U.S. patent application number 13/337014 was filed with the patent office on 2012-09-27 for ferritic stainless steel and method of manufacturing the same.
This patent application is currently assigned to POSCO. Invention is credited to Sang-Seok Kim, Bo-Sung Seo, Do-leal Yoo.
Application Number | 20120241052 13/337014 |
Document ID | / |
Family ID | 46856732 |
Filed Date | 2012-09-27 |
United States Patent
Application |
20120241052 |
Kind Code |
A1 |
Kim; Sang-Seok ; et
al. |
September 27, 2012 |
FERRITIC STAINLESS STEEL AND METHOD OF MANUFACTURING THE SAME
Abstract
The present disclosure relates to a ferritic stainless steel and
fabrication method of a ferritic stainless steel comprising, by
weight %, C: above 0 wt % to 0.01 wt % or less, Si: above 0 wt % to
0.5 wt % or less, Mn: above 0 wt % to 2.0 wt % or less, P: 0 wt %
or more to 0.04 wt % or less, S: 0 wt % or more to 0.02 wt % or
less, Cr: 12 wt % or more to 19 wt % or less, Mo: 0 wt % or more to
1.0 wt % or less, W: 2 wt % of more to 7 wt % or less, Ti: 0 wt %
or more to 0.3 wt % or less, Nb: above 0 wt % to 0.6 wt % or less,
N: above 0 wt % to 0.01 wt % or less, Al: 0 wt % or more to 0.1 wt
% or less; and the balance of Fe and other inevitable
impurities.
Inventors: |
Kim; Sang-Seok; (Pohang-si,
KR) ; Yoo; Do-leal; (Pohang-si, KR) ; Seo;
Bo-Sung; (Pohang-si, KR) |
Assignee: |
POSCO
|
Family ID: |
46856732 |
Appl. No.: |
13/337014 |
Filed: |
December 23, 2011 |
Current U.S.
Class: |
148/506 ;
148/325; 148/505; 148/507; 148/609 |
Current CPC
Class: |
C22C 38/26 20130101;
C22C 38/22 20130101; C21D 2211/005 20130101; C21D 6/002 20130101;
C22C 38/28 20130101; C21D 8/0273 20130101; C21D 8/0473 20130101;
C22C 38/001 20130101; C22C 38/02 20130101; C22C 38/06 20130101;
C22C 38/04 20130101 |
Class at
Publication: |
148/506 ;
148/325; 148/609; 148/505; 148/507 |
International
Class: |
C21D 8/00 20060101
C21D008/00; C21D 11/00 20060101 C21D011/00; C22C 38/28 20060101
C22C038/28; C22C 38/38 20060101 C22C038/38; C22C 38/22 20060101
C22C038/22; C22C 38/26 20060101 C22C038/26 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 25, 2011 |
KR |
10-2011-0027104 |
Mar 25, 2011 |
KR |
10-2011-0027105 |
Claims
1. A ferritic stainless steel comprising: by weight %, C: above 0
wt % to 0.01 wt % or less, Si: above 0 wt % to 0.5 wt % or less,
Mn: above 0 wt % to 2.0 wt % or less, P: 0 wt % or more to 0.04 wt
% or less, S: 0 wt % or more to 0.02 wt % or less, Cr: 12 wt % or
more to 19 wt % or less, Mo: 0 wt % or more to 1.0 wt % or less, W:
2 wt % or more to 7 wt % or less, Ti: 0 wt % or more to 0.3 wt % or
less, Nb: above 0 wt % to 0.6 wt % or less, N: above 0 wt % to 0.01
wt % or less, Al: 0 wt % or more to 0.1 wt % or less; and the
balance of Fe and other inevitable impurities.
2. The ferritic stainless steel according to claim 1, wherein Mo is
0.8 wt % or less, by weight %.
3. The ferritic stainless steel according to claim 2, wherein a
hot-annealed structure of the ferritic stainless steel comprises a
sigma phase of 5% or less.
4. The ferritic stainless steel according to claim 1, wherein W is
3 wt % or more to 6 wt % or less, by weight %.
5. The ferritic stainless steel according to claim 1, wherein Mo wt
%+0.83W wt % is 3.5 wt % or more to 5 wt % or less, by weight
%.
6. The ferritic stainless steel according to claim 1, wherein [(Ti
wt %+1/2Nb wt %)/(C wt %+N wt %)] is 19.5 or more to 32 or
less.
7. The ferritic stainless steel according to claim 1, wherein
ductile-brittleness transition temperature (DBTT) is 90.degree. C.
or less.
8. The ferritic stainless steel according to claim 1, wherein the
ferritic stainless steel satisfies the following equation:
-184.6+3.2(Cr wt %)+27.5(Mo wt %)+4243.4(C wt %+N wt %)-295.6(Al wt
%)+0.9[Nb wt %/(C wt %+N wt %)] 90.
9. A fabrication method of a ferritic stainless steel comprising,
by weight %, C: above 0 wt % to 0.01 wt % or less, Si: above 0 wt %
to 0.5 wt % or less, Mn: above 0 wt % to 2.0 wt % or less, P: 0 wt
% or more to 0.04 wt % or less, S: 0 wt % or more to 0.02 wt % or
less, Cr: 12 wt % or more to 19 wt % or less, Mo: 0 wt % or more to
1.0 wt % or less, W: 2 wt % or more to 7 wt % or less, Ti: 0 wt %
or more to 0.3 wt % or less, Nb: above 0 wt % to 0.6 wt % or less,
N: above 0 wt % to 0.01 wt % or less, Al: 0 wt % or more to 0.01 wt
% or less; and the balance of Fe and other inevitable impurities,
wherein the fabrication method of the ferritic stainless steel
sheet comprises: providing a slab; heating the slab; hot annealing;
cold annealing; and cold rolling.
10. The fabrication method of a ferritic stainless steel according
to claim 9, wherein Mo is 0.8 wt % or less, by weight %.
11. The fabrication method of a ferritic stainless steel according
to claim 9, wherein W is 3 wt % or more to 6 wt % or less, by
weight %.
12. The fabrication method of a ferritic stainless steel according
to claim 9, wherein: Mo wt %+0.83W wt % is 3.5 wt % or more to 5 wt
% or less, by weight %.
13. The fabrication method of a ferritic stainless steel according
to claim 9, wherein: [(Ti wt %+1/2Nb wt %)/(C wt %+N wt %)] is 19.5
or more to 32 or less.
14. The fabrication method of a ferritic stainless steel according
to claim 9, wherein: ductile-brittleness transition temperature
(DBTT) is 90.degree. C. or less.
15. The fabrication method of a ferritic stainless steel according
to claim 9, wherein the ferritic stainless steel satisfies the
following equation: -184.6+3.2(Cr wt %)+27.5(Mo wt %)+4243.4(C wt
%+N wt %)-295.6(Al wt %)+0.9[Nb wt %/(C wt %+N wt %)]<90.
16. The fabrication method of a ferritic stainless steel according
to claim 9, wherein grain size is ASTM No. 3 or more.
17. The fabrication method of a ferritic stainless steel according
to claim 9, wherein heating the slab has slab heating temperature
that is 1180.degree. C. or more to 1240.degree. C. or less, with
respect to slab temperature.
18. The fabrication method of a ferritic stainless steel according
to claim 9, wherein hot annealing has a hot-annealing temperature
that is 1020.degree. C. or more to 1070.degree. C. or less, with
respect to strip temperature.
19. The fabrication method of a ferritic stainless steel according
to claim 9, wherein cold annealing has a cold-annealing temperature
that is 1030.degree. C. or more to 1080.degree. C. or less, with
respect to strip temperature.
20. The fabrication method of a ferritic stainless steel according
to claim 9, wherein (cold-annealing temperature)/(hot-annealing
temperature) is 1.0 or more to 1.1 or less, and wherein the
cold-annealing temperature is the temperature in cold-annealing and
the hot-annealing temperature is the temperature in hot annealing.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims benefit of
priority of Korean Patent Application No. 10-2011-0027104 filed on
Mar. 25, 2011, and Korean Patent Application No. 10-2011-0027105
filed on Mar. 25, 2011. The entire contents of both applications
are incorporated herein by reference.
BACKGROUND
[0002] 1. Technical Field
[0003] The present disclosure relates to a ferritic stainless steel
which can be used for the exhaust manifold of vehicles and a method
of manufacturing a ferritic stainless steel which can be used for
the exhaust manifold of vehicles.
[0004] 2. Description of the Related Art
[0005] Recently, the laws about discharging toxic substances in
exhaust gases have been in force in many countries, against
seriousness of environmental problems due to the exhaust gases from
vehicles. A technology for improving performance of purifying the
exhaust gas, using a catalyst, in consideration of the trend, has
been the focus. The higher the temperature, the more the purifying
reaction of NOx, HC, and CO increases. Therefore, in order to
reduce discharging of pollutants, it has been a trend to
continuously increase the temperature of the exhaust gas, and
accordingly, it is strongly required to improve high-temperature
characteristics of the parts constituting an exhaust system
controlling the exhaust gas.
[0006] An exhaust manifold is a part that collects an exhaust gas
from the cylinders in an engine and discharges the exhaust gas to
exhaust pipes. Since the temperature of the exhaust gas reaches up
to 900.degree. C., the exhaust manifold is a part requiring
oxidation resistance, high-temperature strength, and thermal
fatigue property. In the related art, although nodular cast iron
has been used as a material for the exhaust manifold, it has been
replaced with ferritic stainless steel by a request for increase in
temperature of an exhaust gas and decrease in weight of parts.
Further, recently, as a turbo is mounted and an engine is downsized
to improve fuel efficiency of vehicles, the temperature of an
exhaust gas is expected to increase 30.degree. C. to 50.degree. C.,
as compared with the existing vehicles.
[0007] Therefore, 429EM, 441, and 444, the type of ferritic
stainless steel used for the exhaust manifold in the related art
cannot satisfy the product quality of customers, such that various
studies about ferritic stainless steel having improved
high-temperature performance have been conducted.
[0008] The present disclosure is directed to overcoming one or more
of the problems set forth above and/or other problems of the prior
art.
SUMMARY
[0009] In one aspect, the present disclosure provides a ferritic
stainless steel comprising, by weight %, C: above 0 wt % to 0.01 wt
% or less, Si: above 0 wt % to 0.5 wt % or less, Mn: above 0 wt %
to 2.0 wt % or less, P: 0 wt % or more to 0.04 wt % or less, S: 0
wt % or more to 0.02 wt % or less, Cr: 12 wt % or more to 19 wt %
or less, Mo: 0 wt % or more to 1.0 wt % or less, W: 2 wt % or more
to 7 wt % or less, Ti: 0 wt % or more to 0.3 wt % or less, Nb:
above 0 wt % to 0.6 wt % or less, N: above 0 wt % to 0.01 wt % or
less, Al: 0 wt % or more to 0.1 wt % or less; and the balance of Fe
and other inevitable impurities.
[0010] In another aspect, the present disclosure provides a
fabrication method of a ferritic stainless steel comprising, by
weight %, C: above 0 wt % to 0.01 wt % or less, Si: above 0 wt % to
0.5 wt % or less, Mn: above 0 wt % to 2.0 wt % or less, P: 0 wt %
or more to 0.04 wt % or less, S: 0 wt % or more to 0.02 wt % or
less, Cr: 12 wt % or more to 19 wt % or less, Mo: 0 wt % or more to
1.0 wt % or less, W: 2 wt % or more to 7 wt % or less, Ti: 0 wt %
or more to 0.3 wt % or less, Nb: above 0 wt % to 0.6 wt % or less,
N: above 0 wt % to 0.01 wt % or less, Al: 0 wt % or more to 0.1 wt
% or less; and the balance of Fe and other inevitable impurities,
wherein the fabrication method of the ferritic stainless steel
comprises: providing a slab; heating the slab; hot annealing; cold
annealing; and cold rolling.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a graph showing a test result of an influence of
Mo and W on high-temperature strength.
[0012] FIG. 2A is an optical microscopic picture of a hot-annealed
structure of Mo-added steel.
[0013] FIG. 2B is an optical microscopic picture of a hot-annealed
structure of Mo+W-added steel.
[0014] FIG. 3 is a graph testing a thermal fatigue property of
ferritic stainless steel according to the addition amount of Mo and
W.
[0015] FIG. 4 is a graph testing high-temperature oxidation
resistance of ferritic stainless steel according to the addition
amount of Mo and W.
[0016] FIG. 5 is a graph testing high-temperature salt corrosion
resistance of ferritic stainless steel according to the addition
amount of Mo and W.
[0017] FIG. 6 is a flowchart schematically illustrating a
fabrication method of a ferritic stainless steel according to one
embodiment of the present invention.
[0018] FIG. 7 is a graph showing the grain size of ferritic
stainless steel according to slab heating temperature.
[0019] FIG. 8 is a graph showing average r-bar values of ferritic
stainless steel according to slab heating temperature.
[0020] FIG. 9 is a graph showing average r-bar values according to
hot-annealing temperature in hot annealing.
[0021] FIG. 10 is a graph showing high-temperature tensile strength
according to cold-annealing temperature in cold annealing.
[0022] FIG. 11 is a graph showing average r-bar values according to
cold-annealing temperature/hot-annealing temperature.
[0023] FIG. 12 is a graph showing high-temperature tensile strength
according to cold-annealing temperature/hot-annealing
temperature.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0024] Features of the present disclosure and methods to achieve
them will be clear from exemplary embodiments described below in
detail and with reference to the accompanying drawings. However,
the present disclosure is not limited to the embodiments described
hereafter and may be implemented in various ways.
[0025] Provided is a ferritic stainless steel comprising, by weight
%, C: above 0 wt % to 0.01 wt % or less, Si: above 0 wt % to 0.5 wt
% or less, Mn: above 0 wt % to 2.0 wt % or less, P: 0 wt % or more
to 0.04 wt % or less, S: 0 wt % or more to 0.02 wt % or less, Cr:
12 wt % or more to 19 wt % or less, Mo: 0 wt % or more to 1.0 wt %
or less, W: 2 wt % or more to 7 wt % or less, Ti: 0 wt % or more to
0.3 wt % or less, Nb: above 0 wt % to 0.6 wt % or less, N: above 0
wt % to 0.01 wt % or less, Al: 0 wt % or more to 0.1 wt % or less;
and the balance of Fe and other inevitable impurities.
[0026] In some embodiments, Mo is 0.8 wt % or less, by weight
%.
[0027] In some embodiments, a hot-annealed structure of the
ferritic stainless steel comprises a sigma phase of 5% or less.
[0028] In some embodiments, W is 3 wt % or more to 6 wt % or less,
by weight %.
[0029] In some embodiments, Mo wt %+0.83W wt % is 3.5 wt % or more
to 5 wt % or less, by weight %.
[0030] In some embodiments, [(Ti wt %+1/2Nb wt %)/(C wt %+N wt %)]
is 19.5 or more to 32 or less.
[0031] In some embodiments, ductile-brittleness transition
temperature (DBTT) is 90.degree. C. or less.
[0032] In some embodiments, the ferritic stainless steel satisfies
the following equation: -184.6+3.2 (Cr wt %)+27.5(Mo wt %)+4243.4(C
wt %+N wt %)-295.6(Al wt %)+0.9[Nb wt %/(C wt %+N wt %)] 90.
[0033] Also provided is a fabrication method of a ferritic
stainless steel comprising, by weight %, C: above 0 wt % to 0.01 wt
% or less, Si: above 0 wt % to 0.5 wt % or less, Mn: above 0 wt %
to 2.0 wt % or less, P: 0 wt % or more to 0.04 wt % or less, S: 0
wt % or more to 0.02 wt % or less, Cr: 12 wt % or more to 19 wt %
or less, Mo: 0 wt % or more to 1.0 wt % or less, W: 2 wt % or more
to 7 wt % or less, Ti: 0 wt % or more to 0.3 wt % or less, Nb:
above 0 wt % to 0.6 wt % or less, N: above 0 wt % to 0.01 wt % or
less, Al: 0 wt % or more to 0.1 wt % or less; and the balance of Fe
and other inevitable impurities,
[0034] wherein, the fabrication method of the ferritic stainless
steel comprises: providing a slab; heating the slab; hot annealing;
cold annealing; and cold rolling.
[0035] In some embodiments, Mo is 0.8 wt % or less, by weight
%.
[0036] In some embodiments, W is 3 wt % or more to 6 wt % of less,
by weight %.
[0037] In some embodiments, Mo wt %+0.83W wt % is 3.5 wt % or more
to 5 wt % or less, by weight %.
[0038] In some embodiments, [(Ti wt %+1/2Nb wt %)/(C wt %+N wt %)]
is 19.5 or more to 32 or less.
[0039] In some embodiments, ductile-brittleness transition
temperature (DBTT(.degree. C.)) is 90.degree. C. or less.
[0040] In some embodiments, the ferritic stainless steel satisfies
the following equation: -184.6+3.2(Cr wt %)+27.5(Mo wt %)+4243.4(C
wt %+N wt %)-295.6(Al wt %)+0.9[Nb wt %/(C wt %+N wt %)] 90.
[0041] In some embodiments, grain size is ASTM No. 3 or more. In
some embodiments, heating the slab has slab heating temperature
that is 1180.degree. C. or more to 1240.degree. C. or less, with
respect to slab temperature.
[0042] In some embodiments, hot annealing has a hot-annealing
temperature that is 1020.degree. C. or more to 1070.degree. C. or
less, with respect to strip temperature.
[0043] In some embodiments, cold annealing has a cold-annealing
temperature that is 1030.degree. C. or more to 1080.degree. C. or
less, with respect to strip temperature.
[0044] In some embodiments, (cold-annealing
temperature)/(hot-annealing temperature) is 1.0 or more to 1.1 or
less, wherein the cold-annealing temperature is the temperature in
cold-annealing and the hot-annealing temperature is the temperature
in hot annealing.
[0045] In some embodiments, cold rolling has a cold rolling
temperature that is room temperature.
[0046] High-temperature oxidation resistance, high-temperature salt
corrosion resistance, high-temperature strength, thermal fatigue
property, and formability of ferritic stainless steel may be
influenced by the elements contained in the ferritic stainless
steel and the addition amount of the element. Hereafter, the
component systems constituting the ferritic stainless steel of the
present disclosure are described in more detail. The following
component systems are expressed by weight %.
[0047] C may be above 0 wt % to 0.01 wt % or less in the ferritic
stainless steel. Since C can increase room-temperature strength of
the ferritic stainless steel, C can be added. On the contrary, when
the amount of C is above 0.01 wt %, the room-temperature strength
of the ferritic stainless steel may be increased, while
high-temperature strength and ductility, machinability, and
toughness at room temperature may be relatively decreased.
Therefore, C may be above 0 wt % to 0.01 wt % or less, such as
0.005 wt % or less.
[0048] Si may be above 0 wt % to 0.5 wt % or less in the ferritic
stainless steel. Si functions as a deoxidizer for a molten metal
state of the ferritic stainless steel. Further, Si may improve
oxidation resistance of the ferritic stainless steel. On the
contrary, when Si is above 0.5 wt %, hardness of the ferritic
stainless steel may be increased by solid solution hardening of Si,
such that elongation and machinability of the ferritic stainless
steel may be decreased. Therefore, Si may be above 0 wt % to 0.5 wt
% or less.
[0049] Mn may be above 0 wt % to 2.0 wt % or less in the ferritic
stainless steel. A scale may be produced at high temperature when
the ferritic stainless steel is used as a material of the exhaust
manifold of vehicles. In this case, the produced scale may be
easily separated and the separated scale may flow into a catalytic
converter and block the passage of the catalytic converter.
Therefore, the ferritic stainless steel may have separation
resistance for the scale, such that it may comprise Mn for the
separation resistance. On the other hand, when Mn is above 2.0 wt
%, MnS may be produced by reaction of Mn and S. MnS may have an
adverse influence on the corrosion resistance of the ferritic
stainless steel. Therefore, Mn may be above 0 wt % to 2.0 wt % or
less.
[0050] Although P can increase the strength of the ferritic
stainless steel, it may decrease the machinability. Further, P may
be considered an impurity in the steelmaking process of the
ferritic stainless steel, so P may be reduced. On the other hand,
excessively decreasing P in processes is inefficient in terms of
refining cost or productivity. Therefore, P may be 0 wt % or more
to 0.04 wt % or less.
[0051] S may be 0 wt % or more to 0.02 wt % or less in the ferritic
stainless steel. S may exist as an inclusion in the ferritic
stainless steel or may function as an impurity that decreases the
corrosion resistance. Therefore, although the amount of S may be
reduced to improve corrosion resistance of the ferritic stainless
steel, it may be inefficient in terms of cost and time to
excessively reduce S in the process. Accordingly, S may be 0 wt %
or more to 0.02 wt % or less, such as 0.003 wt % or less.
[0052] Cr may be 12 wt % or more to 19 wt % or less in the ferritic
stainless steel. Cr is an alloy element that may improve corrosion
resistance and oxidation resistance of the ferritic stainless
steel. The ferritic stainless steel may not have sufficient
corrosion resistance when the amount of Cr is low, therefore Cr may
be 12 wt % or more. On the other hand, when the amount of Cr is
above 19 wt %, the corrosion resistance of the ferritic stainless
steel may be improved, whereas the strength may be excessively
increased, such that the elongation and a shock property may be
decreased. Therefore, Cr may be 12 wt % or more to 19 wt % or
less.
[0053] Ti may be 0 wt % or more to 0.3 wt % or less in the ferritic
stainless steel. Ti is an alloy element that may be added to
improve the high-temperature strength and intergranular corrosion
resistance of the ferritic stainless steel. When the amount of Ti
in the ferritic stainless steel is above 0.3 wt %, steelmaking
inclusion may increase and a surface defect, such as scab, may be
generated and a nozzle may be clogged in continuous casting, such
that the process efficiency may be decreased. Further, with the
increase of solid solution Ti, the elongation and low-temperature
shock property of the ferritic stainless steel may be decreased.
Further, when Nb is added with Ti in the ferritic stainless steel
and the ferritic stainless steel is used at high temperature for a
long period of time, Fe3Nb3C carbide may be educed and coarsening
may occur, such that high-temperature deterioration may be caused.
Therefore, Ti may be 0 wt % or more to 0.3 wt % or less.
[0054] N may be above 0 wt % to 0.01 wt % or less in the ferritic
stainless steel. Although N, similar to C, can increase the
strength of the ferritic stainless steel, it may decrease ductility
and machinability. Therefore, N may be above 0 wt % to 0.01 wt % or
less to ensure sufficient elongation and machinability of welded
portions, such as 0.007 wt % or less of N.
[0055] Mo may be 0 wt % or more to 1.0 wt % or less in the ferritic
stainless steel, such as 0.8 wt % or less of Mo. When Mo is 0.8 wt
% or less, the hot-annealed structure of the ferritic stainless
steel may comprise a sigma phase of 5% or less.
[0056] W may be 2 wt % or more to 7 wt % or less in the ferritic
stainless steel, such as 3 wt % or more to 6 wt % or less.
[0057] Various studies and efforts, such as addition of Mo, have
been made in order to affect the high-temperature strength of
ferritic stainless steel. In a method of addition of Mo, when Mo in
ferritic stainless steel is 3 wt % or more, a sigma phase of
ferritic stainless steel is produced. The sigma phase may not only
cause a defect in manufacturing ferritic stainless steel, but may
cause a decline in durability when ferritic stainless steel is used
for the exhaust manifold of vehicles. It is possible to prevent a
sigma phase from being produced by reducing Mo in the ferritic
stainless steel according to the present disclosure. Further, the
amount of Mo may be 1 wt % or less in the ferritic stainless steel
according to the present disclosure to ensure the high-temperature
strength. Mo may be 0.8 wt % or less.
[0058] In the steelmaking process of the ferritic stainless steel,
since the steelmaking process is performed in large quantities, it
is not easy to control the amount of substances within very small
level, such that an effort for controlling the amount may be
inefficient. On the other hand, since the elements, such as Mo, are
expensive raw materials controlling the addition amount of Mo may
reduce the manufacturing cost. Therefore, it may be possible to
keep the properties of the ferritic stainless steel and improve the
process efficiency by controlling the amount of Mo at 0.8 wt % or
less.
[0059] When the amount of W is less than 2 wt %, the produced
amount of nano-sized fine extracts, such as Fe2W, and the solid
solution amount of W in a matrix may be decreased, such that it may
be difficult to provide the ferritic stainless steel with
sufficient high-temperature strength and thermal fatigue property.
Further, when the amount of W is above 7 wt %, the cost of raw
materials of the ferritic stainless steel may increase and a large
amount of Fe2W may be produced in the ferritic stainless steel,
which may be disadvantageous in line threading and may reduce
production efficiency, and may decrease weldability and
formability. The ferritic stainless steel shows tensile strength of
40 MPa or more in a high-temperature tensile strength test at
900.degree. C. by further comprising W, such that it can be used
for the exhaust manifold of vehicles which require high strength at
high temperature.
[0060] When the amount of Mo is 1.0 wt % or less, such as 0.8 wt %
or less, the ferritic stainless steel may further comprise W in
order to ensure high-temperature oxidation resistance,
high-temperature salt corrosion resistance, high-temperature
strength, and a thermal fatigue property. Considering the influence
on the high-temperature strength of the ferritic stainless steel,
the relationship between the two elements, Mo and W, may be
expressed as Mo wt %+0.83W wt %=3.5 wt % or more to 5 wt % or less.
When Mo wt %+0.83W wt % is less than 3.5 wt %, the properties,
high-temperature strength, high-temperature fatigue lifespan,
high-temperature oxidation resistance, and high-temperature salt
corrosion resistance, of the ferritic stainless steel may be
decreased, and when it is above 5 wt %, the high-temperature
properties may be excellent, but the elongation, which is a factor
of room-temperature machinability, may be decreased, and toughness
of the welded portions and the mother material may also be
decreased.
[0061] The ferritic stainless steel may comprise Mo wt %+0.83W wt %
of 3.5 wt % or more to 5 wt % or less. For Mo and W in the ferritic
stainless steel, when the value of Mo wt %+0.83W wt % is less than
3.5 wt %, it may be difficult to provide the ferritic stainless
steel with sufficient high-temperature strength and thermal fatigue
property in order to be used for the exhaust manifold of vehicles.
When Mo wt %+0.83W wt % of less than 3.5 wt % is in the ferritic
stainless steel, the maximum available temperature of an exhaust
manifold of a vehicle which is made of the ferritic stainless steel
may be 900.degree. C. or less, such that it is not available at
higher temperature. Further, when the value of Mo wt %+0.83W wt %
is above 5 wt %, a problem occurs in the line threading of the
ferritic stainless steel, such that productivity may be decreased,
and formability and weldability may also be decreased.
[0062] Ti may be 0 wt % or more to 0.3 wt % or less, Nb may be
above 0 wt % to 0.6 wt % or less, N may be above 0 wt % to 0.01 wt
% or less, and Al may be 0 wt % or more to 0.1 wt % or less, in the
ferritic stainless steel. The relationship between the elements,
[(Ti wt %+1/2Nb wt %)/(C wt %+N wt %)], may be 19.5 or more to 32
or less.
[0063] The ferritic stainless steel may comprise a predetermined
amount of Ti and Nb. When the amount of Ti and Nb is less than a
predetermined level, granular corrosion may occur at welding
heat-influenced portions, or the high-temperature strength and
thermal fatigue property may be decreased. Therefore, the amount of
Ti and Nb can be controlled such that [(Ti wt %+1/2Nb wt %)/(C wt
%+N wt %)] may be 19.5 or more. On the other hand, when
[(Ti+1/2Nb)/(C+N)] is above 32, it may be advantageous in
high-temperature properties of the ferritic stainless steel, the
amount of solid solution Nb is excessively increased, such that
room-temperature elongation, toughness, and machinability may be
decreased. Therefore, [(Ti wt %+1/2Nb wt %)/(C wt %+N wt %)] may be
19.5 or more to 32 or less.
[0064] Hereafter, the present disclosure is described with
reference to embodiments and comparative embodiments, which is for
illustrating the present disclosure. The present disclosure is not
limited to the following embodiments and comparative
embodiments.
1. Manufacturing of Sample
[0065] Table 1 shows chemical components of samples used in the
embodiments and comparative embodiments. Referring to Table 1, the
embodiments and comparative embodiments comprise Fe-15 wt % Cr as a
basic composition, and ferritic stainless steels were manufactured
by changing the addition amount of Mo, W, and Nb, and [(Ti wt
%+1/2Nb wt %)/(C wt %+N wt %)]. Hot annealing was partially
performed with 20 mmt and 5 mmt, and coils having a thickness of
2.0 mm and samples having 20 mmt bar were manufactured by a hot
annealing process and a cold annealing process. The samples
manufactured as described above were the first Embodiments 1 to 7
and Comparative embodiments 1 to 4 in Table 1.
TABLE-US-00001 TABLE 1 ((Ti + Mo + 1/2Nb)/ C Si Mn P S Cr Mo W Ti
Nb N Al Mo + W 0.83W (C + N)) invention 0.005 0.285 0.985 0.027
0.0014 14.9 0.51 4.54 0.105 0.43 0.005 0.07 5.05 4.28 32.00 steel 1
invention 0.0043 0.136 0.988 0.0281 0.0003 14.923 0.74 3.816 0.0921
0.442 0.0063 0.065 4.556 3.91 29.54 steel 2 invention 0.006 0.149
0.976 0.0238 0.0014 14.94 0.522 4.599 0.106 0.384 0.0077 0.072
5.121 4.34 21.75 steel 3 invention 0.005 0.282 0.923 0.027 0.0002
14.7 0.503 5.17 0.102 0.42 0.005 0.07 5.673 4.79 31.20 steel 4
invention 0.0066 0.189 0.976 0.0272 0.0014 14.8 0.704 5 0.0898
0.381 0.0076 0.062 5.704 4.85 19.74 steel 5 invention 0.005 0.21
1.01 0.027 0.0013 15.1 0.76 3.5 0 0.504 0.0064 0.081 4.26 3.67
22.11 steel 6 invention 0.0046 0.294 0.961 0.028 0.0014 14.95 0.7
3.6 0.11 0.41 0.0068 0.058 4.3 3.69 27.63 steel 7 comparative 0.006
0.298 1 0.028 0.0012 15 0.495 0 0.107 0.42 0.0049 0.05 0.495 0.50
29.08 steel 1 comparative 0.005 0.301 1 0.028 0.0013 15.23 1.51 0
0.108 0.43 0.0049 0.056 1.51 1.51 32.63 steel 2 comparative 0.005
0.298 0.986 0.027 0.00134 15 0.552 1.02 0.106 0.43 0.0048 0.061
1.572 1.40 32.76 steel 3 comparative 0.0076 0.439 0.884 0.0231
0.0006 18.24 1.94 0 0.125 0.463 0.0062 0.043 1.94 1.94 25.83 steel
4
2. Property Test of Ferritic Stainless Steel at Room Temperature
and High Temperature.
[0066] As shown in Table 2, a room-temperature tensile strength
test and thermal fatigue lifespan, oxidation resistance, and salt
corrosion resistance were performed at high temperature in order to
check high-temperature properties of the components, in Embodiments
1 to 7 and Comparative embodiments 1 to 4, which were manufactured,
as shown in Table 1.
[0067] First, thermal fatigue samples were manufactured by
machining the samples according to Table 1. Thermal fatigue
lifespan was tested within the temperature range of 200-900.degree.
C. and a confinement factor of 0.3 by using the thermal fatigue
samples manufactured as described above. Further, Embodiments 1 to
7 and Comparative embodiments 1 to 4 were heated at 1000.degree. C.
for 200 hours to test the oxidation resistance. Changes in weight
were measured and the oxidation resistance at high temperature were
checked, after cleaning and removing an oxidized scale produced by
heating, with acid. Solution of 26% NaCl was made to test the salt
corrosion resistance. Impregnating the samples according to
Embodiments 1 to 7 and Comparative embodiments 1 to 4 with the
solution of 26% NaCl for 5 minutes after keeping at 500.degree. C.
for 2 hours were performed ten times, and then reduction of weight
was measured and the salt corrosion resistance at high temperature
was tested.
[0068] The following Table 2 shows the test results of the
embodiments and the comparative embodiments according to Table 1 of
room-temperature tensile strength, r-bar value, thermal fatigue
lifespan, high-temperature oxidation resistance, and
high-temperature salt corrosion resistance.
TABLE-US-00002 TABLE 2 room- temperature thermal tensile fatigue
high-temperature high-temperature salt strength lifespan oxidation
resistance corrosion resistance % r-bar value (cycle) (mg/cm2)
judgment (g/mm2) judgment invention 35.2 0.906 2250 8.077
.smallcircle. 2.996 .smallcircle. steel 1 invention 32.6 0.945 2260
10.1 .smallcircle. 2.823 .smallcircle. steel 2 invention 38.5 1.015
1860 8 .smallcircle. 2.996 .smallcircle. steel 3 invention 32.3
0.91 7.93 .smallcircle. 2.6 .smallcircle. steel 4 invention 31.5
0.932 2520 7.8 .smallcircle. 2.61 .smallcircle. steel 5 invention
31.1 1.01 10.56 .smallcircle. 2.6 .smallcircle. steel 6 invention
31.9 1.019 10.4 .smallcircle. 2.7 .smallcircle. steel 7 comparative
37.4 1.318 12.1 x 20.2 x steel 1 comparative 37.8 1.166 1320 4.603
x 18.288 x steel 2 generating scale comparative 38.4 1.175 1310
5.21 x 18.1 x steel 3 generating scale comparative 30 0.955 1542
11.118 x 14.378 x steel 4
[0069] Referring to Tables 1 and 2, it could be seen that the
room-temperature tensile strength and r-bar values that are factors
estimating ease of forming both satisfied predetermined desired
values in Embodiments 1 to 7 and Comparative embodiments 1 to 4. On
the other hand, it could be seen that although the thermal fatigue
lifespan, high-temperature oxidation resistance, and
high-temperature salt corrosion resistance, that are factors for
testing properties at high temperature satisfied predetermined
desired values for the exhaust manifold of a vehicle in Embodiments
1 to 7, the test items failed to be satisfied in Comparative
embodiments 1 to 4.
[0070] All of Embodiments 1 to 7 according to the present
disclosure comprise Mo of which the addition amount is 0.8 wt % or
less. Further, all of Embodiments 1 to 7 comprise W of 3 wt % or
more to 6 wt % or less. In this case, it could be seen that the
room-temperature and the r-bar value were excellent, at 31% or more
and 0.9 or more, respectively, which are items that makes it
possible to test formability of ferritic stainless steel, in the
embodiments of the present disclosure. Further, it could be seen
that the thermal fatigue lifespan of Embodiments 1-3 and 5
according to the present disclosure were 2250, 2260, 1860, and
2520, respectively, which all satisfied 188 cycle or more.
[0071] In the test of high-temperature oxidation resistance in
Embodiments 1 to 7, it could be seen that the maximum of weight
changes was 10.56 mg/cm2 (Embodiment 6) and the minimum was 7.8
mg/cm2 (Embodiment 5). On the other hand, it could be seen that
scale separation occurred in Comparative embodiments 2 and 3 of
Comparative embodiments 1 to 4.
[0072] For the high-temperature salt corrosion resistance, it could
be seen that the maximum was 2.996 g/mm2 and the decrease in weight
was 3 g/mm2 in Embodiments 1 to 7, whereas the maximum was 20.2
g/mm2 and the minimum was 14.378 g/mm2 in Comparative embodiments 1
to 4 (Embodiment 4), that is, the decrease in weight was increased
in comparison to Embodiments 1 to 7.
[0073] According to the test results, it could be seen that the
room-temperature tensile strength and r-bar value, which are
properties of formability at room temperature, show substantially
equivalent values in the embodiments and the comparative
embodiments, whereas the thermal fatigue property, high-temperature
oxidation resistance, and high-temperature salt corrosion
resistance of the comparative embodiments, which are
high-temperature properties, were decreased, as compared with the
embodiments. For the high-temperature oxidation resistance in
Embodiments 1 to 7, the change in weight was 11 mg/cm2 or less and
all of the samples satisfied the high-temperature oxidation
resistance. On the other hand, for the comparative embodiments, the
high-temperature oxidation resistance failed to satisfy
predetermined desired conditions in Comparative embodiments and 4,
and in addition, a scale is separated in Comparative embodiments 2
and 3. Further, in the high-temperature salt corrosion resistance
repetitively tested, it could be seen that the decrease in weight
was 3 g/mm2 or less in Embodiments 1 to 7, while the maximum of the
decrease in weight was 20.2 g/mm2 in the comparative embodiments,
such that all the four comparative embodiments failed to satisfy
the high-temperature salt corrosion resistance.
[0074] It could be seen that all Embodiments 1 to 7 according to
the present disclosure satisfied the properties at high
temperature, in addition to formability at room temperature.
Therefore, the ferritic stainless steel according to the present
disclosure is available at high temperature and the formability can
also satisfy predetermined conditions. Therefore, it could be seen
that the embodiments can be used for the exhaust manifold of
vehicles which requires formability.
[0075] The ferritic stainless steel according to the present
disclosure comprises a smaller amount of Mo that is an expensive
raw material and has properties suitable for an exhaust manifold,
such that it is possible to reduce the manufacturing cost. On the
other hand, in the comparative embodiments that fail to satisfy the
content of Mo and W, it could be seen that the thermal fatigue
lifespan, high-temperature oxidation resistance, and
high-temperature salt corrosion resistance, which are properties at
high temperature, were not satisfied.
[0076] Further, referring to Tables 1 and 2, it was shown that the
samples manufactured by controlling the value of Mo wt %+0.83W wt %
at 3.5% or more to 5% or less and the value of [(Ti wt %+1/2Nb wt
%)/(C wt %+N wt %)] at 19.5 or more to 32 or less satisfy all the
properties, because the high-temperature properties and
high-temperature formability are both excellent.
[0077] FIG. 1 is a graph showing a test result of an influence of
Mo and W on high-temperature strength.
[0078] Referring to FIG. 1, by weight %, C: 0.005 wt %, N: 0.0006
wt %, Cr: 15 wt %, Nb: 0.4 wt % and Ti: 0.1 wt % were contained in
all the cases, and the influence of Mo and W on the
high-temperature strength of ferritic stainless steel was tested
while changing the addition amount of Mo and W.
[0079] First, the addition amounts of Mo-added steel and Mo+W-added
steel were changed and the high-temperature tensile strength was
tested at 900.degree. C., in order to test influence of Mo and W on
the high-temperature strength. X-axis shows the addition amount of
Mo (Mo-added steel) or the addition amount of Mo+W (Mo+W-added
steel) and Y-axis shows the corresponding high-temperature tensile
strength in the graph, and the relational formula was acquired. The
relationship with the addition amount of Mo or Mo+W that influences
the high-temperature strength was shown by using a regression
equation. As a result, a relational formula of high-temperature
tensile strength at 900.degree. C. (MPa)=22.4+4.7Mo+3.9W could be
acquired, and accordingly, it could be seen that the contribution
degree of W influencing the high-temperature strength was 83%
(W/Mo=3.9/4.7=0.83).
[0080] FIG. 2A is an optical microscopic picture of a hot-annealed
structure of Mo-added steel and FIG. 2B is an optical microscopic
picture of a hot-annealed structure of Mo+W-added steel.
[0081] FIGS. 2A and 2B are optical microscopic pictures comparing
sigma phases in the hot-annealed structures of the Mo-added steel
and the Mo+W-added steel, respectively. FIG. 2A shows a
hot-annealed structure of ferritic stainless steel containing Mo of
3 wt %, wherein the sigma phase existed up to 20% in the
hot-annealed structure of the ferritic stainless steel. FIG. 2B
shows ferritic stainless steel containing Mo of 0.5 wt % and W of
4.5 wt %, wherein the hot-annealed structure of the ferritic
stainless steel includes a sigma phase of 5% or less. Referring to
FIGS. 2A and 2B, it could be seen that it is possible to control
the sigma phase of the hot-annealed structure at a lower percentage
in the Mo+W-added steel than the Mo-added steel.
[0082] FIG. 3 is a graph testing thermal fatigue properties of
ferritic stainless steel according to the addition amount of Mo and
W.
[0083] Referring to FIG. 3, it could be seen that the thermal
fatigue lifespan at 900.degree. C. was excellent in the embodiments
in comparison to the comparative embodiments. That is, the X-axis
is the high-temperature tensile strength in MPa at 900.degree. C.
and Y-axis is the progress cycle of the thermal fatigue lifespan,
it could be seen that the values for the embodiments are all
positioned at the upper right side in the graph, whereas the values
for the comparative embodiments are positioned at the lower left
side in the graph. That is, it could be seen that the
high-temperature tensile strength and thermal fatigue lifespan at
900.degree. C. were excellent in the embodiments, while they were
relatively low in the comparative embodiments.
[0084] FIG. 4 is a graph testing high-temperature oxidation
resistance of ferritic stainless steel according to the addition
amount of Mo and W.
[0085] Referring to FIG. 4, two items with a relatively small
change in weight generated scale on the surface in the comparative
embodiments, and it could be seen that a larger change in weight
was shown in the high-temperature oxidation resistance result in
other comparative embodiments, except for the above comparative
embodiments, as compared with the embodiments. That is, it could be
seen that the embodiments showed excellent high-temperature
oxidation resistance, as compared with the comparative
embodiments.
[0086] FIG. 5 is a graph testing high-temperature salt corrosion
resistance of ferritic stainless steel according to the addition
amount of Mo and W.
[0087] FIG. 5 is a graph showing the test result of
high-temperature salt corrosion resistance according to the
addition amount of Mo or Mo+W, and corresponding changes in weight.
It could be seen that as the addition amount of Mo or Mo+W
increases, the change in weight was about 11 g/cm2 or less at about
4 wt %, while when the addition amount of Mo or Mo+W is about 2 wt
% or less, the change in weight was about 14 g/cm2 or more. That
is, it could be seen that the addition amount of Mo+W can influence
the high-temperature salt corrosion resistance of the ferritic
stainless steel while the change in weight for the high-temperature
salt corrosion resistance is better in the embodiments compared
with the comparative embodiments.
[0088] (Manufacturing Method)
[0089] FIG. 6 is a flowchart schematically illustrating a
fabrication method of a ferritic stainless steel according to one
embodiment of the present disclosure.
[0090] Referring to FIG. 6, a fabrication method of a ferritic
stainless steel comprising, by weight %, C: above 0 wt % to 0.01 wt
% or less, Si: above 0 wt % to 0.5 wt % or less, Mn: above 0 wt %
to 2.0 wt % or less, P: 0 wt % or more to 0.04 wt % or less, S: 0
wt % or more to 0.02 wt % or less, Cr: 12 wt % or more to 19 wt %
or less, Mo: 0 wt % or more to 1.0 wt % or less, W: 2 wt % or more
to 7 wt % or less, Ti: 0 wt % or more to 0.3 wt % or less, Nb:
above 0 wt % to 0.6 wt % or less, N: above 0 wt % to 0.01 wt % or
less, Al: 0 wt % or more to 0.1 wt % or less; and the balance of Fe
and other inevitable impurities may comprise providing a slab (S1),
heating the slab (S2), hot annealing (S3), cold annealing (S4), and
cold rolling (S5).
[0091] Heating the slab (S2) may be performed at a slab heating
temperature that is 1180.degree. C. or more to 1240.degree. C. or
less, with respect to slab temperature. When the heating
temperature is less than 1180.degree. C. in heating the slab (S2),
sticking, wherein the ferritic stainless steel sticks to a roll and
the surface of the ferritic stainless steel is removed, may occur
in the hot rolling. Further, when the heating temperature of the
slab is above 1240.degree. C., the grain size of the ferritic
stainless steel may be increased, such that toughness and r-bar
value may be reduced. Therefore, it is possible to make the grain
of the ferritic stainless steel fine size by controlling the
heating temperature of the slab at 1180.degree. C. or more to
1240.degree. C. or less, such that it is possible to ensure
formability and machinability by improving the toughness and r-bar
value.
[0092] In hot annealing (S3), the hot-annealing temperature may be
1020.degree. C. or more to 1070.degree. C. or less, with respect to
strip temperature.
[0093] In hot annealing (S3), the hot-annealing temperature may be
within the range where the ferritic stainless steel is
recrystallized in annealing. The hot annealing may be at as low
temperature as possible, within the temperature range where
recrystallization occurs. The lower the hot-annealing temperature,
the more the recrystallized grains of the ferritic stainless steel
may be implemented in fine size after hot annealing, and
accordingly, the r-bar value of the ferritic stainless steel that
has finally undergone cold annealing may show excellent
characteristic. When the hot-annealing temperature is less than
1020.degree. C. in hot annealing (S3), recrystallization of the
ferritic stainless steel may be insufficient, such that formability
and elongation may be decreased. Further, when the hot annealing
temperature is above 1070.degree. C., toughness of the ferritic
stainless steel may decrease after hot annealing, such that a plate
may be broken in the manufacturing process, or the grain size of
the cold-annealed ferritic stainless steel may increase, such that
a defect of orange peel may occur in forming. Therefore, it is
possible to improve the toughness and r-bar value of the ferritic
stainless steel by performing hot annealing (S3) with hot-annealing
temperature at 1020.degree. C. or more to 1070.degree. C. or less,
with respect to strip temperature.
[0094] The grain size of the ferritic stainless steel may be ASTM
No. 3 or more. As value in ASTM No. increases, the ferritic
stainless steel has a more fine grain size. Therefore, the larger
the grain size, the more the fine grain size is provided. When the
grain size is less than 3 in ASTM No., the grain size of the
ferritic stainless steel increases, such that the ferritic
stainless steel may become easily brittle in hot annealing and a
defect, such as plate break, may be generated.
[0095] In cold annealing (S4), the cold-annealing temperature may
be 1030.degree. C. or more to 1080.degree. C. or less, with respect
to strip temperature.
[0096] In cold annealing (S4), when the cold-annealing temperature
is less than 1030.degree. C., recrystallization may be insufficient
in cold annealing, such that the elongation and formability of the
ferritic stainless steel may be decreased. Further, when the
cold-annealing temperature is above 1080.degree. C., the grain size
of the ferritic stainless steel may increase and a defect of orange
peel may be generated in forming. Therefore, in order to improve
the high-temperature strength by making an extract of the ferritic
stainless steel fine, the cold-annealing temperature may be
1030.degree. C. or more to 1080.degree. C. or less with respect to
the strip temperature.
[0097] The hot-annealing temperature of the hot annealing (S3) and
the cold-annealing temperature of the cold annealing (S4),
(cold-annealing temperature)/(hot-annealing temperature) may be 1.0
or more to 1.1 or less.
[0098] The manufacture of a ferritic stainless steel according to
the present disclosure may comprise hot annealing (S3) and cold
annealing (S4) and the temperature of the hot annealing (S3) and
the temperature of the cold annealing (S4) may influence each
other. Although the formability and r-bar value may be improved and
the high-temperature strength may be improved when the
cold-annealing temperature increases, when the (cold-annealing
temperature)/(hot-annealing temperature) is less than 1.0 in the
relationship of the cold-annealing temperature and the
hot-annealing temperature, the r-bar value of the cold-annealed
ferritic stainless steel sheet may decrease and the formability may
be decreased. Further, when the cold-annealing temperature
increases and the (cold-annealing temperature)/(hot-annealing
temperature) is above 1.1, the grain size may increase and a defect
of orange peel may be generated in forming the ferritic stainless
steel. Therefore, (cold-annealing temperature)/(hot-annealing
temperature) may be 1.0 or more to 1.1 or less to increase the
r-bar value and the high-temperature tensile strength.
[0099] Cold rolling (S5) may be performed at room temperature.
[0100] DBTT(.degree. C.) may be 90.degree. C. or less in the
ferritic stainless steel.
[0101] The value T which is defined as by "T=-184.6+3.2(Cr wt
%)+27.5(Mo wt %)+4243.4(C wt %+N wt %)-295.6(Al wt %)+0.9[Nb wt
%/(C wt %+N wt %)]" may be 90 or less in the ferritic stainless
steel.
[0102] The DBTT(.degree. C.) and T are factors that are more
advantageous as they become lower, when DBTT(.degree. C.) is above
90.degree. C. or T is above 90, the ferritic stainless steel can be
easily broken, such that a defect, such as plate break, may be
generated.
[0103] In the ferritic stainless steel, when DBTT(.degree. C.) is
above 90.degree. C. or T is above 90, plate break may occur at
welded portions that may be formed by laser welding and seam
resistance welding in hot annealing (S3) and cold annealing (S4) by
deterioration of the toughness in the manufacturing process.
Further, since non-pressed portions and plate break may occur by
deterioration of toughness at room temperature in cold rolling (S5)
in the ferritic stainless steel, DBTT(.degree. C.) may be
90.degree. C. or less and T may be 90 or less to prevent the above
and ensure the toughness of the stainless steel.
[0104] In one aspect, the present disclosure provides a ferritic
stainless steel having thermal resistance at temperature of
900.degree. C. or more, high-temperature tensile strength of at
least 40 MPa, and formability equivalent to or more of 444 steel.
In another aspect, the present disclosure provides a fabrication
method of a ferritic stainless steel sheet having thermal
resistance at temperature of 900.degree. C. or more,
high-temperature tensile strength of at least 40 MPa, and
formability equivalent to or more of 444 steel.
[0105] The high-temperature tensile strength of ferritic stainless
steel can be improved due to solid solution hardening, by adding
elements having relatively large atomic radius, such as Nb, Mo, W,
Ta, and Hf. The one having the most excellent high-temperature
tensile strength in the ferritic stainless steels used for thermal
resistance is 444 steel, which contains Mo of 2 wt % or less. In
the ferritic stainless steel where only Mo is added, the
improvement effect in high-temperature strength is not so large
even if the addition amount of Mo is increased up to 3 wt % or
more. Further, when the addition amount of Mo is increased, a sigma
phase is easily extracted in the manufacturing process of the
ferritic stainless steel, such that a defect, such as plate break
or orange peel, is increased by deterioration of toughness of the
mother material and the welded portions, thereby reducing
efficiency in manufacturing.
[0106] Therefore, in one aspect, the present disclosure provides an
alloy design that can use fine laves phases and is based on
ferritic stainless steel of which the addition amount of Mo is
decreased and the addition amount of W is increased, in
consideration of rapid extraction of laves phases as compared with
Mo. In yet another aspect, the present disclosure provides a
fabrication method of a ferritic stainless steel that prevents a
defect in the manufacturing process which may be caused by
deterioration of welded portions or the welded portion of a coil,
and includes hot-annealing, cold-annealing, and cold rolling.
[0107] DBTT (.degree. C.) was acquired by machining the samples
(2.0 t) of Embodiments 1 to 7 and Comparative embodiments 1 to 4
according to Table 1 into V-notch impact samples and performing
impact tests. It could be seen that the DBTT (.degree. C.) of
Embodiments 1 to 7 according to Table 1 was 90.degree. C. or
less.
3. Property Test of Ferritic Stainless Steel According to
Temperature
[0108] A sample was manufactured by using the ferritic stainless
steel according Embodiment 2 of Table 1 and changing the heating
temperature in the step of heating a slab to 1230.degree. C. and
1280.degree. C.
[0109] FIG. 7 is a graph showing the grain size of ferritic
stainless steel according to slab heating temperature and FIG. 8 is
average r-bar values of ferritic stainless steel according to slab
heating temperature.
[0110] FIG. 7 shows a graph showing the grain size of ferritic
stainless steel that was manufactured at different heating
temperatures in a step of heating a slab and then hot-annealed at
1050.degree. C. It could be seen that the grain size of the
hot-annealed ferritic stainless steel according to the slab heating
temperature may be influenced by the heating temperature of the
slab. In the hot-annealed ferritic stainless steel, the grain size
is large when the slab heating temperature was at 1280.degree. C.,
above 1240.degree. C., and the grain size was small when the slab
heating temperature was lower, at 1230.degree. C.
[0111] FIG. 8 is a graph showing average r-bar values of ferritic
stainless steel that was manufactured at different heating
temperatures in a slab heating step, hot-annealed at 1050.degree.
C. and then cold annealed. It could be seen that as the heating
temperature of the slab was decreased, the average r-bar value of
the cold-annealed ferritic stainless steel was increased.
[0112] On the basis of the above description, it could be seen that
when the heating temperature was decreased in the slab heating
step, the grain size of the ferritic stainless steel was decreased
and the average r-bar value was increased.
[0113] FIG. 9 is a graph showing average r-bar values according to
hot-annealing temperature in hot annealing and FIG. 10 is a graph
showing high-temperature tensile strength according to
cold-annealing temperature in cold annealing.
[0114] Referring to FIG. 9, the average r-bar value of the ferritic
stainless steel was measured while changing the hot-annealed
temperature to 1040.degree. C. and 1280.degree. C. in the step of
hot annealing. As the result of measuring, it could be seen that as
the lower the temperature in the step of hot annealing, the larger
the r-bar value.
[0115] FIG. 10 shows the result of measuring the high-temperature
tensile strength of the ferritic stainless steel according to the
cold-annealing temperature in the step of cold annealing. The
high-temperature strength was measured at 900.degree. C. and the
cold-annealing temperature was changed to 1030.degree. C. and
1060.degree. C. Referring to the result, it could be seen that when
the cold-annealing temperature was 1060.degree. C., the
high-temperature tensile strength was increased in the ferritic
stainless steel.
[0116] FIG. 11 is a graph showing average r-bar values according to
cold-annealing temperature/hot-annealing temperature and FIG. 12 is
a graph showing high-temperature tensile strength according to
cold-annealing temperature/hot-annealing temperature.
[0117] Referring to FIG. 11, it could be seen that as the
cold-annealing temperature/hot-annealing temperature was increased,
the average r-bar value of the ferritic stainless steel increased,
which was measured by changing the cold-annealing
temperature/hot-annealing temperature after cold annealing. That
is, the average r-bar value increased, as the cold-annealing
temperature to the hot-annealing temperature increased. On the
other hand, it could be seen that when the cold-annealing
temperature/hot-annealing temperature was above 1.1, a defect of
orange peel was generated in the ferritic stainless steel.
[0118] FIG. 12 shows values of high-temperature tensile strength
measured by changing the cold-annealing temperature/hot-annealing
temperature. The high-temperature tensile strength was measured at
900.degree. C. It could be seen that the high-temperature tensile
strength was increased, as the cold-annealing
temperature/hot-annealing temperature, which was an annealing
temperature ratio in the step of hot annealing and the step of cold
annealing, became higher in the ferritic stainless steel.
[0119] It will be understood to those skilled in the art that the
present disclosure may be implemented in various ways without
changing the spirit of the present disclosure. Accordingly, the
disclosure described herein should not be limited based on the
described embodiments.
* * * * *