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 0.20-0.23%;
Mn: 0.5- 1.0%, preferably 0.70-0.80%;
S: up to 0.04% max, preferably 0.007-0.020%;
P: up to 0.04% max, preferably 0.010-0.020%;
Si: 0.10-0.30%, preferably 0.20-0.25%;
Cr: >=0.3%, Preferably 0.35-0.43%;
Cu: >=0.2%, Preferably 0.22-0.27%
CRE (Cr+Cu+Ni+Mo+P): >0.4%, preferable 0.55-0.70% including essentially said selective levels of Cr and Cu with reduced levels of the said P with Ni & Mo being balance CRE content in trace amounts of <0.005%;
N: up to 0.012% max and balance Fe, having a microstructure of an outer rim of peripheral tempered Martensite phase and a core of Ferrite and Pearlite phase ensuring high YS>600 MPa and UTS/YS>1.06 with minimum 10% elongation and uniform elongation 5% min.

2. The high strength thermo-mechanically treated(TMT) steel rebar as claimed in claim 1 comprising high strength in the range of YS 640 to 670 MPa, UTS 750 to 780 MPa, total elongation 12-16 % and uniform elongation 6 to 10 %.

3. The high strength thermo-mechanically treated(TMT) steel rebar as claimed in anyone of claims 1 to 2 having an average grain size of 15-60 micron and comprising %volume of said peripheral tempered martensite phase is 28-35, %volume of said core ferrite-pearlite phase is 50-60 and %volume of transition Bainite phase being 12-18.

4. A high strength thermo-mechanically treated(TMT) steel rebar as claimed in anyone of claims 1 to 3 wherein said rebar diameter is upto 40 mm and thickness of the said outer rim of peripheral tempered Martensite phase is in the range of 4 mm to 6 mm.

5. A high strength thermo-mechanically treated(TMT) steel rebar as claimed in anyone of claims 1 to 4 wherein the chemical element Cr and Cu are included selectively as corrosion resistant element based on required strength and elongation and corrosion resistant property of the steel bar.

6. A process for manufacture of high strength thermo-mechanically treated(TMT) steel rebar as claimed in claims 1 to 5 comprising:
(i) providing said steel having composition in wt % comprising:
C : 0.15-0.3%, preferably 0.20-0.23%;
Mn: 0.5- 1.0%, preferably 0.70-0.80%;
S: up to 0.04% max, preferably 0.007-0.020%;
P: up to 0.04% max, preferably 0.010-0.020%;
Si: 0.10-0.30%, preferably 0.20-0.25%;
Cr: >=0.3%, Preferably 0.35-0.43%;
Cu: >=0.2%, Preferably 0.22-0.27%
CRE (Cr+Cu+Ni+Mo+P): >0.4%, preferable 0.55-0.70% including essentially said selective levels of Cr and Cu with reduced levels of the said P with Ni & Mo being balance CRE content in trace amounts of <0.005%;
N: up to 0.012% max and balance Fe,, and continuously cast into billets;
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 said microstructure of an outer rim of peripheral tempered Martensite phase and a core of Ferrite and Pearlite phase ensuring high YS>600 MPa and UTS/YS>1.06 with minimum 10% elongation and uniform elongation 5% min. ;

7. The process as claimed in claim 6 comprising:
(i) including said Cu, Cr as corrosion resistant elements in producing the steel through primary steel making in ConArc furnace/LD converter followed by ladle heating, having composition in wt % comprising:
C : 0.15-0.3%, preferably 0.20-0.23%;
Mn: 0.5- 1.0%, preferably 0.70-0.80%;
S: up to 0.04% max, preferably 0.007-0.020%;
P: up to 0.04% max, preferably 0.010-0.020%;
Si: 0.10-0.30%, preferably 0.20-0.25%;
Cr: >=0.3%, Preferably 0.35-0.43%;
Cu: >=0.2%, Preferably 0.22-0.27%
CRE (Cr+Cu+Ni+Mo+P): >0.4%, preferable 0.55-0.70% including essentially said selective levels of Cr and Cu with reduced levels of the said P with Ni & Mo being balance CRE content in trace amounts of <0.005%;
N: up to 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 following normal water flow rate under controlled quenching.

8. The process as claimed in claims 6 or 7 wherein said peripheral tempered Martensite layer is formed without altering the normal/conventional water flow in said water quenching.

9. The process as claimed in anyone of claims 6 to 8 wherein said step of controlled water quenching includes quenching in water boxes with water flow rate in the range of 2400 to 2600 m3/Hr depending on mill speed and said self-tempering comprise temperature in the range of 500 to 550°C.

10. The process as claimed in anyone of claims 6 to 9 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 2400-2600 m3/hr for cooling in water box according to the diameter of the rebar.

Dated this the 25th day of March, 2021
Anjan Sen
Of Anjan Sen & Associates
(Applicants 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 :
HIGH STRENGTH CORROSION RESISTANT THERMO-MECHANICALLY-TREATED(TMT) REBARS HAVING YIELD STRENGTH OF 600MPa(Min) AND A PROCESS FOR ITS PRODUCTION.



2 APPLICANT (S)

Name : JSW STEEL LIMITED.

Nationality : An Indian Company.

Address : Dolvi Works, Geetapuram, Dolvi, Taluka Pen, Dist. Raigad, Maharashtra-402107,India; Having the Registered Office at
JSW CENTRE,BANDRA KURLA COMPLEX,BANDRA(EAST), MUMBAI-400051, STATE OF MAHARASHTRA,INDIA.




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:

Present invention relates to provide high-strength and corrosion-resistant Thermo-mechanically-treated(TMT) rebar having yield strength of 600MPa (Min)for RCC application. More particularly, the present invention is directed to provide high-strength and corrosion-resistant TMT rebar of diameter 40mm 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 Chromium(Cr) and Copper(Cu) as a micro-alloy to achieve desired microstructure, mechanical properties and corrosion-resistant characteristics.

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 constructions whether they are high rise buildings, bridges or engineering projects. TMT rebar is naturally attacked by oxidizing agents like water & chlorides which corrode the steel and lead to the immature failure of the structure. The inherent corrosion-resistant characteristics which is acquired by the formation of oxide layers of phosphorus, copper & chromium on surface, provides the protection to steel surface from such corrosive attack. However, with addition of Phosphorous (P), brittle phase Fe3P is formed which actually brittle in nature.

The TMT rebar is produced by thermo mechanical treatment which includes, rolling through a sequence of rolling stands comprising roughing, intermediate, and finishing stands which progressively reduce the billet to the final size and shape of the reinforcing bar. In the final rolling pass the bar is ribbed which leads to a good bonding 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 remains higher than that of the surface. This allows heat to flow from the core to the surface, resulting in tempering of the surface, giving a microstructure called ‘Tempered Martensite’ which is strong and tough as well. Upon cooling the core, theTMT rebar converts to (Ferrite + Pearlite) microstructure. The allied properties- “Corrosion resistant” is achieved by addition of phosphorus, chromium and copper in the chemistry. These elements make a passive oxide layer on the steel surface which protects it from corrosion.

As per IS 1786:2008, 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 1786:2008 to provide higher strength rebar for concrete reinforcement application. The required mechanical properties as per this standard are:
YS: 600MPa (Min)
UTS: 660MPa (Min)
UTS / YS ratio: = 1.06
Elongation: 10%
There is no such requirement of uniform elongation i.e elongation at maximum force in IS 1786:2008 Fe600 grade.
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.

CN103849820A- “High-strength corrosion-resistant rebar containing Cr and rolling process thereof” by WUHAN IRON & STEEL GROUP CORP- The rebar comprises the following components by mass percent: 0.05-0.12% of C, 1-3% of Cr, 0.1-0.3% of Cu, 0.1-0.15% of Ni, 0.5-1% of M, no more than 0.015% of N, no more than 0.03% of P, no more than 0.035% of S, 0.2-0.5% of Si, and the balance of Fe. The yield strength l of the produced rebar is greater than or equal to 400MPa, the tensile strength is greater than or equal to 540MPa, the elongation is greater than or equal to 16%, and the corrosion resistance is 2-3 times of the common HR400 rebar. Cooling bed temperature is 850-950.
In this particular invention with such high Mn and alloying addition (Cr and Cu) the mechanical properties is complying to IS 1786 Fe415 only because the material is produced through thermo- mechanical rolling but not quenching as cooling bed temperature is very high. Although it is not mentioned, but the expected microstructure will be ferrite + Pearlite only across the whole diameter. Because of the only ferrite- pearlite microstructure, the elongation is bit higher and strength is very low. Because of the absence of tempered martensite layer at the periphery, the corrosion resistance property is expected to be lower. Higher corrosion resistance property is due to presence of Cr and Cu.

CN109295390A: Ultrahigh-strength corrosion-resistant rebar and producing method thereof. - By CHENGDE BRANCH CO HBIS CO LTD The ultrahigh-strength corrosion-resistant rebar is prepared from, by mass percent, 0.21% to 0.27% of C, 0.50% to 0.80% of Si, 1.10% to 1.30% of Mn, 0.06% to 0.20% of V, 0.01% to 0.02% of Nb, 0.010% to 0.017% of N, 0.02% to 0.10% of Als, 0.01% to 0.07% of Ce, 0.20% to 0.43% of Cr, 0.20% to 0.45%of Cu, and/or 0.10% to 0.70% of Mo and the balance Fe. Tensile strength >=850MPa, yield strength >=700MPa, elongation after fracture >=13%. This high strength is achieved by very rich chemical composition consists of high Mn, Si, V, Nb, N which is not economical for commercial production.

IN214263A1: “A PROCESS OF MANUFACTURING OF SUPERIOR CORROSION RESISTANT COPPER-MOLYBDENUM TMT REBARS” by STEEL AUTHORITY OF INDIA LTD. Process for manufacture of improved corrosion resistant TMT rebars, in particular, copper-molybdenum TMT rebars having improved strength and corrosion resistant properties suitable for applications in chloride containing environment. The process comprises providing steel ingots having compositions comprising 0.14 to 0.18 wt% C 1.0 to 1.2 wt% Mn, 0.2 to 0.3 wt% Si, up to 0.03 wt% S, up to 0.03 wt% P, 0.3 to 0.4 wt% Cu and 0.15 to 0.2 wt% Mo, the balance being iron.

The present invention is thus directed to develop a high strength Fe600 corrosion resistant grade in C-Mn steel with low P and no microalloying addition except Cr and Cu added for enhanced corrosion resistance. Its processing involves alloy design, 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 and corrosion resistant reinforcement TMT Rebars with minimum of 600 MPa yield strength and UTS/YS ratio greater than 1.06 with minimum elongation of 10 percent conforming 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 rebars having uniform elongation greater than 5 percent in TMT Rebars comprising C-Mn steel composition with Cr and Cu as micro-alloying which ensures better inherent corrosion resistance to rebar.
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, elongation and corrosion-resistant 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 of 40 mm 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, min. total elongation, elongation at maximum force and improved corrosion-resistance.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to provide a high strength thermo-mechanically treated(TMT) steel rebar for concrete reinforcement having steel composition comprising by weight%:
C : 0.15-0.3%, preferably 0.20-0.23%;
Mn: 0.5- 1.0%, preferably 0.70-0.80%;
S: up to 0.04% max, preferably 0.007-0.020%;
P: up to 0.04% max, preferably 0.010-0.020%;
Si: 0.10-0.30%, preferably 0.20-0.25%;
Cr: >=0.3%, Preferably 0.35-0.43%;
Cu: >=0.2%, Preferably 0.22-0.27%
CRE (Cr+Cu+Ni+Mo+P): >0.4%, preferable 0.55-0.70% including essentially said selective levels of Cr and Cu with reduced levels of the said P with Ni & Mo being balance CRE content in trace amounts of <0.005%;
N: up to 0.012% max and balance Fe, having a microstructure of an outer rim of peripheral tempered Martensite phase and a core of Ferrite and Pearlite phase ensuring high YS>600 MPa and UTS/YS>1.06 with minimum 10% elongation and uniform elongation 5% min.

A further aspect of the present invention is directed to said high strength thermo-mechanically treated(TMT) steel rebar comprising high strength in the range of YS 640 to 670 MPa, UTS 750 to 780 MPa, total elongation 12-16 % and uniform elongation 6 to 10 %.

A still further aspect of the present invention is directed to said high strength thermo-mechanically treated(TMT) steel rebar having an average grain size of 15-60 micron and comprising %volume of said peripheral tempered martensite phase is 28-35, %volume of said core ferrite-pearlite phase is 50-60 and %volume of transition Bainite phase being 12-18.

A still further aspect of the present invention is directed to said high strength thermo-mechanically treated(TMT) steel rebar wherein said rebar diameter is upto 40 mm and thickness of the said outer rim of peripheral tempered Martensite phase is in the range of 4 mm to 6 mm.

A still further aspect of the present invention is directed to said high strength thermo-mechanically treated(TMT) steel rebar wherein the chemical element Cr and Cu are included selectively as corrosion resistant element based on required strength and elongation and corrosion resistant property of the steel bar.

Another aspect of the present invention is directed to a process for manufacture of high strength thermo-mechanically treated(TMT) steel rebar as described above comprising:
(i) providing said steel having composition in wt % comprising:
C : 0.15-0.3%, preferably 0.20-0.23%;
Mn: 0.5- 1.0%, preferably 0.70-0.80%;
S: up to 0.04% max, preferably 0.007-0.020%;
P: up to 0.04% max, preferably 0.010-0.020%;
Si: 0.10-0.30%, preferably 0.20-0.25%;
Cr: >=0.3%, Preferably 0.35-0.43%;
Cu: >=0.2%, Preferably 0.22-0.27%
CRE (Cr+Cu+Ni+Mo+P): >0.4%, preferable 0.55-0.70% including essentially said selective levels of Cr and Cu with reduced levels of the said P with Ni & Mo being balance CRE content in trace amounts of <0.005%;
N: up to 0.012% max and balance Fe,, and continuously cast into billets;
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 said microstructure of an outer rim of peripheral tempered Martensite phase and a core of Ferrite and Pearlite phase ensuring high YS>600 MPa and UTS/YS>1.06 with minimum 10% elongation and uniform elongation 5% min. ;

Yet another aspect of the present invention is directed to said process comprising:
(i) including said Cu, Cr as corrosion resistant elements in producing the steel through primary steel making in ConArc furnace/LD converter followed by ladle heating, having composition in wt % comprising:
C : 0.15-0.3%, preferably 0.20-0.23%;
Mn: 0.5- 1.0%, preferably 0.70-0.80%;
S: up to 0.04% max, preferably 0.007-0.020%;
P: up to 0.04% max, preferably 0.010-0.020%;
Si: 0.10-0.30%, preferably 0.20-0.25%;
Cr: >=0.3%, Preferably 0.35-0.43%;
Cu: >=0.2%, Preferably 0.22-0.27%
CRE (Cr+Cu+Ni+Mo+P): >0.4%, preferable 0.55-0.70% including essentially said selective levels of Cr and Cu with reduced levels of the said P with Ni & Mo being balance CRE content in trace amounts of <0.005%
N: up to 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 following normal water flow rate under controlled quenching.
.
A 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 2400 to 2600 m3/Hr depending on mill speed and said self-tempering comprise 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 2400-2600 m3/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(a)-(d): Comparative illustration of Macrostructure of the inventive high strength TMT rebars of Fe 600 grade corrosion resistant steel having Cu and Cr as micro alloying element with improved uniform elongation.

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>10%, high uniform elongation (more than 5%) for concrete reinforcement application and method of producing the same. It is already known that with increasing yield strength ductility i.e. total elongation and uniform elongation at maximum force decreases. Also with increase in diameter, total elongation and uniform elongation tends to decrease and this happens particularly in corrosion resistant rebar because of the presence of corrosion resistant elements such as Cu, Cr, P etc. In this invention chemistry has been designed in such a way that even at this higher diameter (40mm) total elongation and uniform elongation is achieved well above the standard requirements.

The present invention relates to development of corrosion resistant steel Fe600 in C-Mn steel with the addition of Cu and Cr as corrosion resistant elements, suitable water quenching and 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 Heating Furnace (LHF) 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 (total elongation and uniform elongation) 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 optimized to get best properties in terms of elongation and uniform elongation. Also the P is used with very low quantity (<0.04) helps to reduce the brittleness which is due to formation of Fe3P.
Main feature of this product is high strength (YS >640 MPa &TS >750 MPa) along with high ductility (i.e. total elongation >12% & uniform elongation at maximum force >6%). This is achieved by proper selection of chemical composition (low P, higher C and optimum corrosion resistant elements such as Cr and Cu) and optimized Cooling Bed Temperature (500-550 Deg C).

Accordingly, the C-Mn steel composition with Cr and Cu 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 Cr Cu N max.
0.15- 0.30 0.5-1.5 0.04 0.04 0.1-0.5 0.3 – 0.7 0.2-0.4 0.012

The balance is Fe and unavoidable impurities.
This combination of chemical composition and water flow gives YS> 600 MPa, % Uniform elongation > 5% and total elongation>10%.
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 range in 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% so that 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 thickness will 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 0.15 to 0.30% and preferably 0.20-0.23%.

Silicon (Si): 0.10% or more and less than 0.50% by weight
Silicon is added as de-oxidizer for purity of Steel and has a strong solid solution strengthening effect. 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 0.10-0.5, preferably 0.20-0.25 % by weight.

Manganese (Mn): 0.5% or more and 1.5% 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 preferably 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 weldability by formation of brittle phase Fe3P. 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 preferably 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 0.007-0.020 %

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.

Copper (Cu) : 0.2 or more and less than 0.50% by weight
Copper is an element that improves the atmospheric corrosion resistance of the structural steel material by formation stable oxide layer at the surface which restricts the non-uniform corrosion. Maximum limit is restricted because of cost. In this present case copper is added intentionally in the range of preferably 0.22-0.27 %.

Chromium (Cr): 0.3% or more and 0.7% or less by weight
Chromium is generally added to steel to increase corrosion resistance and oxidation resistance. Cr along with Cu forms the stable oxide layer which restricts the non-uniform corrosion. Chromium also increases hardenability and improves high temperature strength. In this present case chromium is not added intentionallyand its maximal limit is restricted to 0.70% because of the cost. Cr is added preferably in the range of 0.35-0.43%.

In the composition of present steel grade, the Corrosion Resistant Elements(CRE) are maintained as follows:
i. CRE(Cr+Cu+Ni+Mo+P) =0.4 as per BIS 1786:2008.
ii. Ni & Mo are not added intentionally in the product. These are trace elements and each may exist in the amount <0.005%.
iii. The wt. percentage range of CRE is preferred as 0.55-0.70% on account of sufficiency of acquired corrosion resistant capability of product and optimum cost of Ferro-alloys.

Details of the process of manufacturing:
Ahigh strength reinforcement steel having the composition described above is prepared by obtaining molten steel through primary & secondary 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 Heating 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.
Figure 1 depicts the sequence of operations followed starting from the primary steel making to the inspection of final product during manufacturing of TMT rebar. Secondary steel making (to maintain final chemistry) and Thermo-mechanical rolling (to maintain optimized cooling bed temperature) are crucial steps for this 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 40 mm
4 Yield Strength 600 MPa min (640-670 MPa)
5 UTS 750-780 MPa
6 UTS/YS >1.06 (1.12-1.20)
7 %Total El >10% ( 12-16 %)
8 %Uniform El Not specified in standard (6.0-10.0 %)
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 as per Table 4 with or without P addition). 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:
Chemistry
%C %Si %S %P %Mn %Cr %Cu N2 ppm Remarks
0.135 0.22 0.015 0.09 0.72 0.42 0.22 55 Conventional
0.218 0.231 0.012 0.017 0.71 0.364 0.22 52 Invention

Mechanical Properties
YS UTS EL TE UTS/YS Remarks
639 679 11.0 5.5 1.06 Conventional
624 680 11.7 5.2 1.09 Conventional
609 700 11.4 5.8 1.15 Conventional
605 685 11.9 5.3 1.13 Conventional
650 756 15.4 7.6 1.16 Invented
664 776 15.6 7.5 1.17 Invented
669 780 15.8 7.6 1.17 Invented
663 765 15.9 7.4 1.15 Invented

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.

It would be apparent from accompanying Figure 2a-d that the microstructure has a peripheral tempered martensite rim and a ferrite and pearlite core and a transition bainitic phase in between them.
Figure 2(a) depicts the macrostructure of the cross section of the rebar consist of tempered martensitic rim at the periphery which contributes in the strength of the TMT rebar. In between outer tempered martensitic rim and ferrite-pearlite core, there is a transition band of bainite phase that formed due to the phase transformation with the different cooling rates. The core of ferrite & pearlite imparts the ductility of the TMT rebar.

Figures 2 (b) to 2 (d), depict the microstructure of each of three phases of outer tempered martensitic rim, inner bainitic transition zone and central ferrite and pearlite. Following conclusions can be drawn from analyzing the microstructure:

i. %volume of peripheral tempered martensite phase is 28-35, %volume of transition Bainite phase is 12-18 & %volume of core ferrite-pearlite phase is 50-60.
ii. Average grain size is 15-60 micron.
iii. Thickness range of tempered martensite rim is 4mm to 6mm.

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 Chromium(Cr)and Copper (Cu) as corrosion resistant elements that is processed through controlled rolling, thermomechanical treatment involving selectively the steps of water quenching and self-tempering to achieve optimum microstructure to ensure desired strength and elongation properties.