Thermo-mechanical Finite Element Model of Shell Behavior in Continuous Casting of Steel

A finite-element model, CON2D, has been developed to simulate temperature, shape, stress, and hot-tear crack development during the continuous casting of steel, both in and below the mold. The stress model features an elastic-viscoplastic creep constitutive equation that accounts for the different responses of the liquid, semi-solid, delta-ferrite, and austenite phases. Temperature and composition-dependent functions are also employed for properties such as thermal linear expansion. A contact algorithm prevents penetration of the shell into the mold wall due to the internal liquid ferrostatic pressure. An efficient two-step algorithm has been developed to integrate these highly non-linear equations. An inelastic strain-based criterion is developed to predict damage leading to hot-tear crack formation, which includes the contribution of liquid flow during feeding of the mushy zone. The model is validated with an analytical solution for temperature and stress in a solidifying plate. It is then applied to predict the maximum casting speed to avoid crack formation due to bulging below the mold during casting of square steel billets. C. Li and B.G. Thomas, MCWASP X, TMS, San Destin, FL, May 25-30, 2003, pp. 385-392. 386 Introduction Computational models are important tools to study complex processes such as the continuous casting of steel. They can help to understand how defects form and to optimize casting conditions to maximize quality and productivity at low cost. Several previous researchers have developed two-dimensional (2-D) thermal-mechanical finite element models to study crack formation in continuous cast steel billets [1-4]. One such model, CON2D, has been developed at the University of Illinois over the past decade [5-7]. It has been applied to simulate shell thinning breakouts [5], ideal taper optimization [5], and meniscus distortion [6, 7]. This paper briefly summarizes the features of this model, including a new hot-tearing criterion, and then describes its recent application to predict the maximum casting speed to avoid crack formation due to bulging below the mold during continuous casting of square billets. Model Description Heat Transfer and Solidification Model The model solves a 2-D finite-element discretization of the transient heat conduction equation in a Lagrangian reference frame that moves down the caster with the solidifying steel shell. It features a spatial averaging method by Lemon to handle latent heat evolution [8] and a threelevel time-stepping method by Dupont [9].