JP2012214881A - Ferritic stainless steel for biofuel supply system parts, and biofuel supply system parts - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide ferritic stainless steel for parts having corrosion resistance to biofuel.SOLUTION: The ferritic stainless steel for biofuel supply system parts contains, by mass, 0.03% or less of C, 0.03% or less of N, more than 0.1% and 1% or less of Si, 0.02% or more and 1.2% or less of Mn, 15% or more and 23% or less of Cr, 0.002% or more and 0.5% or less of Al, and one or both of Nb and Ti, and satisfies the following formulas (1): 8(C+N)+0.03≤Nb+Ti≤0.6 and (2): Si+Cr+Al+[Nb+Ti-8(C+N)]≥15.5 (wherein the symbols of elements show the content (mass%) of the respective elements). The ferritic stainless steel has a remaining portion consisting of Fe and inevitable impurities, and is formed on the surface with an oxide film containing 30% or more of Cr, Si, Nb, Ti and Al in total of cation fraction.

Description

  The present invention relates to ferritic stainless steel and biofuel supply system parts suitable for automobile fuel supply system parts for supplying biofuel such as bioethanol and biodiesel. In particular, the present invention relates to a ferritic stainless steel suitable for biofuel supply system parts that are close to the engine and are likely to become high temperature, such as fuel injection system parts.

  In recent years, in the automotive field, exhaust gas regulations have been further strengthened due to increasing awareness of environmental issues, and efforts are being made to reduce carbon dioxide emissions. In addition to efforts to further reduce weight and install exhaust gas treatment devices such as EGR, DPF, and urea SCR systems, efforts from the fuel side such as bioethanol and biodiesel fuel are also being implemented.

  Bioethanol is a fuel in which ethanol is mixed with gasoline as a fuel for a gasoline engine. Biodiesel fuel is obtained by mixing a fatty acid methyl ester with light oil as a fuel for a diesel engine. Here, ethanol is obtained by using corn and sugar cane as raw materials, and fatty acid methyl ester is obtained by esterifying 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 impact on the usage performance of various components that make up fuel system parts has been investigated in advance, but there is a need for a more reliable material from manufacturers that guarantee an ultra long life And stainless steel is one candidate.

Among the fuel system parts, the following is known as a prior art in which stainless steel is applied to a fuel tank and 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%, B: ≤0.0100% included, or Mo: ≤3.0%, Ni: ≤2.0%, Cu if necessary A ferritic stainless steel sheet containing one or more of: ≦ 2.0% and Al: ≦ 4.0%, having an elongation at break of 30% or more and a Rankford value of 1.3 or more is disclosed.

  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%, N: ≦ 0.04%, Cr + 3.3Mo ≧ 18 Satisfying the relational expression, satisfying the relational expression of Nb: ≦ 0.8%, Ti: ≦ 1.0%, 18 ≦ Nb / (C + N) + 2Ti / (C + N) ≦ 60 A ferritic stainless steel sheet containing a ferrite crystal grain number of 6.0 or more and an average r value of 2.0 or more is disclosed.

  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%, N: ≦ 0. The latter half that occurs when it is deformed by 25% by uniaxial tension, including 04%, Nb: 0.01-0.8%, Ti: 0.01-1.0% A ferritic stainless steel sheet having a surface waviness height of 50 μm or less is disclosed.

JP 2003-27792 A JP 2002-285300 A JP 2002-363712 A

However, the patent document deals with the corrosion resistance against ordinary gasoline. As will be described later, since the corrosivity of biofuels is significantly different from that of gasoline, these technologies are not sufficiently corrosive 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.

  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.

The gist of the present invention aimed at solving the above problems is as follows.
[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, containing one or two of Nb and Ti, shown below (Formula 1) and (Formula 2) ), The remainder is made of Fe and inevitable impurities, and the surface is formed with an oxide film containing Cr, Si, Nb, Ti, and Al with a total cation fraction of 30% or more. Ferritic stainless steel for supply system parts.
8 (C + N) + 0.03 ≦ Nb + Ti ≦ 0.6 (Formula 1)
Si + Cr + Al + {Nb + Ti-8 (C + N)} ≧ 15.5 (Formula 2)
In (Formula 1) and (Formula 2), an element symbol represents content (mass%) of each element.

[2] Further, by mass%, Ni: 2% or less, Cu: 1.5% or less, Mo: 3% or less, Sn: 0.5% or less, any one or two or more types The ferritic stainless steel for biofuel supply system parts according to claim 1.
[3] Further, 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, Ca: 0.002% or less, REM: 0.01% or less, and one or more of them are contained. Ferrite for biofuel supply system parts according to claim 1 or 2 Stainless steel.

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

  As described above, according to the present invention, it is possible to provide a 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 are likely to become high temperature, such as injection system parts.

Hereinafter, embodiments of the present invention will be described in detail.
The present inventors are commonly used in Europe, E10 and E22 mixed with 10% and 22% of bioethanol commonly used in North America in gasoline, E100 of bioethanol 100%, and Europe. RME (RapeSeed Methyl Ester) obtained by methyl esterifying rapeseed oil, which is a biodiesel fuel, was obtained, and detailed investigation and analysis were performed with respect to oxidation deterioration behavior and corrosiveness to stainless steel in comparison with ordinary gasoline.

First, the oxidation stability of E10, E22, E100, and RME was compared with that of gasoline according to JIS K2287 used in the evaluation method of oxidation stability of gasoline. These fuels were sealed in an autoclave and oxygen at 7 atmospheres was introduced, and then the temperature was maintained at 100 ° C., and the behavior of the pressure decreasing as oxygen was used to oxidize the fuel was measured.
As a result, it became clear that E10 and E100 are less susceptible to oxidative degradation than gasoline, while 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 the fatty acids, first, stainless steel cold-rolled steel sheets were immersed in oxidized RME and gasoline to examine the presence or absence of corrosion. Then, corrosion was not recognized in any 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.

Accordingly, when 10 vol% water was added to each of the oxidized RME and gasoline and the stainless cold-rolled steel sheet was exposed, corrosion occurred in both cases of RME and gasoline.
From this, it was confirmed that the 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 mainly depends on the fatty acid species and the fatty acid concentration in the oxidized fuel and the distribution behavior of the fatty acid between the fuel and the water phase. Among these, fatty acid distribution behavior is affected by temperature, and the higher the temperature, the easier the fatty acid is distributed from the fuel to the water phase.

  In addition, the pH of the aqueous phase at this time has a difference of 0.9 between R2.1 for pH and pH 3.0 for gasoline, both of which differ by about 100 times when converted to fatty acid concentration. It corresponds to. Conventionally, corrosion tests on oxidatively deteriorated gasoline have been conducted with a concentration of formic acid + acetic acid in water of about 100 to 1000 ppm, but for biofuels such as RME, the concentration of formic acid + acetic acid is about 100 times that of gasoline It was found that it was necessary to increase to 1% to 10% corresponding to the concentration.

Further, the temperature of the fuel injection system close to the engine rises to about 90 to 100 ° C., and the fatty acid is easily distributed from the fuel to the water phase together with the temperature itself, and the corrosive environment becomes severe. This is a severe condition as compared to a corrosion test temperature of 40 to 50 ° C. for 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 particularly in stainless steel.

As described above, since the corrosiveness of biofuel is higher than that of ordinary 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, 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, and Cr, Si, Nb, Ti, and Al are cations on the surface. It was found that when an oxide film containing 30% or more in total ((Cr + Si + Nb + Ti + Al) / total cations) was formed, excellent corrosion resistance was exhibited in a high-temperature acidic fatty acid environment.

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

  It should be noted that Nb and / or Ti contained in the stainless steel does not exist in the form of a solid solution, but exists in a state in which a part thereof is fixed to C and N. And among Nb and / or Ti contained in stainless steel, Nb and / or Ti in a solid solution state not fixed to C and N is concentrated in the passive film by heat treatment, and in the oxide film formed after heat treatment Contributes to corrosion prevention. 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.

Furthermore, an oxide film having the above composition is formed in consideration of process conditions such as heat treatment and pickling.
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, some fuel injection system parts such as delivery tubes and common rails are parts in which members are brazed and joined. As heat treatment conditions during brazing and joining for manufacturing such a component, in an environment containing N ₂ , in a vacuum atmosphere of 800 to 1200 ° C. and 10 ^{−2 to 1} torr or an H ₂ atmosphere, 0.5 to 30 minutes. By holding, an oxide film having a desired composition can be suitably formed. Here, simply by heat-treating 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 reaches the desired cation fraction. do not do. For example, after pulling a vacuum of 10 ⁻² torr or less, N ₂ is introduced and heat treatment is performed at 10 ^{−2 to 1} torr, whereby 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 (Nb, Ti) carbonitrides are formed on the surface of the steel material by heat treatment in an environment containing N ₂ , which promotes the reduction of Fe oxide. There is a possibility.
The content of N ₂ in the heat treatment atmosphere is preferably 0.001 to 0.2%, and more preferably 0.005 to 0.1%.
As heat treatment conditions, in order to form an oxide film in which Cr, Si, Nb, Ti, and Al having a total cation fraction of 30% or more are concentrated, holding at 1000 to 1200 ° C. for 5 to 30 minutes is possible. preferable. The holding temperature is preferably 1050 to 1150 ° C., and the holding time is more preferably 10 to 20 minutes.

As described above, the oxide film having the cation fraction can be formed by heat treatment in brazing and joining the members made of steel having the 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 manufacturing a part that is not brazed, in order to form the oxide film having the above cation fraction, in an environment containing N ₂ , at a temperature of 800 ° C. to 1200 ° C. and 10 ^{−2 to 1} torr. A heat treatment process may be performed for 5 to 30 minutes. In order to simplify the manufacturing process and improve productivity, the above heat treatment process is not added, and an oxide film is formed in the manufacturing process of steel materials and parts. It is good also as an oxide film of a desired cation fraction by adjusting suitably the conditions of the heat processing performed, and the conditions of the pickling from which an oxide film is removed.

When forming the oxide film having the cation fraction in the manufacturing process of steel materials and parts, specifically, for example, in the final finishing annealing of the manufacturing process of steel materials, N ₂ —H ₂ having a dew point of −45 to −75 ° C. The method of hold | maintaining at 800-1100 degreeC for 0.5 to 5 minutes in mixed gas atmosphere is mentioned. In this case, the subsequent pickling is omitted.

Here, in order to obtain even more excellent corrosion resistance, it is preferable to contain Cr, Si, Nb, Ti, Al in a total of 40% or more of the cation fraction, and Cr, Si, Nb, Ti, Al Of these, the most important Cr is preferably 20% or more. More preferably, Cr, Si, Nb, Ti, and Al have a total cation fraction of 50% or more.
Moreover, it is preferable that the film thickness of an oxide film shall be 15 nm or less, and 10 nm or less is more preferable. The increase in film thickness leads to a decrease in the fraction of cations such as Cr, Si, Nb, Ti, and Al occupied per unit volume, leading to a decrease in corrosion resistance. There is a possibility that the carbonitride of (Nb, Ti) generated by heat treatment in an environment containing N ₂ suppresses an increase in film thickness.

  In addition to the above knowledge, the present invention provides a ferritic stainless steel for fuel supply system parts that is made in consideration of workability required as a material for biofuel supply system parts and has excellent corrosion resistance against biofuels. The gist of the present invention is as described in the claims.

  Hereinafter, the reason why each composition of ferritic stainless steel for biofuel supply system parts is limited will be described. In the following description, unless otherwise specified,% of each component represents mass%.

(C: 0.03% or less)
Since C reduces intergranular corrosion resistance and workability, it is necessary to keep the content low. Therefore, the C content is set to 0.03% or less. However, since excessively reducing the scouring cost, the C content is preferably 0.002% or more. More preferably, it is 0.002 to 0.02%.

(N: 0.03% 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 set to 0.03% or less. However, since excessively reducing the scouring cost, the N content is preferably 0.002% or more. More preferably, it is 0.002 to 0.02%.
Moreover, it is preferable to make the total content of C and N 0.015% or more from the viewpoint of suppressing crystal grain coarsening during heat treatment by carbonitride and suppressing strength reduction.

(Si: more than 0.1%, 1% or less)
Si needs to be at least more than 0.1% in order to concentrate in the surface film after heat treatment and contribute to improving the corrosion resistance of stainless steel. Si is useful as a deoxidizing element. However, excessive addition reduces workability, so the Si content is 1% or less. Preferably it is 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 it is necessary to contain at least 0.02% or more. However, if it is excessively contained, the corrosion resistance is deteriorated, so the Mn content is set to 1.2% or less. Preferably, it is 0.05 to 1%.

(Cr: 15% or more, 23% or less)
Cr is an element that is fundamental for ensuring corrosion resistance in biofuel, and it is necessary to contain at least 15% or more. The corrosion resistance can be improved as the Cr content is increased. However, excessive addition reduces workability and manufacturability, so the Cr content is 23% or less. Preferably it is 17 to 20.5%.

8 (C + N) + 0.03 ≦ Nb + Ti ≦ 0.6 (Formula 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. Therefore, the total of Nb and Ti (Nb + Ti) is 8 times the (C + N) amount. It is necessary to contain above. Further, in order to concentrate on the surface film of stainless steel after heat treatment and contribute to the improvement of corrosion resistance, it is necessary to contain at least 0.03% or more as Nb and / or Ti in a solid solution state not fixed to C and N. Therefore, the lower limit of Nb + Ti is set to 8 (C + N) + 0.03%. However, excessive addition of Nb and / or Ti reduces workability and manufacturability, so the upper limit of the content of Nb + Ti is set to 0.6%. The content of Nb + Ti is preferably 10 (C + N) +0.03 to 0.6%.

  Here, among Nb and Ti, Ti concentrates on the surface film of stainless steel and contributes to the improvement of corrosion resistance, but has the effect of inhibiting brazing. In order to obtain good brazing properties when producing a brazing structure biofuel supply system part, 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 is required to be contained in an amount of 0.002% or more in order to concentrate on the surface film of stainless steel after heat treatment and contribute to the improvement of corrosion resistance. Al is an element useful for scouring because it has a deoxidizing effect and the like, and has an effect of improving moldability. However, since excessive addition deteriorates toughness, the Al content is set to 0.002 to 0.5%. Preferably it is 0.005 to 0.1%.

(Ni: 2% or less)
Ni can be contained in an amount of 2% or less as required in order to improve the corrosion resistance. The Ni content that provides a stable effect is 0.2% or more. Ni can improve the corrosion resistance as the content is increased, but the addition of a large amount makes it harder and lowers the workability and increases the cost because it is expensive. Therefore, it is preferable to contain 0.2 to 2% of Ni. More preferably, it is 0.2 to 1.2%.

(Cu: 1.5% or less)
Cu can be contained in an amount of 1.5% or less as required in order to improve the corrosion resistance. The Cu content that provides a stable effect is 0.2% or more. Although Cu can improve corrosion resistance, so that the content increases, addition of a large amount hardens and reduces workability. Therefore, it is preferable to contain 0.2 to 1.5% of Cu. More preferably, it is 0.2 to 0.8%.

(Mo: 3% or less)
Mo can be contained in an amount of 3% or less as required in order to improve the corrosion resistance. The Mo content that provides a stable effect is 0.3% or more. Mo can improve the corrosion resistance as the content is increased, but the addition of a large amount makes it harder and lowers workability and is expensive, leading to an increase in cost. Therefore, it is preferable to contain 0.3 to 3% of Mo. More preferably, 1 is 0.5 to 2.0%.

(Sn: 0.5% or less)
Sn can be contained in an amount of 0.5% or less as required in order to improve the corrosion resistance. The Sn content that provides a stable effect is 0.01% or more. Sn can improve the corrosion resistance as the content is increased, but addition of a large amount makes it harder and lowers the workability. Therefore, it is preferable to contain Sn 0.01 to 0.5%. More preferably, it is 0.05 to 0.4%.

(V: 1% or less)
V can be contained in an amount of 1% or less as required in order to improve the corrosion resistance. The content of V that provides a stable effect is 0.05% or more. However, excessive addition of V deteriorates processability and increases the cost because it is expensive. Therefore, V is preferably contained in an amount of 0.05 to 1%.

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

(B: 0.005% or less)
B can be contained in an amount of 0.005% or less as required in order to improve workability, particularly secondary workability. In order to obtain a stable effect, it is preferable to contain 0.0001% or more of B. More preferably, it is 0.0002 to 0.001%.

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

(Co: 0.2% or less)
Co can be contained in an amount of 0.2% or less as required in order to improve secondary workability and toughness. 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 a deoxidizing effect and the like, and it is effective in improving the workability and toughness by refining the structure. Therefore, it can be contained in an amount of 0.002% or less as necessary. . 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 a deoxidizing effect and the like, and can be contained in an amount of 0.002% or less as required. In order to obtain a stable effect, it is preferable to contain 0.0002% or more of Ca.

(REM: 0.01% or less)
Since REM has a deoxidizing effect and the like, it is an element useful for scouring, and can be contained in an amount of 0.01% or less as required. In order to obtain a stable effect, it is preferable to contain REM 0.001% or more.

  Of the inevitable impurities, P is preferably 0.04% or less, more preferably 0.035% or less from the viewpoint of weldability. Moreover, about S, it is preferable to set it as 0.02% or less from a viewpoint of corrosion resistance, More preferably, it is 0.01% or less.

The stainless steel of the present invention is, for example, a molten steel having the above chemical composition in a converter or electric furnace, scoured in an AOD furnace, a VOD furnace, or the like, and made into a steel piece by a continuous casting method or an ingot forming method, during rolling - annealing - pickling - cold rolling - recrystallization annealing - performs pickling step, after which _{H 2} atmosphere containing ¹⁰ -2 ^~1torr vacuum atmosphere or _{N 2,} including _{N 2,} 800 ° C. ~ It is manufactured by a method of forming an oxide film having the above cation fraction by performing a heat treatment step of holding at a temperature of 1200 ° C. for 0.5 to 30 minutes. If necessary, annealing of the hot-rolled sheet may be omitted, or cold rolling-finish annealing-pickling may be repeated. Examples of product forms include plates, tubes, bars, and wires.
In addition, as above-mentioned, the stainless steel of this invention may be manufactured by performing said heat processing process, after passing through the process of cold rolling-finish annealing-pickling, but heat processing processes are other than a manufacturing process. You may manufacture by the method performed in the step.

Next, the biofuel supply system component of the present invention will be described. The biofuel supply system component of the present invention is made of the stainless steel of the present invention.
The biofuel supply system component of the present invention is preferably manufactured by 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 producing a biofuel supply system component of the present invention 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.
Moreover, it is preferable that the heat treatment process also serves as a process of brazing and joining 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 the present invention may be made of the stainless steel of the present invention, and is not limited to a brazed joint.

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

  Molten steel having the composition shown in Tables 1 and 2 was melted in a 150 kg vacuum melting furnace, cast into a 50 kg steel ingot, formed into a steel piece, and hot rolled to a plate thickness of 4 mm at a heating temperature of 1200 ° C. Thereafter, hot-rolled sheet annealing was performed at 850 to 950 ° C., the scale was removed by pickling and pickling in a nitric hydrofluoric acid solution, and first cold-rolled to a sheet thickness of 2 mm. After performing intermediate annealing again in the same temperature range, the scale was removed by the same pickling method and cold rolled to a plate thickness of 0.8 mm. This was subjected to finish annealing at 880 to 1000 ° C., and material No. A to N cold-rolled steel sheets were used.

(Corrosion test 1)
Material No. From the cold rolled steel sheets A to N, test pieces of 25 W × 100 L were cut out, and the entire surface was wet-polished to # 320 with emery paper.
Subsequently, the material No. The test pieces A to N were subjected to heat treatment under the following condition 1, and Nos. Test pieces of 1 to 10, 101 to 103, 106, 201 to 203 were used. After evacuation at 10 ⁻³ torr, N ₂ was introduced to prepare 10 ⁻¹ to 10 ⁻² torr. Thereafter, the temperature was raised, held at 1100 ° C. for 10 minutes, and then cooled to room temperature in the furnace. In addition, it hold | maintained at 10 ^{< -1} > -10 ^<-2 > torr also during temperature rising and 1100 degreeC holding | maintenance. In addition, the material No. The D, F, and J test pieces were heat-treated at 1100 ° C. for 10 minutes in 100% H ₂ with a dew point of −65 ° C. This heat treatment condition was defined as condition 2, and No. 1 in Table 3 was obtained. It was set as the test piece of 11-13.

For comparison, the material No. About the test piece of D and F, the heat processing on another condition was also performed. Material No. For the test piece of D, the temperature was raised after evacuation at 10 ⁻³ torr, held at 1100 ° C. for 10 minutes, then cooled to room temperature in the furnace (condition 3). 104 test pieces were obtained. Material No. About the test piece of F, after hold | maintaining at 700 degreeC and 30 minute (s) in air | atmosphere, it air-cools to normal temperature (condition 4). 105 test pieces were obtained.

No. in Table 3 Corrosion tests were performed on the test pieces 1 to 101, 101 to 106, and 201 to 203 under the conditions shown in Table 3.
No. In Nos. 1 to 13 and 101 to 106, an aqueous solution in which NaCl was dissolved so that the total concentration of formic acid and acetic acid was 1% to 10% and the Cl ion concentration was 100 ppm was used as the test solution. The test temperature was 95 ° C. and the test time was 168 hr. In addition, No. In 201-203, the test was also conducted for the conditions corresponding to the conventional gasoline in which the total concentration of formic acid + acetic acid was less than 1% and the temperature was 45 ° C. for reference. About test conditions other than these, it applied to JASO-M611-92-A.

  The test piece after the corrosion test was subjected to derusting treatment using nitric acid, and then subjected to corrosion weight loss measurement and observation of local corrosion. Corrosion weight loss was calculated by measuring the mass of the test piece before and after the test using a direct balance capable of measuring up to 0.0001 g, and dividing the mass loss calculated from the change amount by the surface area of the test piece before the test. . The local corrosion was observed using an optical microscope with a magnification of 200 times on the entire surface of the test piece regardless of the gas phase, liquid phase, or gas phase / liquid phase boundary.

A case where corrosion weight loss of 0.5 g · m ^-2 or more equivalent to the detection limit or a corrosion mark exceeding the detection limit of 10 μm by the depth of focus method is detected is defined as “local corrosion” and is rejected. The case where the corrosion weight loss was less than 0.5 g · m ⁻² and local corrosion was not observed was determined to be acceptable (◯). The results are shown in Table 3.

(Corrosion test 2)
In Table 1 and Table 2, the material No. A to N cold-rolled steel sheets were cut out and the entire surface was wet-polished to # 320 with emery paper, and then formed into a cup having an inner diameter of 50 mm and a depth of 35 mm. Next, this was heat-treated in the same manner as in corrosion test 1 under the above conditions 1 to 4. 45 mL of RME is put in one of the cups after the heat treatment, 45 mL of E22 is put in the other, and 5 mL of water in which formic acid + acetic acid and chloride ions are dissolved in the concentrations shown in Table 3 are added to the two cups and sealed. It was left to stand in a constant temperature bath at 168 hours (Nos. 1 to 13, 101 to 106 in Table 3). In addition, some tests were implemented in a 45 degreeC thermostat corresponding to gasoline conditions (No. 201-203 of Table 3). After completion of the test, the corrosive liquid was discharged and the inside of the cup was washed with acetone, and then the presence or absence of corrosion marks was visually observed. The results are shown in Table 3.

(Surface analysis)
No. in Table 3 During the heat treatment of the corrosion test specimens 1 to 13, 101 to 106, 201 to 203, the sample for surface analysis is also heat treated in parallel, and the oxide film on the surface is analyzed by X-ray photoelectron spectroscopy (XPS). The cation fraction (A value) in the oxide film was calculated. XPS was manufactured by ULVAC-PHI, Inc., using a mono-AlKα ray as an X-ray source, and measured under the conditions of an X-ray beam diameter of about 100 μm and an extraction angle of 45 degrees. The results are shown in Table 3.

From the test results shown in Table 3, no. Since 1-13 is in the range of the present invention, it showed excellent corrosion resistance.
On the other hand, Comparative Example No. In 101 to 103, since the Cr content and the value of Si + Cr + Al + {Nb + Ti-8 (C + N)} are outside the range of the present invention, satisfactory corrosion resistance is not obtained. Comparative Example No. No. 106 has a value of Si + Cr + Al + {Nb + Ti-8 (C + N)} outside the range of the present invention, so that satisfactory corrosion resistance is not obtained.

  Reference Example No. 201-203 was a mild condition with a total concentration of formic acid and acetic acid of less than 1% and a temperature of 45 ° C., despite the Cr content not satisfying the conditions of the present invention. Indicated.

In addition, No. 2 was heat-treated only in a vacuum without introducing N ₂ . 104 has an A value of 0.22 and No. 104 heat-treated in air. The A value of 105 is 0.17, and although the composition is within the range of the present invention, the A value does not satisfy the range of the present invention and the corrosion resistance is poor.

  The ferritic stainless steel of the present invention having excellent corrosion resistance against biofuel is suitable for fuel supply system parts, particularly parts in parts that are likely to be close to the engine and become hot like a fuel injection system.

Claims (4)

% 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,
Contains one or two of Nb and Ti,
An oxide film satisfying (Equation 1) and (Equation 2) shown below, the balance being Fe and inevitable impurities, and containing Cr, Si, Nb, Ti, Al on the surface in a total of 30% or more of the cation fraction Ferritic stainless steel for biofuel supply system parts characterized by being formed.
8 (C + N) + 0.03 ≦ Nb + Ti ≦ 0.6 (Formula 1)
Si + Cr + Al + {Nb + Ti-8 (C + N)} ≧ 15.5 (Formula 2)
In (Formula 1) and (Formula 2), an element symbol represents content (mass%) of each element. Furthermore, in mass%,
It contains any one or more of Ni: 2% or less, Cu: 1.5% or less, Mo: 3% or less, and Sn: 0.5% or less. Ferritic stainless steel for biofuel supply system parts. 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, Ca: 0.002% The ferritic stainless steel for biofuel supply system parts according to claim 1 or 2, wherein one or more of REM: 0.01% or less are contained.   A biofuel supply system component comprising the ferritic stainless steel for the biofuel supply system component according to any one of claims 1 to 3.