrite phase, which is a soft phase, is promoted, and mechanical properties such as toughness are reduced. For this reason, the content of Si is set to 0.05 to 1.0%. A preferred lower limit of the Si content is 0.10%, and a more preferred lower limit is 0.15%. A preferred upper limit of the Si content is 0.80%, and a more preferred upper limit is 0.60%.

Mn: 0.35 to 3.0%
Mn is an element effective for improving the hardenability of steel. To obtain this effect, it is necessary to contain at least 0.35% of Mn. On the other hand, even if Mn is contained in excess of 3.0%, the effect is saturated. For this reason, the content of Mn is set to 0.35 to 3.0%. A preferred lower limit of the Mn content is 0.40%, and a more preferred lower limit is 0.50%. A preferred upper limit of the Mn content is 2.0%, and a more preferred upper limit is 1.5%.

P: not more than 0.025% P segregates at the crystal grain boundaries and lowers toughness and hydrogen gas embrittlement resistance. For this reason, the P content needs to be limited to 0.025% or less. It is desirable that the P content be as small as possible.

S: 0.01% or less S also segregates at the grain boundary similarly to P, and decreases toughness and hydrogen gas embrittlement resistance. Further, S generates sulfide-based inclusions and reduces the fatigue life in a high-pressure hydrogen gas environment. Therefore, the S content needs to be 0.01% or less. The S content is preferably as small as possible, and the upper limit of the content is desirably 0.005%, more desirably 0.003%.

Al: 0.005 to 0.10%
Al is an element effective for deoxidizing steel. The effect cannot be obtained if the Al content is less than 0.005%. On the other hand, even if the content of Al exceeds 0.10%, the effect is saturated. Therefore, the content of Al is set to 0.005 to 0.10%. A preferred lower limit of the Al content is 0.01%, and a more preferred lower limit is 0.02%. A preferred upper limit of the Al content is 0.07%, and a more preferred upper limit is 0.05%. The Al content in the present invention refers to the content in acid-soluble Al (“Sol. Al”).

B: 0.0003-0.01%
B is an element that is effective for enhancing the hardenability of steel with a small amount of inclusion, increasing the martensite ratio and improving the durability ratio in the atmosphere and in hydrogen. .0003% or more. On the other hand, even if B is contained in an amount exceeding 0.01%, the above effect is saturated. Therefore, the B content is set to 0.0003 to 0.01%. However, as for the content of B, in order to secure sufficient hardenability, Fn2 represented by the above-mentioned formula [2], which is an index of solid solution B effectively acting on hardenability, satisfies 0.0003 or more. Must be quantity.

Ti: 0 to 0.5%
Ti, like Zr and Hf, is one of the elements having strong ability to form carbonitride, and forms carbonitride by combining with N (nitrogen) and C in steel. Since B is the element having the next highest nitride forming ability after Ti, Zr and Hf, when solid solution N remains, B forms nitride (BN) and forms a solid solution which effectively works for quenching. The amount of B decreases. Therefore, Ti is contained in order to prevent the formation of BN. However, even if Ti is contained in an amount exceeding 0.5%, the above effect is saturated and the material cost is increased. Therefore, the upper limit of the Ti content is set to 0.5%. In order to sufficiently secure the effect of improving the hardenability of B, the Fn1 and Fn2 represented by the above formulas [1] and [2] are 0.008 or more and 0.0003, respectively. The amount must also satisfy the above. Further, the content of Ti is set to Fn1 represented by the above-mentioned formula [1] in order to suppress the generation of a large amount of coarse carbonitride-based inclusions which are the starting points of fatigue fracture in a high-pressure hydrogen gas environment. Must be less than 0.05.

Zr: 0 to 1.0%
Zr, like Ti and Hf, is one of the elements having strong carbonitride forming ability, and combines with N and C in steel to form carbonitride. Since B is the element having the next highest nitride forming ability after Ti, Zr and Hf, when solid solution N remains, B forms nitride (BN) and forms a solid solution which effectively works for quenching. The amount of B decreases. Therefore, in order to prevent the formation of BN, Zr is contained. However, even if Zr is contained in an amount exceeding 1.0%, the above effect is saturated and the material cost is increased. Therefore, the upper limit of the Zr content is set to 1.0%. The content of Zr is such that Fn1 and Fn2 represented by the above formulas [1] and [2] are 0.008 or more and 0.0003, respectively, in order to sufficiently secure the effect of improving the hardenability of B. The amount must also satisfy the above. Further, the content of Zr is set to Fn1 represented by the above-mentioned formula [1] in order to suppress the generation of a large amount of coarse carbonitride-based inclusions which are the starting points of fatigue fracture in a high-pressure hydrogen gas environment. Must be less than 0.05.

Hf: 0 to 2.0%
Hf, like Ti and Zr, is one of the elements having a strong carbonitride forming ability, and forms carbonitride by combining with N and C in steel. Since B is the element having the next highest nitride forming ability after Ti, Zr and Hf, when solid solution N remains, B forms nitride (BN) and forms a solid solution which effectively works for quenching. The amount of B decreases. Therefore, Hf is contained in order to prevent the formation of BN. However, even if Hf is contained in an amount exceeding 2.0%, the above effect is saturated and the material cost is increased. For this reason, the upper limit of the Hf content is set to 1.0%. In order to sufficiently secure the effect of improving the hardenability of B, the content of Hf should be 0.008 or more and 0.0003 or more for Fn1 and Fn2 represented by the above formulas [1] and [2], respectively. The amount must also satisfy the above. Further, the content of Hf is set to Fn1 represented by the above-mentioned formula [1] in order to suppress the generation of a large amount of coarse carbonitride-based inclusions which are the starting points of fatigue fracture in a high-pressure hydrogen gas environment. Must be less than 0.05.

  As for Ti, Zr and Hf, Fn1 represented by the above formula [1] satisfies 0.008 to 0.05, and Fn2 represented by the above formula [2] is 0.0003. If the above is satisfied, it is not necessary to contain two or more kinds in combination, and they may be contained alone.

O (oxygen): 0.005% or less O is present in steel as an impurity, and when its content exceeds 0.005%, forms a coarse oxide, mechanical properties such as toughness and high pressure hydrogen gas. Reduces fatigue life in the environment. Therefore, the O content is set to 0.005% or less. The O content is preferably as small as possible, and the upper limit of the content is preferably 0.004%, more preferably 0.003%.

N (nitrogen): 0.008% or less N is present in steel as an impurity, and when the content exceeds 0.008%, coarse nitrides are formed, and mechanical properties such as toughness and high-pressure hydrogen gas environment Decrease the fatigue life. Furthermore, a large amount of N forms BN, and reduces the effect of improving the hardenability of B. Therefore, the N content is set to 0.008% or less. The N content is preferably as low as possible, and the upper limit of the content is preferably 0.006%, and more preferably 0.005%.

Cr: 0 to 5.0%
Cr has the effect of generating undissolved fine carbides during quenching, refining crystal grains, and improving the durability ratio (in other words, fatigue limit) in high-pressure hydrogen gas. Further, Cr also has an effect of improving the hardenability of steel. Therefore, Cr may be contained as necessary. However, even if Cr is contained in an amount exceeding 5.0%, the above-described effect is saturated and the material cost is increased. Therefore, the upper limit of the Cr content when it is contained is set to 5.0%. The upper limit of the Cr content is preferably 3.0%, and more preferably 1.2%. In order to stably obtain the above effects, the lower limit of the Cr content is preferably 0.1%, and more preferably 0.3%.

Mo: 0 to 1.5%
Mo has the effect of generating undissolved fine carbides during quenching, refining the crystal grains, and improving the durability ratio in high-pressure hydrogen gas. Mo also has the effect of improving the hardenability of steel. For this reason, Mo may be contained as needed. However, even if Mo is contained in an amount exceeding 1.5%, the above effect is saturated and the material cost is increased. Therefore, when Mo is contained, the upper limit of the Mo content is set to 1.5%. The upper limit of the Mo content is preferably 1.0%, and more preferably 0.8%. In order to stably obtain the above effects, the lower limit of the Mo content is preferably 0.1%, and more preferably 0.2%.

V: 0 to 1.0%
V has the effect of generating undissolved fine carbides during quenching, refining the crystal grains, and improving the durability ratio in high-pressure hydrogen gas. V also has the effect of improving the hardenability of steel. Therefore, V may be contained as necessary. However, even if V is contained in an amount exceeding 1.0%, the above effect is saturated and the material cost is increased. Therefore, the upper limit of the V content when it is contained is set to 1.0%. The upper limit of the V content is preferably 0.7%, and more preferably 0.5%. In order to stably obtain the above-described effects, the lower limit of the V content is preferably 0.01%, and more preferably 0.03%.

W: 0-3.0%
W has the effect of producing undissolved fine carbides during quenching, making the crystal grains finer, and improving the durability ratio in high-pressure hydrogen gas. W also has the effect of improving the hardenability of steel. For this reason, W may be contained as needed. However, even if V is contained in an amount exceeding 3.0%, the above effect is saturated and the material cost is increased. Therefore, the upper limit of the W content when it is contained is set to 3.0%. The upper limit of the W content is preferably 2.0%, and more preferably 1.5%. In order to stably obtain the above effects, the lower limit of the W content is preferably 0.01%, and more preferably 0.1%.

  When two or more selected from the above-mentioned Cr, Mo, V and W are combined and contained, the total amount is preferably 5.0% or less.

Nb: 0 to 0.1%
Nb has the effect of producing undissolved fine carbides during quenching, making crystal grains finer, and improving the durability ratio in high-pressure hydrogen gas. In addition, Nb is effective for fixing N since it binds with Ti, Zr and Hf to form carbonitride, and contributes to the effect of improving the hardenability of B. Therefore, Nb may be contained as needed. However, even if Nb is contained in an amount exceeding 0.1%, the above-described effect is saturated and the material cost is increased. Therefore, the upper limit of the Nb content when it is contained is set to 0.1%. The upper limit of the Nb content is preferably 0.08%, and more preferably 0.05%. In order to stably obtain the above effects, the lower limit of the Nb content is preferably 0.001%, and more preferably 0.005%.

Ta: 0 to 0.2%
Ta has the effect of generating undissolved fine carbides during quenching, making crystal grains finer, and improving the durability ratio in high-pressure hydrogen gas. Further, Ta bonds with Ti, Zr and Hf to form a carbonitride, so that Ta is effective for fixing N and contributes to the effect of improving the hardenability of B. For this reason, Ta may be contained as needed. However, even if Ta is contained in an amount exceeding 0.2%, the above-described effect is saturated and the material cost is increased. Therefore, the upper limit of the Ta content when it is contained is set to 0.2%. The upper limit of the Ta content is preferably 0.1%, more preferably 0.05%. In order to stably obtain the above-mentioned effects, the lower limit of the Ta content is preferably 0.001%, and more preferably 0.005%.

  The total amount of the above-mentioned Nb and Ta when combined is preferably 0.2% or less.

Ni: 0 to 5.0%
Ni is an element that improves the hardenability of steel. For this reason, Ni may be contained as needed. However, if the content of Ni exceeds 5.0%, the production cost increases greatly. For this reason, the upper limit of the Ni content when it is contained is 5.0%. The upper limit of the Ni content is preferably 3.0%, and more preferably 1.0%. In order to stably obtain the above effects, the lower limit of the Ni content is preferably 0.1%, and more preferably 0.3%.

Cu: 0 to 3.0%
Cu is an element that improves the hardenability of steel. Therefore, Cu may be contained as necessary. However, when the Cu content exceeds 3.0%, the hot workability decreases. Therefore, the upper limit of the Cu content when it is contained is set to 3.0%. The upper limit of the Cu content is preferably 2.0%, and more preferably 1.0%. In order to stably obtain the above effects, the lower limit of the Cu content is preferably 0.1%, and more preferably 0.3%.

Co: 0 to 3.0%
Co is an element that improves the hardenability of steel. For this reason, you may make it contain Co as needed. However, when the Co content exceeds 3.0%, the production cost increases greatly. Therefore, the upper limit of the Co content when it is contained is set to 3.0%. The upper limit of the Co content is preferably 2.0%, and more preferably 1.0%. In order to stably obtain the above effects, the lower limit of the Co content is preferably 0.1%, and more preferably 0.3%.

  When two or more selected from the above Ni, Cu and Co are combined and contained, the total amount is preferably 5.0% or less.

Ca: 0 to 0.01%
Ca combines with S in steel to form sulfides, and has the effect of improving the shape of inclusions and improving mechanical properties such as toughness. For this reason, Ca may be contained as necessary. However, even if Ca is contained in an amount exceeding 0.01%, such an effect is saturated. For this reason, the upper limit of the Ca content when it is contained is set to 0.01%. The upper limit of the Ca content is preferably 0.005%, more preferably 0.003%. In order to stably obtain the above-described effects, the lower limit of the Ca content is preferably 0.0001%, and more preferably 0.0003%.

Mg: 0 to 0.01%
Mg combines with S in steel to form a sulfide, and has the effect of improving the shape of inclusions and improving mechanical properties such as toughness. Therefore, Mg may be contained as necessary. However, even if Mg is contained in an amount exceeding 0.01%, such an effect is saturated. For this reason, the upper limit of the Mg content when it is contained is set to 0.01%. The upper limit of the Mg content is preferably 0.005%, more preferably 0.003%. In order to stably obtain the above-described effects, the lower limit of the Mg content is preferably 0.0001%, and more preferably 0.0003%.

REM: 0-0.50%
REM combines with S in steel to form sulfides and has the effect of improving the shape of inclusions and improving mechanical properties such as toughness. For this reason, REM may be contained as needed. However, even if REM is contained in excess of 0.50%, such an effect is saturated. For this reason, the upper limit of the REM content when it is contained is set to 0.50%. The upper limit of the REM content is preferably 0.40%, more preferably 0.30%. In order to stably obtain the above effects, the lower limit of the REM content is preferably 0.0001%, more preferably 0.0005%.

  In the present invention, “REM” refers to a total of 17 elements of Sc, Y, and lanthanoid, and “REM content” means the content of REM when one kind is used, and the content when two or more kinds are used. Refers to the total content. REM is also generally supplied as misch metal, which is an alloy of a plurality of types of REM. For this reason, one or two or more individual elements may be added so that the amount of REM is within the above range, or, for example, by adding in the form of misch metal, the amount of May be contained.

  When two or more kinds selected from the above Ca, Mg and REM are combined and contained, the total amount is preferably 0.50% or less.

  The low-alloy steel material for high-pressure hydrogen, the low-alloy steel tube for high-pressure hydrogen, and the container for high-pressure hydrogen according to the present invention include the above-described elements, the balance being Fe and impurities, and represented by the above formula [1]. It has a chemical composition in which Fn1 is 0.008 to 0.05 and Fn2 represented by the above formula [2] is 0.0003 or more.

  Here, “impurities” are components that are mixed in due to various factors in the ore, scrap and other raw materials and the production process when industrially producing a steel material, and are allowable as long as they do not adversely affect the present invention. Means what is done.

Fn1: 0.008 or more and 0.05 or less Fn1 represented by the following formula [1] is 0.008 for the low-alloy steel material for high-pressure hydrogen, the low-alloy steel tube for high-pressure hydrogen, and the container for high-pressure hydrogen according to the present invention. Above is 0.05 or less.
Fn1 = Ti + 0.5Zr + 0.25Hf [1]
However, Ti, Zr and Hf in the formula [1] mean the contents (% by mass) of the respective elements in steel, and are zero when they are not contained.

  When Fn1 is less than 0.008, solid solution N remains in the steel, B forms a nitride (BN), and the amount of solid solution B effective for quenching decreases, so that the hardenability of B The improvement effect may not be sufficiently obtained. On the other hand, even if Fn1 contains Ti, Zr and Hf in an amount exceeding 0.05, the effect of improving the hardenability of B is saturated, and coarse Ti— which becomes a starting point of fatigue fracture under a high-pressure hydrogen gas environment is used. Since a large amount of Zr-Hf carbonitride-based inclusions is generated, fatigue resistance is reduced. A preferred lower limit of Fn1 is 0.01, and a more preferred lower limit is 0.012. A preferred upper limit of Fn1 is 0.03, and a more preferred upper limit is 0.02.

Fn2: 0.0003 or more The low-alloy steel material for high-pressure hydrogen, the low-alloy steel tube for high-pressure hydrogen, and the container for high-pressure hydrogen according to the present invention have an Fn2 represented by the following formula [2] of 0.0003 or more. is there.
Fn2 = B− (11/14) N + (11/48) Ti + (11/91) Zr + (11/178) Hf [2]
However, B, N, Ti, Zr and Hf in the formula [2] mean the contents (% by mass) of the respective elements in the steel, and Ti, Zr and Hf are zero when they are not contained.

  Fn2 is an index of solid solution B effectively acting on hardenability to secure sufficient hardenability. When this value is 0.0003 or more, good hardenability is obtained in combination with Fn1. Can be provided. A preferred lower limit of Fn2 is 0.0005, and a more preferred lower limit is 0.001. A preferred upper limit of Fn2 is 0.002.

2. Inclusions:
The carbon nitride-based inclusions strongly influence the fatigue life in high-pressure hydrogen gas. In particular, a carbonitride-based inclusion having a particle size of 10 μm or more under a stress condition equal to or lower than the durability ratio acts as a starting point of fatigue fracture, and causes a great decrease in fatigue life in a high-pressure hydrogen gas environment. However, if the number of coarse carbonitride-based inclusions having a particle size of 10 μm or more is 10/100 mm ² or less in cross-sectional observation, irregular reduction in fatigue life does not occur. For this reason, the low-alloy steel material for high-pressure hydrogen, the low-alloy steel tube for high-pressure hydrogen, and the container for high-pressure hydrogen according to the present invention have a number of carbonitride-based inclusions having a particle size of 10 μm or more in a cross-sectional observation of 10/100 mm ² or less. And The number of particle size 10μm or more carbonitride inclusions is a cross-sectional observation, preferably 8 / 100mm ² or less, more preferably not more than 5 pieces / 100mm ² or less.

The “particle size” of the carbonitride-based inclusions is defined as the longest dimension of each inclusion, a major axis a (μm), the width perpendicular to the major axis is the width, and the longest value of the width Is the minor diameter b (μm), (a × b) indicates a value of ^0.5 .

The number condition of the carbonitride-based inclusions having a particle size of 10 μm or more may be satisfied at a portion where high stress where fatigue fracture is a problem is satisfied. Measurement of inclusions at the site corresponding to the final solidification position (for example, at the center of the thickness of a thick plate material, near the inner surface of a seamless steel pipe, etc.) of an ingot or a continuously cast slab where the inclusions tend to become coarse. Should be done. On the other hand, excessively reducing the number of inclusions leads to an increase in steelmaking costs. Therefore, in order to suppress an increase in steelmaking cost, the number of carbonitride-based inclusions having a particle size of 10 μm or more may be 3/100 mm ² or more in cross-sectional observation.

  To reduce coarse carbonitride-based inclusions, it is also effective to reduce the N content. Furthermore, since the inclusions are aggregated and coarsened in the process of cooling the molten steel, it is also effective to increase the cooling rate of the molten steel. Desirably, the average cooling rate in the temperature range of 1500 to 1000 ° C. is 50 ° C./min or more. In addition to this, it is also effective to use various steelmaking methods for lifting and removing coarse inclusions during steelmaking, for example, using a tundish heater.

  For the identification of carbonitride-based inclusions, energy-dispersive X-rays attached to non-metallic inclusions having a particle size of 10 μm or more observed with an optical microscope when observed with a scanning electron microscope (hereinafter, referred to as “SEM”). The content (mass ratio) of each metal element including Fe other than the light elements (S, O, N, B and C) may be determined by the analyzer. In the present invention, inclusions containing Ti, Zr, Hf, Nb and Ta in a total amount of 80% by mass or more were identified as carbonitride inclusions by this method.

3. Endurance ratio:
If low-alloy steel is used in a high-pressure hydrogen gas environment and the fatigue life does not decrease irregularly, the higher the durability ratio, the more stable and reliable the setting of the working stress condition can be performed. Therefore, the durability ratio of the low-alloy steel material for high-pressure hydrogen, the low-alloy steel tube for high-pressure hydrogen, and the container for high-pressure hydrogen according to the present invention is 0.45 or more.

4. Old austenite grains:
The grain size of the steel material affects the improvement of the durability ratio (σ _W / σ _B ) in high-pressure hydrogen gas, in other words, the improvement of the fatigue limit (σ _W ). In order to ensure a high durability ratio of 0.45 or more and to use the member under conditions of higher stress, the low-alloy steel material for high-pressure hydrogen, the low-alloy steel tube for high-pressure hydrogen, and the container for high-pressure hydrogen according to the present invention include: It is desirable that the prior-austenite crystal grains have an ASTM grain size number of 9.0 or more. In an industrial production process, the upper limit of the particle size number is about 12.0.

  The ASTM particle size number is measured by embedding a sample collected from a steel material in a resin, polishing the cross section of the sample to a mirror surface, then corroding (etching) with a saturated aqueous solution of picric acid containing a surfactant, and observing the sample with an optical microscope. be able to. From the crystal orientation diagram of the martensite structure obtained by using the electron backscatter diffraction (EBSD), the crystal orientation of the prior austenite grains before martensite transformation was determined during martensite transformation. By reconstructing by using the specific crystal orientation relation (for example, Kurdjumov-Sachs relation) of the austenite, the ASTM grain number of the old austenite crystal grain can also be obtained.

  The endurance ratio in high-pressure hydrogen gas is also strongly affected by the homogeneity of the matrix. In order to secure a high durability ratio of 0.45 or more, it is desirable that the structure of the steel material be a structure mainly composed of a tempered martensite phase. Although it is difficult to measure the martensite ratio from the structure after tempering, the martensite ratio is reflected in the hardness of the as-quenched steel material. The martensite ratio in order not to lower the durability ratio in a high-pressure hydrogen gas environment is 80% or more, and in order to secure this, the hardness as-quenched is determined by the Rockwell C scale hardness (hereinafter referred to as “HRC”). ) Is more than “50C + 26 (where“ C ”represents the content of C in steel (% by mass))” or more.

5. Tensile strength:
The high-pressure hydrogen container of the present invention has a tensile strength of 850 MPa or more. With this tensile strength, it can be stably used as a 70 MPa class high-pressure hydrogen container. The upper limit of the tensile strength is about 1000 MPa from the viewpoint of securing sufficient hydrogen gas embrittlement resistance.

6. Production method:
The container for high-pressure hydrogen of the present invention can be produced, for example, by the following method, but is not limited to this method.

  The low alloy steel having the chemical composition described above is melted and then cast into an ingot or a slab. When producing a seamless steel pipe, a cast piece having a circular billet shape for pipe production may be formed by a so-called “round CC” method. In order to reduce coarse carbonitride-based inclusions, the average cooling rate of the molten steel in a temperature range of 1500 to 1000 ° C is preferably set to 50 ° C / min or more. It is also effective to utilize various steelmaking methods for floating and removing coarse inclusions during steelmaking, such as a tundish heater.

  As the next step, the cast ingot or slab is subjected to hot working such as slab rolling, hot rolling, hot extrusion, hot forging, etc. to produce a thick plate, a seamless steel pipe, etc. The material is formed into a predetermined high-pressure hydrogen container shape.

  After the above forming process, a quenching-tempering process is performed. Desirable heating temperature for quenching is 880 to 950 ° C. If the heating temperature during quenching is lower than 880 ° C., quenching becomes insufficient, the martensite ratio decreases, and the durability ratio in air and high-pressure hydrogen gas may decrease. When the heating temperature for quenching exceeds 950 ° C., the austenite crystal grains become coarse and fall below the ASTM particle size number of 9.0, so that the durability ratio may decrease in the air and in high-pressure hydrogen gas. The holding time in the above temperature range depends on the size and shape of the high-pressure hydrogen container, but is preferably 5 to 90 minutes, and more preferably 15 to 60 minutes.

  The above-mentioned quenching after heating and holding is performed by, for example, water quenching, oil quenching, or the like, so that the average cooling rate in the temperature range of 800 to 500 ° C is 2 ° C / sec or more in order to secure a martensite ratio of 80% or more. It is desirable to perform so that Note that as the cooling equipment, appropriate equipment such as an immersion equipment, a mist cooling equipment, and a shower cooling equipment may be used. The cooling rate during quenching is the slowest cooling rate of the molded material in the shape of a high-pressure hydrogen container (in the case of double-sided quenching, the central part of the plate thickness or wall thickness; It is sufficient to attach a K-type sheath thermocouple to the surface) and calculate from the actually measured temperature history.

After the quenching, a tempering treatment is performed. The temperature is desirably set to a temperature lower than the Ac ₁ point and as high as possible to obtain a tensile strength of 850 MPa or more. The time for the tempering treatment is not particularly limited, but is preferably 10 to 120 minutes from the viewpoint of uniformly heat treating the entire container.

  Hereinafter, the present invention will be described more specifically with reference to Examples, but the present invention is not limited to these Examples.

  After ingoting 1.5 to 3 tons of low alloy steels A to W having the chemical compositions shown in Table 1, each was ingot by casting. In addition, in the steels A to V, the average cooling rate in the temperature range of 1500 to 1000 ° C. after smelting was all 50 ° C./min or more. For steel W, the cooling rate was intentionally reduced by performing a heat retention treatment after melting, so that the average cooling rate in the temperature range of 1500 to 1000 ° C was 20 ° C / min.

Steels A to Q and steel W in Table 1 are steels whose chemical compositions are within the range specified in the present invention. On the other hand, steels R to V are steels whose chemical compositions deviate from the conditions specified in the present invention. In Table 1, the calculated value of the Ac ₁ point represented by the following equation is also shown for each steel. In addition, each element symbol in a formula shows content (mass%) of each element.
_{Ac 1 (℃) = 751-16.3C +} 34.9Si-27.5Mn-5.5Cu-15.9Ni + 12.7Cr + 3.4Mo

  Each of the above-mentioned ingots was subjected to hot forging and hot rolling to finish a plate material having a thickness of 15 to 60 mm.

  Next, under the conditions shown in Table 2, the plate members of each thickness were quenched. The quenching was performed by changing the quenching medium and the stirring conditions, and the cooling rate during quenching was measured by inserting a K-type sheath thermocouple at the center of the plate thickness.

  For each steel, a test piece for measuring ASTM grain size number of old austenite crystal grains and a test piece for HRC measurement at the center of the sheet thickness were collected from the sheet material of each thickness as-quenched.

  That is, a test piece was sampled from a direction perpendicular to the rolling direction of the as-quenched sheet material, filled with a resin so that the cross section perpendicular to the rolling direction was the observation surface, mirror-polished, and then added with picric acid to which a surfactant was added. The sample was corroded (etched) by a saturated aqueous solution, observed with an optical microscope, and the ASTM particle size number of the prior austenite crystal grains (hereinafter, referred to as “previous austenite grains”) was measured.

  Further, from the as-quenched sheet material, a test piece having a size of 20 mm x 20 mm was collected from each sheet thickness, and the HRC at the center of the sheet thickness was measured. 50C + 26 ".

  Further, for each steel, the as-quenched remaining sheet material of each thickness was tempered under the conditions shown in Table 2, and a test piece for inclusion measurement, a tensile test piece, and a fatigue test piece were collected.

That is, for each steel, a test piece for measuring inclusions having a size of 10 mm × 10 mm × 10 mm is sampled from the center of the thickness of the sheet material after tempering, and a surface (so-called longitudinal direction) with respect to the rolling direction of the sheet (so-called “long side”). , "L section") is filled with a resin so as to be an observation surface, and is polished to a mirror surface. The number of carbonitride-based inclusions having a particle size of 10 μm or more in an area of 10 mm × 10 mm (100 mm ² ) is observed by an optical microscope. Was measured.

The particle size of the inclusions is such that the longest dimension of each inclusion is the major axis a (μm), the width perpendicular to the major axis is the width, and the maximum value of the width is the minor axis b (μm). , (A × b) ^0.5 . In addition, 10 specimens for inclusion measurement were collected for each test number, and the number of carbonitride-based inclusions having a particle size of 10 μm or more measured as described above using these 10 specimens was arithmetically calculated. On average, the number of inclusions of the test number was used.

As described above, the identification of carbonitride-based inclusions was performed by using an attached energy dispersive X-ray analyzer (non-metallic inclusions having a particle diameter of 10 μm or more observed with an optical microscope) when the particles were again observed with an SEM. The content (mass ratio) of each metal element including Fe other than the light elements (S, O, N, B, and C) may be determined using JXA-8530F (manufactured by JEOL Ltd.) for both the main body and the detector. For this reason, in this example, inclusions containing Ti, Zr, Hf, Nb and Ta in total of 80% by mass or more were identified as carbonitride-based inclusions. The surface of the sample was mirror-polished, the magnification at the time of analysis was 3000 times, the acceleration voltage was 15 kV, the irradiation current was 5 nA, and the degree of vacuum in the sample chamber was 2 × 10 ⁻⁴ Pa. The spectrum processing was performed with background processing and ZAF correction.

In addition, a round bar tensile test piece having a parallel part diameter of 6 mm was sampled from the center of the tempered plate in the direction parallel to the rolling direction in a direction parallel to the rolling direction, and subjected to a tensile test at room temperature to obtain a tensile strength (σ _B ). (Unit: MPa) was determined.

Further, a rod-shaped fatigue test piece having a circular cross section was sampled from the center of the sheet thickness of the tempered sheet material in a direction parallel to the rolling direction, and subjected to a stress ratio (minimum stress) at room temperature in the atmosphere and in a high-pressure hydrogen gas environment of 90 MPa. A uniaxial tensile / compression fatigue test was performed under the condition that the ratio to the maximum stress was “R” (−1) (that is, swinging). At this time, the frequency was set to 1 Hz, and the test was performed by changing the stress amplitude (σ _a ) (unit: MPa) variously, and the fatigue life (N _f ), that is, the number of repetitions of the stress until the fatigue fracture occurred, was measured. did. For each steel, the ratio “σ _a / σ _B ” between the stress amplitude (σ _a ) and the tensile strength (σ _B ) determined in the tensile test was determined.

Both in the above atmosphere and a high-pressure hydrogen gas, fatigue test was carried out up to 10 ⁷ times to 10 ⁷ times endurance criteria as "infinite life". The upper limit of the stress amplitude (σ _a ) at which the infinite life is reached is the fatigue limit (σ _W ), and the ratio “σ _W / σ _B ” between the (σ _W ) and the tensile strength (σ _B ) is the durability ratio . Under the stress condition (stress amplitude condition) at which the durability ratio is obtained, the fatigue life of each steel was determined by using five rod-shaped fatigue test pieces and further performing the above-described fatigue test in high-pressure hydrogen gas. .

Table 2 also shows the results of the above tests. In Table 2, the shortest fatigue life value was described as “fatigue life” for the results of a fatigue test performed in high-pressure hydrogen gas under stress conditions that resulted in a durability ratio. In other words, in the case of a fatigue test performed in a high-pressure hydrogen gas under a stress condition that provides a durability ratio, the fatigue life when no irregular decrease in the fatigue life was observed for all five test pieces was " 10 ⁷ times or more (≧ 1 × 10 ⁷ in the description in the table) ”, and the fatigue life when an irregular result is obtained is a specific number of times“ less than 10 ⁷ times ”. FIG. 1 shows a test number 1 using steel A as an example of a case where the fatigue life (N _f ) is extremely reduced under a stress condition that provides a durability ratio (σ _W / σ _B ) in a high-pressure hydrogen gas environment. (In white circles and white triangles, the data in "atmosphere" and "in hydrogen", respectively). The data (in hydrogen) of Test No. 23 using steel W in comparison with the data (in hydrogen) Black triangle).

From Table 2, Test Nos. 1 to 17 satisfying the conditions specified in the present invention have a fatigue life of 10 ⁷ times or more (no break at 10 ⁷ times) under the stress condition of the durability ratio in the atmosphere and high-pressure hydrogen gas. Yes, it is clear that it has good fatigue resistance characteristics. Further, the test numbers 1 to 17 of the above examples of the present invention show that the ASTM particle number of the prior austenite grains is 9.0 or more, and that the as-quenched HRC is sufficiently larger than the value of “50C + 26” and the martensite ratio is high. A high durability ratio of 0.45 or more is obtained in the atmosphere and high-pressure hydrogen gas.

  On the other hand, the test numbers 18 to 26 of the comparative examples show that the fatigue life under the stress condition that is the durability ratio in the high-pressure hydrogen gas is short and the fatigue resistance is inferior, or the durability ratio in the atmosphere and the high-pressure hydrogen gas is high. Was low, less than 0.45.

Test No. 18 has a fatigue life of 10 ⁷ times or more under a stress condition that provides a durability ratio in the atmosphere and high-pressure hydrogen gas. However, since the B content of the used steel R is as low as 0.0001% and the as-quenched HRC is smaller than the value of “50C + 26” and the martensite ratio is low, the durability ratio in both the atmosphere and the high-pressure hydrogen gas is low. 0.35.

Test No. 19 also has a fatigue life of 10 ⁷ times or more under stress conditions that provide an endurance ratio in the atmosphere and high-pressure hydrogen gas. However, the value of Fn2 of the steel S used was “−0.0005”, N was combined with B to form BN, and an effective amount of solid solution B for quenching could not be secured. Since the martensite ratio was smaller than the value of “50C + 26” and low in martensite ratio, the durability ratio was 0.4 in both air and high-pressure hydrogen gas.

Test No. 20, the durability ratio both in the air and the high-pressure hydrogen gas to is 0.5, the fatigue life of the stress conditions to be durable ratio in the atmosphere more than 10 ⁷ times, good fatigue resistance can be obtained ing. However, since the Ti content and Fn1 value of the used steel T deviated from the conditions specified in the present invention, many coarse carbonitride-based inclusions were generated. Therefore, the number of carbonitride-based inclusions having a particle size of 10 μm or more in an area of 100 mm ² , which is a starting point of fatigue fracture in high-pressure hydrogen gas, deviates from the condition specified in the present invention. The fatigue life under a stress condition that gives a durability ratio of is as short as 5.1 × 10 ⁴ times, and the fatigue resistance is poor.

Test No. 21, the durability ratio both in the air and the high-pressure hydrogen gas to is 0.5, the fatigue life of the stress conditions to be durable ratio in the atmosphere more than 10 ⁷ times, good fatigue resistance can be obtained ing. However, since the Zr content and the value of Fn1 of the steel U used deviated from the conditions specified in the present invention, many coarse carbonitride-based inclusions were generated. Therefore, the number of carbonitride-based inclusions having a particle size of 10 μm or more in an area of 100 mm ² , which is a starting point of fatigue fracture in high-pressure hydrogen gas, deviates from the condition specified in the present invention. The fatigue life under a stress condition that provides a durability ratio of 1.5 × 10 ⁴ times is short, and the fatigue resistance is poor.

Test No. 22, the durability ratio both in the air and the high-pressure hydrogen gas to is 0.5, the fatigue life of the stress conditions to be durable ratio in the atmosphere more than 10 ⁷ times, good fatigue resistance can be obtained ing. However, since the N content of the used steel V deviated from the conditions specified in the present invention and was high, many coarse carbonitride-based inclusions were generated. Therefore, the number of carbonitride-based inclusions having a particle size of 10 μm or more in an area of 100 mm ² , which is a starting point of fatigue fracture in high-pressure hydrogen gas, deviates from the condition specified in the present invention. The fatigue life under a stress condition that gives a durability ratio of is as short as 6.0 × 10 ⁴ times, which is inferior in fatigue resistance.

Test No. 23 using steel W whose chemical composition is within the range specified in the present invention has an endurance ratio of 0.5 both in the atmosphere and in high-pressure hydrogen gas, and under a stress condition where the endurance ratio is attained in the atmosphere. The fatigue life is 10 ⁷ times or more, and good fatigue resistance is obtained. However, since the cooling rate immediately after melting the steel W was slow, the carbonitrides became coarse in the process of cooling the molten steel, and the number of carbonitride-based inclusions having a particle size of 10 μm or more existing in an area of 100 mm ² was determined by the present invention. The fatigue life under a stress condition that provides a durability ratio in a high-pressure hydrogen gas, which is out of the conditions specified in the above, is as short as 3.1 × 10 ⁴ times, and is inferior in fatigue resistance characteristics.

Using steel A whose chemical composition is within the range specified in the present invention, the number of carbonitride-based inclusions having a particle diameter of 10 μm or more in an area of 100 mm ² and satisfying the conditions specified in the present invention is also the test number 24. And a fatigue life of 10 ⁷ times or more under a stress condition that provides a durability ratio in the atmosphere and high-pressure hydrogen gas. However, the heating temperature of quenching is 1000 ° C., and the austenite crystal grains are coarsened and fall below the ASTM particle size number of 9.0, so that the durability ratio is 0.4 in both air and high-pressure hydrogen gas and high-pressure hydrogen gas. Was.

Using steel A whose chemical composition is within the range specified in the present invention, the number of carbonitride-based inclusions having a particle size of 10 μm or more in an area of 100 mm ² and the number of test pieces 25 satisfying the conditions specified in the present invention are also used. And a fatigue life of 10 ⁷ times or more under a stress condition that provides a durability ratio in the atmosphere and high-pressure hydrogen gas. However, the heating temperature of quenching was 850 ° C., and the as-quenched HRC was smaller than the value of “50C + 26” and the martensite ratio was low, so that the durability ratio was 0.35 in both the atmosphere and the high-pressure hydrogen gas. .

Similarly, using steel A having a chemical composition within the range specified by the present invention, the number of carbonitride-based inclusions having a particle size of 10 μm or more in an area of 100 mm ² satisfying the conditions specified by the present invention. No. 26 also has a fatigue life of 10 ⁷ times or more under stress conditions that provide a durability ratio in the atmosphere and high-pressure hydrogen gas. However, since the average cooling rate in the temperature range of 800 to 500 ° C. during quenching is 10 ° C./sec and the as-quenched HRC is smaller than the value of “50C + 26” and the martensite ratio is low, the hydrogen in the atmosphere and high-pressure hydrogen The durability ratio was 0.4 in both gases.

Durability ratio and the high-pressure hydrogen gas in the atmosphere is at both 0.45 or more, yet, in the fatigue life of the stress condition for its durability ratio of 10 ⁷ times or more, good fatigue resistance is above that obtained Next, the hydrogenated gas embrittlement resistance was investigated using the tempered plate materials prepared in Test Nos. 1 to 17. As a result, it was confirmed that in each case, the hydrogen gas embrittlement resistance was excellent.

According to the present invention, it is possible to design a life based on a durability ratio under a high-pressure hydrogen gas environment, have a high tensile strength of 850 MPa or more, and have excellent fatigue resistance characteristics under a high-pressure hydrogen gas environment. A container for high-pressure hydrogen having good hydrogen gas embrittlement resistance, a low-alloy steel material for high-pressure hydrogen and a low-alloy steel tube for high-pressure hydrogen suitable for use as a material of the container can be obtained. Further, according to the method of the present invention, such a high-pressure hydrogen container can be stably obtained.

Claims (6)

In mass%,
C: 0.20 to 0.60%,
Si: 0.05 to 1.0%,
Mn: 0.35 to 3.0%,
P: 0.025% or less,
S: 0.010% or less,
Al: 0.005 to 0.10%,
B: 0.0003-0.01%,
Ti: 0 to 0.5%,
Zr: 0 to 1.0%,
Hf: 0 to 2.0%,
O: 0.005% or less,
N: 0.008% or less Cr: 0 to 5.0%,
Mo: 0 to 1.5%,
V: 0 to 1.0%,
W: 0 to 3.0%,
Nb: 0 to 0.1%,
Ta: 0 to 0.2%,
Ni: 0 to 5.0%,
Cu: 0 to 3.0%,
Co: 0 to 3.0%,
Ca: 0 to 0.01%,
Mg: 0 to 0.01%,
REM: 0 to 0.50%,
The balance: Fe and impurities, and
Fn1 represented by the following formula [1] is 0.008 to 0.05, and
Fn2 represented by the following formula [2] is 0.0003 or more;
Has a chemical composition,
The number of carbonitride-based inclusions having a particle size of 10 μm or more is 10/100 mm ² or less in cross-sectional observation,
The durability ratio is 0.45 or more;
Low alloy steel for high pressure hydrogen.
Fn1 = Ti + 0.5Zr + 0.25Hf [1]
Fn2 = B− (11/14) N + (11/48) Ti + (11/91) Zr + (11/178) Hf [2]
However, Ti, Zr, Hf, B and N in the above formula mean the contents (% by mass) of the respective elements in the steel, and Ti, Zr and Hf are zero when they are not contained. The chemical composition is expressed in mass%;
Cr: 0.1-5.0%,
Mo: 0.1-1.5%,
V: 0.01 to 1.0%,
W: 0.01 to 3.0%,
Nb: 0.001 to 0.1%,
Ta: 0.001 to 0.2%,
Ni: 0.1 to 5.0%,
Cu: 0.1 to 3.0%;
Co: 0.1-3.0%,
Ca: 0.0001-0.01%,
Mg: 0.0001 to 0.01%, and
REM: 0.0001-0.50%,
The low-alloy steel material for high-pressure hydrogen according to claim 1, comprising at least one member selected from the group consisting of: The prior austenite grains having an ASTM grain size number of 9.0 or more;
The low-alloy steel material for high-pressure hydrogen according to claim 1. It consists of a low alloy steel material for high pressure hydrogen according to any one of claims 1 to 3,
Low alloy steel pipe for high pressure hydrogen. It consists of a low alloy steel pipe for high pressure hydrogen according to claim 4,
A tensile strength of 850 MPa or more;
High pressure hydrogen container. A method for producing a high-pressure hydrogen container according to claim 5,
After forming the low-alloy steel pipe for high-pressure hydrogen into a predetermined shape, after heating and holding at 880 to 950 ° C, quenching is performed at an average cooling rate of 2 ° C / sec or more in a temperature range of 800 to 500 ° C, Then, tempering,
Manufacturing method of high-pressure hydrogen container.

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