Stainless steel material and production method therefor

Abstract

In this invention, a ferritic stainless steel material containing, in mass%, 16% to 30% of Cr and 0.03% to 1% of Ti, is used as a raw material. A first heating step for heating the same to a predetermined temperature T1 under vacuum, a second heating step for, after reaching the predetermined temperature T1, introducing nitrogen gas, continuing to heat further to a predetermined temperature T2 in a nitrogen gas atmosphere, and maintaining the predetermined temperature T2 for a predetermined time, and also a rapid cooling step for rapid cooling, are provided in the stated order. As a result, the stainless steel material has an austenitic phase structure and has a structure in which austenitic phases and titanium nitride phases are distributed in the form of films at least on the surface layer, and the material has both excellent corrosion resistance in high-temperature, very corrosive environments and excellent conductivity equivalent to that of gold plating films.

Description

Stainless steel material and its manufacturing method

[0001] The present invention relates to a stainless steel material suitable for use as a separator in a polymer electrolyte fuel cell, for example, and a method for producing the same, and particularly to improving corrosion resistance and electrical conductivity.

[0002] Austenitic stainless steel materials have satisfactory properties in terms of strength and workability, but there is room for improvement in terms of corrosion resistance. Stainless steel has high corrosion resistance due to the passive film on its surface, but when it is immersed in water containing a certain amount of Cl, － When ions (chlorine ions) are present, the passive film is locally destroyed, resulting in localized corrosion such as pitting corrosion, crevice corrosion, and stress corrosion cracking, depending on the conditions. Localized corrosion is a corrosion phenomenon unique to stainless steels, and corrosion resistance varies depending on the stainless steel grade. Austenitic stainless steel materials generally have a certain level of corrosion resistance, but the aforementioned corrosion occurs in corrosive environments. Nitrogen is an austenite-stabilizing element known to increase the pitting corrosion index, which is an indicator of the corrosion resistance of stainless steels. Adding nitrogen to ferritic stainless steels is expected to austenitize them and improve their corrosion resistance. In response to this issue, 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 contacted with an inert gas containing nitrogen gas at 800°C or higher to austenitize the entire product or a portion of the product, forming a two-phase structure of ferrite and austenite. According to the method for manufacturing a stainless steel product described in Patent Document 1, it is possible to reduce the processing costs of a stainless steel product having satisfactory properties in terms of strength and corrosion resistance.

[0003] Patent Document 2 describes a method for manufacturing a stainless steel product in which a ferritic stainless steel containing, by mass%, 18 to 24% Cr and 0 to 4% Mo is used as a raw material, and nitrogen absorption treatment is performed to manufacture a stainless steel product that does not contain Ni. The method for manufacturing a stainless steel product described in Patent Document 2 is said to produce a stainless steel product with excellent workability and corrosion resistance.

[0004] Patent Document 3 describes a separator for a polymer electrolyte fuel cell that uses stainless steel as a 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, thereby improving acid resistance. Furthermore, the stainless steel is made highly clean by reducing the oxygen content through appropriate combined deoxidation with Al and Ca and by reducing the sulfur content through sufficient desulfurization, thereby reducing the content of non-metallic compounds. This improves plastic workability and provides the corrosion resistance and acid resistance required for the separator to withstand the operating environment of a polymer electrolyte fuel cell.

[0005] Patent Document 4 describes a method for manufacturing a cooking container, which includes the steps of heating a ferritic stainless steel substrate to 1000°C or higher and 1250°C or lower in a nitrogen-containing gas atmosphere and holding the substrate to form an austenite phase in a surface layer of the substrate, cooling the heated substrate at a cooling rate of 1°C / second or higher, 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 an inner layer of the substrate, is formed in the surface layer of the ferritic stainless steel substrate, 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 nitrogen gas atmosphere at 1100 to 1250°C in a heating furnace to allow the ferritic stainless steel to absorb nitrogen, and then quenching the nitrogen-absorbed material to austenitize part or all of the ferritic stainless steel. The technology described in Patent Document 5 is said to enable production of stainless steel with high corrosion resistance and durability, which is effective for use in separators of polymer electrolyte fuel cells.

[0007] Patent Documents 6, 7, and 8 describe stainless steel sheets that have a composition containing, by mass, 20 to 26% Cr and 0.6 to 2.0% N, and on the surface of which a Cr nitride layer that combines electrical conductivity and corrosion resistance is formed, and that have 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 to 8, a stainless steel slab having a composition containing, by mass, 20 to 26% Cr and 0.1% or less N is hot-rolled and cold-rolled to form a thin-rolled steel sheet, which is then annealed and cooled in a nitrogen-containing gas atmosphere and further pickled in a solution containing a non-oxidizing acid to obtain a stainless steel sheet with an N content of 0.6 to 2.0% by mass.

[0008] US2006 / 037669A1 US2007 / 186999A1 JP2008-186601A JP2010-68886A JP2012-92413A JP2020-111805A JP2020-111806A US2020 / 283867A1

[0009] However, in the stainless steel materials manufactured by the techniques described in Patent Documents 1 to 8, for example, pitting corrosion occurs and corrosion resistance decreases, particularly in severe corrosive environments at high liquid temperatures, in the ferric chloride corrosion test specified in JIS G 0578. Therefore, a further improvement in corrosion resistance is desired. Therefore, an object of the present invention is to propose a stainless steel material that has excellent corrosion resistance and electrical conductivity and can be used even in severe corrosive environments at high temperatures, and a method for manufacturing the same.

[0010] First, in order to achieve the above-mentioned object, the inventors conducted extensive research into the effects of material composition and nitrogen absorption treatment conditions on corrosion resistance. As a result, they found that when a Ti-containing ferritic stainless steel material is used as the material, nitrogen absorption treatment can be used to convert part or all of the material's structure into an austenite phase, and at least the surface layer has a structure in which the austenite phase and titanium nitride phase are distributed, thereby significantly improving corrosion resistance and electrical conductivity.

[0011] The present invention was completed based on these findings and further investigations. That is, the present invention provides a stainless steel material having an austenite phase structure, in which 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] The present invention also provides a stainless steel structure made of the above stainless steel material.

[0013] Furthermore, the present invention provides a preferred method for producing the above-mentioned stainless steel material, which comprises using a ferritic stainless steel material containing Ti as a raw material, heating the material to a predetermined temperature T １ a first heating step of heating the material to the predetermined temperature T １ After reaching a predetermined temperature T ２ Heating is continued until the predetermined temperature T ２ The present invention provides a method for manufacturing a stainless steel material, which comprises, in this order, a second heating step of holding the material at 400 K for a predetermined period of time, and a rapid cooling step of rapidly cooling the material.

[0014] FIG. 1 is a field emission scanning electron microscope micrograph of the plate thickness cross section of the stainless steel material of the present invention. FIGS. 2(a) and 2(b) are graphs showing the results of X-ray diffraction near the surface layer of the stainless steel material of the present invention. FIG. 3 is a graph showing the element distribution in the depth direction near the surface layer of the stainless steel material of the present invention. FIG. 4 is a graph showing an example of hardness measurement of a corrosion protection film. FIG. 5 is a graph schematically showing an example of a heating process and a rapid cooling process suitable for the present invention. FIG. 6 is a graph showing the results of a ferric chloride corrosion test conducted in the examples. FIG. 7 is an explanatory diagram showing an outline of the contact resistance measurement test used in the examples. FIG. 8 is a graph showing the standard free energy of formation ΔG of various nitrides. 0 10 is a graph showing the relationship between the temperature and the temperature.

[0015] The stainless steel material of the present invention is a stainless steel material having an austenite phase structure. At least the surface layer of the stainless steel material preferably 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. As used herein, "titanium nitride" refers to a compound represented by TiN. The crystalline phases (austenite phase and titanium nitride phase) distributed in the surface layer are preferably mixed together and distributed in the form of a film (hereinafter also referred to as an anticorrosion film) with a thickness of preferably about 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.

[0016] The corrosion-resistant coating of the stainless steel material of the present invention preferably has a 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. With increasing depth, Ti and N gradually decrease and Fe increase, while Cr is slightly low near the outermost surface but remains almost constant thereafter. For example, as shown in FIG. 3 , which shows the results for stainless steel material No. 1 of the present invention, which will be described later, the compositional proportions of Ti and N are highest slightly deeper than the outermost surface of the stainless steel material and gradually decrease with increasing depth from the surface. At the outermost surface of the stainless steel material, the compositional proportions of Ti and N are preferably higher than those of Cr and Fe, and the compositional proportion of N is preferably higher than that of Ti. The compositional proportion of Cr is preferably lowest at the outermost surface of the stainless steel material, gradually increases with increasing depth from the surface, and remains constant beyond a certain distance from the surface. At the outermost surface of the stainless steel material, the compositional proportion of Cr is preferably lower than those of N, Ti, and Fe. After the Cr composition ratio reaches a certain value, it is preferable that the Cr composition ratio be lower than the Fe composition ratio and higher than the Ti and N composition ratios. It is preferable that the Fe composition ratio be lowest at the outermost surface of the stainless steel material and rapidly increase as the depth from the surface increases. It is preferable that the Fe composition ratio be lower than the Ti and N composition ratios and higher than the Cr composition ratio at the outermost surface of the stainless steel material. After the Fe composition ratio rapidly increases, it is preferable that the Fe composition ratio be higher than the Cr, Ti, and N composition ratios. Having such an element depth profile in the corrosion protection film significantly improves local corrosion resistance in severe corrosion environments and also improves corrosion resistance in strongly acidic environments. It is preferable that the corrosion protection film be a very hard phase with a Vickers hardness equivalent of about 1700 HV.

[0017] The stainless steel material of the present invention preferably has the above-described structure and contains, by mass%, 16% to 30% Cr, 0.03% to 1% Ti, 0.3% to 1.5% N, 1.0% to 1.0% Si, 1.0% to 1.0% Mn, and 0.1% to 0.1% C, with the balance being Fe and unavoidable impurities, and is Ni-free. In addition to the above-described composition, the stainless steel material may further contain 3% or less Mo, or, in addition to Ti, one or two of Nb and Al, with a total of 0.1% to 1% Ti, Nb, and Al. As used herein, "Ni-free" means excluding intentional addition of Ni. The presence of trace amounts of Ni (0.6% or less in the stainless steel material of the present invention) that are unavoidably contained in the steel manufacturing process and trace amounts of Ni that cannot be removed from the raw materials used and remain (0.6% or less in the stainless steel material of the present invention) is permitted.

[0018] Next, the reasons for limiting the composition of the stainless steel material of the present invention will be explained. Hereinafter, mass% in the composition will be simply expressed as %. Cr: 16% or more and 30% or less Cr is limited to the above range in order to ensure that the ferritic stainless steel material that is the raw material for the stainless steel material of the present invention absorbs nitrogen and converts the structure of the steel material to an austenitic phase. Outside the above range, it becomes difficult to contain the desired amount of N. Preferably, Cr is 20% or more and 24% or less.

[0019] N: 0.3% or more and 1.5% or less N is absorbed into the ferritic stainless steel material, which is the raw material, and changes the structure of the stainless steel material from ferrite phase to austenite phase, contributing to improved corrosion resistance. To achieve this effect, a 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 need to particularly limit the upper limit of the N content, but a content exceeding 1.5% requires a long nitrogen absorption treatment, resulting in reduced productivity. For this reason, N is limited to a range of 0.3% or more and 1.5% or less. Preferably, N is 0.8% or more and 1.2% or less.

[0020] Ti: 0.03% or more and 1% or less Ti combines with nitrogen at high temperatures to form (distribute) a film of titanium nitride (titanium nitride) on the surface, contributing to improved corrosion resistance. To achieve this effect, a content of 0.03% or more is required. On the other hand, if the content exceeds 1%, excess nitrides are formed in the ferritic stainless steel material, which is the raw material, and ductility decreases. For this reason, the Ti content is limited to 0.03% or more and 1% or less. Preferably, the Ti content is 0.3% or more and 0.4% or less.

[0021] In the present invention, in addition to the Ti described above, one or two of Nb and Al may be contained in a total amount of 0.1% to 1%. Like Ti, Nb and Al combine with nitrogen to form nitrides, some of which are distributed in the surface layer and contribute to improving corrosion resistance. Therefore, in addition to Ti, one or two of Nb and Al may be contained. However, if the total amount of Ti, Nb, and Al exceeds 1%, excess nitrides are formed in the ferritic stainless steel material, resulting in reduced ductility and poor workability. Therefore, when Nb and Al are contained in addition to Ti, it is preferable to limit the total amount of Nb and Al to 0.1% to 1%. More preferably, it is 0.2 to 0.5%.

[0022] Mo: 3% or less Mo is an element that improves corrosion resistance and can be contained as needed. To obtain this effect, it is desirable to contain 1% or more, but a large content exceeding 3% leads to a rise in manufacturing costs. Therefore, if Mo is contained, it is preferable to limit it to 3% or less.

[0023] In addition to the above-mentioned components, Si, Mn, and C, which are normally contained in the raw material ferritic stainless steel, can be contained in the ranges of Si: 1.0% or less, Mn: 1.0% or less, and C: 0.1% or less, in order to increase strength and improve ductility. The balance other than the above-mentioned components consists of Fe and unavoidable impurities. As unavoidable impurities, P: 0.030% or less and S: 0.030% or less are allowable.

[0024] Of the elemental amounts contained in the stainless steel material of the present invention, C and S can be measured using a carbon-sulfur analyzer. First, two to three 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 small pieces are high-frequency melted in the crucible of a carbon-sulfur analyzer (EMIA-920V2, manufactured by Horiba, Ltd.). The gas generated from the crucible is quantified by infrared light, and the elemental amounts of C and S contained in the stainless steel material are measured.

[0025] Furthermore, of the amounts of 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 of a size that can fit into the measurement holder of the X-ray fluorescence analyzer (S8 TIGER 4KW, manufactured by Bruker AXS K.K.) is taken from the stainless steel material of the present invention. Next, quantitative analysis of the test piece is performed by a calibration curve method using a standard substance, and the amounts of elements other than C and S contained in the stainless steel material are measured.

[0026] The stainless steel material of the present invention preferably has a thickness of 1 mm or less, more preferably 0.05 to 0.8 mm, from the viewpoint of diffusion of N. The term "steel material" includes steel plates, steel bars, wire rods, etc.

[0027] The above-described stainless steel material of the present invention can be subjected to appropriate post-processing, such as press forming, to produce a stainless steel structure having a predetermined size and shape, and can be used for applications such as separators for polymer electrolyte fuel cells, separators for secondary batteries or water electrolysis, and press-formed products for use in corrosive environments such as the ocean or coast. For example, when used as a separator for polymer electrolyte fuel cells, a stainless steel sheet of the present invention (size: approximately 300 x 200 mm) having a thickness of 0.2 mm or less, preferably 0.08 to 0.1 mm, is preferably press-formed to form grooves of a predetermined depth to produce a separator of a predetermined size and shape. The method for producing such a stainless steel material will be described later. In particular, when a ferritic stainless steel material, which is the raw material for the stainless steel material of the present invention, is processed into a predetermined shape after absorbing nitrogen, the thickness of the stainless steel material during processing is preferably less than 0.1 mm, and more preferably 0.05 to 0.08 mm. Having the stainless steel thickness within this range facilitates post-processing, even for stainless steel materials that have 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 upon 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 at least 1 / 10 of the thickness of the ferritic stainless steel material; for example, when the plate thickness is 0.1 mm, the thickness of the corrosion-resistant film is preferably at least 0.01 mm. By providing a corrosion-resistant film with a constant thickness, the appearance of a film-like ferrite phase that could serve as a starting point for corrosion when nitrogen absorption treatment is performed is suppressed, making it possible to ensure corrosion resistance.

[0028] The method for manufacturing a stainless steel material of the present invention will be described below. First, a ferritic stainless steel material containing Ti is prepared as a raw material. The ferritic stainless steel material to be prepared preferably contains, by mass%, 16% to 30% Cr, 0.03% to 1% Ti, 1.0% or less Si, 1.0% or less Mn, 0.1% or less C, the balance being Fe and unavoidable impurities, and is Ni-free. Allowable unavoidable impurities include P: 0.030% or less, S: 0.030% or less, and N: 0.1% or less. In addition to the above composition, Mo may be 3% or less, or in addition to Ti, one or two of Nb and Al may be contained in a total of 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. 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.

[0029] Next, in the present invention, the Ti-containing ferritic stainless steel material having the above-mentioned composition is subjected to a heat treatment and a quenching treatment. The heat treatment and the quenching treatment include a first heating step, a second heating step, and a quenching treatment step in this order.

[0030] The heating and quenching steps are outlined in Figure 5. First, the prepared material is preferably degreased and cleaned before the heating step. The degreased and cleaned material is then placed in a vacuum heating device (vacuum heat treatment furnace) and vacuum-suctioned to remove impurities such as oxygen and moisture from the heating device. If oxygen or moisture remains in the heating device, it reacts with the alloying elements, such as Cr, Ti, and Nb, during the heat treatment to form a thick oxide film, which inhibits the diffusion of nitrogen into the ferritic stainless steel material. Therefore, it is preferable to repeatedly vacuum-suction the heating device twice, for example, with nitrogen gas replacement between cycles. After sufficient vacuum-suction has been performed, the heating device is subjected to heat treatment consisting of the first and second heating steps.

[0031] The first heating step is to heat the material to a predetermined temperature T １ The term "under vacuum" used in this specification means a process of heating the material up to 10 ―２ It is preferable that the pressure is 0.1 Pa or less. １ " is preferably set to a temperature in the range of 800°C to 1150°C. １ The rate of temperature rise up to the temperature of the charged material does not need to be particularly limited as long as the temperature of the charged material can be raised uniformly, but it is preferably in the range of 5°C / min to 20°C / min. Note that, for uniform heating, a treatment of holding the temperature at a predetermined temperature may be carried out during the temperature rise.

[0032] By heating the material (stainless steel material) in a vacuum, Si, Mn, and Cr, which have high vapor pressures, are released from the surface and undergo a de-elementation phenomenon, but Ti, which has a low vapor pressure, is difficult to de-element and tends to remain on the outermost surface. For this reason, it is thought that it reacts with the nitrogen gas introduced in the subsequent second heating step, making it easier to form a titanium nitride phase on the surface. For reference, the standard free energy of formation ΔG of the main nitrides is 0 The relationship between ΔG and temperature is shown in Figure 8. 0 The lower the value of ΔG, the more stable the compound is and the easier it is to form nitrides. 0is the lowest and most stable, followed by AlN, then Nb nitride, and Cr nitride, and it can be inferred that nitrides other than TiN are difficult to form.

[0033] The second heating step is performed at a predetermined temperature T １ After reaching a predetermined temperature T ２ Heating is continued until the predetermined temperature T ２ The nitrogen gas is preferably introduced until the nitrogen gas pressure reaches a range of 100 Pa to 200 kPa. The nitrogen gas to be introduced is preferably nitrogen gas (purity: 99.999% or higher) produced by vaporizing liquid nitrogen.

[0034] If the nitrogen gas pressure is less than 100 Pa, the nitrogen gas pressure is too low and the desired titanium nitride phase cannot be formed in the surface layer. On the other hand, if the nitrogen gas pressure exceeds 200 kPa, the cleanliness of the furnace is affected and the material (stainless steel material) adsorbs impurity gases. For this reason, it is preferable to limit the nitrogen gas pressure in the nitrogen gas atmosphere to a range of 100 Pa or more and 200 kPa or less. ２ " is the temperature T １ <Temperature T ２ From the viewpoint of nitrogen absorption rate and prevention of grain coarsening, a temperature in the range of 1100°C to 1250°C is preferred, and a temperature in the range of 1180°C to 1220°C is more preferred.

[0035] "Predetermined temperature T ２ The holding time at "predetermined temperature T" is preferably determined appropriately depending on the plate thickness of the target stainless steel material, etc. ２ By holding the temperature at ", the stainless steel material absorbs nitrogen and transforms from the ferrite phase to the austenite phase. ２After the temperature reaches 100°C, it is preferable to increase the nitrogen gas pressure to promote the diffusion of nitrogen. In particular, when a stainless steel material that has high strength and hardness due to nitrogen absorption is to be processed after production, 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 anticorrosion 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.

[0036] Next, in the quenching process, the material is subjected to the heat treatment under a nitrogen gas atmosphere as described above, followed by rapid cooling. The quenching process is performed to maintain the austenite phase structure obtained in the second heating process at room temperature. The cooling rate of the quenching is preferably 100°C / min or higher. Note that a cooling rate of less than 100°C / min results in slow cooling, leading to unintended transformation from the austenite phase to the ferrite phase, making it impossible to stably maintain the austenite phase structure at room temperature. The quenching process is preferably performed by blowing a cooling gas at a predetermined pressure into the heating device after the heating process, circulating the gas within the device, and then discharging the gas. The cooling gas is preferably an inert gas, such as nitrogen gas or a rare gas such as Ar gas. To further increase the cooling rate, a water-cooled cooling pipe installed in the device may be used. Note that the thickness of the resulting stainless steel material remains substantially unchanged before and after nitrogen absorption.

[0037] The stainless steel material of the present invention obtained by the above-mentioned manufacturing method is subjected to appropriate post-processing depending on the intended use. For example, when used as a separator for a polymer electrolyte fuel cell, it is preferable to form grooves of a predetermined depth by press molding. Note that, instead of the above-mentioned post-processing, appropriate pre-processing may be performed before the above-mentioned heat treatment to form a structure of a predetermined shape.

[0038] The present invention will be further described below based on examples. Ferritic stainless steel plates (equivalent to SUS445J1 and SUS443J1) having the compositions shown in Table 1 were used as materials A to C (plate thickness: 0.1 mm, size: 80 mm x 80 mm). After degreasing and cleaning, they were placed in a vacuum heating device. The vacuum heating device used had multiple water-cooled cooling pipes installed in the chamber.

[0039]

[0040] Next, the inside of the vacuum heating device is heated to a pressure of 10 ―２ This vacuum suction was repeated twice with nitrogen gas replacement in between to remove impurities from the device, and then the temperature inside the heating device was raised to T １ The sample was subjected to a heat treatment in which the sample was heated to a temperature of 1100° C. (first heating step). The temperature was increased at a rate of 10° C. / min.

[0041] The temperature inside the heating device is T １ When the temperature reached 1100°C, nitrogen gas (purity: 99.999% or more) generated by vaporizing liquid nitrogen was supplied into the heating device until the pressure inside the device reached 100 Pa in nitrogen gas pressure, and heating was continued at a temperature increase rate of 10°C / min (second heating step). During heating, nitrogen gas was circulated inside the heating device to maintain the pressure inside the heating device at 100 Pa in nitrogen gas pressure. ２ When the temperature inside the heating device reached T (1200°C), it was held at that temperature for 0.5 hours. ２ When this temperature reached 100°C, the nitrogen gas pressure in the heating device was increased to 90 kPa.

[0042] Next, nitrogen gas was forced into the heating device as a cooling gas and then discharged, and cooling water was introduced into a cooling pipe installed in the heating device to forcibly cool the inside of the heating device. This resulted in a quenching process for rapidly cooling the material, producing stainless steel materials (No. 1, No. 3, No. 4) (quenching 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 the stainless steel material No. 3 was 0.1 mm.

[0043] For comparison, material No. A (plate thickness: 0.1 mm, size: 80 mm x 80 mm) having the composition shown in Table 1 was placed in a vacuum heating device, and the inside of the heating device was heated at a pressure of 10 ―２ Vacuum suction was performed until the pressure inside the heating device reached 0.1 MPa or less. Next, nitrogen gas (purity: 99.999% or more) generated by vaporizing liquid nitrogen was supplied into the heating device until the pressure inside the device reached 0.1 MPa in terms of nitrogen gas pressure, and then heating was started. During heating, nitrogen gas was circulated inside the heating device to maintain the pressure inside the heating device at 0.1 MPa in terms of nitrogen gas pressure. When the temperature inside the heating device reached T ２ After the temperature reached 1200°C, the material was held at that temperature for 0.5 hours. After the temperature was held, nitrogen gas (pressure: 0.5 MPa) was forced into the heating device as a cooling gas and then discharged, and cooling water was introduced into a cooling pipe disposed in the heating device to forcibly cool the inside of the heating device and rapidly cool the material, thereby producing a stainless steel material (No. 2: Comparative Example).

[0044] Visual observation of the obtained stainless steel materials No. 1 and No. 3 (inventive examples) revealed that the entire surface was uniformly golden, indicating the formation of a titanium nitride (TiN) phase. Stainless steel materials No. 2 and No. 4 (comparative examples), which are outside the scope of the present invention, did not exhibit a golden color. First, test specimens were taken from the base metal of the obtained stainless steel materials No. 1 to No. 4, and their structures were analyzed using an X-ray diffractometer using the following method. Peaks from the austenite phase were observed in all of stainless steel materials No. 1 to No. 4, confirming that the nitrogen absorption treatment had transformed the ferrite phase into a stainless steel material with an austenite phase structure. When the structure was analyzed using an X-ray diffractometer using the same method as described above, it was confirmed that a titanium nitride phase was formed in both stainless steel materials No. 1 and No. 3.

[0045] [Analysis of Stainless Steel Material Structure] First, test pieces (size: 2 × 2 cm) 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 multifunctional X-ray diffractometer (RINT-Ultrma III, manufactured by Rigaku Corporation), and the structure was analyzed in thin film measurement mode.

[0046] Furthermore, the raw material portions of the obtained stainless steel materials Nos. 1 to 4 were subjected to a component analysis. Test pieces were collected from the stainless steel materials, and the proportions of carbon (C) were measured using a carbon-sulfur analyzer (EMIA-920V2, manufactured by Horiba, Ltd.), and the proportions of alloy elements and nitrogen (N) were measured using an X-ray fluorescence analyzer (S8 TIGER 4KW, manufactured by Bruker AXS Co., Ltd.), both by the calibration curve method. The results are shown in Table 2.

[0047]

[0048] The obtained stainless steel materials No. 1, No. 3 (invention examples), No. 2, and No. 4 (comparative examples) all had an increased N content compared to the raw material, containing up to 1.1 mass% N.

[0049] Next, a thickness direction cross-sectional sample was prepared from the obtained stainless steel material by ion milling (Ar ion beam) to suppress sagging, and the sample was observed using a field emission scanning electron microscope (FE-SEM: JSM-7800 Prime, manufactured by JEOL Ltd.), resulting in a scanning electron micrograph of the thickness direction cross-section, as shown in Figure 1. From Figure 1, it can be seen that stainless steel material No. 1 of the present invention has a phase (anticorrosion film: thickness: 0.2 to 0.3 μm) distributed in the form of a thin film in the outermost layer. Although not shown, similar results were also obtained for stainless steel material No. 3 of the present invention. On the other hand, no such anticorrosion film was observed in the comparative examples outside the scope of the present invention.

[0050] Next, for stainless steel material No. 1 of the present invention, the amounts of Cr, Fe, Ti, and N were analyzed in the depth direction from the surface using a glow discharge optical emission spectrometer (GD-OES), and the element depth profile was determined using the following method. The results are shown in Figure 3. As can be seen from Figure 3, Ti had the highest composition ratio of approximately 22 mass% at a position approximately 0.02 μm deep from the surface of the stainless steel material, and gradually decreased as the depth from the surface increased. N had the highest composition ratio of approximately 20 mass% at a position approximately 0.01 μm deep from the surface of the stainless steel material, and gradually decreased as the depth from the surface increased. Furthermore, at the outermost 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 the composition ratio of Ti. The Cr content was lowest at about 4 mass% at the outermost surface of the stainless steel material, gradually increasing with increasing depth from the surface, and reaching a constant content of about 22 mass% at a depth of about 0.05 μm from the surface. The Cr content was also lower than the N, Ti, and Fe content at the outermost surface of the stainless steel material. The Fe content was lowest at about 6 mass% at the outermost surface of the stainless steel material, and rapidly increased with increasing depth from the surface. The Fe content at the outermost surface of the stainless steel material was lower than the Ti and N content but higher than the Cr content. After the Fe content rapidly increased, the Fe content became higher than the Cr, Ti, and N content, reaching a content of about 72 mass% at a depth of 0.5 μm from the surface. Although not shown, similar results were also obtained for stainless steel material No. 3 of the present invention.

[0051] [Measurement of elemental depth profile of stainless steel material] First, test pieces (size: 2 cm x 2 cm) were collected from the stainless steel materials No. 1 and No. 2 of the present invention. Next, the test pieces were set in the holder of the measurement section of a Marcus-type glow discharge optical emission spectrometer (GD-Profiler, manufactured by Horiba, Ltd.), and cathode sputtering was performed in the glow discharge region. The light emission of the sputtered atoms in the Ar plasma was spectroscopically measured, and the elemental depth profile of the stainless steel material was measured.

[0052] Next, an X-ray diffraction test piece was taken from the stainless steel material No. 1 of the present invention, and the film-like phase distributed on the surface layer was investigated using an X-ray diffractometer in a thin film measurement mode in which X-rays are incident at an acute angle. The results are shown in Figure 2(a). From the obtained results (diffraction pattern), austenite (γ) phase and diffraction peaks clearly different from those of the γ phase (diffraction peaks that matched or were close to the diffraction peaks of TiN) were observed. This confirmed that the film-like phase distributed on the surface layer was an austenite phase and a titanium nitride phase. Furthermore, the stainless steel material obtained in the stainless steel material No. 3 of the present invention was also investigated by X-ray diffraction for the film-like phase distributed on the surface layer. The results are shown in Figure 2(b).

[0053] EDS analysis was performed on the surface of stainless steel material No. 1 of the present invention. As a result, for stainless steel material No. 1 of the present invention, the following mass% (semi-quantitative) values ​​were obtained at a depth of approximately 1 μm from the surface of the stainless steel material, which is the reachable range of the electron beam, as measured by EDS analysis: 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. Furthermore, while Al may form compounds such as nitrides, the detected amount of Al was so small that it is believed not to exist in a layered form. Therefore, the presence of Al (compounds) is believed not to affect the corrosion resistance or electrical 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 they are assumed to be contained in the stainless steel material, the amounts are so small that it is believed that they have almost no effect on the corrosion resistance or electrical conductivity of the stainless steel material. For the 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, its content was less than 1%, below the detection limit, and therefore was excluded.

[0054] The composition ratio of elements contained in the surface portion of the sample was measured by the following method. EDS analysis was performed using a scanning electron microscope and energy dispersive X-ray analyzer (SEM-EDS, manufactured by JEOL Ltd.). EDS analysis was performed within a 0.1 mm x 0.1 mm area 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) generated by the detected elements was measured, and the ratio of the elements contained in the surface portion of the sample was calculated from the characteristic X-ray intensity unique to the elements.

[0055] For the stainless steel material No. 1 of the present invention, a thin film hardness tester (HM500, manufactured by Fisher Instruments) was used to press an indenter into the surface, and the plastic hardness (H ＩＴ ) is calculated to be 7345 N / mm 2 This corresponds to a Vickers hardness of 1762 HV. It was confirmed that the film-like phase distributed in the surface layer is a very hard phase. Although not shown, similar results were also obtained for the stainless steel material No. 3 of the present invention.

[0056] From these findings, it was confirmed that the stainless steel material of the present invention has a structure in the outermost layer in which crystalline phases (austenite phase and titanium nitride phase) are distributed in a mixed state in the form of a thin film, and furthermore, that no pinholes or the like are formed, that the stainless steel material is continuously connected to the base material, that it has high hardness, high corrosion resistance, and excellent electrical conductivity.

[0057] Next, the corrosion resistance and electrical conductivity of the stainless steel material of the present invention were investigated. (1) Corrosion Resistance Test (Ferric Chloride Corrosion Test) Plate-shaped test pieces (size: width 20 mm x length 30 mm) were taken from the obtained stainless steel material No. 1 (invention example) and No. 2 (comparative example). After polishing and cleaning the surface, a ferric chloride corrosion test was carried out in accordance with the provisions of JIS G 0578 to evaluate the corrosion resistance. The ferric chloride aqueous solution was prepared by dissolving FeCl in a 0.05 mol / L HCl aqueous solution. ３ ・6H ２ The concentration was adjusted by dissolving O. The Fe concentration was set to 6%.

[0058] The test specimens, whose surfaces had been polished and cleaned, were immersed in aqueous ferric chloride solutions at temperatures of 35°C, 50°C, 65°C, and 80°C in that order for 24 hours each, i.e., for a continuous 96-hour period. The masses of the test specimens were measured before immersion, after 24 hours, 48 ​​hours, 72 hours, and 96 hours, respectively, to calculate the corrosion weight loss. For reference, similar test specimens (as-is) were collected from a ferritic stainless steel plate (SUS445J1) and common austenitic stainless steel plates (SUS304 and SUS316) (cold-rolled plates), and the same ferric chloride corrosion test was performed as described above. The mass changes of the test specimens after immersion were measured, and the corrosion weight loss was calculated. The results are shown in Table 3 and FIG. 6.

[0059]

[0060] The present invention example (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 (corrosion critical temperature: 50°C), and slight corrosion occurred when immersed at 65°C and 80°C. For reference, test pieces No. R1 (SUS304), No. R2 (SUS316), and No. R3 (SUS445J1), which were raw materials, showed corrosion when immersed at 35°C (corrosion critical temperature: 35°C), and corrosion weight loss increased with increasing aqueous solution temperature. Although not shown in Table 3 or Figure 6, similar results were obtained for the stainless steel material obtained as stainless steel material No. 3 of the present invention. From these results, it is clear that the stainless steel material of the present invention has superior corrosion resistance, even compared to austenitic stainless steel materials such as SUS304 and SUS316, which have excellent corrosion resistance.

[0061] (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 a four-terminal method using the contact resistance measurement circuit outlined in Figure 7. For reference, a sample (No. R3) without nitrogen absorption treatment was taken from Material A, and the contact resistance between the sample and carbon paper was measured in the same manner. Similarly, a sample (No. R1) without nitrogen absorption treatment was taken from a general austenitic stainless steel plate (SUS304; plate thickness: 0.1 mm), and the contact resistance between the sample and carbon paper was measured. Furthermore, samples (No. R1A, No. R3A) with gold plating on the surface of Material A and SUS304 steel plate were taken, and the contact resistance between the sample and carbon paper was measured in the same manner.

[0062] As shown in Figure 7, a load cell was fixed to the bottom of a vise, and a stainless steel plate, a Cu plate, carbon paper (CP) and sample, Cu plate, and stainless steel plate were stacked on top of the load cell in this order, and the contact resistance was measured using a milliohm meter with a stress of 1 MPa applied to the sample. Note that in measurement circuit A shown in Figure 7, the stainless steel jig, Cu plate, CP, sample, CP, Cu plate, and stainless steel jig were stacked in this order; in measurement circuit B, the stainless steel jig, Cu plate, CP, CP, CP, Cu plate, and stainless steel jig were stacked in this order; and in measurement circuit C, the stainless steel jig, Cu plate, CP, CP, Cu plate, and stainless steel jig were stacked in this order.

[0063] The contact resistance between the carbon paper (CP) and the sample was measured as follows: Resistance RA was measured using measurement circuit A shown in Figure 7, resistance RB was measured using measurement circuit B, and resistance RC was measured using measurement circuit C. From these resistance values, the contact resistance R between the sample and carbon paper (CP) was calculated using the following formula: R = (RA + RC - 2RB) / 2. The results are shown in Table 4.

[0064]

[0065] For No. R3 (Reference Example), which was made of raw material A (equivalent to SUS445J1) without nitrogen absorption treatment, and No. R1 (Reference Example), which was made of raw SUS304, the contact resistance R between the sample and carbon paper (CP) was approximately 80 mΩ. However, the contact resistance R between sample No. 1 (Invention Example) and carbon paper (CP) was a low value below 10 mΩ, which is close to the contact resistance values ​​measured for gold-plated material A (No. R3A) and gold-plated SUS304 (No. R1A), both of which were measured for reference. In other words, the stainless steel material (Invention Example) having a structure in which crystalline phases (austenite phase and titanium nitride phase) are distributed in a thin film-like mixture on the surface layer has conductivity approximately equivalent to that of a gold-plated film. Although not shown in Table 4, similar results were obtained for the stainless steel material obtained from the present invention, No. 3.

[0066] For these reasons, the stainless steel material of the present invention has superior corrosion resistance and conductivity compared to the general austenitic stainless steel material SUS304 and the ferritic stainless steel material SUS445J1, and can be said to be suitable for use as separators in solid polymer fuel cells.

[0067] According to the present invention, a stainless steel material having excellent corrosion resistance can be obtained, and is of great industrial value, since it exhibits excellent corrosion resistance, particularly in a high-temperature, severely corrosive environment such as a ferric chloride corrosion test in accordance with JIS G 0578. Furthermore, the stainless steel material of the present invention has excellent electrical conductivity equivalent to that of a gold plating film, and has the effect that structures made of the stainless steel material of the present invention can be used, for example, as separators for polymer electrolyte fuel cells.

Claims

1. A stainless steel material having an austenitic phase structure, wherein at least a surface layer of the stainless steel material has a structure in which an austenitic phase and a titanium nitride phase are distributed.

2. The stainless steel material according to claim 1, having a composition containing, by mass%, Cr from 16% to 30%, Ti from 0.03% to 1%, N from 0.3% to 1.5%, Si from 1.0%, Mn from 1.0%, C from 0.1%, the balance being Fe and unavoidable impurities, and not containing Ni.

3. A stainless steel material according to claim 2, further comprising, in addition to the above composition, one or two of Nb and Al, the total of Ti, Nb and Al being 0.1% or more and 1% or less by mass%.

4. A stainless steel material according to claim 2 or 3, further comprising, in addition to the above composition, 3% or less of Mo by mass.

5. A stainless steel structure comprising the stainless steel material according to claim 1.

6. The stainless steel structure according to claim 5, which is a separator for a polymer electrolyte fuel cell.

7. The method for manufacturing the stainless steel material according to claim 1, comprising the steps of: heating a ferritic stainless steel material containing Ti under a vacuum at a predetermined temperature T １ A first heating step of heating the material to the predetermined temperature T １ After reaching a temperature of 100° C., nitrogen gas is introduced and the temperature is further increased to a predetermined temperature T ２ Heating is continued until the predetermined temperature T ２ The method for manufacturing a stainless steel material comprises, in this order, a second heating step of holding the material at this temperature for a predetermined period of time, and a quenching treatment step of quenching the material.

8. The above vacuum is 10 ―２ The method for producing a stainless steel material according to claim 7, wherein the pressure is 0.1 Pa or less.

9. The method for producing a stainless steel material according to claim 7 or 8, wherein the nitrogen gas atmosphere is an atmosphere having a nitrogen gas pressure of 100 Pa or more and 200 kPa or less.

10. A method for producing a stainless steel material as set forth in claim 7 or 8, wherein the rapid cooling is a process for cooling at a cooling rate of 100°C / min or more.

11. The above-mentioned specified temperature T １ is a temperature in the range of 800° C. to 1150° C., and the above-mentioned predetermined temperature T ２ However, at a temperature T １ <Temperature T ２ The method for producing a stainless steel material according to claim 7 or 8, 12. A method for producing a stainless steel material as described in claim 7 or 8, wherein the Ti-containing ferritic stainless steel material has a composition that contains, by mass%, 16% or more and 30% or less of Cr, 0.03% or more and 1% or less of Ti, 1.0% or less of Si, 1.0% or less of Mn, 0.1% or less of C, the balance being Fe and unavoidable impurities, and does not contain Ni.

13. A method for producing a stainless steel material as described in claim 12, further comprising, in addition to the above composition, containing, by mass%, one or two of Nb and Al, with the total of Ti, Nb and Al being 0.1% or more and 1% or less.

14. The method for producing a stainless steel material according to claim 12, further comprising, in addition to the above composition, containing, by mass%, 3% or less of Mo.

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