Field of Invention
The present invention relates to a thermo-mechanically treated steel rebar for seismic resistant applications. More specifically it is related to a boron micro-alloyed steel rebar.
Background of Invention
Steel rebars form the backbone of construction segment, and must possess suitable mechanical properties so that they can exhibit high tolerances to various damaging forces acting on the building members such as foundations, beams, columns and slabs. These damaging forces could be under various loads like the dead load of the building, live load of the occupants and their belongings, wind/storm load or earthquake loads which act in different ways at different times. There are several regions in the world which lie in the seismic zones IV and V and have to combat the odds of earthquakes. With the globe undergoing a series of climatic changes, strong and damage resistant rebars are a call of the day, especially for construction purposes in these high earthquake prone zones.
There are certain primary criteria that steel rebars should fulfill to serve effectively for earthquake resistant applications. First of all, the rebars should have adequate yield strength so that it can withstand a large amount of load without permanent deformation. Secondly, these rebars should have a high UTS/YS ratio so that they can absorb a large amount of energy in the plastic region and hence able to handle the large pressures of an earthquake before it enters into the zone of plastic instability. Thirdly, they must have good ductility (elongation values) and toughness so that effective deformation takes place before failure.
Thermo-mechanically treated (TMT) rebars are developed through the conventional "Tempcore" process where steel billets of suitable composition are hot rolled from the austenitisation temperature and subjected to heat treatment in three successive stages - quenching, self-tempering and atmospheric cooling. The rebars have a hard tempered martensitic rim, which majorly contributes to

the strength, with a softer ferrite-pearlite core contributing to the ductility of the rebars. Rebars developed through this technique exhibit UTS/YS ratio of 1.1 -1.22 which is not satisfactorily suited for the earthquake resistant applications especially in seismic zones IV and V.
There have been recent developments of rebars produced by air cooling route. In this process, small amount of alloying elements like vanadium is added, which promotes strength by precipitation hardening and grain refinement. Micro alloyed rebars are air cooled after hot rolling in the austenitic zone and have a ferrite-pearlite structure with fine precipitates of vanadium nitrides and carbo-nitrides. Unlike the TMT rebars, micro alloyed rebars have a homogeneous cross section in terms of microstructure, strength and ductility. The micro-alloyed rebars exhibit a high strength ratio of UTS/YS > 1.25 and have good resistance to corrosion.
However, certain hurdles are encountered in this process. First of all, in micro-alloyed rebars, strength is produced by precipitation of fine vanadium nitrides and carbo-nitrides. Thus, nitrogen content of steel becomes a pre-requisite. Due to the practical constraints of the existing LD technology, it becomes difficult to produce high nitrogen heats. Secondly, it increases the cost sharply as vanadium is extremely costly alloying element. Thirdly, since the rebars are produced by air cooling route, there is some difficulty in handling these rebars on the cooling bed immediately after they are hot rolled.
There is a huge demand for rebar in the construction sector which provides suitable mechanical properties for earthquake resistance applications. This requirement is even more imminent considering that all of North America, Japan, China, parts of Australia and even 55% of the land area in India fall under the earthquake prone zones. The microalloyed rebar serve the purpose. However they are costly owing to vanadium micro-alloying and pose many operational problems during production.
Indian Patent with application number 1124/KOL/2006- discloses a method of producing 500 MPa strength variants of rebars with minimum UTS/YS ratio of 1-20, total elongation 18-20% and uniform elongation 10-12%. Indian Patent with

patent number 245043 discloses a method of manufacturing 8-40 mm diameter copper-phosphorus bearing corrosion and earthquake resistant TMT rebar of minimum yield strength 415/500 MPa with adequate tensile to yield ratio and charphy impact toughness.
However, the current scenario demands a rebar with minimum UTS/YS ratio of 1.25 in 500 MPa strength variant.
Objects of the Invention
With reference to the above mentioned prior art, there is a need to develop a new cost effective alternative to the existing microalloyed grades of rebar. Another objective is the improvisations in the existing technology to develop rebar which would have superior mechanical properties of yield strength higher than 500 MPa, UTS/ YS ratio must be higher than 1.25 with uniform elongation > 8% and total elongation £ 20% which should be applicable in earthquake resistance constructions.
SUMMARY OF THE INVENTION
In accordance with various embodiments of the invention a process for making steel rebar for earthquake resistance applications comprises steps of casting billet / bloom, the billet / bloom having chemical composition of 0.05 - 0.3 wt.% carbon, 0.4 - 1.65 wt.% manganese, 0.1 - 0.8 wt.% silicon, up to 0.05 wt.% niobium, up to 0.05 wt.% vanadium, up to 0.05 wt.% titanium, up to 0.05 wt.% aluminum, up to 0.04 wt.% Sulphur and phosphorous, up to 0.012 wt.% of nitrogen and 0.0005 to 0.01 wt.% boron, the balance being iron and impurities, and thermo-mechanical rolling of the billet / bloom into rebar with FRT of 980-1100 deg. C and equalization temperature of 600-680 deg. C for 6-12mm diameter rebar and 540-640 deg. C for 16 - 40 mm diameter.
In another embodiment, the invention provides a steel rebar having chemical composition: 0.05 - 0.3 wt.% carbon; 0.4 - 1.65 wt.% manganese; 0.1 - 0.8 wt.% silicon; upto 0.05 wt.% niobium; upto 0.05 wt.% vanadium; upto 0.05 wt.% titanium; upto 0.05 wt.% aluminum; up to 0.04 wt.% Sulphur and

phosphorous; up to 0.012 wt.% of nitrogen; 0.0005 - 0.01 wt.% boron, and the balance being iron and impurities.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING:
FIG. 1 shows a process flow diagram for manufacturing of a rebar.
FIGS. 2a and 2b show the average thickness of a rim of rebar of diameter 12mm and 16mm respectively.
FIGS. 3a and 3b shows cooling curves adopted for 12 mm and 16 mm diameter rebar respectively.
FIG. 4 shows microstructure of core and rim of rebar.
FIG. 5 shows the comparison of hardness profile across diameter for conventional rebar and rebar in accordance with an embodiment of the invention.
Detailed Description of Invention
Various embodiments of the invention provide process for making steel rebar. The process comprises steps of casting billet / bloom having a chemical composition of 0.05 - 0.3 wt.% carbon, 0.4 - 1.65 wt.% manganese, 0.1 - 0.8 wt.% silicon, up to 0.05 wt.% niobium, up to 0.05 wt.% vanadium, up to 0.05 wt.% titanium, up to 0.05 wt.% aluminum, up to 0.04 wt.% Sulphur and phosphorous, up to 0.012 wt.% of nitrogen and 0.0005 to 0.01 wt.% boron, the balance being iron and impurities and thermo-mechanical rolling of the billet / bloom into rebar with FRT of 980-1100 deg. C and equalization temperature of 600-680 deg. C for 6-12mm diameter rebar and 540-640 deg. C for 16 - 40 mm diameter.
TMT process produces rebar through the conventional "Temp-core" route. The rebar have a tempered martensitic rim and a ferrite pearlite core. This invention is triggered from a simplistic idea of relative importance of tempered martensite rim and ferrite pearlite core. Yield strength (YS) of rebar depends more on strength of the rim, as the hard phase constraints the deformation of softer core. There is no plastic deformation in core till yield point, whereas at UTS, plastic deformation spreads over the entire cross-section and is more sensitive to core mechanical

properties. It is understood through the research that rim of the TMT bar mainly affects the yield strength (YS) and core affects the ultimate tensile strength and provides the ductility. Hence certain modifications could be brought about in the chemistry and the process so that the core of the rebar can achieve a higher hardness while the hardness of the rim remains the same to achieve the yield strength. The core of the TMT rebar with higher hardness values will increase the UTS above the current values which shoots up the UTS/YS ratio.
The hardness of the core can be increased by addition of Boron in steel in small amount, which forms harder phases like acicular ferrite, bainite and fine pearlite at the core. Boron increases the hardenability by segregating to the austenite grain boundaries during cooling after austenitisation and reduce the grain boundary energy. Grain boundary is a high energy region since it can be considered to be an array of dislocations and have a large elastic strain field associated with it. As a result they act as a heterogeneous nucleation sites for ferrite / pearlite in hypo-eutectoid steels. When free boron segregates to the grain boundary, being an interstitial element they interact with the lattice defects, get entrapped by the dislocations and thus reduce the strain energy making it less favorable for the heterogeneous nucleation of pro-eutectoid ferrite. Retardation of ferrite nucleation due to boron addition, delays the time taken for austenite to transform into ferrite and therefore, pearlite formation will also be delayed with the concurrent effect that pearlitic bay will also be shifted to the right, along with the ferritic C-curve (i.e. diffusive bay of the TTT diagram), which results in increased chance of bainite formation.
However, the above mentioned mechanism requires boron to be free in solute form so that they are available to segregate to the grain boundary. The hardenability effect of boron is shadowed by the formation of boron nitride and Fe23(CB)6 which do not leave free boron for segregation. Hence, Titanium is added in stoichiometric amount to fix up the nitrogen to prevent the formation of boron nitride. Borocarbides form at a very slow cooling rate and in the presence of higher concentrations of carbon and boron. They can be avoided to form by

tying up carbon with suitable carbide formers like niobium or by employing a faster cooling rate.
With reference to above mentioned idea of core affecting the ultimate tensile strength, steps of process (100) have been formulated for manufacturing of rebar having diameter 6-12mm and 16-40mm. These rebars should have yield strength higher than 500 MPa, UTS/YS ratio higher than 1.25 with uniform elongation > 8% and total elongation > 20% as shown in FIG. 1.
At step (102) steel billets / bloom which can be cast either from LD or Electric Arc Furnace (EAF) having the chemical composition as shown in Table 1.


Carbon (0.05% - 0.3%) is one of the most effective and economical strengthening elements. Carbon is generally found in two forms, either it is in solid solution in austenite / ferrite phase or it is found as carbide. The carbide form can be in iron carbide (Fe3C, known as cementite), or it can be a carbide of an alloying element such as titanium, niobium and vanadium. Carbon combines with Nb to form carbides or carbonitrides which bring about precipitation strengthening. Carbon increases the chance of formation of second phase.
Manganese (0.40% -1.65%) not only imparts solid solution strengthening to the ferrite but it also lowers the austenite to ferrite transformation temperature thereby refining the ferrite grain size. Mn fixes sulfur in the steel as manganese sulfide (MnS) to prevent hot embrittlement. However, the Mn level cannot be increased to beyond 1.65% as at such high levels it enhances centerline segregation during continuous casting.
Sulphur (0.04% max.) content has to be limited otherwise it results in a very high inclusion level that deteriorates formability.
Phosphorus (0.040% max.) is restricted to 0.04% maximum as higher phosphorus levels can lead to reduction in toughness and weldability due to segregation of P into grain boundaries.
Silicon (0.1 - 0.8%) like manganese is a very efficient solid solution strengthening element. Silicon contributes to hardening of the ferritic phase. However, additions of Si should be restricted to maintain Mn/Si >3.5 for better castability of steel in continuous billet caster.
Niobium (0.05% max) is the most potent micro alloying element for grain refinement even when it is added in very small amounts by increasing the no-recrystallization temperature or Tnr. In solid solution, it lowers the austenite to ferrite transformation temperature which not only refines the ferrite grain size but also promotes the formation of lower transformation products like bainite. Niobium added in synergy with boron helps in increasing hardenability manifolds by reacting with carbon to forming Nb(CN) which allows boron to be in free form and not form detrimental Fe23CB6.

Boron (0.0005 - 0.01%) is most important hardenability enhancing element that improves deformability and machinability. Boron is added to fully killed steel and only needs to be added in very small quantities to have a hardening affect. Additions of boron are most effective in low carbon steels. Boron also helps in slip transfer across grain boundaries which in turn improve ductility. Boron added in steel needs to be in uncombined (free) form to depict its hardening effect. The action of boron arises from its ability to diffuse to austenite grain boundaries and reduce the possibility of ferrite nucleation by reducing the interracial energy of the parent austenite grains. Boron addition for optimum hardening effect should be restricted to 10 to 30 ppm. Beyond this limit formation of Fe23(CB)6 accelerates and efficacy of hardening by boron deteriorates.
Aluminium (0.05% max) is used as a deoxidizer and killing of steel. It limits growth of austenite grains.
Titanium (0.05% max) improves strength by limiting austenite grain size. Titanium forms carbides and nitrides which are stable at high temperature. Addition of titanium to steel helps in fixing nitrogen allowing boron to be free to show its hardenability effect. Titanium inhibits VN or Nb(CN) formation and avoids cracking at bends during continuous casting.
Vanadium (0.05% max) like titanium and niobium can produce stable carbides that increase strength at high temperatures by formation of precipitates of carbides and nitrides. By promoting a fine grain structure, ductility and toughness is enhanced. High solubility of V (C, N) allows V to be in solution at normal reheating temps thus energy friendly rolling process.
Nitrogen (0.012% max), reducing nitrogen levels positively affects ageing stability and toughness in the heat-affected zone of the weld seam, as well as resistance to inter-crystalline stress-corrosion cracking. Increasing N content also raises the dissolution temperature of Nb(CN) and hence reduces the effectiveness of Nb. Thus Nb levels should be preferably below 0.05%.
The preferable composition of the steel rebar in accordance with an embodiment of the present invention is shown in Table 2.


After the casting the billet / bloom, the billet / bloom is reheated and thermo-mechanically rolled into rebar with Finished Rolling Temperature (FRT) of 980-1100 deg. Cat step (104).
Between the steps (102 and 104), the billet / bloom is reheated in the preheat zone maintaining a temperature of 950 to 1050°C at step (102a). The billet is again heated in the heating zone maintaining a temperature of 1050 to 1150°C at step (102b). Finally the billet is reheated in a soaking zone at step (102c) maintained at temperature 1200 to 1250°C.

The entire reheating of the billet / bloom (at preheating zone, heating zone and the soaking zone) is done in a reheating furnace.
The entire reheating time for billet (at preheating zone, heating zone and the soaking zone) is maintained between 1.5 to 3 hours. The temperature of reheating is calculated from the amount of micro-alloying added in the steel.
At step (106) the hot rolled billet / bloom is short intensively cooled. This short intensive cooling is done by means of water as they pass through cooling system after last rolling stand. Water pressure in the cooling system is preferably maintained between 5 to 25 kg/cm2.
The equalization temperature for 6-12 mm diameter rebar is maintained at 600-680 deg. C and for 16-40 mm diameter rebar is 540-640 deg. C. The equalization temperature should lie in between BS(T) +70 and Bs -70 (T) wherein BS(T) stands for Bainite Start temperature.
The reheated billets are rolled to TMT rebar finishing at a rolling speed of between 3 to 55 m/sec. The thermo-mechanical rolling of billets in the invention is done by using 15 to 30 rolling stands comprising of roughing, intermediate and finishing stands.
The reduction in temperature obtained at step (106) while cooling the TMT rebar converts the surface region of the rebar to a hardened structure (martensite).
At step (108) the TMT rebar is subsequently air cooled. Air cooling is done so that the temperature between core which remains hot and the cooled surface region (rim) is equalized and rim later gets tempered by the heat from the core.
The resulting structure of the rebar is a self-tempered martensite zone at the rim and a mixture of acicular ferrite, bainite and fine pearlite at the core with phase fractions of 10%, 25% and 65% respectively.
The average thickness of a rim (10) is 10±2% of the diameter of a rebar (14) of 12mm and 16mm respectively as shown in FIGS. 2a and 2b. The average thickness of the rim is 1.10mm (9.16%) of 12mm rebar and 1.4mm (8.75%) of the 16mm rebar. Microstructural examination of a core (18) also confirms the presence of hard phases like acicular ferrite, bainite and fine pearlite as

compared to ferrite-pearlite core in conventional TMT rebar. Rim (10) is the peripheral area of the rebar (14).
The cooling curves adopted for 12 mm and 16 mm diameter rebar is shown in FIGS. 3a and 3b respectively. The microstructure of 12 mm and 16 mm rebar contains a 100% tempered martensitic rim with rim thickness of 10+2% of the diameter of the rebar. Microstructure of core and rim of 12 mm and 16 mm rebar are shown in FIG. 4 respectively.
A steel rebar is obtained from above mentioned process having chemical composition 0.05 - 0.3 wt.% carbon; 0.4 - 1.65 wt.% manganese; 0.1 - 0.8 wt.% silicon; upto 0.05 wt.% niobium; upto 0.05 wt.% vanadium; upto 0.05 wt.% titanium; upto 0.05 wt.% aluminum; up to 0.04 wt.% Sulphur and phosphorous; up to 0.012 wt.% of nitrogen; 0.0005 - 0.01 wt.% boron, and the balance being iron and impurities.
Advantages:
The invention provides a cost effective alternative to the existing microalloyed grades of rebar. Further it also develops rebar which have superior mechanical properties of yield strength higher than 500 MPa, UTS/ YS ratio must be higher than 1.25 with uniform elongation > 8% and total elongation > 20%.
Tensile tests performed as per the standards on the rebar obtained from the invention are all found to have yield strength higher than 500 MPa, UTS/YS ratio higher than 1.25 with uniform elongation > 8% and total elongation > 20% as shown in Table 3. A case to core hardness profile displays a considerable rise in the core hardness of rebar of the invention as compared to the core of conventional TMT rebar which further confirms the presence of hard phases.


The above mentioned process for making rebar having minimum tensile strength of 500 MPa and UTS/YS > 1.25 and a total elongation of > 20% and uniform elongation > 8% can also be validated by the following examples. The following examples should not be construed to limit the scope of invention.

One heat of 150 tons of the composition according to the invention is continuously casted in LD-billet caster. The heat is rolled into rebar of 12 mm and 16 mm sections. However the water-box cooling for both sections were different. For rebar of 12 mm section, shallow water-box cooling is done with equalization temperature of 610-630°C, whereas for rebar of 16 mm a steeper water-box cooling with lower equalization temperature of 590-630°C is used. Core shows acicular ferrite, bainite with fine pearlite whereas rim shows tempered martensite structure.
FIG. 5 shows the comparison of hardness profile across diameter for conventional rebar and complex microstructure rebar of the invention. Case to core hardness profile comparison between normal TMT and complex microstructure earthquake resistant rebar of the invention shows considerable rise in core hardness. It can be concluded that the hardness of core for complex microstructure rebar is greater than conventional TMT rebar. However, the rim hardness remains almost the same. Thus, the hardness profile confirms that improvement in core hardness improves the UTS more and consequently UTS/YS ratio. It is clear from the mechanical properties and the microstructures achieved, that the target properties are achieved for both 12 mm and 16 mm rebar.

We claim:
1) A process for manufacturing steel rebar, the process comprising
steps of:
casting billet / bloom, the billet / bloom having chemical composition of 0.05 - 0.3 wt.% carbon, 0.4 - 1.65 wt.% manganese, 0.1 - 0.8 wt.% silicon, upto 0.05 wt.% niobium, upto 0.05 wt.% vanadium, upto 0.05 wt.% titanium, upto 0.05 wt.% aluminum, up to 0.04 wt.% Sulphur and phosphorous, up to 0.012 wt.% of nitrogen and 0.0005 - 0.01 wt.% boron, the balance being iron and impurities; and
thermo-mechanical rolling of the billet / bloom into rebar with FRT of 980-1100 deg. C. and equalization temperature of 600-680 deg. C for 6-12mm diameter rebar and 540-640 deg. C for 16 -40 mm diameter rebar.
2) The process as claimed in claim 1, wherein the billet / bloom is reheated in a preheating zone before the billet / bloom is being thermo-mechanically rolled, the preheating zone being maintained at temperature 950 to 1050°.
3) The process as claimed in claim 2, wherein the billet / bloom is reheated in a heating zone after reheating the billet / bloom in the preheating zone, the heating zone being maintained at temperature 1050 to 1150°C.
4) The process as claimed in claim 3, wherein the billet / bloom is reheated in a soaking zone after reheating the billet / bloom in the heating zone, the soaking zone being maintained at temperature 1200 to 1250°.

5) The process as claimed in claims 2, 3 and 4, wherein the reheating of the billet / bloom at the preheating zone, heating zone and the soaking zone is done in a reheating furnace.
6) The process as claimed in claim 2, 3 and 4, wherein the reheating of the billet / bloom at the preheating zone, heating zone and the soaking zone is done between 1.5 to 3 hours.
7) The process as claimed in claim 1, wherein the billet / bloom after being thermo-mechanically rolled into rebar is short intensively cooled by means of water.
8) The process as claimed in claim 7, wherein the water pressure for cooling is 5kg/cm2 - 25 kg/cm2.
9) The process as claimed in claim 7, wherein after short intensively cooling of the rebar, the rebar is air cooled.

10) The process as claimed in claim 1, wherein the thermo-mechanical rolling of the billet / bloom is done at rolling speed at the delivery stand of 3 - 55 m/sec.
11) The process as claimed in claim 1, wherein the thermo-mechanical rolling of the billet / bloom is done by using 15 to 30 rolling stands.
12) The process as claimed in claims 1-11, wherein the steel rebar have yield strength ≥ 500 MPa.
13) The process as claimed in claim 1-11, wherein the steel rebar have UTS/YS ≥ 1.25.
14) The process as claimed in claim 1-11, wherein the steel rebar have a total elongation of ≥ 20%.

15) The process as claimed in claim 1-11, wherein the steel
rebar have a uniform elongation > 8%.
16) A steel rebar having chemical composition:
0.05 - 0.3 wt.% carbon;
0.4 -1.65 wt.% manganese;
0.1-0.8 wt.% silicon;
upto 0.05 wt.% niobium;
upto 0.05 wt.% vanadium;
upto 0.05 wt.% titanium;
upto 0.05 wt.% aluminum;
up to 0.04 wt.% Sulphur and phosphorous;
up to 0.012 wt.% of nitrogen;
0.0005 - 0.01 wt.% boron, and
the balance being iron and impurities.