Precipitation Effect on Mechanical Properties and Phase Stability of High Manganese Steel
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TRODUCTION
RECENTLY, high manganese (Mn) steels have received considerable attention for structural applications due to their excellent combination of strength and elongation. The attractive mechanical properties of high Mn steels are known to be closely related to their plasticity behaviors including dynamic strain aging (DSA) and mechanical twinning.[1–6] Consequently, high Mn steels are expected to be a candidate for next-generation high-strength steels. However, there are still a few obstacles to overcome before high Mn steels can be used in automobile body parts. One of the obstacles limiting the immediate use of high Mn steels is their relatively low yield strength, especially when compared with existing advanced high-strength steels (AHSS).[7,8] High yield strength is particularly important in automotive structures used for anti-intrusion (crash) assemblies where it is essential. A reasonable yield strength target for an automotive application is generally considered to be 600 to 700 MPa.[9,10] According to the research of Bouaziz, this value in Mn austenite steels can be achieved by grain refinement if the grain size is reduced below 1 lm.[7] Unfortunately, a minimum grain size of about
CHEOLJUN BAE, RASA KIM, and JONGRYOUL KIM are with the Department of Metallurgy and Material Science, Hanyang University, 55, Hanyangdaehak-ro, Sangrok-gu, Ansan-si, Gyeonggido 15588 Republic of Korea. Contact e-mail: [email protected] UN-HAE LEE is with the Technical Research Laboratories Gwangyang Research Lab., POSCO, 8, Pokposarang-gil, Gwangyang-si, Jeollanam-do 57807, Republic of Korea. Manuscript submitted December 4, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS A
only 2.5 lm is achievable, due to practical process limitations in conventional rolling methods.[11,12] Thus, the formation of fine precipitates in an austenite matrix can be an alternative method to improve the yield strength. Raabe investigated the precipitation hardening mechanism in high Mn steels with a low carbon content and both strength and total elongation were improved by the formation of shearable nano-sized-carbide.[13–15] In addition, the precipitation strengthening by Cu-rich phase was investigated in austenitic heat-resistant steel.[16] Although extensive researches have been carried out on precipitation hardening mechanism in austenitic steel, the effect of microalloying elements such as Ti, Nb, and V on strengthening by MX precipitation has not been studied in detail. In addition, the stacking fault energy (SFE) of austenite can be reduced by lowering the carbon concentration, which induces the formation of e- or a’-martensite phase during plastic deformation.[17–20] It is well known that the formation of the eor a’-martensite phase aggravates the formability of high Mn steels during deformation. Hence, in order to increase the yield strength without any significant loss of formability, it is critical to control the microstructures and carbon contents in high Mn steels. In this study, the precipitation hardening effect in high Mn steels was
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