Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A HYDROGEN EMBRITTLEMENT RESISTANCE COATED STEEL
The invention relates to a steel substrate having hydrogen embrittlement
resistance
and a method for manufacturing the same and particularly to a coated steel
substrate having good resistance against Hydrogen embrittlement.
High strength steels such as dual phase (DP) steels, advanced high strength
steels
(AHSS), Ultra high strength steels (UHSS) or Martensitic Steel (MS) are
characterized by having a high tensile strength. Because of these properties
the use
of such steels in the manufacture of automobiles has increased in response to
the
demands placed on the automotive industry to reduce the weight of motor
vehicles
without sacrificing passenger safety particularly for structural components
such as
a pillar, and reinforcing components such as a bumper and an impact beam, are
required to further increase the strength thereof.
In addition, the all abovementioned steels to be used in automobiles are also
required to be resistant to the occurrence of hydrogen induced delayed
fracture
which is commonly known as hydrogen embrittlement resistance steel. The
hydrogen embrittlement generally refers to as the embrittlement caused by
hydrogen generated during processing such electroplating, electrolytic
cleaning or
during application of end product in a corrosive environment or in atmosphere
contents high moisture. This hydrogen diffuses into defective areas, such as
zo dislocations, holes and grain boundaries, in the steel sheet, to
embrittle the defective
areas and cause deterioration in ductility and rigidity of the steel sheet,
and thereby
causing fracture under that static or dynamic stress.
Hence the purpose of the present invention is to solve these problems by
making
available a method and a coated steel substrate that is suitable to be used in
automobile industry and that has an Hydrogen Embrittlement ratio of less than
30%
and preferably less than 25% and more preferably less than 22%.
In a preferred embodiment, the steel substrate can have:
- an ultimate tensile strength greater than or equal to 900 MPa and preferably
above 980,
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- a yield strength greater than or equal to 700 MPa and preferably above 800
MPa,
Another object of the present invention is also to make available a method for
the
manufacturing of these substrates that is compatible with conventional
industrial
applications while being robust towards manufacturing parameters shifts.
The term "coated steel substrate" for the purpose of the present invention
includes
a hot rolled steel strip, cold rolled steel sheet, flat steel product, tailor
welded blank,
blank substrate containing one or more from C, Al, Si, and Mn as alloying
elements
and having a Ni-MoS2 layer thereon.
The present invention remedies the problem of Hydrogen Embrittlement by
coating
the steel with a layer of Ni-MoS2 having at least 0.3% of MoS2 particles by
weight
percentage with a thickness of the layer equal to or more than 0.1 micron.
The Ni-MoS2 layer of the present invention is able to withstand welding
process so
that the Ni-MoS2 layer of the present invention can be welded for
manufacturing of
automobiles.
The method is specifically explained herein for the appreciation of the
invention. The
method can be according to the invention can be produced by the method
consists
zo of successive steps mentioned herein:
For the purpose of demonstration of the present invention martensitic steel is
taken
as a preferred embodiment steel which will be manufactured into a cold rolled
steel
sheet to demonstrate the beneficial effects of the present invention. The use
of
martensitic steel must not be considered as a limitation of the present
invention and
method of present invention can be implemented on any steel having any one or
more, C, Mn, Al and Si as it is alloying element.
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A coated steel substrate according to the invention can be produced by any
following
method. A preferred method consists in providing a semi-finished casting of
steel
with a chemical composition of the according to the invention. The casting can
be
done either into ingots or continuously in form of thin slabs or thin strips,
i.e. with a
.. thickness ranging from approximately 220mm for slabs up to several tens of
millimeters for thin strip.
For example, a slab having the chemical composition of the steel is
manufactured
by continuous casting wherein the slab optionally underwent the direct soft
reduction
during the continuous casting process to avoid central segregation and to
ensure a
ratio of local Carbon to nominal Carbon kept below 1.10. The slab provided by
continuous casting process can be used directly at a high temperature after
the
continuous casting or may be first cooled to room temperature and then
reheated
for hot rolling.
The temperature of the slab, which is subjected to hot rolling, is at least
1000 C and
at least 1280 C. It is preferred to have the temperature of the slab more than
1150
C, as below this temperature excessive load is imposed on a rolling mill and,
further,
the temperature of the steel may decrease to a Ferrite transformation
temperature
during finishing rolling, whereby the steel will be rolled in a state in which
transformed Ferrite contained in the structure. Therefore, the temperature of
the
zo slab is preferably sufficiently high so that hot rolling can be
completed in the
temperature range of Ac3 to Ac3+100 C and final rolling temperature remains
above
Ac3. Reheating at temperatures above 1280 C must be avoided because they are
industrially expensive.
A final rolling temperature range from Ac3 to Ac3+100 C is preferred to have a
structure that is favorable to recrystallization and rolling. It is necessary
to have final
rolling pass to be performed at a temperature greater than 850 C, because
below
this temperature the steel sheet exhibits a significant drop in rollability.
The sheet
obtained in this manner is then cooled at a cooling rate above 30 C/s to the
coiling
temperature which below 650 C . Preferably, the cooling rate will be less than
or
equal to 200 C/s.
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The hot rolled steel sheet is then coiled at a coiling temperature below 650 C
to
avoid ovalization and preferably below 625 C to avoid scale formation. The
preferred range for such coiling temperature is from 400 C to 625 C. The
coiled hot
rolled steel sheet is cooled down to room temperature before subjecting it to
optional
hot band annealing.
The hot rolled steel sheet may be subjected to an optional scale removal step
to
remove the scale formed during the hot rolling before optional hot band
annealing.
The hot rolled sheet may then have subjected to an optional Hot Band Annealing
at
temperatures from 400 C to 750 C for at least 12 hours and not more than 96
hours,
the temperature remaining below 750 C to avoid transforming partially the hot-
rolled
microstructure and, therefore, losing the microstructure homogeneity.
Thereafter, an
optional scale removal step of this hot rolled steel sheet may performed
through, for
example, pickling of such sheet. This hot rolled steel sheet is subjected to
cold rolling
to obtain a cold rolled steel sheet with a thickness reduction from 35 to 90%.
The
cold rolled steel sheet is then obtained.
Thereafter the cold rolled steel is sent to continuous annealing cycle for
heat
treatment which will impart the steel of present invention with requisite
properties
and microstructure.
In annealing of the cold rolled steel sheet, the cold rolled steel sheet is
heated at a
zo heating rate which is greater than 2 C/s and preferably greater than 3
C/s, to a
soaking temperature from Ad 1 to Ac3+100 C wherein Ad 1 and Ac3 for the
composite steel sheet is calculated by experimental dilatometer study.
The cold rolled steel sheet is held at the soaking temperature during 10
seconds to
500 seconds to ensure a complete recrystallization of the strongly work
hardened
initial structure. The cold rolled steel sheet is then cooled at a cooling
rate greater
than 5 C/s to a temperature less than 550 C and preferably less than 500 C and
optionally holding the cold rolled steel sheet during 10 seconds to 1000
seconds
from 150 C to 500 C to impart the requisite microstructure to the present
invention,
then cool the cold rolled steel sheet to obtain cold rolled steel substrate.
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Then the cold rolled steel substrate is dipped in an acidic pickling solution
during 5
seconds to 100seconds at a temperature range from 30 C to 100 C to activate
the
surface for electroplating.
Ni-MoS2 layer is then coated by electroplating on the surface of the cold
rolled steel
5 substrate. Ni-MoS2 layer is made of a Nickel matrix in which the MoS2
particles are
embedded. The MoS2 particles must be more than 0.3% by weight percentage of
the total coated layer to impart the coated steel substrate with adequate
hydrogen
embrittlement resistance and preferably 0.4% or more and more preferably equal
to
or more than 0.5%. In a preferred embodiment, the presence of MoS2 may be
restricted to 3% due to the economic reasons.
Ni-MoS2 layer is electroplated by coating an electroplate solution containing
NiSO4
and MoS2 wherein the concentration of NiSO4 is from 100g/I to 500g/I and the
concentration of MoS2 is from 1g/I to 15g/I to obtain a hydrogen embrittlement
resistance on the cold rolled steel substrate. The concentration of the MoS2
is kept
from 1g/I to 15g/I because the presence of MoS2 above 15g/I in electroplating
process decreases the efficiency of Ni deposition due to enhancement of
hydrogen
evolution reaction during electroplating. Concentration range of NiSO4 is
optimized
to obtain enough Ni deposition and embedding the MoS2 particles in the
deposited
Ni matrix during electroplating. The preferred concentration of the MoS2 is
from 2g/I
zo to 14g/land more preferably from 3g/I to 12g/I. The preferred
concentration of NiSO4
is from 100g/I to 400g/I and more preferably from 150g/I to 400g/I.
A current density from 15 A/dm2 to 45 A/dm2 is applied during 30 to 300
seconds
during electroplating to embed the MoS2 particles with 0.3% or more by weight
percentage in the nickel matrix of the Ni-MoS2 layer and to have thickness of
at
least 0.1 micron for Ni-MoS2 layer. It is preferable to have a layer thickness
of more
than 0.2 micron and more preferably more than 0.3 micron. If the current
density is
less than 15 A/dm2 the MoS2 particles with 0.3% or more by weight percentage
will
not be embedded in the Ni-Matrix, thereby the final layer having Ni-MoS2 will
not
form. The temperature for electroplating the cold rolled steel substrate is
usually
maintained from 30 C to 90 C while the pH of the electroplating solution is
maintained from 2 to 6. A preferred range for current density during
electroplating
from 15 A/dm2 to 40 A/dm2 and more preferably from A/dm2 to 38 A/dm2 .The
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preferred time for electroplating is from 50 to 250 seconds and more
preferably from
60 seconds to 200 seconds.
Thereafter, the cold rolled steel substrate is rinsed with any appropriate
solvant, like
ethanol for instance and dried using, for example, hot air to obtain a coated
steel
substrate.
The coated steel substrate then may be optionally coated by any of the known
industrial processes such as Electro-galvanization, JVD and PVD etc.
Then an optional post batch annealing may be done at a temperature from 150 C
to 300 C during 30 minutes to 120 hours.
In a preferred embodiment, the chemical composition of the steel substrate to
be
used in the method according to the invention is as follows:
Carbon is present in from 0.05% to 0.5%. Carbon is an element necessary for
increasing the strength of the Steel of present invention by producing a low-
temperature transformation phases such as Martensite, Bainite further Carbon
also
plays a pivotal role in Austenite stabilization, hence, it is a necessary
element for
securing Residual Austenite. Therefore, Carbon plays two pivotal roles, one is
to
increase the strength and another in Retaining Austenite to impart ductility.
But
zo Carbon content less than 0.05% will not be able to stabilize Austenite
in an adequate
amount required by the steel of present invention. On the other hand, at a
Carbon
content exceeding 0.5%, the steel exhibits poor spot weldability, which limits
its
application for the automotive parts.
Manganese is present in the steel of present invention from 0.2 % to 5%. This
element is gammagenous. The purpose of adding Manganese is essentially to
obtain a structure that contains Austenite. Manganese is an element which
stabilizes
Austenite at room temperature to obtain Residual Austenite. An amount of at
least
about 0.2% by weight of Manganese is mandatory to provide the strength and
hardenability to the Steel of the present invention as well as to stabilize
Austenite.
Thus, a higher percentage of Manganese is preferred by presented invention
such
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as 2% or more. But when Manganese content is more than 5% it produces adverse
effects such as it retards transformation of Austenite during cooling after
annealing
which retards he formation of other microstructural constituents. In addition,
Manganese content of above 5% also deteriorates the weldability of the present
steel as well as the ductility targets may not be achieved.
Silicon content of the steel of present invention is from 0.1% to 2.5%.
Silicon is a
constituent that can retard the precipitation of carbides during overaging,
therefore,
due to the presence of Silicon Austenite is stabilized at room temperature.
Further
io due to poor solubility of Silicon in carbide it effectively inhibits or
retards the
formation of carbides, hence, also promote the formation of low density
carbides in
Bainitic structure which impart the Steel of present invention with its
essential
mechanical properties such as tensile strength. However, disproportionate
content
of Silicon does not produce the mentioned effect and leads to problems such as
temper embrittlement. Therefore, the concentration is controlled within an
upper limit
of 2.5%.
The content of the Aluminum is from 0.01% to 2%. in the present invention
Aluminum removes Oxygen existing in molten steel to prevent Oxygen from
forming
zo a gas phase during solidification process. Aluminum also fixes Nitrogen
in the steel
to form Aluminum nitride so as to reduce the size of the grains. Higher
content of
Aluminum, above 2%, increases Ac3 point to a high temperature thereby lowering
the productivity. Aluminum content from 0.8% to 1% can be used when high
Manganese content is added in order to counterbalance the effect of Manganese
on
transformation points and Austenite formation evolution with temperature.
Sulfur is not an essential element but may be contained as an impurity in
steel and
from point of view of the present invention the Sulfur content is preferably
as low as
possible but is 0.09% or less from the viewpoint of manufacturing cost.
Further if
higher Sulfur is present in steel it combines to form Sulfides especially with
Manganese and reduces its beneficial impact on the present invention.
Phosphorus constituent of the Steel of present invention is from 0.002% to
0.09%,
Phosphorus reduces the spot weldability and the hot ductility, particularly
due to its
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tendency to segregate at the grain boundaries or co-segregate with Manganese.
For these reasons, its content is limited to 0.09 % and preferably lower than
0.06%.
Nitrogen is limited to 0.09% in order to avoid ageing of material and to
minimize the
precipitation of Aluminum nitrides during solidification which are detrimental
for
mechanical properties of the steel.
Chromium content of the composite coil of steel of present invention is from
0% to
1%. Chromium is an essential element that provide strength and hardening to
the
steel but when used above 1% impairs surface finish of steel. Further Chromium
content under 1% coarsen the dispersion pattern of carbide in Bainitic
structures,
hence, keep the density of Carbide low in Bainite.
Nickel may be added as an optional element in an amount of 0% to 1% to
increase
the strength of the steel and to improve its toughness. A minimum of 0.01% is
required to get such effects. However, when its content is above 1%, Nickel
causes
ductility deterioration.
Copper may be added as an optional element in an amount of 0% to 1% to
increase
the strength of the steel and to improve its corrosion resistance. A minimum
of
zo 0.01% is required to get such effects. However, when its content is
above 1%, it can
degrade the surface aspects.
Molybdenum is an optional element that constitutes 0% to 0.5% of the Steel of
present invention; Molybdenum plays an effective role in improving
hardenability of
the steel. However, the addition of Molybdenum excessively increases the cost
of
the addition of alloy elements, so that for economic reasons its content is
limited to
0.4%.
Niobium is present in the Steel of present invention from 0% to 0.1% and
suitable
for forming carbo-nitrides to impart strength of the Steel of present
invention by
precipitation hardening. Niobium will also impact the size of microstructural
components through its precipitation as carbo-nitrides and by retarding the
recrystallization during heating process. Thus, finer microstructure formed at
the end
of the holding temperature and as a consequence after the complete annealing
will
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lead to the hardening of the product. However, Niobium content above 0.1% is
not
economically interesting as a saturation effect of its influence is observed
this means
that additional amount of Niobium does not result in any strength improvement
of
the product.
Titanium is added to the Steel of present invention from 0 % to 0.1% same as
Niobium, it is involved in carbo-nitrides so plays a role in hardening. But it
is also
forming Titanium-nitrides appearing during solidification of the cast product.
The
amount of Titanium is so limited to 0.1% to avoid the formation of coarse
Titanium-
nitrides detrimental for formability. In case the Titanium content below
0.001% does
not impart any effect on the steel of present invention.
Calcium content in the steel of present invention is from 0.001% to 0.005%.
Calcium
is added to steel of present invention as an optional element especially
during the
inclusion treatment. Calcium contributes towards the refining of the Steel by
arresting the detrimental Sulfur content in globular form thereby retarding
the
harmful effect of Sulfur.
Vanadium is effective in enhancing the strength of steel by forming carbides
or
carbo-nitrides and the upper limit is 0.1% from economic points of view. Other
zo elements such as Cerium, Boron, Magnesium or Zirconium can be added
individually or in combination in the following proportions: Cerium 0.1%,
Boron
0.003%, Magnesium 0.010% and Zirconium 0.010%. Up to the maximum
content levels indicated, these elements make it possible to refine the grain
during
solidification. The remainder of the composition of the steel consists of iron
and
inevitable impurities resulting from processing.
The microstructure of the coated steel substrate may comprise any one or more
than one from Residual austenite, martensite, tempered martensite, tempered
bainite, ferrite and Bainite. Theses micro-constituents may comprise 90% or
more
of the microstructure of the coated steel substrate of present invention. In
addition
to the above-mentioned microstructure, the microstructural components such as
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pearlite and cementite may also be present in the coated steel substrate but
limited
upto a maximum of 10%in total.
EXAMPLES
The following tests, examples, figurative exemplification and tables which are
5 presented herein are non-restricting in nature and must be considered for
purposes
of illustration only and will display the advantageous features of the present
invention.
Steel with different compositions is gathered in Table 1 which shows the two
example steel composition Steel A and Steel B, wherein the Table 2 shows
io parameters implemented for the coating NiMoS2. Thereafter Table 3
gathers the
microstructures of the steel sheet obtained during the trials and table 4
gathers the
result of evaluations of obtained for hydrogen embrittlement and mechanical
properties.
Table 1
Samples C Mn Si Al Cr Nb S P N B Ti
A 0.25 0.50 0.20 0.050 0.50 0.025 0.004 0.008
0.005 15 ppm 0.025
B 0.30 0.50 0.20 0.043 0.50 0.025 0.001 0.006
0.005 15 ppm 0.025
Table 2
Table 2 gathers the coating parameters implemented on steels of table 1 to be
zo coated on the steels to become a hydrogen embrittlement resistant steel.
The Steel
compositions 11 to 16 serve for the manufacture of hydrogen embrittlement
resistant
steel according to the invention. This table also specifies the reference
steel which
are designated in table from R1 to R4. Before coating the Steels both
Inventive and
reference steels were hot rolled with hot rolled finishing temperature of 890
C and
then coiled at 620 C thereafter cod rolled with a reduction of 60%. The cold
rolled
steel is annealed at a temperature 880 C and then cooled to room temperature
to
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obtained annealed cold rolled steel sheet which is coated with a coating of
NiMoS2
according to the conditions mentioned in table 2 to obtain a hydrogen
embrittlement
resistant steel.
The table 2 is as follows:
Steel Current pH Temper Time,
NiSo4 MoS2
Samples Trials density ature, C sec
(g/1) (g/1)
(A/dm2)
A 11 300 5 20 3 50 180
A 12 300 5 25 4 55 180
A 13 300 8 30 5 45 180
B 14 300 5 20 3 50 180
B 15 300 5 25 4 55 180
B 16 300 8 30 5 45 180
A R1 300 5 10 4 60 180
A R2 300 8 10 6 50 180
B R3 300 5 10 4 60 180
B R4 300 8 10 6 50 180
i = according to the invention; R = reference; underlined values: not
according to the
invention.
Table 3 exemplifies the results of the tests conducted for clearly elucidating
the
inventive feature of the method of the present invention, wherein key
parameters of
the NiMoS2 layers were determined by measuring with SEM cross section, the
Concentration of MoS2 being measured by GDOES method. All trials
microstructure
was fully martensitic.
Table 3
Trials Thickness of MoS2
electroplated (wt%) in
layer ( m) Ni-MoS2
11 0.3 0.5
12 0.4 0.5
13 0.7 1.0
14 0.7 1.0
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15 0.7 1.0
16 0.7 1.0
R1 0.4 0
R2 0.7 0
R3 0.7 0
R4 0.7 0
1= according to the invention; R = reference; underlined values: not according
to the
invention.
Table 4 exemplifies the results of the tests conducted to demonstrate the
mechanical properties and the hydrogen embrittlement resistance properties are
measured in Hydrogen embrittlement ratio for the inventive and reference
steels in
accordance with method published in a journal publication titled as "Graphene
coating as a protective barrier against hydrogen embrittlement" in the
international
journal of hydrogen energy of 39(2014) from page number 11810 to 11817.
The results are stipulated herein:
Hydrogen
embrittlement ratio TS YS
Trials (cyo ) (MPa) (MPa)
11 0 1621 1355
12 0 1650 1358
13 0 1660 1360
14 19 1795 1560
20 1808 1555
16 19 1787 1546
R1 85 1370 1350
R2 85 1350 1350
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Hydrogen
embrittlement ratio TS YS
Trials (cyo ) (MPa) (MPa)
R3 43 1790 1545
R4 85 1570 1350
I = according to the invention; R = reference; underlined values: not
according to the
invention.
