Mechanism of Strengthening by Vanadium Carbide Precipitation in Pearlite in Microalloyed Steels

Abstract

In microalloyed steel, industrial candidate for application to parts of automobiles, we investigated strengthening mechanism by vanadium carbide (VC) precipitation in the pearlite. The specimens, containing 0.1–0.65% carbon (C) and 0–0.4% vanadium (V), were quenched from 1200 to 650–550°C and kept at this temperature. The more V and the lower transformation temperature, the higher strength can be obtained. The maximum 0.2% proof strength is determined to be 1450 MPa for 0.65%C–0.4%V steel and 1200 MPa for 0.65%C–0.2%V steel. VC is found to precipitate at interphase boundaries between the austenite and the ferrite in the pearlite. When the transformation temperature is lowered, the VC size becomes finer and the precipitation density increases. These results suggest that the strengthening mechanism by VC is the Orowan-type. The amount of precipitation strengthening by VC was calculated by Ashby–Orowan equation. Both the calculated and the experimental results were determined to have good correlation. Strengthening mechanism by VC and that by pearlite lamellar would be competitive, however the amount of precipitation strengthening by VC in pearlite was close to that in ferrite.