Rare crack morphology exist: crack parts of the tube on the surface, the appearance of cracks is one of the rare defects in the tube production in the S355. Dendritic, intermittent appearance oxidation. Further microscopic cracks within the iron oxide, black or dark gray.
Analysis: the main reason of crack for the smelting, such as the inclusion aggregation, composition segregation, casting exception. Casting billet appearance of oscillation marks at the crack and corner crack is the main reason for the appearance of cracks in tube; In addition, the peritectic solidification of the molten steel will result in uneven solidified shell and copper wall contact heat transfer mitigation, uneven solidified shell, the local thin point source of cracks occurred in; mold level fluctuation, surface instability, but also makes casting billet appearance of oscillation marks intensified, resulting in the subsequent rolling process of crack.
To this end, the corresponding control measures S355 the tube surface cracks: a C content of the control of large diameter thick-walled tube. Large diameter thick-walled tube to take place in the high-temperature solidification process of the peritectic transformation, volume shrinkage occurs, said outlet contraction. WC 0.10% -0.17%, the average linear shrinkage of 25 ° C below the solidus temperature significantly increased, leading to mold air gap formed between the primary solidified shell and copper wall. Therefore, it should control the composition of large diameter thick-walled tube smelting, to ensure that wC> 0.17% to reduce the stress of contraction of peritectic transformation. An appropriate increase in Mn content, adhere to the Mn / S ratio, thus effectively reducing the slab exterior crack. Optimize the device parameters, to reduce the mold level fluctuation. Timely maintenance of the mold level braking system problems, and further optimize the device parameters, the amplitude of the mold is controlled within ± 2.5mm, and adhere to the continuous casting machine casting speed constant, reducing mold level shake. Optimize maintenance the slag parameters to ensure the maintenance of slag has good mobility, increase the heat transfer capability of the maintenance of slag, to reduce the mold within the air gap formation, slowing the slab surface oscillation marks occur.