Evolution of cementite composition along the processing of cold-rolled and annealed Dual-Phase steels

Abstract

Cementite composition in a ferrite-pearlite steel dedicated to the industrial production of Dual-Phase steels has been investigated after hot-rolling and during annealing. By coupling TEM investigations and local field composition modeling using THERMOCALC and DICTRA softwares, its progressive enrichment in manganese is explained.This enrichment begins during hot-rolling when austenite first decomposes into ferrite and pearlite. Measured enrichments are consistent with a pearlite transformation process governed by two simultaneous local equilibria at the growing fronts between ferrite, cementite and austenite. During subsequent coiling operation and annealing, cementite enrichment in manganese continues. Observed composition profiles can be reproduced considering diffusion simulations with local equilibrium at ferrite/cementite interface. Nevertheless, the known diffusion coefficient of manganese in cementite has been reassessed to explain observed composition gradients.Both thermodynamic approach of pearlite phase transformation and diffusion simulation must be coupled to follow seamlessly manganese composition gradients in cementite all along the manufacturing process. Such calculation is the key to understand the inheritance effects of hot-rolling on final properties on Dual-Phase steels.