Stainless steel material and method for manufacturing the same

Abstract

To provide stainless steel material having excellent corrosion resistance and electrical conductivity, and a method for manufacturing the same.SOLUTION: A method for manufacturing stainless steel material comprises the following step in the undermentioned order: a first heating step in which ferritic stainless steel material containing, by mass%, 16% or more and 30% or less of Cr, and 0.03% or more and 1% or less of Ti is used as raw material, and the raw material is heated to a predetermined temperature T1 under vacuum; a second heating step in which after the raw material is heated to the predetermined temperature T1, nitrogen gas is introduced and heating the raw material to a predetermined temperature T2 under nitrogen gas atmosphere is continued, and the raw material is held at the predetermined temperature T2 for a predetermined time; and a quenching step of quenching the raw material. Thereby, the raw material is formed into stainless steel material which has austenite phase structure, and has such structure that an austenite phase and a titanium nitride phase are distributed filmily at least on a surface layer, and the stainless steel material is obtained which has both excellent corrosion resistance under high-temperature severe corrosive environment, and excellent electrical conductivity equivalent to a gold plating film.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a stainless steel material and a method for manufacturing the same, which is suitable, for example, for use as a separator in a polymer electrolyte fuel cell, and more particularly to improvements in corrosion resistance and electrical conductivity. [Background technology]

[0002] Austenitic stainless steel materials possess properties that are quite satisfactory in terms of strength and workability, but there was room for improvement in terms of corrosion resistance. Stainless steel has high corrosion resistance due to the passive film on its surface, but if water contains more than a certain amount of Cl - When ions (chloride ions) are present, the passive film is locally destroyed, and depending on the conditions, localized corrosion such as pitting, crevice corrosion, and stress corrosion cracking occurs. Localized corrosion is a corrosion phenomenon unique to stainless steel, and corrosion resistance varies depending on the type of stainless steel. Generally, austenitic stainless steel has a certain level of corrosion resistance, but the aforementioned corrosion occurs in corrosive environments. Nitrogen is an austenite-stabilizing element and is also known to increase the pitting index, which is an indicator of the corrosion resistance of stainless steel. Adding nitrogen to ferritic stainless steel is expected to austenitize it and improve its corrosion resistance. In response to these issues, a method for manufacturing stainless steel products using nitrogen absorption treatment has been proposed. For example, Patent Document 1 describes a method for manufacturing stainless steel products, in which a bulk ferritic stainless steel product processed into a desired shape is brought into contact with an inert gas containing nitrogen gas at 800°C or higher, causing the entire product or a portion of it to austenitize, thereby forming a two-phase structure of ferrite and austenite. According to the method for manufacturing stainless steel products described in Patent Document 1, it is possible to keep the processing cost low for stainless steel products that have satisfactory properties in terms of strength and corrosion resistance.

[0003] Patent Document 2 describes a method for manufacturing stainless steel products that do not contain nickel, using ferritic stainless steel containing 18-24% Cr and 0-4% Mo by mass as the raw material and performing a nitrogen absorption treatment. According to the method for manufacturing stainless steel products described in Patent Document 2, stainless steel products with excellent workability and corrosion resistance can be obtained.

[0004] Patent Document 3 describes a separator for a polymer electrolyte fuel cell using stainless steel as the base material. In the technology described in Patent Document 3, the stainless steel used as the base material is made austenitic by increasing the nitrogen content during the melting stage to improve acid resistance, and a high-clean stainless steel is produced by reducing nonmetallic compounds through appropriate combined deoxidation of Al and Ca to reduce oxygen content and sufficient desulfurization treatment to reduce sulfur content. This improves plasticity and provides corrosion resistance and acid resistance that can withstand the operating environment of a polymer electrolyte fuel cell separator.

[0005] Patent Document 4 describes a method for manufacturing a cooking container, comprising the steps of: heating and maintaining a substrate made of ferritic stainless steel in a nitrogen-containing atmosphere at 1000°C to 1250°C to form an austenite phase on the surface of the substrate; cooling the heated substrate at a cooling rate of 1°C / second or more; and processing the cooled substrate into a predetermined shape to produce a container body. According to the technology described in Patent Document 4, an austenite phase, which is a metal phase with higher corrosion resistance than the inner layer of the substrate, is formed on the surface of a substrate made of ferritic stainless steel, resulting in a cooking container with excellent corrosion resistance.

[0006] Patent Document 5 describes a method for producing nickel-free austenitic stainless steel, which includes the steps of heating ferritic stainless steel in a heating furnace in a nitrogen gas atmosphere at 1100 to 1250°C to allow the ferritic stainless steel to absorb nitrogen, and then rapidly cooling the nitrogen-absorbed material to austenitize part or all of the ferritic stainless steel. According to the technology described in Patent Document 5, stainless steel with high corrosion resistance and durability can be produced, and is effective for use as a separator in polymer electrolyte fuel cells.

[0007] Patent documents 6, 7, and 8 describe stainless steel sheets having a composition containing Cr: 20-26% and N: 0.6-2.0% by mass, with a Cr nitride layer formed on the surface that combines conductivity and corrosion resistance, possessing corrosion resistance, low contact resistance, and excellent press workability, and that can be processed into separators for polymer electrolyte fuel cells. In the techniques described in Patent documents 6-8, a stainless steel slab with a composition containing Cr: 20-26% and N: 0.1% or less by mass is hot-rolled and cold-rolled to obtain a thin rolled steel sheet, which is then annealed and cooled in a gas atmosphere containing nitrogen, and further pickled with a solution containing a non-oxidizing acid to obtain a stainless steel sheet with an N content of 0.6-2.0% by mass. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2004-68115 [Patent Document 2] Japanese Patent Publication No. 2006-316338 [Patent Document 3] Japanese Patent Publication No. 2008-186601 [Patent Document 4] Japanese Patent Publication No. 2010-68886 [Patent Document 5] Japanese Patent Publication No. 2012-92413 [Patent Document 6] Japanese Patent Publication No. 2020-111805 [Patent Document 7] Japanese Patent Publication No. 2020-111806 [Patent Document 8] International Publication No. 2019-058409 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] However, in stainless steel materials manufactured using the technologies described in Patent Documents 1 to 8, pitting corrosion occurs and corrosion resistance decreases, particularly under harsh corrosive environments with high liquid temperatures, as shown in the ferric chloride corrosion test specified in JIS G 0578. Therefore, there has been a demand for further improvement in corrosion resistance. Therefore, the present invention aims to propose a stainless steel material with excellent corrosion resistance and electrical conductivity, which can be used even in harsh, high-temperature corrosive environments, and a method for manufacturing the same. [Means for solving the problem]

[0010] First, in order to achieve the above-mentioned objectives, we diligently investigated the effects of material composition and nitrogen absorption treatment conditions on corrosion resistance. As a result, it was found that by using a ferritic stainless steel material containing Ti as the base material and performing nitrogen absorption treatment, part or all of the material's structure becomes austenite, and at least the surface layer has a structure in which austenite and titanium nitride phases are distributed, corrosion resistance is significantly improved, as is electrical conductivity.

[0011] This invention was completed based on the above findings and further investigations. Specifically, this invention provides a stainless steel material having an austenite phase structure, wherein at least the surface layer of the stainless steel material has a structure in which the austenite phase and the titanium nitride phase are distributed.

[0012] Furthermore, the present invention provides a stainless steel structure made of the above-mentioned stainless steel material.

[0013] Furthermore, the present invention provides a preferred method for manufacturing the above stainless steel material, comprising: a first heating step of heating a ferritic stainless steel material containing Ti to a predetermined temperature T1 under vacuum; a second heating step of, after reaching the predetermined temperature T1, introducing nitrogen gas and continuing heating to a further predetermined temperature T2 under a nitrogen gas atmosphere, and holding at the predetermined temperature T2 for a predetermined time; and a quenching treatment step of quenching, in this order.

Advantages of the Invention

[0014] According to the present invention, a stainless steel material with excellent corrosion resistance can be obtained. In particular, it exhibits excellent corrosion resistance even in a severe high-temperature corrosion environment such as the ferric chloride corrosion test conforming to JIS G 0578, achieving remarkable industrial effects. In addition, the stainless steel material of the present invention has excellent conductivity equivalent to that of a gold plating film, and as a structure made of the stainless steel material of the present invention, for example, it can be used as a separator for a solid polymer fuel cell.

Brief Description of the Drawings

[0015] [Figure 1] It is a field emission scanning electron microscope micrograph of the cross-section in the thickness direction of the stainless steel material of the present invention. [Figure 2] [[ID=-18]]It is a graph showing the X-ray diffraction results near the surface layer of the stainless steel material of the present invention. [Figure 3] It is a graph showing the element distribution in the depth direction near the surface layer of the stainless steel material of the present invention. [Figure 4] It is a graph showing an example of the hardness measurement of the anticorrosion film. [Figure 5] It is a graph schematically showing an example of the heating step and the quenching treatment step, which are suitable in the present invention. [Figure 6] It is a graph showing the results of the ferric chloride corrosion test conducted in the examples. [Figure 7] It is an explanatory diagram showing the outline of the contact resistance measurement test used in the examples. [Figure 8] It is a graph showing the relationship between the standard free energy of formation ΔG0 of various nitrides and temperature. Note: There seems to be a minor error in the original text where the tag for the X-ray diffraction graph description has an incorrect "-" in front of "18". It should be just "". The translation has been adjusted accordingly. [Modes for carrying out the invention]

[0016] The stainless steel material of the present invention is a stainless steel material having an austenite phase structure. Preferably, at least the surface layer of the above stainless steel material has a structure in which an austenite phase (a crystalline phase having a face-centered cubic crystal structure) and a titanium nitride phase (a crystalline phase having a titanium nitride crystal structure) are distributed. Here, "titanium nitride" refers to a compound represented by TiN. Preferably, the crystalline phases distributed on the surface layer (austenite phase, titanium nitride phase) are mixed and distributed on the surface layer in a film-like manner with a thickness of preferably 0.1 to 15 μm, more preferably 0.1 to 5 μm, even more preferably 0.1 to 1 μm, and even more preferably 0.1 to 0.5 μm (hereinafter also referred to as a corrosion-resistant film).

[0017] The corrosion-resistant film of the stainless steel material of the present invention preferably has an elemental depth profile in which Ti and N are unevenly distributed at the very surface (0.05 μm from the surface), and Cr and Fe are also present in that area, and as you go deeper, Ti and N gradually decrease and Fe increases, while Cr is somewhat low near the outermost surface but remains almost constant thereafter. For example, as shown in Figure 3, which is the result for stainless steel material No. 1 of the present invention described later, it is preferable that the composition ratio of Ti and N is highest in the portion slightly deeper than the outermost surface of the stainless steel material, and gradually decreases as the depth from the surface increases. At the outermost surface of the stainless steel material, it is preferable that the composition ratio of Ti and N is higher than that of Cr and Fe, and that the composition ratio of N is higher than that of Ti. Preferably, the composition ratio of Cr is lowest at the outermost surface of the stainless steel material, gradually increases with increasing depth from the surface, and becomes constant once the distance from the surface exceeds a certain range. Preferably, at the outermost surface of the stainless steel material, the composition ratio of Cr is lower than that of N, Ti, and Fe. After the composition ratio of Cr reaches a certain value, it is preferable that the composition ratio of Cr is lower than that of Fe and higher than that of Ti and N. It is preferable that the proportion of Fe is lowest at the outermost surface of the stainless steel material and increases rapidly as the depth from the surface increases. At the outermost surface of the stainless steel material, it is preferable that the proportion of Fe is lower than the proportions of Ti and N, and higher than the proportion of Cr. After the proportion of Fe increases rapidly, it is preferable that the proportion of Fe is higher than the proportions of Cr, Ti, and N. The presence of such elemental depth profiles in the protective film significantly improves localized corrosivity in harsh corrosive environments and enhances corrosion resistance in highly acidic environments. The corrosion protection film is preferably a very hard phase with a Vickers hardness of approximately 1700 HV.

[0018] The stainless steel material of the present invention preferably has the above-described structure and contains, by mass%, 16% to 30% Cr, further containing 0.03% to 1% Ti, 0.3% to 1.5% N, 1.0% or less Si, 1.0% or less Mn, and 0.1% or less C, with the remainder being Fe and unavoidable impurities, and preferably having a composition that does not contain Ni. In addition to the above composition, it may further contain 3% or less Mo, or in addition to Ti, one or two of Nb and Al, in a total of 0.1% to 1% of Ti, Nb and Al. In this specification, "Ni-free" means excluding the intentional addition of Ni. The presence of trace amounts of Ni that are inevitably included in the steel manufacturing process (0.6% or less in the stainless steel material of the present invention) and trace amounts of Ni that remain because they cannot be removed from the raw materials used (0.6% or less in the stainless steel material of the present invention) is acceptable.

[0019] Next, the reasons for limiting the composition of the stainless steel material of the present invention will be explained. Hereafter, mass% in the composition will simply be expressed as %. Cr: 16% or more and 30% or less The amount of Cr (chromium) is limited to the above-mentioned range in order to ensure that nitrogen is reliably absorbed into the ferritic stainless steel material that forms the base material of the stainless steel material of the present invention, and to establish the austenite phase of the steel structure. If it falls outside the above-mentioned range, it becomes difficult to contain the desired amount of N (nitrogen). Preferably, the amount of Cr is 20% or more and 24% or less.

[0020] N: 0.3% or more and 1.5% or less Nitrogen (N) is absorbed by the ferritic stainless steel material, changing the structure of the stainless steel from the ferrite phase to the austenite phase, thereby contributing to improved corrosion resistance. To obtain this effect, a N content of 0.3% or more is required. If the N content is less than 0.3%, it becomes difficult to obtain an austenite phase in which N is uniformly diffused. On the other hand, there is no particular upper limit to the N content, but a content exceeding 1.5% requires a long nitrogen absorption treatment, leading to a decrease in productivity. For this reason, the N content is limited to a range of 0.3% to 1.5%. Preferably, the N content is 0.8% to 1.2%.

[0021] Ti: 0.03% or more and 1% or less Ti combines with nitrogen at high temperatures, forming a titanium nitride film on the surface, which contributes to improved corrosion resistance. To obtain this effect, the Ti content must be 0.03% or more. On the other hand, if the content exceeds 1%, excess nitride is formed in the ferritic stainless steel material, reducing its ductility. For this reason, the Ti content is limited to 0.03% to 1%. Preferably, the Ti content is 0.3% to 0.4%.

[0022] In the present invention, in addition to the Ti described above, one or two of Nb and Al may be further included in a total amount of 0.1% to 1% of Ti, Nb, and Al. Nb and Al, like Ti, combine with nitrogen to form nitrides, some of which are distributed on the surface and contribute to improved corrosion resistance. Therefore, in addition to Ti, one or two of Nb and Al may be included. However, if the total amount of Ti, Nb, and Al exceeds 1%, excess nitrides will be formed in the ferritic stainless steel material, reducing ductility and worsening workability. For this reason, when Nb and Al are included in addition to Ti, it is preferable to limit the amount of one or two of Nb and Al to 0.1% or more and 1% or less in total. More preferably, it is 0.2 to 0.5%.

[0023] Mo: 3% or less Mo is an element that improves corrosion resistance and can be included as needed. While it is desirable to include 1% or more to achieve this effect, including amounts exceeding 3% leads to increased manufacturing costs. Therefore, when including Mo, it is preferable to limit the amount to 3% or less.

[0024] In addition to the components mentioned above, Si, Mn, and C, which are normally contained in the ferritic stainless steel material, may be included in the following ranges: Si: 1.0% or less, Mn: 1.0% or less, and C: 0.1% or less, in order to increase strength, improve ductility, etc. The remainder of the components other than those mentioned above consists of Fe and unavoidable impurities. As for unavoidable impurities, P: 0.030% or less and S: 0.030% or less are acceptable.

[0025] Of the elements contained in the stainless steel material of the present invention, C and S can be measured using a carbon-sulfur analyzer. First, 2 to 3 small pieces measuring 2 to 3 mm square are taken from a test piece (size: 5 x 5 cm) of the stainless steel material of the present invention. Next, the taken pieces are high-frequency melted in a crucible of a carbon-sulfur analyzer (EMIA-920V2, manufactured by Horiba, Ltd.). The gas generated from the crucible is quantified using infrared radiation to measure the amount of C and S elements contained in the stainless steel material.

[0026] Furthermore, among the elements contained in the stainless steel material of the present invention, Cr, N, Ti, Nb, Al, Mo, Si, Mn, Fe, and P, other than C and S, can be measured using the following X-ray fluorescence analyzer. First, a test piece small enough to fit into the measuring holder of a fluorescent X-ray analyzer (S8 TIGER 4kW, manufactured by Bruker AXS Co., Ltd.) is taken from the stainless steel material of the present invention. Next, a quantitative analysis of the test piece is performed using a calibration curve method with standard materials to measure the amount of elements other than C and S contained in the stainless steel material.

[0027] From the viewpoint of nitrogen diffusion, the stainless steel material of the present invention is preferably a steel material with a wall thickness of 1 mm or less, and more preferably a wall thickness of 0.05 to 0.8 mm. The term "steel material" includes steel plates, steel bars, wire rods, etc.

[0028] The stainless steel material of the present invention described above can be subjected to appropriate post-processing, such as press forming, to form a structure having predetermined dimensions and shape, which can be applied to applications such as separators for polymer electrolyte fuel cells, separators for secondary batteries and water electrolysis, and press-formed products used in corrosive environments such as marine or coastal areas. For example, when used as a separator for polymer electrolyte fuel cells, it is preferable to press-form a stainless steel sheet of the present invention (size: approximately 300 x 200 mm) with a thickness of 0.2 mm or less, preferably 0.08 to 0.1 mm, to form grooves of a predetermined depth to create a separator of predetermined dimensions and shape. The manufacturing method of such stainless steel material will be described later, but in particular, when processing the ferritic stainless steel material, which is the material for the stainless steel material of the present invention, into a predetermined shape after absorbing nitrogen, it is preferable to have a plate thickness of less than 0.1 mm, and more preferably 0.05 to 0.08 mm, during processing. Having the plate thickness of the stainless steel material within this range makes post-processing easy, even for stainless steel material that has high strength and hardness due to nitrogen absorption treatment. As described above, the thickness of the corrosion-resistant film is preferably 0.1 to 15 μm. The stainless steel material of the present invention transforms from a ferrite phase to a highly corrosion-resistant austenite phase by nitrogen absorption and forms a corrosion-resistant film. From the viewpoint of ensuring the corrosion resistance of the stainless steel material of the present invention, the thickness of the obtained corrosion-resistant film is preferably 1 / 10 or more of the thickness of the ferritic stainless steel material. For example, if the plate thickness is 0.1 mm, the thickness of the corrosion-resistant film is preferably 0.01 mm or more. Having a corrosion-resistant film of a certain thickness suppresses the appearance of a film-likely distributed ferrite phase that can become a starting point for corrosion when nitrogen absorption treatment is performed, thereby ensuring corrosion resistance.

[0029] The method for manufacturing stainless steel materials according to the present invention will be described below. First, a ferritic stainless steel material containing Ti is prepared as the base material. Preferably, the ferritic stainless steel material to be prepared has a composition in mass% of 16% to 30% Cr, 0.03% to 1% Ti, 1.0% or less Si, 1.0% or less Mn, and 0.1% or less C, with the remainder being Fe and unavoidable impurities, and is Ni-free. As unavoidable impurities, P: 0.030% or less, S: 0.030% or less, and N: 0.1% or less are acceptable. In addition to the above composition, Mo may be further added in an amount of 3% or less, or in addition to Ti, one or two of Nb and Al may be further added, with the total amount of Ti, Nb and Al being 0.1% to 1%. Examples of such ferritic stainless steel materials include SUS430LX, SUS430J1L, SUS436L, SUS436J1L, SUS443J1, SUS444, and SUS445J1, with SUS443J1 and SUS445J1 being particularly preferred. Furthermore, the thickness of the ferritic stainless steel material to be prepared is preferably 0.05 to 2 mm, more preferably 0.05 to 0.3 mm, and even more preferably 0.05 to 0.1 mm.

[0030] Next, in the present invention, a ferritic stainless steel material containing Ti of the above composition is subjected to a heat treatment and a rapid cooling treatment. The heat treatment and rapid cooling treatment consist of a first heating step, a second heating step, and a rapid cooling step, in this order.

[0031] Figure 5 shows an overview of the heating and rapid cooling processes. First, it is preferable to degrease and wash the prepared material before the heating process described above. Then, the degreased and washed material is placed in a vacuum heating device (vacuum heat treatment furnace) and vacuum suction is performed to remove impurities such as oxygen and moisture from the heating device. If oxygen and moisture remain in the heating device, they will react with the alloying elements such as Cr, Ti, and Nb contained in the material during the heat treatment, forming a thick oxide film, which will inhibit the diffusion of nitrogen into the ferritic stainless steel material. For this reason, it is preferable to repeat the vacuum suction in the heating device about twice, for example, with nitrogen gas replacement in between. After sufficient vacuum suction in the heating device, the heat treatment consisting of the first heating step and the second step is performed.

[0032] The first heating step is a step of subjecting the material to a heat treatment by heating it to a predetermined temperature T1 under vacuum. In this specification, "under vacuum" means 10 ―2 It is preferable to maintain a pressure of Pa or less. Furthermore, the "predetermined temperature T1" as used herein is preferably a temperature within the range of 800°C to 1150°C. The heating rate from room temperature to the predetermined temperature T1 does not need to be particularly limited, as long as the charged material is heated uniformly, but it is preferably in the range of 5°C / min to 20°C / min. In addition, a process to maintain the predetermined temperature during the heating process may be performed to ensure uniform heating.

[0033] When the material (stainless steel) is heated under vacuum, Si, Mn, and Cr, which have high vapor pressures, detach from the surface and undergo deelementation. However, Ti, which has a low vapor pressure, is less likely to deelement and tends to remain on the outermost surface. Therefore, it is thought that it reacts with the nitrogen gas introduced in the second heating step to easily form a titanium nitride phase on the surface. For reference, the standard free energy of formation ΔG of major nitrides is shown below.0 Figure 8 shows the relationship between ΔG and temperature. 0 The lower the value, the more stable the compound is and the more likely it is to form nitrides. TiN is ΔG 0 TiN is the least stable, followed by AlN, then Nb nitrides, and finally Cr nitrides, suggesting that nitrides other than TiN are difficult to form.

[0034] The second heating step involves introducing nitrogen gas after reaching a predetermined temperature T1 in the first heating step, continuing heating in a nitrogen gas atmosphere to a further predetermined temperature T2, and holding the temperature at T2 for a predetermined time. The introduction of nitrogen gas is preferably limited to a nitrogen gas pressure in the range of 100 Pa to 200 kPa. The nitrogen gas introduced is preferably nitrogen gas produced by vaporizing liquid nitrogen (purity: 99.999% or higher).

[0035] If the nitrogen gas pressure is less than 100 Pa, the nitrogen gas pressure is too low to form the desired titanium nitride phase on the surface. On the other hand, if the nitrogen gas pressure exceeds 200 kPa, it affects the cleanliness of the furnace and the material (stainless steel) adsorbs impurity gases. For this reason, it is preferable to limit the nitrogen gas pressure in a nitrogen gas atmosphere to a range of 100 Pa to 200 kPa. Furthermore, the "predetermined temperature T2" ​​is preferably in the range of 1100°C to 1250°C, provided that temperature T1 < temperature T2, from the viewpoint of nitrogen absorption rate and prevention of grain coarsening. More preferably, it is between 1180°C and 1220°C.

[0036] The holding time at the "predetermined temperature T2" ​​is preferably determined appropriately based on the plate thickness of the stainless steel material being processed. By holding the material at the "predetermined temperature T2" ​​in a nitrogen gas atmosphere, the stainless steel material absorbs nitrogen and transforms from the ferrite phase to the austenite phase. After reaching the predetermined temperature T2, it is preferable to increase the nitrogen gas pressure to promote nitrogen diffusion. In particular, when processing a stainless steel material that has been manufactured with high strength and hardness obtained by nitrogen absorption, the holding time is preferably 5 minutes or more and 20 minutes or less, and more preferably 5 minutes or more and 10 minutes or less. By keeping the holding time within the above range, the thickness of the protective film obtained when nitrogen is absorbed can be reduced, and the soft ferrite phase can be left in the stainless steel material of the present invention, making post-processing easier.

[0037] Next, in the rapid cooling process, the material is heated under the nitrogen gas atmosphere described above, followed by rapid cooling. The rapid cooling process is performed to maintain the austenite phase structure obtained in the second heating process even at room temperature. The cooling rate for rapid cooling is preferably 100°C / min or higher. If the cooling rate is less than 100°C / min, the cooling is slow, and the austenite phase will unintentionally transform into the ferrite phase, making it impossible to stably maintain the austenite phase structure at room temperature. Rapid cooling is preferably performed by blowing a cooling gas at a predetermined pressure into the heating device after the heating process, circulating it within the device, and then discharging it. The cooling gas is preferably a variety of inert gases, such as nitrogen gas or argon gas. Furthermore, to increase the cooling rate, a water-cooled cooling system installed within the device may be used. Furthermore, the thickness of the resulting stainless steel material remains substantially unchanged before and after nitrogen absorption.

[0038] The stainless steel material of the present invention obtained by the manufacturing method described above is subjected to appropriate post-processing depending on the application. For example, when used as a separator for polymer electrolyte fuel cells, it is preferable to press-form it to create grooves of a predetermined depth. Alternatively, instead of the above post-processing, appropriate pre-processing may be performed before the above-described heat treatment or the like to create a structure of a predetermined shape. [Examples]

[0039] The present invention will be further described below based on the following examples. Ferritic stainless steel sheets (equivalent to SUS445J1 and SUS443J1) with the compositions shown in Table 1 were used as materials A to C (thickness: 0.1 mm, size: 80 mm x 80 mm), degreased and washed, and then placed in a vacuum heating device. The vacuum heating device used had multiple water-cooled cooling tubes installed inside the chamber.

[0040] [Table 1]

[0041] Next, inside the vacuum heating device, pressure: 10 ―2 Vacuum suction was performed until the pressure dropped below Pa. This vacuum suction was repeated twice, with nitrogen gas replacement in between, to remove impurities from the apparatus. Then, under the same vacuum, the apparatus was heated until the temperature inside reached T1 (1100°C) (first heating step). The heating rate was set to 10°C / min.

[0042] When the temperature inside the heating device reached T1 (1100°C), nitrogen gas (purity: 99.999% or higher) produced by vaporizing liquid nitrogen was supplied into the heating device until the internal pressure reached 100 Pa in nitrogen gas pressure, and heating was continued at a heating rate of 10°C / min (second heating step). During heating, nitrogen gas was circulated inside the heating device to maintain the internal pressure at 100 Pa in nitrogen gas pressure. When the temperature inside the heating device reached T2 (1200°C), it was held at that temperature for 0.5 hours. Once the temperature inside the heating device reached T2, the nitrogen gas pressure inside the heating device was increased to 90 kPa.

[0043] Next, nitrogen gas was injected and discharged into the heating device as a cooling gas, and cooling water was introduced into cooling pipes arranged inside the heating device to forcibly cool the inside of the heating device and rapidly cool the material, thereby producing stainless steel materials (No. 1, No. 3, No. 4) (rapid cooling process). The cooling rate was 100°C / min or more. The thickness of the obtained stainless steel material No. 1 was 0.1 mm. The thickness of stainless steel material No. 3 was 0.1 mm.

[0044] For comparison, material No. A (thickness: 0.1 mm, size: 80 mm x 80 mm) with the composition shown in Table 1 was placed inside a vacuum heating device, and the inside of the heating device was heated to a pressure of 10 ―2 Vacuum suction was applied until the pressure dropped below Pa. Next, nitrogen gas (purity: 99.999% or higher) produced by vaporizing liquid nitrogen was supplied into the heating device until the internal pressure of the device reached 0.1 MPa (nitrogen gas pressure), and then heating was started. During heating, nitrogen gas was circulated within the heating device to maintain the internal pressure at 0.1 MPa (nitrogen gas pressure). Once the temperature inside the heating device reached T2 (1200°C), it was held at that temperature for 0.5 hours. After holding, nitrogen gas (pressure: 0.5 MPa) was injected into and discharged as a cooling gas into the heating device, and cooling water was introduced into cooling pipes installed inside the heating device to forcibly cool the inside of the heating device, rapidly cooling the material and producing stainless steel material (No. 2: comparative example).

[0045] Visual inspection of the obtained stainless steel materials No. 1 and No. 3 (examples of the present invention) revealed that both exhibited a uniform golden color across their entire surface, indicating the formation of a titanium nitride (TiN) phase. Stainless steel materials No. 2 and No. 4 (comparative examples), which fall outside the scope of the present invention, did not exhibit a golden color. First, test specimens were taken from the base material of the obtained stainless steel materials No. 1 to No. 4, and their microstructure was analyzed using an X-ray diffractometer in the following manner. In all of the stainless steel materials No. 1 to No. 4, a peak from the austenite phase was observed, confirming that the stainless steel material had an austenite phase structure after nitrogen absorption treatment, transitioning from the ferrite phase. When the microstructure was analyzed using an X-ray diffractometer in the same manner as described above, it was confirmed that a titanium nitride phase had formed in both stainless steel materials No. 1 and No. 3.

[0046] [Analysis of the microstructure of stainless steel materials] First, test pieces (2 x 2 cm in size) were taken from the obtained stainless steel materials No. 1 to No. 4. Next, the test pieces were fixed to a jig of a build-up type multi-function X-ray diffractometer (RINT-UltrmaIII, manufactured by Rigaku Corporation), and the microstructure was analyzed in thin film measurement mode.

[0047] Furthermore, component analysis was performed on the raw material portions of the obtained stainless steel materials No. 1 to 4. Test pieces were taken from the stainless steel materials, and the proportion of carbon (C) was measured using a carbon-sulfur analyzer (EMIA-920V2, manufactured by Horiba, Ltd.), and the proportion of alloying elements and nitrogen (N) was measured using an X-ray fluorescence analyzer (S8 TIGER 4KW, manufactured by Bruker AXS Co., Ltd.), both using the calibration curve method. The obtained results are shown in Table 2.

[0048] [Table 2]

[0049] In the obtained stainless steel materials No. 1, No. 3 (example of the present invention), No. 2, and No. 4 (comparative example), the N content increased compared to the raw material, and all contained up to 1.1 mass% N.

[0050] Next, cross-sectional samples in the thickness direction were prepared from the obtained stainless steel material by ion milling (Ar ion beam) to suppress sagging, and observed using a field emission scanning electron microscope (FE-SEM: JSM-7800 Prime, manufactured by JEOL Ltd.). Scanning electron microscope images of the cross-section in the thickness direction, as shown in Figure 1, were obtained. From Figure 1, it can be confirmed that stainless steel material No. 1 of the present invention has a thin film-like phase (corrosion-preventive film: thickness: 0.2~0.3 μm) distributed on the outermost layer. Similar results were obtained for stainless steel material No. 3 of the present invention, although not shown. On the other hand, such a corrosion-preventive film was not observed in comparative examples that fall outside the scope of the present invention.

[0051] Next, the amount of Cr, Fe, Ti, and N in stainless steel material No. 1 of the present invention was analyzed in the depth direction from the surface using a glow discharge emission spectrometer (GD-OES), and the elemental depth profiles were determined by the following method. The results are shown in Figure 3. As can be seen from Figure 3, Ti showed the highest composition ratio of approximately 22 mass% at a depth of approximately 0.02 μm from the surface of the stainless steel material, and gradually decreased as the depth from the surface increased. N showed the highest composition ratio of approximately 20 mass% at a depth of approximately 0.01 μm from the surface of the stainless steel material, and gradually decreased as the depth from the surface increased. Also, at the surface of the stainless steel material, the composition ratios of Ti and N were higher than those of Cr and Fe, and the composition ratio of N was higher than that of Ti. Cr showed the lowest composition ratio of approximately 4 mass% at the surface of the stainless steel material, and gradually increased as the depth from the surface increased, showing a constant composition ratio of approximately 22 mass% beyond a depth of approximately 0.05 μm from the surface. Furthermore, at the outermost surface of the stainless steel material, the composition ratio of Cr was lower than that of N, Ti, and Fe. Fe showed the lowest composition ratio at the outermost surface of the stainless steel material, at approximately 6 mass%, and increased sharply with increasing depth from the surface. At the outermost surface of the stainless steel material, the composition ratio of Fe was lower than that of Ti and N, and higher than that of Cr. After the composition ratio of Fe increased sharply, it became higher than that of Cr, Ti, and N, showing a composition ratio of approximately 72 mass% at a depth of 0.5 μm from the surface. Similar results were also obtained for stainless steel material No. 3 of the present invention, although not shown in the figures.

[0052] [Measurement of elemental depth profiles in stainless steel materials] First, test pieces (size: 2cm x 2cm) were taken from stainless steel materials No. 1 and No. 2 of the present invention. Next, the test pieces were set in the holder of the measurement unit of a Marcus-type glow discharge emission spectrometer (GD-Profiler, manufactured by Horiba, Ltd.), and cathode sputtering was performed in the glow discharge region. The emission of sputtered atoms in the Ar plasma was measured spectroscopically to determine the elemental depth profile of the stainless steel material.

[0053] Next, X-ray diffraction specimens were taken from stainless steel material No. 1 of the present invention, and the phases distributed in a film-like manner on the surface were investigated using an X-ray diffractometer in a thin-film measurement mode with X-rays incident at a sharp angle. The results are shown in Figure 2. From the obtained results (diffraction patterns), an austenite (γ) phase and a diffraction peak clearly different from the γ phase (a diffraction peak that matched or was close to the diffraction peak of TiN) were observed. This confirmed that the phases distributed in a film-like manner on the surface were an austenite phase and a titanium nitride phase. Although not shown in the figures, similar results were obtained for stainless steel material No. 3 of the present invention.

[0054] EDS analysis was performed on the surface portion of stainless steel material No. 1 of the present invention. As a result, for stainless steel material No. 1 of the present invention, the mass% (semi-quantitative) values ​​obtained in the depth direction of approximately 1 μm from the surface of the stainless steel material, which is the reach range of the electron beam, were N: 8.9%, Ti: 10.1%, Cr: 21.6%, Fe: 48.8%, Al: 3.1%, Nb: 1.1%, O: 2.7%, C: 2.8%, and Si: 0.9%. Note that O and C may reflect oxides or contaminated layers present on the outermost surface of the stainless steel material. In addition, Al may form compounds such as nitrides, but since the amount of Al detected is trace, it is considered that it does not exist in a layered manner. Therefore, it is considered that the presence of Al (compound) does not affect the corrosion resistance or conductivity of the stainless steel material. The detected amounts of Nb and Si are close to the detection limit (1%) of EDS analysis, and even if we assume that they are contained in the stainless steel material, the amount is trace, so it is considered that they have almost no effect on the corrosion resistance or conductivity of the stainless steel material. For stainless steel material No. 3 of the present invention, the mass% (semi-quantitative) values ​​obtained were N: 11.9%, Ti: 13.4%, Cr: 31.7%, Fe: 37.2%, and Si: 0.9%. Although Al was detected, it was below 1% and therefore below the detection limit, and was excluded.

[0055] The compositional ratio of the elements contained in the sample surface portion was measured by the following method. EDS analysis was performed using a scanning electron microscope and an energy-dispersive X-ray analysis (SEM-EDS, manufactured by JEOL Ltd.) apparatus. EDS analysis was carried out within an area of 0.1 mm × 0.1 mm on the surface of the stainless steel material of the present invention. An electron beam was irradiated, and the characteristic X-ray intensity (count number) of the detected elements was measured, and the ratio of the elements contained in the sample surface portion was calculated from the unique characteristic X-ray intensity of the elements.

[0056] For the stainless steel material No.1 of the present invention, using a thin film hardness tester (HM500, manufactured by Fischer Instruments), a indenter was pressed into the surface, and from the load-indentation depth relationship obtained as shown in Fig. 4, the plastic hardness (H IT ) was found to be 7345 N / mm 2 and when converted to Vickers hardness, it was 1762 HV. It was confirmed that the phase distributed in a film shape in the surface layer was a very hard phase. Although not shown, the same results were obtained for the stainless steel material obtained from the stainless steel material No.3 of the present invention.

[0057] From this, it was confirmed that the stainless steel material of the present invention has a structure in which crystal phases (austenite phase and titanium nitride phase) are thinly distributed in a mixed state in the outermost layer, and moreover, no pinholes or the like are formed and it is continuously connected to the base material, and it has high hardness, high corrosion resistance, and further excellent conductivity.

[0058] Next, the corrosion resistance and conductivity of the stainless steel material of the present invention were investigated. (1) Corrosion resistance test (ferric chloride corrosion test) From the obtained stainless steel materials No.1 (example of the present invention) and No.2 (comparative example), plate-shaped test pieces (size: width 20 mm × length 30 mm) were taken, and after polishing and cleaning the surface, a ferric chloride corrosion test conforming to the provisions of JIS G 0578 was carried out to evaluate the corrosion resistance. The ferric chloride aqueous solution was adjusted by dissolving FeCl3·6H2O in a 0.05 mol / L HCl aqueous solution. The concentration of Fe was set to 6%.

[0059] Test specimens, whose surfaces had been polished and cleaned, were immersed in a ferric chloride aqueous solution for 24 hours at temperatures of 35°C, 50°C, 65°C, and 80°C, respectively, for a total of 96 hours of continuous immersion. The mass of the test specimens was measured before immersion, after 24 hours, after 48 hours, after 72 hours, and after 96 hours, and the corrosion loss was calculated. For reference, test specimens (in their raw state) similar to those described above were taken from ferritic stainless steel sheets (SUS445J1) and general austenitic stainless steel sheets (SUS304 and SUS316) (cold-rolled sheets), and the same ferric chloride corrosion test was performed. Similarly, the change in mass of the test specimens after immersion was measured, and the corrosion loss was calculated for each. The results obtained are shown in Table 3 and Figure 6.

[0060] [Table 3]

[0061] The example of the present invention (stainless steel material No. 1) showed no mass loss due to corrosion even when immersed at 80°C, demonstrating excellent corrosion resistance. Stainless steel material No. 2 did not corrode when immersed at 35°C, but slight corrosion occurred when immersed at 50°C (critical corrosion temperature: 50°C), and some corrosion occurred when immersed at 65°C and 80°C. For reference, the raw material test pieces No. R1 (SUS304), No. R2 (SUS316), and No. R3 (SUS445J1) showed corrosion when immersed at 35°C (critical corrosion temperature: 35°C), and the corrosion loss increased with increasing aqueous solution temperature. Although not shown in Table 3 and Figure 6, similar results were obtained for the stainless steel material No. 3 of the present invention. From these results, it can be seen that the stainless steel materials of the present invention have superior corrosion resistance even compared to austenitic stainless steel materials such as SUS304 and SUS316, which have excellent corrosion resistance.

[0062] (2) Conductivity test A sample (size: 10 × 10 mm) was taken from the obtained stainless steel material No. 1 (example of the present invention), and the contact resistance between the sample and carbon paper was measured using the four-terminal method with a contact resistance measurement circuit schematically shown in Figure 7. For reference, a sample (No. R3) of material A, which had not undergone nitrogen absorption treatment, was taken, and the contact resistance between the sample and carbon paper was measured in the same manner. Similarly, a sample (No. R1) of a general austenitic stainless steel sheet (SUS304; thickness: 0.1 mm) was taken, and the contact resistance between the sample and carbon paper was measured. Furthermore, samples (No. R1A, No. R3A) of material A and SUS304 steel sheet with gold plating applied to the surface, respectively, were taken, and the contact resistance between the samples and carbon paper was measured in the same manner.

[0063] As shown in Figure 7, the contact resistance was measured using a milliohmmeter with a load cell fixed to the bottom of a vise, and layers of stainless steel plate, copper plate, carbon paper (CP), sample, copper plate, and stainless steel plate stacked on top of it, with a stress of 1 MPa applied to the sample. In measurement circuit A shown in Figure 7, the layers were stacked in the order of stainless steel jig, copper plate, CP, sample, CP, copper plate, and stainless steel jig. In measurement circuit B, the layers were stacked in the order of stainless steel jig, copper plate, CP, CP, CP, copper plate, and stainless steel jig. In measurement circuit C, the layers were stacked in the order of stainless steel jig, copper plate, CP, CP, copper plate, and stainless steel jig.

[0064] The method for measuring the contact resistance between carbon paper (CP) and the sample was as follows. Resistance RA is measured using measurement circuit A as shown in Figure 7, resistance RB using measurement circuit B, and resistance RC using measurement circuit C. From these resistance values, the following equation is obtained. R = (RA + RC - 2RB) / 2 The contact resistance R between the sample and carbon paper (CP) was determined using this method. The results obtained are shown in Table 4.

[0065] [Table 4]

[0066] In the case of No. R3 (reference example), which is material A (equivalent to SUS445J1) without nitrogen absorption treatment, and No. R1 (reference example), which is SUS304 material in its original state, the contact resistance R between the sample and carbon paper (CP) is approximately 80 mΩ. However, the contact resistance R between the sample and carbon paper (CP) of No. 1 (inventive example) shows a low contact resistance value of less than 10 mΩ, which is close to the contact resistance value of gold-plated material A (No. R3A) and gold-plated SUS304 (No. R1A), which were measured for reference. In other words, the stainless steel material (inventive example) having a structure in which crystalline phases (austenite phase and titanium nitride phase) are distributed in a thin film-like manner in a mixed state on the surface has conductivity almost equivalent to that of a gold-plated film. Although not shown in Table 4, similar results were obtained for the stainless steel material No. 3 of the present invention.

[0067] For these reasons, the stainless steel material of the present invention has superior corrosion resistance and conductivity compared to general austenitic stainless steel material SUS304 and ferritic stainless steel material SUS445J1, and is therefore suitable for use as a separator in polymer electrolyte fuel cells.

Claims

1. A stainless steel material having an austenite phase structure, The above stainless steel material has a composition in which, by mass%, Cr is 16% or more and 30% or less, Ti is 0.03% or more and 1% or less, N is 0.3% or more and 1.5% or less, Si is 1.0% or less, Mn is 1.0% or less, and C is 0.1% or less, with the remainder being Fe and unavoidable impurities, and is Ni-free. A stainless steel material wherein at least the surface layer of the stainless steel material has a structure in which an austenite phase and a titanium nitride phase are distributed.

2. The stainless steel material according to claim 1, further comprising, in addition to the above composition, one or two of Nb and Al in mass%, in an amount of 0.1% or more and 1% or less in total for Ti, Nb, and Al.

3. A stainless steel structure made of the stainless steel material described in claim 1 or 2.

4. A stainless steel structure according to claim 3, which is a separator for a polymer electrolyte fuel cell.

5. A method for manufacturing stainless steel according to claim 1 or 2, Using a ferritic stainless steel material containing Ti, a predetermined temperature T is reached under vacuum. １ A first heating step to heat up to the predetermined temperature T １ After reaching a certain temperature, nitrogen gas is introduced and the area is then heated to a predetermined temperature T under a nitrogen gas atmosphere. ２ Continue heating until the predetermined temperature T ２ A method for manufacturing stainless steel, comprising, in this order, a second heating step of holding the material in place for a predetermined time, and a rapid cooling step of rapidly cooling the material.

6. Under the above vacuum, 10 ―２ A method for manufacturing stainless steel material according to claim 5, wherein the pressure is Pa or less.

7. The method for manufacturing stainless steel according to claim 5 or 6, wherein the above-mentioned nitrogen gas atmosphere is an atmosphere in which the nitrogen gas pressure is 100 Pa or more and 200 kPa or less.

8. The method for manufacturing stainless steel according to any one of claims 5 to 7, wherein the rapid cooling described above is a process of cooling at a cooling rate of 100°C / min or more.

9. The above-mentioned specified temperature T １ is a temperature within the range of 800°C or higher and 1150°C or lower, and further, the above-mentioned specified temperature T ２ is a temperature within the range of 1100°C or higher and 1250°C or lower, and the temperature T １ <temperature T ２ is as described in any one of claims 5 to 8, a method for manufacturing a stainless steel material.

10. A method for producing a stainless steel material according to any one of claims 5 to 9, wherein the ferritic stainless steel material containing Ti has a composition in which it contains, by mass%, 16% to 30% Cr, further containing 0.03% to 1% Ti, 1.0% or less Si, 1.0% or less Mn, and 0.1% or less C, with the remainder being Fe and unavoidable impurities, and is Ni-free.

11. In addition to the above composition, one or two of Nb and Al are further added in mass%, Ti, A method for producing stainless steel material according to claim 10, wherein the total amount of Nb and Al is 0.1% or more and 1% or less.

12. A method for producing stainless steel according to claim 10 or 11, further comprising the above composition plus 3% or less of Mo by mass%.

Images