JIS SK5及JIS S60C鋼之變韌鐵組織與其它組織的性質比較

Abstract

本研究目的在探討JIS SK5及JIS S60C兩種鋼材經不同熱處理後之顯微組織及機械性質。此外也對其上、下變韌鐵的破壞行為進行解析，並觀察變韌鐵變態的過程及其相變態的晶體學。 實驗結果顯示，SK5及S60C兩種鋼材之上、下變韌鐵組織的延、韌性較同硬度的回火麻田散鐵組織者佳。與下變韌鐵組織硬度相同之回火麻田散鐵，由於發生回火麻田散鐵脆性（TME），其韌性遠低於下變韌鐵，其破斷面呈沿晶破裂的模式，經歐傑能譜儀對破斷面分析，發現有不純物磷偏析至晶界造成晶界弱化。碳化物是影響所有組織機械性質最主要的因素，於變韌鐵組織中，含碳量高者，其受碳化物分散強化的影響越大，使材料之強度、硬度提升，而韌性下降。 上、下變韌鐵的破斷面皆呈穿晶劈裂破壞，其劈裂刻面尺寸與變韌鐵束尺寸間有對應關係。經EBSD”直接分析法”量測發現劈裂面晶體方位趨向{001}α、{112}α及{123}α等面上，這些晶面都是破裂裂紋傳遞時偏好的低表面能劈裂路徑。理論計算結果顯示變韌鐵束界面大多屬於高角度晶界，因此裂紋會在變韌鐵束界面作路徑的偏折，證實變韌鐵束尺寸即為劈裂路徑的單位尺寸。 上、下變韌鐵之肥粒鐵/雪明碳鐵間呈Bagaryatskii方位關係，組織中的雪明碳鐵會析出於 面上。而沃斯田鐵/肥粒鐵的方位關係則介於K-S與N-W之間。變韌肥粒鐵長條之兩組平行四邊形切面分別座落於 ~ 及 面上。結合變韌鐵中沃斯田鐵/肥粒鐵/雪明碳鐵三者方位關係及恆溫變態的組織觀察，可將變韌鐵之成長過程描述為：（a）變韌肥粒鐵次平板（subunit）或次次平板（sub subunit）在沃斯田鐵相的(111)γ

面上生核且沿著 ~ 方向生長。（b）變韌鐵次平板會叢聚一起，開始了擴散行為，使 面傾向於 面。變韌肥粒鐵於 晶癖面上析出雪明碳鐵，兩相間具有Bagaryatskii方位關係；（c）隨恆溫變態時間增加，肥粒鐵次平板不斷生核成長且叢聚成變韌鐵束，將雪明碳鐵包埋在其中。

This study investigates the microstructures and mechanical properties of various structures in JIS SK5 and JIS S60C steels. Strength falls and ductility raises as carbon content in steels increases for all structures. At equivalent hardness, the toughness and ductility of bainite are superior to those of tempered martensite. Tempered martensite embrittlement (TME) occurred in the ~310℃ tempered specimens, whose hardness is equal to lower bainite one. It is caused by grain boundary segregation of phosphorus and grain boundary precipitation of carbide during tempering. The fracture mode of TME is dominated by intergranular failure. The fracture surfaces of both lower and upper bainite structures exhibit transgranular cleavage. The size of the cleavage facet of bainite was demonstrated to be correlated with the width of the bainite sheaf. The results of EBSD analysis indicates not only that the sheaf boundary is a high-angle boundary, but also that the cleavage crack travels along the {001}α, {112}α and {123}α ferrite planes, whose surface energy are low. According to TEM diffraction analysis, the orientation relationship of cementite / bainitic ferrite satisfies the Bagaryatskii relation, and the habit plane of cementite precipitated in the bainite sheaf locates on

. The orientation relationship of austenite / bainitic ferrite satisfies between K-S and N-W relation. Two sets of parallelogram cross-section of binitic ferrite were found to locate on ~ and respectively. As isothermal holding time increases, one set of parallel plane inclines to via a diffusional interface migration and bainite subunits gradually coalesce.