FIELD OF THE INVENTION
The present invention relates to a heat resistance steel. More practically, the present invention relates to fire resistance steel adaptable to structural
application and the process of producing fire resistance steel for structural
application through deformation of sheet of Cu added low carbon steel
encouraging precipitation of copper during breakout of fire attaining a
temperature of 500°C or above.
BACKGROUND OF THE INVENTION
All commonly used structural materials lose some of their strength when exposed
to fire. The development of fire resistant steel for building is aimed at increasing
steel's elevated temperature strength while assuring other properties equal to or
better than those of temperature strength while assuring other properties equal
to or better than those of conventional steels for building. Mild steel structural is
safe up to 400°C and beyond this the yield strength decreases faster. Thermo
mechanically processed steel with combined addition of molybdenum, niobium
and or chromium increases the elevated temperature yield strength so that two
thirds of room temperature yield strength can be retained at 600°C. Ni, V and Ti
are also added as micro alloying elements to obtain specified ambient
temperature and elevated temperature yield strength.

Hence, there is a need to provide a product from fore resistance point of view by
modifying the chemistry such that when subjected to fire, the steel should retain
minimum 2/3 of its room temperature yield strength.
The proposed invention has been developed to solve the difficulties of using
space consuming fire coating elements or costlier alloying steel by providing
economical grade of steel sheet for structural application on developing.
OBJECTS OF THE INVENTION
It is therefore, an object of the present invention to propose a fire resistance
steel adaptable to structural steel which eliminates the disadvantages in the prior
art.
Another object of the present invention is to propose a fire resistance steel
adaptable to structural steel and the process of producing the same which
modifies the chemistry of steel making such that the add cost does not exceed
Rs 1000 per tone.
A further object of the present invention is to propose a fire resistance steel
adaptable to structural steel and the process of producing the same which can be
widely used in building construction including multi-storey open car parks, sport
facility,railway stations, external steel frames and many other structure.

A still further object of the present invention is to propose a fire resistance steel
adaptable to structural steel and the process of producing the same which
successfully reduces the amount of fire protection and structural members may
even be imported in cases where the steel temperature would not exceeds
600°C. The intention of fire proofing a structure is to save lives, allow safe
evacuation provide safe access for fire fighter and protect the building from
structural failure.
SUMMARY OF THE INVENTION
The beneficial role of Cu has recently received renewed attention due to potential
of developing post heat treatment steel which combine high strength and good
elongation. The precipitation of copper in both the iron and steel is usually
observed in temperature range of 450°C to 650°C. Since, during the fire in a
building, the materials can attain a temperature of 600oC,/the chemistry was
designed as shown in Table 1 with the addition of 0.65-10% copper. The loss in
yield strength during fire as in the case of conventional steel will be compensated
by precipitation of copper during the aging temperature encountered in the event
of fire breakout in steel intensive building.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
Fig-1: Comparative hardness of FR and reference steel at elevated temperature.
Fig-2: Stress-strain curve for reference steel at 600°C for 2 hours.
Fig-3: Stress-strain curve for FR steel at 600°C for 2 hours.
Fig-4: TEM Micro graph of aged sample showing precipitation after 2 hr at
600°C.
Fig-5: EDS Spectra showing precipitates composed of Cu.
DETAILED DESCRIPTION OF THE INVENTION
The heat was melted in 25 kg air induction furnace and cast in to ingot. The
ingot was forged in to 150x300x45mm plate. The forged plates were soaked at (1170oC-1190oC) hot-rolled in to 2.8 mm thick with a finishing temperature of
890°C and air cooled. Samples were cut from the hot band for tensile test as well
as laboratory heat treatment for aging (15 minutes, 30 minutes, 1 hr and 2 hrs
at 600°C) in order to study the copper precipitates in hot band. The samples
after soaking were water quenched to retain the precipitates. Simulated hot
tensile

tests at 600°C were conducted in Gleeble 1500 to determine the loss in yield
strength.
Changed in hardness for different holding times at 600°C for copper added steel
as well as reference steel (without copper while other elements remaining the
same) is shown in Figure 1. It may be seen that there is no increase in the
hardness of reference material where as copper added steel is showing the
increase in hardness due to copper precipitation. The peak hardness is attained
within 15 minutes. It is also interesting to see that there is marginal reduction in
hardness even at holding for 2 hrs at 600°C. This indicates that the loss in
strength at elevated temperature is compensated due to pecipitation of copper in
the fire resistant steel.
The strength of steels at room temperature and 600°C, using simulated test with
Gleeble is shown in table 2. The % ratio of yield strength at 600°C to room
temperature yield strength increased with the addition of copper. It is evident
from the Fig 2 and 3 that yield strength drops to as much as 170 Mpa for
reference steel where as in the case of FR steel it drops to only 290Mpa
respectively. The yield ratio is about 80% for FR steel.
TEM micrograph of hot- rolled and aged sample (soaked at 600°C for 2 hrs) as
shown in Figure 4 reveals the dislocation and sub grain structure. The copper
precipitates, located on dislocation are spherical and disturbed homogeneously.

EDS spectra as shown in Figure 5 confirmed the presence of copper in
precipitates.


WE CLAIM:
1. A fire resistance steel adaptable to structural appliances comprises:-
a steel having a composition (wt%)
C -0.10-0.15
Mn - 0.45- 0.65
Si -0.10 max
S - 0.020 max
P - 0.025 max
Cu -0.60-1.0
Al - 0.02-0.06
and rest Fe
characterized in the post heat treatment steel contains copper
precipitation in the hot band of steel which in turn compensate the loss
in yield strength during fire.
2. The process of producing fire resistance steel as claimed in claim 1
comprising
- melting of steel in Induction furnace at 1600°C-1650°C;
- liquid steel cast into ingot;
- forging of ingot to form a size 150 x 300 x 45 mm plate;
- soaking the plate at 1170°C;
- hot-rolling the plate to make 2.8mm thick with a finishing
temperature 890°C;
- air cooling the plate from 890°C to room temperature;
- heat treating for aging at 600°C for a soaking period 15 minutes,
30 minutes, 1 hour and 2 hours time in order to study the copper
precipitates in hot band;

- water quenching after soaking to retain the precipitates;
characterized in that the said steel should retain minimum 2/3rd times
of its room temperature yield strength at high temperature.
3. The fire resistance steel as claimed in claim 1, wherein the yield ratio
is about 80% is achieved at 600°C.
4. The fire resistance steel as claimed in claim 1, wherein the copper
precipitates at 600°C.

ABSTRACT

A FIRE RESISTANT STEEL ADAPTABLE TO STRUCTURAL APPLICATIONS
AND THE PROCESS OF PRODUCTING THEREOF
The present invention is provided with a fire resistant steel adaptable to
structural appliances comprises a steel having a composition (wt%): C - 0.10 -
0.15, Mn - 0.45 - 0.65, Si - 0.10 max, S - 0.020 max, P - 0.025 max, Cu - 0.60
- 1.0, Al - 0.02 - 0.06 and rest Fe; characterized in that post heat treatment
steel contains copper precipitation in the hot band of steel which in turn
compensate the loss in yield strength during fire.