Refinement of the Solidification Structure of Austenitic Fe-Mn-C-Al TWIP Steel
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TWIP steel is expected to be a promising material for lightweight automotive bodies because of its high strength, high ductility and high forming ability.[1,2] It is well known that the property advantage of TWIP steel is induced by dynamic twinning in the austenite matrix with high manganese content.[3,4] Nowadays, high manganese steel can be commercially provided worldwide by a few steel companies for car body parts, damping plates, cryogenic vessels, the military industry, etc.[5–7] The market and demand for high manganese steel have grown significantly, and increased attention has been paid to the composition design, microstructure control and property improvement in recent years.[8–12]
GEN LI, PENG LAN, and JIAQUAN ZHANG are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 10083, China. Contact e-mail: [email protected]; [email protected] GUIXUAN WU is with GTT Technologies, Kaiserstraße 103, Herzogenrath 52134, Germany and also with the Institute of Energy and Climate Research, Microstructure and Properties of Materials (IEK-2), Forschungszentrum Ju¨lich GmbH, Leo-Brandt-Straße 1, Ju¨lich 52425, Germany. Manuscript submitted August 7, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
However, high alloying in TWIP steels leads to inhomogeneous solute distribution and anisotropic local deformation. In a heat-treated sheet of Fe-29Mn-0.3C TWIP steel by strip casting, the stacking fault energy ranges roughly from 18 to 30 mJ/m2 because of solute microsegregation with uneven grain size distribution.[13,14] Daamen and Hirt found that the stacking fault energy of a recrystallized Fe-17Mn-0.6C-1.5Al TWIP steel is within the range of 20 to 50 mJ/m2. The deformation twins are only observed in the solute-depleted zone.[15,16] In Jo and Sohn’s study, transformation martensite and mechanical twins are observed in solute-depleted and -enriched zones, respectively, in Fe-0.5C-15Mn-1Si-1Al-0.3Mo-0.5V TWIP steel,[17] and the recrystallized grain sizes in these two regions are also different.[18] The inhomogeneous solute distribution even leads to the early fracture of TWIP steel.[19,20] In addition, high manganese and carbon in TWIP steel also promote the formation of casting defects.[14,21] Columnar dendrites are readily grown in Fe-Mn-C TWIP steel under regular cooling conditions,[13,14] and the solute microsegregation and microporosity are quite serious.[22,23] In our previous work, the hot ductility of Fe-22Mn-0.7C TWIP steel was very poor within 700 C to 1350 C.[24] Bleck and Hirt,[25] Kang and Mintz,[26] Mejia and Salas-Reyes,[27–29] Wang and Yang,[30,31] Liu and Shen,[32] Senk and Steenken[33,34] and Kwon and De Cooman[35] reached the similar conclusion that high
manganese steel is apt to crack during casting. Coarse dendritic grains and severe microsegregation are regarded as the crucial factors.[24,25,27,32,35] Now it is still a big challenge to develop an efficient method to improve matrix homogeneity and enhance the processing propert
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