Simulation of Thermal Mechanical Behavior during Initial Solidification

Mathematical models have been developed to predict temperature, stress, and shape development during initial solidification. The heat transfer model was run for typical casting conditions in the mold for typical thin slab and conventional continuous slab casters. The calculated temperatures were input to an elastic-viscoplastic finite-element stress model of the solidifying steel shell. This model features an efficient algorithm to integrate the highly nonlinear constitutive behavior of steel at high temperature. The stress model includes the temperature and composition-dependent effects of phase transformation on both the thermal linear expansion / contraction behavior, creep behavior, and pseudo-strain due to flow in the liquid. Stress and strain distributions are calculated along a line through the shell thickness, assuming no shell bending or sticking to the mold. Results are compared for 0.044%C and 0.1%C steels and for both cooling conditions. The results provide insight into the formation of longitudinal surface cracks in continuous-cast steel. 2 Introduction Most of the surface defects in continuous cast steel initiate during the early stages of solidification in the mold. These include surface depressions, longitudinal and transverse surface cracks. Although a body of empirical knowledge and theory exists to understand how they form, the exact mechanisms for many of these problems are still unclear. Several studies have been made to investigate how cracks form during continuous casting of steel. It is well-known that the middle carbon or “peritectic” steels containing 0.1 0.2% C are more prone to depressions and longitudinal surface cracks during casting than other grades. [13] This was initially attributed to an inherent lower ductility of these grades. However, H. Suzuki et al. [4] performed isothermal tensile tests on in-situ melted and resolidified samples which showed that ductility decreases slowly but steadily with increasing carbon and residual alloy content. There was no special embrittlement problem with middle carbon steels. Embrittlement was attributed to the drop in solidus temperature caused by microsegregation of the alloying elements. This is consistent with the findings of Ye et al. [5] that middle carbon steel shells have a 2% macroscopic strain to failure, which is greater than other steels. Thus, the increased surface cracking tendency of middle carbon steels is now attributed to the peritectic reaction, and the phase transformation contraction from delta-ferrite to austenite. Clearly, an accurate calculation of stress and strain during initial solidification that included the effect of temperature and composition on the steel properties would be useful. Model description A transient, thermal-elastic-viscoplastic finite-element model, CON2D [6, 7] has been developed to follow the thermal and mechanical behavior of a section of the solidifying steel shell, as it moves down the mold at the casting speed. It is applied in this work to simulate stress and strain development in a typical 1-D slice, pictured in Figure 1. Liquid Steel Solid Steel In te rf a c e a n d m o ld y x uy = constant along this edge