Evaluation of microstructures and mechanical properties of deltha trip steel with different vanadium contents

Abstract

This research studies the effect of adding 0.12 and 0.25 wt% vanadium on the microstructure and mechanical properties of a delta-TRIP steel. Optical microscopy and scanning electron microscope (SEM) were employed to investigate the microstructure of the steel. In addition, hardness measurements and tensile testing were used for the evaluation of the mechanical properties. The X-ray diffraction (XRD) analysis was also applied to analyze the present phases. The structure of the three studied steels before applying the heat treatment cycle consisted of delta-ferrite, allotriomorphic ferrite, martensite, and pearlite. The heat treatment cycle led to the stability of the austenite phase. The addition of 0.12 wt% V resulted in the enhancement of the mechanical properties so that the best combination of strength (866 MPa) and elongation percentage (41 %) was achieved in this steel. Nonetheless, the addition of 0.25 wt% V deteriorated the mechanical properties since the increase in the vanadium content promoted the formation of martensite, decreased the percentage of the retained austenite, and weakened the mechanical properties. However, adding 0.12 wt% V improved the mechanical properties since it increased the strength through solid-solution strengthening and precipitation hardening; no brittle martensite was formed, and lamellar δ-ferrite was achieved. The steel containing 0.12 wt% V, exhibiting a formability index of 35.7 GPa%, is in the range of the third-generation advanced high-strength steels.

Keywords

• Delta-TRIP steel
• Vanadium
• Heat treatment
• Formability index