A PROCESS TO MANUFACTURE 8-40 MM D1A. COPPER-PHOSPHORUS
BEARING CORROSION AND EARTHQUAKE RESISTANT TMT REBAR OF YS
415 / 500 MPa (MIN.) WITH ADEQUATE TENSILE TO YIELD RATIO AND
CHARPY IMPACT TOUGHNESS.
FIELD OF THE INVENTION
This invention relates to a thermo mechanical process to manufacture 8-40 mm
diameter copper-phosphorus bearing corrosion and earthquake resistant Thermo
Mechanically Treated (TMT) rebar of YS 415 500 MPa (min) with adequate len.-siic to
yield ratio and Charpy Impact toughness.
PRIOR ART AND DRAWBACKS
Reinforcement bars and wire rods are used in construction industry. These bars
and wire rods are produced by Thermo Mechanically Treated (TMT) process.
There are two main types of reinforcing bars, which are commonly used. These
are Cold Twisted Deformed (CTD) bars and Thermo Mechanically Treated (TMT) bars.
Cold twisted deformed bars: CTD bars are produced by cold twisting of ribbed
bars o\' special profile, to develop required strength level as per Indian standard. Though
carbon content in the bars is restricted to some extent, these bars have an inherent
problem of poor weldability and ductility as certain minimum carbon content is essential
to achieve desired strength. Besides an additional operation of twisting is invoked in
their production, due to which the bars are subjected to torsion stresses and become less
corrosion resistant.
Thermo Mechanically Treated bars: TMT is a process in which the ribbed bar is
subjected to an on-line mechanical and thermal treatment during the production process
itself. The finished bar has a grain structure comprising a hardened and tough outer layer
of "Tempered Martensite" and a ductile core of "Ferrite-Pearlite." The combination of
such microstructural features in the cross-section of the bar izives excellent vield strength
to the bar along with superior tensile to yield ratio and ductility.. Since the desired level
of strength in Thermo Mechanically Treated (TMT) bar is achieved on line at a lower
carbon level than CTD bar. it offers excellent weldability. ductility and earthquake
resistance. Due to the stability of the microstructure at high temperatures, these bars
exhibit good fire resistance too.
In order to impart adequate corrosion resistance to the bars wire rods in concrete
structure copper and phosphorus are used as alloying elements. These elements form an
amorphous layer on the surface of rebar wire rod that has superior corrosion resistance
compared to conventional Thermo Mechanically Treated (TMT) rebar in which copper
and phosphorus of 0.25° o and 0.07° o min. respectively are not intentionally added.
OBJECT OF INVENTION
The main objective ol' the invention is to produce a copper-phosphorus bearing
corrosion and earthquake resistant Thermo Mechanically Treated (TMT) rebar wire rod
of diameter 8-40 mm having minimum yield stress 415 MPa or 500 Mpa. with adequate
tensile to yield ratio and Charpy impact toughness by thermo mechanical treatment.
Another object of the invention is lo produce Thermo Mechanically Treated
(TMT) rcbar/wire rod with adequate tensile lo yield stress ratio.
Another object of the invention is to produce Thermo Mechanically Treated
(TMT) rebar wire rod with good elongation.
Another object of the invention is to produce 'Thermo Mechanically Treated
(TMT) rebar wire rod with good bend and re-bend properties.
A further object of the invention is to produce Thermo Mechanically Treated
(TMT) rebar wire rod with good weldability.
Another object of the invention is lo produce Thermo Mechanically Treated
(TMT) rebar, wire rod with good corrosion resistance.
These and other objects of the invention will be clear from the following
paragraphs.
BACKGROUND OF THE INVENTION
All the above objects of the invention have been achieved by prolonged study and
investigation of corrosion and earthquake resistant TMT rebars/wire rod. The production
of corrosion and earthquake resistant TMT rebars wire rod required different processing
sequence: particularly alloy chemistry formulation and adjustment of finishing rolling
temperature, water pressure and equalization temperature in the (TMT) line.
BRIEF DETAILS OF THE INVENTION
According to this invention there is a new process for the manufacture of copper
phosphorous bearing corrosion and earthquake resistant Thermo Mechanically Treated
(TMT) rebar which comprises the following steps:
(i) melting a steel composition (by weight %) containing: C-0.25 max.: Mn-1.50
max.: S-0.04 max.: P- 0.07-0.12%: Si-0.50 % max.: Cr-rMo-Nb 0.50% max :
Cu-0.25 min. and the balance being Fe in a furnace :
(ii) tapping the melt in a preheated ladle al 1630 = 10'C. provided with Ferro
alloy of quantity Fe-Mn 1.0- 3.0 T: Fe-Si 0.5-1.5T: Fe-P 0.5-2.0 T and copper
cathodes
(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 1300 ~ 20"C as required:
(vi) processing the soaked ingots into billets, where after:
(vii) the billets are reheated at temperature o\' 1230 = 20"C and processed through a
known TMT line.
In this thermo mechanical process the rolling takes place in controlled manner to a
finishing rolling temperature between 950 -1050 :C.
The Thermo mechanically treated process involves cooling the rebar by pressurized
water as it emerges from the finishinu stand at a cooliim rale hiuher than 200CC s inside a
THERMEX ' water cooling installation so that a thin layer of martensite up to 4 mm
thick forms on the surface while the core is still austenite. On emergence out of the
thermex unit, the bar is allowed to cool in the still air.
It is to be noticed that the water pressure in the Thermo Mechanically Treated (TMT)
line varied between 7-12 kg/cm" for 8 to 12 mm diameter wire rod bar and between 15-
25 Kg/ cm" for 16 to 40 mm diameter wire rod bar. The equalization temperature (not
supported by above steps) varied between 450-700'C for bar products and between 500-
700JC for wire rod.
DETAILS OF THE INVENTION AND FIGURES 1, 1A, 2, 3 & 4 OF THE
ACCOMPANYING DRAWINGS
The invention provides a thermo mechanical process for producing high strength
corrosion and earthquake resistant reinforcement bar/wire rod. which comprises melting
the steel in a furnace and tapping the same in a preheated ladle at 1630 = 10~C. The Ferro
alloy of quantity Fe-Mn 1.0- 3.0 T: Fe-Si 0.5-1.5T: Fe-P 0.5-2.0 T and copper cathode
plate are kept in the ladle prior to tapping of steel. The liquid steel is homogenized with
purging of argon and cast as ingots of weight nine tones each or continuously cast as 100
x 100 mm billet. The ingots were soaked at 1300 ~ 20CC for six hours and are processed
to billets of size 105x105 mm which were subsequently processed to TMT bar wire rod
as shown in figure 1. The chemical composition of corrosion and earthquake resistant
bar/wire rod is shown in table 1.
The thermo mechanical process was established after extensive trials in plants.
The billets were reheated at 123Cfc 20CC for two hours and processed through a Thermo
Mechanically Treated (TMT) line as shown in figure 2. The Thermo Mechanically
Treated (TMT) line for 16-40 mm diameter bar is known as Thermex unit and consists of
four tubes through which pressurized water is circulated to cool the surface of bar to
lower temperature for specific metallurgical changes. I or lower diameter wire rod. the
Thermo Mechanically Treated (TMT) line is known as Stellmore unit and consists of five
tubes through which pressurized water is circulated for same purpose as bar product. The
finishing rolling temperature varied between 950-1050=C.
In this thermo mechanical process the rolling takes place in controlled manner. The
finishing rolling temperature in the Thermo Mechanically Treated (TMT) line is carried
out at temperature between 950 - 1050CC. Thermo mechanical finish rolling refers to
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
lo obtain a favorable 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 2()0~C s inside a
THERMEXIM water cooling installation so that a thin layer of martensite up to 4 mm
thick forms on the surface while the core is still austenite. On emergence out of the
thermex unit, the bar is allowed to cool in the still air.
The water pressure varied between 7-12 kg cm" and 15- 25 kg cm" for 8 to 12 mm
and 16 to 40 mm diameter wire rod bar respectively. The equalization temperature varied
between 450-700 =C for bar products and between 500-700°C for wire rod.
The typical macrostructure of the bar wire rod is shown in figure 3 that shows a
distinct tempered martensite rim layer and a central ductile core of predominantly ferrite
and pearlite. By polishing in Alumina and etching for 3 minutes in 2°o nital etc ham. the
said macrostructure of the barwire rod was analyzed. The thickness of the rim is 3 to 4
mm. 2 to 3 mm and 0.5 to 2 mm for 32 to 40mm. 16 to 28 mm and 8 to 12 mm dia.
rebar/wire rod respectively. -" . -: '
The typical mechanical properties of corrosion and earthquake resistant bar/wire
rod were determined as per IS 1786-1985 and are given in fable 2. One important
development of present study is establishment of a novel technique of Charpy impact test
of 32 mm diameter rebar. Charpy impact testing is used for evaluation of impact
toughness o( a variety of mass-produced materials such as plate, forging, bar product,
welded construction etc. In this subsize Charpy specimen (5x10x55 mm) were machined
keeping the tempered martensite rim intact as shown in figure 4.
EXPERIMENTAL VERIFICATION OF THE PROPERTIES
* -
The weldability tests were carried out to establish safe welding procedure for
corrosion and earthquake resistant barwire rod as per IS 9417-1979 specification. The
bar/wire rod can be safely welded using properly baked electrode conforming to A \Y S H
7018-1 and A W S E 7016 specifications respectively. Pre-heating is not required due to
low Carbon equivalent. (CE=C-Mn 6- Cr-Mo 5- Cu 15) o\' products. Interpass
temperature may be 100-150 =C. The heat affected zone exhibited a bainitic
microstructure with maximum hardnes 265 VHN.
The elevated temperature (600 °C) yields strength of 32 mm diameter corrosion
and earthquake resistant rebar showed yield strength which is 54°-% of room temperature
yield strength i.e. 236 MPa at 600 CC and 436 MPa at room temperature (25 CC) of the
same bar.
The corrosion resistance of the rebar was determined by electrochemical method,
salt spray test and static immersion test in 3.5% NaCl solution at ambient temperature.
The corrosion resistance of the rebar was 3- ! .5 to 2.3 times higher than conventional
Thermo Mechanically Treated (TMT) rebar without copper and higher phosphorus
content. The scale layer on the rebar was studied by Auger spectroscopy and showed
presence of phosphorus in the scale.
The invented thermo mechanical process is cost effective as it enables
achievement of higher strength, adequate tensile to yield stress ratio, good elongation (as
illustrated in Table 2) and has been made suitable for implementation using existing
equipment of steel plants without the need of additional facilities.

While this invention has been described with an emphasis upon preferred
embodiments, it will be obvious to those of ordinary skill in the art that variations in the
preferred methods may be used and that it is intended that the invention may be practiced
otherwise than as specifically described herein. Accordingly, this invention includes all
modifications encompassed within the spirit and scope of the invention as defined by the
claims that follow.
WE CLAIM
1. A thcrmo mechanical process for the manufacture of copper phosphorous bearing
corrosion and earthquake resistant Thermo Mechanically Treated (TMT) rebar which
comprises the following steps:
(i) melting a steel composition (by weight %) containing: C-0.25 max.: Mn-1.50
max.: S-0.04 max.: P- 0.07-0.12°.»: Si-0.50 % max.: Cr-Mo+Nb 0.50% max :
Cu-0.25 min. and the balance being He in a furnace :
(ii) tapping the melt in a preheated ladle at 1630 - 10"C. provided with Herro
alloy of quantity Fe-Mn 1.0- 3.0 T: He-Si 0.5-1.5T: He-P 0.5-2.0 T and copper
cathodes
(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 1300 ± 20°C as required:
(vi) processing the soaked ingots into billets, where after:
(vii) the billets are reheated at temperature of 1230 = 20°C and processed through a
known TMT line.
2. A thermo mechanical process as claimed in claim 1. wherein the thermo mechanical
process further comprises of the step in which finishing rolling temperature in the
Thermo Mechanically Treated (TMT) line is carried out at temperature between 950 -
1050°C.
3. A thermo mechanical process as claimed in claim land 2. wherein the thermo
mechanically treated rebar emerging from finishing stand is cooled by the water
pressure in the Thermo Mechanically Treated (TMT) line varied between 7-12
kg.-cm2 for 8 to 12 mm diameter wire rod bar.
4. A process as claimed in preceding claims 1 and 2. wherein the water pressure in the
Thermo Mechanically Treated (T\1T) line varied between 15-25 kg cm" for 16 to 40
mm diameter wire rod bar.
5. A thermo mechanical process as claimed in claim 1 to 4. wherein further the
equalization temperature varied between 450-700cC for bar products.
6. A thermo mechanical process as claimed in claim 1 to 4. wherein the equalization
temperature varied between 500-700"C for wire rod.
7. A thermo mechanical process as claimed in claim 1 to 6. wherein the final
composition of corrosion and earthquake resistant TMT bar wire rod is: Carbon 0.06-
0.15 wt.%: Manganese 0.50-1.50 wt.°%: Silicon 0.05-0.50 wt.%: Sulphur 0.04 max
wt.%: Phosphorus 0.07-0.12 wt.°%. Chromium-Molybdenum-Niobium 0.50 wl.%
max and Copper 0.25-0.50 wt.%.
8. A thermo mechanical process for the manufacture of copper phosphorus bearing
corrosion and earthquake resistant TMT bar substantially as herein described.

The present invention relates to a thermo mechanical process for the manufacture of
copper phosphorous bearing corrosion and earthquake resistant Thermo Mechanically
Treated (TMT) rebar which comprises the steps of melting a steel composition (by
weight %) containing: C-0.25 max.; Mn-1.50 max.; S-0.04 max.; P- 0 07-0.12%; Si-0.50
% max.; Cr+Mo+Nb 0.50% max ; Cu-0.25 min. and the balance being Fe in a furnace ;
tapping the melt in a preheated ladle at 1630 ± 10°C provided with Ferro alloy of
quantity Fe-Mn 1.0- 3.0 T; Fe-Si 0.5-1.5T; Fe-P 0.5-2.0 T and copper cathode
homogenizing the steel in the ladle by purging of argon; casting the homogenized steel
either into ingots or as billet by continuous casting; soaking the ingots at 1300 ± 20°C as
required; processing the soaked ingots into billets, where after; the billets are reheated at
temperature of 1230 ± 20°C and processed through a known TMT line.