Medium-Alloy Manganese-Rich Transformation-Induced Plasticity Steels
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RODUCTION
WORK by Miller established the possibility of alloys containing a medium manganese concentration, Fe-0.11C-5.7 Mn wt pct, in order to generate a mixed microstructure of ferrite and austenite by intercritical annealing. The plasticity obtained during the deformation-induced martensitic transformation of austenite (TRIP) led to impressive values of total elongation (34 to 19 pct) for tensile strengths in the range 875 to 1200 MPa. The time periods for intercritical annealing were in the range 1 to 16 hour. In an effort to make the process more amenable to industrial production, Suh and co-workers[2] added up to 3 wt pct of aluminum to the steel, on the basis that this solute increases the free energy change associated with the austenite to ferrite transformation.[3] This has the effect of increasing the temperature at which the steel can be intercritically annealed, thus permitting the time period of annealing to be reduced to just two minutes.[2] However, it is considered that 3 wt pct is too large a concentration from the point of view of steel manufacture by continuous casting because of the possibility of nozzle clogging.[4] The purpose of the present work was to see if a similar alloy system can be designed with a [1]
DONG WOO SUH, Assistant Professor, JOO HYUN RYU and MIN SUNG JOO, Graduate Students, are with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Kyungbuk 790-784, Republic of Korea. Contact e-mail: dongwoo1@ postech.ac.kr HONG SEOK YANG, Senior Researcher, and KYOOYOUNG LEE, Principal Researcher, are with the Technical Research Laboratories, POSCO, Pohang 790-300, Republic of Korea. H.K.D.H. BHADESHIA, Director, is with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, and also Professor with the Materials Science and Metallurgy Department, University of Cambridge, Cambridge CB2 3QZ, U.K. Manuscript submitted April 25, 2012. Article published online September 15, 2012 286—VOLUME 44A, JANUARY 2013
reduced aluminum concentration of 2 wt pct, a content which is considered acceptable from a steelmaking point of view. It may be possible to compensate for the reduction in aluminum concentration by altering the carbon and manganese concentrations, and this formed the motivation for the present work.
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EXPERIMENTAL METHOD
Table I lists the chemical compositions of the alloys. Compared to the previous work based on Fe-0.12C5.8Mn-0.47Si-3.1Al wt pct,[2] the aluminum content is reduced to 2 wt pct. The carbon and manganese concentrations were adjusted systematically using phase diagram calculations in order to obtain comparable austenite fractions during intercritical annealing. Figure 1 shows the change of equilibrium fraction of austenite estimated using the Thermo-Calc software[5] with TCFE 6 database.[6] It clearly is possible in principle to maintain the austenite content in the temperature range of intercritical annealing. Ingots 300 9 150 9 100 mm were prepared by vacuum-induction melting, after which they we
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