從合金鋼及極低碳鋼之顯微結構探討硬度-強度關係因子之研究

Abstract

在前人的研究中曾以數學方法證明強度與硬度有一關係式H=k ，而k值為硬度與強度的關係因子。許多研究對此值作出分析，因強度值或應力應變圖要獲得需經由拉伸試驗，其中存在耗費時間和材料的缺點，如能夠直接從硬度得到該材料的強度特質，則較為簡便且快速。然而，材料的強度和硬度與材料的顯微結構息息相關，而顯微結構的探討也需耗費時間製備試片，尤其是電子顯微鏡試片。前人常針對k值與材料的對應關係作討論，而對其顯微結構則較少涉略。因此本研究對於其顯微結構與k值作分析研究，以期能對其顯微結構與k值的關係作探討。 本研究首先證明k值具有不確定性，且嘗試在四種材料當中找出規律性。對HT780、HT620、SS400和IF鋼作拉伸強度與硬度測試，得到HT780的k值~3.6，HT620的k值~3.7，SS400之k值~4.7，IF鋼之k值~4.5，觀察四種鋼材的金相，可以發現具有高析出物密度的k值會較小，而晶粒大小和波來鐵相對k值的影響較小。為了證明差排密度會影響k值和晶粒大小對k值的影響較小，對IF鋼板作軋延，從3mm軋延至2.5mm、2mm、1mm，分別量測其降伏強度與硬度，計算出k值，2.5mm~3.8、2mm~3.6、1mm~3.1，發現高軋延量的鋼材其k值會較小。再來利用四種厚度鋼板的TEM照片估計出差排密度，差排密度與k值會呈一負相關。比較晶粒大小與k值之關係時，先將1mm IF steel作不同溫度下的沃斯田鐵化熱處理，發現950℃之k值~2.8，1050℃之k值~4.1，1150℃之k值~5.0，而晶粒大小分別為20 、80 和40 ，證明之前的假設，晶粒大小與k值沒有趨勢關係。最後在Umemoto的研究中，記錄了麻田散鐵之k值為3.51，回火麻田散鐵之k值為3.17，與差排密度越高k值越小的實驗結果相違背，於是對SS400作淬火麻田散鐵化處理，並以200℃、400℃、600℃作回火，測定k值與觀察晶相，發現淬火後之k值為4.8，200℃回火~4.0， 400℃回火~3.5，回火600℃回火~4.2，回火後的k值下降，與Umemoto的結果相同。

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.