Note: Descriptions are shown in the official language in which they were submitted.
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High chromium martensitic heat-resistant steel with combined high creep
rupture strength and oxidation resistance
The invention relates to martensitic high chromium heat-resistant steels for
components operating at elevated temperatures i.e. between 550 and 750 C and
high
stresses. The steel according to the invention can be used in power
generation,
chemical and petrochemical industry.
STATE OF THE ART
The ferritic/martensitic high Cr steel materials are widely used in the modern
power
plants as reheater/superheater tubes and as steam pipes. Further improvement
of the
net efficiency of thermal power plants will require an increase of the steam
parameters pressure and temperature. Therefore, the realization of more
efficient
power plant cycles will require stronger materials with improved steam-side
oxidation
resistance. The known efforts to develop new martensitic high chromium steel
that
combines excellent creep properties and superior oxidations resistance have
failed so
far due to the formation of the so called Z-phase. Z-phase is a complex
nitride that
coarsens quickly thereby consuming the surrounding strengthening MX
precipitates, M
being: Nb, V and X being: C, N.
The expression high chromium steel material generally means steels with more
than
9wt.-% of Cr. Elevated Cr contents i.e. containing more than 9wt.-% of Cr,
which are
essential for good steam oxidation resistance, however, increase the driving
force for
Z-phase formation and also enhance the coarsening rate of chromium carbide
precipitates. Both, the loss of the microstructure stabilizing effect of MX
and chromium
carbide precipitates are responsible for the drop in the long-term creep
rupture
strength of martensitic high Cr heat-resistant steel grades. Hence, the major
challenge
for future steel developments is to resolve the apparent contradiction between
the
creep rupture strength and oxidation resistance.
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Currently, for high-temperature applications, that is applications with
temperatures of
service higher than 550 C, ASTM Grades 91 and 92 are widely used, both
containing 9
wt.-%Cr with creep rupture strengths after 105h at 600 C at 90 and 114MPa
respectively. The main difference between the two steels is that Grade 92
contains W
in the range of 1.8 wt.-% and reduced Mo of 0.4 wt.-% compared to 1 wt.-% in
case of
Grade 91. Additionally, Grade 92 contains small amounts of B below 0.005 wt.-
%.
Both steels suffer from insufficient oxidation resistance in steam atmospheres
at
temperatures above 600 C, which is limiting the application temperature range
significantly. Especially in boiler components with heat transfer, the oxide
scale acts as
thermal insulator thereby increasing the metal temperature and consequently
reducing lifetime of corresponding components. Additionally, the oxide scales,
if
spalled off during operation, will cause erosion damage on the following steam
carrying components or after entering the steam turbine on turbine blades and
guiding
vanes. SpaIled oxide scales may cause tube blockage especially in the region
of bends,
impeding the steam flow often resulting in local overheating and catastrophic
failure.
X20CrMoV11-1 is a well established high Cr ferritic/martensitic steel for high
temperature applications containing 0.20wt.-% C, 10.5-12 wt.-percent Cr, 1 wt.-
% Mo
and 0.2wt.-% V. This steel exhibits oxidation properties which are better than
that of
ASTM steel grades 91 and 92 due to higher Cr contents, but poor creep rupture
strength (creep rupture strength after 105h at 600 C being around 59MPa).
Additionally the hot-workability and weldability are deteriorated due to high
C content
of 0.20 wt.-%. ASTM Grade 122 contains 10-12%Cr, 1.8%W, 1%Cu and also V, Nb
and N
additions to induce the precipitation of MX strengthening particles. The creep
rupture
strength is significantly below that of ASTM Grade 92 that presents a creep
rupture
strength of 98MPa after 105h at 600 C.
Also hot-workability issues due to elevated Cu contents are present.
Another steel with 11 to 12 wt.-% of Cr exists. it is mainly used as thin-
walled tube, and
is called VM12-SHCsteels that combines good steam-side oxidation resistance
and the
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creep rupture strength at the level of ASTM Grade 91.Such steel concept is
known
from patent application W002081766 disclosing a steel for high temperature use
containing by weight : 0.06 to 0.20% of C, 0.10 to 1.00% of Si, 0.10 to 1.00%
of Mn, not
more than 0.010% of S, 10.00 to 13.00% of Cr, not more than 1.00% of Ni, 1.00
to
1.80% of W, Mo such that (W/2+Mo) is not more than 1.50%, 0.50 to 2.00% of Co,
0.15
to 0.35% of V, 0.040 to 0.150% of Nb, 0.030 to 0.12% of N, 0.0010 to 0.0100%
of B and
optionally up to 0.0100% of Ca, the rest of the chemical composition
consisting of iron
and impurities or residues resulting from or required for preparation
processes or steel
casting. The chemical constituent contents preferably verify a relationship
such that
the steel after normalizing heat treatment between 1050 and 1080 C and
tempering
has a tempered martensite structure free or practically free of delta ferrite.
Compared
to this steel, creep rupture strength can still be improved while keeping the
other
properties such as corrosion resistance and mechanical properties unaffected.
OBJECT AND SOLUTION
The object of the present invention is therefore to provide a seamless tubular
product
in a martensitic heat-resistant steel with substantially better creep rupture
strength
than ASTM Grade 92 steel for pipes and tubes, and with hot corrosion and steam
oxidation behavior comparable or better than X20CrMoV11-1 and VM12-SHC steels,
described in the state of the art.
A further object of the invention is to obtain a steel exhibiting martensitic
microstructure with a limitation of the delta ferrite, also known as 6-
ferrite, content to
5 vol.-% in average.
Another object of the invention was to provide a steel that allows the
fabrication of
small or large diameter seamless tubular products such as seamless tubes or
seamless
pipes, and a steel suitable for the fabrication of welded tubes and pipes,
forgings and
plates using the known and established manufacturing processes.
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The steel is suited as a production material for whole variety of components
operating
under stress at elevated temperatures, particularly as seamless and welded
tubes/pipes, forgings and plates in power generation, chemical and
petrochemical
industry. In addition, the steel according to the invention is temper
resistant, after long
tempering times up to 30 hours at 800 C, the yield strength is above or equal
440 MPa,
the tensile stress above or equal 620 MPa and toughness at 20 C is above or
equal 40 J
when tested in longitudinal direction and 27 J when tested in transverse
direction.
In accordance with the present invention, the object can be achieved by a
seamless
tubular product for high-temperature applications in a steel having the
following
chemical composition in weight percent:
C: 0.10 to 0.16%
Si: 0.20 to 0.60%
Mn: 0.30 to 0.80%
P <0.020%
S <0.010%
Al <0.020%
Cr: 10.50to 12.00%
Mo: 0.10 to 0.60%
V: 0.15 to 0.30%
Ni: 0.10 to 0.40%
B: 0.008 to 0.015%
N: 0.002 to 0.020%
Co: 1.50 to 3.00%
W: 1.50 to 2.50%
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Nb: 0.02 to 0.07%.
Ti: 0.001 to 0.020%, the balance of said steel being iron and unavoidable
impurities.
Preferably, the ratio of boron and nitrogen is such that: BAT 1.5to achieve
hot
workability.
Preferably, the following equation is satisfied:
1.00 % Mo+0.5W 1.50 % (in wt %),
In another preferred embodiment, the following equation is satisfied (in wt.-
%):
B _ (11/14)(N ¨ 10- (1/2.45)=(/ogB+6.81) (14/) 48N .
Ti) 0.007
In another preferred embodiment, the following equation is satisfied (in wt.-
%):
2.6 < 4 = (Ni + Co + 0.5 = Mn) ¨ 20 = (C + N) < 11.2
In a preferred embodiment, the carbon content is between 0.13 and 0.16%.
In another preferred embodiment, the Mo content is between 0.20 and 0.60%.
Preferably, B content is between 0.0095 and 0.013%.
In a preferred embodiment, the Ti content is between 0.001 and 0.005%.
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In another preferred embodiment, the microstructure comprises in average at
least 95
% of tempered martensite, the balance being delta ferrite.
In an even more preferred embodiment, the microstructure comprises in average
at
least 98 % of tempered martensite, the balance being delta ferrite.
In the most preferred embodiment, the microstructure is martensitic and free
of delta
ferrite.
The invention also relates to a method of production comprising the following
steps:
-casting a steel with a chemical composition according to the invention,
-hot forming said steel,
-heating said steel and holding said steel for a time between 10 and 120
minutes in the
temperature range between 1050 C and 1170 C,
-cooling said steel down to room temperature,
-reheating said steel and holding said steel up to a tempering temperature TT
that is
between 750 C and 820 C for at least one hour,
-cooling said steel down to room temperature.
Preferably, the cooling step is done using air cooling or water cooling.
The cooling step after reheating step may be done using water cooling.
The cooling step after heating step may be done using water cooling.
The invention may also concern the production of a welded tube, pipe or plate
using
the same steel as the one according to the seamless tubular product of the
invention
or the process according to the invention.
Figure 1 shows the schematic of mass gain due to oxidation plotted versus
chromium
content.
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SUBJECT MATTER OF THE INVENTION
In accordance with the present invention, a martensitic high chromium heat-
resistant
steel is created having the following chemical composition:
(1) C: 0.10 to 0.16%,
C needs to be added to at least 0.10 % to obtain sufficient carbide
precipitation.
Additionally C is also an austenite stabilizing element. C contents below
0.10% would
imply more 5-ferrite in the microstructure. The upper limit for carbon is
0.16% because
excess C addition limits the toughness and weldability properties.
(2) Si: 0.20 to 0.60%,
Si is used for deoxidation during the steel making process. Additionally, it
is one of key
elements, which determines the oxidation behavior in steels. In order to
achieve the
full oxidation improving effect of Si additions an amount of at least 0.20 %
is necessary.
The upper Si level shall preferably be limited to 0.60 %, because the excess
Si addition
accelerates the coarsening of precipitates and decreases toughness. Preferably
the
lower limit is 0.25 %.
(3) Mn: 0.30 to 0.80%,
Mn is an effective deoxidation element. It ties up sulphur and reduces the 6-
ferrite
formation. At least 0.30% Mn may be added. The upper limit shall be 0.8%,
since
excessive additions reduce the strength of steels at elevated temperatures.
(4) P < 0.020%,
P is a grain-boundary active element, which reduces the toughness properties
of
steels. The content has to be limited to 0.020% in order to avoid the negative
impact of
P on toughness properties. P may be present in an amount equal to or greater
than
0.00% as it may be unavoidable as an impurity.
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(5) S < 0.010%,
S forms sulfides and reduces the toughness and hot-workability properties of
steels. A
limitation of upper S content to 0.010 prevents the defect formation during
hot-
working operation and the negative impact on toughness. S may be present in an
amount equal to or greater than 0.00% as it may be unavoidable as an impurity.
(6) Al < 0.020%,
Al is a potent deoxidation element used during the steel making process.
Excess Al
addition above 0.02% can induce AIN formation, thereby reducing the amount of
strengthening MX (M being: Nb, V and X being: C, N) nitride precipitates in
steel and
consequently the creep strength properties. Al may be present in an amount
equal to
or greater than 0.00% as it may be unavoidable as an impurity.
(7) Cr: 10.5 to 12.00%,
Cr forms carbides that form at boundaries of the martensitic microstructure.
Chromium carbides are essential for stabilization of the martensitic
microstructure
during exposure at elevated temperatures. Cr improves the high temperature
oxidation behavior of steels. Contents of at least 10.5% are necessary to
unfold the full
oxidation improving effect of Cr additions. Cr contents above 12% result in
increased 6-
ferrite formation.
(8) Mo: 0.10 to 0.60%,
Mo is an important element for improvement of creep rupture strength that is
also
responsible for solid solution strengthening. This element is incorporated in
carbides
and intermetallic phases as well. Mo content of 0.10 % may be added. The Mo
additions above 0.60 % will deteriorate toughness and induce increase of 6-
ferrite
content. Note that M and W contents shall satisfy the relationship (in weight
%) 1
Mo-i-0.5 x W 1.5, in order to ensure the sufficient precipitation of
carbides and
intermetallic phases.
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(9) V: 0.15 to 0.30%,
V combines with N to form coherent MX nitrides (M being: Nb, V and X being :
C, N),
which contribute to enhancement of long-term creep properties. Contents below
0.15% are not sufficient to achieve this long-term creep improving property
effect
while contents above 0.30% decrease the toughness and increase the danger for
6-
ferrite contents above 5% in average volume.
(10) Ni: 0.10 to 0.40%,
Ni is an important toughness improving element. Therefore, a minimum content
of
0.10 % is necessary. However, it reduces Ac1 temperature and tends to reduce
the
creep rupture strength, if added in contents above 0.40 %.
(11) B: 0.008 to 0.015%,
B is a decisive element responsible for stabilization of M23C6 carbides and
delay of
recovery of the martensitic microstructure. It strengthens the grain
boundaries and
improves the long-term stability of creep rupture strength. In addition, B is
responsible
for remarkable improvement of creep rupture ductility. For achievement of
maximum
strengthening effect additions of at least 0.008% are necessary. Contents
above
0.015%, however, reduce substantially the maximum processing temperature of
steels
and are regarded as detrimental. B and N additions shall satisfy the
relationship
B/I\11.5 to enable transformation using known hot-working processes. Indeed,
this
B/N relationship allows the fabrication of small or large diameter seamless
and welded
tubes, pipes and plates using manufacturing process according to the
invention.
Preferably, the B content should be between 0.0095 and 0.0130 (wt %).
(12) N: 0.002 to 0.020%,
Nitrogen is necessary for formation of MX (M being: Nb, V and X being: C, N)
nitrides
and carbonitrides responsible for achievement of creep rupture strength. At
least
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0.002% may be added. Excessive N additions i.e. above 0.020%, however, result
in
enhanced BN formation, thereby reducing the strengthening effect of B
additions.
Preferably, B and N contents (in weight %) shall satisfy the following
relationship:
B _ (11/14)(N _ 10-(1/2.45)=(/ogB+6.81) (14/48) . Ti) 0.007
(13) Co: 1.50 to 3.00%,
Co is a very effective austenite forming element and useful in limiting 6-
ferrite
formation. Moreover, it has only a weak effect on Ac1 temperature.
Additionally, it is
an element that improves creep strength properties by reducing the size of
initial
precipitates after heat treatment. Therefore, a minimum content of 1.50% shall
be
added. Preferably the minimum content is 1.75%. However, Co in excessive
additions
may induce embrittlement due to enhanced precipitaton of intermetallic phases
during high temperature operation. At the same time Co is very expensive.
Hence, a
limitation of additions to 3.00%, preferably to 2.50%, is necessary.
It is preferable that the Ni, Co, Mn, C and N contents (in weight %) are in
accordance
with the following equation: 2.6 < 4 = (Ni + Co + 0.5 = Mn) ¨ 20 = (C + N) <
11.2.(14) W: 1.50 to 2.50%,
W is known as an effective solution strengthener. At the same time it is
incorporated
in carbides and forms C14 Laves phase, which may contribute to creep strength
enhancement as well. Therefore, a minimum content of 1.50% is needed. However,
this element is expensive, strongly segregating during steel making and
casting process
and it forms intermetallic phases that lead to significant embrittlement.
Hence, the
upper limit for W additions may be set to 2.50%. Note that Mo and W contents
(in
weight %) shall satisfy the relationship 1.00 Mo+0.5W 1.50 in order to ensure
the
sufficient precipitation of carbides and intermetallic phases.
(15) Nb: 0.02 to 0.07%.
Nb forms stable MX carbonitrides important not only for creep properties but
also
austenite grain size control. A minimum content of 0.02% may be added. Nb
contents
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above 0.07% result in formation of coarse Nb carbides that may reduce the
creep
strength properties. Therefore the upper limit is set to 0.07%.
(16) Ti: 0.001-0.020%
Ti is a strong nitride forming element. It is helpful to protect free B by
forming nitrides.
Minimum content of 0.001% is needed for this purpose. Excessive Ti content
above
0.020%, however, can reduce toughness properties due to formation of large
blocky
TiN precipitates.
The balance of the steel comprises iron and ordinary residual elements coming
from
steel making and casting process. The casting techniques used are the one
known from
the skilled man. By impurities we mean elements such as tantalum, zirconium
and any
other elements that can't be avoided. It is to be mentioned that Tantalum and
zirconium are not intentionally added to the steel, however may be present in
less
than 50 ppm overall as unavoidable impurities.
In an embodiment of the steel, the unavoidable impurities may comprise one or
more
of copper (Cu), Arsenic (As), tin (Sn), antimony (Sb) and lead (Pb).
Cu may be present in a content equal or less than 0.20 %.
Element As may be present in a content equal or less than 150 ppm; Sn may be
present in a content equal or less than 150 ppm; Sb may be present in a
content equal
or less than 50 ppm; Pb may be present in a content equal or less than 50 ppm
and the
total content As + Sn + Sb + Pb is equal or less than 0.04 % in mass.
The steel is normalized for a period of about 10 to about 120 minutes in the
temperature range between 1050 C and 1170 C and cooled down in air or water
to
room temperature, and then tempered for at least one hour in the temperature
range
between 750 C and 820 C.
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It has been found out that the resulting steel possesses remarkable and
absolutely
excellent elevated temperature strength and superior steam-oxidation
resistance.
Moreover, it was found that by Creq./Nieq. ratio being less than 2.3, the
average 6-
ferrite content can be limited to less than 5 vol.% to avoid toughness issues,
wherein
Creq. and Nieq. are defined as Cr+6Si+4Mo+1.5W+11V+5Nb+8Ti and
40C+30N+2Mn+4Ni+2Co+Cu, respectively. Surprisingly, it was found that the B/N
ratio
equal or less than 1.5 has to be kept in order to enable the hot-working
operation with
known transformation processes.
The delta ferrite content shall not exceed 5 vol.-% since contents above 5vo1.-
% will
impair the toughness properties.
By hot forming processes, it is meant: hot rolling, pilgering, hot drawing,
forging, plug
mill, push-bench process where the mandrel rod pushes the elongated hollow
through
several in-line roll stands to produce a hollow, continuous rolling, and other
rolling
processes known. The steel according to the invention is able to be formed in
the
shape of tubes and pipes. Numerous attempts have been made with steels
exhibiting
satisfactory properties such as oxidation behavior, creep resistance but these
steels
failed in giving a satisfactory formed product through these hot forming
processes. In
particular, it was even sometime not possible to obtain seamless tubes or
pipes. The
steel of the invention enables having seamless tubular products with
satisfactory
properties and the possibility of obtaining seamless tubular products or
plates by hot
forming processes, these products being into dimensional requirements.
EXAMPLES
The benefits of the steel of the present invention will be explained in more
detail on
the basis of the following examples. Steels in accordance with the present
invention
(Steel 1, Steel 2, Steel 3) and also comparative example steels (Steel 4,
Steel 5), having
the chemical composition indicated in Table 1, have been cast to 100 kg ingots
using
vacuum induction melting furnace, then hot-rolled to plates (13-25mm
thickness) and
subsequently normalized and tempered. The normalizing heat-treatment was
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performed in the temperature range of 1060 C to 1100 C for 30 minutes,
followed by
air cooling to room temperature. The tempering was done at 780 C for 120
minutes,
again followed by cooling in air.
Comparative example steels 4 and 5 have B contents below 0.008 and are
therefore
not in accordance with the invention.
In case of steel 4, the Ni, Co, Mn, C and N additions do not comply with
equation
2.6 4 = (Ni + Co + 0.5 = Mn) ¨ 20 = (C + N) 11.2 (in wt.-%).
The steel 5 does not fulfill the following formula:
B _ (11/14)(N ¨ 10- (1/2.45)=(/ogB+6.81) (14/)
48 = Ti) > 0.007 (in wt. %) either.
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Table 1
Steel 1 Steel 2 Steel 3 Steel 4* Steel 5*
Element (wt.%) (wt.%) (wt.%) (wt.%) (wt.%)
C 0.15 0.148 0.148 0.158 0.152
Si 0.39 0.52 0.29 0.49 0.39
Mn 0.3 0.67 0.65 0.42 0.35
P 0.001 0.015 0.015 0.005 0.001
S 0.002 0.001 0.002 0.001 0.002
A 0.007 <0.002 0.007 0.007 0.006
Cr 11.19 11.4 11.3 11.36 10.85
Mo 0.49 0.46 0.25 0.31 0.49
/ 0.27 0.21 0.2 0.25 0.25
Ni 0.3 0.25 0.3 0.23 0.31
B 0.0145 0.011 0.0100 0.0040 0.0052
N 0.011 0.0088 0.0103 0.042 0.015
Co 1.77 1.9 1.9 0.88 1.72
W 1.91 1.6 1.8 1.46 1.95
Nb 0.048 0.038 0.033 0.038 0.043
Ti 0.001 0.003 0.001 0.001 0.001
*) Comparitive steels
For the two example steels (Steel 1, Steel 2, Steel 3) the results presented
in table 2
were obtained at room temperature for tensile strength, yield stress,
elongation,
reduction of area and Charpy V notch impact energy.
Table 2
Steel 1 Steel 2 Steel 3 P92
Rp0.2 (MPa) 653 683 682 540
Rm (M PA) 840 855.5 859.5 710
A5 ( oh, ) 20.5 22 21 23
Z(%) 64 64 60 65
= (J) - RT 72 52 56 140
Creep tests, performed in accordance to ISO DIN EN 204, on the specimens of
the two
example steels showed furthermore a remarkable improvement of the creep
rupture
strength. This is reflected in rupture times being at least almost two times
more than
that of state-of-the-art steels like P91, P91, VM12-SHC, P122 and X20CrMoV11-1
during long-term creep testing at 130MPa and 100MPa. The results are displayed
in
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Table 3. Also the comparative example steels does not reach the creep rupture
strength of the steels according to the invention.
Table 3
Rupture time in h at 650 C for stresses
Steel grade
130 MPa 100MPa
Steel 1 6470 23844
Steel 2 1824 13867
Steel 3 2194 7552
Steel 4 not tested 5900
Steel 5 526 3354
VM12-SHC 517 2828
P91* 44 498
P92* 686 4682
P122 (single phase)** 533 4572
X20CrMoV11-1* 55 210
*) Average values calculated from strength values indicated in ECCC data sheet
**) K. Kimura et al.. Proc. of ASME PVP Conference (PVP2012), 2012, Toronto,
Canada
Figure 1 shows the schematic of mass gain due to oxidation in water vapor
atmosphere
at elevated temperatures plotted versus chromium content. The basis for the
construction of the schematic is the oxidation tests in water vapor atmosphere
performed according to ISO 21608:2012.
In the figure 1, three regions displaying different steam oxidation behavior
have been
defined as follows:
(I.) Non-protective behavior for mass gain above 10mg/cm2 after 5,000h
(II.) Intermediate behavior for mass gain in the range 5-10mg/cm2
(III.) Protective behavior for mass gains below 5mg/cm2.
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Correspondingly, the classification of different high Cr martensitic heat-
resistant steels
with respect to oxidation behavior was performed in the table 4 below. Regions
I, ll
and Ill correspond to mass gains as described in Figure 1. The two example
steels
clearly outperform P91, P92, P122 and X20CrMoV11-1 with respect to steam
oxidation
resistance. The invention exhibits behavior comparable to VM12-SHC.
Table 4
Mass gain (mg/cm2)
Test temperature ( C) 600 C 650 C
VM12-SHC III III
P92 I I
X20CrMoV11-1 Ill I
P122 (single phase) III ll
Invention III III
According to the invention it is possible to provide a high chromium
martensitic heat-
resistant steel with enhanced creep properties and steam oxidation resistance
that can
be used to produce tubes, forgings, pipes and plates operating at high
temperature in
the power generation, chemical and petrochemical industry.
