A physically based model for predicting microstructural evolution has been developed. The model is based on a physical description of dislocation density evolution, where the generation and recovery of dislocations determine the flow stress and also the driving force for recrystallization. In the model, abnormally growing subgrains are assumed to be nuclei of recrystallized grains and recrystallization starts when the subgrains reach a critical size and configuration. To verify that the model is able to describe dynamic, static and metadynamic recrystallization of C‐Mn steels, hot compression tests combined with relaxation were performed at various temperatures, strains and strain rates. The model showed reasonable agreement with the experimental data for the compression tests performed at temperatures ranging from 850°C to 1200°C and strain rates ranging from 0.1 to 10 s−1. Similarly, the calculations of the stress relaxation tests were in accordance with experimental data. A validation of the model was done by calculating a multi‐step test where good agreement with both flow‐stress values and grain sizes was obtained. The main purpose of the model is to predict the microstructural evolution during hot rolling and this investigation presents very promising results.
The microstructure evolution of a martensitic Stainless steel subjected to hot compression is simulated with a physically based model. The model is based on coupled sets of evolution equations for dislocations, vacancies, recrystallization, and grain growth. The advantage of this model is that with only a few experiments, the material-dependent parameters of the model can be calibrated and used for a new alloy in any deformation condition. The experimental data of this work are obtained from a series of hot compression, and subsequent stress relaxation tests performed in a Gleeble thermo-mechanical simulator. These tests are carried out at various temperatures ranging from 900 to 1200°C, strains up to 0.7, and strain rates of 0.01, 1, and 10 s À1. The grain growth, flow stress, and stress relaxations are simulated by finding reasonable values for model parameters. The flow stress data obtained at the strain rate of 10 s À1 were used to calibrate the model parameters and the predictions of the model for the lower strain rates were quite satisfactory. An assumption in the model is that the structure of second phase particles does not change during the short time of deformation. The results show a satisfactory agreement between the experimental data and simulated flow stress, as well as less than 5 pct difference for grain growth simulations and predicting the dominant softening mechanisms during stress relaxation according to the strain rates and temperatures under deformation.
Specimens from split Hopkinson pressure bar experiments, at strain rates between ~ 1000–9000 s− 1 at room temperature and 500 °C, have been studied using electron backscatter diffraction. No significant differences in the microstructures were observed at different strain rates, but were observed for different strains and temperatures. Size distribution for subgrains with boundary misorientations > 2° can be described as a bimodal lognormal area distribution. The distributions were found to change due to deformation. Part of the distribution describing the large subgrains decreased while the distribution for the small subgrains increased. This is in accordance with deformation being heterogeneous and successively spreading into the undeformed part of individual grains. The variation of the average size for the small subgrain distribution varies with strain but not with strain rate in the tested interval. The mean free distance for dislocation slip, interpreted here as the average size of the distribution of small subgrains, displays a variation with plastic strain which is in accordance with the different stages in the stress-strain curves. The rate of deformation hardening in the linear hardening range is accurately calculated using the variation of the small subgrain size with strain
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