Claims:CLAIMS

1. A process for the manufacturing high strength Seismic Resistant Thermo Mechanically Treated (TMT) bar of 32mm diameter, which comprises the following steps:

(i) melting a steel composition (by weight %) containing: C-0.25-0.26 : Mn-1.56 –

1.58 : S-0.024 – 0.026 : P- 0.033 – 0.036 : Si-0.58-0.064 : Cr-0.20 max and the balance being Fe in Twin Hearth Furnace/ Basic Oxygen Furnace (BOF);

(ii) tapping the melt at 1645-1655°C;

(iii) homogenizing the steel in the ladle by purging of argon;

(iv) casting the homogenized steel either into ingots or as billet by continuous casting;

(v) soaking the ingots at 1320oC max.;

(vi) processing the soaked ingots into billets of 105x105mm size;

(vii) the billets are reheated at temperature of' 1180 +10oC and processed through a known TMT line;

(viii) hot rolling heated billet to rebar in a merchant mill; (ix) accelerated cooling the rolled rebar; and
(x) further cooling in atmosphere.

2. A process for the manufacturing high strength Seismic Resistant Thermo Mechanically Treated (TMT) bar as claimed in claim 1, wherein the process further comprises of the step in which finishing rolling temperature in the Thermo Mechanically Treated (TMT) line is carried out at temperature between 1095-
1125°C.

3. A process as claimed in claim 1, wherein the hot rolled rebar emerging from finishing stand is cooled by the water pressure 9.8 - 10.3 kg/cm2.

4. A process as claimed in preceding claims 1 to 3, wherein the cooling water flow is maintained at 695 – 704 Nm3/hr to attain a cooling rate of 20oC/s.

5. A process as claimed in preceding claims 1 to 4, wherein further the equalization temperature is varied between 530 - 570oC for 32mm rod products.

6. A process as claimed in claim 1 to 6, wherein a thin layer of martensite up to

2.7 mm thick forms on the outer surface.

7. A process as claimed in claim 1 to 7, wherein the core is changed to predominantly acicular ferrite/ bainite along with pearlite.

8. A process as claimed in claim 1 to 7, wherein the TMT bar has yield strength of minimum of 500 MPa with UTS/YS ratio of > 1.25.

9. A process as claimed in any of the preceding claims wherein, no micro-alloying element is added in the alloy mix.

Dated: this 02nd day of April, 2016.

(N. K. Gupta) Patent Agent,
Of NICHE,
For SAIL.

To,
The Controller of Patents,
The Patent Office, Kolkata.
, Description:

FORM 2

THE PATENTS ACT, 1970

(39 of 1970)
COMPLETE SPECIFICATION (Section 10 and rule 13)

TITLE

PROCESS FOR PRODUCTION OF SEISMIC RESISTANT HIGH STRENGTH FE500 S GRADE THERMO MECHANICALLY TREATED (TMT) BAR

APPLICANT

STEEL AUTHORITY OF INDIA LIMITED, A GOVT. OF INDIA ENTERPRISE,
RESEARCH & DEVELOPMENT CENTRE FOR IRON & STEEL, DORANDA, RANCHI-834002, STATE OF JHARKHAND

The following specification particularly describes the nature of the invention and the manner in which it is to be performed

PROCESS FOR PRODUCTION OF SEISMIC RESISTANT HIGH STRENGTH FE500 S GRADE THERMO MECHANICALLY TREATED (TMT) BAR

FIELD OF THE INVENTION

The present invention relates to a process of production of seismic resistant high strength Fe500 S grade Thermo Mechanically Treated (TMT) bar, particularly for reinforcements for concrete or welded mesh, deformed by hot rolling and accelerated cooling. More particularly, the present invention is directed to developing TMT bars having higher production of high strength (YS: 500 MPa minimum) TMT bar in 28 – 32 mm bars.

BACKGROUND OF THE INVENTION Processing of TMT bars
Thermo mechanical processing, also known as thermo-mechanical treatment (TMT), is a metallurgical process that integrates work hardening and heat- treatment into a single process. A description of its application in rebar steel follows.

The quenching process produces a high strength bar from inexpensive low carbon steel. The process quenches the surface layer of the bar, which pressurizes and deforms the crystal structure of intermediate layers, and simultaneously begins to temper the quenched layers using the heat from the bar's core (Fig. 1)

Steel billets 125mm² ("pencil ingots") are heated to approximately 1100°C in a reheat furnace. Then, they are progressively rolled to reduce the billets to the final size and shape of reinforcing bar. After the last rolling stand, the billet moves through a quench box. The quenching converts the billet's surface layer to martensite, and causes it to shrink. The shrinkage pressurizes the core, helping to form the correct crystal structures. The core remains hot, and austenitic. A microprocessor controls the water flow to the quench box, to manage the temperature difference through the cross-section of the bars. The correct temperature difference assures that all processes occur, and bars have the

necessary mechanical properties. Fig. 2 shows the schematic layout of the merchant mill at Bhilai Steel Plant.

The bar leaves the quench box with a temperature gradient through its cross section. As the bar cools, heat flows from the bar's centre to its surface so that the bar's heat and pressure correctly tempers an intermediate ring of martensite and bainite. The equalization temperature is defined as the temperature at which the surface and core temperatures are equal (Fig. 3).

Finally, the slow cooling after quenching automatically tempers the austenitic core to ferrite and pearlite on the cooling bed.

These bars therefore exhibit a variation in microstructure in their cross section, having strong, tough, tempered martensite in the surface layer of the bar, an intermediate layer of martensite and bainite, and a refined, tough and ductile ferrite and pearlite core.

When the cut ends of TMT bars are etched in Nital (a mixture of nitric acid and methanol), three distinct rings appear: 1. A tempered outer ring of martensite, 2. A semi-tempered middle ring of martensite and bainite, and 3. a mild circular core of bainite, ferrite and pearlite. This is the desired micro structure for quality construction rebar.

In contrast, lower grades of rebar are twisted when cold, work hardening them to increase their strength. However, after thermo mechanical treatment (TMT), bars do not need more work hardening. As there is no twisting during TMT, no torsional stress occurs, and so torsional stress cannot form surface defects in TMT bars. Therefore TMT bars resist corrosion better than cold, twisted and deformed (CTD) bars.

OBJECTS OF THE INVENTION

The main objective of the invention is to produce a seismic resistant carbon – manganese bearing high strength and economical cost effective Thermo Mechanically Treated (TMT) rebar / rod of diameter 28-32 mm having minimum yield stress 500 MPa with adequate tensile to UTS/YS ratio of 1.25 by thermo mechanical treatment.

Another object of the invention is lo produce Thermo Mechanically Treated (TMT)

rebar / rod with adequate tensile to yield stress ratio.

Another object of the invention is to produce Thermo Mechanically Treated (TMT)

rebar / rod with good elongation.

Another object of the invention is to produce 'Thermo Mechanically Treated (TMT)

rebar / rod with good bend and re-bend properties.

A further object of the invention is to produce Thermo Mechanically Treated (TMT)

rebar / rod with good weldability.

Another object of the invention is lo produce Thermo Mechanically Treated (TMT)

rebar / rod with good corrosion resistance.

These and other objects of the invention will be clear from the following paragraphs.

SUMMARY OF THE INVENTION

Development of TMT rebar with minimum guaranteed YS of 500 MPa and UTS/YS

ratio of 1.25

TMT rebar with YS of 500 MPa minimum and UTS/YS ratio >1.25 ensures safe mode of failure in the unfortunate event of an earthquake. High UTS/YS ratio improves the capability to absorb more plastic energy. A high ratio of Ultimate Tensile Strength to Yield Strength confers the plastic energy absorption capacity to the rebar and hence the rebar even after yielding can absorb a greater amount of energy before fracture ( Figs.4 &5). The use of these rebar in the construction of building and structures, therefore, make these structures more resistant to instantaneous collapse in the unfortunate event of earthquakes. This delays the eventual collapse of the structures and gives sufficient time for the safe evacuation of these buildings.

There is a large demand of Earthquake Resistant (EQR) TMT bars from the market. These steels find a wide range of applications in the construction sector. In view of the stringent requirement for seismic resistant steel bars, it is necessary to produce these grades of steel with the enhanced performance criteria. Bhilai Steel

Plant (BSP) has the necessary facilities and infrastructure to produce and supply these grades of steel. RDCIS in collaboration with BSP has developed TMT Fe
500S grade of TMT rebar in 32 mm diameter by optimising the chemistry and processing parameters.

Therefore such as herein described, there is provided a process for the manufacturing high strength Seismic Resistant Thermo Mechanically Treated (TMT) bar of 32mm diameter, which comprises the following steps: (i) melting a steel composition (by weight %) containing: C-0.25-0.26 : Mn-1.56 – 1.58 : S-0.024
– 0.026 : P- 0.033 – 0.036 : Si-0.58-0.064 : Cr-0.20 max and the balance being Fe in Twin Hearth Furnace/ Basic Oxygen Furnace (BOF); (ii) tapping the melt in a preheated ladle at 1200°'C; (iii) homogenizing the steel in the ladle by purging of argon; (iv) casting the homogenized steel either into ingots or as billet by continuous casting; (v) soaking the ingots at 1040 – 1050oC; (vi) processing the soaked ingots into billets of 100x100mm size; (vii) the billets are reheated at
temperature of' 530 - 570oC and processed through a known TMT line; (viii) hot rolling heated billet to rebar in a merchant mill; (ix) accelerated cooling the rolled rebar; and (x) further cooling in atmosphere.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Fig. 1 illustrates a schematic diagram of the on line quenching and self-tempering process in general;

Fig. 2 illustrates schematic layout of merchant mill, Bhilai Steel Plant in general;

Fig. 3 illustrates the Temperature gradient in the bars during and after quenching in general;

Fig. 4 illustrates the Stress strain diagram showing the various stages of loading in accordance with the present invention;

Fig. 5 illustrates the Strength elongation diagram showing the effect of UTS/YS ratio on the plastic energy absorption capacity in accordance with the present invention;

Fig. 6 illustrates typical microphotographs of sample from the core of 32 mm diameter TMT Fe 500S rebar, showing ferrite pearlite structure in accordance with the present invention;

Fig. 7 illustrates typical microphotograph of sample from the interface of 32 mm diameter TMT Fe 500S rebar, showing transition from ferrite pearlite structure to tempered martensite in accordance with the present invention;

Fig. 8 illustrates typical microphotograph of sample from the rim of 32 mm diameter TMT Fe 500S rebar showing tempered martensite in accordance with the present invention.

DETAILED DESCRIPTION

As herein disclosed, the present invention relates to a Seismic Resistant ribbed Thermo Mechanically Treated (TMT) bar with high strength and for improved higher adherence with surrounding cement or grout material. The high bond strength TMT bar according to the invention ensures high composite action ensure favouring wide scale application such as reinforcement bar in RCC structures or as rock bolts for effective roof support in underground mines or tunnel construction. The invention provides a thermo mechanical process for producing low cost high strength reinforcement bar/ rod which comprises melting the steel in a furnace and tapping the same in a preheated ladle at around 1050°C. The Alloy was designed with C – Mn chemistry for the developmental heats. No micro-alloying element was added to keep the cost of production competitive. However, manganese content of the steel was kept moderately on lower side (0.05). Heat (liquid steel) was made through Basic Oxygen Furnace with the following Chemical composition and
100x100mm billets were produced out of this steel through Continuous casting process route at Durgapur Steel Plant. The chemical composition of the disclosed low cost high strength rebar/ rod is shown in table 1 and balance essentially Iron.

Table 1 Chemical Composition (wt. %) of the heat with optimized chemistry

Bath/Product C Mn P S Si Cr
Bath 0.042 0.05 0.021 0.022 - -
Product 0.25-0.26 1.56-1.58 0.033-

0.036 0.024-

0.026 0.058-

0.064 0.20

Fe 500 S is a high strength TMT rebar with a minimum guaranteed YS of 500 MPa and UTS/YS ratio of 1.25. These rebar are produced in various diameters ranging from 4mm nominal diameter to 50 mm nominal diameter. A project was undertaken for the development of Fe 500S grade of TMT rebar of 32 mm nominal diameter. Apart from chemistry, processing variables, viz., temperature at which cooling is
started after rolling( depends on the 11th stand temperature), linear speed, water

pressure, water flow rate that determine the equalization temperature are critical factors that are important for achieving the desirable mechanical properties in the rebar. The chemistry was finalized and a number of trials were undertaken with different sets of chemistry and processing conditions leading to various equalization temperatures. It was found that the processing conditions comprising
Linear Speed - 8.80 m/sec Water Pressure - 19.6 kg/cm2 Water Flow Rate – 990

Nm3/hr 11th stand Temp - 1095-1126oC Equalization Temp - 530-539oC led to the achievement of UTS/YS ratio of > 1.25 in 32 mm diameter TMT rebar.

? Process Route : THF – Ingot casting – Blooming and Billet Mill – Merchant

Mill

Table 2 Processing conditions

11th

Stand

Temp. 0C Equalizatio

n temp. 0C Linear Speed

m/sec Water pressure

Kg/cm2 Water flow rate

Nm3/hr
1040-

1050 530-570 10.0-10.7 9.8-10.3 695-704

Microstructures

Conventional TMT rebar possesses tempered martensite structure at the periphery and ferrite pearlite structure in the core. However, accelerated cooling in the newly developed rock bolt rebar, thickness of martensite rim increased and core changed to predominantly acicular ferrite/ bainite along with pearlite (Fig. 6 - 8).

The excellent combination of strength and ductility has been found to be due to the microstructural evolution in the bar. The core microstructure of acicular ferrite+

pearlite along with martensitic rim with increased rim thickness has resulted in this combination of properties.

The thermo mechanical process was established after extensive trials in plants.

The billets were reheated at 560 - 570°C for two hours and processed through a Thermo Mechanically Treated (TMT) line. In this thermo mechanical process the rolling takes place in controlled manner with a speed of 10.0 – 10.7 m/s. The finishing rolling temperature in the Thermo Mechanically Treated (TMT) line is carried out at temperature between 1040 - 1050°C. Thermo mechanical finish rolling refers to accelerated cooling of the material to be rolled for all of the existing cross-sections to a comparatively low rolling temperature, so that finish-rolling is always at the same range of rolling temperature independent of the dimensions of the material to be rolled, in order to obtain a favourable structure.

The Thermo mechanically treated process invokes cooling the rebar by pressurized water as it emerges from the finishing stand at a cooling rate higher than 20°C/s inside a water cooling installation so that a thin layer of martensite up to 2.7 mm thick forms on the surface while the core is still bainite. On emergence out of the cooling unit, the bar is allowed to cool in the still air. The cooling water pressure provided is in the range of: 9.8 - 10.3 kg/cm2 and the water flow is maintained at: 695 - 704 Nm3/hr.

The equalization temperature varied between 530-570°C for 32mm rods. The typical microstructure of the bar rod as shown in figure 6 - 8 discloses a significant increase in distinct tempered martensite rim layer and a central ductile core of predominantly ferrite and pearlite.

The intense water quenching for a short duration transforms the surface austenite into martensite whereas the core remains austenite when the bar leaves the water cooling pipes. Subsequently, the heat from core flows towards surface and tempers the hard martensite. The tempered martensite thus produced is responsible for high strength. Finally, the austenite in the core transforms into low temperature product generally ferrite-pearlite or acicular ferrite / bainite providing high ductility and toughness by Thermo Mechanical Treatment.

Conventional TMT rebar possesses tempered martensite structure at the periphery and ferrite pearlite structure in the core. However, accelerated cooling in the newly developed rock bolt rebar, thickness of martensite rim increased and core changed to predominantly acicular ferrite/ bainite along with pearlite.

Table 3 Tensile properties in the rebar; F: Front; M: Middle; B: Back

Sample YS, MPa UTS, MPa % Elong.

UTS/YS
F1 576 768 20 1.33
F2 541 723 21 1.34
F3 569 772 19 1.36
M1 584 766 18 1.31
M2 565 754 18 1.33
B1 623 784 18 1.26
B2 585 736 20 1.26
B3 569 757 21 1.33
F1 552 732 21 1.33
F2 531 706 25 1.33
F3 562 751 21 1.34
F4 546 715 21 1.31
F5 567 754 22 1.33
F6 598 795 17 1.33
M1 536 707 21 1.32
M2 532 719 19 1.35
M3 579 759 18 1.31
M4 535 705 21 1.32
M5 579 767 18 1.32
M6 573 754 17 1.32
B1 565 747 18 1.32
B2 636 785 16 1.23
B3 590 770 19 1.31
B4 589 763 19 1.30
B5 579 755 20 1.30
B6 550 706 21 1.28

HIGHLIGHTS OF THE INNOVATIVE STEP

? Design of innovative steel chemistry; addition of ~0.20%Cr in steel having C

0.25-0.26%, Mn : 1.56-1.58%, Si : 0.058-0.064% to achieve higher UTS/YS

ratio (> 1.25) in TMT rebar

? Optimization of process parameters in Merchant Mill like Linear Speed, Water Pressure, Water Flow Rate, 11th stands Temp and Equalization Temp. to achieve consistently superior properties in TMT rebar of 32 mm dia. like YS : 531-589 MPa, UTS : 706-795 MPa, %El : 18-25% & UTS/YS :
1.28-1.35

? The rebar processed as per designed chemistry and optimised processing conditions manifested rim thickness of ~2.7 mm. The rebar revealed a microstructure of ferrite pearlite in the core, tempered martensite in the rim, while there was a transition from ferrite pearlite to martensite at the interface

? Process technology for the production of high strength (YS > 500 MPa)

earthquake resistant TMT rebar with UTS/YS > 1.25 has been established.

Although the foregoing description of the present invention has been shown and described with reference to particular embodiments and applications thereof, it has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the particular embodiments and applications disclosed. It will be apparent to those having ordinary skill in the art that a number of changes, modifications, variations, or alterations to the invention as described herein may be made, none of which depart from the spirit or scope of the present invention. The particular embodiments and applications were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such changes, modifications, variations, and alterations should therefore be seen as being within the scope of the present invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.