Estudo do comportamento mecânico, tribológico e fratura da chapa de aço AISI 430 sob dobramento e estiramento

Abstract

One of the great challenges in the Sheet Metal Forming (SMF) area is the possibility of stamping parts without material failure. Such failures are often referred to as shear fractures and can occur unpredictably in regions with small bending radii and little apparent necking in contrast to tensile fractures. This has been a barrier for metal alloys such as Ferritic Stainless Steels (AIF) to be widely used. Techniques such as failure criteria, Finite Element Method (FEM) and Conformation Limit Curve (CLC) have been shown to be ineffective in predicting the shear fracture of this class of material. In this context, experimental research is a very useful tool. Therefore, the present work aimed to investigate the mechanical, tribological and fracture behavior of AISI 430 steel sheet under the bending and stretching condition. For this, a tribo-simulator was designed and built, which made it possible to perform three different types of conformability tests: Bending Under Tension test (BUT), Strip-Tension Test (STT) and Draw-Bend-Fracture (DBF). The main mechanical quantities involved were measured during the experimental tests, such as the speed and displacement of the metallic strip, the torque induced by friction, the acting forces and the temperature variation. In addition, the effect of other process parameters, such as pin radius and roughness, strip direction and microstructure, relative elongation and lubrication condition were also evaluated in the proposed test groups. The results of group 2 showed that the tool roughness, relative elongation, test direction and lubrication condition symptomatically influenced the mechanical and tribological behavior of the AISI 430 steel strip. The greater capacity of plane-strain and strip ridging contributed to the increase of the coefficient of friction. The results of group 3 revealed that the fracture site moves and, at the same time, the morphological aspect changes as the radius of curvature increases. In addition, the fracture limit strain on the outer surface, thickness and wall stretch also increased with increasing bending radius, while temperature decreased and the friction coefficient changed its behavior from a critical radius/thickness ratio. The analysis of variance confirmed that the shear fracture occurs for smaller radii, while the tensile fracture for larger radii.