Note: Descriptions are shown in the official language in which they were submitted.
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Low density cold rolled and annealed steel sheet, method of production
thereof and use of such steel to produce vehicle parts
The present invention deals with a low density steel sheet and in particular a
duplex microstructure. The steel sheet according to the invention is
particularly well
suited for the manufacture of inner or outer panels for vehicles such as land
motor
vehicles.
Environmental restrictions are forcing automakers to continuously reduce the
CO2 emissions of their vehicles. To do that, automakers have several options,
whereby their principal options are to reduce the weight of the vehicles or to
improve
the efficiency of their engine systems. Advances are frequently achieved by a
combination of the two approaches. This invention relates to the first option,
namely
the reduction of the weight of the motor vehicles. In this very specific
field, there is
a two-track alternative:
The first track consists of reducing the thicknesses of the steels while
increasing their levels of mechanical strength. Unfortunately, this solution
has its
limits on account of a prohibitive decrease in the rigidity of certain
automotive parts
and the appearance of acoustical problems that create uncomfortable conditions
for
the passenger, not to mention the unavoidable loss of ductility associated
with the
increase in mechanical strength.
The second track consists of reducing the density of the steels by alloying
them with other, lighter metals. Among these alloys, the low-density ones have
attractive mechanical and physical properties while making it possible to
significantly
reduce the weight.
In particular, EP3421629 is patent that claims for a high strength cold-rolled
and heat-treated steel strip, sheet, blank or hot formed product having a
bimodal
microstructure comprising the steps of producing and casting a melt into a
slab or
cast strip having the following composition;0.05 to 0.50 wt% C;0.50 to 8.0
wt.% Mn;
0.05 - 6.0 wt.% Al_tot; 0.0001 - 0.05 wt.% Sb; 0.0005 - 0.005 wt.% of (Ca +
REM);
5 - 100 ppm N; 0 -2.0 wt.% Si; 0 - 0.01 wt.% S; 0 -0.1 wt. % P; 0 - 1.0 wt.%
Cr; 0 -
2.0 wt.% Ni;0 -2.0 wt.% Cu; 0- 0.5 wt.% Mo; 0- 0.1 wt.% V; 0-50 ppm B; 0- 0.10
wt.% Ti wherein the component has a bimodal grain microstructure consisting of
a
ferritic matrix phase consisting of delta-ferrite and alpha-ferrite, wherein
the delta-
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ferrite has a grain size of between 5 and 20 m, wherein the alpha-ferrite has
a
grainsize at most 3 pm and a second phase consisting of one or more of or
bainite,
martensite and retained austenite with a grain size of at most 3 m. But the
steel of
EP3421629 does not demonstrate the low density steel as well as contains hard
faces such as Martensite and Bainite.
The purpose of the invention therefore is to provide a steel sheet presenting
a relative density below 7.3, an ultimate tensile strength of at least 600
MPa, and an
uniform elongation of at least 17.5%.
In a preferred embodiment, the steel sheet according to the invention
presents a relative density equal or below 7.2, a yield strength of at least
450 MPa.
Other characteristics and advantages of the invention will become apparent
from the following detailed description of the invention.
Carbon content is from 0.12% to 0.25%, more preferably from 0.13% to 0.2%
by weight. Carbon is a Gamagenous element which plays a significant role in
the
formation of residual austenite and also imparts the strength and ductility.
The
carbon content is advantageously from 0.13% to 0.2% to obtain simultaneously
high
strength, elongation and stretch flangeability.
Manganese content is present from 3% to 10% by weight. Manganese is an
important alloying element in this system, mainly due to the fact that
alloying with
very high amounts of manganese stabilizes the austenite down to room
temperature, which can assist in reaching the target properties such as
elongation
and yield strength. Manganese, along with Carbon, control the formation of
carbides
at grain boundaries at high temperature and thereby controls the hot
shortness. If
the Manganese is present above 10% it may lead to central segregation which is
detrimental for the ductility of the steel of present invention. Manganese
when
present below 3% will not stabilize the residual austenite at room temperature
in an
adequate amount. Preferred limit for the presence of Manganese is from 4% to
9%
and more preferably from 4% to 8%.
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Aluminum content is present from 3.5% to 6.5% by weight. Aluminum
addition to the steel of present invention effectively decreases its density.
Aluminum
is an alphagenous element and therefore tends to promote the formation of
ferrite
and in particular of delta ferrite. The aluminum has a relative density of 2.7
and has
an influence on the mechanical properties. As the aluminum content increases,
the
mechanical strength and the elastic limit also increase although the uniform
elongation decreases, due to the decrease in the mobility of dislocations.
Below
3.5%, the density reduction due to the presence of aluminum becomes less
beneficial. Above 6.5%, the presence of ferrite increases beyond the expected
limit
and affects the present invention negatively. Moreover the presence of Al
above
6.5% may forms intermetallics such as Fe-Al, Fe3-Al and other (Fe,Mn)AI
intermetallics which will impart brittleness to the product that can cause
cracking of
the steel during cold rolling and may also be detrimental for the toughness of
the
steel. Preferably, the aluminum content will be limited to strictly less than
6.5% to
prevent the formation of brittle intermetallic precipitation, hence the
preferred limit is
from 4% to 6% and more preferably from 5% to 6%.
Silicon is an optional element that makes it possible to reduce the density of
the steel, and effective in solid solution hardening. Nevertheless, its
content is
limited to 2% by weight because above that level this element has a tendency
to
form strongly adhesive oxides that generate surface defects. The presence of
surface oxides impairs the wettability of the steel and may produces defects
during
a potential hot-dip galvanizing operation. Therefore, the Si content will
preferably be
limited below 1.5%.
Sulfur and phosphorus are impurities that embrittle the grain boundaries.
Their respective contents must not exceed 0.03% and 0.1% by weight so as to
maintain sufficient hot ductility.
Nitrogen content must be 0.1% or less by weight so as to prevent the
precipitation of AIN and the formation of volume defects (blisters) during
solidification.
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Niobium may be added as an optional element in an amount of 0.01% to 0.03%
by weight to the steel of present invention to provide grain refinement. The
grain
refinement allows obtaining a good balance between strength and elongation.
But,
niobium had a tendency to retard the recrystallization during hot rolling and
annealing hence the limit is kept till 0.03%.
Titanium may be added as an optional element in an amount of 0.01% to
0.2% by weight to the steel of present invention for grain refinement, in a
similar
manner as niobium.
Copper may be added as an optional element in an amount of 0.01% to 2.0%
by weight to increase the strength of the steel and to improve its corrosion
resistance. A minimum of 0.01% is required to get such effects. However, when
its
content is above 2.0%, it can degrade the surface aspect.
Nickel may be added as an optional element in an amount of 0.01 to 3.0% by
weight 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
3.0%,
nickel causes ductility deterioration.
Molybdenum is an optional element that is present from 0% to 0.5% by weight
in the steel of present invention; Molybdenum plays an effective role in
improving
hardenability and hardness, when added in an amount of at least 0.01%. Mo is
also
beneficial for the toughness of the hot rolled product resulting to an easier
manufacturing. 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 0.5%. The preferable limit for Molybdenum is from 0% to 0.4% and
more
preferably from 0 % to 0.3%.
Chromium is an optional element of the steel of present invention, is from 0%
to 0.6% by weight. Chromium provides strength and hardening to the steel, but
when
used above 0.5 % impairs surface finish of the steel. The preferred limit for
chromium is from 0.01% to 0.5% and more preferably from 0.01% to 0.2%.
Other elements such as cerium, boron, magnesium or zirconium can be
added individually or in combination in the following proportions by weight:
Ce
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0.1%, B0.01, Ca0.005, Mg 0.005 and Zr0.005. Up to the maximum content
levels indicated, these elements make it possible to refine the ferrite grain
during
solidification.
Additionally some trace elements such as Sb, Sn can come from processing
5 of the steel. The maximum limit up to which these elements are acceptable
and are
not detrimental for the steel of present invention is 0.05% by weight
cumulatively or
alone, It is preferred by the steel of present invention to have the content
of these
elements as low as possible and preferably less than 0.03%.
The microstructure of the steel sheet according to the invention comprises,
in area fractions, delta ferrite from 60% to 90%, alpha ferrite from 1% to 10%
and
residual austenite from 8% to 30% and optionally from 0% to 2% kappa
precipitates.
The delta ferrite matrix is present as a primary phase of the steel of the
present invention and is present from 60% to 90% by area fraction in the steel
of the
present invention and preferably from 65% to 90% by area fraction and more
preferably from 80% to 90%. Delta ferrite is formed during the solidification
of the
slab from liquid iron and has generally a coarse grain size. The delta ferrite
of the
present invention preferably has an average grain size less than 10 pm and
more
preferably less than 9 m. The presence of the delta ferrite matrix in the
present
invention imparts the steel with strength. But the presence of delta ferrite
content in
present invention above 90% may have negative impacts due to the fact that
with
the rise in temperature solubility of carbon increases in ferrite. However,
carbon in
solid solution is highly embrittling for low-density steels because it reduces
the
mobility of dislocations, which is already low on account of the presence of
aluminum. Hence a balance between delta ferrite content and austenite, is very
important to impart the present invention with requisite mechanical
properties.
Residual Austenite is present in the steel of present invention from 8 to 30%
wherein the Residual Austenite of the present invention has an average grain
size
from 0.6 micron to 2 microns. The preferred average grain size of residual
austenite
is between 0.6 micron to 1.2 microns. Residual Austenite is known to have a
higher
solubility of carbon than ferrite and acts as effective Carbon trap. The
Carbon
percentage in Austenite is from 0.7% to 1.5% in weight. Austenite present at a
level
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above 30% produces a negative impact on the present invention by impairing the
stretch flangeability. Austenite contributes to the present invention in a
very versatile
manner depending upon the choice of the temperature of annealing and
composition of steel, Austenite of the present invention depicts diverse
functionalities such as providing formability and ductility due to TRIP
effect. The
preferable limit for the Residual Austenite is from 9% to 29% in area
fraction.
Alpha-Ferrite of the present invention is present from 1% to 10% in area
fraction. Alpha ferrite is generated by partial transformation of the
austenite during
cooling after hot rolling and after intercritical annealing and has an average
grain
size from 0.6 micron to 1.85 microns. The preferred average grain size of
alpha-
ferrite is from 0.6 micron to 1.2 microns. The alpha ferrite of the present
invention
imparts the present steel with ductility and elongation. The preferred limit
for Alpha-
ferrite is from 2% to 10% in area fraction.
Kappa precipitates in the invention is defined by precipitates whose
stoichiometry is (Fe,Mn)3A1Cx, where x is strictly lower than 1. The area
fraction of
Kappa precipitates can go up to 2%. Above 2%, the ductility decreases and
uniform
elongation above 17.5% is not achieved. In addition, uncontrolled
precipitation of
Kappa around the ferrite grain boundaries may occur, increasing, therefore,
the
efforts during hot and/or cold rolling. Preferentially, the area fraction of
Kappa
precipitates should be less than 1%.
In addition to the above-mentioned microstructure, the microstructure of the
low density cold rolled and annealed steel is free from microstructural
components,
such as Pearlite, Bainite and Martensite.
The 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, which
comprises
the following steps:
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The steel sheets according to the present invention are preferably produced
through a method in which a semi product, such as slabs, thin slabs, or strip
made
of a steel according to the present invention having the composition described
above, is cast, the cast input stock first to cooled to room temperature and
then
reheated to a temperature above 1000 C, preferably above 1150 C and more
preferably above1200 C or the casted semi-finished product can be used
directly
at such a temperature without intermediate cooling. The semi-finished product
for
the present process is considered as a slab.
The reheated slabs are then undergoing hot rolling. The hot-rolling finishing
temperature must be above 750 C and preferably above 770 C.
After the hot-rolling, the strip must be coiled at a temperature below 720 C
and preferably from 350 C to 720 C and more preferably the coiling is
performed
from 700 C to 400 C.
The hot rolled steel strip is cooled to room temperature and then pickling is
performed or any other scale removal process is performed.
Then the hot-rolled steel strip is subjected to cold-rolling with a reduction
rate
between 30% and 90%, preferably between 40% and 90%.
After the cold rolling, the cold rolled steel sheet is annealed by heating the
sheet
up to an annealing temperature comprised from 840 C to 1000 C and preferably
from 850 C to 975 C and more preferably from 850 C to 925 C with a heating
rate
of at least 1 C/s and preferably more than 3 C/s , holding it at such
annealing
temperature during less than 1000 seconds and preferably less than 600 seconds
and cooling it at a rate of at least 3 C/s, more preferably of at least 5 C/s
and even
more preferably of at least 10 C/s. Preferably, this annealing is carried out
continuously.
By controlling the annealing temperature and time, a two-phase structure can
be obtained during the soaking.
After such annealing step, the steel sheet is cooled to a temperature between
room temperature and 480 C and can be optionally held from 100 C to 480 C to
be
overaged during 1 hour or less and preferably less than 20 minutes and more
preferably less than 10 minutes. Thereafter it can be cooled to room
temperature.
After annealing, the steel sheet may optionally be submitted to a metallic
coating operation to improve its protection against corrosion. The coating
process
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used can be any process adapted to the steel of the invention. Electrolytic or
physical vapor deposition can be cited, with a particular emphasis on Jet
Vapor
Deposition. The metallic coating can be based on zinc or on aluminium, for
example.
Preferably, the aluminum-based coating comprises less than 15% Si, less
than 5.0% Fe, optionally 0.1% to 8.0% Mg and optionally 0.1% to 30.0% Zn, the
remainder being Al.
Advantageously, the zinc-based coating comprises 0.01-8.0% Al, optionally
0.2-8.0% Mg, the remainder being Zn.
Examples
The following tests, examples, figurative exemplification and tables which are
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 sheets made of steels with different compositions are gathered in Table
1 wherein the presence of Phosphorus is always less than 100ppm for all the
steels,
where the steel sheets are produced according to process parameters as
stipulated
in Table 2, respectively. Thereafter Table 3 gathers the microstructures of
the steel
sheets obtained during the trials and table 4 gathers the result of
evaluations of
obtained properties.
Table 1 - Compositions
Table one shows
Trials C Mn Al P S N Si
1 0.15 4.2 5.6 0.01 0.01 0.001 0
2 0.15 4.0 6.0 0.01 0.01 0.001 0
3 0.15 4.1 5.95 0.01 0.001 0.001 0
4 0.15 4.0 6.0 0.01 0.01 0.001 0.15
5 0.15 4.0 6.0 0.01 0.01 0.001 0
6 0.15 4.1 6.8 0.01 0.001 0.001 0
underlined values: not according to the invention.
Table 2 ¨ Process parameters
0
t..)
The inventive steels and the reference steels are reheated at 1200 C and are
air cooled with cooling rate as 9 C/s after hot rolling =
t..)
t..)
till the coiling temperature.
t..)
(...)
t..)
hot rolling Heating rate
Holding Cooling
coiling cold rolling Annealing
Time for
finish for time for rate after
Trials temperature reduction temperature
TOA ( C) overaging
temperature Annealing
annealing annealing
( C) (07) ( C)
(s)
( C) ( C/s) (s) ( C/s)
11 1 777 650 76 10 900 136
25
12 2 784 650 76 10 900 136
25 P
13 3 781 650 76 10 900 136
23 250 75 0
0
14 4 784 650 76 10 900 136
25 .3
R1 5 790 650 76 10 830 136
25 0
,
0
R2 fi 767 650 76 10 900 136
25
iL
od
n
1-i
5
,..,
=
,..,
=
-a
c,
-,
-,
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The resulting samples were then analyzed and the corresponding
microstructure elements and mechanical properties were respectively gathered
in
table 3 and 4.
Table 3 gathers the results of test conducted in accordance of standards on
5 different microscopes such as EBSD, XRD or any other microscope for
determining
microstructural composition of both the inventive steel and reference trials.
The area
fractions Delta ferrite and Alpha-Ferrite are measured using EBSD. For a given
steel
sample, an EBSD analysis of at least 4 images corresponding to a magnification
of
1000 allows to identify the ferrite grains, their location and size. All
grains which
10 grain size is below the cut-off value of 1.85 pm and are adjacent to
austenite grains
are counted as alpha ferrite and the corresponding area fraction of such
grains is
determined. The remaining ferrite grains are counted as delta ferrite and the
corresponding area fraction of such grains is determined. The average grain
sizes
of Delta Ferrite, Residual Austenite and Alpha-Ferrite are also measured by
using
EBSD. The Residual Austenite area fraction is measured using XRD which are
demonstrated in table 3.
Table 3
Average Average Average
Grain Grain Grain
Delta Residual
Alpha Size of Size of
Size of
Trials Ferrite Austenite
Ferrite ( /o) Delta alpha Residual
(0/0) (0/0)
Ferrite Ferrite Austenite
(rim) (rim) (rim)
11 85.6 3.6 10.8 3.17 0.80 0.80
12 83.6 1.4 18.0 5.06 0.82 0.77
13 86.1 1.8 12.1 6.27 0.72 0.84
14 82.1 3.8 14.1 7.30 0.80 0.78
R1 79.4 5.3 15.4 4.97 0.50 0.49
R2 92.0 0.7 7.3 19.07 0.70 0.81
I = according to the invention; R = reference; underlined values: not
according
to the invention.
It can be seen from the table above that the trials according to the invention
all meet the microstructure targets.
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Table 4 gathers the mechanical and surface properties of both the inventive
steel and reference steel.
Table 4 : mechanical properties of the trials
The yield strength YS, the tensile strength TS and the Uniform elongation UE
are measured according to ISO standard ISO 6892-1, published in October 2009.
To determine the relative density of the steel, the volume of a steel sample
is
measured by Gas Displacement Pycnometry using helium on one side and its
corresponding mass is measured on another side. The mass per volume ratio of
the
steel in g/cm3 can then by calculated and further divided by the mass per
volume
ratio of water at 4 C which amounts to 1g/cm3. The resulting value, which is
without
a unit, is the relative density of the steel.
YS (in TS (in Relative
Trials M Pa) MPa) UE(in %) density
II 463 673 24.5 7.20
12 507 652 22.1 7.19
13 475 643 21.8 7.19
14 493 665 20.4 7.19
RI 509 644 17.0 7.19
R2 511 664 14.0 7.13
I = according to the invention; R = reference; underlined values: not
according
to the invention.
The examples show that the steel sheets according to the invention are the
only one to show all the targeted properties thanks to their specific
composition and
microstructures.
