The Influence of Microstructural Characteristics on Austenite Formation Kinetics in a Plain Carbon Steel
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E (DP) steels are widely used in the automotive industry because of the need to lower vehicle weight with the final goal to improve fuel efficiency while maintaining other desirable properties such as crashworthiness. DP steels with a microstructure consisting of hard martensite islands embedded in a soft and ductile ferrite matrix possess an outstanding combination of strength and formability. The mechanical properties of these steels are highly dependent on their microstructural characteristics, such as volume fraction, morphology, and distribution of constituents, which, in turn, are determined by the austenite condition at the intercritical temperature. Because of the significant influence on the
M.S. MOHSENZADEH is with Department of Materials and Metallurgical Engineering, University of Gonabad, Gonabad, Iran. M. MAZINANI is with Department of Materials and Metallurgical Engineering, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran. Contact e-mail: [email protected] Manuscript submitted February 22, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
final microstructure and mechanical properties of DP steels, formation of austenite in the intercritical temperature range has been widely studied during the previous several decades.[1–8] One of the typical processing routes used to produce DP steels involves intercritical annealing of the steel after cold rolling followed by quenching to room temperature. Since several processing parameters, including heating rate, intercritical temperature, holding time, and cooling rate, are usually variable in an industrial production line, many research works have been conducted to evaluate the effects of these parameters on austenite formation during different stages of processing.[9,10] Furthermore, various steel microstructures can be produced during different stages of steel production, which are considered to be the starting microstructures for the subsequent intercritical annealing stage. It has been clearly seen that the type of steel microstructure prior to intercritical annealing treatment has a significant influence on austenite formation.[11,12] Increasing the heating rate for the case of cold-rolled (CR) steels causes a strong interaction between ferrite recrystallization and austenite formation stages, affecting significantly the kinetics of austenite formation by
changing the way austenite nucleates and grows.[1,3,9,13] At low heating rates, e.g., between 1 and 10 °C/s, ferrite is almost completely recrystallized before austenite formation and, hence, austenite nucleation and growth occur at the ferrite grain boundaries as well as the pearlite colonies. However, at high heating rates, e.g., 100 °C/s, ferrite recrystallization overlaps with austenite formation and, therefore, austenite nucleation and growth take place only at pearlite colonies.[3] The spatial distribution of carbon in the steel microstructure also affects the kinetics of austenite formation as well as the austenite distribution and morphology. Austenite formation has been extensively stu
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