# Mechanical Properties of ASTM A572 Grades 50 and 60 Steels at High Temperatures
## 3 Results and Discussion
### Mechanical Properties
#### 3.1.1 Stress-Strain Curve
60 steel, the stress-strain curves at each target temperature were summarized, as shown in Figure 7. Figure 6(a) shows the stress-strain curve up to the initial 0.05 (5%) strain level. The stress was measured at temperatures higher than the room-temperature strength due to temperature hardening from 200 to 300 \({}^{\circ}\)C. Gr. 60 steels in the room temperature to 300 \({}^{\circ}\)C range were more likely to fail at lower strains (6-8%) than steels at other target temperatures due to their brittle behavior, as shown in Figure 6(b). However, this type of steel exhibited ductile behavior at a target temperature of 400 \({}^{\circ}\)C or higher, with an increase in elongation and the occurrence of ductile fracture. Above 700 \({}^{\circ}\)C, the strain increased at low stress, as shown in Figure 6(b),  
Figure 6: Stress–strain curve for Gr. 50 steel: (a) 0.05 strain level (initial part); (b) 0.2 strain level. for Gr. 60 steel. At 1000 \({}^{\circ}\)C, the deformation of the steel material increased rapidly when the high-temperature extensometer exceeded the limit of the measuring range. Based on the stress-strain curve in Figure 7, the yield strength, tensile strength, and elastic modulus values for the major strain levels are summarized in Table 7. The values in Table 7 were divided by the values at room temperature, which are summarized in the form of a reduction factor of less than 1.0, as listed in Table 8. At 100 \({}^{\circ}\)C, the strength was similar to that at room temperature, but the strength increased in the temperature range from 200 to 300 \({}^{\circ}\)C. As the temperature increased above 400 \({}^{\circ}\)C, there was a gradual decrease in the strength of the steel. The increase in the strength of the steel in the target temperature range from 200 to 300 \({}^{\circ}\)C results in a temperature hardening phenomenon that occurs only in structural steels, which has been reported in several previous studies [31]. As the strain level increased from 0.2% to 20%, there was an increase in strength at each target temperature; however, the strength reduction factor decreased at a strain level of 20%. The strength reduction rate was the smallest at a 0.5% strain level, which was evaluated at a higher strength compared with the other strain levels. The decrease in strength at a 0.2% strain level was up to 10% in the temperature range from 100 to 300 \({}^{\circ}\)C, as listed in Table 8; however, at a 0.5% strain level and tensile strength (\(f_{u}/f_{u,25}\)), the strength was, at most, 9%, an increase of 10%. This may be due to strain hardening. In addition, in Eurocode 1993-1-2 (Annex A) [9], when the temperature of the steel is less than 400 \({}^{\circ}\)C, the tensile strength (ultimate strength) reduction factor is specified to account for strain hardening. At 300 \({}^{\circ}\)C, the tensile strength increased by up to 10%; above 400 \({}^{\circ}\)C, there was a decrease in the tensile strength. The elastic modulus decreased at a target temperature of 100 to 200 \({}^{\circ}\)C, increased again at 300 to 400 \({}^{\circ}\)C, and showed a tendency to decrease above 500 \({}^{\circ}\)C. At 500 \({}^{\circ}\)C, for strain levels of 0.2% and 20%, the strength reduction factor decreased by 41% and 56%, respectively, relative to room temperature, while the elastic modulus reduction factor decreased by 43%. Additionally, at 600 \({}^{\circ}\)C, for strain levels of 0.2% and 20%, the strength reduction factor decreased by 61% and 71%, respectively, relative to room temperature, while the elastic modulus reduction factor decreased by 59%. This shows that, similar to the Gr. 50 steel, the strength reduction rate varies with the strain level.