More Like this

1. Microstructure-Based Modeling of the Effect of Inclusion on the Bendability of Advanced High Strength Dual-Phase Steels

   https://doi.org/10.3390/met11030431  

   Liu, Yu ; Fan, Dongwei ; Arróyave, Raymundo ; Srivastava, Ankit ( March 2021 , Metals)

   null (Ed.)

   Advanced high strength dual-phase steels are one of the most widely sought-after structural materials for automotive applications. These high strength steels, however, are prone to fracture under bending-dominated manufacturing processes. Experimental observations suggest that the bendability of these steels is sensitive to the presence of subsurface non-metallic inclusions and the inclusions exhibit a rather discrete size effect on the bendability of these steels. Following this, we have carried out a series of microstructure-based finite element calculations of ductile fracture in an advanced high strength dual-phase steel under bending. In the calculations, both the dual-phase microstructure and inclusion are discretely modeled. To gain additional insight, we have also analyzed the effect of an inclusion on the bendability of a single-phase material. In line with the experimental observations, strong inclusion size effect on the bendability of the dual-phase steel naturally emerge in the calculations. Furthermore, supervised machine learning is used to quantify the effects of the multivariable input space associated with the dual-phase microstructure and inclusion on the bendability of the steel. The results of the supervised machine learning are then used to identify the contributions of individual features and isolate critical features that control the bendability of dual-phase steels. 

   more » « less
2. Phase determination in dual phase steels via HREBSD‐based tetragonality mapping

   https://doi.org/10.1111/jmi.12980  

   Adams, Derrik ; Miles, Michael P. ; Homer, Eric R. ; Brown, Tyson ; Mishra, Raj K. ; Fullwood, David T. ( April 2021 , Journal of Microscopy)

   Electron Backscatter Diffraction (EBSD) is a widely used approach for characterising the microstructure of various materials. However, it is difficult to accurately distinguish similar (body centred cubic and body centred tetragonal, with small tetragonality) phases in steels using standard EBSD software. One method to tackle the problem of phase distinction is to measure the tetragonality of the phases, which can be done using simulated patterns and cross‐correlation techniques to detect distortion away from a perfectly cubic crystal lattice. However, small errors in the determination of microscope geometry (the so‐called pattern or projection centre) can cause significant errors in tetragonality measurement and lead to erroneous results. This paper utilises a new approach for accurate pattern centre determination via a strain minimisation routine across a large number of grains in dual phase steels. Tetragonality maps are then produced and used to identify phase and estimate local carbon content. The technique is implemented using both kinetically simulated and dynamically simulated patterns to determine their relative accuracy. Tetragonality maps, and subsequent phase maps, based on dynamically simulated patterns in a point‐by‐point and grain average comparison are found to consistently produce more precise and accurate results, with close to 90% accuracy for grain phase identification, when compared with an image‐quality identification method. The error in tetragonality measurements appears to be of the order of 1%, thus producing a commensurate ∼0.2% error in carbon content estimation. Such an error makes the technique unsuitable for estimation of total carbon content of most commercial steels, which often have carbon levels below 0.1%. However, even in the DP steel for this study (0.1 wt.% carbon) it can be used to map carbon in regions with higher accumulation (such as in martensite with nonhomogeneous carbon content).

   Electron Backscatter Diffraction (EBSD) is a widely used approach for characterising the microstructure of various materials. However, it is difficult to accurately distinguish similar (BCC and BCT) phases in steels using standard EBSD software due to the small difference in crystal structure. One method to tackle the problem of phase distinction is to measure the tetragonality, or apparent ‘strain’ in the crystal lattice, of the phases. This can be done by comparing experimental EBSD patterns with simulated patterns via cross‐correlation techniques, to detect distortion away from a perfectly cubic crystal lattice. However, small errors in the determination of microscope geometry (the so‐called pattern or projection centre) can cause significant errors in tetragonality measurement and lead to erroneous results. This paper utilises a new approach for accurate pattern centre determination via a strain minimisation routine across a large number of grains in dual phase steels. Tetragonality maps are then produced and used to identify phase and estimate local carbon content. The technique is implemented using both simple kinetically simulated and more complex dynamically simulated patterns to determine their relative accuracy. Tetragonality maps, and subsequent phase maps, based on dynamically simulated patterns in a point‐by‐point and grain average comparison are found to consistently produce more precise and accurate results, with close to 90% accuracy for grain phase identification, when compared with an image‐quality identification method. The error in tetragonality measurements appears to be of the order of 1%, thus producing a commensurate error in carbon content estimation. Such an error makes an estimate of total carbon content particularly unsuitable for low carbon steels; although maps of local carbon content may still be revealing.

   Application of the method developed in this paper will lead to better understanding of the complex microstructures of steels, and the potential to design microstructures that deliver higher strength and ductility for common applications, such as vehicle components.

    

   more » « less
3. Oxide Stability Analysis and Characterization in a Galvannealed Dual-Phase Steel Exhibiting Coating Defects

   https://doi.org/10.33313/377/186  

   De Moor, E. ; Plessinger, R. ; Seetharaman, S. ( January 2019 , 2019 AISTech Conference Proceedings)

   As more emphasis is placed on thinner gauges and stronger steel grades for more fuel efficient, and crash-worthy vehicles, corrosion becomes a more important consideration. This demand has led to the development of advanced high strength steel (AHSS) grades such as dual-phase (DP) and transformation-induced plasticity (TRIP) aided steels. To protect these steels from corrosion, a hot-dip galvanizing (HDG) or galvannealing (GA) process is employed. However, alloying with Mn and Si can cause complex surface oxides during annealing, which may result in defects in the zinc coating1-8. In order to increase AHSS galvanizability, selective oxidation of alloying elements in DP and TRIP-aided steel needs to be understood. Depending on annealing atmosphere, the mechanism (internal or external) of oxidation can change, as well as the thermodynamic stability of the oxide. Wagner’s theory of oxidation enables prediction of the mechanism of oxidation for binary oxide systems9. The defects can be present after pickling and fluxing, causing wettability issues during the HDG process. Coating defects may also form or be further exacerbated by the galvannealing operation. For this work, oxidation mechanism predictions were performed using Wagner’s theory of oxidation. In the present study a 0.07C-1.9Mn-0.2Si-0.3Cr galvannealed dual-phase steel which exhibited streaking defects in the coating was analyzed. Light optical metallography (LOM) and scanning electron microscopy (SEM) were performed to characterize defects within the galvannealed coating. Static spectra and 3-D profiling using time of flight secondary ion mass spectrometry (TOF-SIMS) was employed. It was found that Mn rich Si containing oxides are present on the steel-coating interface. It does not appear that these oxides affect wettability, however they may have an influence on intermetallic growth during galvannealing. This, combined with an influence from the micro-grooves of the sink roll in the Zn pot, may lead to the streaking defects observed. 

   more » « less
4. Austenite Stability and Strain Hardening in C-Mn-Si Quenching and Partitioning Steels

   https://doi.org/10.1007/s11661-020-05666-8  

   Finfrock, Christopher B. ; Clarke, Amy J. ; Thomas, Grant A. ; Clarke, Kester D. ( February 2020 , Metallurgical and Materials Transactions A)

   Quenching and partitioning (Q&P) processing of third-generation advanced high strength steels generates multiphase microstructures containing metastable retained austenite. Deformation-induced martensitic transformation of retained austenite improves strength and ductility by increasing instantaneous strain hardening rates. This paper explores the influence of martensitic transformation and strain hardening on tensile performance. Tensile tests were performed on steels with nominally similar compositions and microstructures (11.3 to 12.6 vol. pct retained austenite and 16.7 to 23.4 vol. pct ferrite) at 980 and 1180 MPa ultimate tensile strength levels. For each steel, tensile performance was generally consistent along different orientations in the sheet relative to the rolling direction, but a greater amount of austenite transformation occurred during uniform elongation along the rolling direction. Neither the amount of retained austenite prior to straining nor the total amount of retained austenite transformed during straining could be directly correlated to tensile performance. It is proposed that stability of retained austenite, rather than austenite volume fraction, greatly influences strain hardening rate, and thus controls strength and ductility. If true, this suggests that tailoring austenite stability is critical for optimizing the forming response and crash performance of quenched and partitioned grades. 

   more » « less
5. New Insights into the Microstructural Changes During the Processing of Dual-Phase Steels from Multiresolution Spherical Indentation Stress–Strain Protocols

   https://doi.org/10.3390/met10010018  

   Khosravani, Ali ; Caliendo, Charles M. ; Kalidindi, Surya R. ( January 2020 , Metals)

   In this study, recently established multiresolution spherical indentation stress–strain protocols have been employed to derive new insights into the microstructural changes that occur during the processing of dual-phase (DP) steels. This is accomplished by utilizing indenter tips of different radii such that the mechanical responses can be evaluated both at the macroscale (reflecting the bulk properties of the sample) and at the microscale (reflecting the properties of the constituent phases). More specifically, nine different thermo-mechanical processing conditions involving different combinations of intercritical annealing temperatures and bake hardening after different amounts of cold work were studied. In addition to demonstrating the tremendous benefits of the indentation protocols for evaluating the variations within each sample and between the samples at different material length scales in a high throughput manner, the measurements provided several new insights into the microstructural changes occurring in the alloys during their processing. In particular, the indentation measurements indicated that the strength of the martensite phase reduces by about 37% when quenched from 810 °C compared to being quenched from 750 °C, while the strength of the ferrite phase remains about the same. In addition, during the 10% thickness reduction and bake hardening steps, the strength of the martensite phase shows a small decrease due to tempering, while the strength of the ferrite increases by about 50% by static aging. 

   more » « less