Progress in continuous casting mold copper plate and surface treatment technology

Abstract: The crystallizer is a key equipment for continuous casting of steel billets. Its design and manufacturing quality directly affect the normality and stability of continuous casting production. This article summarizes and analyzes the current development status of copper plate materials and copper plate material surface treatment technologies used in continuous casting molds. It is pointed out that in view of the fact that the narrow-faced copper plate of the slab mold is prone to deformation and wear at high temperatures, dispersion-strengthened copper materials with high strength and high thermal conductivity can be used to extend the maintenance cycle of the mold and improve production efficiency. At the same time, based on the advantages and disadvantages of the existing crystallizer copper plate surface modification technology, new alloy coating and plating technologies are developed to further improve the hardness, wear resistance and corrosion resistance of the coating and plating layers.

Keywords: continuous casting mold; copper alloy plate; dispersion strengthened copper; surface technology; coating

Continuous casting is currently the most commonly used method for billet production. The copper mould tube is the core component in continuous casting production. Its purpose is to shape the poured molten steel and generate a solid shell of sufficient thickness so that the ingot belt will not cause steel running when it moves to the secondary cooling zone. In continuous casting It plays the role of solidifying molten steel to form a billet shell. In the continuous casting production process, the copper mould tube is the first step to transform molten steel from liquid to solid. Not only that, in the process of continuous casting billet shell formation, its surface quality control is closely related to the heat transfer conditions of the mold. In particular, the control of longitudinal cracks, transverse cracks and star-shaped cracks on the surface of the cast billet is not only affected by protective slag and steel type. In addition to the influence of factors such as composition design, the selection of parameters of the copper mould tube itself, coating materials and processing levels, and usage conditions are also significantly affected.

copper mould tubes can generally be divided into two types: tubular copper mould tubes and slab copper mould tubes, of which slab copper mould tubes are more commonly used. As the key equipment for forming the primary green shell of continuous casting billet, the quality of copper mould tube design and manufacturing will directly affect the normality and stability of continuous casting production. The continuous casting mold is made of copper alloy (deoxidized copper, CuAg alloy, CuCrZr alloy), plus a cooling water jacket. During the continuous casting process, the working surface of the mold copper plate is in contact with the molten steel at 1530~1570 knots, and the cooling water at 30~40 knots passes through the back of the copper plate, resulting in a large temperature gradient and thermal stress. The molten steel passes through the copper mould tube and crystallizes into a billet under the action of external cooling water, and is pulled out of the mold by the dummy rod. After continuous billet pulling, the copper mould tube is seriously worn. With the development of large-scale continuous casting equipment, high casting speed and online width adjustment technology, the operation rate of continuous casting has been greatly improved, and the heat load of the mold copper plate has been greatly increased, especially on the surface of the mold copper plate near the meniscus. The temperature can reach 300~350 knots. Due to the repeated thermal stress of rapid cooling and rapid heating, the meniscus is prone to thermal cracks, and the entire copper plate will also cause fan-shaped deformation of the wide-sided copper plate and width shrinkage of the narrow-sided copper plate due to thermal deformation. Therefore, in the current continuous casting machines with high operating rate and high pouring speed, high temperature wear, hot cracks, and thermal deformation have become the main reasons for the replacement of the mold copper plate.

1 copper mould plate material

With the high-speed and large-scale continuous casting and the development of online width adjustment, flow control, breakout prediction and other technologies, the working conditions of the mold copper plate have become more demanding. A summary of its performance requirements is as follows: ① Good thermal conductivity, allowing the molten steel to form a shell of a certain thickness as quickly as possible in the copper mould tube. ②High strength, especially high yield strength. The copper plate of the copper mould tube has to withstand huge temperature gradients, and the repeated thermal stress is very large. If the copper plate does not have sufficient strength, it will undergo large deformation, and cracks will occur on the surface of the copper plate or the bottom of the cooling water tank, which may even cause the cooling water to come into contact with the molten steel, causing an accident. ③Sufficient hardness, especially high softening temperature. This is to ensure that the copper plate does not soften at high temperatures, has sufficient wear resistance, and reduces strain and width adjustment scars. ④Good resistance to deformation. In addition to bearing huge thermal load (thermal shock), the copper plate also bears the mechanical load caused by the vibration of the crystallizer and the billet drawing. Therefore, fan-shaped deformation caused by creep of wide-faced copper plates and width shrinkage of narrow-faced copper plates at the meniscus should be prevented. ⑤High surface precision. This is required to ensure the flatness of wide and narrow copper plates, ensure small gaps in combination, and reduce width adjustment scars.

1. 1 TP2 (phosphorus deoxidized copper)

The working conditions of the copper mould tube require that the bulk material must have good thermal conductivity, so the initial research on the copper mould tube mainly used deoxidized copper as the material of the copper mould tube. Although the deoxidized copper mould tube can meet the thermal conductivity of the material during use, it shrinks and wears during use, which seriously hinders the normal growth of the molten steel solidification shell in the copper mould tube. This further induces steel running and becomes the main factor affecting the operation of continuous casting equipment. At present, deoxidized copper is generally used in billet continuous casting molds. The cumulative amount of steel processed is relatively low, generally only about 3,000 to 5,000 tons.

1.2 CuAg plate

CuAg alloy is made by adding 0.08~0.12% silver to copper. The addition of silver can significantly increase the softening temperature (recrystallization temperature), hardness and creep strength, while rarely reducing the electrical conductivity, thermal conductivity and plasticity of copper. The softening temperature can rise to 350°C after adding Ag. Silver copper has high plasticity and is generally cold work hardened to increase strength. It has good wear resistance and corrosion resistance. The thermal conductivity of CuAg alloy is very high, close to that of copper. The hardness of CuAg alloy will decrease above 300℃. Therefore, CuAg plates are suitable for low-speed continuous casting machines (not more than 1m/min) where the crystallizer surface temperature is lower than 300°C. When working at temperatures above 300°C for a long time, the internal stress will disappear and the material strength will also decrease significantly.

1.3 CuCrZr plate

CuCrZr plate has high strength and hardness, and the recrystallization temperature can reach 500°C, which effectively avoids the decrease in mechanical properties and deformation of the copper plate caused by recrystallization. The Zr and Cr elements added to CuCrZr during the smelting process precipitate uniformly finely dispersed second-phase metal compounds (such as ZrCu3, Cu2Zr+Cr) in the pure copper matrix. The particles dispersed in the matrix become obstacles to dislocations and grain boundary movement. That is, precipitation strengthening; at the same time, the severe plastic deformation and grain refinement produced by the CuCrZr plate after forging and rolling processes further improve the deformation resistance of the metal crystal.

Compared with CuAg plates, the strength and hardness of precipitation hardening materials are significantly improved. It has the advantages of high thermal conductivity and strength at high temperatures, and is not prone to creep deformation. CuCrZr is superior to CuAg alloy in terms of high temperature strength and hardness. The working temperature of CuAg is about 270℃~300℃, while the working temperature of CuCrZr can reach 430℃. Under the same working environment, the service life of CuCrZr alloy copper mould tube is 4 to 5 times longer than that of CuAg alloy. Therefore, CuCrZr alloy is the preferred material for the copper mould tube of continuous casting machine.

1.4 ODSCu board

ODSCu (alumina copper or dispersion strengthened copper) is a new structural and functional material with high strength and good resistance to high temperature softening, as well as excellent electrical and thermal conductivity. The strengthening particles in the dispersed copper material structure are nanoscale aluminum oxide particles generated in situ. It is different from the intermetallic compound particles precipitated during aging of precipitation-strengthened copper alloys. At a temperature close to the melting point of the copper matrix, it still retains its original particle size and interparticle spacing, does not cause remelting and deformation of the strengthening phase, and can effectively hinder the movement of dislocations and the slip of grain boundaries. While improving the room temperature and high temperature strength of the alloy, it still has good electrical conductivity, and has good wear resistance and corrosion resistance.

The properties of dispersed copper materials vary with the content of alumina. As the alumina content increases, the strength and hardness of the dispersed copper material increases, and the conductivity decreases accordingly. Generally speaking, dispersed copper materials with medium and high aluminum content have higher strength and hardness, lower elongation, and slightly worse cold working properties. Dispersed copper materials with low aluminum content have slightly lower strength and hardness, but have high electrical conductivity and elongation, and have good cold working properties. For example, the ODS15 grade low-aluminum dispersed copper material produced by China Shipbuilding Industry Corporation’s 725th Research Institute has a yield strength of 458MPa and a conductivity of 90% IACS. The ODS35 grade low aluminum dispersion copper material has a yield strength of 575MPa and a conductivity of 83% IACS.

The narrow-faced copper plate of the slab mold not only suffers from thermal cracking and peeling near the liquid surface, but also because the thermal expansion is restricted by the wide-face clamping force, the narrow-faced copper plate is affected by compressive stress and undergoes creeping deformation. Adjust the taper, the wear will be more serious. Normally, the service life of narrow-face copper plates is much lower than that of wide-face copper plates. The service life of the crystallizer is determined based on the failure cycle of the weakest parts. When the narrow-face copper plates reach the offline cycle, no matter how good the condition of the wide-face copper plates is, The crystallizer must be taken offline for inspection or dismantled for repair. Therefore, the service life of narrow-face copper plates is generally shorter than that of wide-face copper plates, and better materials should be selected.

As can be seen from Table 1 and Table 2, ODSCu has the highest strength and hardness, and has strong resistance to high temperature softening, with a softening temperature greater than 900°C. The linear expansion coefficient is slightly lower than other copper alloys and closer to Ni-based alloy plating. Compared with CuCrZr plates, dispersed copper materials have high yield strength and hardness, and their softening temperature is greater than 900°C. They have superior resistance to high-temperature softening and can reduce creep deformation under compressive stress conditions of narrow-faced copper plates. At the same time, the hardness of the dispersed copper material is also higher than that of the CuCrZr plate, and it is wear-resistant and can significantly reduce the wear of the narrow-faced copper plate. This allows it to achieve a service life and lifespan comparable to that of wide-face copper plates, reduce the number of repairs of mold copper plates, and improve production efficiency. The United States has used dispersed copper materials for continuous casting machine mold linings.

Table 1 Mechanical properties of copper plate at room temperature (section 20)

MaterialRm /MPaRp0.2 /MPa A /% Hardness /HB
TP2> 250> 200> 15> 80
CuC rZ r> 400> 280> 15> 110
CuAg> 280> 265> 17> 90
OD SCu> 480> 465> 15> 137

Table 2 Physical properties of copper plates

MaterialElectrical conductivity/%IACSThermal conductivity/W/(m.k)Linear expansion coefficient/×10-6/℃Softening temperature/℃
TP2≥85≥34017.1
CuCuZr≥80≥32017.0≥450
CuAg≥95≥37717.5
ODSCu≥90≥35016.6≥900

2 Crystallizer surface and coating technology

The working conditions of the copper mould tube require that the copper plate of the copper mould tube must have high thermal conductivity, high deformation resistance, high strength, high surface precision, high hardness and high wear resistance. The surface of the copper mould tube is key to its performance. Under the combined action of high-temperature molten steel and cooling water, the copper plate of the mold is subjected to hot cracks caused by high-temperature oxidation and thermal fatigue, and deformation caused by excessive temperature gradients. Chemical corrosion caused by cooling water and mold slag, cavitation caused by high-temperature steam, friction and wear caused by ingots, billet drawing and vibration, as well as scratches caused by taper adjustment and online width adjustment, etc. In order to meet the needs of the development of efficient continuous casting, the surface of the mold copper plate must have high hardness and high wear resistance to resist the wear of the billet shell at high pulling speeds. At present, the hardness of the bare copper plate of the mold cannot withstand the wear of the shell, so the surface of the mold needs to be modified to increase its hardness. Surface modification of continuous casting molds generally uses methods such as electroplating, chemical plating, electroforming, composite plating, thermal spraying and high temperature self-propagation. It can improve its surface properties to a great extent, thereby achieving the purpose of improving the quality of continuous casting billets, extending the life of the mold and reducing production costs.

The functions of the coating after surface modification of the copper mould tube copper plate are: ①To avoid star-shaped cracks in the copper mould tube copper plate. ②Prevent copper from penetrating into the slab and increase the copper content on the surface of the slab. ③ Prevent the crack sensitivity of stainless steel, high carbon steel, and high alloy steel; ④ Improve the quality of the cast slab, enhance lubricity, and avoid steel sticking; ⑤ Improve the surface hardness and wear resistance of the copper plate. Using methods such as electroplating or thermal spraying to treat the surface of the copper mould tube can greatly improve the wear and corrosion resistance of the copper mould tube, improve the quality and productivity of the slab, save costs, and extend the service life of the copper mould tube.

2. 1 Cr plating

The Cr content is generally greater than 99.9%, and its hardness is high, with a microhardness greater than 700HV. The coating has good chemical stability, can prevent molten steel from splashing, reduce scratches, and has good wear resistance. The disadvantages are: ① The thickness of the coating is limited, only between 0.06 ~ 0.12mm; ② The coating is easy to peel off, has cracks, and has poor corrosion resistance. ③ The linear expansion coefficient and thermal conductivity of the coating and the mold copper plate are quite different. The coating is prone to tearing and peeling during operation, causing copper infiltration and other defects in the casting slab. It is now rarely used, and is only used in the repair of copper plates in tubular molds such as billets.

2.2 Ni plating

Ni plating has good chemical stability, strong sealing ability, and can be plated to 3 ~ 8mm. Although the coating has good bonding force with the mold copper plate, it can effectively prevent star-shaped cracks in the casting billet and reduce wear. However, the hardness of the coating is not high, only 180~250HV, which cannot meet the wear requirements of continuous cast steel billets, and the coating is prone to thermal cracks at the meniscus. Especially when used for narrow-face mold copper plates, the corrosion resistance is not good, so the coating life is not long.

2.3 Ni-Cr coating

The plating structure is first plating 1~3mm pure nickel, and then plating 0.03~0.05mm chromium. This composite coating also has the comprehensive performance of NCr coating, but the electroplating process is complicated. During the wear process, the Ni coating mainly relies on the wear resistance. At the same time, potential corrosion easily occurs in the lower part of the crystallizer copper plate. Therefore, the corrosion resistance and wear resistance of the entire coating are not very ideal.

2.4 Ni-Fe plating

Ni-Fe alloy coating is a coating that has been studied more in recent years. The Fe content in the coating is generally controlled at 3% to 12%. As the Fe content increases, the control of the plating solution becomes more difficult, and the properties of the coating become worse and worse. Many researchers believe that 3% to 8% is more appropriate, and some control it at 3% to 5%. The hardness of Ni-Fe alloy is 320 ~ 420HV at room temperature. The wear resistance of Ni-Fe alloy coating is good, which is 1.5 ~ 3.0 times that of Ni coating. At the same time, it has good bonding force with the crystallizer copper plate, and the hardness of the coating can be changed by adjusting the Fe content. But the disadvantages are: ① Compared with the nickel layer, the chemical stability is reduced, especially under high temperature conditions, thermal corrosion or ablation expands rapidly and cracks are prone to occur. At normal temperature, slow corrosion can be observed when the Fe content is 8%; when the Fe content is more than 12%, the corrosion phenomenon appears quickly. Therefore, the resistance to potential corrosion of Ni-Fe coating is very poor. ② Compared with the nickel layer, the hardness is increased, but the brittleness of the coating is significantly increased, and the bonding strength with the base metal is significantly reduced. Therefore, the Ni-Fe coating has poor heat resistance. In addition, the Ni-Fe alloy plating solution is difficult to control, and if the control is not good, the yield will be reduced. Therefore, Ni-Fe alloy coating is not an ideal slab mold coating.

2.5 Ni-Co or Co-Ni plating

Using Ni as the base is called Ni-Co plating, and using Co as the base is called Co-Ni plating. Replacing Fe with Co solves the instability of divalent Fe ions in electroplating and acts as a stress reliever for the coating. Further reducing the internal stress of the coating, its ability to resist sudden temperature changes is greatly improved, thus solving the problem of cracks in the coating. At the same time, because the thermal expansion coefficients of the Ni-based coating and the Cu matrix are close, the bonding force between the coating and the Cu matrix is strong, and adding Co can ensure that the hardness of the coating does not decrease at high temperatures. Its main advantages are: ① The hardness is significantly improved, and the hardness is also very high at high temperatures; ② Good chemical stability, especially thermal stability. ③The oxide film formed at high temperature has excellent wear resistance, and the friction coefficient between the coating and the cast slab becomes smaller. ④Good thermal fatigue resistance and hot corrosion resistance. Ni-Co alloy coating fundamentally overcomes the peeling, cracking and other defects of previous generations of coatings (Cr, Ni, Ni-Fe alloy), and can increase the service life of the crystallizer to 2,000 furnaces. As can be seen from Table 3, compared with Cr plating, Ni plating, and Ni-Fe plating, Ni-Co or Co-Ni plating has the best overall performance. Therefore, most slab molds currently use Ni-Co or Co-Ni coating.

Table 3 Coating materials and their properties

MaterialHardness/HVThermal conductivity/W/(m.k) Linear expansion coefficient/×10-6/℃ Adhesion strength/MPa
Cr≥60060~667
Ni≥14076~8414~16.7≥220
Ni-Fe≥25063~8814≥220
Ni-Co≥280~37075~8414≥240
Co-Ni≥22080~8414≥240

2.6 N i-Cr alloy spray coating

In the use of slab molds, the wear of narrow-face copper plates is more serious than that of wide-face copper plates. In order to improve the service life of narrow-face copper plates, Japan has developed narrow-face copper plate supersonic spraying technology. That is, a layer of Ni-Cr alloy is sprayed on the surface of the narrow-faced copper plate with a supersonic flame to replace the electroplating layer. Its service life can be the same as that of the Co-Ni plating of the wide-faced copper plate. The Ni-Cr alloy thermal spray mold developed by Japan’s Mishima Co., Ltd. is prepared by spraying a Ni-Cr alloy containing 14% to 17% Cr and other elements on the surface of the continuous casting mold. The high-temperature hardness of the sprayed coating drops very little in the range of 600 to 650 knots. At a temperature of 300 knots, the wear resistance of the Ni-Cr alloy spray coating is 5 to 7 times higher than that of the Ni plating layer, and its service life is 3 to 6 times that of the original Ni electroplated crystallizer. Baosteel’s slab continuous caster uses Co-Ni coating for wide-surface copper plates and Ni-Cr spray coating for narrow-surface copper plates. The steel capacity can reach 200,000 tons at a time.

Thermal spraying technology has stable process specifications and simple operation. The coating after spraying is smooth and flat, and the coating thickness can be controlled more accurately. However, compared with electroplating technology, thermal spraying technology has low bonding strength between the coating and the base material, and because the deposition environment is a high-temperature environment, it is easy to produce thermal stress on the workpiece, causing thermal deformation of the workpiece. In order to compensate for the deficiencies in bonding strength and thermal stress, the copper plate must undergo a lengthy heat treatment process. It is difficult to correct the deformation of copper plates that have undergone high-temperature heat treatment. The base material is damaged, and the number of fatigue layers that need to be repaired and cut increases, which reduces the service life of the copper plates. Since the area of the wide-face copper plate is much larger than that of the narrow-face copper plate, these existing problems will be amplified. Therefore, how to solve the problem of matching between the spraying process and the mold copper plate still requires in-depth research.

2.7 New plating/coating

The friction coefficient of Ni-P alloy coating is small due to the dispersion and precipitation of Ni3P phase. After 400 knots of aging, the wear resistance of Ni-P alloy is better than that of electroplated nickel, hard chromium and other coatings. This coating is used to strengthen the inner wall of the crystallizer, and its expansion coefficient is close to that of the copper substrate, making the coating less likely to fall off prematurely. Moreover, the Ni-P alloy coating has good red hardness, wear resistance and thermal fatigue resistance at the operating temperature, which has important application value for improving the service life of the crystallizer.

Ni-Fe-W-Co coating is developed on the basis of Ni, Ni-Fe, Ni-Fe-W, Ni-Fe-Co and other alloy coatings. This coating combines the advantages of the above-mentioned alloy coatings while avoiding its shortcomings. It has been successfully applied to slab molds, and its life span exceeds that of Ni and Ni-Fe coatings imported from Japan and Ni coatings imported from Germany. The coating has strong bonding force, strong resistance to potential corrosion, and good thermal alternating resistance, making it highly competitive.

Jefrey K. Browe introduced in the literature that the hardness of ceramic coatings is 700 ~ 1200HV. Although ceramic coatings have high hardness, they are limited to applications in thin plate molds and narrow surfaces of large-section molds. Domestic Fu Hanguang and others combined self-propagating high-temperature synthesis technology and centrifugal casting technology, coated the inner surface of copper tubes with ceramics, and applied them to tubular crystallizers.

In order to meet the needs of the development of efficient continuous casting, the copper plate coating of the mold must have high hardness and high wear resistance to resist the wear of the billet shell at high pulling speeds. Strength, hardness, wear resistance, thermal conductivity, and thermodynamic properties are the main indicators to measure the quality of the copper mould tube coating. Therefore, we should start from all aspects and explore composite coatings that meet the excellent performance requirements of continuous casting molds through composite electroplating of several metals with high strength, high wear resistance, high hardness, and high thermal conductivity.

3 Conclusion

With the development of continuous casting technology, the working conditions of mold copper plates have become more stringent, and higher requirements have been put forward for mold copper plates and coating technology. At present, the narrow-faced copper plate of the slab mold is prone to softening, deformation, wear, etc., which shortens the maintenance period of the mold and limits the service life of the mold. Therefore, dispersion copper plates with stronger resistance to high-temperature softening can be used instead of chromium-zirconium copper plates for the narrow-faced copper plates of the copper mould tube. Based on the advantages and disadvantages of existing coating technology, develop new alloy coating and coating technologies to further improve the hardness, wear resistance and corrosion resistance of the coating.

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