The casting billet with a liquid core coming out of the mold, under the action of bending, straightening or wheel pressure, produces cracks at the solidifying, very fragile solid-liquid interface, called internal cracks. Such internal cracks can be severely visible to the naked eye through acid leaching and sulfur mark tests of cast billet samples.
Internal cracks can be divided into the following types:
(1) Straighten cracks.
It is caused by the deformation of the cast slab with liquid core during straightening that exceeds the allowable deformation rate. This kind of crack can be eliminated by multi-point bending straightening and compression casting technology.
(2) Depress cracks.
It is due to the excessive pressure of the pull-straightening wheel that causes cracks in the solid-liquid two-phase zone of the solidified slab. Such cracks can be prevented by means of hydraulically controlled tension and straightening mechanisms or setting limit blocks.
(3) Intermediate cracks.
The main change is caused by the thermal stress caused by the uneven cooling and the large temperature rise when the cast slab passes through the secondary cooling zone. In addition, this kind of cracks can also be caused by the bulging of the billet or the improperly acting on the solid-liquid interface of the arc.
(4) Corner cracks.
It is caused by the deformation stress caused by uneven cooling of the finisher, acting on the corners of the cast slab. Such cracks can be prevented if the cooling inside the crystallizer is as uniform as possible.
(5) Subcutaneous cracks.
The small cracks within 3~10mm from the surface of the cast slab are mainly caused by multiple phase transitions due to repeated changes in the surface temperature of the cast slab, and the cracks are formed along the interface of the two structures.
(6) Central crack.
The visible gap in the center of the cross section of the slab is accompanied by positive segregation of P and S in the center. He was caused by the bulging of the billet at the end of solidification.
(7) Star-shaped cracks.
the cracks in the center of the billet’s cross-section are radial. The billet cools strongly in the secondary cooling zone, and then the temperature rises to cause the solidified layer to swell, causing the central viscous zone of the billet to be damaged by tensile stress .
(8) Diagonal cracks.
Cracks produced along the diagonal of the cross-section of the billet. This is caused by uneven secondary cooling, which causes the casting to be distorted. Preventing slab rhombohedral deformation can eliminate this kind of cracks.