JPH04120247A - Fe-cr-al alloy excellent in manufacturability and having high strength at high temperature and high heat resistance - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a heat-resistant stainless steel foil used as a catalyst carrier for a combustion exhaust gas purification device. More specifically, it relates to heat-resistant stainless steel foil, which not only has excellent oxidation resistance and manufacturability, but also has excellent strength at high temperatures, so when used in the honeycomb body of the catalyst, it has a great effect on improving the structural durability of the catalyst. .

(Conventional technology) Ceramic honeycombs have traditionally been used in combustion exhaust gas purification devices for automobiles, etc., but by replacing them with heat-resistant stainless steel foil, it is possible to reduce the thickness of the honeycomb walls, reducing ventilation resistance. As disclosed in, for example, Japanese Patent Laid-Open No. 50-92286, No. 51-48473, and No. 57-71898, it is possible to improve engine performance and save expensive catalyst precious metals by reducing heat capacity. A technique has been proposed in which this honeycomb body is constructed from FeCr-Al based heat-resistant metal foil.

In this case, oxidation resistance and oxide film adhesion are attracting attention as the characteristics required of the alloy, and therefore, the material has generally been used as a material with excellent oxidation resistance and oxide film adhesion. Based on the Fe-CrAl alloy, which has been widely used as high-temperature parts in electric wires and heating equipment, it is highly resistant to oxidation and activated alumina (which is a direct support for catalysts).
r-A l z 03) Foil with improved adhesion to the coating layer is used. All of the techniques disclosed in the above-mentioned publications utilize Y as a means to improve the oxidation resistance of the material, but since Y is an extremely expensive element, its range of use is limited.

On the other hand, in JP-A-58-177437, Fe-Cr
-1 series alloy mainly contains 0.002 to 0.05% by weight of La, Ce, Pr, to prevent peeling of the oxide film.
Contains rare earth elements of the group consisting of Nd, total amount 0.063
Alloys containing up to 1% by mass of rare earth elements, Zr added to stabilize the alloys, and Nb added to ensure creep strength at high temperatures within specific relationship ranges with the amounts of C and N. Alloys have been proposed. According to this patent, alloys with a total rare earth element content exceeding 0.06% by weight not only have little improvement in oxidation resistance compared to alloys containing less than 0.06% by weight, but also cannot be processed at normal hot working temperatures. It states that

In JP-A No. 63-45351, Fe-
Since the addition of Y in alloys based on Cr-^l system is expensive, Ln or La without Ce is added.
It is proposed to add only 0.05 to 0.2% by weight. This is because the presence of Ce is the cause of the decrease in hot workability due to the addition of Ln, and furthermore, Ce has the effect of decreasing oxidation resistance.
It states that this is based on the knowledge that adding Ln excluding only Ln makes hot working possible and improves oxidation resistance.

However, Ln is a chemically active element,
Since their mutual chemical properties are similar, individual separation is not easy, and therefore, although substantially pure La is cheaper than Y, it is less expensive than Mitsushmetal, which is a common mixture of Ln. remains very expensive. Similarly, separating and removing only Ce also inevitably increases the price. Furthermore, JP-A No. 63-42356 by the same applicant describes a Fe-Cr-Al alloy having excellent oxidation resistance and peeling resistance of oxide scale, containing a total of 0 La, Ce, Pr and Nd. .01%
Although alloys containing 0.30% or less have been disclosed above,
No studies have been conducted on the hot workability of this alloy.

In addition, these conventional techniques mainly examine the adhesion and oxidation resistance of the oxide film, but the structural durability of the honeycomb body, which is a practically important property required for the foil that constitutes the honeycomb body of the catalyst, has been studied. The influence of the high temperature strength of the foil material on the properties has not been sufficiently investigated.

[Problem to be solved by the invention]

However, in the case of catalyst carriers in automobiles, for example, it is rare for the catalyst carriers to reach the end of their service life due to the lack of oxidation resistance of the foil in normal use environments, and rather the need for heating and cooling in conjunction with driving conditions. In most cases, the product breaks due to thermal fatigue caused by repeated cycles and reaches the end of its service life. That is, during acceleration, the honeycomb body is rapidly heated from the inside by the high-temperature, high-speed exhaust gas flow, while it is forcibly cooled from the outside by the traveling wind, resulting in a sharp temperature gradient inside the honeycomb body and large thermal distortion. occurs. This thermal strain is not uniformly distributed in the radial direction of the honeycomb body, but is concentrated several layers inside from the outermost periphery. This is because the outer layer material temperature is low and the yield strength of the foil is high at that point, and the inner layer is high temperature but has a small temperature gradient, so the yield strength of the ferritic stainless steel foil that makes up the honeycomb is low. This is because the temperature range in which the temperature begins to drop significantly and the region in which the steepest temperature gradient occurs coincide with each other several layers from the outermost periphery. Furthermore, even when running at a constant speed, since there is cooling from the outside by the running wind, the degree of distortion is alleviated, but the distortion still remains concentrated in a region several layers from the outermost periphery.

Furthermore, during deceleration or idle running, relatively low-temperature gas flows, so the honeycomb body is cooled from the inside as well as the outside, and the area several layers inside from the outermost periphery is at its highest temperature, so heat is generated in this area. Distortion is concentrated.

11111 The honeycomb body of the catalyst carrier is exposed to such heating and
In most cases, due to the accumulation of thermal strain generated inside the cell due to repeated cooling, the cell collapses or becomes severely deformed, reaching the end of its structural life. In such cases, proof stress is more important than creep strength as the mechanical properties of the foil at high temperatures, and in particular, as described above, the present invention According to their measurements, the yield strength of the foil material in the temperature range of 600 to 850 °C is high, and the degree of decrease in yield strength due to temperature above 600 °C is as small as possible. It is effective in improving the structural life of the In addition, the metal structure of the foil changes during use at such high temperatures,
In some cases, the strength at high temperatures gradually decreases, and it is important to prevent this as much as possible in order to improve the structural durability of the catalyst carrier.

Furthermore, for general use such as in passenger cars, for example, it is desired that the material be inexpensive and stably supplied, and therefore, it is desired that the material has not only low component cost but also excellent manufacturability.

Furthermore, since the foil is exposed to high-temperature exhaust gas with a significantly large surface area relative to its volume, it must naturally have excellent oxidation resistance.

In view of these current problems, the present inventors conducted various studies to develop a method for constructing a catalyst carrier having the above-mentioned characteristics, and arrived at the present invention.

[Means to solve the problem]

That is, in order to improve the oxidation resistance while keeping the component cost of the foil as low as possible, it is necessary to contain Ln exceeding 0.06%.
It is effective to add 0.06% or less, and the oxidation resistance is dramatically improved compared to the case of 0.06% or less. Such a decrease in hot workability does not occur at all if P is contained in combination with Ln.

Furthermore, as mentioned above, in order to improve the structural durability of the catalyst carrier due to heating and cooling, it is necessary to
Improving the yield strength at temperatures between 00 and 850°C is important, and as a result of various studies for this purpose, we found that the combined addition of Ti and Nb improves the yield strength, especially at high temperatures exceeding 700"C, and maintains its strength even after long-term use. It became clear that the decrease in strength was extremely small.

Furthermore, as a result of investigating the toughness of hot-rolled sheets, which is a problem in the production of this type of ferritic stainless steel foil, it was found that it was necessary to keep the C and N contents low, and that Ti had a certain relationship with the C and N contents. It has become clear that when added in a relatively small amount, significant improvements can be made, and that it can be raised to a level that can be sufficiently mass-produced using normal stainless steel manufacturing processes.

That is, the present invention is based on the above study results, and has been developed to improve the oxidation resistance of the foil, the adhesion of the film, and the adhesion of the film even in high-temperature exhaust gas.
Another purpose is to provide a ferritic heat-resistant stainless steel foil that not only has excellent manufacturability such as hot workability, but also has excellent high-temperature strength, which is essential for improving the structural durability of catalyst carriers. It has been achieved. However, the specific means are as follows. That is, the material component of the foil of the present invention is Cr: 1 in weight%.
8 to 28%, A1: 4.5 to 6.5%, P: 31/233X (Ln+0.021) to 0.1
%, Ln: more than 0.06 to 0.15%, Ti: 0.02 to (0,03+ 4 x C+ 2
4/7 x N)%, Nb: (0,1+93/12
×C+93/14×N) more than 2.0%, and as impurities C: 0.02% or less, N: 0.02% or less, and C+N: 0.03% or less, S: 0-003% or less Si: 0.5% or less Mn: 1.0% or less Ni: 0.3% or less The remainder consists essentially of Fe-Cr-Aj! It is composed of a series alloy.

[Function] Next, the reasons for limiting the components in the present invention and their functions will be explained in detail. It should be noted that all chemical compositions in this specification are in percent by weight.

(1) Cr: Cr is a basic element that ensures corrosion resistance of stainless steel. In the present invention, the main component of oxidation resistance is AffiZ (h
Although it is present in the film, if Cr is insufficient, its adhesion and protective properties will decrease. On the other hand, when Cr becomes excessive, the toughness of the hot rolled sheet decreases, so the range is 18% or more and 28% or less.

(2) Ax: A1 is a basic element that ensures oxidation resistance in the present invention, and if it is less than 4.5χ, the protection of the oxide film in the exhaust gas will be poor in the case of foil, and it will easily cause abnormal oxidation. It cannot withstand its use as a catalyst carrier. On the other hand, 6.5
If the content exceeds χ, the toughness of the hot rolled sheet will be extremely reduced and manufacturability will be impaired, and the coefficient of thermal expansion of the foil will increase, resulting in heat loss due to repeated heating and cooling when used as a catalyst carrier. Fatigue increases. Therefore, in the present invention, the range of A1 is 4.5% or more and 6.5% or less.

(3) P: In the present invention, P is an important element for improving hot workability in relation to Ln.

That is, in the present invention, it is possible to significantly improve the oxidation resistance of the foil by adding Ln in the above-mentioned range, but in the past, addition of such a relatively large amount of Ln deteriorated hot workability. This made it difficult to manufacture using the normal mass production process for stainless steel sheets using hot-rolled coils. The reason for this was thought to be that Ce, which is the main component when Mitshu metal is added, tends to form an intermetallic compound with Fe, which has a low melting point. However, when adding a large amount of Ln, if P is added in combination, for example, C
A part of e and La come to exist in the mesh as relatively fine, granular, high melting point phosphides of 3 μm or less, and no deterioration in hot workability occurs. According to the study by the present inventors, the P content necessary for this is Ln of 0.0
In the range of more than 6% and less than 0.15%, it is assumed that large steel ingots are mass-produced in factories with large segregation.

On the other hand, since P has the effect of reducing the toughness of ferritic stainless steel, Fe-Cr-
Am! For stainless steels, the amount added is limited from this point of view, and in the present invention, the amount is 0.1%. Further, addition of P in such a range does not have an adverse effect on oxidation resistance.

(4) L n (Lanthanoid): First, L n (Lanthanoid) is a general term for 166 elements from La to Lu in the periodic table, and in the case of the present invention, as an actual additive raw material, cheaper So-called Mitsushmetal can be used. At this time, La and Ce are detected as a result of the analysis.

There are four elements, Pr and Nd, and the other elements are in extremely small amounts and can be ignored. Therefore, Ln in the present invention refers to a mixture of the above four elements, and the additive raw material is Mitshu metal.

Now, as mentioned above, Ln has the effect of firstly improving the foil's resistance to abnormal oxidation occurring in exhaust gas, and the life of the foil until abnormal oxidation occurs in exhaust gas is as long as Ln is 0.
When it exceeds 0.6%, it increases significantly compared to the case below, but when it exceeds 0.15%, it starts to decrease again. Therefore, the range is limited to more than 0.06% and 0.15% or less.

(5) Ti, Nb: In the present invention, Ti and Nb are the most important additive elements especially for improving high-temperature proof stress.

As mentioned above, in order to improve the structural durability of the catalyst carrier, it is important to first improve the high-temperature yield strength of the foil, and also to suppress the decline in yield strength even after being heated for a long time. However, in such cases, for example, JP-A-58-
As disclosed in Japanese Patent No. 177437, the addition of Zr or Nb alone can improve the creep strength as well as the yield strength at high temperatures, which is most important for the structural durability of the catalyst carrier. Nb
The strengthening caused by the addition of carbon is mainly caused by the precipitation of carbonitrides that combine with C and N. Therefore, during long-term use at high temperatures, these carbonitrides gradually aggregate and coarsen, resulting in a strengthening effect. decreases.

Therefore, precipitation strengthening by adding Zr, Nb, etc. causes a decrease in strength due to changes in metallographic structure during use, and this remains a problem.

However, by adding a small amount of Ti and then adding a considerable amount of Nb, C and N are precipitated as Ti-based carbonitrides. Conceivable. Therefore, a relatively large strengthening effect can be stably obtained, especially on the high temperature side. Furthermore, since almost no change in metallographic structure occurs even when heated at high temperatures and for long periods of time, this strengthening effect hardly deteriorates over time. Furthermore, as mentioned above, Ti is a solid solution of C,
Since N is fixed, the toughness of the hot rolled sheet is improved.

The combined addition amount of Ti and Nb was determined from this point of view, and according to the study results of the present inventors, in order to obtain a sufficient solid solution strengthening effect, Ti should be 0.02% or more, and Nb should be (0,
It is necessary to add more than 1+93/12 x C+93/14 x N)%. However, when Ti is added in an excessive amount, it forms many coarse angular Ti-based precipitates exceeding 10 μm in size, and Nb forms Nb-based intermetallic compounds, which affect hot workability and hot-rolled sheets. toughness decreases. The amount of Ti and Nb added is limited from this point, and the upper limit is (0,03+4 x C+24/4 x N)%, respectively.
, 2.0%. Note that the addition amounts of Ti and Nb within this range do not have any adverse effect on the oxidation resistance of the foil.

Further, it is desirable that Ti be added before the melted 11Nb is added.

(5) C, N: In the present invention, both C and N need to be kept low because they significantly reduce the toughness of the hot rolled sheet.

In addition, this negative effect can be further suppressed by the action of Tc, but if C exceeds 0.02%, N exceeds 0.02%, or the total amount of C + N is 0.
．． If it exceeds 0.3%, it becomes difficult to recover the toughness. Therefore, from this point, the ranges are C: 0.02% or less, N: 0.02% or less, and C+N: 0.03% or less. Further, the preferable range is C1N: 0.015% or less.

(7) Other impurities: S: Like P, S tends to form a compound with a high melting point with Ln, but at the same time it lowers oxidation resistance, so in the present invention, it should be suppressed to 0.003% or less. is desirable.

Si: Si is an element that improves oxidation resistance, but at the same time thickens (decreases) the toughness of hot rolled sheets.High A7! ferritic stainless steel like the one of the present invention inherently has excellent oxidation resistance, so Si is an element that improves oxidation resistance. From the viewpoint of toughness, it is desirable to suppress the amount to a small amount, and the range is 0.5% or less.

Mn: In the present invention, Mn is particularly concentrated in the very early stage of the oxide film, harming the subsequent formation of the AfzO:+ film, and contributing to the persistence of structural defects in the film. It is desirable to limit it to 1.0% or less. Furthermore, it is preferably 0.3% or less.

Ni: Ni is AI! It is an element with a strong bonding force with F
In the present invention, the content is set to 0.3% or less since it significantly embrittles the e-Cr-Aj2 alloy.

Fe-Cr-Aj! of the present invention having such a configuration! The alloy can be manufactured into a foil of approximately 50 μm by combining melting, hot rolling, and cold rolling processes similar to the mass production process of normal ferritic stainless steel, and an appropriate annealing process as needed. In addition, foil manufactured in this way,
And the exhaust gas purification catalyst carrier and the catalyst device constructed using this foil not only have extremely high resistance to occurrence of abnormal oxidation even in a high-temperature combustion exhaust gas atmosphere, but also have a honeycomb structure because the foil has high proof strength at high temperatures. It has high resistance to thermal fatigue, and has excellent structural durability even under usage conditions of repeated heating and cooling.

〔Example〕

Next, the effects of the present invention will be explained in more detail with reference to Examples.

Example 1 Table 1 shows the chemical composition of the steel used to evaluate the hot workability, oxidation resistance of the foil, and yield strength at high temperatures of the steel related to the present invention. In addition, the chemical composition of Minschmetal, which is an additive raw material for Ln used during melting, is Ce: 49-54%,
La: 19-27%, Nd: 16-24%, Pr:
5-8%, S+Il: 0.2% or less, and all other Ln were below the detection limit.

All of these steels were vacuum melted, cast into 40kg ingots, held at 1150°C for 1 hour, immediately hot rolled, finished to a length of 4m, and immediately water-cooled to bring the surface temperature of the plate to 400°C. When the temperature reached 350°C, it was charged into a heating furnace, kept for 1 hour, and then cooled in the furnace. When observing the occurrence of cracks during this hot rolling, it was found that in the comparative example Q6, many severe transverse cracks had already occurred in the ears and on the surface in the first pass, so rolling was stopped midway.

In addition, cracks were observed in the edges of the Q3 hot-rolled sheet.

On the other hand, in all other cases, hot rolled sheets with good shapes were obtained. The results are shown in the hot workability column of Table 2 with XX marks for Q6, X marks for Q3, and Q marks for other samples whose hot workability was judged to be good. As is clear from this example, Q6 containing an excessive amount of Ln is difficult to hot-work, and
In Q3 containing an excessive amount of b, hot workability is decreased.

Two sub-sized notched Charpy test pieces were taken from the hot-rolled sheets thus obtained, and the toughness was investigated. The results are shown in the column of hot-rolled sheet toughness in Table 2. As a judgment index, the average value of the three points of impact absorption energy at - test temperature is 3k
The temperature exceeds gf-va/c4, and those whose temperature is 50°C or less are marked O, those whose temperature exceeds 50°C but not more than 100°C are marked Δ, and those whose temperature exceeds 100°C are marked x. Those marked O can be relatively easily mass-produced in a factory, and those marked Δ may require some heat treatment, but basically they can be mass-produced. On the other hand, it can be determined that the items marked with an "X" are practically difficult to produce in a factory. It was determined that all of Mokuto Aiko could be produced in factories.

Example 2 Next, Q3. obtained in Example 1. Q6. The hot rolled sheets except Q7 were descaled, cold rolled to a thickness of 0.8 m, annealed at 900°C, and further rolled to a foil of 50 μm thick.

The foil thus produced was repeatedly heated to 1150° C. for 25 hours in a heating furnace into which exhaust gas from a gasoline engine was introduced until abnormal oxidation occurred in the foil. In addition,
At this time, the presence or absence of abnormal oxidation was determined visually. Each of the test foils had a thickness of 50±2 μι, and three specimens were tested for each component system, and the average value was taken as the abnormal oxidation life of the component cylinder. If this value is less than 150 hours, mark x, 150 to 2
For less than 00 hours, mark x; for less than 200 to 250 hours, mark Δ; for 250 hours or more, mark O;
It is shown in the column of abnormal oxidation life of foil in the table.

The foils of the invention examples all had a lifespan of 250 hours or more, but the comparative example had a Ql of less than 200 hours, and Q5 also had a short life of less than 150 hours.

Example 3 The hot-rolled sheet obtained in Example 1 was cold-rolled to a thickness of 1.5 mm in the same manner as in Example 2, and then rolled at 1200'C in vacuum.
A plate-shaped tensile test piece was taken from the sample heat-treated for 0 minutes, and the yield strength at 600.700°C and 800°C was measured. The results are shown in the high temperature strength column of Table 2.

The average value of each three bodies is taken as the proof strength at that temperature, and the judgment criteria are 20 kgf/mm” or more at 600°C, and 20 kgf/mm” or more at 700°C.
At 13kgf/kaku2 or more, at 800°C it is 4.
5 kg f/m" or more, and those that cleared these standards are marked with a circle, and those that did not are marked with an While Ti has high high-temperature yield strength,
Comparative examples Ql and Q2, in which the amount of Nb added was small, did not achieve an improvement in yield strength at high temperatures.

Next, in order to investigate the degree of strength reduction during long-term use, a plate-shaped tensile test piece prepared in the same manner as above was
After aging at °C for 900 hours, high temperature yield strength was measured. The results are shown in the column of proof strength after aging in Table 2. Although Q4 with the sole addition of Nb has a high initial high temperature yield strength,
During long-term use, the yield strength tends to decrease, especially at high temperatures, but products containing Ti and Nb within the scope of the present invention, along with the P series and Q series, have high yield strength at high temperatures and hardly decrease in yield strength due to aging. do not have.

From the above results, the steel within the scope of the present invention has excellent hot workability and oxidation resistance, and at the same time has high yield strength at high temperatures, which does not deteriorate even after long-term use at high temperatures.

Example 4 Next, the results of an investigation into the structural durability of the catalyst carrier will be explained.

Using the hot-rolled sheet obtained in Example 1, descaling, cold rolling, and annealing were repeated to produce 50 tons! A foil of s was produced. A sinusoidal corrugated wave with a period of 3.5 degrees and an amplitude of 3.2 degrees is wrapped around these foils by overlapping a processed (corrugated plate) and an unprocessed foil (flat plate) band, resulting in an apparent outer diameter of 110 m. length 110
A honeycomb-shaped cylindrical body with an outer diameter of 110 mm,
The catalyst carrier was inserted into a cylinder (outer cylinder) made of type 434 ferrite stainless steel with a length of 110 ann and a plate thickness of 1.7 mm, and each contact point was brazed in a vacuum using a commercially available Ni-based brazing material. was created.

・Next, these catalyst carriers are attached to the exhaust gas path directly connected to the light oil burner, so that the entire amount of exhaust gas from the burner passes through the inside of this honeycomb body, and the gas salt near the inlet end face at this time is heated to 1000°C. The honeycomb body was heated in the range of ~1030°C for 3 minutes. Next, the burner was stopped, cold air was immediately introduced to cool the honeycomb body for 3 minutes, and then heating and cooling were repeated in the same manner, and a structural durability test of the carrier was conducted 750 times. Table 3 shows the results of observing the damage to the end face of the honeycomb body on the gas-inlet side at this time.

All of the carriers made from the P series within the scope of the present invention exhibited relatively good external shapes, although some cell deformation and foil breakage were observed after the above tests, and the damage was judged to be relatively minor. On the other hand, the comparative example, which was made from a material with low high-temperature yield strength, suffered from partial blockage due to cell collapse, foil breakage due to large deformation of the cell wall, and damage where part of the end surface protruded in the upstream direction of the gas flow. A phenomenon in which a portion of the end face of the honeycomb body was missing (chip) was also observed.

From the above, it is clear that the foil according to the present invention is highly effective in improving the structural durability of the carrier.

Table 3 [Effects of the invention] As is clear from the examples, FeCr-A according to the present invention
/ type stainless steel foil not only has high resistance to abnormal oxidation in engine exhaust gas, but also has excellent manufacturability such as hot workability, and has extremely high proof strength at high temperatures. It has high resistance to thermal fatigue as a honeycomb body, and is therefore highly effective in improving the structural durability of the catalyst carrier. Due to these effects, the stainless steel foil of the present invention is suitable for forming a catalyst carrier for automobiles and the like.

Claims (1)

[Claims] 1. Material components are as follows in weight%: Cr: 18-28%, Al: 4.5-6.5%, P: 31/233×(Ln+0.021)-0.1 %,
Ln: more than 0.06 to 0.15%, Ti: 0.02 to (0.03+4×C+24/7×N)
%, Nb: (0.1+93/12×C+93/14×N
), and contains impurities of C: 0.02% or less, N: 0.02% or less, and C+N: 0.03% or less, S: 0.003% or less, and Si: 0. .5% or less Mn: 1.0% or less Ni: 0.3% or less A Fe-Cr-Al alloy with excellent manufacturability and high temperature and high strength and high heat resistance, characterized in that the balance is substantially Fe.