JP2017150016A - Ferritic stainless steel and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ferritic stainless steel excellent in corrosion resistance after brazing heat treatment when used for a heat exchange part of EGR cooler or the like and a heat exchanger such as a secondary heat exchanger, and a manufacturing method therefor.SOLUTION: There is provided a ferritic stainless steel having a chemical composition of a base material containing C:0.002-0.03%, Si:0.01-0.6%, Mn:0.02-2.0%, P:0.05% or less, S:0.01% or less, Cr:15.0-30.0%, Al:0.001-0.10%, Nb:0.20-2.0%, N:0.05% or less and the balance:Fe with inevitable impurities, and having an oxidized coating on a surface with cation fraction of Nb contained in the oxidized coating of 16.0% or more by atom%.SELECTED DRAWING: None

Description

  The present invention relates to a ferritic stainless steel and a method for producing the same, and more particularly to a ferritic stainless steel having excellent corrosion resistance after brazing heat treatment and a method for producing the same.

In recent years, in the automobile field, various components have been strengthened because various components such as CO ₂ , NO _x , and SO _x contained in exhaust gas cause environmental pollution. Therefore, for the purpose of reducing CO ₂ emissions and improving fuel efficiency of automobiles, not only the improvement of engine efficiency by high-efficiency combustion or idling stop, etc., but also weight reduction by material replacement, hybrid vehicles (HEV), biofuels, hydrogen / fuel cells Improvement by energy diversification of vehicles (FCV), electric vehicles (EV), etc. is required.

  Among them, efforts are being made to improve fuel efficiency by attaching a heat exchanger that recovers exhaust heat, so-called exhaust heat recovery. The exhaust heat recovery device transfers the exhaust gas heat to the cooling water by heat exchange, recovers and reuses the heat energy, and raises the cooling water temperature, thereby improving the heating performance of the passenger compartment and reducing the engine warm-up time. It is a system that shortens and improves fuel efficiency, and is also called an exhaust heat recirculation system.

In addition, efforts are being made to install an exhaust gas recirculation device that recirculates exhaust gas. Examples of the exhaust gas recirculation device include an EGR (Exhaust Gas Recirculation) cooler. EGR cooler after the exhaust gas of the engine is cooled by the engine cooling water or air, to lower the combustion temperature by causing afterburning back to intake side is a device to reduce the detrimental gases NO _x.

Furthermore, the example where a heat exchanger is applied also in the hot water supply equipment field is increasing. In gas water heaters, in order to recover latent heat from high-temperature exhaust gas at about 150 to 200 ° C., a latent heat recovery type gas water heater to which a secondary heat exchanger made of stainless steel is added has been widespread. In addition, in the electric water heater, 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) is progressing, and a heat exchanger is also used here. .

  Good heat efficiency and heat conductivity are required for such a heat exchanger such as an exhaust heat recovery unit, an EGR cooler, or a heat exchanger such as a secondary heat exchanger. In addition, since the above components are in contact with exhaust gas, excellent corrosion resistance is required for exhaust gas condensed water. In particular, since engine cooling water flows in these parts, there is a risk of serious accidents when holes are formed due to corrosion. In addition, the material used is thin in order to increase the heat exchange efficiency. For these reasons, a material used for a heat exchanger such as an EGR cooler or a heat exchanger such as a secondary heat exchanger is required to have better corrosion resistance than an exhaust system downstream member.

  Conventionally, among the exhaust system downstream members mainly composed of a muffler, ferritic stainless steel containing 17% or more of Cr, such as SUS430LX, SUS436J1L, and SUS436L, is used for a part that particularly requires corrosion resistance. Corrosion resistance equal to or higher than these is required for the material of the collector and the EGR cooler.

  Here, a heat exchanger such as an EGR cooler or a heat exchanger such as a secondary heat exchanger is generally assembled by brazing. At the time of brazing joining, the material is subjected to vacuum heat treatment such as brazing heat treatment. At this time, the heat resistance may improve or decrease the corrosion resistance of the material. Therefore, care must be taken when selecting materials, brazing materials, and heat treatment conditions.

  For example, the corrosion resistance of the material of the material may be reduced due to sensitization of the base metal by brazing heat treatment or corrosion of the bonded brazing itself. The cause may be an error in selecting a material or brazing material, or inappropriate brazing heat treatment conditions. On the other hand, a metal oxide in the material is mainly oxidized during vacuum heat treatment such as brazing heat treatment, so that a unique oxide film is formed on the surface, and as a result, the corrosion resistance of the material may be improved.

  For example, Patent Documents 1 to 4 disclose techniques for forming the above oxide film on the steel surface.

  Patent Documents 1 and 2 disclose ferritic stainless steel bright annealed materials that require workability, weather resistance and beautiful metallic luster in a humid atmospheric environment such as interior and exterior materials of buildings and automobile molding materials. . Patent Document 3 discloses ferritic stainless steel suitable for an exhaust heat recovery unit in which a heat exchange part is assembled by brazing. Further, Patent Document 4 discloses a ferritic stainless steel suitable for automobile fuel supply system parts for supplying biofuel such as bioethanol and biodiesel.

Japanese Unexamined Patent Publication No. 07-180001 Japanese Patent Laid-Open No. 08-109443 JP 2012-214880 A JP2012-214881A

  According to Patent Document 1, the stainless steel having the above composition is subjected to bright annealing at an annealing temperature of 850 ° C. or higher in a hydrogen-nitrogen mixed gas atmosphere having a dew point of −35 ° C. or lower. An oxide film with excellent inertia is generated on the surface. However, only in the case of a combined addition of Al and Ti, the effect of increasing the weather resistance is exhibited, and there remains room for improvement.

  In Patent Document 2, bright annealing is performed to generate an oxide film with high Al content and Nb content and excellent weather resistance on the surface. However, only in the case of a combined addition of Al and Nb, the effect of improving weather resistance is exhibited, and there is room for improvement.

  Furthermore, Patent Document 3 describes that the pore resistance is improved by forming an oxide film containing Cr, Nb, Si, Al, and Ti with a total cation fraction of 40% or more on the surface during brazing. In Patent Document 4, it is possible to obtain excellent corrosion resistance against biofuels by forming an oxide film containing Cr, Nb, Si, Al, and Ti with a total cation fraction of 30% or more on the surface during brazing. It is stated that it can be done. However, Patent Documents 3 and 4 specify the total cation concentration of the above five elements, and the cation concentration of each element is not specified. Further, only the Cr concentration is mentioned, but it is described that it is desirable to contain 20% or more, and it is desirable that it is concentrated on the surface.

  Thus, Patent Documents 1 to 4 all improve the corrosion resistance of the material by performing vacuum heat treatment such as brazing heat treatment, but in order to improve the corrosion resistance, several kinds of elements must be included in the stainless steel. There is no mention of which elements are most effective.

  The present invention provides a ferritic stainless steel having excellent corrosion resistance after brazing heat treatment and a method for producing the same when used in a heat exchanger such as an EGR cooler or a heat exchanger such as a secondary heat exchanger. The purpose is to do.

  In order to solve the above-mentioned problems, the present inventors made stainless steels having various chemical compositions and investigated the corrosion resistance after brazing heat treatment. As a result, it was found that the corrosion resistance is improved when an oxide of Nb is formed on the extreme surface. And it became clear that when elements, such as Al and Ti, were contained excessively, the surface concentration of Nb with high corrosion resistance was disturbed.

Specifically, the content of Nb in the cations excluding O, C and N contained in the oxide film formed by brazing heat treatment (hereinafter also referred to as “cation fraction”) is a predetermined amount or more. By controlling to be high, high corrosion resistance can be obtained. Nb ₂ O ₅ , which is an oxide of Nb, is a stable oxide, and is stable even in a low pH environment where Cr ₂ O _3, which is a main constituent material of a passive film of stainless steel produced in the atmosphere or the like, dissolves. . The reason why the steel having an oxide film having a high Nb cation fraction has very high corrosion resistance is considered to be that the steel surface is uniformly covered with this stable Nb ₂ O ₅ .

  The present invention has been made on the basis of the above findings, and the gist thereof is the following ferritic stainless steel and a method for producing the same.

(1) The chemical composition of the base material is mass%,
C: 0.002 to 0.03%,
Si: 0.01 to 0.6%,
Mn: 0.02 to 2.0%,
P: 0.05% or less,
S: 0.01% or less,
Cr: 15.0-30.0%,
Al: 0.001 to 0.10%,
Nb: 0.20 to 2.0%,
N: 0.05% or less,
The remainder: Fe and inevitable impurities
Ferritic stainless steel with an oxide film on the surface,
Ferritic stainless steel in which the content of Nb in the cations other than O, C, and N contained in the oxide film is 16.0% or more in atomic%.

  (2) The content of each element in the cations excluding O, C and N contained in the oxide film is atomic%, Cr: 30.0% or less, Si: 15.0% or less, Al: 40 The ferritic stainless steel according to the above (1), which is 0.0% or less and Ti: 10.0% or less.

  (3) Ferritic stainless steel according to (1) or (2) above, wherein the content of Nb present as Nb (C, N) in the base material is 1.0% or less by mass%. .

(4) The chemical composition of the base material is further mass%,
Ni: 3.0% or less,
Cu: 1.5% or less,
Mo: 3.0% or less,
Ti: 0.02% or less,
W: 1.0% or less,
V: 0.5% or less,
Sn: 0.5% or less,
Sb: 0.5% or less, and Mg: 0.003% or less,
The ferritic stainless steel according to any one of (1) to (3) above, which contains one or more selected from:

(5) The chemical composition of the base material is further mass%,
B: 0.003% or less,
Ca: 0.010% or less,
Zr: 0.3% or less,
Co: 0.3% or less,
Ga: 0.01% or less,
Ta: 0.01% or less, and REM: 0.2% or less,
The ferritic stainless steel according to any one of (1) to (4) above, which contains one or more selected from:

  (6) The ferritic stainless steel according to any one of (1) to (5), which is used as a high-temperature member of at least one of an exhaust heat recovery device, an EGR cooler, and a heat exchanger.

(7) For the steel having the chemical composition described in (1), (4) or (5) above, a post-cold rolling annealing step is performed in which annealing is performed after cold rolling,
In the post-cold rolling annealing step, the ferritic stainless steel is kept at a temperature range of 950 ° C. or higher for 5 s or more, and then cooled under the condition that the average cooling rate to 650 ° C. is 8.0 ° C./s or more. Production method.

(8) A heat treatment step is further provided after the post-cold rolling annealing step,
In the heat treatment step, after the vacuum or the total pressure is 0.1 Pa or less, N ₂ is introduced to 1 to 10 Pa, and the heat treatment is performed again in an atmosphere controlled so that the total pressure is 0.1 Pa or less. The manufacturing method of the ferritic stainless steel as described in said (7).

  According to the present invention, a ferritic stainless steel having excellent corrosion resistance after brazing heat treatment can be obtained. Therefore, the ferritic stainless steel according to the present invention can be suitably used for a heat exchange section such as an EGR cooler or a heat exchanger such as a secondary heat exchanger.

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

1. Chemical composition of base material The reasons for limitation of each element are as follows. In the following description, “%” for the content means “% by mass”.

C: 0.002 to 0.03%
C decreases the intergranular corrosion resistance and workability, so its content needs to be kept low. Therefore, the C content is 0.03% or less. However, excessive reduction promotes crystal grain coarsening and sensitization during brazing and increases the scouring cost, so the C content needs to be 0.002% or more. The C content is preferably 0.004% or more, and preferably 0.02% or less.

Si: 0.01 to 0.6%
Si is useful as a deoxidizing element, but if contained excessively, it is preferentially oxidized and concentrated on the surface, and relatively reduces the cation fraction of Nb on the surface. Therefore, the Si content is set to 0.01 to 0.6%. The Si content is preferably 0.1% or more, and preferably 0.5% or less.

Mn: 0.02 to 2.0%
Mn is useful as a deoxidizing element, but if it is excessively contained, the corrosion resistance is deteriorated. Therefore, the Mn content is 0.02 to 2.0%. The Mn content is preferably 0.05% or more, and preferably 1.0% or less.

P: 0.05% or less P is an element that deteriorates workability, weldability, and corrosion resistance, and its content needs to be limited. Therefore, the P content is 0.05% or less. The P content is preferably 0.03% or less.

S: 0.01% or less Since S is an element that deteriorates corrosion resistance, its content needs to be limited. Therefore, the S content is 0.01% or less. The S content is preferably 0.005% or less.

Cr: 15.0-30.0%
Cr is an important element for securing corrosion resistance in an air environment, a cooling water environment, and an exhaust gas condensed water environment, and it is necessary to contain 15.0% or more. As the Cr content is increased, the corrosion resistance is improved, but the workability and manufacturability are lowered, so the Cr content is 30.0% or less. The Cr content is preferably 16.0% or more, and preferably 23.0% or less.

Al: 0.001 to 0.10%
Al is an element useful for scouring, such as a deoxidizing effect, and has an effect of improving moldability. In order to obtain this effect, the Al content needs to be 0.001% or more. However, if it is excessively contained, it is preferentially oxidized and concentrated on the surface, and the cation fraction of Nb on the surface is relatively lowered, so the Al content is made 0.10% or less. The Al content is preferably 0.003% or more, and preferably 0.070% or less.

Nb: 0.20 to 2.0%
Nb is the most important element in the present invention. Corrosion resistance is drastically improved when an oxide film containing 16.0% or more of Nb in the cation fraction is formed on the surface by oxidation during brazing heat treatment. Further, Nb carbonitride is an important element from the viewpoint of suppressing the coarsening of crystal grains due to heating during brazing and suppressing the strength reduction of the member. It is also useful for improving high temperature strength and intergranular corrosion of welds. However, if contained excessively, processability and manufacturability are reduced. Therefore, the Nb content is 0.20 to 2.0%. The Nb content is preferably 0.30% or more, and preferably 1.0% or less.

N: 0.05% or less N is an element useful for pitting corrosion resistance, but its content needs to be kept low in order to reduce intergranular corrosion resistance and workability. Therefore, the N content is 0.05% or less. The N content is preferably 0.03% or less.

  The ferritic stainless steel of the present invention contains the above-described elements from C to N, and the remainder has a chemical composition consisting of Fe and inevitable impurities.

  Here, “inevitable impurities” are components mixed in due to various factors of raw materials such as ores and scraps and manufacturing processes when steel is industrially manufactured, and are allowed within a range that does not adversely affect the present invention. Means what will be done.

  In addition to the above elements, the ferritic stainless steel according to the present invention has the following amounts of Ni, Cu, Mo, Ti, W, V, Sn, Sb, Mg, B, Ca, Zr, You may contain 1 or more types selected from Co, Ga, Ta, and REM.

Ni: 3.0% or less Since Ni is an element that improves corrosion resistance, Ni may be contained as necessary. However, the cost increases when the Ni content exceeds 3.0%. Therefore, the Ni content is 3.0% or less. The Ni content is preferably 2.0% or less. In order to stably obtain the above effect, the Ni content is preferably 0.01% or more, and more preferably 0.05% or more.

Cu: 1.5% or less Since Cu is an element that improves corrosion resistance, it may be contained as necessary. However, the cost increases when the Cu content exceeds 1.5%. Therefore, the Cu content is 1.5% or less. The Cu content is preferably 1.0% or less. In order to obtain the above effect stably, the Cu content is preferably 0.05% or more, and more preferably 0.1% or more.

Mo: 3.0% or less Since Mo is an element that improves the resistance to condensed water corrosion, it may be contained as necessary. However, if the Mo content exceeds 3.0%, the cost increases. Therefore, the Mo content is 3.0% or less. The Mo content is preferably 2.5% or less. In order to obtain the above effect stably, the Mo content is preferably 0.1% or more, and more preferably 0.5% or more.

Ti: 0.02% or less Since Ti is an element having an effect of preventing the formation of Cr ₂₃ C ₆ by bonding with C, Ti may be contained as necessary. However, when the Ti content exceeds 0.02%, it is preferentially oxidized and concentrated on the surface, and the cation fraction of Nb on the surface is relatively lowered. Therefore, the Ti content is 0.02% or less. The Ti content is preferably 0.005% or less. In order to stably obtain the above effect, the Ti content is preferably 0.001% or more.

W: 1.0% or less Since W is an element that improves corrosion resistance, it may be contained if necessary. However, if the W content exceeds 1.0%, the cost increases. Therefore, the W content is 1.0% or less. The W content is preferably 0.8% or less. In order to stably obtain the above effect, the W content is preferably 0.01% or more, and more preferably 0.02% or more.

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

Sn: 0.5% or less Since Sn is an element that improves corrosion resistance, it may be contained as necessary. However, the cost increases when the Sn content exceeds 0.5%. Therefore, the Sn content is 0.5% or less. The Sn content is preferably 0.3% or less. In order to obtain the above effect stably, the Sn content is preferably 0.005% or more, and more preferably 0.01% or more.

Sb: 0.5% or less Since Sb is an element that improves the overall corrosion resistance, it 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.

Mg: 0.003% or less Mg is an element useful for refining the structure and improving workability and toughness, and may be contained as necessary. However, if the Mg content exceeds 0.003%, the corrosion resistance decreases. Therefore, the Mg content is 0.003% or less. The Mg content is preferably 0.001% or less. In order to obtain the above effect stably, the Mg content is preferably 0.0001% or more.

  In addition, when 1 or more types selected from Ni, Cu, Mo, Ti, W, V, Sn, Sb, and Mg are contained in combination, the total content is 6.0 from the viewpoint of economy. % Or less is preferable.

B: 0.003% or less B is an element useful for improving secondary workability, and may be contained as necessary. However, when the B content exceeds 0.003%, the castability deteriorates. Therefore, the B content is 0.003% or less. The B content is preferably 0.001% or less. In order to obtain the above effect stably, the B content is preferably 0.0002% or more, and more preferably 0.0005% or more.

Ca: 0.010% or less Since Ca is an element having a desulfurization action, Ca may be contained as necessary. However, when the Ca content exceeds 0.010%, CaS which is a water-soluble inclusion is excessively generated and the corrosion resistance is lowered. Therefore, the Ca content is 0.010% or less. The Ca content is preferably 0.005% or less. In order to obtain the above effect stably, the Ca content is preferably 0.0002% or more.

Zr: 0.3% or less Since Zr is an element that improves corrosion resistance, it 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.

Co: 0.3% or less Since Co is an element that improves secondary workability and toughness, it may be contained as necessary. However, the cost increases when the Co content exceeds 0.3%. Therefore, the Co content is 0.3% or less. The Co content is preferably 0.2% or less. In order to obtain the above effect stably, the Co content is preferably 0.01% or more, and more preferably 0.02% or more.

Ga: 0.01% or less 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: 0.01% or less Since Ta is an element that improves corrosion resistance, it may be contained if 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.

REM: 0.2% or less REM has a deoxidizing effect and the like and is an element useful for scouring. However, the cost increases when the REM content exceeds 0.2%. Therefore, the REM content is 0.2% or less. The REM content is preferably 0.1% or less. In order to obtain the above effect stably, the REM content is preferably 0.001% or more, and more preferably 0.002% or more.

  Here, REM refers to a total of 17 elements of Sc, Y, and lanthanoid, and the content of REM refers to the total content of these elements.

2. Oxide film The ferritic stainless steel according to the present invention is provided with an oxide film on the surface by brazing heat treatment. And in order to improve corrosion resistance, especially condensed water corrosion resistance, it is necessary to make the cation fraction of Nb contained in an oxide film into 16.0% or more in atomic%. The cation fraction of Nb is preferably 20.0% or more. On the other hand, the upper limit of the cation fraction of Nb is not particularly limited, but it is difficult to excessively concentrate Nb in the oxide film because of the relationship with the Nb content contained in the steel. Therefore, the cation fraction of Nb is preferably 60.0% or less.

Cr forms a passive film mainly composed of Cr ₂ O ₃ to improve the corrosion resistance. However, Cr is dissolved in the solution as Cr ^{3+ in} a low pH environment having a pH of about 2 to 4. Therefore, if Cr is excessively concentrated in the film, the corrosion resistance in a low pH condensed water environment may be reduced. Therefore, the cation fraction of Cr contained in the oxide film is preferably 30.0% or less in atomic%, and more preferably less than 20.0%.

  Furthermore, when Si, Al, and Ti are excessively concentrated in the film, the surface Nb cation fraction is relatively lowered. Therefore, the cation fraction of each of the above elements contained in the oxide film may be atomic%, Si: 15.0% or less, Al: 40.0% or less, and Ti: 10.0% or less. preferable. The cation fraction of Si is more preferably 13.5% or less, the cation fraction of Al is more preferably 30.0% or less, and the cation fraction of Ti is 8.0% or less. Is more preferable.

  The thickness of the oxide film is not particularly limited, but in order to ensure high corrosion resistance, it is preferably 1.0 nm or more, and more preferably 1.5 nm or more.

3. Precipitates In order to form Nb ₂ O ₅ on the surface during brazing heat treatment, it is desirable that the amount of solute Nb in the base material is high. This is because Nb present as Nb (C, N) in the base material is less likely to form an oxide film on the surface during brazing heat treatment than solid solution Nb. Therefore, the content of Nb present as Nb (C, N) in the base material is preferably 1.0% or less, and more preferably 0.7% or less, in mass%.

4). Manufacturing Method The manufacturing method of the ferritic stainless steel according to the present invention is not particularly limited and can be manufactured by a general process. For example, using a converter or an electric furnace, steel having the above chemical composition is made into molten steel and refined in an AOD furnace or a VOD furnace. Then, after making into a steel piece by a continuous casting method or an ingot-making method, it manufactures through the process of hot rolling-annealing of a hot-rolled sheet-pickling-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.

However, in order to form Nb ₂ O ₅ on the surface during the brazing heat treatment as described above, it is desirable that the amount of the solid solution Nb in the base material is high. In order to increase the amount of solute Nb, in the annealing process after cold rolling, in which annealing is performed after cold rolling, the final annealing temperature is increased to dissolve the Nb carbonitride or the Laves phase, and the annealing is performed. It is effective to increase the cooling rate during subsequent cooling to prevent the precipitation of Nb carbonitride or Laves phase.

  Specifically, in the post-cold rolling annealing step, final annealing is performed for 5 seconds or more in a temperature range of 950 ° C. or higher, and then cooling is performed under an average cooling rate of 8.0 ° C./s or higher to 650 ° C. It is preferable to carry out. The final annealing temperature is more preferably 1000 ° C. or more, and the holding time is more preferably 10 s or more. The average cooling rate from the final annealing temperature to 650 ° C. is more preferably 10.0 ° C./s or more.

The brazing heat treatment conditions are not particularly limited, but it is preferable to appropriately control the atmosphere of the brazing heat treatment in order to form a film having high corrosion resistance. Specifically, when brazing is performed, first, in vacuum or after controlling to an atmosphere in which the total pressure becomes 0.1 Pa or less, N ₂ is introduced to 1 to 10 Pa, and the vacuum is again reduced to 0.1 Pa or less. By pulling, it becomes an ideal atmosphere in which Nb which is more easily oxidized than Cr is preferentially oxidized. This is because if the atmospheric pressure during brazing heat treatment is high, the air-generated film present on the stainless steel surface may not be reduced. Therefore, it is preferable to maintain an atmosphere of 0.1 Pa or less even during brazing heat treatment.

In the above, when controlling to an atmosphere where the total pressure is 0.1 Pa or less, the first atmosphere may be in the air or in a hydrogen atmosphere. Other atmospheres may be used as long as N ₂ can be introduced to 1 to 10 Pa after controlling to an atmosphere of 0.1 Pa or less.

  Brazing heat treatment is performed in the above atmosphere. The brazing heat treatment conditions are not particularly limited, but the brazing temperature is preferably 800 ° C. or higher. This is because, when the temperature is low, the solid solution Nb does not diffuse and an oxide film may not be formed on the surface. The brazing temperature is more preferably 1000 ° C. or higher, and further preferably 1050 ° C. or higher.

  The heat treatment time is preferably 3 to 30 minutes. This is because if the brazing heat treatment time is lengthened, the crystal grains of the stainless steel are coarsened and the mechanical properties may be lowered. In particular, in the present invention, since the Nb carbonitride or the Laves phase is dissolved as much as possible, the factor for pinning the crystal grains is very small, and the brazing heat treatment for a long time greatly reduces the mechanical properties. There is a risk. The brazing heat treatment time is more preferably 5 min or more, and more preferably 20 min or less.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

Steel having the chemical composition shown in Table 1 was melted and hot-rolled to 4 mm. Then, after performing 1 minute annealing at 1050 degreeC, it pickled. Further, after cold rolling to 1 mm, annealing was performed under the conditions shown in Table 2, and then cooled to obtain a cold-rolled annealed plate. Thereafter, pickling was performed under the conditions of HF 20 g / L, HNO ₃ 50 g / L, and 50 ° C. to produce a steel plate having a plate thickness of 1.0 mm.

After cutting a test piece having a width of 10 mm and a length of 10 mm from this steel plate, heat treatment was performed at 1150 ° C. for 10 minutes in various atmospheric pressures including N ₂ as shown in Table 2 as heat treatment simulating brazing heat treatment. . In the following description, the heat treatment is referred to as brazing heat treatment. About the test piece after heat processing, while performing surface analysis by X-ray photoelectron spectroscopy (XPS: X-ray Photoelectron Spectroscopy), depth analysis was also performed by using Ar sputtering together.

The oxide film has a depth at which the peak intensity of the O1s peak appearing at 530 eV is half the maximum value, and the concentration of each element in the oxide film is the average value of the concentration from the outermost surface to the depth at which the oxide film disappears. . In addition, the base material was set to a depth at which the value of the 2p3 / 2 peak of metal Fe appearing at 706.7 eV becomes a constant value. For Nb, quantitative calculation was performed with the 3d5 / 2 peak appearing at 203 eV being the metal Nb, the peak appearing at 204 eV being NbC and NbN, and the peak appearing at 207.5 eV being Nb ₂ O ₅ . The chemical state of Nb from the peak obtained by XPS from the outermost surface of the coating to 3nm was found to be almost _Nb 2 _{O 5.}

  After cutting out a test piece having a width of 25 mm and a length of 100 mm from the steel plate, the same brazing heat treatment as described above was performed. Condensed water corrosion resistance was evaluated by conducting a semi-immersion test of this test piece.

The simulated condensed water used for the semi-immersion test was adjusted to 100 ppm Cl ⁻ +1000 ppm SO ₄ ²⁻ +1000 ppm SO ₃ ²⁻ using hydrochloric acid, sulfuric acid, and ammonium sulfite as reagents. The simulated condensed water was adjusted to pH 2.0 by further adding aqueous ammonia after the reagent addition. The test piece was semi-immersed in this solution heated to 80 ° C. using a jig adjusted so that the test piece was approximately half immersed at about 55 °. The test was conducted for 168 h and the solution was renewed almost every day.

  The maximum pitting depth was used for corrosion evaluation. After completion of the test, the corrosion product was removed using an aqueous solution of ammonium dihydrogen citrate, and the depth of the most corroded portion of the test piece was determined by the depth of focus method. The criterion for the semi-immersion test was 100 μm, at which pitting corrosion growth was remarkable. A sample having a maximum pitting depth of 100 μm or more was designated as “X” and a sample having a maximum pitting depth of less than 100 μm was designated as “◯”. Among them, those having excellent corrosion resistance and a maximum pitting corrosion depth of 50 μm or less were designated as “◎”.

  These results are summarized in Table 2. From Table 2, it can be seen that the steels of the present invention examples (test numbers 1 to 18) that satisfy all the provisions of the present invention have good condensed water corrosion test results after heat treatment. However, the steel of Test No. 18 in which the cation fraction of Cr contained in the oxide film exceeds 30.0% resulted in slightly inferior condensation water corrosion resistance.

  On the other hand, the steel of the comparative example failed the condensed water corrosion test result after heat processing. Among steel components whose composition is outside the specified range, Test Nos. 19, 21 to 24, and 28 show that the cation fraction of Nb in the oxide film is within the range of the present invention, but the corrosion resistance in condensed water is sufficient. The result of the condensed water corrosion test failed.

  In Test Nos. 20, 25, 26, and 29, since the contents of Si, Cr, Al, and Ti were excessive, the cation fraction of Nb in the oxide film was less than 16.0%. Corrosion resistance was not sufficient and the condensed water corrosion test result was rejected. From the measurement result by XPS, it was found that each element was preferentially oxidized and concentrated on the surface, so that the cation fraction of Nb was relatively lowered.

  Further, in Test No. 27 where the Nb content is lower than the specified range and Test Nos. 31 to 33 where the final annealing conditions were inappropriate, the amount of solute Nb was not sufficient, so the cation fraction of Nb in the oxide film was low. The result was less than 16.0%, the corrosion resistance in the condensed water was not sufficient, and the condensed water corrosion test result was rejected.

  Test numbers 34 and 35 indicate that the cation fraction of Nb in the oxide film was less than 16.0% due to the atmospheric pressure during brazing heat treatment exceeding 0.1 Pa, and the corrosion resistance in reduced water Was not sufficient, and the condensed water corrosion test result failed.

  According to the present invention, a ferritic stainless steel having excellent corrosion resistance after brazing heat treatment can be obtained. Therefore, the ferritic stainless steel according to the present invention can be suitably used for a heat exchange section such as an EGR cooler or a heat exchanger such as a secondary heat exchanger.

Claims (8)

The chemical composition of the base material is
C: 0.002 to 0.03%,
Si: 0.01 to 0.6%,
Mn: 0.02 to 2.0%,
P: 0.05% or less,
S: 0.01% or less,
Cr: 15.0-30.0%,
Al: 0.001 to 0.10%,
Nb: 0.20 to 2.0%,
N: 0.05% or less,
The remainder: Fe and inevitable impurities
Ferritic stainless steel with an oxide film on the surface,
Ferritic stainless steel in which the content of Nb in the cations other than O, C, and N contained in the oxide film is 16.0% or more in atomic%.   The content of each element in the cations excluding O, C and N contained in the oxide film is atomic%, Cr: 30.0% or less, Si: 15.0% or less, Al: 40.0% The ferritic stainless steel of Claim 1 which is below and Ti: 10.0% or less.   The ferritic stainless steel according to claim 1 or 2, wherein a content of Nb existing as Nb (C, N) in the base material is 1.0% or less by mass%. The chemical composition of the base material is further mass%,
Ni: 3.0% or less,
Cu: 1.5% or less,
Mo: 3.0% or less,
Ti: 0.02% or less,
W: 1.0% or less,
V: 0.5% or less,
Sn: 0.5% or less,
Sb: 0.5% or less, and Mg: 0.003% or less,
The ferritic stainless steel according to any one of claims 1 to 3, comprising at least one selected from the group consisting of: The chemical composition of the base material is further mass%,
B: 0.003% or less,
Ca: 0.010% or less,
Zr: 0.3% or less,
Co: 0.3% or less,
Ga: 0.01% or less,
Ta: 0.01% or less, and REM: 0.2% or less,
The ferritic stainless steel according to any one of claims 1 to 4, comprising at least one selected from the group consisting of:   The ferritic stainless steel according to any one of claims 1 to 5, which is used as a high temperature member of at least one of an exhaust heat recovery unit, an EGR cooler, and a heat exchanger. The steel having the chemical composition according to claim 1, claim 4, or claim 5, comprising a post-cold rolling annealing step in which annealing is performed after cold rolling,
In the post-cold rolling annealing step, the ferritic stainless steel is kept at a temperature range of 950 ° C. or higher for 5 s or more, and then cooled under the condition that the average cooling rate up to 650 ° C. is 8.0 ° C./s or more. Production method. A heat treatment step is further provided after the post-cold rolling annealing step,
In the heat treatment step, after the vacuum or the total pressure is 0.1 Pa or less, N ₂ is introduced to 1 to 10 Pa, and the heat treatment is performed again in an atmosphere controlled so that the total pressure is 0.1 Pa or less. The manufacturing method of the ferritic stainless steel of Claim 7.