Title

Chemistry and Properties of Medium-Mn Two-Stage TRIP Steels

Abstract

Eight medium manganese steels ranging from 10 to 15 wt pct Mn have been produced with varying levels of aluminum, silicon, and carbon to create steels with varying TRIP (transformation-induced plasticity) character. Alloy chemistries were formulated to produce a range of intrinsic stacking fault energies (ISFE) from − 2.2 to 13.3 mJ/m2 when calculated at room temperature for an austenitic microstructure having the nominal alloy composition. Two-stage TRIP behavior was documented when the ISFE of the γ-austenite phase was 10.5 mJ/m2 or less, whereas an ISFE of 11.9 mJ/m2 or greater exhibited TWIP (twin-induced plasticity) with single-stage TRIP to form α-martensite. Properties were measured in both hot band (hot rolled) and batch annealed (hot rolled, cold rolled, and annealed) conditions. Hot band properties were influenced by the Si/Al ratio and this dependence was related to incomplete recovery during hot working for alloys with Si/Al ratios greater than one. Batch annealing was conducted at 873 K (600 °C) for 20 hours to produce ultrafine-grained microstructures with mean free slip distances less than 1 µm. Batch-annealed materials were found to exhibit a Hall—Petch dependence of the yield strength upon the mean free slip distance measured in the polyphase microstructure. Ultimate tensile strengths ranged from 1450 to 1060 MPa with total elongations of 27 to 43 pct. Tensile ductility was shown to be proportional to the sum of the products of volume fraction transformed times the volume change associated for each martensitic transformation. An empirical relationship based upon the nominal chemistry was derived for the ultimate tensile strength and elongation to failure for these batch-annealed steels. Two additional alloys were produced based upon the developed understanding of these two-stage TRIP steels and tensile strengths of 1150 MPa with 58 pct total elongation and 1400 MPa and 32 pct ductility were achieved.

Department(s)

Materials Science and Engineering

Comments

This work was supported by the Peaslee Steel Manufacturing Research Center (PSMRC). Companies directly involved in this work include AK Steel, ArcelorMittal, Nucor Steel, and U. S. Steel. The FEI Helios NanoLab dual beam FIB was obtained with a Major Research Instrumentation Grant from the National Science Foundation under Contract DMR-0723128. The FEI Tecnai F20 scanning/transmission electron microscope was obtained through a major research instrumentation Grant from the National Science Foundation under Contract DMR-0922851.

Keywords and Phrases

Annealing; Austenitic transformations; Cold rolling; Ductility; Elongation; Hot working; Martensitic transformations; Metal cladding; Microstructure; Plasticity; Tensile strength, Alloy compositions; Austenitic microstructure; Empirical relationships; Intrinsic stacking fault; Polyphase microstructure; Transformation induced plasticity; Ultimate tensile strength; Ultra fine grained microstructure, High strength steel

International Standard Serial Number (ISSN)

1073-5623; 1543-1940

Document Type

Article - Journal

Document Version

Citation

File Type

text

Language(s)

English

Rights

© 2018 The Minerals, Metals & Materials Society and ASM International, All rights reserved.

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