Application of SIMS Nano-Analysis to the Development of New High Strength Steels - SIMSSteel
Coordinating Institution:
CRP Gabriel Lippmann
Other Partner(s):
Arcelor Research
From: 01/04/2006
To: 31/12/2008
Budget: 165,000.00€
Contact(s):
Migeon Henri-Noël
,
Valle Nathalie
Summary Report
The carbon distribution in high strength steels is a key factor controlling the in-use properties. Due to the small size (<1micron) of the microstructural components and the very low C concentrations (a few ppm) of certain phases, analysis tools combining high analytical and spatial resolution are required. New methodologies of C quantification based on the SIMS nano-analysis technique were developed. Some calibration curves were carried out for both the high strength steels and lightweight steels (Fe-Al-C alloys). These analytical developments were applied to establish some links between mechanical behavior, formability and microstructure.
To get a better understanding of the tempering mechanisms in DP steels further analyses have been carried out using NanoSIMS and TEM. The results have shown that the carbon concentration of the martensite is heterogeneous in the quenched state. For tempering temperatures below 200°C the carbon dispersion increases due to the carbon diffusion towards the defects. Above 200°C these clusters induce the precipitation of carbides identified in TEM as cementite (Fe3C). As long as the precipitation does not occur, the mechanical properties remain stable. This is why, the temperings performed below 200°C are of interest. They improve the formability without any variations of mechanical properties.
The calibration curves established to quantify the carbon in the Fe-Al-C alloys have allowed the clarification of the carbon effect on the plasticity of these alloys. The NanoSIMS analyses combined with TEM analyses have shown that a link exists between the presence of carbon in solid solution in the ferrite and the dislocation mobility. This lack of mobility produces a twinning mechanism at the origin of the brittleness. However, it has been shown that the carbon in solid solution is not the only brittleness factor. A strong brittleness of the material is observed after welding even without any carbon in solid solution. This result indicates that the vacancies are also detrimental to the plasticity and the ductility.
Refereed Scientific Publications:
Application of SIMS nano-analysis to the development of new metallurgical solutions, N. Valle, J. Drillet, A. Perlade, H.-N. Migeon, Applied Surface Science 255 (2008) 1569-1571.
Figure 1 : Distribution in the as-quenched DP steel
Figure 2: TEM image of a FeAIC alloy after welding, showing the dislocation accumulation in particular plans