Effect of Microsegregation and Bainitic Reaction Temperature on the Microstructure and Mechanical Properties of a High-Carbon and High-Silicon Cast Steel

Abstract

Bainitic microstructures obtained in high-carbon (HC) and high-silicon (HSi) steels are currently of great interest. Microstructural evolution and the bainitic transformation kinetics of a high-carbon and high-silicon cast steel held at 280, 330, and 380 °C was analyzed using dilatometry, X-ray diffraction, optical and scanning electron microscopy, and electron backscatter diffraction (EBSD). It is shown that the heterogeneous distribution of silicon (Si), manganese (Mn), and chromium (Cr) associated to microsegregation during casting has a great impact on the final microstructure. The transformation starts in the dendritic zones where there is a lower Mn concentration and then expands to the interdendritic ones. As Mn reduces the carbon activity, the interdendritic areas with a higher Mn concentration are enriched with carbon (C), and thus, these zones contain a greater amount of retained austenite plus martensite, resulting in a heterogeneous microstructure. Higher transformation temperatures promote higher amounts of residual austenite with poor thermal/mechanical stability and the presence of martensite in the final microstructure, which has a detrimental effect on the mechanical properties. Tensile tests revealed that the ultra-fine microstructure developed by the transformation at 280 °C promotes very high values of both tensile and yield stress (≈1.8 GPa and 1.6 GPa, respectively), but limited ductility (≈2%).

Controlling the Content and Morphology of Phase Constituents in Nanobainitic Steel Containing 0.6%C to Obtain the Required Ratio of Strength to Plasticity

“…Through the thickness of the plate there are sequentially distributed bands with different chemical composition and morphology of microstructure components. In nanobainitic steels, the elements that tend to segregate during solidification are Mo, Mn, and Cr [34,35]. Changes in the content of elements in neighboring segregated zones are not sharp, and the boundaries between them are characterized by intermediate chemical compositions between the maximum and minimum content of each segregating element.…”

“…In particular, differences in element content affect the Ms temperature and the phase transformation kinetics for specific values of heat treatment parameters, for example the austenitizing temperature, cooling rate, and isothermal heating temperature and time, determined by dilatometric tests. As a result of the application of constant isothermal transformation temperature, zones of material with different morphology of microstructure and hardness were found which corresponded to segregation zones [34,35]. The properties of the material, especially yield strength and plasticity, depend on the constituents of the microstructure which are critically weak for particular types of external load.…”

“…In the area of critical constituents microcracks or local undesirable phenomena associated with the initiation of plastic deformation and/or phase transformation can occur. In nanobainitic steels, the occurrence of primary segregation of a certain intensity (described as a difference in the content of key elements: Mo, Cr, Mn, and Si and distance of neighboring bands) may lead to instability of mechanical properties, especially to a decrease in ductility [35].…”

Bainite plate thickness reduction and microstructure tailoring by double austempering of Al-rich 3Mn steel

“…This is due to the large addition of Al in the steel (1.6 wt%). Al in bainitic TRIP-aided steels is basically added to reduce the harmful effect of carbides on RA stabilization [5,47]. However, Al is also a strong α phase stabilizer, which increases the driving force for bainitic transformation and therefore accelerates the transformation time itself [48,49].…”

“…Single regions poorer in bainite relative to the rest of the microstructure can be indicated (yellow ellipses). However, it may also be the result of the stochastic nature of bainite nucleation [1], as it is highly limited compared to the typical distribution of solute at the dendritic scale [47]. No privileged nucleation regions that may be associated with macrosegregation (bands) [42] were observed.…”