# Mechanical Properties of ASTM A572 Grades 50 and 60 Steels at High Temperatures
## 3 Results and Discussion
### Mechanical Properties
#### 3.1.2 Yield Strength  
Using the 0.2% offset method, previous studies (Table 2) reporting the strength reduction factor and the test data for ASTM A572 Gr. 50 and 60 were compared, as shown in Figure 8a. The maximum difference in the yield strength reduction between the Gr. 50 (black line) and 60 (red line) steels was 0.1 (10%) at a target temperature of 200 \({}^{\circ}\)C. In general, the reduction in the yield strength of the Gr. 50 and 60 steels was similar at the main temperatures of 500 to 600 \({}^{\circ}\)C, where there was a rapid decrease in the strength of the steel material. In addition, the values reported in the literature, listed in Table 2, did not meet the  
Figure 7: Stress–strain curve for Gr. 60 steel: (**a**) 0.05 strain level (initial part); (**b**) 0.2 strain level. standard (Eurocode, AASC) below 400 \({}^{\circ}\)C, as shown in Figure 8a. Therefore, at a 0.2% strain level, for steel materials ranging from room temperature to 400 \({}^{\circ}\)C, engineers should consider the value of the yield strength reduction factor. When comparing the NIST regression equation (green dashed line) with the Gr. 50 and 60 test lines, a similar strength reduction factor was obtained up to 400 \({}^{\circ}\)C, but differences occurred between 500 and 700 \({}^{\circ}\)C. As a result of analyzing the experimental results and the literature (Table 2), the 0.2% strain level yield strength reduction factor should be divided based on the target temperature of 400 \({}^{\circ}\)C. At a 0.5% strain level, previous studies (Table 2) on the strength reduction factor and test data for the 0.5% strain level for ASTM A572 Gr. 50 and 60 were compared, as shown in Figure 8b. The graphs of Gr. 50 (black line) and A572 Gr. 50\(\_\)Kodur (Reg.) below a target temperature of 400 \({}^{\circ}\)C show a similar strength reduction even at a strain level of 0.5%; however, a difference occurred between 500 and 700 \({}^{\circ}\)C. The strength of Gr. 60 (red line) increased due to temperature strain hardening from 100 to 300 \({}^{\circ}\)C. As with the 0.2% strain level, Figure 8b shows that the distribution of data is lower than the reference for a target temperature of 400 \({}^{\circ}\)C. The NIST regression equation (green dashed line) and Gr. 50 (black line) showed a similar decrease in the strength below a target temperature of 400 \({}^{\circ}\)C, but both were different with respect to Gr. 60 (red line). The Gr. 50 and 60 test lines and EN1993-1-2, AASC, and NIST graphs showed similar strength reductions above a target temperature of 500 \({}^{\circ}\)C. Therefore, as the reduction in the yield strength is different based on 400 \({}^{\circ}\)C, a separate regression equation should be proposed. In addition, the range of the strain level at 1.0 to 2.0% is the plastic flow region of the steel material, where there is no significant difference because the steel has a constant strength regardless of the strain level (see Figures A2-A4). The regression equations from the literature (Table 2) and the test data for ASTM A572 Gr. 50 and 60 were expanded to a 20% strain level and compared, as shown in Figure 8c. In EN1993-1-2, the stress-strain curve of steel is presented up to 20% (0.2) to obtain a comparison. However, as there are no previous studies analyzing the strength reduction factor at the 20% strain level, the equations were compared using the ASTM A572 Gr. 50 and 60 test data and regression equations from Eurocode, AASC, NIST, and the existing literature (Table 2). As shown in Figure 8c, the strength of the Gr. 60 (red line) steel increased due to temperature strain hardening at a target temperature of 300 \({}^{\circ}\)C. Above a target temperature of 400 \({}^{\circ}\)C, the strengths of both the ASTM A572 Gr. 50 and 60 steels decreased sharply from 500 \({}^{\circ}\)C. In addition, the two steels were significantly lower in strength than the EN1993-1-2, AASC, and NIST. These results show that the strength of steel can be significantly lowered at a high strain level (20%).