Prediction of hole expansion ratio for automotive grade steels Surajit Kumar Paul ⇑ , Monideepa Mukherjee, Saurabh Kundu, Sanjay Chandra R&D, Tata Steel Limited, Jamshedpur 831 001, India article info Article history: Received 21 November 2013 Received in revised form 18 March 2014 Accepted 21 March 2014 Keywords: Hole expansion ratio Uniaxial tensile test True fracture strain Auto grade steels Finite element simulation abstract The objective of this work is to characterize experimentally and predict analytically the hole expansion ratio of steel sheets from uniaxial tensile properties. Firstly, finite element analysis is performed to understand the deformation modes (i.e. stress state) at the edge of the hole during its expansion by a con- ical punch. Finite element simulation study confirms that during the hole expansion process, the edge of the hole undergoes uniaxial tensile deformation. Secondly, a fracture based failure criterion is employed to predict the hole expansion ratio, considering that the test stop criteria of the hole expansion process is the formation of a through thickness crack. Next, a methodology is proposed to compute the true fracture strain during uniaxial tensile test, following which a model is proposed to predict the hole expansion ratio analytically from the true fracture strain. The proposed model is validated with eight different steel grades. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Among the various auto-components, the weight reduction achievable by substituting mild steel with high strength steels is found to be the largest for auto-wheels [1]. Furthermore, the corre- sponding energy saving is estimated to be 1.2–1.3 times the amount possible for nonrotating parts [2]. As a result, in the past few years, a large amount of research has been dedicated to the development of high strength steels for automotive wheel applica- tions, which can serve as potential replacements for the tradition- ally used mild steel. The automotive wheel consists of a disc and a rim. While the disc is press formed, the rim is flared and then roll formed after flash butt welding. Additionally the wheel disc has hub-holes which are produced by punching a plate followed by expansion of the holes. Therefore, excellent bulk sheet formability and edge formability are necessary to manufacture the wheel disc success- fully. Bulk sheet formability can be described as the ability of a sheet to be formed into a part without failure occurring within the body of the part, due to lack of ductility. It can be determined using forming-limit diagrams, cup-drawing experiments and mea- surement of ductility from conventional uniaxial tensile tests. Edge formability on the other hand, can be described as the ability of the sheet to be stamped into a part without failure by fracture at a sheared edge or hole. The edge formability is evaluated using hole-expansion tests. Further, the hole expanding process itself affects the fatigue resistance and vehicle safety critically since defect generation in the hub hole during manufacturing drastically decreases the load bearing capacity and hence service life of the wheel. Therefore, determination of the edge forming limit of the hole and prediction of material failure during the hole expansion process are also essential for improving the load bearing capacity of the wheel as well as the design safety of the vehicle. In addition, three other factors which have significant influence on hole expansion in steel sheets are non-metallic inclusions, the condition of the hole edge and the steel microstructure [3]. The hole-expansion performance is significantly reduced by the pres- ence of elongated-manganese sulphide inclusions and burrs associ- ated with severely cold-worked material at the edge of a hole. Levy and Tyne [4,5] and Chung et al. [6] have shown that the hole expansion ratio or HER is considerably affected by the condition of the hole edge and that drilled holes (on samples made from the same steel sheet) exhibit better HER than punched holes when tested under similar conditions [6]. Bhattacharya and Patil [7] have examined the effects of inclusions on edge formability of hot rolled HSLA steel. Their test results were very sensitive to the sulphur content, as hole-expansion performance decreased with increasing sulphur content. Low sulphur (<0.008 wt.%) and sulphides with low aspect ratios (globular shaped) led to the best edge formability determined using edge stretch tests [7]. Fang et al. [8,9] have observed that the strength difference between hard and soft phases in a microstructure has a major influence on the hole http://dx.doi.org/10.1016/j.commatsci.2014.03.040 0927-0256/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +91 657 2148919/13, mobile: +91 08092085723; fax: +91 657 2271748. E-mail addresses: paulsurajit@yahoo.co.in, surajit.paul@tatasteel.com (S.K. Paul). Computational Materials Science 89 (2014) 189–197 Contents lists available at ScienceDirect Computational Materials Science journal homepage: www.elsevier.com/locate/commatsci