U.S. patent application number 16/762078 was filed with the patent office on 2020-10-29 for steel section having a thickness of at least 100mm and method of manufacturing the same.
The applicant listed for this patent is ArcelorMittal. Invention is credited to Boris DONNAY, Christophe HOUYOUX, Valerie RINALDI, Liudmila WEBBER.
Application Number | 20200340073 16/762078 |
Document ID | / |
Family ID | 1000004955243 |
Filed Date | 2020-10-29 |
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United States Patent
Application |
20200340073 |
Kind Code |
A1 |
HOUYOUX; Christophe ; et
al. |
October 29, 2020 |
STEEL SECTION HAVING A THICKNESS OF AT LEAST 100MM AND METHOD OF
MANUFACTURING THE SAME
Abstract
A steel section has a web central portion connected on each side
to a flange portion having a thickness of at least 100 mm. The
steel section microstructure includes at least one kind of vanadium
precipitates possibly comprising also one or more metal chosen
among chromium, manganese and iron, the precipitates being chosen
among nitrides, carbides, carbo-nitrides or any combination of
them, more than 70% of such precipitates having a mean diameter
below 6 nm. It also deals with a manufacturing method thereof.
Inventors: |
HOUYOUX; Christophe;
(Etalle, BE) ; RINALDI; Valerie; (Esch Sur
Alzette, LU) ; DONNAY; Boris; (Nandrin, BE) ;
WEBBER; Liudmila; (Yutz, FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ArcelorMittal |
Luxembourg |
|
LU |
|
|
Family ID: |
1000004955243 |
Appl. No.: |
16/762078 |
Filed: |
December 12, 2018 |
PCT Filed: |
December 12, 2018 |
PCT NO: |
PCT/IB2018/059909 |
371 Date: |
May 6, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 2211/002 20130101;
C22C 38/04 20130101; C21D 2211/009 20130101; C21D 2211/008
20130101; C21D 9/0068 20130101; C21D 6/005 20130101; C22C 38/44
20130101; C22C 38/42 20130101; C22C 38/48 20130101; C22C 38/46
20130101; C22C 38/50 20130101; C21D 6/004 20130101; C22C 38/002
20130101; C21D 8/005 20130101; E04C 3/04 20130101; C22C 38/001
20130101; C21D 2211/004 20130101; C21D 6/008 20130101; C21D
2211/005 20130101; C22C 38/06 20130101; C22C 38/02 20130101 |
International
Class: |
C21D 9/00 20060101
C21D009/00; C22C 38/50 20060101 C22C038/50; C22C 38/48 20060101
C22C038/48; C22C 38/46 20060101 C22C038/46; C22C 38/44 20060101
C22C038/44; C22C 38/42 20060101 C22C038/42; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00; C21D 8/00 20060101
C21D008/00; C21D 6/00 20060101 C21D006/00; E04C 3/04 20060101
E04C003/04 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 18, 2017 |
IB |
PCT/IB2017/058055 |
Claims
1-14. (canceled)
15. A steel section comprising a web central portion connected on
each side to flange portions having a thickness of at least 100 mm,
the 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% Mo: 0.001-0.20% V:
0.06-0.12% N: 0.0050%-0.0200% Al.ltoreq.0.040% P.ltoreq.0.040%
S.ltoreq.0.030% and optionally including at least one of the
following elements, in weight percentage: Ti<0.005%
Nb.ltoreq.0.05% a reminder being iron and impurities resulting from
processing; a microstructure of the steel structure including
vanadium precipitates, the vanadium precipitates optionally
including at least one metal chosen from the group consisting of
chromium, manganese and iron, the vanadium precipitates being
chosen among nitrides, carbides, carbo-nitrides or any combination
of them, more than 70% of the vanadium precipitates having a mean
diameter below 6 nm.
16. The steel section as recited in claim 15 wherein such section
composition is such that the following relationship is fulfilled:
0.4.ltoreq.CEV.ltoreq.0.6 with
CEV=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15
17. The steel section as recited in claim 15 wherein the ratio of
vanadium to nitrogen amounts is between 2.5 and 7.
18. The steel section as recited in claim 15 wherein the
microstructure of the flanges portions includes, from surface to
core, a hardened zone including tempered martensite and optionally
bainite and a core zone including ferrite and pearlite.
19. The steel section as recited in claim 15 wherein such steel
section includes portions having a mean density of the vanadium
precipitates of at least 500 precipitates per mm.sup.2.
20. The steel section as recited in claim 15 wherein at least part
of the vanadium precipitates is arranged in regularly spaced
bands.
21. The steel section as recited in claim 20 wherein more than 80%
of such regularly spaced precipitates have a mean diameter below 3
nm.
22. The steel section as recited in claim 21 wherein such regularly
spaced precipitates include at least vanadium and chromium.
23. The steel section as recited in claim 15 wherein at least part
of such precipitates is randomly distributed in a ferrite phase
located in a core zone of the steel section.
24. The steel section as recited in claim 23 wherein more than 80%
of the randomly distributed vanadium precipitates have a mean
diameter between 3.5 and 6 nm.
25. The steel section as recited in claim 24 wherein the randomly
distributed vanadium precipitates include at least vanadium,
chromium and iron.
26. The steel section as recited in claim 15 wherein the vanadium
precipitates are located in a core zone of the steel section.
27. The steel section as recited in claim 15 wherein the flange
portions have a thickness of at most 140 mm.
28. A method of manufacturing of a steel section comprising the
following steps: feeding a steel semi-product 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% Mo: 0.001-0.20% V:
0.06-0.12% N: 0.0050%-0.0200% Al.ltoreq.0.040% P.ltoreq.0.040%
S.ltoreq.0.030% and optionally including at least one of the
following elements, in weight percentage: Ti.ltoreq.0.005%
Nb.ltoreq.0.05% a reminder being iron and impurities resulting from
processing; reheating the steel semi-product at a temperature above
1000.degree. C. and hot rolling the steel semi-product with a final
rolling temperature of at least 850.degree. C., to obtain a hot
rolled steel section; and cooling the hot rolled steel section so
as to produce martensitic or bainitic quenching of the surface
layer of all or part of the product, a non-quenched portion of the
rolled product remaining at a temperature high enough to enable a
self-tempering of the quenched surface layer of martensite or
bainite and to transform the austenite into ferrite and carbides in
a core part of the section during the subsequent cooling, a maximum
temperature of the tempered surface of the product after quenching
being 450 to 650.degree. C.
Description
[0001] 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 100 mm. 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.
BACKGROUND
[0002] 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.
SUMMARY OF THE INVENTION
[0003] It is an object of the invention 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.
[0004] In the 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.
[0005] It is an object of the present invention to provide a steel
section, comprising a web central portion connected on each side to
a flange portion having a thickness of at least 100 mm, such steel
section having a composition comprising, in weight percentage:
[0006] C: 0.06-0.16% [0007] Mn: 1.10-2.00% [0008] Si: 0.10-0.40%
[0009] Cu: 0.001-0.50% [0010] Ni: 0.001-0.30% [0011] Cr:
0.001-0.50% [0012] Mo: 0.001-0.20% [0013] V: 0.06-0.12% [0014] N:
0.0050%-0.0200% [0015] Al.ltoreq.0.040% [0016] P.ltoreq.0.040%
[0017] S.ltoreq.0.030% [0018] and comprising optionally one or more
of the following elements, in weight percentage: [0019]
Ti<0.005% [0020] Nb.ltoreq.0.05% [0021] the reminder being iron
and impurities resulting from elaboration, and said steel section
microstructure including 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 having a mean diameter below 6 nm.
[0022] Another object of the present invention is to provide a
method of manufacturing of a steel section comprising the following
steps: [0023] feeding a steel semi-product which composition is
according to claims 1 to 3, [0024] reheating such steel
semi-product at a temperature above 1000.degree. C. and hot rolling
it with a final rolling temperature of at least 850.degree. C., to
obtain a hot rolled steel section, [0025] 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.degree. C.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] Other characteristics and advantages of the invention will
become apparent from the following detailed description of the
invention and the drawings:
[0027] FIG. 1: shows an electron micrograph illustrating randomly
distributed precipitates in the core of the flange of the heavy
section,
[0028] FIG. 2: shows an electron micrograph illustrating
precipitates, arranged in regularly spaced bands, and
[0029] FIG. 3: shows schematically the web portion and flanges of
the steel section.
DETAILED DESCRIPTION
[0030] 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.
[0031] Manganese is an element which increases hardenability. The
manganese content of the grade according to the invention is
between 1.10 and 2.00%. Manganese content below 1.10% will not
result in a sufficient level of mechanical resistance. On the
opposite side of the range, 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.
[0032] 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 side of the range, silicon contents above 0.40%
would result in the formation of oxides, reducing welding
properties of the steel.
[0033] 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 side of
the range, copper contents above 0.50% would result in increasing
the carbon equivalent and thus deteriorating the weldability or
impacting the hot shortness of the steel during hot deformation,
caused by penetration of the Cu-enriched phase into grain
boundaries.
[0034] 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 side of the range, nickel contents above 0.30% would lead
to high alloying costs.
[0035] 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 side of the range, chromium
contents above 0.50% would result in generating coarse chromium
carbides or carbo-nitrides that may deteriorate the toughness of
the steel
[0036] 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 side of the range,
molybdenum contents above 0.20% would result in reducing the
toughness of the steel.
[0037] 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 side of the range,
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.
[0038] 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 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 side of the range, 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.
[0039] 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.
[0040] 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 AlN
will occur in preference to VN, and AlN being bigger in size than
VN, it will be not as efficient for pinning of austenite grain
boundaries as VN.
[0041] 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.
[0042] 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.
[0043] 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 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%.
[0044] In a preferred embodiment, the carbon, manganese, chromium,
molybdenum, vanadium, nickel and copper contents of the grade are
such that
0.4.ltoreq.CEV.ltoreq.0.6 with
CEV=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15
[0045] 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.ltoreq.0.5%.
[0046] The steel section comprises a web central portion 100
connected on each side to a flange portion 102, 104, as shown
schematically in FIG. 3.
[0047] 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.
[0048] 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.
[0049] The microstructure of each zone depends on the steel section
thickness and on the thermal path it is subjected to.
[0050] 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.
[0051] The first zone can, for example, extend up to 10 mm under
the surface of the flange portion.
[0052] 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.
[0053] In a preferred embodiment, the mean density of those
precipitates is of at least 500 precipitates per mm.sup.2,
preferably of at least 1000 precipitates per mm.sup.2. Those
precipitates have a beneficial effect on strength, known as being
increased with precipitates size decrease and precipitates content
increase.
[0054] 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.
[0055] In a preferred embodiment, two types of precipitates are
preferably present in the core of the flange of the steel section:
[0056] precipitates randomly distributed inside ferrite and [0057]
precipitates arranged in regularly spaced bands, forming thus
parallel sheets densely populated with particles.
[0058] The randomly distributed precipitated are bigger than the
one arranged in regularly spaced bands.
[0059] In a preferred embodiment, such regularly spaced
precipitates include at least vanadium and chromium.
[0060] In another preferred embodiment more than 80% of the
randomly distributed precipitates have a mean diameter between 3.5
and 6 nm. Such precipitates preferably include at least vanadium,
chromium and iron.
[0061] The steel section according to the invention can be produced
by any appropriate manufacturing method and one of skill 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.
[0062] The method according to the invention comprises the
following steps: [0063] feeding a semi-product which composition is
according to the invention [0064] reheating such semi-product at a
temperature above 1000.degree. C. and hot rolling it with a final
rolling temperature of at least 900.degree. C., to obtain a hot
rolled steel section, [0065] 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.degree. C. and even 550-650.degree. C.
[0066] 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.degree. C., preferably above
1050.degree. C. and more preferably above 1100.degree. C. or
1150.degree. C. or used directly at such a temperature after
casting, without intermediate cooling. Such temperatures allow full
dissolution of vanadium carbonitrides, which will further
participate in precipitation strengthening mechanism.
[0067] The final hot-rolling step is performed at a temperature
above 850.degree. C. The end-of-rolling temperature is above or
equal to 850.degree. 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.
[0068] 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.
[0069] The hot-rolled product obtained by the process described
above is then cooled using preferably a quenching and
self-tempering process.
[0070] The so-called quenching and self-tempering process (QST)
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.
[0071] 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.
[0072] 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.
[0073] 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 the
maximum time available for performing surface quenching to the
desired depth.
[0074] 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.
[0075] 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
[0076] 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-US-00001 TABLE 1 Compositions 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
[0077] The tested compositions are gathered in the following table
wherein the element content are expressed in thousands of weight
percent:
[0078] Trial 1 is a comparative example and trial 2 is an example
according to the invention.
TABLE-US-00002 TABLE 2 Process parameters Hot rolling Quenching and
self-tempering Hot Self- Flange rolling Specific tempering
thickness Reheating finish T water flow Temperature Trial (mm) T
(.degree. C.) (.degree. C.) (l/m.sup.2/s) (.degree. C.) 1 80 1150
868 46 640 2 125 1170 893 46 600
[0079] Steel semi-products, as cast, were processed under the
following conditions:
[0080] The resulting samples were then analyzed and the
corresponding microstructure elements and mechanical properties
were respectively gathered in table 3 and 4.
TABLE-US-00003 TABLE 3 Microstructure and precipitates Hardened
zone Core zone Tempered Regular Acicular Pearlite + Trial
martensite Bainite Ferrite ferrite Bainite 1 33.4% 66.6% 76.7% 5.5%
17.7% 2 56.7% 43.3% 72.4% 0.4% 27.2%
[0081] The phase percentages of the microstructures of the obtained
steel section were determined:
[0082] The phase percentages in both zones, especially in the core
zone, of section no 1 are quite similar to section no 2, showing
that the impact of the vanadium precipitation strengthening is
observed at smaller microstructural scale.
[0083] 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.
[0084] 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.
[0085] FIG. 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 FIG. 2 that the microstructure consists of parallel
sheets densely populated with vanadium particles. The sheets appear
with a regular spacing.
Regularly Spaced Precipitates
TABLE-US-00004 [0086] Precipitates characteristics % of
precipitates with a Mean density in Mean diameter Trial mean
diameter below 6 nm ferrite (per mm.sup.2) (nm) 1 0% .ltoreq.100 --
2 98% 1213 3.2
Randomly Distributed Precipitates
TABLE-US-00005 [0087] TABLE 4 Mechanical properties Precipitates
characteristics % of precipitates with Mean density in Mean
Repartition of metallic a mean diameter ferrite diameter elements
in precipitates, % Trial below 6 nm (per mm.sup.2) (nm) V Cr Mn Fe
1 0% <100 -- -- -- -- -- 2 70% 880 5.7 66.6 21.9 4.2 9.3
[0088] Mechanical properties of the tested steel were determined
and gathered in the following table:
TABLE-US-00006 Yield Strength Tensile strength Trial (MPa) (MPa)
CEV (%) 1 398 450 0.32 2 495 653 0.43
[0089] 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.
[0090] 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.
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