HC-HPT 및 CEM 공정으로 제작된 금속 소재의 기계적 성질 분석

Abstract

The aims of this thesis consist of two specific goals. First goal of this thesis is application of SPD processes to industries. To achieve this purpose, SPD process which able to manufacture large specimens compared to other SPD processes are developed, such as hollow cone high-pressure torsion (HC-HPT) and new application field which need strengthened metallic materials with small size is suggested, such as biomedical structural materials. Both experiments and FEM simulation are performed to evaluate mechanical, corrosion, and bio properties of manufactured specimens and deformation mechanisms of new developed SPD process. Second goal of this thesis is optimization of conventional process based on plastic deformation. In this case, compact endless cast & rolling mill (CEM) process, whichis a new steel strip method made by POSCO is selected due to problems of low-carbon steel strip products, especially yield point phenomenon (YPP). During forming process, stretcher strain induced by YPP occur, lower surface quality of final products, and force additional processes which increase production costs of final products and decrease efficiency of CEM process. This YPP is originated fromadislocation locking by interstitial elements such as carbon and nitrogen which tend to diffuse intodislocation core whose atmosphere is energetically higher compared to other regions. Especially, incase of low carbon steel. In this thesis, quantitative analysis on YPE with dislocation density andinterstitial atoms, which originate YPP are performed and solution for the CEM process suggested which could prohibit YPP of CEM low carbon steel products.