Claims:We Claim:

1. A high strength thermo-mechanically treated(TMT) steel rebar for concrete reinforcement having steel composition comprising by weight%:
C: 0.15-0.3%, preferably 0.18-0.22%;
Mn: 0.5- 1.0%, preferably 0.70-0.80%;
S: upto 0.04% max, preferably 0.007-0.015%;
P: upto 0.04%max, preferably 0.010-0.020%;
Si: 0.10-0.30%, preferably 0.15-0.20%;
Nb: 0.010 – 0.030%, preferably 0.015-0.020%;
N: upto 0.012% max and balance Fe,
having a microstructure of an outer peripheral tempered Martensite layer and a core of Ferrite and Pearlite structure ensuringhigh YS>600 MPa and UTS/YS>1.06 with minimum 15% elongation and uniform elongation 8% min.

2. A high strength thermo-mechanically treated(TMT) steel rebar as claimed in claim 1 wherein Nb 0.010 – 0.030% as a micro-alloy provide grain refinement and strength to the steel comprising high strength in the range of 630 MPa to 640 MPa with the uniform elongation in the range of 8.5% to 9.5%.

3. A high strength thermo-mechanically treated(TMT) steel rebar as claimed in anyone of claims 1 or 2 having yield strength in the range of 630-640 MPa and UTS 740-750 MPa.

4. A high strength thermo-mechanically treated(TMT) steel rebar as claimed in anyone of claims 1 to 3 wherein the UTS/YS ratio is > 1.06 preferable in the range of 1.12-1.20, %Elongation is >15% preferably in the range of 16-18 % and uniform elongation> 8% preferably in the range of 8.5-9.5%.

5. A high strength thermo-mechanically treated(TMT) steel rebar as claimed in anyone of claims 1 to 4 wherein said rebar diameter comprises 25mm or 28mm or 32 mm.

6. A high strength thermo-mechanically treated(TMT) steel rebar as claimed in anyone of claims 1 to 5 wherein the micro alloy Niobium content is maintained based on desired strength and elongation of the rebar by grain refinement throughout the cross section of the bar.

7. A process for manufacture of high strength thermo-mechanically treated(TMT) steel rebar as claimed in claims 1 to 6 comprising:
(i) providing said niobium microalloyed steel having composition in wt % comprising:
C: 0.15-0.3%, preferably 0.18-0.22%;
Mn: 0.5- 1.0%, preferably 0.70-0.80%;
S: upto 0.04%, max, preferably 0.007-0.015%;
P: upto 0.04%, max, preferably 0.010-0.020%;
Si: 0.10-0.30%, preferably 0.15-0.20%;
Nb: 0.010 – 0.030%, preferably 0.015-0.020%;
N:upto 0.012% max and balance Fe;
and continuously cast into billets;
(ii) subjecting said billets to processing including reheating in furnace, rough rolling and bar rod rolling in stages and water quenching and self-tempering such as to develop a controlled peripheral tempered martensite and a ferrite-pearlite structure in the core;
and
(iii) enabling grain refinement in the presence of said micro alloy Niobium favouring increasing the elongation and further strength such as to achieve desired peripheral tempered Martensite layer thickness and Ferrite and Pearlite core combination and generating ofsaid reinforced steel bar having YS > 600 MPa, % Uniform Elongation>8 % and % elongation> 15 %.

8. A process as claimed in claim 7 comprising:
(i) providing said niobium microalloyed steel produced through primary steel making in ConArc furnace/LD converter followed by ladle refining, having composition in wt % comprising:
C: 0.15-0.3%, preferably 0.18-0.22%;
Mn: 0.5- 1.0%, preferably 0.70-0.80%;
S: upto 0.04%, max, preferably 0.007-0.015%;
P: upto 0.04%, max, preferably 0.010-0.020%;
Si: 0.10-0.30%, preferably 0.15-0.20%;
Nb: 0.010 – 0.030%, preferably 0.015-0.020%;
N:upto 0.012% max and balance Fe;
and continuously cast into billets;
(ii) subjecting said billets to processing including reheating in furnace, rough rolling and bar rod rolling in stages and controlled water quenching and self-tempering to develop a peripheral tempered martensite and a ferrite-pearlite structure in the core;
(iii) controlling said peripheral tempered Martensite layer thickness by normal water flow rate under controlled quenching and
(iv) grain refinement due to said micro alloy Niobium content increasing the elongation and further strength such as to achieve desired peripheral tempered Martensite layer thickness and Ferrite and Pearlite core combination favoring generation of said reinforced steel bar having YS > 600 MPa, % Uniform Elongation >8 % and % elongation > 15 %.

9. A process as claimed in claims 7 or 8 wherein said peripheral tempered Martensite layer is formed without altering the normal/conventional water flow in said water quenching.

10. A process as claimed in anyone of claims 7 to 9 wherein said step of controlled water quenching includes quenching in water boxes with water flow rate in the range of 1200 to 1500 m3/hr depending on mill speed and diameter of rebarand said self-temperingcomprise temperature in the range of 500 to 550°C.

11. A process as claimed in anyone of claims 7 to 10 comprising said bar rod rolling involving the parameters comprising:

Furnace temperature (soaking) in the range of 1050-11500C;
Finishing rolling temperature < 10000C; and
Cooling bed temperature in the range of 500-5500C;
Controlled cooling involving thermo-mechanical treatment under water flow in the range of 1200-1500m3/hr for cooling in water box according to the diameter of the rebar.

Dated this the 18th day of July, 2018
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)
IN/PA-199
, Description:FIELD OF THE INVENTION:

Present invention relates to provide high strength Thermo-mechanically-treated(TMT)rebars having yield strength of 600MPa (Min)for concrete reinforcement application. More particularly, the present invention is directed to provide high strength TMT rebars conforming to IS 1786 Fe600 grade without compromising with the uniform elongation, for concrete reinforcement application under high tensile load condition and method of producing the same comprising selectively the steps of water quenching and self-tempering. This can be produced in the form of straight bars and can be used for reinforcement in concrete and similar application. The high strength steel according to present invention is obtained by normal C-Mn steel composition with the addition of Niobium(Nb) as a micro-alloy to achieve desired microstructure and mechanical properties.

BACKGROUND OF THE INVENTION
In concrete reinforcement, steel rebar is used to impart tensile strength whereas concrete takes the compressive loads. Therefore, steel rebar is a vital material in any construction industry whether they are high rise buildings, bridges or engineering projects. TMT rebar is produced by thermo mechanical treatment which includes, rolling througha sequence of rolling stands comprising roughing, intermediate, and finishing stands which progressively reducethe billet to the final size and shape of the reinforcing bar. In the final rolling pass the bar is ribbed whichenables providing a good joint strength between steel and concrete. The high strength and moderate elongation is achieved in TMT rebar by a process called QST (Quench & Self Tempering). After the hot rolling is finished the hot rebar is “Quenched” by passing through a series of water boxes which convertthe surface layer of the bar to “Martensite” whereas the core remains as “Austenite”. In the second stage of ‘Self Tempering’ when the bar leaves the water box with a temperature gradientthrough its cross section, the temperature of the core remainshigher than that of the surface. This allows heat to flow fromthe core to the surface, resulting in tempering of the surface,giving a microstructure called ‘Tempered Martensite’ which is strong and tough. Upon cooling the core, theTMT rebar converts to (Ferrite + Pearlite) microstructure.
As per IS 1786, there are several grades of high strength deformed steel bars for various applications in construction field. Depending upon the requirement of mechanical properties (mainly yield strength, YS), grades are designated as Fe415, Fe500, Fe550, Fe600, Fe650 and Fe700 (numbers such as 415, 500 etc. denote the minimum YS requirement) grades.
Fe600 is the grade incorporated in IS 1786 to provide higher strength rebarsfor concrete reinforcement application. The required mechanical properties are:
YS: 600MPa (Min)
UTS / YS ratio: = 1.06
%Elongation: 10%
There is no such requirement of uniform elongation.
For the real estate projects which generally have less space for logistics, with use of high strength rebar such as Fe600, reduction in storage space can be achieved. Man hour spent on procurement and construction will also cut down. Fe 600 ensures reduction in reinforcement coefficient and thus a value addition in cost can be achieved. Since, the strength of the steel is high, the weight of steel will cut down by 8 to 10%, transportation and related costs, viz fuel charges will come down. It leads to optimum usage of resources.

Some of the prior patents in the related field tried to improve upon the properties of existing grades of TMT which include a patent by Steel Authority of India Limited with publication number IN211442A1filed in 2003titled “Process for manufacturing micro alloyed thermo-mechanically treated rebar with improved sub-zero impact toughness”. Chemical composition of the invented steel was 0.05 to 0.1 wt% carbon, 0.3to 0.5 wt% manganese, 0.05 to 0.2 wt% silicon, up to 0.035 wt% sulfurand at least 0.008 wt% of micro-alloying elements comprising niobium and phosphorus wherein the level of phosphorus is in the range of 0.08to 0.1% when niobium is not present or up to 0.04 wt% when niobium ispresent in the range of 0.005 to 0.015 wt%,Various metallurgical and mechanical tests were performed to evaluate theproperties of the TMTrebars manufactured by the process of the saidinvention and they were found to possess yield stress of at least 415 MPa,ultimate tensile strength of at least 485 MPa,% elongation of 18 to 20%.

Another patent by Steel Authority of India Limited for high strength TMT rebar having publication numberIN285954A1 in 2010 achieved YS greater than 640 MPa but they restricted their diameter to 20 mm, 22 mm and 25 mm.The alloy design comprising C-Mn chemistryC:0.18 to 0.3 wt % preferably 0.2wt%, Mn:0.8 to 1.5 wt% preferably 1.1wt% Si: 0.05 to 0.10 wt%, S:0.025 to 0.042 wt %, P:0.025 to 0.042 wt% and nomicro-alloying element (such as Nb/V/Ti) is added.
Mechanical properties achieved
YS(MPa) 640 to 740 and preferably 650;
UTS(MPa) 750 to 850 preferably 780;
UTS/YS: 1.12 to 1.2 preferably1.2; and
% El: 18 to 24
In both the above cited patents, uniform elongation was not reported.
The present invention is thus directed to develop a high strength Fe600 grade in C-Mn steel with Niobium (Nb) as micro-alloy. Its processing involves suitable water quenching + self-tempering after thermo-mechanical rolling to achieve desired microstructure along with specified range of YS, UTS and elongation properties.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide high strength reinforcement TMT Rebars with minimum of 600 MPa yield strength and UTS/YS ratio greater than 1.06 with minimum elongation of 15 percentconforming to IS 1786 Fe600 grade for construction applications and a method of its production.
Another object of the present invention is to provide high strength reinforcement TMT Rebar shaving uniform elongation greater than 8 percent in TMT Rebars comprising C-Mn steel composition with Nb as micro-alloying which ensures greater strength without compromising with the uniform elongation.
A further object of the present invention is to providea process of producing the said grade of reinforced steel rebar following selective process steps and parameters to achieve desired microstructure to ensure the required strength and elongation properties as per the applicable standard.
A still further object of the present invention is to providea process of producing the said grade of reinforced steel rebarwithin the same set up such as mill speed and with minor adjustment in water flow so that mill set up is not hampered.
A still further object of the present invention is to providea process of producing the said grade of reinforced steel rebar wherein billets casted to 165mm x 165 mm from steel melting shop are reheated in the reheating furnace and are subjected to different reduction ratios up to final diameter as per customer requirement in the Bar Rod Mill.
A still further object of the present invention is to providea process of producing the said grade of reinforced steel rebar wherein selection of proper chemistry within BIS specification with combination of controlled cooling is the key to achieve the high yield strength along with specified range of UTS/YS & minimum percent elongation and percent uniform elongation.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to provide ahigh strength thermo-mechanically treated(TMT) steel rebar for concrete reinforcement having steel composition comprising by weight%:
C: 0.15-0.3%, preferably 0.18-0.22%;
Mn: 0.5- 1.0%, preferably 0.70-0.80%;
S: upto 0.04% max, preferably 0.007-0.015%;
P: upto 0.04%max, preferably 0.010-0.020%;
Si: 0.10-0.30%, preferably 0.15-0.20%;
Nb: 0.010 – 0.030%, preferably 0.015-0.020%;
N:upto 0.012% max and balance Fe,
having a microstructure of an outer peripheral tempered Martensite layer and a core of Ferrite and Pearlite structure ensuringhigh YS>600 MPa and UTS/YS>1.06 with minimum 15% elongation and uniform elongation 8% min.

A further aspect of the present invention is directed to said high strength thermo-mechanically treated(TMT) steel rebar wherein Nb 0.010 – 0.030% as a micro-alloy provide grain refinement and strength to the steel comprising high strength in the range of 630 MPa. to 640 MPa with the uniform elongation percent in the range of 8.5 to 9.5.

A still further aspect of the present invention is directed to said high strength thermo-mechanically treated(TMT) steel rebar having yield strength in the range of 630-640 MPa and UTS 740-750 MPa.

A still further aspect of the present invention is directed to said high strength thermo-mechanically treated(TMT) steel rebar wherein the UTS/YS ratio is > 1.06 preferable in the range of 1.12-1.20, %El is >15% preferably in the range of 16-18% and uniform elongation> 8% preferably in the range of 8.5-9.5%.

Another aspect of the present invention is directed to said high strength thermo-mechanically treated(TMT) steel rebar wherein said rebar diameter comprises 25mm or 28mm or 32 mm.

Yet another aspect of the present invention is directed to said high strength thermo-mechanically treated(TMT) steel rebar wherein the micro alloy Niobium content is maintained based on desired strength and elongation of the rebar by grain refinement throughout the cross section of the bar.

A further aspect of the present invention is directed to a process for manufacture of high strength thermo-mechanically treated(TMT) steel rebar as described abovecomprising:
(i) providing said niobium microalloyed steel having composition in wt % comprising:
C: 0.15-0.3%, preferably 0.18-0.22%;
Mn: 0.5- 1.0%, preferably 0.70-0.80%;
S: upto 0.04%, max, preferably 0.007-0.015%;
P: upto 0.04%, max, preferably 0.010-0.020%;
Si: 0.10-0.30%, preferably 0.15-0.20%;
Nb: 0.010 – 0.030%, preferably 0.015-0.020%;
N: upto 0.012% max and balance Fe;
and continuously cast into billets;
(ii) subjecting said billets to processing including reheating in furnace, rough rolling and bar rod rolling in stages and water quenching and self-tempering such as to develop a controlled peripheral tempered martensite and a ferrite-pearlite structure in the core;
and
(iii) enabling grain refinement in the presence of said micro alloy Niobium favouring increasing the elongation and further strength such as to achieve desired peripheral tempered Martensite layer thickness and Ferrite and Pearlite core combination and generating ofsaid reinforced steel bar having YS > 600 MPa, % Uniform Elongation>8 and % elongation> 15 %.

A still further aspect of the present invention is directed to said process comprising:
(i) providing said niobium micro alloyed steel produced through primary steel making in ConArc furnace/LD converter followed by ladle refining, having composition in wt % comprising:
C: 0.15-0.3%, preferably 0.18-0.22%;
Mn: 0.5- 1.0%, preferably 0.70-0.80%;
S: upto 0.04%, max, preferably 0.007-0.015%;
P: upto 0.04%, max, preferably 0.010-0.020%;
Si: 0.10-0.30%, preferably 0.15-0.20%;
Nb: 0.010 – 0.030%, preferably 0.015-0.020%;
N:upto 0.012% max and balance Fe;
and continuously cast into billets;
(ii) subjecting said billets to processing including reheating in furnace, rough rolling and bar rod rolling in stages and controlled water quenching and self-tempering to develop a peripheral tempered martensite and a ferrite-pearlite structure in the core;
(iii) controlling said peripheral tempered Martensite layer thickness by normal water flow rate under controlled quenching and
(iv) grain refinement due to said micro alloy Niobium content increasing the elongation and further strength such as to achieve desired peripheral tempered Martensite layer thickness and Ferrite and Pearlite core combination favoring generation of said reinforced steel bar having YS > 600 MPa, % Uniform Elongation>8 % and % elongation> 15 %.

A still further aspect of the present invention is directed to said process wherein said peripheral tempered Martensite layer is formed without altering the normal/conventional water flow in said water quenching.

A still further aspect of the present invention is directed to said process wherein said step of controlled water quenching includes quenching in water boxes with water flow rate in the range of 1200 to 1500 m3/hr depending on mill speed and diameter of rebarand said self-temperingcomprise temperature in the range of 500 to 550°C.

A still further aspect of the present invention is directed to said process comprising said bar rod rolling involving the parameters comprising:

Furnace temperature (soaking) in the range of 1050-11500C;
Finishing rolling temperature < 10000C; and
Cooling bed temperature in the range of 500-5500C;
Controlled cooling involving thermo-mechanical treatment under water flow in the range of 1200-1500m3/hr for cooling in water box according to the diameter of the rebar.

The 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
Figure 1: Process flow chart for Fe600 grade steel rebars according to present invention.

Figure 2: Comparative illustration of Macrostructure of normal TMT Rebar Fe 600 without micro alloy versus the inventive high strength TMT rebras of Fe 600 grade having Niobium as micro alloying element with improved uniform elongation.

Figure 3: Showing the microstructure comparison of core (Ferrite + Pearlite) wherein normal TMT rebars Fe600 without Niobium having coarser core area as compared to Fe600 grade with Niobium obtained according to present invention having grain refined core giving more strength and elongation to the core.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS
The present invention is directed to provide high strength TMT Rebars conforming to IS 1786 Fe600 grade with minimum of 600 MPa yield strength and improved elongation>15%, high uniform elongation (more than 8%) for concrete reinforcement application and method of producing the same. It is already known that as the yield strength increases the uniform elongation decreases. The micro alloy Niobium element is added in normal C-Mn composition to increase the strength of the TMT rebar without compromising with the uniform elongation.

The present invention relates to development of Fe600 in C-Mn steel with the addition of micro alloying element Niobium, suitable water quenching + self-tempering after thermo mechanical rolling. The billets casted to 165mm x 165 mm from steel melting shop are reheated in the reheating furnace and are subjected to different reduction ratios up to final diameter as per customer requirement in the Bar Rod Mill. Selection of proper chemistry within BIS specification with combination of controlled cooling is the key to achieve uniform elongation along with specified range of YS & min. elongation.
This new steel reinforced grade is made through ConArc furnace steel making and Ladle Refining Furnace (LRF) route. It is further cast into billets through continuous casting process. These billets are processed through reheating furnace and Bar Rolling Mill(BRM) followed by controlled Quenching. The hot rolled reinforced bars are inspected manually. Samples are collected from the bars. These samples are tested in laboratory for cleanliness of steel and mechanical properties.
Normally in TMT rebar, the strength is achieved by the peripheral tempered martensitic rim developed by fast water quenching in the water boxes and self-tempering in the cooling bed and ductility is achieved by ferrite-pearlite structure in the core. Martensite rim thickness increases with the increased water flow or by increase in the retention time in the water boxor by decreasing the speed of the bar. More is the martensite rim thickness, more is the yield strength and less is the ductility because of this hard martensite phase. To increase the uniform elongation, martensite layer thickness should be decreased which results in decrease of the yield strength. In this invention, the martensite thickness is decreased as compared to the Fe 600 C–Mn steel without micro-alloying but the micro alloy addition in the form of Niobium helps in grain refinement and thus retards the reduction of uniform elongation. Niobium added as micro alloy element providesbetter uniform elongation without deteriorating the strength of the rebar thus in addition to meeting the standards of FE 600 it also improves the uniform elongation. The carbon content <= 0.25 wt % also ensures excellent weldability.
Accordingly, the C-Mn steel composition with Niobium as micro alloying element of new reinforced bar grade in weight % selected for the present invention is as presented in following table 1:

Table 1:

C Mn S max. P max. Si Nb N max.
0.15-0.30 0.50-1.0 0.04 0.04 0.10-0.30 0.010-0.030 0.012

The balance is Fe and unavoidable impurities.
This combination of chemical composition and water flow gives YS> 600 MPa, % Uniform elongation> 8%and %elongation>15%.
Now, the essential components of the steel grade for high strength TMT rebars according to present invention are described hereinafter with reasoning for selecting the respective concentration rangein weight percent.

Carbon (C) :0.15% or more and less than 0.30% by weight
Carbon is an essential element that provides strength and hardness to steel further maintains ferrite and pearlite structure in the core. Carbon content is useful in achieving better balance between the strength and the elongation property.
The carbon content needs to be more than 0.20wt% sothat increased Carbon content will increase the hardness of tempered martensite and hence high YS will be achieved. Moreover, the hardenability of the steel increases with the increase in carbon content.
The carbon content needs to be less than 0.30 as increased C content will reduce the martensite start temperature and thus less martensite rim thicknesswill be formed for the same amount of water flow ratewhich results in low YS.
The carbon content needs to be below 0.30% following (i) IS 1786 Fe600 standard requirement and also, (ii) Weldability will be poor. Accordingly, C content is kept in the range of 0.18-0.22

Silicon (Si): 0.10% or more and less than 0.30% by weight
Silicon is added as de-oxidizer for purity of Steel and has a strong solid solution strengthening effect hence acts to reinforce steel. It retards softening during tempering and thus aids in the removal of quenching stresses without appreciable decrease in hardness.
Silicon needs to be contained within 0.30 % as excess of Silicon will deteriorate toughness and weldability of Steel. Further if, excessive silicon will deteriorate the surface quality of the rebar by making sticky scale during reheating of billet and difficult to remove during descaling. Accordingly, the Si content is kept 0.10-0.3, preferably 0.15-0.20 % by weight.

Manganese (Mn): 0.5% or more and 1% or less by weight
Mn acts as solid solution strengthener. Further Manganese improves hardenability and access to critical alloying elements forming precipitate and acts pearlite stabilizer. Hence to ensure required strength, Mn content is kept as 0.5 % minimum. Increasing Mn content will increase the cost of the production. Also increased Mn has a high tendency of segregation so its content should not be very high. In this present invention, Mn content is kept as 0.7-0.8%

Phosphorus (P): 0.04% max or less by weight
Phosphorus improves the atmospheric corrosion resistance of the structural steel material, acts as solid solution strengthener and improves the shape-forming workability of the steel, such as rib forming in rebars. But Phosphorous, when added in large amount deteriorates the toughness and rollability. In addition, the segregation of phosphorus at grain boundaries has been found to result in brittleness of the steel bar. For these reasons, the upper limit of phosphorus content in the present steel composition is kept as 0.04%wt by weight maximum. This is also requirement of IS1786 Fe600 grade. P is kept in the range of 0.010-0.020 %

Sulphur (S): 0.04% max or less by weight
As per standard requirement, sulfur content should not exceed 0.04%. It also causes hot shortness to the steel. Accordingly, the S content is restricted to less than 0.04% by weight.S is kept in the range of 0.007-0.015 %

Niobium (Nb): 0.010% or more and 0.030 % or less by weight
Niobium acts as grain refinement in the steel which aids in high yield strength and toughness. Adding a small amount of Nb can significantly increase the yield strength. Upper limit of Nb is restricted as further increase will firstly not affect the increase in strength because of its lower solubility with increased C, typical of rebar and secondly, it increases the cost of production. So Nb in this case kept in the range of 0.015-0.020 %

Nitrogen (N): not more than 0.012% by weight
Nitrogen acts as solid solution strengthener. It also combines with micro alloy such as Nb/V/Ti to form Nitride/ Carbonitride precipitates and thus increase strength. Excess nitrogen causes a large amount of nitride to precipitate, thereby deteriorating ductility and hardenability and induces the phenomenon of room temperature ageing, which will cause the change of the mechanical properties of the steel and the restoration of the yield point elongation of the steel.Therefore, the amount of nitrogen should be no more than 0.012%. This is also requirement of IS1786 normal Fe600 grade.

Details of the process of manufacturing:
Ahigh strength reinforcement steel having the composition described above is prepared by obtaining molten steel through steel making, followed by continuous casting in billets. To produce a high strength reinforcement steel having desired properties, the billet is subjected to reheating, descaling, rough rolling, bar rod rolling and thermal mechanical treatment, details of which will be described hereinafter.

The process route followed to produce the high strength TMT Rebars according to present invention comprising the steps of:
a) Primary Steel Making by ConArc or similar furnace
b) Secondary steel making: Ladle Refining Furnace.
c) Continuous billet of casting into (165mm x 165 mm) sections
d) Re-heating, bar rod rolling, controlled cooling and cut to length with set optimum processing parameters.

Accompanying Figure 1 illustrates the process flow chart for producing Fe600 grade TMT Rebars according to present invention.

The Bar rod rolling parameters specified for processing are presented in the following Table 2:
Table 2:
1 Furnace Temperature (Soaking) 1050 – 1150 °C
2 Finish Rolling Temperature < 1000 °C
3 Controlled cooling Thermo Mechanical Treatment
4 Cooling bed Temperature 500-550 °C

The material specifications of the TMT Rebars produced according to above process of present invention in supply conditions are as presented in the following
Table 3:
1 Chemical Composition As per Table 1
2 Rebar Diameter 8-32 mm
4 Yield Strength 600 MPa min (630-640 MPa)
5 UTS 740-750 MPa
6 UTS/YS >1.06 (1.12-1.20)
7 %Elongation >15% ( 16-18 %)
8 %Uniform Elongation >8% (8.5-9.5 %)
9 Weight per meter As per sample diameter
10 Bend Test OK
11 Rebend Test OK
112 Microstructure Tempered Martensite + (Ferrite + Pearlite)

Trials were conducted with various steel composition with the Fe600 grade with Nbvis-a-vis the Fe 600 grade without Nb (C-Mn route) according to present invention and the comparison of Normal and Invented Chemistry and Mechanical properties are presented in the following Table 4 wherein the samples marked as “invented” in remarks column conform to the specification and properties according to IS 1786 Fe600 grade:
Table 4:
Dia
mm YS
MPa UTS
MPa % U EL %El UTS/YS Chemistry Water Flow Normal Mill Speed
%
32 630 744 9.1 18 1.18 With Nb Microlloying Normal Normal
32 632 746 8.5 16 1.18
32 635 750 9.2 16 1.18
32 635 749 8.5 16 1.18

32 607 718 7.3 16.4 1.18 Without microalloying, (only C-Mn) Normal 10% lower
32 641 751 6.2 16.0 1.17 10% Higher 10% lower

Mechanical Tests and Metallography
The tensile properties (yield strength and ultimate tensile strength) are measured using 600 mm long and gauge length = 5 d (where d is the nominal diameter of the rebar) test specimens on a universal testing machine. Bend and rebend tests are also conducted as per the standard requirement of IS1786. All tests are performed at room temperature.

Metallographic analysis is carried out to rate the cleanliness of steel. Metallographic samples prepared are polished and etched with 5% nital. A simple light optical microscope is used to record the core and outer microstructure of the rebar.

Accompanying Figure 2 shows the comparative illustration of macrostructure of high strength Fe 600 TMT Rebar without Niobium versus the inventive highstrength TMT rebars of Fe600 grade with Nb, where(a)Without Niobium TMT Rebar Fe 600 having Martensite layer thickness comparatively higher, while (b) Invented TMT rebar(Fe600) with Niobium having lower martensite layer thickness.

Accompanying Figure 3 showing the microstructure comparison of core (Ferrite + Pearlite) wherein (a)Without Niobium TMT Fe 600 rebars are having grain coarser as compared to (b) Fe600 grade obtained according to present invention having grain refined core giving more strength.

It is thus possible by way of the present invention to provide high strength Thermo-mechanically-treated(TMT) rebars having yield strength of 600MPa (Min)conforming to IS 1786 Fe600 grade without compromising with the uniform elongation, for concrete reinforcement application under high tensile load condition and method of producing the same comprising providing normal C-Mn steel composition with the addition of Niobium (Nb) as a micro-alloy that is processed through controlled rolling, thermomechanical treatment involving selectively the steps of water quenching and self-temperingto achieve desired microstructure of narrower martensite rim and fine grained ferrite plus pearlitic core to ensure desired strength and elongation properties.