Fe-Si강의 열간압연 과정에서 발생하는 edge crack 예방을 위한 고온 연성에 관한 연구

Abstract

The analysis of the edge crack in the hot-rolled Fe-Si steels (Si≤1.5) was carried out by the hot tensile test, examination of the defects, microstructure and finite element method (FEM). The hot ductility of solution treated steels was investigated at temperatures between 800 and 1150℃. Steels were prepared by laboratory vacuum induction melting and then hot rolled into 30-mm-thick plates. Samples for mechanical testing were obtained from hot-rolled plates. Critical transformation temperatures on heating and cooling are also determined using dilatometer. A continuous decrease in ductility was observed from 850℃ to about 1000~1050℃ in Fe-Si samples. After that, the ductility was recovered. Fe-Si steels have two phase region in high temperature (≥950℃) which can present problems during hot-rolling because of reduced hot ductility. The results show that, on cooling, formation of ferrite starts at about 950℃. These results suggest that the low tensile ductility exhibited by these materials at temperatures near 1000~1050℃ can be attributed to strain localization at grain boundary nucleated ferrite grains. Rapid growth of microvoids at ferrite films resulted in tensile failure by microvoids coalescence. The results are supported by microstructural observations obtained by electron backscatter diffraction (EBSD) and results of finite element method. The results are also used to discuss the loss of ductility during high temperature deformation of Fe-Si steels and compare to the hot tensile properties of high carbon steel.