Main objective of this dissertation was to obtain a method to define the optimum cooling rate for the industrial size cooling as-cast steel especially for continuous casting steel production.
An FEM algorithm developed with the ANSYS codes was introduced in this dissertation to simulate the cooling of as-cast steel from any temperature below solidification temperature. The algorithm is capable to be customized to simulate the thermodynamic behavior of as-cast steel microstructure with any chemical composition and any casting geometry for a desired cooling method.
The economical advantage of NDT tests over DT, i.e., microstructural analysis tests, is obvious, but their disadvantages are the level of accuracy and the strength or detail of the information. Furthermore, detailed information cannot be extracted from two-dimensional ultrasonic test directly and ultrasonic tests from different angles are more costly. The ultrasonic NDT post image processing developed in this dissertation coupled with the DT microstructural investigation, provides information to decode the information embedded in the NDT tests images. The Microsoft Excel program is coupled with a commercial software ultrasonic image processing process and analyzes TOF results to provide the tools to specify the type of the anomalies in the microstructure and the objective Cartesian coordinate position of the defect in the microstructure and, thereby, to assess each type of anomaly present in the microstructure of the continuously cast slabs cooled at different conditions. The proposed NDT image decoding
elaborates to perceive the behavior of the microstructure during cooling with higher precision by considering the existence of the major defects in the models for industrial applications.
The effect of the composition of the inclusion on the distribution of the stress concentration zone in the matrix was studied by two dimensionless factors, as defined in this paper. These factors were introduced as the stress concentration factors and the inclusion rigidity factor. Variation of the rigidity of the inclusion in a given matrix shows local extremum points within the interface and matrix contact surface at the interface. However, the average stress concentration inside the inclusion is higher for more rigid inclusions. The defined properties for the interface govern the average stress concentration around soft inclusions but the stress concentration around the inclusions with higher rigidity factor is related to the properties of the matrix. From the results of the simulation of the interaction of the inclusion in the steel matrix, it can be deduced that the smaller inclusions localize the residual stresses and create a higher stress concentration. Investigation of the size effect of the inclusion on the level of the stress concentration reveals a critical size of the inclusion where the stress concentration does not increase with the growth of the inclusion for a given applied load and boundary conditions.
The cooling rates of three different grades of steel were obtained experimentally. The numerical results for grade steel 1010* was computed to show a good estimation for experimental results using FEM code developed in this project using FEM numerical methods.
Certain observations were made during the experiments. The FEM model is capable of computing the cooling rate considering the simplification and assumptions made in the model.
The laboratory cooling experiments showed that cooling rates higher than critical cooling rates may produce a defect in the microstructure due to solid-solid phase transformation.
The simulations of the cooling models, based on the proposed algorithm, provided proof that there exists a specific critical cooling rate for cooling each grade of steel from the super-critical and inter-super-critical temperature ranges due to solid-solid phase transformation in which any cooling rate above the critical one may cause a crack or flaw in the continuously cast steel.
Results of simulations also showed that although all five sources of the stress generations listed in this dissertation increase the residual stress of the microstructure, compared to solid-solid phase transformation, other sources of stress generation influence the optimum cooling rate on a smaller scale. The models presented in this research work demonstrated the potential concentration of residual stresses around two phase material such as iron-carbon alloys produced from solid-solid phase transformation and, on a smaller scale, due to the thermal gradient within the microstructure developed by the cooling process (e.g. around inclusions in the steel matrix).Since the maximum stress concentration zone usually appears at the interface of two phases in the microstructural configuration. The models were designed for thermally induced stresses (such as those resulting from cooling) rather than mechanically applied stress models.
The properties of the grain boundaries (GB) play important roles in the intensity of the accumulated residual stresses due to phase transformation in steel slabs with accelerated cooling.
This fact can be observed by comparing the models with four intergranular interaction methods introduced in this work. These four methods of the building interfacial region are;
Interfaces formed by glued grains (without any grain boundary properties or structure)
Thin layer bodies as such as grain boundaries with thermo-dynamic material properties
Contact element interface method
Cohesive zone interface method
The algorithm can simulate the cooling and solid-solid phase transformation processes for any grain shape. It was observed from the results of the numerical computation that the shape of the grains changes the distribution of stress concentration zones. The collection of the numerical simulations, resulting from different steps of the accelerated cooling simulation, indicates that the stress concentration zones generated by solid-solid phase transformation were stored in the microstructure. Thus, it indicates that further cooling below the transformation temperature with any cooling rate may not result in complete relaxation of regions of accumulated residual stress. From simulations it can be concluded that the critical cooling rate depends on the following
Cleanliness of the microstructure (pre-existing flaws and voids)
Initial stress state of the microstructure (i)
Chemical composition of the steel (c%)
Thermodynamic material properties of each phases
Microstructural configuration (single phase or multi phase)
Grain size and grain shape (Dc)
Size of the slabs (V ) s
Cooling procedure (Ts i1,...,n(Tsci1,...,n))
Shape of the slabs
Initial temperature of the as-cast slab at the start of the accelerated cooling process (T ) t
Grain boundary properties (coh)