JP2012107332A - Steel for storing high-pressure hydrogen - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel having high strength of ≥550 MPa tensile strength, which is excellent in toughness and suitable to be used in a high-pressure hydrogen environment, such as a hydrogen storing vessel and an accumulator installed in a hydrogen station or the like.SOLUTION: This steel contains, by mass, 0.05-0.12% C, 0.01-0.50% Si, >0.6 and ≤1.8% Mn, ≤0.02% P, ≤0.003% S, 0.01-0.08% Al, optionally ≤0.3% Cr, optionally ≤0.3% Mo, and optionally one or two or more kinds selected from Cu, Ni, Nb, V, Ti, B and Ca, with the remainder comprising Fe and incidental impurities, wherein a metal structure is a structure mainly composed of bainite whose area fraction is ≥90%, and cementite having an average particle size of ≤50 nm and an average aspect ratio of ≤3 is dispersed and deposited in the bainite.

Description

  The present invention relates to a steel material used in a high-pressure hydrogen environment such as a hydrogen storage container or a pressure accumulator installed in a hydrogen station or the like, and particularly to a steel material having a high tensile strength of 550 MPa or more and excellent toughness.

In order to prevent global warming, technology for utilizing hydrogen energy that does not generate CO ₂ due to combustion has been actively studied in recent years, and development of fuel cells and hydrogen storage, transport, and supply systems has been promoted in national projects. In order to supply a necessary and sufficient amount of hydrogen, a major technical problem is to store and supply hydrogen safely under a high pressure of 35 MPa or more, particularly about 70 MPa or more. It is required to use a material that does not deteriorate in quality due to hydrogen that penetrates into the steel from a high-pressure hydrogen atmosphere, that is, does not easily deteriorate ductility or toughness.

  Conventionally, Cr-Mo steel has been widely used for low-pressure hydrogen storage containers, but many low-alloy steels including Cr-Mo steel are significantly deteriorated by hydrogen, so high-pressure hydrogen of about 35 MPa or more. The material for the storage container is limited to austenitic stainless steel such as SUS316, which has little material deterioration due to hydrogen.

  However, stainless steel such as SUS316 has low strength. For example, when the hydrogen pressure is increased to 70 MPa, the thickness of the storage container becomes extremely thick and the container weight increases. For this reason, there is a problem that not only the size of the storage container is limited and the amount of hydrogen to be stored is limited, but also the material cost becomes too high and the economy is inferior.

  As such a high-pressure hydrogen storage material, many studies have been made to apply a low alloy steel having a lower material cost instead of the conventional austenitic stainless steel. Patent Document 1 proposes a steel for high pressure hydrogen environment that uses MnS, Ca-based inclusions, or VC as non-diffusible hydrogen as a hydrogen trap site in steel and suppresses embrittlement due to diffusible hydrogen. Yes.

  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 V-Mo-based carbides are utilized and the tempering temperature is increased to improve hydrogen embrittlement resistance, and in Patent Document 5, Mo and V are combined. Proposed steel for high-pressure hydrogen gas storage vessel with excellent hydrogen resistance by adding a large amount and applying stress-relief annealing for a long time after the normalizing treatment during steel plate production. Has been.

Japanese Patent Laying-Open No. 2005-2386 JP 2009-46737 A JP 2009-275249 A JP 2009-74122 A JP 2010-37655 A

  However, since the steel material proposed in Patent Document 1 contains a large amount of MnS, Ca-based inclusions, etc., although diffusible hydrogen can be reduced, it may be inferior in the base material toughness required as a basic characteristic of structural steel materials. It is not suitable for parts that are expected easily and require high safety such as high-pressure hydrogen storage containers.

  The low alloy steels proposed in Patent Documents 2 and 3 are lower in cost and higher in strength than conventional SUS316 and the like, but it is necessary to manage the tensile strength of the steel material very strictly. There is a high probability that the strength will be lost due to variations in the heat treatment temperature, and there is a concern that productivity will be inferior due to a decrease in yield.

  The low alloy steel proposed in Patent Document 4 requires the addition of a large amount of Mo and V in order to utilize Mo-V carbide, which not only increases the raw material cost but also increases the coarse Mo- A large amount of V carbide is generated and the toughness of the base material deteriorates. Further, the steel material proposed in Patent Document 5 also uses the same Mo—V carbide as in Patent Document 4, so that not only the base material toughness is inferior, but also long-time stress relief annealing is required and the productivity is inferior.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a low-alloy steel suitable for use in a high-pressure hydrogen storage container of about 70 MPa and having high strength and high toughness and little material deterioration due to hydrogen penetration. To do.

The present inventors have obtained the following knowledge by investigating in detail the relationship between the hydrogen intrusion behavior into steel by high-pressure hydrogen and the relationship between the ductility reduction phenomenon of the steel material and the microscopic microstructure of the steel material.
(1) In a high-pressure hydrogen atmosphere, hydrogen penetrates into the steel and causes embrittlement and ductility reduction of the steel material. Therefore, it is important to suppress the amount of hydrogen entering the steel from the steel material surface.
(2) The amount of hydrogen that penetrates into the steel varies greatly depending on the microscopic structure of the steel material, and Nb, Ti, V, and other carbonitrides often used for precipitation strengthening are NaCl-matched precipitates. Therefore, the hydrogen absorption capacity is high and more hydrogen is absorbed in high-pressure hydrogen. Therefore, in order to suppress embrittlement in high-pressure hydrogen, it is important to suppress the addition of Nb, Ti, V to a certain amount or less.
(3) On the other hand, cementite is an inconsistent precipitate, and its hydrogen absorption capacity is lower than that of NaCl-type carbonitride, but its particle size should be smaller than a certain size and have a lower aspect ratio and a more spherical shape. Thus, it is possible to further suppress the amount of hydrogen intrusion, and to suppress embrittlement and ductility reduction due to hydrogen.
(4) FIG. 1 shows the results of measuring the amount of hydrogen intrusion into steel when exposed to 70 MPa high-pressure hydrogen for 500 hours by thermal desorption analysis. The test steels are two types of steel (a) and steel (b) having the structure shown in FIG. 2, and steel (a) in which fine cementite is dispersed with a small aspect ratio is the amount of hydrogen absorbed by high-pressure hydrogenation. It can be seen that the amount of hydrogen absorption is increased in the steel (b) having a low C, low cementite, and a large aspect ratio.
(5) Since Mo and Cr added as selective elements when improving the strength coarsen cementite, the amount of each additive and the total amount thereof are suppressed to a certain amount or less. By making the average particle size of cementite below a certain size (that is, making the carbide size finer), not only hydrogen intrusion from the high-pressure hydrogen atmosphere is suppressed, but also the toughness of the base material is increased and hydrogen embrittlement is prevented. It is suppressed.

The present invention has been made on the basis of the above findings and further studies, that is,
1. In mass%, C: 0.05 to 0.12%, Si: 0.01 to 0.50%, Mn: more than 0.6 to 1.8%, P: 0.02% or less, S: 0.0. 003% or less, Al: 0.01 to 0.08%, the balance is Fe and inevitable impurities, the metal structure is a bainite main structure with an area fraction of 90% or more, and the average particle size is 50 nm in bainite. Hereinafter, a steel material for high-pressure hydrogen storage, wherein cementite having an average aspect ratio of 3 or less is dispersed and precipitated .
2. Further, by mass%, Cr: 0.3% or less, Mo: 0.3% or less, Cu: 0.5% or less, Ni: 1.0% or less, Nb: 0.005 to 0.05%, V: 0.01-0.10%, Ti: 0.005-0.03%, B: 0.0005-0.002%, Ca: 0.0005-0.005%, one or more 2. The steel material for high-pressure hydrogen storage according to 1, which contains

  According to the present invention, a steel material with high strength and toughness required as a steel material used in a high-pressure hydrogen environment, such as a hydrogen storage container or a pressure accumulator installed in a hydrogen station or the like, and a small material deterioration due to hydrogen can be obtained, It is extremely useful in industry.

The figure which shows the result of having measured the hydrogen penetration | invasion amount into steel when it exposes to 70 Mpa high pressure hydrogen for 500 hours by temperature-programmed desorption analysis. FIG. 1 shows two types of microstructures obtained from the results shown in FIG. 1; a steel (a) in which fine cementite is dispersed with a small aspect ratio and a steel (b) in which cementite is coarsened and the aspect ratio is increased. The figure which shows a microstructure.

In the present invention, the chemical composition and metal structure of steel are defined.

1. In the description of chemical components, all of the component% means mass%.

C: 0.05 to 0.12%
C is an element necessary for increasing the strength of the steel material, and is finely dispersed in the steel as cementite, thereby suppressing embrittlement due to hydrogen intrusion from the high-pressure hydrogen atmosphere and at the same time improving the base material toughness. However, if it is less than 0.05%, sufficient strength cannot be ensured, and if it exceeds 0.12%, the amount of cementite increases, resulting in embrittlement due to hydrogen intrusion, leading to a decrease in base material toughness. 0.12%. In order to obtain more stable performance, the content is preferably 0.05 to 0.10%.

Si: 0.01 to 0.50%
Si is added for deoxidation. This effect is exhibited at 0.01% or more. On the other hand, if it exceeds 0.50%, the toughness deteriorates, so the content is made 0.01 to 0.50%.

Mn: more than 0.6 to 1.8%
Mn is added to improve the strength and toughness of the steel. If it is 0.6% or less, the effect is not sufficient. On the other hand, if it exceeds 1.8%, the toughness deteriorates.

P: 0.02% or less P is an unavoidable impurity element, which does not have a great influence on the strength of the steel material, but is an element that deteriorates toughness, so is 0.02% or less. Preferably, it is 0.015% or less.

S: 0.003% or less S is an unavoidable impurity element, and generally becomes a MnS-based inclusion in steel, leading to deterioration of toughness, particularly a decrease in Charpy absorbed energy. When higher performance is required, it is effective to further reduce the amount of S, preferably 0.002% or less.

Al: 0.01 to 0.08%
Al is added as a deoxidizer. This effect is exhibited at 0.01% or more, but if it exceeds 0.08%, the ductility is deteriorated due to a decrease in cleanliness, so 0.01 to 0.08%.

  The above is the basic component composition of the present invention. In order to further improve desired characteristics, the following elements can be added as selective elements.

Cr: 0.3% or less Cr is an element that is extremely effective in increasing the hardenability of steel and increasing the strength. However, if added over 0.3%, cementite becomes coarse and the amount of hydrogen intrusion in a high-pressure hydrogen atmosphere increases. The increase increases the embrittlement due to hydrogen. Therefore, when adding Cr, it is 0.3% or less. Preferably it is 0.2% or less.

Mo: 0.3% or less Mo, like Cr, is an element that increases the hardenability of steel materials and is extremely effective for increasing the strength. However, if added over 0.3%, cementite becomes coarser in a high-pressure hydrogen atmosphere. Since the amount of hydrogen intrusion increases, embrittlement by hydrogen is promoted. Therefore, when adding Mo, it is 0.3% or less. Preferably it is 0.2% or less.

  When Cr and Mo are added at the same time, if the total amount of these added is large, cementite coarsening is promoted, which may result in toughness deterioration during high-pressure hydrogenation. In order to obtain more stable performance in high-pressure hydrogen, the total amount of Cr and Mo is preferably 0.5% or less.

Cu: 0.5% or less Cu is an element effective for improving toughness and increasing strength, but if added over 0.5%, surface cracks of the slab during casting occur. Therefore, when adding Cu, it is 0.5% or less.

Ni: 1.0% or less Ni is an element effective for improving toughness and increasing strength, but if added over 1.0%, not only the material cost increases but also causes cracking during casting. Become. Therefore, when adding Ni, it is 1.0% or less.

Nb: 0.005 to 0.05%
Nb suppresses grain growth during hot rolling, and improves toughness by refinement. Furthermore, it is a hardenable element and is an extremely effective element for increasing the strength. If it is less than 0.005%, these effects are small. On the other hand, even if added over 0.05%, the increase in strength is saturated. Nb is an element that forms a carbide of NaCl structure in steel, and excessive addition of Nb increases the amount of NaCl-type carbide and increases hydrogen absorption in high-pressure hydrogen, leading to hydrogen embrittlement. . Therefore, when Nb is added, the content is made 0.005 to 0.05%. Preferably, the content is 0.005 to 0.035%.

V: 0.01-0.10%
V is an element effective for improving strength and toughness in the same manner as Nb. To obtain these effects, addition of 0.01% or more is necessary. On the other hand, addition exceeding 0.10% saturates the effect of improving the strength and causes deterioration of toughness. Similarly to Nb, V is an element that forms a NaCl-structure carbide in steel, and excessive addition of V causes embrittlement in high-pressure hydrogen. Therefore, when adding V, it is 0.01 to 0.10%. Preferably, it is 0.01 to 0.05%.

Ti: 0.005 to 0.03%
Ti forms a composite carbonitride to prevent coarsening of crystal grains during slab heating, and contributes to the improvement of toughness by refining the structure. However, if it is less than 0.005%, the effect is not sufficient. On the other hand, if it exceeds 0.03%, coarse TiN increases and toughness deteriorates. Ti, like Nb and V, is an element that forms a carbide having a NaCl structure in steel, and excessive addition of Ti causes embrittlement in high-pressure hydrogen. Therefore, when Ti is added, the content is made 0.005 to 0.03%. Preferably, the content is 0.005 to 0.02%.

B: 0.0005 to 0.002%
B is an element that enhances hardenability and is extremely effective in increasing strength. In order to obtain the effect, addition of 0.0005% or more is necessary, but even if added over 0.002%, the effect is saturated. Therefore, when adding B, it is made into 0.0005 to 0.002%.

Ca: 0.0005 to 0.005%
Ca is an effective element for controlling the form of sulfide inclusions and improving ductility. In order to obtain the effect, addition of 0.0005% or more is necessary, but even if added over 0.005%, the effect is saturated, but rather the toughness is deteriorated due to a decrease in cleanliness. Therefore, when adding Ca, it is set as 0.0005 to 0.005% of range.

  The balance of the steel of the present invention is Fe and inevitable impurities.

2. Metal structure Area fraction of bainite: 90% or more Bainite is a metal structure necessary for obtaining the strength of a steel material, and in order to obtain a tensile strength of 550 MPa or more, the steel of the present invention has an area fraction of 90% or more. A bainite-based organization is adopted. As the remaining metal structure other than bainite, ferrite, pearlite, martensite and the like may be contained.

Cementite average particle size: 50 nm or less, and average aspect ratio: 3 or less Cementite that precipitates and forms in the bainite structure is an extremely important structure factor in the present invention. By setting the average particle size to 50 nm or less, high pressure hydrogen It suppresses the amount of hydrogen entering from the atmosphere and contributes to the improvement of strength and toughness. However, when the average particle size of cementite exceeds 50 nm, the amount of hydrogen penetration increases and the toughness due to hydrogen decreases. Therefore, the average particle diameter of cementite is specified to be 50 nm or less. In order to obtain more stable performance, the thickness is preferably 30 nm or less.

  Cementite is usually an inconsistent precipitate and has a smaller hydrogen absorption capacity than Ti and Nb carbonitride matched precipitates, but the interface area increases when the form is elongated (the aspect ratio is large). However, the amount of hydrogen absorption increases and the toughness decreases.

  Therefore, in the present invention, the average aspect ratio of cementite is specified to be 3 or less so as to reduce the hydrogen absorption amount and suppress the decrease in toughness during high-pressure hydrogenation. In order to obtain more stable performance, it is preferably 2 or less. The method for obtaining the average particle size and the average aspect ratio will be described later.

  By having the above-described chemical components and metal structure, the present invention suppresses toughness reduction under a high-pressure hydrogen atmosphere, and a tensile strength of 550 MPa or more is obtained, which can be applied to a high-pressure hydrogen storage container or the like. .

  The steel material for high-pressure hydrogen storage according to the present invention can be applied to any production method without restricting the production conditions, but in the case of steel pipe production in a seamless process or steel plate production in a hot rolling process, the following conditions are used. It is desirable to manufacture with.

Heating temperature of slab: 1000-1250 ° C
If the slab heating temperature of billets, slabs, etc. is less than 1000 ° C., diffusion of impurity elements such as C, P, S, etc., which are microsegregated is insufficient, and a homogeneous material cannot be obtained. The grains become too coarse and the toughness deteriorates. Therefore, the slab heating temperature is preferably 1000 to 1250 ° C.

Hot rolling end temperature: Ar ₃ temperature or higher After the slab is reheated, hot rolling is performed to a desired tube thickness or plate thickness. The hot rolling end temperature is the Ar ₃ temperature, which is the ferrite formation temperature. The above is preferable. This is because in the case of a process (DQ process) in which direct quenching is performed after hot rolling, strength is reduced due to the formation of a soft ferrite phase.

Since the Ar ₃ temperature varies depending on the alloy component of the steel, the transformation temperature may be measured by experiment for each steel, but it can also be obtained from the component by the following formula (1).
Ar ₃ (° C.) = 910-310C (%)-80Mn (%)-20Cu (%)-15Cr (%)-55Ni (%)-80Mo (%) (1)
Each alloy element has a content (mass%).

Quenching temperature: 900 ° C. or higher When performing the quenching process (RhQ process) by reheating after hot rolling, it is preferable to heat to 900 ° C. or higher. In the process of directly quenching after hot rolling (DQ process), the rolling is finished at the Ar ₃ temperature or higher, and a quenching process is immediately performed.

Tempering temperature: 500-720 ° C
In order to recover the lattice strain (dislocation) introduced by the martensitic transformation or bainite transformation by the above quenching treatment and to disperse and precipitate C dissolved in supersaturation as cementite to obtain the necessary base material toughness, Tempering is essential. If the tempering temperature is less than 500 ° C, a sufficient toughness recovery effect cannot be obtained. If the tempering temperature exceeds 720 ° C, ferrite is generated and the strength is reduced, so the tempering temperature is preferably in the range of 500 to 720 ° C.

Heating rate during tempering: 1 ° C./sec or higher The heating method in the tempering treatment is not particularly limited, but heating is performed at a temperature rising rate of 1 ° C./sec or more in order to obtain fine cementite having an average particle size of 50 nm or less. It is preferable to do. In order to make cementite fine, it is preferable to set the temperature rising rate during tempering to 3 ° C./sec or more, and it is suitable to use an induction heating apparatus as a heating method.

  Since the induction heating device can set the shape of the induction heating coil in accordance with the shape of the material to be heated, it is possible to perform a rapid heat treatment without reducing productivity even with a steel pipe or a steel plate.

  Steel having the chemical composition shown in Table 1 (steel types A to J) was made into a slab by continuous casting or vacuum melting, and then a steel plate having a thickness of 16 to 25 mm was manufactured by hot rolling, and the metal structure and machine of the steel plate Physical properties (strength) were investigated. All the steel types A to F were directly quenched after hot rolling and then tempered at 600 to 680 ° C. by induction heating. Further, the steel sheet of steel type G was subjected to accelerated cooling to 630 ° C. after hot rolling, and then subjected to isothermal heating treatment at 630 ° C. for 2 hours. Steel type H and steel type I are steel materials corresponding to JIS / SCM435 and JIS / SUS316L. Steel type J finished hot rolling at 700 ° C., then was subjected to isothermal heat treatment at 620 ° C. for 2 hours, and then air-cooled.

The embrittlement characteristics in high-pressure hydrogen were obtained by collecting round bar tensile test specimens from these steel sheets, ^10-6 low strain rate tensile test in high-pressure hydrogen with a hydrogen pressure of 70 MPa, and low strain under the same conditions in the atmosphere. A speed tensile test was carried out and evaluated by the ratio of the drawing value in high-pressure hydrogen to the drawing value in air. These tests were performed at room temperature. In the tensile test in high-pressure hydrogen, hydrogen gas is introduced after setting the test piece in the high-pressure chamber, and the tensile test is started immediately after reaching 70 MPa, and the test is performed so that the hydrogen pressure is constant during the test. Carried out. In the above tests, the range of the present invention was set to a ratio of the drawing value in high-pressure hydrogen to the drawing value in the atmosphere: 90% or more, the yield strength in the atmosphere ≧ 480 MPa, and the tensile strength ≧ 550 MPa.

  As the metal structure, the main structure was identified by an optical microscope, and the average particle size and the average aspect ratio as the size and shape of cementite were obtained from images of multiple fields of view by transmission electron microscope observation and analyzed by image analysis. The cementite particle size was determined as the equivalent circle diameter.

  Table 2 shows the observation results of the metal structure, the tensile properties, and the embrittlement properties in high-pressure hydrogen. All of the steel types A to E, which are examples of the present invention, show almost no decrease in the drawing value in high-pressure hydrogen, are less brittle due to hydrogen, and are excellent in toughness.

  On the other hand, in the steel type F, the cementite was coarsened, and the average particle size of the cementite exceeded the range of the present invention, so that the drawing value in high-pressure hydrogen decreased. In steel types G and J, the metal structure is mainly composed of ferrite, and high strength is obtained because carbides such as Nb, V, and Ti are precipitated, but a reduction in drawing value was observed in high-pressure hydrogen.

  The steel type H is SCM435, the chemical composition and the metal structure are outside the scope of the present invention, and the reduction of the drawing value is observed in high-pressure hydrogen. Steel type I is SUS316L, and the reduction of the drawing value in high-pressure hydrogen is not observed, but the tensile strength is below the target 550 MPa of the present invention.

Claims (2)

  In mass%, C: 0.05 to 0.12%, Si: 0.01 to 0.50%, Mn: more than 0.6 to 1.8%, P: 0.02% or less, S: 0.0. 003% or less, Al: 0.01 to 0.08%, the balance is Fe and inevitable impurities, the metal structure is a bainite main structure with an area fraction of 90% or more, and the average particle size is 50 nm in bainite. In the following, a steel material for high-pressure hydrogen storage, wherein cementite having an average aspect ratio of 3 or less is dispersed and precipitated.   Furthermore, in mass%, Cr: 0.3% or less, Mo: 0.3% or less, Cu: 0.5% or less, Ni: 1.0% or less, Nb: 0.005 to 0.05%, V: Contains one or more of 0.01 to 0.10%, Ti: 0.005 to 0.03%, B: 0.0005 to 0.002%, Ca: 0.0005 to 0.005% The steel material for high-pressure hydrogen storage according to claim 1, wherein: