FIELD OF THE INVENTION
The present invention relates to developing high strength (YS:640MPa Min.) Thermo
Mechanically Treated (TMT) bar and a process for its production. More particularly, the
present invention is directed to developing TMT bars having yield strength above 640MPa,
UTS/YS ratio of 1.15 Min., with super toughness (18% elongation Min.) properties of said
TMT rebars through selective alloy design and optimizing process parameters. The alloy
design of the high strength TMT rebars of the invention is obtained with C-Mn chemistry for
the developmental heats wherein manganese content of steel is maintained on moderately
higher side in the range of 0.8-1.3 wt %, and no micro-alloying element is added so that
cost of production remains competitive. Importantly, the process parameters for the
instantaneous quenching by accelerated cooling of the hot deformed rebars according to the
present invention are adapted to provide thick martensitic structure at the periphery and
the core containing predominantly acicular ferrite/bainite structure along with pearlite
ensuring desired end properties. The high strength TMT bar according to the invention thus
ensuring high strength to weight ensure favouring wide scale application such as
reinforcement bar in RCC structures or as rock bolts for effective roof support in under
ground mines or tunnel construction.
BACKGROUND ART
It is well known in the related art that thermo mechanically treated steel reinforcement bars
of Fe 500 grade are being used conventionally as rock bolts for active roof support system
in underground coal mines as well as tunnel construction. It has also been experienced in
the art that considering the safety aspect of such construction, there had been a need to
develop steel materials with even higher strength to weight ratio with same section for the
TMT bars associated with higher toughness properties to suit application in RCC/tunnel
structures.
It is thus the intent of the present invention to overcome the limitation of the existing TMT
bars used for RCC structures and rock bolts for mines or roof support system or the like, by
developing a new grade of steel rebar capable of ensuring higher strength and toughness as
also the desired improved strength to weight ratio of said TMT rebars. Such high strength
rebars (YS:640MPa Min.) is developed and produced by selectively controlling on one hand
the chemistry of the steel material and on the other hand optimizing the process
parameters for rolling and instantaneous quenching by accelerated cooling of the hot
deformed rebars to thereby obtain desired microstructure favouring achieving desired
superior strength/physical properties in the end products.
OBJECTS OF THE INVENTION
The basic object of the present invention is thus directed to developing a high strength (YS:
640 MPa minimum) Thermo Mechanically Treated (TMT) rebar for application as
reinforcement bar in RCC structure and as rock bolt bar in roof support system for
underground mines and tunnel construction and the like.
According to yet another object of the present invention is directed to developing a TMT
rebar with desired higher physical strength properties wherein C-Mn chemistry for
developmental heats for steel material maintained with Mn in the moderately higher range.
A further object of the present invention is directed to developing a TMT rebar which would
be capable to providing high strength, high toughness with moderate elongation properties
suitable for RCC reinforcement or rock bolts in tunnel structures.
A still further object of the present invention is directed to developing a TMT rebar with
desired higher physical strength properties obtained by selective control of chemistry as well
as optimizing the process parameters for rolling and rapid cooling of hot rolled bars such as
controlling cooling water flow, inlet water temperature, retention time in cooling chamber
(cooling chamber length and mill speed) etc. which is selectively varied according to the size
of the bar.
A still further object of the present invention is directed to developing a TMT rebar with
desired higher physical strength properties wherein a preferred microstructure is obtained
by control of chemistry and process parameters to ensure very hard peripheral rim of rebars
with higher thickness of martensite with core relatively soft comprising ferrite/bainite
alongwith pearlite thereby ensuring excellent combination of strength and ductiliy.
A still further object of the present invention is directed to developing a TMT rebar with
desired higher physical strength properties wherein no micro-alloying element is added to
keep the cost of production competitive.
A still further object of the present invention is directed to developing a TMT rebar with
desired higher physical strength properties wherein the developed steel rebar ensure higher
safety of RCC or tunnel structure using such TMT rebars and favour cost advantage by way
of material saving on account of ensuring improved strength to weight ratio for same steel
sections.
SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to High strength (YS;640MPa Min.) TMT
bars comprising:
a chemical composition
C:0.18 to 0.3 wt % preferably 0.2;
Mn:0.8 to 1.5 wt% preferably 1.1;
Si: 0.05 to 0.10 wt %;
S:0.025 to 0.042 wt %;
P:0.025 to 0.042 wt%;
& rest Fe.
Another aspect of the present invention is directed to said High strength TMT bars
comprising:
YS(MPa) 640 to 740 and preferably 650;
UTS(MPa) 750 to 850 preferably 780;
UTS/YS: 1.12 to 1.2 preferablyl.2; and
%EI: 18 to 24;
A further aspect of the present invention is directed to said High strength TMT bars having
hard martensite rim of higher thickness at periphery and acicular ferrite/bainite along with
relatively soft pearlite at core ensuring excellent combination of strength and ductility.
A still further aspect of the present invention is directed to said High strength TMT bars
wherein hardness profile of the bar comprises hardness of martensitic rim of around 290
VHN and core area hardness around 240 VHN.
According to yet another aspect of the present invention is directed to a process for
producing high strength TMT bar comprising the steps of:
(i) producing the steel maintaining C-Mn chemistry free of micro alloying such as to
obtain steel composition comprising
C:0.18 to 0.3 wt % preferably 0.2;
Mn:0.8 to 1.5 wt% preferably 1.1;
Si: 0.05 to 0.10 wt %;
S:0.025 to 0.042 wt %;
P:0.025 to 0.042 wt%; & rest Fe.
(ii) producing billet therefrom;
(iii) hot rolling with controlled parameters to produce TMT bars from billet; and
(iv) instantaneous quenching of hot deformed bars under controlled cooling parameters
such as to achieve desired microstructure and combination of strength with yield
strength of 640MPa min and super tough elongation 18% min.
A still further aspect of the present invention is directed to a process for producing high
strength TMT bar wherein, said rolling parameters comprising
soaking temperature: 1100-1280°C;
bar dia:20,22,25mm;
mill speed:4.8-5.2m/s;
water Pressure:9.5-10.7kg/cm2;
water Flow:535-550m3/hr;
equalization temperature:480-520°C.
A still further aspect of the present invention is directed to a process for producing high
strength TMT bar wherein cooling parameters for instantaneous quenching of hot deformed
bars comprising cooling water flow 520 to 560 m3/nr; inlet water temperature 15 to 30°C;
retention time in cooling chamber 1.8 to 2.5 s which are varied according to the size of the
bar.
The present invention and its objects and advantages are described in greater details with
reference to the following accompanying non limiting illustrative figures and example.
BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
Figure 1: is the process flow chart showing the processing route involving the basic steps for
developing the TMT bars.
Figure 2a: is the image of micro structure of the core of the TMT rebar developed according
to the process of the invention showing acicular Ferrite/Bainite structure at core.
Figure 2b: is the image of micro structure showing Ferrite/Pearlite interface between core
and rim of TMT bars developed according to the present invention.
Figure 2c: is the micrograph showing dendritic tempered Martensite structure at the
rim/periphery of TMT bars developed according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE
ACCOMPANYING FIGURES
The present invention is directed to developing high strength (YS: 640 Mpa Min.) super
tough (Elongation 18% Min.) TMT rebars for application in reinforcement of RCC structure or
as rock bolts used in tunnel structures. The TMT bars is developed following the process
according to the invention involving selective alloy design comprising C-Mn chemistry with
moderately high Mn content in the range of 0.8-1.3 wt% and avoiding any micro alloying
and also involving controlling/optimizing process parameters for hot rolling as well as
accelerated cooling/instantaneous quenching of hot deformed rebars to achieve desired
microstructure in said TMT rebars comprising hard martensite rim of increased thickness
with relatively soft and tough core formed of acicular ferrite/bainite along with pearlite.
The objects and advantageous aspects of the high strength TMT rebars and the process for
its production is described in further details with reference to the following non limiting
figures and example.
Reference is first invited to the accompanying Figure 1 that illustrate the basic steps
involved in the developments of TMT bars. It is clearly apparent from the process route
shown in the accompanying Figure 1, that in the first step making of steel having desired
chemistry is carried out in Twin hearth furnace or Basic Oxygen furnace. Heat making for
steel production is adopted wherein alloy design with C-Mn chemistry is maintained for the
developmental heats. No micro alloying is added to keep the cost of production competitive.
The manganese content is maintained relatively on higher side preferably in the range of
0.8-1.3 wt%.
The steel is then cast into billet in continuous casting mill or made in ingot casting route.
The cast billet obtained is then subjected to hot rolling in Merchant mill for reduction of
section through successive roll passes to desired TMT bar section, preferably obtained in the
sizes of 20mm, 22mm and 25mm bar diameters. Rolling parameters and the water
quenching of the hot deformed bars comprised the variables like the soaking temperature,
mill speed, cooling water pressure and flow rate, inlet water temperature, retention time in
cooling chamber equalization temperature. The parameter values selected/varied based on
the size of the bars. The parameters are optimized through trials and standardized till a
desired micro structure is achieved ensuring the desired strength (YS: 640 MPa min) and
toughness properties(18% El min) of the rolled and quenched TMT bars. Necessary testing
of the rolled and quenched TMT rebars is carried out to ensure desired micro structure and
strength properties.
The steps involved in the process according to the invention for production of TMT bars of
desired strength properties is further illustrated with the help of following example.
Example I:
1. Alloy design for steel manufacturing is maintained with C-Mn chemistry for the
developmental heats, free of any micro alloying and Mn content at relatively higher
range. The composition of steel obtained in Twin hearth/Basic Oxygen furnace is as
follows:
C:0.18 to 0.3 wt %;
Mn:0.8 to 1.3 wt%;
Si: 0.05 to 0.10 wt %;
S:0.025 to 0.042 wt %;
P:0.025 to 0.042 wt%;& rest Fe.
2. The steel so produced is cast into billets either in concast (continuous casting) plant
or through ingot casting route.
3. The cast billet is then subjected to hot rolling in Merchant mill following the selective
rolling parameters as follows:
Soaking temperature: 1100-1280°C;
Bar dia.: 20mm, 22mm and 25mm;
Mill speed:4.8 to 5.2 m/s;
4. Hot deformed rebars are subjected to instantaneous quenching following the cooling
parameters comprising cooling water flow, inlet water temperature, retention time in
cooling chamber determined by cooling chamber length and mill speed and such
aparemeters are varied according to the size of the bar. The cooling parameters are
as follows:
Water pressure: 9.5 to 10.7 kg/cm2;
Water flow rate: 535 to 550 m3/hr;
Inlet water temperature: 15 to 30°C;
Equalisation temperature:480-520°C;
Mill speed:4.8-5.2m/s;
Cooling chamber length: 10m approx;
Retention time in cooling chamber: 1.8 to 2.5 sec;
5. Accelerated cooling of hot rolled TMT rebars according to the process of the present
invention ensure developing a micro structure with increased thickness of very hard
martensite rim at periphery of bars and a relatively soft and tough core having
predominantly acicular ferrite/ bainite structure along with pearlite. Accompanying
Figure 2a shows the micro structure of the core of the TMT rebar having acicular
Ferrite/Bainite structure at core. Accompanying Figure 2b shows the micro structure
having Ferrite/Pearlite interface between core and rim of TMT bars and Figure 2c is
the micrograph showing dendritic tempered Martensite structure at the rim/periphery
of TMT bars of increased thickness developed following the process of the present
invention.
6. The testing of the TMT bars obtained with controlled C-Mn chemistry and optimized
rolling/ cooling parameters are carried out to determine and ensure the desired
physical properties to meet the objective of application in RCC reinforcement or rock
bolts for tunnel/roof support structure.
7. Test Results obtained are as follows:
a. Yield strength of bar is above 640 Mpa with 18% El and UTS/YS ratio of
1.15min.
b. Tensile properties:
Guaranteed: YS(MPa)-640Min.;UTS(MPa)-736Min.; %EI 15 min.;
Typical: YS(MPa):666; UTS(MPa)-793; %EI 19;
c. Hardness profile of the bar shows that hardness of martensite rim is of the
order of 290VHN whereas the core area has attained a hardness of around
240VHN.
d. The excellent combination of strength and ductility obtained due to the
microstructure evolution in the bar such as the core microstructure of acicular
ferrite plus pearlite combined with martensitic rim of increased thickness has
resulted in the favoured combination properties of strength and toughness as
well as ductility.
It is thus possible by way of the present invention to developing a high strength (YS: 640
MPa min.) TMT rebar with improved toughness and moderate elongation (18% El)
properties for application as reinforcement in RCC structures or as rock bolts in roof support
system and a process for its production involving selective C-Mn composition with relatively
high Mn content in the range of 0.8 to 1.3 wt% and optimizing rolling as well as water
quenching parameters to ensure desired micro structure and strength properties in the
resulting hot rolled and quenched TMT rebars. Importantly, the process of production of the
TMT rebars according to the invention eliminates any micro alloying element addition to
keep the cost of production low. The high strength TMT rebars according to the present
invention is advantageously providing improved weight to strength ratio favoring material
saving and resultant cost saving while ensuring safety and security of support structures.
The instantaneous cooling of the hot deformed bars as of the invention results in evolution
of a micro structure of said rebars comprising acicular ferrite/balnite along with pearlite at
core and a very hard dendritic martensite of higher thickness at the periphery favoring
excellent strength and ductility combination in the TMT rebars and thus favoring wide scale
application of this grade of steel rebars in variety of structures and industrial application of
the process technology for their production in steel plants.
We claim:
1. High strength (YS;640MPa Min.) TMT bars comprising:
a chemical composition
C:0.18 to 0.3 wt % preferably o.2;
Mn:0.8 to 1.5 wt% preferably 1.1;
Si: 0.05 to 0.10 wt %;
S:0.025 to 0.042 wt %;
P:0.025 to 0.042 wt%; & rest Fe.
2. High strength TMT bars as claimed in claim 1 comprising:
YS(MPa) 640 to 740 and preferably 650;
UTS(MPa) 750 to 850 preferably 780;
UTS/YS: 1.12 to 1.2 preferably 1.2; and
%EI: 18 to 24;
3. High strength TMT bars as claimed in anyone of claims 1 or 2 having very hard
martensite rim of higher thickness at periphery and acicular ferrite/bainite along with
relatively soft pearlite at core ensuring excellent combination of strength and
ductility.
4. High strength TMT bars as claimed in anyone of claims 1 to 3 wherein hardness
profile of the bar comprises hardness of martensitic rim of around 290 VHN and
core area hardness around 240 VHN.
5. A process for producing high strength TMT bar as claimed in anyone of claimsl to 4
comprising the steps of:
(i) producing the steel maintaining C-Mn chemistry free of micro alloying such as to
obtain steel composition comprising
C:0.18 to 0.3 wt % preferably 0.2;
Mn:0.8 to 1.5 wt% preferably 1.1;
Si: 0.05 to 0.10 wt %;
S:0.025 to 0.042 wt %;
P:0.025 to 0.042 wt%; & rest Fe.
(ii) producing billet therefrom;
(iii) hot rolling with controlled parameters to produce TMT bars from billet; and
(iv) instantaneous quenching of hot deformed bars under controlled cooling parameters
such as to achieve desired microstructure and combination of strength with yield
strength of 640MPa min and super tough with eleongation 18% min.
6. A process for producing high strength TMT bar as claimed in claim 5 wherein, said rolling
parameters comprising
soaking temperature: 1100-1280°C;
bar dia:20,22,25mm;
mill speed:4.8-5.2m/s;
water Pressure:9.5-10.7kg/cm2;
water Flow:535-550m3/hr;
equalization temperature:480-520°C.
7. A process for producing high strength TMT bar as claimed in claims 5 wherein cooling
parameters for instantaneous quenching of hot deformed bars comprising cooling Water
flow rate;535 to 550 m3/hr; inlet water temperature: 15 to 30oC; retention time in cooling
chamber: 1.8 to 2.5 sec which are varied according to the size of the bar.
8. High strength (YS: 640MPa Min.) and super tough (Elongation 18% Min.) TMT bars and a
process for its production as hereindescribed with reference to the accompanying non
limiting examples.

The present invention relates to developing high strength (YS:640MPa Min.) Thermo
Mechanically Treated (TMT) bar and a process for its production. The TMT bars of the
invention is having yield strength above 640MPa, UTS/YS ratio of 1.15 Min., with super
toughness (18% elongation Min.) properties of said TMT rebars through selective alloy
design and optimizing process parameters. The alloy design comprising C-Mn chemistry
wherein manganese content of steel is maintained in the range of 0.8-1.3 wt %, and no
micro-alloying element is added so that cost of production remains competitive. The process
parameters for the instantaneous quenching by accelerated cooling of the hot deformed
rebars are adapted to provide thick martensitic structure at the periphery and the core
containing predominantly acicular ferrite/bainite structure along with pearlite ensuring
desired combination of strength and ductility properties, favoring wide scale application in
RCC structures or as rock bolts for effective roof support in tunnel construction and the like.