High corrosion-resistant stainless steel components and their manufacturing methods, heat treatment methods for stainless steel components, and rolling bearings and their manufacturing methods.

Abstract

This invention provides a highly corrosion-resistant stainless steel component made of martensitic stainless steel, which does not contain ferrite in its surface layer, thus achieving both high corrosion resistance and high hardness. The highly corrosion-resistant stainless steel component contains, by weight, 0.35%–0.43% C, less than 0.5% Si, less than 0.5% Mn, less than 0.04% P, less than 0.04% S, 15%–17% Cr, 0.1%–0.3% W, 1.5%–3.0% Mo, 0.001%–0.005% B, and 0.12%–0.18% N. The remainder is composed of martensitic stainless steel formed by Fe and unavoidable impurities. The matrix structure of the entire outer surface layer of this highly corrosion-resistant stainless steel component is a duplex structure containing retained austenite and martensite. The surface hardness of this highly corrosion-resistant stainless steel component is HRC57 or higher.

Description

Technical Field

[0001] This invention relates to a highly corrosion-resistant stainless steel component with excellent corrosion resistance. Background Technology

[0002] Generally, martensitic stainless steels, such as SUS440C, are used as bearing materials in rolling bearings requiring high corrosion resistance. However, although SUS440C contains 16% to 18% wt% chromium to improve corrosion resistance, its carbon content is also high (0.95% to 1.2% wt%) to ensure hardness. This results in the formation of many chromium carbides around 20 μm in diameter, thus its corrosion resistance is not very high. Therefore, it is not suitable for use in severely corrosive environments such as exposure to strong alkaline disinfectants, seawater, or rainwater. Furthermore, ferritic and austenitic stainless steels have superior corrosion resistance compared to martensitic stainless steels, but they have low strength. For example, even after cold working, the hardness of austenitic stainless steel is only around HRC40, making it almost unusable in rolling bearings.

[0003] Therefore, as a martensitic stainless steel that combines high corrosion resistance and high hardness, a high corrosion-resistant martensitic stainless steel, as described in Patent Document 1, has been developed. This stainless steel contains nitrogen and molybdenum to reduce carbon content, thereby achieving both high corrosion resistance and high hardness.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent No. 5368887 Summary of the Invention

[0007] The problem that the invention aims to solve

[0008] The high-corrosion-resistant martensitic stainless steel disclosed in Patent Document 1 contains a large amount of nitrogen in solid solution. Such martensitic stainless steel with a high nitrogen content is quenched in a vacuum furnace to achieve the desired hardness. Chromium and molybdenum are elements that promote ferrite formation, while nitrogen is an austenite stabilizing element that inhibits ferrite formation. Therefore, during vacuum quenching, as nitrogen detaches from the surface layer, the nitrogen concentration decreases, thereby weakening the ferrite inhibition effect. Sometimes, ferrite forms in the surface layer, preventing the achievement of the desired hardness. JIS B1511:1993, the standard for rolling bearings, requires the hardness of the bearing rings to be in the range of HRC57 to 65. However, the inventors have confirmed that by generating ferrite, the hardness of the surface layer (approximately 50 μm deep from the surface) can sometimes be only slightly less than HRC55.

[0009] Furthermore, ferrite has a body-centered cubic lattice structure, resulting in a low solid solution limit for carbon. The solid solution limit for carbon in ferrite is only about 0.02% by weight at 727°C. Therefore, when cooling from the austenite temperature range causes ferrite to precipitate at the surface, carbon is expelled from the ferrite. Consequently, carbon accumulates around the ferrite, forming chromium carbide. Since the chromium around the ferrite is used in the carbides, it becomes a chromium-deficient layer, resulting in problems such as decreased corrosion resistance around the ferrite.

[0010] The present invention was made in view of the above circumstances, and its object is to provide a high corrosion-resistant stainless steel component that does not contain ferrite in the surface layer, thereby achieving both high corrosion resistance and high hardness.

[0011] Solution for solving the problem

[0012] The inventors have discovered that for highly corrosion-resistant martensitic stainless steel with a large amount of nitrogen dissolved in it, quenching it in a nitrogen environment with a nitrogen partial pressure of 1000 Pa or higher but less than 10000 Pa at a temperature ranging from 1050°C to 1120°C can suppress the detachment of dissolved nitrogen from the surface layer, and prevent the formation of ferrite structure in the surface layer.

[0013] This invention is based on the above-mentioned understanding and is a high corrosion-resistant stainless steel component as follows: The high corrosion-resistant stainless steel component contains, by weight, 0.35%–0.43% C, less than 0.5% Si, less than 0.5% Mn, less than 0.04% P, less than 0.04% S, 15%–17% Cr, 0.1%–0.3% W, 1.5%–3.0% Mo, 0.001%–0.005% B, and 0.12%–0.18% N, with the remainder composed of a high corrosion-resistant martensitic stainless steel formed by Fe and unavoidable impurities. The matrix structure of the entire outer surface layer of the high corrosion-resistant stainless steel component is a duplex structure containing retained austenite and martensite, and the surface hardness of the high corrosion-resistant stainless steel component is HRC57 or higher. Here, "surface layer" refers to the area from the surface to a depth of approximately 50 μm.

[0014] In the high corrosion-resistant stainless steel component of the present invention, the matrix structure of the entire outer surface layer exhibits a dual-phase mixed structure containing retained austenite and martensite. Therefore, the ferrite area ratio of the surface layer is zero, i.e., ferrite is absent. As a result, a high surface hardness of HRC57 or higher can be obtained. Furthermore, since there is no ferrite in the surface layer, carbon does not locally accumulate, thus suppressing the formation of a chromium-deficient layer due to the formation of chromium carbide, thereby improving corrosion resistance.

[0015] Another feature of the invention is a rolling bearing, wherein the outer ring and / or inner ring of the rolling bearing are made of the aforementioned highly corrosion-resistant stainless steel component. Yet another feature of the invention is an assembly comprising a plurality of individual components, wherein at least one of the individual components is the aforementioned highly corrosion-resistant stainless steel component.

[0016] Another feature of the present invention is a heat treatment method for highly corrosion-resistant stainless steel components, comprising the following steps: preparing an intermediate component, wherein the intermediate component contains, by weight, 0.35% to 0.43% C, less than 0.5% Si, less than 0.5% Mn, less than 0.04% P, less than 0.04% S, 15% to 17% Cr, 0.1% to 0.3% W, 1.5% to 3.0% Mo, 0.001% to 0.005% B, and 0.12% to 0.18% N, the remainder being composed of highly corrosion-resistant martensitic stainless steel formed by Fe and unavoidable impurities; and quenching the intermediate component by heating it to a temperature in the range of 1050°C to 1120°C in a nitrogen environment with a nitrogen partial pressure of 1000 Pa or higher and less than 10000 Pa. Another feature of the present invention is a method for manufacturing highly corrosion-resistant stainless steel components, comprising the above-described heat treatment method for highly corrosion-resistant stainless steel components.

[0017] Next, the reasons for limiting the ingredients in this invention will be explained. It should be noted that, unless otherwise specified, "%" in the following description refers to "weight %".

[0018] •C: 0.35%~0.43%

[0019] C is effective in ensuring the hardness (wear resistance) of steel components, but as it is also an austenite-forming element, excessive addition can easily lead to the formation of eutectic carbides, which can cause cracking. Furthermore, excessive addition can also degrade corrosion resistance; therefore, 0.43% is set as the upper limit for achieving good corrosion resistance. And 0.35% is set as the lower limit for achieving a hardness of HRC57 or higher without forming ferrite in the surface layer after heat treatment.

[0020] Si: less than 0.5%

[0021] Excessive Si content can significantly reduce toughness and is detrimental to hot workability. Therefore, Si content should be low, and considering manufacturing costs, it should be set below 0.5%.

[0022] Mn: less than 0.5% by weight

[0023] Mn is an austenite stabilizing element. Excessive addition will increase the amount of residual austenite, thus reducing the hardness after heat treatment and deteriorating corrosion resistance. In addition, it is prone to dimensional changes over the years. Therefore, Mn should be used in small amounts, and considering manufacturing costs, its content is set below 0.5%.

[0024] • P: below 0.04%

[0025] P is a component that can precipitate at grain boundaries, causing cold brittleness. Therefore, to avoid cold brittleness, it is ideal to have as little P as possible. To balance manufacturing costs, its content is set to below 0.04%.

[0026] •S: below 0.04%

[0027] S can degrade corrosion resistance or hot workability, therefore its content is set to be below 0.04%.

[0028] ·Cr: 15%–17%

[0029] For stainless steel, chromium (Cr) forms a strong insulating coating, making it an indispensable element for achieving high corrosion resistance, and thus requiring a significant addition. In salt spray tests, as discussed later, when the Cr content is less than 15%, even with sufficient nitrogen (N) content, good corrosion resistance cannot be achieved; therefore, 15% is set as the lower limit. However, Cr can also lead to ferrite formation, hindering martensiticization. When the Cr content exceeds 17%, ferrite forms on the surface after quenching, resulting in decreased hardness; therefore, 17% is set as the upper limit.

[0030] Mo: 1.5%–3.0%

[0031] Mo has the effect of increasing the solid solution limit of N and improving corrosion resistance and hardenability. To achieve these effects, more than 1.5% Mo needs to be added. However, excessive addition will lead to a decrease in toughness and the formation of ferrite near the surface, so 3.0% is set as the upper limit.

[0032] • N: 0.12%–0.18%

[0033] Nitrogen (N) is a highly effective element for improving the surface hardness and corrosion resistance of martensitic stainless steel after heat treatment. To achieve this effect, the N content needs to be at least 0.12%. On the other hand, the solution limit for producing martensitic stainless steel that does not produce blow-like bubbles in atmospheric dissolution (which is more economical than pressurized dissolution) and is suitable for practical applications is 0.18%, therefore, 0.18% is set as the upper limit. This controls manufacturing costs.

[0034] • B: 0.001%~0.005%

[0035] When boron (B) is added, boron nucleus (BN) is precipitated, which effectively improves strength and hardenability. To achieve this effect, more than 0.001% of boron needs to be added. On the other hand, excessive addition will lead to a decrease in toughness, so the upper limit for the amount added is set below 0.005%.

[0036] • W: 0.1%–0.3%

[0037] W is a component that improves corrosion resistance and acts as a solid solution strengthening element, thus contributing to increased strength. To achieve this effect, an addition of 0.1% or more of W is required. On the other hand, excessive addition can lead to a decrease in toughness, so an upper limit of 0.3% is set to ensure that the desired performance is achieved without hindrance.

[0038] ·Matrix tissue

[0039] Ideally, the matrix structure is a duplex microstructure consisting of retained austenite comprising less than 13% by volume and the remainder being martensite. By suppressing the soft retained austenite to less than 13% by volume and setting the remainder as martensite, a hardness of HRC57 or higher can be ensured. It should be noted that the matrix structure refers to the microstructure of the matrix (matrix) excluding carbides, nitrides, and inclusions.

[0040] Invention Effects

[0041] According to the present invention, a high corrosion-resistant stainless steel component is provided that does not contain ferrite in its surface layer, thereby achieving both high corrosion resistance and high hardness. Attached Figure Description

[0042] Figure 1 This is a cross-sectional view showing a rolling bearing according to an embodiment of the present invention.

[0043] Figure 2 This is a cross-sectional view showing the outer ring (A) and inner ring (B) of the rolling bearing in the embodiment.

[0044] Figure 3 This is a cross-sectional view of an example of the outer ring (A) and inner ring (B) of a comparative rolling bearing.

[0045] Figure 4 This is a cross-sectional view of an example of the outer ring (A) and inner ring (B) of a rolling bearing, representing another comparative example.

[0046] Figure 5 These are microscope images of metallographic structures according to embodiments of the present invention. Detailed Implementation

[0047] Figure 1This is a cross-sectional view of a rolling bearing (deep groove ball bearing, assembly) 10 according to an embodiment of the present invention. Figure 1 As shown, the rolling bearing 10 has an outer ring 1 and an inner ring 2 as raceways. An arc-shaped raceway 1a is formed on the inner circumferential surface of the outer ring 1, and an arc-shaped raceway 2a is formed on the outer circumferential surface of the inner ring 2. A plurality of balls 3 are arranged at equal intervals along the circumferential direction between the raceways 1a and 2a as rolling elements. The plurality of balls 3 are respectively held in a plurality of pockets of a cage 4. The cage 4 can be formed, for example, from a resin such as polyamide or polyetheretherketone, or from a metal. Furthermore, the type of cage 4 is not particularly limited; any shape such as a crown cage, a cut cage, or a corrugated cage can be selected. Figure 1 The fourth type of cage is a crown cage.

[0048] The bearing space 5 between the outer ring 1 and the inner ring 2 is sealed by a metal sealing member 6 (metal shield). The sealing member 6 is not limited to a metal shield; non-contact or contact rubber seals can also be used. Furthermore, grease is sealed into the bearing space 5 as a lubricant. The grease used is selected according to the application of the rolling bearing 10. Representative greases include lithium soap grease and urea grease, but are not limited to these.

[0049] The outer ring 1 and inner ring 2 are formed of highly corrosion-resistant martensitic stainless steel. Furthermore, the heat treatment of the present invention, including quenching, cryogenic treatment, and tempering, is performed on the outer ring 1 and inner ring 2. The matrix structure of the surface layer throughout the entire surface of the outer ring 1 and inner ring 2 consists of martensite and less than 13% by volume of retained austenite, with no ferrite formed.

[0050] That is, the ferrite area ratio of the surface layer is zero across the entire surface of both the outer ring 1 and the inner ring 2. As a result, the surface and internal hardness are increased to HRC57 or higher. It should be noted that, depending on the application, sometimes only the outer ring or the inner ring requires high corrosion resistance. In such cases, the high corrosion-resistant stainless steel component of this invention can be used only on the outer ring or the inner ring. For example, in the rolling bearings supporting sliding doors of automobiles, the outer ring is primarily exposed to rainwater and mud, therefore, higher corrosion resistance is required on the outer ring side.

[0051] The ball 3 can be made of metal or ceramic. It should be noted that the rolling elements of a rolling bearing are not limited to the spherical ball 3; cylindrical rollers can also be used to make the rolling bearing a roller bearing. When the ball 3 is made of metal, its material can be the same as that of the outer ring 1 and inner ring 2—a highly corrosion-resistant martensitic stainless steel. This results in a ball 3 with the same or higher level of corrosion resistance and hardness as the outer ring 1 and inner ring 2. However, if the operating environment is not severely corrosive, grease can provide some degree of rust prevention for the ball 3. Therefore, the ball 3 can also be made of bearing steel with lower corrosion resistance than highly corrosion-resistant martensitic stainless steel (e.g., SUJ2), or conventional bearing martensitic stainless steel (e.g., SUS440C).

[0052] Figure 2 The outer ring 1 and inner ring 2 of this embodiment after heat treatment according to the present invention are shown. Figure 2 As shown, in this embodiment, no ferrite is formed in the surface layer of the entire surface of the outer ring 1 and inner ring 2. On the other hand, Figure 3 The outer ring 1 and inner ring 2 of the comparative example are shown after undergoing the same heat treatment. In the outer ring 1 and inner ring 2 of the comparative example, ferrite is formed on the surface layer of the entire surface. After heat treatment, the end face, outer cylindrical surface (outer diameter surface) and raceway 1a of the outer ring 1 and the end face, inner cylindrical surface (inner diameter surface) and raceway 2a of the inner ring 2 are finished by grinding.

[0053] Figure 4 It shows the Figure 3 The outer and inner rings are subjected to finishing grinding to remove the ferrite layer from the surface. For example... Figure 4 As shown, the end face, outer cylindrical surface, and raceway 1a of the outer ring 1, and the end face, inner cylindrical surface, and raceway 2a of the inner ring 2 are finished by grinding, thus removing the ferrite layer on the surface of these parts. However, in the cylindrical surfaces located axially outside raceways 1a and 2a, the sealing grooves for assembling the sealing member 6, and the chamfered portions, which were not ground during finishing, a ferrite layer still remains on the surface after finishing. As mentioned above, the ferrite layer on the surface leads to a decrease in corrosion resistance and hardness. Therefore, using... Figure 4 The rolling bearings shown, with outer ring 1 and inner ring 2, contain residual portions with poor corrosion resistance and hardness, which is undesirable. Furthermore, the surface of raceway 1a requires ultra-precision machining to achieve a satisfactory finish. Figure 4 In this state, excessive removal of ferrite is difficult and leads to increased manufacturing costs. Therefore, in order to control manufacturing costs, it is also important to prevent the formation of a ferrite layer on the surface during heat treatment.

[0054] Next, the heat treatment conditions for the rolling bearing used to obtain the embodiment will be described.

[0055] Ideally, after forming the outer and inner rings through machining, the rings are quenched in a heat treatment furnace with a nitrogen environment of 1000 Pa to 1120 Pa, heated to a temperature ranging from 1050°C to 1120°C. Next, they are cryogenically cooled to a temperature ranging from -30°C to -90°C, followed by tempering at a temperature ranging from 150°C to 200°C. This is because cryogenic treatment is effective in reducing the amount of retained austenite and increasing hardness.

[0056] Nitrogen partial pressure during quenching

[0057] When the nitrogen partial pressure is less than 1000 Pa, the nitrogen concentration in the surface layer decreases during quenching, leading to the formation of ferrite. On the other hand, when the nitrogen partial pressure is above 10000 Pa, in the case of the martensitic stainless steel of this invention, there is a risk that nitrogen will dissolve in the surface layer, resulting in an excessively high nitrogen concentration. External nitrogen dissolution increases the amount of residual austenite formed after quenching and tempering, leading to a decrease in tempering hardness. Furthermore, while adding nitrogen can generate nitrides, excessive addition due to external dissolution results in a greater decrease in toughness compared to an increase in hardness, promoting brittle fracture.

[0058] Therefore, to prevent nitrogen from detaching from the surface and to avoid nitrogen dissolving from the outside, it is ideal to have a nitrogen partial pressure of 1000 Pa or more but less than 10000 Pa. To obtain such a nitrogen environment, it is preferable to introduce nitrogen gas after reducing the furnace pressure from atmospheric pressure to below 200 Pa, more preferably below 100 Pa. By sufficiently reducing the pressure inside the furnace before introducing nitrogen gas, the amount of gases and moisture other than nitrogen can be reduced, thereby preventing unintended reactions with the metal.

[0059] Quenching temperature

[0060] If the quenching temperature is below 1050℃, the formation of martensite due to rapid cooling (oil or water quenching) is insufficient, making it difficult to obtain a hardness above HRC57. On the other hand, when the quenching temperature exceeds 1120℃, the coarsening of the old austenite grains and the formation of dissolved carbides also make it difficult to obtain a hardness above HRC57. Therefore, ideally, the quenching temperature should be between 1050℃ and 1120℃.

[0061] It should be noted that the present invention is not limited to the rings and rolling elements of rolling bearings, but can also be applied to all highly corrosion-resistant stainless steel parts used as bolts, nuts and other mechanical components.

[0062] Example

[0063] Table 1 shows the composition of the examples and comparative examples of martensitic stainless steels by weight % (%). Furthermore, the ideal content range of the present invention is referred to as the effective range and is shown.

[0064] 1. Hardness and corrosion resistance survey

[0065] By machining martensitic stainless steel bars with the composition shown in Table 1, intermediate parts with an outer diameter of 13 mm, an inner diameter of 11.54 mm, and a height of 4 mm were produced. The parts were then quenched in a heat treatment furnace under the nitrogen partial pressure and quenching temperature conditions shown in Table 1, and subjected to low-temperature treatment in the temperature range of -30℃ to -90℃. After that, they were tempered in the temperature range of 150℃ to 200℃, thereby obtaining ring-shaped samples.

[0066] The hardness of the thus obtained sample at a depth of 20 μm from the surface was measured. Furthermore, after mirror polishing of the sample cross-section, etching was performed, and the microstructure of three regions with a depth of 50 μm and a width of 100 μm from the surface was observed using a metal microscope. Then, the... Figure 5 The tissue images shown were analyzed, and the area fraction (area %: area percentage) of ferrite per 50 μm × 100 μm region was calculated. The average value of this area fraction is shown in Table 1. Furthermore, the amount of retained austenite (retained γ) was measured using an X-ray stress measuring apparatus (PROTO, model iXRD), and the volume fraction (volume %) based on X-ray diffraction was determined.

[0067] In addition, plates with a length of 50 mm, a width of 20 mm, and a thickness of 2 mm were produced from martensitic stainless steel bars with the compositions shown in Table 1 by machining, and were heat-treated under the same conditions as described above. For the obtained samples, a 96-hour neutral salt spray test was conducted according to JIS Z2371, and the grading values ​​were evaluated based on the grading method of JIS Z2371:2015. Corrosion resistance with a grading value of 9.8 or higher was judged as good and rated "A", while corrosion resistance with a grading value less than 9.8 was evaluated as insufficient and rated "B". The above measurement and test results, along with the material composition of each sample, are shown in Table 1.

[0068] [Table 1]

[0069]

[0070] As shown in Table 1, Examples 1-5 all meet the composition range required for essential components of the present invention, and also meet the ideal ranges for nitrogen partial pressure and quenching temperature. As a result, in the surface layer, the ferrite area ratio is zero and ferrite is absent, forming a matrix structure consisting of a duplex microstructure comprising 8.3 vol% to 12.2 vol% of retained austenite and martensite. Furthermore, while an increase in carbides and inclusions with a major diameter exceeding 10 μm dispersed in the matrix can negatively impact corrosion resistance, in Examples 1-5, over 95% of the carbides and inclusions in the duplex microstructure dispersed in the matrix have a major diameter of 10 μm or less. Therefore, in all examples, the corrosion resistance rating is 9.8 or higher, and the corrosion resistance is rated as "A" (good). Moreover, the surface layer hardness is HRC57 or higher, meeting the hardness requirements for rolling bearing races specified in JIS B1511:1993.

[0071] In contrast, in Comparative Example 1, because the nitrogen partial pressure during quenching was set to 1000 Pa, no ferrite was formed in the surface layer. However, since the C content was less than 0.35%, the surface hardness was HRC55, which does not meet the JIS B1511:1993 standard for rolling bearings. In Comparative Example 2, because the nitrogen partial pressure during quenching was set to 2000 Pa, no ferrite was formed in the surface layer. However, since the N content was less than 0.12%, the hardness was only HRC56.

[0072] In Comparative Example 3, since the nitrogen partial pressure during quenching was set to 1000 Pa, the same as in Comparative Example 1, no ferrite was formed in the surface layer. However, because the quenching temperature was less than 1050 °C, the formation of martensite was insufficient, resulting in a hardness of only HRC56. In Comparative Example 4, since the nitrogen partial pressure during quenching was 2000 Pa, no ferrite was formed in the surface layer. However, because the quenching temperature exceeded 1120 °C, the hardness was only HRC56 due to the coarsening of the old austenite grains and the solid solution of carbides.

[0073] In Comparative Example 5, since the nitrogen partial pressure during quenching was only 70 Pa, the ferrite content in the surface layer was as high as 28% of the area, and the hardness was only HRC51. In Comparative Example 6, since the nitrogen partial pressure during quenching was 700 Pa, the ferrite content in the surface layer reached 19% of the area, and the hardness was only HRC52.

[0074] In Comparative Example 7, although the nitrogen partial pressure during quenching was 1000 Pa, the ferrite content in the surface layer was 6% by area and the hardness was HRC54. It is believed that in Comparative Example 7, the Cr content, a ferrite-forming element, exceeded 17%, thus ferrite was formed in the surface layer after quenching, leading to a decrease in hardness.

[0075] In Comparative Example 8, although the nitrogen partial pressure during quenching was 1000 Pa, the ferrite content in the surface layer was 4% by area and the hardness was HRC53. This is attributed to the fact that in Comparative Example 8, the C content was less than 0.35%, resulting in insufficient austenite formation and the presence of residual ferrite, and the N content was less than 0.12%.

[0076] In Comparative Example 9, since the nitrogen partial pressure during quenching was 2000 Pa, ferrite was not present in the surface layer. However, because the C content exceeded 0.43%, the grade value was 7, and therefore, sufficient corrosion resistance could not be said to be achieved; the corrosion resistance evaluation was "B" (insufficient). In Comparative Example 10, since the nitrogen partial pressure during quenching was 2000 Pa, no ferrite was formed. However, because the N content was low at 0.10%, the grade value was 8, and the corrosion resistance evaluation was "B".

[0077] In Comparative Example 11, because the nitrogen partial pressure during quenching was 2000 Pa, no ferrite was formed. However, due to the low Cr content of 14.73%, sufficient corrosion resistance could not be obtained, resulting in a rating of 8 and a corrosion resistance evaluation of "B". In Comparative Example 12, because the nitrogen partial pressure during quenching was 2000 Pa, no ferrite was formed. However, due to the low Mo content of 1.11%, the rating was 8, and the corrosion resistance evaluation was "B".

[0078] It should be noted that, for comparison purposes, the composition and test results of SUS440C are recorded in Table 1. Table 1 clearly shows that SUS440 has a hardness of HRC57, which is suitable for use in rolling bearings, but its grade is 5, and its corrosion resistance is rated "B," therefore it is not durable in severely corrosive environments.

[0079] 2. Organizational observation

[0080] The following is a detailed description of the microstructure observations conducted on Examples 1-3, where the ferrite content was within the effective range and there was no ferrite in the surface layer (i.e., the ferrite area ratio was zero), and Comparative Examples 5 and 6, where the ferrite content was within the effective range but ferrite was present in the surface layer. Table 2 shows the amount of ferrite at three locations in the surface layer of each sample by ferrite area ratio (area %). Table 3 shows the average value of the ferrite area ratio at three locations and the average value of the Rockwell C hardness (HRC) at three locations 20 μm deep from the surface of each sample. It should be noted that the numbers to the left of the hyphens on the sample labels in Table 2 (“70” to “7000”) represent the nitrogen partial pressure (in Pa) during quenching.

[0081] In the tissue observation, after mirror polishing of the sample cross-section, etching was performed with nitric acid ethanol etching solution, and three areas with a depth of 50 μm and a width of 100 μm from the surface were photographed using a metal microscope. Ferrite is difficult to etch and appears white, so this part was blackened by image processing and the area fraction was measured. Figure 5 The microstructure of the surface layer obtained through image processing is shown. For samples 70-1 to 70-3 and 700-1 to 700-3, which had a nitrogen partial pressure of less than 1000 Pa during quenching, the upper part of the surface layer is shown as black in the microstructure images after image processing, clearly indicating the formation of ferrite near the sample surface. For samples with a nitrogen partial pressure of 1000 Pa or more during quenching, no part is shown as black after image processing, indicating that no ferrite has formed. For each sample, the area ratio of ferrite relative to a region with a depth of 50 μm and a width of 100 μm from the surface was calculated using the microstructure images obtained through image processing, and the average value of this area ratio was set as the ferrite area ratio of the surface layer of each sample.

[0082] It should be noted that the ferrite area fraction was calculated using the same method for Comparative Examples 7 and 8. As shown in Table 2, in Comparative Example 5, where the nitrogen partial pressure during quenching was 70 Pa, ferrite with an area fraction of 26% or more was formed in the surface layer, and ferrite with an area fraction of 16% to 23% was formed at 700 Pa. Furthermore, when the nitrogen partial pressure during quenching was 1000 Pa or more, no ferrite was formed, and the ferrite area fraction was zero. In addition, as shown in Table 3, when the nitrogen partial pressure during quenching was 1000 Pa or more, the hardness of the portion with a depth of 20 μm from the surface was HRC59 or more. Therefore, it can be concluded that setting the nitrogen partial pressure during quenching to 1000 Pa or more is effective in preventing ferrite formation in the surface layer.

[0083] [Table 2]

[0084]

[0085] [Table 3]

[0086]

[0087] 3. Life test

[0088] Single-row deep groove ball bearings were fabricated using the materials from Examples 1-5 and Comparative Examples 5-8 for both the inner and outer rings, serving as experimental rolling bearings. The outer ring had an outer diameter of 13 mm, an inner diameter of 11.54 mm, and a width of 4 mm; the inner ring had an outer diameter of 9 mm, an inner diameter of 7 mm, and a width of 4 mm. The balls had a diameter of 1.588 mm and were made of DD400 (martensitic stainless steel, hardness HRC60). A crown-type cage made of polyamide was used as the cage.

[0089] The test rolling bearing was supported for rotation while maintaining a horizontal orientation by mounting its outer ring to a holder and fixing its inner ring to one end of the shaft, with the other end of the shaft inserted into a test apparatus. The shaft was then rotated at 5400 rpm while a radial load of 431 N (44 kgf) was applied to the holder in the vertical direction until the test rolling bearing mounted on the holder locked (the shaft stopped rotating). The elapsed time from the start of the test to the locking of the test rolling bearing is defined as the locking time, and the average locking time of ten locking times is defined as the evaluation index. The results are shown in Table 4.

[0090] In Table 4, for ease of understanding, the numbers of the embodiments and comparative examples of the rolling bearings are set to be the same as the numbers of the embodiments and comparative examples of the materials. For example, the rolling bearing using the material of Embodiment 1 is referred to as Embodiment 1. Furthermore, to confirm the effect of the ferrite layer, in the comparative examples, the inner and outer rings were precision machined while the surface hardness of the raceway surface was insufficient, in other words, a ferrite layer remained in the surface portion. Therefore, the surface portion of the inner and outer rings of the comparative examples after dimensional precision machining is in a state such as... Figure 3 (A) and Figure 3 The state shown in (B). In addition, ten rolling bearings of each embodiment and each comparative example were assigned sample numbers 1 to 10.

[0091] [Table 4]

[0092]

[0093] As shown in Table 4, the average lock-up time of the rolling bearings in Examples 1-5 was 46 to 66 hours, while the average lock-up time of the rolling bearings in Comparative Examples 5-8, where ferrite was present in the surface layer, was only 3 to 4 hours. These results confirm that the rolling bearings of the present invention have a long service life because they do not contain ferrite in the surface layer and have sufficient hardness.

[0094] Industrial availability

[0095] This invention can be used in the field of highly corrosion-resistant stainless steel components such as rolling bearings, and can be appropriately used in the field of highly corrosion-resistant stainless steel components intended for use in particularly severe corrosive environments. Furthermore, while the above embodiments illustrate a rolling bearing equipped with highly corrosion-resistant stainless steel components, the invention is not limited thereto; the highly corrosion-resistant stainless steel components of this invention can be used in assemblies intended for use in particularly severe corrosive environments.

[0096] Explanation of reference numerals in the attached figures

[0097] 1: Outer ring (high corrosion-resistant stainless steel component); 1a: Raceway; 2: Inner ring (high corrosion-resistant stainless steel component); 2a: Raceway; 3: Ball (rolling element); 4: Cage; 5: Bearing space; 6: Sealing component; 10: Rolling bearing (assembly).

Claims

1. A high corrosion resistance stainless steel member, wherein the high corrosion resistance stainless steel member is composed of a high corrosion resistance martensitic system stainless steel containing, in terms of weight ratio, 0.35 to 0.43% of C, 0.5% or less of Si, 0.5% or less of Mn, 0.04% or less of P, 0.04% or less of S, 15 to 17% of Cr, 0.1 to 0.3% of W, 1.5 to 3.0% of Mo, 0.001 to 0.005% of B, 0.12 to 0.18% of N, and the balance of Fe and inevitable impurities, a matrix structure of a surface layer portion of the entire outer surface of the high corrosion resistance stainless steel member is a duplex mixed structure composed of only residual austenite and martensite, and a surface hardness of the high corrosion resistance stainless steel member is HRC 57 or more, the surface layer portion is a range from the entire outer surface to a depth of 50 μm, the duplex mixed structure contains 8.3% by volume or more and 15% by volume or less of residual austenite, the high corrosion resistance stainless steel member is quenched by being heated to a temperature in the range of 1050°C to 1120°C in a nitrogen atmosphere having a nitrogen partial pressure of 1000 Pa or more and less than 10000 Pa.

2. The high corrosion resistance stainless steel member according to claim 1, wherein the duplex mixed structure contains 8.3% by volume or more and 13% by volume or less of residual austenite.

3. The high corrosion resistance stainless steel member according to claim 1 or 2, wherein a length diameter of carbides is 10 μm or less for 95% or more of the number of carbides dispersed in the duplex mixed structure.

4. The high corrosion resistance stainless steel member according to claim 1 or 2, wherein a classification number after a neutral salt spray test based on JIS Z2371 standard for 96 hours is 9.8 or more.

5. The high corrosion resistance stainless steel member according to claim 1 or 2, wherein the high corrosion resistance stainless steel member is tempered after the quenching by being cooled to a temperature in the range of -30°C to -90°C and then being heated to a temperature in the range of 150°C to 200°C.

6. The high corrosion resistance stainless steel member according to claim 1 or 2, wherein the high corrosion resistance stainless steel member is a race of a rolling bearing.

7. A rolling bearing in which a plurality of rolling elements are arranged between an inner ring and an outer ring of the rolling bearing, wherein at least the outer ring or the inner ring is the race according to claim 6.

8. A rolling bearing in which a plurality of rolling elements are arranged between an inner ring and an outer ring of the rolling bearing, wherein the inner ring and the outer ring are the races according to claim 6.

9. An assembly comprising a plurality of individual members, wherein at least one of the individual members is the high corrosion resistance stainless steel member according to claim 1 or 2.

10. A heat treatment method of a high corrosion resistance stainless steel member for manufacturing the high corrosion resistance stainless steel member according to claim 1, comprising the steps of: the high corrosion resistance martensitic system stainless steel is heated to a temperature in the range of 1050°C to 1120°C in a nitrogen atmosphere having a nitrogen partial pressure of 1000 Pa or more and less than 10000 Pa, An intermediate member is prepared from a high corrosion-resistant martensitic stainless steel containing, by weight, 0.35 to 0.43% C, 0.5% or less Si, 0.5% or less Mn, 0.04% or less P, 0.04% or less S, 15 to 17% Cr, 0.1 to 0.3% W, 1.5 to 3.0% Mo, 0.001 to 0.005% B, 0.12 to 0.18% N, and the balance of Fe and unavoidable impurities; and The intermediate member is quenched by heating to a temperature in the range of 1050 to 1120°C in a nitrogen atmosphere having a nitrogen partial pressure of 1000 Pa or more and less than 10000 Pa.

11. The heat treatment method of high corrosion-resistant stainless steel parts according to claim 10, wherein, comprising the steps of: After the quenching, a low-temperature treatment is performed in which the intermediate member is cooled to a temperature in the range of -30 to -90°C; and After the low-temperature treatment, tempering is performed by heating to a temperature in the range of 150 to 200°C.

12. The heat treatment method of a high corrosion-resistant stainless steel member according to claim 10 or 11, wherein the stainless steel member is a race of a rolling bearing.

13. A manufacturing method of a high corrosion-resistant stainless steel member, wherein comprising the heat treatment method of a high corrosion-resistant stainless steel member according to claim 10 or 11.

14. A manufacturing method of a rolling bearing configured with a plurality of rolling elements between an inner ring and an outer ring, wherein at least the inner ring or the outer ring is manufactured by the manufacturing method of a high corrosion-resistant stainless steel member according to claim 13.

Images