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| Niżnik B., Pietrzyk M. |
| RHEOLOGICAL MODEL FOR MICROALLOYED STEEL IN THE TWO-PHASE REGION OF TEMPERATURES, ACCOUNTING FOR THE INFLUENCE OF DEFORMATION AND PRECIPITATION ON THE |
Abstract
Results of physical and numerical modelling of phase transformation kinetics for microalloyed niobium steel, with precipitation of Nb(C,N) and deformation of austenite taken into account, are presented. Physical modelling included plastometric and dilatometric tests. The numerical model based on Finite Element Method solution determines the flow stress. The primary analysis based on the inverse approach to plastometric tests and application of the rule of mixture in the two-phase region gave the results, which do not agree well with the experiments. It was assumed that accounting for influence of precipitates and dislocation density on transformation kinetics should improve the accuracy of the model. Strain induced precipitation is a key phenomenon that controls the microstructure evolution in microalloyed steels. The model describing influence of precipitates on the phase transformation is one of the objectives of the research. Dutta-Sellars model is used to describe the kinetics of precipitation of Nb(C,N) on dislocations during isothermal holding following deformation. The size of precipitates after continuous cooling of experimental steel was calculated using the additivity rule. The analysis of precipitation kinetics process was performed in the deformed and undeformed austenite. Description of nucleation kinetics and precipitation growth under conditions of variable temperature is presented in continous - cooling – precipitation (CCP) diagram form. Development of the model, which predicts dislocation density at the beginning of transformation, is the next objective of this work. The results from both models are presented in the paper. This objectives are reached in two steps. Dilatometric tests for steel containing an addition of niobium were performed first, and parameters of the phase transformation model for this steel were determined. The dilatometric tests were carried out at cooling rates 0.04 – 76oC/s. Austenitizing temperature prior to cooling was 1180oC. Tests were performed for the austenite deformed by 20% and 50% at the temperature 920oC. The second step included the plastometric tests, which were performed for the investigated steel in the temperature range between 550 – 1100oC and three strain rates 0.1, 1.0 and 10 s-1 were investigated. The coefficients in the flow stress model were evaluated by performing the inverse analysis of the stress – strain curves determined in plastometric tests.
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| Keywords: niobium microalloyed steels|carbonitride precipitation process|dislocation density model| |
| No 2 (2007), p. 179-189 |
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