CN102925815A - Martensite stainless steel for reactor internals of nuclear power station - Google Patents

Abstract

The invention provides a martensite stainless steel for reactor internals of a nuclear power station, belonging to the technical field of material for the nuclear power station. The martensite stainless steel comprises chemical components by weight percentage: 0.05-0.18% of C, less than or equal to 0.60% of Si, 0.20-0.80% of Mn, less than or equal to 0.020% of P, less than or equal to 0.020% of S, 12.0-15.0% of Cr, 0.3-1.5% of Mo, less than or equal to 0.05% of Co, less than or equal to 0.005% of B, wherein the oxygen content is no more than 20ppm, and the balance of Fe and unavoidable impurities. The martensite stainless steel has the advantages of being less in impurity, and high in room temperature and high-temperature strength.

Description

A kind of martensitic stainless steel for nuclear power plant internal components

technical field

The invention belongs to the technical field of nuclear power materials, and in particular relates to a martensitic stainless steel for nuclear reactor internal components, which is mainly applicable to the fields of energy, metallurgy, machinery, chemical industry and the like. the

Background technique

Stainless steel refers to the general term for steels with certain chemical stability in the atmosphere, water, acid, alkali and salt solutions, or other corrosive media. The good corrosion resistance of stainless steel is due to the addition of chromium to the iron-carbon alloy. Although other elements, such as copper, aluminum, and silicon, nickel, molybdenum, etc. can also improve the corrosion resistance of steel, the effect of these elements is limited without the presence of chromium. There are many types of stainless steel, according to my country's national standard GB/T13304-1991 "Steel Classification" and the international general classification method is divided into five categories according to the metallographic structure of steel, namely austenitic stainless steel, austenitic- Ferritic duplex stainless steel, ferritic stainless steel, martensitic stainless steel and precipitation hardening stainless steel. In addition, according to the alloying elements in steel, it can be divided into chromium stainless steel, chromium-nickel stainless steel, chromium-nickel-molybdenum stainless steel, chromium-manganese-nickel (nitrogen) stainless steel, as well as low-carbon stainless steel, ultra-low carbon stainless steel and high-purity stainless steel, etc. . The chemical composition of austenitic stainless steel is based on chromium and nickel, adding elements such as molybdenum, tungsten, niobium and titanium. Due to its face-centered cubic structure, it has high strength and creep properties at high temperatures; duplex stainless steel consists of two phases of austenite and ferrite, and chromium and nickel are two important alloys in duplex stainless steel Element, nickel can expand the γ phase zone in α+γ duplex stainless steel. When the chromium content in the steel is 5%, the yield strength of the steel reaches the highest value. When the nickel content is 10%, the strength of the steel reaches the maximum. Value; The chemical composition of ferritic stainless steel is characterized by 11% to 30% chromium, in which niobium and titanium are added. When the chromium content is less than 25%, the ferrite structure will inhibit the formation of martensite structure, so its strength will decrease with the increase of chromium content. When the chromium content is higher than 25%, the strength is slightly improved due to the solid solution strengthening effect of the alloy; precipitation hardening stainless steel is a type of stainless steel with ultra-high strength, which can be divided into three types according to its structure, precipitation hardening martensitic stainless steel, Precipitation hardening semi-austenitic stainless steel, precipitation hardening austenitic stainless steel. Some also classify the first category as martensitic stainless steel, and the second and third categories as austenitic stainless steel. After solid solution treatment, maraging stainless steel always exists in martensitic structure when it is cooled to room temperature, and then undergoes aging treatment from the solid solution state to produce phase precipitation and strengthen. Austenitic precipitation hardening stainless steel has a stable austenite structure, and heat treatment cannot change the structure. Therefore, it can only be strengthened by adding precipitation strengthening elements and aging treatment; martensitic stainless steel has the same properties as ordinary alloy steel. Quenching achieves hardening properties, so a wide range of different mechanical properties can be obtained by selecting grades and heat treatment conditions. Martensitic stainless steel can be divided into martensitic chromium stainless steel and martensitic chromium-nickel stainless steel. For martensitic chromium stainless steel under quenching-tempering conditions, increasing the chromium content can increase the ferrite content, which will reduce the hardness and tensile strength. Since the addition of chromium can improve the hardenability of iron-carbon alloys, it is widely used in steels that need to be quenched. Martensitic stainless steel is generally represented by 13%Cr steel. This steel has poor welding performance and needs to be preheated before welding. The structure of the welding heat-affected zone is usually hard and brittle. Only by post-weld heat treatment can its toughness and plasticity be restored. . the

With the continuous development of human society, the demand for energy is also increasing. Due to the continuous consumption of fossil raw materials such as coal and petroleum and the increasing damage to the environment caused by the use of these raw materials, people are constantly looking for clean and green energy. A nuclear power plant is a power plant that uses the energy released by nuclear fission or nuclear fusion reactions to generate electricity. Nuclear power plants currently in commercial operation use nuclear fission reactions to generate electricity. According to the type of reactor, nuclear power plants can be divided into pressurized water reactor nuclear power plants, boiling water reactor nuclear power plants, heavy water reactor nuclear power plants and fast neutron breeder reactors. Currently, pressurized water reactor nuclear power plants are the most widely used in the world and in China. The structure of the pressurized water nuclear power plant reactor is basically composed of the following parts, the reactor core, the reactor internals, the reactor pressure vessel and the top cover, and the control rod drive mechanism. The internal components of the reactor play an important role in the operation of the entire nuclear power plant, such as supporting and fixing the core assembly, centering the drive lines, guiding the movement of the control rods, and at the same time serving as coolant channels. The flow is reasonably distributed to reduce invalid flow, provide heat shielding for pressure vessels, reduce neutron gamma ray exposure, provide installation and fixing conditions for in-pile measurements, and provide a place to store samples for pressure vessel material irradiation supervision tests. Structurally, the internal components are composed of the lower support member of the core and the upper support member of the core. Shield, irradiated sample tube, and secondary support assembly. The upper support member of the core is composed of the guide cylinder support plate, the core upper grid plate, the control rod guide cylinder, the support column, the thermocouple and the compression spring. The compression spring is located between the flange of the hanging basket and the support plate of the guide cylinder, and is a ring forging of martensitic stainless steel, which compresses the lower and upper stack internals on the pressure vessel support platform. The compression spring can compensate the machining error of the flange and provide sufficient compression force, and can also compensate the compression deformation and thermal expansion of the internal components of the stack. When the pressure vessel top cover is installed, the compression spring is compressed to limit the axial displacement of the upper and lower core support assemblies, so the compression spring is an important part of the reactor internals. the

China's nuclear power development has experienced a development process from scratch, from low to high. Through the combination of independent innovation and digestion and absorption of foreign advanced nuclear power technology, my country's nuclear power technology has now possessed research and development capabilities close to the world's advanced level. China already has the independent design capability of 300,000-600,000-kilowatt pressurized water reactor nuclear power plants, and basically has the design capability of the second-generation 1,000,000-kilowatt nuclear power plants. China's self-innovated second-generation pressurized water reactor nuclear power technology has started construction in China. In 2006, China introduced the world's most advanced third-generation pressurized water reactor nuclear power plant AP1000 from the United States. The introduction of this project has brought my country's nuclear power industry Pushed to a new peak. Whether it is the second-generation CPR1000 or the third-generation AP1000, the internal components of the reactor include the compression spring as an important part. In the second-generation technology, the compression spring is made of martensitic stainless steel. The material used for the tight spring is still martensitic stainless steel. the

Contents of the invention

The purpose of the present invention is to provide a martensitic stainless steel for nuclear reactor internal components, which has higher purity, room temperature strength and high temperature strength than the same type of steel. the

The martensitic stainless steel of the present invention is smelted in a vacuum induction furnace, and the weight percentage of its chemical composition is: C: 0.05-0.18%, Si: ≤0.60%, Mn: 0.20-0.80%, P: ≤0.020%, S: ≤ 0.020%, Cr: 12.0~15.0%, Mo: 0.3~1.5%, Co: ≤0.05%, B: ≤0.005%, wherein the oxygen content is not more than 20ppm (ie ≤0.002%). The balance is Fe and unavoidable impurities. the

The key of the invention lies in: through reasonable optimization of important alloy elements in steel and reasonable control of gas elements, a martensitic stainless steel for nuclear power plant internal components with less inclusions and high strength is obtained. the

Common alloying elements in steel are C (carbon), Si (silicon), Mn (manganese), Cr (chromium), Ni (nickel), Mo (molybdenum), W (tungsten), Co (cobalt), Cu (copper ), Nb (niobium), Al (aluminum), Ti (titanium), B (boron), N (nitrogen), RE (rare earth), etc. Rare earth elements are usually composed of La (lanthanum) and Ce (cerium). S (sulfur) and P (phosphorus) are impurity elements in steel. Carbon is an indispensable element in steel. Carbon not only expands the austenite phase region in steel, but also is a constituent element of high-strength carbides. Its strengthening effect is related to temperature. As the temperature increases, the strengthening effect decreases due to the aggregation of carbides. The carbon content is low, the strength is insufficient, the carbon content is too high, the plasticity is insufficient and the welding performance is not good; silicon is a beneficial element for high temperature corrosion resistance in heat-resistant steel. At high temperature, a protective layer will be formed on the surface of heat-resistant steel containing silicon. Good _SiO2 film. When the silicon content in the steel reaches 1%, there is an obvious anti-oxidation effect. For example, when the silicon content in Cr5Mo steel increases from 0.2% to 1%, the high temperature oxidation resistance of the steel is significantly improved; manganese can eliminate or Reduce the hot embrittlement caused by sulfur, thereby improving the hot workability of steel. Manganese is also a carbide forming element, which enters the carbide to replace a part of the iron atoms. Manganese is beneficial to the improvement of the high-temperature instantaneous strength of steel, but it does not help much to improve the high-temperature long-term performance; Cr is an important alloying element in stainless steel, and more than 12% of Cr in steel will make the steel have good corrosion resistance. Cr in steel will react with oxygen during the oxidation process, forming a dense Cr ₂ O ₃ film on the surface of the steel, preventing further reaction between oxygen and the matrix, and playing the role of anti-oxidation and corrosion resistance. Therefore, Cr in stainless steel The content should not be too low, and the Cr content in the present invention is 12.0~15.0%; molybdenum is also an important alloying element in stainless steel, and like chromium, molybdenum is also an element that forms and stabilizes ferrite and expands the ferrite phase region, and molybdenum forms iron The capacity of the element body is comparable to that of chrome. Molybdenum can be dissolved in ferrite, austenite and carbide in stainless steel, and has the effect of solid solution strengthening. When the content of molybdenum is low, it can form composite carbides with iron and carbon, and can also improve the stability of carbides. When the content is high, it can form special carbides of molybdenum. In addition, molybdenum can improve the hardenability of martensitic stainless steel and improve the strength of steel. When molybdenum exists together with chromium, manganese, etc., it can reduce or inhibit the temper brittleness caused by other impurity elements. Cobalt is also a commonly used element in stainless steel, but in the primary circuit of nuclear power plants, since stainless steel is directly exposed to the radiation of nuclear fuel, cobalt will form new cobalt isotopes after receiving radiation, and become a radioactive source, and the half-life is very long. If its content is not limited, it will inevitably affect the normal operation of nuclear power plants. The hazards of boron in nuclear power components are similar to those of cobalt and must be limited. Oxygen is a harmful element in steel, and it is easy to form oxide inclusions with oxide-forming elements, which affects the purity of steel. The lower the oxygen content, the higher the production cost and the higher the requirements for equipment. On the premise of meeting the performance requirements of the steel, determining an appropriate oxygen content is not only a requirement to improve the purity of the steel, but also a requirement to ensure the production cost.

Compared with prior art, the beneficial effect of the present invention is:

Through the reasonable optimization and matching of key elements in martensitic stainless steel, the produced martensitic stainless steel has the characteristics of high purity and high strength. the

Detailed ways

The present invention will be further described below in conjunction with a typical embodiment. the

In this embodiment, the specific composition of the martensitic stainless steel used and the composition of the comparative material SUS403 stainless steel are shown in Table 1. The specific technological process of the new martensitic stainless steel is as follows: select the fine material, use vacuum induction furnace to smelt, the smelted steel ingot is opened to form a billet, the billet after billet is surface polished, corner cracks and surface defects are removed, and the furnace is heated . The billet forging temperature is 1120~1160°C, the final forging temperature is not less than 900°C, and it is slowly cooled after forging. The forged bar is used for component analysis, inclusion evaluation, room temperature and high temperature mechanical performance testing after heat treatment, etc. the

The comparison of the non-metallic inclusions of the embodiment of the present invention and the comparison material SUS403 stainless steel is shown in Table 2. Because the oxygen content in the steel of the present invention is low, the non-metallic inclusions in the steel, especially the oxide level, are lower than SUS403, showing that the steel of the present invention has a relatively high High purity with low inclusion levels. the

The room temperature tensile properties of the examples of the present invention and the comparative materials are compared in Table 3, from which it can be seen that under the same heat treatment system, the room temperature tensile strength of the present invention is higher than that of the comparative materials. the

The high-temperature tensile properties of the examples of the present invention and the comparative materials are shown in Table 4, from which it can be seen that the high-temperature tensile strength of the present invention is higher than that of the comparative materials at 350° C. under the same heat treatment system. the

The room temperature properties of the examples of the present invention under other heat treatment regimes are shown in Table 5, from which it can be seen that the examples of the present invention have relatively high room temperature mechanical properties. the

Table 1 Chemical composition of the embodiment of the present invention and comparative material composition (wt%)

Table 2 Inclusion Levels of Examples of the Invention and Comparative Materials

Table 3 Comparison of tensile properties at room temperature of the examples of the present invention and comparison materials at room temperature

Table 4 Comparison of the room temperature tensile properties of the examples of the present invention and the high temperature tensile properties of the reference materials

Table 5 Mechanical properties at room temperature of the examples of the present invention

Claims (1)

1. A martensitic stainless steel for nuclear power plant internal components, belonging to the technical field of nuclear power plant materials. The weight percentage of its chemical composition is: C: 0.05~0.18%, Si: ≤0.60%, Mn: 0.20~0.80%, P: ≤0.020%, S: ≤0.020%, Cr: 12.0~15.0%, Mo: 0.3 ~1.5%, Co: ≤0.05%, B: ≤0.005%, of which the oxygen content is not more than 20ppm; the balance is Fe and unavoidable impurities.