Analysis on the Manufacture and Restoration Technology of High Efficiency Continuous Casting Mold Copper Tube

 Through the introduction and comparison of the continuous casting mold copper tube processing and repair process, the complexity and pros and cons of the manufacturing process, precision, quality and cost of the continuous casting mold copper tube under two different processing techniques are analyzed.

Key words: continuous casting mold copper tube; mechanical cutting extrusion forming and repair; explosive forming and repair

The core purpose of the continuous casting machine to achieve high-efficiency continuous casting is to increase the casting speed on the premise of ensuring the quality of the slab, thereby increasing the production capacity. The multi-taper mold copper tube is the heart of the continuous casting machine and is responsible for the future. The main task of solidifying molten steel into the required billet shape, and the mold copper tube has the problems of high rigidity, not easy to deform, high manufacturing cost, difficult to repair and other advantages and disadvantages.

How to ensure that the manufacturing method of mold copper tube can adapt to the needs of modern high-efficiency continuous casting, and has the advantages of simple production, low cost, cheap repair and easy disassembly, etc., is an important research topic of modern mold copper tube forming technology.

The current domestic mold copper tube manufacturing process

The current domestic mold copper tube manufacturing methods mainly include: mechanical cutting extrusion molding and explosive molding.

Brief introduction of crystallizer copper tube

The crystallizer copper tube is the core of the crystallizer, mainly made of pure copper ingot or copper alloy (such as silver-copper alloy containing 0.08~0.12% silver), and the inner layer is a high-performance metal coating (0.08~0.1 hard chromium or Nickel-plated alloy), the length is about 700~1000mm (generally controlled between 850mm and 900mm), the wall thickness is generally controlled at 8~10% of the body specification, generally it is more reasonable to choose according to the lower limit, and the inner fillet radius is generally controlled at 6~8mm, the inner cavity is mostly single taper, double taper and multi-taper, etc., because the solidification and shrinkage law of the high-speed stretching continuous casting machine in the copper tube of the mold is very different from the shape of the inner cavity of the copper tube of the multi-taper mold. Therefore, the current design mostly uses multi-taper mold copper tubes to increase the casting speed of the continuous casting machine, especially in multi-steel casting, which can effectively improve the production efficiency of continuous casting casting.

As an important part of the continuous casting machine, the mold copper tube should have the following characteristics: ①Good thermal conductivity, wear-resistant inner surface, multi-taper geometry; ②Good structural rigidity: high strength, high hardness, Good heat dissipation; ③It has good performance of high temperature resistance, corrosion resistance and thermal stress resistance; ④The quality should be relatively light, easy to disassemble and adjust, and easy to process and manufacture.

Basic principles of mechanical cutting extrusion molding

Mechanical cutting and extrusion molding mainly uses convex and concave dies to extrude on horizontal or vertical presses (above 300 tons). For crystallizer copper tubes with large cross-sections, the pressure of the press is usually required to reach at least 800 tons. And multi-pass extrusion molding is required. Process flow: raw copper ingot → heating → extrusion → cold drawing → annealing → orthopedic → finished product.

The key to the mold copper tube manufacturing process is the fit between the copper tube and the mold, that is, the inner cavity size of the copper tube must fully conform to the manufacturing accuracy of the mold. There are several key technical difficulties in the machining process of mechanical cutting extrusion molding. There are two main factors: ①The selection of wall thickness deformation. If the deformation amount of the copper tube is selected to be small during the extrusion process, the inner surface of the copper tube cannot produce sufficient plastic deformation, causing the copper tube to not stick to the mold; The wall is straightened, so choosing the appropriate amount of extrusion deformation is the key to the accuracy of the inner cavity of the copper tube; ② Stress and deformation control of copper tube forming. For copper tubes with larger wall thickness, the deformation of the inner and outer surfaces is different during extrusion deformation; the metal on the outer surface is pulled very long, while the deformation of the inner surface is small or even partially undeformed, which causes copper There is a lot of internal stress in the tube. During the subsequent machining of the copper tube, these stresses will be released and cause the deformation of the copper tube. Therefore, appropriate technological measures should be taken during the forming process of the copper tube to eliminate the internal stress and obtain a qualified product.

Basic principle of explosive forming

Explosive forming refers to the processing operation of using controllable chemical explosion energy to process metal into the desired shape. Explosive molding can be traced back to 1876, and it has a history of nearly 150 years. At first, it used gunpowder explosion to study the behavior of iron plates and steel plates under impact pressure. Later, it used engraving templates and steel mesh modules to engrave patterns on metal surfaces, etc. Both are early examples of humans engaging in explosive formation. At present, the scope of metal explosive forming mainly includes forming, shaping, bulging, flanging, engraving, pressing, powder pressing, welding, surface hardening and cutting.

The explosion in the field of metal explosion forming is a controlled explosion form under limited conditions, which has a unique theoretical system and engineering conditions. Process flow: raw copper plate → heating → extrusion welding → mold loading → explosive forming → demoulding → finished product.

Using explosive forming technology to process continuous casting mold copper tube is to use the huge energy released by the explosive detonation laid on the surface of the blank material to load the mold copper tube blank, so that the tube blank material can obtain enough energy to the mandrel. The surface is impacted at high speed. When the tube blank material and the core mold are completely attached, the high-speed movement of the tube blank material stops, and the inner cavity of the mold copper tube tube blank forms a cavity that is completely consistent with the shape of the core mold. This explosive molding is a kind of explosive compression molding in which the blank material undergoes plastic flow under a three-dimensional compressive stress state.

Table 1 Comprehensive comparison of mechanical cutting extrusion molding and explosive molding methods

Method item Comparison contentMechanical Cut ExtrusionExplosive molding
Production MethodUse convex and concave dies to extrude on a horizontal or vertical press. Generally, the pressure of the press is required to be about 800 tons, and multiple extrusions are required.The controllable chemical explosion energy is used to make the material of the tube blank and the mandrel completely fit, so that the inner cavity of the copper tube tube blank of the crystallizer forms a cavity that is completely consistent with the shape of the mandrel.
Process comparisonThe internal stress caused by the different deformation of the inner and outer surfaces of the extrusion will cause the deformation of the copper tube during the subsequent processing of the copper tube. It is necessary to take appropriate technological measures such as annealing process during the forming process of the copper tube to eliminate the internal stress deformation caused by the forming of the workpiece.It can give full play to the stretchability of metal, save the intermediate annealing process required by mechanical forming, and effectively avoid the local stress concentration phenomenon that may occur during mechanical forming.
quality analysisMultiple extrusion corrections are required and proper extrusion deformation must be controlled well, otherwise it is easy to cause insufficient plastic deformation and the copper pipe will not stick to the mold, or excessive extrusion resistance will cause the pipe wall to straighten.The processing methods that are difficult to realize by mechanical processing methods can ensure strict manufacturing tolerances and obtain better surface finish.
Repair methodOn the basis of annealing and other technological processes, the mandrel is installed into the copper pipe by a hydraulic machine, and the wall is reduced by the die to extend the multi-pass extrusion, and the machine is trimmed and electroplated. The disadvantages are low production accuracy and high cost of consumables.The standard mandrel is used to press into the inner cavity of the damaged crystallizer, and explosives are used as the energy source, and the copper tube of the crystallizer is repaired by explosive forming technology. The advantage is that the hardness and service life of the copper tube are improved, and the processing cost is low.

The repair process of current domestic crystallizer copper tube

The copper tube of the mold also has a service life. After reaching a certain amount of steel, the inner layer of the copper tube of the mold will cause defects such as wear and scratches on the inner surface of the inner cavity of the copper tube, multi-tapered inner surface of the copper tube, and deformation of the cross section, which will affect the drawing. Billet speed and quality effects must be replaced or repaired to ensure the speed and quality of the continuous casting machine.

The current methods of mold copper tube repair mainly rely on “mechanical cutting extrusion repair molding” and “explosive repair molding”.

Basic principles of mechanical cutting extrusion repair molding

Process flow: material selection → inspection → pickling and chromium removal → annealing → hole expansion → closing → extrusion → grinding → trimming → electroplating → forming. On the basis of material selection, pickling, chromium removal, annealing and other technological processes, the mandrel is installed into the copper pipe with a hydraulic machine, and the wall is reduced by the die for extension and extrusion, machining and trimming, and electroplating to form. The disadvantage is that the production accuracy is low and the consumables are large.

Basic principle of explosive forming repair

Process flow: material selection→inspection→pickling and chromium removal→molding→explosive molding→demoulding→electroplating→forming. Explosive forming repair technology has theoretical basis and practical experience for the repair of copper tubes in continuous casting molds with complex inner cavity shapes. It has been improved to a certain extent, effectively enhancing the service life of the tube blank, and can better reduce the cost of repairing and processing compared with the machining method.

 Comprehensive comparison of mechanical cutting extrusion molding and explosive molding methods

According to the relevant experimental data query and summary, the explosive forming and repair mainly have the following characteristics:

Advantages

(1) High energy rate and high performance: The energy released by the explosion of a unit gram of explosives reaches several thousand joules, and the explosion time of explosives is mostly on the order of microseconds, so releasing such high energy in a short period of time can form an effective period of time. The high-energy rate process produces a pressure of more than ten gigapascals, and the temperature can reach as high as thousands of degrees, forming high-temperature and high-pressure material formation conditions, which in turn affect the material form of the material and the change of metal crystals, creating new processes and new properties for the material. Under certain conditions, explosive forming can effectively strengthen the hardness of the workpiece and prolong the service life of the workpiece.

(2) High benefit and high efficiency: Due to the high pressure and short time generated by the explosion, the material forming rate is fast and the efficiency is high; explosive forming generally does not require complicated installation equipment, which can effectively save production costs and repair costs.

Deficiencies

(1) Explosive forming is restricted by the explosive effect of explosives, which involves the control of the amount of explosives, the distribution of explosive points, the surrounding environment and other factors.

(2) Explosive molding requires a vacuum between the copper tube and the mandrel, so sealing devices, vacuuming devices and corresponding procedures need to be installed.

(3) The scope of application of explosive forming: ① Processing parts with complex shapes, especially parts with complex shapes and sizes, high strength and hardness that are difficult to process effectively, and the required processing pressure is high, and the turning process is cumbersome. High-strength heat-resistant materials with particularly high capacity requirements; ② Realize processing methods that are difficult to achieve by certain mechanical processing methods and ensure strict manufacturing tolerances to obtain better surface finish; ③ By adjusting the overall explosion pressure distribution on the surface of the workpiece , can give full play to the ductility of the metal, save the intermediate annealing, turning correction and other processes required for mechanical forming, and effectively avoid the local stress concentration that may occur during mechanical forming.

Conclusion

The use of explosive forming technology to produce and repair crystallizer copper tubes has the characteristics of high efficiency, energy saving, low cost, and high precision. The application of this technology no longer uses large-scale pressure equipment and large-scale CNC processing machines, but only on special explosive forming devices. Using explosives as energy sources, mold copper tubes with complex shapes, high dimensional accuracy and surface finish requirements can be realized.

In the era of the development of new materials and new processes, explosive forming has an important role and great potential that cannot be replaced by traditional processing techniques; making full use of and making good use of the high temperature and high pressure generated by explosive forming is the driving force and condition for realizing explosive forming. The different changes that occur under high temperature and high pressure are the direct reflection of various effects produced by explosive molding. Explosive molding is a high-tech industry that intersects and penetrates multiple disciplines. It should have broad application prospects in the future.

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