Metallurgical Science and Technology Award Series Report · First Prize Basic research and engineering application of nano phase deep hydrogen trap in high strength and toughness steel

Background and problems of the research

With the depletion of fossil energy, the search for clean and alternative energy has become the focus of global attention. Hydrogen energy has been widely promoted around the world due to its abundant reserves, green environment protection, ideal caloric value and high utilization rate. In the new era of carbon peak and carbon neutrality, hydrogen energy is an important strategic direction to achieve carbon neutrality and sustainable development. In addition, to meet the growing social and market demand and the development needs of related industries, it is the national industrial policy to develop high-strength and tough-resistant hydrogen brittle steel and extend its service life. However, the hydrogen embrittlement problem of high strength steel has always been an important bottleneck restricting its development and application, whether in the process of smelting, rolling, heat treatment, welding, electroplating and other production and preparation, or in the process of storage, transportation and service. The higher the strength of high strength steel, the greater the sensitivity of hydrogen embrittlement, the more serious the harm.

At present, the scientific mechanism of hydrogen embrittlement and hydrogen damage has been relatively clear, but the means of engineering hydrogen removal are still limited to raw material control, vacuum degassing of liquid steel and slow cooling of piling, which can remove diffusible hydrogen to a certain extent. However, during the service of high strength steel, hydrogen will enter the system and eventually cause serious harm. Therefore, the essential problem of hydrogen embrittlement has not been completely solved, especially for high-strength steel used in major equipment. How to solve the bottleneck problem of hydrogen embrittleness and hydrogen damage of high strength steel from the fundamental problem of steel material design and preparation? It is of great scientific significance and engineering value to construct deep hydrogen trap. Developing new methods and ideas, exploring and developing high-strength steel which can improve both strength and anti-hydrogen embrittlement, has important engineering significance for the development and utilization of resources and energy and national defense security, and has important theoretical value for the development and improvement of anti-hydrogen embrittlement research.

Aiming at the hydrogen embrittlement problem faced by high strength steel, this project systematically characterized the parameters of shallow hydrogen trap and deep hydrogen trap in high strength steel through the characterization of hydrogen trap and the observation and analysis of the microstructure in steel. It was concluded that in order to improve the anti-hydrogen embrittlement energy, high density deep hydrogen trap should be designed and prepared, and hydrogen should be uniformly dispersed in the grain. The physical nature of the deep hydrogen trap at the semi-coherent interface of nano-phase has been studied comprehensively, systematically and deeply by means of atomic observation under high resolution transmission electron microscopy, first-principles simulation and hydrogen desorption experiments. It has been revealed that the mismatching dislocation at the semi-coherent interface is the root of the deep hydrogen trap, and the hydrogen embrittlement of high-strength steel has been inhibited by the design of the precipitated deep hydrogen trap in nano-phase. In combination with the design of multiple microalloy composition and content, this project adopts the method of local microsupply to obtain the multi-component nano-phase strengthened steel with excellent hydrogen brittleness, which provides an effective and feasible scientific concept and technical route for the development of high strength and toughness anti-hydrogen brittleness steel. The high strength and toughness anti-hydrogen embrittleness wheel steel, spring steel and Marine equipment steel series products developed by this project have many varieties, complete specifications and good surface quality, which have created significant economic and social benefits.

Ideas and technical solutions for solving problems

This project is based on the scientific basis of hydrogen trapping in deep hydrogen traps, and focuses on the key problem of hydrogen embrittlement of high strength steel caused by large amounts of hydrogen entering the material during service, aiming at the current situation of raw material control, vacuum degassing of liquid molten steel and slow cooling in stack. A design concept was gradually formed to enhance the anti-hydrogen brittleness energy by constructing a large number of high density and dispersed deep hydrogen traps in the crystal.

Figure 1 Main research scheme of this project

The main scheme and technical route of the project are shown in Figure 1. By revealing the interaction mechanism between the nano phase and hydrogen atomic level in high strength and toughness steel, the irreversible deep hydrogen trap was designed and prepared, which effectively inhibited the hydrogen embrittlement of high strength steel. In the actual production, the coupling design of multiple trace elements and precise process control were broken through, and the controllable preparation of a large number of dispersing deep hydrogen traps was realized. By controlling the addition of multiple microalloys and the convective intensity of liquid steel in the industrialization process, the stable batch production of high strength and high toughness steels with hydrogen embrittleness resistance is realized.

This project combines the interaction between the second phase of multiple nanostructures and hydrogen in high-strength steel, the coupling mechanism of multiple trace elements, multi-point regional trace supply, formation and control of nanostructures in melt and other core and key technologies to realize the technological innovation of nanostructures toughening high-strength steel and extending its service life. For the first time, the project team completely independently proposed the basic research on the deep hydrogen trap of nano phase and the formation mechanism and control theory of nano phase in high strength and toughness steel, and formed nano phase in the melt and solidification process of high strength and toughness steel. This technology changes the removal of large particles in the second phase of high strength steel production. The traditional idea of reducing hydrogen content makes the second phase of high strength steel nano-sized and evenly dispersed in the matrix, which solves the scientific problem of cracking caused by hydrogen in high strength and toughness steel by mechanism, greatly improves the anti-hydrogen brittleness and strength and toughness of the material, and is successfully used in the stable batch production of anti-hydrogen brittleness and high strength and toughness steel for train wheel steel, high strength spring steel and Marine equipment.

Major innovative achievements

In order to meet the important requirements of high strength, toughness and anti-hydrogen embrittance of steel used in advanced rail transit and Marine equipment, this project broke through the key technologies of multi-element and micro-element coupling alloy design, precision process control, nano-phase construction and semi-congruent interface of iron matrix, and solved the major problems of anti-hydrogen embrittance of high strength and toughness steel from three dimensions of basic research, key technology and product development. High strength and toughness anti-hydrogen embrittlement steel for major equipment has been developed. The main innovations are as follows:

First, the mechanism of interaction between the nanophase and hydrogen atoms in high-strength and toughened steel is revealed, and the physical nature of semi-coherent interface mismatch as a deep hydrogen trap is clarified.

This project revealed that the physical nature of nanophase as a deep hydrogen trap is the mismatch dislocation of semi-coherent interface, solved the scientific problem of hydrogen cracking of high strength and toughness steel from the mechanism, and provided a theoretical basis and engineering practice method for improving the anti-hydrogen brittleness of high strength and toughness steel. By designing the composition and content of multiple microalloys, the semi-coherent interface between the nano-phase and the iron matrix was constructed as a deep hydrogen trap, and the hydrogen brittleness resistance of the high-strength and toughened steel was improved.

Second, it breaks through the design of multi-trace element coupling alloy and the precision process control technology, and realizes the controllable preparation of a large number of dispersing deep hydrogen traps.

Based on the coupled thermodynamic analysis of multiple trace elements, this project revealed the coupling mechanism of solid melting and precipitation of multiple trace elements in different temperature zones. The effect of interfacial concentration, growth rate and growth time of oxide nanoparticles on particle size in steel was discovered, and the precise process control technology of formation of nano-phase in steel was invented. According to the results of solid melting and precipitation, the temperature and time of heat treatment process were adjusted to achieve uniform and diffuse precipitation of nanometer phase in the special steel such as large round rod and medium thick plate, and nanoscale carbon nitride deep hydrogen trap was constructed during rolling and heat treatment. The multi-element alloy system is designed scientifically, and the flow field, concentration field and force field of melt are controlled, so that a large number of dispersing nano phases are kept in the whole process, and the controllable preparation of a large number of dispersing deep hydrogen traps is realized in engineering.

Thirdly, by controlling the addition of multiple microalloys and the convective strength of liquid steel in the process of industrialization, the batch and stable production of high strength and high toughness steel with hydrogen embritzability has been realized.

Based on the above theories and technologies, this project developed hydrogen embrittlement resistant high-strength and toughened steel. By cooperating with the design of multiple trace elements to control the quantity, microstructure and properties of nanoparticles, a large number of dispersing nanoparticle precipitates were formed in the temperature separation zone of high strength and toughened steel during melt, solidification and heat treatment, and hydrogen embrittlement resistant high-strength and toughened steel was obtained. This project has laid the theoretical and application foundation for the development of new products and the transformation and upgrading of products. The relevant enterprises have led and participated in the formulation and revision of national standards and group standards, achieved stable production and supply in batches, created significant economic and social benefits, and met the major demand for high-strength, tough and anti-hydrogen brittle metal materials for major equipment.

Application and effect

Figure 2 High-strength and high-toughness anti-hydrogen embrittlement wheel steel, spring steel and Marine equipment steel developed by this project

As shown in Figure 2, this project has developed a series of products of high strength and toughness anti-hydrogen embrittleage wheel steel, spring steel and Marine equipment steel, with various varieties, complete specifications and good surface quality. Due to their excellent comprehensive properties, they have been widely used, while creating significant economic and social benefits, and playing a leading role in the demonstration and technological progress of high strength and toughness anti-hydrogen embrittleage steel industry. The project team's development of heavy haul wheel steel solves the white point problem of high strength wheel steel, realizes mass export to North America, Australia and other heavy haul freight highly developed regions, expands the overseas product market, and supports the implementation of the major strategies of our railway going abroad. The spring steel developed in this project eliminates the shear crack phenomenon at the end of the high strength and toughness spring steel, solves the key problem of hydrogen corrosion in the shipping process, reduces the customer's product quality objections and claims, and expands the overseas product market of spring steel. The steel for Marine equipment developed solves the hydrogen embrittlement problem of high-strength and tough-steel for deep-sea oil fields in key projects. 30CrNi2MoV and other materials fill the domestic gap and completely replace imported products. The research results have realized large-scale production and application. A variety of products developed by the project broke the foreign blockade, conquered the "bottleneck" problem, gained the right of discourse in the domestic and foreign markets, realized the upgrading of related industries, promoted the progress of the metallurgical industry and manufacturing industry in China, promoted the development of manufacturing technology in rail transit, Marine equipment and other fields, and ensured the safety of the industrial chain of our strategic advantages.