JP6871045B2 - Ferritic stainless steel and Ni brazed joint members with excellent brazing and corrosion resistance - Google Patents

Description

The present invention relates to ferritic stainless steel and Ni brazed joint members having excellent brazing properties and corrosion resistance used for members assembled by brazing joints. In particular, it targets parts that are assembled by brazing joining using Ni wax and Cu wax, and for automobiles, EGR (Exhaust Gas Recirculation) coolers, oil coolers, exhaust heat recovery devices, and fuel delivery parts are listed. Be done. Further, in the field of water heaters, there are secondary heat exchangers of latent heat recovery type gas water heaters and heat exchangers of CO ₂ refrigerant heat pump type water heaters (commonly known as EcoCute (registered trademark)).

In recent years, in the automobile field, due to the growing awareness of environmental issues, exhaust gas regulations have been tightened and efforts to control carbon dioxide emissions have been promoted. In addition to efforts from the fuel side such as bioethanol and biodiesel fuel, efforts such as further weight reduction and installation of exhaust gas treatment equipment such as EGR, DPF (Diesel Particulate Filter) and urea SCR (Selective Catalytic Reduction) system. Is being implemented. Furthermore, for the purpose of improving fuel efficiency, exhaust heat recovery devices that recover heat from exhaust heat are also beginning to be installed.

Among these, the EGR cooler aims to lower the combustion temperature and reduce NOx, which is a toxic gas, by cooling the exhaust gas of the engine with engine cooling water and then returning it to the intake side and reburning it. .. The exhaust heat recovery device is a system that heats engine cooling water with exhaust gas and utilizes it for warming up a heater or an engine, and is also called an exhaust heat recirculation system. As a result, in hybrid vehicles, the time from cold start to engine stop is shortened, which contributes to improved fuel efficiency, especially in winter.

Furthermore, in the field of hot water supply equipment, the application of heat exchangers is expanding with the spread of environment-friendly equipment. In gas water heaters, latent heat recovery type gas water heaters with a secondary heat exchanger made of stainless steel have become widespread in order to recover latent heat from high-temperature exhaust gas of about 150 to 200 ° C, which was conventionally exhausted as it is. I'm out. In addition, the electric water heater was also a type with a built-in heater in the past, but the CO ₂ refrigerant heat pump type water heater that can reduce the electric energy to 1/3 or less; is being switched to the so-called EcoCute (registered trademark). A heat exchanger is also used here.

Such a heat exchanger may be assembled by welding, but since the structure of the heat exchange portion is complicated, it is often assembled by brazing. Therefore, the material of the heat exchange part assembled by brazing joining needs to have good brazing property.

Further, the heat exchanger is required to have high thermal efficiency, good thermal conductivity, and corrosion resistance and workability. Damage to the member due to lack of corrosion resistance leads to impairing the function of the heat exchanger, and lack of workability adversely affects the manufacture of the heat exchanger member having a complicated shape.

Austenitic stainless steels such as SUS304 and SUS316L are often used as the material used for heat exchangers by taking advantage of their corrosion resistance and strength, but recently, the use of inexpensive ferritic stainless steels with a small coefficient of thermal expansion is increasing. I'm doing it.

On the other hand, brazed joints are also required to have good corrosion resistance, but Ni brazing is superior to Cu brazing in this respect, and is therefore suitable for applications in which corrosion resistance is important. In addition, the joint portion is required to have strength and toughness, but there is a problem that the brazing joint portion formed of Ni brazing is inferior in toughness as compared with Cu brazing.

In Patent Document 1, a phosphorus-containing nickel alloy is coated on the surface of a heat exchanger component made of stainless steel by electroless plating, and then the phosphorus-containing nickel film is melted in a high-temperature vacuum to be used as a brazing material. The process is disclosed. SUS304 is disclosed as an example of the stainless steel used.

As a brazing joint member using austenitic stainless steel, Patent Document 2 discloses a tubular structure that is a part of an engine exhaust gas purification device and accommodates a metal carrier carrying an exhaust gas purification catalyst. There is. Patent Document 3 discloses a common rail for low-pressure fuel. Neither Patent Document 2 nor Patent Document 3 discloses the steel grade. Similarly, Patent Document 4 discloses a heat transfer tube for a heat exchanger of an EGR gas cooling device, and discloses SUS304, SUS304L, SUS316, and SUS316L as austenitic stainless steels used for the corrugated fin structure of the heat transfer tube. ..

Patent Document 5 describes C: 0.080% or less, Si: 1.2 to 3.0%, Mn: 0.4 to 2.0%, P: 0.03% or less, S: 0.003%. Hereinafter, Ni: 6.0 to 12.0%, Cr: 16.0 to 20.0%, Cu: 0.2 to 3.0%, Mo: 0.1 to 1.0%, Al: 0. Excellent corrosion resistance and brazing resistance satisfying 002 to 0.10%, N: 0.030 to 0.150%, and 1.6 ≦ Cu × Si ≦ 4.4 and 0.16 ≦ 2N + Mo ≦ 1.0. Austenite-based stainless steel is disclosed.

Patent Document 6 describes C: 0.03% or less, Si: 1% or less, Mn: 1% or less, Cr: 10.5 to 13.5%, Mo: 1.25% or less, Al: 0.10. % Or less, N: 0.050% or less, and a ferritic stainless steel having good brazing property containing at least one of Ti, Ta, and Nb is disclosed.

Patent Document 7 describes C: 0.03% or less, Si: 0.02 to 1.5%, Mn: 0.02 to 1.5%, Cr: 10 to 22%, Al: 0.5% or less. , N: 0.05% or less, C + N: 0.015% or more, and further satisfying Ti-3N ≤ 0.03, 10 (Ti-3N) + Al ≤ 0.5, and excellent brazing property. Ferritic stainless steel is disclosed.

Patent Document 8 describes C: 0.03% or less, Si: 3% or less, Mn: 2% or less, P as a ferritic stainless steel suitable as a heat exchanger member used for Ni brazing or Cu brazing. : 0.005% or less, S: 0.03% or less, Cr: 11 to 30%, Nb: 0.15 to 0.8%, N: 0.03% or less, and further Nb- (C ×) A ferritic stainless steel satisfying 92.9 / 12 + N × 92.9 / 14) ≧ 0.10.

Patent Document 9 describes C: 0.03% or less, Si: 0.1 or more to 3%, Mn: 0.1 to 2%, Cr: 10 to 35%, Nb: 0.2 to 0.8%. , N: 0.03% or less, and a ferritic stainless steel material for brazing having a partially recrystallized structure in which the area ratio of recrystallized grains generated by heating after cold working is 10 to 80% is disclosed. ing.

Patent Document 10 describes C: 0.001 to 0.1%, Si: 1.5 or more to 4.0%, Mn: 0.05 to 4.0%, Cr: 10.5 to 30%, Ni. : 35% or less, Ti: 0.002 to 0.030% and / or Al: 0.002 to 0.10%, N: 0.001 to 0.4%, Si / (Ti + Al) ≧ 40 A stainless steel having excellent brazing property is disclosed.

In Patent Document 11, by adding less than 1 to 10% of a metal powder selected from Ni, Cr, Ni—Cr alloy, and stainless steel to powdered Ni wax, wettability is good and continuous embrittlement is performed. A Ni brazing material that can prevent the occurrence of cracks without forming a chemical phase is disclosed.

Japanese Unexamined Patent Publication No. 2004-205059 Japanese Unexamined Patent Publication No. 2004-100988 Japanese Unexamined Patent Publication No. 2005-171938 Japanese Unexamined Patent Publication No. 2008-202846 Japanese Unexamined Patent Publication No. 2012-207259 Japanese Unexamined Patent Publication No. 57-60056 Japanese Unexamined Patent Publication No. 2009-174046 Japanese Unexamined Patent Publication No. 2009-299182 Japanese Unexamined Patent Publication No. 2010-285683 International Publication No. 2016/152854 Japanese Unexamined Patent Publication No. 11-114692

As the materials used for heat exchangers are switched from austenitic stainless steel to ferritic stainless steel, there is an increasing demand for improving the brazing property of ferritic stainless steel. When comparing various stainless steels with respect to brazing properties, it cannot be said that the brazing properties of ferritic stainless steels are better than those of austenitic stainless steels. This point has been one of the problems when adopting a ferrite system for heat exchangers such as an EGR cooler and an exhaust heat recovery device. Further, with the spread of such heat exchangers, the price of parts has been reduced, and there is a demand for ferritic stainless steel which is inexpensive and has excellent brazing property. Further, there is a demand for a ferritic stainless steel having good properties not only in brazing property but also in corrosion resistance, workability, and toughness of Ni brazed joint portion.

The present invention has been proposed in view of such circumstances, and can be suitably used for members assembled by brazing joints such as heat exchangers, and has brazing properties, corrosion resistance, workability, and Ni brazing joints. An object of the present invention is to provide a ferritic stainless steel and a Ni brazed joint member having excellent toughness.

The gist of the present invention for solving the above problems is as follows.
[1] In mass%, C: 0.020% or less, Si: 1.0 or more to 3.5%, Mn: 0.02 to 0.80%, Cr: 10.5 to less than 15.0%, 2. Contains Nb: 0.03 to 0.60%, Al: 0.050% or less, N: 0.025% or less, has a chemical composition in which the balance is Fe and unavoidable impurities, and is at 30 ° C. A ferritic stainless steel having excellent brazing and corrosion resistance, characterized in that the pore corrosion potential V'c100 in a 5 mass% NaCl aqueous solution is 150 mV or more.
[2] The ferritic stainless steel having excellent brazing property and corrosion resistance according to [1], which is characterized by satisfying 16% or more with Cr + 1.8Si in terms of mass%.
[3] The ferritic stainless steel having excellent brazing and corrosion resistance according to [1], which has an oxide film satisfying Cr ≧ 11%, Si ≧ 1%, and Nb ≧ 1% in terms of cation fraction. steel.
[4] Further, in terms of mass%, Sn: 0.001 to 0.5%, Co: 0.01 to 0.5%, Bi: 0.001 to 0.01%, B: 0.0002 to 0. The first group consisting of any one or more of 005%,
Ni: 0.1 to 0.8%, Mo: 0.1 to 2%, W: 0.1 to 1%, V: 0.05 to 0.5%, Cu: 0.1 to 0.8% , Sb: 0.001 to 0.5%, Zr: 0.001 to 0.3%, Ga: 0.0001 to 0.01%, Ta: 0.0001 to 0.01%. Or the second group consisting of two or more types,
Of the third group consisting of any one or more of Ca: 0.0002 to 0.005%, Mg: 0.0002 to 0.005%, and REM: 0.005 to 0.1%. The ferrite-based stainless steel having excellent brazing property and corrosion resistance according to any one of [1] to [3], which is characterized by containing at least one of the groups.
[5] The brazing property and corrosion resistance according to any one of [1] to [4], which are brazed and joined using Ni brazing or Cu brazing, and are used for brazing joining members. Excellent ferritic stainless steel.
[6] When Ni brazing is performed and the cooling rate from the brazing temperature to 900 ° C. is set to 20 ° C./min or less, the Ni-rich phase ratio of the adjacent Ni brazing portion is 40% or more. The ferritic stainless steel having excellent brazing property and corrosion resistance according to any one of [1] to [4].
[7] The ferritic stainless steel having excellent brazing property and corrosion resistance according to any one of [1] to [6], which is characterized by being used in a heat exchanger.
[8] The brazing property and corrosion resistance according to any one of [1] to [6], which are used for EGR coolers, exhaust heat recovery devices, or fuel delivery parts, which are automobile parts. Excellent ferritic stainless steel.
[9] _{One of [1] to [6], which is characterized in that it is used as a CO 2} refrigerant heat pump type water heater, a secondary heat exchanger of a latent heat recovery type water heater, or a plate type heat exchanger. Ferrite-based stainless steel with excellent waxability and corrosion resistance as described.
[10] A Ni brazed joint member obtained by Ni brazing ferritic stainless steel having excellent brazing property and corrosion resistance according to any one of [1] to [4], and adjacent Ni brazing portions. A Ni brazed joint member characterized by having a Ni-rich phase ratio of 40% or more.

According to the present invention, it is possible to provide a ferritic stainless steel having excellent brazing property and corrosion resistance for a member assembled by brazing joining. The ferrite-based stainless steel of the present invention is used for EGR coolers, oil coolers, exhaust heat recovery devices, fuel delivery parts, etc. among automobile parts, and for hot water supply-related heat exchangers, latent heat recovery type water heaters for gas. It is suitable for secondary heat exchangers, electric plate type heat exchangers of EcoCute (registered trademark), and other members assembled by brazing with Ni brazing or Cu brazing.

About the evaluation method of a brazed part, it is a figure which shows before brazing heat treatment, (A) is a cross-sectional view taken along the line AA, (B) is a plan view. It is a figure which shows after the brazing heat treatment about the evaluation method of a brazing part, (A) is a cross-sectional view taken along the line AA, (B) is a plan view.

Hereinafter, embodiments of the present invention will be described in detail.

The present invention relates to a ferritic stainless steel having excellent brazing property and corrosion resistance. The brazing is intended for brazing using Ni brazing or Cu brazing, and is performed at 950 to 1200 ° C. in a vacuum or in a hydrogen atmosphere. At this time, argon gas, nitrogen gas, or the like may be used in combination for controlling or replacing the brazing atmosphere. Brazing is the process in which brazing is joined by wetting the base metal and filling a gap. If an oxide film is present on the surface of the base material during brazing, it becomes difficult for the wax to get wet, which hinders the brazing property.

A Cr-rich (Fe, Cr) oxide film (passivation film) is formed on the surface of the stainless steel, thereby exhibiting excellent corrosion resistance. It is necessary to remove this oxide film to ensure wettability, and brazing is performed under conditions of low vacuum or low dew point in order to reduce the oxide film. Specifically, it is carried out at a brazing temperature under a condition lower than the degree of vacuum at which Cr and Cr ₂ O _{3 are in equilibrium or the dew point.} Therefore, it was considered that the brazing property could be improved if the elements effective for the brazing property could be concentrated on the surface in the brazing atmosphere in which the oxide film was reduced. As a result, it was found that in the case of ferritic stainless steel having a relatively low Cr content, it is effective for brazing property to contain more than 1.0% of Si. The brazing property is improved as the amount of Si in the steel is increased, but the effect is saturated even if the content of Si exceeds 3.5%. The reason why Si improves brazing property has not been clarified, but Si has the effect of lowering the interfacial tension between ferritic stainless steel and brazing, and Si is an element that is easily dissolved in Ni brazing and Cu brazing. It is presumed that this is one of the factors that improved the brazing property.

Next, the corrosion resistance was examined. Although the outer surface side of the heat exchangers targeted in the present invention is exposed to a salt damage environment, it was determined that at least SUS430LX level salt damage corrosion resistance is required. Therefore, pitting potential measurement was adopted as a method for easily evaluating the order of salt damage corrosion resistance, and the effect of Si was examined. In order to obtain the SUS430LX level at the pitting potential V'c100 in a 3.5 mass% NaCl aqueous solution at 30 ° C., 150 mV or more was set when Ag / AgCl containing saturated KCl as an internal solution was used as a reference electrode. It is preferably 160 mV or more, and more preferably 170 mV or more. The measurement was performed in accordance with JIS G0577, and the ^{most noble potential with a current value exceeding 100 μA / cm 2} was defined as the pitting potential V'c100.
When the steel material before brazing heat treatment contains more than 1% Si, the pitting potential is improved not only by increasing the Cr content but also by increasing the Si content, and the effect is about 1. It was found that there were eight times as many. Therefore, in order to make the pitting corrosion potential 150 mV or more, this value can be satisfied if Cr + 1.8Si is 16% or more.

In order to investigate the reason why the pitting potential was improved by the increase in Si content, the passivation film on the surface of the steel before the brazing heat treatment was analyzed by X-ray photoelectron spectroscopy (XPS). A passivation film in which Si was concentrated was formed on the surface, and it was considered that the pitting potential was improved as a result of the improvement in the protection of the passivation film.

The above is related to the corrosion resistance of the material, but since it is practically used in a brazed state, the corrosion resistance after the brazing heat treatment is also important. It is preferable that both the steel material before the brazing heat treatment and the steel after the brazing heat treatment satisfy the above-mentioned pore corrosion potential, but either the material before the brazing heat treatment or the steel after the brazing heat treatment is used. It suffices to satisfy the pitting potential of 150 mV or more only by itself.
Heat treatment was performed for 10 minutes in a vacuum atmosphere of 50 Pa containing nitrogen at 1130 ° C., and evaluation was performed using the same pitting potential as described above. As a result, when the amounts of Cr and Si in the steel material satisfy 16% or more with Cr + 1.8Si, an oxide film having a composition satisfying Cr ≧ 11%, Si ≧ 1%, and Nb ≧ 1% in terms of cation fraction is surfaced. A pitting potential of 150 mV or more and equal to or higher than that of the material was obtained. If the atmosphere of the brazing heat treatment is 100 Pa or less, the pitting potential of the material before the heat treatment and the steel after the heat treatment when Cr + 1.8Si ≧ 16% is satisfied while containing the component of the present invention [1] described later. Can be 150 mV or more. However, even if the amounts of Cr and Si in the steel material are Cr + 1.8Si and less than 16%, the pitting potential of the steel after the brazing heat treatment can be set to 150 mV or more by devising the brazing heat treatment conditions. .. Specifically, when the atmosphere of the brazing heat treatment is 100 Pa or less, an oxide film having a composition satisfying Cr ≧ 11%, Si ≧ 1%, and Nb ≧ 1% in terms of cation fraction is formed on the surface, and a predetermined value is obtained. Can be the pitting potential of.

Furthermore, room temperature elongation was examined as one index of workability. Although it is not possible to evaluate workability only by normal temperature elongation, it was judged that at least 25% of normal temperature elongation is necessary. Since the room temperature elongation of 25% or more was obtained by setting Cr + 4Si to 26% or less, it is desirable to set Cr + 4Si to 26% or less.

Finally, the toughness of the brazed joint will be described. In the case of Ni wax, the Ni-rich primary phase (hereinafter referred to as "Ni-rich phase") first crystallizes from the boundary with the base metal during brazing, and then the Ni-rich phase and Cr-rich phase crystallize by the eutectic reaction. To form a eutectic part. The latter eutectic portion is brittle and serves as a starting point for fracture, and cracks occur during brazing, and as a result, the toughness tends to decrease. Further, if cracks occur during brazing, the cracked portion becomes a starting point of corrosion and becomes a factor of deteriorating corrosion resistance. Therefore, the toughness is ensured by minimizing the volume of the brazed portion by controlling the space between the brazed portions to be normally narrowly controlled. However, depending on the shape of the component, it may be difficult to control the clearance interval narrowly. Therefore, the inventors considered to increase the Ni-rich phase ratio of the brazed portion by increasing the Ni-rich phase of the primary crystal to ensure toughness. As described above, Si is an element that is easily dissolved in Ni wax, but it was found from the analysis results of the brazing part that Si is particularly easily dissolved in the Ni-rich phase. That is, by setting the Si content in the base metal to the content specified in the present invention, Si in the base metal diffuses into the Ni brazing material and dissolves in the Ni brazing material to increase the Ni-rich phase. This is to contribute to improving the toughness of the joint. The Ni-rich phase ratio of the primary crystal so that cracks do not occur during brazing is preferably 40% or more, and more preferably 45% or more. The amount of Si in the Ni-rich phase is preferably 0.7 × (Si amount of Ni brazing material) + 0.5% or more, and is 0.7 × (Si amount of Ni brazing material) + 1% or more. Is more preferable.

The present invention provides a ferritic stainless steel having excellent brazing property, which has been made in consideration of the above studies, and the gist thereof is as described in the claims.

Hereinafter, the reasons for limiting each composition of the ferritic stainless steel having excellent brazing property and corrosion resistance will be described. In the following description, unless otherwise specified,% of each component represents mass%.

(C: 0.020% or less)
C is an element useful for ensuring strength, but since excessive addition reduces intergranular corrosion resistance, the content of C is set to 0.020% or less. It is preferably 0.002% or more and 0.018% or less.

(Si: over 1.0 to 3.5%)
Si is the most important element in the present invention and improves brazing property and corrosion resistance. It is also effective in oxidation resistance, and it is necessary to contain it in excess of 1.0%. It is preferably 1.1% or more, more preferably 1.2% or more. However, excessive addition saturates the effect on brazing property and lowers weldability and workability, so the Si content is set to 3.5% or less. It is preferably 3.2% or less, more preferably 2.9% or less.

(Mn: 0.02 to 0.80%)
Mn is a useful element as a deoxidizing element, and it is necessary to contain it at least 0.02% or more. Preferably, it is 0.05% or more. However, if it is excessively contained, the corrosion resistance is deteriorated, so the Mn content is set to 0.80% or less. It is preferably 0.70% or less, more preferably 0.60% or less.

(Cr: 10.5 to less than 15.0%)
Cr is an element that is a basis for ensuring corrosion resistance. Therefore, the Cr content is required to be at least 10.5% or more. It is preferably 11.0% or more, more preferably 11.5% or more, still more preferably 12.5% or more. Corrosion resistance can be improved as the Cr content is increased, but it is set to less than 15.0% in order to reduce processability. It is preferably 14.8% or less, more preferably 14.5% or less.

(Nb: 0.03 to 0.60%)
Nb is contained in an amount of 0.03% or more because C and N are fixed, the intergranular corrosion resistance of the welded portion is improved, and the high temperature strength is improved. It is preferable to contain 8 or more in Nb / C + N, and more preferably 10 or more in Nb / C + N. However, since excessive addition reduces weldability, the upper limit of the Nb content is set to 0.60%. It is preferably 0.50% or less, more preferably 0.45% or less.

(Al: 0.050% or less)
Al is an element useful for refining because it has a deoxidizing effect and the like, but its content needs to be limited to 0.050% or less in order to deteriorate the brazing property which is the most important in the present invention. It is preferably 0.002 or more and 0.03% or less, more preferably 0.003% or more and 0.015% or less.

(N: 0.025% or less)
N is an element useful for strength and pitting corrosion resistance, but the content of N is 0.025% or less because excessive addition reduces intergranular corrosion resistance. It is preferably 0.002 to 0.023%, more preferably 0.003 to 0.020%.

Further, if necessary, it is preferable to contain the following components.

(Sn: 0.001 to 0.5%)
Sn can be contained in an amount of 0.001% or more, if necessary, in order to improve the brazing property. The addition of Sn is also effective in improving corrosion resistance. It is more preferably 0.01% or more, still more preferably 0.05% or more. However, excessive addition lowers manufacturability and toughness, so it is preferable to add 0.5% or less. It is more preferably 0.3% or less, still more preferably 0.25% or less.

(Co: 0.01-0.5%)
Co can be contained in an amount of 0.01% or more, if necessary, in order to improve the brazing property. More preferably, it is 0.03% or more. Excessive addition leads to cost increase, so it is preferable to contain 0.5% or less. More preferably, it is 0.4% or less.

(Bi: 0.001 to 0.01%)
Bi can be contained in an amount of 0.001% or more, if necessary, in order to improve the brazing property. More preferably, it is 0.002% or more. Since excessive addition reduces the manufacturability, it is preferable to contain 0.01% or less. More preferably, it is 0.008% or less.

(B: 0.0002 to 0.005%)
B can be contained in an amount of 0.0002% or more, if necessary, in order to improve the brazing property. The addition of B is also effective in improving the secondary workability. More preferably, it is 0.0004% or more. However, since excessive addition lowers the intergranular corrosion resistance, it is preferable to contain 0.005% or less. More preferably, it is 0.004% or less.

(Ni: 0.1 to 0.8%)
Ni can be contained in an amount of 0.1% or more and 0.8% or less, if necessary, in order to improve the corrosion resistance. Excessive addition increases costs. It is preferably 0.2% or more and 0.7% or less, more preferably 0.25% or more and 0.6% or less.

(Mo: 0.1 to 2%)
Mo can be contained in an amount of 0.1% or more and 2% or less, if necessary, in order to improve the strength and corrosion resistance. Excessive addition increases costs. It is preferably 0.2% or more and 1.5% or less, more preferably 0.3% or more and 0.9% or less.

(W: 0.1 to 1%)
W can be contained in an amount of 0.1% or more and 1% or less, if necessary, in order to improve the strength and corrosion resistance. Excessive addition increases costs. It is preferably 0.2% or more and 0.9% or less.

(V: 0.05 to 0.5%)
V can be contained in an amount of 0.05% or more, if necessary, in order to improve the corrosion resistance. Excessive addition deteriorates processability and is expensive, which leads to cost increase. Therefore, it is preferable to add 0.5% or less.

(Cu: 0.1 to 0.8%)
Cu can be contained in an amount of 0.1% or more, if necessary, in order to improve the corrosion resistance. It is preferably 0.2% or more, more preferably 0.3% or more. Excessive addition deteriorates processability, so it is preferable to add 0.8% or less. It is preferably 0.7% or less, more preferably 0.6% or less.

(Sb: 0.001 to 0.5%)
Since Sb is an element that improves the total corrosion resistance, 0.001% or more may be contained as necessary. However, if the Sb content exceeds 0.5%, the cost increases. Therefore, the Sb content is set to 0.5% or less. The Sb content is preferably 0.3% or less. In order to stably obtain the above effects, the Sb content is preferably 0.005% or more, and more preferably 0.01% or more.

(Zr: 0.001 to 0.3%)
Since Zr is an element that improves corrosion resistance, 0.001% or more may be contained as needed. However, if the Zr content exceeds 0.3%, the cost increases. Therefore, the Zr content is set to 0.3% or less. The Zr content is preferably 0.2% or less. In order to stably obtain the above effects, the Zr content is preferably 0.01% or more, and more preferably 0.02% or more.

(Ga: 0.0001 to 0.01%)
Since Ga is an element that improves corrosion resistance and hydrogen embrittlement resistance, it may be contained if necessary. However, if the Ga content exceeds 0.01%, the cost increases. Therefore, the Ga content is set to 0.01% or less. The Ga content is preferably 0.005% or less. In order to stably obtain the above effects, the Ga content is preferably 0.0001% or more, and more preferably 0.0005% or more.

(Ta: 0.0001 to 0.01%)
Since Ta is an element that improves corrosion resistance, it may be contained if necessary. However, if the Ta content exceeds 0.01%, the cost increases. Therefore, the Ta content is set to 0.01% or less. The Ta content is preferably 0.005% or less. In order to stably obtain the above effects, the Ta content is preferably 0.0001% or more, and more preferably 0.0005% or more.

(Ca: 0.0002 to 0.005%)
Ca is an element useful for refining such as a deoxidizing effect and is effective for hot workability, so that it can be contained in an amount of 0.0002% or more and 0.005% or less, if necessary. Preferably, it is 0.0005% or more. Further, it is preferably 0.003% or less.

(Mg: 0.0002 to 0.005% or less)
Since Mg has a deoxidizing effect and is an element useful for refining, it can be contained in an amount of 0.0002% or more and 0.005% or less, if necessary. It is preferably 0.0004% or more. Further, it is preferably 0.002% or less.

(REM: 0.005 to 0.1%)
REM is an element useful for refining because it has a deoxidizing effect and the like, and is also useful for brazing property and oxidation resistance. Therefore, it is contained in an amount of 0.005% or more and 0.1% or less as necessary. be able to. It is preferably 0.008% or more. Further, it is preferably 0.08% or less.

Of the unavoidable impurities, P is preferably 0.05% or less, more preferably 0.04% or less, from the viewpoint of weldability. Further, S is preferably 0.02% or less, more preferably 0.01% or less, from the viewpoint of corrosion resistance.

In addition to the elements described above, they can be contained within a range that does not impair the effects of the present invention. It is preferable to reduce Zn, Pb, Se, H, etc. as much as possible, including the above-mentioned P and S, which are general impurity elements. On the other hand, the content ratio of these elements is controlled to the extent that the problem of the present invention is solved, and if necessary, one or more of Zn ≦ 100 ppm, Pb ≦ 100 ppm, Se ≦ 100 ppm, and H ≦ 100 ppm can be used. contains.

The ferritic stainless steel of the present invention is basically manufactured by taking a general process for manufacturing stainless steel. For example, molten steel having the above chemical composition is prepared in an electric furnace, refined in an AOD furnace or a VOD furnace to form steel pieces by a continuous casting method or an ingot method, and then hot-rolled-annealed hot-rolled plate-acid. Manufactured through the steps of washing-cold rolling-finish annealing-pickling. If necessary, annealing of the hot-rolled sheet may be omitted, or cold rolling-finish annealing-pickling may be repeated.

Since the ferrite-based stainless steel of the present invention is particularly brazed, it is preferable to braze it with Ni brazing or Cu brazing.
The ferritic stainless steel of the present invention has a Ni-rich phase ratio of 40% or more in the adjacent Ni brazed portion when Ni brazing is performed and the cooling rate from the brazing temperature to 900 ° C. is 20 ° C./min or less. This is preferable because the toughness of the Ni brazed joint member after brazing becomes good.
Since the ferrite-based stainless steel of the present invention is excellent in brazing property, corrosion resistance, and workability by Ni brazing and Cu brazing, heat exchangers, EGR coolers which are automobile parts, exhaust heat recovery devices, fuel delivery parts, CO ₂ It is preferable to use it for the purpose of a refrigerant heat pump type water heater, a secondary heat exchanger of a latent heat recovery type water heater, or a plate type heat exchanger.
The Ni brazed joint member of the present invention is preferable because the toughness of the Ni brazed joint member after brazing becomes good when the Ni-rich phase ratio of the adjacent Ni brazed portion is 40% or more.

Hereinafter, the effects of the present invention will be made clearer by examples. The present invention is not limited to the following examples, and can be appropriately modified and implemented without changing the gist thereof.

Ferrite-based stainless steel having the composition shown in Table 1 is melted in a 180 kg vacuum melting furnace, cast into a 45 kg ingot, and then subjected to the steps of hot rolling-hot rolling plate annealing-shot-cold rolling-finish annealing, and the plate thickness is 1 mm. Cold-rolled steel sheet was produced. The hot-rolled plate was produced by rolling to a plate thickness of 5 mm at a material thickness of 50 mm and a heating temperature of 1200 ° C. and air-cooling. The hot-rolled sheet annealing and finish annealing conditions were air-cooled for × 1 minute. A test piece was cut out from the obtained cold-rolled steel sheet and used for evaluation of wax spreadability and measurement of pitting corrosion potential.

[Pitting potential]
15 mm in width from the cold-rolled steel sheet (No. 1 to 18) and the plate (No. A1 to A18, hereinafter referred to as "brazing heat-treated steel sheet") obtained by subjecting the cold-rolled steel sheet to the same heat treatment as the brazing spreadability evaluation below. A test piece having a length of 20 mm was cut out and wet-polished to # 600 with emery paper. The periphery was coated with resin so that the central portion 10 mm × 10 mm of the plate was exposed, and used as an electrode for measurement. The pitting potential (V'c100: the most noble potential with a ^{current value exceeding 100 μA / cm 2} ) was measured in a 3.5 mass% NaCl aqueous solution at 30 ° C. according to JIS G0577. As the reference electrode, Ag / AgCl containing saturated KCl as an internal solution was used, and the sweep rate of the potential was set to 20 mV / min. The number of test pieces was 5, and the average value of the obtained pitting potentials was used for evaluation.

[Normal temperature tensile test]
After collecting JIS 13B tensile test pieces from a direction parallel to the rolling direction of the cold-rolled steel sheets (No. 1 to 18), a normal temperature tensile test was performed to measure the total elongation.

[Deaf spreadability]
A test piece having a width of 40 mm and a length of 40 mm was cut out from the cold-rolled steel sheet and wet-polished to # 600 with emery paper. After degreasing with an organic solvent, 0.1 g of Ni wax (BNi-5 system) and Cu wax (BCu-1) were placed in the center of the plate, placed in a vacuum furnace, and heated at 1130 ° C. for 10 minutes. Nitrogen was used as the carrier gas, and No. A1 to No. The degree of vacuum of A16 is about 50 Pa, No. No. which is the same material as A1. The degree of vacuum of A17 was about 500 Pa, and the degree of vacuum of No. A18 was about 5 Pa.
After the heating was completed, the material was cooled, and the wax area after the heat treatment was determined by image analysis. Based on the obtained wax area, the wax spread coefficient was calculated from the following formula. The number of test pieces was set to 3, and evaluation was performed using the average wax spread coefficient.
Wax spread coefficient = wax area after heat treatment / initial wax area

[Oxide film analysis]
The oxide film on the surface of the plate (brazing heat-treated steel sheet) subjected to the same heat treatment as the wax spreadability evaluation was analyzed by X-ray photoelectron spectroscopy (XPS). XPS was manufactured by ULVAC PHI, and was carried out under the conditions that the X-ray source used was mono-AlKα, the X-ray beam diameter was about 200 μm, and the extraction angle was 45 degrees. From the results of quantitative analysis on the outermost surface, the cation fractions of Cr, Si and Nb in the oxide film were determined. Here, the cation fractions of Cr, Si and Nb were taken as the cation fractions in the oxide state.

Table 2 shows the results of the pitting corrosion potential and total elongation of the cold-rolled steel sheets (Nos. 1 to 18), showing the pitting corrosion potential, the brazing spread coefficient, and the oxide film of the brazing heat-treated steel sheets (No. A1 to A18). The cation fraction was shown. Here, as the achievement target of the wax spread coefficient in the present invention, it is set to 15 or more in the case of Ni wax and 10 or more in the case of Cu wax. The target for total growth is 25% or more.

From Table 2, No. 1 within the scope of the present invention. 1-No. 14, No. 17 (cold-rolled steel sheet, however, as shown in Tables 1 and 2, No. 1 and No. 17 have the same components and the same characteristics) has good characteristics in pitting potential and room temperature elongation (total elongation). Shown.

From Table 3, No. 1 within the scope of the present invention. A1 to No. Since A14 (brazing heat-treated steel sheet) has a good brazing spread coefficient, No. 1 within the scope of the present invention. 1-No. It can be confirmed that 14 (cold-rolled steel sheet) has good brazing spreadability when brazed and heat-treated. Further, it can be confirmed that the pitting potential after the brazing heat treatment is also good by performing the heat treatment under appropriate brazing heat treatment conditions (atmosphere with a vacuum degree of about 50 Pa).

From Tables 2 and 3, No. 1 in which Si is below the lower limit of the scope of the present invention. 15 (cold-rolled steel sheet), No. It can be seen that A15 (brazing heat-treated steel sheet) is inferior in pitting corrosion resistance because it has a low pitting potential. In addition, No. Reference numeral 15 (cold-rolled steel sheet) is No. 15 in Table 3. Since the brazing coefficient of A15 (brazing heat-treated steel sheet) is low, it can be seen that the brazing property is inferior.

From Tables 2 and 3, No. 1 in which Si is above the upper limit of the present invention. It can be seen that 16 (cold-rolled steel sheet) has good brazing spreadability and pitting corrosion resistance before and after brazing heating, but is inferior in room temperature elongation.

No. 17 (cold-rolled steel sheet) satisfies Cr + 1.8Si ≧ 16%, so that No. 17 (cold-rolled steel sheet) is No. Since it is the same as 1 (cold-rolled steel sheet), the pitting corrosion potential satisfies the present invention. On the other hand, because the brazing heat treatment conditions are inappropriate (atmosphere with a vacuum degree of about 500 Pa), No. In A17 (brazing heat-treated steel sheet), the Si and Nb cation fractions in the oxide film are lowered, and as a result, the pitting corrosion potential is out of the range of the present invention. A17 (brazing heat-treated steel sheet) is used as a reference example.

Since No. 18 (cold-rolled steel sheet) has Cr + 1.8Si <16%, the pitting potential is inferior to the specification of the present invention at the stage of the material (cold-rolled steel sheet), so it is used as a reference example. , No. In A18 (brazing heat-treated steel plate), the pitting corrosion potential is improved as a result of improving the Cr, Si, and Nb cation fractions in the oxide film by optimizing the brazing heat treatment conditions (atmosphere with a degree of vacuum of about 5 Pa). The present invention is satisfied after brazing heat treatment.

[Cross-section observation of brazed part]
No. 1 and No. A large plate 1 having a width of 30 mm and a length of 50 mm and a small plate 2 having a width of 15 mm and a length of 30 mm were cut out from 15 cold-rolled steel plates one by one. Both the large plate 1 and the small plate 2 were wet-polished to # 600 using emery paper and then degreased with an organic solvent. As shown in FIG. 1, a stainless foil 3 having a thickness of 100 μm was laminated between the large plate 1 and the small plate 2, and a gap 4 having a thickness of 100 μm was formed between the large plate 1 and the small plate 2. In this state, 5 g of Ni wax 5 (BNi-5 system) was applied to the long side of the small plate 2. Then, it was put in a vacuum furnace and heated at 1130 ° C. for 10 minutes. The degree of vacuum was about 50 Pa, and the cooling rate from 1130 ° C to 900 ° C was 20 ° C / min. Of these, No. For No. 1, the cooling rate from 1130 ° C. to 900 ° C. was also performed under the condition of 22 ° C./min. After the heat treatment was completed, as shown in FIG. 2, the cross section of the gap portion where the wax was wet and spread (see FIG. 2 (A)) was observed, and the presence or absence of voids and cracks in the wax spread portion 6 and the Ni-rich phase ratio were determined. The wax spreading portion 6 has a primary crystal Ni-rich phase and a eutectic portion, and the Ni-rich phase ratio (area ratio) was obtained by binarizing the brazing portion 6 by image analysis.

As a result of cross-sectional observation, when the cooling rate from 1130 ° C. to 900 ° C. was 20 ° C./min, No. The Ni-rich phase ratio in No. 1 was 45%, which was within the range of the present invention, and no voids or cracks were observed. Good toughness with no voids or cracks. On the other hand, No. The Ni-rich phase ratio at 15 was 37%, and voids and cracks were observed. On the other hand, No. 1 having a cooling rate of 22 ° C./min from 1130 ° C. to 900 ° C. The Ni-rich phase ratio in 1 was 36%, and voids and cracks were observed.

The ferrite-based stainless steel with excellent brazing property of the present invention includes EGR coolers and exhaust heat collectors for automobile parts, secondary heat exchangers for latent heat recovery type water heaters, and plate heat exchangers from EcoCute (registered trademark). It is suitable as a material for heat exchangers assembled by brazing.

1 Large plate 2 Small plate 3 Stainless steel foil 4 Clearance 5 Ni wax 6 Wax spread part

Claims (15)

By mass%, C: 0.020% or less, Si: 1.0 or more to 3.5%, Mn: 0.02 to 0.80%, Cr: 10.5 to less than 15.0%, Nb: 0 .03 to 0.60%, Al: 0.050% or less, N: 0.025% or less, the balance has a chemical composition consisting of Fe and unavoidable impurities, 3.5% by mass at 30 ° C. The perforation potential V'c100 in the aqueous NaCl solution was 150 mV or more when Ag / AgCl containing saturated KCl as the internal solution was used as the reference electrode.
A ferritic stainless steel having excellent brazing and corrosion resistance, which has an oxide film satisfying Cr ≧ 11%, Si ≧ 1%, and Nb ≧ 1% in terms of cation fraction.
Here, the cation fraction means the ratio (atomic %) of cations existing in the oxide state to all cations. The ferritic stainless steel having excellent brazing property and corrosion resistance according to claim 1, wherein Cr + 1.8Si is satisfied with 16% or more in mass%. Further, in mass%, Sn: 0.001 to 0.5%, Co: 0.01 to 0.5%, Bi: 0.001 to 0.01%, B: 0.0002 to 0.005%. The first group consisting of any one or more of them,
Ni: 0.1 to 0.8%, Mo: 0.1 to 2%, W: 0.1 to 1%, V: 0.05 to 0.5%, Cu: 0.1 to 0.8% , Sb: 0.001 to 0.5%, Zr: 0.001 to 0.3%, Ga: 0.0001 to 0.01%, Ta: 0.0001 to 0.01%. Or the second group consisting of two or more types,
Of the third group consisting of any one or more of Ca: 0.0002 to 0.005%, Mg: 0.0002 to 0.005%, and REM: 0.005 to 0.1%. The ferrite-based stainless steel having excellent brazing property and corrosion resistance according to claim 1 or 2, wherein it contains at least one of the groups. The ferrite having excellent brazing property and corrosion resistance according to any one of claims 1 to 3, which is brazed and joined using Ni brazing or Cu brazing and is used for a brazed joining member. Ferritic steel. By mass%, C: 0.020% or less, Si: 1.0 or more to 3.5%, Mn: 0.02 to 0.80%, Cr: 10.5 to less than 15.0%, Nb: 0 .03 to 0.60%, Al: 0.050% or less, N: 0.025% or less, the balance has a chemical composition consisting of Fe and unavoidable impurities, 3.5% by mass at 30 ° C. The perforation potential V'c100 in the aqueous NaCl solution was 150 mV or more when Ag / AgCl containing saturated KCl as the internal solution was used as the reference electrode.
Brazing is characterized in that when Ni brazing is performed and the cooling rate from the brazing temperature to 900 ° C. is 20 ° C./min or less, the Ni-rich phase ratio of the adjacent Ni brazing portion is 40% or more. Ferritic stainless steel with excellent properties and corrosion resistance. The ferritic stainless steel having excellent brazing property and corrosion resistance according to claim 5, wherein Cr + 1.8Si is satisfied with 16% or more in mass%. The ferrite-based stainless steel according to claim 5, which has an oxide film satisfying Cr ≧ 11%, Si ≧ 1%, and Nb ≧ 1% in terms of cation fraction, and has excellent brazing and corrosion resistance.
Here, the cation fraction means the ratio (atomic %) of cations existing in the oxide state to all cations. Further, in mass%, Sn: 0.001 to 0.5%, Co: 0.01 to 0.5%, Bi: 0.001 to 0.01%, B: 0.0002 to 0.005%. The first group consisting of any one or more of them,
Ni: 0.1 to 0.8%, Mo: 0.1 to 2%, W: 0.1 to 1%, V: 0.05 to 0.5%, Cu: 0.1 to 0.8% , Sb: 0.001 to 0.5%, Zr: 0.001 to 0.3%, Ga: 0.0001 to 0.01%, Ta: 0.0001 to 0.01%. Or the second group consisting of two or more types,
Of the third group consisting of any one or more of Ca: 0.0002 to 0.005%, Mg: 0.0002 to 0.005%, and REM: 0.005 to 0.1%. The ferrite-based stainless steel having excellent brazing property and corrosion resistance according to any one of claims 5 to 7, which contains at least one of the groups. The ferrite-based stainless steel having excellent brazing property and corrosion resistance according to any one of claims 1 to 8, characterized in that it is used for a heat exchanger. The ferrite having excellent brazing property and corrosion resistance according to any one of claims 1 to 8, which is used for an EGR cooler, an exhaust heat recovery device, or a fuel delivery type component which is an automobile part. Ferritic steel. The wax according to any one of claims 1 to 8, characterized in that it is used as a secondary heat exchanger of a CO _{2 refrigerant heat pump type water heater, a latent heat recovery type water heater, or a plate type heat exchanger.} Ferritic stainless steel with excellent adhesion and corrosion resistance. By mass%, C: 0.020% or less, Si: 1.0 or more to 3.5%, Mn: 0.02 to 0.80%, Cr: 10.5 to less than 15.0%, Nb: 0 .03 to 0.60%, Al: 0.050% or less, N: 0.025% or less, the balance has a chemical composition consisting of Fe and unavoidable impurities, 3.5% by mass at 30 ° C. The pore corrosion potential V'c100 in the NaCl aqueous solution is 150 mV or more when Ag / AgCl with saturated KCl as the internal solution is used as the reference electrode. A Ni brazing joint member which is a brazing joint member and has a Ni-rich phase ratio of 40% or more in an adjacent Ni brazing portion. The Ni brazed joint member according to claim 12, wherein the ferritic stainless steel satisfies 16% or more of Cr + 1.8Si in terms of mass%. The Ni brazed joint member according to claim 12, wherein the ferritic stainless steel has an oxide film satisfying Cr ≧ 11%, Si ≧ 1%, and Nb ≧ 1% in terms of cation fraction.
Here, the cation fraction means the ratio (atomic %) of cations existing in the oxide state to all cations. The ferrite-based stainless steel further contains, in terms of mass%, Sn: 0.001 to 0.5%, Co: 0.01 to 0.5%, Bi: 0.001 to 0.01%, B: 0. The first group consisting of any one or more of 0002 to 0.005%,
Ni: 0.1 to 0.8%, Mo: 0.1 to 2%, W: 0.1 to 1%, V: 0.05 to 0.5%, Cu: 0.1 to 0.8% , Sb: 0.001 to 0.5%, Zr: 0.001 to 0.3%, Ga: 0.0001 to 0.01%, Ta: 0.0001 to 0.01%. Or the second group consisting of two or more types,
Of the third group consisting of any one or more of Ca: 0.0002 to 0.005%, Mg: 0.0002 to 0.005%, and REM: 0.005 to 0.1%. The Ni brazed joining member according to any one of claims 12 to 14, characterized in that it contains at least one of the groups.

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