JPH10273759A - Fe-cr-al ferritic stainless steel excellent in high temperature strength, high temperature oxidation resistance and diffusion joinability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an Fe-Cr-Al ferritic stainless steel excellent in high temp. strength, high temp. oxidation-resistance and diffusion joinability by specifying the compsn. composed of C, Si, Mn, P, S, Cr, N, Al, Mo, Nb, rare earth metals and Fe in a steel, annealing the steel in a reducing atmosphere and thereafter subjecting it to cold rolling to regulate its roughness into specified one. SOLUTION: A stainless steel having a compsn. contg., by weight, <=0.03% C, <=1% Si, <=1% Mn, <=0.04% P, <=0.003% S, 15 to 25% Cr, <=0.03% N, 3.5 to 6.0% Al, 0.1 to <2.0% Mo, 0.01 to 0.3% Nb, 0.01 to 0.2% rare earth metals, and the balance substantial Fe, in which Al and Mo also lie within the region obtained by connecting (a), (b), (c) and (d) in two-dimensional co-ordinates, and the relationships of 280>14.6Al+24.2Mo+481Nb%>=100 and Al+10Nb<=7.0 are satisfied is annealed in an atmosphere of >=90 vol.% reducing gas and is thereafter subjected to cold rolling. The surface 10 point average roughness Rz in a direction orthogonal to the rolling direction of the obtd. cold rolled steel sheet is regulated to <=1.0 μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an Fe—Fe—alloy excellent in high-temperature strength, high-temperature oxidation resistance and diffusion bonding property, which is suitable for use in a high-temperature environment such as an automobile exhaust gas purifying carrier.
It relates to a Cr-Al ferritic stainless steel.

[0002]

2. Description of the Related Art Conventionally, ceramics have been frequently used as a material for a vehicle exhaust gas purifying carrier. In recent years, compared to this ceramic, the exhaust resistance is small and the impact resistance is excellent,
Attention has been paid to a carrier made of a metal material that can be easily reduced in size and weight. The metal carrier is generally 50 μm thick
The cross section is a honeycomb-shaped cylindrical structure obtained by alternately stacking a metal foil plate and a corrugated plate of about m in length and spirally winding them. Fe-Cr-A is used for the metal foil as the metal carrier material.
1 Ferritic stainless steel is used.

[0003]

A material for a metal carrier for purifying an automobile exhaust gas has been developed because of the structure of the carrier and the severe use environment.
The following characteristics are required. Since it is used in exhaust gas, which is a severe oxidizing atmosphere, excellent high-temperature oxidation resistance is required. Further, during operation of the automobile, the central portion of the cylindrical carrier is exposed to high-temperature exhaust gas and its temperature rises. However, since the outer peripheral portion of the carrier is cooled by the outside air, the temperature is not as high as that of the central portion. Therefore, the carrier material is subjected to thermal stress due to the temperature difference between the central part and the outer peripheral part, and is also subjected to repeated stress due to the heat cycle of heating and cooling accompanying repetition of operation and shutdown. Is done.

Further, the metal carrier is generally formed by alternately stacking a flat plate and a corrugated plate and spirally winding the flat plate and the corrugated plate. Conventionally, the flat plate and the corrugated plate are joined by brazing using Ni solder. It has been. However, Ni brazing is not only expensive, but also because the Ni content of the brazing material is added to the carrier,
i-brazing is not desirable from the viewpoint of manufacturing cost and weight reduction of the vehicle body. As a joining method instead of brazing,
Diffusion bonding in which metal foils themselves are bonded to each other without using a brazing material has been proposed. Diffusion bonding is a bonding method utilizing the interdiffusion phenomenon of metal atoms at a high temperature, and solves the above-mentioned problem. Therefore, a metal carrier material in the future is required to have excellent diffusion bonding properties.

As a material for a metal carrier, Fe--Cr--Al
A ferrite stainless steel to which a rare earth element is added and Mo, W, and Nb are added is disclosed in
No. 28345. However, JP
The stainless steel disclosed in -128345 contains expensive Mo and W in total of 2% by weight or more, and is not always satisfactory from the viewpoint of cost.
Further, when Mo is contained in a large amount, the toughness of the hot-rolled sheet is reduced, and the productivity is impaired. Further, as a result of the study by the present inventors, since Nb content promotes the generation of defects in the Al oxide layer, excessive addition is not always preferable for high-temperature oxidation resistance.

[0006] In diffusion bonding, it is known that the properties of the metal surface greatly affect the bonding properties, and in particular, the effect of an oxide film formed on the metal surface is great. Since Fe-Cr-Al ferritic stainless steel used as a metal carrier material contains Al, which has a high affinity for oxygen, these elements combine with oxygen to form a strong oxide film and diffuse metal elements. And it was difficult to obtain sufficient bonding strength.

SUMMARY OF THE INVENTION The present invention has been made to solve these problems, and it is an object of the present invention to provide an automobile exhaust gas purifying carrier Fe-Cr- excellent in high-temperature strength, high-temperature oxidation resistance and diffusion bonding property.
Provide Al ferritic stainless steel.

[0008]

SUMMARY OF THE INVENTION The object of the present invention is to provide a method for measuring the weight percentage
And C: 0.03% or less, Si: 1% or less, Mn: 1%
Below, P: 0.04% or less, S: 0.003% or less, C
r: 15 to 25%, N: 0.03% or less, Al: 3.5
To 6.0%, Mo: 0.1 to less than 2.0%, Nb: 0.1 to 2.0%.
01-0.3%, rare earth element (REM): 1 type or 2
Containing at least 0.01% to 0.2% of the seed,
e, and Al and Mo are within the above-mentioned ranges, respectively, and coordinate points represented by two-dimensional coordinates (Al%, Mo%), (3.5, 2.0), (4.5, 0.
0), (6.0, 0.0) and (5.0, 1.5) in or on the boundary region connecting the four points, and between the components, 280> 14.6 × Al% + 24.2 × Mo% + 481
× Nb% ≧ 100 and Al% + 10 × Nb% ≦ 7.0 2
A stainless steel that satisfies the formula, is annealed in an atmosphere containing at least 90% by volume of a reducing gas, and then cold-rolled. Fe having excellent high-temperature strength, high-temperature oxidation resistance and diffusion bonding properties, characterized in that the Rz is 1.0 μm or less.
-Achieved by Cr-Al ferritic stainless steel.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have conducted intensive studies and as a result,
The following points were clarified. When Fe-Cr ferritic stainless steel is applied to the carrier material, the addition of Al and Mo significantly improves the high-temperature oxidation resistance, but has the problem that the toughness is reduced and the productivity is deteriorated. Therefore, using the abnormal oxidation time as an index of high-temperature oxidation resistance and the bendable angle of a hot-rolled sheet as an index of manufacturability, the addition of Al and Mo to these properties of the Fe-Cr ferritic stainless steel is considered. As a result of examining the influence of the quantity, the coordinate points represented by two-dimensional coordinates (Al%, Mo%), (3.5, 2.0), (4.5, 0.
0), (6.0, 0.0), and (5.0, 1.5), in the case of a composition within or on a boundary region connecting the four points, high-temperature oxidation resistance, manufacturability, Was found to be compatible.

Further, as an element for improving the high-temperature strength, N
b, Mo and Al are known. These elements are
Although a certain amount of addition is required in terms of high-temperature strength, an excessive addition lowers the toughness of the steel, so that its addition is limited in terms of manufacturability. Therefore, the amounts of these elements to be added should be determined in consideration of the required high-temperature strength as a carrier and the productivity. Incidentally, the high-temperature strength required for a carrier is, as disclosed in Japanese Patent Application Laid-Open No. 4-128345, a strength against strain deformation caused by a difference in thermal expansion caused by a temperature difference in the carrier. It is evaluated by the proof stress. Nb, Mo and Al
As a result of examining the effect on the proof stress at high temperature, that is, the effect on the high-temperature strength, among these elements, the effect of Nb is large, and the effect is maximum at 700 ° C. 700 ° C
In order to obtain high-temperature strength exceeding conventional levels, it is necessary to add Nb, Mo, and Al to satisfy the following expression. 280> 14.6 × Al% + 24.2 × Mo% + 481
× Nb% ≧ 100

[0011] Next, diffusion bonding is a method in which metal atoms that have been brought into close contact with each other by applying an appropriate pressure are kept at a high temperature, whereby metal atoms are mutually diffused and bonded. However, if an oxide film is present on the metal surface, interdiffusion of atoms does not proceed smoothly, so that sufficient bonding strength cannot be obtained. Therefore, the diffusion bonding is usually performed in an atmosphere having a reduced oxygen partial pressure so as not to form an oxide film on the surface of the metal material. However, as described above, Al and Nb contained from the requirement as a material for a metal carrier have a high affinity for oxygen, and even under a low oxygen partial pressure atmosphere,
An oxide film that is not preferable for diffusion bonding is easily formed. Therefore, in the present invention, the gas composition of the atmosphere in the annealing performed before the final finish cold rolling is assumed to contain a reducing gas of 90% by volume or more, and the formation of an oxide film is suppressed as much as possible, and the high-temperature oxidation resistance and High temperature strength,
Al and Nb that can also obtain good diffusion bonding properties
As a result of investigation to clarify the range of addition of
% + 10 × Nb% ≦ 7.0, the object was achieved.

Furthermore, since diffusion bonding is based on mutual diffusion movement of atoms at the bonding interface, the larger the effective area of the bonding interface, that is, qualitatively, the smoother the contact interface, the stronger the bonding strength. . However, in order to smooth the metal surface, there are methods such as rolling, mechanical or chemical polishing.
To obtain a smoother surface requires more time and money. Therefore, it is industrially feasible,
As a result of examining the surface roughness with which sufficient bonding strength can be obtained, the surface ten-point average roughness Rz in the direction perpendicular to the rolling direction on the surface obtained by cold rolling is 1.0 μm.
It was found that a value of m or less was sufficient. The cold rolling conditions for setting the surface ten-point average roughness Rz to 1.0 μm or less are not particularly limited, but can be achieved by selecting the surface roughness and the material of the cold rolled finishing roll. For example, the surface roughness of the roll used for the final pass of the cold rolling is set to # 600 or less, and the roll material is made of WC, which is hardly deformed during rolling and can obtain a relatively smooth steel strip surface. And so on.

The reasons for limiting the composition of the steel of the present invention will be described below. C: 0.03% by weight or less If the content is large, abnormal oxidation is likely to occur, and the toughness of the slab or hot-rolled sheet is deteriorated, and the productivity is reduced.
The upper limit is set to 0.03% by weight. Si: 1% by weight or less Excessive content deteriorates toughness, so the upper limit is set to 1% by weight.

Mn: not more than 1% by weight Excessive content produces Mn oxides, hinders formation of a dense Al ₂ O ₃ layer and adversely affects high-temperature oxidation resistance.
The upper limit is 1% by weight. P: 0.04% by weight or less Excessive content adversely affects high-temperature oxidation resistance and toughness of a hot-rolled sheet, so the upper limit is made 0.04% by weight. S: 0.003% by weight or less Bonds with rare earth elements (REM) in steel to form non-metallic inclusions and deteriorates the surface properties of steel. In addition, the upper limit is set to 0.003% by weight in order to reduce the substantially effective amount of REM that is beneficial for high-temperature oxidation resistance by bonding with REM.

Cr: 15 to 25% by weight It is essential as an element for improving high-temperature oxidation resistance. In order to obtain excellent high-temperature oxidation resistance, it must be contained in an amount of 15% by weight or more. However, an excessive content degrades the toughness of the slab or hot-rolled sheet, so the upper limit is set to 25% by weight. N: 0.03% by weight or less Since it combines with Al in steel and becomes a starting point of abnormal oxidation, it is desirable that the content is small, and the upper limit is 0.03% by weight.

Al: 3.5-6.0% by weight Like Cr, it is an essential element for improving high-temperature oxidation resistance. Excellent high-temperature oxidation resistance is obtained by a dense Al oxide layer formed on the steel surface.
In order to form an oxide layer, the content must be 3.5% by weight or more. However, an excessive content degrades the toughness of the slab or hot-rolled sheet, so the upper limit is set to 6.0% by weight. Mo: 0.1 to less than 2.0% by weight It is an element effective for improving high-temperature oxidation resistance and high-temperature strength, and 0.1% by weight or more for obtaining these effects. Must be included. However, an excessive content degrades toughness and adversely affects manufacturability, so the upper limit is regulated to less than 2.0% by weight.

Nb: 0.01 to 0.3% by weight In addition to improving high-temperature strength, it has the effect of significantly improving toughness by combining with C and N in steel. The content of Nb is effective for a material used for a member subjected to a thermal cycle. These N
In order to obtain the effect of b, the content needs to be 0.01% by weight or more. However, an excessive content deteriorates the toughness and is not preferable for the diffusion bonding property. Therefore, the upper limit is set to 0.3% by weight.

Rare earth element (REM): 0.01 to 0.2
0% by weight It is an important element for improving high-temperature oxidation resistance. La, C
REs such as e stabilize the oxide film of Al ₂ O ₃ formed on the surface of the metal foil and improve the adhesion between the metal foil substrate and the oxide film, thereby improving the high-temperature oxidation resistance. Conceivable. This effect is obtained by containing 0.01% by weight or more. However, an excessive content degrades hot workability and toughness, and also generates inclusions that serve as starting points of abnormal oxidation, and conversely lowers high-temperature oxidation resistance. Therefore, the upper limit is set to 0.20% by weight. .

[0019]

The present invention will be described below with reference to examples. Table 1
Shows the chemical component values of the test materials. The test material was melted in a 30 kg vacuum melting furnace, subjected to forging, cutting, and hot rolling, and thereafter, was repeatedly subjected to annealing and cold rolling to finally have a thickness of 50 μm.
Was subjected to a high-temperature oxidation test and a diffusion bonding test.
The ten-point average roughness Rz of the foil surface was measured by scanning a stylus type surface roughness meter in a direction perpendicular to the rolling direction.

Here, the sample numbers A1 to A4 both fall within the scope of the present invention in terms of the chemical components and the values of the limiting formulas. On the other hand, in sample numbers B1 to B9, one or both of the chemical components and the values of the limiting formulas fall outside the scope of the present invention. In addition, in the middle of the process of manufacturing each test material shown in Table 1 into a foil, that is, a 3.5 mm-thick plate as hot-rolled was sampled and subjected to a bending test in the C direction. From hot-rolled annealed plate, width 12.5mm, parallel part length 105mm
Was taken and subjected to proof stress measurement at a temperature of 700 ° C.

In the high-temperature oxidation test, a time when abnormal oxidation occurred in a 50 μm-thick foil in a high-temperature atmosphere at 1050 ° C. in the air was measured. The time when abnormal oxidation occurs is 2
The high-temperature oxidation resistance is good when the time is 00 hours or more. The case of less than 200 hours was evaluated as poor in high-temperature oxidation resistance. FIG. 1 is a diagram in which the results of the high-temperature oxidation test are arranged in terms of Al% and Mo%. It can be seen that the high-temperature oxidation resistance is excellent when the content is higher than the linear portion shown in FIG. 1, that is, when the content of Al% and Mo% is larger than a certain value.

[0022]

[Table 1]

In the bending test, when cracks occurred at a bending angle of 90 degrees or more (180 degrees in the case of close contact bending), the manufacturability was determined to be good. The case where cracking occurred at a bending angle of less than 90 degrees was regarded as poor manufacturability. FIG. 2 is a diagram in which the bending test results are arranged in Al% and Mo%. It can be seen that bendability is excellent when the content is lower than the linear portion shown in FIG. 2, that is, when the content of Al% and Mo% is smaller than a certain value.

FIG. 3 shows the results of the high-temperature oxidation test shown in FIG. 1 together with the results of the bending test shown in FIG. Therefore, (Al%, Mo%) shown by the two-dimensional coordinates of Al% and Mo% shown in FIG.
5,2.0), (4.5,0.0), (6.0,0.
(0), (5.0, 1.5), when it is within or on the boundary obtained by connecting the four points, both the high-temperature oxidation resistance and the bending property are good.

In the diffusion bonding test, a 10 × 25 mm sample was collected from the 50 μm-thick metal foil prepared as described above, and two foils having the same sample number were overlapped with each other with a margin of 10 × 10 mm. A load of 0.6 kg was applied to make the substrates adhere to each other, and the substrates were heated at 1250 ° C. for 1 hour in a vacuum of 10 ⁻⁴ Torr to perform diffusion bonding. Evaluation of diffusion bonding properties
The peel test was performed. A peeling test was performed by applying a tensile load while holding both ends of the sample after the diffusion bonding. As a result of the peeling test, one that was broken from the foil base material was evaluated as having good bonding properties, and one that was peeled from the bonded part was evaluated as having poor bonding properties.

Table 2 shows the results of the high-temperature tensile test and the diffusion bonding test together. In the present invention, the improvement target of the high temperature strength is set to 1.3 times or more of the conventional steel type,
It was determined that a proof stress of 00 N / mm ² or more could be obtained. As shown in Table 2, Al, Mo, and Nb correspond to the formula (1) 2
80> 14.6 × Al% + 24.2 × Mo% + 481 ×
When Nb% ≧ 100 is satisfied, the desired high-temperature strength is obtained.

FIG. 4 shows 1% by weight of Al and Mo.
And the effect on the proof stress per 0.1% by weight of Nb. According to FIG.
The effect of the Al, Mo, and Nb contents on the proof stress per 1% by weight at 1 ° C. is 14.6, 2
4.2 and 481 N / mm ² . Therefore, 7
The above formula (1) is obtained as a requirement for satisfying the target proof stress of 100 N / mm ² at 00 ° C.

Although Nb has a remarkable effect in improving the high-temperature strength, the relationship between the Nb content in steel and the abnormal oxidation occurrence time is shown in FIG. The abnormal oxidation occurrence time is shortened, and particularly when it exceeds 0.3% by weight, the decrease is remarkable.
It can be seen that the content has an upper limit.

Table 2 also shows the results of the diffusion bonding test.
High concentration of H ₂ in the atmospheric gas at the time of final annealing, and Rz
It was found that the bondability was good when was small.
FIG. 6 shows the effect of Al% and Nb% on the diffusion bonding property. For Al and Nb having a large affinity for oxygen, the requirement of the formula (2) Al% + 10 × Nb% ≦ 7.0 is satisfied. In this case, excellent diffusion bonding properties are obtained.

[0030]

[Table 2]

Further, in order to further clarify the conditions for obtaining excellent diffusion bonding properties, the atmosphere gas composition of the final annealing and the final cold rolling were performed using the intermediate rolled plate of Sample A1 shown in Table 1 above. By changing the conditions variously, sample numbers C1 to
A foil of C6 having a thickness of 50 μm was prepared, and these samples were subjected to a diffusion bonding test in the same manner as described above. FIG. 7 and Table 3 show the results of the final annealing atmosphere, the surface ten-point average roughness Rz, and the diffusion bondability test. As shown in FIG. 7 and Table 3, the annealing atmosphere gas contains 90% by volume or more of H ₂ ,
In addition, when Rz in a direction perpendicular to the rolling direction of the foil is in the range of 1.0 μm or less, excellent diffusion bonding properties can be obtained.

[0032]

[Table 3]

[0033]

As described above, as described above, the Fe-
Cr-Al ferritic stainless steel has excellent high-temperature strength and high-temperature oxidation resistance, and also has excellent diffusion bonding properties. Further, the steel of the present invention can provide an inexpensive material excellent in manufacturability by suppressing the content of expensive Mo and the content of Al and Nb. The steel of the present invention is suitable as a metal carrier material that is repeatedly subjected to a heat cycle of heating and cooling by high-temperature exhaust gas.

[Brief description of the drawings]

FIG. 1 is a view showing the influence of Al and Mo on the time when abnormal oxidation occurs in a high-temperature oxidation test.

FIG. 2 is a diagram showing the influence of Al and Mo on bending characteristics.

FIG. 3 is a diagram showing a range of (Al%, Mo%) that satisfies both high-temperature oxidation resistance and bending characteristics.

FIG. 4 is a diagram showing the effect of Al, Mo, and Nb on proof stress at high temperatures.

FIG. 5 is a view showing the influence of Nb on abnormal oxidation occurrence time in a high-temperature oxidation test.

FIG. 6 is a diagram showing the effect of Al and Nb on diffusion bonding.

FIG. 7: H ₂ gas concentration in atmosphere during final annealing and R of foil
The figure which shows the influence which z has on diffusion bonding property.

Claims (1)

[Claims] 1. In weight%, C: 0.03% or less, Si: 1
%, Mn: 1% or less, P: 0.04% or less, S:
0.003% or less, Cr: 15 to 25%, N: 0.03
% Or less, Al: 3.5-6.0%, Mo: 0.1-2.
0%, Nb: 0.01 to 0.3%, rare earth element (R
EM): 0.01 to 0.2% of one or more kinds, the balance substantially consisting of Fe, and Al and Mo
Are within the above ranges, and are represented by two-dimensional coordinates (Al%, Mo%), (3.5, 2..
0), (4.5, 0.0), (6.0, 0.0),
280> 14.6 × Al% + 24.2 × Mo% + 481 in the boundary region or on the boundary line connecting the four points of (5.0, 1.5).
× Nb% ≧ 100, Al% + 10 × Nb% ≦ 7.0 A stainless steel which satisfies the expression 2;
The ten-point average surface roughness Rz in a direction perpendicular to the rolling direction of a cold-rolled steel sheet obtained by performing cold rolling after annealing in an atmosphere containing a reducing gas having a volume percentage of not less than 1.0 μm. Fe-Cr-Al ferritic stainless steel excellent in high-temperature strength, high-temperature oxidation resistance, and diffusion bonding property characterized by the following characteristics.