KR20210092292A - Cr-based stainless steel sheet with excellent hydrogen embrittlement resistance - Google Patents

Abstract

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, Nb: 0.5% or less, Ti: 0.5% or less, containing one or two types, characterized in that the texture on the plate surface satisfies the following (i) and (ii) Cr-based stainless steel sheet with excellent hydrogen embrittlement resistance. (i) The area ratio of crystal grains ({211}±10° orientation grains) whose angle difference between the normal direction of the steel plate surface and the {211} plane orientation on the plate surface is within 10° is less than 30% (ii) {211 } In the ±10° orientation grain, the length in the rolling direction and the length in the sheet width direction are both less than 0.15 mm on 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 more particularly, to a Cr-based stainless steel sheet suitable as a metal material for a high-pressure hydrogen gas device.

In recent years, suppression of generation|occurrence|production of the greenhouse gas mainly carbon dioxide is calculated|required from the abnormal weather in which global warming becomes one factor. As a part of this, development of automobiles and transportation equipment using fuel cells as power sources is progressing. Since the fuel cell generates electric power using hydrogen as a fuel, it does not generate carbon dioxide and has high energy conversion efficiency, so it is positioned as a powerful electric power source.

In a fuel cell using hydrogen as a fuel, or an apparatus including a hydrogen station for supplying hydrogen thereto, constituent parts are exposed to a hydrogen gas environment. As a metal material exposed to a hydrogen gas environment, there is known a phenomenon in which mechanical properties such as tensile strength, elongation, or drawing are deteriorated due to hydrogen penetrating into the material. These phenomena are called hydrogen embrittlement. From this problem of hydrogen embrittlement, both austenitic stainless steel SUS316L and aluminum alloy 6061, both for the high-pressure hydrogen container for automobiles with a pressure of 35 MPa in JARIS001, and for the high-pressure hydrogen container for automobiles with a pressure of 70 MPa in KHKS0128, according to the Japanese Automotive Research Institute technical standard JARIS001 The use of -T6 is prescribed.

In the example standard of the general high-pressure gas security rule, Ni equivalent (Ni+) of austenitic stainless steel sheets (SUS316 and SUS316L) prescribed in JIS G 4304 and JIS G 4305 for hydrogen infrastructure equipment with a pressure of 20 MPa or more and 82 MPa or less 0.65Cr+0.98Mo+1.05Mn+0.35Si+12.6C) is stipulated to use a higher material (for example, Ni equivalent ≥28.5). The operating temperature is -45°C or higher and 250°C or lower. In these austenitic stainless steels, for example, Patent Document 1 or Patent Document 2 discloses a stainless steel for improving economic efficiency by increasing the strength of SUS316L or reducing expensive Mo.

In the above general high-pressure gas security rules, material restrictions for hydrogen devices with a pressure of 20 MPa or less were abolished by the revision in 2016. With these deregulations, the demand for the use of highly economical stainless steel sheets also in high-pressure hydrogen gas is increasing, and evaluation of hydrogen embrittlement resistance in high-pressure hydrogen gas is desired in various steel materials. Ferritic and martensitic stainless steel sheets (hereinafter, collectively referred to as "Cr-based stainless steel sheets") have excellent economic efficiency compared to austenitic stainless steel sheets since they contain almost no Ni, which is a rare metal. Conventionally, for example, Non-Patent Document 1 discloses hydrogen embrittlement characteristics evaluated in room temperature/high pressure hydrogen gas for general steel materials including stainless steel. It has been reported that SUS304, which is a typical austenitic stainless steel, and Cr-based stainless steel are prone to hydrogen embrittlement. Therefore, it is generally recommended to use SUS316L or SUS316 even in high-pressure hydrogen gas at a pressure of about 20 MPa. Moreover, the Cr-type stainless steel which has a body-centered cubic structure also has the subject (low-temperature brittleness) that toughness falls at the low temperature of room temperature or less compared with the austenitic stainless steel of a face-centered cubic structure.

For the purpose of expanding materials that can be used in a high-pressure hydrogen gas environment, materials coated with Al or an Al alloy having excellent hydrogen embrittlement resistance have also been devised. Patent Document 3 discloses a pressure vessel for high-pressure hydrogen gas and a pipe for high-pressure hydrogen gas coated with Al or an Al alloy. In the examples, film formation on austenitic stainless steel and two-phase stainless steel containing an austenitic phase are targeted, and film formation and hydrogen intrusion in steel materials prone to hydrogen embrittlement, for example, Cr-based stainless steel characteristics are not shown.

Moreover, in patent document 4, with respect to the steel material which is easy to become hydrogen embrittled, by single-piece|unit, hot-dip plating using the Al-Si type alloy which made the addition amount of Si 1 to 5 % is performed, and hydrogen which forms a hydrogen permeation film by this. A substrate for a device is disclosed. It is said that the steel material of a base material shall be carbon steel, low alloy steel, and Cr-type stainless steel, and hydrogen embrittlement can be prevented and manufacturing cost can be lowered also. However, the embodiment is limited to SUS304, SUS630 (15Cr-4Ni-3Cu) and SCM435 (low alloy steel). As for the highly economical Cr-based stainless steel sheet, neither its hydrogen embrittlement characteristics nor its use is mentioned at all.

Japanese Patent Laid-Open No. 2014-114471 Japanese Patent Laid-Open No. 2016-183412 Japanese Patent Laid-Open No. 2004-324800 International Publication No. 2015-098981

PVP2007-26820 Michihiko Nagumo “The Foundation of Hydrogen Brittleness” Rokakuho Uchida (December 2008)

The stainless steels described in Patent Documents 1 to 4 described above are limited to austenitic, two-phase, and SUS630 (precipitation hardening type), and the Cr-based stainless steel disclosed in Non-Patent Document 1 is prone to hydrogen embrittlement and is used in high-pressure hydrogen gas applications. It does not have hydrogen embrittlement resistance. Cr-based stainless steel also has a subject of low-temperature brittleness.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a Cr-based stainless steel sheet having hydrogen embrittlement resistance for use in high-pressure hydrogen gas and excellent hydrogen embrittlement resistance suitable as a metal material for high-pressure hydrogen gas equipment. do. In addition, it makes it a subject to realize coexistence with low temperature brittleness resistance.

In order to solve the said subject, this invention employ|adopts the following structures.

[1] 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, Nb: 0.5% or less, Ti: 0.5% or less, containing one or two types, Sn: 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 A Cr-based stainless steel sheet comprising impurities and having a texture on the surface of the sheet satisfying the following (i) and (ii).

(i) The area ratio of crystal grains (hereinafter referred to as “{211}±10° orientation grains”) whose angle difference between the normal direction of the steel plate surface and the {211} plane orientation on the plate surface is within 10° is 30% under

(ii) In the {211}±10° orientation grain defined in (i), the average length in the rolling direction and the length in the sheet width direction is less than 0.15 mm

[2] Further, by mass%, Sn: 0.001 to 0.3%, B: 0.005% or less,

Cr-based stainless steel sheet of the present invention, characterized in that it satisfies the following formula (1).

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: 0.1% or less, Hf: 0.1% or less, REM: 0.1% or less Or the Cr-based stainless steel sheet of the present invention, characterized in that it contains two or more.

[4] The Cr-based stainless steel sheet of the present invention, which is used as a metal material for a device for high-pressure hydrogen gas.

ADVANTAGE OF THE INVENTION According to this invention, while being excellent in hydrogen embrittlement resistance, the Cr-type stainless steel sheet excellent also in low-temperature toughness can be provided. Moreover, the Cr-type stainless steel sheet of this invention can be used suitably as a metal material of the apparatus for high-pressure hydrogen gas.

In order to solve the above problems, the present inventors earnestly study the influence of alloy elements and texture on hydrogen embrittlement resistance and low temperature embrittlement resistance in a Cr-based stainless steel sheet, obtain the following new knowledge, and provide the present invention came to achieve

(a) As mentioned above, there exist hydrogen embrittlement resistance and low temperature embrittlement resistance in the characteristic requested|required of the metal material of the apparatus for high-pressure hydrogen gas. In the Cr-based stainless steel sheet, compared with the austenitic stainless steel sheet, the amount of hydrogen entering the steel material from the high-pressure hydrogen gas is reduced due to the crystal structure, but having hydrogen embrittlement resistance suitable for high-pressure hydrogen gas applications is not obtained. 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 material destruction progresses due to the interaction between hydrogen and plastic deformation, which has penetrated into steel materials from the high-pressure hydrogen gas. From recent research results, the hydrogen-induced strain-induced vacancy theory, in which the mechanism of hydrogen embrittlement promotes the generation of vacancy lattice defects in steel by the interaction of hydrogen and plastic strain, and progresses destruction [Non-patent literature 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, in the present invention, the amount of Cr is suppressed to 18% or less. Further, the present inventors have found that it is preferable to control the addition amounts of Si, Mn, P, Ti, and Nb within a predetermined range.

(b) The present inventors also found that, when a low strain rate tensile test was performed in high pressure hydrogen gas, the method for measuring crystal orientation had an effect on the occurrence of cracks due to the interaction between hydrogen and plastic deformation. When hydrogen embrittlement becomes present, the frequency of cracks occurring and advancing within crystal grains increases. The cracks in the grains are not {111} oriented grains that are recrystallized grains (crystal grains with {111} plane orientation toward the normal direction of the steel sheet surface), but {211} oriented grains that are rolled texture ({211} plane orientation of the steel sheet surface). It can be seen that there are many cases that occur in grains facing the normal direction). From these facts, it is presumed that the {211} orientation grains are easy to introduce and accumulate due to the interaction of hydrogen and plastic deformation. And, in {211} orientation grain, it is estimated that it acted as a crack generation site by the generation|occurrence|production of the said porosity lattice defect becoming active. In order to suppress hydrogen embrittlement proceeding by such a mechanism, it is effective to reduce the area ratio and size of the {211} orientation grains in addition to adjusting the range of the alloy elements described above, and the critical value has been found.

(c) Further, hydrogen that has penetrated into the steel material from the high-pressure hydrogen gas moves through the grain boundary as a main diffusion path. Trace addition of Sn and B, which are grain boundary segregation elements, serves as a diffusion barrier at the grain boundary of hydrogen and reduces the interaction between hydrogen and plastic deformation. In the conventional Cr-based stainless steel, impurity elements such as P or S segregate at grain boundaries, which tends to promote low-temperature brittleness. Therefore, the present inventors paid attention to the trace addition of Sn and B, and, by containing these elements in a predetermined range, suppressed adverse effects, such as P and S, discovered that coexistence of hydrogen embrittlement resistance and low temperature embrittlement resistance was anticipated.

The summary of this invention made based on the knowledge of said (a)-(c) is as follows.

Cr-based stainless steel sheet of this embodiment, 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, Nb: 0.5% or less, Ti: 0.5% or less, containing one or two types, the balance being Fe and impurities, and the aggregate on the plate surface It is a Cr-based stainless steel sheet excellent in hydrogen embrittlement resistance and low temperature embrittlement resistance, characterized in that the structure satisfies the following (i) and (ii).

(i) The area ratio of crystal grains ({211}±10° orientation grains) whose angle difference between the normal direction of the steel plate surface and the {211} plane orientation on the plate surface is within 10° is less than 30%

(ii) In the {211}±10° orientation grain defined in (i), the average length in the rolling direction and the length in the sheet width direction is less than 0.15 mm

Moreover, it is preferable that the Cr-type stainless steel sheet of this embodiment contains Sn:0.001-0.3% and B:0.005% or less by mass %, and satisfy|fills following formula (1).

In the above formula, the element symbol means the content (mass %) of the element.

In addition, the Cr-based stainless steel sheet of this embodiment, 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 1 type, or 2 or more types may be contained.

Moreover, it is preferable that the Cr-type stainless steel plate of this embodiment is used as a metal material of the apparatus for high-pressure hydrogen gas.

Hereinafter, each requirement of this invention is demonstrated in detail. In addition, the indication of content "%" of each element means "mass %".

C: 0.020% or less

C increases work hardening of steel by solid solution and precipitation of carbides, deteriorates hydrogen embrittlement resistance, further lowers toughness and deteriorates low-temperature embrittlement resistance, so its content is small, and the upper limit is 0.020% or less . However, in order to reduce the amount of C, the refining process becomes complicated and the cost increases. Therefore, it is preferable that the amount of C shall be 0.001% or more. A preferable range in consideration of the refining cost is 0.003 to 0.015%, and a more preferable range is 0.003 to 0.010%.

Si: 1.00% or less

While Si is effective as a deoxidation element, when it contains excessively, since solid solution strengthening and work hardening will raise and the fall of hydrogen embrittlement resistance and low temperature embrittlement resistance will be caused, an upper limit is made into 1.00 % or less. In order to ensure deoxidation capability, it is preferable to make a minimum into 0.01 % or more. A preferable range is 0.05 to 0.50% in consideration of manufacturability and characteristics, and 0.05 to 0.30% may be sufficient.

Mn: 1.00% or less

Mn is effective as a deoxidation element, and is also an effective element in order to fix S, improve toughness, and obtain low-temperature brittleness resistance. On the other hand, when Mn is contained excessively, since work hardening will raise and the fall of hydrogen embrittlement resistance and low temperature toughness will be caused, an upper limit is made into 1.00 % or less. In order to ensure the action of deoxidation and S fixation, it is preferable that the lower limit be 0.01% or more. A preferable range is 0.05 to 0.50%, and 0.05 to 0.30% may be sufficient in consideration of an effect and manufacturability.

P: 0.040% or less

P is an element which segregates grain boundary and reduces low-temperature brittleness, and since the content is so good, it makes an upper limit 0.040% or less. However, since excessive reduction leads to an increase in refining cost, it is preferable that the lower limit be 0.005% or more. A more preferable range is 0.010 to 0.030% in consideration of manufacturing cost and characteristics, and 0.010 to 0.020% may be sufficient.

S: 0.0030% or less

Since S forms sulfides in grain boundary segregation or steel and deteriorates low-temperature brittleness, the smaller the content, the better, and the upper limit is made 0.0030% or less. However, since excessive reduction leads to an increase in raw material and refining costs, it is preferable that the lower limit be 0.0001% or more. A more preferable range is 0.0002 to 0.0015% in consideration of manufacturing cost and characteristics, and 0.0002 to 0.0008% may be sufficient.

Cr: 10.0 to 18.0%

Cr is a basic element of the Cr-based stainless steel of this embodiment, and is an essential element in order to hold|maintain hydrogen embrittlement resistance and low temperature embrittlement resistance in addition to the corrosion resistance of steel. In order to acquire the said characteristic which assumed the high-pressure hydrogen gas use of this embodiment, a minimum is made into 10.0 % or more. An upper limit shall be 18.0 % or less from a viewpoint of making hydrogen embrittlement resistance and low temperature embrittlement resistance compatible. When Cr having a high hydrogen trapping ability exceeds 18.0%, the amount of hydrogen entering the steel from a high-pressure hydrogen gas environment increases and hydrogen embrittlement resistance deteriorates, and the texture may deviate from the range suitable for the present invention. The more preferable range of Cr may be 11.0 to less than 17.0 %, and 12.0 to 15.0 % may be sufficient as it.

N: 0.020% or less

As with C, N increases work hardening of steel by solid solution and precipitation of carbides and deteriorates hydrogen embrittlement resistance, further lowering toughness and worsening low-temperature embrittlement resistance. Therefore, the smaller the content, the better the upper limit is 0.020% below. However, in order to reduce the amount of N, the refining process becomes complicated and the cost increases. Therefore, it is preferable that the amount of N shall be 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 a very effective element as a deoxidation element. On the other hand, while reducing the toughness of steel and deteriorating low-temperature brittleness, since a texture may deviate from the range suitable for this invention, an upper limit is made into 0.10 % or less. The lower limit is preferably 0.005% or more in consideration of the deoxidation effect. A preferable range is 0.01 to 0.07% in consideration of characteristics and manufacturability, and 0.01 to 0.05% may be sufficient.

Nb: 0.5% or less, Ti: 0.5% or less 1 type or 2 types

Nb and Ti have an effect of suppressing grain boundary segregation of P and S by segregating at grain boundaries, and improving low-temperature brittleness resistance. Moreover, in Nb and Ti, work hardening of steel is suppressed by the action|action as a stabilizing element which fixes C, N, P, and S, and improvement of hydrogen embrittlement resistance is also anticipated. Both Nb and Ti become effective elements for the improvement of the hydrogen embrittlement resistance and low temperature embrittlement resistance which are the target of this invention in order to exhibit these two actions. When it contains, it is preferable to set it as 0.01 % or more in which the effect is expressed, respectively. However, excessive content increases work hardening and leads to a decrease in hydrogen embrittlement resistance and an increase in alloy cost, and also, toughness decreases and low temperature embrittlement resistance deteriorates, and the texture may deviate from the scope suitable for the present invention. , the upper limit is set to 0.5% or less, respectively. A preferable range is made into 0.05 to 0.5% with respect to the total of 1 type or 2 types of Nb and Ti in consideration of the said characteristic improvement effect and alloy cost. A more preferable range is 0.08 to 0.4%, and 0.1 to 0.3% may be sufficient with respect to the total of 1 type or 2 types.

More preferably, Sn and B are contained in the following content ranges.

Sn: 0.001 to 0.3%

Sn is an effective element in order to improve the hydrogen embrittlement resistance and low temperature embrittlement resistance which are the target of this invention. Sn, which is a grain boundary segregation element, serves as a diffusion barrier at the grain boundary of hydrogen and reduces the interaction between hydrogen and plastic deformation. Moreover, segregation of P and S is suppressed in a crystal grain boundary, and the bad influence of low-temperature-resistance brittleness is also relieved. Since coexistence of hydrogen embrittlement resistance and low temperature embrittlement resistance is anticipated by containing Sn in a predetermined range, it is preferable to make it contain in 0.001 to 0.5% of range in this invention. By containing 0.001% or more of Sn, the said effect is expressed and hydrogen embrittlement resistance improves. However, since excessive content increases Sn concentration in the grain boundary and causes low-temperature brittleness resistance and a fall of manufacturability, an upper limit is made into 0.5 % or less. Preferably it is 0.005 to 0.3 %, and 0.010 to 0.2 % may be sufficient.

B: 0.005% or less

B is a grain boundary segregation element, is an element which improves hydrogen embrittlement resistance and low temperature embrittlement resistance similarly to Sn, and it is effective to make it contain in the Cr-type stainless steel of this embodiment. In this invention, in order to aim at the improvement of hydrogen embrittlement resistance, it is preferable to set it as 0.0003 % or more. However, excessive B content causes a decrease in elongation and productivity, so the upper limit is made 0.005% or less. Preferably it is 0.0005 to 0.002%, and 0.001 to 0.002% may be sufficient.

Si, Mn, P, Nb, and Ti are each within the range of the above content, and in order to improve the hydrogen embrittlement resistance and low temperature embrittlement resistance that are the targets of the present invention, and further satisfy the following formula (1) it is preferable

In the above formula, the element symbol means the content (mass %) of the element.

In order to improve the said characteristic aimed at this invention, it is preferable that Formula (1) shall be less than 2.00, and it is preferable that a lower limit shall be 0.05 from a viewpoint of a characteristic and manufacturability. A preferred range is 0.35 to 1.80, and a more preferred range is 0.50 to 1.50.

Except for the above-mentioned elements, it consists of Fe and an impurity. However, in the range which does not impair the effect exhibited by the technical characteristic of this invention, the element described below other than the above can be selectively contained. The reason for limitation is 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 elements effective in improving corrosion resistance, and Ni and Cu are also effective in improving low-temperature toughness. In order to exhibit this effect, you may contain Ni, Cu, and Mo in 0.05% or more of ranges, respectively. Excessive content increases solid solution strengthening and work hardening of stainless steel and causes a decrease in hydrogen embrittlement resistance, so the upper limit is set to 1% or less, respectively. More preferably, they are 0.1 % or more and 0.8 % or less, respectively, More preferably, they are 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 are elements effective for improving corrosion resistance and suppressing grain boundary segregation of P and S, and improving low-temperature brittleness, and they are contained as needed. In particular, Sb is a strong grain boundary segregation element, and similarly to Sn and B, it has an effect of excluding grain boundary segregation of impurity elements such as P and S. When containing these elements, it is preferable to set it as 0.01 % or more in which the effect is expressed, respectively. Since excessive content leads to the fall of manufacturability and low-temperature brittleness resistance, Sb shall be 0.2 % or less, and V, W, Zr, and Co shall be 0.5 % or less, respectively. More preferably, the range of Sb is 0.02 to 0.15%, more 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 more 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 deoxidizer, and also acts as a crystallization nucleus of TiN. TiN becomes the solidification nucleus of the ferrite phase in the solidification process, and by accelerating the crystallization of TiN, a fine ferrite phase can be formed during solidification. By refining the solidified structure, low-temperature brittleness can also be improved. When making it contain, it is preferable to set it as 0.0001 % or more which expresses these effects. However, since manufacturability and corrosion resistance will deteriorate when Mg exceeds 0.005 %, an upper limit is made into 0.005 % or less. Preferably it is 0.0003 to 0.002%, More preferably, it is 0.0003 to 0.001%.

Ca: 0.005% or less

Ga: 0.020% or less

Ca and Ga are elements which improve the cleanliness of steel, and in order to suppress a raise of work hardening and to improve hydrogen embrittlement resistance, they are contained as needed. When making it contain, in order to express these effects, it is preferable to set it as 0.0003 % or more, respectively. However, since excessive containing leads to deterioration of manufacturability and corrosion resistance, Ca makes 0.005 % or less and Ga makes an upper limit into 0.020 % or less. 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 are elements which improve the cleanliness of steel like Ca and Ga, and in order to suppress a raise of work hardening and to improve hydrogen embrittlement resistance, you may contain as needed. When making it contain, since an effect is expressed, it is preferable to set it as 0.001 % or more, respectively. However, since excessive content leads to an increase in alloy cost and deterioration in manufacturability, the upper limit is made 0.1% or less, respectively. Preferably it is 0.001 to 0.05%, respectively, More preferably, it is set as 0.001 to 0.03%.

REM (rare earth element) refers to the general name of 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 independently, and a mixture may be sufficient as them.

In addition, the impurity contained in a remainder means that it is allowed in the limit which solves the subject of this invention by mixing from ore as a raw material, scrap, or a manufacturing environment, when manufacturing steel industrially. If necessary, Ta: 0.1% or less, Bi: 0.01% or less, Zn: 0.05%, or H: 0.0005% or less may be contained. The Cr-based stainless steel of the present embodiment may contain crystal grains of ferrite or may contain crystal grains of martensite.

Next, the texture of the Cr-based stainless steel sheet of the present embodiment will be described. In the Cr-based stainless steel sheet of the present embodiment, the texture on the surface of the sheet satisfies the following (i) and (ii).

(i) The area ratio of crystal grains ({211}±10° orientation grains) in which the angle difference between the normal direction of the steel plate surface and the {211} plane orientation on the plate surface is within 10° is less than 30%

(ii) In the {211}±10° orientation grain defined in (i), the length in the rolling direction and the length in the sheet width direction are both less than 0.15 mm on average

Here, the {211} plane orientation means the normal direction of the {211} plane.

The {211} orientation is called α-fiber, and is a rolled texture that is integrated in cold rolling. In this invention, in order to improve these hydrogen embrittlement resistance, it discovered that it is effective to control the area ratio and size of the {211}±10 degree orientation grain with high frequency used as the crack generation site in the plate surface. The area ratio of the {211}±10° orientation grains is less than 30%, and by increasing the abundance ratio of the {111} orientation, which is a recrystallized texture, on the plate surface, it can contribute to the improvement of hydrogen embrittlement resistance. From the viewpoint of hydrogen embrittlement resistance and manufacturability, the preferable range of the area ratio of the {211}±10° orientation grains is 5 to 20%, and the more preferable range is 3 to 15%.

In addition, in the plate surface, the size of the {211} +/-10 degree orientation grain shall be less than 0.15 mm in the average of both the length of a rolling direction and a board width direction (rolling perpendicular direction). By subdividing the size of the {211}±10° azimuth grains, the introduction and accumulation of strain to the {211}±10° azimuth grains are alleviated, contributing to the improvement of hydrogen embrittlement resistance. From the viewpoint of hydrogen embrittlement resistance and manufacturability, the preferable size of the {211}±10° azimuth grain 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 refers to 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° orientation grains refer to crystal grains having a crystal orientation within 10° of the angle difference between the normal direction of the steel plate surface and the {211} plane orientation on the plate surface.

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

Hydrogen embrittlement resistance shall be evaluated by a relatively low strain rate tensile test with a relatively small strain rate, and it is preferable that the ^{strain rate be 10 -5 /s.} In the case of a relatively large strain rate of 10 ^-4 /s or more, the intrusion and diffusion of hydrogen into the steel does not proceed, and hydrogen embrittlement of the steel may be reduced. On the other hand, in the case of a small strain rate of 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 tensile strength and elongation at break in the above-described low strain rate tensile test, and compared with tensile strength or elongation at break in air or inert gas, the value in high-pressure hydrogen gas is difficult to decrease. . Here, the value obtained by dividing the tensile strength in the high-pressure hydrogen gas by the tensile strength in the atmosphere or in an inert gas is referred to as “relative tensile strength”. The value obtained by dividing the elongation at break in the high-pressure hydrogen gas by the elongation at break in the atmosphere or in an inert gas is called a “relative elongation”. It is preferable that the Cr-type stainless steel sheet of this embodiment has a relative tensile strength of 0.98 or more and a relative elongation of 0.75 or more. A more preferable range is a relative tensile strength of 0.98 to 1.05 and a relative elongation of 0.85 to 1.05.

The low-temperature brittleness resistance shall be evaluated by the Charpy impact test based on JISZ2242, for example, the absorbed energy is measured using the V-notch 2mm-thick test piece. Low-temperature brittleness resistance is evaluated by the energy transition temperature based on Annex D of the said JIS, and it is so favorable that the energy transition temperature is low. The energy transition temperature is a temperature corresponding to a value of 1/2 of the absorbed energy at a temperature at which the fracture surface ratio due to ductile fracture becomes 100%. As for the Cr-type stainless steel sheet of this embodiment, it is preferable that the energy transition temperature is -10 degreeC or less in consideration of use in the outdoors or in vehicle-mounted hydrogen equipment. More preferably, it is -40 degrees C or less in consideration for use in a cold region.

Next, the manufacturing method of the Cr-type stainless steel plate of this embodiment is demonstrated.

If the Cr-based stainless steel sheet of this embodiment meets the above chemical composition, even if it is manufactured under normal process conditions such as casting, hot rolling, cold rolling, etc., the target hydrogen embrittlement resistance and low temperature embrittlement resistance of the present invention can be secured. In some cases.

In order that the Cr-type stainless steel sheet of this embodiment forms the texture of this invention and improves hydrogen embrittlement resistance, while satisfying the said chemical component, the following manufacturing methods are preferable.

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

Cold rolling may be implemented with a reversible 20-stage senjimere rolling mill, a 6-stage, or a 12-stage rolling mill, and may be implemented with a tandem rolling mill which rolls multiple passes|passes continuously. In order to form the texture of the present invention, the larger diameter of the work roll is preferable. Therefore, it is preferable that the diameter of a work roll shall be 200 mm or more. Such large-diameter roll rolling is preferably performed at the time of primary cold rolling (initial cold rolling in the case of repeatedly performing cold rolling several times). As a result, {111} orientation grains that are recrystallized textures develop, and the area ratio of {211}±10° orientation grains that are rolled textures is reduced, which is effective for forming the target texture of the present invention. It is preferable to perform cold rolling at a reduction ratio of 40% or more. When the cold rolling ratio is less than 40%, the area ratio and size of the {211}±10° orientation grains in the recrystallized texture tend to rise, which may lead to a decrease in hydrogen embrittlement resistance. From the viewpoint of hydrogen embrittlement resistance and manufacturability, a preferable range of the reduction ratio is 40 to 90%, and a more preferable range is 50 to 80%.

In the finish annealing after cold rolling, in order to develop {111} orientation grains and reduce the area ratio and size of {211} orientation grains, it is preferable to heat-process more than 900 degreeC. In order that excessive temperature rise raises the size of the {211} +/-10 degree orientation grain by crystal grain growth, it is preferable that the upper limit of the finish annealing temperature is 1050 degreeC. In addition, although the atmosphere in particular at the time of process annealing is not prescribed|regulated, it is preferable that it is an atmosphere of LNG fuel in air|atmosphere, and BA atmosphere.

The soaking time of the heat treatment (finish annealing) is preferably set to 10 seconds to 10 minutes. If the soaking time is 10 seconds or more, it is preferable because softening of the material for cold rolling is attained. In addition, when the cracking time is 10 minutes or less, the growth of {211}±10° orientation grains is suppressed, the size of the crystal grains can be suppressed small, and a texture effective for hydrogen embrittlement resistance can be secured.

Example

Hereinafter, an embodiment of the present invention will be described.

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

It hot-rolled by heating to a heating temperature of 1150-1250 degreeC, and manufactured the hot-rolled steel plate with a plate|board thickness of 5.0 mm. The hot-rolled steel sheet was annealed after hot rolling in the range of 700 to 900°C, and cold-rolled to a sheet thickness of 1.5 to 2.5 mm after pickling to obtain a cold rolled steel sheet. Cold rolling conditions are shown in Table 2. Cold rolling is performed with a senjimere mill and a tandem mill of different work roll diameters, the former being a small-diameter roll (60 mm) (indicated by “S” in Table 2), and the latter being a large-diameter roll (200 mm) (in Table 2, “ L") was used. The cold-rolled steel sheet was subjected to finish annealing and pickling at 920 to 1020°C to prepare a Cr-based stainless steel sheet.

Collective tissue was analyzed using EBSD. The crystal orientation group contributing to hydrogen embrittlement resistance was quantified by displaying a crystal orientation map divided into {211}±10° orientation grains on the plate surface and other regions. That is, in the plane parallel to the surface of the steel plate in the range of t/8 of the plate thickness t of the steel plate from the surface of the steel plate, EBSD was measured at a magnification of 100 in the measurement area of 850 µm in the plate width direction and 2250 µm in the rolling direction, and the steel plate A crystal orientation map of crystal grains (i.e., {211} ± 10° orientation grains) whose angle difference between the direction of the normal line parallel to the surface and the orientation of the {211} plane is within 10° is displayed, and the grain boundaries are also displayed; The area ratio and average grain diameter (rolling direction and plate width direction) of the crystal grains were measured. In Table 2, the notation in the "size" column of the {211}±10° orientation grain means "rolling direction/plate width direction". In addition, about some comparative examples, the measurement result in the plate|board thickness center (t/2) was also written together as reference. A region in which the crystal orientation differs by 15° or more was used as the grain boundary.

The obtained Cr-type stainless steel sheet was used for evaluation of hydrogen embrittlement and low temperature embrittlement. For hydrogen embrittlement resistance, commercially available 2 mm thick SUS316L steel sheets (17.5% Cr-12% Ni-2% Mo) and SUS316 steel sheets (17.5% Cr-10% Ni-2% Mo) were used for evaluation as comparative materials.

Evaluation of hydrogen embrittlement was performed in the following procedure.

A tensile test piece having a width of 4 mm and a length of 20 mm was prepared in parallel, and immediately before the tensile test in high-pressure hydrogen gas, the surface was polished with dry #600 sandpaper and then degreased and cleaned with an organic solvent. As shown in Table 1, the tensile test in high-pressure hydrogen gas was conducted 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. A comparative tensile test was performed in 0.1 MPa nitrogen at -40°C. The tensile strength in high-pressure hydrogen gas was divided by the tensile strength in 0.1 MPa nitrogen to obtain a relative tensile strength, and the elongation at break in high-pressure hydrogen gas was divided by the elongation at break in 0.1 MPa nitrogen to obtain a relative elongation. Hydrogen embrittlement resistance was evaluated using relative tensile strength and relative elongation as evaluation indexes. The evaluation criteria were made as follows. A and B were set as pass.

A: A relative tensile strength of 0.98 or more and a relative elongation of 0.85 or more are satisfied.

B: Other than the above, the relative tensile strength of 0.98 or more and the relative elongation of 0.75 or more are satisfied.

X: either or both of a relative tensile strength of less than 0.98 or a relative elongation of less than 0.75.

Here, in the case of a hydrogen gas pressure of 45 MPa and a test temperature of -40°C, the SUS316L steel sheet has a relative elongation of less than 0.75, and the evaluation becomes X. In addition, in the case of a hydrogen gas pressure of 20 MPa and a test temperature of -40°C, the SUS316 steel sheet has a relative elongation of less than 0.75, and the evaluation becomes X.

Evaluation of low-temperature brittleness was performed by the Charpy impact test based on JISZ2242. The test piece was made into the V-notch shape of 1.5-2.5 mm thickness x 10 mm width x 55 mm length, and the test temperature was made into the range of -100 degreeC - room temperature (20 degreeC). Low-temperature embrittlement resistance calculated|required said energy transition temperature from the absorbed energy measured by the Charpy test, and was set as the evaluation parameter|index. The evaluation criteria were made as follows. A and B were set as pass.

A: It satisfies the energy transition temperature of -40 degrees C or less.

B: The energy transition temperature exceeds -40°C and meets -10°C or less.

X: The energy transition temperature is higher than -10°C.

Table 2 summarizes the test results and shows them.

No. All of 1-11 are Cr-type stainless steel sheets which have the chemical composition and texture within the range of this invention, and hydrogen embrittlement resistance and low temperature embrittlement resistance were favorable. In particular, No. in the range of preferred components and texture. In 5, 6, 9, and 10, hydrogen embrittlement resistance index|index was "B" or "A" in the pressure of 45 MPa of hydrogen gas, and the hydrogen embrittlement resistance was a high position compared with SUS316L. Also, No. 6, 8, and 10 are those in which {211}±10° azimuth grains are reduced by using large-diameter rolls, with the same chemical composition and No. Compared to 5, 7, and 9, the hydrogen embrittlement resistance was further improved.

No. All of 12 to 20 are Cr-based stainless steel sheets that do not have a chemical composition within the range of the present invention, and cannot form a texture within the range of the present invention, and either or both of hydrogen embrittlement resistance or low temperature embrittlement resistance are inferior. Also, No. 17, 19 and 20, the area ratio of the {211}±10° orientation grains at the center of the plate thickness is less than 30%, but the area ratio in the plate surface exceeds 30%, and hydrogen embrittlement resistance and low temperature embrittlement resistance It turns out that it is important to control the area ratio in the plate|board surface in order to obtain all of them.

From the above evaluation results, the hydrogen embrittlement resistance of the Cr-based stainless steel sheet was higher than that of SUS316 on the market by having the components and texture within the range of the present invention. In addition, it was found that by having a preferable component and controlling it to a preferable texture using a large-diameter roll, the hydrogen embrittlement resistance surpassing that of SUS316L was obtained.

Claims (4)

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;
In addition, Nb: 0.5% or less, Ti: 0.5% or less of one or two types,
Sn: 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 consists of Fe and impurities, and the texture on the surface of the plate satisfies the following (i) and (ii), A Cr-based stainless steel sheet.
(i) The area ratio of crystal grains (hereinafter referred to as “{211}±10° orientation grains”) whose angle difference between the normal direction of the steel plate surface and the {211} plane orientation on the plate surface is within 10° is less than 30%
(ii) In the {211}±10° orientation grain defined in (i), the average length in the rolling direction and the length in the plate width direction is less than 0.15 mm The composition according to claim 1, further comprising Sn: 0.001 to 0.3% and B: 0.005% or less by mass%,
A Cr-based stainless steel sheet satisfying the following formula (1).

In the above formula, the element symbol means the content (mass %) of the element. The method according to claim 1 or 2, 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: 0.1% or less;
Hf: 0.1% or less;
REM: 0.1% or less
Cr-based stainless steel sheet, characterized in that it contains one or two or more of. The Cr-based stainless steel sheet according to any one of claims 1 to 3, which is used as a metal material for an apparatus for high-pressure hydrogen gas.