Proposal: Quenched and Partitioned Steel Strength/Ductility versus Volume Fraction of Retained Austenite

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Abstract

3rd generation advanced high strength steels (3GAHSS) offer auto makers lighter than before vehicles that maintain structural integrity in a cost effective manner. An integrated computational materials engineering (ICME) framework is proposed to develop 3GAHSS. From the electronic scale to structural scale, this steel can be studied computationally and produced commercially. Future vehicles will benefit from having better fuel economy with no diminished crashworthiness. Introduction to 3rd Generation Advanced High Strength Steels (3GAHSS) Automakers are always looking for ways to lower a vehicle’s weight. The challenge is balancing weight reduction with crashworthiness [1]. Any structural lightweight material on a car must perform well during a crash event. 3rd generation advanced high strength steels seek to offer such a balance in their improved strength and ductility. Higher mechanical properties in 3GAHSS mean less material required in a vehicle; thus, a vehicle can lose weight. Not to mention, these steels also offer better energy absorption which is crucial for structural integrity during high rate phenomena (car crash).

Figure 1: Steel generation banana chart show. MS is martensitic steels. DP is dual phase. CP is complex phase. TRIP is transformation induced plasticity. HSLA is high strength low alloy. FB is ferritic-bainitic. CMn is carbon manganese. BH is bake hardenable. IF is interstitial free. IF-HS is interstitial free high strength. TWIP is twinning induced plasticity. SS is stainless steel.

Background on 3GAHSS

Figure 1 shows a steel banana chart diagram listing established steel generations. The maroon swathe is 1st generation steels such as martensitic (MS), dual phase (DP), and transformation induced plasticity (TRIP). These have either high strength or high ductility but not both. The tradeoff between strength and ductility led to research regarding improved steels. The result – 2nd generation steels such as austenitic and twinning induced plasticity (TWIP). These steels have much higher strength and ductility than 1st generation but are costly to produce. The alloying elements, manganese, nickel, and chromium are very expensive, and auto makers needed steel that was cost effective.


Current research focuses on 3rd generation advanced high strength steels through heat treatments such as quenching and partitioning (QP). These steels are heated up to austenitization and rapidly quenched to a temperature between martensitic start and finish. Next, the steel is heated to a partitioning temperature (PT), held isothermally at PT, and rapidly quenched to room temperature [2]. The idea is to have steel fully austenitic undergo phase transformation to martensite but retain pockets of austenite. Then, carbon diffusion from saturated martensite stabilizes the retained austenite [1]. Figure 2 shows this heat treatment schedule from start to finish. The resulting microstructure improves the strength and ductility of the steel in a cost effective manner. Carbon is much cheaper than rare earth metals and has been theorized to stabilize face-centered cubic (FCC) austenite [2]. Understanding this thermodynamic process through integrated computational can lead to new steels developed without trial and error conventional methods.

Figure 2: Quenching and partitioning heat treatment of steel beginning with full austenitization and rapid quenching, followed by heating to partitioning temperature and rapid quenching to room temperature. MS is martensitic start. MF is martensitic finish. QT is quench temperature. PT is partition temperature [3].

Electronic scale

Designing 3GAHSS steels begins by downscaling to the electron scale. Through density functional theory (DFT), surface energy, stacking fault energy, and elastic moduli information can be calculated and calibrated. This is important information for the next step, atomistic modeling.

Atomistic scale

Modified embedded atom method (MEAM) can create iron – carbon and tertiary potentials that describe atom interactions within 3GAHSS. These potentials can be calibrated through the up scaled electron information, and then, dislocation mobility information can then be passed up to the nanoscale.

Nanoscale

Dislocation dynamics and molecular dynamics (MD) are important for developing a crystal plasticity model. Different dislocations and dislocation mechanisms are crucial to understanding martensitic twining and crystal slip.

Microscale

Next, microstructure formation and evolution of QP steels can be found through phase field modeling. Here phase transformation and interaction can be studied from hundreds of different chemistries and processing routes. The volume fraction of retained austenite can be found and validated through optical microscopy, electron back scatter diffraction, and energy dispersive spectroscopy of 3GAHSS. Phase field modeling offers a faster and cheaper route than picking one chemistry and processing route at a time.

Macroscale

A crystal plasticity model and internal state variable model can now be developed from dislocation dynamics and phase field modeling. This culmination will feed into finite element analysis (FEA) models simulating 3GAHSS in tension, compression, and torsion. Mechanical testing at quasi-static, intermediate, and high rate can validate the FEA model. Entire components can then be manufactured through the optimal chemistry and processing route from FEA simulation and tested for fatigue or fracture environments. Validation of components can then be obtained through experiments.

Conclusion

The end result of 3rd generation advanced high strength steels developed through ICME is a future vehicle using 3GAHSS in its structure. This vehicle will be lighter weight than previous vehicles and have better fuel economy. Structural integrity is not lost due to improved strength and ductility, and crashworthiness is maintained through better energy absorption properties of 3GAHSS. Also, this steel is cost effective to produce both in production and simulation.

References

[1] N. Limited, “AHSS 101 - Final report.pdf - Lumin PDF.” [Online]. Available: https://www.luminpdf.com/viewer/cwR9mwZj7hxzWAksD. [Accessed: 28-Nov-2016].

[2] J. Speer, D. K. Matlock, B. C. De Cooman, and J. G. Schroth, “Carbon partitioning into austenite after martensite transformation,” Acta Mater., vol. 51, no. 9, pp. 2611–2622, May 2003.

[3] D. V. Edmonds, K. He, F. C. Rizzo, B. C. De Cooman, D. K. Matlock, and J. G. Speer, “Quenching and partitioning martensite—A novel steel heat treatment,” Mater. Sci. Eng. A, vol. 438–440, pp. 25–34, Nov. 2006.

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