Effects of Nb, Ti and V on recrystallization kinetics of austenite in microalloyed steels

Purpose: The work presents research results of impact of Nb, Ti and V microadditions on flow stress, recrystallization kinetics and microstructure of newly elaborated steels assigned for production of forged machine parts, using the method of thermo-mechanical treatment. Design/methodology/approach: The study was performed with the use of Gleeble 3800 simulator. Stress-strain curves were determined during continuous compression test in a temperature range from 900 to 1100°C and at a strain rate of 1, 10 and 50 s-1. In order to determine recrystallization kinetics of plastically deformed austenite, discontinuous compression tests of specimens were done with a given strain at the rate of 10 s-1., in a temperature range from 900 to 1100°C, with isothermal holding of samples between successive stages of deformation for 2 to 100 s. Recrystallization kinetics of plastically deformed austenite was described using the Johnson-Mehl-Avrami equation. The observations of microstructures of thin foils were done using JEOL JEM 3010 transmission electron microscope. Findings: Basing on the analysis of the form and the course of curves obtained in the compression test, it was found that in the studied range of parameters of hot plastic deformation, the decrease of strain hardening of studied steels is caused by the process of continuous dynamic recrystallization. This is also confirmed by calculation results of activation energy of plastic deformation process. Performed two-stages compression tests revealed that microadditions introduced into steel considerably influence the kinetics of static recrystallization. Research limitations/implications: It was found that the time necessary for a total course of recrystallization of austenite is too long to be accepted in the production process of forgings. Practical implications: Executed hot compression tests will contribute to establishing conditions of forging with the method of thermo-mechanical treatment. Originality/value: Strain-stress curves and recrystallization kinetics curves of newly elaborated microalloyed steels have been determined.