FIELD OF THE INVENTION
The present invention relates to an improved method of producing and testing of
hot-rolled high strength steel (600 MPa minimum) for fire resistant structural
applications. The invention further relates to a fire resistant steel product,
produced by the improved method.
BACKGROUND OF THE INVENTION
When there is a fire, the rise in temperature causes elongation and deformation
of the structural steel members of a structure which may lead to failure of the
load bearing structure. For instance, in the case of a frame structure the
elongation of the beams adversely affect the safety of the columns and the
entire structure becomes susceptible to collapse. Examples for use of fire-
resistant steel exist in Japan, China and Germany. These include a multitier car
parking space, a sports arena, a railway station, and an office building, among
others. While some data exists regarding the performance and material
properties of fire-resistant steel, additional research is needed to determine the
advantages and disadvantages including cost impacts associated with such
applications. Additionally, research is necessary to understand how and when
such steels can be appropriately applied in structural design practice.
To protect human lives, therefore, it is necessary to ensure that load bearing
structure remains substantially stable even during a fire. The fire resistance of a
structure is an extremely complex issue which involves many characteristics
properties of the material forming the structure and is highly influencable with
the type of material being employed.

To deal with these trends, it was previously taught in Japanese Patent
Publication (A) No.277523 that a building should use low yield ratio steel and
steel material superior in fire resistance and a method of production of the same.
The gist of this prior art is to make the yield point at 600°C, which is 70% or
more of that of the normal temperature of steel making by adding Mo and Nb to
improve the high temperature strength. The high temperature strength of steel
materials was set at 600°C, based on the findings that such a temperature range
is enabled to strike an economic balance between rise in cost of the steel
materials due to the alloy elements and the cost of fire resistant protection of the
same. The H-section steel developed by prior art process is characterized by a
reduction in the carbon content and addition of a small amount of Nb, B, and Cu
so as to produce a low carbon bainite structure. Such composition retained at
least by 2/3rd of the room temperature the yield strength at 600°C.
For the purpose of improving the brittleness at locations such as the fillets of H-
sertion steel, Japanese Patent Publication (A) 9-137218 discloses a building
structure which comprises. H-section steel including Mo, Cu, and Ni to reduce the
variation in material quality.
Japanese Patent Publication (A) No. 10-072620 discloses a method of production
of H-section steel reduced in variation of material quality and superior in
waldability.

Low cost conventional construction steel are generally adopted for structural
purpose but their elevated temperature behavior is unsatisfactory. Hence the
conventional construction steel need adequate protective coatings to ensure
compliance with Fire Regulations. The use of protective coating leads to an
increase in the total cost of construction thereby reducing the competitive edge
of the steel. Further, it is always not possible to apply such coating.
JP 52-16021 discloses use of hollow beam with forced cooling in order to offset
the low strength of the base material. However, the associated cost as well as
the operating and instrumental problems involved with such cooling system,
makes the product commercially unviable.
Japanese patent JP 55-41960 teaches the use Cr-Mo steels. Although this may
have good elevated temperature strength, weldability is not satisfactory which is
an important feature in respect of construction steel. Moreover, the cost of this
steel is considerably high.
EP 347156 describes a steel product having improved properties attained by the
addition of Mo and Nb to a low carbon, low manganese composition. However,
the plate/sheet after finish rolling was subjected to direct quenching by
accelerated cooling, followed by a dual phase, or lamellarizing, heat treatment
and tempering to produce a mixed microstructure of bainite and ferrite.
Moreover, the minimum Mo addition to get high temperature strength is 0.80%
in steel which exorbitantly increase the cost. Accordingly, there is a strong need
for an improved process and product which provides a better mechanical
properties, particularly at elevated temperature to the produced steel. This need
is particularly pronounced in the case of small and medium structure where
thickness are less than typically 12mm, and which are quickly susceptible to high

temperature than the structures where thickness higher. Moreover, the steel
produced thereof must possess very good weldability.
OBJECTS OF THE INVENTION
It is therefore, an object of the present invention to propose a method of
producing hot-rolled high strength steel for fire-resistant structural applications,
which eliminates the disadvantages of prior art.
A further object of the present invention is to propose a method of producing
hot-rolled high strength steels for fire-resistant structural applications which
increases the yield strength of the steel by addition of hardening elements for
example, Ti and Mo.
A still another object of the present invention is to propose a method of
producing hot-rolled high strength steels for fire resistant structural applications,
which generates stable precipitates of (TiMo) carbides at elevated temperature.
A further object of the present invention is to propose a method of producing
hot-rolled high strength steels for fire-resistant structural applications, which
yields 66% or more than the room temperature yield strength (600 MPa) at
elevated temperature such as 600 deg centigrade.
A still further object of the present invention is to propose a method of producing
hot-rolled high strength steels for fire resistant structural applications, which
achieves an excellent weldability for construction.

SUMMARY OF THE INVENTION
According to the invention, there is provided a high strength resistant steel
superior in fire resistance having less variation in material quality and exhibiting
a yield strength of 2/3rd or more of that at ordinary temperature even at 600°C
and a method of producing a fire resistant steel characterized by containing, by
mass % C: 0.04 to 0.08%, Mn: 0.8 to 1.3%, Si: 0.2% or less, Mo.0.1% or less,
Ti: 0.005 to 0.03%, N: 0.006% or less, Al: 0.05% or less, P:0.03% or less, and
S:0.02% or less, and comprising a balance quantity consisting of Fe and
unavoidable impurities. The steel has an yield strength at 600°C of 66% or more
of the yield strength at room temperature.
More specifically, the present invention provides a process of producing a fire
resistant structural steel which is subjected to a very small loss in strength on
exposure of fire than in the case with other similar structural steel through
deformation of low carbon steel sheet containing Ti and Mo as main precipitation
strengthening elements possessing strong stabilization of precipitates at higher
temperature.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Shows a microstructure of steel showing single phase ferrite
according to the invention.
Figure 2 - Shows a schematic diagram of a sample of the inventive product in
an elevated temperature test in Gleeble.

Figure 3 - Graphically illustrated a Gleeble simulation in terms of temperature
versus time.
Figure 4 - Shows a stress-strain curve for multiple elevated temperatures
between 500°C to 700°C.
Figure 5 - Shows an yield strength vs. elevated temperature after quenching.
Figure 6 - Schematically shows an weld configuration of the steel components
produced according to the invention.
Figure 7 - Graphically illustrates the microhardness across the weld of Figure
6.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
OF THE INVENTION
According to the invention, a process of preparing high strength steel is
proposed. The process comprising the steps of :-
making liquid steel in a basic oxygen furnace, transferring the liquid steel to a
ladle furnace wherein the heat is degassed to restrict the nitrogen content in
order to achieve maximum precipitates of 71, Mo carbide.

The final composition in wt% of the liquid steels achieved before tapping is as
follows :
C-0.04 to 0.08 Mn-1.15 to 1.3 Si-0.1 to 0.2
S- 0.008 max P - 0.025 max Al- 0.01 to 0.07
N-0.005 max Mo-0.08 to 0.12 Ti-0.04 to 0.05
The liquid steel melt having such composition is continuously cast into a slab of
1100 mm width and 210 mm thickness with a casting speed of 1.0-1.30
metre/minute. The casting speed is optimized to avoid any break out during the
slab casting. The slabs are hot rolled into a strip of 1100 m width and 3.2 mm
thickness at FRT of 850-920°C with a coiling temperature of 500-630°C
respectively. The step of alloying and processing of the steel is done in such a
way that the transformation of bainite to ferrite is delayed and transferred to the
end of the run out table. Samples were cut from coil dividing line in straightened
condition from transverse direction for characterization. As shown in Figure 1
the microstructure is a single phase ferrite. Samples for hot tensile test in
thermomechanical simulator (Gleeble 1500) were prepared according to Figure 2.
The test was performed for room temperature as well as at elevated temperature
as per schematic diagram shown in Figure 3 and the stess-strain curves for
various elevated temperature starting from 500 to 700 degree centigrades are
shown in Figure 4. It may be seen from figure 4 that even after 2 hours of

heating at elevated temperature, the yield strength is much more than 2/3 rd of
room temperature strength.
In order to check the stability of the precipitates, the samples were heated and
soaked in furnace for 2 hours at 500, 550, 600 and 700 degree centigrade
followed by quenching in water. The results of tensile test carried out on the
quenched samples are shown in Figure 5 which clearly reveals an increase in
yield strength in place of deterioration as expected in the event of a fire
breakout. Hence, this grade is highly suitable for fire resistance application.
Within the range of thickness involved it has been found experimentally that
from a low carbon steel alloyed especially with Ti and Mo, a steel is obtained
with properties that remain virtually unaltered after exposure to fire.
The high temperature strength in 600 MPa class of steel of the invention is
attained due to retardation of dislocation recovery. The precipitates of (TiMo)
carbides introduce the coherent strain by increasing the dislocation density and
as a consequence, the significant increase in long range resistance to dislocation
motion in the temperature range up to 700°C.
The invented steel have good weldability for the particular field of application
where consideration of shielding or cooling parameters including protection of
the material are not that important. For testing weldability of the product, sheets
were cut to 300 x 150 mm size for welding into butt joint configuration with a
single side V-grove with an angle of 60° as shown in Figure 1. A gap of ~ 1mm
was maintained in between two sheets for full penetration of the weld metal and
good bonding between the two sheets. The welding was carried out under a gas
shielding of 100% argon at a flow rate of 15 1/min using 1.2 mm diameter filler

wire direct current electrode positive (DCEP) and adapting a joint set up as
shown in Figure 6. The stick out distance from the electrode tip to base metal
was maintained at 12 mm for getting stable arc. The welding torch was set at an
angle of 20° in the welding direction and also 20° with the edge (joining face) of
the sheets. Welding was carried out transverse to the rolling direction.
Metallographically polished and etched specimens were used to measure
hardness of different zones across the weld. Hardness values were measured in
a Vicker's hardness testing machine using 5kg load as shown in Figure 7. From
the microhardness data, it can be seen that no HAZ (Heat Affected Zone)
softening occurs. Welding was perfect and the weld was free from any defects
like porosities, slag inclusion, under cut or lack of fusion. All tensile samples
failed from either HAZ or Base. Those samples failed at HAZ also break at a
stress level greater than 90% of Base metal. Thus the material is weldable
without any special precaution.

WE CLAIM :
1. A fire resistant steel product characterized by comprising by mass %,
C:0.04 to 0.08%, Mn: 0.8 to 1.3%, Si: 0.2% or less, Mo: 0.1% or less,
Ti: 0.005 to 0.3%, N:0.006% or less, A1: 0.05% or less, P:0.03% or
less, and S:0.02% or less, and comprising a balance quantity
consisting of Fe and unavoidable impurities, and having a yield
strength at 600°C. of 66% or more of the yield strength at room
temperature.
2. A method of producing a fire resistant steel comprising the steps of
heating a steel slab containing, by mass %, C:0.04 to 0.08%, Mn: 0.8
to 1.3%, Si: 0.2% or less, Mo: 0.1% or less, Ti: 0.005 to 0.03%,
N:0.006% or less, Al: 0.05% or less, P: 0.03% or less and S: 0.02%
or less, including a balance quantity consisting of Fe and unavoidable
impurities, to a temperature range of 1150 to 1250 degree C, rolling
and, after finish rolling, cooling at an cooling at an average rate of 10-
20 degree C/s or more in a temperature range of 890.degree. C. to
600.degree.C.
3. A method of producing a fire resistant steel as claimed in claim 2
wherein the step of rolling comprises producing a sheet/strip.
4. The method as claimed in claim 2, wherein the synergic effect of the
composition and treatment of the steel transforms the steel as a fire
resistant steel.

5. The method as claimed in any of the preceding claims, wherein the
produced steel developes an improved weldability characterstics,
precise thermomechanical, high resistivity against softening at 600°C,
and sufficient toughness in heat affected zone (HAZ).
6. The fire-resistant steel product as claimed in claim 1, comprising
reducing need of coating protectivity parameters.
7. The method as claimed in claim 1, wherein the method provides a
high resistivity against softening at 600°C, and sufficient toughness in
heat affected zone (HAZ) to the produced steel.
8. A fire resistant steel product as substantially described and illustrated
herein with reference to the accompanying drawings.
9. A method of producing a fire resistant steel as substantially described
and illustrated herein with reference to the accompanying drawings.

The invention relates a fire resistant steel product characterized by
comprising by mass %, C:0.04 to 0.08%, Mn: 0.8 to 1.3%, Si: 0.2% or less,
Mo: 0.1% or less, Ti: 0.005 to 0.3%, N:0.006% or less, A1: 0.05% or less,
P:0.03% or less, and S:0.02% or less, and comprising a balance quantity
consisting of Fe and unavoidable impurities, and having a yield strength at
600°C. of 66% or more of the yield strength at room temperature.