Nanobainitic steels are microstructures presenting high mechanical strength, good ductility and toughness; characteristics which make them attractive for agricultural, mining, railway, and defense applications, among others [1]. In recent years, the study of nanobainitic steels with a high content of silicon (and carbon) has called the attention of the industrial sector, since it inhibits the precipitation of carbides during the bainitic reaction and increases the strength, enhancing the mechanical properties. This microstructure is obtained by an austempering heat treatment, which consists of austenitization followed by quenching in a salt bath at a temperature between the bainite onset (Bs) and martensite onset (Ms) temperatures. The final microstructure is a mixture of bainitic ferrite plates and retained austenite, the latter appearing with two morphologies, thin films of C-enriched retained austenite and block austenite [2]. This microstructure confers to the steels high ultimate tensile strength (UTS) values above 2.0 GPa, high yield strength (YS) above 1.5 GPa, total elongation of 5-20%, and fracture toughness of 45-90 MPa/ m1/2 [3,4].

The development of these microstructures has focused on their production from formed steels and there is very little information related to bainitic transformation from cast steels [5-7]. However, casting makes it possible to obtain parts with complex finished or near-finished geometries, which sometimes cannot be manufactured by other processes. Therefore, it is possible to take advantage of the current knowledge on nanostructured bainitic steels and transfer it to cast steels, which opens up a range of possibilities that would allow the application of nanobainitic steels to be expanded while improving the mechanical properties of cast steels.

However, this transfer implies facing some challenges, as there are differences between cast and formed steels, which should be studied with a view to future industrial application. The main microstructural differences between these two types of steels are the distribution of non-metallic inclusions and the degree of micro-segregation (Figure 1), which can affect the final microstructure of the product, the kinetics of the bainitic transformation and the mechanical properties. This work explores these fronts in depth, so that a significant contribution can be made to the use of nanostructured bainitic steels for the fabrication of castings for applications where excellent mechanical properties are required.

References

[1] A. Saccocco et.al; Bainitic steels for new rail materials, 2006.

[2] H. Bhadeshia et.al; Bainite in Steels: Theory and Practice, 2015.

[3] C.Garcia-Mateo et.al; Ultra-high strength bainitic steels, 2005, 45, 1736-1740.

[4] L.Morales-Rivas et.al; Ductility of Nanostructured Bainite, 2016, 6, 302.

[5] R. Voigt et.al; Development of Austempered High Silicon Cast Steels, 1985, 93, 453-462.

[6] J. Son et.al; Effects of austempering conditions on the microstructures and mechanical properties in Fe-0.9%C-2.3%Si-0.3%Mn Steel, 2010, 16, 357-361.

[7] J. Cornide et.al; An assessment of the contributing factors to the nanoscale structural refinement of advanced bainitic steels, 2013, 577, S43-S47.