Effect of retained austenite stability of Si-Mn TRIP steel on the product of strength and ductility

摘要

In order to study the characteristic behaviour of the stability of retained austenite in the course of tensile test, a low-carbon Si-Mn TRIP-assisted steel (0.22%C-1.50%Si-1.65%Mn) was subject to different heat treatments so as to obtain retained austenite with different levels of stability. The TRIP steel specimens were annealed in the intercritical range. Three groups of samples were heated to 760, 780 and 800℃ respectively. After 5 minutes of annealing in the intercritical zone, the samples were held at a temperature of 400℃ for 5 minutes. Each group of specimens with identical heat treatment was divided into two subgroups. One was subject to a tensile test until fracture occurred, whereas the other group was tested without fracture, in order to obtain different elongations. The amount of retained austenite in each tensile specimen was analyzed by means of X-ray diffraction. The product of strength and ductility of the specimen is proportional to the amount of retained austenite transformation in a specific strain range. The specimens annealed at a temperature of 780℃ showed an optimum value of strength and ductility product, their retained austenite is characterized by the highest transformation rate in the strain range of 7.8 to 14.5%. The study also shows that this particular strain range coincides with the range of elongation where most void nu-cleation occurs. Therefore, specimens having the highest transformation rate during void nucleation postpone necking in the course of tensile test very effectively.