Dilatometric study of phase transformations in advanced high-strength bainitic steel
A. Grajcar
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W. Zalecki
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P. Skrzypczyk
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A. Kilarski
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A. Kowalski
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S. Koodziej
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A. Kilarski General Motors, 1 Adama Opla Street, Gliwice,
Poland
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W. Zalecki Institute for Ferrous Metallurgy
, 12-14 K. Miarki Street, 44-100 Gliwice,
Poland
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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.
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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)