Study On Simulation Of Surface Cracks Growth For Continuous Casting Q235Slab

Surface cracks always have been a difficult and stubborn obstacle that impairs slabquality along with the development of continuous casting. Slab with mild surface cracksneeds to be finished while worse cracks may lead to breakout, which reduces productionefficiency and slab quality. These are also the main reason why many metallurgicalresearchers focus on mechanism of crack formation and influence factors. However, thelimitation of production field and equipment make it difficult to measure surface stressdistribution let alone monitoring crack growth. So, it is difficult to understand themechanism of crack growth thus unable to deeply analyze crack extension behaviours.In order to fully quantitatively understand the pattern and dynamic process thecrack growth, this dissertation aims to analyze crack growth in slab with different crackpositions during bending and straightening by combining high temperature fracturetoughness experiment with macro-and meso-scopical simulation analysis. Based onabove study, it is able to understand crack extension behariours and lay the foundationof crack research. Specifically, the detailed results are listed below.①Based on fracture mechanism, using J-integral as the crack extension criterionto establish Q235slab fracture toughness experiment. In macro research, XFEM is usedto simulate crack growth process during bending/straightening. Pressures of bendingand straightening rolls, thermal stress, steel static pressure and simplifiedbending/straightening strain are considered in this model. Longitudinal crack, transversecrack and oblique crack are analyzed. On the other hand, in mesoscopic research,coupled cohesive zone model (CCZM) used to simulate debonding phenomena betweengrain and grain, grain and second phase particle. Thiessen polygon method is used tobuild polycrystal model and simulated parameters are acquired through literatures.②Experimental results show that with the temperature rising, the plasticdeformation work, fracture load and fracture toughness are decreasing. Data achieved inexperiment can be used in numerical simulation. Through macro observation, fracturesurface ban be divided into four area and crack growth area can be clearly observed athigh temperature.③Macro simulation results show that growth has not happen in longitudinal crackcase yet crack surface has deformed or small expand. Transverse cracks at outer arc andouter corner growth largely during bending while transverse cracks at inner arc and inner corner growth a lot during straightening. Transverse cracks grow along with thespecified direction. Because the maximum strain is along with casing direction, so thegrowth direction of oblique crack，whose below45°, are perpendicular to castingdirection.75°oblique crack grows along the specified direction.④Mesoscopic simulation results show that model size has small influence ongrowth path, which is determined by load and the way it works. With load enlarges, thenumber of debonding element in interface is increasing. Number of debonding elementin grain boundary is13, but sider and inner model has no debonding element in bending.In addition, debonding element in grain model with second phase particles is more thanthe model without second phase particles in the same load condition. Debondingphenomena are observed at different second phase particles surrounding.