The cleavage initiation of notched specimens of a low alloy hot rolled steel （16 MnR） with carbides and inclusions at -196℃ and -130℃ is studied in this paper. The results of tensile tests, four point bending tests of single and double notched specimens, microscopic observations and finite element method calculations show that two types of cleavage initiation sites in notched specimens can be identified, one is related to the inclusions ahead of the notch root （IC type） and the others related to the inclusions located ahead of string cracks far from the notch root （SIC type）. The types of initiation sites are influenced strongly by temperature, changing from IC type at -196℃ to SIC type at -130℃. In both IC and SIC initiation mechanisms, the crack nucleation is induced by inclusions and the final fracture is controlled by the propagation of a ferrite grain-sized crack into the matrix grain. The cleavage fracture of IC initiation type in notched specimens can be described by a dual-criterion model, i.e. a critical plastic strain εP≥εpc for initiating a crack nucleus and a critical tensile stress σyy≥σf for its propagation. While for SIC initiation type, the dual-criterion model is to be modified with the expression of εp＋εps≥εpc and σyy＋σyys≥σf%研究了低合金热轧钢16MnR缺口试样在-196℃和-130℃的解理断裂机理．拉伸试验、单、双缺口四点弯曲实验、断口形貌观察以及有限元分析结果表明，缺口试样发生解理断裂时均起裂于夹杂物粒子，一种位于缺口根部前端（IC型），另一种位于距缺口根部较远的条形裂纹前端（SIC型）；且随温度升高，起裂源的类型从-196℃下的IC型转变为-130℃下的SIC型．微裂纹均形核于夹杂物，最终的断裂由铁素体晶粒尺寸的微裂纹扩展控制．缺口试样IC型解理断裂遵循裂纹形核条件εP≥εpc。和裂纹扩展条件σyy≥σf，而SIC型解理断裂条件则演化为εp＋εps≥εpc和σyy＋σyys≥σf．
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