The automotive industry is always seeking materials that offer the combined advantages of being lightweight, having a high strength and extended ductility. AHSS is one of the potential material candidates that allows high volume, cost-effective production steel sheets with an ultra-high strength which are superior to other engineering materials.
During development, the precipitation of nano-sized carbides or carbon nitride (V, Nb, Ti) during the transformation of austenite to ferrite imparts a relatively high yield strength to AHSS – a limitation for this material due to which it offers only moderate formability. These precipitates can be refined through the addition of Mo, although the transformation kinetics is completely characterized. The role Mo plays during the steel forming process can be understood better using the Murano heating stage.
Materials and Methods
Tata Steel supplied two samples of low-carbon V-bearing steel, one with and one without 0.2 wt.% Mo addition. After heat-treating the steel samples, they were shifted to a Murano hot stage mounted within a Zeiss Gemini FEG-SEM. The heating rate and target temperature set were 100 °C/minute and 925 °C, respectively. At this point, the Oxford Instruments EBSD Nordlys system confirmed the growth of austenitic grains.
For the two samples, a series of high-resolution EBSD maps were acquired after holding times of 600, 1800, and 4200 s using the Murano stage that maintained a stable target temperature of 925 °C.
Summary
The role of Mo in the growth of austenitic grains and the kinetics of transformation of austenite to ferrite can be directly evaluated using in-situ characterization by hot-stage EBSD. The growth of austenitic grains is restricted in V-bearing low carbon steel upon addition of 0.2 wt.% of Mo to it, thus providing insight into how the addition of Mo yields AHSS.
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Figure 1. Maps a – c show the steel sample without Mo, maps d – f show steel with added Mo. White arrows in maps show grains that were at later stages of treatment consumed by a progressing high angle boundaries of large neighboring grain that is growing selectively (yellow arrow). No such events were observed in the sample that included Mo.
Gatan Instrument Used
The Murano™ heating stage is a compact stage specially developed for use with electron backscatter diffraction (EBSD), enabling high-resolution in-situ analysis of dynamic changes taking place within a sample. Oxford Instruments’ Nordlys detector was utilized in this experiment.
Credit(s)
A special thanks to Vit Janik, WMG, University of Warwick. For further information see Janik, Vit, et al. “Application of In-Situ Material Characterization Methods to Describe Role of Mo during Processing of Vbearing Micro-Alloyed Steels.” HSLA Steels 2015, Microalloying 2015 & Offshore Engineering Steels 2015: Conference Proceedings. John Wiley & Sons, Inc., 2015.
This information has been sourced, reviewed and adapted from materials provided by Gatan Inc.
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