Investigation of Abnormal Mechanical Behaviour of a Low Alloy Spring Steel containing 0.5wt%C-1.7wt%Si Under Bainitic- Martensitic- Retained Austenitic Condition

In recent years, more attention has been paid to the development of advanced micro-compositic high strength steels. Multiphase steels are a new family of these steels. It is possible to create the micro-composite phases containing bainite, martensite and retained austenite in their microstructures by selecting the appropriate chemical composition and heat treatment cycles in order to achieve the optimum engineering properties. In the present research work, high silicon steel sheets were selected and then heat treated to achieve triple phase steels. The heat treatment cycles included austenitizing at 900oC for 5 min, quick transferring to 350oC salt bath, holding for different times between 5 seconds to 5 hours and finally quenching in water in order to achieve the triple phase microstructures. The mechanical properties and microstructural investigations were carried out by using macro-hardness tester, tensile test, optical and scanning electron microscopes, respectively. The observation of resulted microstructures showed the various amounts of bainite, martensite and retained austenite phases have been formed in the specimens that are responsible for sharp and uncommon changes in tensile and macrohardness properties in comparison with those of third generation of advanced high strength steels. The results indicated the ultimate tensile strength was increased from 1850 to 2052MPa for the specimens treated from 5s to 50s and then decreased to 1386 MPa in 300s and again increased to 1460MPa for 1 hour specimens. These changes in properties were analyzed on the basis of the volume fractions and different mechanical behavior of bainite, martensite and retained austenite phases. The best condition for achieving the highest toughness which can be calculated by the product of tensile strength and elongation (25.9 GPa%) was obtained in the specimens heat treated in 350oC salt bath for 200 seconds.