High strength IF steels

These steels have been designed to provide an excellent combination of drawability and mechanical strength based on their specific interstitial-free (IF) metallurgy.

These steels are hardened by adding manganese, silicon and phosphorous in solid solution to the ferrite. The metallurgy of IF steels optimises their drawability. Their low YS/UTS ratio and high strain-hardening coefficient or n-value gives them excellent suitability for deep-drawing and good strain distribution. Their high anisotropy coefficient or R-value translates into good necking behaviour, which also beneficial to deep-drawing.

These steels are particularly suitable for complex parts requiring high mechanical strength at the same time, such as wheel arches, toe-boards, reinforcements, etc. Their high strain-hardening potential during forming gives them good dent resistance on deep-drawn parts luggage compartments, tailgates, doors, linings, wheel arches, etc.). CR180IF to CR240IF grades can be used, with certain coatings, to manufacture visible parts such as door panels. CR270IF grade is designed for more complex structural parts (longitudinal beams, cross members, suspension and chassis components, etc.).

With their high mechanical strength ensuring good fatigue and impact resistance, these steels are intended for structural parts (longitudinal beams, cross members, B-pillars, etc.) as well as for skin parts, where they provide good dent resistance.

Compared to conventional drawing grades, the weight reduction potential of these products increases with drawing depth.

While the ArcelorMittal grades conform perfectly well to the indicated EN standards, ArcelorMittal grades generally offer tighter mechanical properties (see table below).

The above indicative table summarizes the European and VDA standards corresponding to the ArcelorMittal product range.

A_80mm %: Percentage elongation after fracture using a specimen with gauge length L₀ = 80 mm (ISO 6892-1 type 2 (EN20x80))
A_50mm %: Percentage elongation after fracture using a specimen with gauge length L₀ = 50 mm (ISO 6892-1 type 1 (ASTM12.5x50) or type 3 (JIS25x50)
A%: Percentage elongation after fracture using a proportional specimen with L₀ = 5.65 (So)^1/2
Ag %: Percentage plastic extension at maximum force
BH₂: Increase in yield strength between a reference condition after a 2% plastic pre-strain and the condition obtained after heat treatment (170°C-20minutes)

IF steels offer excellent drawability for their mechanical strength level. This behaviour is associated with very good elongation at rupture, very good strain-hardening coefficient and normal anisotropy.

The diagram opposite shows examples of forming limit curves for the IF family of steels in 1.00 mm thickness.

ArcelorMittal has an extensive database on the forming of IF steels. To incorporate these steels as from the design stage, a team of experts is available to carry out specific forming studies based on modelling or shop-floor experience.

IF steels can be readily welded by all welding processes.