FIELD OF INVENTION
The present invention relates to the designing of easily formable and weldable
high strength low alloy (HSLA), steel having excellent elevated temperature
strength and the manufacturing method for the same.
BACKGROUND OF THE INVENTION
The fire safety of high rise buildings, airport structures, stadium and auditorium
has become a prime focus in last few decades. The structural members like
columns, trusses, beams made up of unprotected plain carbon structural steel
defined as per IS10748, BS EN 10219-1,ASTM A6-11/A6M standard, significantly
loses its load bearing ability when exposed to temperatures above 350°C .The
yield strength of such steel is reduced to 25-30% of their room temperature yield
strength at 600°C. This limitation of plain carbon steel defines the need of
development of steel capable of retaining significant strength at high
temperature, especially for the smaller and thinner structures (with thickness less
than 12 mm ) where the temperature builds up at rapid pace as compared to the
thicker sections.
Application of fire protective paint on the steel surface is a most common
measure to attain fire resistant properties in structural steels. These fire
protective paints form a thin, hard and smooth film. When exposed to a high
temperature of about 400°C swells up to form a thermo insulating layer. This
insulating layer because of its low thermal conductivity, significantly delays the
temperature build up in the steel structures.
Fire protective coatings intended for the protection of steel structures vary in
thickness from 250 urn to several millimeters. The use of fire protection coating
on steel structures may prolong the fire resistance time from 15 to 120 minutes

depending upon the composition and thickness of the coating applied. The
synthesis and manufacturing of such coating involves various types of chemicals
that may be hazardous to the environment. The alternate method of achieving
the fire resistance is by providing fire-proofing mortar overlay or further
protecting the formed heat insulating layer further with a thin metal sheet such
as aluminium or stainless steel sheets. The use of such fire proofing measures on
ordinary structural steel can double up the cost per ton of steel construction,
thus reducing the competitive edge of steel. Detail product design and
information on fire resistant properties of low Mo steel (<0.20 wt %) is not
available.
Mo is usually considered to be the most effective element for improving the high-
temperature strength of steel. The fire resistance steel with high Mo of up to 1.5
%has been produced by NIPPON STEEL (JP3240918). The higher Mo content in
the steel leads to the weld embrittlement due to the increased in hardenebility of
steel and thus lowers the toughness of steel in the 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. 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. 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.
KOBE Steel ltd reported the invention of FR steel in patent JP2002249845, using
Cu precipitates having excellent fire resistance which exhibits high strength even
at high temperatures. Such a high content of Cu (>1 wt.%) in steel causes the
problem of hot shortness while hot forming/rolling of steel and is prone to
surface cracking.
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.
OBJECTS OF THE INVENTION
An object of this invention is to propose a lean composition of steel capable of
imparting fire resistant properties with the fire resistance ratio greater than 0.5
at 600°C for a minimum period of one hour.
Another object of the present invention is to propose the chemistry for a fire
resistance steel, ofwhich ferrite potential is >1.05.
Another object of the present invention is to propose a composition with carbon
equivalence (as defined by IIW) less than 0.35 that will enable the steel to
exhibit excellent weldability.

Still another object of this invention is to propose a thermo mechanical
processing that is effective in optimizing the room temperature 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 duringthe tube processing by Electric resistance welding (ERW)
process.
Another object of present invention is to propose the fire resistant steel with
total micro alloying content (Molybdenum + vanadium + Nitrogen) less than
0.25.
SUMMARY OF THE INVENTION
The present invention is aimed to develop the fire resistant steel capable of
retaining at least one half of its room temperature yield strength for a minimum
duration of one hour and also meet the room temperature properties of high
strength structural steel as per IS10748 Grade 6 and BS EN10219-1. In
accordance the target room temperature properties for the present invention
were aimed as YS 355-450 MPa, UTS 490-540 MPa and elongation values greater
than 20%.

The maximum yield strength of the steel was restricted to 450 MPa to limit the
spring back phenomenon during the course of cold forming employed for ERW
tube making process. The spring back phenomenon due to the increased yield
strength results in geometrical imperfections and impairs the quality of structural
tube.
The steel was designed with ferrite potential in excess of 1.05 to ensure that the
composition of the designed steel remains non-peritectic.The steel with
peritectic composition have the tendency to breakouts and cracking of slab
during solidification in the caster. And finally the steel composition was designed
to aim for carbon equivalence to be less than 0.3 to make the hot rolled steel
readily weldable, and to enable the usage of steel in various shape profiles for
constructional purpose.
In the present invention, the fire resistant properties in steel were attained with
leaner micro alloyed composition coupled with controlled thermo mechanical
processing. Room temperature strength was achieved with the presence of
pearlite as a second phase, solid solution strengthening derived from C, N, Mn
and Si. Whereas the high temperature strength was obtained by the presence of
fine precipitates of vanadium. These precipitates are partly formed during hot
rolling of the steel and remaining are formed during the event of fire, when the
steel is heated up to a temperature of 400°C or more. The thermal stability of
precipitates was ensured with the addition of alloying elements like Mn and Mo.
The steel of the current invention is capable of maintaining yield strength greater
than 180 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: <0.12 %
Mn: <1.2%
Mo: <0.25%
V: <0.15%
Si: <0.50%
N :<0.020%
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.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 is the schematic illustrating the thermo mechanical processing employed
for the development of fire resistant steel.
Figure 2 illustrates the change in room temperature properties and fire resistant
ratio with the variation in coiling temperature for steel 1. Figure 3 illustrate the
change in room temperature properties and fire resistant ratio with the variation
in coiling temperature for steel 2.
Figure 4 shows Bright and Dark field TEM micrographs of vanadium
nitride/carbonitrides precipitates interacting with dislocations.
Figure 5 shows TEM micrograph of fine dispersed precipitates and their
interaction with dislocations when exposed to a temperature of 650°C for a
period of 60 minutes.
Figure 6 shows TEM micrograph revealing the interphase precipitation of
vanadium nitrides/carbonitrides in the ferrite matrix for steel 2 coiled at a
temperature of 650°C.

Figure 7 shows the stress strain curve for the steel at room temperature and
high temperature for a holding period of 60 minutes.
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 formable and welding friendly fire resistant steel. The developed steel
exhibits yield strength of one half or more at a temperature of 600°C compared
to the room temperature yield strength. The developed hot rolled steel is
hot/cold formed and welded to produce structural tubes of various cross
sectional shape profiles that are being employed in the construction of multi
storey buildings for offices, shopping malls, auditoriums, stadiums etc.
The detailed research was carried out to study the effect and role of alloying
elements on the room temperature and high temperature properties and
accordingly the composition was designed The steel composition of fire resistant
steel in the present invention consists of V, Mo, Mn, N, besides C, Si, Cr, P and S.
The proposed chemical composition, in weight percent of the steel, consists of
C: <0.12%
Mn: <1.2%
Mo: <0.25%
V: <0.15%
Si: <0.50%
N :< 0.020%
S<0.005%
P<0.030
Table 1 provides a few examples of the composition of steel as per the current
invention


* Ferrite Potential ** Carbon Equivalence
The ferrite potential for a given composition of steel is a parameter that defines
whether steel composition is peritectic or non-peritectic. 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
Carbon Equivalent (IIW)
Figure 1 shows the schematic that defines the thermo mechanical processing
employed for the production of hot rolled strips of the designed chemistry. The
cast slab with the following composition in weight percent
C: <0.12 %
Mn: <1.2%
Mo: <0.25%
V: <0.15%
Si: <0.50%
N :<0.020%
S<0.005%
P<0.030 is heated to a temperature of 1200 to 1350° 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 600-650°C with the average cooling rate of 0.5-20°C/s. Figure 2
illustrates the change in room temperature properties and fire resistant ratio with
the variation in coiling temperature for steel 1. As evident from the figure the fire
resistant ratio increases with increase in coiling temperature. This is because the
steel at a coiling temperature of 650°C undergoes inter-phase precipitation of
micro alloying elements in ferrite phase. The fine precipitates offer greater
resistance to dislocation movement and grain boundary motion .The spatial
distribution of precipitates and thus help in retaining the strength at higher
temperatures
Salient features on the role primary alloying elements in the present invention for
fire resistant steel development are described below:
C:<0.12 wt%: The preferable range for the carbon in the steel is 0.05-0.08%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: <1.2 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 vanadium is enhanced with the increasing Mn
content because manganese lowers the austenite-to-ferrite transformation
temperature, and results in a finer precipitate dispersion.

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 and is also effective in
limiting the chemistry to non peritectic composition. The presence of Si becomes
more significant when the steel is being rolled to higher sheet thicknesses. Si is
also being employed as a deoxidizing element. However the Si content in the
steel is restricted to 0.5% in order to prevent the formation of surface scales.
Also the higher Si content will impair the weldability of the steel.
V: 0.05-0.15 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 carbo
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. Figure 4 shows Bright and Dark field TEM micrographs of
vanadium nitride/carbonitrides precipitates interacting with dislocations Figure 6
shows TEM micrograph revealing the interphase precipitation of vanadium
nitrides/carbonitrides in the ferrite matrix for steel 2 coiled at a temperature of
650°C. Such a fine dispersion of precipitates in a array like spatial arrangement
offers a long range internal strain that results in greater resistance to dislocation
motion.
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
0.0070-0.020 %wt Nitrogen in steel combines with the vanadium and carbon to
form nitrides/carbonitirdes. These precipitates have a high thermal stability i.e.
resistance to the coarsening compared to carbides of vanadium or any other
alloying element. The lower Nitrogen content would severely affect the
precipitation potential of vanadium and would deteriorate the high temperature
properties of 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 steels developed using the process and composition as per the current
invention were tested for various properties such as YS, UTS, EI and FR.
Table 2 summarizes the micro structural features of steell and its variation with
the coiling temperature. Although the higher coiling temperature leads to coarser
grain size, still higher strength is achieved for the steel coiled at a temperature of
650°C as evident from the room temperature and high temperature properties
shown in table 3. The increase in the room temperature and the fire resistant
properties of this steel is attributed to the interphase precipitation of vanadium
precipitates as nitrides/carbonitrides in the ferrite matrix. Cooling the steel
rapidly to a lower coiling temperature suppresses the precipitation formation in
the steel which not only lowers the room temperature properties but also leads
to a lower fire resistant ratio.
Table 2: Micro structural description of steel 1 coiled at different coiling

Table 4 summarizes the room temperature and high temperature properties of
steel 2 for the different coiling temperatures. Similar trend in the performance of
steel 2 was observed, as in case of steell.

Table 3: Room temperature tensile properties and fire resistant ratio for
steel 1

The room temperature as well as fire resistant ratio for steel 2 was higher than
the steel 1 for almost identical levels of Mo and V in steel for the same coiling
temperature. The increased performance of steel 2 was due to (a) increase in Mn
content and (b) elimination of Cr from the steel 2.
Chromium being a strong nitride former consumes the nitrogen present in the
ferrite and reduces the nitrogen content present as interstitials. This lowers room
temperature properties compared to the Cr free steel i.e. steel2.
Table 5 summarizes the room temperature and high temperature properties of
steel 3. The steel composition for steel 3 was made Mo free and rich in Mn
content. Though the room temperature target properties were successfully met
but the fire resistant properties significantly fell off the target.
Table 4: Room temperature tensile properties and fire resistant ratio for steel 2


It is evident from the examples of the steels described above that current
invention provides steel with fire resistance properties with a leaner micro
alloying composition. Further, the steels as per the current invention exhibit
desired room temperature properties with YS: 355-450 MPa and UTS: 490-
540MPa. The steel of the current invention is capable of cold formed and
weldable to form structural tubes of different cross sectional profiles like square,
circular, rectangular using ERW tube processing.
Figure 7 shows the stress strain curve for the steel at room temperature and
high temperature for a holding period of 60 minutes and it is evident from the
curve that steel as per the current invention retains significant strength at higher
temperature.

WE CLAIM:
1. A fire resistance hot-rolled steel for structural applications, the steel
comprising, in terms of weight %:
C: < 0.12%;
Mn: <1.2 %;
Mo: <0.25 %;
V: <0.15%;
Si: < 0.50%;
N: <0.020%;
S<0.005 %; and P<0.030%,
where in ferrite potential of the fire resistance steel composition is greater than
1.05 and fire resistance ratio is greater than 0.5 at 600°C.
2. The fire resistance steel as claimed in claim 1, wherein one or more of the
following elements are present in the fire resistance steel in weight %:
C: 0.05-0.07 %
Mn: 0.7 -1.2 %
Mo: 0.10-0.15 %
V: 0.05-0.10%
Si: 0.05-0.20%
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 %EI> 20.
4. The fire resistance steel as claimed in claim 1, wherein the total micro
alloying content (Molybdenum + vanadium + Nitrogen) is less than 0.25
wt. %.

5. The fire resistance steel according to one or more of claims 1-4 further
comprising optionally 0.25 wt. % of chromium.
6. The fire resistance steel as claimed in claim 1, wherein the fire resistance
steel is hot- rolled to a thickness of up to 12 mm.
7. The fire resistance steel as claimed in claim 1, wherein pearlite volume
fraction is less than 10%.
8. The fire resistance steel as claimed in claim 1, wherein ferrite grain size
varies in the range 6 to 12 µm.
9. A structural tube produced from the fire resistance steel as claimed in
claim 1.
10. Method of production of the fire resistant steel, the method comprising:
reheating a steel slab with a composition in weight %
C: < 0.12%;
Mn: <1.2%;
Mo: <0.25 %;
V: <0.15%;
Si: < 0.50%;
N: <0.020%;
S<0.005 %; and P<0.030%/
to a temperature of 1200 to 1350° C; hot rolling to a finish rolling temperature of
870 to 910°C; and cooling at a temperature from 600 to 650°C with cooling rate
from 0.5 to 20cC/s.

ABSTRACT

The present invention is related to a fire resistance steel composition and
process for preparing the same. The fire resistance steel of the current invention
has a fire resistance yield ratio of 0.5 or more at a temperature of 600°C for a
minimum duration of 60 minutes. In accordance the target room temperature
properties for the present invention were aimed as YS 355-450 MPa minimum,
UTS 490-540 MPa and elongation values greater than 20%. The steel is designed
such that it is readily hot/cold formed and welded to form structural tubes of
various cross sectional shape profiles that are being employed in the construction
of multi storey buildings for offices, shopping malls, auditoriums, stadiums etc.