Assessment of quench and partitioning heat-treated high-strength low-alloy reinforcing steel ASTM A706 grade 60 at quasi-static strain rates for seismic safety

Abstract

In this study, low-carbon steel reinforcing bars (rebars) grade 60 commonly used in concrete structures of residential buildings were subjected to quenching and partitioning ( Q & P ) heat treatment to improve their strength for seismic applications. The novelty of this work is the optimization of Q & P heat treatment for commercially used grade 60 rebars to develop a tailored multiphase microstructure that enhances strength and ductility while significantly improving the UTS/YS ratio, achieving a 54 % increase, which is important for seismic applications. Key mechanical properties, including tensile strength, yield strength, and total elongation, were evaluated at various austenitization temperatures under quasi-static strain rates. Optical microscopy was used to analyze the microstructure, while micro-Vickers hardness testing determined the hardness of individual phases. Q & P treatment at austenitization temperatures of 1,020 °C, 820 °C, and 785 °C increased the UTS/YS ratio from 1.4 for the as-received rebars to 2.16 and 1.86, respectively. The highest mechanical properties were achieved at 785 °C, resulting in an ultimate tensile strength (UTS) of 814 MPa, representing a 60 % increase, and a total elongation of 16 %, significantly exceeding the UTS of commercial grade 60 rebars (500 MPa) while maintaining good ductility.