Microstructure-Property Relationship in the Thermomechanically Processed C-Mn-Si-Nb-Al-(Mo) Transformation-Induced Plast
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TRODUCTION
THE increased demand worldwide for high strength steels for automotive applications is driven by the high oil prices, new emissions compliance regimes, and higher safety requirements. The higher strength product could offer equivalent strength at proportionally reduced thickness, and therefore reduced weight. Transformation-induced plasticity (TRIP) steel is a possible candidate for automotive applications, as they demonstrate a high ultimate tensile strength (~900 to 1100 MPa) without sacrificing ductility (30 to 40 pct).[1] There are two processing routes for TRIP steels: one involves cold rolling and intercritical annealing (IA), whereas the other is controlled by thermomechanical processing (TMP). The conventional composition of TRIP steels is Fe-0.15-0.2 wt pct C-1.5 wt pct Mn-1.5 wt pct Si with possible substitution of Si by Al or addition of Mo, Cu, and P.[2–5] The substitution of Si by Al is driven by the need for automotive sheet galvanizing, as high Si content degrades the adhesion of Zn by formation of a thin surface oxide layer. Mo increases the hardenability of steel and also assists its galvanizing.[4] Microalloying additions of carbide or carbo-nitride forming elements, such as Nb or Ti, are used for refinement of I.B. TIMOKHINA, Senior Research Academic, is with the Centre for Material and Fibre Innovation, Deakin University, VIC 3216, Australia. Contact e-mail: [email protected] M. ENOMOTO, Professor, is with the Department of Materials Science and Engineering, Ibaraki University, Hitachi 316-8511, Japan. M.K. MILLER, Professor, is with the Microscopy Group, Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6136. E.V. PERELOMA, Professor, is with the School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, NSW 2522, Australia. Manuscript submitted September 1, 2011. Article published online March 20, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
the microstructure and further strength increases.[6] Simultaneous additions of Nb and Mo lead to even further strength increases due to NbMoC precipitation hardening.[7,8] The microstructure of a thermomechanically processed TRIP steel consists of polygonal ferrite (PF), bainite (B), retained austenite (RA), and possibly some martensite (M) or carbide-containing B, typically found in commercial alloys.[9] To a large extent, the properties of TRIP steels are determined by the interaction of the phases present in the microstructure, as well as by the stability of RA, which transforms continuously to M during straining.[10,11] This results in a localized increase of the strain-hardening coefficient that delays the onset of necking, leading to high elongation without compromising strength.[12] During car manufacturing, the automotive body is formed, paint coated, and then baked at temperatures of 423 to 473 K (150 to 200 C) for 20 to 30 minutes.[13] During this process, the steel is strain aged. This results in changes in microstructure and mechanical properties (i.e., a yiel
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