Research on Technology and Application of 20Cr1Mo1VNbTiB High Temperature and High Strength Steel

20Cr1Mo1VNbTiB (also known as "Zhengqi No. 1 Steel") is a pearlitic heat-resistant steel independently developed in China, specifically designed for fasteners operating in high-temperature and high-pressure environments. Through precise alloying and advanced heat treatment processes, this steel achieves an exceptional balance of high-temperature strength, anti-relaxation properties, and structural stability, making it a benchmark for domestically produced alternatives to imported materials. According to the DZ2.16.4-2020 standard, this material is primarily used in high-temperature fasteners and key components in the energy and power, petrochemical, and aerospace industries.

The development of 20Cr1Mo1VNbTiB steel stems from China's urgent need for high-performance, high-temperature bolt materials. In industries like thermal power generation and the petrochemical industry, high-temperature fasteners operate in harsh environments for extended periods, enduring extremely high temperatures and stresses. Conventional materials are prone to relaxation, creep, and structural instability at temperatures above 570°C, leading to reduced equipment reliability and increased maintenance costs. The successful development of 20Cr1Mo1VNbTiB overcomes these technical challenges, surpassing the performance of earlier Soviet ЭИ723 steels in all aspects. When used below 570°C, it even approaches the performance level of 12% chromium steels ЭИ993 and ЭИ802.

Standard DZ2.16.4-2020 is a technical specification for steel used in high-temperature fasteners, covering the material's chemical composition, mechanical properties, heat treatment processes, inspection methods, and quality requirements. This standard ensures the reliability and safety of 20Cr1Mo1VNbTiB steel under high-temperature conditions, providing a technical basis for design, material selection, manufacturing, processing, and maintenance. 20Cr1Mo1VNbTiB steel meeting this standard has been fully utilized in major projects such as China's Hualong One nuclear power project and ultra-supercritical thermal power units, breaking the monopoly of foreign technology and achieving domestic substitution.

The alloy system of 20Cr1Mo1VNbTiB steel is based on chromium (Cr), molybdenum (Mo), and vanadium (V), supplemented by microalloying elements such as niobium (Nb), titanium (Ti), and boron (B), forming a scientific and reasonable chemical composition ratio. According to the requirements of the DZ2.16.4-2020 standard, the content range and control requirements of each chemical element are shown in the following table:

All alloying elements in steel work synergistically to ensure the material's comprehensive performance. Chromium (Cr) forms a dense oxide film on the steel surface, effectively protecting it from corrosive media like air and water, ensuring long-term, stable service in conventional environments. Molybdenum (Mo) is the key element in improving steel's high-temperature performance. It not only enhances its strength, hardness, and toughness, but also significantly improves its creep resistance in high-temperature environments, ensuring its stability under high-temperature, high-pressure conditions.

Vanadium (V) and niobium (Nb) form stable carbonitrides, refining grain size and inhibiting high-temperature softening. These compounds remain stable at high temperatures, effectively preventing grain growth and dislocation movement, thereby improving the material's high-temperature strength and creep resistance. Titanium (Ti) combines with carbon to form titanium carbide, mitigating the negative effects of carbon in steel while also helping to refine grain size and enhance the steel's toughness and fatigue resistance.

Boron (B), despite its extremely low content (0.001-0.004%), plays a crucial role. Even a tiny amount of boron can significantly improve the hardenability of steel, allowing it to achieve a deeper hardened layer during quenching, significantly enhancing its overall mechanical properties. To ensure the effective effect of boron, an appropriate amount of titanium (Ti) is added to protect it. Titanium's stronger bonding with nitrogen prevents the boron from forming boron nitride, which can lead to its ineffectiveness.

The DZ2.16.4-2020 standard has extremely strict requirements for the control of impurity elements, stipulating that sulfur (S) and phosphorus (P) contents must be ≤0.025% and ≤0.020%, respectively, to ensure the purity and uniformity of the material. This strict control improves the steel's corrosion resistance and resistance to stress corrosion cracking, especially in oil and gas environments containing hydrogen sulfide.

20Cr1Mo1VNbTiB steel exhibits **excellent mechanical properties** at both room and high temperature environments, making it an ideal choice for high-temperature fasteners and critical components. According to the requirements of the DZ2.16.4-2020 standard, the mechanical properties of this steel must meet the following requirements:

20Cr1Mo1VNbTiB steel offers exceptional high-temperature performance, with a sustained strength of ≥100 MPa at 560°C, surpassing comparable 12% chromium steels (such as ЭИ993). It maintains high preload at 570°C, exhibiting significant anti-relaxation properties, extending bolt life threefold compared to conventional materials. This characteristic is crucial for fasteners operating in high-temperature environments, ensuring long-term operation without leaks or failures caused by loosening fasteners.

The structural stability of steel is key to its long-term, reliable operation at high temperatures. 20Cr1Mo1VNbTiB steel exhibits no significant σ phase precipitation after long-term high-temperature service, eliminating the risk of brittle fracture. This characteristic is due to sophisticated alloy design and rigorous heat treatment processes, which enable the material to maintain a stable microstructure at high temperatures.

20Cr1Mo1VNbTiB steel also exhibits excellent toughness and impact resistance. According to the DZ2.16.4-2020 standard, different impact toughness requirements apply to bolts of different sizes: for bolts smaller than M52, ak ≥ 78.4 J/cm²; for bolts equal to or larger than M52, ak ≥ 58.8 J/cm². This excellent toughness prevents brittle fracture when subjected to impact loads, improving the safety of equipment and components.

20Cr1Mo1VNbTiB steel also exhibits excellent corrosion and oxidation resistance. The addition of chromium imparts a degree of corrosion resistance to the steel, maintaining excellent stability in air, water, and some mildly corrosive media. This characteristic is particularly important in corrosive environments such as petrochemicals and marine engineering, effectively extending the service life of equipment and fasteners.

The excellent performance of 20Cr1Mo1VNbTiB steel depends largely on its scientific heat treatment process and strict microstructure control. According to the requirements of the DZ2.16.4-2020 standard, the heat treatment of this steel needs to follow specific process parameters and quality control standards to ensure the final product has stable and consistent performance.

1)Heat treatment process

The standard heat treatment process for 20Cr1Mo1VNbTiB steel includes the following key steps:

Forging process: Heating to 1100-1150℃, final forging temperature ≥850℃, and sand cooling for more than 48 hours after forging to eliminate stress. This process requires strict control of heating temperature and cooling rate to avoid excessive internal stress and structural defects.

Quenching: Oil or water quenching at 1020-1040°C. This step aims to achieve a high-strength martensite or bainite structure, preparing for subsequent tempering. The heating rate, holding time, and cooling rate during the quenching process all require precise control to ensure uniform and consistent microstructure.

Tempering: Temper at 700-740°C for at least 6 hours, followed by air cooling. The purpose of tempering is to eliminate internal stresses generated during quenching, improve the toughness and stability of the material, and obtain a tempered bainite structure. Tempering temperature and time significantly affect the final properties and must be strictly controlled according to standard requirements.

2) Microstructural characteristics

Properly heat-treated 20Cr1Mo1VNbTiB steel exhibits a typical tempered bainite structure, which offers excellent stability and performance retention at high temperatures. Carbide-forming elements (such as vanadium, niobium, and titanium) in the microstructure form fine, stable carbonitride compounds, which are difficult to aggregate and grow at high temperatures. This effectively prevents grain boundary migration and dislocation motion, improving the material's high-temperature strength and creep resistance.

The DZ2.16.4-2020 standard has strict requirements on the grain size of materials, and it is necessary to limit coarse grains (≤ level 5) to avoid performance degradation caused by improper hot working. Grain size control is mainly achieved through the following aspects:

Alloy design: By adding elements such as niobium, titanium, and vanadium, carbonitrides are formed to prevent grain growth during heat treatment.

Hot working control: Strictly control the forging temperature and deformation amount to further refine the grains through deformation-induced precipitation.

Optimization of heat treatment parameters: Control quenching heating temperature and holding time to prevent excessive grain growth.
 
3) Relationship between tissue stability and performance

The long-term performance of 20Cr1Mo1VNbTiB steel depends largely on its structural stability. During long-term service at high temperatures, the material may undergo structural changes, such as the aggregation and growth of carbides and the precipitation of new phases. These changes can lead to performance degradation or even failure.

The DZ2.16.4-2020 standard requires that 20Cr1Mo1VNbTiB steel maintains stable performance after long-term high-temperature exposure. This is mainly achieved through the following mechanisms:

Stable carbide formation: Carbon and nitrogen compounds formed by elements such as vanadium, niobium, and titanium have extremely low solubility and diffusion rate at high temperatures, making it difficult for them to aggregate and grow, thus maintaining long-term stability.

Inhibit the precipitation of harmful phases: Through precise control of chemical composition, the precipitation of harmful phases such as σ phase and Laves phase is inhibited to avoid increased brittleness and decreased toughness.

Grain boundary strengthening: The segregation of boron at the grain boundary can reduce the grain boundary energy, improve the grain boundary stability, and inhibit grain boundary sliding and void formation.

These mechanisms work together to ensure that 20Cr1Mo1VNbTiB steel can maintain stable performance and organizational integrity during long-term service at high temperatures, meeting the strict requirements of the DZ2.16.4-2020 standard.

20Cr1Mo1VNbTiB steel is widely used in various industrial fields due to its excellent high-temperature performance and good comprehensive mechanical properties. According to the requirements of the DZ2.16.4-2020 standard, this material is mainly used in key components and fasteners in high-temperature, high-pressure and corrosive environments.

1)Energy and Power Sector

In the thermal power generation sector, 20Cr1Mo1VNbTiB steel is primarily used to manufacture key components such as high-temperature bolts, turbine rotors, and main steam valves, operating below 570°C. It replaced the earlier-used Soviet IE723 steel, extending component life by over 50%. In ultra-supercritical thermal power units, this material is used to manufacture high-temperature fasteners, capable of withstanding higher temperatures and pressures, improving power generation efficiency and reliability.

In the nuclear power equipment sector, 20Cr1Mo1VNbTiB steel is used to manufacture radiation-resistant fasteners, with a long-term operating temperature of up to 510°C. This material has been fully implemented in China's Hualong One nuclear power project, breaking the foreign technology monopoly and achieving domestic substitution. The DZ2.16.4-2020 standard sets stricter requirements for steel used in nuclear power plants, and 20Cr1Mo1VNbTiB steel fully meets these requirements, ensuring the safety and reliability of nuclear power equipment.

2) Petrochemical Industry

In the petrochemical industry, 20Cr1Mo1VNbTiB steel is used to manufacture key components such as high-temperature pipe connectors, reactor bolts, and pressure vessel fasteners. This material exhibits excellent resistance to stress corrosion cracking (SCC) in oil and gas environments containing hydrogen sulfide, significantly improving the safety and service life of equipment.

High-temperature, high-pressure pipelines in petrochemical equipment place extremely high demands on the sealing and reliability of connectors. The high-temperature relaxation resistance and structural stability of 20Cr1Mo1VNbTiB steel ensure that bolt preload is maintained during long-term operation, preventing leakage accidents. According to the DZ2.16.4-2020 standard, materials used in these applications undergo more rigorous inspection and testing to ensure their performance meets design requirements.

3) Aerospace

In the aerospace industry, 20Cr1Mo1VNbTiB steel is used to manufacture critical parts such as turbine disk fastening bolts and jet engine components. These components must meet high-cycle fatigue strength requirements at 560°C, posing extremely high challenges to the material's overall performance.

Aerospace applications place extremely high demands on material reliability and consistency, and the DZ2.16.4-2020 standard sets strict quality control requirements.

20Cr1Mo1VNbTiB steel meets these stringent requirements through precise chemical composition control, rigorous heat treatment processes, and comprehensive performance testing, ensuring the safety and reliability of aerospace equipment.

4) Heavy Equipment Manufacturing

In heavy equipment manufacturing, 20Cr1Mo1VNbTiB steel is used as a nitriding steel in the manufacture of valve stems, gears, and other components, achieving a surface hardness exceeding 600 HV. These components must withstand high loads, high wear, and high temperatures, placing high demands on the material's strength, hardness, and wear resistance.

The DZ2.16.4-2020 standard clearly stipulates the performance and quality of steel for heavy equipment. Through reasonable heat treatment process and surface treatment technology, 20Cr1Mo1VNbTiB steel can meet these requirements, thereby improving the reliability and service life of heavy equipment.

The following is a summary table of the main application areas of 20Cr1Mo1VNbTiB steel:

The DZ2.16.4-2020 standard is a professional technical specification for steel used in high-temperature fasteners. It sets clear requirements for the chemical composition, mechanical properties, heat treatment process, inspection methods, and quality requirements of 20Cr1Mo1VNbTiB steel. Materials meeting this standard undergo a rigorous quality control process to ensure reliability and safety in high-temperature and high-pressure environments.

1) Standard technical requirements

The main technical requirements of DZ2.16.4-2020 standard for 20Cr1Mo1VNbTiB steel include:

Chemical composition: specifies the precise control range of each alloying element, especially the control requirements of trace elements (boron, niobium, titanium, etc.), as well as strict restrictions on impurity elements (sulfur, phosphorus).

Mechanical properties: Specifies indicators such as strength, toughness, hardness and endurance strength at room temperature and high temperature to ensure the performance reliability of the material under service conditions.

Heat treatment process: The temperature range, holding time and cooling method of heat treatments such as quenching and tempering are clarified to ensure the consistency of material structure and performance.

Inspection methods: Specifies standards for chemical composition analysis, mechanical property testing, non-destructive testing, and metallographic inspection to ensure the verifiability of product quality.

Quality requirements: Clear requirements are put forward for the surface quality, internal defects, dimensional deviation and other aspects of the material to ensure the overall quality of the product.

2) Quality Control Measures

To ensure that 20Cr1Mo1VNbTiB steel meets the requirements of DZ2.16.4-2020 standard, the following quality control measures need to be taken during the production process:

Smelting Control: Utilizing a dual process of electric furnace smelting, vacuum degassing (VD), and electroslag remelting (ESR), we strictly control sulfur (S) and phosphorus (P) content to ≤ 0.025% to ensure material purity. This process requires precise control of the amount and timing of alloying element addition to ensure uniformity and accuracy of composition.

Hot working control: Strictly control the forging heating temperature (1100-1150°C), final forging temperature (≥850°C), and cooling method (sand cooling) to ensure uniformity of the forging blank structure and fine grain size. For large-sized forgings (such as round steel with a diameter of ≥200mm), special control of grain uniformity is required.

Heat treatment control: An intelligent heat treatment system monitors tempering temperature in real time (accuracy ±1°C) to reduce performance fluctuations. The heat treatment process requires precise control of heating rate, holding time, and cooling rate to ensure uniformity and consistency of the structure.

Inspection and testing: In accordance with the requirements of DZ2.16.4-2020 standard, comprehensive performance testing and organizational analysis are carried out, including chemical composition analysis, mechanical property testing, endurance strength testing, impact testing, hardness testing, metallographic analysis and non-destructive testing. These tests ensure that the product performance meets the standard requirements.

3) Certification and Compliance

The production process and final product of 20Cr1Mo1VNbTiB steel are subject to rigorous certification and compliance assessment to ensure compliance with the requirements of the DZ2.16.4-2020 standard. This includes:

Material certification: Obtain material certification through comprehensive performance testing and evaluation of materials by a third-party organization.

Process Certification: Evaluate the production process, quality control system and testing capabilities to ensure the stability and consistency of the production process.

Product Certification: Each batch of products is inspected and tested, and a certificate of conformity is issued to ensure that the products meet the requirements of the DZ2.16.4-2020 standard.
 
These certifications and compliance assessments ensure the reliability and safety of 20Cr1Mo1VNbTiB steel in high-temperature and high-pressure environments, and provide technical support for design, material selection, and equipment manufacturing.

In summary, 20Cr1Mo1VNbTiB steel, as a high-performance high-temperature fastener steel, meets the technical requirements and quality control measures of the DZ2.16.4-2020 standard, ensuring its reliability and safety in high-temperature and high-pressure environments. This material has been successfully applied in numerous major Chinese projects, breaking the monopoly of foreign technology, achieving domestic substitution, and making a significant contribution to the development of China's manufacturing industry.