HK1237380B - Stainless steel material for diffusion bonding - Google Patents

Description

Stainless steel materials for diffusion bonding

Technical Field

The present invention relates to a multiphase stainless steel material used in diffusion-bonded molded products.

Background Art

Diffusion bonding is one method for joining stainless steel materials. Stainless steel diffusion-bonded products assembled by diffusion bonding are used in a variety of applications, including heat exchangers, mechanical components, fuel cell components, home appliance components, equipment parts, decorative components, and building materials. Diffusion bonding methods include the "insert material insertion method," which inserts an insert material into the bonding interface and bonds through solid-phase diffusion or liquid-phase diffusion, and the "direct method," which directly brings the surfaces of the two stainless steel materials into contact.

The insert material insertion method is advantageous in that it can achieve reliable diffusion bonding relatively easily. However, it is disadvantageous compared to the direct method in that the use of the insert material increases costs and the fact that the joining portion is formed of a different metal than the base material sometimes reduces corrosion resistance.

On the other hand, the direct method is generally difficult to obtain sufficient bonding strength compared to the insert material insertion method. However, since the direct method has the potential to reduce manufacturing costs and is advantageous, various methods have been studied.

For example, Patent Document 1 discloses the following technology: by reducing the S content in stainless steel to 0.01% by weight or less and performing diffusion bonding in a non-oxidizing atmosphere at a predetermined temperature, deformation of the material is avoided and the diffusion bonding of the stainless steel material is improved. Patent Document 2 discloses a method of using a stainless steel foil material whose surface is given an uneven surface by pickling. Patent Document 3 discloses a method of using stainless steel with a suppressed Al content as a bonded material to make it difficult to form an aluminum oxide film that hinders diffusion bonding during diffusion bonding. Patent Document 4 discloses a method of promoting diffusion using stainless steel foil that has been deformed by cold working. Patent Documents 5 and 6 describe ferritic stainless steel for direct diffusion bonding with optimized component composition.

Prior art literature

Patent Literature

Patent Document 1: Japanese Patent Application Laid-Open No. 62-199277

Patent Document 2: Japanese Patent Application Laid-Open No. 2-261548

Patent Document 3: Japanese Patent Application Laid-Open No. 7-213918

Patent Document 4: Japanese Patent Application Laid-Open No. 9-279310

Patent Document 5: Japanese Patent Application Laid-Open No. 9-99218

Patent Document 6: Japanese Patent Application Laid-Open No. 2000-303150

Patent Document 7: Japanese Patent Application Laid-Open No. 2013-103271

Patent Document 8: Japanese Patent Application Laid-Open No. 2013-173181

Patent Document 9: Japanese Patent Application Laid-Open No. 2013-204149

Patent Document 10: Japanese Patent Application Publication No. 2013-204150

Summary of the Invention

Problems to be solved by the invention

Diffusion bonding of stainless steel materials can also be implemented by using the direct method through the above-mentioned joining technology. However, in industry, the direct method has not yet been characterized as the mainstream method for diffusion bonding of stainless steel materials. The main reason is that it is difficult to take into account the following two issues: ensuring the reliability of the joint such as bonding strength and tightness and suppressing the manufacturing load such as the bonding device and bonding time. According to previous technical insights, in order to ensure sufficient reliability of the joint manufactured by the direct method, it is necessary to adopt a process with a high manufacturing load such as a process of setting the joining temperature to a high temperature of more than 1100°C or a process of applying high surface pressure through hot pressing, HIP, etc., and the resulting cost increase cannot be avoided. At present, if an attempt is made to implement diffusion bonding of stainless steel materials using the direct method with the same workload as the conventional insert material insertion method, it is difficult to fully ensure the reliability of the joint.

Therefore, a manufacturing method has been proposed that utilizes the driving force of the ferrite to austenite phase transformation (Patent Document 7) and the driving force of grain growth (Patent Document 8) during diffusion bonding, thereby eliminating the need for high-temperature heating and high surface pressure and enabling the manufacturing of diffusion-bonded products using a direct method with a workload equivalent to that of the insert material insertion method. Furthermore, methods have been proposed that minimize the surface oxides of the stainless steel material used for diffusion bonding to improve diffusion bonding properties (Patent Documents 9 and 10). In order to ensure good bonding properties, these methods require limiting the surface roughness of the stainless steel material used before bonding. Therefore, there is a demand for further improved bonding properties of the stainless steel material used in diffusion-bonded products.

An object of the present invention is to provide a stainless steel material suitable for diffusion-bonding molded products, which has improved diffusion bonding properties without being affected by the degree of surface roughness.

Solutions to Problems

The present inventors have discovered that, for a multiphase stainless steel material having a multiphase structure composed of at least two or more of the following phases: ferrite, martensite, and austenite, by controlling the average grain size, γmax value, and creep elongation before diffusion bonding, it is possible to achieve excellent diffusion bonding properties without being affected by the surface roughness of the steel material, thereby creating a stainless steel material for diffusion bonding. Specifically, the present invention provides the following solutions.

(1) The present invention is a stainless steel material for diffusion bonding, which is a multiphase stainless steel material having a metal structure before diffusion bonding having a multiphase structure composed of at least two or more phases of a ferrite phase, a martensite phase, or an austenite phase, wherein the average crystal grain size of the multiphase structure is 20 μm or less, γmax represented by the following formula (a) is 10 to 90, and the creep elongation when a load of 1.0 MPa is applied at 1000° C. for 0.5 h is 0.2% or more.

γmax＝420C-11.5Si+7Mn+23Ni-11.5Cr-12Mo+9Cu-49Ti-47Nb-52Al+470N+189…(a) formula

The element symbols in the above formula (a) represent the content (mass %) of each element.

(2) The present invention is a stainless steel material for diffusion bonding as described in (1) above, wherein the stainless steel material contains, in mass%, C: 0.2% or less, Si: 1.0% or less, Mn: 3.0% or less, P: 0.05% or less, S: 0.03% or less, Ni: 10.0% or less, Cr: 10.0 to 30.0%, N: 0.3% or less, Ti: 0.15% or less, Al: 0.15% or less, and the balance contains Fe and unavoidable impurities, and the total amount of Ti and Al is 0.15% or less.

(3) The present invention is a stainless steel material for diffusion bonding as described in (1) or (2) above, wherein the stainless steel material further contains, in mass%, one or more of the following: Nb: 4.0% or less, Mo: 0.01 to 4.0%, Cu: 0.01 to 3.0%, and V: 0.03 to 0.15%.

(4) The present invention is the stainless steel material for diffusion bonding according to any one of (1) to (3) above, wherein the stainless steel material further contains, by mass%, 0.0003 to 0.01% B.

Effects of the Invention

According to the present invention, a multiphase stainless steel having a multiphase structure composed of at least two or more of the ferrite, martensite, and austenite phases is provided by optimizing the average grain size and γmax before diffusion bonding, as well as the creep elongation at the bonding temperature, thereby providing a stainless steel material with excellent diffusion bondability. Consequently, diffusion-bonded molded products exhibiting a good bonding interface can be provided. Furthermore, by controlling the combined content of Ti and Al, diffusion-bonded molded products with improved diffusion bondability can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a measurement specimen used in a zygosity test.

DETAILED DESCRIPTION

Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to these descriptions.

According to conventional methods, direct diffusion bonding of stainless steel materials is believed to be accomplished by the following three processes occurring in parallel: (i) the unevenness of the bonding surface deforms and closes, increasing the bonding area of the bonded portion; (ii) the surface oxide film of the steel materials before bonding disappears in the close-fitting portion; and (iii) the residual gas in the gaps in the unbonded portion reacts with the base material.

In the past, the inventors have focused on process (ii) above, restricting the base material composition, the components contained in the passivation film, and the surface roughness of the joining surface to avoid the reduction in productivity that would become an industrial obstacle. However, even if process (ii) above is controlled, it is sometimes difficult to ensure stable industrial bondability. Therefore, considering process (i) above, they have conducted extensive research on steel materials to achieve stable bondability. As a result, they discovered that when the stainless steel used for diffusion bonding is a multiphase stainless steel with a multiphase structure, a method of reducing the crystal grain size before diffusion bonding is extremely effective.

1. Multiphase organization

Generally speaking, stainless steel can be classified into austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, and other categories based on their metallurgical structure at room temperature. The "multiphase structure" herein refers to a structure comprising at least two or more of the following phases: ferrite, martensite, and austenite. The "multiphase stainless steel material" herein refers to a steel material having such a multiphase structure, and refers to steel that forms a dual-phase structure of austenite + ferrite in the joining temperature range. Such dual-phase stainless steels may also include those classified as ferritic or martensitic stainless steels.

In the present invention, in order to achieve diffusion bonding using a direct method at low temperatures and low surface pressures, a multiphase stainless steel having a multiphase structure consisting of at least two of the following phases: a ferrite phase, a martensite phase, and an austenite phase is used as a stainless steel material for diffusion bonding. In the temperature range where diffusion bonding is performed, a portion of the ferrite phase and the martensite phase of the stainless steel transforms into the austenite phase, forming a dual-phase structure of the austenite phase + the ferrite phase. The phases in this dual-phase structure inhibit grain growth that occurs at high temperatures, thereby maintaining a fine structure and easily generating creep deformation that is presumably caused by grain boundary sliding. As a result, easy deformation is promoted in the concave and convex portions of the bonding surface, and the bonding area of the bonding portion is increased, thereby enabling diffusion bonding using a direct method at low temperatures and low surface pressures.

The multiphase stainless steel material of the present invention can be used for either or both of the stainless steel materials that are in direct contact and integrated by diffusion bonding. In addition to the stainless steel material of the present invention, other duplex steel grades, austenitic steel grades that form a single austenite phase in the diffusion bonding heating temperature range, and ferritic steel grades that form a single ferrite phase can also be used as the integrated counterpart.

2. Ingredients

The multiphase stainless steel to which the present invention is applicable is not particularly limited in terms of component elements other than Ti and Al from the perspective of diffusion bonding, and various compositions can be employed as needed. However, since the present invention targets an austenite + ferrite dual-phase structure within the diffusion bonding temperature range, it is necessary to employ a steel composition in which γmax, as expressed by the following formula (a), satisfies a value of 10 to 90. Specific composition ranges include the following.

In terms of mass%, it contains C: less than 0.2%, Si: less than 1.0%, Mn: less than 3.0%, P: less than 0.05%, S: less than 0.03%, Ni: less than 10.0%, Cr: 10.0-30.0%, N: less than 0.3%, Ti: less than 0.15%, Al: less than 0.15%, and the balance contains Fe and unavoidable impurities, and the total amount of Ti and Al is less than 0.15%.

Furthermore, the alloy may contain, by mass%, one or more of Nb: 4.0% or less, Mo: 0.01 to 4.0%, Cu: 0.01 to 3.0%, and V: 0.03 to 0.15%. Furthermore, the alloy may contain, by mass%, B: 0.0003 to 0.01%.

The components contained in the stainless steel material will be described below.

C improves the strength and hardness of steel by solid solution strengthening. On the other hand, if the C content increases, the workability and toughness of the steel decrease. Therefore, the C content is preferably 0.2% by mass or less, more preferably 0.08% by mass or less.

Si is an element used for deoxidation of steel. However, excessive Si content reduces the toughness and workability of the steel. Furthermore, it forms a strong surface oxide film, hindering diffusion bonding. Therefore, the Si content is preferably 1.0% by mass or less, and more preferably 0.6% by mass or less.

Mn is an element that improves high-temperature oxidation resistance. On the other hand, if the Mn content is too high, it will cause steel work hardening and reduce the cold workability of the steel. Therefore, the Mn content is preferably 3.0% by mass or less.

P is an unavoidable impurity that increases intergranular corrosion resistance and reduces the toughness of steel. Therefore, the P content is preferably 0.05% by mass or less, more preferably 0.03% by mass or less.

S is an unavoidable impurity that reduces the hot workability of steel. Therefore, the S content is preferably 0.03 mass% or less.

Nickel is an austenite-forming element and also improves the corrosion resistance of steel in reducing acid environments. On the other hand, excessive Ni content stabilizes the austenite phase, failing to suppress ferrite crystal growth. Consequently, a stable austenite single phase is formed, inhibiting ferrite crystal growth. Therefore, the Ni content is preferably 10.0% or less.

Cr is an element that forms a passive film and imparts corrosion resistance. If the Cr content is less than 30.0 mass%, the corrosion resistance effect is insufficient. If it exceeds 10.0 mass%, workability is reduced. Therefore, the Cr content is preferably 10.0 to 30.0 mass%.

N is an inevitable impurity and deteriorates cold workability, so its content is preferably 0.3 mass % or less.

Ti has the function of fixing C and N, and is therefore an effective element in improving corrosion resistance and workability. Al is mostly added as a deoxidizer. On the other hand, since Ti and Al are easily oxidizable elements, the Ti oxide and Al oxide contained in the oxide film on the surface of the steel material are not easily reduced during the heat treatment of vacuum diffusion bonding. Therefore, if there are many Ti oxides and Al oxides, the process of (ii) above may sometimes be hindered during diffusion bonding. Therefore, the Ti content is 0.15% by mass or less, and the Al content is preferably 0.15% by mass or less, more preferably 0.05% by mass or less. In addition, the total content of Ti and Al is preferably 0.15% by mass or less, more preferably 0.05% by mass or less.

Nb is an element that forms carbides or carbonitrides, refines the grains of steel, and improves toughness. On the other hand, if the Nb content is too high, the workability of the steel will be reduced, so the Nb content is preferably 4.0% by mass or less.

Mo is an element that improves corrosion resistance without reducing strength. If the Mo content is too high, the workability of the steel will be reduced. Therefore, the Mo content is preferably 0.01 to 4.0 mass%.

Cu is effective in improving corrosion resistance and is also an element that functions to form a ferrite phase. On the other hand, if the Cu content is too high, the workability of the steel decreases, so the Cu content is preferably 0.01 to 3.0 mass %.

V is an element that helps improve the workability and toughness of steel by fixing dissolved C as carbides. On the other hand, excessive V content reduces manufacturability, so the V content is preferably 0.03 to 0.15%.

B is an element that contributes to improving corrosion resistance and workability by fixing N. On the other hand, if excessive B is contained, the hot workability of the steel decreases, so the B content is preferably 0.0003 to 0.01%.

As the multiphase stainless steel having the above chemical composition, steel having a γmax of 10 to 90 represented by the following formula (a) can be particularly preferably used.

γmax＝420C-11.5Si+7Mn+23Ni-11.5Cr-12Mo+9Cu-49Ti-47Nb-52Al+470N+189…(a) formula

In the above formula (a), element symbols such as C and Si represent the content (mass %) of each element.

γmax is an indicator of the amount (volume %) of austenite formed when heated and held at approximately 1100°C. A γmax of 100 or greater indicates a single-phase austenite steel. A γmax of 0 or less indicates a single-phase ferrite steel.

In the multiphase stainless steel of the present invention, when γmax is 10 to 90, a dual phase of austenite and ferrite is formed in the diffusion-bonding temperature range. This dual phase mutually inhibits grain growth at high temperatures, effectively achieving a fine crystalline structure. A γmax of 50 to 80 is even more preferred.

3. Average grain size before bonding

The finer the grain structure of the multiphase stainless steel of the present invention, the faster the process (i) can proceed. Therefore, the average grain size before joining is preferably 20 μm or less, more preferably 10 μm or less.

4. Surface roughness

The multiphase stainless steel having fine grains of the present invention rapidly progresses through process (i) above, resulting in minimal impact from process (ii) above, and the likelihood of bondability being restricted by the degree of surface roughness Ra is low. However, increasing the surface roughness of the stainless steel material used for diffusion bonding tends to slow the disappearance of the oxide film during process (ii) above. Therefore, the surface of the stainless steel material is preferably smooth, with a surface roughness Ra of preferably 0.3 μm or less.

5. Manufacturing method of diffusion bonded products

The stainless steel material of the present invention can be subjected to vacuum diffusion bonding using a direct method, resulting in a diffusion-bonded product with excellent bondability. Specifically, diffusion bonding can be achieved by heating and holding the components at 900-1100°C in a furnace with a pressure of 1.0× ^10-2 Pa or less, preferably 1.0× ^10-3 Pa or less, and a dew point of -40°C or less, while in direct contact at a contact pressure of 0.1-1.0 MPa. The holding time can be adjusted within a range of 0.5-3 hours.

Example

The present invention is not limited to the following examples, and can be implemented with appropriate modifications within the scope of the invention.

Stainless steel having the chemical composition shown in Table 1 was melted using a 30 kg vacuum melt. The resulting steel ingot was forged into a 30 mm thick plate. This was then hot-rolled at 1230°C for 2 hours to produce a 3.0 mm thick hot-rolled plate. This was then annealed, pickled, and cold-rolled to produce a 1.0 mm thick cold-rolled plate. This cold-rolled plate was then subjected to the annealing treatment described below to produce a cold-rolled annealed plate, which was used as test material.

Table 1

Table 1 shows various steels. FM-1 to FM-4 steels are dual-phase steels (α+M phase) whose metal structures before diffusion bonding are ferrite + martensite. FA-1 and FA-2 steels are dual-phase steels (α+γ phase) whose metal structures before diffusion bonding are ferrite + austenite. F-1 steel is a single-phase steel (α phase) whose metal structure before diffusion bonding is ferrite. A-1 steel is a single-phase steel (γ phase) whose metal structure before diffusion bonding is austenite. M-1 steel is a single-phase steel (M phase) whose metal structure before diffusion bonding is martensite.

For each steel sheet, the annealing temperature after cold rolling was varied between 900°C and 1200°C to produce test materials with varying average grain sizes. Furthermore, to investigate the influence of surface roughness, some steel sheets were used and the finishing treatment of the cold-rolled and annealed sheets was varied to produce test materials with varying surface roughness Ra.

1) Average crystal particle size

The average grain size (μm) of the steel sheet before diffusion bonding is measured using the quadrature method as shown below. The metallographic structure of the sheet is observed over a continuous cross section of at least 1 ^mm² , parallel to the cold rolling direction. The number of grains per unit area is calculated using the quadrature method. The average area per grain is then calculated and raised to the 1/2 power, which is used as the average grain size.

2) Surface roughness

The surface roughness Ra (μm) was measured using a surface roughness measuring device (SURFCOM2900DX, manufactured by Tokyo Seimitsu Co., Ltd.) to measure the surface roughness Ra in a direction perpendicular to the rolling direction.

3) Creep elongation

The creep elongation is measured according to the method shown below. A JIS13B test piece is cut out from each steel plate, and a hole is opened in the center of one of the gripping parts. After marking a line of 50 mm between marks on the test piece, the test piece is mounted on a high-temperature tensile testing machine in such a way that the above-mentioned gripping part with the hole is at the bottom. The temperature is raised until the temperature in the above-mentioned mark space reaches 1000°C. After heating stably at this temperature for 15 minutes, a SUS310S metal wire (which has a weight, converted to apply a stress of 1.0 MPa) is installed in the hole of the gripping part and maintained for 0.5 hours. Then, the SUS310S metal wire is removed from the test piece and then cooled to room temperature by air cooling. Then, the length L between the above-mentioned marks is measured, and (L-50)/50×100 is calculated as the creep elongation (%).

4) Zygosity test

A 20 mm × 20 mm flat test piece was cut out from each steel plate and diffusion-bonded according to the following method. Two test pieces of the same steel material were stacked in such a way that their surfaces were in contact with each other. Using a fixture with a weight, the surface pressure applied to the contact surfaces of the two test pieces was adjusted to 0.1 MPa. Hereinafter, the stacked flat test pieces will be referred to as "steel material". The stacked state of the steel materials will be referred to as a "laminate". Next, the fixture and the laminate were inserted into a vacuum furnace, evacuated to an initial vacuum degree of 1.0 × 10 ^-3 to 1.0 × 10 ^-4 Pa, and then heated to 1000 ° C over about 1 hour and maintained at this temperature for 2 hours. Then, it was moved to a cooling chamber for cooling. During the cooling, the above-mentioned vacuum degree was maintained until 900 ° C, and then Ar gas was introduced and cooled to below about 100 ° C in an Ar gas atmosphere of 90 kPa. For the laminate that has completed the above-mentioned heat treatment, an ultrasonic thickness meter (Model 35DL, manufactured by Olympus Corporation) was used. As shown in Figure 1, the thickness was measured at 49 measuring points set at 3mm intervals on the surface of the 20mm×20mm laminate. The probe diameter is 1.5mm. When the plate thickness measurement value at a certain measuring point represents the total plate thickness of two steel materials, it can be considered that at the interface position of the two steel materials corresponding to the measuring point, the two steel materials are integrated due to the diffusion of atoms. On the other hand, when the plate thickness measurement value is different from the total plate thickness of the two steel materials, it can be considered that there is an unbonded portion (defect) at the interface position of the two steel materials corresponding to the measuring point. The correspondence between the cross-sectional structure of the laminate after heat treatment and the measurement results obtained by the measurement method was investigated, and the results confirmed that the area ratio of the bonding portion to the contact area can be evaluated with good accuracy using the value obtained by dividing the number of measuring points where the measurement result is the total plate thickness of the two steel materials by the total number of measurements of 49 (hereinafter referred to as "bonding rate"). Therefore, the diffusion bonding is evaluated according to the following evaluation criteria.

A: Bonding rate 100% (excellent)

B: Bonding rate 90-99% (good)

C: Bonding rate 60-89% (slightly good)

D: Bonding rate 0 to 59% (poor)

As a result of various studies, evaluations A and B were determined to be acceptable because the strength of the diffusion bonded portion was sufficiently ensured and the sealing performance between the two components (no gas leakage through the communicating defects) was also good.

Table 2 shows the average grain size and γmax of each steel after cold rolling and annealing, surface roughness, creep elongation, and bondability evaluation results.

Table 2

As shown in Table 2, Examples 1-6 of the present invention achieved a bonding rate of 90% or higher, demonstrating excellent diffusion bonding even at relatively low temperatures of 1000°C and low surface pressures of 0.1 MPa. Furthermore, Examples 1-6 of the present invention exhibited excellent diffusion bonding regardless of the surface roughness Ra, demonstrating no apparent effect of surface roughness. The multiphase stainless steel material having the composition of the present invention exhibits no decrease in diffusion bonding even with increased surface roughness, demonstrating that its diffusion bonding is not constrained by the steel material's surface properties.

In contrast, the average grain size, γmax, and creep elongation of Comparative Examples 1 to 10 were outside the ranges of the present invention. Consequently, deformation of the concave and convex portions of the joint surface in the dual-phase high-temperature region was minimal, and the joint area at the joint site did not increase. Consequently, the bonding ratio was often less than 80%, indicating slightly poor or poor performance.

Furthermore, for the ferritic single-phase steels of Comparative Examples 5 and 7 and the austenitic single-phase steels of Comparative Examples 8 and 9, the changes in bonding rate relative to surface roughness Ra show that Comparative Examples 7 and 9, which have extremely low surface roughness, exhibit bonding rates exceeding 90%. On the other hand, the bonding rates for the other comparative examples are lower due to their high surface roughness. This demonstrates that for single-phase steels, high surface roughness leads to poor bonding rates, and that diffusion bonding is restricted by surface roughness.

Claims (5)

1. A diffusion-bonded stainless steel material, wherein the metal microstructure prior to diffusion bonding has a multiphase microstructure consisting of at least two of ferrite, martensite, or austenite phases. The average crystal grain size of the multiphase structure is less than 20 μm. The γmax expressed by equation (a) below is 10–90. The creep elongation rate when a load of 1.0 MPa is applied at 1000℃ for 0.5 h is greater than 0.2%. γmax＝420C-11.5Si+7Mn+23Ni-11.5Cr-12Mo+9Cu-49Ti-47Nb-52Al+470N+189…(a) formula In equation (a), the element symbols represent the content of each element in terms of mass%. 2. The stainless steel material for diffusion bonding according to claim 1, The stainless steel material contains, by mass percent, less than 0.2% C, less than 1.0% Si, less than 3.0% Mn, less than 0.05% P, less than 0.03% S, less than 10.0% Ni, less than 10.0% Cr, 10.0-30.0% N, less than 0.3% Ti, less than 0.15% Al, with the remainder being Fe and unavoidable impurities, and the combined amount of Ti and Al being less than 0.15%. 3. The stainless steel material for diffusion bonding according to claim 1 or 2, The stainless steel material further contains, by mass percent, one or more of the following: Nb: less than 4.0%, Mo: 0.01 to 4.0%, Cu: 0.01 to 3.0%, and V: 0.03 to 0.15%. 4. The stainless steel material for diffusion bonding according to claim 1 or 2, The stainless steel material further contains B by weight: 0.0003 to 0.01%. 5. The stainless steel material for diffusion bonding according to claim 3, The stainless steel material further contains B by weight: 0.0003 to 0.01%.