JP5856878B2 - Ferritic stainless steel for exhaust heat recovery and exhaust heat recovery - Google Patents

Description

  The present invention relates to a ferritic stainless steel for an exhaust heat recovery device of an automobile and an exhaust heat recovery device. Especially, it is related with the ferritic stainless steel suitable for the waste heat recovery device with which a heat exchange part is assembled by brazing joining.

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

  Among them, efforts are being made to improve fuel consumption by attaching a heat exchanger that recovers exhaust heat mainly so-called hybrid vehicles, so-called exhaust heat recovery. 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 exchanging part of the exhaust heat recovery unit is required to have good thermal efficiency and good thermal conductivity, and to have excellent corrosion resistance against exhaust gas condensed water because it contacts exhaust gas. 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, but the exhaust heat recovery device that can lead to a serious accident of leakage of cooling water is required to have even greater safety. Greater corrosion resistance is required.

  Conventionally, ferritic stainless steels containing 17% or more of Cr, such as SUS430LX, SUS436J1L, and SUS436L, have been used for parts that require particularly corrosion resistance among exhaust system downstream members mainly composed of mufflers. The heat recovery material is required to have a corrosion resistance equivalent to or higher than these.

  Moreover, since the structure of the heat exchange part is complicated, it may be assembled by welding joint, but may be assembled by brazing joint. The material of the heat exchange part assembled by brazing and joining needs to have good brazing properties. In addition, the exhaust heat recovery unit is often installed downstream of the catalytic converter under the floor, so that the exhaust gas on the inlet side is heated and the exhaust gas is forcibly cooled by heat exchange. Become.

  In Patent Document 1, 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% Hereinafter, Ni: 0.70% or less, Cr: 18.00 to 25.50%, Cu: 0.70% or less, Mo: 2 / (Cr-17.00) to 2.50%, N: 0.00. 020% or less, Ti: 0.50% or less, and Nb: 0.50% or less, and (Ti + Nb) ≧ (7 × (C + N) +0.05), with the balance being Fe and An automobile exhaust heat recovery device is disclosed that is made of ferritic stainless steel, which is an inevitable impurity, as a material. In the ferritic stainless steel described in Patent Document 1, corrosion resistance against exhaust gas condensed water is ensured by adding Mo to 18% or more of Cr.

  In Patent Document 2, 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%, N: 0 A ferritic stainless steel sheet containing 0.05% or less, containing Nb carbonitride of 5 μm or less, and having a surface roughness Ra of 0.4 μm or less and excellent in crevice corrosion resistance is disclosed. In the ferritic stainless steel sheet described in Patent Document 2, Ni and Cu are added in combination to 20% or more of Cr to ensure crevice corrosion resistance.

  In Patent Document 3, 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 to 25%, Nb: 0.05 to 0.2%, Ti: 0.05 to 0.5%, N: 0.025% or less, Al: 0.02 to 1.0%, and Ni: 0.1 An automobile exhaust gas flow path member made of ferritic stainless steel containing at least 0.6% by Ni + Cu and at least one of -2.0% and Cu: 0.1-1.0% is disclosed. In the ferritic stainless steel described in Patent Document 3, an appropriate amount of Ni and Cu are added together to realize inexpensive and good corrosion resistance without using expensive Mo.

  In Patent Document 4, Cr: 16 to 30%, Ni: 7 to 20%, C: 0.08% or less, N: 0.15% or less, Mn: 0.1 to 3%, S: 0.008 %, Si: 0.1 to 5%, and satisfying Cr + 1.5Si ≧ 21 and 0.009Ni + 0.014Mo + 0.005Cu− (0.085Si + 0.008Cr + 0.003Mn) ≦ −0.25 Stainless steel for heat pipes is disclosed. The technique described in Patent Document 4 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 4 discloses an austenitic stainless steel suitable for a heat pipe.

JP 2009-228036 A JP 2009-7663 A JP 2007-92163 A JP 2010-24527 A

  The exhaust heat recovery device is required to have corrosion resistance equivalent to or higher than that of 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.

  The present invention has been proposed in view of such conventional circumstances, and can be suitably used for a heat exchanging portion assembled by brazing, in particular, exhaust heat having excellent corrosion resistance against exhaust gas condensed water. An object is to provide a ferritic stainless steel sheet for a collector.

The gist of the present invention aimed at solving the above problems is as follows.
[1] By mass%, C: 0.03% or less, N: 0.05% 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, Ni or Cu, Mo or two or three of Ni: 0.25% or more, 1.5% or less Cu: 0.25% or more, 1% or less, Mo: 0.5% or more, 2% or less, containing one or two of Nb and Ti, shown below (Formula 1) and (Formula 2) ), The balance is made of Fe and inevitable impurities, and an oxide film containing Cr, Nb, Si, Al 2 and Ti in 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 (Formula 1)
Si + Cr + Al + {Nb + Ti-8 (C + N)} ≧ 17.5 (Expression 2)
In (Formula 1) and (Formula 2), an element symbol represents content (mass%) of each element. In (Formula 2), Nb + Ti-8 (C + N) is 0 or more.

[2] 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, Ni or Cu, Mo or two or three of Ni: 0.25% or more, 1.5% or less Cu: 0.25% or more, 1% or less, Mo: 0.5% or more, 2% or less, containing one or two of Nb and Ti, shown below (Formula 1) and (Formula 2) ) meet, the balance being Fe and inevitable impurities, by heat treatment in a vacuum atmosphere or an _{H 2} atmosphere of ¹⁰ -2 ^~1torr containing _{N 2,} on the surface, the cationic component Cr, Nb, Si, Al, and Ti An exhaust film containing 40% or more of the total rate is formed. Light stainless steel.

8 (C + N) + 0.03 ≦ Nb + Ti ≦ 0.6 (Formula 1)
Si + Cr + Al + {Nb + Ti-8 (C + N)} ≧ 17.5 (Expression 2)
In (Formula 1) and (Formula 2), an element symbol represents content (mass%) of each element. In (Formula 2), Nb + Ti-8 (C + N) is 0 or more.

[3] 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, REM: 0.01% or less, containing any one or more of [1] or [2] Ferritic stainless steel for exhaust heat recovery device.

[4] A heat exchange part in which members are assembled by brazing joining is provided, and the heat exchange part is in mass%, C: 0.03% or less, N: 0.05% or less, Si: 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, Ni, Cu, 2 or 3 types of Mo: Ni: 0.25% to 1.5%, Cu: 0.25% to 1%, Mo: 0.5% to 2%, Nb, Ti 1 or 2 of the above, satisfying (Equation 1) and (Equation 2) shown below, the balance consisting of Fe and inevitable impurities, and Cr, Nb, Si, Al , Ti on the surface as a cation component It is made of ferritic stainless steel on which an oxide film containing 40% or more in total is formed. Waste heat recovery device.

8 (C + N) + 0.03 ≦ Nb + Ti ≦ 0.6 (Formula 1)
Si + Cr + Al + {Nb + Ti-8 (C + N)} ≧ 17.5 (Expression 2)
In (Formula 1) and (Formula 2), an element symbol represents content (mass%) of each element. In (Formula 2), Nb + Ti-8 (C + N) is 0 or more.

 [5] 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: 0.00%. 5% or less, Co: 0.2% or less, Mg: 0.002% or less, Ca: 0.002% or less, REM: 0.01% or less, containing one or more of them. The exhaust heat recovery device according to [4], which is characterized.

  As described above, according to 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 as a heat exchange part assembled by brazing joining. The ferritic stainless steel of the present invention can be suitably used as an exhaust heat recovery member, particularly a heat exchange member.

Hereinafter, embodiments of the present invention will be described in detail.
The important corrosion damage to be considered when applying ferritic stainless steel to the exhaust heat recovery device is the resistance to pitting caused by pitting corrosion and crevice corrosion, as in the exhaust system downstream member mainly composed of a muffler. 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, and excellent perforation resistance is required compared to 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 pH from neutral to weak. It may change to acidity and the corrosive environment may become severe.
In view of such a background, the present inventors diligently studied to improve the perforation resistance of stainless steel in an exhaust gas condensed water environment.

As a result, it has been found that it is necessary to combine the following (1) and (2) in order to improve the perforation resistance due to 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 Cr, Nb, Si, Al, Ti with a total cation fraction {(content of Cr, Nb, Si, Al and Ti contained in the oxide film) (Total) / (Content of all cation elements contained in oxide film) × 100 (%)}.

In order to improve the perforation resistance due to pitting corrosion and crevice corrosion of stainless steel, 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 rich in Cr is formed on the surface.

  Furthermore, when brazing is performed in an environment with a low oxygen partial pressure such as in a vacuum or a hydrogen atmosphere, elements such as Nb, Si, and Al contained in the steel material are concentrated in the passive film, and Cr, Si, Nb are formed on the surface. An oxide film rich in Al and Ti 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 was found that it acts effectively on development.

In order to form such an oxide film, it is necessary to satisfy the following (Formula 2) as the chemical composition of the steel material.
Si + Cr + Al + {Nb + Ti-8 (C + N)} ≧ 17.5 (Expression 2)
In (Formula 2), an element symbol represents content (mass%) of each element. Nb-8 (C + N) is 0 or more.

  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. Further, among Nb and / or Ti contained in stainless steel, Nb in a solid solution state that is not fixed to C and N is concentrated in the passive film at the time of brazing, and prevents corrosion in the oxide film formed after brazing. Contributes to action. 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 17.5% or more. There is a need to. Further, when the stainless steel does not contain Nb and / or Ti or does not contain 8 times or more of the (C + N) amount, the total content of Si, Cr and Al needs to be 17.5% or more. .

On the other hand, as a heat treatment condition for forming the oxide film at the time of brazing, it is maintained in an environment containing N ₂ at 1000 to 1200 ° C., 10 ^{−2 to 1} torr in a vacuum atmosphere or H ₂ atmosphere for 5 to 30 minutes. Is preferred. If the heat treatment is simply performed in a vacuum of 10 ⁻² torr or less, the total cation fraction of Cr, Nb, Si, Al and Ti in the formed oxide film does not reach the desired cation fraction. For example, after pulling a vacuum of ¹⁰ -2 torr, by heat treatment in an atmosphere which is a ¹⁰ -2 ^~1torr by introducing _{N 2,} more than 40% in total of said cationic fraction Cr, Si, Nb It becomes possible to form an oxide film in which Al , Ti and Ti are concentrated. 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) carbonitrides 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%, and more preferably 0.005 to 0.1%.
As heat treatment conditions, in order to form an oxide film in which Cr, Si, Nb, Al , and Ti with a total cation fraction of 40% or more are concentrated, it is held at 1050 to 1150 ° C. for 5 to 30 minutes. More preferred. The holding time is more preferably 10 to 20 minutes.
In addition, Cr is the most important among Cr, Si, Nb, Al 2 and Ti contained in the oxide film, and it is preferable to contain 20% or more in terms of cation fraction. 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 cations such as Cr, Si, Nb, Ti, and Al occupying 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.

On the other hand, the present inventor has focused on Ni, Cu, and Mo from the viewpoint of the growth suppression effect. The reason why the perforation resistance is improved by incorporating two or more of Ni, Cu, and Mo into stainless steel is estimated as follows.
Accompanying the occurrence of corrosion, chloride concentrates in the pits or gaps, and the pH decreases. In many cases, the material is actively dissolved in such an environment, 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 affects the ease of stopping corrosion (repassivation) as well as the dissolution reaction (anodic 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 increases the cathode current, and Cu contributes to promoting repassivation by making the potential noble. On the other hand, Mo has the effect of strengthening the passivation and suppressing the reoccurrence of corrosion. It is presumed that the hole resistance of stainless steel has been improved by combining these different effects of Ni, Cu and Mo.

  The present invention is made in consideration of the thermal fatigue characteristics and workability required as an exhaust heat recovery member, in addition to the above knowledge about the perforation resistance, and for an exhaust heat recovery device having excellent corrosion resistance against exhaust gas condensed water. Ferritic stainless steel is provided, the gist of which is as described in the claims.

  Hereinafter, the reason which limited each composition of the ferritic stainless steel for exhaust heat recovery devices is demonstrated. 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: 0.05% or less)
N is an element useful for pitting corrosion resistance, but its content needs to be kept low in order to reduce intergranular corrosion resistance and workability. Therefore, the N content is set to 0.05% 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%. Furthermore, it is preferable that the total content of C and N is 0.015% or more ((C + N) ≧ 0.015%) from the viewpoint of suppressing grain coarsening during brazing.

(Si: more than 0.1%, 1% or less)
Si is required to be contained in an amount of 0.1% or more in order to concentrate on the surface film of stainless steel after brazing and contribute to the improvement of corrosion resistance. Si is useful as a deoxidizing element. However, excessive addition reduces workability, so the Si content is 1% or less. More 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. More preferably, it is 0.05 to 1%.

(Cr: 17% or more, 23% or less)
Cr is an element that is fundamental in securing exhaust gas condensed water and salt corrosion resistance of stainless steel, and it is necessary to contain at least 17% or more. The corrosion resistance can be improved as the Cr content is increased, but a large amount of addition is required to obtain the same effect as Ni, Cu, and Mo with respect to the perforation resistance of the gap. Moreover, since excessive addition reduces workability and manufacturability, the content of Cr is set to 23% or less. Preferably they are 17% or more and 20.5% 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 brazing and contribute to 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 0.003 to 0.1%.

In the present invention, the stainless steel needs to contain any two or three 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 after containing either Cu or Mo is 0.25% or more. As the Ni content is increased, the corrosion resistance can be improved. However, the addition of a large amount makes it harder and lowers the workability, and also increases the cost because it is expensive. Therefore, the Ni content is set to 1.5% or less. Preferably it is 0.25 to 1.2%. More preferably, it is 0.25 to 0.6%.

(Cu: 0.25% to 1%)
Cu, together with Ni and Mo, is an important element in improving the corrosion resistance, particularly the perforation resistance. The content of Cu that provides a stable effect after containing either Ni or Mo is 0.25% or more. The corrosion resistance can be improved as the content of Cu is increased, but the addition of a large amount makes it harder and lowers the workability. Therefore, the Cu content is set to 1% or less. Preferably, it is 0.25 to 0.8%. More preferably, it is 0.25 to 0.6%.

(Mo: 0.5% or more, 2% or less)
Mo, together with Ni and Cu, is an important element in improving the corrosion resistance, particularly the perforation resistance. The content of Mo that can provide a stable effect after 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 makes it harder and lowers the workability, and also increases the cost because it is expensive. Therefore, the Mo content is set to 2% or less. As described above, Mo is a more important element because it improves the perforation resistance by an action different from Ni and Cu. Therefore, it is preferable to contain 0.7% or more and 2% or less. More preferably, it is 0.9% or more and 2% or less.

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 brazing 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, 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 improves the high-temperature strength, and thus is useful for a member that requires high-temperature fatigue characteristics by cooling high-temperature exhaust gas such as an exhaust heat recovery device.

(V: 0.5% or less)
V can be contained in an amount of 0.5% or less as required in order to improve the corrosion resistance. A stable effect is obtained at 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 0.5%.

(W: 1% or less)
W can be contained in an amount of 1% or less as required for improving the corrosion resistance. It is 0.3% or more that a stable effect can be obtained. 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 0.0015%.

(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.

(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. In order to obtain a stable effect, it is preferable to contain 0.01% or more of Sn.

(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. . 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 can be produced by performing a general process for producing ferritic stainless steel. For example, molten steel having the above chemical composition in a converter or electric furnace, scoured in an AOD furnace or VOD furnace, etc. to form a steel piece by a continuous casting method or an ingot method, It is manufactured through the steps of annealing, pickling, cold rolling, finish annealing, and pickling. If necessary, annealing of the hot-rolled sheet may be omitted, or cold rolling-finish annealing-pickling may be repeated.

Next, the exhaust heat recovery device of the present invention will be described. The exhaust heat recovery device of the present invention is provided with a heat exchange part in which members are assembled by brazing and joining. In the heat exchange part of the exhaust heat recovery device of the present invention, an oxide film containing Cr, Nb, Si, Al and Ti in total of the cation fraction of 40% or more is formed on the surface of the ferritic stainless steel of the present invention. It consists of what is.

The manufacturing method of the exhaust heat recovery device of the present invention includes, for example, a process of forming a member made of the ferritic stainless steel of the present invention by performing a general processing step, and 10 ^{−2 to 1} torr including N ₂ as a member. And an assembly step of forming a heat exchange part by heat-treating in a vacuum atmosphere or H ₂ atmosphere and brazing and joining. By performing such an assembling process, a heat exchange part in which an oxide film containing Cr, Nb, Si, Al , and Ti with a total cation fraction of 40% or more is formed on the surface of a member made of ferritic stainless steel Is obtained.

  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.

  The molten steel having the chemical composition shown in Table 1 and Table 2 below is melted in a 30 kg vacuum melting furnace to produce a 17 kg flat steel ingot, and then hot rolled to a thickness of 4.5 mm at a heating temperature of 1200 ° C. Hot-rolled sheet annealing was performed at 1030 ° C. The scale was removed by alumina shot and after cold rolling to a plate thickness of 1 mm, finish annealing was performed at 950 to 1050 ° C. Using the cold-rolled steel sheets of Material Examples 1 to 17 thus obtained, the corrosion resistance was evaluated and the surface film was analyzed.

Test pieces of material examples 1 to 17 having a width of 25 mm and a length of 100 mm were cut out from each cold-rolled steel sheet, and the entire surface was wet-polished to # 320 with emery paper. Next, the atmosphere at the time of brazing was simulated and heat treatment was performed under the following conditions to obtain test pieces of Experimental Examples 1 to 17 in Table 3. 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.
Further, only the test piece of material example 1 was evacuated at 10 ⁻³ torr and then heated, held at 1100 ° C. for 10 minutes, cooled to room temperature in the furnace, and the test piece of experimental example 18 in Table 3 did.
Further, for the material Examples 1-3 in 100% _{H 2} having a dew point of -65 ° C., a heat treatment was carried out 10 minutes holding at 1100 ° C.. This was designated as Experimental Examples 19 to 21 in Table 3.

The following corrosion tests were performed on the test pieces of Experimental Examples 1 to 21 in Table 3. A solution of 100 ppm Cl ⁻ +1000 ppm SO ₄ ² + +1000 ppm SO ₃ ²⁻ was prepared using hydrochloric acid, sulfuric acid, and ammonium sulfite as reagents, and then adjusted to pH 3.5 using aqueous ammonia. The solution was put in a sealed glass container that can prevent evaporation and concentration of the solution, and the test piece was semi-immersed. This was kept at 80 ° C. and a 500 hour corrosion test was conducted. 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. The case where the maximum corrosion depth was 400 μm or less was evaluated as good corrosion resistance.
The results are shown in Table 3.

  During the heat treatment of the corrosion test pieces of Experimental Examples 1 to 21 in Table 3, the surface analysis sample was also heat-treated in parallel, and the surface oxide film was analyzed by X-ray photoelectron spectroscopy (XPS). The cation fraction (A value) of Cr, Nb, Si, Al, and Ti was calculated. XPS was manufactured by ULVAC-PHI, Inc., using a mono-AlKα ray as an X-ray source, and was carried out under the conditions of an X-ray beam diameter of about 100 μm and a take-off angle of 45 degrees.

From the test results shown in Table 3, the steels of Experimental Examples 1 to 12 and 19 to 21 within the scope of the present invention have an A value of 40% or more and good corrosion resistance in exhaust gas simulated condensed water.
On the other hand, experimental examples 13 to 15 which are comparative examples containing only one kind of Ni, Cu and Mo, and experimental example 17 which is a comparative example in which the Cr content and the A value are out of the scope of the present invention are simulated exhaust gas condensation. Corrosion resistance in water is poor.
Experimental example No. which is a comparative example in which the cation fraction in the oxide film formed after the brazing simulated heat treatment does not satisfy the scope of the present invention. No. 16 has an A value of less than 40% and is inferior in corrosion resistance.
In Experimental Example 18 in which heat treatment was performed only in a vacuum without introducing N ₂ , the A value was less than 40%, and the corrosion resistance in exhaust gas simulated condensed water was poor.

  The ferritic stainless steel for exhaust heat recovery device of the present invention, which has excellent corrosion resistance against exhaust gas condensate, is a member for exhaust heat recovery device (exhaust heat recirculation system), especially heat exchange member of exhaust heat recovery device Is preferred. In addition, it is also suitable for exhaust gas path members that are exposed to exhaust gas condensed water such as EGR and mufflers.

Claims (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,
2 or 3 types of Ni, Cu, and Mo are Ni: 0.25% or more, 1.5% or less,
Cu: 0.25% or more, 1% or less,
Mo: 0.5% or more, 2% 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, Nb, Si, Al , and Ti in a total of 40% or more of the cation fraction on the surface A ferritic stainless steel for exhaust heat recovery device, characterized in that it is formed.
8 (C + N) + 0.03 ≦ Nb + Ti ≦ 0.6 (Formula 1)
Si + Cr + Al + {Nb + Ti-8 (C + N)} ≧ 17.5 (Expression 2)
In (Formula 1) and (Formula 2), an element symbol represents content (mass%) of each element. In (Formula 2), Nb + Ti-8 (C + N) is 0 or more. % 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,
2 or 3 types of Ni, Cu, and Mo are Ni: 0.25% or more, 1.5% or less,
Cu: 0.25% or more, 1% or less,
Mo: 0.5% or more, 2% or less,
Contains one or two of Nb and Ti,
By satisfying (Equation 1) and (Equation 2) shown below, the balance being Fe and inevitable impurities, and heat-treating in a vacuum atmosphere or H ₂ atmosphere of 10 ^{−2 to 1} torr containing N ₂ , Cr is formed on the surface. A ferritic stainless steel for exhaust heat recovery device, characterized in that an oxide film containing 40% or more of Nb, Si, Al 2 and Ti in total of the cation fraction is formed.
8 (C + N) + 0.03 ≦ Nb + Ti ≦ 0.6 (Formula 1)
Si + Cr + Al + {Nb + Ti-8 (C + N)} ≧ 17.5 (Expression 2)
In (Formula 1) and (Formula 2), an element symbol represents content (mass%) of each element. In (Formula 2), Nb + Ti-8 (C + N) is 0 or more.   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% 3 or less, Mg: 0.002% or less, Ca: 0.002% or less, REM: 0.01% or less, any 1 type or 2 types or more are contained. Ferritic stainless steel for waste heat recovery equipment. It has a heat exchange part in which members are assembled by brazing joint,
The heat exchange part is in 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 %: Cr: 17% or more, 23% or less, Al: 0.002% or more, 0.5% or less, Ni: Cu, Mo, or two or three of Ni: 0.25% or more. 5% or less, Cu: 0.25% or more, 1% or less, Mo: 0.5% or more, 2% or less, containing one or two of Nb and Ti, shown below (Formula 1) and From ferritic stainless steel satisfying (Equation 2), the balance being Fe and inevitable impurities, and having an oxide film containing Cr, Nb, Si, Al , and Ti with a total cation fraction of 40% or more on the surface An exhaust heat recovery device characterized by comprising:
8 (C + N) + 0.03 ≦ Nb + Ti ≦ 0.6 (Formula 1)
Si + Cr + Al + {Nb + Ti-8 (C + N)} ≧ 17.5 (Expression 2)
In (Formula 1) and (Formula 2), an element symbol represents content (mass%) of each element. In (Formula 2), Nb + Ti-8 (C + N) is 0 or more.   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, REM: 0.01% or less. The exhaust heat recovery device according to claim 4.