線材における表面疵発生挙動: 表面疵の発生を判断する解析手法の確立

摘要

If surface defects on hot rolled wire rod subsequently remained, it might become the starting point of cracks in the forging process. Therefore, the surface quality for hot rolled wire rod is strongly required. In order to avoid surface defects, it is important to establish methods for predicting the positions and the criteria of the defects as a function of rolling conditions. In this paper, we assume that the defect is a kind of plastic instability on the surface. We analyze the criteria of the defects by using a geometrical velocity field. By using FEM, we calculate the velocity vector and the geometry of the surface during rolling. The angle between the normal vector and the velocity vector on the surface is constantly changing during rolling. In order to analyze the phenomena, we introduce the new parameters η = (→v)·(→n) and ψ = dη/dθ. Comparing with the experimental results of the position of defects, the velocity gradient (ψ) is a good index for prediction of the defect position. In that case, ψ is locally changing near the inlet on the roll bite. A plastic instability might occur on the surface, as a result of this condition.%熱間圧延された線材は，熱処理，酸洗，引抜きの後，種々の形状に鍛造される。線材の表面に疵があると，その箇所が鍛造工程での割れの起点となる。近年，鍛造加工の多様化や高精度化，工程省略の促進に伴って要求される表面品質もますます厳しくなつてきており，よりいつそうの表面疵の低減が求められている。