FIELD OF THE INVENTION
The present invention relates to the development of cold formed, resistance welded
micro alloyed structural steel tubes with superior formability and weldability having
excellent elevated temperature strength and the manufacturing method for the same.
BACKGROUND OF THE INVENTION:
Structural tubes used in the construction of high rise building, stadiums, auditoriums,
shopping malls has emerged as a first choice construction material for the architects
and engineers because of their high strength to weight ratio, aesthetic appeal and high
resistance to torsional loading. However plain carbon structural tube members like
columns, trusses, beams as per IS10748, BS EN 10219-1, standard, significantly loses
its load bearing ability when exposed to temperatures above 400°C. The yield strength
of such steel is reduced to 25-30% of their room temperature yield strength at 600°C.
This limits the service temperature of plain carbon structural steel tubes when it is used
for fire resistance applications.
High temperature functional abilities of plain carbon structural tubes is commonly
enhanced by providing the insulation to the steel surface by the usage of gypsum
boards, concrete encasement , insulating coatings composed of mineral aggregates and
fibers in a cement slurry. However the usage of such fire proofing measures not only
reduces effective utilization of interior space and but also impairs the aesthetics of steel
structures.
Intumescent coatings applied on a steel surface are thin films composed of a mixture of
binders, resins, ceramics and refractory fillers. These films expands when exposed to
rising temperatures and form cellular foam layer which acts as an insulator and restricts

the temperature build up in steel substrate. The steel surface in this case has to be
supplemented with a base coat followed by a top coat prior to the application of
intumescent coating. Hence, the application of these coatings is not only tedious, labour
intensive but also requires special skill. More importantly these coatings are very costly
and significantly escalates the project cost.
Mo is being traditionally used as most effective element in improving fire resistant
properties of steel. Nippon steel (JP2001262269) in their development of fire resistant
steel and welded steel tube revealed the usage of Mo content to a level as high as 1 %(
in mass%) in conjunction with the other alloying elements like Ti,Cu ,Nb. Though such
levels of Mo yields excellent fire resistant properties in steel but significantly increases
the mill load during the hot rolling of the steel and may challenge the rolling capability
of hot rolling mills. The higher Mo content also increases the hardenability of the steel
which may lead to the issues of the weld embrittlement causing the deterioration steel
toughness in the weld zone.
Nippon Steel has produced another Mo free (JP2000239792) low yield ratio type fire
resistant hot rolled steel sheets. The steel composition revealed that Mo free fire
resistant steel employs the use of titanium and Niobium. The titanium combines with
the nitrogen to form titanium nitrides at a temperature in the region of 1300°C. Since
the nitrides are formed at such a high temperature, they undergo coarsening to form
coarse TiN precipitates in the hot rolled steel. The presence of these coarse precipitates
impairs the toughness of the heat affected weld zone.
An US patent US2010/0065168 reported the invention of the fire resistance steel (Mo
free) excellent in high temperature strength, and reheating embrittlement resistance
used for structural member. The composition of the invented steel was (by mass %), C

0.001-0.03, Mn 0.4-2, Nb 0.03-0.5, Ti 0.005-0.04, Ti/N ratio of 2-12. The minimum
yield ratio achieved in the invented steel is 0.59.
In a patent JP2002249845, KOBE Steel ltd reported the invention of fire resistant steel,
using Cu precipitates having excellent fire resistance which exhibits high strength even
at high temperatures. Because of such a high content of Cu ( in excess of 1% by
weight) the steel becomes vulnerable to the issues of hot shortness that causes
cracking of steel surface while hot forming/rolling of steel slab.
While there are abundant use of high alloyed fire resistant steels, but the availability of
low alloy fire resistant steel with fire resistant ratio greater than 0.66 remains an area
to be addressed. This opens up the opportunity to improve on the high temperature
strength of the structural steel tube with minimal alloying and rendering it with the fire
resistant properties combined with excellent formability and weldabilty.
OBJECTS OF THE INVENTION
An object of this invention is to propose a leaner steel composition and thermo
mechanical processing of hot rolled strips that enables to achieve inherent fire resistant
properties in cold formed and resistance welded structural tube, with the fire resistance
ratio greater than 0.66 at 600°C for a minimum period of one hour.
Another object of the present invention is to propose non-peritectic steel composition
for fire resistance structural steel tube, for which ferrite potential is >1.05.
Another object of the present invention is to propose a steel composition with carbon
equivalence (as defined by IIW) less than 0.35 to ensure that the steel exhibits
excellent weldability during the process of tube manufacturing and at end application.

Still another object of this invention is to propose a thermo mechanical processing that
is effective in optimizing the room temperature properties (YS, UTS and more
specifically ductility) and high temperature properties of the designed steel.
Further object of present invention is to propose the micro alloyed steel with room
temperature properties YS: 355-450 MPa, UTS: 490-540MPa and percentage elongation
in excess of 20.
Another object of the present invention is to limit the yield strength of hot rolled fire
resistant steel to 450 MPa to prevent the spring back phenomenon encountered during
the cold forming process of tube manufacturing.
Another object of present invention is to propose the fire resistant steel composition
that restricts the total micro alloying content (Molybdenum + vanadium + Nitrogen) to
less than 0.25.
Another object of present invention is to propose a steel composition for fire resistance
structural tube that is compatible with respect to the composition of conventional
welding electrodes, to ensure the fire resistant properties across the welded joints.
Another object of present invention is to propose a steel composition that allows the
processing of steel through conventional Continuous casting & hot strip route and thin
slab casting route of hot strip manufacturing.
SUMMARY OF THE INVENTION
The present invention is aimed to develop the cold formed resistance welded fire
resistant structural steel tubes capable of retaining at least two third of its room
temperature yield strength for a minimum duration of two hours and also meet the
specification laid in IS10748 Grade 6 and BS EN10219-1 standards for its room

temperature properties. In accordance, the target room temperature properties in the
hot rolled strip used as input material for the tube manufacturing are aimed as YS 355-
450 MPa, UTS 490-540 MPa and elongation values greater than 22%.
The room temperature yield strength of the steel was restricted to maximum of 460
MPa to restrict spring back phenomenon that is encountered during the process of cold
forming and electric resistance welding. The phenomenon of spring back, due to the
increased yield strength, only leads to impairing the quality of tube because of
geometrical imperfections. Further, spring back phenomenon adds to the mill load. .
The non-peritectic steel composition for the current steel is designed by ensuring ferrite
potential in excess of 1.05. The steel with peritectic composition have the tendency to
breakouts and surface cracking during the solidification process of slab in the caster.
The steel with peritectic composition are thus casted with slower casting speed which
significantly hampers the mill productivity. Moreover, casting of steel with peritectic
composition becomes practically impossible through TSCR route, where the casting
speed is much higher i.e. almost 2-3 times higher than convention continuous casting o
steel.
The steel composition is designed to aim for carbon equivalence to be less than 0.35 to
make the structural steel tube readily weldable, and enable its application in various
shape profiles used for construction purpose.
In the present invention, the fire resistant properties in steel are attained with leaner
micro alloyed composition with total micro alloying content restricted to less than 0.25,
coupled with controlled thermo mechanical processing.

The thermo mechanical processing is designed to ensure the retention of micro alloying
elements in the solid solution in the finished product so as to allow the precipitation to
occur when the component is exposed to the increasing temperature (in excess of
400°C) during the event of fire. The presence of alloying elements like Mo and Mn
ensures the thermal stability of precipitates by preventing the precipitate coarsening.
In the present invention, the microstructure of steel consists of ferrite and pearlite as
second phase with volume fraction less than 10%. The targeted room temperature
strength was achieved with the presence of pearlite, solid solution strengthening
derived from interstitials like C, N and substitution solid solution strengtheners like Mn
and Si. The high temperature strength in the present invention was attributed to the
fine precipitates of vanadium nitrides/carbo nitrides that nucleates in large number
when steel is exposed to a temperature in excess of 400°C. These precipitates as a
nature of their fineness and spatial distribution act as a strong pinning agent to the
movement of dislocations and the grain boundaries, thereby prevents the loss of
strength even at higher temperatures.
The structural steel tube of the current invention is capable of maintaining yield
strength greater than 230 MPa at a temperature of 600°C for an exposure time of two
hours.
The proposed chemical composition, in weight percent of the steel consists of
C: <0.10 %
Mn: <1.5 %
Mo: <0.25%
V: <0.10%
Si: <0.50%

N :<0.020%
Al: <0.04%
S<0.005%
P<0.030, remaining Fe along with the unavoidable impurities.
The proposed steel essentially consists of ferritic microstructure with pearlite volume
fraction less than 10% and ferrite grain size of 6-12 urn. The steel consists of fine
precipitates dispersed in the ferrite matrix. The precipitate density in the ferrite matrix
in as rolled condition is approximately 5.0 x1014/cm3,which increases approximately to
25x1014/cm3 when steel is exposed to fire. This increase in precipitate density increases
the precipitate dislocation interaction and thus limits the softening of steel at higher
temperatures.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 is the schematic illustrating the designed thermo mechanical processing for the
manufacturing of hot rolled strips used for the processing of fire resistant structural
tubes.
Figure 2 is a schematic showing the critical process steps employed in the processing of
fire resistant structural steel tube.
Figure 3 shows the optical microstructure of fire resistant steel rolled to 5.86 and 7.5
mm
Figure 4 shows Bright and Dark field TEM micrographs reveal the restricted precipitation
of vanadium nitride/carbonitrides precipitates in the ferrite matrix.
Figure 5 shows the intense precipitation of fine precipitates of vanadium
nitrides/carbonitrides when steel is exposed to 600°C for holding time of two hours.

Figure 6 shows Cross sectional profiles for various geometries of Fire resistant structural
tube
Figure 7 shows the fire resistant structural tubes of varying cross sectional profiles
processed from 3.5 mm hot rolled strips through ERW process of tube making.
Figure 8 shows the yield strength and tensile strength variation of fire resistant tube
with increasing temperature.
Figure 9 shows the stress strain behavior of fire resistant structural tube at elevated
temperatures.
Figure 10 shows the stress strain behavior of welded FR Steel tube at room
temperature and elevated temperature.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to the designing of the chemical composition of steel
coupled with the controlled thermo mechanical processing method to develop cold
formed structural tubes with excellent formability, weldability and fire resistant
properties. The developed structural steel tubes exhibits yield strength of two third or
more at a temperature of 600°C compared to the room temperature yield strength for a
minimum period of two hours. The fire resistant structural steel tubes are developed in
various cross sectional shape profiles and sizes. The said structural steel tubes are used
in the construction of multi storey buildings for offices, shopping malls, auditoriums,
stadiums etc.
The detailed research is carried out to define the role of alloying elements on the room
temperature and high temperature properties. The steel composition , thermo-
mechanical processing parameters (e.g. FRT,CT etc. ) are designed and optimized by

conducting multiple plant scale trials through conventional hot strip mill and thin slab
casting route of steel processing .The steel composition of fire resistant structural steel
tube in the present invention comprise V, Mo, Mn, N, besides C, Si, Cr, P and S. The
proposed chemical composition, in weight percent of the steel, comprises:
C: <0.10 %
Mn: <1.5 %
Mo: <0.25%
V: <0.10%
Si: <0.50%
N :<0.020%
Al: <0.04%
S<0.005%
P<0.030


# Ferrite Potential; ## Carbon Equivalence
The ferrite potential for a given composition of steel is a parameter that defines
whether the steel composition is peritectic or non-peritectic. Ferrite potential (FP) is
calculated by the following empirical formula:

Figure 1 shows the schematic that defines the thermo mechanical processing employed
for the production of hot rolled strips of the designed chemistry used for manufacturing
of fire resistant structural tube. The cast slab with the following composition in weight
percent
C: <0.10 %
Mn: <1.5 %
Mo: <0.25%
V: <0.10%
Si: <0.50%
N :<0.020%
Al: <0.04%
S<0.005% &P<0.030
is heated to a temperature of 1050 to 1250° C to homogenize the cast structure and
ensure that the micro alloying elements are dissolved as solid solution. The reheated
slab is subsequently hot rolled with finish hot rolling at 870-910°C and finally water

cooled over the run out table to a coiling temperature of 550-600°C with the average
cooling rate of 0.5-20°C/s.
Alloying additions: Salient features on the role primary alloying elements in the
present invention for fire resistant structural steel tube development are described
below:
C:<0.10 wt%: The preferable range for the carbon in the steel is 0.04-0.08%wt. C is
added to derive the strength in steel through solid solution strengthening, second phase
formation along with the formation of precipitates in the form carbides/carbonitrides.
The presence of carbon in the present steel also helps in improving the high
temperature properties as a result of Mo-C cluster formation.
Mn: <1.5 wt%: The preferable range for the Mn in the steel is 0.7-1.2 %wt. Manganese
other than imparting solid solution strengthening effect plays an important role in
controlling the precipitate size that ensures the retention of steel strength at high
temperature for a longer duration of time. The precipitation strengthening by vanadium
is enhanced with the increasing Mn content because manganese lowers the austenite-
to-ferrite transformation temperature, and results in a finer precipitate dispersion. Also,
increase in Mn content increase the solubility product of vanadium nitride in austenite at
a given temperature and thus favors the precipitation of vanadium nitrides at much
lower temperatures that lead to a dispersion of finer precipitates in the ferrite matrix
and imparts higher strengthening effects at room temperature and more so on high
temperature properties.


Si: 0.05-0.50 wt%: The preferable range for the Si in the steel is 0.05-0.30 %wt. Silicon
imparts the solid solution strengthening effect like Mn . The role of Si in present
invention is key in limiting the designed chemistry to non peritectic composition. Si is
also being employed as a deoxidizing element. However in order to prevent the
formation of surface scales ,the Si content in the steel is restricted to a maximum
content of 0.5% . Also the higher Si content increases the carbon equivalence and
impairs the weldability of the steel.
V: <0.10% The preferable range for the vanadium in the steel is 0.03-0.08 %wt
Vanadium in the present invention plays a key role in attaining the high temperature
strength in the steel. Vanadium combines with the nitrogen in the steel to form
vanadium nitrides. Hence the nitrogen content in the steel is deliberately kept high. The
higher nitrogen content for the same vanadium level leads to finer and denser
precipitate formation. The extensive mutual solubility of carbides and nitrides results in
the formation of fine carbon nitrides in the ferrite matrix. These fine precipitates of
vanadium nitrides/carbonitrides offers a greater resistance to the dislocation motion and
causes the pinning of grain boundaries and thus limits the softening of steel at a higher
temperature. The following features of vanadium is being utilized in designing the
composition for fire resistant structural steel tube
a) The precipitates that are formed during the casting of steel can be re-dissolved
during reheating prior to hot rolling.
b) No significant increase in hardness during hot rolling and no decrease in hot
ductility as vanadium remain in the solid solution state to fairly low
temperatures. Thus steel is being rolled with lower rolling load compared to steel
comprising of Nb/Ti.

c) Delayed precipitation of vanadium precipitates during finishing and coiling
ensures the dispersion fine precipitates unlike Ti/Nb that precipitates out at
much higher temperature and is more prone to the precipitate coarsening.
d) Ability of vanadium to be restricted in solid solution, to ensure precipitation of
vanadium nitrides/carbo nitrides as a secondary precipitation while steel is being
exposed to rising temperature of fire.
Mo :< 0.25 wt%: Presence of Mo in steel reduces the carbon activity as a result of Mo-
C cluster formation. The reduced activity of carbon in austenite makes carbon less
available for vanadium to form carbides. Thus help in retaining higher amount
vanadium in solid solution during the rolling in austenitic rolling. The upper limit of Mo
in the present steel was restricted to 0.25 by weight percent. Further, increase in the
Mo content leads to significant escalation in the price of the steel. Moreover, increased
hardenebility because of high Mo content in the steel causes the weld embrittlement
and deteriorates the toughness of weld heat affected zone.
Nitrogen < 0.020 wt%: The preferable range for the nitrogen in the steel is 0.0070-
0.015 %wt Nitrogen in the present steel is key element as it combines with the
vanadium and carbon to form nitrides/carbonitirdes. These precipitates have a higher
thermal stability i.e. resistance to the coarsening compared to carbides of vanadium or
any other alloying element. The lower Nitrogen content would severely impact the
precipitation potential of vanadium and would deteriorate the high temperature
properties in the present steel. However, increasing the nitrogen content above 0.010%
may lead to the embrittlement of the heat affected zone (HAZ) of weld joints.

Sulphur: <0.005 by wt%: Sulphur has to be limited to 0.005% to avoid high level of
inclusions which induces the in homogeneity in the steel and deteriorates the
formability of the steel.
Phosphorous: <0.03 wt%: The Phosphorous content should be restricted to 0.03%
maximum as higher phosphorus content leads to the reduction in toughness and
weldability of steel as a result of phosphorus segregation at the grain boundaries.
Production process: The method of manufacturing of fire resistant structural steel
tube according to the present invention consists of a casting of steel with designed
composition as mentioned above, followed by a hot rolling ,controlled cooling and
coiling to produce hot rolled strips which are further slit into pre-defined dimensions
and subsequently subjected to tube processing comprising of cold forming and seam
welding though high frequency induction welding (HFIW) . The various processing steps
are described in their respective order below:
Casting: In the present invention, the steel of the specified composition is first cast
either through conventional continuous caster or a thin slab casting route. Non
peritectic steel composition is specifically designed to enable the smooth casting of steel
through either route.
Reheating: After casting the slab with the specified composition, the slabs are
reheated to a temperature greater than 1050°C (preferably in the range of 1100 to
1200°C) for a duration of 20 minutes to 2 hours. The reheating temperature should be
above 1050°C, to ensure complete dissolution of any precipitates Vanadium
carbide/carbonitrides that may have formed in the preceding processing steps. A
reheating temperature greater than 1250°C is also undesirable as it may lead to grain
coarsening of austenite and lead to yield loss due to excessive scale formation.

Hot Rolling: After casting and reheating the steel slab with the specified composition,
it is hot-rolled. The hot rolling should constitute of a roughing step above the
recrystallization temperature and a finishing step below the recrystallization
temperature, when rolling is done in a conventional hot strip mill. In case a CSP is used
for producing this steel, where there is no separate roughing mill, the deformation
schedule should be designed in order to break the cast structure during the initial
stands of hot rolling and finishing is done below the recrystallization temperature. More
specifically the finish rolling in either set up should be done at a temperature (FRT )
given by Ae3 - 50 (°C) ≤ FRT ≤ Ae3 + 50 (°C). The preferred FRT temperature lies in
the range of 870-910°C.
Laminar cooling on the Run-Out-Table (ROT) and Coiling: Post finish rolling, the
hot rolled steel sheet is subjected to laminar cooling with near group (NG) cooling
strategy on the ROT, at a cooling rate of 0.5 -20°C/s to attain a coiling temperature in
a range of 550-600°C in order to suppress the micro alloyed precipitation in the hot
rolled product.
Tube processing comprising of cold forming and welding:
The hot rolled strips are further slit as per the predefined dimensions set in accordance
to the final size of tube. The slit coils are then subjected to cold forming followed by
seam welding through high frequency induction welding to produce structural tube.
The cold forming operation increases the dislocation density in the parent material
owing to the strain hardening effect. The dislocation induced act as nucleation site for
the precipitation of vanadium carbonitrides/nitrides when the steel is being heated
above the temperature range of 350°C and above.

The fire resistant structural tube developed using the process and composition as per
the current invention were tested for various properties such as YS, UTS, EI and FR.

HSM-Hot strip mill; TSCR- Thin slab casting route.
Table 3 details microstructural description of hot rolled strips rolled out in different
thicknesses through conventional hot strip mill and thin slab casting /Compact strip
processing route. Figure 3 shows the optical microstructure of fire resistant steel rolled
to 5.86 and 7.5 mm through thin salb casting/ compact strip processing route.
The steel consists of fine precipitates dispersed in the ferrite matrix. Figure 4 shows
bright and dark field TEM micrographs reveals the restricted precipitation of vanadium
nitride/carbonitrides precipitates in the ferrite matrix. The precipitates were observed to
be in a size of few nano meters. Figure 5 shows the intense precipitation of vanadium
nitrides/carbonitrides when steel is exposed to 600°C for holding time of two hours. The
precipitate density in the ferrite matrix in as rolled condition is approximately 5.0 x1014
/cm3 , which increases approximately to 25 x1014 / cm3 when steel is exposed to fire.

This increase in precipitate density increases the precipitate dislocation interaction and
thus limits the softening of steel at higher temperatures.

Figure 6 shows cross sectional profiles for various geometries of fire resistant structural
tube. As an example in present invention figure 7 shows the fire resistant structural
tubes of varying cross sectional profiles processed from 3.5 mm hot rolled strips
through ERW process of tube making.
The high temperature strength of the weld joints of fire resistant structural tube is
equally critical for the structural integrity of building/structure exposed to fire. It is most
likely that the welded joint may fail under the conditions of fire than the developed fire
resistant steel. Considering the above mentioned fact research in the prevent invention
was carried to design steel composition compatible to the commonly available micro

alloyed welding electrodes. The developed fire resistant structural tube was welded with
the compatible welding electrodes and the weld joints were evaluated for high
temperature properties. Figure 10 shows the high temperature stress strain behavior of
structural tube across the weld joints.
The fire resistant structural steel tubes are developed in various cross sectional shape
profiles and sizes as demonstrated in table 4, 5 and 6.
The present invention produces structural steel tube with inherent fire resistant
properties, achieved with minimal micro alloying content (micro alloying content <0.25,
with Mo content <0.15). Further, steel with non peritectic and low carbon equivalence
composition (CE<0.3) ensures the ease of manufacturing through conventional
continuous casting and thin slab casting route and subsequent processing and
Eliminates the fire proofing of steel structures under load, where temperature is not
expected to rise beyond 600°C.

WE CLAIM:
1. A fire resistance steel tube with yield strength of at least 350 MPa, UTS of at least 490
MPa, fire resistance ratio of at least 0.66, and chemical composition comprising in weight
% C: < 0.10%; Mn: <1.5 %;Mo: <0.25 %;V: <0.10%; Si: < 0.50%; N: <0.020%; S<0.005
%; and P<0.030%.
2. The fire resistance steel tube as claimed in claim 1, wherein the process of developing
fire resistance steel comprises:
Reheating a steel slab with a composition in weight % C: < 0.10%; Mn: <1.2
%;Mo: <0.25 %;V: <0.10%; Si: < 0.50%; N: <0.020%; S<0.005 %; and
P<0.030%, to a temperature in the range of 1050 to 1250° C;
hot rolling the heated steel slab to a thickness range of 1 to 12 mm at a finish
rolling temperature range of 870 to 910°C, followed by cooling at ROT to a
coiling temperature in a range of 550 to 600°C with cooling rate from 0.5 to
20°C/s;
slitting of the hot rolled strips based upon predetermined dimensions;
cold forming to convert the strips in tubular shapes; and
seam welding via high frequency induction welding process.
3. The fire resistance steel tube as claimed in claim 1, wherein the ferrite potential of the
fire resistance steel composition is greater than 1.05.
4. The fire resistance steel tube as claimed in claim 1, wherein the fire resistance ratio is
greater than 0.66 at 600°C for minimum time period of two hours.

5. The fire resistance steel tube as claimed in claim 1, wherein the fire resistance steel has
UTS preferably in the range 490 to 540 MPa and % Elongation is at least 20.
6. The fire resistance steel tube as claimed in claim 1, wherein the total micro alloying
content (Molybdenum + vanadium + Nitrogen) is less than 0.25 wt. %.
7. The fire resistance steel tube as claimed in claim 1 further comprising optionally 0.25 wt.
% of chromium, Nb < 0.025 %, and Cu < 0.25%.
8. The fire resistance steel tube as claimed in claim 1 or claim 2, wherein the fire resistant
steel is capable of being processed through conventional continuous casting and thin slab
casting route of steel processing.
9. The fire resistance steel tube as claimed in claim 1, wherein microstructure of fire
resistance steel tube comprises around 90% ferrite and pearlite volume fraction less than
10% with traces of bainite.
10. The fire resistance steel tube as claimed in claim 9, wherein ferrite grain size varies in the
range 4 to 12 µm.
11. The fire resistance steel tube as claimed in claim 9, wherein ferrite grain size preferably
varies in the range 6 to 12 µm.
12. The fire resistance steel tube as claimed in claim 1, wherein the fire resistance steel
preferably comprises: C 0.04-0.08 wt.%, Mn 0.7-1.2 wt. %, Si 0.05-0.30 wt. %,
vanadium 0.03-0.08 wt. %, and nitrogen 0.0070-0.015 wt. %.