JP2017014547A - High Mn steel for high-pressure hydrogen gas and pipes, containers, valves and joints made of the steel - Google Patents

Abstract

[PROBLEMS] To provide a high-Mn steel material that is cheaper than SUS316 steel with a controlled Ni equivalent, has TS ≧ 800MPa, has excellent mechanical properties in a high-pressure hydrogen gas environment, and has good hydrogen gas embrittlement resistance. To do. SOLUTION: Chemical composition is mass%, C <0.40%, Si: 0.05-1.0%, Mn: 20.0-60.0%, N <0.10%, Ni: 0-5%, Cu: 0-5%, Co: 0-5%, Al: 0-1%, Cr: 0-5%, Mo: 0-3%, W: 0-6%, V: 0-1.0%, Nb: 0-1.0%, Ti : 0-1.0%, Zr: 0-1.0%, Hf: 0-1.0%, Ta: 0-1.0%, B: 0-0.020%, Ca: 0-0.0050%, Mg: 0-0.0050%, REM: 0 to 0.50%, balance: Fe and impurities, P, S, and O as impurities are P ≦ 0.050%, S ≦ 0.050%, O ≦ 0.020%, and 0.05 ≦ C + 4N ≦ 0.40, and the metal structure of the matrix is , Volume ratio, fcc structural phase: 90-100%, bcc structural phase: 0-10% and hcp structural phase: 0-10%, total of the above structural phases: 100%, crystal grain aspect ratio: over 2.0 And high Mn steel for high pressure hydrogen gas with TS ≧ 800MPa. [Selection figure] None

Description

  The present invention relates to a high-Mn steel material for high-pressure hydrogen gas and a pipe, container, valve and joint made of the steel material. More specifically, the present invention relates to a high-Mn steel material having excellent mechanical properties in a high-pressure hydrogen gas environment, and further, a fuel cell vehicle (hereinafter simply referred to as “fuel cell”) made of the above-described steel material and running on hydrogen as fuel. It also relates to piping, containers, valves and fittings for high pressure hydrogen gas used in hydrogen stations that supply hydrogen to the fuel cell vehicle.

  In recent years, development of fuel cell vehicles and practical application research of hydrogen stations have been promoted, and austenitic stainless steel is the main metal material used in these.

  This is because the austenitic stainless steel having a face-centered cubic (fcc) structure as a crystal structure is generally a body-centered cubic (bcc) structure or a body-centered tetragonal (bct) structure (hereinafter, these are summarized in this specification). This is because it has excellent hydrogen gas embrittlement resistance compared to carbon steel and low alloy steel of “bcc structure”.

  However, in a high-pressure hydrogen gas environment, austenitic stainless steel may also be embrittled by hydrogen gas (hereinafter referred to as “hydrogen gas embrittlement”).

  For example, among austenitic stainless steels, metastable austenitic stainless steels such as SUS304 produce strain-induced martensite (α ′ martensite) having a bcc structure that is highly susceptible to hydrogen embrittlement due to plastic deformation. Prone to hydrogen gas embrittlement.

  On the other hand, stable austenitic stainless steels such as SUS316 and SUS316L have excellent hydrogen gas embrittlement resistance because they hardly undergo phase transformation at room temperature.

  However, even in the above-mentioned SUS316 and SUS316L, when the temperature is lower than normal temperature, strain-induced martensite is generated even when the content of Cr, Ni, etc. is low even within these component specifications, causing hydrogen gas embrittlement. .

  Furthermore, in order to improve the cruising distance of the fuel cell vehicle, the pressure of the hydrogen tank mounted on the fuel cell vehicle is 70 MPa, which is higher than the conventional 45 MPa in recent years.

For this reason, non-patent document 1 describes a chemical composition as austenitic stainless steel for high-pressure hydrogen gas environment based on a parameter formula called Ni equivalent represented by the following formula [1] according to the operating temperature. It is proposed to use a strictly controlled version.
Ni equivalent = 12.6C + 0.35Si + 1.05Mn + Ni + 0.65Cr + 0.98Mo (1).
However, C, Si, Mn, Ni, Cr and Mo in the formula [1] mean the content (mass%) of each element in steel.

  In the example of compressed hydrogen container for automobiles stipulated in the High Pressure Gas Safety Law, the lower limit of the Ni equivalent is defined as an austenitic stainless steel that is less prone to hydrogen gas embrittlement, and the use of SUS316 and SUS316L satisfying the condition is permitted. It has been. In actuality, SUS316 and SUS316L containing a large amount of expensive Ni up to an amount close to the upper limit of the component standard are used so as to satisfy a predetermined Ni equivalent value.

  On the other hand, at present, reduction of manufacturing costs is the biggest issue for fuel cell vehicles and hydrogen stations. Therefore, using a material containing a large amount of expensive Ni is a great resistance to reducing the cost of fuel cell vehicles and hydrogen stations. For this reason, there is an extremely high demand for development of austenitic stainless steel that is less prone to hydrogen gas embrittlement and cheaper than SUS316 and SUS316L.

  Further, the austenitic stainless steel is required to have higher strength than before so that it can withstand a high hydrogen tank pressure for improving the cruising distance of the fuel cell vehicle. For this reason, in particular, there is a great demand for development of an inexpensive high-strength austenitic stainless steel having a tensile strength of 800 MPa or more.

  An element having an austenite stabilizing action and cheaper than Ni is Mn. As an austenitic high Mn steel, for example, AISI type 205 stainless steel and C-Mn based Hadfield's Steel, which are Cr-Mn-Ni high N steels, are well known.

  Patent Documents 1 to 3 disclose various austenitic high Mn steels.

JP-A-53-096912 International Publication No. 2015/012357 JP 2007-126688 A

Toshihiro Yamada, Hideo Kobayashi: High-pressure gas, Vol. 49 (2012) No. 10, pp. 885-893 Takeo Yazawa et al .: Iron and steel, Vol. 83 (1997) no. 1, pp. 60-65

  As described above, austenitic stainless steel, which has low austenite stability such as SUS304 and generates strain-induced martensite by plastic deformation, easily causes hydrogen gas embrittlement and cannot be used in a high-pressure hydrogen gas environment.

  Further, SUS316 and SUS316L containing a large amount of expensive Ni up to an amount close to the upper limit of the component standard in order to satisfy the specified value of the Ni equivalent represented by the formula [1] are less prone to hydrogen gas embrittlement. Things are expensive.

  On the other hand, existing AISI type 205 stainless steel and hadfield steel have high austenite stability and do not cause strain-induced martensite even when plastically deformed. Furthermore, these steels are cheaper than the above-mentioned SUS316 and SUS316L. However, when the present inventors evaluated the hydrogen gas embrittlement characteristics in a high-pressure hydrogen gas environment of 70 MPa or more, all of them easily cause hydrogen gas embrittlement, and these steels cannot be used in the high-pressure hydrogen gas environment. found.

  The iron alloy disclosed in Patent Document 1 is said to have “resistance to hydrogen embrittlement”. However, when the present inventors evaluated the hydrogen gas embrittlement property in a high-pressure hydrogen gas environment of 70 MPa or more, the iron alloy of Patent Document 1 has a particularly high tensile strength even if it has a chemical composition within the specified range. When the thickness is 800 MPa or more, hydrogen gas embrittlement may occur, and it has been found that it cannot be stably used in the high-pressure hydrogen gas environment as described above.

  Since the steel material disclosed in Patent Document 2 has excellent resistance to sulfide stress cracking (SSC), it is suitable as a steel material for high-strength wells. The above-mentioned “SSC” is one type of hydrogen embrittlement that results in fracture due to the synergistic action of the diffusion of hydrogen generated on the steel material surface in the corrosive environment into the steel and the stress applied to the steel material. However, when the present inventors evaluated the hydrogen gas embrittlement characteristics of the above steel material in a high-pressure hydrogen gas environment of 70 MPa or more, the steel material of Patent Document 2 has a chemical composition within the specified range. It has been found that gas embrittlement may occur. For this reason, the steel material proposed in Patent Document 2 has room to be improved for use in a high-pressure hydrogen gas environment of 70 MPa or more.

  The steel disclosed in Patent Document 3 certainly has a higher resistance to hydrogen embrittlement than SUS316 steel, and is therefore suitable for use in a high-pressure hydrogen environment. However, since the steel of Patent Document 3 has a high Cr content of 10 to 20%, there is room for improvement from the viewpoint of reducing the steel material cost.

  As described above, there has been no metal material that has a tensile strength of 800 MPa or more and a good hydrogen gas embrittlement resistance in a high-pressure hydrogen gas environment of 70 MPa or more and is also excellent in economy.

  The present invention has been made in view of the above situation, and is cheaper than SUS316 and SUS316L in which Ni equivalent is controlled and has a tensile strength of 800 MPa or more and a high-pressure hydrogen gas environment (in particular, a high-pressure hydrogen gas environment of 70 MPa or more). An object of the present invention is to provide a high Mn steel material having excellent mechanical properties under the above and having good hydrogen gas embrittlement resistance. Furthermore, it is also an object of the present invention to provide pipes, containers, valves and joints for high-pressure hydrogen gas made of the above steel materials.

  In order to solve the above-mentioned problems, the present inventors conducted an investigation to ensure sufficient hydrogen gas embrittlement characteristics in a high-pressure hydrogen gas environment. In this study, various chemical compositions were adjusted based on existing AISI type 205 stainless steel and Hadfield steel based on various chemical compositions, and on the conventional austenitic high Mn steel. High Mn steel was used. As a result, the following findings (a) to (f) were obtained.

  (A) By containing a large amount of Mn, austenite can be stabilized without containing expensive Ni. However, the high Mn steel may not be able to stably obtain a tensile strength of 800 MPa or more with the solution heat treatment.

(B) The high Mn steel having Fn1 represented by the following formula [1] of 0.05 or more has a tensile strength of 800 MPa or more stably by plastic deformation, particularly cold working.
Fn1 = C + 4N ... [1].
However, C and N in the formula [1] mean the content (mass%) of each element in steel.

  (C) AISI type 205 stainless steel and Hadfield steel are susceptible to hydrogen gas embrittlement in a high-pressure hydrogen gas environment of 70 MPa or higher, although strain-induced martensite is not generated by plastic deformation. The reason for this is considered to be that a large amount of solid solution element N (nitrogen) and / or C (carbon), which is an element that lowers the stacking fault energy, is contained. That is, AISI type 205 stainless steel generally contains 0.32 to 0.40% N and 0.12 to 0.25% C in mass%, while the Hadfield steel generally contains mass%. And containing 0.9-1.2% C. For this reason, in the above steel types containing a large amount of N or / and C, stacking faults are likely to occur during plastic deformation, and deformation is localized due to the generation of stacking faults, and hydrogen gas embrittlement occurs at the site where the deformation is localized. It is thought that susceptibility will increase.

  (D) Further, in the cold-worked high Mn steel, the effect of the contents of C and N on hydrogen gas embrittlement is increased. In particular, a high Mn steel having Fn1 expressed by the above formula [1] exceeding 0.40 is likely to cause hydrogen gas embrittlement in a high-pressure hydrogen gas environment of 70 MPa or more by cold working.

  (E) Austenitic high Mn steel having low austenite stability and undergoing cold working to increase the volume fraction of ferrite or / and α ′ martensite in the bcc structure in the metal structure is 70 MPa or more. It is extremely easy to cause hydrogen gas embrittlement in a high-pressure hydrogen gas environment.

  (F) Similarly, an austenitic high Mn steel having a low austenite stability and undergoing cold working to have a high volume fraction of ε-martensite having an hcp structure in the metal structure has a high pressure of 70 MPa or more. Hydrogen gas embrittlement easily occurs in a hydrogen gas environment.

  The present invention has been completed on the basis of the above contents, the gist of which is the following high-pressure hydrogen gas Mn steel material and high-pressure hydrogen gas pipe, container, valve and It is in the joint.

(1) Chemical composition is mass%, C: less than 0.40%, Si: 0.05 to 1.0%, Mn: 20.0 to 60.0%, N: less than 0.10%, Ni : 0-5%, Cu: 0-5%, Co: 0-5%, Al: 0-1%, Cr: 0-5%, Mo: 0-3%, W: 0-6%, V: 0 to 1.0%, Nb: 0 to 1.0%, Ti: 0 to 1.0%, Zr: 0 to 1.0%, Hf: 0 to 1.0%, Ta: 0 to 1.0 % B: 0 to 0.020%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, REM: 0 to 0.50%, the balance being Fe and impurities, P as impurities, S and O are P: 0.050% or less, S: 0.050% or less and O: 0.020% or less, and Fn1 represented by the following formula [1] is 0.05 ≦ Fn1 ≦ 0.40,
The metal structure of the matrix is, by volume, fcc structural phase: 90 to 100%, bcc structural phase: 0 to 10% and hcp structural phase: 0 to 10%, and the total of the above structural phases: 100%, The aspect ratio of the grains is over 2.0,
The tensile strength is 800 MPa or more,
High Mn steel for high pressure hydrogen gas.
Fn1 = C + 4N [1]
However, C and N in the formula [1] mean the content (mass%) of each element in steel.

  (2) The composition according to (1) above, containing 1% or more selected from Ni: 0.1 to 5%, Cu: 0.1 to 5%, and Co: 0.1 to 5% by mass%. High Mn steel for high pressure hydrogen gas.

  (3) By mass%, Al: 0.005 to 1%, Cr: 0.1 to 5%, Mo: 0.1 to 3%, W: 0.1 to 6%, V: 0.01 to 1 0.0%, Nb: 0.01 to 1.0%, Ti: 0.001 to 1.0%, Zr: 0.001 to 1.0%, Hf: 0.001 to 1.0%, Ta: The high Mn steel material for high-pressure hydrogen gas according to the above (1) or (2), which contains one or more selected from 0.001 to 1.0% and B: 0.0001 to 0.020%.

  (4) by mass%, containing at least one selected from Ca: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050% and REM: 0.0001 to 0.50%. The high Mn steel for high-pressure hydrogen gas according to any one of (1) to (3) above.

  (5) The high Mn steel material for high-pressure hydrogen gas according to any one of (1) to (4), wherein the matrix metal structure has a volume ratio of fcc structural phase: 100%.

  (6) A pipe, container, valve and joint for high-pressure hydrogen gas, comprising the high-Mn steel material for high-pressure hydrogen gas according to any one of (1) to (5) above.

  According to the present invention, it is cheaper than SUS316 and SUS316L in which Ni equivalent is controlled, has a tensile strength of 800 MPa or more, and has mechanical properties under a high-pressure hydrogen gas environment (in particular, a high-pressure hydrogen gas environment of 70 MPa or more). A high Mn steel with excellent hydrogen gas embrittlement resistance can be obtained. Moreover, piping, containers, valves and joints made of this high Mn steel material are excellent in durability in the high-pressure hydrogen gas environment.

  Hereinafter, each requirement of the present invention will be described in detail.

1. Chemical composition:
The reasons for limiting the chemical composition of the high-Mn steel for high-pressure hydrogen gas of the present invention are as follows. In the following description, “%” for the content of each element means “mass%”.

C: Less than 0.40% C is an element effective for stabilizing austenite and increasing strength after cold working. However, the excessive C content promotes the generation of stacking faults in the high Mn steel by cold working and greatly reduces the hydrogen gas embrittlement resistance. Therefore, the C content is less than 0.40%. The upper limit with preferable C content is 0.30%, and a more preferable upper limit is 0.10%. In addition, as for C content, Fn1 represented by said Formula [1] needs to also satisfy | fill 0.05 <= Fn1 <= 0.40.

Si: 0.05-1.0%
Si is an element effective for deoxidation of steel, and to obtain this effect, it is necessary to contain 0.05% or more. On the other hand, even if it contains Si exceeding 1.0%, said effect is saturated. For this reason, content of Si shall be 0.05-1.0%. The minimum with preferable Si content is 0.1%, and a preferable upper limit is 0.5%.

Mn: 20.0-60.0%
Mn is an important element in the present invention. Mn is inexpensive and has the effect of stabilizing austenite. In order to obtain this effect sufficiently, it is necessary to contain 20.0% or more of Mn. On the other hand, even if Mn is contained excessively exceeding 60.0%, the above effects are saturated and productivity such as hot workability is lowered. For this reason, content of Mn shall be 20.0-60.0%. The minimum with preferable Mn content is 30.0%, and a more preferable minimum is 35.0%. The upper limit with preferable Mn content is 50.0%, and a more preferable upper limit is 45.0%.

N: Less than 0.10% N is an element effective for stabilizing austenite and increasing strength after cold working. However, the excessive N content promotes the generation of stacking faults in the high Mn steel by cold working, and greatly reduces the resistance to hydrogen gas embrittlement. Therefore, the N content is less than 0.10%. The upper limit with preferable N content is 0.08%, and a more preferable upper limit is 0.05%. The N content needs to satisfy that Fn1 represented by the formula [1] satisfies 0.05 ≦ Fn1 ≦ 0.40.

Ni: 0 to 5%
Ni is an element effective for stabilizing austenite and preventing hydrogen gas embrittlement. Ni is an element effective for improving toughness. For this reason, you may contain Ni as needed. However, when Ni is contained in a large amount, the material cost increases. Therefore, the upper limit of the Ni content when contained is 5%. The upper limit of the Ni content is preferably 3%. In order to obtain the above-described effect, the lower limit of the Ni content is preferably 0.1%, and more preferably 0.5%.

Cu: 0 to 5%
Cu is an element effective for stabilizing austenite and preventing hydrogen gas embrittlement. For this reason, you may contain Cu as needed. However, when Cu is contained in a large amount, the material cost is increased, and the productivity such as hot workability is also decreased. Therefore, the upper limit of the Cu content when contained is 5%. The upper limit of the Cu content is preferably 3%. In addition, in order to acquire the above-mentioned effect, it is preferable that the minimum of Cu content is 0.1%, and it is more preferable that it is 0.5%.

Co: 0 to 5%
Co is an element effective for stabilizing austenite and preventing hydrogen gas embrittlement. Co is an element effective for improving toughness. For this reason, you may contain Co as needed. However, if Co is contained in a large amount, the material cost increases. Therefore, the upper limit of the Co content when it is contained is set to 5%. The upper limit of the Co content is preferably 3%. In addition, in order to acquire the above-mentioned effect, it is preferable that the minimum of Co content is 0.1%, and it is more preferable that it is 0.5%.

  The total amount in the case where two or more selected from Ni, Cu and Co described above are combined and contained is preferably 5% or less.

Al: 0 to 1%
Al is a ferrite stabilizing element. On the other hand, Al is an element effective for deoxidation of steel. For this reason, you may contain Al as needed. However, the effect is saturated even if Al is contained exceeding 1%. In addition, if the Al content exceeds 1%, the formation of ferrite is promoted to deteriorate the hydrogen gas embrittlement resistance, and oxides are more easily formed, which adversely affects toughness and the like. There is. For this reason, the quantity of Al in the case of making it contain shall be 1% or less. The amount of Al is preferably 0.5% or less. On the other hand, the amount of Al is preferably 0.005% or more, and more preferably 0.02% or more, in order to stably express the effect of Al described above. The Al content of the present invention refers to the content of acid-soluble Al (so-called “Sol. Al”).

Cr: 0 to 5%
Cr is a ferrite stabilizing element. On the other hand, Cr is an element effective for ensuring general corrosion resistance as stainless steel, such as weather resistance and acid resistance. For this reason, you may contain Cr as needed. However, even if Cr is contained in a large amount, the above effects are saturated, leading to an increase in material cost, and further, the formation of ferrite may be promoted to deteriorate the hydrogen gas embrittlement resistance. Therefore, the upper limit of the Cr content when contained is 5%. The upper limit of the Cr content is preferably 3%. In order to obtain the above-described effect, the lower limit of the Cr content is preferably 0.1%, and more preferably 0.5%.

Mo: 0 to 3%
Mo is a ferrite stabilizing element. On the other hand, Mo is an element effective for ensuring general corrosion resistance as stainless steel, such as weather resistance and acid resistance. For this reason, you may contain Mo as needed. However, even if Mo is contained in a large amount, the above effect is saturated, resulting in an increase in material cost, and further, the formation of ferrite may be promoted to deteriorate the hydrogen gas embrittlement resistance. Therefore, the upper limit of the Mo content in the case of inclusion is set to 3%. The upper limit of the Mo content is preferably 2%. In addition, in order to acquire the above-mentioned effect, it is preferable that the minimum of Mo content is 0.1%, and it is more preferable that it is 0.5%.

W: 0-6%
W is a ferrite stabilizing element. On the other hand, W is an element effective for ensuring general corrosion resistance as stainless steel, such as weather resistance and acid resistance. For this reason, you may contain W as needed. However, even if a large amount of W is contained, the above effects are saturated, leading to an increase in material cost, and further, the formation of ferrite may be promoted to deteriorate the hydrogen gas embrittlement resistance. Therefore, the upper limit of the W content in the case of inclusion is 6%. The upper limit of the W content is preferably 3%. In addition, in order to acquire the above-mentioned effect, it is preferable that the minimum of W content is 0.1%, and it is more preferable that it is 0.5%.

V: 0 to 1.0%
V is a ferrite stabilizing element. On the other hand, V is an element that forms alloy carbonitrides, refines crystal grains, and contributes to toughness improvement. For this reason, you may contain V as needed. However, even if V is contained in a large amount, the above effects are saturated, leading to an increase in material cost, and further, the formation of ferrite may be promoted to deteriorate the hydrogen gas embrittlement resistance. Therefore, the upper limit of the V content when contained is 1.0%. The upper limit of V content is preferably 0.5%. In addition, in order to acquire the above-mentioned effect, it is preferable that the minimum of V content is 0.01%, and it is more preferable that it is 0.1%.

Nb: 0 to 1.0%
Nb is a ferrite stabilizing element. On the other hand, Nb is an element that forms alloy carbonitrides, refines crystal grains, and contributes to toughness improvement. For this reason, you may contain Nb as needed. However, even if Nb is contained in a large amount, the above effects are saturated, leading to an increase in material cost, and further, the formation of ferrite may be promoted to deteriorate the hydrogen gas embrittlement resistance. Therefore, the upper limit of the Nb content when contained is 1.0%. The upper limit of the Nb content is preferably 0.5%. In addition, in order to acquire the above-mentioned effect, it is preferable that the minimum of Nb content is 0.01%, and it is more preferable that it is 0.1%.

Ti: 0 to 1.0%
Ti is a ferrite stabilizing element. On the other hand, Ti is an element that forms alloy carbonitrides, refines crystal grains, and contributes to toughness improvement. For this reason, you may contain Ti as needed. However, even if Ti is contained in a large amount, the above effects are saturated, leading to an increase in material cost, and further, the formation of ferrite may be promoted to deteriorate the hydrogen gas embrittlement resistance. Therefore, the upper limit of the Ti content when contained is 1.0%. The upper limit of the Ti content is preferably 0.5%. In order to obtain the above-described effect, the lower limit of the Ti content is preferably 0.001%, and more preferably 0.1%.

Zr: 0 to 1.0%
Zr is a ferrite stabilizing element. On the other hand, Zr is an element that forms alloy carbonitrides, refines crystal grains, and contributes to toughness improvement. For this reason, you may contain Zr as needed. However, even if Zr is contained in a large amount, the above effects are saturated, leading to an increase in material cost, and further, the formation of ferrite may be promoted to deteriorate the hydrogen gas embrittlement resistance. Therefore, the upper limit of the Zr content when contained is 1.0%. The upper limit of the Zr content is preferably 0.5%. In addition, in order to acquire the above-mentioned effect, it is preferable that the minimum of Zr content is 0.001%, and it is more preferable that it is 0.1%.

Hf: 0 to 1.0%
Hf is a ferrite stabilizing element. On the other hand, Hf is an element that forms alloy carbonitrides, refines crystal grains, and contributes to toughness improvement. For this reason, you may contain Hf as needed. However, even if Hf is contained in a large amount, the above effects are saturated, leading to an increase in material cost, and further, the formation of ferrite may be promoted to deteriorate the hydrogen gas embrittlement resistance. Therefore, the upper limit of the Hf content when contained is 1.0%. The upper limit of the Hf content is preferably 0.5%. In addition, in order to acquire the above-mentioned effect, it is preferable that the minimum of Hf content is 0.001%, and it is more preferable that it is 0.1%.

Ta: 0 to 1.0%
Ta is a ferrite stabilizing element. On the other hand, Ta is an element that forms alloy carbonitrides, refines crystal grains, and contributes to toughness improvement. For this reason, you may contain Ta as needed. However, even if Ta is contained in a large amount, the above effects are saturated, leading to an increase in material cost, and further, the formation of ferrite may be promoted to deteriorate the hydrogen gas embrittlement resistance. Therefore, the upper limit of the Ta content when contained is 1.0%. The upper limit of the Ta content is preferably 0.5%. In order to obtain the above-described effect, the lower limit of the Ta content is preferably 0.001%, and more preferably 0.1%.

B: 0 to 0.020%
B is a ferrite stabilizing element. On the other hand, B is an element that refines the austenite crystal grain size and contributes to toughness improvement. For this reason, you may contain B as needed. However, even if a large amount of B is contained, the above effects are saturated, leading to an increase in material cost, and the formation of ferrite may be promoted to deteriorate the hydrogen gas embrittlement resistance. For this reason, the quantity of B in the case of making it contain shall be 0.020% or less. The amount of B is preferably 0.01% or less. On the other hand, in order to stably express the effect of B described above, the amount of B is preferably 0.0001% or more, and more preferably 0.0005% or more.

  The total amount in the case where two or more selected from Al, Cr, Mo, W, V, Nb, Ti, Zr, Hf, Ta, and B are combined and contained is preferably 8% or less. More preferably, it is 6% or less.

Ca: 0 to 0.0050%
Ca has the effect of preventing solidification cracking during casting. For this reason, you may contain Ca as needed. However, when Ca is contained in a large amount, hot workability may be deteriorated. For this reason, the upper limit of Ca content in the case of making it contain shall be 0.0050%. The upper limit of the Ca content is preferably 0.0030%. In addition, in order to acquire the above-mentioned effect, it is preferable that the minimum of Ca content is 0.0001%, and it is more preferable that it is 0.0005%.

Mg: 0 to 0.0050%
Mg has the effect of preventing solidification cracking during casting. For this reason, you may contain Mg as needed. However, when Mg is contained in a large amount, hot workability may be deteriorated. For this reason, the upper limit of Mg content in the case of making it contain shall be 0.0050%. The upper limit of the Mg content is preferably 0.0030%. In order to obtain the above effect, the lower limit of the Mg content is preferably 0.0001%, and more preferably 0.0005%.

REM: 0 to 0.50%
REM has the effect of preventing solidification cracking during casting. For this reason, you may contain REM as needed. However, when a large amount of REM is contained, the hot workability may be deteriorated. For this reason, the upper limit of REM content in the case of making it contain shall be 0.50%. The upper limit of the REM content is preferably 0.30%. In addition, in order to acquire the above-mentioned effect, it is preferable that the minimum of REM content is 0.0001%, and it is more preferable that it is 0.0005%.

  In the present invention, “REM” refers to a total of 17 elements of Sc, Y, and lanthanoid, and “REM content” refers to the content when REM is 1 type, and the content thereof when 2 or more types are included. Refers to the total content. REM is also supplied as misch metal, which is generally an alloy of a plurality of types of REM. For this reason, one or more individual elements may be added and contained so that the amount of REM is in the above range. For example, the amount of REM may be added in the form of misch metal. You may make it contain so that it may become this range.

  In the case where two or more selected from the above-mentioned Ca, Mg and REM are combined and contained, the total amount is preferably 0.50% or less when REM is included, and when REM is not included Is preferably 0.0050% or less.

  The high Mn steel for high-pressure hydrogen gas of the present invention comprises the above-mentioned elements, the balance being Fe and impurities, and P, S and O as impurities are P: 0.050% or less, S: 0.00. 050% or less and O: 0.020% or less, and Fn1 represented by the formula [1] has a chemical composition satisfying 0.05 ≦ Fn1 ≦ 0.40.

  Here, “impurities” are components that are mixed due to various factors of raw materials such as ores and scraps and manufacturing processes when industrially producing steel materials, and are permitted within a range that does not adversely affect the present invention. Means something.

P: 0.050% or less P is an element that segregates at grain boundaries and adversely affects mechanical properties such as toughness. For this reason, P content needs to be limited to 0.050% or less. It is desirable that the P content is as low as possible.

S: 0.050% or less S, like P, is an element that adversely affects mechanical properties such as toughness of steel. For this reason, S content needs to be limited to 0.050% or less. It is desirable that the S content is as low as possible.

O: 0.020% or less O (oxygen), like S and P, is an element that adversely affects mechanical properties such as toughness of steel. For this reason, O content needs to be limited to 0.020% or less. It is desirable that the O content be as low as possible.

Fn1: 0.05 or more and 0.40 or less In the high Mn steel for high-pressure hydrogen gas of the present invention, Fn1 represented by the following formula [1] is 0.05 ≦ Fn1 ≦ 0.40.
Fn1 = C + 4N [1]
However, C and N in the formula [1] mean the content (mass%) of each element in steel.

  Fn1 is an index for stabilizing austenite, increasing strength after cold working, and ensuring hydrogen gas embrittlement resistance in the high Mn steel for high pressure hydrogen gas of the present invention. When Fn1 is 0.05 or more and 0.40 or less, stabilization of austenite and high strength after cold working are achieved, and good hydrogen gas embrittlement resistance is ensured. The upper limit with preferable Fn1 is 0.30, and a more preferable upper limit is 0.10. If Fn1 is 0.05 or more, it is desirable that Fn1 is closer to 0.05.

  For example, a high-Mn steel material having the above-described chemical composition obtained by the method described in “4. Manufacturing method” described later has a tensile strength of 800 MPa or more and generally has mechanical properties in a high-pressure hydrogen gas environment. Excellent hydrogen gas embrittlement characteristics. However, there may be a case where a good hydrogen gas embrittlement characteristic in a high-pressure hydrogen gas environment of 70 MPa or more cannot be stably ensured only by defining the chemical composition. Therefore, in the high Mn steel for high-pressure hydrogen gas of the present invention, the metal structure of the matrix described below is also defined.

2. Matrix microstructure:
In the high Mn steel for high pressure hydrogen gas of the present invention, the matrix metal structure has a volume ratio of fcc structural phase: 90 to 100%, bcc structural phase: 0 to 10%, and hcp structural phase: 0 to 10%. And the total of the said structural phase: It is 100%, and the aspect-ratio of a crystal grain is more than 2.0.

  The high-Mn steel material for high-pressure hydrogen gas of the present invention that satisfies the above-mentioned chemical composition regulations and the metal structure of the matrix has a tensile strength of 800 MPa or more and mechanical properties in a high-pressure hydrogen gas environment of 70 MPa or more. And excellent hydrogen gas embrittlement resistance can be stably ensured.

  The volume fraction of the fcc structural phase in the metal structure is preferably 95% or more, and most preferably 100%. The volume fraction of the bcc structural phase in the metal structure is preferably 5% or less, and most preferably 0%. Similarly, the volume fraction of the hcp structure phase in the metal structure is preferably 5% or less, and most preferably 0%.

  As described above, the “bcc structure” in this specification refers to a combination of the bcc structure and the bct structure.

The volume ratio of each structural phase in the metal structure of the matrix of the high Mn steel for high pressure hydrogen gas of the present invention can be determined by, for example, processing in the following order (1) to (4).
(1) Collect a test piece having a thickness of 2 mm, a width of 10 mm, and a length of 10 mm.
(2) Polish the test piece up to 1200 emery paper.
(3) The polished test piece is immersed in a mixed solution of hydrogen peroxide and oxalic acid at room temperature to remove the processed layer on the surface.
(4) X-ray diffraction measurement is performed on the test piece from which the processed layer has been removed.

  The high Mn steel for high-pressure hydrogen gas of the present invention can stably have a tensile strength of 800 MPa or more as long as the aspect ratio of the crystal grains in the metal structure exceeds 2.0. In the present specification, the “aspect ratio of crystal grains” means the average value a of the major axis in the rolling direction of the steel material or the direction parallel to the forging axis (hereinafter collectively referred to as “longitudinal direction”). And the ratio “a / b” to the average value b of the major axis in a direction perpendicular to the direction (hereinafter referred to as “thickness direction”). The aspect ratio of the crystal grains is preferably 4.0 or more.

The aspect ratio of the crystal grains in the metal structure of the matrix of the high Mn steel for high pressure hydrogen gas of the present invention can be determined by, for example, processing in the following order <1> to <4>.
<1> A test piece having a thickness of 10 mm, a width of 10 mm and a length of 20 mm is collected.
<2> The test piece is filled with resin so that a cross section having a thickness of 10 mm and a length of 20 mm becomes a test surface, and is mirror-polished.
<3> Corrode (etch) with Virella solution, and observe a visual field of an appropriate area with a magnification of 100 using an optical microscope.
<4> By the intercept method, the average value a of the major axis in the longitudinal direction and the average value b of the major axis in the thickness direction are obtained for the crystal grains, and “a / b” is calculated.

3. Tensile strength:
The tensile strength (TS) of the high Mn steel material for high pressure hydrogen gas according to the present invention and the strength of pipes, containers, valves, and joints for high pressure hydrogen gas made of the steel material is 800 MPa or more. If the tensile strength is 800 MPa or more, for example, it can stably withstand a high hydrogen tank pressure for improving the cruising distance of a fuel cell vehicle. In the present invention, “tensile strength” refers to “tensile strength in the atmosphere”.

4). Production method:
The high-Mn steel material for high-pressure hydrogen gas of the present invention and the pipe, container, valve, and joint for high-pressure hydrogen gas made of the steel material can be produced, for example, by the following method, but are limited to this method. Not.

  After melting the high Mn steel having the chemical composition described above, it is made into an ingot or slab by casting. The cast ingot or slab is finished into a steel material having a required rough shape such as a thick plate, a thin plate, a round bar, or a seamless steel pipe by hot working such as hot rolling, hot extrusion, hot forging, or the like. Thereafter, a solution heat treatment is performed.

  The conditions for the solution heat treatment may be any temperature and time conditions that can sufficiently dissolve the precipitates and the like. Specifically, although it depends on the size of the steel material having the above-mentioned rough shape, it is sufficient to keep the soaking for 10 minutes or more in a temperature range of 1000 to 1200 ° C. In the solution heat treatment, the preferred lower limit of the soaking temperature is about 1050 ° C., the preferred upper limit is about 1100 ° C., and the preferred upper limit of the soaking time is about 60 minutes. After soaking, it is desirable to cool at a cooling rate higher than oil cooling.

  The steel material having the required rough shape subjected to the solution heat treatment is then subjected to cold working to finish the steel material having the required shape, and further to a member having a predetermined shape such as a pipe and a container. The cold working method in this case is not particularly specified, and for example, cold working may be performed so that the cross-section reduction rate is 20% or more. When the above cold working is performed, the aspect ratio of the crystal grains exceeds 2.0. In the cold working, a preferable lower limit of the cross-section reduction rate is about 40%.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  High Mn steels A to Z having the chemical compositions shown in Table 1 were melted using a 50 kg vacuum melting furnace and cast into ingots.

  Steels A to S, Steel X, and Steel Z in Table 1 are steels whose chemical compositions are within the range defined by the present invention.

  On the other hand, steels T to W and steel Y in Table 1 are steels whose chemical compositions deviate from the conditions defined in the present invention.

  Each of the above ingots was hot forged into a block having a thickness of 80 mm, and this block was further hot-rolled to finish a plate material having a thickness of 15 to 60 mm.

  Each of the above plate materials of 15 to 60 mm were held at 1100 ° C. for 60 minutes and then cooled with water and subjected to solution heat treatment.

  About each steel, it cold-rolled by the cross-sectional reduction rate of 20 to 80% shown in Table 2 using the board | plate material which carried out the said solution heat treatment, and calculated | required the volume ratio of each structural phase. Specifically, a specimen having a thickness of 2 mm in the longitudinal direction, a width of 10 mm, and a length of 10 mm is collected from the central portion in the thickness direction of the plate material, polished to 1200-th emery paper, and then subjected to room temperature excess. The surface processed layer was removed by dipping in a mixed solution of hydrogen oxide and oxalic acid. Next, X-ray diffraction measurement (Cu anti-cathode, tube voltage 30 kV, tube current 100 mA) was performed on the test piece from which the processed layer had been removed by a method based on Non-Patent Document 2, and each structural phase in the metal structure of the matrix was measured. The volume ratio of was determined.

  Specifically, for the fcc structure phase, the peak intensity of (111), (200) and (220), for the bcc structure phase, the peak intensity of (110), (200) and (211), and for the hcp structure phase Based on the peak intensities of (100), (101) and (102), the volume fraction of each structural phase was identified.

  For each steel, a test piece having a thickness of 10 mm in the longitudinal direction, a width of 10 mm, and a length of 20 mm was sampled from the thickness direction center of the cold-rolled plate. Next, in order to observe both the major axis in the longitudinal direction and the major axis in the thickness direction in the crystal grain, the resin is buried so that a cross section having a thickness of 10 mm and a length of 20 mm becomes the test surface. After polishing, corrosion (etching) was performed with a Virella solution, and the aspect ratio of the crystal grains in the metal structure of the matrix was determined by the following method.

  Using a cross section having a thickness of 10 mm and a length of 20 mm as a test surface, a 500 μm × 500 μm field of view was randomly observed at a magnification of 100 using an optical microscope, and a section method was applied to the crystal grains. The average value a (μm) of the major axis in the longitudinal direction and the average value b (μm) of the major axis in the thickness direction were determined. Next, “a / b”, that is, the aspect ratio of the crystal grains was calculated.

  Furthermore, about each steel, the round bar tensile test piece whose parallel part diameter is 2.5 mm was extract | collected from the thickness direction center part of the cold-rolled board | plate material.

Using the above round bar tensile test piece, a tensile test was conducted at room temperature and at a strain rate of 3 × 10 ⁻⁶ / s, yield strength (0.2% proof stress, YS1), tensile strength (TS1) The elongation at break (EL1) was measured. In addition, using the above round bar tensile test piece, a tensile test was performed at a strain rate of 3 × 10 ⁻⁶ / s in the high-pressure hydrogen gas at a normal temperature of 85 MPa, and the elongation at break (EL2) was measured. .

The effect of hydrogen is prominent in the reduction of ductility. Therefore, the relative elongation at break was calculated from the elongation at break (EL1) in the atmosphere and the elongation at break (EL2) in high-pressure hydrogen gas using the following equation [2]. If the relative elongation at break obtained in this way is 50% or more, the hydrogen embrittlement is suppressed and good hydrogen gas embrittlement resistance is obtained, and if it is 70% or more, extremely good. It can be judged that it has a good hydrogen gas embrittlement resistance.
(EL2 / EL1) × 100 (2).

  Table 2 summarizes the results of each of the above investigations.

  Test numbers 1 to 19 in Table 2 are examples of the present invention. It is clear that all of the above test numbers with high strength at which TS exceeds 800 MPa have a good hydrogen gas embrittlement resistance with a relative elongation at break exceeding 50%. In all of the above examples of the present invention, the metal structure of the matrix was volume ratio and the fcc structure phase was 100%.

  Test No. 20 is a reference example, and the relative elongation at break is as large as 92% and has good hydrogen gas embrittlement resistance. However, since this test number 20 is 0.033 where Fn1 of the steel T used deviates from the chemical composition condition defined in the present invention, TS is as low as 705 MPa even when cold rolled, and the lower limit defined in the present invention. The value of 800 MPa was not reached.

  Test numbers 21 to 26 are comparative examples. All of the above comparative examples had a relative elongation at break of less than 50% and were inferior in hydrogen gas embrittlement resistance.

  Test No. 21 was inferior in hydrogen gas embrittlement resistance because the C content and Fn1 of steel U used were 0.453% and 0.525, respectively, both of which deviated from the chemical composition conditions defined in the present invention. It was.

  Test No. 22 has a Mn content of steel V used as low as 19.82% and deviates from the chemical composition condition defined in the present invention. The volume fraction in the metal structure of the matrix is 26% for the fcc structural phase and the hcp structural phase. However, since it did not satisfy the regulations at 74%, the hydrogen gas embrittlement resistance was inferior.

  Test No. 23 was inferior in hydrogen gas embrittlement resistance because Fn1 of the steel W used was 0.495 which deviates from the chemical composition conditions defined in the present invention.

  In test number 24, the chemical composition of steel X used is within the range specified in the present invention, but the volume ratio in the metal structure of the matrix is specified as 30% for the fcc structural phase and 70% for the hcp structural phase. Since it did not satisfy | fill, it was inferior to the hydrogen gas embrittlement characteristic.

  Test number 25 was inferior in hydrogen gas embrittlement resistance because Fn1 of steel Y used was 0.470, which deviates from the chemical composition conditions defined in the present invention, as in test number 23.

  In test number 26, the chemical composition of the steel Z used is within the range specified in the present invention, but the volume fraction in the metal structure of the matrix satisfies the specification at 65% for the fcc structural phase and 35% for the bcc structural phase. Therefore, the hydrogen gas embrittlement resistance was poor.

According to the present invention, it is cheaper than SUS316 and SUS316L in which Ni equivalent is controlled, has a tensile strength of 800 MPa or more, and has mechanical properties under a high-pressure hydrogen gas environment (in particular, a high-pressure hydrogen gas environment of 70 MPa or more). A high Mn steel with excellent hydrogen gas embrittlement resistance can be obtained. Moreover, piping, containers, valves and joints made of this high Mn steel material are excellent in durability in the high-pressure hydrogen gas environment.

Claims (6)

Chemical composition is mass%, C: less than 0.40%, Si: 0.05 to 1.0%, Mn: 20.0 to 60.0%, N: less than 0.10%, Ni: 0 to 5%, Cu: 0 to 5%, Co: 0 to 5%, Al: 0 to 1%, Cr: 0 to 5%, Mo: 0 to 3%, W: 0 to 6%, V: 0 to 1 0.0%, Nb: 0 to 1.0%, Ti: 0 to 1.0%, Zr: 0 to 1.0%, Hf: 0 to 1.0%, Ta: 0 to 1.0% B: 0 to 0.020%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, REM: 0 to 0.50%, the balance being Fe and impurities, and P, S and O as impurities P: 0.050% or less, S: 0.050% or less, and O: 0.020% or less, and Fn1 represented by the following formula [1] is 0.05 ≦ Fn1 ≦ 0.40 And
The metal structure of the matrix is, by volume, fcc structural phase: 90 to 100%, bcc structural phase: 0 to 10% and hcp structural phase: 0 to 10%, and the total of the above structural phases: 100%, The aspect ratio of the grains is over 2.0,
The tensile strength is 800 MPa or more,
High Mn steel for high pressure hydrogen gas.
Fn1 = C + 4N [1]
However, C and N in the formula [1] mean the content (mass%) of each element in steel.   The high-pressure hydrogen gas according to claim 1, comprising at least one selected from Ni: 0.1 to 5%, Cu: 0.1 to 5%, and Co: 0.1 to 5% by mass%. For high Mn steel.   In mass%, Al: 0.005 to 1%, Cr: 0.1 to 5%, Mo: 0.1 to 3%, W: 0.1 to 6%, V: 0.01 to 1.0% Nb: 0.01 to 1.0%, Ti: 0.001 to 1.0%, Zr: 0.001 to 1.0%, Hf: 0.001 to 1.0%, Ta: 0.001 The high Mn steel material for high-pressure hydrogen gas according to claim 1 or 2, comprising at least one selected from -1.0% and B: 0.0001-0.020%.   It contains at least one selected from Ca: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050%, and REM: 0.0001 to 0.50% by mass%. To 3 for high-pressure hydrogen gas for high-pressure hydrogen gas.   The high Mn steel material for high-pressure hydrogen gas according to any one of claims 1 to 4, wherein the metal structure of the matrix is, by volume ratio, fcc structural phase: 100%. A pipe, container, valve and joint for high-pressure hydrogen gas, comprising the high-Mn steel for high-pressure hydrogen gas according to any one of claims 1 to 5.