STRUCTURE FORMATION OF HIGH STRENGTH NITROGEN-BEARING STEEL UNDER HOT DEFORMATION

Deformation resistance characteristics of nitrogen-bearing steel of Cr-Ni-Mn alloying composition have been defined with the help of universal research complex Gleeble 3800 at various temperatures between 1200 and 800 ^:C depending on the strain rate. Temperature and deformation conditions of dynamic recrystallization depending on the strain rate have been determined by analyzing the deformation diagrams, namely that of the deformation limit beginning the process. Optimum temperature regimes for industrial hot stamping, forging and rolling have been recommended. It has been found that at a degree of true strain e = 0.9 the dynamic recrystallization of steel occurs in the range of 10~² + 2 s^_1 of strain rate at temperatures not lower than 900°C. The experimental data have been confirmed by metallographic research. Studies have shown that the processes of structure formation during isothermal deformation of steel at different speeds have some differences at temperatures above 900°C. The higher the temperature and the lesser the strain rate, the greater is the development of relaxation. It was found that at a strain rate of 0.01 s^_1 corresponding to hot stamping, dynamic recrystallization in the temperature range of 1200 - 1100°C deformation starts at values equal to e = 0.1 (about 10 % of relative reduction). When the temperature lowers to 1000 and 900°C it is necessary to accumulate deformation to 20 and 30 %, respectively. Increasing the strain rate to 0.1 s^_1 (when forging) leads to the fact that at temperatures above 1100°^:C dynamic recrystallization begins at a degree of deformation about 20 %; at temperatures of 1000 and 900°C about 28 and 35 %, respectively. When the strain rate is  1-2 s^_1 (when rolling) dynamic recrystallization starts when the temperature range is 1100 -1000°C and degree of deformation equals to 30 %. Lowering the temperature to900°C, as well as increasing to1200°C, increases the limit degree to 36 %.

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