Development and Application of a Thermal Microstructure Model of Laminar Cooling of an API X70 Microalloyed Steel

Abstract

A thermal microstructure model of laminar cooling of X70 microalloyed steel skelp was developed to predict the effect of the laminar cooling temperature profile on the through thickness skelp microstructure. Plant trials using infrared video imaging were undertaken to establish the laminar cooling conditions prevalent in the industrial cooling system. The infrared video temperature measurements were used to develop a finite element thermal model of the skelp transiting the entire laminar cooling system. Dilatometer testing of the X70 steel with cooling rates ranging from 1 °C/s to 120 °C/s was undertaken to develop the CCT curve and to quantify austenite decomposition. The predicted thermal profile from the finite element model and the phase transformation behaviour were combined into a thermal microstructural model capable of predicting the phases that would develop through the skelp thickness as a function of the laminar cooling profile. The predicted through thickness microstructures were verified from electron backscattered diffraction (EBSD) phase analysis of industrially produced API X70 skelp.