FIELD OF THE INVENTION
The present invention relates to steel reinforcement bars and, in particular, to high strength earthquake resistant rebars intended for RCC (Reinforced concrete cement) structures and the process of producing it. The invention is directed to provide adequate added mechanical properties to the existing kind of rebars obtainable in market as thermo-mechanically treated (TMT) bars, covered under the Indian Standard specification IS: 1786-1985, that provide usual resistance to tensile forces to which a concrete structure may be subjected. The present invention rebars achieves selective earthquake resistant characteristics by way of its findings on the relevance of selective chemistry, process conditions, UTS/YS ratio and uniform elongation in imparting additional earthquake resistant properties to rebars and the like. The invention would ensure resistance of the rebars to unforeseen seismic load variations on RCC structures and thereby favour wide scale and advantageous use and application of the rebars of the invention especially in RCC structures in earthquake prone zones/areas.
BACKGROUND AND PRIOR ART
It is a well known art in the building of concrete structures, to make use of reinforcement steel bars under concrete cover to provide tensile resistance to a RCC structure subjected to various patterns of external loading, other than exclusively axially compressive nature of loading to which concrete has fairly high inherent resistance. Although the RCC construction was started about a century ago, the recent trend in construction sector with rapid and massive development in real estate and infrastructure, reinforced cement concrete structures are substituting many conventional steel structures due to simpler and faster construction with RCC, selectively with pre-fabricated modular RCC structures, wherever possible. Now, for these upcoming structures, RCC being seen as a better alternative to steel, although steel structures provide
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better protection against earthquakes. Moreover, with the increased incidences of natural calamities like earthquakes, cyclone and tsunami in the recent years, there has been more emphasis laid on earthquake proof structures, especially in quake prone areas. This has led to the requirement of reinforcing steel rebars of higher strength and elongation properties that makes RCC structures resistant to seismic vibrations and associated load variations on the structures. The need for higher strength of RCC has made way for new generation of reinforcing steel bars.
The state of the art in the segment include development of High Strength Cold Twisted Rebars and at a later stage the thermo-mechanically treated rebars covered under the Indian Standard IS: 1786-1985 which specifies three grades of rebars viz Fe-415, Fe-500 and Fe-550 where Fe stands for minimum specified yield strength in MPa. Importantly, such known standards, however, do not take care of or suggest standards, which could be specifically relevant to constitute an important criterion for earthquake resistant steels requiring enhanced capacity to absorb plastic energy, and high uniform elongation whereby higher capacity of steel to accommodate more plastic strain during an earthquake could be readily provided.
In view of the emerging threats in relation to the perceived subjection of the massive RCC structures to natural devastations especially earthquakes, an obvious need exists in the art to develop and provide steel rebars which would be earthquake resistant and capable of absorbing the stress and strains required in steel rebar to meet such adverse natural disaster conditions such as earthquakes and the like and save valuable life and infrastructure.
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OBJECTS OF THE INVENTION
It is thus the basic object of the present invention to provide for steel rebars which would be earthquake resistant and capable of absorbing the stress and strains required in steel rebar to meet such adverse natural disaster conditions such as earthquakes and the like and save valuable life and infrastructure.
Another object of the present invention to develop and produce standards for earthquake resistant variety of steel rebar having the required properties to resist adverse effect of earthquake on RCC structures.
Another object of the invention is directed to identifying the key criteria to decide on the suitability of a particular grade of steel for its resistance to earthquake, and generate high values of these parameters for enhanced capacity of the steel produced to absorb energy plastically and also elastic strain, before actual failure takes place, that truly makes it resistant to earthquake.
Yet another object of the present invention is to identify and improve upon the earthquake resistant strength characteristics and values in the steel rebar by way of selective microstructure of the rebars
involving selective chemistry and processing conditions for obtaining quality earthquake resistant steel rebars.
Another object of the present invention is directed to providing steel rebars while complying to the minimum YS criteria as per standard specifications and also achieve process enabled induction of higher uniform elongation in those grades of steel so that these can be effectively used as earthquake resistant rebars.
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SUMMARY OF THE INVENTION
Thus according to the basic aspect of the present invention there is provided high strength earthquake resistant rebars comprising
hard tempered martensite preferably atleast 8% at the surface and soft and tough ferrite-pearlite/ bainite core exhibiting
high UTS/YS ratio of 1.25 min., high uniform elongation of min. 12% , high total elongation of 20% min. and Yield Strength of Min. 415 MPa min.
According to another aspect of the present invention there is provided high strength earthquake resistant rebars comprising
hard tempered martensite preferably atleast 20% at the surface and soft and tough ferrite-pearlite/ bainite core exhibiting
high UTS/YS ratio of 1.20 min. and high uniform elongation of min. 10% , high total elongation of 18 % min. and Yield Strength of Min. 500 MPa min.
It is found by way of the present invention that the UTS/YS ratio and uniform elongation constitutes critical parameters for earthquake resistant steels. Importantly, high UTS/YS ratio is found to ensure enhanced capacity to absorb plastic energy and high uniform elongation ensured higher capacity to accommodate more plastic strain during earthquake.
In accordance with an aspect of the invention, the high strength earthquake resistant rebars comprise selectively high strength cold twisted rebars and thermo-mechanically treated rebars.
It is thus possible by way of the above disclosed steel rebars of the invention to obtain earthquake resistant rebars with desired properties by adopting selective steel composition and processing conditions. Importantly, the steel rebars of the invention are imparted with selective microstructure to archive
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the combination of the high UTS/YS ratio and high elongation and uniform elongation. Advantageously in the steel rebars of the invention, the identified criteria for the UTS/YS ratio and uniform elongation has been introduced to ensure earthquake resistant properties in the rebars. Thus the elongation level of such rebars achieved is found to be much higher than conventional rebars.
In accordance with another aspect of the present invention there is provided a process for the manufacture of high strength earthquake resistant rebars comprising:
providing steel billets with composition comprising C:0.20 wt.% max., Mn:2.00 wt. % max., Si:0.50 wt.%, S:0.040 wt. % max., P:0.040 wt.% max., microalloying elements : 0.30 wt.% max. and other alloying elements :1.5 wt.% max.;
reheating at a temperature of 1250-1300°C ;
hot rolling into bars through number of rolling stands;
subjecting the hot rolled rebars to intense water cooling to thereby provide the hard tempered martensite at the surface and the soft and tough ferrite and pearlite/ bainite core.
Importantly, in the above process of the invention the desired high UTS/YS ratio and uniform elongation is achieved by way of the selective steel composition and the processing conditions such as to generate the tempered martensite at the surface and the soft and tough ferrite and banite core. In particular, this is achieved by way of the intense water-cooling after hot rolling into bars through number of rolling stands. The intense water cooling was carried out basically through a series of water cooling pipes having controlled water header pressure and variable valve opening for different
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pipes to control the volume of water flowing through the individual cooling pipes.
Thus following the above process of the invention it is possible to obtain the much required high strength earthquake resistant rebars having hard tempered martensite at the surface and soft and tough ferrite-pearlite/ bainite core exhibiting high UTS/YS ratio of 1.25 min., high uniform elongation of min. 12%, high total elongation of 22% min. and Yield Strength of Min. 415 MPa min.
Also, the above process of the invention further enables obtaining of high strength earthquake resistant rebars having hard tempered martensite at the surface and soft and tough ferrite-pearlite/ bainite core exhibiting high UTS/YS ratio of 1.20 min. and high uniform elongation of min. 10%, high total elongation of 18 % min. and Yield Strength of Min. 500 MPa min.
The process for the manufacture of high strength earthquake resistant rebars of the invention further adds flexibility to add earthquake resistant benefit characteristics to steel rebars wherein the hot rolling and water cooling parameters are selectively determined based on the desired microstructure and the tensile properties of the earthquake resistant rebars targeted.
The high strength earthquake resistant steel rebars of the invention are intended to be used as reinforcement in RCC structures so that the structure becomes resistant to earthquake or similar other seismic load variation, which is not presently covered under any of the steel rebars presently available for similar use.
As discussed hereinbefore, the properties to ascertain the earthquake resistance of the reinforcement steel rebars are determined by using the indicators of high values of the UTS/YS ratio and the total and uniform elongation of the steel produced by applying the process.
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Also, the earthquake resisting steel rebars are characterized through optimization of chemical composition of the billet as starting material, which is subjected to selective hot rolling and controlled water cooling
According to one preferred aspect of the invention, the starting billet
materials can have a selective composition comprising
C: 0.20% max, Mn: 2.00%max, Si: 0.50%max, S: 0.040%max,
P: 0.040%max and
Micro-alloying elements: 0.30%max,
Other alloying elements: 1.5%max .
Preferably, the billets are reheated to a temperature of 1250-1300°C and are subsequently hot rolled into rebars through a number of rolling stands, the number of stands and the number of roll passes are being selectively determined to produce optimum results in terms of end properties.
The intense water cooling can be carried out through a series of water jets from coolant water pipes, to result in a specialized and favourable microstructures in the steel rebars produced, that include a hard tempered martensite at the surface of the bars and a soft and tough core of the bar composed of fine ferrite and pearlite/ bainite structures. The deciding parameters for water cooling, like the water header pressure and water cooling pipe valve opening etc. are determined for different alternative chemical compositions of the steel billets subjected to hot rolling for optimization of values for the preferred properties.
Importantly, it has also been observed by way of the present invention that the volume fraction of tempered martensite formed at the surface of the rebar after intense water cooling, has a linear relationship with the tensile properties. Accordingly, it was established by computations that a minimum of 8 and 20% tempered martensite were preferred to be produced at the
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surface of rebars to achieve desired yield strength of 415 MPa and 500 MPa respectively for the two grades Fe 415 and Fe 500. The simultaneous requirement of the desired properties essential for earthquake resistance is determined based on the UTS/YS ratio required in excess of 1.25 and 1.20 to achieve high values of compatible total and uniform elongation.
The details of the invention, its objects and advantages are explained hereunder in greater detail in relation to non-limiting exemplary illustrations as per the following Figures and Examples:
BRIEF DESCRIPTION OF FIGURES:
Figure 1: Illustrates the schematic process of intense water cooling of hot rolled rebars of the present invention.
Figure 2: Illustrates a typical section of earth quake resisting rebars of the present invention showing the distribution of hard tempered martensite at surface and soft and tough pearlite/bainite core.
EXAMPLES Example: IA
Under this Example a high strength rebar was obtained following the conventional route.
In particular a standard IS: 1786-1985 grade Fe-415 rebar was obtained following the composition and process as discussed hereunder:
For such purposes the billet composition used comprised:
C: 0.20 ; Mn: 0.72 ; Si: 0.20 , S: 0.035 P: 0.031; and Other alloying elements: Nil
The billet as above was subjected to conventional processing as detailed hereunder:
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For the purpose the above selective billet composition was reheated at a temperature of 1300 to 1320°C and subsequently hot rolled into rebars through a number of rolling stands. The hot rolled rebars are subjected to intense water cooling through a series of water cooling pipes have water header pressure of 1.2 MPa and valve opening of 100% and 100% for the two pipes only resulting in hard tempered martensite at the surface and soft and tough ferrite - pearlite/ bainite core.
Example: IB
Upder this Example a high strength rebar was obtained following the conventional route.
In particular a standard IS: 1786-1985 grade Fe-500 rebar was obtained following the composition and process as discussed hereunder:
For such purposes the billet composition used comprised:
C: 0.23 ; Mn: 0.86 ; Si: 0.23 , S: 0.035; P: 0.032; and Other alloying elements: Nil .
The billet as above was subjected to conventional processing as detailed hereunder:
For the purpose the above selective billet composition was reheated at a temperature of 1300 to 1320°C and subsequently hot rolled into rebars through a number of rolling stands. The hot rolled rebars are subjected to intense water cooling through a series of water cooling pipes have water header pressure of 1.2 MPa and valve opening of 100% ,100% and 50% for the two pipes only resulting in hard tempered martensite at the surface and soft and tough ferrite - pearlite/ bainite core.
The process of intense water cooling of the hot rolled rebars as described above involving series of water cooling pipes are illustrated in the accompanying Figure 1. The intense water cooling was carried out basically through a series of water cooling pipes having controlled water header
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pressure and variable valve opening for different pipes to control the volume of water flowing through the individual cooling pipes.
Thus following the above process hot rolling and intense water cooling, of the invention it is possible to obtain the much required high strength earthquake resistant rebars having hard tempered martensite at the surface and soft and tough ferrite-pearlite/ bainite core exhibiting high UTS/YS ratio. A typical section of earth quake resisting rebars of the present invention showing the distribution of hard tempered martensite at surface and soft and tough pearlite/bainite core has been illustrated in the accompanying Figure 2.
Example: 2A
Under this Example a high strength rebar was obtained following the selective composition and processing conditions in accordance with the present invention.
In particular a grade Fe-415 rebar was obtained following the selective composition and process in accordance with the present invention as discussed hereunder:
For such purposes the selective billet composition used comprised:
C:0.18 ; Mn: 0.83 ; Si: 0.30 , S: 0.028; P: 0.028 ; and Other alloying elements: 0.75.
The billet as above was subjected to controlled processing in accordance with the present invention as detailed hereunder:
For the purpose the above selective billet composition was reheated at a temperature of 1250 to 1300°C and subsequently hot rolled into rebars through a number of rolling stands. The hot rolled rebars are subjected to intense water cooling through a series of water cooling pipes have water header pressure of 1.0 MPa and valve opening of 50% each for the two pipes
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only resulting in hard tempered martensite at the surface and soft and tough ferrite - pearlite/ bainite core.
Example: 2B
Under this Example a high strength rebar was obtained following the selective composition and processing conditions in accordance with the present invention.
In particular a grade Fe-500 rebar was obtained following the selective composition and process in accordance with the present invention as discussed hereunder;
For such purposes the selective billet composition used comprised:
C:0.18 ; Mn: 0.83 ; Si: 0.30 , S: 0.028; P: 0.028 ; and Other alloying elements: 0.75.
The billet as above was subjected to controlled processing in accordance with the present invention as detailed hereunder:
For the purpose the above selective billet composition was reheated at a temperature of 1250 to 1300°C and subsequently hot rolled into rebars through a number of rolling stands. The hot rolled rebars are subjected to intense water cooling through a series of water cooling pipes have water header pressure of 1.0 MPa and valve opening of 50% each for the three pipes resulting in hard tempered martensite at the surface and soft and tough ferrite - pearlite/ bainite core.
The UTS and YS , Elongation % and Uniform Elongation % obtained from the above varieties of steel rebars produced under Examples Ito 3 above were noted and the results are reproduced hereunder in Table I:
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Table 1 above clearly illustrates the properties of the existing grades of high strength steel rebars (Examples 1A,1B,2A and 2B) conforming to IS: 1786-1985and IS: 1786-2005. These rebars in general have the specified minimum Yield strength of the steel rebars which is 415 MPa for Fe-415 & 415 D grades and 500 MPa for the Fe-500 & Fe-500 D grades respectively under this specification. These steels exhibit minimum UTS of 485 MPa for Fe-415 and Fe-415 D grades whereas the values of minimum UTS for Fe-500 & Fe-500 D grades are 545 MPa and 565 MPa respectively. The total elongations are 14.5% and 18.0% for the Fe-415 & 415 D grades whereas the same for
j
Fe-500 & 500 D grades are 12.0% and 16.0% respectively. Also, such conventional steel rebars are not known to involve any critical consideration
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for the UTS/YS ratio and accordingly the values for such UTS/YS under the conventional steel rebar specifications are not shown. Moreover, the values for the Ultimate Tensile Strength for these existing grades of rebars (Examples 1A, IB, 2A and 2B) are observed to be not much higher than that the respective Yield Strength values and as such having relatively lower capacity to absorb energy in the elastic range as well as in the plastic deformation zone, when loaded. As a result these grades are not considered to be capable to withstand load conditions that may result due to severe seismic vibrations.
The steel rebars obtained following the findings of the present invention on the criticality in the UTS/YS ratio based on the selective composition and the processing conditions enabled the development of an improved variety of steel rebars which exhibit higher UTS values, higher UTS/YS ratio and higher total and uniform elongation values that makes the resultant grades of reinforcing rebars resistant to earthquake or seismic load variations when used in RCC structures.
Thus the process of the invention and steel rebars obtained thereof under Examples 3A and 3B provided for a new grade of steel rebars as reflected in the characteristics achieved for such Fe-415 and Fe-500' varieties shown in Table 1 above. Importantly, such rebars of the invention involving high UTS/YS ratio of 1.25 min. and 1.20, achieve high total elongation of 20% and 18% (minimum) and high uniform elongation of 12% and 10%(minimum) as an improvement on the properties of Fe 415 and Fe 500 grades having a yield strength level of 415 MPa min. and 500 MPa min., respectively (Refer table 1) prevalent under conventional standards (Examples 1A,1B, 2A and 2B). Thus the invention achieves the higher values for the UTS as 540 MPa min. and 620 MPa min., favouring the ratio UTS/YS to be 1.25 and 1.20 minimum, for the two grades, respectively derived under the present process.
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Thus, in order to achieve these earthquake resistant properties, the Higt Strength Earthquake Resisting Rebars are produced through optimization oi chemistry of the starting billet material and the processing conditions used ir the hot rolling and controlled water-cooling processes.
Importantly, as discussed earlier, the starting billet materials should have the
selective chemistry (on weight basis) indicatively as follows:
C: 0.20% max, Mn: 2.00%max, Si: 0.50%max, S: 0.040%max,
P: 0.040%max and
Micro-alloying elements: 0.30%max,
Other alloying elements: 1.5%max .
To produce high strength earthquake resistant steel rebars by the invented process, the billets having composition conforming to preferred specified limits as given above, are reheated to a temperature of 1250-1300°C and are subsequently hot rolled into rebars through a number of rolling stands, the number of stands and the number of roll passes are being selectively determined to produce optimum results in terms of desired end properties.
The hot rolled rebars are then subjected to intense water cooling through a series of water jets from coolant water pipes, to result in a specialized and favourable microstructures in the steel rebars produced, that include a hard tempered martensite at the surface of the bars and a soft and tough core of the bar composed of fine ferrite and pearlite/ bainite structures. This preferred distribution of various microstructures in the resulting rebars induce the properties of higher UTS/YS ratio and with simultaneous higher total and uniform elongations, making them fit for use as earthquake resistant steel rebars. The deciding parameters for water cooling, like the water header pressure and water cooling pipe valve opening etc. are selectively determined based on the chemical compositions of the steel billets subjected to hot rolling for optimization of values for the preferred properties.
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The present invention is thus directed to selectively specify the parameters for the hot rolling and water cooling operations for the invented process determined based on the desired microstructure of the rebars and their co-relation with tensile properties. It has been observed that the volume fraction of tempered martensite formed at the surface of the rebars after intense water-cooling, has a linear relationship with the tensile properties. Accordingly, it was established by computations that a minimum of 8 and 20% tempered martensite were required to be produced at the surface of rebars to achieve desired yield strength of 415 MPa and 500 MPa respectively for the two improvised variants of grades Fe 415 arid Fe 500, as per Examples 3A and 3B. The simultaneous requirement of the desired properties essential for earthquake resistance was achieved by way of attaining the UTS/YS ratio well in excess of 1.25 and 1.20, respectively and compatible with high values of total elongations (20% and 18%) and uniform elongations (12 and 10%) as demonstrated under Table 1 above.
Thus the present invention would serve the much required need to provide for steel rebars which would be earthquake resistant and capable of absorbing the stress and strains required in steel rebar to meet such adverse natural disaster conditions such as earthquakes and the like and save valuable life and infrastructure.
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WE CLAIM:
1. High strength earthquake resistant rebars comprising
hard tempered martensite preferably atleast 8% ;at the surface and soft and tough ferrite-pearlite/ bainjte core exhiDiting high UTS/YS ratio of 1.25 min., high uniform elongation of min. 12%, high total elongation of 20% min. and Yield Strength of Min. 415 MPa min.
2. High strength earthquake resistant rebars comprising
hard tempered martensite preferably atleast 20% at the surface and soft and tough ferrite-pearlite/ bainite core exhibiting high UTS/YS ratio of 1.20 min. and high uniform elongation of min. 10% , high total elongation of 18 % min. and Yield Strength of Min. 500 MPa min.
3. High strength earthquake resistant rebars as claimed in anyone of
claims 1 or 2 comprising thermo-mechanically treated rebars.
4. A process for the manufacture of high strength earthquake resistant
rebars as claimed in anyone of claims 1 to 3 comprising:
providing steel billets with composition comprising C:0.20 wt.% max., Mn:2.00 wt. % max., Si:0.50 wt.%, S:0.040 wt! % max., P:0.040 wt.% max., microalloying elements : 0.30 wt.% max. and other alloying elements :1.5 wt.% max.;
reheating at a temperature of 1250-1300°C ;
hot rolling into bars through number of rolling stands;
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subjecting the hot rolled rebars to intense water cooling to thereby provide the hard tempered martensite at the surface and the soft and tough ferrite and pearlite/ bainite core.
5. A process for the manufacture of high strength earthquake resistant
rebars as claimed in claim 4 wherein the said step of intense water
cooling comprises passing of hot rolled rebars through a series of
water cooling pipes wherein cooled water flows at a high pressure.
6. A process for the manufacture of high strength earthquake resistant
rebars as claimed in anyone of claims'4 or 5 wherein said hot rolling
into bars through number of rolling stands comprises roughing,
intermediate and finish rolling wherein the rectangular billet is
deformed to achieve final desired shape and size.
7. A process for the manufacture of high strength earthquake resistant
rebars as claimed in anyone of claims 4 or 5 carried out such as to
obtain the rebars having hard tempered martensite at the surface and
soft and tough ferrite-pearlite/ bainite core exhibiting high UTS/YS
ratio of 1.25 min., high uniform elongation of min. 12% , high total
elongation of 20% min. and Yield Strength of Min. 415 MPa min.
8. A process for the manufacture of high strength earthquake resistant
rebars as claimed in anyone of claims 4 or 5 carried out such as to
obtain the rebars having hard tempered martensite at the surface and
soft and tough ferrite-pearlite/ bainite core exhibiting high UTS/YS
ratio of 1.20 min. and high uniform elongation of min. 10% , high
total elongation of 18 % min. and Yield Strength of Min. 500 MPa
min.
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9. A process for the manufacture of high strength earthquake resistant
rebars as claimed in anyone of claims 4 to 8 wherein the hot rolling
and water cooling parameters are selectively determined based on the
desired microstructure and the tensile properties of the earthquake
resistant rebars targeted.
10. High strength earthquake resistant rebars and its process of
manufacture substantially as herein described and illustrated with
reference to the accompanying examples.