JP2018172735A - FERRITIC STAINLESS STEEL EXCELLENT IN BRAZABILITY AND CORROSION RESISTANCE, AND Ni BRAZED JOINT MEMBER - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ferritic stainless steel that can suitably be used as a member to be assembled by brazing joining such as a heat exchanger or the like, and is excellent in brazability, corrosion resistance, processability and toughness of Ni brazed joint portion, and to provide a Ni brazed joint member.SOLUTION: Provided is a ferritic stainless steel excellent in brazability and corrosion resistance characterized in having a chemical composition containing, by mass%, C: 0.020% or less, Si: 1.0 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, and N: 0.025% or less and the balance is composed of Fe and unavoidable impurities, and in which the pitting potential V'c100 in a 3.5 mass% NaCl aqueous solution at 30°C is 150 mV or more.SELECTED DRAWING: None

Description

The present invention relates to a ferritic stainless steel and a Ni brazed joint member that are excellent in brazeability and corrosion resistance used for members assembled by brazing joint. In particular, it is intended for parts that are assembled by brazing joints using Ni brazing and Cu brazing. For automobiles, EGR (Exhaust Gas Recirculation) coolers, oil coolers, exhaust heat recovery devices, and fuel delivery parts are listed. It is done. In the field of water heaters, secondary heat exchangers for latent heat recovery type gas water heaters and heat exchangers for CO ₂ refrigerant heat pump water heaters (commonly known as Ecocute (registered trademark)) can be mentioned.

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

  Among these, the EGR cooler aims to reduce NOx, which is a toxic gas, by cooling the exhaust gas of the engine using engine cooling water and then returning it to the intake side to re-combust it to lower the combustion temperature. . The exhaust heat recovery unit is a system that heats engine cooling water with exhaust gas and uses it to warm up the heater and the 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 consumption, especially in winter.

Furthermore, in the field of hot water supply equipment, the application of heat exchangers has been expanded with the spread of environment-friendly equipment. In gas water heaters, in order to recover latent heat from high-temperature exhaust gas of about 150 to 200 ° C., which has been exhausted as it is, a latent heat recovery type gas water heater to which a secondary heat exchanger made of stainless steel is added has been widely used. It is out. In addition, the electric water heater has also been a type with a built-in heater in the past, but switching to a CO ₂ refrigerant heat pump water heater that can reduce electric energy to 1/3 or less; commonly known as Ecocute (registered trademark), A heat exchanger is also used here.

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

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

  As materials used for heat exchangers, austenitic stainless steels such as SUS304 and SUS316L are often used taking advantage of their corrosion resistance and strength, but recently, the use of inexpensive ferritic stainless steels with a low coefficient of thermal expansion is increasing. I am doing.

  On the other hand, good corrosion resistance is also required for brazed joints. However, since Ni brazing is superior to Cu brazing in this respect, it is suitable for applications in which corrosion resistance is regarded as important. In addition, the joint portion is required to have strength and toughness, but the brazed joint portion formed by Ni brazing has a problem that it is inferior in toughness compared to Cu brazing.

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

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

  In Patent Document 5, C: 0.080% or less, Si: 1.2-3.0%, Mn: 0.4-2.0%, 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: 0.0. 002-0.10%, N: 0.030-0.150%, 1.6 ≦ Cu × Si ≦ 4.4 and 0.16 ≦ 2N + Mo ≦ 1.0 Austenitic stainless steel is disclosed.

  In Patent Document 6, 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 brazeability containing at least one of Ti, Ta, and Nb is disclosed.

  In Patent Document 7, C: 0.03% or less, Si: 0.02-1.5%, Mn: 0.02-1.5%, Cr: 10-22%, Al: 0.5% or less , N: 0.05% or less, C + N: 0.015% or more, and further excellent ferrite brazing properties satisfying Ti-3N ≦ 0.03 and 10 (Ti-3N) + Al ≦ 0.5 Stainless steel is disclosed.

  In Patent Document 8, as ferritic stainless steel suitable as a heat exchanger member used for Ni brazing and Cu brazing, C: 0.03% or less, Si: 3% or less, Mn: 2% or less, P : 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 Nb- (C × 92.9 / 12 + N × 92.9 / 14) Ferritic stainless steel satisfying ≧ 0.10 is disclosed.

  In Patent Document 9, C: 0.03% or less, Si: more than 0.1 to 3%, Mn: 0.1 to 2%, Cr: 10 to 35%, Nb: 0.2 to 0.8% , N: 0.03% or less, and disclosed is 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% ing.

  Patent Document 10 includes 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, Ti: 0.002 to 0.030% and / or Al: 0.002 to 0.10%, N: 0.001 to 0.4%, Si / (Ti + Al) ≧ 40 Stainless steel excellent in brazing property that satisfies the requirements 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 a powdered Ni solder, wettability is good and continuous brittleness is achieved. A Ni brazing material capable of preventing the occurrence of cracks without causing a chemical phase is disclosed.

JP 2004-205059 A Japanese Patent Laid-Open No. 2004-1000059 JP 2005-171938 A JP 2008-202846 A JP 2012-207259 A Japanese Patent Laid-Open No. 57-60056 JP 2009-174046 A JP 2009-299182 A JP 2010-285683 A International Publication No. 2016/152854 Japanese Patent Application Laid-Open No. 11-114692

  As materials used for heat exchangers are switched from austenitic stainless steel to ferritic stainless steel, there is an increasing demand for improved brazing performance of ferritic stainless steel. When compared with various stainless steels for brazing, it can be said that the brazing of ferritic stainless steel is not as good as that of austenitic stainless steel. This was one of the problems when adopting a ferrite system for heat exchangers such as EGR coolers and exhaust heat recovery devices. In addition, with the widespread use of such heat exchangers, cost reduction of components has been directed, and there is a demand for ferritic stainless steel that is inexpensive and has excellent brazeability. Furthermore, ferritic stainless steel having good properties not only for brazing but also for corrosion resistance, workability, and toughness of Ni brazed joints is required.

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

The gist of the present invention aimed at solving the above problems is as follows.
[1] By mass%, C: 0.020% or less, Si: more than 1.0 to 3.5%, Mn: 0.02 to 0.80%, Cr: 10.5 to less than 15.0%, 2. Nb: 0.03 to 0.60%, Al: 0.050% or less, N: 0.025% or less, with the balance being Fe and an unavoidable impurity, and a chemical composition of 30 ° C. A ferritic stainless steel excellent in brazing and corrosion resistance, characterized in that the pitting corrosion potential V′c100 in a 5 mass% NaCl aqueous solution is 150 mV or more.
[2] Ferritic stainless steel excellent in brazeability and corrosion resistance according to [1], characterized by satisfying 16% or more of Cr + 1.8Si by mass%.
[3] Ferritic stainless steel excellent in brazeability and corrosion resistance according to [1], characterized by having an oxide film satisfying Cr ≧ 11%, Si ≧ 1%, and Nb ≧ 1% in cation fraction steel.
[4] Further, by mass%, Sn: 0.001 to 0.5%, Co: 0.01 to 0.5%, Bi: 0.001 to 0.01%, B: 0.0002 to 0. 1st group consisting of any one or more of 005%,
Ni: 0.1-0.8%, Mo: 0.1-2%, W: 0.1-1%, V: 0.05-0.5%, Cu: 0.1-0.8% , 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 a second group consisting of two or more species,
Of the third group consisting of one or more of Ca: 0.0002-0.005%, Mg: 0.0002-0.005%, REM: 0.005-0.1%, The ferritic stainless steel having excellent brazeability and corrosion resistance according to any one of [1] to [3], comprising at least one of the groups.
[5] Brazing and corrosion resistance according to any one of [1] to [4], wherein the brazing joint member is brazed using Ni brazing or Cu brazing. Excellent ferritic stainless steel.
[6] Characteristically, 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. The ferritic stainless steel excellent in brazeability and corrosion resistance according to any one of [1] to [4].
[7] The ferritic stainless steel having excellent brazeability and corrosion resistance according to any one of [1] to [6], which is used for a heat exchanger.
[8] Brazing and corrosion resistance according to any one of [1] to [6], characterized by being used for an EGR cooler, an exhaust heat recovery unit, or a fuel delivery system part that is an automobile part Excellent ferritic stainless steel.
[9] Any one of [1] to [6], characterized by being 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. Ferritic stainless steel with excellent brazing and corrosion resistance.
[10] A Ni brazed joint member obtained by brazing the ferritic stainless steel excellent in brazing and corrosion resistance according to any one of [1] to [4], and adjacent Ni brazed portions Ni brazing joining member characterized by having a Ni-rich phase ratio of 40% or more.

  ADVANTAGE OF THE INVENTION According to this invention, the ferritic stainless steel excellent in brazing property and corrosion resistance can be provided for the members assembled by brazing joining. The ferritic stainless steel of the present invention is an EGR cooler, oil cooler, exhaust heat recovery device and fuel delivery system component among automotive parts, and as a heat exchanger related to hot water supply, a latent heat recovery type water heater is used for gas. In terms of electricity, it is suitable for other members that are assembled by brazing joint using Ni brazing or Cu brazing, such as an Ecocute (registered trademark) plate heat exchanger.

It is a figure which shows before brazing heat processing about the evaluation method of a brazing part, (A) is AA arrow sectional drawing, (B) is a top view. It is a figure which shows after brazing heat processing about the evaluation method of a brazing part, (A) is AA arrow sectional drawing, (B) is a top view.

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

  The present invention relates to a ferritic stainless steel having excellent brazeability 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 a process in which the solder is joined to the base metal by filling the gap. If an oxide film is present on the surface of the base material during brazing, the brazing will be difficult to wet and the brazing property will be hindered.

On the surface of stainless steel, a Cr-rich (Fe, Cr) oxide film (passive film) is formed, thereby exhibiting excellent corrosion resistance. In order to ensure wettability, it is necessary to remove this oxide film. In order to reduce the oxide film, brazing is performed under conditions of a vacuum degree or a low dew point. Specifically, it is carried out under a condition lower than the degree of vacuum or the dew point at which at least Cr and Cr ₂ O ₃ equilibrate at the brazing temperature. Therefore, it was considered that the brazing property can be improved if an element effective for brazing can be concentrated on the surface in the brazing atmosphere in which the oxide film is reduced. As a result, it has been found that in the case of a ferritic stainless steel having a relatively low Cr content, it is effective for brazing to contain Si exceeding 1.0%. The brazing property is improved as the amount of Si in the steel is increased, but the effect is saturated even if Si exceeding 3.5% is contained. The reason why Si improves brazeability is not clarified, but Si has an 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 reasons for improving brazing.

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 judged that at least SUS430LX level salt corrosion resistance is necessary. Therefore, pitting potential measurement was adopted as a method for easily evaluating the order of salt corrosion resistance, and the influence of Si was examined. In order to obtain a SUS430LX level at a pitting corrosion potential V′c100 in a 3.5 mass% NaCl aqueous solution at 30 ° C., 150 mV or more was obtained when Ag / AgCl containing saturated KCl as an internal solution was used as a reference electrode. Desirably, it is 160 mV or more, and more desirably 170 mV or more. The measurement was performed according to 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% of Si, the pitting corrosion potential is improved not only by the increase of Cr content but also by the increase of Si content. It was found that there were 8 times. Therefore, in order to set the pitting corrosion potential to 150 mV or more, this value can be satisfied by setting Cr + 1.8Si to 16% or more.

  In order to examine the reason why the pitting potential was improved by increasing the Si content, the passive film on the surface of the steel before brazing heat treatment was analyzed by X-ray photoelectron spectroscopy (XPS). A passivated film enriched with Si was formed on the surface, and it was thought that the pitting potential was improved as a result of improving the protective property of the passivated film.

The above is related to the corrosion resistance of the material, but since it is used in a brazed state in practice, the corrosion resistance after brazing heat treatment is also important. It is preferable that both the steel material before brazing heat treatment and the steel after brazing heat treatment satisfy the above pitting potential, but either the material before brazing heat treatment or the steel after brazing heat treatment is used. It is sufficient that the pitting potential is 150 mV or more.
A heat treatment was performed for 10 minutes in a vacuum atmosphere of 50 Pa containing nitrogen at 1130 ° C., and the pitting corrosion potential was evaluated in the same manner as described above. As a result, when the amount of Cr and Si in the steel material satisfies Cr + 1.8Si of 16% or more, an oxide film having a composition satisfying a cation fraction of Cr ≧ 11%, Si ≧ 1%, Nb ≧ 1% is obtained. In addition, a pitting corrosion potential of 150 mV or higher and equal to or higher than that of the material was obtained. If the atmosphere of brazing heat treatment is 100 Pa or less, it contains the component of the present invention [1] described later, and when satisfying Cr + 1.8Si ≧ 16%, the material before heat treatment and the pitting potential of steel after heat treatment Can be set to 150 mV or more. However, even if the amount of Cr and Si in the steel material is Cr + 1.8Si and less than 16%, the pitting corrosion potential of the steel after brazing heat treatment can be made 150 mV or more by devising the brazing heat treatment conditions. . Specifically, if the atmosphere of brazing heat treatment is 100 Pa or less, an oxide film having a composition satisfying Cr ≧ 11%, Si ≧ 1%, and Nb ≧ 1% in the cation fraction is formed on the surface, Of pitting corrosion potential.

  Furthermore, room temperature elongation was examined as one index of workability. Although workability cannot be evaluated only by room temperature elongation, it was judged that at least 25% was necessary as room temperature elongation. By setting Cr + 4Si to 26% or less, a room temperature elongation of 25% or more was obtained, so Cr + 4Si is preferably 26% or less.

  Finally, the toughness of the brazed joint will be described. In the case of Ni brazing, a Ni-rich initial phase (hereinafter referred to as “Ni-rich phase”) is first crystallized from the boundary with the base material at the time of brazing, and then Ni-rich phase and Cr-rich phase are crystallized by eutectic reaction. To form a eutectic part. The latter eutectic part is brittle and becomes the starting point of fracture, cracking occurs during brazing, and as a result, the toughness tends to decrease. Further, when a crack occurs during brazing, the crack becomes a starting point of corrosion and becomes a factor of deteriorating corrosion resistance. For this reason, the toughness is ensured by minimizing the volume of the brazed portion by controlling the gap between the gap portions to be brazed normally. However, it may be difficult to control the clearance gap narrowly depending on the shape of the part. Therefore, the inventors considered increasing the Ni-rich phase of the brazing portion by increasing the Ni-rich phase of the primary crystal and ensuring toughness. As described above, Si is an element that is easily dissolved in Ni solder, but it has been found from the analysis result of the brazed portion that it is particularly easily dissolved in the Ni-rich phase. That is, by setting the Si content in the base material to the content specified in the present invention, Si in the base material diffuses and dissolves in the Ni brazing during brazing, thereby increasing the Ni-rich phase. It contributes to the improvement of the toughness of the joint. The Ni-rich phase ratio of the primary crystal that does not cause cracks during brazing is preferably 40% or more, and more preferably 45% or more. Further, the amount of Si in the Ni-rich phase is preferably 0.7 × (Si amount of Ni brazing material) + 0.5% or more, and 0.7 × (Si amount of Ni brazing material) + 1% or more. It is more preferable.

  The present invention provides a ferritic stainless steel excellent in brazeability made in consideration of the above examination, and the gist thereof is as described in the claims.

  Hereinafter, the reason why each composition of the ferritic stainless steel excellent in brazing property and corrosion resistance is limited 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 excessive addition reduces intergranular corrosion resistance, so the C content is 0.020% or less. Preferably they are 0.002% or more and 0.018% or less.

(Si: more than 1.0 to 3.5%)
Si is the most important element in the present invention, and improves brazeability and corrosion resistance. The oxidation resistance is also effective, and it is necessary to contain more than 1.0%. Preferably it is 1.1% or more, More preferably, it is 1.2% or more. However, excessive addition saturates the effect on brazeability and lowers weldability and workability, so the Si content is 3.5% or less. Preferably it is 3.2% or less, More preferably, it is 2.9% or less.

(Mn: 0.02 to 0.80%)
Mn is an element useful as a deoxidizing element, and it is necessary to contain 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 made 0.80% or less. Preferably it is 0.70% or less, More preferably, it is 0.60% or less.

(Cr: 10.5 to less than 15.0%)
Cr is an element that is fundamental in securing corrosion resistance. Therefore, the Cr content is required to be at least 10.5%. Preferably it is 11.0% or more, More preferably, it is 11.5% or more, More preferably, it is 12.5% or more. Although the corrosion resistance can be improved as the Cr content is increased, the content is made less than 15.0% in order to reduce the workability. Preferably it is 14.8% or less, More preferably, it is 14.5% or less.

(Nb: 0.03-0.60%)
Nb fixes C and N, improves the intergranular corrosion resistance of the weld, and improves the high-temperature strength. Therefore, Nb is contained in an amount of 0.03% or more. Nb / C + N is preferably 8 or more, and Nb / C + N is preferably 10 or more. However, excessive addition reduces weldability, so the upper limit of the Nb content was 0.60%. Preferably it is 0.50% or less, More preferably, it is 0.45% or less.

(Al: 0.050% or less)
Al is an element useful for scouring because it has a deoxidizing effect or the like, but its content needs to be limited to 0.050% or less in order to degrade the most important brazing property in the present invention. Preferably they are 0.002 or more and 0.03% or less, More preferably, they are 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. However, excessive addition reduces intergranular corrosion resistance, so the N content is 0.025% or less. Preferably it is 0.002-0.023%, More preferably, it is 0.003-0.020%.

  Furthermore, it is preferable to contain the following components as required.

(Sn: 0.001 to 0.5%)
Sn can be contained in an amount of 0.001% or more as required in order to improve brazing properties. The addition of Sn is also effective for improving the corrosion resistance. More preferably, it is 0.01% or more, More preferably, it is 0.05% or more. However, since excessive addition reduces manufacturability and toughness, it is preferable to contain 0.5% or less. More preferably, it is 0.3% or less, More preferably, it is 0.25% or less.

(Co: 0.01-0.5%)
Co can be contained in an amount of 0.01% or more as required in order to improve brazing properties. More preferably, it is 0.03% or more. Since excessive addition leads to cost increase, 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 as required in order to improve brazing properties. More preferably, it is 0.002% or more. Excessive addition lowers manufacturability, so 0.01% or less is preferable. 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 as required in order to improve brazing properties. The addition of B is also effective for improving secondary workability. More preferably, it is 0.0004% or more. However, excessive addition reduces intergranular corrosion resistance, so 0.005% or less is preferable. More preferably, it is 0.004% or less.

(Ni: 0.1-0.8%)
Ni can be contained in an amount of 0.1% or more and 0.8% or less as necessary to improve the corrosion resistance. Excessive addition increases costs. Preferably, they are 0.2% or more and 0.7% or less, More preferably, they are 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 as required in order to improve strength and corrosion resistance. Excessive addition increases costs. Preferably they are 0.2% or more and 1.5% or less, More preferably, they are 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 as required in order to improve strength and corrosion resistance. Excessive addition increases costs. Preferably they are 0.2% or more and 0.9% or less.

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

(Cu: 0.1-0.8%)
Cu can be contained in an amount of 0.1% or more as required in order to improve the corrosion resistance. Preferably it is 0.2% or more, More preferably, it is 0.3% or more. Since excessive addition degrades workability, it is preferable to contain 0.8% or less. Preferably it is 0.7% or less, More preferably, it is 0.6% or less.

(Sb: 0.001 to 0.5%)
Since Sb is an element that improves the overall corrosion resistance, 0.001% or more may be contained as necessary. However, the cost increases when the Sb content exceeds 0.5%. Therefore, the Sb content is 0.5% or less. The Sb content is preferably 0.3% or less. In order to obtain the above effect stably, 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 the corrosion resistance, 0.001% or more may be contained as necessary. However, the cost increases when the Zr content exceeds 0.3%. 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 effect, the Zr content is preferably 0.01% or more, and more preferably 0.02% or more.

(Ga: 0.0001 to 0.01%)
Ga is an element that improves corrosion resistance and hydrogen embrittlement resistance, and may be contained as necessary. However, the cost increases when the Ga content exceeds 0.01%. Therefore, the Ga content is 0.01% or less. The Ga content is preferably 0.005% or less. In order to obtain the above effect stably, 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 the corrosion resistance, it may be contained as necessary. However, when the Ta content exceeds 0.01%, the cost increases. Therefore, the Ta content is 0.01% or less. The Ta content is preferably 0.005% or less. In order to stably obtain the above effect, 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 scouring, such as a deoxidizing effect, and is effective for hot workability. Therefore, it can be contained in an amount of 0.0002% to 0.005% as 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 the like and is an element useful for scouring, it can be contained in an amount of 0.0002% to 0.005% as necessary. Preferably it is 0.0004% or more. Further, it is preferably 0.002% or less.

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

  Of the inevitable impurities, P is preferably 0.05% or less, more preferably 0.04% or less from the viewpoint of weldability. Moreover, about S, it is preferable to set it as 0.02% or less from a viewpoint of corrosion resistance, More preferably, it is 0.01% or less.

  In addition to the elements described above, the elements of the present invention can be contained within a range not impairing the effects of the present invention. It is preferable to reduce as much as possible Zn, Pb, Se, H, etc. as well as the aforementioned P and S, which are general impurity elements. On the other hand, the content ratio of these elements is controlled within the limits to solve the problems of the present invention, and if necessary, one or more of Zn ≦ 100 ppm, Pb ≦ 100 ppm, Se ≦ 100 ppm, and H ≦ 100 ppm. 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 in an electric furnace, scouring in an AOD furnace or VOD furnace, etc. to form a steel piece by a continuous casting method or an ingot forming method, and then hot rolling-annealing of a hot rolled sheet-acid It is manufactured through the steps of washing, cold rolling, finish annealing, and pickling. If necessary, annealing of the hot-rolled sheet may be omitted, or cold rolling-finish annealing-pickling may be repeated.

The ferritic stainless steel of the present invention is preferably brazed using Ni brazing or Cu brazing because brazing spreads in particular.
The ferritic stainless steel of the present invention performs Ni brazing, and when 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. Therefore, the toughness of the Ni brazed joint member after brazing becomes favorable, which is preferable.
Since the ferritic stainless steel of the present invention is excellent in brazing, corrosion resistance, and workability with Ni brazing and Cu brazing, heat exchangers, EGR coolers that are automotive parts, exhaust heat recovery units, fuel delivery parts, CO _second refrigerant heat pump water heater, it is preferably used in applications latent heat recovery type water heater of the secondary heat exchanger or plate heat exchanger.
The Ni brazing joint member of the present invention is preferable since the Ni-rich phase ratio of the adjacent Ni brazing portions is 40% or more, so that the toughness of the Ni brazing joint member after brazing is improved.

  Hereinafter, the effects of the present invention will be made clearer by examples. In addition, this invention is not limited to a following example, In the range which does not change the summary, it can change suitably and can implement.

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

[Pitting corrosion potential]
A width of 15 mm from the cold-rolled steel plate (No. 1 to 18) and a plate (No. A1 to A18, hereinafter referred to as “brazed heat-treated steel plate”) subjected to the same heat treatment as the following evaluation of the solderability of the cold-rolled steel plate A 20 mm long test piece was cut out and wet-polished to # 600 with emery paper. The periphery was covered with a resin so that the central portion 10 mm × 10 mm of the plate was exposed to obtain an electrode for measurement. The pitting corrosion potential (V′c100: the most noble potential with a current value exceeding 100 μA / cm ² ) was measured in accordance with JIS G0577 in a 3.5 mass% NaCl aqueous solution at 30 ° C. Note that Ag / AgCl with saturated KCl as an internal solution was used for the reference electrode, and the potential sweep rate was 20 mV / min. The number of test pieces was set to 5, and the average value of the obtained pitting corrosion potential was evaluated.

[Normal temperature tensile test]
After collecting a JIS 13B tensile test piece from a direction parallel to the rolling direction of the cold-rolled steel sheet (Nos. 1 to 18), a normal temperature tensile test was performed to measure the total elongation.

[Wax spreading]
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 using an organic solvent, 0.1 g of Ni brazing (BNi-5 series) and Cu brazing (BCu-1) were respectively 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. A1-No. The vacuum degree 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.
It cooled after completion | finish of a heating, and calculated | required the brazing area after heat processing by image analysis. Based on the obtained wax area, the wax spread coefficient was calculated from the following equation. Note that the number of test pieces was 3, and evaluation was performed using an average wax spreading coefficient.
Brazing coefficient = brazing area after heat treatment / initial brazing area

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

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

  From Table 2, No. which is within the scope of the present invention. 1-No. 14, no. No. 17 (cold-rolled steel sheet, but as described in Tables 1 and 2, No. 1 and No. 17 have the same components and the same characteristics) are good in pitting potential and room temperature elongation (total elongation). Show.

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

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

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

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

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

[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 using an organic solvent. As shown in FIG. 1, a stainless foil 3 having a thickness of 100 μm was sandwiched between the large plate 1 and the small plate 2 to form a 100 μm gap 4 between the large plate 1 and the small plate 2. In this state, 5 g of Ni solder 5 (BNi-5 series) was applied to the long side of the small plate 2. Then, it put into the vacuum furnace and heated at 1130 degreeC 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, the cross-section of the gap where the wax spread and wet (see FIG. 2A) as shown in FIG. Although the Ni-rich phase and the eutectic part of the primary crystal exist in the solder spreading part 6, the Ni-rich phase ratio (area ratio) was obtained by binarization by image analysis.

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

  The ferritic stainless steel of the present invention with excellent brazeability is an EGR cooler for automobile parts, an exhaust heat recovery device, a secondary heat exchanger for a latent heat recovery type water heater, an Ecocute (registered trademark) plate type heat exchanger, etc. It is suitable as a material for heat exchangers assembled by brazing and joining.

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

Claims (10)

  In mass%, C: 0.020% or less, Si: more than 1.0 to 3.5%, Mn: 0.02 to 0.80%, Cr: 10.5 to less than 15.0%, Nb: 0 0.03 to 0.60%, Al: 0.050% or less, N: 0.025% or less, with the balance being Fe and an inevitable impurity chemical composition, 3.5% by mass at 30 ° C A ferritic stainless steel excellent in brazing and corrosion resistance, characterized by having a pitting potential V'c100 in an aqueous NaCl solution of 150 mV or more.   The ferritic stainless steel having excellent brazeability and corrosion resistance according to claim 1, wherein Cr + 1.8Si in mass% satisfies 16% or more.   2. The ferritic stainless steel having excellent brazing and corrosion resistance according to claim 1, comprising an oxide film satisfying Cr ≧ 11%, Si ≧ 1%, and Nb ≧ 1% in terms of cation fraction. Furthermore, by 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% 1st group which consists of any 1 type or 2 types or more of them,
Ni: 0.1-0.8%, Mo: 0.1-2%, W: 0.1-1%, V: 0.05-0.5%, Cu: 0.1-0.8% , 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 a second group consisting of two or more species,
Of the third group consisting of one or more of Ca: 0.0002-0.005%, Mg: 0.0002-0.005%, REM: 0.005-0.1%, The ferritic stainless steel excellent in brazeability and corrosion resistance according to any one of claims 1 to 3, comprising at least one group.   The ferrite having excellent brazeability and corrosion resistance according to any one of claims 1 to 4, wherein the ferrite is used for a brazed joint member that is brazed using Ni brazing or Cu brazing. Stainless steel.   The Ni-rich phase ratio of an adjacent Ni brazing portion is 40% or more when Ni brazing is performed and the cooling rate from the brazing temperature to 900 ° C is 20 ° C / min or less. The ferritic stainless steel excellent in brazeability and corrosion resistance according to any one of claims 1 to 4.   The ferritic stainless steel excellent in brazeability and corrosion resistance according to any one of claims 1 to 6, wherein the ferritic stainless steel is used for a heat exchanger.   The ferrite having excellent brazeability and corrosion resistance according to any one of claims 1 to 6, which is used for an EGR cooler, an exhaust heat recovery unit or a fuel delivery system part which is an automobile part. Stainless steel. The wax according to any one of claims 1 to 6, which is a use of a CO ₂ refrigerant heat pump type hot water heater, a secondary heat exchanger of a latent heat recovery type hot water heater or a plate type heat exchanger. Ferritic stainless steel with excellent adhesion and corrosion resistance.   A Ni brazed joint member obtained by brazing the ferritic stainless steel excellent in brazeability and corrosion resistance according to any one of claims 1 to 4, wherein Ni is rich in adjacent Ni braze portions. A Ni brazed joint member having a phase ratio of 40% or more.

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