FIELD OF THE INVENTION:
The present invention relates to the development of lean chemistry micro-alloyed
steels having elevated temperature (up to 600°C) yield strength of greater than
half the room temperature yield strength.
BACKGROUND OF THE INVENTION:
Modern high rise buildings such as residential, offices, shopping malls, offshore
structures are increasingly looking for fire resistance (FR) steel structural tubes,
where the strength at elevated temperatures is of significant consideration. The
hot rolled general purpose and structural steels covered in IS10748, BS EN
10219-1, ASTM A6-11/A6M, JIS G3101 have a tendency to lose its yield strength
(YS) when exposed to temperatures exceeding 350°C. The yield strength (YS) at
600°C is reduced to about 25% of the YS at room temperature at 600°C. This
limits the service temperature of the conventional structural steels included in
the aforesaid standards to 350°C.
The fire resistance property is incorporated in conventional constructional steels
by applying an insulating coating of mineral fiber or vermiculite in a cement
slurry, slag wool, rock wool, glass wool or asbestos of sufficient thickness on
steel structures as stated in US patent 5147474. This patent also indicates the
alternate method of achieving fire resistance by applying a fire-proof mortar
overlay or further protecting the heat insulating layer with a metal thin sheet,
such as an aluminium or stainless steel sheet. The steels with fire proofing
coatings increase the cost of the steel and thus the overall cost of the building
construction. Further, the coating is temporary in nature and need to be applied
at regular intervals.

The steels with chemical composition which offer inherent fire resistance with
less or no protective coatings are preferred for fire proof constructions. Fire
resistance steels as per the Australian (AS15304), European and North American
(ASTM El 19-82) design codes are defined as the steels which exhibit at least one
half of the room temperature YS at elevated temperature of 550°C. As per the
Japanese code (JIS A 1304), the fire resistance steels must withstand at least
two thirds of the room temperature YS at 600°C.
Fire resistant steels have been developed with high amount of Mo (> 0.25 mass
%) in combination with other alloying elements such as Cr, Ni, Cu, Nb, V, Ti and
N. Another US patent US 2009/0087335 disclosed the optimized chemical
composition of fire resistance steels, to achieve yield ratio of 0.6 (min). The
major alloying elements of the invented steels contain C 0.01-0.03, Mn 0.2-1.7,
Cu-0.7-2.0, Mo-0.8 or less, Nb-0.01-0.3, Ti-0.005-0.03, V-0.2 or less, Cr-1.0 or
less.
The fire resistance steels containing Mo exhibits fine Mo carbide precipitates
along the grain boundaries which obstruct dislocation movement within the
crystal. The same is presented in the Japanese patents A5-186847, A7-300618
and A9-241789. The invention in these patents claims that sufficient amount Mo
carbides offer higher resistance to dislocations than Mo as solid solutes in the
matrix.
Nippon Steel Technical Report (Article No. 90, July 2004) has reported about
590MPa class fire resistance steel for building structures. It contains 0.51 Mo,
0.02 Nb and a carbon equivalent (CE) of 0.43. Such high CE steels are difficult to
weld by the electric resistance welding process used for tube making. Also, high
Mo content makes the product very costly. In similar lines, Kawasaki Steel report
(No. 29, Nov. 1993) has mentioned about fire resistance steel of 400MPa and

490MPa tensile grades with a CE of 0.35 for both the grades. However, the
details of Mo and other micro alloying elements have not been reported.
An US patent US2010/0065168 reported the invention of Mo free fire resistance
steel with excellent high temperature strength. The composition of the invented
steel by mass % is 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 (i.e. YS at 600°C/YS at room temperature)
achieved in the invented steel was 0.59.
In the present invention the chemistry of the fire resistance steel is aimed to
meet the room temperature tensile properties of the benchmark, structural steel
of YS 355MPa as per BS EN10219-1. The room temperature ultimate tensile
strength to be achieved is 475-525MPa and the elongation of minimum 20%.
Effort was made to design chemistries that avoid the peritectic region; otherwise
there would be tendency to experience surface cracks during solidification at the
slab casting stage. The yield ratio obtained for the fire resistance steels is greater
than 50% (minimum). The results are based high temperature tensile testing,
wherein the samples were held for lh at the specified temperature of 600°C and
then pulled.
In the present invention, the aforesaid properties were achieved with the
addition of Mo less than 0.25 mass%. Given the low C in the steel, part of the
Mo forms carbide precipitates (based on stoichiometry) and the other part
remains in solution and exerts solute drag effect at elevated temperature. The
Mo content used in the present invention is significantly lower compared to the
other fire resistance steels developed, which makes it technically and
commercially attractive. The other elements present in the fire resistance steels
of the current invention are Mn, Si, S, P, Cr, V and N.

OBJECTS OF THE INVENTION:
An object of the present invention is to propose lean chemistry micro-alloyed
steels with carbon equivalent [CE(IIW)] less than 0.25 with fire resistance
property of yield ratio greater than 0.5 up to 600°C of temperature.
Another object of the present invention is to propose micro-alloyed steels with a
low carbon equivalent [CE(IIW)] of less than 0.30, which will make it suitable for
manufacturing of tubes by the electric resistance welding (ERW) process.
Another object of the present invention is to propose micro-alloyed steels with
room temperature YS of 355MPa (min), UTS of 475-525MPa, and Elongation of
22% (min).
Another object of the present invention to propose micro-alloyed fire resistance
steels with low alloy additions (CE<0.25), which has a lower cost of production
than some of the richer chemistry fire resistant steels.
SUMMARY OF THE INVENTION:
The present invention deals with the development of a leaner chemistry micro-
alloyed steels with fire resistance property upto 600°C for structural tube
application having a chemical composition (mass %) consisting of:
C: 0.10 max
Mn: 0.50 max;
Cr + Mo + V + N= 0.50 max;
Si: 0.15 (max);
Al: 0.15 (max);

The hot rolled steel microstructure contains carbide and nitride precipitates,
which resists thermal softening and improves fire resistance property in the form
of higher yield strength at elevated temperature.
DESCRIPTION OF THE ACCOMPANYING FIGURES:
Fig. 1 Time-temperature plot used in Gleeble thermo-mechanical simulation
corresponding to hot strip mill run out table (ROT) and coiling temperature (CT)
strategy.
Fig. 2 TEM image of Mo and Cr carbide precipitates in the hot rolled and ROT
simulated condition. These carbides ensure superior elevated temperature yield
strength compared to non-Mo fire resistance steels.
Fig. 3 Yield strength of fire resistance and CMn plain carbon structural steels at
different temperatures. The plain carbon steel considered for the comparison is
Grade 4 CMn steel as per IS 10748 with composition of C 0.22% max, Mn 1.6%
max, Si 0.55% max, S 0.03% max and P 0.055% max. The figure shows that the
yield strength decreases as the test temperature increases. The conventional
plain carbon structural steel exhibits lower YS compared to the fire resistance
steels invented in the present work. The complex precipitates along the grain
boundaries and the alloyed matrix offers resistance to the movement of
dislocations at elevated temperatures in case of fire resistance steels.
Fig. 4 Yield strength ratio versus temperature for fire resistance and plain carbon
structural steels. The figure presents the yield strength ratio (yield strength
at600°C/yield strength at room temperature) for fire resistance and conventional
plain carbon steels. For the yield ratio calculation, room temperature tensile

properties and elevated temperature tensile properties were used. The results
show that the yield ratio is greater than 0.5, which the inventors have set as the
objective of the present invention.
Fig. 5 Tensile elongation and reduction area plot vs. temperature for fire
resistance and plain carbon structural steels. It presents the variation of
elongation (El) and reduction in area (RA) for the fire resistance and plain carbon
steels as a function of the testing temperature. The ductility increases as the
testing temperature is raised for all the steels, as expected. The increased
ductility is due to softening of the steels at elevated temperatures.
Fig. 6 Yield strength ratio for the fire resistance steel grades mentioned in the
present work. It compares the yield strength ratio (the ratio between the YS at
600°C and YS at room temperature) for the fire resistance steels disclosed in the
present invention. It can be noted that yield strength ratio of greater than 0.50
has been achieved in all the designed and experimented fire resistance steels.
DETAILED DESCRIPTION OF THE INVENTION:
The present invention is related to the development of fire resistance steel,
which has an elevated temperature yield strength greater than 0.5 of the room
temperature yield strength. The steels invented can be used as the sheets, tubes
and other profiled shapes like rectangles and square hollow section.
To attain the fire resistance property, the inventors engaged in a detailed
research in designing the alloy chemistry. The fire resistance steels in the
present invention contain alloying elements such as Mo, Cr, V and N, besides Si,
Mn, P and S. Table 1 presents the composition of the steels, and Table 2 shows

the mechanical properties of the steels included in the present invention. The
composition of the steels was varied, particularly Mn and micro-alloy
constituents, by controlling addition of various alloying elements. Fire resistance
property was evaluated for each of the chemistries.

Note: CE (IIW) = C + Mn/6 + (Cr+MO+V)/10

It can be seen from Table 1 that Steel 3 and 4 has a leaner chemistry than Steel
1 and 2. The presence of Mo was found to be effective in retaining the yield

strength at elevated temperature. The coherent and semi coherent strains from
the fine alloy precipitates increase the long range resistance to dislocation
motion at 600°C, which results in higher yield strength ratio at elevated
temperature as compared to non-Mo steels.
Also, the synergic effect of V and Mo are more effective in achieving high yield
ratio than that of the steels with only Mo or V. Moreover, the solute drag effect
arising from solute Mo and V retards the recovery of dislocations and reduction
in dislocation density at higher temperatures. It is important to note that Mo is
more effective in retaining YS at high temperatures rather than at ambient
temperature. Chromium helps in forming complex carbides along the grain
boundaries and inhibits grain boundary movement.
Vanadium forms V(C,N) precipitates at the grain boundaries which prevent the
grain coarsening during exposure to fire temperatures (600°C). The chemistry of
the fire resistance steels in the present invention also contains the solid solution
elements, besides the micro-alloying elements discussed above. Si is a
substitutional solid solution strengthening element which increases the strength
of ferrite. S and P presence in the fire resistance steels is considered to be
unavoidable impurities, though P increases the strength of the steel by solid
solution strengthening.
Another important solid solution strengthening element in the fire resistance
steels is Mn, which basically increases the room temperature strength and
ductility. Very high room temperature yield strength resulting from excess Mn
content would reduce the yield strength ratio corresponding to 600°C.
The fire resistance steels of the present invention were made in the laboratory
using an air induction melting furnace. The ingots were forged and then

subsequently hot rolled to 1.5-3.8 mm thickness using a single strand, double
action hot rolling mill. The steels in the present study were produced after
soaking the forged flats at temperatures of 1040°- 1080°C, and then rolled at a
finish rolling temperature (FRT) of 880°-900°C.
Subsequently, ROT simulation (Fig. 1) was done using a Gleeble thermo-
mechanical simulator with the finish rolling temperature (FRT), coiling
temperature (CY) and cooling rates prevalent in the hot strip mill practice. The
simulation included the following steps: (i) heating to a temperature of 880°C
and holding for 2 min; (ii) cool to the an intermediate temperature of 750°C and
hold for 2 min; (iii) cool to CT of 600°C.
The inventors have observed that the microstructure of the steels consists of
ferrite and pearlite with traces of low temperature transformation products such
as bainite. It is well known that thermo-mechanically controlled hot rolling of the
micro alloyed steels results in the formation of precipitates which contributes to
the strength at room and elevated temperatures. The inventors have
characterized the precipitates in the fire resistance steels using the scanning and
transmission electron microscopy.
These precipitates are largely responsible for the elevated temperature
properties of these steels. From the results observed, the inventors have
concluded that it is possible to obtain yield strength ratio of greater than 0.5 with
leaner chemistry steels (like Steel 3 and 4), unlike adopting a rich chemistry
containing higher levels of Mo and V.

WE CLAIM:
1. Micro-alloyed steel with fire resistance property upto 600°C for structural
tube application having a lean chemical composition (mass %) consisting
of:
C = 0.10 max
Mn = 0.50 max;
Cr + Mo + V + N= 0.50 max;
Si = 0.15 (max);
Al = 0.15 (max);
CE <0.30
balance being iron with impurities and
characterized in that the microstructure of the micro-alloyed steel exhibits
formation of (Cr, Mo, V) carbides, VN and complex carbo-nitrides (Cr, Mo,
V) (C, N), which increases the resistance of thermal softening and
improves fire resistance property in the form of higher yield strength at
elevated temperature.
2. The micro-alloyed steel as claimed in claim 1 wherein the hot rolled steel
sheets having yield strength of 355MPa minimum, ultimate tensile
strength of 475-525 MPa range and % elongation of 20 min at room
temperature (RT).
3. The micro-alloyed steel as claimed in claim 1, wherein excellent fire
resistance property shows at least 50% of the RT yield strength up to
600°C.

4. The micro-alloyed steel as claimed in claim 1 wherein Cr+Mo+V+N is 0.50
mass % max.
5. The micro-alloyed steel as claimed in claim 1 wherein the hot rolling
parameters corresponding to finish rolling temperature (FRT) of 880°C -
910°C, coiling temperature (CT) of 600°C-700°C.
6. The micro-alloyed steel with excellent fire resistance property up to 600°C
as substantially described and illustrated herein with accompanying
figures.

ABSTRACT

The present invention deals with the development of a leaner chemistry micro-alloyed steels with fire resistance property upto 600°C for structural tube
application having a lean chemical composition (mass %) consisting of:
C: 0.10 max
Mn: 0.50 max;
Cr + Mo + V + N= 0.50 max;
Si: 0.15 (max);
Al: 0.15 (max);
CE <0.30
balance being iron with impurities and characterized in that the microstrucre of the micro-alloyed steel exhibits formation of (cr.mo,v)carbides,vn and complex carbo-nitrides (cr,mo,v) (c,n),which increases the resistance of thermal softening and improves fire resistance property in the form of higher yield strength at elevated temperature.