U.S. patent application number 11/981933 was filed with the patent office on 2008-03-20 for high a1 stainless steel sheet and double layered sheet, process for their fabrication, honeycomb bodies employing them and process for their production.
This patent application is currently assigned to Nippon Steel Corporation. Invention is credited to Tooru Inaguma, Shogo Konya, Hiroaki Sakamoto, Motonori Tamura.
Application Number | 20080069717 11/981933 |
Document ID | / |
Family ID | 39188810 |
Filed Date | 2008-03-20 |
United States Patent
Application |
20080069717 |
Kind Code |
A1 |
Inaguma; Tooru ; et
al. |
March 20, 2008 |
High A1 stainless steel sheet and double layered sheet, process for
their fabrication, honeycomb bodies employing them and process for
their production
Abstract
The present invention provides an Fe--Cr--Al based stainless
steel sheet and double layered sheet having a high Al content of
greater than 6.5%, honeycomb bodies employing the stainless steel
sheet or double layered sheet, and a process for fabrication of the
steel sheet or double layered sheet. The sheet is a high
Al-containing Fe--Cr--Al based stainless steel sheet or high
Al-containing double layered sheet characterized by comprising, by
weight, Cr: 10-30% and Al: >6.5%-15%. Preferably, the steel
sheet further comprises either or both Ti: 0.02-0.1% and Nb:
0.02-0.3%, and also comprises La: 0.01-0.1%, Ce: 0.01-0.1% and P:
0.01-0.05%. It also preferably comprises Cu: 0.01-1.0% by weight,
and preferably further comprises Mg: 0.001-0.1% by weight. There is
also provided a honeycomb body fabricated using the Fe--Cr--Al
based stainless steel sheet.
Inventors: |
Inaguma; Tooru; (Futtsu-shi,
JP) ; Konya; Shogo; (Futtsu-shi, JP) ;
Sakamoto; Hiroaki; (Futtsu-shi, JP) ; Tamura;
Motonori; (Futtsu-shi, JP) |
Correspondence
Address: |
KENYON & KENYON LLP
ONE BROADWAY
NEW YORK
NY
10004
US
|
Assignee: |
Nippon Steel Corporation
Tokyo
JP
|
Family ID: |
39188810 |
Appl. No.: |
11/981933 |
Filed: |
October 31, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10535602 |
Dec 12, 2005 |
|
|
|
PCT/JP03/14832 |
Nov 20, 2003 |
|
|
|
11981933 |
Oct 31, 2007 |
|
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|
Current U.S.
Class: |
420/34 ;
420/62 |
Current CPC
Class: |
C22C 38/02 20130101;
C22C 38/26 20130101; C22C 38/28 20130101; C22C 38/06 20130101; C22C
38/04 20130101 |
Class at
Publication: |
420/034 ;
420/062 |
International
Class: |
C22C 38/18 20060101
C22C038/18 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 20, 2002 |
JP |
2002-336048 |
Nov 20, 2002 |
JP |
2002-336049 |
Claims
1-20. (canceled)
21: A high Al-containing Fe--Cr--Al based stainless steel sheet,
characterized by comprising, by weight, Cr: 10-30% and Al:
>6.5-15%, remainder Fe and unavoidable impurities, and
characterized in that the thickness t of said steel sheet is 10-40
.mu.m, the thermal expansion coefficient .alpha. from 20.degree. C.
to 1000.degree. C. is 15-23 .mu.m/m/.degree. C. and the 0.2% proof
strength .sigma. (N/mm.sup.2) measured at 900.degree. C., the steel
sheet thickness t (.mu.m) and the thermal expansion coefficient
.alpha. (.mu.m/m/.degree. C.) are in a relationship satisfying the
following inequality <1>, and the steel sheet is used in an
exhaust gas purification catalyst-carrying honeycomb body,
.sigma..gtoreq.(-9.0875.times..alpha..sup.2+4.2913.times.10.sup.2-
.times..alpha.-3.84215.times.10.sup.3)/t <1>.
22: A high Al-containing Fe--Cr--Al based stainless steel sheet
according to claim 21, characterized in that said steel sheet
further comprises, by weight, Si: 0.1-1.0% and Mn:
.ltoreq.0.5%.
23: A high Al-containing Fe--Cr--Al based stainless steel sheet
according to claim 21, characterized in that said steel sheet
further comprises, by weight, either or both Ti: 0.02-0.1% and Nb:
0.02-0.3%, as well as La: 0.01-0.1%, Ce: 0.01-0.1% and P:
0.01-0.05%.
24-42. (canceled)
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a stainless steel sheet,
and particularly it relates to a high Al Fe--Cr--Al based stainless
steel sheet and a high Al double layered sheet, and to a process
for their fabrication. The invention further relates to a high Al
Fe--Cr--Al based stainless steel sheet used for production of an
exhaust gas catalyst-carrying honeycomb structure with excellent
heat resistance and oxidation resistance. The invention still
further relates to honeycomb bodies using the steel sheet or double
layered sheet and to a process for their production.
BACKGROUND ART
[0002] Metal carriers comprising heat resistant alloy honeycomb
structures housed in similarly heat resistant alloy jackets have
recently come into common use as exhaust gas purifying catalyst
carriers for internal combustion engines in automobiles and the
like. A honeycomb structure is commonly formed by alternately
laminating a flat foil with a thickness of about 50 .mu.m with a
corrugated foil obtained by corrugating the flat foil, and may be
used in the form of the alternate laminate of the flat foil and
corrugated foil, or as a spiral coil of stacked bands of the flat
foil and corrugated foil.
[0003] In a conventional ceramic carrier, the temperature of the
catalyst is too low for activation during the initial engine
start-up period, and therefore most of the harmful components of
the exhaust gas (HC, NOx, CO, etc.) are released during the initial
engine start-up period. In contrast, a metal carrier offers
numerous advantages, such as low heat capacity compared to
conventional ceramic carriers, such that the heat energy of the
exhaust gas itself produces rapid heating to a temperature at which
the catalyst functions, for superior exhaust gas purification
during the initial engine start-up period. With the increasingly
rigorous restrictions on automobile exhaust gases in the U.S.,
Europe and Japan in recent years, demand is rising for even more
rapid activation of catalysts. For this reason, there is a need for
further reduction in the heat capacity of metal carriers, and this
has created a demand for foil materials which are even thinner than
the conventional 50 .mu.m thickness for metal carrier foils.
[0004] The compositions employed for foil materials are commonly
Fe--Cr--Al based alloys such as Fe-20 wt % Cr-5 wt % Al, as
described in Japanese Examined Patent Publication HEI No. 6-84868,
for example. The alloy in this composition forms a dense
Al.sub.2O.sub.3 film on the surface when exposed to a
high-temperature oxidizing atmosphere, and formation of the
Al.sub.2O.sub.3 film inhibits the rate of oxidation and is
therefore highly advantageous from the viewpoint of oxidation
resistance.
[0005] As mentioned above, the need for reduced heat capacity of
catalyst carriers has led to a demand for construction of metal
carrier honeycombs with foils thinner than 30 .mu.m and with lower
heat capacity. On the other hand, a smaller foil thickness results
in a lower absolute retention of the Al which supports oxidation
resistance in the Fe--Cr--Al based stainless steel sheet, thereby
reducing the oxidation resistance of the foil. The Al content
therefore preferably should exceed 6.5% in order to form a metal
carrier with excellent oxidation resistance, particularly when
using a foil material with a thickness of less than 30 .mu.m.
[0006] Ordinary metal honeycomb bodies are subjected to brazing
with brazing materials at all or portions of the joints between
foils, and in stainless steel sheets comprising Al at 6.5% or
greater, an alumina film forms on the steel sheet surface during
the brazing treatment and significantly impairs the wettability of
the brazing material.
[0007] When a foil material is produced in mass in an ordinary
steelmaking/rolling process, an Al content of greater than 6.5% in
an Fe--Cr--Al steel sheet will impair the hot rolled workability
and hot rolled sheet toughness, and therefore the greater number of
passes required will result in the disadvantage of increased
production cost. Thus, measures for improved oxidation resistance
by simple increase in Al cannot be adopted in the conventional
process. A demand therefore exists for a process wherein cost is
not increased even by conventional means.
[0008] In a metal catalyst carrier, the Al in the metal foil is
oxidized in the high-temperature exhaust gas, forming alumina
(Al.sub.2O.sub.3) and consuming the Al in the foil. The Cr is
oxidized next to chromium oxides, and iron chromium oxide is formed
on the foil surface so that the oxidation resistance is maintained,
but depletion of the Al results in deformation of the metal foil,
more loss of the oxides, breaking of the foil and loss of the
function as a carrier.
[0009] In order to prevent depletion of Al in the metal foil and
extend the durable life of the catalyst carrier, it is effective to
increase the amount of Al in the foil. In particular, a smaller
metal foil thickness requires an increase in the Al concentration
of the foil to ensure an absolute amount of Al. However, special
processing steps are necessary when the amount of Al in stainless
steel exceeds 6 wt %, while at greater than 8.0 wt % the
workability is notably impaired, making rolling of the foil
difficult. Especially in the case of stainless steel foils with a
sheet thickness of 60 .mu.m or smaller, materials with Al contents
exceeding 7.0 wt % are poorly suited for mass production in terms
of workability, and even when foil rolling is possible, corrugation
results in numerous cracks and hence it is difficult to form
honeycomb bodies.
[0010] In Japanese Examined Patent Publication HEI No. 4-51225
there is disclosed a process for fabrication of an exhaust gas
purification catalyst wherein the surface of a stainless steel
sheet with an Al content of no greater than 6.0% is plated with Al
and subjected to foil rolling, and the foil is used to form a
honeycomb body, after which heat treatment is carried out in a
non-oxidizing atmosphere. Cold rolling and honeycomb working can be
accomplished since the Al content is no greater than 6.0% at the
steel sheet stage, and the subsequent heat treatment in the
non-oxidizing atmosphere actively dissolves the plated Al into the
steel sheet, thereby ensuring an amount of Al necessary for
oxidation resistance.
[0011] For production of a metal carrier, a stainless steel flat
foil and corrugated foil are alternately wound or laminated to
produce a honeycomb body form, and then the points of contact
between the flat and corrugated foils are brazed for bonding. For
this purpose, a brazing metal is coated onto the stainless steel
foil surface either after the honeycomb body has been formed or
before it is formed, and the honeycomb body is heated at high
temperature to melt the brazing metal and create brazed joints at
the foil contact sections.
[0012] When a honeycomb body is formed using a metal foil
comprising a stainless steel foil coated with Al on the surface, as
described in Japanese Examined Patent Publication HEI No. 4-51225,
the Al on the foil surface undergoes vaporization loss after the
honeycomb body has been formed, specifically during the
high-temperature heat treatment for diffusion of the foil surface
Al into the stainless steel or during the high-temperature heat
treatment for brazing, and in some cases the Al content of the
stainless steel cannot be adequately increased. Also, the Al in the
stainless steel foil surface and the brazing metal will sometimes
react before brazing, during the temperature increase period prior
to brazing, thus producing high melting point intermetallic
compounds and impairing the bonding property at the brazed
sections.
[0013] As explained above, a catalyst-carrying catalyst converter
is situated in the exhaust gas path for the purpose of purifying
exhaust gas from internal combustion engines. A carrier supporting
a catalyst in the same manner may also be used in
methanol-converting devices which perform water vapor conversion of
hydrocarbon compounds such as methanol to produce hydrogen-rich
gas, CO-removing devices which convert CO to CO.sub.2 for its
removal, or H.sub.2-burning devices which burn H.sub.2 to H.sub.2O
for its removal. Such catalyst carriers comprise numerous cells
through which the gas passes, with the catalyst being coated on the
walls of each cell, thereby allowing contact between the passing
gas and the catalyst over a wide contact area.
[0014] Catalyst carriers which can be used for this purpose include
ceramic catalyst carriers and metal catalyst carriers. For a metal
catalyst carrier, a heat-resistant alloy-containing flat foil with
a thickness of several micrometers and a corrugated foil are
alternately wound or laminated to make a cylindrical metal
honeycomb body, and the metal honeycomb body is inserted into a
cylindrical metal jacket to make a metal carrier. A
catalyst-supporting layer comprising the catalyst impregnated is
formed on the metal foil surface of the cells of the honeycomb body
which serve as the gas pathways of the metal carrier, to produce a
catalyst carrier. The contacting portions of the flat foil and
corrugated foil of the honeycomb body comprising the wound and
laminated flat foil and corrugated foil are bonded by means such as
brazing, to produce the honeycomb body as a firm structure.
[0015] The catalyst may be supported on the metal foil surface of
the honeycomb body by a method in which the metal foil surfaces of
the cells of the honeycomb body which serve as the gas pathways of
the metal carrier are coated with a porous .gamma.-alumina layer
known as a wash coat layer and then a catalyst comprising a rare
metal or the like is impregnated into the wash coat layer, or a
method in which a wash coat layer containing the catalyst is coated
onto the metal honeycomb body. The method for forming the wash coat
layer on the cell surface of the metal honeycomb body may be a
method in which the honeycomb body is immersed in the wash coat
solution to attach the wash coat solution onto the cell surfaces of
the honeycomb body, and is then dried to form a wash coat layer on
the cell surfaces.
[0016] Japanese Examined Patent Publication HEI No. 8-197 describes
a method of forming a honeycomb body using an Al-containing
stainless steel metal foil, subsequently heat treating it in air
and utilizing the Al in the steel to produce .alpha.-alumina
whiskers on the stainless steel surface, and coating the
needle-like crystals with .gamma.-alumina, for the purpose of
improving cohesion between the metal foil surface and wash coat
layer of the metal honeycomb body. A method of heat treating
Al-containing stainless steel in a CO.sub.2 atmosphere beforehand
is described in Japanese Unexamined Patent Publication SHO No.
57-71898, as a method of accelerating production of the
.alpha.-alumina whiskers.
[0017] For .alpha.-alumina whiskers to be produced on the metal
foil surface it is necessary for the honeycomb body to be heat
treated in air or in a specific atmosphere. Since the foil material
is the source of Al for the .alpha.-alumina whiskers, the Al
concentration of the foil material decreases due to the
.alpha.-alumina whiskers. As a result, the original oxidation
resistance of the foil cannot be exhibited.
[0018] For exhaust gas purification using a catalyst carrier, the
catalyst reaction is accelerated, and exhaust gas purification
efficiency improved, by more active substance movement between the
exhaust gas passing through the honeycomb body cells and the
catalyst on the cell surfaces.
[0019] Incidentally, the wash coat layer (.gamma.-Al.sub.2O.sub.3)
is formed on the foil surface of the honeycomb body first, after
which a rare metal catalyst is loaded to produce the catalyst
carrier. The loadability and high temperature stability of the wash
coat on the metal carrier is important for maintaining and
improving the catalyst purification performance, and various types
of treatment are currently being combined for this purpose.
[0020] The wettability is poorer between stainless steel foil
surfaces and wash coat layers of metal carriers, as compared to
ceramic carriers such as cordierite, and therefore the loadability
of the wash coat is insufficient, such that a surfactant or the
like must be used for pretreatment.
[0021] The high temperature stability of the wash coat is important
to maintain the specific surface ratio (a specific surface ratio of
about 80-160 m.sup.2/g with 0.5-40 .mu.m micropores) and increase
the reaction efficiency. The .gamma.-Al.sub.2O.sub.3 used for the
wash coat undergoes a phase transition to .alpha.-Al.sub.2O.sub.3
from about 900.degree. C. This leads to breakup of the
microstructure of the micropores, thereby notably reducing the
specific surface ratio. Thus, in order to increase the phase
transition temperature and increase the thermal stability of the
wash coat, a rare earth oxide such as CeO.sub.2 is dispersed in the
wash coat.
[0022] The wash coat also importantly acts as a co-catalyst,
adsorbing oxygen to supplement the catalytic action, and since
CeO.sub.2 is also effective for this purpose it is often added in a
large amount.
[0023] A metal foil with satisfactory wettability with the wash
coat is preferably used for the metal carrier, since this will
result in satisfactory loadability of the wash coat without
pre-treatment using a surfactant or the like. Also, the metal foil
of the metal carrier preferably has the power to improve the high
temperature stability or oxygen-storing effect of the wash coat,
since this will eliminate the need to disperse a rare earth oxide
such as CeO.sub.2 in the wash coat. It is currently the case that
heavy metals such as Cr or Ni, which are added to stainless steel
foils and effectively improve the workability or corrosion
resistance of the foils, have an oxygen-storing effect but also
sometimes accelerate .alpha.-transition of .gamma.-Al.sub.2O.sub.3,
such that improvement is not easily achieved by addition of large
amounts of Cr or Ni.
[0024] In a catalytic carrier for exhaust gas purification, the
catalyst reaction is accelerated when the catalyst carrier reaches
a temperature above its ignition point. Since the temperature of
the catalyst carrier is low when the engine is started, the
temperature of the exhaust gas passing through raises the
temperature of the catalyst carrier and the catalyst reaction
begins only after the temperature exceeds the ignition point. The
time period from starting of the engine to initiation of the
catalyst reaction is preferably minimized because it is during this
period that the emitted exhaust gas is discharged without being
purified by the catalyst. It is therefore important to increase the
catalyst carrier temperature elevation rate during engine startup
to improve the purification performance immediately after
start-up.
[0025] The following methods have been disclosed for increasing the
catalyst carrier temperature elevation rate during engine startup
to improve the purification performance immediately after
start-up.
[0026] Japanese Unexamined Patent Publication HEI No. 6-997976
describes an invention which is a tandem-type metal carrier wherein
the sheet thickness of the honeycomb body at the exhaust gas
upstream end is made smaller than the sheet thickness of the
honeycomb body at the downstream end, thereby reducing heat
conduction in the radial direction of the honeycomb body at the
upstream end, and tending to form a heat spot. It is stated that
the catalyst carrier temperature elevation time is effectively
shortened by further reducing the foil thickness of the honeycomb
body.
[0027] Japanese Unexamined Patent Publication HEI No. 6-320014
describes an invention wherein slits are formed in the sheet of the
honeycomb at the engine-end.
[0028] Japanese Unexamined Patent Publication HEI No. 6-327973
describes an invention wherein a heating coil is provided around
the carrier to allow an induction current to flow, in order to
increase the temperature of the catalyst by induction heating.
[0029] Also, Japanese Unexamined Patent Publication HEI No.
9-192503 describes an invention wherein a flat foil with a
thickness of no greater than 30 .mu.m and a corrugated foil are
used to construct 100-400 cells per square inch, and the outermost
periphery of the honeycomb body is covered with an elastic
retaining member with a heat-insulating property. The
heat-insulating mechanism in combination with the thinner foil
prevents heat loss from the outer periphery of the honeycomb body
and improves the temperature elevation property.
[0030] The prior art inventions described above are aimed at
increasing the temperature elevation rate of the catalyst carrier
to improve the purification performance immediately after engine
start-up. They therefore involve reducing the sheet thickness,
adding slits to the flat sheet or adding a secondary heating
mechanism or heat-insulating mechanism.
[0031] However, reducing the thickness of the foil lowers both the
honeycomb body strength and the oxidation resistance. Foil
thickness reduction is achieved when applying the invention
described in Japanese Unexamined Patent Publication HEI No.
8-168680, but only up to a certain limit. Adding slits to the flat
sheet will unavoidably reduce the strength of the flat sheet at
those sections, while cost is also increased by the slit forming
step. Furthermore, addition of a secondary heating or
heat-insulating mechanism also increases the overall volume and
raises costs.
[0032] Incidentally, the heat capacity of the honeycomb body can be
lowered to improve the temperature elevation speed during engine
start-up by reducing the thickness of the metal foil of the
honeycomb body. It is known that reducing the foil thickness lowers
the oxidation resistance, and methods have been proposed for
increasing the Al concentration in the metal foil of the honeycomb
body. However, apart from the oxidation resistance with smaller
thickness, it is important to guard against problems such as
flaking of the honeycomb body due to the high-temperature,
high-pressure exhaust gas during use, or its collapse or break up
under thermal stress.
[0033] In Japanese Unexamined Patent Publication HEI No. 5-27737
there is disclosed an invention wherein an inexpensive Y misch
metal is added to an Fe--Cr--Al alloy to ensure oxidation
resistance, and one or more metals from among Nb, Ta, Mo and W is
further added to improve the high-temperature proof strength, the
body being able to withstand a cold-heat durability test with
exhaust gas at 900-1000.degree. C.
[0034] Also, Japanese Unexamined Patent Publication HEI No. 6-389
discloses an invention wherein the metal carrier is composed of a
honeycomb made of a stainless steel foil material having a
high-temperature proof strength at 600.degree. C. and 700.degree.
C. of 22 kgf/mm.sup.2 or greater and 11 kgf/mm.sup.2 or greater,
respectively, with durability that can withstand a cold-heat
durability test with exhaust gas at 900-1000.degree. C.
[0035] Japanese Unexamined Patent Publication HEI No. 8-168679
discloses a honeycomb body wherein all of the points of contact
between the flat sheet and foil sheet of the honeycomb body are
bonded, which has excellent durability with an elastic modulus of
no greater than 200 kg/mm.sup.2 in the radial direction.
[0036] Also, Japanese Unexamined Patent Publication HEI No.
8-168680 describes a honeycomb body have a foil thickness of
between 17 .mu.m and 40 .mu.m, a proof strength of 350/t
(kgf/mm.sup.2) or greater at 700.degree. C., and a specified
relationship between the Al and Cr content and the foil thickness
t.
[0037] All of these prior art technologies are aimed at improving
the high-temperature durability of the honeycomb body, and were
developed for the purpose of increasing the proof strength at high
temperature and reducing the partial elastic modulus in the
honeycomb body. Methods which improve proof strength invariably
lower the material workability, and also increase working costs for
rolling and the like. Moreover, the effects of methods which
locally reduce the honeycomb body elastic modulus are insufficient
when the foil temperature increases above 1000.degree. C.
[0038] Incidentally, it has been attempted in recent years to
construct honeycombs with thin foils of 30 .mu.m and smaller for
increased heat capacity of catalyst carriers, since the heat
capacity is too high with the conventional thickness of 50 .mu.m.
On the other hand, since a small foil thickness leads to lower
absolute retention of Cr and Al which maintain the oxidation
resistance, the oxidation resistance of a foil is proportional to
its thickness, given the same chemical composition. Consequently,
since the oxidation resistance of thin foils is reduced in most
cases, and particularly with a thin foil of 30 .mu.m or smaller,
the alloy design must be such as to maximize the oxidation
resistance above that of conventional foils. A thin foil of 30
.mu.m or smaller preferably has an Al content of 6 wt % or
greater.
[0039] When such a high-Al thin foil is used for mass production of
a foil material in an ordinary steel-making, hot-rolling or
cold-rolling process, the amount of Al which can be added to the
Fe--Cr--Al based alloy is limited by problems such as the rolling
property, and means intended to improve the oxidation resistance
simply by increasing the amount of Al in an ordinary process tend
to raise the rolling costs.
[0040] Published Japanese translation of PCT international
publication for patent application HEI No. 11-514929 discloses a
method wherein either the flat foil or corrugated foil of the
honeycomb structure is an Fe--Cr--Al based alloy and the other is a
layered structure comprising an Fe--Cr based alloy and an
Al-containing layer, and diffusion treatment is carried out. In
this method, however, Al is not enriched in the Fe--Cr--Al based
alloy without the Al-containing layer, and it is difficult to
obtain an Al concentration of 7 wt % or greater as a whole.
[0041] U.S. Pat. No. 4,602,001 discloses a method wherein the cell
walls of a honeycomb structure composed of a steel foil with an Al
content of no greater than 1 wt % are coated with Al powder, and
heat treated. However, since this method employs alloy steel with
an Al content of 1 wt % as the starting material, coating
irregularities occurring during coating of the Al powder tend to
promote abnormal oxidation at those portions.
[0042] It is an object of the present invention to solve the
problems described above. Specifically, it is an object of the
invention to provide an Fe--Cr--Al based stainless steel sheet and
double layered sheet comprising Al at greater than 6.5% and having
satisfactory hot rolled sheet ductility and excellent wettability
for brazing metals, as well as a process for their fabrication,
[0043] an Fe--Cr--Al based stainless steel sheet which makes it
possible to achieve improved cohesion between the metal surface and
wash coat layer of the metal honeycomb body of an exhaust gas
purification catalyst carrier, and an Fe--Cr--Al based stainless
steel sheet or double layered sheet with excellence in terms of
wash coat loading property, high temperature stability and oxygen
storage property,
[0044] an Fe--Cr--Al based stainless steel sheet for fabrication of
a catalyst carrier with an excellent temperature elevating property
after engine start-up, i.e. with excellent exhaust gas purification
performance, without radically reducing the sheet (foil) thickness
and without the need for slit formation, a heating mechanism or a
heat insulating mechanism,
[0045] an Fe--Cr--Al based stainless steel sheet wherein the foil
material has excellent high-temperature durability, allowing its
use under stringent conditions exceeding 1000.degree. C.,
[0046] an exhaust gas purification catalyst carrier honeycomb body
employing the aforementioned Fe--Cr--Al based stainless steel sheet
and double layered sheet, and
[0047] a process for fabrication of a low heat capacity metal
honeycomb body with sufficient oxidation resistance and excellent
structural durability, even as a honeycomb body composed of an
ultrathin foil.
[0048] The stainless steel sheet and double layered sheet of the
invention both also encompass foils.
SUMMARY OF THE INVENTION
[0049] The present invention has been accomplished with the aim of
solving the problems referred to above, and its gist is as
follows.
[0050] (1) A high Al-containing Fe--Cr--Al based stainless steel
sheet characterized by comprising, by weight, Cr: 10-30% and Al:
>6.5%-15%, with the remainder consisting of Fe and unavoidable
impurities.
[0051] (2) A high Al-containing Fe--Cr--Al based stainless steel
sheet according to (1), characterized in that the steel sheet
further comprises, by weight, Si: 0.1-1.0% and Mn:
.ltoreq.0.5%.
[0052] (3) A high Al-containing Fe--Cr--Al based stainless steel
sheet according to (1) or (2), characterized in that the steel
sheet further comprises, by weight, either or both Ti: 0.02-0.1%
and Nb: 0.02-0.3%, as well as La: 0.01-0.1%, Ce: 0.01-0.1% and P:
0.01-0.05%.
[0053] (4) A high Al-containing Fe--Cr--Al based stainless steel
sheet according to any one of (1) to (3), characterized in that the
steel sheet further comprises, by weight, Cu: 0.01-1.0%.
[0054] (5) A high Al-containing Fe--Cr--Al based stainless steel
sheet according to any one of (1) to (4), characterized in that the
steel sheet further comprises, by weight, Mg: 0.001-0.1%.
[0055] (6) A high Al-containing Fe--Cr--Al based stainless steel
sheet according to any one of (1) to (5), characterized in that the
total of Zn, Sn, Sb, Bi and Pb in the steel sheet is limited to no
greater than 0.05% by weight.
[0056] (7) A high Al-containing Fe--Cr--Al based stainless steel
sheet according to any one of (1) to (6), characterized in that the
thickness of the steel sheet is 10-40 .mu.m.
[0057] (8) A high Al-containing double layered sheet characterized
by comprising Al or an Al alloy adhering to the surface of a
stainless steel sheet with a thickness of 5 .mu.m to 2 mm, wherein
the average composition is the composition of a high Al-containing
Fe--Cr--Al based stainless steel sheet according to any one of (1)
to (6).
[0058] (9) A high Al-containing double layered sheet according to
(7), characterized in that the Al or Al alloy comprises at least
one from among Si, Ca, Sr, Y, Zr, La, Ba, Mg, Ce, Hf and Ta.
[0059] (10) A high Al-containing double layered sheet according to
(8) or (9), characterized in that the sheet thickness is 10-40
.mu.m.
[0060] (11) A high Al-containing Fe--Cr--Al based stainless steel
sheet according to (1), characterized in that the surface of the
steel sheet has protrusions with a height of 1 .mu.m or greater at
a density of at least 100/cm.sup.2, and a sheet thickness of no
greater than 100 .mu.m, and is used in an exhaust gas purification
catalyst-carrying honeycomb body.
[0061] (12) A high Al-containing Fe--Cr--Al based stainless steel
sheet according to (11), characterized in that the protrusions are
made of metal, and the Al concentration in the protrusions is
higher than the Al concentration in the steel sheet.
[0062] (13) A high Al-containing Fe--Cr--Al based stainless steel
sheet according to (11) or (12), characterized in that the steel
sheet further comprises, by weight, Si: 0.1-1.0% and Mn:
.ltoreq.0.5%.
[0063] (14) A high Al-containing Fe--Cr--Al based stainless steel
sheet according to (13), characterized by further comprising, by
weight, either or both Ti: 0.02-0.1% and Nb: 0.02-0.3%, as well as
La: 0.01-0.1%, Ce: 0.01-0.1% and P: 0.01-0.05%.
[0064] (15) A high Al-containing Fe--Cr--Al based stainless steel
sheet according to (1), characterized in that the steel sheet has
isolated gaps in the interior and is used in an exhaust gas
purification catalyst-carrying honeycomb body.
[0065] (16) A high Al-containing Fe--Cr--Al based stainless steel
sheet according to (15), characterized in that the gaps are at
positions within t/7 from the steel sheet surface in the sheet
thickness direction of the steel sheet, where t is the thickness of
the steel sheet.
[0066] (17) A high Al-containing Fe--Cr--Al based stainless steel
sheet according to (15) or (16), characterized in that the sizes of
the gaps are between 0.1 and 5 .mu.m.
[0067] (18) A high Al-containing Fe--Cr--Al based stainless steel
sheet according to any one of (15) to (17), characterized in that
the thickness of the steel sheet is 10-40 .mu.m.
[0068] (19) A high Al-containing Fe--Cr--Al based stainless steel
sheet according to any one of (15) to (18), characterized in that
the steel sheet further comprises, by weight, Si: 0.1-1.0% and Mn:
.ltoreq.0.5%.
[0069] (20) A high Al-containing Fe--Cr--Al based stainless steel
sheet according to (19), characterized in that the steel sheet
further comprises, by weight, either or both Ti: 0.02-0.1% and Nb:
0.02-0.3%, as well as La: 0.01-0.1%, Ce: 0.01-0.1% and P:
0.01-0.05%.
[0070] (21) A high Al-containing Fe--Cr--Al based stainless steel
sheet according to (1), characterized in that the thickness t of
the steel sheet is 10-40 .mu.m, the thermal expansion coefficient
.alpha. from 20.degree. C. to 1000.degree. C. is 15-23
.mu.m/m/.degree. C. and the 0.2% proof strength .sigma.
(N/mm.sup.2) measured at 900.degree. C., the steel sheet thickness
t (.mu.m) and the thermal expansion coefficient .alpha.
(.mu.m/m/.degree. C.) are in a relationship satisfying the
following inequality <1>, and the steel sheet is used in an
exhaust gas purification catalyst-carrying honeycomb body.
.sigma..gtoreq.(-9.0875.times..alpha..sup.2+4.2913.times.10.sup.2.times..-
alpha.-3.824215-.times.10.sup.3)/t <1>
[0071] (22) A high Al-containing Fe--Cr--Al based stainless steel
sheet according to (21), characterized in that the steel sheet
further comprises, by weight, Si: 0.1-1.0% and Mn:
.ltoreq.0.5%.
[0072] (23) A high Al-containing Fe--Cr--Al based stainless steel
sheet according to (21) or (22), characterized in that the steel
sheet further comprises, by weight, either or both Ti: 0.02-0.1%
and Nb: 0.02-0.3%, as well as La: 0.01-0.1%, Ce: 0.01-0.1% and P:
0.01-0.05%.
[0073] (24) A process for fabrication of a high Al-containing
double layered sheet, characterized by adhering Al or an Al alloy
to the surface of a stainless steel sheet with a thickness of 5
.mu.m to 2 mm, wherein the average composition is a composition
comprising Cr: 10-30% and Al: >6.5%-15%, with the remainder
consisting of Fe and unavoidable impurities.
[0074] (25) A process for fabrication of a high Al-containing
double layered sheet according to (24), characterized in that the
average composition of the high Al double layered sheet further
comprises, by weight, Si: 0.1-1.0% and Mn: .ltoreq.0.5%.
[0075] (26) A process for fabrication of a high Al-containing
double layered sheet according to (24) or
[0076] (25), characterized in that the average composition of the
high Al double layered sheet further comprises, by weight, either
or both Ti: 0.02-0.1% and Nb: 0.02-0.3%, as well as La: 0.01-0.1%,
Ce: 0.01-0.1% and P: 0.01-0.05%.
[0077] (27) A process for fabrication of a high Al-containing
double layered sheet according to any one of (24) to (26),
characterized in that the average composition of the high Al double
layered sheet further comprises, by weight, Cu: 0.01-1.0%.
[0078] (28) A process for fabrication of a high Al-containing
double layered sheet according to any one of (24) to (27),
characterized in that the average composition of the high Al double
layered sheet further comprises, by weight, Mg: 0.001-0.1%.
[0079] (29) A process for fabrication of a high Al-containing
double layered sheet according to any one of (24) to (28),
characterized in that the total of Zn, Sn, Sb, Bi and Pb in the
average composition of the high Al double layered sheet is limited
to no greater than 0.05% by weight.
[0080] (30) A process for fabrication of a high Al-containing
double layered sheet according to any one of (24) to (29),
characterized in that the adhering Al or Al alloy comprises at
least one from among Si, Ca, Sr, Y, Zr, Ba, La, Mg, Ce, Hf and
Ta.
[0081] (31) A process for fabrication of a high Al-containing
Fe--Cr--Al based stainless steel sheet, characterized by subjecting
a high Al-containing double layered sheet obtained by a process
according to any one of (24) to (30) to foil rolling.
[0082] (32) A process for fabrication of a high Al-containing
Fe--Cr--Al based stainless steel sheet, characterized by subjecting
a high Al-containing double layered sheet obtained by a process
according to any one of (24) to (30) to diffusion heat
treatment.
[0083] (33) A process for fabrication of a high Al-containing
Fe--Cr--Al based stainless steel sheet, characterized by subjecting
a high Al-containing double layered sheet obtained by a process
according to any one of (24) to (30) to diffusion heat treatment
and then to foil rolling.
[0084] (34) A process for fabrication of a high Al-containing
Fe--Cr--Al based stainless steel sheet, characterized by subjecting
the double layered sheet obtained by a process according to any one
of (24) to (30) to foil rolling and then to diffusion heat
treatment.
[0085] (35) A process for fabrication of a high Al-containing
Fe--Cr--Al based stainless steel sheet according to any one of (31)
to (34), characterized in that the thickness of the steel sheet is
no greater than 40 .mu.m.
[0086] (36) An exhaust gas purification catalyst-carrying honeycomb
body, characterized by being fabricated using a high Al-containing
Fe--Cr--Al based stainless steel sheet according to any one of (1)
to (7) or a high Al-containing double layered sheet according to
any one of (8) to (10).
[0087] (37) An exhaust gas purification catalyst-carrying honeycomb
body, characterized by being fabricated using a high Al-containing
Fe--Cr--Al based stainless steel sheet according to any one of (11)
to (23).
[0088] (38) An exhaust gas purification catalyst-carrying honeycomb
body, characterized by being fabricated using a high Al-containing
double layered sheet obtained by a process according to any one of
(24) to (30), or an Fe--Cr--Al based stainless steel sheet obtained
by a process according to any one of (31) to (35).
[0089] (39) A process for fabrication of an exhaust gas
purification catalyst-carrying honeycomb body, characterized by
constructing a honeycomb body from an Fe--Cr--Al based stainless
steel sheet comprising, by weight, Cr: 10-30% and Al: .ltoreq.6.5%,
with the remainder consisting of Fe and unavoidable impurities,
coating the surface of the steel sheet of the honeycomb body with
Al powder, and then subjecting the steel sheet to diffusion heat
treatment.
[0090] (40) A process for fabrication of an exhaust gas
purification catalyst-carrying honeycomb body according to (39),
characterized in that the steel sheet further comprises, by weight,
Si: 0.1-1.0% and Mn: .ltoreq.0.5%.
[0091] (41) A process for fabrication of an exhaust gas
purification catalyst-carrying honeycomb body according to (39) or
(40), characterized in that the steel sheet further comprises, by
weight, either or both Ti: 0.02-0.1% and Nb: 0.02-0.3%, as well as
La: 0.01-0.1%, Ce: 0.01-0.1% and P: 0.01-0.05%.
[0092] (42) A process for fabrication of an exhaust gas
purification catalyst-carrying honeycomb body according to any one
of (39) to (41), characterized in that the coated Al powder
comprises at least one from among Si, Ca, Sr, Y, Zr, Ba, La, Mg,
Ce, Hf and Ta.
BRIEF DESCRIPTION OF THE DRAWINGS
[0093] FIG. 1 is a cross-sectional view showing a state of Al
powder accumulated on the cell walls of a honeycomb body.
[0094] FIG. 2.is a cross-sectional view showing a state of Al
powder accumulated on the cell walls and brazed sections of a
honeycomb body.
[0095] FIG. 3 is a perspective view showing an embodiment of a step
in which a honeycomb body is immersed in a binder solution or Al
powder slurry.
[0096] FIG. 4 is a schematic view showing an embodiment of a step
in which excess binder or Al powder slurry is removed using an air
blower.
[0097] FIG. 5 is a schematic view showing an embodiment of a step
in which excess binder or Al powder slurry is removed by applying
centrifugal force.
[0098] FIG. 6 is a schematic view showing an embodiment of a step
in which Al powder is dispersed in a honeycomb body.
[0099] FIG. 7 is a cross-sectional schematic view showing the
condition of Al diffused in the base material during the heat
treatment step.
[0100] FIG. 8(a) is a perspective view showing protrusions formed
in the cell walls of a honeycomb body.
[0101] FIG. 8(b) is a cross-sectional view showing protrusions
formed in the cell walls of a honeycomb body.
DETAILED DESCRIPTION OF THE INVENTION
[0102] The reasons for the component ranges in the Fe--Cr--Al based
stainless steel sheet of the invention will now be explained. The
units are weight percentages.
[0103] If the Al content exceeds 6.5% it will be possible to ensure
oxidation resistance as a catalyst carrier even if the honeycomb
body employs a thin steel sheet of 30 .mu.m or smaller. However,
the upper limit is 15% since an Al content of greater than 15%
results in a fragile steel sheet.
[0104] A Cr content of 10% or greater produces oxidation
resistance, and therefore the lower limit is 10%. An amount of
greater than 30% results in a fragile steel sheet, and therefore
the upper limit is 30%.
[0105] The Fe--Cr--Al based stainless steel sheet of the invention
comprises the aforementioned alloy components, with the remainder
consisting substantially of Fe and unavoidable impurities. As
examples of unavoidable impurities there may be mentioned C:
.ltoreq.0.01% and S: .ltoreq.0.005%.
[0106] The Fe--Cr--Al based stainless steel sheet of the invention
also preferably comprises the following components.
[0107] A Mn content of no greater than 0.5% will ensure oxidation
resistance of the steel sheet, and therefore Mn is preferably
present at no greater than 0.5%.
[0108] Addition of Si at 0.1% or greater will improve the oxidation
resistance, and therefore the lower limit is preferably 0.1%. If
the Si content exceeds 1.0% the steel sheet will tend to become
fragile, and therefore the upper limit is preferably 1.0%.
[0109] Either or both Ti: .gtoreq.0.02% and Nb: .gtoreq.0.02% will
exhibit an effect of improving the hot rolled sheet ductility. On
the other hand, amounts greater than Ti: 0.1% or Nb: 0.3% will tend
to impair the oxidation resistance, and therefore these values are
preferably the upper limits.
[0110] La and Ce both have an effect of improving the oxidation
resistance. Oxidation resistance can be guaranteed if each is
present at 0.01% or greater. A content of either exceeding 0.1%
will result in segregation at the grain boundary and will adversely
affect the hot workability, and therefore the upper limit is
preferably 0.1%.
[0111] A P content of 0.01% or greater will allow formation of
phosphides with La and Ce, with an effect of inhibiting La and Ce
grain boundary segregation and improving the hot workability. The
lower limit for the content is 0.01%, while the upper limit is
preferably 0.05% since the oxidation resistance may be inferior at
greater than 0.05%.
[0112] In an Fe--Cr--Al based stainless steel sheet with an Al
content exceeding 6.5%, an oxidation film is formed during brazing
treatment, thus impairing the wettability of the brazing material.
A Cu content of 0.01% or greater will produce an effect of
improving the brazing material wettability, and therefore Cu is
preferably added at 0.01% or greater. On the other hand, a Cu
content of greater than 1.0% will impair the hot workability, and
therefore the upper limit is 1.0%. The range for the Cu content is
more preferably 0.03% to 0.5%, and even more preferably 0.05% to
0.5%.
[0113] If the Mg content is 0.001% or greater, the low vapor
pressure of Mg will result in fly off of Mg vapor during the
brazing treatment, and this will provide an effect of breaking up
the oxidation film and improving the brazing property. The Mg
content is therefore preferably 0.001% or greater. On the other
hand, excessive addition of Mg will impair the hot rolled sheet
ductility of the steel sheet, and therefore the upper limit is
0.1%.
[0114] The Zn, Sn, Sb, Bi and Pb components may be present as
impurities in the Fe--Cr--Al based stainless steel sheet. These
elements have low melting points, and tend to segregate at the
grain boundary to result in grain boundary cracking during slab
solidification or hot rolling. In particular, an Al content of
greater than 6.5% in the steel will increase the sensitivity to
cracking of these elements. The total of Zn, Sn, Sb, Bi and Pb is
preferably no greater than 0.05% to reduce the crack
sensitivity.
[0115] As described in Japanese Examined Patent Publication HEI No.
6-8486, an Al content exceeding 6.5% in a stainless steel sheet
forming a metal carrier will lead to fine cracks in the coating
upon repeated heating of the steel sheet by exhaust gas. According
to the invention it was discovered that cracking can be prevented
by limiting the thickness of the stainless steel sheet to no
greater than 40 .mu.m. Therefore, in a metal carrier employing an
Fe--Cr--Al based stainless steel sheet of the invention comprising
Al at greater than 6.5%, cracking will be avoided if the stainless
steel sheet thickness is 40 .mu.m or smaller, even when the metal
carrier is subjected to repeated heating. In addition, limiting the
sheet thickness to no greater than 40 .mu.m also reduces the heat
capacity of the honeycomb structure, and therefore allows the
light-off performance and purification performance to be improved
when the honeycomb structure is used as a catalyst converter.
[0116] The high Al-containing double layered sheet of the invention
will now be explained.
[0117] The high Al-containing double layered sheet of the invention
is obtained by adhering Al or an Al alloy onto the surface of an
Fe--Cr--Al based stainless steel sheet made of a base material to
form an Al coating, and the average composition of the Fe--Cr--Al
based stainless steel sheet base material and the Al coating is
adjusted to match the composition of the aforementioned high
Al-containing Fe--Cr--Al based stainless steel sheet. Thus, the
reasons for the limitations on the amounts of Cr and Al in the
double layered sheet, the amounts of Si and Mn added as optional
components, the amounts of Ti, Nb, La and Ce, the amount of P, the
amount of Cu added to improve the brazing material wettability, the
amount of Mg added to improve the brazing property, and the total
amount of Zn, Sn, Sb, Bi and Pb which is restricted in order to
reduce the crack sensitivity, are the same as explained for the
high Al-containing Fe--Cr--Al based stainless steel sheet described
above, and will not be explained again.
[0118] Since, as mentioned above, the Al content of the Fe--Cr--Al
based stainless steel as the base material of the double layered
sheet may be adjusted to an average composition of 6.5-15% by the
adhered Al coating, it may be a relatively low content of no more
than 8%, or even 6.5%.
[0119] The thickness of the double layered sheet is not
particularly restricted and may be appropriately selected according
to the purpose of use. If it is too thin, the rigidity of the
double layered sheet will be significantly reduced to the point of
limiting its use, and therefore the thickness is preferably at
least 5 .mu.m. If it is too thick, however, the amount of adhered
Al will need to be increased and peeling of the Al coating may
become a problem, and therefore the thickness is preferably no
greater than 2 mm. For use as the steel sheet of a honeycomb body
in a metal carrier, it is preferably no greater than 40 .mu.m, and
more preferably 10-40 .mu.m, as explained for the Fe--Cr--Al based
stainless steel sheet described above.
[0120] The high Al-containing double layered sheet of the invention
comprises Al or an Al alloy adhered to one or both sides of the
surface of the Fe--Cr--Al based stainless steel sheet as the base
material.
[0121] The Al or Al alloy adhered to the surface preferably
comprises at least one from among Si, Ca, Sr, Y, Zr, Ba, La, Mg,
Ce, Hf and Ta.
[0122] If the coating of the Al or Al alloy adhered to the surface
comprises at least one from among Si, Ca, Sr, Y, Zr, Ba, La, Mg,
Ce, Hf and Ta, improvement will be realized in the wash coat
loading property, high temperature stability and oxygen storage
property when an automobile exhaust gas purification catalyst is
loaded in the honeycomb structure. Addition of these elements in an
Al-based metal film comprising at least 40 wt % Al improves
wettability with the wash coat solution, compared to the surface of
stainless steel, and is effective for producing a more uniform wash
coat. Furthermore, when these elements diffuse in the wash coat and
form oxides, they effectively prevent .alpha. transformation of
Al.sub.2O.sub.3 and enhance the oxygen storing effect. Since the
wash coat layer is formed on both sides of the steel sheet, it is
suitable for a double layered steel sheet having Al or an Al alloy
on both sides. The content of each of these elements in the outer
layer is preferably 0.01-15 wt %. The effect described above may
not be exhibited at less than 0.01 wt %, while the steel sheet will
tend to become brittle at greater than 20 wt %.
[0123] The Al or Al alloy adhered to the surface preferably
comprises Fe at 1% or greater. This will inhibit vaporization of
Al, thereby inhibiting Al loss and improving the oxidation
resistance during the heat treatment for brazing of the honeycomb
body or during the heat treatment for uniform diffusion of Al.
[0124] When the double layered sheet is subjected to heat treatment
or foil rolling to diffuse the Al or Al alloy on the surface and
produce a high Al-containing Fe--Cr--Al based stainless steel sheet
as described hereunder, the Al or the aforementioned elements
contained in the Al alloy on the surface are also present in the
Fe--Cr--Al based stainless steel sheet, and therefore a similar
effect can be obtained.
[0125] A high Al-containing stainless steel sheet of the invention
will now be explained wherein the surface of the Fe--Cr--Al based
stainless steel sheet (foil) comprises protrusions with a height of
1 .mu.m or greater at a density of at least 100/cm.sup.2.
[0126] If protrusions with a height of 1 .mu.m or greater are
present on the surface of the honeycomb body steel sheet at a
density of at least 100/cm.sup.2, the wash coat layer will have a
better taper and the cohesion between the metal foil surface and
wash coat layer will be improved, when the wash coat layer is
formed on the honeycomb body. Since protrusions are present on the
surface of the steel sheet forming the cell surfaces of the
honeycomb body, the cell surfaces will be irregular, thus promoting
turbulence of the gas passing through the cells, and this
turbulence effect will accelerate replacement of the gas contacting
with the cell surfaces, thereby promoting the catalyst reaction.
The protrusions referred to here are wart-like projections
extending from the steel sheet surface.
[0127] The heights of the protrusions formed on the steel sheet
surface must be at least 1 .mu.m. If they are smaller than 1 .mu.m,
an effect of increased wash coat adhesion will not be achieved. The
protrusion heights are more preferably 2 .mu.m or greater. The
density of the protrusions must be at least 100/cm.sup.2. This can
reliably increase the ameliorating effect on cohesion of the wash
coat layer bearing the protrusions. The reason for the upper limit
of 100 .mu.m for the thickness of the steel sheet of the invention
is that a thickness of greater than 100 .mu.m will lead to
excessive pressure loss upon formation of the honeycomb body.
[0128] Incidentally, if the surface roughness Ra of the honeycomb
body, or the steel sheet composing the honeycomb body, is 2 .mu.m
or greater, it will be possible to improve the cohesion between the
steel sheet surface and the wash coat layer when the wash coat
layer is formed on the honeycomb body. Also, since irregularities
are present on the surface of the steel sheet forming the cell
surfaces of the honeycomb body, turbulence of the gas passing
through the cells is promoted, and the turbulence effect
accelerates replacement of the gas contacting with the cell
surfaces, thereby promoting the catalyst reaction. The surface
roughness is the value of Ra as specified by JIS S2000.
[0129] The surface roughness Ra of the steel sheet surface is
preferably 2 .mu.m or greater because this will allow the cohesive
property of the wash coat layer to be exhibited. The surface
roughness Ra is more preferably 4 .mu.m or greater.
[0130] The steel sheet of the invention has protrusions on the
surface and a prescribed surface roughness, to allow the
aforementioned effect to be obtained when the steel sheet is used
to form a honeycomb body.
[0131] The steel sheet or honeycomb body-forming steel sheet of the
invention may be a steel sheet comprising, by weight, the
components Al: >6.5%-15% and Cr: 10-30%, with the remainder
consisting of Fe and unavoidable impurities. For improved oxidation
resistance, the steel sheet used is preferably one whose components
are Si: 0.1-1.0%, Mn: .ltoreq.0.5%, Al: >6.5%-15% and Cr:
10-30%, with the remainder consisting of Fe and unavoidable
impurities.
[0132] Specifically, Mn is limited to no greater than 0.5% in order
to ensure that the steel sheet will have oxidation resistance.
[0133] An Si content of 0.1% or greater can improve the oxidation
resistance of the steel sheet. However, the steel sheet may become
brittle if the Si content exceeds 1.0%, and therefore the upper
limit is 1.0%.
[0134] An Al content of greater than 6.5% can also improve the
oxidation resistance of the steel sheet. However, since the steel
sheet may become brittle if the Al content exceeds 15%, the upper
limit is 15%.
[0135] A Cr content of 10% or greater can also improve the
oxidation resistance of the steel sheet. However, since the steel
sheet may become brittle if the Cr content exceeds 30%, the upper
limit is 30%.
[0136] The steel sheet or honeycomb body-forming steel sheet of the
invention also preferably comprises either or both Ti: 0.02-0.1%
and Nb: 0.02-0.3%, as well as La: 0.01-0.1%, Ce: 0.01-0.1% and P:
0.01-0.05%.
[0137] By including either or both Ti at 0.02% or greater and Nb at
0.02% or greater, it is possible to improve the ductility of the
steel sheet. However, the upper limits are 0.1% for Ti and 0.3% for
Nb, because exceeding these limits may adversely affect the
oxidation resistance of the steel sheet.
[0138] By including La at 0.01% or greater and Ce at 0.01% or
greater, it is possible to improve the oxidation resistance of the
steel sheet. However, the upper limits are 0.1% for La and 0.1% for
Ce, because exceeding these limits can lead to cracking during hot
rolling.
[0139] A P content of 0.01% or greater has the effect of preventing
cracking during hot rolling when La and Ce are present. However,
the upper limit for the P content is 0.05%, because exceeding this
limit can lead to inferior oxidation resistance.
[0140] A high Al-containing Fe--Cr--Al based stainless steel sheet
(foil) according to the invention having isolated gaps in the
interior of the Fe--Cr--Al based stainless steel sheet (foil) will
now be explained.
[0141] The Fe--Cr--Al based stainless steel sheet (foil) of the
invention is a stainless steel sheet (foil) comprising Al, and it
is characterized by having isolated gaps in the interior of the
steel sheet. The presence of the isolated gaps can increase the
heat insulating property of the steel sheet. The result of the gaps
formed in the interior of the steel sheet is a reduction in the
heat conductivity near those areas. When a catalyst carrier is
constructed using a honeycomb body comprising a steel sheet
according to the invention, the heat of the exhaust gas during the
initial engine start up period raises the temperature of the
catalyst first before raising the temperature near the center of
the steel sheet thickness, thereby increasing the temperature
elevation rate of the catalyst itself and providing the catalyst
carrier with excellent purification performance.
[0142] The high Al-containing stainless steel of the invention is
preferably Fe--Cr--Al based stainless steel comprising Al:
>6.5%-15% and Cr: 10-30%.
[0143] If the formed gaps are at positions within t/7 from the
steel sheet surface, where t is the thickness of the steel sheet,
an effect of reduced surface heat conductivity will be obtained
according to the invention. Forming gaps toward the center end of
t/7 of the sheet thickness will not provide a heat insulating
effect on the steel sheet surface to a degree which will increase
the temperature elevation rate of the catalyst. More preferably,
formation of gaps within t/10 from the steel sheet surface will
further reduce the heat conduction of the steel sheet surface,
thereby further improving the temperature elevating property of the
catalyst.
[0144] If the sizes of the gaps are smaller than 0.1 .mu.m, it will
be difficult to effectively reduce the heat conduction. They are
preferably not larger than 5 .mu.m because the strength near those
areas will be reduced. The sizes of the gaps are therefore
specified as being between 0.1 .mu.m and 5 .mu.m. The sizes are
more preferably between 1 .mu.m and 4 .mu.m in order to facilitate
the effect of the invention.
[0145] The effect of the invention will be achieved if the
distances between the centers of the gaps according to the
invention are preferably greater than L and no greater than 20 L,
where L is defined as the sizes of the gaps. This is because if the
distances between the gaps are less than L, the gaps may become
connected and in some cases too large to the point of reducing the
steel sheet strength. If they are greater than 20 L, the effect of
the invention of reducing the heat conductivity of the steel sheet
surface layer will be less prominent. The distances between the
gaps are more preferably no greater than 10 L for greater
effectiveness.
[0146] The thickness of the steel sheet of the invention is
preferably between 10 .mu.m and 40 .mu.m. If the thickness of the
steel sheet is less than 10 .mu.m, the honeycomb body fabricated
using the steel sheet will lack strength. If the thickness of the
steel sheet exceeds 40 .mu.m, the heat capacity of the steel sheet
itself will increase, thus diluting the effect of the
invention.
[0147] The steel sheet and honeycomb body-forming steel sheet of
the invention may be an Fe--Cr--Al based steel sheet (foil)
comprising, by weight, the components Al: >6.5%-15% and Cr:
10-30%, with the remainder consisting of Fe and unavoidable
impurities. For improved oxidation resistance, the steel sheet used
is preferably one whose components are Si: 0.1-1.0%, Mn:
.ltoreq.0.5%, Al: >6.5%-15% and Cr: 10-30%, with the remainder
consisting of Fe and unavoidable impurities.
[0148] An Al content of greater than 6.5% can improve the oxidation
resistance of the steel sheet. However, since the steel sheet may
become brittle if the Al content exceeds 15%, the upper limit is
15%.
[0149] A Cr content of 10% or greater can also improve the
oxidation resistance of the steel sheet. However, since the steel
sheet may become brittle if the Cr content exceeds 30%, the upper
limit is 30%.
[0150] Mn is limited to no greater than 0.5% in order to ensure
that the steel sheet will have oxidation resistance.
[0151] An Si content of 0.1% or greater can improve the oxidation
resistance of the steel sheet. However, the steel sheet may become
brittle if the Si content exceeds 1.0%, and therefore the upper
limit is 1.0%.
[0152] The steel sheet and honeycomb body-forming steel sheet of
the invention also preferably comprises either or both Ti:
0.02-0.1% and Nb: 0.02-0.3%, as well as La: 0.01-0.1%, Ce:
0.01-0.1% and P: 0.01-0.05%.
[0153] By including either or both Ti at 0.02% or greater and Nb at
0.02% or greater, it is possible to improve the ductility of the
steel sheet. However, the upper limits are 0.1% for Ti and 0.3% for
Nb, because exceeding these limits may adversely affect the
oxidation resistance of the steel sheet.
[0154] By including La at 0.01% or greater and Ce at 0.01% or
greater, it is possible to improve the oxidation resistance of the
steel sheet. However, the upper limits are 0.1% for La and 0.1% for
Ce, because exceeding these limits can lead to cracking during hot
rolling.
[0155] A P content of 0.01% or greater has the effect of preventing
cracking during hot rolling when La and Ce are present. However,
the upper limit for the P content is 0.05%, because exceeding this
limit can lead to inferior oxidation resistance.
[0156] A honeycomb body constructed using a steel sheet with gaps
according to the invention, when used as a catalyst carrier
supporting a catalyst on the honeycomb cell surfaces, can serve as
a catalyst carrier with an increased temperature elevation rate of
the catalyst itself during initial engine startup, and therefore
excellent purification performance.
[0157] A high Al-containing Fe--Cr--Al based stainless steel sheet
according to the invention will now be explained, wherein the proof
strength condition required for an Fe--Cr--Al based stainless steel
sheet (foil) used in an exhaust gas purification catalyst carrier
is specified by the relationship between the thickness and the
thermal expansion coefficient of the steel sheet (foil).
[0158] A catalyst carrier which is performing exhaust gas
purification treatment has a temperature gradient in which the
temperature is higher at the center and lower at the periphery, in
the radial direction. Moreover, a temperature gradient also exists
in which the temperature of the exhaust gas is high at the inlet
end and lower toward the outlet end, in the axial direction. The
thermal expansion of the catalyst carrier differs due to this
temperature gradient, and thermal stress acts inside the catalyst
carrier as a result. In other words, a larger thermal expansion
coefficient of the steel sheet used results in a greater thermal
stress in the catalyst carrier during treatment.
[0159] The present invention was accomplished with this in mind,
and it is characterized in that the proof strength condition
required for the steel sheet is specified by the relationship
between the thickness and the thermal expansion coefficient of the
steel sheet. This allows the most preferred proof strength range to
be specified for the actual thermal expansion coefficient of the
steel sheet, to fabricate a steel sheet and honeycomb body having
excellent high temperature durability even under stringent
conditions exceeding 1000.degree. C.
[0160] Specifically, the thickness t of the steel sheet of the
invention is 10-40 .mu.m, the thermal expansion coefficient .alpha.
from 20.degree. C. to 1000.degree. C. is 15-23 .mu.m/m/.degree. C.
and the 0.2% proof strength a (N/mm.sup.2) measured at 900.degree.
C., the steel sheet thickness t (.mu.m) and the thermal expansion
coefficient .alpha. (.mu.m/m/.degree. C.) are in a relationship
satisfying the following inequality <1>.
.sigma..gtoreq.(-9.0875.times..alpha..sup.2+4.2913.times.10.sup.2.times..-
alpha.-3.82415.times.10.sup.3)/t <1>
[0161] As mentioned above, the thickness, thermal expansion
coefficient and high temperature proof strength of the steel sheet
are controlled in order to improve the cold-heat durability of the
honeycomb body (the durability in an environment with an
alternately repeating high temperature/low temperature atmosphere),
and provide a honeycomb body with excellent high temperature
durability in a steel sheet temperature range exceeding
1000.degree. C. Inequality <1> above is derived from numerous
experimental data, and the reasons for the limits are as
follows.
[0162] A thickness t of less than 10 .mu.m will lead to buckling
and crumbling, and therefore the lower limit is 10 .mu.m. At
greater than 40 .mu.m, the back pressure of the honeycomb body will
be high and the resistance will increase against gas passing
through the honeycomb body, and therefore the upper limit is 40
.mu.m.
[0163] The lower limit for the thermal expansion coefficient
.alpha. is 15 .mu.m/m/.degree. C. because at less than 15
.mu.m/m/.degree. C. the cold-heat properties will depend on the
proof strength and the relational inequality of the invention will
not apply. The lower limit for the thermal expansion coefficient
.alpha. is preferably 16 .mu.m/m/.degree. C. Also, if it exceeds 23
.mu.m/m/.degree. C., the heat stress will be too great and
sufficient durability will not be achieved even if the sheet has
the proof strength specified by the relational inequality of the
invention, and therefore the upper limit is 23 .mu.m/m/.degree.
C.
[0164] The range of inequality <1> was specified because when
the proof strength .sigma. was less than the value at the right of
inequality <1>, steel sheet tearing and collapsing occurred
more frequently if the environment was repeatedly alternated from a
high-temperature to low-temperature atmosphere, and therefore
adequate durability was clearly not achieved. A larger thermal
expansion coefficient of the steel sheet resulted in increased
thermal stress on the honeycomb body when the catalyst carrier was
used, and therefore the lower limit for the required proof strength
is also a large value.
[0165] The steel sheet and honeycomb body-forming steel sheet of
the invention may be an Fe--Cr--Al based steel sheet (foil)
comprising, by weight, the components Al: >6.5%-15% and Cr:
10-30%, with the balance consisting of Fe and unavoidable
impurities. For improved oxidation resistance, with a proof
strength .sigma. value satisfying inequality <1> above, the
steel sheet used is preferably one whose components are Si:
0.1-1.0%, Mn: .ltoreq.0.5%, Al: >6.5%-15% and Cr: 10-30%, with
the remainder consisting of Fe and unavoidable impurities.
[0166] An Al content of greater than 6.5% can improve the oxidation
resistance of the steel sheet. However, since the steel sheet may
become brittle if the Al content exceeds 15%, the upper limit is
10%.
[0167] A Cr content of 10% or greater can also improve the
oxidation resistance of the steel sheet. However, since the steel
sheet may become brittle if the Cr content exceeds 30%, the upper
limit is 25%.
[0168] Mn is limited to no greater than 0.5% in order to ensure
that the steel sheet will have oxidation resistance.
[0169] An Si content of 0.1% or greater can improve the oxidation
resistance of the steel sheet, and therefore the lower limit is
0.1. However, the steel sheet may become brittle if the Si content
exceeds 1.0%, and therefore the upper limit is 1.0%.
[0170] The steel sheet and honeycomb body-forming steel sheet of
the invention also preferably comprise either or both Ti: 0.02-0.1%
and Nb: 0.02-0.3%, as well as La: 0.01-0.1%, Ce: 0.01-0.1% and P:
0.01-0.05%.
[0171] By including either or both Ti at 0.02% or greater and Nb at
0.02% or greater, it is possible to improve the ductility of the
steel sheet. However, the upper limits are 0.1% for Ti and 0.3% for
Nb, because exceeding these limits may adversely affect the
oxidation resistance of the steel sheet.
[0172] By including La at 0.01% or greater and Ce at 0.01% or
greater, it is possible to improve the oxidation resistance of the
steel sheet. However, the upper limits are 0.1% for La and 0.1% for
Ce, because exceeding these limits can lead to cracking during hot
rolling.
[0173] A P content of 0.01% or greater has the effect of preventing
cracking during hot rolling when La and Ce are present. However,
the upper limit for the P content is 0.05%, because exceeding this
limit can lead to inferior oxidation resistance.
[0174] The steel sheet of the invention is preferably a steel sheet
for a metal catalyst carrier with excellent high temperature
durability.
[0175] By using a honeycomb body constructed using the steel sheet
of the invention as a catalyst carrier supporting a catalyst on the
cell surfaces of the honeycomb, it is possible to obtain a catalyst
carrier having excellent high temperature durability which can be
used under stringent conditions exceeding 1000.degree. C.
[0176] An exhaust gas purification catalyst-supporting honeycomb
body constructed using a high Al-containing Fe--Cr--Al based
stainless steel sheet according to the invention will now be
explained.
[0177] A honeycomb body is formed using the high Al-containing
Fe--Cr--Al based stainless steel sheet, or an alternately stacked
laminate of a high Al-containing flat double layered sheet and a
corrugated sheet obtained by corrugating the flat sheet, or a wound
cylinder comprising the flat sheet alternately stacked with the
corrugated sheet. Such honeycomb bodies are constructed by housing
these forms in a jacket and fitting and anchoring the honeycomb
body therein, or by bonding the points of contact between the flat
sheet and corrugated sheet of the honeycomb body.
[0178] Such steel sheets (foils) used may, of course, be fabricated
by the method of the invention described below.
[0179] Also, needless to mention, a honeycomb body may be obtained
in the same manner described above using a high Al-containing
Fe--Cr--Al based stainless steel sheet of the invention having
protrusions on the surface, a high Al-containing Fe--Cr--Al based
stainless steel sheet of the invention having gaps in the interior
or a high Al-containing Fe--Cr--Al based stainless steel sheet of
the invention having the proof strength specified by the
relationship between the steel sheet (foil) thickness and thermal
expansion coefficient.
[0180] When such a honeycomb body is used to make an exhaust gas
purification catalyst carrier, a wash coat layer is usually formed
on the honeycomb body surface for support of the catalyst, and the
catalyst is supported therein.
[0181] A process for fabrication of a high Al-containing Fe--Cr--Al
based stainless steel sheet of the invention with an Al content
exceeding 6.5% and a high Al-containing double layered sheet will
now be explained.
[0182] Since Fe--Cr--Al stainless steel having an Al content
exceeding 6.5% has a low hot workability and low hot rolled
ductility, it cannot be satisfactorily rolled in an ordinary
steel-making/rolling process without increasing the number of
passes during rolling. On the other hand, increasing the number of
passes naturally increases the rolling cost.
[0183] According to the invention, the Al content in the stainless
steel sheet is a low value prior to hot rolling, and an Al or Al
alloy coating is formed on the surface of the steel sheet at least
after completion of the hot rolling or after completion of cold
rolling and after foil rolling, to prepare a high Al-containing
double layered sheet wherein the average composition between the
base stainless steel sheet (base material) and the surface Al
coating is the composition of Fe--Cr--Al stainless steel according
to the invention.
[0184] The double layered sheet of the invention may be obtained by
adhering Al or an Al alloy on the surface of the Fe--Cr--Al
stainless steel sheet as the base material to form an Al coating,
by publicly known means including plating such as hot dipping or
electrolytic plating, or a dry process such as vapor deposition,
ion plating, sputtering, CVD or the like. The coating may be formed
on one side or both sides of the base material surface. Rolling can
be carried out without very high cost if the Al concentration of
the stainless steel sheet prior to the Al coating formation is no
greater than 8%, but rolling costs can be further reduced by
limiting the concentration to no greater than 6.5%. For use as a
double layered sheet, therefore, a high Al-containing double
layered sheet may be fabricated by rolling the base material used
to an appropriate sheet thickness, such as up to 40 .mu.m, to
prepare a steel sheet as the base material, and then forming an Al
coating thereover.
[0185] After forming the Al coating of the double layered sheet, or
after further rolling, the double layered sheet may be subjected to
diffusion annealing to diffuse the surface Al into the stainless
steel, to produce a stainless steel sheet having an Al content
according to the invention. Since the Al content is low during hot
rolling, it is possible to carry out satisfactory hot rolling
without increasing the number of passes for rolling. If cold
rolling or foil rolling is carried out prior to the diffusion
annealing, it will be possible to accomplish satisfactory rolling
without increasing the number of passes even for the cold rolling
or foil rolling.
[0186] In this case, the thickness of the stainless steel sheet
prior to the Al coating formation is preferably 0.005-2 mm. At less
than 0.005 mm, the rigidity of the sheet will be notably reduced,
making it difficult to form a honeycomb body. The upper limit is 2
mm because if the sheet thickness is greater than 2 mm, it will be
necessary to increase the Al coating thickness, leading to such
problems as peeling of the Al coating.
[0187] The process for fabricating the stainless steel sheet using
a double layered sheet of the invention or the process for
fabricating the honeycomb body may be selected from among the
following modes.
[0188] As a first fabrication process, the Al coating-adhered
stainless steel sheet may be fabricated as a high Al-containing
double layered sheet of the invention. This may be used directly to
form a honeycomb body, and then the honeycomb body may be subjected
to diffusion annealing for diffusion of Al into the stainless steel
sheet.
[0189] As a second fabrication process, a double layered sheet
comprising an Al coating-adhered stainless steel sheet may be
subjected to foil rolling to fabricate a high Al-containing
Fe--Cr--Al based stainless steel sheet of the invention. The steel
sheet (foil) may then be used to form a honeycomb body, and the
honeycomb body subjected to diffusion annealing for diffusion of Al
into the stainless steel sheet.
[0190] As a third fabrication process, a double layered sheet
comprising an Al coating-adhered stainless steel sheet may be
subjected to diffusion annealing for diffusion of Al into the
stainless steel sheet to fabricate a high Al-containing Fe--Cr--Al
based stainless steel sheet of the invention. The stainless steel
sheet may then be used to form a honeycomb body.
[0191] As a fourth fabrication process, a high Al-containing double
layered sheet comprising an Al coating-adhered stainless steel
sheet may be subjected to diffusion annealing for diffusion of Al
into the stainless steel sheet, and then the stainless steel sheet
subjected to foil rolling to obtain a high Al-containing Fe--Cr--Al
based stainless steel sheet of the invention. The steel sheet
(foil) may then be used to form a honeycomb body.
[0192] As a fifth fabrication process, a high Al-containing double
sheet comprising an Al coating-adhered stainless steel sheet may be
subjected to foil rolling, and then the foil subjected to diffusion
annealing for diffusion of Al into the stainless steel sheet to
obtain a high Al-containing Fe--Cr--Al based stainless steel sheet
of the invention. The steel sheet (foil) may then be used to form a
honeycomb body.
[0193] A process for fabrication of a high Al-containing Fe--Cr--Al
based stainless steel sheet (foil) having protrusions with a height
of 1 .mu.m or greater at a density of at least 100/cm.sup.2
according to the invention will now be explained.
[0194] The means for adhering Al onto the stainless steel sheet
surface may be a method of applying Al paint onto the steel sheet
surface. A paint comprising Al powder, a resin and a solvent is
prepared. The Al powder used may have a mean particle size of about
0.1-50 .mu.m. Using flakes as the Al powder can provide a more
preferred effect. The resin is necessary for anchorage onto the
cell wall surfaces after the solvent has evaporated by drying. The
resin used may be a common one such as an ethyl cellulose or phenol
resin. The solvent used may be industrial kerosene or xylene. The
amount of solvent used is important for management of the viscosity
of the paint. A satisfactory result is achieved if the viscosity of
the paint is kept between 10-5000 cp. The paint is applied onto the
steel sheet surface. The application method may be a method
involving immersion of the honeycomb body in the paint solution.
The paint-coated steel sheet is then heat treated. The heat
treatment atmosphere may be air or an inert atmosphere, but it is
preferably an inert atmosphere. In order to melt the Al powder, the
heat treatment temperature must be 600.degree. C. or higher. When
the resin is included in the paint, the heat treatment will result
in thermal decomposition and removal of the resin.
[0195] The heat treatment of the paint-coated steel sheet results
in melting of the Al powder in the paint and formation of numerous
molten Al droplets on the steel sheet surface. The components in
the steel sheet diffuse into the molten Al droplets from the
sections of the steel sheet in contact with the droplets, and these
components form an alloy with Al in the droplets and solidify. If
the temperature of the steel sheet is lowered in this state, the
droplets will form protrusions. The protrusions formed in this
manner are composed of metal. The composition of the protrusions
has a higher Al concentration than the Al concentration of the
steel sheet.
[0196] The protrusions on the steel sheet surface formed by the
method described above can have heights of 1 .mu.m or greater, and
the density of the protrusions may be 100/cm.sup.2 or higher.
[0197] A portion of the Al powder in the paint applied onto the
steel sheet surface forms numerous molten Al droplets on the steel
sheet surface as described above, eventually forming protrusions.
The rest of the Al powder melts during the heat treatment and
diffuses into the steel sheet, forming an alloy with the
constituents of the steel sheet.
[0198] For fabrication of a high Al-containing stainless steel
sheet according to the invention, generally a continuous cast slab
is subjected to hot rolling and then to cold rolling to obtain a
steel sheet. If the Al content of the cast slab before hot rolling
is greater than 6.5%, the hot rolling workability will be impaired,
and satisfactory rolling will not be possible. According to the
invention, the Al content of the cast slab before hot rolling was
kept to no greater than 6.5%, paint comprising Al powder was
applied onto the surface at the steel sheet stage, and heat
treatment accomplished diffusion of Al into the steel sheet,
thereby producing a steel sheet with an Al content of greater than
6.5%. An Al content exceeding 6.5% will allow the steel sheet to
exhibit very satisfactory oxidation resistance.
[0199] The application of Al powder onto the steel sheet surface
may be carried out on the steel sheet prior to formation of the
honeycomb body, as explained above, but it is more preferably
carried out on the steel sheet after formation of the honeycomb
body. The application of Al powder onto the surface of the steel
sheet forming the honeycomb body may be accomplished as described
above, by immersing the honeycomb body in a paint comprising the Al
powder, resin and solvent. Alternatively, an adhesive may be
applied onto the surface of the steel sheet forming the honeycomb
body, and then Al powder sprinkled on the honeycomb body to adhere
the Al powder onto the adhesive-coated sections of the steel sheet
surface. By heat treating the honeycomb body after application of
the Al powder, it is possible to form protrusions on the surface of
the steel sheet forming the honeycomb body, in a similar manner as
the steel sheet described above, while simultaneously increasing
the Al content of the steel sheet. The heat treatment may be
conducted under the same conditions as the heat treatment for the
steel sheet described above, or it may be combined with the heat
treatment for brazing of the steel sheet contact sections of the
honeycomb body.
[0200] A process for fabrication of a steel sheet having a steel
sheet surface roughness Ra of 2 .mu.m or greater will now be
explained.
[0201] Al is vacuum vapor deposited on the surface of the steel
sheet. The vapor deposition thickness may be an average thickness
of 1 .mu.m or greater under conditions with a film-forming speed of
at least 0.5 .mu.m/min. The steel sheet is heat treated after the
Al vapor deposition. Heat treatment of the steel sheet having an Al
coating on the surface can roughen the surface of the steel sheet
and provide a roughness degree Ra of 2 .mu.m or greater. The heat
treatment conditions may be, for example, heat treatment for 2
hours in a vacuum atmosphere at about 1000.degree. C. The heat
treatment may be carried out on the steel sheet prior to honeycomb
body formation, or after formation of the Al-deposited steel sheet
into a honeycomb body.
[0202] The Al coating formed on the steel sheet surface has an
effect of increasing the surface roughness of the steel sheet as
mentioned above, but the diffusion of Al into the steel sheet
during heat treatment also has an effect of increasing the Al
content of the steel sheet. Thus, by limiting the Al content before
hot rolling to no greater than 6.5% and heat treating the steel
sheet after forming the Al coating, it is possible to increase the
Al content of the steel sheet to above 6.5%.
[0203] The means for forming the Al coating on the surface of the
steel sheet may be vacuum vapor deposition of Al on the steel sheet
surface, or Al plating on the surface of the metal sheet before it
is foil rolled, followed by foil rolling.
[0204] This method may be employed to increase the surface
roughness Ra of the steel sheet to 2 .mu.m or greater.
[0205] A process for fabrication of a high Al-containing Fe--Cr--Al
based stainless steel sheet (foil) of the invention having isolated
gaps in the interior will now be explained.
[0206] When an Al alloy is coated onto the stainless steel sheet
serving as the base material and heat treatment is carried out to
diffuse the Al on the surface into the stainless steel sheet base
material, careful formulation of the alloy composition will allow
formation of gaps in the steel sheet interior after diffusion. The
gaps are formed by the difference in the diffusion rate of each
element composing the Al alloy on the surface into the base
material, and they are known as Kirkendall voids, which are formed
near the bonding interface between the base material and the Al
alloy. The present inventors succeeded in fabricating a steel sheet
having isolated gaps in the interior according to the invention by
controlling formation of the gaps.
[0207] The stainless steel sheet serving as the base material
comprises Cr at 10-30%, with an Al content of no greater than 6.5
wt %. It may also be free of Al. By coating the base material
surface with an Al alloy and then subjecting it to diffusion heat
treatment, Al will diffuse into the steel sheet, yielding the
prescribed Al content of, for example, greater than 6.5% and no
greater than 15%. The Al content of the base material is limited to
no greater than 6.5 wt % because a content of no greater than 6.5%
will allow hot rolling and cold rolling to be satisfactorily
carried out.
[0208] The components of the Al alloy applied to the surface of the
base material are selected based on the following concept.
Specifically, gaps according to the invention can be effectively
formed during the diffusion heat treatment by using an Al alloy
composition wherein the inward diffusion rate of each atom of the
applied Al alloy into the steel sheet is as different as possible
from the outward diffusion rate of each atom of the base material
toward the steel sheet surface. In order to form gaps closer to the
surface of the steel sheet, it is sufficient if the inward
diffusion rate is greater than the outward diffusion rate. For this
purpose, an element may be included which is an element from the
applied Al alloy having a relatively fast rate of diffusion in Al,
and mixing relatively uniformly with low segregation in the base
material. For example, Si, Be, Co, Cr, Mg, Zr and the like are
highly effective for forming gaps according to the invention. An Al
alloy containing these added elements can be applied onto the base
material relatively easily.
[0209] The base material may be used directly in the form of a flat
steel sheet, or it may be used in the form of a honeycomb body. The
Al alloy is applied to the surface of the base material by hot
dipping, electrolytic plating, powder coating, a dry process or the
like. The thickness of the application is determined based on the
Al composition of the base material and on the target Al
composition to be finally obtained as a result of the heat
treatment diffusion. Diffusion heat treatment of the base material
with the Al alloy applied to the surface results in formation of
isolated gaps inside the steel sheet.
[0210] Since the positions in which the gaps are formed in the
direction of thickness of the steel sheet are near the interface
between the base material and the Al alloy, the thickness of the
base material and the thickness of the applied Al alloy may be
controlled for adjustment. When the positions of the gaps are
positioned near the surface of the steel sheet, the Al content of
the base material may be increased and the applied Al alloy
thickness reduced. In this manner, it is possible to control the
positions of the gaps in the sheet thickness direction, while
maintaining the target Al content after diffusion heat treatment.
Alternately repeating the Al alloy application and the diffusion
heat treatment will allow control of the thickness of the gap layer
in the direction of the sheet thickness. Thus, it is possible to
fabricate a steel sheet having gaps at preferred positions in the
direction of thickness, as explained above.
[0211] The gaps of the invention are formed by diffusion heat
treatment after application of the Al alloy into the base material,
and the sizes of the gaps may be controlled by adjusting the
temperature and time of the diffusion heat treatment. If the heat
treatment is carried out in a temperature range wherein the inward
diffusion rate of each atom of the applied Al alloy is greater than
the outward diffusion rate of each element in the base material,
the sizes of the gaps can be relatively easily controlled by the
heat treatment time. When finer gaps are to be formed, the control
will be accomplished more easily by approximately matching the
inward diffusion rate and the outward diffusion rate. The distances
between the gaps are controlled mainly by the heat treatment time.
The heat treatment time may be shortened in order to obtain
narrower distances, and the heat treatment time may be lengthened
in order to obtain wider distances.
[0212] The thickness of the base material for application of the Al
alloy may be about 10 .mu.m to about 500 .mu.m. If the thickness of
the base material before Al application is greater than 70 .mu.m,
it may be reduced to the prescribed thickness by rolling or the
like with the Al alloy applied to the base material, and heat
treatment carried out to form the gaps while diffusing the Al.
Formation of the prescribed gaps by heat treatment may be followed
by rolling or the like in order to reduce the sheet thickness to a
prescribed value, but in this case the rolling percentage is
preferably no greater than about 50% so as to avoid crushing the
formed gaps.
[0213] A process for fabrication of a high Al-containing Fe--Cr--Al
based steel sheet (foil) according to the invention will now be
explained, wherein the proof strength condition required for an
Fe--Cr--Al based steel sheet (foil) to be used in a honeycomb body
of an exhaust gas purification catalyst carrier is specified by the
relationship between the thickness and the thermal expansion
retention of the steel sheet (foil).
[0214] It is known that for fabrication of an Fe--Cr--Al based
stainless steel sheet, hot rolling becomes difficult to accomplish
if the Al content before steel sheet rolling is greater than 6.5%.
The preferred component range of the steel sheet of the invention
is an Al content of >6.5%-15%, but it is difficult to produce a
steel sheet having such a high Al concentration by direct hot
rolling from a cast slab comprising the components described
above.
[0215] According to the invention, a steel sheet with an Al content
of less than 6.5% (hereinafter referred to as "base material") or a
honeycomb body employing the steel sheet is formed, Al is adhered
to the steel sheet of the base material or the steel sheet surface
of the honeycomb body, and then diffusion heat treatment is carried
out to increase the Al content in the steel sheet, in order to
yield Fe--Cr--Al based stainless steel comprising Al as required
for the invention. The Al adhesion is carried out during the cold
rolled sheet stage before foil rolling, and it may be diffused at
that stage, or the Al-adhered cold rolled sheet may be subjected to
foil rolling. The steel sheet before Al adhesion has an Al content
of less than 6.5%, and is therefore suitable for hot rolling. The
relationship between the Al adhesion thickness and the base
material thickness may be determined based on the difference
between the amount of Al in the base material and the target amount
of Al after heat diffusion, so that the difference in the Al
amounts matches the amount of Al enriched by diffusion.
[0216] When the steel sheet is used for fabrication of a honeycomb
body to be employed as a catalyst carrier, flat and corrugated
sheets of the steel sheet are alternately wound or laminated into a
honeycomb form, and the points of contact between the flat and
corrugated sheets of the honeycomb body are brazed to form a firm
honeycomb body. The bonding strength at the brazed sections of the
honeycomb body of the invention is preferably 5 t (N/cm) or greater
per centimeter of bonding line, where t (.mu.m) is the thickness of
the steel sheet used. The bonding method used may be a diffusion
bonding method instead of a brazing method.
[0217] On the other hand, if Al is adhered on the surface of the
stainless steel sheet serving as the base material, a multilayer
structured steel flat sheet and corrugated sheet are wound or
laminated to form a honeycomb body and the points of contact
between the flat and corrugated sheets are bonded so that the Al
and brazing material on the stainless steel sheet surface react
during the high-temperature heat treatment for brazing, producing
high-melting-point intermetallic compounds which often impair the
bondability at the brazed sections.
[0218] According to the invention, one of the following three
processes a) to c) is selected as the process for fabrication of
the honeycomb, in order to allow fabrication of a honeycomb body
with excellent bonding strength at the brazed sections.
[0219] a) According to this process, a base material with a low Al
content is brought through to the cold rolling stage, an Al coating
is formed on the cold rolled sheet surface, and the cold rolled
sheet is heat treated in a high-temperature atmosphere for
diffusion of Al into the base material, after which foil rolling is
carried out, the steel sheet is used to form a honeycomb body, and
the points of contact between the steel sheet of the honeycomb body
are brazed. Since the steel sheet has a low Al content up to the
cold rolling stage, it is possible to satisfactorily carry out hot
rolling. Also, since an Al coating is formed on the cold rolled
steel sheet surface prior to diffusion treatment, it is possible to
ensure a suitable Al content according to the invention in the
steel sheet. The Al coating is not present on the steel sheet
surface during honeycomb body formation, and therefore firm bonded
sections can be formed by brazing in the subsequent step. The
method for forming the Al coating on the surface of the cold rolled
steel sheet may be a method of hot dipping, electrolytic plating,
powder coating, a dry process (vapor deposition, etc.) or the
like.
[0220] b) According to this process, a base material with a low Al
content is brought through to the foil rolling stage, an Al coating
is formed on the surface of the steel sheet base material, and on
the exterior there is formed a metal layer, such as an Fe layer,
which reacts with the brazing material at a higher temperature than
Al and can be brazed. The steel sheet having such a multilayer
structure is used to form a honeycomb body, and then the points of
contact between the steel sheets are bonded by brazing. Since the
exterior of the Al coating is coated with an Fe layer so that the
surface of the Al coating is not exposed, the brazing material at
the bonding sections does not contact directly with the Al coating,
and reaction between the brazing material and Al can thereby be
controlled.
[0221] c) According to this process, first a steel sheet is
prepared using a base material with a low Al content, the steel
sheet is used to form a honeycomb body, and the points of contact
between the steel sheet of the honeycomb body are brazed. Al powder
is then adhered to the surface of the steel sheet of the honeycomb
body, and the entire honeycomb body is subjected to
high-temperature heat treatment for diffusion of the Al into the
steel sheet. Since no Al coating is present on the surface of the
steel sheet when the steel sheet is used to form the honeycomb
body, and the points of contact between the steel sheet are
maintained in a satisfactory state of contact during the brazing
stage, it is possible to accomplish brazing in a sufficient manner.
The Al powder is subsequently adhered onto the surface of the steel
sheet and subjected to diffusion heat treatment, and therefore the
steel sheet of the honeycomb body has an Al content which is
suitable for the invention. The method of adhering the Al powder
may be carried out by immersing the honeycomb body in a paint
comprising the Al powder and a solvent, or by applying an adhesive
onto the cell surfaces of the honeycomb body and then sprinkling
the Al powder onto the honeycomb structure to adhere the Al powder
onto the adhesive-applied sections of the cell surfaces.
[0222] In process c) above, the Al content of the stainless steel
sheet before the honeycomb body formation is no greater than 6.5%,
and the Cr content is 10-30%.
[0223] Also, the Fe--Cr--Al based stainless steel sheet (foil) of
the honeycomb body prior to formation of the Al-adhered layer
further comprises, by weight, Si: 0.1-1.0% and Mn: .ltoreq.0.5%, or
may further comprise either or both Ti: 0.02-0.1% and Nb:
0.02-0.3%, as well as La: 0.01-0.1%, Ce: 0.01-0.1% and P:
0.01-0.05%, as explained above.
[0224] The adhering Al powder may also contain at least one from
among Si, Ca, Sr, Y, Zr, Ba, La, Mg, Ce, Hf and Ta.
[0225] Process c) will now be explained in greater detail.
[0226] As shown in FIG. 1, in this method the metal Al powder is
adhered onto the surface of the Fe--Cr--Al based alloy steel sheet
composing the honeycomb body, and the Al is alloyed with the base
material components during the heating process so that it diffuses
into the base material to increase the Al concentration of the base
material. The important aspect of the invention is that metal Al is
adhered to the honeycomb body after the honeycomb body has been
constructed from at least the flat and corrugated steel sheets, or
from corrugated steel sheets.
[0227] Although heat treatment is usually carried out for a
honeycomb body in order to bond the flat sheet and corrugated
sheet, the adhesion of metal Al is carried out before or after the
bonding heat treatment. When the bonding method is brazing, and the
Al adhesion is carried out after the bonding heat treatment, the
metal Al may also be adhered in the same manner to the brazing
sections, as shown in FIG. 2. When it is carried out before the
bonding heat treatment, the metal Al may be adhered after the
brazing material has been situated at the brazing sections.
[0228] The honeycomb body fabrication process may be divided
generally into a binder coating step, a powder adhering step, a
drying step and a heat treatment step. Of these, the binder coating
step and powder adhering step are accomplished by a single combined
step of applying an Al powder paint. The drying step and heat
treatment step may also be accomplished by a single combined step
of drying included within the heat treatment step.
[0229] 1) Binder Coating Step (FIG. 3)
[0230] This is a step of coating the binder to adhere the Al powder
onto the honeycomb body. The binder used may be a binder solution
comprising a binder component which can affix the powder after
evaporation of the solvent, such as an aqueous solution of PVA or
an acrylic acid-based polymer, or an organic binder comprising
ethyl cellulose dissolved in an organic solvent.
[0231] As one example of a coating method, the honeycomb is
immersed in a PVA aqueous solution to adhere the binder onto the
cell walls of the honeycomb, after which the excess binder solution
in the cells must be removed. The removal method is preferably, as
shown in FIG. 4, a method of casting a high-speed gas flow against
the honeycomb body in the direction of the cell lengths and air
blowing the excess binder solution, or, as shown in FIG. 5, a
method of subjecting the honeycomb to centrifugal force in the
direction of the cell lengths to spin off the excess binder
solution.
[0232] 2) Al Powder Dispersing Step
[0233] This is a step of adhering the powder onto the cell walls of
the honeycomb body coated with the binder solution. As shown by the
example in FIG. 6, the powder is dispersed from at least one side
of the honeycomb. If the form of the powder used, and particularly
the form of the Al powder used, is a flake powder having a particle
size/thickness ratio of 10 or greater and a particle size of 1
.mu.m or greater based on spherical size, the powder will aggregate
less easily and therefore the flow property of the powder will be
satisfactory and uniform coating will be accomplished on the
coating surface, thereby giving a desirable result. With a particle
size exceeding 50 .mu.m based on spherical size, the flow property
will be adequate but the powder will be large with respect to the
sheet thickness, thereby increasing the heat capacity of the
honeycomb. If possible, the particle size based on spherical size
is more preferably no greater than the sheet thickness.
[0234] 3) Al Powder Paint Application Step
[0235] This is a process in which steps 1) and 2) are carried out
simultaneously, with the advantage of shortening the process.
Specifically, this is realized by dispersing the Al powder in a
solvent to prepare a paint, dipping the honeycomb in the paint in
the same manner as step 1) shown in FIGS. 4 to 6, and removing the
excess slurry in the cells. The Al paint is generally composed of
Al powder, a resin (ethyl cellulose, acryl, phenol or the like) and
a solvent. A satisfactory result can be obtained if the viscosity
of the Al paint is kept between 10-5000 mPas. If the paint used
comprises flaky Al powder with a particle size/thickness ratio of
10 or greater, it will be possible to regularly and uniformly cover
the cell wall surfaces of the honeycomb body with the Al powder,
for a suitable result. The Al particle size is preferably 1 .mu.m
or greater based on spherical size. For the same reason mentioned
above, the upper limit is no greater than 50 .mu.m and preferably
no greater than the sheet thickness.
[0236] The method for removing the excess paint is preferably a
method using an air blower, or a method of subjecting the honeycomb
to centrifugal force in the direction of the cell lengths to spin
off the excess binder solution.
[0237] 4) Drying Step and Heat Treatment Step
[0238] This is a step wherein the honeycomb body in which the Al
powder has been accumulated on the cell walls using a combination
of steps 1) and 2) above, or step 3), is dried and heat treated.
The drying step is a step of evaporation or thermal decomposition
of the binder or slurry solvent component and the binder component,
and it may be included within the heat treatment step. The heat
treatment step is a step of dispersing the Al into the steel sheet
of the honeycomb body to increase the Al concentration of the steel
sheet.
[0239] During the course of the heat treatment, Al enrichment
proceeds in the following manner, as illustrated in FIG. 7. First,
the Al adhered to the cell walls melts. Next, the components in the
base material elute into the molten Al and increase the Fe and Cr
concentration in the molten Al. The amounts of Fe and Cr which can
elute into the liquid phase Al are respectively limited, and once
they have eluted to their limits, intermetallic compounds are
formed at the interface between the liquid phase and the base
material, this region gradually spreads, and the intermetallic
compounds eventually spread to the areas in which the Al powder was
originally present.
[0240] The final Al content of the steel sheet composing the
honeycomb body after Al enrichment is preferably, on average,
greater than 6.5 wt % and no greater than 15 wt %. It is not
possible to achieve sufficient oxidation resistance at 6.5 wt % or
less in a steel sheet with a thickness of less than 40 .mu.m.
[0241] Specifically, the amount of Al powder which must be adhered
is determined by the following formula. 0.06.ltoreq.{(weight of
honeycomb body).times.(Al wt % of base material)/100+(weight of
coated Al)}/{(weight of honeycomb body)+(weight of coated
Al)}.ltoreq.0.15
[0242] For control of the Al concentration in the base material it
is necessary to adequately control the amount of adhering Al. For
this purpose, the Al amount is controlled by the air flow from the
air blower or by the degree of centrifugal force, as described
above, and by managing the viscosity of the Al paint or binder.
[0243] Another reason to control the amount of adhering Al is to
prevent the unbonded sections from bonding when the Al melts. A
metal honeycomb body used as an automobile exhaust gas purification
catalyst normally has a construction wherein only the necessary
points of contact between the steel sheet are bonded while the
other regions are not bonded, in order to inhibit internal
deformation when an internal thermal gradient is produced. However,
when excess Al is supplied to the regions near the contacts between
the steel sheet, the steel sheet strips become bonded together, as
disclosed in U.S. Pat. No. 4,602,001. If the average Al content
exceeds 15 wt %, more of the points of contact between the steel
sheets which are not meant to be bonded will be bonded, and the
bonded structure designed to control deformation will not be
realized.
[0244] Another means for preventing bonding of the sections which
are designed to be non-contact regions employs flaky Al powder with
a particle size/thickness ratio of 10 or greater. In particular,
when an Al paint is used to adhere the Al powder, the paint will
tend to pool near the points of contact between the steel sheets,
increasing the Al adhesion near the points of contact. Using flaky
Al powder allows the Al to evenly adhere to the cell walls and
thereby alleviate notable adhesion of Al at the points of contact
between the steel sheets, in order to eliminate the drawback of
bonding at the sections which are not to be bonded. The average Al
content can therefore exceed 15 wt %.
[0245] The Al content in the base material before Al enrichment is
preferably 2 wt % or greater. At less than 2 wt %, irregularities
in the Al powder application will prevent the Al from adequately
diffusing into the base material, resulting in a low Al
concentration in those regions and potentially leading to partial
poor oxidation resistance, but an Al content of 2 wt % or greater
in the base material before Al enrichment will result in formation
of a strong alumina coating on the surface during the initial use
at high temperature, even in those regions. Oxidation after
formation of an alumina coating proceeds very slowly. Although Al
is certainly consumed as oxidation proceeds in those regions, Al
diffuses at a faster rate from the regions of high Al concentration
during use so that the Al consumed in those regions is replaced,
and therefore oxidation resistance can be maintained even with
coating irregularities. Conversely, if the Al content in the base
material before Al enrichment exceeds 8 wt % the base material
fabrication cost will increase, and therefore the upper limit is 8
wt %.
[0246] This process is particularly suited for a honeycomb body
composed of an Fe--Cr--Al based alloy steel sheet with a thickness
of no greater than 40 .mu.m. For a honeycomb body formed from a
steel sheet with a thickness of greater than 40 .mu.m, it is
suitable in most cases to simply use a steel sheet with an Al
content obtained by the prior art, without any special treatment.
However, for use under the particularly severe oxidizing condition
immediately under an engine, the Al content obtained by the prior
art is inadequate even if a steel sheet with a thickness of greater
than 40 .mu.m is used, and in such cases the present invention is
highly useful. If the sheet thickness is less than 5 .mu.m, the
rigidity of the steel sheet is reduced and mass production of the
honeycomb body becomes difficult, and therefore the lower limit for
the sheet thickness is preferably 5 .mu.m.
[0247] The present invention is a process for accumulating Al
powder on the surface of a steel sheet composing a honeycomb body
and converting it to an alloy, and when the steel sheet of the
honeycomb body is Al-enriched by this process, protrusions are
formed on the surface which reflect the shape of the Al powder, as
shown in FIG. 8(a) and FIG. 8(b). Although a steel sheet formed by
rolling is ordinarily flat, the process of the invention forms
protrusions on the surface and the protruding sections improve the
cohesion of the wash coat (.gamma.-alumina) and increase the
surface area of the honeycomb body cell walls, thereby providing
the advantage of allowing effective use of the catalyst. Even when
flaky Al powder is used, the flakes are converted to spheres by
surface tension when the Al powder is melted during the heat
treatment step, and therefore similar protrusions are formed. The
protrusions are effective for preventing detachment of the
.gamma.-alumina when the catalyst is loaded onto the metal
honeycomb body.
[0248] Incidentally, a honeycomb body comprising a laminated
stainless steel sheet is formed by stacking and winding a flat
stainless steel sheet with a corrugated sheet obtained by
corrugating the flat sheet, or alternately laminating them. All or
a portion of the contact sections between the flat sheet and
corrugated sheet are bonded by brazing. A honeycomb body formed in
this manner is fitted into an jacket made of the same stainless
steel, and all or a portion of the contact sections between the
honeycomb body and the jacket are also bonded by brazing to obtain
a metal carrier. A catalyst supporting layer is formed, wherein the
surface of the steel sheet of the honeycomb body cells serving as
the gas passageways in the metal carrier is impregnated with the
catalyst, to produce a catalyst carrier.
[0249] When the catalyst carrier produced in this manner is used as
an internal combustion engine exhaust gas purification catalyst
carrier, and particularly when used directly under an engine, the
temperature of the catalyst carrier increases to about 1100.degree.
C. at the exhaust gas inlet end of the catalyst carrier. In order
to achieve a long usable life of the honeycomb body in such a
high-temperature environment, the Al content of the Fe--Cr--Al
based stainless steel sheet forming the honeycomb body is
preferably greater than 6.5 wt % and no greater than 15 wt %. The
Al content is preferably greater than 6.5 wt % because sufficient
oxidation resistance cannot be obtained with a lower content, and
it is preferably no greater than 15 wt % because at a higher
content the foil becomes brittle and tends to break easily.
[0250] At the exhaust gas outlet end of the catalyst carrier,
however, the temperature of the catalyst carrier only increases up
to about 1000.degree. C. at most. At this temperature level, an Al
content of 3 wt % or greater and lower than at the inlet end in the
Fe--Cr--Al based stainless steel sheet forming the honeycomb body
will ensure adequate oxidation resistance. As a result of lowering
the Al content, the thermal expansion coefficient of the stainless
steel sheet at the exhaust gas outlet end can be lowered. The Al
content is 3 wt % or greater because oxidation resistance must be
ensured even at the exhaust gas outlet end, and the content is less
than at the inlet end because exceeding it will excessively
increase the thermal expansion coefficient and lead to breakage of
the honeycomb body.
[0251] The temperature distribution in the radial direction of the
catalyst carrier for exhaust gas purification is a temperature
distribution which is higher at the center and lower at the outer
periphery. The flow rate distribution of the exhaust gas tends to
be non-uniform in the radial direction, with a higher exhaust gas
flow rate at the center and therefore increased catalytic reaction
activity and increased temperature at the center. On the other
hand, since the exhaust gas flow rate is relatively lower at the
periphery, while heat is emitted outward through the jacket, the
temperature at the outer periphery is lower than at the center.
When the thermal expansion of the catalyst carrier in the axial
direction of the catalyst carrier is considered, the thermal
expansion in the axial direction is non-uniform in the radial
direction due to the temperature distribution in the radial
direction, and therefore the degree of thermal expansion at the
center is high while the degree of thermal expansion at the outer
periphery is low. This non-uniformity of the thermal expansion in
the radial direction produces thermal stress in the catalyst
carrier. In a catalyst carrier having a high Al content throughout
the entire honeycomb body as according to the prior art, the high
Al content results in a high thermal expansion coefficient, such
that the thermal stress due to temperature non-uniformity is also a
large value, and consequently the honeycomb body is prone to
breakage due to thermal stress.
[0252] According to the invention, the Al content of the stainless
steel sheet at the exhaust gas outlet end is a low value as
explained above, and therefore the thermal expansion coefficient of
the honeycomb body as a whole is low, and the value of the thermal
stress can be reduced. As a result, breakage of the honeycomb body
due to thermal stress caused by temperature non-uniformity can be
prevented.
[0253] The distribution of the Al content in the axial direction of
the honeycomb body may be such that the Al content changes in a
linear fashion from the exhaust gas inlet end to the exhaust gas
outlet end, or it may be such that the Al content changes in a
curved fashion from the exhaust gas inlet end to the exhaust gas
outlet end. Alternatively, the Al content may be high only near the
exhaust gas inlet end. The Al content may also change in a stepwise
fashion from the exhaust gas inlet end to the exhaust gas outlet
end. When the change is in a stepwise fashion, it may be in two,
three or more stages. If in two stages, the stepwise change in the
Al content may be at a position 1-50% from the exhaust gas inlet
end, where 100% is the length in the axial direction of the
catalyst carrier.
[0254] The Al content of the stainless steel sheet before formation
of the honeycomb body is a value lower than the value at the
sections with the lowest Al content of the Al content in the
honeycomb body to be used as a catalyst carrier. Also, if the Al
content of the steel sheet exceeds 6.5%, productivity may be
compromised by a greater number of passes required for rolling, and
therefore the Al content of the stainless steel sheet prior to
formation of the honeycomb body is preferably no greater than
6.5%.
[0255] Creation of a different Al content in the stainless steel
sheet composing the honeycomb body at the exhaust gas inlet end and
exhaust gas outlet end may be accomplished by adhering Al onto the
surface of the stainless steel sheet after formation of the
honeycomb body and producing a different amount of Al adhesion at
the exhaust gas inlet end and exhaust gas outlet end. If the
honeycomb body is heat treated after Al adhesion, the adhered Al
will melt and diffuse into the stainless steel sheet, thereby
increasing the Al content in the stainless steel sheet. Also, a
difference in Al content in the stainless steel sheet may be
produced based on the difference in amounts of Al adhesion at
different locations.
[0256] The Al may be adhered to the surface of the stainless steel
sheet by a method of applying Al paint onto the steel sheet
surface. A paint comprising Al powder, a resin and a solvent is
prepared. The Al powder used may have a mean particle size of about
0.1-50 .mu.m. Using flakes as the Al powder can provide a more
desirable effect. The resin is necessary for anchorage onto the
cell wall surfaces after the solvent has evaporated by drying. The
resin used may be a common one such as an ethyl cellulose or phenol
resin. The solvent used may be industrial kerosene or xylene. The
amount of solvent used is important for management of the viscosity
of the paint. A satisfactory result is achieved if the viscosity of
the paint is kept between 10-5000 mPas. The paint is applied onto
the steel sheet surface. The application method may be a method
involving immersion of the honeycomb body in the paint solution.
The paint-coated honeycomb body is then subjected to centrifugation
or air blowing to remove the excess paint. The removal is carried
out so that the paint flows from the exhaust gas outlet end toward
the exhaust gas inlet end. As a result, the paint adhesion
thickness at the exhaust gas outlet end is reduced, while the paint
adhesion thickness at the exhaust gas inlet end is increased. The
paint is dried, and then the honeycomb body is heat treated. The
heat treatment atmosphere may be air or an inert atmosphere, but it
is preferably an inert atmosphere. In order to melt the Al powder,
the heat treatment temperature must be 600.degree. C. or higher.
When the resin is included in the paint, the heat treatment will
result in thermal decomposition and removal of the resin. The Al
adhering to the surface during heat treatment diffuses into the
stainless steel sheet, thereby increasing the Al content of the
stainless steel sheet in correspondence to the amount of Al
adhesion. The heat treatment may be conducted at the same time as
the heat treatment for brazing of the honeycomb body steel sheet
contact sections.
[0257] For stepwise change in the Al concentration in the axial
direction of the honeycomb body, two different Al paints with
different Al concentrations are prepared, and the exhaust gas inlet
end part of the honeycomb body is immersed in the high
Al-concentration Al paint while the exhaust gas outlet end part of
the honeycomb body is immersed in the low Al-concentration Al
paint. Alternatively, the exhaust gas inlet section alone may be
immersed in the paint.
[0258] The method employed to adhere the Al to the stainless steel
sheet surface may instead be vapor deposition. This method involves
dissolution and vaporization with an electron gun for adhesion of
Al onto the stainless steel sheet set above the vaporization
source. Utilizing the fact that the Al vaporization rate is fastest
directly above the electron gun while the vaporization rate is
slower further from the area directly above the electron gun in the
horizontal direction, it is possible to form the coating thicker
directly above and thinner around the peripheral areas.
Specifically, a steel sheet having a width approximately equal to
the length of the honeycomb body may be placed with the section
corresponding to the exhaust gas inlet end situated directly above
the vaporization source, and the section corresponding to the
outlet end situated away from the vaporization source, in order to
fabricate a steel sheet having a different Al content in the
widthwise direction.
[0259] If the steel sheet fabricated by this process is corrugated
and wound together with a flat sheet and the honeycomb body
subjected to brazing treatment, it will be possible to produce a
honeycomb body having a higher Al content at the exhaust gas inlet
end than at the outlet end.
[0260] An additional effect of the present invention is that, since
the Al-enriched sections are mainly at the exhaust gas inlet end,
the manufacturing cost can be advantageously reduced compared to
processes wherein the entire honeycomb body is Al-enriched.
EXAMPLES
[0261] The present invention will now be explained in greater
detail by examples.
Example 1
[0262] A 30 .mu.m thick steel sheet composed of Fe--Cr--Al based
stainless steel having the composition shown in Table 1 was
prepared and used to form a honeycomb body.
[0263] A 50 kg ingot was melted and subjected to hot rolling, cold
rolling and foil rolling to obtain a 30 .mu.m thick steel sheet.
For a steel sheet with an Al content of 8.0 wt % or less, the
components of the ingot were matched to the components of the
desired steel sheet, and rolling was carried out to complete the
steel sheet. For a steel sheet with an Al content of greater than
8.0%, an Al content of 8.0% was used for the ingot, the contents of
the components other than Al were matched to the target contents of
those components in the steel sheet, and the ingot was rolled to a
sheet thickness of 30 .mu.m, after which Al was adhered to the
surface of the steel sheet by vacuum vapor deposition followed by
diffusion annealing, and finally rolling was carried out slightly
to prepare a 30 .mu.m thick steel sheet. The amount of adhesion of
the vapor deposited Al was selected so that the Al content of the
steel sheet after diffusion annealing was according to the
composition shown in Table 1.
[0264] Flat and corrugated forms of the steel sheet prepared above
were stacked and wound to prepare a honeycomb body with a honeycomb
body length of 80 mm and a honeycomb diameter of 60 mm. The cell
density was 400 cpsi (wave pitch: 2.5 mm, wave height: 1.25 mm).
The honeycomb body was fitted into a jacket and subjected to
brazing. At the exhaust gas inlet end edge, the contact sections
between the flat and corrugated sheets were brazed to a depth of 20
mm. At the exhaust gas outlet end edge, the contact sections
between the honeycomb body core and jacket were brazed to a depth
of 25 mm. The outer periphery of the honeycomb body had a
peripheral reinforcing layer formed by brazing the contact sections
between the flat and corrugated sheets to a 3-layer portion from
the periphery, to form a "portal" structure.
[0265] The hot rolling workability of the steel sheet was judged
based on the hot rolling success rate during the hot rolling. The
success rate was judged by hot rolling a 50 kg ingot 20 times, with
hot rolling success defined as successful rolling to a 3 mm
thickness without cracks during the hot rolling. A hot rolling
success rate of 80% was designated as satisfactory.
[0266] For the brazing property of the honeycomb body, the brazed
metal carrier was mounted in an engine and an engine heating cycle
test was carried out with 1000 cycles, where each cycle consisted
of 10 minutes of engine running with the exhaust gas temperature at
the metal carrier inlet end at 1000.degree. C., followed by 10
minutes of engine rest, upon which the degree of displacement of
the core toward the exhaust gas outlet end was noted.
[0267] The results are shown in Table 1.
[0268] Sheet Nos. 1-13 of the invention and comparison sheet No. 14
all exhibited satisfactory hot rolling success rates. Comparison
sheet No. 15 exceeded the upper limit for the Cu content, and
exhibited a hot rolling success rate of 0%. Comparison sheet No. 16
exceeded the upper limit for the total of the Zn, Sn, Sb, Bi and Pb
contents, and exhibited a hot rolling success rate of 0%.
[0269] As regards improvement in wettability during brazing, Sheet
Nos. 1-7 of the invention which contained Cu, sheet Nos. 8-10, 12
and 13 which contained Cu and Mg and sheet No. 11 which contained
Mg all exhibited satisfactory brazing properties. Comparison sheet
No. 14 which contained no Mg and had a Cu content below the lower
limit had an unsatisfactory brazing property. Comparison sheet Nos.
15 and 16 which had 0% hot rolling success rates could not be
evaluated in the brazing test. TABLE-US-00001 TABLE 1 Stainless
steel sheet components (wt %) No. C N Cr Al Si Mn P S Ti Nb Present
1 0.0045 0.0065 21.0 7.31 0.24 0.42 0.034 0.0009 0.021 0.001
invention 2 0.0031 0.0074 21.4 7.21 0.45 0.41 0.046 0.0008 0.023
0.034 sheet No. 3 0.0024 0.0054 19.5 6.58 0.29 0.20 0.046 0.0012
0.034 0.005 4 0.0072 0.0073 22.5 7.75 0.35 0.31 0.038 0.0004 0.085
0.046 5 0.0068 0.0065 26.2 8.54 0.37 0.25 0.037 0.0013 0.067 0.032
6 0.0057 0.0076 17.9 7.54 0.54 0.25 0.036 0.0007 0.059 0.057 7
0.0054 0.0082 22.5 6.57 0.47 0.21 0.037 0.0009 0.045 0.034 8 0.0072
0.0126 23.4 8.14 0.38 0.34 0.024 0.0008 0.034 0.085 9 0.0165 0.0034
23.4 9.17 0.95 0.25 0.026 0.0008 0.034 0.092 10 0.0064 0.0075 21.4
7.69 0.79 0.25 0.038 0.0004 0.034 0.04 11 0.0055 0.0047 20.8 10.24
0.74 0.34 0.016 0.0001 0.015 0.048 12 0.0091 0.0046 21.7 9.68 0.61
0.45 0.024 0.0007 0.034 0.001 13 0.0087 0.0084 23.5 8.54 0.36 0.25
0.038 0.0006 0.001 0.010 Comparison 14 0.0089 0.0072 18.6 12.67
0.85 0.21 0.038 0.0006 0.034 0.001 sheet No. 15 0.0032 0.0064 20.5
8.45 0.82 0.12 0.024 0.0006 0.054 0.024 16 0.0045 0.0052 19.7 10.64
0.84 0.39 0.016 0.0006 0.034 0.047 Stainless steel sheet components
(wt %) Hot Zn + Sn + rolling Sb + Bi + success Brazing No. La Ce Cu
Mg Pb rate (%) property Present 1 0.015 0.045 0.012 <0.001 0.022
100 4 mm invention 2 0.030 0.048 0.032 <0.001 0.032 100 3 mm
sheet No. 3 0.040 0.046 0.053 <0.001 0.015 100 1 mm 4 0.053
0.038 0.102 <0.001 0.007 100 1 mm 5 0.027 0.050 0.334 <0.001
0.025 95 1 mm 6 0.047 0.024 0.572 <0.001 0.030 90 1 mm 7 0.021
0.050 0.785 <0.001 0.042 80 1 mm 8 0.034 0.064 0.007 0.002 0.025
100 1 mm 9 0.030 0.081 0.005 0.009 0.030 100 0 mm 10 0.072 0.012
0.002 0.013 0.045 95 1 mm 11 0.046 0.025 <0.001 0.084 0.019 100
1 mm 12 0.020 0.035 0.095 0.005 0.047 95 0 mm 13 0.017 0.033 0.125
0.013 0.023 100 0 mm Comparison 14 0.046 0.050 0.007 <0.001
0.031 100 7 mm sheet No. 15 0.024 0.034 1.270 <0.001 0.048 0 --
16 0.076 0.050 0.187 0.005 0.055 0 --
Example 2
[0270] Stainless steel comprising, by weight, C: 0.007%, Si: 0.3%,
Mn: 0.3%, P: 0.03%, S: 0.001%, Al: 5%, Ti: 0.03%, Cr: 20%, Nb:
0.03%, La: 0.05%, Ce: 0.05%, N: 0.007% was subjected to hot rolling
and cold rolling to obtain a 30 .mu.m thick stainless steel
sheet.
[0271] For Example 2-1, a paint comprising Al powder was applied
onto the stainless steel sheet to various thicknesses, and heat
treatment was carried out for 4 hours at a temperature of
1000.degree. C. The viscosity of the Al powder-comprising paint was
adjusted to 100 cp by addition of xylene, based on a 50 wt %
content of Al powder with a mean particle size of 10 .mu.m and a 50
wt % content of ethyl cellulose. As a result, protrusions formed on
the surface of the stainless steel sheet. The density of
protrusions with a height of 1 .mu.m or greater was 10, 50, 100 and
200/cm.sup.2 for sheet Nos. 1, 2, 3 and 4, respectively.
[0272] The amount of Al in each stainless steel sheet having
protrusions formed on the surface was at least 7%.
[0273] For Example 2-2, Al was vapor deposited on the stainless
steel sheet to form an Al coating with a thickness of 0.5-4 .mu.m,
prior to heat treatment for 2 hours at a temperature of
1000.degree. C. As a result, the surface roughness Ra of the
stainless steel sheet was 1, 2, 3 and 4 .mu.m for sheet Nos. 5, 6,
7 and 8, respectively.
[0274] The amount of Al in each stainless steel sheet having a
surface roughness Ra of 2 .mu.m or greater was at least 7%.
[0275] For Example 2-3 (sheet No. 9), no treatment was carried out
for formation of an Al coating. The surface roughness Ra of the
stainless steel sheet was 0.8 .mu.m, and no protrusions were
formed.
[0276] Each of the prepared stainless steel sheets was directly
used as a flat sheet or as a corrugated sheet obtained by
corrugating the stainless steel sheet, and the flat and corrugated
sheets were alternately wound in a spiral fashion to produce a
metal honeycomb body, which was then inserted into an identical
stainless steel jacket 4 to prepare a metal carrier. The diameter
of the metal carrier was 100 mm, the length was 110 mm, the
corrugated sheet wave height was 1.25 mm and the corrugation pitch
was 2 mm.
[0277] The metal carrier was immersed in a wash coat solution and
then dried to form a wash coat layer with an average thickness of
25 .mu.m in the cells. A catalyst comprising a rare metal was
impregnated into the wash coat layer to complete the metal catalyst
carrier.
[0278] The cohesion of the wash coat layer was examined by an
engine heat cycle test. Each heat cycle consisted of 10 minutes of
engine running with the temperature at the catalyst carrier exhaust
gas inlet end at 1000.degree. C., followed by 10 minutes of engine
rest, and evaluation was based on the presence of any peeling of
the wash coat.
[0279] For the exhaust gas performance, a metal catalyst carrier
fabricated in the manner described above was set in an automobile
exhaust gas system and the HC emission was evaluated in mode
11.
[0280] The results are shown in Table 2. TABLE-US-00002 TABLE 2
Protrusion Surface Wash coat HC density roughness layer emission
No. (/cm.sup.2) Ra (.mu.m) cohesion (g/km) Example 2-1 1 10 -- x
1.56 comparison sheet Al powder 2 50 -- x 1.53 comparison sheet
applied 3 100 -- .smallcircle. 1.47 present invention 4 200 --
.smallcircle. 1.43 present invention Example 2-2 5 -- 1 x 1.56
comparison sheet Al vacuum 6 -- 2 .smallcircle. 1.48 present
invention vapor 7 -- 3 .smallcircle. 1.46 present invention
deposited 8 -- 4 .smallcircle. 1.46 present invention Example 2-3 9
-- 0.8 x 1.55 comparison sheet
[0281] As shown in Table 2, the catalyst carriers using the steel
sheets with protrusion density according to the present invention
had satisfactory wash coat cohesion and acceptable HC emission.
[0282] As also shown in Table 2, the catalyst carriers using the
steel sheets with surface roughness Ra of 2 .mu.m or greater had
satisfactory wash coat cohesion and acceptable HC emission.
Example 3
[0283] As stainless steel sheets there were prepared a steel sheet
having gaps according to the invention and a comparison steel sheet
with the same components and same sheet thickness but containing no
gaps as a comparison, and then the two steel sheets were
simultaneously placed in engine-simulated exhaust gas and the
temperature elevation rates of the surface temperatures of both
were compared.
Example 3-1
[0284] A comparison test was carried out with a 30 .mu.m thick
stainless steel sheet comprising, by weight, C: 0.007%, Si: 0.3%,
Mn: 0.3%, P: 0.03%, S: 0.001%, Al: 7.5%, Ti: 0.03%, Cr: 20%, Nb:
0.03%, La: 0.05%, Ce: 0.05%, N: 0.007%.
[0285] Steel sheet No. 1 according to the invention had an Al
content of 5% and a content of components other than Al as listed
above, having a 27.6 .mu.m thick stainless steel sheet as the base
material with an Al alloy comprising Al-10% Si plated onto both
surfaces of the base material to a thickness of 1.2 .mu.m on each
side, prior to heat treatment in a vacuum at 1200.degree.
C..times.2 hours. The sheet thickness after heat treatment was 30
.mu.m, the Al content of the steel sheet was 7.5%, and numerous
0.2-0.5 .mu.m gaps were confirmed near a region 1.2 .mu.m from the
surface of the steel sheet, at an average spacing of 6 .mu.m.
[0286] Comparison steel sheet No. 2 was produced by the same method
as sheet No. 1 of the invention, i.e., using a stainless steel
sheet with an Al content of 5% and a content of components other
than Al as listed above as the base material, plating both sides
with Al and then subjecting it to heat treatment, to obtain a steel
sheet with a thickness of 50 .mu.m and an Al content of 7.5%. The
surface was polished, including the gaps present near the surface
of the steel sheet, to make a thickness of 30 .mu.m.
[0287] The two steel sheets were simultaneously placed in
engine-simulated exhaust gas and the temperature elevation rates of
the surface temperatures of both were compared, yielding the
results shown in Table 3. As clearly shown by these results, the
temperature elevation rate for the temperature of the steel sheet
of the invention was greater than for the comparison steel sheet.
The temperature difference was particularly notable up to about the
first 10 seconds, corresponding to build-up immediately after
starting of the engine. TABLE-US-00003 TABLE 3 Elapsed time (sec)
No. 5 10 15 20 25 30 Present 1 Foil surface 454 612 635 663 667 675
invention temperature (K) Comparison 2 Foil surface 413 565 603 643
652 662 sheet temperature (K)
Example 3-2
[0288] A comparison test was carried out with a 42 .mu.m thick
stainless steel sheet comprising, by weight, C: 0.007%, Si: 0.3%,
Mn: 0.3%, P: 0.03%, S: 0.001%, Al: 12.4%, Ti: 0.03%, Cr: 20%, Nb:
0.03%, La: 0.05%, Ce: 0.05%, N: 0.007%.
[0289] Steel sheet No. 3 according to the invention had an Al
content of 8.1% and a content of components other than Al as listed
above, having a 36.2 .mu.m thick stainless steel sheet as the base
material with an Al alloy comprising Al-10% Si plated onto both
surfaces of the base material to a thickness of 2.9 .mu.m on each
side, prior to heat treatment in a vacuum at 1200.degree.
C..times.2 hours. The sheet thickness after heat treatment was 42
.mu.m, the Al content of the steel sheet was 12.4%, and numerous
0.2-0.7 .mu.m gaps were confirmed near a region 2.9 .mu.m (0.7t/10)
from the surface of the steel sheet, at an average spacing of 8
.mu.m.
[0290] Steel sheet No. 4 according to the invention had an Al
content of 0.45% and a content of components other than Al as
listed above, having a 32.4 .mu.m thick stainless steel sheet as
the base material with an Al alloy comprising Al-10% Si plated onto
both surfaces of the base material to a thickness of 4.2 .mu.m on
each side, prior to heat treatment in a vacuum at 1200.degree.
C..times.2 hours. The sheet thickness after heat treatment was 42
.mu.m, the Al content of the steel sheet was 12.4%, and numerous
0.3-0.7 .mu.m gaps were confirmed near a region 4.8 .mu.m (0.8t/7)
from the surface of the steel sheet, at an average spacing of 8
.mu.m.
[0291] The base material for sheet No. 3 of the invention was
produced by the same method as sheet No. 1 of the invention, i.e.,
using a stainless steel sheet with an Al content of 5% and a
content of components other than Al as listed above as the base
material, plating both sides with Al and then subjecting it to heat
treatment, to obtain a steel sheet with a thickness of 50 .mu.m and
an Al content of 8.1%. The surface was then polished, including the
gaps present near the surface of the steel sheet, to make a
thickness of 30 .mu.m.
[0292] Comparison steel sheet No. 5 contained no Al and a had
content of components other than Al as listed above, with a 30
.mu.m thick stainless steel sheet as the base material and an Al
alloy comprising Al-10% Si plated onto both surfaces of the base
material to a thickness of 7 .mu.m on each side, prior to heat
treatment in a vacuum at 1200.degree. C..times.2 hours. The sheet
thickness after heat treatment was 42 .mu.m, the Al content of the
steel sheet was 12.4%, and numerous 0.2-0.6 .mu.m gaps were
confirmed near a region 7.2 .mu.m (1.2t/7) from the surface of the
steel sheet, at an average spacing of 8 .mu.m.
[0293] The three steel sheets were simultaneously placed in
engine-simulated exhaust gas and the temperature elevation rates of
the surface temperatures were compared, yielding the results shown
in Table 4. As clearly seen in Table 4, the temperature elevation
rate for steel sheets No. 3 and No. 4 which had gaps within t/7 (6
.mu.m) from the surface was greater than for the 30 .mu.m thick
steel sheet No. 2 of Example 3-1, despite a larger sheet thickness
of 42 .mu.m. In contrast, the gaps in the comparison steel sheet
No. 5 did not significantly contribute to surface temperature
elevation of the steel sheet. TABLE-US-00004 TABLE 4 Gap Elapsed
time (sec) No. position 5 10 15 20 25 30 Present 3 0.7t/10 Foil
surface 430 610 622 645 651 655 invention (2.9 .mu.m) temperature
(K) Present 4 0.8t/7 Foil surface 420 595 610 630 635 640 invention
(4.8 .mu.m) temperature (K) Comparison 5 1.2t/7 Foil surface 392
540 575 611 620 630 sheet (7.2 .mu.m) temperature (K)
Example 3-3
[0294] A comparison test was carried out with a 62.2 .mu.m thick
stainless steel sheet comprising, by weight, C: 0.007%, Si: 0.3%,
Mn: 0.3%, P: 0.03%, S: 0.001%, Al: 7.0%, Ti: 0.03%, Cr: 20%, Nb:
0.03%, La: 0.05%, Ce: 0.05%, N: 0.007%.
[0295] Steel sheet Nos. 7-12 according to the invention and
comparison sheet Nos. 6 and 13 had an Al content of 7.0% and a
content of components other than Al as listed above, having a 50
.mu.m thick stainless steel sheet as the base material with an Al
alloy comprising Al-10% Si plated onto both surfaces of the base
material to a thickness of 6.1 .mu.m on each side, prior to heat
treatment in a vacuum at 1000-1200.degree. C..times.1-10 hours. The
sheet thickness after heat treatment was 62.2 .mu.m, and the Al
content of the steel sheet was 7.0%.
[0296] Numerous gaps were formed near a region about 6 .mu.m from
the surface of the steel sheet. The sizes of the gaps were as shown
in Table 5, each differing due to the heat treatment conditions,
allowing creation of sizes from 0.05 .mu.m for comparison sheet No.
6 and 0.19 .mu.m for sheet No. 7 of the invention, to 4.7 .mu.m for
sheet No. 12 of the invention and 5.8 .mu.m for comparison sheet
No. 13. The average spacing between the gaps was in the range of
about 10 L to 14 L for each of the samples, where L is the gap
size.
[0297] The eight steel sheets were simultaneously placed in
engine-simulated exhaust gas and the surface temperature elevation
rates were compared, yielding the results shown in Table 5. The
steel sheet surface temperature elevation was low for steel sheet
No. 6 which had smaller gaps than the range of the invention. The
elevation rates were relatively high for steel sheet Nos. 7 to 12
which were in the range of the invention. The elevation rate was
high for the comparison steel sheet No. 13, but cracks were
introduced in the surface near the gaps during a tensile test, and
therefore the strength was reduced. TABLE-US-00005 TABLE 5 Elapsed
time (sec) No. Gap size 5 10 15 20 25 30 Comparison 6 0.05 Foil
surface 370 510 551 592 603 611 sheet temperature (K) Present 7
0.19 Foil surface 415 570 605 645 655 665 invention temperature (K)
Present 8 0.3 Foil surface 420 573 609 650 660 667 invention
temperature (K) Present 9 1.2 Foil surface 430 580 615 655 662 669
invention temperature (K) Present 10 2.3 Foil surface 437 584 620
661 662 670 invention temperature (K) Present 11 3.5 Foil surface
440 590 627 663 665 671 invention temperature (K) Present 12 4.7
Foil surface 450 597 635 665 669 673 invention temperature (K)
Comparison 13 5.8 Foil surface 452 615 643 667 671 675 sheet
temperature (K)
Example 4
[0298] For each of the following examples there were prepared a
steel sheet flat sheet and a corrugated sheet obtained by
corrugating the stainless steel sheet, and the flat and corrugated
sheets were alternately wound in a spiral fashion to produce a
metal honeycomb body, which was then inserted into an identical
stainless steel jacket to prepare a metal carrier. The diameter of
the metal carrier was 100 mm, the length was 110 mm, the corrugated
sheet wave height was 1.25 mm and the corrugation pitch was 2 mm.
The formed honeycomb body was coated with a brazing material and
the honeycomb body was subjected to high temperature heat treatment
for brazing of the contact sections between the flat and corrugated
sheets of the metal honeycomb body.
[0299] The metal carrier was immersed in a wash coat solution and
then dried to form a wash coat layer in the cells. A catalyst
comprising a rare metal was impregnated into the wash coat layer to
complete the metal catalyst carrier.
[0300] The metal catalyst carrier was subjected to a cold-heat
durability test. A catalyst carrier sample was mounted directly
under the exhaust manifold of a 3000 cc gasoline engine, and an
engine test was conducted by an engine bench test with 1200
repeated cold/hot cycles each comprising 5 minutes full-throttle at
5000 rpm and 10 minutes of engine rest/cooling, inspecting the
catalyst carrier every 50-100 cycles, and evaluating the degree of
displacement of the honeycomb body and any abnormal oxidation.
Example 4-1
[0301] The steel sheet thickness was 20 .mu.m, and the components
of the steel sheet of the honeycomb body, the thermal expansion
coefficient .alpha., the proof strength .sigma. and the value of
the right side of inequality <1> were as shown in Table
1.
[0302] First, different Fe--Cr--Al based alloys with Al contents of
no greater than 5% were melted and subjected to hot and cold
rolling to produce 0.4 mm thick cold rolled steel sheets as base
materials. The steel sheets were then passed through a 90 wt %
Al-10 wt % Si plating bath melted to a temperature of 660.degree.
C., for adhesion of the Al--Si alloy onto the surface. The wiping
flow rate was varied to adjust the plating thickness, and the
plating thickness formed was based on the difference between the Al
content of the base material and the target Al content after heat
diffusion. The diffusion of Al into the steel was accomplished by
heat treatment in a vacuum, and this was followed by cold rolling
to obtain a 20 .mu.m thick steel sheet. A high temperature tensile
test piece (#13-B) and a thermal expansion coefficient measurement
test piece were cut out from part of the obtained steel sheet, and
used to determine the 0.2% proof strength at 900.degree. C. and the
thermal expansion coefficient with temperature increase from
20.degree. C. to 1000.degree. C. The strain rate in the tensile
test was constant at 0.3%/min, and the temperature elevation rate
during measurement of the thermal expansion coefficient was
constant at 10.degree. C./min.
[0303] Steel sheet Nos. 1-6 of the invention and comparison steel
sheet Nos. 7 and 9 were produced by the hot dip plating method
described above, while comparison steel sheet No. 8 was produced by
melting and hot/cold rolling of the Fe--Cr--Al based alloy.
[0304] A corrugated sheet obtained by corrugating the
aforementioned steel sheet was combined with the flat sheet and
partially bonded by brazing to manufacture a honeycomb body. The
bonding strength per centimeter of the brazed sections was 100 N or
greater in all cases, and satisfactory brazing was confirmed.
[0305] The results of the cold-heat durability test are shown in
Table 6. TABLE-US-00006 TABLE 6 Thermal expansion Proof Right side
of Cold-heat Steel sheet components (wt %) coefficient strength
inequality <1> durability No. Cr Al Ti Nb La Ce
(.mu.m/m/.degree. C.) (N/mm.sup.2) (N/mm.sup.2) test result Present
1 20 6.0 0.06 -- 0.04 0.04 16.5 38.8 24.1 OK invention 2 19 7.0
0.06 -- 0.04 0.04 17.2 42.1 28.4 OK 3 19 8.4 0.06 -- 0.04 0.04 18.3
50.2 34.3 OK 4 18 10.3 0.06 -- 0.04 0.04 20.0 56.1 41.2 OK 5 18 7.0
-- 0.05 0.04 0.04 17.3 65.3 29.0 OK 6 15 7.1 -- 0.05 0.04 0.04 17.3
59.8 29.0 OK Comparison 7 12 6.5 0.06 -- 0.04 0.04 16.9 24.8 26.6
displacement sheet at 1100 cycles 8 10 2.0 0.06 -- 0.04 0.04 14.3
15.3 7.7 abnormal oxidation at 50 cycles 9 17 12.6 0.06 -- 0.04
0.04 24.3 62.4 47.2 displacement at 1000 cycles
[0306] For sheet Nos. 1-6 according to the invention, the thermal
expansion coefficients .alpha. and the proof strength .sigma. were
all within the ranges of the invention, and the cold-heat
durability test results were satisfactory.
[0307] The proof strength of comparison sheet No. 7 failed to
satisfy inequality <1>, and displacement of the honeycomb
body occurred at 1100 cycles of the cold-heat durability test. The
proof strength failed to satisfy inequality <1> because of
the low Cr content of 12% among the steel sheet components.
[0308] The thermal expansion coefficient of comparison sheet No. 8
was less than the lower limit, and abnormal oxidation occurred at
50 cycles of the cold-heat durability test. The reason for the low
thermal expansion coefficient and the abnormal oxidation was that
the Al content of the steel sheet was a low 2%.
[0309] The thermal expansion coefficient of comparison sheet No. 9
exceeded the upper limit of the range of the invention, and the
high thermal stress causes displacement of the honeycomb body at
1000 cycles. The reason for the high thermal expansion coefficient
was that the Al content was a high 12.6%.
Example 4-2
[0310] The thickness of the steel sheet was 30 .mu.m, and the
components of the steel sheet of the honeycomb body, the thermal
expansion coefficient .alpha., the proof strength .sigma. and the
value of the right side of inequality <1> were as shown in
Table 7.
[0311] Steel sheets with a thickness of 30 .mu.m were produced by
the same process as in Example 4-1, and the steel sheets were used
to fabricate honeycomb bodies. The bonding strength per centimeter
of the brazed sections of the honeycomb bodies was 150 N or greater
in all cases, and satisfactory brazing was confirmed.
[0312] The results of the cold-heat durability test are shown in
Table 7. TABLE-US-00007 TABLE 7 Thermal expansion Proof Right side
of Cold-heat Steel sheet components (wt %) coefficient strength
inequality <1> durability No. Cr Al Ti Nb La Ce
(.mu.m/m/.degree. C.) (N/mm.sup.2) (N/mm.sup.2) test result Present
10 20 6.1 0.06 -- 0.04 0.04 16.6 38.3 16.5 OK invention 11 19 7.1
0.06 -- 0.04 0.04 17.3 42.5 19.3 OK 12 19 8.5 0.06 -- 0.04 0.04
18.4 51.3 23.2 OK 13 18 10.0 0.06 -- 0.04 0.04 19.8 57.2 27.0 OK
Comparison 14 12 2.3 0.06 -- 0.04 0.04 14.5 23.5 6.3 abnormal sheet
oxidation at 300 cycles 15 17 13.0 0.06 -- 0.04 0.04 25.0 65.4 30.8
displacement at 400 cycles
[0313] For sheet Nos. 10-13 according to the invention, the thermal
expansion coefficients .alpha. and the proof strength .sigma. were
all within the ranges of the invention, and the cold-heat
durability test results were satisfactory.
[0314] The thermal expansion coefficient of comparison sheet No. 14
was less than the lower limit of the range of the invention, and
abnormal oxidation occurred at 300 cycles of the cold-heat
durability test. The reason for the low thermal expansion
coefficient and the abnormal oxidation was that the Al content of
the steel sheet was a low 2.3%.
[0315] The thermal expansion coefficient of comparison sheet No. 15
exceeded the upper limit of the range of the invention, and the
high thermal stress caused displacement of the honeycomb body at
4000 cycles. The reason for the high thermal expansion coefficient
was that the Al content was a high 13.0%.
INDUSTRIAL APPLICABILITY
[0316] The present invention provides an Fe--Cr--Al based stainless
steel sheet and double layered sheet having an Al content of
greater than 6.5%, and a honeycomb body employing the stainless
steel sheet or double layered sheet, wherein inclusion of Cu and/or
Mg results in satisfactory wettability of the brazing material. In
addition, appropriate control of the content of Cu or impurity
elements can produce satisfactory hot rolled sheet ductility. There
is further provided a process for fabrication of a high
Al-containing stainless steel sheet suitable for hot rolling,
whereby a steel sheet with a low Al content is rolled, an Al layer
is adhered to the surface and the Al diffuses into the stainless
steel sheet by diffusion annealing.
[0317] Furthermore, by forming protrusions with a height of 1 .mu.m
or greater on the surface of the steel sheet composing the
honeycomb body, it is possible to improve the cohesion of the wash
coat layer, and accelerate the catalyst reaction by a turbulence
effect.
[0318] Moreover, by producing a surface roughness Ra of 2 .mu.m or
greater in the steel sheet composing the honeycomb body, it is
possible to improve the cohesion of the wash coat layer, and
accelerate the catalyst reaction by a turbulence effect.
[0319] The Al-containing stainless steel sheet of the invention may
also have isolated gaps formed in the interior of the steel sheet
to reduce the thermal conductivity. Consequently, when a catalyst
carrier is constructed with a honeycomb body employing a steel
sheet according to the invention, it is possible to increase the
temperature of the catalyst before the heat of the exhaust gas
raises the temperature near the center of the steel sheet
thickness, thereby increasing the temperature elevation rate of the
catalyst itself and yielding a catalyst carrier with excellent
purification performance.
[0320] The present invention also specifies the proof strength
condition required for a steel sheet by the relationship between
the thickness and thermal expansion coefficient of the steel sheet,
while also specifying the preferred range for the thermal expansion
coefficient, and therefore permitting fabrication of a steel sheet
and honeycomb body having excellent high-temperature durability to
allow its use under severe conditions at temperatures exceeding
1000.degree. C.
* * * * *