Thin–Walled Structures xxx (xxxx) xxx Please cite this article as: Xia Yan, Thin–Walled Structures, https://doi.org/10.1016/j.tws.2020.107293 0263-8231/© 2020 Elsevier Ltd. All rights reserved. Post-fre mechanical properties of advanced high-strength cold-formed steel alloys Xia Yan a , Yu Xia b , Hannah B. Blum b , Thomas Gernay a, * a Johns Hopkins University, Baltimore, USA b University of Wisconsin-Madison, USA A R T I C L E INFO Keywords: AHSS Cold-formed steel Post-fre behavior Material properties Residual testing Structures in fre ABSTRACT An experimental investigation was conducted on the post-fre mechanical properties of advanced high-strength cold-formed steel alloys. Tensile specimens made of two dual-phase steels and two martensitic steels with nominal yield stress from 340 MPa to 1200 MPa were tested after exposure to temperatures up to 700 ◦ C. The stress-strain curves and mechanical properties were obtained from the tests and post-fre retention factors were derived. Comparison with available test data on hot-rolled and conventional cold-formed steels showed the infuence of cold-working and steel grade on the post-fre mechanical properties. The permanent reduction in strength after exposure to elevated temperature is more severe and starts at lower temperatures for cold-formed steels compared with hot-rolled steels. Furthermore, cold-formed steels made of advanced high-strength grade exhibit proportionally larger permanent reductions in yield stress and ultimate stress than those made of con- ventional grade. A model for the post-fre properties of advanced high-strength cold-formed steels was proposed based on calibration of a simple three-coeffcient equation developed through committee work with the Amer- ican Iron and Steel Institute. 1. Introduction With recent progress in the control of chemical composition and multiphase microstructure resulting from careful alloy selection and precise heating and cooling procedure, advanced high-strength steels (AHSS) can be manufactured with nominal yield stress up to 1200 MPa and ultimate stress up to 1600 MPa [1,2]. This new category of steels contains one or more phases, such as martensite, austenite, or bainite, other than ferrite, pearlite, or cementite phase. The AHSS family in- cludes dual phase (DP), martensitic (MS), transformation-induced plasticity (TRIP), and some other complex-phase steels [3]. AHSS can be used in hot-rolled or cold-formed members. These sophisticated materials exhibit a range of strength, toughness, ductility, and fatigue performance, which meets the challenge of structural safety, durability, and aesthetic requirements. The increased strength-to-weight ratio and unique mechanical properties of these AHSS make them valuable for the construction of the next generation infrastructure. An important requirement for structural steel is the performance under conditions resulting from fre. As new types of steel alloys are made available, their behaviors at elevated temperature and after cooling down need to be characterized to enable structural fre design [4] and post-fre safety assessment [5]. The material properties of con- ventional grade steel at elevated temperature can be found in design codes such as the European code (Eurocode3 part 1–2 [6]) and the American standard (AISC Specifcation 360 [7]). However, design guidance on the post-fre behavior of structural steels is limited. The Annex B of British Standard 5950 part 8 [8] provides suggestions on the reuse of structural steels after exposure to elevated temperature if the degradation of mechanical properties and member distortion are within appropriate engineering criteria. It specifes that hot rolled S235 and S275 steels can retain at least 90% of their original mechanical prop- erties regardless of the exposure temperature. The S355 steels can be assumed to retain at least 75% of their original strength when the exposure temperature is higher than 600 ◦ C. The cold-formed steels with grades up to Z35 (BS EN 10143) can retain 90% of the original strength. However, there is no specifc guidance in Eurocode 3 [6]. In the research literature, experimental investigations on the mechanical properties of various types of steels are presented both at elevated temperature [9–17] and in the post-fre situation [10,18–25], but the data for high strength steels and AHSS is limited. Further, predictive models can be found for cold-formed steels with nominal yield stress up to 900 MPa [21], but no model on the post-fre properties of AHSS steels is available. * Corresponding author. E-mail address: tgernay@jhu.edu (T. Gernay). Contents lists available at ScienceDirect Thin-Walled Structures journal homepage: http://www.elsevier.com/locate/tws https://doi.org/10.1016/j.tws.2020.107293 Received 15 June 2020; Received in revised form 26 September 2020; Accepted 9 November 2020