JP6129140B2 - Stainless steel for diffusion bonding - Google Patents

Description

    The present invention relates to a duplex stainless steel material used for a molded article to be diffusion bonded.

  One of the joining methods of stainless steel materials is diffusion bonding. Stainless steel diffusion bonding products assembled by diffusion bonding are heat exchangers, machine parts, fuel cell parts, home appliance parts, plant parts, decorative components. It is applied to various uses such as building materials. The diffusion bonding method includes an “insert material insertion method” in which an insert material is inserted into the bonding interface and bonded by solid phase diffusion or liquid phase diffusion, and a “direct method” in which the surfaces of both stainless steel materials are in direct contact with each other. There is.

The insert material insertion method is advantageous in that reliable diffusion bonding can be realized relatively easily. However, the use of the insert material is disadvantageous compared to the direct method in that the cost is increased and the corrosion resistance may be lowered due to the dissimilar metal in the joint portion.
On the other hand, it is generally difficult for the direct method to obtain sufficient bonding strength compared to the insert material insertion method. However, since there is a possibility that it is advantageous in terms of reduction in manufacturing cost, various methods have been examined with respect to the direct method. For example, Patent Document 1 discloses that the amount of S in steel is 0.01% by weight or less and diffusion bonding is performed in a non-oxidizing atmosphere at a predetermined temperature, so that deformation of the material is avoided and the diffusion bonding property of the stainless steel material is improved. Technology is disclosed. Patent Document 2 discloses a method of using a stainless steel foil material having irregularities on the surface by pickling treatment. Patent Document 3 discloses a method of using, as a material to be joined, stainless steel in which the Al content is suppressed so that an alumina film, which is an impediment to diffusion bonding, is difficult to form during diffusion bonding. Patent Document 4 discloses that diffusion is promoted using a stainless steel foil that has been deformed by cold working. Patent Documents 5 and 6 describe ferritic stainless steel for direct diffusion bonding with an optimized composition.

JP 62-199277 A JP-A-2-261548 JP 7-213918 A JP-A-9-279310 JP-A-9-99218 JP 2000-303150 A JP 2013-103271 A JP 2013-173181 A JP2013-204149A JP2013-204150A

  Due to the above technique, diffusion bonding of stainless steel materials has become possible by the direct method. However, industrially, the direct method has not been established as the mainstream of the diffusion bonding method of stainless steel materials. The main reason is that it is difficult to ensure the reliability (bonding strength and adhesion) of the joint and to suppress the production load. According to the conventional knowledge, in order to ensure the reliability of the joint part by the direct method, the process of making the joint temperature higher than 1100 ° C. or applying a high surface pressure by hot press, HIP or the like is a heavy load process. It is necessary to employ it, and the cost increase by it is inevitable. If diffusion bonding of stainless steel material is performed with a work load equivalent to that of a normal insert material insertion method, it is difficult to sufficiently secure the reliability of the joint.

  Therefore, by utilizing the driving force when the ferrite phase is transformed into the austenite phase during diffusion bonding (Patent Document 7) or by using the driving force of crystal grain growth (Patent Document 8), There has been proposed a method for manufacturing a diffusion bonded product that can be carried out with a work load equivalent to that of the insert material insertion method without applying a high surface pressure. In addition, a method (Patent Documents 9 and 10) has been proposed in which the surface oxide of a stainless steel material used for diffusion bonding is reduced as much as possible to enhance diffusion bonding properties. In these methods, it is necessary to regulate the surface roughness before joining of the used stainless steel material in order to ensure good joining properties. Therefore, a further improvement in bondability is required for stainless steel materials used in diffusion bonding products.

  It is an object of the present invention to provide a stainless steel material suitable for a diffusion bonding molded product having further improved diffusion bonding properties without being affected by the degree of surface roughness.

  The present inventors have controlled the average crystal grain size, γmax amount, and creep elongation before diffusion bonding for a duplex stainless steel material having a duplex structure consisting of at least two of a ferrite phase, a martensite phase, and an austenite phase. As a result, it was found that good diffusion bondability was obtained without being affected by the surface roughness of the steel material, and the present invention was completed as a stainless steel material for diffusion bonding. Specifically, the present invention provides the following.

(1) The present invention is a dual-phase stainless steel material in which the metal structure before diffusion bonding has a multi-phase structure composed of at least two of a ferrite phase, a martensite phase, or an austenite phase, and the average of the multi-phase structure The crystal grain size is 20 μm or less, γmax represented by the following formula (a) is 10 to 90, and the creep elongation is 0.2% or more when a 1.0 MPa load is applied at 1000 ° C. for 0.5 h. It is a stainless steel material for diffusion bonding.
γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr-12Mo + 9Cu-49Ti-47Nb-52Al + 470N + 189 (a) Formula
Here, the element symbol in the above formula (a) means the content (% by mass) of each element.

(2) In the present invention, the stainless steel material is 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 The balance is the stainless steel material for diffusion bonding according to (1), wherein the balance is made of Fe and inevitable impurities, and the total amount of Ti and Al is 0.15% or less.

  (3) In the present invention, the stainless steel material is further in mass%, Nb: 4.0% or less, Mo: 0.01 to 4.0%, Cu: 0.01 to 3.0%, V: The stainless steel material for diffusion bonding according to (1) or (2) above, containing one or more of 0.03 to 0.15%.

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

  According to the present invention, a duplex stainless steel having a duplex structure composed of at least two of a ferrite phase, a martensite phase, and an austenite phase is obtained by creeping at an average crystal grain size and γmax before diffusion bonding, and at a bonding temperature. By providing the elongation within the optimum range, a stainless steel material having excellent diffusion bonding properties is provided, and thus a diffusion bonding molded product exhibiting a good bonding interface is provided. Furthermore, by suppressing the total content of Ti and Al, a diffusion bonded molded article with improved diffusion bonding properties can be obtained.

It is a figure which shows the measurement test body used by the bondability test.

  Hereinafter, embodiments of the present invention will be described.

  According to the conventional method, diffusion bonding by a direct method of stainless steel material is (i) a process in which the unevenness of the joint surface is deformed and brought into close contact, and the joint area of the joined part increases, and (ii) joining at the tight part It is considered that the process in which the surface oxide film of the front steel material disappears and (iii) the process in which the residual gas in the void which is an unjoined portion reacts with the base material proceed in parallel.

  The inventors have so far focused on the process (ii) above to regulate the base material components, the components contained in the passive film, and the surface roughness, and avoid the industrial productivity drop. I have been considering it. However, even if the step (ii) is controlled, it may be difficult to ensure industrially stable bondability. With regard to the steel material for obtaining stable bondability in consideration of the step (i), Various studies have been conducted. As a result, it has been found that when the stainless steel used for diffusion bonding is a dual phase stainless steel having a double phase structure, it is extremely effective to make the crystal grain size before diffusion bonding fine.

[Multiphase structure]
Stainless steel is generally classified into austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, and the like based on the metal structure at room temperature. The “multiphase structure” of the present invention has a metal structure composed of at least two of a ferrite phase, a martensite phase, and an austenite phase. The “multi-phase stainless steel material” of the present invention has such a multi-phase structure, and refers to a steel having an austenite + ferrite two-phase structure in the joining temperature range. Such a two-phase stainless steel may include a stainless steel classified as a ferritic stainless steel or a martensitic stainless steel.

  In the present invention, in order to realize diffusion bonding by a direct method under low temperature and low surface pressure, the stainless steel material used for diffusion bonding has a multiphase structure composed of at least two types of ferrite phase, martensite phase, and austenite phase. Use duplex stainless steel. In this stainless steel, in the temperature range where diffusion bonding proceeds, the ferrite phase and martensite phase are partly transformed to austenite phase to form a two-phase structure of austenite phase + ferrite phase, and each phase is produced at high temperature. By suppressing grain growth, a fine structure can be maintained and creep deformation presumed to be caused by grain boundary sliding can easily occur. As a result, easy deformation is promoted in the concavo-convex portion of the bonding surface, and the bonding area of the bonded portion increases, thereby enabling diffusion bonding by a direct method at a low temperature and low surface pressure.

  The duplex stainless steel material of the present invention can be used for both or one of the stainless steel materials that are brought into direct contact and integrated by diffusion bonding. As a counterpart material to be integrated, the stainless steel material of the present invention can be applied, other two-phase steel types, austenitic steel types that become austenite single phase in the heating temperature range of diffusion bonding, ferritic steel types that become ferrite single phase Etc. can be applied.

[Ingredient composition]
The duplex stainless steel to be applied in the present invention does not need to be particularly concerned with respect to the component elements other than Ti and Al from the viewpoint of diffusion bonding, and may employ various component compositions depending on the application. it can. However, in the present invention, an austenite + ferrite two-phase structure is an object in the temperature range where diffusion bonding proceeds, and it is necessary to employ steel having a component composition satisfying γmax of 10 to 90 represented by the following formula (a). Specific examples of the component composition range include the following.

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% Hereinafter , Cr: 10.0 to 30.0%, N: 0.3% or less, Ti: 0.15% or less, Al: 0.15% or less, the balance is made of Fe and inevitable impurities, Ti And the total amount of Al is 0.15% or less.

  Furthermore, in mass%, Nb: 4.0% or less, Mo: 0.01-4.0%, Cu: 0.01-3.0%, V: 0.03-0.15% Two or more types can be included. Furthermore, it can contain B: 0.0003-0.01% by the mass%.

  Hereinafter, the components contained in the stainless steel material will be described.

  C improves the strength and hardness of the steel by solid solution strengthening, but when the content increases, the workability and toughness of the steel decrease, so the C content is set to 0.2% or less. Preferably, it is 0.08% or less.

  Si is used for deoxidation of steel, but if it is excessive, toughness and workability are reduced. Moreover, in order to form a strong surface oxide film and inhibit diffusion bonding properties, the content was made 1.0% or less. Preferably, it is 0.6% or less.

  Mn is an element that improves high-temperature oxidation characteristics. However, if it is excessive, Mn is set to 3.0% or less because it is hardened by work and decreases cold workability.

  P is an inevitable impurity, and increases grain boundary corrosion and causes a decrease in toughness. Therefore, P is preferably 0.05% or less, and more preferably 0.03% by mass or less.

  S is an unavoidable impurity and is preferably 0.03% or less in order to reduce hot workability.

  Ni is an austenite-generating element and has an effect of improving the corrosion resistance in a reducing acid environment. However, if it is excessive, the austenite phase becomes stable and the growth of ferrite crystals cannot be suppressed. In order to form a stable austenite single phase and suppress the growth of ferrite crystals, the content was made 10.0% or less.

Cr is an element that forms a passive film and imparts corrosion resistance. If it is less than 10.0%, the effect is not sufficient. 3 If it exceeds 0.0%, workability deteriorates. Therefore, the Cr content is set to 10.0 to 30.0%.

  N is an unavoidable impurity and is preferably 0.3% or less in order to deteriorate the cold workability.

Ti has an effect of fixing C and N, and is therefore an effective element for improving corrosion resistance and workability. Al is often added as a deoxidizer. On the other hand, since Ti and Al are easily oxidizable elements, Ti oxide and Al oxide contained in the oxide film on the surface of the steel material are difficult to be reduced in the heat treatment of vacuum diffusion bonding. Therefore, if these Ti oxides and Al oxides are large, the progress of the process (ii) may be hindered during diffusion bonding. Therefore, the Ti content is 0.15% by mass or less, and the Al content is 0.15 mass% or less is preferable, and 0.05 mass% or less is more preferable. Then, the total content of Ti and Al is preferably set to 0.15 mass% or less, and more preferably not more than 0.05 mass%.

  Nb has the effect of forming carbides or carbonitrides and increasing the toughness by refining the crystal grains of the steel, but if it is excessive, it causes a decrease in workability, so 4.0% by mass or less is preferable.

  Mo has the effect of improving the corrosion resistance without reducing the strength. If it is excessive, the workability is lowered, so 0.01 to 4.0% by mass is preferable.

  Cu is effective for improving the corrosion resistance and has an effect of generating a ferrite phase. However, if it is excessive, the workability is lowered, so 0.01 to 3.0% by mass is preferable.

  V is an element that contributes to improvement of workability and toughness by fixing solute C as a carbide. However, if excessively contained, it causes a decrease in manufacturability, so 0.03 to 0.15%. preferable.

  B is an element that contributes to the improvement of corrosion resistance and workability by fixing N. However, if contained excessively, the hot workability is lowered, so 0.0003 to 0.01% is preferable.

As the duplex stainless steel having the above chemical composition, steel having a γmax of 10 to 90 represented by the following formula (a) can be applied.
γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr-12Mo + 9Cu-49Ti-47Nb-52Al + 470N + 189 (a) Formula
Here, element symbols such as C and Si in the above formula (a) mean the content (% by mass) of each element.

γmax is an index representing the amount (volume%) of the austenite phase that is generated when heated and held at about 1100 ° C. When γmax is 100 or more, it can be regarded as a steel type that becomes an austenite single phase, and when γmax is 0 or less, it can be regarded as a steel type that becomes a ferrite single phase.
In the duplex stainless steel of the present invention, when γmax is 10 to 90, the two phases become austenite + ferrite in the temperature range where diffusion bonding proceeds, and these two phases suppress grain growth at high temperatures. It is effective for obtaining a fine crystal structure. More preferably, γmax is 50-80.

[Average crystal grain size before bonding]
As the duplex stainless steel of the present invention has a finer grain structure, the process (i) can be rapidly advanced. Therefore, the average crystal grain size before bonding is preferably 20 μm or less, and more preferably 10 μm or less.

[Surface roughness]
In the multi-phase stainless steel having fine crystal grains according to the present invention, since the process (i) proceeds rapidly, the influence of the process (ii) is small, and the bondability is limited by the degree of the surface roughness Ra. It will never be done. However, when the surface roughness of the stainless steel material used for diffusion bonding increases, the disappearance of the oxide film in the process (ii) tends to be delayed. Therefore, the surface of the stainless steel material is preferably smooth, and the surface roughness Ra is preferably 0.3 μm or less.

[Diffusion bonding product manufacturing method]
The stainless steel material of the present invention provides a diffusion bonded product with good bondability by performing vacuum diffusion bonding by a direct method. As a specific diffusion bonding treatment, for example, a direct contact is made at a contact surface pressure of 0.1 to 1.0 MPa, and the pressure is 1.0 × 10 ⁻² Pa or less, preferably 1.0 × 10 ⁻³ Pa. Hereinafter, diffusion bonding is advanced by heating and maintaining at 900 to 1100 ° C. in a furnace having a dew point of −40 ° C. or less. The holding time may be adjusted in the range of 0.5 to 3 h.

  Examples of the present invention will be described below, but the present invention is not limited to the following examples, and can be implemented with appropriate modifications within the scope of the gist of the invention.

  The stainless steel having the chemical composition shown in Table 1 was melted by 30 kg of vacuum melting, and the obtained steel ingot was forged into a 30 mm thick plate, followed by hot rolling at 1230 ° C. for 2 hours and 3.0 mm. A thick hot-rolled sheet was obtained. Subsequently, annealing, pickling, and cold rolling were performed to obtain a cold-rolled sheet having a thickness of 1.0 mm. Then, the cold-rolled sheet was annealed as described later to produce a cold-rolled sheet, which was used as a test material.

For FM-1 steel to FM-4 steel, the metal structure before diffusion bonding is ferrite + martensite dual phase steel (α + M phase), and for FA-1 steel and FA-2 steel, the metal structure before diffusion bonding is ferrite + Austenitic dual-phase steel (α + γ phase), F-1 steel has a ferrite single-phase steel (α phase) as the metal structure before diffusion bonding, and A-1 steel has an austenitic single-phase steel (γ as the metal structure before diffusion bonding). Phase). M-1 steel is martensitic single phase steel (M phase) in the metal structure before diffusion bonding.
Each steel plate obtained the test material from which an average crystal grain diameter differs by changing the annealing temperature after cold rolling between 900 degreeC-1200 degreeC. Moreover, in order to investigate the influence of surface roughness, the test material from which surface roughness Ra differs was obtained by changing the finishing process of a cold-rolled annealing board using some steel plates.

(Average crystal grain size)
The average crystal grain size (μm) before diffusion bonding of the steel plate is observed within 1 mm ² or more of the metal structure of the plate thickness section parallel to the cold rolling direction, and is included in the unit area using the quadrature method. The number of crystal grains was calculated, and a value obtained by raising the average area per crystal grain to the power of 1/2 was used.

(Surface roughness)
The surface roughness Ra (μm) was measured with a surface roughness measuring device (manufactured by Tokyo Seimitsu Co., Ltd .; SURFCOM 2900DX) in a direction perpendicular to the rolling direction.

(Creep elongation)
Creep elongation was measured by the method shown below. A JIS 13B test piece was cut out from each steel plate, and a hole of φ5 mm was formed in the center of one gripping part. After marking the test piece with a mark of 50 mm between the marks, the test piece was attached in a high-temperature tensile tester so that the gripping part having a hole would be downward. The temperature between the gauge points is raised to 1000 ° C., soaked at that temperature for 15 minutes, and then a SUS310S wire having a weight calculated so as to apply a stress of 1.0 MPa is attached to the hole in the gripping portion. And held for 0.5 h. Thereafter, the SUS310S wire was removed from the test piece and further cooled to room temperature by air cooling. The length L between the gauge points was measured, and (L-50) / 50 × 100 was calculated as the creep elongation (%).

(Jointability test)
A plate test piece of 20 mm × 20 mm was taken out from each steel plate and diffusion bonded by the following method. Two test pieces of the same steel material are laminated so that the surfaces are in contact with each other, and using a jig having a weight, the surface pressure applied to the contact surface of these two test pieces is 0.1 MPa. Adjusted as follows. Hereinafter, the laminated flat plate test piece is referred to as “steel material”. A state in which the steel materials are laminated is referred to as a “laminated body”. After that, the jig and the laminated body are inserted into a vacuum furnace, and evacuation is performed to obtain an initial vacuum degree of 1.0 × 10 ^{−3 to} 1.0 × 10 ⁻⁴ Pa, and then the temperature is increased to 1000 ° C. in about 1 h. After warming and holding at that temperature for 2 h, it was transferred to a cooling chamber and cooled. The vacuum was maintained at 900 ° C., and then Ar gas was introduced to cool to about 100 ° C. or less in a 90 kPa Ar gas atmosphere. For the laminate after the heat treatment, 49 measurement points provided at a 3 mm pitch on the surface of the laminate of 20 mm × 20 mm as shown in FIG. 1 using an ultrasonic thickness meter (manufactured by Olympus; Model 35DL) The thickness was measured at. The probe diameter was 1.5 mm. If the measured thickness at a given measurement point indicates the total thickness of the two steel materials, it is assumed that both steel materials are integrated by diffusion of atoms at the interface position of both steel materials corresponding to the measurement point. Can do. On the other hand, when the plate thickness measurement value is different from the total plate thickness of both steel materials, it can be considered that an unjoined portion (defect) exists at the interface position of both steel materials corresponding to the measurement point. When the correspondence between the cross-sectional structure of the laminate after the heat treatment and the measurement results obtained by this measurement method was examined, the number of measurement points at which the measurement results were the total plate thickness of both steel materials was 49 in total. It was confirmed that the area ratio of the joint portion occupying the contact area can be accurately evaluated by the value obtained by dividing (hereinafter referred to as “joining ratio”). Therefore, diffusion bonding properties were evaluated according to the following evaluation criteria.
A: Joining rate 100% (excellent)
○: Joining rate 90 to 99% (good)
Δ: Joining rate 60-89% (slightly good)
X: Joining rate 0 to 59% (defect)
As a result of various investigations, the strength of the diffusion bonding portion is sufficiently secured in the evaluation, and the sealing property between the two members (the property that gas does not leak through the communicating defect) is also good. The above was determined to be acceptable.

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

As shown in Table 2, Examples 1 to 6 of the present invention have a bonding rate of 90% or more, and exhibit good diffusion bonding even at a relatively low temperature of 1000 ° C. and a low surface pressure of 0.1 MPa. It was. In addition, Examples 1 to 6 of the present invention showed good diffusion bonding properties regardless of the degree of the surface roughness Ra, and no influence by the surface roughness was observed. In the multiphase stainless steel material having the configuration of the present invention, the diffusion bondability does not decrease even when the surface roughness is increased, so that it can be understood that the diffusion bondability is not restricted by the surface property of the steel material. In addition, the underline attached | subjected to the numerical value of Table 2 shows that it is outside the scope of the present invention.

On the other hand, in Comparative Examples 1 to 10, since the average crystal grain size, γmax, and creep elongation were out of the scope of the present invention, the deformation of the uneven portion of the joint surface in the two-phase high temperature region was small, The bonding area did not increase. Therefore, most of the joining ratios were slightly poor or poor at less than 80%.
Moreover, about the ferrite single phase steel of Comparative Examples 5-7 and the austenite single phase steel of Comparative Examples 8-9, when the change of the joining rate by surface roughness Ra is seen, the comparative example 7 and comparative example whose surface roughness is very small While 9 showed a bonding rate of 90% or more, the other comparative examples had large surface roughness and a low bonding rate. Thus, it can be seen that in a single phase system, if the surface roughness is large, the bonding rate becomes poor, and the diffusion bonding property is restricted by the surface roughness.

Claims (3)

A metal structure before diffusion bonding is a duplex stainless steel material having a duplex structure consisting of at least two of a ferrite phase, a martensite phase or an austenite phase,
The average crystal grain size of the multiphase structure is 20 μm or less (excluding 1 μm or less) ;
Γmax represented by the following formula (a) is 10 to 90,
Creep elongation when applying a 1.0 MPa load at 1000 ° C. for 0.5 h is 0.2% or more,
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% Hereinafter, Cr: 10.0 to 30.0%, N: 0.3% or less, Ti: 0.15% or less, Al: 0.15% or less, the balance is made of Fe and inevitable impurities, Ti and the total amount of Al Ri der 0.15% or less,
A stainless steel material for diffusion bonding having a surface roughness Ra of 0.22 μm or more .
γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr-12Mo + 9Cu-49Ti-47Nb-52Al + 470N + 189 (a) Formula Here, the element symbol in the formula (a) means the content (mass%) of each element. To do.   The stainless steel material is further in mass%, Nb: 4.0% or less, Mo: 0.01-4.0%, Cu: 0.01-3.0%, V: 0.03-0.15 The stainless steel material for diffusion bonding according to claim 1, comprising 1 type or 2 types or more of%.   The stainless steel material for diffusion bonding according to claim 1 or 2, wherein the stainless steel material further includes B: 0.0003 to 0.01% by mass%.

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