Cold Deformation Behavior Of Ultra-high Strength Spring Steel 60Si2CrVAT For Cold Coiling Process

Ultra high strength spring steel 60Si2 Cr VAT is a type of dedicated spring steel for bogies which carrying capacity is strong,impact resistance is good and the comprehensive performance is excellent.To adapt to energy conservation and emissions reduction,cold coiled spring was designed to satisfy the requirement for green manufacture.But there are still many problems for ultra-high strength spring steel's cold coiling process in terms of the current production status,such as severe wear of the die because of bad cold coiling performance of the spring steel,which may also results in cracking during cold-forming process.All of which has restricted the further development of high strength spring steel's cold coiling process.It is theoretical and practical significance to make a systematic study on the cold-forming performance of 60Si2 Cr VAT and to develop new heat treatment process more suitable for it.It will drive the course of the green manufacturing for spring.In this context,from the factors influencing the cold-forming behavior,such as strength,hardness,strain-hardening exponent,strain-hardening rate and product of strength and elongation,the relationship between cold-forming performance and the change rule of microstructure and mechanical properties of the ultra-high strength steel treated by different heat treatment process was studied and the cold-forming mechanisms of the material with different microstructures were deeply analyzed in this dissertation by OM,XRD,SEM,TEM,HRTEM,EBSD and in situ tensile test.The main research contents and conclusions are as follows:(1)Based on the quenching and tempering(Q&T)process,the strain-hardening mechanism of the ultra-high strength spring steel with martensite and carbides was systematically studied.It is noted that the strain-hardening behavior of this kind of material has the relationship with the initial dislocation densities in the martensite matrix and the amount and the sustainable multiplication ability of the geometrically necessary dislocations(GNDs).The dislocation density of the matrix and the morphology,numbers,distribution and the volume fraction of secondary phase particles will all affect the change of GNDs.Furthermore,whether the GNDs can continuously growth or not are the key factors to influence the cold-forming performance for materials with dual phase microstructures.(2)Isothermal transformation dynamics of short time quenching and isothermal treatment in the process of quenching-isothermal-quenching-tempering)was studied for the ultra-high strength spring steel 60Si2 Cr VAT.The corresponding Isothermal transformation kinetics diagram(TTT curve)was established.And the theoretical foundation was built for further in-depth study.(3)It is first time to systematically study the strain-hardening mechanism of 60Si2 Cr VAT with triple phase including martensite,bainite and retained austenite based on Q-I-Q-T process.Results show that the strain-hardening behavior of materials with such kind of microstructures is determined by the interactions between the matrix and the retained austenite.Furthermore,the strain-hardening ability is mainly determined by the matrix at the beginning stage of the uniform plastic deformation and it is determined by the morphology,quantity,size and the distribution of the retained austenite.(4)Based on the cold coiling process of the spring in practice,the change rules of the microstructures and mechanical properties of 60Si2 Cr VAT with different microstructures after treated by pre-deformation.It was found that triple phase materials with a higher volume fraction of retained austenite have more outstanding cold-forming ability.(5)Based on the theory of fracture toughness and combined with the strain-hardening exponent,the distribution of the stress and the strain were studied by DEFORM software for 60Si2 Cr VAT with different microstructures during the cold coiled process.The probability of damage accumulation was predicted which provides additional evidence for better heat treatment process for the tested steel.In conclusion,some of the key problems of the cold-forming behavior of the ultra-high strength steel 60Si2 Cr VAT have been systematically studied and analyzed.and the corresponding cold-forming mechanisms were proposed.More suitable heat treatment regulation of the tested steel was established.Finally,the experimental and theoretical basis of the green production for the ultra-high strength steel 60Si2 Cr VAT was provided.