Advanced High-Strength Steels: Nanostructures and Precipitates
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 8:30 AM
March 1, 2017
Room: 17A
Location: San Diego Convention Ctr

Session Chair: Matthias Militzer, The University of British Columbia; Cem Tasan, Massachusetts Institute of Technology

8:30 AM  Invited
Extraordinary Crack Resistance in Metastable Multi-phase Nanolaminated Steels: Cem Tasan1; 1MIT
    Interestingly, although it is a materials-phenomenon, fatigue is mostly dealt by structural engineering approaches and by employing large safety-ratios in engineering design. New materials are rarely considered for fatigue-prone components, since it is extremely rare to come across new materials that exhibit very significant improvements that would compensate for the intrinsic statistical variations in materials response to cyclic loading. Here we present a materials design approach to address this trend. A novel metastability-assisted multi-phase nanolaminated steel microstructure is designed that exhibits extraordinary crack propagation resistance. High fatigue limit and life are successfully obtained in this concept, and surprisingly, the superior fatigue resistance is very robust against variation of stress amplitude, from the fatigue limit to near tensile strength. This unexpected behavior arises due to the underlying materials design strategy that aimed at the well-orchestrated activation of two different crack closure mechanisms: roughness-induced crack closure and transformation-induced crack closure.

9:00 AM  
Advanced High Strength Steel Based on Vanadium Carbide Precipitation: William Rainforth1; Arjan Rijkenberg2; David Hanlon2; Peng Gong1; Alfonce Chamisa2; Andrew Patterson1; Francis Sweeney1; 1The University of Sheffield; 2Tata Steel Europe
    Advanced high strength steels are being developed for light-weight automotive applications. We report on a fine-grained ferritic steel that exhibits high strength through a fine dispersion vanadium carbide precipitates. A high number density of precipitates was achieved through interphase precipitation. The steel had high strength (UTS ~1000MPa), high ductility (19% elongation) and excellent formability. A particular focus of the work has been on the role of Mo additions. Detailed TEM and atom probe tomography was used to show that the precipitates were coherent with the matrix exhibiting predominantly a Baker Nutting orientation relationship, although a Nishiyama-Wassermann orientation relationship was also observed. All precipitates were found to contain molybdenum as well as vanadium. The role of Mo in achieving optimum microstructure is discussed.

9:20 AM  
Appication of Nano-sized Precipiation in Strengthening Low Alloy Dual Phase Steel: Tadashi Furuhara1; Elango Chandiran1; Naoya Kamikawa2; 1Tohoku University; 2Hirosaki University
    In this study, strengthening of ferrite by interphase precipitation of nano-sized vanadiun carbide (VC) was applied to ferrite-martensite dual phase (DP) steels and their effects on tensile behaviors were examined. Both the yield stress and the ultimate tensile strength are significantly increased by VC precipitation. For ferrite volume fractions of 20–50% a dispersion of VC results in only a small change in the elongation, whereas for ferrite volume fractions of above 50% both uniform and post-uniform elongations are decreased by the VC dispersion. Digital image correlation (DIC) analysis demonstrates that the ferrite phase is more deformed than the martensite phase in both VC-free and VC-dispersed DP samples, but that such strain partitioning is less pronounced in the VC dispersion-hardened samples. It is suggested that dispersion of nano-precipitates in ferrite is an effective approach to simultaneously improve the strength and the strength-ductility balance of DP steels.

9:40 AM  
Design of a Core-Shell Structure Carbide for Enhancing Toughness of UHS Steels: Wei Xiong1; Ye Tian2; Oleg Kontsevoi2; Gregory Olson2; 1University of Pittsburgh; 2Northwestern University
    At ambient temperature service, Zener pinning grain refiners often act as microvoid nucleation sites, participating in the ductile fracture process. In order to reduce the microvoid softening effects due to these residual carbides, it is desirable to strengthen the interface bonding between matrix and these MC carbides, and thus delay the microvoid softening. We employ the highly-precise FLAPW method to compute the interfacial adhesion between matrix Fe and different transition metal carbides. The calculated separation energy clearly demonstrates the sequence of cohesion energy by substituting different species in the metal sublattice of MC. Post heat treatment was designed employing a CALPHAD database, which leads to a successful demonstration of designed core-shell Zener pinning particles according to the DFT calculations. STEM validates the particle dispersion with low volume fraction and mean size between 50 and 200 nm, ideally suited to act as a Zener pinning particle dispersion with improved adhesion energy.

10:00 AM  
Influences of Thermomechanical Treatments on the Microstructure Evolution and Mechanical Properties of Nano-precipitates Strengthened Steels: Yu Zhao1; Songsong Xu1; Hao Guo1; Junpeng Li1; Z.W. Zhang1; 1Harbin Engineering University
    Nano-precipitates strengthened steels exhibit excellent combination of high strength, high toughness and good weldability. In this study, the optimized themomechanical processing and aging treatments are used to control simultaneously the matrix microstructure and nano scale preciptiates. The size and number density of nano scale precipitates are characterized by the small-angle neutron scattering (SANS) and atom probe tomography. The matrix microstructure evolution is observed by transmission electron microscopic (TEM). It is found that the low tempereature properties are significantly influenced by themomechanical processing. The aging temperature plays a key role to control the low temperature toughness. The effects of alloying elements and thermomechanical processing on the microstructure evolution and mechanical properties are discussed.

10:20 AM Break

10:40 AM  
Ab-initio Investigation of the Interaction of Hydrogen with Carbides in Advanced High-strength Steels: Poulumi Dey1; Tobias Timmerscheidt2; Jrg von Appen2; Tilmann Hickel1; Richard Dronskowski2; Jrg Neugebauer1; 1Max-Planck-Institut fr Eisenforschung GmbH; 2Institute of Inorganic Chemistry, Chair of Solid-State and Quantum Chemistry, RWTH Aachen University
    Controlling the role of hydrogen in advanced high-strength steels is decisive for their application in automotive industry. Ab initio calculations can provide pathways to adjust the amount of trapping sites by modifying the chemical composition of the steels. Here, we focus on the role played by homogeneously distributed nano-sized precipitates of κ carbides in Al-rich high strength steels. The analysis based on density functional theory (DFT) reveals that the C concentration is a decisive parameter for the solubility of hydrogen inside the carbides as well as in the interface to the austenitic matrix. The hydrogen solubility, kinetics and its impact on the strength of the interface is compared to the situation in other carbides, in order to assess the role of κ carbides for the hydrogen embrittlement of these steels.

11:00 AM  
Effect of B2 Morphology on the Mechanical Properties of Dispersion Strengthened Lightweight Steels: A. Zargaran1; C. Nam1; S.-H. Kim1; Nack J. Kim1; 1Graduate Institute of Ferrous Technology (GIFT) and CAAM, Pohang University of Science and Technology (POSTECH)
    There has been growing interest in development of lightweight steels with a high strength and reduced density for automotive applications. Recently, a new type of lightweight steels has been developed possessing much higher strength and work hardening rate than conventional lightweight steels. Excellent properties of these lightweight steels mainly comes from a presence of B2 particles, which are dispersed in three morphologies in austenite matrix; fine particles formed along austenite grain boundaries and within austenite matrix, and band type particles elongated along rolling direction. It is expected that these B2 particles with different morphologies would have different effect on mechanical properties of dispersion strengthened lightweight steels, which is the main objective of the present study. In the present study, two steels with nominal compositions of Fe-10Al-15Mn-0.8C-5Ni and Fe-8Al-17Mn-1C-5Ni (in wt.%) were subjected to microstructural analyses and mechanical testing including tensile tests as well as hole expansion ratio and bendability tests.

11:20 AM  
Interaction of VC-Precipitation and Phase Transformation Kinetics in Mo-containing Nano-steels: Chrysoula Ioannidou1; Zaloa Arechabaleta1; Arjan Rijkenberg2; Ad van Well3; Erik Offerman1; 1Delft University of Technology; 2Tata Steel Research, Development and Technology; 3Reactor Institute Delft
    Nano-steels, i.e fully ferritic steels combined with nanometer-sized precipitates, are a new generation of Advanced High Strength Steels (AHSS), extremely suitable for lightweight automotive applications. The ferrite matrix provides ductility and flangebility, whereas the nanometer-sized precipitates significantly increase the strength level. These steels rely on microalloy additions, including Nb, V, Mo and Ti, which form the essential nano-precipitates. An efficient use of these microalloy elements, crucial for commercial implementation, requires further knowledge on the precipitation kinetics of the specific carbides and carbo-nitrides, and a better understanding of the interaction of these nano-precipitates with the austenite to ferrite phase transformation. In this work, the VC-precipitation kinetics of C-Mn-V and C-Mn-V-Mo steels and its interaction with phase transformation is analysed during isothermal holding at different temperatures. Dilatometry experiments are combined with advanced characterisation techniques, such as Small Angle Neutron Scattering (SANS) and Transmission Electron Microscopy (TEM), to disentangle both effects.

11:40 AM  
Effects of Solid Solution Treatment on the Microstructure and Mechanical Properties in the Ultra-high Strength Steel Strengthened by Nanoscale Particles: Songsong Xu1; Yu Zhao1; Hao Guo1; Mingxing Qiu1; Jing Zhang1; Junpeng Li1; Zhongwu Zhang1; 1Harbin Engineering University
    The microstructure and mechanical properties of the ultra-high strength steel by nanoscale particles are studied in detail with different solid solution treatment. The results indicated that the microstructure has been transformed from ferrite bands to sorbite and polygonal ferrite with the solid solution’s temperature increasing from 600 to 950℃. The microhardness has three part which are ascent, descent stage and peak value, what is more, the tensile strength is corresponding to it. The relationship of the microstructure and tensile strength has been investigated and discussion with solid solution in this paper. This work was supported by the NSFC Funding (51371062 and U1460102), NSFHLJ (ZD201411), the Scientific Research Foundation for the Returned Overseas Chinese Scholars (Heilongjiang Province), the Project for Innovative Talents of Science and Technology of Harbin (2014RFXXJ006).