Structure formation in high-strength nitrogen-bearing steel on hot deformation

Abstract

The resistance to deformation of nitrogen-bearing Cr–Ni–Mn steel at 800–1200°C is investigated by means of the Gleeble 3800 system. By analysis of the deformation diagrams—in particular, determination of the threshold strain for dynamic recrystallization—the temperature and strain corresponding to the onset of dynamic recrystallization are established as a function of the strain rate, and optimal temperatures for hot stamping, forging, and rolling are recommended for industrial conditions. With true strain e = 0.9, the dynamic recrystallization in the steel at strain rates of 10–2–2 s–1 occurs at temperatures no lower than 900°C. Metallographic data confirm the experimental results and show that the structure formation in the steel on isothermal deformation at different rates is different above 900°C. With increase in temperature and decrease in strain rate, relaxation processes are more developed. At a strain rate of 0.01 s–1 (stamping on a press), dynamic recrystallization begins at around e = 0.1 (relative reduction around 10%) in the range 1100–1200°C. Strain of around 20 and 30%, respectively, is required with decrease in temperature to 1000 and 900°C. With increase in strain rate to 0.1 s–1 (forging), dynamic recrystallization begins with around 20% strain above 1100°C, 28% at 1000, and 35% at 900°C. At a strain rate of 1–2 s–1 (rolling), dynamic recrystallization begins at around 30% strain in the range 1000–1100°C. In that case, the threshold strain is 36% at both 900 and 1200°C.