Note: Descriptions are shown in the official language in which they were submitted.
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COLD ROLLED AND COATED STEEL SHEET AND A METHOD OF
MANUFACTURING THEREOF
The present invention relates to cold rolled and coated steel sheets suitable
for use
as steel sheet for automobiles.
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:
US20140234657 is a patent application that claims for a hot-dip galvanized
steel
sheet having a microstructure, by volume fraction, equal to or more than 20%
and
equal to or less than 99% in total of one or two of martensite and bainite, a
residual
structure contains one or two of ferrite, residual austenite of less than 8%
by
volume fraction, and pearlite of equal to or less than 10% by volume fraction.
Further US20140234657 reaches to a tensile strength of 980 MPa but unable to
reaches the elongation of 25%.
2
US8657969 claims for high strength galvanized steel sheet has a Tensile
Strength of 590 MPa or
more and excellent processability. The component composition contains, by mass
%, C: 0.05% to
0.3%, Si: 0.7% to 2.7%, Mn: 0.5% to 2.8%, P: 0.1% or lower, S: 0.01% or lower,
Al: 0.1% or lower,
and N: 0.008% or lower, and the balance: Fe or inevitable impurities. The
microstructure contains,
in terms of area ratio, ferrite phases: 30% to 90%, bainite phases: 3% to 30%,
and martensite
phases: 5% to 40%, in which, among the martensite phases, martensite phases
having an aspect
ratio of 3 or more are present in a proportion of 30% or more.
The purpose of the present invention is to solve these problems by making
available cold-rolled steel
and coated sheets that simultaneously have:
- an ultimate tensile strength greater than or equal to 600 MPa and
preferably above 620 MPa,
- an total elongation greater than or equal to 31% and preferably above
33%.
In a preferred embodiment, the steel sheets according to the invention may
also present a yield
strength 320 MPa or more.
In a preferred embodiment, the steel sheets according to the invention may
also present a yield
strength to tensile strength ratio of 0.6 or more.
Preferably, such steel can also have a good suitability for forming, in
particular for rolling with good
weldability and coatability.
In accordance with another aspect, the disclosure relates to a cold rolled
steel sheet having a
composition comprising the following elements, expressed in percentage by
weight:
0.13% Carbon 0.18%
1.1 % Manganese 1.8%
0.5 % Silicon 0.9 %
0.6% Aluminum 1%
0.002 % Phosphorus 0.02 %
0 % Sulfur ).003 %
0 To 5 Nitrogen 5 0.007%
and can contain one or more of the following optional elements
0.05% Chromium 1 %
0.001% Molybdenum 0. 5%
0.001% Niobium 0.1%
0.001 Titanium 0.1%
Date Recue/Date Received 2022-05-17
2a
0.01% 5 Copper 5 2%
0.01% 5 Nickel 5 3%
0.0001 5 Calcium 5 0.005%
0 % 5 Vanadium 5 0.1%
0 % 5 Boron 5 0.003%
0 % 5 Cerium 5 0.1%
0 % 5 Magnesium 50.010%
0 % 5 Zirconium 50.010%
the remainder composition being composed of iron and unavoidable impurities
caused by
processing, the microstructure of said steel sheet comprising in area
fraction, 60% to 75% Ferrite,
20% to 30% Bainite, 10% to 15% Residual Austenite, and 0% to 5% Martensite,
wherein the
cumulated amounts of Residual Austenite and Ferrite is between 70% and 80%.
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 cold rolled and heat treated steel sheet of the present invention may
optionally be coated with
zinc or zinc alloys, or with aluminium or aluminium alloys to improve its
corrosion resistance.
Carbon is present in the steel between 0.13% and 0.18%. Carbon is an element
necessary for
increasing the strength of the steel sheet by producing low-
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temperature transformation phases such as bainite, further Carbon also plays a
pivotal role in Austenite stabilization hence a necessary element for securing
Residual Austenite. Therefore, Carbon plays two pivotal roles one in
increasing the
strength and another in retaining austenite to impart ductility. But Carbon
content
less than 0.13% will not be able to stabilize Austenite in an adequate amount
required by the steel of present invention. On the other hand, at a Carbon
content
exceeding 0.1 8%, the steel exhibits poor spot weldability which limits its
application
for the automotive parts.
Manganese content of the steel of present invention is between 1.1 % and 1.8%.
This element is gammagenous. The purpose of adding Manganese is essentially
to obtain a structure that contains Austenite and impart strength to the
steel. An
amount of at least 1.1% by weight of Manganese has been found in order to
provide
the strength and hardenability of the steel sheet as well as to stabilize
Austenite.
But when Manganese content is more than 1.8% it produces adverse effects such
as it retards transformation of Austenite to Bainite during the over-aging
holding
for Bainite transformation. In addition the Manganese content of above 1.8%
also
reduces the ductility and also deteriorates the weldability of the present
steel hence
the elongation targets may not be achieved. A preferable content for the
present
invention may be kept between 1.2% and 1.8%, further more preferably 1.3% and
1.7%.
Silicon content of the steel of present invention is between 0.5% and 0.9%.
Silicon
is a constituent that can retard the precipitation of carbides during
overageing,
therefore, due to the presence of Silicon, carbon rich Austenite is stabilized
at room
temperature. Further, due to poor solubility of Silicon in carbide it
effectively inhibits
or retards the formation of carbides, hence also promotes the formation of
Bainitic
structure which is sought as per the present invention to impart steel with
its
essential features. However, disproportionate content of Silicon does not
produce
the mentioned effect and leads to a problem such as temper embrittlement.
Therefore, the concentration is controlled within an upper limit of 0.9%. A
preferable
content for the present invention may be kept between 0.6% and 0.8%
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Aluminum is an essential element and is present in the steel between 0.6% and
1%. Aluminum is an alphagenous element and imparts total elongation to the
steel
of present invention. A minimum of 0.6% of Aluminum is required to have a
minimum Ferrite thereby imparting the elongation to the steel of present
invention.
Aluminum is also used for removing oxygen from the molten state of the steel
to
clean steel of present invention by and it also prevents oxygen from forming a
gas
phase. But whenever the Aluminum is more than 1% it forms AIN which is
detrimental for the steel of Present invention therefore preferable range for
the
presence of the Aluminum is between 0.6% and 0.8%.
Phosphorus constituent of the steel of present invention is between 0.002% and
0.02%. 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 0.02 % and preferably
lower
than 0.014%.
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 is 0.003% or less from the viewpoint of manufacturing cost.
Further
if higher Sulfur is present in steel it combines to form Sulfides especially
with
zo Manganese and reduces its beneficial impact on the steel of present
invention.
Nitrogen is limited to 0.007% in order to avoid ageing of material and to
minimize
the precipitation of nitrides during solidification which are detrimental for
mechanical properties of the Steel.
Chromium is an optional element for the present invention. Chromium content
may
be present in the steel of present invention is between 0.05% and 1%. Chromium
is an essential element that provides strength and hardening to the steel but
when
used above 1% it impairs surface finish of steel. Further Chromium contents
under
1% coarsen the dispersion pattern of carbide in Bainitic structures, hence;
keep
the density of carbides low in Bainite.
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Molybdenum is an optional element that constitutes 0.001% to 0,5% of the Steel
of
present invention; Molybdenum plays an effective role in determining
hardenability
and hardness, delays the appearance of Bainite and avoids carbides
precipitation
in Bainite. However, the addition of Molybdenum excessively increases the cost
of
5 the addition of alloy elements, so that for economic reasons its content
is limited to
0.5%.
Niobium is an optional element for the present invention. Niobium content may
be
present in the steel of present invention between 0.001 and 0.1% and is added
in
the Steel of present invention for forming carbo-nitrides to impart strength
of the
lo 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
completion of annealing that will lead to the hardening of the Steel of
present
invention. 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.
Titanium is an optional element and may be added to the Steel of present
invention
between 0.001% and 0.1%. As Niobium, it is involved in carbo-nitrides
formation
.. so plays a role in hardening of the Steel of present invention. In addition
Titanium
also forms Titanium-nitrides which appear during solidification of the cast
product.
The amount of Titanium is so limited to 0.1% to avoid formation of coarse
Titanium-
nitrides detrimental for formability. In case the Titanium content is below
0.001% it
does not impart any effect on the steel of present invention.
Copper may be added as an optional element in an amount of 0.01% to 2% to
increase the strength of the steel and to improve its corrosion resistance. A
minimum of 0.01% of Copper is required to get such effect. However, when its
content is above 2%, it can degrade the surface aspects.
Nickel may be added as an optional element in an amount of 0.01 to 3% to
increase
.. the strength of the steel and to improve its toughness. A minimum of 0.01%
is
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required to produce such effects. However, when its content is above 3%,
Nickel
causes ductility deterioration.
Calcium content in the steel of present invention is between 0.0001% and
0.005%.
Calcium is added to steel of present invention as an optional element
especially
s during the inclusion treatment. Calcium contributes towards the refining
of Steel by
arresting the detrimental Sulfur content in globular form, thereby, retarding
the
harmful effects of Sulfur.
Vanadium is effective in enhancing the strength of steel by forming carbides
or
carbo-nitrides and the upper limit is 0.1% due to the economic reasons. Other
1.0 elements such as Cerium, Boron, Magnesium or Zirconium can be added
individually or in combination in the following proportions by weight: Cerium
0.1%, Boron 0.003%, Magnesium 0.010% and Zirconium 0.010%. 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
15 .. consists of iron and inevitable impurities resulting from processing.
The microstructure of the Steel sheet comprises:
Ferrite constitutes from 60% to 75% of microstructure by area fraction for the
Steel
of present invention. Ferrite constitutes the primary phase of the steel as a
matrix.
In the present invention, Ferrite cumulatively comprises of Polygonal ferrite
and
zo acicular ferrite Ferrite imparts high strength as well as elongation to
the steel of
present invention. To ensure an elongation of 31% and preferably 33% or more
it
is necessary to have 60% of Ferrite. Ferrite is formed during the cooling
after
annealing in steel of present invention. But whenever ferrite content is
present
above 75% in steel of present invention the strength is not achieved.
zs .. Bainite constitutes from 20% to 30% of microstructure by area fraction
for the Steel
of present invention. In the present invention, Bainite cumulatively consists
of Lath
Bainite and Granular Bainite, To ensure tensile strength of 620 MPa and
preferably
630 MPa or more it is necessary to have 20% of Bainite. Bainite is formed
during
over-aging holding.
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Residual Austenite constitutes from 10% to 15% by area fraction of the Steel.
Residual Austenite is known to have a higher solubility of Carbon than Bainite
and,
hence, acts as effective Carbon trap, therefore, retarding the formation of
carbides
in Bainite. Carbon percentage inside the Residual Austenite of present
invention is
preferably higher than 0.9% and preferably lower than 1.1%. Residual Austenite
of
the Steel according to the invention imparts an enhanced ductility.
Martensite is an optional constituent and may be present between 0% and 5 % of
microstructure by area fraction and found in traces. Martensite for present
invention
includes both fresh martensite and tempered martensite. Present invention form
martensite due to the cooling after annealing and get tempered during
overaging
holding. Fresh Martensite also form during cooling after the coating of cold
rolled
steel sheet. Martensite imparts ductility and strength to the Steel of present
invention when it is below 5%. When Martensite is in excess of 5 % it imparts
excess strength but diminishes the elongation beyond acceptable limit. The
preferable limit for martensite is between 0% and 3%.
A total amount of Ferrite and Residual Austenite must always be between 70%
and
80% to have total elongation of 31% and a minimum of 70% is required to ensure
the total elongation above 31% while having a tensile strength of 600M Pa.
Ferrite
and residual austenite are soft phase in comparison to martensite and bainite
zo therefore imparts for elongation and ductility but whenever the
cumulative
presence is more than 80% the strength drops beyond the acceptable limits.
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 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
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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, isat least
1150 C
and must be below 1280 C. In case the temperature of the slab is lower than
1150
C, excessive load is imposed on a rolling mill Therefore, the temperature of
the
slab is preferably sufficiently high so that hot rolling can be completed in
the
temperature range of Ad 1 +50 C to Ad +250 C and preferably between Ad +50 C
and Ac1+200 C while always having final rolling temperature remains above
Ac1+50 C. Reheating at temperatures above 1280 C must be avoided because
they are industrially expensive.
A final rolling temperature range between Ad 1 +50 C to Ad +250 C is preferred
to
have a structure that is favorable to recrystallization and rolling. It is
necessary to
zo have final rolling pass to be performed at a temperature greater than Ad
1 +50 C,
because below this temperature the steel sheet exhibits a significant drop in
rollability. The sheet obtained in this manner is then cooled at a cooling
rate above
30 C/s to the coiling temperature which must be below 625 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 625 C
to
avoid ovalization and preferably below 600 C to avoid scale formation. The
preferred range for such coiling temperature is between 350 C and 600 C. The
coiled hot rolled steel sheet may be cooled down to room temperature before
subjecting it to optional hot band annealing.
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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 subjected to an optional Hot Band Annealing at
temperatures between 400 C and 750 C for 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 sheet may
performed through, for example, pickling of such sheet. This hot rolled steel
sheet
is subjected to cold rolling to obtain a cold rolled steel 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.
In the annealing, the cold rolled steel sheet subjected to two steps of
heating to
reach the soaking temperature between Ad +30 C and Ac3 wherein Ad 1 and Ac3
.. for the present steel is calculated by using the following formula:
Ad 1 = 723- 10,7[Mn] - 16[Ni] + 29,1[Si] + 16,9[Cr] + 6,38[W] + 290[As]
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[AI] + 120[As] + 400[Ti]
wherein the elements contents are expressed in weight percent.
zo In step one cold rolled steel sheet is heated at a heating rate between
10 C/s and
40 C/s to a temperature range between 550 C and 650 C. Thereafter in
subsequent second step of heating the cold rolled steel sheet is heated at a
heating
rate between 1 C/s and 5 C/s to the soaking temperature of annealing.
Then the cold rolled steel sheet is preferably held at the soaking temperature
during
10 to 500 seconds to ensure at least 30% transformation to Austenite
microstructure of the strongly work-hardened initial structure. Then the cold
rolled
steel sheet is then cooled in two step cooling to an over-aging holding
temperature.
In step one of cooling the cold rolled steel sheet is cooled at cooling rate
less than
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5 C/s and preferably less than 3 C/s to a temperature range between 600 C and
720 C and preferably between 625 C and 720 C. During this step one of cooling
ferrite matrix of the present invention is formed. Thereafter in a subsequent
second
cooling step the cold rolled steel sheet is cooled to an overaging temperature
range
5 between 250 C and 470 C at a cooling rate between 10 C/s and 100 C/s.
Then
the cold rolled steel sheet is held in the over-aging temperature range during
5 to
500 seconds. The cold rolled steel sheet is then brought to the temperature to
a
coating bath temperature range of 400 C and 480 C to facilitate coating of the
cold
rolled steel sheet. Then the cold rolled steel sheet is coated by any of the
known
10 industrial processes such as Electro-galvanization, JVD, PVD, Hot
dip(GI) etc.
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
zo evaluations of obtained properties.
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Table 1
Other
Sample c
Mn Si Al P S N elements
Steels present
A 0.155 1.54 0.696 0.728
0.014 0.002 0.003
B 0.157 1.54 0.690 0.721
0.014 0.002 0.003
C 0.148 1.54 0.698 0 0.013 0.0027 0.0044
D 0.114 1.62 0.293 0.031 0.027 0.0028 0.005 -
Ni:0.025, Cr:
0.345
underlined values: not according to the invention.
Table 2
Table 2 gathers the annealing process parameters implemented on steels of
Table 1.
The Steel compositions A and B serve for the manufacture of sheets according
to the
invention. This table also specifies the reference steels which are designated
in table
as C and D . Table 2 also shows tabulation of Ad 1 and Ac3. These Ad 1 and Ac3
are
defined for the inventive steels and reference steels as follows:
Ad 1 =723 - 10,7[Mn] - 16[Ni] + 29,1[Si] + 16,9[Cr] + 6,38[W] + 290[As]
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[AI] + 120[As] + 400[Ti]
wherein the elements contents are expressed in weight percent.
All sheets were cooled at a cooling rate of 34 C/s after hot rolling and were
finally
brought at a temperature of 460 C before coating. All the sheets have a cold
rolled
reduction of 65%.
The table 2 is as follows :
Table 2
0
t.)
Step one
Step two =
N
=
--
=
Heating rate for Slow
HeatingSoaking u,
Steel Reheating HR Finish HR Coiling Fast heating Rate
before Soaking time
00
Trial fast heating before
Temperature N
Sample T ( C) T ( C) T ( C) stop T ( C).
annealing (s)
annealing (0 C/s)
( C/s) ( C)
11 A 1200 850 500 12 600 1.6
770 179
12 B 1200 870 520 19 600 3.1
800 110
13 A 1200 850 500 12 600 1.9
800 179
R1 A 1200 850 500 12 600 1.9
800 293
R2 C 1200 850 500 12 600 1.9
800 179
R3 D 1200 920 585 9 600 1.2
770 238 P
.
.
.,
1--,
N -
Step one Step two
,õ
Slow cooling Slow Fast Fast cooling
temperature
rate after cooling stop stop
Holding Ac3 Ad1
Trial cooling for overaging
time (s) ( C) ( C) .
annealing temperature rate (0C/s) temperature
( C)
( C/s) ( C/s) ( C)
11 0.6 700 30 410 410 129
1117 727
12 1.5 700 39 460 460 79
1112 727
13 0.9 700 31 400 400 129
1112 727
R1 _ 34 460 460 129
1117 727
-o
R2 0.9 700 31 400 400 129
827 727 en
-i
R3 0.5 700 27 350 350 172
835 720 --,
FJ
=
.-
rA
I = according to the invention; R = reference; underlined values: not
according to the invention. -1
-.1
sz,
,..
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Table 3
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.
The results are stipulated herein:
Ferrite
Sample Ferrite Bainite Residu.al Martensite +Residual
Steels (%o) (0/ Austenite 0) (0/0) Austenite
(%)
(0/0)
11 67 22 11 0 78
12 65 24 10 1 75
13 63 27 10 0 73
RI 53 37 10 0 63
R2 62 32 5 1 67
R3 62 33 5 0 67
I = according to the invention; R = reference; underlined values: not
according to the
invention.
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Table 4
Table 4 exemplifies the mechanical properties of both the inventive steel and
reference
steels. In order to determine the tensile strength, yield strength and total
elongation,
tensile tests are conducted in accordance of JIS Z2241 standards.
The results of the various mechanical tests conducted in accordance to the
standards
are gathered
Table 4
Sample Tensile Strength YS Total
YS/TS
Steels (MPa) (MPa) Elongation(%)
Ii 634 386 0.61 34.7
12 672 401 0.60 33.2
13 633 411 0.65 36.1
RI 677 389 0.57 27.7
R2 602 365 0.61 28.6
R3 622 343 0.55 22.5
lo 1 = according to the invention; R = reference; underlined values: not
according to the
invention.
