Automobile, engine manufacturing and railway industries are the main users of spring steel. Due to the harsh working conditions of the spring, the requirements for spring steel are very strict. In recent years, with the lightweight and high-performance of automobiles, it is urgently required to increase the strength of spring steel and increase the design stress of springs; with the continuous improvement of domestic trains, especially the development of trucks in the direction of high speed and heavy load, the same It is required to increase the strength of the suspension spring of the train bogie and extend the fatigue life. In view of this, a lot of research work has been carried out in recent years at home and abroad to further improve the strength level and service life of spring steel.
1. High strength and development trend of spring steel
Since the mass of the suspension spring is proportional to the square of the design stress, the most effective way to reduce the spring mass is to increase the design stress of the spring. Under the premise of the same spring performance, with the increase of design stress, the wire diameter and effective number of coils of the spring are reduced, and the weight of the spring can be reduced by 40% to 50%. Therefore, with the lightweight of automobiles and the high-speed and heavy-load of railway trains, the design of springs has been continuously improved. For example, the design stress of car suspension springs was 900 MPa in the 1970s and increased to 1000-1100 MPa in the 1980s. At present, the spiral suspension springs of foreign cars have begun to generally adopt a new generation of ultra-high strength spring steel with a design stress of 1200 MPa.[2] . This kind of spring steel will be the mainstream of spring steel for cars in the future. Japan and South Korea also successfully developed new spring steels such as UHS2000 and ND250S (RK360) with the design stress ranking first in the world in 1991 and 1996 respectively. However, due to the high cost, these new spring steels are currently only Limited to high-end cars.
In order to improve the design stress of the spring, it is necessary to increase the hardness (strength) of the spring steel. For example, in order to obtain a design stress of 1200MPa, the hardness of the spring steel is required to reach HRC53. However, for existing spring steels such as high Si-Mn series SUP7, the fatigue strength and spring resistance will drop sharply at such a high level of hardness. The current development trend of spring steel is towards economy and high performance. The existing foreign spring steel grades are relatively complete, and the mechanical properties, hardenability and fatigue properties can basically meet the current production and use requirements. At present, on the one hand, it is to give full play to the potential of existing spring steel, such as improving the production process, adopting new technologies, and making certain adjustments to the composition, to further improve its performance and expand the scope of application, such as the proposed high-performance valve for engines Ultra-pure spring steel; on the other hand, the research and development of new steel grades are carried out. Since the two most important factors that affect the improvement of spring design stress are fatigue resistance and elastic reduction, these two factors have become the research and development of spring steel grades today. For example, the recent development of UHS1900, UHS2000, ND120S and other corrosion-resistant fatigue-resistant high-strength spring steels and SRS60, ND250S and other high-strength spring steels with excellent elasticity resistance. It is worth noting that the development of new high-strength spring steels Therefore, it is necessary to improve the mechanical properties and application performance of steel while taking into account its economic efficiency in order to be accepted by the majority of users.
2. Research progress of spring steel alloying
As mentioned earlier, the strength level of traditional spring steel is difficult to meet the requirements of modern industrial development. An important way to solve this problem is how to give full play to the role of alloying elements to achieve the best alloying effect.
Carbon is the main strengthening element in steel, and its influence on the properties of spring steel often exceeds that of other alloying elements. Spring steel requires higher strength and fatigue limit, and is generally used in the state of quenching + medium temperature tempering to obtain a higher elastic limit. To ensure the strength, the spring steel must contain enough carbon. But with the increase of carbon content in steel, the plasticity and toughness of steel will drop sharply. At present, the carbon content of spring steel widely used in various countries in the world is mostly between 0.45% and 0.65%. In order to overcome the problem of reduced toughness and plasticity after the strength of spring steel is increased, there is also a tendency to reduce the carbon content. Among the spring steels currently included in the standard with lower carbon content are Japan’s SUP10 (0.47%～0.55%), the United States’ 6150 (0.48%～0.53%), Germany’s 38Si7 (0.35%～0.42%), and France’s 45C4 ( 0.41%～0.48%) etc. In-depth research on low-carbon martensitic spring steel has been conducted in China [5], and a series of low-carbon spring steels have been developed, such as 28SiMnB, 35SiMnB, 26Si2MnCrV, etc., with a carbon content of about 0.30%. The research results show that these spring steels can be used under low temperature tempered lath martensite structure, and have sufficient strength and excellent comprehensive mechanical properties, especially excellent plasticity and toughness. Several high-strength spring steels recently developed in Japan, such as UHS1900, UHS2000, ND120S, ND250S, etc., have a carbon content of about 0.40%, as shown in Table 3 [3].
It can be seen that reducing the carbon content in spring steel is an important means to research and develop a new generation of ultra-high strength spring steel. At this time, the decrease in strength and hardness caused by the decrease in carbon content can be achieved by optimizing alloying elements and reducing the tempering temperature.