**7. Mechanism of slab broadening**

576 Numerical Simulation – From Theory to Industry

the continuous caster is composed of three parts:

this part is focused on in our experiments.

broadening of the molten steel core.

standard width for assessing slab broadening.

stagnant slab, which is cooled down continuously by secondary cooling water. The slab in

1. For the fully solidified part during casting, the slab broadening is the sum of

2. For the part solidified during the stopping period, because it reveals the slab broadening at specific position, and corresponds to the real broadening amount of slab,

3. For the unsolidified part until restarting the casting, the slab broadening continues during subsequent casting, as the molten steel core still exists in the slab shell.

Using the square-root law of solidification, the fully solidified normal position and stagnant position can be derived, and thus the above three parts of the slab could be determined.

The slab was Q235 steel, the casting speed was 0.0167 m·s-1, the cross section of the slab was 2.050 m × 0.230 m, the upper width of the mold was 2.0813 m, the lower width of the mold was 2.0675 m, the casting temperature was 1533°C, *T*l was 1513°C, and *T*s was 1546°C.

The width of the front slab was also traced. It remained at 2.040 m, indicating that nearly no broadening of the slab happened at this position. It may be because it was cooled so rapidly that there was no time for broadening. Therefore, the width of the front slab was used as the

The absolute broadening of the slab was derived from the slab width, which was measured while the slab was pushed through the exit of the continuous caster, subtracting the width

**Figure 15.** (a) Absolute broadening of slab in the first strand of stagnant slab; (b) Absolute broadening

(a) (b)

Slab broadening mainly happened in the front 6 segments (before 12.6 m). In these sectors, the broadening increases linearly with the distance from the meniscus. At the position of

of slab in the second strand of stagnant slab.( FU JianXun et al.2011(a))

of the front slab. The broadening values of the slab are shown in Figure 15(a) and (b).

However, because the slab shell is very thick, little broadening happens.

The static pressure of molten steel deforms the slab shell. The coupled thermo-mechanical viscoelastic-plastic 3D finite element model was built with the secondary development of the commercial software MSC.Marc. The calculated and measured results of slab width are shown in Figure 16.

The figure reveals that the calculated deformation agrees very well with the measured deformation. Slab broadening is the result of slab deformation under the pressure of static melting at high temperature. The deformation of the slab in the direction of thickness is shown in Figure 17(a). The temperature field of the slab is shown in Figure 17(b).

**Figure 16.** Simulated and measured widths of slab;( FU JianXun et al.2011(a))

The on-site investigation, force analysis, calculation from Maxwell creep model, and numerical simulation from the coupled thermo-mechanical viscoelastic-plastic 3D finite element model reveal that the slab broadening is due to slab deformation under the static pressure of molten steel. The slab shell deforms without constraints on the narrow side.

Creep deformation appears when the material plastic gradually deforms with time under certain conditions. Plastic deformation only happens when the stress exceeds the elastic limit. However, creep deformation happens when the acting time of stress is sufficiently long, even if the stress is very small. The creep deformation of metal is obvious only if the temperature is over the creep temperature (about 0.3 *T*m). The slab deforms for a long time

under the pressure of static molten steel at high temperature. The creep rate depends on the composition of the compound metal, and the processes of refining and thermal treatment. Creep deformation causes slab broadening because it makes the material keep stress relaxed, reduces hardness, and enhances plasticity.

Numerical Simulation of Slab Broadening in Continuous Casting of Steel 579

reinforce steel. Thus, micro-alloying of steel could enhance the hardness of the slab and

In summary, higher casting speed, lower intensity of secondary cooling, thinner slab shell, larger static pressure of molten steel, and lower hardness of steel at high temperature

1. The mechanism of slab broadening is that the slab with high temperature exposes to no constraint at the direction of narrow face, and because of the static pressure of molten

2. Slab broadening is a common problem in continuous casting. The average RUB for the three grades of steel studied was in the range of 1.27%~3.00%, with a maximum of 4.4%. 3. Stagnant slab measurement experiments reveal that slab broadening happens in the 6 front segments, and that roller compaction is not responsible for slab broadening. 4. The agreement between the calculated results from the Maxwell model and the measured results illustrates that the Maxwell model is able to reveal the deformation

5. Higher casting speed, lower intensity of secondary cooling, thinner slab shell, larger static pressure of molten steel, and lower hardness of steel at high temperature increase slab broadening. The micro-alloying of steel improves the hardness of the slab and

*Research Center for Energy Technology and Strategy & Department of Materials Science and* 

The authors gratefully acknowledge the guidance of Prof. Jingshe Li, Prof. Hui Zhang, Prof. Xingzhong Zhang, et al. The authors would like to thank the National Science Council of

Chen J. Hand Book of Continuous Casting (in Chinese). Beijing: Metallurgical Industry

Fu JianXun, Li Jingshe, Zhang Hui, Zhang Xing-zhong. Industrial Research on broadening of slab in continuous casting. JOURNAL OF RON AND STEEL RESEARC

*Engineering, National Cheng Kung University, Tainan 701, Taiwan* 

Taiwan (NSC100-2221-E-006-091-MY3) for funding this work.

INTERNATIONAL. 2010. 17(8):20-24,

reduce slab broadening.

increase slab broadening.

steel, the slab deforms in this direction.

behavior of a slab at high temperature.

reduces slab broadening.

Jian-Xun Fu and Weng-Sing Hwang

**Author details** 

**Acknowledgement** 

**9. References** 

Press, 1990,

**8. Conclusion** 

**Figure 17.** (a) Deformation of slab in the direction of thickness at 230 mm; (b) Temperature field of slab at 1150 s. ( FU JianXun et al. 2010(c))

The amount of broadening depends on the forces acting on the slab and the properties of the slab material, especially those at high temperature. Specifically, it depends on the static pressure of molten steel, the high-temperature mechanical properties of steel, the composition of the slab material, the thickness of the slab shell, secondary cooling intensity, casting speed, and the constitution of the caster.

The static pressure of molten steel is the driving force for the deformation of the slab shell. It is related to the type and constitution of caster. At present, vertical-bending casters are most common. For these casters, the static pressure is related to the density of molten steel and the height of the caster.

Under the conditions of high casting speed and constant cooling water, the fully solidified zone extend, the length of molten core increases, and the shell becomes thinner. Because of the higher temperature, the slab shell also has lower yield strength and better malleability. Consequently, the slab broadening increases. However, if the cooling water supply is changed when the casting speed is increased, the problem will become sophisticated.

The effects of steel grade on the broadening result from differences in material properties at high temperature, and hence differences in resistance to plastic deformation and creep deformation. With an increase in the carbon percentage, the ratio of ferrolite and austenite in the two phase regions changes. The increase in austenite is helpful to the reduction of slab broadening.

Intracell dislocation climb and intercell slide are two forms of creep deformation. Solution strengthening, precipitation strengthening, and dispersion strengthening insert a lot of defects into the crystal structure of steel, which hinder dislocation movement and thus reinforce steel. Thus, micro-alloying of steel could enhance the hardness of the slab and reduce slab broadening.

In summary, higher casting speed, lower intensity of secondary cooling, thinner slab shell, larger static pressure of molten steel, and lower hardness of steel at high temperature increase slab broadening.
