Application progress of continuous casting mold copper plate and coating

Abstract: Under normal cooling conditions, the operating temperature of the inner wall of the copper mould tube is 250~350℃. The copper plate of the copper mould tube should have good thermal conductivity and deformation resistance, high high temperature strength, surface precision and wear resistance. The service life of Ag-Cu and Cr-Zr-Cu plates is better than that of deoxidized copper and red copper. The function of the mold copper plate coating is to avoid star-shaped cracks in the mold copper plate; prevent copper from penetrating into the casting; and improve lubricity and wear resistance. Currently, Ni+Cr and Ni-Co coatings and other newly developed alloy coatings are mainly used. This article analyzes the characteristics and applications of copper plates and coatings.

Keywords: continuous casting mold; copper mould plate; plating

With the continuous development of high-efficiency continuous casting technology, the copper mold base material cannot meet its usage requirements. And copper is a harmful element to most steel types. The copper washed into the billet by the molten steel will cause star-shaped cracks on the surface of the billet, leading to quality defects. In recent years, the base material of the copper mould tube copper plate has gradually developed from pure copper plate to strengthened copper alloy plate, and the copper plate working surface has gradually developed from bare copper plate to copper plate alloy plating, thus improving the surface properties of the copper mould tube. To achieve the purpose of improving the quality of continuous casting billets, extending the life of the mold and reducing production costs.

1 Factors affecting the life of the copper mould tube copper plate

During the use of the copper mould tube, there are main problems such as edge wear, wide-surface thermal cracks, narrow-surface shrinkage, corrosion, etc. The surface of the crystallizer is the key to its performance. Under the combined action of high-temperature molten steel and cooling water, the mold copper plate is subjected to high-temperature oxidation, hot cracks caused by thermal and cold fatigue, and deformation caused by excessive temperature gradients. Chemical corrosion of cooling water and protective slag components, cavitation caused by high-temperature steam, friction and wear caused by spindle drawing, billet drawing, and vibration, as well as scratches caused by taper adjustment and online width adjustment, etc. Therefore, the basic requirements for the performance of the mold copper plate are: (1) Good thermal conductivity and deformation resistance. (2) Higher high temperature strength and surface accuracy; (3) Higher hardness and wear resistance; (4) Longer working life; (5) Lower cost per ton of steel.

2 Selection of copper mould tube copper plate base material

Continuous casting copper mould tubes initially used red copper plates or deoxidized copper plates. Although they have good thermal conductivity, they have low strength and hardness, poor wear resistance, and even lower strength at high temperatures. Today’s copper mould tubes are generally made of copper alloy plates such as Ag-Cu, Cr-Zr-Cu, Cr-Zr-As-Cu, Mg-Zr-Cu, Cr-Zr-Mg-Cu. At present, Ag-Cu and Cr-Zr-Cu alloys are more commonly used in China (Table 1). The purpose of using Ag-Cu alloy is mainly to increase the recrystallization temperature of copper plates. When the silver content is 0.08% to 0.10%, the recrystallization temperature is 318 to 326°C, which is about 50°C higher than the recrystallization temperature of ordinary cold-rolled copper plates. However, the hardness of the Ag-Cu plate decreases above 300°C.

Table 1 Physical and chemical properties of copper mould tube copper plate base material

MaterialChemical composition/%Thermal expansion coefficient/(10-6.℃-¹)Mechanical behavior
σa/MPaσb/MPaδ/%Hardness (HB)
Ag-CuCu>99.5Ag=0.07~0.1016.5245290≥15≥70
Cr-Zr-CuCu>98Cr=0.50~1.50 Zr=0.08~0.3017.4~17.828030010~ 20≥100

The Cr-Zr-Cu plate matrix has high strength and hardness, and the recrystallization temperature is 480 to 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 Cr-Zr-Cu during the smelting process precipitate (precipitate) uniform, fine, and dispersed second-phase metal compounds (such as ZrCu₃, Cu₂Zr+Cr) in the α matrix of pure copper. The particles dispersed in the matrix become obstacles for dislocations and grain boundary movement, that is, precipitation strengthening. At the same time, the severe plastic deformation and grain refinement of the Cr-Zr-Cu plate after forging and rolling processes further improve the anti-deformation ability of the metal crystal.

Table 2 The usage status of copper mould tubes in some domestic steel companies

factoryLife/heats MaterialTypePulling speed/(m·min-¹)End face size/(mm×mm)
WISCO600Ag-Cuwide pressure narrow fitting0.9~1.2210×1500
Baosteel250Cr-Zr-CuStraight combination1.3~1.5900×1900
Shougang140Cr-plated copperTubular2.4-2.8125×125
Angang Steel28Deoxidized copperTubular1.8136×136
Magang70red copperTubular3.090×90

As can be seen from Table 2, deoxidized copper has the shortest lifespan as a copper mould tube material, followed by red copper, and Ag-Cu and Cr-Zr-Cu have relatively longer lifespans. Under normal cooling conditions, the operating temperature of the inner wall of the copper mould tube is 250-350°C. If there is scale, the inner wall temperature will reach over 400°C. Therefore, the copper mould tube is required to have high strength, high thermal conductivity and high recrystallization temperature in the range of 250 to 400°C. Phosphorus deoxidized copper, Ag-Cu, Cr-Zr-Cu have higher thermal conductivity and recrystallization temperature. However, the first two have lower high-temperature strength. Therefore, for copper mould tubes with higher operating temperatures, Cr-Zr-Cu is preferred as the material if economic conditions permit.

The deformation resistance and thermal conductivity of Cu-Ni-Co are better than those of Ag-Cu and Cr-Zr-Cu, making it an ideal material for copper mould tube copper plates. In actual use, combined with comprehensive factors such as the performance and cost of copper plates, Cr-Zr-Cu should be the first choice.

For high-speed continuous casting machines, appropriate surface treatment technologies (such as electroplating, thermal spraying, etc.) should be used. The bonding strength between the coating and the base alloy directly affects the service life of the copper mould tube. The higher the pulling speed, the higher the bonding strength required. The quality of the bonding performance depends on the difference in linear expansion coefficient between the coating and the base alloy. Therefore, the high-speed continuous casting machine mold should try to choose a coating with a small linear expansion coefficient and a small difference between the base alloy and good strength and hardness.

The narrow-faced copper plate of the copper mould tube 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 creep deformation. , the wear is more serious. Therefore, generally narrow-face copper plates have a shorter service life than wide-face copper plates, and better materials should be used.

3 Selection and design of copper plate plating

3.1 Selection of coating material

The functions of the mold copper plate plating are: (1) to prevent star-shaped cracks in the mold copper plate; (2) to prevent copper from penetrating into the cast slab and increase the copper content on the surface of the cast slab. (3) Prevent the crack sensitivity of stainless steel, high carbon steel, and high alloy steel; (4) Improve the quality of the cast billet, enhance lubricity, and avoid steel sticking; (5) Improve the surface hardness and wear resistance of the copper plate.

The characteristics and usage of several coatings used in production are shown in Table 3. The number of times the steel is cast after using several coatings is roughly:. 100 to 150 times of traditional Cr plating, 300 times of Ni plating of the same thickness, 500 times of step-by-step Ni plating, 800 times of gradient Ni and Ni-Fe plating, and 2,000 times of Ni-Co plating. Considering the stability of the electroplating solution and the performance of the coating, the Ni-Co alloy layer is the preferred coating on the crystallizer until there is a coating with superior performance.

Table 3 Characteristics and usage of several mold coatings

Coating typeFeaturesUsage
Single Cr, Ni platingThe chromium plating layer has high hardness and good chemical stability. The disadvantages are: the safe thickness is limited, and cracks will exist regardless of the thickness of the coating. The hardness decreases rapidly as the temperature increases. When combined with copper, the linear expansion coefficient and thermal conductivity are too different, and the coating is easy to peel off at high temperatures. Therefore, the chromium plating layer affects the life of the copper mould tube, and its use is subject to certain restrictions.The nickel plating layer has good chemical stability, strong sealing ability, and can be plated to 3 to 8 mm. However, its hardness (around 200 HV) cannot withstand the wear of continuous cast steel billets, so the coating life is not long. Using thick nickel plated copper mould tube, the service life is relatively prolonged.All major steel companies no longer use continuous bending molds with hard Cr layers. Ni electroplating process quickly replaced Cr electroplating process.
Ni-Cr The surface of the copper plate is first plated with nickel of 1 to 4 mm, and then chromium plated after processing. Nickel plays an intermediate transition role and can improve the bonding strength between the chromium layer and the matrix, while the hard chromium on the outer surface improves its wear resistance. Although the coating properties such as hardness and wear resistance can meet the requirements, network cracks can be eliminated and the life of the crystallizer can be extended. However, the double plating process is relatively complex. Since the thermal expansion coefficients of chromium and nickel are nearly 2 times different, the bonding strength between the chromium layer and the nickel layer is still low.Thermal sprayed Ni-Cr layer has high hardness (600 HV), high temperature corrosion resistance and high temperature oxidation resistance, which can significantly increase the service life of the crystallizer. However, the thermal spray process is relatively complex in operation.At present, most copper crystallizers adopt double plating with Ni plating on the inner layer and Cr on the outer layer. Such as Baosteel, etc.
Ni-Fe The melting point of domestically developed Ni-Fe alloy coatings is generally 1400°C, with strong high temperature resistance and good thermal shock resistance.Due to the addition of iron, its hardness (500 HV) is higher than that of the nickel plating layer, its wear resistance is twice that of the nickel plating layer, and the mold life is twice as long as that of a single nickel plating layer. However, the resistance to potential corrosion and heat exchange of Ni-Fe coating is very poor, and the Ni-Fe alloy plating solution is difficult to control. If the control is not good, the yield will be reduced.It is widely used in medium and low speed continuous casting. Large domestic steel companies are gradually replacing it with electroplated nickel and its alloy layers.
Ni-CoThe surface of the Ni-Co alloy coating becomes ceramic after being heated, which has good lubricity and can avoid bonding. It has a high softening temperature and can form a stable high-hardness wear-resistant surface at higher temperatures. The friction coefficient is low, and the copper plate coating is not prone to thermal cracks. . It has strong bonding force with the base material and is suitable for copper plates with high copper plate surface temperatures and slow cooling molds, which can increase the service life of the crystallizer to more than 2,000 heats.It has been widely used in the United States, Australia, Canada and other countries.

In recent years, new alloy coatings developed at home and abroad mainly include: Ni-P, Ni-W-Fe, Ni-W-P, Ni-Co-W, Ni-Fe-W-Co, RE-Ni-W-P-SiC, ceramics Coatings, nanocomposite coatings, etc. The new alloy coating has strong bonding force, high hardness, good wear resistance and oxidation resistance, which significantly improves the service life of the high-speed continuous casting mold and the surface quality of the cast slab.

Surface treatment of domestic mold copper plates is mostly limited to the use of Cr, Ni, Ni-Fe, and Ni-Cr plating. Among them, the inevitable defects of electroplated Cr and Ni-Cr layers limit its application. The Ni-Fe layer is also gradually being replaced by Ni and Ni-Co layers due to its shortcomings. Electroplating Ni layer is a coating with relatively mature technology and many applications. Domestic research on Ni-Co layers has achieved certain results and has good application prospects. The electroplated Ni-W-P layer has advantages in terms of coating performance, operating technology and production cost, and is the most promising coating.

3.2 Coating design

Because the heat flux density is the largest in the meniscus area of the copper mould tube, the meniscus and the upper area of the copper mould tube are in best contact with the billet shell, and the heat transfer coefficient of the coating is smaller than that of the base material. Therefore, the coating should not be too thick to prevent steel leakage from the bonding of the billet shell. The lower part of the mold is mainly affected by the alternating effects of the air gap generated by the shrinkage of the billet shell and the static pressure of the molten steel, which causes uneven contact between the billet and the copper plate of the mold, greatly reducing the heat flow density in the lower part. Secondly, the shell at the lower mouth of the mold is thicker and harder, causing the copper plate coating at the lower mouth to wear and peel more seriously than at the upper mouth. Therefore, the coating of the lower port is thicker. On the one hand, it can slow down the heat transfer and promote the uniform contact between the blank shell and the copper mould tube. On the other hand, it can improve the wear resistance of the lower port and protect the copper plate. For this purpose, the alloy plating on the surface of the mold copper plate base material can be designed in a trapezoidal shape. According to the situation that the narrow-side copper plate of the slab mold has greater wear than the wide-side copper plate, the upper limit size can be adopted for the coating thickness of the narrow-side copper plate to ensure that the life of the narrow-side copper plate is synchronized with that of the wide-side copper plate. Narrow-surface copper plate plating should also be made of better materials (such as Ni-Co).

copper mould tube coating types include: single coating (single coating of Cr, Ni or Ni series alloy), composite coating (Ni-Cr, Ni-Ni series alloy-Cr). The surface of copper plates is generally plated with Ni or Ni-based alloys, mainly because the thermal expansion coefficients of Cu and Ni (1.65×10-⁵/℃, respectively) are relatively close. The purpose of Cr plating on the outer layer is to improve the smoothness of the inner surface of the copper plate and the wear resistance of the surface. The use of composite plating can significantly increase the life of copper plates.

4 Conclusion

(1) copper mould tubes of different materials and coatings should be used according to different casting machines and different casting speeds. At present, copper plate base materials mainly include deoxidized copper, red copper, Ag-Cu, Cr-Zr-Cu alloy, etc. In terms of comprehensive considerations such as performance and cost, Cr-Zr-Cu material should be the first choice for high-speed casting machines.

(2) The plating layers mainly include Ni, Ni-Fe, Ni-Cr, Ni-Co layers, etc. The electroplating Ni layer process is relatively mature and has many applications. The Ni-Co layer has excellent performance and has good application prospects. In order to meet the needs of the development of high-speed continuous casting molds, research on new composite coatings and electroplating methods should be strengthened and put into practical use as soon as possible.

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