The crystallizer is the core of the continuous casting machine. It is a device that injects molten steel into it, and after cooling, the outer layer of the molten steel forms a continuous hard shell to form a cast slab, and the cast slab is gradually pulled out. The copper plates in the crystallizer are prone to thermal cracks, wear, and corrosion due to the working conditions they are used. Improving the high-temperature hardness, wear resistance, and thermal fatigue resistance, and extending the service life of the crystallizer copper plate surface is an extremely important research topic in the current metallurgical industry.

Copperplate crystallizer background

The crystallizer is the core of the continuous casting machine. It is a device that injects molten steel into it, and after cooling, the outer layer of the molten steel forms a continuous hard shell to form a cast slab, and the cast slab is gradually pulled out. The copper plates in the crystallizer are prone to thermal cracks, wear, and corrosion due to the working conditions they are used. Improving the high-temperature hardness, wear resistance, and thermal fatigue resistance, and extending the service life of the crystallizer copper plate surface is an extremely important research topic in the current metallurgical industry.

At present, large domestic steel mills and research institutions prepare wear-resistant coatings on the surface of mold copper plates through surface treatment methods such as electroplating and thermal spraying to improve their wear resistance. Although these process methods are relatively low-cost, the coating and the substrate are mechanically bonded, the bonding force is weak, the coating thickness is limited, and defects such as peeling off and cracks are prone to occur during use, and the surface strengthening effect is not ideal. Laser cladding technology can be used to prepare a wear-resistant coating that is metallurgically bonded with the copper plate. The coating has a strong bonding force, controllable coating thickness, longer service life, and no pollution to the environment. Therefore, laser cladding technology is used to crystallize the coating. Surface strengthening of copper plates has good benefits.

Laser cladding process

1. Technical difficulties

• Copper has excellent thermal conductivity, and it is difficult to form a molten pool whether it is ordinary welding or laser cladding.
• Copper materials, especially red copper materials, are highly reflective to lasers, with a reflectivity of more than 80% for lasers of general wavelengths.
• Coating materials are difficult to fully meet the technical requirements for copper plate work.
• Coating crack control is difficult.

The cladding powder is HR-Ni-S14 (Ni60), and the ingredients are shown in Table 1.

Copperplate: chromium zirconium copper plate.

Equipment: fiber laser equipment, three-point nozzle, spot 3-5mm, powder utilization rate 65-75%.

The process parameters are shown in Table 3.

2. Test results

1) Surface forming:
Cladding is performed on the copper plate, and the cladding-forming effect is shown in Figure 3.27. The cladding surface is well formed, and flat, with small fluctuations, no obvious powder staining, and good wettability, and there are no defects such as cracks on the colored flaw detection surface.

2) Metallographic examination:
After cladding, cut a 25mm long sample on a plane perpendicular to the cladding direction for metallographic examination, as shown in Figure 3.28. From a macroscopic observation, the surface undulations are small, the cladding layer has a certain thickness, the cladding interface is relatively clear and wavy, and the penetration depth is present, which is proof of the metallurgical combination of the cladding layer and the copper plate. From microscopic observation, the cladding layer is well combined with the base material, and the interface is blurred. The two materials are embedded and melted into each other to a certain extent, which is a good metallurgical combination. The cladding layer is dense and defect-free, with a thickness of 0.5-0.7mm.

3)Microhardness testing:
Use a Vickers hardness tester with a load of 0.2kg to detect the hardness of the base material and cladding layer, as shown in Figure 3.10. A total of 8 points are tested from the base material to the heat-affected zone and the cladding layer. The test results are shown in Table 4.

4) Thermal shock test

The thermal shock test was carried out according to the national standard GB/T 5270-2005/ISO2819:1980, and two cooling methods were used in the test. The first method is to heat it to 250°C and keep it for 10 minutes, then immerse the side of the sample without the cladding layer in water, but not to the cladding layer, wait until it is completely cooled to water temperature, then take it out and reheat, repeat 10 times in total. After completion, another cooling method is performed. The difference from the previous 10 times is that after the sample is heated, the entire sample including the cladding layer is immersed in water and quenched. After completion, the coloring flaw detection method is used to detect whether there are any cracks on the surface.

The flaw detection results are good, with no cracks, peeling and other defects.

3. Application

The use of laser cladding technology to produce wear-resistant coatings for crystallizer copper plates is an emerging strengthening technology. Compared with other strengthening methods, it has great advantages in coating performance, lifespan, environmental protection, process conditions, material selection, etc. It can improve the surface properties of the mold to a great extent, and achieve the purpose of improving the quality of continuous casting billets, extending the life of the mold, and reducing production costs.