Claims:WE CLAIM:

1. Fire resistant steel composition comprising :
C (0.01 to 0.20%);
Si (0.1 to 0.5%);
Mn (0.5 to 2%);
S (up to 0.05%);
P (up to 0.05%);
Cr (0.1 to 1.0%);
Mo (0.1 to 0.5) ;
Ti (0.01 to 0.05%);
Al (up to 0.004%); and
balance being Fe.
retaining atleast 2/3rd of room temperature yield strength at 6000C.

2. Fire resistant steel composition as claimed in claim 1 including predominantly polygonal ferrite and having a fine ferritic 5 to 10 µm and bainitic grain microstructure.

3. Fire resistant steel composition as claimed in anyone of claims 1 or 2 including MoC and TiC precipitation on the grain boundary for required high temperature strength.

4. Fire resistant steel composition as claimed in anyone of claims 1 to 3 having mechanical properties:

Before Fire Exposure:
• Yield strength in the range of 350 MPa to 450 MPa, ultimate tensile strength in the range of 550 MPa to 650 MPa and elongation in the range of 20 to 30% at room temperature condition;
• Yield strength in the range of 290 MPa to 350 MPa and ultimate tensile strength in the range of 300 MPa to 350 MPa at 600°C for 2 min soaking condition;
• Yield strength in the range of 280 MPa to 350 MPa and ultimate tensile strength in the range of 300 MPa to 350 MPa at 600°C for 3 hours soaking condition.
• Yield strength in the range of 270 MPa to 350 MPa and ultimate tensile strength in the range of 300 MPa to 350 MPa at 600°C for 5 hours soaking condition.

After Fire exposure
c) Yield strength in the range of 350 MPa to 450 MPa, ultimate tensile strength in the range of 550 MPa to 650 MPa and elongation in the range of 30 to 38% at room temperature condition;
d) Yield strength in the range of 290 MPa to 350 MPa and ultimate tensile strength in the range of 300 MPa to 350 MPa at 600°C for 2 min soaking condition.

5. Fire resistant steel composition as claimed in anyone of claims 1 to 4 having load endurance of 2 to 3 times superior to plain carbon steel under standard fire exposure as per international standard [IS: 3614 (Part 2) – 1992, BS: 476 (Part 20 & 22) – 1987 and ASTM E 119].

6.A process for the manufacture of fire resistant steel composition as claimed in anyone of claims 1 to 5 comprising:

i) providing steel composition including :
C (0.01 to 0.20%);
Si (0.1 to 0.5%);
Mn (0.5 to 2%);
S (up to 0.05%);
P (up to 0.05%);
Cr (0.1 to 1.0%);
Mo (0.1 to 0.5) ;
Ti (0.01 to 0.05%);
Al (up to 0.004%); and
balance being Fe.
ii) processing the steel composition to produce steel including steps of

• reheating the steels blooms and soaking in a reheating furnace to 1200-1300°C for 2 to 3 preferably 3 hours.
• rolling the blooms in to steel structural at a rolling speed of about 2 to 3.5 m/s preferably 2.4 m/s ; and
• finishing rolling temperature of about 950 – 1050°C.

7. A process as claimed in claim 6 wherein said steel is produced through BOF-LF-BRC route and said blooms rolled in to steel structural using 16 stands mill at a rolling speed of about 2 to 3.5 m/s preferably 2.4 m/s and said finishing rolling temperature at stand 16 is maintained at about 950°C – 1050°C.

8. A process as claimed in anyone of claims 6 or 7 wherein the entry temperature in stand 1 is kept in the range of 1150 to 1200°C preferably around 1180°C and finishing rolling temperature at stand 16 is kept about 960 to 1000°C preferably about 980°C and said structural sections were rolled to 11 to 12 meter length respectively.

9. A process as claimed in anyone of claims 6 to 8 involving selectively manganese restricted to 1% with carbon 0.10 to 0.20% as hardening enhancing element and said chromium for forming (FeCr)3C in the cementite of pearlite and with Molybdenum for desired elevated temperature yield strength forming carbide (Fe, Mo)3C in the cementite of pearlite resisting softening on prolonged exposure in a fire.

10. A process as claimed in anyone of claims 6 to 9 comprising involving said selective levels of Chromium in small percent forms (FeCr)3C in the cementite of pearlite and providing of bainite as a structure having a high dislocation density effective in enhancing and stabilizing elevated temperature strength.

Dated this the 30th day of January, 2021
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199
, Description:FORM 2
THE PATENT ACT 1970
(39 OF 1970)
&
The Patent Rules, 2003
COMPLETE SPECIFICATION
(See Section 10 and Rule 13)

1 TITLE OF THE INVENTION :
FIRE RESISTANT STEEL AND A PROCESS FOR PRODUCTION THEREOF.



2 APPLICANT (S)

Name : STEEL AUTHORITY OF INDIA LIMITED.

Nationality : Indian.

Address : Research & Development Centre for Iron & Steel,
Doranda, Ranchi, Jharkhand, India. PIN-834002.






3 PREAMBLE TO THE DESCRIPTION

COMPLETE

The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OF THE INVENTION
The present invention relates to a Fire Resistant steel and a process for production thereof. More particularly, the present invention is directed to a Fire resistant steel composition comprising : C (0.01 to 0.20%); Si (0.1 to 0.5%); Mn (0.5 to 2%); S (up to 0.05%); P (up to 0.05%); Cr (0.1 to 1.0%); Mo (0.1 to 0.5) ;Ti (0.01 to 0.05%); Al (up to 0.004%); and balance being Fe., selectively processed to produce structural sections that retain atleast 2/3rd of room temperature yield strength at 6000C. The elevated temperature yield stress depends mainly on the stability of microstructure at higher temperatures. Chromium in small percent forms carbide in the cementite of pearlite. This carbide steel prevents excessive dropping of yield stress at 600°C. Mo and Ti have added advantages due to their precipitation hardening potential and the loss of strength at higher temperature is minimal. Fire resistant steel have finer grain size of ferrite and higher content of bainite. The bainite is a structure having a high dislocation density, is effective in enhancing and stabilizing elevated temperature strength. It has been observed that the bainite and fine grains are mainly beneficial to the elevated temperature, which in turn achieved by micro alloying of Mo and Ti.

BACKGROUND OF THE INVENTION

In view of the growing requirement for steel structures in the country, demand for fire resistant steel(FRS) has increased. At present, fire protection of such structures is being met through other means, like fire protection coating, fire resistant packing, etc. which are very costly options and require regular maintenance. The problem with unprotected carbon–manganese mild steel is its poor strength at temperatures above ~ 350°C, which can make a structure unsafe after a major fire. In case of short duration fire, it is rather difficult to assess the damage to structures caused by fire and may call for demolition/renovation of the structures. At high temperatures, both the flexural strength and tensile strength of steel decreases, as does the modulus of elasticity. Most of the research efforts relating to construction in fire sensitive areas were directed towards development of steels that can retain adequate strength after prolonged exposure in fire. The building codes of some specifications require the steel to have a minimum of two-thirds of room temperature yield strength at 600°C.

Recent awareness about fire resistant building has led to the demand of fire resistance structural in market. At present, fire protection of such structure is being met through other means like coatings, which is very costly and requires regular maintenance. Recently many studies focused on processing, structure and different combinations of alloying elements for developing the fire resistant steels.

.
The for manufacturing Fire resistant structural steel according to present invention with enhanced high temperature property at 600 °C for prolonged exposure is very different from the existing prior art . Most of the prior art on the subject are related to the novelty in:
(a) High temperature strength property was achieved with micro-alloying of Mo, V, Nb and Ti combination in low carbon steel through ingot route. (EP0470055A3, EP 2 065 481 A1, US20090087335, Indian Patent No. : 265742)
(b) High temperature strength property was achieved with micro-alloying of Nb and Ti combination in low carbon steel(WO2006118339A1, JPH10176237A).
(c) High temperature strength property was achieved with micro-alloying of Mo and Nb combination in low carbon steel(CA1320110C).
(d) High temperature strength property was achieved with micro-alloying of Mo, Nb and V combination in low carbon steel(JPH10237583A).
(e) High temperature strength property was achieved with micro-alloying of Mo, Nb, V, Ti, B, Cu and Ni combination in low carbon steel(JP3550721B2).
(f) High temperature strength property was achieved by having Nb>=0.08+7.75C-1.98Ti+6.64N and also by controlled cooling process(JP3559455B2).
(g) High temperature strength property was achieved with micro-alloying of Mo and Nb combination in low carbon steel and achieved in hot rolled strips(CA1320110C).
(h) High temperature strength property was achieved with micro-alloying of Mo and Ni combination in low carbon steel and also Ca-Si treatment(JPH10237583A).
(i) High temperature strength property was achieved with micro-alloying vanadium:nitrogen ratio of at least 4.5 in low carbon steel(GB2388845A).
(j) High temperature strength property was achieved with micro-alloying of Mo, Nb, V, Ti, B, Cu and Ni combination in low carbon steel(JP5079793B2).
(k) High temperature strength property was achieved in low carbon steel by ageing treatment(JP2760713B2).
(l) High temperature strength property was achieved with micro-alloying of Cu and heat treating steel to get copper precipitate (Indian Patent No. 255307).
(m) High temperature strength property was achieved with micro-alloying of Cr and V in low carbon steel and also with accelerated cooling the rolled steel(US8323561).
(n) High temperature strength property was achieved with micro-alloying of Mo, W, Ta, Ti, V, Ni, Co and B combination in low carbon steel and also need to satisfies C-0.06×(Mo+0.5W)?0.01 and Mn+0.69×log(Mo+0.5 W+0.01)?0.60 composition(US20010035235).
(o) High temperature strength property was achieved with micro-alloying of Nb, Ti, N, and B combination in low carbon steel and also need to needs to have Ti/N of 2 to 8, the value C-Nb/7.74 of 0.02%(CN101379209A0).
(p) High temperature strength property was achieved with micro-alloying of Cu, Mo and Nb combination in low carbon steel(CN101397627A).

In the present work, a new fire resistance structural steel was developed having atleast 2/3rd of its specified room temperature yield strength when tested at 600°C. Also the performance evaluation of the fire resistance steel was carried out under simulated fire condition as per international standards at Fire Resistant Laboratory.

The present invention provides viable prospect of being utilized/commercialized manufacturing of Fire resistant steel structural with enhanced high temperature property at 600 °C for prolonged exposure.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide a Fire Resistant steel structural with enhanced high temperature property at 600 °C after prolonged exposure and a process for producing the same.

A further object of the present invention is directed to provide Fire Resistant steel having composition comprising selective concentration of carbon, manganese, chromium, titanium and molybdenum selectively processed to ensure a minimum of two-thirds of room temperature yield strength at 600°C.

A still further object of the present invention is directed to provide saidFire Resistant steel with stability of microstructure at higher temperatures.

A still further object of the present invention is directed to provide said Fire Resistant steel have finer grain size of ferrite and higher content of bainite wherein the bainite structure having a high dislocation density, effective in enhancing and stabilizing elevated temperature strength achieved by micro alloying of Mo and Ti.

A still further object of the present invention is directed to provide said Fire Resistant steel having manganese restricted to 1% with carbon 0.10 to 0.20% as hardening enhancing element and chromium for forming (FeCr)3C in the cementite of pearlite and with Molybdenum for desired elevated temperature yield strength forming carbide (Fe, Mo)3C in the cementite of pearlite resisting softening on prolonged exposure in a fire.

SUMMARY OF THE INVENTION

The basic aspect of the present Fire resistant steel composition comprising:
C (0.01 to 0.20%);
Si (0.1 to 0.5%);
Mn (0.5 to 2%);
S (up to 0.05%);
P (up to 0.05%);
Cr (0.1 to 1.0%);
Mo (0.1 to 0.5);
Ti (0.01 to 0.05%);
Al (up to 0.004%); and
balance being Fe.
retaining atleast 2/3rd of room temperature yield strength at 6000C.

A further aspect of the present invention is directed to Fire resistant steel composition including predominantly polygonal ferrite and having a fine ferritic 5 to 10 µm and bainitic grain microstructure.

A still further aspect of the present invention is directed to Fire resistant steel composition including MoC and TiC precipitation on the grain boundary for required high temperature strength.

A still further aspect of the present invention is directed to Fire resistant steel composition having mechanical properties:

Before Fire Exposure:
• Yield strength in the range of 350 MPa to 450 MPa, ultimate tensile strength in the range of 550 MPa to 650 MPa and elongation in the range of 20 to 30% at room temperature condition;
• Yield strength in the range of 290 MPa to 350 MPa and ultimate tensile strength in the range of 300 MPa to 350 MPa at 600°C for 2 min soaking condition;
• Yield strength in the range of 280 MPa to 350 MPa and ultimate tensile strength in the range of 300 MPa to 350 MPa at 600°C for 3 hours soaking condition.
• Yield strength in the range of 270 MPa to 350 MPa and ultimate tensile strength in the range of 300 MPa to 350 MPa at 600°C for 5 hours soaking condition.

After Fire exposure
a) Yield strength in the range of 350 MPa to 450 MPa, ultimate tensile strength in the range of 550 MPa to 650 MPa and elongation in the range of 30 to 38% at room temperature condition;
b) Yield strength in the range of 290 MPa to 350 MPa and ultimate tensile strength in the range of 300 MPa to 350 MPa at 600°C for 2 min soaking condition.

Another aspect of the present invention is directed to Fire resistant steel composition having load endurance of 2 to 3 times superior to plain carbon steel under standard fire exposure as per international standard [IS: 3614 (Part 2) – 1992, BS: 476 (Part 20 & 22) – 1987 and ASTM E 119].

A still further aspect of the present invention is directed to a process for the manufacture of fire resistant steel composition comprising:

i) providing steel composition including :
a) C (0.01 to 0.20%);
b) Si (0.1 to 0.5%);
c) Mn (0.5 to 2%);
d) S (up to 0.05%);
e) P (up to 0.05%);
f) Cr (0.1 to 1.0%);
g) Mo (0.1 to 0.5) ;
h) Ti (0.01 to 0.05%);
i) Al (up to 0.004%); and
j) balance being Fe.
ii) processing the steel composition to produce steel including steps of

• reheating the steels blooms and soaking in a reheating furnace to 1200-1300°C for 2 to 3 preferably 3 hours.
• rolling the blooms in to steel structural at a rolling speed of about 2 to 3.5 m/s preferably 2.4 m/s ; and
• finishing rolling temperature of about 950 – 1050°C.

Yet another aspect of the present invention is directed to a process wherein said steel is produced through BOF-LF-BRC route and said blooms rolled in to steel structural using 16 stands mill at a rolling speed of about 2 to 3.5 m/spreferably 2.4 m/s and said finishing rolling temperature at stand 16 is maintained at .about 950 – 1050°C.

A still further aspect of the present invention is directed to a process wherein the entry temperature in stand 1 is kept in the range of 1150 to 1200°C preferably around 1180°C and finishing rolling temperature at stand 16 is kept about 960 to 1000°C preferably about 980°C and said structural sections were rolled to 11 to 12 meter length respectively.

Another aspect of the present invention is directed to a process involving selectively manganese restricted to 1% with carbon 0.10 to 0.20% as hardening enhancing element and said chromium for forming (FeCr)3C in the cementite of pearlite and with Molybdenum for desired elevated temperature yield strength forming carbide (Fe, Mo)3C in the cementite of pearlite resisting softening on prolonged exposure in a fire.

Yet another aspect of the present invention is directed to a process comprising involving said selective levels of Chromium in small percent forms (FeCr)3C in the cementite of pearlite and providing of bainite as a structure having a high dislocation density effective in enhancing and stabilizing elevated temperature strength.
Above and other objects and advantages of the present invention are described hereunder in greater details with reference to following accompanying non limiting illustrative drawings.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.1: Photograph of Fire resistant steel rolled to different sections of 11 to 12 meter length.
Fig. 2: The complete furnace assembly consists of four walls and the open roof- top for specimen mounting as well as for simulating if necessary, the loading condition of the specimen under evaluation.
Fig. 3: Temperature profile as per IS: 3614 (Part 2) – 1992 and BS: 476 (Part 20 & 22) – 1987 / ASTM E119.
Fig. 4: Experimental Set up.
Fig. 5: CCT curve for fire resistant steel for (a) 5 and (b) 1 degree/ sec cooling rate.
Fig. 6: Microstructure of (a) plain carbon steel showing ferrite-pearlite microstructure microsture, (b) Fire resistant steel showing Ferrite and asicular ferrite microstructure.
Fig. 7: TEM Micrograph of Fire resistant steel showing lath boundaries confirming formation of bainitic structure and TiC precipitates.
Fig. 8: High temperature tensile behaviour of plain carbon steel and fire resistant steel at 600 oC.
Fig. 9: Tensile behaviour of plain carbon steel and fire resistant steel at room temperature and prolonged exposure at 600 oC.
Fig. 10: Load Endurance behavior of Fire resistant steel and Plain carbon steel under standard fire exposure as per international standard.
Fig. 11: Optical micrograph of (a) before and (b) after fire exposed Plain carbon steel showing grain coarsening due to exposure to fire.
Fig. 12: Optical micrograph of (a) before and (b) after fire exposed fire resistant steel showing coarsening of grains due to exposure to fire.
Fig. 13: TEM micrograph of after fire exposed fire resistant steel showing MoC and TiC precipitation.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS
The present invention is directed to provide a new fire resistance structural steel having atleast 2/3rd of its specified room temperature yield strength when tested at 600°C. Following description explains the salient features of the Fire Resistant steel according to present invention:

The novelty of present invention lies in formulation of a comprehensive methodology for manufacturing Fire resistance steel with superior high temperature strength for prolonged exposure at 600oC

Alloy composition:

Fire resistance steel composition of C (0.01 to 0.20%), Si (0.1 to 0.5%), Mn (0.5 to 2%), S (up to 0.05%), P (up to 0.05%), Cr (0.1 to 1.0%), Mo (0.1 to 0.5) Ti (0.01 to 0.05%), Al (up to 0.004% and balance being Fe.

Processing: Manufacturing method of claimed Fire resistance steel, comprising following steps:
• The steels are produced through BOF-LF-BRC route.
• The steels blooms are reheated and soaked in a reheating furnace to 1200-1300°C for 3 hours.
• Blooms were rolled in to steel structural using 16 stands mill at a rolling speed of about 2.4 m/s.
• Finishing rolling temperature at stand 16 is about 950 – 1050°C

Microstructure:
• The claimed fire resistance steel compositions has a fine ferritic (5 to 10 µm) and bainitic grain microstructure
• The claimed fire resistance steel compositions has fine MoC and TiC precipitation on the grain boundary improves the high temperature strength.

Mechanical properties:

Before Fire Exposure
• The claimed fire resistance steel compositions has a yield strength in the range of 350 MPa to 450 MPa, ultimate tensile strength in the range of 550 MPa to 650 MPa and elongation in the range of 20 to 30% at room temperature condition.
• The claimed fire resistance steel compositions has a yield strength in the range of 290 MPa to 350 MPa and ultimate tensile strength in the range of 300 MPa to 350 MPa at 600°C for 2 min soaking condition.
• The claimed fire resistance steel compositions has a yield strength in the range of 280 MPa to 350 MPa and ultimate tensile strength in the range of 300 MPa to 350 MPa at 600°C for 3 hours soaking condition.
• The claimed fire resistance steel compositions has a yield strength in the range of 270 MPa to 350 MPa and ultimate tensile strength in the range of 300 MPa to 350 MPa at 600°C for 5 hours soaking condition.

After Fire exposure
• The claimed fire exposed fire resistance steel compositions has a yield strength in the range of 350 MPa to 450 MPa, ultimate tensile strength in the range of 550 MPa to 650 MPa and elongation in the range of 30 to 38% at room temperature condition.
• The claimed fire exposed fire resistance steel compositions has a yield strength in the range of 290 MPa to 350 MPa and ultimate tensile strength in the range of 300 MPa to 350 MPa at 600°C for 2 min soaking condition.

Fire performance:
• Load endurance of claimed fire resistance steel compositions has 2 to 3 times superior to plain carbon steel under standard fire exposure as per international standard.
• Critical failure temperature of claimed fire resistance steel compositions was about 600 – 700 °C

Also the performance evaluation of the fire resistance steel was carried out under simulated fire condition as per international standards at Fire Resistant Laboratory.

One heat made with unique chemistry to meet the requirements of FR-Fe490 grade fire resistant through BOF-LF-BRC route. The heat was cast into 12 blooms of 350 x 240 BRC cast of 9 meter length weighing about 105 T. Blooms were rolled in to structurals of different sections in Medium structural mill of integrated steel plant as shown in accompanying Fig. 1. 4nos. of blooms were rolled in each section. The Blooms were soaked to 1240-1260°C for 3 hours and rolled in 16 stands of MSM at a rolling speed of about 2.4 m/s. The entry temperature in stand 1 was kept around 1180°C and finishing rolling temperature at stand 16 is about 980°C. Structural sections were were rolled to 11 to 12 meter length respectively.

Metallographic specimens were sectioned in longitudinal transverse direction from flange portion of the structure. The precipitation behavior was studied using Transmission electron microscopy (TEM). Zeol. The room tensile test specimens were machined and prepared to dimensions with 50 mm gauge length as per ASTM A370-07a.

The high temperature tensile testing of the steels was performed using Gleeble 3500 C thermo-mechanical simulator at 600°C. The samples were electrical resistance heated at 5 °C/s and soaked for 2 mins at the preset temperature before hot uniaxial tensile test. A thermocouple was spot welded onto the surface of sample in order to measure and control the temperature. The test was carried out by subjecting sample to tension test at a crosshead speed of 2 mm/min.

Both FRS and plain carbon (PC) steels were tested extensively under load (with three side fire exposure) and in absence of load (with four side fire exposure) under fire conditions as per IS: 3614 (Part 2) – 1992, BS: 476 (Part 20 & 22) – 1987 and ASTM E119 in a floor furnace as shown in accompanying Fig. 2.

Beams (FRS & PC) is subjected to a single point loading in reaction frame during fire exposure as per standard heating conditions as shown in Fig.3. A constant load of 10.5 T was maintained during the entire fire exposure period i.e. upto failure. Deflection was measured at the centre of the beam using LVDT. The rate of deflection for both FRS and PC beams was recorded with respect to time as shown in accompanying Fig.4. The time at which the rate of deflection crosses the serviceability limit (= l2/9000d i.e. 4.14 mm/min) was considered the failure time/ fire resistance rating of the beam. Based on this failure criterion the temperature corresponding to failure time is taken as the critical temperature for failure for four side fire exposure conditions.

In a structural steel, the composition will be determined by the requirement of strength, toughness and weldability. The chemical composition of the developed fire resistant steel is given in followingTable 1.

Table 1: Composition of Fire resistant steel in wt%
Grade C Mn Si S P Cr Mo Ti Al CE*
Plain carbon 0.18 1.10 0.38 0.025 0.024 --- --- --- --- 0.36
Fire resistance steel 0.15 1.18 0.38 0.025 0.024 0.46 0.24 0.026 0.002 0.49

Lower carbon in hot-rolled steel improves notch toughness and weldability. Manganese was restricted to 1% in fire resistant structural with carbon 0.10–0.20%, where manganese is a potential hardenability enhancing element. Chromium in small percent forms (FeCr)3C in the cementite of pearlite. It is a ferrite stabilizer and increases the hardenability. However, its potential as a solid solution strengthening element is small due to its high affinity to carbon. Molybdenum increases elevated temperature yield strength even when present in small percent (~ 0×25%) in solid solution in low carbon steel. It also forms carbide (Fe, Mo)3C in the cementite of pearlite resisting softening on prolonged exposure in a fire. The CCT curve was drawn using J-MAT PRO software for the above composition for different cooling rate to identify the different phases during processing. From accompanying Fig. 5 it is clear that within the cooling rate of 1 to 5 per second, the steel tends to form > 50% of bainitic structure.

Typical optical microstructures of steels in as-rolled condition are shown in accompanying Fig.6. The steel plain carbon steel showed ferrite-pearlite microstructure, where as Fire resistant steel predominantly polygonal ferrite. At higher magnification and in TEM, two steels also showed upper bainiteas in accompanying Fig. 7. The grain size of ferrite in plain carbon steel is about 10 – 15 µm. In case of fire resistant steel, it showed bi-modular ferritic grains ranging from 5 to 10 µm. Finer grains in fire resistant steel are mainly due to addition of Mo. It is well known that the recrystallization and austenite transformation of low-carbon steel is strongly influenced by the addition of small amount of Ti and Mo. The increase in recrystallization temperature and the decrease in austenite transformation temperature during hot rolling caused by the effect of solute drag Mo and Ti in solid solution and the pinning effect of fine precipitates such as TiC and MoC. Thus, the Mo and Ti is beneficial to the bainite transformation and the grain refinement.

The mechanical properties of both plain carbon fire resistant steel and steels at room temperature (RT) shown in following Table 2. FRS steel shows higher YS, UTS and elongation than that of PC steel. The higher strength is mainly due to formation of bainitic structure and also the carbide precipitation as seen TEM micrograph. The bainite is a structure having a high dislocation density, is effective in enhancing and stabilizing strength.

Table 2: Room temperature tensile test
Steel Yield strength in MPa Ultimate tensile strength in MPa YS/UTS ratio % of Elongation
Plain Carbon 354 500 0.69 30
Fire Resistant steel 495 610 0.81 21

Accompanying Fig. 8 shows the mechanical properties of the two steels at 600°C. From figure it is clear that fire resistant steel showed better tensile behavior than Plain carbon steel. Plain carbon steel the yield strength drastically falls down to 187 MPa. But the yield strength of the developed fire resistant steel retains the two-thirds of their room temperature yield strength at 600°C as in following Table 3.

Table 3: High Temperature Tensile test
Steel Condition Yield strength in MPa Ultimate tensile strength in MPa
Plain Carbon 600 C; 2 min. soaking 187 226
Fire Resistant 600 C; 2 min. soaking 297 333

Plain Carbon 600 C; 3 hrs soaking 169 197
Fire Resistant 600 C; 3 hrs soaking 285 334

Plain Carbon 600 C; 5 hrs soaking 104 185
Fire Resistant 600 C; 5 hrs soaking 275 308

In order to study the stability of the material under fire condition, the high temperature tensile test was carried out for different hours of soaking at 600°C. It is clear from accompanying Fig. 9 and Table 3 above that yield strength of the fire resistant steel does not change (~300 Mpa) even after 5 hours of exposure to 600°C, which is greater than that of the two-third of yield strength of the material at room temperature. But in case of plain carbon steel, yield strength drastically decreases from 180 to 100 MPa, which is less than that of two-third of yield strength of the material at room temperature.

The elevated temperature yield stress depends mainly on the stability of microstructure at higher temperatures. Chromium in small percent forms (FeCr)3C in the cementite of pearlite. A higher amount of chromium in steel prevents excessive dropping of yield stress at 600°C. Mo and Ti have added advantages due to their precipitation hardening potential and the loss of strength at higher temperature is minimal. Fire resistant steel have finer grain size of ferrite and higher content of bainite. The bainite is a structure having a high dislocation density, is effective in enhancing and stabilizing elevated temperature strength. So we can conclude that the bainite and fine grains are mainly beneficial to the elevated temperature, which in turn achieved by micro alloying of Mo and Ti.

Based on these studies, following were the salient findings from fire performance evaluation:
• Yield strength of plain carbon steel decreased significantly lower than specified level both at 600°C and after fire exposure whereas for fire resistant steel, it remained significantly higher than specified level and no deterioration observed after fire exposure.
• Rate of deflection for both beams (PC and FRS) were observed with respect to time with increase in temperature.
• Load endurance of FRS was around two times superior to PC steel under standard fire exposure as per international standard mentioned earlier and as shown in accompanying Fig. 10.
• Average Temperature for FRS at the time of failure was about 600°C whereas it was only 400°C for PC steel.

Accompanying Fig. 11 and Fig. 12 shows the microstructures of before and after fire exposure of FRS and PC steels. From microstructure it is clear that both steels showed grain coarsening. FRS steel showed a coarsen ferrite and distinct bainitic structure. TEM micrograph as in accompanying Fig. 13 clearly shows the formation of MoC precipitates along the grain boundary and TiC in the grains which is responsible for improving the high temperature property of the claimed fire resistance steel.

The room temperature mechanical properties of three side and four side fire exposed steels for different condition and location are shown in following Table 4. Tensile testing of post fire exposed FRS and PC steels revealed a drop of about 50 MPa in yield strength of PC steel owing to grain coarsening, where as no such drop in yield strength was observed for FRS underlining its superior high temperature performance under fire (Table 4). This also clearly brought out that structures made using fire resistant steel will have no adverse effect on life even after exposure to fire.
Table 4: Room temperature tensile properties of PC and FRS steels at different condition of fire exposure
Steel Yield strength in MPa Ultimate tensile strength in MPa % of Elongation
Plain Carbon in loading condition
Top End 308 500 35.6
Top Center 315 508 35.7
Bottom End 306 508 36.0
Bottom Center 306 504 33.0
Plain Carbon in unloading condition
Top 315 504 33.0
Bottom 323 502 36.6
Fire Resistant steel in loading condition
Top End 411 563 30.4
Top Center 414 557 32.6
Bottom End 413 550 35.0
Bottom Center 414 552 35.0
Fire Resistant steel in unloading condition
Top 410 556 32.0
Bottom 423 554 31.6

Following Table 5 shows the mechanical properties of three side and four side fire exposed steels for different condition and location are shown in at 600°C. From Table 5, it is clear that FRS steels showed no drop in yield strength, where as there is a significant drop in yield strength for PC steel. The yield strength of the FRS still retains the two-thirds of their room temperature yield strength at 600°C (> 233 MPa, as per fire resistant application) even after prolonged fire exposure. The elevated temperature yield stress depends mainly on the stability of microstructure at higher temperatures.

Table 5: High temperature tensile properties of post fire exposed PC and FRS steels at different conditions
Steel Yield strength in MPa Ultimate tensile strength in MPa % of Elongation
Plain Carbon in loading condition
Top End 128 184 52.2
Top Center 119 176 51.2
Bottom End 159 211 64.7
Bottom Center 162 213 67.7
Plain Carbon in unloading condition
Top 168 210 61.8
Bottom 146 216 58.9
Fire Resistant steel in loading condition
Top End 298 330 44.1
Top Center 258 298 48.1
Bottom End 289 318 47.8
Bottom Center 279 319 48.9
Fire Resistant steel in unloading condition
Top 294 325 45.4
Bottom 288 321 46.4

Fire exposed FRS steel shows distinct bainitic grains and these bainite is effective in enhancing and stabilizing elevated temperature strength. Thus, it can be concluded that bainatic microstructure helps FRS steel to perform better under fire as well as under extended exposure at 600°C and steel structures made by fire resistant steel (FRS) will remain in its original condition even after fire hazard.