JP7618508B2 - Austenitic stainless steel materials and hydrogen equipment - Google Patents

Description

The present invention relates to austenitic stainless steel materials and hydrogen equipment.

In recent years, hydrogen energy has been attracting attention as a clean energy source that does not emit greenhouse gases such as carbon dioxide. To utilize hydrogen energy, there is a need to establish hydrogen-related technologies such as hydrogen production, storage, and transportation.

However, there are various problems in establishing hydrogen-related technologies. One of these is the problem of hydrogen embrittlement. Hydrogen energy uses hydrogen gas as a fuel source. For this reason, for example, when metal materials are used in related equipment such as hydrogen production equipment and storage equipment, the hydrogen gas causes the materials to become embrittled, resulting in the problem of so-called hydrogen embrittlement.

Austenitic stainless steel is one of the metallic materials used in the above-mentioned related equipment from the viewpoints of manufacturing cost, strength, and corrosion resistance. Therefore, in order to suppress hydrogen embrittlement, austenitic stainless steel with improved hydrogen embrittlement resistance has been developed.

For example, Patent Documents 1 and 2 disclose high-Mn austenitic stainless steels that are excellent in hydrogen embrittlement resistance at low temperatures and are economical. The austenitic stainless steels disclosed in Patent Documents 1 and 2 improve both economical efficiency and hydrogen embrittlement resistance by adjusting the chemical composition to a specified amount.

JP 2019-143227 A JP 2019-143228 A

In addition, in recent years, weather resistance has also been required for austenitic stainless steel materials used in hydrogen environments. This is because, as the environments in which they are used have become more diverse, it has become necessary to suppress deterioration in environments where material deterioration is likely to progress, such as outdoors. However, the austenitic stainless steel materials disclosed in Patent Documents 1 and 2 do not consider weather resistance at all. For this reason, there is room for improvement in the weather resistance of the above steel materials.

As such, it is difficult to achieve both hydrogen embrittlement resistance and weather resistance in austenitic stainless steel materials used as components and parts of hydrogen-related equipment.

The present invention aims to solve the above problems and provide an austenitic stainless steel material with improved hydrogen embrittlement resistance and weather resistance.

The present invention was made to solve the above problems, and the gist of the invention is the following austenitic stainless steel material.

(1) An austenitic stainless steel material having a passive film,
The chemical composition, in mass%, is
C: 0.20% or less,
Si: 2.0% or less,
Mn: 6.0-20.0%,
P: 0.060% or less,
S: 0.0080% or less,
Cr: 10.0-18.0%,
Ni: 4.0 to 12.0%,
N: 0.01-0.30%,
Cu: 4.0% or less,
Mo: 3.0% or less,
Al: 0-0.20%,
Ca: 0-0.01%,
B: 0-0.01%,
Mg: 0 to 0.01%,
Nb: 0 to 1.0%,
Ti: 0 to 1.0%,
V: 0 to 1.0%,
W: 0 to 2.0%,
Zr: 0 to 1.0%,
Co: 0-2.0%,
Ga: 0 to 0.10%,
Hf: 0-0.10%,
REM: 0-0.10%,
The balance is Fe and impurities.
The A value calculated by the following formula (i) is 30.0 to 60.0,
The austenitic stainless steel material, wherein the passive film satisfies the following formulas (ii) to (iv):
A value=3.2Mn+0.7Cr+6.2Ni+38.7N+4.8Cu+9.3Mo
-53 ... (i)
I _Fe /I _O ≦1.0 (ii)
0.3<I _Cr /I _O ≦2.0 (iii)
I _Mn /I _O <0.3...(iv)
Here, each element symbol in the above formula (i) represents the content (mass%) of each element contained in the steel, and when no element is contained, it is set to zero, and each symbol in the above formulas (ii) to (iv) represents the intensity of each element measured by glow discharge optical emission spectrometry, and is defined as follows:
I _Fe : Fe intensity (cps) at the depth position where the O intensity is half the O intensity at the surface
I _O : O intensity (cps) that is 1/2 of the O intensity on the surface
I _Cr : Cr intensity (cps) at the depth position where the O intensity is half that of the surface O intensity
I _Mn : Mn intensity (cps) at the depth position where the O intensity is half the O intensity at the surface

(2) Vickers hardness is 200 to 500 HV1;
In the crystal structure, 97% or more of the area ratio is an fcc structure.
The austenitic stainless steel material according to (1) above.

(3) The chemical composition is, in mass%,
Al: 0.01-0.20%,
Ca: 0.001-0.01%,
B: 0.0002-0.01%,
Mg: 0.0002-0.01%,
Nb: 0.01-1.0%,
Ti: 0.01 to 1.0%,
V: 0.01-1.0%,
W: 0.01-2.0%,
Zr: 0.01 to 1.0%,
Co: 0.01-2.0%,
Ga: 0.01-0.10%,
Hf: 0.01-0.10%, and REM: 0.01-0.10%,
The austenitic stainless steel material according to the above (1) or (2), comprising one or more selected from the following:

(4) An austenitic stainless steel material according to any one of (1) to (3) above, which is used in a high-pressure hydrogen gas environment.

(5) Hydrogen equipment including the austenitic stainless steel material described in any one of (1) to (4) above.

(6) The hydrogen equipment described in (5) above, wherein the hydrogen equipment is a tank body, a tank nozzle, a liner, piping, a valve, or a heat exchanger.

According to the present invention, it is possible to obtain an austenitic stainless steel material with improved hydrogen embrittlement resistance and weather resistance.

FIG. 1 is a schematic diagram showing an example of the formation state of a passive film.

The inventors conducted research to obtain an austenitic stainless steel material that has improved strength in addition to hydrogen embrittlement resistance and weather resistance, and obtained the following findings (a) to (d).

(a) In cold working during steel manufacturing, by promoting plastic deformation due to twinning, high strength can be obtained through work hardening and hydrogen embrittlement resistance can be improved. In order to generate plastic deformation due to twinning, it is effective to control the A value, which is an index of the state of plastic deformation in steel. This is because twinning deformation can be promoted by controlling the A value. On the other hand, if elements such as Ni, N, Cu, and Mo are contained in excess, plastic deformation due to twinning may be suppressed. Therefore, within the range of the chemical composition of the steel, it is effective to increase the content of Mn, which is economical. and then control the A value.

(b) Furthermore, in the cold working process used to manufacture the steel, it is desirable that the production of processing-induced martensite, which is susceptible to hydrogen embrittlement, is minimal, and that the steel has an austenitic single-phase metal structure (face-centered cubic: fcc). This prevents a decrease in hydrogen embrittlement resistance. In addition, from the standpoint of hydrogen embrittlement resistance, it is also desirable that the Vickers hardness be in the range of 200 to 500 HV1.

(c) As mentioned above, cold working is necessary to increase strength, but the passive film formed after cold working tends to have a higher ratio of Fe oxide and be thicker than when cold working is not performed. On the other hand, it is effective to reduce the ratio of Fe and Mn oxide and increase the ratio of Cr oxide near the interface between the passive film and the base material in order to improve weather resistance. For this reason, in order to control the oxides on the surface, it is preferable to remove the passive film after cold working and then repassivate the passive film.

(d) In order to perform the above-mentioned repassivation, it is desirable to mechanically polish or electrolytically polish the surface of the steel material after cold working. In particular, by controlling the conditions of electrolytic polishing, it is possible to reduce the ratio of Fe and Mn oxides and increase the ratio of Cr oxides on the surface of the steel material after cold working. As a result, it is possible to improve weather resistance.

One embodiment of the present invention has been made based on the above findings. Each requirement of this embodiment will be explained in detail below.

1. Chemical composition The reasons for limiting the content of each element are as follows. In the following description, "%" for the content means "mass %".

C: 0.20% or less C is an element effective in stabilizing the austenite phase and improves hydrogen embrittlement resistance. However, excessive C content promotes grain boundary precipitation of Cr-based carbides, reducing weather resistance and hydrogen embrittlement resistance after cold working. For this reason, the C content is set to 0.20% or less. The C content is preferably set to 0.15% or less, and more preferably set to 0.10% or less. On the other hand, in order to obtain the above effect, the C content is preferably set to 0.01% or more.

Si: 2.0% or less Si is an effective element for deoxidation, and contributes to improving hydrogen embrittlement resistance and strength through solid solution strengthening. However, excessive Si content promotes the formation of intermetallic compounds such as σ phase, and reduces cold workability. For this reason, the Si content is set to 2.0% or less. The Si content is preferably set to 1.0% or less. On the other hand, in order to obtain the above effect, the Si content is preferably set to 0.1% or more.

Mn: 6.0-20.0%
Mn is an element that stabilizes the austenite phase, promotes the cold working and the subsequent occurrence of deformation twins, and contributes to improving strength and hydrogen embrittlement resistance. Therefore, the Mn content is set to 6.0% or more. The Mn content is preferably set to 8.0% or more. However, However, excessive Mn content promotes the formation of the ε phase, which is highly susceptible to hydrogen embrittlement, and actually reduces hydrogen embrittlement resistance. For this reason, the Mn content is set to 20.0% or less. The content is preferably 15.0% or less, and more preferably 10.0% or less.

P: 0.060% or less P is an element contained in steel as an impurity, but it may become concentrated in the final solidification process, lower the melting point of the steel, and promote solidification cracking (hot cracking). For this reason, the P content is set to 0.060% or less. From the viewpoint of improving hot cracking resistance, the P content is preferably set to 0.030% or less, and more preferably set to 0.025% or less. On the other hand, excessive reduction of P leads to an increase in manufacturing costs, so the P content is preferably set to 0.005% or more.

S: 0.0080% or less S is an element contained in steel as an impurity, and like P, may promote solidification cracking (hot cracking). For this reason, the S content is set to 0.0080% or less. The S content is preferably set to 0.0030% or less, and more preferably set to 0.0020% or less. However, excessive reduction of S increases the manufacturing cost. For this reason, the S content is preferably set to 0.0001% or more.

Cr:10.0~18.0%
Cr is an element necessary for improving the corrosion resistance of stainless steel. Therefore, the Cr content is set to 10.0% or more. The Cr content is preferably set to 13.0% or more. From the viewpoint of weather resistance, the Cr content is preferably 15.0% or more. However, Cr is a ferrite forming element. Therefore, if Cr is contained in excess, the austenite phase is insufficient. It stabilizes the alloy and reduces hydrogen embrittlement resistance. It also reduces cold workability. For this reason, the Cr content is set to 18.0% or less. The Cr content is set to 17.0% or less. It is preferable to set the above.

Ni: 4.0-12.0%
Ni, like Mn, stabilizes the austenite phase and promotes the cold working and the subsequent occurrence of deformation twins, thereby increasing strength and improving hydrogen embrittlement resistance. The Ni content also improves weather resistance. For this reason, the Ni content is set to 4.0% or more. The Ni content is preferably set to 6.0% or more, and more preferably set to 7.0% or more. However, excessive Ni content suppresses deformation twins after cold working, resulting in a decrease in strength and hydrogen embrittlement resistance. For this reason, the Ni content is set to 12.0% or less. The Ni content is preferably 10.0% or less, and more preferably 9.0% or less.

N: 0.01-0.30%
Like Mn and Ni, N stabilizes the austenite phase and promotes the cold working and the subsequent occurrence of deformation twins, thereby increasing strength and improving hydrogen embrittlement resistance. In order to improve the pitting corrosion resistance and weather resistance, the N content is set to 0.01% or more. In order to positively utilize the effects of N, the N content is set to 0.10% or more. However, if N is contained in excess, internal defects such as blowholes may occur, and hot workability, cold workability, and hydrogen embrittlement resistance are deteriorated. Therefore, the N content is set to 0.30% or less, and preferably to 0.25% or less.

Cu: 4.0% or less Cu is an element mixed in from raw materials such as scrap, and stabilizes the austenite phase to improve hydrogen embrittlement resistance. Cu also improves weather resistance. On the other hand, excessive Cu content not only reduces manufacturability, but also suppresses the formation of deformation twins after cold working, reducing strength and hydrogen embrittlement resistance. For this reason, the Cu content is set to 4.0% or less. The Cu content is preferably set to 3.0% or less, and more preferably set to 1.5% or less. However, excessive reduction of Cu leads to restrictions on the melting raw materials and increases manufacturing costs. For this reason, the Cu content is preferably set to 0.01% or more.

Mo: 3.0% or less Mo is an element that is mixed in from raw materials such as scrap and is effective in hydrogen embrittlement resistance. On the other hand, excessive Mo content promotes the formation of δ ferrite phase and reduces manufacturability. It also suppresses the formation of deformation twins after cold working, reducing strength and hydrogen embrittlement resistance. For this reason, the Mo content is 3.0% or less. It is preferable that the Mo content is 2.0% or less. However, excessive reduction of Mo leads to restrictions on melting raw materials and increases manufacturing costs. For this reason, the Mo content is preferably 0.01% or more.

In addition to the above elements, one or more elements selected from Al, Ca, B, Mg, Nb, Ti, V, W, Zr, Co, Ga, Hf, and REM may be contained within the ranges shown below. The reasons for limiting each element are explained below.

Al: 0-0.20%
In addition to being an effective deoxidizing element, Al has the effect of suppressing the grain boundary segregation of low melting point elements and strengthening the grain boundaries. As a result, manufacturability and the like are improved, and cold workability is improved. Therefore, Al may be added as necessary. However, excessive Al content causes AlN to precipitate, which actually reduces manufacturability. It also reduces the solubility of N and reduces hydrogen resistance. Therefore, the Al content is set to 0.20% or less. The Al content is preferably set to 0.08% or less. On the other hand, in order to obtain the above effect, the Al content is preferably set to 0.08% or less. The amount is preferably at least 0.01%.

Ca: 0-0.01%
Ca has the effect of suppressing the grain boundary segregation of low melting point elements and strengthening the grain boundaries. As a result, manufacturability and cold workability are improved. For this reason, it is added as necessary. However, if an excessive amount of Ca is contained, inclusions are formed, which in turn reduces manufacturability and corrosion resistance. Therefore, the Ca content is set to 0.01% or less. To this end, the Ca content is preferably 0.001% or more.

B: 0 to 0.01%
B strengthens grain boundaries, improves strength, and improves hot workability and cold workability. Therefore, it may be contained as necessary. However, excessive B content promotes grain boundary precipitation of boron compounds (BN, BC, Cr ₂ B), reducing workability and corrosion resistance. Therefore, the B content is set to 0.01% or less. On the other hand, in order to obtain the above effects, the B content is preferably set to 0.0002% or more.

Mg: 0-0.01%
Mg is an element that has a deoxidizing effect and has the effect of improving manufacturability. Therefore, Mg may be contained as necessary. However, if Mg is contained in excess, manufacturability may be deteriorated during refining, etc. Therefore, the Mg content is set to 0.01% or less. On the other hand, in order to obtain the above effects, the Mg content is preferably set to 0.0002% or more. , and more preferably, 0.0005% or more.

Nb: 0 to 1.0%
Nb has the effect of forming carbonitrides, refining crystal grains, and strengthening grain boundaries. As a result, it contributes to improving strength. Therefore, it may be contained as necessary. However, if Nb is contained in excess, the solid solution N concentration decreases, which leads to a decrease in hydrogen embrittlement resistance and weather resistance, as well as a decrease in hot workability and cold workability. Therefore, the Nb content is set to 1.0% or less. The Nb content is preferably set to 0.50% or less. On the other hand, in order to obtain the above effect, the Nb content is preferably set to 0.01% or more.

Ti: 0 to 1.0%
Ti has the effect of forming carbonitrides, refining crystal grains, and strengthening grain boundaries. As a result, it has the effect of improving strength. For this reason, it may be contained as necessary. However, if Ti is contained in excess, the solid solution N concentration decreases, and hydrogen embrittlement resistance and weather resistance decrease, as well as hot workability and cold workability decrease. Therefore, the Ti content is set to 1.0% or less. The Ti content is preferably set to 0.50% or less. On the other hand, in order to obtain the above effect, the Ti content is preferably set to 0.01% or more.

V: 0 to 1.0%
V has the effect of improving strength by dissolving in steel or precipitating as carbonitride. Therefore, it may be contained as necessary. However, if an excessive amount of V is contained, an excessive amount of carbonitride is formed, and hot workability and cold workability are deteriorated. Therefore, the V content is set to 1.0% or less. On the other hand, in order to obtain the above effect, the V content is preferably set to 0.01% or more.

W: 0 to 2.0%
W has the effect of improving strength and weather resistance. Therefore, it may be contained as necessary. However, if an excessive amount of W is contained, the manufacturability and raw material cost increase, so the W content is set to 2.0% or less. On the other hand, in order to obtain the above effect, the W content is preferably set to 0.01% or more.

Zr: 0 to 1.0%
Zr has a deoxidizing effect. It also has an effect of improving weather resistance. Therefore, it may be contained as necessary. However, if Zr is contained in excess, toughness and workability decrease. Therefore, the Zr content is set to 1.0% or less. The Zr content is preferably set to 0.50% or less. On the other hand, in order to obtain the above effect, the Zr content is preferably set to 0.01% or more.

Co: 0 to 2.0%
Co has the effect of improving corrosion resistance and stabilizing the austenite phase. Therefore, it may be contained as necessary. However, since excessive Co content increases the manufacturing cost, the Co content is set to 2.0% or less. On the other hand, in order to obtain the above effect, the Co content is preferably set to 0.01% or more.

Ga: 0-0.10%
Ga has the effect of improving hot workability. Therefore, it may be contained as necessary. However, if Ga is contained in excess, manufacturability decreases. Therefore, the Ga content is On the other hand, in order to obtain the above-mentioned effects, the Ga content is preferably 0.01% or more.

Hf: 0-0.10%
Hf has a deoxidizing effect and an effect of improving weldability, so it may be contained as necessary. However, if Hf is contained in excess, manufacturability such as refining decreases. Therefore, the Hf content is set to 0.10% or less. On the other hand, in order to obtain the above effects, the Hf content is preferably set to 0.01% or more.

REM: 0~0.10%
REM has a deoxidizing effect and is effective in improving manufacturability. It also has an effect of improving corrosion resistance. For this reason, it may be contained as necessary. However, excessive REM content When the REM content is too low, not only does the effect saturate, but manufacturability, such as refining, decreases. Therefore, the REM content is set to 0.10% or less. On the other hand, in order to obtain the above effect, the REM content is set to It is preferable that the content is 0.01% or more.

REM refers to a total of 17 elements, including Sc, Y, and lanthanides, and the REM content above refers to the total content of these elements. In industry, REM is often added in the form of misch metal.

In the chemical composition of this embodiment, the balance is Fe and impurities. Here, "impurities" refer to components that are mixed in due to various factors in the raw materials, such as ores and scraps, and manufacturing processes when industrially manufacturing austenitic stainless steel materials, and are acceptable within the range that does not adversely affect this embodiment.

In the chemical composition of this embodiment, the A value, which is an index of the stability of the austenite phase and an index of the plastic deformation mechanism, is set to 30.0 to 60.0. The A value is calculated by the following formula (i).

A value=3.2Mn+0.7Cr+6.2Ni+38.7N+4.8Cu+9.3Mo
-53 ... (i)
In the above formula (i), each element symbol represents the content (mass%) of each element contained in the steel, and when the element is not contained, it is set to zero.

If the A value is less than 30.0, the stability of the austenite phase is low and deformation-induced martensite is formed. This results in an increase in crystal grains that are not only face-centered cubic (fcc) but also body-centered cubic (bcc), reducing hydrogen embrittlement resistance. Furthermore, deformation-induced martensite with a high amount of strain acts as a starting point for corrosion, reducing weather resistance. For this reason, the A value must be 30.0 or more.

However, if the A value exceeds 60.0, the appearance of deformation twins after cold working is suppressed, leading to a decrease in strength and elongation in a high-pressure hydrogen gas environment. As a result, hydrogen embrittlement resistance decreases. In addition, raw material costs increase, leading to a decrease in manufacturability. For this reason, the A value is set to 60.0 or less. From the viewpoints of strength after cold working, hydrogen embrittlement resistance, economic efficiency, and manufacturability, it is preferable that the A value be in the range of 35.0 to 50.0.

2. Passive Film In the austenitic stainless steel material according to this embodiment, the ratios of Fe oxide, Cr oxide, and Mn oxide in the passive film are set to the following ranges in order to improve weather resistance.

The passive film satisfies the following formulas (ii) to (iv).
I _Fe /I _O ≦1.0 (ii)
0.3<I _Cr /I _O ≦2.0 (iii)
I _Mn /I _O <0.3...(iv)
In the above formulas (ii) to (iv), each symbol represents the intensity of each element measured by glow discharge optical emission spectrometry, and is defined as follows:

I _Fe : Fe intensity (cps) at the depth position where the O intensity is half the O intensity at the surface
I _O : O intensity (cps) that is 1/2 of the O intensity on the surface
I _Cr : Cr intensity (cps) at the depth position where the O intensity is half that of the surface O intensity
I _Mn : Mn intensity (cps) at the depth position where the O intensity is half the O intensity at the surface

Here, in the austenitic stainless steel material according to this embodiment, it is desirable to lower the abundance ratio of Fe oxide and Mn oxide in the passive film, while increasing the abundance ratio of Cr oxide. The passive film is formed on the surface of the base material as shown in FIG. 1. In GDS, the distribution of each element is examined in the depth direction from the surface. The concentration distribution of each element varies, but for example, the O concentration decreases as it moves from the surface to the base material in the depth direction. The passive film is the region up to the depth position where the intensity of O at the surface, i.e., the maximum intensity of O, is 1/2. In other words, the interface between the passive film and the base material (base steel sheet) is the position where the intensity of O is 1/2 of the intensity of O at the surface.

The austenitic stainless steel material of this embodiment must satisfy the above formulas (ii) to (iv).

If the value on the left side of equation (ii) exceeds 1.0, the amount of Fe oxide in the passive film becomes excessive, and weather resistance decreases. For this reason, the value on the left side of equation (ii) is set to 1.0 or less. There is no particular lower limit for the value on the left side of equation (ii), but from the viewpoints of hydrogen embrittlement resistance and manufacturability, it is preferably set to 0.3, and more preferably to 0.5.

Furthermore, if the side value in formula (iii) is 0.3 or less, the ratio of Cr oxides is small, and weather resistance is reduced. On the other hand, it is difficult for the side value in formula (iii) to exceed 2.0 in the steel material with the chemical composition of this embodiment. Furthermore, it becomes necessary to contain an excess of Cr, which not only requires surface treatment but also leads to reduced hydrogen embrittlement resistance and strength. For this reason, the side value in formula (iii) is set to 2.0 or less. The side value in formula (iii) is preferably in the range of 0.3 to 1.5, and more preferably in the range of 0.5 to 1.0.

Furthermore, if the value of the left side of equation (iv) is 0.3 or more, the amount of Mn oxide in the passive film becomes excessive, and weather resistance decreases. For this reason, the value of the left side of equation (iv) is set to less than 0.3. There is no particular lower limit for the value of the left side of equation (iv), but from the viewpoints of hydrogen embrittlement resistance and manufacturability, it is preferably set to 0.05, and more preferably to 0.1.

Here, the intensity of each of the above elements can be measured by glow discharge optical emission spectrometry (hereinafter referred to as "GDS"). The measurement conditions for GDS are, for example, a pulse sputtering mode with an output of 20 W and a pressure of 600 Pa, and a sputtering depth of 0.1 μm. By using the pulse sputtering mode of GDS, the measurement accuracy of O, Fe, Cr, and Mn in the passive film with a thickness of 0.001 to 0.01 μm can be maintained.

3. Vickers Hardness The Vickers hardness of the austenitic stainless steel material in this embodiment becomes hard due to work hardening. For this reason, the Vickers hardness is preferably set to 200 to 500 HV1. If the Vickers hardness is less than 200 HV1, it becomes difficult to obtain the desired strength of 800 MPa or more. For this reason, the Vickers hardness is preferably set to 200 HV1 or more. On the other hand, if the Vickers hardness exceeds 500 HV1, the tensile strength tends to exceed 1700 MPa, and the elongation of the material itself is greatly reduced. As a result, it becomes difficult to maintain the elongation in a high-pressure hydrogen gas environment.

In addition, when a processing-induced martensite phase (body-centered cubic: bcc) is generated during cold working and subsequent plastic deformation, and the proportion of austenite phase (face-centered cubic: fcc) decreases, this leads to a decrease in hydrogen embrittlement resistance. For this reason, it is preferable for the Vickers hardness to be 500 HV1 or less. From the standpoint of high strength and hydrogen embrittlement resistance, it is more preferable for the Vickers hardness to be in the range of 250 to 450 HV1.

The Vickers hardness should be measured in accordance with JIS Z 2244:2020 under the following conditions. Specifically, a cross-sectional resin-embedded sample is prepared, and a load of 1 kg (9.8 N) is applied near t/2, and the Vickers hardness is measured using a Vickers hardness tester. The test force should be held for 20 seconds.

4. Crystal structure In the austenitic stainless steel material according to the present embodiment, it is desirable to suppress the generation of processing-induced martensite after processing in order to suppress the deterioration of hydrogen embrittlement resistance. For this reason, it is preferable that 97% or more of the crystal structure is an fcc structure in terms of area ratio.

If the area ratio of crystal grains with an fcc structure is less than 97%, a large amount of δ-ferrite and processing-induced martensite with a bcc structure is formed, which is undesirable from the viewpoint of hydrogen embrittlement resistance. For this reason, it is preferable that the area ratio is 97% or more of the fcc structure. Note that some grains with a bcc structure, such as δ-ferrite, may be unavoidably formed, but even in such cases, if the area ratio of crystal grains with a bcc structure is less than 3%, it is acceptable because the effect on hydrogen embrittlement resistance is small.

The crystal structure can be measured by the following procedure. Specifically, X-ray diffraction is performed on the measurement surface of the EBSP described above, and the area ratio of fcc structure grains and bcc structure grains is calculated. The conditions for X-ray diffraction are CuKα radiation, an applied voltage of 40 kV, and 2θ in the range of 30°≦2θ≦90°.

The austenitic stainless steel material according to this embodiment is preferably used in hydrogen equipment that requires light weight and compactness due to high strength in a high pressure hydrogen environment. Examples of hydrogen equipment include the main body of a tank that stores hydrogen gas and liquid hydrogen, tank nozzles, liners, piping that serves as a hydrogen gas flow path, valves, heat exchangers, etc.

6. Manufacturing method A preferred manufacturing method for the austenitic stainless steel material according to this embodiment will be described. The austenitic stainless steel material according to this embodiment can obtain the effects described above regardless of the manufacturing method, but can be stably manufactured, for example, by the following manufacturing method.

For the sake of simplicity, in the following explanation, the shape of the steel material will be described as a steel plate, but the shape of the steel material is not particularly limited. For example, the steel material may be in the shape of a plate, a rod, or a tube. A processing method appropriate for each shape can be selected.

The steel adjusted to the above-mentioned chemical composition is melted and cast by conventional methods to obtain a slab for hot rolling. Then, hot rolling is performed by conventional methods. There are no particular restrictions on the conditions during hot rolling, but it is usually preferable that the heating temperature of the slab is 1050 to 1250°C and the rolling reduction is in the range of 20 to 99%. After hot rolling, annealing and pickling may be performed as necessary. There are no particular restrictions on the annealing temperature in this case, but it may be in the range of 1050 to 1100°C, for example.

Next, cold rolling and annealing are performed. The annealing immediately prior to the final cold rolling is called solution treatment, and is also performed in the same manner. Pickling is also performed as necessary. Cold rolling is performed, for example, at a rolling reduction rate of 20 to 90%, and the subsequent annealing is preferably performed by isothermal holding at 1000 to 1150°C for 1 to 600 seconds. Cold rolling, annealing, and pickling may be repeated multiple times to obtain steel of the final required thickness.

Solution treatment is performed immediately before the final finish cold rolling for work hardening. In solution treatment, heat treatment is preferably performed at 1000 to 1150°C and isothermal holding for 1 to 600 seconds. If the solution treatment temperature is less than 1000°C, recrystallization will be insufficient and twin deformation will not be sufficient during finish cold rolling. For this reason, the solution treatment temperature is preferably 1000°C or higher. On the other hand, if the solution treatment temperature exceeds 1150°C, the crystal grains will become coarse (more than 0.1 mm) and the strength will decrease. For this reason, the solution treatment temperature is preferably 1150°C or lower.

Finally, final cold rolling is performed to produce work-hardened steel. The desired strength can be obtained by performing final cold rolling after solution treatment or after the subsequent sub-zero treatment. For this reason, it is preferable to perform final cold rolling with a rolling reduction ratio in the range of 10 to 80% after solution treatment. If the rolling reduction ratio is less than 10%, sufficient hardness and strength cannot be obtained. For this reason, it is preferable for the final cold rolling reduction ratio to be 10% or more.

On the other hand, if the final cold rolling reduction exceeds 80%, the strength increases too much and hydrogen embrittlement resistance decreases. For this reason, the final cold rolling reduction is preferably 80% or less. In order to achieve a hardness in the range of 250 to 450 HV1, the final cold rolling reduction is preferably in the range of 20 to 70%. Note that, although a plate-shaped steel plate has been used as an example, for example, in the case of a rod-shaped or tubular steel material, the reduction in area may be in the range of 10 to 80%. In other words, the cold working rate may be adjusted to be in the range of 10 to 80%. In addition, the above range of conditions and other conditions may be adjusted to control the metal structure to be an austenitic stainless steel.

After the final cold rolling, it is desirable to remove as much Fe and Mn oxide as possible from the surface to form a passive film with a high ratio of Cr oxide. This is because weather resistance is improved as a result. In order to form such a passive film, it is preferable to mechanically polish or electrolytically polish the surface of the steel material. To further improve weather resistance, electrolytic polishing is preferable. The surface roughness after the above-mentioned polishing is preferably 0.1 μm or less in average roughness Ra.

Polishing is preferably performed from the surface to a depth of 10 μm in the direction perpendicular to the plate thickness. Mechanical polishing may be performed using abrasive paper or abrasive stones, or a solid abrasive may also be used. In addition, electrolytic polishing may be performed using a commercially available electrolytic polishing solution. Electrolytic polishing is preferably performed at a temperature of 40 to 60°C.

If the electrolytic polishing temperature is less than 40°C, dissolution is difficult to proceed, the ratio of Cr oxide in the passive film is low, and the formula (iii) and/or (iv) may not be satisfied. As a result, weather resistance is reduced. For this reason, the electrolytic polishing temperature is preferably 40°C or higher. On the other hand, if the electrolytic polishing temperature exceeds 60°C, excessive Fe oxide is generated in the passive film, and the formula (ii) and/or (iii) may not be satisfied. For this reason, the electrolytic polishing temperature is preferably 60°C or lower. Other conditions include a current density of 1 to 20 dm ² and a time of 1 to 10 minutes.

In order to control the amount of oxide in the passive film within an optimal range and reduce surface roughness, it is more preferable to carry out electrolytic polishing at 40 to 50°C, with a current density of 1 to 10 ^dm2 and a time period of 5 to 10 minutes. After electrolytic polishing, it is preferable to form a passive film by washing with water and drying at 120°C or less. Specifically, the steel material after washing with water is preferably inserted into a drying furnace at 50 to 100°C.

Through these steps, the austenitic stainless steel material according to this embodiment can be obtained.

The following provides a more detailed explanation of the austenitic stainless steel material according to this embodiment, but the present embodiment is not limited to these examples.

A slab with the composition shown in Table 1 was melted, heated to 1200°C, and then hot rolled to a thickness of 5.0 mm. The hot rolled sheet was then annealed and pickled at 1050-1100°C, and cold rolled to a thickness of 2 mm to produce a cold rolled sheet. The cold rolled sheet was then solution-treated by heat treatment at 1050-1100°C for 30 seconds, and then pickled. The resulting steel sheet was subjected to final cold rolling in the range of 10-80% to a thickness of 0.4-1.8 mm. The surface was then finished by mechanical polishing and electrolytic polishing to prepare the test material.

The mechanical polishing was wet polishing using a grindstone of #320. The electrolytic polishing was performed under different conditions. Electrolysis 1 in Table 2 indicates that electrolytic polishing was performed using a commercially available electrolytic polishing solution S-CLEAN EP at 40 to 50°C, a current density of 3 to 5 ^dm2 , and 5 minutes. Electrolysis 2 indicates that electrolytic polishing was performed using a similar electrolytic polishing solution at 60°C, a current density of 3 to 5 ^dm2 , and 1 minute. Electrolysis 3 indicates that electrolytic polishing was performed using a similar electrolytic polishing solution at 30°C, a current density of 3 to 5 ^dm2 , and 10 minutes. After washing with water, the material was inserted into a drying furnace at 80°C for 30 minutes. For comparison, a cold-rolled material without polishing was also evaluated.

The strength, hardness, and crystal structure of each element in the passive film of the obtained test materials were examined, and their resistance to hydrogen embrittlement was evaluated, using the following procedure.

(Strength of each element in the passive film)
The test material was subjected to elemental analysis from the surface to the depth direction by GDS to determine the intensities of O, Fe, Cr, and Mn. The GDS conditions were the above-mentioned pulse sputtering mode with an output of 20 W and a pressure of 600 Pa, and the sputtering depth was 0.1 μm.

(Hardness measurement)
In addition, a resin-embedded specimen was prepared for the cross section of the test material, and a load of 1 kg (9.8 N) was applied near t/2, and a hardness test was performed in accordance with JIS Z 2244: 2020. Specifically, a resin-embedded specimen was prepared for the cross section, and a load of 1 kg (9.8 N) was applied near t/2, and a Vickers hardness tester was used to perform a hardness test with a test force holding time of 20 seconds.

(Identification of crystal structure)
Regarding the crystal structure, X-ray diffraction was performed on the measurement surface of the EBSP, and the area ratio of the fcc structure grains and the bcc structure grains was calculated. The conditions of the X-ray diffraction were CuKα radiation, an applied voltage of 40 kv, and 2θ in the range of 30°≦2θ≦90°. In the results in Table 2, the area ratio of bcc was more than 3%, and it was described as "present".

(Evaluation of Hydrogen Embrittlement Resistance)
Hydrogen embrittlement resistance was measured by the following procedure. A tensile test piece with a parallel portion of 4 mm width x 20 mm length was taken. Next, the tensile test piece was subjected to a slow strain rate tensile test (hereinafter referred to as "SSRT test") at a strain rate of 10 ^-5 /s at -40°C, in 70 MPa hydrogen and 0.1 MPa nitrogen. The SSRT test was evaluated by measuring the tensile breaking strength and tensile breaking elongation. Hydrogen embrittlement resistance was evaluated using a value calculated by the following formula.
Hydrogen embrittlement resistance evaluation value={(tensile breaking strength or breaking elongation in 70 MPa hydrogen)/(tensile breaking strength or breaking elongation in 0.1 MPa nitrogen)}×100(%) (a)

When the hydrogen embrittlement resistance evaluation value of the tensile breaking strength calculated from the above formula is 95% or more and the hydrogen embrittlement resistance evaluation value of the tensile breaking elongation is 85% or more, it is deemed to have good hydrogen embrittlement resistance and is marked with "Good". On the other hand, when the hydrogen embrittlement resistance evaluation value is less than the above values, it is deemed to have poor hydrogen embrittlement resistance and is marked with "Poor".

(Weather resistance)
Weather resistance was measured by the following procedure. A test piece measuring 50 mm wide x 100 mm long was taken. The test piece was then subjected to a combined cycle test (JASO M609) according to the Japan Automotive Engineering Society standard. One cycle consisted of spraying a 5% NaCl aqueous solution (35°C, 2 h), drying (60°C, 4 h), and wetting (50°C, 95% RH, 2 h), and the SARN was measured from the appearance after 30 cycles.

When the SARN was equivalent to that of SUS304 (18Cr-8Ni), it was deemed to have good weather resistance and marked with a "◯". Furthermore, when the SARN was higher than that of SUS304, it was deemed to have particularly excellent weather resistance and marked with a "◎". On the other hand, when the SARN was lower than that of SUS304, it was marked with a "△", and when the SARN was the lowest or lower, it was deemed to have poor weather resistance and marked with an "×". The results are summarized below in Table 2.

Test materials No. 2 to 11, which satisfy the chemical composition, A value, and passive film requirements of this embodiment, had the desired hydrogen embrittlement resistance and weather resistance. In particular, No. 3, 4, 6, 8, 10, and 11 had the preferred strength ranges of Fe, Cr, and Mn oxides in the passive film, and were rated as excellent in weather resistance, rated ◎. In addition, compared to No. 2, 7, and 9, No. 3, 8, and 10 had a higher ratio of Cr oxides through electrolytic polishing, and similarly had excellent weather resistance.

On the other hand, Nos. 1 and 12 to 19 did not meet any of the requirements of this embodiment for chemical composition, A value, and passive film, and both or either of hydrogen embrittlement resistance and weather resistance were reduced. No. 1 was not polished after cold working and had slightly poor weather resistance, earning a △ rating. Nos. 12 to 19 had components and A value outside the specified range, and hydrogen embrittlement resistance was reduced. In addition, Nos. 20 and 21 had electrolytic polishing conditions outside the preferred range, so at least one of formulas (i) to (iii) did not meet the requirements of this embodiment, and weather resistance was reduced.

The austenitic stainless steel material according to this embodiment has high strength, good resistance to hydrogen embrittlement, and good weather resistance. For this reason, it is suitable for steel plates, bars, and tubes that are cold worked and used in a high-pressure hydrogen gas environment, and is preferably applied to hydrogen equipment that requires weather resistance in addition to being lightweight and compact. The austenitic stainless steel material according to this embodiment contributes to the thinning and weight reduction of hydrogen equipment and parts through the high strength achieved by cold working and the subsequent formation of a passive film.

1. Passive film 2. Base material

Claims (6)

An austenitic stainless steel material having a passive film,
The chemical composition, in mass%, is
C: 0.20% or less,
Si: 2.0% or less,
Mn: 6.0 to 20.0%,
P: 0.060% or less,
S: 0.0080% or less,
Cr: 10.0-18.0%,
Ni: 4.0 to 12.0%,
N: 0.01-0.30%,
Cu: 4.0% or less,
Mo: 3.0% or less,
Al: 0-0.20%,
Ca: 0-0.01%,
B: 0-0.01%,
Mg: 0 to 0.01%,
Nb: 0 to 1.0%,
Ti: 0 to 1.0%,
V: 0 to 1.0%,
W: 0 to 2.0%,
Zr: 0 to 1.0%,
Co: 0-2.0%,
Ga: 0 to 0.10%,
Hf: 0-0.10%,
REM: 0-0.10%,
The balance is Fe and impurities.
The A value calculated by the following formula (i) is 30.0 to 60.0,
The austenitic stainless steel material, wherein the passive film satisfies the following formulas (ii) to (iv):
A value=3.2Mn+0.7Cr+6.2Ni+38.7N+4.8Cu+9.3Mo
-53 ... (i)
I _Fe /I _O ≦1.0 (ii)
0.3<I _Cr /I _O ≦2.0 (iii)
I _Mn /I _O <0.3...(iv)
Here, each element symbol in the above formula (i) represents the content (mass%) of each element contained in the steel, and when no element is contained, it is set to zero, and each symbol in the above formulas (ii) to (iv) represents the intensity of each element measured by glow discharge optical emission spectrometry, and is defined as follows:
I _Fe : Fe intensity (cps) at the depth position where the O intensity is half the O intensity at the surface
I _O : O intensity (cps) that is 1/2 of the O intensity on the surface
I _Cr : Cr intensity (cps) at the depth position where the O intensity is half that of the surface O intensity
I _Mn : Mn intensity (cps) at the depth position where the O intensity is half the O intensity at the surface Vickers hardness is 200 to 500 HV1,
In the crystal structure, 97% or more of the area ratio is an fcc structure.
The austenitic stainless steel material according to claim 1. The chemical composition, in mass%,
Al: 0.01-0.20%,
Ca: 0.001-0.01%,
B: 0.0002-0.01%,
Mg: 0.0002-0.01%,
Nb: 0.01-1.0%,
Ti: 0.01 to 1.0%,
V: 0.01-1.0%,
W: 0.01-2.0%,
Zr: 0.01 to 1.0%,
Co: 0.01-2.0%,
Ga: 0.01-0.10%,
Hf: 0.01-0.10%, and REM: 0.01-0.10%,
The austenitic stainless steel material according to claim 1 or 2, comprising one or more selected from the following: The austenitic stainless steel material according to any one of claims 1 to 3, which is used in a high-pressure hydrogen gas environment. Hydrogen equipment comprising the austenitic stainless steel material according to any one of claims 1 to 4. 6. The hydrogen equipment according to claim 5, wherein the hydrogen equipment is a tank body, a tank cap, a liner, a pipe, a valve, or a heat exchanger.