Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Coated steel sheet and high strength press hardened steel part and method
of manufacturing the same
The present invention relates to coated steel sheets and to high strength
press hardened steel parts having good bendability properties.
High strength press-hardened parts can be used as structural elements in
automotive vehicles for anti-intrusion or energy absorption functions.
In such type of applications, it is desirable to produce steel parts that
combine high
mechanical strength, high impact resistance and good corrosion resistance.
Moreover, one of major challenges in the automotive industry is to decrease
the
weight of vehicles in order to improve their fuel efficiency in view of the
global
environmental conservation, without neglecting the safety requirements.
This weight reduction can be achieved in particular thanks to the use of steel
parts with a martensitic or bainitic/martensitic microstructure.
The publication W02016104881 relates to a hot press forming part used as a
structural part of a vehicle or the like, requiring impact resistance
characteristics,
and more particularly, having a tensile strength of 1300 MPa or greater and a
method for manufacturing the same by heating a steel material to a temperature
at
which an austenite single phase may be formed, and quenching and hot forming
thereof using a mold. To obtain such properties, the base steel sheet
comprises a
thin ferrite layer lower than 50 pm at the surface, and the carbides size and
density
should be controlled. This ferrite layer in the substrate allow to inhibit the
propagation of the fine cracks formed on the plating layer to the base but
leads to a
low bendability with bending angle lower than 70 .
The publication W02018179839 relates to a hot-pressed part obtained by hot
pressing a steel sheet having a microstructure changing in the thickness
direction,
with a soft layer made of at least 90% of ferrite, a transition layer made of
ferrite and
martensite and a hard layer mainly martensitic and has both high strength and
high
bendability. To obtain such properties, the cold rolled steel sheet is
annealed in an
atmosphere with a dew point temperature comprises from 50 C to 90 C, which
could
be harmful to aluminium alloy coating.
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The purpose of the invention therefore is to solve the above-mentioned
problem and to provide a press hardened steel part having a combination of
high
mechanical properties with the tensile strength TS above or equal to 1500 MPa
and
bending angle higher than 70 . Preferably, the press hardened steel part
according
to the invention has yield strength YS above or equal to 1250 MPa.
Another purpose of the invention is to obtain a coated steel sheet that can be
transformed by hot forming into such a press hardened steel part.
The object of the present invention is achieved by providing a steel sheet
according to claim 1. Another object is achieved by providing the method
according
to claim 2. Another object of the present invention is achieved by providing a
press
hardened steel part according to claim 3. The steel part can also comprise
characteristics of anyone of claims 4 to 6. Another object is achieved by
providing
the method according to claim 7.
The invention will now be described in detail and illustrated by examples
without introducing limitations, with reference to the appended figures:
- Figure la illustrates a schematic section of the coated steel sheet of
trial 4,
which is not according to the invention
- Figure lb represents a schematic section of the press hardened steel part
from trial 4 which is not according to the invention
- Figure 2a illustrates a schematic section of the coated steel sheet of
trial 3,
which is not according to the invention
- Figure 2b represents a schematic section of the press hardened steel part
from trial 3 which is not according to the invention
- Figure 3a illustrates a schematic section of the coated steel sheet of
trial 2,
which is according to the invention
- Figure 3b represents a schematic section of the press hardened steel part
from trial 2 which is according to the invention
- Figure 4a illustrates a schematic section of the coated steel sheet of
trial 1,
which is according to the invention
- Figure 4b represents a schematic section of the press hardened steel part
from trial 1 which is according to the invention
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- Figure 5a illustrates a schematic section of the coated steel sheet
of trial 5,
which is not according to the invention
- Figure 5b represents a schematic section of the press hardened steel part
from trial 5 which is not according to the invention
The composition of the steel according to the invention will now be described,
the content being expressed in weight percent.
According to the invention the carbon content is comprised from 0.26% to
0.40 % to ensure a satisfactory strength. Above 0.40% of carbon, weldability
and
bendability of the steel sheet may be reduced. If the carbon content is lower
than
0.26%, the tensile strength will not reach the targeted value.
The manganese content is comprised from 0.5% to 1.8 %. Above 1.8% of
addition, the risk of central segregation increases to the detriment of the
bendability.
Below 0.5% the hardenability of the steel sheet is reduced. Preferably the
manganese content is comprised from 0.5% to 1.3%.
According to the invention, silicon content is comprised from 0.1% to 1.25%.
Silicon is an element participating in the hardening in solid solution.
Silicon is added
to limit carbides formation. Above 1.25%, silicon oxides form at the surface,
which
impairs the coatability of the steel. Moreover, the weldability of the steel
sheet may
be reduced. Preferably, the silicon content is from 0.2% to 1.25%. More
preferably
the silicon content is from 0.3% to 1.25%. More preferably, the silicon
content is
from 0.3% to 1%.
The aluminium content is comprised from 0.01% and 0.1% as it is a very
effective element for deoxidizing the steel in the liquid phase during
elaboration.
Aluminium can protect boron if titanium content is not enough. The aluminium
content is lower than 0.1% to avoid oxidation problems and ferrite formation
during
press hardening. Preferably the aluminium content is comprised from 0.01% to
0.05%.
According to the invention, the chromium content is comprised from 0.1% to
1.0 %. Chromium is an element participating in the hardening in solid solution
and
must be higher than 0.1%. The chromium content is below 1.0% to limit
processability issues and cost.
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The titanium content is comprised from 0.01% to 0.1% in order to protect
boron from formation of BN. Titanium content is limited to 0.1% to avoid TiN
formation.
According to the invention, the boron content is comprised from 0.001% to
0.004%. Boron improves the hardenability of the steel. The boron content is
not
higher than 0.004% to avoid a risk of breaking the slab during continuous
casting.
Some elements can optionally be added.
Nickel may be added as an optional element up to 0.5% as it can substantially
reduce the sensitivity to delayed fracture.
Molybdenum content can optionally be added up to 0.40%. As boron,
molybdenum improves the hardenability of the steel. Molybdenum is not higher
than
0.40% to limit cost.
According to the invention, niobium can optionally be added up to 0.08% to
improve ductility of the steel. Above 0.08% of addition, the risk of formation
of NbC
or Nb(C,N) carbides increases to the detriment of the bendability. Preferably
the
niobium content is below or equal to 0.05%.
Calcium may be also added as an optional element up to 0.1%. Addition of
Ca at the liquid stage makes it possible to create fine oxides which promote
castability of continuous casting.
The remainder of the composition of the steel is iron and impurities resulting
from the smelting. In this respect, P, S and N at least are considered as
residual
elements which are unavoidable impurities. Their content is less than 0.010 %
for
S, less than 0.020 % for P and less than 0.010 % for N.
The microstructure of the coated steel sheet according to the invention will
now be described.
A section of a coated steel sheet of the invention is schematically
represented
on Fig 3a and Fig 4a. The coated steel sheet comprises a bulk (2) topped by a
decarburized layer (3) comprising in upper part a ferrite layer having a
thickness
from 1 m to 100 pm (4), and a coating layer (1). Preferably, the thickness of
the
ferrite layer is comprised from 20 pm to 100 m. More preferably, the
thickness of
the ferrite layer is from 25 pm to 100 m. More preferably the thickness of
the ferrite
layer is from 30 pm to 80 m.
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The bulk of the coated steel sheet (2) has a microstructure comprising, in
surface fraction, from 60% to 90% of ferrite, the rest being martensite-
austenite
islands, pearlite or bainite.
5 This
ferrite is formed during the intercritical annealing of the cold rolled steel
sheet. The rest of the microstructure is austenite at the end of the soaking,
which
transform into martensite-austenite islands, pearlite or bainite during the
cooling of
the steel sheet.
The decarburized layer present on top of the bulk is obtained during the
annealing of the cold rolled steel sheet thanks to the control of the
atmosphere in
the furnace to set a dew point temperature strictly higher than -10 C and
below or
equal to 20 C.
The coated steel sheet according to the invention can be produced by any
appropriate manufacturing method and the man skilled in the art can define
one. It
is however preferred to use the method according to the invention comprising
the
following steps:
A semi-product able to be further hot-rolled, is provided with the steel
composition described above. The semi product is reheated at a temperature
comprised from 1150 C to 1300 C.
The steel sheet is then hot rolled at a finish hot rolling temperature
comprised
from 800 C to 950 C.
The hot-rolled steel is then cooled and coiled at a temperature Tcon lower
than
670 C, and optionally pickled to remove oxidation.
The coiled steel sheet is then optionally cold rolled to obtain a cold rolled
steel
sheet. The cold-rolling reduction ratio is preferably comprised from 20% to
80%.
Below 20%, the recrystallization during subsequent heat-treatment is not
favored,
which may impair the ductility of the steel sheet. Above 80%, there is a risk
of edge
cracking during cold-rolling.
The steel sheet is then annealed in an HNx atmosphere with from 0% to 15%
of H2, to an annealing temperature TA comprised from 700 C to 850 C and
maintained at said annealing temperature TA for a holding time tA comprised
from
lOs to 1200s, in order to obtain an annealed steel sheet.. Below 700 C, the
kinetic
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of formation of the decarburized layer is too slow to obtain a ferrite layer
in its upper
part. The holding time tA is above or equal to 10 s to allow the ferrite layer
to form,
and below or equal to 1200s in order to limit the thickness of this ferrite
layer.
During this annealing, the atmosphere in the furnace is controlled to have a
dew point temperature TDP1 strictly higher than -10 C and below or equal to
+20 C
in order to form a decarburized layer according to the invention. If TDP1 is
below or
equal to -10 C, the formation of the decarburized layer is slowed down and the
ferrite
layer is not formed in its upper part. The bendability of the steel part will
be too low.
If TDP1 is higher than 20 C, the surface of the steel sheet may be completely
oxidized, impairing coatability and mechanical properties of the sheet
In an embodiment of the invention the annealed steel sheet is heated to an
annealing temperature T2 comprised from 700 C to 850 C and maintained at said
temperature T2 for a holding time t2 comprised from lOs to 1200s, the
atmosphere
having a dew point TDP2 strictly higher than -10 C and below or equal to +20
C.
The steel sheet is then coated with an aluminium alloy coating.
The microstructure of the press hardened steel part according to the invention
will now be described. A section of the press hardened steel part is
schematically
represented on Fig 3b and Fig 4b.
The steel part comprises successively from the bulk to the surface of the
steel
part:
- a bulk (7) having a microstructure comprising, in surface fraction, more
than 95% of martensite and less than 5% of bainite,
- a ferritic interdiffusion layer (6),
- a coating layer (5) based on aluminium.
During the heating of the steel blank cut out of the steel sheet according to
the invention, all microstructural elements of the bulk are transformed into
austenite,
and the ferrite of the decarburized layer is transformed into austenite with
wider grain
size than the austenite of the bulk. After hot forming, the steel part is then
die-
quenched. The interdiffusion layer grows from the former wide grain size
austenite
layer, thus having larger grain width than prior austenitic grain size in the
bulk. The
ratio between the ferritic grain width in the interdiffusion layer GWint over
prior
austenite grain size in the bulk PAGSbuik, satisfies following equation:
(GWint / PAGSbuik ) -1 30%
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in order to improve bendability of the steel sheet, without deteriorating
mechanical
properties.
The ferritic grain width is the average distance between two parallel grain
boundaries of the interdiffusion layer, grain boundaries being oriented in the
direction of the thickness of the sheet. The combination of annealing
temperature
TA, annealing time tA and dew point temperature TDP1 according to the
invention
promotes the formation of large grain width GWint in the interdiffusion layer.
Moreover, the thermal treatment of the steel blank before the press forming,
rules
the austenitic grain growth and so the FAGS in the bulk.
In an embodiment, the press hardened steel part may further comprise a
martensite layer with a carbon gradient between the bulk and the
interdiffusion layer,
as represented by (8) in Fig 4b. During the heating of the steel blank, carbon
diffuses
from the bulk to the surface. The ferrite upper part of the decarburized layer
is then
transformed in a layer of austenite with a gradient of carbon. During the die-
quenching, this layer of austenite with a gradient of carbon is transformed in
a layer
of martensite with a carbon gradient.
The press hardened steel part according to the invention has a tensile
strength TS above or equal to 1500 MPa and a bending angle higher than 700.
The
bending angle has been determined on press hardened parts according to the
method VDA238-100 bending Standard (with normalizing to a thickness of 1.5
mm).
In a preferred embodiment of the invention, the yield strength YS is above or
equal to 1250 MPa. TS and YS are measured according to ISO standard ISO 6892-
1.
The press hardened steel part according to the invention can be produced by
any appropriate manufacturing method and the man skilled in the art can define
one.
It is however preferred to use the method according to the invention
comprising the
following steps:
A coated steel sheet according to the invention is cut to a predetermined
shape to obtain a steel blank. The steel blank is then heated to a temperature
comprised from 880 C to 950 C during 1Os to 900s to obtain a heated steel
blank.
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The heated blank is then transferred to a forming press before being hot
formed and
die-quenched.
The invention will be now illustrated by the following examples, which are by
no way !imitative.
Example
6 grades, which compositions are gathered in table 1, were cast in semi-
products and processed into steel sheets, then steel parts, following the
process
parameters gathered in table 2.
Table 1 - Compositions
The tested compositions are gathered in the following table wherein the
element contents are expressed in weight percent.
Steel C Mn Si Al Cr Ti B P Ni
Mo Nb
A 0.34 0.6 0.56
0.043 0.32 0.017 0.002 0.009 0.0018 0.0027 0.43 0.19 0.052
B 0.32 0.7 0.35 0.03
0.5 0.026 0.002 0.014 0.0022 0.0059 0.028 0.21
C 0.36 0.6 0.53 0.035 0.34
0.014 0.003 0.011 0.001 0.0034 0.4 0.20 0.049
D 0.33 0.6 0.53 0.028 0.33
0.013 0.003 0.013 0.001 0.0041 0.4 0.17 0.046
E 0.36 1.3 0.28 0.003 0.25 0.02
0.003 0.01 0.002 0.006 -
002
0.21 1.2 0.27 0.018 0.15 0.037 0.002 0.01
0. 0.006 0.012
Steels A-D are according to the invention.
Underlined values: not corresponding to the invention
Table 2 - Process parameters
Steel semi-products, as cast, were reheated at 1200 C, hot rolled with a
finish hot rolling temperature comprised from 800 to 950 C, coiled at 550 C
and cold
rolled with a reduction rate of 60%. Steel sheets are then heated to a
temperature
TA and maintained at said temperature for a holding time tA, in an HNx
atmosphere
with 5% of H2, having a controlled dew point. The steel sheets were then
cooled
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down to a temperature from 560 to 700 C and then hot dip coated with an
aluminium-silicon coating comprising 10% of silicon.
Samples 1,2,5 and 6 did undergo a second annealing at a temperature T2
before coating, the steel sheet being maintained at said T2 temperature for a
holding
time t2, in an HNx atmosphere with 5% of H2 and a controlled dew point. The
following specific conditions were applied:
Trial Steel Soaking Second soaking
tA (s) TA ( C) TDP1 ( C) t2 (s) T2 ( C) TDP2 ( C)
1 A 135 850 10 70 800 0
2 B 135 850 10 70 800 0
3 C 240 755 -10 - - -
4 D 30 730 -40 - - -
5 E 10800 700 0 70 750 0
6 F 135 850 10 70 800 0
Underlined values: not corresponding to the invention
The coated steel sheets were analyzed, and the corresponding properties of
decarburized layer are gathered in table 3.
Table 3 ¨ Properties of the decarburized layer of the coated steel sheet
Trial Presence of Thickness of the
ferrite
decarburized layer upper part of the
decarburized layer (pm)
1 yes 60
2 yes 35
3 yes - _
4 no -
_
5 yes 140
6 yes 60
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Underlined values: not corresponding to the invention
The coated steel sheets were then cut to obtain a steel blank, heated at
5 900 C during 6 minutes and hot-formed. The steel parts were analyzed and the
corresponding microstructure, ferritic grain width in interdiffusion layer
GWint, and
prior austenite grain size in the bulk PAGSbuik are gathered in table 4.
Mechanical
properties are gathered in Table 5.
10 Table 4 ¨ Microstructure of the press hardened steel part
Trial Bulk (GWInt / PAGSbulk )-1 GWInt (tim) PAGSbuik
microstructure (illia)
1 100% martensite 83% 9.9 5.4
2 100% martensite 128% 16.9 7.4
3 100% martensite 279/ 6.6 5.2
4 100% martensite -2% 4.9 5
5 10% ferrite
70% bainite
20% martensite n.d 8.3 n.d
6 100% martensite 45% 8.9 7.8
Underlined values: not corresponding to the invention
n.d: non determined
The surface fractions, ferritic grain width in the interdiffusion layer and
FAGS
are determined through the following method: a specimen is cut from the press
hardened steel part, polished and etched with a reagent known per se, to
reveal the
microstructure. The section is afterwards examined through optical or scanning
electron microscope, for example with a Scanning Electron Microscope with a
Field
Emission Gun ("FEG-SEM") at a magnification greater than 5000x, coupled to a
BSE (Back Scattered Electron) device.
Table 5 ¨ Mechanical properties of the press hardened steel part
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Mechanical properties of the tested samples were determined and gathered
in the following table:
Trial TS (MPa) Bending angle at YS (MPa)
1.5mm ( )
1 1691 77 1361
2 1649 73 1296
3 1922 36 1488
4 1917 42 1527
5 1566 34 1183
6 1410 65 1060
Underlined values: do not match the targeted values
The examples show that the steel parts according to the invention, namely
examples 1-2 are the only one to show all the targeted properties thanks to
their
specific compositions and microstructures.
Figure 3a represents a schematic section of the coated steel sheet of trial 2.
The combination of process parameters of the invention, annealing temperature
TA,
annealing time tA and dew point temperature TDP1 allow to obtain a
decarburized
layer (3), in which a layer of ferrite is formed in the upper part (4).
The coated steel sheet is then hot formed. Fig 3b represents a schematic
section of the press hardened steel part of trial 2.
The grain width of ferrite formed in the interdiffusion layer (5) is a
heritage of
the pure ferrite layer in which austenite formation takes place during
heating, with
larger grain size. The interdiffusion layer grows on this large austenite
grain size.
The grain width of ferrite in the interdiffusion layer (6) is then larger than
prior
austenite grain size in the bulk (7), leading to good bendability with bending
angle
higher than 700
.
Figure 4a represents a schematic section of the coated steel sheet of trial 1.
The
combination of process parameters of the invention, annealing temperature TA,
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annealing time tA and dew point temperature TDP1 allow to obtain a
decarburized
layer (3), in which a layer of ferrite is formed in the upper part (4),
thicker than in trial
1 due to the higher C content.
The coated steel sheet is then hot formed. Fig 4b represents a schematic
.. section of the press hardened steel part of trial 1.
The grain width of ferrite formed in the interdiffusion layer (6) is a
heritage of
the pure ferrite layer in which austenite formation takes place during
heating, with
larger grain size. The interdiffusion layer grows on this large austenite
grain size .
The grain width of ferrite in the interdiffusion layer (6) is then larger than
prior
.. austenite grain size in the bulk (7), leading to good bendability with
bending angle
higher than 700. Moreover, due to the thick ferrite layer (4) in the coated
steel sheet,
a layer of martensite with a carbon gradient is formed between the bulk and
the
interdiffusion layer in the press hardened steel part, leading to tensile
strength higher
than 1500 MPa.
In trial 3, the coated steel sheet has a decarburized layer, without ferrite
layer in its
upper part, as represented schematically in Fig 2a. The absence of ferrite
layer is
due to the low dew point temperature TDP1 of -10 C, which slow down the
kinetics
of the decarburization.
The coated steel sheet is then hot formed. Figure 2b represents a schematic
section of the press hardened steel part from trial 3. Due to the absence of
the ferrite
layer, the ferritic grain width in the interdiffusion layer (6) is then
equivalent to prior
austenite grain size in the bulk (7), leading to a low bending angle below 70
.
.. In trial 4, the low dew point temperature TDP1 of -40 C implies an absence
of the
decarburized layer and ferrite layer in the coated steel sheet.
Fig la represents a schematic section of the coated steel sheet of this trial,
with the
coating layer (1) and the bulk (2).
The coated steel sheet is then hot formed. Figure lb represents a schematic
section
.. of the press hardened steel part from trial 4. Due to the absence of the
ferrite layer,
the ferritic grain width in the interdiffusion layer (6) is then equivalent to
prior
austenite grain size in the bulk (7), leading to a low bending angle below 70
.
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In trial 5, the steel sheet is maintained during 10800s at soaking
temperature, which
form in the coated steel sheet, a thicker ferrite layer in the decarburized
layer than
previous trials. Fig 5a represents a schematic section of the coated steel
sheet of
trial 5, with the coating layer (1) the decarburized layer (3), the thicker
ferrite layer
.. (4) with coarser grain size, and the bulk (2).
The coated steel sheet is then hot formed and Fig 5b represents a schematic
section
of the press hardened steel part from trial 5. During the heating of the steel
part, the
microstructure of the bulk is austenitic, and the thick ferrite layer is
transformed in a
layer of austenite with gradient of carbon. But due to the thickness of the
ferrite
layer higher than 100 m, a layer of ferrite remains present between the
interdiffusion layer and the layer of austenite with gradient of carbon.
During die quenching of the steel part, the ferrite layer is still present and
the layer
of austenite with carbon gradient transforms into a martensite layer with
gradient of
carbon, leading to a multi-phased layer. This triggers a decrease of yield
strength.
In trial 6, the steel sheet has a low carbon level of 0.21%. This low carbon
content
combined to the process parameters, leads to a decarburized layer in the
coated
steel sheet with the ferrite layer. Nevertheless, the yield strength and
tensile strength
of the press hardened steel part are not achieved because of the low level of
carbon.
