制御圧延した棒鋼の靱性におよぼす組織と集合組織の影響

抄録

Texture and mechanical properties of low carbon microalloyed steel bars control-rolled in an oval/round process have been studied. Strength can be increased by decreasing rolling temperature without any ductility loss. This can be explained in terms of the refinement of ferrite grains as well as the work hardening by rolling in the γ/α duplex phase region. Ductile /brittle transition temperature (VTS) in Charpy impact test is also remarkably decreased by lowering rolling temperature, e. g, VTS<-196°C for the material with the finishing rolling temperature of 644°C. Such a marked improvement of toughness mostly arises from the refinement of microstructure although a significantly strong texture development promotes the occurrence of separations as well. The main and suborientations of texture developed are an unusual {100} <001> and a {111}{211} <011>, respectively. Fractographic examinations revealed that cleavage cracks either perpendicular to rolling direction or parallel to the rolling plane initiate within relatively coarse grains of the cube orientation elongated to rolling direction and that the propagation of such cracks is arrested by the surrounding extremely fine grained area of different orientations such as {111}{211} <011>. The initiation of separations cannot be explained by the CHAO'S model considering the plastic anisotropy due to the above textures.