U.S. patent application number 13/254923 was filed with the patent office on 2012-01-26 for stainless steel material having outstanding high-temperature strength, and a production method therefor.
Invention is credited to Woo-Sang Jung, Deong-Ryung Kim, Dong-Ik Kim, Dong-Hee Lee, Seung-Cheol Lee, Young-Su Lee, Dae-Bum Park, Jae-Hyeok Shim.
Application Number | 20120018054 13/254923 |
Document ID | / |
Family ID | 42710141 |
Filed Date | 2012-01-26 |
United States Patent
Application |
20120018054 |
Kind Code |
A1 |
Lee; Seung-Cheol ; et
al. |
January 26, 2012 |
STAINLESS STEEL MATERIAL HAVING OUTSTANDING HIGH-TEMPERATURE
STRENGTH, AND A PRODUCTION METHOD THEREFOR
Abstract
Provided are a stainless steel having excellent high-temperature
strength and a method of manufacturing the same, and more
particularly, an austenitic stainless steel having excellent
high-temperature and creep strength as well as excellent corrosion
resistance able to be used in high-temperature corrosive
environments such as power plants and a method of manufacturing the
same. The stainless steel of the present invention may have a
precipitation index of 1.5 to 2.5.
Inventors: |
Lee; Seung-Cheol;
(Gyeonggi-do, KR) ; Park; Dae-Bum; (Seoul, KR)
; Jung; Woo-Sang; (Seoul, KR) ; Kim; Dong-Ik;
(Seoul, KR) ; Shim; Jae-Hyeok; (Seoul, KR)
; Lee; Young-Su; (Seoul, KR) ; Kim;
Deong-Ryung; (Gyeongsangnam-do, KR) ; Lee;
Dong-Hee; (Gyeongsangnam-do, KR) |
Family ID: |
42710141 |
Appl. No.: |
13/254923 |
Filed: |
March 8, 2010 |
PCT Filed: |
March 8, 2010 |
PCT NO: |
PCT/KR10/01436 |
371 Date: |
October 10, 2011 |
Current U.S.
Class: |
148/505 ;
148/326 |
Current CPC
Class: |
C21D 6/004 20130101;
C22C 38/46 20130101; C22C 38/06 20130101; B82Y 30/00 20130101; C22C
38/48 20130101; C22C 38/04 20130101; C21D 2211/001 20130101; C22C
38/001 20130101; C22C 38/52 20130101; C21D 8/0205 20130101; C21D
2211/004 20130101; C22C 38/02 20130101; C22C 38/42 20130101; C22C
38/44 20130101 |
Class at
Publication: |
148/505 ;
148/326 |
International
Class: |
C21D 6/02 20060101
C21D006/02; C21D 11/00 20060101 C21D011/00; C22C 38/58 20060101
C22C038/58; C22C 38/48 20060101 C22C038/48; C22C 38/52 20060101
C22C038/52; C22C 38/42 20060101 C22C038/42; C22C 38/46 20060101
C22C038/46 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 6, 2009 |
KR |
10-2009-0019174 |
Claims
1. A stainless steel with excellent high-temperature strength
having a precipitation index of about 1.5 to about 2.5 expressed by
the following Equation 1: (C/12+N/14)/(Nb/91+V/51): 1.5 to 2.5
[Equation 1] where carbon (C), nitrogen (N), niobium (Nb), and
vanadium (V) denote contents (wt %) of corresponding components,
respectively.
2. The stainless steel with excellent high-temperature strength of
claim 1, wherein the stainless steel comprises about 0.1 wt % to
about 1.0 wt % of Nb and about 0.1 wt % to about 1.0 wt % of V.
3. The stainless steel with excellent high-temperature strength of
claim 1, wherein a content of V is about 10% or less than that of
Nb in an atomic fraction in precipitates of the steel before being
used.
4. The stainless steel with excellent high-temperature strength of
claim 1, wherein the stainless steel has a composition including
about 0.01 to 0.1 wt % of carbon (C), about 0.1 to 1.0 wt % of
silicon (Si), about 0.1 to 2.0 wt % of manganese (Mn), about 16 to
20 wt % of chromium (Cr), about 7 to 15 wt % of nickel (Ni), about
0.1 to 1.0 wt % of niobium (Nb), about 0.1 to 1.0 wt % of vanadium
(V), about 0.1 to 0.3 wt % of cobalt (Co), about 2 to 5 wt % of
copper (Cu), about 0.03 wt % or less of aluminum (Al), about 0.01
to 0.25 wt % of nitrogen (N), residual iron (Fe), and unavoidable
impurities.
5. A method of manufacturing a stainless steel with excellent
high-temperature strength, the method comprising: heating a steel
satisfying the composition of claim 1, and subjected to hot rolling
and/or cold rolling to about 1200.degree. C. or more, and cooling
the heated stainless steel at a cooling rate of about 10.degree.
C./s or more to a temperature of about 500.degree. C. or less.
6. The stainless steel with excellent high-temperature strength of
claim 2, wherein a content of V is about 10% or less than that of
Nb in an atomic fraction in precipitates of the steel before being
used.
7. The stainless steel with excellent high-temperature strength of
claim 2, wherein the stainless steel has a composition of claim 2,
wherein the stainless steel has a composition including about 0.01
to 0.1 wt % of carbon (C), about 0.1 to 1.0 wt % of silicon (Si),
about 0.1 to 2.0 wt % of manganese (Mn), about 16 to 20 wt % of
chromium (Cr), about 7 to 15 wt % of nickel (Ni), about 0.1 to 1.0
wt % of niobium (Nb), about 0.1 to 1.0 wt % of vanadium (V), about
0.1 to 0.3 wt % of cobalt (Co), about 2 to 5 wt % of copper (Cu),
about 0.03 wt % or less of aluminum (Al), about 0.01 to 0.25 wt %
of nitrogen (N), residual iron (Fe), and unavoidable
impurities.
8. A method of manufacturing a stainless steel with excellent
high-temperature strength, the method comprising: heating a steel
satisfying the composition of claim 2, and subjected to hot rolling
and/or cold rolling to about 1200.degree. C. or more, and cooling
the heated stainless steel at a cooling rate of about 10.degree.
C./s or more to a temperature of about 500.degree. C. or less.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a stainless steel having
excellent high-temperature strength and a method of manufacturing
the same, and more particularly, to an austenitic stainless steel
having excellent high-temperature and creep strength as well as
excellent corrosion resistance able to be used in high-temperature
corrosive environments such as power plants and a method of
manufacturing the same.
[0003] 2. Description of the Related Art
[0004] A power plant having a high-temperature operating
environment, such as a thermal power plant or a nuclear power
plant, may have various limitations, in that high-temperature steam
may corrode plant facilities or the like. Also, ultra super
critical boilers with increased steam temperature and pressure have
recently been installed globally in order to improve the efficiency
of facilities, and high-temperature strength properties are
required, because the pressures acting on pipes under the foregoing
ultra high critical pressure are very high in comparison to the
case of a typical boiler.
[0005] Therefore, austenitic stainless steels having excellent
corrosion resistance at high temperatures have been widely used in
such plant facilities. Austenitic stainless steel has excellent
high-temperature strength and high-temperature corrosion resistance
in comparison to ferritic steel, and thus, austenitic stainless
steel largely replaces ferritic steel in temperature regions of
about 650.degree. C. or more.
[0006] However, even in the case of austenitic stainless steel,
steam oxidation scales are formed at contact areas between the
steel and steam at high temperatures and a phenomenon may be
generated in which the formed scales are delaminated. Various
methods have conventionally been suggested to resolve the foregoing
phenomenon. The methods are widely classified as a method of
increasing the contents of corrosion resistant elements such as Cr
and Ni in the entirety or a portion of the steel, a method of
improving a surface concentration of Cr by the refinement of grains
in the entirety or a portion of the steel, etc. Most of the
foregoing methods may be regarded as methods of improving corrosion
resistance by controlling the Cr content to be higher at a surface
portion.
[0007] However, manufacturing costs may be increased because large
amounts of alloying elements may be required to be added or special
surface treatments may have to be performed in order to increase
the content of elements such as Cr and Ni. Also, the method of
refining grains may have a limitation in that grains may become
coarse again by a high-temperature treatment such as
high-temperature processing and heat treatment, and thus, the
limitations of a material for high-temperature use may be
considered as not having been completely resolved. In particular,
when grain refinement is promoted by precipitates, the precipitates
generally may not perform pinning action preventing grain growth
because the precipitates are redissolved at high temperatures or
become coarse by the so-called `Ostwald ripening` in which fine
precipitates are dissolved and diffused to be absorbed by large
precipitates.
[0008] Further, a stainless steel pipe or the like used in high
temperature and high pressure environment, such as a power plant
boiler as described above, should have great resistance with
respect to a creep phenomenon according to the continuous action of
stress, as well as high strength, as the stainless steel pipe is
required to to endure high pressure at high temperatures; however,
a clear solution with respect to the foregoing limitations has not
been suggested to date.
SUMMARY OF THE INVENTION
[0009] An aspect of the present invention provides a stainless
steel having greatly improved high-temperature and creep strength
as well as not greatly increasing the amounts of added alloying
elements and not recoarsening the refined grains.
[0010] According to an aspect of the present invention, there is
provided a stainless steel having a precipitation index of 1.5 to
2.5 which is expressed by the following Equation 1:
(C/12+N/14)/(Nb/91+V/51): 1.5 to 2.5 [Equation 1]
[0011] where, carbon (C), nitrogen (N), niobium (Nb), and vanadium
(V) denote contents (wt %) of corresponding components,
respectively.
[0012] At this time, 0.1 wt % to 1.0 wt % of niobium (Nb) and 0.1
wt % to 1.0 wt % of vanadium (V) may be included.
[0013] Also, V may be present at a level of 10% or less of Nb in an
atomic ratio in precipitates of steel before being used.
[0014] The stainless steel, for example, may have a composition
comprising : 0.01 to 0.1 wt % of carbon (C), 0.1 to 1.0 wt % of
silicon (Si), 0.1 to 2.0 wt % of manganese (Mn), 16 to 20 wt % of
chromium (Cr), 7 to 15 wt % of nickel (Ni), 0.1 to 1.0 wt % of
niobium (Nb), 0.1 to 1.0 wt % of vanadium (V), 0.1 to 0.3 wt % of
cobalt (Co), 2 to 5 wt % of copper (Cu), 0.03 wt % or less of
aluminum (Al), 0.01 to 0.25 wt % of nitrogen (N), residual iron
(Fe), and unavoidable impurities.
[0015] According to another aspect of the present invention, there
is provided a method of manufacturing stainless steel having
high-temperature strength including: heating a steel satisfying Nb
and V compositions of the foregoing stainless steel and subjected
to hot rolling and/or cold rolling to 1200.degree. C. or more; and
cooling the heated stainless steel at a cooling rate of 10.degree.
C./s or more to 500.degree. C. or less.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The above and other aspects, features and other advantages
of the present invention will be more clearly understood from the
following detailed description taken in conjunction with the
accompanying drawings, in which:
[0017] FIG. 1 is a graph comparing distribution frequencies for the
sizes of precipitates after the high-temperature creep tests, in
which (a) is a frequency distribution graph for the precipitate
sizes of Comparative Steel 1, and (b) is a frequency distribution
graph for the precipitate sizes of Inventive Steel 2;
[0018] FIG. 2 shows volume ratios occupied according to the size
distributions of precipitate particles, in which (a) is a volume
distribution graph for the particle sizes of Comparative Steel 1,
and (b) is a volume distribution graph for the particle sizes of
Inventive Steel 2;
[0019] FIG. 3 shows the results of investigating precipitates in
Comparative Steel 1, in which (a) is an electron micrograph showing
each region, (b) is a result of energy-dispersive X-ray
spectroscopy (EDX) on the precipitate marked in number 1 in (a),
and (c) is a result of EDX on the precipitate marked in number 2 in
(a); and
[0020] FIG. 4 shows the results of investigating precipitates in
Inventive Steel 2, in which (a) is an electron micrograph showing
each region, (b) is a result of EDX on the precipitate marked in
number 1 in (a), and (c) is a result of EDX on the precipitate
marked in number 2 in (a).
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0021] Hereinafter, the present invention is described in
detail.
[0022] From the results of in-depth research on a method of
improving high-temperature and creep strength of austenitic
stainless steel without the deterioration of corrosion resistance
at the same time, the inventors of the present invention discovered
that when niobium (Nb)-based precipitates are stably formed in the
steel, a creep phenomenon may not only be prevented even in the
case in which stress acts at high temperatures, but
high-temperature strength may also be improved and the pinning
effect of grains may be maintained, and thus, have completed the
present invention.
[0023] That is, the present invention is characterized by the fact
that the precipitation condition of vanadium (V)-based precipitates
is controlled in order to stably form Nb precipitates in austenitic
stainless steel with Nb and V added.
[0024] That is, according to the result of research undertaken by
the present inventors, finely distributed Nb-based precipitates are
in stable phases in a general material for room-temperature use and
are very favorable in order to secure strength, but the finely
distributed Nb-based precipitates may be changed into coarse and
concentrated Nb precipitates by means of the foregoing Ostwald
ripening phenomenon in which the steel is heated to high
temperatures and thus, do not exhibit any function of improving
strength or creep strength. However, control of the foregoing
phenomenon is very important because the coarsening of the Nb
precipitates may be prevented as much as possible when the
interfaces of the Nb-based precipitates are stabilized by
minimizing the surface energy acting on the interfaces between the
Nb precipitates and the steel.
[0025] For this purpose, the Nb precipitates in the present
invention are stabilized by forming V precipitates on the surfaces
of the Nb precipitates during a process of performing a heat
treatment after hot and cold processing.
[0026] That is, since a V atom is relatively smaller than an Nb
atom, the surfaces of the Nb precipitates may be greatly stabilized
when the V atoms precipitate on the surfaces of the Nb
precipitates. However, when V precipitates are excessively formed,
even in the steel first manufactured, the V precipitates are not
formed on the surfaces of the Nb precipitates but there is a
concern that separate V precipitates may be formed. In this case,
the effect intended in the present invention is difficult to
obtain, because the V source, which is for precipitating on the Nb
precipitates at high temperatures, may be insufficient.
[0027] That is, in the present invention, a large amount of V
precipitates are not formed in unused austenitic stainless steel at
room temperature, but V acts to stabilize the Nb precipitates by
precipitating on the surfaces of the Nb precipitates. According to
the result of research undertaken by the present inventors, a
precipitation phenomenon of the V precipitates on the Nb
precipitates is considered to occur due to the fact that voids are
formed between the Nb precipitates and the matrix structures of the
stainless steel and the precipitation of the fine V precipitates in
portions in which the voids are formed is facilitated even under
conditions of low pressure. Also, creep fracture stress is
increased by reprecipitation of the residual V dissolved in the
matrix under high-temperature stress.
[0028] The foregoing phenomenon is not always generated by the
addition of Nb and V, and even when the foregoing phenomenon is
generated, the V precipitates may not precipitate sufficiently to
such a degree that the Nb precipitates may be stabilized without
being dissolved at high temperatures. Therefore, an appropriate
condition by which the Nb precipitates are stabilized is therefore
required.
[0029] In order to find the foregoing condition, the present
inventors investigated the changes in the composition of austenitic
stainless steel and the composition of precipitates when the stress
acts at high temperatures. As a result, it may be confirmed that
Nb, V, C, and N, major components constituting the precipitates,
may satisfy the following Equation 1:
(C/12+N/14)/(Nb/91+V/51): 1.5 to 2.5 [Equation 1]
[0030] where C, N, Nb, V denote contents (wt %) of corresponding
components, respectively.
[0031] That is, since the precipitates of Nb generally have a
carbide, nitride, or carbonitride form, which are compounds of
carbon or nitrogen, and the precipitates of V may also have a
carbide, nitride, or carbonitride form, the V precipitates may be
formed on the Nb precipitates while the stress acts at high
temperatures when the Equation 1 between these precipitates is
properly controlled. In the present invention,
(C/12+N/14)/(Nb/91+V/51) expressed in Equation 1 is briefly
referred to as a `precipitation index`.
[0032] The precipitation index value may be in a range of 1.5 to
2.5 as described in Mathematical Equation 1. When the value is less
than 1.5, precipitates may not be formed because the contents of
carbon and nitrogen are insufficient, and when the value is more
than 2.5, sufficient precipitates are difficult to be formed with
respect to input amounts because the surplus amounts of Nb and V
are too large.
[0033] That is, the stainless steel having excellent
high-temperature strength of the present invention is austenitic
stainless steel having a (C/12+N/14)/(Nb/91+V/51) value satisfying
a range of 1.5 to 2.5.
[0034] In the stainless steel having the foregoing condition, a
sufficient precipitation hardening effect may be obtained even at
high temperatures because finely distributed Nb precipitates are
stabilized. Therefore, high-temperature strength and
high-temperature creep strength are very good. In addition,
high-temperature corrosion resistance may also be effectively
secured according to the fine grains because a grain growth pinning
effect by means of the Nb precipitates may also be anticipated.
Sufficient corrosion resistance may also be secured when the grains
are refined, because the fact that Cr required for securing
corrosion resistance diffuses to a surface through crystal
interfaces is a general opinion.
[0035] Since one of the main features of the present invention is
that Nb precipitates are initially formed, and then V precipitates
precipitate on the Nb precipitates as described above, it is first
necessary to set a condition of forming sufficient Nb precipitates.
In order to form sufficient Nb precipitates, a content of Nb may be
additionally limited in addition to the foregoing condition.
According to the result of research undertaken by the inventors of
the present invention, Nb may be added to the steel in an amount of
0.1 wt % or more. When the content of Nb is excessively high, the
precipitates become too coarse and may adversely affect the
physical properties of the steel. Thus, an upper limit of the Nb
content is determined as being 1.0 wt %. Therefore, the content of
Nb included in the stainless steel of the present invention may be
in a range of 0.1 wt % to 1.0 wt %.
[0036] Also, the stainless steel having excellent high-temperature
strength has additional features, in which the V precipitates are
formed around the Nb precipitates during a solution heat treatment
after hot and cold processing to form stable Nb precipitates, and
creep strength is improved by forming precipitates from the
residual V dissolved in the matrix after being subjected to a
high-temperature creep test under a high-temperature creep
condition of continuously applying stress of 200 MPa for 1200
hours. That is, the content of V in an atomic fraction has a ratio
of 10% or less of Nb during energy-dispersive X-ray spectroscopy
(EDX) compositional analysis of the Nb precipitates before the
high-temperature creep test. However, for example, additional V may
precipitate in order that the content of V in an atomic fraction
included in the foregoing precipitates becomes 15% or more of Nb
after the high-temperature creep test in which tensile stress of
200 MPa is applied for 1200 hours or more.
[0037] Any stainless steel may be included in the scope of the
present invention, so long as the stainless steel has the foregoing
compositional features. That is, the present invention has the most
important feature in which the Nb precipitates are stabilized
during high-temperature creep by using the range of the foregoing
relation and thus, any stainless steel may be included in the scope
of the present invention if the stainless steel is austenitic
stainless steel using the foregoing feature.
[0038] Since the basic strength is determined by a component system
of stainless steel, the foregoing condition may give more effect
when the basic strength condition is satisfied. The desirable
composition of the stainless steel of the present invention
discovered through a great deal of research by the inventors of the
present invention is characterized in that C, Si, Mn, Cr, Ni, Mo,
W, Cu, Al, Co, Nb, V, or N are added in the following content
ranges. Hereinafter, the component system of the stainless steel is
briefly described.
[0039] Carbon (C): 0.01 wt % to 0.1 wt %
[0040] Since C is a very useful element for securing
high-temperature tensile strength and high-temperature creep
strength required in austenitic stainless steel for
high-temperature use, C may be added in an amount of 0.01 wt % or
more. However, C combines with Cr to form a carbide such as
Cr.sub.23C.sub.6 when a content of C is too high, and in this case,
a Cr-deficient layer having insufficient Cr is formed around the
carbides so that it is unfavorable to secure corrosion resistance
and weldability may also decrease. Therefore, an upper limit of the
C content is determined as 0.1 wt %.
[0041] Silicon (Si): 0.1 wt % to 1.0 wt %
[0042] Silicon is useful for preventing oxidation of the steel by
steam, as well as an element for deoxidation, and may be added in
an amount of about 0.1 wt % or more. However, since the
processability of steel deteriorates when a Si content is too high,
the Si content may be controlled to 1.0 wt % or less.
[0043] Manganese (Mn): 0.1 wt % to 2.0 wt %
[0044] Mn combines with an impurity, S, included in the steel, to
form MnS and improves hot processability of the steel, but the
processability markedly deteriorates and weldability also decreases
when Mn is added excessively. Therefore, the Mn content may be in a
range of 0.1 wt % to 2 wt %.
[0045] Chromium (Cr): 16 wt % to 20 wt %
[0046] Cr is an element that improves corrosion resistance,
especially high-temperature corrosion resistance, of the steel, and
may be added in an amount of 16 wt % or more. However, an
austenitic structure becomes unstable when a Cr content is too
high, and a large amount of Ni must to be added to compensate for
this, so that manufacturing costs of the steel will be increased.
Therefore, Cr may be added in an amount of 20 wt % or less.
[0047] Nickel (Ni): 7 wt % to 15 wt %
[0048] Ni is an element stabilizing an austenitic structure and is
also an important element for securing corrosion resistance. Also,
Ni needs to be added in an amount of 7 wt % or more, in order to
obtain the austenitic structure by compensating for the effect of
Cr on the austenitic structure. However, the Ni content may be
limited to 15 wt % or less, because any further increase in the
effect of the Ni addition is difficult to be anticipated and the
manufacturing costs of the steel are also unnecessarily increased
when Ni is added excessively.
[0049] Niobium (Nb): 0.1 wt % to 1.0 wt %
[0050] Nb is an element that acts to improve high-temperature
strength and high-temperature creep strength by precipitating in
the steel as described above, and thus, needs to be added in an
amount of 0.1 wt % or more. However, since there is a concern that
precipitates become coarse when the Nb content is too high, Nb may
be added in an amount of 1.0 wt % or less.
[0051] Vanadium (V): 0.1 wt % to 1.0 wt %
[0052] V is another important element in the present invention
which stabilizes Nb precipitates by forming precipitates on the Nb
precipitates when the steel is under stress during hot rolling.
Therefore, V may be added in an amount of 0.1 wt % or more in order
to obtain the foregoing effect. However, V may be added in an
amount of 1.0 wt % or less because coarse precipitates will be
formed by increasing the grain sizes of the precipitates when a V
content is too high.
[0053] Cobalt (Co): 0.1 wt % to 0.3 wt %
[0054] Co is a solid solution strengthening element that
contributes to improving the strength of the steel and may be added
in an amount of 0.1 wt % or more. However, an upper limit of the Co
content is determined as 0.3 wt % because any further increase in
the effect of the Co addition is difficult to anticipate and there
may be a burden of cost increase according to the addition of an
alloying element when Co is added excessively.
[0055] Copper (Cu): 2 wt % to 5 wt %
[0056] Cu is a useful element for securing the strength of the
steel and is added in an amount of 2 wt % or more in the present
invention. However, since Cu may adversely affect the strength or
toughness when Cu is added excessively, an upper limit of the Cu
content is limited to 5 wt %.
[0057] Aluminum (Al): 0.03 wt % or less
[0058] Al is an element that generates a large amount of hard
inclusions and adversely affects the characteristics of the steel
when Al is added excessively. Therefore, Al may not be added if
possible. However, since there may be a burden on steel making
processes when the addition amount of Al is too strictly limited,
it is effective that an upper limit of the Al content is limited to
0.03 wt %.
[0059] Nitrogen (N): 0.01 wt % to 0.25 wt %
[0060] N reacts with Nb or V to form a nitride or a carbonitride
and is a useful element for securing high-temperature strength and
high-temperature creep strength of the steel by precipitation
hardening. Therefore, N may be added in an amount of 0.01 wt % or
more in order to obtain the foregoing effect. However, N may be
added 0.25 wt % or less because there is a concern that the steel
may be weaken due to the formation of coarse nitrides, especially
nitrides in a bulk form when N is added excessively.
[0061] A remaining portion in addition to the foregoing additive
elements is iron (Fe) and some impurities may be unavoidably
included.
[0062] Therefore, the stainless steel, for example, may include
0.01 to 0.1 wt % of C, 0.1 to 1.0 wt % of Si, 0.1 to 2.0 wt % of
Mn, 16 to 20 wt % of Cr, 7 to 15 wt % of Ni, 0.1 to 1.0 wt % of Nb,
0.1 to 1.0 wt % of V, 0.1 to 0.3 wt % of Co, 2 to 5 wt % of Cu,
0.03 wt % or less of Al, 0.01 to 0.25 wt % of N, residual Fe, and
unavoidable impurities.
[0063] Also, in order to improve the strength of the steel, one or
more elements selected from the group consisting of W and Mo may be
further added to the foregoing stainless steel composition.
However, since toughness, ductility, or processability of the steel
may be adversely affected when large amounts of W and Mo are added,
W and Mo may have contents in ranges of 0.05 wt % to 3.0 wt % and
0.05 wt % to 3.0 wt %, respectively.
[0064] The typical unavoidable impurities included in the steel of
the present invention may be phosphorus (P) and sulfur (S), and if
possible, contents of P and S may be reduced because P and S
degrade the properties of the steel. However, since a burden in a
steel making process or the like may be caused when impurity
elements are excessively limited, the contents of P and S are
limited to 0.040 wt % or less.
[0065] Even in the case in which the stainless steel of the present
invention is manufactured by using a typical manufacturing method
of stainless steel, stainless steel having high high-temperature
strength and high-temperature creep strength as well as corrosion
resistance may be obtained. However, in order to more effectively
obtain high-temperature strength and high-temperature creep
strength of the steel, the stainless steel may be manufactured
through performing a heat treatment by the following method.
[0066] Hereinafter, a method of manufacturing stainless steel is
described.
[0067] That is, the method of manufacturing the stainless steel of
the present invention includes heating a hot rolled and/or cold
rolled steel to 1200.degree. C. or more by a typical method and
cooling the heated stainless steel at a cooling rate of 10.degree.
C./s or more to 500.degree. C. or less. Each operation is described
in more detail below.
[0068] Heating temperature: 1200.degree. C. or more
[0069] The stainless steel may be heated to a temperature of
1200.degree. C. or more. The heating temperature is determined to
dissolve V precipitates so as to more easily precipitate on the
surfaces of Nb precipitates later. In consideration of the
foregoing effect, the heating temperature may be 1200.degree. C. or
more. An upper limit of the heating temperature is not necessarily
limited as long as the steel is not deformed or melted. However, in
consideration of energy costs required for heating the steel and
excessive grain growth, the heating temperature may be limited to
1300.degree. C. or less.
[0070] Cooling rate: 10.degree. C./s or more
[0071] Even in the case in which the stainless steel is heated to
the foregoing temperature or more to redissolve all the V
precipitates, coarse V precipitates may be formed again when a
cooling rate in a subsequent cooling operation is too slow.
Therefore, the cooling rate may be 10.degree. C./s or more in order
to prevent the foregoing phenomenon. An upper limit of the cooling
rate is not particularly limited in consideration of the foregoing
effect, but the cooling rate, for example, may be limited to
100.degree. C./s or less, because distortion of the steel may be
generated due to excessive cooling.
[0072] Cooling stop temperature: 500.degree. C. or less
[0073] When the cooling is stopped at too high temperature, there
is a concern that a large amount of V precipitates may be formed
again. Therefore, it is necessary to stop the cooling at least at
500.degree. C. or less.
MODE FOR INVENTION
[0074] Hereinafter, the stainless steel of the present invention
will be described in more detail according to the Example. However,
it has to be noted that the following Example only exemplifies the
present invention and does not limit the scope of the present
invention. Therefore, the scope of the present invention is defined
by the appended claims and the details reasonably inferred
thereform.
EXAMPLE
[0075] Stainless steel having a composition described in the
following Table 1 was melted and casted, and then hot rolled and
cold rolled by using typical methods. Thereafter, specimens were
manufactured by heating at a temperature of 1200.degree. C. for 30
minutes and performing a cooling treatment to 100.degree. C. at a
cooling rate of 100.degree. C./s.
TABLE-US-00001 TABLE 1 Precipitation C Si Mn Cr Ni Mo W Cu Al Co Nb
V N index Comparative 0.06 0.27 0.48 18.4 9.47 0.46 -- 2.95 0.0274
0.212 0.282 0.1 0.14 2.96 Steel 1 Inventive 0.05 0.30 0.5 18.6 9.33
0.46 -- 3 0.0144 0.202 0.3 0.305 0.16 1.68 Steel 1 Inventive 0.07
0.27 0.48 18.4 9.47 0.46 -- 2.95 0.0274 0.212 0.282 0.286 0.19 2.23
Steel 2 Inventive 0.06 0.3 0.49 19 9.35 0.44 -- 3 0.015 0.202 0.297
0.29 0.24 2.47 Steel 3 Comparative 0.07 0.3 0.48 18.3 9.46 0.48 --
-- 0.0047 0.196 0.3 0.288 0.14 1.77 Steel 2 Comparative 0.05 0.3
0.5 18.7 9.35 0.45 -- 2.9 0.0117 0.203 0.298 0.515 0.13 1.01 Steel
3 Comparative 0.06 0.28 0.5 18.7 9.35 0.44 -- 3 0.01 0.202 0.306
0.488 0.19 1.44 Steel 4 Inventive 0.07 0.3 0.49 18.8 9.36 0.45 -- 3
0.0151 0.201 0.309 0.518 0.25 1.75 Steel 4 Comparative 0.06 0.3 0.5
18.2 9.5 0.48 -- -- 0.0065 0.201 0.306 0.518 0.16 1.22 Steel 5
Comparative 0.08 0.32 0.49 19 9.4 -- 0.5 3 0.0781 0.214 0.302 0.507
0.24 1.80 Steel 6 Comparative 0.08 0.26 0.8 19 8.5 0.4 0.08 2.9 --
0.136 0.5 0.07 0.12 2.22 Steel 7 Comparative 0.047 0.44 1.44 17.4
9.12 0.19 0.02 0.479 0.0058 0.14 0.05 0.05 0.017 3.35 Steel 8
[0076] As shown in Table 1, Comparative Steel 2 represents a case
in which a Cu content is insufficient in the present invention,
Comparative Steel 6 represents a case in which an Al content is
excessive, and Comparative Steels 7 and 8 represent a case in which
V contents are insufficient. Also, component systems of Comparative
Steels 1, 3, 4, and satisfied the more desirable composition of the
austenitic stainless steel suggested in the present invention, but
precipitation indices were out of a range of 1.5 to 2.5 which was
defined in the present invention.
[0077] Also, all of Inventive Steels 1, 2, 3, and 4 not only
satisfied the foregoing precipitation index range, but also
satisfied the more desirable composition defined in the present
invention.
[0078] High-temperature tensile tests and creep tests were
performed on the foregoing steels. The conditions of the
high-temperature tensile tests and creep tests, and the results
thereof are presented together in Table 2. The creep tests were
performed at 700.degree. C. in a non-oxidizing atmosphere under
various loads from a high load of 350 MPa to 200 MPa. It has to be
noted that units of strength described in Table 2 are all mega
pascals (MPa).
TABLE-US-00002 TABLE 2 Room Temperature 3000 hours Category (RT)
500 600 700 (creep) Comparative 641.23 490.43 456.418 399.14 167.9
Steel 1 Inventive 669.7 522.74 479.29 421.72 183.68 Steel 1
Inventive 692.21 530.35 487.017 438.78 173 Steel 2 Inventive 714.14
536.28 513.152 463.82 203.58 Steel 3 Comparative 690.24 521.78
476.15 413.21 146.73 Steel 2 Comparative 656.15 514.91 476.159
433.77 166.7 Steel 3 Comparative 678.39 520.24 507 425.36 170.8
Steel 4 Inventive 683.39 539.4 516.43 462.84 172 Steel 4
Comparative 710.77 515.49 469.84 426.46 149.12 Steel 5 Comparative
682.04 516.57 503.79 449.24 166.9 Steel 6 Comparative 663 456.64
409.22 325.46 63.28 Steel 8
[0079] As shown in Table 2, there were no big differences between
Inventive Steels and Comparative Steels with respect to the room
temperature strength of each steel, excluding Comparative Steel 1,
but considerably large differences were generated as the foregoing
steels were heated to high temperatures. In particular, Comparative
Steel 8, having insufficient V, exhibited considerably low
high-temperature strength and high-temperature creep strength, and
it is considered that a precipitation hardening effect due to Nb
precipitates was no longer obtained because a redissolution
phenomenon of the Nb precipitates was not prevented. Also, the
component systems of Comparative Steels 1, 3, 4, and 5 having
insufficient precipitation indices corresponded to a desirable
range, but high high-temperature and creep strength may not be
achieved because a sufficient precipitation hardening effect may
not be obtained. It may be found that high-temperature physical
properties of other Comparative Steels were also insufficient.
[0080] The foregoing result may also be found through particle size
distributions between Comparative Steel 1 and Inventive Steel 2
shown in FIGS. 1 and 2. FIG. 1 is a graph comparing distribution
frequencies for the sizes of precipitates after the foregoing heat
treatments, in which (a) represents a frequency distribution for
the sizes of Comparative Steel 1, and (b) represents a frequency
distribution for the sizes of Inventive Steel 2. Also, FIG. shows
volume ratios occupied according to the size distributions of
precipitate particles, in which (a) represents a volume
distribution for the particle sizes of Comparative Steel 1, and (b)
represents a volume distribution for the particle sizes of
Inventive Steel 2.
[0081] As shown in each graph, the particle size of Comparative
Steel 1 mostly exceeded 100 nm even in the case of the smallest
precipitate particle. On the other hand, Inventive Steel 2,
satisfying the conditions of the present invention, had a minimum
particle size of about 10 nm and it may be understood that the size
of a particle having maximum frequency was determined in a range of
10 nm to 20 nm as shown in FIG. 1(b). It means that a large number
of very fine particles may be distributed in Inventive Steel 2 in
comparison to Comparative Steel 1, having a maximum frequency
particle size of 200 nm. Therefore, according to the present
invention, it may be confirmed that high-temperature strength and
high-temperature creep properties may be improved by the dispersion
and distribution of fine precipitates in the steels.
[0082] In order to confirm a phenomenon of forming V precipitates
around Nb precipitates during a heat treatment, micrographs showing
precipitates existing after the heat treatment (a heat treatment of
the foregoing condition), micrographs showing precipitates existing
after performing a creep test, and the results of energy-dispersive
X-ray spectroscopy (EDX) compositional analyses in Comparative
Steel 1 and Inventive Steel 2 were shown in FIGS. 3 and 4,
respectively. In FIGS. 3 and 4, (a) is electron micrographs showing
each region, (b) is a result of EDX on the precipitate marked in
number 1 in (a), and (c) is a result of EDX on the precipitate
marked in number 2 in (a). As shown in FIGS. 3 and 4, a content of
the V precipitates before the heat treatment was less than 10% of
Nb precipitates based on an atomic content, but the content of the
V precipitates after the heat treatment was 15% or more of the
atomic content of the Nb precipitates. Therefore, it may be
confirmed that a large amount of the V precipitates may be formed
around the Nb precipitates during the creep test.
[0083] While the present invention has been shown and described in
connection with the exemplary embodiments, it will be apparent to
those skilled in the art that modifications and variations can be
made without departing from the spirit and scope of the invention
as defined by the appended claims.
[0084] According to the present invention, since high-temperature
and creep strength of stainless steel are greatly improved, effects
may be obtained, in which the use amount of steel may be decreased
even under the action of large stress and durability may also be
improved.
[0085] While the present invention has been shown and described in
connection with the exemplary embodiments, it will be apparent to
those skilled in the art that modifications and variations can be
made without departing from the spirit and scope of the invention
as defined by the appended claims.
* * * * *