Advanced High-Strength Steels: Microstructure Property Relationship
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

Wednesday 2:00 PM
March 1, 2017
Room: 17A
Location: San Diego Convention Ctr

Session Chair: Matous Mrovec, ICAMS - Ruhr University Bochum; Amy Clarke, Colorado School of Mines

2:00 PM  Invited
Influence of the Initial Microstructure on the Reverse Transformation Kinetics and Microstructural Evolution in TRIP-assisted Steel: Jeong In Kim1; Joo Hyun Ryu2; Sea Woong Lee2; Kyooyoung Lee2; Dong Woo Suh1; 1Pohang University of Science and Technology; 2POSCO
    The transformation induced plasticity steels is classified as one of advanced high strength steel. However, relatively poor stretch-flangeability has been an obstacle for more active application. The stretch-flangeability is strongly affected by microstructure. Fine, homogenous microstructure is thought to be beneficial to obtain enhanced stretch-flangeability. Even though it was reported that the fine lath-type morphology could be generated by starting from the martensite as initial microstructure for heat treatment, the mechanism which governs the evolution of the lath-type reverse transformed microstructure upon heating to an intercritical temperature is not fully understood. The present study was aimed to examine this unclear issue on the critical condition to obtain fine lath-type microstructure in the TRIP-assisted steels. The influence of initial microstructure was investigated with respect to the reverse transformation kinetics and microstructural evolution upon heating to intercritical temperature. In particular, the role of austenite retained in the initial microstructure is clarified.

2:30 PM  
Observation of Low Cycle Fatigue Dislocation Structures in a TWIP, TRIP and MBIB Steel, Using Electron Channelling Contrast Imaging (ECCI): Dayong An1; Stefan Zaefferer1; 1Max-Planck-Institut für Eisenforshung GMBH
    Different high Mn steels showing the TRIP and TWIP effect as well as planar glide and kappa carbides have been submitted to a low-cycle fatigue experiment using about 0.5 % of cyclic strain. Subsequently the samples were prepared for ECCI observations and the dislocation structures in the different materials were compared for a number of different orientation. ECCI allows to observe and quantify the density and arrangement of dislocation veins and channels and to directly image the dislocations inside the veins and channels in the bulk samples. It is possible to determine the Burgers vector of the dislocations and distinguish between geometrically necessary (GND) and statistically stored (SSD) dislocations. Our observations allow to distinguish the role of the different stacking fault energies and of the presence or absence of kappa carbides.

2:50 PM  
Microstructural Evolution and Mechanical Behavior of Medium Mn Steels Intercritical Annealed from Different Starting Structure: Binhan Sun1; Fateh Fazeli2; Colin Scott2; Stephen Yue1; 1McGill University; 2CanmetMATERIALS, Natural Resources Canada
    The characteristics (e.g. fraction, stability and homogeneity) of retained austenite remain critical in controlling the mechanical properties of medium Mn steels. This work attempts to maximize the fraction of retained austenite in the intercritical annealed medium Mn steels (with 7 and 10 wt. % Mn), by applying different prior heat treatments before cold rolling. Several different structures were created through these treatments after cold rolling, including pearlite/bainite structure, martensite structure, and martensite, ferrite and retained austenite multi-phase structure. The microstructural evolution and mechanical behavior of the steels intercritical annealed from these different starting structures are investigated. Excellent combination of strength-ductility balance of ~55,000 MPa pct can be achieved. The results showed that the intercritical annealed austenite fraction could be influenced by the initial structure, however, the overall mechanical performance was only slightly affected, which indicates the dominant roles of other factors such as phase compositions and the extent of recrystallization.

3:10 PM  
In-situ Synchrotron X-ray Diffraction Investigation on Strain Hardening Behavior of Fe-17Mn-1.5Al-0.3C Steel: Yan Ma1; Wenwen Song1; Wolfgang Bleck1; 1RWTH Aachen University
    The quantitative characterization of the deformation microstructure evolution in high-Mn austenitic TRIP/TWIP steel is of great importance for the understanding of its strain hardening behavior. Stacking fault energy (SFE) exhibits the decisive role to control the strain hardening behavior in high-Mn austenitic steels. In the present work, the in-situ high energy synchrotron X-ray diffraction (SYXRD) was applied to investigate the TRIP and TWIP effect during the deformation of Fe-17Mn-1.5Al-0.3C steel. The in-situ tensile test tracked by high-energy synchrotron X-ray diffraction was carried out and the 2D diffraction patterns were taken in every 10 seconds. The strain hardening behavior in different deformation stages was discussed with the variation of the ε-martensite and α´-martensite phase fraction, stacking faults and twin faults probability, and dislocation density. Finally, the stacking fault energy was calculated based on the synchrotron measurement and further compared with the thermodynamic calculation using subregular model.

3:30 PM Break

3:50 PM  
Effect of Microstructure on Formability and Micro Fracture Mechanism in DP Steel for Automotive Outer Panel: Yeon-sang Ahn1; Chang-hyo Seo1; Sang-Ho Han1; In-Shik Suh1; John Speer2; 1POSCO Technical Research Laboratories; 2Colorado School of Mines
    Application of high strength steel to automotive outer panels such as doors, hoods, fenders and deck-lids has accelerated due to the need for superior dent resistance and overall vehicle weight reduction. DP steel with 490MPa minimum tensile strength has recently been developed to replace bake hardening steel with 340MPa tensile strength. In the near future, DP grades for exposed application are expected to be extended to 590 MPa minimum tensile strength, to achieve even further light-weighting. For the application of higher strength of DP steel to exposed outer panels, excellent formability is required to prevent cracking during cold forming, along with surface quality and other critical characteristics. In this research, the effect of microstructure on formability and micro fracture mechanisms in DP steel for automotive outer panels has been studied. Especially, key microstructural factors to improve formability and to delay ductile micro-fracture will be discussed in relation to alloy composition, heat treatment process details and phase transformation behavior.

4:10 PM  
High Speed Tensile Test with Infrared Thermography and Microstructure Analysis on a High Mn TWIP Steel: Sebastian Wesselmecking1; Harald Hofmann2; Thorsten Beier2; Thorsten Rösler3; Maximillian Nagel3; Klaus Unruh4; Wolfgang Bleck1; 1RWTH Aachen; 2ThyssenKrupp Steel Europe; 3Hoesch Hohenlimburg GmbH; 4Faurecia Autositze GmbH
    Due to the increasing demand for lightweight design in the automotive industry, twinning induced plasticity (TWIP) steels are receiving extended attention, as they combine high strength with outstanding plasticity. Especially crash relevant material parameters like strain rate sensitivity and the influence of adiabatic deformation conditions on the strain hardening behavior are of great importance for design proposes. In the presented study, high strain rate as well as isothermal tensile tests were conducted on a 0.4C-19Mn-2Cr-1Al-V TWIP steel. Adiabatic heating is quantified with high speed infrared thermography and correlated with the mechanical properties as well as the microstructure. It can be pointed out that the dynamic temperature change with increasing strain leads a decrease of strain hardening potential. Besides a thermal softening effect, it is found that the dynamic change of the stacking fault energy affects also the twinning behavior of the steel, also decreasing the materials strength.

4:30 PM  
Microstructure and Mechanical Properties of a 0.2C-5Mn TRIP Steel after Continuous Intercritical Annealing: Wei Ding1; Yan Li1; 1Inner Mongolia University of Science and Technology
    A 0.2C-5Mn TRIP steel after cold rolling was intercritical annealed between 600 and 700 °C with different intercritical time, simulating a continuous annealing process line. Before annealing, the microstructure mainly consists of tempered martensite and large fraction of cementite. During annealing, martensite partially transforms to austenite, which may retain or transform to martensite after final cooling to room temperature, depending on its stability. With increasing the interitical annealing temperature and time, less cementite could be observed. The transformation of martensite to austenite with the dissolution of cementite was simulated using the DICTRA software. With increasing the annealing temperature from 650 to 700 °C, the time required to achive the optimal mechanical property is decreased. The optimal mechanical property was obtained after annealing at 675 °C for 5 min, with an ultimate tensile strength of 1000 MPa and a total elongation of 29%.