PROCESS FOR PRODUCTION OF 25MM HIGH STRENGTH ROCK BOLT TMT BAR
FIELD OF INVENTION
The present invention relates to a high strength ribbed rock bolt Fe 600 grade Thermo
Mechanically Treated (TMT) More particularly, particularly for reinforcements for
concrete or welded mesh, deformed by hot rolling and accelerated cooling, the
present invention is directed to developing rock bolt grade TMT bars having higher
production of high strength (YS: 600 MPa minimum) TMT bar in 25mm diameter for
application as rock/roof bolt bar.
BACKGROUND ART
Reinforcement bars and rods are used in construction industry. These bars and 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 of
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 provides

excellent yield 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 ensures better weldability.
Conventional Thermo Mechanically Treated (TMT) steel reinforcement bars of Fe 500
grade are being used in Indian underground coalmine and tunnel construction as rock
bolts for active roof support system. Considering the safety aspect of such
construction and to accrue the benefits of material with higher strength to weight ratio,
it has become imperative to use higher strength TMT rebars of rock bolt grades for
such applications. In the present work high strength (Yield Strength 600 MPa min.)
rock bolt grade TMT rebars in 25mm diameter have been developed by innovative
alloy design and optimising process parameter and with modified rib design.
This high strength bars not only enhance load bearing capability due to its higher
strength but also maximise bond strength by virtue of its unique rib design. The newly
developed rock bolt also has better shear restrain capability which is beneficial
considering less lateral displacement of rocks. The excellent combination of strength
and ductility has been found to be due to the microstructural evolution in the bar.
OBJECTS OF THE INVENTION
The main objective of the invention is to produce a carbon - manganese bearing high
strength and economical cost effective Thermo Mechanically Treated (TMT) rebar /
rod of diameter 25 mm having minimum yield stress 600 MPa with adequate tensile to
yield ratio and Charpy impact toughness (of 15% elongation minimum) 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.
These and other objects of the invention will be clear from the following paragraphs.
SUMMARY OF THE INVENTION
All the above objects of the invention have been achieved by prolonged study and
investigation of low cost and high strength TMT rebar / rod. The production of said
TMT rebar / rod required different processing sequence: particularly alloy chemistry
formulation and adjustment of finishing rolling temperature, water pressure and
equalization temperature in the production line of merchant mill.
According to this invention there is a new process for the manufacture of C-Mn
bearing high strength rock bolt grade Thermo Mechanically Treated (TMT) rebar which
comprises the following steps:
(i) melting a steel composition (by weight %) containing: C-0.18-0.30 : Mn-0.8-1.5 : S-
0.04 max.: P- 0.04 max : Si- 0.10-0.40 : S+P- 0.075 max and the balance being Fe in
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 1100 - 1280°C as required:
(vi) processing the soaked ingots into billets of 100x100mm size, where after:
(vii) the billets are reheated at temperature of 1200°C and processed through a
known TMT line.
In this thermo mechanical process the rolling takes place in controlled manner with a
mill speed of 5.0 - 6.5m/s. The Thermo mechanically treated process involves cooling
the rebar by pressurized water as it emerges from the finished stand at a cooling rate
higher than 20°C/s inside a 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 the
emergence out of the cooling unit, the rod is allowed to further cool in the still air.
It is to be noticed that the water pressure in the Thermo Mechanically Treated (TMT)
line is varied between 9.0 - 10.7 kg/cm2 for 25 mm diameter rod. The equalization
temperature varied between 400-500°C for rod products.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1(a) illustrates a microstructure representation of tempered Martensite Rim of
TMT rebars in accordance with the present invention;
Figure 1(b) illustrates microstructure representation of Acicular Ferrite / Bainite Core of
TMT rebars in accordance with the present invention.
DETAILED DESCRIPTION
As herein disclosed, the present invention relates to a 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 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 1200°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 higher
side (0.8 -1.5). 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.

The liquid steel is homogenized with purging of argon and cast as ingots continuously
cast as 100 x 100 mm billet. The ingots were soaked at 1100 - 1280°C for two hours
which were subsequently processed to TMT bar.
In the present work high strength (Yield Strength 600 MPa min.) rockbolt grade TMT
rebars have been developed by innovative alloy design and optimising process
parameter and special rib geometry. Yield strength of the bar is above 600 MPa with
15% El and UTS/YS ratio of 1.15min.

Microstructures
Conventional TMT rebars possesses tempered martensite structure at the periphery
and ferrite pearlite structure in the core. However, accelerated cooling in the newly
developed rock bolt rebars, thickness of martensite rim increased and core changed to
predominantly acicular ferrite/ bainite along with pearlite. (Fig. 1a &1b).
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 1200° 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 5.0 to 6.5 m/s. The finishing rolling
temperature in the Thermo Mechanically Treated (TMT) line is carried out at
temperature between 950 - 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 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 120° C/s
inside a 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. The cooling water pressure
provided is in the range of: 9.0 -10.7 kg/cm2 and the water flow is maintained at: 360 -
500 m3/hr On emergence out of the cooling unit, the bar is allowed to cool in the still
air.
The equalization temperature varied between 400-500°C for 25mm rods. The typical
microstructure of the bar rod is shown in figure 1 that shows a significant increase in
distinct tempered martensite rim layer and a central ductile core of predominantly
acicular ferrite and bainite. By polishing in Alumina and etching for 3 minutes in 2%
nital etchant the said microstructure of the rod was analyzed. The thickness of the rim
is 2.5 to 4 mm.
Rolling Parameters
Soaking temperature: 1100 - 1280°C
Bardia.: 25mm

Rolling Mill Speed (Hot): 5.0 to 6.5 m/s
Finishing Rolling temperature: 950 - 1050° C
Cooling Water Pressure: 9.0 -10.7 kg/cm2
Cooling Water Flow: 360 - 500 m3/hr
Equalisation Temperature: 400 - 500°C
Rolling of Billets:
Cold Billets were heated to around 1200°C in the reheating furnace of merchant mill
for about two hours. Billets were rolled to 25mm diameter rockbolt TMT in the
merchant mill. Rib profiles were cut to the rolls of finishing stand as per design to
obtain desired rib pattern (indentation) on rockbolt TMT during rolling as per table 3.

Cooling parameters was varied as required for instantaneous quenching of hot
deformed rebars by maintaining cooling water flow, inlet water temperature, retention
time in cooling chamber (cooling chamber length and mill speed) etc.
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 rebars possesses tempered martensite structure at the periphery
and ferrite pearlite structure in the core. However, accelerated cooling in the newly
developed rock bolt rebars, thickness of martensite rim increased and core changed to
predominantly acicular ferrite/ bainite along with pearlite.
An innovative process technology has been developed by suitable alloy design and
optimizing process parameters for production of high strength (of 600 MPa yield
strength) and tough (of 15% elongation minimum) rock-bolt TMT bar in 25mm
diameter.
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.

WE CLAIM
1. A process for the manufacturing high strength ribbed rockbolt grade Thermo
Mechanically Treated (TMT) bar of 25mm diameter, which comprises the following
steps:
(i) melting a steel composition (by weight %) containing: C-0.18-0.30 : Mn-0.8-1.5 : S-
0.04 max.: P- 0.04 max : Si-0.10-0.40 : S+P 0.075 max and the balance being Fe in 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 1100 - 1280° C;
(vi) processing the soaked ingots into billets of 100x100mm size;
(vii) the billets are reheated at temperature of 1200° C and processed through a
known TMT line;
(viii) hot rolling heated billet to rebar in a merchant mill;
(ix) accelerated cooling the rolled rebar;
(x) further cooling in atmosphere.
2. A process for the manufacturing high strength ribbed 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 950-1050° C.
3. A process as claimed in claim 1, wherein, the hot rolled rebar emerging from
finishing stand is cooled by the water pressure 9.0 -10.7 kg/cm2.
4. A process as claimed in preceding claims 1 to 3, wherein, the cooling water flow is
maintained at 360 - 500 m3/hr to attain a cooling rate of 120° C/s.

5. A process as claimed in preceding claims 1 to 4, wherein further the equalization
temperature is varied between 400-500° C for 25mm rod products.
6. A process as claimed in claim 1 to 5, wherein, the rolling takes place in controlled
manner with a speed of 5.0 to 6.5 m/s.
7. A process as claimed in claim 1 to 6, wherein a thin layer of martensite up to 4 mm
thick forms on the outer surface.
8. A process as claimed in claim 1 to 7, wherein the core is changed to predominantly
acicular ferrite/ bainite along with pearlite.
9. A process as claimed in claim 1 to 7, wherein the TMT bar has a yield strength of
minimum of 600 MPa with a minimum of 15% elongation.
10. A process as claimed in any of the preceding claims wherein, no micro-alloying
element is added in the alloy mix.

ABSTRACT

The present invention relates to a high strength ribbed rock bolt Fe 600 grade Thermo
Mechanically Treated (TMT) for reinforcements for concrete or welded mesh,
deformed by hot rolling and accelerated cooling, and it is directed to developing rock
bolt grade TMT bars having higher production of high strength (YS: 600 MPa
minimum) TMT bar in 25mm diameter for application as rock/roof bolt bar.