Cause Analysis and Improvement of Hornet's Honeycomb Cracks in Class E Ship Plates

In order to analyze the cause of the hornet's honeycomb cracks on the surface of the steel plate after the continuous casting slab of the E -class ship plate steel was rolled , the chemical composition and metallographic characteristics of the cracks and the nearby parts of the E -class ship plate were analyzed by spectrometer, metallographic microscope, scanning electron microscope and energy dispersive spectrometer . The microstructure and microscopic morphology were tested and analyzed. The results show that subcutaneous air bubbles or tiny cracks already exist in the continuous casting slab, and brittle secondary oxidation particles are formed after heating. In the subsequent rolling process, the defect area cannot participate in simultaneous deformation, and the cracks not only cannot be rolled together, but the opening expands and expands. , eventually forming hornet-like cracks. By improving the quality of the continuous casting slab, the hornet's honeycomb cracks are significantly improved, and the quality of the steel plate is improved.
【Key words】 E -class ship plate; hornet's honeycomb crack; subcutaneous air bubbles; secondary oxidation particles

E -class ship plate is a relatively common defect in the production process of steel ship plate. There are many types of crack defects on the surface of the steel plate, such as large longitudinal cracks, small longitudinal cracks, transverse cracks, network cracks, etc. Although hornet’s honeycomb cracks are not very common, serious cracks that cannot be repaired can directly lead to the degradation of the steel plate or even the scrapping of the steel plate. It affects the execution of production plan and the delivery of orders, and increases the quality loss. Therefore, it is of great significance to realize the continuous and stable production of ship plates and improve product benefits to study the causes of hornet's honeycomb cracks and take targeted measures to control them.

• Crack Macroscopic Morphology

By observing the macroscopic morphology of the two hornet's honeycomb cracks on the E3 ship plate, the distribution is very irregular, generally with larger openings and smaller crack lengths.

• crack microscopic characteristics
• E Ship Plate Composition Inspection

of E -class ship plates was removed on site, and the steel plates with defects were cut into small samples by sawing machine, and then polished by automatic grinding and polishing equipment, and the samples after grinding and polishing were processed by direct reading spectrometer Composition detection was carried out, and the test results showed that the smelting composition inquired about the steelmaking process according to the furnace number was basically consistent with the laboratory spectral analysis composition, indicating that there was no abnormal change in the composition near the crack.

• Sample metallographic analysis

Cut along the rolling direction at the hornet's honeycomb crack of the steel plate, mechanically grind, polish and corrode the cross-section parallel to the rolling direction, and observe the metallographic structure. The metallographic structure of E3 ship plate is ferrite + pearlite structure, pearlite The body is distributed in bands. The hornet's honeycomb crack has a large opening, extending from the surface into a trumpet shape, and a severe decarburization zone can be observed on the side of the crack near the surface, while on the other side of the crack, there is only slight decarburization near the crack, and it can be observed A small amount of pearlite structure. The crack openings are filled with gray oxide. Observing another deeper hornet's honeycomb crack and its surrounding metallographic structure, there is a slight decarburization on both sides of the crack, and there are obvious differences in the structure of the two sides. The pearlite on the side of the crack near the surface The structure is diffusely distributed, and the ferrite grains are equiaxed, showing signs of growth; while in the matrix on the other side of the crack, the pearlite is distributed in a distinct band, and the crack opening and crack propagation direction are parallel to the band-shaped pearlite. The distribution direction of body tissue.

• Sample Scanning Electron Microscopy Analysis

Scanning electron microscope analysis and energy spectrum analysis were carried out on the crack and around the crack of the sample. It can be seen that there are granular substances in the crack and its vicinity. The energy spectrum analysis of the particles shows that these granular substances are mainly oxide particles containing Si and Mn . In addition, it can be found that the density and size of oxide particles near the crack are significantly higher than those near the crack.
The formation of secondary oxidation particles must meet three conditions: First, the material is in a high temperature state, which should be above 900 for steel ; second, the matrix is first decarburized and has an oxygen atmosphere; third, it must have a long time , if the time is short, even at high temperature, iron oxide scale can only be formed on the surface layer, and secondary oxide particles cannot be regenerated under the iron oxide scale [1-2] . The specific reaction process is: during the high-temperature heating process, the oxide skin on the surface of the crack undergoes a reduction reaction, the oxygen in the iron oxide is replaced, and the carbon in the matrix reacts with the replaced oxygen or the oxygen infiltrated from the outside to form carbon dioxide or carbon monoxide. At this time, the oxygen content in the furnace is extremely low, and there is a relatively anoxic environment. After decarburization, Si and Mn in the matrix will preferentially react with a small amount of oxygen to form secondary oxidation particles. The formation of secondary oxidation particles is a relatively slow process, which requires sufficient time. In addition, the surrounding oxygen content should not be too high, otherwise a protective film of iron oxide will be formed on the surface quickly to prevent the entry of oxygen, and secondary oxidation particles cannot be produced .

• Analysis and Discussion of Class E Ship Plate

According to the metallographic morphology of the two different cracks, it can be observed that there is a serious decarburization zone near the surface of the crack, and there is a serious decarburization phenomenon at the opening, which should be the crack in the slab. In the case of cracking during rolling, the decarburized layer around the crack should only exist on the surface of the crack. There are obvious differences in the morphology of the two sides of the crack. The pearlite structure on the side of the crack near the surface is diffusely distributed, while the other side of the crack has a complete banded structure, and the crack opening and propagation direction are similar to the banded pearlite structure. Parallel, which means that the cracks already exist before the banded structure is formed. It is precisely because of the cracks that have existed for a long time that the stress state of the cracks in the rolling process is changed, and the crack openings are no longer subjected to tensile stress parallel to the rolling direction, so the banded structure near the surface is not obvious; For the cracking during rolling, the banded structure already exists during rolling, and the banded structure at the crack opening should be approximately perpendicular to the crack, rather than the observed approximately parallel relationship.
From the scanning electron microscope and energy spectrum analysis at and around the crack, it is observed that a large number of secondary oxidation particles exist around the crack. Although the rolling temperature is high, the duration is short, and there is no condition for the formation of subcutaneous secondary oxidation particles. The rolling process The iron oxide protective film is formed rapidly on the surface of the medium steel plate, and a large amount of secondary oxidation particles will not be produced during the rolling process.
the above analysis, it can be seen that the hornet's honeycomb cracks in Class E ship plates did not occur during the rolling process, but already existed in the slab. Since no mold flux components were observed at the opening of the crack, there are two possible causes of the crack: one is that very fine cracks are generated in the crystallizer, or exist in the form of bubbles [3] without involving the mold flux, After high-temperature heating in the steel rolling heating furnace, a large number of brittle secondary oxide particles and iron oxide scales in the cracks are formed. During the rolling process, the cracks cannot be welded, but the openings expand and expand; thermal cracks in the.

• improvement measures

1)Strengthen the refining method to further control the content of O , N , H and other gases in the steel. Control the entry of H from the source , ensure that the raw materials used are used after drying, strengthen the protective pouring of LF , VD , continuous casting and other links, and avoid air inhalation.
2)Avoid unsteady pouring in the continuous casting process, and do a good job of off-line trial cleaning for unsteady billets.
3)Strengthen the control of the continuous casting process, avoid nozzle clogging, and make the slab overcooled evenly.
4)To ensure a good running state of the casting machine equipment, strengthen the maintenance of the bending section and the straightening section, and ensure the alignment and opening.
5 Epilogue
E -class ship plate are not produced in the rolling process, but already exist in the slab. The continuous casting slab already has subcutaneous air bubbles or tiny cracks, which form brittle secondary oxidation particles after heating. During the subsequent rolling process, the defect area cannot participate in simultaneous deformation. Hornet's nest of cracks. By strengthening the process control of continuous casting and refining, high-quality continuous casting slabs were obtained, which reduced the honeycomb cracks of Jinan Iron and Steel Co. The timely delivery of the E -class ship plate provides guarantee.

---