The traditional manufacturing method of large forgings is to use large steel ingots for blanking and free forging. However, with the continuous development of major equipment, the requirements for large forgings are getting higher and higher, not only the specifications and sections are getting larger and larger, but also the internal quality is constantly improving, and the traditional manufacturing methods are difficult to meet the requirements. In order to meet the needs of high-end equipment and achieve the goal of combining large-scale forgings, it is urgent to change the manufacturing method. To this end, extreme manufacturing methods such as homogenization of blanks, integrated manufacturing, and die forging, represented by the development of traditional steel ingot manufacturing technology and the development of new additive blanking technology, have emerged.
Large forgings are the basic components of electric power, metallurgy, petrochemical, shipbuilding, mining, aerospace, military and other equipment. They are economically driven and cover a wide range, which is an indispensable part of the equipment manufacturing industry chain.
The traditional manufacturing method of large forgings is free forging, and the "fat head big ear" and "stupid big black thick" used to be synonymous with large forgings. In order to reverse the passive situation of low utilization rate, high cost and unstable quality of large forging materials, it is necessary to innovate ideas, challenge “impossible”, and carry out transformation and upgrading of large forgings. Through homogenization, integration, and die forging, the goal of extreme manufacturing is achieved.
The quality of large forgings is mainly manifested in three aspects of purity, uniformity and compactness. To improve the quality of large forgings, reduce manufacturing costs, and achieve extreme manufacturing, it is necessary to start the development work from the "three characteristics" of forgings. The relationship between different processes of hot working and the "three characteristics" of forgings.
In the hot processing of large forgings, ingot/casting is a key part of the “three natures” of forgings. Therefore, forging suppliers at home and abroad are paying more and more attention to the preparation of forging blanks. In order to obtain a homogenized billet, in addition to the integrated innovation of the traditional billet process (smelting, ingot casting, billeting), various additive blanking methods are also eager to try.
Large, super large steel ingot preparation
In order to improve the uniformity and purity of large and super large steel ingots, China has integrated a series of smelting and ingot casting technologies.
(1) Low silicon controlled aluminum steel smelting technology.
In order to reduce inclusions and obtain intrinsic fine grain steel, a low silicon controlled aluminum steel manufacturing technique was invented. It not only improves the purity of the molten steel, but also obtains the intrinsic fine-grain steel. It also effectively controls the segregation of the components of the ultra-large steel ingot and reduces the inclusions and gas content in the ingot. The low-silicon controlled aluminum steel smelting and pouring method produces high-quality forgings with uniform composition and few inclusions.
(2) Protection pouring.
1) New tundish.
In order to reduce the steel slag being involved in the ingot mold during the ingot casting process, the experience of “retaining the wall” and “blocking the dam” in the continuous casting process of the metallurgical industry was used to invent the new middle with the “retaining wall” and “blocking dam”. package. Numerical simulations and engineering practices have shown that the inclusion content in the ultra-large steel ingots cast by the new tundish is greatly reduced compared to the traditional round tundish.
2) Long nozzle protection pouring.
The injection of air in the pouring process is an important reason for the secondary oxidation of molten steel. The secondary oxidation of molten steel not only causes the formation of harmful oxide inclusions, but also causes the forgings to be scrapped, and also increases the content of gases (H, O, N) in the ingots. The high gas content is one of the main causes of defects in ultra-large forgings. In order to avoid the secondary oxidation of molten steel, based on the experience of the metallurgical industry, the long nozzle protection casting technology was developed to effectively avoid the secondary oxidation of molten steel.
3) Secondary filling.
Segregation is the inevitable result of the selective crystallization of molten steel and the solidification process of steel ingot. The larger the steel ingot, the more serious the defects such as segregation and shrinkage. When China developed a support roll with a 459t steel ingot with an average C content of 0.62%, it broke at the roll body near the riser end. After analyzing the macroscopic morphology of the fracture site, it was found that the secondary shrinkage cavity was serious, and the C content in the lower part of the riser was as high as 1.16%, which was close to 2 times of the standard value. In order to solve this problem, the secondary infusion technology of steel ingots was invented, and the C content in the lower part of the riser was reduced to about 0.8%, and the ultra-large steel ingots for the 5m and 5.5m support rolls were successfully manufactured.
Although large forging suppliers at home and abroad continue to improve the purity of large steel ingots, the inherent defects of steel ingots cannot be eradicated. In addition, the traditional steel ingot preparation and blanking method (Fig. 3) leads to a low material utilization rate of large forgings. After the steel ingot removes water, riser and uplift and long fire, the billet utilization rate of the billet is 70%. about.
At present, all countries in the world have taken additive manufacturing as a new growth point for future industrial development, and strive to seize the future high point of technology and industry. The development stage of China's additive manufacturing industry has shifted from research and development to industrial application. New equipment, new technologies, new materials and new applications are constantly being introduced. More and more enterprises are taking additive manufacturing as the direction of industrial upgrading and technological transformation.
Additive manufacturing technology is the use of materials to gradually add a method to manufacture solid parts, compared to the traditional material removal - cutting technology, is a "bottom-up" manufacturing method. Academician Guan Guan proposed the concept of “generalized” and “narrow sense” additive manufacturing. The “narrow sense” additive manufacturing refers to the technical system of combining different energy sources with CAD/CAM technology and layering and accumulating materials; Additive manufacturing is based on the accumulation of materials as a basic feature, a large-scale technology group that aims to directly manufacture parts. If classified according to the type and manner of processing materials, it can be divided into metal forming, non-metal forming, and biomaterial forming.
Due to the room for improvement in the purity and uniformity of large forgings, some metal blank preparation technologies related to additive manufacturing have entered the stage of development and application.
(1) 3D printing.
As an application form of AM, 3D printing is characterized by easy forming and difficult modification. At present, the metal 3D printing cladding technology for industrial application has the advantages of laser cladding deposition and fused additive manufacturing. The common point is that the formed metal is as-cast structure, and the compactness is poor compared with the forging, and the formed part exists inside. The pores and pore morphology are regular spherical or spheroidal, and the distribution is random. Therefore, limited by factors such as manufacturing cost, compactness, etc., 3D printing is currently only suitable for direct forming of thin section metal parts.
(2) Spray forming.
Spray forming is a process in which a high-pressure inert gas atomizes an alloy liquid stream into fine droplets, flies and cools under a high-speed air stream, and is deposited into a blank before it is completely solidified. It has become a new material development and application in the world. hot spot. However, subject to the conditions of rapid solidification, manufacturing cost, etc., the spray forming technique is currently only applicable to the manufacture of small and thin section blanks or parts, and cannot be applied to the manufacture of ingots/blanks of super large and thick sections.
(3) "Covered" ingots.
In order to solve the problem of segregation of large steel ingots, a foreign research institute invented the "cladding" technology for manufacturing ESR ingots. Because it is "coating" layer by layer, and each layer is electroslag remelting, it is also called additive manufacturing.
(4) "No trace construction".
The basis of "no trace construction" is the diffusion connection. Under the action of high vacuum, high temperature, high pressure, large deformation and other factors, the same or dissimilar metals form a strong metal bond on the bonding surface, and a small amount of microscopic holes and bonding layers disappear further under diffusion, so that the bonding interface and the substrate are Completely consistent in composition, organization and performance. Diffusion bonding technology has played an active role in composite sheet rolling. Since the joint structure of the joints after diffusion bonding between the same materials is basically the same as that of the base material, it is easier to make the "scar-free" blank of the same material.
Although the "face diffusion" and "body diffusion" of the blanks produced by "no trace construction" can be solved by forging (similar to "kneading") and high temperature maintenance of the blank during forging (similar to "awake"), large blanks The two major problems of the difficult-to-deformation zone at the end and the tensile stress zone in the middle of the upsetting process need to be taken seriously. The difficult-to-deformation zone during upsetting can be improved by adding insulation pads up and down, while the tensile stress zone can be avoided by increasing the strain rate or optimizing the shape of the blank before upsetting.
In addition to homogenized blanks and integrated manufacturing, the highest goal of extreme forging of large forgings is the extremely important content of dieless forging without cutting.
Forgings to be die-formed
Compared with the free forging of other forging suppliers at home and abroad, the material utilization rate of the overall water chamber head in China has been improved by more than 30%, but there is still room for optimization compared with the finished forgings. In order to realize the die forging of oversized shaped forgings, the die forging forming scheme of the CAP1400 reactor pressure vessel integrated connecting pipe section and the steam generator water chamber head was studied.
China has re-compressed the rolls and shafts of large and super large shaft forgings such as support rolls and rotors, and forged the rolls and journals. Table 2 shows the cost comparison of the different forming methods of the 2250mm continuous rolling mill support roll of 45Cr4NiMoV. It can be seen from Table 2 that the continuous casting billet is forged and formed by die forging and traditional steel ingot blanking. Compared with free forging, it has the advantages of material saving, energy saving and greatly reduced manufacturing cost.
The goal of ultra-large forgings is to have both "shape and spirit", both to obtain the shape of the casting, but also to maintain the structure and performance of the forging. In order to achieve the goal of "shaping the spirit and the spirit", large forgings need to meet the requirements of "shape", "granule" and "force" at the same time. "Shape" is near net shape; "grain" means that the grains are uniform and fine; "force" is formed under the condition of three-direction compressive stress.
In order to realize additive blanking and die forging, and to ensure the development of “shape”, “grain” and “force” for ultra-large forgings, it is necessary to develop a forging press and free forging in terms of pressure, space and speed. The special super press with the advantage of machine and extruder is determined to be 1500MN; 10m×10m×12m. The following transformation and upgrade can be achieved by constructing a 1500 MN press.
(1) Change the forming method.
If the innovation of the last round of forging manufacturing focuses on the large-scale and integration of the single scale, then the future innovation of "Made in China 2025" will be the individualization and refinement of the shape of the forging, and the high alloying of the material. The toughness and high density of the texture, to achieve the above innovations, non-super presses.
(2) Change the blanking method.
Adding material blanks to replace traditional ingots is a revolutionary process innovation that can gradually eliminate the smelting and ingots of the ultra-large forgings manufacturing industry. It not only solves the outstanding problems of the smelting and ingot operators of the forgings manufacturing industry, but also solves the worldwide problem of uniformity and poor compactness of super large steel ingots, and can also reduce the manufacturing cost of ultra-large forgings by more than 50% (cancellation of water) , risers, reduce forging fires, etc.).
(3) Castings forgings.
Practice has proved that with the continuous improvement of major equipment requirements, the casting of product parts has become a development trend. Castings such as nuclear power main pipes and pump casings have been changed to forgings because they cannot meet the requirements. Turbine blades have also evolved from cast forming to forging. The forged casting of parts such as the upper crown of the turbine, the turbine block, and the bearing housing in the rolling equipment will also be not far away.
(1) Low Si-controlled Al steel smelting and long nozzle protection casting technology are the first choice to ensure the purity and uniformity of super large steel ingots.
(2) Additive blanking is a development direction that greatly improves the homogeneity and material utilization rate of large forgings.
(3) Integrated manufacturing and die forging are the only way to manufacture extreme forgings.
(4) The super large press is the guarantee for the homogenization of large forgings, integrated manufacturing and die forging, and is also a necessary and sufficient condition for the transformation and upgrading of large forgings.