JP7600970B2 - Ferritic stainless steel for copper phosphorus brazing and water heater using same - Google Patents

Description

The present invention relates to a ferritic stainless steel for copper phosphorus brazing and a water heater using the same, and to a ferritic stainless steel that exhibits good brazing properties when brazing with copper phosphorus brazing material.

Copper is used in large quantities in the field of water heaters, including gas water heaters and electric water heaters. For example, copper pipes are widely used for the piping that carries the water and heat transfer medium used in these devices. Furthermore, from the perspective of protecting the global environment, environmentally friendly water heaters, which have become increasingly popular in recent years, are fitted with heat exchangers, which also use copper.

In recent years, the demand for copper for electrical wires and conductors has increased due to growing demand for power infrastructure in emerging countries and the shift to electric vehicles, resulting in a tight supply and demand situation for copper. Demand for copper is expected to continue to increase, and reducing copper usage in response to rising copper prices has become an important issue.

In light of this, the water heater industry is moving away from copper to other materials, particularly ferritic stainless steel, which is relatively inexpensive and has excellent corrosion resistance.

The piping and heat exchanger parts of water heaters are mainly made of Cu-containing brazing materials, and in particular phosphorus copper brazing materials with an appropriate brazing temperature of 600 to 950°C. These materials are therefore required to have good brazing properties for phosphorus copper brazing (hereinafter simply referred to as brazing properties).

In general, the brazeability of ferritic stainless steel is inferior to that of copper (Cu), so improvements are required.

For example, Patent Document 1 describes a stainless steel having excellent brazability, which contains, by mass%, C: 0.001 to 0.1%, Si: more than 1.5 to 4.0%, Mn: 0.05 to 4.0%, Cr: 10.5 to 30%, Ni: 35% or less, N: 0.001 to 0.4%, and further contains one or both of Ti: 0.002 to 0.030% and Al: 0.002 to 0.10%, with the balance being Fe and unavoidable impurities, the contents of Si, Ti and Al satisfying (Formula 1), and an oxide film having a composition that satisfies (Formula 2) is formed on the surface.
Si _m /(Ti _m + Al _m )≧40 (Formula 1)
1.2×Si _m /Fe _m ≦ _Sif /Fe _f ≦5×Si _m /Fe _m ... (Formula 2)
In (Formula 1) and (Formula 2), the element name with the subscript f represents the content of each element in the oxide film in atomic %, and the element name with the subscript m represents the content of each element in the base steel in mass %.

Patent Document 2 describes a ferritic stainless steel having excellent Cu brazeability, which contains, by mass%, C: 0.01 to 0.03%, Si: 1.00% or less, Mn: 1.50% or less, P: 0.04% or less, S: 0.03% or less, Ni: 0.6% or less, Cr: 17 to 25%, Mo: 2.50% or less, Cu: 0.10 to 0.50%, N: 0.030% or less, Nb: 7×(C+N)% or more, 0.80% or less, Al: 0.022% or less, with the balance being Fe and unavoidable impurities, and which satisfies the following formula (1):
C+6Si-4Al-72Ti≧1.5 (1) where C, Si, Al, and Ti in formula (1) represent the mass percentage of each element.

International Publication No. 2016/152854 JP 2018-145497 A

The ferritic stainless steel described in Patent Document 1 contains a certain amount of Si, and ensures brazability by forming an oxide film in which Si is concentrated. The ferritic stainless steel described in Patent Document 2 contains a certain amount of Nb, and ensures brazability by utilizing the carbides formed in the steel.

The ferritic stainless steels disclosed in Patent Documents 1 and 2 have room for improvement in terms of brazing properties. Both of the ferritic stainless steels disclosed in Patent Documents 1 and 2 are suitable for brazing at 1000 to 1200°C, but there was an issue that sufficient brazing properties could not be obtained, particularly when brazing at 600 to 950°C using copper phosphorus brazing filler metal.

The present invention was developed in consideration of the above circumstances, and aims to provide a ferritic stainless steel for copper phosphorus brazing that exhibits good brazing properties when brazing is performed using a copper phosphorus brazing material.

In this case, good brazeability means that when the copper phosphorus brazing filler metal is placed on the surface of the stainless steel sample, the spreadability of the copper phosphorus brazing filler metal is 150% or more when the copper phosphorus brazing filler metal is heated. The spreadability of the copper phosphorus brazing filler metal is calculated by the following formula.
Spreadability of phosphorus copper brazing material (%) = [equivalent circle diameter (mm) of phosphorus copper brazing material on the surface of the test material after heating] ÷ [equivalent circle diameter (mm) of phosphorus copper brazing material on the surface of the test material before heating] × 100
Regarding the heating conditions of the phosphorus copper brazing material, etc., please refer to the description of the examples described later.

Brazing is a method of joining base materials by melting a brazing filler metal and utilizing the metallic bond that occurs between the brazing filler metal and the base material. However, an oxide film is formed on the surface of the base material during brazing heat treatment, which inhibits the formation of a metallic bond between the brazing filler metal and the base material and the wetting of the brazing filler metal. For this reason, in order to improve brazing properties, it is necessary to prevent the formation of this oxide film. As a method for this, it has been considered to suppress the formation of oxide films by reducing the elements contained in stainless steel that are particularly prone to forming oxide films (hereinafter also referred to as easily oxidizable elements).

However, when brazing using copper phosphorus brazing filler metal, sufficient brazing properties cannot be obtained simply by reducing the amount of easily oxidized elements.

The inventors therefore conducted extensive research into a method for suppressing the formation of an oxide film on the surface of the base material and ensuring good brazing properties when brazing using copper phosphorus brazing filler metal.

As a result, they discovered that by strictly regulating the content of easily oxidizable elements and adding a certain amount of Cu, it is possible to suppress the formation of an oxide film and obtain good brazing properties.

The inventors believe that the mechanism by which the inclusion of Cu inhibits the formation of an oxide film is as follows. Cu concentrates on the surface of the base material during brazing. The concentration of Cu on the surface of the base material reduces the diffusion of easily oxidized elements to the surface of the base material, inhibiting the formation of an oxide film of these elements (easily oxidized elements). On the other hand, as Cu concentrates, oxidation of Cu progresses and Cu oxide is formed, but because Cu oxide has a high affinity with phosphorus copper brazing filler metal, Cu oxide does not inhibit phosphorus copper brazing. Therefore, by including an appropriate amount of Cu, it is possible to inhibit the formation of an oxide film that inhibits brazing and obtain good brazing properties.

The present invention was completed after further investigation based on the above findings.

That is, the gist and configuration of the present invention are as follows.
[1] In mass%,
C: 0.003-0.020%,
Si: 0.01-0.60%,
Mn: 0.01 to 0.40%,
P: 0.040% or less,
S: 0.020% or less,
Cr: 16.0-27.0%,
Ni: 0.01 to 2.50%,
Al: 0.010% or less,
Cu: 0.40-0.80%,
Nb: 0.10 to 0.50%,
A ferritic stainless steel for copper phosphorus brazing containing 0.020% or less N, with the balance being Fe and unavoidable impurities.
[2] Further, in mass%,
Mo: 0.01-2.50%,
Co: 0.01-0.70%,
W: 0.01-1.00%,
The ferritic stainless steel for phosphorus copper brazing according to [1], which contains one or more selected from the following:
[3] Further, in mass%,
Ti: 0.01 to 0.10%,
V: 0.01-0.20%,
Zr: 0.01 to 0.10%,
Mg: 0.0005-0.0050%,
Ca: 0.0005-0.0050%,
B: 0.0005-0.0050%,
REM (rare earth metal): 0.001-0.100%,
Sn: 0.001-0.300%,
Sb: 0.001 to 0.100%,
The ferritic stainless steel for phosphorus copper brazing according to [1] or [2], which contains one or more selected from the following:
[4] A water heater assembled using the ferritic stainless steel for copper phosphorus brazing according to any one of [1] to [3] above, with at least one or more joints being brazed using a copper phosphorus brazing material.

According to the present invention, it is possible to provide a ferritic stainless steel for phosphorus copper brazing that exhibits good brazing properties when brazing is performed using a phosphorus copper brazing material.

The present invention will be described in detail below.

First, we will explain the reason why the composition of the steel is limited to the above range in the present invention. Note that the unit of the content of elements in the composition of the steel is "mass %", but hereinafter, unless otherwise specified, it will be simply indicated as "%".

C: 0.003-0.020%
A high C content improves strength, while a low C content improves workability. Here, in order to obtain sufficient strength, the C content must be 0.003% or more. If the C content exceeds 0.020%, the workability is significantly reduced, and Cr carbides are precipitated at the grain boundaries, causing sensitization and reducing the corrosion resistance. The C content is preferably 0.004% or more. The C content is preferably 0.015% or less, and more preferably 0.010% or less. .

Si: 0.01~0.60%
Silicon is a useful element as a deoxidizer, and this effect can be obtained with a silicon content of 0.01% or more. However, if the silicon content exceeds 0.60%, silicon oxides are generated during brazing heat treatment. The Si content is therefore set to the range of 0.01 to 0.60%. The Si content is preferably 0.01 to 0.60%. The Si content is preferably 0.10% or more. The Si content is preferably 0.55% or less, and more preferably 0.50% or less.

Mn: 0.01-0.40%
Mn has a deoxidizing effect, and this effect can be obtained with a Mn content of 0.01% or more. However, if the Mn content exceeds 0.40%, Mn concentrates are formed on the steel sheet surface during brazing heat treatment. As a result, brazing properties are reduced. Therefore, the Mn content is set to the range of 0.01 to 0.40%. The Mn content is preferably 0.10% or more, and more preferably 0.15% or more. % or more. The Mn content is preferably 0.25% or less, and more preferably 0.20% or less.

P: 0.040% or less P is an element that is inevitably contained in steel, and excessive content reduces weldability and makes intergranular corrosion more likely to occur. This tendency becomes more pronounced when the P content exceeds 0.040%. Therefore, the P content is set to 0.040% or less. Preferably, the P content is 0.030% or less. The lower limit of the P content is not particularly limited. However, since excessive de-P leads to an increase in costs, the P content is preferably 0.010% or more.

S: 0.020% or less S is an element that is inevitably contained in steel, and an S content of more than 0.020% promotes the precipitation of MnS and reduces corrosion resistance. Therefore, the S content is set to 0.020% or less. Preferably, the S content is 0.010% or less. The lower limit of the S content is not particularly limited. However, since excessive de-S leads to an increase in costs, the S content is preferably 0.0005% or more.

Cr:16.0~27.0%
Cr is effective in ensuring corrosion resistance. If the Cr content is less than 16.0%, sufficient corrosion resistance cannot be obtained after brazing. However, if the Cr content exceeds 27.0%, the brazing process During this process, a Cr oxide film forms on the surface of the steel sheet, which deteriorates the brazing property. Therefore, the Cr content is set to the range of 16.0 to 27.0%. The Cr content is preferably 18.0%. The Cr content is preferably 23.0% or less, and more preferably less than 22.5%.

Ni: 0.01-2.50%
Ni is effective in ensuring corrosion resistance and toughness, and this effect can be obtained at a content of 0.01% or more. On the other hand, if the Ni content exceeds 2.50%, the workability decreases. The amount of Ni is 0.01 to 2.50%. The Ni content is preferably 0.05% or more, and more preferably 0.10% or more. .60% or less, and more preferably 0.40% or less.

Al: 0.010% or less Al is a useful element for deoxidation. However, if the Al content exceeds 0.010%, Al-enriched substances such as Al oxides and Al nitrides are easily formed on the steel sheet surface during brazing heat treatment, which reduces brazing properties. Therefore, the Al content is set to 0.010% or less. The Al content is preferably 0.005% or less. The lower limit of the Al content is not particularly limited. However, in order to obtain the deoxidation effect by adding Al, the Al content is preferably 0.001% or more.

Cu: 0.40-0.80%
Cu is one of the important elements in the present invention. As described above, by properly adjusting the composition and adding an appropriate amount of Cu, it is possible to prevent the formation of an oxide film caused by other easily oxidized elements. In order to obtain such an effect, the Cu content is set to 0.40% or more. However, if the Cu content exceeds 0.80%, The hot workability and corrosion resistance are reduced. Therefore, when Cu is contained, the Cu content is set to the range of 0.40 to 0.80%. The Cu content is preferably 0.70% or less. and more preferably 0.60% or less.

Nb: 0.10-0.50%
Nb is an element that inhibits the deterioration of corrosion resistance (sensitization) caused by the precipitation of Cr carbonitrides by combining with C and N. These effects are obtained when the Nb content is 0.10% or more. On the other hand, if the Nb content exceeds 0.50%, the hot workability decreases and manufacturability becomes difficult. Therefore, the Nb content is set in the range of 0.10 to 0.50%. The Nb content is preferably 0.30% or more.

N: 0.020% or less If the N content exceeds 0.020%, the corrosion resistance and workability are significantly reduced. Therefore, the N content is set to 0.020% or less. Preferably, the N content is set to 0.015% or less. More preferably, the N content is set to 0.010% or less. Although there is no particular restriction on the lower limit of the N content, since excessive denitrification leads to an increase in costs, it is preferable that the N content is set to 0.005% or more.

The above describes the basic components of the ferritic stainless steel of the present invention. The components other than those mentioned above in the composition of the present invention are Fe and unavoidable impurities.

The ferritic stainless steel of the present invention may further contain one or more elements selected from Mo, Co, and W, each in the ranges below.

Mo: 0.01~2.50%
Mo stabilizes the passivation film of stainless steel to improve corrosion resistance. This effect is obtained when the Mo content is 0.01% or more. However, when the Mo content exceeds 2.50%, Surface defects are likely to occur during hot rolling. Therefore, when Mo is contained, the Mo content is set to the range of 0.01 to 2.50%.

Co:0.01~0.70%
Co is an element that enhances corrosion resistance. This effect can be obtained when the Co content is 0.01% or more. However, when the Co content exceeds 0.70%, the workability is reduced. When Co is contained, the Co content is in the range of 0.01 to 0.70%. When Co is contained, the Co content is more preferably 0.05% or more. When Co is added, the Co content is more preferably 0.10% or less.

W: 0.01~1.00%
W is an element that enhances corrosion resistance. This effect can be obtained when the W content is 0.01% or more. However, when the W content exceeds 1.00%, the workability decreases. When W is contained, the W content is set to the range of 0.01 to 1.00%. When W is contained, the W content is more preferably 0.50% or less.

The ferritic stainless steel of the present invention may further contain one or more elements selected from Ti, V, Zr, Mg, Ca, B, REM, Sn, and Sb, each within the range below.

Ti: 0.01~0.10%
Ti combines with C and N contained in steel and has the effect of preventing sensitization. This effect is obtained when the Ti content is 0.01% or more. On the other hand, Ti is active against oxygen. Ti is an element, and if the content of Ti exceeds 0.10%, a Ti oxide film is formed on the surface of the steel during brazing treatment, which reduces brazeability. Therefore, when Ti is contained, the Ti content should be 0. The range of Ti is 0.01 to 0.10%. When Ti is contained, the Ti content is preferably 0.05% or less.

V:0.01~0.20%
Like Ti, V bonds with C and N contained in the steel and prevents sensitization. These effects are obtained when the V content is 0.01% or more. If it exceeds 20%, the workability decreases. Therefore, if V is contained, the V content is set to the range of 0.01 to 0.20%. More preferably, it is 0.10% or less.

Zr: 0.01~0.10%
Zr, like Ti and Nb, is an element that bonds with C and N contained in the steel and suppresses sensitization. This effect is obtained when the Zr content is 0.01% or more. If the Zr content exceeds 0.10%, the workability decreases. Therefore, if Zr is contained, the Zr content is set to the range of 0.01 to 0.10%. The Zr content is more preferably 0.03% or more. Also, the Zr content is more preferably 0.05% or less.

Mg: 0.0005-0.0050%
Mg acts as a deoxidizer. This effect can be obtained when the Mg content is 0.0005% or more. However, if the Mg content exceeds 0.0050%, the toughness of the steel decreases, and manufacturability decreases. Therefore, when Mg is contained, the Mg content is set to the range of 0.0005% to 0.0050%. When Mg is contained, the Mg content is more preferably set to 0.0020% or less. be.

Ca: 0.0005-0.0050%
Ca improves the penetration of welds and improves weldability. This effect is obtained when the Ca content is 0.0005% or more. However, when the Ca content exceeds 0.0050%, S When Ca is contained, it is combined with Ca to generate CaS, which reduces the corrosion resistance. Therefore, when Ca is contained, the Ca content is set to the range of 0.0005 to 0.0050%. When Ca is contained, the Ca content is , more preferably 0.0010% or more. In addition, in the case where Ca is contained, the Ca content is more preferably 0.0020% or less.

B: 0.0005-0.0050%
B is an element that improves secondary work embrittlement. This effect is manifested when the B content is 0.0005% or more. However, when the B content exceeds 0.0050%, ductility is reduced by solid solution strengthening. Therefore, when B is contained, the B content is set to the range of 0.0005 to 0.0050%. When B is contained, the B content is more preferably set to 0.0020%. The following is the result.

REM (rare earth metal): 0.001-0.100%
REM (rare earth metals: elements with atomic numbers 57 to 71, such as La, Ce, and Nb) are effective elements for deoxidization. This effect can be obtained when the REM content is 0.001% or more. However, If the REM content exceeds 0.100%, the hot workability decreases. Therefore, if REM is contained, the REM content is set to the range of 0.001 to 0.100%. In the case where REM is contained, the REM content is more preferably 0.010% or more. In the case where REM is contained, the REM content is more preferably 0.050% or less. REM is Sc, Y, REM is a general term for 15 elements ranging from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content here refers to the total content of these elements.

Sn: 0.001-0.300%
Sn is an element that is effective in suppressing roughness of the surface caused by processing. This effect can be obtained when the Sn content is 0.001% or more. However, when the Sn content exceeds 0.300%, the hot workability is deteriorated. Therefore, when Sn is contained, the Sn content is set to the range of 0.001 to 0.300%. When Sn is contained, the Sn content is more preferably 0.050% or less.

Sb: 0.001-0.100%
Sb, like Sn, is an element that is effective in suppressing roughness of the processed surface. This effect is obtained when the Sb content is 0.001% or more. However, when the Sb content exceeds 0.100%, the surface roughness of the processed steel is reduced. Therefore, when Sb is contained, the Sb content is set to the range of 0.001 to 0.100%. When Sb is contained, the Sb content is more preferably set to 0.050% or less. It is.

If the content of Mo, Co, W, Ti, V, Zr, Mg, Ca, B, REM, Sn, or Sb described above as optional components is less than the lower limit value, the component is considered to be included as an unavoidable impurity.

Ferritic means that the structure is mainly composed of ferrite phase, specifically, ferrite phase occupies 80% or more, preferably 90% or more, more preferably 95% or more of the area of the entire structure, and the remaining structure (other than the ferrite phase). Examples of the remaining structure include martensite phase and retained austenite phase. It may be a single ferrite phase (the area ratio of the ferrite phase to the entire structure is 100%).

Here, the area ratio of the ferrite phase is measured as follows. That is, a test piece for cross-sectional observation is prepared from the stainless steel sample. Next, the test piece is etched with aqua regia, and then observed with an optical microscope at a magnification of 200x in 10 fields of view, and the martensite phase, ferrite phase, and retained austenite phase are distinguished from each other based on the structure shape and etching strength. After that, the area ratio of the ferrite phase is determined for each field of view by image processing, and the arithmetic average value for the 10 fields of view is calculated.

The shape of the ferritic stainless steel according to one embodiment of the present invention is not particularly limited, but examples include plate (steel plate), tubular (steel pipe), and rod (round bar, square bar, etc.). The thickness (outer diameter in the case of round bar) is preferably 0.1 to 15.0 mm.

Next, a preferred method for producing ferritic stainless steel according to one embodiment of the present invention will be described using an example of producing plate-shaped ferritic stainless steel (ferritic stainless steel plate).

First, a steel material (steel slab) having the above-mentioned composition is hot-rolled to obtain a hot-rolled steel sheet of a predetermined thickness. Then, the hot-rolled steel sheet is optionally annealed to obtain a hot-rolled annealed steel sheet. Then, the hot-rolled steel sheet or the hot-rolled annealed steel sheet is cold-rolled to obtain a cold-rolled steel sheet of a predetermined thickness. Then, the cold-rolled steel sheet is annealed to obtain a cold-rolled annealed steel sheet. In this way, a plate-shaped ferritic stainless steel (ferritic stainless steel sheet) having the above-mentioned composition can be manufactured. Note that the conditions for hot rolling, cold rolling, hot-rolled annealing, and cold-rolled annealing are not particularly limited, and may be conventional.

For example, in the steelmaking process in which steel is melted, it is preferable to perform secondary refining of steel melted in a converter or electric furnace, etc., by the VOD method or the like to produce steel having the above-mentioned composition. The produced molten steel can be made into a steel material by a known method, but from the viewpoint of productivity and quality, it is preferable to use the continuous casting method. Next, the steel material is heated, preferably to 1050 to 1250°C, and hot-rolled into a hot-rolled steel sheet of a predetermined thickness by hot rolling. Next, the above-mentioned hot-rolled steel sheet is optionally subjected to continuous annealing at a temperature of 900 to 1150°C, and then optionally descaled by pickling or the like to produce a hot-rolled annealed steel sheet. Furthermore, scale may be removed by shot blasting before pickling.

The hot-rolled steel sheet or hot-rolled annealed steel sheet may be subjected to cold rolling. The number of cold rolling may be one, but from the viewpoint of productivity and required quality, it may be two or more with intermediate annealing in between. The total reduction in cold rolling is preferably 60% or more, more preferably 70% or more. Next, the cold-rolled steel sheet obtained by cold rolling is optionally subjected to continuous annealing (finish annealing) at a temperature of preferably 900 to 1150°C, more preferably 950 to 1150°C, and then optionally pickled to obtain a cold-rolled annealed steel sheet. Note that continuous annealing may be performed by bright annealing, and pickling may be omitted. Furthermore, depending on the application, after finish annealing, skin pass rolling or the like may be performed to adjust the shape, surface roughness, and material of the steel sheet.

In the above example, a manufacturing method for forming a plate shape was described as an example, but it is also possible to perform hot working and cold working in addition to forming a plate shape. In this case, the conditions for hot working and cold working are not particularly limited, and may be in accordance with conventional methods.

The ferritic stainless steel according to one embodiment of the present invention is particularly suitable for use as a base material for phosphorus copper brazing. For example, phosphorus copper brazing is preferably performed using a vacuum furnace or an atmosphere furnace in a vacuum atmosphere or a nitrogen carrier atmosphere with a degree of vacuum of 4.0×10 ⁻³ Pa to 60 Pa or less. The brazing temperature is not particularly limited, but is preferably 600 to 950° C. The phosphorus copper brazing material is a Cu alloy for brazing that contains 50 mass % or more of Cu and 4 to 8 mass % of P, and has a melting point adjusted by adding other elements. The shape of the phosphorus copper brazing material is not particularly limited, but is preferably, for example, a paste-like brazing material (a powder brazing material mixed with a binder composed of an organic substance).

When the ferritic stainless steel of the present invention described above is brazed using copper phosphorus brazing filler metal, the Cu contained in the steel acts to suppress the formation of an oxide film caused by other easily oxidizable elements, improving the wettability of the brazing filler metal.

The ferritic stainless steel of the present invention described above is suitable for use in water heaters such as electric water heaters and latent heat recovery water heaters, in which at least one joint is assembled by brazing using a copper phosphorus brazing material.

Steel having the composition shown in Table 1 was melted in a vacuum melting furnace, heated at 1100 to 1200°C for 1 hour, and then hot-rolled to produce a hot-rolled sheet having a thickness of 4.0 mm. After hot-rolled sheet annealing at 950 to 1100°C, the sheet was mechanically ground to remove scale and cold-rolled to a thickness of 1.0 mm. The surface of the cold-rolled annealed sheet obtained by finish annealing at 950 to 1100°C was polished with emery paper up to No. 800 and degreased with acetone.
The cold-rolled and annealed sheets thus treated were brazed with a copper phosphorus brazing filler metal as described below, and the brazeability was evaluated.

(Evaluation of brazing ability)
A test piece having a width of 50 mm and a length of 50 mm was cut out from the cold-rolled annealed sheet subjected to the above treatment, and 0.1 g of phosphorus copper brazing material (BCuP-2 (JIS Z 3264: 1998), Cu-7.2 mass% P) was placed in a circle having a diameter of 5 mm on the surface. The test piece was placed horizontally in a vacuum furnace, and after evacuation, nitrogen gas was introduced and heated at 830°C for 10 minutes in a nitrogen atmosphere of 40 Pa. After cooling to room temperature, the circle equivalent diameter of the phosphorus copper brazing material on the surface of the test piece was measured, and the spread ratio of the phosphorus copper brazing material was calculated according to the following formula.
Spreadability of phosphorus copper brazing material (%)=[equivalent circle diameter (mm) of phosphorus copper brazing material on the surface of the test piece after heating]/[equivalent circle diameter (mm) of phosphorus copper brazing material on the surface of the test piece before heating]×100
Based on the spreadability of the obtained copper phosphorus brazing filler metal, the brazeability of each steel sheet was evaluated as follows.
◎ (Pass, particularly excellent): 250% or more 〇 (Pass): 150% or more but less than 250% × (Fail): Less than 150%

As can be seen from Table 1, all of the inventive examples had good brazing properties when using phosphorus copper brazing filler metal. In particular, in inventive examples Nos. 1-2, 6-11, and 13, the content of easily oxidized elements (Cr and Al) was appropriately regulated, and the Cu content was appropriate, resulting in excellent brazing properties. In contrast, in comparative examples Nos. 14-18, whose component compositions were outside the appropriate range, good brazing properties were not obtained.

More specifically, in Comparative Example No. 14, the Si content exceeded the upper limit of the present invention, and therefore good brazeability was not obtained.
In addition, in Comparative Example No. 15, the Mn content exceeded the upper limit of the present invention, and therefore good brazeability was not obtained.
In addition, in Comparative Example No. 16, the Cr content exceeded the upper limit of the present invention, and therefore good brazeability was not obtained.
In addition, in Comparative Example No. 17, the Al content exceeded the upper limit of the present invention, and therefore good brazeability was not obtained.
In addition, in Comparative Example No. 18, the Cu content was below the lower limit of the present invention, and therefore good brazeability was not obtained.

The ferritic stainless steel of the present invention exhibits good brazing properties when brazed using a copper phosphorus brazing material. For this reason, it is particularly suitable for use in water heaters such as electric water heaters and latent heat recovery water heaters, in which at least one joint is assembled by brazing using a copper phosphorus brazing material.

Claims (4)

In mass percent,
C: 0.003-0.020%,
Si: 0.01-0.50 %,
Mn: 0.01-0.20 %,
P: 0.040% or less,
S: 0.020% or less,
Cr: 16.0-23.0 %,
Ni: 0.01 to 2.50%,
Al: 0.001% or more and 0.005 % or less,
Cu: 0.40-0.70 %,
Nb: 0.10-0.50%,
A ferritic stainless steel for copper phosphorus brazing containing 0.020% or less N, with the balance being Fe and unavoidable impurities. Further, in mass%,
Mo: 0.01-2.50%,
Co: 0.01-0.70%,
W: 0.01-1.00%,
2. The ferritic stainless steel for copper phosphorus brazing according to claim 1, which contains one or more selected from the following: Further, in mass%,
Ti: 0.01 to 0.10%,
V: 0.01-0.20%,
Zr: 0.01 to 0.10%,
Mg: 0.0005-0.0050%,
Ca: 0.0005-0.0050%,
B: 0.0005-0.0050%,
REM (rare earth metal): 0.001-0.100%,
Sn: 0.001-0.300%,
Sb: 0.001 to 0.100%,
3. The ferritic stainless steel for copper phosphorus brazing according to claim 1, which contains one or more selected from the following: A water heater assembled using the ferritic stainless steel for copper phosphorus brazing according to any one of claims 1 to 3, with at least one joint being brazed using a copper phosphorus brazing material.