Dilatometric study of phase transformations in advanced high-strength bainitic steel

Journal of Thermal Analysis and Calorimetry, Nov 2014

The work deals with dilatometric studies of a new-developed advanced high-strength bainitic 3Mn–1.5Al steel. Ferritic, bainitic and martensitic phase transformations are investigated in detail in respect of their temperature range forming and microstructures produced under various conditions of both continuous and isothermal cooling. The equilibrium temperatures of A e1 and A e3 and phase composition of the investigated steel were initially calculated whereas critical temperatures of A c1 and A c3 as well as the decomposition of retained austenite were determined upon heating. The major tests consisted of controlled cooling of undeformed or plastically deformed austenite using the dilatometer within the cooling rate range of 2–0.5 °C s−1. The effects of the cooling rate and deformation at temperatures of 900 and 1,050 °C on the phase transformation behaviour and microstructure were explained. The final experiment was carried out using a thermo-mechanical simulator under conditions of multi-step deformation and isothermal holding of the steel at 400 °C. Microstructural features were revealed using light microscopy and scanning electron microscopy techniques.

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Dilatometric study of phase transformations in advanced high-strength bainitic steel

A. Grajcar 0 1 2 W. Zalecki 0 1 2 P. Skrzypczyk 0 1 2 A. Kilarski 0 1 2 A. Kowalski 0 1 2 S. Koodziej 0 1 2 0 A. Kilarski General Motors, 1 Adama Opla Street, Gliwice, Poland 1 W. Zalecki Institute for Ferrous Metallurgy , 12-14 K. Miarki Street, 44-100 Gliwice, Poland 2 A. Grajcar (&) P. Skrzypczyk A. Kowalski S. Koodziej Institute of Engineering Materials and Biomaterials, Silesian University of Technology , 18a Konarskiego Street, 44-100 Gliwice, Poland The work deals with dilatometric studies of a new-developed advanced high-strength bainitic 3Mn-1.5Al steel. Ferritic, bainitic and martensitic phase transformations are investigated in detail in respect of their temperature range forming and microstructures produced under various conditions of both continuous and isothermal cooling. The equilibrium temperatures of Ae1 and Ae3 and phase composition of the investigated steel were initially calculated whereas critical temperatures of Ac1 and Ac3 as well as the decomposition of retained austenite were determined upon heating. The major tests consisted of controlled cooling of undeformed or plastically deformed austenite using the dilatometer within the cooling rate range of 2-0.5 C s-1. The effects of the cooling rate and deformation at temperatures of 900 and 1,050 C on the phase transformation behaviour and microstructure were explained. The final experiment was carried out using a thermo-mechanical simulator under conditions of multistep deformation and isothermal holding of the steel at 400 C. Microstructural features were revealed using light microscopy and scanning electron microscopy techniques. - The beneficial combination of high strength, ductility and technological formability of steel sheets for the automotive industry can be achieved using advanced high-strength steels (AHSS). They consist of different soft and hard structural constituents in various proportions, which enable to obtain a very wide range of mechanical and technological properties. The microstructure of dual phase (DP) steel contains soft ferrite and hard martensite whereas the multiphase microstructure of TRIP (Transformation Induced Plasticity) steel consists of ferrite, bainite and retained austenite [16]. New demands of the automotive industry for relatively low-cost steel sheets characterized by tensile strength above 1,000 MPa require further searching of new chemical composition strategies. Advanced ultra-high-strength steels contain a higher fraction of hard phases, i.e. acicular ferrite, bainite or martensite [710] compared to AHSS containing polygonal ferrite as a matrix. A key microstructural constituent of advanced multiphase steels is retained austenite with the amount from 10 to 30 %. This phase ensures a required ductility level by its strain-induced martensitic transformation during cold forming operations. Recently, a high amount of retained austenite is obtained in different bainitic alloys containing from 1.5 up to 8 mass% Mn, being a main austenite stabilizer [814]. These steels are dedicated to the automotive industry for different crash-relevant elements, especially in the side zone of a car (B-pillars, roof rails, side-impact beams, etc.). Their wide use requires improved forming technologies and special welding procedures. Monitoring of phase transformations and the knowledge of continuous cooling transformation (CCT) diagrams are of primary importance for proper design of bainiteaustenite microstructures with an optimal morphology. Decomposition of retained austenite on heating or cooling from the c region is often monitored by dilatometry, differential thermal analysis (DTA) or differential scanning calorimetry (DSC) [11, 1517]. Results of these investigations have often to be confirmed by detailed microscopic research because phase transformations in multiphase steels are very complex. In medium-Mn steels, the bainite is particularly difficult for unequivocal identification because it can contain carbides or may form carbide-free bainite [8, 1822]. Films of retained austenite instead of carbides occur between laths of bainitic ferrite. The destabilization of the austenite can happen during heating and cooling through precipitation of carbides, martensitic transformation, etc. Monitoring the volume fraction of all microstructural constituents and their morphology is a key to obtain optimal mechanical properties of multiphase steels. Beneficial mechanical properties and formability of steels with a bainiticaustenitic mixture are obtained for fine, homogeneous bainite microstructures. Carbide precipitates and a bimodal morphology of fine and coarse bainite are detrimental for fracture toughness and ductility of steel products [19, 20, 23]. The novelty of the present work includes the identification of temperatures of phase transformations for a newdeveloped steel containing 3 % Mn, which was cooled from the plastically deformed recrystallized and non-recrystallized austenite regions. The research has (...truncated)


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A. Grajcar, W. Zalecki, P. Skrzypczyk. Dilatometric study of phase transformations in advanced high-strength bainitic steel, Journal of Thermal Analysis and Calorimetry, 2014, pp. 739-748, Volume 118, Issue 2, DOI: 10.1007/s10973-014-4054-2