Modelling of microstructure evolution in advanced high strength steels

摘要

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 thefohnson-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.