JP6202065B2 - Steel structure for hydrogen - Google Patents

Description

  The present invention relates to a hydrogen steel structure such as a hydrogen pressure accumulator and a hydrogen line pipe, and more particularly to a hydrogen steel structure having excellent hydrogen permeation resistance in a high-pressure hydrogen environment.

  In recent years, hydrogen has attracted a great deal of attention worldwide as a clean energy source and from the viewpoint of energy diversification. In particular, there are high expectations for fuel cell vehicles that use high-pressure hydrogen gas as the fuel source, and research related to the development of fuel cell vehicles has been widely promoted worldwide, and some have already been put into practical use. .

  Since a fuel cell vehicle travels with hydrogen stored in a tank instead of gasoline, a hydrogen station for refueling is required instead of a gas station in order to spread the fuel cell vehicle. In the hydrogen station, hydrogen is filled from a hydrogen pressure accumulator, which is a hydrogen container for storing hydrogen at a high pressure, into an on-vehicle hydrogen fuel tank. The maximum filling pressure in an on-vehicle hydrogen tank is currently 35 MPa, but it is expected that the maximum filling pressure is 70 MPa in order to make the cruising range comparable to that of a gasoline vehicle. Therefore, it is required to store and supply hydrogen safely.

  On the other hand, the pressure of the hydrogen pressure accumulator used in the hydrogen station is currently required to be 40 MPa. However, when the maximum filling pressure of the on-vehicle hydrogen tank is increased to 70 MPa, the pressure of the hydrogen pressure accumulator of the hydrogen station is increased. Is expected to require 82 MPa, in which case the hydrogen accumulator at the hydrogen station will be exposed to an ultra-high pressure hydrogen environment of 82 MPa.

  Examples of such a material for hydrogen gas environment include low alloy steel, stainless steel, aluminum alloy and the like. Among these, it is known that particularly low alloy steels exhibit significant hydrogen embrittlement such as a reduction in drawing when hydrogen enters the material.

  For this reason, if the hydrogen pressure is up to about 40 MPa, there is a case where a low alloy steel having a sufficient thickness is used, but at a pressure higher than that, the risk of causing hydrogen embrittlement during use increases. Utilization of austenitic stainless steel SUS316L and aluminum alloy A6061-T6, which are less prone to hydrogen embrittlement than low alloy steels, is being studied.

  However, SUS316L and A6061-T6 are not only high in material cost, but also low in strength. Therefore, in order to design to withstand a hydrogen pressure of 82 MPa, the wall thickness becomes very thick, and a hydrogen pressure accumulator The price itself is very expensive. Therefore, it is desired to develop a hydrogen accumulator for a hydrogen station that can withstand a pressure of 82 MPa at a lower cost.

  Various techniques for solving the above problems and applying low alloy steel to a high pressure hydrogen accumulator have been studied. In Patent Document 1, as a trap site for hydrogen in steel, MnS, Ca-based inclusions, or VC is used to make hydrogen in steel non-diffusible hydrogen and suppress the embrittlement due to diffusible hydrogen. Steel has been proposed. In Patent Documents 2 and 3, the tensile strength is controlled to an extremely narrow range of 900 to 950 MPa by tempering at a relatively high temperature in the tempering treatment of Cr—Mo steel, and it has excellent high-pressure hydrogen environment embrittlement resistance. Low alloy high strength steels have been proposed. Patent Document 4 proposes a low-alloy steel for high-pressure hydrogen environment in which hydrogen-environment embrittlement characteristics are improved by using V-Mo carbide and increasing the tempering temperature. In Patent Document 5, a large amount of Mo and V is added, and a long time stress-relief annealing is performed after the normalizing treatment at the time of manufacturing the steel sheet, so that (Mo, V) C is precipitated in a large amount and excellent in hydrogen resistance. Steel for high-pressure hydrogen gas storage containers has been proposed. In Patent Document 6, a technique for reducing hydrogen penetration by further reducing cementite and further improving the toughness of the base metal to suppress hydrogen embrittlement is disclosed in Patent Document 6. In Patent Document 7, coarse cementite and island martensite ( There has been proposed a technique for suppressing hydrogen embrittlement by suppressing generation of MA) to suppress hydrogen penetration and ductility reduction. These Patent Documents 1 to 7 are techniques for improving the hydrogen embrittlement resistance of the material itself by controlling the composition, structure, precipitates, inclusions, and the like of the material.

  On the other hand, in Patent Document 8, after the container body is formed of an appropriate steel material, a hydrogen intrusion prevention metal film is formed on the inner surface thereof, thereby reducing the cost and the high-pressure hydrogen container for high-pressure hydrogen excellent in hydrogen embrittlement resistance. A technology has been proposed. Note that the fatigue crack growth characteristics in high-pressure hydrogen gas for ordinary low alloy steels are described in Non-Patent Documents 1 and 2, etc.

Japanese Patent Laying-Open No. 2005-2386 JP 2009-46737 A JP 2009-275249 A JP 2009-74122 A JP 2010-37655 A JP 2012-107332 A JP 2012-107333 A JP 2006-9982 A

Wada Yoryu: “Hydrogen Energy System”, Vol. 35, no. 4 (2010), p. 38-44 Taisuke Miyamoto et al .: “The Transactions of the Japan Society of Mechanical Engineers (Part A)”, 78, 788 (2012), p. 531 to 546

  From the viewpoint of cost and strength, low-alloy steel is desired for hydrogen steel structures such as hydrogen accumulators and hydrogen line pipes used in high-pressure hydrogen environments. Many studies have been made to improve the hydrogen embrittlement resistance of alloy steels.

  The above prior art is a technique for improving the hydrogen embrittlement resistance of the material itself by mainly controlling the composition, structure, precipitates, inclusions, and the like of the material. On the other hand, unlike such conventional approaches, it is conceivable to suppress hydrogen embrittlement of the steel structure for hydrogen by reducing the amount of hydrogen permeating the material itself. Ideally, if the amount of hydrogen penetrating into the material can be kept below the amount at which hydrogen embrittlement occurs, it will be possible to increase the service life and suppress hydrogen embrittlement. Standing studies are extremely limited, such as Patent Document 8.

  The present invention has been made in view of the above circumstances, and suppresses hydrogen permeation itself into the material and has excellent hydrogen permeation resistance and hydrogen steel such as a hydrogen accumulator and a hydrogen line pipe. The object is to provide a structure.

  The present inventors have studied to form an appropriate film on the surface of a steel material by plating or the like as a means for suppressing the penetration of hydrogen into the material. First, the present inventors carefully examined the influence of various platings on the hydrogen permeation performance of a steel material in high-pressure hydrogen gas. As a result, by forming an aluminum film such as an aluminum plating film on the surface of the steel material, it becomes possible to significantly reduce the amount of hydrogen intrusion into the steel material compared to the case where such a film is not formed. It has been found that hydrogen steel structures such as hydrogen accumulators and hydrogen line pipes having excellent hydrogen permeation characteristics can be obtained.

  The present invention has been made on the basis of such new findings and has been further studied.

  [1] A steel structure for hydrogen, comprising an aluminum film having a thickness of 0.10 μm or more on the surface of a steel material in contact with hydrogen gas.

  [2] The steel structure for hydrogen according to [1], wherein the aluminum film has a thickness of 50.00 μm or less.

  [3] The steel structure for hydrogen according to [1] or [2], wherein an iron oxide film is provided between the steel material and the aluminum film, and an aluminum oxide film is provided on the aluminum film. .

  [4] The steel structure for hydrogen according to any one of [1] to [3], wherein the steel structure for hydrogen is a hydrogen pressure accumulator or a hydrogen line pipe.

  According to the present invention, it is possible to obtain a hydrogen steel structure such as a hydrogen pressure accumulator or a hydrogen line pipe, which is extremely superior in hydrogen permeation resistance in high-pressure hydrogen gas as compared with the prior art, which is extremely useful industrially.

  Hereinafter, the present invention will be specifically described.

  The steel structure for hydrogen of the present invention has an aluminum film having a thickness of 0.10 μm or more on the surface of a steel material in contact with hydrogen gas.

  The steel structure for hydrogen of the present invention preferably has an iron oxide film between the steel material and the aluminum film, and has an aluminum oxide film on the aluminum film.

  In the aluminum film, the diffusion coefficient of hydrogen is remarkably small, so when the aluminum film is formed on the surface of the steel material, the time until the hydrogen reaches the steel material is significantly longer than when the aluminum film is not formed. Become. Furthermore, the dissociation efficiency of hydrogen molecules into hydrogen atoms on the aluminum film is remarkably reduced as compared with the efficiency on the surface of the steel material, so the amount of hydrogen entering the steel material through the aluminum film is remarkably reduced.

  In addition, when the steel material, the iron oxide film, the aluminum film, and the aluminum oxide film are laminated in this order, the interface between the iron oxide film and the aluminum film and the interface between the aluminum film and the aluminum oxide film are hydrogen. This significantly reduces the amount of hydrogen entering the steel material. In addition to the effect obtained by adding the effect of the aluminum film, an effect that hydrogen permeation resistance in hydrogen gas is remarkably excellent is exhibited. In the present invention, the hydrogen permeation resistance is a characteristic that the hydrogen permeability is low and the penetration of hydrogen into the steel material is suppressed. The steel structure for hydrogen of the present invention is excellent in hydrogen permeation resistance. And it can be expected that the hydrogen embrittlement resistance is excellent by suppressing the intrusion of hydrogen into the steel material in the high-pressure hydrogen gas.

  In the present invention, the aluminum film is, for example, an aluminum plating film obtained by performing aluminum plating. In addition, even if inevitable impurities other than aluminum are included, the effect is not affected.

  In order to appropriately express the effect of the aluminum film as described above and to obtain excellent hydrogen permeation resistance in high-pressure hydrogen gas, the thickness of the aluminum film formed on the surface of the steel material in contact with the hydrogen gas is set to 0. It is necessary to be 10 μm or more. In order to obtain excellent hydrogen permeation resistance, it is important that the aluminum film on the surface of the steel material is uniform without gaps. If the thickness of the aluminum film is less than 0.10 μm, it may be difficult to form the aluminum film uniformly on the surface of the steel material without any gap when forming the aluminum film by applying aluminum plating, etc. The property is not sufficiently secured. The thickness of the aluminum film is preferably 0.40 μm or more, more preferably 1.00 μm or more. On the other hand, if the thickness of the aluminum film exceeds 50.00 μm, the effect on the hydrogen permeation resistance is saturated, the effect on the hydrogen permeation resistance is hardly affected, and the time required for forming the aluminum film becomes longer. , Production costs increase. Therefore, the thickness of the aluminum film is preferably 50.00 μm or less. From the viewpoint of cost, since the aluminum film is preferably thin, the thickness of the aluminum film is more preferably 30.00 μm or less, and further preferably 25.00 μm or less. Even if the thickness of the film is 30.00 μm or less, or 25.00 μm or less, a sufficient effect can be obtained in improving the hydrogen permeation resistance. That is, the thickness of the aluminum film is 0.10 μm or more. The thickness is preferably 0.40 to 50.00 μm, more preferably 1.00 to 30.00 μm, and still more preferably 1.00 to 25.00 μm.

  In the present invention, it is only necessary to have an aluminum film on the surface of the steel material that comes into contact with hydrogen gas, and the surface that does not come into contact with hydrogen gas may or may not have an aluminum film. There is no particular limitation. Moreover, in order to avoid peeling of the film during construction of the steel structure, the aluminum film is preferably formed on the surface of the steel material after the shape of the steel structure for hydrogen is used, but is not particularly limited. As long as the effect of the film intended at the time of use can be obtained, it may be formed on the surface of the steel material before the shape of the steel structure for hydrogen.

  The method for forming the aluminum film is not particularly limited. For example, an aluminum plating film can be formed as an aluminum film on the surface of the steel material by performing molten aluminum plating. Moreover, an aluminum film can also be formed by performing the process by physical vapor deposition (PVD), chemical vapor deposition (CVD), or cold spray.

  When the aluminum film is formed by hot-dip aluminum plating, the plating bath composition is preferably pure aluminum, for example, and the plating conditions are preferably immersed in molten pure aluminum for 10 seconds or longer, for example.

  In the present invention, the aluminum film may be an aluminum plating film, or a film formed by PVD, CVD, or cold spray.

  In the present invention, as described above, the steel structure for hydrogen preferably has a laminated structure in which a steel material, an iron oxide film, an aluminum film, and an aluminum oxide film are laminated in this order.

  An iron oxide film is an oxide film formed on the surface of a steel material, and exists in a lower layer (steel material side) than the aluminum film.

  In the present invention, the aluminum oxide film is, for example, an oxide film formed on the surface of an aluminum film, such as an aluminum oxide film formed on an aluminum plating film. In addition, even if it contains inevitable impurities other than aluminum, the effect is not affected.

  It is important that the iron oxide film and the aluminum oxide film are uniform without gaps. The thicknesses of the iron oxide film and the aluminum oxide film are each preferably 0.1 to 20 nm. If the thickness is less than 0.1 nm, the presence of a region where no oxide film is formed does not exhibit superior performance of hydrogen permeation resistance, and if it exceeds 20 nm, the oxide film is greatly produced. This is because it takes time. More preferably, the thickness of the iron oxide film and the aluminum oxide film is 0.2 to 15 nm.

  The method for forming the iron oxide film and the aluminum oxide film is not particularly limited. For example, an iron oxide film can be formed by performing a heat treatment on a steel material before plating. Moreover, an aluminum oxide film can be formed by heat-treating the aluminum plating film after the aluminum plating treatment. When heat treatment is applied, preferable heat treatment conditions are an atmospheric furnace, a holding temperature: 200 to 600 ° C., and a holding time: 1 to 24 hours. This is because if the holding temperature is less than 200 ° C., the amount of oxide film produced is small, and if the holding temperature exceeds 600 ° C., the influence on the material, such as strength reduction, becomes large. Furthermore, if the holding time is less than 1 h, the amount of oxide film produced is small, and if the holding time exceeds 24 h, the influence on the material, such as a decrease in strength, becomes large.

  Further, as described above, the hydrogen pressure accumulator which is the steel structure for hydrogen of the present invention is a pressure accumulator used in a hydrogen station or the like, for example, a type using only a type 1 steel material, or type 2 and This is a type in which a carbon fiber reinforced plastic (CFRP) is wound around a type 3 steel material. Here, Type 1, Type 2, and Type 3 are the standards for compressed natural gas automobile fuel containers, ISO 11439, ANSI (American National Standards Institute) / NGV (Natural Gas Vehicle), and container safety regulations of the High Pressure Gas Safety Act. It is a classification about the structure of the container described in Exhibit Criteria Attachment 9 and the like. Further, the pressure of the stored hydrogen is about 40 MPa or about 82 MPa. Moreover, the hydrogen line pipe which is the steel structure for hydrogen of the present invention is a seamless type or UOE type steel pipe, and the hydrogen pressure is 3 MPa or more.

  As described above, the steel material used in the steel structure for hydrogen of the present invention is preferably a low alloy steel, but the same effect can be expected if an aluminum film as described above can be formed on the surface. It is not particularly limited to low alloy steel. Furthermore, the steel structure for hydrogen of the present invention is a steel structure for hydrogen using various steel materials such as thin plates, thick plates, pipes, shaped steel, and steel bars as they are, or a steel structure for hydrogen formed into a predetermined shape. May be.

  In addition, as a low alloy steel used for the steel structure for hydrogen of this invention, C: 0.05-0.35%, Si: 0.05-0.50%, Mn: 0.0. 5 to 2.0%, Al: 0.01 to 0.10%, N: 0.0005 to 0.008%, P: 0.05% or less, S: 0.01% or less, O: 0.01 %, With the balance being a steel having a composition composed of Fe and inevitable impurities, or in addition to the above composition, Cu: 0.05 to 1.0%, Ni: 0.00%. 05-10.0%, Cr: 0.1-2.5%, Mo: 0.05-2.0%, Nb: 0.005-0.5%, V: 0.005-1.0% Ti: 0.005 to 0.5%, W: 0.05 to 2.0%, B: 0.0005 to 0.005%, or Nd: 0.005 to 1 0%, Ca: 0.0005~0.005%, Mg: 0.0005~0.005%, REM: a 0.0005 to 0.005 percent of one or steel containing two or more. In addition, the low alloy steel used for the steel structure for hydrogen of this invention is not limited to such a low alloy steel.

The steel structure of the steel structure for hydrogen of the present invention is not particularly specified. For example, the steel structure may be bainite, martensite, tempered martensite, or a structure made of ferrite and pearlite. On the other hand, according to the study by the present inventors, by setting the steel structure of the steel material to a predetermined steel structure, even if hydrogen penetrates into the steel material, the fatigue crack growth rate in a high-pressure hydrogen environment is reduced. Therefore, a hydrogen steel structure having excellent hydrogen embrittlement resistance can be obtained. Such a preferable steel structure can exert its effect particularly when the steel material is used in a severe environment such as an ultra-high pressure hydrogen environment.
Hereinafter, from such a viewpoint, a preferable steel structure and steel composition will be described.

  The steel structure of the steel structure for hydrogen is 1) the area ratio of bainite is 10 to 95% and the balance is substantially made of ferrite, or 2) the area ratio of martensite is 10 to 95% and the balance is By making the steel structure substantially composed of ferrite, or 3) having a steel structure having a pearlite area ratio of 10 to 95% and the balance being substantially composed of ferrite, the rate of progress of fatigue cracks is reduced. It is preferable because a hydrogen steel structure having excellent hydrogen embrittlement resistance can be obtained. In the steel structure as described above, soft ferrite and hard phase are dispersed, and at those interfaces, fatigue cracks are stagnated and detoured and / or branched. For this reason, the growth rate of fatigue cracks is reduced, and a hydrogen steel structure having excellent hydrogen embrittlement resistance can be obtained.

  For the measurement of the tissue fraction, for example, the microstructure is revealed by nital etching, the structure is photographed using an optical microscope or SEM (Scanning Electron Microscope), each structure is identified, and the area ratio is determined. You can ask for.

  Below, it demonstrates concretely according to the steel structure of the structure for hydrogen, such as 1) bainite and ferrite, 2) martensite and ferrite, and 3) pearlite and ferrite.

1) Steel structure consisting of bainite and the balance substantially ferrite One of the preferable steel structures of the steel structure for hydrogen is a steel structure where the area ratio of bainite is 10 to 95% and the balance is substantially made of ferrite. is there. The steel structure of the hydrogen steel structure is a steel structure in which soft ferrite and hard bainite are dispersed. In the steel structure for hydrogen having such a steel structure, the fatigue crack is stagnated near the interface between the soft ferrite and hard bainite that are dispersed, and the fatigue crack progresses due to the detour and branching effect. The speed is reduced and it has excellent hydrogen embrittlement resistance. The soft ferrite referred to here means polygonal ferrite, and the hardness is approximately HV10 and a structure of 70 to 150. Further, the hard bainite may be either upper bainite (BI, BII, BIII type) or lower bainite, and means a structure having a hardness of HV10 and 150 to 300. Moreover, HV10 is the Vickers hardness calculated | required by the test force 98N according to the test method of the Vickers hardness test of JISZ2244: 2009.

  Such an effect is an area ratio with respect to the entire structure. The area ratio of the bainite structure is 10 to 95%, and the balance is basically ferrite. That is, the steel structure is mainly composed of ferrite and bainite. By doing so, an obvious effect is recognized. For this reason, the steel structure of the said steel structure for hydrogen is made into 10 to 95% of the area ratio of a bainite structure, and the remainder becomes a ferrite structure substantially. Preferably, the area ratio of bainite is 20 to 95%, more preferably 25 to 95%. The area ratio of bainite is more preferably 30 to 70%, and the area ratio of bainite is preferably 40 to 60%. Here, when the area ratios of the ferrite structure and the bainite structure are substantially the same, the fatigue crack growth rate is the lowest. That is, the ratio of the area ratio of the bainite structure to the total area ratio of the ferrite structure and the bainite structure, that is, the ratio of the bainite area ratio [bainite area ratio ratio: (area ratio of bainite structure) / ((area ratio of ferrite structure) + ( When the area ratio of the bainite structure))] is 0.3 to 0.7, the fatigue crack growth rate decreases most. For this reason, it is preferable that a bainite area ratio ratio shall be 0.3-0.7. More preferably, the bainite area ratio is 0.4 to 0.6. The balance other than the bainite structure is substantially ferrite. However, if the total area ratio of the structure other than bainite and ferrite (for example, pearlite and martensite) is 2% or less, fatigue cracks may occur. Since there is no substantial influence on the progress rate, it may be contained. That is, other structures may be included as long as the total area ratio of bainite and ferrite is 98% or more. “The balance is substantially ferrite” means that the ferrite content of the balance is 60% or more.

  Next, a preferable steel composition of the steel structure for hydrogen having the steel structure in which the area ratio of the bainite is 10 to 95% and the balance is substantially made of ferrite will be described. Although the preferable component composition of the low alloy steel used for the steel structure for hydrogen of the present invention has been described above, the components described below among the components included in the above-described component composition are in the case of the present steel structure. Is more preferably the following content. Hereinafter, “%” indicating the component composition means “% by mass” unless otherwise specified.

C: 0.05-0.20%
C is contained in order to ensure an appropriate hardenability, but if it is less than 0.05%, its effect is insufficient. For this reason, C content shall be 0.05% or more. Preferably, the C content is 0.08% or more. In particular, in order to make it easy to ensure the above-described area ratio of bainite, the C content is preferably set to 0.10% or more. On the other hand, if the C content exceeds 0.20%, the toughness of the base metal and the weld heat affected zone deteriorates, and the weldability deteriorates remarkably. For this reason, C content shall be 0.20% or less. Preferably, the C content is 0.17% or less. In particular, in order to make it easy to ensure the above-described area ratio of bainite, the C content is preferably 0.15% or less. Therefore, the C content is limited to 0.05 to 0.20%.

Si: 0.05 to 0.50%
Si is contained as a deoxidizing material in the steelmaking stage and as an element for ensuring hardenability, but if it is less than 0.05%, its effect is insufficient. For this reason, Si content shall be 0.05% or more. Preferably, the Si content is 0.08% or more. In particular, in order to make it easy to ensure the above-described area ratio of bainite, the Si content is preferably set to 0.10% or more. On the other hand, if the Si content exceeds 0.50%, the grain boundaries become brittle and the low temperature toughness is deteriorated. For this reason, Si content shall be 0.50% or less. Preferably, the Si content is 0.45% or less. In particular, in order to make it easy to ensure the above-described area ratio of bainite, the Si content is preferably 0.40% or less. Therefore, the Si content is limited to 0.05 to 0.50%.

Mn: 0.5 to 2.0%
Mn is contained as an element for ensuring hardenability, but if less than 0.5%, the effect is insufficient. For this reason, Mn content shall be 0.5% or more. Preferably, the Mn content is 0.8% or more. In particular, in order to make it easy to ensure the above-described area ratio of bainite, the Mn content is preferably 1.0% or more. On the other hand, when Mn contains more than 2.0%, the grain boundary strength decreases and the low temperature toughness deteriorates. For this reason, Mn content shall be 2.0% or less. Preferably, the Mn content is 1.8% or less. In particular, in order to make it easy to ensure the above-described area ratio of bainite, the Mn content is preferably 1.5% or less. Therefore, the Mn content is limited to 0.5 to 2.0%.

Al: 0.01-0.10%
Al is added as a deoxidizer and at the same time, pinning austenite grains during heating as Al-based nitride fine precipitates, and has the effect of suppressing grain coarsening, but less than 0.01% Is not effective enough. For this reason, Al content shall be 0.01% or more. Preferably, the Al content is 0.02% or more. On the other hand, if the Al content exceeds 0.10%, surface flaws of the steel sheet are likely to occur. For this reason, Al content shall be 0.10% or less. Preferably, the Al content is 0.08% or less. Therefore, the Al content is limited to 0.01 to 0.10%.

N: 0.0005 to 0.008%
N forms fine precipitates by forming nitrides with Nb, Ti, Al, etc., and pinning austenite grains during heating, thereby suppressing the grain coarsening and improving the low temperature toughness Add for. If the content is less than 0.0005%, the effect of refining the structure is not sufficiently brought about. For this reason, N content shall be 0.0005% or more. Preferably, the N content is 0.002% or more. On the other hand, the content exceeding 0.008% impairs the toughness of the base metal and the weld heat affected zone because the amount of dissolved N increases. For this reason, N content shall be 0.008% or less. Preferably, the N content is 0.006% or less. Therefore, the N content is limited to 0.0005 to 0.008%.

P: 0.05% or less P, which is an impurity element, is easily segregated at grain boundaries, and if it exceeds 0.05%, it lowers the bonding strength of adjacent crystal grains and degrades low-temperature toughness. Therefore, the P content is limited to 0.05% or less. Preferably, the P content is 0.03% or less.

S: 0.01% or less S, which is an impurity element, easily segregates at the crystal grain boundaries and easily generates MnS, which is a non-metallic inclusion. When it exceeds 0.01%, the bonding strength of adjacent crystal grains decreases, the amount of inclusions increases, and the low-temperature toughness deteriorates. Therefore, the S content is limited to 0.01% or less. Preferably, the S content is 0.005% or less.

O: 0.01% or less O affects the moldability of a material by forming an oxide with Al or the like. Inclusions exceeding 0.01% increase inclusions and impair formability. Therefore, the O content is limited to 0.01% or less. Preferably, the O content is 0.006% or less.

2) Steel structure consisting of martensite and the balance substantially ferrite One of the preferable steel structures of the steel structure for hydrogen is a steel whose martensite area ratio is 10 to 95% and the balance is substantially made of ferrite. It is an organization. The steel structure of this hydrogen steel structure is a steel structure in which soft ferrite and hard martensite are dispersed. In a steel structure for hydrogen with such a steel structure, fatigue cracks are stagnated near the interface between soft ferrite and hard martensite that exist in a dispersed state, and the effect of diverting and branching is the cause of fatigue cracks. Progression speed is reduced and has excellent hydrogen embrittlement resistance. The soft ferrite referred to here means polygonal ferrite, and the hardness is approximately HV10 and a structure of 70 to 150. The hard martensite may or may not contain cementite, and means a structure having a hardness of approximately HV10 and 200 to 600.

  Such an effect is an area ratio with respect to the entire structure. The area ratio of the martensite structure is 10 to 95%, and the remainder is basically made of ferrite. That is, the steel structure is mainly composed of martensite and ferrite. A clear effect can be seen in the organization. For this reason, the steel structure of the steel structure for hydrogen has a martensite structure area ratio of 10 to 95%, and the balance is substantially a ferrite structure. Preferably, the area ratio of martensite is 20 to 95%, more preferably 25 to 95%. The area ratio of martensite is more preferably 30 to 70%, and the area ratio of martensite is preferably 40 to 60%. Here, when the area ratios of the ferrite structure and the martensite structure are substantially the same, the fatigue crack growth rate is most reduced. That is, the ratio of the area ratio of the martensite structure to the total area ratio of the ferrite structure and the martensite structure, the ratio of the martensite area ratio [martensite area ratio: (area ratio of the martensite structure) / ((of the ferrite structure When the area ratio) + (the area ratio of the martensite structure))] is 0.3 to 0.7, the fatigue crack growth rate decreases most. For this reason, the martensite area ratio is preferably 0.3 to 0.7. More preferably, the martensite area ratio is 0.4 to 0.6. The remainder other than the martensite structure is substantially made of ferrite, but if the structure other than martensite and ferrite (for example, pearlite or bainite) is 2% or less in terms of the total area ratio, fatigue cracks are caused. It may be contained because it has no effect on the growth rate of. That is, other structures may be included as long as the total area ratio of martensite and ferrite is 98% or more. “The balance is substantially ferrite” means that the ferrite content of the balance is 60% or more.

  Next, the preferable steel composition of the steel structure for hydrogen which has the steel structure which the above-mentioned martensite area ratio is 10 to 95% and the remainder substantially consists of a ferrite is demonstrated. Although the preferable component composition of the low alloy steel used for the steel structure for hydrogen of the present invention has been described above, the components described below among the components included in the above-described component composition are in the case of the present steel structure. Is more preferably the following content. In addition, as above-mentioned,% which shows a component composition means the mass% unless there is particular notice.

C: 0.05 to 0.35%
C is contained in order to ensure an appropriate hardenability, but if it is less than 0.05%, its effect is insufficient. For this reason, C content shall be 0.05% or more. Preferably, the C content is 0.08% or more. In particular, in order to easily secure the above-described martensite area ratio, the C content is preferably set to 0.10% or more. On the other hand, if the C content exceeds 0.35%, the toughness of the base metal and the weld heat affected zone deteriorates, and the weldability deteriorates remarkably. For this reason, C content shall be 0.35% or less. Preferably, the C content is 0.27% or less. In particular, in order to easily secure the above-described martensite area ratio, the C content is preferably 0.25% or less. Therefore, the C content is limited to 0.05 to 0.35%.

Si: 0.05 to 0.50%
Si is contained as a deoxidizing material in the steelmaking stage and as an element for ensuring hardenability, but if it is less than 0.05%, its effect is insufficient. For this reason, Si content shall be 0.05% or more. Preferably, the Si content is 0.08% or more. In particular, in order to easily secure the above-described martensite area ratio, the Si content is preferably set to 0.10% or more. On the other hand, if the Si content exceeds 0.50%, the grain boundaries become brittle and the low temperature toughness is deteriorated. For this reason, Si content shall be 0.50% or less. Preferably, the Si content is 0.45% or less. In particular, in order to easily secure the above-described martensite area ratio, the Si content is preferably set to 0.40% or less. Therefore, the Si content is limited to 0.05 to 0.50%.

Mn: 0.5 to 2.0%
Mn is contained as an element for ensuring hardenability, but if less than 0.5%, the effect is insufficient. For this reason, Mn content shall be 0.5% or more. Preferably, the Mn content is 0.8% or more. In particular, the Mn content is preferably 1.0% or more in order to easily secure the above-described martensite area ratio. On the other hand, when Mn contains more than 2.0%, the grain boundary strength decreases and the low temperature toughness deteriorates. For this reason, Mn content shall be 2.0% or less. Preferably, the Mn content is 1.8% or less. In particular, the Mn content is preferably 1.5% or less in order to easily secure the above-described martensite area ratio. Therefore, the Mn content is limited to 0.5 to 2.0%.

Al: 0.01-0.10%
Al is added as a deoxidizer and at the same time, pinning austenite grains during heating as Al-based nitride fine precipitates, and has the effect of suppressing grain coarsening, but less than 0.01% Is not effective enough. For this reason, Al content shall be 0.01% or more. Preferably, the Al content is 0.02% or more. On the other hand, if the Al content exceeds 0.10%, surface flaws of the steel sheet are likely to occur. For this reason, Al content shall be 0.10% or less. Preferably, the Al content is 0.08% or less. Therefore, the Al content is limited to 0.01 to 0.10%.

N: 0.0005 to 0.008%
N forms fine precipitates by forming nitrides with Nb, Ti, Al, etc., and pinning austenite grains during heating, thereby suppressing grain coarsening and improving low-temperature toughness Add for. If the content is less than 0.0005%, the effect of refining the structure is not sufficiently brought about. For this reason, N content shall be 0.0005% or more. Preferably, the N content is 0.002% or more. On the other hand, the content exceeding 0.008% impairs the toughness of the base metal and the weld heat affected zone because the amount of dissolved N increases. For this reason, N content shall be 0.008% or less. Preferably, the N content is 0.006% or less. Therefore, the N content is limited to 0.0005 to 0.008%.

P: 0.05% or less P, which is an impurity element, is easily segregated at grain boundaries, and if it exceeds 0.05%, it lowers the bonding strength of adjacent crystal grains and degrades low-temperature toughness. Therefore, the P content is limited to 0.05% or less. Preferably, the P content is 0.03% or less.

S: 0.01% or less S, which is an impurity element, easily segregates at the crystal grain boundaries and easily generates MnS, which is a non-metallic inclusion. When it exceeds 0.01%, the bonding strength of adjacent crystal grains decreases, the amount of inclusions increases, and the low-temperature toughness deteriorates. Therefore, the S content is limited to 0.01% or less. Preferably, the S content is 0.005% or less.

O: 0.01% or less O affects the moldability of a material by forming an oxide with Al or the like. Inclusions exceeding 0.01% increase inclusions and impair formability. Therefore, the O content is limited to 0.01% or less. Preferably, the O content is 0.006% or less.

3) Steel structure consisting of pearlite and the balance substantially ferrite One of the preferred steel structures of the steel structure for hydrogen is a steel structure in which the area ratio of pearlite is 10 to 95% and the balance is substantially made of ferrite. is there. The steel structure of the steel structure for hydrogen is a steel structure in which soft ferrite and hard pearlite are dispersed. In a steel structure for hydrogen with such a steel structure, the fatigue crack growth rate is stagnated near the interface between the dispersed soft ferrite and hard pearlite, detouring, and branching. And has excellent hydrogen embrittlement resistance. The soft ferrite referred to here means polygonal ferrite, and the hardness is approximately HV10 and a structure of 70 to 150. The hard pearlite may be in any form in which ferrite and pearlite are lamellar or pearlite is dispersed in a lump in the ferrite, and means a structure having a hardness of about HV10 and 150 to 300.

  Such an effect is an area ratio with respect to the entire structure. The area ratio of the pearlite structure is 10 to 95%, and the remainder is basically made of ferrite. That is, the steel structure is mainly composed of a two-phase structure mainly composed of pearlite and ferrite. By doing so, an obvious effect is recognized. For this reason, the steel structure of the steel structure for hydrogen has an area ratio of pearlite structure of 10 to 95%, and the remainder substantially has a ferrite structure. Preferably, the area ratio of pearlite is 20 to 95%, more preferably 25 to 95%. The area ratio of pearlite is more preferably 30 to 70%, and the area ratio of pearlite is preferably 40 to 60%. Here, when the area ratios of the ferrite structure and the pearlite structure are substantially the same, the fatigue crack growth rate is most reduced. That is, the ratio of the area ratio of the pearlite structure to the total area ratio of the ferrite structure and the pearlite structure, the ratio of the pearlite area ratio [the ratio of the pearlite area ratio: (the area ratio of the pearlite structure) / ((the area ratio of the ferrite structure) + ( When the area ratio of the pearlite structure))] is 0.3 to 0.7, the fatigue crack growth rate decreases most. For this reason, the pearlite area ratio is preferably 0.3 to 0.7. More preferably, the pearlite area ratio is 0.4 to 0.6. The remainder other than the pearlite structure is substantially made of ferrite. However, if the total area ratio of the structure other than pearlite and ferrite (for example, bainite and martensite) is 2% or less, fatigue cracks will occur. Since there is no influence on the progress rate, it may be contained. That is, as long as the total area ratio of pearlite and ferrite is 98% or more, other structures may be contained. “The balance is substantially ferrite” means that the ferrite content of the balance is 60% or more.

  Next, a preferable steel composition of the steel structure for hydrogen of the present invention having a steel structure in which the area ratio of the pearlite is 10 to 95% and the balance is substantially made of ferrite will be described. Although the preferable component composition of the low alloy steel used for the steel structure for hydrogen of the present invention has been described above, the components described below among the components included in the above-described component composition are in the case of the present steel structure. Is more preferably the following content. In addition, as above-mentioned,% which shows a component composition means the mass% unless there is particular notice.

C: 0.05-0.10%
C is contained in order to ensure an appropriate hardenability, but if it is less than 0.05%, its effect is insufficient. For this reason, C content shall be 0.05% or more. Preferably, the C content is 0.06% or more. In particular, in order to make it easy to ensure the above-described area ratio of pearlite, the C content is preferably set to 0.07% or more. On the other hand, if it exceeds 0.10%, the toughness of the base metal and the weld heat affected zone deteriorates, and the weldability deteriorates remarkably. For this reason, C content shall be 0.10% or less. Preferably, the C content is 0.09% or less. In particular, in order to make it easy to ensure the above-described area ratio of pearlite, the C content is preferably set to 0.08% or less. Therefore, the C content is limited to 0.05 to 0.10%.

Si: 0.05 to 0.50%
Si is contained as a deoxidizing material in the steelmaking stage and as an element for ensuring hardenability, but if it is less than 0.05%, its effect is insufficient. For this reason, Si content shall be 0.05% or more. Preferably, the Si content is 0.08% or more. In particular, in order to make it easy to ensure the above-described area ratio of pearlite, the Si content is preferably set to 0.10% or more. On the other hand, if the Si content exceeds 0.50%, the grain boundaries become brittle and the low temperature toughness is deteriorated. For this reason, Si content shall be 0.50% or less. Preferably, the Si content is 0.45% or less. In particular, in order to make it easy to ensure the above-described area ratio of pearlite, the Si content is preferably 0.40% or less. Therefore, the Si content is limited to 0.05 to 0.50%.

Mn: 0.5 to 2.0%
Mn is contained as an element for ensuring hardenability, but if less than 0.5%, the effect is insufficient. For this reason, Mn content shall be 0.5% or more. Preferably, the Mn content is 0.8% or more. In particular, in order to make it easy to ensure the above-described area ratio of pearlite, the Mn content is preferably 1.0% or more. On the other hand, when Mn contains more than 2.0%, the grain boundary strength decreases and the low temperature toughness deteriorates. For this reason, Mn content shall be 2.0% or less. Preferably, the Mn content is 1.8% or less. In particular, in order to make it easy to ensure the above-described area ratio of pearlite, the Mn content is preferably 1.5% or less. Therefore, the Mn content is limited to 0.5 to 2.0%.

Al: 0.01-0.10%
Al is added as a deoxidizer and at the same time, pinning austenite grains during heating as Al-based nitride fine precipitates, and has the effect of suppressing grain coarsening, but less than 0.01% Is not effective enough. For this reason, Al content shall be 0.01% or more. Preferably, the Al content is 0.02% or more. On the other hand, if the Al content exceeds 0.10%, surface flaws of the steel sheet are likely to occur. For this reason, Al content shall be 0.10% or less. Preferably, the Al content is 0.08% or less. Therefore, the Al content is limited to 0.01 to 0.10%.

N: 0.0005 to 0.008%
N forms fine precipitates by forming nitrides with Nb, Ti, Al, etc., and pinning austenite grains during heating, thereby suppressing grain coarsening and improving low-temperature toughness Add for. If the content is less than 0.0005%, the effect of refining the structure is not sufficiently brought about. For this reason, N content shall be 0.0005% or more. Preferably, the N content is 0.002% or more. On the other hand, the content exceeding 0.008% impairs the toughness of the base metal and the weld heat affected zone because the amount of dissolved N increases. For this reason, N content shall be 0.008% or less. Preferably, the N content is 0.006% or less. Therefore, the N content is limited to 0.0005 to 0.008%.

P: 0.05% or less P, which is an impurity element, is easily segregated at grain boundaries, and if it exceeds 0.05%, it lowers the bonding strength of adjacent crystal grains and degrades low-temperature toughness. Therefore, the P content is limited to 0.05% or less. Preferably, the P content is 0.03% or less.

S: 0.01% or less S, which is an impurity element, easily segregates at the crystal grain boundaries and easily generates MnS, which is a non-metallic inclusion. When it exceeds 0.01%, the bonding strength of adjacent crystal grains decreases, the amount of inclusions increases, and the low-temperature toughness deteriorates. Therefore, the S content is limited to 0.01% or less. Preferably, the S content is 0.005% or less.

O: 0.01% or less O affects the workability of the material by forming an oxide with Al or the like. Inclusions exceeding 0.01% increase inclusions and impair processability. Therefore, the O content is limited to 0.01% or less. Preferably, the O content is 0.006% or less.

Furthermore, in the case of any structure of 1) bainite and ferrite, 2) martensite and ferrite, and 3) pearlite and ferrite, the balance of the above component composition is a steel composition composed of Fe and inevitable impurities. Is preferred. Further, it is more preferable that the following components i) and ii) are further appropriately contained individually or simultaneously depending on the desired properties.
i) Cu: 0.05-1.0%, Ni: 0.05-10.0%, Cr: 0.1-2.5%, Mo: 0.05-2.0%, Nb: 0. 005 to 0.5%, V: 0.005 to 1.0%, Ti: 0.005 to 0.5%, W: 0.05 to 2.0%, B: 0.0005 to 0.005% One or more of.
ii) One or two types of Nd: 0.005-1.0%, Ca: 0.0005-0.005%, Mg: 0.0005-0.005%, REM: 0.0005-0.005% that's all.

Cu: 0.05 to 1.0%
Cu has the effect | action which improves hardenability. If it is less than 0.05%, the effect is not sufficient. On the other hand, if it exceeds 1.0%, hot cracking is likely to occur during heating of the steel slab or welding. Therefore, when adding Cu, the content is limited to 0.05% or more and 1.0% or less.

Ni: 0.05-10.0%
Ni, like Cu, has an effect of improving hardenability, and further has an effect of improving toughness. If it is less than 0.05%, the effect is not sufficient. On the other hand, if it exceeds 10.0%, the economy is inferior. Therefore, when adding Ni, the content is limited to 0.05% or more and 10.0% or less.

Cr: 0.1 to 2.5%
Cr is contained as an element for ensuring hardenability. However, if it is less than 0.1%, its effect is insufficient. On the other hand, if it exceeds 2.5%, weldability deteriorates. Therefore, when adding Cr, the content is limited to 0.1% to 2.5%.

Mo: 0.05-2.0%
Mo has an effect of improving the hardenability, but if it is less than 0.05%, the effect is insufficient. On the other hand, if it exceeds 2.0%, the economy is inferior. Therefore, when adding Mo, the content is limited to 0.05% or more and 2.0% or less.

Nb: 0.005 to 0.5%
Nb has the effect of improving hardenability, and also pinns austenite grains during heating as fine precipitates of Nb-based carbonitrides, thereby suppressing grain coarsening. If the content is less than 0.005%, the effect is insufficient. On the other hand, if the content exceeds 0.5%, the toughness of the heat affected zone is deteriorated. Therefore, when adding Nb, the content is limited to 0.005% or more and 0.5% or less.

V: 0.005-1.0%
V has the effect of improving hardenability, and also pinns austenite grains during heating as fine precipitates of V-based carbides, and suppresses coarsening of the grains. If the content is less than 0.005%, the effect is insufficient. On the other hand, if the content exceeds 1.0%, the toughness of the heat affected zone is deteriorated. Therefore, when adding V, the content is limited to 0.005% or more and 1.0% or less.

Ti: 0.005 to 0.5%
Ti has the effect of improving the hardenability and has the effect of pinning austenite grains during heating as a fine precipitate of Ti-based carbonitride to suppress grain growth. If the content is less than 0.005%, the effect is insufficient. On the other hand, if the content exceeds 0.5%, the toughness of the heat affected zone is deteriorated. Therefore, when adding Ti, the content is limited to 0.005% or more and 0.5% or less.

W: 0.05-2.0%
W has an effect of improving the hardenability, but if it is less than 0.05%, its effect is insufficient. On the other hand, if it exceeds 2.0%, the weldability deteriorates. Therefore, when adding W, the content is limited to 0.05% or more and 2.0% or less.

B: 0.0005 to 0.005%
B is contained as an element for ensuring hardenability. However, if it is less than 0.0005%, its effect is insufficient. On the other hand, if it exceeds 0.005%, the toughness is deteriorated. Therefore, when adding B, the content is limited to 0.0005% or more and 0.005% or less.

Nd: 0.005 to 1.0%
Nd has the effect of incorporating S as inclusions, reducing the amount of S grain boundary segregation, and improving low-temperature toughness and hydrogen embrittlement resistance. If the content is less than 0.005%, the effect is insufficient. On the other hand, if the content exceeds 1.0%, the toughness of the heat affected zone is deteriorated. Therefore, when adding Nd, the content is limited to 0.005% or more and 1.0% or less.

Ca: 0.0005 to 0.005%
Ca forms CaS and acts to control the form of sulfide inclusions to CaS, which is a spherical inclusion that is difficult to expand by rolling, instead of MnS, which is an inclusion that is easy to expand by rolling. Have. If the content is less than 0.0005%, the effect is not sufficient. On the other hand, if the content exceeds 0.005%, the cleanliness is lowered, and thus materials such as toughness deteriorate. Therefore, when adding Ca, the content is limited to 0.0005% or more and 0.005% or less.

Mg: 0.0005 to 0.005%
Mg may be used as a hot metal desulfurization material. If the content is less than 0.0005%, the effect is not sufficient. On the other hand, if the content exceeds 0.005%, the cleanliness decreases. Therefore, when adding Mg, the content is limited to 0.0005% or more and 0.005% or less.

REM: 0.0005 to 0.005%
REM improves the SR cracking resistance by reducing the amount of solid solution S at the grain boundaries by producing sulfide as REM (O, S) in steel. If the content is less than 0.0005%, the effect is not sufficient. On the other hand, if the content exceeds 0.005%, REM sulfide is remarkably accumulated in the precipitation crystal zone, leading to deterioration of the material. Therefore, when adding REM, the content is limited to 0.0005% or more and 0.005% or less. Note that REM is an abbreviation for Rare Earth Metal and is a rare earth metal.

  The production method for obtaining the steel structure for hydrogen having the preferred steel structure is not particularly limited. For example, when producing a hydrogen line pipe as a hydrogen steel structure, the steel structure is adjusted by hot rolling and accelerated cooling of the steel material having the above preferred component composition, or by direct quenching and tempering. Thus, the preferred steel structure can be obtained. The hydrogen pressure accumulator, which is a steel structure for hydrogen, is a steel structure in which, for example, a steel material having a predetermined component composition is formed into a predetermined shape, that is, a desired hydrogen pressure accumulator shape, reheated, quenched, and tempered. By adjusting the above, it is possible to have the preferable steel structure. In addition, after the steel structure is adjusted in this way, the steel structure for hydrogen of the present invention can be obtained by forming a predetermined aluminum film by performing aluminum plating as described above.

  Below, the hydrogen production pipe structure and hydrogen pressure accumulator which are the steel structure for hydrogen of this invention are illustrated, and a preferable manufacturing method is demonstrated about the steel structure for hydrogen which has the above-mentioned preferable steel structure.

  In addition, the temperature regulation in the following manufacturing conditions shall be at the center of the steel material, and the thin plate, thick plate, pipe, and shape steel shall be the center of the plate thickness, and the center of the steel plate in the radial direction. However, the vicinity of the center portion has substantially the same temperature history, and is not limited to the center itself.

  The hydrogen line pipe, which is the steel structure for hydrogen of the present invention having the above-described preferable steel structure, is produced by hot rolling and accelerated cooling of a steel material or direct quenching and tempering as described above. be able to.

Steel material The steel material used for the production of the hydrogen line pipe of the present invention is cast from molten steel adjusted to a preferred component composition according to the preferred steel structure. Here, it is not necessary to limit the casting conditions in particular, and a steel material manufactured under any casting conditions may be used. A method for producing a slab from molten steel and a method for producing a slab by rolling the slab are not particularly specified. Steel melted by a converter method, an electric furnace method, etc., or a steel slab produced by a continuous casting / ingot-making method can be used.

Production by Accelerated Cooling The steel material is heated to the Ac ₃ transformation point or higher, hot rolled to a predetermined plate thickness, and continuously from the Ar ₃ transformation point to 600 ° C./s with a cooling rate of 1 to 200 ° C./s. Accelerate cooling to below ℃. When the heating temperature is less than the Ac ₃ transformation point, a part of untransformed austenite remains, so that a desired steel structure cannot be obtained after hot rolling and accelerated cooling. Therefore, the heating temperature before hot rolling is set to Ac ₃ transformation point or more. Preferably, the heating temperature is (Ac ₃ +50) ° C. or higher. In addition, it is preferable that this heating temperature shall be 1250 degrees C or less from a viewpoint of excessive coarsening suppression of an initial austenite particle size, and a production efficiency improvement. Further, if the start temperature of cooling after hot rolling is less than the Ar ₃ transformation point, a part of austenite transformation occurs before the start of cooling, and thus a desired steel structure cannot be obtained after accelerated cooling. For this reason, after hot rolling, cooling is started from the Ar ₃ transformation point or higher. Preferably, cooling is started from (Ar ₃ +50) ° C. or higher. In addition, it is preferable that the temperature which starts cooling shall be 1000 degrees C or less from balance with hot rolling. The cooling rate from the Ar ₃ transformation point or higher is set to 1 ° C./s or more and 200 ° C./s or less in order to obtain a desired structure. The cooling rate is an average cooling rate at the center of the plate thickness. Further, in order to stably obtain a steel structure in which the area ratio of bainite is 10 to 95% and the balance is substantially made of ferrite, the cooling rate may be 5 ° C./s or more and less than 20 ° C./s. preferable. Further, in order to stably obtain a steel structure in which the area ratio of martensite is 10 to 95% and the balance is substantially made of ferrite, the cooling rate should be 20 ° C./s or more and 200 ° C./s or less. Is preferred. Further, in order to stably obtain a steel structure in which the area ratio of pearlite is 10 to 95% and the balance is substantially composed of ferrite, the cooling rate may be 1 ° C./s or more and less than 5 ° C./s. preferable. The cooling means is not particularly limited and may be performed by water cooling or the like. Further, when the cooling is stopped at a temperature exceeding 600 ° C., the desired transformation is not completed, so that a desired steel structure cannot be obtained. For this reason, accelerated cooling is performed to a temperature of 600 ° C. or lower. Preferably, accelerated cooling is performed to 550 ° C. or lower. In addition, it is preferable that the temperature which stops this cooling shall be 300 degreeC or more from the relationship with a transformation behavior.

Production by direct quenching and tempering The above steel material is heated to the Ac ₃ transformation point or higher, and after hot rolling, is subsequently quenched from the Ar ₃ transformation point to a temperature of 250 ° C. or lower at a cooling rate of 1 to 200 ° C./s. Temper at a temperature below ₁ transformation point. When the heating temperature is less than the Ac ₃ transformation point, a part of untransformed austenite remains, and thus a desired steel structure cannot be obtained after hot rolling, quenching, and tempering. Therefore, the heating temperature before hot rolling is set to Ac ₃ transformation point or more. Preferably, the heating temperature is (Ac ₃ +50) ° C. or higher. In addition, it is preferable that this heating temperature shall be 1250 degrees C or less from a viewpoint of excessive coarsening suppression of an initial austenite particle size, and a production efficiency improvement. Further, if the start temperature of quenching after hot rolling is less than the Ar ₃ transformation point, a partial transformation of austenite occurs before quenching, so that a desired steel structure cannot be obtained after quenching and tempering. For this reason, after hot rolling, cooling is started from the Ar ₃ transformation point or higher, and quenching is performed. Preferably, cooling is started from (Ar ₃ +50) ° C. or higher. In addition, it is preferable that the starting temperature of hardening shall be 1000 degrees C or less from the balance with hot rolling. The cooling rate at the time of quenching from the Ar ₃ transformation point or higher is set to 1 ° C./s or higher and 200 ° C./s or lower in order to obtain a desired structure. The cooling rate is an average cooling rate at the center of the plate thickness. Further, in order to stably obtain a steel structure in which the area ratio of bainite is 10 to 95% and the balance is substantially made of ferrite, the cooling rate may be 5 ° C./s or more and less than 20 ° C./s. preferable. Further, in order to stably obtain a steel structure in which the area ratio of martensite is 10 to 95% and the balance is substantially made of ferrite, the cooling rate should be 20 ° C./s or more and 200 ° C./s or less. Is preferred. Further, in order to stably obtain a steel structure in which the area ratio of pearlite is 10 to 95% and the balance is substantially composed of ferrite, the cooling rate may be 1 ° C./s or more and less than 5 ° C./s. preferable. The cooling means is not particularly limited and may be performed by water cooling or the like. Further, when the quenching is stopped at a temperature exceeding 250 ° C., the desired transformation is not completed, so that a desired steel structure cannot be obtained after tempering. For this reason, it shall temper to the temperature of 250 degrees C or less. Preferably, quenching is performed to a temperature of 200 ° C. or lower. In addition, it is preferable that the quenching stop temperature is 100 ° C. or higher from the viewpoint of improving production efficiency. After quenching, tempering is continued at a temperature below the Ac ₁ transformation point. When the tempering temperature exceeds the Ac ₁ transformation point, a part of the steel is transformed into austenite, so that a desired steel structure cannot be obtained after tempering. _Preferably, tempering at a temperature of _(Ac 1 -20) ℃ or less. The tempering temperature is preferably 300 ° C. or higher in order to achieve the purpose of recovering toughness and the like.

  As described above, the hydrogen pressure accumulator which is the steel structure for hydrogen of the present invention is reheated after forming a steel material having a predetermined component composition into a predetermined shape, that is, a desired hydrogen pressure accumulator shape, for example. It can be manufactured by quenching and tempering.

Production by reheating, quenching, and tempering After forming a steel material having the above-described composition into a predetermined shape, the steel material is heated to the Ac ₃ transformation point or higher and subsequently cooled from the Ar ₃ transformation point or higher at a cooling rate of 0.5 to 100 ° C./s. Quenching to a temperature of 250 ° C. or lower, and subsequently tempering at a temperature of the Ac ₁ transformation point or lower. Here, the steel material to be heated to the Ac ₃ transformation point or higher may have a component composition corresponding to the desired steel structure of the hydrogen pressure accumulator, and the steel structure of the steel material need not be specified. If the heating temperature after forming into a predetermined shape is less than the Ac ₃ transformation point, a part of untransformed austenite remains, and thus a desired steel structure cannot be obtained after hot rolling, quenching, and tempering. Therefore, the heating temperature is set to Ac ₃ transformation point or more. Preferably, the heating temperature is (Ac ₃ +50) ° C. or higher. In order to suppress excessive coarsening of the initial austenite grain size and improve production efficiency, the heating temperature is preferably 1250 ° C. or lower. Further, if the quenching start temperature after heating is less than the Ar ₃ transformation point, a partial transformation of austenite occurs before cooling, so that a desired steel structure cannot be obtained after quenching and tempering. For this reason, after the heating, cooling is started from the Ar ₃ transformation point or higher, and quenching is performed. Preferably, cooling is started from (Ar ₃ +50) ° C. or higher. In consideration of hot rolling, the quenching start temperature is preferably 1000 ° C. or lower. The cooling rate at the time of quenching from the Ar ₃ transformation point or higher is set to 0.5 ° C./s or more and 100 ° C./s or less in order to obtain a desired structure and prevent quench cracking. The cooling rate is an average cooling rate at the center of the plate thickness (wall thickness of the accumulator). Further, in order to stably obtain a steel structure in which the area ratio of bainite is 10 to 95% and the balance is substantially made of ferrite, the cooling rate may be 5 ° C./s or more and less than 20 ° C./s. preferable. Further, in order to stably obtain a steel structure in which the area ratio of martensite is 10 to 95% and the balance is substantially made of ferrite, the cooling rate should be 20 ° C./s or more and 100 ° C./s or less. Is preferred. Further, in order to stably obtain a steel structure in which the area ratio of pearlite is 10 to 95% and the balance is substantially made of ferrite, the cooling rate is 0.5 ° C./s or more and less than 5 ° C./s. It is preferable. The cooling means is not particularly limited and may be performed by oil cooling or water cooling. Further, when the quenching, that is, the cooling is stopped at a temperature exceeding 250 ° C., the desired transformation is not completed, and thus a desired steel structure cannot be obtained after tempering. For this reason, it shall temper to the temperature of 250 degrees C or less. Preferably, quenching is performed to a temperature of 200 ° C. or lower. In order to improve production efficiency, the quenching stop temperature is preferably set to 100 ° C. or higher. After quenching, tempering is continued at a temperature below the Ac ₁ transformation point. When the tempering temperature exceeds the Ac ₁ transformation point, a part of the steel is transformed into austenite, so that a desired steel structure cannot be obtained after tempering. Preferably, tempering temperature is _(Ac 1 -20) ℃ or less. In addition, in order to achieve the objectives, such as recovering toughness etc., it is preferable that tempering temperature shall be 300 degreeC or more.

In the present invention, the method for obtaining the Ac ₃ transformation point (° C.), the Ar ₃ transformation point (° C.), and the Ac ₁ transformation point (° C.) is not particularly specified. For example, Ac ₃ = 854-180C + 44Si-14Mn-17 _{.8Ni-1.7Cr, Ar 3 = 910-310C} -80Mn-20Cu-15Cr-55Ni-80Mo, can be obtained as Ac 1 = 723-14Mn + 22Si-14.4Ni + 23.3Cr. In the above formula, each element symbol is the content (mass%) of each element in steel, and 0 is set for elements not contained.

  Depending on the above conditions, it has a steel structure having a desired amount of bainite and the balance being substantially ferrite, or having a desired amount of martensite and the balance being substantially ferrite, or A hydrogen line pipe or a hydrogen pressure accumulator which is a steel structure for hydrogen having a steel structure having a desired amount of pearlite and the balance being substantially ferrite is obtained.

  Examples in which the effects of the present invention are verified will be described below.

  Table 1 shows the chemical composition, steel structure and tensile strength of the low alloy steel subjected to the verification experiment. The steel structure shown in Table 1 is 3 vol. % Nital etching reveals the microstructure, and the position of the thickness 1/4 of the cross section parallel to the rolling direction is observed with an optical microscope at an appropriate magnification of 200 to 400 times, and the type of the structure is visually observed. When the tissue is identified and has two or more phases, the area ratio of each tissue is obtained by image processing. Further, the tensile strength is evaluated by performing a tensile test in accordance with JISZ 2241 using a full thickness tensile test piece in which the rolling direction in accordance with JISZ 2201 (1980) is the longitudinal direction (tensile direction).

  Using these low-alloy steel plates shown in Table 1, disk-shaped samples for high-pressure hydrogen permeation experiments having a plate thickness of 2 mm or 3 mm were prepared from 1/4 positions in the plate thickness direction, as shown in Table 2. A test piece without plating and a test piece with plating of several levels of thickness were prepared.

  As the test piece subjected to plating, a test piece in which aluminum (Al) plating having several levels of thickness shown in Table 2 was applied to form an aluminum plating film was prepared.

  Here, aluminum plating was performed only on one side of the test piece. Moreover, the aluminum plating shown in Table 2 was performed by hot dipping using pure aluminum, and the aluminum plating conditions were immersion in molten pure aluminum. By plating in this way, an aluminum plating film as an aluminum film was formed on the surface of the low alloy steel as a test piece. Moreover, the aluminum plating film thickness was changed by adjusting the immersion time in the molten pure aluminum. The thickness of the aluminum plating film (the thickness of the aluminum film) was measured by observing the cross section of the test piece with an SEM.

  For sample Nos. (AL12 to AL21), a first heat treatment (holding temperature: 300 ° C., holding time: 12 to 24 h) was performed before aluminum plating to form an iron oxide film. For (AL12 to AL19, AL21, AL22), after the aluminum plating, a second heat treatment (holding temperature: 300 ° C., holding time: 12 to 24 h) was performed to form an aluminum oxide film.

  The thickness of the iron oxide film and the aluminum oxide film was measured by TEM for the cross section of the test piece. The measurement results are shown in Table 2.

One side of these test pieces (aluminum-plated sample is the aluminum-plated side) is the hydrogen gas pressure / temperature shown in Table 2 ("Hydrogen gas pressure" and "Hydrogen permeation measurement temperature" in Table 2). Immediately after the start of hydrogen introduction, hydrogen coming out from the opposite side of hydrogen introduction was continuously analyzed by Q-mass (Quadrupole Mass Spectrometer) to obtain a hydrogen permeation curve. From the obtained hydrogen permeation curves, the time (h) from the start of hydrogen measurement to the start of hydrogen permeation and the hydrogen permeation amount (H ₂ / s) when the steady state was reached were analyzed. The unit of hydrogen permeation is the number of hydrogen molecules that permeate the sample per second. Moreover, the hydrogen permeation curve increased with time, and after a certain period of time, a state in which the hydrogen permeation curve almost did not change with respect to time was judged as a steady state. Table 2 shows the aluminum plating thickness (aluminum film thickness) obtained as described above, the hydrogen permeation start time which is the time from the start of hydrogen measurement to the start of hydrogen permeation, and the hydrogen permeation amount in a steady state.

  Also, regarding the hydrogen permeation start time, the hydrogen permeation start time (h) and the plating in the case of plating (with plating) under the same steel material, the same disk-shaped sample thickness, and the same hydrogen introduction conditions The ratio of hydrogen permeation start time (h) when not applied (without plating) (= (hydrogen permeation start time with plating) / (hydrogen permeation start time without plating)) was obtained as a hydrogen permeation start time ratio, It is shown in 2. The hydrogen permeation start time ratio was set to 2.0 or more. As for the hydrogen permeation amount, the ratio of the hydrogen permeation amount when plating is applied to the hydrogen permeation amount when plating is not performed under the same steel material, the same disk-shaped sample thickness, and the same hydrogen introduction conditions. The hydrogen permeation ratio (= (hydrogen permeation with plating) / (hydrogen permeation without plating)) was determined and shown in Table 2. The hydrogen permeation ratio was set to 0.5 or less. When both the hydrogen permeation start time ratio target and the hydrogen permeation amount ratio target are satisfied, the hydrogen permeation resistance is excellent.

  Sample No. shown in Table 2 AL13 to AL15 and AL17 to AL22 have an aluminum plating thickness of 0.10 μm or more, satisfying the scope of the present invention, and the hydrogen permeation start time is 2.0 times or more of the case without plating, and The hydrogen permeation amount is 0.5 times or less that in the case of no plating, indicating that the hydrogen permeation resistance is excellent.

  Furthermore, when either the iron oxide film or the aluminum oxide film is below the lower limit of 0.1 to 20 nm (sample Nos. AL20 and AL22), the hydrogen permeation start time ratio is slightly small, and the hydrogen permeation suppression ability is slightly Inferior.

  On the other hand, sample No. In AL12, the thickness of the aluminum plating is smaller than the lower limit of the range of the present invention, and the hydrogen permeation start time ratio does not reach the target value. Furthermore, sample no. In AL16, the thickness of the aluminum plating is smaller than the lower limit of the range of the present invention, and neither the hydrogen permeation start time ratio nor the hydrogen permeation amount ratio has reached the target value.

  As is clear from the above results, the example of the present invention satisfies the hydrogen permeation start time ratio of 2.0 or more and the hydrogen permeation amount ratio of 0.5 or less, and is excellent in hydrogen permeation resistance. According to the present invention, it is understood that a hydrogen steel structure such as a hydrogen pressure accumulator or a hydrogen line pipe having excellent hydrogen permeation resistance can be obtained.

  In this example, the influence of plating on fatigue crack growth characteristics in high-pressure hydrogen of a hydrogen line pipe and a hydrogen pressure accumulator was evaluated.

  CT specimens according to ASTM E 647 were taken from the low alloy steel plate shown in Table 1 so that the load direction was parallel to the rolling direction, and the fatigue crack length was measured by the compliance method using a clip gauge. Was measured, and the fatigue crack growth rate in 90 MPa high-pressure hydrogen gas was determined. In addition, the test piece of thickness 10mm was extract | collected from the board thickness 1/2 position as a CT test piece. In addition, when the test piece was mirror-polished on both the front and back sides and plated, both the front and back sides were mirror-polished, and then the entire test piece was plated. The aluminum plating conditions shown in Table 3 were applied by hot dipping using pure aluminum and immersed in molten pure aluminum. By plating in this way, an aluminum plating film as an aluminum film was formed on the surface of the low alloy steel as a test piece. Moreover, the aluminum plating film thickness was changed by adjusting the immersion time in the molten pure aluminum. The thickness of the aluminum plating film (the thickness of the aluminum film) was measured by observing the cross section of the test piece with an SEM.

  For sample Nos. (PAa to PAg, PAi to PAk), the first heat treatment (holding temperature: 300 ° C., holding time: 12 to 24 h) before aluminum plating was performed. About (PAa-PAj), the 2nd heat processing (holding temperature: 300 degreeC, holding time: 12-24h) was given after aluminum plating.

  The thickness of the iron oxide film and the aluminum oxide film was measured by TEM for the cross section of the test piece. The measurement results are shown in Table 3.

As a comparison, a specimen without plating was also subjected to a fatigue crack growth test. At this time, the fatigue crack growth rate (m / cycle) in the stress intensity factor range ΔK = 25 (MPa · m ^1/2 ) was defined as the fatigue crack growth rate as a stable growth region where the Paris law is established. Furthermore, in order to clarify the effect of the presence or absence of a plating film, the ratio of the fatigue crack growth rate with plating to the fatigue crack growth rate without plating, that is, the fatigue crack growth rate with plating / The fatigue crack growth rate without plating was also obtained.

  When either the iron oxide film or the aluminum oxide film is below the lower limit of 0.1 to 20 nm (sample No. PAh, PAk), the fatigue crack growth rate is slightly large, and the acceleration suppression effect of fatigue crack growth Slightly inferior.

Sample No. PAa to PAk all satisfy a fatigue crack growth rate ratio of 0.8 or less. Ferrite area ratio 35% -bainite area ratio 65% (PAb), ferrite area ratio 25% -martensite area ratio 75% (PAj), ferrite area ratio 85% -pearlite area ratio 15% (PAg), bainite area The rate of fatigue crack growth in the stress intensity factor range ΔK = 25 (MPa · m ^1/2 ) is 1.0 × 10 ^{−6 when the} rate is 35% and the martensite area rate is 65% (PAf). It shows very excellent characteristics of less than (m / cycle).

  As is clear from the above results, the examples of the present invention are excellent in hydrogen permeation resistance, so that the penetration of hydrogen into the steel material in high-pressure hydrogen gas is suppressed, and as a result, the hydrogen embrittlement resistance is further improved. It turned out to be a thing.

Claims (3)

Have a thickness 0.10μm or more of the aluminum film on the surface of the steel material in contact with the hydrogen gas,
A steel structure for hydrogen having an iron oxide film of 0.04 to 20 nm between the steel material and the aluminum film, and an aluminum oxide film on the aluminum film .   The steel structure for hydrogen according to claim 1, wherein the aluminum film has a thickness of 50.00 μm or less. The hydrogen steel structure according to claim 1 or 2 , wherein the hydrogen steel structure is a hydrogen pressure accumulator or a hydrogen line pipe.