FIELD OF INVENTION:
The present invention relates to the designing of easily formable and weldable high
strength micro alloyed steel having excellent elevated temperature strength and the
manufacturing method for the same.
BACKGROUND OF THE INVENTION:
Structural tubes in the recent times has emerged as the first choice construction
material for the architects and engineers, to be used in the construction of stadiums,
auditoriums amphitheaters, high rise buildings etc. owing to their high strength to
weight ratio, aesthetic appeal and exceptional resistance to the torsional loading.
However the conventional 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 capabilities of such steels are usually 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
The most modern methods of fire protection of steel structures involves the application
of Intumescent coatings on to the steel surface. These coatings 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, labor intensive but also requires special skill. More importantly these coatings
are very costly and drastically increase the cost of the project.
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.

A 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.
KOBE Steel ltd reported the invention of fire resistant steel in patent JP2002249845,
using Cu precipitates having excellent fire resistance which exhibits high strength even
at high temperatures. Because of such a high content of Cu (>1 wt.%) the steel
becomes vulnerable to the issues of hot shortness that causes cracking of steel surface
while hot forming/rolling of the steel slab.
A patent 203/KOL/2014 filed by Tata Steel Pvt. Ltd reports the development of micro
alloyed fire resistant steel using the combination of Mo and V in steel composition.
However, the alternatives to such a lean composition were not reported elsewhere.
In light of the above discussed prior art, there is a need of a steel composition which
overcomes the limitations of prior art and possess properties with respect to fire
resistance coupled with superior formability and weldability.
OBJECTS OF THE INVENTION:
An object of this invention is to propose a micro alloyed steel with the leaner
composition and thermo mechanical processing to roll out hot rolled strips with inherent
fire resistant properties with fire resistance ratio greater than 0.66 at 600°C for a
minimum period of two hours.
Another object of the present invention is to propose non-peritectic steel composition
for fire resistance structural steel, for which ferrite potential is greater1.05.

Another object of the present invention is to propose a steel composition with carbon
equivalence (as defined by IIW) less than 0.30 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: 355-460 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 460 MPa to limit 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 + Niobium +Titanium+
Nitrogen) to less than 0.25.
SUMMARY OF THE INVENTION:
The present invention is related to developing the fire resistant steel 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. Hot rolled steel strip as per
the current invention exhibit room temperature properties 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 employed in tube making process. The
phenomenon of spring back due to the increased yield strength not only impairs the
quality of tube because of geometrical imperfections but is also taxing on the mill on
account of increased mill load.
The non-peritectic steel composition is designed for fire resistant steel by ensuring
ferrite potential in excess of 1.05.
And finally the steel composition was designed to aim for carbon equivalence to be less
than 0.35 to make the structural steel readily weldable, and enable its applications as
structural tube in various shape profiles used for the construction purpose.
In the present invention, the fire resistant properties in steel were attained with leaner
micro alloyed composition with total micro alloying content restricted to less than 0.25,
coupled with controlled thermo mechanical processing.
Room temperature strength was achieved with the presence of bainite as a second
phase, solid solution strengthening derived from C, N, Mn and Si. The high temperature
strength in the present invention was attributed to the fine precipitates of Niobium
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 fire resistant steel in the present 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 chemical composition, in weight percent of the steel consists of
C: < 0.08 %;
Mn: <1.25 %;
Mo: <0.20 %;
Nb: <0.10%;
Ti:<0.020%;
Al:<0.1%;
Si: < 0.50%;
N: <0.020%;
S<0.005 %; and P<0.030%,
remaining Fe along with the unavoidable impurities.
The proposed steel essentially consists of ferritic microstructure with bainite as a
second phase with a volume fraction less than 10% and ferrite grain size of 4-10 µm.
The steel consists of fine precipitates dispersed in the ferrite matrix. 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 through either hot strip mill route or thin slab casting
route.

Figure 2 shows the optical and scanning electron micrograph revealing predominantly
ferrite matrix with bainite as second phase in steel1
Figure 3 shows the optical and Scanning electron micrograph revealing predominantly
ferrite matrix with bainite as second phase in steel 2.
Figure 4 shows Bright and Dark field TEM micrographs reveals the interphase
precipitation of Nb (CN) in the ferrite phase
Figure 5 shows the bright field TEM micrograph revealing the presence of bainite in the
microstructure for steel 1 and steel 2
Figure 6 shows the stress strain behavior of fire resistant steel at room temperature and
elevated temperatures
DETAILED DESCRIPTION OF THE INVENTION:
The present invention relates to chemical composition of steel coupled with the controlled
thermo mechanical processing method to develop the fire resistant steel with excellent ductility,
weldability and fire resistant properties. The developed fire resistant steel in the present
invention exhibits yield strength of two third or more at a temperature of 600°C compared to
the room temperature yield strength for minimum period of two hours. The fire resistant steel
developed finds application as structural steels in the construction of multi storey buildings for
offices, shopping malls, auditoriums, stadiums etc. The fire resistance steel of the current
invention is hot- rolled to a thickness of up to 15 mm.
The detailed research was carried out to study the effect and role of alloying elements
on the room temperature and high temperature properties. The steel composition of
fire resistant hot rolled steel in the present invention consists of Nb, Mo,Ti,Mn, N,
besides C, Si, Cr, P and S. The proposed chemical composition, in weight percent of the
steel, consists of

C: < 0.08 %;
Mn: <1.25 %;
Mo: <0.25 %;
Nb: <0.10%;
Ti:<0.020%;
Al:<0.1%;
Si: < 0.40%;
N: <0.020%;
S<0.005 %; and P<0.030%,
Table 1 provides a few examples of the composition of steel as per the current
invention

# Ferrite Potential(FP); ## 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. For a steel composition to
be non-peritectic , the value of FP should be either less than 0.85 or greater than 1.05.

Ferrite potential (FP) is calculated by the following empirical formula:
FP = 2.5 * (0.5 - Ceq), where Ceq is defined by following equation
Ceq = C + 0.04*Mn +0.1*Ni+0.7*N-0.14*Si-0.04*Cr-0.1*Mo-0.24*Ti-0.7*S

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: < 0.08 %;
Mn: <1.25 %;
Mo: <0.25 %;
Nb: <0.10%;
Ti:<0.020%;
Al:<0.1%;
Si: < 0.40%;
N: <0.020%;
S<0.005 %; and 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 remain
in a 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 10-25°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:<0.0.08 wt%: The preferable range for the carbon in the steel is 0.02-0.07%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 of carbides/carbonitrides.
The presence of carbon in the present steel plays a significant role in improving the
high temperature properties of the proposed steel, as a result of Mo-C cluster
formation. However, in order to avoid peritectic regime during the casting and limit the
carbon equivalence below 0.35, the carbon content is restricted to less than 0.07.
Mn: <1.25 wt%: The preferable range for the Mn in the steel is 0.7-1.2 %wt. The
preferable range for the Mn in the steel is 0.7-1.0 %wt. Manganese apart from
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 Niobium is enhanced with
the increasing Mn content because manganese lowers the austenite-to-ferrite
transformation temperature, and results in a dispersion of fine precipitates in the ferrite
matrix. Manganese at higher level enhances the centerline segregation during the
process of continuous casting. Moreover it leads to the higher number of MnS inclusions
which bring anisotropy in the steel 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.
Nb:<0.10: The preferable range for the Nb in the steel is 0.05-0.08 %wt. Niobium in
steel helps in the grain refinement because of its solute drag effect and allows to lower
the carbon content in the steel. Niobium plays significant role in enhancing the high
temperature properties of steel as it forms the fine carbonitrides precipitates in the
ferrite matrix which interacts with the dislocations and restricts the grain boundary
movement and thus limits the softening of steel at higher temperatures. However,
niobium content in excess of 0.10% can significantly increase the mill load which may
drastically reduce the life of the rolls in the rolling mill or in some cases it may be
beyond the capacity of rolling mills.
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 < 0.015 wt%: The preferable range for the nitrogen in the steel is 0.0070-
0.020 %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.
The fire resistant steel developed as per the current invention were tested for various
properties such as YS, UTS, EI and FR.

Table 3 details microstructural description of hot rolled strips rolled out in different
thicknesses

The steel consists of fine precipitates dispersed in the ferrite matrix along with bainite
as a second phase. Figure 2 shows the optical and SEM micrographs revealing
predominantly ferrite matrix with bainite as second phase The precipitates were
observed to be in a size of few nano meters. Figure 3 shows the interphase
precipitation of fine carbo-nitrides in the ferrite matrix. The interspacing between the
rows of precipitates was about few hundreds of nanometer.

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 thin slab or compact strip casting route, where the
casting speeds are much higher i.e. almost 2-3 times higher than convention continuous
casting o steel.

WE CLAIM:
1. A fire resistance hot-rolled steel for structural applications, the steel comprising,
in terms of weight %:
Carbon ( C ) : < 0.08 %;
Manganese (Mn) : <1.25 %;
Molybdenum (Mo): <0.20 %;
Niobium (Nb): <0.10%;
Titanium (Ti):<0.020%;
Aluminum (Al):<0.1%;
Silicon (Si): < 0.40%;
Nitrogen (N): <0.020%;
Sulphur (S) <0.005 %; and Phosphorus (P) <0.030%,
wherein ferrite potential of the fire resistance steel composition is greater than
1.05 and the fire resistant ratio is greater than 0.66 at 600°C for minimum period
of two hours.
2. The fire resistance steel as claimed in claim 1, wherein the steel comprises
carbon ( C ), Manganese (Mn), Molybdenum (Mo), Niobium (NB), Titanium (Ti),
Silicon (Si) Nitrogen (N) preferably, in terms of weight %:
C: 0.02-0.07 %
Mn: 0.5 -1.2 %
Mo: 0.10-0.15 %
Nb: 0.05-0.08%
Ti: 0.015-0.020%

Si: 0.05-0.40%
N: 0.0070-0.0150%.
3. The fire resistance steel as claimed in claim 1, wherein the fire resistance steel at
room temperature has YS of at least 355 MPa, UTS 490 to 540 MPa and %El>
20.
4. The fire resistance steel as claimed in claim 1, wherein the total micro alloying
content (Molybdenum + Niobium +Titanium+ Nitrogen) is less than 0.25 wt. %.
5. The fire resistance steel As per one or more preceding claims further comprising
optionally 0.25 wt. % of chromium, V < 0.05 %, Cu < 0.25% with Cu/Ni in steel
ratio >0.5.
6. The fire resistance steel as claimed in claim 1, wherein the fire resistance steel is
hot- rolled to a thickness of up to 15 mm.
7. The fire resistance steel as claimed in claim 1, wherein the bainite volume
fraction is less than 10%.
8. The fire resistance steel as claimed in claim 1, wherein ferrite grain size varies in
the range 3 to 10 μm.
9. The fire resistance steel as claimed in claim 1, wherein the steel exhibits
weldability with carbon equivalence <0.35.

10. Fire resistant structural steel tubes of made up of fire resistant steel as claimed
in any of the preceding claims.
11. Method of production of fire resistant structural steel, the method comprising:
reheating a steel slab with a composition in weight %
C: < 0.08 %;
Mn: <1.25 %;
Mo: <0.20 %;
Nb: <0.10%;
Ti:<0.020%;
Al:<0.1%;
Si: < 0.40%;
N: <0.020%;
S<0.005 %; and P<0.030%,
to a temperature of 1050 to 1250° C; hot rolling to a finish rolling temperature of
870 to 910°C; and cooling at a rate of 10 to 25°C/s to a coiling temperature in a
range of 550 to 600°C.