**6.7. Industrial experiments of stagnant slab (FU JianXun et al.2011(a))**

If the static pressure of molten steel is the main reason for the broadening of a slab, the broadening must happen at the forepart of the continuous caster where the slab has a high temperature and a thin shell. If the stress of the rollers is the main reason for broadening, the broadening must happen at the middle part of the continuous caster, specifically the position near the completely solidified zone. Because the molten steel is fluidic before this part(Lin Q Y et al ,2004), decreasing the roller gap does not broaden the slab.

Since the continuous caster is a vertical bow type, it is very dangerous to keep close to it, and thus it is impossible to measure the width of the slab directly. Therefore, when the caster stopped to for the tundish replacement, the width of the stagnant slab was measured to determine where slab broadening happens. When the tundish is to be replaced, the casting speed gradually slows down to zero. This process takes about 4-5 min to form a

stagnant slab, which is cooled down continuously by secondary cooling water. The slab in the continuous caster is composed of three parts:

Numerical Simulation of Slab Broadening in Continuous Casting of Steel 577

12.6 m, the slab broadening was at its maximum, and then decreased slowly with distance from the meniscus. These results confirm that the stress of the roller is not the main reason for broadening. Otherwise, the slab broadening will happen before the slab is fully solidified near the 9th and 10th segments. The trend of slab broadening is consistent with that of the static pressure of molten steel, which confirms that the slab broadening is dependent on the

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

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

**0 5 10 15 20 25 30 35 40**

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

**Distance from meniscus, m**

**Simulated result Measured result**

shown in Figure 17(a). The temperature field of the slab is shown in Figure 17(b).

static pressure of molten steel.

shown in Figure 16.

**7. Mechanism of slab broadening** 

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

**Width of slab, mm**


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 standard width for assessing slab broadening.

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

**Figure 15.** (a) Absolute broadening of slab in the first strand of stagnant slab; (b) Absolute broadening of slab in the second strand of stagnant slab.( FU JianXun et al.2011(a))

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 12.6 m, the slab broadening was at its maximum, and then decreased slowly with distance from the meniscus. These results confirm that the stress of the roller is not the main reason for broadening. Otherwise, the slab broadening will happen before the slab is fully solidified near the 9th and 10th segments. The trend of slab broadening is consistent with that of the static pressure of molten steel, which confirms that the slab broadening is dependent on the static pressure of molten steel.
