Joint and manufacturing method thereof

Abstract

A joint having excellent corrosion resistance and can be produced easily and at low cost by air brazing stainless steel is provided. The Sn content of the joint is set to 40 mass% or more, and the deepest point of the Cr-enriched surface layer in the heated region of the base material is set to 300 nm or less.

Description

[Technical Field]

[0001] The present invention relates to a bonded body and a method for producing the same. [Background technology]

[0002] Stainless steel is a material with excellent corrosion resistance, and is widely used in the form of steel plates and pipes for various heat exchangers in automobiles, air conditioners, and other applications.

[0003] In recent years, with the rise in copper prices, there has been a trend toward switching from copper to stainless steel as the material for copper heat exchangers, and at the same time, there is a strong demand for switching to stainless steel for the copper used in the piping that transports fluids to heat exchangers. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-349443 Summary of the Invention [Problem to be solved by the invention]

[0005] Brazing is often used to join parts together in the manufacture of heat exchangers. The two main brazing methods used in the manufacturing process of heat exchangers are furnace brazing, in which parts are heated in an atmospheric furnace to join them simultaneously at multiple points (hereafter also referred to as furnace brazing), and air brazing, in which the joining parts are heated with a torch or the like to join them in the atmosphere (hereafter also referred to as air brazing). Of these brazing methods, an appropriate technique is used depending on the product part and the assembly stage.

[0006] Generally, the manufacture of a heat exchanger involves the steps of manufacturing a can body that performs heat exchange, and then connecting the can body to piping that serves as a fluid transport path. In the manufacture of the can body, components such as fins and pipes are primarily joined by furnace brazing to improve heat exchange efficiency. Meanwhile, air brazing is primarily used to join the can body to piping and to each other (hereinafter also referred to as piping joining).

[0007] The selection of joining techniques for manufacturing such heat exchangers is common regardless of the material. Therefore, when changing the material of a heat exchanger from copper to stainless steel, joining of the stainless steel in air is required. However, joining of stainless steel by air brazing is difficult.

[0008] Furthermore, Patent Document 1 states: "A method for joining stainless steel and an object to be joined to the stainless steel, comprising the steps of: bringing a bonding agent made of solder and a bonding metal into contact between the stainless steel and the object to be joined; and performing a heat treatment while bringing the bonding agent into contact with the stainless steel and the object to be joined." has been disclosed.

[0009] However, the bonded body obtained by the bonding method of Patent Document 1 may have reduced corrosion resistance. Furthermore, the bonding method of Patent Document 1 requires the placement of a bonding metal, specifically, Ni foil or Ni particles, along with the solder between the bonding surfaces of the stainless steel and the bonded object. In other words, the bonding method of Patent Document 1 requires the use of expensive Ni, which increases manufacturing costs. Furthermore, the additional manufacturing steps reduce productivity.

[0010] Therefore, there is a current demand for the development of a joined body that can be produced easily and at low cost by joining stainless steels by air brazing and that has excellent corrosion resistance.

[0011] The present invention has been developed in view of the above-mentioned current situation, and aims to provide a joined body that can be produced easily and at low cost by joining stainless steels by air brazing and that has excellent corrosion resistance. Another aim of the present invention is to provide a suitable method for producing the joined body.

[0012] Here, excellent corrosion resistance means that the pitting potential Vc'100 of the heated region of the base material of the joined body is 200 mV (vs. SCE) or more. The pitting potential Vc'100 is measured in accordance with Method B specified in JIS G 0577:2014. However, the test surface of the test piece is not dry-polished. Details are as described in the Examples below.

[0013] In addition, in this disclosure, any numerical range expressed using "to" means a range that includes the numerical values written before and after "to" as the lower limit and upper limit, respectively. [Means for solving the problem]

[0014] The present inventors have conducted extensive research to achieve the above object and have concluded that it is desirable to use Sn and Sn alloys such as solder as the joining metal when joining stainless steel by air brazing (hereinafter, these will be collectively referred to simply as Sn alloys).

[0015] One of the reasons why joining stainless steel by air brazing is difficult is that oxygen in the air oxidizes the stainless steel when heated during joining, forming an oxide film on the surface of the stainless steel. This oxide film reduces the wettability of the joining metals. Furthermore, during brazing, the constituent elements of the joining metals diffuse slightly at the interface between the base materials, resulting in bonding. However, the oxide film prevents the diffusion of the constituent elements of the joining metals.

[0016] Therefore, when joining stainless steel by air brazing, it is necessary to prevent the formation of the oxide film. The higher the temperature, the more likely the oxide film is to form. Here, Sn alloys have a lower melting point than Ni, Cu, Ag, and the like used in brazing stainless steel, and are also advantageous in terms of cost. For these reasons, the inventors have come to the conclusion that it is desirable to use Sn alloys as the joining metal when joining stainless steel by air brazing.

[0017] Based on the above idea, the inventors used an Sn alloy as the joining metal and performed air brazing joining of stainless steel under various conditions.

[0018] As a result, the inventors have discovered the following: Namely, the Cr enrichment formed in the heated region during joining of stainless steels is limited to the surface vicinity, and in particular, the deepest point of the Cr-enriched layer at the surface of the heated region is set to 300 nm or less. This makes it possible to obtain a joined body with excellent corrosion resistance, even when joining stainless steels by air brazing.

[0019] Here, the surface Cr-enriched layer is a region from the surface of the heated region (position of depth 0) to a position of depth 5 μm, where A(d) satisfies the relationship of the following formula (1). A(d)>(B+C)×0.6 (1) During the ceremony, A(d) is [Cr] / ([Cr]+[Fe]) at depth d, B is the maximum value of [Cr] / ([Cr]+[Fe]) from the surface of the heated area to a depth of 5 μm, C is the average value of [Cr] / ([Cr]+[Fe]) from the 10 μm depth position to the 20 μm depth position in the heated area, [Cr] is the element concentration of Cr (at%) and [Fe] is the elemental concentration of Fe (at%) is.

[0020] Furthermore, the inventors have conducted further studies and found that the above bonded body can be obtained by simultaneously satisfying the following conditions: The maximum temperature in the heating area of the materials to be joined must be 350°C or less. The residence time of the materials to be joined in the heating area at temperatures above 100°C should be between 5 and 300 seconds. The relationship in the following equation (2) must be satisfied. t≦300×10.5 / [Cr0]×400 / T (2) During the ceremony, T: Maximum temperature reached in the heating area of the joined material (℃) t: Residence time (seconds) in the heated area of the workpiece at a temperature of 100°C or higher; [Cr0]: The Cr content (mass%) of the stainless steel that forms the heating region, between the first and second stainless steels that are to be welded. is.

[0021] The present invention has been completed based on the above findings and further investigations. That is, the gist and configuration of the present invention are as follows.

[0022] 1. A joined body having a first stainless steel and a second stainless steel as base materials, and a joined portion between the first stainless steel and the second stainless steel, The Sn content of the joint is 40 mass% or more, A joined body, wherein the deepest point of the surface Cr-enriched layer in the heated region of the base material is 300 nm or less. Here, the surface Cr-enriched layer is a region from the surface of the heated region to a depth of 5 μm, where A(d) satisfies the relationship of the following formula (1). A(d)>(B+C)×0.6 (1) During the ceremony, A(d) is [Cr] / ([Cr]+[Fe]) at depth d, B is the maximum value of [Cr] / ([Cr]+[Fe]) from the surface of the heated area to a depth of 5 μm, C is the average value of [Cr] / ([Cr]+[Fe]) from the 10 μm depth position to the 20 μm depth position in the heated area, [Cr] is the element concentration of Cr (at%) and [Fe] is the elemental concentration of Fe (at%) is.

[0023] 2. The joint described in 1 above, wherein the joint is a lap joint or a T-joint.

[0024] 3. A method for producing the conjugate according to 1 or 2 above, comprising: The method comprises: a joining step in which a first stainless steel and a second stainless steel are used as joined materials, the joined materials are heated while melting a joining metal, and the joined materials are joined; the joining metal is Sn or an Sn alloy having an Sn content of 40 mass% or more, In the joining step, The maximum temperature reached in the heating region of the workpiece is 350°C or less, The residence time of the workpieces in a temperature range of 100°C or higher in the heating region is 5 seconds or more and 300 seconds or less, A method for producing a bonded body, which satisfies the relationship of the following formula (2): t≦300×10.5 / [Cr0]×400 / T (2) During the ceremony, T: Maximum temperature reached in the heating area of the joined material (℃) t: Residence time (seconds) in the heated area of the workpiece at a temperature of 100°C or higher; [Cr0]: The Cr content (mass%) of the stainless steel that forms the heating region, between the first and second stainless steels that are to be welded. is. [Effects of the Invention]

[0025] According to the present invention, a joined body having excellent corrosion resistance can be produced at low cost and simply by joining stainless steels by air brazing. Furthermore, the joined body of the present invention can be produced by air brazing. Therefore, the joined body of the present invention is particularly advantageous for heat exchangers and piping used in various devices such as water heaters, particularly in household gas water heaters, which are becoming increasingly compact to save space and have complex shapes with multiple branches. [Brief explanation of the drawings]

[0026] [Figure 1] FIG. 2 is a schematic diagram of an example of a joint (lap joint) according to one embodiment of the present invention. [Figure 2] FIG. 1 is a schematic diagram of an example of a junction (T-junction) according to one embodiment of the present invention. [Figure 3] 1 is an example of an optical microscope photograph of a cross section of a joint (lap joint). [Figure 4] FIG. 2 is a schematic diagram showing an example of the positional relationship of materials to be joined in a method for manufacturing a joined body (lap joining) according to one embodiment of the present invention. [Figure 5] FIG. 2 is a schematic diagram showing an example of the positional relationship of materials to be joined in a method for manufacturing a joined body (T-jointing) according to one embodiment of the present invention. [Figure 6] 1 shows an example of concentration profiles of [Cr], [Fe], and [Cr]+[Fe] in the depth direction in the heated region (Example Test No. 1-6). DETAILED DESCRIPTION OF THE INVENTION

[0027] [1] Zygote A bonded body according to one embodiment of the present invention will be described below. The X, Y, and Z directions in Figures 1 to 5 are as follows: X direction: joining direction Y direction: direction perpendicular to the joining (direction perpendicular to the joining direction (X direction) and perpendicular to the thickness direction (Z direction) described below) Z direction: the thickness direction of the joined material (joined body) (hereinafter simply referred to as the thickness direction).

[0028] [1-1] The first and second stainless steels that serve as base materials The first and second stainless steels that serve as base materials are derived from the stainless steels (first and second stainless steels) that serve as the materials to be joined. The shapes of the first and second stainless steels are, for example, plate-like (stainless steel plate) or tubular (stainless steel pipe). Note that the term "plate-like" here includes not only flat plates but also curved plates (curved plates). The thicknesses (plate thickness or tubular thickness) of the first and second stainless steels are not particularly limited. From the viewpoint of heat transfer during joining, the thicknesses of the first and second stainless steels are preferably 0.1 mm or more, more preferably 0.2 mm or more, and even more preferably 0.3 mm or more. Furthermore, the thicknesses of the first and second stainless steels are preferably 4.0 mm or less, more preferably 2.0 mm or less, and even more preferably 1.0 mm or less.

[0029] When the first stainless steel and the second stainless steel are in the form of plates, the size (length x width) of the plates is not particularly limited. For example, from the viewpoint of heat dissipation after joining, the length and width of the plates are preferably 30 mm or more. Furthermore, the length and width of the plates are preferably 2000 mm or less.

[0030] When the first stainless steel and the second stainless steel are tubular, there are no particular limitations on the size (outer diameter and length) of the tube. For example, from the viewpoint of heat dissipation after joining, the outer diameter of the tube is preferably at least four times the tube thickness (wall thickness). The outer diameter of the tube is more preferably at least 5 mm. The outer diameter of the tube is preferably at most 500 mm. The length of the tube is preferably at least 10 mm, more preferably at least 30 mm. The length of the tube is preferably at most 2000 mm.

[0031] The chemical compositions of the first and second stainless steels are not particularly limited, and may be any of the typical stainless steel compositions. For example, the first and second stainless steels may be iron-based alloys containing 10.5% by mass or more of Cr and 50% by mass or more of Fe. The Cr content is preferably 15% by mass or more, more preferably 17% by mass or more. The Cr content is preferably 25% by mass or less, more preferably 23% by mass or less. The Fe content is preferably 60% by mass or more. The Fe content is preferably 85% by mass or less. The total content of the remaining elements other than Cr and Fe in the chemical compositions of the first and second stainless steels (hereinafter also referred to as the total content of the remaining elements) is preferably 10% by mass or less, more preferably 8% by mass or less. The total content of the remaining elements may be 0%. Examples of the remaining elements include C, Si, Mn, Ni, Al, Cu, Nb, Ti, V, Mo, W, B, Ca, and Mg. Furthermore, the chemical compositions of the first stainless steel and the second stainless steel may contain unavoidable impurities. Examples of unavoidable impurities include P and S. A total content of unavoidable impurities of 1.0 mass% or less is acceptable. The total content of unavoidable impurities may be 0 mass%. Note that elements that are intentionally added to the first stainless steel and the second stainless steel are not considered unavoidable impurities. The chemical compositions of the first stainless steel and the second stainless steel may, for example, consist of 10.5 to 25 mass% Cr, 50 to 85 mass% Fe, and the remainder being 10 mass% or less and unavoidable impurities.

[0032] Examples of suitable stainless steels include austenitic stainless steel plates, austenitic-ferritic stainless steel plates, ferritic stainless steel plates, martensitic stainless steel plates, and precipitation-hardened stainless steel plates, as well as processed products thereof, as specified in JIS G 4305:2021. Also suitable are stainless steel sanitary pipes, general piping pipes, piping pipes, and boiler / heat exchanger pipes, as specified in JIS G 3447:2015, JIS G 3448:2016, JIS G 3459:2021, JIS G 3463:2019, and JIS G 3468:2021. The surface finish of the stainless steel plates is not particularly limited. As an example, stainless steel sheets with various surface finishes can be used, including No. 2B finish (annealed, pickled, skin-pass finish), No. 2D finish (annealed, pickled finish), No. 4 finish (polished finish), No. 8 finish (mirror-polished finish), BA finish (bright annealed finish), HL (hairline) finish, dull finish, embossed finish, and blast finish.

[0033] [1-2]Joint part The joint is a portion where the first stainless steel and the second stainless steel are joined together. In a joined body according to one embodiment of the present invention, it is important that the Sn content of the joint is 40 mass% or more.

[0034] Sn content in joint: 40% by mass or more The joint is formed by melting and then solidifying the joining metal. Therefore, the chemical composition of the joint is basically the same as that of the joining metal. Here, to prevent excessive temperature rise in the stainless steel during joining and to sufficiently melt the joining metal, the joining metal needs to be Sn or an Sn alloy with an Sn content of 40% by mass or more. That is, if the Sn content of the joining metal is less than 40% by mass, a large amount of heat is required to melt the joining metal. As a result, the joining metal does not melt sufficiently, and the stainless steel is not joined. Therefore, the Sn content of the joint is 40% by mass or more, preferably 90% by mass or more. Furthermore, from the viewpoint of preventing a decrease in brittleness in extremely cold regions, the Sn content of the joint is preferably 99.5% by mass or less, more preferably 97% by mass or less.

[0035] The composition of the joint contains, for example, 60% by mass or less, preferably 10% by mass or less, of metal elements other than Sn. The content of metal elements other than Sn is preferably 0.5% by mass or more, more preferably 3.0% by mass or more. Examples of metal elements other than Sn include Ag, Cu, Sb, Pb, Bi, In, and Zn. However, Ni is not included in the metal elements other than Sn, i.e., the composition of the joint. Suitable contents of each metal element other than Sn are, for example, 5.5% by mass or less of Ag, 4.0% by mass or less of Cu, 6.0% by mass or less of Sb, 60% by mass or less of Pb, 60% by mass or less of Bi, 55% by mass or less of In, and 10.0% by mass or less of Zn. More details are as described in [2-2] below. The composition of the joint may contain unavoidable impurities. Examples of unavoidable impurities include elements such as Pb, Sb, Bi, Cd, Cu, Au, In, Ag, Al, As, Cd, Fe, Ni, Zn, Ge, P, and Ga. The total content of unavoidable impurities is allowable if it is 2.0% by mass or less. The total content of unavoidable impurities may also be 0% by mass. Note that elements that are intentionally added to the joining metals are not considered unavoidable impurities. For example, the component composition of the joining part may consist of 40% by mass or more of Sn, 60% by mass or less of metal elements other than Sn, and unavoidable impurities.

[0036] Examples of the form of the joint include a lap joint (lap joint) and a T-shaped joint (T-joint). In a lap joint, at least a portion of a first stainless steel and a second stainless steel are overlapped and joined. In this case, for example, as shown in Figure 1, at least a portion of one surface of the first stainless steel and at least a portion of one surface of the second stainless steel are joined by a joint (lap joint).

[0037] In a T-joint, a first stainless steel and a second stainless steel are combined and joined in a T-shape. In this case, for example, as shown in FIG. 2, the end face of the first stainless steel and the surface of the second stainless steel are joined by the T-joint. Hereinafter, among the stainless steels joined by the T-joint, the stainless steel arranged like the first stainless steel in FIG. 2 (the stainless steel that contacts the joint at the end face) is referred to as the "standing plate," and the stainless steel arranged like the second stainless steel in FIG. 2 (the stainless steel that contacts the joint at the surface) is referred to as the "lower plate." In addition, in the case of a T-joint, the direction perpendicular to the surface of the stainless steel that corresponds to the lower plate is referred to as the thickness direction (Z direction). However, in a T-shaped joint in which a portion of one stainless steel pipe (inserted pipe) is inserted into another stainless steel pipe (inserted pipe) having a hole in its side with a diameter roughly equal to the outer diameter of the other stainless steel pipe (inserted pipe) described below, and the joint is formed in a T shape, the longitudinal direction of the inserted pipe is defined as the thickness direction (Z direction), and the direction perpendicular to the joining direction (X direction, circumferential direction of the inserted pipe) and perpendicular to the thickness direction, for example, the longitudinal direction of the inserted pipe, is defined as the joint perpendicular direction (Y direction).

[0038] The average thickness of the joint is preferably 0.50 mm or less, more preferably 0.20 mm or less, and is preferably 0.01 mm or more, more preferably 0.02 mm or more.

[0039] The average width of the joint (average length of the joint in the direction perpendicular to the joint (Y direction)) is preferably 1.0 mm or more, more preferably 2.0 mm or more, and even more preferably 5.0 mm or more. In the case of a T-shaped joint between stainless steel pipes, the average width of the joint is preferably 0.2 mm or more, and more preferably 50% or more of the thickness of the first stainless steel or the second stainless steel to be joined.

[0040] The length of the joint (the length of the joint in the direction of welding (X direction)) is preferably 50% or more, and more preferably 80% or more, of the overlap length between the first stainless steel and the second stainless steel in the direction of welding (X direction).

[0041] In the case of a T-shaped joint, the joint angle (the angle between the first stainless steel and the second stainless steel in a plane (YZ plane) perpendicular to the joining direction (X direction)) is preferably, for example, 40 to 140 degrees.

[0042] Here, the weld is defined, for example, as follows. First, a cross-sectional sample of the weld in the thickness direction (a sample with a cross section on a plane (YZ plane) perpendicular to the welding direction (X direction)) as shown in Figure 3 is prepared with a mirror-polished finish. Next, the cross-sectional sample is observed with an optical microscope or SEM at 100x magnification. Then, based on the difference in color tone (contrast) of each structure and the contrast of the interface observed in the obtained image, the interface between the weld and the first stainless steel (the base material) and the interface between the weld and the second stainless steel (the base material) are determined, and the weld is defined. Then, based on the defined weld, the thickness and width of the weld in the cross-sectional sample are measured. This measurement is performed on four cross-sectional samples cut at random from the welded body, and the thickness and width of the weld in each cross-sectional sample are determined. Next, the average values are calculated and used as the thickness and width of the weld in the welded body.

[0043] In the above cross-sectional sample, if the thickness of the joint is not uniform, for example, if there are portions that are not parallel to the interface between the joint and the first stainless steel and the interface between the joint and the second stainless steel, the thickness of the joint in the cross section is calculated as follows: That is, in the joint defined as above, the thickness of the joint is measured at five equally spaced locations, including both ends. The average value of the thicknesses of the joint measured at the five locations is then taken as the thickness of the joint in the cross-sectional sample.

[0044] The Sn content of the weld is measured at a position halfway through the thickness of the weld. For example, the Sn content of the weld can be measured as follows. First, a cross-sectional sample in the thickness direction (a sample with a cross section perpendicular to the X direction, which is the direction of welding (YZ plane)) is prepared as shown in Figure 3. The cross section is mirror-polished and then etched using an etching solution (5 mL of 38% by mass iron (II) chloride aqueous solution, 15 mL of 36% by mass hydrochloric acid, and 120 mL of distilled water). The obtained cross-sectional sample is then observed using an SEM at 100x magnification, followed by SEM-EDS analysis. In this analysis, an EDS point scan is performed on the cross-sectional weld to measure the Sn content (mass%). The EDS scan points are 10 randomly selected at half-thickness positions of the weld (the midpoint in the thickness direction (Z direction) between the interface between the first stainless steel and the weld and the interface between the second stainless steel and the weld). The Sn content (mass%) measured at each point was then averaged to obtain the Sn content of the joint of the cross-sectional sample. This measurement was performed on four cross-sectional samples randomly taken from the joint, and the average value of the Sn content of the joint of each cross-sectional sample was used to obtain the Sn content of the joint.

[0045] For example, if the end of the first stainless steel or the end of the second stainless steel is chamfered on the mating surface side to facilitate the flow of the joining metal into the mating surface of the joined materials during joining, the thickness of the joint may become uneven. When chamfering is performed, it is preferable that the angle between the surface formed by the chamfering and the surface of the other joined material is 5 to 10 degrees. In addition, the length of the chamfering is preferably 1.0 to 3.0 mm in the direction perpendicular to the joining.

[0046] Furthermore, if the width of the joint is not uniform, for example, if the width is different near the first stainless steel and the second stainless steel, that is, if the width of the joint varies depending on the position in the thickness direction, the minimum length of the joint in the direction perpendicular to the joint at the position in the thickness direction between the overlapping surfaces of the joined materials shall be taken as the width of the joint.

[0047] A part of the joint may extend beyond the overlapping surfaces of the workpieces in the direction perpendicular to the joint. In this case, the thickness and width of the joint are measured excluding the part that extends beyond the overlapping surfaces of the workpieces.

[0048] Furthermore, the joints may be connected by caulking or by fitting, depending on the strength and airtightness required.

[0049] [1-3] Heated area In a joined body according to one embodiment of the present invention, it is important that the Cr enrichment in the heated region formed during joining of stainless steels is limited to the vicinity of the surface, and in particular, that the deepest point of the Cr enriched surface layer in the heated region of the base material is 300 nm or less.

[0050] Deepest point of surface Cr enriched layer in heated area: 300nm or less By limiting the Cr enrichment in the heated region formed during joining of stainless steels to the surface, particularly by limiting the deepest point of the Cr-enriched layer at the surface of the heated region of the base metal to 300 nm or less, excellent corrosion resistance can be obtained even when joining is performed by air brazing. Therefore, the deepest point of the Cr-enriched layer at the surface of the heated region is set to 300 nm or less, preferably 100 nm or less. The deepest point of the Cr-enriched layer at the surface of the heated region is not particularly limited. Furthermore, from the viewpoint of ensuring the wettability and spreadability of the joining metals, it is preferable to ensure a certain amount of heat. Therefore, the deepest point of the Cr-enriched layer at the surface of the heated region is preferably 60 nm or more.

[0051] Here, the heated region refers to the region of the first and second stainless steels, which are the base materials, that is heated during joining (for example, by contact with the heat-generating part of a heating device) and is not covered by the joining area (joining metal). In other words, the heated region can be said to originate from the heated area of the joined materials during joining. The heated region may be formed on only one of the first and second stainless steels, or on both. The heated region can be visually defined, for example, based on the presence or absence of a temper color. Note that if visual definition is difficult, the heated region may be defined as the region extending 20 mm from the inflow starting point (on the opposite side of the joining area) in the direction perpendicular to the joining (Y direction). In this case, the inflow starting point is the position in the direction perpendicular to the joining (Y direction) where the joining metal starts to flow between the first and second stainless steels during joining. The inflow starting point can be identified based on the shape of the joining area. For example, in the case of Figure 1, the end of the first stainless steel (the end closest to the heated area) is the inflow starting point. Also, in the case of Figure 2, the surface of the first stainless steel, which is a vertical plate (the surface closest to the heated area), is the inflow starting point. When the heated area is formed on the first stainless steel, which is a vertical plate, in a T-joint joint, the heated area can be defined as the area extending 20 mm in the thickness direction (Z direction) from the inflow starting point (on the opposite side from the joint). In this case, the end of the first stainless steel (the end closest to the second stainless steel) is the inflow starting point.

[0052] The deepest point of the Cr-enriched surface layer in the heated region is measured by X-ray photoelectron spectroscopy (XPS). Specifically, a 5 mm square (5 mm × 5 mm) sample is cut out from the heated region of the base metal of the joined body, with the center at a position 4 mm away from the inflow starting position (on the opposite side of the weld) in the direction perpendicular to the weld (Y direction). In a T-joint joint, when the heated region is formed in the first stainless steel, which is a vertical plate, a 5 mm square (5 mm × 5 mm) sample is cut out from the center at a position 4 mm away from the inflow starting position (the end of the first stainless steel (the end closest to the second stainless steel)) in the thickness direction (Z direction). However, if the center position is covered by the weld, a 5 mm square (5 mm × 5 mm) sample is cut out from the center at a position 3 mm away from the center position in the direction perpendicular to the weld or in the thickness direction. Next, X-ray photoelectron spectroscopy (XPS) is performed at the center of the sample, extending from the surface to a depth of at least 20 μm, preferably 50 μm, to determine the elemental concentrations (at%) of Cr and Fe. The measurements are performed at 20 nm intervals from the surface to a depth of 1 μm, and at 1 μm intervals from a depth of 1 μm to a depth of 50 μm. From the elemental concentrations (at%) of Cr and Fe determined at each depth, A(d), which is [Cr] / ([Cr] + [Fe]) at depth d, is calculated, and B and C are determined. The deepest point satisfying the relationship of Equation (1) above is then derived from A(d), B, and C. The above measurements are performed on samples cut from any five locations, and the maximum value of the deepest points satisfying the relationship of Equation (1) derived for the five samples is taken as the deepest point of the Cr-enriched surface layer in the heated region. If the heated region is formed in both the first and second stainless steels, the above measurements are performed on both the first and second stainless steels, and the larger value is taken as the deepest point of the Cr-enriched surface layer in the heated region. For reference, Figure 6 shows an example of the concentration profiles of [Cr], [Fe], and [Cr] + [Fe] in the heated region in the depth direction.

[0053] The joined body according to one embodiment of the present invention may be either a plate-like body (including not only a flat plate but also a curved plate (a curved plate)) or a tubular body. When it is a tubular body, it is a joined body of stainless steel pipes.

[0054] In a joined body in which the joint is a lap joint, the following combinations of the first and second stainless steels are possible: In these combinations, a portion of one stainless steel pipe can be inserted into the other stainless steel pipe, or a portion of the other stainless steel pipe can be inserted into the first stainless steel pipe, and the two pipes can be joined. A combination of one stainless steel pipe and another stainless steel pipe, where the outer diameter of one stainless steel pipe is roughly equal to the inner diameter of the other stainless steel pipe. A combination of one stainless steel pipe and another stainless steel pipe whose end has been expanded so that the outer diameter is roughly equal to that of the first stainless steel pipe. A combination of one stainless steel pipe and another stainless steel pipe whose end has been reduced in diameter so that the inside diameter is roughly equal to that of the first stainless steel pipe, etc.

[0055] In a joined body in which the joint is a T-shaped joint, for example, the end of one stainless steel pipe can be butted against the side surface of the other stainless steel pipe, or the first stainless steel pipe (inserted pipe) can be joined by inserting a portion of the first stainless steel pipe (inserted pipe) into the other stainless steel pipe (inserted pipe) which has a hole in its side surface with a diameter roughly equal to the outer diameter of the first stainless steel pipe.

[0056] Furthermore, a bonded body according to one embodiment of the present invention includes a bonded body having a plurality of bonded portions, at least one of which is the above-described bonded portion.

[0057] [2] Manufacturing method of the bonded body Next, a method for producing a bonded body according to one embodiment of the present invention will be described. The method for producing a bonded body according to one embodiment of the present invention is a suitable method for producing the above-mentioned bonded body.

[0058] [2-1] Material to be joined The first and second stainless steels to be welded respectively constitute the first and second stainless steels that will be the base materials of the welded body after welding. The preferred thickness, shape, and chemical composition of the first and second stainless steels to be welded are as explained in [1-1] above.

[0059] [2-2] Joining metal The joining metal is Sn or an Sn alloy with an Sn content of 40 mass % or more, for the reasons explained in [1-2] above.

[0060] An example of an Sn alloy having an Sn content of 40% by mass or more is solder. In particular, solders having an Sn content of 40% by mass or more among the solders described in JIS Z 3282:2017 can be preferably used. The joining metal may contain flux (so-called resin-cored solder may also be used).

[0061] Examples of the Sn alloy are given below. (Sn-Ag alloy) Ag: 4.5 to 5.5 mass %, with the remainder being Sn and unavoidable impurities. (Sn-Cu alloy) Cu: 2.0 to 4.0 mass %, with the remainder being Sn and unavoidable impurities. (Sn-Sb alloy) Sb: 4.0 to 6.0 mass %, with the remainder being Sn and unavoidable impurities. (Sn-Ag-Cu alloy) It contains 0.5 to 4.5 mass % of Ag and 0.2 to 1.0 mass % of Cu, with the remainder being Sn and unavoidable impurities. (Sn-Pb alloy) Pb: 3.0 to 60.0 mass % with the remainder being Sn and unavoidable impurities. (Sn-Ag-Bi-Cu alloy) It contains 2.0 to 3.0 mass % of Ag, 0.5 to 1.5 mass % of Bi, and 0.2 to 1.0 mass % of Cu, with the remainder being Sn and unavoidable impurities. (Sn-In-Ag-Bi alloy) It contains 3.0 to 9.0 mass % In, 3.0 to 4.0 mass % Ag, and 0.2 to 1.0 mass % Bi, with the remainder being Sn and unavoidable impurities. (Sn-Zn alloy) Zn: 8.0 to 10.0 mass % is contained, and the remainder is Sn and unavoidable impurities. (Sn-Zn-Bi alloy) It contains 7.0 to 9.0 mass % of Zn, 2.5 to 3.5 mass % of Bi, and the remainder being Sn and unavoidable impurities. (Sn-Bi alloy) The content is 55.0 to 60.0 mass % of Bi, with the remainder being Sn and unavoidable impurities. (Sn-In alloy) The content is 50.0 to 55.0 mass % of In, with the remainder being Sn and unavoidable impurities.

[0062] Thus, the composition of the joining metal contains, for example, 60% by mass or less, preferably 10% by mass or less, of metal elements other than Sn. The content of the metal elements other than Sn is preferably 0.5% by mass or more, more preferably 3.0% by mass or more. Examples of metal elements other than Sn include Ag, Cu, Sb, Pb, Bi, In, and Zn. However, Ni is not included in the metal elements other than Sn, i.e., the composition of the joining metal. Suitable contents of the metal elements other than Sn are, for example, 5.5% by mass or less of Ag, 4.0% by mass or less of Cu, 6.0% by mass or less of Sb, 60% by mass or less of Pb, 60% by mass or less of Bi, 55% by mass or less of In, and 10.0% by mass or less of Zn. More details are as described above. The composition of the joining metal may contain unavoidable impurities. Examples of unavoidable impurities include elements such as Pb, Sb, Bi, Cd, Cu, Au, In, Ag, Al, As, Cd, Fe, Ni, Zn, Ge, P, and Ga. The total content of unavoidable impurities is allowable if it is 2.0% by mass or less. The total content of unavoidable impurities may be 0% by mass. Note that elements that are intentionally added to the joining metal are not considered unavoidable impurities. For example, the component composition of the joining metal is composed of 40% by mass or more of Sn, 60% by mass or less of metal elements other than Sn, and unavoidable impurities.

[0063] The melting point of the joining metal is preferably 100 to 350°C. The melting point of the joining metal is more preferably 190°C or higher. The melting point of the joining metal is more preferably 300°C or lower. Here, the melting point of the joining metal, T' (°C), is a temperature calculated from the solidus temperature Ts (°C) and liquidus temperature Tl (°C) of the joining metal by the following formula: T´=(Ts+Tl) / 2

[0064] [2-3]Joining process In the joining process, for example, while moving a heating device in the joining direction, the heating element of the heating device is brought into contact with the workpieces, which are a first stainless steel and a second stainless steel overlapped as shown in Figure 4, or a first stainless steel and a second stainless steel combined in a T-shape as shown in Figure 5. This heats the workpieces and melts the joining metal, joining the first stainless steel and the second stainless steel, which are the workpieces. It is important to satisfy the following conditions during this process. Hereinafter, the joining configurations shown in Figures 4 and 5 will be referred to as lap joining and T-jointing, respectively.

[0065] Maximum temperature reached in the heating area of the joined material (hereinafter simply referred to as maximum temperature reached): 350°C or less In a method for manufacturing a joined body according to one embodiment of the present invention, it is important to suppress the formation of an oxide film on the surface of the stainless steel to be joined and to limit Cr enrichment in the heated region of the joined materials to the surface vicinity. In particular, if the maximum temperature exceeds 350°C, an oxide film is rapidly formed on the surface of the stainless steel to be joined, making it easier for the joining metal to detach from the stainless steel. Furthermore, Cr enrichment in the heated region of the joined materials cannot be limited to the surface vicinity, resulting in a decrease in corrosion resistance. Therefore, the maximum temperature is set to 350°C or less, preferably 300°C or less. Furthermore, the maximum temperature is preferably set to 200°C or more.

[0066] Residence time in the heating zone of the workpiece at a temperature of 100°C or higher (hereinafter simply referred to as residence time): 5 seconds or more and 300 seconds or less, and satisfying the relationship of the above formula (2) In a method for manufacturing a joined body according to one embodiment of the present invention, it is important to suppress the formation of an oxide film on the surface of the stainless steel to be joined and to limit Cr enrichment in the heated region of the joined materials to the surface. Here, if the residence time is less than 5 seconds, the joining metal does not sufficiently flow into the overlapping surface of the joined materials, hindering joining. On the other hand, if the residence time exceeds 300 seconds, a thick oxide film forms on the surface of the stainless steel to be joined, making the joining metal more likely to detach from the stainless steel. Furthermore, corrosion resistance decreases. Additionally, a longer residence time promotes Cr enrichment in the heated region of the joined materials. In this regard, by satisfying the relationship of formula (2) above, i.e., by appropriately controlling the residence time according to the maximum temperature reached and the Cr content of the joined materials, it is possible to achieve good joining of the joined materials while limiting Cr enrichment in the heated region of the joined materials to the surface. Therefore, the residence time is set to 5 to 300 seconds and satisfies the relationship of formula (2). The residence time is preferably 15 seconds or longer. The residence time is preferably 180 seconds or less, more preferably 60 seconds or less.

[0067] The heating region of the workpieces may be formed in only one of the first stainless steel and the second stainless steel, or may be formed across both. When the heating region of the workpieces is formed across both the first stainless steel and the second stainless steel, [Cr0] in the above formula (2) is set to the larger value of the Cr content (mass%) of the first stainless steel or the Cr content (mass%) of the second stainless steel.

[0068] In measuring the temperature of the workpieces to derive the maximum temperature and residence time, the surfaces of the first and second stainless steels to be joined may be measured overall using, for example, thermography, etc. Also, depending on the heating area by the heating device, the surface temperature of the stainless steel in the vicinity of the heating area may be measured using a thermocouple.

[0069] For example, when a heated region is formed in the second stainless steel in a lap joint as shown in Figure 4 or a T-shaped joint as shown in Figure 5, the surface temperature of the second stainless steel can be measured at predetermined intervals (for example, about 5 mm) in the direction of welding progress (X direction) within a range of 20 mm from the flow start position (on the opposite side of the weld) in the direction perpendicular to the weld (Y direction). In this case, if the range in the direction perpendicular to the weld (Y direction) is as described above, the surface temperature during welding will be approximately the same, so the temperature measurement position in the direction perpendicular to the weld (Y direction) can be determined from the above range depending on the structure of the welded materials, etc.

[0070] Furthermore, when a heated region is formed in the first stainless steel, which is a vertical plate in a T-shaped joint, the surface temperature of the first stainless steel can be measured at predetermined intervals (for example, about 5 mm) in the direction of welding progress (X direction) within a range of 20 mm from the flow start position in the thickness direction (Z direction) (on the opposite side from the joint). In this case, if the range in the thickness direction (Z direction) is as described above, the surface temperature during welding will be approximately the same, so the temperature measurement position in the thickness direction (Z direction) can be determined from the range as described above depending on the structure of the materials to be joined.

[0071] The maximum temperature and residence time can be controlled by adjusting, for example, the heating position, the joining speed (the speed at which the heating device moves in the joining direction), the distance between the heating device and the materials to be joined, and the output of the heating device. For example, when a soldering iron is used as the heating device, the maximum temperature and residence time can be controlled within the above ranges by adjusting the joining speed in the range of 30 to 200 mm / min and the output (heat amount) of the heating device in the range of 100 to 500 W depending on the type, size, and arrangement of the materials to be joined.

[0072] The conditions other than those mentioned above are not particularly limited and may be those according to conventional methods.

[0073] For example, the joining metal may be applied or placed between the overlapping surfaces of the workpieces, on the first stainless steel end or the second stainless steel end, prior to the joining process, or may be continuously supplied to the workpieces during joining (for example, by inserting a filler rod to serve as the joining metal).

[0074] The separation distance of the workpieces (the gap between the first and second stainless steel) is, for example, preferably 0.50 mm or less, more preferably 0.20 mm or less. The separation distance of the workpieces is preferably 0.01 mm or more, more preferably 0.02 mm or more. In lap joining as shown in FIG. 4, the overlap width L of the workpieces (the width in the direction perpendicular to the welding) is preferably, for example, 5 to 20 mm. In T-jointing as shown in FIG. 5, the joint angle (the angle between the first and second stainless steel in a plane (YZ plane) perpendicular to the welding direction (X direction)) is preferably, for example, 40 to 140 degrees.

[0075] The heating device used is not particularly limited. For example, a heating device may have a heat generating element that can heat the materials to be joined by contacting the heat generating element with the materials to be joined. A soldering iron is particularly suitable. Examples of the heat generating method of the heat generating element include electric heating (mains power or battery power) and gas heating. The output (heat amount) of the heating device is preferably 100 W or more. In other words, if the amount of heat generated is small, it takes a long time to heat the materials to be joined, which may lead to the formation of an oxide film on the stainless steel surface. Furthermore, the tip diameter of the heat generating element (the tip diameter of the iron tip in the case of a soldering iron) is preferably 1 to 5 mm. If the surface of the heat generating element oxidizes due to its own heat generation, it is preferable to polish it with a metal file or the like to remove the oxide before starting joining.

[0076] Additionally, from the viewpoint of improving the wetting spread of the joining metals and improving workability, it is preferable to apply a flux to the joining target portions of the materials to be joined and their vicinity before the joining process. A typical flux for stainless steel is suitable as the flux. For example, it is preferable to use an aqueous solution containing 30 to 50 mass% zinc chloride, 1 to 10 mass% ammonium chloride, 1 to 10 mass% hydrogen chloride, 30 to 70 mass% water, with the remainder consisting of unavoidable impurities. The flux may be applied to both the joining target portions of the first stainless steel and the second stainless steel, which are the joining materials, and their vicinity, or it may be applied to the joining target portion of either one of them and its vicinity. Furthermore, the flux may be applied only to the joining target portions or only to a portion of the joining target portions. [Example]

[0077] Example 1 The first and second stainless steels were cut into 120 mm square pieces from 1.0 mm thick stainless steel plates (various stainless steels specified in JIS G 4305:2021) listed in Table 1. Then, in Test Nos. 1-1 to 1-21 and 1-23 to 1-26, the first and second stainless steels were overlapped as shown in Figure 4 to obtain the welded materials. The overlap width L was 10 mm in all cases. In Test No. 1-22, the first and second stainless steels were arranged in a T-shape as shown in Figure 5 to obtain the welded materials. In all Test Nos., spacers were placed between the first and second stainless steels at both ends of the overlapping portion of the welded materials in the welding direction so that the separation distance between the first and second stainless steels was 0.04 mm, and the welded materials were then clamped with clips. Furthermore, flux (BS-45 manufactured by Taiyo Electric Industries) was applied in advance to the surfaces of the materials to be joined, at the overlapping and abutting surfaces of the materials to be joined and at positions in the vicinity thereof.

[0078] Next, in Test Nos. 1-1 to 21 and 1-23 to 26, thermocouples were attached to the surface of the second stainless steel as shown in Fig. 4. That is, the thermocouples were attached to the surface of the second stainless steel at positions 10 mm away from the end of the first stainless steel, which was the flow starting point, toward the planned heating area in the direction perpendicular to the joining (Y direction), and at intervals of 5 mm in the joining direction, and these attachment positions were used as temperature measurement positions.

[0079] In Test No. 1-22, thermocouples were attached to the surface of the second stainless steel (bottom plate) as shown in Figure 5. That is, thermocouples were attached to the surface of the second stainless steel at positions 10 mm away from the surface of the first stainless steel (upright plate), which was the flow starting point, toward the planned heating area in the direction perpendicular to the joining (Y direction), and at intervals of 5 mm in the joining direction, and these attachment positions were used as temperature measurement positions.

[0080] Next, a φ2.0 bar solder (various solders specified in JIS Z 3282:2017) listed in Table 1 was inserted between the first and second stainless steels, and the soldering iron was moved in the direction of joining, contacting the workpieces and the solder to heat them. In both cases, joining was performed so that the heated area (heated area) was formed only on the second stainless steel. The soldering iron used was an SS-152J (150W) sheet metal soldering iron manufactured by Ishizaki Electric Mfg. Co., Ltd. Note that, under all conditions, joining was performed in an air atmosphere. For conditions not specified, the general description and standard procedures were followed. During joining, the temperatures of the workpieces were measured and recorded at 0.1-second intervals using the thermocouple and temperature logger. The temperature history of the workpieces was obtained from the output temperature log. From the obtained temperature history of the workpieces, (I) Maximum temperature reached (II) Residence time The results are shown in Table 1.

[0081] Note that (I) and (II) were derived for each temperature measurement point where a thermocouple was installed. Table 1 lists only the maximum value of (I) and the minimum and maximum values of (II) at each temperature measurement point. In the present invention example, (I) and (II) satisfied the above ranges at all temperature measurement points.

[0082] Four cross-sectional samples were cut out from the bonded body thus obtained in the same manner as above, and the width of the bonded portion of each cross-sectional sample was measured. The feasibility of bonding was then judged based on the following criteria. The results are also shown in Table 1. For samples that were judged to be unbondable, the measurements (III) and (IV) described below and the evaluation of corrosion resistance were not carried out. Pass: The width of the joint is 1.0 mm or more in all four cross-sectional samples. Unacceptable: The width of the joint is less than 1.0 mm in at least one cross-section specimen.

[0083] Also, in the above manner, (III) Sn content of the joint (mass%) (IV) The deepest point of the surface Cr enriched layer in the heated area (nm) The results are shown in Table 1.

[0084] Here, the width of the joint was measured using an optical microscope, DSX510, manufactured by Olympus Corporation (now Evident Co., Ltd.). Measurement of (III) was performed using a scanning electron microscope (SEM), Miniscope® TM3030plus, manufactured by Hitachi High-Tech Corporation, and an energy dispersive X-ray spectrometer (EDS), AZtecOne, manufactured by Oxford Instruments. Measurement of (IV) was performed using an X-ray photoelectron spectrometer (XPS), QuanteraSXM, manufactured by ULVAC-PHI, Inc.

[0085] In addition, corrosion resistance was evaluated as follows. Specifically, a 15 mm square (15 mm × 15 mm) test piece was taken from the heated area of the bonded body. Next, the area of the taken test piece other than the test surface (10 mm × 10 mm) was covered with a sealant. Next, the pitting potential Vc'100 was measured in accordance with Method B specified in JIS G 0577:2014. Then, corrosion resistance was evaluated according to the following criteria. Pass (Excellent): Pitting potential Vc'100 is 200mV (vs. SCE) or more Fail (bad): Pitting potential Vc'100 is less than 200mV (vs. SCE)

[0086] [Table 1]

[0087] As shown in Table 1, in all of the invention examples, a cost-effective Sn alloy was used as the joining metal, and a joined body having excellent corrosion resistance was obtained by air brazing of stainless steel.

[0088] On the other hand, in the comparative examples, the first stainless steel and the second stainless steel were not joined together, or sufficient corrosion resistance was not obtained. That is, in the comparative example of Test No. 1-23, the maximum temperature reached exceeded the appropriate range, so the deepest point of the Cr-enriched layer at the surface of the heated area was deep, and sufficient corrosion resistance was not obtained. In the comparative examples of Test Nos. 1-24 and 1-25, the residence time exceeded the appropriate range, so the deepest point of the Cr-enriched layer on the surface of the heated area became deep, and sufficient corrosion resistance was not obtained. In the comparative example of Test No. 1-26, the residence time was less than the appropriate range, and therefore the first stainless steel and the second stainless steel were not joined. In the comparative example of Test No. 1-27, the Sn content of the joining metal was below the appropriate range, so the joining metal was not melted sufficiently, and the first stainless steel and the second stainless steel were not joined.

[0089] Example 2 Stainless steel pipes (hereinafter referred to as "first stainless steel pipe" and "second stainless steel pipe") with the outer diameter, thickness (wall thickness), and steel type listed in Table 2 were cut to a length of 300 mm for the first and second stainless steels, respectively. Next, a hole with a diameter 0.1 mm larger than the outer diameter of the second stainless steel pipe (inserted pipe) was drilled in the longitudinal center (150 mm from the end) of the first stainless steel pipe (inserted pipe). The second stainless steel pipe was inserted into the hole drilled in the first stainless steel pipe to obtain a welded material. In other words, this welded material can be considered a T-shaped combination of the first and second stainless steel pipes. The same flux as in Example 1 was previously applied to the outer surface of the second stainless steel pipe at and near the insertion point of the welded material.

[0090] Next, thermocouples were attached to the surface of the second stainless steel pipe (inserted pipe). That is, thermocouples were attached to the outer surface of the second stainless steel pipe at positions 10 mm away from the outer surface of the first stainless steel pipe (inserted pipe), which was the inflow starting point, in the direction perpendicular to the joining (Y direction, the longitudinal direction of the second stainless steel pipe), and at intervals of 5 mm in the joining direction (circumferential direction of the second stainless steel pipe), and these attachment positions were used as temperature measurement positions.

[0091] Next, in the insertion portion of the workpiece, a rod-shaped solder (Sn96.5Ag3Cu0.5 as specified in JIS Z 3282:2017) having a diameter of 1.0 mm as shown in Table 2 was wound around the outer periphery of the second stainless steel pipe (insertion pipe) and installed.

[0092] Next, the soldering iron was moved in the direction of joining (circumferential direction of the second stainless steel pipe) while being applied to the materials to be joined and the solder, and these were heated. Here, in all cases, joining was carried out so that the heated area (heated area) was formed only on the second stainless steel pipe (inserted pipe). The same soldering iron as in Example 1 was used. Note that in all conditions, joining was carried out in an air atmosphere. Furthermore, for conditions not specified, the descriptions in the general description section above and common methods were followed. Then, during joining, the temperatures of the materials to be joined were measured and recorded at 0.1 second intervals using the above thermocouple and temperature logger, and the temperature history of the materials to be joined was obtained from the output temperature log. From the obtained temperature history of the materials to be joined, (I) Maximum temperature reached (II) Residence time The results are shown in Table 2.

[0093] Note that (I) and (II) were derived for each temperature measurement point where a thermocouple was installed. Table 2 lists only the maximum value of (I) and the minimum and maximum values of (II) at each temperature measurement point. In the present invention example, (I) and (II) satisfied the above ranges at all temperature measurement points.

[0094] Four cross-sectional samples were cut out from the bonded body thus obtained in the same manner as above, and the width of the bonded portion of each cross-sectional sample was measured. The feasibility of bonding was then judged based on the following criteria. The results are also shown in Table 2. Note that for samples judged to be unbondable, the measurements (III) and (IV) described below and the evaluation of corrosion resistance were not carried out. Pass: The width of the joint is 0.3 mm or more in all four cross-sectional samples. Unacceptable: The width of the joint is less than 0.3 mm in at least one cross-section sample.

[0095] Also, in the same manner as in Example 1, (III) Sn content of the joint (mass%) (IV) The deepest point of the surface Cr enriched layer in the heated area (nm) The results are shown in Table 2.

[0096] In addition, corrosion resistance was evaluated as follows. Specifically, a 15 mm square (15 mm × 15 mm) test piece was taken from the heated area of the bonded assembly. Next, the area of the taken test piece other than the test surface (10 mm × 10 mm) was covered with a sealant. Note that since the taken test piece was curved, the surface was protected and then straightened to make it flat. The above dimensions of the test piece and test surface are those after straightening. Next, the pitting potential Vc'100 was measured in accordance with Method B specified in JIS G 0577:2014. Then, corrosion resistance was evaluated according to the following criteria. Pass (Excellent): Pitting potential Vc'100 is 200mV (vs. SCE) or more Fail (bad): Pitting potential Vc'100 is less than 200mV (vs. SCE)

[0097] [Table 2]

[0098] As shown in Table 2, in all of the invention examples, a cost-effective Sn alloy was used as the joining metal, and a joined body with excellent corrosion resistance was obtained by air brazing of stainless steel pipes.

[0099] On the other hand, in the comparative examples, the first stainless steel pipe and the second stainless steel pipe were not joined together, or sufficient corrosion resistance was not obtained. That is, in the comparative example of Test No. 2-13, the maximum temperature reached exceeded the appropriate range, so the deepest point of the Cr-enriched layer at the surface of the heated area was deep, and sufficient corrosion resistance was not obtained. In the comparative examples of Test Nos. 2-14 and 2-15, the residence time exceeded the appropriate range, so the deepest point of the Cr-enriched layer on the surface of the heated area became deep, and sufficient corrosion resistance was not obtained. In the comparative example of Test No. 2-16, the residence time was below the appropriate range, and therefore the first stainless steel pipe and the second stainless steel pipe were not joined. [Industrial Applicability]

[0100] The bonded article according to one embodiment of the present invention is suitable for application to a variety of products, including heat exchanger piping, hot water supply piping, water supply piping, gas hot water heater piping, air conditioner piping, housing equipment piping, electronic equipment parts, and household electrical appliances.

Claims

1. A joined body having first and second stainless steels as base materials, and a joined portion between the first and second stainless steels, The Sn content of the joint is 40 mass% or more, A joined body, wherein the deepest point of the surface Cr-enriched layer in the heated region of the base material is 300 nm or less. Here, the surface Cr-enriched layer is a region from the surface of the heated region to a depth of 5 μm, where A(d) satisfies the relationship of the following equation (1). A(d)>(B+C)×0.6...(1) During the ceremony, A(d) is [Cr] / ([Cr]+[Fe]) at depth d; B is the maximum value of [Cr] / ([Cr]+[Fe]) from the surface of the heated region to a depth of 5 μm, C is the average value of [Cr] / ([Cr]+[Fe]) from a position at a depth of 10 μm to a position at a depth of 20 μm in the heated region, [Cr] is the element concentration of Cr (at%), and [Fe] is the element concentration of Fe (at%) is.

2. The joint of claim 1 , wherein the joint is a lap joint or a T-joint.

3. A method for producing the bonded body according to claim 1 or 2, comprising the steps of: The method comprises: a joining step in which a first stainless steel and a second stainless steel are used as joined materials, the joined materials are heated while melting a joining metal, and the joined materials are joined; the joining metal is Sn or an Sn alloy having an Sn content of 40 mass% or more, In the joining step, The maximum temperature reached in the heating region of the workpiece is 350°C or less, The residence time of the workpieces in the heating region at a temperature of 100°C or higher is 5 seconds or more and 300 seconds or less, A method for producing a bonded body, which satisfies the relationship of the following formula (2): t≦300×10.5 / [Cr ０ ]×400 / T ・・・(2) During the ceremony, T: Maximum temperature reached in the heating area of the workpiece (°C), t: residence time (seconds) in the heating area of the workpiece at a temperature of 100°C or higher; [Cr ０ ]: Cr content (mass%) of the stainless steel in which the heated region is formed, of the first stainless steel and the second stainless steel that are the materials to be joined is.

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