고강도 열연판재의 석출 거동 및 기계적 물성에 미치는 가공 열처리 변수의 영향

Abstract

To reduce weight and improve the structural performance, demands for new materials development have been emphasized. High-strength hot-rolled sheet steels used for bridge, building, pressure vessel, facilities, line-pipes, and heavy construction equipment have steadily required to have higher strength as well as enhanced toughness and improved weldability. To achieve all requirements, active studies to develop high performance steels by controlling the thermo-mechanical controlled processing (TMCP) schedule and micro-alloying elements have been conducted. Generally, the TMCP schedules are strongly dependent on the chemical compositions of the materials, it is essential to find the optimum TMCP schedules of hot-rolled sheet steel.In this study, the effects of TMCP on tensile, Charpy impact, and low cycle fatigue (LCF) properties of a new hot-rolled (Ti,Mo) micro-alloyed steel were investigated. Moreover, the effects of TMCP on the interphase precipitation hardening characteristics were also investigated. Part I: The goal is to investigate the effect of the coiling temperature (CT) on microstructures and mechanical properties of hot-rolled sheet steels. For this purpose, high-strength hot-rolled sheet steels were fabricated by controlling TMCP schedules such as final rolling temperature (FRT: 880 or 1050 oC) and CT (570, 620, and 670 oC), and then tensile and Charpy impact tests were conducted. Microstructural factors, such as ferrite grains and precipitates were quantitatively and statistically analysed to explain the mechanical properties. The FRT 880 oC steels had the higher tensile strength and toughness than the FRT 1050 oC steels because their fine ferrite grain size. For same FRT, CT 620 oC steels showed the highest tensile strength due to their higher volume fraction of fine precipitates. The FRT 880 oC and CT 620 oC steel showed the best combination of high strength and toughness due to the fine grain size and well developed low angle grain boundaries. Part II: Effects of FRT (1050, 960, 880, 800, and 750 oC) on microstructure and mechanical properties (yield strength (YS), impact toughness, LCF) of high-strength hot-rolled sheet steels were investigated in this study. Microstructural factors, such as ferrite grains and precipitates were quantitatively and statistically analysed to explain the mechanical properties. The tensile test results indicated that the FRT 960 oC steel had the highest YS than the other steels due to the fine grain size and well developed low angle grain boundaries. The FRT in dual phase region (800 and 750 oC) had the lower energy transition temperature and higher LCF resistance than the FRT in austenite region (1050, 960, and 880 oC) because of their fine grain size and low YS. The FRT 880 oC steel showed the best combination of high strength, toughness, and LCF properties due to the fine grain size, fine precipitates, and well developed low angle grain boundaries. Part III: The abnormal interphase precipitation hardening characteristics of Ti and Mo micro-alloyed hot-rolled sheet steel was investigated. The different prior  grain structures depending on FRT introduced a significant difference in the interphase precipitation characteristics and consequently caused a totally different precipitation hardening, in spite of the same CT. It is concluded that the CT should be modified by considering not only chemical composition but also FRT to improve precipitation hardening.