# Mechanical Properties of ASTM A572 Grades 50 and 60 Steels at High Temperatures
## 1 Introduction  
An elevated temperature of a structure exposed to fire can cause temperature-dependent effects on the building materials, such as concrete and steel [1]. Generally, a structure can experience large deflection with the reduction in both strength and stiffness at elevated temperatures, as mentioned in Chapter 1, MOP, ASCE [1]. Therefore, it is necessary to know the mechanical properties, including the Young's modulus reduction, yield strength, and tensile strength reduction, under various stress levels at elevated temperatures. The stress-strain relationships must be acquired to draw out the mechanical properties at elevated temperatures. Both steady-state and transient-state tests are being used to induce the mechanical properties at high temperatures currently. As we know, the steady-state test has been widely used to evaluate the stress-strain relationship of steel members because of its simplicity and practicality [1, 2, 3]. The steady-state tests provide the stress-strain results directly [4]; however, the transient-state tests result in a temperature-strain curve that needs to be modified into a stress-strain curve. In a transient-state test, a series of works are required to convert a temperature-strain curve into a stress-strain curve. There is the possibility of data being missed or differing due to the modification of the analytical processes on the results [2]. Thus, many of the research results regarding the stress-strain curve and strength reduction factors at elevated temperatures are reliant on the steady-state test. Most of the steady-state tests disregard the creep effects for the following reasons. First, the creep effects are offset due to the heating of the steel member at the jig setting for a short period of ten minutes or for a long period of more than one hundred minutes (because both ends of the steel are fixed by the jig). Secondly, data differences can easily arise notfrom the heating rate but from the loading rate [5; 6]. In other words, the steady-state test implicitly considers creep effects through loading rates under constant-temperature conditions [1]. Thirdly, the creep effect is disregarded during the tension test with a short period of time of approximately two to three minutes at elevated temperatures [7; 8]. If it is necessary to determine the creep effect, a separate creep test is needed [8]. The creep effect also implicitly is considered in EN 1993-1-2 [9] in the steel material [1; 4]. In this paper, the steady-state test was selected and carried out because the purpose of the research was to provide the stress-strain curve and determine the difference in the mechanical properties of mild steel (MS) and high-strength steel (HSS) under high-temperature conditions. Among the various steel materials in building construction, the high-strength low alloy (HSLA) provides better strength, weldability, corrosion, and weather resistance compared to conventional steel materials [10]. Most of the HSLA are ASTM A572 Grade 50 (Gr. 50) and have the same characteristics as the HSLA 355 in Europe. Because Gr. 50 steel is increasingly used in buildings and bridges, it is increasingly important to know its thermal properties. There are many existing studies on the Gr. 50 steel; however, very few are available on the ASTM A 572 Grade 60 (Gr. 60) in comparison with the Gr. 50. As mentioned earlier, additional studies on high-strength steel (Gr. 60) are necessary at elevated temperatures in accordance with the increase in demand. It is called high-strength steel (HSS) if the normal yield strength exceeds 460 N/mm\({}^{2}\) based on the EN 1993-1-2 [9]. The ASCE MOP [1] also states that not only is the yield strength of high-strength steel (HSS) between 490 N/mm\({}^{2}\) and 690 N/mm\({}^{2}\) but also the yield strength of very-high-strength steel (VHSS) is more than 690 N/mm\({}^{2}\). In the paper, the strength categories are presented in Table 1 for various steel materials, including mild steel, HSS, and VHSS, for convenience [11; 12].