INVESTIGATION OF FLUID FLOW AND STEEL CLEANLINESS I

Metallurgical and Materials Transactions B, Vol. 38B, No. 1 (Feb.), 2007, pp. 63-83.

INVESTIGATION OF FLUID FLOW AND STEEL CLEANLINESS IN THE

CONTINUOUS CASTING STRAND

Authors:

Prof. Lifeng Zhang (Correspondence author)

Department of Materials Science and Engineering

Norwegian University of Science and Technology (NTNU)

Høgskoleringen 8, Alfred Getz vei2

7491 Trondheim, Norway.

Tel: Fax:

Email: @

Dr. Subo Yang

Technical Research Center, Panzhihua Iron and Steel Company

Panzhihua, Sichuan Province,

Prof. Kaike Cai, Mr. Jiying Li, Mr. Xiaoguang Wan

School of Metallurgy, University of Science & Technology Beijing

Beijing 100083,

Prof. Brian G. Thomas

Department of Mechanical and Industrial Engineering

University of Illinois at Urbana-Champaign

1206 W. Green St., Urbana, IL 61801, USA

Phone number: 1-217-333-6919

Fax number: 1-217-244-6534

Email: bgthomas@

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Abstract:Fluid flow in the mold region of the continuous slab caster at Panzhihua Steel is investigated

with 0.6-scale water model experiments, industrial measurements, and numerical simulations. In

the water model, multiphase fluid flow in the submerged entry nozzle (SEN) and the mold with

gas injection is investigated. Top surface level fluctuations, pressure at the jet impingement point,

and the flow pattern in the mold are measured with changing submergence depth ,SEN geometry,

mold width, water flow rate, and argon gas flow rate. In the industrial investigation, the top

surface shape and slag thickness are measured, and steel cleanliness including inclusions and the

total oxygen content are quantified and analyzed, comparing the old and new nozzle designs.

Three kinds of fluid flow pattern are observed in the SEN: bubbly flow, annular flow, and an

intermediate critical flow structure. The annular flow structure induces detrimental asymmetrical

flow and worse level fluctuations in the mold.. The SEN flow structure depends on the liquid

flow rate, the gas flow rate, and the liquid height in the tundish. The gas flow rate should be

decreased at low casting speed in order to maintain stable bubbly flow, which produces desirable

symmetrical flow. Two main flow patterns are observed in the mold: single roll and double roll.

The single roll flow pattern is generated by large gas injection, small SEN submergence depth

and low casting speed. To maintain a stable double-roll flow pattern, which is often optimal, the

argon should be kept safely below a critical level. The chosen optimal nozzle had 45mm inner

bore diameter, downwards 15oport angle, 2.27 port-to-bore area ratio, and a recessed-bottom.

The pointed bottom SEN generates smaller level fluctuations at the meniscus, larger

impingement pressure, deeper impingement, and more inclusion entrapment in the strand than

the recess-bottom SEN. Mass balances of inclusions in the steel slag from slag and slab

measurements shows that around 20% of the alumina inclusions are removed from the steel into

the mold slag. However, entrainment of the mold slag itself is a critical problem. Inclusions in

the steel slabs increase two-fold during ladle changes and ten-fold during the start and end of a

sequence. All the findings in the current study are important for controlling slag entrainment.

Key Words:Continuous Casting, SEN, Mold, Fluid flow, Level Fluctuation, Water Model, Inclusions, Total

Oxygen, Industrial Investigation, Slag Entrainment

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UCTIONFluid flow in the Submerged Entry Nozzle (SEN) and the continuous casting mold is important

due to its effect on many phenomena related to the steel quality, such as the transport of argon

bubbles and inclusions, transient waves and fluctuations of the top surface, the transport of

superheat, meniscus freezing, shell thinning from the jet impinging upon the solidifying shell,

thermal stress and crack formation. The entrainment of mold slag due to excessive surface

velocities and level fluctuations is one of the most important causes of defects found in steel

products.[1-9]The main flow-related phenomena that cause slag entrainment and surface quality problems are

shown in Figure 1[10]. If the jet from the SEN outport strongly impinges on the narrow face and

splits to flows upwards along the narrow face, it will lift the level of the molten steel, changing

its profile, and also generating large level fluctuations near the meniscus. This also pushes slag

away from the narrow face, leading to surface quality problems.

[10-17] Direct jet impingement of

the jet onto the steel-slag interface associated with a single roll flow pattern, such as induced by

excessive gas bubble injection,[18] is even more detrimental. Excessive velocity of the molten

steel across the top surface may shear off fingers of slag into the steel

[19-27] Turbulence and level

fluctuations at the top surface can induce slag entrainment at the meniscus, and surface defects.

Flow problems such as uneven flow discharge from opposite ports of the SEN[10], may cause

asymmetric and unsteady flow in the mold ,

[28] and / or periodic oscillations of the level. High

speed surface flows or asymmetrical flow in the mold may also induce vortices near the SEN,

[10,

29]. These are other important causes of slag entrainment. Emulsification of the slag / steel

interface, such as caused by the rupture of bubbles floating to this interface

[16, 30] is also

dangerous. The slag foam is easily entrained into the steel flow. Alternatively, slag may be

sucked down along the SEN wall due to flow recirculation and the low pressure region just

above the SEN port exits.

[10] Mold slag may enter the upper portion of the ports and cause

clogging problems, or become entrained into the jet, and cause serious slag entrainment

problems. Thus, the fluid flow pattern in the mold and level fluctuations are of great importance

to slag entrainment quality problems,

[21] and large level fluctuations correlate with more surface

defects in the steel product.

[22]3

In the current paper, fluid flow in the SEN and the mold of the slab continuous caster at

Panzhihua Steel () is investigated using water models, numerical simulations and

industrial measurements. First, the water model is used to investigate the fluid flow pattern in the

SEN, which is one of the sources of asymmetrical flow in the mold. Then, flow in the mold is

quantified by measuring the magnitude of the top surface level fluctuations, the pressure near the

jet impingement point on the narrow face, and the flow pattern shape for different SEN geometry,

submergence depth, mold width, water flow rate, and gas flow rate. The single roll and double

roll flow pattern, and the top surface emulsification induced by bubbles are noted. In the

mathematical simulation, three-dimensional fluid flow and particle transport are calculated in the

water model of the tundish, the SEN and the mold. In the industrial trial, the thickness of the

liquid slag layer, and the inclusion content in the steel and slag are measured, to determine the

extent of inclusion removal in the mold, relative to inclusions entrapped in the steel product. The

effect of the SEN well shape on steel cleanliness is also investigated. Finally, improved design

and operating conditions are chosen to improve fluid flow and steel cleanliness in the . INVESTIGATION METHODS

rity Criterion of Water Model experiments

Extensive past work has employed physical water models to investigate fluid flow phenomena in

the mold region of the continuous casting process.

[19, 31-46] The first study was carried out by

Afanaseva et al

[31] for a straight bore nozzle system. Heaslip et al extensively studied the fluid

flow in SENs under stopper-rod control and slide-gate control

[35, 36]. Gupta investigated the

residence time distribution

[37], asymmetry and oscillation of the fluid flow pattern

[38, 39], and slag

entrainment

[40, 41]. Tanaka et al

[42] and Wang et al

[44] studied the influence of wettability on the

behavior of argon bubbles and fluid flow. Teshima et al

[19] and Iguchi et al

[43] studied slag

entrainment. However, there are few papers with measurements of level fluctuations at the

meniscus

[19, 47]. Only a few papers investigate impingement pressure on the narrow face, and

believe that lower pressure is better.

[48, 49]In the current work, the Weber-Froude similarity criterion was used to design the water model

for the gas-water two phase fluid flow phenomena of interest. For the high-velocity flow

conditions present in a steel continuous caster, fully-developed turbulent flow conditions are

4

always produced, so achieving Reynold’s similarity by matching the ratio of the momentum and

diffusion forces was judged to be less important, so long as fully-turbulent flow conditions are

maintained.

First, invoking Froude similarity to ensure equal ratios of the momentum and buoyancy forces in

the water model (w) and steel caster (s) gives:

§U2·§U2·Fr

¨¸

¨¸

©gL¹w©gL¹s (1)

where Fr is the Froude number, U is a characteristic velocity (m/s), g is the gravitational

acceleration rate (m/s2), L is a characteristic length (m). Substituting the geometry scale factor,

O=Lw/Ls, into Eq.(1) gives:

Uw

O

Us (2)

Second, applying Weber similarity to match the ratio of the momentum and surface tension

forces implies

§UU2L·§UU2L·We

¨¸

¨¸

VV©¹w©¹s, (3)

whereWeis the Weber number,

U is the liquid density, 7020kg/m3 for molten steel and 998

kg/m3 for water, and

V is the surface tension, 1.6 for molten steel and 0.073N/m for water.

Combining Eqs. (2) & (3) gives:

§USVw·O

¨¨UV¸¸©wS¹1/2~ 0.6, (4)

Thus, a 0.6-scale water model can satisfy both Froude similarity and Weber similarity

simultaneously. An additional requirement is to scale down the water flow rate (Qw) relative to

the molten steel throughput ( QS) (m3/hour) according to:

QwUwL22.5w

O

0.279

2QsUsLs (5)

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The casting speed (measured for the solid strand) is related to the liquid steel flow rate from the

following mass balance, which considers the effect of steel solidification on the

QSS§1hour·§7020·¨¸¨¸

60min7800©¹©¹ (6)

whereVC is the steel casting speed (m/min), QS is the liquid steel flow rate (m3/hr),S is the cross-section area of the strand (m2), and 7800/7020 is the ratio of solid to liquid steel densities.

Combining Eq.(5) and (6) gives the relationship between casting speed in the real caster and flow

rate in the water model (Table I). For a 200mmu900mm strand,

(7)

VC(m/min)= 0.298 Qw(m3/hr),

and for a 200mmu1250mm strand,VC= 0.214 Qw. (8)

A suggested argon flow rate to use in the steel continuous casting is found by matching the

modified Froude numbers in the water model and steel caster,

2UArVArFrc

, (9)

gL