Note: Descriptions are shown in the official language in which they were submitted.
1
Steel section having a thickness of at least 100mm and method of manufacturing
the
same
The present invention deals with a steel section comprising a web central
portion
connected on each side to a flange portion having a thickness above 100mm. The
steel section
according to the invention is particularly well suited for the manufacture of
columns for high-rise
buildings, long span, transfer and belt trusses, outriggers and bridge
girders.
The development of new modern structural steel grades is always driven by the
users'
requirements towards higher mechanical properties such as yield strength and
toughness, as well
as excellent technological properties, ensuring an efficient fabrication
technology at workshop
and on site.
The purpose of the invention therefore is to provide a steel heavy section
reaching a high
yield strength of at least 485 MPa and a high tensile strength of at least 580
MPa with excellent
weldability.
In practice of structural steel manufacturing, it is known that in order to
improve strength
and toughness it is preferable to refine the structure through hot rolling at
lower temperatures or
to add some alloying elements for austenite grain refining. Both solutions are
not sufficient for
heavy structural steel manufacturing, because in case of lower hot rolling
temperatures the
overheating of the rolls is inevitable. At the same time, when the alloyed
elements are added in
high amounts, the weldability of the steel deteriorates.
Broadly stated, in some embodiments, the present disclosure is related to a
steel section,
comprising a web central portion connected on each side to a flange portion
having a thickness
between 100 mm to 140 mm, said steel section having a composition comprising,
in weight
percentage:
C: 0.06 - 0.16 %
Mn: 1.10 - 2.00 %
Si: 0.10 - 0.40 %
Cu : 0.001 - 0. 50 %
Ni : 0.001 - 0.30 %
Cr : 0.001 - 0. 50 %
Date Recue/Date Received 2021-10-18
1a
Mo : 0.001 - 0.20 %
V: 0.06 - 0.12 %
N : 0.0050% - 0.0200 %
Al 0.040%
P 0.040%
S 0.030%
and comprising optionally one or more of the following elements, in weight
percentage:
Ti < 0.005 %
Nb 5 0.05 %
the reminder being iron and impurities resulting from elaboration, wherein the
ratio of vanadium
to nitrogen amounts is between 2.5 and 7 and said steel section microstructure
including at least
one kind of vanadium precipitates , said precipitates being chosen among
nitrides, carbides,
carbo-nitrides or any combination thereof, more than 70% of such precipitates
having a mean
diameter below 6 nm, wherein the section composition is such that the
following relationship is
fulfilled: 0.4% CEV ).6 % with CEV= C + Mn/6 + ( Cr + Mo + V )/5 + ( Ni + Cu
)/15.
In some embodiments, the steel section may further have one or more of the
following
features:
= the steel section microstructure includes at least one kind of vanadium
precipitates
comprising also one or more metals chosen among chromium, manganese and iron;
= the microstructure of said flanges portions includes, from surface to
core, a hardened zone
comprising tempered martensite and a core zone comprising ferrite and
pearlite.
= the hardened zone comprising tempered martensite also comprises bainite.
= having a mean density of said precipitates of at least 500 precipitates
per mm2 in said core
zone.
= at least part of said precipitates comprises of regularly spaced
precipitates arranged in
regularly spaced bands.
= more than 80% of said regularly spaced precipitates have a mean diameter
below 3 nm.
Date Recue/Date Received 2021-10-18
lb
= said regularly spaced precipitates include at least vanadium and
chromium.
= at least part of said precipitates is randomly distributed in the ferrite
phase, located in the
core of the steel section.
= more than 80% of said randomly distributed precipitates have a mean
diameter between
3.5 and 6nm.
= said randomly distributed precipitates include at least vanadium,
chromium and iron.
= said precipitates are located in said core zone.
Broadly stated, in some embodiments, the present disclosure is related to a
method of
manufacturing of a steel section comprising the following steps:
- feeding a steel semi-product having a composition as described herein,
- reheating such steel semi-product at a temperature above 1000 C and hot
rolling it
with a final rolling temperature of at least 850 C, to obtain a hot rolled
steel section,
- cooling the hot rolled steel section so as to produce martensitic and/or
bainitic
quenching of the surface layer of all or part of the product, the non-quenched
portion
of the rolled product remaining at a temperature high enough to make it
possible to
cause a self-tempering of the quenched surface layer of martensite and/or
bainite and
to transform the austenite into ferrite and carbides in the core part of the
section during
the subsequent cooling, the maximum temperature of the tempered surface of the
product after quenching being 450 to 650 C.
Other characteristics and advantages of the invention will become apparent
from the
following detailed description of the invention and the drawings:
- Figure 1: shows an electron micrograph illustrating randomly distributed
precipitates in the core
of the flange of the heavy section,
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- Figure 2 : shows an electron micrograph illustrating precipitates,
arranged in regularly spaced bands.
All compositional percentages are given in weight percent (wt.%),
unless indicated otherwise. Regarding the chemical composition of the steel,
carbon plays an important role in the formation of the microstructure and
reaching of the targeted mechanical properties. Its main role is to provide
strengthening through hardening of the martensite/bainite phases but also
through formation of carbides and/or carbo-nitrides of metallic elements of
the
steel. The carbon content of the grade according to the invention is between
0.06 and 0.16 % weight. Carbon content below 0.06% will not result in a
sufficient level of mechanical resistance, leading to yield strengths value
below
485 MPa. On the opposite, carbon contents above 0.16% would result in
reducing ductility and the weldability of the steel. Preferably, the carbon
content is between 0.08 and 0.14 %, so as to obtain sufficient strength and
weldability.
Manganese is an element which increases hardenability. The
manganese content of the grade according to the invention is between 1.10
and 2.00 /0. Manganese content below 1.10 /:. will not result in a
sufficient
level of mechanical resistance. On the opposite, manganese content above
2.00 % would result in decreased weldability or would promote the formation of
hard martensite-austenite constituents, also negatively impacting the
toughness of the steel.
Silicon is a deoxidizing element and contributes to improving strength.
Silicon content below 0.10% will not result in a sufficient level of
mechanical
resistance nor a good deoxidation. On the opposite, silicon contents above
0.40% would result in the formation of oxides, reducing welding properties of
the steel.
Copper is an element contributing to improving the strength of the steel
by hardenability improvement and precipitation strengthening. Copper content
below 0.001% will not result in a sufficient level of mechanical resistance.
On
the opposite, copper contents above 0.50% would result in increasing the
carbon equivalent and thus deteriorating the weldability or impacting the hot
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shortness of the steel during hot deformation, caused by penetration of the Cu-
enriched phase into grain boundaries.
Nickel is an element contributing to improving the strength and
toughness of the steel. Nickel content below 0.001% will not result in a
sufficient level of mechanical resistance. On the opposite, nickel contents
above 0.30% would lead to high alloying costs.
Chromium is an element contributing to improving the strength of the
steel by improving hardenability through solution hardening but also through
precipitation hardening. Chromium content below 0.001% will not result in a
sufficient level of mechanical resistance. On the opposite, chromium contents
above 0.50% would result in generating coarse chromium carbides or carbo-
nitrides that may deteriorate the toughness of the steel
Molybdenum is an element contributing to improving the strength of the
steel by improving hardenability. Molybdenum content below 0.001% will not
result in a sufficient level of mechanical resistance. On the opposite,
molybdenum contents above 0.20% would result in reducing the toughness of
the steel.
Vanadium is an important element that is used to achieve hardening
and strengthening by precipitation of nitrides, carbo-nitrides or carbides but
also through grain refining. The formation of vanadium precipitation limits
the
austenite grain coarsening, by resulting in ferrite grain decrease and
improved
strength by precipitation in ferrite phase. Vanadium would also prevent the
chromium and manganese migration in the cementite, resulting in their
application in small precipitation formation. Vanadium content below 0.06 %
will not result in a sufficient level of mechanical resistance. On the
opposite,
vanadium contents above 0.12 % would result in a risk that an excessive
precipitation may cause a reduction in toughness, which has to be avoided. In
a preferred embodiment, vanadium addition is limited to 0.09% to improve
further the toughness of the steel.
Nitrogen is an important element to form nitrides and carbo-nitrides of
metallic elements like vanadium , niobium aluminum and titanium . Their size,
distribution density and stability have a significant effect to mechanical
strengthening. Nitrogen content below 0.0050% will not result in a sufficient
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level precipitation and grain size control. To further improve those
properties, a
minimum level of 0.0060%, or even of 0.0070% or even better of 0.0080% is
preferred. On the opposite, nitrogen contents above 0.0200 % would result in
the presence of free nitrogen in the steel, which is known as having a
negative
impact on toughness in the Heat Affected Zone after welding.
During hot rolling, part of the vanadium will combine with nitrogen in
order to form VN particles for austenite grain boundaries pinning. The
remaining vanadium, in solution, will then precipitate in form of fine
precipitates
during cooling of the steel, thus making an important contribution to final
strength. The inventors have found that the precipitation strengthening can be
enhanced by optimizing the vanadium to nitrogen ratio in the steels section to
approach the stoichiometric ratio of 4:1. In a preferred embodiment, the ratio
of
V to N is comprised between 2.5 and 7, and even comprised between 3 and 5.
Aluminium can be added in the steel for deoxidizing effect and removing
of the oxygen from the steel. If other deoxidizing elements are added in the
steel, the aluminum content is 0.005% and lower. Otherwise, the aluminum
content is between 0.005% and 0.040%. If the aluminum content is too high,
the formation of AIN will occur in preference to VN, and AIN being bigger in
size than VN, it will be not as efficient for pinning of austenite grain
boundaries
as VN.
Sulfur and phosphorus are impurities that embrittle the grain boundaries
and lead to the formation of center and micro-segregation. Their respective
contents must not exceed 0.030 and 0.040% so as to maintain sufficient hot
ductility and to avoid deterioration in welding properties.
Niobium is an element that may optionally be used to achieve hardening
and strengthening by precipitation of nitrides, carbo-nitrides or carbides. It
suppresses the growth of austenite grains during rolling, by refining them,
thus
resulting in improvement of strength and low-temperature toughness.
However, when its amount is above 0.05%, could deteriorate toughness in the
Heat Affected Zone due to martensite hardening. On the other hand, when
niobium amount is 0.05% and higher, it will pin to available nitrogen and thus
impairing nitrogen from forming vanadium precipitates that assures the
strengthening of the ductile core of the section.
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Titanium is an element that may optionally be used to achieve
hardening and strengthening by precipitation of nitrides, carbo-nitrides or
carbides. However, when its amount is above or equal to 0.005%, there is a
risk of TiN formation rather than VN. Moreover, TiN being cuboids particles
5 may react as
stress concentrators thus negatively impacting the toughness
and fatigue properties of the steel. In a preferred embodiment, the maximum
amount of titanium is set to 0.003% and even to 0.001%.
In a preferred embodiment, the carbon, manganese, chromium,
molybdenum, vanadium, nickel and copper contents of the grade are such that
0.4 CEV 0.6
with CEV = C + Mn/6 + ( Cr + Mo + V )/5 + ( Ni + Cu )/15
Respecting these values ensures that the hardenability of the steel
section will be in suitable ranges through sufficient formation of bainite,
while
maintaining a good weldability of the steel sections. The reduced carbon
equivalent allows avoiding weld processing steps such as preheating (when
acceptable) and also results in reduction of fabrication costs. In a preferred
embodiment, CEV 0.5%.
The steel section comprises a web central portion connected on each
side to a flange portion.
The thickness of the flange of the steel section according to the
invention is set above 100 mm, allowing the use of such beam for high-rise
building structures, notably. Its thickness is preferably below 140 mm as a
sufficient cooling rate to ensure the requested tensile and toughness
properties is difficult to obtain.
According to the invention, the web and the flanges of the heavy section
are composed of a hardened zone, resulting from the water cooling of the
surface and a non-hardened zone, in the core of the product. Each zone of the
steel section can have a specific microstructure that can include one or more
phases among tempered martensite, bainite, ferrite and pearlite. Ferrite can
be
present under the form of acicular ferrite or of regular ferrite.
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The microstructure of each zone depends on the steel section thickness
and on the thermal path it is submitted to.
In a preferred embodiment, the microstructure of the flanges portions
include, from surface to core, a first zone comprising tempered martensite and
possibly bainite and a second zone comprising ferrite and pearlite.
The first zone can, for example, extend up to 10 mm under the surface
of the flange portion.
An essential characteristic of the invention is the presence, in the steel
section microstructure, of at least one kind of vanadium precipitates possibly
comprising also one or more metal chosen among chromium, manganese and
iron, said precipitates being chosen among nitrides, carbides, carbo-nitrides
or
any combination of them, more than 70% of such precipitates and preferably
more than 80%, having a mean diameter below 6 nm. The mean diameter
determination was done in the following way: the surface of each detected
precipitate was measured and applied to the corresponding circle, from which
the diameter was extracted, giving then the mean diameter size for all
detected
precipitates.
In a preferred embodiment, the mean density of those precipitates is of
at least 500 precipitates per mm2, preferably of at least 1000 precipitates
per
mm2. Those precipitates have a beneficial effect on strength, known as being
increased with precipitates size decrease and precipitates content increase.
Such precipitates are preferably present in the core zone of the flange
of the section, mainly in the ferrite phase. At least 70% of such precipitates
and preferably at least 80%, have a mean diameter below 6 nm. The reduced
.. size of such precipitates increases their hardening effect and hence the
tensile
strength of the steel section.
In a preferred embodiment, two types of precipitates are preferably
present in the core of the flange of the steel section:
- precipitates randomly distributed inside ferrite and
- precipitates arranged in regularly spaced bands, forming thus parallel
sheets densely populated with particles.
The randomly distributed precipitated are bigger than the one arranged
in regularly spaced bands.
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In a preferred embodiment, such regularly spaced precipitates include
at least vanadium and chromium.
In another preferred embodiment more than 80% of the randomly
distributed precipitates have a mean diameter between 3.5 and 6nm. Such
precipitates preferably include at least vanadium, chromium and iron.
The steel section according to the invention can be produced by any
appropriate manufacturing method and the man skilled in the art can define
one. It is however advisable to use a process ending by an accelerated
cooling, in that case quenching and self-tempering of the surface layer after
hot-rolling step.
The method according to the invention comprises the following steps:
- feeding a semi-product which composition is according to the
invention
- reheating such semi-product at a temperature above 1000 C and
hot rolling it with a final rolling temperature of at least 900 C, to
obtain a hot rolled steel section,
- cooling the hot rolled steel section so as to produce martensitic
and/or bainitic quenching of the surface layer of all or part of the
product, the non-quenched portion of the rolled product remaining at
a temperature high enough to make it possible to cause a self-
tempering of the quenched surface layer of martensite and/or bainite
and to transform the austenite into ferrite and carbides in the core
part of the section during the subsequent cooling, the maximum
temperature of the tempered surface of the product after quenching
being 450 to 650 C and even 550-650 C
The steel sections according to the present invention are preferably
produced through a method in which a semi product made of a steel according
to the present invention having the composition described above, is cast, the
cast input stock is heated to a temperature above 1000 C, preferably above
1050 C and more preferably above 1100 C or 1150 C or used directly at such
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a temperature after casting, without intermediate cooling. Such temperatures
allow full dissolution of vanadium carbonitrides, which will further
participate in
precipitation strengthening mechanism.
The final hot-rolling step is performed at a temperature above 850 C.
The end-of-rolling temperature is above or equal to 850 C in order to assure
the austenite grains refining and thus the formation of a thinner
microstructure
after transformation, which is known to enhance the toughness and strength
properties.
During hot-rolling, it is preferable to use managed combination of rolling
steps and controlling the rolling temperature. The aim is to create fine
grained
microstructure by grain refinement during the subsequent recristallization
during rolling.
The hot-rolled product obtained by the process described above is then
cooled using preferably a quenching and self-tempering process.
The so-called quenching and self-tempering process (OST) consists in
subjecting a hot rolled steel section emerging from the finishing stand of the
rolling mill to cooling by means of a fluid so as to produce martensitic
and/or
bainitic quenching of the surface layer of all or part of the product.
Moreover, at
the outlet of the fluid cooling zone, the non-quenched portion of the rolled
product is at a temperature high enough to permit, during subsequent air
cooling, tempering of the surface layer of martensite and/or bainite to take
place.
The cooling fluid employed for carrying out the quenching and self
tempering step is usually water with or without conventional additives, or
aqueous of mineral salts, for example. The fluid may be a mist, for example
obtained by suspending water in a gas, or it may be a gas, such as steam.
From a practical view point, desired cooling of the rolled products
depends on the cooling devices used, and on suitable choice of the length and
the flow rate characteristics of the cooling means.
The dimensions of the product are known as well as the composition of
the steel, and thus its continuous cooling transformation diagram, making it
possible to determine the conditions to apply for an adequate treatment of the
steel section, among which, the temperature at which martensite is formed and
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the maximum time available for performing surface quenching to the desired
depth.
Based on curves of the temperature gradients in the core and the skin
of the rolled steel section, the amount of heat to be removed can be as well
as
the characteristics of the cooling devices and the flow rates of the fluid
applied
by the cooling devices.
To monitor the formation of the desired microstructures in the different
zones of the steel section, the evolutions of the skin temperature of the
steel
section starting from the end of the martensitic and/or bainitic quenching are
being measured. After quenching, the skin temperature rises while the
temperature at the core continuously decreases after the section has emerged
from the last stand of the rolling mill. The skin temperature and the core
temperature in a given cross-section converge towards a time from where the
two curves continue substantially parallel to one another. The skin
temperature
at this point is called the "equalization temperature".
Examples
Two grades, which compositions are gathered in table 1, were cast in
semi-products and processed into steel sections following the process
parameters gathered in table 2, going through heating, controlled hot rolling
and subsequent water cooling, achieved by quenching and self-tempering.
Table 1 - Compositions
The tested compositions are gathered in the following table wherein the
element content are expressed in thousands of weight percent:
Trial C Mn Si Cu Ni Cr Mo V N Ti Nb
Al P S CEV
1 82 1059 171 170 162 129 49 34 9.6 1 1 3 13 23
0.32
2 98 1559 191 193 117 122 35 74 16.2 1 1 13 17
29 0.43
Trial 1 is a comparative example and trial 2 is an example according to the
invention.
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Table 2 ¨ Process parameters
Steel semi-products, as cast, were processed under the following conditions:
Flange Hot rolling Quenching and self-tempering
Trial thickness Specific
Reheating Hot rolling Self-tempering
(mm) T ( C) finish T ( C) water2 flow
Temperature ( C)
1 80 1150 868 46 640
2 125 1170 893 46 600
5
The resulting samples were then analyzed and the corresponding
microstructure elements and mechanical properties were respectively gathered
in table 3 and 4.
10 Table 3¨ Microstructure and precipitates
The phase percentages of the microstructures of the obtained steel
section were determined:
Hardened zone Core zone
Trial
Tempered martensite Bainite Regular
Ferrite Acicular ferrite Pearlite
+Bainite
1 33.4% 66.6% 76.7% 5.5% 17.7%
2 56.7 % 43.3 % 72.4 % 0.4 % 27.2 %
The phase percentages in both zones, especially in the core zone, of
section n 1 are quite similar to section n 2, showing that the impact of the
vanadium precipitation strengthening is observed at smaller microstructural
scale.
Precipitation analysis done by TEM examination of carbon extraction
replicas taken from the core zone of the flange thickness of the section,
showed the presence of vanadium precipitates. Fine precipitates analysis was
performed through TEM thin foil method, which allowed quantifying the mean
size and the density of the precipitates.
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It was found that precipitates participating in mechanical strengthening
of the section were located in the core zone of the steel sections, in
particular
inside the ferrite phase.
Figure 1 shows the vanadium precipitates mostly having spherical
shape, with bigger or smaller size. The bigger size precipitates (typical size
about 6 nm in diameter) were mostly randomly distributed. But the fine
precipitates (typical size about 3 nm in diameter) were arranged in regularly
spaced bands. It can be seen on figure 2 that the microstructure consists of
parallel sheets densely populated with vanadium particles. The sheets appear
with a regular spacing.
Regularly spaced precipitates
Trial
Precipitates characteristics
% of precipitates with a Mean density in Mean
diameter (nm)
mean diameter below 6 nm ferrite (per mm2)
1 0% <100
2 98% 1213 3.2
Randomly distributed precipitates
Trial
Precipitates characteristics Repartition of metallic
elements in precipitates, A
c'/0 of precipitates with Mean density in Mean V Cr Mn Fe
a mean diameter ferrite diameter
below 6 nm (per mm2) (nm)
1 0% <100
2 70% 880 5.7
66.6 21.9 4.2 9.3
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Table 4 ¨ Mechanical properties
Mechanical properties of the tested steel were determined and gathered
in the following table:
T Yield Strength Tensile strength
0/
CEV ( rial
(MPa) (MPa) 0)
1 398 450 0.32
2 495 653 0.43
The examples show that the steel sections according to the invention
are the only one to show all the targeted properties thanks to their specific
composition and microstructures.
Steel sections according to the present invention show excellent values
of high strength, toughness and good weldability, which is nowadays not easily
achievable. With the steel grade as per the invention, design and construction
teams involved in large-scale construction projects can benefit from more
efficient structural solutions. The steel section's higher yield strength
enables
weight savings and lower transportation and fabrication costs than other
commonly-used structural steel grades. And thus, the present invention makes
an extremely significant contribution to construction industry.
