Thermal Stability of Retained Austenite in Advanced TRIP Steel with Bainitic Ferrite Matrix for Automotive Industries

Abstract:

Multiphase steels consisting of retained austenite and martensite/bainite microstructures such as TRIP, low-temperature-bainite, and Q&P steels are attractive candidates for the new-generation of AHSS. These steels exhibit a remarkable combination of strength and toughness which is essential to meet the objective of weight reduction of engineering-components, while maintaining the compromise of tough-safety requirements. Such good mechanical properties are due to the enhanced work hardening rate caused by austenite-to-martensite transformation during deformation and the strengthening contribution of martensite/bainite. The retained austenite can thermally decompose into more thermodynamically stable phases as a consequence of temperature changes, which is referred to as the thermal stability of retained austenite. TRIP-aided steel is an effective candidate for automotive parts because of safety and weight reduction requirements. The strength–ductility balance of high strength steel sheets can be remarkably improved by using transformation induced plasticity behavior of retained austenite. In manufacturing hot rolled TRIP-aided sheet steels, austenite transforms into bainite during the coiling process. Because black hot coils cool slowly after the coiling process, they are exposed at about 350–450°C for a few hours or days. Therefore, the metastable residual austenite can be decomposed into other phases. This decomposition of residual austenite can produce serious deteriorate of mechanical properties in hot rolled TRIP-aided sheet steels. The present work identified the decomposition behavior and study the thermal stability of retained austenite in the TRIP-aided steel with bainitic/ferrite matrix depending on coiling temperatures and holding times by means of DSC and XRD analysis.

• Periodical: Materials Science Forum (Volume 1016)
• Pages: 429-434
• DOI: https://doi.org/10.4028/www.scientific.net/MSF.1016.429
• Online since: January 2021
• Authors: Eman El-Shenawy*, Hoda Refaiy, Hoda Nasr El-Din
• Keywords: Austempering Heat Treatment, Bainitic Steel, Retained Austenite, Thermal Stability, TRIP-Aided Steel, Ultra-High Strength Steel

References

[1] B.C.‏ De Cooman: Structure-properties relationship in TRIP steels containing carbide-free bainite, Current Opinion in Solid State and Materials Science, Vol.8‏ (2004), 285-303.

DOI: 10.1016/j.cossms.2004.10.002

[2] J. Chiang: Effect of microstructure on retained austenite stability and tensile behavior‏ in an aluminum-alloyed TRIP steel, Master thesis, Queen's University, Canada,‏ (2012).

[3] H. N. El-Din , E. A. Showaib , N. Zaafarani and H. Refaiy : Structure properties relationship in TRIP type bainitic ferrite steel austempered at different temperatures, International Journal of Mechanical and Materials Engineering , DOI 10.1186/s40712-017-0071-9, (2017) , 1-9.

DOI: 10.1186/s40712-017-0071-9

[4] K. Sugimoto, K. Nakano, S. Song and T. Kashima: Retained austenite characteristics and stretch-flangeability of high-strength low-alloy TRIP type bainitic sheet steels, ISIJ International, Vol. 42 (2002), No. 4, 450-455.

DOI: 10.2355/isijinternational.42.450

[5] K. Sugimoto, K. Nakano, S. Song and T. Kashima: Retained austenite characteristics and stretch-flangeability of high-strength low-alloy TRIP type bainitic sheet steels, ISIJ International, Vol. 42 (2002), No. 4, 450-455.

DOI: 10.2355/isijinternational.42.450

[6] E. Emadoddin, H. Asmari, A. Zadeh: Formability evaluation of TRIP-aided steel sheets with different microstructures: polygonal ferrite and bainitic ferrite matrix, International Journal of Material Forming, Vol. 2 (2009), 781-784.

DOI: 10.1007/s12289-009-0455-3

[7] S.w. Lee, S. Lee, B. C. De Cooman: Austenite stability of ultrafine-grained transformation-induced plasticity steel with Mn partitioning, S. Lee et al. / Scripta Materialia 65 (2011) 225–228.

DOI: 10.1016/j.scriptamat.2011.04.010

[8] S.v.d. ZWAAG, L. ZHAO, S. O. KRUIJVER and J. SIETSMA: Thermal and Mechanical Stability of Retained Austenite in Aluminum-containing Multiphase TRIP Steels, ISIJ International, Vol. 42 (2002), No. 12, p.1565–1570.

DOI: 10.2355/isijinternational.42.1565

[9] H. J. Jun, S.H. Park , S.D. Choi, C.G. Park: Decomposition of retained austenite during coiling process of hot rolled TRIP-aided steels, Materials Science and Engineering A 379 (2004) 204–209.

DOI: 10.1016/j.msea.2004.01.029

[10] K. Sugimoto, M. Tsunezawa, T. Hojo, S. Ikeda: Ductility of 0.1–0.6C–1.5Si–1.5Mn ultra-high-strength TRIP-aided sheet steels with bainitic ferrite matrix, ISIJ International, Vol. 44 (2004), No. 9, 1608-1614.

DOI: 10.2355/isijinternational.44.1608

[11] K. Sugimoto, T. Muramatsu, S. Hashimoto, Y. Mukaid: Formability of Nb bearing ultra-high-strength TRIP-aided sheet steels, Journal of Materials Processing Technology, Vol. 177 (2006), 390-395.

DOI: 10.1016/j.jmatprotec.2006.03.186

[12] K. Hausmann, D. Krizan, K. Spiradek-Hahn, A. Pichler, E. Werner: The influence of Nb on transformation behavior and mechanical properties of TRIP-assisted bainitic-ferritic sheet steels, Materials Science & Engineering A,‏ Vol. 588 (2013), 142-150.

DOI: 10.1016/j.msea.2013.08.023

[13] A.S. Podder, I. Lonardelli, H. K. D. H. Bhadeshia, A. Molinari: Thermal stability of retained austenite in bainitic steel: an in situ study, Proc. R. Soc. A (2011) 467, 3141–3156.

DOI: 10.1098/rspa.2011.0212

[14] H. Park, J. Seol, N. Lim, S. Kim, C. Park: Study of the decomposition behavior of retained austenite and the partitioning of alloying elements during tempering in CMnSiAl TRIP steels, Materials & Design 82 (2015) 173–180.

DOI: 10.1016/j.matdes.2015.05.059