# Mechanical Properties of ASTM A572 Grades 50 and 60 Steels at High Temperatures
## 4 Conclusions  
This study presents a detailed experimental study on the mechanical properties of ASTM A572 Grade 50 (Gr. 50) and Grade 60 (Gr. 60) steels at high temperatures. In addition, extensive literature data on the yield strength, tensile strength, and elastic modulus of mild steel (MS), high-strength steel (HSS), and very-high-strength steel (VHSS) are provided. Both ASTM A572 Gr. 50 and 60 steels were used in the experimental tests under steady-state fire conditions. The yield strength reduction factor was analyzed by expanding the strain up to 20%. Based on the test and existing literature data, regression equations for yield strength, tensile strength, and elastic modulus were proposed. The test data and the regression equation were compared with the reduction factors suggested by the standards, including Eurocode, AISC, and NIST. Based on the results of this study, the following conclusions can be drawn:  
* When comparing the proposed MS Equations (1)-(3) of the yield strength reduction with the standard (EN 1993-1-2 and AISC 2016), the maximum difference of 24%  
Figure 16: A comparison of the reduction factors for the tensile strength with the literature values and standards. occurred at 400 \({}^{\circ}\)C. On the other hand, the proposed MS Equations (1)-(3) did not differ from the ones of the NIST 1907 (0%). This indicates that the proposed MS Equations (1)-(3) and NIST equations differ from the ones of the standards (EN 1993-1-2 and AISC 2016) based on experimental data of various mild steels. * Eurocode and AISC propose a yield strength reduction factor of 1.0 for mild steel at 400 \({}^{\circ}\)C, but the proposed MS Equations (1)-(3) and the NIST equations indicate a value of 0.76. Although the point at which the strength of steel material rapidly decreases is known to be 538 \({}^{\circ}\)C, this study suggests that the yield strength reduction starts from 400 \({}^{\circ}\)C for MS.
* The proposed Equations (4)-(6) of the yield strength reduction for the HSS at 400 \({}^{\circ}\)C are different from the ones of the standard (EN 1993-1-2, AISC 2016) by 7%, and differ by 16% from the NIST. The MS Equations (1)-(3) showed a difference of 5% from the ones of the NIST equation at 700 \({}^{\circ}\)C. The HSS Equations (4)-(6) were 16% different from the ones of the NIST at 700 \({}^{\circ}\)C.
* These results indicate that the HSS Equations (4)-(6) are closer to the yield strength reduction factor suggested by the standards (Eurocode and AISC) than the MS Equations (1)-(3) at 400 \({}^{\circ}\)C. However, it does not work at 700 \({}^{\circ}\)C such that the yield strength reduction factor of the HSS increases greatly compared with the standards (Eurocode and AISC). * In the case of HSS, it is better to make the yield strength reduction factors lower than the ones of current design standards to ensure a safe design. Moreover, the strength reduction factors of the MS at both 400 \({}^{\circ}\)C and 700 \({}^{\circ}\)C agreed well with the NIST code rather than the standards, including the Eurocode and AISC. * The differences in the elastic modulus reduction factor of the Eurocode and AISC to MS are 30% and 26%, respectively. However, the elastic modulus factors of the NIST are in good agreement under 5% based on the experimental works for both MS and HS. In the case of the elastic modulus reduction factor for the HSS, the decrement difference was approximately 25% from the one of Eurocode. As with the yield strength results, the proposed elastic modulus Equations (7) and (8) were close to the ones of the NIST compared to Eurocode or AISC. * As a result of analyzing the tensile strength, there was a negligible difference in the reduction factor depending on the steel type compared to the yield strength and the elastic modulus. Because the minor difference occurred in the 300-700 \({}^{\circ}\)C range, it is necessary to perform careful investigation for much higher strength levels of steel.