Cracks caused by developed columnar crystals on the billet surface and serious burning

Crack Formation in Hot-rolled Slab Surface due to Heavy Pebble Formation

Cracks in hot-rolled slab surface due to heavily developed pebbles can be attributed to several factors, including the mechanical deformation of the steel during the rolling process, metallurgical changes in the steel, and the onset of oxidation. The mechanical deformation of the steel during the rolling process results in the reduction of the exposed surface area, which causes the pebbles to become more densely packed. This compaction of the steel on the slab surface increases the risk of cracks due to the greater stress on the slab surface.

The metallurgical changes in the steel caused by the rolling process can cause significant changes in the surface microstructure, such as a decrease in the percentage of austenite-cemented grains. This decrease in the austenite-cemented grains can lead to a decrease in creep strength, which can then cause the steel slab to become susceptible to cracking. The changes in the surface microstructure also cause a decrease in surface hardness, which increases the risk of cracks due to increased local yielding in the steel.

The onset of oxidation due to prolonged exposure of the steel slab to high temperatures during the rolling process can also lead to the formation of cracks. Oxidation produces an increase in the volume of the steel surface, which can increase the strain on the steel slab and thus increase the risk of cracking. Oxidation also changes the overall surface chemistry, which can lead to the formation of oxides on the surface of the steel slab. These oxides can act as stress concentrators at the grain boundaries of the steel slab and increase the likelihood of slab cracking.

The combination of these various factors can produce cracks in the hot-rolled slab surface due to heavily developed pebbles. In order to prevent such cracking in the slab surface, it is important to ensure that the steel has undergone sufficient mechanical deformation during the rolling process, that the exposed surface area of the steel is sufficiently reduced, and that the steel is sufficiently cooled during the rolling process. Additionally, it is important to minimize the amount of oxidation occurring on the steel slab, either by minimizing the exposure time of the steel slab to high temperatures or by applying a protective coating to the hot-rolled slab surface. These measures can help minimize the risk of cracking due to heavily developed pebbles in the hot-rolled slab surface.

Introduction

Steel billet surface columnar grains are burned and cracked seriously in the application of continuous casting. The crack phenomenon is known as “turtle crack”. It is named for the surface of the steel billet after being torn apart by the serious burning.

Cause of the Turtle Crack

Turtle crack is mostly caused by inadequate cooling of the continuous casting billet. The surface of the billet is cooled quickly and severely, while the interior of the billet is cooled slowly and only slightly. The rapid cooling will cause the columnar crystal grains on the surface of the billet to shrink dramatically and expand inwardly, while the slow cooling makes the inner crystal grains difficult to shrink, forming a great pressure to cause the billet to crack. Besides, if the cooling water used is not clean enough, material attachment may occur. This can also cause turtle cracking.

Precautions

To prevent turtle cracking, the most important thing is to ensure the cooling water used is clean and without clogging. When continuous casting, it is necessary to make the section of billet smaller, reduce the speed of casting in order to prevent the too quickly cooling so as to reduce the probability of cracking. Meanwhile, it is necessary to reduce the shrinkage caused by too serious pulling, reduce the speed of the crystallization and properly increase the thickness of the billet.

Conclusion

Turtle cracking easily occur in continuous casting. In order to avoid this problem, it is necessary to pay attention to the cleanness of the cooling water, reduce the section of the billet, reduce the speed of casting, reduce the shrinkage caused by too-hard pulling and properly increase the thickness of the billet.