Claims:We Claim:

1. A reinforcement steel comprising of steel composition having

C: 0.08 to 0.25 wt%;
Mn: 0.50 to 1.50 wt%;
S: 0.05 wt% Max;
P: 0.05 wt% Max;
Si: 0.10 to 0.60 wt%;
V: 0.005 to 0.06 wt%;
Cr: 0.05 to 0.70 wt%;
Cu: 0.05 to 0.50 wt%;
Ni: 0.01 to 0.50 wt;
N: 0.012 wt% Max;
and rest is iron and impurities;

and Yield Strength (YS): 500 N/mm2 (Min); Ultimate Tensile Strength (UTS): 625 N/mm2 (Min); and UTS/YS ratio >1.25.

2. A reinforcement steel as claimed in claim 1 having microstructure with uniform ferrite-pearlite structure in the bar free of tempered martensite ring favoring plastic deformation before failure.

3. A reinforcement steel as claimed in anyone of claims 1 or 2 in the form of bars, coils and wires.

4. A reinforcement steel as claimed in anyone of claims 1 to 3 which is seismic resistant reinforcement steel in wire rod form with thickness in the range of 6 - 12 mm and bar rod form with thickness in the range of 8 – 40 mm.

5. A process for manufacture of reinforcement steel as claimed in anyone of claims 1 to 4 comprising:

involving a steel composition including alloying elements selectively including chromium, nickel and copper and having therein :

C: 0.08 to 0.25 wt%;
Mn: 0.50 to 1.50 wt%;
S: 0.05 wt% Max;
P: 0.05 wt% Max;
Si: 0.10 to 0.60 wt%;
V: 0.005 to 0.06 wt%;
Cr: 0.05 to 0.70 wt%;
Cu: 0.05 to 0.50 wt%;
Ni: 0.01 to 0.50 wt;
N: 0.012 wt% Max;
and rest is iron and impurities;

rolling of the billet / bloom involving said composition into rebar with finished rolling temperature of 900 – 1050 OC and with cooling bed temperature of the rebar at 800 - 900 OC for 8 – 40 mm in diameter such as to achieve Yield Strength (YS): 500 N/mm2 (Min); Ultimate Tensile Strength (UTS): 625 N/mm2 (Min); and UTS/YS ratio >1.25.

6. A process as claimed in claim 5 comprising:
I. primary steel making involving blowing in LD converter/ Electric Arc Furnace and tapping into steel ladles; followed by

II. secondary steel making in ladle heating furnace to obtain a castable composition including said composition comprising
C: 0.08 to 0.25 wt%;
Mn: 0.50 to 1.50 wt%;
S: 0.05 wt% Max;
P: 0.05 wt% Max;
Si: 0.10 to 0.60 wt%;
V: 0.005 to 0.06 wt%;
Cr: 0.05 to 0.70 wt%;
Cu: 0.05 to 0.50 wt%;
Ni: 0.01 to 0.50 wt;
N: 0.012 wt% Max;
and rest is iron and impurities;

III. Continuous casting said steel composition into billet ;

IV. Reheating the billets followed by descaling, intermediate rolling, bar rod rolling, controlled cooling with air and processing such as to provide high strength uniform ferritic-pearlitic structure with desired plastic deformation characteristics.

7. A process as claimed in claim 5 to 6 wherein said process including maintaining
Furnace Temperature (Soaking): 1050 – 1150 oC;
Finish Rolling Temperature < 1000 oC;
Controlled cooling involving air cooling of bars.

8. A process as claimed in anyone of claims 5 to 7, comprising air cooling with desired cooling bed temperature at 800 - 900 OC.

9. A process as claimed in anyone of claims 5 to 8, involving a combination of precipitation strengthening using Vanadium to form vanadium nitrides and Vanadium carbides and rolling with predominantly air cooling for developing a high strength uniform ferritic-pearlitic structure thereby increasing the plastic deformation before failure.

Dated this the 10th day of November, 2016
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)

, Description:FIELD OF THE INVENTION
The present invention relates to reinforcement steel rebars with high UTS/YS ratio, excellent corrosion resistant properties and method for producing the same. More particularly, the present invention is directed to provide corrosion resistant and seismic resistant steel rebars for reinforcement of concrete structures produced in the form of bars, coil and wires etc. for advantageous use in reinforced concrete in seismic resistant zone. The new grade of reinforced steel bar according to the present invention are directed to improved mechanical properties, high corrosion resistance and seismic resistant reinforced steel rebars with minimum of 500 MPa yield strength and improved UTS/YS ratio of more than 1.25 especially suited for construction applications. The steel rebars obtained are targeted at high strength, uniform ferritic-pearlitic structure thereby increasing the plastic deformation before failure. Importantly, the corrosion and seismic resistant reinforcement steel rebars having greater energy absorption capability before failure and larger deformations experienced which could serve as a visible warning to the building occupants prior to failure or collapse suitable for wide range of applications.

BACKGROUND OF THE INVENTION
Reinforced bar, abbreviated as rebar, is used in the construction industry to impart tensile strength to concrete structures which by nature is very brittle. Rebar is, therefore, a vital material in modern high rise buildings and engineering projects. Concrete is one of the most important building materials since its properties include good formability and resistance to weathering and fire. It can also withstand high compressive stresses but unfortunately almost no tensile and shear stresses. Steel is considered as the best material to reinforce the concrete. The rebar is mainly ribbed to provide a good joint between these two materials. There are several grades of high strength deformed steel wires for various applications in construction field. The material strength properties such as Ultimate Tensile Strength and Yield Strength are individually important as they influence the behaviour of structures during seismic excitation. Both the mechanical properties taken together as UTS/YS ratio, known as the strain hardening value, indicate the ductile capacity of the structural members. The larger the UTS/YS ratio, the better is for the structure. A higher UTS/YS ratio refers to the greater energy absorption capability before failure. Larger deformations are experienced which could serve as a visible warning to the building occupants prior to failure or collapse.

In general, reinforced bars rolled in the steel industry have good mechanical properties like yield strength and tensile strength but do not have better UTS/YS ratio for its application at seismic conditions. The reinforced steel bars which are used in the seismic zones require mainly yield strength of 500 MPa and higher UTS/YS ratio, i.e., more than 1.25. Normally in bar and wire rolling, the strength is achieved by the peripheral martensitic rim developed by fast water quenching in the water boxes. However due to this two layer structure (martensite at periphery and ferrite-pearlite in core) the plastic deformation is restricted leading to lower UTS/YS ratio. To increase the UTS/YS ratio a single phase structure is desired.

There has been therefore a need in the related field for developing seismic resistant reinforcement steel rebars for use in concrete reinforced structures which would have improved strength and UTS/YS ratio to enable higher capacity to absorbing higher amount of energy plastically and undergo greater deformation before failure under severe seismic environment thus providing an early warning favouring opportunity for evacuation of habitats in such concrete structure and ensuring safety of occupants.

The durability of the rebars is very important in considering its long term use for critical structures where money and safety of the people are concerned. The mechanical and corrosion resistance properties command rebars durability when embedded in concrete. The corrosion of the rebars deteriorates their mechanical properties which consequently prematurely damages the whole structure. It involves not only the monetary loss but also the safety of the living beings. The life of steel reinforcement is normally better when embedded in the dense concrete as a result of the pore solution phase being sufficiently alkaline causing formation of a passive layer (thin protective oxide film) over the reinforcing steel surface. The passive layer provides chemical protection while the physical protection of steel rebar is through the retarding access of oxygen, moisture, and various aggressive species to the steel/concrete interface. However, the breakdown of the passive film and initiation of corrosion takes place most frequently in the presence of chloride ions for example, when the steel rebar is exposed to deicing salt, coastal atmospheric conditions, tidal wetting or flowing sea water. The aggressive chloride ions can also be originated from the use of contaminated mix ingredients and/or from the surrounding environment in the hardened state.

Once the rebar under concrete structure comes in contact with corrosive solution by any means, the corrosion of reinforcement develops. Carbonation of concrete or penetration of acidic gases into the concrete, are the other causes of reinforcement corrosion. Besides these there are few more factors, some related to the concrete quality, such as water/cement (w/c) ratio, cement content, impurities in the concrete ingredients, presence of surface cracks, etc. and others related to the external environment such as moisture, oxygen, humidity, temperature, bacterial attack, stray currents, etc., which affect reinforcement corrosion. The buildup of corrosion products at the rebars results in loss of reinforcement cross-section. The large volume of corrosion products (iron oxides) generates tensile stresses on the concrete from inside to outward direction leading to cracking and spalling of the concrete. Due to the formation of corrosion products, rebars looses interfacial bond with concrete. The rebars corrosion also results in reduction in cross-sectional area of the steel bar and consequently reducing the load carrying capacity of reinforced concrete structures which may further result in structure collapse, monetary loss and the loss of people. The corrosion related issues of rebars are a particular problem where the concrete is exposed to salt water, as in bridges where salt is applied to roadways in winter, or in marine applications or near the building of coastal area where the humidity with chloride content is very high. Efforts have been continuously made to protect the rebar corrosion by different means such as coating on steel rebars either by organic resin or zinc coating, modifying the concrete mix or by addition of anti corrosive elements during steel making. The coated material performs well with respect to corrosion resistance behavior even in very aggressive corrosive environment. However, the limitations of applying such coating becomes worst if the surface coating of rebars break or falls by any means.

The present invention is thus directed to develop a high corrosion resistant and seismic resistant steel rebar by alloying and processing with optimum thermo-mechanical treatment of steel to perform well inside concrete with the required mechanical and corrosion resistance properties.

The uniform ferrite-pearlite structure on the surface helps in improving the corrosion resistance of TMT bar than that of the rebar having martensite ring on the surface. As martensite, a distorted stressed structure is far more prone to corrosion than that of the ferrite-pearlite structure, the self corrosion of martensite is almost 2-3 times higher than ferrite. When the steel is alloyed and processed to obtain ferrite-pearlite structure, the intactness of the passive layer improves and results in higher corrosion protection than martensite ring structured TMT bars. In present invention, the alloying element Chromium, Nickel and Copper are added during steel making process to obtain ferrite-pearlite structure throughout from the surface to the core of rebar where the alloying element helped in forming intact passive layer under corrosive environment resulting in improved corrosion resistance property of seismic resistant TMT bars. The improved corrosion resistance property of seismic resistant TMT bar is expected to enhance the life cycle of the rebar and safety of the structure and human life.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide a reinforcement steel rebar and more specifically to seismic and corrosion resistant reinforcement steel rebars with higher UTS/YS ratio and a method for manufacturing the same.

A further object of the present invention is directed to seismic and corrosion resistant reinforcement steel rebars having minimum of 500 MPa yield strength, improved UTS/YS ratio of more than 1.25 and excellent corrosion resistance for construction applications at seismic prone areas.

A still further object of the present invention is directed to seismic and corrosion resistant reinforcement steel rebars with higher UTS/YS ratio having selective addition of micro alloying and corrosion resistance elements to chemical composition for desired microstructure, improved mechanical properties, higher UTS/YS ratio and excellent corrosion resistant properties.

A still further object of the present invention is directed to seismic and corrosion resistant reinforcement steel rebars with higher UTS/YS ratio wherein unique combination of precipitation strengthening with vanadium microalloying, corrosion resistant elements like chromium, copper, nickel and predominantly air cooling is adopted to achieve a high strength uniform ferritic - pearlitic structure thereby increasing the plastic deformation before failure.

A still further object of the present invention is directed to seismic and corrosion resistant reinforcement steel rebars with higher UTS/YS ratio wherein loss of strength due to air cooling is compensated by the precipitation strengthening with the use of vanadium whereby vanadium forms fine vanadium nitrides and vanadium carbides which get dispersed homogenously across the steel bar and improves the strength.

SUMMARY OF THE INVENTION

Thus according to the basic aspect of the present invention there is provided a reinforcement steel comprising of steel composition having

C: 0.08 to 0.25 wt%;
Mn: 0.50 to 1.50 wt%;
S: 0.05 wt% Max;
P: 0.05 wt% Max;
Si: 0.10 to 0.60 wt%;
V: 0.005 to 0.06 wt%;
Cr: 0.05 to 0.70 wt%;
Cu: 0.05 to 0.50 wt%;
Ni: 0.01 to 0.50 wt;
N: 0.012 wt% Max;
and rest is iron and impurities;

and Yield Strength (YS): 500 N/mm2 (Min); Ultimate Tensile Strength (UTS): 625 N/mm2 (Min); and UTS/YS ratio >1.25.

According to another aspect there is provided a reinforcement steel having microstructure with uniform ferrite-pearlite structure in the bar free of tempered martensite ring favoring plastic deformation before failure.

According to yet further aspect of the present invention there is provided a reinforcement steel in the form of bars, coils and wires.

According to yet further aspect of the present invention there is provided a reinforcement steel as above which is seismic resistant reinforcement steel in wire rod form with thickness in the range of 6 - 12 mm and bar rod form with thickness in the range of 8 – 40 mm.

According to yet further aspect of the present invention there is provided a process for manufacture of reinforcement steel as above comprising:
involving a steel composition including alloying elements selectively including chromium, nickel and copper and having therein :
C: 0.08 to 0.25 wt%;
Mn: 0.50 to 1.50 wt%;
S: 0.05 wt% Max;
P: 0.05 wt% Max;
Si: 0.10 to 0.60 wt%;
V: 0.005 to 0.06 wt%;
Cr: 0.05 to 0.70 wt%;
Cu: 0.05 to 0.50 wt%;
Ni: 0.01 to 0.50 wt;
N: 0.012 wt% Max;
and rest is iron and impurities;

rolling of the billet/bloom involving said composition into rebar with finished rolling temperature of 900 – 1050 OC and with cooling bed temperature of the rebar at 800 - 900 OC for 8 – 40 mm in diameter such as to achieve Yield Strength (YS): 500 N/mm2 (Min); Ultimate Tensile Strength (UTS): 625 N/mm2 (Min); and UTS/YS ratio >1.25.

According to another aspect of the present invention there is provided a process as above comprising:
(i) primary steel making involving blowing in LD converter/ Electric Arc Furnace and tapping into steel ladles; followed by
(ii) secondary steel making in ladle heating furnace to obtain a castable composition including said composition comprising
C: 0.08 to 0.25 wt%;
Mn: 0.50 to 1.50 wt%;
S: 0.05 wt% Max;
P: 0.05 wt% Max;
Si: 0.10 to 0.60 wt%;
V: 0.005 to 0.06 wt%;
Cr: 0.05 to 0.70 wt%;
Cu: 0.05 to 0.50 wt%;
Ni: 0.01 to 0.50 wt;
N: 0.012 wt% Max;
and rest is iron and impurities;
(iii) Continuous casting said steel composition into billet ;
(iv) Reheating the billets followed by descaling, intermediate rolling, bar rod rolling, controlled cooling with air and processing such as to provide high strength uniform ferritic-pearlitic structure with desired plastic deformation characteristics.

Yet further aspect of the present advancement relates to a process as above wherein said process included maintaining
Furnace Temperature (Soaking): 1050 – 1150 oC;
Finish Rolling Temperature < 1000 oC;
Controlled cooling involving air cooling of bars.

According to a further aspect of the present advancement there is provided a process as above comprising air cooling with desired cooling bed temperature at 800 - 900 OC.

According to yet another aspect of the present invention there is provided a process as above involving a combination of precipitation strengthening using Vanadium to form vanadium nitrides and Vanadium carbides and rolling with predominantly air cooling for developing a high strength uniform ferritic-pearlitic structure thereby increasing the plastic deformation before failure.

The above and other objects and advantages of the present invention are described hereunder in greater details with reference to the following accompanying non limiting illustrative drawings.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1: shows the process flow chart for production of seismic and corrosion resistant reinforcement steel rebars according to the present invention.

Figure 2: is the image showing the Macrostructure of conventional reinforcement grade steel grade.

Figure 3: is the image showing the Macrostructure of seismic and corrosion reinforcement grade according to the present invention.

Figure 4: is the image showing the Microstructure of conventional Reinforcement steel illustrating TMT Surface with tempered martensite.

Figure 5: is the image showing the Microstructure of conventional Reinforcement steel illustrating TMT core with Ferrite + Pearlite.

Figure 6: is the image showing the Microstructure of Seismic and corrosion resistant steel according to present invention illustrating Ferrite + pearlite structure throughout the cross section.

Figure 7: illustrates the Load Vs Displacement graph of conventional reinforcement grade.

Figure 8: illustrates the Load Vs Displacement graph for Seismic and corrosion resistant steel.

Figure 9: illustrates comparison of corrosion resistance of steel (Fe500S) rebar according to present invention with conventional Fe500D grade rebars.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

The present invention is directed to provide seismic and corrosion resistant reinforcement steel grade(Fe500S) with minimum of 500 MPa yield strength and improved UTS/YS ratio of more than 1.25 for construction applications at seismic prone zones and a process for producing the said steel in the form of bars, coil and wire rod.

The present invention relates to a new grade of reinforced steel bar with addition of micro alloying and corrosion elements to chemical composition for desired microstructure, improved mechanical properties, and higher UTS/YS ratio and excellent corrosion resistant properties. The billets casted to 165x165 mm from steel melting shop are reheated in the reheating furnace and are subjected to different reduction ratios in 10 to 30 stands up to final diameter as per customer requirement in the bar / wire rod mill. The deformed bar is subjected to air cooling for desired microstructure and mechanical properties.

The composition of the seismic and corrosion resistant steel rebars according to the present invention having constituent elements as in the following Table 1:

Chemical Composition of seismic and corrosion resistant reinforced steel rebar grade comprises;

Table: 1:

Constituent elements Wt%
C 0.08 to 0.25 wt%;
Mn 0.50 to 1.50 wt%;
S 0.05 wt% Max;
P 0.05 wt% Max;
Si 0.10 to 0.60 wt%;
V 0.005 to 0.06 wt%;
Cr 0.05 to 0.70 wt%;
Cu 0.05 to 0.50 wt%;
Ni 0.01 to 0.50 wt;
N 0.012 wt% Max; and
rest is iron and impurities.

Now, the essential components of the steel grade of the present invention according to a working embodiment are described hereunder with considerations for selecting the respective concentrations range of components in mass percentage:

Carbon (C): 0.08% or more and less than 0.25% by weight

Carbon is an essential element that provides strength and hardness to steel further maintains Pearlite structure. The carbon content needs to be below 0.25% to ensure the required strength and hardness for construction applications as higher content refrains the invention from gaining seismic resistant properties due to low elongation. Further concentration of Carbon above 0.25% may result in poor weldability. Accordingly, the C content is 0.25% max and preferably in a range of 0.08 to 0.25 % by weight.

Silicon (Si): 0.1% or more and less than 0.6% by weight

Silicon is added as deoxidiser for purity of steel and has a strong solid solution strengthening effect hence acts to reinforce the steel. Silicon needs to be below 0.6 % as excess of Si will deteriorate toughness and weldability of Steel. Further if, excessive silicon is present in the steel billet upon heated, the oxide skin thereof may become highly viscous and it is difficult to descale after the steel billet exiting from furnace, thereby resulting in red oxide skins on the steel billet after rolling, i.e. deterioration in surface quality. Accordingly, the Si content is 0.1% or more and is preferably in a range of 0.10 to 0.60 % by weight.

Manganese (Mn): 0.5% or more and less than1.5% by weight
Increasing the content of Mn is the most inexpensive and immediate way to compensate for the strength loss caused by the reduction of carbon content due to its solid solution strengthening characteristics. But Mn has a high segregation tendency, so its content should not be very high, generally, no more than 2.0% in low-carbon micro-alloyed steel. Hence, to ensure the required strength 1.2% or more of Mn needs to be contained to ensure the required strength and to be controlled within 1.5%, therefore the preferable range of 0.50 to 1.50 % by weight. Further, Mn is improving hardenability and access to critical alloying elements forming precipitate and acts as pearlite stabilizer.

Phosphorus (P): not more than 0.05% by weight
Phosphorus is an element that improves the atmospheric corrosion resistance of the structural steel material, further adding Phosphorus into the steel will improve the precipitation of solid solutions and the shape-forming workability of the steel, such rib forming in Construction steel. Phosphorous, when in large amount is added to the present steel composition, the toughness and rollability of the steel sheet has been found to deteriorate. In addition, the segregation of phosphorus at grain boundaries of the present composition 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 about 0.05% by weight.

Sulphur (S): not more than 0.05% by weight
Sulfur content should not exceed 0.05% otherwise it results in high sulphide inclusions and may impair formability and mechanical properties. Accordingly, the S content is restricted to 0.05% or less and preferably less than 0.02% by weight.

Nitrogen (N): not more than 0.012% by weight
Nitrogen in seismic resistant reinforcement steel is mainly combined with vanadium into vanadium nitrides and carbides for precipitation strengthening. Further, Nitrogen contributes towards grain refinement. Excess nitrogen causes a large amount of nitride to precipitate, thereby deteriorating ductility 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%, preferably less than 0.008% by weight.

Vanadium (V): 0.005% or more and less than 0.06% by weight
Vanadium is added to improve the strength, ductility, bendability, ease of welding, mechanical joining and insensitivity to strain aging. Further, vanadium is a very strong carbide and nitride forming element. Vanadium has a significant amount of precipitation strengthening when added to the chemical composition hence increases the yield strength and the tensile strength of the steel. The high solubility of vanadium carbo-nitrides in austenite minimizes the risk of cracking during continuous casting and permits the use of economical hot rolling practices compared to the other microalloying choices. Vanadium-nitride is completely dissolved by heating the steel above the temperature 1150 °C. Further, by proper control over rolling, rolling-temperature and cooling, grain refinement is promoted wherein the vanadium-nitride precipitates upon cooling and is dispersed homogenously across the steel bar, which results in improved strength and maintain a high UTS/YS ratio. Accordingly, the V content is 0.005% or more and is preferably in a range of 0.005 to 0.06% by weight.

Copper (Cu): 0.05% or more and less than 0.5% by weight
Copper improves the atmospheric corrosion resistance of the structural steel. In present invention, copper is added intentionally and its maximal limit is restricted to 0.50% by weight.

Chromium (Cr): 0.05% or more and less than 0.7% by weight
Chromium is generally added in steel to increase corrosion and oxidation resistance. Chromium also increases hardenability and improves high temperature strength. Chrmoium improves wear resistance and toughness of steel. Chroimum in steel resists staining and corrosion under corrosive environment. In present invention, chromium is added intentionally and its maximal limit is restricted to 0.70% by weight.

Nickel (Ni): 0.05% or more and less than 0.5% by weight
Nickel in steel increases strength, impact strength & toughness, and resistance to oxidation & corrosion. It also increases toughness at low temperatures when added in small amount. In present invention, nickel is added intentionally and its maximal limit is restricted to 0.50% by weight.

Method of manufacturing the steel grade according to the invention:

The step wise implementation of the process of the invention to obtain the reinforcement steel rebars according to the present invention and test procedure to ascertain the properties of the steel grade produced is presented in the following Example I starting with a specific basic steel composition as given in Table 2.

Example I:

Table 2:

Constituent elements Wt%
(Conventional TMT) Wt% (Invention)
C 0.13 0.14
Mn 0.70 1.40
S 0.03 0.03
P 0.03 0.03
Si 0.20 0.20
V 0.001 0.05
Cr 0.05 0.45
Cu 0.04 0.30
Ni 0.01 0.10
N 0.008 0.008

Processing steps:
The steel composition of the invention as mentioned in Table 2 is achieved by addition of alloying elements during steel making. The molten steel is cast in billet shape through continuous casting.

The cast billets are loaded into reheating furnace to heat the billets to a temperature range of 1060 to 1160 0C for hot rolling. The said billets are discharged out of furnace and are subjected to descaling for removal of oxides / scales present on the surface of billet with a descaling pressure of 50 to 180 bars. The descaled billets are passed through roughing, intermediate and finishing stands to produce the bar of diameter 8 to 40 mm. The finishing temperature of hot rolled bars is less than 1000 0C and the bars temperature drops down to 800 – 900 0C while reaching to cooling bed. The bars are left on the cooling bed for air cooling.

The air cooling of bars with above mentioned chemical composition from austenitic region results in uniform microstructure comprising of ferrite and pearlite. In conventional cooling, the finished hot rolled bars are subjected to water cooling with high jet pressure which result in martensitic structure on surface and ferrite plus pearlite in the core. The surface martensite is further self tempered with the residual heat transfer from the core to the surface of the bar.

The air cooled bars with above mentioned chemical composition will have minimum yield strength of 500 MPa, minimum ultimate tensile strength to yield strength ratio of 1.25 and a minimum elongation of 20 percent whereas a conventional processed rebar will be having minimum yield strength of 500 MPa, maximum ultimate tensile strength to yield strength ratio of 1.18 and a maximum elongation of 18 percent.

A seismic and corrosion resistant reinforcement steel rebars having the composition as described above is produced by obtaining molten steel through steel making, followed by ingot making or continuous casting in billets. To produce a seismic and corrosion resistant reinforcement steel having desired properties, the billet is subjected to reheating, descaling, intermediate rolling, Bar/wire rod rolling, Thermal mechanical treatment(TMT) and bundling, details of which will be described hereinafter. Accompanying Figure 1 shows the process flow chart for production of seismic and corrosion resistant reinforcement steel rebars according to the present invention.

It is apparent that the process route comprising the following basic steps:
(i) Steel making by LD Converter/ Electric Arc Furnace;
(ii) Secondary steel making: Ladle Heating Furnace;
(iii) Continuous billet casting into (165 x 165 mm);
(iv) Re-heating, bar rod rolling, controlled cooling and cut to length with set optimum processing parameters.

This new steel reinforced grade is made through Basic Oxygen Furnace (BOF) steel making/ Electric Arc Furnace (EAF) and Ladle Heating Furnace (LHF) route. It is further cast into billets through continuous casting process. These billets are processed through reheating furnace and hot wire rod rolling followed by controlled cooling. 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. The Bar rod rolling parameters specified for processing are presented in following Table 3.

Table 3: Rolling parameters

Furnace temperature (soaking) 1050 – 1150 oC
Cooling bed temperature 800 - 900 oC
Finish rolling temperature < 1000 oC
Controlled cooling Thermo-mechanical treatment (Partial water cooling in water box)

Mechanical Tests and Metallography:

The tensile properties (yield strength and ultimate tensile strength) are measured using 600 mm long and gauge length of 5.65*(A)½, where A is the area of cross section of the test specimen on a universal testing machine. All tests are performed at room temperature conforming to the requirement of IS 1608 standard.

Tensile test was performed using 250 kN Zwick tensile testing machine. Tensile test condition such as pre-load of 1 MPa, speed in yield range 60 MPa/s and test speed 0.008 cm/s were maintained while performing tensile test.

The microstructural examinations of rebar steel samples were carried out using standard metallographic procedure. The rebar samples were cut and mounted using bakelite powder. The mounted samples were polished using various grades of SiC paper (220, 320, 400, 600, 800 grit size respectively). Subsequently, the sample was polished on velvet cloth using 0.1 µm diamond paste and cleaned using water and ethanol to obtain mirror finish. The etching of the polished samples was done using 5% Nital solution. The Carl Zeiss (Model: Axiovert 40 MAT) inverted light microscope equipped with quantitative metallographic software was used for the examination of structures.

Normally in bar and wire rolling, the strength is achieved by the peripheral martensitic rim developed by water quenching in the water boxes. However, due to this two layer structure (martensite at periphery and ferrite-pearlite in core) the plastic deformation is restricted leading to lower UTS/YS ratio. To increase the UTS/YS ratio a single phase structure is desired which can be attained by air cooling, but it lowers the strength.

The Electro-chemical impedance spectroscopy (EIS) studies were performed using a potentiostat/galvanostat/frequency response analyzer of AUTOLAB instruments (model:PGSTAT). EIS study was conducted by applying sinusoidal signal amplitude of 10mV and the electrode response was analyzed in the frequency range between 10,000 and 0.01 Hz at their respective open circuit potential. The electrolyte solution used for the EIS studies was 10,000ppm of chloride solution with 0.04 N NaOH which has close simulation prevailing in concrete under actual condition. Before performing the corrosion studies, the rebar samples were cut, degreased using acetone and rinsed with de-ionized water. The samples were hanged in the beaker to dip the required surface area for studies. The cleaned samples formed the working electrode while a silver-silver chloride (Ag-AgCl) electrode and a platinum wire served as the reference and auxiliary electrodes, respectively. The polarization resistance value was measured from the Nyquist plots and given in Table 4. Potentiodynamic polarization tests were carried out at a scan rate of 1mVsec-1 from -100 to +100 mV vs Ag-AgCl electrode with respect to the open circuit potential (OCP). The corrosion potential (Ecorr), corrosion current density (icorr), polarization resistance (Rp) and corrosion rate were determined from the polarization curves using Tafel extrapolation method and given in Table 5.

Table 4: Polarization resistance obtained from Electrochemical Impedance Spectroscopy
Material Rp (Ohm)
Fe 500 D (Conventional TMT) 181
Fe500S-CRS 350.44

Table 5: Corrosion rate (mm/yr) determined using Tafel Extrapolation Method and Potentiodynamic Polarisation Test
Material Icorr (µA/cm2) Ecorr (mV) Rp (Ohm) C.R (mm/yr)
Fe 500 D (Conventional TMT) 8.18 -534 297 0.095
Fe500S-CRS 4.61 -583 489 0.054

Where, Icorr : Corrosion Current Density
Ecorr : Corrosion Potential
Rp : Polarisation Resistance
C.R : Corrosion Rate

Test Results:

Accompanying Figure 2 shows the macrostructure of conventional reinforcement grade steel grade, Figure 3 shows the macrostructure of invention seismic reinforcement grade steel and Figure 4 shows the microstructure of conventional reinforcement steel illustrating TMT Surface with tempered martensite while Figure 5 shows the microstructure of conventional reinforcement steel illustrating TMT core with ferrite & pearlite.

Accompanying Figure 6 shows the image showing the microstructure of seismic and corrosion resistant steel according to present invention illustrating with ferrite & pearlite throughout the cross section of the bar.

Accompanying Figure 7 illustrates the Load Vs Displacement graph of conventional reinforcement grade. Figure 8 illustrates the Load Vs Displacement graph for seismic and corrosion resistant steel according to present invention, and

Accompanying Figure 9 illustrates the comparison of corrosion resistance behavior of the present invention with conventional TMT in saline solution where (a) Nyquist plot and (b) Tafel’s plot. The Material Specifications for the invented steel grade in supply conditions are presented in Table 6.

Table 6: Material specification

Chemical Composition As per Table 1
Wire Rod Thickness 8 - 40 mm
Yield Strength 500 (min) N/mm2
UTS 625 (min) N/mm2
UTS/YS >1.25
Weight per meter As per sample diameter
Bend Test OK
Rebend Test OK
Microstructure (Ferrite + Pearlite)

In the present invention to avoid the martensitic ring formation, the water flow in the water box is restricted so that the temperature after cooling is in the range of 800 - 900 oC and the bar is primarily cooled by air cooling. This resulted in increase in the cooling bed temperature of the bars from 620 oC to 800-900 oC. The loss of strength was compensated by the precipitation strengthening with the use of vanadium, chromium. Vanadium forms fine vanadium nitrides and vanadium carbides which gets dispersed homogenously across the steel bar and improves the strength. This unique combination of vanadium strengthening and low water rolling or predominantly air cooling helped in developing a high strength uniform ferritic - pearlitic structure thereby increasing the plastic deformation before failure. This increased the UTS/YS ratio to >1.25.

It is thus possible by way of the present invention to provide seismic and corrosion resistant reinforced steel rebars with minimum of 500 MPa yield strength and improved UTS/YS ratio of more than 1.25 for construction applications at seismic and corrosive conditions and a process for producing the said steel rebars, wherein unique combination of vanadium strengthening and air cooling helped in developing a high strength uniform ferritic-pearlitic structure thereby increasing the plastic deformation before failure which ensure significant seismic resistance for steel reinforced concrete structure.