Effects of Austenitizing Temperature and Cooling Rate on the Phase Transformation Texture in Hot Rolled Steels

Abstract

The effects of austenitizing temperature and cooling rate on the microstructures and textures produced by phase transformations in high strength hot rolled Fe-C-Mn steel plates are investigated using orientation imagining microscopy. Samples machined from the plates are austenitized at temperatures between 820-950°C during 30 minutes and quenched in either iced-water, water or oil. Finally, the quenched samples are tempered at 450°C during 30 minutes. Characterization of microstructure and textures produced by these heat treatments was performed by conventional metallography using a reflected light microscope and orientation imaging microscopy using backscattered-electron diffraction patterns in a scanning electron microscope with thermo-ionic electron source.

The results show that the microstructure and texture produced under a given combination of austenitizing temperature and cooling rate are strongly dependent on the mechanism involved in the phase transformation of the austenite (γ). High austenitizing temperatures and cooling rates promote martensitic transformation and development of textures containing significant volume fractions of Br, Cu, transformed-Cu and transformed-Br orientation components. In this case, the austenite and martensite phases are clearly related through the Kurdjumov-Sachs orientation relationship. In contrast, low temperatures and low cooling rates result in a complex mixture of transformation products, such as polygonal ferrite, Widmanstäten ferrite, martensite, bainite and pearlite. The textures formed under these conditions are quite different and contain significant volume fractions of cube, rotated-cube, Goss and rotated-Goss components, following the Bain orientation relationship.