|
In
the last 15-20 years demand for higher strength deformed bars has been
increasing. The most common of these bars is of strength grade Fe 415. Strength
grade means the value of yield stress or the 0.2% proof stress.
You
may here recall the definitions of yield stress & proof stress;
which are
as below :
Yield
Stress: Stress
(i.e load per unit cross-sectional area) at which elongation
first occurs in the test piece without increasing the load in tensile
test. In
case of steel with no definite yield point, proof stress is applicable.
0.2%
Proof Stress: The
stress at which the non proportional elongation equal to 0.2%
of the original gauge length takes place.
In
India,
deformed bars for concrete reinforcement are being produced
for many
years. The main processes for their production are hot
rolling or hot
rolling followed by cold twisting. The increased yield strength was
being first
achieved by raising carbon & manganese and to a great extent by
cold
twisting.
These
cold
twisted bars, though in extensive use at present, continue to have
inherent problem of inferior ductility, weldability and increased rate of
corrosion due to presence of residual stresses and higher carbon
content.
Additionally, cold twisting being labour intensive, enhances cost of
production
with limitation on production rates.
Increasingly
varied & innovative applications have resulted in growing
demand for larger
diameter bars with similar strength, elongation, weldability and
bendability as
the small size bars. Along with this, there is also a need for these
steel bars
to be welded and fabricated on the site easily. For this strength
&
ductility have to be achieved at the lowest possible carbon content.
The most
challenging requirement is to achieve all these superior properties at
relatively lower cost.
|
