interstitial free steels preparation , properties and it's applications

1. By
Y UMAMAHESH(212315011)
B PRAVALLIKA(212315018)

2. Introduction:
 Commercially developed in Japan in 1970
 The term interstitial steel or IF steel refers to the fact that there are no
interstitial solute atoms to strain the solid iron lattice, resulting in very soft
steel.
 We can reduce C amount by Vacuum Degassing up to 40-70ppm
 By adding stabilizing elements such Ti & Nb we can reduce C amount to
below 30ppm by forming subsequent carbides.
 Also termed as clean steels as the total volume fraction of precipitates is very
less.

3. Composition &Types
 Typical IF steel composition:
Types of IF steels:
1. Titanium stabilized IF steels.
2. Niobium stabilized IF steels.
3. Ti & Nb stabilized IF steels.
C Si Mn P S N Al Nb Ti
0.002 0.01 0.15 0.01 0.01 0.0025 0.04 0.016 0.025

4. Properties:
Low yield strength
High plastic Strain ratio( r-value)
High strain rate sensitivity
Good formability
High strain hardening coefficient
Low YS/UTS ratio
Age hardening resistance

5. Importance:
In contrast to that of conventional drawing quality steels , the weight reduction
potential of these steel products increases with the depth of drawing.
IF steels has ability to form crystal orientations favourable to deep drawing by
continuous annealing.
YS/UTS ratio-----Excellent deep drawability, Good strain distribution
High strain ratio(r)---good deformation behaviour

6. Production Sequence:

7. Vacuum Degassing:
 With the increase demand for ultra low carbon steel, it becomes essential
to establish an effective refining process for ultra low carbon steel.
Mechanism of RH Vacuum treatment:

8. Steps in RH Process:
Preheating of Reacting vessel to 900-15000C
Reaction vessel lowered to liquid steel
Reaction vessel is evacuated so that liquid steel rise in the vessel
Lifter gas (usually Ar) introduced. It will create buoyant force
to increase the speed of the liquid steel rising.

9. Liquid steel in the reaction vessel is degassed
Degassed steel flows back through
outlet snorkel in to steel ladle
Buoyant force created due to density
difference stirs the bath
Alloy addition depends up on the super heat of liquid
steel

10. Process characteristics and developments:
RH process will depends on the following factors:
 Rate of steel recirculation
 Snorkel diameter
 Radius of the equipment
 Position and number of tuyers.
Other developments in RH process:
 use of oxygen during RH treatment is to enhance the decarburization reaction
this is process.
Eg: RH-OB

11. Addition of C Stabilizers
If titanium is used alone as the stabilizing agent then the aim Ti content is
calculated from the formula:
Ti= (4×%C)+(3.42×%N)+(1.5%×%S)+0.02
It Nb is also added along with Ti then
Ti=(3.42×%N)+(1.5×%S)
and
Nb= 7.75× %C

12. Precipitates
Precipitates formed due to the addition of Ti

13. Bake hardening of IF steels:
Bake hardening is a diffusion controlled process involving the migration of
solute carbon atoms within the iron lattice. yield strength will increase due to
bake hardening process.
These steels don’t exhibit bake hardening as they have no interstitial elements
in solution.
However the chemistry and processing of these steels, can be adjusted to leave
15-25ppm carbon in solution to give bake hardening effect for increasing
strength.

14. Applications:
Automotive body parts.
House hold appliances
Deep drawn parts
 trunks
 tail gates
 doors
 Linings
 wheel arches

16. Conclusion:
 The automotive industry aims to reduce the weight of outer-body car
panels while maintaining strength , formability and dent resistance.
However conventional high strength steels have insufficient formability to
meet drawing requirements of today’s complex outer body car panels. In
the recent years low and ultra low carbon steels like interstitial free,
interstitial free high strength carbon steels are extensively used for the
auto bodies.

17. References:
 http://www.intechopen.com/books/recrystallization/physical-
metallurgy-and-drawability-of-extra-deep-drawing-and-interstitial-free-
steels
 http://ispatguru.com/interstitial-free-steels/
 http://ispatguru.com/rh-vacuum-degassing-technology/
 http://apac.totalmateria.com/page.aspx?ID=Articles&LN=EN