A Novel Approach for Controlling the Band Formation in Medium Mn Steels
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TRODUCTION
THE formation of ferrite/pearlite microstructural bands in hot-rolled low-alloy steels has been one of the main challenges in the development of medium Mn steels for low-weight automotive applications.[1–3] The presence of the ferrite/pearlite microstructural banding leads to an undesirable reduction of the mechanical properties perpendicular to the rolling direction.[4–10] The band formation is the result of significant partitioning of alloying elements during solidification which generally cannot be undone sufficiently during the solid-state homogenization treatments preceding the hot rolling and subsequent thermomechanical processing steps. Not surprisingly, the topic of band formation and its relation to the imposed thermal trajectory when cooling down from the hot rolling temperature has been researched for more than 50 years.[11–16] In micro-chemically banded Fe-C-Mn-Si steels, the spatial variation of substitutional alloying element concentrations locally reduces (in regions with high
H. FARAHANI and S. VAN DER ZWAAG are with the Novel Aerospace Materials Group, Faculty of Aerospace Engineering, Delft University of Technology, Delft, The Netherlands. Contact e-mail: [email protected] W. XU is with the State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang, 110819 China. Manuscript submitted November 1, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
levels of austenite forming elements such as Mn) or raises (in regions with high levels of ferrite forming elements such as Si) the A3 transition temperature. Upon cooling from the austenite, the ferrite starts to nucleate in regions with a high A3 transition temperature as the required undercooling for ferrite nucleation is reached there first. With the formation of the early ferrite, C is injected into the remaining neighboring austenite with a higher Mn content, reducing the local A3 transition temperature even more. Upon further cooling, the composition in these remaining austenitic regions reaches eutectoid levels and the austenite decomposition continues with pearlite formation in a banded arrangement.[17] Hence, formation of the ferrite/pearlite microstructural bands is a matter of balancing nucleation and growth during austenite decomposition.[5,18] In a standard approach, formation of microstructural bands can be suppressed via fast cooling after hot rolling. The severe undercooling from austenitization temperature results in simultaneous nucleation of ferrite in both rich and poor Mn-containing regions. Although fast cooling prevents band formation, but it leads to formation of ferrite with a fine average grain size and a higher hardness and yield strength which is not always desirable. Recently, the ‘Cyclic Partial Phase Transformation’ (CPPT) approach has been proposed as an scientifically interesting intercritical annealing route to investigate the effect of alloying elements on interface migration kinetics.[19,20] In a CPPT experiment, the temperature is
cycled between two temperatures inside the austenite-ferr
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