MATETPRO - Matériaux et Procédés pour Produits Performants

Development of 3rd generation duplex steels for automotive applications – MeMnAl Steels

3rd AHSS steel generation for automotive applications

The European regulations restrict CO2 emissions for the automotives to 130g/km. Weight saving is a major contribution to this target. Steels are a good candidate for safety parts, combining high resistance and ductility. New high resistance grades are thus to be developed with an ultimate strength higher than 1000 MPa, an elongation over 15%, and a potential decrease in density by 5 to 10%. They constitute the 3rd AHSS generation.

Development of an efficient tool to design new steel grades, based on the knowledge of the physical mechanisms, allowing a weight reduction of 20% of final parts

The main issue is to develop high strength steels allowing a weight reduction over 20% of final parts. The forming of these grades has to be at the same order as the present commercial alloys. The developed steels have also to be easily processed in large quantities with the current industrial tools<br />Former studies designed duplex grades with a medium content in manganese and aluminum. Their chemical composition and microstructure give rise to targeted mechanical properties and to a potential decrease in the alloy density. However, such a development and optimization are still a metallurgical challenge which has to be overpassed. In fact, these compositions are not yet included in the thermodynamic data bases, and modeling of their behavior requires a joint development of models for microstructure evolutions and mechanical properties.<br />The project aims at developing a model to speed up the industrialization of these new high strength steels. It allows the optimal properties to be estimated, so the final cost of industrial compromises can be evaluated. Such a way also helps the improvement of the production route robustness.<br />The main deliverable is thus an efficient tool to design the new Fe-Mn-Al-C alloys, based on modeling of the microstructure genesis and quantitative relations between microstructures and mechanical properties. In this way, the project relies on numerical simulation to capitalize the knowledge, to favour its transferability, and to bridge the link between the processing conditions and the final product.

Depending on the Mn and Al content, very different microstructures and mechanical behavior can be obtained. Moreover, it is needed to locate the obtained properties with respect to an optimal structure. It is thus proposed to couple physical metallurgy and mechanical metallurgy in order to guide the steel grade developments.
From the first part of the project, it is expected to get quantitative information regarding the possible microstructures and their chemical composition as a function of alloy chemistry and thermomechanical treatments. Based on these results, the second part of the project aims at predicting the mechanical properties and fracture process from the knowledge of the microstructure characteristics and of the deformation and damage mechanisms. The project will be based both on experimental approaches and numerical simulations. The accurate alloy design of Fe-Mn-Al-C 3rd generation alloys is based on the modeling of:
1) genesis of the microstructures through:
- an accurate definition of phase diagrams,
- a careful study of the influence of alloying elements on the austenite formation and stability.
2) relation between the microstructure and mechanical properties, namely:
- effect of alloying elements on the deformation mechanisms in austenite and in ferrite,
- influence of the microstructural parameters on the mechanical properties and damage.
The two parts of the project are managed in parallel. To start the mechanical modeling, advantage is taken from the first trials made by ArcelorMittal to define a couple of reference cases that are first studied. In a parallel way, the genesis of the microstructure is performed following both ab initio calculation and thermodynamic data. This will allow predicting the phase equilibrium and phase composition. This part is essential to predict the deformation behavior within the austenitic phase, the damage development and the fracture occurrence.

First results, June 2015:
- Synthetic casts were performed by ArcelorMittal and lead to the definition of typical compositions for microstructural and mechanical studies (one with a medium content in Mn and a low content in Al, two others with a medium content in Al). After cold rolling, 5 annealing temperatures were defined between 740 and 780°C to obtain the duplex structure with various degrees of austenite stability. The 5 grades were studied from a microstructural and mechanical aspects (deformation and fracture).
- A first approach using ab initio calculation, accounting for magnetism, leads to the estimation of a Hamiltonian function for Fe-Mn-Al systems with a centered cubic structure. These results will allow the completion and extrapolation of data for thermodynamic data bases issued from experiments.
- In situ TEM tensile tests were performed on synthetic Fe-17at.%Al alloys with 19 and 126ppm C. The dislocation behavior was observed at different temperatures between 100 and 520 K. The first results confirm the large difference in mobility between the screw and edge components of dislocations, depending on the temperature and alloying additions. The effect of Al addition is close to the influence of Cr as studied formerly. The dynamic aging in Fe-Al appears at lower temperatures compared to Fe-Cr in agreement with the easiness of Al diffusion.

Prospective (July2015):
On next months, studies will be devoted to the prediction of microstructure evolutions as a function of the chemical composition and the transformation processes.
Experiments will validate and complete the data bases; ab initio calculations will lead to a Hamiltonian function for Fe-Mn-Al on a fcc lattice and to the introduction of carbon effect.
The kinetics studies will be enlarged to account for the cementite particles size and for temperature and heating rate.
Finally, microstructural and mechanical characterization will be extended to medium Mn and Al grades which have been recently elaborated. These experimental characterizations will lead to the first modeling approaches of mechanical behavior.

None at the present date (Aug. 31st, 2015)

Next European regulations on CO2 emissions for automotive transportation (130 g/km by 2015) will lead to a reduction of car weight of about 20%. Car parts’ lightening has to be made for equivalent functions and similar safety requirements, and to be manufactured with a same productivity. Financial penalties, foreseen in case of the CO2 emission target is not fulfilled, make car makers ready to accept a slight increase in the material cost to obtain such substantial weight reductions. This allows new solutions to be proposed as the use of light alloys, the proposal of new designs… Regarding these new conditions, steel has still important assets for customers: large availability, moderate price, easy recyclability, formability and usability easy to manage, superior crash properties for safety design.
A 1st generation of very high strength steels (HSS) gave rise to high mechanical properties resulting from design and control of the microstructure. These are essentially ferritic-bainitic matrix with a fraction of metastable carburized phases: Dual-Phase and TRIP steels are two of the typical grades. A 2nd HSS generation is under development based on the TWIP mechanism; however, the industrial manufacturing is not straightforward.
The aimed properties (UTS above 1,000 MPa, uniform elongation larger than 15%, and, when possible, the density decreased by 5 to 10% with respect to standard carbon steel) require the development of a new steel generation. Their metallurgy is based on medium Mn, medium Al, steel grades (Mn between 5 and 8 wt%, Al lower than 8 wt%). A few laboratory trials show that fine duplex ferrite-austenite microstructures can be reached. Depending on the chemical composition and the thermomechanical processing, the volume fractions of each phase can be tailored, and the mechanical behaviour of the austenite is controlled over a wide range including the expected targets. Adding aluminium decreases the density and favours the hardening of the ferritic phase. It is already established that such steel grades can be produced on the present industrial lines. They are also easily recyclable in a common steel process.
The MeMnAl Steels project is devoted to understanding the physical mechanisms involved in the development of these new steels, in particular those governing microstructural evolutions and deformation. Based on the expected results, these microstructural evolutions and the final properties will be modelled. Physical metallurgy and mechanical metallurgy approaches will be combined to map the final capabilities of these steel grades, to predict their ultimate behaviour, and to assist in defining the main stages of the industrial processing. The different teams involved in the project have complementary experimental skills and facilities, and modelling competencies: physical metallurgy, thermodynamics and kinetics, mechanical metallurgy and damage-fracture... These approaches are performed in a multi-scale and multi-physics framework.
This 4-years project is divided in two strongly interacting domains: (i) modelling of microstructure genesis, and (ii) modelling of the relations between resulting microstructures and mechanical properties.
i. For the first domain, thermodynamics and kinetics tools will be developed to predict the actual phases and their volume fraction. Ab initio models and CALPHAD approach will be used in this way.
ii. The relations between the microstructures and the mechanical properties will be predicted using crystal plasticity modelling which links the macroscopic behaviour with the grain behaviour, accounting for interfaces between various constituents.
The ultimate goal is to capitalize the whole knowledge acquired during the project to build a model supporting the developments of these new steel grades. It will help to define, in an easy and reliable way, the composition domains and the optimal processing schedules to reach the mechanical behaviour and, thus, to speed-up industrial developments.

Project coordination

Jean-Hubert SCHMITT (MSSMat/Centre Natioanl de la Recherche Scientifique)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

ARMINES - Centre des Matériaux ARMINES
CEMES CNRS - CEMES
IM2NP Institut Matériaux Microélectronique Nanoscience de Provence
SIMaP Laboratoire Science Ingénierie des Matériaux & Procédés
UMET Université de Lille1 - UMET
AMMR ArcelorMittal Maizières Research SA
CNRS-MSSMat MSSMat/Centre Natioanl de la Recherche Scientifique

Help of the ANR 1,199,501 euros
Beginning and duration of the scientific project: December 2013 - 48 Months

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