Austenite decomposition during continuous cooling of an HSLA-80 plate steel

Abstract

Decomposition of fine-grained austenite (10-µm grain size) during continuous cooling of an HSLA-80 plate steel (containing 0.05C, 0.50Mn, 1.12Cu, 0.88Ni, 0.71Cr, and 0.20Mo) was evaluated by dilatometric measurements, light microscopy, scanning electron microscopy, transmission electron microscopy, and microhardness testing. Between 750 °C and 600 °C, austenite transforms primarily to polygonal ferrite over a wide range of cooling rates, and Widmanstatten ferrite sideplates frequently evolve from these crystals. Carbon-enriched islands of austenite transform to a complex mixture of granular ferrite, acicular ferrite, and martensite (all with some degree of retained austenite) at cooling rates greater than approximately 5 °C/s. Granular and acicular ferrite form at temperatures slightly below those at which polygonal and Widmanstatten ferrite form. At cooling rates less than approximately 5 °C/s, regions of carbon-enriched austenite transform to a complex mixture of upper bainite, lower bainite, and martensite (plus retained austenite) at temperatures which are over 100 °C lower than those at which polygonal and Widmanstatten ferrite form. Interphase precipitates of copper form only in association with polygonal and Widmanstatten ferrite. Kinetic and microstruc-tural differences between Widmanstatten ferrite, acicular ferrite, and bainite (both upper and lower) suggest different origins and/or mechanisms of formation for these morphologically similar austenite transformation products.