Chemistry and Properties of Medium-Mn Two-Stage TRIP Steels
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INTRODUCTION
A. Design and Processing of 3rd Generation Advanced High-Strength Steels
MICROSTRUCTURAL engineering is central to the development of 3rd generation advanced high-strength steel for automotive application. Quench and partitioning of martensitic steels is an established approach to obtain the desired combination of a-martensite and c-austenite with partitioning of carbon to stabilize the c-austenite at room temperature. Transformation-induced plasticity (TRIP) in quench and partitioned steel results from the transformation of c-austenite to a-martensite, which results in both greater formability
DANIEL M. FIELD, JINGJING QING, 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 date January 16, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
and improved crashworthiness. Quench and Partitioning steels exhibit ultimate tensile strengths in excess of 1500 to 1700 MPa and elongations to failure of 20 to 10 pct.[1–3] Medium manganese (5 to 12 wt pct Mn) steels are also potential 3rd generation advanced high-strength steels where fine-grained microstructures of a-ferrite and c-austenite are created by first cold working and isothermal annealing. Partitioning of substitutional alloying elements (e.g., Al, Mn) and interstitial carbon stabilize the c-austenite, i.e., Al is partitioned to the a-ferrite for solid solution strengthening and Mn and C are partitioned to the c-austenite to produce TRIP during straining. TRIP behavior may be proceeded by twin-induced plasticity (TWIP) or may exhibit two-stage TRIP where c-austenite first transforms to e-martensite and subsequently transforms to a-martensite. These medium-Mn steels demonstrate ultimate tensile strengths ranging from 700 to 1550 MPa and total elongations of 65 to 10 pct.[4–9] Processing of medium manganese steels includes hot rolling to form a hot band thickness of 1.8 to 3.3 mm,[10]
cold rolling the hot band, and intercritical annealing within a temperature range of 873 K to 1023 K (600 C to 750 C) to obtain a metastable c-austenite that TRIP’s to a-martensite. The stability and volume fraction of retained c-austenite is controlled by the time and temperature at which annealing is performed. Zhang et al.[11] investigated a 7 wt pct Mn steel annealed at 893 K (620 C) for times ranging from 3 minutes to 96 hours and found that the increased time at temperature coarsened the c-austenite from 300 to 940 nm, but noted that the volume fraction of c-austenite was relatively constant at ~ 40 vol pct. During tensile testing, the c-austenite transformed to a-martensite. Luo et al. reported[12] on two 5 wt pct Mn steels where the intercritical annealing temperature was varied to manipulate the Mn and C content of the c-austenite. A maximum in the measured retained c-austenite at ambient temperature was obtained by intercritical annealing at 943 K (670 C) for 10 minutes. Zhang et al.[13] investigated a 0.2 C to 5
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