Influence of Heat Treatment on Microstructure Stability and Mechanical Properties of a Carbide-Free Bainitic Steel

The ever-rising demands on fuel efficiency and safety in the automotive industry have resulted in the inception of advanced high strength steels. The newest candidate of advanced high strength steels achieves considerable mechanical attributes by implementing a clear-cut heat treatment to a low-alloyed steel.

Carbide-free bainitic steels appear to be preferable candidates for automotive industry to satisfy the high demands of increasing safety and decreasing vehicle weight. Their fine-grained multi-phase microstructure leads to a balanced blend of ductility and high strength. The high strength is referenced to a fine-grained martensitic or bainitic matrix, while an increased fraction of retained austenite with suitable stability leads to improved ductility.

Addition of approximately 1.5m% silicon suppresses the formation of carbides leaving carbon available for the stabilization of the remaining austenite. Nanometer-sized austenite films with high carbon content are situated in between subunits of bainite while the austenite between sheaves of bainite shows a blocky morphology. Nevertheless, the austenite stability is affected by its chemical composition and the combination of size, distribution and morphology.

A comprehensive understanding of the relationship between heat treatment, mechanical properties, and microstructure is needed in view of the microstructural evolution. Therefore, researchers led by Dr. Christina Hofer from the Montanuniversität Leoben in Austria studied carbide-free bainitic steel to describe the occurring phases in detail and evaluated their effect on mechanical attributes. Metallographic as well as high-resolution methods were applied to identify and characterize all phases in dependence of the isothermal transformation temperature. Their research work is published in Advanced Engineering Materials.

The authors used cold-rolled sheet samples which were heat-treated for microstructural analysis. The specimens were heated to a 900°C austenitization, annealed and finally, quenched. The authors applied annealing times between 100s and 1000s. During this time, the austenite partially transformed to carbide-free bainite. The research team observed that LePera etching could be successfully applied to distinguish between martensite and bainite and was appropriate for determining the end of the phase transformation. They then determined the austenite phase fraction magnetically.

The partial transformation of austenite to martensite as the samples cooled to room temperature led to the formation of martensite-austenite islands. As these islands transformed last, they had a high defect density and could be observed based on their lower image quality in electron backscatter diffraction maps.

Atom probe tomography measurements of every phase indicated an inhomogeneous carbon distribution. Bainitic ferrite was depleted in carbon while retained austenite was enriched in carbon to different extent dictated by its morphology. The austenite fraction increased with the isothermal transformation temperature; therefore, the deformability initially increased as compared to lower transformation temperatures.

For more quantities of retained austenite, its stability decreased owing to the limited amount of carbon causing an early transformation in the course of plastic straining. Additionally, the increasing fraction of martensite-austenite islands reduced the deformability.