JP6933096B2 - Nickel steel for high pressure hydrogen - Google Patents

Description

The present invention relates to a nickel-containing steel material (hereinafter, also referred to as “Ni steel material”) suitable for use in an environment containing high-pressure hydrogen.

In recent years, expectations for the use of hydrogen as clean energy have increased. Fuel cell vehicles that use hydrogen are expected to become widespread, and along with this, hydrogen stations that supply hydrogen to fuel cell vehicles are being constructed. Furthermore, in order to supply hydrogen to the station, facilities for storing a large amount of hydrogen and the use of hydrogen for power generation are being considered. Normally, hydrogen existing as a gas has a lower energy density than liquid fuel such as gasoline, and hydrogen compressed to a high pressure is stored in a fuel cell vehicle or the like in order to secure a load capacity. Such high pressure hydrogen is known to reduce the properties of various materials. This phenomenon is commonly referred to as hydrogen embrittlement. For this reason, it is required to use a special material that does not easily cause hydrogen embrittlement for steel materials used for high-pressure hydrogen tanks for automobiles or hydrogen stations, piping, and the like. Therefore, Ni steel materials are attracting attention.

As described in Patent Document 1, for example, the Ni steel material is being developed for use in an extremely low temperature environment such as a storage tank for liquefied natural gas (LNG). Ni steel for low temperature is positively retained austenite by subjecting the steel after hot rolling to a special heat treatment (generally called "intermediate heat treatment") in order to improve the toughness at an extremely low temperature such as -162 ° C. The volume fraction of is increased.

On the other hand, regarding the high-pressure hydrogen tank for automobiles, in Non-Patent Document 1, only austenitic stainless steels SUS316 and SUS316L are certified as usable steel materials, and the Ni equivalent of the following formula (1) is limited by the operating temperature. ing.
Ni equivalent (%) = Ni + 12.6C + 1.05Mn + 0.35Si + 0.65Cr + 0.98Mo ... (1)

SUS316, which conforms to this example standard, has sufficient hydrogen embrittlement resistance, but has a yield stress of about 300 MPa (JIS standard: 175 MPa or more) at room temperature, which is a low class as a structural steel material. When applying SUS316, which has a relatively low strength, to a high-pressure hydrogen tank, a very large wall thickness is required. For this reason, in fuel cell vehicles where weight reduction is important, there are cases where this steel material is used for some parts such as valves and bosses, but there are no cases where it is used for the tank body. In addition, a very large wall thickness is required for piping, and the amount of steel used increases, resulting in an increase in weight and cost.

For example, Patent Document 2 proposes an austenitic stainless steel having excellent hydrogen embrittlement resistance and high strength, similar to SUS316.

Non-Patent Document 2 describes a Cr-Mo system mainly composed of a crystal structure (hereinafter, also referred to as “bcc structure”) of a body-centered cubic lattice (hereinafter, also referred to as “bcc”) such as martensite and bainite. It is stated that low alloy steel (SCM435) can be used in high pressure hydrogen tanks for hydrogen stations up to 35 MPa.

Non-Patent Document 3 reports the results of investigating the mechanical properties of Fe-9Ni-4Co steel, which is a Ni steel mainly composed of bcc structure, in high-pressure hydrogen. It is classified as a material that has reduced properties and is strongly affected by hydrogen.

Japanese Unexamined Patent Publication No. 2014-125678 International Publication No. 2012/132992

High Pressure Gas Safety Association Standard "Technical Standard for Containers for 70MPa Compressed Hydrogen Automotive Fuel Equipment" KHK S0128 Japan Automobile Research Institute Standards "Technical Standards for Containers for Compressed Hydrogen Vehicle Fuel Equipment" JARI S001 W. T. Chandler and R. J. Walter, “Hydrogen embrittlement testing”, ASTM STP543, ASTM, (1974), p. 170.

The purpose of containing a large amount of Ni in a low temperature Ni steel material as in Patent Document 1 is to ensure fracture toughness at extremely low temperatures such as LNG temperature. In this low-temperature Ni steel material, after increasing the proportion of the austenite phase in the metal structure, Ni is concentrated in the austenite phase to stabilize the austenite phase and realize higher fracture toughness at extremely low temperatures. ing. In order to realize such a metallographic structure, a complicated heat treatment is performed in which hot rolling is performed, quenching is performed, intermediate heat treatment is performed, and then tempering is performed. However, according to the studies by the present inventors, it has been found that the steel material obtained by performing the heat treatment including such an intermediate heat treatment deteriorates the hydrogen embrittlement resistance.

Since the steel material of Patent Document 2 contains an extremely large amount of alloying elements, there is a problem that the cost is extremely high.

Here, in a hydrogen station that supplies high-pressure hydrogen, it is necessary to enable filling within 3 minutes. It is known that when hydrogen, which is a gas, is filled in a short time, the temperature rises due to adiabatic compression and the tank becomes hot. On the other hand, it is stipulated that the temperature of the tank does not exceed 85 ° C when filled because the material of the safety valve required for high-pressure equipment is made of a material whose upper limit temperature for use is limited. Therefore, the hydrogen station solves the problems of short-time filling and temperature rise at the same time by supplying high-pressure hydrogen that has been cooled to about −50 ° C. in advance. Therefore, the material for high-pressure hydrogen used in fuel cell vehicles needs to assume a temperature change between -50 ° C and 85 ° C.

Austenitic stainless steel mainly consists of an austenitic phase (hereinafter, also referred to as “fcc structure”) having a face-centered cubic lattice (hereinafter, also referred to as “fcc”) crystal structure (hereinafter, also referred to as “fcc structure”). However, it is generally known that a steel material having an fcc structure tends to have a larger thermal expansion rate than a steel material having a bcc structure such as a ferrite phase. If the coefficient of thermal expansion is large, a large thermal stress may be generated due to the temperature change, and when a large temperature change is expected as described above, it is more desirable to use a bcc structure-based steel material in which the generated thermal stress is small. I can say.

However, as described in Non-Patent Document 3, a steel material having a bcc structure such as martensite, bainite and ferrite has a problem that it is more easily hydrogen embrittled than an austenitic stainless steel having an fcc structure.

SCM435 described in Non-Patent Document 2 causes hydrogen embrittlement due to the influence of high-pressure hydrogen. In particular, as the strength is increased, hydrogen embrittlement becomes more remarkable. Therefore, when used in a high-pressure hydrogen environment, the heat treatment conditions must be changed to lower the strength as compared with that generally used. However, hydrogen embrittlement cannot be avoided even if the strength is lowered. Further, SNCM439, which has higher hardenability than SCM435 and is easy to secure strength even if the wall thickness is increased with a member having a higher pressure, is similar to SCM435 in that it is affected by hydrogen. When applying a steel material having a bcc structure that causes so-called hydrogen embrittlement, in which the material deteriorates due to the influence of hydrogen, to high-pressure hydrogen equipment, a large safety factor is taken and it is better than using it for high-pressure gas other than hydrogen. It is necessary to consider such as designing with a large plate thickness.

Even in in-vehicle equipment and hydrogen station equipment, not all members are necessarily exposed to high-pressure hydrogen of 35 MPa or 70 MPa, and there are containers and pipes for storing lower-pressure hydrogen. Further, for applications not related to fuel cells, it is assumed that hydrogen having a pressure of about 20 MPa is used. Therefore, by finding a high-strength bcc steel that does not become hydrogen embrittled up to about 25 MPa, a high-strength steel that can be designed with the characteristics in the atmosphere as it is can be obtained, which is a great merit.

In view of the above circumstances, the present invention has been made to provide a nickel steel material that can be used in a high-pressure hydrogen environment of more than 20 MPa and 25 MPa or less, which simultaneously solves the problems of hydrogen embrittlement resistance, strength, and cost. It is a thing.

The reason why the bcc structure is more susceptible to hydrogen than the fcc structure in high-pressure hydrogen is that the hydrogen dissolution limit of the fcc structure is generally larger than that of the bcc structure, and that the hydrogen in the fcc structure is more susceptible to hydrogen. The diffusion rate is extremely slow compared to the bcc tissue.

Based on the above-mentioned search results of Non-Patent Document 3, it is not possible to improve the hydrogen embrittlement resistance of a steel material mainly composed of a bcc structure simply by increasing the Ni content.

In order to use it as a structural steel material that composes hydrogen equipment, it is most important that it does not become hydrogen embrittled, but depending on the form provided such as plate materials and pipes and the type of member that will be finally used. The required strength and toughness are also important mechanical properties.

Therefore, the present inventors have conducted a number of studies on a means for suppressing a decrease in mechanical properties called hydrogen embrittlement in a high-pressure hydrogen environment, targeting a steel material mainly composed of a bcc structure.

First, the present inventors focused on the behavior of Ni in the steel material and repeated studies in order to enhance the hydrogen embrittlement resistance. Ni is an element that stabilizes the austenite phase, and in steel materials containing 3.5% by mass or more of Ni, when an austenite phase having an fcc structure is mixed in the bcc structure, it tends to be concentrated on the austenite phase side. be. However, the present inventors did not improve the hydrogen embrittlement resistance of the entire steel even if Ni was concentrated in the austenite phase, which was originally not easily affected by hydrogen, but rather contained a large amount of Ni in the bcc structure which is the matrix phase. It has been found that the hydrogen embrittlement resistance of the bcc structure is improved by the solid solution, and the hydrogen embrittlement resistance of the entire steel material including the austenite phase is improved.

Next, the present inventors focused on the metal structure and repeated studies in order to stably obtain hydrogen embrittlement resistance. Hydrogen is relatively easy to accumulate at the former austenite grain boundaries, and hydrogen embrittlement is relatively easy to occur compared to other sites. Therefore, when tensile stress acts on the steel material in high-pressure hydrogen, crack-like fracture is likely to occur at the old austenite grain boundaries. The present inventors have found that if the particle size of the old austenite is reduced, the propagation of cracks is suppressed, and as a result, the hydrogen embrittlement resistance of the steel material is stabilized.

From the above examination results, specifically, by satisfying the following two conditions at the same time, a Ni steel material mainly having a bcc structure having excellent hydrogen embrittlement resistance can be obtained even in a high-pressure hydrogen environment of 25 MPa or less. I found that.
(B) The Ni content of the steel material shall be 3.5% by mass or more, and the Ni content in the bcc structure mainly composed of martensite as the parent phase shall be secured at 2.8% by mass or more.
(B) The old austenite particle size of steel materials shall be 12.0 μm or less.

Furthermore, as a result of examining the relationship between the mechanical properties of the steel material and the hydrogen embrittlement resistance, assuming that it is applied to members such as high-pressure hydrogen containers in the above Ni steel material, the tensile strength was set to 930 MPa or less. , It has been found that it exhibits excellent hydrogen embrittlement resistance of 0.80 or more in relative drawing value in high-pressure hydrogen of 25 MPa or less.

The above relative drawing value is an index of hydrogen embrittlement resistance of steel materials, and the drawing value of steel materials during a tensile test in high-pressure hydrogen is drawn in an inert gas (or atmosphere) such as helium. It is the value divided by the value. Therefore, the closer the relative aperture value is to 1, the better the hydrogen embrittlement resistance. Practically, if the relative drawing value of the steel material is 0.80 or more, there is no problem and it can be used in a high-pressure hydrogen environment.

The present invention has been made based on the above findings, and the gist thereof is as follows.

(1) The chemical composition is mass%.
Ni: 3.5% or more and less than 9.0%,
C: 0.003 to 0.080%,
Si: 0.03 to 0.30%,
Mn: 0.10 to 2.00%,
Mo: 0.10 to 0.50%,
Al: 0.010 to 0.060%,
N: 0.0015 to 0.0050%,
O: 0.0030% or less,
P: 0.0070% or less,
S: 0.0040% or less,
Cu: 0-0.50%,
Cr: 0 to 0.50%,
Nb: 0 to 0.020%,
Ti: 0-0.020%,
V: 0 to 0.50%,
B: 0 to 0.0020%,
Ca: 0-0.0040%,
Rare earth elements: 0-0.0050%,
Remaining: Fe and impurities,
The metal structure is mainly composed of the body-centered cubic lattice structure.
The amount of Ni solid solution in the structure of the body-centered cubic lattice structure is 2.8% or more.
The old austenite particle size is 12.0 μm or less,
The tensile strength is 930 MPa or less.
Nickel steel for high-pressure hydrogen.

(2) The chemical composition is mass%.
V: Contains 0.10% or more,
The nickel steel material for high-pressure hydrogen described in (1) above.

(3) The tensile strength is 560 MPa or more.
The Ni steel material for high-pressure hydrogen according to (1) or (2) above.

(4) The tensile strength is 690 MPa or more.
The Ni steel material for high-pressure hydrogen according to (1) or (2) above.

(5) In the metal structure, the volume fraction of the austenite phase is 5.0% or less.
The nickel steel material for high-pressure hydrogen according to any one of (1) to (4) above.

According to the present invention, it is possible to provide a Ni steel material having sufficient hydrogen embrittlement resistance and high strength even in a high-pressure hydrogen environment of up to 25 MPa. This Ni steel material is used in high-pressure hydrogen applications such as tanks, pipes, valves for storing high-pressure hydrogen, hydrogen storage facilities, hydrogen supply facilities, fuel cell vehicles, hydrogen-utilizing power generation facilities, etc. in a high-pressure hydrogen environment. Are suitable. Therefore, if this Ni steel material is used for high-pressure hydrogen equipment, the plate thickness can be made thinner and lighter, and the amount of steel material used can be reduced as compared with SUS316-based austenitic stainless steel having a high Ni equivalent. In addition, the cost can be reduced as compared with high-strength austenitic stainless steel. Therefore, according to the present invention, it is possible to reduce the weight and cost of high-pressure hydrogen equipment, and improve the fuel efficiency of fuel cell vehicles. As described above, the industrial contribution of the present invention is extremely remarkable.

Hereinafter, the nickel steel material for high-pressure hydrogen according to this embodiment will be described. First, the chemical composition of nickel steel for high-pressure hydrogen will be described. The content "%" means "mass%" unless otherwise specified.

1. 1. Chemical composition Ni: 3.5% or more and less than 9.0% Ni is an essential element for ensuring hydrogen embrittlement resistance. If the Ni content is less than 9.0%, the drawing in a high-pressure hydrogen environment with a maximum of 25 MPa deteriorates, and the relative drawing value may drop to less than 0.80. In particular, when the hydrogen pressure rises or the operating temperature falls, the relative throttle value tends to decrease. Therefore, the Ni content is set to 3.5% or more. The Ni content should be increased in order to secure the relative throttle value, but assuming use in a high-pressure hydrogen environment with a maximum of 25 MPa, the merit of containing 9.0% or more of expensive Ni is small. .. Therefore, the Ni content is set to less than 9.0%. The preferred upper limit is 8.0%, the more preferred upper limit is 7.0%, and the more preferred upper limit is 6.0%.

C: 0.003 to 0.080%
C is an element that increases the yield stress at room temperature and also contributes to the formation of martensite and austenite. If the C content is less than 0.003%, the strength cannot be secured. Therefore, the C content is set to 0.003% or more. On the other hand, if the C content exceeds 0.080%, the strength becomes too high and the relative aperture value decreases, so the upper limit is set to 0.080%. The upper limit of the preferable C content is 0.070%, more preferably 0.060%, and the upper limit of the more preferable C content is 0.055%. In order to obtain a tensile strength of 560 MPa or more, the lower limit of the C content is preferably 0.005%, more preferably 0.010% or more, and further preferably 0.030% or more. preferable. In particular, in order to obtain a tensile strength of 690 MPa or more, the lower limit of the C content is preferably 0.040%.

Si: 0.03 to 0.30%
Si is an element that increases the yield stress. If the Si content is less than 0.03%, the effect of improving the yield stress at room temperature is small. Therefore, the Si content is set to 0.03% or more. The preferable lower limit of the Si content is 0.04%, and the more preferable lower limit is 0.05%. On the other hand, when the Si content exceeds 0.30%, the cementite at the former austenite grain boundaries tends to be coarsened, and the toughness is lowered. Therefore, the Si content is set to 0.30% or less. The upper limit of the preferable Si content is 0.20%, more preferably 0.15%, and the upper limit of the more preferable Si content is 0.10%.

Mn: 0.10 to 2.00%
Mn is an element that increases the yield stress at room temperature. If the Mn content is less than 0.10%, the strength cannot be ensured, and the toughness required as a structural material may decrease due to the formation of coarse bainite or the like. Therefore, the Mn content is set to 0.10% or more. The preferable lower limit of the Mn content is 0.20%. On the other hand, when the Mn content exceeds 2.00%, the Mn segregated at the old austenite grain boundaries and the MnS coarsely precipitated cause fracture at the grain boundaries, resulting in a decrease in toughness. Further, since Mn is also an austenite stabilizing element, if it is contained in a large amount, an austenite phase is excessively formed, and it becomes difficult to dissolve a sufficient amount of Ni in the bcc structure. Therefore, the Mn content is set to 2.00% or less. The upper limit of the preferable Mn content is 1.50%.

Mo: 0.10 to 0.50%
Mo is an element that increases the yield stress and also has the effect of suppressing grain boundary embrittlement. Therefore, the Mo content is set to 0.10% or more. The preferable lower limit of the Mo content is 0.15%, and the more preferable lower limit is 0.20%. On the other hand, Mo is an expensive element, and if the Mo content exceeds 0.50%, the economic efficiency is impaired. Further, since Mo is also an austenite stabilizing element, if it is added in a large amount, an austenite phase is excessively formed, and it becomes difficult to dissolve a sufficient amount of Ni in the bcc structure. Therefore, the Mo content is set to 0.50% or less. The preferable upper limit of the Mo content is 0.40%.

Al: 0.010 to 0.060%
Al is an element mainly used for deoxidation, and also contributes to the miniaturization of the metal structure and the reduction of the solid solution N that lowers the toughness by forming AlN. When the Al content is less than 0.010%, the deoxidizing effect, the metal structure refining effect, and the solid solution N reducing effect are small. Therefore, the Al content is set to 0.010% or more. The lower limit of the Al content is preferably 0.015%, more preferably 0.020%. However, if the Al content exceeds 0.060%, the toughness decreases. Therefore, the Al content is set to 0.060% or less. The preferred upper limit of the Al content is 0.050%, more preferably 0.040%.

N: 0.0015 to 0.0050%
N contributes to the formation of nitrides. When the N content is reduced to less than 0.0015%, fine AlN that suppresses the coarsening of the austenite particle size during the heat treatment may be insufficient, and the austenite particles may be coarsened to reduce the toughness. Therefore, the N content is set to 0.0015% or more. It is preferably 0.0020% or more. On the other hand, when the N content exceeds 0.0050%, the solid solution N increases and the AlN becomes coarse, so that the toughness decreases. Therefore, the N content is set to 0.0050% or less. The preferred upper limit of the N content is 0.0045%, more preferably 0.0040%.

O: 0.0030% or less O is an impurity. If the O content exceeds 0.0030%, _{the clusters of Al 2} O ₃ may increase and the toughness may decrease. Therefore, the upper limit of the O content is set to 0.0030% or less. The upper limit of the preferable O content is 0.0025%, more preferably 0.0020%, still more preferably 0.0015%. It is desirable that the O content is low, but reducing the O content to less than 0.0007% may be accompanied by an increase in cost. Therefore, the O content is preferably 0.0007% or more.

P: 0.0070% or less P is an element that causes grain boundary embrittlement at the former austenite grain boundaries and is harmful to toughness. Therefore, it is desirable that the P content is low. If the P content exceeds 0.0070%, the toughness may decrease. Therefore, the P content is set to 0.0070% or less. The upper limit of the P content is preferably 0.0050%, more preferably 0.0040%, still more preferably 0.0030%. P may be mixed as an impurity from scrap or the like during the production of molten steel, but the lower limit thereof does not need to be particularly limited, and the lower limit is 0%.

S: 0.0040% or less
S may be the starting point of brittle fracture as MnS, and is an element harmful to toughness. If the S content exceeds 0.0040%, the toughness may decrease. Therefore, the S content is set to 0.0040% or less. The upper limit of the S content is preferably 0.0030%, more preferably 0.0020%, and even more preferably 0.0010%. S may be mixed as an impurity from scrap or the like during the production of molten steel, but the lower limit thereof does not need to be particularly limited, and the lower limit is 0%.

Cu: 0-0.50%
Cu is an element that increases the yield stress at room temperature, and may contain Cu. However, if the Cu content exceeds 0.50%, the toughness decreases, so the upper limit is set to 0.50% or less. The upper limit of the Cu content is preferably 0.40% or less, more preferably 0.30% or less, still more preferably 0.20% or less. Cu may be mixed as an impurity from scrap or the like during the production of molten steel, but the lower limit thereof does not need to be particularly limited, and the lower limit is 0%. In order to obtain the above effect, the lower limit of Cu is preferably 0.01%, more preferably 0.03%, and even more preferably 0.04%.

Cr: 0 to 0.50%
Cr is an element that increases the yield stress at room temperature, and Cr may be contained. However, if the Cr content exceeds 0.50%, the toughness decreases. Therefore, the Cr content is set to 0.50% or less. The upper limit of the Cr content is preferably 0.30%, more preferably 0.20%, and even more preferably 0.10%. Cr may be mixed as an impurity from scrap or the like during the production of molten steel, but the lower limit thereof does not need to be particularly limited, and the lower limit is 0%. In order to obtain the above effect, the lower limit of Cr is preferably 0.01%.

Nb: 0 to 0.020%
Nb is an element that increases the yield stress at room temperature, and also has the effect of improving toughness by refining the metal structure, so Nb may be contained. However, if the Nb content exceeds 0.020%, the toughness decreases. Therefore, the Nb content is set to 0.020% or less. The upper limit of the Nb content is preferably 0.015%, more preferably 0.010%. Nb may be mixed as an impurity from scrap or the like during the production of molten steel, but the lower limit thereof does not need to be particularly limited, and the lower limit is 0%. In order to obtain the above effect, the lower limit of Nb is preferably 0.005%.

Ti: 0-0.020%
Ti may be contained because it forms TiN and contributes to the miniaturization of the metal structure and the reduction of the solid solution N that lowers the toughness. However, if the Ti content exceeds 0.020%, the toughness decreases. Therefore, the Ti content is set to 0.020% or less. The upper limit of the preferable Ti content is 0.015%, and the more preferable upper limit is 0.010%. Ti may be mixed as an impurity from scrap or the like during the production of molten steel, but it is not necessary to limit the lower limit thereof, and the lower limit is 0%. In order to obtain the above effect, the lower limit of Ti is preferably 0.008%.

V: 0 to 0.50%
V is an element that increases the yield stress at room temperature, and also has the effect of generating carbides (VC) to trap hydrogen that has entered from the environment and detoxify it, so V may be contained. .. However, if it is contained in excess of 0.50%, the toughness decreases. Therefore, the V content is set to 0.50% or less. The upper limit of the V content is preferably 0.40%. V may be mixed as an impurity from scrap or the like during the production of molten steel, but the lower limit thereof does not need to be particularly limited, and the lower limit is 0%. In order to obtain the above-mentioned hydrogen trap effect, the lower limit of V is preferably 0.001%, more preferably 0.005%, more preferably 0.010%, and 0. It is more preferably 10%.

B: 0 to 0.0020%
B is an element that increases the yield stress at room temperature, and also contributes to the reduction of solid solution N that forms BN and reduces toughness, and therefore B may be contained. However, if B is contained in excess of 0.0020%, the toughness decreases. Therefore, the B content is set to 0.0020% or less. The upper limit of the B content is preferably 0.0015%, more preferably 0.0012%, and even more preferably 0.0010%. B may be mixed as an impurity from scrap or the like during the production of molten steel, but the lower limit thereof does not need to be particularly limited, and the lower limit is 0%. In order to obtain the effect of B, the lower limit thereof is preferably 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%.

Ca: 0 to 0.0040%
Ca may be contained in Ca because it is effective in improving toughness by spheroidizing MnS as CaS, which is easily stretched by hot rolling to increase the harmfulness to toughness. However, when the Ca content exceeds 0.0040%, the acid sulfide containing Ca becomes coarse and the toughness decreases. Therefore, the Ca content is set to 0.0040% or less. The upper limit of the preferable Ca content is 0.0030%. Ca may be mixed as an impurity from scrap or the like during the production of molten steel, but the lower limit thereof does not need to be particularly limited, and the lower limit is 0%. In order to obtain the effect of Ca, the lower limit thereof is preferably 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%.

Rare earth element: 0-0.0050%
Like Ca, rare earth elements (REM) are effective in improving toughness by spheroidizing MnS, which is easily stretched by hot rolling and is likely to be harmful to toughness, as an acid sulfide of REM. May be contained. However, when the REM content exceeds 0.0050%, the acid sulfide containing REM becomes coarse and the toughness decreases. Therefore, the REM content is set to 0.0050% or less. The upper limit of the preferred REM content is 0.0047%. REM may be mixed as an impurity from scrap or the like during the production of molten steel, but the lower limit thereof does not need to be particularly limited, and the lower limit is 0%. In order to obtain the effect of REM, the lower limit thereof is preferably 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%.

The REM in the present specification contains at least one of 17 elements of Sc, Y, and lanthanoids (Atomic number 57 La to 71 Lu), and the REM content of these elements is the same. The total content.

The nickel steel material for high-pressure hydrogen according to this embodiment contains each of the above elements, and the balance is iron and impurities. Here, the impurity is a component mixed by various factors in the manufacturing process, including raw materials such as ore and scrap, when steel is industrially manufactured, and does not adversely affect the present invention. Means what is acceptable in the range. However, in the present invention, among the impurities, it is necessary to specify the upper limit for O, P and S as described above. In addition to the above components, the high-pressure hydrogen steel according to the present embodiment may contain the following alloying elements as impurities from auxiliary raw materials such as scrap.

Since Sb impairs toughness, the Sb content is preferably 0.005% or less, more preferably 0.003% or less, and most preferably 0.001% or less.

Since Sn impairs toughness, the Sn content is preferably 0.005% or less, more preferably 0.003% or less, and most preferably 0.001% or less.

Since As impairs toughness, the As content is preferably 0.005% or less, more preferably 0.003% or less, and most preferably 0.001% or less.

Further, Co, Zn and W are each preferably 0.010% or less, and more preferably 0.005% or less.

It is not necessary to limit the lower limit of Sb, Sn, As, Co, Zn and W, and the lower limit of each element is 0%. Further, even if an impurity element (for example, P, S) is intentionally added, an optional additive element (Cu, Cr, Mo, Nb, V, Ti, B, Ca, and REM) is mixed as an impurity. However, if the content is within the range specified in the claims, the Ni steel material for high-pressure hydrogen is interpreted as being within the range of the present invention.

2. Metal structure The nickel steel material for high-pressure hydrogen according to this embodiment mainly has a body-centered cubic lattice structure, that is, a bcc structure. The term "main body" means that most of the metal structure is a bcc structure, and specifically, the volume fraction is preferably 90.0% or more. A more preferable volume fraction is 95.0%. The bcc structure according to the present embodiment is mainly composed of martensite, but may contain bainite in a part thereof. The proportion of martensite in the bcc tissue may be 50.0% or more in terms of volume fraction.

As will be described later, in order to increase the amount of Ni solid solution in the bcc structure, it is desirable to reduce the austenite phase in which Ni tends to concentrate as much as possible. Therefore, the austenite phase preferably has a volume fraction of 5.0% or less, and more preferably 3.0% or less. More preferably, the volume fraction of the austenite phase is set to 0%, and the bcc structure is a single phase. However, the volume fraction of the austenite phase may exceed 5.0% as long as a sufficient amount of Ni solid solution in the bcc structure can be secured. The austenite phase referred to here is an austenite phase present in the Ni steel material after the heat treatment, unlike the old austenite. The volume fraction of the austenite phase is measured by X-ray diffraction.

Ni solid solution amount of bcc tissue: 2.8 mass% or more <br/> austenite phase is originally because less susceptible to hydrogen, resistance of the entire nickel steel even Ni is concentrated in the austenite phase Although the hydrogen brittle property is not improved, when a large amount of Ni is dissolved in the bcc structure which is the matrix, the hydrogen brittle property of the bcc structure is improved and the hydrogen brittle property of the entire nickel steel material is also improved. In particular, in order to obtain excellent hydrogen embrittlement resistance under the following high-pressure hydrogen environment 25MPa is, Ni solid solution amount of bcc tissue needs to be 2.8 mass% or more. The preferred lower limit is 3.0% by mass, more preferably 3.2% by mass. The larger the amount of Ni solid solution, the better the hydrogen embrittlement resistance, so it is not necessary to specify it. Substantially, it depends on the upper limit of the amount of Ni added and is less than 9.0% by mass.

Old austenite grain size: 12.0 μm or less When the old austenite grain size exceeds 12.0 μm, the crack-like fracture that occurs at the packet boundary of the old austenite grain boundary and the martensite transformed from the old austenite grain causes the destruction of the entire steel material. It becomes large enough to cause it, and it may become a starting point to accelerate the fracture of the entire steel material and lower the relative drawing value, which is an index of hydrogen embrittlement in hydrogen. Therefore, the particle size of the old austenite is set to 12.0 μm or less. The preferred upper limit is 10.0 μm. The smaller the particle size of the old austenite is, the more preferable it is. Considering the industrial manufacturing process, the lower limit of the old austenite particle size is preferably 3.0 μm. The old austenite particle size can be determined by burying a sample collected from a steel material in a resin, mirror-polishing the cross section, etching with a saturated aqueous solution of picric acid to which a surfactant is added, and magnifying and observing with an optical microscope or the like. Can be measured.

3. 3. Physical properties Tensile strength: 930 MPa or less Generally, it is known that hydrogen embrittlement of a steel material is more likely to occur as the strength of the steel material is increased, and the nickel steel material for high-pressure hydrogen according to the present embodiment is no exception. Even when the amount of Ni solid solution in the bcc structure is increased, if the tensile strength exceeds 930 MPa, the hydrogen embrittlement phenomenon is likely to occur, and the characteristics in high-pressure hydrogen are deteriorated. Therefore, the tensile strength of the steel material is set to 930 MPa or less. On the other hand, the lower the strength of the steel material, the less likely it is that hydrogen embrittlement will occur, but if the strength of the steel material is low, the plate thickness required for designing structures and equipment will increase, which is not preferable because the weight of the structure will increase. Therefore, the tensile strength of the steel material is preferably 560 MPa or more, more preferably 690 MPa or more.

4. Manufacturing Method Next, a steel sheet manufacturing method will be described as an example of the manufacturing method of the Ni steel material for high-pressure hydrogen according to the present embodiment.

The method for melting steel is, for example, after adjusting the element content in a state where the molten steel temperature is 1650 ° C. or less, the molten steel O concentration is 0.01% or less, and the molten steel S concentration is 0.02% or less. , Manufacture steel pieces by continuous casting. The obtained steel pieces are heated, hot-rolled, hardened by forced cooling such as water cooling, and then tempered in sequence. Hereinafter, preferable production conditions in each step will be described.

The heating temperature of the steel pieces to be hot-rolled is preferably 950 ° C. or higher and 1100 ° C. or lower. If the heating temperature is lower than 950 ° C., AlN may become coarse and the toughness may decrease. If the heating temperature exceeds 1100 ° C., the austenite particle size becomes coarse during heating, and the hydrogen embrittlement resistance and toughness may deteriorate. The upper limit of the more preferable heating temperature is 1050 ° C. The heating holding time is preferably 30 minutes to 180 minutes.

In hot rolling, when the rolling reduction at 950 ° C or lower is less than 85%, the austenite grains are not sufficiently finely divided by recrystallization of austenite during rolling, and some of the austenite grains after rolling become coarse. .. As a result, hydrogen embrittlement resistance and toughness may decrease. Therefore, the reduction rate at 950 ° C. or lower is preferably 85% or more. A more preferable reduction rate is 90% or more. Homogeneous granulation of old austenite grains by recrystallization of rolling is important for ensuring hydrogen embrittlement resistance and toughness, and strict regulation of rolling temperature and rolling reduction is preferable. If the reduction rate at 950 ° C. or lower exceeds 95%, the rolling time becomes long and a problem may occur in productivity. Therefore, the reduction rate at 950 ° C. or lower is preferably 95% or less.

If the end temperature of hot rolling is lower than 580 ° C., the water cooling start temperature may decrease, and it may not be possible to satisfy the preferable water cooling start temperature described later. Therefore, it is preferable that the end temperature is 580 ° C. or higher. A more preferable hot rolling end temperature is 600 ° C. or higher. On the other hand, when the end temperature of hot rolling exceeds 770 ° C., the dislocations introduced by rolling are reduced by recovery, and the grain size of the old austenite becomes larger than 12.0 μm. As a result, the hydrogen embrittlement resistance may be lowered, the toughness may be lowered, or the yield stress at room temperature may be insufficient. Therefore, the end temperature is preferably 770 ° C. or lower. A more preferable hot rolling end temperature is 750 ° C. or lower.

After hot rolling, it is preferable to water-cool to around room temperature. The water cooling start temperature is preferably 580 ° C. or higher and 770 ° C. or lower. When the water cooling start temperature is lower than 580 ° C., coarse bainite may be partially formed. Since this coarse bainite tends to remain as relatively coarse bainite even after heat treatment, it is difficult to control the particle size of the old austenite to 12.0 μm or less, and the hydrogen embrittlement resistance and toughness are deteriorated. There is. In addition, since it contains bainite transformed at high temperature, the yield stress at room temperature tends to decrease. Therefore, the water cooling start temperature is preferably 580 ° C. or higher, and the more preferable water cooling start temperature is 600 ° C. or higher. The upper limit of the water cooling start temperature does not need to be particularly regulated, and water cooling may be started immediately after the completion of hot rolling. 770 ° C., which is a preferable upper limit of the rolling end temperature, is a preferable upper limit of the water cooling start temperature.

Tempering is carried out for the purpose of adjusting the strength. The heating temperature is preferably 490 ° C. or higher and 590 ° C. or lower. If the tempering heating temperature (tempering temperature) is lower than 490 ° C., the tensile strength of the steel material may become too high and the toughness may decrease. A more preferable tempering temperature is 510 ° C. or higher. On the other hand, when the tempering temperature exceeds 590 ° C., the austenite phase at room temperature increases, and as a result, the amount of Ni solid solution in the bcc structure decreases, and the hydrogen embrittlement resistance may decrease. Therefore, the preferred tempering temperature is 590 ° C. or lower, more preferably 570 ° C. or lower, and even more preferably 560 ° C. or lower. The tempering retention time is preferably 20 to 180 minutes. As a cooling method at the time of tempering, it is desirable to perform water cooling in order to avoid tempering embrittlement.

As described above, in the case of Ni steel for low temperature, intermediate heat treatment is usually performed between water cooling (that is, quenching) and tempering after hot rolling. In this intermediate heat treatment, a part of the hardened structure is reverse-transformed into austenite, Ni is concentrated and stabilized, and stable retained austenite is increased in the steel material after the final heat treatment. This stable retained austenite phase plays a role in improving fracture toughness at extremely low temperatures. However, as a result of increasing the austenite phase and concentrating Ni, the amount of Ni solid solution in the bcc structure of the matrix phase, which is important for hydrogen embrittlement resistance, decreases. Therefore, it is not preferable to carry out the intermediate heat treatment in the Ni steel material for high pressure hydrogen according to the present embodiment.

In the above-mentioned manufacturing method, a method for manufacturing a thick steel sheet has been described as an example. However, the steel material of the present invention has the same steel pipe and other shapes. That is, a steel piece is manufactured, the heated steel piece is hot-worked, and a heat treatment is performed in which quenching and tempering are sequentially performed.

Hereinafter, examples will be shown as an example of the present invention, but the present invention is not limited to the examples described below.

Steel was melted by a converter, and slabs with a thickness of 100 mm to 240 mm were produced by continuous casting. Tables 1 and 2 show the chemical components of steel types A1 to A21. These slabs were heated, rolled hot, and then quenched by water cooling as they were, and further subjected to tempering heat treatment to produce a steel sheet. As a comparative example, some steel grades were subjected to an intermediate heat treatment. The holding time for heating in hot rolling was 30 to 120 minutes, and the holding time for intermediate heat treatment and tempering heat treatment was 20 to 60 minutes. A sample was taken from the steel sheet, and the metallographic structure, mechanical properties (tensile strength), and relative drawing value were evaluated.

The particle size of the old austenite was measured with the plane (L plane) parallel to the rolling direction at the center of the plate thickness as the observation plane. The particle size of the former austenite was determined in accordance with JIS G 0551. First, the observation surface of the sample is corroded with a saturated aqueous solution of picric acid to reveal the old austenite grain boundaries, and then with a scanning electron microscope (hereinafter, also referred to as “SEM”) at 1000 times or 2000 times, 5 fields or more. I took a picture. After identifying the prior austenite grain boundaries using microstructure photographs, the circle-equivalent particle size (diameter) was determined for at least 20 former austenite grains by a cutting method, and the average value thereof was taken as the particle size of the former austenite.

In the steel of the present invention, in order to suppress the destruction of the grain boundaries of the old austenite, the grain size of the old austenite is made finer and the P content is suppressed. Therefore, it may be difficult to identify the old austenite grain boundaries due to corrosion. In such a case, after heating to 450 to 490 ° C. and performing a heat treatment for holding for 1 hour or more, the particle size of the old austenite was measured by the above-mentioned method. If it is difficult to identify the old austenite grain boundaries even after heat treatment at 450 to 490 ° C, take a Charpy test piece from the heat-treated sample, perform an impact test at -196 ° C, and destroy it at the old austenite grain boundaries. The sample was used. In this case, a cross section of the fracture surface is prepared on a plane parallel to the rolling direction (L plane), and after corrosion, the old austenite grain boundaries of the fracture surface cross section at the center of the plate thickness are identified by SEM, and the old austenite grain size is determined. It was measured. When the former austenite grain boundaries are made brittle by heat treatment, minute cracks are generated in the former austenite grain boundaries due to the impact load during the Charpy test, so that the former austenite grain boundaries can be easily identified.

The volume fraction of the austenite phase was measured by X-ray diffraction method by taking a sample parallel to the plate surface at the center of the plate thickness. Using CrKα rays, the volume fraction of each phase was identified from the ratio of the integrated intensities of the diffraction peaks of austenite (fcc structure (111)) and tempered martensite (bcc structure (110)).

Regarding the amount of Ni solid solution in the bcc structure, first, the amount of Ni in the austenite phase was measured, and the amount of Ni solid solution in the bcc structure was calculated by using the relationship with the volume fraction of the austenite phase. To measure the amount of Ni in the austenite phase, a thin film sample with a thickness of 0.1 μm is taken from a steel material, and a transmission electron microscope (hereinafter, also referred to as “TEM”) is used to determine the portion of the thin film that is thought to be the austenite phase. After observing with a dark field image and confirming that it is an austenite phase by a diffraction pattern, an energy dispersion type X-ray analyzer (hereinafter, also referred to as "EDS") attached to the TEM is used to determine the Ni content in the portion. Measured using. This was carried out at 5 places in the thin film, and it was obtained from the average value.

Tensile strength was evaluated as a mechanical property. A No. 1A full-thickness tensile test piece specified in JIS Z 2241 whose longitudinal direction is parallel to the rolling direction (L direction) was sampled from each steel material and evaluated at room temperature by the method specified in JIS Z 2241. The target value of tensile strength is 930 MPa or less.

Furthermore, the relative aperture value was evaluated. First, a No. 4A round bar tensile test piece having a parallel portion diameter of 6 mm specified in JIS Z 2241 whose longitudinal direction is the direction parallel to the rolling direction (L direction) as the drawing value at room temperature in the atmosphere is collected from each steel material. It was evaluated by the method specified in JIS Z 2241. Next, the throttle value at room temperature in hydrogen was measured in high-pressure hydrogen at 25 MPa by collecting test pieces collected by the same method as in the atmosphere from each steel material, and subjected to a tensile test for evaluation. At this time, in order to make it possible to sufficiently evaluate the influence of hydrogen, the strain rate was 8 × ^10-5 (1 / s), which was slower than the specification of JIS Z 2241. Then, the diaphragm value obtained in hydrogen was divided by the diaphragm value obtained in the atmosphere to obtain a relative diaphragm value. The target value of the relative aperture value is 0.80 or more, preferably 0.90 or more. When the relative drawing value was 0.80 or more, it was determined that the hydrogen embrittlement resistance was excellent.

Tables 3 and 4 show the plate thickness, manufacturing method, mechanical properties (tensile strength), and metallographic structure of steel materials (manufacturing conditions No. 1-32) manufactured using slabs having chemical components of steel types A1 to A21. show.

As shown in Table 3, the manufacturing condition No. In 1 to 18, the intensity at room temperature and the relative aperture value satisfy the target values. However, the production condition No. 1 in which the Ni content of the steel material is low and the Ni solid solution amount in the bcc structure is low. Of 2 to 6, No. In 2, 3 and 5, the relative aperture value is within the target value, but is slightly lower. On the other hand, even if the Ni content of the steel material is low, the production condition No. 1 in which V is contained in an amount of 0.10% or more. For 4 and 6, high relative aperture values were shown. In addition, the manufacturing condition No. In No. 9, the water cooling start temperature is low and the old austenite particle size is within a preferable range but slightly larger, and the relative aperture value is within the target value but slightly lower. In addition, No. No. 10 has a high tempering temperature, and No. Since the C content of No. 17 is small, the tensile strength of each of 17 is lower than that of other examples of the present invention. No. In No. 18, the volume fraction of the austenite phase was as high as 5.2%, but the relative drawing value was high because the amount of Ni solid solution in the bcc structure was as high as 4.5%.

On the other hand, as shown in Table 4, the manufacturing condition No. In No. 19, since the C content is high, the strength is high and the relative aperture value is low. Manufacturing condition No. In Nos. 20 and 21, since the Ni content is low, the Ni content in the bcc structure is insufficient, and the relative drawing value is lowered. Manufacturing condition No. In 22 and 23, the Mn content and the Mo content are high, respectively, and the volume fraction of the austenite phase is high, so that the amount of Ni solid solution in the bcc structure is insufficient, and the relative drawing value is lowered. Manufacturing condition No. In No. 24, since the N content is high, the strength is high, the volume fraction of the austenite phase is high, and the amount of Ni solid solution in the bcc structure is also low, so that the relative drawing is lowered.

Manufacturing condition No. 25 to 32 are examples in which manufacturing conditions deviating from the preferable range are adopted. Manufacturing condition No. In No. 25, the tempering temperature is low and the strength is high, so that the relative drawing value is lowered. Manufacturing condition No. In Nos. 26 to 28, since the intermediate heat treatment was performed, the amount of Ni solid-solved in the austenite phase increased, and as a result, the amount of Ni solid-solved in the bcc structure decreased, so that the relative drawing value decreased. Manufacturing condition No. In No. 29, the heating temperature at the time of rolling is high, the particle size of the old austenite grains is large, and the relative drawing value is lowered. Manufacturing condition No. In No. 30, the reduction rate at 950 ° C. or lower is low, the particle size of the old austenite is large, and the relative drawing value is low. Manufacturing condition No. In No. 31, the rolling end temperature is high, the grain size of the old austenite is large, and the relative drawing value is low. Manufacturing condition No. In No. 32, the heating temperature and the rolling end temperature of hot rolling are low, the particle size of the old austenite is large, and the relative drawing value is low.

Claims (5)

The chemical composition is mass%,
Ni: 3.5% or more and less than 9.0%,
C: 0.003 to 0.080%,
Si: 0.03 to 0.30%,
Mn: 0.10 to 2.00%,
Mo: 0.10 to 0.50%,
Al: 0.010 to 0.060%,
N: 0.0015 to 0.0050%,
O: 0.0030% or less,
P: 0.0070% or less,
S: 0.0040% or less,
Cu: 0-0.50%,
Cr: 0 to 0.50%,
Nb: 0 to 0.020%,
Ti: 0-0.020%,
V: 0 to 0.50%,
B: 0 to 0.0020%,
Ca: 0-0.0040%,
Rare earth elements: 0-0.0050%,
Remaining: Fe and impurities,
The metal structure and the body-centered cubic lattice structure have a volume fraction of 90.0% or more .
The amount of Ni solid solution in the structure of the body-centered cubic lattice structure is 2.8 % by mass or more.
The old austenite particle size is 12.0 μm or less,
The tensile strength is 930 MPa or less.
Nickel steel for high-pressure hydrogen. When the chemical composition is mass%,
V: Contains 0.10% or more,
The nickel steel material for high-pressure hydrogen according to claim 1. The tensile strength is 560 MPa or more.
The nickel steel material for high-pressure hydrogen according to claim 1 or 2. The tensile strength is 690 MPa or more.
The nickel steel material for high-pressure hydrogen according to claim 1 or 2. In the metal structure, the volume fraction of the austenite phase is 5.0% or less.
The nickel steel material for high-pressure hydrogen according to the deviation from claims 1 to 4.