Segregation in Continuously Cast Steel Billets Investigated using Thermo-Calc and DICTRA

Steel billets with text that reads Thermo-Calc and DICTRA used for investigating segregation in continuously cast steel billets.

A paper published in IOP Conference Series: Materials Science and Engineering investigates various fundamental concepts that are at play during the solidification of continuously cast steel billets at different cooling rates. The simulations in the paper, Continuous Casting of High Carbon Steel: How Does Hard Cooling Influence Solidification, Micro and Macro Segregation?, were performed with the Thermo-Calc software package to investigate solidification as well as micro and macro segregation of steel billets from continuous casting. Thermo-Calc, together with the TCFE database, was used to calculate different steel properties and perform Scheil solidification simulations to compare simplified C80D steels with no carbon diffusion and fast carbon diffusion. The calculated steel properties were then used in the solidification simulation in CHILL (developed by the SMS Group). With information from the CHILL simulations, the diffusion equations were solved using the Diffusion module (DICTRA) together with the free mobility database MFEDEMO. Thermo-Calc was also used to calculate the driving forces for diffusion with the free database FEDEMO.

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The authors state that the ambition of this project is to generate an in-depth description of the billet solidification and that the aim is to outline the linkage of different readily available simulation tools to elucidate a real-life industrial example from several angles. The next step, according to the authors, is to expand and adapt the outlined models and apply them to the industrial process.

This paper is written by A. Nicholas Grundy, Steve Münch, Stephan Feldhaus and Johan Bratberg.

Nicholas Grundy and Johan Bratberg are Thermo-Calc Software employees.

Abstract

One technology that is often employed in continuous casting of high-carbon steel billets to minimize centre- (or macro) segregation is hard secondary cooling. Investigations unanimously show, that hard cooling significantly reduces macro-segregation, but a mechanism for the reduced segregation is rarely given. In this paper the solidification of high carbon tire cord grade C80D cast as a 150×150 mm billet is calculated using the proprietary SMS Group solidification simulation package CHILL using steel properties calculated with the Thermo-Calc Software package and TCFE steels database. The obtained cooling rates in the billet for hard and soft secondary cooling are used to run solidification simulations considering solute redistribution using the diffusion module DICTRA. It is shown that for cooling rates achieved in continuous casting the steel solidifies far away from equilibrium. The solidification profile and solidus temperature lie in between the Scheil solidification model and the para-equilibrium Scheil model with carbon defined as a fast diffusing element. The calculated cooling rates and temperature gradients are used to simulate the solidification microstructure 20 mm from the billet surface using the phase field approach and the MICRESS® software package linked to Thermo-Calc through the TQ interface. This model clearly shows, that the most probable mechanism by which hard cooling reduces segregation is trapping of solutes between the intricately branched dendrite microstructure that results from the steep temperature gradients achieved when applying hard secondary cooling.

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