Carbon partitioning in nanostructured ferritic phases: kinetics and microstructures – CAP NANO

Original in situ experiments
Compared to recent works exposed in the literature, the project intends to investigate the kinetics aspects of the partitioning process mainly using in situ techniques. We hence propose for the first time the investigation of this process (phase fraction and local mean carbon contents) thanks to high energy X-ray diffraction (synchrotron radiation facility - TGIR). The possible interface mobility will be studied in situ by confocal microscopy.

Investigations of the microstructures at the nanoscale
The project will benefit from the leading-edge and reference technology to measure carbon concentration profiles at the nanoscale: 3D Atom Probe Tomography. Transition carbides and cementite often neglected in the literature will also be the subject of particular attention and will be explored thanks to recent Transmission Electron Microscopes (TEM) offering micro-diffraction and breakthrough analytical capabilities (Energy Electron Loss Spectroscopy (EELS) with a nanometric resolution, Energy Filtered TEM (EFTEM) or High Angle Annular Dark Field (HAADF)). More conventional techniques (Scanning TEM and Electron BackScattered Diffraction – Scanning Electron Microscope) will also be used by the industrial partner to investigate the nature and the morphology of the phases and to quantify the microstructure at all the relevant scales.

Unified mean-field model
The key innovation proposal concerns the kinetics modeling that will explicitly take into account carbide precipitation and carbon segregations as possible competitive mechanisms to partitioning. Bainite or isothermal martensitic formation that cannot be ruled out will also be considered as secondary competitive mechanisms.

(update 05/09/2016)

The Q&P treatment on high C TRIP steel (Fe-0.3C-2.5 Mn-1.5Si-0.8Cr in wt.%) has been studied by the means of in situ X-Ray diffraction experiments using a synchrotron source. The experiments have been carried out on the ESRF ID15B line in the ETMT Instron device (powder diffraction configuration in transmission). The high energy monochromatic beam (E = 87 keV) enables high acquisition rates (10 Hz) adapted to study ‘real-time’ processes on bulk samples and for the first time during first reheating and cooling sequences.
The main findings of our study are the following:
• Absence of microstructure evolution during reheating (at about 30°C/s) after the first quenching step (below 370°C for the studied steels),
• Significant increase in ferritic phase fraction during the partitioning step. In the literature, this evolution is interpreted as a bainitic transformation, as the mobility of martensite/austenite interface during partitioning can be ruled out,
• Evidence of heterogeneous carbon distribution in austenite at the beginning of the partitioning step. Carbon enrichment in austenite and bainitic transformations are strongly related.
• High residual stresses in retained austenite at RT which are favorable to a TRIP effect due to eigenstrains resulting from the differences of thermal expansions between austenite and martensitic phases during final cooling,
• Local carbon mass balances permitted by in situ experiments show that a large fraction of carbon remains trapped in martensite laths (segregations on dislocations, carbides). Unfortunately, our experiments have not yet permitted to identify the nature of carbides and to quantify their relative fractions.

(update 05/09/2016)

(Poster) PTM 2015, Vancouver, Canada
(Oral Presentation) TMS 2016, Nashville, USA
(Poster) Colloque IMT 2016 « Matériaux : Réalités et Nouvelles Frontières », Paris, France
(Oral Presentation) Conférence Nationale du RNM « Métallurgie : Quel avenir ? », St Etienne, France

The proposed project is devoted to provide a better fundamental understanding of the quench and partitioning (Q&P) process, in the aim of optimizing the production of the next generation of advanced high strength steels (AHSS). Q&P is a new route proposed to produce the third generation of AHSS. It relies on the generation of a mixed martensite (a’) and austenite (g) microstructure by a quench (Q) followed by a carbon (C) redistribution from supersaturated a’ to g (P). These steels have the best potential weight reduction requirements needed to meet regulations for reduction of the CO2 production in ground transportation, imposed by the European Commission by 2021.

This process, proposed in 2003, has been adopted by the main Asian steelmakers. The industrial parameters are derived from ‘end properties’ oriented investigations, with limited basic knowledge. As a consequence, there is room to optimize the process, provided deeper understanding of the Q&P metallurgy is gained. This is the aim of this project in the ‘industrial renewing’ framework: based on a microstructural analysis at the scale of the phases and an accurate thermo-kinetics description of the process, provide a simulation tool to optimize the industrial processing of Q&P steels, with ArcelorMittal, a major steel producer for the automotive industry, as an industrial partner.

Up to date, the thermo-kinetics modelling of the Q&P process relies on simple arguments, making predictions still inaccurate. The current limitations are clearly identified. They are related to the description of the thermodynamics of a’, to the local conditions at the a’/g interfaces, to C segregation and precipitation, and to the stability criteria of C enriched g. The project is built to address these points, and to provide a more accurate description of the whole Q&P process. The thermodynamical description of the C supersaturated a’, including C diffusion and segregation to lattice defects, is the first point to be drastically improved. This will be conducted in the framework of the CALPHAD approach. The local thermo-kinetics conditions at the a’-g interface, together with C mobility in both phases, will govern the C redistribution kinetics, and are key-points to optimize the industrial process. They will be treated according to the Darken’s approach, which was shown to be adapted in this case. The influence of other potential mechanisms influencing C redistribution, namely the precipitation of iron carbides and the formation of bainite, will also be investigated and accounted for.

These previous refinements must be sustained by experimental data, in particular the nature and amount of C redistribution, from the meso to the nanometer scale. It is first proposed to perform in situ X-Ray diffraction, using a high energy synchrotron source, giving access to real time evolution of C partitioning and to the nature of the phases that form during Q&P process. In addition to this global survey, a more accurate investigation of C distribution at the nanometer scale will be conducted by atom probe tomography, which provides C concentrations at any site of interest (structural defects, a’/g interfaces, C enriched g). These techniques will be complemented by electron microscopy to get structural information in particular as far as iron carbides precipitation is involved. Special attention will be paid on the comparison and coherency of the information obtained with the different techniques. It is therefore expected to get major advances in the metrology of C in steels.

At last, the integration of the predicted final microstructures in a model developed at ArcelorMittal to predict the tensile properties of Q&P steels will fit the so-called Product-Line-Management long-term program developed at ArcelorMittal, aiming at improving product quality and process efficiency to reduce costs.