JP5561583B2 - High pressure hydrogen components - Google Patents

Description

The present invention relates to a member for high-pressure hydrogen using a high-strength γ ′ phase precipitation strengthened austenitic alloy having excellent hydrogen embrittlement resistance.

In recent years, hydrogen energy that does not emit carbon dioxide during use has attracted attention from the viewpoint of protecting the global environment. As an example, the development of vehicles equipped with high-pressure hydrogen containers such as fuel cell vehicles has been studied. The pressure of the onboard container is currently 35 MPa, and the cruising distance by this container is inferior to that of a gasoline automobile. In order to obtain a cruising range similar to that of a gasoline vehicle, it is necessary to increase the amount of hydrogen gas loaded, and further, the pressure of hydrogen gas filling is being increased by developing a pressure vessel and related equipment compatible with 70 MPa.
A Coriolis flow meter (an example of a high-pressure hydrogen flow meter) is used as a dispenser for supplying high-pressure hydrogen gas to a vehicle-mounted container at a hydrogen station serving as a base for hydrogen energy infrastructure. In order to supply hydrogen gas at high pressure, the material used for the Coriolis flow meter is required to have high strength and excellent ductility at room temperature and high pressure hydrogen.
Generally, in many alloy systems, hydrogen embrittlement tends to occur more easily as the strength is increased. However, as disclosed in Patent Document 1, as an alloy having high strength and less hydrogen embrittlement, γ The possibility of using a 'phase-strengthened FeNi-based alloy has been suggested. In addition, the γ 'precipitation strengthened Fe-based superalloy, A286 alloy (JIS SUH660 equivalent alloy), which has a proven record in high-temperature applications, is known as a high-strength alloy having excellent hydrogen embrittlement resistance. Used for members that require strength.

JP2008-144237

In general, an ordinary austenitic Fe-based alloy SUS316L is an alloy having excellent hydrogen embrittlement resistance, but the strength is low in order to realize high pressure of supplied hydrogen. However, it is difficult to use it for a thin-walled device such as a Coriolis flow meter of a dispenser that maintains sensitivity. In addition, a Fe-based alloy having a low tensile strength at room temperature, such as SUS316L, has insufficient fatigue strength to be used in a device that receives a vibration load during use, such as a Coriolis flow meter. There was a problem that it was difficult to use for the purpose.
In addition, the A286 alloy having high strength also has a tensile strength of around 1100 MPa, and there is a risk that the strength may be insufficient for the demand for downsizing and high-pressure infrastructure equipment for high-pressure hydrogen.
An object of the present invention is to provide a member for high-pressure hydrogen using a hydrogen -brittle high-strength austenitic alloy that has both high strength and hydrogen-brittle resistance.

The inventors can increase the strength by increasing the γ ′ phase by increasing the amount of Ni, but since Ni itself is an element that is easily hydrogen embrittled, it is possible to achieve both high strength and hydrogen embrittlement resistance with Fe groups. The present invention has been achieved by investigating the amount of Ni and adjusting the composition to an optimum composition capable of finely dispersing (Nb, Ti) composite carbides that can serve as hydrogen trap sites.
That is, the present invention is a high-pressure hydrogen member using a hydrogen brittle high strength austenitic alloy, wherein the hydrogen brittle high strength austenitic alloy is C: 0.01 to 0.10% by mass, Si : 0.01-0.8%, Mn: 0.01-0.8%, Cr: 14-17%, Mo: 3.5% -5.0%, Al: 1.6-2.5% , Ti: 1.5-3.0%, Nb: 0.5-2.0%, Ni: 50-60%, B: 0.001-0.015%, Mg: 0.001-0.015 %, The balance consists of Fe and impurities, and in atomic%, the following A value is 2.0 to 4.0, B value is 0.5 to 0.6, C value is 6.4 to 7.6, and room temperature. Is a member for high-pressure hydrogen having a tensile strength of 1180 MPa or more and an elongation of 19% or more.
A value 0.293 [Ni] -0.513 [Cr] -1.814 [Mo]
B value [Al] / ([Al] + [Ti] + [Nb])
C value [Al] + [Ti] + [Nb]
[] Represents atomic%.
Preferably, the high-pressure hydrogen member has a circle equivalent diameter of 25 μm or less of the (Nb, Ti) composite carbide dispersed in the austenite base of the hydrogen-brittle high-strength austenitic alloy.

According to the present invention, it is possible to provide a high-pressure hydrogen member using a hydrogen -brittle high-strength austenitic alloy that has high strength and is difficult to be hydrogen-brittle.

It is a figure which shows a rotation bending fatigue test result.

The reason why each chemical composition is specified in the hydrogen brittle high strength austenitic alloy used for the high-pressure hydrogen member of the present invention is as follows. Unless otherwise specified, the chemical composition is expressed in mass%.
C: 0.01 to 0.10%
C combines with Nb and Ti to form MC-type carbides and refines the crystal grains, thereby contributing to high strength and improved hydrogen embrittlement resistance. When C is less than 0.01%, the amount of MC-type carbide produced is small, and the effect of crystal grain refinement cannot be sufficiently obtained. On the other hand, when it exceeds 0.10%, the size and amount of MC-type carbide formed are large. Thus, hydrogen embrittlement resistance and fatigue strength may be reduced, so C was made 0.01 to 0.10%. The preferable lower limit of C is 0.02%, and the preferable upper limit is 0.08%.
Si: 0.01 to 0.8%
Si needs to be added in an amount of 0.01% or more for deoxidation, but if it exceeds 0.8%, the toughness may decrease, so Si was made 0.01 to 0.8%. A preferable upper limit of Si is 0.5%.
Mn: 0.01 to 0.8%
Like Si, Mn needs to be added in an amount of 0.01% or more for deoxidation, but if it exceeds 0.8%, the toughness may be lowered. %. A preferable upper limit of Mn is 0.5%.

Cr: 14-17%
Cr is an essential element for maintaining the corrosion resistance necessary for the high-pressure hydrogen member , and has an effect of increasing the tensile strength at room temperature by solid solution in the austenite base and solid solution strengthening. In order to maintain the corrosion resistance, addition of 14% or more is necessary. On the other hand, if adding over 17%, the austenite structure becomes unstable and it becomes difficult to maintain stable corrosion resistance. It was. The preferable lower limit of Cr is 15%, and the preferable upper limit is 16.5%.
Mo: 3.5% to 5.0%
Mo has the effect of improving the corrosion resistance necessary for the member for high-pressure hydrogen as well as the effect of increasing the tensile strength at room temperature by solid solution strengthening in the austenite base. If Mo is less than 3.0%, sufficient high strength at room temperature cannot be obtained. On the other hand, if it exceeds 5.0%, the solid solution strengthening becomes excessive and the austenite structure becomes unstable. Since the interworkability is lowered or the ductility at room temperature is lowered, Mo is set to 3.5% to 5.0%. A preferable upper limit of Mo is 4.6%.

Al: 1.6-2.5%
Al is an indispensable element for finely precipitating the γ ′ phase together with Ni, Ti, and Nb by aging treatment to obtain high strength at room temperature, and at least 1.6% is required. If added in excess of%, hot workability and weldability may be deteriorated, so Al was made 1.6 to 2.5%. A preferable upper limit of Al is 2.1%, and a more preferable upper limit is 1.9%.
Ti: 1.5-3.0%
Ti is indispensable for obtaining high strength at normal temperature by forming MC type carbide with C and Nb to refine the austenite crystal grains and finely precipitating γ 'phase with Ni, Al and Nb by aging treatment. It is an element and requires addition of 1.5% or more. On the other hand, if adding over 3.0%, hot workability and weldability may be deteriorated, so Ti was made 1.5 to 3.0%. The preferable lower limit of Ti is 1.8%, the preferable upper limit is 2.5%, and the more preferable upper limit is 2.3%.

Nb: 0.5-2.0%
Nb is effective in forming MC type carbide with C and Ti to refine austenite crystal grains and finely precipitating γ 'phase with Ni, Al and Ti by aging treatment to obtain high strength at normal temperature. It is an element and requires addition of 0.5% or more. On the other hand, if added over 2.0%, coarse MC-type carbides may be formed and hot workability may be reduced, so Nb was made 0.5 to 2.0%. The preferable lower limit of Nb is 0.8%, and the preferable upper limit is 1.6%. A more preferred lower limit is 1.0%, and a more preferred upper limit is 1.4%.
Ni: 50-60%
Ni stabilizes the austenite base and sufficiently dissolves intermetallic compounds such as the γ 'phase during the solution treatment, and sufficiently dissolves Mo that contributes to solid solution strengthening, and also finely precipitates during the aging treatment. It is an important element indispensable for improving the tensile strength at room temperature by precipitation strengthening. If Ni is less than 50%, the austenite structure becomes unstable, and the precipitation of the γ 'phase becomes insufficient, resulting in a decrease in tensile strength at room temperature. On the other hand, if it exceeds 60%, hot workability is reduced. Or hydrogen embrittlement is likely to occur, so Ni was made 50 to 60%. The preferable lower limit of Ni is 52%, and the preferable upper limit is 58%. A more preferred lower limit is 54%, and a more preferred upper limit is 56%.

B: 0.001 to 0.015%
B is segregated at the austenite grain boundary by adding a small amount to strengthen the grain boundary and improve the hot workability. However, when B is added in an amount of 0.001% or more, the effect is obtained, but it exceeds 0.015%. When added, the melting point of the grain boundary part where B segregates is locally lowered, and conversely, hot workability is adversely affected, so B was made 0.001 to 0.015%.
Mg: 0.001 to 0.015%
Mg forms a sulfide together with S and has an effect of preventing deterioration of hot workability due to segregation of grain boundaries of S. However, if less than 0.001%, a sufficient effect cannot be obtained, while 0, If over 0.15% is added, a compound with a low melting point is formed, which impairs hot workability, so Mg was made 0.001 to 0.015%.
The balance is Fe and impurities The balance is Fe and impurities, but Fe is an element that adjusts the content of each of the above elements. Moreover, in this invention, when manufacturing a high intensity | strength Fe base superalloy, the impurity inevitably mixed can be included. In particular, the following elements may be contained within the ranges shown below.
P ≦ 0.04%, S ≦ 0.015%, O ≦ 0.015%, N ≦ 0.05%, Cu ≦ 1.0%

In the present invention, in order to have good hydrogen embrittlement resistance, it is necessary to use an austenitic structure having a large allowable solid solution amount of hydrogen as a basic structure. In addition, in order to obtain high strength and good corrosion resistance, it is necessary to dissolve as much solid solution Mo as possible.
Therefore, by specifying the value of A value expressed in atomic%: 0.293 [Ni] −0.513 [Cr] −1.814 [Mo] to 2.0 to 4.0, high strength is achieved. And good corrosion resistance. If the A value is less than 2.4, it is difficult to obtain a stable austenite structure containing a specified amount of Mo. On the other hand, if it exceeds 4.0, the solid solution strengthening is insufficient and the strength at room temperature may be insufficient.
Furthermore, in the present invention, the Al ratio in the γ ′ phase and the total amount of Al, Ti, and Nb in the γ ′ phase generating element are also defined as follows.
B value [Al] / ([Al] + [Ti] + [Nb])
C value [Al] + [Ti] + [Nb]
[] Represents atomic%.
The B value represents the ratio of Al in the γ ′ phase. If the Al ratio is low and the B value is less than 0.5, the strength at room temperature becomes too high and the ductility decreases. On the other hand, if the Al ratio is high and the B value is greater than 0.6, precipitation strengthening due to the γ 'phase will occur. Since the effect is lowered and the strength at room temperature is lowered, the B value is set to 0.5 to 0.6.
The C value represents the total amount of Al, Ti, and Nb in the γ ′ phase generating element. If the C value is less than 6.4, the amount of γ ′ phase that undergoes aging precipitation decreases and high tensile strength at room temperature cannot be obtained. On the other hand, if the C value exceeds 7.6, the amount of γ ′ phase that undergoes aging precipitation increases. The C value was set to 6.4 to 7.6 because the ductility at room temperature was decreased.

After adjusting the amount of added elements so as to satisfy the above-mentioned chemical component regulations, by performing an appropriate heat treatment, that is, an appropriate solution treatment and an aging treatment, a tensile strength at room temperature of 1180 MPa or more and 19% or more Elongation at room temperature can be obtained.
If the tensile strength is 1180 MPa or more, it is possible to obtain room temperature tensile strength higher than that of the A286 alloy showing high strength among the existing hydrogen brittle materials, and at the same time maintain good ductility. It is also possible to adjust the tensile strength to 1210 MPa or more, and it is more preferable.

In the present invention, it is preferable that the equivalent circle diameter of the composite carbide of (Nb, Ti) dispersed in the austenite base is 25 μm or less.
The MC type composite carbide of (Nb, Ti) dispersed in the austenite base serves as a hydrogen trap site and may become a starting point of hydrogen embrittlement. If the composite carbide of (Nb, Ti) is larger than 25 μm in equivalent circle diameter, there is a risk that hydrogen is localized in the carbide and hydrogen embrittlement is likely to occur. Therefore, the composite carbide of (Nb, Ti) The equivalent circle diameter is preferably 25 μm or less.

An alloy having a composition defined in the present invention (referred to as an alloy of the present invention) and a comparative alloy were melted by vacuum melting, and a bar having a diameter of 16 to 50 mm was obtained through hot working.
Table 1 shows the alloy No. of the present invention. 1-6 and comparative alloy no. Chemical components 11 to 13 are shown. Here, comparative alloy No. 11 and 12 are conventional material A286 alloy.

Invention alloy No. 1-6 and comparative alloy no. No. 13 was subjected to aging treatment at 750 ° C. after solution treatment at 1050 ° C. 11 and 12 were subjected to aging treatment at 720 ° C. after solution treatment at 900 ° C. Thereafter, a plate tensile test piece having a thickness of 1 mm and a rotating bending fatigue test piece having a parallel part diameter of 8 mm were collected.
For tensile properties at room temperature, a tensile test was performed at room temperature without using a sheet tensile test piece, and after confirming the tensile properties, hydrogen charging was performed on some alloys, and then The tensile properties at were confirmed.

About the hydrogen charge to a plate tension test piece, hydrogen charge was performed by the high-pressure hydrogen charge method or cathode charge method which used the autoclave.
In the high-pressure hydrogen charging method, the amount of hydrogen was changed under the conditions of a temperature of 300 to 400 ° C. and a hydrogen pressure of 2 to 20 MPa. The amount of occluded hydrogen was analyzed by hydrogen thermal desorption analysis. In the cathode charging method, a test piece, a Pt electrode, and a thermocouple are placed in an electrolyte solution having 0.05 M H ₂ SO ₄ and 0.01 M KSCN (potassium thiocyanate), and the test piece is a negative electrode and a Pt electrode is used. As a positive electrode, hydrogen was occluded by flowing a constant current of 200 mA / cm ² at 50 to 60 ° C. The amount of occluded hydrogen in this case was analyzed by an inert gas melting method. The tensile test is performed at room temperature at a strain rate of 2.5 × 10 ⁻⁴ / s, and the value obtained by dividing the elongation at break with a hydrogen-charged specimen by the elongation at break with a specimen not charged with hydrogen is hydrogen embrittled. As an index, the degree of hydrogen embrittlement was evaluated. That is, when the hydrogen embrittlement index is close to 1, it indicates that the material is less prone to hydrogen embrittlement.

The rotating bending fatigue test piece was charged with hydrogen by the cathodic charging method.
A test piece, a Pt electrode, and a thermocouple are placed in an electrolyte solution having 0.05 M H ₂ SO ₄ and 0.01 M KSCN, and the test piece is a negative electrode and the Pt electrode is a positive electrode, and the current is 200 mA at 50 to 60 ° C. Hydrogen was occluded by applying a constant current of / cm ² . The amount of occluded hydrogen in this case was analyzed by an inert gas melting method. The rotating bending fatigue test was performed at room temperature in accordance with JIS Z2274, and the test was stopped if it did not break even after exceeding 10 ⁷ times.

  Further, the alloys of the present invention and comparative alloy No. For No. 11, a block-shaped test piece is collected as it is, and the carbide is extracted by the acid extraction method. The equivalent circle diameter of the carbide is measured by SEM observation, and the estimated maximum equivalent circle diameter is obtained by the extreme value statistical processing. Asked.

Table 2 shows the results of the plate tension test.
Invention alloy No. Nos. 1 to 6 have a tensile strength at room temperature of 1180 MPa or more and an elongation of 19% or more. In addition, the alloy No. of the present invention. 1 and 2 show that when hydrogen is occluded, there is a tendency for the elongation to decrease slightly due to hydrogen embrittlement, but the decrease in the hydrogen embrittlement index is not so large and is still maintained at a relatively large value of 0.76 or more, And comparative alloy No. The tensile strength of 1180 MPa or higher, which is higher than that of 11-13, is maintained.
On the other hand, Comparative Alloy No. 11 and 12 are A286 equivalent alloys, which are conventional alloys, and are materials that have a small decrease in hydrogen embrittlement index and are difficult to hydrogen embrittle, but have a lower tensile strength than the alloys of the present invention. Comparative alloy No. No. 13 is a comparative alloy no. Although the tensile strength is higher than 11 and 12, the tensile strength is lower than that of the alloy of the present invention and the degree of decrease in the hydrogen embrittlement index is the same as that of the alloy of the present invention. is there.

In FIG. 2 and comparative alloy no. 11 shows the results of a rotating bending fatigue test of 11 (A286 equivalent alloy).
The hydrogen amount of the hydrogen-charged test piece used for the rotating bending fatigue test was about 12 ppm, and analysis was performed before and after the test to confirm that there was no change.
As shown in FIG. 1, the fatigue strength of the alloy of the present invention is higher than that of the comparative alloy, and even if hydrogen is charged, the fatigue strength is not lowered as in the comparative alloy. Therefore, it is considered that good fatigue strength can be exhibited even if hydrogen is occluded under repeated stress.

Further, the alloys of the present invention and comparative alloy No. 11 had a maximum carbide size (equivalent circle diameter) of 25.0 μm or less in the case of the alloy of the present invention. 11 was 19.7 μm and 31.2 μm. The carbide of the alloy of the present invention is a composite carbide of (Ti, Nb). Since 11 carbide was Ti carbide, there is a possibility that the difference in the composition of MC type carbides affects the carbide size. Although the amount of carbide in the alloy of the present invention has not been determined, the comparative alloy No. Compared to the amount of 11 carbide, it was significantly less.
From the above, the alloy of the present invention is considered to have a small amount of hydrogen trapped by the carbide because the composite MC type carbide of (Nb, Ti) is finely dispersed, and there is little localization of hydrogen at the carbide interface, Since the absorbed hydrogen is distributed uniformly, it is estimated that hydrogen embrittlement is small for high strength.

  As described above, a high-strength Fe-based alloy with less hydrogen embrittlement can be provided by the present invention, and if used for a high-pressure hydrogen infrastructure member, it can contribute to weight reduction and longer life of the member.

Claims (2)

A member for high-pressure hydrogen using a hydrogen brittle high strength austenitic alloy, wherein the hydrogen brittle high strength austenitic alloy is C: 0.01 to 0.10% by mass, Si: 0.01 to 0.8%, Mn: 0.01-0.8%, Cr: 14-17%, Mo: 3.5% -5.0%, Al: 1.6-2.5%, Ti: 1. 5 to 3.0%, Nb: 0.5 to 2.0%, Ni: 50 to 60%, B: 0.001 to 0.015%, Mg: 0.001 to 0.015%, the balance being Fe The following A value is 2.0 to 4.0, B value is 0.5 to 0.6, C value is 6.4 to 7.6, and tensile strength at room temperature is atomic percent. A member for high-pressure hydrogen characterized by having 1180 MPa or more and an elongation of 19% or more.
A value 0.293 [Ni] -0.513 [Cr] -1.814 [Mo]
B value [Al] / ([Al] + [Ti] + [Nb])
C value [Al] + [Ti] + [Nb]
[] Represents atomic%. 2. The member for high-pressure hydrogen according to claim 1, wherein an equivalent circle diameter of the composite carbide of (Nb, Ti) dispersed in the austenite base of the hydrogen-brittle high-strength austenitic alloy is 25 μm or less.