U.S. patent application number 15/126827 was filed with the patent office on 2017-03-30 for ferritic stainless steel and production method therefor (as amended).
This patent application is currently assigned to JFE STEEL CORPORATION. The applicant listed for this patent is JFE STEEL CORPORATION. Invention is credited to Mitsuyuki Fujisawa, Kunio Fukuda, Tomohiro Ishii, Chikara Kami, Akito Mizutani.
Application Number | 20170088912 15/126827 |
Document ID | / |
Family ID | 54144136 |
Filed Date | 2017-03-30 |
United States Patent
Application |
20170088912 |
Kind Code |
A1 |
Fukuda; Kunio ; et
al. |
March 30, 2017 |
FERRITIC STAINLESS STEEL AND PRODUCTION METHOD THEREFOR (AS
AMENDED)
Abstract
Provided is a ferritic stainless steel that has excellent
corrosion resistance and displays good brazing properties when
brazing is carried out at high temperature using a Ni-containing
brazing metal. These effects are obtained as a result of the steel
having a chemical composition containing, in mass %: 0.003% to
0.020% of C; 0.05% to 1.00% of Si; 0.10% to 0.50% of Mn, 0.05% or
less of P; 0.01% or less of S; 16.0% to 25.0% of Cr; 0.05% to 0.35%
of Ti; 0.005% to 0.05% of Al; and 0.005% to 0.025% of N, the
balance being Fe and incidental impurities, and as a result of a
nitrogen-enriched layer being created that has a nitrogen
concentration peak value of 0.05 mass % to 0.30 mass % at a depth
of within 0.05 .mu.m of a surface of the steel.
Inventors: |
Fukuda; Kunio; (Chiyoda-ku,
Tokyo, JP) ; Fujisawa; Mitsuyuki; (Chiyoda-ku, Tokyo,
JP) ; Mizutani; Akito; (Chiyoda-ku, Tokyo, JP)
; Ishii; Tomohiro; (Chiyoda-ku, Tokyo, JP) ; Kami;
Chikara; (Chiyoda-ku, Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE STEEL CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
JFE STEEL CORPORATION
Tokyo
JP
|
Family ID: |
54144136 |
Appl. No.: |
15/126827 |
Filed: |
February 25, 2015 |
PCT Filed: |
February 25, 2015 |
PCT NO: |
PCT/JP2015/000954 |
371 Date: |
September 16, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/20 20130101;
C22C 38/24 20130101; C22C 38/48 20130101; C22C 38/28 20130101; C22C
38/004 20130101; C22C 38/32 20130101; C21D 9/46 20130101; C22C
38/42 20130101; C22C 38/26 20130101; C23C 8/02 20130101; C21D 8/005
20130101; C22C 38/00 20130101; C22C 38/46 20130101; C22C 38/04
20130101; C22C 38/54 20130101; C22C 38/22 20130101; C22C 38/44
20130101; C23G 1/081 20130101; C22C 38/002 20130101; C22C 38/50
20130101; C21D 6/004 20130101; C22C 38/001 20130101; C23C 8/80
20130101; C23G 1/086 20130101; C22C 38/02 20130101; C23G 1/00
20130101; C21D 1/74 20130101; C21D 1/06 20130101; C21D 6/002
20130101; C23C 8/26 20130101; C22C 38/06 20130101 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C21D 6/00 20060101 C21D006/00; C21D 1/06 20060101
C21D001/06; C21D 1/74 20060101 C21D001/74; C22C 38/54 20060101
C22C038/54; C22C 38/50 20060101 C22C038/50; C22C 38/48 20060101
C22C038/48; C22C 38/46 20060101 C22C038/46; C22C 38/44 20060101
C22C038/44; C22C 38/42 20060101 C22C038/42; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00; C23C 8/02 20060101
C23C008/02; C23C 8/26 20060101 C23C008/26; C23C 8/80 20060101
C23C008/80; C21D 8/00 20060101 C21D008/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 20, 2014 |
JP |
2014-058880 |
Claims
1. A ferritic stainless steel comprising a chemical composition
containing, in mass %: 0.003% to 0.020% of C; 0.05% to 1.00% of Si;
0.10% to 0.50% of Mn; 0.05% or less of P; 0.01% or less of S; 16.0%
to 25.0% of Cr; 0.05% to 0.35% of Ti; 0.005% to 0.05% of Al; and
0.005% to 0.025% of N, the balance being Fe and incidental
impurities, wherein a nitrogen-enriched layer is present that has a
nitrogen concentration peak value of 0.05 mass % to 0.30 mass % at
a depth of within 0.05 .mu.m of a surface of the steel.
2. The ferritic stainless steel of claim 1, wherein the chemical
composition further contains, in mass %, one or more of: 0.05% to
0.50% of Ni; 0.10% to 3.00% of Mo; 0.10% to 0.60% of Cu; 0.01% to
0.50% of V; 0.01% to 0.15% of Nb; 0.0003% to 0.0040% of Ca; and
0.0003% to 0.0100% of B.
3. A production method for the ferritic stainless steel of claim 1,
comprising subjecting a slab having the chemical composition of
claim 1 to hot rolling, subsequent hot band annealing as required,
and a subsequent combination of cold rolling and annealing to
produce the ferritic stainless steel, wherein in final annealing of
the annealing, treatment for creating a nitrogen-enriched layer is
performed at a temperature of 800.degree. C. or higher in an
atmosphere having a dew point of -20.degree. C. or lower and a
nitrogen concentration of 5 vol % or greater.
4. A production method for the ferritic stainless steel of claim 2,
comprising subjecting a slab having the chemical composition of
claim 2 to hot rolling, subsequent hot band annealing as required,
and a subsequent combination of cold rolling and annealing to
produce the ferritic stainless steel, wherein in final annealing of
the annealing, treatment for creating a nitrogen-enriched layer is
performed at a temperature of 800.degree. C. or higher in an
atmosphere having a dew point of -20.degree. C. or lower and a
nitrogen concentration of 5 vol % or greater.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to a ferritic stainless steel
having excellent corrosion resistance and displaying good brazing
properties when brazing is carried out at high temperature using a
Ni-containing brazing metal, and to a production method for the
ferritic stainless steel.
BACKGROUND
[0002] In recent years, there has been demand for further
improvement of automobile fuel efficiency and exhaust gas
purification from a standpoint of environmental protection.
Consequently, adoption of exhaust heat recovery units and EGR
(Exhaust Gas Recirculation) coolers in automobiles continues to
increase.
[0003] An exhaust heat recovery unit is an apparatus that improves
fuel efficiency by, for example, using heat from engine coolant for
automobile heating and using heat from exhaust gas to warm up
engine coolant in order to shorten warming-up time when the engine
is started up. The exhaust heat recovery unit is normally located
between a catalytic converter and a muffler, and includes a heat
exchanger part formed by a combination of pipes, plates, fins, side
plates, and so forth, and entry and exit pipe parts. Exhaust gas
enters the heat exchanger part through the entry pipe, transfers
its heat to a coolant via a heat-transfer surface such as a fin,
and is discharged from the exit pipe. Bonding and assembly of
plates, fins, and so forth forming the heat exchanger part of an
exhaust heat recovery unit such as explained above is mainly
carried out by brazing using a Ni-containing brazing metal.
[0004] An EGR cooler includes a pipe for intake of exhaust gas from
an exhaust manifold or the like, a pipe for returning the exhaust
gas to a gas intake-side of an engine, and a heat exchanger for
cooling the exhaust gas. The EGR cooler more specifically has a
structure in which a heat exchanger including both a water flow
passage and an exhaust gas flow passage is located on a path along
which exhaust gas is returned to the gas intake-side of the engine
from the exhaust manifold. Through the structure described above,
high-temperature exhaust gas at the exhaust-side is cooled by the
heat exchanger and the cooled exhaust gas is returned to the gas
intake-side such as to lower the combustion temperature of the
engine. Accordingly, this structure forms a system for inhibiting
NO.sub.x production, which tends to occur at high temperatures.
Furthermore, the heat exchanger part of the EGR cooler is formed by
overlapping thin plates in a fin shape for reasons such as
improving compactness, and reducing weight and cost. Bonding and
assembly of these thin plates is mainly carried out by brazing
using a Ni-containing brazing metal.
[0005] Since bonding and assembly for a heat exchanger part in an
exhaust heat recovery unit or an EGR cooler such as described above
are carried out by brazing using a Ni-containing brazing metal,
materials used in the heat exchanger part are expected to have good
brazing properties with respect to the Ni-containing brazing metal.
Moreover, a heat exchanger part such as described above is expected
to be highly resistant to oxidation caused by high-temperature
exhaust gas passing through the heat exchanger part. The exhaust
gas includes small amounts of nitrogen oxides (NO.sub.x), sulfur
oxides (SO.sub.x), and hydrocarbons (HC) that may condense in the
heat exchanger to form a strongly acidic and corrosive condensate.
Therefore, materials used in a heat exchanger part such as
described above are expected to have corrosion resistance at normal
temperatures. In particular, because brazing heat treatment is
carried out at high temperature, it is necessary to prevent
formation of a Cr depletion layer due to preferential reaction of
Cr at grain boundaries with C and N, which is referred to as
sensitization, in order to ensure that corrosion resistance is
obtained.
[0006] For the reason described above, heat exchanger parts of
exhaust heat recovery units and EGR coolers are normally made using
an austenite-based stainless steel such as SUS316L or SUS304L that
has a reduced carbon content and is resistant to sensitization.
However, austenite-based stainless steels suffer from problems such
as high cost due to having high Ni content, and also poor heat
fatigue properties at high temperatures and poor fatigue properties
when used in an environment in which constraining force is received
at high temperature and with violent vibration, such as when used
as a component located peripherally to an exhaust manifold.
[0007] Therefore, steels other than austenite-based stainless
steels are being considered for use in heat exchanger parts of
exhaust heat recovery units and EGR coolers.
[0008] For example, PTL 1 discloses, as a heat exchanger component
of an exhaust heat recovery unit, a ferritic stainless steel that
has added Mo, Ti, or Nb and that has reduced Si and Al content. PTL
1 discloses that addition of Ti or Nb prevents sensitization by
stabilizing C and N in the steel as carbonitrides of Ti and Nb and
that reduction of Si and Al content improves brazing
properties.
[0009] PTL 2 discloses, as a component for a heat exchanger of an
exhaust heat recovery unit, a ferritic stainless steel having
excellent condensate corrosion resistance in which Mo content is
defined by Cr content, and Ti and Nb content is defined by C and N
content.
[0010] Furthermore, PTL 3 discloses, as a material for an EGR
cooler, a ferritic stainless steel in which added amounts of
components such as Cr, Cu, Al, and Ti satisfy a certain
relationship.
[0011] Additionally, PTL 4 and 5 disclose, as a component of an EGR
cooler and a material for a heat exchanger part of an EGR cooler, a
ferritic stainless steel containing 0.3 mass % to 0.8 mass % of Nb
and a ferritic stainless steel containing 0.2 mass % to 0.8 mass %
of Nb.
CITATION LIST
Patent Literature
[0012] PTL 1: JP H7-292446 A
[0013] PTL 2: JP 2009-228036 A
[0014] PTL 3: JP 2010-121208 A
[0015] PTL 4: JP 2009-174040 A
[0016] PTL 5: JP 2010-285683 A
[0017] PTL 6: JP 2842787 B
SUMMARY
Technical Problem
[0018] However, there is a presumption that brazing of the steel
disclosed in PTL 1 is carried out using a copper brazing metal
having a low brazing temperature and inadequate brazing may,
therefore, occur in a situation in which a Ni-containing brazing
metal (for example, BNi-2 or BNi-5 stipulated by Japanese
Industrial Standards (JIS Z 3265)) having a high brazing
temperature is used.
[0019] In the case of the steel disclosed in PTL 2, in particular
steel containing Ti, a problem of reduced brazing properties may
occur as a result of a thick Ti oxide film being formed such that
spreading of the brazing metal is decreased when brazing is carried
out at a temperature that is high, even among brazing metals in
which a Ni-containing brazing metal is used.
[0020] Furthermore, although the chemical composition of the steel
disclosed by PTL 3 takes into account inhibition of Ti or Al oxide
film formation during brazing at high temperature using a
Ni-containing brazing metal, this inhibitive effect is not thought
to be sufficient. Consequently, it has not necessarily been
possible to achieve adequate brazing properties due to, for
example, unsatisfactory joint strength or unsatisfactory brazing
metal infiltration into a joint gap between overlapping parts when
overlapping steel is brazed.
[0021] In relation to this point, steel disclosed in PTL 4 and 5
has a high Nb content in order to inhibit coarsening of crystal
grains during brazing using a Ni-containing brazing metal and
prevent reduction in toughness, and a certain degree of improvement
of brazing properties is obtained in a situation in which Ti and Al
are not contained in the steel.
[0022] However, the high Nb content leads to a higher
recrystallization temperature, which causes growth of a thicker
oxide film, referred to as a scale, during final annealing.
Consequently, descaling properties in a descaling process performed
after the annealing are negatively affected, which is problematic
because it makes it difficult to adopt an efficient production
process (high-speed pickling process) using a normal carbon steel
production line as disclosed in PTL 6. Nb is also expensive, which
is problematic in terms of production costs.
[0023] The present disclosure is the result of development
conducted in order to solve the problems described above and an
objective thereof is to provide a ferritic stainless steel that has
excellent corrosion resistance, displays good brazing properties
when brazing is carried out at high temperature using a
Ni-containing brazing metal, and can be produced by a highly
efficient production process, and also to provide a production
method for this ferritic stainless steel.
Solution to Problem
[0024] The inventors decided to use Ti as a stabilizing element for
C and N due to the fact that, unlike Nb addition, Ti addition does
not lead to a higher recrystallization temperature. The inventors
conducted diligent investigation in which they produced
Ti-containing ferritic stainless steel using various different
chemical compositions and production conditions, and investigated
various properties thereof, particularly brazing properties when
brazing is carried out at high temperature using a Ni-containing
brazing metal.
[0025] However, no matter how the chemical composition was adjusted
in production of the Ti-containing ferritic stainless steel
described above, it was not possible to satisfactorily inhibit
formation of an oxide film of Ti, Al, or the like, which negatively
affects spreading of brazing metal, during brazing carried out at
high temperature using a Ni-containing brazing metal. As a result,
desired brazing properties--specifically, brazing metal
infiltration into a joint gap between overlapping parts when
overlapping steel is brazed and brazed part joint strength--could
not be adequately obtained.
[0026] Therefore, the inventors conducted further investigation
with an objective of effectively inhibiting formation of an oxide
film of Ti, Al, or the like when brazing is carried out at high
temperature using a Ni-containing brazing metal.
[0027] As a result of this investigation, the inventors discovered
that it is possible to prevent formation of an oxide film of Ti,
Al, or the like during brazing by subjecting the steel to heat
treatment in a controlled atmosphere prior to brazing such that a
specific nitrogen-enriched layer is formed in a surface layer part
of the steel. It was also discovered that through formation of this
nitrogen-enriched layer, good brazing properties can be
satisfactorily obtained even when brazing is carried out at high
temperature using a Ni-containing brazing metal.
[0028] The inventors also realized that steel having a
nitrogen-enriched layer formed therein as described above is also
extremely advantageous in terms of production efficiency because an
efficient production process is applicable thereto.
[0029] Based on these findings, the inventors conducted further
investigation which eventually led to the present disclosure.
[0030] Specifically, the primary features of the present disclosure
are as follows.
[0031] 1. A ferritic stainless steel comprising
[0032] a chemical composition containing (consisting of), in mass
%:
[0033] 0.003% to 0.020% of C;
[0034] 0.05% to 1.00% of Si;
[0035] 0.10% to 0.50% of Mn;
[0036] 0.05% or less of P;
[0037] 0.01% or less of S;
[0038] 16.0% to 25.0% of Cr;
[0039] 0.05% to 0.35% of Ti;
[0040] 0.005% to 0.05% of Al; and
[0041] 0.005% to 0.025% of N,
[0042] the balance being Fe and incidental impurities, wherein
[0043] a nitrogen-enriched layer is present that has a nitrogen
concentration peak value of 0.05 mass % to 0.30 mass % at a depth
of within 0.05 .mu.m of a surface of the steel.
[0044] 2. The ferritic stainless steel described above in 1,
wherein
[0045] the chemical composition further contains, in mass %, one or
more of:
[0046] 0.05% to 0.50% of Ni;
[0047] 0.10% to 3.00% of Mo;
[0048] 0.10% to 0.60% of Cu;
[0049] 0.01% to 0.50% of V;
[0050] 0.01% to 0.15% of Nb;
[0051] 0.0003% to 0.0040% of Ca; and
[0052] 0.0003% to 0.0100% of B.
[0053] 3. A production method for the ferritic stainless steel
described above in 1 or 2, comprising
[0054] subjecting a slab having the chemical composition described
above in 1 or 2 to hot rolling, subsequent hot band annealing as
required, and a subsequent combination of cold rolling and
annealing to produce the ferritic stainless steel, wherein
[0055] in final annealing of the annealing, treatment for creating
a nitrogen-enriched layer is performed at a temperature of
800.degree. C. or higher in an atmosphere having a dew point of
-20.degree. C. or lower and a nitrogen concentration of 5 vol % or
greater.
Advantageous Effect
[0056] According to the present disclosure, a ferritic stainless
steel can be obtained that has excellent corrosion resistance and
that displays good brazing properties when brazing is carried out
at high temperature using a Ni-containing brazing metal.
[0057] Moreover, the presently disclosed ferritic stainless steel
can be produced by a highly efficient production process and is,
therefore, extremely advantageous in terms of productions
costs.
BRIEF DESCRIPTION OF THE DRAWINGS
[0058] In the accompanying drawings:
[0059] FIG. 1 is a schematic view illustrating a test material used
to evaluated joint gap infiltration by a brazing metal; and
[0060] FIG. 2 schematically illustrates a tensile test piece used
to evaluate joint strength of a brazed part, wherein FIG. 2A
illustrates one side of the tensile test piece prior to brazing and
FIG. 2B illustrates the entire tensile test piece after
brazing.
DETAILED DESCRIPTION
[0061] The following provides a specific description of the present
disclosure.
[0062] First, the reasons for limiting the chemical composition of
the steel to the aforementioned range in the present disclosure are
explained. Hereinafter, the unit "%" relating to the content of
elements in the chemical composition of the steel refers to "mass
%" unless specified otherwise.
C: 0.003% to 0.020%
[0063] C is an element contained incidentally in the steel.
Strength of the steel improves with increasing C content whereas
workability of the steel improves with decreasing C content.
Herein, the C content is required to be 0.003% or greater in order
to obtain sufficient strength. However, if the C content is greater
than 0.020%, workability noticeably decreases and sensitization
tends to occur more easily due to Cr carbide precipitation at grain
boundaries. Accordingly, the C content is in a range of 0.003% to
0.020%. Furthermore, although low C content is preferable from a
viewpoint of corrosion resistance, if the C content is set too low,
refining becomes time consuming, leading to increased costs.
Accordingly, the C content is preferably in a range of 0.010% to
0.020%.
[0064] Si: 0.05% to 1.00%
[0065] Si is a useful element as a deoxidizer. This effect is
obtained through Si content of 0.05% or greater. However, if Si
content is greater than 1.00%, workability noticeably decreases and
forming becomes difficult. Furthermore, application of a high-speed
pickling process using a normal carbon steel production line as
described in PTL 6 becomes difficult if the Si content is greater
than 1.00%. Accordingly, the Si content is in a range of 0.05% to
1.00%. The Si content is preferably in a range of 0.10% to 0.50%.
Moreover, an upper limit for the Si content is more preferably
0.40%, and particularly preferably 0.30%.
[0066] Mn: 0.10% to 0.50%
[0067] Mn has a deoxidizing effect that is obtained through Mn
content of 0.10% or greater. However, excessive Mn addition leads
to loss of workability due to solid solution strengthening.
Furthermore, excessive Mn decreases corrosion resistance by
promoting precipitation of MnS, which acts as a starting point for
corrosion. Therefore, Mn content of 0.50% or less is appropriate.
Accordingly, the Mn content is in a range of 0.10% to 0.50%. The Mn
content is preferably in a range of 0.15% to 0.50%. Moreover, an
upper limit for the Mn content is more preferably 0.35%, and
particularly preferably 0.25%.
[0068] P: 0.05% or less
[0069] P is an element that is incidentally included in the steel.
However, excessive P content reduces weldability and facilitates
grain boundary corrosion. This trend is noticeable if the P content
is greater than 0.05%. Accordingly, the P content is 0.05% or less.
The P content is preferably 0.03% or less.
[0070] However, since excessive dephosphorization leads to
increased refining time and costs, the P content is preferably
0.02% or greater.
[0071] S: 0.01% or less
[0072] S is an element that is incidentally contained in the steel,
and that promotes MnS precipitation and decreases corrosion
resistance if S content is greater than 0.01%. Accordingly, the S
content is 0.01% or less. The S content is preferably 0.007% or
less.
[0073] Cr: 16.0% to 25.0%
[0074] Cr is an important element for ensuring corrosion resistance
of the stainless steel. Adequate corrosion resistance after brazing
is not obtained if Cr content is less than 16.0%. However,
excessive addition of Cr causes deterioration of workability.
Accordingly, the Cr content is in a range of 16.0% to 25.0%. The Cr
content is preferably in a range of 18.0% to 23.0%.
[0075] Ti: 0.05% to 0.35%
[0076] Ti is an element that prevents the precipitation of Cr
carbonitride, which decreases corrosion resistance (sensitization),
since Ti combines with C and N preferentially. This effect is
obtained through Ti content of 0.05% or greater. However, Ti is not
a particularly preferable element from a viewpoint of brazing
properties. The reason for this is that Ti is an active element
with respect to oxygen and thus brazing properties are decreased as
a result of a dense and continuous Ti oxide film being formed
during brazing. Ti oxide film formation is prevented in the present
disclosure through creation of a nitrogen-enriched layer in a
surface layer of the steel, but it is not possible to adequately
prevent Ti oxide film formation if Ti content is greater than
0.35%. Accordingly, the Ti content is in a range of 0.05% to 0.35%.
The Ti content is preferably in a range of 0.10% to 0.25%, and is
more preferably in a range of 0.10% to 0.20%.
[0077] Al: 0.005% to 0.05%
[0078] Al is a useful element for deoxidization, which is obtained
as an effect through Al content of 0.005% or greater. However, in
the same way as Ti, Al is not a particularly preferable element
from a viewpoint of brazing properties. The reason for this is
that, in the same way as Ti, Al causes formation of a dense and
continuous Al oxide film (Al.sub.2O.sub.3 film) at the surface of
the steel during brazing and therefore negatively affects brazing
properties as a result of the Al oxide film hindering spreading and
adhesion of the brazing metal. Al oxide film formation is prevented
in the present disclosure through creation of the nitrogen-enriched
layer in the surface layer of the steel, but it is not possible to
adequately prevent Al oxide film formation if Al content is greater
than 0.05%. Accordingly, the Al content is in a range of 0.005% to
0.05%. The Al content is preferably in a range of 0.01% to
0.03%.
[0079] N: 0.005% to 0.025%
[0080] N is an important element for preventing Ti or Al oxide film
formation and improving brazing properties through creation of the
nitrogen-enriched layer. N content is required to be 0.005% or
greater in order to create the nitrogen-enriched layer. However, N
content of greater than 0.025% facilitates sensitization and
reduces workability. Accordingly, the N content is in a range of
0.005% to 0.025%. The N content is preferably in a range of 0.007%
to 0.020%.
[0081] In addition to the basic components described above, the
chemical composition in the present disclosure may appropriately
further contain the following elements as required.
Ni: 0.05% to 0.50%
[0082] Ni is an element that effectively contributes to improving
toughness and to improving crevice corrosion resistance when
contained in an amount of 0.05% or greater. However, Ni content of
greater than 0.50% increases stress corrosion crack sensitivity.
Furthermore, Ni is an expensive element that leads to increased
costs. Accordingly, in a situation in which Ni is contained in the
steel, the Ni content is in a range of 0.05% to 0.50%. The Ni
content is preferably in a range of 0.10% to 0.30%.
[0083] Mo: 0.10% to 3.00%
[0084] Mo improves corrosion resistance by stabilizing a
passivation film of the stainless steel. In the case of an exhaust
heat recovery unit or an EGR cooler, Mo has an effect of preventing
inner surface corrosion by a condensate and outer surface corrosion
by a snow-melting agent or the like. Furthermore, Mo has an effect
of improving high-temperature heat fatigue properties and is a
particularly preferable element in a situation in which the steel
is used in an EGR cooler attached directly below an exhaust
manifold. These effects are obtained through Mo content of 0.10% or
greater. However, Mo is an expensive element that leads to
increased costs. Furthermore, Mo content of greater than 3.00%
reduces workability. Accordingly, in a situation in which Mo is
contained in the steel, the Mo content is in a range of 0.10% to
3.00%. The Mo content is preferably in a range of 0.50% to
2.50%.
[0085] Cu: 0.10% to 0.60%
[0086] Cu is an element that enhances corrosion resistance. This
effect is obtained through Cu content of 0.10% or greater. However,
Cu content of greater than 0.60% reduces hot workability.
Accordingly, in a situation in which Cu is contained in the steel,
the Cu content is in a range of 0.10% to 0.60%. The Cu content is
preferably in a range of 0.20% to 0.50%.
[0087] V: 0.01% to 0.50%
[0088] V combines with C and N contained in the steel and prevents
sensitization in the same way as Ti. V also has an effect of
creating the nitrogen-enriched layer by combining with nitrogen.
These effects are obtained through V content of 0.01% or greater.
On the other hand, V content of greater than 0.50% reduces
workability. Accordingly, in a situation in which V is contained in
the steel, the V content is in a range of 0.01% to 0.50%. The V
content is preferably in a range of 0.05% to 0.40%.
[0089] Nb: 0.01% to 0.15%
[0090] Nb combines with C and N contained in the steel and prevents
sensitization in the same way as Ti. Nb also has an effect of
creating the nitrogen-enriched layer by combining with nitrogen.
These effects are obtained through Nb content of 0.01% or greater.
On the other hand, Nb content of greater than 0.15% raises the
recrystallization temperature such that an efficient high-speed
pickling process such as described in PTL 6 cannot be adopted.
Accordingly, in a situation in which Nb is contained in the steel,
the Nb content is in a range of 0.01% to 0.15%. The Nb content is
preferably in a range of 0.01% to 0.10%.
[0091] Ca: 0.0003% to 0.0040%
[0092] Ca improves weldability by improving penetration of a welded
part. This effect is obtained through Ca content of 0.0003% or
greater. However, Ca content of greater than 0.0040% decreases
corrosion resistance by combining with S to form CaS. Accordingly,
in a situation in which Ca is contained in the steel, the Ca
content is in a range of 0.0003% to 0.0040%. The Ca content is
preferably in a range of 0.0005% to 0.0030%.
[0093] B: 0.0003% to 0.0100%
[0094] B is an element that improves resistance to secondary
working brittleness. This effect is exhibited when B content is
0.0003% or greater. However, B content of greater than 0.0100%
reduces ductility due to solid solution strengthening. Accordingly,
in a situation in which B is contained in the steel, the B content
is in a range of 0.0003% to 0.0100%. The B content is preferably in
a range of 0.0005% to 0.0030%.
[0095] Through the above description, the chemical composition of
the presently disclosed ferritic stainless steel has been
explained.
[0096] In the chemical composition according to the present
disclosure, components other than those listed above are Fe and
incidental impurities.
[0097] In the presently disclosed ferritic stainless steel, it is
vital that the chemical composition of the steel is appropriately
controlled such as to be in the range described above and that a
nitrogen-enriched layer such as described below is created in the
surface layer part of the steel by performing heat treatment in a
controlled atmosphere prior to brazing. Nitrogen concentration peak
value at depth of within 0.05 .mu.m of surface: 0.05 mass % to 0.30
mass %
[0098] In the presently disclosed ferritic stainless steel, a
nitrogen-enriched layer is created that has a nitrogen
concentration peak value of 0.05 mass % to 0.30 mass % at a depth
of within 0.05 .mu.m of the surface of the steel in a depth
direction. This nitrogen-enriched layer can prevent formation of a
continuous and dense oxide film of Ti, Al, or the like at the
surface and, as a result, can improve brazing properties when a
Ni-containing brazing metal is used.
[0099] N in the nitrogen-enriched layer described above combines
with Ti, Al, V, Nb, Cr, and the like in the steel. The following
describes a mechanism which the inventors consider to be
responsible for the nitrogen-enriched layer inhibiting formation of
a Ti or Al oxide film.
[0100] Specifically, formation of the nitrogen-enriched layer
causes Ti and Al present in the surface layer part of the steel to
combine with N such that the Ti and Al cannot diffuse to the
surface of the steel. Furthermore, Ti and Al present inward of the
nitrogen-enriched layer cannot diffuse to the surface of the steel
because the nitrogen-enriched layer acts as a barrier. According,
formation of a Ti or Al oxide film is inhibited as a result of Ti
and Al in the steel not diffusing to the surface.
[0101] Herein, formation of a Ti or Al oxide film at the surface
cannot be adequately prevented if the nitrogen concentration peak
value is less than 0.05 mass %. On the other hand, the surface
layer part hardens if the nitrogen concentration peak value is
greater than 0.30 mass %, making defects more likely to occur, such
as fin plate cracking due to hot vibration of an engine or the
like.
[0102] Therefore, the nitrogen concentration peak value at a depth
of within 0.05 .mu.m of the surface has a value in a range of 0.05
mass % to 0.30 mass %. The nitrogen concentration peak value is
preferably in a range of 0.07 mass % to 0.20 mass %.
[0103] Note that the nitrogen concentration peak value at a depth
of within 0.05 .mu.m of the surface referred to herein can for
example be calculated by measuring nitrogen concentration in the
steel in a depth direction by glow discharge optical emission
spectroscopy, dividing a maximum value for nitrogen concentration
at a depth of within 0.05 .mu.m of the steel surface by a measured
value for nitrogen concentration at a depth of 0.50 .mu.m, and
multiplying the resultant value by the nitrogen concentration of
the steel obtained though chemical analysis.
[0104] Furthermore, the nitrogen-enriched layer described herein
refers to a region in which nitrogen is enriched due to permeation
of nitrogen from the surface of the steel. The nitrogen-enriched
layer is created in the surface layer part of the steel and more
specifically in a region spanning for a depth of approximately
0.005 .mu.m to 0.05 .mu.m in the depth direction from the surface
of the steel.
[0105] The following describes a suitable production method for the
presently disclosed ferritic stainless steel.
[0106] Molten steel having the chemical composition described above
is prepared by steelmaking through a commonly known method such as
using a converter, an electric heating furnace, or a vacuum melting
furnace, and is subjected to continuous casting or ingot casting
and blooming to obtain a semi-finished casting product (slab).
[0107] The semi-finished casting product is hot rolled to obtain a
hot-rolled sheet either directly without prior heating or after
heating at 1100.degree. C. to 1250.degree. C. for 1 hour to 24
hours. The hot-rolled sheet is normally subjected to hot band
annealing at 800.degree. C. to 1100.degree. C. for 1 minute to 10
minutes, but depending on the intended use, this hot band annealing
may be omitted.
[0108] Thereafter, the sheet is subjected to a combination of cold
rolling and annealing to obtain a product steel sheet.
[0109] The cold rolling is preferably performed with a rolling
reduction rate of 50% or greater in order to improve shape
correction, extensibility, bendability, and press formability.
Furthermore, the cold rolling and annealing process may be repeated
two or more times.
[0110] Herein, it is necessary to create the above-described
nitrogen-enriched layer in order to obtain the presently disclosed
ferritic stainless steel. Treatment for creating the
nitrogen-enriched layer is preferably performed during final
annealing (finish annealing) carried out after the cold
rolling.
[0111] Note that treatment for creating the nitrogen-enriched layer
can be performed in a separate step to annealing, such as, for
example, after a component has been cut from the steel sheet.
However, it is advantageous in terms of production efficiency to
create the nitrogen-enriched layer during the final annealing
(finish annealing) carried out after the cold rolling because this
allows the nitrogen-enriched layer to be created without increasing
the number of production steps.
[0112] The following describes conditions in treatment for creating
the nitrogen-enriched layer.
[0113] Dew point: -20.degree. C. or lower
[0114] If the dew point is higher than -20.degree. C., a
nitrogen-enriched layer is not created because nitrogen from the
surrounding atmosphere does not permeate into the steel due to
formation of an oxide film at the surface of the steel.
Accordingly, the dew point is -20.degree. C. or lower. The dew
point is preferably -30.degree. C. or lower.
[0115] Treatment atmosphere nitrogen concentration: 5 vol % or
greater
[0116] If the nitrogen concentration of the treatment atmosphere is
less than 5 vol %, a nitrogen-enriched layer is not created because
an insufficient amount of nitrogen permeates into the steel.
Accordingly, the nitrogen concentration of the treatment atmosphere
is 5 vol % or greater. The nitrogen concentration of the treatment
atmosphere is preferably 10 vol % or greater. The remainder of the
treatment atmosphere, besides nitrogen, is preferably one or more
selected from hydrogen, helium, argon, neon, CO, and CO.sub.2.
[0117] Treatment temperature: 800.degree. C. or higher
[0118] If the treatment temperature is lower than 800.degree. C., a
nitrogen-enriched layer is not created because nitrogen in the
treatment atmosphere does not permeate into the steel. Accordingly,
the treatment temperature is 800.degree. C. or higher. The
treatment temperature is preferably 850.degree. C. or higher.
However, the treatment temperature is preferably 1050.degree. C. or
lower because a treatment temperature of higher than 1050.degree.
C. (particularly 1100.degree. C. or higher) leads to deformation of
the steel. The treatment temperature is more preferably
1000.degree. C. or lower, and is particularly preferably
950.degree. C. or lower.
[0119] The treatment time is preferably in the range of 5 seconds
to 3600 seconds. The reason for this is that nitrogen in the
treatment atmosphere does not sufficiently permeate into the steel
if the treatment time is shorter than 5 seconds, whereas the
effects of treatment reach saturation if the treatment time is
longer than 3600 seconds. The treatment time is preferably in a
range of 30 seconds to 300 seconds.
[0120] Through the above description, conditions in treatment for
creating the nitrogen-enriched layer have been explained.
[0121] Although descaling may be performed after final annealing
(finish annealing) by normal pickling or polishing, from a
viewpoint of production efficiency, it is preferable to perform
descaling by adopting the high-speed pickling process described in
PTL 6 in which mechanical grinding is performed using a brush
roller, a polishing powder, shot blasting, or the like, and
pickling is subsequently performed in a nitrohydrochloric acid
solution.
[0122] In a situation in which treatment for creating the
nitrogen-enriched layer is performed during final annealing (finish
annealing), care should be taken to adjust the amount of pickling
or polishing in order that the nitrogen-enriched layer that has
been created is not removed.
EXAMPLES
[0123] Steels having the chemical compositions shown in Table 1
were each prepared by steelmaking using a 50 kg small vacuum
melting furnace. Each resultant steel ingot was heated to
1150.degree. C. in a furnace purged with Ar gas and was
subsequently subjected to hot rolling to obtain a hot-rolled sheet
having a thickness of 3.5 mm. Next, each of the hot-rolled sheets
was subjected to hot band annealing at 950.degree. C. for 1 minute
and shot blasting of the surface thereof with glass beads was
performed. Thereafter, descaling was performed by carrying out
pickling in which the sheet was immersed in a 200 g/1 sulfuric acid
solution at a temperature of 80.degree. C. for 120 seconds and was
subsequently immersed in a mixed acid of 150 g/1 of nitric acid and
30 g/1 of hydrofluoric acid at a temperature of 55.degree. C. for
60 seconds.
[0124] Next, cold rolling was performed to reach a sheet thickness
of 0.8 mm and annealing was performed under the conditions shown in
Table 2 to obtain a cold-rolled and annealed sheet. Note that in a
situation in which the external appearance of the sheet was deep
yellow or blue, it was judged that a thick oxide film had been
formed and +20 A/dm.sup.2.fwdarw.-20 A/dm.sup.2 electrolytic
picking was performed twice, with different electrolysis times, in
a mixed acid solution of 150 g/1 of nitric acid and 5 g/1 of
hydrochloric acid at a temperature of 55.degree. C.
[0125] Evaluation of (1) ductility and measurement of (2)
nitrogen-enriched layer nitrogen concentration were performed as
described below for each cold-rolled and annealed sheet obtained as
described above.
[0126] Furthermore, brazing was carried out for each cold-rolled
and annealed sheet using a Ni-containing brazing metal and the
cold-rolled and annealed sheet was evaluated after brazing in terms
of (3) corrosion resistance and (4) brazing properties. The
evaluation of (4) brazing properties was performed as described
below for (a) joint gap infiltration of the brazing metal and (b)
joint strength of a brazed part.
[0127] (1) Ductility evaluation
[0128] A JIS No. 13B tensile test piece was sampled at a right
angle to the rolling direction from each of the cold-rolled and
annealed sheets described above, a tensile test was carried out in
accordance with JIS Z 2241, and ductility was evaluated using the
following standard. The evaluation results are shown in Table
2.
[0129] Good (pass): Elongation after fracture of 20% or greater
[0130] Poor (fail): Elongation after fracture of less than 20%
[0131] (2) Measurement of Nitrogen-Enriched Layer Nitrogen
Concentration
[0132] The surface of each of the cold-rolled and annealed sheets
was analyzed by glow discharge optical emission spectroscopy
(hereinafter referred to as GDS). First, samples with different
sputtering times from the surface layer were prepared and
cross-sections thereof were observed by SEM in order to prepare a
calibration curve for a relationship between sputtering time and
depth.
[0133] Nitrogen concentration was measured while performing
sputtering from the surface of the steel to a depth of 0.50 .mu.m.
Herein, the measured values of Cr and Fe are fixed at the depth of
0.50 .mu.m and thus a measured value for nitrogen concentration at
the depth of 0.50 .mu.m was taken to be the nitrogen concentration
of the base material (steel substrate).
[0134] A highest peak value (greatest value) among measured
nitrogen concentration values within 0.05 .mu.m of the steel
surface was divided by the measured nitrogen concentration value at
the depth of 0.50 .mu.m and the resultant value was multiplied by a
nitrogen concentration of the steel obtained by chemical analysis
to give a value that was taken to be a nitrogen concentration peak
value at a depth of within 0.05 .mu.m of the surface. Nitrogen
concentration peak values that were obtained are shown in Table
2.
[0135] (3) Evaluation of Corrosion Resistance
[0136] After brazing was carried out for each of the cold-rolled
and annealed sheets, a 20 mm square test piece was sampled from a
part to which brazing metal was not attached, and the test piece
was covered by a sealing material, but leaving a 11 mm square
measurement surface. Thereafter, the test piece was immersed in a
3.5% NaCl solution at 30.degree. C. and a corrosion resistance test
was conducted in accordance with HS G 0577 with the exception of
the NaCl concentration. Pitting corrosion potentials V.sub.c'100
that were measured are shown in Table 2.
[0137] When usage conditions of a heat exchanger part of an exhaust
heat recovery unit or an EGR cooler are taken into account, a
pitting corrosion potential V.sub.c'100 of 150 (mV vs SCE) or
greater can be judged to indicate excellent corrosion
resistance.
[0138] (4) Evaluation of Brazing Properties
(a) Infiltration of Brazing Metal into Joint Gap
[0139] As illustrated in FIG. 1, a 30 mm square sheet and a 25
mm.times.30 mm sheet were cut out from each of the cold-rolled and
annealed sheets and these two sheets were overlapped and clamped in
place using a clamp jig with a fixed torque force (170 kgf). Next,
1.2 g of a brazing metal was applied onto an end surface of one of
the sheets and brazing was carried out. After the brazing, the
degree to which the brazing metal had infiltrated between the
sheets was visually confirmed from a side surface part of the
overlapped sheets and was evaluated using the following standard.
The evaluation results are shown in Table 2. Note that in the
drawings, the reference sign 1 indicates the cold-rolled and
annealed sheet and the reference sign 2 indicates the brazing
metal.
[0140] Excellent (pass, particularly good): Brazing metal
infiltration to opposite end relative to application end
[0141] Satisfactory (pass): Brazing metal infiltration over at
least 50% and less than 100% of the overlapping length of the two
sheets
[0142] Unsatisfactory (fail): Brazing metal infiltration over at
least 10% and less than 50% of the overlapping length of the two
sheets
[0143] Poor (fail): Brazing metal infiltration over less than 10%
of the overlapping length of the two sheets
[0144] (b) Joint Strength of Brazed Part
[0145] As illustrated in FIG. 2, portions of a JIS No. 13B tensile
test piece that had been split at the center thereof were
overlapped by 5 mm and were clamped in place using a clamp jig.
Next, brazing was carried out by applying 0.1 g of a brazing metal
to an overlapping part of one of the portions. After the brazing, a
tensile test was conducted at normal temperature and joint strength
of the brazed part was evaluated using the following standard. The
evaluation results are shown in Table 2. Note that in the drawings,
reference sign 3 indicates the tensile test piece.
[0146] Excellent (pass, particularly good): No brazed part fracture
even at 95% or greater of tensile strength of base material (base
material part fracture)
[0147] Satisfactory (pass): Brazed part fracture at 95% or greater
of tensile strength of base material
[0148] Unsatisfactory (fail): Brazed part fracture at 50% or
greater and less than 95% of tensile strength of base material
[0149] Poor (fail): Brazed part fracture at less than 50% of
tensile strength of base material
[0150] In each evaluation of brazing properties described above,
the brazing metal was a representative Ni-containing brazing metal
BNi-5 (19% Cr and 10% Si in a Ni matrix) stipulated by Japanese
Industrial Standards. The brazing was carried out in a sealed
furnace. Furthermore, brazing was carried out in a high vacuum
atmosphere of 10.sup.-2 Pa and was also carried out in an Ar
carrier gas atmosphere by enclosing Ar with a pressure of 100 Pa
after forming a high vacuum. A temperature pattern of the heat
treatment involved performing treatment with a heating rate of
10.degree. C/s, a first soaking time (step of equilibrating overall
temperature) of 1800 s at 1060.degree. C., a heating rate of
10.degree. C/s, and a second soaking time (step of actually
carrying out brazing at a temperature equal to or higher than the
melting point of the brazing metal) of 600 s at 1170.degree. C.,
followed by cooling of the furnace and purging of the furnace with
external air (atmosphere) once the temperature had fallen to
200.degree. C.
TABLE-US-00001 TABLE 1 Steel Chemical composition (mass %) symbol C
Si Mn P S Cr Ti Al N Ni Mo Cu V Nb Ca B Remarks A 0.012 0.12 0.22
0.03 0.0011 21.5 0.220 0.006 0.011 -- -- -- -- -- -- -- Conforming
steel B 0.010 0.09 0.18 0.02 0.0010 22.4 0.082 0.021 0.013 -- 1.05
-- -- 0.125 -- -- Conforming steel C 0.011 0.13 0.21 0.03 0.0013
21.5 0.124 0.041 0.010 -- -- -- -- -- -- -- Conforming steel D
0.009 0.20 0.19 0.03 0.0010 21.6 0.050 0.015 0.007 -- -- -- -- --
-- -- Conforming steel E 0.015 0.20 0.21 0.04 0.0020 21.6 0.105
0.008 0.012 0.12 -- 0.45 0.201 0.125 0.0023 -- Conforming steel F
0.008 0.22 0.22 0.03 0.0007 19.2 0.100 0.030 0.007 0.11 1.96 --
0.302 0.105 -- 0.0004 Conforming steel G 0.006 0.11 0.23 0.02
0.0010 16.5 0.066 0.035 0.007 0.21 1.15 -- 0.152 0.105 0.0020 --
Conforming steel H 0.015 0.20 0.19 0.02 0.0010 21.7 0.102 0.005
0.013 0.19 -- 0.48 0.225 0.085 -- 0.0005 Conforming steel I 0.007
0.10 0.22 0.03 0.0021 18.5 0.098 0.050 0.013 0.18 -- 0.49 0.223
0.095 -- -- Conforming steel J 0.008 0.26 0.21 0.03 0.0018 17.2
0.105 0.006 0.009 -- -- -- 0.220 -- 0.0032 0.0007 Conforming steel
K 0.007 0.23 0.22 0.02 0.0020 21.9 0.420 0.050 0.007 -- -- -- -- --
-- -- Comparative steel L 0.012 0.22 0.13 0.03 0.0011 19.3 0.382
0.030 0.016 0.09 1.86 0.42 -- 0.192 -- 0.0005 Comparative steel M
0.012 0.23 0.23 0.02 0.0010 21.5 0.041 0.015 0.014 0.21 -- 0.44
0.162 0.008 -- -- Comparative steel N 0.011 0.21 0.19 0.03 0.0016
21.5 0.105 0.070 0.008 0.15 -- 0.51 0.124 0.089 -- -- Comparative
steel O 0.007 0.21 0.19 0.03 0.0021 14.5 0.090 0.020 0.008 0.15 --
-- 0.094 0.068 -- -- Comparative steel
TABLE-US-00002 TABLE 2 Annealing conditions Measurement and
evaluation results (nitrogen-enriched layer Nitrogen creation
treatment conditions) concentration Pitting Atmosphere peak value
of corrosion Dew Treatment Treatment Post- nitrogen- potential
Steel H.sub.2 N.sub.2 point temperature time annealing Ductility
enriched layer V 100 No symbol (vol %) (vol %) (.degree. C.)
(.degree. C.) (s) pickling evaluation (mass %) (mV vs SCE) 1 A 5 95
-30 890 60 Yes Good 0.05 221 2 A 75 25 -55 950 30 No Good 0.25 212
3 D 10 90 -45 890 90 Yes Good 0.10 208 4 C 20 80 -25 860 60 Yes
Good 0.08 215 5 B 75 25 -50 900 60 No Good 0.23 285 6 B 5 95 -35
890 30 Yes Good 0.08 292 7 E 80 20 -50 890 60 No Good 0.19 208 8 F
75 25 -55 860 30 No Good 0.18 268 9 G 10 90 -35 880 60 Yes Good
0.06 276 10 H 5 95 -30 880 30 Yes Good 0.08 211 11 I 30 70 -40 860
60 Yes Good 0.08 192 12 J 10 90 -55 880 30 Yes Good 0.11 187 13 K
10 90 -45 950 30 Yes Good 0.11 205 14 L 10 90 -30 890 30 Yes Poor
0.09 267 15 M 75 25 -55 950 60 No Good 0.29 108 16 N 75 25 -55 890
60 Yes Good 0.22 212 17 O 10 90 -40 890 30 Yes Good 0.10 87 18 A 10
90 -10 890 60 Yes Good 0.02 211 19 A 100 0 -35 890 30 Yes Good 0.03
205 20 C 10 90 -45 750 60 Yes Poor 0.03 199 Measurement and
evaluation results Brazing properties evaluation Brazing properties
evaluation (brazing in high vacuum) (brazing in Ar atmosphere)
Brazing metal Brazed part Brazing metal Brazed part No infiltration
joint strength infiltration joint strength Remarks 1 Satisfactory
Satisfactory Satisfactory Satisfactory Example 2 Satisfactory
Excellent Satisfactory Satisfactory Example 3 Excellent Excellent
Excellent Excellent Example 4 Satisfactory Satisfactory
Satisfactory Satisfactory Example 5 Excellent Satisfactory
Excellent Satisfactory Example 6 Satisfactory Excellent Excellent
Satisfactory Example 7 Excellent Excellent Excellent Satisfactory
Example 8 Excellent Excellent Excellent Satisfactory Example 9
Excellent Satisfactory Satisfactory Satisfactory Example 10
Excellent Satisfactory Satisfactory Satisfactory Example 11
Excellent Satisfactory Satisfactory Satisfactory Example 12
Excellent Satisfactory Satisfactory Satisfactory Example 13 Poor
Unsatisfactory Poor Poor Comparative example 14 Unsatisfactory Poor
Poor Poor Comparative example 15 Excellent Excellent Excellent
Satisfactory Comparative example 16 Unsatisfactory Poor Poor Poor
Comparative example 17 Excellent Satisfactory Satisfactory
Satisfactory Comparative example 18 Poor Poor Poor Poor Comparative
example 19 Poor Unsatisfactory Poor Poor Comparative example 20
Unsatisfactory Poor Poor Poor Comparative example indicates data
missing or illegible when filed
[0151] Table 2 shows that for each of Examples 1-12, infiltration
of the brazing metal into the joint gap was good and joint strength
of the brazed part was good. Accordingly, it was demonstrated that
Examples 1-12 display good brazing properties even when a
Ni-containing brazing metal is used. Furthermore, Examples 1-12 had
good corrosion resistance and ductility.
[0152] In contrast, good brazing properties and/or good corrosion
resistance were not obtained in Comparative Examples 13-20 for
which the chemical composition or the nitrogen concentration peak
value was outside of the appropriate range.
INDUSTRIAL APPLICABILITY
[0153] The present disclosure enables a ferritic stainless steel to
be obtained that can be suitably used for heat exchanger components
and the like of exhaust heat recovery units and EGR coolers that
are assembled by brazing, and is therefore extremely useful in
industry.
REFERENCE SIGNS LIST
[0154] 1 cold-rolled and annealed sheet
[0155] 2 brazing metal
[0156] 3 tensile test piece
* * * * *