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Microscopic investigation of the strain rate hardening for auto-body steel sheet(차체강판 변형률속도 경화의 미시적 관찰)
The high strain rate properties of auto-body steel have been studied for accurate crashworthiness of vehicles. The mechanical properties at high strain rates show different characteristics from those at the quasi-static state. In order to seek for the physical phenomenon of the strain rate hardening, a microscopic investigation has been conducted for the texture evolution and dislocation behavior of auto-body steel. Tensile tests were performed at strain rates ranged from 0.001/sec to 100/sec which is the common range in practical vehicle crashes to investigate the microstructural variation and dislocation behavior. Then, samples were extracted from elongated tensile specimens to investigate the microstructural variation and dislocation behavior of deformed materials. Microstructural changes such as the size, shape, and aspect ratio of grains in the deformation textures were investigated at various strain rates using an electron backscattered diffraction (EBSD) experiment. The effect of the dislocation density, behavior, and multiplication with the variation of strain rates was also observed to clarify the mechanism of the strain rate hardening with transmission electron microscopy (TEM) experiments. This research provides a method of constructing a general constitutive equation regarding the microscopic behavior of texture and dislocations at various strain rates. The investigation was mainly focused on the dislocation behavior and mechanism. At a low strain rate of 0.001/sec, the dislocation density is low compared to that at high strain rates and dislocations show the half-loop and V-shape since they are influenced significantly by the other dislocations and precipitations. At a high strain rate of 100/sec, the dislocation density increases abruptly and the dislocation mechanism constructs the dislocation crossing-net with the specific angle based on the slip system of the BCC structure. Results show that the dislocation shape and behavior are not influenced significantly by dislocation interaction and precipitations while they can induce the strain rate hardening by acting as dislocation sources or obstacles. A large number of crossing dislocations and precipitations accelerate the dislocation tangling and activate the strain rate hardening. Experiments with TRIP steels, which have low strain rate sensitivity, were conducted to observe different results from the strain rate hardening of SPCC steel. The dislocation structure in a specimen at each strain rate shows similar characteristics regardless of strain rates since hard second phases of TRIP steel restrict the dislocation movement in ferrite grains as dislocation obstacles and barriers with deformation.