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I Steelmaking

Development of Ultrahigh-Basicity Mold Fluxes for Peritectic Steel Continuous Casting

Xiao Long, Shengping He, Qian Wang and P. Chris Pistorius

Abstract In the continuous casting of peritectic steels, mold fluxes play significant roles in providing defect-free surfaces of slabs. In this work, a traditional high-basicity mold flux was studied. Slag film samples were solidified from molten flux, with an improved water-cooled copper probe and using different probe immersion times. The film microstructures, interfacial roughness and porosity were studied. Based on the results, ultrahigh-basicity mold fluxes (CaO wt%/SiO2 wt% = 1.5 * 1.8) were developed. A typical ultrahigh-basicity mold flux (CaO wt %/SiO2 wt% = 1.74) was analyzed in the laboratory and applied in industrial processes. The laboratory experiments and industrial results indicate that the novel mold fluxes developed in this work have excellent performance in balancing heat transfer control by the solid film and lubrication by molten flux.

Keywords Mold flux • Peritectic steel • Solid slag film • Ultrahigh basicity

Introduction

Mold fluxes are widely used in the continuous casting of steels to improve the surface quality of slabs and the efficiency of casting [1]. In the production of peritectic steel slabs, the volume shrinkage caused by the peritectic reaction in the cooling of the initial steel shells can lead to longitudinal cracks on the surface and sub-surface of slabs. The usual method to decrease the cracking tendency on the slab surface is to form a less intensive cooling environment for initial shells near the meniscus [2]. Thus, the mold flux adopted in peritectic steel casting must provide

X. Long (H) • S. He • Q. Wang

College of Materials Science and Engineering,

Chongqing University, Chongqing 400044, China e-mail: This email address is being protected from spam bots, you need Javascript enabled to view it

X. Long • P.C. Pistorius

Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA

© The Minerals, Metals & Materials Society 2017

A. Allanore et al. (eds.), Materials Processing Fundamentals 2017,

The Minerals, Metals & Materials Series, DOI 10.1007/978-3-319-51580-9_1

sufficient retardation of heat transfer through the slag film (the thermal resistance between the steel shell and the water-cooled copper mold). In the industrial process of peritectic steel continuous casting, high basicity (1.2 < (%CaO)/(%SiO2) < 1.4) mold fluxes are widely used to ease the crack tendency of slabs. The mechanisms by which such mold fluxes control heat transfer are widely considered to be as follows [3-6]:

  • (1) High basicity increases the solidification temperature (break temperature) of liquid slag, which increases the thickness of solid slag film between mold and steel shell for heat flux control.
  • (2) The crystallization in the glass layer of solid slag film (devitrification) increases the roughness of film surfaces in contact with the copper mold, which increases the thermal contact resistance between mold and slag film.
  • (3) Compared with low basicity mold fluxes, the crystals precipitated by solidification in high basicity slag films decrease the thermal conductivity of solid slag film.

Although high-basicity mold fluxes decrease the cracking tendency of peritectic steel slabs, their use may cause lubrication problems like sticking or even breakouts (suggested to be related to the thicker solid and thinner liquid slag film [3, 4]). The most commonly used method to resolve this contradiction (between heat transfer control and lubrication) is to decrease the casting speed. To provide more information to solve this problem, an improved water-cooled copper probe was used to solidify slag films from a typical traditional high-basicity flux (TB), using different probe immersion times. Ultrahigh-basicity mold fluxes developed by Qian Wang et al. of Chongqing University [7-9] to overcome the limitations of traditional high-basicity films were also tested.

 
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