CB2 cast steel is a heat-resistant steel developed for 630 ℃ ultra supercritical units in the European joint research project cost522 R & D plan. After the development of 620 ℃ high-efficiency ultra supercritical units in China, CB2 cast steel has been used as the manufacturing material for large steel castings (cylinders, high-temperature valves, etc.) of steam turbines. Its chemical composition is shown in the table.
| C | Si | Mn | P | S | Cr | Mo | Nb | Co | B | N | V | NI | Al |
| 0.12 | 0.21 | 0.79 | 0.0096 | 0.0084 | 9.15 | 1.74 | 0.079 | 0.98 | 0.0012 | 0.015 | 0.18 | 0.16 | 0.0099 |
Before 2017, CB2 steel castings used in China were mainly imported from Voestalpine in Austria, Sande in Germany and JSW in Japan. Since 2017, with the continuous research and research of domestic heavy machinery plants, they have mastered the key manufacturing technology of CB2 steel castings, and gradually realized the localized manufacturing and promotion and application of CB2 steel castings. Their domestic manufacturers mainly include China Erzhong Group, Ningxia shared Steel Casting Co., Ltd., Dongfang turbine Co., Ltd. (casting Division), Shanghai Honggang power station equipment casting and Forging Co., Ltd, At present, Shanghai Honggang power station equipment casting and Forging Co., Ltd. has been shut down for environmental reasons. In the past two years, with the continuous maturity of the manufacturing technology of domestic CB2 steel castings, CB2 steel castings have basically achieved more than 70% of domestic supply.
In the manufacturing process of large CB2 steel castings, the key processes are mainly casting process design, smelting and pouring, performance heat treatment, repair welding and post weld heat treatment. The operation and control level of these key processes directly affect the final product quality of CB2 steel castings.
CB2 steel castings have complex structure, large wall thickness gap, obvious stress concentration, and are prone to slag inclusion, shrinkage porosity, cracks and other defects during casting. Therefore, the design of casting process is very important in the manufacturing process of CB2 steel castings, including the design of molding scheme, gating system and the selection of casting process parameters. Among them, casting process design is the first link of casting production. Relying solely on experience and manual calculation may lead to problems such as unstable casting quality and high scrap rate; By using casting simulation software (such as ProCAST, magma, AnyCasting, Huazhu CAE, etc.), the gating system, riser design, temperature field and solidification cooling process of large castings are simulated and analyzed, and the casting defects such as shrinkage cavity and porosity are predicted in time, so as to optimize the casting process, reduce casting defects, improve the casting quality, and provide technical guidance for the implementation of casting process.
Smelting and pouring are the key processes to ensure the metallurgical quality of CB2 steel castings. The chemical composition of CB2 steel is complex, with many alloy elements, including polluting element Co, easily oxidized element B and element n, and the requirements are narrow. The technical difficulties of smelting are:
1) Precise control of chemical components, especially C, Si, N, B and other elements;
2) Final deoxidation control;
3) Co pollution. By optimizing raw and auxiliary materials, adopting electric furnace smelting, ladle vacuum refining, argon blowing and other technological measures, we can ensure that the liquid steel is fully dephosphorized, desulfurized, deoxidized, and the inclusions in the steel are removed to ensure the purity of the liquid steel;
Through the reasonable design of smelting process flow, measures are taken in the sequence of slagging, deoxidation and alloying to strengthen the deoxidization ability of molten steel, improve the nitrogen fixation ability, and realize the accurate control of chemical composition of N, B and co elements [9]. Before pouring of CB2 steel castings, argon is blown into the mold cavity, and the pouring temperature is strictly controlled by controlling the time from LF furnace tapping ladle to pouring and argon blowing time, so as to prevent low-temperature pouring caused by too fast cooling of molten steel.
The heat treatment of CB2 steel castings is carried out in stages. Among them, the preliminary heat treatment is the homogenization treatment of CB2 steel castings at 1100 ℃, which aims to refine the grain, uniform the structure, eliminate the internal stress, reduce the component segregation in the steel castings, and prepare for the subsequent performance heat treatment. Performance heat treatment adopts normalizing + high temperature tempering method. Normalizing is to rapidly cool CB2 steel castings after heat preservation at 1100~1130 ℃ to obtain strip martensite structure, while high temperature tempering is to fully temper the hard brittle martensite structure obtained by normalizing, so as to ensure that CB2 steel has high strength and good plastic toughness, so as to obtain the best comprehensive mechanical properties. Therefore, performance heat treatment is the key process to determine the mechanical properties of CB2 steel castings.
Common casting defects of CB2 steel castings are shrinkage, porosity, sand inclusion, cracks and other defects. The difficulty of repair welding is that the structure of large steel castings is complex, especially the steam turbine cylinder block, which has large stiffness and high dimensional accuracy. It is required that the temperature and deformation caused by preheating, repair welding and post welding stress relief during the repair welding of steel castings are small; It is required to completely remove the defective parts and prevent new defects after repair welding; Minimize welding deformation; Repair welds shall have certain thermal fatigue resistance, oxidation resistance and thermal stability. CB2 repair welding treatment mainly adopts homogeneous welding material WB2 for repair welding. Except for the slightly higher Ni content, the chemical composition of WB2 welding material is completely consistent with that of CB2. The preheating temperature should reach 200 ℃ before welding, and after welding, the whole body should be put into the furnace for stress relief treatment (730 ± 10 ℃).
As CB2 steel castings are new heat-resistant steels developed abroad, there are no relevant technical standards to rely on in China at present. At this stage, major turbine manufacturers only conduct product acceptance of CB2 steel castings according to the formulated enterprise standards. However, there are obvious differences in the requirements of key technical indicators in the enterprise standards of various factories, such as the control range of some elements is inconsistent, the requirements of important mechanical property indicators are not unified, and the content of non-metallic inclusions and grain size are not specified in the standards. Due to the lack of unified industry standards, great obstacles have been caused to the power station users in the quality acceptance and operation supervision of CB2 steel castings in the project construction.