In this study a new method is introduced to produce bimodal grain structures in low carbon steels. This method is based on cold rolling of dual phase structures and appropriate annealing treatments. The difference in the recrystallization behaviour of ferrite and martensite yields a heterogeneous microstructure with a distribution of coarse and fine grains. These types of microstructures are of interest for optimizing the balance of strength and uniform elongation in ultra-fine grained low carbon steels.

In this study, austenite formation from hot-rolled (HR) and cold-rolled (CR) ferrite-pearlite structures in a plain low-carbon steel was investigated using dilation data and microstructural analysis. Different stages of microstructural evolution during heating of the HR and CR samples were investigated. These stages include austenite formation from pearlite colonies, ferrite-to-austenite transformation, and final carbide dissolution. In the CR samples, recrystallization of deformed ferrite and spheroidization of pearlite lamellae before transformation were evident at low heating rates. An increase in heating rate resulted in a delay in spheroidization of cementite lamellae and in recrystallization of ferrite grains in the CR steel. Furthermore, a morphological transition is observed during austenitization in both HR and CR samples with increasing heating rate. In HR samples, a change from blocky austenite grains to a fine network of these grains along ferrite grain boundaries occurs. In the CR samples, austenite formation changes from a random spatial distribution to a banded morphology.

In this study, ultrafine grained dual phase structures have been developed in a plain low carbon steel, 0.17C and 0.74Mn (wt pct). The approach is based on rapid heating of a very fine ferrite-carbide aggregate into the intercritical annealing region followed by water quenching. This rapid heat treatment results in an ultrafine grained dual phase steel with improved properties. The effect of thermomechanical processing parameters such as heating rate and intercritical annealing time on the microstructure and mechanical properties have been examined. The key factors contributing to the grain refinement are uniform distribution of nanosize cementite particles acting as potential sites for austenite nucleation as well as the limited time available for coarsening of the microstructure. The mechanical properties of the present ultrafine grained dual phase steel show an excellent combination of strength and uniform elongation because of considerable work hardening.

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There is renewed interest in the investigation of austenite formation due to the development and increased use of advanced high strength steels for automotive applications. Intercritical annealing is an essential processing step for cold rolled and coated steel products with multi-phase microstructures. During intercritical annealing the initial ferrite-pearlite microstructure transforms partially to austenite. Models for the austenite formation are critical to predict the austenite fraction as a function of the thermal cycle thereby facilitating the design and control of robust processing paths. Modelling the austenite formation is challenging because of the morphological complexity of this transformation. Phase field models are a powerful tool to describe the evolution of microstructures with complex morphologies, e.g. formation of finger-type features during austenite formation. The present paper gives an overview of model approaches for the austenite formation. Phase field simulations are presented for two scenarios: (i) austenite formation from a fully pearlitic structure with a lamellar arrangement of carbide aggregates and (ii) austenite formation from ferrite-pearlite microstructures. Simulation results are compared with experimental observations for pearlitic steels. The challenges are delineated for the development of austenite formation models with predictive capabilities.