Austenite Stability Effects on Tensile Behavior of Manganese-Enriched-Austenite Transformation-Induced Plasticity Steel
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RESEARCH on new advanced high strength steels (AHSS) has led to the development of steel grades with improved property ranges as required by new automotive designs optimized for fuel efficiency and safety.[1,2] Current ‘‘first generation’’ AHSS grades are based predominantly on ferritic microstructures with the addition of low-temperature transformation products (bainite, martensite, and carbon-enriched austenite) to increase strength; these steel grades include dual-phase (DP), transformation-induced plasticity (TRIP), complex-phase, and martensitic steels. While the properties displayed by these steels are impressive, there is a desire to develop steels with improved formability at a given strength level.[1] Austenitic steels, including stainless steels and recently developed twinning-induced plasticity steels (TWIP), exhibit excellent combinations of strength and ductility and constitute a group of steels referred to as the ‘‘second generation’’ AHSS. However, stabilization of the fully austenitic structure requires high alloy levels, and thus, the steels are expensive and have received limited use in the commercial automotive industry.
An opportunity exists to develop a new family of steels with properties between the first and second generation steels addressing the limitations of each. These ‘‘third generation’’ AHSS grades are of great interest and there is considerable research effort being focused on their development.[2] In comparison to the first generation steels, it is anticipated that these steels will have increased amounts of retained austenite with controlled stability against strain-induced transformation to martensite.[3,4] One approach to develop microstructures of interest uses lean alloys (5 to 8 wt pct Mn) and intercritical annealing in the ferrite-austenite region to enrich austenite in Mn, thus stabilizing it to room temperature. Significant austenite fractions (20 to 40 pct) have been obtained in this way depending on processing and Mn content.[9–18] A methodology for identifying heat treatment conditions for optimal Mn enrichment has been developed based on an equilibrium thermodynamic analysis.[5,6] Predictions of austenite amount and composition as a function of annealing temperature were made using THERMO-CALC* software with the TCFE2 database *THERMO-CALC is a trademark of Thermo-Calc, Stockholm.
P.J. GIBBS, Graduate Student, E. DE MOOR, Research Faculty, and J.G. SPEER and D.K. MATLOCK, Professors, are with the Advanced Steel Processing and Products Research Center, Colorado School of Mines, Golden, CO 80401. Contact e-mail: [email protected] M.J. MERWIN, Research Consultant, is with the United States Steel Corporation Research and Technology Center, Munhall, PA 15120. B. CLAUSEN, SMARTS Instrument Scientist, is with the Los Alamos Neutron Science Center, Los Alamos National Laboratory, Los Alamos, NM 87545. Manuscript submitted January 13, 2011. Article published online April 27, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
for a 0.1-C 7.1-Mn 0.1-C steel, and the predicted austeni
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