Advanced High-Strength Steels: Recent Developments in High-/Medium Mn Steels
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Phase Transformations Committee
Program Organizers: Tilmann Hickel, Max-Planck-Institut fuer Eisenforschung GmbH; Wolfgang Bleck, RWTH Aachen; Amy Clarke, Colorado School of Mines ; Young-Kook Lee, Yonsei University; Matthias Militzer, The University of British Columbia

Monday 2:00 PM
February 27, 2017
Room: 17A
Location: San Diego Convention Ctr

Session Chair: Jörg Neugebauer, Max-Planck-Institut für Eisenforschung; Young-Kook Lee, Yonsei University

2:00 PM Introductory Comments

2:05 PM  Invited
Strain Path Dependence of Retained Austenite Mechanical Stability in a Medium Manganese Steel Stamping: Louis Hector Jr1; Yu-wei Wang2; Wei Wu2; Feng Zu2; Panagiotis Makrygiannis2; Fadi Abu-Farha3; Xin Sun4; Xioahua Hu4; Yang Ren5; 1General Motors; 2AK Steel; 3Clemson University; 4Pacific Northwest National Laboratory; 5Argonne National Laboratory
    The extent to which mechanical stability of retained austenite in Generation Three Advanced High Strength steels is dependent upon strain path during plastic deformation is an open question in the material science of these multiphase materials. This presentation will discuss a novel experimental approach aimed at answering this question for a medium manganese TRIP steel. A laboratory stamping process produced T-shaped components that emulate the upper portion of an automotive body B-pillar. During plastic deformation, sheet material was deformed under both linear and non-linear strain paths and subject to fracture. Local strain fields in each stamping were measured with a digital image correlation technique. Retained austenite volume fraction (RAVF) was then measured in small squares extracted at locations of linear and non-linear strain paths using synchrotron X-ray diffraction. Strain path dependence was quantified by coupling the DIC strains in each square with the corresponding RAVF in a major-minor strain space.

2:35 PM  
Segregation Engineering in Medium Manganese Steels: Dirk Ponge1; Margarita Kuzmina1; Alisson Kwiatkoski1; Meimei Wang1; Stefanie Sandlöbes1; Michael Herbig1; Dierk Raabe1; 1Max-Planck-Institut für Eisenforschung GmbH
    In medium manganese steel austenite reversion is of high interest to produce austenite islands to provide a TRIP or TWIP effect to raise strain hardening rate and ductility. We will discuss our results from atom probe tomography and TEM showing that equilibrium segregation of austenite stabilizing elements to dislocations or grain boundaries can enhance formation of embryos or nucleii of austenite. On the other hand, segregation of ferrite forming elements to grain boundaries can be used to avoid formation of austenite. This is of technical interest for heat treatments for very high strength materials like maraging steels where formation of soft austenite during precipitation hardening is degrading the mechanical properties. The effect of Mn segregation to grain boundaries is more complicated: For short heat treatments it can result in grain boundary embrittlement. At longer times the Mn segregation can lead to austenite formation and a dramatic increase in impact toughness.

2:55 PM  
High Strength Nb-bearing Medium Mn Steel for Warm Stamping: Jae-Hoon Nam1; Jeongho Han2; Young-Kook Lee1; 1Yonsei University; 2Max-Planck-Institut für Eisenforschung
    Hot stamping is a technology to produce automobile parts (e.g. A,B-pillar, roof rail, bumper, etc.) that require high strength and high formability. Applications of hot stamping are expanding rapidly because of its advantages of precise dimension, high rigidity and complex geometry of the formed parts. However, hot stamping shows limitations in process; it needs high temperature for forming (over A3 temperature), rapid quenching for hard martensite phase at room temperature, and additional tempering for better toughness. Recently, the medium Mn steel with 3-10 wt.% Mn have been actively studied due to its excellent tensile strength (< 1.2 GPa) and ductility (< 40%). In present study, we studied the Nb-bearing medium Mn steel for the warm stamping process. The low A3 temperature and martensitic transformation occurring even during slow cooling of medium Mn steel could solve the disadvantages of present hot stamping, and Nb precipitation produces additional hardening.

3:15 PM  
High Strain Rate Deformation of High-Mn and Medium-Mn TWIP-TRIP Steel: Jake Benzing1; Whitney Poling2; Dean Pierce3; Kip Findley2; Dirk Ponge4; Dierk Raabe4; James Wittig1; 1Vanderbilt University; 2Colorado School of Mines; 3Oak Ridge National Laboratory; 4Max-Planck-Institut für Eisenforschung
    Servo-hydraulic tension was utilized to study the effects of strain rate (0.02–200 strain/s) on mechanical properties and deformation mechanisms in twinning and transformation-induced plasticity [TWIP-TRIP] steels. A fully austenitic Fe-25Mn-3Al-3Si alloy possesses an intermediate stacking fault energy [SFE] of 21±3 mJm-2 such that a combination of TWIP and TRIP is promoted, which provides obstacles to subsequent dislocation motion. To correlate TWIP-TRIP distributions to changes in strain rate, (at the same interrupted strain), dark field transmission electron microscopy, electron channeling contrast imaging and electron backscattered diffraction were used to quantify the spacing and thickness of planar defects. A medium-Mn alloy, (Fe-12Mn-3Al-0.06C), has been designed via Thermo-Calc predictions to optimally balance composition, phase fraction/stability, SFE and intercritical annealing. High strain rate mechanical testing of the already processed medium-Mn alloy is currently in progress. Additional characterization involves x-ray diffraction and scanning-TEM energy-dispersive spectroscopy to respectively quantify phase fraction and alloy segregation.

3:35 PM Break

3:50 PM  
Effect of Retained Austenite Transformation Holding Time and Temperature on the Microstructural Development and Properties of a Medium Mn Third Generation Advanced High Strength Steel: Kazi Bhadon1; Joseph McDermid1; Elizabeth McNally1; Frank Goodwin2; 1McMaster University; 2International Zinc Association
    Third Generation Advanced High Strength Steels (3G-AHSS) are promising candidates for automotive weight reduction, but must be compatible with the thermal processing capabilities of the continuous galvanizing process to be successfully employed in automotive applications. A prototype medium-Mn 3G-AHSS TRIP-aided steel of approximate composition 0.2wt% C, 6wt% Mn, 1.5wt% Si and 0.5 wt% Al with a martensitic starting microstructure was subjected to a variety of thermal cycles, comprising an Intercritical Anneal and a Retained Austenite Transformation (RAT) at temperatures ranging from 400C to 460C for a variety of times. The effect of the RAT processing parameters on the sample microstructures were analysed at a variety of length scales and the effect of the heat treatments on mechanical properties assessed via conventional tensile testing. This contribution will discuss the microstructural evolution of the prototype 3G-AHSS composition within the context of their effect on retained austenite stability and impact on mechanical properties.

4:10 PM  
Effect of Starting Microstructure and Intercritical Annealing Parameters on Mechanical Properties of a Medium-Mn Third-generation Advanced High Strength Steel: Daniella Pallisco1; Joseph McDermid1; Elizabeth McNally1; Frank Goodwin2; 1McMaster University; 2International Zinc Association
    Third generation advanced high strength steels (3G AHSS) are relatively cost-effective alloys with superior combinations of high strength and ductility that would facilitate vehicle light-weighting. The present work investigates the mechanical properties and microstructural evolution of a prototype 0.15C-6Mn-2Al-1Si 3G AHSS arising from heat treatments that are compatible with the continuous galvanizing process. This paper will discuss the effects of starting microstructure and thermal processing parameters on the microstructural evolution of the alloy, focussing on retained austenite volume fraction, and resultant tensile properties within the context of established targets.

4:30 PM  
Influence of Cooling and Strain Rate on the Hot Ductility Behavior of High Manganese Steels within the System Fe-Mn-C: Bernhard Steenken1; Dieter Senk1; Joao L. L. Rezende1; 1RWTH Aachen
    The hot ductility is a measure to determine the susceptibility to cracking during continuous casting (CC). Especially fully austenitic high manganese steel grades show low ductility values in the relevant temperature range of 700–1000°C. Due to the varied conditions in a CC-machine, the hot ductility of high manganese steel grades with varying manganese contents of 12, 18, 24, 30wt.-% and a carbon content of 0,3wt.-% for different strain rates(0.01 1/s and 0.001 1/s) and varying cooling rates (-3K/s and -7K/s) have been measured. With this aim the insitu hot tensile testing unit of the Department of Ferrous Metallurgy has been used. The specimens were investigated by using the SEM, EDX and light microscopy to determine the influence on the hot ductility as well as on the precipitates and phase formation. Also the thermodynamic modeling using ThermoCalc® has been conducted, in order to perform a comparison with the experimental results.

4:50 PM  
Austenite Formation along Dislocations in Medium Manganese Steels: Margarita Kuzmina1; Dirk Ponge1; Stefanie Sandlöbes1; Michael Herbig1; Dierk Raabe1; 1Max-Planck-Institut für Eisenforschung GmbH
    In a binary martensitic Fe-9wt%Mn alloy with very low carbon content (75 wt ppm) we observe a serrated appearance of the stress-strain curve at 450 °C and discontinuous yielding after tempering at 450°C. These phenomena have so far mainly been observed in the Fe-C system, where they are referred to as dynamic and static strain ageing, respectively. Correlative TEM/atom probe tomography experiments give direct evidence for pronounced equilibrium segregation of Mn (up to a factor of 3) and the absence of C at dislocations. The tubular-like Mn enriched zone has the chemical composition of austenite when in thermodynamic equilibrium with the present ferrite phase (partitioning). In conjunction with the thermodynamically calculated radius of a subcritical metastable particle of the second phase, we hence suggest that some of the dislocation segments might have undergone segregation-induced phase transformation to austenite.

5:10 PM  
Ultrahigh Strength and Excellent Ductility Achieved by Grain Refinement in Low-carbon High-manganese Steels: Hung-Wei Yen1; Yu-Han Huang1; Ching-Yuan Huang2; Steve Ooi3; 1National Taiwan University; 2China Steel Corporation; 3University of Cambridge
    The annealed microstructure and mechanical properties of low-carbon high-manganese steels have been investigated by using transmission electron microscopy, X-ray analysis, and electron backscattered diffraction in this work. It was found that annealing at a lower temperature leads to both high tensile strength and excellent elongation in spite of its extremely small grains. The reason is that refinement of austenite grain size due to lowering annealing temperature could significantly inhibits martensite transformation during quenching to room temperature. Hence, during plastic deformation, tiny and metastable austenite transformed into ε martensite and α΄ martensite, producing high strength and excellent ductility via TRIP effects.