JP5846868B2 - Manufacturing method of stainless steel diffusion bonding products - Google Patents

Description

  The present invention relates to a method of manufacturing a stainless steel diffusion bonding product in which stainless steel materials are diffusion bonded without an insert material.

  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.

  As an insert material insertion method, for example, a method using duplex stainless steel as an insert material (Patent Document 1), and a liquid phase diffusion bonding method using a foil-like insert material having the same composition as a workpiece to be plated with Ni and Au by several μm Many techniques are conventionally known, such as (Patent Document 2) and a method of using austenitic stainless steel containing a large amount of Si in a range of 11.5% or less as an insert material (Patent Document 3). Further, “brazing” using a nickel-based brazing material (for example, JIS: BNi-1 to 7) or a copper-based brazing material as an insert material can be regarded as one type of liquid phase diffusion bonding. These insert material insertion methods are 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 4 discloses a technique for improving the diffusion bonding property of a stainless steel material by avoiding deformation of the material by diffusion bonding in a non-oxidizing atmosphere at a predetermined temperature with an S amount in steel of 0.01% or less. It is disclosed. Patent Document 5 discloses a method of using a stainless steel foil material having irregularities on the surface by pickling treatment. Patent Document 6 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 that becomes an impediment to diffusion bonding is difficult to form during diffusion bonding. Patent Document 7 discloses that diffusion is promoted using a stainless steel foil that has been deformed by cold working. Patent Document 8 describes a ferritic stainless steel for direct diffusion bonding having an optimized composition.

JP-A-63-119993 Japanese Patent Laid-Open No. 4-294484 Japanese Patent Publication No.57-4431 JP 62-199277 A JP-A-2-261548 JP 7-213918 A JP-A-9-279310 JP-A-9-99218 JP 2000-303150 A

  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 yet been established as the mainstream diffusion bonding method for stainless steel materials. The main reason is that it is difficult to ensure the reliability of the joint (joint strength and sealability) and 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.

  The present invention intends to provide a diffusion bonding product of stainless steel material having excellent joint reliability by a “direct method” that can be carried out with a work load equivalent to that of a conventional insert material insertion method.

  As a result of detailed research by the inventors, when using the grain boundary movement when the ferrite phase transforms to the austenite phase during diffusion bonding, it is not possible to apply special high-temperature heating or high surface pressure at the boundary between stainless steel materials. It was found that diffusion was promoted. The present invention uses the growth of crystal grains accompanying such phase transformation (movement of phase boundary) to diffuse and join stainless steel materials without using an insert material.

That is, in the present invention, when stainless steel materials are brought into direct contact and integrated by diffusion bonding, at least one of both stainless steel materials to be brought into contact has an austenite transformation start temperature Ac ₁ point in the temperature rising process at 650 to 950 ° C. Applying a two-phase steel having an austenite + ferrite two-phase temperature range of 880 ° C. or higher, with a contact surface pressure of 0.03 MPa to 1.0 MPa and a heating temperature of 880 to 1080 ° C. There is provided a method for producing a stainless steel diffusion bonding product, in which diffusion bonding proceeds while accompanying grain boundary movement when the ferrite phase of the steel is transformed into an austenite phase.

In particular, at least one of the two stainless steel materials to be contacted can be applied to a two-phase steel having a chemical composition of the following (A) and having an austenite + ferrite two-phase temperature range of 880 ° C. or more.
(A) By mass%, C: 0.0001 to 0.15%, Si: 0.001 to 1.0%, Mn: 0.001 to 1.0%, Ni: 0.05 to 2.5% Cr: 13.0 to 18.5%, Cu: 0 to 0.2%, Mo: 0 to 0.5%, Al: 0 to 0.05%, Ti: 0 to 0.2%, Nb: It consists of 0 to 0.2%, V: 0 to 0.2%, B: 0 to 0.01%, N: 0.005 to 0.1%, the balance Fe and inevitable impurities, and the following formula (1) The X value indicated by is 650-950.
X value = 35 (Cr + 1.72Mo + 2.09Si + 4.86Nb + 8.29V + 1.77Ti + 21.4Al + 40.0B-7.14C-8.0N-3.28Ni-1.89Mn-0.51Cu) +310 (1)

Generally stainless steel austenitic stainless steels based on the metal structure at room temperature, ferritic stainless steels are classified into such martensitic stainless steel, referred to herein as "2-phase steels" refers Ac ₁ or more points Is a steel having a two-phase structure of austenite + ferrite in the temperature range. Such a two-phase steel includes stainless steels classified as ferritic stainless steel and martensitic stainless steel.

About the combination of both stainless steel materials used for diffusion bonding, the following three patterns can be illustrated.
[Pattern 1] Both stainless steel materials used for diffusion bonding are two-phase steels having the chemical composition (A).
[Pattern 2] When one of the stainless steel materials used for diffusion bonding is a two-phase steel having the chemical composition (A) and the other is a steel having the chemical composition (B) below.
[Pattern 3] When one of the stainless steel materials used for diffusion bonding is a two-phase steel having the following chemical composition (A) and the other is a steel having the following chemical composition (C).

(B) By mass%, C: 0.0001 to 0.15%, Si: 0.001 to 4.0%, Mn: 0.001 to 2.5%, P: 0.001 to 0.045% , S: 0.0005 to 0.03%, Ni: 6.0 to 28.0%, Cr: 15.0 to 26.0%, Mo: 0 to 7.0%, Cu: 0 to 3.5 %, Nb: 0 to 1.0%, Ti: 0 to 1.0%, Al: 0 to 0.1%, N: 0 to 0.3%, B: 0 to 0.01%, V: 0 -0.5%, W: 0-0.3%, Ca, Mg, Y, REM (rare earth elements) total: 0-0.1%, balance Fe and inevitable impurities.

(C) By mass%, C: 0.0001 to 0.15%, Si: 0.001 to 1.2%, Mn: 0.001 to 1.2%, P: 0.001 to 0.04% S: 0.0005 to 0.03%, Ni: 0 to 0.6%, Cr: 11.5 to 32.0%, Mo: 0 to 2.5%, Cu: 0 to 1.0%, Nb: 0 to 1.0%, Ti: 0 to 1.0%, Al: 0 to 0.2%, N: 0 to 0.025%, B: 0 to 0.01%, V: 0 to 0 0.5%, W: 0 to 0.3%, Ca, Mg, Y, REM (rare earth elements) total: 0 to 0.1%, balance Fe and inevitable impurities.

  Here, (B) is a composition range including austenitic stainless steel, and (C) is a composition range including ferritic stainless steel.

In the two-phase steel having the above composition (A), the degree of freedom of the diffusion bonding conditions is further expanded by applying a steel having a γmax of 20 to less than 100 represented by the following formula (2). When such steel is provided, diffusion bonding may be advanced in the condition range of contact surface pressure of 0.03 to 0.8 MPa and holding temperature of 880 to 1030 ° C.
γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr-12Mo + 9Cu-49Ti-50Nb-52Al + 470N + 189 (2)

  The value of the content of the element expressed by mass% is substituted for the component element in the above formulas (1) and (2). For elements not contained, 0 (zero) is substituted.

  The stainless steel diffusion bonding structure according to the present invention is excellent in bonding strength and sealing performance, and is effective in avoiding a decrease in corrosion resistance due to contact with dissimilar metals (particularly metals containing Cu) since no insert material is used. . In addition, compared with conventional diffusion bonding of stainless steel materials by direct method, the contact surface pressure and temperature can be reduced, and general diffusion bonding equipment applied to the insert material insertion method can be used. For this reason, the manufacturing cost reduction effect by not using insert material is not offset by the increase in work load. Therefore, the present invention contributes to the spread of highly reliable stainless steel diffusion bonding products.

The cross-sectional structure | tissue photograph of the joint interface vicinity which tried diffusion bonding at 900 degreeC using the duplex-phase steel of this invention object for both steel materials. The figure which shows the position of the thickness measurement point by the ultrasonic thickness meter on the surface of the laminated body which tried diffusion bonding. The cross-sectional structure | tissue photograph (example of this invention) of the laminated body used for the diffusion bonding test. The cross-sectional structure | tissue photograph (comparative example) of the laminated body used for the diffusion bonding test. The cross-sectional structure | tissue photograph (example of this invention) of the laminated body used for the diffusion bonding test. The cross-sectional structure | tissue photograph (comparative example) of the laminated body used for the diffusion bonding test. The cross-sectional structure | tissue photograph (example of this invention) of the laminated body used for the diffusion bonding test. The cross-sectional structure | tissue photograph (comparative example) of the laminated body used for the diffusion bonding test. The cross-sectional structure | tissue photograph (comparative example) of the laminated body used for the diffusion bonding test. The cross-sectional structure | tissue photograph (comparative example) of the laminated body used for the diffusion bonding test. The cross-sectional structure | tissue photograph (comparative example) of the laminated body used for the diffusion bonding test. The cross-sectional structure | tissue photograph (comparative example) of the laminated body used for the diffusion bonding test. The cross-sectional structure | tissue photograph (comparative example) of the laminated body used for the diffusion bonding test. The figure which showed typically the size shape of each stainless steel material which is a to-be-joined material used for Example 2, and the external shape of a diffusion bonding product.

In general, it is considered that the following processes are required until diffusion bonding is completed in a state where stainless steel materials are in direct contact with each other.
[1] A process in which the unevenness of the contact surface is deformed and closely adheres to increase the contact area.
[2] A process in which the surface oxide films of both steel materials existing on the contact surface decompose and diffuse and disappear.
[3] A process in which interdiffusion of atoms and growth of crystal grains occur.
[4] A process in which the residual gas in the void interposed in the contact surface disappears due to the reaction with the metal substrate.
Since the surface oxide film of the stainless steel material is a strong passive film, it is necessary to maintain a high contact surface pressure and high temperature for a long time, in particular, in order to complete the process [2]. This is a factor that hinders the industrial spread of the direct diffusion bonding method of stainless steel.

  As a result of intensive studies, the inventors have found a technique for realizing direct diffusion bonding of stainless steel materials without requiring completion of the process [2]. The essence of this method is to perform diffusion bonding using the driving force of the phase transformation that occurs in the vicinity of the contact surfaces of both stainless steel materials. According to this, industrial direct diffusion bonding can be performed under conditions of lower temperature and lower contact surface pressure than before, and the reliability of the bonding surface is improved.

FIG. 1 exemplifies a cross-sectional structure in the vicinity of a joint interface when diffusion bonding at 900 ° C. is attempted using a two-phase steel having the chemical composition of the present invention for both steel materials. This two-phase steel is a steel corresponding to D-2 in Table 1 described later, and the metal structure before diffusion bonding is a ferrite phase + M ₂₃ C ₆ (M is a metal element such as Cr) -based carbide. A 2D finishing material having a thickness of 1.0 mm is used as a sample, the surfaces having a surface roughness Ra of 0.21 μm are brought into direct contact with each other, the contact surface pressure is set to 0.3 MPa, and a pressure of 10 ⁻³ Pa is obtained by evacuation. In the chamber, the temperature of the sample was raised from room temperature to 900 ° C. with a heater in about 1 h, held at that temperature after reaching 900 ° C., taken out of the furnace when a predetermined holding time passed, and rapidly cooled to obtain a cross-sectional structure. Is a survey. FIG. 1A shows a stage where the holding time is 10 min, and FIG. 1B shows a stage where the holding time is 50 min.

In this specification, a position where both members are in contact before diffusion bonding is referred to as an “interface position”.
As can be seen in FIG. 1A, crystal grains straddling the interface position are generated at the stage of the holding time of 10 min. These crystal grains correspond to the austenite phase generated by transformation from the ferrite phase and carbide before the temperature rise (the photograph is taken after quenching from the holding temperature, so the austenite phase is a martensite phase) . In this example, since the metal structure before diffusion bonding is a ferrite phase + carbide, the transformation to the austenite phase occurs from the carbide. The generated austenite crystal grows while expanding grain boundaries in the ferrite phase. That is, austenite crystal grains grow while accompanying grain boundary movement when the ferrite phase transforms into the austenite phase.

  In the example of FIG. 1, since both steel materials are the same two-phase steel, when the austenite phase generated from the carbide of one of the steel materials existing near the interface position grows, the austenite crystal grains Part of the side steel crystal is taken in and grown as one austenite crystal grain, and diffusion bonding proceeds. In the stage of FIG. 1A, many unjoined portions remain at the interface position.

  Even if the steel on the other side is a steel that becomes an austenite single phase or a ferritic single phase at the holding temperature of diffusion bonding, austenite grains generated from carbides near the interface position of the two-phase steel grow. At this time, it has been confirmed that it also grows into the crystal of the counterpart material across the interface position.

The metal structure of the two-phase steel that is the subject of the present invention is a ferrite phase + carbide, a ferrite phase + martensite phase, or a martensite single phase, depending on the chemical composition and steel sheet production conditions. In order to utilize the diffusion of atoms accompanying the grain boundary migration when the ferrite phase transforms to the austenite phase during diffusion bonding, the ferrite phase is present in an amount of 50% by volume or more at the time when austenite begins to be generated by heating the diffusion bonding. It is desirable. When the two-phase steel used for diffusion bonding has a martensitic single-phase metal structure, it is effective to anneal it in advance to obtain a ferrite phase + martensitic phase structure. As the annealing, for example, the condition of holding the material at 600 ° C. to Ac ₁ point + 50 ° C. can be adopted, but usually the annealing effect is obtained in the temperature rising process of diffusion bonding, and the austenite phase is in the structure state where the ferrite phase exists. The generation start can be reached.

  The austenite crystal is generated starting from carbide in a two-phase steel having a ferrite phase + carbide structure, and starting from and generating a martensite phase in a two-phase steel having a ferrite phase + martensitic structure. In either case, the austenite crystal in the duplex stainless steel grows while moving the grain boundary into the surrounding ferrite phase. At that time, at the interface position between the two steel materials, the crystal grain boundary moves toward the bonding partner material without waiting for the complete disappearance of the oxide serving as a diffusion barrier.

  When a sufficient holding time at the temperature at which diffusion bonding proceeds is secured, the increase in the volume fraction of the austenite phase is completed, and grain boundary migration also occurs in the remaining ferrite crystal grains. Then, as shown in FIG. 1B, ferrite crystal grains grown over the interface position are observed. At this stage, there are very few unbonded portions remaining at the interface position, and it can be considered that both steel materials are integrated by diffusion bonding. It can be confirmed that most portions of the interface position are joined by measurement with an ultrasonic thickness meter, which will be described later.

[Dual phase steel]
In the present invention, in order to realize diffusion bonding by a direct method under low temperature and low contact surface pressure, at least one of both stainless steel materials used for diffusion bonding is austenite + ferrite two-phase structure in a temperature range where diffusion bonding proceeds. The following steel (duplex steel) is applied. Specifically, a two-phase steel having an austenite transformation start temperature Ac ₁ point in the temperature rising process at 650 to 950 ° C. and an austenite + ferrite two-phase temperature range in the range of 880 ° C. or more is a suitable target. Here, if the Ac ₁ point is 880 ° C. or higher, it will inevitably have a two-phase temperature range in the range of 880 ° C. or higher. However, if the Ac ₁ point is too high, the lower limit of the two-phase temperature range is associated with it. As the temperature rises, the lower limit of the heating temperature is also increased, and diffusion bonding is performed without using an insert material at a relatively low temperature by using the grain boundary movement when the ferrite phase of the two-phase steel transforms to the austenite phase. The merit of the present invention cannot be utilized. As a result of various studies, it is effective to apply steel having an Ac ₁ point in the range of 950 ° C. or lower, and steel of 900 ° C. or lower is more preferable.

  The duplex stainless steel applied in the present invention may be a steel type classified as a so-called “martensitic stainless steel” as long as it exhibits an austenite + ferrite dual phase structure in a temperature range in which diffusion bonding proceeds. Absent. Martensitic stainless steel is a steel type that obtains a martensite structure by quenching from a high-temperature austenite single-phase region such as 1050 ° C. or more. There is also a martensitic stainless steel having a composition capable of diffusion bonding utilizing the movement of. Therefore, such martensitic stainless steel is also treated as a duplex stainless steel in this specification.

Examples of the specific component composition of the duplex stainless steel targeted in the present invention include those satisfying the following (A).
(A) By mass%, C: 0.0001 to 0.15%, Si: 0.001 to 1.0%, Mn: 0.001 to 1.0%, Ni: 0.05 to 2.5% Cr: 13.0 to 18.5%, Cu: 0 to 0.2%, Mo: 0 to 0.5%, Al: 0 to 0.05%, Ti: 0 to 0.2%, Nb: It consists of 0 to 0.2%, V: 0 to 0.2%, B: 0 to 0.01%, N: 0.005 to 0.1%, the balance Fe and inevitable impurities, and the following formula (1) The X value indicated by is 650-950.
X value = 35 (Cr + 1.72Mo + 2.09Si + 4.86Nb + 8.29V + 1.77Ti + 21.4Al + 40.0B-7.14C-8.0N-3.28Ni-1.89Mn-0.51Cu) +310 (1)

Here, the above-mentioned X value accurately estimates the austenite transformation start temperature Ac ₁ point (° C.) in the temperature rising process in the two-phase steel having the austenite + ferrite two-phase temperature range of 880 ° C. or more. It is an indicator that can.

As the two-phase steel having the chemical composition (A), steel having a γmax represented by the following formula (2) of 20 to less than 100 can be applied.
γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr-12Mo + 9Cu-49Ti-50Nb-52Al + 470N + 189 (2)
γmax is an index that represents the amount (volume%) of the austenite phase that is produced when heated to about 1100 ° C. When γmax is 100 or more, the steel can be regarded as a steel type that becomes an austenite single phase at a high temperature. In steels where γmax is less than 20 to 100, it is easy to set the temperature to avoid the γ single-phase region, and the degree of freedom of appropriate conditions is expanded to the lower temperature and lower contact pressure side. It is more preferable to apply steel having γmax of 50-80.

[Joint type]
As the counterpart material to be integrated by diffusion bonding with the steel material composed of the above-mentioned two-phase steel, in addition to the above-mentioned two-phase steel, an austenitic steel type or a ferrite single phase that becomes an austenite single phase in the heating temperature range of diffusion bonding can be used. A ferritic steel grade can be applied. Even if a material other than a dual phase steel is used as the counterpart material, the austenite phase that grows by transformation in one of the dual phase steels also grows from the interface position toward the counterpart material. It is possible to construct a simple diffusion joint.

  As the austenitic or ferritic steel types, various existing steel types can be applied depending on the application, and there is no need to pay particular attention to the component composition from the viewpoint of diffusion bonding properties. Specific component composition ranges include the following (B) as an austenitic steel type and the following (C) as a ferritic steel type.

(B) By mass%, C: 0.0001 to 0.15%, Si: 0.001 to 4.0%, Mn: 0.001 to 2.5%, P: 0.001 to 0.045% , S: 0.0005 to 0.03%, Ni: 6.0 to 28.0%, Cr: 15.0 to 26.0%, Mo: 0 to 7.0%, Cu: 0 to 3.5 %, Nb: 0 to 1.0%, Ti: 0 to 1.0%, Al: 0 to 0.1%, N: 0 to 0.3%, B: 0 to 0.01%, V: 0 -0.5%, W: 0-0.3%, Ca, Mg, Y, REM (rare earth elements) total: 0-0.1%, balance Fe and inevitable impurities.

(C) By mass%, C: 0.0001 to 0.15%, Si: 0.001 to 1.2%, Mn: 0.001 to 1.2%, P: 0.001 to 0.04% S: 0.0005 to 0.03%, Ni: 0 to 0.6%, Cr: 11.5 to 32.0%, Mo: 0 to 2.5%, Cu: 0 to 1.0%, Nb: 0 to 1.0%, Ti: 0 to 1.0%, Al: 0 to 0.2%, N: 0 to 0.025%, B: 0 to 0.01%, V: 0 to 0 0.5%, W: 0 to 0.3%, Ca, Mg, Y, REM (rare earth elements) total: 0 to 0.1%, balance Fe and inevitable impurities.

[Diffusion bonding conditions]
The contact surface pressure between the two members used for diffusion bonding is 1.0 MPa or less. If the contact surface pressure is 1.0 MPa or less, diffusion bonding using no insert material can be performed with relatively simple equipment. What is necessary is just to set the contact surface pressure which is sufficient for diffusion joining to advance according to the steel grade to apply, heating holding temperature, and holding time in the range of 1.0 Mpa or less. In particular, when a steel having a γmax of less than 100 is applied as the two-phase steel, good results can be easily obtained at a contact surface pressure of 0.8 MPa or less. On the other hand, when the contact surface pressure is extremely low, the heating and holding time becomes long and the productivity is lowered. Industrially, it is preferable to ensure a contact surface pressure of 0.03 MPa or more, and the contact pressure may be controlled to be 0.1 MPa or more. In addition, as for the surface used as the joining surface of the stainless steel material used for diffusion joining, it is desirable for Ra to be 0.30 micrometer or less. The surface finish may be pickling, bright annealing, or polishing.

  The heating temperature for diffusion bonding is 880 ° C. or higher. According to the study by the inventors, when using the grain boundary movement due to the transformation of the above-mentioned two-phase steel, diffusion bonding between stainless steel materials can be performed without holding at a high temperature as in the prior art. When the pressure is 1.0 MPa or lower, heating at 880 ° C. or higher is desired. A temperature of 900 ° C. or higher is more preferable from the viewpoint of promoting diffusion.

  However, it is necessary to heat and hold the two-phase steel austenite + ferrite two-phase temperature range applied to both or one member. By maintaining the temperature range in which these two phases coexist, newly grown austenite crystal grains easily grow over the interface position toward the contact partner material. Although there are many unclear points about the mechanism at this time, the following can be considered. When the austenite + ferrite two-phase coexisting temperature region is heated in a state where the ferrite phase is present in the two-phase steel, austenite crystals are first generated in the two-phase steel starting from the carbide or martensite phase. The grain boundary between the generated austenite crystal and the surrounding ferrite crystal is in a state of being very mobile due to the “transformation driving force” that tries to bring the ratio of the ferrite phase to the austenite phase closer to the equilibrium state. Utilizing this transformation driving force, the austenite crystal grows while moving the grain boundary into the adjacent ferrite crystal in the duplex phase steel. At that time, the austenite crystal grains that are growing facing the interface with the contact partner material are moved to expand into the crystal grains of the contact partner material in order to become a more stable form of energy. As a result, austenite crystal grains straddling the interface position are formed. When the interface position of both members is partially connected by austenite crystal grains, the crystal grains of both members facing the interface position in the vicinity of these austenite crystal grains also cause grain boundary movement due to diffusion and diffusion. Joining proceeds.

  When steel that becomes a ferrite single phase at 880 ° C or higher or steel that becomes an austenite single phase is used instead of the above two-phase steel, a low contact surface with a contact surface pressure of 1.0 MPa or less and a heating temperature of 880 to 1080 ° C. It has been confirmed that unbonded portions (defects) are likely to remain at the interface position under pressure and low temperature heating conditions. From this, it can be said that using the transformation driving force of the above-mentioned two-phase steel is extremely advantageous in promoting the progress of diffusion bonding between stainless steel materials at low contact surface pressure and low temperature heating.

  The duplex stainless steel having a chemical composition in which the γmax in the formula (2) is 100 or less has a temperature range in which austenite + ferrite two-phase structure is formed in a temperature range lower than about 1100 ° C. According to the study by the inventors, when using a dual-phase steel whose γmax is adjusted to less than 20 to 100, diffusion bonding is performed in a condition range of a contact surface pressure of 0.03 to 0.8 MPa and a heating temperature of 880 to 1030 ° C. It is possible to proceed, which is advantageous for lowering the contact surface pressure and lowering the temperature of the diffusion bonding conditions. In particular, when a duplex stainless steel having a γmax of 50 to 80 is used, the degree of freedom of diffusion bonding conditions is further expanded, and conditions suitable for a contact surface pressure of 0.03 to 0.5 MPa and a heating temperature of 880 to 1000 ° C. Can be found. In this case, the upper limit of the heating temperature may be set in a low temperature range of 980 ° C. or lower, for example.

As in the case of diffusion bonding between stainless steel materials using a conventional general insert material, the heat of diffusion bonding is to heat and hold the member to be bonded in an atmosphere in which the pressure is approximately 10 ⁻³ Pa or less by evacuation. Can be done by. However, the insert material is not used, and the stainless steel materials to be joined are brought into direct contact with each other. As described above, the contact surface pressure is set within a range of 1.0 MPa or less. As a heating method, in addition to a method of uniformly heating the entire member in the furnace with a heater, a method of heating the vicinity of the contact portion to a predetermined temperature by resistance heating by energization can be adopted. What is necessary is just to set the heating holding time in the range of 30-120 min.

[Example 1]
Steel plates having the chemical composition shown in Table 1 were prepared. D-1 to D-3 are steels having a γmax of less than 100, M-1 to M-2 are steels classified as so-called martensitic stainless steels, F-1 is a ferritic single phase steel, and A-1 is an austenitic single steel. It is a phase steel. Table 1 also lists the plate thickness, surface finish, and surface roughness Ra of the steel plate. The metallographic structure of each prepared steel sheet is as follows: D-1 to D-3 are ferrite phase + carbide, M-1, M-2 is ferrite phase + martensite phase, M-3 is martensite single phase, F-1 is A ferrite single phase, A-1, is an austenite single phase.

A plate test piece of 20 mm × 20 mm was cut out from each steel plate, and two test pieces were overlapped by the following method to attempt diffusion bonding.
The surface pressure applied to the contact surface of these two test pieces using a jig utilizing the principle of the two test pieces that are to be diffusion-bonded so that the surfaces are in contact with each other. (Contact surface pressure) was adjusted to a predetermined magnitude. This jig is attached to a carbon composite column with a carbon composite arm rotatable around a fixed axis in the horizontal direction, and a load is applied to the specimens stacked by the gravity of the weight suspended on the arm. It is given. In other words, this jig uses the fixed axis position of the arm as a fulcrum, the position where the load is applied to the stacked test pieces as the action point, and the position where the weight is suspended as the force point. The gravity applied to the weight is amplified and acts on the contact surface of the test piece. Hereinafter, the two test pieces that are laminated are referred to as “steel material 1” and “steel material 2”, and a state in which the steel material 1 and the steel material 2 are laminated is referred to as a “laminate”.

The diffusion bonding of the steel materials 1 and 2 was tried by the following heat treatment. After a predetermined load is applied to the laminate by the jig, the jig and the laminate are placed in a vacuum furnace and evacuated to a vacuum of 10 ⁻³ to 10 ⁻⁴ Pa. The temperature was raised to a predetermined heating temperature set in a range of 880 ° C. or higher in about 1 h, held at that temperature for 2 h, then transferred to a cooling chamber and cooled. The above-mentioned degree of vacuum was maintained at a holding temperature of −100 ° C., and then Ar gas was introduced to cool to about 100 ° C. or less in a 90 kPa Ar gas atmosphere.

About the laminated body which completed the said heat processing, 49 measurement provided in 3 mm pitch on the surface of a 20 mm x 20 mm laminated body as shown in FIG. 2 using the ultrasonic thickness meter (Olympus company make; Model35DL). Thickness measurements were made at points. The probe diameter was 1.5 mm. When the measured thickness at a certain measurement point indicates the total thickness of steel materials 1 and 2, it is considered that both steel materials are integrated by diffusion of atoms at the interface position of both steel materials corresponding to the measurement point. be able to. On the other hand, when the plate thickness measurement value is less than the total plate thickness of the steel materials 1 and 2, there is an unjoined portion (defect) at the interface position between the two steel materials corresponding to the measurement point. The inventors examined in detail the correspondence between the cross-sectional structure of the laminate after the heat treatment and the measurement result obtained by this measurement method, and the measurement result was the total thickness of the steel materials 1 and 2. It was confirmed that the area ratio of the joint portion occupying the contact area can be accurately evaluated by a value obtained by dividing the number of measurement points by the total number of measurements 49 (referred to as “joining ratio”). Therefore, diffusion bonding properties were evaluated according to the following evaluation criteria.
A: Joining rate 100% (diffusion bonding; excellent)
○: Joining rate 90 to 99% (diffusion bonding; good)
Δ: Joining rate 60-89% (diffusion bonding; somewhat poor)
×: Joining rate 0 to 59% (diffusion bonding property; poor)
As a result of various investigations, the strength of the diffusion bonding part is sufficiently ensured 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. Was determined to be acceptable.
The evaluation results are shown in Table 2.

  Moreover, about the laminated body which finished the said heat processing, structure | tissue observation was performed with the optical microscope about the area | region containing the interface position in the cross section parallel to a thickness direction. As a result, it was confirmed that the degree of progress of diffusion bonding observed by microscopic observation (that is, the degree of disappearance of the unbonded portion at the interface position) and the value of the above-mentioned “bonding rate” had a good correspondence. . 3 to 13 illustrate cross-sectional structure photographs for some examples. The structural photographs shown here were taken by intentionally selecting locations where as many unbonded portions as possible remained at the interface positions, except for those evaluated as ◎. Table 2 shows which test No. these cross-sectional structure photographs correspond to.

  As can be seen from Table 2, in the example of the present invention in which the two-phase steel according to the present invention is used for both or one of the steel materials, diffusion bonding is allowed to proceed under conditions of a contact surface pressure of 1.0 MPa or less and a heating temperature of 1080 ° C. or less. And a healthy diffusion joint was obtained. Since it is not necessary to set the heating temperature to a high temperature such as 1100 ° C. or higher, the coarsening of crystal grains is suppressed, and the mechanical properties of the diffusion bonded product are improved. When two-phase steels (D-1 to D-3) having a γmax of 100 or less are adopted, an appropriate diffusion bonding condition range extends in a direction where both the contact surface pressure and the holding temperature are low. In particular, the use of a two-phase steel (D-1, D-2) whose γmax is adjusted to 50 to 80 further expands the appropriate condition range, which is advantageous in terms of manufacturing cost.

  On the other hand, Comparative Examples No. 9, 17, 20, 22, and 25 were coarse in crystal grains because of the high heating temperature (see FIGS. 4, 6, and 8). Nos. 21 and 24 employ martensitic stainless steel with a γmax of 100 or more as a two-phase steel, and the driving force for transformation to an austenite phase in a two-phase temperature range is less than that of a steel type with a γmax of less than 100. It was thought that the temperature was small, and diffusion bonding did not proceed when the holding temperature was lowered to 1000 ° C. No. 27 and No. 28 were unable to find appropriate diffusion bonding conditions by applying martensitic stainless steels that were considered to have no temperature range that would be an austenite + ferrite two-phase structure. No. 32 and No. 33 were obtained by attempting diffusion bonding between steels having a ferrite single-phase structure and an austenite single-phase structure at 880 to 1080 ° C., respectively. Diffusion bonding at the heating temperature could not be realized.

[Example 2]
The steel having the chemical composition shown in Table 3 is melted and hot rolled to obtain a hot rolled sheet having a thickness of 3 to 4 mm, and the thickness of the sheet is determined by sequentially performing annealing, pickling, cold rolling, finish annealing, and pickling. A 1.0 mm test steel plate was used. D-11 to D-15 are two-phase steels according to the present invention, F-11 is a ferrite single-phase steel, and A-11 is an austenite single-phase steel. Table 3 also lists the plate thickness, surface finish, and surface roughness Ra of the steel plate. As for the metal structure of each steel plate, D-11 to D-15 are a ferrite phase + carbide, F-11 is a ferrite single phase, and A-11 is an austenite single phase.

  A 100 mm square steel material (hereinafter referred to as “flat plate material”) was produced from each test steel plate of steels D-11 to D-15 by cutting. Further, a steel material (hereinafter referred to as “frame material”) constituted by a frame having a width of 5 mm was produced by cutting the center of a 100 mm square plate by cutting from test steel plates of all steel types. At that time, burrs are not removed. In the flat plate member and the frame member, 6 mmφ holes were formed at two locations near the upper end of the diagonal line. In FIG. 14A, [1] and [5] schematically show the dimensions and shape of the flat plate, and [2] to [4] schematically show the dimensions and shape of the frame material. As shown in FIG. 14, these three steel members are stacked in the stacking order of [1] to [5] shown in FIG. Then, a 5 mmφ pin made of Alloy 600 was inserted into the hole communicating with each steel material, and a weight of 5 kg was placed on the upper surface of the horizontally placed laminate, and subjected to vacuum diffusion bonding. At this time, a contact pressure of about 0.05 MPa is applied to the contact surface between the steel materials.

The combinations of steel materials [1] to [5] were the following two patterns.
Pattern A; [1] to [5] are all the same two-phase steel grades of the present invention.
Pattern B; [1] [3] [5] are the same two-phase steel types of the subject of the present invention, and [2] [4] corresponding to the counterpart material are the same austenitic or ferritic same steel types.

In diffusion bonding, the laminate is charged in a vacuum furnace and evacuated to a pressure of 10 ⁻³ Pa or less, and then heated to a heating temperature set in the range of 900 to 1100 ° C. and then heated to that temperature for 60 min. It hold | maintained and it carried out by the method of cooling in a furnace after that.

[Diffusion bonding reliability evaluation]
The above stainless steel diffusion bonding product (in the shape of FIG. 14B) was subjected to a heating test at 800 ° C. for 24 hours in the atmosphere. Then, it cut | disconnected in the lamination direction in the position of aa 'of FIG.14 (b), and the presence or absence of oxidation of the internal cavity surface (inner surface) was investigated visually. If there is a gap connected to the outside in the diffusion bonding part or if the diffusion bonding part is damaged during the heat treatment, the inner surface after the heating test is oxidized because oxygen penetrates into the inside. The metallic luster is lost. On the other hand, when the soundness of the diffusion bonded portion is maintained and the inside is kept in a high vacuum state, the inner surface after the heating test exhibits a metallic luster unique to stainless steel. Therefore, a stainless steel diffusion bonding product whose inner surface maintained the original metallic luster was evaluated as “◯” (diffusion bonding portion reliability: good), and the others were evaluated as “x” (diffusion bonding reliability: poor). The results are shown in Table 4.

  As can be seen from Table 4, a diffusion bonded product having excellent reliability at a low temperature of 880 to 1000 ° C. can be obtained by applying the two-phase steel of the present invention to at least one of both steel materials in the diffusion bonded portion. It was.

  On the other hand, No. 49 and 50 which are comparative examples did not use a duplex steel, and a diffusion bonded product excellent in reliability could not be obtained even at 1100 ° C.

Claims (6)

When stainless steel materials are brought into direct contact and integrated by diffusion bonding, at least one of both stainless steel materials to be brought into contact has an austenite transformation start temperature Ac ₁ point in the temperature rising process at 650 to 950 ° C. and austenite + ferrite two phases Applying a two-phase steel having a temperature range of 880 ° C. or more, the ferrite phase of the two-phase steel is austenite under the conditions of a contact surface pressure of 0.03 MPa to 1.0 MPa and a heating temperature of 880 to 1080 ° C. A method for producing a stainless steel diffusion bonding product, in which diffusion bonding proceeds while accompanying grain boundary movement during transformation to a phase. When stainless steel materials are brought into direct contact and integrated by diffusion bonding, at least one of both stainless steel materials to be brought into contact has a chemical composition of the following (A), and the austenite + ferrite two-phase temperature range is in the range of 880 ° C. or more. Grain boundary movement when the ferrite phase of the two-phase steel transforms to the austenite phase under the conditions of a contact surface pressure of 0.03 MPa to 1.0 MPa and a heating temperature of 880 to 1080 ° C. A method of manufacturing a stainless steel diffusion bonding product, in which diffusion bonding is advanced while accompanying the above.
(A) By mass%, C: 0.0001 to 0.15%, Si: 0.001 to 1.0%, Mn: 0.001 to 1.0%, Ni: 0.05 to 2.5% Cr: 13.0 to 18.5%, Cu: 0 to 0.2%, Mo: 0 to 0.5%, Al: 0 to 0.05%, Ti: 0 to 0.2%, Nb: It consists of 0 to 0.2%, V: 0 to 0.2%, B: 0 to 0.01%, N: 0.005 to 0.1%, the balance Fe and inevitable impurities, and the following formula (1) The X value indicated by is 650-950.
X value = 35 (Cr + 1.72Mo + 2.09Si + 4.86Nb + 8.29V + 1.77Ti + 21.4Al + 40.0B-7.14C-8.0N-3.28Ni-1.89Mn-0.51Cu) +310 (1) When stainless steel materials are brought into direct contact and integrated by diffusion bonding, one of the stainless steel materials to be brought into contact has the chemical composition of the following (A) and has an austenite + ferrite two-phase temperature range of 880 ° C. or more. Two-phase steel and the other stainless steel having the chemical composition (B) below are applied to each other, and the above-mentioned two-phase steel under the conditions of a contact surface pressure of 0.03 MPa to 1.0 MPa and a heating temperature of 880 to 1080 ° C. A method for producing a stainless steel diffusion bonding product, in which diffusion bonding proceeds while accompanying grain boundary movement when the ferrite phase of the steel transforms into an austenite phase.
(A) By mass%, C: 0.0001 to 0.15%, Si: 0.001 to 1.0%, Mn: 0.001 to 1.0%, Ni: 0.05 to 2.5% Cr: 13.0 to 18.5%, Cu: 0 to 0.2%, Mo: 0 to 0.5%, Al: 0 to 0.05%, Ti: 0 to 0.2%, Nb: It consists of 0 to 0.2%, V: 0 to 0.2%, B: 0 to 0.01%, N: 0.005 to 0.1%, the balance Fe and inevitable impurities, and the following formula (1) The X value indicated by is 650-950.
X value = 35 (Cr + 1.72Mo + 2.09Si + 4.86Nb + 8.29V + 1.77Ti + 21.4Al + 40.0B-7.14C-8.0N-3.28Ni-1.89Mn-0.51Cu) +310 (1)
(B) By mass%, C: 0.0001 to 0.15%, Si: 0.001 to 4.0%, Mn: 0.001 to 2.5%, P: 0.001 to 0.045% , S: 0.0005 to 0.03%, Ni: 6.0 to 28.0%, Cr: 15.0 to 26.0%, Mo: 0 to 7.0%, Cu: 0 to 3.5 %, Nb: 0 to 1.0%, Ti: 0 to 1.0%, Al: 0 to 0.1%, N: 0 to 0.3%, B: 0 to 0.01%, V: 0 -0.5%, W: 0-0.3%, Ca, Mg, Y, REM (rare earth elements) total: 0-0.1%, balance Fe and inevitable impurities. When stainless steel materials are brought into direct contact and integrated by diffusion bonding, one of the stainless steel materials to be brought into contact has the chemical composition of the following (A) and has an austenite + ferrite two-phase temperature range of 880 ° C. or more. Two-phase steel, and stainless steel having the chemical composition (C) below is applied to the other, and the above-mentioned two-phase steel under the conditions of contact surface pressure of 0.03 MPa to 1.0 MPa and heating temperature of 880 to 1080 ° C. A method for producing a stainless steel diffusion bonding product, in which diffusion bonding proceeds while accompanying grain boundary movement when the ferrite phase of the steel transforms into an austenite phase.
(A) By mass%, C: 0.0001 to 0.15%, Si: 0.001 to 1.0%, Mn: 0.001 to 1.0%, Ni: 0.05 to 2.5% Cr: 13.0 to 18.5%, Cu: 0 to 0.2%, Mo: 0 to 0.5%, Al: 0 to 0.05%, Ti: 0 to 0.2%, Nb: It consists of 0 to 0.2%, V: 0 to 0.2%, B: 0 to 0.01%, N: 0.005 to 0.1%, the balance Fe and inevitable impurities, and the following formula (1) The X value indicated by is 650-950.
X value = 35 (Cr + 1.72Mo + 2.09Si + 4.86Nb + 8.29V + 1.77Ti + 21.4Al + 40.0B-7.14C-8.0N-3.28Ni-1.89Mn-0.51Cu) +310 (1)
(C) By mass%, C: 0.0001 to 0.15%, Si: 0.001 to 1.2%, Mn: 0.001 to 1.2%, P: 0.001 to 0.04% S: 0.0005 to 0.03%, Ni: 0 to 0.6%, Cr: 11.5 to 32.0%, Mo: 0 to 2.5%, Cu: 0 to 1.0%, Nb: 0 to 1.0%, Ti: 0 to 1.0%, Al: 0 to 0.2%, N: 0 to 0.025%, B: 0 to 0.01%, V: 0 to 0 0.5%, W: 0 to 0.3%, Ca, Mg, Y, REM (rare earth elements) total: 0 to 0.1%, balance Fe and inevitable impurities. The stainless steel diffusion bonding according to any one of claims 2 to 4, wherein a steel having a γmax expressed by the following formula (2) of 20 to less than 100 is applied as the two-phase steel having the chemical composition (A). Product manufacturing method.
γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr-12Mo + 9Cu-49Ti-50Nb-52Al + 470N + 189 (2) As the two-phase steel having the chemical composition (A), a steel having a γmax of 20 to less than 100 represented by the following formula (2) is applied, the contact surface pressure is 0.03 to 0.8 MPa, and the heating temperature is 880. The stainless steel diffusion bonding product according to any one of claims 2 to 4, wherein diffusion bonding proceeds while accompanying grain boundary movement when the ferrite phase of the two-phase steel transforms to an austenite phase in a condition range of -1030 ° C. Manufacturing method.
γmax = 420C-11.5Si + 7Mn + 23Ni-11.5Cr-12Mo + 9Cu-49Ti-50Nb-52Al + 470N + 189 (2)