Modeling of Hot Ductility During Solidification of Steel Grades in Continuous Casting – Part I

Abstract

Hot cracking during solidification in continuous casting is one of the quality problems due to the development of local stresses and strains in the CC strand which exceeds the material strength. Strains result from alternating motion of the steel shell due to thermal and mechanical contraction expension and ferrostatic pressure, primarily in the transition region between the geometrical strong edge and the more flexible wide side of a slab. [1] The formation of hot cracks is often related to the reduced ductility of steel in the mushy zone, caused by thin liquid layers between dendrites resulting from microsegregation. Those liquid layers between dendrite arms exist up to a high fraction of solid, just until complete solidification; [2] applied strain in the mushy zone in the solid fraction range between 0.9 and 0.99 can initiate brittle cracks. The temperature at which the solid brittle fraction is 0.9 is known as liquid impenetrable temperature (LIT) while the temperature at a solid fraction of 0.99 is the zero ductility temperature (ZDT). The region between both temperatures will be referred to as brittle temperature range. Reported critical strains are varying from 0.2 to 3.8% [3][4][5][6] for different carbon contents and strain rates, while the strains analyzed in any location of the caster are 0.3% at the most. [7] In the past it was common practice to describe the high temperature properties of steels according to tensile strength vs. temperature or reduction-of-cross-section-area (RoA) vs. temperature determined by hot tensile tests following theThe present paper gives an overview of the simultaneous research work carried out by RWTH Aachen University and ThyssenKrupp Steel Europe AG. With a combination of sophisticated simulation tools and experimental techniques it is possible to predict the relations between temperature distribution in the mould, solidification velocity, chemical steel composition and, furthermore, the mechanical properties of the steel shell. Simulation results as well as experimentally observed microstructure parameters are used as input data for hot tearing criteria. A critical choice of existing hot tearing criteria based on different approaches, like critical strain and critical strain rate, are applied and developed. The new ''damage model'' is going to replace a basic approach to determine hot cracking susceptibility in a mechanical FEM strand model for continuous slab casting of ThyssenKrupp Steel Europe AG. Critical strains for hot cracking in continuous casting were investigated by in situ tensile tests for four steel grades with carbon contents in the range of 0.036 and 0.76 wt%. Additionally to modeling, fractography of laboratory and industrial samples was carried out by SEM and EPMA and the results are discussed.94

Calculation of Initial Stage of Solidified Shell Deformation during <i>γ</i> to <i>δ</i> Transformation in Mold

“…Figure 13 shows the ratio of the strain ε within R1 (at x = 0) at the inner edge of the solidified shell (corresponding to the solidliquid interface) calculated by the mechanical model to the critical strain ε c for both q MF-A and q MF-B as a function of time. The material parameters proposed by Senk et al 35) (φ = 0.00427, m* = 0.4151 and n* = 0.9979) were employed to determine ε c . The value of ΔT B was calculated from the equilibrium phase diagram using THERMOCALC 18) based on a Fe-0.1 wt.%C alloy; this value is especially sensitive to composition.…”

Phase-field based simulation of microstructure evolution in technical alloy grades

“…Recent work describes modeling of the solidification of technical steel grades [64,65] and also addresses aspects like hot ductility during solidification of steel grades in continuous casting processes [66,67].…”

Key Lubrication Concepts to Understand the Role of Flow, Heat Transfer and Solidification for Modelling Defect Formation during Continuous Casting

“…Numerical modelling was tion of volume and finite element techniques (PHYSICA) 20) and b) a fully-coupled, thermal-metallurgical-mechanical, finite-element analysis coupled to a Navier-Stokes solver to calculate the stress-strain during solidification through FEM (THERCAST). 21) Finally, significant efforts have been made by Beckerman et al, 22) Gandin et al, 23) Senk et al 24) and others to bring microstructural modelling to usable scales for industrial application in CC. However; despite all the advances described, the prediction of some particular problems (e.g.…”