JP2019090069A - Nickel steel for high-pressure hydrogen - Google Patents

Abstract

To provide a nickel steel that can be used in a high-pressure hydrogen environment of 100 MPa or less.SOLUTION: A nickel steel for high-pressure hydrogen comprises, in mass%, Ni: 14.0% or more and less than 20.0%, C: 0.003-0.080%, Si: 0.03-0.30%, Mn: 0.10-0.50%, Mo: 0.10-0.50%, Al: 0.010-0.060%, N: 0.0015-0.0050%, O: 0.0030% or less, P: 0.0070% or less, S: 0.0040% or less, Cu: 0-0.50%, Cr: 0-0.50%, Nb: 0-0.020%, Ti: 0-0.020%, V: 0-0.50%, B: 0-0.0020%, Ca: 0-0.0040%, a rare earth element: 0-0.0050%, with the balance being Fe and impurities, wherein, the nickel steel is mainly composed of a body-centered cubic lattice structure, the Ni solid solution amount in the body-centered cubic lattice structure is 12.5% or more, the old austenite grain size is 12.0 μm or less, and the tensile strength is 930 MPa or less.SELECTED DRAWING: None

Description

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

  In recent years, expectations for the use of hydrogen as clean energy are increasing. With the prospect of widespread use of fuel cell vehicles using hydrogen, construction of a hydrogen station for supplying hydrogen to fuel cell vehicles is being carried out. In addition, facilities for storing a large amount of hydrogen and use of hydrogen for power generation etc. are being considered for hydrogen supply to stations. In general, hydrogen existing as a gas has a lower energy density than liquid fuel such as gasoline, and fuel cell vehicles and the like store hydrogen compressed to a high pressure in order to secure a load capacity. Such high pressure hydrogen is known to degrade the properties of various materials. This phenomenon is generally called hydrogen embrittlement. For this reason, it is required to use a special material which is less likely to cause hydrogen embrittlement as a steel material used for high pressure hydrogen tanks for vehicles or hydrogen stations, piping and the like. Therefore, Ni steel is attracting attention.

  As Ni steel materials, for example, as described in Patent Document 1, development for use in a cryogenic environment such as a storage tank of liquefied natural gas (LNG) is in progress. In order to improve the toughness at cryogenic temperatures such as -162 ° C, Ni steels for low temperatures are subjected to special heat treatment (generally called "intermediate heat treatment") on steel materials after hot rolling to actively retain retained austenite. Volume fraction of

On the other hand, with regard to high-pressure hydrogen tanks for vehicles, in Non-Patent Document 1, only austenitic stainless steels SUS316 and SUS316L are recognized as usable steel materials, and further, the Ni equivalent of the following formula (1) is limited depending on the use temperature. ing.
Ni equivalent (%) = Ni + 12.6C + 1.05Mn + 0.35Si + 0.65Cr + 0.98Mo (1)

  Although SUS316 conforming to this example standard has sufficient resistance to hydrogen embrittlement, the yield stress at room temperature is about 300 MPa (JIS standard: 175 MPa or more), which is a low class as a steel material for construction. In the case where such a relatively low strength SUS 316 is applied to a high pressure hydrogen tank, a very large thickness is required. For this reason, in fuel cell vehicles where weight reduction is important, there is an example in which this steel material is adopted for some parts such as valves and bosses, but there is no example adopted for the tank main body. In addition, also in piping, a very large wall thickness is required, and since the amount of steel material used increases, weight and cost increase.

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

  In Non-Patent Document 2, 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 or bainite It is described that low alloy steel (SCM 435) can be used for high pressure hydrogen tanks for hydrogen stations up to 35 MPa.

  Non-Patent Document 3 reports the results of investigation of the mechanical properties of Fe-9Ni-4Co steel, which is a bcc structure-based Ni steel, in high pressure hydrogen, and this steel exhibits significant hydrogen embrittlement. It is classified as a material whose properties are degraded and which is strongly affected by hydrogen.

JP 2014-125678 A International Publication No. 2012/132992

High Pressure Gas Safety Association standard "Technical standard of 70MPa compressed hydrogen automobile fuel container" KHK S0128 Automobile research institute standard "Technical standard of container for compressed hydrogen automobile fuel system" JARI S001 W. T. Chandler and R. J. Walter, "Hydrogen embrittlement testing", ASTM STP 543, ASTM (1974), p.

  In the low-temperature Ni steel material as described in Patent Document 1, the purpose of containing a large amount of Ni is to secure fracture toughness at cryogenic temperatures such as the LNG temperature. In this low-temperature Ni steel, 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 complex heat treatment is performed, in which after hot rolling, hardening is performed and then intermediate heat treatment is performed and then tempering. However, according to the study of the present inventors, it has been found that the steel obtained by performing the heat treatment including such an intermediate heat treatment is deteriorated in the resistance to hydrogen embrittlement.

  The steel material of Patent Document 2 has a problem that the cost is extremely high because it contains a very large number of alloying elements.

  Here, in the hydrogen station which supplies high pressure hydrogen, it is necessary to enable filling within 3 minutes. It is known that when hydrogen 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 specified that the temperature of the tank does not exceed 85 ° C. at the time of filling because materials for which the upper limit temperature of use is limited are used for parts of the safety valve required for high pressure equipment. Therefore, the hydrogen station solves the problem of short time filling and temperature rise at the same time by supplying high pressure hydrogen previously cooled to about -50 ° C. Therefore, materials for high pressure hydrogen used in fuel cell vehicles need to assume temperature changes between -50 ° C and 85 ° C.

  Austenitic stainless steel is mainly composed of an austenitic phase (hereinafter, also referred to as “fcc structure”) having a crystal structure (hereinafter, also referred to as “fcc structure”) of a face-centered cubic lattice (hereinafter, also referred to as “fcc”). However, in general, it is known that a steel material of fcc structure tends to have a larger coefficient of thermal expansion than a steel material mainly 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 a temperature change, and if a large temperature change is assumed as described above, a bcc-based steel material having a smaller thermal stress is more desirable. It can be said.

  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 likely to be hydrogen-embrittled than an austenitic stainless steel having an fcc structure.

  SCM 435 described in Non-Patent Document 2 causes hydrogen embrittlement due to the influence of high pressure hydrogen. In particular, since hydrogen embrittlement becomes more remarkable as the strength is increased, when used in a high pressure hydrogen environment, the heat treatment conditions are changed to lower the strength than generally used, and the use must be made. However, even if the strength is reduced, hydrogen embrittlement can not be avoided. In addition, SNCM 439 or the like, which has higher hardenability than SCM 435 and can easily secure strength even if made thicker with a higher pressure member, has been studied, but it is similar to SCM 435 in that it is affected by hydrogen. When applying a steel material with a bcc structure that causes so-called hydrogen embrittlement, the quality of which decreases due to the influence of hydrogen, to a high-pressure hydrogen device, the safety factor is large and it is better to use for high-pressure gas other than hydrogen. Also, consideration should be given such as designing with a large thickness.

  Among the hydrogen devices for 70 MPa, for the in-vehicle devices, it is necessary to consider a pressure fluctuation of 25%, and it is required not to embrittle with about 88 MPa of hydrogen. Therefore, by finding a steel material having a high strength bcc structure that does not hydrogen embrittle to about 100 MPa, a high strength steel material that can be designed as it is in the atmosphere is obtained, which is a great merit.

  The present invention has been made in view of the above-mentioned circumstances and provides a nickel steel material usable in a high pressure hydrogen environment of more than 20 MPa and 100 MPa or less, which simultaneously solves the problems of hydrogen embrittlement resistance and strength, and cost. It is

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

  According to the investigation result of Non-Patent Document 3 mentioned above, in the steel having a bcc structure as a main component, the hydrogen embrittlement resistance can not be improved simply by simply increasing the Ni content.

  In order to be used as a structural steel material that constitutes equipment for hydrogen, it is most important not to embrittle hydrogen, but it depends on the type of provided material such as plate material and pipe and the kind of member finally used. The required strength and toughness are also important mechanical properties.

  Therefore, the present inventors conducted a number of studies on means for suppressing the deterioration of mechanical properties called hydrogen embrittlement in a high pressure hydrogen environment, for steel materials mainly having a bcc structure.

  First, in order to enhance the resistance to hydrogen embrittlement, the present inventors repeatedly studied by focusing on the behavior of Ni in steel materials. Ni is an element that stabilizes the austenite phase, and in a steel containing 3.5% by mass or more of Ni, when the austenite phase of the fcc structure is mixed in the bcc structure, it tends to be concentrated on the austenite phase side is there. 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 is originally not easily influenced by hydrogen, but rather, it is more Ni in the matrix bcc structure. It was found that the hydrogen embrittlement resistance of the bcc structure is improved by solid solution, and the hydrogen embrittlement resistance of the entire steel material including the austenite phase is improved.

  Next, in order to stably obtain the resistance to hydrogen embrittlement, the present inventors repeated studies focusing on the metal structure. The former austenite grain boundaries are relatively easy to accumulate hydrogen, and are relatively susceptible to hydrogen embrittlement as compared with other portions. For this reason, when tensile stress acts on the steel material in high pressure hydrogen, it is likely to cause fracture like fracture at the prior austenite grain boundary. The inventors of the present invention found that if the prior austenite grain size 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, by satisfying the following two conditions simultaneously, Ni steels mainly composed of bcc structure excellent in resistance to hydrogen embrittlement can be obtained even under a high pressure hydrogen environment of 100 MPa or less. I found that.
(I) After the Ni content of the steel material is 14.0 mass% or more, securing 12.5 mass% or more of the Ni content in the bcc structure mainly composed of martensite as a matrix.
(B) Making the prior austenite grain size of the steel material 12.0 μm or less.

  Furthermore, assuming that the above-described Ni steel material is applied to a member such as a high-pressure hydrogen container, the relationship between the mechanical properties of the steel material and the resistance to hydrogen embrittlement is examined, and the tensile strength is 930 MPa or less It has been found that it exhibits excellent resistance to hydrogen embrittlement with a relative squeeze value of not less than 0.80 in high pressure hydrogen at 100 MPa or less.

  The above relative squeeze value is an index of hydrogen embrittlement resistance of the steel material, and the squeeze value of the steel material at the time of the tensile test in high pressure hydrogen is squeezed in an inert gas (or the atmosphere) such as helium. It is the value divided by the value. Therefore, the closer the relative reduction value to 1, the better the resistance to hydrogen embrittlement. In practice, if the relative reduction value of the steel material is 0.80 or more, it can be used in a high pressure hydrogen environment without any problem.

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

(1) The chemical composition is in mass%,
Ni: 14.0% or more and less than 20.0%,
C: 0.003 to 0.080%,
Si: 0.03 to 0.30%,
Mn: 0.10 to 0.50%,
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 to 0.50%,
Cr: 0 to 0.50%,
Nb: 0 to 0.020%,
Ti: 0 to 0.020%,
V: 0 to 0.50%,
B: 0 to 0.0020%,
Ca: 0 to 0.0040%,
Rare earth elements: 0 to 0.0050%,
Remainder: Fe and impurities,
The metallographic structure is mainly composed of a body-centered cubic lattice structure,
The solid solution content of Ni in the structure of the body-centered cubic lattice structure is at least 12.5%,
The prior austenite grain 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 in mass%,
V: containing at least 0.10%,
The nickel steel material for high pressure hydrogen according to the above (1).

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

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

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

  According to the present invention, it is possible to provide a Ni steel material having sufficient resistance to hydrogen embrittlement and high strength even under a high pressure hydrogen environment of up to 100 MPa. This Ni steel material is used under high pressure hydrogen environment in high pressure hydrogen applications, for example, tanks for storing high pressure hydrogen, piping, valves, hydrogen storage facilities, hydrogen supply facilities, fuel cell vehicles, hydrogen power generation facilities, etc. Is suitable. Therefore, when this Ni steel material is used for high-pressure hydrogen equipment, it is possible to reduce the thickness and weight and to reduce the amount of steel material used, as compared to a high Ni equivalent SUS316 austenitic stainless steel. In addition, cost can be reduced as compared to high strength austenitic stainless steel. Therefore, according to the present invention, weight reduction and cost reduction of high pressure hydrogen devices can be promoted, and fuel efficiency of fuel cell vehicles can be improved. Thus, the present invention is extremely significant for industrial contribution.

  Hereinafter, the high-pressure hydrogen nickel steel material according to the present embodiment will be described. First, the chemical composition of the high-pressure hydrogen nickel steel material will be described. In addition, "%" of content means "mass%", unless there is particular explanation.

1. Chemical composition Ni: 14.0% or more and less than 20.0% Ni is an essential element for securing hydrogen embrittlement resistance. If the Ni content is less than 9.0%, the reduction in a high pressure hydrogen environment at a maximum of 100 MPa may be deteriorated, and the relative reduction value may be reduced to less than 0.80. In particular, when the pressure of hydrogen increases or the operating temperature decreases, the relative throttle value tends to decrease. Therefore, the Ni content is 14.0% or more. The Ni content should be increased in order to secure the relative squeeze value, but assuming use in a high-pressure hydrogen environment of up to 100 MPa, the merit of containing 20.0% or more of expensive Ni is small . Therefore, the Ni content is less than 20.0%. The upper limit is preferably 19.0%, more preferably 18.0%, and still more preferably 17.0%.

C: 0.003 to 0.080%
C is an element that raises 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 can not be secured. Therefore, the C content is made 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 made 0.080%. The upper limit of the C content is preferably 0.070%, more preferably 0.060%, and still more preferably 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 raises 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 made 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%, cementite in the prior austenite grain boundaries tends to be coarsened, and the toughness is lowered. Therefore, the Si content is 0.30% or less. The upper limit of the Si content is preferably 0.20%, more preferably 0.15%, and the upper limit of the Si content is more preferably 0.10%.

Mn: 0.10 to 0.50%
Mn is an element that raises the yield stress at room temperature. If the Mn content is less than 0.10%, the strength can not be secured, and the formation of coarse bainite or the like may lower the toughness required as a structural material. Therefore, the Mn content is 0.10% or more. The preferable lower limit of the Mn content is 0.20%. On the other hand, when the Mn content exceeds 0.50%, fracture at grain boundaries occurs due to Mn segregated at prior austenite grain boundaries and MnS precipitated coarsely, and the toughness decreases. Further, since Mn is also an austenite stabilizing element, if a large amount of Mn is contained, an austenite phase is excessively formed, and it becomes difficult to form a solid solution of a sufficient amount of Ni in the bcc structure. Therefore, the Mn content is made 0.50% or less. The upper limit of the preferable Mn content is 0.40%.

Mo: 0.10 to 0.50%
Mo is an element that raises the yield stress, and also has the effect of suppressing intergranular embrittlement. Therefore, the Mo content is 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 economy is impaired. Further, since Mo is also an austenite stabilizing element, if a large amount of Mo is added, 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 made 0.50% or less. The preferred 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 forms AlN to contribute to the refinement of the metal structure and the reduction of solid solution N that lowers the toughness. If the Al content is less than 0.010%, the effect of deoxidation, the effect of refining the metal structure, and the effect of reducing solid solution N are small. Therefore, the Al content is made 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 upper limit of the Al content is preferably 0.050%, and more preferably 0.040%.

N: 0.0015 to 0.0050%
N contributes to the formation of nitride. When the N content is reduced to less than 0.0015%, fine AlN that suppresses the coarsening of the austenite grain size during heat treatment may be insufficient, and the austenite grains may be coarsened to lower the toughness. Therefore, the N content is made 0.0015% or more. Preferably, it is 0.0020% or more. On the other hand, when the N content exceeds 0.0050%, solid solution N increases or AlN coarsens, so the toughness decreases. Therefore, the N content is made 0.0050% or less. The upper limit of the N content is preferably 0.0045%, more preferably 0.0040%.

O: 0.0030% or less O is an impurity. When the O content exceeds 0.0030%, Al ₂ O ₃ clusters may increase and the toughness may decrease. Therefore, the upper limit of the O content is made 0.0030% or less. The upper limit of the O content is preferably 0.0025%, more preferably 0.0020%, and still more preferably 0.0015%. While it is desirable for the O content to be low, reducing the O content to less than 0.0007% may be accompanied by cost increases. Therefore, the O content is preferably made 0.0007% or more.

P: 0.0070% or less P causes intergranular embrittlement at prior austenite grain boundaries and is a harmful element to toughness. Therefore, the lower the P content, the better. If the P content exceeds 0.0070%, the toughness may decrease. Therefore, the P content is made 0.0070% or less. The upper limit of the P content is preferably 0.0050%, more preferably 0.0040%, and still more preferably 0.0030%. P may be mixed as impurities from scraps or the like during molten steel production, but the lower limit is not particularly limited, and the lower limit is 0%.

S: 0.0040% or less
S, which may be a starting point of brittle fracture as MnS, is an element harmful to toughness. If the S content exceeds 0.0040%, the toughness may decrease. Therefore, the S content is made 0.0040% or less. The upper limit of the S content is preferably 0.0030%, more preferably 0.0020%, and still more preferably 0.0010%. S may be mixed as impurities from scraps or the like during molten steel production, but the lower limit is not particularly limited, and the lower limit is 0%.

Cu: 0 to 0.50%
Cu is an element that raises the yield stress at room temperature, and may contain Cu. However, since toughness will fall when Cu content exceeds 0.50%, an upper limit is made into 0.50% or less. The upper limit of the Cu content is preferably 0.40% or less, more preferably 0.30% or less, and still more preferably 0.20% or less. Although Cu may be mixed as impurities from scraps or the like at the time of production of molten steel, the lower limit thereof need not be particularly limited, and the lower limit is 0%. In order to obtain the above effects, the lower limit of Cu is preferably 0.01%, more preferably 0.03%, and still more preferably 0.04%.

Cr: 0 to 0.50%
Cr is an element that raises the yield stress at room temperature, and may contain Cr. However, if the Cr content exceeds 0.50%, the toughness decreases. Therefore, the Cr content is made 0.50% or less. The upper limit of the Cr content is preferably 0.30%, more preferably 0.20%, still more preferably 0.10%. Cr may be mixed as impurities from scraps or the like at the time of production of molten steel, but the lower limit thereof need not 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 raises 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, when the Nb content exceeds 0.020%, the toughness decreases. Therefore, the Nb content is made 0.020% or less. The upper limit of the Nb content is preferably 0.015%, more preferably 0.010%. Nb may be mixed as impurities from scraps or the like at the time of production of molten steel, but the lower limit is not 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 to 0.020%
Since Ti forms TiN and contributes to the refinement of the metal structure and the reduction of solid solution N which lowers the toughness, Ti may be contained. However, if the Ti content exceeds 0.020%, the toughness decreases. Therefore, the Ti content is made 0.020% or less. The upper limit of the Ti content is preferably 0.015%, and more preferably 0.010%. Although Ti may be mixed as impurities from scraps or the like at the time of production of molten steel, the lower limit thereof need not be particularly limited, 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 has the effect of forming carbides (VC) and trapping hydrogen harmed from the environment, so V may be contained. . However, if the content is more than 0.50%, the toughness is reduced. Therefore, the V content is 0.50% or less. The upper limit of the V content is preferably 0.40%. V may be mixed as impurities from scraps or the like at the time of production of molten steel, but the lower limit thereof need not be particularly limited, and the lower limit is 0%. In order to obtain the aforementioned hydrogen trapping effect, the lower limit of V is preferably 0.001%, more preferably 0.005%, still more preferably 0.010%, and 0. 0%. More preferably, it is 10%.

B: 0 to 0.0020%
B is an element that raises the yield stress at room temperature, and forms BN, which also contributes to the reduction of solid solution N that lowers toughness. Therefore, B may be contained. However, when the B content exceeds 0.0020%, the toughness is lowered. Therefore, the B content is made 0.0020% or less. The upper limit of the B content is preferably 0.0015%, more preferably 0.0012%, and still more preferably 0.0010%. B may be mixed as impurities from scraps or the like at the time of production of molten steel, but the lower limit thereof need not 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 still more preferably 0.0005%.

Ca: 0 to 0.0040%
Ca is effective for improving toughness by spheroidizing MnS, which is likely to increase the harmfulness to toughness by hot rolling, as CaS, and may contain Ca. However, if the Ca content exceeds 0.0040%, the Ca-containing acid sulfide is coarsened to lower the toughness. Therefore, the Ca content is made 0.0040% or less. The upper limit of the preferred Ca content is 0.0030%. Ca may be mixed as impurities from scraps or the like during molten steel production, but the lower limit is not particularly limited, and the lower limit is 0%. In order to obtain the effect of Ca, the lower limit thereof is preferably set to 0.0001%, more preferably set to 0.0005%, and still more preferably set to 0.0010%.

Rare earth element: 0 to 0.0050%
The rare earth elements (REM), like Ca, are effective in spheroidizing MnS, which is likely to increase the harmfulness to toughness by hot rolling, as REM oxysulfide and improve the toughness. May be contained. However, when the REM content exceeds 0.0050%, the acid sulfide containing REM is coarsened to lower the toughness. Therefore, the REM content is made 0.0050% or less. The upper limit of the preferred REM content is 0.0047%. Although REM may be mixed as impurities from scraps or the like at the time of production of molten steel, the lower limit thereof need not be particularly limited, and the lower limit is 0%. In order to obtain the effect of REM, the lower limit thereof is preferably set to 0.0001%, more preferably set to 0.0005%, and still more preferably set to 0.0010%.

  Note that REM in the present specification contains at least one or more of Sc, Y, and 17 elements of lanthanoids (La from 57 to 71), and the REM content of these elements is It is the total content.

  The high-pressure hydrogen nickel steel material according to the present embodiment contains the above-described elements, with the balance being iron and impurities. Here, the impurities are components which are mixed due to various factors of the manufacturing process, including raw materials such as ore and scraps when industrially manufacturing steel, and do not adversely affect the present invention It means what is permitted in the range. However, in the present invention, among the impurities, O, P and S need to have upper limits as described above. In addition to the above components, the steel for high pressure hydrogen 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.

  As 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, each of Co, Zn and W is preferably 0.010% or less, more preferably 0.005% or less.

  It is not necessary to limit the lower limits of Sb, Sn, As, Co, Zn and W, and the lower limit of each element is 0%. Also, even if an impurity element (for example, P, S) is intentionally added, any 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 defined in the claims, the high-pressure hydrogen Ni steel material is interpreted as within the scope of the present invention.

2. Metallographic Structure The nickel steel material for high pressure hydrogen according to the present embodiment mainly has a structure of a body-centered cubic lattice structure, that is, a bcc structure. Being mainly composed means that the majority in the metallographic structure is a bcc tissue, and specifically, it is preferable that the volume fraction is 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 bainite may be partially contained. The proportion of martensite in the bcc structure may be 50.0% or more in volume fraction.

  As described later, in order to increase the amount of solid solution of Ni in the bcc structure, it is desirable to reduce the austenite phase in which Ni is easily concentrated as much as possible. Therefore, the austenite phase is preferably 5.0% or less by volume fraction, and more preferably 3.0% or less. More preferably, the volume fraction of the austenite phase is 0%, and the bcc structure is single phase. However, the volume fraction of the austenite phase may exceed 5.0% as long as the Ni solid solution amount in the bcc structure can be sufficiently secured. In addition, the austenite phase here is an austenite phase which exists in Ni steel materials after heat treatment unlike prior austenite. The volume fraction of the austenite phase is measured by X-ray diffraction.

Ni solid solution amount in bcc structure: 12.5% by mass or more Since the austenite phase is originally hardly affected by hydrogen, even if Ni is concentrated in the austenite phase, the hydrogen embrittlement resistance of the entire nickel steel is Although it does not improve, when a large amount of Ni is dissolved in the matrix phase bcc structure, the hydrogen embrittlement resistance property of the bcc structure is improved, and the hydrogen embrittlement resistance property of the entire nickel steel material is also improved. In particular, in order to obtain excellent hydrogen embrittlement resistance under a high pressure hydrogen environment of 100 MPa or less, the amount of Ni solid solution in the bcc structure needs to be 12.5% by mass or more. The lower limit is preferably 13.0% by mass, more preferably 13.5% by mass. The amount of Ni solid solution is not particularly limited because it improves the resistance to hydrogen embrittlement as the amount increases. Substantially, depending on the upper limit of the amount of Ni to be added, it is less than 20.0%.

Old austenite grain size: 12.0 μm or less If the former austenite grain size exceeds 12.0 μm, the crack-like fracture generated at the former austenite grain boundary or at the packet boundary of martensite transformed from the former austenite grain fractures the entire steel material It may become large to cause it, and as a starting point, it may accelerate the destruction of the entire steel material and reduce the relative reduction value, which is an indicator of hydrogen embrittlement in hydrogen. Therefore, the prior austenite grain size is 12.0 μm or less. The preferred upper limit is 10.0 μm. The former austenite grain size is preferably as small as possible, but the grain refinement is accompanied by an increase in manufacturing cost, such as an increase in the number of heat treatments. In consideration of an industrial manufacturing process, the lower limit of the prior austenite grain size is preferably 3.0 μm. The former austenite grain size is obtained by burying a sample collected from a steel material in a resin and mirror polishing the cross section, and then etching with a picric acid saturated aqueous solution to which a surfactant is added, for example, and enlarging observation with an optical microscope. It can be measured.

3. Physical Properties Tensile Strength: 930 MPa or Less Generally, hydrogen embrittlement of steel materials is known to occur more easily as the strength of the steel materials increases, and the nickel steel materials for high pressure hydrogen according to this embodiment are no exception. Even when the solid solution amount of Ni in the bcc structure is increased, if the tensile strength exceeds 930 MPa, the hydrogen embrittlement phenomenon easily occurs, and the characteristics in high pressure hydrogen deteriorate. Therefore, the tensile strength of the steel material is 930 MPa or less. On the other hand, hydrogen embrittlement is less likely to occur as the strength of the steel material is lower, but if the strength of the steel material is lower, the thickness of the plate necessary for designing a structure or equipment is increased. Therefore, the tensile strength of the steel material is desirably 560 MPa or more, and more desirably 690 MPa or more.

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

  The method of melting steel materials is, for example, after adjusting the content of elements 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 , To produce billets by continuous casting. The obtained billet is heated, subjected to hot rolling, and quenched by forced cooling such as water cooling, and then heat treatment is sequentially applied to tempering. Hereinafter, preferable manufacturing conditions in each step will be described.

  It is preferable that the heating temperature of the steel piece which performs hot rolling sets it as 950 degreeC or more and 1100 degrees C or less. When the heating temperature is lower than 950 ° C., AlN may be coarsened and the toughness may be reduced. When the heating temperature exceeds 1100 ° C., the austenite grain size becomes coarse during heating, and the hydrogen embrittlement resistance and toughness may be lowered. The upper limit of the heating temperature is more preferably 1050 ° C. The holding time of heating is preferably 30 minutes to 180 minutes.

  In hot rolling, when the rolling reduction at 950 ° C. or less is less than 85%, the austenite grain refinement due to recrystallization of austenite during rolling becomes insufficient and a part of austenite grains after rolling becomes coarse . As a result, the hydrogen embrittlement resistance and toughness may be reduced. Therefore, the rolling reduction at 950 ° C. or less is preferably 85% or more. A more preferable rolling reduction is 90% or more. Homogenous grain refinement of prior austenite grains by recrystallization of rolling is important in securing hydrogen embrittlement resistance and toughness, and strict control of rolling temperature and rolling reduction is preferred. If the rolling reduction at 950 ° C. or less exceeds 95%, rolling time will be long, which may cause problems in productivity. Therefore, the rolling reduction at 950 ° C. or less is preferably 95% or less.

  When the end temperature of the hot rolling is below 580 ° C., the water cooling start temperature may be lowered, and it may not be possible to meet the preferable water cooling start temperature described later. Therefore, the end temperature is preferably 580 ° C. or higher. The more preferable completion | finish temperature of hot rolling is 600 degreeC or more. On the other hand, when the finish temperature of hot rolling exceeds 770 ° C., the dislocations introduced by rolling decrease due to recovery, and the grain size of the prior austenite becomes larger than 12.0 μm. As a result, the hydrogen embrittlement resistance may be lowered, the toughness may be lowered, and the yield stress at room temperature may be insufficient. Therefore, the end temperature is preferably 770 ° C. or less. A more preferable hot rolling finish temperature is 750 ° C. or less.

  After hot rolling, it is preferable to carry out water cooling to room temperature vicinity. The water cooling start temperature is preferably 580 ° C. or more and 770 ° C. or less. 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 if it is subsequently subjected to heat treatment, it becomes difficult to control the former austenite grain size to 12.0 μm or less, and the hydrogen embrittlement resistance and toughness deteriorate. There is. In addition, since it contains bainite transformed at high temperature, yield stress at room temperature is likely to be reduced. Accordingly, the water cooling start temperature is preferably 580 ° C. or higher, and more preferably 600 ° C. or higher. The upper limit of the water cooling start temperature does not have to be particularly limited, and the water cooling may be started immediately after the end of the hot rolling. The preferable upper limit of the rolling end temperature, 770 ° C., is the preferable upper limit of the water cooling start temperature.

  Tempering is performed for the purpose of adjusting the strength. The heating temperature is preferably 490 ° C. or more and 590 ° C. or less. When the heating temperature (tempering temperature) of tempering is lower than 490 ° C., the tensile strength of the steel material may be too high, and the toughness may be reduced. A more preferable tempering temperature is 510 ° C. or more. 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 may decrease, and the hydrogen embrittlement resistance may decrease. Therefore, a preferable tempering temperature is 590 ° C. or less, more preferably 570 ° C. or less, and still more preferably 560 ° C. or less. The retention time of tempering is preferably 20 minutes to 180 minutes. It is desirable that the cooling method during tempering be water cooling to avoid temper embrittlement.

  In addition, as above-mentioned, in the case of Ni steel materials for low temperatures, intermediate heat treatment is usually implemented between the water-cooling (namely, hardening) and temper after hot rolling. This intermediate heat treatment reversely converts a part of the quenched structure to austenite, as well as enriches and stabilizes Ni, and increases stable retained austenite in the steel material after final heat treatment. This stable retained austenite phase plays a role in improving the fracture toughness at cryogenic temperatures. However, as the austenite phase is increased and Ni is enriched, the amount of Ni solid solution in the bcc structure of the parent phase, which is important for hydrogen embrittlement resistance, decreases. Therefore, in the Ni steel material for high pressure hydrogen according to the present embodiment, it is not preferable to carry out the intermediate heat treatment.

  In the above-described manufacturing method, a method of manufacturing a thick steel plate has been described as an example. However, the steel material of the present invention is similar to a steel pipe or other shapes. That is, a billet is manufactured, and the heated billet is hot-worked, and heat treatment is carried out in which quenching and tempering are sequentially performed.

  Hereinafter, although an Example is shown as an example of this invention, this invention is not restrict | limited to the Example described below.

  The steel was melted by a converter, and a slab of 100 mm to 240 mm in thickness was manufactured by continuous casting. Tables 1 and 2 show chemical components of steel types A1 to A21. These slabs were heated, hot rolled, and then subjected to heat treatment such as quenching and tempering by water cooling to produce steel plates. As a comparative example, intermediate heat treatment was applied to some steel types. The holding time of the hot rolling heating was 30 to 120 minutes, and the holding time of the intermediate heat treatment and the heat treatment of the tempering was 20 to 60 minutes. A sample was taken from the steel plate, and the metal structure, mechanical properties (tensile strength), and relative throttling value were evaluated.

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

  In the steel of the present invention, in order to suppress the destruction of the grain boundaries of the former austenite, the grain refinement of the former austenite grain size and the suppression of the P content are carried out. For this reason, it may be difficult to identify former austenite grain boundaries due to corrosion. In such a case, after heat treatment at 450 to 490 ° C. and heat treatment for holding for 1 hour or more, the grain size of the prior austenite was measured by the method described above. If identification of the former austenite grain boundaries is difficult even after heat treatment at 450 to 490 ° C, Charpy specimens are collected from the sample after heat treatment, subjected to an impact test at -196 ° C, and broken at the former austenite grain boundaries Samples were used. In this case, a cross section of the fractured surface is prepared in a plane (L plane) parallel to the rolling direction, and after corrosion, the former austenite grain boundary of the fractured surface cross section at the center of thickness is identified by SEM. It was measured. When the prior austenite grain boundaries are embrittled by heat treatment, minute cracks are generated in the former austenite grain boundaries due to the impact load at the Charpy test, so that the former austenite grain boundaries are easily identified.

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

  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. In the measurement of the amount of Ni in the austenite phase, a thin film sample of 0.1 μm thickness is taken from a steel material, and using a transmission electron microscope (hereinafter, also referred to as “TEM”) The energy dispersive X-ray analyzer (hereinafter also referred to as "EDS") attached to the TEM after observing the dark field image and confirming the austenite phase from the diffraction pattern is attached to the TEM. It measured using. This was carried out at five places in the thin film, and the average value was obtained.

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

Furthermore, the relative aperture was evaluated. First, a 4A round bar tensile test specimen with a parallel part diameter of 6 mm specified in JIS Z 2241 whose longitudinal direction is the reduction value at room temperature in the atmosphere parallel to the rolling direction (L direction) is taken from each steel material, It evaluated by the method of prescription to JISZ2241. Next, test pieces of the squeeze value at room temperature in hydrogen were taken from each steel material in a high pressure hydrogen of 100 MPa in the same manner as in the air, and were evaluated by a tensile test. At this time, in order to be able to sufficiently evaluate the influence of hydrogen, the strain rate was 8 × 10 ⁻⁵ (1 / s) slower than the definition of JIS Z 2241. Then, the squeeze value obtained in hydrogen was divided by the squeeze value obtained in the atmosphere to obtain a relative squeeze value. The target value of the relative aperture value is 0.80 or more, preferably 0.90 or more. It was determined that the hydrogen embrittlement resistance was excellent when the relative throttling value was 0.80 or more.

  Table 3 and Table 4 show the plate thickness, manufacturing method, mechanical properties (tensile strength) and metal structure of steel products (manufacturing conditions No. 1 to 32) manufactured using slabs having chemical compositions 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, in the production condition No. 4 where the Ni content of the steel 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 slightly lower than the target value. On the other hand, even if the Ni content of the steel material is low, manufacturing conditions No. 1 in which V is contained 0.10% or more. For 4 and 6, high relative aperture values were shown. Moreover, manufacturing condition No. In No. 9, although the water-cooling start temperature is lower and the prior austenite grain size is within the preferable range, it is slightly larger, and the relative reduction value is slightly within the target value, though it is smaller. Also, no. No. 10 has a high tempering temperature. In No. 17 in all, the tensile strength is lower than those of the present invention examples because the C content is small. No. No. 18 had a high austenitic volume fraction of 5.3%, but had a high relative squeeze value because the Ni solid solution content in the bcc structure was as high as 14.8%.

  On the other hand, as shown in Table 4, the manufacturing condition No. Since No. 19 has a high C content, the strength is high and the relative throttling value is low. Manufacturing condition No. Since No. 20 and 21 had small Ni content, Ni content in bcc structure | tissue runs short, and the relative contraction value is falling. Manufacturing condition No. In each of Nos. 22 and 23, the Mn content and the Mo content are high, and the volume fraction of the austenite phase is high, so the amount of Ni solid solution in the bcc structure is insufficient, and the relative reduction value is reduced. Manufacturing condition No. Since No. 24 has a high N content, 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 the relative reduction is low.

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

Claims (5)

The chemical composition is in mass%,
Ni: 14.0% or more and less than 20.0%,
C: 0.003 to 0.080%,
Si: 0.03 to 0.30%,
Mn: 0.10 to 0.50%,
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 to 0.50%,
Cr: 0 to 0.50%,
Nb: 0 to 0.020%,
Ti: 0 to 0.020%,
V: 0 to 0.50%,
B: 0 to 0.0020%,
Ca: 0 to 0.0040%,
Rare earth elements: 0 to 0.0050%,
Remainder: Fe and impurities,
The metallographic structure is mainly composed of a body-centered cubic lattice structure,
The solid solution content of Ni in the structure of the body-centered cubic lattice structure is at least 12.5%,
The prior austenite grain size is 12.0 μm or less,
The tensile strength is 930 MPa or less
Nickel steel for high pressure hydrogen. The chemical composition is, in mass%,
V: containing at least 0.10%,
A 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 austenite phase is 5.0% or less
A nickel steel material for high pressure hydrogen according to any one of claims 1 to 4.