Since the first half of 2018, the crystallizer copper plate has experienced more local copper anomalies, mainly reflected in the failure of the copper plate near the lower inlet, which has an oval shape and is more concentrated at the cooling water inlet nozzle of the copper plate. There are both wide and narrow sides, and the steel passing capacity is 40,000 to 50,000 tons, which is far lower than the agreed steel passing target. Since no changes were found in the copper plate manufacturer’s plating material and process, it was initially determined through communication with the copper plate manufacturer and on-site analysis that it was caused by corrosion of the copper plate.
Keywords: continuous casting mold; copper plate, corrosion
Mechanism of copper plate corrosion
This type of corrosion of copper plates is defined as galvanic corrosion in electrochemical corrosion. It is a kind of corrosion that inevitably occurs when dissimilar metals are in direct contact and in the presence of electrolyte. During the continuous casting process, there is a certain gap between the casting slab and the copper plate at the lower port, and the water vapor generated by the lower spray can easily penetrate into the gap, causing a corrosion cell to form between the casting slab and the copper plate. This corrosion galvanic cell uses the copper plate coating as the anode and the cast blank as the cathode. The anode dissolves, so corrosion occurs.
Analysis of the causes of gaps
This type of corrosion is caused by the existence of a certain gap between the cast slab and the copper plate, and the water vapor generated by the lower spray entering the gap. This article mainly analyzes the reasons for the gap between the lower opening of the copper plate from the aspect of equipment. It is verified by on-site analysis that the gap between the lower opening of the copper plate and the casting billet is mainly caused by the following four reasons.
The shrinkage of the wide side foot roller of the crystallizer is too large
When the foot roller is centered, the wide foot roller is higher than the bottom of the crystallizer (that is, the opening of the wide foot roller is smaller than the width of the lower opening of the narrow copper plate of the crystallizer). The opening of the wide-side foot roller of Meishan Steel’s Voestalpine continuous casting machine has shrunk by 1 mm relative to the lower opening of the mold, so the wide-side foot roller of the mold is 0.5mm higher. Since the outer arc of the mold is aligned through the template, and the inner arc is aligned through the opening, taking into account the problem of the corner seam of the mold, the actual value of the wide-side foot roller of the inner arc is higher than the bottom of the crystallizer.
The flatness of the mold copper plate is out of tolerance
In addition to the copper plate itself, the flatness of the copper plate is also affected by the back plate and the mold frame. If the local flatness error is large after the copper plate is assembled, it will affect the local protrusion of the wide-side foot roller.
The straightness and runout values of the wide-side foot roller of the crystallizer are out of tolerance.
Excessive straightness and runout values can also cause excessive local roller protrusion and create gaps. On-site inspection of the foot rollers of the off-line crystallizer revealed that the straightness and runout values of some rollers exceeded the standard, and some rollers exceeded 0.2mm. After disassembly and inspection, it was found that it was mainly caused by the deformation of the roller core shaft.
Unreasonable design of copper plate cooling water tank
The unreasonable design of the copper plate cooling water tank causes uneven temperature distribution in the width direction of the copper plate, causing local deformation of the copper plate. Due to the low cooling intensity at the stud bolts of the copper plate, thermal analysis shows that the temperature is nearly 30°C higher than that at the adjacent water tank, resulting in inconsistent thermal deformation in the width direction of the copper plate. The low-temperature part is far away from the casting gap.
On-site improvement measures
This article mainly makes the following improvement measures from the perspective of how to reduce the gap between the lower opening of the mold copper plate and the slab and reduce the entry of water vapor.
Reduce water vapor from entering the gap between the wide-side foot roller and the copper plate
Since the gap between the lower opening of the copper plate and the slab is difficult to completely eliminate, it is necessary to minimize the entry of water vapor into the gap. Install a water baffle at the bottom of the wide-side copper plate backing plate of the crystallizer. The thickness of the water baffle is about 10 mm to reduce some water vapor from entering the lower opening of the copper plate. After the crystallizer is assembled, check the spray angle of the foot roller. Since the spray angle is determined by the installation position of the spray rack, it can be fine-tuned on site. Lowering the angle of the spray rack as much as possible without the nozzle rolling can also reduce the entry of water vapor. In addition to the angle of the spray frame, the angle of the individual nozzles is equally important. On-site inspection found that the water seam installation of some nozzles was not horizontal. The mouth of the nozzle was tilted. The high point of the tilt corresponded to the high point of corrosion of the copper plate. In order to make the mouth of the nozzle straight when installed. Positioning grooves are added to the nozzle and spray frame to ensure that the water seam of the nozzle is straight.
Improve the positioning accuracy of the wide-side foot roller of the crystallizer
At present, the wide-side foot rollers on the outer arc side of the crystallizer are aligned using a template, and the inner arc side is aligned using an inner micrometer. The ideal state of the wide-side foot roller of the mold is that the inner and outer arcs shrink the same amount relative to the lower mouth of the mold. However, it is difficult to achieve this arc alignment method at present. Based on this idea, a positioning device for the wide-side foot roller of the crystallizer is designed (see Figure 1). This device can be used to position the wide-side foot roller of the crystallizer and detect the flatness and shape of the copper plate of the crystallizer after installation. The device is mainly composed of the internal shape simulation block of the crystallizer, the wide-side foot roller positioning block, the left connecting plate, the right connecting plate, the horizontal plate and other components. The slab produced by continuous casting is determined by the internal shape of the crystallizer, which is determined by the combination of 4 copper plates (2 wide-side and 2 narrow-side copper plates). The two wide-side copper plates are two flat surfaces. The sides of the two narrow-side copper plates are in close contact with the wide-side copper plates. The outer dimensions of the narrow-side copper plates determine the thickness of the slab. The distance between the two narrow-side copper plates determines the thickness of the slab. width. Therefore, the internal shape of the crystallizer is mainly determined by the shape of the narrow-side copper plate of the crystallizer. Design a simulation block for the internal shape of the crystallizer. The cross-sectional shape of the simulation block completely matches the lower 180 mm part of the narrow-side copper plate of the crystallizer. The processing accuracy is slightly higher than that of the narrow-edge copper plate. The processing dimensional tolerance is negative to ensure that the simulation block can smoothly enter the interior of the crystallizer. The length of the simulation block is 1000 mm. The spatial dimensions of the crystallizer after assembly can be easily detected through the internal shape simulation block of the crystallizer, providing data support for overhaul of the crystallizer frame. Two wide-side foot roller positioning blocks are welded to the bottom of the internal shape simulation block of the crystallizer. A positioning reference plane is processed on the positioning blocks. The positioning of the wide-side foot roller of the crystallizer can be achieved through spacers and positioning plates. The positioning amount can be adjusted through gaskets according to process requirements, and the positioning is fast and accurate. A horizontal plate is welded at the upper end of the connection between the two pieces. The distance between the lower surface of the horizontal plate and the lower surface of the internal shape simulation block of the crystallizer is 900mm, which is opposite to the height of the narrow-side copper plate of the crystallizer. After the horizontal plate is in contact with the surface of the narrow-side copper plate of the crystallizer, the automatic positioning of the wide-side foot roller of the crystallizer can be started. The process of positioning the wide-side foot roller of the crystallizer:
1) Fine-tuning the transverse horizontality of the horizontal plate through fine-tuning screws, fixing the device and longitudinal horizontal fine-tuning through fixing bolts and fixing the device to the wide-side copper plate of the crystallizer;
2) Use a feeler gauge to detect the gap between the internal shape simulation block of the crystallizer and the inner and outer arc copper plates of the crystallizer, and judge the shape accuracy of the internal space of the crystallizer through the gap;
3) Push the wide-side foot roller to the positioning plate and stick it tightly. Use a feeler gauge to check the straightness of the wide-side foot roller. If the straightness exceeds the standard, replace the wide-side foot roller;
4) If the straightness of the wide-side foot roller meets the requirements, directly tighten the wide-side foot roller connecting bolt to complete the positioning of the wide-side foot roller. After completing the positioning of the wide-side foot roller on one side, use the same steps to complete the positioning of the wide-side foot roller on the other side.
Figure 1 Crystallizer wide side foot roller positioning device
Optimize the copper plate cooling water seam
At present, Meishan Steel’s wide-edge copper plates have straight water seams. The main drawback of this water seam is that the distance between the two water seams at the stud bolts is large and the cooling is not even enough. After on-site inspection, it was found that the center of the copper plate where the lumpy wear occurred was not only related to the position of the nozzle, but also had a certain correlation with the position of the copper plate, and was often located in the middle of the two stud bolts. In order to improve the cooling condition of the copper plate of the crystallizer and make the heat distribution on the surface of the copper plate more uniform, the structure of the wide-side copper plate of the crystallizer was improved, and the water seam design of the copper plate of the crystallizer was optimized. The original straight water seam was changed to a curved water seam. The original water seam diagram of the copper plate and the optimized water seam diagram are shown in Figure 2 and Figure 3. The beneficial effect of the curved water seam is that the wide-surface copper plate of the mold has higher cooling intensity, while improving the cooling uniformity at the meniscus and bolts, ensuring rapid heat transfer between the copper plate and the high-temperature casting billet, and achieving The uniformity of heat exchange ensures that the heat load of the copper plate is uniform, thereby achieving the purpose of improving the quality of the slab, extending the service life of the copper plate and reducing production costs.
Figure 2 The original water seam of the copper plate
Figure 3 Water seam after optimization of copper plate
Through the above improvement measures, the corrosion problem of the crystallizer copper plate has been significantly improved. The steel throughput of the crystallizer has been increased to about 100,000 tons, but it is still far from the target of 15 tons.
Since copper exposure on copper plates is generally not caused by a single corrosion, but may be caused by wear and corrosion at the same time, the next step is to try improvement measures in aspects such as adjusting the composition of the copper plate coating and optimizing the on-site process, so that the crystallizer can reach the target steel throughput as soon as possible.