Note: Descriptions are shown in the official language in which they were submitted.
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HEAT TREATED COLD ROLLED STEEL SHEET AND A METHOD OF
MANUFACTURING THEREOF
The present invention relates to cold rolled steel sheet with high strength
and high formability having tensile strength of 980 MPa or more and a total
elongation of more than 14% which is suitable for use as a steel sheet for
vehicles.
Automotive parts are required to satisfy two inconsistent necessities, viz.
ease of forming and high strength but in recent years a third requirement of
improvement in fuel consumption is also bestowed upon automobiles in view of
global environment concerns. Thus, now automotive parts must be made of
material having high formability in order that to fit in the criteria of ease
of fit in the
intricate automobile assembly and at same time have to improve strength for
vehicle crashworthiness and durability while reducing weight of vehicle to
improve
fuel efficiency.
Therefore, intense Research and development endeavors are put in to
reduce the amount of material utilized in car by increasing the strength of
material.
Conversely, an increase in strength of steel sheets decreases formability, and
thus
development of materials having both high strength and high formability is
necessitated.
Earlier research and developments in the field of high strength and high
zo formability steel sheets have resulted in several methods for producing
high
strength and high formability steel sheets, some of which are enumerated
herein
for conclusive appreciation of the present invention:
US 9074 272 describes steels that have the chemical composition: 0.1-
0.28% C, 1.0-2.0% Si, 1.0-3.0% Mn and the remainder consisting of iron and the
inevitable impurities. The microstructure includes residual austenite between
5 to
20%, bainitic ferrite 40 to 65%, polygonal ferrite 30 to 50% and less than 5%
martensite. US 9074 272 refers to a cold rolled steel sheet with excellent
elongation
but the invention described in it fails to achieve the strength of 900 MPa
which is a
mandate for reducing the weight while keeping the complex automotive part
robust.
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The known prior art related to the manufacture of high strength and high
formability steel sheets is inflicted by one or the other lacuna : hence there
lies a
need for a cold rolled steel sheet having high strength and high formability
and a
method of manufacturing the same.
The purpose of the present invention is to solve these problems by making
available cold-rolled steel sheets that simultaneously have:
- an ultimate tensile strength greater than or equal to 950 MPa and
preferably
above 980 or even above 1000 MPa,
- a total elongation greater than or equal to 14% and preferably greater
than
or equal to 15%.
In a preferred embodiment, the steel sheet according to the invention may have
a yield strength value greater than or above 540 MPa or even better above 550
MPa.
Preferably, such steel can also have a good suitability for forming, in
particular for rolling with good weldability and coatability.
Another objective of the present invention is also to make available a
method for the manufacturing of these sheets that is compatible with
conventional
industrial applications while being robust towards manufacturing parameters
shifts.
Other characteristics and advantages of the invention will become apparent
zo from the following detailed description of the invention.
Carbon is present in the steel from 0.05% to 0.15%. Carbon is an element
necessary for increasing the strength of a steel sheet by producing a low-
temperature transformation phase such as martensite. Further carbon also plays
a
pivotal role in austenite stabilization. A content less than 0.05% would not
securing
the formation of martensite, thereby decreasing strength. On the other hand,
at a
carbon content exceeding 0.15%, a weld zone and a heat-affected zone are
significantly hardened, and thus the mechanical properties of the weld zone
are
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impaired. Hence the preferable limit is from 0.07% to 0.12% and more
preferably
from 0.08% to 0.11%.
Manganese content of the steel of present invention is from 1.8% to 2.7%.
Manganese is an element that imparts strength to the steel by solid solution
strengthening. An amount of at least about 1.8 % by weight of manganese is
needed in order to provide the strength and hardenability of the steel sheet
as well
as to form ferrite. Thus, a higher percentage of Manganese such as 1.9% to
2.5%
is preferred and more preferably 2.1% to 2.5%. But when manganese is more than
2.7%, this produces adverse effects such as slowing down the transformation of
austenite during cooling after annealing, leading to a reduction of ductility.
Moreover, a manganese content above 2.7% would also reduce the weldability of
the present steel.
Silicon content of the steel of present invention is from 0.1% to 1%. Silicon
imparts the strength to the steel of present invention by solid solution
strengthening. Silicon promotes the ferrite transformation. However, adding
more
than 1% of silicon does not improve the mentioned effect and leads to problems
such as hot rolling embrittlement. Therefore, the concentration is controlled
within
an upper limit of 1%. A preferable limit for the presence of silicon is kept
from 0.2%
to 0.9% and more preferably from 0.3% to 0.7%.
The content of aluminum of the steel of the present invention is from 0.01 to
0.8%. Within such range, aluminum bounds nitrogen in the steel to form
aluminum
nitride so as to reduce the size of the grains. But, whenever the content of
aluminum exceeds 0.8% in the present invention, it will increase the Ac3
point,
thereby lowering the productivity. Hence the preferable range for aluminum is
kept
from 0.01% to 0.7% and more preferably from 0.01% to 0.6%.
In a preferred embodiment, the cumulated amounts of Silicon and Aluminum
is at least 0.6% because both the elements are ferrite phase-generating
element,
thereby participating to the formation of ferrite that is favorable for both
the
elongation and ductility.
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Chromium content of the steel of present invention is from 0.1% to 0.9%.
Chromium is an essential element that provide strength and hardening to the
steel,
but when used above 0.9% impairs surface finish of the steel. Hence to achieve
the effects of chromium optimally the preferred limit is between 0.2% and 0.8%
and
more preferably from 0.2% to 0.7%.
Titanium is an essential element which may be added to the steel of the
present invention from 0.0001% to 0.1% and preferably from 0.01% to 0.08%,
Similarly to niobium, it is involved in carbo-nitrides so plays a role in
hardening. But
it is also involved to form TiN appearing during solidification of the cast
product.
The amount of Ti is so limited to 0.1% to avoid coarse TiN detrimental for
hole
expansion. In case the titanium content is below 0.0001% it does not impart
any
effect on the steel of present invention.
Boron is an essential element for the present invention and is added in very
small amount and is added from 0.0005% to 0.003%. Boron imparts hardenability
and strength to the steel of present invention. However, when boron is added
more
than 0.003% the rollability of the steel sheet is found to be significantly
lowered.
Further boron the segregation may happen at grain boundaries which is
detrimental for the formability.
Niobium is an essential element that can be added to the steel from 0.01%
zo to 0.1%, preferably from 0.01% to 0.06%. It is suitable for forming
carbonitrides to
impart strength to the steel according to the invention by precipitation
hardening.
Because niobium delays the recrystallization during the heating, the
microstructure
formed at the end of the holding temperature and as a consequence after the
complete annealing is finer, this leads to the hardening of the product. But,
when
the niobium content is above 0.1% the amount of carbo-nitrides is not
favorable for
the present invention as large amount of carbo-nitrides tend to reduce the
ductility
of the steel.
Vanadium is an optional element which may be added to the steel of the
present invention up to 0.2%, preferably from 0.001% to 0.01%. As niobium, it
is
involved in carbo-nitrides so plays a role in hardening. But it is also
involved to form
VN appearing during solidification of the cast product. The amount of V is so
limited
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to 0.2% to avoid coarse VN detrimental for hole expansion. In case the
vanadium
content is below 0.001% it does not impart any effect on the steel of present
invention.
Phosphorus content of the steel of present invention is limited to 0.09%.
Phosphorus is an element which hardens in solid solution and also interferes
with
formation of carbides. Therefore, a small amount of phosphorus, of at least
0.002%
can be advantageous, but phosphorus has its adverse effects also, such as a
reduction of the spot weldability and the hot ductility, particularly due to
its tendency
to segregation at the grain boundaries or co-segregation with manganese. For
these reasons, its content is preferably limited a maximum of 0.02%.
Sulfur is not an essential element but may be contained as an impurity in
steel. The sulfur content is preferably as low as possible but is 0.09% or
less and
preferably less than 0.03%, from the viewpoint of manufacturing cost. Further
if
higher sulfur is present in steel it combines to form sulfide especially with
Mn and
Ti and reduces their beneficial impact on the present invention.
Nitrogen is limited to 0.09% to avoid ageing of material and to minimize the
precipitation of Aluminum nitrides during solidification which are detrimental
for
mechanical properties of the steel.
Molybdenum is an optional element that constitutes from 0% to 0.2% of the
zo Steel of present invention; Molybdenum improves hardenability and
hardness,
delays the appearance of Bainite hence promote the formation of Martensite,
when
added in an amount of at least 0.01%. Molybdenum also facilitate the formation
of
Ferrite. 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
.. 0.2%. The preferable limit for Molybdenum is from 0.01% to 0.2%
Nickel may be added as an optional element in an amount of 0% to 2% to
increase the strength of the steel present invention and to improve its
toughness.
A minimum of 0.01% is preferred to get such effects. However, when its content
is
above 2%, Nickel causes ductility deterioration.
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Copper may be added as an optional element in an amount of 0% to 2% to
increase the strength of the of Steel of present invention and to improve its
corrosion resistance. A minimum of 0.01% is preferred to get such effects.
However, when its content is above 2%, it can degrade the surface aspects.
Calcium is an optional element which may be added to the steel of present
invention up to 0.005%, preferably from 0.0001% 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 sulphur content in globularizing it.
lo Other elements such as cerium, magnesium or zirconium can be added
individually or in combination in the following proportions: Ce < 0.1%, Mg <
0.05%
and Zr < 0.05%. 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 steel sheet according to the invention comprises
in area fraction, 40% to 60% of martensite, 5% to 40% of Inter-critical
Ferrite, a
cumulated amount of 10 to 35% of transformed ferrite and bainite and 0% to 5%
of
residual austenite.
Martensite constitutes 40% to 60% of microstructure by area fraction.
Martensite can notably be formed during the cooling after annealing and
particularly after crossing the Ms temperature and particularly between Ms-10
C
and 20 C or during cooling after overaging. Martensite imparts strength to the
.. present invention. Preferable limit for Martensite is between 42% and 58%
and
more preferably between 43% and 56%.
Inter-critical ferrite constitutes between 15% and 40% of microstructure by
area fraction of the steel of present invention. This inter-critical ferrite
imparts the
.. steel of present invention with a total elongation of at least 14%. The
intercritical
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ferrite results from the annealing at a temperature below Ac3. The
intercritical
ferrite is different from the ferrite that could be created after the
annealing, named
hereinafter "transformed ferrite", that will be described below. Contrarily to
the
transformed ferrite, the intercritical ferrite is polygonal. Besides, the
transformed
ferrite is enriched in carbon and manganese, i.e. has carbon and manganese
contents which are higher than the carbon and manganese contents of the
intercritical ferrite. The intercritical ferrite and the transformed ferrite
can therefore
be differentiated by observing a micrograph with a SEM microscope using
secondary electrons, after etching with 2% Nital etching agent. On such
micrograph, the intercritical ferrite appears in medium grey, whereas the
transformed ferrite appears in dark grey, owing to its higher carbon and
manganese
contents. It is preferable to have inter-critical ferrite from 20% to 40% and
more
preferably from 25% to 38%.
The total amount of transformed ferrite and bainite constitutes from 10% to
35% of microstructure by area fraction for the steel of present invention.
Transformed Ferrite of present invention constitutes of Ferrite formed during
the
cooling after annealing and the steel according to the present invention
always
contains transformed ferrite that is the presence of transformed ferrite is
always
more than 0%. Transformed Ferrite imparts high strength as well as elongation
to
zo the steel of present invention. Transformed Ferrite of the present steel
is rich in
carbon and Manganese as compared to the inter-critical ferrite and it is
mandatory
to have transformed ferrite in the steel. Bainite forms during the averaging
holding
especially between 400 C and 480 C. To ensure an elongation of 14% it is
necessary to have 10% of transformed ferrite and bainite. But whenever the
total
amount is present above 35% in steel of present invention it is not possible
to have
both tensile strength and the total elongation at the same time. The preferred
limit
for transformed ferrite and bainite for the present invention is between 15%
and
30%.
Residual Austenite is an optional microstructure and can be present
between 0% and 5% in the steel.
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In addition to the above-mentioned microstructure, the microstructure of the
cold rolled and heat treated steel sheet is free from microstructural
components,
such as pearlite, tempered martensite and cementite without impairing the
mechanical properties of the steel sheets.
A steel sheet according to the invention can be produced by any suitable
method. A preferred method consists in providing a semi-finished casting of
steel
with a chemical composition 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 above-described chemical composition 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 must be below 1280 C. In case the temperature of the slab is lower
zo than 1000 C, 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. Reheating at temperatures
above
1280 C must be avoided because they are industrially expensive. Therefore, the
finish rolling temperature of the slab is above Ac3 and preferably
sufficiently high
so that hot rolling can be completed in the temperature range of Ac3 + 150 C
to
Ac3+250 C
A final rolling temperature range between Ac3 to Ac3+200 C is necessary
to have a structure that is favorable to recrystallization and rolling. It is
preferred
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that the final rolling pass to be performed at a temperature greater than 850
c and
better of at least 950 C.
The hot rolled steel obtained in this manner is then cooled at a cooling rate
of at least 30 C/s to the coiling temperature. Preferably, the cooling rate
will be less
than or equal to 200 C/s.
The hot rolled steel is then coiled at a temperature between 475 C and
650 C to avoid ovalization and preferably between 475 C and 625 C to avoid
scale
formation. A more preferred range for such coiling temperature is between 500
C
and 625 C. The coiled hot rolled steel is then cooled down to room temperature
before subjecting it to optional hot band annealing.
The hot rolled steel 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 subjected to an optional Hot Band Annealing at,
for
example, temperatures between 400 C and 750 C for preferably 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 may performed through, for example, pickling of such sheet.
This hot rolled steel is subjected to cold rolling to obtain a cold rolled
steel
zo sheet with a thickness reduction between 35 to 90%. The cold rolled
steel sheet
obtained from cold rolling process is then subjected to annealing to impart
the steel
of present invention with microstructure and mechanical properties.
To anneal the said cold rolled steel sheet, it is heated up to the soaking
temperature between Ad 1 +60 C and Ac3, preferably at a heating rate of at
least
3 C/s, then the annealing is performed at that temperature during 5 to 500
seconds,
preferably during 50 to 250 seconds. In a preferred embodiment, the heating is
at
least 10 C/s and more preferably at least 15 C/s. During this annealing Inter-
critical
ferrite forms.
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The preferred annealing soaking temperature is between Ad 1 + 70 C and
Ac3 and more preferably between Ad 1 + 80 C and Ac3 - 30 C.
In a preferred embodiment, the time and temperature of soaking are
selected to ensure that the microstructure of the steel sheet at the end of
the
soaking contains at least 50% of Austenite and more preferably at least 60% of
austenite.
Then the cold rolled steel is cooled in a two-step cooling process wherein
the first step starts from soaking temperature to a temperature Ti between 550
C
and 650 C, at a cooling rate CR1 which is least 3 C/s, preferably at least 5
C/s
and more preferably at least 10 C/s. During this step the transformed ferrite
forms.
The cold rolled steel is then held at Ti during 1 s to 20 s and preferably
between 2
s and 15 s and more preferably between 5 s and 12 s.
Thereafter the second step starts from cooling further the cold rolled steel
sheet from Ti to overaging temperature T2 between 400 C and 480 C, at a
cooling
rate CR2 of at least 3 C/s, preferably at least 5 C/s and more preferably at
least
7 C/s..
Then overaging is being performed at T2 during 5 to 100 seconds. During
overaging, some bainite gets formed. The preferred temperature T2 for
overaging
is between 420 C and 475 C. The preferred time for overaging temperature
during
zo 15 to 75 seconds and more preferably 20 and 75 seconds.
The cold rolled steel sheet can then either be cooled down to room
temperature or can be brought to the temperature of a hot dip coating bath
between
420 C and 680 C, depending on the nature of the coating, to facilitate hot dip
coating of the cold rolled steel sheet.
In either case, the final cooling down to room temperature is done at a
cooling rate of at least 5 C/s and preferably at least 9 C/s to ensure the
formation
of fresh martensite in the steel of present invention.
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The cold rolled steel sheet can also be coated by any of the known industrial
processes such as Electro-galvanization, JVD, PVD, etc, which may not require
bringing it to the above-mentioned temperature range before coating.
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EXAMPLES
The following tests and examples 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 and expound the significance of
the parameters chosen by inventors after extensive experiments and further
establish the properties that can be achieved by the steel according to the
invention.
Samples of the steel sheets according to the invention and to some
comparative grades were prepared with the compositions gathered in table 1 and
the processing parameters gathered in table 2. The corresponding
microstructures
of those steel sheets were gathered in table 3 and the properties in table 4.
Table 1 : composition of the trials
Table 1 depicts the steels with the compositions expressed in percentages by
weight.
Samples C Mn Si Al Cr Ti B Nb P S N Mo Ca
Ad 1 Ac3
A 0.10 2.29
0.60 0.04 0.53 0.028 0.0016 0.017 0.012 0.004 0.0069 0.003 0.0002 728
865
B 0.09 2.23
0.59 0.19 0.52 0.012 0.0017 0.016 0.003 0.004 0.0069 0.003 0.0002 718
863
C 0.10 2.31
0.60 0.19 0.53 0.001 0.0016 0.015 0.012 0.004 0.0056 0.003 0.0002 728
883
D 0.09 2.30
0.62 0.31 0.53 0.001 0.0018 0.012 0.014 0.004 0.0056 0.003 0.0002 730
904
Table 1 also shows Ad 1 and Ac3 temperature points which are calculated by
dilatometry.
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Table 2 : Process parameters
Table 2 gathers the annealing process parameters implemented on steel samples
of Table 1, that were all reheated at 1230 C, hot-rolled with a finish rolling
temperature of 875 C, coiled at 550 C and cold rolled with a 50% reduction
rate
before undergoing annealing and two-steps cooling scheme including overaging:
Trials Samples Annealing Annealing CRI Ti ( C) Holding CR2
Overaging Overaging Final
temperature time C/s time at Ti C/s Temperature
time (s) cooling
( C) (s) (s) ( C)
rate( C/s)
11 C 810 156 15 620 6 12 463 30
22
12 D 810 233 10 620 9 8 463 47
14
13 D 810 233 10 620 9 8 463 80
14
RI A 777 156 10 - - - - 463 30
22
R2 B 777 156 10 - - - - 463 30
22
underlined values: not according to the invention.
Table 3 gathers the results of test conducted in accordance of standards on
different microscopes such as Scanning Electron Microscope for determining
microstructural composition of both the inventive steel and reference trials.
Table 3 : microstructures of the trials
Trials Martensite Intercritical Transformed
ferrite ferrite + bainite
11 44 34 22
12 53 36 11
13 44 36 20
RI 46 45 9
R2 39 49 12
underlined values: not according to the invention.
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Table 4 gathers the mechanical properties of both the inventive steel and
reference steel. The tensile strength, yield strength and total elongation
tests are
conducted in accordance with JIS Z224I standards.
Table 4 : mechanical properties of the trials
Tensile Strength Total Yield Strength
Trials
(MPa) elongation (%) (MPa)
11 1017 18.0 595
12 986 19.4 582
13 979 18.7 543
RI 967 8.2 516
R2 910 7.5 475
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.
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