Note: Descriptions are shown in the official language in which they were submitted.
WO 2016/162525 PCT/EP2016/057831
A method of producing a tube of a duplex stainless steel
TECHNICAL FIELD
The present disclosure relates to a method of producing a tube of a duplex
stainless steel,
in particular a duplex stainless steel suitable for use in fuel injection
systems for injection
of fuel into the combustion chamber of a combustion engine.
BACKGROUND
In connection to the design of Gasoline Direct Injection (GDI) systems for the
automotive
industry, it has been suggested to use duplex stainless steel for the rails
used for conducting
fuel to be injected into the combustion chamber of a combustion engine.
The requirements on a tube to be used as a GDI-rail are several, and must be
considered
when designing the duplex stainless steel to be used in such an application.
It is thus of
importance to select a chemical composition of the duplex stainless steel
that, in
combination with a properly chosen tube manufacturing process, results in a
predetermined
austenite/ferrite ratio, a requested corrosion resistance (resistance against
general corrosion
as well as against pitting corrosion), a microstructure essentially free from
intermetallic
phases, in particular sigma phase and chromium nitrides, a predetermined
impact
toughness, a predetemiined tensile strength and a predetermined fatigue
strength.
Furthermore, the mechanical properties of the duplex steel should be such that
the obtained
tube will present a predetermined burst pressure, i.e. internal pressure until
failure, which
is high enough for the envisaged application, also when the wall thickness of
the tube is
relatively small, thereby enabling a GDI-rail that requires less space and
weight. The
corrosion and fatigue properties should guarantee the endurance of the tube
over time.
Designing of a duplex stainless steel and the process of producing a tube
thereof assumed
to meet the requirements of a GDI-rail is therefore a complex task. The
selected chemical
composition and the production process parameters must be tuned with regard to
each
other. Accordingly, once a nominal chemical composition has been decided for
the duplex
stainless steel, the production process parameters must also be selected with
regard thereto.
The chemical composition of the duplex stainless steel should also promote a
cost efficient
Date Recue/Date Received 2022-06-02
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production process. In other words, the chemical composition should not be
such that it
will require excessively complicated, energy-consuming or time-consuming
production
steps.
The aspect of the present disclosure is to present a method of producing a
tube of a duplex
stainless steel that enables the production of a tube of said duplex stainless
steel presenting
properties making the tube suitable to applications in which there are high
requirements on
corrosion resistance (resistance against general corrosion as well as against
pitting
corrosion), a predetermined impact toughness, a predetermined tensile strength
and a
predetermined fatigue strength.
One such application is a GDI-rail for conducting fuel to be injected into the
combustion
chamber of a combustion engine. The duplex stainless steel of said tube should
present a
microstructure essentially free from intermetallic phases, in particular sigma
phase and
chromium nitrides. The chemical composition of the duplex stainless steel
shall enable
cost-efficient production of a tube thereof in terms of promoting the use of
cost-efficient
process steps.
SUMMARY
The aspects mentioned above are achieved by the present disclosure which
provides a
method of producing a tube of duplex stainless steel comprising the following
composition, in weight% (wt%),
max 0.06;
Cr 21-24.5;
Ni 2.0-5.5;
Si max 1.5;
Mo 0.01-1.0
Cu 0.01-1.0;
Mn max 2.0;
N 0.05-0.3;
max 0.04;
max 0.03; and
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balance Fe and unavoidable impurities, and with a PRE-value of at least 23.0,
wherein the method comprises the steps of:
a) providing a melt of the duplex stainless steel;
b) casting a body of the duplex stainless steel from the melt;
c) foiiiiing a bar of the body;
d) faiming a tube of the bar by generating a hole therein;
e) reducing the diameter and/or wall thickness of the tube by hot extrusion;
f) further reducing the diameter and/or wall thickness of the tube by cold
deformation, and
g) annealing the cold deformed tube;
wherein after step g), the duplex stainless steel of the obtained tube
consists of 40-60%
austenite and 40-60% ferrite and wherein step g) comprises subjecting said
tube to a
temperature in the range of from 950 C-1060 C for a time period of from 0.3-10
minutes
and to an atmosphere consisting of a gas mixture comprising 1-6 vol% nitrogen
gas and
the remainder is H2 or an inert gas.
Thus, it has been found that to reach optimal material properties, the
annealing
temperature, the annealing time and the annealing atmosphere. It has been
found that the
annealing temperature should be in the range of from 950 to 1060 C and the
atmosphere
should comprise a gas mixture of 1-6 vol% nitrogen and the remainder is
selected from H2
or an inert gas and the annealing should be performed in a time period of from
0.3-10
minutes
If lower annealing temperatures are used, there is a risk of forming un-wanted
precipitates,
such as intermetallic phases. Additionally, the recrystallization will be
slower and therefore
an increased soaking time will be required for completing the
recrystallization, thus having
a negative impact on productivity.
In principal, the upper temperature limit for the annealing step is set by the
temperature at
which the duplex stainless steel will start to melt. However, there are also
practical reasons
for why the annealing temperature shall be further restricted. At temperatures
higher than
the provided interval, the duplex stainless steel will become softer, which
will increase the
risk of damages during the annealing step. Also, at high temperatures, the
grain growth
will increase making it more difficult to obtain a good process and grain size
control.
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It is also very important to use an annealing temperature which will balance
the phase
fraction, a too low temperature will cause too low ferrite content and a too
high
temperature will provide too high ferrite content. The temperature of the
annealing step
will also influence the chemical composition of the ferrite and the austenite
phase, so the
annealing temperature needs to be balanced together with the chemical
composition to
ensure that both these phases will have good corrosion resistance.
The time period for which the tube is subjected to the annealing temperature
should be
between 0.3 to 10 minutes, such as 0.3 to 5 minutes, such as 0.3 to 2.5
¨minutes. This time
period needs to be long enough to ensure complete recrystallization. However,
if said time
period is too long, the obtained tube will have a coarse structure which will
have a negative
impact on the mechanical properties. The larger the thickness of the tube
wall, the longer
the annealing time. Wall thicknesses of from about 1 mm up to about 5 mm are
conceived.
Furthermore, the atmosphere of the annealing step is very important. An
atmosphere
comprising nitrogen will affect the content of nitrogen in the surface of the
duplex stainless
steel. Hence, the role of nitrogen in the atmosphere is to maintain the
nitrogen content of
the material at the surface. At the annealing temperature of the present
method, nitrogen
will diffuse into and out from the material. The nitrogen content should be
selected so that
the nitrogen content in the surface is maintained. It has been found that too
low nitrogen
content in the atmosphere where the annealing is perfoiniedwill result in a
net loss of
nitrogen in the surface, which will affect the corrosion resistance and the
mechanical
properties of the duplex stainless steel as defined hereinabove or hereinafter
negatively. It
has also been found that too high nitrogen levels in the atmosphere where
annealing is
performed will result in an increase of nitrogen in the surface of the
material during
annealing and as nitrogen is a strong austenite former, a change in the
nitrogen content
may therefore influence the phase balance. Hence, a high content of nitrogen
in the
atmosphere will provide for the formation of austenite in the surface. The
nitrogen content
in the surface of the material will also influence the structure stability
with respect to the
sensitivity of forming precipitates, such as chromium nitrides. The formation
of
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precipitates will have a negative impact on the corrosion resistance of the
duplex stainless
steel as defined hereinabove or hereinafter.
The pitting corrosion resistance equivalent PRE is defined as
PRE=Cr(wt%)+3.3Mo(wt%)+16N(wt%). A PRE of at least about 23.0 indicates that,
with
the above-defined composition, all three of chromium, molybdenum and nitrogen
are not
allowed to be at their minimum simultaneously but must be combined such that
the defined
PRE-value is obtained. According to another embodiment, the PRE-value is at
least about
24Ø The term "about" as used hereinabove and hereinafter indicates +/- 10%
of an
integer.
According to one embodiment, the temperature range of the annealing step (step
g) is of
from 970 C to1040 C. According to yet another embodiment, said temperature
range is of
from 1000 C to1040 C.
According to one embodiment, said annealing step comprises subjecting said
tube to said
temperature for a time period of from 0.5-5 minutes, such as of from 0.5 to
1.5 minutes.
According to one embodiment, the inert gas is argon or helium or a mixture
thereof.
According to one embodiment, the content of nitrogen gas in the gas mixture is
equal to or
less than 4 vol%. According to another embodiment the content of nitrogen gas
in said gas
mixture is equal to or less than 3 vol%. According to yet one embodiment, the
content of
nitrogen gas in said gas mixture is equal to or above 1.5 vol%.
According to one embodiment, said hot extrusion step (step e) comprises
subjecting said
tube to hot extrusion at a temperature in the range of from 1100 C-1200 C and
a cross-
sectional area reduction thereof in the range of from 92-98%. According to one
embodiment, said hot extrusion step (step e) comprises subjecting said tube to
hot
extrusion at a temperature in the range of from 1100 C-1170 C and a cross-
sectional area
reduction thereof in the range of from 92-98%. The cross-sectional area
reduction is
defined as (cross-sectional area (of tube) before extrusion minus cross-
sectional area after
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extrusion)/(cross-sectional area before extrusion). The extrusion temperature
and
deformation degree is chosen with regard to the chemical composition of the
duplex
stainless steel such that it will not have a detrimental effect on the
microstructure of the
duplex stainless steel or will result in cracks or the like therein that would
be detrimental to
the mechanical properties of the final product.
According to one embodiment, the cold deformation step (step f) comprises
subjecting the
tube to cold deformation without pre-heating the tube. According to one
embodiment, said
cold deformation step (step f) comprises subjecting said tube to a cross
sectional area
reduction thereof in the range of 50-90%. Cross-sectional area reduction is
defined as
(cross-sectional area (of tube) before pilgering minus cross-sectional area
after
pilgering)/(cross-sectional area before pilgering). The chemical composition
of the duplex
stainless steel is selected to enable such cold deformation thereof without
unwanted crack-
generation in the material or any detrimental negative effects on the
microstructure of the
material.
According to one embodiment of the method as defined hereinabove or
hereinafter, the
cold deformation is either pilgering or cold drawing.
According to one embodiment, when the cold defoiniation is pilgering, the
relationship
between the wall thickness reduction and the outer diameter reduction of the
tube is
expressed as the Q-value, wherein
Q-value = (Wallh ¨ Wallt)*(0dh ¨ Wallh)/Wallh ((Odh ¨ Wallh) ¨ (Odt ¨ Wallt)),
wherein
Wallh=hollow wall=the thickness of the wall before pilgering
Wallt=tube wall=the thickness of the wall after pilgering
Odh=hollow OD=the diameter of the tube before pilgering
Odt=tube OD=the diameter of the tube after pilgering,
and wherein Q is in the range of 0.5-2.5. If the area reduction is too high,
the force will be
too high and the material might crack.
According to yet another embodiment, Q is in the range of from 0.9-1.1.
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According to one embodiment, said duplex stainless steel has the following
composition,
in weight%:
0.01-0.025;
Si 0.35-0.6;
Mn 0.8-1.5;
Cr 21-23.5;
Ni 3.0-5.5;
Mo 0.10-1.0;
Cu 0.15-0.70;
0.090-0.25;
less than or equal to 0.035;
less than or equal to 0.003;
balance Fe and unavoidable impurities.
A duplex stainless steel with this chemical composition is particularly
suitable to be
subjected to the above-mentioned process steps with the above-mentioned
process
parameters. In other words, the process steps and parameters as defined
hereinabove or
hereinafter are selected to be particularly suitable on a duplex stainless
steel with this
chemical composition and to result in a tube with properties that makes it
particularly
suitably in an application as GDI-rail for conduction of a fuel in a fuel
injection system for
injecting fuel into the combustion chamber of a combustion engine.
According to another embodiment, the tube is a tube for conduction of a fuel
in a fuel
injection system for injecting fuel into the combustion chamber of a
combustion engine.
The present disclosure may, as an alternative, be defined as a process of
producing a fuel
conductor in a fuel injection system for injecting fuel into the combustion
chamber of a
combustion engine, wherein said process comprises the method defined
hereinabove
and/or hereinafter for producing a tube of duplex stainless steel. Such a
process includes
attaching the tube of duplex stainless steel to a further structural member of
said
combustion engine by means of brazing. The further structural member may be
metal,
typically austenitic or duplex steel. The method of producing the tube,
including the
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selection of the chemical composition of the duplex stainless steel, also aims
at achieving a
tube with advantageous brazing properties, in particular a low susceptibility
to liquid metal
induced embrittlement (LMIE) caused by liquid metal penetration. The brazing
includes
copper brazing, possibly in a continuous furnace at temperature in the range
of from
1100 C-1140 C.
According to one embodiment, the tube has an outer diameter in the range of
from 15-35
mm after said pilgering step. According to one embodiment, this tube is used
as a GDI-rail
in a fuel injection system for conducting fuel to be injected into the
combustion chamber of
a combustion engine.
According to another embodiment, the tube has an outer diameter of from 7-10
mm after
said pilgering step. According to one embodiment, this tube is used as a fuel
line in a fuel
injection system for conducting fuel to be injected into the combustion
chamber of a
combustion engine.
The functions and effects of essential alloying elements of the duplex
stainless steel
defined hereinabove and hereinafter will be presented in the following
paragraphs. The
listing of functions and effects of the respective alloying elements is not to
be seen as
complete, but there may be further functions and effects of said alloying
elements.
However, it provides a view of the underlying knowledge that should be
considered when
designing the duplex stainless steel as well as the process parameters of a
method for the
production of a tube of said duplex stainless steel, in particular a duplex
stainless tube
aimed for conduction of a fuel in a fuel injection system for injecting fuel
into the
combustion chamber of a combustion engine.
Carbon, C, has an austenite stabilizing effect and counteracts the
transformation from
austenitic to martensitic structure upon deformation of the duplex stainless
steel. C has a
positive effect on the strength of the duplex stainless steel. Therefore, the
content of C
should be equal to or above 0.01 wt%. However, at too high levels, carbon
tends to form
unwanted carbides with other alloying elements. The content of C should
therefore not be
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above 0.06 wt%. According to one embodiment, the content of C should not be
above
0.025 wt%.
Chromium, Cr, has strong impact on the corrosion resistance of the duplex
stainless steel,
especially pitting corrosion. According to the present disclosure, the PRE-
value is above
23Ø Moreover, Cr improves the yield strength, and counteracts transformation
of
austenitic structure to martensitic structure upon deformation of the duplex
stainless steel.
Therefore, the content of Cr should be equal to or above 21.0 wt%. At high
levels, an
increasing content of Cr results in a higher temperature for unwanted stable
sigma phase
and a more rapid generation of sigma phase. Therefore, the content of Cr is
equal to or less
than 24.5 wt%. Cr also has a ferrite-stabilizing effect on the duplex
stainless steel.
According to one embodiment the content of Cr is equal to or less than 23.5
wt%.
Nickel, Ni, has a positive effect on the resistance against general corrosion.
Ni also has a
strong austenite-stabilizing effect and counteracts transformation from
austenitic to
martensitic structure upon deformation of the duplex stainless steel. The
content of Ni is
therefore equal to or more than 2.0 wt%. According to another embodiment the
content of
Ni is equal to or more than 3.5 wt%. To some extent the austenite-stabilizing
effect of Ni
may be compensated for by adjusting the Cr content. The content of Ni should,
however,
not be more than or equal to 5.5 wt%.
Silicon, Si, is often present in the duplex stainless steel since it may have
been used for
deoxidization of the steel melt. Si is a ferrite stabilizer but also
counteracts transformation
of austenite to martensite in connection to deformation of the duplex
stainless steel. It may
also improve the corrosion resistance in some environments. However, Si
reduces the
solubility of nitrogen and carbon and may form unwanted silicides if present
at too high
levels. Therefore, according to one embodiment, the content of Si in the
duplex stainless
steel is not more than 1.5 wt%. According to one embodiment, the content of Si
in the
duplex stainless steel is not more than 0.6 wt%. According to one embodiment
the content
of Si may be as low as about 0 wt%. According to one embodiment, the content
of Si
should be equal to or more than 0.35 wt%.
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Molybdenum, Mo, has a strong influence on the corrosion resistance of the
duplex
stainless steel. It heavily influences the PRE thereof. Mo is added in amount
of equal to or
more than 0.01 wt%. It also has a ferrite-stabilizing effect on the duplex
stainless steel.
According to one embodiment, the content of Mo is above 0.10 wt%. Mo also
increases
the temperature at which unwanted sigma-phases are stable and promotes the
rate of
generation thereof. It is also a relatively expensive alloying element.
Therefore, the content
of Mo should be equal to or less than 1.0 wt%.
Copper, Cu, has a positive effect on the corrosion resistance. Cu also
counteracts
transformation of austenite to martensite upon deformation of the duplex
stainless steel. It
is thus optional to purposively add Cu to the duplex stainless steel. Often,
Cu is present in
scrapped goods used for the production of steel, and is allowed to remain in
the steel at
moderate levels. According to one embodiment, the content of Cu may be equal
to or more
than 0.01 wt%. According to another embodiment, the content of Cu is equal to
or more
than 0.15 wt%. According to one embodiment, the content of Cu is equal to or
less than 1.0
wt%. According to another embodiment, the content of Cu is equal to or less
than 0.7 wt%.
Manganese, Mn, has a deformation hardening effect on the duplex stainless
steel, and it
counteracts the transfoimation from austenitic to martensitic structure upon
deformation of
the duplex stainless steel. Mn also has an austenite stabilizing effect.
According to one
embodiment, the content of Mn in the duplex stainless steel should be equal to
or above 0.8
wt%. However, Mn has a negative impact on the corrosion resistance in acids
and chloride-
containing environments, and it increases the tendency to generation of
intermetallic
phases. Therefore, the maximum content of Mn should not be above 2.0 wt%.
According
to one embodiment, the content of Mn is equal to or less than 1.0 wt%.
Nitrogen, N, has a positive effect on the corrosion resistance of the duplex
stainless steel
and also contributes to deformation hardening. It has a strong effect on the
pitting
corrosion resistance equivalent PRE. It also has a strong austenite
stabilizing effect and
counteracts transfaimation from austenitic structure to martensitic structure
upon plastic
deformation of the duplex stainless steel, and is therefore added in an amount
of 0.05 wt%
or higher. According to one embodiment, the content of N should be equal to or
more
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above 0.090 wt%. At too high levels, N tends to form chromium nitrides in the
duplex
stainless steel, which should be avoided due to its negative effect on
ductility and corrosion
resistance. Therefore, the content of N should be equal to or lower than 0.3
wt%.
According to one embodiment, the content of N is equal to or less than 0.25
wt%.
Phosphorus, P, is an impurity contained in the duplex stainless steel and it
is well known
that P affects the hot workability negatively. Accordingly, the content of P
is set at 0.03
wt% or less.
Sulphur, S. is an impurity contained in the austenitic stainless steel and it
will deteriorate
the hot workability. Accordingly, the allowable content of S is less than or
equal to 0.03
wt%, such as less than or equal to 0.005 wt%.
The duplex stainless steel as defined hereinabove or herein after may
optionally comprise
one or more of the following elements selected from the group of Al, V, Nb,
Ti, 0, Zr, Hf,
Ta, Mg, Ca, La, Ce, Y and B. These elements may be added during the
manufacturing
process in order to enhance e.g. deoxidation, corrosion resistance, hot
ductility or
machinability. However, as known in the art, the addition of these elements
has to be
limited depending on which element is present. Thus, if added the total
content of these
elements is less than or equal to 1.0 wt%.
The term "impurities" as referred to herein is intended to mean substances
that will
contaminate the duplex stainless steel when it is industrially produced, due
to the raw
materials such as ores and scraps, and due to various other factors in the
production
process, and are allowed to contaminate within the ranges not adversely
affecting the
duplex stainless steel as defined hereinabove or hereinafter.
The present disclosure is further illustrated by the following non-limiting
examples.
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EXAMPLES
Two melts were made having the following compositions: Fe is the balance for
both
No C Si Mn P S Cr Ni Mo Cu
1 0.02 0.5 1.5 <0.035 <0.010 22.2 3.3 0.25 0.25 0.15
2 0.01 0.53 1.09 0.026 <0.003 22.88 3.15 0.12 0.21 0.25
The obtained melts were then processed accordingly:
They were casted to bodies by using continuous casting.
Round bars were then formed by forging and the tubes were then formed by
boring a hole
therein. The diameter of the tubes was then reduced by by using hot extrusion
at a
temperature in the range of from 1120 C-1150 C, the obtained tubes had a cross-
sectional
area reduction of 96-98%. The hot extrusion was followed by pickling to remove
glass
beads.
The diameter was further reduced by pilgering and subjecting the tubes to a
cross sectional
area reduction thereof in the range of 80-86%.
The pilgered tubes were then annealed in an atmosphere consisting of a gas
mixture
.. comprising about 2% nitrogen gas and remainder argon gas and subjecting the
tubes to a
temperature of about 1030 C for a time period of about 1 minute.
In the pilgering step Q is about 1Ø
After annealing, the obtained tubes were subjected to a straightening step.
Straightening
was performed in a roll straightening machine with a combination of bending
and
ovalization. The tubes were passed through a series of angled rollers which
rotated the tube
and applied to it a series of bending movements. During straightening the
yield strength is
exceeded in order to get a permanent change in shape to obtain a straight
tube.
The obtained tubes had on outer diameter in the of 30 mm aand the tubes are to
be used as
a GDI-rail in a fuel injection system for conducting fuel to be injected into
the combustion
chamber of a combustion engine.
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One additional tube of melt 1 was also manufactured according to the method
disclosed
above. This tube had an outer diameter of from 8 mm after the pilgering step.
This tube
was also used as a fuel line in a fuel injection system for conducting fuel to
be injected into
the combustion chamber of a combustion engine.
