HIGH STRENGTH AND HIGH DUCTILITY OF IF STEEL FOR AUTOMOBILE APPLICATION

Abstract

Interstitial free (IF) steels have excellent formability [plastic anisotropy (rm) ? 1.8 and strain - hardening exponent (n)? 0.221 in light of the nonattendance (low amount) of interstitial atoms (0.005wt% C and 0.009wt% N) in the matrix. Absence (or very low amount) of these elements inhibits very low strength in steel which is non-desirable in automotive industry due to its poor impact resistance, low yield strength, and other mechanical properties. To improve the microstructure and mechanical properties of the steel, 0.005wt% carbon IF steel were systematically optimized by a series of heat treatment processes at various temperatures in Gleeble® 3800 thermo mechanical simulator. The processes include annealing, normalizing and water quenching after heating the sample within (a+y) and single phase yregion. The microstructural analyses were performed using X-Ray Diffraction, light optical microscopy and scanning electron microscopy. The formation of carbide phases may not be seen and grain refinement also appears to be negligible within 3-5 min of isothermal holding during heat treatment. However an increase in hardness value of the as-prepared (VI-IN 89) sample to VHN 146 for annealed sample and VHN 173.42 for water quenched under 5 Kg load, raising a further interest that the IF steel may be an important contender for the use in automobile sector, provided microstructure analysis is performed in a systematic way. From the present investigation it is most likely that acicular ferrite formation is the key for the increase in hardness and along with the deformation band a possible signature of the martensitic transformation.