In situ measurement of austenite grain growth and recrystallization using laser ultrasonics

Abstract

The development of next generation process models and advanced high-strength steel products for thin slab casting and direct rolling requires quantification of microstructure evolution during thermomechanical processing. Laser ultrasonics is a non-contact in-situ method to record grain growth, recrystallization and phase transformations in metals and alloys. Here, we will present an improved experimental design that facilitates a continuous microstructure measurement through the various stages of simulated hot rolling from reheating to runout table cooling using a Gleeble thermomechanical simulator equipped with a laser ultrasonics for metallurgy (LUMet) system. Austenite grain growth and static recrystallization after hot deformation are quantified based on attenuation of the ultrasound waves whereas austenite decomposition can be recorded with the changes in ultrasound velocity during the phase transformation. Further, the LUMet results for a microalloyed low carbon steel are validated with conventional techniques including optical and electron microscopy as well as double-hit tests. These experimental studies demonstrate the capabilities of laser ultrasonics in the identification of both normal and abnormal grain growth, non-recrystallization temperature, recrystallization, and austenite decomposition kinetics in a single test for a given processing path, as well as its potential for accelerated optimization of process control under industrial rolling conditions.