U.S. patent application number 15/523882 was filed with the patent office on 2017-11-09 for stainless steel material for diffusion bonding.
The applicant listed for this patent is NISSHIN STEEL CO., LTD.. Invention is credited to Yoshiaki HORI, Kazunari IMAKAWA, Kazuyuki KAGEOKA, Manabu OKU, Atsushi SUGAMA.
Application Number | 20170321311 15/523882 |
Document ID | / |
Family ID | 55908971 |
Filed Date | 2017-11-09 |
United States Patent
Application |
20170321311 |
Kind Code |
A1 |
SUGAMA; Atsushi ; et
al. |
November 9, 2017 |
STAINLESS STEEL MATERIAL FOR DIFFUSION BONDING
Abstract
Provided is a stainless steel material suitable for diffusion
bonded moldings in which diffusion bondability has been further
improved without being affected by the extent of surface roughness.
The present invention is a stainless steel material for diffusion
bonding in which the metal structure before diffusion bonding has a
multi-phase structure obtained from two or more of a ferrite phase,
a martensite phase and an austenite phase, wherein: the mean
crystal grain diameter in the multi-phase structure is not more
than 20 .mu.m; .gamma.max represented by formula (a) is 10-90; and
creep elongation when a 1.0 MPa load is applied at 1000.degree. C.
for 0.5 his at least 0.2%.
.gamma.max=420C-11.5Si+7Mn+23Ni-11.5Cr-12Mo+9Cu-49Ti-47Nb-52Al+470N+189
. . . Formula (a) The element notations in formula (a) represent
the contents (mass %) of the respective elements.
Inventors: |
SUGAMA; Atsushi; (Sakai-shi,
Osaka, JP) ; KAGEOKA; Kazuyuki; (Shunan-shi,
Yamaguchi, JP) ; HORI; Yoshiaki; (Shunan-shi,
Yamaguchi, JP) ; IMAKAWA; Kazunari; (Shunan-shi,
Yamaguchi, JP) ; OKU; Manabu; (Sakai-shi, Osaka,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NISSHIN STEEL CO., LTD. |
Chiyoda-ku, Tokyo |
|
JP |
|
|
Family ID: |
55908971 |
Appl. No.: |
15/523882 |
Filed: |
October 16, 2015 |
PCT Filed: |
October 16, 2015 |
PCT NO: |
PCT/JP2015/079342 |
371 Date: |
May 2, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/00 20130101;
C22C 38/001 20130101; C22C 38/06 20130101; C21D 2211/005 20130101;
C21D 2211/008 20130101; C21D 2211/001 20130101; C22C 38/50
20130101; C22C 38/58 20130101; C22C 38/02 20130101 |
International
Class: |
C22C 38/50 20060101
C22C038/50; C22C 38/02 20060101 C22C038/02; C22C 38/00 20060101
C22C038/00; C22C 38/06 20060101 C22C038/06 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 5, 2014 |
JP |
2014-225576 |
Claims
1. A dual-phase stainless steel material for diffusion bonding, a
metal structure before diffusion bonding having a dual-phase
structure composed of at least two phases of a ferrite phase, a
martensite phase, or an austenite phase, wherein the dual-phase
structure has an average crystal grain size of 20 .mu.m or less,
.gamma.max represented by the formula (a) mentioned below is 10 to
90, and creep elongation is 0.2% or more when a load of 1.0 MPa is
applied at 1,000.degree. C. for 0.5 hour:
.gamma.max=420C-11.5Si+7Mn+23Ni-11.5Cr-12Mo+9Cu-49Ti-47Nb-52Al+470N+189
Formula (a) where an element symbol in the formula (a) mentioned
above denotes the content (% by mass) of each element.
2. The stainless steel material for diffusion bonding according to
claim 1, comprising, in % by mass: C: 0.2% or less, Si: 1.0% or
less, Mn: 3.0% or less, P: 0.05% or less, S: 0.03% or less, Ni:
10.0% or less, Cr: 10.0 to 30.0%, N: 0.3% or less, Ti: 0.15% or
less, and Al: 0.15% or less, with the remainder being Fe and
inevitable impurities, wherein the total amount of Ti and Al is
0.15% or less.
3. The stainless steel material for diffusion bonding according to
claim 1, further comprising, in % by mass: one or two or more
elements of Nb: 4.0% or less, Mo: 0.01 to 4.0%, Cu: 0.01 to 3.0%,
and V: 0.03 to 0.15%.
4. The stainless steel material for diffusion bonding according to
claim 1, further comprising, in % by mass: B: 0.0003 to 0.01%.
Description
TECHNICAL FIELD
[0001] The present invention relates to a dual-phase stainless
steel material used in a molding, which is allowed to undergo
diffusion bonding.
BACKGROUND ART
[0002] One method of bonding stainless steel materials to each
other includes a diffusion bonding method. A stainless steel
diffusion bonded product assembled by diffusion bonding has been
applied in various applications such as heat exchangers, machine
components, fuel cell components, home appliance components, plant
components, ornament constituent members, and building materials.
The diffusion bonding method includes an "insert material inserting
method" of inserting an insert material into a bonding interface,
and performing bonding by solid phase diffusion or liquid phase
diffusion; and a "direct method" of directly bringing surfaces of
both stainless steel materials into contact with each other, and
performing diffusion bonding.
[0003] The insert material inserting method is advantageous in that
it is capable of realizing certain diffusion bonding in a
relatively simple manner. However, this method becomes
disadvantageous as compared with a direct method for the following
reasons. That is, an insert material is used, thus leading to an
increase in costs, and also a bonding portion is formed of metal
which is different from that forms a base material, thus leading to
deterioration of corrosion resistance. On the other hand, it is
commonly said to be difficult for the direct method to obtain
sufficient bonding strength as compared with the insert material
inserting method. However, this direct method includes the
possibility to become advantageous in that it can reduce production
costs, so that various methods have been studied. For example,
Patent Document 1 discloses technology in which the amount of S in
a stainless steel is set at 0.01% by weight or less and also
diffusion bonding is performed in a non-oxidizing atmosphere at a
predetermined temperature, thereby avoiding deformation of the
material, thus leading to an improvement in diffusion bondability
of a stainless steel material. Patent Document 2 discloses a method
using a stainless steel foil material whose surface is imparted
with unevenness by a pickling treatment. Patent Document 3
discloses a method using, as a material to be bonded, a stainless
steel whose Al content is suppressed so that an alumina film, which
causes inhibition of diffusion bonding, is less easily to be formed
during diffusion bonding. Patent Document 4 discloses a method in
which diffusion is promoted using a stainless steel foil imparted
with deformation by cold working. Patent Documents 5 and 6 describe
a ferritic stainless steel for direct diffusion bonding, the
component composition of which is optimized.
[0004] Patent Document 1: Japanese Unexamined Patent Application,
Publication No. S62-199277
[0005] Patent Document 2: Japanese Unexamined Patent Application,
Publication No. H02-261548
[0006] Patent Document 3: Japanese Unexamined Patent Application,
Publication No. H07-213918
[0007] Patent Document 4: Japanese Unexamined Patent Application,
Publication No. H09-279310
[0008] Patent Document 5: Japanese Unexamined Patent Application,
Publication No. H09-99218
[0009] Patent Document 6: Japanese Unexamined Patent Application,
Publication No. 2000-303150
[0010] Patent Document 7: Japanese Unexamined Patent Application,
Publication No. 2013-103271
[0011] Patent Document 8: Japanese Unexamined Patent Application,
Publication No. 2013-173181
[0012] Patent Document 9: Japanese Unexamined Patent Application,
Publication No. 2013-204149
[0013] Patent Document 10: Japanese Unexamined Patent Application,
Publication No. 2013-204150
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0014] The above-mentioned bonding technology enabled
implementation of diffusion bonding of a stainless steel material
even when using a direct method. However, from the industrial point
of view, the direct method is yet to be taken root as the
mainstream of a diffusion bonding method of the stainless steel
material. The main reason is the fact that it is difficult to
achieve both two issues, for example, security of reliability in
the bonding portion, such as bonding strength or adhesiveness, and
suppression of a load in the production, such as bonding device or
bonding time. According to conventional technical knowledge, in
order that the bonding portion to be produced by the direct method,
there is a need to employ a step requiring a large production load,
such as a step in which a bonding temperature is set at high
temperature of higher than 1,100.degree. C., or a step in which
high surface pressure is imparted by hot press, HIP, or the like,
so that it was impossible to avoid an increase in costs due to the
step. When an attempt is made to carry out diffusion bonding of a
stainless steel material by the direct method under the same
workload as in a conventional insert material inserting method, it
is difficult to sufficiently secure reliability of the bonding
portion in the current situation.
[0015] Thus, there has been proposed a method for producing a
diffusion bonded product by a direct method, which can be carried
out under the same workload as in a conventional insert material
inserting method without applying special high-temperature heating
or high surface pressure by making use of a driving force when a
ferrite phase is transformed into an austenite phase during
diffusion bonding (Patent Document 7) or a driving force of crystal
grain growth (Patent Document 8). There has also been proposed a
method in which an amount of a surface oxide of a stainless steel
material to be allowed to undergo diffusion bonding is reduced as
much as possible, thereby enhancing diffusion bondability (Patent
Documents 9 and 10). To secure good bondability, there is a need
for these methods to regulate surface roughness before bonding of a
stainless steel material to be used. Therefore, there is a need to
further improve bondability in a stainless steel material to be
used in a diffusion bonded product.
[0016] An object of the present invention is to provide a stainless
steel material suitable for diffusion bonded molding, diffusion
bondability of which is further improved without being influenced
by the extent of surface roughness.
Means for Solving the Problems
[0017] The present inventors have found that, by controlling an
average crystal grain size before diffusion bonding, an amount of
.gamma.max, and creep elongation of a dual-phase stainless steel
material having a dual-phase structure composed of at least two
phases of a ferrite phase, a martensite phase, and an austenite
phase, good diffusion bondability can be obtained without being
influenced by surface roughness of the steel material. Thus, the
present invention has been completed as a stainless steel material
for diffusion bonding. Specifically, the present invention provides
the followings.
[0018] (1) The present invention is directed to a dual-phase
stainless steel material for diffusion bonding, a metal structure
before diffusion bonding having a dual-phase structure composed of
at least two phases of a ferrite phase, a martensite phase, or an
austenite phase, wherein the dual-phase structure has an average
crystal grain size of 20 .mu.m or less, .gamma.max represented by
the formula (a) mentioned below is 10 to 90, and creep elongation
is 0.2% or more when a load of 1.0 MPa is applied at 1,000.degree.
C. for 0.5 hour:
.gamma.max=420C-11.5Si+7Mn+23Ni-11.5Cr-12Mo+9Cu-49Ti-47Nb-52Al+470N+189
Formula (a)
where an element symbol in the formula (a) mentioned above denotes
the content (% by mass) of each element.
[0019] (2) The present invention is directed to the stainless steel
material for diffusion bonding according to (1), including, in % by
mass: C: 0.2% or less, Si: 1.0% or less, Mn: 3.0% or less, P: 0.05%
or less, S: 0.03% or less, Ni: 10.0% or less, Cr: 10.0 to 30.0%, N:
0.3% or less, Ti: 0.15% or less, and Al: 0.15% or less, with the
remainder being Fe and inevitable impurities, wherein the total
amount of Ti and Al is 0.15% or less.
[0020] (3) The present invention is directed to the stainless steel
material for diffusion bonding according to (1) or (2), further
including, in % by mass: one or two or more elements of Nb: 4.0% or
less, Mo: 0.01 to 4.0%, Cu: 0.01 to 3.0%, and V: 0.03 to 0.15%.
[0021] (4) The present invention is directed to the stainless steel
material for diffusion bonding according to any one of (1) to (3),
further including, in % by mass: B: 0.0003 to 0.01%.
Effects of the Invention
[0022] According to the present invention, a dual-phase stainless
steel having a dual-phase structure composed of at least two phases
of a ferrite phase, a martensite phase, and an austenite phase is
provided with an average crystal grain size and .gamma.max before
diffusion bonding, and creep elongation at a bonding temperature in
an optimum range, whereby, a stainless steel material having
excellent diffusion bondability is provided, thus providing a
diffusion bonded molding which exhibits a good bonding interface.
The total content of Ti and Al is suppressed, thereby obtaining a
diffusion bonded molding having improved diffusion bondability.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a drawing showing a measurement test piece used in
a bondability test.
PREFERRED MODE FOR CARRYING OUT THE INVENTION
[0024] Embodiments of the present invention will be described
below. The present invention is not limited to the description
thereof.
[0025] It is considered that diffusion bonding by a direct method
of a stainless steel material is completed by simultaneous
proceeding of three types of processes, for example, a process (i)
in which unevenness of a bonding surface undergoes deformation
leading to adhesion, thus increasing a bonding area of the bonded
position, a process (ii) in which a surface oxide film of the steel
material before bonding disappears at the adhered position, and a
process (iii) in which a residual gas in voids as the unbonded
portion reacts with a base material, according to a conventional
technique.
[0026] Heretofore, the present inventors have studied so as to
avoid deterioration of productivity, which creates an industrial
obstacle, by regulating a base material component, components
included in a passive film, and surface roughness of a bonding
surface, focusing attention on the process (ii) mentioned above.
However, it is sometimes difficult to secure industrially stable
bondability even when the step (ii) mentioned above is controlled.
Therefore, numerous studies have been performed on a steel material
for obtaining stable bondability considering the step (i) mentioned
above. As a result, it has been found that, when a stainless steel
to be allowed to undergo diffusion bonding is a dual-phase
stainless steel having a dual-phase structure, it is extremely
effective to reduce a crystal grain size before diffusion
bonding.
[Dual-Phase Structure]
[0027] Stainless steels are commonly classified into an austenitic
stainless steel, a ferritic stainless steel, a martensitic
stainless steel, and the like based on a metal structure at normal
temperature. A "dual-phase structure" of the present invention has
a metal structure composed of at least two phases of a ferrite
phase, a martensite phase, and an austenite phase. The "dual-phase
stainless steel material" of the present invention means a steel
which has such a dual-phase structure, and exhibits an
austenitic-ferritic two-phase structure within a bonding
temperature range. Stainless steels classified into a ferritic
stainless steel and a martensitic stainless steel are sometimes
included in such a two-phase stainless steel.
[0028] In the present invention, in order to realize diffusion
bonding by a direct method at low temperature under low surface
pressure, a dual-phase stainless steel having a dual-phase
structure composed of at least two phases of a ferrite phase, a
martensite phase, and an austenite phase is used as a stainless
steel material to be allowed to undergo diffusion bonding.
Regarding this stainless steel, within a temperature range where
diffusion bonding proceeds, a ferrite phase and a martensite phase
are partially transformed into an austenite phase to form a
two-phase structure composed of an austenite phase and a ferrite
phase. There will easily take place creep deformation which is
considered to cause grain boundary sliding as a result of
maintenance of a fine structure due to suppression of crystal grain
growth of each phase in the two-phase structure at high
temperature. As a result, easy deformation is promoted at the
unevenness portion of a bonding surface, leading to an increase in
a bonding area of the bonded portion, thus enabling diffusion
bonding by a direct method at low temperature under low surface
pressure.
[0029] The dual-phase stainless steel material of the present
invention can be used as both or one of stainless steel materials
which are directly brought into contact with each other and
integrated by diffusion bonding. It is possible to apply, as a
mating material to be integrated, in addition to the stainless
steel material of the present invention, other types of two-phase
steels, types of austenitic steels in which an austenite
single-phase is formed within a heating range of diffusion bonding,
types of ferritic steels in which a ferrite single-phase is formed
within the heating range, and the like.
[Component Composition]
[0030] In the dual-phase stainless steel which is an application
object in the present invention, there is no need to be particular
about component elements other than Ti and Al from the viewpoint of
diffusion bondability, and it is possible to employ various
component compositions according to the uses. The present invention
is directed to an austenitic-ferritic two-phase structure within a
temperature range where diffusion bonding proceeds, so that there
is a need to employ a steel having a component composition in which
.gamma.max represented by the formula (a) mentioned below satisfies
a range of 10 to 90. It is possible to exemplify, as a specific
component composition range, the followings.
[0031] Component composition including, in % by mass: C: 0.2% or
less, Si: 1.0% or less, Mn: 3.0% or less, P: 0.05% or less, S:
0.03% or less, Ni: 10.0% or less, Cr: 10.0 to 30.0%, N: 0.3% or
less, Ti: 0.15% or less, and Al: 0.15% or less, with the remainder
being Fe and inevitable impurities, wherein the total amount of Ti
and Al is 0.15% or less.
[0032] Component composition further comprising, in % by mass: one
or two or more elements of Nb: 4.0% or less, Mo: 0.01 to 4.0%, Cu:
0.01 to 3.0%, and V: 0.03 to 0.15%. Component composition further
including, in % by mass: B: 0.0003 to 0.01%.
[0033] Components included in the stainless steel material will be
described below.
[0034] C improves strength and hardness of a steel by solid
solution strengthening. Meanwhile, an increase in C content causes
deterioration of workability and toughness of the steel, so that
the C content is preferably 0.2% by mass or less, and more
preferably 0.08% by mass or less.
[0035] Si is an element used for deoxidation of the steel.
Meanwhile, excessive Si content causes deterioration of toughness
and workability of the steel. Thus, a firm surface oxide film is
formed to inhibit diffusion bondability. Therefore, the Si content
is preferably 1.0% by mass or less, and more preferably 0.6% by
mass or less.
[0036] Mn is an element which improves high-temperature oxidation
properties. Meanwhile, excessive Mn content allows the steel to
undergo work hardening, leading to deterioration of cold
workability of the steel. Therefore, the Mn content is preferably
3.0% by mass or less.
[0037] P is an inevitable impurity element and enhances
intergranular corrosion properties and also causes deterioration of
toughness of the steel. Therefore, the P content is preferably
0.05% by mass or less, and more preferably 0.03% by mass or
less.
[0038] S is an inevitable impurity element and causes deterioration
of hot workability of the steel. Therefore, the S content is
preferably 0.03% by mass or less.
[0039] Ni is an austenite formation element and has a function of
improving corrosion resistance of the steel in a reducing acid
environment. Meanwhile, excessive Ni content makes an austenite
phase stable, thus failing to suppress the growth of a ferrite
crystal, so that a stable austenite single-phase is formed to
suppress the growth of the ferrite crystal. Therefore, the Ni
content is preferably 10.0% or less.
[0040] Cr is an element which forms a passive film to impart
corrosion resistance. The Cr content of less than 30.0% by mass
does not exert a sufficient effect of imparting corrosion
resistance. The Cr content exceeding 10.0% by mass causes
deterioration of workability. Therefore, the Cr content is
preferably 10.0 to 30.0% by mass.
[0041] N is an inevitable impurity element and causes deterioration
of cold workability, so that the content thereof is preferably 0.3%
by mass or less.
[0042] Ti has a function of fixing C and N and is therefore an
element effective in improving corrosion resistance and
workability. Al is often added as a deoxidizing agent. Meanwhile,
Ti and Al are easily oxidizable elements, so that Ti oxide and Al
oxide included in an oxide film on a surface of the steel material
are less likely to be reduced in a heat treatment of vacuum
diffusion bonding. Therefore, numerous Ti oxide or Al oxide may
cause prevention of proceeding of the process (ii) mentioned above
during diffusion bonding, so that the Ti content is preferably
0.15% by mass or less, while the Al content is preferably 0.15% by
mass or less, and more preferably 0.05% by mass. The total content
of Ti and Al is preferably set at 0.15% by mass or less, and more
preferably 0.05% by mass or less.
[0043] Nb is an element which forms carbide or carbonitride to
refine crystal grains of the steel, thus exerting the effect of
enhancing the toughness. Meanwhile, excessive Nb content causes
deterioration of workability of the steel, so that the Nb content
is preferably 4.0% by mass or less.
[0044] Mo is an element which has a function of improving corrosion
resistance without reducing the strength. Excessive Mo content
causes deterioration of workability of the steel, so that the Mo
content is preferably 0.01 to 4.0% by mass.
[0045] Cu is an element which is effective in improving corrosion
resistance, and also has a function of forming a ferrite phase.
Meanwhile, excessive Cu content causes deterioration of workability
of the steel, so that the Cu content is preferably 0.01 to 3.0% by
mass.
[0046] V is an element which contributes to an improvement in
workability and toughness of the steel by fixing solid-soluted C as
carbide. Meanwhile, excessive content of a V element causes
deterioration of productivity, so that the V content is preferably
0.03 to 0.15%.
[0047] B is an element which contributes to an improvement in
corrosion resistance and workability by fixing N. Meanwhile,
excessive content of a B element causes deterioration of hot
workability of the steel, so that the B content is preferably
0.0003 to 0.01%.
[0048] It is possible to apply, as a dual-phase stainless steel
having the chemical composition mentioned above, a steel in which
.gamma.max represented by the formula (a) mentioned below is 10 to
90:
.gamma.max=420C-11.5Si+7Mn+23Ni-11.5Cr-12Mo+9Cu-49Ti-47Nb-52Al+470N+189
Formula (a)
where an element symbol of C, Si, and the like in the above formula
(a) means the content (% by mass) of each element.
[0049] .gamma.max is an indicator which represents an amount (% by
volume) of an austenite phase formed when heated and retained at
about 1,100.degree. C. When .gamma.max is 100 or more, it is
possible to regard as types of austenitic steels in which an
austenite single-phase is formed. When .gamma.max is 0 or less, it
is possible to regard as types of ferrite steels in which a ferrite
single-phase is formed. Regarding the dual-phase stainless steel of
the present invention, when .gamma.max is 10 to 90, an
austenitic-ferritic two-phase is formed within a temperature range
where diffusion bonding proceeds, and two phases mutually suppress
crystal grain growth at high temperature, so that it is effective
for obtaining a fine crystal structure. .gamma.max is more
preferably 50 to 80.
[Average Crystal Grain Size Before Bonding]
[0050] The more the grain structure of the dual-phase stainless
steel of the present invention becomes fine, more quickly the
process (i) mentioned above can be allowed to proceed. Therefore,
the average crystal grain size before bonding is preferably 20
.mu.m or less, and more preferably 10 .mu.m or less.
[Surface Roughness]
[0051] Regarding the dual-phase stainless steel including fine
crystal grains of the present invention, the process (i) mentioned
above quickly proceeds, so that the process (ii) mentioned above
exerts a small influence and there is low possibility that
bondability is restricted by the extent of surface roughness Ra. If
surface roughness of the stainless steel material to be allowed to
undergo diffusion bonding increases, disappearance of an oxide film
in the process (ii) mentioned above tends to become late.
Therefore, a surface of the stainless steel material is preferably
smooth, and surface roughness Ra is preferably 0.3 .mu.m or
less.
[Method for Producing Diffusion Bonded Product]
[0052] Regarding the stainless steel material of the present
invention, a diffusion bonded product having good bondability is
obtained by performing vacuum diffusion bonding using a direct
method. Specific diffusion bonding treatment is as follows, for
example, diffusion bonding can be allowed to proceed by heating and
retaining in a furnace under the conditions of a pressure of
1.0.times.10.sup.-2 Pa or less (preferably 1.0.times.10.sup.-3 Pa
or less) and a dew point of -40.degree. C. or lower at 900 to
1,100.degree. C. in a state of being directly contacted under a
contact surface pressure of 0.1 to 1.0 MPa. The retention time can
be adjusted within a range of 0.5 to 3 hours.
Examples
[0053] Examples of the present invention will be described below.
The present invention is not limited to the following Examples, and
can be carried out within the scope of the present invention by
making appropriate modifications.
[0054] A stainless steel with the chemical composition shown in
Table 1 was melted by vacuum melting (30 kg). The steel ingot thus
obtained was forged into a 30 mm thick plate and then hot-rolled at
1,230.degree. C. for 2 hours to obtain a 3.0 mm thick hot rolled
sheet. Then, annealing, pickling, and cold rolling was performed to
obtain a 1.0 mm thick cold rolled sheet. Thereafter, the cold
rolled sheet was subjected to an annealing treatment mentioned
below to produce a cold rolled annealed sheet, which was used as a
test material.
TABLE-US-00001 TABLE 1 Steel Phase material C Si Mn P S Ni Cr Cu Mo
Al Ti Nb V B N .alpha. + M FM-1 0.064 0.54 0.31 0.01 0.002 1.90
16.37 0.04 0.04 0.004 0.004 -- -- -- 0.011 FM-2 0.095 0.16 0.50
0.02 0.003 0.10 16.28 -- -- 0.007 -- -- -- -- 0.010 FM-3 0.080 0.20
0.44 0.03 0.005 0.11 17.02 0.02 0.01 0.090 0.033 -- -- 0.0015 0.015
FM-4 0.018 0.38 0.49 0.02 0.004 0.09 16.82 0.01 0.01 0.002 0.001 --
-- -- 0.009 .alpha. + .gamma. FA-1 0.010 0.44 0.57 0.02 0.004 6.55
23.55 0.46 3.21 0.055 -- 0.18 0.08 -- 0.109 FA-2 0.013 0.48 0.62
0.01 0.009 6.44 24.54 0.46 2.88 0.080 -- -- 0.06 0.0020 0.150
.alpha. F-1 0.009 0.33 0.99 0.02 0.010 0.13 18.32 0.17 2.00 0.017
0.010 0.61 0.05 -- 0.009 .gamma. A-1 0.060 0.44 1.04 0.02 0.003
8.06 18.05 -- 0.11 -- 0.010 -- -- -- 0.015 M M-1 0.133 0.45 0.60
0.03 0.011 0.09 12.34 0.06 0.02 0.001 -- -- -- 0.0009 0.014
(.alpha.: Ferrite phase M: Martensite phase .gamma.: Austenite
phase)
[0055] Plural steel materials are shown in Table 1. The metal
structure before diffusion bonding of each of FM-1 steel to FM-4
steel is composed of a ferritic-martensitic two-phase (.alpha.+M
phase). The metal structure before diffusion bonding of each of
FA-1 steel and FA-2 steel is composed of a ferritic-austenitic
two-phase (.alpha.+.gamma. phase). The metal structure before
diffusion bonding of F-1 steel is composed of a ferrite
single-phase (.alpha. phase). The metal structure before diffusion
bonding of A-1 steel is composed of an austenite single-phase
(.gamma. phase). The metal structure before diffusion bonding of
M-1 steel is composed of a martensite single-phase (M phase). By
changing an annealing temperature of each steel sheet after cold
rolling within a range of 900.degree. C. to 1,200.degree. C., test
materials each having a different average crystal grain size were
obtained. To examine an influence of surface roughness, test
materials each having different surface roughness Ra were obtained
by changing a finishing treatment of a cold rolled annealed sheet
using a part of a steel sheet.
(Average Crystal Grain Size)
[0056] An average crystal grain size before diffusion bonding
(.mu.m) of a steel sheet was measured by a quadrature procedure as
mentioned below. A metal structure of a sheet thickness
cross-section parallel to a cold rolling direction was observed
with respect to a continuous area of 1 mm.sup.2 or more, and then
the number of crystal grains included in a unit area was calculated
using a quadrature procedure. Thereafter, an average area per one
crystal grain was determined and a value obtained by raising
variable the average area to the power of 1/2 was used as an
average crystal grain size.
(Surface Roughness)
[0057] Regarding surface roughness Ra (.mu.m), surface roughness Ra
in a direction perpendicular to a rolling direction was measured
using a surface roughness measuring instrument (SURFCOM2900DX;
manufactured by TOKYO SEIMITSU CO., LTD.).
(Creep Elongation)
[0058] Creep elongation was measured by the method mentioned below.
A JIS13B test piece was cut out from each steel sheet and a .phi.5
mm hole was made at the center of one grip. A making-off line (50
mm in length, between gauge marks) was formed on the test piece,
and then the test piece was attached to a high temperature tensile
testing machine so that the grip with a hole faces downward. After
temperature rise until the temperature between the gauge marks
becomes 1,000.degree. C. and soaking at the same temperature for 15
minutes, a wire made of SUS310S provided with a weight calculated
so as to apply stress of 1.0 MPa was attached to the hole of the
grip, followed by retaining for 0.5 hour. The wire made of SUS310S
was removed from the test piece and cooled to normal temperature by
air cooling. Then, the length L between gauge marks was measured
and (L-50)/50.times.100 was calculated as creep elongation (%).
(Bondability Test)
[0059] Plane test pieces measuring 20 mm.times.20 mm were cut out
from each steel sheet and diffusion bonding was performed by the
following method. Two test pieces made of the same steel material
were laminated in a state where surfaces of the test pieces come
into contact with each other. Using a jig with a weight, surface
pressure to be applied to a contact surface of these two test
pieces was adjusted to 0.1 MPa. Hereinafter, the plane test piece
thus laminated is referred to as a "steel material". Those in which
the steel materials are laminated are referred to as a "laminate".
Then, the jig and the laminate were placed in a vacuum furnace.
Vacuuming was performed until the pressure reaches initial vacuum
degree of 1.0.times.10.sup.-3 to 1.0.times.10.sup.-4 Pa and the
temperature was raised to 1,000.degree. C. over about 1 hour,
followed by retaining at the same temperature for 2 hours. After
transferring to a cooling chamber, cooling was performed. During
cooling, the vacuum degree was maintained up to 900.degree. C. and
then an Ar gas was introduced, followed by cooling to about
100.degree. C. or lower in an Ar gas atmosphere under 90 kPa.
Regarding the laminate after completion of the heat treatment,
using a ultrasonic thickness gage (manufactured by OLYMPUS
CORPORATION; Model 35DL), the thickness was measured at 49
measurement points formed at 3 mm pitch on a laminate surface
measuring 20 mm.times.20 mm as shown in FIG. 1. A probe diameter
was set at 1.5 mm. When a measured value of the sheet thickness at
certain measurement point exhibits the total sheet thickness of two
steel materials, it is possible to consider that both steel
materials are integrated with each other by diffusion of atoms at
the position of an interface between both steel materials
corresponding to the measurement point. Meanwhile, when a measured
value of the sheet thickness is different from the total sheet
thickness of two steel materials, it is possible to consider that
the unbonded portion (defect) exists at the position of an
interface between both steel materials corresponding to the
measurement point. A correspondence relation between a
cross-sectional structure of the laminate after a heating treatment
and the measurement results obtained by this measurement technique
was examined. As a result, it has been confirmed that it is
possible to accurately evaluate an area ratio of the bonded portion
in a contact area by the value obtained by dividing the number of
measurement points where the measurement results exhibited the
total sheet thickness of both steel materials by the total number
of measurement 49 (hereinafter this is referred to as a "bonding
ratio"). Diffusion bondability was evaluated by the following
evaluation criteria.
A: Bonding ratio of 100% (excellent) B: Bonding ratio of 90 to 99%
(good) C: Bonding ratio of 60 to 89% (fairly good) D: Bonding ratio
of 0 to 59% (bad) As a result of various studies, sufficient
strength of the diffusion bonded portion was secured and also
sealability (property not causing leakage of a gas through
communicating defects) between both members is good in ratings A
and B, so that ratings A and B were considered as passing.
[0060] An average crystal grain size and .gamma.max after cold
rolling annealing of each steel, surface roughness, creep
elongation, and bondability are shown in Table 2.
TABLE-US-00002 TABLE 2 Average crystal grain Creep Surface Steel
size elongation roughness Ra Remarks Category material .gamma. max
(.mu.m) (%) (.mu.m) Bondability (phase) Inventive FM-1 71.9 9 1.42
0.40 A .alpha. + M Example 1 Inventive FM-1 71.9 15 0.80 0.56 B
Example 2 Inventive FM-2 50.0 18 0.42 0.22 B Example 3 Inventive
FM-3 31.0 11 0.95 0.12 B Example 4 Inventive FA-1 77.5 12 1.11 0.33
A .alpha. + .gamma. Example 5 Inventive FA-1 77.5 16 0.65 0.49 B
Example 6 Comparative FM-1 71.9 35 0.11 0.28 C .alpha. + M Example
1 Comparative FM-4 8.3 16 0.12 0.43 C Example 2 Comparative FA-1
77.5 26 0.14 0.27 C .alpha. + .gamma. Example 3 Comparative FA-2
95.1 18 0.09 0.15 C Example 4 Comparative F-1 -60.0 15 0.08 0.41 D
.alpha. Example 5 Comparative F-1 -60.0 41 0.05 0.32 D Example 6
Comparative F-1 -60.0 41 0.05 0.05 B Example 7 Comparative A-1
199.5 12 0.17 0.31 D .gamma. Example 8 Comparative A-1 199.5 25
0.13 0.04 B Example 9 Comparative M-1 110.9 35 0.12 0.54 D M
Example 10 (Underlined numerical value shows a value deviating from
the scope of the present invention.)
[0061] As shown in Table 2, in Inventive Examples 1 to 6, a bonding
ratio was 90% or more and good diffusion bondability was exhibited
even at comparatively low temperature, for example, 1,000.degree.
C. under low surface pressure, for example, 0.1 MPa. In Inventive
Examples 1 to 6, good diffusion bondability was exhibited
regardless of the extent of surface roughness Ra, and there was no
influence of surface roughness. Since dual-phase stainless steel
material having a structure of the present invention does not cause
deterioration of diffusion bondability even when surface roughness
increases, it is apparent that diffusion bondability thereof is not
restricted to surface property of the steel material.
[0062] To the contrary, in Comparative Examples 1 to 10, an average
crystal grain size, .gamma.max, and creep elongation deviated from
the scope of the present invention, leading to small deformation of
the unevenness portion of the bonding surface within a two-phase
high temperature range, thus failing to increase the bonding area
at the bonded position. Therefore, numerous bonding ratios are less
than 80% and rated fairly bad or bad. Regarding ferrite
single-phase steels of Comparative Examples 5 to 7 and austenite
single-phase steels of Comparative Examples 8 to 9, according to a
change in bonding ratio depending on the surface roughness Ra,
Comparative Example 7 and Comparative Example 9 with very small
surface roughness exhibited a bonding ratio of 90% or more.
Meanwhile, other Comparative Examples exhibited large surface
roughness, and a bonding ratio decreased. As is apparent from the
above results, in a single-phase steel, large surface roughness
leads to bad bonding ratio, so that diffusion bondability is
restricted by surface roughness.
* * * * *