Materials Technology Effect of Bake-Hardening on the Structure-Property Relationship of Multiphase Steels for the Automotive Industry I.B. Timokhina 1) , P.D. Hodgson 1) , S.P. Ringer 2) , R.K. Zheng 2) and E.V. Pereloma 3) 1) Centre for Material and Fibre Innovation, Deakin University, Geelong, Vic 3217, Australia, Ilana.timokhina@eng.monash.edu.au 2) Australian Key Centre for Microscopy and Microanalysis, University of Sydney, NSW 2006, Australia 3) School of Mechanical, Material and Mechatronics Engineering, University of Wollongong, Wollongong, NSW 2522, Australia The effect of a bake-hardening (BH) treatment on the microstructure and mechanical properties has been studied in C-Mn-Si TRansformation Induced Plasticity (TRIP) and Dual Phase (DP) steels after: (i) thermomechanical processing (TMP) and (ii) intercritical annealing (IA). The steels were characterized using X-ray diffraction, transmission electron microscopy (TEM) and three-dimensional atom probe tomography (APT). All steels showed high BH response. However, the DP and TRIP steels after IA/BH showed the appearance of upper and lower yield points, while the stress-strain behavior of the TRIP steel after TMP/BH was still continuous. This was due to the higher volume fraction of bainite and more stable retained austenite in the TMP/BH steel, the formation of plastic deformation zones with high dislocation density around the “as-quenched” martensite and “TRIP” martensite in the IA/BH DP steel and IA/BH TRIP steel, respectively. Keywords: bake-hardening , TRIP steel, DP steel, thermomechanical processing, intercritical annealing, TEM, APT DOI: 10.2374/SRI08SP050; submitted on 27 January 2009, accepted on 23 February 2009 Introduction The search for high strength steels of improved formability was stimulated by the need for weight savings in automobiles while improving the safety aspects. Past efforts have concentrated on achieving an adequate strength ductility balance since an increase in strength is generally accompanied by a reduction in ductility. The search for new materials has led to the development of new multiphase steels such as dual phase (DP) steels and Transformation Induced Plasticity (TRIP) steels [1, 2]. In these new steels, the complex microstructure can introduce defects, encourage transformation during deformation, or increase the work hardening rate in other ways [2]. There are two main routes for the production of DP and TRIP steels: (i) an intercritical annealing after cold rolling (IA) and (ii) thermomechanical processing (TMP) [3, 4]. The aim of any production route is to decrease the martensite start transformation temperature below room temperature through carbon enrichment of the retained austenite. The microstructure of DP steels consists of a soft, ductile polygonal ferrite matrix with hard martensite islands and, in some cases, a small amount of retained austenite [5]. The TRIP steel microstructure contains polygonal ferrite, bainite, retained austenite and a minor quantity of martensite [6]. Both steels showed continuous yielding behavior, a low yield point and a high strain- hardening coefficient [7, 8]. It has recently been proposed to use a bake-hardening (BH) treatment for both types of steels to provide additional strengthening [9, 10]. This should potentially lead to an increase in strength for the same thickness of steel sheet. In industrial processing, the bake-hardening of the automotive body is conducted at temperatures of 150- 200 o C for 20-30 min after forming [9]. The increase in the strength is attributed to the pinning of dislocations by interstitial atoms, such as carbon [11]. The yield strength increase is accompanied by the return of the yield point, a slight increase in the tensile strength and a decrease in the elongation [12, 13]. It has recently been proposed [14, 15] to study the bake- hardening behaviour in the DP and TRIP steels after TMP and intercritical annealing using Atom Probe Tomography (APT). This technique has the high spatial resolution required to provide accurate determination of the solute content in ferrite, martensite, retained austenite and bainite and measure the local composition of nanosized microstructural features such as solute clusters and segregations at dislocations. The aim of the current study is to study the bake- hardening behaviour in the DP and TRIP steels after different processing routes using advanced analytical techniques such as TEM and APT. Experimental Procedure Two TRIP steels after a standard cold rolling and intercritical annealing (TRIP 1) and directly after controlled hot rolling (TRIP 2) and one DP steel produced by a standard cold rolling and intercritical annealing were studied (Table 1). Table 1. Chemical composition of the investigated steels. Steel C Si Mn Al Mo Cr Nb wt% 0.12 1.77 1.39 0.031 - 0.02 - TRIP 1 at% 0.55 3.44 1.38 0.062 - 0.02 - wt% 0.21 1.18 1.52 0.57 0.29 - 0.036 TRIP 2 at% 0.95 2.3 1.51 1.15 0.16 - 0.02 wt% 0.04 1.07 1.08 0.018 - 0.083 - DP at% 0.17 2.09 1.08 0.037 - 0.088 - steel research int. 80 (2009) No. 7 507