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The formation of oscillation marks in the continuous casting of steel slabs

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Abstract

The formation of oscillation marks on the surface of continuously cast slabs has been studied by metallographically examining slab samples and by performing a set of mathematical analyses of heat flow, lubrication, and meniscus shape in the meniscus region of the mold. The metallographic study has revealed that, in agreement with previous work, the oscillation marks can be classified principally according to the presence or absence of a small “hook” in the subsurface structure at the base of individual oscillation marks. The depth of the oscillation marks exhibiting subsurface hooks varies with the carbon content, reaching a maximum at about 0.1 pct carbon, while the oscillation marks without hooks show no carbon dependence. The analysis of heat flow at the meniscus, which is based on a measured mold heat-flux distribution, indicates that depending on the level of superheat, the meniscus may partially freeze within the period of a typical mold oscillation cycle. Lubrication theory has shown that, owing to the geometry of the mold flux channel between the solidifying shell at the meniscus and the straight mold wall, significant pressure gradients capable of deforming the meniscus can be generated in the flux by the reciprocating motion of the mold relative to the shell. A force balance on the interface between the steel and the mold flux has been applied to compute the shape of the meniscus as a function of the pressure developed in the lubricating flux at different stages in the mold oscillation cycle. This has demonstrated that the “contact” point between the meniscus and mold moves out of phase with (by π/2), and has a greater amplitude than, the mold displacement so that just at, or near, the end of the negative strip time molten steel can overflow at the meniscus. From these studies a reasonable mechanism of oscillation-mark formation emerges which involves interaction between the oscillating mold and the meniscusvia pressure gradients in the mold flux, meniscus solidification, and overflow. The mechanism is consistent with industrial observations.

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Abbreviations

a 2 :

capillary constant (−)

C pf , C ps :

specific heat of mold flux and steel, respectively(J/g °C)

f :

frequency of mold oscillation (cycle/s)

f s :

fraction solid [= (T liq −T s)/(@#@ Tliq -T sol @#@)]

h(x) :

width of flux channel (cm)

h j , h f :

width of flux channel at inlet and outlet, respectively (cm)

h s− f :

heat transfer coefficient between steel and mold flux (W/cm2 °C)

l :

pitch of oscillation marks (cm)

l f :

length of flux channel (cm)

P(x) :

axial distribution of pressure in mold flux (dyne/cm2)

ΔP :

pressure difference (dyne/cm2)

Pi,Pf:

pressure at inlet and outlet of flux channel, respectively (dyne/cm2)

q (x) o :

heat-flux distribution along mold wall (W/cm2)

Q R :

relative consumption of mold flux (cm3/s)

R(x) :

integration of pressure in flux channel fromx = 0 tox = x (dyne/cm)

r :

distance normal to meniscus (cm)

S :

stroke of mold oscillation (cm)

s :

distance along meniscus (cm)

T f , T s :

temperature of flux and steel, respectively (°C)

T fi ,T si :

initial temperature of flux and steel, respectively (°C)

Tliq,T sol :

liquidus and solidus temperature, respectively (°C)

t N , t p :

negative and positive strip time, respectively (s)

u x :

relative velocity of mold flux (cm/s)

V f :

velocity of mold flux (cm/s)

v m :

velocity of mold (cm/s)

v s :

velocity of slab (cm/s)

x c :

contact point of meniscus with mold wall (cm)

λf, λs :

thermal conductivity of mold flux and steel, respectively (W/cm °C)

μf:

viscosity of mold flux (poise)

σ interfacial tension between mold flux and steel:

(dyne/cm) τ shear stress (dyne/cm2)

ϕ :

contact angle (radian)

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Authors and Affiliations

  1. Department of Metallurgical Engineering, The University of British Columbia, V6T 1W5, Vancouver, B.C., Canada

    E. Takeuchi (Graduate Student) & J. K. Brimacombe (Stelco Professor of Process Metallurgy)

Authors
  1. E. Takeuchi
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  2. J. K. Brimacombe
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E. TAKEUCHI, on study leave from Nippon Steel Corporation

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Takeuchi, E., Brimacombe, J.K. The formation of oscillation marks in the continuous casting of steel slabs. Metall Trans B 15, 493–509 (1984). https://doi.org/10.1007/BF02657380

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