Papers by Author: Matthias Militzer

Abstract: Several studies have shown that recrystallization of cold rolled martensite results in low carbon steels with very fine microstructures. Correspondingly, these materials exhibit promising combinations of strength and elongation. Most of the work on this processing route has focused on low carbon steels (0.1-0.2wt% carbon) where the interstitial content may play an important role in the microstructure refinement. In this note we describe experiments performed on a low interstitial stainless steel containing 0.02wt%C. It has been possible to achieve materials with high strengths (UTS > 1 GPa) and significant uniform elongation (> 8%), however, the microstructures associated with these properties are very different from those previously reported for low carbon steels.

Abstract: The microstructural evolution has been studied for hot rolling of a dual-phase steel with a lean C-Mn-Si chemistry. This study includes the investigation of austenite grain growth during reheating, constitutive behaviour and static recrystallization kinetics of austenite, and austenite decomposition during simulated run-out table cooling conditions. To develop and validate the microstructure models for these phenomena, experimental studies have been carried out in the laboratory using a Gleeble 3500 thermomechanical simulator. The hyperbolic sine relationship between flow stress and Zener-Hollomon parameter is employed to describe the constitutive behaviour. The Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory is used to predict the static recrystallization kinetics. Ferrite transformation start is described with an approach that considers early growth of corner nucleated ferrite. The fraction of ferrite transformed from austenite during continuous cooling is described using the JMAK approach in combination with the additivity rule. The ferrite grain size is quantified as a function of the transformation start temperature. The overall microstructure model has been validated based on a number of laboratory simulations of the entire hot strip rolling and controlled cooling process with an emphasis on industrially relevant run-out table cooling strategies.

Abstract: Using physical concepts, an integrated transformation model to describe the kinetics of ferrite and bainite formation from work-hardened austenite has been developed for a Mo-TRIP steel. The ferrite sub-model assumes a mixed-mode kinetics under paraequilibrium condition and accounts explicitly for the effect of alloying elements by considering their interaction with the moving ferrite-austenite interface. To predict the onset of bainite formation, which corresponds to the cessation of ferrite reaction along a given cooling path, a criterion based on a critical driving pressure is formulated. Regarding the kinetics of the subsequent bainite reaction, the proposed model adopts the Zener-Hillert diffusional approach. The proposed integrated model has been employed to describe the continuous cooling transformation kinetics for a 0.19C-1.5Mn-1.6Si- 0.2Mo (wt%) TRIP steel that had previously been subjected to a systematic experimental study. The predictive capabilities of the model and the challenges for further model improvements are delineated.

Abstract: Recently, there has been a large interest in the development of low carbon steels with ultra fine grain structure using lean chemistries. Although these steels typically have superior strength, the lack of work hardening capability limits the uniform elongation and thus the formability of these kinds of steels. It has been reported by Tsuji and co-workers (2002) that straining of martensite as an initial structure can yield an ultra fine grain structure with good combination of strength and ductility. However, the detailed mechanism of the grain refinement has not yet been clarified. In the present work, the annealing behavior of a low carbon martensitic structure with and without deformation at room temperature has been systematically studied. It is proposed that the process of concurrent softening due to recovery and recrystallization and precipitation of carbides is different for the deformed and undeformed materials. Further, preliminary results have been found on the role of substitutional alloying elements such as Mo or Cr on the kinetics of the softening processes.

Abstract: Controlled cooling on the runout table is a crucial component in the production of highly tailored steels since it has a strong influence on the final mechanical properties. High efficiency heat transfer in impinging jet cooling makes this an important method for heat transfer enhancement. The purpose of this study is to develop an experimental database for modelling of boiling heat transfer for bottom jet impingement that occurs during runout table cooling in a steel mill. Experiments have been carried out on a pilot scale runout table using stationary plates, with focus on the effect of water flow rate and nozzle inclination to the overall heat transfer rates. Volumetric flow rates and inclination angles are in the range of 35-55 l/min and 0-30º, respectively. Temperatures on the test plates are measured internally very close to the surface during cooling for the purpose of reducing thermal lag and receiving better data responsiveness. These measurements are taken at the impingement point and several streamwise distances from the impingement point. From the above measurements transient cooling data on the hot steel plate by bottom jet impingement has been analysed.

Abstract: There is an increasing emphasis to develop novel hot-rolled high strength steels with fine and ultra fine grain sizes for structural and other applications. Traditionally the concept of microalloying has been employed to refine microstructures thereby obtaining increased strength levels. For example, employing an alloying strategy with Nb, Ti and Mo is promising to attain yield strength levels of 700MPa and beyond. In the present study, the transformation behaviour is investigated for a HSLA steel containing 0.05wt%C-1.65wt%Mn-0.20wt%Mo-0.07wt%Nb- 0.02wt%Ti. The ferrite formation from work-hardened austenite has been studied for simulated run-out table cooling conditions employing a Gleeble 3500 thermomechanical simulator equipped with a dilatometer. The effects of cooling rate and initial austenite microstructure, i.e. austenite grain size and degree of work hardening, on the austenite decomposition kinetics and resulting ferrite grain size have been quantified. Based on the experimental results, a phenomenological transformation and ferrite grain size model is proposed for run-out table cooling conditions. The transformation model includes submodels for transformation start and ferrite growth. The latter is described using a Johnson-Mehl-Avrami-Kolmogorov approach. The degree of work hardening is incorporated by introducing an effective austenite grain size as a function of the strain applied under no-recrystallization condition. The ferrite grain size can be predicted as a function of the transformation start temperature. Increasing both cooling rate and amount of work hardening can optimize ferrite grain refinement. In the present steel, ferrite grain sizes of as low as 2µm have been obtained in this way. The results observed for the present steel are compared to the transformation behaviour in previously studied Nb-Ti HSLA steels of similar strength levels.

Abstract: Cu interconnects are essential in advanced integrated circuits to minimize the RC delay. In manufacturing these devices, Cu is deposited electrochemically using a plating bath containing organic additives. The as-deposited nanocrystalline Cu films undergo self-annealing at room temperature to form a micronsized grain structure by abnormal grain growth. Systematic experimental studies of self-annealing kinetics on model Cu films deposited on a Au substrate suggest that the rate of grain size evolution depends primarily on the initial grain size of the asdeposited film. A model for the observed abnormal grain growth process is proposed. Assuming that desorption of the organic additives leads to mobile grain boundaries, the onset of abnormal grain growth is attributed to a sufficiently low additive concentration such that a full coverage of all grain boundaries cannot be maintained. The incubation time of abnormal growth is then a logarithmic function of the initial grain size. The probability to find a growing grain is proportional to the number of grains per unit volume. This assumption is seen to be in good agreement with the experimental observations for subsequent abnormal grain growth rates. The limitations of the proposed model and the challenges to obtain further insight into the complex microstructure mechanisms during self-annealing are delineated.