Hole expansion characteristics of ultra high strength steels

The hole expansion ratio is a key indicator to evaluate stretch flanging performance of steel sheets, which is usually obtained by hole expanding test using cylindrical or conical punch. According to ISO 16630-2009(Metallic materials - Sheet and strip - Hole expanding test), hole expanding tests of 15 types of steel sheet were conducted using conical punch. The results indicated that there was obvious correlation between hole expansion ratio and tensile strength. While the tensile strength of steel is less than 700 MPa, hole expansion ratio of steel decreases linearly with the increase of tensile strength. While the tensile strength is greater than 700 MPa, hole expansion ratio tends to a constant value (about 30 similar to 40%). Steels with tensile strength of 700 MPa or more, including DP780, DP980, MS1180, have a common characteristic that they have relatively high volume of the hard phase (martensite phase). Therefore, it can be considered that, with large different deformation capacity during the forming process, the two-phase (ferrite & martensite) interface of these steels is prone to crack and expand. When the hard phase (martensite) reaches a certain volume fraction, there may be a limit of the damage energy during the microscopic damage of the two-phase interface. (C) 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).