Claims:We Claim:

1. A seismic resistant reinforcement steel comprising of steel composition including
C: 0.16 to 0.25 wt%;
Mn: 1.2 to 1.5 wt%;
S: 0.04wt% Max.;
P: 0.04 wt% Max.;
Si:0.2 to 0.6 wt%;
V: 0.02 to 0.06wt%;
N: 0.012 wt% Max.
and rest is iron;
and having
Yield Strength(YS): 500 N/mm2(Min);
Ultimate Tensile Strength(UTS): 625 N/mm2(Min);
UTS/YS >1.25.

2. A seismic resistant reinforcement steel as claimed in claim 1 having microstructure comprising tempered Martensite on the rib surface with uniform ferrite-pearlite structure in the rod free of any martensite ring favouring plastic deformation before failure.

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

4. A seismic resistant reinforcement steel as claimed in anyone of claims 1 to 3 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 seismic resistant reinforcement steel as claimed in anyone of claims 1 to 4 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 comprising
C: 0.16 to 0.25 wt%;
Mn: 1.2 to 1.5 wt%;
S: 0.04wt% Max.;
P: 0.04 wt% Max.;
Si: 0.2 to 0.6 wt%;
V: 0.02 to 0.06wt%;
N: 0.012 wt% Max.
and rest is iron.
(iii) Continuous casting said steel composition into billet ;
(iv) Re-heating the billets followed by descaling, rough rolling, bar rod rolling, controlled cooling and processing such as to provide high strength uniform ferritic-pearlitic structure with desired plastic deformation characteristics .

6. A process as claimed in claim 5 wherein said process including maintaining
Furnace Temperature (Soaking): 1050 – 1150 oC;
Finish Rolling Temperature < 1000 oC;
Controlled cooling involving Thermo Mechanical Treatment with partial water cooling in water box and primarily air cooling of bars.

7. A process as claimed in anyone of claims 5 or 6, comprising controlled cooling with water box cooling water maintained so that the temperature after cooling is the range of 800 - 870 0C. and preferably at 830 0C with the bar primarily cooled by air cooling.

8. A process as claimed in anyone of claims 5 to 7, where in cooling bed temperature of the bars is maintained between 800- 870oC and preferably at 830 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 low water rolling with predominantly air cooling for developing a high strength uniform ferritic-pearlitic structure thereby increasing the plastic deformation before failure.

Dated this the 17th day of Match, 2016
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)

, Description:FIELD OF THE INVENTION

The present invention relates to the seismic resistant reinforcement steel rebars and method for producing the same. More particularly, the present invention is directed to provide seismic resistant steel rebars for reinforcement of concrete structures produced in the form of bars, coil and wires etc. for advantageous use as seismic resistant reinforcement steel in reinforced concrete. The new grade of reinforced steel bar according to the present invention is obtained with addition of micro alloying elements to chemical composition for desired microstructure, improved mechanical properties and higher UTS/YS ratio. The billets from steel melting shop are cast into billets, 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 and thermo mechanically treated and selectively cooled. The steel rebars obtained are having high strength, uniform ferritic-pearlitic structure thereby increasing the plastic deformation before failure. Importantly, the seismic resistant reinforcement steel rebars according to the present invention is having yield strength of 500MPa(min) and enhanced Ultimate Tensile Strength (UTS) to Yield Strength (YS) ratio of more than 1.25 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 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 to 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.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide Seismic Resistant Reinforced Steel (Fe 500-S grade) rebars with higher UTS/YS ratio and a method for manufacturing the same.

A further object of the present invention is directed to Seismic Resistant Reinforced Steel (Fe 500-S grade) rebars having minimum of 500 MPa yield strength and improved UTS/YS ratio of more than 1.25 for construction applications at seismic conditions.

A still further object of the present invention is directed to Seismic Resistant Reinforced Steel (Fe 500-S grade) rebars with higher UTS/YS ratio having selective addition of micro alloying elements to chemical composition for desired microstructure, improved mechanical properties and higher UTS/YS ratio.

A still further object of the present invention is directed to Seismic Resistant Reinforced Steel (Fe 500-S grade) rebars with higher UTS/YS ratio wherein unique combination of precipitation strengthening with vanadium microalloying and low water cooling or predominantly air cooling is adopted to achieve a high strength uniform ferritic-pearlitic structure and with partial tempered martensite on the ribs thereby increasing the plastic deformation before failure.

A still further object of the present invention is directed to Seismic Resistant Reinforced Steel (Fe 500-S grade) 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 gets dispersed homogenously across the steel bar and improves the strength.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to a seismic resistant reinforcement steel comprising of steel composition including
C: 0.16 to 0.25 wt%;
Mn: 1.2 to 1.5 wt%;
S: 0.04wt% Max.;
P: 0.04 wt% Max.;
Si: 0.2 to 0.6 wt%;
V: 0.02 to 0.06wt%;
N: 0.012 wt% Max.
and rest is iron;
and having
Yield Strength(YS): 500 N/mm2(Min);
Ultimate Tensile Strength(UTS): 625 N/mm2(Min);
UTS/YS >1.25.

A further aspect of the present invention is directed to said seismic resistant reinforcement steel having microstructure comprising tempered Martensite with uniform ferrite-pearlite structure free of any martensite ring favouring plastic deformation before failure.

Advantageously, said seismic resistant reinforcement steel according to present invention is obtained in the forms of bars, coils and wires.

A further aspect of the present invention is directed to said seismic resistant reinforcement steel which is obtained in wire rod form with thickness in the range of 6-12 mm and in bar rod form with thickness in the range of 8 – 40 mm.

A still further aspect of the present invention is directed to a process for manufacture of seismic resistant reinforcement steel as described 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 comprising
C:0.16 to 0.25 wt%;
Mn: 1.2 to 1.5 wt%;
S: 0.04wt% Max.;
P: 0.04 wt% Max.;
Si: 0.2 to 0.6 wt%;
V: 0.02 to 0.06wt%;
N: 0.012 wt% Max.
and rest is iron.
(iii) Continuous casting said steel composition into billet ;
(iv) Re-heating the billets followed by descaling, rough rolling, bar rod rolling, controlled cooling and processing such as to provide high strength uniform ferritic-pearlitic structure with desired plastic deformation characteristics .

Another aspect of the present invention is directed to said process wherein said process including maintaining
Furnace Temperature (Soaking): 1050 – 1150 oC;
Finish Rolling Temperature < 1000 oC;
Controlled cooling involving Thermo Mechanical Treatment with partial water cooling in water box and primarily air cooling of bars.

Yet another aspect of the present invention is directed to a process comprising controlled cooling with water box cooling water maintained to achieve temperature in the range of 800 0C to 870 0C and preferably at 830 0C with the bar primarily cooled by air cooling.

A further aspect of the present invention is directed to said process, wherein cooling bed temperature of the bars is maintained between 800- 870 oC and preferably at 830 oC.

A still further aspect of the present invention is directed to said process involving a combination of precipitation strengthening using Vanadium to form vanadium nitridesvanadium Carbides and low water 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 resistant reinforcement steel rebars(Fe500 S grade steel) 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 reinforcement grade (Fe500 S grade steel) 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 resistant steel(Fe 500 S grade) according to present invention illustrating with Partial martensite on rib surface and Ferite + pearlite in core.
Figure 7: illustrates the Load Vs Displacement graph of conventional reinforcement grade.
Figure 8: illustrates the Load Vs Displacement graph for Seismic resistant (Fe500S grade) steel.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

The present invention is directed to provide Seismic Resistant Reinforced Steel (Fe 500-S grade) composition/grade with minimum of 500 MPa yield strength and improved UTS/YS ratio of more than 1.25 for construction applications at seismic conditions and a process for producing the said steel in the form bars, coil and wire rod.

The present invention relates to a new grade of reinforced steel bar with addition of micro alloying elements to chemical composition for desired microstructure, improved mechanical properties and higher UTS/YS ratio. The billets casted to 165x165 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. The deformed bar is subjected to partial water cooling for desired microstructure and mechanical properties.

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

Table 1: Chemical Composition of seismic resistant reinforced steel rebar grade, (mass %)

C Mn S max. P max. Si V N max.
0.16-0.25 1.2-1.5 0.04 0.04 0.2-0.6 0.02-0.06 0.012

The balance is Fe and unavoidable 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) : 0.16% 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. Carbon content is useful in achieving better balance between the strength and the elongation property. The carbon content needs to be below 0.25% to ensure a 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 0.16 to 0.25 % by weight.

Silicon (Si): 0.2% 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 steel. Silicon needs to be contained within 0.6 % as excess of Silicon 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. the surface quality is poor; Accordingly, the Si content is 0.2% or more and is preferably in a range of 0.2 to 0.6% by weight.

Manganese (Mn) : 1.2% or more and less than1.5% by weight
Increasing the content of Manganese 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 Manganese 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 a required strength and to be controlled within 1.5%, therefore the preferable in range of 1.2 to 1.5 % by weight. Further Manganese is improving hardenability and access to critical alloying elements forming precipitate and acts pearlite stabilizer.

Phosphorus (P) : not more than 0.04% 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 0.04% by weight.

Sulphur (S): not more than 0.04% by weight
Sulfur content should not exceed 0.04%, accordingly, the S content is restricted to 0.04% 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 Nitride 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 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%, preferably less than 0.008%.

Vanadium (V) : 0.02% or more and less than 0.06% by weight
Vanadium is added for high strength , because it offers the best combination of high strength, good ductility, bendability, easy of welding, mechanical joining and insensitivity to strain aging, further Vanadium is a very strong carbide and nitride forming element. V 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 minimises the risk of cracking during continuous casting, and permits the use of economical hot rolling practices compared to the other microalloying choices. V-N is completely dissolved by heating the present steel composition above the temperature 1150 °C. Further by proper control over rolling, rolling temperature and cooling temperature grain refinement is promoted to form ferrite and pearlite phase wherein the V-N precipitates upon cooling and is dispersed homogenously across the steel bar, which results in improved strength, both the yield and tensile strength and is also conducive to maintain a high tensile strength and yield ratio. Also, vanadium rebars do not require sophisticated cooling lines, and the required microstructures and mechanical properties are achieved directly during air cooling after rolling. Accordingly, the V content is 0.02% or more and is preferably in a range of 0.02 to 0.06% by weight.

Copper (Cu) : 0% or more andnot more than 0.30% by weight
Copper is an element that improves the atmospheric corrosion resistance of the structural steel material. In this present case copper is not added intentionally and its maximal limit is restricted to 0.30%.

Chromium (Cr) : 0% or more andnot more than 0.5% by weight
Chromium is generally added to steel to increase corrosion resistance and oxidation resistance. Chromium also increases hardenability and improves high temperature strength. In this present case chromium is not added intentionally and its maximal limit is restricted to 0.50%.

Nickel (Ni) : 0 % or more and not more than 0.3% by weight
Nickel addition to the chemical composition of steel improves the red hot strength and corrosion resistance properties. In this present case nickel is not added intentionally and its maximal limit is restricted to 0.30%.

Method of manufacturing the steel grade according to the invention:

A seismic resistant reinforcement steel having the composition described above is produced by obtaining molten steel through steel making, followed by ingot making or continuous casting in billets. To produce a seismic resistant reinforcement steel having desired properties, the billet is subjected to reheating, descaling, rough rolling, Bar rod rolling, Thermal mechanical treatment and bundling, details of which will be described hereinafter. Accompanying Figure 1 shows the process flow chart for production of seismic resistant reinforcement steel rebars (Fe500 S grade steel) 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 2.

Table 2:

1 Furnace Temperature (Soaking) 1050 – 1150 oC
2 Cooling bed Temperature 800 - 870 oC
3 Finish Rolling Temperature < 1000 oC
4 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 5.65vA, where A is Area of cross section test specimens on a universal testing machine. All tests are performed at room temperature.

Metallographic analysis is carried out 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 comprising the material.

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 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. 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 resistant steel (Fe 500 S grade) according to present invention illustrating with Partial martensite on rib surface and Ferrite + pearlite in core.

Accompanying Figure 7 illustrates the Load Vs Displacement graph of conventional reinforcement grade and accompanying Figure 8 illustrates the Load Vs Displacement graph for Seismic resistant (Fe500S grade) steel according to present invention.

The Material Specifications for the invented steel grade(Fe 500S grade) in supply conditions are presented in following Table 3.

Table 3:
1 Chemical Composition As per Table 1
2 Wire Rod Thickness 8 - 40 mm
4 Yield Strength 500 (min) N/mm2
5 UTS 625 (min) N/mm2
6 UTS/YS >1.25
7 Weight per meter As per sample diameter
8 Bend Test OK
9 Rebend Test OK
10 Microstructure Tempered Martensite + (Ferrite + Pearlite)

The Comparison of Chemistry for conventional/Normal and inventive steel grade vis-à-vis the Mechanical properties achieved, as established through experiments under the present invention are presented in the following Table 4.

Table 4:

%-C %-Mn %-S max. %-P max. %-Si max %-N, max. %-V Cooling Bed Temperature Water box LPM YS UTS UTS/YS Remarks
0.231 0.79 0.008 0.017 0.159 0.0049 -- 620 7372 570 690 1.21
0.234 0.83 0.024 0.021 0.169 0.0045 -- 630 7321 548 665 1.21
0.215 0.75 0.015 0.015 0.174 0.0041 -- 625 7825 545 670 1.23
0.226 0.75 0.008 0.016 0.159 0.0053 -- 641 7903 550 679 1.23
0.215 0.77 0.008 0.017 0.166 0.0038 -- 615 8281 566 680 1.20
0.23 0.77 0.008 0.022 0.192 0.0052 -- 625 8356 566 690 1.22
0.228 0.76 0.009 0.018 0.158 0.0038 -- 622 8272 555 688 1.24
0.229 1.43 0.012 0.017 0.407 0.0056 0.05 820 3800 533 689 1.29 Invention
0.229 1.43 0.012 0.017 0.407 0.0056 0.05 825 3800 545 702 1.28 Invention
0.229 1.43 0.012 0.017 0.407 0.0056 0.05 840 3200 510 670 1.32 Invention
0.229 1.43 0.012 0.017 0.407 0.0056 0.05 845 3200 525 696 1.32 Invention

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 - 870 0C and the bar is primarily cooled by air cooling. This resulted in increase in the cooling bed temperature of the bars from 620oC to 830oC. The loss of strength was compensated by the precipitation strengthening with the use of vanadium. 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 Resistant Reinforced Steel (Fe 500-S grade) rebars with minimum of 500 MPa yield strength and improved UTS/YS ratio of more than 1.25 for construction applications at seismic conditions and a process for producing the said steel rebars, wherein 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 which ensure significant seismic resistance for steel reinforced concrete structure.