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Metals 11(3), 467 (2021). doi:10.3390/met11030467, In this work, the microstructural evolution during the dynamic transformation of austenite to bainite was directly observed by in-situ high energy synchrotron X-ray diffraction measurements during warm uniaxial compression performed at the P07 beamline of PETRA III, DESY (Deutsches Elektronen-Synchrotron). Plastic deformation triggers the phase transformation, which is continuously stimulated by the introduction of dynamic dislocations into the austenite. This scenario accelerates the kinetics of bainite formation in comparison with conventional isothermal treatment. No mechanical stabilization of austenite was observed during dynamic transformation. Evidence of carbon partitioning between phases during plastic deformation was obtained. Further post-process investigations suggest that the bainitic microstructure developed during compression is oriented perpendicular to the loading direction. The findings open up new possibilities to design carbide-free bainitic microstructures directly via thermomechanical processing., Published by MDPI, Basel

A non-linear relation between fracture strain and temperature is observed in Split Hopkinson Tension Bar (SHTB) tests for free-cutting steel 50SiB8 at high strains and high temperatures, due to blue-brittleness. A fourth-degree polynomial function for the thermal parts of the original and modified Johnson-Cook (JC) fracture strain constitutive equation is suggested and the corresponding parameters are identified. Fracture modelling of 50SiB8 is not reported in literature. 50SiB8 is characterized under quasi-static, isothermal, high temperature and high strain rate conditions using smooth and notched tension specimens. For the calculation of the original and modified JC fracture parameters, two different parameter identification procedures, so-called forward and backward parameter identification methods are performed, compared with each other and explained in detail. For the stress triaxiality factor of the original and modified JC fracture model, the forward parameter identification method is suggested for the original JC fracture model, while backward parameter identification method is proposed for the modified JC fracture model. The effect of blue-brittleness in the temperature factor and the corresponding parameters are presented. Furthermore, FEM simulations of the original and the blue-brittleness including JC fracture model are compared with experiments. In the blue-brittle temperature range, the new adjusted JC model shows more accurate results, regarding fracture diameter and strain. Additionally, this work introduces a fracture model combining the Bai-Wierzbicki stress triaxiality dependent model, JC fracture models and the nonlinear temperature factor with blue-brittleness effect, while further experiments are required in order to determine its parameters., Theoretical and Applied Fracture Mechanics, 112, ISSN:0167-8442

The effects of hot deformation on the bainitic transformation of a low carbon steel during continuous cooling were comprehensively studied through in situ high-energy synchrotron X-ray diffraction, dilatometry, and ex situ microstructural characterizations. The obtained results indicated that the prior deformation of austenite at 950 °C accelerates the bainite formation at the early stages. During the ongoing of the transformation, both the overall kinetics of bainite and carbon enrichment of austenite are lower in deformed austenite. The bainitic microstructure developed from deformed austenite is more refined and presents the same retained austenite content at room temperature with slightly lower carbon content when compared with the undeformed sample. Besides, a significant higher dilatation strain was measured during the bainitic transformation in the deformed sample, which can be explained by the crystallographic texture in hot deformed austenite. The evolution of the peak broadening of the {220}γ and {211}α reflections during bainitic transformation are discussed in detail.

Graphite inclusions in steel are easing chip breakage and act as in situ lubricants. Therefore, graphitized steels are claimed to perform excellent in machining processes. They are considered as environmentally friendly alternatives to widely used Pb-alloyed steels. However, the effectiveness of graphite inclusions in different machining processes is not well understood. With the support of computer simulation techniques, a deeper understanding about chip formation and the role of internal friction (or lubrication) can be obtained. To enter into numerical analysis of material flow and chip formation, the quality of the applied constitutive law is crucial for close-to-reality simulation. In the present work, a most accurate determination of the Johnson-Cook constitutive law of partially graphitized steel 50SiB8 (hardness 214 HB) is aimed for. Johnson-Cook parameters are identified from dynamical compression tests, performed at strain rates up to 20s−1 and temperatures up to 800 °C. The material parameters are derived from two different numerical methods. The first standard method is based on minimizing the sum of squares of the differences between calculation and measurement. The parameters identified from this method are used as initial values for the second method, an inverse material modeling. The second method is based on a multi-case polynomial metamodel optimization in combination with finite element techniques. By applying inverse material modeling, the maximum discrepancy between measurement and calculation is reduced from 12% (regression with standard method) to 1%. A reliable parameter set for the Johnson-Cook model for graphitic steel 50SiB5 as input for future modeling of the chip formation is now available.

Simulation of the rolling and controlled cooling sequence for bainitic steel rods was the general objective of the paper. The main focus was put on exploring possibility of prediction of the retained austenite occurrence in TRIP assisted bainitic steels. Existing discrete phase transformation models require long computing times and their application to optimization of industrial processes is limited. Therefore, a model based on the modified JMAK equation was proposed. The occurrence of the retained austenite was predicted by carbon distribution calculations in the austenite during bainite transformation. This model was implemented into the FE software for simulation of cooling of rods. The model was verified by comparison of results with the physical simulations during rolling in the pilot mill and during cooling. The first part of the paper contains thermal-mechanical-microstructural simulations of rod hot rolling process. The objective of this part was to determine temperature and grain size distribution at the rod cross section at the beginning of phase transformations. FE simulations of the cooling were performed next. Correlation between cooling parameters and the volume fraction of the retained austenite in rod was determined.

Products and manufacturing processes of long-product producers are subject to constantly changing requirements, characterized by increasing demands on product quality and the efficient use of resources in production, combined with permanent cost pressures. For these reasons steel wire producers must constantly improve their innovation capacity to be able to meet increasing customer demands more flexibly and more efficiently. This awareness has been anchored at Swiss Steel AG, part of the Schmolz + Bickenbach Group, for many years and leads to more and new solutions in all corporate divisions.This article focuses on practical examples from the areas of process and product innovation. It discusses options for and the potentials and challenges of meeting current and future market needs.The successful implementation of new solutions requires an in-depth understanding as well as the application of knowledge throughout the entire process chain from development, production, sales to further processing and actual use of a product. The technical challenges are addressed as well as the opportunities presented when new approaches are sought.

The presence of banding in a hot-rolled air-cooled bainitic steel has been thoroughly characterized. The banded microstructure was observed due to distinctive sensitivities to the etching agent. Microstructural and crystallographic studies by means of Scanning Electron Microscopy and Electron Backscatter Diffraction did not reveal any substantial differences between bands and matrix. However, solute segregation of some alloying elements was detected by Wavelength- Dispersive Spectroscopy, it being found that bands are enriched in Cr, Mo and Si and depleted in Mn. Finally, a set of theoretical calculations suggested that the solidification mode is responsible for such unusual partitioning behavior, which is beneficial in terms of the bainitic transformation.

Chemical segregation of alloying elements during solidification of steel grades leads to development of a banded microstructure, causing a degree of anisotropy that can be detrimental to the mechanical behavior under service conditions. It is well-known that the presence of strongly orientated martensite bands in carbide-free bainitic microstructures, associated to inhomogeneous Mn redistribution during solidification, leads to a remarkable deterioration in toughness in advanced high strength bainitic steels. In this study, while bands were clearly visible on light optical micrographs of continuously cooled carbide-free bainitic steels, scanning electron microscopy examination revealed only a gradual transition between matrix and bands, both with a granular bainitic structure. Electron backscatter diffraction was used to quantify the bainitic packet size and volume fraction of martensite/austenite constituent between and within the bands, after a process of optimization of the analysis settings in order to minimize the inherent difficulties linked to submicrometric and minority phase indexation. The quantitative microstructural results showed negligible morphological differences between bainitic structure bands and matrix, only solute segregation of Cr and Mo was detected by energy-dispersive X-ray spectroscopy within bands, which must be responsible for a stronger resistance against metallographic etching in those regions

Graphitized steels are claimed to perform excellent in machining processes. They therefore can be considered as environmental friendly alternatives to the widely used Pb-alloyed steels. Due to liquid metal embrittlement and in-situ lubrication Pb improves machinability in a narrow tool-chip interface temperature window corresponding to low machining speeds. Although graphite inclusions are also supposed to generate in-situ lubrication, the mechanism and the corresponding optimum working zone is not very clear. The present work applies a new test methodology (including in-situ tribology, analysis of material flow and chip formation, optimum working zone analysis) to investigate the effects of graphite inclusions on turning and drilling operations. A Pb-alloyed low carbon free-cutting steel and Pb-alloyed case hardening steel were used as reference steels.

An alternative procedure for indirect and automatic measurement of the prior austenite grain size (PAGS) in bainite/martensite is proposed in this work. It consists in the determination of an effective grain size (EGS) by means of statistical post-processing of electron backscatter diffraction (EBSD) data. The algorithm developed for that purpose, which is available on-line, has been applied to simulated EBSD maps as well as to both a nanocrystalline bainitic steel and a commercial hot-rolled air-cooled steel with a granular bainitic microstructure. The new proposed method has been proven to be robust and results are in good agreement with conventional PAGS measurements. The added value of the procedure comes from its simplicity, as no parent reconstruction is involved during the process, and its suitability for low-magnification EBSD maps, thus allowing a large step-size and coverage of a substantially broader area of the sample than the previous methods reported., Spanish Ministry of Economy and Competitiveness for funding this research under the contract IPT-2012-0320-420000. L.M.-R. also acknowledges the Spanish Ministry of Economy and Competitiveness for financial support in the form of a PhD research grant (FPI-Ref. BES-2011-044186).

Oxide inclusions formed during steelmaking processes influence the machinability of steel products. At moderate and high cutting speeds, the tool life is dominated by chemical wear. However this wear can be suppressed by engineering exogenous and indigenous glassy oxide inclusions in steel. The present work demonstrates a method to engineer glassy oxide inclusions in a low carbon free cutting steel applying a new thermodynamic model for deoxidation control of steel based on slag-melt as well as melt-oxide inclusion equilibration. The model is used online in an industrial production line for the controlled production of glassy inclusions. These inclusions are shown to improve machinability by lubricating the tool-chip interface during machining of the steel at high cutting speeds. Using an inclusion engineered work piece, the crater wear of an uncoated P10 tool is significantly improved and the tool life is tripled at cutting speeds in the range between 200 and 400 m/min. The industrial results show that thermodynamic modelling is a powerful tool to produce free cutting steels with consistently good machinability behaviour.

The influence of bainite morphology on the impact toughness behaviour of continuously cooled cementite-free low carbon bainitic steels has been examined. In these steels, bainitic microstructures formed mainly by lath-like upper bainite, consisting of thin and long parallel ferrite laths, were shown to exhibit higher impact toughness values than those with a granular bainite, consisting of equiaxed ferrite structure and discrete island of marteniste/austenite (M/A) constituent. Results suggest that the mechanism of brittle fracture of cementite-free bainitic steels involves nucleation of microcracks in M/A islands but is controlled by the bainite packet size, Spanish Ministry of Science and Innovation for financial support in the form of a PhD research grant (FPI grant under the project no. MAT2007-63873)