Note: Descriptions are shown in the official language in which they were submitted.
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A COLD ROLLED MARTENSITIC STEEL AND A METHOD OF MARTENSITIC STEEL
THEREOF
The present invention relates to a method of manufacturing of a cold rolled
martensitic steel
suitable for automotive industry and particularly to Martensitic steels having
tensile strength 1280
MPa or more.
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
3.0 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:
The steel sheet of W02017/065371 is manufactured through the steps of: rapidly
heating a
material steel sheet for 3 to 60 seconds to an Ac3 transformation point or
higher and maintaining
the material steel sheet, the material steel sheet containing 0.08 to 0.30 wt%
of C, 0.01 to 2.0
wt% of Si, 0.30 to 3.0 wt% of Mn, 0.05 wt% or less of P and 0.05 wt% or less
of S and the
remainder being Fe and other unavoidable impurities; rapidly cooling the
heated steel sheet to
100 C/s or higher with water or oil; and rapidly tempering to 500 C to Al
transformation point for
3 to 60 seconds including heating and maintaining time. But the steel of
W02017/065371 not able
to surpass the tensile strength of 1300 MPa and do not mention about hole
expansion ratio even
having a tempered martensite single phase structure.
W02010/036028 relates to a hot dip galvanized steel sheet and a manufacturing
method thereof.
The hot dip galvanize steel sheet includes a steel sheet including a
martensitic structure as a
matrix, and a hot dip galvanized layer formed on the steel sheet. The steel
sheet includes C of
2
0.05 wt % to 0.30 wt %, Mn of 0.5 wt % to 3.5 wt %, Si of 0.1 wt % to 0.8 wt
A), Al of 0.01 wt A)
to 1.5 wt %, Cr of 0.01 wt % to 1.5 wt %, Mo of 0.01 wt % to 1.5 wt %, Ti of
0.001 wt % to 0.10
wt %, N of 5 ppm to 120 ppm, B of 3 ppm to 80 ppm, an impurity, and the
remainder of Fe. But
the steel of W02010/036028 does not mentions hole expansion ratio.
.. A purpose of the present invention is to solve these problems by making
available cold-rolled
martensitic steel sheets that simultaneously have:
- an ultimate tensile strength greater than or equal to 1280 MPa and
preferably above 1300
MPa,
- a yield strength greater than or equal to 1100 MPa and preferably
above 1150 MPa
- a hole expansion ratio of more the 40% and preferably above 50%
According to a first aspect, a cold rolled martensitic steel sheet is provided
comprising of the
following elements, expressed in percentage by weight:
0.1 % C 0.2 %;
1.5 % ^ Mn 2.5 %;
0 .1% ^ Si 0.25 %;
0.1% Cr 1 %;
0.01% -5õ, Al 0.1 %;
0.001% -5,, Ti 0.1%;
0% S 0.09%;
0% P 0.09%;
0% N 0.09%;
and can contain one or more of the following optional elements:
0% Ni 1%;
0% Cu 1%;
0% Mo 5- 0.4%;
0% -5õ Nb -5- 0.1%;
0% 0.1%;
0% -5õ B -5õ 0.05%;
0% -5õSn-5õ 0.1%;
P b 0.1%;
0% Sb-5_, 0.1%;
Date Recue/Date Received 2023-03-21
2a
0.001% Ca 0.01%;
the remainder composition being composed of iron and unavoidable impurities
caused
by processing, the microstructure of said steel comprising, by area
percentage, at least
95% of martensite, a cumulated amount of ferrite and bainite between 1 % and
5%, and
an optional amount of residual austenite between 0% and 2%, wherein said
martensite
includes tempered martensite and fresh martensite.
Preferably, such steel can also have a good suitability for forming, for
rolling with good weldability
and coatability.
According to another aspect, the invention is related to a method of
production of a cold rolled
martensitic steel sheet comprising the following successive steps:
- providing a semi-finished product having a steel composition as
described herein;
- reheating said semi-finished product to a temperature between 1000 C and
1280 C;
- hot rolling the said semi-finished product in the austenitic range wherein
the hot
rolling finishing temperature is between Ac3 and Ac3 + 100 C to obtain a hot
rolled steel sheet;
- cooling the hot rolled steel sheet at a cooling rate of at
least 20 C/s to a coiling
temperature which is below 650 C; and coiling the hot rolled sheet;
- cooling the hot rolled sheet to room temperature;
- optionally performing scale removal process on said hot rolled
steel sheet;
- optionally annealing may be performed on the hot rolled steel
sheet;
- optionally performing a scale removal process on said hot
rolled steel sheet;
- cold rolling the hot rolled steel sheet with a reduction rate
between 35 and 90%
to obtain a cold rolled steel sheet;
- then heating the cold rolled steel sheet at a rate of at least 2 C/s to a
soaking
temperature Tsoak between Ac3 and Ac3+100 C where it is held during 10 to 500
seconds;
- then cooling the cold rolled steel sheet in a two step cooling
process wherein:
o a first step of cooling the cold rolled steel sheet starts from Tsoak down
to
a temperature Ti between 650 C and 750 C, with a cooling rate CR1
between 15 C/s and 150 C/s;
Date Recue/Date Received 2023-03-21
2b
o
a second step of cooling starts from Ti down to a temperature T2 between
Ms-10 C and 20 C, with a cooling rate CR2 of at least 50 C/s,
- then reheating the said cold rolled steel sheet at a rate of at least 1 C/s
to a
tempering temperature Ttemper between 150 C and 300 C where it is held during
100 to 600 seconds;
- then cooling to room temperature with a cooling rate of at least 1
C/s to obtain a
cold rolled martensitic steel sheet.
According to another aspect the invention is related to a use of a steel sheet
described herein or
a steel sheet manufactured according to the method described herein, for
manufacturing a
structural part of a vehicle.
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.
The above object and other advantages of the present invention will become
more apparent by
describing in detail the preferred embodiment of the present invention.
The chemical composition of the cold rolled martensitic steel comprises of the
following
elements:
Carbon is present in the steel of present invention is between 0.1% and 0.2%.
Carbon is an
element necessary for increasing the strength of the Steel of present
invention by producing a
low-temperature transformation phases such as Martensite, Therefore, Carbon
plays two pivotal
roles, one is to increase the strength. But Carbon content less than 0.1% will
not be able to impart
the tensile strength to the steel of present invention. On the other hand, at
a Carbon content
exceeding 0.2%, the steel exhibits poor spot weldability which limits its
application for the
automotive parts. A preferable content for the present invention may be kept
between 0.11% and
0.19% and more preferably between 0.12% and 0.18%.
Manganese content of the steel of present invention is between 1.5 % and 2.5%.
This element
is gammagenous. Manganese provides solid solution strengthening and suppresses
the ferritic
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transformation temperature and reduces ferritic transformation rate hence
assist in the formation
of martensite. An amount of at least 1.5% is required to impart strength as
well as to assist the
formation of Martensite. But when Manganese content is more than 2.5% it
produces adverse
effects such as it retards transformation of Austenite to Martensite during
cooling after annealing.
Manganese content of above 2.5% can get excessively segregated in the steel
during
solidification and homogeneity inside the material is impaired which can cause
surface cracks
during a hot working process. The preferred limit for the presence of
Manganese is between 1.6%
and 2.4% and more preferably between 1.6% and 2.2%.
Silicon content of the steel of present invention is between 0.1% and 0.25%.
Silicon is an element
that contributes to increasing the strength by solid solution strengthening.
Silicon is a constituent
that can retard the precipitation of carbides during cooling after annealing,
therefore, Silicon
promotes formation of Martensite. But Silicon is also a ferrite former and
also increases the Ac3
transformation point which will push the annealing temperature to higher
temperature ranges that
is why the content of Silicon is kept at a maximum of 0.25%.Silicon content
above 0.25% can also
temper embrittlement and in addition silicon also impairs the coatability. The
preferred limit for the
presence of Silicon is between 0.16% and 0.24% and more preferably between
0.18% and 0.23%.
Chromium content of the composite coil of steel of present invention is
between 0.1% and 1%.
Chromium is an essential element that provide strength to the steel by solid
solution strengthening
and a minimum of 0.1% is required to impart the strength but when used above
1% impairs
surface finish of steel. The preferred limit for the presence of Chromium is
between 0.1% and
0.5%.
The content of the Aluminum is between 0.01% and 1%. in the present invention
Aluminum
removes Oxygen existing in molten steel to prevent Oxygen from forming a gas
phase during
solidification process. Aluminum also fixes Nitrogen in the steel to form
Aluminum nitride to reduce
the size of the grains. Higher content of Aluminum, above 1%, increases Ac3
point to a high
temperature thereby lowering the productivity. The preferred limit for the
presence of Aluminium
is between 0.01% and 0.05%
Titanium is added to the Steel of present invention between 0.001 % to 0.1%.
It forms Titanium-
nitrides appearing during solidification of the cast product. The amount of
Titanium is so limited
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to 0.1% to avoid the formation of coarse Titanium-nitrides detrimental for
formability. In case the
Titanium content below 0.001% does not impart any effect on the steel of
present invention.
Sulfur is not an essential element but may be contained as an impurity in
steel and from point of
view of the present invention the Sulfur content is preferably as low as
possible but 0.09% or less
from the viewpoint of manufacturing cost. Further if higher Sulfur is present
in steel it combines
to form Sulfides especially with Manganese and reduces its beneficial impact
on the present
invention.
Phosphorus constituent of the Steel of present invention is between 0% and
0.09%, Phosphorus
reduces the spot weldability and the hot ductility, particularly due to its
tendency to segregate at
the grain boundaries or co-segregate with Manganese. For these reasons, its
content is limited
to 0.09 % and preferably lower than 0.06%.
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 0% to 0.4% of the Steel of
present invention;
Molybdenum plays an effective role in improving hardenability and hardness,
delays the
appearance of Bainite hence promote the formation of Martensite, in particular
when added in an
amount of at least 0.001% or even of at least 0.002%. 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 0.4%.
Niobium is present in the Steel of present invention between 0% and 0.1% and
suitable for forming
carbo-nitrides to impart strength of the Steel of present invention by
precipitation hardening.
Niobium will also impact the size of microstructural components through its
precipitation as carbo-
nitrides and by retarding the recrystallization during heating process. Thus,
finer microstructure
formed at the end of the holding temperature and as a consequence after the
complete annealing
will lead to the hardening of the product. However, Niobium content above 0.1%
is not
economically interesting as a saturation effect of its influence is observed
this means that
additional amount of Niobium does not result in any strength improvement of
the product.
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Vanadium is effective in enhancing the strength of steel by forming carbides
or carbo-nitrides and
the upper limit is 0.1% from economic points of view.
Nickel may be added as an optional element in an amount of 0% to 1% to
increase the strength
5 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 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 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 1%, it
can degrade the surface
aspects.
Boron is an optional element for the steel of present invention and may be
present between 0%
and 0.05%. Boron forms boro-nitirides and impart additional strength to steel
of present invention
when added in an amount of at least 0.0001%.
Calcium can be added to the steel of present invention in an among between
0.001% and
0.01%%. 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 binding the
detrimental Sulfur content in globular form thereby retarding the harmful
effect of Sulfur.
Other elements such as Sn , Pb or Sb can be added individually or in
combination in the following
proportions: Sn 0.1%, Pb 0.1% and Sb 0.1%. 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 martensitic steel sheet will now be described in
details, all
percentages being in area fraction.
Martensite constitutes at least 95% of the microstructure by area fraction.
The martensite of the
present invention can comprise both fresh and tempered martensite. However,
fresh martensite
is an optional microconstituent which is limited in the steel at an amount of
between 0% an 4%,
preferably between 0 and 2% and even better equal to 0%. Fresh martensite may
form during
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cooling after tempering. Tempered martensite is formed from the martensite
which forms during
the second step of cooling after annealing and particularly after below Ms
temperature and more
particularly between Ms-10 C and 20 C.Such martensite is then tempered during
the holding at
a tempering temperature Ttemper between 150 C and 300 C. The martensite of the
present
invention imparts ductility and strength to such steel. Preferably, the
content of martensite is
between 96% and 99% and more preferably between 97% and 99%.
The cumulated amount of ferrite and bainite represents between 1% and 5% of
the microstructure.
The cumulative presence of bainite and ferrite does not affect adversely to
the present invention
till 5% but above 5% the mechanical properties may get impacted adversely.
Hence the preferred
limit for the cumulative presence ferrite and bainite is kept between 1% and
4% and more
preferably between 1% and 3%.
Bainite forms during the reheating before tempering. In a preferred
embodiment, the steel of
present invention contains 1 to 3% of bainite. Bainite can impart formability
to the steel but when
present in a too big amount, it may adversely impact the tensile strength of
the steel.
Ferrite may form during the first step of cooling after annealing but is not
required as a
microstructural constituent. Ferrite formation must be kept as low as possible
and preferably less
than 2% or even less than 1%.
Residual Austenite is an optional microstructure that can be present between
0% and 2% in the
steel.
In addition to the above-mentioned microstructure, the microstructure of the
cold rolled martensitic
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.
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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.
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 1280 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.
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 between 475 C and 625 C to avoid scale formation, with an even
prefererred
range for such coiling temperature between 500 C and 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
have subjected to an optional hot band annealing. In a preferred embodiment,
such hot band
annealing is performed at temperatures between 400 C and 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.
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This hot rolled steel sheet is then subjected to cold rolling to obtain a cold
rolled steel sheet with
a thickness reduction between 35 to 90%.
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 heated at a heating rate which is of at least 2
C/s and preferably
greater than 3 C/s, to a soaking temperature Tsoak between Ac3 and Ac3+100 C
and preferably
between Ac3+10 C and Ac3+100 C, 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 [Ma] +
13.1[W] ¨ 30 [Mn]
¨ 11 [Cr] ¨ 20 [Cu] + 700[P] + 400 [AI] + 120 [As] + 400 [Ti]
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 15 C/s and 150 C/s, to a temperature Ti which is in a range between
650 C and 750 C.
In a preferred embodiment, the cooling rate CR1 for such first step of cooling
is between 20 C/s
and 120 C/s. The preferred Ti temperature for such first step is between 660 C
and 725 C.
In the second step of cooling, the cold rolled steel sheet is cooled down from
Ti to a temperature
T2 which is between Ms-10 C and 20 C, at a cooling rate CR2 of at least 50
C/s. In a preferred
embodiment, the cooling rate CR2 for the second step of cooling is at least
100 C/s and more
preferably at least 150 C/s. The preferred T2 temperature for such second step
is between Ms-
50 C and 20 C.
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Ms for the steel sheet is calculated by using the following formula:
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] ¨ 14[A/] ¨ 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 1 C/s and preferably of at
least 2 Cis and more
of at least 10 C/s during 100 s and 600 s.The preferred temperature range for
tempering is
between 200 C and 300 C and the preferred duration for holding at Ttemper is
between 200 s
and 500 s.
Then, the cold rolled steel sheet is cooled down to room temperature to obtain
a cold rolled
martensitic steel.
The cold rolled martensitic 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.
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, 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
1,4
Steels C Mn Si Al Cr Nb S P N Mo Cu Ni
V B Ti tsa
A 0.148 1.884 0.204 0.028 0.176 0.0005 0.019 0.0183 0.0041 0.0022 0.012
0.015 0.0022 0.0001 0.0235 oo
oo
0.153 1.882 0.202 0.025 0.182 0.0012 0.002 0.0116 0.0047 0.0026 0.016 0.016
0.0018 0.0001 0.0206
0.141 1.888 0.203 0.024 0.184 0.0005 0.008 0.0139 0.0043 0.0027 0.027 0.016
0.0018 0.0001 0.0227
according to the invention; R = reference; underlined values: not according to
the invention.
rs,
tsi
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Table 2
Table 2 gathers the hot rolling and annealing process parameters implemented
on cold rolled
steel sheets to impart the steels of table 1 with requisite mechanical
properties to become a cold
rolled martensitic steel.
The table 2 is as follows:
0
Hot rolling CR Annealing Step 1 Cooling Step 2
Cooling 1,4
w
Trials
Steel Reheat FRT Cooling rate to Coiling Reduction Heating
rate to Tsoak Tsoak ( C) Annealing Ti CR1 ( C/s)' T2 CR2
,
tsa
t..)
ing ( C) coiling ( C/s) ( C)
(%) ( C/s) time(s) ( C) ( C) ( C/s)
oo
oo
( C)
11 A 1245 895 30 530 40 6.5
888 290 692 36 25 690
12 B 1245 895 30 530 60 10
890 165 690 72 25 1030
13 C 1245 895 30 530 50 7
890 245 675 48 25 750
R1 A 1245 895 30 530 40 6.5
880 290 600 34 25 600
R2 A 1245 895 30 530 40 6.5
882 290 643 35 25 636
i 1
P
.
i-
..
.
1-'
.1
n)
t:)
n)
_ Tempering
i-
,
i-
,
Trials Steel Heating rate to Ttemper
Holding time Ms Ac3 i-
i-
Ttemper ( C/s) ( C) (s) ( C) ( C)
11 A 12 230 290 403 816
7
12 B 20 230 165 400 808
I-
13 C 15 230 245 405 812
R1 A 12 230 290 403 816
ti
n
R2 A 12 230 290 403 816
tt
i . according to the invention; R = reference; underlined values: not
according to the invention. w
t..)
=
,
=
tm
tsi
Y>
Y>
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Table 3 exemplifies the results of the tests conducted in accordance with the
standards on
different microscopes such as Scanning Electron Microscope for determining the
microstructures
of both the inventive and reference steels in terms of area fraction. The
results are stipulated
herein:
Table 3 :
Trials Steel Tempered Fresh
Ferrite + Residual
Martensite Martensite Bainite Austenite
Ii A 98% 0 2% 0
12 B 98% 0 2% 0
13 C 98% 0 2% 0
R1 A 8 9 /0 0 1 1 % 0
R2 A 93.5% 0 6.5% 0
I = according to the invention; R = reference; underlined values: not
according to the invention.
Table 4
The results of the various mechanical tests conducted in accordance to the
standards are
gathered. For testing the ultimate tensile strength and yield strength are
tested in accordance of
J1S-Z2241. To estimate hole expansion, a test called hole expansion is
applied, in this test sample
is subjected to punch a hole of 10mm and deformed after deformation we measure
the hole
diameter and calculate HER%. 100*(Df-Di)/Di
Tensile Yield HER
Trials Steels Strength Strength
(MPa) (MPa) (%)
Ii A 1321 1160 68
12 B 1344 1207 70
13 C 1334 1208 50
R1 A 1190 983 20
R2 A 1262 1071 35
I = according to the invention; R = reference; underlined values: not
according to the invention.
