Evolution Behavior Of Cracks And Inclusions In Slab During Rolling

A further requirement is needed to improve the quality of rolled products with the rapid development of steel industry in recently years. And the cracks and the inclusions in rolled steel are the main factors that affect the quality of rolled products. In this thesis, the simulation researches on the initiation, the propagation, and the closure of cracks and the evolution behavior of inclusions in slabs during rolling have been carried out by using the finite element method (FEM) and the experiments. The research contents and results are as follows:1. The geometric updating method was proposed for FE simulation of the slab deformation behavior during multi-pass rolling. With this method, the deformation behavior of slabs during multi-pass vertical-horizontal rolling was carried out. The non-uniform shape of the slab head and tail, the metal flow at the slab corner and the strain distribution in slab after every pass during rolling were analyzed under various vertical roll shapes, etc. The numerical results were in good agreement with the physical simulation ones.2. The evolution behavior of transversal cracks on slab corner during multi-pass vertical-horizontal rolling was simulated using the geometric updating method. The contact pressure on crack surfaces was firstly used for analyzing the crack closure under various rolling conditions. The crack shape, the contact pressure on crack surfaces, and the crack tip stress during rolling were investigated which were adopted to analyze the influence of the vertical roll shape, the friction coefficient, and the initial crack size on the growth and closure of cracks. The calculated crack shapes after every pass during rolling are in good agreement with that obtained by physical simulation. At the same time, the evolution behavior of longitudinal cracks on the slab surface during multi-pass vertical-horizontal rolling was also simulated with the same method, and the projection of crack shapes in different sections were analyzed. And the evolution mechanism of surface cracks on slabs during rolling was analyzed.3. The evolution behavior of internal longitudinal cracks in slabs during vertical-horizontal rolling was simulated with the three-dimensional FEM. The crack shapes at different stages in rolling deformation zone were obtained, and the contact pressure on crack surfaces during rolling were adopted to analyze the effect of the vertical roll shape, and the crack location in slab on the growth and closure of cracks.4. The physical simulation was carried out on the healing of internal cracks in slab during hot rolling. The SEM micrographs of the crack shape, the SEM micrographs of crack fracture surface and the shear strength on the crack healing surface zone were analyzed under various rolling reductions. Meanwhile, a series of experiments were carried out for the internal crack healing by using the thermo-mechanical simulator. The SEM micrographs of crack shapes were obtained under different the heating rate, the heating temperature, the holding time, the strain rate, the reduction, and the deformation passes, and the influences of the above-mentioned factors on the crack healing were discussed. The mechanisms of internal crack healing during rolling were proposed based on the experimental results.5. The inclusion shape in strips was analyzed by SEM, and the chemical compositions of inclusions were analyzed by the energy spectrum analysis. Based on the experimental results, the evolution behavior of inclusions in strips during multi-pass rolling was carried out with the three-dimensional numerical simulation when the inclusions are combined with the matrix. The inclusion shape, the strain distribution around inclusion and the relative position of inclusions in strips after rolling were obtained considering a variety of inclusion sizes, positions, work roll diameters, etc. The influence of deformation behavior of inclusions on the crack initiation was discussed.6. The strain gradient around inclusions gradually increases with increasing the hard inclusions sizes during rolling. The evolution of inclusions and the initiation of cracks during rolling were simulated by using a two-dimensional numerical model when inclusions are separated with the matrix. The influence of inclusion size, shape, and position on the crack initiation around inclusions after rolling was analyzed.Analysis of the evolution behavior of cracks and inclusions in slab during rolling is important for understanding the mechanism of defects, and the results are useful for guiding the production.