In transformation-induced plasticity (TRIP) steels, Si and/or Al delay cementite precipitation during bainite transformation leading to a transformation product that is usually referred to as ...carbide-free bainite. The present work proposes an approach to calibrate the parameters of a multi-phase field model to simulate the austenite decomposition into ferrite and bainite in a TRIP steel. Ferrite nuclei are introduced at austenite grain boundaries and suitable interfacial mobilities are selected to reproduce experimental ferrite formation kinetics. Bainite nucleation occurs for a sufficiently high undercooling at available interface sites (i.e. austenite grain boundaries, austenite-ferrite interfaces) and/or within austenite grains. For simplicity, the formation of carbide-free bainite is considered and a suitable anisotropy approach is proposed for the austenite-bainite interface mobility. The proposed model is benchmarked and validated with experimental continuous cooling transformation data.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
The nickel-base superalloy, Inconel 718, is widely used in aircraft engines. The control of its final microstructure requires detailed knowledge on recrystallization, grain growth and second phase ...evolution during thermomechanical processing. The present study focuses on the in-situ evaluation of heterogeneous grain growth in the super-δ-solvus domain (above 1050 °C) using laser ultrasonics. This technology is dedicated to the ultrasonic characterization of microstructure evolution during thermomechanical processing of metals and alloys. The real time measurement of the ultrasonic attenuation provides the evolution of a representative grain size in the material. A criterion is developed to determine the onset and completion of heterogeneous grain growth stages associated with the dissolution of precipitates. The laser ultrasonic methodology is validated with ex-situ metallographic observations and quantitative characterization of the grain size heterogeneity.
•The laser ultrasonic technology is used to examine grain growth in situ in Inconel 718.•Physically based correlation is developed between ultrasonics attenuation and mean grain size.•Real time ultrasound measurements are used to quantify the onset and completion of heterogeneous grain growth.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
•Systematic experiments were carried out on a pilot scale run-out table using industrial size jets.•A heat transfer model is proposed for jet impingement cooling of steel plates.•The proposed model ...maps boiling curves at different positions on the plate surface.
The cooling pattern employed on the run-out table during steel processing highly affects the final microstructure of hot-rolled steel products and thus their final mechanical properties. This paper describes a strategy to develop a mechanistic heat transfer model for cooling of steel based on systematic experimental studies. Experiments were carried out on a pilot scale run-out table facility to investigate the cooling behavior of stationary steel plates. Based on the experimental results, a model is proposed that takes into account boiling heat transfer mechanisms to calculate heat fluxes on stationary plates during jet impingement boiling.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Models of microstructure evolution in steels are reviewed. The emphasis of the review is on low carbon sheet steels both hot-rolled and cold-rolled and annealed. First the state-of-the-art on ...industrial microstructure process models is presented. The individual model concepts for grain growth, recrystallization, precipitation and phase transformations are briefly discussed. The development from empirically-based models to physically-based models is identified as a key issue to have increased predictive capabilities for these models over a wider range of steel grades and operational conditions. The challenges in the development of the next generation of models are delineated. In particular, new aspects of microstructure evolution associated with novel processing routes and advanced high strength steels are evaluated. Further, the majority of the currently employed models are on the macro-scale but future microstructure models will increasingly be meso-scale models that predict actual microstructures rather than a number of average parameters (e.g. grain size, fraction transformed) to describe microstructure evolution.
Dual-phase (DP) steels are advanced high-strength steels used in automotive design. To achieve optimal mechanical properties the control of phase transformations during processing is paramount, e.g. ...for hot-rolled DP steels a desired ferrite fraction is required to form during run-out table cooling. Thus, sensor technologies such as laser ultrasonics (LUS) are of considerable interest that can in-situ monitor ferrite formation. In this study, the ferrite formation kinetics in a laboratory DP steel were measured by LUS during stepped cooling treatments which were designed to simulate the cooling paths on the run-out table in hot strip mills. LUS measurements were first validated with well-established dilatometry measurements during continuous cooling. For the stepped cooling tests, the fractions transformed obtained from the ultrasonic velocity changes agree with the ferrite phase fractions as characterized by ex-situ metallography. Further, the velocity changes are described by the JMAK approach using parameters that are consistent for the austenite-to-ferrite transformation in low-carbon steels.
Accelerated run-out table cooling has become a key technology that determines microstructure and resulting mechanical properties of thermo-mechanically controlled processed (TMCP) steels. The present ...study quantifies the heat transfer mechanisms during bottom jet cooling of a stationary steel plate with systematic pilot-scale experiments. The emphasis of the study is to quantify the effect of process parameters, i.e. jet impingement velocity, water temperature and nozzle orientation, on heat extraction rates. Experimental results are described and quantitatively analyzed, adding to the database for run-out table cooling.
Microalloyed low-carbon steels are used for line pipe applications as they combine high strength and acceptable fracture toughness with good weldability. During multi-pass welding, the strength and ...impact toughness of the material in the heat-affected zone (HAZ) is potentially degraded, in particular the regions where the thermal fields from multi-pass welds overlap (for example: the intercritically reheated coarse grain heat-affected zone, ICCGHAZ). Using a Gleeble thermomechanical simulator, bulk microstructures were produced that are representative for the ICCGHAZ for two high-strength X80 line pipe steels. Here, the first thermal cycle produces a bainitic microstructure that is characteristic of the coarse grain heat-affected zone (CGHAZ) and the second cycle involves intercritical annealing of this region to form microstructures representative of the ICCGHAZ. The effect of the intercritical austenite fraction and the resulting martensite–austenite (M/A) constituents on the tensile properties and the ductile-brittle transition temperature (DBTT) has been quantified for two steels with different carbon contents,
i.e
., 0.063 and 0.028 wt. pct. Detailed fractography studies have been conducted to evaluate the fracture mechanisms with respect to the microstructural features. Upon intercritical annealing (relevant to ICCGHAZ), the ductile–brittle transition temperature was above room temperature when a nearly continuous necklace of M/A formed on the prior austenite grain boundaries (for M/A ≈ 10 pct). Finally, the role of carbon content on the yield strength and plasticity of martensite has been considered for the tensile fracture behavior and ductile–brittle transition temperature. It is proposed that as the average carbon content of the M/A decreases (both due to (i) a decrease in the bulk carbon content of the steel and (ii) an increase in the volume fraction of austenite formed during intercritical annealing), martensite plasticity was possible which reduced nucleation of voids or cracks at the M/A–bainite interface.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
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Niobium is a widely used micro-alloying element in steels that can retard the austenite-to-ferrite transformation primarily by solute drag when Nb remains in solution. It is critical to develop ...quantitative models to predict the effect of Nb on the transformation kinetics. In the present work, dedicated continuous cooling transformation (CCT) studies were performed for a low-carbon steel microalloyed with 0.047 wt pct Nb starting from fully recrystallized austenite states with the same grain size but different amounts of Nb in solution. The austenite-to-ferrite transformation kinetics is described from a fundamental perspective by assuming a mixed-mode reaction including solute drag of Nb. Using the solute drag model of Fazeli and Militzer, the intrinsic interface mobility, trans-interface diffusivity of Nb, and its binding energy to the interface have been determined from the CCT data. The interfacial parameters are critically analyzed and compared with independent measurements of diffusion and grain boundary segregation.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Accelerated cooling (ACC) is one of the key processing steps in the production of Advanced High Performance Steels. In order to obtain thermo-mechanically controlled processed (TMCP) steel products ...with desired microstructures and mechanical properties, it is necessary to properly adjust the processing parameters of the cooling facility, and therefore it is critically important to quantify the physical process of heat removal by applying water jets on the hot surface of steel. In the present study we propose a mechanistic model for top jet cooling of a moving plate with circular and planar nozzles. The simulation model has been developed based on the extensive experimental database generated with pilot scale runout table tests, and it provides a potentially powerful tool for simulation of cooling of steel strips and plates over the entire length of the cooling facility.
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