Title of Invention	PROCESS FOR MANUFACTURING MICROALLOYED THERMO-MECHANICALLY TREATED REBAR WITH IMPROVED SUB-ZERO IMPACT TOUGHNESS.
Abstract	A process for manufacturing high strength microalloyed TMT rebar with improved sub-zero impact toughness is provided by suitably selecting the levels of microalloying elements in the steel. The process comprises providing steel billet having a composition of 0.05 to 0.1 wt% carbon, 0.3 to 0.5 wt% manganese, 0.05 to 0.2 wt% silicon, upto 0.035 wt% sulfur and atleast 0.008 wt% of microalloying elements comprising niobium and phosphorus wherein the level of phosphorus is in the range of 0.08 to 0.1% when niobium is not present or upto 0.04 wt% when niobium is present in the range of 0.005 to 0.015 wt%, the balance being iron, heating said steel billet at a temperature between 1200 to 1250 degree C, rolling heated billet to rebar, short intensive cooling the rolled rebar and further cooling in atmosphere. The rebars of the present invention have wide applications in the construction sector such as general concrete reinforcement in buildings, bridges and various other concrete structures and especially high rise buildings.

Title of Invention

PROCESS FOR MANUFACTURING MICROALLOYED THERMO-MECHANICALLY TREATED REBAR WITH IMPROVED SUB-ZERO IMPACT TOUGHNESS.

Abstract

A process for manufacturing high strength microalloyed TMT rebar with improved sub-zero impact toughness is provided by suitably selecting the levels of microalloying elements in the steel. The process comprises providing steel billet having a composition of 0.05 to 0.1 wt% carbon, 0.3 to 0.5 wt% manganese, 0.05 to 0.2 wt% silicon, upto 0.035 wt% sulfur and atleast 0.008 wt% of microalloying elements comprising niobium and phosphorus wherein the level of phosphorus is in the range of 0.08 to 0.1% when niobium is not present or upto 0.04 wt% when niobium is present in the range of 0.005 to 0.015 wt%, the balance being iron, heating said steel billet at a temperature between 1200 to 1250 degree C, rolling heated billet to rebar, short intensive cooling the rolled rebar and further cooling in atmosphere. The rebars of the present invention have wide applications in the construction sector such as general concrete reinforcement in buildings, bridges and various other concrete structures and especially high rise buildings.

Full Text	Field of invention The present invention relates to a process for manufacturing of high strength weldable microalloyed TMT rebar with improved sub-zero impact toughness. TMT rebars have wide applications in the construction sector such as general concrete reinforcement in buildings, bridges and various other concrete structures and especially high rise buildings. Background of the invention Thermo-mechanically treated (TMT) rebars are extra high strength reinforcing bars which eliminate any form of cold twisting. Conventionally TMT rebars are manufactured by processes where the steel rebars receive a short, intensive cooling as they pass through a cooling system preferably a water cooling system after the last rolling stand. This reduction in temperature converts the surface region of the steel bar to a hardened structure. This phase of intensive cooling is followed by further cooling in atmosphere so that the temperature between the core which remains hot and the cooled surface region (rim) is equalized and the rim later gets tempered by the heat from the core. The resulting structure is a tempered martensite/bainite zone at the rim periphery and a fine grain ferrite/pearlite structure in the core. Due to the improved properties of high strength, combined with toughness and ductility, microalloyed TMT rebars are preferred for construction purposes over mild steel plain and CTD rebars. Traditionally high strength carbon-manganese TMT rebars which are widely used have carbon and manganese in the levels of 0.20 to 0.25% and 1.0 to 1.5% respectively. The levels of carbon and manganese can be reduced improving weldability and sub-zero impact toughness keeping the strength intact by employing microalloying technique. Earlier, in microalloyed TMT rebars, the previous investigator used niobium between 0.02 to 0.06% or vanadium between 0.07 to 0.15%. Sub-zero impact toughness is impact toughness below zero degree centigrade and steel bars having high sub-zero impact toughness are desirable in colder climatic regions. The known microalloyed TMT rebars possess the required sub- zero impact toughness for such purposes at reasonably higher percent of niobium or vanadium. Objects of the invention Therefore the main object of the present invention is to provide a microalloyed TMT rebar which has high sub-zero impact toughness as well as high yield stress, ultimate tensile strength and ductility at lower cost by the use of low niobium or by use of phosphorus. A further object of the present invention is to provide a microalloyed TMT rebar which possesses low impact transition temperature. The applicants have now found that microalloyed TMT rebar having substantially higher sub-zero impact toughness is achieved by suitably selecting the levels of microalloying elements in the steel which are different from the previous investigators. Summary of the invention Accordingly, the present invention provides a process for manufacturing of high strength microalloyed TMT rebar with improved sub-zero impact toughness comprising the steps of: (i) providing steel billet having a composition of 0.05 to 0.1 wt% carbon, 0.3 to 0.5 wt% manganese, 0.05 to 0.2 wt% silicon, upto 0.035 wt% sulfur and atleast 0.008 wt% of microalloying elements comprising niobium, and phosphorus wherein the level of phosphorus is in the range of 0.08 to 0.1% when niobium is not present or upto 0.04 wt% when niobium is present in the range of 0.005 to 0.015 wt%, the balance being iron; (ii) heating said steel billet at a temperature between 1200 to 1250°C; (iii) rolling heated billet to rebar; (iv) short intensive cooling the rolled rebar; (v) further cooling in atmosphere . Detailed description of the invention According to the present invention the sub-zero impact toughness of micro-alloyed TMT rebars has been improved by selecting the level of micro-alloying elements specially that of phosphorus and niobium in the initial steel compositions. For phosphorus based TMT rebars where niobium is not present, the preferred level of phosphorus is selected between 0.08 to 0.1 wt%. For niobium based TMT rebars the niobium is preferably 0.01 wt% while the phosphorus content is kept low i.e. up to 0.04wt%. Preferred levels of other elements in the initial steel composition is 0.08wt% carbon, 0.4% manganese, 0.1% silicon and up to 0.03% sulfur. The process for manufacture of the improved TMT rebar comprises preparing ingots having the above-mentioned steel composition. The ingots are cast according to conventional methods such as in twin-hearth furnace with tap temperature preferably at 1620±10°C and using ladles. The cast ingots are thereafter soaked in a soaking pit at temperatures between 1280 to 1300°C for around 4-5 hours. The soaked steel is then rolled into billets, which is fed into a reheating furnace. In the reheating furnace the billets are heated at a temperature preferably between 1200 and 1250°C for a period of around two hours. The reheated billets are rolled to TMT rebars finishing at temperature between 980 to 1020°C and at a rolling speed of between 4 to 10 m/sec. The next and most essential step in the process for manufacturing TMT rebars is the short and intensive cooling of the steel. The cooling is preferably carried out in THERMEX water cooling system into which the steel is fed after the last rolling mill stand. The equalization temperature at cooling bed is maintained between 500 to 700°C. The water in the cooling system is preferably maintained between 15 to 22 kg/cm2. This is followed by a further cooling in atmosphere so that the temperature between the core and the rim layer is equalized and the rim layer gets tempered by the heat from the core. The steel is then available as TMT rebars for application in construction industry. The process of the present invention results in TMT rebars having low carbon bainitic steel structure in the rim and acicular ferrite steel structure in the core. Various metallurgical and mechanical tests were performed to evaluate the properties of the TMT rebars manufactured by the process of the present invention and they were found to possess yield stress of at least 415 Mpa, ultimate tensile strength of at least 485 Mpa, elongation of 18 to 20%. The rebars have excellent bend and rebend properties. The rebar also has excellent Charpy impact toughness and weldability due to low carbon equivalent of around 0.2%. The present invention will now be demonstrated with respect to the following non- limiting examples: EXAMPLES Example - 1: Process for manufacturing of phosphorus based TMT rebars with improved sub-zero impact toughness. Steel having composition of 0.08 wt% carbon, 0.4 wt% manganese, 0.1 wt% silicon, 0.01 wt% sulfur and 0.09 wt% phosphorus with balance iron was cast into ingots using a twin hearth furnace. The ingots were then soaked in a soaking pit at a temperature of 1280°C for five hours. The soaked ingots were then rolled into 100 x 100 mm billets. The billets were reheated in reheating furnace at a temperature 1250°C for two hours. The reheated steel was finish rolled at a temperature of 1000°C and at rolling speed of 8 m/sec to 32mm diameter rebar. The rolled steel was then passed through THERMEX water cooling system after the last rolling mill stand. The equalization temperature is maintained at about 600°C with water pressure of 20kg/cm2. After the phase of intensive cooling the steel is further cooled in atmosphere so that the temperature between the core and the rim layer is equalized and the rim layer is tempered by the heat from the core. Example - 2: Process for manufacturing of niobium based TMT rebars with improved sub-zero impact toughness. Steel having composition of 0.08 wt% carbon, 0.4 wt% manganese, 0.1 wt% silicon, 0.01 wt% sulfur and upto 0.04 wt% phosphorus, 0.01 wt% niobium with balance iron was cast into ingots using twin hearth furnace. The ingots were then soaked in a soaking pit at a temperature of 1280°C for five hours. The soaked ingots were then rolled into 100 x 100 mm billets. The billets were reheated in reheating furnace at a temperature 1250°C for two hours. The reheated billets were finish rolled at a temperature of 1000°C and at rolling speed of 8 m/sec to 32mm diameter TMT rebar. The rolled steel was then passed through THERMEX water cooling system after the last rolling mill stand. The equalization temperature is maintained at about 600°C with water pressure of 20kg/cm2. After the phase of intensive cooling the steel is further cooled in atmosphere so that the temperature between the core and the rim layer is equalized and the rim layer is tempered by the heat from the core. Several metallurgical and mechanical tests were performed and it was found that the phosphorus and the niobium based steels have yield stress of 450 Mpa, ultimate tensile strength of 510 Mpa, elongation of 18 to 20%, bend property of 3.d and rebend property of 4.d. The Charpy impact toughness was found to be 100J for the phosphorus based TMT steel and 135J for niobium based TMT steel at room temperature compared to 60J for conventional TMT rebars. It was also found that the rebars possess impact transition temperature of -42°C for phosphorus based steel and below -60°C for the niobium based steel compared to -10°C for conventional TMT rebars. The rebars were also found to possess excellent weldability due to their low carbon equivalent of 0.2%. We claim: 1. A process for manufacturing microalloyed TMT rebar with improved sub-zero impact toughness comprising the step of: (i) providing steel billet having a composition of 0.05 to 0.1 wt% carbon, 0.3 to 0.5 wt% manganese, 0.05 to 0.2 wt% silicon, upto 0.035 wt% sulfur and atleast 0.008 wt% of microalloying elements comprising niobium and phosphorus wherein the level of phosphorus is in the range of 0.08 to 0.1% when niobium is not present or upto 0.04 wt% when niobium is present in the range of 0.005 to 0.015 wt%, the balance being iron; (ii) heating said steel billet at a temperature between 1200 to 1250°C; (iii) rolling heated billet to rebar; (iv) short intensive cooling the rolled rebar; (v) further cooling in atmosphere. 2. A process as claimed in claim 1, wherein level of carbon in said steel of step (i) is preferably 0.08 wt%. 3. A process as claimed in claim 1, wherein the level of manganese in said steel of step (i) is preferably 0.4 wt%. 4. A process as claimed in claim 1, wherein the level of silicon in said steel of step (i) is preferably 0.1 wt%. 5. A process as claimed in claim 1, wherein the level of sulfur in said steel of step (i) is preferably upto 0.03 wt%. 6. A process as claimed in claim 1, wherein the level of phosphorus in said steel of step (i) is preferably between 0.08 to 0.1 wt% when niobium is not present. 7. A process as claimed in claim 6, wherein said level of phosphorus is 0.09 wt%. 8. A process as claimed in claim 1, wherein said steel of step (i) preferably comprises 0.01 wt% niobium and upto 0.04wt% phosphorus. 9. A process as claimed in claim 1, wherein said steel of step (i) is prepared in twin hearth furnace and cast into ingots. 10. A process as claimed in claim 9, wherein the tap temperature during preparation of said steel in twin hearth furnace is maintained at 1620°C. 11. A process as claimed in claim 9, wherein said ingots are preferably soaked in soaking pit at a temperature of between 1280 to 1300°C. 12. A process as claimed in claim 11, wherein said soaking is carried out for 4 to 5 hours. 13. A process as claimed in any of the preceding claims wherein the said steel after soaking is preferably rolled into billets and said steel of step (i) is in the form of billets. 14. A process as claimed in claim 1, wherein said temperature in step (ii) is maintained at 1250°C. 15. A process as claimed in claim 1, wherein said heated steel of step (ii) is rolled to rebar and optionally finished at a temperature between 980 to 1020°C. 16. A process as claimed in claim 15, wherein said finishing temperature of rebar is preferably 1000°C. 17. A process as claimed in claim 1, wherein said short intensive cooling in step (iv) is carried out in a water cooling system. 18. A process as claimed in any of the preceding claims wherein equalization temperature at cooling bed in step (iv) is maintained between 500 and 700°C. 19. A process as claimed in claim 18, wherein said temperature is maintained at 600°C. 20. A process as claimed in any of the preceding claims wherein the water pressure in said water cooling system is maintained between 15 to 22 kg/cm2. 21. A process as claimed in claim 20, wherein said water pressure is maintained at 20 kg/cm2. 22. A process as claimed in any of the preceding claim wherein said further cooling in atmosphere is carried out till the temperature of the TMT rebar is about100°C. A process for manufacturing high strength weldable microalloyed TMT rebar with improved sub-zero impact toughness is provided by suitably selecting the levels of microalloying elements in the steel. The process comprises providing steel billet having a composition of 0.05 to 0.1 wt% carbon, 0.3 to 0.5 wt% manganese, 0.05 to 0.2 wt% silicon, upto 0.035 wt% sulfur and atleast 0.008 wt% of microalloying elements comprising niobium and phosphorus wherein the level of phosphorus is in the range of 0.08 to 0.1% when niobium is not present or upto 0.04 wt% when niobium is present in the range of 0.005 to 0.015 wt%, the balance being iron, heating said steel billet at a temperature between 1200 to 1250°C, rolling heated billet to rebar, short intensive cooling the rolled rebar and further cooling in atmosphere. The rebars of the present invention have wide applications in the construction sector such as general concrete reinforcement in buildings, bridges and various other concrete structures and especially high rise buildings.

Full Text

Field of invention
The present invention relates to a process for manufacturing of high strength
weldable microalloyed TMT rebar with improved sub-zero impact toughness. TMT
rebars have wide applications in the construction sector such as general concrete
reinforcement in buildings, bridges and various other concrete structures and
especially high rise buildings.
Background of the invention
Thermo-mechanically treated (TMT) rebars are extra high strength reinforcing
bars which eliminate any form of cold twisting. Conventionally TMT rebars are
manufactured by processes where the steel rebars receive a short, intensive
cooling as they pass through a cooling system preferably a water cooling system
after the last rolling stand. This reduction in temperature converts the surface
region of the steel bar to a hardened structure. This phase of intensive cooling is
followed by further cooling in atmosphere so that the temperature between the
core which remains hot and the cooled surface region (rim) is equalized and the
rim later gets tempered by the heat from the core. The resulting structure is a
tempered martensite/bainite zone at the rim periphery and a fine grain
ferrite/pearlite structure in the core. Due to the improved properties of high
strength, combined with toughness and ductility, microalloyed TMT rebars are
preferred for construction purposes over mild steel plain and CTD rebars.
Traditionally high strength carbon-manganese TMT rebars which are widely used
have carbon and manganese in the levels of 0.20 to 0.25% and 1.0 to 1.5%
respectively. The levels of carbon and manganese can be reduced improving
weldability and sub-zero impact toughness keeping the strength intact by
employing microalloying technique. Earlier, in microalloyed TMT rebars, the
previous investigator used niobium between 0.02 to 0.06% or vanadium between
0.07 to 0.15%.
Sub-zero impact toughness is impact toughness below zero degree centigrade
and steel bars having high sub-zero impact toughness are desirable in colder
climatic regions. The known microalloyed TMT rebars possess the required sub-
zero impact toughness for such purposes at reasonably higher percent of niobium
or vanadium.
Objects of the invention
Therefore the main object of the present invention is to provide a microalloyed
TMT rebar which has high sub-zero impact toughness as well as high yield stress,
ultimate tensile strength and ductility at lower cost by the use of low niobium or by
use of phosphorus.
A further object of the present invention is to provide a microalloyed TMT rebar
which possesses low impact transition temperature.
The applicants have now found that microalloyed TMT rebar having substantially
higher sub-zero impact toughness is achieved by suitably selecting the levels of
microalloying elements in the steel which are different from the previous
investigators.
Summary of the invention
Accordingly, the present invention provides a process for manufacturing of high
strength microalloyed TMT rebar with improved sub-zero impact toughness
comprising the steps of:
(i) providing steel billet having a composition of 0.05 to 0.1 wt% carbon, 0.3 to
0.5 wt% manganese, 0.05 to 0.2 wt% silicon, upto 0.035 wt% sulfur and
atleast 0.008 wt% of microalloying elements comprising niobium, and
phosphorus wherein the level of phosphorus is in the range of 0.08 to 0.1%
when niobium is not present or upto 0.04 wt% when niobium is present in
the range of 0.005 to 0.015 wt%, the balance being iron;
(ii) heating said steel billet at a temperature between 1200 to 1250°C;
(iii) rolling heated billet to rebar;
(iv) short intensive cooling the rolled rebar;
(v) further cooling in atmosphere .
Detailed description of the invention
According to the present invention the sub-zero impact toughness of micro-alloyed
TMT rebars has been improved by selecting the level of micro-alloying elements
specially that of phosphorus and niobium in the initial steel compositions. For
phosphorus based TMT rebars where niobium is not present, the preferred level
of phosphorus is selected between 0.08 to 0.1 wt%.
For niobium based TMT rebars the niobium is preferably 0.01 wt% while the
phosphorus content is kept low i.e. up to 0.04wt%.
Preferred levels of other elements in the initial steel composition is 0.08wt%
carbon, 0.4% manganese, 0.1% silicon and up to 0.03% sulfur.
The process for manufacture of the improved TMT rebar comprises preparing
ingots having the above-mentioned steel composition. The ingots are cast
according to conventional methods such as in twin-hearth furnace with tap
temperature preferably at 1620±10°C and using ladles. The cast ingots are
thereafter soaked in a soaking pit at temperatures between 1280 to 1300°C for
around 4-5 hours. The soaked steel is then rolled into billets, which is fed into a
reheating furnace. In the reheating furnace the billets are heated at a temperature
preferably between 1200 and 1250°C for a period of around two hours. The
reheated billets are rolled to TMT rebars finishing at temperature between 980 to
1020°C and at a rolling speed of between 4 to 10 m/sec.
The next and most essential step in the process for manufacturing TMT rebars is
the short and intensive cooling of the steel. The cooling is preferably carried out in
THERMEX water cooling system into which the steel is fed after the last rolling
mill stand. The equalization temperature at cooling bed is maintained between
500 to 700°C. The water in the cooling system is preferably maintained between
15 to 22 kg/cm2. This is followed by a further cooling in atmosphere so that the
temperature between the core and the rim layer is equalized and the rim layer
gets tempered by the heat from the core. The steel is then available as TMT
rebars for application in construction industry.
The process of the present invention results in TMT rebars having low carbon
bainitic steel structure in the rim and acicular ferrite steel structure in the core.
Various metallurgical and mechanical tests were performed to evaluate the
properties of the TMT rebars manufactured by the process of the present
invention and they were found to possess yield stress of at least 415 Mpa,
ultimate tensile strength of at least 485 Mpa, elongation of 18 to 20%. The rebars
have excellent bend and rebend properties. The rebar also has excellent Charpy
impact toughness and weldability due to low carbon equivalent of around 0.2%.
The present invention will now be demonstrated with respect to the following non-
limiting examples:
EXAMPLES
Example - 1: Process for manufacturing of phosphorus based TMT rebars with
improved sub-zero impact toughness.
Steel having composition of 0.08 wt% carbon, 0.4 wt% manganese, 0.1 wt%
silicon, 0.01 wt% sulfur and 0.09 wt% phosphorus with balance iron was cast into
ingots using a twin hearth furnace. The ingots were then soaked in a soaking pit
at a temperature of 1280°C for five hours. The soaked ingots were then rolled into
100 x 100 mm billets. The billets were reheated in reheating furnace at a
temperature 1250°C for two hours. The reheated steel was finish rolled at a
temperature of 1000°C and at rolling speed of 8 m/sec to 32mm diameter rebar.
The rolled steel was then passed through THERMEX water cooling system after
the last rolling mill stand. The equalization temperature is maintained at about
600°C with water pressure of 20kg/cm2. After the phase of intensive cooling the
steel is further cooled in atmosphere so that the temperature between the core
and the rim layer is equalized and the rim layer is tempered by the heat from the
core.
Example - 2: Process for manufacturing of niobium based TMT rebars with
improved sub-zero impact toughness.
Steel having composition of 0.08 wt% carbon, 0.4 wt% manganese, 0.1 wt%
silicon, 0.01 wt% sulfur and upto 0.04 wt% phosphorus, 0.01 wt% niobium with
balance iron was cast into ingots using twin hearth furnace. The ingots were then
soaked in a soaking pit at a temperature of 1280°C for five hours. The soaked
ingots were then rolled into 100 x 100 mm billets. The billets were reheated in
reheating furnace at a temperature 1250°C for two hours. The reheated billets
were finish rolled at a temperature of 1000°C and at rolling speed of 8 m/sec to
32mm diameter TMT rebar. The rolled steel was then passed through THERMEX
water cooling system after the last rolling mill stand. The equalization temperature
is maintained at about 600°C with water pressure of 20kg/cm2. After the phase of
intensive cooling the steel is further cooled in atmosphere so that the temperature
between the core and the rim layer is equalized and the rim layer is tempered by
the heat from the core.
Several metallurgical and mechanical tests were performed and it was found that
the phosphorus and the niobium based steels have yield stress of 450 Mpa,
ultimate tensile strength of 510 Mpa, elongation of 18 to 20%, bend property of 3.d
and rebend property of 4.d. The Charpy impact toughness was found to be 100J
for the phosphorus based TMT steel and 135J for niobium based TMT steel at
room temperature compared to 60J for conventional TMT rebars. It was also
found that the rebars possess impact transition temperature of -42°C for
phosphorus based steel and below -60°C for the niobium based steel compared
to -10°C for conventional TMT rebars.
The rebars were also found to possess excellent weldability due to their low
carbon equivalent of 0.2%.
We claim:
1. A process for manufacturing microalloyed TMT rebar with improved sub-zero
impact toughness comprising the step of:
(i) providing steel billet having a composition of 0.05 to 0.1 wt% carbon, 0.3
to 0.5 wt% manganese, 0.05 to 0.2 wt% silicon, upto 0.035 wt% sulfur
and atleast 0.008 wt% of microalloying elements comprising niobium
and phosphorus wherein the level of phosphorus is in the range of 0.08
to 0.1% when niobium is not present or upto 0.04 wt% when niobium is
present in the range of 0.005 to 0.015 wt%, the balance being iron;
(ii) heating said steel billet at a temperature between 1200 to 1250°C;
(iii) rolling heated billet to rebar;
(iv) short intensive cooling the rolled rebar;
(v) further cooling in atmosphere.
2. A process as claimed in claim 1, wherein level of carbon in said steel of step
(i) is preferably 0.08 wt%.
3. A process as claimed in claim 1, wherein the level of manganese in said steel
of step (i) is preferably 0.4 wt%.
4. A process as claimed in claim 1, wherein the level of silicon in said steel of
step (i) is preferably 0.1 wt%.
5. A process as claimed in claim 1, wherein the level of sulfur in said steel of
step (i) is preferably upto 0.03 wt%.
6. A process as claimed in claim 1, wherein the level of phosphorus in said steel
of step (i) is preferably between 0.08 to 0.1 wt% when niobium is not present.
7. A process as claimed in claim 6, wherein said level of phosphorus is 0.09
wt%.
8. A process as claimed in claim 1, wherein said steel of step (i) preferably
comprises 0.01 wt% niobium and upto 0.04wt% phosphorus.
9. A process as claimed in claim 1, wherein said steel of step (i) is prepared in
twin hearth furnace and cast into ingots.
10. A process as claimed in claim 9, wherein the tap temperature during
preparation of said steel in twin hearth furnace is maintained at 1620°C.
11. A process as claimed in claim 9, wherein said ingots are preferably soaked in
soaking pit at a temperature of between 1280 to 1300°C.
12. A process as claimed in claim 11, wherein said soaking is carried out for 4 to
5 hours.
13. A process as claimed in any of the preceding claims wherein the said steel
after soaking is preferably rolled into billets and said steel of step (i) is in the
form of billets.
14. A process as claimed in claim 1, wherein said temperature in step (ii) is
maintained at 1250°C.
15. A process as claimed in claim 1, wherein said heated steel of step (ii) is rolled
to rebar and optionally finished at a temperature between 980 to 1020°C.
16. A process as claimed in claim 15, wherein said finishing temperature of rebar
is preferably 1000°C.
17. A process as claimed in claim 1, wherein said short intensive cooling in step
(iv) is carried out in a water cooling system.
18. A process as claimed in any of the preceding claims wherein equalization
temperature at cooling bed in step (iv) is maintained between 500 and 700°C.
19. A process as claimed in claim 18, wherein said temperature is maintained at
600°C.
20. A process as claimed in any of the preceding claims wherein the water
pressure in said water cooling system is maintained between 15 to 22 kg/cm2.
21. A process as claimed in claim 20, wherein said water pressure is maintained
at 20 kg/cm2.
22. A process as claimed in any of the preceding claim wherein said further
cooling in atmosphere is carried out till the temperature of the TMT rebar is
about100°C.
A process for manufacturing high strength weldable microalloyed TMT rebar with
improved sub-zero impact toughness is provided by suitably selecting the levels of
microalloying elements in the steel. The process comprises providing steel billet
having a composition of 0.05 to 0.1 wt% carbon, 0.3 to 0.5 wt% manganese, 0.05 to
0.2 wt% silicon, upto 0.035 wt% sulfur and atleast 0.008 wt% of microalloying
elements comprising niobium and phosphorus wherein the level of phosphorus is in
the range of 0.08 to 0.1% when niobium is not present or upto 0.04 wt% when
niobium is present in the range of 0.005 to 0.015 wt%, the balance being iron,
heating said steel billet at a temperature between 1200 to 1250°C, rolling heated
billet to rebar, short intensive cooling the rolled rebar and further cooling in
atmosphere. The rebars of the present invention have wide applications in the
construction sector such as general concrete reinforcement in buildings, bridges and
various other concrete structures and especially high rise buildings.

Documents:

00551-kol-2003-abstract.pdf

00551-kol-2003-claims.pdf

00551-kol-2003-correspondence.pdf

00551-kol-2003-description (complete).pdf

00551-kol-2003-form 1.pdf

00551-kol-2003-form 18.pdf

00551-kol-2003-form 2.pdf

00551-kol-2003-form 3.pdf

00551-kol-2003-letter patent.pdf

00551-kol-2003-pa.pdf

00551-kol-2003-reply f.e.r.pdf

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Patent Number

211442

Indian Patent Application Number

551/KOL/2003

PG Journal Number

44/2007

Publication Date

02-Nov-2007

Grant Date

29-Oct-2007

Date of Filing

24-Oct-2003

Name of Patentee

STEEL AUTHORITY OF INDIA LIMITED.

Applicant Address

RESEARCH & DEVELOPMENT CENTRE FOR IRON & STEEL, DORANDA, RANCHI-834002

Inventors:

#	Inventor's Name	Inventor's Address
1	PANIGRAHI BIMAL KUMAR	RESEARCH & DEVELOPMENT CENTRE FOR IRON & STEEL, STEEL AUTHORITY OF INDIA LTD., DORANDA, RANCHI-834002
2	SINGH VIJAY KUMAR	-DO-
3	SINGH SHOBH NATH	BHILAI STEEL PLANT, STEEL AUTHORITY OF INDIA LTD., BHILAI
4	PRASAD TALLURY CHANDRA SEKHARA	-DO-
5	SINGH RAM BRIKSH	-DO-

PCT International Classification Number

B 05 D 7/24

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA