U.S. patent application number 10/507069 was filed with the patent office on 2005-07-21 for high-grade duplex stainless steel with much suppressed formation of intermetallic phases and having an excellent corrosion resistance, embrittlement resistance castability and hot workability.
Invention is credited to Kim, Soon-Tae, Lee, In-Sung, Park, Yong-Soo.
Application Number | 20050158201 10/507069 |
Document ID | / |
Family ID | 28450068 |
Filed Date | 2005-07-21 |
United States Patent
Application |
20050158201 |
Kind Code |
A1 |
Park, Yong-Soo ; et
al. |
July 21, 2005 |
High-grade duplex stainless steel with much suppressed formation of
intermetallic phases and having an excellent corrosion resistance,
embrittlement resistance castability and hot workability
Abstract
Formation of intermetallic phases such as sigma (.sigma.) and
khi (.chi.) shows detrimental effects on the corrosion and
mechanical properties of high-grade duplex stainless steel. The
present invention provides high-grade duplex stainless steel with
much suppressed formation of intermetallic phases, of which the
chemical composition consists essentially, on a weight basis, of:
Cr: 21.0%.about.38.0%, Ni: 3.0%.about.12.0%, Mo: 1.5%.about.6.5%,
W: 6.5% or less, Si: 3.0% or less, Mn: 8.0% or less, N:
0.2%.about.0.7%, C: 0.1% or less, at least one element selected
from the group consisting of Ba: 0.0001.about.0.6% and one or more
elements of Mischmetal (MM) and Y: 0.0001.about.1.0% in total, and
a balance of Fe and incidental impurities. The pitting resistance
equivalent has a value of 40.ltoreq.PREW.ltoreq.67 defined by the
following formula (1): PREW=wt. % Cr+3.3(wt. % Mo+0.5 wt. % W)+30
wt. % N ((1) The present high-grade duplex stainless steel exhibits
an excellent corrosion resistance, embrittlement resistance,
castability and hot workability.
Inventors: |
Park, Yong-Soo; (Seoul,
KR) ; Kim, Soon-Tae; (Seoul, FR) ; Lee,
In-Sung; (Seoul, KR) |
Correspondence
Address: |
VOLENTINE FRANCOS, & WHITT PLLC
ONE FREEDOM SQUARE
11951 FREEDOM DRIVE SUITE 1260
RESTON
VA
20190
US
|
Family ID: |
28450068 |
Appl. No.: |
10/507069 |
Filed: |
March 22, 2005 |
PCT Filed: |
March 24, 2003 |
PCT NO: |
PCT/KR03/00568 |
Current U.S.
Class: |
420/40 ; 420/46;
420/47; 420/49; 420/52 |
Current CPC
Class: |
C22C 38/001 20130101;
C22C 38/44 20130101; C21D 2211/001 20130101; C22C 38/58 20130101;
C22C 38/42 20130101; C21D 2211/005 20130101; C22C 38/002 20130101;
C22C 38/005 20130101 |
Class at
Publication: |
420/040 ;
420/046; 420/047; 420/049; 420/052 |
International
Class: |
C22C 038/44 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 25, 2002 |
KR |
10-2002-0016214 |
Claims
1. High-grade duplex stainless steel with high corrosion
resistance, embrittlement resistance, castability and hot
workability which suppresses formation of intermetallic phases,
consisting essentially of 21.0 to 38.0% of Cr, 3.0, to 12.0% of Ni,
1.5 to 6.5% of Mo, 0 to 6.5% of W, 3.0% or less of Si, 8.0% or less
of Mn, 0.2 to 0.7% of N, 0.1% or less of C, 0.0001 to 0.6% of Ba,
and a balance of Fe and incidental impurities on a weight basis, a
pitting resistance equivalent (PREW) defined by following formula
{circle over (1)} satisfying 40.ltoreq.PREW.ltoreq.67: PREW=wt %
Cr+3.3(wt % Mo+0.5wt % W)+30 wt % N {circle over (1)}
2. The high-grade duplex stainless steel of claim 1, further
containing 0.0001 to 1.0% of mischmetal (MM) and/or Y in total.
3. The high-grade duplex stainless steel of claim 2, wherein Ba is
added within the range of 0.001 to 0.2%.
4. High-grade duplex stainless steel with high corrosion
resistance, embrittlement resistance, castability and hot
workability which suppresses formation of intermetallic phases,
consisting essentially of 21.0 to 38.0% of Cr, 3.0 to 12.0% of Ni,
1.5 to 6.5% of Mo, 0 to 6.5% of W, 3.0% or less of Si, 8.0% or less
of Mn, 0.2 to 0.7% of N, 0.1 % or less of C, 0.0001 to 1.0% of MM
and/or Y in total, and a balance of Fe and incidental impurities on
a weight basis, a pitting resistance equivalent (PREW) defined by
following formula {circle over (1)} satisfying
40.ltoreq.PREW.ltoreq.67: PREW=wt % Cr+3.3 (wt % Mo+0.5 w1 % W)+3
Owt % N {circle over (1)}
5. The high-grade duplex stainless steel of one of claims 2 to 4,
wherein a value of [MM and/or Y+Al].multidot.[O+S] which is an
equation of solubility products of MM and/or Y, and Al, O and S of
steel ranges from 0.001.times.10.sup.-5 to
30000.times.10.sup.-5[%].sup.2.
6. The high-grade duplex stainless steel of claim 5, wherein, in
the case of a cast product, the value of the equation of the
solubility products ranges from 1.times.10.sup.-5 to
5000.times.10.sup.-5[%].sup.2.
7. The high-grade duplex stainless steel of claim 5, wherein, in
the case of a hot working product, the value of the equation of the
solubility products ranges from 0.1.times.10.sup.-5 to
2000.times.10.sup.-5[%].sup.2- .
8. The high-grade duplex stainless steel of one of claims 2 to 4,
wherein a total amount of MM and/or Y ranges from 0.01 to 0.6%.
9. The high-grade duplex stainless steel of claim 8, wherein the
total amount of MM and/or Y ranges from 0.2 to 0.5%.
10. The high-grade duplex stainless steel of one of claims 1 to 4,
further containing at least one element selected from the group
consisting of 0.5% or less of Ca, 0.5% or less of Mg, 1.0% or less
of Al, 0.5% or less of Ta, 0.5% or less of Nb, 1.5% or less of Ti,
1.0% or less of Zr, 1.0% or less of Sn and 1.0% or less of In.
11. The high-grade duplex stainless steel of one of claims 1 to 4,
further containing 0.1% or less of B.
12. The high grade duplex stainless steel of one of claims 1 to 4,
further containing one or more among 3.0% or less of Cu and 3.0% or
less of Co.
13. The high-grade duplex stainless steel of one of claims 1 to 4,
wherein a value of [PREW(.gamma.)-PREW(.alpha.)] which is a
corrosion resistance balance of austenitic phase and ferritic phase
ranges from -5 to 10.
14. The high-grade duplex stainless steel of one of claims 1 to 4,
wherein a volume fraction of ferritic phase ranges from 20 to 70%,
and a volume fraction of austenitic phase ranges from 30 to 80% on
a volume basis.
15. The high-grade duplex stainless steel of claim 10, further
containing 0.1% or less of B.
16. The high grade duplex stainless steel of claim 10, further
containing one or more among 3.0% or less of Cu and 3.0% or less of
Co.
17. The high grade duplex stainless steel of claim 11, further
containing one or more among 3.0% or less of Cu and 3.0% or less of
Co.
Description
TECHNICAL FIELD
[0001] The present invention relates to duplex stainless steel
having excellent corrosion resistance, and more particularly to,
high-grade duplex stainless steel having excellent corrosion
resistance, embrittlement resistance, castability and hot
workability by suppressing formation of intermetallic phases, such
as sigma (.theta.) and khi (.chi.), formed during the production
(casting, hot rolling or welding).
BACKGROUND ART
[0002] Duplex stainless steel where austenite (.gamma.) phase
providing high workability and ferrite (.alpha.) phase providing
high corrosion resistance are minutely combined has higher strength
than austenitic stainless steel by at least 1.7 times, and also
shows high pitting resistance and high stress corrosion cracking
(SCC) resistance. Commercial high-grade duplex stainless steels
having a pitting resistance equivalent (PREW=wt % Cr+3.3(wt %
Mo+0.5wt % W)+30wt % N) of about 46, such as SAF 2507 (UNS S32750),
UR 52N+ (UNS 32550) and ZERON 100 (UNS 32760) have been used for
various purposes since 1990s. Quality of duplex stainless steel has
improved due to development of refining processes, and thus it has
been increasingly used in various fields for a few years.
[0003] However, as compared with commercial PREW 38-level duplex
stainless steel such as SAF 2205, the PREW 46-level high-grade
duplex stainless steel contains a large amount of Cr, Mo and W
which are major elements of sigma and khi phases deteriorating
mechanical properties and corrosion resistance, and thus easily
forms precipitation phases thereof during the production or
application. Actually, embrittlement by the precipitation phases
has been observed in cooling after continuous casting of duplex
stainless steel, slow cooling after hot rolling, slow cooling of a
heat affected zone after welding, and slow cooling of an ingot
center unit after casting. In addition, Mo for improving local
corrosion and SCC resistances among the added alloying elements is
a high-priced element facilitating formation of sigma phases and
475.degree. C. brittleness, and thus restrictively used. Sigma
phase is a very brittle intermetallic compound formed from
temperature of 650.degree. C. to 1000.degree. C. More than 1 vol. %
of sigma phase can remarkably reduce impact toughness and corrosion
resistance of duplex stainless steel.
[0004] Accordingly, a lot of research and development have been
made to suppress formation of sigma phase during the production or
application of duplex stainless steel. But, the conventional
research and development have the following problems. 1) When 1 to
3% of Al or Al and Nb are added to ferritic stainless steel
containing 39% of Cr, a formation speed of sigma phase is lowered,
a formation temperature range of sigma phase is reduced, and thus a
precipitation speed of sigma phase is lowered (K. Permachandra et
al., Materials Science and Technology, Vol. 8, p. 2477 (1997)).
However, it is not relevant to duplex stainless steel containing
austenite and ferrite.
[0005] 2) When Zr is added to stainless steel, a formation speed of
sigma phase is lowered. However, alloying elements such as Al or Zr
are ferrite former which reduce an austenite phase fraction and
form different kinds of intermetallic compounds containing N, to
deteriorate corrosion resistance and mechanical properties (M. B
Cotrie et al., Metallurgical and Materials Transaction 28A (1997)
2477).
[0006] 3) When Sn is added to ferritic stainless steel containing
43 to 46% of Cr, Sn is precipitated in a nucleation area of sigma
phase such as a grain boundary or grain boundary triple point, to
reduce a formation speed of sigma phase. When an alloy is exposed
to a high temperature over 232.degree. C., the ferritic stainless
steel may be cracked due to a low melting point (232.degree. C.) of
Sn. It is not relevant to duplex stainless steel either (Costa et.
Al., Physica Status Solidi, A 139(1993)83).
[0007] 4) Okamoto et al. disclosed that DP3W (UNS S39274) which was
high-grade duplex stainless steel containing 3% Mo+2% W could delay
a precipitation speed of sigma phase more than commercial
high-grade duplex stainless steels containing 3.8% Mo, such as SAF
2507, UR 52N+ and ZERON 100 by adding W in aging heat-treatment for
10 minutes at 850.degree. C. However, when a large-sized ingot and
slab are hot-rolled, or a large-sized product is molten and cast,
corrosion resistance and mechanical properties are deteriorated due
to precipitation of khi and sigma phases showing high brittleness
(H. Okamoto et al., 4.sup.th International Conferences on Duplex
Stainless Steels, (1994) Paper91 and U.S. Pat. No. 5,298,093).
[0008] Especially, in accordance with U.S. Pat. No. 5,298,093,
although a large amount of W (1.5 to 5.0%) is added to improve
corrosion resistance, formation of intermetallic phases is not
accelerated. Therefore, W is positively added, S and O are fixedly
used, and at least one element selected from the group consisting
of 0.02% or less of Ca, 0.02% or less of Mg, 0.02% or less of B,
and 0.2% or less of at least one REM in total is added to improve
hot workability by fixing S and O. In the case that Ca, B, Mg and
REM exceed their upper limits, a lot of oxides and sulfides are
formed. The non-metallic inclusions such as oxides and sulfides are
operated as the pitting point, which reduces corrosion
resistance.
[0009] In addition, U.S. Pat. No. 5,733,387 suggests duplex
stainless steel containing 0.03% or less of C, 1.0% or less of Si,
2.0% or less of Mn, 0.04% or less of P, 0.004% or less of S, 2.0%
or less of Cu, 5.0 to 8.0% of Ni, 22 to 27% of Cr, 1.0 to 2.0% of
Mo, 2.0 to 5.0% of W, 0.13 to 0.30% of N, at least one element
selected from the group consisting of a certain amount of Ca, Ce, B
and Ti, and a balance of Fe. The aforementioned patent reduces the
content of Mo that facilitates formation of intermetallic phases,
and increases the content of W in order to improve corrosion
resistance. However, as confirmed in PREW formula that will later
be described, effects of Mo for improving pitting resistance are
twice as many as W. It is thus inefficient to decrease the content
of Mo.
[0010] On the other hand, rapid cooling is essential in
heat-treatment of duplex stainless steel to suppress formation of
intermetallic phases having high brittleness. When duplex stainless
steel is cooled from a heat-treatment temperature, it passes
through a precipitation temperature of intermetallic phases. If the
cooling speed is not sufficiently high in the temperature zone,
intermetallic phases are rapidly precipitated. When intermetallic
phases are precipitated at a high speed in the slow cooling, duplex
stainless steel becomes embrittled and also shows low corrosion
resistance. Accordingly, another conventional methods for
suppressing precipitation of intermetallic phases are intended to
control a cooling process during heat-treatment.
[0011] In accordance with Japan Patent Laid-Open Publication No.
5-271776, in order to suppress precipitation of intermetallic
phases, duplex stainless steel is cooled to a temperature just
below the lowest temperature of precipitation zone of intermetallic
phases at a much higher speed than a precipitation cooling speed of
intermetallic phases during heat-treatment, and maintained for 5
minutes at a temperature lower than the lowest precipitation
temperature zone of intermetallic phases by over 200.degree. C.
[0012] In addition, Japan Patent Publication No. 62-6615 suggests a
method for suppressing formation of intermetallic phases when
duplex stainless steel is manufactured as a mechanical component by
casting. In general, when the mechanical component is manufactured
by using duplex stainless steel, molten steel is poured into a sand
mold, solidified and left at a room temperature. However, when a
cast product is manufactured using high-grade duplex stainless
steel where intermetallic phases are easily precipitated, some
ferritic phase is transformed into sigma and austenitic phases
during the cooling process to the room temperature after casting,
and thus sigma phase includes embrittlement. In order to suppress
precipitation of sigma phase, the aforementioned Japan Patent
Publication teaches a method for removing a mold when the
temperature is over 1,000.degree. C. and rapidly cooling the
product. If the cooling speed is not sufficiently high in passing
the precipitation temperature zone, sigma phase is rapidly
precipitated. That is, when sigma phase is precipitated during the
cooling process, stainless steel is embrittled and also shows low
corrosion resistance.
[0013] However, the above-described methods for adding the third
alloying elements or controlling the cooling process during the
heat-treatment cannot sufficiently suppress sigma phase in
high-grade duplex stainless steel.
DISCLOSURE OF THE INVENTION
[0014] The main object of the present invention is to remove
brittleness and improve corrosion resistance by reducing a
precipitation speed and amount of brittle intermetallic phases, by
delaying diffusion and precipitation of intermetallic phases by
adding appropriate amounts of Ba, Y, Ce, La, Nd, Pr, Ta, Zr and Ti
atoms having a large atomic diameter, and additionally blocking
diffusion of Cr, Mo, Si and W by using minute Rare Earth compounds
or Ba oxides.
[0015] Another object of the invention is to prevent individual
formation of Al.sub.2O.sub.3 and MnS inclusions which have
detrimental effects on properties of steel by performing proper
preliminary deoxidation according to a common method using Ti, Mg,
Ca, Al and Ca+Al, as well as adding MM (Mischmetal: rare-earth
metallic mixtures consisting of atoms with atomic numbers from 57
to 71, containing at least 50% or more of Ce, a certain amount of
La, Nd and Pr, minute amounts of Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er,
Tm, Yb, Lu and Sc, and 1% or less of Fe. Hereinafter, the detailed
description and embodiment of the present invention uses MM
containing major elements of 51% Ce-26% La-15.5% Nd-5.5% Pr, minute
amounts of Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Sc, and
1% or less of Fe) and/or Y.
[0016] Yet another object of the invention is to control solubility
products in molten steel of rare-earth metal elements of MM and/or
Y (REM, hereinafter, referred as `RE` in compound formulae) within
a certain range, supplying heterogeneous nucleation sites to make
fine and minute structure during dendrite formation of the
solidification, and controlling segregation of the solute elements
such as Cr, Mo, W, Ni, Mn and Si by forming a rare-earth metallic
compound mixture (RExOy or (RE,Al)xOy+RExOyS+RExSy) having a
diameter below 5 .mu.m in molten steel, resulting in improving
mechanical properties, physical properties and corrosion
resistance.
[0017] Yet another object of the invention is to remarkably
suppress formation of intermetallic phases such as sigma in duplex
stainless steel by adding new alloying elements, and to improve the
production yield during mass production.
[0018] Yet another object of the invention is to considerably
increase the production yield in casting and hot working, by
improving embrittlement resistance and preventing cracks by
lowering a precipitation speed of intermetallic phases such as
sigma.
[0019] Yet another object of the invention is to improve corrosion
resistance and mechanical properties and upgrade durability of
equipments, by suppressing precipitation of sigma and khi phases
deteriorating corrosion resistance and mechanical properties in a
casting state, and controlling precipitation of such phases in
heat-affected zone after welding when equipment components are
necessarily welded in various application fields.
[0020] In order to achieve the above-described objects, the summary
of the invention will now be explained:
[0021] (1) High-grade duplex stainless steel with high corrosion
resistance, embrittlement resistance, castability and hot
workability which suppresses formation of intermetallic phases,
consisting essentially of 21.0 to 38.0% of Cr, 3.0 to 12.0% of Ni,
1.5 to 6.5% of Mo, 0 to 6.5% of W, 3.0% or less of Si, 8.0% or less
of Mn, 0.2 to 0.7% of N, 0.1% or less of C, 0.0001 to 0.6% of Ba,
and a balance of Fe and incidental impurities on a weight basis, a
pitting resistance equivalent (PREW) defined by following formula
{circle over (1)} satisfying 40.ltoreq.PREW.ltoreq.67:
PREW=wt % Cr+3.3(wt % Mo+0.5wt % W)+30 wt % N {circle over (1)}
[0022] (2) The high-grade duplex stainless steel of (1), further
containing 0.0001 to 1.0% of MM and/or Y in total.
[0023] (3) The high-grade duplex stainless steel of (2), wherein Ba
is added within the range of 0.001 to 0.2%.
[0024] (4) High-grade duplex stainless steel with high corrosion
resistance, embrittlement resistance, castability and hot
workability which suppresses formation of intermetallic phases,
consisting essentially of 21.0 to 38.0% of Cr, 3.0 to 12.0% of Ni,
1.5 to 6.5% of Mo, 0 to 6.5% of W, 3.0% or less of Si, 8.0% or less
of Mn, 0.2 to 0.7% of N, 0.1% or less of C, 0.0001 to 1.0% of MM
and/or Y in total, and a balance of Fe and incidental impurities on
a weight basis, a pitting resistance equivalent (PREW) defined by
following formula {circle over (1)} satisfying
40.ltoreq.PREW.ltoreq.67:
PREW=wt % Cr+3.3(wt % Mo+0.5 wt % W)+30 wt % N {circle over
(1)}
[0025] (5) The high-grade duplex stainless steel of one of (2) to
(4), wherein a value of [MM and/or Y+Al].multidot.[O+S] which is a
equation of solubility products of MM and/or Y, and Al, O and S of
steel ranges from 0.001.times.10.sup.-5 to
30000.times.10.sup.-5[%].sup.2.
[0026] (6) The high-grade duplex stainless steel of (5), wherein,
in the case of a cast product, the value of the equation of the
solubility products ranges from 1.times.10.sup.-5 to
5000.times.10.sup.-5[%].sup.2.
[0027] (7) The high-grade duplex stainless steel of (5), wherein,
in the case of a hot working product, the value of the equation of
the solubility products ranges from 0.1.times.10.sup.-5 to
2000.times.10.sup.-5[%].sup.2.
[0028] (8) The high-grade duplex stainless steel of one of (2) to
(4), wherein a total amount of MM and/or Y ranges from 0.01 to
0.6%.
[0029] (9) The high-grade duplex stainless steel of (8), wherein
the total amount of MM and/or Y ranges from 0.2 to 0.5%.
[0030] (10) The high-grade duplex stainless steel of one of (1) to
(4), further containing at least one element selected from the
group consisting of 0.5% or less of Ca, 0.5% or less of Mg, 1.0% or
less of Al, 0.5% or less of Ta, 0.5% or less of Nb, 1.5% or less of
Ti, 1.0% or less of Zr, 1.0% or less of Sn and 1.0% or less of
In.
[0031] (11) The high-grade duplex stainless steel of one of (1) to
(4), further containing 0.1% or less of B.
[0032] (12) The high-grade duplex stainless steel of one of (1) to
(4), further containing one or more among 3.0% or less of Cu and
3.0% or less of Co.
[0033] (13) The high-grade duplex stainless steel of one of (1) to
(4), wherein a value of [PREW(.gamma.)-PREW(.alpha.)] which is a
corrosion resistance balance of austenitic phase and ferritic phase
ranges from -5 to 10.
[0034] (14) The high-grade duplex stainless steel of one of (1) to
(4), wherein a volume fraction of ferritic phase ranges from 20 to
70%, and a volume fraction of austenitic phase ranges from 30 to
80% on a volume basis.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIGS. 1A to 1F are pictures showing microstructures of
invention steel 4 (FIG. 1A), invention steel 10 (FIG. 1B) and
invention steel 36 (FIG. 1C) aging heat-treated at 850.degree. C.
for 30 minutes, comparative steel 47 (FIG. 1D), and UR 52N+ (FIG.
1E) and SAF 2507 (FIG. 1F) which are commercial steels;
[0036] FIGS. 2A to 2D are graphs showing X-ray diffraction test
results of invention steel 4 (FIG. 2A) aging heat-treated at
850.degree. C. for 30 minutes, comparative steel 47 (FIG. 2B), and
UR 52N+ (FIG. 2C) and SAF 2507 (FIG. 2D) which are commercial
steels;
[0037] FIGS. 3A to 3D are pictures showing macrostructures of
invention steel 10 (FIG. 3A) and comparative steel 47 (FIG. 3B) in
a middle portion of an ingot (.O slashed.110 mm.times.L550 mm), and
microstructures of invention steel 10 (FIG. 3C) and comparative
steel 47 (FIG. 3D);
[0038] FIG. 4 is a graph showing anodic polarization resistance
test results of invention steels and commercial steel in a casting
state in 50.degree. C. deaerated 0.5N HCl+1.0N NaCl solution;
[0039] FIG. 5 is a graph showing critical pitting temperature test
results of invention steels and commercial steels in 6% FeCl.sub.3
solution;
[0040] FIGS. 6A, 6B and 6C are graphs showing anodic polarization
resistance test results of invention steels (FIG. 6A) solution
heat-treated at 1130.degree. C., commercial high-grade duplex
stainless steels (FIG. 6B), and commercial high-grade austenitic
stainless steels (FIG. 6C) in 70.degree. C. deaerated 0.5N HCl+1.0N
NaCl solution;
[0041] FIGS. 7A and 7B are graphs showing anodic polarization
resistance test results of invention steels (FIG. 7A) aging
heat-treated at 850.degree. C. for 10 minutes and commercial
high-grade duplex stainless steels (FIG. 7B) in 50.degree. C.
deaerated 0.5N HCl+1.0N NaCl solution; and
[0042] FIGS. 8A and 8B are graphs showing anodic polarization
resistance test results of invention steels (FIG. 8A) aging
heat-treated at 850.degree. C. for 30 minutes and commercial
high-grade duplex stainless steels (FIG. 8B) in 50.degree. C.
deaerated 0.5N HCl+1.0N NaCl solution.
BEST MODE FOR CARRYING OUT THE INVENTION
[0043] High-grade duplex stainless steel having excellent corrosion
resistance, embrittlement resistance, castability and hot
workability which suppresses formation of intermetallic phases in
accordance with the present invention will now be described in
detail with reference to the accompanying drawings.
[0044] Based on the fact that, even if corrosion resistance and
mechanical properties are remarkably improved in a thin laboratory
size of mother alloy manufactured by optimal alloy design, special
conditions should be satisfied to increase the production yield of
thick cast products and hot working products in the mass
production, and to improve corrosion resistance and mechanical
properties thereof, the present inventors have thoroughly
researched into mechanism of intermetallic phases such as sigma
(.theta.) and khi (.chi.) showing detrimental effects on corrosion
resistance, embrittlement resistance, castability and hot
workability, and reached the following results.
[0045] That is, the present inventors found out that, when alloying
elements such as Ba, MM (Ce, La, Nd, Pr) and/or Y which have a much
larger atomic diameter than basic alloying elements such as Fe, Cr,
Mo, Ni, W, Mn and Si which composed duplex stainless steel
containing Cr, Mo, Si and W facilitating formation of intermetallic
phases were added, the alloying elements atoms having a larger
atomic diameter filled atomic vacancies operating as a diffusion
path for Cr, Mo, Si and W composing sigma and khi phases,
especially filled atomic vacancies in austenitic and ferritic phase
boundaries and crystal grains of ferritic phase, to lower a
formation speed of intermetallic phases at a temperature ranging
from 1000 to 650.degree. C.
[0046] In addition, the present inventors discovered that, because
the alloying elements having a large atomic diameter have much
lower free energy for thermodynamically forming oxides or
oxy-sulfides than Fe, Cr, Mo, W, Ni, Mn and Si, and thus could form
minute and uniform oxides and oxy-sulfides having a diameter below
5 .mu.m. Those minute rare-earth metallic mixtures or Ba oxides
could additionally block diffusion of Cr, Mo, Si and W at a
temperature ranging from 1000 to 650.degree. C., to lower a
precipitation speed of intermetallic phases.
[0047] The present inventors also found out that MnS non-metallic
inclusion was generally operated as a starting point of corrosion
due to its lower corrosion resistance than a matrix, but rare-earth
non-metallic inclusion was not operated as a starting point of
corrosion due to its higher corrosion resistance than the
matrix.
[0048] That is, the present invention suppresses formation of
intermetallic phases by adding 0.0001 to 0.6% of Ba (2.18 .ANG.)
(number of bracket represents atomic diameter) having a larger
atomic diameter than Fe (1.24 .ANG.), Cr (1.25 .ANG.), Mo (1.36
.ANG.), W (1.37 .ANG.), Ni (1.25 .ANG.), Mn (1.12 .ANG.) and Si
(1.17 .ANG.) which are major alloying elements of commercial duplex
stainless steel.
[0049] Moreover, the present invention actively suppresses
formation of intermetallic phases by adding MM (comprising major
elements such as Ce: 1.83 .ANG., La: 1.88 .ANG., Nd: 1.82 .ANG. and
Pr: 1.83 .ANG., minute amounts of Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er,
Tm, Yb, Lu and Sc, and 1% or less of Fe) and/or Y (1.82 .ANG.)
having a larger atomic diameter than Fe (1.24 .ANG.), Cr (1.25
.ANG.), Mo (1.36 .ANG.), W (1.37 .ANG.), Ni (1.25 .ANG.), Mn (1.12
.ANG.) and Si (1.17 .ANG.) which are major alloying elements of
commercial duplex stainless steel, or adding it with Ba. Here, in
order to facilitate such effects, [MM and/or Y+Al].multidot.[O+S]
which is a equation of solubility products of MM and/or Y, and Al,
O and S of steel should exist within the range of
0.001.times.10.sup.-5 to 30000.times.10.sup.-5.
[0050] In addition, when an appropriate amount of at least one
alloying element of Ca (1.97 .ANG.), Mg (1.6 .ANG.), Al (1.43
.ANG.), Ta (1.43 .ANG.), Nb (1.43 .ANG.), Ti (1.47 .ANG.), Zr (1.62
.ANG.), Sn (1.51 .ANG.) and In (1.68 .ANG.) which have a larger
atomic diameter than the alloying elements is added, formation of
sigma and khi phases is more efficiently suppressed.
[0051] When B which has a much smaller atomic diameter than Fe, Cr,
Mo, W, Ni, Mn and Si to fill the spaces of the alloying elements
having a large atomic diameter is added with the alloying elements,
B serves to lower a precipitation speed of sigma and khi phases
with the alloying elements.
[0052] At least one alloying element of Cu and Co can be
additionally used to improve acid resistance and strength.
[0053] The role of alloying elements added to duplex stainless
steels in accordance with the present invention, and the reasons
for restricting chemical composition ranges thereof will now be
described.
[0054] Cr: 21.0 to 38.0%
[0055] Cr is a basic important element for maintaining corrosion
resistance of stainless steel. At least 12% of Cr is required to
maintain corrosion resistance. In the present invention, the alloy
needs to have austenite-ferrite duplex structure, and thus at least
21% of Cr must be used in consideration of Cr.sub.eq and Ni.sub.eq
defined in the following formula and austenite/ferrite phase ratio
determined by them. So as to manufacture duplex stainless steel by
balance of C, N, Ni, Mo, W, Si, Mn and Cu, the upper limit of Cr is
set up 38%, more preferably, 24 to 28%.
Cr.sub.eq=% Cr+2% Si+1.5% Mo+0.75% W+5% V+5.5% A1+1.75% Nb+1.5% Ti
{circle over (2)}
Ni.sub.eq=% Ni+0.5% Mn+30% C+0.3% Cu+25% N+% Co {circle over
(3)}
Austenitic phase fraction (vol
%)=100-[55.times.(Cr.sub.eq/Ni.sub.eq)-66.1- ] {circle over
(4)}
Ferritic phase fraction (vol %)=55.times.(Cr.sub.eq/Ni.sub.eq)-66.1
{circle over (5)}
[0056] In addition, a range of phase ratios for maximizing
corrosion resistance of duplex stainless steel is obtained by the
following examples of the invention. The ferritic ratio ranges from
20 to 70 vol. % (30 to 80 vol. % in austenitic phase fraction).
[0057] Ni: 3to 12%
[0058] At least 3% of Ni is required because it is an austenitic
stabilizer for increasing uniform corrosion resistance. 3.0 to
12.0%, more preferably 6 to 9% of Ni is used in consideration of
Cr.sub.eq, Ni.sub.eq, phase ratio and its high cost.
[0059] Mo: 1.5 to 6.5%
[0060] Mo is an important element for maintaining corrosion
resistance of the alloy like Cr. Mo serves to stabilize ferritic
phase. Since the alloy of the invention needs to have
austenite-ferrite duplex structure, at least 1.5% of Mo should be
added in consideration of Cr.sub.eq, Ni.sub.eq and phase ratio.
Especially, when Mo is added with Cu, it can remarkably improve
corrosion resistance in high density SO.sub.4.sup.2- and Cl.sup.-
environment. Mo is very useful to improve mechanical properties and
corrosion resistance in an annealing state, but forms intermetallic
phases having detrimental effects such as sigma in aging
heat-treatment, hot rolling or welding. Accordingly, 6.5% or less
of Mo is used in consideration of Cr.sub.eq, Ni.sub.eq, corrosion
resistance and phase stability. As confirmed in PREW formula,
effects of Mo for improving pitting resistance are twice as many as
W. Thus, the content of Mo is more preferably over 2% to obtain
superior pitting resistance.
[0061] W: 0 to 6.5%
[0062] W is a ferritic stabilizer and a homologous alloying element
having similar chemical properties to Mo. W improves corrosion
resistance in high density SO.sub.4.sup.2- and Cl.sup.-
environment, and also improves corrosion resistance and mechanical
properties by delaying a precipitation speed of brittle sigma and
khi phases after sensitization heat-treatment or welding. However,
W is a high-priced alloying element, and if a large amount of W is
used, it facilitates formation of intermetallic compounds.
Therefore, 6.5% or less, more preferably 4.0% or less of W is used
in consideration of phase stability, mechanical properties and
corrosion resistance.
[0063] Si: 3% or Less
[0064] Si is a ferritic stabilizer which has deoxidation effects in
refining, and which increases fluidity of molten steel and reduces
surface defects in cast production. When Si is used over 3%, it
increases a precipitation speed of brittle intermetallic phases,
and reduces ductility of steel. 3.0% or less, more preferably, 1.0%
or less of Si is used in consideration of corrosion resistance.
[0065] Mn: 8% or Less
[0066] Mn is an austenitic stabilizer which can replace high-priced
Ni. Mn serves to increase solid solubility of N and reduce high
temperature deformation resistance. In order to improve corrosion
resistance by increasing the content of N, an appropriate amount of
Mn is essentially used. It has deoxidation effects in dissolution
and refining. However, a large amount of Mn deteriorates corrosion
resistance, and facilitates formation of brittle intermetallic
phases. Accordingly, the content of Mn is set up 8% or less, more
preferably 1.0 to 3.0%.
[0067] N: 0.2 to 0.7%
[0068] N is very useful to improve pitting resistance, which is
more effective than Cr by about 30 times. N is a strong austenitic
stabilizer, and also is one of the most important elements for
improving corrosion resistance. When N exists with Mo, it can
considerably improve corrosion resistance. When the content of C is
reduced to improve grain boundary corrosion resistance, N can
compensate for mechanical properties. In addition, N suppresses
formation of Cr carbides, and improves tensile strength and yield
strength without reducing elongation. The content of N must be
controlled in consideration of balance with C, Cr, Ni, Mo and W and
austenitic-ferritic phase ratio. At least 0.2% of N is preferably
used in respect of corrosion resistance. However, when N is used
over 0.7%, it may reduce castability (blowhole, shrinkage) and
rollability. More preferably, the content of N ranges from 0.32 to
0.45%.
[0069] C: 0.1% or Less
[0070] C is a representative element for stabilizing austenite
phase and an important element for maintaining mechanical strength.
However, if a large amount of C is used, it precipitates carbides
and thus reduces corrosion resistance. Therefore, 0.1% or less,
preferably 0.05% or less of C is used, and more preferably, 0.03%
or less of C is used to improve corrosion resistance in aging.
[0071] PREW Value: 40 to 67
[0072] In addition to that the contents of Cr, Mo, W and N are
restricted as described above, the value of the PREW defined by
following formula {circle over (1)} satisfies
40.ltoreq.PREW.ltoreq.67:
PREW=wt % Cr+3.3(wt % Mo+0.5wt % W)+30 wt % N {circle over (1)}
[0073] When the value of the PREW is lower than the lowest limit,
corrosion resistance is not sufficiently obtained, and when the
value of the PREW is higher than the upper limit, formation of
intermetallic phases is facilitated. Preferably, the value of the
PREW is greater than 45.
[0074] Moreover, [PREW(.gamma.)-PREW(.alpha.)] which is a corrosion
resistance balance of phases for maximizing corrosion resistance of
duplex stainless steel preferably ranges from -5 to 10 according to
the examples of the invention which will later be discussed.
[0075] Ba: 0.0001 to 0.6%
[0076] As described above, Ba is one of the most important elements
of the invention. Ba has an atomic diameter of 2.18 .ANG.. Ba
having a much larger atomic diameter than the other alloying
elements (Fe, Cr, Mo, W, Ni, Mn, Si, etc.) of duplex stainless
steel is operated as a barrier for blocking diffusion of Cr, Mo and
W which are main constituents of brittle intermetallic phases, and
thus effective to reduce a diffusion speed, a precipitation speed
and a precipitation amount. In addition, Ba is coupled with solute
atoms and oxygen to form oxides, thereby lowering a precipitation
speed of sigma and khi phases. In order to obtain the
aforementioned effects, 0.6% or less of Ba is required. When Ba is
used over 0.6%, it is not economically advantageous. Furthermore, a
large amount of Ba is precipitated in grain boundaries, to reduce
grain boundary strength at a high temperature and offset
improvements of high temperature cracking sensitivity. Accordingly,
the upper limit of Ba is set up 0.6%. On the other hand, if Ba is
used below 0.0001%, addition effects thereof are not
expectable.
[0077] MM and/or Y: 0.0001 to 1.0%
[0078] MM (Mischmetal: rare-earth metallic mixtures consisting of
atoms with atomic numbers from 57 to 71, containing at least 50% or
more of Ce, a certain amount of La, Nd and Pr, minute amounts of
Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Sc, and 1% or less
of Fe. As described above, the detailed description and embodiment
of the present invention uses MM containing major elements of 51 %
Ce-26% La-15.5% Nd-5.5% Pr, minute amounts of Pm, Sm, Eu, Gd, Tb,
Dy, Ho, Er, Tm, Yb, Lu and Sc, and 1% or less of Fe) and/or Y is
one of the most important alloying elements which can be added
with/without Ba. When MM and/or Y is added, it can prevent
individual formation of Al.sub.2O.sub.3 and MnS non-metallic
inclusions which have detrimental effects on general properties of
steel, form a rare-earth metallic compound mixture (RExOy or
(RE,Al)xOy+RExOyS+RExSy) having a diameter below 5 .mu.m in molten
steel, operate as heterogeneous nucleation sites to make a
solidified structure fine and minute in solidification, and control
segregation of solute elements, to improve mechanical properties,
physical properties and corrosion resistance.
[0079] In addition, Y, MM(Ce, La, Nd, Pr, etc.), Ba, Zr and Ti
having a large atomic diameter maintained in steel in an atomic
state are very efficient to delay a precipitation speed of brittle
intermetallic phases. MM and/or Y is a very important element for
improving weldability, high temperature oxidation resistance,
machinability and high temperature workability. The content of MM
and/or Y ranges from 0.0001 to 1.0%. If MM and/or Y is used over
1.0%, the addition is not economically advantageous, and the
excessive amounts have detrimental effects on general properties of
steel. When MM and/or Y is used below 0.0001%, the aforementioned
addition effects cannot be obtained.
[0080] Moreover, in order to obtain micro-uniformity of a
segregation area of solute elements resulting from fine and minute
structure induced by heterogeneous nucleation during dendrite
formation of solidification of a rare-earth metallic compound
mixture (RExOy or (RE,Al)xOy+RExOyS+RExSy), and reduction of a
precipitation speed of intermetallic phases by allowing Y, MM(Ce,
La, Nd, Pr, etc.), Ba, Zr and Ti to block diffusion of Cr, Mo, Si
and W facilitating formation of intermetallic phases, [MM and/or
Y+Al].multidot.[O+S] which is a equation of solubility products of
MM and/or Y in steel, and Al, O and S of steel is supposed to range
from 0.001.times.10.sup.-5 to 30000.times.10.sup.-5[%].sup.2. When
the value of the equation of the solubility products is below
0.001.times.10.sup.-5, it is difficult to control the solidified
structure, decrease segregation of solute elements and suppress
formation of intermetallic phases. If the value of the equation of
the solubility products is over 30000.times.10.sup.-5[%].sup.2,
rare-earth metallic compound mixtures are excessively formed, to
deteriorate mechanical properties, physical properties and
corrosion resistance of steel. More preferably, in the case of a
cast product, the value of the equation of the solubility products
ranges from 1.times.10.sup.-5 to 5000.times.10.sup.-5[%].sup.2, and
in the case of a hot working product, the value of the equation of
the solubility products ranges from 0.1.times.10.sup.-5 to
2000.times.10.sup.-5[%].sup.2.
[0081] The content of MM preferably ranges from 0.01 to 0.6%, more
preferably 0.2 to 0.5%.
[0082] Ca: 0.5% or Less
[0083] Ca is a deoxidation element for improving embrittlement
resistance, and reducing high temperature deformation resistance
and machinablitiy resistance. When a large amount of Ca is used, it
reduces purity and corrosion resistance of steel. Preferably, 0.5%
or less of Ca is used.
[0084] Al, O and S
[0085] Al is a ferritic stabilizer for improving oxidation
resistance and embrittlement resistance. When Al is added to steel,
it increases purity of steel by deoxidation effects, and reduces
high temperature deformation resistance. Preferably, 1.0% or less
of Al is used.
[0086] In addition, steel essentially contains O and S which
generate cracks during solidification process and decrease
ductility after production. Accordingly, O and S which generate
brittleness should be restrictively used. In the case of a cast
product, 200 ppm or less of O and 50 ppm or less of S should be
used, and in the case of wrought product, 100 ppm or less of O and
20 ppm or less of S should be used.
[0087] Ti: 1.5% or Less
[0088] Ti shows deoxidation effects in refining process, and forms
titanium sulfides to improve mechinability. In order to improve
intergranular corrosion resistance, the content of Ti is determined
in consideration of an amount of C. 1.5% or less of Ti is used to
improve corrosion resistance in environments including chloride
ions after sensitization heat-treatment.
[0089] Mg: 0.5% or Less, Ta: 0.5% or Less, Nb: 0.5% or Less, Zr:
1.0% or Less, Sn: 1.0% or Less, In: 1.0% or Less
[0090] As disclosed by the present inventors, in addition to Ca
(1.97 .ANG.), Al (1.43 .ANG.and Ti (1.47 .ANG.) which have a larger
atomic diameter than Fe, Cr, Mo and W, Mg (1.6 .ANG.), Ta (1.43
.ANG.), Nb (1.43 .ANG.), Zr (1.62 .ANG.), Sn (1.51 .ANG.) and In
(1.68 .ANG.) are efficient to suppress sigma and khi phases.
Therefore, 0.5% or less of Mg, 0.5% or less of Ta, 0.5% or less of
Nb, 1.0% or less of Zr, 1.0% or less of Sn and 1.0% or less of In
are used.
[0091] When the aforementioned alloying elements exceed their upper
limits, they are not economically advantageous, and generate grain
boundary embrittlement to deteriorate castability and hot
workability.
[0092] B:0.1% or Less
[0093] B is useful to improve embrittlement resistance and reduce
high temperature deformation resistance, and prevent high
temperature cracks in welding. When B is used with N, boron
nitrides having a low melting point are formed to improve
machinability. Especially, B has a much smaller atomic diameter
than Fe, Cr, Mo, W, Ni, Mn and Si, and thus fills minute gaps.
Accordingly, when B coexists with alloying elements having a large
atomic diameter, it can enhance blocking effects to decrease a
precipitation speed of sigma and khi phases. Preferably, 0.1% or
less of B is used.
[0094] Cu: 3% or Less
[0095] Cu is an austenitic stabilizer for improving corrosion
resistance. Especially, when Cu is used with Mo, it considerably
increases corrosion resistance in acid environments of concentrated
sulfuric acid and hydrochloric acid. Cu also induces substitutional
solid solution hardening effects to improve tensile strength and
yield strength.
[0096] If an appropriate amount of Cu is not used in consideration
of phase ratio, Cr and Mo, Cu may reduce pitting resistance. In
addition, Cu is an important element for improving machinability by
lowering a working hardening speed. When Cu is over 3%, it
generates hot shortness. Therefore, 3% or less of Cu is used.
[0097] Co: 3.0% or Less
[0098] Co is an austenitic stabilizer which can replace Ni. Co is
very effective to improve corrosion resistance and strength, but
its cost is high. 3.0% or less of Co is used in consideration of
phase ratio and corrosion resistance balance.
EXAMPLE 1
Method for Manufacturing and Testing Steel of Invention
[0099] A method for designing and manufacturing optimal alloys in
accordance with the present invention will now be explained. The
method for designing the alloy is obtained by optimally combining
alloy design factors such as PREW of equation {circle over (1)},
[PREW(.gamma.)-PREW(.alpha.)] for corrosion resistance balance of
phases, Cr.sub.eq of equation {circle over (2)}, and Ni.sub.eq of
equation {circle over (3)}, and resultant values are shown in Table
2.
[0100] Cr.sub.eq and Ni.sub.eq were calculated by the equation
{circle over (2)} and {circle over (e)} to determine the
composition, the alloying elements are melted in high frequency
induction furnace using commercial pure grade of Fe, Cr, Mo, Ni, W,
Cu, Si, Mn and Fe--Cr--N as recited in the claims of the invention,
and deoxidized according to a common method such as Ti, Mg, Al, Ca
or Al+Ca composite deoxidation, samples for casting melted in air,
and samples for wrought products were melted in vacuum and nitrogen
gas atmosphere. As shown in Table 4, the value of
[PREW(.gamma.)-PREW(.alpha.)] for corrosion resistance balance of
phases of Table 2 was obtained by analyzing Cr, Mo, W and N
elements composing austenitic and ferritic phases, and introducing
the resultant values to the PREW equation {circle over (1)}.
[0101] According to another aspect of the invention, molten steel
containing the elements as recited in the claims of the invention
was preliminarily deoxidized according to a common method such as
Al, Ca or Al+Ca composite deoxidation, and Ba and/or MM and/or Y
was added to molten steel to form Ba oxides or rare-earth metallic
compound mixtures (RExOy or (RE,Al)xOy+RExOyS+RExSy), so that
solubility products could satisfy `[Ba and/or MM and/or
Y+Al].multidot.[O+S]=0.001.times.10.sup.-5 to
30000.times.10.sup.-5[%].sup.-2`.
[0102] Thereafter, 25 Kg of plate-type cast product (9 mm thick)
was manufactured by pouring molten metal into a plate type ceramic
mold, and 30 Kg of ingot was manufactured by pouring molten metal
into a preheated rectangular steel mold. In the case of the ingot
for wrought products, it was processed in a proper size using a
grinding or machining process, soaked at 1250.degree. C., and hot
rolled to a thickness of 6 mm. The solution heat-treatment was
performed for both the cast product and 6 mm thick--hot rolled
product at a temperature ranging from 1050 to 1150.degree. C. Table
1 shows the chemical compositions of the solution heat-treated
steels of the invention in comparison with comparative and
commercial steels.
[0103] In order to evaluate general properties of the solution
heat-treated product and the product aging heat-treated at
850.degree. C. for 10 minutes, microstructures, X-ray diffraction
test results, anodic polarization test results, critical pitting
temperature, critical crevice corrosion temperature, and mechanical
properties thereof were measured.
[0104] The samples were polished to 2000 grit using SiC polishing
paper, finally polished using alumina, processed in Murakami
solution (30 g K.sub.3Fe(CN).sub.6+30 g KOH+100 ml distilled water)
at 80.degree. C., etched, and washed ultrasonically in acetone and
distilled water. Then, the microstructures of the samples were
observed using an optical microscope.
[0105] The X-ray diffraction test was performed to confirm sigma
and khi phases precipitated according to the aging heat-treatment
at 850.degree. C. for 30 minutes. Rikagu D/MAX-B was used as a test
device, the samples were analyzed at an accelerating voltage of 35
kv and a current of 35 mA, and Ni filter was used with Cu
target.
[0106] According to the observation results, the samples where a
lot of phases had been precipitated were analyzed at angles ranging
from 30 to 120.degree. with a speed of 12.degree./min., and
analyzed precisely again at a angles ranging from 40 to 50.degree.
where peak concentration of the precipitated phases are observed
with a speed of 1.degree./min.
[0107] The anodic polarization test was performed per ASTM G5 in 50
and 70.degree. C. deaerated 0.5N HCl+1.0N NaCl solution with a
scanning speed of 1 mV/sec.
[0108] The critical pitting resistance temperature was measured per
ASTM G 48A-92, and the critical crevice corrosion temperature was
measured per ASTM G 48D.
[0109] The samples were polished to 600 grit, and the hardness of
the samples was measured in C-scale by using Rockwell hardness
tester.
EXAMPLE 2
Comparison of Microstructures of Aged Products
[0110] FIGS. 1A to 1F are pictures of microstructures showing
precipitation of brittle intermetallic phases deteriorating
corrosion resistance and mechanical properties, such as sigma and
khi of invention steel 4 (FIG. 1A), invention steel 10 (FIG. 1B)
and invention steel 36 (FIG. 1C) aging heat-treated at 850.degree.
C. for 30 minutes, comparative steel 47 (FIG. 1D), and UR 52N+
(FIG. 1E) and SAF 2507 (FIG. 1F) which are commercial high-grade
duplex stainless steels. Bright parts show austenitic phase, and
dark parts show ferritic phase decomposed into sigma
phase+austenite phase in the aging heat-treatment. The degree of
precipitation of the intermetallic phases was `invention steel
4=invention steel 10=invention steel 36<<commercial steel UR
52+<commercial steel SAF 2507<<comparative steel 47`. As a
result, invention steels 4, 10 and 36 suppressed precipitation of
the intermetallic phases more than commercial steels UR 52+ and SAF
2507 and comparative steel 47, to remarkably improve embrittlement
resistance.
EXAMPLE 3
X-ray Diffraction Analysis Test
[0111] FIGS. 2A to 2D are graphs of X-ray diffraction analysis test
results showing precipitation of brittle intermetallic phases
deteriorating corrosion resistance and mechanical properties, such
as sigma and khi of invention steel 4 (FIG. 2A) aging heat-treated
at 850.degree. C. for 30 minutes, comparative steel 47 (FIG. 2B),
and UR 52N+ (FIG. 2C) and SAF 2507 (FIG. 2D) which are commercial
high-grade duplex stainless steels. As compared with comparative
steel 47 and commercial steels UR 52+ and SAF 2507, invention steel
4 did not precipitate sigma phase, and precipitated little khi
phase, to considerably improve embrittlement resistance.
EXAMPLE 4
Comparison of Macrostructures and Microstructures in Cast State
[0112] FIGS. 3A to 3D are pictures showing macrostructures (FIGS.
3A and 3B) and microstructures (FIGS. 3C and 3D) in a middle
portion of an ingot (.O slashed.110 mm.times.L550 mm) of invention
steel 10 manufactured according to the method for controlling
solidified structures, segregation of solute elements, and
formation of intermetallic phases, and comparative steel 47 in a
casting state.
[0113] As compared with the macrostructure (FIG. 3B) of comparative
steel 47 (0.015% O, 0.007% S) where a value of solubility products
was zero because MM and Al were not added, the macrostructure (FIG.
3A) of invention steel 10 (0.09% MM, 0.02% Al, 0.025% O and 0.007%
S) where [MM+Al].multidot.[O+S], a value of solubility products of
MM (Ce, La, Nd, Pr) of molten steel, and/or Y, and Al, O and S was
352.0.times.10.sup.-5 [%].sup.2 was a minute equiaxed crystal
structure where growth of columnar crystals was restricted, had
dense solidified structures, and did not generate V segregation and
inverse V segregation.
[0114] In addition, as compared with the microstructure (FIG. 3D)
of comparative steel 47, the microstructure (FIG. 3C) of invention
steel 10 remarkably suppressed precipitation of intermetallic
phases deteriorating corrosion resistance and mechanical properties
such as sigma and khi, and reduced a size of austenitic and
ferritic phases.
EXAMPLE 5
Anodic Polarization Test Results in Cast State
[0115] FIG. 4 is a graph showing anodic polarization resistance
test results of invention steels 4, 10, 26 and 36 which are not
solution heat-treated and comparative steel 47 in a cast state. The
degree of pitting resistance was `invention steel 10>invention
steel 4>invention steel 36.gtoreq.invention steel
26>comparative steel 47`.
EXAMPLE 6
Results of Critical Pitting and Crevice Corrosion Temperature
Tests
[0116] FIG. 5 is a graph showing a critical pitting temperature of
solution heat-treated invention steels 4, 10, 26 and 36, UR 52N+,
SAF 2507 and ZERON 100 which are commercial high-grade duplex
stainless steels, SAF 2205 which is commercial duplex stainless
steel, SR-50A which is commercial high-grade austenitic stainless
steel, and AISI 316L which is commercial austenitic stainless
steel. When the critical pitting temperature increased, the pitting
resistance improved. The degree of corrosion resistance of
invention steels and commercial steels was `invention steel
10=invention steel 26=invention steel 36>commercial steel
SR-50A>invention steel 4>commercial steel UR 52N+=commercial
steel ZERON 100>commercial steel SAF 2507>commercial steel
SAF 2205>commercial steel AISI 316L`.
[0117] Invention steels 10, 26 and 26 showed higher pitting
resistance than commercial high-grade duplex stainless steels UR
52N+, SAF 2507 and ZERON 100, and higher corrosion resistance than
SR-50A which was commercial high-priced austenitic stainless steel.
Invention steels had a much higher critical pitting temperature
than comparative and commercial steels, and thus had a higher
critical crevice corrosion temperature as shown in Table 2, thereby
remarkably improving crevice corrosion resistance. Table 2 shows
the critical pitting temperature and the critical crevice corrosion
temperature of the steels.
EXAMPLE 7
Anodic Polarization Test Results of Solution Heat-Treated
Product
[0118] FIGS. 6A to 6C are graphs showing anodic polarization
resistance test results of solution heat-treated invention steels
4, 10, 26 and 36 (FIG. 6A), commercial high-grade duplex stainless
steels UR 52N+, SAF 2507 and ZERON 100 (FIG. 6B) and commercial
high-grade austenitic stainless steels AL-6XN, SR-50A and 254SMO
(FIG. 6C). The degree of pitting resistance was `invention steel
26=invention steel 36=commercial steel SR-50A>invention steel
10>invention steel 4>commercial steel AL-6XN>commercial
steel 254SMO>commercial steel UR 52N+=SAF 2507=ZERON 100`.
[0119] In Example 6, invention steels had a much higher critical
pitting temperature and critical crevice corrosion temperature than
comparative and commercial steels, and thus had a high pitting
potential in the anodic polarization test (refer to Table 2). That
is, the three tests showed similar results.
EXAMPLE 8
Anodic Polarization Test Results of Aged Product (850.degree.
C.times.10 min.)
[0120] FIGS. 7A and 7B are graphs showing anodic polarization
resistance test results of invention steels 4, 10, 26 and 36 (FIG.
7A) aging heat-treated at 850.degree. C. for 10 minutes and
commercial high-grade duplex stainless steels UR 52N+, SAF 2507 and
ZERON 100 (FIG. 7B). The degree of pitting resistance was
`invention steel 4=invention steel 10=invention steel
26>invention steel 36>commercial steel ZERON
100>commercial steel SAF 2507>commercial steel UR 52N+`.
[0121] As a result, as compared with commercial steels UR52N+, SAF
2507 and ZERON 100, invention steels 4, 10 and 26 lowered a
precipitation speed of intermetallic phases such as sigma and khi
during the aging heat-treatment, to improve pitting resistance.
EXAMPLE 9
Anodic Polarization Test and Hardness Measurement Results of Aged
Product (850.degree. C..times.30 min.)
[0122] FIGS. 8A and 8B are graphs showing anodic polarization
resistance test results of invention steels 4, 10, 26 and 36 (FIG.
8A) aging heat-treated at 850.degree. C. for 30 minutes and
commercial high-grade duplex stainless steels UR 52N+, SAF 2507 and
ZERON 100 (FIG. 8B). The degree of pitting resistance was
`invention steel 10>invention steel 4>invention steel
36=invention steel 26=commercial steel SAF 2507>commercial steel
ZERON 100>commercial steel UR 52N+`.
[0123] Accordingly, as compared with commercial steels UR52+, SAF
2507 and ZERON 100, invention steels 4 and 10 lowered a
precipitation speed of intermetallic phases such as sigma and khi
during the aging heat-treatment, to remarkably improve pitting
resistance, and invention steels 36 and 26 showed pitting
resistance similar to or higher than commercial steels.
[0124] Table 2 shows differences of hardness values
(.DELTA.H=H.sub.A-H.sub.SA) obtained by subtracting hardness values
H.sub.S.A of solution heat-treated invention steels from hardness
values HA of aging heat-treated invention steels at 850.degree. C.
for 30 minutes. In general, when sigma and khi phases having high
brittleness increased, .DELTA.H also increased, to seriously reduce
corrosion resistance, strength, elongation and impact strength. As
shown in Table 2, .DELTA.H of invention steels ranged from 0.1 to
3.7 due to delay of a precipitation speed of intermetallic phases,
.DELTA.H of comparative steels ranged from 10.3 to 16.2, and
.DELTA.H of commercial steels ranged from 5.6 to 6.2. That is,
invention steels showed more excellent embrittlement resistance
than comparative steels.
EXAMPLE 10
Mechanical Properties
[0125] Table 3 shows yield strength, tensile strength and
elongation after solution heat-treatment of cast product at
1130.degree. C. and performing a tensile test thereon. Invention
steels had high strength due to interstitial solution hardening
effects by high N addition, and fixed crystal grain boundaries by
Ba, and rare-earth oxides or sulfuric oxides (<5 .mu.m), to
simultaneously improve strength and elongation. Therefore,
invention steels have much better mechanical properties than
comparative steels.
EXAMPLE 11
Properties of Hot Rolled Samples
[0126] Table 5 shows critical pitting temperature, mechanical
properties and hot workability of hot-rolled samples after casting
in vacuum and nitrogen atmosphere. The hot-rolled samples showed
better mechanical properties and microstructures than invention
steels cast in air by over 10%, and showed corrosion resistance
similar to them.
[0127] The hot-rolled samples generated less cracks on the edge
than comparative products during the hot rolling, to maintain high
hot workability.
1TABLE 1 Chemical Composition of Invention Steel, Comparative Steel
and Commercial Steel (wt. %) Classi- fication.sup.1) C Cr Ni Mo W N
Cu Mn Si MM.sup.2) or Y Ba Ta Zr B Others STEELS .smallcircle. 1
0.015 29.0 7.0 1.7 3.0 0.43 0.01 3.0 0.82 Y: 0.45 -- -- 0.02 0.0015
Sn: 0.01, Nb: 0.01, Mg: 0.01, Co: 0.01, In: 0.01, Al: 0.02
.smallcircle. 2 0.014 26.9 7.3 2.5 3.2 0.34 0.4 2.0 0.17 0.01
0.0015 Al: 0.03 .smallcircle. 3 0.020 25.3 7.1 3.9 0.7 0.27 0.80
0.8 0.32 MM: 0.05 0.01 -- 1.0 0.0021 Al: 0.01 .smallcircle. 4 0.017
27.0 7.4 2.4 3.1 0.40 0.02 1.6 0.55 MM: 0.11 -- -- -- 0.0028 Al:
0.02 .smallcircle. 5 0.012 27.5 7.1 2.5 3.24 0.34 2.1 0.22 MM:
0.008 0.002 0.0020 Al: 0.02 .smallcircle. 6 0.015 21.0 7.0 6.4 3.2
0.36 0.05 2.1 0.37 MM: 0.08 0.04 -- -- -- Nb: 0.04, Mg: 0.02, Al:
0.03 .smallcircle. 7 0.014 27.1 7.3 2.7 2.9 0.33 0.04 1.7 0.36 MM:
0.08 0.01 -- -- 0.0050 Al: 0.02 .smallcircle. 8 0.013 26.8 6.8 2.5
3.2 0.35 1.2 2.1 0.2 0.01 0.0015 Al: 0.03 .smallcircle. 9 0.012
26.6 5.2 2.6 3.9 0.34 2.30 2.2 0.34 MM: 0.4 0.02 -- 0.01 0.0019 In:
0.42, Al: 0.03 .smallcircle. 10 0.021 26.3 7.3 2.6 3.0 0.36 0.08
1.4 0.59 MM: 0.09 0.01 -- -- 0.0024 Al: 0.02 .smallcircle. 11 0.015
27.1 6.6 2.5 3.2 0.36 1.2 2.0 0.21 MM: 0.01 0.0018 Al: 0.02
.smallcircle. 12 0.014 30.0 9.6 2.8 1.8 0.44 0.04 2.8 0.60 Y: 0.3,
0.02 0.004 0.05 -- Sn: 0.56, MM: 0.2 Al: 0.03 .smallcircle. 13
0.017 27.3 7.4 2.5 3.0 0.41 0.06 1.6 0.55 MM: 0.12 -- -- -- 0.08
Ti: 0.04, Al: 0.01 .smallcircle. 14 0.014 26.7 7.3 2.5 3.3 0.35 1.2
2.2 0.25 MM: 0.009 0.001 0.0016 Al: 0.03 .smallcircle. 15 0.012
27.0 7.0 2.4 3.2 0.35 0.07 2.2 0.32 MM: 0.02 -- -- 0.20 -- Mg:
0.48, Ti: 0.7, Al: 0.02 .smallcircle. 16 0.014 27.2 7.1 2.2 2.8
0.36 0.09 1.8 0.40 MM: 0.22 0.01 -- -- -- Mg: 0.01, Al: 0.03
.smallcircle. 17 0.012 27.5 7.2 2.5 3.2 0.36 2.0 0.24 0.0015 Al:
0.02, Ca: 0.01 .smallcircle. 18 0.016 27.1 7.3 2.3 3.1 0.40 0.01
1.8 0.57 MM: 0.22 -- 0.26 -- 0.0500 Co: 0.02, Al: 0.02
.smallcircle. 19 0.012 26.8 6.9 2.5 3.0 0.32 0.05 2.0 0.31 MM: 0.04
0.03 -- -- 0.0030 Al: 0.02 .smallcircle. 20 0.014 35.0 10.0 1.8 5.9
0.50 0.03 1.2 0.36 MM: 0.86 -- -- -- -- Al: 0.03 .smallcircle. 21
0.014 27.0 6.8 2.5 3.2 0.34 1.2 2.1 0.17 0.0017 Al: 0.03, Ca: 0.01
.smallcircle. 22 0.023 26.2 8.5 5.4 3.3 0.22 2.70 0.7 0.97 -- 0.60
-- -- -- Al: 0.02 .smallcircle. 23 0.032 23.6 8.2 5.7 0.02 0.3 0.01
0.04 0.59 MM: 0.15 0.02 -- -- 0.0001 Al: 0.005, V: 0.024, Ti:
0.015, Ca: 0.001 .smallcircle. 24 0.016 27.6 7.2 2.6 3.2 0.35 2.1
0.23 MM: 0.025 0.0015 Al: 0.03 .smallcircle. 25 0.020 27.2 6.3 2.6
3.2 0.36 1.50 2.0 0.38 MM: 0.1 0.01 -- -- 0.0021 Mg: 0.02, Co:
0.02, Al: 0.02 .smallcircle. 26 0.017 27.3 7.2 4.5 -- 0.39 0.04 2.0
0.64 MM: 0.32 -- -- -- 0.0020 Al: 0.02 .smallcircle. 27 0.020 25.1
7.0 3.8 0.2 0.26 0.03 0.5 0.25 MM: 0.17 0.005 -- -- -- Mg: 0.04,
Co: 0.06, Al: 0.02 .smallcircle. 28 0.028 24.1 8.7 5.7 0.2 0.25
0.06 1.0 0.48 MM: 0.46 0.50 -- -- 0.0050 Al: 0.03 .smallcircle. 29
0.034 24.0 7.6 5.5 -- 0.26 2.00 1.2 0.75 MM: 0.62 -- -- 0.04 0.0050
Ti: 0.80, Sn: 0.02, In: 0.04, Al: 0.03, .smallcircle. 30 0.025 24.2
8.7 5.3 0.3 0.26 0.07 1.2 0.80 -- 0.05 0.45 -- 0.0080 Ti: 0.02, Al:
0.02 .smallcircle. 31 0.020 24.0 8.7 5.7 -- 0.24 0.08 0.9 0.54 MM:
0.21 -- -- -- 0.0030 Nb: 0.5, Co: 0.04, Al: 0.02 .smallcircle. 32
0.024 24.0 8.1 5.6 0.2 0.23 0.04 1.4 0.44 MM: 0.32 -- -- 0.50
0.0040 Al: 0.02 .smallcircle. 33 0.015 23.5 10.0 5.4 0.2 0.25 0.02
1.1 0.45 MM: 0.46 -- -- -- 0.0020 Co: 0.45, In: 0.01, Al: 0.03
.smallcircle. 34 0.017 23.7 8.7 5.5 -- 0.25 0.01 2.8 0.53 MM: 0.6
0.01 0.01 0.01 0.0010 Sn: 0.01, In: 0.01, Mg: 0.01, Co: 0.01, Nb:
0.01, Al: 0.03 .smallcircle. 35 0.047 26.8 8.4 5.0 0.3 0.35 0.06
2.7 0.73 MM: 0.4, 0.10 -- 0.02 0.0040 Al: 0.02 Y: 0.1 .smallcircle.
36 0.014 26.7 7.4 4.6 0.2 0.34 0.07 1.6 0.43 MM: 0.06 0.01 -- --
0.0015 Al: 0.02 .smallcircle. 37 0.018 34.5 10.0 1.5 0.4 0.37 0.05
1.1 0.41 MM: 0.8, -- 0.02 -- 0.0022 Co: 0.04, Y: 0.1 Nb: 0.02, Al:
0.03 .smallcircle. 38 0.020 27.1 6.4 4.5 -- 0.36 1.60 2.2 0.56 MM:
0.03 0.01 -- -- 0.0015 Al: 0.02 .smallcircle. 39 0.019 25.7 3.2 2.8
3.9 0.40 2.4 2.7 1.0 MM: 0.04 0.01 -- 0.01 -- Sn: 0.8, Co: 1.4, Al:
0.01 .smallcircle. 40 0.022 34.0 12.0 2.1 6.5 0.50 0.03 1.2 2.9 MM:
0.03 0.02 -- -- 0.0200 In: 0.9, Al: 0.01 .smallcircle. 41 0.014
23.6 7.0 5.4 0.22 0.25 0.02 1.2 2.2 MM: 0.04 0.01 0.01 -- -- Co:
2.9, Al: 0.01 .smallcircle. 42 0.018 37.5 12.0 1.52 0.41 0.37 0.05
1.6 1.1 MM: 0.02 0.02 -- -- 0.0800 Mg: 0.47, Al: 0.02 .smallcircle.
43 0.015 25.3 7.1 3.7 0.21 0.27 0.04 0.6 0.3 MM: 0.02 0.01 -- -- --
Ti: 1.4, Al: 0.01 x 44 0.020 34.9 9.8 1.5 2.1 0.49 0.02 1.3 0.45 --
-- -- -- -- -- x 45 0.015 29.3 7.2 1.9 3.2 0.45 0.02 2.8 0.70 -- --
-- -- -- -- x 46 0.018 22.1 7.3 6.5 3.3 0.34 0.03 2.3 0.45 -- -- --
-- -- -- x 47 0.018 23.0 8.1 5.6 0.1 0.40 0.05 1.2 0.54 -- -- -- --
-- -- x 48 0.030 34.8 9.8 1.6 0.1 0.35 1.40 2.0 0.60 -- -- -- -- --
-- x 49 0.014 29.4 9.5 3.7 0.2 0.45 0.01 2.5 0.65 -- -- -- -- -- --
x 50 0.013 27.2 7.4 4.2 0.2 0.35 0.02 1.8 0.45 -- -- -- -- -- -- UR
52N+ 0.016 25.2 6.3 3.8 -- 0.25 1.70 1.0 0.38 -- -- -- -- -- -- SAF
2507 0.014 24.8 6.9 3.9 -- 0.26 -- 0.4 0.30 -- -- -- -- -- -- ZERON
100 0.012 25.2 6.9 3.8 0.7 0.26 0.70 0.9 0.31 -- -- -- -- -- -- SAF
2205 0.016 22.3 5.2 3.2 -- 0.18 -- 0.6 0.24 -- -- -- -- -- --
AL-6XN 0.018 21.2 24.5 6.3 -- 0.21 0.50 0.8 0.33 -- -- -- -- -- --
SR-50A 0.015 23.2 21.3 6.3 -- 0.25 -- 1.1 0.28 -- -- -- -- -- --
254SMO 0.016 20.1 18.2 6.3 -- 0.21 0.70 1.2 0.31 -- -- -- -- -- --
AISI 316L 0.018 17.2 12.3 2.5 -- -- -- 1.2 0.34 -- -- -- -- -- --
(Notes) .sup.1)Invention Steel .smallcircle.; Comparative Steel x
.sup.2)MM(Mischmetal) represent total weight of Ce, La, Nd, Pr,
etc. added into Invention Steel.
[0128]
2TABLE 2 Properties of Invention Steel, Comparative Steel and
Commercial Steel (Cast Product) Corrosion Resistance Pitting
Differences Austenite Ferrite Balance of Phases Potential of
Hardnesses Classification.sup.1) PREW (vol. %) (vol. %)
[PREW(.gamma.) - PREW(.alpha.)] (mV.sub.SCE) CPT(.degree. C.)
CCCT(.degree. C.) (.DELTA.H = H.sub.A - H.sub.S.A) STEELS
.smallcircle. 1 52.5 67.2 32.8 9.7 2) 80 40 0.32 .smallcircle. 2
50.7 59.5 40.5 5.0 2) 90 50 2.1 .smallcircle. 3 47.4 48.3 51.7 5.9
60 70 40 0.24 .smallcircle. 4 52.0 66.1 33.9 7.6 2) 85 45 0.10
.smallcircle. 5 51.4 55.3 44.7 6.0 2) 95 60 2.3 .smallcircle. 6
58.2 59.9 40.1 1.8 2) 80 40 0.12 .smallcircle. 7 50.7 54.9 45.1 7.1
2) 90 50 0.10 .smallcircle. 8 50.8 59.8 40.2 5.4 2) 95 55 2.5
.smallcircle. 9 51.8 47.8 52.2 9.6 2) 85 45 0.28 .smallcircle. 10
50.6 61.3 38.7 6.8 2) 95 60 0.10 .smallcircle. 11 51.5 59.2 40.8
6.0 2) 90 50 2.2 .smallcircle. 12 55.4 76.0 24.0 7.1 2) 85 45 0.21
.smallcircle. 13 52.8 66.2 33.8 7.9 2) 90 50 1.00 .smallcircle. 14
51.0 62.9 37.1 4.5 2) 95 60 2.4 .smallcircle. 15 50.7 55.4 44.6 8.2
2) 90 50 0.44 .smallcircle. 16 49.9 60.8 39.2 7.7 2) 90 50 0.52
.smallcircle. 17 52.0 58.7 41.3 6.0 2) 90 50 2.8 .smallcircle. 18
51.9 66.1 33.9 7.7 2) 85 45 0.90 .smallcircle. 19 49.6 53.2 46.8
7.3 2) 90 50 0.60 .smallcircle. 20 65.7 65.9 34.1 10.0 2) 80 40
0.64 .smallcircle. 21 50.8 57.9 42.1 5.4 2) 90 50 2.9 .smallcircle.
22 56.1 31.8 68.2 4.7 2) 85 45 0.72 .smallcircle. 23 51.4 55.1 44.9
3.5 2) 95 60 3.7 .smallcircle. 24 51.9 57.8 42.2 5.8 2) 90 50 3.1
.smallcircle. 25 51.9 57.6 42.4 7.7 2) 85 55 0.86 .smallcircle. 26
53.9 60.9 39.1 8.2 2) 95 60 1.40 .smallcircle. 27 45.4 46.1 53.9
6.5 -10 70 40 1.10 .smallcircle. 28 53.4 52.2 47.8 1.5 2) 90 50
1.81 .smallcircle. 29 52.1 48.3 51.7 3.6 2) 95 55 1.43
.smallcircle. 30 51.9 53.2 46.8 2.3 2) 90 50 1.21 .smallcircle. 31
53.3 46.1 53.9 2.8 2) 90 50 1.32 .smallcircle. 32 52.7 45.8 54.2
2.2 2) 90 55 1.61 .smallcircle. 33 48.8 64.7 35.3 -0.7 2) 90 50
1.14 .smallcircle. 34 52.7 58.4 41.6 0.6 2) 85 45 1.63
.smallcircle. 35 53.8 66.8 33.2 3.7 2) 85 45 1.42 .smallcircle. 36
52.1 55.0 45.0 7.1 2) 95 55 0.82 .smallcircle. 37 51.1 63.9 36.1
10.0 2) 85 45 1.32 .smallcircle. 38 52.8 56.7 43.3 8.0 2) 95 55
1.90 .smallcircle. 39 53.4 55.0 45.0 10.0 2) 90 50 0.87
.smallcircle. 40 66.7 64.0 36.0 9.8 2) 95 55 0.63 .smallcircle. 41
49.3 50.0 50.0 2.5 2) 90 50 1.21 .smallcircle. 42 54.3 63.2 36.8
10.0 2) 95 55 1.59 .smallcircle. 43 46.0 39.2 60.8 9.9 2) 75 40
1.06 x 44 58.0 72.6 27.4 11.5 2) 75 35 13.40 x 45 54.4 68.7 31.3
9.6 2) 80 40 10.40 x 46 59.2 55.4 44.6 3.8 2) 90 50 16.20 x 47 53.5
73.1 26.9 3.8 2) 95 55 14.10 x 48 50.1 64.7 35.3 9.0 2) 80 40 12.20
x 49 55.1 76.8 23.2 7.4 2) 80 40 10.30 x 50 51.6 57.2 42.8 7.6 2)
90 50 12.00 UR 52N+ 45.2 41.5 58.5 7.5 -32 70 35 6.20 SAF 2507 45.5
43.2 56.8 7.5 -14 65 35 5.60 ZERON 100 46.7 44.4 55.6 6.7 57 70 40
-- SAF 2205 38.3 19.7 80.3 14.3 -- 35 -- -- AL-6XN 48.3 100 0 --
518 85 35 -- SR-50A 51.5 100 0 -- 2) 90 40 -- 254SMO 47.2 100 0 --
204 75 30 -- AISI 316L 25.5 100 0 -- -- 10 -10 -- (Notes)
.sup.1)Invention Steel .smallcircle.; Comparative Steel x 2) Above
equilibrium oxygen evolution potential: No pitting generated.
[0129]
3TABLE 3 Mechanical Properties of Invention Steel and Comparative
Steel (Cast Product) Yield Strength Tensile Strength
Classification.sup.1) (MPa) (MPa) Elongation(%) .smallcircle. 1 712
816 40 .smallcircle. 2 650 809 33 .smallcircle. 3 696 812 40.5
.smallcircle. 4 720 840 44 .smallcircle. 5 645 811 32 .smallcircle.
6 714 836 36 .smallcircle. 7 734 880 38 .smallcircle. 8 655 815 35
.smallcircle. 9 712 820 40.4 .smallcircle. 10 680 820 46
.smallcircle. 11 638 825 31 .smallcircle. 12 720 860 40.6
.smallcircle. 13 702 814 36 .smallcircle. 14 655 807 35
.smallcircle. 15 708 810 38 .smallcircle. 16 724 846 41.2
.smallcircle. 17 661 825 32 .smallcircle. 18 712 842 41
.smallcircle. 19 704 832 40 .smallcircle. 20 620 785 32
.smallcircle. 21 659 818 31 .smallcircle. 22 612 808 38
.smallcircle. 23 660 842 36 .smallcircle. 24 647 819 33
.smallcircle. 25 684 882 32.4 .smallcircle. 26 724 842 34
.smallcircle. 27 682 814 34 .smallcircle. 28 580 798 38.6
.smallcircle. 29 690 806 38 .smallcircle. 30 688 812 39
.smallcircle. 31 702 830 40.6 .smallcircle. 32 604 826 40.8
.smallcircle. 33 704 822 40 .smallcircle. 34 720 806 34.1
.smallcircle. 35 696 798 34 .smallcircle. 36 680 822 44
.smallcircle. 37 660 814 32.2 .smallcircle. 38 690 880 30.2
.smallcircle. 39 685 890 34 .smallcircle. 40 618 782 32
.smallcircle. 41 709 831 37 .smallcircle. 42 601 817 39.2
.smallcircle. 43 687 820 35 x 44 624 804 24 x 45 620 786 27.2 x 46
596 780 24 x 47 591 598 18 x 48 612 814 22 x 49 598 800 24.5 x 50
614 790 26.4 (Notes) .sup.1)Invention Steel .smallcircle.;
Comparative Steel x
[0130]
4TABLE 4 Corrosion Resistance Balances of
Austente(.gamma.)/Ferrite(.alpha.) Phases of Invention Steel,
Comparative Steel and Commercial Steel Corrosion Resistance
Balances of Phases PREW (.gamma.) PREW(.alpha.) [PREW(.gamma.) -
PREW(.alpha.)] Classification.sup.1) No. Cr Mo W N PREW Cr Mo W N
PREW 30N 16N STEELS .smallcircle. 1 28.35 1.44 2.46 0.62 55.6 30.33
2.23 4.11 0.05 46.0 9.7 1.7 .smallcircle. 2 26.25 2.12 2.58 0.5
52.5 28.08 3.28 4.32 0.05 47.5 5.0 -0.6 .smallcircle. 3 24.42 3.04
0.52 0.51 50.5 26.13 4.71 0.87 0.05 44.6 5.9 -0.5 .smallcircle. 4
26.37 2.02 2.53 0.58 54.6 28.22 3.14 4.22 0.05 47.0 7.6 0.2
.smallcircle. 5 26.75 2.08 2.56 0.53 53.8 28.63 3.22 4.27 0.05 47.8
6.0 0.1 .smallcircle. 6 20.43 5.24 2.52 0.57 58.9 21.86 8.13 4.21
0.05 57.1 1.8 -5.5 .smallcircle. 7 26.27 2.16 2.23 0.56 53.9 28.11
3.35 3.72 0.05 46.8 7.1 -0.1 .smallcircle. 8 26.15 2.09 2.61 0.51
52.8 27.99 3.23 4.36 0.05 47.4 5.4 -0.3 .smallcircle. 9 25.66 2.02
2.89 0.66 56.8 27.46 3.13 4.82 0.05 47.2 9.6 1.1 .smallcircle. 10
25.61 2.14 2.38 0.56 53.3 27.40 3.32 3.98 0.05 46.4 6.8 -0.2
.smallcircle. 11 26.44 2.10 2.60 0.53 53.6 28.29 3.25 4.34 0.05
47.7 6.0 0.0 .smallcircle. 12 29.5 2.47 1.55 0.56 57.1 31.57 3.83
2.59 0.05 50.0 7.1 -0.1 .smallcircle. 13 26.67 2.11 2.45 0.59 55.5
28.54 3.27 4.09 0.05 47.6 7.9 0.3 .smallcircle. 14 26.11 2.16 2.72
0.49 52.5 27.94 3.34 4.55 0.05 48.0 4.5 -0.9 .smallcircle. 15 26.18
1.93 2.46 0.59 54.4 28.01 2.99 4.11 0.05 46.2 8.2 0.6 .smallcircle.
16 26.47 1.81 2.22 0.56 52.9 28.33 2.81 3.7 0.05 45.2 7.7 0.6
.smallcircle. 17 26.82 2.12 2.59 0.54 54.2 28.69 3.28 4.32 0.05
48.2 6.0 0.0 .smallcircle. 18 26.47 1.94 2.53 0.58 54.4 28.32 3.00
4.22 0.05 46.7 7.7 0.3 .smallcircle. 19 25.95 1.99 2.28 0.56 53.0
27.77 3.08 3.81 0.05 45.7 7.3 0.2 .smallcircle. 20 34.19 1.52 4.80
0.73 69.1 36.58 2.35 8.02 0.05 59.1 10.0 0.5 .smallcircle. 21 26.32
2.09 2.56 0.51 52.8 28.16 3.24 4.28 0.05 47.4 5.4 -0.3
.smallcircle. 22 25.01 3.93 2.26 0.59 59.3 26.76 6.09 3.78 0.05
54.6 4.7 -2.8 .smallcircle. 23 22.87 4.6 0.02 0.5 53.0 24.47 7.12
0.03 0.05 49.5 3.5 -2.8 .smallcircle. 24 26.90 2.12 2.58 0.53 54.0
28.78 3.29 4.31 0.05 48.3 5.8 -0.1 .smallcircle. 25 26.42 2.11 2.49
0.59 55.1 28.27 3.27 4.16 0.05 47.4 7.7 0.2 .smallcircle. 26 26.57
3.70 0 0.61 57.0 28.43 5.74 0 0.05 48.9 8.2 0.3 .smallcircle. 27
24.19 2.93 0.15 0.51 49.3 25.88 4.54 0.25 0.05 42.8 6.5 0.1
.smallcircle. 28 23.32 4.51 0.15 0.43 51.5 24.95 7.0 0.25 0.05 50.0
1.5 -3.9 .smallcircle. 29 23.16 4.28 0 0.48 51.8 24.78 6.64 0 0.05
48.2 3.6 -2.4 .smallcircle. 30 23.43 4.22 0.23 0.44 51.1 25.07 6.53
0.38 0.05 48.8 2.3 -3.2 .smallcircle. 31 23.13 4.4 0 0.46 51.5
24.75 6.81 0 0.05 48.7 2.8 -3.0 .smallcircle. 32 23.12 4.31 0.15
0.44 50.9 24.74 6.69 0.25 0.05 48.7 2.2 -3.3 .smallcircle. 33 22.93
4.52 0.16 0.36 48.9 24.54 7.01 0.27 0.05 49.6 -0.7 -5.0
.smallcircle. 34 23.03 4.48 0 0.39 49.6 24.64 6.94 0 0.05 49.0 0.6
-4.3 .smallcircle. 35 26.19 4.23 0.25 0.5 55.5 28.02 6.55 0.41 0.05
51.8 3.7 -2.6 .smallcircle. 36 25.89 3.69 0.15 0.58 55.6 27.7 5.72
0.26 0.05 48.5 7.1 -0.2 .smallcircle. 37 33.65 1.25 0.32 0.55 54.8
36.0 1.94 0.54 0.05 44.8 10.0 3.0 .smallcircle. 38 26.30 3.63 0 0.6
56.2 28.14 5.63 0 0.05 48.2 8.0 0.3 .smallcircle. 39 24.92 2.24 3.0
0.69 57.9 26.66 3.48 5.0 0.05 47.9 10.0 1.1 .smallcircle. 40 33.16
1.75 5.24 0.75 70.2 35.49 2.72 8.75 0.05 60.4 9.8 0.0 .smallcircle.
41 22.80 4.23 0.16 0.45 50.6 24.4 6.56 0.28 0.05 48.0 2.5 -3.1
.smallcircle. 42 36.56 1.26 0.33 0.56 58.0 39.12 1.96 0.55 0.05
48.0 10.0 2.9 .smallcircle. 43 24.27 2.77 0.15 0.61 52.0 25.97 4.30
0.25 0.05 42.1 9.9 2.1 x 44 34.24 1.3 1.77 0.66 61.2 36.64 2.02
2.96 0.05 49.7 11.5 3.0 x 45 28.67 1.62 2.65 0.63 57.3 30.68 2.51
4.42 0.05 47.8 9.6 1.4 x 46 22.58 4.88 0.08 0.53 54.7 24.16 7.56
0.14 0.05 50.8 3.8 -5.4 x 47 22.58 4.88 0.08 0.53 54.7 24.16 7.56
0.14 0.05 50.8 3.8 -2.9 x 48 33.96 1.34 0.08 0.51 53.9 36.34 2.08
0.14 0.05 44.9 9.0 2.5 x 49 28.93 3.28 0.17 0.57 57.2 30.96 5.09
0.29 0.05 49.7 7.4 0.2 x 50 26.41 3.40 0.16 0.57 55.1 28.26 5.27
0.26 0.05 47.6 7.6 0.2 UR 52N+ 24.21 2.88 0 0.53 49.6 25.9 4.46 0
0.05 42.1 7.5 0.8 SAF 2507 23.85 2.97 0 0.54 49.7 25.52 4.61 0 0.05
42.2 7.5 0.7 ZERON 100 24.26 2.91 0.51 0.52 50.4 25.95 4.51 0.85
0.05 4.37 6.7 0.0 SAF 2205 21.11 2.22 0 0.71 49.8 22.59 3.44 0 0.05
35.4 14.3 5.1 (Notes) .sup.1)Invention Steel .smallcircle.;
Comparative Steel x
[0131]
5TABLE 5 Properties of Hot-Rolled Broad Plank of Invention Steel
and Comparative Steel Mechanical Properties Yield Strength Tensile
Strength Elongation Evaluation to Classification.sup.1)
CPT(.degree. C.) (MPa) (MPa) (%) Hot-Rolling.sup.2) STEELS
.smallcircle. 2 85 663 879 44 .smallcircle. .smallcircle. 5 90 681
877 37 .smallcircle. .smallcircle. 8 90 721 894 38 .smallcircle.
.smallcircle. 11 85 711 892 36 .smallcircle. .smallcircle. 14 90
673 833 35 .smallcircle. .smallcircle. 17 85 682 851 34
.smallcircle. .smallcircle. 21 85 673 843 38 .smallcircle.
.smallcircle. 23 90 693 887 36 .smallcircle. .smallcircle. 24 85
716 902 34 .smallcircle. .smallcircle. 36 90 691 837 35
.smallcircle. x 39 95 620 632 36 x x 41 90 643 812 38 x (Notes)
.sup.1)Invention Steel .smallcircle.; Comparative Steel x
.sup.2)Evaluation: Good(No Crack) .smallcircle.; Normal(Few Crack)
.DELTA.; Bad(Many Crack) x
INDUSTRIAL APPLICABILITY
[0132] As described above, the present invention removes
brittleness and improves corrosion resistance by reducing the
precipitation speed and amount of intermetallic phases having high
brittleness, by delaying diffusion and precipitation of
intermetallic phases by using an appropriate amount of Ba, Y, Ce,
La, Nd, Pr, Ta, Zr and Ti atoms having a large atomic diameter, and
additionally blocking diffusion of Cr, Mo, Si and W by using minute
RE metallic compound mixtures or Ba oxides.
[0133] In addition, the present invention prevents individual
formation of Al.sub.2O.sub.3 and MnS non-metallic inclusions which
have detrimental effects on general properties of steel by
performing proper preliminary deoxidation according to the common
method using Ti, Mg, Ca, Al and Ca+Al, and adding MM and/or Y. For
this, the present invention improves mechanical properties,
physical properties and corrosion resistance by forming the
rare-earth metallic compound mixture (RExOy or
(RE,Al)xOy+RExOyS+RExSy) having a diameter below 5 .mu.m, supplying
heterogeneous nucleation sites to make the solidified structure
fine and minute during dendrite formation of the solidification,
and controlling segregation of the solute elements such as Cr, Mo,
W, Ni, Mn and Si, by using the solubility product equation `[MM
and/or Y+Al].multidot.[O+S]=0.- 001.times.10.sup.-5 to
30000.times.10.sup.-5[&].sup.2`.
[0134] Accordingly, the present invention provides the method for
remarkably suppressing formation of intermetallic phases such as
sigma in duplex stainless steel by adding new alloying elements,
and improving the production yield during mass production.
[0135] Moreover, the present invention increases the production
yield in casting and hot working, by improving embrittlement
resistance and preventing cracks by lowering a precipitation speed
of intermetallic phases such as sigma.
[0136] Furthermore, the present invention considerably improves
corrosion resistance and mechanical properties and upgrades
durability of equipments, by suppressing precipitation of sigma and
khi phases deteriorating corrosion resistance and mechanical
properties in a casting state, and also controlling precipitation
of such phases in a heat-affected zone when equipment components
are necessarily welded in various application fields.
* * * * *