금형강의 동적 및 정적 재결정 거동과 미세조직 변화 예측에 관한 연구

Abstract

Evaluation of microstructural changes during open die forging of heavy ingots is important for process control. The objective of the control of forging parameters, such as shape of the dies, reduction, temperature and sequence of passes, is to maximize the forging effects and to minimize inhomogeneities of mechanical properties.

A numerical analysis was performed to predict flow curves and dynamic and static recrystallization behaviors of die steel(0.36%C, 1.1%Mn and 1.21%Cr) from hot compression test results. The hot compression tests were carried out in the ranges of temperature 950∼1150℃ and strain rate 0.01∼1.0sec-1. The modeling equation for flow curves was a function of strain, strain rate and temperature. Models for predicting the evolution of microstructure in die steel during thermomechanical processing was developed in terms of dynamic, static recrystallization and grain growth phenomena. The microstructure model was combined with rigid visco-plastic finite element modeling to predict microstructure development. Predicted microstructure is consistent with results obtained in multiple compression tests.

For the grain growth evolution, hot compression test carried out the temperature of 950 and 1150℃, and the strain rate of 0.01 and 1.0sec-1. The specimens were compressed 10%, 20% and 30% in height, unloaded and held for holding times at isothermal condition resulting in static and metadynamic recrystallization in specimens.

The softening occurred during the holding time between the first and the second compression was calculated from the experimental result. Predicted grain size and load are consistent with experiment results from double compression test with constant velocity. Thereforce, the usefulness of the program is verified.