Precipitation in Microalloyed Steel by Model Alloy Experiments and Thermodynamic Calculations
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INTRODUCTION
MICROALLOYED steels have low alloying contents of elements such as niobium, vanadium, titanium, molybdenum, zirconium, boron, rare-earth metals, carbon, and nitrogen, that are used to tailor the microstructure and properties of the steel. Precipitation processes are important, and nanosized carbides, nitrides, or carbonitrides are often critical in the microstructure development. The experimental characterization of the low alloying contents and small precipitates is challenging, and computational predictions are therefore of interest. Thermodynamic calculations are often used as a tool in the development of new steel grades, and as a guide to understand the microstructure development. The reliability of the calculations is determined by the thermodynamic parameters that are used, and depends on the availability and quality of the experimental information used for the assessment of parameters. For the case of microalloyed steel, it is not only important to have an accurate description of the partitioning of the alloying elements between the matrix and the precipitates, and between ferrite and austenite, but also of the composition and stability range of the precipitates. The partitioning of the alloying elements between different precipitates, for example carbonitrides and cementite, influences the amount of alloying elements that remain
KARIN FRISK, Research Leader, is with Swerea KIMAB, Box 7047, 164 07 Kista, Sweden. Contact e-mail: [email protected] ULRIKA BORGGREN, Research Engineer, is with SSAB Europe, 781 84 Borla¨nge, Sweden. Manuscript submitted November 9, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
in the matrix and is therefore critical information. In microalloyed steel, the most commonly observed multicomponent carbides, nitrides, and carbonitrides can, under some conditions, separate into precipitates of the same crystal structure but distinctly different compositions. This phase separation can be described by thermodynamic modeling,[1,2] and is an important feature of the microstructure development, thus accurate descriptions are needed. New experiments were therefore performed in the present work to complement available information. The need for computational predictions of precipitation in microalloyed steel was also recently stressed by Xu et al.,[3] who developed an equilibrium model, based on solubility products. In the present work, thermodynamic modeling based on a full description of the Gibbs energies of all underlying alloying systems is used. The advantage of this modeling approach is the predictive capacity, as will be demonstrated in the present work by comparing with results from recent experimental studies. Further developments of the thermodynamic modeling based on the new dedicated experimental studies, is also described. With recent developments of high-resolution experimental techniques, it is today possible to study nanoprecipitates in detail, giving new insights in the effects of alloying on precipitation. Some of these recent studies provide information that
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