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Introduction

In continuous casting scaling results in an uneven increased heat transfer between the air/mist-nozzles and the strand surface. Similar observations are reported for hot rolling. In both cases the uneven cooling of the surface results in the formation of unfavourable phases. Adherent scale layers will result in surface defects. A good or bad removability of the scale is mainly influenced by the chemistry of the steel-slag-interfacial layer.

Objectives and Motivation

  • Heat transfer in the secondary cooling zone - influence of scale formation
  • Influence of process parameters on the formation of interface oxide layers in steel processing

Methodology

A so called nozzle-measuring-stand (NMS) is used to determine local HTC’s with the option on variation of a number of nozzle- and casting parameters. Currently these HTC’s measurements are carried out with a non-oxidizing sample material.

To gather more information about the influence of the surface roughness on the HTC, the present sample material should be taken for measurements with an artificial modified surface, with definite and reproducible roughness.

In secondary cooling the use of water and/or water-air nozzles leads to a “wet” atmosphere, which enhances scale formation, which will be investigated under near process conditions that are also given at tests on the NMS.  

The investigations on scale formation and the characterisation of scale-steel-interface layers analytical techniques used will encompass SEM, EDX, TEM, Auger-spectroscopy and microprobe. High temperature laser scanning confocal microscopy will be employed for in-situ studies of the scale formation on cast slabs during reheating.

In order to investigate the influence of different amounts of chemical elements in the steel with respect to their tendency to increase or decrease the amount of interface layer created, thermochemical simulations are to be employed. Software packages applied for these simulations will be FactSage and other tools as necessary.

Properties of the interface layer during pickling will be investigated using a laboratory pickling setup using a digital camera/microscope system for monitoring the pickling process.

Results and application

The impact of nozzle parameter variations, overlapping nozzles, and the oxide scale will be quantified in more detail.

The project aims on the formation of a reproducible surface roughness on a non-oxidizing material with comparable surface roughness to the strand material. The clear separation of surface roughness and oxide scale formation (composition, bond with strand material) is assessed as an important step.

Furthermore the possibility to represent the oxide scale formation during the continuous casting (CC) process by simple laboratory test assemblies will be revealed and the dominating impact factor on the HTC will be shown. 

The project also focuses on the clear evaluation if surface roughness and oxide scale formation have to be considered as one single impact factor on the HTC.