The present invention relates to a Method for producing corrosion
resistant steel reinforcing bars.
BACKGROUND OF THE INVENTION:
Reinforcing bars find wide application in concrete structure for
taking up the load of the structure during the whole of its life
span. The corrosion of the reinforcing bars, also called the
rebars, resulting into concrete failure is one of the primary
causes of premature deterioration of the concrete structure. In
fact, the largest challenge faced by civil engineers in the world
is the deterioration of concrete infrastructure. The corrosion
product, which has a volume increase exerts pressure resulting in
cracking of the concrete. The loss of steel due to corrosion and
the cost of repairing are quite substantial. Therefore, it
becomes extremely necessary to improve the material
characteristics in terms of strength and corrosion protection.
To meet these properties, the optimal choice is to use corrosion
resistant rebars having microstructure and chemistry controlled
steel. Ideally, the corrosion protection should be provided right
at the time of manufacturing of the steel rebar before it is
encased in the concrete.
Pitting corrosion studies of corrosion resistant steel rebars
produced by known Methods reveal that the corrosion resistant
steel have to be Modified for inducing additional pitting
corrosion resistance.
Further, the corrosion resistant steel rebars are Manufactured
through the ingot route. This expensive process had to be
replaced by a More cost effective route, like the continuous
casting process.
SUMMARY OF THE INVENTION :
Thus, one abject of the present invention is to produce a low
carbon, high strength, high ductility rebar which has a superior
pitting corrosion resistance.
Another object of the present invention is to produce corrosion
resistant steel rebars through a More cost effecting steel
processing route like continuous casting billets.
A further object of the present invention is to produce corrosion
resistant steel rebars that creates a dense, crack-free adhering
oxide layer on the surface of the rebar delaying the
depassivation process which precedes the actual corrosion of the
rebars.
Yet another object of the present invention is to provide
corrosion resistant steel rebars with improved properties, like
Mechanical strength, fire resistance, fatigue and seismic
resistance, besides corrosion resistance.
These and other objects of the invention are achieved by
producing the corrosion resistante steel rebar through a
combination of steel processing route, such as continuous casting
billets and thermo-mechanically treated (TUT) rolling.
Any rebar embedded in concrete oust fulfill the required
properties such as Mechanical, corrosion resistance and seismic
resistance.
In the present invention, THT - CRS rebars are produced using a
judicious selection of corrosion resistant elements complemented
by a special thermo-mechanical treatment (THT) route.
Thus, the present invention provides a method for producing
corrosion resistant steel rebars comprising the steps of:
judicious selection of steel billets through the continuous
casting route for better weldability and corrosion resistance
keeping the chroaiua level within 0.2X for better pitting
resistance in oxygen deficient conditions; keeping the silicon
content within 0.18 to 0.35% for improved corrosion resistance;
keeping the tundish teaperature to at least 156 °C; reheating
the billets in respective reheating furnace; laying head
temperature between 630 to 440 °C for disasters upto 12 as and
580°C to 610°C for disasters froa 16 to 40 mm; and treating
thermo-mechanically through tempeore process giving an outer rim
of tempered and ferrite-peartite core structure.
The aethod for producing corrosion resistant steal reinforcing
bars of the present invention will now be described in detail
with the help of accoapanying drawings and tables where :
Fig. 1 shows pitting potential of mild steal and CRS;
Fig. 2 shows effect of fluoride ion on pitting
The cheaistry of steel is optimized in the present invention in
such a way that it could be continuously cast without ladle
furnace and at the same time giving the finished product a
superior corrosion resistance capability over aild steel.
A judicious selection of steel composition is made containing
corrosion resistant elements as well as elements for iaprovidng
other properties like aechanical strength, better weldability and
fatigue, fire and seismic resistance.
The pitting tendency of corrosion resistant steel rebar in
simulated condition at concrete enviroment was determined by
cyclic polarization technique (ASTHB 61). To simulate the actual
condition in steel rebar embedded in concrete, a chloride
solution that is saturated with sodium hydroxide was used. The
pitting potential of mild steel and THT CRS rebar in 0.01 N NaOH
having 500 pom chloride + 500 ppm sulphate and 1000 ppm chloride
+ 500 ppm sulphate and 10 ppm of fluoride respectively wera
measured. In absence of any fluoride the pitting potential of CRS
rebar is 10 to 12 times higher than mild steel while with 10 ppm
fluoride, it is more than five times as shown in Figs. 1 and 2.
In the mechanism of imparting corroosion resistance when steel is
alloyed with copper and phosphorus, the oxide layer formed is
dense with very little pores/cracks and thus cuts down tha
ingress of aggressive species to the steel surface. This initial
oxide layer, being flawless, also helps in promoting better
bonding between concrete and rebar. Copper reacts with chloride
ions electrochemically forming copper chloride capper hydroxide
[CuCl2 3Cu(0H)2 ] layer which deposits on the steel surface as
insoluble product and thus retarding further corrosion. Copper
also plugs the pore in the rust. In industrial atmosphere where
sulphur dioxide is prevailing, copper forms an adhesive layer of
capper sulphate capper hydroxide [CuSO4.3 Cu(OH)2 ]. Phosphorus
acts as an inhibitor and forms a complex oxide responsible for
protection. The combination of capper and phosphorus gives a
synergistic effect on the reduction of corrosion rate by acting
as strong oxygen — depolariser.
The chromium content should be drastically reduced in the
existing chemistry to avoid pitting corrosion. In the presence of
chlorine ions, the corrosion product of chromium are coarse and
the potential difference between the base metal and the corrosion
product is large leading to increase in active area possessing
higher pitting corrosion propensity.
Proper selection of carbon, silicon, copper, phosphorus and
chromium contents can give better weldability and corrosion and
pitting resistance to the steel rebars produced. The chromium
level has to be reduced to the maximum in order to have better
pitting corrosion resistance in an oxygen deficient situation
such as the one prevailing inside the concrete. The chromium
level should be reduced to 0.2% or less. The silicon content
should be kept within a range of 0.18 to 0.39% for improving the
corrosion resistance.
The following additions can be made :
Cu - 500 kg in vessel
FeP - 400 kg in ladle during tapping
FeCr - 200 kg in ladle during tapping
The thermo-mechanical treatment involves a tempeore process
giving an outer rim of tempered and ferrite-peartite core
structure. The other parameters used in the process are shown
below :
1) Tundish temperature 1560 °C minimum
2) 100X billets of 127 mm2 through a continuous casting (CC)
route.
3) Reheating of billets in respective reheating furnaces of
ware rod sill (WRH) and M mill.
4) Laying head temperature of 630- 640°C for 8, 10, 12 mm in
WRM.
5) Laying a head temperature of 900°C to 610°C in M mill for
16 mm to 40 mm.
The chemical composition of the corrosion resistant steel rebars
produced by the method of the present invention can be shown as
in Table I.
The bond strength of rebar embedded in concrete was found to be
more than 90% over plain bar at 0.29 slip (as par IS 1786t, it
should be mort than 80%).
These rebars ara mora suitable for use in places prona to fira
hazards bacausa of high thermal stability of tha haat treated
structura and tha total absence of cold worked structural zona.
TMT—CR8 bars ramain unaffactad even hours of axposura to
tasparatura of 450 to 500.
Tha rabars producad Mara tastad in a wide range of corrosiva
medium in accordance with ASTM standards for corrosiva
resistance. Tha tasts included a salt spray, sulfur dioxide
chamber, alternate immersion, atmospheric exposure near seaside
and industrial environment and electrochemical tests. The results
obtained are shown in table 3 below which clearly reveals the
superior corrosion resistance compared to conventional mild steel
CTD rebars.
In concluding the present invention produces * Method of
production of thereto Mechanically treated corrosive resistant
steel rebars with Modified cheMistry through the continuous
casting (CC) route without ladle furnace. The entire gamut of
rebar from 8 mm to 40 mm diameter are produced through the
continuous casting and thermo Mechanical treatetent process.
Design of cheMistry is amenable to the CC route and at the same
tiMe providing low carbon equivalent for better weldability, high
ductality suitable for seisMic zones and less prone to pitting
corrosion.
The cheMistry is optimized so that additional properties can be
obtained by varying the process parameters.
Although the invention has been described with a certain degree
of particularity, the details are to be considered in all
respects as illustrative and not restrictive. It is understood
that various changes can be made by one skilled in the art
without departing from the spirit and scope of the invention.
Studies conducted on concrete beam column Joints reinforced with
CRS bars to evaluate its perforMance under repeated reversed
loading with inelastic strains as encountered during earthquake,
has revealed the superior seisMic resistance property of TMT -
CRS rebars of the present invention.
WE CLAIM :
1. A method for producing corrosion resistant steel rebars
comprising the steps of :
- judicious selection of steel billets through the continuous
casting route for better weldability and corrosion resistance)
- keeping the chromium level within 0.2 X for better pitting
resistance in oxygen deficient conditions!
- keeping the silicon content within 0.18 to 0.39 % for
improved corrosion resistance)
- keeping the tundish temperature to at least 1560°C ;
- reheating the billets in respective reheating furnace ;
- laying head teaperature between 630 to 640°C for diameters
upto 12 mm and 980 to 610°C for diameters from 16 to 40 mm) and
- treating thermo—mechanically through tempeore process
giving an outer rim of tempered and ferrite-peartite core
structure.
2. A method for producing corrosion resistant steel rebars
substantially as herein described and illustrated.

The invention provides a method for producing corrosion resistant steel rebars.
Steel billets are judiciously selected through the continuous casting route for
obtaining better weldability and corrosion resistance. The chromium level is kept
within 0.2% for better pitting resistance in oxygen deficient conditions. The
silicon content is kept within 0.18 to 0.35 % for improved corrosion resistance.
The tundish temperature is kept to at least 1560°C and the billets are reheated in
respective reheating furnaces. The laying head temperature is kept between 630
to 640 for diameters up to 12 mm and 580 to 610°C for diameters from 16 to 40
mm. The thermo mechanical treatment through tempeore process gives an
outer rim of tempered and ferrite-peartite structure.