Note: Descriptions are shown in the official language in which they were submitted.
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1
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 950 MPa or more and a hole
expansion ratio of more than 56% 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 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
strength and high formability steel sheets, some of which are enumerated
herein
for conclusive appreciation of the present invention:
JP2012111978 is a patent application having a composition
C: 0.05-0.3%, Si: 0.01-3.0%, Mn:0.5-3%, Al: 0.01-0.1%, and the balance Fe and
incidental impurities and having a component composition consisting of,
ferrite and
tempered martensite as a main component of the high-strength cold rolled steel
sheet but such steel is not able to reach more than 50% of hole expansion
ratio.
2
EP2971209 is patent that relates to a high strength hot dip galvanised complex
phase steel strip having improved formability to be used in the automotive
industry
having an mandatory elemental
composition
C: 0.13 - 0.19 %, Mn :1.70 - 2.50 % Si: 0- 0.15 %, Al: 0.40 - 1.00 %, Cr: 0.05
- 0.25
%, Nb : 0.01 - 0.05 %,P : 0- 0.10 %, Ca: 0-0.004%, S: 0- 0.05 %, N : 0- 0.007
% the
balance being Fe and inevitable impurities, wherein 0.40 % < Al + Si < 1.05 %
and
Mn + Cr > 1.90 %, and having a complex phase microstructure, in volume
percent,
comprising 8-12 % retained austenite, 20 - 50 % bainite, less than 10 %
martensite,
the remainder being ferrite but the granted patent is unable to reach the
tensile
strength beyond 900MPa.
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 MPa, or even above 1000 MPa,
- a total elongation greater than or equal to 8%.
- a hole expansion ratio of 56% or more and preferably 57% or more.
According to one aspect the present invention relates to a heat treated and
cold rolled
steel sheet having a composition comprising of the following elements,
expressed in
percentage by weight:
0.09% 5 Carbon _5 0.15 %
1.8% 5 Manganese 5 2.5%
0.2 % 5 Silicon 5 0.7 %
0.01% 5 Aluminum 5 0.1%
0 % 5 Phosphorus 5 0.09 %
0 % 5 Sulfur 5 0.09 %.
0 % 5 Nitrogen 5 0.09%
0% 5 Niobium 5 0.1%
Date Recue/Date Received 2023-09-25
3
0% 5 Titanium 50.1%
0% 5 Chromium 5 1%
0 % 5 Molybdenum 5 1%
0% 5 Vanadium 50.1%
0 % 5 Calcium 5 0.005%
0 % 5 Boron 5 0.01%
0 % 5 Cerium 5 0.1%
0 % 5 Magnesium ...- 0.05%
0 % 5 Zirconium -- 0.05%
the remainder of the composition being composed of iron and unavoidable
impurities , a microstructure of said heat treated and cold rolled steel sheet
comprising in area fraction, 65 to 85% Tempered Martensite, 0% to 5%
Residual Austenite and a cumulative presence of Ferrite and Bainite between
and 35%,wherein said heat treated and cold rolled steel sheet has an
15 ultimate
tensile strength of at least 950 MPa, a total elongation of at least 8%
and a hole expansion ratio greater than 55%, wherein while the ultimate
tensile strength and the total elongation are measured in accordance with ISO
6892 standard and the hole expansion ratio is measured in accordance with
IS016630:2009 standard.
In a preferred embodiment, the steel sheet according to the invention may have
a
yield strength value greater than or above 750 MPa.
Preferably, such steel can also have a good suitability for forming, in
particular
for rolling with good weldability and coatability.
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.
According to another aspect, the invention relates to a method of production
of heat
treated and cold rolled steel sheet comprising the following successive steps:
-
providing a semi-finished steel product with a composition of the type
described herein;
Date Recue/Date Received 2023-09-25
3a
- reheating said semi-finished product to a temperature between 1000 C
and 1250 C;
- rolling said semi-finished product in a temperature range between Ac3
and Ac3 +100 C wherein a hot rolling finishing temperature is above
Ac3 to obtain a hot rolled steel sheet;
- cooling the hot rolled steel sheet at a cooling rate of at least 30 C/s
to
a coiling temperature which is below 600 C; and coiling said cooled hot
rolled steel sheet to obtain a coiled hot rolled sheet;
- cooling said coiled hot rolled steel sheet to room temperature;
- optionally performing a first scale removal process on said coiled hot
rolled steel sheet;
- optionally annealing the coiled hot rolled steel sheet between 400 C
and 750 C;
- optionally performing a second scale removal process on said coiled
hot rolled steel sheet;
- cold rolling said coiled hot rolled steel sheet with a reduction rate
between 35 and 90% to obtain a cold rolled steel sheet;
- annealing said cold rolled steel sheet in two-step heating process
wherein:
o a first step starts from heating the cold rolled steel sheet to a
temperature HT1 between 600 C and 650 C, with a heating rate
HR1 of at least 10 C/s, and
o a second step starts from heating further the cold rolled steel
sheet from HT1 to an annealing temperature range between Ac3
and Ac3 +200 C, with a heating rate HR2 of at least 1 C/s , HR2
being lower than HR1,
- then perform annealing at an annealing temperature in the annealing
temperature range during 5 to 1000 seconds,
- then cooling the cold rolled steel sheet in a three-step cooling process
wherein:
o a first step starts from cooling the steel sheet from the annealing
temperature to a temperature CT1 between 675 C and 725 C,
with a cooling rate CR1 of at most 10 C/s ,
Date Recue/Date Received 2023-09-25
3b
o a second step starts from cooling further the steel sheet from
CT1 to CT2 between 450 C and 550 C, with a cooling rate CR2
of at least 30 C/s,
o a third step starts from cooling further the steel sheet from CT2
to CT3 between Ms-50 C and 20 C, with a cooling rate CR2 of
at least 200 C/s,
- heating said cold rolled steel sheet at a heating rate of at least 10 C/s
to a tempering temperature between 300 C and 380 C and tempering
said cold rolled steel sheet during 100s to 1000 seconds,
- cooling the cold
rolled steel sheet to room temperature range to obtain
a heat treated and cold rolled steel sheet.
Other characteristics and advantages of the invention will become apparent
from the following detailed description of the invention.
Carbon is present in the steel between 0.09% and 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. A content less than 0.09% would not
allow
securing adequate amount of martensite, thereby decreasing strength as well as
ductility. 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 impaired. The preferred limit for Carbon is between 0.1%
and
0.14%, more preferably 0.1% and 0.13%.
Manganese content of the steel of present invention is between 1.8% and 2.5%.
Manganese is an element that imparts strength by solid solution strengthening.
An
amount of at least about 1.8 % by weight of manganese has been found in order
to
provide the strength and hardenability of the steel sheet. Thus, a higher
percentage
of Manganese such as 1.9% to 2.4% is preferred and more preferred limit is
between
2.0% and 2.3%. But when manganese is more than 2.5%, this produces adverse
effects such as slowing down the transformation of austenite to martensite
during the
cooling after annealing, leading to a reduction of strength. Moreover, a
manganese
content above 2.5% would also reduce the weldability of the present steel.
Date Recue/Date Received 2023-09-25
3c
Silicon is an essential element for the steel of present invention, Silicon is
present
between 0.2% and 0.7%. 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
formation of martensite. But whenever the silicon content is more than 0.7%,
surface
properties and weldability of steel is deteriorated, therefore the Silicon
content is
preferred between 0.3% and 0.7% and more preferably between 0.4% and 0.6%.
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Aluminum content of the present invention is between 0.01% and 0.1%.
Aluminum is added to de-oxidise the steel of present invention. Aluminum is an
alphageneous element and also retarding the formation of carbides. This can
increase the formability and ductility of steel. In order to obtain such an
effect,
Aluminum content is required at 0.01% 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 between 0.01% and
io 0.08% and more preferably 0.01% and 0.05%.
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
is 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
zo preferably at most 0.01%, from the viewpoint of manufacturing cost.
Further if
higher sulfur is present in steel it combine 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
forms the nitrides which impart strength to the steel of present invention by
25 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
upper
limit for nitrogen is 0.01%.
Niobium is an optional element that can be added to the steel up to 0.1%,
30 preferably between 0.001% and 0.1%. It is suitable for forming
carbonitrides to
impart strength to the steel according to the invention by precipitation
hardening.
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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
5 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.
Chromium content of the steel of present invention is between 0 ./c. and 1%.
Chromium is an optional element that provide strength and hardening to the
steel,
but when used above 1 /0 impairs surface finish of the steel.
Molybdenum is an optional element that constitutes between 0% and 1% of the
zo
Steel of present invention; Molybdenum increases the hardenability 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 1%.
Vanadium is an optional element which may be added to the steel of the
present invention up to 0.1%, preferably between 0.001% and 0.01%. 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 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.
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Calcium is an optional element which may be added to the steel of present
invention up to 0.005%, preferably between 0.001% and 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
<
0.01%, 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.
Present
invention does not intend to add Copper and Nickel but these elements may be
present as residuals up to 0.1% either severaly or cumulatively.
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 fractions 65% to 85% of Tempered Martensite, 0% and 5% of residual
austenite and cumulative amount of bainite and ferrite between 15% and 35%.
Tempered martensite constitutes the matrix phase for the steel of present
invention
Tempered Martensite constitutes between 65% and 85% of the microstructure by
zo area fraction. Tempered martensite is formed from the martensite which
forms
during the second step of cooling after annealing and particularly below Ms
temperature and more particularly between Ms-50 C and 20 C. Such martensite is
then tempered during the holding at a tempering temperature Temper between
150 C and 400 C. The martensite of the present invention imparts ductility and
strength to such steel. Preferably, the content of martensite is between 65%
and
80% and more preferably between 68% and 78%.
Bainite and Ferrite are cumulatively present in the steel between 15% and
35%. In a preferred embodiment, the range for cumulated amount of ferrite and
bainite is between 20% and 35% and more preferably between 22% and 32%.
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Ferrite constituent improves the properties of the steel of the present
invention, in particular regarding elongation and hole expansion ratio as
ferrite is a
soft and intrinsically ductile constituent. This ferrite is mainly formed
during the first
step of cooling after annealing. In a preferred embodiment, ferrite can be
present
at least in an amount of 15%.
Bainite can impart strength to the steel but when present in a large amount
it may adversely impact the hole expansion ratio and elongation of the steel.
Bainite
forms during the reheating before tempering. In a preferred embodiment, the
bainite content is kept between 0% and 10%more preferably below 8% and even
3.0 more preferably below 5%.
Residual Austenite is an optional phase that can be present between 0% and 5%
in the steel, but is preferably not present.
In a preferred embodiment the steel sheet according to the invention may
be obtained by any appropriate method. It is however preferred to use the
process
according to the preferred embodiments of the invention, which comprises the
following successive steps:
Such process includes providing a semi-finished product of steel with a
zo chemical composition according to the invention. The semi-finished
product can be
casted 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.
For the purpose of simplification of the present invention, a slab will be
considered as a semi-finished product. A slab having the above-described
chemical composition is manufactured by continuous casting wherein the slab
preferably underwent a direct soft reduction during casting to ensure the
elimination of central segregation and porosity reduction. 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.
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The temperature of the slab which is subjected to hot rolling is at least
1000 C, preferably above 1100 C and must be below 1250 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. Further,
the
temperature must not be above 1250 C as there would be a risk of formation of
rough ferrite grains resulting in coarse ferrite grain which decreases the
capacity
of these grains to re-crystallize during hot rolling. The larger the initial
ferrite grain
size, the less easily it re-crystallizes, which means that reheat temperatures
above
1250 C must be avoided because they are industrially expensive and unfavorable
in terms of the recrystallization of ferrite.
The temperature of the slab is preferably sufficiently high so that hot
rolling
can be completed entirely in the austenitic range, the finishing hot rolling
temperature remaining above Ac3 and preferably above Ac3 + 50 C. It is
necessary that the final rolling be performed above Ac3, because below this
temperature the steel sheet exhibits a significant drop in rollability. A
final rolling
temperature is preferably above Ac3 +50 C to have a structure that is
favorable to
recrystallization and rolling.
The sheet obtained in this manner is then cooled down at a cooling rate of
at least 30 Cis to the coiling temperature which is below 600 C. Preferably,
the
cooling rate will be less than or equal to 65 C/s and above 35 C/s. The
coiling
temperature is preferably of at least 350 C to avoid the transformation of
austenite
into ferrite and pearlite and to contribute in forming an homogenous bainite
and
martensite microstructure.
The coiled hot rolled steel sheet may be cooled down to room temperature
before subjecting it to an optional hot band annealing or may be send to an
optional
hot band annealing directly.
Hot rolled steel sheet may be subjected to an optional pickling to remove
the scale formed during the hot rolling, if needed. The hot rolled sheet is
then
subjected to an optional hot band annealing at a temperature between 400 C and
750 C, preferably during 1 to 96 hours.
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Thereafter, pickling of this hot rolled steel sheet may be performed if
necessary to remove the scale.
The hot rolled steel sheets are then cold rolled with a thickness reduction
between 35 to 90%. The cold rolled steel sheet is then subjected to annealing
to
impart the steel of present invention with targeted microstructure and
mechanical
properties.
To anneal the cold rolled steel sheet, the cold rolled steel sheet is heated
in a two-
step heating process, in step one the cold rolled sheets is heated to a
temperature
HT1 between 600 C and 650 C at a heating rate HR1 of at least10 C/s. Then, in
step two, the cold rolled sheet is heated from HT1 to an annealing temperature
between Ac3 and Ac3 +200 C at a heating rate HR2 of at least1 C/s and
preferably
at least 2.0 C/sHR1 is always higher than HR2.
The preferred HR1 is at least 15 C/s and the preferred HT1 temperature
range is between 600 C and 630 C. The preferred range for annealing
temperature
is between Ac3 +10 C and Ac3 +150 C and more preferably between Ac3 +20 C
and Ac3 +100 C.
Then the cold rolled steel sheet is held at the annealing temperature during
at least 5s and not more than 1000s. The temperature and time are selected to
zo ensure 100% re-crystallization i.e. to obtain a percentage of 100%
austenite at the
end of the annealing.
The sheet is then cooled in a three-step cooling process. In step one, the
cold rolled sheet is cooled from the annealing temperature to a temperature
CT1
between 675 C and 725 C at a cooling rate CR1 of 10 Cis or less. Then, in step
two, the cold rolled sheet is cooled from CT1 to CT2 between 450 C and 550 C
at
a cooling rate CR2 of at least 30 C/s. Then, in step three, the cold rolled
sheet is
cooled from CT2 to CT3 between Ms-50 C and 20 C at a cooling rate CR3 which
is at least 200 C/s.
In a preferred embodiment, the cooling rate CR1 is 5 C/s or less and CT1
is preferably between 685 C and 720 C and more preferably 685 C and 700 C.
The preferred range for CR2 is at least 40 C/s and the preferred range for CT2
is
between 450 C and 525 C and more preferably between 460 C and 510 C. The
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preferred range for CR 3 is at least 300 C/s and more preferably at least 400
C/s.
Preferred limit for CT3 is between Ms-80 C and 20 C and more preferably
between
Ms-100 C and 20 C.
5 Then the cold rolled steel sheet at a heating rate of at least 10 C/s,
or better
of at least 20 C/s and to a tempering temperature between 300 C and 380 C and
held at tempering temperature during at least 100 s but not more than 1000 s.
to
obtain tempered martensite, conferring the steel of present invention with
good
mechanical properties. The preferred tempering temperature range is between
io 320 C and 360 C and more preferably is 330 C and 350 C.
The cold rolled steel sheet is then cooled to room temperature, preferably
at a cooling rate of 200 C/s or less.
An optional skin pass operation with a reduction rate below 1% may be
performed at that stage or an optional tension leveling operation.
The heat treated cold rolled sheet may then be optionally coated by
electrodeposition or vacuum coating or any other suitable process.
An optional post batch annealing, preferably done at 170 to 210 C during
12h to 30h can be done optionally after annealing on uncoated product or after
coating on coated product in order to reduce hardness gradient between phases
and ensure degasing for coated products.
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
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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
Samples C Mn Si Al P S N Nb Ti Cr Mo B
A
0.116 2.150 0.482 0.031 0.020 0.011 0.002 0.011 0.017 0.036 0.002 0.003
B
0.117 2.120 0.495 0.033 0.02 0.022 0.002 0.011 0.015 0.040 0.002 0.004
C
0.114 2.180 0.458 0.032 0.017 0.019 0.002 0.018 0.025 0.033 0.002 0.003
Table 2 gathers the annealing process parameters implemented on steels
of Table 1.
Table 2 also shows Ac3 and Martensite transformation Ms temperatures of the
1.0 steel samples. The calculation of Ac3 and Ms is done by using following
formulas:
1
Ac3(Andrews) = 910 - 203[C]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]
Ms(Barbier) = 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[A/] - 16.3 [Cu] - 361 [Nb]
- 2.44[Ti] - 3448[B]
Further, the samples were heated to a temperature between 1000 C and
1250 C and then subjected to hot rolling with finish temperature 890 C and
thereafter were coiled at a temperature below 600 C. The hot rolled coils were
then
processed as claimed and cold rolled with a thickness reduction between 35 to
90%.
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Table 2 : process parameters of the trials
Table 2a
ANNEALING and COOLING
Trial Steel HR1 HT1 HR2 Annealing Annealing CT1 CR1 CT2 CR2 CT3 CR3
( C/s) ( C) ( C/s) T ( C) time (s) ( C) ( C/s) ( C) ( C/s) ( C) ( C/s)
11 A 22 610 2.à 870 112 688 2.1 462 64 20 587
12 B 18 600 2.6 870 130 691 t8 507 44 20 559
R1 A 23 590 3.5 870 102 702 2.1 487 66 20 677
R2 A 19 640 2.1 870 137 679 1.8 440 55 20 457
R3 C 18 610 2.4 875 137 660 2.1 460 46 20 478
R4 B 18 600 2.6 870 130 680 1.9 505 43 20 556
underlined values: not according to the invention.
Table 2b
TEMPERING
Trial Heating Tempering Tempering Ms Ac3( C)
rate for temp( C) time(s) ( C)
ternpering
( C/s)
11 24 340 197 409 829
12 21 340 228 409 829
R1 27 340 180 409 829
R2 20 340 240 409 829
R3 19 320 240 409 829
R4 21 340 228 409 829
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 Tempered Ferrite + Residual
Sample Martensite Bainite Austenite
11 74.9 25.1 0
12 72.4 27.6 0
R1 88.4 11.6 0
R2 63.3 36.7 0
R3 56.8 43.2 0
R4 63.4 36.6 0
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
test are
conducted in accordance with ISO 6892 standards, whereas to estimate hole
expansion, a test called hole expansion is applied according the standard
IS016630:2009. In this test, sample is subjected to punching to form a hole of
lOmm (=Di) and deformed. After deformation, the hole diameter Df was measured
and the hole expansion ratio (HER) is calculated using the under formula:
HE R /0= 100*(Df-Di)/Di
Table 4 : mechanical properties of the trials
Tensile Yield Hole
Total Elongation
Sample Strength (in Strength Expansion
(in 0/0)
Steels MPa) (in MPa) Ratio(in %)
Ii 1030 852 8.2 57
12
1006 822 9.4 74
111
1136 994 6.2 63
R2
945 739 9.9 56
R3
894 678 13.0 54
R4
937 735 11.1 55
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.
