International Journal of Metallurgical Engineering 2013, 2(2): 161-178 DOI: 10.5923/j.ijmee.20130202.08 Structure-Property Correlation in TRIP Aided Steels Shiv Brat Singh 1,* , Tanmay Bhattacharyya 1,2 , Ravi Ranjan 1 , Sandip Bhattacharyya 2 , De bas his h Bhattacharjee 2 1 Department of Metallurgical and Materials Engineering, IIT Kharagpur 2 R&D and SS, Tata Steel Limited Abstract The need for light-weight auto bodies with improved crash resistance and other safety parameters has encouraged the development of a new family of multi-phase steels having higher strength and better formability. High strength TRIP aided steel is the ideal material for such applications, but developing a grade that has good formability, coatability and weldability is a major challenge. The present work has been undertaken to address the issues of coatability and weldability as well as to target a high strength - ductility balance. The work includes Thermo Calc calculations to study the phase relationships, thermo-mechanical simulation, prediction of microstructure through artificial neural network and laboratory experiments in hot-dip galvanising simulator. Routine characterisation was done for the assessment of microstructure and properties. Three experimental steels were prepared in a vacuum induction furnace. The as-cast ingots were forged and then hot and cold rolled. Laboratory salt bath heat treatment of the cold rolled samples was carried out following the standard two-step heat treatment cycle consisting of intercritical annealing and isothermal bainitic transformation to obtain the desired microstructure and target mechanical properties. As a part of weldability assessment, th e heat treated samples were spot welded. A difficulty with TRIP steels is their poor wettability during galvanising due to the formation of oxides of silicon on the surface. Two of these three test grades had aluminium as the replacement for silicon to improve wettability. Dew point during thermal processing plays a critical role and affects the wettability. The two -step heat treatment described above followed by hot dip galvanising was simulated with varying dew points in hot dip galvanising (Rhesca) simulator to assess the coatability of the samples. Very good strength-elongation balance was obtained for all the three steels and the samples with lower silicon and higher dew point showed better wettability and coatability. Weldability studies revealed that the introduction of post weld tempering cycles improves the weld nugget geometry, breaking load and failure mode under tensile shear loading. Tensile studies at high strain rate revealed a satisfactory performance of the steel even at 100 s -1 suggesting the fitness to be used at the crash prone zones of automotives. Keywords TRIP (Transformation Induced Plasticity), Wettability, Weldability, Crash Resistance Crumpling 1. Introduction The need for addressing stringent environment and safety norms has compelled the automakers to design light-weight auto bodies with enhanced crash resistance. A new family of multi-phase steels having higher strength and better formability, named Transformation induced plasticity (TRIP) aided steel is gaining popularity to satisfy the demand as it has been adjudged as the most promising solution for the production of cars with low body mass because of the combination of high strength and large uniform elongation that they offer. The TRIP effect arises from deformation induced transformation of retained austenite to martensite[1]. It is accompanied by an invariant plane strain shape deformation as well as a volume expansion. This results in a higher strain hardening rate that delays the onset of necking, * Corresponding author: sbs22@metal.iitkgp.ernet.in (Shiv Brat Singh) Published online at http://journal.sapub.org/ijmee Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved eventually resulting into higher uniform and total elongation [2]. An important feature of steels of this genre is enhanced ductility at a very high strength level[3]. TRIP effect enhances the mechanical properties by two mechanisms[4, 5]: (i) composite strengthening via the formation of hard martensite particles dispersed in the ferrite matrix and (ii) formation of dislocations around newly formed martensite regions as a result of the volumetric expansion during the austenite to martensite transformation. This steel is ideal for passive safety structural applications like bumper reinforcement, door impact beam etc., due to its high strain hardening rate and dynamic energy absorption capacity[6]. The microstructure of conventional TRIP aided steels comprises ferritic matrix (~55-65%) along with bainite (~25-35%) and metastable retained austenite (~5-20%)[7]. The TRIP effect depends on the amount of retained austenite and its stability to deformation induced transformation. Conventionally, TRIP aided steels contain about 1.5 wt% silicon which enhances the volume fraction and stability of the retained austenite by suppressing cementite formation during the isothermal bainitic transformation[8]. The