# Mechanical Properties of ASTM A572 Grades 50 and 60 Steels at High Temperatures
## 1 Introduction
There are not many mechanical property datasets specific to both the ASTM A572 Gr. 50 and 60 steels for fire design purposes. In addition, most of the studies provided only the data sheet of the tested steel, and there were few comparison targets with other steel types. Since the mechanical properties are different according to the steel type, it is necessary to analyze the data and propose various expected equations for each type of steel. Therefore, previous test results were analyzed to propose various reduction factors for mild steel (MS) and high-strength steel (HSS), and additional tests were conducted to compare these to proposed results groups using ASTM A572 Gr. 50 and 60 steels. Table 2 lists the yield strength and tensile strength values of each study that examined the stress-strain relationship at elevated temperatures. Figures 1 and 2 summarize the results of the yield strength reduction factor and tensile strength reduction factor for MS, HSS, and VHSS. First, Figure 1 shows the MS, HSS, and VHSS yield strength reduction factor graph. Despite using the same steel type, there was a difference in the yield strength reduction factor, as shown in Figure 1a, according to the strain level. In particular, the classification of MS, HSS, and VHSS according to steel type is prominent in Figure 1b. EN1993-1-2 (blue fine line) and AISC_2016 (black dotted-dashed fine line) are drawn in the MS zone. In contrast, the regression equation represented by NIST (NIST_2016, green dashed fine line) is drawn in the HSS zone up to 450 \({}^{\circ}\)C and in the MS zone above 450 \({}^{\circ}\)C. There is not a significant amount of data on VHSS, but it is distributed below in the MS and HSS regions based on S960. As the temperature increased, the strength of HSS and VHSS decreased sharply compared with MS, as shown in Figure 1. These analyses indicate that the yield strength reduction factor depends on the steel type, test method, and strain level. In general, the Eurocode and AISC propose a strength reduction factor in the MS range. As the temperature increases, the yield strength reduction of HSS and VHSS is greater than that of MS, such that the strength reduction factor of HSS and VHSS at high temperatures should be applied more conservatively than the standard. Therefore, the yield strength reduction factor should be separately presented according to the steel type (MS, HSS, and VHSS). In this study, a yield strength reduction factor formula for MS and HSS is proposed. Figure 2 shows the MS, HSS, and VHSS tensile strength reduction factor graph. The tensile strength reduction factor indicates the maximum value of the yield strength regardless of the strain level. As shown in Figure 2b, EN1993-1-2 (blue fine line) shows a conservative tensile strength reduction factor compared with other studies, but AISC (black dotted-dashed line) was relatively overestimated. In addition, the tensile strength reduction factor may have a value greater than 1.0 at 100-300 \({}^{\circ}\)C, which is a phenomenon due to temperature hardening and has been reported in several studies [22]. Therefore, more research is required to apply and analyze the tensile strength reduction factor above HSS to the design. These figures show that the reduction in the yield and tensile strength of HSS decreases compared with MS. Thus, engineers must be careful when using the strength reduction for HSS at temperatures between 500 \({}^{\circ}\)C and 600 \({}^{\circ}\)C. Figure 3 summarizes the reduction factors for the elastic modulus of previous tests for MS, HSS, and VHSS. The elastic modulus reduction factors are also reduced further than those for MS at elevated temperatures. Based on the results of previous studies, the strength reduction in HSS must be investigated further for fire design purposes. To fill this knowledge gap, a series of experimental studies on the mechanical characteristics at elevated  
temperatures are performed for ASTM A572 steels with the objective of generating the strength reduction factors at various strain levels.