This article describes the morphology characteristics, formation mechanism and main factors of longitudinal cracks on the surface of continuous casting slabs.
Key words; continuous casting; longitudinal crack; mechanism
Morphology and characteristics of longitudinal cracks
Since mold slag film and iron oxide scale are often present on the surface of the slab, small longitudinal cracks are difficult to observe with the naked eye, and coarse longitudinal cracks are easy to identify. Surface longitudinal cracks are divided into surface groove longitudinal cracks and surface flat longitudinal cracks. Surface groove longitudinal cracks refer to longitudinal cracks with grooves on the surface. The cracks are thicker and longer, and are generally filled with slag, as shown in Figure 1. It is easy to find on the surface of the cold billet;
Fig.1 Hook and groove longitudinal crack
Flat longitudinal cracks refer to straight, thinner, shorter longitudinal cracks with a smooth surface, which are difficult to find when the billet is cold, and generally need to be pickled to see the surface, as shown in Figure 2.
Figure 2 Flat longitudinal crack
Mechanism of Longitudinal Crack Formation
Studies have shown that the longitudinal cracks on the surface of the slab are due to the uneven cooling of the mold, the uneven thickness of the primary slab shell, the stress concentration in the thin part of the slab shell, and the stress on the slab shell exceeds the tensile strength of the slab shell near the meniscus. Micro cracks are formed, and after entering the secondary cooling zone, the tiny cracks expand into obvious longitudinal cracks.
It is considered that the main reason for groove cracks is that the thickness of the primary billet shell in the crystallizer is not uniform due to factors such as steel characteristics, equipment, technology, and operation, and stress concentration occurs in the thin shell; while the main cause of flat cracks is casting The bulging of the billet destroys the stress balance and forms tiny cracks in the weak part of the billet shell.
The main factors affecting the occurrence of longitudinal fissures
The root cause of longitudinal cracks is the uneven thickness of the primary shell in the crystallizer, and the main reason for the uneven shell at the meniscus is:
(1) Peritectic transformation occurs.
(2) Equipment and process factors affect the uneven growth of the billet shell.
The copper plate of the crystallizer has pits, scratches, cracks, or the coating is worn or even peeled off seriously in some parts during use, as shown in Figure 3, resulting in the primary shell of the copper plate in the mold not being in good contact with the copper plate at the defective part. The effect is poor, the thickness of the billet shell is uneven, and longitudinal cracks are easy to occur at the corresponding positions.
Fig. 3 Condition of the inner wall copper plate of the crystallizer with severe longitudinal cracks
Measures to reduce longitudinal cracks on slab surface
The reasons for cracks in continuous casting slabs are very complicated. To prevent longitudinal cracks, it is necessary to ensure that the thickness of the primary slab shell at the meniscus of the crystallizer is uniform. Among them, the high-temperature mechanical properties determined by the chemical composition of steel are the internal causes of cracks, and the equipment, process conditions and operating level are the external causes of cracks. From the above analysis, the measures that are conducive to reducing the longitudinal cracks on the surface of the slab mainly include: :
(1) On the premise that the composition of the steel grade is allowed, [C]≥0.15% in the steel is controlled to widen the two-phase region during the solidification of molten steel and avoid the scope of the peritectic region.
(2) Make sure that the mold copper plate of the online crystallizer has no obvious abnormality and the coating is intact.
(3) Improve the accuracy of casting machine maintenance to ensure the casting machine is in good condition.
(4) Ensure that the ladle temperature of the molten steel is within the target range and have good castability, creating conditions for continuous casting at constant speed.
(5) Ensure that the flow field in the crystallizer is reasonable and stable, and the submerged depth of the tundish nozzle is controlled at 100-150 mm.
(6) Cooperate with the manufacturer to conduct optimization tests on the existing mold powder for different steel types in different sections.