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.
Automotive parts are required to satisfy two inconsistent necessities, viz.
ease of forming and 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
formability steel sheets have resulted in several methods for producing high
zo strength and high formability steel sheets, some of which are enumerated
herein
for conclusive appreciation of the present invention:
EP3187608 is high-strength hot-dip galvanized steel sheet having a tensile
strength (TS) of 1,300 MPa or more and excellent in ductility and in-plane
uniformity
of material properties is provided, and a method for manufacturing the steel
sheet
is also provided. The high-strength hot-dip galvanized steel sheet has a
specific
composition including C, Si, Mn, etc. In this chemical composition, the
content of
Ti [Ti] and the content of N [N] satisfy [Ti] > 4[N]. The high-strength hot-
dip
galvanized steel sheet has a microstructure including martensite at an area
fraction
of 60% or more and 90% or less, polygonal ferrite at an area fraction of more
than
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5% and 40% or less, and retained austenite at an area fraction of less than 3%
(including 0%). The average hardness of the martensite is 450 or more and 600
or
less in terms of Vickers hardness, and the average crystal grain diameter of
the
martensite is 10 pm or less. The standard deviation of the crystal grain
diameters
of the martensite is 4.0 pm or less. EP3187608 is able to provide the tensile
strength above 980MPa but does not have an elongation of 8% or more.
EP3473741 is a steel sheet having a tensile strength of 950 MPa or more and
good toughness and a method for manufacturing the same. The steel sheet has a
specific composition and a metallographic structure containing: a ferrite area
fraction of 30% or less (including 0%), a tempered martensite area fraction of
70%
or more (including 100%), and a retained austenite area fraction of 4.5% or
less
(including 0%), wherein the average aspect ratio of an iron based carbide,
precipitated in tempered martensite grains, having a grain size in the largest
10%
is 3.5 or more. But the steel of EP3473741 is not able to provide the ultimate
tensile strength of 950 or more in both rolling as well as the transversal
direction.
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 from 980 MPa to 1150MPa in both transversal
direction as well as rolling direction and preferably from 980 MPa to
1150MPa in both transversal direction as well as rolling direction.
In a preferred embodiment, the total elongation of the steel sheet is greater
than or equal to 8%,
In a preferred embodiment, a yield strength from 700 MPa to 850MPa in both
transversal direction as well as rolling direction and preferably from 720 MPa
to
850MPa in both transversal direction as well as rolling direction.
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Preferably, such steel can also have a good suitability for forming, in
particular for rolling with good weldability and coat ability.
Another object 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
from the following detailed description of the invention.
Carbon is present in the steel from 0.1% to 0.2%. Carbon is an element
necessary
for increasing the strength of a steel sheet by producing a low-temperature
transformation phase such as martensite. A content less than 0.1% would not
allow
the formation of martensite there by tempered martensite, thereby decreasing
strength as well as ductility. On the other hand, at a carbon content
exceeding
0.2%, a weld zone and a heat-affected zone are significantly hardened, and
thus
the mechanical properties of the weld zone are impaired. The preferred limit
for
Carbon is from 0.12 to 0.19% and more preferably is from 0.13 to 0.17%.
Manganese content of the steel of present invention is from 1.2% to 2.2%.
Manganese is an element that imparts strength. An amount of at least about
1.2%
by weight of manganese has been found in order to provide the strength and
zo hardenability of the steel sheet. Thus, a higher percentage of Manganese
such as
1.3% to 2.1% is preferred. But when manganese is more than 2.2%, this produces
adverse effects such as slowing down the transformation of austenite to
ferrite
during the slow cooling after annealing, leading to a reduction of ductility.
Moreover,
a manganese content above 2.2% would also reduce the weldability of the
present
steel. Hence the preferred limit for the steel of present invention is from
1.3% to
2.1% and more preferably from 1.6% to 2.0%.
Silicon is an essential element for the steel of present invention, Silicon is
present
from 0.05% to 0.6%. Silicon is added to the steel of present invention to
impart
strength by solid solution strengthening. Silicon plays a part in the
formation of the
microstructure by preventing the precipitation of carbides and by promoting
the
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formation of martensite. But whenever the silicon content is more than 0.6%,
surface properties and weldability of steel is deteriorated, therefore the
Silicon
content is preferred from 0.1% to 0.5% and more preferably 0.1% to 0.4%.
Aluminum content of the present invention is from 0.001% to 0.1%.
Aluminum is added to de-oxidise the steel of present invention. Aluminum is an
alphageneous element. This can increase the formability and ductility of
steel. In
order to obtain such an effect, Aluminum content is required at 0.001% or
more.
However, when the Aluminum content exceeds 0.1%, Ac3 point increases beyond
acceptable, austenite single phase is very difficult to achieve industrially
hence hot
rolling in complete austenite region cannot be performed. Therefore, Aluminum
content must not be more than 0.1% .The preferable limit for the presence of
Aluminum is from 0.001% to 0.09% and more preferably 0.001% to 0.06%.
Chromium content of the steel of present invention is from 0.01% to 0.5%.
Chromium is an essential element that provide 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 0.1% to 0.4% and more preferably 0.1% to 0.3%.
Phosphorus content of the steel of present invention is limited to 0.09%.
zo 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 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 0.09%.
Sulfur is not an essential element but may be contained as an impurity in
steel up to 0.09%. The sulfur content is preferred as low as possible, but
between
0.001% and 0.03% is preferred 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% in order to avoid ageing of material. Nitrogen
can form nitrides or carbonitrides together with carbon, that can impart
strength to
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the steel of present invention by precipitation strengthening with Vanadium
and
Niobium but whenever the presence of nitrogen is more than 0.09% it can form
high amount of Aluminum Nitrides which are detrimental for the present
invention
hence the preferable limit for the nitrogen is between 0.001% and 0.01%.
5 .. Molybdenum is an optional element that constitutes from 0% to 0.5% of the
Steel
of present invention; Molybdenum increases the hardenabty of the steel of
present invention and influences the transformation of austenite to Ferrite
and
Bainite during cooling after annealing. 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%.
Niobium is an optional element that can be added to the steel up to 0.1%,
preferably between 0.0010 and 0.1%. 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.
Titanium is an optional element which may be added to the steel of the
present invention up to 0.1%, preferably between 0.001% and 0.1%. As 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 0.001% it does not impart any effect on the steel of
present invention.
Vanadium is an optional element which may be added to the steel of the
present invention up to 0.1%, 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 0.1% 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.
Nickel may be added as an optional element in an amount of 0% to 10/ to
increase
.. the strength of the steel and to improve its toughness. A minimum of 0.01%
is
required to produce 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
1.0 0.01% is required to produce such effects. However, when its content is
above 1%.
copper causes hot ductility deterioration during hot rolling.
Calcium is an optional element which may be added to the steel of present
invention up to 0.005%, preferably 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 sulphur content in globularizing it.
Other elements such as cerium, boron, magnesium or zirconium can be
added individually or in combination in the following proportions: Ce < 0.1%,
B <
0.05%, Mg < 0.05% and Zr < 0.05%. Up to the maximum content levels indicated,
zo 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
60% to 85% of tempered martensite, 0% to 5% of residual austenite, 0% to 5% of
fresh martensite and cumulative amount of ferrite and bainite of 15% to 38% in
area fractions, Tempered Martensite constitutes the matrix phase for the steel
of
present invention
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Tempered Martensite constitutes from 60% to 85% of the microstructure by area
fraction. Tempered martensite is formed from the martensite which forms during
the second step of cooling after annealing and particularly when the
temperature
drops below Ms temperature and more particularly from Ms-10 C to 15 C.Such
martensite is then tempered during the holding at a tempering temperature
Temper
from 150 C to 300 C. The martensite of the present invention imparts ductility
and
strength to such steel. Preferably, the content of martensite is from 62% to
80%
and more preferably from 62% to 75%.
Fresh martensite is an optional microconstituent which is limited in the steel
at an amount of from 0% to 5%, preferably from 0 to 2% and even better equal
to
0%. Fresh martensite may form during the final cooling after tempering.
The cumulated amount of ferrite and bainite represents from 15% to 38% of the
microstructure. The cumulated amounts of bainite and ferrite is greater than
15%
is mandatory to ensure a balance between strength and elongation in which
is presence of Bainite impart tensile strength of 980 MPa and Ferrite
ensure the
elongation. Bainite forms during the reheating before tempering. Bainite can
impart
strength to the steel but when present in a too big amount, it may adversely
impact
the yield strength of the steel. Ferrite imparts elongation as well as
formability to
the steel of the present invention. To ensure an elongation of 8% and
preferably
zo 9% or more it is preferred to have 10% of Ferrite. Ferrite is formed
during the first
step of cooling after annealing. But when the cumulative presence of bainite
and
ferrite are present above 38% the mechanical properties may get impacted
adversely specifically the tensile strength and yield strength in transverse
direction.
Hence the preferred limit for the cumulative presence ferrite and bainite is
kept
25 from 20% to 37% and more preferably from 25% to 36%.
Residual Austenite is an optional microstructure that can be present from 0%
to
5% in the steel. The presence of Residual austenite till 5% is not detrimental
to the
mechanical properties. Up to 5% Residual austenite imparts ductility and
elongation to the steel. It is preferred residual austenite between 0% and 3%
and
30 more preferably from 0% to 2%.
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In addition to the above-mentioned microstructure, the microstructure of the
cold
rolled steel sheet is free from microstructural components such as pearlite
and
cementite.
The steel according to the invention can be manufactured by any suitable
methods.
It is however preferable to use the method according to the invention that
will be
detailed, as a non-limitative example.
Such preferred method consists in providing a semi-finished casting of steel
with a
chemical composition of the prime steel 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 according to the invention
is
manufactured by continuous casting wherein the slab optionally underwent a
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.
zo The temperature of the slab, which is subjected to hot rolling, must be
at least
1000 C and must be below 1280 C. In case the temperature of the slab is lower
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. Therefore, the temperature of
the
slab must be high enough so that hot rolling should be completed in the
temperature range of Ac3 to Ac3+100 C. Reheating at temperatures above
1280 C must be avoided because they are industrially expensive.
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The sheet obtained in this manner is then cooled at a cooling rate of at least
20 C/s
to the coiling temperature which must be below 650 C. Preferably, the cooling
rate
will be less than or equal to 200 C/s.
The hot rolled steel sheet is then coiled at a coiling temperature below 650 C
to
avoid ovalization and preferably from 475 C to 625 C to avoid scale formation,
with an even prefererred range for such coiling temperature from 500 C to 625
C.
The coiled hot rolled steel sheet is then 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 be subjected to an optional hot band annealing.
In
a preferred embodiment, such hot band annealing is performed at temperatures
from 400 C to 750 C, preferably for at least 12 hours and not more than 96
hours,
the temperature preferably remaining below 750 C to avoid transforming
partially
the hot-rolled microstructure and, therefore, possibly losing the
microstructure
homogeneity. Thereafter, an optional scale removal step of this hot rolled
steel
sheet may be performed through, for example, pickling of such sheet.
This hot rolled steel sheet is then subjected to cold rolling to obtain a cold
rolled
steel sheet with a thickness reduction from 35 to 90%.
zo Thereafter the cold rolled steel sheet is being heat treated which will
impart the
steel of present invention with requisite mechanical properties and
microstructure.
The cold rolled steel sheet is then heated in a two steps heating process
wherein
the first step of heating starts from room temperature, the cold rolled steel
sheet
being heated, at a heating rate HR1 of at least 10 C/s, to a temperature HT1
which
is in a range from 550 C to 750 C. In a preferred embodiment, the heating rate
HR1 for such first step of heating is at least 12 C/s and more preferably at
least
15 C/s. The preferred HT1 temperature for such first step is from 575 C to 725
C
and more preferably from 575 C to 700 C.
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In the second step of heating, the cold rolled steel sheet is heated from HT1
to an
annealing temperature Tsoak which is from Ac3 to Ac3 + 100 C, preferably from
Ac3 +10 C to Ac3 + 100 C, at a heating rate HR2 which is from 1 C/s to 15 C/s.
In a preferred embodiment, the heating rate HR2 for the second step of heating
is
5 from 1 C/s to 8 C/s and more from 1 C/s to 4 C/s, wherein Ac3 for the
steel sheet
is calculated by using the following formula:
Ac3 = 910 ¨ 203[C]"(1/2) ¨ 15.2 [Ni] + 44.7[Si] + 104[V] + 31.5[Mo] + 13.1[W]
¨ 30[Mn] ¨ 11[Cr] ¨ 20[Cu] + 700[P] + 400[A/] + 120[As]
+ 400 [Ti]
10 wherein the elements contents are expressed in weight percentage of the
cold
rolled steel sheet.
The cold rolled steel sheet is held at Tsoak during 10 seconds to 500 seconds
to
ensure a complete recrystallization and full transformation to austenite of
the
strongly work hardened initial structure.
The cold rolled steel sheet is then cooled in a two steps cooling process
wherein
the first step of cooling starts from Tsoak, the cold rolled steel sheet being
cooled
down, at a cooling rate CR1 between 1 C/s and 15 C/s, to a temperature Ti
which
is in a range from 630 C to 685 C. In a preferred embodiment, the cooling rate
CR1 for such first step of cooling is from 1 C/s to 10 C/s and more preferably
from
zo 1 C/s to 4 C/s. The preferred Ti temperature for such first step is from
640 C to
685 C and more preferably from 650 C to 685 C.
In the second step of cooling, the cold rolled steel sheet is cooled down from
Ti to
a temperature T2 which is from Ms-10 C to 15 C, at a cooling rate CR2 of at
least
100 C/s. In a preferred embodiment, the cooling rate CR2 for the second step
of
cooling is at least 200 C/s and more preferably at least 300 C/s. The
preferred T2
temperature for such second step is from Ms-20 C to 20 C and more preferably
from Ms-50 C to 20 C.
Ms for the steel sheet is calculated by using the following formula:
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Ms = 545 ¨ 601.2 * (1 ¨ EXP(-0.868[C])) ¨ 34.4[Mn] ¨ 13.7[Si] ¨ 9.2 [Cr]
¨ 17 .3[Ni] ¨ 15.4[Mo] + 10.8[V] + 4.7[Co] ¨ 1.4[Al] ¨ 16.3[Cu]
¨ 361[Nb] ¨ 2.44 [Ti] ¨ 3448[B]
Thereafter the cold rolled steel sheet is reheated to a tempering temperature
Ttemper between 150 C and 300 C with a heating rate of at least 5 C/s and
preferably of at least 10 C/s and more preferably 12 C/s or more during 100 s
to
600 s. The preferred temperature range for tempering is from 175 C to 280 C
and
the preferred duration for holding at Ttemper is from 200 s to 500 s.
According to the present invention the tempering temperature is selected such
that
the difference between Ti and Ttemper is from 415 C to 455 C. AT is determined
as follows:
AT = (Ti ¨ Ttemper)
When AT is less than 415 C then the cumulative amount of bainite and ferrite
exceeds 38% which is detrimental for the mechanical properties specifically
the
tensile strength in transversal direction. When AT is greater than 455 C then
the
amount of tempered martensite is too high, thereby the steel of present
invention
in rolling direction exceeds 1150 MPa. The preferred AT is between 420 C and
zo 440 C
Then, the cold rolled steel sheet is cooled down to room temperature to obtain
a
heat treated cold rolled steel sheet.
The heat treated cold rolled steel sheet of the present invention may
optionally be
coated with zinc or zinc alloys, or with aluminum or aluminum alloys to
improve its
corrosion resistance.
The heat treated cold rolled steel sheet can also be coated by any of the
known
industrial processes such as Electro-galvanization, JVD, PVD, etc,.
Then an optional post batch annealing may be done at a temperature between
150 C and 300 C during 30 minutes to 120 hours.
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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 depicts the steels with the compositions expressed in percentages
by weight.
Table 1 : composition of the trials
Steel C Mn Si Al Cr P 5 N Mo Ti Nb V Ni
B
1 0.15 1.90 0.19 0.04 0.18 0.0031 0.001 0.0034 0.002 0.02 0..001 0.06 0.015
0.001
2 0.15 1.90 0.18 0.04 0.18 0.0031 0.001 0.004 0.0021 0.02 0.001 0.06 0.015
0.001
3 0.15 1.89 0.21 0.03 0.19 0.0025 0.0013 0.005 0.0031 0.025 0.001 0.03 0.02
0.001
4 0.15 1.88 0.20 0.03 0.18 0.0018 0.0012 0.0045 0.0022 0.025 0.001 0.02 0.015
0.001
5 0.15 1.91 0.20 0.03 0.19 0.0016 0.0025 0.004 0.0035 0.025 0.001 0.02 0.017
0.001
6 0.15 1.87 0.21 0.027 0.18 0.0018 0.0023 0.004 0.0043 0.025 0..001 0.02 0.015
0.001
7 0.14 1.87 0.21 0.027 0.18 0.0018 0.0023 0.004 0.0043 0.025 0..001 0.02 0.015
0.001
8 0.15 1.88 0.20 0.025 0.20 0.0023 0.0021 0.004 0.0043 0.025 0..001 0.03 0.015
0.001
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Table 2 gathers the annealing process parameters implemented on steels
of Table 1.
Table 2 : process parameters of the trials
All the examples and counter examples are reheated to a temperature of
1200 C and then hot rolled wherein the hot rolled finishing temperature is 890
C
thereafter the hot rolled steel strip is cooled at a rate of 80 C/s and coiled
at 530 C
and cold rolled reduction for all examples and counter examples is 50%.
Table 2a
Trial Steel HRI HTI HR2 T soak Soaking
CR1
Ti ( C)
Sample ( C/s) ( C) ( C/s) ( C)
time (s) ( C/s)
II 1 17 640 1.8 850 154 680 1.5
12 2 15 620 1.8 849 164 680 1.4
13 3 15 600 1.9 850 164 664 1.5
14 4 20 600 2.7 850 117 660 2.1
RI 5 15 530 3.1 865 137 690 1.7
R2 6 15 560 2.3 855 154 667
1.6s
R3 7 15 600 2.3 855 154 665 1.6
R4 8 12 560 2.2 850 189 670 1.2
underlined values: not according to the invention
lo Table 2b
Heating
T2 CR2 rate to Ttemper Tempering A T Ms
Trial
Ac3 ( C)
( C) ( C/s) Ttemper ( C) time (s) ( C) ( C)
( C/s)
II 20 650 14 260 270 420 402 826
12 20 598 13 260 289 420 400 825
13 20 583 11 230 289 434 400 820
14 20 812 15 229 206 431 402 815
RI 20 728 13 230 241 460 399 813
R2 20 625 13 260 271 407 405 818
R3 20 623 13 255 271 410 405 818
R4 20 510 11 260 333 410 402 818
underlined values: not according to the invention.
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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
Steel Martensite Ferrite + Residual Fresh
Sample (tempered) Bainite Austenite martensite
11 71 29 0 0
12 74 26 0 0
13 67 33 0 0
14 64 36 0 0
RI 87 13 0 0
R2 61 39 0 0
R3 53 47 0 0
R4 57 43 0 0
underlined values: not according to the invention.
Table 4 gathers the mechanical properties of both the inventive steel and
reference steel. The tensile strength, yield strength and total elongation
test are
conducted in accordance with NF EN ISO 6892 standards,
Table 4 : mechanical properties of the trials
Tensile Tensile Yield Yield Total
Strength (MPa) Strength Strength Strength Elongation
Trials TD (MPa) RD (MPa) TD (MPa) RD (0/0)
11 1069 1064 798 807 8.0
12 1072 1064 789 797 9.4
13 1044 1035 719 744 9.4
14 1076 1076 737 767 9.8
R1 1177 1159 887 895 8.7
R2 960 958 653 670 11.5
R3 959 954 652 662 10.4
R4 971 966 658 732 9.1
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underlined values: not according to the invention.
The examples show that the steel sheets according to the invention are the
5 only one to show all the targeted properties thanks to their specific
composition
and microstructures.
