INTERSTITIAL FREE STEELS

ByY UMAMAHESH(212315011)B PRAVALLIKA(212315018)

Introduction:Commercially developed in Japan in 1970The 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 40-70ppmBy 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.

Composition &TypesTypical 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

Properties:Low yield strengthHigh plastic Strain ratio( r-value)High strain rate sensitivity Good formabilityHigh strain hardening coefficient Low YS/UTS ratioAge hardening resistance

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

Production Sequence:

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:

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.

Liquid steel in the reaction vessel is degassed

Degassed steel flows back throughoutlet snorkel in to steel ladle

Buoyant force created due to densitydifference stirs the bath

Alloy addition depends up on the super heat of liquid steel

Process characteristics and developments:

RH process will depends on the following factors:Rate of steel recirculationSnorkel diameterRadius of the equipmentPosition 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

Addition of C StabilizersIf 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)+0.02It Nb is also added along with Ti then

Ti=(3.42×%N)+(1.5×%S) and

Nb= 7.75× %C

Precipitates

Precipitates formed due to the addition of Ti

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.

Applications:Automotive body parts.House hold appliancesDeep drawn parts

trunks tail gates doors Linings wheel arches

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.

References:http://www.intechopen.com/books/recrystallization/physical-m

etallurgy-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