Modelling of microstructure evolution in advanced high strength steels

Abstract

There is currently a significant development of new families of steels, i.e. advanced high strength steels,
in response to the demands of the automotive and construction industries for materials with improved property
characteristics. The austenite-ferrite transformation is the key metallurgical tool to tailor the properties
of steels. The design of processing paths that will lead to the desired microstructures is increasingly been aided
by computer simulations. The present paper illustrates state-of-the-art microstructure modelling approaches
for low carbon steels considering three important processing aspects: (i) run-out table cooling of hot-rolled
steels, (ii) intercritical annealing of cold-rolled sheets, (iii) girth welding of linepipe steels.
Phenomenological models based on the Johnson-Mehl-Avrami-Kolmogorov (JMAK) approach incorporating
additivity are now available to describe phase transformations during run-out table cooling of microalloyed
steels. Strengths and limitations of this approach will be discussed. Process models for intercritical annealing
require an accurate description of the austenite formation kinetics where morphological complexities can be
captured using the phase field approach. During girth welding the control of the microstructure in the heat
affected zone (HAZ) is of paramount importance. The HAZ experiences rapid thermal cycles and steep
temperature gradients. Phase field modelling is an excellent tool to describe the role of these spatial
constraints as will be illustrated for austenite grain growth.