Morphology of Transverse Cracks at the Corner of Slabs
The location of corner cracks in slabs varies depending on the steel grade. For most steel grades, corner cracks are distributed on the inner curved surface of the slab. For some steel grades, corner cracks are distributed on two narrow faces near the inner curved surface. For others, corner cracks appear on the outer curved surface. For some steel grades with severe abnormalities, cracks appear on both the inner and outer curved surfaces and on the two narrow faces. For still others, corner cracks appear as longitudinal cracks along the casting direction. The vast majority of corner cracks in low-carbon steel slabs occur near the corner on the inner curved surface and are generally called transverse corner cracks or corner fissures.
Figure 1. Corner Transverse Cracks
The occurrence of corner cracks in slabs is closely related to the steel grade. Low-carbon Nb and V microalloyed steel slabs are particularly prone to corner cracks. The main corner crack morphologies are shown in Figure 1. Longitudinal corner cracks are generally caused by deviations in the arrangement of the short-side copper plates and foot rolls in the crystallizer, as well as uneven cooling along the narrow side of the crystallizer. Ensuring the precision of the crystallizer equipment and the uniformity of the primary cooling water can largely eliminate longitudinal corner cracks. Transverse corner cracks are relatively easy to occur in low-carbon, medium-carbon, and high-carbon steels, especially in low-carbon microalloyed steels. The severity of transverse corner crack defects varies among different steel grades, with differences in the number, length, and depth of cracks at the slab corners.
Characteristics of Corner Transverse Cracks in Slabs
The distribution characteristics of corner transverse crack defects in continuously cast slabs can be measured by the following aspects: the location of the corner transverse cracks on the slab, such as the upper surface, lower surface, subsurface, or surface; the crack opening degree; and the crack length and depth. Based on these characteristics, the degree of corner cracking in slabs is generally classified into three categories.
For steel grades with a carbon content below 0.1% and no Nb or V microalloying elements, the transverse corner cracks in the slab are generally mild. They are mainly distributed within 50mm of the edge on the upper surface. Because the cracks are mild and generally distributed within 10mm below the surface, with a crack opening of less than 2mm and a maximum length of no more than 30mm, they are usually not directly visible on the surface. They can only be discovered after manually removing the surface oxide slag from the corner of the slab. The number of cracks is relatively small, generally about 2-3 per meter on one side along the casting length. For this type of defect, since there is no significant extension in the depth direction, the defect can usually be removed by flame cleaning.
Steel grades with a carbon content between 0.1% and 0.2% and no Nb or V microalloying elements exhibit severe transverse corner cracking defects in slabs. Obvious cracks are typically found at the corners of the slab, and milder cracks perpendicular to the casting direction are usually present on narrow faces of the slab. The cracks are densely distributed, typically approximately 10-20 mm per meter on each side along the casting length. The depth of the cracks generally does not exceed 20 mm, and the transverse crack length along the slab length is generally within 50 mm. Since the crack propagation in the width and thickness directions of the slab is not severe, the defects can usually be eliminated by corner cutting or flame cleaning.
Steel grades with a carbon content between 0.09% and 0.16% and containing Nb and V microalloying elements exhibit even more severe transverse corner cracking defects in slabs under the same equipment and process conditions. Typically, these transverse corner cracks on slabs are very densely distributed at the corners of the cast slab. Most cracks extend from the top surface to the side, and in severe cases, cracks can be found on both the top and bottom corners, sometimes even extending very long along the width of the slab. Most slabs require thorough cleaning of the corners and sides to eliminate these corner cracks.
FAQ
Why do corner transverse cracks always occur at the bottom of “oscillation marks”?
Answer: This is a typical stress concentration phenomenon.
Geometric factors: Oscillation marks are periodic ripples left by the vibration of the crystallizer, and their roots (the bottom of the V-shape) are natural stress concentration points.
Microstructure factors: The billet shell at the bottom of the oscillation mark cools fastest in the crystallizer, easily forming coarse columnar crystals or austenite grains.
Consequence: Under the action of straightening stress or thermal stress, these fragile “V”-shaped grooves are the first to fracture, forming transverse cracks.
How do microalloying elements (such as Nb, V, Ti) induce corner transverse cracks?
Answer: These types of steel (such as high-strength low-alloy steel) experience a third type of brittle zone at high temperatures (usually between 700℃ and 900℃).
Precipitation effect: Carbonitrides of niobium, vanadium, and titanium will disperse and precipitate at the austenite grain boundaries. These tiny precipitates hinder grain boundary sliding, significantly reducing the elongation (plasticity) of molten steel.
Crack Formation: If the corner temperature of the slab falls within this brittle range during straightening, the grain boundaries will crack due to their inability to withstand deformation stress.
Why does the “Chamfered Mold” technology effectively reduce transverse corner cracks?
A: Traditional right-angle molds cause the slab corners to be subjected to intense cooling in two directions (two-sided heat dissipation), resulting in excessively rapid temperature drops at the corners.
Heat Dissipation Optimization: By changing the corner geometry, the chamfered mold transforms “two-sided heat dissipation” into a more uniform heat transfer mode, slowing down the corner cooling rate.
Stress Distribution: The chamfered design makes the thermal stress distribution at the corners more uniform, avoiding excessive stress concentration at sharp corners, thereby inhibiting crack formation.
