Advanced High Strength Steels IV: Session I
Sponsored by: TMS Structural Materials Division, TMS: Steels Committee
Program Organizers: Ana Araujo, Vesuvius USA; Mary O'Brien, Los Alamos National Laboratory; Tilmann Hickel, Bam Federal Institute For Materials Research And Testing; Amy Clarke, Los Alamos National Laboratory; Kester Clarke, Los Alamos National Laboratory; C. Tasan, Massachusetts Institute of Technology; MingXin Huang, University of Hong Kong
Tuesday 8:30 AM
February 25, 2020
Room: Mission Hills
Location: Marriott Marquis Hotel
Session Chair: Amy Clarke, Colorado School of Mines; Rachael Stewart, AK Steel
8:30 AM
An Energy Absorbing Medium Mn Steel for Industry: Thomas Kwok1; Xin Xu1; David Dye1; 1Imperial College London
Medium Mn steels have been of large research interest recently due to their high energy absorbed upon deformation. We present a new medium Mn steel with 8 wt% Mn and a microalloy content of <0.1 wt% V. The steel is designed to be processed on existing machinery that manufactures dual phase steels and is more cost-effective than DP800 which is currently used in automotive crash assemblies. In the annealed condition, it was found that the steel deforms via the TWIP+TRIP mechanism. When cold rolled, the TWIP effect can be selectively consumed to provide a large increment in strength while the TRIP effect can still be triggered to provide ductility and strain hardening during deformation. SEM and EBSD were used to elucidate the change in microstructure between annealed and cold rolled conditions. TEM was used to characterise the V precipitates which contribute to the strengthening of the steel.
8:50 AM
Annealing Time Dependence of Tensile Properties of 8 wt% Mn Steel: Xin Xu1; Thomas Kwok1; David Dye1; 1Dept of Materials, Imperial College London
The automobile industry is interested in medium Mn steels due to their high strength, high ductility and potentially higher cost efficiency of production compared to high Mn steels. Their high performance arises from deformation twinning, strain-induced transformation, or both mechanisms. These mechanisms are significantly affected by the thermodynamic stability and stacking fault energy of austenite, which are largely influenced by its chemical composition. Intercritical annealing is commonly used to tune the composition and volume fraction of austenite. Therefore, the temperature and duration are critical. In this work, we systematically studied the effect of intercritical annealing time on element partitioning and resultant deformation mechanisms in an 8 wt.% Mn steel. A significant annealing time dependence of tensile properties has been revealed. Generally, as annealing time prolongs, the engineering tensile strength and strain hardening rate decrease but elongation increases. The relation of the annealing time, chemical composition and deformation mechanisms is discussed.
9:10 AM
Microstructural Origins of Lüders Banding in Medium Manganese Steels: Rama Srinivas Varanasi1; Dirk Ponge1; Dierk Raabe1; 1Max Planck Institute for Iron Research
The combination of high strength and ductility makes medium manganese steels promising prospects for the automobile industry. The only pinprick is the presence of Lüders bands, due to its adverse effect on the surface finish during forming operations. The present study is aimed at understanding the complex microstructural origins of Lüders band in medium manganese steels and design the processing routes to eliminate such localized deformation. We performed quasi in-situ deformation studies with electron channeling contrast imaging (ECCI) to study the discontinuous yielding. In our current work, we discuss the relevance of various mechanisms influencing Lüders band formation, namely: Cottrell atmosphere, strain partitioning in the dual phase microstructure, transformation-induced plasticity (TRIP) in the metastable retained austenite, the morphology of the retained austenite, segregation at the interfaces and the grain size. We will show how Lüders banding can be suppressed by tailoring the microstructure.
9:30 AM
TBF Steels Produced by Partial Austenitization: Ersoy Erisir1; Oğuz Gürkan Bilir1; Yunus Emre Sözer2; Özge Ararat1; 1Kocaeli University; 2Erdemir (Eregli Iron and Steel Works Co)
Carbide free bainitic steels or TRIP aided bainitic ferrite steels (TBF) are produced by two steps as controlled cooling to bainitic formation temperature from an austenitization temperature and an isothermal holding. During the bainitic transformation, austenite is expected to be stabilized by carbon partitioning. In this work, it is studied to enhance austenite stability by Mn partitioning at partial austenitization before carbon partitioning stage at the bainitic transformation. A martensitic transformation is applied to samples before the partial austenitization to assure grain refinement of fresh formed austenite and ferrite during the partial austenitization stage. Baehr dilatometer is used to reveal transformation temperatures and ThermoCalc and DICTRA are used to examine transformation behavior. XRD and FE-SEM is carried out for characterization of austenite. Tensile test was also applied for mechanical properties. A final microstructure of bainitic ferrite and austenite is possible by applying the partial austenitization.
9:50 AM
Effects of Prior Austenite Grain Size on Microstructure of Bainite and Retained Austenite in TRIP Steel: Miku Watanabe1; Goro Miyamoto1; Tadashi Furuhara1; 1Tohoku University
In TRIP steel austenite is stabilized by carbon enrichment during bainitic transformation. The stability depends on its size and morphology, which should be affected by prior austenite grain size. Thus, the purpose of this study is to reveal the effects of prior austenite grain size on microstructure and mechanical property in TRIP steel. Fe-2Mn-1.5Si-0.4C (mass%) alloy was used. Large (348 microns) and Small (12.5 microns) austenite grain sizes were obtained through austenitization at 1150 degC and 830 degC, respectively. Following to austenitization, samples were isothermally transformed at 400 degC for various reaction time. Quantitative analysis based on SEM images reveals that large-grained specimen contains a high density of film-like austenite and a few coarse blocky austenite while untransformed austenite is mostly fine blocky austenite in the small-grained specimen. FE-EPMA measurement clarifies that carbon content in austenite increases with decreasing austenite grain size, which further stabilizes fine untransformed austenite.
10:10 AM Break
10:30 AM
Improving Microstructure of Ferritic-bainitic Steel used in Automotive Industry: Mostafa Tawfeek1; Ayman Fathy1; Ahmed Abdelaziz1; 1German University in Cairo
This study is concerned with Ferritic-Bainitic steel of chemical composition of 0.2 wt% C, 0.5wt% Si, 1.2wt% Mn. The metallic casted block was cut then, hot rolled and heat treated at 3 different heat treatment programs. Heat treatment program applied was heating up sample to 850℃ , 900℃ and 950℃ then quenching it in a salt bath for 30 minutes to (450℃ – 500℃ – 550℃) .The sample was shaped to apply metallurgical test. The best results were shown for tensile and yield strength took place when samples are heated to 850℃ and then quenched to 450℃ , while the best results reached for toughness when samples are heated to 850℃ and then quenched to 550℃ . However elongation percentages are almost the same during all heat treatment programs. Microstructure investigation also showed a reduction in grain size after heat treatment applied compare to hot rolled samples only.
10:50 AM
End to End Simulation and Genetic Optimization for the Design of Medium Mn Steels: Ioannis Aristeidakis1; Gregory Haidemenopoulos1; 1University of Thessaly, Department of Mechanical Engineering
Medium Mn TRIP steels have received considerable attention as candidates for the 3rd generation of AHSS. Limited attempts were made to optimize compositions and processing conditions leading to desirable properties. In the present study, the FeCMnAlSi system, was investigated using CALPHAD based thermodynamic and kinetic calculations, coupled with evolutionary and gradient based optimization techniques, to improve formability via TRIP and TWIP. A short list of compositions and intercritical annealing parameters were identified using thermodynamics, by solving a constrained multi-objective optimization problem via the NSGA-II algorithm, resulting in optimal combinations of quantity and stability of retained austenite. Kinetic calculations followed, to simulate the entire process chain including solidification, homogenization and cooling, as well as intercritical annealing and quenching. Favorable heat treatment processes were determined by solving additional optimization problems. The method could be used to accelerate the alloy design by reducing the experimental effort required for alloy development.
11:10 AM Cancelled
A Novel High-strength Oxidization-resistant Press Hardening Steel Sheet Requiring No Al-Si Coating: Shuoshuo Li1; Haiwen Luo1; 1University of Science and Technology Beijing
Press hardened steels (such as 22MnB5) have been widely used in automobile structure for light weighting. However, these steels have relatively low elongation (< 8 %), and particularly are severely oxidized if an Al-Si coating is not applied during the hot forming process. A new medium-Mn TRIP steel is designed to have good ductility and high resistance to oxidation without the need of Al-Si coating for protection. The relatively low austenitization temperature led to very fine prior austenite grains (~ 1μm), and the final microstructure was composed of martensite as matrix, about 10 vol.% fraction of retained austenite and carbide precipitates. As a consequence, this medium-Mn steel possessed 1420 MPa yield strength, 1700 MPa ultra-tensile strength and 11.8 % elongation after press hardening. Furthermore, the thickness of oxidation layer was less than 3 μm, which was much smaller than that in the 22MnB5 steel after the conventional press hardening.