# Mechanical Properties of ASTM A572 Grades 50 and 60 Steels at High Temperatures
## 1 Introduction
ASTM A992 steel has a specified minimum yield stress of 345 MPa (50 ksi) and specified maximum yield stress of 450 MPa (65 ksi). The estimation of the stress-strain relation with their detailed model showed results more similar to EC3 and NIST. [PERSON] et al. [19] also pointed out that the cooling methods can affect the residual strength after a fire. [PERSON] et al. [10] examined the effect of temperature on creep deformations in high-strength, low-alloy ASTM A572 steel. They suggested that creep deformations of A572 steel are not significant until 500 \({}^{\circ}\)C and become predominant at temperatures above 500 \({}^{\circ}\)C. They also specified the creep effect on steel members, where creep deformation becomes predominant at temperatures above 500 \({}^{\circ}\)C.  
[PERSON] et al. [20] investigated the reduction in the strength of ultra-high-strength (grade 1200) steel tubes after cooling from fire temperatures of up to 600 \({}^{\circ}\)C, which does not occur to the same extent for HSS and MS. HSS did not experience significant strength reductions after cooling from fire temperatures of up to 600 \({}^{\circ}\)C. The difference in residual strength after cooling of MS, HSS, and UHSS was derived through microstructure observation using SEM. [PERSON] et al. [21] tested ASTM A572 Gr. 50 steel to obtain mechanical property data, including tensile strength (stress-strain response during heating) and residual strength (residual stress-strain response after cooling) evaluations at various temperatures. If A572 steel is cooled after heating up to 600 \({}^{\circ}\)C, it recovers almost 100% of its yield strength at room temperature. The equation of ASTM A572 Gr. 50 was proposed and compared with A992 and S690 steel. However, this study did not provide a comparison with ASTM A572 Gr. 60 steel, and a comparative analysis for mild steel and high-strength steel was not conducted. [PERSON] et al. [12] analyzed the mechanical properties of HSS and UHSS at elevated temperatures and after the cooling state. They also discussed the fact that the most recent codes have conservatively reported on the reductions in the yield strength for MS; however, this is not the case for HSS. [PERSON] et al. [22] examined the mechanical properties of high-strength structural steel (S460N) at elevated temperatures. The recommendations for the mechanical properties of structural steel for current European and American steels were mainly obtained from MS. Compared with the current European, American, Australian, and British steel structure design standards, it was deduced that high-strength SN460N steel is not safe for fire-resistant design. [PERSON]'s study emphasizes the need for more research on the mechanical properties at high temperatures of all high-strength steel grades used in construction. [PERSON] et al. [23] also analyzed the post-fire mechanical properties of high-strength Q690 structural steel. Their study found that high-temperature treatments and cooling methods can significantly affect the yield strength, ultimate strength, and elongation at 500 \({}^{\circ}\)C. A review of the literature indicates that there is a lack of information on HSS at elevated temperatures. Most tests have reported that the mechanical properties are different from those of conventional carbon steels [24; 25]. Moreover, the mechanical properties are mentioned in the MOP Sec.8.2.1.2, ASCE MOP 138 in terms of stress-strain relationships for structural steel at elevated temperatures [1]. It is mentioned that the most important mechanical properties of structural steel all decrease as the temperature increases. Nevertheless, it is not mentioned whether the types of structural steel are mild steel or high-strength steel. In this research, we focus on the difference in the strength reduction factor between mild steel (MS) and high-strength steel (HSS). In addition, ASCE MOP states that caution should be exercised because high-strength steels may experience greater strength loss at elevated temperatures than mild steels [1].