AUSTENITE DECOMPOSITION DURING CONTINUOUS COOLING OF AN HSLA-80 PLATESTEEL

Decomposition of fine-grained austenite (10-mu m grain size) during co ntinuous 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 meas urements, light microscopy, scanning electron microscopy, transmission electron microscopy, and microhardness testing. Between 750 degrees C and 600 degrees C, austenite transforms primarily to polygonal ferrit e over a wide range of cooling rates, and Widmanstatten ferrite sidepl ates frequently evolve from these crystals. Carbon-enriched islands of austenite transform to a complex mixture of granular ferrite, acicula r ferrite, and martensite (all with some degree of retained austenite) at cooling rates greater than approximately 5 degrees C/s. Granular a nd acicular ferrite form at temperatures slightly below those at which polygonal and Widmanstatten ferrite form. At cooling rates less than approximately 5 degrees C/s, regions of carbon-enriched austenite tran sform to a complex mixture of upper bainite, lower bainite, and marten site (plus retained austenite) at temperatures which are over 100 degr ees C lower than those at which polygonal and Widmanstatten ferrite fo rm. Interphase precipitates of copper form only in association with po lygonal and Widmanstatten ferrite. Kinetic and microstructural differe nces between Widmanstatten ferrite, acicular ferrite, and bainite (bot h upper and lower) suggest different origins and/or mechanisms of form ation for these morphologically similar austenite transformation produ cts.