| Summary: | 碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 101 === In old research, there was a mathematical proof of related equation H=k for strength and hardness, and k value is the strength-hardness correlation factor. There were many analyses on k value because value of strength or stress-strain curve is obtained by tensile test, which is time-wasted and material-wasted. If we could obtain strength by hardness, the process is easy and fast. However, the hardness and strength of materials highly related to microstructures, and investigation of microstructures also needs a lot of time to prepare specimen, especially TEM specimen. Previous research usually focused on relationship between k value and specific material, and seldom involved microstructure. Therefore, the study analyzed the microstructure and k value, and investigated their relationship.
The study first proved different materials possibly with different k value, and tried to find out any regular pattern. The tensile and hardness test were done for HT780, HT620, SS400 and IF steel, whose k value was 3.6, 3.7, 4.7 and 4.5 respectively. Then by observation of optical microstructure, it shows that high precipitation density has small k value and not large enough pearlite volume fraction and grain size may not change k value sharply. Great strength effect induce low k value is our hypotheses of k value. In order to prove dislocation density influenced k value and grain size less affected k value, we rolled IF steel plate from 3mm to 2.5mm, 2mm, 1mm, and calculated k=4.5, 3.8, 3.6, 3.1, respectively. We found out high rolling capacity relating to low k value and high rolling capacity representing high dislocation density. When comparing grain size and k value, we set 950℃, 1050℃, 1150℃ for grain growth, and grain size was 20 , 80 and 40 . The final k value is 2.8, 4.1, 5.0, respectively, which prove that grain size less affected k value. Moreover, in study of Umemoto showed that tempered martensite had lower k value than martensite that conflict with previous study about dislocation density. The specimens are prepared by SS400 quenching and tempering to 200℃, 400℃, 600℃.The result of martensite ~4.8, 200℃ tempered martensite~4.0, 400℃ tempered martensite~3.5 and 600℃ tempered martensite~4.2. The k value decreased when tempering.
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