JP2024103391A - Stainless steel for Ni brazing, joints, heat exchangers and water heaters - Google Patents

Abstract

[Problem] To provide a stainless steel with excellent Ni brazing properties. [Solution] The component composition is appropriately adjusted, and the relationship of the following formula (1) is satisfied: Ni/(Si+Mn+0.08×Cr+Al)≧2.30 ... (1) In formula, Si, Mn, Cr, Ni, and Al are the contents (mass%) of Si, Mn, Cr, Ni, and Al in the component composition, respectively. [Selected Figure] None

Description

The present invention relates to stainless steel for Ni brazing (hereinafter simply referred to as stainless steel), joints, heat exchangers and water heaters.

Copper is used in large quantities in 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 water heaters. Also, from the perspective of protecting the global environment, environmentally friendly water heaters are becoming more and more popular. These environmentally friendly water heaters use secondary heat exchangers. Copper is also used in these secondary heat exchangers.

In recent years, the demand for copper for electrical wires and conductors has increased due to growing demand for power infrastructure in various 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 the associated rise in copper prices. For this reason, there is a strong demand to reduce the amount of copper used in water heaters.

In light of this, there is currently discussion about replacing the copper used in water heaters with other materials, particularly stainless steel, which is relatively inexpensive and has excellent corrosion resistance.

Here, the pipes and heat exchanger parts of the water heater (hereinafter also referred to as the pipes of the water heater) are mainly bonded and assembled by brazing. In general, the brazeability of stainless steel is inferior to that of copper. Therefore, stainless steel used in water heaters is required to have the same level of brazeability as copper.

As a technique related to the brazing property of stainless steel, for example, Patent Document 1 discloses:
"A fuel rail for a direct injection internal combustion engine, in which an injector cup is copper brazed to a stainless steel pipe,
The stainless steel pipe,
Formula I (Si%)≦3.75(C%)+0.37 (provided that (C%)≦0.08) (wherein the combination of atomic symbol and % means the mass % concentration of each atom in the steel)
A fuel rail for a direct injection internal combustion engine, characterized in that it is made of austenitic stainless steel having a Si concentration (Si%) and a C concentration (C%) that satisfy the above.
has been disclosed.

In Patent Document 2,
"By weight, C: 0.06% or less, Si: 1.0-5.0%, Mn: 1.0% or less, P: 0.045% or less, S: 0.003% or less, Ni: 8.0-17.0%, Cr: 16.0-20.0%, Mo: 0.5-2.5%, Cu: 4.0% or less, N: 0.1% or less, and A
At least one of the following is added: Si: 0.2% or less; Nb: 0.2% or less; Ti: 0.2% or less
+ Cr + Mo ≧ 20%, with the balance being Fe and unavoidable impurities, and characterized by excellent resistance to Cu grain boundary penetration during Cu brazing.
has been disclosed.

In Patent Document 3,
"In mass%, C: 0.05% or less, Si: 1% or less, Mn: 1% or less, P: 0.045% or less, S: 0.005% or less, Ni: 1-6%, Cr: 18-24%, N: 0.05% or less,
Nb: 7×(C+N) to 0.7%, the balance being Fe and unavoidable impurities, and the following (1)
A stainless steel for brazing in which the content of each element on the right side of formula (1) is adjusted so that the M value on the left side of the formula may be 1 to 25 when the T value of the first term on the right side of the formula is varied in the range of 950 to 1200.
M=-0.22T+34.5Ni+10.5Mn+13.5Cu-17.3Cr-17.3S
i-18Mo+475.5...(1)
However, the content value of the element expressed in mass% is substituted for each element symbol on the right side of formula (1), and here, 0 (zero) is substituted for Cu and Mo.
has been disclosed.

JP 2020-2839 A JP 2015-200023 A JP 2010-65278 A

The piping of water heaters, etc., is required to be corrosion-resistant, and in particular, high corrosion resistance is required depending on the region in which it is installed. For example, in regions with low water purification capacity, tap water contains corrosive substances. In such cases, there is a high risk of corrosion occurring inside the water heater. In addition, when water heaters are used in environments where chlorides adhere, such as coastal areas, the shortening of their lifespan due to corrosion can be an issue.

Therefore, when brazing pipes for water heaters, etc., it is desirable to use a brazing filler metal that has the same corrosion resistance as the base material, stainless steel, such as a brazing filler metal that contains Ni (hereinafter also referred to as Ni brazing filler metal).

In this regard, the techniques disclosed in Patent Documents 1 and 2 provide good brazing properties when using Cu brazing material, but the brazing properties when using Ni brazing material (hereinafter also referred to as Ni brazing properties) are not sufficient.

The technology disclosed in Patent Document 3 also did not provide sufficient Ni brazing properties. There were also problems with manufacturability, such as the need for a long heating process during production.

The present invention has been developed in view of the above-mentioned circumstances, and has an object to provide a stainless steel having excellent Ni brazeability.
Another object of the present invention is to provide a joint, a heat exchanger and a water heater using the above stainless steel.

Here, "excellent Ni brazeability" means that when the Ni brazing material placed on the surface of the stainless steel sample is heated, the spreadability of the Ni brazing material is 150% or more. The spreadability of the Ni brazing material is calculated by the following formula.
[Spreadability of Ni brazing filler metal (%)] = [Circle equivalent diameter (mm) of Ni brazing filler metal on the surface of the test material after heating] ÷ [Circle equivalent diameter (mm) of Ni brazing filler metal on the surface of the test material before heating] × 100
The heating conditions for the Ni brazing material are as described in the examples below.

In order to achieve the above object, the present inventors have conducted extensive research and have obtained the following findings.
(1) 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 a base metal such as stainless steel (hereinafter simply referred to as base material). Brazing is generally performed in a vacuum or in an atmosphere using an inert gas such as nitrogen or argon as a carrier gas. However, it is difficult to completely remove oxygen from the atmosphere. Therefore, an oxide film composed of oxides (of elements contained in the base material) is formed on the surface of the stainless steel base material. This oxide film inhibits the formation of a metallic bond between the brazing filler metal and the base material and the wetting of the brazing filler metal.

(2) Based on this, the inventors first considered suppressing the formation of an oxide film by reducing elements such as Al that are particularly prone to forming an oxide film (hereinafter also referred to as easily oxidized elements) among the elements contained in the stainless steel base material. However, sufficient Ni brazeability could not be obtained by only reducing the easily oxidized elements.

(3) Therefore, the inventors conducted further experiments and studies on the composition of the stainless steel base material, and as a result, they discovered that excellent Ni brazeability can be obtained by optimizing the composition by reducing easily oxidizable elements, as well as by adding a certain amount of Ni, particularly by appropriately controlling the Ni content so as to satisfy the relationship of the following formula (1) according to the Si, Mn, Cr and Al contents.
Ni/(Si+Mn+0.08×Cr+Al)≧2.30 (1)
In the formula, Si, Mn, Cr, Ni and Al are the Si, Mn, Cr, Ni and Al contents (mass%) in the component composition, respectively.

The inventors consider the above reasons as follows.
The wetting phenomenon can be understood from the bond energy between solid (base material) atoms and the bond energy between liquid (brazing material) atoms. In other words, the closer the bond energy between solid atoms and the bond energy between liquid atoms are, the better the liquid will wet the solid. These values are mainly determined by the composition of the material. In this regard, by optimizing the composition of the stainless steel base material and satisfying the relationship of the above formula (1), the bond energy between solid atoms and the bond energy between liquid atoms will become closer. As a result, excellent Ni brazing properties can be obtained.
The present invention was completed based on the above findings and through further investigation.

In other words, the gist of the present invention is as follows:

1. In mass percent,
C: 0.001-0.050%,
Si: 0.01 to 0.50%,
Mn: 0.01 to 0.50%,
P: 0.050% or less,
S: 0.010% or less,
Cr: 16.00-22.00%,
Ni: 3.00 to 15.00%,
Al: 0.100% or less and N: 0.001 to 0.300%,
and the balance being Fe and unavoidable impurities,
A stainless steel for Ni brazing that satisfies the following formula (1).
Ni/(Si+Mn+0.08×Cr+Al)≧2.30 (1)
In the formula, Si, Mn, Cr, Ni and Al are the Si, Mn, Cr, Ni and Al contents (mass%) in the component composition, respectively.

2. The composition further comprises, in mass %,
Mo: 3.00% or less,
Cu: 3.00% or less,
Ti: 0.50% or less,
Nb: 0.50% or less,
V: 0.20% or less,
Zr: 0.10% or less,
Sn: 0.50% or less,
B: 0.0100% or less,
Ca: 0.0100% or less,
Mg: 0.0100% or less,
2. The stainless steel for Ni brazing according to 1 above, containing one or more selected from Ta: 0.10% or less and REM: 0.10% or less.

3. The stainless steel for Ni brazing according to 1 or 2 above,
A metal material;
a joint between the stainless steel for Ni brazing and the metal material, and a Ni brazing material;
A zygote that has the following functions.

4. A heat exchanger having the joint described in 3.

5. A water heater having the joint described in 3.

According to the present invention, a stainless steel with excellent Ni brazing properties can be obtained. In addition, it is possible to replace much of the copper used in water heaters, heat exchangers, etc. with the stainless steel of the present invention. As a result, it is possible to manufacture water heaters, heat exchangers, etc. that use significantly less copper, which has a significant industrial effect.

[1] Stainless Steel The stainless steel of the present invention will be described based on the following embodiments.
First, the composition of the stainless steel according to one embodiment of the present invention will be described. Note that the unit of the content of elements in the composition is "mass %", and hereinafter, unless otherwise specified, it will be simply indicated as "%".

C: 0.001-0.050%
The higher the C content, the higher the strength of the steel, and the lower the C content, the better the workability. Here, the C content must be 0.001% or more to obtain sufficient strength. If the C content exceeds 0.050%, the workability decreases significantly. In addition, Cr carbides precipitate at the grain boundaries, causing sensitization and decreasing the corrosion resistance. Therefore, the C content is set to 0. The C content is preferably 0.003% or more. The C content is preferably 0.030% or less, and more preferably 0.020% or less. The following is the result.

Si: 0.01~0.50%
Silicon is a useful element as a deoxidizer. Such an effect can be obtained with a silicon content of 0.01% or more. However, if the silicon content exceeds 0.50%, oxidation occurs during brazing. A coating is formed, and brazing properties are reduced. Therefore, the Si content is set to the range of 0.01 to 0.50%. The Si content is preferably 0.35% or less.

Mn: 0.01-0.50%
Mn is an element that has a deoxidizing effect. Such an effect can be obtained with a Mn content of 0.01% or more. However, if the Mn content exceeds 0.50%, Mn oxidation occurs during brazing. The Mn content is therefore set to the range of 0.01 to 0.50%. The Mn content is preferably 0.25% or less. , and more preferably 0.20% or less.

P: 0.050% or less P is an element that is inevitably contained in steel. Excessive P content reduces weldability and makes intergranular corrosion more likely to occur. This tendency becomes more pronounced when the P content exceeds 0.050%. Therefore, the P content is set to 0.050% or less. The P content is preferably 0.040% or less, and more preferably 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: 0.010% or less S is an element that is inevitably contained in steel. Excessive S content, particularly S content exceeding 0.010%, promotes the precipitation of MnS and reduces corrosion resistance. Therefore, the S content is set to 0.010% or less. Preferably, the S content is 0.005% 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 0.001% or more.

Cr: 16.00-22.00%
Cr is an element effective in ensuring corrosion resistance. If the Cr content is less than 16.00%, sufficient corrosion resistance cannot be obtained after brazing. However, if the Cr content exceeds 22.00%, the brazing During brazing, a Cr oxide film is formed on the surface of the stainless steel, which reduces the brazeability. Therefore, the Cr content is set to the range of 16.00 to 22.00%. The Cr content is preferably within the range of 16.00 to 22.00%. The Cr content is preferably 19.50% or less, and more preferably 19.00% or less.

Ni: 3.00-15.00%
Ni is an important element in the present invention. As described above, by appropriately controlling the Ni content, the bond energy between the solid (base stainless steel) and the liquid (Ni brazing material) can be adjusted. The value of the bond energy between atoms approaches that of the alloy. As a result, excellent Ni brazing properties are obtained. Therefore, the Ni content is set to 3.00% or more. On the other hand, when the Ni content exceeds 15.00%, If the Ni content is too high, the hot workability and cold workability will decrease. Therefore, the Ni content is set to the range of 3.00 to 15.00%. The Ni content is preferably 4.00% or more, and more preferably 1.00% or more. The Ni content is preferably 6.00% or more. The Ni content is preferably 10.00% or less, and more preferably 7.00% or less.

Al: 0.100% or less Al is an element useful for deoxidization. Such an effect is preferably obtained by containing 0.001% or more of Al. However, if the Al content exceeds 0.100%, Al-enriched substances such as Al oxides and Al nitrides are likely to be formed on the surface of the stainless steel during brazing. As a result, brazing properties are reduced. Therefore, the Al content is set to 0.100% or less. The Al content is preferably 0.040% or less, and more preferably 0.015% or less.

N: 0.001-0.300%
N enhances corrosion resistance and strength. To obtain these effects, the N content is set to 0.001% or more. On the other hand, if the N content exceeds 0.300%, the excess N may cause problems during casting and N is a factor in the generation of blowholes during welding. Therefore, the N content is set to the range of 0.001 to 0.300%. The N content is preferably 0.003% or more. is preferably 0.010% or less, and more preferably 0.005% or less.

Ni/(Si+Mn+0.08×Cr+Al)≧2.30 (1)
In the formula, Si, Mn, Cr, Ni and Al are the Si, Mn, Cr, Ni and Al contents (mass%) in the component composition, respectively.
As described above, in order to improve the Ni brazeability, it is necessary to appropriately control the Ni content so as to satisfy the relationship of the above formula (1) according to the Si, Mn, Cr and Al contents. In other words, by optimizing the composition of the stainless steel as the base material as described above and satisfying the above formula (1), the bond energy between solid atoms and the bond energy between liquid atoms can be calculated. As a result, excellent Ni brazing properties are obtained. Therefore, the composition of the stainless steel as the base material is optimized, particularly the Si, Mn, Cr, Ni and Al contents. With the above range, Ni/(Si+Mn+0.08×Cr+Al) is set to 2.30 or more. Ni/(Si+Mn+0.08×Cr+Al) is preferably 2.50 or more, and more preferably 4. It is preferably 0.00 or more, and more preferably 5.00 or more.

The basic component composition of the stainless steel according to one embodiment of the present invention has been described above. In addition, one or more elements selected from the following elements may be further contained.
Mo: 3.00% or less,
Cu: 3.00% or less,
Ti: 0.50% or less,
Nb: 0.50% or less,
V: 0.20% or less,
Zr: 0.10% or less,
Sn: 0.50% or less,
B: 0.0100% or less,
Ca: 0.0100% or less,
Mg: 0.0100% or less,
Ta: 0.10% or less and REM: 0.10% or less

Mo: 3.00% or less Mo stabilizes the passivation film of stainless steel to improve corrosion resistance. In order to obtain such an effect, it is preferable that the Mo content is 0.01% or more. On the other hand, if the Mo content exceeds 3.00%, the high-temperature strength increases and the hot workability is likely to decrease. Furthermore, surface defects are likely to occur during hot rolling. Therefore, when Mo is contained, its content is preferably 3.00% or less. The Mo content is more preferably 0.60% or less, and further preferably 0.40% or less.

Cu: 3.00% or less Cu has the effect of increasing corrosion resistance. In order to obtain such an effect, it is preferable that the Cu content is 0.01% or more. The Cu content is more preferably 0.30% or more, and even more preferably 0.50% or more. On the other hand, if the Cu content exceeds 3.00%, the high-temperature strength increases and the hot workability tends to decrease. Therefore, when Cu is contained, the content is preferably 3.00% or less. The Cu content is more preferably 1.50% or less, and even more preferably 1.20% or less.

Ti: 0.50% or less Ti has the effect of increasing the strength of steel. In addition, Ti also has the effect of preventing sensitization by bonding with C and N contained in steel. In order to obtain such an effect, it is preferable that the Ti content is 0.01% or more. The Ti content is more preferably 0.03% or more, and even more preferably 0.05% or more. On the other hand, if the Ti content exceeds 0.50%, an oxide film may be formed on the surface of the stainless steel during brazing, and the brazing property may be reduced. Therefore, when Ti is contained, the content is preferably 0.50% or less. The Ti content is more preferably 0.20% or less, and even more preferably 0.10% or less.

Nb: 0.50% or less Like Ti, Nb has the effect of increasing the strength of steel. In addition, Nb also has the effect of preventing sensitization by bonding with C and N contained in steel. In order to obtain such an effect, it is preferable that the Nb content is 0.01% or more. The Nb content is more preferably 0.03% or more, and even more preferably 0.05% or more. On the other hand, if the Nb content exceeds 0.50%, an oxide film may be formed on the surface of the stainless steel during brazing, and the brazing property may be reduced. Therefore, when Nb is contained, the content is preferably 0.50% or less. The Nb content is more preferably 0.20% or less, and even more preferably 0.10% or less.

V: 0.20% or less Like Ti and Nb, V combines with C and N contained in the steel and has the effect of preventing sensitization. In order to obtain such an effect, it is preferable that the V content is 0.01% or more. On the other hand, if the V content exceeds 0.20%, the workability is likely to decrease. Therefore, when V is contained, the content is preferably 0.20% or less. The V content is more preferably 0.10% or less.

Zr: 0.10% or less Like Ti and Nb, Zr combines with C and N contained in the steel and has the effect of preventing sensitization. In order to obtain such an effect, it is preferable that the Zr content is 0.01% or more. On the other hand, if the Zr content exceeds 0.10%, the workability is likely to decrease. Therefore, when Zr is contained, the content is preferably 0.10% or less. The Zr content is more preferably 0.05% or less.

Sn: 0.50% or less Sn is an element effective in suppressing roughness during rolling. In order to obtain such an effect, it is preferable to set the Sn content to 0.01% or more. On the other hand, if the Sn content exceeds 0.50%, the hot workability is likely to decrease. Therefore, when Sn is contained, the content is preferably 0.50% or less. The Sn content is more preferably 0.20% or less, and further preferably 0.10% or less.

B: 0.0100% or less B is an element that improves hot workability. In order to obtain such an effect, it is preferable that the B content is 0.0003% or more. On the other hand, if the B content exceeds 0.0100%, there is a risk of causing a decrease in corrosion resistance. Therefore, when B is contained, the content is preferably 0.0100% or less. The B content is more preferably 0.0030% or less.

Ca: 0.0100% or less Ca improves the penetration of the welded portion and improves the weldability. In order to obtain such an effect, it is preferable that the Ca content is 0.0005% or more. On the other hand, if the Ca content exceeds 0.0100%, Ca and S are combined to generate CaS, which may reduce the corrosion resistance. Therefore, when Ca is contained, the content is preferably 0.0100% or less. The Ca content is more preferably 0.0050% or less, and further preferably 0.0020% or less.

Mg: 0.0100% or less Mg acts as a deoxidizer. In order to obtain such an effect, it is preferable that the Mg content is 0.0005% or more. On the other hand, if the Mg content exceeds 0.0100%, the toughness of the steel decreases and the manufacturability decreases. Therefore, when Mg is contained, the content is preferably 0.0100% or less. The Mg content is more preferably 0.0050% or less, and further preferably 0.0020% or less.

Ta: 0.10% or less Ta is an element that improves hot workability. In order to obtain such an effect, it is preferable that the Ta content is 0.01% or more. On the other hand, if the Ta content exceeds 0.10%, the corrosion resistance may decrease. Therefore, when Ta is contained, the content is preferably 0.10% or less. The Ta content is more preferably 0.05% or less.

REM: 0.10% or less REM is an element that improves hot workability. In order to obtain such an effect, it is preferable that the REM content is 0.01% or more. On the other hand, if the REM content exceeds 0.10%, the corrosion resistance may decrease. Therefore, when REM is contained, the content is preferably 0.10% or less. The REM content is more preferably 0.05% or less. Note that REM means Sc, Y, and lanthanoid elements (elements with atomic numbers of 57 to 71, such as La, Ce, Pr, Nd, and Sm). The REM content here is the total content of these elements.

The balance other than the above elements in the composition of the stainless steel according to one embodiment of the present invention is Fe and unavoidable impurities. Note that for the above-mentioned Mo, Cu, Ti, Nb, V, Zr, Sn, B, Ca, Mg, Ta and REM, if the content of each element is less than the respective preferred lower limit, the element is treated as an unavoidable impurity.

The shape of the 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 0.3 to 5.0 mm, and more preferably 0.8 to 2.0 mm.

In addition, the stainless steel according to one embodiment of the present invention is a stainless steel for Ni brazing, that is, a stainless steel used as a base material (joined material) for Ni brazing. Here, Ni brazing is brazing using Ni brazing material as a brazing material. The Ni brazing material is an alloy having a composition of Ni: 50% or more by mass. The Ni brazing material is preferably an alloy having a composition of Ni: 70% or more by mass, more preferably Ni: 80% or more by mass. The composition of the Ni brazing material may contain other elements, for example, at least one element selected from Cr, B, Si, Fe, C and P, for the purpose of adjusting the melting point. A more specific composition of the Ni brazing material may be, for example, the composition of nickel brazing material specified in JIS Z 3265:1998. The form of use of the Ni brazing material is not particularly limited, but is preferably, for example, a paste (a mixture of powdered Ni brazing material and a binder composed of an organic substance). In the paste-like Ni brazing material, the mass mixing ratio of the binder to the Ni brazing material (= [mass of binder] / [mass of Ni brazing material] x 100) is, in mass%, preferably 5 to 30%, and more preferably 10 to 20%. Note that the binder, which is made of an organic substance, volatilizes during Ni brazing.

The stainless steel according to one embodiment of the present invention is particularly suitable for use in water heaters and heat exchangers, as described below.

[2] Joint, Water Heater, and Heat Exchanger Next, a joint, a water heater, and a heat exchanger according to one embodiment of the present invention will be described.

The joint according to one embodiment of the present invention comprises the stainless steel [1] above, a metal material, and a joint (hereinafter simply referred to as the joint) of Ni brazing material that joins the stainless steel and the metal material.

Here, the stainless steel in [1] above is the base material.

The metallic material is the base material (hereinafter, simply referred to as the mating material) of the stainless steel of [1] above, which is the base material. In other words, the metallic material is a material that is joined to the stainless steel of [1] above, and examples of such materials include stainless steel, copper, alloys such as copper alloys and nickel alloys, carbon steel, and alloy steel. Among these, stainless steel, copper, and copper alloys are preferable. The metallic material may also be the stainless steel of [1] above. In other words, the joint may be a joint between the stainless steels of [1] above.

The joint is formed by solidifying the Ni brazing material that is molten during brazing. Therefore, the joint can be said to have a composition of Ni: 50 mass% or more, similar to the composition of the Ni brazing material described above. In addition to Ni, the composition of the joint may also contain at least one element of Cr, B, Si, Fe, C, and P, or elements derived from the stainless steel base material or the metallic material that is the mating material.

In addition, when a joint according to one embodiment of the present invention has multiple joints, at least one of them should be a Ni brazing material joint.

The water heater and heat exchanger according to one embodiment of the present invention have a joint according to one embodiment of the present invention, for example, as a joint between pipes or a joint between a pipe and a tube sheet. Here, the pipes and the pipe and the tube sheet correspond to the stainless steel and metal material (base material and mating material) in [1] above, respectively.

Examples of water heaters include gas water heaters, electric water heaters, and latent heat recovery water heaters. Examples of heat exchangers include water heaters, exhaust heat recovery devices, and heat exchangers in exhaust gas recirculation devices.

[3] Method for Producing Stainless Steel Next, a preferred method for producing stainless steel according to one embodiment of the present invention will be described taking as an example the case of producing a plate-shaped stainless steel (hereinafter also referred to as a stainless steel plate).
First, a steel material (steel slab) having the above-mentioned composition is hot-rolled to obtain a hot-rolled steel sheet having a predetermined thickness. Then, the hot-rolled steel sheet is optionally subjected to hot-rolled annealing 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 having a predetermined thickness. Then, the cold-rolled steel sheet is subjected to cold-rolled annealing to obtain a cold-rolled annealed steel sheet. In this way, a stainless steel sheet having the above-mentioned composition can be manufactured. Note that the conditions of hot rolling, cold rolling, hot-rolled annealing, cold-rolled annealing, etc. are not particularly limited, and may be in accordance with a conventional method.

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. In terms of productivity and quality, the continuous casting method is preferable. 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. From the viewpoint of productivity and required quality, the number of cold rolling 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 hot working and cold working can also be used for other shapes besides plate shapes. In this case, the conditions for hot working and cold working are not particularly limited, and may be in accordance with conventional methods.

[4] Ni brazing Next, the joined body according to the embodiment of the present invention described above in [2], and Ni brazing performed when manufacturing (assembling) a water heater and a heat exchanger will be described.

Ni brazing is preferably performed in a vacuum atmosphere or nitrogen atmosphere with a degree of vacuum of 4.0×10 ⁻³ Pa to 60 Pa or less using, for example, a vacuum furnace or an atmosphere furnace. The temperature during brazing is not particularly limited, but 900 to 1100° C. is preferable. If the holding time at the brazing temperature is too short, the brazing material does not spread sufficiently. On the other hand, if the holding time at the brazing temperature is too long, an oxide film may grow on the surface of the stainless steel as the base material, and the brazing property may be reduced. Therefore, the brazing time is preferably 5 to 60 minutes. The Ni brazing material, the stainless steel as the base material [1] above, and the metal material as the mating material are as described above. In this way, by Ni brazing the stainless steel as the base material [1] and the metal material as the mating material, the joint according to one embodiment of the present invention as well as the water heater and the heat exchanger as described above [2] can be manufactured. Other manufacturing steps may be performed according to ordinary methods.

A steel having the composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted in a vacuum melting furnace. The steel was then heated at 1100-1200°C for 1 hour and hot rolled to obtain a hot-rolled steel sheet with a thickness of 4.0 mm. The hot-rolled steel sheet was then annealed at a temperature range of 950-1100°C to obtain a hot-rolled annealed steel sheet. The hot-rolled annealed steel sheet was then mechanically ground to remove scale, and then cold rolled to obtain a cold-rolled steel sheet with a thickness of 1.0 mm. The cold-rolled steel sheet was then finish-annealed at a temperature range of 950-1100°C to obtain a cold-rolled annealed steel sheet. The surface of the obtained cold-rolled annealed steel sheet was polished with emery paper up to No. 800 and degreased with acetone to obtain a stainless steel specimen.

The stainless steel thus obtained was evaluated for Ni brazing properties as follows. The composition of the stainless steel test materials was substantially the same as the composition of each steel type listed in Table 1, and all of the inventive examples satisfied the above range of composition.

(Evaluation of Ni brazing ability)
A test piece having a width of 50 mm and a length of 50 mm was cut out from the above stainless steel (cold-rolled annealed steel sheet). Then, 0.1 g of a paste-like Ni brazing material was placed in a circle having a diameter of 5 mm on the surface of the test piece. The paste-like Ni brazing material was a mixture of powdered Ni brazing material (BNi-2 (JIS Z 3265:1998)) and a binder composed of an organic substance. The mass mixing ratio of the binder to the Ni brazing material was 10% by mass. Then, the test piece was placed horizontally in a vacuum furnace, and the vacuum furnace was evacuated. Then, nitrogen gas was introduced into the vacuum furnace. Then, the vacuum furnace was heated, and the test piece was held in a nitrogen atmosphere of 1050 ° C. and 40 Pa for 10 minutes. Then, it was cooled to room temperature, and the circle equivalent diameter of the Ni brazing material on the surface of the test piece was measured. The spreadability of the Ni brazing material was calculated by the following formula, and the Ni brazing property was evaluated according to the following evaluation criteria. The evaluation results are shown in Table 1.
[Spreadability of Ni brazing filler metal (%)] = [Circle equivalent diameter (mm) of Ni brazing filler metal on the surface of the test piece after heating] ÷ [Circle equivalent diameter (mm) of Ni brazing filler metal on the surface of the test piece before heating] × 100
Evaluation criteria: Excellent (pass): The spread rate of the Ni-containing brazing filler metal is 150% or more. Insufficient (fail): The spread rate of the Ni-containing brazing filler metal is less than 150%.

As shown in Table 1, all of the stainless steels in the invention examples showed excellent Ni brazing properties.

On the other hand, in the comparative examples, sufficient Ni brazeability was not obtained.
That is, in Comparative Example No. 19, the Si content exceeded the upper limit, and therefore sufficient brazeability was not obtained.
In Comparative Example No. 20, the Cr content exceeded the upper limit, and therefore sufficient brazeability was not obtained.
In Comparative Example No. 21, the Al content exceeded the upper limit, and therefore sufficient brazeability was not obtained.
In Comparative Example No. 22, the Mn content exceeded the upper limit, and therefore sufficient brazeability was not obtained.
In Comparative Example No. 23, the Ni content was below the lower limit and did not satisfy the above formula (1), so sufficient brazeability was not obtained.
In Comparative Example No. 24, the above formula (1) was not satisfied, and therefore sufficient brazeability was not obtained.

The stainless steel according to one embodiment of the present invention has excellent Ni brazing properties. Therefore, the ferritic stainless steel according to one embodiment of the present invention is particularly suitable for use in water heaters and heat exchangers that are assembled using Ni brazing.

Claims (5)

In mass percent,
C: 0.001-0.050%,
Si: 0.01 to 0.50%,
Mn: 0.01 to 0.50%,
P: 0.050% or less,
S: 0.010% or less,
Cr: 16.00-22.00%,
Ni: 3.00 to 15.00%,
Al: 0.100% or less and N: 0.001 to 0.300%,
and the balance being Fe and unavoidable impurities,
A stainless steel for Ni brazing that satisfies the following formula (1).
Ni/(Si+Mn+0.08×Cr+Al)≧2.30 (1)
In the formula, Si, Mn, Cr, Ni and Al are the Si, Mn, Cr, Ni and Al contents (mass%) in the component composition, respectively. The composition further comprises, in mass%,
Mo: 3.00% or less,
Cu: 3.00% or less,
Ti: 0.50% or less,
Nb: 0.50% or less,
V: 0.20% or less,
Zr: 0.10% or less,
Sn: 0.50% or less,
B: 0.0100% or less,
Ca: 0.0100% or less,
Mg: 0.0100% or less,
The stainless steel for Ni brazing according to claim 1, containing one or more selected from the group consisting of Ta: 0.10% or less and REM: 0.10% or less. The stainless steel for Ni brazing according to claim 1 or 2,
A metal material;
a joint between the stainless steel for Ni brazing and the metal material, and a Ni brazing material;
A zygote that has the following functions. A heat exchanger having the joint described in claim 3. A water heater having the joint described in claim 3.