WO2012133506A1 - Ferritic stainless steel for biofuel supply system part, biofuel supply system part, ferritic stainless steel for exhaust heat recovery unit, and exhaust heat recovery unit - Google Patents

Abstract

According to one embodiment, ferritic stainless steel comprises, by mass%, 0.03% or less of C, 0.03% or less of N, more than 0.1% but 1% or less of Si, between 0.02% and 1.2% of Mn, between 15% and 23% of Cr, between 0.002% and 0.5% of Al, and one or both of Nb and Ti, with the balance being Fe and inevitable impurities; formulas (1) and (2) are satisfied; and an oxide film comprising 30% or more of Cr, Si, Nb, Ti, and Al in terms of the total cation percentage is formed on the surface. 8(C+N)+0.03≤Nb+Ti≤0.6 …(1) Si+Cr+Al+{Nb+Ti-8(C+N)}≥15.5 …(2)

Description

Ferritic stainless steel for biofuel supply system parts, biofuel supply system parts, ferritic stainless steel for exhaust heat recovery equipment, and exhaust heat recovery equipment

The present invention relates to ferritic stainless steel and biofuel supply system parts suitable for automobile fuel supply system parts for supplying biofuels such as bioethanol and biodiesel. In particular, the present invention relates to a ferritic stainless steel suitable for a biofuel supply system component that is close to an engine and tends to be hot, such as a fuel injection system component.
The present invention also relates to a ferritic stainless steel for an exhaust heat recovery device of an automobile and an exhaust heat recovery device. In particular, the present invention relates to a ferritic stainless steel suitable for an exhaust heat recovery unit in which a heat exchange part is assembled by brazing joint.
This application is filed in Japanese Patent Application No. 2011-071372 filed in Japan on March 29, 2011, Japanese Patent Application No. 2011-0771812 filed in Japan on March 29, 2011, and in Japan on March 14, 2012. Claimed priority based on Japanese Patent Application No. 2012-057362 filed and Japanese Patent Application No. 2012-057363 filed in Japan on March 14, 2012, the contents of which are incorporated herein by reference.

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.
Fuels such as bioethanol and biodiesel fuel, in addition to efforts to further reduce weight and install exhaust gas treatment equipment such as EGR (Exhaust Gas Recirculation), DPF (Diesel Particulate Filter), and urea SCR (Selective Catalytic Reduction) systems Efforts using are also being implemented.

Bioethanol is ethanol produced from biomass, and bioethanol is mixed with gasoline and used as fuel for gasoline engines. Biodiesel fuel is a fuel in which fatty acid methyl ester is mixed with light oil, and is used as a fuel for diesel engines. Here, ethanol is produced using corn and sugar cane as raw materials. Fatty acid methyl esters are produced by esterification using vegetable oil and waste oil such as rapeseed oil, soybean oil, and palm oil as raw materials.

Biofuels such as bioethanol and biodiesel fuel are considered to be more corrosive than metal materials. In using these, the influence on the use performance of various members constituting the fuel system parts has been investigated in advance. Manufacturers that guarantee an ultra-long life expectancy demands a more reliable material, and stainless steel is one of the candidates.

Among the fuel system parts, the following techniques are known as conventional techniques for applying stainless steel to a fuel tank or a fuel supply pipe.
In Patent Document 1, in mass%, C: ≦ 0.015%, Si: ≦ 0.5%, Cr: 11.0 to 25.0%, N: ≦ 0.020%, Ti: 0.05 -0.50%, Nb: 0.10-0.50%, and B: ≦ 0.0100%, and Mo: ≦ 3.0%, Ni: ≦ 2.0%, Cu if necessary A ferritic stainless steel sheet containing one or more selected from: ≦ 2.0% and Al: ≦ 4.0% is disclosed. The breaking elongation of the steel sheet is 30% or more, and the Rankford value is 1.3 or more.

Patent Document 2 includes mass%, C: ≦ 0.01%, Si: ≦ 1.0%, Mn: ≦ 1.5%, P: ≦ 0.06%, S: ≦ 0.03%, Cr: 11 to 23%, Ni: ≦ 2.0%, Mo: 0.5 to 3.0%, Al: ≦ 1.0%, and N: ≦ 0.04%, Cr + 3.3Mo ≧ 18 A ferritic stainless steel sheet satisfying the following relational expression is disclosed. The steel sheet further satisfies one or both of Nb: ≦ 0.8% and Ti: ≦ 1.0%, satisfying the relational expression of 18 ≦ Nb / (C + N) + 2Ti / (C + N) ≦ 60. contains. The grain number of the ferrite crystal grains of the steel sheet is 6.0 or more, and the average r value is 2.0 or more.

In Patent Document 3, in mass%, C: ≦ 0.01%, Si: ≦ 1.0%, Mn: ≦ 1.5%, P: ≦ 0.06%, S: ≦ 0.03%, Al: ≦ 1.0%, Cr: 11-20%, Ni: ≦ 2.0%, Mo: 0.5-3.0%, V: 0.02-1.0%, and N: ≦ 0 A ferritic stainless steel sheet containing 0.04% and containing either one or both of Nb: 0.01 to 0.8% and Ti: 0.01 to 1.0% is disclosed. In the steel sheet, the surface undulation height generated when deformed by 25% by uniaxial tension is 50 μm or less.

However, Patent Documents 1 to 3 are techniques dealing with corrosion resistance against ordinary gasoline. As will be described later, since the corrosiveness of biofuels is significantly different from that of gasoline, these technologies have insufficient corrosion resistance to biofuels.
Conventionally, the details of the corrosiveness of biofuels to stainless steel are not necessarily clarified, and it is difficult to say that the corrosion resistance of various stainless steel types to biofuels is necessarily clarified.

In addition to the above-mentioned efforts from the fuel side, efforts to improve fuel efficiency by installing a heat exchanger that recovers exhaust heat mainly from hybrid vehicles, a so-called exhaust heat recovery device, have been made as an approach to environmental problems in the automobile field. Yes. The exhaust heat recovery device is a system that heats engine cooling water with exhaust gas and uses it to warm 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 consumption, especially in winter.

The heat exchange part of the exhaust heat recovery device is required to have good thermal conductivity in order to obtain good thermal efficiency. Further, since the heat exchanging portion is in contact with the exhaust gas, excellent corrosion resistance is required for the exhaust gas condensed water. On the other hand, the outer surface of the exhaust heat recovery device is also required to have excellent corrosion resistance against salt damage. Such corrosion resistance is also required for exhaust system downstream members mainly composed of mufflers. However, corrosion of the exhaust heat recovery device may lead to a serious accident such as leakage of cooling water. Therefore, the exhaust heat recovery device is required to have higher safety and better corrosion resistance.

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

Also, since the structure of the heat exchange part is complicated, it may be assembled by welding joint, but it may be assembled by brazing joint. The material of the heat exchange part assembled by brazing and joining needs to have good brazing properties. Furthermore, since the exhaust heat recovery unit is often installed downstream of the catalytic converter under the floor, the exhaust gas on the inlet side is heated. Further, the exhaust gas is forcibly cooled by heat exchange. Accordingly, the exhaust heat recovery device also requires good thermal fatigue characteristics.

Patent Document 4 discloses an automobile exhaust heat recovery device made of ferritic stainless steel. The ferritic stainless steel has C: 0.020% or less, Si: 0.05 to 0.70%, Mn: 0.05 to 0.70%, P: 0.045% or less, S: 0.005 %: Ni: 0.70% or less, Cr: 18.00 to 25.50%, Cu: 0.70% or less, Mo: 2 / (Cr-17.00) to 2.50%, and N: Contains 0.020% or less. The ferritic stainless steel further includes one or both of Ti: 0.50% or less and Nb: 0.50% or less, and a relationship of (Ti + Nb) ≧ (7 × (C + N) +0.05) The formula is satisfied, and the balance is Fe and inevitable impurities. In the ferritic stainless steel described in Patent Document 4, corrosion resistance against exhaust gas condensed water is secured by adding Mo to 18% or more of Cr.

In Patent Document 5, C: 0.05% or less, Si: 0.02 to 1.0%, Mn: 0.5% or less, P: 0.04% or less, S: 0.02% or less, Al : 0.1% or less, Cr: 20 to 25%, Cu: 0.3 to 1.0%, Ni: 0.1 to 3.0%, Nb: 0.2 to 0.6%, and N: A ferritic stainless steel sheet containing 0.05% or less and having excellent crevice corrosion resistance is disclosed. The steel sheet has Nb carbonitride of 5 μm or less, and the steel sheet has a surface roughness Ra of 0.4 μm or less. In the ferritic stainless steel sheet described in Patent Document 5, Ni and Cu are added together with 20% or more of Cr to ensure crevice corrosion resistance.

Patent Document 6 discloses an automobile exhaust gas passage member made of ferritic stainless steel. The ferritic stainless steel has C: 0.015% or less, Si: 2.0% or less, Mn: 1.0% or less, P: 0.045% or less, S: 0.010% or less, Cr: 16 Containing 25 to 25%, Nb: 0.05 to 0.2%, Ti: 0.05 to 0.5%, N: 0.025% or less, and Al: 0.02 to 1.0%, The steel further contains one or both of Ni: 0.1 to 2.0% and Cu: 0.1 to 1.0% in total (Ni + Cu) of 0.6% or more. In the ferritic stainless steel described in Patent Literature 6, by adding 0.6% or more of Ni and Cu in total, inexpensive and good corrosion resistance is realized without using expensive Mo.

In Patent Document 7, Cr: 16-30%, Ni: 7-20%, C: 0.08% or less, N: 0.15% or less, Mn: 0.1-3%, S: 0.008 %, And Si: 0.1-5%, high temperature exhaust heat recovery satisfying Cr + 1.5Si ≧ 21 and 0.009Ni + 0.014Mo + 0.005Cu− (0.085Si + 0.008Cr + 0.003Mn) ≦ −0.25 Stainless steel for the heat pipe of the apparatus is disclosed. The technique described in Patent Document 7 relates to an exhaust heat recovery device using heat transfer means called a heat pipe, not a heat exchanger that exchanges heat between exhaust heat and cooling water. Patent Document 7 discloses an austenitic stainless steel suitable for a heat pipe.

排 Waste heat recovery equipment is required to have corrosion resistance equivalent to or better than ferritic stainless steel containing 17% or more of Cr. However, in the conventional ferritic stainless steel containing 17% or more of Cr, the corrosion resistance after brazing has not been considered. For this reason, when existing ferritic stainless steel is applied to an exhaust heat recovery device, the corrosion resistance after brazing cannot be sufficiently ensured due to the change in the metal structure of the brazed portion and the progress of oxidation of the steel surface.

JP 2003-277792 A JP 2002-285300 A JP 2002-363712 A JP 2009-228036 A JP 2009-7663 A JP 2007-92163 A JP 2010-24527 A

The present invention has been proposed in view of such a conventional situation, and an object of the present invention is to provide a ferritic stainless steel for biofuel supply system parts having corrosion resistance to biofuel in particular.
Another object of the present invention is to provide a ferritic stainless steel sheet for an exhaust heat recovery device that can be suitably used for a heat exchanging part assembled by brazing and has excellent corrosion resistance against exhaust gas condensed water. .

The gist of the first aspect of the present invention aimed at solving the above problems is as follows.
[1] In mass%, C: 0.03% or less, N: 0.03% or less, Si: more than 0.1%, 1% or less, Mn: 0.02% or more, 1.2% or less, Cr: 15% or more, 23% or less, Al: 0.002% or more, 0.5% or less, and any one or both of Nb and Ti, and the balance is composed of Fe and inevitable impurities. A biofuel characterized by satisfying the formulas (1) and (2) and having an oxide film containing Cr, Si, Nb, Ti, and Al in total of 30% or more in total on the surface thereof Ferritic stainless steel for supply system parts.
8 (C + N) + 0.03 ≦ Nb + Ti ≦ 0.6 (1)
Si + Cr + Al + {Nb + Ti-8 (C + N)} ≧ 15.5 (2)
In formula (1) and formula (2), the element symbol represents the content (mass%) of each element.

[2] Further, by mass%, Ni: 2% or less, Cu: 1.5% or less, Mo: 3% or less, and Sn: 0.5% or less are contained. The ferritic stainless steel for biofuel supply system parts as described in [1] above.
[3] Further, by mass, V: 1% or less, W: 1% or less, B: 0.005% or less, Zr: 0.5% or less, Co: 0.2% or less, Mg: 0.002 % Or less, Ca: 0.002% or less, and REM: One or more selected from the group consisting of 0.01% or less, The biofuel supply according to the above [1] or [2] Ferritic stainless steel for system parts.

[4] A biofuel supply system component comprising the ferritic stainless steel for a biofuel supply system component as described in any one of [1] to [3].

The gist of the second aspect of the present invention aimed at solving the above problems is as follows.
[5] By mass%, C: 0.03% or less, N: 0.05% or less, Si: more than 0.1%, 1% or less, Mn: 0.02% or more, 1.2% or less, Cr: 17% or more, 23% or less, Al: 0.002% or more, 0.5% or less, and any one or both of Nb and Ti, Ni: 0.25% or more 5% or less, Cu: 0.25% or more, 1% or less, and Mo: 0.5% or more, containing 2 or 3 types selected from the group consisting of 2% or less, the balance being Fe and inevitable impurities And satisfying the following formulas (3) and (4), and an oxide film containing Cr, Si, Nb, Ti, and Al with a total cation fraction of 40% or more is formed on the surface. Ferritic stainless steel for waste heat recovery equipment.
8 (C + N) + 0.03 ≦ Nb + Ti ≦ 0.6 (3)
Si + Cr + Al + {Nb + Ti-8 (C + N)} ≧ 17.5 (4)
In Formula (3) and Formula (4), an element symbol represents content (mass%) of each element. In the formula (4), the value of Nb + Ti-8 (C + N) is 0 or more.

[6] By mass%, C: 0.03% or less, N: 0.05% or less, Si: more than 0.1%, 1% or less, Mn: 0.02% or more, 1.2% or less, Cr: 17% or more, 23% or less, Al: 0.002% or more, 0.5% or less, and any one or both of Nb and Ti, Ni: 0.25% or more 5% or less, Cu: 0.25% or more, 1% or less, and Mo: 0.5% or more, containing 2 or 3 types selected from the group consisting of 2% or less, the balance being Fe and inevitable impurities consists, satisfies expression (3) and (4) shown below, by heat treatment in a _{H 2} atmosphere including a vacuum atmosphere or _{N 2} of ¹⁰ -2 ~ 1 torr comprising _{N 2,} on the surface, Cr, Si An oxide film containing 40% or more of the total cation fraction of Nb, Ti, and Al is formed. Heat recovery dexterity ferritic stainless steel characterized by Rukoto.
8 (C + N) + 0.03 ≦ Nb + Ti ≦ 0.6 (3)
Si + Cr + Al + {Nb + Ti-8 (C + N)} ≧ 17.5 (4)
In Formula (3) and Formula (4), an element symbol represents content (mass%) of each element. In the formula (4), the value of Nb + Ti-8 (C + N) is 0 or more.

[7] Further, in mass%, V: 0.5% or less, W: 1% or less, B: 0.005% or less, Zr: 0.5% or less, Sn: 0.5% or less, Co: 0 1 or more selected from the group consisting of 2% or less, Mg: 0.002% or less, Ca: 0.002% or less, and REM: 0.01% or less [5 ] Or ferritic stainless steel for exhaust heat recovery device according to [6].

[8] A heat exchange part in which members are assembled by brazing joining is provided, and the heat exchange part is made of ferritic stainless steel, and the ferritic stainless steel is C: 0.03% or less in mass%. N: 0.05% or less, Si: more than 0.1%, 1% or less, Mn: 0.02% or more, 1.2% or less, Cr: 17% or more, 23% or less, Al: 0. 002% or more, 0.5% or less, and any one or both of Nb and Ti, Ni: 0.25% or more, 1.5% or less, Cu: 0.25% or more, 1% or less And Mo: 2 or 3 selected from the group consisting of 0.5% or more and 2% or less, with the balance being Fe and inevitable impurities, and the following formulas (3) and (4) the filled, the surface, Cr, Si, Nb, Ti, and Al a cationic fraction Exhaust heat recovery device, wherein the oxide film containing a total of more than 40% is formed.
8 (C + N) + 0.03 ≦ Nb + Ti ≦ 0.6 (3)
Si + Cr + Al + {Nb + Ti-8 (C + N)} ≧ 17.5 (4)
In Formula (3) and Formula (4), an element symbol represents content (mass%) of each element. In the formula (4), the value of Nb + Ti-8 (C + N) is 0 or more.

[9] The ferritic stainless steel further includes, in mass%, V: 0.5% or less, W: 1% or less, B: 0.005% or less, Zr: 0.5% or less, Sn: 0.00. It contains at least one selected from the group consisting of 5% or less, Co: 0.2% or less, Mg: 0.002% or less, Ca: 0.002% or less, and REM: 0.01% or less. The exhaust heat recovery device as described in [8] above.

According to the first aspect of the present invention, it is possible to provide ferritic stainless steel having excellent corrosion resistance against biofuel. This ferritic stainless steel can be suitably used for biofuel supply system parts. In particular, this ferritic stainless steel is suitable for biofuel supply system parts that are close to the engine and tend to be hot, such as injection system parts.

According to the second aspect of the present invention, it is possible to provide a ferritic stainless steel for exhaust heat recovery equipment having corrosion resistance against exhaust gas condensed water after brazing. This ferritic stainless steel can be suitably used as a member for an exhaust heat recovery device. In particular, this ferritic stainless steel can be suitably used for a heat exchange part assembled by brazing joint.

Hereinafter, embodiments of the present invention will be described in detail.
(First embodiment)
The present inventors obtained E10, E22, and E100 of fuels containing bioethanol commonly used in North America and RME (Rapeseed Methylester) of biodiesel fuel commonly used in Europe. did. E10 and E22 are fuels in which bioethanol is mixed with gasoline at a ratio of 10% and 22%, respectively, and E100 is 100% bioethanol. RME is a fuel produced by methyl esterifying rapeseed oil. A detailed investigation and analysis of these oxidative degradation behaviors and corrosiveness to stainless steel were conducted in comparison with ordinary gasoline.

First, the oxidation stability of E10, E22, E100, and RME was evaluated according to JIS K2287 used in the method for evaluating the oxidation stability of gasoline, and compared with the oxidation stability of gasoline. These fuels were sealed in an autoclave, 7 atmospheres of oxygen were introduced, and then the temperature was raised to 100 ° C. and held. In this state, the change in pressure was measured, and the behavior of the pressure decreasing as oxygen was used to oxidize the fuel was evaluated.
As a result, the following matters became clear. (1) E10 and E100 are less susceptible to oxidative degradation than gasoline. (2) E22 and RME are more susceptible to oxidative degradation than gasoline, and the degree of oxidative degradation of RME is the greatest.

When the fuel is oxidized, fatty acids such as formic acid, acetic acid, and propionic acid are produced. In order to know the corrosiveness of fatty acids, first, stainless cold-rolled steel sheets were immersed in oxidized RME and gasoline to examine the presence or absence of corrosion. As a result, no corrosion was observed in either case.
This is because the fatty acid that is an oxidation product exists as a dimer in the fuel medium. In order for fatty acids to develop corrosive properties, they must dissociate and release hydrogen ions, and for this purpose, the existence of water was considered indispensable. In an actual environment, water is generated by condensation of moisture in the air, so it is extremely important to consider the coexistence of the aqueous phase.

Therefore, 10 vol% water was added to each of the oxidized RME and gasoline, and stainless steel cold-rolled steel sheets were immersed in these. As a result, corrosion occurred in both cases of RME and gasoline.
From this, it was confirmed that coexistence of water is indispensable for the oxidation-degraded fuel to exhibit corrosivity, and the corrosivity is manifested only after the fatty acid in the fuel is distributed to the aqueous phase. Since the corrosive substance in the aqueous phase is hydrogen ions, the corrosiveness is expressed by the hydrogen ion concentration. The hydrogen ion concentration in water depends mainly on the type of fatty acid in the oxidized fuel, the concentration of the fatty acid, and the partitioning behavior of the fatty acid between the fuel and the aqueous phase. Among these, the distribution behavior of the fatty acid is affected by the temperature, and the higher the temperature, the easier the fatty acid is distributed from the fuel to the water phase.

Also, in the case of RME, the pH of the aqueous phase is 2.1, and in the case of gasoline, the pH of the aqueous phase is 3.0, and there is a difference of 0.9 between the two. When this difference is converted into the fatty acid concentration, it corresponds to a difference of about 100 times. Conventionally, corrosion tests using oxidized and deteriorated gasoline have been conducted with a concentration of formic acid and acetic acid in water of about 100 to 1000 ppm. For this reason, it was found that in the corrosion test with biofuels such as RME, it is necessary to increase the concentration of formic acid + acetic acid to 1% to 10% corresponding to a concentration about 100 times that of gasoline.

In addition, the temperature of fuel injection system parts close to the engine rises to about 90 to 100 ° C., and as the temperature rises, fatty acids are easily distributed from the fuel to the water phase, and the corrosive environment becomes severe. This corrosive environment is a harsh condition compared to a temperature of 40 to 50 ° C. in a corrosion test with oxidized and deteriorated gasoline.
Furthermore, the bioethanol in the fuel moves to the aqueous phase and enlarges the aqueous phase, and becomes a factor that hinders maintaining a passive state (passive state) particularly in stainless steel.

As described above, since the corrosiveness of biofuel is higher than that of normal gasoline, the material used for the biofuel supply system parts is required to have better corrosion resistance.
Therefore, the present inventors diligently investigated the corrosion resistance in a high-temperature acidic fatty acid environment. As a result, the following matters were discovered. (1) It is most important to maintain a passive state and suppress the occurrence of corrosion by forming a stable oxide film on the surface of stainless steel. (2) On the surface, an oxide film containing Cr, Si, Nb, Ti, and Al with a total cation fraction ({(Cr + Si + Nb + Ti + Al) / (total cation content)} × 100) of 30% or more was formed. In some cases, it exhibits excellent corrosion resistance in an acidic fatty acid environment at high temperatures.

In order to form such an oxide film, first, the chemical composition of the steel material needs to satisfy the following formula (2).
Si + Cr + Al + {Nb + Ti-8 (C + N)} ≧ 15.5 (2)
In Formula (2), an element symbol represents content (mass%) of each element.

Note that the entire amount of Nb and / or Ti contained in the stainless steel does not exist as a solid solution, but a part of the Nb and / or Ti is fixed to C and N. Of the Nb and / or Ti contained in the stainless steel, the solid solution Nb and / or Ti that is not fixed to C and N is concentrated in the passive film (oxide film) by the heat treatment. Nb and Ti contribute to the corrosion preventing action in the oxide film formed by the heat treatment. Among Nb and / or Ti contained in stainless steel, the amount of Nb and / or Ti that is fixed to C and N and does not enter into a solid solution state is Nb atomic weight 93, C atomic weight 12, N atomic weight 14 From the ratio, it is considered that the total amount of C and N (C + N) is approximately 8 times. Therefore, in order to form the above oxide film that suppresses the occurrence of corrosion, the total content of Si, Cr, Al, and {Nb + Ti-8 (C + N)} contained in the stainless steel is 15.5% or more. It is necessary to make it 17.5% or more.

Further, heat treatment, by adjusting the process conditions such as pickling to form an oxide film of the above composition.
An example of the heat treatment for forming the oxide film having the above cation fraction on the surface of the steel material having the above chemical composition is a heat treatment for brazing and joining members to be parts. For example, fuel injection system parts such as delivery tubes and common rails are parts manufactured by brazing and joining members. As heat treatment conditions at the time of brazing for manufacturing such parts, in an _{H 2} atmosphere containing ¹⁰ vacuum atmosphere -2 ~ 1 torr (reduced pressure atmosphere) or _{N 2} containing _{N 2,} temperature of 800 ~ 1200 ° C. And a condition of holding for 0.5 to 30 minutes. Under these conditions, an oxide film having a desired composition can be suitably formed. Here, simply by heat treatment in a vacuum of 10 ⁻² torr or less, the total cation fraction of Cr, Si, Nb, Ti, and Al in the formed oxide film is the desired cation fraction. Not reach. For example, a vacuum of 10 ⁻² torr or less is evacuated, and then N ₂ is introduced to adjust the pressure to 10 ⁻² to 1 torr. By performing heat treatment in this atmosphere, an oxide film having a desired composition can be obtained. On the other hand, N ₂ may be introduced in the H ₂ atmosphere, but it is not particularly necessary to introduce N _2, and an oxide film having a desired composition can be obtained even with N ₂ remaining in the atmosphere.

The reason for this is not clear, but heat treatment in an environment containing N ₂ produces (Nb, Ti) carbonitrides on the surface of the steel, which promotes reduction of Fe oxide. It may have been done.
The content of N ₂ in the heat treatment atmosphere is preferably 0.001 to 0.2%, more preferably 0.005 to 0.1%.
The heat treatment condition is to hold at 1000 to 1200 ° C. for 5 to 30 minutes in order to form an oxide film enriched with Cr, Si, Nb, Ti and Al with a total cation fraction of 30% or more. Is preferred. The holding temperature is more preferably 1050 to 1150 ° C., and the holding time is more preferably 10 to 20 minutes.

Thus, the oxide film which has the said cation fraction can be formed by the heat processing at the time of brazing and joining the member which consists of a steel material of the said chemical composition. Therefore, the heat treatment step for forming the oxide film having the cation fraction can also serve as a step of brazing and joining a member made of a steel material having the above chemical composition.
In the case of producing a part that is not brazed, in order to form an oxide film having the above cation fraction, it is 800 to 1200 ° C. in an environment containing N ₂ and a pressure of 10 ⁻² to 1 torr. You may perform the heat processing process hold | maintained at temperature for 0.5 to 30 minutes. In addition, in order to simplify the manufacturing process and improve productivity, the above heat treatment process is not added, and in the manufacturing process of steel materials and parts, conditions for heat treatment for forming an oxide film and pickling that removes the oxide film are removed. In this way, an oxide film having a desired cation fraction may be formed.

When forming an oxide film having the above cation fraction in the manufacturing process of steel materials and parts, specifically, for example, N _{2 having} a dew point of −45 to −75 ° C. in final finish annealing in the manufacturing process of steel materials. a mixed gas atmosphere of _{H 2,} and a method of holding 0.5 to 5 minutes at 800 ~ 1100 ° C.. In this case, the subsequent pickling is omitted.

Here, in order to obtain much more excellent corrosion resistance, the oxide film preferably contains Cr, Si, Nb, Ti, and Al in a total of 40% or more of the cation fraction. Further, it is preferable to contain 20% or more of the most important Cr among Cr, Si, Nb, Ti, and Al in terms of cation fraction (the ratio of Cr content to the content of all cations in the oxide film). . The total of the cation fractions of Cr, Si, Nb, Ti, and Al is more preferably 50% or more.
The film thickness of the oxide film is preferably 15 nm or less, more preferably 10 nm or less. The increase in film thickness leads to a decrease in the cation fraction of Cr, Si, Nb, Ti, and Al per unit volume, leading to a decrease in corrosion resistance. There is a possibility that (Nb, Ti) carbonitride generated by heat treatment in an environment containing N ₂ suppresses an increase in film thickness.

In addition to the above knowledge, this embodiment is made in consideration of workability required as a material for biofuel supply system parts, and provides ferritic stainless steel for fuel supply system parts having excellent corrosion resistance against biofuels. To do. The summary is shown below.

Hereinafter, the reason why each composition component of ferritic stainless steel for biofuel supply system parts is limited will be described. In addition, the ferritic stainless steel of this embodiment comprises a steel main body and an oxide film provided on the surface of the steel main body. Since the thickness of the oxide film is very thin compared to the thickness of the steel body, the composition of the steel material before the oxide film is formed is substantially the same as the composition of the steel body (steel material) after the oxide film is formed. Are identical. The composition of the steel body (steel material) will be described below. In the present specification, unless otherwise specified, the unit “%” indicating the content of a component represents mass%.

(C: 0.03% or less)
Since C reduces intergranular corrosion resistance and workability, it is necessary to keep the content low. For this reason, content of C shall be 0.03% or less. However, excessively lowering the C content increases the scouring cost, so the C content is preferably 0.002% or more. The C content is more preferably 0.002 to 0.02%.

(N: 0.03% or less)
N is an element useful for pitting corrosion resistance, but it is necessary to keep the N content low in order to reduce intergranular corrosion resistance and workability. For this reason, content of N shall be 0.03% or less. However, excessively reducing the N content increases the scouring cost, so the N content is preferably 0.002% or more. The N content is more preferably 0.002 to 0.02%.
Moreover, it is preferable that the total content of C and N is 0.015% or more from the viewpoint of suppressing coarsening of crystal grains during heat treatment by carbonitride and suppressing a decrease in strength.

(Si: more than 0.1%, 1% or less)
Si concentrates in the surface film after heat treatment and contributes to the improvement of the corrosion resistance of stainless steel. In order to obtain this effect, at least 0.1% of Si is required. Si is useful as a deoxidizing element. However, excessive addition of Si decreases workability, so the Si content is 1% or less. The Si content is preferably more than 0.1% to 0.5%.

(Mn: 0.02% or more, 1.2% or less)
Mn is an element useful as a deoxidizing element and needs to contain at least 0.02% or more of Mn. However, if an excessive amount of Mn is contained, the corrosion resistance deteriorates, so the Mn content is set to 1.2% or less. The Mn content is preferably 0.05 to 1%.

(Cr: 15% or more, 23% or less)
Cr is an element serving as a basis for ensuring corrosion resistance in biofuel, and needs to contain at least 15% or more of Cr. The corrosion resistance can be improved as the content of Cr is increased. However, the addition of an excessive amount of Cr decreases the workability and manufacturability, so the Cr content is 23% or less. The Cr content is preferably 17 to 20.5%.

8 (C + N) + 0.03 ≦ Nb + Ti ≦ 0.6 (1)
In addition, in Formula (1), an element symbol represents content (mass%) of each element.
Nb and Ti are elements useful for fixing C and N and improving the intergranular corrosion resistance of the weld. In order to obtain this effect, it is necessary to contain Nb and Ti so that the total amount of Nb and Ti (Nb + Ti) is eight times or more of the total amount of C and N (C + N). Further, Nb and Ti are concentrated in the surface film of stainless steel after the heat treatment and contribute to the improvement of corrosion resistance. In order to obtain this effect, at least 0.03% or more of Nb and / or Ti in a solid solution state that is not fixed to C and N needs to be contained. Therefore, the lower limit of Nb + Ti is set to 8 (C + N) + 0.03%. However, excessive addition of Nb and / or Ti decreases workability and manufacturability, so the upper limit of Nb + Ti is set to 0.6%. Nb + Ti is preferably {10 (C + N) +0.03} or more and 0.6% or less.

Here, of Nb and Ti, Ti concentrates on the surface film of stainless steel and contributes to improvement of corrosion resistance. However, Ti has an effect of inhibiting brazing properties. In order to obtain good brazing properties when producing biofuel supply system parts by brazing, it is preferable to limit the amount of Ti so that the value of Ti-3N is 0.03% or less.

(Al: 0.002% or more, 0.5% or less)
Al concentrates in the surface film of stainless steel after heat treatment and contributes to the improvement of corrosion resistance. In order to acquire this effect, it is necessary to contain 0.002% or more of Al. In addition, Al has an effect such as a deoxidizing effect, and thus is an element useful in scouring and has an effect of improving moldability. However, since the addition of an excessive amount of Al deteriorates toughness, the Al content is set to 0.002 to 0.5%. The Al content is preferably 0.005 to 0.1%.

(Ni: 2% or less)
In order to improve the corrosion resistance, 2% or less of Ni may be included as necessary. The Ni content that provides a stable effect is 0.2% or more. The corrosion resistance can be improved as the Ni content is increased. However, the addition of a large amount of Ni hardens the steel and reduces workability. Moreover, since Ni is expensive, it leads to a cost increase. Therefore, the Ni content is preferably 0.2 to 2%, more preferably 0.2 to 1.2%.

(Cu: 1.5% or less)
In order to improve the corrosion resistance, 1.5% or less of Cu may be included as necessary. The Cu content that provides a stable effect is 0.2% or more. The corrosion resistance can be improved as the Cu content is increased. However, the addition of a large amount of Cu hardens the steel and decreases the workability. Therefore, Cu content is preferably 0.2 to 1.5% and more preferably 0.2 to 0.8%.

(Mo: 3% or less)
In order to improve corrosion resistance, you may contain 3% or less of Mo as needed. The Mo content that provides a stable effect is 0.3% or more. The corrosion resistance can be improved as the content of Mo is increased. However, the addition of a large amount of Mo hardens the steel and decreases the workability. Moreover, since Mo is expensive, it leads to a cost increase. Therefore, the Mo content is preferably 0.3 to 3%, more preferably 1 to 0.5 to 2.0%.

(Sn: 0.5% or less)
In order to improve the corrosion resistance, 0.5% or less of Sn may be included as necessary. The Sn content that provides a stable effect is 0.01% or more. As the Sn content is increased, the corrosion resistance can be improved. However, the addition of a large amount of Sn hardens the steel and decreases the workability. Therefore, the Sn content is preferably 0.01 to 0.5%, more preferably 0.05 to 0.4%.

(V: 1% or less)
In order to improve the corrosion resistance, 1% or less of V may be contained as necessary. The content of V that provides a stable effect is 0.05% or more. However, the addition of an excessive amount of V deteriorates workability. Moreover, since V is expensive, it leads to an increase in cost. Therefore, the V content is preferably 0.05 to 1%.

(W: 1% or less)
In order to improve the corrosion resistance, 1% or less of W may be contained as necessary. The W content that provides a stable effect is 0.3% or more. However, the addition of an excessive amount of W deteriorates workability. Moreover, since W is expensive, it leads to an increase in cost. Therefore, the W content is preferably 0.3 to 1%.

(B: 0.005% or less)
In order to improve workability, particularly secondary workability, 0.005% or less of B may be contained as necessary. In order to obtain a stable effect, it is preferable to contain 0.0001% or more of B. The B content is more preferably 0.0002 to 0.001%.

(Zr: 0.5% or less)
In order to improve the corrosion resistance, 0.5% or less of Zr may be contained as necessary. In order to obtain a stable effect, it is preferable to contain 0.05% or more of Zr.

(Co: 0.2% or less)
In order to improve secondary workability and toughness, 0.2% or less of Co may be included as necessary. In order to obtain a stable effect, it is preferable to contain 0.02% or more of Co.

(Mg: 0.002% or less)
Mg is an element useful for scouring because it has effects such as a deoxidation effect. Further, Mg has an effect on the miniaturized improvement in workability and toughness tissue. For this reason, you may contain 0.002% or less of Mg as needed. In order to obtain a stable effect, it is preferable to contain 0.0002% or more of Mg.

(Ca: 0.002% or less)
Ca is an element useful for scouring because it has an effect such as a deoxidation effect. For this reason, you may contain 0.002% or less of Ca as needed. In order to obtain a stable effect, it is preferable to contain 0.0002% or more of Ca.

(REM: 0.01% or less)
REM is an element useful for scouring because it has effects such as a deoxidation effect. For this reason, you may contain 0.01% or less of REM as needed. In order to obtain a stable effect, it is preferable to contain REM 0.001% or more.

Of the inevitable impurities, the P content is preferably 0.04% or less from the viewpoint of weldability, and the P content is more preferably 0.035% or less. Moreover, about S, it is preferable to make S content into 0.02% or less from a corrosion-resistant viewpoint, More preferably, S content is 0.01% or less.

The stainless steel of this embodiment is manufactured by the following method, for example.
Molten steel having the above chemical composition is converted into a converter or electric furnace, the molten steel is refined in an AOD furnace, a VOD furnace, or the like, and then formed into a steel piece by a continuous casting method or an ingot forming method. The steel slab is subjected to the steps of hot rolling, annealing, pickling, cold rolling, finish annealing, and pickling. Thereafter, _{H 2} atmosphere including a vacuum atmosphere or _{N 2} of ¹⁰ -2 ~ 1 torr comprising _{N 2,} performs heat treatment step of holding 0.5 to 30 minutes at a temperature of 800 ℃ ~ 1200 ℃. Thereby, an oxide film having the cation fraction is formed. If necessary, annealing of the hot-rolled sheet may be omitted, or cold rolling, finish annealing, and pickling may be repeated. Examples of product forms include plates, tubes, bars, and wires.
In addition, as described above, the stainless steel of the present embodiment may be manufactured by a method in which the heat treatment step is performed after the steps of cold rolling, finish annealing, and pickling. However, the stainless steel of this embodiment may be manufactured by a method in which the heat treatment process is performed at another stage of the manufacturing process.

Next, the biofuel supply system component of this embodiment will be described.
The biofuel supply system component of this embodiment is made of the stainless steel of this embodiment.
The biofuel supply system component of this embodiment is preferably manufactured by a method of performing the step of forming a member having the above chemical composition and the above heat treatment step. The heat treatment step in the method for manufacturing a biofuel supply system component of the present embodiment may be performed before being processed into a shape as a component, or may be performed after being processed into a shape as a component. When the heat treatment step is performed after being processed into a shape as a part, it is preferable that the shape is processed without removing the oxide film on the surface and reducing the corrosion resistance.
The heat treatment step preferably also serves as a step of brazing the members. In this case, compared with the case where the heat treatment step and the brazing step are performed separately, the biofuel supply system component can be efficiently manufactured.
In addition, the biofuel supply system component of this embodiment should just consist of stainless steel of this embodiment, and is not limited to what was brazed and joined.

(Second Embodiment)
When ferritic stainless steel is applied to an exhaust heat recovery device, it is necessary to consider corrosion damage as in the case of application to an exhaust system downstream member mainly composed of a muffler. This important corrosion damage is pitting due to pitting corrosion or crevice corrosion. Similarly to the exhaust system downstream member mainly composed of a muffler, it is necessary to prevent leakage of internal fluid due to perforation in the exhaust heat recovery device. Furthermore, in the exhaust heat recovery device, it is necessary to prevent leakage of cooling water in addition to the exhaust gas. Therefore, the exhaust heat recovery device requires better perforation resistance than a muffler or the like. In addition, there is a need to reduce the thickness of the heat exchanging portion for the purpose of improving thermal efficiency, and excellent perforation resistance is also required in this respect.

Of the heat exchange part of the exhaust heat recovery unit, the exhaust gas side is required to have corrosion resistance against the exhaust gas condensed water. With the diversification of fuels, exhaust gas condensate has also diversified, increasing chloride ions and sulfuric acid ions (SO ₃ ²⁻ , SO ₄ ²⁻ ), which have a significant effect on corrosion resistance, and a neutral to weak pH. It may change to acidity and the corrosive environment may become severe.
In view of such a background, the present inventors have intensively studied on improving the perforation resistance of stainless steel in an exhaust gas condensed water environment.

As a result, it was found that the following (1) and (2) must be combined to improve the perforation resistance against pitting corrosion and crevice corrosion and to obtain stainless steel having excellent corrosion resistance.
(1) It is effective to contain Ni, Cu, and Mo, and two or more of these should be contained in combination.
(2) At the time of brazing, the film formed on the surface is composed of Cr, Si, Nb, Ti, and Al with a total cation fraction ({(Contains Cr, Si, Nb, Ti, and Al contained in the oxide film) (Total amount) / (content of all cation elements contained in oxide film)} × 100 (%)).

In order to improve the pitting resistance of stainless steel against pitting corrosion and crevice corrosion, it is effective to make improvements from both aspects of the occurrence and growth of corrosion.
First, it is effective to contain Cr for suppressing the occurrence of corrosion. By containing an appropriate amount of Cr in stainless steel, a passive film (oxide film) rich in Cr is formed on the surface.

Furthermore, when brazing is performed in a vacuum or in an environment with a low oxygen partial pressure such as a hydrogen atmosphere, elements such as Nb, Si, and Al contained in the steel material are concentrated in the passive film, and Cr, Si, An oxide film rich in Nb, Ti, and Al is formed. The present inventor has found that the oxide film formed on the surface of stainless steel contains these elements in a total of 40% or more of the cation fraction, and is particularly corrosive among the pore resistance in the exhaust gas condensed water environment. It has been found that it acts effectively on the suppression of the occurrence of selenium.

In order to form such an oxide film, the chemical composition of the steel material needs to satisfy the following formula (4).
Si + Cr + Al + {Nb + Ti-8 (C + N)} ≧ 17.5 (4)
In Formula (4), an element symbol represents content (mass%) of each element. The value of Nb + Ti-8 (C + N) is 0 or more.

Note that the entire amount of Nb and / or Ti contained in the stainless steel does not exist as a solid solution, but a part of the Nb and / or Ti is fixed to C and N. Of the Nb and / or Ti contained in the stainless steel, the solid solution Nb that is not fixed to C and N is concentrated in the passive film (oxide film) during brazing. And Nb contributes to the corrosion prevention effect in the oxide film formed by brazing. Among Nb and / or Ti contained in stainless steel, the amount of Nb and / or Ti which is fixed to C and N and does not enter into a solid solution state is Nb atomic weight 93, C atomic weight 12, N atomic weight 14 From the ratio, it is considered that the total amount of C and N (C + N) is approximately 8 times. Therefore, in order to form the above oxide film that suppresses the occurrence of corrosion, the total content of Si, Cr, Al, and {Nb + Ti-8 (C + N)} contained in the stainless steel is 17.5% or more. There is a need to.

On the other hand, the heat treatment conditions in which the oxide film during brazing is formed in an _{H 2} atmosphere containing ¹⁰ vacuum atmosphere -2 ~ 1 torr (reduced pressure atmosphere) or _{N 2} containing _{N 2,} temperature of 1000 ~ 1200 ° C. The condition of holding for 5 to 30 minutes at is preferable. If the heat treatment is simply performed in a vacuum of 10 ⁻² torr or less, the total cation fraction of Cr, Si, Nb, Ti, and Al in the formed oxide film does not reach the desired cation fraction. For example, a vacuum of 10 ⁻² torr or less is evacuated, and then N ₂ is introduced to adjust the pressure to 10 ⁻² to 1 torr. By heat-treating in this atmosphere, it is possible to form an oxide film in which Cr, Si, Nb, Ti, and Al are concentrated so that the total cation fraction is 40% or more. On the other hand, in an H ₂ atmosphere is not particularly necessary to introduce N _2, it is possible to obtain an oxide film having a desired composition with N ₂ remaining in the atmosphere.

The reason for this is not clear, but heat treatment in an environment containing N ₂ produces (Nb, Ti) carbonitride on the surface of the stainless steel, which reduces the reduction of Fe oxide. It may have been promoted.
The content of N ₂ in the heat treatment atmosphere is preferably 0.001 to 0.2%, more preferably 0.005 to 0.1%.
The heat treatment condition is to hold at 1050 to 1150 ° C. for 5 to 30 minutes in order to form an oxide film enriched with Cr, Si, Nb, Ti and Al having a total cation fraction of 40% or more. Is more preferable. The holding time is more preferably 10 to 20 minutes.

Thus, the oxide film which has the said cation fraction can be formed by the heat processing at the time of brazing and joining the member which consists of a steel material of the said chemical composition. Therefore, the heat treatment step for forming the oxide film having the cation fraction can also serve as a step of brazing and joining a member made of a steel material having the above chemical composition.
In the case where brazing is not performed, in order to form an oxide film having the above cation fraction, 0.5 is contained at a temperature of 800 to 1200 ° C. in an environment containing N ₂ and a pressure of 10 ⁻² to 1 torr. A heat treatment step of holding for ˜30 minutes may be performed. In addition, in order to simplify the manufacturing process and improve productivity, the above heat treatment process is not added, and in the manufacturing process of steel materials and parts, conditions for heat treatment for forming an oxide film and pickling that removes the oxide film are removed. In this way, an oxide film having a desired cation fraction may be formed.

When forming an oxide film having the above cation fraction in the manufacturing process of steel materials and parts, specifically, for example, N _{2 having} a dew point of −45 to −75 ° C. in final finish annealing in the manufacturing process of steel materials. a mixed gas atmosphere of _{H 2,} and a method of holding 0.5 to 5 minutes at 800 ~ 1100 ° C.. In this case, the subsequent pickling is omitted.

Of the Cr, Si, Nb, Ti, and Al contained in the oxide film, Cr is the most important, and the cation fraction (the ratio of the Cr content to the total cation content in the oxide film) is 20%. It is preferable to contain the above Cr. More preferably, Cr, Si, Nb, Ti, and Al are 50% or more in total of the cation fraction.
The film thickness of the oxide film is preferably 15 nm or less, more preferably 10 nm or less. The increase in the film thickness leads to a decrease in the cation of Cr, Si, Nb, Ti, and Al per unit volume, leading to a decrease in corrosion resistance. There is a possibility that (Nb, Ti) carbonitride generated by heat treatment in an environment containing N ₂ suppresses an increase in film thickness.

On the other hand, from the viewpoint of the effect of inhibiting the growth of corrosion, the present inventor has focused on Ni, Cu, and Mo. The reason why the perforation resistance is improved by combining stainless steel with two or more selected from Ni, Cu, and Mo is estimated as follows.
With the occurrence of corrosion, chloride concentrates in the pits or in the gaps, and the pH decreases. In many of these environments, the material is actively dissolved, but Ni, Cu, and Mo are all effective in reducing the active dissolution rate. Further, since the exhaust heat recovery device is used in an environment in which wetting and drying are repeated, the progress and stop of corrosion are repeated. In this case, it is effective for the perforation resistance that the corrosion is likely to stop (repassivation is easy) and the corrosion is less likely to reoccur. It is considered that the cathodic reaction influences the ease of stopping the corrosion (repassivation) as well as the dissolution reaction (anode reaction). Ni and Cu having an effect of promoting the cathode reaction are considered to contribute to the promotion of repassivation. Here, it is considered that Ni mainly contributes to the promotion of repassivation by increasing the cathode current. Cu is thought to contribute to the promotion of repassivation by the action of making the potential noble. Meanwhile, Mo has the effect of strengthening the passive, suppresses re-occurrence of corrosion. By combining such effects of Ni, Cu, and Mo, it is estimated that the perforation resistance of stainless steel is improved.

This embodiment is made in consideration of the thermal fatigue characteristics and workability required as a member of the exhaust heat recovery device in addition to the above knowledge about the perforation resistance, and exhaust heat with excellent corrosion resistance against exhaust gas condensed water. Providing ferritic stainless steel for collectors. The summary is shown below.

Hereinafter, the reason for limiting each composition component of the ferritic stainless steel for the exhaust heat recovery device will be described. In addition, the ferritic stainless steel of this embodiment comprises a steel main body and an oxide film provided on the surface of the steel main body. Since the thickness of the oxide film is very thin compared to the thickness of the steel body, the composition of the steel material before the oxide film is formed is substantially the same as the composition of the steel body (steel material) after the oxide film is formed. Are identical. The composition of the steel body (steel material) will be described below. In the present specification, unless otherwise specified, the unit “%” indicating the content of a component represents mass%.

(C: 0.03% or less)
Since C reduces intergranular corrosion resistance and workability, it is necessary to keep the content low. For this reason, content of C shall be 0.03% or less. However, excessively lowering the C content increases the scouring cost, so the C content is preferably 0.002% or more. The C content is more preferably 0.002 to 0.02%.

(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. For this reason, content of N shall be 0.05% or less. However, excessively reducing the N content increases the scouring cost, so the N content is preferably 0.002% or more. The N content is more preferably 0.002 to 0.02%.
Furthermore, from the viewpoint of suppressing crystal grain coarsening during brazing, the total content of C and N is preferably 0.015% or more ((C + N) ≧ 0.015%).

(Si: more than 0.1%, 1% or less)
Si concentrates in the surface film of stainless steel after brazing and contributes to the improvement of corrosion resistance. In order to obtain this effect, 0.1% or more of Si is necessary. Si is useful as a deoxidizing element. However, excessive addition of Si decreases workability, so the Si content is 1% or less. The Si content is more preferably more than 0.1% to 0.5%.

(Mn: 0.02% or more, 1.2% or less)
Mn is an element useful as a deoxidizing element and needs to contain at least 0.02% or more of Mn. However, if an excessive amount of Mn is contained, the corrosion resistance deteriorates, so the Mn content is set to 1.2% or less. The Mn content is more preferably 0.05 to 1%.

(Cr: 17% or more, 23% or less)
Cr is an element serving as a basis for ensuring corrosion resistance and salt damage resistance to the exhaust gas condensate of stainless steel, and needs to contain at least 17% or more of Cr. The corrosion resistance can be improved as the content of Cr is increased. However, a large amount of Cr needs to be added in order to obtain the same effect as Ni, Cu, and Mo with respect to the perforation resistance of the gap. Moreover, since addition of an excessive amount of Cr decreases workability and manufacturability, the Cr content is set to 23% or less. The Cr content is preferably 17% or more and 20.5% or less.

(Al: 0.002% or more, 0.5% or less)
Al is concentrated in the surface film of stainless steel after brazing and contributes to the improvement of corrosion resistance. In order to acquire this effect, it is necessary to contain 0.002% or more of Al. In addition, Al has an effect such as a deoxidizing effect, and thus is an element useful in scouring and has an effect of improving moldability. However, since the addition of an excessive amount of Al deteriorates toughness, the Al content is set to 0.002 to 0.5%. The Al content is preferably 0.003 to 0.1%.

In the present embodiment, the stainless steel needs to contain two or three selected from the group consisting of Ni, Cu, and Mo.
(Ni: 0.25% to 1.5%)
Ni, together with Cu and Mo, is an important element for improving the corrosion resistance, particularly the perforation resistance. The content of Ni that provides a stable effect in a state containing either Cu or Mo is 0.25% or more. The corrosion resistance can be improved as the Ni content is increased. However, the addition of a large amount of Ni hardens the steel and reduces workability. Moreover, since Ni is expensive, it leads to a cost increase. Therefore, the Ni content is 1.5% or less. The Ni content is preferably 0.25 to 1.2%, more preferably 0.25 to 0.6%.

(Cu: 0.25% to 1%)
Cu, together with Ni and Mo, is an important element for improving the corrosion resistance, particularly the perforation resistance. The Cu content that provides a stable effect in a state containing either Ni or Mo is 0.25% or more. The corrosion resistance can be improved as the Cu content is increased. However, the addition of a large amount of Cu hardens the steel and reduces workability. Therefore, the Cu content is 1% or less. The Cu content is preferably 0.25 to 0.8%, more preferably 0.25 to 0.6%.

(Mo: 0.5% or more, 2% or less)
Mo, together with Ni and Cu, is an important element for improving the corrosion resistance, particularly the perforation resistance. The Mo content that provides a stable effect in a state containing either Ni or Cu is 0.5% or more. The corrosion resistance can be improved as the content of Mo is increased. However, the addition of a large amount of Mo hardens the steel and reduces workability. Moreover, since Mo is expensive, it leads to a cost increase. Therefore, the Mo content is 2% or less. As described above, Mo is a more important element because it improves the perforation resistance by an action different from that of Ni and Cu. Therefore, it is preferable to contain Mo in a range of 0.7% to 2%. The Mo content is more preferably 0.9% or more and 2% or less.

8 (C + N) + 0.03 ≦ Nb + Ti ≦ 0.6 (3)
In the equation (3), the element symbols represent the content of respective elements (mass%).
Nb and Ti are elements useful for fixing C and N and improving the intergranular corrosion resistance of the weld. In order to obtain this effect, it is necessary to contain Nb and Ti so that the total amount of Nb and Ti (Nb + Ti) is 8 times or more the total amount of C and N (C + N). Moreover, Nb and Ti concentrate on the surface film of stainless steel after brazing and contribute to improving corrosion resistance. In order to obtain this effect, at least 0.03% or more of Nb and / or Ti in a solid solution state that is not fixed to C and N needs to be contained. Therefore, the lower limit of Nb + Ti is set to 8 (C + N) + 0.03%. However, addition of an excessive amount of Nb and / or Ti decreases workability and manufacturability, so the upper limit of the content of Nb + Ti is set to 0.6%. Nb + Ti is preferably {10 (C + N) +0.03} or more and 0.6% or less.

Here, of Nb and Ti, Ti concentrates on the surface film of stainless steel and contributes to improvement of corrosion resistance. However, Ti has an effect of inhibiting brazing properties. In order to obtain good brazing properties, it is preferable to limit the amount of Ti so that the value of Ti-3N is 0.03% or less. On the other hand, Nb has the effect of improving the high temperature strength. The exhaust heat recovery device is required to have thermal fatigue characteristics in order to cool high temperature exhaust gas. Thus, when stainless steel is applied to a member that requires thermal fatigue characteristics, the stainless steel preferably contains Nb.

(V: 0.5% or less)
In order to improve the corrosion resistance, 0.5% or less of V may be contained as necessary. The content of V that provides a stable effect is 0.05% or more. However, the addition of an excessive amount of V deteriorates workability. Moreover, since V is expensive, it leads to an increase in cost. Therefore, the V content is preferably 0.05 to 0.5%.

(W: 1% or less)
In order to improve the corrosion resistance, 1% or less of W may be contained as necessary. The W content that provides a stable effect is 0.3% or more. However, the addition of an excessive amount of W deteriorates workability. Moreover, since W is expensive, it leads to an increase in cost. Therefore, the W content is preferably 0.3 to 1%.

(B: 0.005% or less)
In order to improve workability, particularly secondary workability, 0.005% or less of B may be contained as necessary. In order to obtain a stable effect, it is preferable to contain 0.0001% or more of B. The B content is more preferably 0.0002 to 0.0015%.

(Zr: 0.5% or less)
In order to improve the corrosion resistance, 0.5% or less of Zr may be contained as necessary. In order to obtain a stable effect, it is preferable to contain 0.05% or more of Zr.

(Sn: 0.5% or less)
In order to improve the corrosion resistance, 0.5% or less of Sn may be included as necessary. In order to obtain a stable effect, it is preferable to contain 0.01% or more of Sn.

(Co: 0.2% or less)
In order to improve secondary workability and toughness, 0.2% or less of Co may be included as necessary. In order to obtain a stable effect, it is preferable to contain 0.02% or more of Co.

(Mg: 0.002% or less)
Mg is an element useful for scouring because it has effects such as a deoxidation effect. Further, Mg has an effect on the miniaturized improvement in workability and toughness tissue. For this reason, you may contain 0.002% or less of Mg as needed. In order to obtain a stable effect, it is preferable to contain 0.0002% or more of Mg.

(Ca: 0.002% or less)
Ca is an element useful for scouring because it has an effect such as a deoxidation effect. For this reason, you may contain 0.002% or less of Ca as needed. In order to obtain a stable effect, it is preferable to contain 0.0002% or more of Ca.

(REM: 0.01% or less)
REM is an element useful for scouring because it has effects such as a deoxidation effect. For this reason, you may contain 0.01% or less of REM as needed. In order to obtain a stable effect, it is preferable to contain REM 0.001% or more.

Of the inevitable impurities, the P content is preferably 0.04% or less from the viewpoint of weldability, and the P content is more preferably 0.035% or less. Moreover, about S, it is preferable to make S content into 0.02% or less from a corrosion-resistant viewpoint, More preferably, S content is 0.01% or less.

The stainless steel of this embodiment is manufactured by the following method, for example.
Molten steel having the above chemical composition is converted into a converter or electric furnace, the molten steel is refined in an AOD furnace, a VOD furnace, or the like, and then formed into a steel piece by a continuous casting method or an ingot forming method. The steel slab is subjected to the processes of hot rolling, hot-rolled sheet annealing, pickling, cold rolling, finish annealing, and pickling. Thereafter, _{H 2} atmosphere including a vacuum atmosphere or _{N 2} of ¹⁰ -2 ~ 1 torr comprising _{N 2,} performs heat treatment step of holding 0.5 to 30 minutes at a temperature of 800 ℃ ~ 1200 ℃. Thereby, an oxide film having the cation fraction is formed. In addition, the said heat processing process can serve as the process of brazing and joining the member which consists of steel materials of the said chemical composition. If necessary, annealing of the hot-rolled sheet may be omitted, or cold rolling, finish annealing, and pickling may be repeated. Examples of product forms include plates, tubes, bars, and wires.

Next, the exhaust heat recovery device of this embodiment will be described.
The exhaust heat recovery device of the present embodiment includes a heat exchange part, and this heat exchange part is formed by assembling members by brazing and joining. The heat exchanging portion is made of the ferritic stainless steel of this embodiment, and this ferritic stainless steel has the above-described chemical composition, and Cr, Si, Nb, Ti, and Al on the surface have a cation fraction. An oxide film containing 40% or more in total is formed.

The manufacturing method of the exhaust heat recovery device of the present embodiment includes a step of forming a member having the chemical composition of the present embodiment and a step of assembling the member by, for example, a general processing process. The members of the assembly process, it is preferable to brazing by heat-treating member in an _{H 2} atmosphere including a vacuum atmosphere or _{N 2} of ¹⁰ -2 ~ 1 torr containing _{N 2.} By performing such an assembly process, an oxide film containing Cr, Si, Nb, Ti, and Al in a total cation fraction of 40% or more is formed on the surface of the member made of ferritic stainless steel. The heat exchange part of this embodiment is obtained by the above.
In the member assembling process, brazing and joining are not necessary. In this case, the ferritic stainless steel of this embodiment having an oxide film on the surface is processed into a shape as a part. Thereby, a member is formed. Subsequently, a heat exchange part is obtained by assembling a member.

Hereinafter, the effects of the present embodiment will be made clearer by examples. In addition, this embodiment is not limited to the following Examples, It can implement by changing suitably in the range which does not change the summary.

[Example 1]
150 kg of molten steel having the composition shown in Tables 1 and 2 was melted in a vacuum melting furnace and cast into a 50 kg steel ingot to obtain a steel piece. Next, the steel slab was hot rolled to a plate thickness of 4 mm at a heating temperature of 1200 ° C. to obtain a hot rolled sheet. Thereafter, the hot-rolled sheet was annealed at 850 to 950 ° C. Next, the scale was removed by shot blasting and pickling in a nitric hydrofluoric acid solution (mixed solution of nitric acid and hydrofluoric acid). Then, the steel plate was cold rolled to a plate thickness of 2 mm. Again, intermediate annealing was performed in the same temperature range as the annealing of the hot-rolled sheet. Thereafter, pickling was performed under the same conditions to remove the scale. Subsequently, the steel plate was cold-rolled to a plate thickness of 0.8 mm. After that, the steel plate was subjected to finish annealing at 880 to 1000 ° C. Cold-rolled steel sheets 1-A to 1-N were obtained.
In Tables 1 and 2, numerical values with an underline are outside the scope of the present embodiment.

(Corrosion test 1)
Material No. Test pieces each having a width (W) of 25 mm and a length (L) of 100 mm were cut out from each of the cold rolled steel sheets 1-A to 1-N, and the entire surface of the test piece was wet-polished using emery paper up to # 320.
Subsequently, the material No. The test pieces 1-A to 1-N were heat-treated under the conditions 1-1 shown below, Test pieces of 1-1 to 1-10, 1-101 to 1-103, 1-106, 1-201 to 1-203 were obtained.
(Condition 1-1)
A test piece was placed in the heating furnace. The inside of the furnace was evacuated at 10 ⁻³ torr and then N ₂ was introduced to adjust the pressure to 10 ⁻¹ to 10 ⁻² torr. The test piece was heated in this atmosphere and held at 1100 ° C. for 10 minutes. Subsequently, it cooled to normal temperature in the furnace. Note that the furnace pressure was maintained at 10 ⁻¹ to 10 ⁻² torr during the temperature increase and also at 1100 ° C.
In addition, the material No. The test pieces 1-D, 1-F, and 1-J were subjected to heat treatment under the conditions 1-2 shown below. Test pieces 1-11 to 1-13 were obtained.
(Condition 1-2)
The specimen was heated in 100% H _{2 with} a dew point of −65 ° C. and held at 1100 ° C. for 10 minutes.

For comparison, the material No. The 1-D and 1-F test pieces were also heat-treated under different conditions. Material No. The test piece of 1-D, and was heat-treated under the conditions 1-3 below, No. of Table 3 A test piece of 1-104 was obtained.
(Condition 1-3)
A test piece was placed in the furnace. The inside of the furnace was evacuated to 10 ⁻³ torr. The test piece was heated in this atmosphere and held at 1100 ° C. for 10 minutes. Subsequently, it cooled to normal temperature in the furnace.
Material No. The test piece of 1-F, and was heat-treated under the conditions 1-4 below, No. of Table 3 A test piece of 1-105 was obtained.
(Condition 1-4)
The test piece was heated in the atmosphere and held at 700 ° C. for 30 minutes. Next, it was air cooled to room temperature.
In Table 3, numerical values underlined are outside the scope of this embodiment.

No. in Table 3 Corrosion tests were performed on the test pieces of 1-1 to 1-13, 1-101 to 1-106, and 1-201 to 1-203 using the aqueous solutions shown in Table 3.
No. In 1-1 to 1-13 and 1-101 to 1-106, the total concentration of formic acid and acetic acid is 1% to 10% and the Cl ion (chloride ion) concentration is 100 ppm as the test solution. An aqueous solution in which NaCl was dissolved was used. The test temperature was 95 ° C. and the test time was 168 hr. In addition, No. In 1-201 to 1-203, for reference, a test was performed under conditions for evaluating the corrosivity of conventional deteriorated gasoline. Specifically, the total concentration of formic acid and acetic acid was less than 1%, and the temperature was 45 ° C. In the corrosion test 1, the other test conditions were in accordance with JASO-M611-92-A.

The test piece after the corrosion test was subjected to a derusting treatment using nitric acid, then measured for corrosion weight loss and observed for the presence of local corrosion.
Corrosion weight loss was calculated as follows. First, it measured using the direct balance which can measure the mass of the test piece before and behind a test to 0.0001g. The decrease in mass calculated from the amount of change was divided by the surface area of the test piece before the test to calculate the corrosion loss. Observation of local corrosion was performed as follows. A magnification of 200 was applied to the entire surface of the test piece regardless of the portion that was in contact with the gas phase (the portion that was not in contact with the aqueous solution), the portion that was in contact with the liquid phase (the portion that was in contact with the aqueous solution), and the gas phase / liquid phase boundary. Observation was carried out using a double optical microscope. Moreover, the corrosion depth was measured by the depth of focus method in the location where local corrosion was observed.

The case where the corrosion weight loss was less than 0.5 g · m ⁻² and no local corrosion was observed was regarded as “Good”. If the corrosion weight loss is 0.5 g · m ^-2 or more equivalent to the detection limit, or if a corrosion mark with a measured depth of corrosion exceeding 10 μm is detected by the depth of focus method Defined as “bad”. The results are shown in Table 3.

(Corrosion test 2)
In Table 1 and Table 2, the material No. Two test pieces were cut out from each of the cold rolled steel sheets 1-A to 1-N, and the entire surface of the test piece was wet-polished using emery paper up to # 320. Thereafter, each test piece was molded into a cup having an inner diameter of 50 mm and a depth of 35 mm. Next, heat treatment was performed in the same manner as in Conditions 1-1 to 1-4 of Corrosion Test 1 described above. 45 mL of RME was put into one of the cups after the heat treatment, and 45 mL of E22 was put into the other cup. An aqueous solution containing formic acid, acetic acid, and chloride ions at a concentration shown in Table 3 was prepared in advance, and 5 mL of this aqueous solution was added to two cups and sealed. Then, the two cups were left in a constant temperature bath at 95 ° C. for 168 hours (Nos. 1-1 to 1-13 and 1-101 to 1-106 in Table 3). Some tests were carried out in a 45 ° C. constant temperature bath corresponding to the conditions for evaluating the corrosiveness of conventional deteriorated gasoline (No. 1-201 to 1-203 in Table 3). After completion of the test, the corrosive liquid was discharged and the inside of the cup was washed with acetone. Thereafter, the presence or absence of corrosion marks was visually observed. The results are shown in Table 3.

(Surface analysis)
Material No. Samples for surface analysis were cut out from cold rolled steel sheets 1-A to 1-N. No. in Table 3 Samples for surface analysis were also heat-treated under the same conditions as the heat treatments for the corrosion test pieces 1-1 to 1-13, 1-101 to 1-106, and 1-201 to 1-203. Subsequently, the surface oxide film was analyzed by X-ray photoelectron spectroscopy (XPS), and the cation fraction (A value) in the oxide film was calculated. XPS was an X-ray photoelectron spectrometer manufactured by ULVAC-PHI, Inc., using a mono-AlKα ray as an X-ray source, an X-ray beam diameter of about 100 μm, and an extraction angle of 45 degrees. The results are shown in Table 3.
In Table 3, “A value” indicates the total of the cation fractions of Cr, Si, Nb, Ti, and Al in the oxide film represented by the following formula.
A value = (Cr + Si + Nb + Ti + Al) / (the total content of cations)

From the test results shown in Table 3, Invention Example No. Since 1-1 to 1-13 had compositions within the range of this embodiment, they exhibited excellent corrosion resistance.
On the other hand, Comparative Example No. For 1-101 to 1-103, the Cr content and the value of Si + Cr + Al + {Nb + Ti-8 (C + N)} were outside the range of the present embodiment, so that satisfactory corrosion resistance was not obtained. Comparative Example No. For 1-106, the value of Si + Cr + Al + {Nb + Ti-8 (C + N)} was outside the range of the present embodiment, so that satisfactory corrosion resistance was not obtained.

Also, Reference Example No. 1-201 to 1-203 showed good corrosion resistance even though the Cr content did not satisfy the conditions of this embodiment. This is because the total concentration of formic acid + acetic acid was less than 1% and the temperature was mild at 45 ° C.

In addition, Comparative Example No. 1 was heat-treated only in vacuum without introducing N ₂ . The A value of 1-104 was 0.22. In addition, Comparative Example No. The A value of 1-105 was 0.17. In either case, the composition was within the range of the present embodiment, but the A value did not satisfy the range of the present embodiment and the corrosion resistance was poor.

[Example 2]
30 kg of molten steel having the chemical composition shown in Table 4 and Table 5 below was melted in a vacuum melting furnace to produce a 17 kg flat steel ingot. Subsequently, the steel ingot was hot rolled to a thickness of 4.5 mm at a heating temperature of 1200 ° C. to obtain a hot rolled sheet. Thereafter, the hot rolled sheet was annealed at 900 to 1030 ° C. The scale was then removed by alumina shot blasting. Thereafter, the steel sheet was cold-rolled to a thickness of 1 mm and then subjected to finish annealing at 950 to 1050 ° C. to obtain cold-rolled steel sheets of Material Examples 2-1 to 2-17. Using this cold-rolled steel sheet, the corrosion resistance was evaluated and the surface film was analyzed.
In Tables 4 and 5, the numerical values underlined are outside the scope of the present embodiment.

Test pieces each having a width of 25 mm and a length of 100 mm were cut out from each of the cold rolled steel sheets of Examples 2-1 to 2-17, and the entire surface of the test pieces was wet-polished with emery paper up to # 320. Next, heat treatment was performed under conditions 2-1 shown below, simulating the atmosphere during brazing, and test pieces of Experimental Examples 2-1 to 2-17 shown in Table 6 were obtained.
(Condition 2-1)
A test piece was placed in the heating furnace. The inside of the furnace was evacuated at 10 ⁻³ torr and then N ₂ was introduced to adjust the pressure to 10 ⁻¹ to 10 ⁻² torr. The test piece was heated in this atmosphere and held at 1100 ° C. for 10 minutes. Subsequently, it cooled to normal temperature in the furnace. Note that the furnace pressure was maintained at 10 ⁻¹ to 10 ⁻² torr during the temperature increase and also at 1100 ° C.
In addition, the specimen 2-1 of the material example was subjected to heat treatment under the following condition 2-2 to obtain a specimen of the experimental example 2-18 shown in Table 6.
(Condition 2-2)
A test piece was placed in the heating furnace. The inside of the furnace was evacuated at 10 ⁻³ torr. The test piece was heated in this atmosphere and held at 1100 ° C. for 10 minutes. Subsequently, it cooled to normal temperature in the furnace.
Further, the test pieces of the material examples 2-1 to 2-3 were subjected to heat treatment under the following conditions 2-3, and experimental examples 2-19 to 2-21 shown in Table 6 were obtained.
(Condition 2-3)
The specimen was heated in 100% H _{2 with} a dew point of −65 ° C. and held at 1100 ° C. for 10 minutes.

Corrosion tests were performed on the test pieces of Experimental Examples 2-1 to 2-2 in Table 6 under the following conditions. An aqueous solution containing 100 ppm Cl ⁻ , 1000 ppm SO ₄ ²⁻ , and 1000 ppm SO ₃ ²⁻ was prepared using hydrochloric acid, sulfuric acid, and ammonium sulfite as reagents, and then the aqueous solution was adjusted to pH 3.5 using aqueous ammonia. Adjusted. The aqueous solution was placed in a sealed glass container that can prevent evaporation and concentration of the aqueous solution, and half of the test piece was immersed in the aqueous solution. This state was maintained at 80 ° C. for 500 hours to conduct a corrosion test. After completion of the test, the corrosion products were removed, and the corrosion depth was measured by the depth of focus method of an optical microscope. When the maximum corrosion depth was 400 μm or less, the corrosion resistance was evaluated as good. The results are shown in Table 6.

Samples for surface analysis were cut out from the cold rolled steel sheets of Material Examples 2-1 to 2-17. The sample for surface analysis was also heat-treated under the same conditions as the heat treatment of the corrosion test pieces of Experimental Examples 2-1 to 2-2 in Table 6, and the surface analysis test pieces of Experimental Examples 2-1 to 2-2-21 were prepared. did. Next, the oxide film on the surface was analyzed by X-ray photoelectron spectroscopy (XPS), and the cation fraction (A ′ value) of Cr, Si, Nb, Ti, and Al in the oxide film was calculated. XPS was an X-ray photoelectron spectrometer manufactured by ULVAC-PHI, Inc., using a mono-AlKα ray as an X-ray source, an X-ray beam diameter of about 100 μm, and an extraction angle of 45 degrees. The results are shown in Table 6.
In Table 6, “A ′ value” indicates a cation fraction in the oxide film represented by the following formula. Moreover, the numerical value with an underline is outside the range of this embodiment.
(A ′ value) = (Cr + Si + Ti + Nb + Al) / (content of all cations)

From the test results shown in Table 6, the steels of Experimental Examples 2-1 to 2-12 and 2-19 to 2-21 within the scope of the present embodiment have an A ′ value of 0.4 or more (40% or more). The corrosion resistance in the exhaust gas simulated condensed water is good.
On the other hand, Experimental Examples 2-13 to 2-15 are comparative examples containing only one of Ni, Cu, and Mo. Experimental Example 2-17 is a comparative example in which the Cr content and the A ′ value are out of the range of the present embodiment. Experimental Examples 2-13 to 2-15 and 2-17 are inferior in corrosion resistance in exhaust gas simulated condensed water.
Experimental Example 2-16 is a comparative example in which the cation fraction (A ′ value) in the oxide film formed by the brazing simulated heat treatment does not satisfy the range of the present embodiment. In this Experimental Example 2-16, the A ′ value is less than 0.4 (less than 40%), and the corrosion resistance is poor.
Experimental Example 2-18 was heat-treated only in a vacuum without introducing N ₂ . In Experimental Example 18, the A ′ value was less than 0.4 (less than 40%), and the corrosion resistance in the exhaust gas simulated condensed water was poor.

Since the ferritic stainless steel for biofuel supply system parts of the first embodiment has excellent corrosion resistance against biofuel, it is suitably applied to fuel supply system parts. In particular, the present invention is suitably applied to parts in the fuel supply system parts that are close to the engine and are likely to become hot, such as fuel injection system parts.
The ferritic stainless steel for exhaust heat recovery device of the second embodiment is suitably used as a member for an exhaust heat recovery device (exhaust heat recirculation system) because it has excellent corrosion resistance against exhaust gas condensed water. In particular, it is suitably used as a member of the heat exchange part of the exhaust heat recovery device. In addition, it is also suitably used as a member of an exhaust gas passage portion exposed to exhaust gas condensed water such as EGR and muffler.

Claims (9)

1. % By mass
   C: 0.03% or less,
   N: 0.03% or less,
   Si: more than 0.1%, 1% or less,
   Mn: 0.02% or more, 1.2% or less,
   Cr: 15% or more, 23% or less,
   Al: 0.002% or more, 0.5% or less, and one or both of Nb and Ti,
   The balance consists of Fe and inevitable impurities,
   The following formulas (1) and (2) are satisfied,
   A ferritic stainless steel for biofuel supply system parts, characterized in that an oxide film containing Cr, Si, Nb, Ti, and Al in a total cation fraction of 30% or more is formed on the surface.
   8 (C + N) + 0.03 ≦ Nb + Ti ≦ 0.6 (1)
   Si + Cr + Al + {Nb + Ti-8 (C + N)} ≧ 15.5 (2)
   In formula (1) and formula (2), the element symbol represents the content (mass%) of each element.
2. Furthermore, it contains at least one selected from the group consisting of Ni: 2% or less, Cu: 1.5% or less, Mo: 3% or less, and Sn: 0.5% or less in mass%. The ferritic stainless steel for biofuel supply system parts according to claim 1.
3. Furthermore, in mass%, V: 1% or less, W: 1% or less, B: 0.005% or less, Zr: 0.5% or less, Co: 0.2% or less, Mg: 0.002% or less, The ferritic stainless steel for biofuel supply system parts according to claim 1 or 2, comprising at least one selected from the group consisting of Ca: 0.002% or less and REM: 0.01% or less. steel.
4. A biofuel supply system component comprising the ferritic stainless steel for a biofuel supply system component according to any one of claims 1 to 3.
5. % By mass
   C: 0.03% or less,
   N: 0.05% or less,
   Si: more than 0.1%, 1% or less,
   Mn: 0.02% or more, 1.2% or less,
   Cr: 17% or more, 23% or less,
   Al: 0.002% or more, 0.5% or less, and one or both of Nb and Ti,
   Furthermore, Ni: 0.25% or more, 1.5% or less,
   Cu: 0.25% or more, 1% or less, and Mo: 0.5% or more, containing 2 or 3 types selected from the group consisting of 2% or less,
   The balance consists of Fe and inevitable impurities,
   Satisfying the following expressions (3) and (4),
   A ferritic stainless steel for exhaust heat recovery device, characterized in that an oxide film containing Cr, Si, Nb, Ti, and Al in a total cation fraction of 40% or more is formed on the surface.
   8 (C + N) + 0.03 ≦ Nb + Ti ≦ 0.6 (3)
   Si + Cr + Al + {Nb + Ti-8 (C + N)} ≧ 17.5 (4)
   In Formula (3) and Formula (4), an element symbol represents content (mass%) of each element. In the formula (4), the value of Nb + Ti-8 (C + N) is 0 or more.
6. % By mass
   C: 0.03% or less,
   N: 0.05% or less,
   Si: more than 0.1%, 1% or less,
   Mn: 0.02% or more, 1.2% or less,
   Cr: 17% or more, 23% or less,
   Al: 0.002% or more, 0.5% or less, and one or both of Nb and Ti,
   Furthermore, Ni: 0.25% or more, 1.5% or less,
   Cu: 0.25% or more, 1% or less, and Mo: 0.5% or more, containing 2 or 3 types selected from the group consisting of 2% or less,
   The balance consists of Fe and inevitable impurities,
   Satisfying the following expressions (3) and (4),
   By heat treatment in a _{H 2} atmosphere including a vacuum atmosphere or _{N 2} of ¹⁰ -2 ~ 1 torr comprising N _2, including surface, Cr, Si, Nb, Ti , and Al more than 40% in total of cationic fraction A ferritic stainless steel for waste heat recovery equipment, characterized in that an oxide film is formed.
   8 (C + N) + 0.03 ≦ Nb + Ti ≦ 0.6 (3)
   Si + Cr + Al + {Nb + Ti-8 (C + N)} ≧ 17.5 (4)
   In Formula (3) and Formula (4), an element symbol represents content (mass%) of each element. In the formula (4), the value of Nb + Ti-8 (C + N) is 0 or more.
7. Further, in mass%, V: 0.5% or less, W: 1% or less, B: 0.005% or less, Zr: 0.5% or less, Sn: 0.5% or less, Co: 0.2% 7. One or more types selected from the group consisting of Mg: 0.002% or less, Ca: 0.002% or less, and REM: 0.01% or less. Ferritic stainless steel for waste heat recovery equipment.
8. It has a heat exchange part in which members are assembled by brazing joint,
   The heat exchange part is made of ferritic stainless steel,
   The ferritic stainless steel is, by mass%, C: 0.03% or less, N: 0.05% or less, Si: more than 0.1%, 1% or less, Mn: 0.02% or more. 2% or less, Cr: 17% or more, 23% or less, Al: 0.002% or more, 0.5% or less, and any one or both of Nb and Ti, and Ni: 0.25% or more 1.5% or less, Cu: 0.25% or more, 1% or less, and Mo: 0.5% or more, 2% or less selected from the group consisting of 2% or less, with the balance being Fe And an oxide film containing at least 40% of the total cation fraction of Cr, Si, Nb, Ti, and Al is formed on the surface satisfying the following formulas (3) and (4). An exhaust heat recovery device characterized by that.
   8 (C + N) + 0.03 ≦ Nb + Ti ≦ 0.6 (3)
   Si + Cr + Al + {Nb + Ti-8 (C + N)} ≧ 17.5 (4)
   In Formula (3) and Formula (4), an element symbol represents content (mass%) of each element. In the formula (4), the value of Nb + Ti-8 (C + N) is 0 or more.
9. The ferritic stainless steel is further, in mass%, V: 0.5% or less, W: 1% or less, B: 0.005% or less, Zr: 0.5% or less, Sn: 0.5% or less Co: 0.2% or less, Mg: 0.002% or less, Ca: 0.002% or less, and REM: containing at least one selected from the group consisting of 0.01% or less The exhaust heat recovery device according to claim 8.