WO2020130060A1

WO2020130060A1 - Cr-based stainless steel having excellent hydrogen embrittlement resistance

Info

Publication number: WO2020130060A1
Application number: PCT/JP2019/049717
Authority: WO
Inventors: 秦野　正治; 佑一田村
Original assignee: 日鉄ステンレス株式会社
Priority date: 2018-12-21
Filing date: 2019-12-18
Publication date: 2020-06-25
Also published as: JP7121142B2; KR20210092292A; EP3901292A1; JPWO2020130060A1; CN113227414B; US20220033944A1; EP3901292A4; CN113227414A; KR102539588B1

Abstract

Provides is a Cr-based stainless steel having excellent hydrogen embrittlement resistance, the Cr-based stainless steel being characterized by comprising one or two kinds of stainless steel having, by mass%: 0.020% or less of C; 1.00% or less of Si; 1.00% or less of Mn; 0.04% or less of P; 0.0030% or less of S; 10.0-18.0% of Cr; 0.0020% or less of N; 0.10% or less of Al; 0.5% or less of Nb; and 0.5% or less of Ti, wherein an aggregate structure on a sheet surface satisfies the following (i) and (ii): (i) on the sheet surface, the area proportion of crystal grains ({211}±10° oriented grains), in which an angle difference between the normal direction of a steel sheet surface and the {211} face orientation is less than 10°, is less than 30%; and (ii) in the {211}±10° orientated grains, both the length in the rolling direction and the length in the sheet width direction are less than 0.15 mm in average.

Description

Cr-based stainless steel sheet with excellent hydrogen embrittlement resistance

The present invention relates to a Cr-based stainless steel sheet having excellent hydrogen embrittlement resistance, and particularly to a Cr-based stainless steel sheet suitable as a metal material for high-pressure hydrogen gas equipment.

In recent years, it has been strongly demanded to suppress the generation of greenhouse gases, mainly carbon dioxide, due to the abnormal weather that contributes to global warming. As part of this, the development of automobiles and transportation equipment that use fuel cells as a power source is under way. Since the fuel cell uses hydrogen as a fuel to generate electric power, it does not generate carbon dioxide and has a high energy conversion efficiency. Therefore, it is positioned as an effective power source.

　In a fuel cell that uses hydrogen as fuel, and in equipment that includes a hydrogen station for supplying hydrogen to it, the components are exposed to a hydrogen gas environment. It is known that, in a metal material exposed to a hydrogen gas environment, mechanical properties such as tensile strength, elongation and drawing are deteriorated by hydrogen that has penetrated into the material. These phenomena are called hydrogen embrittlement. Due to such a problem of hydrogen embrittlement, both austenitic stainless steel is used for the high-pressure hydrogen container for automobiles having a pressure of 35 MPa in the Japan Automotive Research Institute Technical Standard JARIS001 and for a high-pressure hydrogen container for automobiles having a pressure of 70 MPa in KHKS0128. It specifies the use of steel SUS316L and aluminum alloy 6061-T6.

According to the example standard of the general high pressure gas safety regulation, the Ni equivalent (Ni+0. It specifies the use of a material (for example, Ni equivalent ≧28.5) in which 65Cr+0.98Mo+1.05Mn+0.35Si+12.6C) is increased. The operating temperature is -45°C or higher and 250°C or lower. Among these austenitic stainless steels, for example, Patent Documents 1 and 2 also disclose stainless steels which are intended to improve the economical efficiency by increasing the strength of SUS316L and lowering expensive Mo.

In the above-mentioned general high pressure gas safety regulation, the regulation of materials for hydrogen equipment with a pressure of 20 MPa or less was abolished by revision in 2016. With the deregulation, there is an increasing demand for the use of highly economical stainless steel sheets even in high-pressure hydrogen gas, and it is desired to evaluate the hydrogen embrittlement resistance of various steel materials in high-pressure hydrogen gas. Ferrite-based and martensitic stainless steel sheets (hereinafter collectively referred to as "Cr-based stainless steel sheets") contain almost no Ni, which is a rare metal, and are therefore more economical than austenitic stainless steel sheets. Conventionally, for example, Non-Patent Document 1 discloses hydrogen embrittlement characteristics evaluated in a high-pressure hydrogen gas at room temperature for all steel materials including stainless steel. It has been reported that SUS304, which is a typical austenitic stainless steel, and Cr-based stainless steel are easily hydrogen embrittled. Therefore, generally, it is recommended to use SUS316L or SUS316 even in a high-pressure hydrogen gas having a pressure of about 20 MPa. Further, the Cr-based stainless steel having a body-centered cubic structure also has a problem (low-temperature brittleness) that the toughness is lowered at a low temperature of room temperature or lower as compared with the austenitic stainless steel having a face-centered cubic structure.

For the purpose of expanding the materials that can be used in a high-pressure hydrogen gas environment, materials coated with Al or Al alloy that have excellent hydrogen embrittlement resistance have been devised. Patent Document 3 discloses a high-pressure hydrogen gas pressure vessel and a high-pressure hydrogen gas pipe coated with Al or an Al alloy. In the examples, the coating of austenitic stainless steels and duplex stainless steels containing an austenitic phase is targeted, and the film formation and hydrogen penetration characteristics in steel materials that are susceptible to hydrogen embrittlement, such as Cr-based stainless steels, are not shown.

Further, in Patent Document 4, a steel material which is apt to be hydrogen embrittled by itself is subjected to hot dip plating using an Al-Si alloy with an added amount of Si of 1 to 5%, thereby forming a hydrogen permeation resistant film. A formed substrate for a hydrogen appliance is disclosed. The base material is carbon steel, low alloy steel, or Cr-based stainless steel to prevent hydrogen embrittlement and also keep manufacturing costs low. However, the examples are limited to SUS304, SUS630 (15Cr-4Ni-3Cu) and SCM435 (low alloy steel). Regarding highly economical Cr-based stainless steel sheets, there is no mention of their hydrogen embrittlement characteristics or their use.

JP, 2014-114471, A Japanese Patent Laid-Open No. 2016-183412 JP-A-2004-324800 International publication WO2015-098981

The stainless steels described in Patent Documents 1 to 4 described above remain only in the austenite-type and two-phase and SUS630 (precipitation hardening type), and the Cr-type stainless steel disclosed in Non-Patent Document 1 is easily hydrogen embrittlement and is high-pressure hydrogen gas. It does not have hydrogen embrittlement resistance for use in applications. Cr-based stainless steel also has a problem of low temperature brittleness.

The present invention has been made in view of the above circumstances, has a hydrogen embrittlement resistance for use in high-pressure hydrogen gas, and is suitable as a metal material for high-pressure hydrogen gas equipment, and is excellent in hydrogen embrittlement resistance Cr-based stainless steel. An object is to provide a steel plate. At the same time, it is an object to realize compatibility with low temperature brittleness.

In order to solve the above problems, the present invention adopts the following configurations.
[1]% by mass, C: 0.020% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.040% or less, S: 0.0030% or less, Cr: 10 0.0 to 18.0%, N: 0.020% or less, Al: 0.10% or less, Nb: 0.5% or less, Ti: 0.5% or less : 0 to 0.3%, B: 0 to 0.005%, Ni: 0 to 1%, Cu: 0 to 1%, Mo: 0 to 1%, Sb: 0.2% or less, V: 0 to 0.5%, W:0 to 0.5%, Zr:0 to 0.5%, Co:0 to 0.5%, Mg:0 to 0.005%, Ca:0 to 0.005%, Ga: 0 to 0.020%, La: 0 to 0.1%, Y: 0 to 0.1%, Hf: 0 to 0.1%, REM: 0 to 0.1%, the balance being Fe and impurities And a texture on the plate surface satisfying the following (i) and (ii).
(I) The area ratio of crystal grains (hereinafter referred to as “{211}±10° oriented grains”) having an angle difference of 10° or less between the normal direction of the steel plate surface and the {211} plane orientation on the plate surface is 30. % (Ii) In the {211}±10° oriented grains defined by (i), the length in the rolling direction and the length in the strip width direction are both less than 0.15 mm on average [2] and further by mass%. Sn: 0.001 to 0.3%, B: 0.005% or less,
The Cr-based stainless steel sheet according to the present invention, which satisfies the following formula (1).
Si+0.5Mn+10P+5Nb+2Ti<2.00 (1) Formula In the above formula, the element symbol means the content (mass %) of the element.
[3] Further, in mass%, Ni: 1% or less, Cu: 1% or less, Mo: 1% or less, Sb: 0.2% or less, V: 0.5% or less, W: 0.5% or less, Zr: 0.5% or less, Co: 0.5% or less, Mg: 0.005% or less, Ca: 0.005% or less, Ga: 0.020% or less, La: 0.1% or less, Y: Cr type|system|group stainless steel plate of this invention characterized by containing 1 type or 2 types or more of 0.1% or less, Hf:0.1% or less, and REM:0.1% or less.
[4] The Cr-based stainless steel sheet according to the present invention, which is used as a metal material for a high-pressure hydrogen gas device.

According to the present invention, a Cr-based stainless steel sheet having excellent hydrogen embrittlement resistance and low temperature toughness can be provided. In addition, the Cr-based stainless steel sheet of the present invention can be suitably used as a metal material for high-pressure hydrogen gas equipment.

In order to solve the above-mentioned problems, the present inventors have earnestly studied the effects of alloying elements and texture on hydrogen embrittlement resistance and low temperature embrittlement resistance in Cr-based stainless steel sheets, and have obtained the following new findings. The present invention has been completed.

(A) As described above, the properties required for the metal material of the high-pressure hydrogen gas device include hydrogen embrittlement resistance and low temperature embrittlement resistance. Cr-based stainless steel sheets reduce the amount of hydrogen penetrating into steel materials from high-pressure hydrogen gas due to the crystal structure, as compared with austenitic stainless steel sheets, but those having hydrogen embrittlement resistance suitable for high-pressure hydrogen gas applications can be obtained. Has not been done. According to Non-Patent Document 2, hydrogen embrittlement is characterized as a decrease in mechanical properties (strength, elongation, drawing) involved in plastic deformation. Therefore, hydrogen embrittlement is an event in which the destruction of a material progresses due to the interaction between hydrogen that has penetrated into the steel material from high-pressure hydrogen gas and plastic deformation. From the recent research results, the mechanism of hydrogen embrittlement is considered to be due to the hydrogen-assisted strain-induced vacancy theory, which promotes the formation of vacancy lattice defects in steel by the interaction between hydrogen and plastic deformation and promotes fracture. [Non-Patent Document 2]. Therefore, in order to realize a Cr-based stainless steel sheet suitable for high-pressure hydrogen gas, it is necessary to reduce the interaction between hydrogen and plastic deformation as much as possible. In particular, since Cr has a large hydrogen trapping ability, the Cr content is suppressed to 18% or less in the present invention. Furthermore, the present inventors have found that it is preferable to control the added amounts of Si, Mn, P, Ti, and Nb within a predetermined range.
(B) Furthermore, the present inventors have found that when a low strain rate tensile test is performed in high-pressure hydrogen gas, the crystal orientation has an influence on the occurrence of cracks due to the interaction between hydrogen and plastic deformation. .. When hydrogen embrittlement becomes apparent, cracks frequently occur and propagate from within crystal grains. The cracks in the crystal grains are not the recrystallized texture {111} oriented grains (the crystal grains whose {111} plane orientation is in the normal direction of the steel plate surface), but the rolling texture {211} oriented grains. It has been found that it often occurs in (crystal grains whose {211} plane orientation is in the direction normal to the surface of the steel sheet). From these facts, it is presumed that the {211} oriented grains are likely to introduce and accumulate strain due to the interaction between hydrogen and plastic deformation. It is speculated that, in the {211}-oriented grains, the generation of the above-mentioned vacancy lattice defects was activated, and thus it acted as a crack generation site. In order to suppress hydrogen embrittlement that proceeds by such a mechanism, it is effective to reduce the area ratio and size of {211} oriented grains in addition to adjusting the range of the alloying element described above. , Came to find that threshold.
(C) Further, the hydrogen that has entered the steel material from the high-pressure hydrogen gas moves through the crystal grain boundaries as the main diffusion path. The addition of a small amount of grain boundary segregation elements, Sn and B, serves as a diffusion barrier at the crystal grain boundaries of hydrogen and reduces the interaction between hydrogen and plastic deformation. In the conventional Cr-based stainless steel, impurity elements such as P and S are segregated at the grain boundaries, which facilitates low temperature brittleness. Therefore, the inventors of the present invention pay attention to a small amount of addition of Sn and B, and by containing these elements in a predetermined range, it is expected that the adverse effects of P, S, etc. are suppressed and hydrogen embrittlement resistance and low temperature embrittlement resistance are compatible. I found that.

The gist of the present invention made based on the above findings (a) to (c) is as follows.
The Cr-based stainless steel sheet of the present embodiment is, in mass %, C: 0.020% or less, Si: 1.00% or less, Mn: 1.00% or less, P: 0.040% or less, S: 0.0. 0030% or less, Cr: 10.0 to 18.0%, N: 0.020% or less, Al: 0.10% or less, Nb: 0.5% or less, Ti: 0.5% or less Alternatively, a Cr-based stainless steel sheet excellent in hydrogen embrittlement resistance and low temperature embrittlement resistance, characterized in that it contains two kinds, the balance consisting of Fe and impurities, and the texture on the plate surface satisfies the following (i) and (ii): is there.
(I) The area ratio of crystal grains ({211}±10° oriented grains) in which the angle difference between the normal direction of the steel sheet surface and the {211} plane orientation on the plate surface is within 10° is less than 30% (ii) In the {211}±10° oriented grains defined in (i), the length in the rolling direction and the length in the strip width direction are both less than 0.15 mm on average. It is preferable that the content of Sn is 0.001 to 0.3% and the content of B is 0.005% or less, and the following formula (1) is satisfied.
Si+0.5Mn+10P+5Nb+2Ti<2.00 (1) Formula In the above formula, the element symbol means the content (mass %) of the element.
Further, the Cr-based stainless steel sheet of the present embodiment is further mass %, Ni: 1% or less, Cu: 1% or less, Mo: 1% or less, Sb: 0.2% or less, V: 0.5% or less. , W: 0.5% or less, Zr: 0.5% or less, Co: 0.5% or less, Mg: 0.005% or less, Ca: 0.005% or less, Ga: 0.020% or less, La : 0.1% or less, Y: 0.1% or less, Hf: 0.1% or less, REM: 0.1% or less, or two or more thereof may be contained.
In addition, the Cr-based stainless steel sheet of the present embodiment is preferably used as a metal material for high-pressure hydrogen gas equipment.

Below, each requirement of the present invention will be explained in detail. The “%” display of the content of each element means “mass %”.

C: 0.020% or less C increases the work hardening of steel due to solid solution and precipitation of carbides to deteriorate hydrogen embrittlement resistance, and further lowers toughness to deteriorate low temperature embrittlement resistance. Is as small as possible, and the upper limit is made 0.020% or less. However, in order to reduce the amount of C, the refining process becomes complicated and the cost increases. Therefore, the C content is preferably 0.001% or more. Considering the refining cost, the preferable range is 0.003 to 0.015%, and the more preferable range is 0.003 to 0.010%.

Si: 1.00% or less Si is effective as a deoxidizing element, but when contained in excess, it causes solid solution strengthening and work hardening to increase, leading to a reduction in hydrogen embrittlement resistance and low temperature embrittlement resistance. It should be 0.000% or less. The lower limit is preferably 0.01% or more in order to secure deoxidizing ability. The preferable range is 0.05 to 0.50% in view of manufacturability and characteristics, and may be 0.05 to 0.30%.

Mn: 1.00% or less Mn is an element effective as a deoxidizing element, and is also an element effective for fixing S to improve toughness and to obtain low temperature brittleness resistance. On the other hand, if Mn is excessively contained, work hardening is increased and hydrogen embrittlement resistance and low temperature toughness are lowered, so the upper limit is made 1.00% or less. In order to secure the effects of deoxidation and S fixing, the lower limit is preferably 0.01% or more. Considering the effect and manufacturability, the preferable range is 0.05 to 0.50%, and may be 0.05 to 0.30%.

P: 0.040% or less P is an element that segregates at grain boundaries to reduce low temperature embrittlement resistance, and the lower the content, the better. Therefore, the upper limit is 0.040%. However, excessive reduction leads to an increase in refining cost, so the lower limit is preferably made 0.005% or more. A more preferable range is 0.010 to 0.030%, and may be 0.010 to 0.020% in consideration of manufacturing cost and characteristics.

S: 0.0030% or less Since S forms grain boundary segregation and sulfides in steel to deteriorate low temperature embrittlement resistance, the smaller the content, the better. The upper limit is 0.0030%. However, excessive reduction leads to an increase in raw materials and refining costs, so the lower limit is preferably made 0.0001% or more. A more preferable range is 0.0002 to 0.0015%, and may be 0.0002 to 0.0008% in consideration of manufacturing cost and characteristics.

Cr: 10.0-18.0%
Cr is a basic element of the Cr-based stainless steel of the present embodiment, and is an essential element for maintaining hydrogen embrittlement resistance and low temperature embrittlement resistance in addition to the corrosion resistance of the steel. The lower limit is made 10.0% or more in order to obtain the above-mentioned characteristics for use in high-pressure hydrogen gas of this embodiment. The upper limit is 18.0% or less from the viewpoint of achieving both hydrogen embrittlement resistance and low temperature embrittlement resistance. When Cr, which has a high hydrogen trapping capacity, exceeds 18.0%, the amount of hydrogen penetrating into the steel from the high-pressure hydrogen gas environment increases, the hydrogen embrittlement resistance deteriorates, and the texture deviates from the preferred range of the present invention. is there. A more preferable range of Cr may be 11.0 to less than 17.0%, or 12.0 to 15.0%.

N: 0.020% or less N, like C, increases work hardening of steel due to solid solution and precipitation of carbides to deteriorate hydrogen embrittlement resistance, and further reduces toughness to deteriorate low temperature embrittlement resistance. Therefore, the smaller the content, the better, and the upper limit is made 0.020% or less. However, in order to reduce the amount of N, the refining process becomes complicated and the cost increases. Therefore, the N content is preferably 0.001% or more. A preferable range is 0.005 to 0.015% in consideration of characteristics and manufacturing cost.

Al: 0.10% or less Al is an extremely effective element as a deoxidizing element. On the other hand, since the toughness of steel is deteriorated to deteriorate the low temperature brittleness resistance and the texture may deviate from the preferred range of the present invention, the upper limit is set to 0.10% or less. The lower limit is preferably 0.005% or more in consideration of the deoxidizing effect. Considering the characteristics and manufacturability, the preferable range is 0.01 to 0.07%, and may be 0.01 to 0.05%.

Nb: 0.5% or less, Ti: 0.5% or less 1 type or 2 types Nb and Ti segregate at grain boundaries to suppress grain boundary segregation of P and S and improve low temperature brittleness resistance. There is an action to try. In addition, Nb and Ti can be expected to improve hydrogen embrittlement resistance by suppressing work hardening of steel due to the action as a stabilizing element that fixes C, N, P, and S. Both Nb and Ti exhibit these two effects, and are effective elements for improving the hydrogen embrittlement resistance and the low temperature embrittlement resistance, which are the goals of the present invention. When it is contained, the content is preferably 0.01% or more so that the respective effects are exhibited. However, excessive content increases work hardening and leads to a decrease in hydrogen embrittlement resistance and an increase in alloy cost, and further, toughness decreases and low temperature embrittlement resistance deteriorates, and the texture may deviate from the preferred range of the present invention. Therefore, the upper limits are set to 0.5% or less, respectively. The preferable range is 0.05 to 0.5% with respect to the total of one or two of Nb and Ti in consideration of the effect of improving the characteristics and alloy cost. A more preferable range is 0.08 to 0.4% for one kind or the sum of two kinds, and it may be 0.1 to 0.3%.

More preferably, Sn and B are contained in the following content ranges.
Sn: 0.001-0.3%
Sn is an element effective for improving the hydrogen embrittlement resistance and the low temperature embrittlement resistance which are the targets of the present invention. Sn, which is a grain boundary segregation element, serves as a diffusion barrier at hydrogen crystal grain boundaries and reduces the interaction between hydrogen and plastic deformation. It also suppresses the segregation of P and S at the crystal grain boundaries and alleviates the adverse effects of low temperature embrittlement resistance. By incorporating Sn in a predetermined range, both hydrogen embrittlement resistance and low temperature embrittlement resistance are expected to be compatible. Therefore, in the present invention, it is preferable to contain Sn in the range of 0.001 to 0.5%. When Sn is contained in an amount of 0.001% or more, the above effect is exhibited and hydrogen embrittlement resistance is improved. However, an excessive content increases the Sn concentration at the crystal grain boundary, and causes low-temperature brittleness and manufacturability to decrease, so the upper limit is made 0.5% or less. It is preferably 0.005 to 0.3%, and may be 0.010 to 0.2%.

B: 0.005% or less B is a grain boundary segregation element and is an element that improves hydrogen embrittlement resistance and low temperature embrittlement resistance like Sn, and it is effective to contain Cr in the Cr-based stainless steel of the present embodiment. is there. In the present invention, in order to improve the hydrogen embrittlement resistance, the content is preferably 0.0003% or more. However, excessive B content causes elongation and a decrease in manufacturability, so the upper limit is made 0.005% or less. It is preferably 0.0005 to 0.002%, and may be 0.001 to 0.002%.

Si, Mn, P, Nb, and Ti each satisfy the following formula (1) in order to improve the hydrogen embrittlement resistance and the low temperature embrittlement resistance, which are the targets of the present invention, in addition to the content ranges described above. It is preferable.
Si+0.5Mn+10P+5Nb+2Ti<2.00... Formula (1)
In the above formula, the element symbol means the content (mass %) of the element.
In order to improve the above-mentioned properties which are the targets of the present invention, it is preferable that the formula (1) is less than 2.00, and the lower limit is 0.05 from the viewpoint of properties and manufacturability. A preferred range is 0.35 to 1.80, and a more preferred range is 0.50 to 1.50.

Other than the above elements, it consists of Fe and impurities. However, the elements described below other than the above can be selectively contained within a range that does not impair the effects exhibited by the technical features of the present invention. The reasons for limitation are described below. The lower limit of these elements is 0%.

Ni: 1% or less Cu: 1% or less Mo: 1% or less Ni, Cu and Mo are effective elements for improving corrosion resistance and Ni and Cu for improving low temperature toughness. In order to exert this effect, each of Ni, Cu, and Mo may be contained in the range of 0.05% or more. Excessive content increases the solid solution strengthening and work hardening of stainless steel, leading to a decrease in hydrogen embrittlement resistance, so the upper limit is made 1% or less. A more preferable range is 0.1% or more and 0.8% or less, and a still more preferable range is 0.2% or more and 0.5% or less.

Sb: 0.2% or less V: 0.5% or less W: 0.5% or less Zr: 0.5% or less Co: 0.5% or less Sb, V, W, Zr, and Co improve corrosion resistance. It is an element effective in suppressing the grain boundary segregation of P and S and improving the low temperature embrittlement resistance, and is contained if necessary. In particular, Sb is a strong grain boundary segregation element and, like Sn and B, has a function of eliminating grain boundary segregation of impurity elements such as P and S. When these elements are contained, the content is preferably 0.01% or more so that the respective effects are exhibited. Excessive content leads to a reduction in manufacturability and low temperature brittleness, so Sb is made 0.2% or less, and V, W, Zr, and Co are made 0.5% or less, respectively. The more preferable range of Sb is 0.02 to 0.15%, and further preferably 0.02 to 0.1% or less. A more preferable range of V, W, Zr, and Co is 0.02 to 0.3%, and a further preferable range is 0.02 to 0.2%.

Mg: 0.005% or less Mg forms Mg oxide together with Al in molten steel and acts as a deoxidizing agent, and also acts as a crystallization nucleus of TiN. TiN serves as a solidification nucleus of the ferrite phase in the solidification process, and promotes crystallization of TiN, so that the ferrite phase can be finely generated during solidification. By refining the solidified structure, the low temperature brittleness resistance can be improved. When it is contained, the content is preferably 0.0001% or more for exhibiting these effects. However, if Mg exceeds 0.005%, manufacturability and corrosion resistance deteriorate, so the upper limit is made 0.005% or less. It is preferably 0.0003 to 0.002%, and more preferably 0.0003 to 0.001%.

Ca: 0.005% or less Ga: 0.020% or less Ca and Ga are elements that improve the cleanliness of steel, and are contained as necessary to suppress an increase in work hardening and increase hydrogen embrittlement resistance. .. When they are contained, the content is preferably 0.0003% or more in order to exhibit these effects. However, since excessive content leads to deterioration in manufacturability and corrosion resistance, the upper limits are set to 0.005% or less for Ca and 0.020% or less for Ga. Preferably, Ca is 0.0003 to 0.0030% and Ga is 0.0030 to 0.015%.

La: 0.1% or less Y: 0.1% or less Hf: 0.1% or less REM: 0.1% or less La, Y, Hf, and REM improve the cleanliness of steel similarly to Ca and Ga. It is an element and may be contained if necessary in order to suppress an increase in work hardening and enhance hydrogen embrittlement resistance. When they are contained, they are preferably contained in an amount of 0.001% or more in order to exhibit the effect. However, excessive contents lead to an increase in alloy cost and deterioration in manufacturability, so the upper limits are made 0.1% or less. It is preferably 0.001 to 0.05%, and more preferably 0.001 to 0.03%.

REM (rare earth element) is a general term for two elements, scandium (Sc) and yttrium (Y), and 14 elements (lanthanoids) from cerium (Ce) to lutetium (Lu) in the periodic table. These elements may be contained alone or in a mixture.

The impurities contained in the balance are those that are mixed in from the ore as a raw material, scrap, or the manufacturing environment when steel is industrially manufactured, and are allowed within the limit of solving the problem of the present invention. Means what is done. If necessary, Ta: 0.1% or less, Bi: 0.01% or less, Zn: 0.05%, H: 0.0005% or less may be contained. The Cr-based stainless steel of the present embodiment contains ferrite crystal grains, and may contain martensite crystal grains.

Next, the texture of the Cr-based stainless steel sheet of this embodiment will be described. In the Cr-based stainless steel sheet of this embodiment, the texture on the plate surface satisfies the following (i) and (ii).
(I) The area ratio of crystal grains ({211}±10° oriented grains) in which the angle difference between the normal direction of the steel sheet surface and the {211} plane orientation on the plate surface is within 10° is less than 30% (ii) In the {211}±10° oriented grains defined in (i), both the length in the rolling direction and the length in the strip width direction are less than 0.15 mm on average, where {211} plane orientation is {211}. It means the direction normal to the surface.

The {211} orientation is called α-fiber and is a rolling texture that accumulates in cold rolling. In the present invention, in order to improve the hydrogen embrittlement resistance, it has been found that it is effective to control the area ratio and size of {211}±10° oriented grains that frequently become sites of crack generation on the plate surface. did. The area ratio of {211}±10° oriented grains is set to less than 30%, and the presence ratio of {111} orientation, which is a recrystallization texture, on the plate surface can be contributed to the improvement of hydrogen embrittlement resistance. From the viewpoint of hydrogen embrittlement resistance and manufacturability, the area ratio of {211}±10° oriented grains is preferably in the range of 5 to 20%, more preferably in the range of 3 to 15%.

∙ The average grain size of {211}±10° grains on the plate surface is less than 0.15 mm in both rolling direction and plate width direction (rolling vertical direction). By subdividing the size of the {211}±10° oriented grains, the introduction/accumulation of strain in the {211}±10° oriented grains is relaxed, which contributes to the improvement of hydrogen embrittlement resistance. From the viewpoint of hydrogen embrittlement resistance and manufacturability, the preferable size of the {211}±10° oriented grains is less than 0.10 mm, more preferably less than 0.07 mm.

In the present invention, the “plate surface” is a region up to t/8 of the plate thickness t of the steel plate, and a region from the surface of the steel plate to a thickness of 1/8 t in the surface direction on both sides of the steel plate. In addition, the {211}±10° oriented grains mean, on the plate surface, crystal grains having a crystal orientation in which an angle difference between the normal line direction of the steel sheet surface and the {211} plane orientation is within 10°.

The above-mentioned texture can be analyzed using electron beam backscattering diffraction method (hereinafter referred to as EBSD). EBSD is to measure and analyze the crystal orientation of each crystal grain in a micro region of the sample surface at high speed. The crystal orientation group that contributes to hydrogen embrittlement resistance is displayed by displaying a crystal orientation map divided into {211}±10° oriented grains and other regions on the plate surface, and the area ratio and grain of the {211}±10° oriented grains are displayed. You can quantify the size. For example, on a surface parallel to the steel plate surface from the steel plate surface to t/8 of the steel plate thickness t, EBSD is measured at a magnification of 100 in a measurement region of a plate width direction of 850 μm and a rolling direction of 2250 μm, and parallel to the steel plate surface. The crystal orientation map of crystal grains (that is, {211}±10° orientation grains) in which the angle difference between the normal direction of the plane and the {211} plane orientation is within 10° is displayed, and the area ratio and the grain size The size (rolling direction, plate width direction) can be quantified. If the range from the steel plate surface to t/8 of the steel plate thickness t is used as the inspection surface, the texture of the plate surface can be evaluated with good reproducibility.

The hydrogen embrittlement resistance is evaluated by a low strain rate tensile test having a relatively low strain rate, and the strain rate is preferably 10 ⁻⁵ /s. If the strain rate is 10 ⁻⁴ /s or more, which is relatively high, hydrogen penetration into the steel and diffusion of hydrogen do not proceed, and the hydrogen embrittlement of the steel may be reduced. On the other hand, when the strain rate is low, 10 ⁻⁶ /s, an excessive test time is required and the effect on the hydrogen embrittlement property is saturated. Hydrogen embrittlement resistance is evaluated by the tensile strength and elongation at break in the low strain rate tensile test described above, and the value in high-pressure hydrogen gas is less likely to decrease in comparison with the tensile strength and elongation at break in air or inert gas. As good. Here, the value obtained by dividing the tensile strength in the high-pressure hydrogen gas by the tensile strength in the atmosphere or the inert gas is referred to as "relative tensile strength". The value obtained by dividing the elongation at break in high-pressure hydrogen gas by the elongation at break in air or an inert gas is called "relative elongation". The Cr-based stainless steel sheet of this embodiment preferably has a relative tensile strength of 0.98 or more and a relative elongation of 0.75 or more. More preferable ranges are a relative tensile strength of 0.98 to 1.05 and a relative elongation of 0.85 to 1.05.

The low temperature brittleness shall be evaluated by the Charpy impact test according to JIS Z 2242, and the absorbed energy shall be measured using, for example, a 2 mm thick test piece with a V notch. The low temperature brittleness resistance is evaluated by the energy transition temperature according to JIS D, and the lower the energy transition temperature, the better. The energy transition temperature is a temperature corresponding to 1/2 of the absorbed energy at the temperature at which the fracture surface ratio due to ductile fracture is 100%. The Cr-based stainless steel sheet of the present embodiment preferably has an energy transition temperature of −10° C. or lower in consideration of the use in outdoor or on-vehicle hydrogen equipment. More preferably, it is -40°C or lower in consideration of use in cold regions.

Next, a method for manufacturing the Cr-based stainless steel sheet of this embodiment will be described.
As long as the Cr-based stainless steel sheet of the present embodiment satisfies the above chemical composition, it can be manufactured under normal process conditions such as casting, hot rolling, cold rolling, etc. In some cases, brittleness can be secured.
In order to improve the hydrogen embrittlement resistance by forming the texture of the present invention, the Cr-based stainless steel sheet of the present embodiment satisfies the above chemical components, and the following manufacturing method is preferable.

It is preferable that after hot rolling the steel having the above-described chemical composition, it is annealed after hot rolling at 900° C. or less, then cold rolling at a reduction rate of 40% or more, and finish annealing at a temperature of more than 900° C. .. The heat treatment after hot rolling (annealing after hot rolling) is 900° C. or lower, more preferably 700 to 900° C., in order to suppress the growth of {211} oriented grains generated in the hot rolling stage.

Cold rolling may be carried out by a reversible 20-high Sendzimir rolling machine, a 6-high rolling mill or a 12-high rolling mill, or a tandem rolling mill that continuously rolls multiple passes. In order to form the texture of the present invention, the work roll diameter is preferably large. Therefore, the work roll diameter is preferably 200 mm or more. Such large-diameter roll rolling is preferably carried out at the time of primary cold rolling (initial cold rolling when cold rolling is repeated a plurality of times). As a result, the {111} oriented grains that are recrystallized textures develop and the area ratio of the {211}±10° oriented grains that are the rolling textures is reduced, which is effective in forming the target texture of the present invention. Is. Cold rolling is preferably carried out at a reduction rate of 40% or more. If the cold rolling ratio is less than 40%, the area ratio and size of the {211}±10° oriented grains in the recrystallized texture tend to increase, and the hydrogen embrittlement resistance may decrease. From the viewpoint of hydrogen embrittlement resistance and manufacturability, the preferable range of the rolling reduction is 40 to 90%, and the more preferable range is 50 to 80%.

In finish annealing after cold rolling, it is preferable to perform heat treatment at over 900°C in order to develop {111} oriented grains and reduce the area ratio and size of {211} oriented grains. Since the excessive temperature rise increases the size of {211}±10° oriented grains due to the grain growth, the upper limit of the finish annealing temperature is preferably 1050°C. Further, the atmosphere at the time of finish annealing is not particularly specified, but the atmosphere, the LNG fuel atmosphere, and the BA atmosphere are preferable.

The soaking time for heat treatment (finish annealing) is preferably 10 seconds to 10 minutes. A soaking time of 10 seconds or more is preferable because the material for cold rolling can be softened. Further, if the soaking time is 10 minutes or less, the growth of {211}±10° oriented grains can be suppressed, the size of the crystal grains can be suppressed small, and a texture effective for hydrogen embrittlement resistance can be secured. ..

Hereinafter, examples of the present invention will be described.

-Cr-based stainless steel having the composition shown in Table 1 was melted. In the contents of Nb, Ti, Sn, and B in Table 1, those described as "0.0" mean that the element is not added.

Heated to a heating temperature of 1150 to 1250°C and hot rolled to produce a hot rolled steel sheet with a thickness of 5.0 mm. The hot rolled steel sheet was annealed after hot rolling in the range of 700 to 900° C., pickled and then cold rolled in the range of sheet thickness of 1.5 to 2.5 mm to obtain a cold rolled steel sheet. Cold rolling conditions are shown in Table 2. Cold rolling was carried out on a Sendzimir rolling machine and a tandem rolling machine with different work roll diameters. The former is a small diameter roll (60 mm) (indicated as “S” in Table 2) and the latter is a large diameter roll (200 mm) (in Table 2). "L" is used). The cold rolled steel sheet was subjected to finish annealing at 920 to 1020° C. and pickling to produce a Cr-based stainless steel sheet.

The organization was analyzed using EBSD. The crystal orientation group contributing to the hydrogen embrittlement resistance was numerically displayed by displaying a crystal orientation map divided into {211}±10° oriented grains and other regions on the plate surface. That is, EBSD was measured at a magnification of 100 in a measurement region of 850 μm in the width direction and 2250 μm in the rolling direction on a plane parallel to the steel plate surface within a range of t/8 of the thickness t of the steel plate from the steel plate surface, and parallel to the steel plate surface. A crystal orientation map of crystal grains (that is, {211}±10° orientation grains) in which the angle difference between the normal direction of the plane and the {211} plane orientation is within 10° is displayed, and the grain boundaries are also displayed. Then, the area ratio of the crystal grains and the average particle diameter (rolling direction and plate width direction) were measured. The notation in the “size” column of {211}±10° oriented grains in Table 2 means “rolling direction/plate width direction”. Further, for some of the comparative examples, the measurement results at the plate thickness center (t/2) are also shown for reference. The site where the crystal orientation differs by 15° or more was defined as a crystal grain boundary.

The obtained Cr-based stainless steel sheet was evaluated for hydrogen embrittlement and low temperature embrittlement. For the hydrogen embrittlement resistance, a commercially available 2 mm thick SUS316L steel plate (17.5%Cr-12%Ni-2%Mo) and SUS316 steel plate (17.5%Cr-10%Ni-2%Mo) were used for evaluation. I was there.

The hydrogen embrittlement was evaluated by the following procedure.
A tensile test piece having a width of 4 mm and a length of 20 mm in the parallel portion was prepared, and immediately before the tensile test in high-pressure hydrogen gas, the surface was polished with dry type #600 emery paper and then degreased and washed with an organic solvent. The tensile test in high-pressure hydrogen gas was carried out at a hydrogen gas pressure of 20 MPa or 45 MPa, a test temperature of −40° C., and a strain rate of 10 ⁻⁵ /s, as shown in Table 1. The comparative tensile test was carried out in 0.1 MPa nitrogen at -40°C. The tensile strength in high-pressure hydrogen gas is divided by the tensile strength in 0.1 MPa nitrogen to obtain the relative tensile strength, and the breaking elongation in high-pressure hydrogen gas is divided by the breaking elongation in 0.1 MPa nitrogen to obtain the relative elongation. did. The hydrogen embrittlement resistance was evaluated using relative tensile strength and relative elongation as evaluation indexes. The evaluation criteria are as follows. A and B were passed.
A: The relative tensile strength is 0.98 or more and the relative elongation is 0.85 or more.
B: Other than the above, the relative tensile strength is 0.98 or more and the relative elongation is 0.75 or more.
X: Either or both of the relative tensile strength of less than 0.98 and the relative elongation of less than 0.75.
Here, when the hydrogen gas pressure is 45 MPa and the test temperature is −40° C., the relative elongation of the SUS316L steel sheet is less than 0.75, and the evaluation is X. When the hydrogen gas pressure is 20 MPa and the test temperature is −40° C., the relative elongation of the SUS316 steel sheet is less than 0.75, and the evaluation is X.

The evaluation of the low temperature brittleness was performed by the Charpy impact test according to JIS Z2242. The test piece had a V-notch shape of 1.5 to 2.5 mm thickness×10 mm width×55 mm length, and the test temperature was in the range of −100° C. to room temperature (20° C.). The low temperature brittleness resistance was used as an evaluation index by obtaining the energy transition temperature from the absorbed energy measured by the Charpy test. The evaluation criteria are as follows. A and B were passed.
A: Energy transition temperature of −40° C. or lower is satisfied.
B: Energy transition temperature of more than −40° C. and below −10° C. is satisfied.
X: Energy transition temperature is higher than −10° C.

The test results are summarized in Table 2.
No. Nos. 1 to 11 were all Cr-based stainless steel sheets having the chemical composition and texture within the scope of the present invention, and had good hydrogen embrittlement resistance and low temperature embrittlement resistance. In particular, No. 1 with the range of preferable components and textures. 5, 6, 9, and 10 had a hydrogen embrittlement resistance index of "B" or "A" at a hydrogen gas pressure of 45 MPa, and the hydrogen embrittlement resistance was higher than that of SUS316L. In addition, No. Nos. 6, 8 and 10 are obtained by reducing the {211}±10° oriented grains using a large-diameter roll and have the same chemical composition as No. The hydrogen embrittlement resistance was further improved as compared with 5, 7, and 9.

No. Nos. 12 to 20 are Cr-based stainless steel sheets that do not have chemical components within the scope of the present invention, cannot form a texture within the scope of the present invention, and either or both of hydrogen embrittlement resistance and low temperature embrittlement resistance are inferior. became. In addition, No. In Nos. 17, 19 and 20, the area ratio of {211}±10° oriented grains in the plate thickness center is less than 30%, but the area ratio on the plate surface exceeds 30%, and hydrogen embrittlement resistance and low temperature embrittlement resistance. It can be seen that it is important to control the area ratio on the plate surface in order to obtain

From the above evaluation results, the hydrogen embrittlement resistance of the Cr-based stainless steel sheet was higher than that of SUS316 in the market by having the components and the texture within the scope of the present invention. Further, it has been found that the hydrogen embrittlement resistance surpassing that of SUS316L can be obtained by controlling the texture to have a preferable texture by using a large diameter roll having a preferable component.

Claims

In mass %,
C: 0.020% or less,
Si: 1.00% or less,
Mn: 1.00% or less,
P: 0.040% or less,
S: 0.0030% or less,
Cr: 10.0 to 18.0%,
N: 0.020% or less,
Al: 0.10% or less,
Further, it contains one or two of Nb: 0.5% or less and Ti: 0.5% or less,
Sn: 0-0.3%,
B: 0 to 0.005%,
Ni: 0 to 1%,
Cu: 0 to 1%,
Mo: 0 to 1%,
Sb: 0.2% or less,
V: 0-0.5%,
W: 0-0.5%,
Zr: 0-0.5%,
Co: 0-0.5%,
Mg: 0 to 0.005%,
Ca: 0 to 0.005%,
Ga: 0 to 0.020%,
La: 0 to 0.1%,
Y: 0 to 0.1%,
Hf: 0-0.1%,
REM: 0-0.1%,
A Cr-based stainless steel sheet characterized in that the balance consists of Fe and impurities, and the texture on the plate surface satisfies the following (i) and (ii).
(I) The area ratio of crystal grains (hereinafter referred to as “{211}±10° oriented grains”) having an angle difference of 10° or less between the normal direction of the steel plate surface and the {211} plane orientation on the plate surface is 30. % (Ii) In the {211}±10° oriented grains defined by (i), the length in the rolling direction and the length in the strip width direction are both less than 0.15 mm on average.
Further, by mass%, Sn: 0.001 to 0.3% and B: 0.005% or less are contained,
The Cr-based stainless steel sheet according to claim 1, wherein the following formula (1) is satisfied.
Si+0.5Mn+10P+5Nb+2Ti<2.00 (1) Formula In the above formula, the element symbol means the content (mass %) of the element.
Furthermore, in mass%,
Ni: 1% or less,
Cu: 1% or less,
Mo: 1% or less,
Sb: 0.2% or less,
V: 0.5% or less,
W: 0.5% or less,
Zr: 0.5% or less,
Co: 0.5% or less,
Mg: 0.005% or less,
Ca: 0.005% or less,
Ga: 0.020% or less,
La: 0.1% or less,
Y: 0.1% or less,
Hf: 0.1% or less,
REM: 0.1% or less of 1 type or 2 types or more of containing, The Cr type|system|group stainless steel plate of Claim 1 or Claim 2 characterized by the above-mentioned.
The Cr-based stainless steel sheet according to any one of claims 1 to 3, which is used as a metal material of a device for high-pressure hydrogen gas.