Dynamic Strain Aging Phenomena and Tensile Response of Medium-Mn TRIP Steel
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MEDIUM manganese steels are of interest for automotive sheet steel, since they can be processed by cold working and intercritical or batch annealing. The resulting grain structures can be submicron with yield strengths in excess of 800 MPa as will be shown in this study. These medium manganese alloys are considered 3rd generation advanced high strength steel with strength and ductility combinations meeting or exceeding goals set by the Department of Energy, defined as strengths of 1200 to 1500 MPa and associated total elongations of 30 to 25 pct. However, many of these medium-Mn steels exhibit static strain aging that is manifested by yield point behavior and yield point elongations that contribute to the overall total elongations.[1–13] Medium manganese steels can also be
DANIEL M. FIELD and DAVID C. VAN AKEN are with the Department of Materials Science and Engineering, Missouri University of Science and Technology Rolla, MO 65409. Contact e-mail: [email protected] Manuscript submitted September 22, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
formulated to produce rapid work hardening as a result of two-stage transformation-induced plasticity (TRIP), but many of these alloys exhibit dynamic strain aging (DSA) and a reduced total elongation to failure, with failure occurring prior to necking. A. Static Strain Aging Yield point elongation (YPE), associated with static strain aging and the formation of Lu¨ders bands, is undesirable when forming complex automotive parts.[14] Inhomogeneous deformation associated with YPE are deleterious to press forming and leads to inconsistencies in the thickness of the final parts. Static strain aging is characterized by an increase in yield and ultimate strengths, a decrease in the total elongation, and a time-dependent return of the YPE or Lu¨ders strain after plastic deformation.[15,16] Baird and Jamieson showed that the addition of N will have a strong effect on the occurrence of static strain aging in ferritic steels.[17] Many of the medium manganese (5 to 10 wt pct)[2,3,5–12,18] steels proposed for 3rd generation advanced high strength steel application exhibit YPE, and it is noted that the degree of YPE can range from 2
to 10 pct depending upon chemistry and processing prior to mechanical testing. Work by Suh et al.[3] on three Fe-0.5Si-5Mn-2Al (wt pct) alloys with carbon contents (0.06 to 0.11 wt pct) exhibited varying degrees of YPE that was dependent upon heat treatment. They observed that an increase in heat treatment temperature from 993 K to 1033 K (720 C to 760 C) leads to a change in YPE from 10 to 4 pct strain for all three alloys independent of carbon content. This significant change in the YPE was not discussed by Suh et al., but is potentially attributed to the volume fraction of a-ferrite/martensite in the microstructure. Specimens with 10 pct YPE had 87 vol pct a-ferrite as compared to the 4 pct YPE specimens with 72 vol pct ferrite. Work by Han et al.[9] on a medium-Mn steel (8.5 wt pct Mn) showed an extensive yield point elongation (11 pct) after cold wor
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