Note: Descriptions are shown in the official language in which they were submitted.
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HOT ROLLED AND STEEL SHEET AND A METHOD OF MANUFACTURING
THEREOF
The present invention relates to hot rolled steel sheet suitable for use as
structural
steel or for manufacturing industrial machinery, yellow goods, green goods and
for
cryogenic applications.
In recent years, efforts have been actively made to reduce the weight of the
equipment
and structures by applyina high-strength steel for the purpose of improving
fuel
efficiency as well as reducing the environmental impact. However, when the
strength
of the steel is increased, the toughness generally deteriorates. Therefore, in
the
development of high-strength steel, it is an important issue to increase the
strength
without deteriorating the toughness.
Intense Research and Development endeavors are put in reducing the amount of
material utilized by increasing the strength of material. Conversely, an
increase in
strength of steel decreases toughness, and thus development of materials
having both
.. high strength and good toughness is necessitated.
Earlier Research and Developments in the field of high strength and good
toughness
steel have resulted in several methods for producing high strength steel, some
of which
are enumerated herein for conclusive appreciation of the present invention:
EP2392681 discloses thick-walled high-strength hot rolled steel sheet having
zo composition which contains by mass% 0.02 to 0.08% C, 1.0% or less Si,
0.50 to 1.85%
Mn, 0.03% or less P, 0.005% or less S, 0.1% or less Al, 0.03 to 0.10% Nb,
0.001 to
0.05% Ti, 0.0005% or less B, optionally one or two kinds or more selected from
a group
consisting of 0.010% or less Ca, 0.02% or less REM, 0.003% or less Mg, 0.5% or
less
V, 1.0% or less Mo, 1.0% or less Cr, 4.0% or less Ni, 2.0% or less Cu, other
unavoidable impurities and Fe as balance. The steel sheet has the structure
formed
of bainitic ferrite phase or a bainite phase in which solid-solution C content
in ferrite
grains is 10ppm or more, and surface layer hardness is 230HV or less in terms
of
Vickers hardness but the steel of EP2392681 is unable to reach the tensile
strength of
700MPa or more.
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EP2971211 discloses a method for fabricating a high manganese steel component
having a composition consisting of: manganese ranging from about 9 to about 20
weight % of the total composition, carbon ranging from about 0.5 to about 2.0
weight
% of the total composition, and the balance iron; and optionally: chromium
ranging
from 0.5 to 30 weight % of the total composition; nickel or cobalt ranging
from 0.5 to
20 weight % of the total composition; aluminum ranging from 0.2 to 15 weight %
of the
total composition; molybdenum, niobium, copper, titanium or vanadium ranging
from
0.01 to 10 weight % of the total composition; silicon ranging from 0.1 to 10
weight %
of the total composition; nitrogen ranging from 0.001 to 3.0 weight % of the
total
io
composition; boron ranging from 0.001 to 0.1 weight % of the total
composition; or
zirconium or hafnium ranging from 0.2 to 6 weight % of the total composition;
heating
the composition to at least about 1000 C; cooling the composition at a rate of
from
about 2 C per second to about 60 C per second, followed by hot rolling the
composition at a temperature in a range of about 700 C to about 1000 C; slowly
cooling or isothermally holding the composition; and quenching or accelerated
cooling
or air cooling the composition from a temperature in a range of from 700 C to
about
1000 C to a temperature in range of from 0 C to about 500 C at a rate of at
least about
10 C per second. But EP2971211 is not able to reach an Impact toughness of
60J/cm2
or more when measured at -40 C
zo
The purpose of the present invention is to solve these problems by making
available
hot-rolled steel that simultaneously have:
- a yield strength 715 MPa or more and preferably equal to or more than
725MPa,
- a tensile strength of 750 MPa or more and preferably 800MPa or more,
- a total elongation greater than or equal to 20%.
- an impact toughness of greater than or equal to 60 J/cm2 when measured at -
40 C.
In a preferred embodiment, the steel sheets according to the invention may
also
present a yield strength to tensile strength ratio of 0.5 or more
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Preferably, such steel can also have a good suitability for forming, in
particular for
rolling with good weldability, bending.
Another object of the present invention is also to make available a method for
the
manufacturing of these steels that is compatible with conventional industrial
processes
while being robust towards manufacturing parameters shifts.
The hot rolled steel sheet of the present invention may optionally be coated
with zinc
or zinc alloys, to improve its corrosion resistance.
Carbon is present in the steel between 0.02% and 0.2%. Carbon is an element
necessary for increasing the strength of the steel by assisting in the
stabilization of
austenite at room temperature. But Carbon content less than 0.02% 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 weldability as well as
it is
detrimental for the impact toughness which limits its application for the
structural parts
of yellow or green goods. A preferable content for the present invention may
be kept
between 0.03% and 0.18%, and more preferably between 0.04% and 0.15%.
Manganese content of the steel of present invention is between 3 % and 9%.
This element is gammagenous and therefore plays an important role in
controlling the
residual austenite fraction as well as enriching the residual austenite with
Manganese
zo to impart hardenability to the steel and impact toughness. An amount of
at least 3% by
weight of Manganese has been found in order to provide the strength and
toughness
to the steel. But when Manganese content is more than 9 % it produces adverse
effects
such as it stabilizes the austenite too much and devoid the steel of present
invention
from TRIP effect. In addition, the Manganese content of above 9% leads to
excessive
central segregation, hence reducingthe formability and also deteriorating the
weldability of the present steel. A preferable content for the present
invention may be
kept between 3.5% and 8.5% and more preferably 4% and 8%.
Silicon content of the steel of present invention is between 0.2% and 1.2%.
Silicon is
solid solution strengthener for the steel of present invention. In addition,
Silicon retards
the precipitation of Cementite and also limits the formation of cementite
although it
often cannot completely eliminate cementite formation. Silicon keeps C in
solid solution
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in austenite, as such lower the Ms temperature to below room temperature. As
such,
Silicon assists in the formation of Residual austenite at room temperature.
However, a
content of Silicon more than 1.2% leads to a problem such as surface defects
which
adversely effects the steel of present invention. Therefore, the concentration
is
controlled within an upper limit of 1.2%. A preferable content for the present
invention
may be kept between 0.3% and 1`)/0 and more preferably between 0.4% and 0.8%.
Aluminum is an essential element and is present in the steel between 0.9% and
2.5%.
Aluminum is an alphagenous element a minimum of 0.9% of Aluminum is required
to
have a minimum Ferrite thereby imparting the elongation and toughness 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 inventionand it also prevents oxygen
from forming
a gas phase. But whenever the Aluminum is more than 2.5% it is difficult to do
casting
because of the surface defects on the slabs such as breakouts. Therefore
preferable
range for the presence of the Aluminum is between 1% and 2.3% and more
preferably
between 1% and 2%.
Phosphorus content of the steel of present invention is between 0% and 0.03%.
Phosphorus reduces the hot ductility and toughness, particularly due to its
tendency to
zo segregate at the grain boundaries or co-segregate with manganese. For these
reasons, its content is limited to 0.03% and preferably lower than 0.015%.
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 0.03% or less from the viewpoint of manufacturing cost.
Further if
higher Sulfur is present in steel it combines to form Sulfides especially with
Manganese
which is detrimental on the steel of present invention, therefore preferred
below 0.01%
Nitrogen is limited to 0.025% 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. Hence the preferable upper limit for nitrogen is
0.02% and more
preferably 0.005%.
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Molybdenum is an optional element that constitutes 0% to 0.6% of the steel of
present
invention. Molybdenum increases the hardenability and allowing the steel of
present
invention to achieve targeted properties for thicker gauges . A minimum of
0.1% of
Molybdenum is required to be beneficial in increasing the hardenability.
However, the
5 addition of Molybdenum excessively increases the cost of the addition of
alloy
elements, so that for economic reasons its content is limited to 0.6%.
Preferable limit
for molybdenum is between 0 % and 0.4% and more preferably between 0 % and
0.3%.
Titanium is an optional element and present between 0% and 0.1% in the steel
of
present invention. Titanium imparts the steel of present invention with the
strength by
forming carbides and control the grain size. But whenever Titanium is present
more
than 0.1%, it imparts excess strength and hardness to the steel of present
invention
which diminishes the toughness beyond the targeted limits. The preferable
limit for
titanium is between 0 % and 0.09% and more preferred limit is 0% and 0.08%.
Boron is an optional element to the steel of present invention and may be
present
between 0.0001% and 0.01%. Boron imparts toughness to the steel of present
invention when added along with Titanium.
Chromium is an optional element for the present invention. Chromium content
may be
zo present in the steel of present invention is between 0% and 0.5%. Chromium
is an
element that provides hardenability to the steel but content of Chromium
higher than
0.5% leads to central co-segregation with Manganese.
Vanadium is an optional element that may be present between 0% and 0.2% of the
steel of present invention. Vanadium is effective in enhancing the strength of
steel by
forming carbides, nitrides or carbo-nitrides and the upper limit is 0.2% due
to economic
reasons and even if Vanadium is present above 0.2% it does not bring any
considerable benefit to the steel of present invention.
Niobium is an optional element for the present invention. Niobium content may
be
present in the steel of present invention between 0% and 0.1 A and is added in
the
steel of present invention for forming carbides or carbo-nitrides to impart
strength to
the steel of present invention by precipitation strengthening. Preferable
limit is between
0% and 0.05%
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Nickel may be added as an optional element in an amount of 0% to 1`)/0 to
increase the
strength of the steel present invention and to improve its toughness. A
minimum of
0.01% is preferred to get such effects. However, the content of Nickel is
restricted to
1% due to economic viability.
Copper may be added as an optional element in an amount of 0% to 1`)/0 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 lead to problems such as copper hot shortness during the
casting
process.
Calcium content in the steel of present invention is below 0.005%. Calcium is
added
to steel of present invention in a preferable amount of 0.0001 to 0.005% as an
optional
element especially during the inclusion treatment, thereby, retarding the
harmful
effects of Sulfur.
Other elements such as, Magnesium can be added in the following proportions by
weight Magnesium 0.0010%. 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 Steels comprises:
zo Martensite is present in the steel of present invention is at least 60%
wherein the
martensite of present invention is comprising of tempered martensite and fresh
martensite wherein tempered martensite is the matrix phase for the steel of
present
invention. The tempered martensite of the steel of present invention
preferably has its
aspect ratio between 4 and 12 preferably and more preferably between 5 and 11.
The
aspect ratio is the ratio between the longest and the shortest dimension
within a single
grain. Tempered martensite is formed from the rflartensite which forms during
the
cooling after hot rolling. Such martensite is then tempered during the
annealing
process. The tempered martensite of the steel of present invention imparts
ductility
and strength. It is preferred that the content of tempered martensite is
between 65%
and 84% and more preferably between 70% and 80% by area fraction of total
microstructure. Fresh martensite can also be optionally present in the steel
of present
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invention. Fresh martensite may form during cooling after annealing from
remaining
unstable residual austenite. Fresh martensite can be present between 0% an
15%,
preferably between 0 and 10% and even better no fresh martensite is present.
Residual Austenite is an essential microstructural constituent of the steel of
present
invention and is present between 15% and 40%. Residual Austenite of the
present
invention imparts toughness to the steel of present invention. Residual
Austenite of the
present invention can only be stable at room temperature when it is enriched
with
Manganese and Carbon. The percentage of Carbon inside the Residual Austenite
higher than 0.8% and lower than 1.1%. The percentage of Manganese in Residual
Austenite is preferably more than 5% and more preferably more than 5.5%.
However,
when the Residual Austenite of present invention is not enriched with Carbon
and
Manganese it will not be stable at room temperature and will lead to formation
of
excess fresh martensite instead of adequate amount of Residual Austenite This
effect
provides excess strength to the steel and is also detrimental to elongation
and
toughness. The preferable limit for the presence of Austenite is between 18%
and 35%
and more preferably between 18% and 30% wherein the preferable Carbon content
limit in austenite is preferred between 0.9% and 1.1% and more preferably
between
0.95% and 1.05%.
Polygonal Ferrite constitutes from 0% to 10% of microstructure by area
fraction for the
zo Steel of present invention. In the present invention Polygonal Ferrite
imparts high
strength as well as elongation to the steel of present invention. Polygonal
Ferrite may
be formed during the soaking and cooling after annealing in steel of present
invention.
But whenever polygonal ferrite content is present above 10% in steel of
present
invention the strength is not achieved.
Bainite and cementite may present in the steel of present invention between 0%
and
5%. Till 5% bainite does not influence the target properties of the steel of
present
invention.
In addition to the above-mentioned microstructure, the microstructure of the
hot rolled
steel is free from microstructural components, such as Pearlite. Carbides of
alloying
elements might be present in the steel of present invention between 0% and 5%
such
as of Niobium, Titanim, Vanadium, Iron and others these carbides impart the
steel of
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present invention with the strength by precipitation strengthening in but
whenever the
presence of carbides is 5% or more carbides consume partly the amount of
Carbon,
which is deterimental for the stablization of residual austenite.
A hot rolled steel 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 thick slabs, thin slabs or thin strips, i.e.
with a thickness
ranging from approximately 220mm to 350mm 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. 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 slab is reheated to a temperature between at least Ac3 + 50 C and 1300 C.
In
case the temperature of the slab is lower than least Ac3 + 50 C, excessive
load is
imposed on the rolling mill. Therefore, the temperature of the slab is
sufficiently high
so that hot rolling can be completed fully in the austenitic range. Reheating
at
zo temperatures above 1300 C must be avoided because it causes productivity
loss and
is also industrially expensive and some segregated parts may melt which may
lead to
breaking of slabs or cracking of slabs. Therefore, the preferred reheating
temperature
is between least Ac3 + 100 C and 1280 C.
Hot rolling finishing temperature for the present invention is at least Ac3
and preferably
between Ac3 and Ac3 + 100 C, more preferably between 840 C and 1000 C and
even
more preferably between 850 C and 990 C.
The hot rolled strip obtained in this manner is then cooled from hot roll
finishing
temperature to a temperature range between Ms and 20 C at a cooling rate
between
1 C/s and 50 C/s. In a preferred embodiment, the cooling rate for this step of
cooling
is between 1 C/s and 20 C/s and more preferably between 5 C/s and 20 C/s.
During
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this step the martensite is formed which will be tempered during the soaking
done
under annealing process to form tempered martensite.
Then the hot rolled strip may optionally be coiled wherein coiling temperature
is
between Ms and 20 C or may optionally be cut to sheets
The hot rolled steel strip, plate or sheet is being heated from a temperature
between
Ms and 20 C up to the annealing temperature Tsoak which is between 550 C and
Ac3,
preferably between 600 C and Ac3 -40 C, such heating being performed at a
heating
rate HR1 of at least 1 C/s.
The hot rolled steel strip, plate or sheet is held at Tsoak during 5 seconds
to 1000
seconds to ensure the targeted transformation to austenite from the initial
structure.
Then, the hot rolled steel is cooled wherein the cooling starts from Tsoak at
a cooling
rate CR1 between 0.1 C/s and 150 C/s, to a cooling stop temperature Ti which
is in
a range between Ms-10 C and 20 C. In a preferred embodiment, the cooling rate
CR1
for such cooling is between 0.1 C/s and 120 C/s. During this cooling the fresh
martensite may form from some remaining unstable austenite.
The hot rolled steel thus obtained preferably has a thickness between 2mm and
100mm and more preferably between 2 mm and 80 mm and even more preferably
between 2mm and 50 mm.
EXAMPLES
zo 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. Ac3 and Ms temperature are determined through thermodynamic
calculations done with the use of a software like Thermo-Calc .
Table 1
0
t..)
o
Steel
t..)
C Mn Si Al Nb S P N Cu Ni V B Ti (...)
Samples
O-
(...)
11 0.12 5 0.5 2.0 0.03 0.002 0.01 0.0042 0 0 0
0 0
o,
.6.
12 0.05 7 0.5 1.5 0.05 0.002 0.01 0.0022 0 0 0
0 0 u,
13 0.05 7 0.5 1.5 0 0.002 0.01 0.0021 0 0 0 0.002
0.03
14 0.05 7 0.5 1.5 0 0.002 0.01 0.0028 0 0 0.2 0
0
R1 0.05 7 0.5 1.5 0 0.002 0.01 0.0021 1.25 1.5 0
0 0
1 = according to the invention; R = reference; underlined values: not
according to the invention.
Table 2
c;
Table 2 gathers the process parameters implemented on steels of Table 1.
ri
.
,.
=
:1
T.'
St Cooling Cooling HR1( C/s)
CR1 Cooling
eel HR
,
.
Reheat! Finish( rate stop Tsoak
Annealing ( C/s) Stop after Ac3 Ms
Sample
ng ( C) C) after HR temperat ( C)
time (s) annealing ( C) ( C)
s
( C) ure( C/s)
( C)
11 1250 980 30 25 10 665
300 2 25 910 365
12 1200 980 35 25 10 640 45
1 25 910 365
13 1200 980 35 25 10 640 45
1 25 910 365
14 1200 980 35 25 10 640 45
1 25 910 365
od
R1 1200 980 35 25 10 640 45 1 25 910 365 n
1-i
5
,..,
=
,..,
-a
u,
-4
.6.
,...,
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Table 3
Table 3 gathers the results of test conducted in accordance of standards on
different
microscopes such as SEM, EPMA, EBSD, XRD or any other microscope for
determining microstructural composition of both the inventive steel and
reference trials.
The area fractions for the carbides is measured on polished samples after
etching them
in 2% Nital etching solution for 10 seconds and observed by an SEM. Polygonal
Ferrite
and tempered martensite are measured using EBSD wherein Electron backscattered
diffraction (EBSD) is a SEM based technique to measure crystal orientations
with a
sub-micron resolution. An electron beam is focused on the 70 tilted specimen
in the
scanning electron microscope (SEM). Electrons that satisfy the Bragg condition
for a
family of planes are channelled and induce kikuchi bands. Electrons strike a
phosphor
screen and produce light, which is detected and digitized by a camera. The
resulting
EBS pattern is analyzed and indexed. This process is realized for each point
analysed.
For a given steel sample, an EBSD analysis of at least 4 images corresponding
to a
magnification of 1000 allows to identify the polygonal ferrite and tempered
martensite
microconstituents, their location and area percentage. The Residual Austenite
area
fraction is measured using XRD which are demonstrated in table 3.
The results are stipulated herein:
Residual Tempered Polygonal Bainite
Samples Austenite Martensite Ferrite (%) Precipitates (area
%)
(%) (%) ( %)
Ii 19 79 1 0 1
12 25 75 0 0 1
13 23 77 0 0 1
14 20 80 0 0 0
RI 44 55 0 0 1
Ii and 12 samples include niobium carbides, 13 sample includes titanium
carbides and
R1 sample includes iron carbides (cementite). No samples were containing any
fresh
martensite or bainite constituents.
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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 NBN EN IS06892-1 standards with
tensile
samples having A25. The toughness is tested by a Charpy test performed
according
to ISO 148-1 .The results of the various mechanical tests conducted in
accordance to
the standards are gathered
Samples TS (M Pa) YS (MPa) Tel (%) CVN -40C
(J/cm2)
11 866 725 25.5 115
12 923 806 23.6 65
13 871 725 22.9 74
14 891 762 21.2 60
R1 835 707 28.1 79
I = according to the invention; R = reference; underlined values: not
according to the
invention.
15
