JP3301845B2 - Manifold converter based on high Al content ferritic stainless steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a manifold converter based on a high Al-containing ferritic stainless steel having excellent resistance to high-temperature oxidation.

[0002]

2. Description of the Related Art Ferritic stainless steel containing high Al has excellent resistance to high-temperature oxidation, and is widely used as a heater, electric heating material, etc., including chimney materials for stoves. Recently, it has also been used as a base material for a catalytic converter of an exhaust gas purification device for automobiles. Conventionally, a catalytic converter using a ceramic base material has been used. However, since it is weak against thermal shock and has a large heat capacity, it takes time to raise the temperature to the catalytic reaction temperature range. In contrast, metallic converters made of stainless steel or the like do not have the disadvantages of ceramic substrates. Although a foil material having a thickness of about 50 μm is used as a base material of the metallic converter, the foil material is very excellent because abnormal oxidation easily occurs and is used in an exhaust gas atmosphere under severe oxidation conditions. High temperature oxidation resistance is required. As a material satisfying such required characteristics, high Cr-containing ferritic stainless steel has attracted attention, for example, 20Cr-5Al.
Ferrite stainless steel to which a rare earth element (REM), Y, or the like is added based on is used.

[0003] As a recent trend, demands for automobile exhaust gas regulations are becoming severer from the viewpoints of prevention of global warming and prevention of pollution. Therefore, in order to quickly raise the temperature from the start of the engine to the catalytic action temperature range, measures are being taken to increase the temperature of the exhaust gas and to mount a converter immediately below the manifold close to the engine. Exhaust gas temperature has been increasing even with higher engine output. As a result, the environment in which the converter substrate is used is becoming increasingly harsh. Conventional steels for metallic converters do not exhibit sufficient high-temperature oxidation resistance in such severe use environments, and have been a bottleneck in developing manifold converters.
Therefore, there is a demand for a material having better high-temperature oxidation resistance than before.

Conventionally, for example, a steel material having a thickness of 50 μm is subjected to an oxidation test at 1150 ° C. for 96 hours, and a steel material which does not cause abnormal oxidation has been used. However, as the use environment becomes severe, the metal foil for a metallic converter is required to have extremely excellent high-temperature oxidation resistance that does not cause abnormal oxidation even when the temperature is maintained at 1150 ° C. for 500 hours or more. High Al content ferritic stainless steels are Cr, A
The high-temperature oxidation resistance improves as the amount of l, REM, Y, or the like increases. For example, Japanese Patent Application Laid-Open No. 4-128344 discloses a high Al-containing ferritic stainless steel in which the high-temperature oxidation resistance is enhanced by adding 0.01 to 0.5% by weight of Y. In addition, Japanese Patent Application Laid-Open No.
No. 45 discloses that in order to improve oxidation resistance under the condition that the component cost is kept as low as possible, 0.06 to 0.15% by weight of a lanthanide group element Ln is added, and in relation to Ln. A high Al-containing ferritic stainless steel having improved hot workability by containing a specified amount of P is introduced.

[0005]

The alloying elements such as REM and Y are effective in improving the high-temperature oxidation resistance of ferritic stainless steel containing high Al. It can have adverse effects. In addition, high Al-containing ferritic stainless steel generally has low toughness of a slab and a hot-rolled sheet, and is inferior in productivity. Therefore, if the content of Cr and Al is increased to improve the high-temperature oxidation resistance, not only the raw material cost is increased, but also the manufacturability is deteriorated due to the deterioration of toughness. Invite a rise. Ferritic stainless steel used as a base material for a metallic converter is processed into a foil having a thickness of about 50 μm. Since this foil is exposed to a heat cycle of repeated heating and cooling by high-temperature exhaust gas, deformation due to heating and deformation becomes a problem. In this regard, the material as the base material for the metallic converter is also required to have excellent high-temperature strength. Particularly, in the case of a manifold converter exposed to a severe use atmosphere, required characteristics are further severed. The present invention
It has been devised to solve such a problem, and by controlling the oxide layer formed on the surface of steel used as a converter base material, it has a significantly higher temperature oxidation resistance than a conventional metallic converter. An object of the present invention is to provide a manifold converter exhibiting characteristics.

[0006]

SUMMARY OF THE INVENTION The manifold converter of the present invention has a Cr: 15-28
A Cr-based oxide layer and an Al-based oxide layer are sequentially formed on the surface of a ferritic stainless steel containing 3% to 8% by weight of Al and 3 to 8% by weight. The base material is a high Al-containing ferritic stainless steel having a thickness ratio of 1 or less, preferably 0.3 or less. The ferritic stainless steel used as the base material is specifically C:
0.03% by weight or less, Si: 0.25% by weight or less, M
n: 0.3% by weight or less, P: 0.04% by weight or less, S:
0.003% by weight or less, N: 0.03% by weight or less, C
r: 15 to 28% by weight, Al: 3 to 8% by weight, Mo:
0.5 to 3% by weight, one or more kinds of rare earth elements (REM) and Y: including 0.01 to 0.15% by weight in total, and REM + Y + Mo / 10 = 0.35 to 0.1 (preferably Satisfies 0.35 to 0.22). This stainless steel further contains Ti and / or V in a total amount of 0.01 to
0.5% by weight.

[0007]

The high-temperature oxidation resistance of the high Al-containing ferritic stainless steel is given by the Al oxide formed on the surface of the steel material. When Y or REM is added to the stainless steel, the adhesion of the oxide film is improved and abnormal oxidation is suppressed. The present inventors have developed 1150 in an oxidizing atmosphere to develop steel that can withstand very harsh oxidizing conditions.
An oxide film formed on the foil surface when heated to 500 ° C. for 500 hours was investigated. When the Fe-Cr-Al-based alloy foil is heated to a high temperature, an Al-based oxide layer is formed on the foil surface.
In an alloy foil having a small thickness, the amount of Al supplied per unit surface area of a steel material is small, and all of the Al in the steel is consumed as a surface oxide in a short time, and the Al in the steel is eliminated. For example, when a 50 μm-thick foil is heated to 1150 ° C.,
Within 00 hours, all the Al in the steel was oxidized. A in steel
When the oxidation further proceeds after 1 is consumed, a Cr-based oxide layer is formed between the Al-based oxide layer and the base steel. If the Cr-based oxide layer is not stable against oxidation, a thick bulk oxide is locally formed, and the oxidation proceeds from the bulk oxide. As a result, Fe of the base steel is oxidized and reaches an oxidized state penetrating in the thickness direction.

In this regard, the conventional materials have improved the adhesion and stability of Al-based oxides by adding REM, Y, etc. for the purpose of suppressing abnormal oxidation. However, in order to further improve the high-temperature oxidation resistance, it is necessary to modify not only the Al-based oxide but also the Cr-based oxide. The present inventors have conducted investigation and research based on such a premise, and as a result, have found that when the Cr-based oxide is not growing in a lump, the high-temperature oxidation resistance is improved. The bulk growth of the Cr-based oxide can be quantitatively expressed by the maximum thickness ratio of the Cr-based oxide layer to the Al-based oxide layer. When the maximum thickness ratio is 1 or less, there is substantially no bulk growth of the Cr-based oxide, and excellent high-temperature oxidation resistance is obtained. Maximum thickness ratio is 1
Is exceeded, large portions of the Cr-based oxide layer formed between the base steel and the Al-based oxide layer are grown in large blocks. Those having such a massive Cr-based oxide layer have reduced high-temperature oxidation resistance.

The bulk growth of the Cr-based oxide layer is performed by using REM, Y
And Mo in combination. Further, Ti,
When V is added, the progress of local oxidation is suppressed.
As a result, a foil exhibiting excellent high-temperature oxidation resistance is obtained.
A 50 μm-thick foil manufactured from steels A and B shown in Table 1 was oxidized at 1150 ° C. for 200 hours. Steel A is a conventional steel used for metallic converters.
Steel B is a steel to which 2% by weight of Mo and a trace amount of REM are added. After Ni plating was performed so that the generated oxide film was not lost, the cross-sectional structure of the surface layer was observed. In the steel A foil, as shown in FIG. 1, the Cr-based oxide grew remarkably locally under the Al-based oxide layer. As shown in FIG. 2, the foil of steel B has a thin Cr-based oxide layer having a uniform thickness.
It was formed under the system oxide layer, and no massive oxide was detected.

[0010]

[Table 1]

When steels A and B were subjected to another oxidation test at 1150 ° C., no abnormal oxidation was detected in steel B even after heating for 1000 hours or more. When the cross section of the steel after heating was observed, growth of the Cr-based oxide layer was hardly observed, and the change in the amount of increase in oxidation after the formation of the initial oxide film was very small. On the other hand, in steel A, partial abnormal oxidation started when heating was continued for 230 hours, and abnormal heating progressed over the entire test piece by subsequent heating. Observation of the cross section of the heated steel revealed that abnormal oxidation occurred starting from the portion where the Cr-based oxide layer grew along the plate thickness direction in a lump. Steel A
From the results of the oxidation tests of A and B, the cause of the abnormal oxidation was A
It can be said that the Cr-based oxide formed under the l-based oxide layer is agglomerated. In addition, agglomeration of the Cr-based oxide is suppressed by adding REM and / or Y in combination with Mo. As a result, a steel material having significantly improved high-temperature oxidation resistance can be obtained.

Therefore, after holding various steel foils having a basic composition of 20Cr-5Al-REM, Y at 1150 ° C. for 200 hours, the sectional structure of the surface layer was observed, and the Al-based oxide layer and the Cr-based oxide layer were observed. Was examined for its thickness. The survey results showed that Al
When the maximum thickness ratio of the Cr-based oxide layer to the system-based oxide layer was 1 or less, it was indicated that the Cr-based oxide layer was not aggregated. The maximum thickness ratio of the Cr-based oxide layer to the Al-based oxide layer is affected by REM + Y + Mo / 10, and as shown in FIG. 3, when REM + Y + Mo / 10 ≧ 0.1, the maximum thickness ratio becomes 1 or less. Was. Based on this finding,
A 50 μm-thick foil material was prepared from the steel shown in Table 2 and
It was subjected to an oxidation test at 50 ° C. Each of the steels listed in Table 2 satisfies REM + Y + Mo / 10 ≧ 0.1.
Abnormal oxidation did not occur even when heated to 50 ° C. for 500 hours or more. In particular, steels E and F with REM + Y + Mo / 10 ≧ 0.22 did not generate abnormal oxidation even when heated for more than 1000 hours. However, as the value of REM + Y + Mo / 10 increases, the toughness of the steel material decreases and the production becomes difficult, so the upper limit of REM + Y + Mo / 10 is set to 0.1.
It is preferably 35.

[0013]

[Table 2]

Mo together with REM and / or Y, and T
The reason why abnormal oxidation is suppressed by adding i and V in combination is not clear. However, it is inferred that the inward diffusion of oxygen is suppressed by Mo or the like dissolved in the Cr-based oxide and oxidation hardly proceeds. In fact,
When the steel to which REM, Y, Mo, and the like are added in combination is kept at a high temperature, the change with time of the increase in oxidation is extremely small after the initial oxide film is formed. This indicates that the rate at which oxygen in the oxide film diffuses inward is very small.

Hereinafter, alloy components, contents, and the like contained in the ferritic stainless steel used as the base material will be described. C: 0.03% by weight or less As the C content increases, abnormal oxidation easily occurs,
The high temperature oxidation resistance deteriorates. Further, in a high Al-containing ferritic stainless steel, the toughness of a slab or a hot coil deteriorates due to an increase in the C content. Therefore,
In the present invention, the upper limit of the C content is specified as 0.03% by weight. Si: 0.25% by weight or less In ordinary stainless steel, it is treated as an effective element for improving high-temperature oxidation resistance, and has been positively added to high-temperature oxidation-resistant stainless steel. However, in a high Al-containing ferritic stainless steel, the lower the Si content, the better the high-temperature oxidation resistance, and the less likely it is for abnormal oxidation to occur. Therefore, in the present invention, the upper limit of the Si content is set to 0.25% by weight.

Mn: 0.3% by weight or less Mn is an element that improves hot workability, but adversely affects high-temperature oxidation resistance. By reducing the Mn content, high-temperature oxidation resistance is improved, and abnormal oxidation is less likely to occur. The toughness also improves as the Mn content decreases. However, Mn
Is mixed from the raw material scrap, so that it is difficult to control it at a low level. Therefore, in the present invention, M
The upper limit of the n content was 0.3% by weight. P: 0.04% by weight or less Since a high temperature oxidation resistance is adversely affected, a lower content is preferable. Further, since the toughness of the hot-rolled sheet is adversely affected, the upper limit of the P content is set to 0.04% by weight. S: 0.003% by weight or less Combined with REM, Y, etc., not only degrades the surface properties of steel as inclusions, but also reduces the effective amount of REM, Y, etc., which are effective in high-temperature oxidation resistance. . This adverse effect becomes remarkable when the S content exceeds 0.003% by weight. Therefore, in the present invention, the S content is set to 0.03% by weight.
Or less, preferably 0.002% by weight or less.

N: 0.03% by weight or less N is an harmful element which reduces toughness and forms AlN which is a starting point of abnormal oxidation. Therefore, the upper limit of the N content is set to 0.03% by weight. Cr: 15 to 28% by weight It is a basic element necessary for improving the high-temperature oxidation resistance. In order to exert its effect, the Cr content is 15% by weight.
It is necessary to do the above. However, when a large amount of Cr is contained in excess of 28% by weight, the toughness of the slab and the hot coil deteriorates, and the manufacturability deteriorates. Al: 3 to 8% by weight Like Cr, it is an alloy element indispensable for maintaining the high-temperature oxidation resistance of steel. Due to the inclusion of Al, A
An l-based oxide layer is formed, and excellent high-temperature oxidation resistance is imparted. Particularly, in a foil material having a thickness of 100 μm or less, abnormal oxidation is likely to occur. In order to grow a sufficient Al-based oxide, an Al content of 3% by weight or more is required. However, when the Al content exceeds 8% by weight, the toughness of the slab and the hot coil decreases, and mass production becomes difficult.

Mo: 0.5 to 3% by weight Mo has been treated as a harmful element that deteriorates the high-temperature oxidation resistance of steel because it easily forms a highly volatile oxide. However, in the steel used in the present invention, the addition of Mo improves the stability of the Cr-based oxide and suppresses the formation of massive oxide. As a result, the high temperature oxidation resistance is significantly improved. Mo addition is also effective in improving high-temperature strength. Such effects become remarkable when the Mo content is 0.5% by weight or more. However, a large amount of Mo exceeding 3% by weight
Is contained, the toughness of the steel deteriorates, and the manufacturability deteriorates. One or more of REM and Y: 0.01 to a total
0.15% by weight It is an important alloy element in improving the high-temperature oxidation resistance of the Fe-Cr-Al-based alloy. RE of La, Ce, Nd, etc.
M and Y remarkably improve the stability of the oxide film formed on the surface of the high Al ferritic stainless steel, and suppress abnormal oxidation that easily occurs in the foil material. REM and Y also have the effect of improving the adhesion of the oxide film. In order to obtain such an effect, it is necessary to include one or more of REM and Y in a total amount of 0.01% by weight or more. Conversely, a large amount of REM and / or
Alternatively, when Y is added, hot workability and toughness are significantly deteriorated, and production becomes difficult. REM, Y added in large amounts
Generates inclusions that serve as starting points for abnormal oxidation, rather deteriorating high-temperature oxidation resistance.

REM + Y + Mo / 10: 0.35 to 0.1% by weight This is a factor that affects the maximum thickness ratio of the Cr-based oxide layer to the Al-based oxide layer. According to the investigation and research by the present inventors, REM + Y + Mo / 10 is 0.1% by weight.
When the content is at least 0.22% by weight, the maximum thickness ratio of the Cr-based oxide layer / Al-based oxide layer becomes 1 or less, and the Cr-based oxide layer is prevented from agglomerating. But R
When EM + Y + Mo / 10 exceeds 0.35% by weight, the productivity is deteriorated due to a decrease in toughness. Ti and / or V: 0.01-0.5% by weight in total The foil material used as a manifold converter is repeatedly subjected to a cooling and heating cycle under an oxidizing atmosphere. That is,
Used in an environment where the oxide film formed on the foil surface is easy to peel off. Although the adhesion of the oxide film is improved by the addition of REM, Y, etc., the addition of a large amount increases the raw material cost.
In this regard, when adding Ti and / or V, REM, Y
An oxide film having very excellent adhesion can be obtained without adding a large amount of the above, and abnormal oxidation does not occur. In order to obtain such an effect, a total of 0.01% by weight or more of T
i and / or V is contained. When Ti and / or V is added, the steel material becomes harder as the content increases, so the upper limit of the content is regulated to 0.5% by weight.

[0020]

EXAMPLES Various steels shown in Tables 3 and 4 were melted in a vacuum, forged, cut and hot rolled, and then repeatedly subjected to annealing and cold rolling.
A 50 μm-thick foil material was manufactured. A test piece cut from this foil material was subjected to an oxidation test at 1150 ° C., and the time when abnormal oxidation occurred was measured. Table 3 and Table 4 show the test results.
Are shown together.

[0021]

[Table 3]

[0022]

[Table 4]

As is clear from Table 3, in the steel of the present invention,
In each case, abnormal oxidation occurred after 500 hours or more, and exhibited extremely excellent high-temperature oxidation resistance. Therefore, the target characteristic required for a material for a manifold converter, which does not cause abnormal oxidation even when the temperature is maintained at 1150 ° C. for 500 hours or more, is satisfied. In particular, REM + Y
Nos. 3, 5 to 11, in which + Mo / 10 ≧ 0.22 and the maximum thickness ratio of the Cr oxide layer / Al oxide layer was 0.3 or less, 1
Even when it was held at 150 ° C. for a long time exceeding 1000 hours, it exhibited extremely excellent characteristics without abnormal oxidation.
On the other hand, as shown in Table 4, each of the comparative steels
Abnormal oxidation occurred within hours, and did not satisfy the target characteristics. For example, No. 12 which does not contain Mo
In No. 14, abnormal oxidation occurred in a short time. Mo, V and R
No. EM containing EM but having REM + Y + Mo / 10 less than 0.1%. No. 15, the Al content is small. No. 16 and No. 16 to which Y and REM were not added. No. 17 is inferior in high temperature oxidation resistance as compared with the steel of the present invention. The oxide film formed when each steel was heated and oxidized at 1150 ° C. for 200 hours was subjected to structural analysis, and the maximum thickness ratio of Cr-based oxide layer / Al-based oxide layer was determined. In the steel of the present invention, as shown in Table 3, the maximum thickness ratio was 1 or less, and a uniform thin Cr-based oxide layer was formed. As shown in Table 4, the comparative steel had a maximum thickness ratio of 1 or more, and localized bulk growth was detected in the Cr-based oxide layer. Also, 200
No. in which abnormal oxidation occurred within hours 12, 13, and 1
In steel foil No. 7, oxidation occurred through the thickness of the steel foil.
The maximum thickness ratio could not be measured.

[0024]

As described above, the manifold converter of the present invention is based on a ferritic stainless steel containing high Al and Mo and having a maximum thickness ratio of Cr-based oxide layer / Al-based oxide layer of 1 or less. The material exhibits excellent high-temperature oxidation resistance characteristics in which abnormal oxidation does not occur even when exposed to heating at 1150 ° C. for 500 hours or more. for that reason,
It is used as a converter that exhibits stable performance over a long period of time even under extremely severe oxidation conditions.

[Brief description of the drawings]

Fig. 1 Cross-section of the surface of a steel material with a Cr-based oxide layer grown partially in bulk

FIG. 2 is a sectional view of a surface layer of a steel material in which bulk growth of a Cr-based oxide layer is suppressed according to the present invention.

FIG. 3 Effect of REM + Y + Mo / 10 on the maximum thickness ratio of Cr-based oxide layer / Al-based oxide layer

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C23C 28/04 C22C 38/00 302 C22C 38/22

Claims (4)

(57) [Claims] 1. Cr: 15 to 28% by weight and Al: 3 to
Cr on the surface of ferritic stainless steel containing 8% by weight
A system-based oxide layer and an Al-based oxide layer are sequentially formed,
A manifold converter based on a high-Al-containing ferritic stainless steel having a maximum thickness ratio of Cr-based oxide layer / Al-based oxide layer of 1 or less. 2. C: 0.03% by weight or less, Si: 0.2
5% by weight or less, Mn: 0.3% by weight or less, P: 0.04
% By weight, S: 0.003% by weight or less, N: 0.03%
% By weight, Cr: 15 to 28% by weight, Al: 3 to 8% by weight, Mo: 0.5 to 3% by weight, rare earth element (REM)
And one or more of Y: 0.01 to 0.15 in total
Wt.%, REM + Y + Mo / 10 = 0.35
The manifold converter according to claim 1, wherein the base material is a ferritic stainless steel satisfying 0.1. 3. The method according to claim 1, wherein Ti and / or V are added in a total of 0.01
The manifold converter based on a ferritic stainless steel according to claim 2, wherein the content of the ferrite stainless steel is from 0.5 to 0.5% by weight. 4. REM + Y + Mo / 10 = 0.35
The high Al-containing ferritic stainless steel according to any one of claims 1 to 3, wherein the maximum thickness ratio of the Cr-based oxide layer / Al-based oxide layer is 0.3 or less at 0.22. Manifold converter.